ITTO940792A1 - ADHESIVE COMPOSITION, RELATED PROCEDURE AND EQUIPMENT. - Google Patents

Abstract

The invention relates to threads or strips formed by a hot melt elastomeric adhesive composition, comprising at least one thermoplastic elastomer and at least one tackifier resin, the thermoplastic elastomer (elastomers) being a styrene / butadiene / styrene block copolymer (SBS ) or a mixture of a styrene / butadiene / styrene copolymer with a styrene / isoprene / styrene block copolymer (SIS) in which SIS is present in an amount equal to or less than 50% by weight of the total of the block copolymer, and in to which the adhesive composition: a) is able to bind, when applied in the molten state, to plastic and or cellulosic materials, with a peel at 90 ° of not less than 0.5 N / cm; b) has a retention of resistance at traction, after 50 cycles, of at least 40%; c) has a viscosity of 120,000 cps or less at 180 ° C and under an applied shear stress of 80 sec-1. The invention also refers to a process for the manufacture of threads or strips and an apparatus for applying the hot melt elastomeric adhesive to a raw material according to a non-linear arrangement (Figure 1).

Description

DESCRIPTION of the industrial invention entitled: "Adhesive composition, related process and equipment",

DESCRIPTION

The present invention relates to an adhesive composition. More particularly, the invention refers to wires and strips formed by a hot melt adhesive with elastic properties in the solid state, but which has a low and in many cases negligible elasticity in the molten state, as shown by its essentially Newtonian behavior at temperature. processing (application). The invention also relates to a process for the production of such threads and strips and to an apparatus for the production of such threads and strips.

In the production of many different types of articles it is necessary to attach an elastic material to a non-elastic substrate. An example is the production of disposable panty diapers, in which elastic strips are used for elasticization around the legs and waist. Typically! the elastic strips consist of rubber bands fixed to the body of the diaper panties by layers of adhesive, eg hot melt adhesive, applied to both surfaces. "

This arrangement involves a number of drawbacks. For example, the elastic strip is often coated with an anti-blocking agent such as talc during manufacture, to prevent the strip from sticking to itself. However, this anti-blocking agent can in turn make it much more difficult to fix the elastic strip to the body of the article. to "affect" the elongation of that part of the elastic to which it is applied. Finally, the use of two materials, rubber and adhesive, is expensive.

A composition which has adhesive properties when applied in the molten state and preferably also has pressure sensitive adhesive properties, and the elasticity of the rubber, while possessing rheological properties suitable for melt processing, is highly desirable as it could replace the rubber. rubber band and adhesive currently employed and would represent a considerable advance in the manufacture of absorbent articles such as panty diapers. Such a composition would also find numerous other applications in many different fields. For many applications, both in the field of absorbent articles and elsewhere, it is necessary that the elastic adhesive be present in the form of threads or strips.

There are many types of compositions on the market such as hot melt adhesives and a much wider range of compositions have been suggested for use in this field. A hot melt adhesive can be defined as a composition which exhibits adhesive properties when applied to a substrate in the molten state. This does not preclude that the composition exhibits pure adhesive properties at room temperature, e.g. pressure adhesive properties. In general, hot melt adhesives are formulated for properties such as adhesion to a variety of surfaces, heat stability and workability properties rather than elasticity, and any attempt to increase elasticity has a negative effect on adhesion or other properties. desirable properties of the composition. So far, none of the hot melt adhesive compositions on the market combine good adhesion with elastic properties comparable to those of rubber.

Natural rubber is generally too viscous to be used as a base for hot melt adhesives and vulcanized rubber cannot be melted. Many hot melt adhesives are based on block thermoplastic elastomers as these can be melted. Many of these elastomers, with a variety of properties, are commercially available. For use as an adhesive, thermoplastic block elastomers must generally be mixed with tackifier resin-type agents to improve adhesion. Other additives such as plasticizers may also be needed, depending on the application.

Certain prior art proposals have attempted to provide hot melt adhesives which also have elastic properties. For example, US-A-4418 123 seeks to provide a self-adhesive rubber band having a combination of elastic and adhesive properties. The composition is broadly defined as a combination of a block copolymer comprising at least one substantially amorphous rubbery polymeric middle block and at least two poly (vinyl arene) glassy end blocks, together with a resin compatible with the middle block and a compatible resin. with terminal blocks. All specific examples (except one comparison example with unsatisfactory properties) are based on styrene-isoprene-styrene block copolymers. Notwithstanding the claims submitted for these, to the best of the present applicants' knowledge, none of the compositions exemplified in US-A-4 418 123 exhibit genuine elastic properties comparable to those of rubber, combined with good adhesion and workability (see example comparison A below).

Similarly EP-A-0 424 295 (HB Fuller France SARL) refers to a thermoplastic elastic particularly intended for the elasticization of panty diapers, which includes:

a) at least one synthetic rubber which is of the block copolymer type, comprising at least one rubbery middle block and at least two glassy end blocks;

b) from 20 to 150% by weight with respect to the block copolymer, of at least one tackifier-type resin, which is compatible with the median block of the copolymer;

c) from 10 to 50% by weight with respect to the block copolymer of at least one tackifier-type resin which is compatible with the end blocks of the copolymer; And

d) from 5 to 35% by weight with respect to the composition, of a mineral oil.

The exemplified compositions in EP-A-0 424 295 are generally based on SIS (styrene-isoprenestyrene) and an example of a composition based on SBS (styrene-butadiene-styrene) shows unsatisfactory properties (see comparative example B below ).

It has now been found that, with a careful selection of the components, it is possible to produce hot melt adhesives having the desirable combination of properties previously indicated, i.e. good adhesion in the molten state (and in many embodiments also at room temperature), elastic properties comparable with those of the rubber and good machinability in the molten state. It has also been found that these adhesives can be made in the form of threads or strips.

The present invention provides threads and strips that are formed from a hot melt elastomeric adhesive composition comprising at least one thermoplastic elastomer and at least one tackifier-type resin, the thermoplastic elastomer (s) being a styrene / butadiene / styrene (SBS) block copolymer or a mixture of styrene / butadiene / styrene with styrene / isoprene / styrene block copolymer (SIS), in which SIS is present in an amount equal to or less than 50% by weight of the total block copolymer, and in which the composition adhesive is characterized by the fact that:

a) is able to bind, when applied in the molten state, to plastic and / or cellulosic materials with a peel force under a 90 ° angle of not less than 0.5 N / cm (as defined here);

b) maintains a tensile strength, after 50 cycles (as defined here), of at least 40%; And

c) has a viscosity of 120,000 cps or less at 180 ° C, and under a shear stress of 80 sec. hot melt adhesive which is capable of bonding to appropriate substrates, typically plastic and / or cellulosic materials, when applied in the molten state. In particular, "capable of binding" indicates that the composition is capable of exhibiting adhesion to plastic and / or cellulosic materials sufficient especially for use in the manufacture of absorbent sanitary articles. When applied in the molten state between two substrates of plastic and / or cellulosic materials, the composition forms a bond with a strength, measured as a peel at 90 °, of not less than 0.5 N / cm. The composition, in a grammage of 5 g / m, is applied between the substrates and 48 hours after the formation of the bond, the resistance to peel is measured under a release angle of 90 ° at 23 ° C and with a separation speed of 300 mm / minute.

As described in more detail below, many compositions such as those described herein also bond to suitable substrates at room temperature, and may exhibit the properties of a pressure sensitive adhesive.

The previously indicated characteristic b) refers to the elastic properties of the composition. The test that is used is described in more detail below and involves measuring the point up to which the elastic properties are maintained after 50 elongation and release cycles. The percentage of elongation to which the composition is subjected relates to the modulus of the composition and the foreseeable degree of elongation of the composition during use. The figure of 40% for maintaining the tensile strength indicates that the elastic properties of the composition are comparable with those of natural rubber and are preferably superior to these. Preferably the retention for the tensile strength is at least 50%, more preferably at least 60%.

The above characteristic c) refers to the workability of the composition and a viscosity of 120,000 cps or less at 180 ° C (shear stress 80 sec X) and indicates that the composition can be applied with conventional equipment used for hot melt adhesives. . Preferably the viscosity is 60,000 cps or less, more preferably 30,000 cps or less. It is also highly desirable that the composition according to the invention exhibit a substantially Newtonian rheological behavior, in particular the viscosity must not vary significantly as the shear stress varies. As discussed in more detail below, many compositions such as those defined here show Newtonian behavior at the expected processing temperatures, i.e. around 180 ° C.

The compositions as defined herein can be formulated with any desired form, depending on the desired end use. However, the modulus has influence on the main properties of the composition and it is convenient to divide the compositions of the invention into low modulus compositions and high modulus compositions.

Low modulus compositions are defined as compositions having a modulus of 0.5 MPa or less at an elongation of 500% (six times the initial sample length), measured at 23 ° C with an elongation rate of 500 mm / minute . Generally, the low modulus compositions have a modulus of between 0.05 and 0.5 MPa, preferably between 0.05 and 0.3 MPa. Low modulus compositions generally exhibit good adhesive properties at room temperature and can also be pressure sensitive adhesives. These compositions are normally drawn immediately after their formation, e.g. after extrusion from the melt as a yarn or strip. Ironing can occur immediately before or during application to an article, so that the compositions are effectively applied in the ironed state. The low modulus compositions are typically used with an elongation between 400% and 1000%.

Since low modulus compositions will normally be stretched immediately after extrusion, it is desirable that they have a relatively high solidification point, so that they solidify quickly after extrusion. Preferably the solidification point (measured with the Dynamic-Mechanical Analysis method described in more detail below) is at least 80 ° C, more preferably at least 100 ° C.

High modulus compositions are defined as compositions having a modulus greater than 0.5 MPa at an elongation of 500% (six times the initial sample length) measured at 23 ° C with an elongation rate of 500 mm / minute. Preferably the high modulus compositions have a modulus comprised between 1 MPa and 10 MPa. Since the pressure sensitive adhesive character is generally inversely proportional to modulus, high modulus compositions are often applied in the molten state, although some may retain a pressure sensitive adhesive character sufficient to allow application at room temperature. Molten application implies that the composition is applied without prior elongation, said elongation generally occurring at the time of use, and this is particularly true for compositions with a modulus of 1 MPa or higher. For this reason, the high solidification temperature is less critical for high modulus compositions, but for convenience these are also preferably formulated to have a solidification point of at least 80 ° C, more preferably at least 100 ° C. High modulus compositions are generally used with a lower degree of ironing. They are capable of providing sufficient spring force with low deformation (typically no more than 50%). However, in cases where the compositions retain a sufficient pressure-sensitive adhesive character, so that they can be applied at room temperature in the stretched state, they can be used with an elongation of up to 400%.

The essential components of the composition, as defined herein, for the production of threads and strips, are a thermoplastic elastomer and a tackifier resin, and these will be described in general terms.

Thermoplastic elastomers are a very interesting chemical and technological class of polymers which are distinguished by their behavioral characteristics. At room temperature they behave like vulcanized rubbers, showing a high elasticity but, unlike vulcanized rubbers / they can be melted and reworked in the same way as thermoplastic materials.

This behavior is determined by a particular chemical structure. Most thermoplastic elastomers are made up of block copolymers, i.e. their molecules are formed by blocks of different nature bound together. Different blocks can alternate along the chain as relatively short blocks (multi-block structure of the type A-B-A-B-A, etc.); or the molecules can have a type A-B-A triblock structure, in which A are the terminal blocks and B is a central block of a different nature (linear triblock copolymers); or the molecules can have a "radial" or "stellar" structure represented as (AB) X, in which all the median blocks B are chemically linked to each other and at a central point, and the terminal blocks A are each arranged radially all the way end of a block B. Structures formed by only two blocks (diblock) of the AB type are ineffective as thermoplastic elastomers due to their elastic behavior.

The chemical nature of the various blocks can be different and the resulting copolymers can be classified, for example, as polyurethanes, polyesters, polyethers, poly-ether-esters amides, etc. However, a common feature is the following: the different blocks are physically incompatible as they are mutually insoluble. The material can thus be considered a heterogeneous system in which different blocks, even if chemically linked in the same molecule, exist as separate entities. Blocks A of different molecules tend to associate with each other in microscopic areas or "domains" and the same happens for blocks B. The material thus formed has a heterogeneous structure of domains A and B, each well separated, with the one present in smaller quantity dispersed microscopically in the other which constitutes a continuous phase. This continuous phase is generally formed by "soft" or rubbery blocks B, which give the material its elastic properties, while the dispersed phase A is formed by "rigid" non-elastomeric blocks. Below the glass transition temperature or the softening point of the rigid blocks, each molecule of the copolymer has its A blocks fixed in at least two points, ie "confined" in the rigid domains. Consequently, the rubbery part of the molecule can withstand an elongation but without sliding with respect to the other molecules, and when the external stretching force is released it returns to its initial position due to the entropic effect.

Thus, in thermoplastic elastomers, the rigid blocks operate as a physical vulcanization, and the advantages of this situation are clear. The chemical bonds that form the vulcanized structure of a standard rubber cannot be destroyed by heating, and at a sufficiently high temperature the rubber simply begins to decompose. On the other hand, in thermoplastic elastomers, heat can actually melt the rigid domains; the material can thus be melted and processed, but the rigid domains that give rise to the pseudovulcanization are reformed again by simple cooling of the material. From the explanation it is obvious that diblocks, which contain only one hard and one soft block, cannot contribute to the elastic properties.

The diblocks can improve workability, but their content in the material must be limited within certain values, in order not to reduce the elasticity in an unacceptable way. Also, the total amount of hard blocks is important; too low a content will impart modest elastic properties (similar to those of insufficiently vulcanized rubber), while too high a content will cause the material to behave like a very hard and super vulcanized rubber, even so with modest elasticity.

Among the block thermoplastic elastomeric copolymers, the so-called styrenic block copolymers (SBC) are well known and widely used in many uses, due to their excellent properties. Styrene block copolymers, as a class, are described e.g. in Thermoplastic Elastomers: A Comprehensive Review, Legge, Holder & Schroeder (Ed.), Hauser Publishers (1987), Chapters 3, 4 and 12 (1). They may have the structures already mentioned, such as:

- multiblock A-B-A-B-A-B- ... etc.

- linear triblock A-B-A

- radial or "star" polymers (AB) X, where x> 2.

A represents a "rigid" block of a polymerized vinylarene monomer, generally styrene or alpha-methyl-styrene; and B represents a "soft intermediate block" generally formed from a rubbery monomer such as polybutadiene, polyisoprene, polyethylene-butylene or polyethylene-propylene.

The content of diblock molecules A-B in such materials can reach up to 80% by weight and in special commercial products it can also constitute the whole of the polymer. The latter materials are used for particular applications since, for the reasons indicated above, they do not have elastic properties or they have very modest them. Diblocks can contribute to workability and improve adhesion properties, but to maintain good elastic characteristics, their content in the thermoplastic elastomeric block copolymer must be kept below 40% by weight.

SBCs are widely used as substitutes for vulcanized rubbers, being their hardness, modulus and general mechanical and elastic properties strongly correlated with the content of rigid blocks, especially formed from polystyrene. They have also found use as base polymers for hot melt adhesives, for their generally good mechanical characteristics, for the ease of developing adhesive properties in their rubbery middle blocks and for the good thermal stability, which makes them superior to traditional bases. for hot melts such as ethylene-vinyl acetate copolymers.

However, the main purpose of the standard compositions was to optimize the adhesive properties without taking into account the maintenance of at least part of the elastic properties typical of the base polymer.

The block elastomeric thermoplastic copolymers known as SBC typically have the following characteristics:

- They are formed by two species of monomers, each polymerized in blocks of the same monomer units, the blocks being distinct even if chemically bonded within the copolymer molecule. Furthermore, the two types of blocks must be mutually incompatible.

- The structure according to which the two types of blocks present are connected in the molecule can be:

- alternating multi-block, such as ... A-B-A-B-A-B

- linear triblock, such as A-B-A

- radial or stellar structure, such as (A-B) x, where x> di 2.

- "A" represents blocks of a polymer derived from a vinyl-arene monomer, typically styrene or alpha-methyl-styrene. They are defined as rigid blocks because at room temperature these types of polymers are rigid, glassy and brittle materials, being below their glass transition temperature (Tg).

Typically useful constituents for rigid blocks have Tg well above ambient temperature, and preferably above 90 ° C.

- "B" represents blocks of a rubbery polymer having a Tg <0 ° C and preferably <-40 ° C.

Typically, these "soft" blocks are formed from rubbers such as polybutadiene and polyisoprene.

In the common technological lexicon, the resulting block thermoplastic elastomeric copolymers are often referred to by the abbreviations SBS and SIS, respectively. As already mentioned with reference to the mechanism of the generation of elastic properties in these types of polymers, and particularly about the function of the rigid blocks in imparting a physical vulcanization to the polymer, useful SBCs contain at least two rigid "A" blocks per molecule and at least one soft block "B". The molecules formed by a block of A and a block of B (the so-called diblocks) should, for use in the present invention, be kept below 40% by weight in the polymer of base.

From the literature it is widely known that there are differences between SIS copolymers and SBS copolymers, which are relevant in the formulation of hot melt adhesives. SBS copolymers generally cost less than similar SIS copolymers, and SBS copolymers can be synthesized to exhibit better elasticity than similar SIS copolymers. However, it has not been possible so far to take advantage of these potentially advantageous properties of SBS copolymers, as a result of the fact that SBS copolymers have generally not shown adequate adhesive properties and SIS copolymers are much more easily modifiable to develop adhesive properties. For this reason, hot melt adhesives have generally been formulated using SIS copolymers as the predominant SBC. Both US-A-4418123 and EP-A-0424295, discussed earlier, clearly prefer SIS, as the SBC on which the compositions are based, and neither document describes an SBS-based composition that has satisfactory properties.

According to the present invention it has been found that it is possible to produce compositions in which the SBS copolymers are the main styrenic block copolymer (s), with satisfactory adhesive properties. At the same time these compositions maintain the typical advantages of SBS copolymers as regards elasticity, which have been mentioned previously. Therefore, compared to compositions based on other SBCs, compositions according to the present invention can be formulated with better elastic properties, faster elastic recovery, flatter stress / strain diagrams even with elongations greater than 1000% and compositions with better workability (rheological behavior more Newtonian). However, it should be noted that a direct comparison between SIS and SBS-based compositions is very difficult, since the compositions have to be formulated differently. Consequently, it will generally not be possible to simply replace an SBS copolymer with a SIS copolymer in a hot melt composition and achieve satisfactory properties, and other adjustments in the formulation will have to be made depending on the nature of the SBC, to achieve optimal results. The way in which compositions according to the invention can be formulated to obtain the desired properties is described in more detail below.

Therefore, the preferred compositions in the form of threads and strips according to the invention are based on SBS copolymers or on a mixture of SBS / SIS, in which SIS is present in an amount equal to or less than 50% by weight of the block copolymer total.

All thermoplastic elastomers can be processed in the molten state by adopting various technologies, various equipment, showing in any case in the solid state properties similar to vulcanized rubber. Virtually all thermoplastic elastomers can be made adhesive. Some adhere quite well in the molten state to different substrates. However, it is clearly very desirable to obtain thermoplastic elastomers which are capable of adhering at room or only moderately elevated temperatures to various substrates.

Thus, while pure thermoplastic elastomers have a certain adhesiveness at high temperatures, this adhesiveness can be conveniently improved both in terms of the strength of the bonds formed with various substrates and in terms of the temperature range within which the bonds are formed.

This improvement is achieved with the use of at least one suitable tackifier resin. More particularly, much better adhesive properties and also adhesive properties at room temperature (pressure tack behavior) can be obtained by mixing thermoplastic elastomers with materials known as tackifier resins which, as a class, are well known in the literature.

When using thermoplastic elastomers at room temperature (e.g. because you want to pre-stretch them in the solid state and fix them under tension), they need to exhibit the behavior typical of true pressure sensitive adhesives, and this generally requires a blend of a thermoplastic elastomer in blocks and a tackifier-type resin. It can be noted that, in order to improve the adhesive properties of the thermoplastic elastomer (both at high temperature and at room temperature), only the soft (rubbery) blocks of its molecule must be modified by means of the tackifier-type resin. Therefore, only the interactions between the soft (rubbery) blocks and a resin, substantially compatible with these, causes the generation of adhesiveness; while any modification of the rigid blocks with a resin never leads to the development of an adhesive behavior.

Not only do the rigid blocks have no adhesive activation, but their eventual modification by means of a tackifier-type resin can "soften" their mechanical strength. This risks compromising their ability to function as "physical vulcanization centers" for the elastomer, thereby destroying the elastic behavior.

Therefore, for the various thermoplastic block elastomers, depending on the chemical nature of their soft and rigid blocks, suitable tackifier-type resins can be identified which must be compatible (i.e. soluble and capable of creating the appropriate physical modification of the system) only with soft or rubbery blocks, while compatibility with rigid blocks must be as low as possible or even zero, to maintain the primary elastic properties of the polymer as much as possible. However, the amount of tackifier type resin must be controlled, as the addition of too large amount of tackifier type resin (or resins), even if the tackifier type resin is only compatible with middle blocks (soft blocks) and totally incompatible with rigid blocks, it could worsen the elastic properties of the resulting formulation. In any case, the addition of the resin constitutes a dilution of the concentration of the rigid block domains, weakening their ability to function as "physical crosslinking" centers for the elastomer. Therefore, both the content of rigid domains in the basic block thermoplastic elastomer and the content of elastomer in the final formulation must be such as to ensure in the formulation a final concentration of rigid domains sufficient to maintain an adequate value of "physical vulcanization" and therefore of elastic properties.

Therefore, on the one hand it is important to control the final concentration of rigid blocks in the composition, and on the other hand the addition of resin (or resins) compatible only with the rigid blocks and their domains, is completely ineffective in developing and / or improve adhesive properties. Resins compatible with rigid blocks will, by swelling the rigid domains, stiffen the composition, increase the modulus and (compared to similar contents of tackifier-type resins compatible with the intermediate block) will tend to increase viscosity. In a system that already contains a tackifier-type resin compatible with soft domains, adding resins compatible with hard domains will also decrease adhesiveness. Consequently, in general terms, only limited quantities of resins compatible with the "rigid blocks" can be used without worsening too much the general properties of the elastic hot melt adhesive. Generally, these will only be used in special cases, eg if an additional modulus increment is needed for certain applications; or (using resins compatible with rigid blocks with a high softening point) if a higher solidification temperature or better temperature resistance is desired.

The compositions thus defined in the form of threads or strips can be used to elasticise structures in which they are applied without the use of any glue, for example structures whose elasticization has been conventionally obtained with an elastic formed from vulcanized rubber. A single material (the hot melt elastic adhesive) can replace the use of two materials (the rubber elastic and the glue for its fixing) with a substantial cost saving. Normally rubber bands are covered with talc to prevent the bands from sticking to themselves in the package. Talc can generate problems in the adhesion phase with glue.

Furthermore, the elastic and thermoplastic hot melt adhesive can be extruded directly into various geometric shapes directly during the manufacture of the products to be elasticized, and the appropriate geometric shape depends on the nature of the product and / or the function of the hot melt composition in that product. As stated above, according to the invention the composition can be extruded in the form of threads or strips. The wires are generally extruded from nozzles having a round cross-section, preferably nozzles with a circular cross-section having a diameter ranging, for example, between about 0.4 mm and up to about 3 mm or more. A certain number of wires can then be extruded at the same time from separate nozzles and, by rotating the entire nozzle head with respect to a common axis, the wires can be collected in the form of a strand. The strips are generally obtained by extrusion from a slit cut nozzle. The nozzle head can contain one or more blades, and strips of different sizes can be obtained by varying the position of the blade (s) in the head.

Besides being able to be applied along linear traces (straight lines), the elasticization can also be applied according to non-linear geometric shapes (curves) which make the anatomical adaptability of the product, including the elasticization, to the user's body particularly good. As explained in more detail below, this is very difficult to achieve with standard rubber threads or bands. The various shapes (threads and strips) in which the hot melt elastic adhesives can be extruded depend on the different products to be manufactured and on the type of elasticization desired. Thus, for example, the strips are suitable for elasticizing the waist and legs of panty diapers (linear path) while threads can be used for application along curved paths.

Under different geometric shapes, the hot melt elastic adhesive can be applied both in the already stretched state and in the unstretched state. In the first case, the extruded melt is cooled immediately after the extrusion die and stretched to the desired elongation. In this case it should have the following properties:

- a relatively high solidification point so that it solidifies immediately after extrusion and can be stretched elastically. An elastic stretch can only be imparted to a solid material, since any force applied to a molten or semi-solid material will only cause a plastic stretch in the direction of the force, without any elastic tension.

- Good pressure adhesion properties, as the adhesive elastic hot melt contacts the substrate (substrates) when it is already cold, i.e. at room temperature.

When the material is applied without any prior elastic stretching, and brought directly into contact with the substrate (s) to which it must adhere at the exit of the extrusion die, the behavior of the pressure adhesive is less important, since the fixing is done when the material is still at a higher temperature than ambient.

All these characteristics are particularly suitable for the elasticization of absorbent sanitary articles, although the potential of the materials according to the invention are clearly not limited to these applications. The use of materials applied in an already stretched condition can replace all the rubber elastics currently used in baby and adult diapers, pads, etc., when it is desired to have parts of the product already under elastic tension that offer, as a result of the their extreme versatility, the possibility of new stretch structures of practically infinite variety. In this case, another advantage of hot melt elastic adhesives over rubber bands is noteworthy. As will be described in more detail below, the preferred hot melt elastic compositions have a stress / strain diagram that is much flatter than that of a rubber band, i.e., even if the material is already under tension, further elongation (p eg due to the movements of the user of the absorbent article) causes a very low increase in modulus and tensile strength which is felt by the user. This is especially true for low modulus compositions.

Compositions such as those defined herein, which are most conveniently applied in the unstretched state, are typically employed to impart springback to structures or products only when the entire final structure or product is subjected to some deformation in use. Normally, in this case, the typical deformations that are imparted during use to an absorbent are very limited, e.g. of the order of 5-50%. It is therefore necessary that the hot melt elastic adhesive contained in these structures is able to respond with a sufficient elastic return force to external deformations even at these low elongations. For these applications, it will generally be more convenient to employ higher modulus formulations.

In summary, the use of block thermoplastic elastomers, in different physical forms and with different application processes, for the elasticization of structures and particularly of hygienic articles, is very advantageous.

The adhesive compositions used according to the present invention to form threads or strips form the subject of our application filed by us on the same date entitled "Adhesive composition". These compositions in the form of foams and processes for their preparation form the subject of an application filed by us on the same date, entitled "Adhesive composition and related preparation process".

The use of adhesive compositions in the elasticization of absorbent articles, such as nappies, panties and absorbent pads, is the subject of our application filed by us on the same date entitled "Absorbent article".

The formulations used here have optimal properties, typical of hot melts, ranging from compositions which can more conveniently be strongly fixed to high temperature substrates between the melting point and about 50 ° C, to compositions which maintain a strong permanent adhesiveness on the surface. most substrates even at room temperature, being authentic pressure adhesives. Furthermore, the compositions are characterized by the fact that they maintain excellent elastic properties deriving from the basic thermoplastic block copolymer, showing all the typical behaviors that define an elastomeric material in the technological sense.

Thus, when they are stretched to the solid state and when the tension is released, they quickly return to their initial length with only minimal permanent (plastic) deformation. The formulations preferably having an evident pressure adhesive character, can also be applied at room temperature for elasticization, both in the unstretched state or preferably in the stretched state, under different geometric shapes in various articles, in particular absorbent hygienic products, such as panty diapers for children and adults or adult incontinence products other than panty or sanitary napkins for female use. The compositions which have lower pressure adhesive characteristics will be more conveniently applied at temperatures above 50 ° C, in the stretched state or preferably in the unstretched state, for the elasticization of the same structures or products. Particularly when applied in the unstretched state, they will work under limited elongation, e.g. up to 50% (i.e. final stretched length = 1.5 times the initial length).

To show even at these low elongations a distinct springback strength, these formulations will generally have a higher modulus than the former, the two types of materials being, in fact, the extremes of a range of formulations which are all excellent hot melt adhesives and they maintain excellent elastic properties, the transition from one to the other being gradual.

As already mentioned, the compositions of the type described here can be divided into "low modulus" compositions and "high modulus" compositions, this distinction being based on the value of their modulus and on the good or modest.

In this way the present invention makes use of a family of compositions based on at least one elastic thermoplastic block copolymer in which the SBS copolymers and the main copolymer (copolymers) and at least one tackifier-type resin essentially compatible with the soft (rubbery) blocks of the aforesaid copolymer, the tackifier-type resin being mainly used to improve the adhesiveness, both at high temperature and at room temperature of the aforesaid copolymer, can be processed to obtain threads or strips. The compositions are extrudable and in the solid state they maintain a distinct elastic behavior typical of the elastomers from which they are derived. These compositions can be extruded and applied in the form of threads and strips. As examples of the applications in the field of absorbent articles, they can be used for the elasticization around the legs of the panty diapers, as an elastic waist band in the same, for the elasticization of sanitary pads and incontinent adult products other than panty diapers, for the internal elasticization of the structures of all the aforementioned products, as a reinforcement under mechanical stress of their absorbent cores and to impart their stretch and elasticity, etc.

A lower modulus generally means that adhesive materials have higher tack, so lower modulus formulations generally have a higher tack, which is typical of pressure adhesives. They are able to form very strong bonds with many substrates by simple contact even at room temperature, or in any case below 50 ° C.

The ratios between hard and soft blocks in the basic thermoplastic elastomeric copolymer are very important in determining the elastic behavior and the mechanical and adhesive properties.

Generally, the higher the content of rigid blocks (which will conventionally be referred to as "styrene content") the higher the modulus, the more evident the elastic properties, the faster the springback after the release of the tension, but as much the lower the adhesiveness and especially the behavior as a pressure adhesive.All this is true as long as the content of rigid blocks does not become so high as to constitute a continuous phase and the material becomes a rigid and no longer elastic material.

Useful SBCs may contain 10 to 50% by weight of styrene. However, when they are modified with the tackifier type resin, the behavior of the resulting composition will be clearly regulated, in terms of all the above properties, by the resulting styrene content, that is, by that of the rigid blocks in the composition; whereby it is determined both by the amount of styrene in the base copolymer (s) and by the content of copolymer (copolymers) in the final composition. Too low block styrene content in the final material will impart modest elastic properties. Too high a content will increase modulus and decrease adhesiveness to an unacceptable level. Increasing the final styrene content in the composition with increasing copolymer content (copolymers) will excessively increase viscosity and decrease workability. Therefore, both the copolymer content (copolymers) in the composition and their styrene content must be chosen so as to optimize the final styrene content, and therefore all the above properties. The optimal ranges will be indicated below.

If desired, the rubbery part of the SBC can also be vulcanized (similar to the vulcanization of natural or synthetic rubbers) using suitable chemical or physical means, in particular known vulcanization systems for synthetic rubbers, which are not activated. from the heat. This will have the effect of increasing the modulus of the final composition.

The tackifier-type resin is mainly added to improve the adhesive properties of the base copolymer (s), even down to the typical behavior of a pressure-sensitive adhesive. Furthermore, it improves the workability of the thermoplastic elastomer, imparting to the composition both lower absolute values of viscosity (compared to the pure block copolymer) and a rheological behavior which, at the indicated resin contents, is practically Newtonian, i.e. such that the viscosity depends only by the temperature and does not change with the applied stress, a property that is very advantageous for the ease of processing. It is known that SBCs, which have several very interesting characteristics, can be difficult to process, as a result of non-Newtonian behavior in the melt state as pure products. This means that not only do they have a very high viscosity but also that, under the influence of temperature alone, they do not even appear to melt even at very high temperatures, close to 200 ° C. They may also begin to thermally decompose before showing a distinct fluid state. To make them flow, and therefore to be able to work them, it is necessary to apply high temperatures and strong mechanical stresses. In any case, the processing of pure SBC is difficult, the viscosities are high and strongly dependent on the combination of temperature and applied stresses.

The basic compositions of SBC and tackifier-type resin are capable of producing materials which, while maintaining very good elastic and adhesive properties, are also easily workable due to their relatively low viscosity and practically Newtonian (or acceptably similar to Newtonian) rheological behavior. ). The latter property is measured as a constant temperature (180 ° C) variation in viscosity by applying two shear stresses, 20 and 80 sec-1. The ratio of these two viscosities is defined in the following "Newtonian Index" (N.I.).

An ideal Newtonian fluid would have N.I. = 1, while a pure SBC can have, under the same conditions, a N.I. also> 6; that is, the change in viscosity, due only to the change in shear stress from 20 to 80 sec-1, is more than six times, which can cause severe problems for easy and regular workability. For good workability, it is necessary that the compositions have, at constant temperature, only limited viscosity variations with the variation of the applied shear stress.

More particularly, it is preferred that the compositions exhibit a Newtonian or almost Newtonian rheological behavior referred to the molten material at 180 ° C, by comparison of the viscosities under a shear stress of 20 and 80 sec. The preferred compositions do not show viscosity variations higher than 50%, i.e. a viscosity ratio (N.I.) not higher than 1.5.

It has been found that the most preferred SBS-based compositions have an almost ideal Newtonian behavior, with an N.I. not exceeding 1.05.

In order to sufficiently maintain the elastic properties of the base polymer it is necessary that the tackifier-type resin is compatible mainly and preferably essentially only with the soft, rubbery blocks of the block copolymer and does not significantly interfere with the rigid blocks. This depends on both the chemical nature of the resin and its molecular weight. The compatibility of the resin with rubber blocks and its incompatibility with hard blocks can be measured, eg, by determining the variation in Tg of soft blocks and hard blocks that results from the addition of resin. In particular, the incompatibility with rigid blocks is considered satisfactory if their Tg (originally at 100 ° C if formed from polystyrene) does not change by more than 15 ° C by adding 100 parts of resin to 100 parts of copolymer. The measurements of the two Tgs require adequate equipment. Therefore, both the experience of the formulators and the technical literature of the resin suppliers must be taken into account to determine which tackifier-type resins are chemically compatible with the soft blocks and incompatible with the rigid blocks of SBC. As used herein, the expression "essentially compatible with soft blocks only" indicates that a tackifier-type resin is compatible with soft copolymer blocks and is incompatible with hard blocks to the extent that the Tg of hard blocks does not is changed significantly and more preferably is not lowered by more than 15 ° C by mixing 100 parts of tackifier resin with 100 parts of copolymer Preferably the Tg of the rigid blocks does not decrease at all.

More specifically, a suitable primary tackifier type resin will be chosen from the following chemical groups which have high compatibility with SBC's soft blocks and low or no compatibility with their hard blocks:

- hydrocarbon resins

- aliphatic resins

- polyiterpene resins

- phenol-terpene resins

- C5 synthetic resins

- synthetic resins C5 / C9

- rosin and rosin esters

as well as their totally or partially hydrogenated derivatives. They can be used as pure resin or also in mixture.

When more than one resin is used, the primary tackifier resin system, defined as the fundamental resin / resin mixture present in a content of at least 50% of the total amount of resin, is characterized by having a point of softening between 85 and 150 ° C and more preferably between 100 and 140 ° C (all softening points being measured with the well known method called "Ring and Ball" (R & B)).

It is considered that tackifier resins that have a softening point below 85 ° C have predominantly a plasticizing effect which can in any case be important for the development of good adhesive and elastic properties, but which must be distinguished from the tackifier effect. . This is due to the fact that in the processing of the present hot melt elastic compositions, rapid solidification of the material after extrusion is desirable, especially for compositions which need to be stretched prior to application to the substrate, which is clearly only possible with solid materials. For this reason it is desirable that the solidification point of these compositions is not less than 80 ° C and more preferably is greater than or equal to 100 ° C.

The solidification point is most accurately determined by adopting the technique known as Dynamic / Mechanical Analysis under sinusoidal stress, which is well known in the science and technology of polymers and adhesives. According to this technique, the three main rheological parameters of the material are determined as a function of temperature:

- elastic or accumulation modulus G '

- viscous or dissipation modulus G "

- angle δ and its tangent, being such the phase shift between G 'and G ”.

G 'is larger than G "when the material is solid; When the material is fluid G" becomes larger than GT. Of course, G' is larger at low temperature and smaller at high temperature. The crossing temperature between G ' and G "is assumed to be the true rheological solidification point (or melting point) of the material.

It is preferable that the crossing temperature for the composition is greater than or equal to 80 ° C and more preferably greater than or equal to 100 ° C.

The location of the cross point depends on many physical parameters of the hot melt. However, the main influence has been found to be the softening point of the main tackifier resin, and a secondary influence is due to the content and molecular weight of the copolymer. Hence, the tackifier resin should preferably have a softening point between 85 and 150 ° C and more preferably between 100 and 140 ° C, provided that in any case the total composition has a real rheological solidification temperature of at least 80 ° C and more preferably at least 100 ° C.

In addition to the thermoplastic elastomeric block copolymer and the main tackifier resin, the compositions may contain other components which enhance specific properties. A more detailed description of the compositions and their main properties is given below.

For practical reasons of clarity of description, further descriptions will refer specifically to "low modulus" compositions and "high modulus" compositions.

The low modulus compositions are elastic, extrudable, adhesive and based on at least one thermoplastic elastomeric block copolymer, suitably modified with an adequate addition of at least one tackifier resin essentially compatible with its soft blocks. The polymer, or at least the polymer present in greater quantity, is a polystyrene / polybutadiene block copolymer. In this embodiment, the compositions have a modulus of 0.5 MPa or less, practically between 0.05 MPa and 0.5 MPa and preferably less than or equal to 0.3 MPa; the modulus is measured at 23 ° C at 500% elongation (six times the initial length of the sample) with an elongation rate of 500 mm / minute. Further, the compositions have viscosities at 180 ° C and under a shear stress of 80 sec. 1 of 120,000 centipoise (cps) or less and preferably 60,000 cps or less and more preferably 30,000 cps or less.

The low modulus compositions will typically contain from 10 to 80% by weight, and more preferably from 15 to 50% by weight, of SBC or a blend of SBC having the following characteristics:

- a molecular structure that can be multi-block, linear or radial (star) provided that it contains, per molecule, at least two rigid blocks formed by a vinyl-arenic polymer and preferably polystyrene or poly-alpha-methyl-styrene, and at least one soft or rubbery block , the soft block of the SBC, or of the SBC present in higher quantity, being polybutadiene. The diblock content in the SBC (or SBCs) must be kept below 40% by weight. - The aromatics content (conventionally referred to below as "block styrene content") of the SBC or SBCs can vary between 10 and 50% by weight, and preferably between 20 and 50% by weight.

However, in order to maintain significant elastic properties, both the quantities of SBC in the final composition and its styrene content must be chosen so as to have a final content of styrene in the composition comprised between 3 and 17% by weight, and preferably between 6 and 15% by weight.

The composition also contains a tackifier-type resin or a mixture of resins, essentially compatible with the soft blocks of SBC.

The preferred resins belong to the chemical groups known as:

- hydrocarbon resins

- aliphatic resins

- polyiterpene resins

- phenol-terpene resins

- C5 synthetic resins

- synthetic resins C5 / C9

- rosin and rosin esters,

as well as their totally or partially hydrogenated derivatives.

The resin or mixture of tackifier-type resins has (or has) an R&B softening point of between 85 and 150 ° C and preferably between 100 and 140 ° C. The content of such resin or mixture of resins in the composition can be between 20 and 90% by weight. However, in a preferred embodiment, the content of resin or mixture of resins described above is comprised between 30 and 55% by weight, the rest being formed by the other components described below, which improve the elastic and / or adhesive properties.

In any case, both the content and the softening points of the tackifier resin or mixture of resins as well as those of the other components described below, will be chosen so that the final composition has a real rheological solidification temperature (measured as temperature of crossing of G 'and G ") not lower than 80 ° C and preferably not lower than 100 ° C.

It has also been found that the adhesive and / or elastic and / or mechanical properties of the tackifier-type binary SBC / resin mixtures can be improved by using other components.

Adhesive properties can be improved by adding limited amounts of high molecular weight rubbers, such as polyisoprene, polybutadiene, polyisobutylene, natural rubber, butyl rubber, styrene / butadiene rubber (SBR) or styrene / isoprene rubber (SIR) and their mixtures. These polymers have high viscosities and, in the uncured state, have modest elastic properties. However, by adding them in amounts up to 15% by weight of the formulation and using polymers with Mooney ML viscosity (1 + 4) at 100 ° C, between 30 and 70, the resulting compositions exhibit better pressure adhesion properties, while still maintaining viscosity. final within the limits of the useful range and without any negative effect on the final elastic properties. A particularly suitable SBR is the product sold by Enichem under the trade name EUROPRENE SOL 1205 and by Fina under the trade name FINAPRENE 1205. This product is described as an SBR in which styrene is partially distributed in blocks. Of the total styrene content equal to 25% by weight, between 15 and 18% have a block structure and the rest is copolymerized in a random way with butadiene.

Plasticization of the composition can have very good effects not only on adhesive properties and viscosity, but can also improve elastic behavior by reducing internal (molecular) friction which dissipates elastic energy during stretching and subsequent release. In general, the composition can contain up to 40% by weight of plasticizer (or plasticizers).

In a preferred embodiment, the compositions contain at least one of the following plasticizers:

up to 40% by weight of a tackifier-type resin with a softening point between 50 and 85 ° C, up to 20% by weight, and preferably up to 15% by weight, of a hydrocarbon resin, rosin ester or resin liquid polytherpenic with a softening point not exceeding 30 ° C,

between 3 and 30% by weight, and preferably between 5 and 15% by weight, of a paraffinic or naphthenic mineral oil having an aromatic content of less than 10% by weight, in order not to interfere with the styrene domains,

up to 15% by weight of a liquid polyisoprene or depolymerized natural rubber or polyisobutylene, polybutenic or polypropylene oils and their liquid copolymers, eg PARAPOL (Exxon), or LIR (by KURARAY).

The quantity of plasticizer must be such that the solidification temperature is not lowered below the limit indicated above. In a preferred embodiment, the total plasticizer content in a low modulus formulation is not less than 10% by weight and not more than 40% by weight.

In low modulus compositions, the use of aromatic resins, which have no effect on the adhesive properties, while interfering with the rigid blocks of SBC and tend to stiffen the composition by increasing its viscosity, is generally undesirable, and the preferred content of resins aromatic is zero. However, limited amounts of an aromatic resin or blend of aromatic resins, e.g. 20% by weight or less, more preferably 10% by weight or less, can be used as a reinforcement for compositions that have a low total content. of styrene (up to 6%) or which include significant amounts of SIS, e.g. 30% by weight or more of SIS based on total SBC. Indeed, SIS copolymers, especially those which have a styrene content of less than 30% by weight, when diluted in the composition by means of resin and other additives, may exhibit an inadequate (too low) modulus and modest springback characteristics, as a result of both the inherently lower modulus of SIS and the low concentration of styrene, which acts as a physical vulcanizing agent. In this case the aromatic resin can both increase the modulus up to a useful value and increase the density of the rigid domains, which are swollen by the resin. The useful aromatic resins have a softening point between 115 and 160 ° C and are chemically identified as derivatives of styrene, alpha-methyl-styrene, vinyl-toluene, coumarone-indene and their copolymers; alkyl-aryl resins, etc.

In addition to the components discussed above, the compositions can contain the normal additives such as antioxidants, U.V. inhibitors, pigments and coloring products, mineral fillers, etc., generally in a total amount up to 20% by weight.

Without limiting what refers to the most suitable processing and use, the low modulus formulations are typically used in the stretched state with typical elongation values of 400-1000%. They are characterized by having a very high elongation at break (over 1100% and often over 1400%) and very good adhesive properties, often with pressure adhesion properties.

To simulate the application of the composition in an absorbent article, the pressure adhesive properties are measured with stickiness values ("Loop tack" or "Quick Stick Tack") and as a peel value at 90 ° according to the standard FINAT Test methods Method N ° 9 for the loop tack and FINAT Test Method N ° 2 for the 90 ° peel, modified as described below. - For both tests the compositions are applied on a polyester film with a grammage of 80 g / m2.

- The adhesive properties, both as a loop tack and as a 90 ° peel, are determined on a polyethylene film fixed on the standard plate.

- Loop tack values are determined as peak values, ignoring the initial peak.

- The 90 ° peel is evaluated after a compression with a 400g roller passed back and forth, ie with two passes, one in each direction, and the measurements are performed 20 minutes after the establishment of the contact between the adhesive and the polyethylene. The compositions according to the invention generally have a loop tack greater than 5 N / cm and a peel at 90 ° greater than 7 N / cm (separation speed = 300 mm / minute). Materials that can usefully be adhered and applied at room temperature in a hygienic product are considered to be those that show a loop tack on polyethylene greater than 2.5 N / cm and a 90 ° peel greater than 3 N / cm.

The high modulus formulations are elastic, adhesive and extrudable compositions similar to those previously described, and having the following characteristics;

1) They have a modulus higher than 0.5 MPa and more preferably not lower than 1 MPa and up to 10 MPa.

2) At 180 ° C, and applying a shear stress of 80 sec ", they have viscosities of 80,000 cps or less, preferably 50,000 cps or less and more preferably 35,000 cps or less.

3) They are based on the same types of SBC as previously indicated, but have a final content of block styrene in the composition between 15 and 30% by weight and preferably between 15 and 25% by weight.

4) The SBCs or mixtures of SBCs that are used have a diblock content not higher than 25% and preferably not higher than 10%. The most preferred polymers are those that do not contain diblocks such as those sold by DEXCO Co under the VECTOR brand.

5) The preferred SBC contains between 20 and 50% by weight of styrene and the preferred content of SBC or mixture of SBC in the composition is between 35 and 75% by weight, provided that it is the styrene content of the SBC that their content in the composition is such as to satisfy the requirements of point 3) above as regards the final styrene content.

6) The tackifier resin or mixture of resins has or have the same chemical and physical characteristics previously indicated. However, the preferred content is between 20 and 40% by weight.

7) The content of higher molecular weight rubbers, such as polyisoprene, polybutadiene, polyisobutylene, natural rubber, butyl rubber, SIR and SBR, should not exceed 10% by weight and preferably be less than 5% by weight of the total composition .

8) The total content of plasticizers as previously indicated, should not exceed 25% by weight.

9) Aromatic resins or their mixtures should preferably be avoided due to their deleterious effect on the adhesive properties and on the stress / strain correlation (more inclined stress / strain diagrams; appearance of a yield point and as a consequence of a non-recoverable plastic deformation) . However, as in the previous case, these materials can be present in the composition with contents up to 20% by weight with acceptable properties, provided that the non-adhesive / non-elastomeric part of the composition does not exceed 50% by weight of the total composition. This non-adhesive / non-elastomeric part is formed by the sum of the total styrene content in the composition plus the content of resin or aromatic resins.

Other characteristics and other possible and further components and additives remain the same. In particular, it is still necessary that the real rheological solidification temperature (measured at the crossing point between G 'and G ") is not lower than 80 ° C and preferably not lower than 100 ° C.

Still without wanting to limit the most suitable processing and use range, these high modulus compositions are often used in the unstretched state, especially those having modulus greater than 1 MPa. This preferred use is due to the fact that they are capable of giving sufficient springback forces even with low deformation (typically not more than 50%) which are often encountered during the use of deformable hygienic articles, which can conveniently be made elastic. and resilient in this way. This is also partly due to the fact that the application of the composition in the stretched state implies the need to apply it at approximately room temperature, and thus requires that it adheres strongly to the substrates even under these conditions (pressure adhesive properties). The pressure-sticking characteristics of adhesives tend to be inversely proportional to their elastic modulus.

However, some of the high modulus compositions still maintain a net and useful pressure adhesive behavior (loop tack on PE> 2.5 N / cm; 90 ° peel on PE> 3 N / cm) and can also be applied to room temperature and in the stretched state with typical elongation up to 400%, being the elongation at break of these compositions typically greater than 900%.

Adhesive properties are measured under the same conditions as low modulus compositions.

The unexpectedly good level of elasticity of the compositions according to the invention can be measured as maintaining the tensile strength after cyclic deformation. This is a test that simulates conditions of use on a hygienic article, in which the user's movements can cause further and subsequent elongation of the elasticized parts which, for optimal behavior, must regain their initial length with a minimum loss of resistance to traction. All the compositions are tested starting from an already stretched state and are brought to a further elongation of about 15% with respect to the initial stretched length, to simulate the movements of the user. The compositions are cyclically stretched and released 50 times from the initial stretch to the stretch obtained by further stretching.

The percentage retention of the tensile strength at an elongation equal to the initial one after 50 ironing cycles at a speed of 500 mm / minute, compared with the initial tensile strength, is taken as a measure of the elasticity of the material. The tests are carried out at room temperature on belts with a width of 2.54 cm.

The low modulus compositions are stretched with an initial elongation typical of the intended application of 800%, and then further cyclically stretched and released 50 times between 800 and 920% (i.e. 9 to 10.2 times the initial sample length) .

The high modulus compositions are tested in the same way but with the initial elongation typical of the expected applications equal to 300% and with 50 cycles of further elongation and release between 300 and 345%.

The term "retention of tensile strength after 50 cycles" used herein refers to the test described above. An elastic in vulcanized natural rubber produced by the company JPS Elastomerics, which is used in the elasticization around the legs of panty diapers and applied with an initial stretch of 220%, is taken as a reference and is cyclically deformed 50 times between 220 and 255%. It has been found that under these conditions the vulcanized natural rubber elastic, after 50 cycles, has an average retention of tensile strength equal to 47% of the initial tensile strength at 220%. This retention value of the tensile strength is considered indicative of a good elastic behavior.

More generally, it has been observed that materials which do not lose more than 60% of their initial tensile strength under these test conditions exhibit good elastic behavior. Maintaining less than 40% of the initial tensile strength represents unsatisfactory elasticity, indicated by slow return to initial length after the release of tension, high permanent plastic deformation, etc.

As will be highlighted in the following examples, the compositions, both low and high modulus, show a good elastic behavior, of the same value or better than that of a vulcanized natural rubber elastic.

More specifically, the high modulus compositions retain up to 67.5% of their initial tensile strength and the low modulus compositions up to 59.8%.

The elastic properties are also judged by the following method: the compositions in the form of ribbons with a width of 2.54 cm, are tested at 23 ° C and with an ironing speed of 1000 iran / minute, with 3 elastic hysteresis cycles between zero and a typical elongation during use of about 800% for low modulus compositions and 300% for high modulus compositions. The elastic energy of each cycle is recorded, evaluated as the area of the cycle, and the ratio between the elastic energy of the third and the first cycle is considered as retention of the elastic energy after 3 hysteresis cycles. For a good elastic behavior it is considered that in these test conditions it is necessary a retention of the elastic energy of not less than 30%.

The present invention also provides a process for preparing yarns or strips of a hot melt elastomeric adhesive composition which comprises extruding the composition, as previously indicated, onto a moving surface, cooling and optionally drawing the yarns or strips.

The present invention involves the use of an apparatus for the production of threads or strips from a hot melt elastomeric adhesive composition as defined herein, which comprises extrusion means, a movable surface arranged so that the threads or strips come into contact of the surface at the exit from the extrusion means, and a cooling system placed in such a way as to cool the threads or strips. After cooling, the threads or strips on the movable surface can be subsequently placed in contact with a continuous ribbon of raw material, so that the threads or strips are transferred onto the raw material. In this basic form, the apparatus of this type in which the adhesive is extruded directly onto the intended substrate is already known, and has been used for the application of conventional hot melt adhesive threads or strips. It is a surprising advantage of the present invention that the elastomeric hot melt adhesives can be produced as threads or strips with the use of the same equipment already used previously for conventional hot melts.

The raw material can be, for example, a material suitable for use in the production of absorbent products such as nappies, panties and absorbent pads.

The transfer of the threads or strips from the movable surface to the raw material, with the raw material moving faster than the movable surface, carries out the transfer itself with an elastic stretching of the threads or strips.

As already mentioned, the process according to the invention produces both threads and strips, however, for convenience, reference will be made in certain points below to threads, although the description, with the appropriate modifications where necessary, also adapts to the Stripes.

The composition is preferably extruded from a hot melt applicator, which applicator is fed from a reservoir containing the composition through a suitable conduit, such as a pipe or a connecting hose. The applicator comprises one or more extrusion heads each having an opening for extruding one or more threads. The movable surface, which is preferably a conveyor belt, passes under the extrusion heads, preferably kept at a distance between about 1 mm for the strips up to about 200 mm for the wires. The conveyor belt can also be in contact with the extrusion head. Alternatively, the movable surface may be a cylinder which is preferably cooled to provide the necessary cooling of the wires or strips as they come into contact with the cylinder.

The extrusion head can be controlled by a mechanical system, and this system adjusts the angular position of the head and allows its movement on the x, y and / or z axes, the y axis for this purpose being the one along which the l 'extrusion. By adjusting the angle of the head around the x-axis (axis along which the wires are transported by the moving surface) the deposition of the wire lines on the moving surface can be controlled, and in particular the distance between the lines can be adjusted. of threads from each head on the moving surface. The adjustment of the angle of the head around the z axis allows to change the extrusion point of the wire with respect to the mobile surface and the adjustment of the angle of the head around the x axis allows to change the distance between the individual wires. Preferably the heads are arranged so that the wire lines are at an adequate distance, which is necessary for the article to be produced. The partial rotation of a series of extrusion openings around a single common axis brings the wires together to the point where they attach to form a strand.

The extrusion heads extrude the composition into wires, e.g., having a diameter between 0.4 mm and about 3 mm or more, preferably between 0.8 and 1.5 mm and more preferably less than or equal to about 1 mm . The diameter of the wire extruded from the head can be changed by changing the diameter of the opening (s) in the head. It will also be noted that the shape of the opening in the head determines whether a wire or a strip is produced. By changing the size of the opening in the head, and thus the diameter of the wire, the adhesion strength of the wire can be changed and also the cooling time of the wire is changed. The larger the wire diameter the slower the cooling will be. The diameter of the wire will thus be varied according to the desired use.

The moving surface, e.g. the conveyor belt, will generally be made of or coated with a material to which the yarn or strip adheres sufficiently to prevent the yarn or strip from stretching while attached to the conveyor. This allows cooling of the wire or strip without plastic deformation. However, the adhesion to the moving surface will be less than the expected substrate adhesion, so that the wire or strip can subsequently be detached from the moving surface and transferred to the substrate with an elastic stretch. For example, the movable surface can be coated with a non-stick material comprising fluorocarbon polymers such as polytetrafluoroethylene, and preferably silicones such as silicone rubbers. In the case of such non-adherent materials, the friction between the movable surface and the wire or strip is generally sufficient to prevent stretching of the yarn or strip while these are in contact with the movable surface.

The movable surface, which is preferably a conveyor belt, can be operated by a mechanism, preferably mechanical, by which the speed of the belt can be varied. The variation of the belt speed is necessary in order to regulate the elastic stretching that occurs between the conveyor belt and the raw product to which the threads or strips are to be applied, this stretching being obtained from a difference in speed between the conveyor belt and the raw product. Varying the speed of the conveyor belt with respect to the extrusion speed of the composition may also be useful, since in this way it is possible to stretch the threads or strips, although, since the threads or strips are still in the molten state, by such stretching only a permanent reduction of the section is obtained. The ratio of web feed speed to extrusion speed must be at least one to one (no ironing) and can be up to e.g. about 1.5: 1, preferably 1.3: 1, plus preferably 1.2: 1, if an ironing effect is required. It is also possible to provide the extrusion head with a device that adjusts the extrusion speed of the hot melt composition with respect to the belt speed to ensure a constant diameter of the wires in case of variation of the belt speed, e.g. variations necessary for adjust the degree of elastic ironing.

The threads are cooled while they are on the conveyor belt. The cooling is preferably obtained by using an air blow system arranged so as to cool the yarns. The blowing system is preferably located below the belt. It has surprisingly been found that a blow molding system is capable of providing sufficient cooling for the yarns which are cooled and solidified very rapidly without the need for prolonged cooling. However, the conveyor belt must be of sufficient length for the wires to have sufficient time to cool down. The length of the tape can be determined according to the available space and the wires can run over practically the entire length of the tape or affect only a part of it, depending on the length of the tape and the cooling time required. In addition, the air blowing system can be adjusted to cool the wires more quickly or slower as needed. This varies the extent to which the inside of the threads and ribbons are cooled as a function of the transfer time of the raw product, and therefore the extent to which the threads or ribbons are subjected to elastic stretching as opposed to plastic deformation.

After cooling and possible ironing, the yarns are transferred to a raw material, e.g. a topsheet for an absorbent article. The transfer can be achieved by means of an idle silicone roller, preferably pneumatically operated. The roller brings the raw material into contact with the threads, the threads adhere to the raw material and are continuously fed from the conveyor to the raw product. As already mentioned, the raw material preferably moves at a higher speed than the conveyor belt, so that the threads are arranged with an elastic drawing at the moment of transfer onto the raw material. Due to the adhesiveness of the threads and the nature of the raw material, the threads remain adherent to this. Optionally, the threads can then be pressed onto the raw material by means of one or more cylinders to ensure that the threads remain attached to the raw material.

This pressing can occur directly on the wires in which case the cylinders must be coated with a non-stick material, eg PTFE or preferably silicone. More preferably the pressure can be applied after applying a second sheet of raw material to the threads, in which case a non-stick coating is no longer required. In either case, the cylinders can be heated to contribute to adhesion, although the temperature must be well below the melting point of the composition in order not to affect its elastic properties. In practice, about 10 to 40 ° C below the rheological solidification temperature of the composition (measured as described here) is a suitable value.

To provide the threads with a non-linear (curved) arrangement, they can be passed through one or more combs before being transferred to the raw material. The comb (combs) adjusts the position of the thread on the raw material. The combs must be able to move in a plane practically parallel to the plane of the raw material. The comb (s) are preferably moved by means of a cam. By moving the comb in a plane practically parallel to the plane of the raw material the threads, which have passed through the comb (combs), can be transferred to the raw material in a curved arrangement.

When transferred onto the raw material, the yarn has a tendency to detach itself from it since it tends to resume a linear position, and the higher the degree of curvature of the yarn, the greater the tendency to detach. To prevent detachment, a roll of non-adherent material of the type indicated above can be passed over the bent threads so as to compress them on the raw material so that the threads remain fixed to the raw material in a curved form.

Curved threads are preferably employed in diaper panties, although they can also be used in sanitary pads. The curved threads allow the panty diaper, eg, to assume a better anatomical shape, which makes it more comfortable for the wearer.

It has surprisingly been found that the yarns of the present invention have considerable advantages with respect to the elasticization with rubber previously used. In particular, the yarns of the present invention can be formed with a smaller diameter than the rubber band of previous use, and can also be stretched to even smaller diameters before being fixed. In particular, the wire may have an initial diameter less than or equal to about 1 irai on the conveyor belt, prior to application to the raw material. As a consequence of the elastic ironing upon application of the yarn on the raw material, the yarn can assume a diameter between about 0.3 and 0.2 mm, which corresponds to an ironing of about 10 to about 25 times the initial length. The modulus of elasticity of the yarn is much lower than that of elasticization using conventional rubber and thus allows the yarn to be stretched more quickly than conventional rubber elastic. Obviously, a single yarn provides a lower spring force than a number of yarns, and hence the number of yarns can be quickly adjusted to provide the necessary spring force.

Another advantage of the yarns of the present invention with respect to conventional rubber elasticization lies in the fact that they adhere to the raw material without the use of glue. This is especially beneficial for curved strands. When applying rubber to a raw product in curved form, a significant amount of glue is required to fix the rubber in curved form, and this can have a negative influence on the elastic properties of the rubber. The use of a smaller amount of glue to avoid this negative effect has the consequence that the retention force of the rubber on the raw material is less than the elastic force of the rubber itself, since a rubber band has a higher modulus than adhesives. hot melt elastic used according to the present invention, and furthermore the elastic hot melt adhesives can be applied in thin threads with respect to rubber bands. For these reasons, the rubber band generally tends to regain its linear shape and therefore quickly detach from the raw material. The hot melt elastic adhesives used according to the present invention have a much lower modulus than that of traditional rubber bands and can be stretched into very thin threads which adhere to the raw material without the need for the addition of glue. Their adhesion force is therefore higher than the detachment force, and the threads remain adherent to the raw material in a curved shape.

It certainly falls within the ability of technicians skilled in this field to ascertain the optimal parameters for the wire in each particular case, and for this, for example, the modulus of the composition, the diameter of the wire, the adhesion of the wire to the material must be taken into account. before and possibly the radius of curvature. The adhesion will depend on the composition adopted and on whether the wire is pressed on the raw material or not.

It has also been surprisingly found that if the yarn breaks during transfer to the raw material, the broken end of the yarn can easily be picked up and transferred to the raw material to rejoin the breaking point, and how this can be carried out is described in more detail below.

The invention will now be described in more detail with reference to a specific embodiment of the apparatus for forming the yarns according to the invention. This equipment is illustrated in the attached drawings, in which:

Figure 1 illustrates an apparatus used to produce curved yarns and to transfer the yarns to a raw material; And

Figure 2 is a schematic view of the apparatus of Figure 1, together with the equipment used to produce a disposable hygienic product, more specifically a disposable baby panty diaper.

Of course, the apparatus illustrated in Figure 2 can be modified to produce other disposable stretch sanitary products.

With reference to Figure 1:

Figure 1 illustrates an apparatus for making wires and in particular illustrates a hot melt applicator generally indicated with (1), which comprises extrusion heads (3,4) fed from a tank through a suitable fitting. For clarity of illustration of the remaining parts of the equipment, the tank and the connecting pipe are not shown. Below the extrusion heads (3,4) and not in contact with them, is the conveyor belt (6) which moves in the direction shown in the figure by an arrow. The speed of advancement of the conveyor belt (6) is adjustable by means of a mechanical speed regulator (2) and the angular position of the extrusion heads can be adjusted with an adjustable support (5). Below the conveyor belt (6) there is a fan (7) which cools the wires (8) which are extruded from the heads (3,4) on the conveyor belt (6). Four threads (8) are shown in the drawing although the number of threads can, of course, be varied according to the particular product to be produced. The wires (8) are bent at the exit from the conveyor belt (6). The threads (8) pass from the conveyor (6), through one or more combs (9), onto a raw material (11) which is brought into contact with the threads (8) by means of a first silicone idle roller (12 ). The curvature is obtained by means of the comb (combs) (9) which is movable in a plane parallel to the plane of the raw material (11). The movement of the combs (9) is obtained by means of a cam (10). The position of the first idle roller (12) is activated pneumatically. The threads (8) adhere to the raw material (11) with the threads (8) in a geometrically curved position as seen in (13). Silicone rolls (14) compress the threads (8) on the surface of the raw material (11).

A second idle roller (15), which is preferably silicone coated, is arranged adjacent to the silicone roller (12) but on the opposite side of the raw material and in contact with the threads which are transferred to the raw material. The first and second idle rollers (12) and (15) are moved by the same pneumatic system. The second idle roller (15) does not compress the threads against the raw product (which is achieved by successive rolls (14)) but simply ensures the correct contact angle between the threads and the raw material during transfer from the conveyor belt (6 ).

When the machine is started, or after an interruption of one or more of the threads that are transferred to the raw material (11), the first idle roller (12) moves towards the conveyor belt (6) and carries the raw material (11) in contact with the wires (8) above this. At the same time, the combs (9) can be lowered in order not to interfere with the movement of the first idle roller (12). The threads (8) thus adhere to the raw material (11) and can be stretched in the normal way when the first idle roller (12) returns to its normal position.

The desired degree of elastic stretching of the yarns at the time of application to the raw material (11) is obtained from the difference in speed between the conveyor belt (6) (which is also the speed of the yarns before application) and the raw material itself. (11). As already mentioned, the speed of the conveyor belt (6) can be adjusted by means of the speed regulator (2).

Figure 2 is a schematic view of the apparatus of Figure 1, illustrating the equipment and materials used to manufacture a disposable baby diaper panty. The same numerical references are used for components represented in both Figure 1 and 2.

As seen in Figure 2, a nonwoven used as a topsheet for the disposable baby panty diaper is fed through the roller (30) as raw material (11). The raw material (11) passes near the conveyor (6), due to the effect of the first idle roller (12), and the threads (not represented in Figure 2) are transferred onto the raw material in the manner described in more detail previously with reference to Figure 1, by means of the hot melt applicator (1). The raw material (11) with the threads is passed to the point (55) where it is joined to the core material by a core feeding device and a cutting unit (50) and to the backsheet by means of a roller (60) to form a composite material. The composite material then passes through an optional crimping unit (70) in which the various elements are pressed in order to improve adhesion particularly around the perimeter. The material then passes through a final cutting unit (90) to form the final product. It can be noted that at various points of this process, conventional hot melt adhesion (HO) is applied to the components of the final product and then passes through vacuum (100).

The upper left of Figure 1 illustrates the hot melt elastic adhesive applied in a curved pattern to form the leg stretch for the panty diaper. The positioning of the absorbent cores on top of the raw material, obtained by the core feeding system and the cutting unit (Figure 2, (50)) is also illustrated in more detail, although for clarity, the core feeding system and the cutting units are omitted from Figure 1, as is the backsheet with its feed roll (Figure 2, (60)).

As already indicated, conventional rubber bands cannot be satisfactorily applied to raw materials along non-linear paths, and it is an important feature of the present invention that hot melt elastomeric adhesives can be applied to a raw material substrate according to non-linear paths, 'eg. curved.

According to another aspect, the present invention provides an apparatus for applying a hot melt elastomeric adhesive to a raw material with a non-linear trend, which comprises:

an extrusion medium for the hot melt elastomeric adhesive;

a movable surface positioned near the extrusion means so that the wires or strips extruded from the extrusion means come into contact with the movable surface at the exit from the extrusion means; means for bringing a continuous layer of raw material into contact with the threads or strips on the movable surface, so that the threads or strips are transferred onto the raw material; And

at least one comb movable in a plane parallel to the raw material, through which the yarns or strips pass during the transfer from the movable surface to the raw material and which allows the yarns or strips to be applied to the raw material according to a non-linear trend.

The following examples illustrate compositions which can be used in the present invention for the preparation of threads or strips. The examples are in no way to be considered as limiting the invention. In the case of registered products, the details on their nature and composition are those provided by the manufacturer.

The compositions of all Examples 1 to 5, when applied in the molten state between plastic and / or cellulosic materials, in an amount corresponding to 5 g / m2 show a strong bond strength, well above 0.5 N / cm .

Example 1

An SBC polymeric system, based on SBS (styrene-butadiene-styrene block copolymer) is formulated as follows:

CARIFLEX TR-4113 S 36% by weight

EUROPRENE SOL 1205 8%

DERCOLYTE A 115 45.8%

FORAL 85-E 6%

HERCOLYN D-E 4%

IRGANOX 1010 0.2%

m which:

CARIFLEX TR-4113 S is an SBS copolymer diluted with oil marketed by SHELL Co, which contains: 68.5% by weight of a linear triblock SBS having a styrene content of 35% by weight and with a diblock content of less than 20 % by weight

31.5% by weight of a naphthenic mineral oil, which acts as a plasticizer, and which contains less than 5% aromatics.

EUROPRENE SOL 1205 is a styrene / butadiene rubber (SBR) produced by ENICHEM. (The FINAPRENE 1205 product produced by FINA is similar, and can equally be used).

It is described as a solution polymerized SBR having a Mooney ML (1 4) viscosity at 100 ° C equal to 47 and a total styrene content of 25% by weight. This styrene is partially (typically between 15 and 18%) distributed in blocks with the remainder randomly copolymerized with butadiene. This randomly copolymerized styrene gives the rubbery parts of the molecule the chemical structure of an amorphous SBR, which contributes to the development of particularly good pressure-sticking characteristics.

DERCOLYTE A 115 (the main tackifier type resin) is produced by DRT. It is a polyterpene resin derived from alpha-pinene having a softening point of 115 ° C.

FORAL 85-E is a tackifier-type resin consisting of a hydrogenated glyceric ester of rosin, produced by HERCULES Co. It has a softening point of 85 ° C.

HERCOLYN D-E is a liquid methyl ester of rosin produced by HERCULES.

IRGANOX 1010 is a phenolic antioxidant produced by CIBA-GEIGY.

The composition was found to have the following properties:

- total content of styrene = 10.6% by weight, of which 10.1% is in blocks

- viscosity at 180 ° C at 80 sec-1 = 20520 cps

- modulus at 500% elongation = 0.182 MPa. (low modulus)

- elongation at break> 1400% (1400% is the maximum measurable elongation with the equipment used for this determination)

- rheological solidification temperature (crossing point of G 'and G ") = 125 ° C

- loop tack on PE = 8.5 N / cm

- 90 ° peel on PE = 16.3 N / cm

- retention of tensile strength after 50 cycles = 59.8%

- retention of elastic energy after 3 hysteresis cycles between zero and 800% = 57.7%

- Newtonian index (N.I.) = 1.05.

The composition exhibits extremely good elastic and adhesive properties and is considered fully suitable for the elasticization of structures, particularly of absorbent sanitary articles. It is easily workable, that is extrudable, solidifies quickly (ironable in production) and has good pressure adhesive characteristics that allow the formation of strong bonds by simple contact with many substrates even at room temperature.

Example 2

The formulation is:

CARIFLEX TR-4113 S 38.8% by weight

FINAPRENE 1205 8.9%

DERCOLYTE At 15 26%

FORAL 85-E 26%

IRGANOX 1010 0.3%

The following properties were measured:

- total content of styrene = 11.5% by weight, of which 10.9% is in blocks

- viscosity at 180 ° C at 80 sec- · '- = 28180 cps

- modulus at 500% elongation = 0.188 MPa (low modulus)

- elongation at break> 1400%

- rheological solidification temperature = 120 ° C - loop tack on PE = 8.6 N / cm

- 90 ° peel on PE = 10.4 N / cm

- retention of tensile strength after 50 cycles between 800 and 920% = 59.1%

- retention of elastic energy after 3 hysteresis cycles between zero and 800% = 48.3%

- Newtonian index (N.I.) = 1.01.

The composition is similar to that of Example 1, the main variation being the fact that about 50% of the high softening point tackifier resin was replaced with a lower softening point resin, and only plasticizer is the oil contained in CARIFLEX TR-4113 S (12.2% by weight of the composition). Nonetheless it has been found that the composition still maintains a very high solidification temperature, solidifies rapidly and has optimum elastic and pressure adhesion properties, as well as excellent workability, whereby it is easy to extrude and iron immediately even up to 800% below. form of very thin threads (diameter = 0.4 mm) which can be applied for the lateral elasticization of an absorbent product.

Example 3

A different system based on radial SBS has been found, more precisely a mixture of an SBS with relatively low hard block content and a SBS with relatively high hard block content. The wording is as follows:

FINAPRENE 415 17.7% by weight

FINAPRENE 401 7.0%

FINAPRENE 1205 8.0%

ZONATAC 115 LITE 45.8%

FORAL 85 ~ E 6.3%

HERCOLYN D-E 4.0%

SHELLFLEX 4510 FC 11.0%

IRGANOX 1010 0.2%

in which:

- FINAPRENE 415 and FINAPRENE 401 are radial SBS copolymers produced by FINA. Both are assumed to contain less than 20% diblock and are formed in a "star" structure of four SB blocks chemically bonded at a central point across the butadiene blocks. FINAPRENE 415 contains 40% by weight of block styrene and FINAPRENE 401 contains 22% of block styrene.

ZONATAC 115 LITE is a hydrocarbon modified terpene tackifier resin, with a softening point of 115 ° C, manufactured by Arizona Co. It is supposed to be based on styrene modified limonene.

SHELLFLEX 4510 FC is a naphthenic mineral oil, produced by SHELL, which is supposed to have an aromatics content of less than 5%.

The following properties were measured:

- total styrene content = 10.6% by weight, of which 10.1% is in blocks

- viscosity at 180 ° C at 80 sec - · '- = 12810 cps

- modulus at 500% elongation = 0.223 MPa (low modulus)

- elongation at break> 1300%

- rheological solidification temperature = 107 ° C <■> - loop tack on PE = 6.4 N / cm

- 90 ° peel on PE = 14 N / cm

- retention of tensile strength after 50 cycles between 800 and 920% - 50%

- retention of elastic energy after 3 hysteresis cycles between zero and 800% = 44.9%

- Newtonian index (N.I.) = 1.04.

The composition shows excellent properties typical of an easily workable extrudable and elastic adhesive material.

Example 4

The following high modulus composition is prepared, according to the formulation:

VECTOR 4461-D 44.8% by weight

ZONAREZ 7115 LITE 37%

FORAL 85-E 5%

ZONAREZ ALPHA 25 3%

PRIMOL 352 10%

IRGANOX 1010 0.2%

in which:

VECTOR 4461-D is a linear SBS copolymer having 43% by weight of styrene and containing no diblock, manufactured by DEXCO Co.

ZONAREZ 7115 LITE is a tackifier-type polytherpene resin, having a softening point of 115 ° C, derived from limonene, produced by ARIZONA ZONAREZ ALPHA 25 is a tackifier-type liquid resin (softening point = 25 ° C) derived from alpha -pinene, having a very good plasticizing effect. It is produced by the ARIZONA Co.

PRIMOL 352 is a paraffinic plasticising mineral oil produced by EXXON, which is believed to contain no aromatics.

The following properties were measured:

- total block styrene content = 19.3% by weight

- viscosity at 180 ° C at 80 sec <-1> = 16810 cps

- modulus at 500% elongation = 1.07 MPa (high modulus)

- elongation at break = 987%

- rheological solidification temperature = 111 ° C

- loop tack on PE = 4.3 N / cm

- 90 ° peel on PE = 6.5 N / cm

- retention of tensile strength after 50 skies between 300 and 345% = 67.5%

- retention of elastic energy after 3 hysteresis cycles between zero and 300% = 63.3%

- Newtonian index (N.I.) = 1.05.

The composition is a good elastic material used especially at low elongations. It shows acceptable pressure semi-adhesive characteristics, so it can be fixed on materials even at room temperature in the stretched state.

Example 5

The formulation is:

VECTOR 4461-D 54.8% by weight

ECR 368 35%

PRIMOL 352 10%

IRGANOX 1010 0.2%

m which:

ECR 368 is a hydrogenated hydrocarbon resin of the tackifier type, produced by EXXON and having a softening point of 100 ° C.

The following properties were measured:

- total block styrene content = 23.56% by weight

- viscosity at 180 ° C at 80 sec <“> l = 34000 cps

- modulus at 500% elongation = 1.61 MPa (high modulus)

- elongation at break = 947%

- rheological solidification temperature = 114 ° C - loop tack on PE = 1.3 N / cm

- 90 ° peel on PE = 2.6 N / cm

- retention of tensile strength after 50 cycles between 300 and 345% = 62.3%

- retention of elastic energy after 3 hysteresis cycles between zero and 300% = 38.3%

- Newtonian index = 1.05.

The composition was made in such a way as to maximize the modulus while maintaining acceptable elasticity and good adhesiveness at temperatures above room temperature. This material is most conveniently applied in the unstretched state and fixed directly at a temperature higher than 50 ° C. It gives good springback forces even at very low elongations, eg modulus at 20% elongation = 0.236 MPa. However, it also works well in the stretched state, e.g. at 300% elongation, and exhibits adhesive properties on PE which are not negligible even at room temperature.

Comparison example A

The formulation is:

KRATON DI107 40.2% by weight

WINGTACK 95 32.9%

CI-145 26.5%

WESTON 618 0.2%

IRGANOX 1010 0.2%

in which:

KRATON DI107 is a linear SIS block copolymer produced by SHELL, containing 14% by weight of styrene.

WINGTACK 95 is a synthetic polypropylene resin, having a softening point of 95 ° C, produced by Goodyear Co.

IC-145 is a coumarone-indene aromatic resin, having a softening point of 145 ° C and produced by the German company VFT.

WESTON 618 is a phosphite based antioxidant manufactured by Borg Warner Co.

IRGANOX 1010 is the one already described in <'> Example 1.

This formulation was prepared according to the teachings of US-A-4 418 123 (Example IV). According to the US patent, the composition results to have completely satisfactory properties from the elastic, adhesive and processing point of view.

The formulation of Comparative Example A has the following differences from Example IV of US-A-4418 123:

1) CUMAR LX-509 coumarone-indene resin is not readily available in Europe, and the equivalent IC-145 resin was used. The resins are chemically similar but IC-145 has a softening point of 145 ° C with respect to the figure of 155 ° C indicated in the US patent for CUMAR LX-509;

2) For practical reasons, the pigment (1.5% titanium dioxide) was not used. The pigment was presumably present in the original formulation to mask the slight brown color, and it is believed that it may have a negligible effect on the adhesive and elastic properties.

The main properties can be summarized as follows:

- total block styrene content = 5.6% by weight

- viscosity at 180 ° C at 80 sec <-1> = 192000 cps

- modulus at 500% elongation = 0.365 MPa (low modulus)

- elongation at break> 1400% (1400% is the maximum elongation obtainable on the machine used for this measurement)

- rheological solidification temperature (crossing point of G 'and G ") = 145 ° C

- loop tack on PE = 5.3 N / cm

- 90 ° peel on PE = 15.7 N / cm

- retention of tensile strength after 50 cycles = 34.3%

- retention of elastic energy after 3 hysteresis cycles between zero and 800% = 19.8%

- Newtonian index (N.I.) = 2.03.

It has been found that the above formulation has good pressure adhesive characteristics, as indicated in the US patent, since, measured on PE under the conditions described above, the loop tack is 5.3 N / cm and the peel at 90 ° is of 15.7 N / cm. However, it has also been found that both the elastic and workability properties for an extrudable material are not satisfactory. In particular:

Workability, especially application in thin strips or threads, is very difficult due to the extremely high viscosity and highly non-Newtonian rheological behavior.

The modulus at 500% of elongation was found to be 0.365 MPa, and the elongation at break was greater than 1400%. However, the elastic properties are not satisfactory. In particular, in cyclic deformation tests between 800 and 920%, it was found that the formulation loses 65.7% of its initial tensile strength after 50 cycles, and loses 80.2% of its elastic energy after 3 cycles of hysteresis between 0 and 800%, making it unsuitable for the elasticization of products such as sanitary absorbent articles that during use are deformed many times, with repeated elongation due to the movements of the user. It is noteworthy that 45% of the loss of tensile strength occurs between the first and second cycle, confirming an easy and non-recoverable plastic deformation.

Comparison example B

The formulation is:

TUFPRENE A 30.0% by weight

ESCOREZ CR 368 55.0%

CATENEX P941 10.0%

KRISTALEX FI00 5.0%

with the addition of 0.2 parts per 100 parts by weight of the aforementioned composition of the antioxidant IRGANOX 1010,

in which:

- TUFPRENE A is a linear SBS manufactured by Asahi Chemical CO. and containing 40% by weight of styrene. The diblock content is not specified by the manufacturer.

ESCOREZ CR 368 is a modified hydrogenated hydrocarbon resin produced by Exxon, having a softening point of 100 ° C.

CATENEX P941 is a parafiin mineral oil produced by Shell, which is supposed to have an aromatics content of less than 5% by weight.

KRISTALEX F 100 is an aromatic resin based on a-methyl styrene and styrene, produced by Hercules, and having a softening point of 100 ° C.

This formulation was produced according to the teachings of EP-A-0424295 (Example V). The formulation of Comparative Example B has the following difference from Example V of EP-A-0424295

0.2 parts per 100 parts by weight of IRGANOX 1010 antioxidant were added to the composition, as in Example V of EP-A-0424295. As is evident to anyone familiar with this subject, attempting to prepare and process the composition exactly as described in EP-A-0424295, i.e. without an antioxidant, would probably have resulted in thermal degradation of the system. To follow the teachings of EP-A-0424295 as closely as possible, the antioxidant used in Comparative Example A of said document was employed. However, the antioxidant was added in the normal amount of 0.2% (this being the value normally used in the previous examples according to the invention) instead of in the unusual (and certainly unnecessary) high amount used in Comparative Example A by EP-A-0424295.

The main properties can be summarized as follows:

- total block styrene = 12% by weight

- viscosity at 180 ° C at 80 sec - · * · = 5620 cps

- modulus at 500% elongation = 0.204 MPa (low modulus)

- elongation at break> 1300%

- rheological solidification temperature (crossing point of G1 and G ") = 106 ° C

- loop tack on PE = 0.7 N / cm

- 90 ° peel on PE = 0.8 N / cm

- retention of tensile strength after 50 cycles = 21.3%

- retention of elastic energy after 3 hysteresis cycles between zero and 800% = 25.0%

- Newtonian index (N.I.) = 1.16.

Processability, as indicated by viscosity and the Newtonian index, was found to be acceptable although the Newtonian index was high for an SBS-based composition. However, the formulation has modest pressure adhesive characteristics and values of 0.7 N / cm for the loop tack and 0.8 N / cm for the 90 ° peel show that it is practically not usable as a low modulus elastic adhesive. intended for use in the stretched condition, in the preparation of sanitary absorbent articles. The elastic properties are not satisfactory, as indicated by the loss of about 80% of the tensile strength of the composition after 50 successive ironing cycles and of 75% of its elastic energy after 3 cycles of hysteresis.

The unsatisfactory properties of the composition can be attributed to the diblock content of TUFPRENE A. This · ηοη is indicated by the manufacturer and direct measurement is difficult. However, the manufacturer's value for permanent plastic deformation, measured according to the ASTM D412 method, is 47%. This compares with much lower values of about 10% or less indicated for SBS copolymers with a diblock content of less than 20% by weight. On the other hand, the SHELL technical literature provides the following data for permanent plastic deformation for copolymers with a high diblock content:

KRATON D-1112 (SIS containing 40% by weight of diblock) = 20%

KRATON D-1118X (SBS containing 80% diblock by weight) = 40%.

From these data it can be assumed that TUFPRENE A probably has a diblock content well above 40%.

It should be noted that the previous results appear not to match the results reported in EP-A-0424295 in at least some respects. Thus, the 90 ° peel of 0.8 N / cm previously indicated, compares with 3.7 N / cm for the 180 ° peel obtainable from EP-A-0424295. This can be explained at least in part both with the different peel angle and with the compression used in the test (400 g instead of 2 kg) since the composition is rigid and the bond is strongly influenced by the compression. It should also be noted that the applied adhesive weight per m2 is not specified in EP-A-0424295 and this is extremely important in determining the bond strength. The test methods adopted according to the present invention provide a realistic measure of the compliance of the compositions for use in the proposed applications. The modest properties of the composition of EP-A-0424295 can also be attributed to the combination of a rigid SB5 (40% styrene) with an aromatic resin and with low amounts of plasticizer (10%).

Example 6

This example refers to the stress / strain diagrams of the compositions of Examples 1 to 5 and Comparative Examples A and B. The hot melt elastic compositions according to the invention are desirable to have a stress / strain diagram which is much flatter than the rubber bands, that is, even if it is already under tension, further stretching (e.g. due to the movements of the user of the absorbent article) causes only a very low increase in modulus and tensile strength, which is felt by the 'user. This is especially true for low modulus compositions, and can be seen by measuring the average modulus increase for a given elongation. The low modulus compositions, which are typically applied in the already stretched state, are evaluated as the average modulus increase for every 100% of the elongation between zero and 800% of elongation (nine times the initial length), i.e. the increase module total divided by 8.

With reference to the low modulus compositions mentioned in the previous examples, the results are the following:

EXAMPLE N ° AVERAGE INCREASE OF THE MODULE FOR 100% ELONGATION 1 0, 044 MPa / 100% elongation 2 0.045 MPa / 100% elongation 3 0.063 MPa / 100% elongation Example of comparison A 0.099 MPa / 100% elongation Example of comparison B 0.079 MPa / 100% elongation The high modulus compositions of Examples 4 and 5, which are generally used in the unstretched state, or in any case with a lower elongation, are evaluated as an average increase in modulus for a 100% increase in elongation between zero and 300% final elongation:

EXAMPLE N ° AVERAGE INCREASE OF THE MODULE

FOR 100% ELONGATION 4 0.169 MPa / 100% elongation 5 0.284 MPa / 100% elongation As a comparison, a vulcanized natural rubber elastic, used for stretch around the legs of panty diapers, even when applied with a much lower stretch ( typically 220%) shows, between zero and 220%, an average modulus increase of 0.89 MPa per 100% elongation.

It is possible to compare the behavior of a natural rubber elastic and of a low modulus composition according to the invention.

In the case of the rubber band, even limited movements of the user causing, for example, a stretching of the elastic parts with a further elongation of only 10%, will cause an average increase in the modulus of about 0.09 MPa. Using a low modulus composition, the increase in modulus will be about 20 times smaller and even with the strongest compositions with a higher modulus it will be many times lower, whereby the user's movements of an absorbent stretch article by means of the compositions described in the present invention are much freer. Consequently, in these applications, low modulus and low modulus variations as a function of voltage are a clear advantage.

Claims (28)

CLAIMS 1. Threads or strips formed from a hot melt elastomeric adhesive composition comprising at least one thermoplastic elastomer and at least one tackifier-type resin, the thermoplastic elastomer (s) being a styrene / butadiene / styrene block copolymer (SBS) or a mixture of styrene / butadiene / styrene copolymer with styrene / isoprene / styrene block copolymer (SIS) in which SIS is present in an amount equal to or less than 50% by weight of the total block copolymer, and in which the adhesive composition is characterized by fact that: a) is able to bind, when applied in the molten state, to plastic and or cellulosic materials, with a 90 ° peel force of not less than 0.5 N / cm (as defined here); b) maintains a retention of tensile strength after 50 cycles (as defined here) of at least 40%; c) has a viscosity of 120,000 cps or less at 180 ° C and under an applied shear stress of 80 sec-1. 2. Threads or strips formed from a hot melt elastomeric adhesive composition, comprising: 1) from 10 to 80% by weight of a styrenic block copolymer comprising at least two styrenic end blocks and at least one rubbery middle block per molecule and containing less than 40% by weight of the total block copolymer of a block copolymer containing only one styrene block and one rubbery block per molecule; 2) from 20 to 90% of a tackifier-type resin essentially compatible only with the rubbery median blocks; 3) from 0 to 40% of a plasticizer; 4) from 0 to 20% of an aromatic resin; the composition being also characterized by the fact that: a) is able to bind, when applied in the molten state, to plastic and / or cellulosic materials with a peel force at 90 ° of not less than 0.5 N / cm (as defined here); b) has a retention of tensile strength after 50 cycles (as defined here) of at least 40%; c) has a viscosity of 120,000 cps or less at 180 ° C, given an applied shear stress of 80 sec-1. Threads or strips according to claims 1 or 2, wherein the composition is a low modulus composition having a modulus of 0.5 MPa or less at 500% elongation, measured at 23 C with an elongation rate of 500 znm / minute. 4. Yarns or strips according to claims 1 or 2, wherein the composition is a high modulus composition having a modulus of more than 0.5 MPa at 500% elongation, measured at 23 <e> C with an elongation rate of 500 mm / minute. 5. Threads or strips according to any one of claims 1 to 4, wherein the composition has a teak loop greater than 2.5 N / cm and a 90 ° peel greater than 3 N / cm (separation speed 300 mm / cm minute) (both values, measured as defined here). Threads or strips according to any one of claims 1 to 5, wherein the composition has a retention of the tensile strength, after 50 cycles, of at least 50%. 7. Threads or strips according to any one of claims 1 to 6, wherein the composition has an actual rheological solidification temperature of 80 ° C or more (as defined herein). Θ. Yarns according to any one of claims 1 to 7, which have a diameter between 0.4 mm and 3 nos in the unstretched state. A method for producing yarns or strips according to any one of claims 1 to 8, which comprises extruding the composition as defined in any one of claims 1 to 8 onto a movable surface, cooling and optional ironing of threads or strips. Method according to claim 9, wherein the movable surface is a conveyor belt or a conveyor roller. 11. A process according to claim 9 or 10, wherein the composition is extruded from a hot melt applicator comprising one or more extrusion heads. Method according to claim 11, wherein the extrusion heads are controlled by a mechanical system which adjusts the angular position of the head. Method according to any one of claims 9 to 12, wherein the speed of the movable surface is variable. 14. A process according to any one of claims 9 to 13, wherein the threads or strips are cooled by means of an air blowing system. Process according to any one of claims 9 to 14, wherein the threads or strips are intended for subsequent application to a raw material and the movable surface is made up of or coated with a material to which the threads or strips are less adherent than not to the raw material. 16. Process according to claim 15, wherein the material is silicone. 17. Process according to claim 15 or 16, wherein the transfer to a raw material is obtained by means of an idle cylinder which brings the raw material into contact with the threads or strips. 18. Process according to any one of claims 15 to 17, wherein the raw material moves at a speed greater than the conveyor belt to perform elastic stretching of the threads or strips during transfer onto the raw material. 19. Process according to any one of claims 9 to 18, wherein the threads or strips pass from the conveyor through at least one reed which moves in a plane parallel to the raw material being transferred onto the raw material, to make the threads or strips describe a curve with respect to the raw material. 20. A method according to claim 19, wherein the or each reed is movable by means of a cam. 21. Equipment for applying a hot melt elastomeric adhesive to a raw material, which includes: an extrusion medium for the hot melt elastomeric adhesive; a movable surface positioned near the extrusion means so that the wires or strips extruded from the extrusion means come into contact with the movable surface at the exit from the extrusion means; a means for bringing a continuous layer of raw material into contact with the threads or strips on the movable surface so that the threads or strips are transferred onto the raw material. 22. Apparatus according to claim 21, further comprising at least one comb movable in a plane parallel to the raw material, through which the threads or strips pass during the transfer from the movable surface to the raw material and which gives the threads or strips a non-linear trend in the application on the raw material. 23. Apparatus according to claim 22, wherein the or each reed is movable by means of a cam. 24. Apparatus according to any one of claims 21 to 23, wherein the movable surface has a non-stick surface. 25. Apparatus according to any one of claims 21 to 24, wherein cooling means are provided for the wires or strips after extrusion. 26. Apparatus according to claim 25, wherein the cooling means is an air blowing system. 27. Apparatus according to any one of claims 21 to 26, wherein the raw material is brought into contact with the wires or strips on the movable surface by means of an idle roller. 28. Apparatus according to any one of claims 21 to 27, wherein the raw material moves at a speed greater than the movable surface to carry out the elastic drawing of the threads or strips during the transfer onto the raw material.