MXPA99010771A - Polypropylene/polystyrene multilayer film structures - Google Patents

Abstract

An oriented co-extruded film suitable for use as a window film for mailing envelopes, having a polyolefin layer and a non-glossy surface layer of polystyrene.

Description

STRUCTURES OF PELI CULAS OF MULTI PLES LAYERS OF POLI PROPI LENO / POLIESTI RENO This invention relates to a transparent or translucent multilayer film having at least one layer of lower gloss, which can be easily glued to paper. The film has a particular utility for windows in envelopes. It is known to use polymeric films for windows in paper envelopes. Such window envelopes save the resources needed to separately direct an envelope on its surface, or apply a label with the address to the envelope. A letter, an invoice or other content can be shaped to adjust, as per dollar, as necessary, to place the address of the intended recipient that appears in the contents in a position so that it is visible through the window comprising the polymer film. Films are known that have applicability as envelope windows. DE-A-4312388.0 discloses a transparent stretched polypropylene film, which has a reduced reflecting surface, which can be bonded. The film comprises polypropylene of at least two layers. The reduced reflection is obtained by incorporating a substantially propylene / ethylene copolymer incorporating linear low density polyethylene and maleic anhydride. The films described are drawn on two axes. DE-A-4312388 0 reports that polystyrene film and PVC film have been used as envelope windows. Biaxially oriented polypropylene films have been used, but such films are reported there as not easily sticking to paper. by means of fast run envelope manufacturing equipment. However, polystyrene surfaces are known to envelope manufacturers. The adhesion products for gluing the window film to envelopes are known and effective. However, polypropylene offers a cost advantage over polystyrene. It is known to prepare films having layers of polypropylene and polyethylene from GB-A-2281 282 and GB-A-21 1 1 908. JP-A-51 -28879 promotes a biaxially stretched film of propylene and laminate of thermoplastic elastomer of styrene butadiene. It is claimed that this film exhibits good heat sealing characteristics. J P-A-55-57428 describes a polystyrene film laminated in polypropylene, which subsequently is biaxially oriented (stretched). Its use is proposed as a suitable tape for wrapping heavy objects. JP-A-55-148155 discloses three-layer films comprising: 1) a layer of styrene and polystyrene-butadiene in block in combination with a solvent for the styrene-butadiene block copolymer, 2) a flexible layer, which it can be polypropylene and ethylene-vinyl acetate co-polymer, and 3) a drag layer, which can be polypropylene. The use of the film is proposed as storage bags that open easily for food, which remain flexible at freezing temperatures, and which can also withstand boiling water. JP-A-55-163164 also discloses a food container formed from a biaxially stretched polystyrene film "onto which a polypropylene layer is emptied or laminated." JP-A-80-23746 describes a food packaging film. of polystyrene, having a polypropylene layer of 1/10 to 1/100 thickness of the polystyrene layer The film of the layer is stretched JP-A-56-27326 describes a laminated sheet of polystyrene / butadiene / styrene and polypropylene, which is biaxially stretched It is proposed that the film is used for adhesive tapes JP-A-89-174324 discloses a biaxially stretched polypropylene film having reduced luster and gloss, as a wrapping material.Polypropylene is coated with polypropylene. a styrene-containing emulsion, after corona discharge treatment JP-A-63-110150 shows the lamination of a polystyrene film and a polyolefin film to be used as bags NL-A-6 9-13608 discloses a stencil film of oriented polypropylene film optionally laminated with polystyrene. US-A-4608284 shows a laminate of a oriented polystyrene film, which may have been printed on the laminated surface, and a polypropylene film, which has not been oriented. An adhesive bonds the films to their interfacial interface surfaces. The films find use as printed labels shrinkable with heat.

US-A-5468563 discloses films that are claimed to have utility as a window film prepared from polypropylene. It is suggested that a reduced reflection layer be ethylene modified with rubber and maleic anhydride, co- and propylene / ethylene / butylene terpolymer having propylene as the main component, or propylene / polyethylene block copolymers. The core layer is comprised of isotactic polypropylene, a random propylene copolymer. A second outer layer is constituted by the materials of the core layer having added anti-block agents. The layers can be co-extruded and then stretched. EP-A-01 22495 discloses biaxially oriented polypropylene films laminated to an olefin polymer film having at least three peaks between 120 ° C and 65 ° C when analyzed by DSC (differential scanning calorimetry). It is said that multiple peaks produce low brightness and a high haze. It is suggested as a final use a decorative wrapping paper having a metallized surface thereon, for example, by rolling. JP-A-51-19079 discloses a multi-layer composite film, in which a layer is formed from a polyolefin (such as, polypropylene) and a layer is a thermoplastic layer obtained by adding 5-30 parts by weight of a styrene-butadiene copolymer copolymer to a polystyrene resin. The multilayer film is manufactured by coextrusion and thermal fusion of the layers. No orientation of the films is made.

US-A-4464439 discloses a co-extruded sheet formed from at least one sheet of crystalline polypropylene and at least one sheet of polymeric material consisting of a mixture of high impact polystyrene, crystalline polypropylene and styrene block copolymer / diene monomer. No guidance was given to any of the films. GB-A-1 357950 discloses a thermoplastic laminate having at least two layers, one of which is a ternary mixture of a homopolymer or copolymer of an aromatic vinyl monomer; a homopolymer or copolymer of an olefin monomer; and a block copolymer having a polyvinyl / aromatic block and a polyolefin block. No orientation was made of the films. Film for envelope windows requires several properties. As noted in US-A-5468563, several important properties include: ability to glue the window film to the envelope paper using conventional bonding agents in the industry, and transparency, so that the address can be read at through the envelope by postal postal machines. Although transparency is important, a window that is transparent and also bright reflects light. The reflected light can damage the efficiency of postal machines and indirectly, the date of arrival of the envelope and contents to the recipient. According to the invention, an oriented polymeric film is provided by co-extruding a first layer having at least 96 percent by weight of a polypropylene and an outer layer having (by weight) a) from 70 to 97% by weight. polystyrene, polystyrene modified with rubber or a mixture thereof, and b) from 3 to 30% of a polypropylene / polystyrene compatibilizer selected from the group of ethylene vinyl acetate copolymer and block or random block copolymers of an aromatic compound of vinyl and an olefin, where the film has a minimum tension ratio of 3 in the machine direction, and 4 in the transverse direction. Polymeric multilayer films can be formed by co-extruding the layers of the film. However, when a film is formed from a two-layer co-extrudate according to the invention, it is found that the co-extruded layers are separable simply by pulling them by hand, for example, by using adhesive tape. office attached to each side of the film. Now it has been discovered that, if the co-extrudate layers are oriented both in the machine direction and in the transverse direction after the film has been co-extruded, then this results in a film, in which the co-extruded layers are extremely difficult to separate. Another advantage of oriented films is that they have a larger modulus (greater stiffness) than non-oriented films. Stiffness (high modulus) is a property required for the window envelope film for two reasons; so that the envelope converter allows an optimum feeding of the window patch unit in the envelope machine, and to minimize the wrinkling / buckling of the window panel in the final envelope.

The "first layer" comprising polypropylene is referred to from time to time as a "core layer", because for economic reasons, it will usually be the predominant layer, and because external layers are often provided in both sides of the polypropylene layer. However, the description "core layer" should not be taken to imply that external layers are necessarily provided on both sides of the core layer. Applicants have observed that polystyrene meets many of the limitations required by the envelope window film. However, polyolefins provide an advantage in terms of cost. The films according to the invention are advantageously co-extruded from a comparatively less expensive core layer of polypropylene. A layer (usually thinner) substantially comprising polystyrene provides the required gloss and paper-bonding capacity when used with conventional glue for window film applications. The polypropylene used in the core layer may be a homopolymer or copolymer and may preferably be predominantly isotactic polypropylene having a density from 0.895 to 0.91 0, and a melt index from 0.5 to 10 g / 10 min ( 230 ° C / 2, 16 kg). Optionally, additional conventional components for the film industry can be added to the polypropylene core layer, for example, slip control additives, such as, erucam ida, estearamide or oleam ida; additives for block control, such as, silica or calcium carbonate; additives for static control, such as ethoxylated fatty acid amides, alkyl benzene sulfonates or polyethylene glycol esters; additives to aid clarity of the film, such as, dibenzylidene sorbitol; and additives to aid in the processing, such as a fluoroelastomer or a polyamide. The amounts of each optional component can vary from zero to one percent by weight. In the aggregate, all optional components generally do not exceed four, preferably no more than three percent by weight of the polypropylene layer. In one embodiment, the amount of polystyrene is from 50% to 95% by weight, and the amount of rubber-modified polystyrene is from 0 to 50% by weight based on the total weight of the outer layer. The polystyrene used in the outer or low gloss / bond layer is preferably general purpose polystyrene (GPPS), preferably GPPS having a melt index of 0.5 to 15 g / 10 min (200 ° C 5 kg weight) . Optionally present in the outer layer or low gloss / capable of being joined, is high impact polystyrene (HIPS). The HIPS may be present in an amount from zero to fifty weight percent of the low gloss layer. The H I PS preferably has a melt index of 0.5 to 15 g / 10 min (200 ° C 5 kg of weight) and a rubber content of 4 to 1 5 percent by weight. The outer layer or low gloss / capable of being joined also includes from three to thirty percent. and preferably 1 5% of a compatibilizing compound useful for compatibilizing polypropylene and polystyrene. A preferred compatibilizing component is an ethylene vinyl acetate copolymer, having a vinyl acetate content of fifteen to thirty percent by weight and a melt index of 0.5 to 25 g / 10 min (1 90 ° C 2. 1 6 kg) of weight). A further preferred compatilizer is a block or random copolymer of an aromatic vinyl compound and an olefin. Suitable compatibilizing components, which are styrenic copolymers include: styrene-butadiene-styrene (SBS); styrene-isoprene-styrene (SIS); styrene-isoprene (SI); styrene-butadiene (SB); styrene-ethylene-butylene-styrene (S E BS) having a melt index of 0.5 to 35 g / 10 min (200 ° C, 5 kg of weight) and a styrene content of 10 to 50 weight percent; and ethylene-styrene interpolymers having a melt index of 0.5 to 40 g / 1 0 min (190 ° C 2.16 kg of weight) and a styrene content of 5 to 80 weight percent. The presence of the compatibilizing component in the outer layer is also an advantage for the disposition of the film, which is not of a suitable grade for sale. Small amounts of the co-extruded film, including the low gloss tie layer, which comprises polystyrene film, can be included with the polypropylene core layer as a "re-ground". In this way, the waste generated in manufacturing can become useful products. The incorporation of the re-ground film is advantageously in a minimum, but up to 25 weight percent of re-ground may be incorporated in the propylene core layer when com- patibilizers are included within the ranges described in the outer layer. . Excess brightness results in reflection of light from the surface of the film. The reduction of brightness usually also leads to opacity of the film, thus making it difficult to read the direction through the film. The brightness level of the film is preferably below 1 05%, more preferably below 1 00%. The film is extruded, preferably, from a circular die or a slot. The orientation of the film occurs after the film is co-extruded. The film can be reheated from 120 ° C to 170 ° C, and oriented in the Machine Direction (MD) to form a film from 3 to 7, preferably from 4 to 6, times the original length and in the transverse direction from 4 to 12, preferably from 5 to 9, alternatively from 6 to 10 times the original width. The orientation of the film can occur simultaneously or sequentially. Different reheating temperatures can be used for the MD and TD orientations. Then, the oriented film can be optionally tempered, for example, at a temperature from 5 to 15 ° C above the orientation temperature. Each step in the orientation process of the film is generally completed in a few seconds, for example, less than three seconds, after the film reaches the chosen temperature. The co-extrudate consists of 2 or more layers. Additional low brightness / ion layers can improve the loss properties of gloss even though the additional low gloss layer may be on the opposite side of the film from the measured surface. It is important for the window film that the film is capable of forming a strong bond to paper. The bond strength is preferably at minus 3N / 2.5 cm, preferably at least 4 N / 2.5 cm. In a second aspect of the invention, there is provided a polymeric film comprising a co-extrudate of a first layer having at least 96 percent by weight of a polypropylene and an outer layer having (by weight) a) from 50% up to 95% polystyrene b) from 0 to 50% rubber modified polystyrene c) from 0 to 30% of a polypropylene / polystyrene compatibilizer, selected from the group that ethylene vinyl acetate copolymer and block or random copolymers of a compound aromatic devinyl and an olefin, where the sum of the components b and c is from 5% to 50% of the outer layer. In an alternative embodiment, said film is oriented as described above, and in the preferred proportions described above. In a third aspect of the invention, there is provided a window envelope having a transparent panel, which is formed from a polymeric film as described above. In a fourth aspect of the invention, the use of a polymeric film as described above is provided as a transparent panel in a window envelope.

The following examples illustrate the invention, but are not intended to limit the scope of the invention. Example 1: A symmetrical three layer film structure of "A-B-A" is produced with a polypropylene core and low gloss outer layers using a cast coextrusion process. The polymer of the core layer and the polymer of the low gloss layer are extruded from separate extruders at a temperature of 225 to 230 ° C. the polymers are combined in a feed block, and then flow in a die-holder type slot die having an opening of 635 μm. The polymer that emerges from the die is then emptied on a cooling roller at 24 ° C. The thickness of the three-layer films in the cooling roller is 345 μm. After extrusion, the film is re-heated to 150 ° C and stretched at a 3 to 1 ratio in the machine direction, and at a 5 to 1 ratio in the transverse direction. The composition of the film and the thickness of each layer are listed in Table 1.

Examples 2-4 Examples 2-4 were prepared in the same manner as in Example 1, but substituting the composition, proportions and post-treatment stated in Table 1.

Comparative Example 1 A polymer film is extruded from a die having a 635 μm aperture onto a cooling roll having a temperature of 24 ° C. The thickness of the extrudate is 500 μm. The film is comprised of 1 00% polypropylene homopolymer with a melt index of 3.0 g / 1 0 min (240 ° C / 2.1 6 kg) after extrusion, the film was stretched at a ratio of 5 to 1 in the direction of the machine and at a ratio of 5 to 1 in the direction transverse to 150 ° C up to a final thickness of 20 μm.

Comparative Example 2 Comparative Example 2 was a biaxially oriented, voided propylene film with a thickness of 36 μm (available from UCB Avenue Louise 326, B-1 050, Brussels, BELGIUM under the tradename RAYOPPM R 439). This film is a biaxially oriented polypropylene film having a polymeric acrylic coating on both sides.

Comparative Example 3 Comparative Example 3 was a 32 μm thick biaxially oriented polystyrene film, commercially available under the trade designation DOW Window Film ™ 6003E, available from Dow Deutschland Inc., and sold for use in envelope windows.

• TABLE 1 *: property measured in the low brightness layer of this film #: property measured in the high brightness layer of this film Before stretching the films of the examples, the co-extruded layers were separable simply by pulling them by hand, using an office adhesive tape attached to each side of the film. After subsequent stretching, the films of the invention were extremely difficult to separate. The films of the examples are evaluated according to standard test methods. The brightness is measured in the direction of the film machine (MD) according to ASTM D2457 with a reflection angle of 60 degrees, using a Dr. Lange reflectometer type LMG 064. Unless noted, the film The particle is measured in both sides and the values are averaged. The secant module 1% is measured according to ASTM D882 using a sample width of 25.4 μm, a pulling speed of 200 mm / min, an initial distance between the jaws of 200 mm. The elongation is measured according to the sample width of ASTM D882, 25.4 mm, pulling speed 501 mm / min, initial distance between the jaws of 51 mm. The bond strength of Examples 1 to 4 and Comparative Examples 1 to 3 is measured as follows: a commercially used glue for applying window films to paper envelopes, HP375 QUAID 8N available from the Chemical Industries of Adhesive, S.A. (QU IADSA, Calle Valdelacueva s / n 28880 Meco, Madrid, SPAIN) is applied to 8 μm thick one half of a paper cut at 8 cm by 1 0 cm. The paper is 70g / m2 paper used for commercial envelopes. A sample of the 8 cm by 1 0 cm film is pressed manually on the paper.

After drying 24 hours at ambient conditions, the laminate is cut into 2.5 cm wide strips. The unbound ends of the paper and film are stapled into a tension tester. The maximum force required to separate the strips was measured at a pulling speed of 10 cm / m in. Examples 5 to 9 were prepared in the same manner as in Example 1, but substituting the composition, proportion of layers and post-treatment as stated in Table 2. Comparative Examples 4 to 5 were prepared in the same manner as in the Example 1, on replacing the composition, layer 5 ratio, and post-treatment as stated in Table 2. The bond strength in the paper of Examples 5 to 9 and Examples 4 to 5 were measured as it was described for Example 1, but with a different glue: ENVAFILM 204 from National Starch & Adhesives GmbH (Im altenschemel 55, 67435 Neustadt, Germany). After the orientation of the films, it was noted that some cracking occurred in the A layer (s) in some films. These films have some "holes" in layer A as a result of the cracking and shrinking of the layer during the orientation process. The percentage of hole area in the layer A or layers of the films was determined by measuring the total hole area in an average representative film surface of approximately 3000 cm2. Table 2 shows that Comparative Example 4 has a very high percentage of holes, incompatible with an economical commercial film production. Comparative Example 5 has a high brightness (above 100%) and a high% hole area. Table 2 shows that Examples 5 to 9 combine low gloss, good adhesion to paper and a very low percentage of hole area (the holes in these examples were located only near the edges of the film, in the vicinity of the orientation staples). This low level of holes is attributed to the presence of the PP / PS compatibilizer in the formulation of layer A of these films, which increases the interfacial adhesion between layers A and B.

TABLE 2 As mentioned before, another advantage of oriented films is that they have a higher modulus (greater stiffness) than non-oriented films. This is shown in Comparative Example 6, which describes the properties of a co-extruded film of the same composition as Examples 1 to 4, but not oriented. The modulus of this film is less than half of one of oriented film of Examples 1 to 4. Stiffness (high modulus) is a property required for the envelope window film for two reasons: to allow a feeding of optimal window patch unit of the envelope converter in the envelope machine, and to minimize the wrinkling / buckling of the window panel in the final envelope. The shared example 6 would be too loose for this application.

TABLE 3

Claims (9)

1. REIVI NDICATIONS 1 . An oriented polymeric film comprising a co-extrudate of a first layer having at least 96 percent by weight of a polypropylene and an outer layer having (by weight) a) from 70 to 97% polystyrene, rubber modified polystyrene or a mixture thereof, and b) from 3 to 30% of a polypropylene / polystyrene compatibilizer selected from the group of ethylene vinyl acetate copolymer and block or random block copolymers of an aromatic vinyl compound and an olefin, wherein the film has a minimum stretch ratio of 3 in the machine direction and 4 in the transverse direction.
2. 2. The polymeric film of claim 1, wherein the amount of polystyrene is from 50% to 95% by weight and the amount of rubber modified polystyrene is from 0 to 50% by weight based on the total weight of the layer external 3.
3. The polymeric film of any of the preceding claims, wherein the stretch ratio in the direction of the machine is from 4 to 6.
4. The polymeric film of any of the preceding claims, wherein the Proportion of stretch in the transverse direction is from 6 to 10.
5. 5. A polymer film comprising a co-extrudate of a first layer having at least 96 percent by weight of a polypropylene and an outer layer having (by weight) a) from 50% to 95% polystyrene b) from 0 to 50% by weight rubber-modified polystyrene c) from 0 to 30% of a polypropylene / polystyrene com pound selected from the group of ethylene vinyl acetate copolymer and block or random block copolymers of an aromatic vinyl compound and an olefin, wherein the sum of the components b and c is from 5% to 50% of the outer layer.
6. 6. The polymeric film of any of the preceding claims, wherein the rubber modified polystyrene contains from four to five percent rubber.
7. 7. The polymeric film of any of the preceding claims, wherein the compatibilizer is a vinyl acetate / ethylene copolymer having a vinyl acetate content of 15% to 30% by weight, and a melt index of 0.5. up to 25g / 10 min (190 ° C 2.16 kg of weight).
8. 8. The polymeric film of any of the preceding claims wherein the compatibilizer is styrene-butadiene-styrene (SB); styrene-isoprene-styrene (SI S); styrene-isoprene (SI); styrene-butadiene (SB); styrene-ethylene-butylene-styrene (SEBS) or an ethylene-styrene interpolymer (ES I).
9. 9. The polymeric film of any of the preceding claims, wherein the outer layer of the film has a brightness of not more than 1 00% according to ASTM D2457. 1. The polymeric film of any of the preceding claims, wherein the film has a paper adhesion of at least 3 N / 2.5 cm. eleven . The polymeric film of claim 5, which is an oriented film having a stretch ratio in the machine direction from 3 to 7, a stretch ratio in the transverse direction from 4 to 1 2, and wherein the The amount of compatibilizer is 15% based on the total weight of the outer layer. 12. A window envelope having a transparent panel, said panel is formed from a polymeric film, as claimed in any of the preceding claims. 3. The use of a polymeric film as claimed in any of claims 1 to 11, as a transparent panel in a window envelope.