AU2022299275A1 - Biodegradable pressure-sensitive adhesive - Google Patents

Description

BIODEGRADABLE PRESSURE-SENSITIVE ADHESIVE

The invention relates to the technical field of pressure-sensitive adhesives, of the kind used in numerous areas of technology for the temporary or permanent bonding of a plethora of substrates, and in medicine as well, for equipping contact materials for bonding on the skin. The invention more specifically proposes a polyhydroxyalkanoate-based pressure-sensitive adhesive whose biodegradability is a marked improvement on that of conventional pressure sensitive adhesives (PSAs).

In almost all areas of life, there have for several years been increasing concerns around greater sustainability. Against this background, an awareness has also grown of the ecological problems associated with the entry of persistent plastics particulates into the environment. Accordingly, there is increasing demand for products which, following their disposal, transform simply and quickly into harmless breakdown products. The greatest attention here is being paid to biodegradable plastics and to materials based on biodegradable polymers.

The term "biodegradable polymers" is used for natural, bioidentical and synthetic polymers which, in contrast to conventional plastics, are broken down by a host of microorganisms in a biologically active environment (compost, digested sludge, soil, wastewater); this does not necessarily take place under customary household conditions (garden composting). A definition of biodegradability is found in European Standard DIN EN 13432 (biodegradation of packaging) and in International Standard ISO 14855-1 (aerobic biodegradability of plastics).

The skilled person distinguishes between disintegration and biodegradability. Disintegration refers to physical breakdown into very small fragments.

Among texts describing how to determine the disintegrability or degree of disintegration of polymers is DIN EN ISO 20200. In this case, the sample under analysis is stored at 58± 20C in a defined artificial solid waste for at least 45 and not more than 90 days. Thereafter the entire sample is passed through a 2 mm sieve and the degree of disintegration D is determined in accordance with equation (1) below:

D= M,-,x100 (1)

Here,

mi is the initial dry mass of the sample material

and

mr is the dry mass of the residual sample material obtained by sieving.

Biodegradability is understood in general as the capacity for a chemical compound or an organic material to be broken down by microorganisms either, in the presence of oxygen, into carbon dioxide, water and salts of other elements present (mineralization), with formation of new biomass, or, in the absence of oxygen, into carbon dioxide, methane, mineral salts and new biomass. The biodegradation is accomplished outside and/or inside the cell by bacteria, fungi and microorganisms and also their enzymes. Standards governing the biodegradability of plastics materials include ISO 14855-1:2012, "Determination of the ultimate aerobic biodegradability of plastic materials under controlled composting conditions", and a document referenced therein, DIN EN 13432, "Requirements for packaging recoverable through composting and biodegradation". Here, the material for testing is subjected to an aerobic degradation test, and within not more than six months, in comparison to a suitable reference substance, a degree of degradation of at least 90% must be achieved. The degree of degradation here is determined by the measured evolution of carbon dioxide. After comminution, the sample is stored with vermiculite or well-working aerated compost as inoculum in a vessel equipped with air supply at 58± 20C and the evolution of C02 is recorded on an ongoing basis. In view of the complex apparatus involved, there are a number of testing institutes which have specialized in the test and which subsequently issue a corresponding certificate.

After the end of testing, the degradation rate Di is obtained in accordance with equation (2) below as:

Dt = (CO2)T - (CO2)B X 100 ThCO 2 (2)

Here,

(CO2)T is the cumulative amount of carbon dioxide formed in each composting vessel containing the test substance, in grams per vessel;

(CO2)B is the average cumulative amount of carbon dioxide formed in the control vessels, in grams per vessel;

ThCO2 is the theoretical amount of carbon dioxide which the test substance can form, in grams per vessel.

As well as the biodegradability, DIN EN 13432 also includes a test for determining the quality of the compost produced by the degradation. This compost must not have any adverse effects on plant growth. Generally speaking, biodegradable components also have a high disintegration rate, whereas the disintegration of a material does not automatically imply its biodegradability.

Examples of further relevant standards relating to determining and evaluating the biodegradability of plastics are ISO 17088, "Plastics - Organic recycling - Specifications for compostable plastics" and ASTM D6400, "Standard Specification for Labeling of Plastics Designed to be Aerobically Composted in Municipal or Industrial Facilities".

Biodegradability in the context described above is also becoming increasingly important for pressure-sensitive adhesive tapes and medical products; in this respect, the possibilities for producing suitable PSAs are themselves very limited. PSAs are amorphous materials with a low glass transition point. According to the foundational European standards as exemplified by DIN EN 13432, the classic scaffold polymers such as natural rubber, styrene block copolymers or polyacrylates are not biodegradable. The same applies to the usual tackifier resins such as rosin derivatives, hydrocarbon resins or terpene-phenol resins. The outstanding ageing stability of silicone PSAs rules them out entirely.

PSAs find a broad spectrum of use in the manufacture of wound management products and for other medical applications. A precondition for this is that such adhesives are skin-friendly. For example, they must contain only very low quantities of residual monomers and are not to exhibit any irritant or allergenic potentials.

Principal functions of the adhesives used are to bond the intended skin contact article reliably to the skin, and, at the end of the usage period, to allow it to be detached and removed from the skin easily and with as little pain as possible. The former property is often regarded as the most important property of a plaster or similar article, but cannot always be achieved, as perspiration and secretion of sebum, skin oil or sweat lead to unwanted instances of premature detachment from the skin. How quickly the bond parts of itself is dependent, generally, on the fluid absorption capacity of the wound contact material and/or of the adhesive, and on the water vapour transmissibility of the article overall. One objective criterion for differentiating the breathability of plaster adhesives is represented by the determination of the water vapour permeability as the moisture vapour transmission rate (MVTR), where a defined amount of water evaporates through an area covered by the plaster, in a period of 24 hours and at a temperature of 35°C, for example, and is quantified by differential weighing. (The MVTR is measured according to various standards, including DIN EN 13726-2, which yield results that are not comparable among one another.)

As well as the above-described requirements which must be met by plasters in use, there are further properties and regulations that must be heeded. They are defined in the current UNEP and ECHA guidelines.

Particularly for medical products that are generally available and that accordingly may be regarded as consumer goods, disposal issues are acquiring ever greater importance. These issues have very great effects on the manufacture and marketing of such products. Sustainable approaches and, in this context, the biodegradability of the products as well are becoming increasingly important to consumers, and for that reason are coming more and more under the spotlight of the manufacturers.

The presence of heteroatoms in the carbon main chain is generally seen to be a structural condition for biodegradability. Owing to the higher polarity of the bond and also to the free electron pairs present in the heteroatoms, this structural feature offers higher reactivity than C C bonds, often resulting in greater amenability to microbial breakdown, through hydrolysis, for example. Advantageous polymers as a basis for biodegradable adhesives are therefore those containing functional groups such as, for example, ester, amide, imine or enamine groups, which are then able to act as points of attack for biological breakdown.

CN 102867459 A describes in this context a biodegradable sticker label comprising a surface material layer, an adhesive layer and a carrier paper, with the adhesive layer forming the middle ply.

The carrier paper consists of glassine paper coated with silicone oil;

the surface material layer comprises the following composition:

- 30 to 70 parts by weight of polyhydroxyalkanoate - up to 150 parts by weight of polylactic acid - up to 50 parts by weight of starch - 1 to 15 parts by weight of plasticizer - 0.3 to 1 part by weight of nucleating agent and - 1 to 10 parts by weight of lubricant; and the adhesive layer comprises the following composition:

- 20 to 60 parts by weight of starch - 90 to 110 parts by weight of water - 5 to 50 parts by weight of polyhydroxyalkanoate - 0.5 to 20 parts by weight of alkaline matter - 0.5 to 10 parts by weight of impregnating agent - 0.1 to 5 parts by weight of crosslinker and - 0.1 to 5 parts by weight of bacteria-inhibiting material.

Complete biological breakdown is said to be achieved without compost treatment.

In an extended context, EP 2 305 324 Al relates to a biocompatible medical implant for soft tissue applications that comprises a poly(4-hydroxybutyrate-co-hydroxyalkanoate) copolymer composition, with the copolymer having a weight-average molecular weight of 10 000 to 10 000 000 daltons.

The overall finding is that there is an ongoing demand for powerful PSAs with a wide spectrum of possible uses and a regard for aspects of sustainability.

It was an object of the invention to provide a pressure-sensitive adhesive which exhibits good technical adhesive parameters and is biodegradable in the sense of the applicable standards. A supplementary object of the invention was to provide a pressure-sensitive adhesive of this kind that can be produced at least in part and ideally as a whole from bio-based raw materials.

A first and general subject of the invention with which the object is achieved is a pressure sensitive adhesive which comprises at least one polyhydroxyalkanoate and which is characterized in that the polyhydroxyalkanoate

- comprises 20% to 75% by weight of structural units deriving from 3-hydroxybutyric acid (3-HB); and - comprises at least one further structural unit deriving from a hydroxyalkanoic acid selected from the group consisting of 4-hydroxybutyric acid (4-HB), 3-hydroxyvaleric acid (3-HV), 4-hydroxyvaleric acid (4-HV), 3-hydroxyhexanoic acid (3-HX) and/or 4 hydroxyhexanoic acid (4-HX).

PSAs of this kind have proved to be powerful and reliably biodegradable.

A "pressure-sensitive adhesive" (PSA) is understood, in agreement with the general understanding, to be a material which possesses the property of entering into a permanent bond to a substrate under just relatively low applied pressure. PSAs generally have a permanent inherent tack at room temperature, meaning that they exhibit a certain viscosity and touch-stickiness. This is attributed in particular to their wetting of a substrate surface with just low applied pressure.

Without wishing to be bound by this theory, it is frequently assumed that a pressure-sensitive adhesive may be regarded as a fluid of extremely high viscosity which has an elastic component and which, accordingly, has characteristic viscoelastic properties leading to the above-described permanent inherent tack and adhesiveness. It is assumed that PSAs on mechanical deformation are subject both to processes of viscous flow and to development of elastic forces of resilience. The proportional viscous flow is used to achieve adhesion, while the proportional elastic forces of resilience are needed in particular to achieve cohesion. The relationships between the rheology and the pressure-sensitive adhesiveness are known in the prior art and described for example in Satas' "Handbook of Pressure Sensitive Adhesive Technology", Third Edition (1999), pages 153 to 203.

To characterize the extent of elastic and viscous components, it is usual to employ the storage modulus (G') and the loss modulus (G"), which can be ascertained by dynamic mechanical analysis (DMA), making use, for example, of a rheometer, as disclosed for example in WO 2015/189323 Al.

In the context of the present invention, an adhesive is preferably deemed to have pressure sensitive qualities and hence to be a pressure-sensitive adhesive when at a temperature of 23°C in the deformation frequency range from 100to 101 rad/sec, G' and G" are each situated at least partly in the range from 103 to 10 Pa.

Polyhydroxyalkanoates, frequently also referred to in abbreviated form as PHAs, are polyesters which are derivable formally, on a monomer basis, from one or more hydroxyalkanoic acids. They have, accordingly, a structure of the formula H-[-O-R-C(O)-]-OH, in which R is a branched or unbranched and functionalized or non-functionalized alkylene radical. From a technical standpoint, however, PHAs are generally not synthesized via polymerization of hydroxyalkanoic acids, but instead are produced biologically by bacterial cultures. The composition of these PHAs is controlled through the biological material made available to the bacteria, and in particular through the selection of the bacteria.

The hydroxyalkanoic acids on which the polyhydroxyalkanoate in the PSA of the invention is formally based are preferably hydroxylated in 3- or 4-position. On its main chain, therefore, in part, the polyhydroxyalkanoate has a pendant functionalization composed of alkyl groups. In accordance with the invention, the polyhydroxyalkanoate comprises 20% to 75% by weight of structural units deriving from 3-hydroxybutyric acid. On these structural units, therefore, the PHA has methyl groups projecting from the main chain.

In addition, the polyhydroxyalkanoate of the PSA of the invention has structural units deriving from 4-hydroxybutyric acid (4-HB), 3-hydroxyvaleric acid (3-HV), 4-hydroxyvaleric acid (4-HV), 3-hydroxyhexanoic acid (3-HX) and/or 4-hydroxyhexanoic acid (4-HX), and so structural units without pendant functionalization, and also further methyl or else propyl side chains projecting from the main chain, are also possible.

The polyhydroxyalkanoate preferably comprises structural units (3-HB) at 30% to 65% by weight, more preferably at 35% to 60% by weight. In this way, a substantial part of the pendant functionalization consists of short alkyl radicals. This is advantageous because pendant functionalization of this kind endows the polymer with an amorphous character.

In one embodiment, the polyhydroxyalkanoate also comprises structural units deriving from (4 HB), and the (3-HB):(4-HB) weight ratio is from 5.5:4.5 to 2:3, preferably from 1.05:0.95 to 0.95:1.05. It has emerged that in this way, polymers with particularly amorphous character and comparatively low glass transition temperature are obtained, which therefore, with their rheological profile, are very suitable for use in PSAs.

All in all, in the sense of the best-possible pressure-sensitive adhesive properties, it is preferred for the at least one polyhydroxyalkanoate of the PSA of the invention to have only very low crystallinity. In this sense, the at least one polyhydroxyalkanoate of the PSA of the invention preferably has an enthalpy of fusion of 15 J/g, more preferably of 10J/g. With very particular preference, the at least one PHA of the PSA of the invention has no crystallinity.

Likewise preferably, the polyhydroxyalkanoate of the PSA of the invention has a glass transition temperature of less than 0°C. This property is beneficial to what is called the flow-on capacity of the PSA and hence ultimately likewise to the technical adhesive performance of the PSA.

The polyhydroxyalkanoate of the PSA of the invention may in principle comprise further structural units, deriving from hydroxyalkanoic acids beyond those so far identified. These structural units, however, are present preferably in minor proportions. Examples of such further structural units are 3-hydroxyheptanoic acid, 3-hydroxyoctanoic acid, 3-hydroxynonanoic acid, 3-hydroxydecanoic acid, 3-hydroxyundecanoic acid, 3-hydroxydodecanoic acid, 4 hydroxyhexanoic acid, 4-hydroxyheptanoic acid, 4-hydroxyoctanoic acid, 4-hydroxynonanoic acid, 4-hydroxydecanoic acid, 4-hydroxyundecanoic acid and 4-hydroxydodecanoic acid. With preference, however, the PHA exclusively comprises structural units deriving from 3 hydroxybutyric acid (3-HB), 4-hydroxybutyric acid (4-HB), 3-hydroxyvaleric acid (3-HV), 4 hydroxyvaleric acid (4-HV), 3-hydroxyhexanoic acid (3-HX) and/or 4-hydroxyhexanoic acid (4 HX).

The polyhydroxyalkanoate of the PSA of the invention is preferably crosslinked. This means that the originally individual macromolecules of the PHA are at least partly connected to one another through chemical bonds so as to constitute a more or less continuous, three dimensional network.

In one preferred embodiment, the crosslinking is radically initiated. Correspondingly, the polyhydroxyalkanoate of the PSA of the invention is preferably radically crosslinked.

The composition to be crosslinked for producing the PSA of the invention in this case preferably comprises one or more initiators which are able to set in train a crosslinking reaction that proceeds via formation of radicals. Suitable initiators are known in principle to the skilled person. Examples of suitable radical sources or initiators are peroxides, hydroperoxides, azo compounds and compounds having benzophenone substructures. The crosslinking is preferably initiated by at least one compound selected from the group consisting of peroxides and compounds having benzophenone substructures. In this sense, the PSA of the invention preferably comprises at least one compound selected from the group consisting of peroxides and compounds having benzophenone substructures. To the skilled person it is clear that this phrasing in the strict sense is addressed more to the composition to be crosslinked for producing the PSA of the invention, whereas in fact the decomposition products of the initiator are more likely in the PSA itself.

The crosslinking is initiated more preferably by a compound selected from the group consisting of dibenzoyl peroxide, cumene hydroperoxide, cyclohexanone peroxide, di-t-butyl peroxide, cyclohexylsulfonyl acetyl peroxide, dicumyl peroxide (DCP) and 4-benzoylphenyl acrylate. In the aforementioned sense, therefore, the PSA of the invention more preferably comprises at least one compound selected from the group consisting of dibenzoyl peroxide, cumene hydroperoxide, cyclohexanone peroxide, di-t-butyl peroxide, cyclohexylsulfonyl acetyl peroxide, dicumyl peroxide (DCP) and 4-benzoylphenyl acrylate.

The PSA, or the composition to be crosslinked for producing the PSA, preferably comprises one or more radical-forming initiators at in total 0.1% to 10% by weight, more preferably at 0.5% to 7% by weight, more particularly at 1% to 5% by weight, based in each case on the total weight of PSA or composition.

Activation of the initiators and hence of the radical crosslinking is accomplished preferably via UV radiation or heat. It may be advantageous, as well as the initiators, to use one or more auxiliaries for preventing or at least reducing chain degradation processes induced by free radicals. These auxiliaries or stabilizers improve the efficiency of the initiators by suppressing unwanted chain degradation reactions. Suitable stabilizers are multifunctional organic compounds with high reactivity towards free radicals. Preferred stabilizers are triallyl isocyanurate (TAIC) and tetraethylthiuram disulfide (TEDS).

In another embodiment, the polyhydroxyalkanoate is chemically crosslinked. This means that the crosslinking is brought about by means of a chemical crosslinker. This crosslinker is a linear or branched compound having at least two reactive centres, i.e. at least two functionalities in the molecule that are able to react with suitable functional groups of the polyhydroxyalkanoate to form chemical bonds. Preferred crosslinkers are compounds containing one or more epoxide groups, isocyanate groups, carbodiimide groups, C-C double bonds, aldehyde groups, OH groups, carboxylic acid groups and/or amino groups.

In the simplest case, such crosslinkers are incorporated by mixing into the composition containing the polyhydroxyalkanoate for crosslinking; subsequently, the crosslinking reaction is initiated. Initiation takes place in general through input of temperature, but may even commence at room temperature. For accelerating or, generally, for enabling the crosslinking reaction, it may be necessary to use a crosslinking catalyst or else a substance with accelerating effect.

In the simplest case, the chemical crosslinking takes place via the terminal functional groups of the polymer chains of the PHA - in general, therefore, via terminal COOH and/or OH groups. It is possible, furthermore, for the polyhydroxyalkanoate to possess functional groups suitable for a crosslinking reaction. In the non-crosslinked state, the PHA may comprise 0.1% to 25% by weight, more preferably 2% to 15% by weight, more particularly 5% to 12% by weight of structural units which contain functional groups suitable for crosslinking.

The functional groups suitable for a crosslinking reaction may also be contained in the pendant functionalizations of the structural units. The functional groups of the structural units of the PHA are preferably selected from the group consisting of C-C double bonds, hydroxyl groups, carboxyl groups and amino groups; more preferably, they are terminal hydroxyl and/or carboxyl groups of the PHA polymer chains.

Illustratively, a number of possible chemical crosslinking processes are described in more detail below.

For polyhydroxyalkanoates functionalized with C-C double bonds, up to 100 g of PHA in 60 to 150 g of solvent, for example in ethyl acetate or a benzine-acetone mixture, may be admixed with 1 to 2 g of radical initiator, e.g. 2,2'-azobis(2-methylbutyronitrile). Crosslinking may be carried out directly with this composition, but it is also possible to add 5% to 10% by weight, based on the PHA for crosslinking, of monoethylenically unsaturated compounds as reactive diluents or co-substrates, examples being (meth)acrylic esters, cyclic ketene acetals or itaconates. The crosslinking reaction is carried out under a nitrogen atmosphere and initiated either by UV radiation or by heating the reaction mixture to 800 C. The mixture is stirred for 5 hours and the crosslinked composition obtained is purified by precipitation or dialysis or else used without further purification.

PHAs functionalized with carboxyl and/or hydroxyl groups and having a weight-average molecular weight Mw of < 50 000 g/mol, preferably of 10 000 to 30 000 g/mol, may be cross linked with a diisocyanate (NCO), as for example with hexamethylene diisocyanate, isophorone diisocyanate and/or methylenediphenyl isocyanate. The amount of diisocyanate to be used here is dependent on the amount of carboxyl and hydroxyl groups in the PHA (OH number). The aim is for an NCO:OH ratio of around 1. In a final crosslinking step, the network obtained may be brought to the desired final degree of crosslinking through the addition of COOH-typical crosslinkers, examples being di- or multi-functional epoxide crosslinkers such as tetraglycidyl-m-xylenediamine. The degree of crosslinking is preferably 0.5% to 1% and is based on the carboxylgroups.

For correspondingly functionalized PHAs having a weight-average molecular weight Mw of > 50 000 g/mol, crosslinking takes place preferably with a polyisocyanate - for example, with trimerized hexamethylene diisocyanate. In this case the aim is for an NCO:OH ratio of 0.6 to 1.

The final crosslinking described may also be accomplished via polycarbodiimides; in particular, this is also possible for PHAs containing amino groups. This is typically done using 0.1% to 2% by weight of polycarbodiimide, based on the total weight of the PHA to be crosslinked.

Crosslinking of the polyhydroxyalkanoate in the PSA of the invention may take place not only by the crosslinking already described but also, in addition, via physical methods, as for example through the formation of blended systems with one or more substances selected from the group consisting of polylactic acid, cellulose, starch, sorbitol, mannose and polyglycerol.

The PSA of the invention may in principle comprise one or more polyhydroxyalkanoates as described above. All statements above relating to "the polyhydroxyalkanoate" of course apply equally, in the event of the PSA containing multiple polyhydroxyalkanoates as described above, to all conceivable fractional amounts and to the entirety of these polyhydroxyalkanoates.

Further to the constituents identified so far, the PSA of the invention may comprise further components, which are preferably likewise biodegradable. The PSA of the invention preferably comprises one or more components selected from the group consisting of UV stabilizers, antioxidants, hydrophilizing agents, tackifiers, fillers, pigments, dyes, flame retardants, foaming agents, antistatic reagents, plasticizers, surfactants, breathability enhancers and sugar esters. More particularly, the PSA of the invention comprises one or more tackifiers and/or plasticizers.

In one embodiment, the PSA of the invention comprises substances with antibacterial (antiseptic) activity and/or skincare substances.

The term "tackifier" or else "tackifier resin" is understood by the skilled person to refer to a resin-based substance which raises the tack of the PSA. Tackifiers are, for example, hydrocarbon resins (for example, polymers based on unsaturated C5 or C9 monomers), terpene-phenol resins, polyterpene resins based on a- and/orR-pinene and/or6-limonene, aromatic resins such as coumarone-indene resins or resins based on styrene or o methylstyrene, and also rosin and derivatives thereof, for example disproportionated, dimerized or esterified resins, examples being reaction products with glycol, glycerol or pentaerythritol. Preferred for use are natural resins such as rosins and derivatives thereof.

The addition of tackifiers in minimal amounts of up to 1% by weight is possible without detracting from the biodegradability of the PSA of the invention. Where larger amounts are added to the PSA, however, it is possible for its biodegradability to be lost. For this reason, tackifiers are preferably forgone.

Suitable miscible plasticizers are, for example, aliphatic and aromatic mineral oils, polyethylene glycol and polypropylene glycol, diesters or polyesters of phthalic acid, citric acid, trimellitic acid or adipic acid, liquid rubbers (for example nitrile or polyisoprene rubbers of low molecular weight), liquid polymers of butene and/or isobutene, acrylic esters, polyvinyl ethers, liquid resins and plasticizer resins based on the raw materials of tackifier resins, wool wax and other waxes, or liquid silicones. Particular preference is given to using plasticizers made from renewable raw materials, such as, for example, bio-based polyoxytrimethylene glycol, plant oils, preferably refined plant oils such as rapeseed oil and soyabean oil, for example, fatty acids or fatty acid esters or epoxidized plant oils, for example epoxidized soyabean oil. Use is made more particularly of biodegradable plasticizers, preferably diesters or polyesters of citric acid or of adipic acid. With further preference, the plasticizer, more particularly the biodegradable plasticizer, is used in amounts of up to 10% by weight (based on total PSA weight), more preferably in amounts of up to 5% by weight (based on total PSA weight), very preferably in amounts of up to 2.5% by weight (based on total PSA weight). As in the case of the tackifiers, it is possible to add any plasticizers in minimal amounts of up to 1% by weight without detracting from the biodegradability of the PSA of the invention. It is the case here as well that, where larger amounts are added to the PSA, it is possible for its biodegradability to be lost. For this reason, plasticizers are preferably forgone, or biodegradable plasticizers are employed.

A "breathability enhancer" is an additive that increases the water vapour transmissibility of the adhesive by comparison with an adhesive of otherwise identical composition. Preferred breathability enhancers are sorbitan esters.

The further additives already stated above are subject to the proviso already expressed for the tackifiers and plasticizers: in minor amounts of up to 1% by weight, the addition even of non biodegradable additives is possible without substantially affecting the biodegradability of the PSA. In the case of larger amounts added to the PSA of the invention, it is possible for the PSA to be no longer sufficiently biodegradable. With preference, therefore, additives, more particularly non-biodegradable additives, are forgone. Biodegradable additives, for example biodegradable fillers, conversely, may be used even in relatively large amounts.

The PSA of the invention may comprise at least one further polymer (B), which is different from the one or more polyhydroxyalkanoates and is likewise a polyester. The polyester (B) may be a further polyhydroxyalkanoate, but may also be a polyester different from a polyhydroxyalkanoate.

The PSA of the invention may be prepared in principle in any way contemplated by the skilled person.

For example, the components for the preparation of the PSA may simply be dispersed and mixed in a solvent, which is subsequently removed.

As has emerged, though, the PSA of the invention may also be obtained from the melt. In that case, the components are melted and mixed in the melt or incorporated in part into the melt of the copolymer. A further subject of the invention, accordingly, is a process for preparing a PSA of the invention which comprises

- melting at least one polyhydroxyalkanoate which o comprises 20% to 70% by weight of structural units deriving from 3 hydroxybutyric acid (3-HB); and o comprises at least one further structural unit deriving from an acid selected from the group consisting of 4-hydroxybutyric acid (4-HB), 3-hydroxyvaleric acid (3 HV), 4-hydroxyvaleric acid (4-HV), 3-hydroxyhexanoic acid (3-HX) and/or 4 hydroxyhexanoic acid (4-HX); - initiating crosslinking of the polyhydroxyalkanoate; and

- shaping the resulting composition into a web.

The process is performed in particular at low temperatures, preferably at less than 60°C, more particularly at less than 50°C and very preferably at less than 40°C. This makes it possible in particular to incorporate even temperature-sensitive substances, such as proteins, for example. The further potential components of the PSA of the invention that are described herein may also be incorporated into the melt in a particularly uniform manner.

A further subject of the invention is a multi-layer composite system which comprises at least one carrier material and a pressure-sensitive adhesive of the invention. The multi-layer composite system of the invention is preferably a contact material for bonding on the skin, or an adhesive tape.

In one embodiment, the multi-layer composite system is a contact material for bonding on the skin (also referred to as a "dressing "or "contact material" below), preferably on the human skin. Contact materials of the invention therefore offer access to biodegradable dressings which satisfy all technical adhesive and medical requirements.

Suitable carrier material for the contact material for bonding on the skin includes in principle all rigid, flexible and elastic sheetlike structures made of synthetic and natural raw materials. The carrier material consists preferably of an air-permeable and water vapour-permeable but water-impermeable polymer layer having a thickness of around 10 to 200 pm. Preferred carrier materials are those which after application of the adhesive can be used such that they meet the requirements imposed on a functional dressing. Illustratively, these include textiles such as woven, knitted, laid-scrim or nonwoven fabrics, additionally nets, films, foams and laminates of the aforesaid materials, and papers. The carrier material is preferably a film or a woven or nonwoven fabric, more preferably having a thickness of 20 to 200 pm, more particularly having a basis weight of 20 to 200 g/m 2 .

Additionally, these materials may be pretreated and/or aftertreated. Common pretreatments are plasma or corona treatment and hydrophobizing; familiar aftertreatments are calendering, thermal conditioning, laminating, diecutting, wrapping and sterilizing.

In principle both natural and synthetic carrier materials are contemplated. The carrier material is preferably selected from the group consisting of cotton; viscose; polypropylene; polyesters, selected more particularly from polyethylene terephthalate (PET), polybutylene terephthalate

(PBT), polybutylene adipate-terephthalate (PBAT), polybutylene succinate (PBS), polyisosorbitol terephthalate (PIT) and copolymers of these; polyamides; PVC; polyethylene; polyurethanes; silicones; and polylactic acid. The carrier material is preferably selected from the group consisting of cotton, viscose, polyesters, polyethylene and polyurethanes.

The carrier material more preferably is a polyurethane film, a polyethylene film, a polyester nonwoven, a woven viscose, polyester or cotton fabric, or a fabric woven from any desired blends of viscose, polyester and/or cotton.

The PSA of the invention may be applied to the carrier material by familiar methods. It is in general lined on one side with the carrier material, and the resulting structure is applied as a composite. The carrier material used can be employed for controlling the water vapour permeability, the firmness of a wound covering, the pressure cushioning, and other physical properties of the dressing.

To ensure ease of handling, the dressing is preferably wrapped with a protective layer, for example a siliconized paper or a siliconized film (release liner).

The dressing may further comprise a substrate for absorbing wound exudate.

The contact material may take the form in principle of a dressing which is self-adhesive over the full area or in partial areas, more particularly the form of a wound pad, plaster, hygiene article, cosmetic and/or dermatological pad, patch, tape, band, colostomy bag, fixative dressing, kinesiology tape, poultice, bandage, surgical covering or mask. The contact material is preferably a plaster. In addition, it may also take the form of a pad or patch with relatively low adhesiveness, for example, in which case the peel adhesion may be tailored to the specific end use.

In the contact material, the PSA of the invention takes the form preferably of a layer having a layer thickness of up to 120 pm, more preferably of up to 100 pm, more particularly of up to 80 pm.

In a further embodiment, the multi-layer composite system of the invention is an adhesive tape.

The general expression "adhesive tape" for the purposes of this invention encompasses all sheetlike structures, such as two-dimensionally extended tapes or tape portions, tapes with extended length and limited width, diecuts, labels and the like. The adhesive tape may be made available in fixed lengths, such as product by the metre, for example, or else as continuous product on rolls (archimedean spiral) or as a cross-wound spool. One or both sides of the carrier material may bear one or possibly more applied layers of adhesive.

Carrier materials used for the adhesive tape may be the carrier materials customary and familiar to the skilled person, such as paper, woven or nonwoven fabric or films, the latter composed for example of polyesters such as polyethylene terephthalate (PET), polyethylene, polypropylene, oriented polypropylene or polyvinyl chloride. The carrier material is based more preferably on renewable raw materials such as paper, woven fabrics composed for example of cotton, hemp, jute, stinging-nettle fibres, or bio-based and/or biodegradable polymers, for example polylactic acid.

The carrier material may be furnished on one or both sides with a PSA. In the case of double sided furnishing with PSA, it is preferred for both PSAs to be PSAs of the invention.

The adhesive tape is preferably formed by application of the PSA partially or to the full area of the carrier. Coating may also take place in the form of one or more stripes in longitudinal direction (machine direction), optionally in transverse direction, but more particularly is over the full area. Moreover, the PSA may be applied in the form of patterned dots via screen printing, in which case the dots of adhesive may also differ in size and/or in distribution; by gravure printing in lands which are coherent in longitudinal and transverse directions; by halftone printing, or by flexographic printing. The PSA may take the form of domes (produced by screen printing) or else of a different pattern such as lattices, stripes or zigzag lines. Further, for example, it may also have been applied by spraying, producing an irregular applied pattern.

The thickness of the PSA coating is preferably between 10 and 200 g/m 2 , more preferably between 15 and 75 g/m 2 and very preferably between 20 and 50 g/m 2 .

There is advantage to using an adhesion promoter, a so-called primer layer, between carrier material and PSA or to physically pretreating the carrier surface to improve the adhesion of the adhesive on the carrier material. The above-described application of the PSA to the carrier accordingly also comprises application to a carrier provided with a primer layer. Primers which can be used are the known dispersion-based and solvent-based systems, examples being those based on isoprene- or butadiene-containing rubber, acrylate rubber, polyvinyl, polyvinylidene and/or cyclo rubber. Isocyanates or epoxy resins as additives improve the adhesion and in part also have the advantageous effect of raising the shear strength of the adhesive tape. The adhesion promoter may also be applied to the carrier film via a co-extruded layer.

Examples of suitable physical surface treatments include flame, corona or plasma treatment.

Furthermore, the carrier material may have undergone an anti-adhesive physical treatment or coating process on the back or top side, i.e. opposite the adhesive side, this treatment more particularly involving being furnished with a release agent or release systems (optionally blended with other polymers). Examples of release layers are layers of stearyl compounds, for example polyvinyl stearyl carbamate, stearyl compounds of transition metals such as Cr or Zr, ureas of polyethylenimine and stearyl isocyanate; or of polysiloxanes. The term "stearyl" stands as a synonym for all linear or branched alkyl or alkenyl radicals having a C number of at least 10 such as an octadecyl radical, for example. Suitable release agents further encompass surfactant-type release systems based on long-chain alkyl groups, such as stearylsulfosuccinates or stearylsulfosuccinamates, but also polymers selected from the group consisting of polyvinylstearyl carbamates, polyethyleniminestearyl carbamides, chromium complexes of C14 to C28 fatty acids and stearyl copolymers. Likewise suitable are release agents based on acrylic polymers with perfluorinated alkyl groups, silicones, based for example on polydimethylsiloxanes, and fluorosilicone compounds.

Furthermore, the carrier material may be otherwise pretreated/aftertreated. Common further pretreatments are hydrophobizing; familiar aftertreatments are calendering, thermal conditioning, laminating, diecutting and wrapping.

The adhesive tape may comprise lamination with a standard commercial release film or release paper, typically constructed of a base material composed of polyethylene, polypropylene, polyester or paper that is coated on one or both sides with polysiloxane. A structure of this kind is commonly also referred to as a release liner.

A further subject of the invention is a medical device which is wearable on the skin, more particularly on the human skin, and which comprises a PSA of the invention and a medical system. A "medical system" here refers to a sequence of elements which in their entirety and in their interaction fulfil a medical function, which may be, for example, the delivery of an active ingredient, the stimulation of defined areas of the body, or the capture of medically relevant data. The skin-wearable medical advice, accordingly, is preferably a device for delivering an active ingredient, for stimulating areas of the body, or for capturing medical data.

The envisaged delivery of an active ingredient may in particular be transdermal; for example, a microneedle may be used to inject patients having type 2 diabetes with insulin, in a three day regimen, for example.

The stimulation is more preferably electronic stimulation, for example electronic muscle stimulation (EMS).

Medical data may be, in particular, body temperature, heart rate, blood pressure, step count or respiratory frequency, and also chemical concentrations such as pH or levels of lactate, glucose or chloride. From this data, the medical system may preferably derive and output particular information such as sleep status, stress level or training status.

The structure of the medical system may comprise, in particular, a protective layer, one or more adhesive interlayers, one or more diecut parts, one or more insulating layers, particularly to insulate against electromagnetic radiation/interference (EMI shielding), and one or more sensors.

A further subject of the invention is the use of a PSA of the invention as a bonding agent for producing adhesive bonds on the skin, more particularly on the human skin. The adhesive of the invention here may be used in particular in a contact material for bonding on the skin or in a skin-wearable medical device, in each case as set out above.

Besides this, the PSA of the invention may also be used as a bonding agent in the production of labels for medical products or for production of adhesive bonds in medical devices.

Examples

Methods of measurement and testing

Method 1 - Determination of molecular weight

The figures for the number-average molar mass M, and the weight-average molar mass Mw in this specification relate to the determination by gel permeation chromatography (GPC), which is known per se. The determination is made on 100 pl of sample having undergone clarifying filtration (sample concentration 4 g/I). The eluent used is tetrahydrofuran with 0.1 vol% of trifluoroacetic acid. The measurement is made at 25°C.

The precolumn used is a PSS-SDV-type column, 5 pm, 103 A, 8.0 mm * 50 mm (statements here and below in the following order: type, particle size, porosity, internal diameter * length; 1 A = 10-10m). Separation takes place using a combination of the columns of type PSS-SDV, 5 pm, 103 A and also 105 A and 106 A each of 8.0 mm * 300 mm (columns from Polymer Standards Service; detection by means of Shodex R171 differential refractometer). The flow rate is 1.0 ml per minute. Calibration is carried out using the commercially available ReadyCal Kit Poly(styrene) high from PSS Polymer Standards Service GmbH, Mainz. The values are converted using the Mark-Houwink parameters K and alpha universally into polymethyl methacrylate (PMMA), and so the data are reported in PMMA mass equivalents.

Method 2 - Peel adhesion to steel

The peel adhesion was determined under test conditions of 23°C +/- 1C temperature and 50% +/- 5% relative humidity. The specimens were trimmed to a width of 20 mm and bonded to a steel plate (ASTM). The steel plate was cleaned and conditioned prior to the measurement. This was done by first wiping the plate with solvent and then leaving it lying in the air for 5 minutes for the solvent to evaporate. The side of the adhesive tape remote from the test substrate was then lined with etched PET film 25 pm thick, to prevent the specimen stretching during the measurement. The test specimen was thereafter rolled onto the substrate. This was done by rolling over the tape five times back and forth using a 4 kg roller at a rolling speed of 10 m/min. 1 min after the rolling, the plate was inserted into a special mount. The peel adhesion was measured using a Zwick tensile testing machine; the specimens were peeled off at an angle of 1800and a speed of 300 mm/min. The results are reported in N/cm and are averaged from five individual measurements.

Method 3 - Determination of holding power

The shear strength was determined under test conditions of 23°C +/- 1C temperature and 50% +/- 5% relative humidity.

The test specimens were trimmed to a width of 13 ±0.2 mm and stored under the conditions for at least 16 h. Testing took place using 50 x 25 mm ASTM steel plates having a thickness of 2 mm with a 20 mm marking line, which prior to bonding were cleaned thoroughly with acetone several times and thereafter left to dry for 10 min. The bond area was 13 x 20 ±0.2 mm. The test strip was applied centrally to the substrate in longitudinal direction, using a wiper to run over it in order to avoid air inclusions, application taking place so that the upper edge of the test specimen was exactly on the 20 mm marking line.

The reverse of the test specimen was taped off with aluminium foil. The free projecting end was taped off with paper. The adhesive strip was then rolled over back and forth twice with a 2 kg roller. After the rolling, a belt loop (weight 5 - 7g) was mounted on the projecting end of the adhesive tape.

A nut and bolt were then used to affix an adapter plaque on the front side of the shear test plate. To ensure that the adapter plaque is seated firmly on the plate, the bolt was tightened forcefully by hand.

Using the adapter plaque, the plate thus prepared was attached to a timer by means of a hook; a weight of 1 kg was then gently suspended in the belt loop.

The adhesion time between rolling and loading was 12 min. Measurements were made of the time in minutes for the bond to fail; the results are averaged from three measurements. A holding power of at least 5000 min is regarded as a good result.

Table 1: Chemicals used

Chemical compound Designation Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) PHACT Al000P (CJ Bio) (P(3HB-4HB)), 69 wt% 3HB, 31 wt% 4HB Sucrose benzoate Tributyl 2-hydroxy-4-oxopentane-1,2,3-tricarboxylate Citroflex A-4 4-Benzoylphenyl acrylate

PSA preparation

The constituents for preparing the PSAs were introduced in the quantities indicated in Table 2 in ethyl acetate (solids content 15%) and stirred at 45°C for 3 hours. The resulting solution was subsequently homogenised overnight, using a roller bed. The dispersions obtained were applied to carriers (etched PET film, 23 pm thickness), the solvent was removed, and the resultant PSAs in web form (layer thickness 25 pm) were left to rest under test conditions for one day. The tests were then performed.

Table 2: PSA composition (amounts in g) and test results

No. PHA Sucrose Citroflex 4-Benzoyl- Peel adhesion Holding benzoate A-4 phenyl acrylate (N/cm) power (min) 1 70 30 3.88 10000+ 2 60 30 10 1.77 10000+ 3 56 30 11 3 1.24 10000+

The pressure-sensitive adhesives of Examples 1-3 were all biodegradable according to DIN EN 13432.

Claims (10)

Claims

1. Pressure-sensitive adhesive comprising at least one polyhydroxyalkanoate,

characterized in that the polyhydroxyalkanoate

o comprises 20% to 75% by weight of structural units deriving from 3 hydroxybutyric acid (3-HB); and o comprises at least one further structural unit deriving from a hydroxyalkanoic acid selected from the group consisting of 4-hydroxybutyric acid (4-HB), 3 hydroxyvaleric acid (3-HV), 4-hydroxyvaleric acid (4-HV), 3-hydroxyhexanoic acid (3-HX) and/or 4-hydroxyhexanoic acid (4-HX).

2. Pressure-sensitive adhesive according to Claim 1, characterized in that the polyhydroxyalkanoate comprises structural units deriving from (4-HB) and the (3 HB):(4-HB) weight ratio is from 5.5:4.5 to 2:3.

3. Pressure-sensitive adhesive according to either of Claims 1 and 2, characterized in that the polyhydroxyalkanoate comprises structural units (3-HB) at 35% to 60% by weight.

4. Pressure-sensitive adhesive according to any of the preceding claims, characterized in that the polyhydroxyalkanoate is crosslinked.

5. Pressure-sensitive adhesive according to Claim 4, characterized in that the polyhydroxyalkanoate is radically crosslinked.

6. Multi-layer composite system, comprising at least one carrier material and a pressure sensitive adhesive according to any of Claims 1 to 5.

7. Multi-layer composite system according to Claim 6, characterized in that the multi-layer composite system is a contact material for bonding on the skin.

8. Multi-layer composite system according to Claim 6, characterized in that the multi-layer composite system is an adhesive tape.

9. Skin-wearable medical device, comprising a pressure-sensitive adhesive according to any of Claims 1 to 5 and a medical system.

10. Use of a pressure-sensitive adhesive according to any of Claims 1 to 5 as a bonding agent for producing bonds on the skin.