Pressure-sensitive vinyl acetate copolymer

Among the different pressure sensitive adhesives, acrylates are unique because they are one of the few materials that can be synthesized to be inherently tacky. Indeed, polyvinylethers, some amorphous polyolefins, and some ethylene-vinyl acetate copolymers are the only other polymers that share this unique property. Because of the access to a wide range of commercial monomers, their relatively low cost, and their ease of polymerization, acrylates have become the dominant single component pressure sensitive adhesive materials used in the industry. Other PSAs, such as those based on natural rubber or synthetic block copolymers with rubbery midblock require compounding of the elastomer with low molecular weight additives such as tackifiers, oils, and/or plasticizers. The absence of these low molecular weight additives can have some desirable advantages, such as ... 

A different approach, although stdl working with essentially non-fiinctional polymers has been exemplified [114,115], in which, a 100% solid (solvent free) hot melt has been irradiated to produce pressure-sensitive adhesives with substantially improved adhesive properties. Acrylic polymers, vinyl acetate copolymers with small amounts of A,A -dimethylaminoethyl methacrylate, diacetone acrylamide, A-vinyl pyrrohdone (NVP) or A A have been used in this study. Polyfunctional acrylates, such as trimethylolpropane trimethacrylate (TMPTMA) and thermal stabilizers can also be used. 

A product is only considered to be totally biodegradable if all its single components can be degraded naturally. Currently, pressure sensitive adhesives (PSA) are mostly based on non-biodegradable synthetic polymers such as polyacrylates, ethylene-vinyl acetate copolymers and styrene block copolymers [124]. Therefore there is a growing demand for the application of biodegradable PSAs on naturally degradable products like paper and cardboard. 

The fundamentals of pressure-sensitive hot-melt adhesives are similar to those of solvent-based systems. Most elastomers and tackifiers are suitable, although ethylene-vinyl acetate copolymers are also used and the conventional rubber types are not. Pressure-sensitive hot melts are dominated by thermoplastic rubbers, which are ideal for use in these applications. Their unique properties arise from their essentially two-phase structure, in which thermoplastic regions of styrene end blocks lock the elastomeric midsections of butadiene or isoprene at room temperature but allow the elastomer to move freely at elevated temperatures or in solvent. This gives the polymer properties that are akin to those of vulcanized rubbers at room temperature, while allowinig it to behave as a thermoplastic when heated or dissolved. This structure is illustrated in Fig. 1. 

Early pressure-sensitive hot-melt adhesives used ethylene-vinyl acetate copolymers as elastomers, but they are seldom used now. Atactic polypropylene is sometimes used on its own or in admixtures. More recently, vinyl ethers and acrylic resins have become available and will probably play an increasingly important role as the technology is developed, especially on polar surfaces. 

Adhesive applications can be widespread including contact cmnents, pressure-sensitive adhesives and hot melts. In solid compounded articles footwear is a large volune application such as molded-ln-place soling or unit soles to be cemented to uppers. Additional uses are in toys, milk tubing, cove base, mats and miscellaneous coatings or molded parts. Potential uses include blends with ethylene-propylene rubber and ethylene-vinyl acetate copolymers to provide ozone-resistant fomulations or blends to improve crack- or impact-resistance of plastics. 

Pressure-sensitive adhesive dispersions are based primarily on special acrylic ester copolymers, again generally in combination with resins. In addition to suitable resins, two polymer bases are used primarily for hot-melt pressure-sensitive adhesives, namely, ethylene-vinyl-acetate copolymers and styrene-butadiene or styrene-isoprene block copolymers, which also are known as thermoplastic rubbers. 

Ethylene-Vinyl Acetate Copolymers (EVAc). The majority of EVAc is used for hard tack-free hot-melt adhesives. Pressure-sensitive properties can be achieved with types containing 40 - 60% vinyl acetate compounded with compatible resins. EVAc-based PSAs are used only for specialty tapes. 

Early hot melt adhesives were based on ethyl cellulose and animal or hide glues. These were later replaced by synthetic resins such as polyamides and ethylene-vinyl acetate copolymers. More recently a new class of compounds, referred to as block copolymers because of their unique chemical structure, have emerged. These latter compounds are copolymers of styrene and butadiene, isoprene, or ethylene-butylene which tend to widen the flexibility property range of hot melt adhesives. They probably represent the fastest growing segment of the hot melt adhesives market at the present time. Their primary application is in hot melt pressure sensitive adhesives. Polymers based on other than polyolefin resins are discussed in other chapters in this handbook. 

NR, styrene-butadiene mbber (SBR), polybutadiene rubber, nitrile mbber, acrylic copolymer, ethylene-vinyl acetate (EVA) copolymer, and A-B-A type block copolymer with conjugated dienes have been used to prepare pressure-sensitive adhesives by EB radiation [116-126]. It is not necessary to heat up the sample to join the elastomeric joints. This has only been possible due to cross-linking procedure by EB irradiation [127]. Polyfunctional acrylates, tackifier resin, and other additives have also been used to improve adhesive properties. Sasaki et al. [128] have studied the EB radiation-curable pressure-sensitive adhesives from dimer acid-based polyester urethane diacrylate with various methacrylate monomers. Acrylamide has been polymerized in the intercalation space of montmorillonite using an EB. The polymerization condition has been studied using a statistical method. The product shows a good water adsorption and retention capacity [129]. 

Ethylhexyl acrylate manufacture represented about 15 percent of domestic consumption of the alcohol. The acrylate is the longest chain acrylate ester produced by esterification of acrylic acid. The monomer is used in acrylic copolymers for pressure sensitive adhesives, PVC impact modifiers, and as a comonomer with vinyl acetate and vinyl chloride in latexes for paints and textiles. Growth over the next 5 years is estimated at 6 percent per year. 

Ethyl and butyl derivatives have found uses as adhesives. Pressure-sensitive adhesive tapes made from poly(vinyl ethyl either) incorporating antioxidants are said to have twice the shelf life of similar tapes made from natural rubber. Copolymers of vinyl isobutyl ether with vinyl chloride, vinyl acetate and ethyl acrylate are also produced. 

More commonly, acrylic polymers have been utilized as the sole or major components of radiation-curable adhesive systems. These acrylics have included a homopolymer and many examples of copolymers. One UV-cured acrylic ester copolymer system cited was claimed to have 180° peel strength of 4.8 to 5.5 pounds/inch. Radiation-curable systems based on copolymers of vinyl acetate and 2-ethylhexyl acrylate have been reported more frequently than other compositions and sometimes have been applied by hot melt techniques. These pressure-sensitive adhesives have displayed 180° peel strengths ranging from about 1.5 to 3 pounds/inch when cured by either UV or EB radiation. Not surprisingly, solutions of acrylic polymers in acrylic and vinyl monomers have also been used as radiation-curable pressure-sensitive adhesives. 

Copolymers of 2-ethylhexyl acrylate, vinyl acetate, and MA, in the range 59-64 35-40 1, have been produced in isopropyl acetate or dichloromethane solvents.Formulation of the materials with epoxy plasticizers, solution applied and cured 3 min at 275°F on poly (vinyl chloride) gave excellent pressure-sensitive adhesive films. The small amount of anhydride lowered both shrinkage and improved the cohesion of the applied films. A version of the same theme, i.e., solution copolymerization of octyl acrylate, ethyl acrylate, vinyl acetate, and MA (70 10 20 7.5) also provides useful pressure-sensitive... 

These empirical relationships along with the glass transition temperature data estabhshed for homopolymers (Table 9.1) allow polymer chemists to determine the optimal position of the glass transition temperature and then design adequate emulsion polymer compositions to fulfill end-users requirements. n-Butyl acrylate (T = -54°C) and 2-ethylhexyl acrylate (Tg = -85°C), for example, are widely used as the major components of water-based pressure-sensitive adhesives. Vinyl acetate and n-butyl acrylate copolymer latexes with a weight ratio of about 80 20 Tg = 8°C) are a primary choice for inte-... 

Copolymerization is an important way to produce properties that are not possible with homopolymers. For example, the homopolymer of vinylidene chloride is highly crystalline, and though it has excellent moisture and oxygen barrier properties, it does not produces very strong film or fiber. Copolymerization with 15 percent vinyl chloride disrupts the regular structure of the homopolymer to produce a stronger, clearer, more flexible material. The copolymer retains much of the barrier properties of the homopolymer and finds wide use for food packaging and filament. Other commercial copolymers include styrene-acrylonitrile, discussed above vinylidene fluoride-hexafluoropropylene, a heat- and oil-resistant elastomer styrene-butadiene rubber ethylene-vinyl acetate hot melt adhesive and 2-ethylhexyl acrylate-vinyl acetate-acrylic acid pressure-sensitive adhesives. 

A hot melt adhesive is heated to a viscosity of approximately 10 000 cP and can be coated at speeds as high as 800 mpm, typically onto papers and films. Most hot melt coating processes apply either pressure-sensitive adhesives or permanent adhesives. Pressure sensitive adhesives are applied within the converting industry in web width ranges over 2 m with a coating thickness up to approximately 100 g/m. The adhesives include ethylene vinyl acetate (EVA) copolymers, sty-rene-isoprene-styrene (SIS) copolymers, styrene-butadiene-styrene (SBS) copolymers, ethylene ethyl acrylate copolymers (EEA), and polyurethane reactive (PUR) pressure sensitive formulations. 

Before adhesion occurs, wetting of the surface must occur, which implies that the molecules of the adhesives must come close with those of the surface to interact. After the solvent evaporates, a permanent bond sets between the surfaces to be joined. Pressure-sensitive adhesives are special nonreacting ones that do not lose their tackiness even when the solvent evaporates. This is because the polymer used is initially in the liquid stage and it remains so even after drying. The most common adhesive used industrially is polymer dispersion of a copolymer of 2-ethyl hexyl acrylate, vinyl acetate, and acrylic acid in water... 

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