Poly -based pressure sensitive adhesives

Poly(isobutene)-based pressure sensitive adhesive 30 (c) - 101... 

Cellulosics. CeUulosic adhesives are obtained by modification of cellulose [9004-34-6] (qv) which comes from cotton linters and wood pulp. Cellulose can be nitrated to provide cellulose nitrate [9004-70-0] which is soluble in organic solvents. When cellulose nitrate is dissolved in amyl acetate [628-63-7] for example, a general purpose solvent-based adhesive which is both waterproof and flexible is formed. Cellulose esterification leads to materials such as cellulose acetate [9004-35-7], which has been used as a pressure-sensitive adhesive tape backing. Cellulose can also be ethoxylated, providing hydroxyethylceUulose which is useful as a thickening agent for poly(vinyl acetate) emulsion adhesives. Etherification leads to materials such as methylceUulose [9004-67-5] which are soluble in water and can be modified with glyceral [56-81-5] to produce adhesives used as wallpaper paste (see Cellulose esters Cellulose ethers). 

Polystyrene-PDMS block copolymers4l2), and poly(n-butyl methacrylate-acrylic acid)-PDMS graft copolymers 308) have been used as pressure sensitive adhesives. Hot melt adhesives based on polycarbonate-PDMS segmented copolymers 413) showed very good adhesion to substrates with low surface energies without the need for surface preparation, such as etching. 

Silicone release liners are used as a nonadhering surface to which adhesive materials, such as, for example, pressure-sensitive adhesives, can be laminated. UV curable release coatings are based on poly(dimethylsiloxane) oligomers functionalized with acrylate or epoxy groups and are mostly coated onto papers or thin films. 

There have been very few studies reported on the viscoelastic properties of rubber-resin pressure sensitive adhesive systems. In 1973, M. Sherriff and co-workers (1) reported on the effect of adding poly (j3-pinene) resin to natural rubber. Based on a G master curve, they showed that the resin shifted the entry to the transition zone to a lower frequency and reduced the modulus in the rubbery plateau. G. Kraus and K.W. Rollman (2) reported in 1977 on their study of resins blended with styrene-isoprene-styrene block copolymers. They showed that the addition of a resin increased the glass transition temperature of the rubbery mid-block and decreased the plateau modulus. Accordingly, a satisfactory tackifying resin should produce these changes. 

Other adhesives such as hot melt adhesives (HMA), poly(vinyl acetate) (PVA, catalyzed or uncatalyzed), pressure-sensitive adhesives (PSA), or elastomeric adhesives (based on natural or synthetic rubbers) are also used in wood bonding (7). However, their use is mostly limited to nonstructural applications (eg, secondary manufacturing processes such as kitchen furniture, interior joinery, decorative paper, and packaging) where strength and water resistance are of limited concern. 

In general two types of adhesives are used for the manufacture of baby diapers, feminine-hygiene, and incontinence products. For simple construction purposes, hot-melt adhesives based on amorphous poly(alpha-olefins) (APAOs) are used. High-quality constructions are produced with adhesives based on styrenic block copolymers. For bonding of elastic materials such as polyurethane foam, lycra and natural rubber strands, and hydrophilic and hydrophobic of nonwovens, pressure-sensitive adhesives (PSAs) based on styrenic block copolymers are used. Hot-melt adhesives are the most common types of adhesive. The most widely used synthetic block copolymers are styrene-butadiene-styrene (SBS), styrene-isoprene-styrene (SIS), and styrene-ethylene-butadiene-styrene (SEES) with different styrene contents. 

Kajtna and Sebenik described the synthesis of pressure-sensitive adhesives based on polyacrylates by suspension polymerization. The monomers used were 2-ethylhexyl acrylate (2-EHA) and ethyl acrylate (EA), and dibenzoyl peroxide (DBF) was used as initiator. Surface-active agents [modified ester of sulfocarboxylic acid (SCA) and ethoxylated oleyl alcohol (EOA)], chain transfer agent (w-dodecanethiol) (CTA) and suspension stabilizer [poly(vinyl alcohol) (PVA)] were also used. Various organically modified montmorillonites were used as fillers. It was observed that the kinetics of suspension polymerization were independent of the presence of the montmorillonite in the system, as shown in Figure 1.8. 

Up to now, poly(methyl methacrylate) and methyl methacrylate copolymers e.g. with styrene, butyl acrylate and dodecyl methacrylate) have been the most widely used acrylic polymers for nanocomposite preparation by emulsion and suspension polymerization. Less research has been based on other acrylic polymers, such as polyacrylonitrile, poly(butyl acrylate), " poly(butyl methacrylate), poly(2-ethylhexyl acrylate), poly(2-hydroxyethyl methacrylate), polyacrylamide, poly(lauryl acrylate)," poly(butyl acrylate-co-styrene)," " poly(acrylonitrile-co-styrene), poly(acrylonitrile-co-meth-acrylate)," poly(ethyl acrylate-co-2-ethylhexyl acrylate)" and poly(2-ethylhexyl acrylate-co-acrylic acid)," and sometimes small amounts of hydophilic acrylic monomers, such as hydroxyethyl methacrylate, methacrylic acid and acrylic acid, have been used as comonomers. " Therefore, it may be stated that, so far, the preparation of acrylic-clay nanocomposites has been based mainly on high glass transition temperature polymers, although nanocomposite materials with lower glass transition temperatures with improved or novel properties, which exhibit a balance of previous antagonistic properties, can also be achieved and are very desirable. Regarding nanocomposites of low glass transition temperature polymers, such as poly(butyl acrylate), poly(ethyl acrylate) and poly(2-ethylhexyl acrylate), which have been utilized as the main components of acrylic pressure-sensitive adhesives, little information is available. 

Standard-grade PSAs are usually made from styrene-butadiene rubber (SBR), natural rubber, or blends thereof in solution. In addition to rubbers, polyacrylates, polymethylacrylates, poly(vinyl ethers), polychloroprene, and polyisobutenes are often components of the system ([198], pp. 25-39). These are often modified with phenolic resins, or resins based on rosin esters, coumarones, or hydrocarbons. Phenolic resins improve temperature resistance, solvent resistance, and cohesive strength of PSA ([196J, pp. 276-278). Antioxidants and tackifiers are also essential components. Sometimes the tackifier will be a lower molecular weight component of the high polymer system. The phenolic resins may be standard resoles, alkyl phenolics, or terpene-phenolic systems ( 198], pp. 25-39 and 80-81). Pressure-sensitive dispersions are normally comprised of special acrylic ester copolymers with resin modifiers. The high polymer base used determines adhesive and cohesive properties of the PSA. 

Unlike all the other classes described above these adhesives do not typically undergo hardening after they have been applied to the substrate surfaces and the joint formed. They are generally in the form of an already polymerized adhesive which is coated onto one or both sides of a backing material such as cellulose, polyester, foamed polyurethane, poly(vinyl chloride), aluminium or lead. The adhesive is usually permanently tacky and based upon natural rubber, styrene-butadiene rubber (random and, more recently, block copolymers), polyisobutylene or an acrylic polymer, but as usual is a complex formulation containing many additives. The adhesive is formulated so that it flows sufficiently, when hand pressure is applied to the joint for a short period of time, to wet the substrate adequately in order to attain a certain, albeit minimal, level of joint strength. Thus, the term pressure-sensitive is often applied to this class of adhesives. 

Thus, acrylics are used for such diverse products as pressure-sensitive and structural adhesives, coatings, and rigid plastics. Because of their low 7, poly(ethyl acrylate) and poly (butyl acrylate) are used in acrylate-based rubbers. 

The chemistry of silicone sealants is much the same as that discussed above for silicone pressure-sensitive and mbber-based adhesives, with the exception of the MQ taddfier addition. The basic polymer for silicone sealants is that based upon poly (dimethyl siloxane)diol. This material can be cured by any one of a number of polyfunctional curing agents, all of which are based upon tetrachlorosilane. Thus, such materials as methyl triacetoxysilane or methyl trimethoxysilane can be used in conjunction with a poly(dimethyl siloxane)diol to yield a room... 

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