Acrylic copolymers adhesive formulations

Adhesives. Acryhc emulsion and solution polymers form the basis of a variety of adhesive types. The principal use is in pressure-sensitive adhesives, where a film of a very low T (<—20 " C) acrylic polymer or copolymer is used on the adherent side of tapes, decals, and labels. Acrylics provide a good balance of tack and bond strength with exceptional color stabiUty and resistance to aging (201,202). AcryUcs also find use in numerous types of constmction adhesive formulations and as film-to-film laminating adhesives (qv). 

An example of this improvement in toughness can be demonstrated by the addition of Vamac B-124, an ethylene/methyl acrylate copolymer from DuPont, to ethyl cyanoacrylate [24-26]. Three model instant adhesive formulations, a control without any polymeric additive (A), a formulation with poly(methyl methacrylate) (PMMA) (B), and a formulation with Vamac B-124 (C), are shown in Table 4. The formulation with PMMA, a thermoplastic which is added to modify viscosity, was included to determine if the addition of any polymer, not only rubbers, could improve the toughness properties of an alkyl cyanoacrylate instant adhesive. To demonstrate an improvement in toughness, the three formulations were tested for impact strength, 180° peel strength, and lapshear adhesive strength on steel specimens, before and after thermal exposure at 121°C. 

However, newer adhesives systems having moderate temperature resistance have been developed with improved toughness without sacrificing other properties. When cured, these structural adhesives have discrete elastomeric particles embedded in the matrix. The most common toughened hybrids using this concept are acrylic and epoxy systems. The elastomer is generally a vinyl- or carboxyl-terminated acrylonitrile butadiene copolymer. These adhesive formulations are discussed in detail in Chaps. 8 and 12. 

One of the most important features of SBC adhesives is their versatility. Because they are thermoplastic materials of carefully controlled molecular weight, they can be applied both as hot melts and from solvent. More importantly, adhesive manufacturers can tailor their properties to match a wide range of applications. The primary competitors for SBCs in the pressure-sensitive adhesive market are acrylic copolymers. While acrylics have better specific adhesion to polar materials, adhesive manufacturers cannot easily formulate them for varied adhesion applications. 

Acrylic structural adhesives have been modified by elastomers in order to obtain a phase-separated, toughened system. A significant contribution in this technology has been made in which acrylic adhesives were modified by the addition of chlorosulfonated polyethylene to obtain a phase-separated structural adhesive (11). Such adhesives also contain methyl methacrylate, glacial methacrylic acid, and cross-linkers such as ethylene glycol dimethacrylate [97-90-5]. The polymerization initiation system, which includes cumene hydroperoxide, IV,AT - dimethyl-p- toluidine, and saccharin, can be applied to the adherend surface as a primer, or it can be formulated as the second part of a two-part adhesive. Modification of cyanoacrylates using elastomers has also been attempted copolymers of acrylonitrile, butadiene, and styrene ethylene copolymers with methylacrylate or copolymers of methacrylates with butadiene and styrene have been used. However, because of the extreme reactivity of the monomer, modification of cyanoacrylate adhesives is very difficult and material purity is essential in order to be able to modify the cyanoacrylate without causing premature reaction. 

A variety of acrylic copolymers are prepared by emulsion polymerization. A number of acrylic adhesives that are called reactive adhesives, modified acrylics, second-generation acrylics, or reactive-fluid adhesives have become available over the years. These formulations polymerize in the glue line and become an integral part of an adhesive assembly. 

Adh6SiVGS. Acrylic emnlsion polymers are nsed in a wide variety of adhesives. Pressure-sensitive Adhesives, which typically have Tg values less than 20° C, are the main type of acrylic adhesive. Acrylic polymers and copolymers find use as PSAs in tapes, decals, and labels. Along with their aforementioned superior chemical resistance properties, acrylics possess an excellent balance of tack, peel, and shear properties which is crucial in the adhesives market (218,219). Other types of adhesives that employ acrylics include construction formulations and film-to-film laminates. 

Acetone and methyl ethyl ketone are components of solvent blends in urethane, nitrile rubber, and neoprene industrial adhesives. Acetone is the primary solvent in resin-type adhesives and pressure sensitive chlorinated rubber adhesives. Methyl isobutyl ketone is a solvent component for nitrile rubber and acrylic adhesives as well as in polyvinyl chloride and polyvinyl chloride-polyvinyl acetate copolymer bonding adhesives. Again, the desired evaporation rate of the adhesive formulation will often determine the ketone selected. 

The hot melt adhesives are based on ethylene-vinyl acetate or ethyl acrylate copolymers or styrene block copolymers, because of the need to adhere to polyester. The starting point formulation for a PET bottle adhesive is given in Table 19 ... 

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. 

Ethylene Copolymers. Ethylene copolymers probably are the most important materials in hot-melt formulations. Ethylene-vinyl acetate and ethylene-ethyl acrylate polymers are very versatile and available in a wide range of grades offering different co-monomer contents and viscosities. The melts are stable and compatible with various modifying resins, waxes, extenders, and fillers. Adhesion to many substrates is good—including the polyolefin plastics, which are difficult to bond with most other types of adhesive unless the surfaces are pre-treated. 

Table 3 Influence of the plasticizer in the coating formulation on the force of adhesion and the glass transition temperature (Tg) of an acrylic resin copolymer (standard deviation in parentheses)...

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