Acrylic adhesives chemistry

These are all examples of soluble polymers. Combinations of soluble with insoluble polymers have also been reported. Polychloroprene or chlorosulfonated polyethylene was eombined with core-shell polymer particles to give an adhesive with improved cold impact resistance [33]. The fascinating chemistry of chlorosulfonated polyethylene in acrylic adhesives will be further discussed in the section on initiators. In many cases chlorosulfonated polyethylene is chemically attached to the acrylic matrix. 

The Chemistry of Structural Adhesives Epoxy, Urethane, and Acrylic Adhesives... 

Curing acrylic adhesives are distinctly different from anaerobics, cyanoacrylates, and acrylic solution adhesives and emulsions. These related chemistries use different formulating materials, cure via different curing mechanisms, and often possess minimal high performance properties over long periods of time, or when exposed to aggressive environments. 

This chapter will cover acrylic chemistry, the various types of acrylic adhesives that have come into existence over the years, advantages and limitations of these systems, properties of adhesives, lap shear strength, what makes these systems unique among other types of adhesives, and examples of performance. 

Acrylic adhesives cure by addition polymerization reactions. These chain reactions are initiated by the formation of free radicals that result in the adhesive curing by way of a very rapid polymer chain growth. This cure chemistry is significantly more rapid than a typical cure curve (i.e., condensation type) found in epoxy and urethane adhesives. A comparison of the cure profile of condensation (epoxy and urethane) versus addition... 

As a consequence, two distinctly unique methods of handling and curing acrylic adhesives follow from their cure chemistry. These are the so-called accelerator lacquer cure and the no-mix or honeymoon cure. These methods of handling acrylics are depicted in Figs. 5 and 6. 

Worldwide use and sales for acrylic adhesives of the types discussed here are difficult to determine, particularly since so many of the chemistries involved are nowadays hybrids of more than one type. Acrylics would, no doubt, not be considered the major chemical family of adhesives, when compared to epoxies and urethanes. Nevertheless, due to the need for the unique handling characteristics and performance properties achievable with acrylics, they maintain and will continue to maintain an important position among high performance structural adhesive types. 

Aromatic tertiary amines can also be used as part of the initiator system [58], Glycidyl methacrylate or methacrylic acid have been used to hybridize acrylic adhesives based on this cure chemistry with epoxies [59]. The epoxy functionality is reactive with the carboxylic acid groups on the monomer. 

Acrylic adhesives result from an entirely different chemistry and a different type of photoinitiator. Curing of acrylic adhesives results from a free radical mechanism. The free radicals are produced by the photoinitiator when it is exposed to UV light. However, the free radicals are consumed in the adhesive cure process, so acrylic adhesives can cure oidy where UV light is delivered. At least one of the components being bonded must be UV transparent to some degree. Another consequence of this cure mechanism is that no shadow cure capability is evident. 

C. W. Boeder, Anaerobic and structural acrylic adhesives, in Structural Adhesives - Chemistry and Technology, S. R. Hartshorn, Ed., Plenum Press, New York, 1985, Chap. 5. 

Structural adhesives are normally categorized or subdivided into subclasses based on the resin chemistries used in their formulation, which can be in the form of solids, liquids, pastes or films. The most important categories of structural adhesives are the thermoset cured adhesives based on phenolic, epoxy and polyurethane or acrylic resins and include phenolic adhesives. Epoxide adhesives, Toughened epoxide adhesives, Polyurethane adhesives, Acrylic adhesives and Toughened acrylic adhesives. Several of the categories can be further subdivided into both one- and two-component adhesives. The one-component structural adhesives, which can be liquids, pastes or solids (films), usually require the inclusion of added energy for activation or to effect cure in the... 

Anaerobic and structural acrylic adhesives (acrylics) are closely related members of the large acrylic adhesive family. They are reactive systems which cure by redox-initiated free radical polymerization. It is difficult to make a clear distinction between the two classes of adhesives. Many aspects of the chemistry, compositions, and terminology overlap. 

Engineering adhesives are solventless, liquid, reactive, durable adhesives for bonding durable substrates. There are six recognized chemical types of engineering adhesives acrylic, anaerobic, cyanoacrylate, epoxy, silicone, and urethane. Strictly speaking, the first three in this list are varieties of acrylic, but only the first will be discussed here. There is, however, considerable overlap in the chemistry and properties of acrylic and anaerobic adhesives. Since considerable progress has been made in acrylic adhesives over the last two decades, those used in engineering applications are now referred to as modified acrylic adhesives. 

The adhesive containers ideally should be stored in dependence of their used chemistry. The same chemistry should be stored together for example, epoxy adhesives should be stored separately from acrylic adhesives. The adhesive containers should be stored above collecting trays to prevent pollution in case of a leakage or spill. Depending on their hazardous potential, adhesives need to be stored in special places ... 

A second type of uv curing chemistry is used, employing cationic curing as opposed to free-radical polymerization. This technology uses vinyl ethers and epoxy resins for the oligomers, reactive resins, and monomers. The initiators form Lewis acids upon absorption of the uv energy and the acid causes cationic polymerization. Although this chemistry has improved adhesion and flexibility and offers lower viscosity compared to the typical acrylate system, the cationic chemistry is very sensitive to humidity conditions and amine contamination. Both chemistries are used commercially. 

Liquid organic rubbers with reactive functionality can be prepared by several methods. End-functional oligomers are preferred. Chains attached to the network at only one end do not contribute as much strength to the network as those attached at both ends [34], Urethane chemistry is a handy route to such molecules. A hydroxy-terminated oligomer (commonly a polyester or a polyether) can be reacted with excess diisocyanate, and then with a hydroxy methacrylate to form a reactive toughener [35]. The methacrylate ends undergo copolymerization with the rest of the acrylic monomers. The resulting adhesive is especially effective on poIy(vinyl chloride) shown in Scheme 2. 

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