Structural Acrylic Adhesives

All acrylic structural adhesives consist basically of a solution or a mixture of polymers and unsaturated, low-molecular-weight, free-radical-polymerizable monomers with other materials added as needed for the particular intended use. In addition, some precursor of the polymerization initiator will be present. 

The choice of monomers that are useful in acrylic structural adhesives is rather limited. Cost always is an important factor, and because acrylic structural adhesives consume only a very small portion of the world s output of acrylic monomer, the formulator usually must rely on acrylic monomers that are made in large quantity for other uses. In addition, the monomers must polymerize readily at room temperature. If a mixture of monomers is to be used, the monomers must copolymerize easily. Finally, the monomers must be good solvents or dispersants for the polymers used in formulating the adhesive. The acrylic monomers finding most use in acrylic structural adhesives are methyl methacrylate and tetrahydrofurfuryl methacrylate. The later, albeit more expensive, has a much higher flashpoint and a generally perceived low odor, yet maintains good solvency. 

The curing reaction of an acrylic structural adhesive is the chain-growth polymerization of the acrylic monomer. The monomer units are not reactive with each other but react only with a growing chain having an active site on one end. In order to begin a chain, one must generate an initiator in the monomer solution. In the case of acrylic structural adhesives, this initiator nearly always is a free radical, a species having an unpaired, reactive electron. 

A technology developed at Du Pont75 combines the use of reactive sites on the oligomers with the initiation reaction. The resulting family of acrylic structural adhesives has become popularly known as second generation acrylics. They consist essentially of solutions of chlorosulfonated polyethylene (Du Pont Hypalon ) in acrylic or methacrylic monomers. The chlorosulfonyl groups present on the polymer will react with... 

Other acrylic structural adhesives can be used on poorly prepared metal surfaces. Inclusion of reactive monomers having phosphate groups can lead to significant improvements in primary bonds to many unprepared metals.76 Kowa American supplies one such monomer, 2-hydroxyethylmethacrylate acid phosphate. 

It is common in acrylic structural adhesives to use oligomers that have a desirable backbone and are terminated with free-radical-polymerizable bonds. A variety of isocyanate-terminated polyurethanes can be adapted to use in acrylic structural adhesives by reacting the terminal isocyanates with a hydroxy functional acrylic monomer such as 2-hydroxyethyl methacrylate.76,92 93... 

Acrylic Adhesives. Acrylic structural adhesives can be classified into three major types the surface-activated acrylics (anaerobics), the surface-activated second-generation acrylics, and the cyanoacrylates. 

Kanerva L, Jolanki R, Leino T, Estlander T. Occupational allergic contact dermatitis from 2-hydroxyethyl methacrylate and ethylene glycol dimethacrylate in a modified acrylic structural adhesive. Contact Dermatitis 1995 33(2) 84-9. 

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. 

Versilok . [Lord] Acrylic structural adhesives to strong, durable bonds between metals, (totics, ceramics, glass, other substrates. 

Urethanes have also been used to toughen vinyl-terminated acrylic adhesives for improved impact resistance. Thus rubber-toughened urethane acrylates [79,80], water-dispersible urethane acrylates [81], and high-temperature-performance urethane-acrylate structural adhesives have been reported [82]. Polyurethanes terminated with acrylic functionality are also used for anaerobic or radiation-cured adhesives with improved toughness [83]. 

D.H. Green and L.E. Toy, "Acrylic Structural Adhesives Technology in Rapid Evolution", Technology Conferences (SPE) - ADHESIVES FOR INDUSTRY, Aug. 24, 1980, El Segundo, CA. 

Anaerobic acrylic sealants or cyanoacrylates are known sensitizers, while the modified acrylic structural adhesives that cure in air more rarely cause allergy. Tosti et al. (1993) reported of three carpenters, each of whom became sensitized to wood paints and glues with butyl acrylate, 2-ethoxyethyl methacrylate, or a phth-late. Epoxy resin compounds are another commonly used group of adhesives, which contain resins (generally included in patch-test screening batteries), hardeners, and reactive diluents, as well as many other potentially harmful ingredients, such as tar, fillers, colorants, and other plastics. 

Kanerva L, Jolanki R, Leino T, Estlander T (1995b) Occupational allergic contact dermatitis from 2-hydroxyethyl methacrylate and ethylene glycol dimethacrylate in a modified acrylic structure adhesive. Contact Dermatitis 33 84-89 Kanerva L, Keskinen H, Autio P, Estlander T, Tuppurainen M, Jolanki R (1995c) Occupational respiratory and skin sensitization caused by polyfunctional aziridine hardener. Clin Exp Allergy 25 432-439... 

This is an area where acrylic structural adhesives are superior to epoxides because of their ability to dissolve oils and greases. 

Acrylic structural adhesives for the aerospace industry are available [33] from 3Ms, Bostik, Henkel Corporation and Permabond. 

The scientific literature on adhesives and adhesion is growing at a rapid pace. There are many texts devoted to adhesion and adhesives. New patents on adhesive compositions and processes are granted almost daily, and there are scientific journals devoted to the science and technology of adhesives and their use. A significant portion of the publications dealing with adhesives are concerned with epoxy, urethane, and acrylic structural adhesives, as they are used in a wide variety of commercially important applications. 

The third method is the one most commonly used with epoxy, urethane, and acrylic structural adhesives. The chemistry, methods and mechanisms for accomplishing this polymerization will be covered in more detail in the following sections. 

All acrylic structural adhesives consist basically of a solution or a mixture of polymers and... 

Another innovative approach to initiating the polymerization of acrylic structural adhesives is illustrated by a family of adhesives that uses the adherend surface itself as part of the chemical reaction that generates the free radical initiator. The compositions contain an acidic material, a sulfonyl halide group, and a transition metal compound. Upon contact with a metal surface such as iron, zinc, copper, cadmium, and their alloys, the components react with the metal surface through a free radical pathway, producing effective initiators for the polymerization of the acrylic monomers. Later extensions of this concept provide two-part adhesives employing... 

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