Second generation acrylic adhesive

An unusual type of two component system has recently been introduced in the adhesive industry (1,). These are called reactive adhesives, second generation acrylics, toughened acrylics, modified acrylics (2) or "honeymoon adhesives" (in Europe). 

Acrylic polymers are one of the important structural adhesives (Fig. 11). These polymers can be classified as anaerobic structural acrylics (60), second generation acrylics (60,61), and aerobic acrylics (61,62). Most anaerobic adhesives are based on dimethacrylic esters, e.g., TEGDMA (triethylene glycol dimethacrylate) (Fig. 12). Anaerobic adhesives (63) containing maleimides have been reported to have improved thermal resistance. 

In this Institute, there are approximately 200 research personnel and one fourth of them are working on adhesives in two laboratories, one pilot-plant, one testing group and one adhesive-film manufacturing and testing facility. Their research and development broadly covered structural adhesives, strain gauge adhesives, wood adhesives, second-generation acrylics, anaerobic adhesives and composite binders. Their products varied from J-01 to J-53. There were at least thirty different kinds of adhesive and sealant, for example, nitrile-phenolics, modified epoxies, acrylics and epoxy-phenolics. 

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

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... 

A second generation of phenolic dispersions, patented by J. S. Fry (33). involved the post dispersion of phenolic resins in a mixture of water and water-miscible solvents. To conform with air pollution regulations, the solvent was held to 20 volume %, or less, of the volatiles. A heat-reactive phenolic resin dispersion (34) and a phenolic-epoxy codispersion have become commercially available based on the above technology. Supplied at 40-45% solids, these products, which have a small particle size (0.75-1.0 ym), are better film formers than the earlier dispersions. Used alone or in blends with other waterborne materials, corrosion-resistant baking coatings may be formulated for coil coating primers, dip primers, spray primer-surfacers, and chemically resistant one-coat systems. Products of this type are also tackifiers for acrylic latexes, and such systems have been employed as contact, heat seal, and laminating adhesives for diverse substrates. 

Two component acrylic systems which are known as second generation acrylics in the U.S. and as "honeymoon adhesives" in Europe are discussed in Chapter 10. Acrylic resins have been... 

Second generation and DH acrylic adhesives were notably different from earlier systems in that they were the first type of acrylic adhesives that could be cured and good bonds to unprepared metals could be obtained. These systems showed the ability not only to bond to unprepared steel and aluminum surfaces, but also to aluminum and steel surfaces that still had varieties of different oils and drawing compounds on them. This ability marked a significant advancement in acrylic adhesives and established them as a unique family of adhesive materials for bonding oily and unprepared metals. 

In the mid-seventies, spurred by developments from the DuPont Corporation, a new kind of two-component acrylic adhesive system was introduced to the joining industry. The DuPont technology has been widely licensed and is now referred to by numerous terms such as "second-generation acrylics", "reactive adhesives", "modified acrylics", "toughened acrylics", etc. These adhesives seemed to answer the need for improvements over some of the major disadvantages found in anaerobic structural adhesives, namely those of cost, the need for scrupulous surface preparation, and the ability to bond plastic surfaces. 

In spite of this impressive list of characteristics the use of reactive acrylic adhesives has been limited to selected high-performance adhesive markets. Their growth otherwise, has been less rapid than expected (10). One of the principal reasons is due to the toxicity characteristics associated with methyl methacrylate and methacrylic acid monomers, major formulation constituents of "second-generation" acrylic adhesives, with regard to inhalation, ingestion and skin contact. Compositions containing large quantities of these monomers have been rejected on certain production lines such as automotive assembly. The low flash point and resultant flammability hazard have also been important contributors to the restricted use of modified acrylic adhesives (9 and 11). 

This article examines a new reactive acrylic adhesive that overcomes most of these disadvantages. This new family of adhesives, called aerobic acrylics, maintains, and in some cases improves upon, the performance advantages of second-generation acrylics. The products are low in odor, essentially nonflammable, and lower in toxicity. Their higher speed of cure and fixture rate of 10 to 60 seconds and their solvent-diluted activators have minimized the other problems associated with acrylic adhesives namely, sensitivity to high concentrations of activator and movements in the substrates prior to fixture. 

Bondable substrates for aerobic and second-generation acrylics include clean as well as oily or "as received" metal polyurethanes, filled polycarbonates, filled nylon, phenolics, thermosets, wood, concrete, ceramics, glass, and other common materials of manufacture. Some thermoplastic materials are bonded better by using a second-generation acrylic adhesive. Examples are unfilled nylon, ABS, and rubber. 

Table 3 also provides a comparison of cured properties of aerobic with second-generation acrylic adhesives. The data shows that, while tensile strengths are similar, the toughness and durability parameters are superior for the aerobics. 

The cost per pound of adhesives is an important, though not a limiting, consideration on the choice of an acrylic adhesive. The cost of assembly usually far outweighs adhesive cost per pound. Generally, cyanoacrylates cost the most per pound, followed by anaerobics, aerobics, and then second-generation acrylics. 

Aerobic adhesives have a wider latitude in formulating for specific end-use requirements than do second-generation acrylic adhesives. Flexibility, opacity, cure speed, and surface compatibility may be controlled over a wide range to produce adhesives for metal, glass, many plastics, ceramics, and hard woods. The cured properties of specific aerobic formulations are scarcely affected by efficient thickening agents such as fumed silica. As a consequence, it is possible for an end-user to pick a set of cured properties and then have the adhesive s viscosity modified to specific requirements. The properties of three representative cured formulations are outlined in Table 5. 

A number of attempts with anaerobic adhesives, epoxies, urethanes, and second-generation acrylic adhesives had proved unsuccessful. The bonded alternator had to... 

Radiation curable adhesives can be based on raw materials that are not greatly different from those used in acrylic adhesives (first, second, and third generations), epoxy adhesives, urethane adhesives, and anaerobic adhesives. The formulation approach, however, is somewhat different. Very important is that no solvents are used to thin the resins a monomer reactive diluent is used instead. The monomer must be matched with the resin to give the desired set of properties with respect to adhesion, substrate, flexibility or stiffness, cure behavior, and durability of the cured product. Most of the radiation curable adhesives have an acrylate (or methacrylate) basis many of the acrylics are modified. 

Their J-39 adhesives are room-temperature curable, second-generation acrylics (16). Methyl methacrylate is the monomer, and a nitrile rubber and Dow ABS resin are used as toughening agents. This family of adhesives have been used in automotive, machinery and aerospace structures. Their J-50 adhesives (17) are two-component, fast-cure acrylics. In addition, they also developed a special adhesive, J-15 structural adhesive, for hydroplanes. 

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. 

Adhesives recommended include modified epoxies, modified phenoUcs, epoxy-phenolics, neoprene-phenolics, second-generation acrylics, cyanoacrylates, silicone rubbers, and vinyl plastisols. Sell has ranked a number of adhesives in the order of decreasing durability with aluminum adherends as follows ... 

Bodied solvent cements are usually used to bond ABS. Adhesives recommended include epoxies, urethanes, second-generation acrylics, vinyls, nitrile-phenolics, and cyanoacrylates. ... 

Ordinarily solvent cementing or thermal welding is used with PMMA. These methods provide stronger joints than adhesive bonding. Adhesives used are cyanoacrylates, second-generation acrylics, and epoxies, each of which provides good adhesion but poor resistance to thermal aging. "... 

In a study of nine commercially available acrylic adhesives, Wilkinson and Tyler found a considerable variation in the resistance to environmental exposure. The tests employed were 1,000-h water soak, 30 days at 125°F/100% RH, and 1,000-h 5% salt spray at 95°F aluminum adherends were used. The nature of the alloy was also a factor in determining joint durability. The authors concluded that excellent durability can be obtained with the proper selection of adhesive and alloy. A similar study by Zalucha using recently developed high-performance acrylic adhesives also indicates excellent performance for many acrylic-bonded assemblies. Minford has conducted weathering tests on both the early formulations and the more recently introduced second generation acrylics." He found that the durability of the earlier products was generally poor in both water soak and... 

Modified acrylics, also referred to as second-generation acrylics or no-mtx adhesives, are composed of a modified acrylic adhesive and a surface activator. Typical modified acrylics are based on cross-linked polymethyl methacrylate grafted to vinyl-terminated nitrile rubber. Carbojqfl-terminated rubbers have also been used. 

Reactive acrylic adhesives (sometimes called tough acrylics, reactive fluids, second-generation acrylics or methyl methacrylate adhesives are based on acrylate and methacrylate monomers, and have been used commercially for >50 years. Reactive acrylic adhesives are based on acrylic and methacrylic monomers, and polymerise via a free-radical route similar to catalysed anaerobic adhesives. However, very significant differences exist. Whereas the monomers in anaerobics are predominantly difunctional or trifunctional to achieve highly crosslinked thermoset systems, reactive acrylics are based mainly on monofunctional monomers e.g., methyl methacrylate, or cyclo hexyl methacrylate ... 

Modified or second generation acrylic engineering adhesives are more complicated than those described above. In modified acrylic adhesives, polymerization between adherends leads to impact resistant plastics with adhesion to the adherends. The following formulation of Brigas and Muschiatti illustrates an early modified acrylic engineering adhesive ... 

Acrylic anaerobics, conventional acrylics, cyanoacrylates, the so-called second generation , toughened acrylics and adhesives having UV-activateable or UV-curing capabilities... 

Small die castings are conveniently bonded with polymethylmethacrylate dissolved in its monomer together with a peroxide initiator. Metal naphthenates are to be avoided as accelerators and hence the so-called second generation or toughened acrylics are not recommended for use with zinc. Other acrylics may be used but are slow curing after evaporation of solvent. Polyester adhesives cured with styrene monomer frequently contain some free phthalic acid as an impurity and this militates against their use. Polyurethane adhesives cured with isocyanate should behave quite satisfactorily. 

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