The ability of an adhesive to absorb energy without catastrophic failure can be increased through toughening. This means enhanced resistance to fracture, impact, and thermal stress with minimal change in the gross properties of the matrix resin. Improved toughness also results in higher peel strength. The adhesive formulator can improve toughness of the adhesive formulation by various means.

A common method of toughening adhesives is blending the primary resin with other polymers, including thermoplastics or elastomers. However, this technique usually combines the good and bad characteristics of each resin system. For example, epoxy-nylon adhesives provide a major improvement in toughness over a pure epoxy formulation, but they have sensitivity to moisture because of the nylon (polyamide) constituent. In these conventional hybrid systems, the added toughening resin reduces the overall glass transition temperature of the system, thereby, reducing elevated temperature performance and environmental resistance of the more brittle constituent.

However, newer adhesives systems have been developed with improved toughness without sacrificing other properties. When cured, these structural adhesives have discrete elastomeric particles embedded in a resin matrix. This can be accomplished in a variety of ways. One approach with epoxy and acrylic adhesives is by incorporating small rubber inclusions into the adhesive formulation. The rubbery polymer is dissolved in the resin binder, and when the adhesive cures, the rubber precipitates as droplets of about one micron in diameter. These rubber inclusions absorb energy and stop a crack from propagating throughout the bond line. This allows enhanced resistance to fracture, impact, and thermal stress with minimal change in the gross properties of the matrix resin.

One such toughened acrylic system is based on a crosslinked network of polymethyl methacrylate chemically grated to vinyl terminated acrylonitrile butadiene copolymer, with the rubber particles existing as the dispersed phase. Similarly, adhesive systems containing epoxy resins with carboxyl-terminated butadiene acrylonitrile (CTBN) or amine-terminated butadiene acrylonitrile (ATBN) liquid polymers are available. These systems provide a balance of shear, peel, and environmental aging performance properties.

Within the past several years, improvements in the toughening of high temperature epoxies and other reactive thermosets, such as cyanate esters and bismaleimides, have been accomplished through the incorporation of engineering thermoplastics. Additions of poly(arylene ether ketone), PEK, and poly(aryl ether sulfone), PES, have been found to improve fracture toughness. Direct addition of these thermoplastics generally improves fracture toughness but results in decreased tensile properties and reduced chemical resistance.

Chemical functionalization of the thermoplastics was found to improve toughness without such detractions. High molecular weight resins based on amine terminated PES oligomers or chain extension of bismaleimide resin with the same amine terminated PES were found to have improved fracture resistance and reduced thermal shrinkage. Also a mechanism was found to toughen cyanate esters by incorporating epoxy resins, which can react with a cyanate ester.

An excellent technical review on toughening approaches for structural adhesives has been developed by a group at Lord Corporation. This treatise describes chemical and physical toughening approaches for thermosetting structural adhesives (epoxy and acrylates), and it contains 36 references.

References

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Andy Extance