Elastomer Modification of Structural Adhesives

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Abstract
An attempt has been made to review the highlights of the chemistry and physical properties of the rubber modification of structural thermosetting polymers that are used as adhesives. The elastomers are added in order to improve the characteristics of these structural thermosets such that they would be more useful as structural adhesives. The addition of an elastomer acts to increase the resistance of the structural thermoset to crack propogation. The resistance to crack propogation is obtained either by plasticization to increase the ductility of the thermoset or by generation of a two-phase system where the structural polymer forms a matrix in which the phase-separated elastomeric particles are imbedded. In the case of flexibilization by plasticization, the increase in peel strength (fracture toughness) is accompanied by a decrease in shear strength (modulus) at high temperatures. In the case of the two phase system, the matrix properties are unaffected for the most part, and increases in peel strength are not accompanied by significant decreases in high-temperature shear strength. In the case of flexibilization, the increase in fracture toughness is accomplished by increasing the ductility of the resin while in the case of the two-phase system, the rubber particles act as stress concentrators to cause conditions of exceeding the yield stress of the matrix near the particles. Exceeding the yield stress increases the amount of plastic deformation of the matrix. We have briefly reviewed the chemistry and physical properties of phenolic, epoxy, acrylic, and polyimide structural adhesives and their modification with vinyl, nitrile, acrylic, siloxane, and other types of elastomers.

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