Toughened adhesives introduction

The various ways in which epoxies can be toughened by introduction of a secondary phase are discussed in separate articles Toughened epoxide adhesives particle formation prior to cure, Toughened epoxide adhesives precipitation during cure and Toughened epoxide adhesives blend synergy. 

The introduction of toughened adhesives has brought major improvements in performance, since they are not prone to brittle failure. Very high shear strengths are now attainable on both collar and pin and lap joint assemblies over an extremely wide temperature range. 

Stainless steel alloys have often proved difficult to bond, but the introduction of the toughened adhesives has alleviated the problem. Both the toughened acrylic and single-part epoxy types will bond these alloys well. Abrasion followed by a solvent wipe appears to be sufficient in most cases, though chemical treatments may be beneficial in more demanding environments. 

Were it not for the recent spectacular increase in the capacity of structural adhesives to cope with the conditions found on most mass-production lines, there would be little justification for urging those involved to reconsider their position. But things have changed — the introduction of the toughened adhesives " has given all concerned the need, and opportunity, to review their design and assembly techniques. 

This tendency for an overloaded joint to fail catastrophically when distorted has been substantially suppressed by the introduction of the toughened adhesives. These adhesives resist crack propagation by incorporating a dispersed rubbery phase within the harder load-bearing portion of the adhesive (Fig. 7.3). When the joint is overloaded... 

Toughened adhesives. Toughened adhesives contain a dispersed, physically separate, though chemically attached, resilient rubbery phase. The toughened concept, in the modem sense, has so far only been successfully applied to acrylic and epoxy based adhesives. Since their introduction, some ten or more years ago, these adhesives have been shown to be very durable and to possess an outstanding resistance to shock loading. 

Many formulations are claimed to be capable of absorbing normal residues of rolling oil on steel surfaces, thus reducing the surface preparation requirements as compared with, say, epoxies. As with epoxies, acrylics can be toughened by the introduction of a rubbery phase within the matrix of the adhesive and they are particularly suitable for polymeric adherends because the surface is eompliant. 

The surface content and the distribution of (ethylene-propylene) copolymer (EP) in toughened polypropylene (PP) resins (PP/EP) have important impact on a lot of properties such as gloss, paint adhesion, hardness,. .. These surface properties are more and more important in the multiple applications of these resins, for instance for paint adhesion in the automotive industry. It has already been shown that the introduction of EP in PP provides better paint adhesion but its role remains speculative (1-4), A major drawback to the understanding of the EP influence on PP/EP surface properties is the lack of knowledge concerning the blends surface morphology (EP content, EP lateral distribution,. ..). This is essentially due to the similar chemical composition of both polymers that prevents surface analysis by classical chemical surface spectroscopies. 

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