Method of Decreasing Internal Stresses in Adhesive-Bonded Joints

All the practically acceptable methods of decreasing the internal stresses in adhesive-bonded joints can be divided into three groups decrease of the adhesive shrinkage in the course of cure increase of the relaxation rate of the internal stresses in the adhesive and decrease of the difference between the coefficients of linear thermal expansion of the adhesive and the substrate. 

Widely used methods for decreasing the internal stresses relate to increasing their rate of relaxation, for example, by decreasing the modulus of elasticity when adding plasticizers to adhesives or when they are combined with elastomers. The residual stresses were found 

A more convenient method is not to decrease the elastic modulus of the whole adhesive but to form an elastic finish coat [182] not less than 30 pm thick [183] on the adhesive—substrate interface. With lower elastic modulus, the mobility of structural elements decreases. Application of an elastic sublayer decreased the internal stresses during the formation of coatings made of unsaturated polyester resins [185], epoxides [186], and polymer solutions [187]. The use of elastic finish coats found comparatively wide application for paint coatings but seems less promising for adhesive-bonded joints because of decrease of the adhesion strength due to the low cohesion strength of the finish coat itself and because of the labor requirements in producing the adhesive-bonded joints. 

High adhesion strength with simultaneous minimization of the internal stresses can be obtained in the formation of adhesive-bonded joints imder conditions in which the rates of internal stress relaxation and pol5rmer formation are compatible. This approach to decreasing 

The thermal stresses in the layers and films of the polymer can be decreased by heat treatment due to the increase in the rate of their relaxation [194], 

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