Chemiml Technology, MOSCOW, U.S.S.R.In considering the materials used as adhesives it is easy to note that almost all of them are high polymers. Thermoplastic and thermosetting resins, natural resins, cellulose derivatives, proteins, and rubbers are known to be used for bonding. It is important that only high molecular substances with their long-chain molecules have pronounced adhesive properties.Only such inorganic compounds as cements and solders can be considered as nonpolymeric adhesives. However, the bond formation with cements and solders takes place in accordance with completely specific mechanism (fusion of crystals, welding) which has little in common with the formation of the usual adhesive bond and can only conventionally be considered as bonding. As far as bonding with sodium silicate is concerned, it is established at present that sodium silicate can be considered as an inorganic high molecular substance.The present paper is a discussion of the mechanism of specific adhesion of polymers to each other. This case of adhesion is of great practical importance. The production of tires, confection of rubber products, production of rubber wear by glueing, as well as bonding and welding of plastics, and sizing of paper are all based on polymer-to-polymer adhesion. The technology of most of the aviation materials and of the artificial leather is also entirely based on the process of polymer .to-polymer bonding. Finally, bonding with polymers is widely used in wood processing and plywood manufacture, civil engineering, during the assembly of airplanes, and in a good number of other branches of engineering. Polymer-to-metal adhesion is not discussed here because of a special character of polymer-metal bond, which is probably chemical in most cases. The adhesion of polymers to inorganic glass is not considered here either though glass represents inorganic polymer and the authors of the present paper have some grounds to believe that the mechanism of polymer-to-glass adhesion in some cases is similar to that of polymer-to-polymer adhesion.Attempts at theoretical generalization of the results obtained were made by the first investigators of bonding.2-6 However, the importance of the earlier investigations in the development of a theoretical approach to the bonding of polymers is depreciated by the fact that in most cases bonding of surfaces (wood, metal, glass, etc.) has been studied by means of the layer of i?.5

In accordance with our concepts, the autohesion of polymers is caused by diffusion of segments of chain molecules from one layer into another layer of the same polymer brought into contact with the first. This conclusion was drawn by us from generally known facts and modern concepts concerning the nature of high-molecular substances. The present work represents an attempt to confirm experimentally the diffusional theory of autohesion. Polyisobutylene with a molecular weight of 150,000 was used to investigate autohesion. Polyisobutylene was chosen as the material for study because its molecules contain no double bonds. This renders them sufficiently stable in regard to oxidation at ordinary temperature. The strength of autohesion was characterized by the stress necessary to delaminate two laminated fabric strips treated before laminating with the polymer solution. For the preparation of splices subjected to delamination in determining the autohesion of polyisobutylene, use was made of strips of unsized technical cotton fabric with a smooth surface. The working length of the specimen was always 10 cm.; its width was usually 5 cm. The polyisobutylene was applied to the specimens in the form of 6.0–7.5 per cent solution. The polyisobutylene solution was made up in a low-boiling fraction of aviation gasoline, which left no dry residue upon evaporation. The quantity of polyisobutylene, in grams per square centimeter of specimen surface was established from the weight of the absolutely dry initial fabric and of the polymer coated material after drying.

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Diffusion of low-molecular substances in high polymers has been the object of study by many Soviet and other investigators during the past two decades. Included is the work of Reitlinger, Zhurkova, Ryskin, Malinskii, and Vasenin in the Soviet Union, and Barrer, Crank, Long, Auerbach, van Amerongen, McCall, Shtamm, and many other authors elsewhere. By way of contrast, the effects of self-diffusion in polymers and diffusion of polymer in polymer (inter-diffusion) have been given little study because of the great experimental difficulties and the complexity of theoretical explanations. Meanwhile, these effects have considerable scientific and practical significance. As will be seen hereafter, very many properties of polymers are associated in some manner with diffusivity of macromolecules or their segments. No less important is the tendency toward self- and inter-diffusion of polymers in processing and use. The aim of this review is to correlate data concerning self- and inter-diffusion of polymers containing elastic molecules (elastomers), available in several studies, and to suggest further paths for investigation of this problem.

A theory of adhesion of polymers to each other is suggested ; according to it adhesion of dissimilar polymers as well as auto-adhesion is explained by the diffusion of chainlike molecules segmentally or otherwise. Starting with this theory, the effects of various factors on the adhesion of polymers to each other are discussed. The factors considered are: duration of contact between adhesive and substrate, temperature, molecular weight of adhesive, molecular shape and polarity.