Interfacial bonding studies have attracted the attention of many workers using a variety of spectroscopic techniques. Evidence for coupling activity has been demonstrated in the cases shown in Table 1.
II. MECHANISM OF ADHESION PROMOTION
Mechanistic theories of adhesion promotion have been described in detail by Rosen [11] with sole reference to silanes, but it is likely that many of the proposed theories apply equally well to coupling agents in general. The proposed mechanisms are described below.
As applied to silanes, the theory postulates that trialkoxysilane groups chemically bond to silanols on the mineral substrate surface by reaction of the hydrolyzed alkoxy group forming interfacial bonds of 50-100 kcal/mol [12] to 50-250 kcal/mol [13]. The organofunctional groups of the silane bond chemically to the polymer molecules. Both reactions were considered to be essential if true coupling is to be achieved. Although originally postulated for silanes, it is equally applicable to other adhesion promoters, including titanates and zirconates. It has been pointed out that although covalent bonds may be formed between polymer and mineral surfaces, some covalent oxane bonds are easily hydrolyzed [12,14], and examples of hydrolytically stable bonding have been achieved in the absence of chemical reaction with the polymer [12,15].