The Coupling Agents

Yet another feature of anionic polymerisation is the possibility of coupling chains together at their living ends . Where the coupling agent is bifunctional... 

Another interesting innovation is that developed by the Malaysian Rubber Producers Research Association. In this case the coupling agent is first joined to a natural rubber molecule involving an ene molecular reaction. The complex group added contains a silane portion which subsequently couples to filler particles when these are mixed into the rubber. 

Fig. 22. Adhesion rating based on flexural sti engths of laminates of poly(styrene) with glass treated with various silane coupling agents vs. solubility parameter of the organo-functional group of the coupling agent. From ref. [117], by permission.

In primer formulations for adhesive bonding of metals, the coupling agents that are most frequently used are those based on epoxy and amine functionalities. Aqueous solutions of aminosilanes have been successfully used for obtaining stable adhesive bonds between epoxy and steel [10] and epoxy and titanium [11,12], while epoxy functional silanes are preferable for applications involving aluminum substrates [13,14], A simple solution of % epoxy functional silane in water is currently used for field repairs of military aircraft [15] where phosphoric acid anodization would be extremely difficult to carry out, and performance is deemed quite acceptable. 

Scheme 3 Reactions in a mixture of PS, PE, and styrene monomer in the presence of a peroxide and the coupling agent.

The organofunctional group (R) in the coupling agent causes the reaction with the polymer. This could be a copolymerization and/or the formation of an interpenetrating network. This curing reaction of a silane-treated substrate enhances the wetting by the resin (Table 9). 

By increasing or decreasing concentration of the coupling agent il/d)c can be controlled. A coupling agent may increase r, which in turn decreases the (// /), ... 

Prior to the chemical reaction of the silane with the silanol-groups on the sUica surface, the silane molecule has to make contact with the sUica surface by adsorption. Then the chemical reaction of silica with an alkoxy-silyl moiety of the coupling agent takes place in a two-step, endothermic reaction. The primary step is the reaction of alkoxy-groups with silanol-groups on the silica filler surface [4]. Two possible mechanisms are reported ... 

In the presence of ZnO, the dump temperature is a parameter of paramount importance in mixing of a silica compound containing a couphng agent with a polysulfidic moiety The sulfur eliminated from the coupling agent causes premature scorch in the presence of ZnO. This problem is partially solved when ZnO is added in the productive mixing step on the two-roU mill. 

R.N. Rothon, The Coupling Agent Index, Intertech, Portland, MN (2000). 

A series of thiol and selenolesters can be prepared from carboxylic acids and the coupling agents CDI, oxalyldiimidazole, or oxalylditriazole via the corresponding carboxylic acid azolides (Tables 3-7 and 3-9). 

Table 3-9. Thiolesters prepared with the coupling agents A, B, and C.

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