Processing aids surface tack

Natural rubber displays the phenomenon known as natural tack. When two clean surfaces of masticated rubber (rubber whose molecular weight has been reduced by mechanical shearing) are brought into contact the two surfaces become strongly attached to each other. This is a consequence of interpenetration of molecular ends followed by crystallisation. Amorphous rubbers such as SBR do not exhibit such tack and it is necessary to add tackifiers such as rosin derivatives and polyterpenes. Several other miscellaneous materials such as factice, pine tar, coumarone-indene resins (see Chapter 17) and bitumens (see Chapter 30) are also used as processing aids. 

A compounding ingredient which enhances the surface tack of uncured rubber compounds. Tackifiers, exemplified by pine tar are classed under softeners and processing aids . 

Some processing aids can either improve or eliminate the surface tackiness of uncured compounds. Tackifying resins, especially phenolic types, are excellent for increasing surface tack, while materials such as primary amides can be used to reduce or eliminate surface tack. The primary amides may not be desirable for use in compounds meant for compression molded products because the resultant surface bloom on the preforms may cause knitting problems during the molding operation. 

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