MOULD FOULING

The main method of crosslinking the homopolymer and copolymer is by use of thioureas, and, as the cure reaction requires basic conditions, an acid acceptor is also added. Litharge, red lead, magnesium oxide and dibasic lead phosphite are commonly used acid acceptors, and the most commonly used thiourea is ethylene thiourea, but this has a tendency to promote mould fouling. 

The Echo S cure system has been commercialised by BFGoodrich and this is said to give improved scorch safety and reduced mould fouling over the use of thioureas. Inorganic acid acceptors other than those based on lead are recommended for use with the Echo S cure system. 

A relatively simple way of testing mould fouling is to use a spiral mould. This enables mould fouling to be observed qualitatively, although with a little development (weighing... 

The well-run factory should plan to minimise the problem of mould fouling in the following ways ... 

The initial objective has been the replacement of the solvent-based top coat. This has been highly successful being less prone to mould fouling or lamination within the rubber matrix than the Chemosil 411/Thixon 530 type solvent systems, due to the fact that the waterborne top coat (Chemosil XW 7484 now Chemosil 7500) is intermediate in its reactivity between Chemosil 220 and Chemosil 411 solvent-based materials. 

Rubbers using this cure-site monomer were not entirely satisfactory as they had to be used with cure systems that had poor processing safety and also gave problems such as sticking on to mill rolls and mould fouling. 

In addition to these developments, work has been carried out to improve the standard range of nitrile rubbers by designing polymers with better colour and reduced mould-fouling characteristics, and an attempt to rationalize the range and thereby simplify polymer choice has also occurred. 

The TMS rheometer from Negretti is effectively a modification of the Mooney viscometer principle for use with elastomeric materials. The elastomer is introduced into a cavity containing a shearing disc by injection, and measurements of torque under fixed or variable conditions of temperature and rotor speed can be made. Effective simulation of a variety of process conditions can be made. The instrument has found particular use in the investigation of rubber-to-metal adhesion behaviour, a problem of importance to mould fouling and extrusion processing. 

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