Natural rubber viscosity reduction-routes to improved processability, productivity and properties?

摘要

Viscosity reduction of Natural Rubber (NR) is one of the oldest processing problems in the rubber industry. The molecular weight of NR as delivered in sheets or bale form is much too high to process it on standard rubber machinery such as roll mills, internal mixers, extruders, or injection moulding machines. Moreover, the gel content of NR can reach levels of 20% and more. Thus the pre-treatment of NR prior to blending with other polymers or addition of fillers is almost mandatory. This process is called mastication. Until about 15 years ago a separate mixing step in order to reduce NR viscosity was widely used in the industry, this step is called pre-mastication and can involve the use of chemicals to accelerate the breakdown of NR to a lower viscosity. Most popular is the use of chemical peptisers, normally a combination of a free radical scavenger and an iron complex to boost oxida-tive chain scission at higher temperatures. Formerly the radical scavenger used was a pentachlorophenol derivative (PCTP), however due to environmental problems it was replaced by Dibenzamido-diphenyl-disulphide (DBD). The scavengers block free radicals at the ends of broken NR chains produced by mechanical rupture mainly at lower temperatures. Oxidative chain scission catalyzed by organic iron complexes occurs mainly above 120℃ and then becomes the dominant process for molecular weight reduction, because mechanical shear forces are low. As an alternative to chemical peptisers, lubricants, often referred to as "physical peptisers" are widely used. Lubricants do not reduce the molecular weight of the NR, and compounds may exhibit both good processability as well as good dynamic properties. However the higher Mooney viscosity obtained using lubricants can often prevent their effective usage in some production mixing or processing equipment. Newer products have been developed which combine the advantages of lubricants together with the free radical scavenging effect of DBD without the disadvantages associated with more aggressive chemical peptisers.