MIXING

The mixing process is the beginning of any rubber plant, whether it makes tyres



or rubber articles.


Why mix at all?



1. The basic purpose of mixing is to mechanically breakdown the rubber


in an attempt to obtain a uniformly homogeneous mass, which is


subsequently formed into slabs or sheets of rubber.


2. In other words the mixing process mixes or blends the rubber with the


various constituents uniformly.


3. The mixing plasticizes the base material, i.e raw rubber.




What all go in for mixing?




Rubber (Natural rubber or Synthetic rubber), Fillers, Vulcanizing agents,


Peptizers, Accelerators, Antioxidants, Plasticizers, Process oil, Pigments, Antiozonant,


Other chemicals (Elastomers, Cobalt, Magnesium oxide, Calcium carbonate, Zinc oxide)


(Refer the section on raw materials for more information)


What are the different types of mixing?



In principle, there are two types of mixing (though there are classifications based


on other parameters also).


1. Continuous mixing.


2. Discontinuous mixing or batch mixing.



Either of the above mentioned processes usually comprise of two stages namely, the


distributive and the dispersive stages.



• The distributive mixing stage yields the master batch after primary mixing and


plasticizing. To get a good mix with the additives, the rubber has to be plastic.


The raw rubber is not plastic and is hard. The first stage mainly deals with the


mastication of rubber i.e. making the elastic rubber plastic. Thus the name


distributive as it churns and tears and distributes the elastic rubber to get a


sticky plastic yielding compound.


• The dispersive stage serves for the final dispersion of the vulcanizing agents

The mixers used in both stages of the continuous mixing process are of the


continuous type. Typically, a continuous mixer consists of screws rotating in cylindrical


chambers, in which the material to be mixed enters and leaves continuously.


On the other hand, in batch mixing the first stage is served by the batch mixer


followed by the second mixing stage in a rolling mill or a continuous mixer. Sometimes,


rolling mills are used in both stages. If such is the case, they operate discontinuously.