Polymer science aspects polymers in the rubber state

6 POLYMER SCIENCE ASPECTS POLYMERS IN THE RUBBER STATE 

In Chapter 1, section 3 there is an account of the behaviour of rubber polymers. The brittle holder is an interesting example of control of Tg by means of the plasticizer content of a PVC composition. One of the important effects of plasticizer is to lower Tg. Unplasticized or rigid PVC, as used in rainwater goods and window frames has a Tg of 80 C. Addition of plasticizer (usually high boiling point esters) lowers Tg progressively, and softens the material. The effect is to impart elastomeric, or rubber, properties. Fig. 3.3 shows this. 

It can now be seen why semi-plasticized (12i%) PVC is a suitable choice for the bristle holder. At its demould temperature it is in its rubbery state, above Tg, and can be easily deformed to accommodate the other parts. It is an elastomer and will thus recover its shape after the deformation. When cool, it is below its Tg, and hence has the rigid properties of a glassy material. 

The protection ring, on the other hand, has a high plasticizer content (30%) and has a Tg far below room temperature. Thus it retains its rubber nature at ordinary temperatures. 

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