Softening in Plastics

Many polymers will not have a specific melting point, which marks the transition from solid to liquid.  Instead, they will gradually soften as the temperature is increased.  This is known as softening. 

The temperature at which a certain extent of softening occurs is called the softening point for that polymer.

The softening temperature for a particular material is determined by the Vicat test.

In this test, a specimen of the polymer to be tested is placed in a Vicat apparatus.  A needle with a cross section area of 1 sq.mm is placed on the specimen with a load of 10 N for the Vicat A test and 50 N for the Vicat B test.

The temperature at which the needle makes an impression 1mm deep is noted as the softening temperature.