CAUSES OF VARIATION IN VST OF PVC PRODUCTS (1)
YASHODHAN KANADE
PVC Technologist and performance improvement consultant with 50 years of experience. Advanced Trainer & mentor. Provide on line training on Quality Management and Performance Improvement in the field of PVC processing..
Temperature related properties of plastic are measured in terms of Tb, Tg, VST, HDT, DTUL, Tm, Tf etc.
Due to costly DSC equipment and requirement of skilled operator for testing Tg, VST, an user friendly method, is often used to interpret Tg in PVC processing industries .
However, unlike Tg which is an inherent property, VST is a conditional property.
VST is defined as a temperature at which a circular flat ended needle having 1 sq. mm. cross section area, penetrates in a specified sample immersed in oil bath, heated @ 50+/-5 Rate A or 120 +/- 5 °C/ Hr Rate B, under the combined load of either 1 Kg or 5 Kg to 1 mm depth.
If any of the conditions is changed or the sample is prepared differently, the result won’t be reproducible. It is necessary to report in the test result – standard used for testing, method of sample preparation, sample thickness, heating rate & load. All these factors affect the result.
It is important to note that, there is a difference in various standard test methods as below -
ASTM D 1525 stipulates a sample of minimum width of 12 mm and a minimum thickness of 3mm. Two layers of thin sheets are allowed to build up thickness of 3mm.
DIN 53 460 stipulates a sample of size 10mm x 10mm and a thickness of 3 to 6.4mm. It allows 3 layers of specimens to build up thickness of 3mm. The test sample should be in as delivered condition.
IS 6307 standard for Rigid PVC sheets stipulates a sample size of 10mm x 10mm and a thickness of 3 to 6 mm. and conditioning at 27± 2 °C and relative Humidity of 65± 5 %.
The result of VST test by these methods can be different. Hence it is necessary to mention about the standard followed in the test result.
If the testing equipment has no automatic adjustment for controlling heating rate, it is necessary to monitor temperature after every five minutes and adjust the heating rate manually as per the requirements.
It is always better to use VST test equipment with automatic heating rate controlling system having 2 – 4 stations.
Unless we want to study effect on various formulations on VST for R & D purposes, we should always prepare a sample from the product itself, following the standard method.