ANOMALY IN IMPACT TEST AS PER IS 4985:2000 FOR UPVC PIPES

Introduction:

Unlike other plastics, mechanical properties in PVC product depends upon the extent of fusion taking place during processing of PVC compound.

Due to shear & temperature, as the level of fusion increases, various mechanical properties are differently affected.

1.      Properties concerned with stiffness - such as tensile strength and resistance to internal pressure shows a continuing improvement, until a maximum value is reached.

2.      Properties concerned with ductile – brittle behaviour such as impact strength and resistance to stress cracking reach an optimum level and then decline with increase in gelation.

For best combination of mechanical properties, the fusion has to be optimum.

Tensile vs. Impact strength:

As the level of fusion increases during processing, the tensile strength increases due to fusion (entanglement).

As the fusion level crosses optimum value, more entanglement is responsible for further improvement in tensile strength.

Even degradation of PVC results in cross linking that adds up to tensile strength.

On the other hand, impact properties vary at various fusion levels -

1.      At low fusion level, voids, imperfections and lack of entanglement bring about low fracture values and hence low impact strength.

2.      At optimum fusion level, increased chain entanglement provides energy absorbing mechanism that improves impact strength.

3.      However, at even higher temperature, with further increase in fusion level, material becomes stiffer and loses ductility.

Thus, maximum impact strength is the key property processors should strive to achieve.

Interpretation of IS 4985:2000:

Impact strength is evaluated in the form of – Resistance to external blow at zero degrees C.

Impact strength of the pipe reduces with reduction in temperature. So it is logical to test the pipe at zero degrees C. Also, the drop impact test is more practical and relevant than Izod or charpy impact test.

It tells us the minimum impact energy required for the pipe to pass this test, but does not measure the real impact strength of the pipe. Two pipes passing in this test may have different impact strengths.

However, measuring of actual impact strength is more important for those who are interested in improving the quality of pipe.

Impact strength by falling weight = mass (kg) x height (m) x g = mgh, at which the pipe breaks at zero deg C.

Observations in impact test criteria as per IS 4985:2000:

1.      The requirement of impact energy increases with increase in diameter of pipe.

By this logic, requirement of impact energy for 110 mm diameter pipe should be more than the requirement of 90 mm diameter pipe. However, it is the same.

Why?

2.      It is not considering the effect of thickness on the impact strength. While 90 mm x 0.4 MPa and 90 mm x 1 MPa pipe has more impact strength, 90 mm x 0.6 MPa pipe has less impact strength. Why?

3.      The criteria for lowest diameter of pipe of 25 mm diameter is 250 g weight dropped from 0.5 m height, while the criteria for 125 mm and above diameter pipe is 1 Kg dropped from 2m height. Why?

If it has considerations of mishandling, then can in actual handling, more than 250g weight not drop on 25 mm diameter pipe or more than 1 Kg not drop on 125 mm diameter and above pipe?

ISO 1452-2:2009

This test method has improvised these aspects and stipulated progressively increasing impact energy in falling weight impact test with increasing diameter. Also, there is more impact energy requirement for high level than medium level.

But still the test is pass or fail.

ASTM D 4226

This test method covers the determination of impact energy required to crack or break rigid PVC sheeting and flat sections used in building products. This test uses the statistical method to calculate the impact energy required for 50% failure of PVC profiles and can be expressed in terms of energy required per mm thickness of profile.

This principle can be adopted for UPVC pipes with the same equipment used for IS 4985:2000, (pipe is a round profile). By progressively increasing the impact energy in terms of height or weight or both, IS 4985 test method can be modified to calculate impact energy required for the 50% failure of the pipe per mm thickness at zero degrees C.