Reversion test


Introduction:
As per IS 4985: 2000 for U PVC pipes and IS 15778:2007 for C – PVC pipes, the longitudinal  shrinkage or reversion shall be less than 5%, when tested at 150° C.
It is left to us to find and interpret:
Why it should not be more than 5%? And what happens if reversion values are 2%, 3%, 4% etc?
Question is whether there is any correlation between reversion and mechanical or thermal properties?
Material exiting the die undergoes:
1.      Swelling
2.      Oxidation of external surface in the air gap
3.      Stretching or orientation through haul off,
4.      Sizing (shaping), and
5.      Cooling well below the glass transition temperature.
All these “micro processes” take place simultaneously.
To understand reversion phenomenon, consider two extreme situations:
Situation 1 -
Lower thickness pipe is produced from bigger die gap by increasing Haul off speed.         
Say 90 x 4 Kg/cm^2 pipe is produced from 90 x 6 Kg/cm^2 die.
In such case reversion will be 8-10% or more.
Pipe will bend on exposure to sunlight.
Pipe will curl during socketting.
Pipe will fail in impact.
Situation 2 -
Higher thickness pipe is produced from lower die gap by reducing the speed.
Say 90 x 6 Kg/cm^2 pipe is produced from 90 x 4 Kg/cm^2 die.
In such case reversion will be even on + ve side.
Inside surface will be rough.
Pipe will fail in impact.
Thus we can observe that pipe fails in impact, if reversion is too high or too low.
This suggests that there must be an optimum reversion; where impact strength will be the best.
This led to research that has shown that the reversion is related to impact strength.
If we plot a reversion versus impact graph, it is observed that the impact strength passes through a maximum at particular reversion for a particular pipe size.
For thicker pipes, optimum reversion is lesser and vice versa.
It is therefore necessary to capture parameters at optimum reversion and include it in the process control plan.
Various parameters affecting optimum reversion are:
1.      Extrudate temperature – depends upon temperatures set for barrel and die.
2.      Distance between die and sizer for vacuum sizing,
3.      Cooling water temperature and efficiency of sprays,
4.      Die gap ( addressed in tool specification)
5.      Die swell (addressed in compound formulation and machine speed)
6.      Speed – more the speed, more the reversion – especially for thinner and lower diameter pipes.
Effect of reversion is better observed in twin die:
If the temperature, thickness and the distance between the die and sizer (air gap) for both extrudates is not uniform, the reversion varies.
It is observed many times that pipe on one side passes in impact while pipe on other side fails in impact.
Orientation in machine direction has following effects –
1.      Tensile strength increases,
2.      Elongation at break reduces,
3.      Hence, impact strength reduces.
4.      VST reduces.
However research shows that falling weight impact strength passes through a maximum at optimum orientation.
The reason:
Below optimum orientation, intimate contact to transfer impact energy to additives is less. This reduces impact strength.
Beyond optimum orientation, impact strength reduces due to strain on entangled (fused) PVC chain and lowering of elongation at break.
This relationship between reversion and falling weight impact strength is more observable in 90 x 6 Kg/cm^2 UPVC pipes, which fails in impact strength many a times, in spite of good formulation and good extruder.
If reversion is not uniform around the circumference of pipe, it will fail in impact at weaker point.

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