PVC Gelation: The Property You Cannot See in a Cross Section

What gelation actually is

Rigid PVC is not a true melt in the way polyethylene is. It arrives at the extruder as a dry blend of porous grains around 100 to 150 microns across, each grain containing resin, stabiliser, lubricant, filler and impact modifier.

Under heat and shear the grains break down, the boundaries between them disappear and the material becomes a continuous melt. That process is gelation, and the degree to which it has happened is expressed as a percentage.

Below about 60 percent the grain boundaries remain visible under a microscope and act as crack paths. The profile looks fine and breaks easily. Above about 90 percent the material is fully fused and the impact strength is high, but the process is running hot and the risk of degradation rises.

This is why two profiles that pass every dimensional check can behave completely differently on a drop test. Dimensions are set by the die and the calibrator. Gelation is set by the screw, the temperature and the screw speed.

How to measure it

The standard method is a solvent test. A weighed sample is immersed in acetone or methylene chloride, the undissolved fraction is dried and weighed, and the result is expressed as a percentage. It is simple, it is comparative, and it takes about an hour.

The second method is differential scanning calorimetry, which measures the residual crystallinity of the PVC and infers gelation from it. It is more precise and requires equipment that most profile plants do not have.

The third is microscopy on a microtomed section, looking for the disappearance of grain structure. It is qualitative but it is fast and it shows the distribution, which the bulk methods do not.

Whichever method is used, the sample has to come from the same place on the profile. Gelation varies across a profile wall because shear and temperature vary, and a sample from the outside skin is not representative of the core.

Table 1 — Ways to measure gelation, and what each is good for.

Impact peaks before full gelation

The chart on the left is gelation against melt temperature for a rigid PVC window profile with a 4 mm wall and a K value of 67. At 175 degrees the gelation is 42 percent, at 185 it is 68, at 195 it is 88 and at 205 it is 97.

The right hand panel is the part that matters. Impact strength rises with gelation from 8 kilojoules per square metre at 42 percent to a peak of 33 at 88 percent, and then falls back to 26 at 97 percent.

The peak is the point where the material is fully fused but has not yet begun to degrade. Running past it costs impact strength, costs energy and shortens the life of the stabiliser system.

So the target is not maximum gelation. It is the gelation at the peak of the impact curve, which for most rigid PVC profiles sits between 75 and 90 percent.

What moves gelation

Melt temperature is the strongest lever. Every 5 degrees of barrel or die temperature moves gelation by roughly 8 to 12 points in the range that matters, which is why temperature control on a PVC line has to be tighter than on a polyolefin line.

Screw speed is the second. Higher speed means more shear, more shear means more energy at the same temperature, and gelation rises. It is a faster lever than temperature and it is also a more dangerous one, because shear heating is local and can degrade the material at the flight land before the bulk temperature moves.

Back pressure and the screen pack are the third. A tighter pack raises the pressure in the metering zone, increases shear and raises gelation. That is useful when a line is running cold, and it is a hidden variable when a pack is changed.

The formulation is the fourth. Lubricant levels, impact modifier type and filler loading all shift the gelation curve. A change of compound supplier can move the same machine from 78 to 65 percent without any setting being touched.

Table 2 — The four variables that move gelation on a given line.

Over-gelation is also a fault

A profile at 97 percent gelation has been through more heat history than it needed. The stabiliser is partly consumed, so the material has less reserve against a hot day or a slow start-up, and the first sign is usually a colour shift rather than a mechanical failure.

It also costs energy. Running a die 15 degrees hotter than necessary is a continuous cost on every metre, and it shortens heater life and increases the thermal load on the calibrator.

And it narrows the process window. A line already at 97 percent has nowhere to go if the compound changes and needs more heat, whereas a line at 82 percent has room in both directions.

The practical target is therefore the lowest gelation that still meets the impact requirement, not the highest the machine can reach.

Setting a gelation target

Start by measuring the gelation of a profile that performs well in service. That is your reference, and it is more useful than any number from a textbook.

Then measure impact strength at three gelation levels by adjusting melt temperature in steps, and find the peak for your compound and your profile. The peak position moves with formulation, so it has to be established rather than assumed.

Set the target 3 to 5 points below the peak, so that normal process variation does not push the line over the top. A target of 85 percent against a peak at 88 is a stable process; a target of 88 is a process that will drift past the peak.

Then control it. Gelation is a QC measurement, not an inline one, so the control is temperature and screw speed held within limits, checked by a solvent test once a shift on a sample from a fixed position.