Ask Dr. Nick: XLPE v. PE heating and cooling cycles

Question: What are typical heating cooling cycles compared between XLPE and PE? 

Dr. Nick: Crosslinkable polyethylene (XLPE) rotomolding grades work in a different way to standard linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) grades.

During the cook stage of rotomolding standard PE grades, two separate things need to be achieved:

• Sintering – ensuring that powder particles melt and fuse together in a solid mass.  For standard roto grades, sintering is typically completed by the time that the Internal Air Temperature (IAT, the air temperature inside the mold) reaches approx 265 degF.
• Consolidation – allowing sufficient time and temperature for the gases in trapped air bubbles to dissolve into the molten polymer matrix.  For standard roto grades, consolidation is typically sufficiently accomplished by the time that the IAT reaches 390 degF.

During the cook stage of rotomolding XLPE grades, the above two mechanisms need to be achieved, followed by another additional one:

• Crosslinking – XLPE grades contain a special package of additives, based on organic peroxides, which form side links and create a network structure from the individual polymer chains.  This network structure provides improved short- and long-term physical properties.  BUT – and it’s a big but – this won’t happen unless sufficient time and temperature is provided.

The requirements for individual XLPE grades may vary, but one general recommendation that I have seen is that, during the final stages of cooking, the IAT should be above 390 degF for several minutes.  An additional processing benefit of the best XLPE grades is that over-cooking does not result in the usual catastrophic loss of impact strength due to chain scission.

I recommend that, if possible, you set up your cook cycles for XLPE using a device that can measure IAT and that you follow the guidelines above, in the absence of anything more specific from your material supplier.  A more rough-and-ready guideline might be to add two or three minutes on to the cook cycle you would use for standard PE,

However –

Depending on the formulation, over-cooking XLPE can result in some undesirable effects, that you will want to avoid.  One common effect is known as coining – the appearance of a locally depressed area on the surface of the part, as though a large coin had been pressed into the surface while the polymer was soft.  Reducing the oven temperature is the usual expedient to eliminate such defects, but then you may affect crosslinking.  Hence my main recommendation, to use available control tools to achieve as much precision in set-up as you possibly can.

Hope that helps; happy rotomolding!

Dr Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has more than 30 years of experience in rotomolding, specializing in the fields of materials development and process control. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.