Processors who run LDPE and LLDPE are careful when choosing higher-viscosity purge options. Usually, these processors can get by using LDPE or LLDPE pe as a purge or look to ldpe or LLDPE-based purge compounds for increased cleaning performance. However, if they opt to purge with something stiffer like HDPE or an HDPE-based purge compound, the concern arises: how efficiently will ldpe or LLDPE displace the HDPE or HDPE-based purge? How much LDPE or LLDPE will be needed to completely displace a lower viscosity polyethylene like hdpe or hdpe-based purge compounds?
As with any purge solution, you must respect and consider the possible differences in viscosity between resin and purge to avoid residue issues from the stiffer materials. Residue from a cleaning agent or purge resin will eat away at the savings provided by a more efficient cleaning method with higher quantities of displacement resin needed, leaving the processor with close to, if not more, costs in material and time.
Fortunately, a little material science can go a long way here. Temperatures up to 450F show that LDPE/LLDPE and HDPE viscosities are similar. Hdpe will maintain viscosity until about 500 to 550F degrees before it becomes “watery” and loses viscosity drastically. However, once you go higher than 450F, LDPE and lldpe’s viscosities will drop like a rock, and you can immediately notice the resins become far softer and lose stiffness, in turn making it considerably more difficult to “push” an HDPE or HDPE-based purge out of the system. With temperatures directly affecting viscosities, 450F is a “magic number” defining optimal purge and displace temperatures between different pe types. At 450 or cooler, a ldpe or LLDPE can more effectively displace an hdpe than it could at temperatures higher than 450F.
This info is useful when the LDPE or LLDPE processor needs the added stiffness of an HDPE or HDPE-based purge to help rid the system of a previous color or when purging for carbon contamination (black specks) with an appropriate HDPE-based purge compound like Asaclean NCR Grade. Keeping 450F in mind as a benchmark when purging ldpe or LLDPE with HDPE or HDPE-based purge will help you avoid residue and more downtime, and your bottom line will thank you for it.
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