Effect of Temperature on Morphology and Mechanical Properties of Terephthalate Co-Polyesters Processed by Lamination Extrusion Coating and Film Casting

PET is very hard in granulate form and its melting point is much higher than PEs.
This means:
1. much more torque from the motor
2. longer time in solid state

These two things mean that if only motors designed for LDPE coating are used, there will be a sure problem in motor loads.

When LDPE and PET are processed, the maximum value of friction is developed in the first zone of the screw and with the polymer melting, the friction becomes lower because the polymer came from the solid state to the melted state. However, the molecular rigidity and consequently the high melting point of PET, results in a significantly higher power requirements as compared to polyethylenes.

Higher temperatures, especially in the feed and in the transition zones of the extruder, can help to minimize this effect, but watch out for high temperatures because they could cause degradation.

To avoid degradation of the melt, it is necessary to gently and quickly convey the polymer after it has been melted, because too much shear or heat can cause deterioration.

Using the screw designed for PET with LDPE is not easy anymore. A good option is to have a compromise but it is not simple.

In addition, it is necessary to take care to the mixes of PET with other Extrusion Coating resin pellets. The reason is connected to the different melting properties. If a mixture of polyester and polyethylene is powered to the extruder, the polyethylene will melt while the polyester remains solid or semi-solid. This means an incompatible mixture that will not convey. If temperatures are > 290oC a PET/LDPE mixture should convey. The extrudate will be cloudy because of the incompatibly of PET and LDPE. However, after the transition between the two resins is complete, the extrudate will once again become clear.

There are some ”transition rules”, related to the transition from LDPE to PET and vice-versa, to correctly set-up the extruder and to avoid troubles in transitions.

These rules are:
1.Insure all transfer lines are clear
2.Run extruder until hopper is empty
3.Clear all LDPE(or PET) pellets from hopper
4.Transfer PET(or LDPE) pellets into hopper
5.Change temperature profile