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Processing of PLA (biopolymer) requires some care

PLA (polymer of lactic acid) is an important biopolymer with a great future. It can be extruded, molded and can also be oriented biaxially. All the processes used for thermoplastics can be essentially used for processing of PLA. However, processing of PLA has certain inherent problems. A few are discussed below:
PLA picks up ambient moisture very rapidly and most of the problems experienced on PLA arise from inadequate drying. Hence, PLA is required to be properly dried before processing. Because it is a condensation polymer, presence of even a very small amount of moisture during melt processing causes degradation of polymer chains and loss of molecular weight and mechanical properties. The drying must be done to the extent when moisture in PLA is at least below 250 PPM level. PLA needs different degrees of drying depending on the grade and how it will be used. Under 200 PPM is better because viscosity is more stable. PLA needs a dehumidifying (desiccant) dryer. Amorphous grades used for heat seal layers in film are dried at 60 degree C for 4 hours. Crystallized grades used to extrude sheet and film are dried at 80 degree C for 4 hours. Processes with long residence times or higher temperatures like fiber spinning need more drying, to less than 50 PPM of moisture. Like PET, PLA requires crystallization so it will not clump together in a dryer.

PLA film can be extruded with a general purpose screw, though output is said to be better with a low shear screw designed specifically for this polymer. Short, low shear PVC screws are probably the best. Polyolefin screws are inadequate in power and give bad performance - hence should not be used. Sheet and foam require their own screw. Xaloy has developed an essentially modified fusion screw. This is special screw for processing PLA, particularly for sheet and foam. A special Turbo Screw has been developed for extruding PLA foam. A mixing section on the screw and a static mixer before the die is recommended for good melt uniformity. All temperatures in the extrusion system must be closely controlled, as excessive heat can be disastrous.

A significant processing cost for PLA film is scrap. Oriented film producers generally can not recycle scrap back into the process, hence it should be repalletized and used for less critical applications. Thermoformed sheet scrap is being granulated for reuse, but care must be taken to keep knife to screen gap constant, so the flake doesn't overheat and degrade or melt.

Winding and roll handling equipment may need to be resized, and hoppers may need reinforcing to handle more weight. PLA has a density of 1.24 g/cc, lighter than PET (1.33), but a lot heavier than PP (0.90) or PS (1.04). That means the same size roll of PLA film weighs a lot more than one of OPP or PS. Because PLA is more dense than OPP, chill rolls need to remove more heat from the same thickness of sheet. Cooling is hindered by the fact that the bio polymer conducts heat about 30% to 40% more slowly than PS or PP.

PLA viscosity is also very sensitive to temperature and moisture, so PLA web tends to stretch and sag unevenly. PLA also shrinks 9% in the transition from melt to solid, going from a melt density of 1.13 g/cc to a solid density of 1.24 g/cc. This shrinkage builds up tension in a roll stack if all three chill rolls turn at the same speed. A clutch and variable speed is recommended on the lowest chill roll to relieve tension.

No biaxially oriented film line has ever been built specifically for PLA, but it is being run on modified BOPET lines because its stretch ratio is similar (about 3.5:1 in both MD and TD). PET processors also have drying capabilities. PLA can run on lines for BOPS film with typical stretch ratios of 2:1 MD and 4.5:1 TD. But PLA is too different from PP, which has a 5:1 MD and 10:1 TD ratio. BOPP film line is not easily adapted to PLA. PLA has a low Tg (135 to 140 C) and high tensile modulus, an unusual pairing that requires special attention in sheet processing.

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