[Degradable plastics] Processing method and Precautions for PLA

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Using corn, cassava and other crops as raw materials, lactic acid (PLA) is prepared by microbial fermentation and extraction, and then refined, dehydrated, oligomerized, decomposed and polymerized at high temperature.

PLA has excellent biodegradability and can be completely degraded by microorganisms in the soil to produce CO2 and water, without causing environmental pollution.

PLA has good mechanical properties, transparent luster and low shrinkage, and is widely used in the production of packaging materials, disposable tableware, household appliance shells, fibers, 3D consumables, etc.

With the promotion of plastic limit, the market scale of degradable plastics expands rapidly. PLA plays a very important role as a mature degradable plastic.

The properties of PLA are compared with other polymer materials

PLA has similar mechanical properties to polypropylene and can replace PP and PET in some fields. It can be processed by a common plastic processing method. Such as injection molding, extrusion, blister, blow molding, spinning and other molding processing technology.
However, the biggest hard damage of PLA is its poor heat resistance. Even at 60℃ or so, ordinary PLA tableware will become soft. However, some modified polylactic acid has overcome this shortcoming, and some modified varieties can reach 120 degrees Celsius in long-term use. PLA also has the disadvantages of easy hydrolysis and poor impact performance, which need to be paid attention to during the processing.

Processing method and precautions of PLA

1.PLA extrusion molding

The screw extruder can be used to produce PLA sheet, film, fiber, pipe and foamed sheet. The main steps are melting → extrusion → cooling → winding and so on.

Matters needing attention
A. The internal cleaning of the extruder shall be ensured before PLA resin is added. Because the compatibility of PLA resin with ordinary plastics is poor, and the processing temperature of PLA is very low.
B. When extrusion processing is carried out on PET equipment, the processing temperature should be raised to the PET processing temperature first. A low-melt, non-thermal cross-linking polypropylene cleaning extruder should be used to keep the temperature in a stable state. After a period of time, the temperature was adjusted to the processing temperature range of PLA.
C. PLA is easy to absorb water, so the water content of PLA resin should be controlled below 0.05% for sheet and film processing.
D. PLA has a high elastic modulus, and the winding equipment must be equipped with a good tension control system.

2.PLA injection molding

Injection molding is the most extensive processing method for PLA. PLA injection molding machines mostly use reciprocating screw injection machines. As injection molding is a very mature processing method, the injection molding of PLA is relatively simple.

Matters needing attention
A. For the injection molding of PLA, it is required that the cooling effect of the mold must be good and the chiller must be equipped.
B. Mold temperature should be controlled between 20-30℃. The wall thickness of the product should be controlled below 4mm as far as possible, in case the forming time is too long and the cooling is not sufficient.
C. PLA raw materials must be dried before injection, and the humidity should be controlled below 0.025%.
D. The injection molding machine needs to be cleaned with low viscosity polystyrene or polyethylene before the PLA resin is injected, and then the temperature is kept in a stable state.

3. The PLA thermoforming

Hot forming is a processing method that uses sheet as raw material to produce products. The main principle is to heat and soften the flat plastic sheet, and then adsorb or press it on the surface of the mold, including falling molding, vacuum molding, pressure molding and so on.

PLA thermoforming is suitable for the production of transparent blister products such as cold drink cups, fruit dish boxes, salad plates, etc. Its products have good transparency and luster, and can also be used for the production of thermostable products such as blister cup lid and blister meal box.

Matters needing attention
A. PLA is A crystalline polymer. The higher the crystallinity, the higher the rigidity, tensile strength and heat resistance of the product. Therefore, PLA thermal forming needs to choose the brand with high crystallinity.
B. The temperature of the hot forming mold shall be heated to 90-100 ℃. It is recommended to use high temperature thermal conduction oil or infrared lamp tube for heating. The resulting PLA products can withstand temperature of more than 90℃ and appear opaque or semi-permeable in appearance.