Processing Making
Processing Making preforms for PLA bottles … Injection moulding of PLA preforms Although the name might suggest otherwise, the PETline injection moulding systems offered by the Swiss manufacturer Netstal for the production of preforms for plastic bottles can also be used for processing materials other than polyethylene terephthalate (PET). This not only applies to other „fossil“ plastics such as polypropylene, but also to plastics made from renewable resources such as PLA. PLA is an industrially compostable material that is well suited for the production of injection-moulded preforms to be used for stretch blow moulding of bottles. The first such bottles have already been launched in the US and European markets. PLA is characterized by very good transparency, high gloss and a good flavour barrier and has oxygen barrier properties similar to those of polypropylene. Injection moulding of preforms and strech blow moulding of bottles (principle); courtesy SIG Corpoplast Nevertheless, the use of PLA for beverage bottles is currently still somewhat limited. The available material has so far had insufficient creep behaviour, which has limited its application to non-carbonated beverages. Since the stretching behaviour of PLA during the production of the bottle is different to that of PET, different blowing process parameters are required. The stretching ratios, on the other hand, are very similar, so that, roughly speaking, the same shapes as with PET preforms can also be achieved with PLA. This means that preform injection moulds designed for PET processing can also be used for processing PLA, even if certain compromises may have to be made. While PLA was relatively expensive when it was initially launched in the market, the price differential between PLA and PET has meanwhile diminished. And since the price of PLA has remained relatively stable, the negative upward development of PET prices is making PLA increasingly attractive. If the prices continue to follow these trends, the demand for PLA is certain to rise and this might even lead to bottlenecks in material supply. Basically, standard equipment can be used to produce preforms from PLA. Some specific characteristics of this material, however, require significant changes in processing conditions. PLA processing properties PLA has a melt temperature of approximately 145- 155°C. Its glass transition temperature is about 55-58°C and its crystallisation temperature about 95-120°C. In its molten state, PLA sticks strongly to textiles, wood and metals. 16 bioplastics [06/02] Vol. 1
Processing PLA bolltes made with preforms from a NETSTAL machine Since PLA is also a hydrophilic material, it must be dried prior to injection moulding. In this respect, the significantly lower processing temperatures compared to PET must be taken into account already during the drying process. The drying process An existing dryer for drying PET can well be used to dry PLA. It is important, however, that PET or other plastics do not contaminate the drying system. The drying temperatures are in the range of approximately 90-100°C, and drying takes no longer than that of PET. Residual moisture content of up to 100ppm is fully sufficient (compared with 50 ppm for PET). The plastifying process Not only the temperature of the drying equipment but also the temperature profile of the plastifying unit has to be significantly lower than for PET: the feed zone is heated to about 180°C, while the compression and the metering zones are heated to approximately 210 - 220°C. „In our case were able to use our standard PET screw that has an ideal shape for avoidance of acetaldehyde formation when processing PET, that it also brought to bear here with its advantages in low shear combined with optimum homogenisation“, says Stefan Bock, Manager Application Technology PET Systems at Netstal. A specific Netstal program called „Intrusion“ additionally supports the design of the screw shape. This means that the screw, due to its storage of melt, can plastify continuously under almost the same conditions, so that the rotation speed of the screw remains almost constant over the entire cycle. This way, the melt is produced in a gentle manner and is very homogenous due to the conditions remaining constant. This condition is of great importance, especially from the aspect of the addition of liquid additives. These additives are distributed very homogeneously in the melt - an advantage from which especially PLA benefits, as PLA can hardly be processed into hollow items without using additives, Stefan Bock observes. The mould As a rule, an existing PET mould can also be used to process PLA. The material‘s stretching ratios are similar to those of PET; merely its density of 1.25g/cm³ is lower. A hot runner with needle shut-off is also suitable for PLA. The mould maker should satisfy himself, however, that operation of the hot runner at the lower temperature of only about 220°C does not result in leakages as a consequence of the lower thermal expansion of the hot runner manifold bars. At cold mould surfaces (temperatures below 25°C), the PLA melt has a marked tendency to separation of lactide. It is therefore essential that the mould is equipped with a cooling system that does not reduce the temperature below this threshold. While this temperature range will inevitably lengthen cycle times, the effect can be partly offset through intensive post-cooling. The post-cooling As a consequence of the relatively high mould temperature of 25°C, the preforms are very susceptible to scratching and tend to stick together immediately after demoulding, a characteristic that is not conducive to the desired short cycle times. „Here, the unique post-cooling system of the Netstal systems ensures extremely good cooling of the preform surface“, explains Stefan Bock further. The preforms are first post-cooled for one cycle by an actively cooled remov- bioplastics [06/02] Vol. 1 17
