>> Thursday, 19 March 2009
Poly(ethylene terephthalate), PET resins have several basic differences follow this notes and find the one which is the best for your application.
1. Intrinsic Viscosity, I.V. - ranging from 0.72 up to 0.85 for injection stretch blow molding.
2. Polymer Type - Homopolymer or Copolymer
3. Residual Acetaldehyde A.A Level - Normal or Reduced
4. Cosmetic Grades - Able to resist crystallization.
1. Intrinsic Viscosity, I.V.
Low I.V. materials tend to be better if you need to blow an intricate shape but will result in a container having less strength and will become more difficult to control in the stretch blow process.
A higher I.V. material will give better overall strength and it will be more easy to control. However, it will be more difficult to stretch into corners meaning that logos and textures will not be so well defined. In severe cases, it may also lead to pearlescence in the finished container.
Some of these difference can be adjusted out by raising or lowering the preform processing temperature.
2. Polymer Type
Most bottles can be blown from either polymer type, differences will only tend to show up when the container design becomes a little marginal. For example, homopolymer will crystallize more easily than copolymer so it may not be suitable for thick preforms. On the other hand, if you require some crystal growth (for heat resistance) then a homopolymer may be the best choice.
Copolymers also have a lower melting temperature than homopolymers and since it is a golden rule of PET processing to keep the melt temperature as low as possible to reduce degradation, then a copolymer again looks more favorable.
As a rule, copolymers tend to be a little more expensive than homopolymers but this can be offset by their (generally) beneficial characteristics.
3. Residual Acetaldehyde A.A. Level
If you are producing a container for mineral water or certain soft drinks, then you probably should choose one of the grades having a reduced level of A.A. These often include the letter 'W' in their grade number to indicate suitability for water containers. These materials are generally a little more expensive than the standard type so if you are producing a non-food container (or a non-sensitive foodstuff container) then it is advisable to go for the cheaper option.
4. Cosmetic Grades
These grades have been specifically manufactured to be virtually non-crystallizable. As a result of the differences in manufacture and the relatively low demand, these can also be extremely expensive, as much as four times the price of "normal" PET. However, exceptional results can be achieved even with very thick preforms (6~10mm).
How do know if I have selected a good material?
Although it is possible to generalize about PET grades as shown above, the actual results on the machine can sometimes be different to those expected. Therefore it is advisable to do a practical test with a sample of the material.
Different containers react to material type in different ways. If a material works well with one container, then it does not always guarantee that it will be good with all other containers. If a company has several machines and different mold designs, then it should sample the new material on all molds before committing to a bulk supply of the sample material.