Polypropylene is a semi-crystalline, colorless and translucent thermoplastic polymer, that can reach high level of transparency, which can be compared to the PET ones, thanks to specific processing technologies. It has good physical and mechanical properties and a very low specific weight, it does not absorb moisture and has a very low water permeability, it maintains its mechanical and chemical resistance properties very well, up to about 120°C. PP containers are used in different sectors of food packaging: fruit and vegetable sector, meat, fish etc. A recurring application, considering its high resistance to heat, is to pack food that has to undergo a pasteurization or sterilization process or to be heated in a microwave. A great advantage of PP, over all other synthetic materials, is that it can be available in various formulations in order to obtain different characteristics for the most varied uses, so to make it comparable to the techno polymers family.
The containers can easily be sealed with PP or PE based film (in this case, the structure of the container will have to be PP-PE) the packaging will be completely sealed and without deformations. The resistance characteristics to oxygen and carbon dioxide are to be considered medium-low barrier and its recyclability is very high also in the PP-PE structure, both being polyolefin materials.
If a longer shelf-life is required, the addition of an intermediate layer of EVOH gives the material higher oxygen barrier properties, making it a high barrier material. The container properties, however, in terms of transparency, would not change and would depend from the transparency level of the film used.
According to Greenpeace, the pyramid shown to the side measures the environmental impact of various polymers. Those at the top are considered of highest impact, and obviously the bio-polymers are at the base of the pyramid. It is important to note that polyolefin, and therefore PP, are immediately above the bio-polymers. Besides being easily recyclable into new products, PP is particularly suitable for reuse in waste to-energy plants due to its high energy content and low emissions.
POLYETHYLENE TEREPHTHALATE (PET)
Polyethylene terephthalate, generally abbreviated to PET, is a linear thermoplastic polymer belonging to the polyester family. With 18% of world market share, it is one of the most widespread of the families of polymeric materials and third for production volumes behind the two main ones: polyolefins PE and PP. Its chemical inertia and its physical properties have made it particularly suitable for various food packaging applications, bottles, trays for various types of foods, films, etc. PET packaging offers an optimum combination of dimensional stability, rigidity, transparency and barrier effect.
A great advantage of PET comes from the fact that, since it is widely used for the production of bottles for mineral water and beverages in general, it is also widely available as waste. Thanks to its excellent recyclability and to the modern systems of material separation in the recycle plants, make it easily identifiable and processable into recovery and recycling operations for the production of new raw material (R-PET). The PET/RPET/PET structure is widely used for making containers of various shapes and depths. PET containers of suitable shapes and sizes are widely used in various food packaging sectors, and, thanks to their particular brilliance and transparency, they are highly valued in the confectionery and delicatessen sectors. PET material, having medium-high barrier properties, is suitable for protective atmosphere packaging. Its sealing properties are not its strength, but with the addition of a PE film, it can be easily sealed on the main top films using specific sealing films; in this case, its recyclability features are of course compromised.