What Is PET Resin?

    Since the 1973 registered patent to make bottles for carbonated drinks using PET, PET starts to become popular. Of the 140 million tons of plastic resin produced worldwide in 2000, PET takes up 25 million tons, although not all PET resin is used to make bottles. Currently, bottle-grade PET resin is suppliedby Voridian, KoSa and other suppliers.

    Resin could be classified into crystalline and non-crystalline types. PET, PP, PE, nylon and PBT are crystalline. PC, ABS, PS, SAN, PPO and PVC are non-crystalline.

    When crystalline film is stretched, crystals will align themselves in the stretch direction, increasing the film strength. On the other hand, non-crystalline resins do not increase their strength when stretched.

    If uni-directional stretching increases the PET film strength in one direction, bi-directional stretching will increase its strength in both directions. The injection stretch blow molding process could stretch the PET film in two directions in the stretch blow process.

    Another function of bi-directional stretching is to reduce wall thickness, which reduces resin usage.

PET material is different than other crystalline materials in that it is crystal clear with light transmission similar to that of glass.

    The advantages of PET bottles are they are less prone to breaking and are light. A 0.7-litre glass bottle weighs more than its content. However, the weight of a 1-litre PET bottle is less than 20% of gross weight. During transportation, spoilage and transportation fee could be reduced.

    PET bottles allow oxygen to migrate in and carbon dioxide to migrate out and therefore could not be used alone to contain beer. Oxygen will oxidize the beer composition, changing the taste of the beer. In general, beer manufacturers request the shelf life of beer to be at least 6 months.There are two classes of methods to overcome this shortcoming: resin barrier and coating barrier.

    PEN (PolyEthylene Naphthalate), nylon and EVOH have a much higher barrier to oxygen than PET. They could be sandwiched between two PET layers.

    Using barrier coating, coating like carbon film is sprayed onto the inner or outer surface of the bottle.

    In the hot processing of PET, e.g. injection molding, when the shearing is too high, PET degrades to AcetAldehyde, which is abbreviated to AA. AA is not odourless. Especially when the bottle contains mineral water which tastes bland, the AA smell becomes unacceptable.

    PET bottles will soften when containing water at 80oC. Many drinks like tea, juice and diary products are hot filled or hot pasteurized afterwards. In such cases, PET is not the right choice.

    The most common solution is to mix for example 25% PEN material into PET to increase its hot fill temperature. Alternatively, the bottle neck is crystallized (and turns white). Crystallized PET bottle neck could stand up to a high hot fill temperature. In the hot fill arena, clarified PP is becoming popular with a hot fill temperature of 95oC, but its clarity is not comparable to that of PET.