Nowadays PET is still widely used for these purposes, but when, in the 1970s a marketing need was identified for larger light-weight, unbreakable bottles to contain carbonated drinks, PET fit the bill perfectly. Unlike simple polymers such as polyethylene, PET is not made by a single stage process, but by the reaction between two chemicals, purified terephthalic acid (PTA) and ethylene glycol (EG). The availability of the first of these has dictated the supply of PET resin in the past, but new capacity coming on stream this year will ensure more than adequate supplies to meet the growing uses of PET over the coming years. Related polyesters are polybutylene terephthalate (PBT) used mainly for engineering applications, and polyethylene naphthalate (PEN). The latter offers significant performance improvements over PET, particularly in terms of barrier properties and heat tolerance. Since PEN can be blended with PET a range of new 'alloys' is becoming available for special packaging applications.
As PET (bottle grade) is a kind of transparent, wear-resisting and corrosion-resisting plastics with high strength and smooth finish, it is widely used for PET bottles of mineral water, juice, edible oil, pharmaceuticals, cosmetics, etc.
• Melting Temperature: 254-2560C
• Crystallinity: >=45%
PET products can be made crystallizable or non-crystallizable through controlling crystallization temperature and cooling speed. Usually it is crystallinity is 0~50%. The higher IV value, the slower crystallization speed.
The key factors that effect PET bottles' molding process and performance are crystallization and orientation. If PET’s moisture level is high, hydrolysis will happen while molding and it is IV value drop which means products' quality be affected.
• Glass Temperature: 820C
• Carboxyl End Group: <=20mol/t
• Acetaldehyde: <=3ppm (Ex-Work, related to drying & molding temperature)
For CSD bottle, <=9ppm required; for mineral water, <=4ppm required.
• Density: 1.38~1.40g/mm3
In the bottle industry, the length of the PET chains is usually described by the resin IV (Intrinsic Viscosity). Bottle grades have IV values of about 0.65 to 0.85 dL/g, or about 100-155 repeating units per chain.
Most bottle grades of PET are copolymers which mean that a few percent of a modifier has been incorporated into the polymer chain. Copolymers are easier to injection mold because the crystallinity behavior is improved.
A remarkable transformation takes place when injection molded PET is stretched at the right temperatures and to the right extent. The long chains undergo strain-hardening and strain-induced crystallization, which gives the properly-made PET bottle exceptional clarity, resistance to internal pressure, uniform wall thickness, toughness, and a host of other features. To achieve these useful properties, however, care must be taken in choosing the right grade of resin, as well as the right perform and bottle designs, and good molding practices.
Some grades of PET have other modifications to improve the bottle barrier properties, the reheat characteristics (for two-stage systems), or the generation of AA (acetaldehyde).