In the bottle industry, the length of the PET chains is usually described by the resin IV (Intrinsic Viscosity).
Bottle grades have 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 means 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 preform 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).
Crystal Clear:
Products look good, pure and healthy. Sparkling PET bottles attract attention.
Brilliant glass-clear presentation of your products.
Pure:
Products taste good. PET complies with international food contact regulations.
Safe:
PET bottles are tough and virtually unbreakable during production, storage and transportation.
If they do fail, they split, not shatter.
Their high impact and tensile strength makes them ideal for carbonated products.
Good Barrier:
The low permeability of PET to oxygen, carbon dioxide and water means that it protects
and maintains the integrity of products giving a good shelf life.
PET also has good chemical resistance.
Lightweight:
10% weight of an equivalent glass pack, PET bottles reduce shipping costs by about 30%,
and because the material in the wall is thinner, shelf utilization is improved by 25% on
volume compared to glass.
High strength, low weight PET bottles can be stacked as high as glass.
No Leakage:
Absolute closure integrity is possible because of the injection molded neck finish.
The absence of a weld line in the base means that PET bottles don't leak.
Design Flexibility:
Suitable for containers of all shapes, sizes, neck finishes, designs and colors.
Recyclable:
Excellent environmental profile due to single material.
Used PET bottles can be washed, granulated into flakes and reshaped as PET bottles
or employed as material for strapping, carpeting, fiber filling, etc.
Specially designed thick-wall bottles can be washed, refilled and reused.
PET is made from the same three elements (carbon, oxygen, and hydrogen) as paper,
and contains no toxic substances. When burned, it produces carbon dioxide gas and water,
leaving no toxic residues.
Good Resistance:
PET offers the best chemical resistance performance of any mainstream
polymer used in packaging today.
Long Shelf-life:
Very good shelf-life performance, especially with the new higher barrier formulations.
Although general-purpose grades are available, as in all walks of
life the best results come from a product specifically adapted for its role.
The fact that PET is used for such a variety of packaging items,
from soft drinks to cosmetics, demonstrates how the qualities of the resin must
match the standards needed to keep the product preserved and safe for travel.
For that reason, the drink or food item and its packaging must be attractive,
sensible and convenient. The resin chosen for a certain PET application has to
fulfil the requirements of the end product.
Homopolymers and copolymers are both used in PET resins.
A homopolymer is PET containing only ethylene glycol and terephthalic acid repeat units
which results in the highest melt point and the best strength properties possible.
A copolymer is PET containing additional monomers, such as isophthalic acid, to disrupt
the polymer chains, resulting in a lower melting point, slower crystallisation and improved
clarity particularly in heavy wall applications.
Different grades of resins are appropriate for different end uses.
Their specific qualities make them ideal for the product which they have been designed
to package. What qualities of PET are important? As mentioned above, the qualities that
are important depend very much on the product for which the bottles is destined.
Strength: The resins in PET bottles used to package carbonated soft drinks need very
specific qualities.
The material must be extremely strong to contain the internal pressures of CO2 without
distortion or expansion.
This is obtained by using a resin which has high IV and lower copolymer levels.
Clarity: Clarity is an important aspect of drink packaging in terms of consumer acceptance.
A low level of crystallisation is needed to achieve clarity.
Colour: Although one of the key qualities of PET is that it can be used as a perfectly
clear material, with soft drinks this property is not essential as many soft drinks themselves
are coloured, while clear soft drinks are packaged in coloured PET bottles.
Some waters are packaged in exactly the same bottles as carbonated soft drinks and
would therefore carry many of the same requirements, but some vary.
Strength: Unlike carbonated drinks, the bottle filled with still water needs only
enough strength to hold water and to survive impact. IV requirement is
reduced to the .74 to .76 range in most cases.
Colour and Clarity: Clarity is one of the most important reasons why PET is used for
packaging water. PET used in water bottles needs to be very clear and maybe even slightly
blue in colour.
A resin with higher levels of copolymer adds to the clarity.
Purity: Because water is a flavourless product, having a plastic that remains tasteless
and odourless is imperative.
In general terms PET doesn’t affect the taste of the product it protects and therefore
is a key material for packaging water.
Juices and many other products such as sport drinks or other high acid drinks are hot filled
into PET bottles.
In these cases the package is heat set in order to improve the temperature resistance of the container.
PET resins with a higher Tg (glass transition) temperature and/or a faster rate of crystalisation are preferred.
Generally lower levels of copolymer are preferred and IV’s of about .80 are acceptable.
Strength and colour/clarity: This factor is not as important with juices as CSD’s or water.
The purity factor is important, as juices would contract the taste of the plastic bottle
if not pure enough.