What is PET plastic?
Why is PET plastic in the beverage industry?
What are the differences of the two states of
PET (amorphous and crystalline)?
How is PET manufactured?
What is Intrinsic Viscosity IV with PET?
What is the difference between hydroscopic and
What is Acetaldehyde (AA) in PET bottles?
How much colorant is typically used in PET
Where can I learn more about stretch blow
molding bottle manufacturing?
Polyethylene Terephthalate (PET) is a substance made from
ethylene glycol and terephthalic acid which react at high
temperature and pressure to create PET plastic and polyethylene
terephthalate bottles (PET bottles). PET is a strong,
lightweight form of clear polyester, which is easily formed into
containers for water, soft drinks, other foods, and other non-food
items such as household cleaners.
PET plastic packaging is widely used for packaging in the drink
industry; the bottles have to answer a wide and expanding range of
expectations. To fulfil these expectations completely, different
manufacturing processes have to be used, as well as chemical
alteration of the material to make it fit for purpose.
PET exists as amorphous (opaque and white) and semi-crystalline
(transparent) thermoplastic materials. Generally, it has good
resistance to mineral oils, solvents and acids but not to bases.
The semi-crystalline PET has good strength, ductility, stiffness and
hardness. The amorphous PET has better ductility but less stiffness
Read more -
PET is a strong, lightweight form of clear polyester, which is
easily formed into containers for water, soft drinks, and other
The sensitivity of some beverages to Ultra Violet Light (less UV
means a longer shelf life), oxygen (less oxygen allows a better
resistance to thermal degradation) and sometimes their carbonation
introduces the need to use a PET blend, creating an adequate
barrier. This can be achieved through multi-layered or
surface-coated bottles, associating different material, each of them
having one or more of the required properties.
However, the ‘ideal’ route is to use a single-layer PET blend
material for reasons of simplicity of the process and lower capital
investments. To achieve this mono-layer, PET-based blend,
oxygen-scavenging agents, barrier improving agents, toners and
nucleating agents are traditionally used. The proportion and the
choice of the additives is an area of expertise, and is still
developing, making more options available to the industry.
PET has different states as a resin is processed during
manufacturing: amorphous and crystalline.
- Crystallisation is a structured organisation of atoms within
the material. Crystallisation can be partial or complete. This
stage produces a clear strong material.
- Amorphous is a term used to define a material lacking a
crystal structure. For example, when a solid melts, it changes
from the crystalline to the amorphous state, in which it remains
until recrystallisation takes place.
There are two main families of manufacturing processes: extrusion
and injection molding.
Extrusion molding is used mainly for five-gallon
(19-litre) bottles with handles, which are going to be used for cold
fill, mainly for dairy and juices but sometimes packaged water. This
process also allows a wide range of different-shaped designs for the
pack. The reason why amorphous extrudable PET grades weren't easily
recyclable lies in the drying process.
A plasticating screw heats the plastic granulates. The melted
polymer goes through a die to form a hollow tube. The hollow tube,
still warm, is cut and enclosed in a mold where air is injected and
the hollow tube takes shape.
Injection molding is used for regular bottle
shapes with less mechanical resistance expectations. Plastic
injection molding is the most common process used on bottling lines
around the world. Also, plastic injection molding is the most
cost-efficient method for producing the majority of the smaller
format packs (33cl to five litres).
Two main types of processes exist:
- One-step injection molding – the melted
polymer is injected into the final shape of the container in a
cold mold, it's then released.
- Two-step injection molding – the first step
is the injection of melted plastic in a mold, resulting in the
creation of a preform. The cold preform is then taken onto
another machine, enclosed in a mold, heated, stretched with a
pin to reach the desired length and then blown in the shape of
the chosen mold.
One of the most important characteristics of PET is referred to
as I.V. (intrinsic viscosity). The I.V. of the material,
measured in deciliters per gram (dl/g) is dependent upon the length
of its polymer, the longer the polymer chains, the stiffer the
material, and therefore the higher the I.V. The average chain length
of a particular batch of resin can be controlled during
An I.V. of about:
0.60 dl/g: Would be appropriate for
0.65 dl/g: Film
0.76-0.84 dl/g: Bottles
0.85 dl/g: Tire cord
- hydroscopic: dictionary has no entry
- hygroscopic: readily taking up and retaining moisture
A hygroscopic polymer has the tendency to absorb moisture from
humid air, hydrophobic means water hating and hydrophilic means
Acetaldehyde is an organic chemical compound
with the formula CH3CHO. It is a colorless
gas at room temperature and has a strong fruity smell. It
occurs naturally in many fruits and other food stuff and is used as
a flavor enhancer for certain products and is produced by plants as
part of their normal metabolism.
Acetaldehyde is also generated during the production and injection
processing of PET material. It can cause an off-taste in bottled
water. Generally, most acetaldehyde problems originate in the
barrel during the injection process. All PET resins have some
residual acetaldehyde after being manufactured; the quantity will
vary according to the grade. Acetaldehyde is only generated while
the PET resin is in its melt condition, therefore it can only be
controlled by adjustments in the barrel (90%) and hot runner (10%)
of the machine. The generation of acetaldehyde is not linked to
moisture content of the material, although in the process of being
dried, acetaldehyde can also be driven off. Therefore, correct
drying is also important for acetaldehyde control.
The amount of colorant loaded into PET bottles can range from
.33% up to 2%. A solid colored bottle will have a colorant
load of approximately 2%. Bottlers will seldom go over a
colorant load of 2%.
The following books offered by PET Planet will provide a good
introduction into stretch blow molding:
Vol 1 – Dressed to Fill (ISBN 3-9807497-0-3)
Vol 2 – Dictionary for the PET Industry (ISBN 3-9807497-1-1)
Vol 3 – Stretch Blow Moulding: A Hands-on Guide (ISBN 3-9807497-2-X)
Vol 4 – Bottles, Preforms, Closures: A Design Guide for PET
Packaging (ISBN 3-9807497-3-8)
Vol 5 – Polyester Bottle Resins: Production, Processing, Properties
and Recycling (ISBN 978-3-9807497-4-9)
These books can be found at
under the PET Books tab.