How Plastic Bottel are made?
Before looking on how bottel are made let's have a look on some of definations that are used in making.
What is PET (PolyEthylene Terephthalate)
PET
(also named PETE) is a kind of polyester material for
fiber, injection molded parts, as well as blow-molded bottles and jars.
Special grades are offered with the required properties
for the
different applications.
PET is linear thermoplastic (long-chain molecule consists of
repeating units shown as figure right), white but bluish
resin made from terephthalic acid and ethylene glycol through
poly-condensation. PET is supplied by the resin manufacturers in
the form of small pellets, each about 0.05 gram. PET came into prominence in the 1950s as a
textile material. Its strength, temperature tolerance and
wear-resistance made it an ideal replacement for, or addition
to natural fibers such as silk, cotton and wool.
It has good antiosmosis, low water absorbability and good
toughness. PET film's tensile strength is similar with aluminum
film's, and is three times that of PC and PA film. PET film is
transparent. It's tensile strength can reach 1/3~1/2 of steel's
if dealed by oriented draw. It's the toughest thermoplastic
film. It will be burnt with yellow flame and will burst when
burning. And it will continue burning when away from fire.
Bottle Grade PET
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's 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's IV value will drop which means products' quality be
affected.
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
Glass Temperature: 820C
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 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).
Main Advantage of PET
-
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.
Select the Right Grade of Resin -- Different Resins
for Bottles of Different Purpose
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. Not only does the product need to be maintained
in good condition, but it also needs to sell. 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. Strength, clarity, barrier
performance and UV resistance are only a few of the factors to be
considered in packaging.
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:
CSD --
Carbonated Soft Drinks
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.
Water
-- The Clear Story
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.
Juice
-- Hot Fill Qualities
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 crystallisation 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.
Beer -- A New Market with New Requirements
Beer is often seen as an extension of the CSD market, but also
often requires heat fill and barrier qualities. Because the
technologies used to make beer bottles are varied and the
applications differ (cold filled or pasteurised), finding a common
denominator is difficult. Making a bottle to meet the desired
requirements has been the most important factor. The types of PET
beer bottles typically used are: non-tunnel pasteurised, one way
tunnel pasteurised and returnable/refillable bottles.
Barrier: Beer needs higher performance in both CO2 and O2
barriers compared to PET used in CSD applications. The level
required depends on the type of beer, container size, distribution
channels and environmental conditions (storage time, temperature and
humidity levels). Improvements in barrier properties can be obtained
via coatings, creating mutli-layer bottles and scavengers.
Colour, clarity and UV protection: Beyond the barrier properties,
most beers also need to be protected from UV light. Protection can
be obtained via colorants or UV additives, which can be added to the
PET at the injection stage. Because of the varied requirements for
beer, resins must be chosen which provide an adequate barrier, UV
protection and clarity.
Strength: Beer bottles do need strength in order to maintain the CO2
pressure over a wide range of temperatures including pasteurisation.
IV’s in the .80 to .84 range are normally favoured. Beer bottles
also tend to use champagne bases rather than footed or “petaloid?
bases. In all of the above cases, the resource efficiency of PET -
due to its light weight - is one of the essential reasons why it is
used with these consumer drink products. Not only does the consumer
benefit, but the transportation costs and energy consumption in
delivery are decreased due to its weight. Because PET bottles are so
much lighter than alternatives, a truck can carry 60% more of the
beverage and 80% less packaging ?a fuel saving of 40% and less air
pollution.
To Produce PET Preform/Bottle
Traditionally PET bottle molding machines could be divided into
two categories - those using the One-Step "hot preform" method
and those using the Two-Step "cold preform" method.
The
One-Step method -- from PET granule to finished bottle --
all processes are
completed on one integrated machine. This means the
injection-molded preform is withdrawn from the injection cavity
while still hot enough to be stretch blown to form the bottle.
No extra heating is required, and since preforms are not
stockpiled to be blown at a later date, but are freshly molded
every time, there is no risk of surface damage from preforms
knocking together during storage or transportation. One-Step
method is highly suited to small and medium scale production
lines.
The Two-Step method uses two separate machines. The preform
is injection molded on the first, then reheated and blown on the
second. The Two-Step system uses two separate machines: an
injection molding machine for making the preforms, and a reheat
blow molding machine to reheat the preforms from cold and blow
the bottles. The requirement for a preform heating system means
the Two-Step process has a lower thermal efficiency. This method
is most suited to medium to large-scale production.
Usually 2-stage method is adopted to produce PET bottle.
- Drying of PET
PET absorbs
moisture from the atmosphere. This must be removed by a
dehumidifying drying before processing.
-
Plasticizing
the PET
Dried PET pellets are compressed and melted
by a rotating screw.
- Injection Molding the PET Preform
Molten PET is injected into the injection cavity and cooled
rapidly to form a “preform? (The test-tube-like form from
which bottles are blown is known as a preform).
-
Heating
the PET Preform
The temperature of the preform is
adjusted to the correct profile for blowing.
-
Stretch
Blow Molding the PET Container
The hot preform is
simultaneously stretched and blown (thereby orienting the
crystals of and strengthening the PET*) into a shaped blow
mold to form a tough, lightweight container. PET that is
heated to a temperature where its chain-like molecules are
sufficiently mobile to uncoil instead of breaking when
extended, can be oriented by stretching. Stretching applied
from two directions at right angles, as in stretch blow
molding, gives biaxial orientation. Oriented PET contains
closely packed chains aligned in the directions of stretch.
The material is stronger because the molecules act together
instead of individually. The tensile strength of oriented PET
is several times that of the unstretched material and the
impact strength, barrier and chemical resistance are also
significantly improved, so bottles can be lighter without
sacrificing performance.
-
PET
Container Ejector
The finished container is ejected.
Acetaldehyde Level
What is Acetaldehyde?
Acetaldehyde (AA) is a compound that exists naturally in many citrus
fruits and other foods. It has a distinctive acidic taste.
Acetaldehyde is also generated during the process of PET bottle
manufacture.
How Acetaldehyde affects the product? With
the growing popularity of PET containers, water companies are becoming
more aware of AA and how it can affect product taste. In most cases,
the flavour of AA is barely detectable during beverage consumption.
Carbonated soft drinks, for example, have very sweet or tangy flavours
that mask the slight taste of the plastic. But the very delicate, or
even imperceptible, taste of pure water is not strong enough to mask
acetaldehyde, and so it is easier for the consumer to notice the
presence of AA. Consumers rank taste as the number one criterion when
selecting a water brand. They look for purity that cannot be found in
tap water. Water companies retain customer loyalty by delivering
consistent, high quality products that fulfil the brand promise. Even
minor differences in taste will compromise brand integrity and affect
sales.
How is AA generated in PET bottles? During bottle manufacture
the resin pellets are dried and melt-processed at high temperatures
(higher than 2600C) and are injection moulded into preforms.
The preforms are then reheated and stretch-blow moulded to form a
container. It is during the melt-processing stage that AA is
generated.
Optimization of the molding conditions by way of
minimizing melt temperature and residence time can aid in minimizing
AA generation. However it cannot be entirely eliminated. AA produced
during processing is retained or trapped in the polymer matrix of the
bottle or container. Since AA boils at below room temperature, it
eventually migrates out of the PET into the atmosphere or into the
packaged contents, affecting taste. The amount of AA that migrates is
directly related to storage time and temperature. There are a number
of ways that AA levels can be significantly reduced so that taste is
not affected. The first step is choosing the right PET resin. Resins
are available with properties that are specifically tailored to the
needs of water containers. For example, Voridian Aqua PET resins 18696
and 20837 are considered leaders in low AA levels. Each has a low
intrinsic viscosity (IV) of 0.72 and 0.76 respectively. The low IV
minimizes AA generation during melt-processing.
Because bottle performance is important, these resins are designed so
that the low molecular weight does not compromise mechanical
properties of the finished bottle.
Certain water markets seek AA levels that are lower than even the most
progressive polymers will allow. In these cases, the low AA resins are
combined with an AA “scavenger? AA scavengers are blended with the
PET as it is melted in the injection-moulding phase. The scavenger
reacts or combines with AA to ‘lock?it into the polymer matrix so
that it cannot migrate into the product. Scavengers can further reduce
AA levels in preforms up to 70-80%.It is important to choose a PET
solution with the proper balance between low AA levels and high
performance. This can yield a bottle with excellent mouldability and
optical qualities. A low-AA resin should blend well with colors and
tinters to produce a package with the look and feel that supports the
brand.
To Recycle PETPET is fully recyclable where facilities exist. It is given the recycling
code 1. Post-consumer recycled PET (PCR PET) can be used for clothing and
carpet fiber, and fiberfill for stuffing articles such as pillows.
Recycled PET can be used to make new bottles for non-food products such as
cleaning products. To make food and beverage containers out of PCR PET, it
must pass through approved processes to ensure it has no contaminants, and
it must retain enough of the original properties to meet the final quality
requirements. For more information about recycling PET.
A simple recycle method popular is to only crush mold PET products, then
mix crushed PET granules into virgin PET resins, but the percentage should
be no more than 20%. High regrind proportion will cause variance on melt's
viscosity, color and odor. Hopper magnet is suggested in injection machine
when using recycled material.
