|Plastics ProjectsPET Preform/Bottle Project|
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.
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|
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.Water -- The Clear Story
Juice -- Hot Fill Qualities
Beer -- A New Market with New Requirements
|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.
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 PET
PET 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, check out the NAPCOR website at www.napcor.com.
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.
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