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About Plastics

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 hygroscopic?

What is Acetaldehyde (AA) in PET bottles?

How much colorant is typically used in PET bottles?

Where can I learn more about stretch blow molding bottle manufacturing?



What is PET plastic?

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 and hardness.

Read more - http://en.wikipedia.org/wiki/Polyethylene_terephthalate

 

Why is PET plastic in the beverage industry?

PET is a strong, lightweight form of clear polyester, which is easily formed into containers for water, soft drinks, and other foods items.

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.

 

What are the differences of the two states of PET (amorphous and crystalline)?

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.

 

How is PET manufactured?

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.

 

What is Intrinsic Viscosity IV with PET?

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 polymerization.

An I.V. of about:

0.60 dl/g: Would be appropriate for
fiber
0.65 dl/g: Film
0.76-0.84 dl/g: Bottles
0.85 dl/g: Tire cord

 

What is the difference between hydroscopic and hygroscopic?
  • 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 water loving.

What is Acetaldehyde (AA) in PET bottles?

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.

How much colorant is typically used in PET bottles?

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%.

Where can I learn more about stretch blow molding bottle manufacturing?

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 www.hbmedia.info under the PET Books tab.