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Polyethylene Terephthalate

We know atoms are basic units or smallest unit of any matter. An element contains same type of atoms. Chemical bonding between atoms leads to the formation of molecule. Molecules can be smaller or bigger in size that depends on number of atoms involve in that molecule. Today we know the molecules like water which is composed of two H atoms and one O atom. At the same time the molecules of proteins are large molecules which are formed by different small molecules and they are also called as polymer.

Polymers are macromolecules which are formed by certain repeating units. These repeating units are called as monomers. Monomers have some functional groups which react with each other to form a long polymeric chain. The chemical bonds between monomer units are usually covalent bonds which are formed due to addition between some unsaturated monomer units or condensation between two different functional groups such as –COOH, $-NH_{2}$, -OH etc. 

The process of formation of polymers from monomer units are called as polymerization process. Polymerization can be because of addition reaction between unsaturated monomer units or condensation between two different monomers with the elimination of certain small molecules such as water, carbon dioxide etc. 

Types of Polymers

On the basis of polymer structure, polymers can be classified as condensation polymers and addition polymers. On the basis of mechanism there are two types of polymerization; step growth and chain growth polymerization. Chain growth polymerization is also called as addition polymerization in which monomer units are bonded with each other through their multiple bonds. For example polymerization of ethylene leads to the formation of polyethylene polymer which is also known as polythene. This polymerization is a three step process. First step is formation of free radical in the presence of some initiator like peroxide. We know that a free-radical is a chemical species with a free electron. The hemolytic cleavage of chemical bond produces free radicals. For example an organic peroxide forms two R-O• free radicals. 

$R-O-O-R \rightarrow 2 R-O.

These free radicals involves initiation steps and form new free radicals with monomer units which begin the polymer chain. The double bond of monomer units breaks and attach with free radical to form new free radicals. 

Propagation step is a repetitive operation of bonding between monomer units which initiated with the free radicals formed in the initiation step. The multiple bonds in successive monomers are opened up to form a reactive polymer chain. Last step is termination which involves the combination of two free radicals to form a molecule. It terminates the propagation of polymeric chain. Formation of polyethylene from ethylene molecules is simplest example of chain growth polymerization.

Polymerisation

Another type of polymerization is step reaction polymerization which produces polymers of lower molecular weight than chain reactions as some small molecules like water eliminate during the reaction.  Step-wise reactions require two different types of di-functional monomers which react with one another to form a long polymeric chain.  Such polymerization reactions are also called as condensation polymerization. For example condensation of hexamethylene diamine and adipic acid forms Nylon-6, 6 polymer. Here the linkage between monomer units is an amide linkage which is formed by condensation between carboxylic group and amino group of two different monomer units. One water molecule eliminates in each step results the formation of long polymeric chain. 

Formation of polymer

In this polymerization, the polymeric chain can grow from either terminal as both sides have some functional groups which can continue the chain reaction. Small polymeric chains are called as oligomers which have 2-10 monomer units. This polymerization is relatively slow and can take up to several hours or days. Unlike chain growth, this polymerization breeds linear chains. No doubt presence of tri-functional monomer may add branch to polymeric chain. Let’s discuss one of the step-growth polymers, known as PET.

 

Polyethylene Terephthalate Definition

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Polyethylene terephthalate is a condensation polymer of ethylene glycol and terephthalic acid. The by-product of the reaction to create this compound is water so it is an example of condensation or step growth polymerization. 

Formation of Polyethylene Terephthalate
There are two groups, terephthalate group from terephthalic acid and ethylene group from ethylene glycole which combine together through –COO- group. 

Polmer

Polyethylene Terephthalate Structure

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In acidic medium, protonation of terephthalic acid occurs that reacts with ethylene glycol to form an intermediate which rearrange and with transfer of OH- ions, forms polyethylene terephthalate polymer. 

Mechanism of Polyethylene Terephthalate

Overall both monomer units are joined together with ester linkage therefore commonly called as polyester. It is a chemical compound that is used to make plastic, and fiber. It is formed by polycondensation of terepthlaic acid and ethylene glycol. Polyethylene terephthalate is a general-purpose plastic which has resistance to heat and cold, a transparent material with good electrical qualities and also chemical and abrasion proof.

How is Polyethylene Terephthalate Made?

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The manufacturing of polyethylene terephthalate polymer initiated with slurry mixture of PTA and ethylene glycol. In appropriate reaction conditions, slurry mixture reacts and esterification process occurs. The un-reacted ethylene glycol can be recovered with water. The pre-polymerization and polymerization process forms PET polymer in the vacuum.  

PET Formation
PET bottles can be made by either One-Step "hot preform" method or Two-Step "cold preform" method. In One-Step method the PET granule are used in one integrated machine. This method is highly suited to small and medium scale production lines.

On the contrary, Two-Step method performs in two separate machines. The preform is injection molded on the first machine, then reheated and blown on the second machine. The injection molding machine and a reheat blow molding machine are used in this process. Since PET can absorb moisture so must be dehydrated by a dehumidifying drying before processing.  These dried PET pellets are compressed and melted with the help of rotating screw.

Polyethylene terephthalate which is also abbreviated as PET / PETE is mainly used to manufacture the packaging material for food products such as fruit and drinks containers. It is lightweight, transparent and also available in some color. It is a member of ester family so also called as polyester. It is a recyclable thermoplastic polymer with good strength, ductility, stiffness and hardness therefore can be processed through vacuum forming, injection moulding, compression moulding and blow moulding. PET / PETE can also be recycled back, to its original elements as well as into polyester fibres. These polyester fibres are used in manufacturing of synthetic carpets, synthetic clothing and other textile products. PET fibres are wrinkle free and less expensive therefore often mixed with natural fibres. It is also used for the manufacture of microwavable trays and for the packaging of microwavable meals, in containers for cosmetic products and pharmaceutical products. Compare to glass, it is very light weight polymer therefore also used to form bottles. 

PET Examples

Polyethylene Terephthalate Properties

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Many different properties of this polyester make it very useful for the different applications. It is a linear thermoplastic with long-chain molecule consists of repeating; terephthalic acid and ethylene glycol. PET is marketed in the form of small pellets in which each pellet is about 0.05 gram. Due to good strength, temperature tolerance and wear-resistance of polyester fibre, it is an ideal replacement for natural fibers such as silk, cotton and wool. PET fibres have good antiosmosis, low water absorbability and good toughness. The PET films have same tensile strength as of aluminum films. Bottle grade PET is used to make larger light-weight; unbreakable bottles which are used t contain carbonated drinks. The preparation of PET is not a single stage process but involves a long chain of chemical reactions. It is mainly used for manufacturing of edible oil, pharmaceuticals, mineral water, juice, cosmetics, etc. The melting point of PET is about 254 °C to -256°C with crystallinity of more than 45%. With the help of control crystallization temperature and cooling speed, crystallizable or non-crystallizable PET products can be made. Some other properties are listed below.
  • Crystal clear polymer- It is a crystal clear polymer with good purity and healthy. You must have seen the sparkling PET bottles with brilliant glass-clear presentation attract us.
  • Purity- The products of PET taste good and comply with international food contact regulations.
  • Safe – The objects made from PET like bottles are tough and virtually unbreakable therefore can be easily used for storage and transportation. This polymer has high impact and tensile strength that makes it ideal for carbonated products.
  • Good barrier- PET products have low permeability to oxygen, carbon dioxide and water therefore it maintains the integrity of products with good shelf life.
  • Lightweight – The light weight of PET products reduce the shipping costs compare to glass products.
  • No Leakage and damage- Due to absence of weld line in the base, PET bottles are free from leakage and damage.
  • Recyclable- PET polymer is recyclable and can be reshaped in different shapes.
  • Good resistance power- PET products have good resistance against different chemicals such as acid, bases etc.

Polyethylene Terephthalate Uses

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  • For manufacturing of shopping bags, water bottles, videotapes
  • For manufacturing of , containers and bags
  • For manufacturing of clothes and housing material
  • For manufacturing of water bottles
  • For manufacturing of microwaves containers
  • For manufacturing of carpets
  • For manufacturing of packaging films
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