PET Full Form

Most people around the globe are very familiar with the acronym PET and it instantly strikes our mind when we see a finished plastic bottle filled with water or any kind of fluids being sold at almost all the commercial stores. Sterilized medical bottles for injections where injection stretch blow moulding is performed by PET, the microfiber towels and cleaning cloths people often use for household cleaning purposes, the PET clamshell packaging used to sell fruits and other food products or the aluminized myler balloons filled with helium that is commonly used as a decorating material for events like birthdays, etc. remind us of PET full name. 

But very few of us actually know what is the full form of PET, its manufacturing, properties, application and significance in today's world as almost every commercial packaging is synthetically manufactured and is cost-effective. Thus in order to gain a brief knowledge about the characteristics and the significance of PET let us first consider the question “what is the full form of PET in science?”

PET Full Name

The full form of PET in science is polyethylene terephthalate which is the most common resin of thermoplastic polymer that belongs to the polyester family. It is mostly used as fibres for clothing, clamshell containers for food packaging, thermoforming for manufacturing and in combination with glass to produce engineering resins. In other words, PET full form is polyethylene terephthalate which is basically a very flexible, tough, shatterproof and clear plastic that is commonly used for the manufacturing of mostly bottles followed by film packaging, fabrics and moulded parts of automobiles etc. 

It is very flexible and crystal clear in its natural state, however, depending on its processing methods, it can turn into rigid or semi-rigid forms. It has a very close similarity to polybutylene terephthalate chemically. Also depending on its thermal history, it can be in an amorphous or semi-crystalline polymer. The semi-crystalline polymer might appear transparent with its molecular size of approximately 500nm or opaque and white with the particle size in some micrometres depending on the crystalline structure and its particle size. Polyethene terephthalate (PET full form) is a polymer that contains a polymerized unit of monomer ethylene terephthalate with repeating units (C10H8O4). This polymer is commonly recycled and is served number one as its resin identification code (RIC). The structural formula of the PET polymer is (C10H8O4)n and has an IUPAC name poly (ethylene terephthalate) and a systematic IUPAC as poly (oxyethyleneoxyterephthaloyl).

 The chemical structure of PET is illustrated below for a better understanding:

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The monomer of this polymer bis(2-hydroxyethyl)terephthalate is synthesized by an esterification process between terephthalic acid and ethylene glycol that produces the monomer with water as its bi-product. This is also known as a condensation reaction. Another method of synthesis of bis(2-hydroxyethyl)terephthalate is by transesterification reaction between dimethyl terephthalate and ethylene glycol with methanol as the byproduct of the complete reaction. Immediately after the esterification or transesterification reaction, the polymerization process needs to be followed by carrying out polycondensation reactions of the monomers to form PET with water as a byproduct.  

Production of PET

Polyethylene terephthalate, which is also a PET full form in plastic, is produced by the polycondensation of the monomer that is produced by reacting ethylene glycol ether with terephthalic acid in esterification or with dimethyl terephthalate in the transesterification method. 

1.Dimethyl terephthalate( DMT) process

In this process, DMT with excess ethylene glycol is first fed into a melt where the reaction between the two is carried out at about 150 - 200 0C with the help of a basic catalyst. Methanol that is formed during this process is removed by distillation in order for the reaction to proceed forward. The excess ethylene glycol that is left is distilled in a vacuum at a very high temperature. The second step of transesterification proceeds at a temperature of 270 - 280 0C with a continuous distillation of the excess ethylene glycol. 

The reaction of the process is as follows:

C6H4(CO2CH3)2 + 2HOCH2CH2OH → C6H4( COCH2CH2OH)2 + 2CH3OH (step 1)

nC6H4( COCH2CH2OH)2 → [(CO)C6H4( COCH2CH2O)]n + n HOCH2CH2OH (step 2)

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1.Terephthalic acid process

This reaction is carried on by the esterification process in which ethylene glycol reacts with terephthalic acid under the normal pressure of 2.7 - 5.5 bar and at a high temperature of 220-260 0C. Water that is forming as a byproduct is removed by distillation and there is a continuous distillation process that is carried on throughout the entire esterification process.

 The reaction that takes place in this process is as follows:

nC6H4(CO2H2)2 +  n HOCH2CH2OH → [(CO)C6H4( COCH2CH2O)]n + n H2O

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Properties of PET

Some of the basic properties of Polyethylene terephthalate (PET long form) has some of the unique properties that make it the most desirable polymer of the polyester family with numerous brand names out of which some of the famous brand names PET are Terylene in the UK, Dacron in USA and Lavsan in Russia.

 Thus some of its basic properties are:

1. Its strength, as well as stiffness, is higher than most of the polymers of the polyester family-like PBT.

2. It has natural shielding property to gas and moisture

3. It is a very good insulator of electricity

4. It has temperature susceptibility as its properties do not change over a wide range of temperatures. Thus it can be used over a temperature that ranges from minus 60 to 1300C.

5. It has a higher heat distortion temperature as compared to the other polymers like PBT of the polyester family

6. As it has a shatter resistance property thus it becomes a very good alternative for glass in many applications.

7. It has been approved as a safe material to be used in food and beverage packaging by FDA. Its contact is non - carcinogenic in nature. 

Some of the other physicochemical properties of PET is as follows:-

One of the most important properties of this polymer is intrinsic viscosity that is measured by extrapolating a graph of relative viscosity at zero concentration to the concentration which is measured in deciliters per gram. Though the intrinsic viscosity directly depends upon the chain length of the polymer, it does not possess any value as it is extrapolated to a zero concentration of the polymer. The relation between the intrinsic viscosity and its polymer is longer the chain of the polymer more will be the entanglement of the chains, the higher will be the viscosity. The average chain length of the polymer is controlled during the polycondensation step. 

Thus the intrinsic viscosity limit of the PET chains is as follows:

Fibre grade: 0.40-0.70 for textiles and 0.72-0.98 for technical tire cord

Film grade: 0.60-0.70 for biaxially oriented PET film and 0.70-1.00 for sheet grade used for  Thermoforming.

Bottle grade: 0.70-0.78 for water bottles and 0.78-0.85 for carbonated soft drink grade

Monofilament: 1.00-2.00 for engineering grade plastic

Degradation of PET

Polyethylene terephthalate or PET bottle full form is subjected to degradation very easily. The degradation of this polymer results in decolouration because of the various chromophoric system that undergoes thermal treatment at a higher temperature for a prolonged time, chain cutting that results in the lowering of its molecular weight, cross-linking that results in gel formation or fish-eye formation or formation of acetaldehyde. This degradation becomes a problem for the polymer where visual expectations are very high for an instant in various grades of food packaging. The thermal oxidation or thermoovidative degradation makes the processability of the polymer difficult and hence lower the performance of the material. 

For instance, acetaldehyde is a colourless volatile polymer with a fruity smell but it often produces an off-taste for bottled water. Acetaldehyde is the result of manhandling of the polymer. PET when exposed at high pressure and high temperature (at 300 0C PET decomposes) and long residence in the barrel decomposes it to acetaldehyde. After its production, some of it remains soluble in the solution and when the polymer solution is moulded into plastic bottles or food-grade packaging, this aldehyde diffuses into the food or liquid stored inside the packaging hence brings unnecessary fruity smell and off-tastes to the products. 

Also, antimony (Sb) is a metalloid element and is often used as the catalyst in the formation of antimony derivatives, present in the form of antimony triacetate or antimony oxide during PET production. Though the compound is detectable and usually found on the surface of the polymer and is removed by washing, many times its residue remains in the polymer and can migrate into food and drinks. When the packet with the food is heated in the microwave, due to the high temperature, its level in the food increases beyond the declared significant contamination level by the US EPA. It has been found in the studies that one species of bacteria in genus Nocardia is capable of degrading the PET with the help of an esterase enzyme. Thus it is also subjected to biodegradation. 

Thus one of the most common and economical ways to prevent its degradation is by the use of copolymers. In the copolymerization process, comonomers such as CHDM or isophthalic acid undergo copolymerization which decreases the melting temperature and hence reduces the degree of crystallinity in PET. Thus, it becomes possible for the polymer to get plastically formed at lower temperature and lower pressure with lower force applied during synthesis. Thus this prevents degradation by reducing the level of acetaldehyde product in the polymer.    

Copolymer of PET

In addition to the homopolymer, the addition of copolymers is also carried out in polyethylene terephthalate (PET full form in English) in order to prevent its degradation. In many cases, the modified version of PET due to copolymerization or the addition of copolymers is desirable for certain applications. For instance, when a copolymer named cyclohexanedimethanol (CHDM) copolymerized, it gets attached to the backbone of PET by replacing the ethylene glycol units. Thus the chain becomes larger than before with the addition of six extra carbon atoms and hence CHDM molecules do not fit in the places from where the ethylene glycol units are removed. Thus the melting point of PET reduces as it interferes with the crystallization of the polymer. 

The new form of PET polymer is referred to as PETG or PET-G. Thus it no longer remains crystalline but forms amorphous thermoplastic that is used as an injection-moulded container, sheet - extruded filament used in 3D printing. Another example of such copolymer is isophthalic acid that replaces the 1,4-para linkage of terephthalate. Thus the new 1,2-(ortho-) or 1,3-(meta-) linkage formed by the isophthalic acid provides an angle in the chain and thus disturbs the crystallinity of the polymer and its melting point decreases. Hence it becomes a very suitable polymer for moulding as well as thermoforming processes.

Uses of PET

Some of the major uses of PET polymer are :

1. It is used for manufacturing water bottles as well as carbonated soft drink bottles.

2. Making packaging trays and boxes for various food grades in different shapes and sizes.

3. To make cosmetic jars, microwave containers as well as other similar packaging.

4. To make certain special types of fibres that are included in printing, filter oil, filter cloths, woven belts etc.

5. To make engine covers, wiper arms as well as gear housings.

6. It is mostly used to create polyester fabrics that are flexible and firm in nature and resist shrinkage as compared to cotton.

7. Most of the recycled PETs are used for making other products like carpet, tote bags, strapping, film sheet, comforter fills etc.