Polyethylene terephthalate (PET):
- Linear polyesters were studied by Carothers during his classical researches for development of Polyamides.
- But it was Whinfield and Dicson who developed PET at Calico Printers Association, England I 1941 (As Fibres, Terylene and Dacron and as Film, Melinex and My lar, with the expiry of the basic ICI patents on PET there was considerable development in terephthalate polymers in the early 1970’s.
- More than a dozen companies introduced PBT as an Engineering Plastics during this time.
- The fiber in the name of Kodel and Film in the name of Kodar was developed by Kodak.
- PET was also the basis of glass filled engineering polymer (Rynite) introduced by DuPont in 1970’s. Towards the end of 1970’s PET was used for the manufacture of biaxial oriented bottles .
Polyethylene terephthalate (sometimes written poly(ethylene terephthalate)), commonly abbreviated PET, PETE, or the obsolete PETP or PET-P, is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination with glass fiber. The term polyethylene terephthalate is a source of confusion because this substance, PET, does not contain polyethylene.
Polyethylene terephthalate produced by the condensation reaction of ethylene glycol with either terephthalic acid (TPA) or dimethyl terephthalate (DMT).
The production of PET resin is two step process.
1. Dimethyl terephthalate is heated with ethylene glycol to form a mixture of dihydroxyethyl terephthalate and higher oligomers.
2. Step two is further heating to 270℃ under vacuum, with the catalytic conversion of dihydroxyethyl terephthalate to PET.
Monomer Ingredients for PET
The ingredients which are used for PET are ethylene glycol and dimethyl teraphthalate.
Chemistry of Preparation of PET
1. The PET polymers is produced by the reaction of ethylene glycol and Terephthalic acid or Dimethyl terephthalate in the presence of metal acetate catalyst. This process is called ester exchange process.
2. In this process, a low molecular weight diester, known as the monomer is first produced by reacting 1 mol. or dimethyle terephthalate with about 2.1 – 2.2 mol ethylene glycol at 150°C in the presence of catalysts, such as antimony trioxide and cobaltous acetate.
- In addition to this diester, di-2-hydroxyethyl terephathalte, some oligomers of following general structure, are also produced.
HO-(CH2 ) 2 -O[O-C- O -COO-(CH2 ) 2 -O] n-H
When n = 1, 2, 3
Methanol formed during the condensation is removed using a distillation column. The monomer is then heated to 270° - 280°C with removal of volatiles under reduced pressure down to 1 mm Hg. Heat and vacuum are employed to increase the molecular weight with concurrent removal of volatile products such as water /alcohol and glycol.
Manufacturing of PET
PET Polymers are manufactured by a stage-wise melt polymerization process consists of transesterification, pre-polymerization and finishing polymerization steps.
Continuous melt polycondensation process for PET manufacture.
- In the transesterification stage dimethyl temphthalate (DMT) or terephthalic acid (TPA) is converted into bishydroxy ethyl terephthalate in the presence of metal acetate catalyst.
- Ethylene glycol (EG) DMT, and catalyst are fed at a molar ratio of 1.7 to 2.0:1 (with TPA this ratio is 1-3 to 1.5:1) and a temperature of 100-180°C. This stage takes 3-4 hr.
- The reaction byproduct methanol (Water When TPA) is used is separated from EG vapors in a reflux column.
- After adding stabilizer and additives, the prepolymer is forced through a superfine fitter to the second (pre-poly condensation) stage.
-The poly condensation reaction is performed under vacuum (15-25 torr) and elevated temperature. (280 – 300°C).
- The resulting EG is removed by a vacuum pump while the prepolycondensation product – a low molecular weight (DP= 30) relatively non-viscosity material (40 poise) is pumped to the finishing poly condensation stage after a residence time of about 2 hrs.
- The finishing polymerization stage, which is operated at 0.5 – 1 torr, requires special wiped film or extruder type reactor in order to handle the high viscosity of the polymer ((DP = 100), few thousand poise).
- Many manufacturer have devised their own proprietary agitation systems to minimize the build-up of the solidifying polymer layers.
- The EG vapours contaminated with oligomers are drawn off through a special condensing system and sent to a recovery unit
- The polymers melt can be sent to a direct spinning plant or transformed into pellets.
Relations of Structure and Properties of PET
- There are various classes of linear polyesters.
- The concentration of ester groups has predominant effect on the properties of linear polyesters.
- For the aromatic classes of linear polyester, decreasing the concentration of ester groups apparently leads to a reduction in melting point.
- However, in such aromatic linear polyester the decrease in concentration of ester group is accompanied by the p-phenylene group concentration.
- It is found from aliphatic polyester that the ester group concentration has little effect on the melting point.
- In fact a decrease in ester group concentration leads to a slight increase in the melting point.
- It is found that in an ester group the in chain ether link –C-O-C- increase the chain flexibility compared with a polyethylene chain to decrease the heat of fusion.
- At the same time there will be some increase in inter chain attraction via the carbonyl group which will decrease the entropy of fusion.
- Since these two effects almost cancel each other out there is almost no change in ester group concentration.
- With all linear series of polyester as the no. of methylene groups in the repeating unit increases so the polymer becomes more like a linear polyethylene (polymethylene).
- It is also observed that Tm (melting point) of a polymer with an odd no. of methylene groups in the aliphatic portion of the repeat unit is lower than for the polymer with one more but an even no. of methylene groups.
- Generally, the highest melting points are obtained where the in-chain aromatic ring is of the p-phenylene type.
- The materials have a disadvantage of sensitivity to water and alkaline solution.
- These materials are crystalline and only proton donors that are capable of inter action with the ester groups are effective solvents.
Characteristics of PET (For Identification)
1. The PET is a semicrystalline material, normally amorphous with low crystallinity and high transparency.
2. It is identified by sweet smell, when burns, orange flame, soot forming and melt drips.
3. It is scratch resistant.
4. Melting point is 220°C.
5. It is having short and long term thermal limit temperature respectively 135°C and 100°C.
6. Outstanding chemical resistance to organic and inorganic liquids
7. Water repellent
8. Sterilizable by ethylene oxide and X-rays
9. Inherent good electrical property
10. Ability to orient
11. Good tensile strength
12. Superior strength and stiffness
13. Excellent dimensional stability
14. Excellent melt strength with slower crystallization rate
15. Good impact strength even at low temperature
16. High tear strength
17. Heat resistant
18. Flame retardant
19. Outstanding clarity
20. High gloss
Properties
1. Outstanding chemical resistance
2. Water repellent
3. Good Tensile strength
4. Superior strength and stiffness
5. Excellent dimensional stability
6. Good impact strength even at low temp.,
7. High heat resistance,
8. Flame retardant,
9. Outstanding clarity
10. High Gloss
APPLICATIONS OF PET
1. Synthetic fibre - Screen and sewing thread
2. Biaxially oriented film - Magnetic tape
- X-ray and photographic film
- Electrical insulation tapes
- Food packaging
3. Blow moulded containers
4. Automotive components
5. Electrical components
6. Power tool housings
7. Appliance bases
8. Medical device packaging
PET/PC BLENDS
1. Improved process stability
2. Excellent impact and chemical resistance
3. Reduce co-efficient of friction with improved dimensional stability
APPLICATIONS
1. Automotive, recreation equipment, components for hydraulic systems
PET/Elastomer BLENDS
1. Good processability, rigidity
2. Excellent weatherability
3. Improved impact strength
APPLICATION
1. Electrical/Electronics, Automotive and hardware applications.
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