polyesters ¤ polyesters are among the more versatile synthetic polymers. ¤ the common methods of...

Polyesters

Polyesters are among the more versatile synthetic polymers. The common methods of synthesising simple esters are used to make polyesters. These include direct esterification (1), transesterification (2) and the reaction of alcohols (3) with acyl chlorides or anhydrides (4).

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Commercially important Polyesters

Poly(ethylene terephthalate) (PET)

Poly(trimethylene terephthalate)

Poly (ethylene napthalate-2,6-dicarboxylate) (PEN)


Commercially important Polyesters

Poly (butylene terephthalate) (PBT)

Poly (4-hydroxybenzonate)

Polycarbonate


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.

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Polyethylene Terephthalate (PET)

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 .


Monomer Ingredients for PET

The ingredients which are used for PET are ethylene

glycol and dimethyl teraphthalate.

HO – CH2 – CH2 – OH Ethylene glycol

Dimetyl terephthalate


Chemistry of Preparation of PET

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.

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.


Chemistry of Preparation of PET

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-HWhen 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.

n CH3OOC - O - COO CH3 + n HO - CH2 - CH2 - OH

O O- [C - O - C - O -CH2- CH2 - O -] n + 2n CH3 OHCORPORATE TRAINING AND PLANNING

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)

The PET is a semicrystalline material, normally

amorphous with low crystallinity and high transparency.

It is identified by sweet smell, when burns, orange flame,

soot forming and melt drips.

It is scratch resistant

Melting point is 220°C

It is having short and long term thermal limit temperature

respectively 135°C and 100°C


Characteristics of PET

Outstanding chemical resistance to organic and inorganic liquids

Water repellent Sterilizable by ethylene oxide and X-rays Inherent good electrical property Ability to orient Good tensile strength Superior strength and stiffness Excellent dimensional stability Excellent melt strength with slower crystallization rate Good impact strength even at low temperature High tear strength Heat resistant Flame retardant Outstanding clarity High gloss


Properties of PET

Properties Units ValuesSpecific gravity ---- 1.40Tensile strength MPa 51Tensile modulus MPa 175Flexural modulus Mpa 204Elongation at break % 7Impact strength izod, Notched, kg.cm/cm 9.25Hardness R 105Deflection temperature under load 0C 205-210 (1.82 Mpa)Vicat softening point °C 75Coefficient of linear expansion mm/mm/ 0C6.0 x 10-5

Water absorption, 24hrs % < 0.1Refractive index 1.62Dielectric strength KV/mm 26Dielectric constant 10-6 Hz 3.3Power factor 0.03Volume resistivity Ohm.m 1013

Melting point 0C 265Glass transition temperature °C 67


Properties of PET

Mechanical PropertiesPET exhibits a high modulus up to 75°C, when it is crystalline and 40°C, when it is amorphous even when they are mechanically stressed for a long period.

Thermal PropertiesPET has excellent dimensional stability.

Electrical PropertiesPET has high dielectric strength and high volume

resistivity. The dielectric properties are adequate for many electronics applications.


Properties of PET

Water Absorption PET is hygroscopic and in its melt states the resin hydrolyses.

Optical Properties

The light transmission of PET differs greatly according to chemical composition.


Properties of PET

Permeability to water vapour and Gases

PET is less permeable than PVC-U.

The permeability of film is 40µm/1.58 mil.

The permeability to water vapour is higher than that of

the polyolefins but lower than that of PC, PA, POM. PET

films are largely odor-proof which leads to their frequent

use in packaging materials in combination with PE films


Properties of PET

Chemical Properties

PET is resistant to weak acids and alkaline solutions, oils, fats, aliphatic and aromatic hyrocarbons, carbon tetrachloride. It is not resistant to strong acids and alkaline solutions, phenol, long exposure to hot water. The crystalline polyalkyle terephthalates are susceptible to hydrolysis which precludes their use in aqueous solution temperatures above 70°C for in steam. PET does not show any pronounced tendency to stress cracking.


Properties of PET

Weathering Resistance

The relatively high resistance to weathering can be increased by the use of carbon black to protect against UV.Resistance to High energy Radiation

PET is relatively resistant to high energy radiation. It is degraded more strongly only at absorbed energies of more than 100 KJ/ Kg. After 7 to 8 MJ/Kg it decomposes into powder.


Properties of PET

FlammabilityPET burns with a sooty orange-yellow flame, it drips and

smells sweetly aromatic. Flame retardant grades donot drip and meet the requirements of UL 94 V-0.

ToxicityPET is suitable for applications involving contact with

food stuffs and is physiologically inert.

SterilizationPET can be sterilized in an ethylene oxide atmosphere

or by irradiation.


Additives of PET

1. Functional additives

2. Fillers

3. Reinforcements


Functional Additives

1. Anti-oxidants

2. UV absorbers

3. Flame retardants


Fillers and Reinforcements

1. Mica - Fillers

2. Thermoplastics glass fiber - Reinforcements


Grades of PET

The PET is available in the following grades.

Injection moulding grade

Extrusion grade

Stretch blow moulding grade


Processing considerations of PET

PET can be processed by the following techniques :

Injection moulding

Stretch blow moulding

Thermoforming

Extrusion


PET resins are primarily for injection moulding.

PET is hygroscopic and in its melt state the resin hydrolyzes.

PET resin supplied in moisture porof containers can have a moisture content upto 0.04%, but hydrolysis can occur at a moisture level above 0.02%.

Therefore, even virgin PET directly from suppliers containers, as well as regrind, should be dried to level of less than 0.02% and maintained at that level.

Dehumidifier hopper is recommended with injection moulded PET.

Drying time values from 4 hrs. for wet resin to 2 hrs. for virgin resin at 135°C.



The moulding must be carried out with dried material and because of the free flowing nature of the melt, restricted nozzles should be used and a back flow valve fitted to screw injection moulding machine.

Cylinder temperature are about 260 – 290°C.and mould temperature as high as 140°C to promote a controlled crystallization.

A significant degree of crystallinity occurs in the mould. Therefore, mould temperature should be selected carefully in order to enhance crystalline formation.

Mould temperature also influence cycle time and products quality, particularly warpage shrinkage, surface gloss depth or colour and chemical resistance.



Although resin grades from the same supplier show the same melt temperature, PET from different suppliers shows different melt temperature ranging from 260 – 290°C .The recommended typical injection moulding cylinder temperature also differ between suppliers.

An interesting feature of PET is that according to the moulding conditions two quite dissimilar products one amorphous, the other transparent, may be obtained, this beings a consequence of having a Tg of about 80°C.

Because of the low melt viscosity injection moulding screw for PET should be fitted with back flow valves and the barrel nozzles should have shut-off-valves.



Surface Finishing of PET

The high chemical resistance of the polyalkylene terephthalates

necessitates the use of special purpose lacquers to ensure good

adhesion.

The same applies to hot embossing (temperature range of the

die 140 - 200°C).

PET can be polished easily. Metallization can be only be

carried out by high vacuum deposition.

Where high gloss surfaces are required on filled and reinforced

product, an intermediate lacquer must be used.


Machineability of PET

CuttingCutting can be possible in PET with suitable equipments in a specified

speed.

Joining

WeldingMoldings and semi-finished products can be welded and bonded.

Unreinforced moldings can be joined by hot gas and heated tool welding. With increasing glass fiber and the flame retardant content, the welded joints decrease in strength. Friction and ultrasonic welding are efficient joining methods. The conditions for heated tool welding are

Heated tool temperature - 260°CContact pressure at start of heating - 0.5 barContact pressure at start of welding - 0.5 barHeating up time (according to wall thickness) - 10 to 30 s


Applications of PET

Appliances

Automotive

Electrical/Electronics

Films

Furniture

Packaging

MiscellaneousCORPORATE TRAINING AND PLANNING

Applications of PET

Appliances : Oven and appliance handles, coil forms for

microwave oven transformers, small appliance structural framers, panels, chassis and housing and instrument covers.


Applications of PET

Automotive: Structural parts such as luggage racks and grills,

fuctional housing such as windshield, wiper motor housing , fuel filters, blade supports and bells, sensor, lamp sockets, relays, switches and solenoids

Grill opening retainer Door moduleCORPORATE TRAINING AND PLANNING

Applications of PET

Electrical/Electronics: Lamps sockets , coil forms for audio/video

transformers, terminal blocks, electrical. Electronic connectors and thrust washers.

Electricals Connectors


Applications of PET

Films: X-ray films, photographic films, magnetic

tapes and printing sheets.

Food packaging


Applications of PET

Furniture:

Pedestal bases, sent pans, chair arms and castes.


Applications of PET

Packaging : Carbonated soft drink containers, distilled

spirits, toiletries, food products, mustard, pickle foods, peanut butter, spices, edible oil cocktail mixer and syrups.

Containers


Applications of PET

Miscellaneous:General purpose pumps housing , mirror

housings three dimensional circuit boards and structural components in office automation equipment, computer housing and structural components, containers, trays, blisters and drinking cups.


Applications of PET

Automobile engine cover Heat shield

Gear box Door washersCORPORATE TRAINING AND PLANNING

Blends of PET

PET/PC

Partially miscible blends (miscibility is controlled by the level of transesterification) with co-continuous morphology show improved processability, chemical resistance, crystallinity, elongation, low temperature impact and tensile strength.

Blends also show decreased shrinkage and high heat deflection temperature.

These blends are especially interesting for reinforced products because the warpage is significantly decreased.


Blends of PET

PET/Elastomers

These blends are formulated for extrusion, injection,

compression, blow and transfer molding.

The blends contain up to 35 wt. of glass fiber.

They show good processability rigidity impact and tensile

strength as well as excellent weatherability.


List of Manufacturers/Suppliers of PET

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polyesters ¤ polyesters are among the more versatile synthetic polymers. ¤ the common methods of...

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