1. Polybutylene Terephthalate (PBT)
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Polybutylene Terephthalate (PBT)
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.
PBT was introduced commercially in 1969 by Celanese.
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Monomer Ingredients for PBT
The ingredients which are used for preparing PBT are 1,4 butanediol and dimethyl terephthelate.
HO – CH2 – CH2- CH2- CH2 – OH
1,4 - butanediol
Dimethyl terephthelate
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4. Chemistry of Preparation of PBT
Polybutylene Terephthelate (PBT) also called polytetramethylene terephthalate (PTMT) resin is a semicrystalline polyester resin. It is produced by the catalytic polycondensation of 1,4 butanediol and dimethyl terephthelate.
n HO-CH2 -CH2 -CH2 CH2 -OH +n CH3 OOC - O COOCH3
o o
-[O-C- 0 -C-O CH2 -CH2 -CH2 -CH2- 0]+ 2n CH3 OH
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Manufacturing of PBT
PBT is manufactured using similar principles of PET manufacturing.
At first step PBT is produced by the catalytic polycondensation of 1,4 butanediol and dimethyl terephthalate, Subsequent post condensation raises transerterified product to the desired molecular weight.
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6. Relations of Structure and Properties of PBT
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.
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7. Relations of Structure and Properties of PBT
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).
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8. Relations of Structure and Properties of PBT
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.

9. Characteristics of PBT ( For Identification)
The PBT is semicrystalline material.
It is having good wear characteristics and good insulating properties.
It is also having good dimensional stability.
It is identified by orange colored sooty flame.
When burns melt drips and gives sweet smell
The melting point is °C
Short term and long term service temperature are respectively 160 and 100°C
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10. Characteristics of PBT
High strength & rigidity
Excellent dimensional stability
Low co-efficient of friction
Good abrasion resistance
Good resistance to creep at ambient & elevated temperature
Low moisture absorption
Resistance to weak acid, bases, alcohols, ketones, oils, fats and aliphatic hydrocarbons
Good resistance to motor oil, gasoline and brake fluids
High temperature resistance
Excellent flame resistance
Good electrical resistance
High arc resistance & dielectric strength
Low dissipation factor
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Properties of PBT
Properties Units Values
Specific gravity
Tensile strength MPa 55
Tensile modulus MPa
Flexural modulus Mpa Elongation at break % 200
Impact strength izod, Notched, J/m 53
Hardness R -72
Deflection temperature under load 0C
(1.82 Mpa)
Coefficient of linear expansion mm/mm/0C x 10-5
Water absorption, 24hrs % 0.1
Dielectric strength KV/mm
Dielectric constant Hz
Power factor
Volume resistivity Ohm.m 4 x 1017
Melting point 0C 225
Glass transition temperature °C 40
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Properties of PBT
Mechanical Properties
The material has very good strength, stiffness, hardness and maintain toughness at low temperature.
Thermal Properties
The melting temperature of PBT is 225°C. It has excellent dimensional stability at variable temperature range.
Electrical Properties
PBT is a good electrical insulator and this is little affected by water absorption. Electrolytic corrosion or contact-impairing deposits are unlikely. The dielectric strength is also independent of temperature.
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Properties of PBT
Water Absorption
The low moisture absorption ensure consistent mechanical performance and dimensional stabiltiy.
Permeability to Water vapour and Gases
Permeability resembles that of PET films, i.e. it is good compared to other polymers.
Chemical Properties
PBT is resistant to water, aqueous solutions (at room temperature), weak acids, many organic solvents, oils, fats, brake fluid, carbon tetrachloride.
It is not resistant to strong acids, alkaline solutions, phenol, ethyl acetate, acetone. Extended contact with water at 60°C leads to hydrolytic decomposition.
PBT is not susceptible to stress cracking.
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Properties of PBT
Resistance to Weathering and High Energy Radiation
The properties are same as discussed for PET.
Flammability
Unreinforced PBT burns slowly with an orange, sooty flame. The melt drips. Glass fiber reinforced material burns more rapidly because there is less tendency to drip.
Toxicity
PBT is suitable for applications involving contact with food stuffs. It is physiologically inert.
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Additives of PBT
1. Functional additives
2. Fillers
3. Reinforcements
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Functional Additives
1. Anti-oxidants
2. UV absorbers
3. Flame retardants
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Fillers and Reinforcements
1. Mica - Fillers
2. Thermoplastics glass fiber - Reinforcements
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Grades of PBT
The PBT is available in the following grades.
Injection moulding grade
Extrusion grade
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Processing of PBT
PBT can be processed by the following techniques
Injection Moulding
Extrusion
Blow Moulding
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20. Processing considerations of PBT
This resin must be dried to a moisture of 0.02% or less before processing to ensure desired quality performance. The drying condition should be 120°C for 2-4 hours.
The injection moulding machine should be cleaned thoroughly with polyolefin or polystyrene to remove any residual material in the cylinder.
The cylinder temperature should be maintained at °C. The recommended mould temperature should be °C.
The maximum recommended regrind / reclaimed material which can be mixed without adverse affect on resin properties is 25%.
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Surface Finishing of PBT
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 °C).
PBT 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.
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Machineability of PBT
Cutting
Cutting can be possible in PBT with suitable equipments in a specified speed.
Joining
Welding
Moldings 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 °C
Contact pressure at start of heating bar
Contact pressure at start of welding bar
Heating up time (according to wall thickness) - 10 to 30 s
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Applications of PBT
Electrical and electronic applications
Automobile applications
Others
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Applications of PBT
Electrical and electronic applications:
Connector, IC Sockets, Terminal blocks, bobbins, push-buttons of telephone set, micro switch, TV parts, hair dryer parts, micro motor housing, glue gums etc.
Automotive plug connector
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Applications of PBT
Automobile applications:
Wiper gear case, outer handle, distributor, coupler, Ignition system parts, bumper, valve of exhaust, gas treating system etc.
Application of PBT in various areas
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Applications of PBT
Air bag cover
Optical connector
Fuse holder
Car seat frame
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Applications of PBT
Iron box
Inhaler
Overhandle
Blood filter
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Blends of PBT
PBT/PET
The injection molding of glass fiber reinforced PBT leads to long directional orientation and anisotropy in the molded part.
And additional deficiencies is the poor surface quality of the molded parts.
But when blended with PET warpage of the part is clearly diminished and surface quality improved.
The blends also show improvements in heat deflection temperature and impact strength.
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Blends of PBT
PBT/Elastomers
The blends developed for molding show good heat deflection temperature processability, electrical properties stiffness, strength, dimensional stability impact strength, solvent and chemical resistance
PBT/ABS (contain upto 30% glass fiber)
The blend shows excellent processability, high gloss, stiffness at high temperature and mechanical strength, heat deflection temperature, low shrinkage, good dimensional stability, impact strength as well as solvent and chemical resistance.
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30. List of Manufacturers/Suppliers of PBT
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