POLYBUTYLENE TEREPHTHALATE (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.
The PBT is available in the following grades.
1. Injection moulding grade
2. Extrusion grade
- 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.
PBT is produced by the catalytic polycondensation of 1, 4-butanediol and dimethyl terepathalate.
Subsequent post-condensation raises transesterified product to the desired molecular weight.
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.
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.
- 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 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 220-230°C
- Short term and long term service temperature are respectively 160 and 100°C.
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.
Processing of PBT:
PBT can be processed by the following techniques
1. Injection Moulding
2. Extrusion
3. Blow Moulding
Processing considerations of PBT
1. 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.
2. The injection moulding machine should be cleaned thoroughly with polyolefin or polystyrene to remove any residual material in the cylinder.
3. The cylinder temperature should be maintained at 230-260°C. The recommended mould temperature should be 60-120°C.
4. The maximum recommended regrind / reclaimed material which can be mixed without adverse affect on resin properties is 25%.
Surface Finishing of PBT:
1. The high chemical resistance of the polyalkylene terephthalates necessitates the use of special purpose lacquers to ensure good adhesion.
2. The same applies to hot embossing (temperature range of the die 140 - 200°C).
3. PBT can be polished easily. Metallization can be only be carried out by high vacuum deposition.
4. Where high gloss surfaces are required on filled and reinforced product, an intermediate lacquer must be used.
Machineability of PBT:
1. Cutting
Cutting can be possible in PBT with suitable equipments in a specified speed.
2. Joining
3. 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
1. Heated tool temperature - 260°C
2. Contact pressure at start of heating - 0.5 bar
3. Contact pressure at start of welding - 0.5 bar
4. Heating up time (according to wall thickness) - 10 to 30 s
Properties of PBT:
Properties Units Values
Specific gravity ---- 1.31
Tensile strength MPa 55
Tensile modulus MPa 2551
Flexural modulus Mpa 2275 (Elongation at break % 200)
Impact strength izod, Notched, J/m 53
Hardness R -72
Deflection temperature under load ℃ 205-210(1.82 Mpa)
Coefficient of linear expansion mm/mm/℃ 7.4 x 10-5
Water absorption, 24hrs % 0.1
Dielectric strength KV/mm 15.8
Dielectric constant 10-6 Hz 3.24
Power factor 0.019
Volume resistivity Ohm.m 4 x 1017
Melting point ℃ 225
Glass transition temperature °C 40
PROPERTIES
1. Good electrical property
2. Superior strength and stiffness
3. Excellent dimensional stability
4. Water repellants
5. Outstanding chemical resistance
6. Sterlisable by ethylene oxide and gamma rays
7. Good abrasion resistance
8. Low co-efficient of friction
9. Excellent flame retardance
10. High strength & rigidity
11. High impact resistance
12. High heat resistance
13. Excellent dimensional stability
14. Excellent flame resistance
15. Good electrical insulation
16. Good abrasion resistance
17. Resistant to fuels, lubricants
18. High gloss, good moulded surface finish
19. Good stress crack resistance
20. Low moisture absorption.
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.
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.
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.
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.
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.
- Connectors
- TV turners
- High voltage components
- Relays
- Ignition coil caps
- Speedometer frames
- Gears
- Lamp sockets
- Toasters
- Switches
- Coil cylinders
- Washing machines Components
PBT/ABS BLENDS
1. Show excellent processability with high gloss
2. Stiffness at high temperature
3. Low shrinkage
4. Resistance to gasoline and motor oils
Additives of PBT:
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
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