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Complete Guide on Polybutylene Terephthalate (PBT)

Complete Guide on Polybutylene Terephthalate (PBT)

Providing innovative solutions that save weight and decrease costs along with other benefits, PBT has made its place in end-markets like automotive, consumer goods as well as electrical & electronics. Explore polybutylene terephthalate (PBT) in detail along with its key properties like mechanical, thermal, electrical, etc. and understand what makes it an ideal choice in high-end engineering applications. Also, learn about few polybutylene terephthalate blends with PC, PET and conditions to process this polymer material. Let’s first start by understanding PBT plastic and how it is made

Overview

What is Polybutylene Terephthalate (PBT)?

What is Polybutylene Terephthalate (PBT)?

Polybutylene Terephthalate (PBT) is a semi-crystalline engineering thermoplastic material. It has similar properties and composition to polyethylene terephthalate (PET). It is a member of polyester family of polymers.

PBT is produced by polycondensation of terephthalic acid or dimethyl terephthalate with 1,4–butanediol using special catalysts.

Polybutylene Terephthalate Structure

Molecular Structure of Polybutylene Terephthalate
Chemical Formula: (C12H12O4)n

Polybutylene terephthalate has gained commercial interest due to its wide range of applications ranging from automotive, electrical and electronics, medical and many more.

The PBT product range includes a wide variety of grades which are great for injection molding. These include:

  • Reinforced
  • Filled
  • Impact-modified, and
  • Flame-retardant

Compared to non-reinforced resin, glass-reinforced PBT exhibits a two-to threefold increase in tensile, flexural and compressive strengths and moduli.

The unfilled PBT grades have a wide range of melt viscosities. This helps to provide processing latitude in injection molding and extrusion techniques (from melt-blowing of PBT fibers to production of rod and slab, fiber optic buffer tubes or brake cable liners).

Flame-retardant Polybutylene Terephthalate and lubricated PBT versions are also available in both unreinforced and reinforced grades. Compared to non-reinforced resin, glass-reinforced grade exhibits a two-to threefold increase in tensile, flexural and compressive strengths and moduli.

The broad use of PBT is also shown by the numerous regulatory approvals held by various grades. These include VDE or UL-approvals for the electrical and electronics market or FDA approval for the nutrition and medical market.

Properties of PBT

Properties of PBT

The polymer displays excellent mechanical and electrical properties. Review the key properties of this polyester material discussed below.

  • It has excellent stain resistant and machining characteristics

  • It enables efficient use of material to reduce weight and cost

  • It delivers excellent short-term mechanical properties, such as high strength, toughness and stiffness as well as good practical impact

  • It provides good creep resistance, dimensional stability and low moisture absorption characteristics

Elongation at Break 5-300%
Elongation at Yield 3.5-9%
Flexibility (Flexural Modulus) 2-4 GPa
Hardness Rockwell M 70-90
Hardness Shore D 90-95
Stiffness (Flexural Modulus) 2-4 GPa
Strength at Break (Tensile) 40-50 MPa
Toughness (Notched Izod Impact at Room Temperature) 27-999 J/m
Toughness at Low Temperature (Notched Izod Impact at Low Temperature) 27-120 J/m
Young's Modulus 2-3 GPa
Coefficient of Linear Thermal Expansion 6-10 x 10-5 /°C
Shrinkage 0.5-2.2%
Water Absorption 24 hours 0.1-0.2%

  • It gives good durability under thermal stress and /or harsh chemical environments, particularly in automotive underhood applications

  • PBT's high heat-deflection temperatures and high temperature index ratings help components withstand both short term thermal excursions and long-term heat exposure

  • Polybutylene Terephthalate helps protect electrical and electronic components with extremely high electrical resistance and high dielectric strength that guard against leakage and breakdown in power circuitry

  • The low dielectric loss minimizes energy absorption in high frequency electronic uses

  • Where needed, appropriate flame-resistant grades are available

Arc Resistance 124-190 sec
Dielectric Constant 2.9-4
Dielectric Strength 15-30 kV/mm
Dissipation Factor 10-200 x 10-4
Volume Resistivity 14-17 x 1015 Ohm.cm
Fire Resistance (LOI) 20-24%
Flammability UL94 HB

The other benefits provided by polybutylene terephthalate include:

  • Good creep resistance over steady and elevated temperatures

  • Good UV radiation resistance and excellent heat aging behavior

  • Compliance with numerous agency and regulatory approvals for food, medical and potable water applications, as well as specialized performance specifications

Density 1.3-1.4 g/cm3
Glass Transition Temperature 55-65°C
Thermal Insulation (Thermal Conductivity) 0.21 W/m.K

It has robust chemical resistance to a wide range of chemicals such as diluted acids, alcohols, aromatic hydrocarbons, ketones etc., solvents, oils and greases. This makes it suitable for use to produce plastic parts that are exposed to organic solvents, gasoline, oils and hence avoid erosion after period of time.

By incorporating fillers, reinforcing materials, and additives during compounding, material properties can be tailored to requirements. A benzotriazole-type UV stabilizer is generally used with PBT because it offers low initial color and good color stability.


Limitations of Polybutylene Terephthalate


  • High mold shrinkage

  • Poor resistance to hydrolysis (sensitive to hot water)

  • Prone to warping due to high differential shrinkage

  • Unreinforced PBT is notch sensitive

  • Low HDT @ 60°C (140°F) compared to competitive materials


Polybutylene Terephthalate Alloys with PC, PET and Other Thermoplastics

Polybutylene Terephthalate Alloys with PC, PET and Other Thermoplastics

While based on the fundamental advantages of PBT, polyester alloys are "purpose-designed" to meet demanding performance specifications. Polyester alloys:

  • Deliver engineering properties in a ductile, chemically-resistant resin.

  • Give designers the ability to provide toughness without compromising mechanical performance and /or environmental resistance.

  • Remain ductile down to -40°C, yet retains functional stiffness in a hot car interior. Specialty grades withstand the difficult performance requirements of safety equipment such as air bag doors and containers.

  • Combine ductility, high electrical resistance, dielectric strength, and good chemical resistance to oils, greases and fuels while meeting demanding heat shock criteria. Possible use in under-the hood power distribution boxes and distributor-less ignition systems, etc.

  • Perform (weatherable grades) in outdoor electrical applications, such as splice cases. Polyester flame retardant technology can also be used in these alloys.

Polycarbonate/Polybutylene Terephthalate (PC/PBT) Blend


Polycarbonate and PBT have comparable tensile strength and flexural strength. In polycarbonate/polybutylene terephthalate (PC/PBT) blend:

  • The polycarbonate component adds resistance to impact, heat and stiffness, whereas
  • The polybutylene terephthalate component provides chemical resistance

Polycarbonate/polybutylene terephthalate blends retain their toughness even at low temperatures and offer improved resistance to fuel and weather. The presence of polycarbonate suppresses the moisture sensitivity of crystalline polybutylene terephthalate in the blend.

  • PC blends show higher modulus than that of PC alone. 
  • Blending small quantity of polyester can improve flexural modulus of PC 
  • Impact strength of PC being higher, this property improves considerably for all polyester rich blends

PC/PBT blends have poor miscibility as compared to other PC/polyester blends.

Strengths Limitations
  • Very high toughness, down to -40°C

  • Good heat resistance, up to 140°C

  • Excellent weatherability

  • Good UV resistance

  • High color retention

  • FDA compliant
  • Not suitable for thin wall design, due to its medium flow properties

  • Opaque

  • High shrinkage

  • Attacked by alkalis, esters, ketones and halogenated hydrocarbons
Strengths and Limitations of PC/PBT Blends 

Because PC/PBT blends offer excellent toughness and good resistance to stress cracking from fuels and lubricating fluids, they are found ideal in applications like automotives/ transportation (bumpers, tailgate outer panels, door handles, tractor hoods), household/ consumer goods (chain saw housings, power drill housings) and items and several others (Electrical enclosures, Double glazing spacer bars, etc.).

PE/PBT Blend


When PBT is alloyed with 15-25% low density polyethylene, processing and mechanical properties can be enhanced as well as the reduction of moisture consumption.

Comparison with Other Plastics

Comparison with Other Plastics

PBT vs Other Plastics

As compared to PET, PBT has:

  • Lower melting point
    • PBT: (223°C [433°F])
    • PET: (255°C [491°F])

  • Lower strength and rigidity

  • Lower glass transition temperature

  • Better impact resistance

PBT is often preferred over Polyethylene Terephthalate (PET). Its property of excellent flow in molten state combined with its rapid crystallization benefit makes it more cost effective in injection molding applications.

In some areas, PBT is replacing other crystalline thermoplastics such as polypropylene, nylon and acetal because of the need for improved performance. PBT can compete with many amorphous engineering materials such as polysulfone and polycarbonate.




Methods of Processing PBT

Methods of Processing PBT

PBT grades are processed mainly by injection molding at temperatures ranging from 230° to 270° C but it can also be processed by extrusion, blow molding.... Unreinforced PBT is also used in special extrusion and fiber spinning processes.

A variety of PBT grades are also offered for other processes including: profile, tube and monofilament extrusion, melt blown and spunbond production.

An optimum cooling temperature of 250-275° F for 2-4 hours will produce the best results.

Ductile / Brittle Transition Temperature -40°C
HDT @0.46 Mpa (67 psi) 115-150°C
HDT @1.8 Mpa (264 psi) 50-85°C
Max Continuous Service Temperature 80-140°C
Min Continuous Service Temperature -40°C

Injection Molding Conditions


  1. Before injection molding, pre-drying is recommended to control moisture content below 0.02%

  2. Mold Temperature: 40-80°

  3. Mold Shrinkage: Higher the mold temperature, greater is the shrinkage
    • Unfilled: 1.4-2.0%
    • Glass-fiber Reinforced: 0.4-0.6%

  4. Injection Pressure: 100-140 MPa / 1000-1400 Bar

  5. Screw: Graded three-zone screw with L/D ratio of 15-20 & compression ratio of 2.5-3.0

PBT can be joined together by a number of different ways; ultrasonic, hot plate, friction, and by hot gas welding. PBT also can be joined by two-component adhesives. Efficiently manage the problem of internal stress while bonding plastics...Learn More!

The key use of polyamide 6 is in carpets and a recycling process for this was initially devised by DuPont in 1944 although recycling a dirty carpet is still a challenge.

Polyamide polymer can be chemically recycled or de-polymerized

De-polymerization method involves breaking down the long polymer chains into monomers which can be then re-polymerized which possibly converts the waste into products having a quality equivalent to that of the “virgin” polymer.

Polyamide 6 can be depolymerized to its monomer – caprolactam, byacidolysis, hydrolysis, aminolysis or catalyzed-de-polymerization in vacuum. Companies recycling polyamide 6 and 6.6 by depolymerization includes: DuPont, AlliedSignal, BASF and Novalis Fibers.

Other recycling method include recovery of polymer components without reaching the monomer level. Includes multiples extraction and separation steps, Mechanical recycling and Thermal recycling or Energy generator.

Polyamide Recycling from Carpets
(Source: AlliedSignal/NCSU)


Find Suitable Polybutylene Terephthalate Grade

View a wide range of PBT grades available in the market today, analyze technical data of each product, get technical assistance or request samples.



Key Applications

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1 Comments on "Complete Guide on Polybutylene Terephthalate (PBT)"
FAIZAL M Mar 25, 2021
Hello! Why PBT are used over PC material for Electrical & Electronic Components Manufacturing? Which advantages gives PBT over PC? Regarads, F.N.Mansuri Axel Polymers Limited

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