Polybutylene Terephthalate (PBT) Material Guide & Properties Info

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 the polyester family of polymers.

PBT has gained commercial interest due to its wide range of applications. These range 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

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

Flame-retardant PBT and lubricated PBT versions are also available in both filled and unfilled grades. When compared to non-reinforced resins, glass-reinforced PBT grades exhibit increased mechanical properties. This means a 2 to 3-fold increase in tensile, moduli, flexural, and compressive strengths.

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
FDA approval for the nutrition and medical market.

How is polybutylene terephthalate made?

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

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

What are the properties of PBT?

Physical properties – PBT provides dimensional stability and low moisture absorption characteristics. It gives good durability under thermal stress and/or harsh chemical environments. Can be particularly seen in automotive underhood applications.

Mechanical properties – PBT delivers high strength, toughness, and stiffness. All over it shows a good practical impact. It provides good creep resistance over steady and elevated temperatures.

Thermal properties – PBT has a high heat-deflection temperature and high-temperature index rating. It helps components withstand both short-term thermal excursions and long-term heat exposure.

Electrical properties – PBT protects electrical and electronic components against discharge. It provides high electrical resistance and dielectric strength. It also guards the components against leakage and breakdown in power circuitry. The low dielectric loss minimizes energy absorption in high-frequency electronic uses.

Chemical resistance – PBT has robust chemical resistance to a wide range of chemicals. These include diluted acids, alcohols, aromatic hydrocarbons, ketones, solvents, oils, and greases. Suitable to produce plastic parts exposed to organic solvents, gasoline, and oils. It also avoids erosion after a period of time. It exhibits good resistance to UV radiation and stain.

Other benefits – PBT has excellent heat aging behavior and machining characteristics. It enables efficient use of material to reduce weight and cost. It is compliant with many agency and regulatory approvals for food, medical, and potable water applications.

What is the value of PBT for several properties?

Mechanical Property
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
Electrical Property
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
Physical property
Shrinkage	0.5-2.2%
Water Absorption 24 hours	0.1-0.2%
Density	1.3-1.4 g/cm³
Glass Transition Temperature	55-65°C
Thermal Property
Coefficient of Linear Thermal Expansion	6-10 x 10^-5 /°C
Thermal Insulation (Thermal Conductivity)	0.21 W/m.K
Fire Resistance (LOI)	20-24%
Flammability UL94	HB
Service Temperature
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
Get more information about polymer properties here »

How can material properties be tailored?

Material properties can be tailored during compounding. This is done by incorporating fillers, reinforcers, and additives to meet end-user requirements.

For example, a benzotriazole-type UV stabilizer is generally used with PBT. It offers low initial color and good color stability.

What happens when PBT alloys with other thermoplastics?

PBT alloys are "purpose-designed" to meet demanding performance specifications. They offer the following benefits:

Deliver engineering properties in a ductile, chemically resistant resin.

Give designers the ability to provide toughness. They do not compromise on 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. These types of equipment include airbag doors and containers.

They show high electrical resistance, dielectric strength, and ductility. They also provide good chemical resistance to oils, greases, and fuels. These properties combine to meet 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.

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

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

PC blends show a higher modulus than that of PC alone.
Blending a small quantity of polyester can improve the flexural modulus of PC.
The impact strength of PC is higher. This property considerably improves 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

PC/PBT blends offer excellent toughness. They also provide good resistance to stress cracking from fuels and lubricating fluids. Hence, they are ideal in applications like:

Automotive/transportation - Bumpers, tailgate outer panels, door handles, tractor hoods
Household/consumer goods - Chain saw housings, power drill housings, and
Others - Electrical enclosures, Double glazing spacer bars, etc.

Polyethylene/Polybutylene Terephthalate (PE/PBT) Blend

When PBT blends with 15-25% low density polyethylene, the following changes can happen:

The processing can increase.
The mechanical properties can enhance.
The moisture consumption can reduce.

What are the benefits of PBT over other thermoplastics?

When 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). It has excellent flow in the molten state combined with its rapid crystallization. This benefit makes it more cost-effective in injection molding applications.

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

What are the methods for processing PBT?

PBT grades are processed mainly by injection molding at 230°C to 270°C temperature range. They can also be processed by extrusion and 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. These processes include profile, tube, and monofilament extrusion. They are also produced by melt-blown and spun-bond processes.

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

Injection Molding Conditions

Before injection molding, pre-drying is recommended to control moisture content below 0.02%
Mold Temperature: 40-80°C
Mold Shrinkage: Higher the mold temperature, greater the shrinkage

Unfilled: 1.4-2.0%
Glass-fiber Reinforced: 0.4-0.6%

Injection Pressure: 100-140 MPa / 1000-1400
Bar Screw: Graded three-zone screw with L/D ratio of 15-20 & compression ratio of 2.5-3.0

PBT can be joined together in a number of different ways. These include ultrasonic, hot plate, friction, and hot gas welding. PBT also can be joined by two-component adhesives.

What are the limitations of PBT?

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

What are the commercially available PBT grades?

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 Properties

	Property	PC/PBT BLEND
Electrical
Dielectric Constant	2.95-3.14
Dissipation Factor x 10^-4	190-300
Volume Resistivity x 10¹⁵, Ohm.cm	16-17
Mechanical
Elongation at Break, %	4-175
Elongation at Yield, %	4.4-4.5
Flexural Modulus, Gpa	1.6-3.9
Toughness (Notched Izod Impact at Room Temperature), J/m	50-960
Toughness at Low Temperature (Notched Izod Impact at Low Temperature), J/m	170-640
Young's Modulus, GPa	1.8-2.3
Physical
Density, g/cm³	1.17-1.3
Shrinkage, %	0.6-1.1
Water Absorption 24 hours, %	0.03-0.5
UV Light Resistance	Fair
Gamma Radiation Resistance	Good
Sterilization Resistance (Repeated)	Poor
Service Temperature
HDT @0.46 Mpa (67 psi), °C	91-118
HDT @1.8 Mpa (264 psi), °C	60-121
Max Continuous Service Temperature, °C	60-121
Thermal
Flammability, UL94	HB

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Complete Guide on Polybutylene Terephthalate (PBT)