Polybutylene Terephthalate (PBT): How to select the right grade?

Definition

Polybutylene Terephthalate (PBT) is a semi-crystalline engineering thermoplastic material. It is a member of the polyester family of polymers. The chemical formula is (C₁₂H₁₂O₄)_n.

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


 

Synthesis

Polybutylene terephthalate (PBT) is produced through a process called polycondensation.

The raw materials used in the manufacturing include:

1. 1,4-Butanediol (BDO)
2. Dimethyl terephthalate (DMT) or Terephthalic acid (TPA)

The esterification process further takes place depending on the starting material.
 

• If using DMT: It reacts with BDO to form bis(4-hydroxybutyl) terephthalate and methanol.
• If using TPA: It reacts directly with BDO to form bis(4-hydroxybutyl) terephthalate and water.

The bis(4-hydroxybutyl) terephthalate further undergoes condensation reactions at high temperatures (typically 230-250°C) and under vacuum. This process forms long polymer chains of PBT while removing excess BDO. 
 

Having defined PBT, it’s important to understand the core material characteristics that underpin its performance.

Knowing the material properties of PBT is crucial to manufacture the desired product of your need. Let's take you through the benefits which PBT provides in your application.
 

Physical properties

It 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 under hood applications.

Mechanical properties

It 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

It 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

It 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 

It 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. 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 and FDA approval for the nutrition and medical market.

Properties of PBT
 

Optimizing PBT material properties

PBT material properties can be tailored during compounding. This is done by incorporating fillers, reinforcers, and additives to meet end-user requirements. The PBT product range includes:
 

1. 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.
2. Glass fiber reinforced PBT grades exhibit increased mechanical properties compared to unfilled grades. This means their tensile, moduli, flexural, and comprehensive strengths increase by 2 to 3 fold.
3. Flame-retardant PBT and lubricated PBT versions are also available in both filled and unfilled grades.
4. Other PBT grades that are available include: Mineral filled, PTFE filled, Glass bead filled, and Carbon fiber filled.

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


 

Typical values of PBT

Advantages and disadvantages of PBT

Knowing the advantages and disadvantages of PBT is important to make your PBT selection process easy.

 

Benefits of PBT over other thermoplastics

Find out the advantages of PBT over other thermoplastics below.

 

Comparison with other thermoplastics	Advantages of PBT
Polyethylene terephthalate (PET)	Lower melting point 223°C [433°F] than PET 255°C [491°F] Lower strength and rigidity Lower glass transition temperature Better impact resistance Excellent flow in molten state combined with rapid crystallization making it more cost effective in injection molding applications
Crystalline thermoplastics like Polypropylene, Nylon, Acetal	Shows improved performance than crystalline thermoplastics
Amorphous engineering materials like Polysulfone and Polycarbonate	Lower heat resistance and better chemical resistance than PSU Low moisture absorption than PC

 

Limitations of PBT

Mentioned below are the disadvantages of Polybutylene terephthalate (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

 

 

 

Beyond base properties, PBT can be modified through blends and alloys to meet specific performance needs.