Modified PBT: Proven Performance in Auto & Electronic Parts

Modified PBT: Proven Performance in Auto & Electronic Parts

Among the family of plastics, there is a material renowned for its exceptional mechanical strength, outstanding heat resistance, and excellent electrical properties, making it a "hardcore player" among engineering plastics—this is PBT. When you use power tools, drive a car, or operate a household appliance, you have likely already been in "close contact" with PBT. This seemingly ordinary yet high-performance engineering plastic quietly underpins the reliable operation of modern industry.

PBT is a crystalline linear saturated polyester produced by the polycondensation of purified terephthalic acid (PTA) or dimethyl terephthalate (DMT) with 1,4-butanediol (BDO). PBT has a density of approximately 1.30-1.32 g/cm³, a melting point between 225-235°C, and exhibits good crystallization rates and short molding cycles, which benefits production efficiency. PBT parts are easily flow-molded with short cycles, helping to reduce production costs. Furthermore, PBT offers advantages such as moisture resistance, wear resistance, oil resistance, and low creep.

Currently, global annual consumption of PBT exceeds 1 million tons. It holds a significant position among the five major general-purpose engineering plastics (PA, PBT, PC, POM, PPO) and is an indispensable key material in fields like electronics/electrical appliances and the automotive industry.

However, PBT has drawbacks such as flammability, significant bleeding of low-molecular-weight compounds upon contact with refrigerants, insufficient dielectric properties, and a tendency for thin-walled parts to warp, which limit its application scope. To compensate for the performance shortcomings of pure PBT resin, various modification studies have been conducted.

I. Current Status of Modification Research for PBT Engineering Plastics

To meet industrial needs, modifying PBT to enhance its functionality is favored by the market. Currently, methods such as copolymerization modification, inorganic filler modification, nanocomposite technology, and blend modification are primarily used domestically and internationally to improve the overall performance of PBT. Research on modifying PBT materials mainly focuses on high strength, high flame retardancy, low warpage, low bleeding, and low dielectric properties.

1. Mechanical Properties

Pure PBT resin has relatively low tensile strength, flexural strength, and flexural modulus, limiting its widespread application in industrial fields. Modification is needed to enhance its mechanical properties. Glass fiber (GF) offers advantages like strong applicability, simple filling processes, and low cost. Adding glass fiber to PBT allows the inherent advantages of PBT resin to be realized while significantly improving the tensile strength, flexural strength, and notched impact strength of PBT products.

2. Flame Retardancy

Pure PBT can only achieve a UL94 HB rating in vertical burning tests. It is flammable, drips continuously while burning (potentially spreading flames), restricting its application in automotive, electronics/electrical, and textile sectors. Halogenated and halogen-free flame retardants are commonly added for flame-retardant modification. Halogenated flame retardants release toxic smoke containing hydrogen halides upon combustion, posing risks to human health and the environment. The EU has banned some halogenated flame retardants. For PBT flame-retardant modification, phosphorus-based and inorganic flame retardants are primarily used.

When using inorganic flame retardants, excessive addition can degrade the material's mechanical properties. Phosphorus-based flame retardants do not share this drawback and offer excellent characteristics like low smoke, low toxicity, and high flame retardancy. They often work synergistically with nitrogen-containing compounds to achieve more efficient flame-retardant systems. During combustion, phosphorus-based flame retardants generate phosphoric anhydride, which causes dehydration and charring of the combustible material. The char layer reduces heat conduction, retards or prevents the generation of flammable gases. Additionally, the phosphoric anhydride forms a molten layer covering the combustible surface, hindering the release of flammable gases.

Beyond traditional flame retardants, adding nanofillers to PBT materials can also improve flame retardancy and anti-dripping performance without compromising processability.

3. Warpage Deformation

PBT material has relatively easy molecular slippage, leading to high orientation and crystallization, which results in high shrinkage rates. This causes warpage deformation in PBT parts, especially large thin-walled ones. For glass-fiber-reinforced PBT, the anisotropic nature of the added glass fibers leads to different shrinkage rates in different directions during injection molding, increasing part warpage. This not only affects the surface quality and assembly performance of plastic products but also impacts their strength.

To address PBT part warpage, besides improving part design, mold design, and processing parameters, modifying the PBT material itself can mitigate warpage. In recent years, warpage deformation in PBT materials has been primarily improved through inorganic filler addition and blending/alloying.

Furthermore, amorphous polymers like polycarbonate (PC), acrylonitrile-butadiene-styrene copolymer (ABS), and styrene-acrylonitrile copolymer (SAN) do not undergo crystallization during injection molding. Blending them with PBT can effectively reduce PBT's shrinkage rate.

4. Bleeding/Exudation Performance

Due to incomplete reactions of raw materials during PBT production, small molecules and oligomers are generated. Products made from unmodified PBT material can exhibit bleeding/exudation under certain conditions, affecting part functionality. When PBT materials are used in components like refrigerator compressor mufflers, motor coil bobbins, or air conditioner insulation frames, the unique operating conditions can cause exudation of large amounts of small molecules. These can dissolve in refrigerants (e.g., Freon, dichlorodifluoromethane) and potentially clog refrigerant lines, leading to cooling failure.

Bleeding substances from PBT mainly consist of small-molecule oligomers from the resin itself and small amounts of additives. Using glass fiber, high-viscosity resins, and filling with a certain amount of adsorbent can reduce PBT's bleeding/exudation.

5. Dielectric Properties

The application of PBT materials in integrated circuits and electromagnetic shielding places importance on its dielectric properties, which affect signal transmission speed and loss. In recent years, stricter requirements have been placed on the dielectric properties of insulating materials, demanding dielectric constants not exceeding 2.8 for insulating resin materials. Pure PBT's dielectric properties cannot meet communication requirements, making the development of PBT materials with low dielectric constant and low dielectric loss significant.

Currently, modifying PBT's dielectric properties mainly involves filling or blending with low-dielectric-constant copolymers. Commonly used fillers include polytetrafluoroethylene (PTFE) powder and hollow glass microspheres. Carbon nanotubes also have a positive impact on PBT's dielectric properties, but excessive addition can increase both dielectric constant and dielectric loss.

With the rapid development of electric vehicles (EVs), high-voltage transmission imposes higher demands on the dielectric strength of connector materials. Although PBT has good arc resistance and is suitable for high-speed molding, its low dielectric strength prevents its use for high-voltage transmission in EVs. Modifying PBT's dielectric properties with conductive or ceramic fillers can improve dielectric strength to ensure EV safety.

II. Applications of Modified PBT Engineering Plastics

1. Automotive Field
With the gradual development of "replacing steel with plastics," more non-ferrous metals and alloys are being replaced by plastics. PBT's good chemical resistance, stress crack resistance, wear resistance, weatherability, aging resistance, and high strength make it widely used in automotive exterior parts. Examples include wiper arm brackets, bumpers, door handles, mirror housings, underbody panels, body side panels, radiator fans, radar-transparent covers, corner grilles, and lighting components.

Due to its good processability and insulation properties, PBT is also extensively used in automotive interior components, such as instrument panels, accelerator/clutch pedals, ashtrays, and interior mirror brackets. Furthermore, due to its good oil resistance, PBT is used in automotive engine system parts like fuel system components and spark plug sub-plates. PBT modified via alloying has been applied in recent years to parts like shock absorber bushings and bearings. Modified PBT, with its advantages of good flame retardancy, dielectric properties, low warpage, and low water absorption, is also widely used in automotive engine components.

2. Electronics/Electrical Appliances Field
Due to its low dielectric properties, low warpage, high flame retardancy, high toughness, aging resistance, and environmental friendliness, PBT is widely used in electronics and electrical appliances. Examples include computer housings, igniters, electrical switches, copiers, transformer bobbins, toaster parts, and iron covers. Moreover, modified PBT with excellent dielectric properties and easy processability can be used for appliance base covers, housings, and bobbins.

Communication equipment also extensively utilizes PBT, e.g., connection boxes, network ports, and mid-frames for mobile phones and laptops. Modified PBT is also used to manufacture energy-saving lamp cap components.

3. Machinery and Equipment Field
Due to its high flame retardancy and heat resistance, PBT is widely used in machinery and equipment, such as cams, gears, camera parts, watch cases, mercury lamp covers, and various buttons. Common coil bobbins require materials with high insulation breakdown strength to avoid electrical breakdown during use. When used in components like refrigerators, low bleeding/exudation is also necessary to prevent small-molecule exudates from causing mechanical component failure.

Due to PBT's excellent flame retardancy, good flowability, low dielectric properties, and easy moldability, it is used in manufacturing cooling fans, such as those for computer CPUs, power supplies, motors, and similar heat sink applications.

4. Communication Field
Due to its good dielectric properties, processability/moldability, dimensional stability, and relatively low linear expansion coefficient, PBT is widely used in the communication field. In wireless communication, Fe₃O₄ nanoparticles are added to PBT composites to increase electromagnetic wave absorption for magnetic shielding, reducing electromagnetic radiation hazards to humans. This makes it suitable as a plastic substrate for basic components in high-power communication equipment.

PBT is also used to produce connectors for signal transmission. Modified PBT not only possesses the required insulation, flame retardancy, and weatherability for connectors but also offers cost advantages and good moldability, making it suitable for connector production. It is widely used in TV and network cable ports, and for connections and transmissions between various unit components in new energy vehicles.

III. Conclusion

Several different modification methods have been developed to address specific performance deficiencies of PBT engineering plastics in various application fields. Through modification methods such as filling, blending, and preparing nanocomposites, PBT can achieve excellent high strength, high flame retardancy, low warpage, low bleeding, and low dielectric properties, meeting requirements in automotive, electronics/electrical, optical fiber, textile, and other fields.

Future efforts should focus on developing PBT materials that align with low-carbon environmental goals and the high-quality material demands of various industries. By advancing modification technologies to accelerate the development of functional PBT products, combining multiple modification strategies can avoid drawbacks associated with single methods, leading to PBT materials with high added functionality. Emphasis should be placed on expanding PBT material applications in thermal conductivity, biotechnology, and electromagnetic shielding, enabling its use in an increasing number of fields.