Polybutylene terephthalate (PBT) resin has superior mechanical properties, heat resistance, and molding properties. Filling it with glass fibers can drastically improve its strength and rigidity. Since it can also easily be made flameproof with the addition of flame retardants, this resin is used extensively for a variety of industrial products in fields ranging from automotive to electrical and electronics. However, in some cases PBT resin could not be used due to concerns about its hydrolysis resistance and the required level of heat shock properties for insert moldings with harsh usage environments. Here we will introduce special grades of DURANEX^® PBT which offer significant improvements on these concerns and can be used for an even broader range of applications.

PBT resin belongs to the same family of polyester resins as polyethylene terephthalate (PET). It has an uninterrupted ester bond comprised of dicarboxylic acid and diol components within its molecular structure. This ester bond is susceptible to molecular chain breakdowns (hydrolysis) due to moisture, particularly in high-temperature environments, which can cause the resin to deteriorate. Therefore, the hydrolysis resistance of PBT is considered one of its key properties for ensuring the long-term reliability of products.

Insert molding that creates composites of resins and metal parts is often used for products that are made of PBT in order to simplify the production process, reduce the number of parts, make products more compact, and other such purposes. Insert molding is an extremely useful method, but when there is large temperature variation in the usage environment of the product, the potential for differences in linear expansion coefficient between the resin and the metal to cause fatigue breakdown of the resin must always be taken into consideration. Materials have the properties of expanding at high temperatures and contracting at low temperatures, but the degree of expansion or contraction (linear expansion coefficient) differs from one material to the next. Resin has a higher linear expansion coefficient than metal (higher rate of expansion and contraction due to temperature changes), so when products are formed with resin enveloping metal, the metal resists the contraction of the resin due to falling temperatures in the usage environment. This causes strain in the inner part of the resin. If temperature repeatedly rises and falls in these circumstances, the damage from strain will accumulate and ultimately fatigue breakdown will occur.

Polyplastics offers DURANEX 201HR and the DURANEX LT Series for products that require these long-term characteristics.

DURANEX 201HR is a material with drastically improved hydrolysis resistance, which is an issue in the molecular structure of resins, while also maintaining the excellent mechanical and molding properties of conventional PBT resins. Also ideal for use in high-humidity environments, DURANEX 201HR is a material that can extend the usage life of our customers' products. It is particularly well-suited for automotive connectors which function in particularly harsh usage environments.

Table 1 : Properties of DURANEX^® PBT 201HR

Density

ISO1183

Charpy impact strength
(notched)

* Nominal strain at break
The values above are representative measured values and are not minimum values for material standards.

3.2 DURANEX^® LT Series - PBT with heat shock and hydrolysis resistance
      (glass fiber-reinforced)

The DURANEX LT Series drastically improves heat shock resistance by improving the toughness of the resin as well as relaxing strain due to heat shock cycle testing. Its performance even surpasses that of polyphenylene sulfide (PPS) resin, which is known to be one of the top resins in terms of heat shock resistance.

The DURANEX LT Series also has highly superior hydrolysis resistance, unlocking the potential for high performance and longer usage life in our customers' products. These grades can also be used in a wide range of parts for which conventional PBT resin was not an option. These materials are well-suited for use in parts for power modules, DC/DC converters, motor insulators, EPS systems, which are subjected to harsh usage environments.

Table 2 : Properties of the DURANEX^® LT Series

Density

ISO1183

Charpy impact strength
(notched)

The values above are representative measured values and are not minimum values for material standards.

Hydrolysis and heat shock are considered to be barriers to long-term reliability. Here we have introduced Polyplastics DURANEX 201HR and LT Series which are designed to solve these problems, with a focus on explaining the mechanisms behind the problems. These special grades are materials with drastically improved long-term characteristics of hydrolysis and heat shock resistance while maintaining the excellent properties of PBT resins. We hope these suggestions can help our customers design even better products and parts.

[Related information]

• DURANEX^® PBT 532AR Delivers Alkali Stress Cracking Resistance Properties

• DURANEX^® PBT Flame and Hydrolysis Resistant Grade 750AM

• Resin materials suited to high-voltage automotive environments

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14 th September 2020