Polybutylene terephthalate (PBT) resin DURANEX^® has an excellent balance of physical properties. The addition of glass fibers dramatically boosts strength, rigidity, and heat-resistance, while the addition of flame retardant allows for easy fireproofing. Since the resin is relatively easy to modify and add functionality to with additives, it has an extensive lineup of grades on the market, and it is widely used in a variety of industrial products in fields such as automotive, electrical and electronics.

Technological development is particularly robust in the automotive field, where in addition to bolstering the basic functions of cars such as driving, turning, and braking, recent years in particular have also seen development of products and parts that focuses on areas such as safety, comfort, and eco-friendliness. DURANEX has excellent electrical properties for use in materials for parts such as ECU cases, sensors, and connectors, and is therefore being used more and more often for the development of these new parts.

An increase in the number of parts has also left insufficient space to install parts, and in more cases various parts including those that use PBT are being installed in lower areas of vehicles such as the chassis section. Parts installed in the lower areas of cars are close to the road, and they easily come into contact with rust since they are surrounded by metal parts that are prone to rust formation from water and mud splashes.

Illustration 1 shows the mechanism by which iron gets rusted. Iron does not only corrode and get rusted, but the alkaline substances arising from the formation of rust inflict damage on resin, which can cause cracking (stress cracking ※) to occur, which could potentially disable the function of the part.

Snow melting agent applied on the ground in snowy areas is particularly liable to destroy the passive film of iron and make it easier for rust to form, so the stress cracking ability of resin is needed.

Polyplastics successfully brought new grade DURANEX 532AR to the market after engaging in studies on the modification of stress cracking resistance properties of PBT in alkali environments. In stress cracking tests in an alkali environment (using our own testing methods), DURANEX 532AR not only had excellent durability than existing grades but also showed outstanding hydrolysis and heat shock resistance.

This is a phenomenon which is also called environmental stress fracture, in which a resin is loaded with either chemicals or stress and experiences no impact, but cracking and breakage occur when loaded with both chemicals and stress. Cracking can also occur due to internal stress occurring due to residual stress and temperature changes during molding.

Next, we will introduce the characteristics of DURANEX 532AR, which is capable of boosting the reliability and lifespan of auto parts.

DURANEX 532AR has successfully reduced the risk of cracking when molded articles come into contact with alkali by preventing alkali from penetrating the inside of the resin, and imparting toughness that helps generate less stress.

Table 1 shows a breakdown of the characteristics of DURANEX 532AR and some existing grades. We have confirmed that in addition to stress cracking resistance properties, DURANEX 532AR also has equal or better functionality than existing grades in terms of hydrolysis and heat shock resistance required in the usage environment of the chassis section. For this reason it is ideal for use in parts installed in the chassis section and lower areas of cars which can be splashed by water and mud, and come into contact with chemicals such as snow melting agent.

Table 1 : Breakdown of DURANEX^® PBT 532AR

Grade name

Characteristic

Mechanical
properties,
heat resistance

Hydrolysis resistance

Heat shock resistance

Alkali resistance

532AR

GF30% Alkali-resistant

●

●

●

●

531HS

GF30% Alkali-resistant

●

●

●

330HR

GF30% Hydrolysis-resistant

●

●

3300

GF30% Standard

●

[ Evaluation results of stress cracking resistance when immersed in alkali ]

The evaluation results of stress cracking resistance when immersed in alkali (using our own testing methods) in Illustration 2 show that cracking occurs within two hours in standard material DURANEX 3300 and in hydrolysis- and heat shock-resistant DURANEX 531HS, but no cracking occurs in DURANEX 532AR even when immersed for up to 200 hours. This shows us that DURANEX 532AR has excellent stress cracking resistance.

<Testing method>

• Cut the weld part of a 80mm × 80mm × 1mmt test piece with holes into strip form

• Load the strip-form test piece with 1% strain, and immerse it in strongly alkaline 10%

  sodium hydroxide solution at room temperature

• Measure the time it takes for cracking to occur in the strain-loaded strip-form test piece

[Hydrolysis resistance and heat shock resistance of DURANEX^® PBT 532AR]

PBT resin has an outstanding balance of properties and is used in a wide variety of auto parts, but it has weakness to hydrolysis which we could consider to be the only weakness of polyester resins. This is when polymer chains break down leading to degradation of materials in very hot and humid environments. DURANEX 532AR is treated with a hydrolysis-resistant formula, and has much more durability than standard materials. (Illustration 3)

Electrical components and sensors installed in the vicinity of the engine are also often metal insert molded, and there are cases when sudden changes in the surrounding temperature when the engine is turned on or off can cause heat shock fracture problems. This is due to the metal and the resin having linear expansion coefficients that differ by roughly a factor of 10. As we can see in Illustration 4 DURANEX 532AR also has outstanding heat shock resistance. It is a superior grade that is capable of withstanding harsh environments.

[ Market outlook ]

As the demands for durability, safety, and reliability in auto parts continue to increase, DURANEX 532AR not only reduces stress cracking from alkaline substances but also includes measures to counteract heat shock fracture and the weakness of PBT resin to hydrolysis. This grade offers a combination of high reliability and long life even in harsh environments for resins such as around the chassis section and inside the engine room of cars.

Our extensive DURANEX grade lineup is capable of offering the optimal grades for sensors and electrical parts such as ECU cases that are needed in the autonomous driving and safety driving technologies our customers are developing.

At Polyplastics our development efforts will continue in order to keep offering producers the grades that meet the needs and applications of the market.

[Related information]

• Design Technology for DURANEX^® PBT (Only for WEB@TSC members)

 Technical data sheet is available online:

 For inquiries about our technologies
 and materials, please contact us via:

17th January 2019