Ultradur^® HR – PBT for challenging environments

Important material properties such as strength, elasticity and impact strength are affected, if PBT is hydrolytically damaged. The figure shows, in the example of tensile strength, how HR modification of Ultradur^® HR grades have a positive effect on the continuous use.

Apart from a slight effect on conditioning at the beginning of storage, the tensile strength remains virtually constant up to the end of the aging test. The reference material without HR modification, however, is clearly damaged and weakened after a third of the time.

Even more rapid damage to PBT may occur under the USCAR2 test conditions, especially in the higher classes 3 to 5. In these tests, the components are exposed to a succession of moisture and heat, in a cycle which is repeated 40 times. In the moisture phase, the component is conditioned and largely saturated with water. The following hot, dry phase simulates the rapid heating of a thoroughly moistened component.

Hydrolytic damage then takes place in the component at high temperatures – and hence with massive acceleration – because the component is unable to rid itself quickly enough of the moisture it earlier stored.

In the highest test class (Class 5), the peak temperature amounts to 175°C. Only PBT materials with excellent stabilization such as Ultradur^® B4330 G3 HR make it to the end of the test (40 cycles) without their properties suffering significantly. Less well-stabilized products show perceptible damage, products with no stabilization even long before the end of the test.

Additives that can be used to improve the hydrolysis resistance are normally also rheological effective; to be more specific, they tend to increase the melt viscosity. At normal processing temperatures, this effect is greater as the residence time increases. Unattractive speckled surfaces on the parts (known as marble effect) are fairly harmless consequences of this.

In extreme cases, for example, hot runners can be blocked which inevitably leads to production interruption and costly cleaning of molds and injection molding machines.

Ultradur^® HR is optimized in such a way that the melt viscosity remains as stable as possible, even with long residence times – ideal conditions for stable and problem-free processing.

In addition to excellent hydrolysis resistance, Ultradur^® B4330 G6 HR also has a much higher resistance to alkaline media that cause stress cracks. Damage due to stress cracks propagates along the emerging microcracks. This also very rapidly leads to a macroscopic fracture.

Unlike purely hydrolytic aging which normally occurs in the entire part, the material changes due to stress cracks primarily appear locally, restricted to the surfaces of the cracks. In fact, a small quantity of a crack-initiating medium can then lead to a (local) failure of the part. A critical alkaline environment can form in metal corrosion processes, for example.

Chiefly at risk are plastic parts that are in direct contact with metal. In the laboratory, stress due to a stress crack-initiating medium is simulated by clamping test bars to a bending jig and coating them with caustic soda solution. Bars that do not have improved alkali resistance completely break under these conditions even after a very short time.

Since stress cracks can occur independently of hydrolytic damage, bars made of HR-modified PBT can also fail in the caustic test. Decisive for the highest possible alkali resistance are additional features such as those in Ultradur^® B4330 G6 HR.

Experience shows that materials that pass the caustic test in the laboratory with good results also perform considerably better in actual applications. So they improve the desired reliability of the part under critical environmental conditions.