Ultradur® HR – PBT for Hot-damp Environments
The main feature of the Ultradur® product range with the suf-fix HR (=hydrolysis resistant) is the extremely high resistance of the PBT polymer (PBT=polybutylene terephthalate) to damage due to water at elevated temperatures.
Contact with water in polyesters, even in the form of atmospheric humidity, leads to hydrolytic cleavage of the polymer chains and thus to a weakening of the material, particularly at elevated temperatures.
Ultradur® HR is based on standard PBT, but incorporates highly effective additives which greatly retard the hydrolytic degradation and can therefore considerably extend the life of a part.
BASF offers a range of HR-modified Ultradur® grades which are notable not only for their high hydrolysis resistance, but also for their advantages in processing.The portfolio shows the following Ultradur® grades with 15% glass-fiber reinforcement (G3) or 30% glass-fiber reinforcement (G6):
Both product groups, Ultradur® B4300 G6 HR and B4330 G6 HR, have a comparable hydrolysis resistance. The B4330 G6 HR grades are also impact-modified. This is an advantage, if there is a danger of stress cracks due to contact with alkaline media. Alkaline media can be formed for example as a result of corrosion processes on metals.
For critical applications such as those in automotive electronics, long life and reliability are basic requirements. It is often possible that the average operating temperature in modern electronic components increases, e.g. due to compacting.
It is also possible that the components are used where they are exposed to the waste heat from the drive system to a considerable extent. Naturally, the plastic parts should be functional in all climate zones on earth, even in damp hot conditions.
If spray water and road salt play a role, this can also increase the demands on the plastic. Therefore, the level and duration of the stress are key factors as to whether or not an application is feasible using a PBT without improved hydrolysis resistance.
Today, the specifications for a number of plastic applications in the automotive sector include tests at elevated temperatures and humidity or tests on changing climatic conditions. These tests can only be passed by HR-modified PBT grades.
Typical applications for Ultradur® HR can be found in auto-motive electronics, e.g.:
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 has 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 suc-cession 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 rheologically effective; to be more specific, they tend to increase the melt viscosity. At normal processing temperatures, this effect is greater as the resdence time increases. Unattractive speckled surfaces on the parts (known as marble effect) are fairly harmless conse-quences of this.
In extreme cases, for example, hot runners can be blocked which inevitably leads to production inter-ruption and costly cleaning of molds and injection molding machines.
Ultradur® HR is optimized in such a way that the melt vis-cosity 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.
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