The challenges for materials in the automotive drive technology include amongst others high temperatures, moisture and vibrations. In order to be able to withstand such an environment over the service life of a car, the plastics used often have to be specially equipped.
This is also true of thermoplastic polyesters such as polybutylene terephthalate (PBT) which are used under the engine hood, for example for plugs, connectors and housings of electronic components. Contact with water – even in the form of atmospheric moisture – leads in the case of polyesters to hydrolytic splitting of the polymer chains, and therefore to a weakening of the material, especially at elevated temperatures.
Ultradur HR – hydrolysis-resistant PBT
With its Ultradur HR grades (HR stands for hydrolysis-resistant), BASF provides optimized PBT materials which are able to withstand hydrolysis under damp environmental conditions. As a result, the material is suitable for producing components which have a particularly long service life and withstand operation under intensive conditions. This is also true of two new Ultradur grades which combine a high level of hydrolysis resistance with integrated flame retardance and laser transparency respectively and therefore help to develop numerous new applications.
USCAR class 5 – test passed
Furthermore, the Ultradur HR grades from BASF pass not only customary tests but also the demanding climatic cycling tests of class 5 according to the applicable USCAR standard. This test standard has lately been anchored in the specification sheets of the automobile industry more and more frequently. Component tests according to this standard are prescribed, for example, for plugs and connectors. The demands of these component tests go far beyond the customary static climate tests. Already in the past, the specially developed Ultradur HR showed that it can withstand damp and hot environmental conditions without problems. Ultradur HR is commercially available with 30% glass-fiber reinforcement under the name Ultradur B4330 G6 HR, and with 15% glass-fiber reinforcement under the name Ultradur B4330 G3 HR. Both grades are substantially more hydrolysis-resistant than products without HR finish and are also ahead of competing HR-modified materials.
Aging in the climatic exposure test cabinet: stable for 6,000 hours
Test methods with constant climatic conditions are often used to verify the hydrolysis resistance. Common conditions for accelerating testing are, for example, a temperature of 85°C and a relative humidity of 85%. Test specimens made of Ultradur HR grades were typically stored in this climate for up to 3,000 hours and examined in detail after fixed intervals. It was found that the tensile strength remained constantly high over the entire test period.
In order to identify the limits of the material, the test was extended to 8,000 hours for Ultradur B4330 G6 HR. This showed that the tensile strength was gradually impaired only after 4,800 to 6,000 hours. Therefore, Ultradur HR is considerably more stable than all other materials under these conditions.
T esting at tightened climatic cycling: USCAR class 5
The important difference in the component test according to USCAR class 5 is that the heat and moisture load does not act statically on the test specimens but in a cyclical way. The test specimens are subjected to various defined temperatures and atmospheric moisture levels over an eight-hour cycle. In the process, the change in these parameters is chosen in such a way that the moisture absorbed by the plastic does not have any chance to escape before a high-temperature phase begins. This simulates a two-phase process: First, the component is thoroughly wetted in standard climatic conditions, then it rapidly heats itself up in a temporary operating process, either by external or inherent heating. This represents an extraordinarily high loading for a material such as PBT. The hydrolytic damage to a conventional PBT would be very high under such circumstances.
This test cycle is repeated 40 times before the specimens are subjected to detailed examination. In this case as well, Ultradur HR shows extreme resistance and only slight changes to the mechanical properties. Whereas the tensile strength of a non-hydrolysis-stabilized comparison material has already dropped to a great extent even after 20 cycles, the tensile strength of Ultradur HR remains virtually constant even after 40 cycles. It is therefore considerably superior to the competing product.
4,000 hours thermal aging without problems
Ultradur HR performs better than other products even when stored under dry and hot conditions. This is demonstrated by tests for thermal aging for which Ultradur B4330 G6 HR was subjected to a constant temperature of 150°C in a thermal exposure test cabinet for 4,000 hours. Subsequent mechanical tests showed that the impact strength remains at a constantly high level. In contrast, the impact strength of a comparison material without HR finish fell rapidly to a low level. A high level of thermal aging resistance is important when components are heated to high temperatures by external or inherent heating over long periods of time. Since Ultradur HR grades withstand permanent loading at high temperatures in the dry state particularly well, they are also very well suited for applications with a high temperature loading, for example for components in the engine compartment.
New application options in electric cars
On account of this considerably expanded property profile, Ultradur HR is suitable for a range of applications for which PBT has not been considered so far. For example, the high thermal stability together with the well-known good electrical properties of PBT may be of interest for components in electric vehicles. Examples include charging plugs, housings of battery stacks or plugs in the high-voltage circuit of electric cars.
Stable and straightforward processing
Ultradur HR is not only designed to be hydrolysis-resistant, but also to have as stable a melt viscosity as possible. Therefore, the material provides good preconditions for processing even at high temperatures and with relatively long residence times. The optimized melt viscosity prevents esthetically unpleasant flecked surfaces on the components, what is known as the marbling effect. In addition, the tendency to form thermally cracked material, so-called black specks, is low, as is the risk of hot runners being blocked, which can lead to an interruption in production.
New grades: flame retardance and laser transparency
BASF is currently working on developing further hydrolysis-stabilized Ultradur grades. The most recent ones include Ultradur B4450 G5 HR. This new development combines hydrolysis resistance with incorporated flame retardance, a high tracking resistance and low smoke density. This property profile is targeted at electrical mobility, amongst others. Since considerably higher currents flow in electric vehicles than in vehicles with a conventional drive train, the safety requirements made of the components involved also have to be reconsidered.
A further new development relates to the combination of hydrolysis stabilization and laser transparency. Ultradur B4300 G6 HR LT has a laser-transparent black color and specified laser transparency for laser transmission welding. Applications for this include, for example, housings for control units or sensors, which not only have to be hydrolysis-resistant but also be laser-welded from several housing parts.