Thermoplastic elastomers bridge the gap

Dr Jurgen Kautt, marketing manager of Advanced Elastomer Systems (AES) in Brussels, describes four new types of TPE grades and some new manufacturing processes too.

Engineered thermoplastic elastomers (TPEs) combine the performance characteristics of` vulcanised rubber with the ease of processing usually associated with thermoplastics. Many grades have been commercialised over the past two decades and a number of new grades have recently been developed.

Thermoplastic elastomers bridge a gap between thermoplastics and thermoset rubbers. TPEs are highly elastic and rubber-like and show properties that are very close to thermoset rubbers. The key benefit of TPEs is that they can be processed on conventional thermoplastic machinery, offering the potential for cost savings when compared to the very complex and labour and cost intensive manufacturing process associated with thermoset rubber.

An additional benefit of TPEs is their versatility – they can be combined with rigid thermoplastic materials such as polypropylene in new designs that allow functional integration, improved aesthetics and lower part cost. What is more, both parts and scrap can be recycled.

The two main groups of TPEs are the block co-polymers and the polymer/elastomer alloys. Thermoplastic vulcanates (TPVs) demonstrate material performance that most closely matches that of thermoset rubber, ie heat ageing, oil resistance, weather resistance, mechanical properties, rubber-like appearance and soft touch. These TPVs are also sometimes referred to as engineered TPEs, elastomeric alloys (EAs) or dynamically vulcanised alloys (DVAs).

TPVs are manufactured by dynamically vulcanising the rubber during the mixing step with thermoplastic. The end product is a two-phase structure with micro-rubber particles as the heterogenous phase distributed in thermoplastic as the continuous phase.

Polyolefin-based TPVs originate from the introduction in 1981 of Santoprene thermoplastic rubber, an EPDM-PP fully vulcanised TPE. Due to the balance between performance and processing, Santoprene rubber is used in many applications as thermoset rubber replacements or as a complement to thermoset rubber parts. In the 1990s, an important area of development has been to improve the processing properties of polyolefin-based TPVs.

Injection moulding TPEs are used in more complex mould designs, such as designs with long flow lengths, thin sections and low pressures for overmoulding or multi-material (2K) processes. Multi-material or dual (2K) injection moulding is the injection of two or more (compatible) materials in any one moulding cycle. These applications are often more demanding, requiring an excellent surface finish together with shorter cycle times.

Recently, four new types of TPE grades have become available: high-flow grades, EPDM bonding moulding grades, polyamide bonding grades and the new Santoprene 8000 rubber series of grades with no hygroscopicity and easy colourability.

The Santoprene rubber M100 series has specifically been designed to have a much higher flow rate, while maintaining the key properties of Santoprene rubber: sealability, good oil and heat resistance and weatherability. In addition, the surface quality has improved. The Santoprene rubber M100 series is currently available in three hardnesses: 50, 62 and 75 Shore A, with a new grade at Shore A hardness about to be launched.

IMPROVED FLOW

The flow behaviour of these M100 series grades represents an important enhancement over Santoprene rubber 111 moulding grades. Both hard and soft combination flapper door seals can be moulded with standard TPEs. However, the M100 series grades demonstrate less warpage, less mould shrinkage and faster injection cycles.

Parts made with Santoprene rubber M100 series offer a good surface appearance without blooming. Thermoplastic processing, combined with high flow, has enlarged the processing window for the critical process of glass encapsulation by overmoulding.

In the thermoset rubber industry, corner moulding of EPDM profiles is a widely used technology. Although the cycle time of rubber moulding can still be optimised, cooling of the profiles and trimming of the corners make thermoset corner moulding a cost and labour intensive process.To benefit from thermoplastic processing where trimming is unnecessary and cycle times can be shorter, Advanced Elastomer Systems (AES) has developed two new Santoprene grades that bond to cured EPDM materials. The bonding strength is comparable to that of a thermoset rubber corner moulding compound.

A key advantage for the processor is that the thermoplastic injection moulding process has a short cycle time as low as 25s. Flash finishing can be avoided and there is none of the blooming that occurs with many EPDM corners. The technology also works well with flocked profiles. The new EPDM bonding grades are available in 65 and 79 Shore A hardness. Applications include glass run channels, end-caps, belt line seals.

Two-component systems are showing strong growth in all markets, where PP/Santoprene rubber combinations are widely used. However, there are many applications where PP does not give the impact strength, rigidity, heat distortion or fluid resistance required. In these cases, PA or ABS are normally used rather than PP.

In 1997, AES introduced a new TPV material that can bond to PA 6 and PA 6,6 or their compounds, without the use of any primers and adhesives, by co-extrusion or 2K moulding. Like other TPEs with the ability to bond, these materials show all the touch and feel characteristics of rubber as well as very good heat resistance (up to 135 degrees C) and fluid resistance.

DESIGN SOLUTIONS

These products offer a solution to designers of power tool handles, thanks to their soft and rubber-like feel, combined with vibration reduction. By eliminating the mechanical interlocks, the 2K moulding solution is often much more cost effective.

PA bonding grades offer typical Santoprene rubber properties such as soft touch, and allow for innovative part design, which leads to improved ergonomics.

In response to new demands from several industries, AES, through a proprietary new technology, has introduced fully vulcanised, non-hygroscopic and easy to colour TPVs – the new Santoprene 8000 rubber series. Besides its ability to be easily coloured, it offers the benefits of high resin purity of low extractables. Being fully cured, it exhibits the same engineered rubber properties as current Santoprene rubber grades.

Apart from new materials, AES, in collaboration with several machine manufacturers, has developed innovative technologies that open up some new opportunities for the use of TPVs. Developed by AES and Gepoc Verfahrenstechnik, a robotic extrusion process has been developed that overcomes the difficulties of producing a structural rigid part with complex or hollow soft seals, especially for large parts. To take advantage of this process, a special Santoprene rubber grade (121-60E500) with the appropriate balance between viscosity and performance was developed.

The extrusion system consists of a 6-axis robot that controls an extrusion head in the x, y and z directions. The Santoprene rubber is extruded through a flexible and heated hose connected to the extruder on one end and to the moving head on the other end. This process allows for extrusion onto or around rigid substrates.

Adhesion to the substrate can be achieved using mechanical fixation, a bonding agent or heat welding.

Applications using this technology include belly pans, car engine encapsulation parts and sun roof profiles in the automobile industry. Mercedes Benz used this technology very successfully for the belly pan on the E-Class.

For the future, conventional rubber parts will be replaced by TPEs or combinations of thermoplastic materials. Interest is also growing in the rubber industry, where TPEs are increasingly considered to be more an opportunity and not a threat.

AES Tel: 0116 289 122