TPE technology enables direct adhesion to EPDM rubber

The automotive industry is the largest and fastest-growing market for thermoplastic elastomers TPEs. Kraiburg TPE has identified an increasing demand for TPE compounds for high-quality exterior applications and has speeded up its application-specific development of efficient TPE solutions for this industry.

The latest developments include TPE compounds featuring adhesion to ethylene propylene diene monomer rubber (EPDM). The materials from the Thermolast K series have been specially developed for two-component parts used for automotive exterior applications and feature good adhesion to EPDM and excellent UV resistance. The new compounds are primarily intended for automotive applications such as window trims and sealing profiles made of EPDM with molded corner joints and end elements made of TPE.

The new TPE technology for two-component parts with EPDM is intended primarily for economical production of window trims and other sophisticated exterior automotive profiles. (Photo: © 2018 Kraiburg TPE)

TPE compounds and EPDM materials show excellent adhesion even with a small contact surface. (Photo: © 2018 Kraiburg TPE)

Grades TC7EAZ and TC7EFZ with adhesion to EPDM combine good adhesion properties to EPDM with excellent long-term resistance to UV radiation and thermal effects with a hardness of 70 Shore A. Weathering tests have shown that the surface quality of  Kraiburg TPE compounds is superior to that of competing materials. In developing the individual compounds, the TPE specialist focused on adhesion (TC7EAZ) and processability as well as on low fogging values (TC7EFZ).

Particularly with large vehicle series, styrene-butadiene compounds (SBC) and crosslinked thermoplastic vulcanized (TPV) rubber have mainly been used for these applications in the past. Because the geometrical designs vary between the vehicle series, cost-effective processing of these materials is limited. Kraiburg TPE’s material solution, with adhesion to EPDM when processed by injection molding, offers new opportunities.

The application of the advanced EPDM adhesion series has been successfully tested in practical collaboration with LWB Steinl, a machine manufacturer, as well as in comprehensive laboratory tests and customer tests. To start with, the focus was on adhesion to EPDM. In addition to adhesion tests of EPDM/TPE specimens in accordance with the common VDI-2019 standard (at ambient temperature and after 1,500 hours at 90°C), tests based on ISO 37 were also performed. Along with tensile stress and percent elongation, the absolute force required to destroy the specimen was determined.

In these adhesion tests, the new materials received good results with both foamed and compact EPDM workpieces. The initial adhesion plays a decisive role in allowing the finished parts to be easily removable from the mold with the shortest possible cycle times. In addition, the test series showed that adhesion mainly depends on the cleanness of the inserted EPDM profiles. To achieve optimal and reliably reproducible results, the profile parts should be cut within as short a time as possible before the TPE component is molded – the fresher the cutting, the better the adhesion.

The key requirements for exterior vehicle components also include lasting heat resistance and weathering resistance even with increased UV radiation. This was simulated by exposing material specimens to hot air in accelerated aging tests (1,500 hours at 90 °C and 120 °C) and to environmental conditions resembling those in Florida (heat and humidity) and the Kalahari Desert (heat and drought), for example, in weathering tests. In all of these weathering tests the AD/EPDM/UV series proved to have stable mechanical properties and featured significantly dry and non- adhesive surfaces.

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