Flame-retardant TPEs with adhesion to polyamides

Complex adhesion mechanism

It has already been found in previous adhesion experiments with non-flame retardant TPE/PA samples that good binding may apparently be impaired by glass fiber accumulations on the cut surface. Impact-resistant modifiers, stabilizers, colors, dispersants, separating agents and processing aids that migrate to the surface and thus impair the wetting and anchoring of the materials are regarded as other potential influencing variables. 

Along with the formulation of the polyamide, the flame retardant’s temperature sensitivity is a major challenge. In multi-component applications, non-flame retardant TPEs are frequently processed at temperatures of 240°C to achieve optimal adhesion.  

Kraiburg TPE’s newly developed flame-retardant materials have shown optimal adhesion at a relatively low melt temperature of 190°C, which accommodates the temperature sensitivity of the flame retardants. 

As a general rule, the evaluation of adhesion quality has to take into account the break pattern along with the value actually measured for peeling resistance. Due to their reduced mechanical properties as compared to adhesion-optimized compounds without flame retardancy, the flameretardant materials show lower adhesion values despite cohesive failure (break pattern D). 

Custom-engineered solutions

The tests have nevertheless shown that, in specific cases, excellent adhesion to flame retardant PA can also be achieved with flame-retardant TPEs within a hardness range starting at 60 Shore A. The key to this lies in testing the materials combinations that are being considered as early as possible, in order to exclude incompatible compound partners even before the prototype phase and to identify more appropriate TPE/PA combinations in a targeted way. Kraiburg TPE not only has the appropriate facilities and development competence for such projects, but can also on request provide a test report on the tested adhesion properties in accordance with relevant standards such as VDI 2019. 

Product Features THERMOLAST K
Color Natural color / according to customer specification
Properties Test method
Density DIN EN ISO 1183-1 1.05g/cm3 1.05g/cm3
Hardness DIN ISO 7619-1 62 Shore A 69 Shore A
Tensile strength1 DIN 53504/ISO 37 3.0MPa 3.0MPa
Elongation at break1 DIN 53504/ISO 37 291% 342%
Resistance to tear propagation ISO 34-1 B (b) (Graves) 17.1N/mm 17.5N/mm
Adhesion to PA62 VDI 2019 ~ 4N/mm (D) ~ 4.9N/mm (D)
Nonflammability UL94 V-0 at wall thicknesses of 3mm
  1. Notwithstanding ISO 37, tested at a feed rate of 200 mm/min
  2. Depending on specific PA formulation, glass fiber content, modifiers and processing aids

Appendix 2 summarizes the key properties of two THERMOLAST compounds that have performed best in the adhesion tests. Both materials are classified as V-0 starting from wall thicknesses of 3mm and provide very good processing properties in injection molding. The color of these compound is natural, they can be individually colored. 

The wide range of potential multi-component applications made of flameretardant TPE/PA combinations in electrical engineering and electronics extends from plug-in connectors, to relay components and circuit breakers, as well as switch boxes and cable bushings. Further possible important areas of application are motor vehicles, commercial vehicles, rail vehicles and aircraft, as well as construction and building technology. 

Kraiburg TPE will once again be presenting itself as the competence leader in the thermoplastic elastomer (TPE) market during the forthcoming K 2019 from 16 to 23 October in Düsseldorf. At Stand C58-4 in the "Gummistraße" in Hall 6 and at the "Innovation Center" Stand E22, also in Hall 6, the manufacturer will address key challenges and trends in the growing TPE business. 


The diverse range of modified polyamides and their different compositions make it harder to select suitable compounds for multi-component applications with TPEs, especially if the material properties required include high nonflammability. In order to find the best possible hart/soft combination with secure adhesion and halogen-free flame protection, thorough compatibility tests are required that are based on a solid understanding of adhesion mechanisms and on how additives affect adhesion. Ideally, processors and end-users should therefore have material suppliers involved at as early as possible a stage during the process of application development, in order to avoid expensive surprises and ensure that their products will be launched on schedule. Kraiburg TPE has extensive expertise in formulating flame-retardant TPEs and making them reliably compatible for creating compounds with polyamides, and also supports its customers with relevant adhesion tests in accordance with the VDI 2019 standard. With this extension it is possible to extend application areas and market opportunities.  

Source: Kraiburg TPE