In a market where "consumer is king", the key factors influencing product design is maintaining consumer interest through innovation and comfort in usage. There is an increasing demand for "soft feel" on several consumer products like toothbrushes, razors, tool products, etc. Newer designs that appeal to consumers are being constantly introduced for "soft feel" in such products. The major factor influencing innovations leading to soft feel is the need to provide higher level of comfort to consumers and preventing a switch in brand preference.

This is possible because several types of TPES are now available with specific formulations to tailor adhesion to different hard polymeric substrates. These TPES are available from all TPE families like TPO, TPU, SEBS/SBS based TPES. The processors should select an appropriate grade with the desired adhesion to specific substrates. Today, several types of polymers like PP, PS, ABS, SAN, Polyamides are used for consumer products. All these different products have different compatibility to the above TPES. For instance, ABS/SAN as well as Polyamide are not very compatible with TPOS or SBS/SEBS based TPES. Special adhesion promoters/compatibilizers are required to be incorporated in these TPES to achieve good adhesion.

SURFACE HARDNESS

The surface hardness of TPE also plays an important role in developing good adhesion. While very soft feel can be achieved by lower hardness material, these softer products may not be able to provide the adequate adhesion. A balance between softness and adhesion is required.

MELT TEMPERATURE

Another important parameter that decides the level of adhesion of the selected grade is melt temperature. Generally lower melt temperatures would not provide sufficient adhesion. Increased melt temperatures would improve adhesion. However, very high melt temperatures would again reduce the adhesion. The processor should establish the optimum range of melt temperatures in consultation with TPE supplier.

PRE DRYING and AIR ENTRAPMENT

For hard substrate like Polyamide that is moisture sensitive, predrying is must. The inherent moisture would always reduce adhesion. Similarly air entrapment during moulding process should be avoided. Any air entrapment would also reduce the adhesion level between soft TPE and hard substrate. The moulder should ensure that good venting system is available in the moulding machine.

THICKNESS OF OVERMOULDING

Thickness of TPE overmoulding also plays an important role. Lower thickness would obviously reduce the adhesion. It is important to have a thickness of at least 1mm of TPE overmoulding. Besides it is necessary to have uniform thickness. The moulder should have a good facility of shut off valve to ensure that after injecting the TPE no further TPE melt passes through the shut off valve.

FLOW BEHAVIOUR

The flow behaviour of TPE also plays an important role in developing good adhesion. Generally flow length/thickness should be higher than 150 at the operating process conditions(shear rate/stress).

SURFACE FINISH

The surface finish of mould also exerts influence on the adhesion. Very smooth surface will interfere with adhesion. On the other hand very rough surface texture would also reduce the adhesion.

ADDITIVES

Generally TPES are required in various colours. The moulder should establish the right type of carrier resin used to colour the masterbatch that would not hamper the adhesion. For instance, PE carrier could adversely affect the adhesion of SBS based TPE on Styrenic substrates.

CHOICE OF SUBSTRATE

Among the hard substrates Polyamide, SAN and even ABS require more careful considerations compared to PP.
The moulder could however produce an aesthetic product with soft feel that would satisfy the most demanding customers if care is taken on these lines to achieve good adhesion and prevent curling, delamination etc due to poor adhesion.