Compounding and extrusion of TPEs; new developments in equipment.

Thermoplastic elastomers (TPEs) belong to a relatively young group of materials that is continuously gaining in importance. The automotive industry, in particular, contributes to a decisive increase in the overall consumption.

In ideal circumstances, TPEs combine the functional properties of rubber with the processing characteristics of thermoplastics, thus filling the gap between elastomers and thermoplastic materials.

The number of possible structures of TPE materials is almost unlimited. The continuous development of new variants, the multitude of currently available TPE types with various structural features and the different material characteristics as compared to plastics and elastomers require new concepts in order to adapt the thermoplastics processing to the specific requirements of TPE materials.

TPEs can be basically subdivided into two main groups--block copolymers and elastomer blends. These are two-phase systems composed of an elastic soft phase and a thermoplastic rigid phase (figure 1). The aforementioned groups can be broken down into additional TPE classes (figure 2).

[FIGURES 1-2 OMITTED]

Compounding and dynamic vulcanization of TPVs

For many years already, lines for the compounding of TPE (incorporation of fillers, reinforcing agents, plasticizers, etc.) represent an important market. Fully developed solutions in terms of machine design and process technology are available for this field.

The reactive compounding comprising the mixing of rubber and thermoplastics and the simultaneous vulcanization of the rubber component on the same machine goes one step further. The following observations refer to completely crosslinked elastomer/thermoplastic blends.

The physical properties of a TPV mainly depend on the achieved phase morphology (figure 3) (ref. 1). This morphology is determined during the compounding process and the simultaneous vulcanization of the elastomer phase usually carried out on a twin-screw extruder.

[FIGURE 3 OMITTED]

In order to convert the elastomer phase (generally a major domain) in smallest possible crosslinked parts, high shear or elongation flow is required. The process involved is referred to as phase inversion, which means that the excessive raw material quantity (elastomer) is broken down, vulcanized and converted from the continuous phase into the discontinuous phase. This is indispensable in order to ensure a subsequent thermoplastic processing of the TPE-V obtained.

However, during the phase inversion, the shear and elongation flow must not reach a level at which a thermal degradation of the starting material is caused. At the same time, it has to be ensured that each volume element experiences an almost identical process in order to avoid domains that have not been subjected to shear and cause undesired specks or gels in the finished TPV.

Studies on the particle formation in shear and elongation flows (ref. 2) have revealed that a viscosity ratio of approximately one between the thermoplastic material and the elastomer (un-crosslinked) is advantageous for a simple dispersion (figure 4). With a pure shear flow, dispersion can then be reached when the critical capillary number is exceeded.

[FIGURE 4 OMITTED]

However, the tests have also demonstrated that dispersion can more easily be realized with an elongation flow. For this reason, it can be assumed that a combination of shear and elongation flows particularly facilitate dispersion and phase inversion.

New shear and elongation mixing elements

The mentioned studies formed the basis for the development of new shear and elongation elements for co-rotating twin screw extruders. Figure 5 shows a new element of this type. The compound is conveyed into a flow channel that is closed by the screw flights on one side and by a shear/elongation flight on the other. The material is thus forced to overcome the shear/elongation flight. This forced flow ensures at the same time that each volume element is subjected to almost the same load. The risk of non-dispersed parts (specks) is thus very low. In addition to this, the forces flow allows the energy input to be reduced to a minimum, which implies that there is also an extremely low risk of thermal degradation.

[FIGURE 5 OMITTED]

Extensive test series with the new screw elements are currently carried out at Berstorff, e.g., for the production of TPV and the continuous mixing of rubber compounds.

Compact TPE

Due to their rheological properties, most TPE materials cannot be processed on conventional machines designed for the processing of rubber.

The product characteristics are mainly determined by the structure and the phase morphology; with elastomer blends, for instance, by particle size and distribution. Apart from the most important factors for homopolymers, such as orientation and crystallization phenomena, the mechanical properties of two-phase systems are also influenced, to a large extent, by the particle size and distribution in the disperse phase.

An additional decisive advantage of TPV materials over conventional crosslinkable elastomers is that they are generally suited for recycling. As all other plastics, TPV materials exhibit a viscosity decrease as the number of process steps increases. In this case, however, the product properties are not affected in a significant manner. This was clearly confirmed by several tests carried out with different TPV materials

Extrusion lines for the production of mono-component or multi-component profiles made of compact TPE or thermo-plastics/TPE offer a big variability of the profile.

Expanded TPE

In addition to the chemical foaming of TPV materials by appropriate recipe components, Berstorff has developed a low-priced and environmentally compatible process for expanding TPV by means of water (ref. 3).

For this purpose, a defined quantity of water is continuously added under high pressure into the homogeneously compounded TPV melt via the extruder barrel. The water is then homogeneously incorporated into the melt that is subsequently cooled down. During the shaping of the profile, the pressure of the melt decreases so that the finely dispersed water can simultaneously expand. Appropriate cooling conditions ensure that the resulting vapor inclusions are fixed by the solidifying melt. This allows expanded TPV profiles with a density of only approximately 200 g/l to be produced. Berstorff lines are already employed for the production of commercial water-foamed TPV profiles that are used in the automotive industry.

Co-extrusion of rubber/TPE mid hybrid die

The use of so-called hybrid profiles, i.e., crosslinked robber and (expanded) TPV, can be considered as an interesting interim solution on the way towards a complete substitution of robber profiles.

Due to the thermoplastic matrix of TPV raw materials and the resulting compression set data, a dimensionally accurate transfer from rubber to TPV is sometimes rather difficult. In these cases, a partial substitution allows the advantages of rubber, such as low compression set, to be combined with the avantages offered by TPV, e.g., its tactile properties and color.

For these applications, Berstorff developed the so-called hybrid die that ensures a thermal separation of both material flow required due to the different processing temperatures of TPV (approximately 220[degrees]C) and robber compounds (approximately 110[degrees]C) (figure 6). The material flows are joined in the extrusion die just before their discharge and led through the vulcanization section in the form of two-component profiles, with the rubber component being vulcanized at approximately 170[degrees]C. A steel insert or the like for reinforcing purposes can also be integrated.

Conclusion and outlook

In view of the almost unlimited number of possible material combinations and the high growth rate, TPVs represent an extremely interesting field. In the future, they will certainly continue to replace conventional rubber products to an increasing extent.

References

(1.) A.Y. Coran; R. Patel, Rubber Chem. Technology 53, 141 (1980).

(2.) H.P. Grace, Chem. Eng. Commun., 14, 225 (1982).

(3.) Deseke, O. et al. EP 0664197 BI (1999).