Continuous mixing of EPDM rubber with a co-rotating twin screw extruder.

In the past few years, Maris has cooperated with some leading companies in the development of an extrusion process that is suitable for making rubber compound continuously.

In the field of compounds for technical applications, an important improvement in terms of productivity, process simplification and quality also comes from the availability of new materials, such as the granular Nordel MG EPDM rubber made by DuPont Dow Elastomers. A reference formulation supplied by Trelleborg Industri AB has been tested using the grade NDR 37060.02, and final blend properties have been compared to standard production.

This type of formulation currently is used in the application of hose extrusion.

The most important tasks were to explore the quality as a function of process parameters, the maximum throughput achievable, the process stability and also the process flexibility.

Many different conditions of temperature profile, screw profile, screw speed and feeding solution have been tested. For each condition, some key feature such as Mooney viscosity, hardness and tensile properties of the blend have been measured.

An important solution to simplify the feeding step of the extruder was accomplished with the benefit of the rubber features. This made it necessary to only require two gravimetric feeders in which to feed the solid raw materials.

The granular rubber can be pre-blended with carbon black and/or oil by using a turbo-mixer. All the powdered chemicals had been pre-blended using the same method.

The trials were performed on a TM 58 HT extruder, which led to excellent

results with regard to productivity and stability of the process. As shown in figure 1, the extruder is able to keep the blend temperature within a safe range, even during the use of higher extruder screw speeds.

[FIGURE 1 OMITTED]

The use of EPDM in granular form leads to significant improvement in the productivity of the machine compared to ground EPDM, so that the specific energy involved is kept lower than 0.12 kW/kg. At the same time, mixing performance is also benefitted.

Both the extruder and screw configuration have been designed to get constant results over a wide range of conditions. Hardness and Mooney viscosity show no significant deviation from reference values. Rheometer tests also give an excellent indication of the process stability and of the machine flexibility. All the tested values stayed within the quality range for the output range from 200 up to 700 kg/h.

Similar results have also been shown for softer and tougher formulations.

Results from this research demonstrate that continuous compounding of rubber for technical goods is a suitable and promising alternative to today's batch mixing process.

This technology works easily and has already been tested with EPDM from ground bales. In addition, the availability of granular rubber Nordel MG is an additional improvement for the process and an advantage for users.

Table 1--reference formulation

   Type           Rate (kg/hr.)         Blend features

Polymer EPDM         130           ML (1+4) @ 100[degrees]C
  NDR 37060.02                              of 55
Filler                67                 Hardness 64
Softener              58                 durometer A
Chemicals             12
Curing agents          2.5