Powdered rubber technology - an equipment concept.

In the late 70s and early 80s, powdered rubber technology was developed considering economical and quality aspects. Compared with the conventional batch mixing process with its well-known disadvantages, powdered rubber technology features a continuous mixing/compounding process with well-known advantages:

* more uniform product quality;

* reduced processing cost.

The industrialization of this technology can only be successful if raw material producers, equipment suppliers and rubber processors work closely together.

Raw material producers - supply raw materials in powder or crumb form to meet the requirements of the equipment supplier and rubber processor.

Equipment suppliers - develop a process and equipment concept together with the raw material supplier and rubber processor in view of quality and economics.

Rubber processors - cooperate in process development and compound reformulation to meet specific compound requirements.

Even though raw material producers and equipment suppliers put considerable effort into the development of powdered rubber technology, no major breakthrough was achieved.

The main reason for this lack of success must be contributed to the availability of powdered or crumbed raw materials. Today most of the elastomers are still being delivered in bales which require an additional, expensive grinding process, making the entire powder rubber process uneconomical.

Equipment concept

The introduced equipment concept was developed in close cooperation with Bayer AG in Germany. The process schematic (figure 1) shows the powdered rubber technology concept, starting with baled rubber, powdered rubber or crumbed rubber directly from the polymerization process.

The following differentiation in powdered rubber technology has been made:

* Direct processing. Using mostly single screw extruders did not lead to satisfactory results. Insufficient mixing and shearing in the extruder must be contributed to the lack of success. However, direct processing using injection molding machines led to positive results.

* Centralized processing. For preparation of a final compound including cure package, the flow chart in figure 2 shows the described process on which this article is concentrated.

After grinding or polymerization, the rubber particles will have a particle size of 40/1,000" to 60/1,000". Based on the recipe of the compound, the individual components will be metered into a high intensity mixer (discontinuous) and thereafter precisely fed to a planetary gear extruder for continuous mixing and plasticizing.

For forming or shaping of the finished mixed rubber compound, the material will be extruded through a die. Since the planetary gear extruder does not have sufficient pressure build up capability, a single screw extruder in series will have to be used.

The finished compound can be shaped into pellets, strips or can inline be transformed into the final shape of profiles or sheets in combination with a calender. Also, straining can be added in line.

This process offers the following advantages:

* improved compound uniformity;

* improved physical properties;

* lower overall energy consumption;

* shorter heat history (time/temperature relationship).

Mixing principle

The described process has been developed using a planetary gear extruder as a continuous mixing and plasticizing device. Planetary gear extruders are widely being used in the plastics industry for continuous mixing and plasticizing of PVC and other temperature sensitive polymers.

The preblended powdered rubber mix is continuously fed to the planetary gear extruder by a volumetric or gravimetric feeder depending on accuracy desired. A feed screw conveys the powder mix into the planetary gear section for intensive mixing and plasticizing.

The mixing and plasticizing section consists of a driven main spindle, a fixed extruder barrel and a predetermined number of planetary spindles located symmetrically between the extruder barrel and the driven main spindle. By rotation of the main spindle, the planetary spindles are forced to rotate. To avoid the spindles from moving in axial direction, a so-called thrust ring is arranged at the barrel discharge side. The main spindle as well as the extruder barrel are intensively heated and cooled by circulating water.

Mixing and plasticizing in the planetary gear extruder can best be compared with the actions of a two-roll mill. Intensive shearing of the rubber is achieved in the gap between the main spindle, extruder barrel and planetary spindles. The compound is continuously rolled into a thin film under high shear and thereafter relaxes in the barrel voids between the planetary spindles. The large surface of the extruder barrel and the main spindle created by the teeth profile as well as intensive surface removal provides for excellent heat transfer and avoids overheating of the compound.

Screw speed and number of planetary spindles determine shear energy input and can be optimized for specific compounds.

The 5 [degrees] helical teeth ensure pressureless conveying and provide for self-cleaning of the planetary section.

The pressureless compound discharged from the planetary gear extruder is consequently transferred in a close coupled hot feed extruder for pressure build up and forming into the following:

* pellets;

* sheets;

* profiles;

* calendering, etc.

Performance data

Figure 3 shows test results which have been achieved with a planetary gear extruder, Type WE 140. Several different compounds have been processed and the results such as output rate, specific energy and compound temperature are plotted as a function of screw speed of the planetary gear extruder.

A total specific energy of the planetary gear extruder WE 140 and discharge extruder GE 150 of approximately 0.16 kwh/kg is required. This compares very favorably with an internal mixer/two-roll mill combination with an energy requirement of 0.26 kwh/kg.

Based on the current available planetary gear extruder sizes, output rates up to approximately 5,000 lbs./hr. are possible (figure 4).

Conclusion

This machine concept has been developed for powdered and crumb rubber technology and even though the results are positive, this process found very little acceptance in the rubber industry.

With the availability of more rubber raw materials in free flowing powder and crumbs, it is expected that this technology may be revived and again become important.

Continuous mixing of elastomers is still a priority being pursued by raw material producers, equipment suppliers and rubber processors.