Advancements in continuous elastomer processing on twin screw extruders. (Process Machinery).

Today's global marketplace continues to push the boundaries of quality and cost economics. Quality is being measured according to both cosmetics and functionality, while cost economics continue to be driven by throughput and off-spec product. Fully co-rotating, intermeshing, twin screw extruder (CRITSE) compounders have been meeting these demanding needs for many years by providing good dispersion, a higher level of formulation consistency and lower overall heat history than any other batch or continuous process. However, since the introduction of continuous processing for rubber compounds on ZSK CRITSE compounders, long L/D process configurations have had to be utilized, typically 32 to 48 L/D (figure 1). This was required to spread out the introduction of the compound components to aid in maintaining lower process temperature and prevent scorching due to the rapid rise in temperature induced by the high shearing action of traditional kneading elements. By spreading out the unit operations of mastication, distributive mixing, dispersive mixing, extending and discharge, the temperature rise was controlled, and successful one pass mixing was achieved. This was accomplished at the expense of rate. The maximum rates achievable were always limited by the shear induced heating, which exceeded the limit placed on process temperature.

[FIGURE 1 OMITTED]

To overcome the shear induced temperature limitation, Krupp Werner & Pfleiderer developed a new type of mixing element that reduced the amount of high strain imparted to the elastomer. The tapered kneading block design maintains the self-wiping feature inherent in a CRITSE compounder, while stalling the flow sufficiently to provide good compounding without introducing the high transient pressure pulses of conventional kneading blocks. This novel design has allowed a wide range of rubber compounds to be processed at significantly higher rates than previously attainable. In the following case studies, we will examine the results comparing standard kneading elements to tapered kneaders.

Carbon black - EPDM

The carbon black reinforced EPDM consisted simply of EPDM and carbon black. The process was developed on a 20:1 L/D ZSK 40 mm compounder, similar to the design shown in figure 2. All of the dry ingredients were metered into barrel 1 by a loss in weight feeder. The final barrel was operated in an open discharge mode. The quality of the mix was determined by visual inspection and by examining for free carbon black.

[FIGURE 2 OMITTED]

Discussion

The total feed rate was set at 52 kg/hr., and the screw speed was varied from 125 to 225 rpm for the two types of kneading elements. Figure 3 shows the effect of the new element design on the energy input to the material. It is important to note that the new element design not only introduces less total energy into the material, but the amount introduced remains virtually flat over the range of screw speeds examined. This opens up a range of processing throughputs to the elastomer industry that were previously unobtainable due to the low screw speeds needed to maintain acceptable shear rates.

[FIGURE 3 OMITTED]

Foamable EPDM weatherstrip

The automotive weatherstrip compound consisted of EPDM, carbon black, whiting, process oil, curatives and an azo blowing agent. The process was developed on a 20:1 L/D ZSK 40 mm compounder (figure 2). All of the dry ingredients were metered into barrel 1 by a loss in weight feeder with the process oil injected in barrel 3. The final barrel was operated in an open discharge mode. Previous attempts at compounding this formulation had required a 44:1 L/D configuration, with the carbon black and temperature sensitive ingredients added downstream to minimize heat buildup, scorching and decomposition of the blowing agent. The quality of the mix was determined by visual inspection and by examining the results from moving die rheometry (MDR) for signs of curing or degradation of the blowing agent.

Discussion

The physical properties of the material made with the equipment in figure 2 were equivalent to those for materials manufactured by traditional batch processes. Therefore, this type of formulation can be effectively transferred to a continuous process through the use of the new tapered kneading elements.

EPDM roofing compound

The goal in compounding the EPDM roofing material was to obtain an improved rate, while maintaining the process temperature below 110 [degrees] C. The initial process for this compound employed conventional kneading elements and had required the use of a 32:1 L/D machine (figure 4). The EPDM, carbon black, cure package and clay were fed into barrel 1. The process oil feed required the use of two gear pumps and was injected separately into barrels 3 and 7. Replacing the conventional kneading elements with the tapered kneaders allowed the process section to be reduced from eight to five barrels, yielding the same configuration employed in the earlier examples (figure 2). As in the earlier examples, the process was monitored for specific mechanical energy (SME) and discharge stock temperature. Product quality was determined by visual inspection and by MDR.

[FIGURE 4 OMITTED]

Discussion

In the standard configuration (figure 4), the discharge temperature was 124 [degrees] C at a screw speed of 153 rpm and a throughput of 100 lbs./hr. Examination of the screw revealed that full mixing was achieved within the first two sets of kneading elements. This prompted reconfiguration to a five-barrel design (figure 2).

A combination of standard and tapered kneading elements decreased the SME from 0.105 kW-hr./kg to 0.089 kW-hr./kg with a resulting drop in discharge stock temperature to 115 [degrees] C (table 1). To further reduce the stock temperature, the remaining standard kneading elements were removed. This resulted in a discharge temperature of 104 [degrees] C with a SME of 0.064 kW-hr./kg. Material was produced at up to 300 lbs./hr. and met the maximum discharge stock temperature criteria of 110 [degrees] C. A typical volume scale up from a rate of 300 lbs./hr. on a ZSK 40 would yield a result of 9,500 lbs./hr. on a ZSK 133.

Compounding of fluoroelastomers

The objective of this study was to explore continuous compounding for fluoroelastomer based compounds. The fluoroelastomer fomulation consisted of fluoroelastomer, carbon black, acid acceptors and a wax. The compounding process was developed on a 29:1 L/D ZSK 30 mm compounder cascaded into a 7:1 L/D 60 mm single screw which could be operated in a pelletizing mode or fit with a slit die to directly extrude test strip specimens. All of the dry ingredients were metered into barrel 1 by means of two loss in weight feeders. Product quality was monitored for dispersive mixing by scanning electron microspectroscopy/energy dispersive spectroscopy (SEM/EDS) with particle size distribution analysis. Formulation consistency was determined by wavelength dispersive x-ray fluorescence (WDXRF) and monitored periodically during extended trials to assess process consistency. Adverse temperature and shear effects of the process on the product were measured by MDR.

Results

Screw configuration and degree of fill significantly affected temperature, particle dispersion and particle size distribution. Optimization of the process resulted in particle size distributions equivalent to those obtainable in two-pass batch operations. The cumulative particle size distribution shows greater than 85% of the granular material to be composed of less than 10 [micro]m particles (figure 5).

[FIGURE 5 OMITTED]

Consistency of formulation is often a concern in a continuous process due to the need to continuously meter the formulation into the compounder. Figure 6 presents the WDXRF results for concentration of the two acid acceptors used in the formulation. Another measure of process consistency for rubber products is the t'50 obtained from the MDR cure curve. Figure 7 presents data for a trial lasting approximately two hours and includes a refill cycle of the gum feeder. Figure 8 presents t'50 results of a four hour run. Only a slight downward trend in the t'50 results was detected. Throughput is always a consideration in reducing the cost of a product, and while a ZSK 30 mm is a laboratory scale machine, it is worthy of note that these data were generated from a process running at 80 lbs./hr. As to the possibilities of higher throughput, table 2 shows the relationship between rpm, throughput and material temperature for a masterbatch material produced on a pilot scale ZSK 58 mm compounder.

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Conclusions

The examples presented herein speak to the level of product quality and throughput that can be readily achieved when processing elastomers with twin screw compounding equipment. As the demand for higher quality products at lower costs continues to weigh on today's production techniques, the differentiation between batch and continuous process will continue to grow.

Table 1 - discharge stock temperature and SME
as a function of kneader design

                                Discharge stock
Screw config.         SME         temperature
(kneader type)     (kW-hr/kg)    ([degrees] C)

5 x STD              0.105            124
1 x STD - 1x BKB     0.089            115
1 x BKB              0.064            104
Table 2 -  discharge stock temperature as a
function of screw speed and throughput

Screw speed   Throughput     Compound
   (rpm)       (lbs/hr)     temperature
                           ([degrees] C)

    190           662           139
    250           828           139
    400          1500           139