Twin screw compounding.

The modern twin-screw extruder differs significantly from the first designs as a result of growing requirements for increased throughput rates and higher quality product. Since being introduced in the 1950s, the twin-screw extruder has become widely accepted as the preferred machine for compounding plastics. It has also shown itself to be a valuable and sometimes indispensable part of manufacturing processes in the chemical and food industries. Today, the vast majority of rubber stock is still batch processed, but with advancements in hardware and improved understanding of unit operations in rubber processing, continuous extrusion compounding of rubber formulations is being given serious consideration.

In the 1940s, Rudolf Erdmenger et. al. designed a fully intermeshing, co-rotating twin-screw extruder. In 1954, the prototype of the first co-rotating twin screw extruder, the ZSK-83/500, was built. This machine had a three-lobe design, utilizing two fully intermeshing screws, one over the other. The machine had hinged barrels (clam-shell type) and the temperature was controlled with oil. Analysis of the performance of this machine led to the design and development of the first commercial twin-screw extruder, introduced in 1957. This machine was available only with an 83 mm screw diameter, a length of 700 mm, and a 60 kW motor.

Modern geometry

The modern ZSK twin screw compounding extruder has evolved significantly from those first designs. Increases in free volume and torque have permitted higher rates for plastics compounding and lower temperature processing for rubber compounding, while maintaining the ability to produce consistently high quality compounds. These improvements have resulted in twice the free volume, twice the torque, five times the screw speed of the earliest designs, and a rate improvement greater than 10 times the original machines. In the 1950s, the ZSK-83 commonly produced 150 Kg/hr. Today, the ZSK-92, the modem version of the ZSK-83, easily produces 1,500 Kg/hr.

Since the early 1960s, the design and construction of the co-rotating twin-screw compounder has been totally modular. Both barrels and elements are assembled for a particular process task. The two screw shafts lie nested side by side within the figure-eight barrel housing.

The barrels are 4/1 length to diameter ratio L/D), are totally interchangeable, and come in various forms which are available to feed materials into the ZSK at any point along the compounder, to allow for liquid injection, again at any point, and for venting or vacuum degassing. All barrels are internally cored for cooling, and are typically electrically heated.

The screw elements come in several designs. The most common units used in rubber compounding are the conveying and the kneading elements. These are mounted on shafts with the appropriate configuration for the particular process task at hand.

Principles of operation

The modular equipment design and the mixing capability of the twinscrew extruder enable the overall process task for a given compound to be broken down to individual unit operations. The unit operations that are normally seen for rubber compounding are:

* meter feeding of ingredients;

* mastication of rubber polymer;

* distributive and dispersive mixing of fillers;

* plasticization;

* discharge and preforming of the finished compound.

These unit operations can be summed up into an overall process configuration. A twin-screw extruder based compounding system for an EPDM automotive weatherstrip compound is shown in figure I (ref. 1). This process configuration is 44/1 LID.

This example is a dense automotive weatherstrip. Dense weatherstrip is used either as an attachment carrier (sponge carrier) for dual durometer weatherstrip or as its own entity. The dense weatherstrip formulation is a typical black compound. In traditional internal batch mixing operations, manufacturers employ a single pass mix, and some a two pass mix. A hybrid mix has the accelerator system added at the mill. The mixing technology of choice is the balance of the real cost of the mixing technique versus the product consistency and scrap rate. Automotive companies have a constant push for quality improvements, both cosmetic and functional. Weatherstripping needs to seal out wind, water and noise, as well as look attractive. This requires consistent compound with superior dispersion to maintain extruded dimensions and defect free surfaces.

The meter feeding of the ingredients is necessary to maintain the formulation recipe and is one of the process variables used to control shear stress. The meter feeding of both the dry and the liquid ingredients use commercially available equipment. Feed locations are available in this example for the dry ingredients at the first, fifth and ninth barrel segments. Oil injection locations are at the third and seventh barrel segments.

Feedstreams

The EPDM used in the dense formulation is a pelletized grade, and easy to feed using standard loss-in-weight feed equipment.

All of the minor ingredients, including the active components, were preblended in a 50 L Drais mixer.

The carbon black was fed as a separate feedstream from the preblend.

The process oil was injected using standard gear pump technology, and using mass flow meters to ensure the correct feed rate.

Feeding rubber polymers can be problematic because the rubber is supplied in bales. However, many grades are currently available in particulate or pelletized form. In situations where the rubber is only supplied in bales, rubber grinders can be used, essentially in-line with the feeder, to provide an agglomerate free particulate.

With rubber formulations having so many dry ingredients, it is not practical to use one feeder for each ingredient. Many of these ingredients can be combined into a preblend, then fed with only one feeder. Shown in figure I are two feeders downstream, one for the carbon black, and one for the balance of the ingredients, including die sulfur cure package. The temperature sensitive ingredients that are used, for example the sulfur and the other components of the cure system, may be added with the preblend, or may be added further downstream to minimize temperature exposure. The residence time of these ingredients in the masticated rubber can be kept to a minimum, usually only 10 seconds, which means a minimal activity reduction resulting from thermal degradation.

The screw configuration is designed around the unit operations that are to occur along the process section of the ZSK twin screw compounder. The mastication of the rubber is the first of the unit operations that is actually performed in the extruder. Conveying elements were used in the first barrel to convey the rubber blend being fed there into three banks of kneading blocks in barrels 2 and 3, to masticate the rubber, and to incorporate the first portion of die process oil that is added at barrel 3. The particular kneading block configuration is used to masticate the rubber, with a relatively slow energy input to be sure small nibs of unmasticated rubber do not float through the balance of the compounder. The first portion of the process oil is added at this point to prevent overheating, and to start extending the rubber. The peak temperature in the process should occur at this point due to the shear heating response of the rubber. This peak temperature is isolated from the remainder of the process, especially from any temperature sensitive ingredients.

The filler preblend and the carbon black are added to the ZSK twin screw compounder at barrel 5, and are being added to the rubber/oil matrix that has already been masticated. This total mass is conveyed to another series of kneading blocks in barrels 7 and 8, where the fillers are both distributively and dispersively mixed into the rubber/oil matrix. The kneading block configuration here must attain the desired dispersion of the fillers without overheating the matrix at this point, and without creating a rate limiting bottleneck. The balance of the oil is added directly over kneading blocks to quickly homogenize it into the matrix.

Compounding of the dense weatherstrip compound formulation is now complete, and in only one pass. This compound was produced at 100 pphr on the ZSK, with a screw speed of 200 rpm. Each of the feedstreams were metered at the appropriate rate to total 100 pphr. The stock temperature at the discharge of the ZSK was 122 [degrees] C. The mechanical energy input to the compound was 0.143 kw-hr./kg. The stock temperature is higher than compound produced on the IA internal mixer, but the residence time is shorter, approximately 45 seconds total.

In this case, the compound is discharged from the ZSK into a single screw extruder for conversion into a strip feed for subsequent processing. This is an integral part of the overall compounding operation that provides an opportunity to create preforms, such as a rope, strip, tube or sheet. This integration of making the preform directly at the compounder further reduces the heat history.

Results

Compound prepared on the ZSK co-rotating twin screw compounding extruder is compared to compound prepared in a 1A internal mixer using standard compounding techniques. This compound is a standard formulation as published by Uniroyal Chemical and appears in table 1.

Table 1 - desne weatherstrip formulation

Ingredients                            PHR
EPDM (IM7200)                         100.0
Carbon black (N650)                   150.0
Whiting                                45.0
Zinc oxide                              4.0
Calcium oxide (in polymeric binder)     8.0
Stearic acid                            1.0
Sunpar 2280                           115.0
MBT                                     1.0
Tuex                                    1.0
Ethyl tellurac                          1.0
Sulfads                                 1.0
Sulfur                                  1.5

The control formulation was mixed in the mixer using a two-pass mixing technique. The first pass generated a masterbatch. The second pass finished the compound with the addition of the active ingredients. The exact formulations for each of the two passes and mixing procedures are listed in table 2.

Table 2- internal mixing procedures

Masterbatch (1st pass)

Ingredient         PHR   Grams/batch
EPDM            100.00     3,220.00
Carbon black    150.00     4,830.00
Whiting          45.00     1,449.00
Zinc oxide        4.00       128.80
Stearic acid      1.00        32.20
Calcium oxide     8.00       257.60
Sunpar 2280     115.00     3,703.00

Finished compound (2nd pass)

Ingredient          PHR   Grams/batch
Masterbatch      423.00    13,366.88
MBT                1.00        31.60
Tuex               1.00        31.60
Ethyl tellurac     1.00        31.60
Sulfads            1.00        31.60
Sulfur             1.50        47.40

Mixing procedure Masterbatch mix High rotor speed, water on; 0' - Load all ingredients, then the EPDM At 210 [degrees] F, raise and scrape ram. At 260 [degrees] F, drop batch.

Finished compound mix Low rotor speed, water on 0' - load one half masterbatch, all ingredients, then balance of masterbatch; At 120 [degrees] F, raise and scrape ram; At 140 [degrees] F, drop batch.

Compound from each of the two compounding processes was extruded into a weatherstripping profile on a continuous extrusion/microwave curing line. This system consisted of a 3.5", 20:1 L/D single screw extruder, a profile die and a three stage on-line curing system. The first stage is a high velocity hot air preheater; the second stage a 12 kw microwave oven with auxiliary hot air; and the third is a hot air final cure oven. The energy input is adjusted as required to attain the proper temperatures to cure the compound. The profile for this evaluation was a flat strip 3" by 0.125". The cure time was three minutes at 205 [degrees] C.

A comparison of the physical and rheological testing has shown that the dense weatherstripping compound on the twin screw compounding extruder is essentially equivalent to the compound generated in the internal mixer in all respects. The physical and rheological testing data appear in table 3.

Table 3 - rheological and physical test results

Rheological testing
Sample                                   1A mixer   Twin screw
ML 1+4 @ 100 [degrees] C                       41           39
Mooney scorch @ 125 [degrees] C (min.)       12.6         12.8
Monsanto ODR, 3 [degrees] arc, 6 min.
 360 [degrees] F, 100 cpm, microdie
[M.sub.L] (min.)                              5.8          5.5
[M.sub.HF](plateau)                          39.5         39.9
[t.sub.s1](scorch 1 [degrees] arc)           0.93         0.95
[t'.sub.c]50 (cure time 50%), min.           1.40         1.40
[t'.sub.v]90 (cire time 90%), min            2.05         2.15

Physical properties
100% modulus (psi)                            370          360
Tensile strength (psi)                      1,500        1,470
% elongation                                  480          470
Hardness, Shore A                              76           73
% compression set                              53           55
(plied discs - 70 hrs @ 70 [degrees] C,
25% deflection, 30 min. recovery)

Surface appearance
As extruded                                smooth       smooth
After cure                                 smooth       smooth

Summary

The state of the art equipment for compounding polymeric formulations is the fully intermeshing co-rotating twin screw extruder. Each of the unit operations necessary to compound rubber formulations can be isolated from the others, and then optimized due to the twin screw's modular design. The feeding of ingredients, and the appropriate dispersion and mixing can be discreetly configured along the extruder for optimal product quality. Preforming operations can be directly intergrated as the last stage of the compounding process. This results in the ability to produce rubber compounds with improved consistency, better dispersion and with a lower heat history.

In the example of dense weatherstripping, the twin screw compounding technology produces a rubber compound in one pass which has the identical characteristics as compound produced in a mixer in two passes.

Reference

[1.] Robert C. Tyler, Donald W. Tredinnick and Frederick Burbank, Continuous Mixing of EPDM Automotive Weatherstrips," paper no. 61 presented at a meeting of the Rubber Division of the ACS, Oct. 1995.