Continuous mixing of powder rubber in a twin-screw extruder. (Tech Service).
Powder rubber compounds based on E-SBR/CB prepared in the kneader revealed a favorable mixing behavior and good properties (refs. 1 and 2). Efforts were made on the development of a continuous mixing process for the systems based on SBR (refs. 2, 5 and 6) and NR (refs. 7 and 8) by using the twin-screw extruder (TSE).
The behavior of the mixing process on the extruder is essentially determined by the screw design. The corresponding elements allow modifications of screw geometry and a great variety of mixing sections longitudinally from the feeding zone towards the screw tip. However, the optimization of the screw assembly and for the continuous mixing process has to be based on principles applied by an internal mixer or on the roll mill. In other words, suitable extrusion concepts require that the powder rubber goes through regions of high shear forces and long duration, alternating with sections of low shear exposure and short residence time. The former can be realized by dispersive and distributive mixing elements.
The degree of filler dispersion, Mooney viscosity and extrudate temperature are important criteria for the assessment of the mixing quality and efficiency of the extruder screw design. Therefore, it is useful to pursue the development of these properties along the extrusion process. The effects of the mixing elements, as well as of the extruder cooling system, can be established. This may lead to a "tailor made" screw design for the simplification of the continuous mixing process.
The aim of tiffs study is to optimize the development of the continuous mixing process for powder rubber compounds based on NR by using the TSE. This should be based on a reliable screw configuration for maximizing output with still high mixing quality and physical properties of the vulcanizates. The influence of various screw elements on the development of the above mentioned properties along the extruder screw is considered.
Material and feeding
Three types of powder rubber based on NR/CB were supplied. The particle size of the granules was 0.5-2.5 mm. Different chemicals such as antioxidants and curatives were premixed and incorporated during the mixing process. The formulation of the powder rubber compounds is given in table 1.
Gravimetric single-screw loss-in-weight feeders (screw diameter 35 mm and 20 mm) were used for proportioning powder rubber and premix 1. A twin-screw loss-in-weight-feeder was used for the addition of the antioxidants and curatives (premix 2).
Twin-screw extruder and screw elements
Experiments were performed by using the TSE equipped with co-rotating screws (ref. 10). Key data on the machine are: Maximum process length--36 L/D: extruder hole diameter--37 mm; diameter ratio--1.55; and maximum screw speed--500 [min..sup.-1].
The extruder barrel consists of nine replaceable parts of four L/D lengths. The first metering zone barrel is cooled by water to prevent material caking. The following eight processing segments can be cooled with water or heated up to temperatures of 370[degrees]C, where open cylinders are suitable for feeding additional ingredients and for degassing. Figure 1 illustrates available screw and mixing elements for optimizing screw configuration.
[FIGURE 1 OMITTED]
All materials produced by extrusion were threaded through a two-roll mill with a large nap (4 mm) at 70[degrees]C, to obtain rolling sheets.
The vulcanization was performed at 150[degrees]C and 200 bar in a press, yielding sheets of 2 mm thickness. The vulcanization time corresponds to [t.sub.90] of the rheometer curve.
The degree of dispersion of filler was quantified by the "DIK-method" based on the light reflectance of razor blade cuts, as described elsewhere (refs. 5 and 11). The Mooney viscosity was recorded to evaluate the processing behavior. Stress-strain measurements were performed on dumbbell specimens (DIN) of the vulcanizates. Tensile strength, elongation at break and hardness were determined.
Results and Discussion
Property development during extrusion
For powder rubber 1, which is based on NR and 47 phr CB (N234) without chemicals, the development of the filler dispersion, Mooney viscosity and temperature along the extruder screw is studied by testing different screw configurations, at constant feed rate (25 kg/h), process length (36 L/D) and set temperature profile (barrel 1 : 80[degrees]C; 2-8: 20[degrees]C).
During the extrusion process, samples were taken off at different stages (after passing the mixing elements), in order to determine the above mentioned properties. The temperature of the material was directly detected by a push-in thermocouple.
In figure 2, three of the tested screw configurations are shown. The corresponding measurement results (dispersion, Mooney and temperature) are given in table 2.
[FIGURE 2 OMITTED]
The screw design "A" (figure 2a), composed of mostly conveying elements, contains one CME in the first zone for dispersive mixing and low-shear polygon elements in the seventh barrel for the distributive mixing of the curatives, which will be added at a later stage in this work. A comparison of the powder rubber properties determined after outgoing the CME with those at the extrude," exit reveals an apparent decrease of the temperature and Mooney viscosity, while a slight rise in the filler dispersion can be observed. However, the characteristics of the extrudate at both stages are insufficient due to the poor mixing effects.
In configuration "B'" (figure 2b), kneading blocks having a staggered arrangement of only forward-pumping are located in the first mastication zone, while the CME is placed in the middle of the screw and the polygons remain as in the previous configuration. The chosen arrangement of the kneading elements provides an insufficient level of mixing quality determined by low filler dispersion degree and high Mooney viscosity, due to low shear stresses (table 2). In the following mixing region, the material is exposed to higher shear forces, consequently, the dispersion degree and the related viscosity value of the extrudate are reduced by passing the CME. The polygon elements contribute to an improvement of the filler dispersion, which remains unsatisfactory.
The screw designs "A" and "B" result in low exit temperatures and require at the feed rate 25 kg/h a relatively low screw speed of 160 rpm and 200 rpm, respectively, i.e. the specific energy input needed is also reasonable.
The screw configuration "C" (figure 2c) also contains kneading blocks in the first plasticizing zone, but with forward- and reverse-pumping effects. The results of this staggered arrangement arc a clearly high mixing temperature, a good dispersion degree and a relatively low Mooney viscosity. This is attributed to a sufficient mastication and intensive mixing caused by high shear stresses and long residence time of the powder rubber in this region. The use of a CME in the middle of the screw slightly enhances the dispersion degree and reduces the viscosity, while the temperature is considerably lower than after the KB. The polygon elements also improve the properties of the outgoing extrudate, where the filler dispersion degree increases. In comparison to the previous configurations, the screw design "C" is apparently suitable for the continuous mixing process; however, a higher screw speed of 250 rpm and a certain increase of the specific energy input are required.
The whole effects observed show that the type, location and staggered arrangement of the screw elements result in different shear rates and residence times. These affect the mixing quality and extrudate temperature. However, the aimed material properties can be largely achieved in the first mixing zone, where necessary characteristics of the mixing elements are required tot the purpose of an optimum powder rubber mixing.
Continuous mixing and vulcanizate properties
Figure 3 illustrates the screw configuration optimized for the performance of the continuous compounding experiments on the different powder rubber types. This screw design is based on the establishment that a primal highly efficient extruder zone is required for a full compaction, plasticizing and an intensive mixing of the powdery rubber/filler batch. The first mixing zone is characterized by an increasing degree of fill, high shear stresses and long residence times. Consequently, high pressures and temperatures are detected.
[FIGURE 3 OMITTED]
The kneading blocks consisting of forward- and reverse-pumping units are placed in the primal mixing zone at a set temperature of 80[degrees]C. They are followed by a cooling section having a high venting and exhaustion capacity to carry off the heat of evaporation generated during the mixing process. The corresponding conveying elements are alternating with turbine elements for a homogeneous oil incorporation, one CME for further dispersive mixing and polygon elements, which are used in the latest stage for the addition of the curative premix. This distributive mixing zone is characterized by relatively low shear stresses, short residence time and reasonable temperatures.
Measurement results corresponding to different properties of the extrudates and vulcanizates are given in table 3.
The overall material feed rate (recipe in table 1) was varied and a high output of 50 kg/h was reached at moderate mixing temperatures. However, this gradual enhancement in output was feasible with a simultaneous increase of the screw speed, to remain within the upper torque limit of the twin-screw extruder. Consequently, the real mass temperature measured for the outgoing extrudate by a push-in thermocouple also gradually increased up to 110[degrees]C for the mix "1" and 115[degrees]C for mix 2. These extrudate temperatures obviously remain under the upper acceptable limit (125-130[degrees]C) and would presumably enable a further increase of the output by increasing the screw speed. This cannot be realized with the extruder used in this work, since its maximum screw speed (500 [min..sup.-1]) is approximately reached at the output 50 kg/h (for powder rubber compounds). The rheometer measurements of the compounds 1 and 2 clearly revealed scorching safety for all samples, where a reasonable scorching time higher than four minutes was recorded. It is assumed that the short residence time of the compounds compensates for the increasing temperature in the shear regions of the polygon mixing elements.
For the cushion compound "3," a higher exit temperature of the extrudate and a short scorching time of ca. one minute is detected for the samples at the corresponding feed rate. This is due to the great difference of the compound composition.
An effect observed on the rheometer curves is a small reduction of the corresponding maximum torque with increasing output and screw speed (table 3). This can be attributed to the slightly decreasing homogeneity of the chemicals in the entire rubber matrix.
The Mooney viscosity of the samples of each compound lies in a good, comparable value range, i.e., the influence of the feed rate on the processing behavior seems to be low.
For the degree of filler dispersion, only a weak decrease can be detected with increasing output. As it depends on screw speed, the residence time of the material in the high shear regions decreases, as does the duration of dispersive and distributive mixing. However, in this instance, the degrees of dispersion also remain in a good range of 95-99%. This is due to the suitability of the screw configuration to yield good dispersion at maximized output.
The above mentioned effects are consistent with the mechanical properties of the various vulcanizates. Increasing process parameters results in very weak differences between the values of the tensile strength, elongation at break and hardness.
The development of the continuous mixing process for powder rubber compounds in the TSE requires the simultaneous pursuit of several mutual aims, including the achievement of a good material quality, as well as a maximization of the output at low temperatures. The basis of this process is an optimum screw configuration containing mixing elements at different locations. As the powder rubber exhibits a good initial filler dispersion, a primal mixing zone having appropriate screw elements is largely capable of providing the desired extrudate properties. For the incorporation of chemicals, a low-shear mixing region is needed. A significant increase in throughput rate is achieved by increasing screw speed.
Table 1--formulation of the powder rubber compounds Ingredients Mix 1 Mix 2 Mix 3 (phr) (phr) (phr) 100 Powder rubber NR 100 100 -- N234 47 -- -- N115 -- 42 55 (Softener N326 3 Enerthene -- -- 7.5 Premix 1 ZnO 4 4 1 Stearic acid 2 2 - Vulkanox 1 1 6.4 Ultrasil VN3 -- -- 3.2 Cofill 11 -- -- -- Premix 2 6PPD 1.5 1.5 1 TMQ 1 1 0.7 TBBS 1 1 6 (Crystex) Sulfur 1.5 1.5 -- CTP 0.15 0.15 2 Manobond -- -- 0.6 Cohedur -- -- Table 2--properties of the extrudates at different zones of the screw assemblies Screw A B C configuration Sample S1 S2 S3 S4 S5 S6 S7 S8 Temperature ([degrees]C) 128 84 118 123 96 162 114 95 Mooney viscosity 112 92 116 97 81 72 68 64 @100[degrees]C (ML1+4) Degree of filler 84 87 84 88 90 95 97 98 dispersion (%) Table 3--properties of powder rubber compounds produced by continuous mixing in twin-screw extruder Parameter and property Mix 1 Feed rate (kg/h) 25 37 50 Screw speed (rpm) 280 380 480 Temperature ([degrees]C) 79 95 110 Mooney viscosity @ 100[degrees]C 48.0 48.5 48.2 Dispersion degree (%) 98.4 97.2 96.5 Torque [S'.sub.max.]-[S'.sub.min.] (dNm) 14.9 14.6 14.4 Scorching time, [T.sub.S1], (min) 4.6 4.2 4.2 Tensile strength, [[sigma].sub.max.] (MPa) 30.1 30.2 30.0 Elongation at break, [[epsilon].sub.b] (%) 526 526 523 200% modulus (MPa) 8.1 8.0 8.2 Hardness (durometer A) 65 64 65 Parameter and property Mix 2 Feed rate (kg/h) 25 37 50 Screw speed (rpm) 240 340 420 Temperature ([degrees]C) 81 96 114 Mooney viscosity @ 100[degrees]C 44.8 45.3 45.1 Dispersion degree (%) 98.1 96.7 95.0 Torque [S'.sub.max.]-[S'.sub.min.] (dNm) 13.5 13.3 12.9 Scorching time, [T.sub.S1], (min) 5.6 5.3 5.2 Tensile strength, [[sigma].sub.max.] (MPa) 29.6 29.4 29.3 Elongation at break, [[epsilon].sub.b] (%) 580 570 588 200% modulus (MPa) 5.4 5.4 5.0 Hardness (durometer A) 63 62 62 Parameter and property Mix 3 Feed rate (kg/h) 25 37 50 Screw speed (rpm) 270 380 480 Temperature ([degrees]C) 86 107 124 Mooney viscosity @ 100[degrees]C 51.1 51.4 51.0 Dispersion degree (%) 97.6 95.7 94.1 Torque [S'.sub.max.]-[S'.sub.min.] (dNm) 34.5 33.8 33.6 Scorching time, [T.sub.S1], (min) 1.16 1.14 1.10 Tensile strength, [[sigma].sub.max.] (MPa) 25.6 24.9 25.0 Elongation at break, [[epsilon].sub.b] (%) 381 370 375 200% modulus (MPa) 12.9 13.1 12.8 Hardness (durometer A) 81 81 80
(1.) U. Gorl and K.H. Nordsiek: Kautsch. Gummi Kunstst., 1998, 51, 250-258.
(2.) U. Gorl, M. Schmitt, A. Amash and M. Bogun, "Rubber/ filler compound systems in powder form" Part I, Kautsch. Gummi Kunstst., 2002, 55, 23-32.
(3.) E.T. Italiaander: Rubber Technology International, 1996, "96, 26-34.
(4.) W.A. Ploski and R.K. Williams, Rubber World, 2002, 225, 23-25, 49.
(5.) R. Uphus, O. Skibba, R.H. Schuster and U. Gorl, Kautsch. Gummi Kunstst., 2000, 53, 279-289.
(6.) R. Uphus, O. Skibba and R.H. Schuster, ACS meeting, Rubber Division, Dallas, April 2000, Paper No. 16.
(7.) A. Amash, M. Bogun, R.H. Schuster, U. Gorl and M. Schmitt: Plastic Rubber Composites, 2001, 30, 401-405.
(8.) A. Amash, M. Bogun, R.H. Schuster and U. Gorl, paper presented at the international conference "Rubber Research in Practice," Paderborn, Jan. 2002.
(9.) A. Amash, M. Bogun, R.H. Schuster and U. Gorl: Kautsch. Gummi Kunstst, in press
(10.) Farrel Instruction Manual for FTX-80 twin-screw extruder.
(11.) R.H. Schuster, "Fullstoffe," in W. Hofmann, H. Gupta (eds.): "Handbuch der Kautschuktechnologie," chap. 8, Dr. Gupta Verlag, Ratingen 2001.
Dr. Robert-Hans Schuster has been director of the Deutsches Institut fur Kautschuktechnolgie (DIK) since 1990. Dr. Ali Amash is manager of the processing department at the DIK. He joined the institute in 1998. Martin Bogun is a Ph.D. student at the DIK, Dr. Udo Gorl is responsible for the powder rubber development at Degussa AG. He joined Degussa in 1988.
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|Date:||Jun 1, 2002|
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