Deposits on extruder dies - plating

Transcription

1 Gummi Fasern Kunststoffe, No. 3, 21, p. 185 Deposits on extruder dies - plating H. Oggermueller & A. Risch Translation submitted by E. A. Inglis Selected from International Polymer Science and Technology, 28, No. 4, 21, reference GK 1/3/185; transl. serial no INTRODUCTION The word plating is used to refer to the unwanted deposits in the flow channel and die orifice during extrusion of rubber mixes. With the passage of time such deposits can result not only in the spoiling of the surface quality of the extrudate but also in a reduction in dimensional stability, thus giving rise to processing rejects and ultimately expensive downtime when the machine has to be stopped for exchanging parts or cleaning. Similar phenomena are observed in injection moulding. The causes of plating can be looked for in two areas: - in the mix formulation, where the nature of the polymer, the fillers and numerous other additives can have an effect, and - in parameters peculiar to the process, such as the tool surface, extruder geometry and processing conditions. 2. OBJECTIVE The present study was intended to work out the corresponding magnitudes of effects in the extrusion process and to find ways of avoiding plating or at least of reducing it. The tests were carried out on EPDM mixes with various fillers and additives or auxiliary materials (but without vulcanisation chemicals), using a specially developed test method. 3. EXPERIMENTAL The extruder employed in the study, Schwabenthan Polytest 3 R, is shown in Fig. 1. Each test was carried out with a 5 kg mix under the conditions described in Table 1. Fig. 1 Schwabenthan Polytest R extruder machine (right) and special measuring attachment (left) International Polymer Science and Technology, Vol. 28, No. 9, 21 T/11

2 Table 1 Conditions of the extrusion test Extruder Screw diameter Processing length Temperature required head/zone 1/zone 2 Cooling (zone 1 and 2) Screw speed Feeding strips Schwabenthan Polytest 3 R 3 mm 45 mm 6/6/6 C 1 / turn u p 1 4 rpm 3 x 6 mm The additional device with the special measuring plate for evaluating the plating effect is shown in Fig. 2. Fig. 3 shows typical results for two pure EPDM mixes, with no plating tendency and a pronounced plating tendency. Table 2 Basic formulation for extrusion tests (phr) EPDM Light filler Corax N55 black (FEF) Paraffin oil Sunpar The characteristics of the EPDM rubbers are presented in Table 3. The corresponding measurement plates at the end of the test (Fig. 4) showed clear differences. The deciding factor was found to be the viscosity (the polymers with medium or lower viscosity showed favourable behaviour), while the molecular mass distribution seems hardly to have any effect. 4. RESULTS AND DISCUSSION 4.1 Influence of EPDM polymers In the basic formulation shown in Table 2 four different types of EPDM mix were compared. The Neuburg silicate was used here in combination with carbon black N55 (FEF) as a light filler. Fig. 2 Diagram of measuring attachment with special measuring plate 4.2 Effect of additives In the unmodified base mix (Table 2) with standard EPDM 1 and as a filler (Fig. 4) we used 2 phr of the additives listed in Table 4. For comparison we also tested an additive-free mix with TESPT-treated silicate Aktisil PF 216. The evaluation of the individual additives is based in this case on a reduction or increase of the plating effect, as was observed for the additive-free mix (Fig. 5). The results are likewise shown in Table 4: there were clear differences observable between the various groups of additives. A surprisingly large number of additives had no appreciable effect on the plating tendency, and some others amplified he effect. The silicate treated with TESPT (Si 69) without other additives had a relatively favourable effect, and low-molecular polymers as additives also showed a positive effect. The clearest improvement was achieved with stearic acid. An additional series of tests with at least 5 phr of plasticiser oil in the standard formulation replaced by silicone oil showed a complete absence of plating (Fig. 6). Table 3 Characteristics of tested EPDM rubbers EPDM 1 type without plating tendency Type Mooney v iscosity, 125 C ML (14) ML (18) Molecular mass distribution Ethylene content ( C ), % 2 Diene content (ENB), % EPDM type with higher plating tendency Fig. 3 Typical results for rubber with and without higher plating tendency EPDM 1 (Standard) EPDM 2 EPDM 3 EPDM 4 ca n.b. = not determined 8 n.b. n.b. n.b. bimodal medium broad narrow T/12 International Polymer Science and Technology, Vol. 28, No. 9, 21

3 Practically no effect Slight positive effect EPDM 1 (standard formulation) High viscosity/bimodal MMD EPDM 3 medium viscosity/ broad MMD Clear negative effect Clear positive effect EPDM 2 High viscosity/medium MMD EPDM 4 Low viscosity/narrow MMD Slight negative effect Fig. 4 Measuring plates after tests with mixes based on four different EPDM rubbers Fig. 5 Evaluation of reducing or reinforcing effect of additives in relation to plating with standard EPDM (Fig. 4 top left) Table 4 Tested additives and their effect Type of agent Material group Chemical name Commercial name Effect Fatty acid Stearic acid Oligomer additive Low-molecular polymer Liquid 1,2-polybutadiene Lithene AH Oligomer additive Low-molecular polymer Liquid 1,2-polybutadiene, silylated Polyvest 25 Oligomer additive Low-molecular polymer Liquid EPDM Trilene 67 Coupling agent Alkoxysilylalkylsulphane TESPT* Si 69 ** Hydrocarbon wax derivative** Perfluoroalkyl wax** Genolubb** Pff6/12 Fatty acid derivative Stearyl alcohol Lorol C 18 Fatty acid derivative Hydroxystearic acid Edenor OSSG Hydrocarbon wax derivative Montanic acid wax Luwax S Metal soap Calcium stearate / Metal soap Zinc stearate /- Quaternary ammonium salt Dimethyldistearylammonium chloride Varisoft TA 1 Glycol Diethyleneglycol (DEG) Fatty acid derivative Stearic acid amide Uniwax 175 Effect on plating: clearly reducing, reducing no appreciable effect - reinforcing, clearly reinforcing * Treating agent based on Neuburg silicate ** Dose of.3 and.3 phr 4.3 Effect of different fillers Using the basic mix with standard EPDM, finally the fillers shown in Table 5 were compared with each other (only in the case of talc the dose was raised from 5 to 53 phr). As Fig. 7 shows in the case of Neuburg silicate and talc, a coarser particle size has a good effect on reduction of the plating effect. Fig. 8 confirms once again the advantageous effect of added stearic acid. As was expected the silicate Aktisil PF 231 treated with stearic acid shows a similar positive effect in that the slight difference is caused by different stearic acid concentrations. International Polymer Science and Technology, Vol. 28, No. 9, 21 T/13

4 - 5 phr of softener replaced by silicone oil - 2 phr of softener replaced by silicone oil - 1 phr softener replaced by silicone oil combination of corpuscular and lamellar primary particles Talc lamellar Whiting predominantly corpuscular CaCO 3 content: 88% (possibly small lamellar fractions) Fig. 6 Influence of silicone oil as partial replacement for paraffinic softener oil on plating tendency Fig. 9 Comparison of the plating effect of fillers with different particle structures Sillitin V 85 Talc 1 Talc 2 Effect of particle size finer Sillitin V 85 coarser Talc 1 finer Talc 2 coarser spherical (corpuscular) plate-like (lamellar) rod-shaped (cylindrical) Fig. 7 Comparison of several light fillers quartz kaolin Sillitin Sillikolloid Aktisil without surface treatment stearic acid added separately Aktisil PF 231 surface treatment with stearic acid Fig. 1 Schematic representation of different particle structures and SEM images of light fillers Fig. 8 Influence of addition of stearic acid to the mix with Neuburg silicate and of pretreatment of the silicate with stearic acid Standard surface (lonbiotudinally ground, R 2 : 5 7 mm) Surface coated with fluoroalkylsilane Surface polished From Fig. 9 it is seen that fillers with a lamellar (platelike) structure give rise to a clearly more marked plating effect than similar fillers with corpuscular particle characteristics (Fig. 1). Fig. 11 Influence of the measuring plate surface on the plating effect 4.4 Supplementary investigations Fig. 11 shows a comparison between normal and polished measuring plate surfaces with a surface coated with fluoroalkylsilane. It is quite obvious that the reduced roughness of a polished extruder channel surface is an advantage for reducing the plating effect. Fig. 12 shows the favourable effect of an admittedly extremely reduced throughput in extrusion. According to Fig. 13 higher extruder temperatures seem similarly to be an advantage. Standard formulation 5 g/min Fig. 12 Influence of extruder throughput on plating effect Standard formulation, extruder head 6 C Standard formulation 5 g/min Standard formulation, extruder head 1 C Fig. 13 Influence of extruder head temperature on plating effect T/14 International Polymer Science and Technology, Vol. 28, No. 9, 21

5 Table 5 Description of characteristics of different light fillers Filler Sillitin Sillitin Aktisil V 85 Z 86 PF 231 Description Coarser fraction of lamellar kaolinite Finer fraction of Neuburg lamellar kaolinite treated with stearic acid Neuburg silicate, natural agglomerate of corpuscular quartz and silicate, natural agglomerate of corpuscular quartz and W hiting Natural high-fineness calcium carbonate (particle size similar to ) T alc 1 Micronised high-fineness talc (particle size similar to ) T alc 2 Micronised high-fineness talc (particle size larger than that of Talc 1) 5. INTERPRETATION OF THE RESULTS On the basis of the facts observed the following hypotheses can be put forward: Lamellar solids are oriented in a shear/stretching flow and become concentrated in the vicinity of the wall in the region of the highest shear rate, and they are thus separated from the mix without being transported further by the shear stress of the flowing mix. In the case of wall/film slippage this phenomenon does not occur or is greatly subdued. Geiger describes in ref. 1 the rheology of highly-filled EPDM mixes. He reaches the conclusion that that below a certain critical wall shear stress wall slippage occurs and above this critical value shear flow with adhesion to the wall occurs. The factors influencing this critical wall shear stress are: - the smoothness (depth of irregularities) of the surface of the die nozzle wall - the plastic-viscoelastic properties of the mix. As a model representation it can be assumed that below the critical wall shear stress the rubber mix glides on a very thin film over the irregularities. In this way it is subjected to only minimal shear. The normal stress differences arising in the sheared plastic-viscoelastic mix tend to expand the gliding mix and hence to fill up the depressions in the wall surface. As long as the shear stress in the direction of flow predominates over the effect of the normal stress differences acting in the direction of the wall, wall slippage takes place. This state can now also be regarded as a type of lubricated solid friction. Here the shear stress corresponds to the friction force F R and the components of the normal stress differences correspond to the normal force F N. The quotient F R /F N formed the coefficient of friction m, which apart from the viscosity of the lubricant (flowing film) depends on the roughness of the surface. If now the depth of the irregularities and hence the friction coefficient is steadily increased with unaltered values of the other factors, a critical point is reached at which no movement at the wall is possible. In the opposite case there will be a corresponding movement from a critical point with the static friction being overcome. This model, applied to the plating-measurement device system, would seem to indicate that reduction of the depth of irregularities of the measurement plate increases the value of the critical wall shear stress (prevalent wall shear stress < critical wall shear stress) and hence favours wall slippage processes. The actual measurement result and opinions of rubber processors show qualitative agreement with this assumption. The reduced coating observed with increased extruder head temperatures can also be explained according to this model. Since the model shows that the formation of plating involves a shear/elastic flow, an increase in temperature has the effect of lowering the viscosity of the mix. The resulting shear stresses for a given extrusion speed (= shear rate) decrease, and approach or go below the critical wall shear stress. The lower extrusion speed at the given temperature leads to the same result of reduction in shear stress. The positive effects found with variations in the formulation could be explained by the fact that under given conditions with the use of other types of rubbers or certain additives that displace the region of wall slippage to higher critical wall shear stresses the stress falls below the critical wall shear stress. 6. CONCLUSIONS According to the results obtained the following measures are effective for reducing of avoiding the plating effect during extrusion of EPDM mixes: In the mix formulation - the addition of at least 5 phr of silicone oil, as an effective preventable measure. - working with rubbers having low viscosity - avoidance of lamellar fillers such as talc (even as a powdering agent of component of release agent bath in batch-off equipment) International Polymer Science and Technology, Vol. 28, No. 9, 21 T/15

6 - avoidance of processing agents with diethyleneglycol (DEG), quaternary ammonium compounds or amide waxes - addition of stearic acid, if possible in increased dosage - addition of polyethyleneglycol 4 (depending on the formulation) - addition of processing aids based on calcium stearate (depending on the formulation) - preferably processing with Sillitin V or Sillitibn N as a filler (less favourable are Sillitin Z or Sillikolloid). In the processing operation - smallest possible depth of irregularities in all surfaces, particularly in the extruder nozzle - higher extruder head temperature - lower extrusion speed - geometry favourable to flow in the nozzle region. REFERENCE 1. K. Geiger, IKT Stuttgart, Rheological characterisation of EPDM rubber mixes by means of capillary rheometer systems, Kautschuk und Gummi Kunststoffe, 42, 1989, p (No date given) T/16 International Polymer Science and Technology, Vol. 28, No. 9, 21