PROFILE EXTRUSION OF WPC S SUPPORTED BY RHEOLOGICAL AND SIMULATION DATA

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1 PROFILE EXTRUSION OF WPC S SUPPORTED BY RHEOLOGICAL AND SIMULATION DATA Cristiano R. de Santi 1, Elias Hage Jr. 1, John Vlachopoulos 2 and Carlos A. Correa 3* 1 PPG-CEM/Univ. Federal de S. Carlos UFSCar, Campus de São Carlos, São Carlos-SP, Brazil 2 Department of Chemical Engineering - McMaster University, Hamilton-ON, Canada 3* Centre for Biocomposites and Biomaterials Processing, University of Toronto, Toronto-ON, Canada corresponding author: carlos.correa@utoronto.ca Rheological-processing correlations in wood-plastic composites (WPC s) have been investigated aiming to providing a better understanding of processing conditions in profile extrusion for this class of thermoplastics composites. The basic formulations prepared for the present study were comprised of 60 wt% HDPE, with and without LLDPE-MAH as coupling agent, compounded with up to 40 wt% of wood flour and some lubricant. Capillary and torque rheometers were employed to measure the rheological properties. A cooled die at the end of a single screw extruder was used for the extrusion of rectangular profiles. Once extrusion steady conditions were achieved, linear output and limiting processing conditions were recorded. Simulation with softwares Profilecad and PolyCad was performed using rheological data to determine velocity and temperatures profiles across the die. It has been shown that linear output for HDPE/wood-plastic extrusion profiles are strongly influenced by a combination of several factors such as, presence of coupling agents and lubricants, processing conditions, die pressure drop and cooling at the die. Keywords: wood-plastic composites; profile extrusion; rheology; die cooling. Introduction Significant research work has been done during the last 20 years to find new applications for inexpensive and abundant natural fillers [1-3] and among them, wood plastic composites (WPC) have attracted increasing research interest as wood waste fillers represent low-cost renewable reinforcements that enhance mechanical properties such as stiffness, strength and heat deflection temperature under load. The growing commercial importance of these materials has expanded efforts for understanding the structure-property relationships and for exploring new methodologies for producing new materials [4-6]. WPC are primarily produced with high filler loading and commercial products typically use wood filler contents of approximately 50 and 60 wt%. The engineering design of extrusion operations requires proper knowledge of the flow mechanism of these highly filled melts. Therefore, WPC formulations require consistent and reliable evaluation of flow performance of the composite melts. Rheological characterization is based upon deep knowledge of both the fundamentals of flow behaviour of highly filled plastics and the practical methods for evaluating the flow performance. Advancing this knowledge for composite materials is important for further developments in scientific and industrial terms [7]. HDPE is one of the most used polymers in WPC extruded profiles, followed by PVC and polypropylene. Productivity and linear output are key issues being addressed by processors of extruded WPC profiles although not much information can be found in the current literature about

2 their processing-property correlations. However, although linear output should be primarily dependent upon screw speed and material flow properties, it also can be influenced by other factors such as die pressure drop, melt temperature, wall slip, profile calibration, cooling rate, and so forth. Some authors [5-7] have conducted comprehensive investigations on wall slip phenomena in woodfilled HDPE. These authors have shown through the Mooney analysis that the shear flow of wood composites in capillary dies is characterized with varying level of wall slip depending on both wood content and species, such as Pine and Maple. Maiti et al. [8] have studied the melt rheological properties of PP/wood flour composites at filler loadings from 3 to 20 wt%. They reported an increase of the melt viscosity and decrease of melt elasticity depending on the filler concentration. The authors discussed that the melt viscosity of PP tends to increase owing to the flow thickening effect caused by irregular-shaped wood flour particles. Other researchers [9,10] also reported a large increase in the viscosity of wood filled composites in comparison to the neat polymers. The adhesion improvement at the interface of cellulosic fillers and thermoplastics matrices in WPC s has also been the focus of current research [11-14]. Cellulosic fillers can be modified by physical and chemical methods. Among various ways of chemical modification for polyolefin/wood flour composites, the functionalized polyolefins are most often tested because of their efficiency and commercial availability and among them, it was found that maleated linear low-density polyethylene (LLDPEgMA) gave maximum tensile and impact strength of the composites, presumably because of better compatibility with the HDPE matrix [15]. Another great concern in WPC processing at high filler content is their processability and to improve it the research with lubricating systems based on ester-type or zinc stearate lubricants has been the focus of studies by Li and Wolcott [16]. In the present work, some rheological and simulation for wood filled HDPE with different particle size distribution and additives were investigated with particular reference of the influence of flow properties on output and properties of extruded WPC profiles. The effects of additives such as a compatibilizer and a lubricating agent and possible interactions were also studied. Experimental High-density polyethylene from Braskem, having MFI 0.3 g/10 min (190 C; 2.16 kg) was employed as the composite matrix. Wood flour from Pine, mesh 40 and 80, and relative moisture 9%, supplied by Pinhopó was used as filler. The particle size distribution of the wood flour was determined by sieving and the results are shown in Figure 1. To reduce the relative moisture below 1.0%, the wood flour was dried at 100 C for 48h prior to compounding.

3 Cumulative Mass (%) WF80 WF Particle Size (microns) Figure 1 Wood flour particle size distribution Other components used in the formulations were an ester-type lubricant agent (Struktol TPW 113) and a maleated coupling agent (maleic anhydride modified LLDPE, from Arkema). The composite formulations are given in Table 1. The components were blended in a co-rotating 30mm twin-screw extruder Werner-Pfleiderer, model ZSK-30, having a temperature profile between 185 C and 200 C, screw speed of 200 rpm and output rate of 8 kg/h. To minimize thermal degradation, the wood flour was fed in the extruder by a side feeder positioned at approximately 2/3 of the barrel s length. Code Table 1 Composite formulations Polymer/compatibilizer 60(wt%) Filler / Lubricant 40(wt%) HDPE (wt%) PE-MA (wt%) WF 40 mesh (wt%) WF 80 mesh (wt%) Lubricant (wt%) W04ML W80ML W04M W80M W W Results and Discussion Rheological Characterization Capillary rheometry was carried out on a Rosand Precision Series 700 rheometer. Shear viscosity in terms of strain rate was determined at 180 C with a 2 mm die, L/D = 16 and 180 entry

4 angle. The results were fitted to a power law fluid and the power law exponent (n) and consistency (m) was determined for all compositions. A slit die of length (L) = 20mm, width (W) = 10mm and gap (H) = 1mm and 180 entry angle was also adapted to the same rheometer. Each formulation was extruded through the slit die at 180 C. Slit die results were also fitted to the power law model and the parameters are shown together with those of the capillary on Table 2. It can be seen that the two set of parameters are very close. Pressure versus wall shear rate data was recorded for further analysis of the die pressure drop. The values of wall shear rate were given by the equation: γ 2Q 2n+ 1 w = 2 H W n where Q = volume flow rate, n = power-law exponent HDPE wood composites flow properties were strongly influenced by the presence of a lubricant agent, and also influenced by the wood flour characteristics as indicated by changes in power-law exponent and melt consistency showed in Table 2. From these data, it can be seen that the lubricant decreases the consistency index but increases the power-law exponent. Larger n means less shear thinning but the viscosity is overall lower due to the smaller value of the consistency index. For the lubricated compounds, wood flour with lower medium particle size (higher mesh size) showed the lowest values of melt consistency. On the other hand, for the compounds without the lubricant and larger mesh size shows a tendency to a slightly increase in melt consistency and the presence of the coupling agent also increase the melt consistency, which agrees with what can be expected from the enhanced interaction between HDPE and wood owing to the presence of the maleated LLDPE. (1) Table 2 Power-law model parameters at 180 C Capillary Rheometry Slit Die Rheometry Code Power-law exponent n Consistency m (Pa.s n ) Power-law exponent n Consistency m (Pa.s n ) W04ML W80ML W04M W80M W W Figure 2 show the dependence of viscosity on shear rate of HDPE melts with the variations in wood particle size and additives. For best visualization of the curves without lubricant, the region marked with a dashed square was enlarged in (b).

5 η (Pa*s) (b) W04ML W80ML W04M W80M W04 W η (Pa*s) 2000 (b) W04ML W80ML W04M W80M W04 W γ (s -1 ) 700 γ (s -1 ) 100 Figure 2 Shear viscosity at 180 C of all formulations. The dashed region (b) is enlarged on the right. It can be seen that all compounds with lubricant had significant lower values of the shear viscosity, and comparing these two compounds at shear rates lower than 200s -1 there is a trend of reduction in viscosity with increasing mesh size. It can be seen in (b) that the compounds without lubricant presented very close viscosity values at shear rates between 20 and 200s -1, and for these compounds the presence of the coupling agent causes an increase in observed shear viscosity. This behavior is similar to the observed change in melt consistency and can also be explained by the enhanced interaction between HDPE and the cellulosic filler owing to the presence of the maleated LLDPE. With regard to the wood particle size, there is a slight trend towards increase in viscosity with increasing mesh size. The latter is in agreement with the results published by Li and Wolcott [7]. The same behavior for viscosity versus wall shear rate was observed for slit die rheometry data. The slit die entrance pressure loss versus wall shear rate data is shown on the Figure 3. Also as observed on capillary rheometry, data for the lubricated formulations showed a non-linear pattern for both low and high shear rates. This could be attributed to the wall slip phenomena. 10 P ent (MPa) W04ML W80ML W04M W80M W04 W Wall Shear Rate (s -1 ) Figure 3 Pressure versus Wall Shear Rate at the Slit Die

6 Torque Rheometry [17] was carried on a Haake Polylab System, mod. Rheocord 300 p/rheomix 600 p. A constant volume of 50 cm 3 for each formulation was loaded in the rheometer chamber and mixed at 180 C, 50 RPM and 20min. Data from torque versus time were recorded for analysis and Figure 4 shows these data for all compounds. The resultant curves confirmed the same tendencies observed on both the capillary and slit die rheometry. The compounds with lubricant required much less torque than the others without lubricant and all the compounds with coupling agent (but without lubricant) displayed the highest torque values n = 50 (1/min) Temp = 180 oc 18 Torque (N.m) W04M W80M W04 W W04ML W80ML Time (min) Figure 4 Torque Rheometry Data Profile Extrusion It is well known from industrial practice that extrusion of wood plastic composites through dies results in severe surface tearing and distortions. This phenomenon has been studied in detail by Hristov et al. [5]. Edges of profiles exhibit the worst tearing phenomena [9,18]. In industrial practice, however, die cooling is frequently used for the production of smooth profile surfaces. This method of distortion-free extrusion has been described in a number of patents [19-21]. By cooling a skin is formed which does not permit the wood fibers to protrude from the extrudate surface and produce the severe surface or edge tearing phenomena mentioned above. In the present work, a single screw extruder (D= 25mm, L/D= 25) and a rectangular die having dimensions 25mm x 6mm were used. Temperature at the extruder barrel ranged from 150 C to 190 C and in the first part of the die the temperature was controlled at 145 C. By cooling the die it was possible to produce a smooth profile as shown in Fig. 5. The die was cooled to 80ºC by circulating water through the coiled tube as shown in Fig. 5. The conditions for all extruded compounds are summarized in Table 3.

7 Formulation (1) Table 3 Profile extrusion conditions Linear output Head pressure Limiting condition rate (m/h) (psi) W04ML 6 ~ 7 42 Surface defects W80ML 6 ~ 7 42 Surface defects W04 < Pressure W80 < Pressure (1) For W04M and W80M the head pressure reached the safety limit of 170 psi, without the successful production of a profile The profiles were extruded at very reasonable output rates, at about 6m/h and a pressure at the die of 40 to 50 psi for compounds with lubricant. For the two compounds with the coupling agent but without lubricant, the profiles could not be extruded due to the excessive pressure build up at the die, above 170 psi, reaching the equipment safety limit. The other two compounds without lubricant and without coupling agent, the profiles still could be extruded, but at output rates lower than 2m/h and pressure at the die up to psi, almost reaching the equipment limit. Such reduction on the productivity was due to the attempt of reaching stability of profile extrusion, keeping the pressure at the die under control. We observed a direct correlation between torque rheometry data and the compound processing performance in the profile extrusion, where all compounds that presented torque values above 16 N m after 20min in the rheometer chamber could not be extruded in profiles due to the high values of pressure in the die. Figure 5 Extruded profile from a cooled die A computer simulation with the assumption of fully developed flow was carried out using the Profilecad finite element software from Polydynamics, Inc. In Fig. 6 the velocity contours are shown. Near the die corners there are large regions of very slow flow, which would be responsible for severe edge tearing in case the profile was not cooled. Figure 6 - Velocity contours in the die

8 Figure 7 Boundary conditions for finite element simulation from the end of the extruder to the die exit For the study of the temperature field a two-dimensional flow package called Polycad 2D from Polydynamics, Inc. was used. It was assumed that flow between two flat plates is a reasonable approximation for the present die. The boundary conditions are shown in Fig. 7 and correspond to the extrusion of compound W80ML at Table 3. The average input velocity was 0.4mm/s and the incoming melt was assumed to have a uniform temperature at 190 C. The converging part of the die was heated to 145 C and the final part was cooled to 80 C. The calculated pressure drop was 0.29 MPa (42 psi) which is the same as the experimentally measured pressure of 42 psi at the end of the extruder. The temperature field is shown in Fig. 8 and the temperature profile at the die exit is shown in Fig. 9. Under these conditions the extruded profile had a smooth surface without any distortions. Figure 8 Temperature field for W80ML simulation at 0.4 mm/s Figure 9 Temperature profile at die exit Increasing slightly the extrusion speed results in the formation of some sort of surface blistering and subsequent interruption of smooth profile production, as shown in Fig. 10. Figure 10 Detail of the W80ML extruded profile showing surface blistering

9 The blistering phenomenon is apparently due to skin softening of the profile as it is demonstrated by flow simulation with the input velocity increased to 1 mm/s, as shown in Figs 11 and 12. In this case the temperature at the middle point of the die exit reaches C while in the former case at lower velocity it only reached C. Figure 11 Temperature field for W80ML simulation at 1 mm/s. Figure 12 Temperature profile at die exit. Conclusions Multicomponent high-density polyethylene (HDPE)/wood flour composites have been investigated in relation to the rheological parameters, processing conditions and their influence on profile output rate, surface quality and properties. It was also found that pressure drop is strongly influenced by the presence of lubricant and increases in the presence of the maleated coupling agent. A direct correlation of torque and capillary rheometry data with profile performance was found during extrusion. Accordingly, all formulations that presented high torque values and high shear viscosity could not be profiled owing to excessive high pressure at the extrusion die. Die cooling was necessary to produce profiles having smooth surfaces and edges. The increase in shear viscosity due to the presence of maleated LLDPE made the profiles extrusion process impossible in the absence of lubricant. The results indicate that a proper combination of factors such as formulation, rheological properties and processing conditions is needed for WPC profile high output rate and good end-use properties. Scaling-up the process shall require optimization of cooling conditions at the die as profile geometry and its bulk volume are key factors to maintaining reasonable output rates without compromising the profile surface finishing and properties.

10 Acknowledgements The authors are grateful to FAPESP Fundação de Amparo à Pesquisa do Estado de São Paulo and CNPq - Brazil for scholarship and financial support, to Braskem and Pinhopó for materials supply and technical support, UFSCar (Brazil), USF (Brazil) and McMaster (Canada) for access to their laboratory facilities and to Dr. Velichko Hristov and Dr. Carlos Razzino for helping the start up of the experimental program. References 1. M. Bengtsson; K. Oksman; N.M. Stark Polym. Comp. 2006, 27, D.N. Saheb; J.P. Jog Adv. Polym. Technol. 1999, 18, M. Xanthos; S.K. Dey; S. Mitra; U. Yilmazer; C. Feng Polym. Comp. 2002, 23, C.A. Correa; C.A. Razzino; E. Hage Jr. J. Thermoplast. Comp. Mat. 2007, 20, V. Hristov; E. Takács; J. Vlachopoulos Polym. Eng. Sci. 2006, 46, T.Q. Li; M.P. Wolcott Composites: Part A, 2004, 35, T.Q. Li; M.P. Wolcott Polym. Eng. Sci. 2005, 45, S.N. Maiti; R. Subbarao; M.N. Ibrahim J. Appl. Polym. Sci. 2004, 91, Z. Charlton in Proceedings of the SPE ANTEC Tech. Conference, 2000, Vol. 1, H. Li; S. Law; M. Sain J. Reinforc. Plast. Compos. 2004, 23, J.M. Felix; P. Gatenholm J. Appl. Polym. Sci. 1993, 50, D. Maldas; B.V. Kokta Compos. Interfaces, 1993, 1, R. Gauthier; C. Joly; A.C. Coupas; H. Gauthier; M. Escoubes Polym. Comp. 1998, 19, J.Z. Lu; Q. Wu; I. I. Negulescu J. Appl. Polym. Sci. 2005, 96, Y. Wang; F. Yeh; S. Lai; H. Chan; H. Shen Polym. Eng. Sci. 2003, 43, T.Q. Li; M.P. Wolcott Polym. Eng. Sci. 2006, 46, C.R. Santi; E. Hage Jr.; C.A. Correa; J. Vlachopoulos Appl. Rheol. 2008, 19, Z. Charlton, Master s Thesis, McMaster University, T.C. Laver, U.S. Patent 5,516,472, S. Nishibori, U.S. Patent 5,725,939, J.J. Muller; R.A.Wittenberg, U.S. Patent 5,851,469, 1998.