Rheology of HDPE wood composites. I. Steady state shear and extensional flow

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1 Composites: Part A 35 (2004) Rheology of HDPE wood composites. I. Steady state shear and extensional flow T.Q. Li*,1, M.P. Wolcott Wood Materials and Engineering Laboratory, College of Engineering and Architecture, Washington State University, P.O. Box , Pullman, WA , USA Received 26 November 2002; revised 3 May 2003; accepted 11 September 2003 Abstract Steady state flow of wood/high density polyethylene (HDPE) composites was studied using capillary rheometry to approach a fundamental understanding of the rheology of wood polymer composite melts. Mooney slip analysis was applied to examine the nature of shear flow, indicating that the flow of wood/hdpe melts consist of contributions from both wall slip and viscous flow. Both simple viscous flow and yield stress behavior were observed, depending on wood species and content. It was observed that the dependence of wall slip velocity on shear stress in maple (Acer spp.) formulations resembles that of neat HDPE. In pine (Pinus spp.) formulations, however, wall slip rate is influenced by wood content. A converging flow technique was used to study the extensional flow. In contrast to shear flow properties, the extensional viscosity was found to depend much less on wood species. The effects of both wood content and species were observed through Trouton ratio. Strong inter-particle interaction was recorded for some filled melts as yield stress in shear flow and apparent strain hardening in extensional flow. q 2003 Elsevier Ltd. All rights reserved. Keywords: A. Polymer matrix composites; A. Wood; B. Rheological properties; Wall slip 1. Introduction Polymer wood composites have attracted increasing attention in the past decade owing to the reinforcing potential of lignocellulosic fibers and the economic advantage of using them as fillers for thermoplastics. The wood fillers offer environmental advantageous because they originate from renewable resources and may add degradability to plastics [1,2]. The commercial markets for wood filled thermoplastics, are most frequently termed wood plastic composites (WPC) in North America. The production of these materials has grown four-fold between the year 1997 and 2000 [3]. Many technical issues regarding WPCs have been well studied. However, the melt rheology is critical to many processing issues and is poorly understood. For application properties and economic considerations, it is desirable to maximize the wood loading. The rheological phenomena * Corresponding author. Tel.: þ address: tieqili@wsu.edu (T.Q. Li). 1 On leave from Guangdong University of Technology, Guangzhou, China. resulting from the high filler contents (e.g. high viscosity and complex stress strain rate dependence) must be understood for proper formulation design and process control. Filler wetting and dispersion and wood matrix polymer interactions are also important factors for WPC, because they strongly contribute to properties [4 11]. Rheology has been widely used to assess the morphology and interfacial status of polymer blends, alloys and filled polymers [11 13] but effective rheological methods have not yet been explored to evaluate wood filler dispersion and interfacial status. In addition, additives are frequently used to modify processing or end properties of WPC formulations, however, their effects have been largely assessed from an applied viewpoint [14,15]. Proper rheological tests may prove useful in characterizing the efficiency of these additives. Rheological properties similar to other suspensions of polymeric matrices [16] are expected from WPC. Increased viscosity and shear thinning was reported for wood flour filled polyethylene [17] and polypropylene (PP) [14,18,19]. Shear thinning behavior was also reported for melts of wood flour and styrene/maleic anhydride copolymer [20] and X/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi: /j.compositesa

2 304 T.Q. Li, M.P. Wolcott / Composites: Part A 35 (2004) PVC/wood [21]. Viscosity of wood flour/pp composites was found to depend on wood loading and believed to rely on the nature of wood fiber to fiber interactions [19,20]. Elastomeric modifiers of 7 15 wt% were found to improve the flow of wood flour/pp composites [22]. Corona treatment of the matrix, wood, or both components was found to reduce viscosity of the wood flour/pp composites. This behavior was attributed to low-molecular weight moieties formed on the surfaces of both the polyethylene and wood fiber during corona treatment. The resulting components were expected to act as lubricants at the interfaces polymer fiber interface [22]. Other behavior known to thermoplastic polymers with inorganic fillers, such as yield stress [16] and wall slip [23 27], have not been reported for the wood filled polymers. From the existing literature, it is also unclear how the flow behavior changes with wood content, species, and morphology. In particular, filler morphology dictates the maximum packing content, one of the most important quantities for polymer suspensions. As this material type continues to grow and diversify, the current lack of rheological data can considerably limit further development and applications. As the first step to understanding the rheology of WPC, steady state shear and extensional flow of wood flour filled high density polyethylene (HDPE) melts were studied. Specifically, the objectives of this research were to determine the influence of wood species (maple and pine) and content (0.4 and 0.6 weight fraction) in binary wood HDPE blends. Mooney analysis [25,28] was applied to determine the existence and contribution of wall slip to the total shear flow. Extensional flow of the wood composites was studied with a hyperbolic die technique [12]. The Trouton ratio, as defined at identical strain rates [12,29 31], was calculated and compared to examine the nature of extensional flow of the typical wood composites. 2. Experimental 2.1. Materials The raw materials used in this research are listed in Table 1. The Equistar LB polymer is a typical HDPE resin in granule form and for applications such as color master batch and filled plastics. As shown in Fig. 1, the differential scanning calorimetry (DSC) curves, obtained using a Mettler Toledo DSC 882e machine, indicate that the melting peak temperature and offset Table 1 Raw materials Materials Producer Grade Description HDPE Equistar LB MFI ¼ 0.1 g/10 min Maple American wood fiber mesh Pine American wood fiber mesh Fig. 1. DSC curves of the matrix HDPE resin in as received granule form (as received) and after 3 min residence at 200 8C and cooling to 40 8C with the 5 8C/min rate (melted). temperature of as-received HDPE resins are and C, respectively. The melting temperatures are and C when the previous thermal history is removed through 3 min melt residence at 200 8C and subsequent cooling with a 5 8C/min rate to 40 8C. Both maple (Acer spp.) and pine (Pinus spp.) wood fibers were received at 6% moisture content and further dried in a ventilated oven at 95 8C overnight before compounding. The particle sizes of the wood fibers were analyzed as shown in Table 2, indicating the particle size is on the order of 0.25 mm for both fillers. The analysis with a digital optical particle size analyzer confirms that the average diameter and the ratio of length over diameter of the maple particles are 0.26 mm and 4.5, respectively, based on analyses on more than a sample of 3000 particles. The wood/hdpe formulations with either 40 or 60% of maple or pine were dry blended and extruded with a 55 mm (CM55, Cincinnati Milacron) counter-rotating, conical extruder (with front/rear diameter of 55/114 mm and length over diameter ratio of 22) through a die with 152 mm by 12.7 mm rectangular cross-section. Extruder barrel temperatures were set with a flat profile of 163 8C. The extrudates were knife milled with a Nelmor Mill into Table 2 Sieve analyses results of the wood fibers Tyler Sieve Maple a Pine a R20 T T R P32 R P35 R P48 R P65 R P100 T 1.0 R, Retain; P, pass; T, Trace (,0.09%); numbers indicate Tyler sieve mesh. a Fractions by weight percentage.

3 T.Q. Li, M.P. Wolcott / Composites: Part A 35 (2004) particles of ca. 4 mm in the longest dimension for capillary tests Capillary shear flow Capillary shear flow tests were performed using a screwdriven capillary rheometer (Rheometric Scientific Acer 2000) with a series of flat entry dies with diameters ðdþ of 1.0, 1.5, 2.0 or 2.5 mm and a length up to 30 mm. All tests were performed at 180 8C. Zero-length dies with a physical length of 0.2 mm were used for end correction. This correction was verified with Bagley plots using dies of 2.0 mm diameter and the length/diameter ratio of 0.1, 5, 10 and 15. A pressure transducer of 0.5% precision and with either 70 or 20 MPa full range was used depending on the measured range of pressure drop. It was observed that for apparent the shear rate range of the concern in this paper, the measured pressure drop always approached a stable level within a comparable time scale to that required for measuring the neat HDPE. The possible effects of discontinuity resulting from the relatively large wood particles, therefore, appear to be averaged over the time scale of the experiment. Despite the relative large particles used in this work, the assumptions for conventional capillary rheometry analyses (e.g. Rabinowitsch procedures and Mooney analysis) are taken for examining the flow properties of these melts highly filled with wood. Mooney analyses were performed on end-corrected data to obtain the wall slip velocity ðu s Þ and the geometryindependent shear rate ð _g c ¼ð4=t 3 Þ Ð t 0 t 2 _g dtþ through the plot of apparent shear rate _g a ¼ð8u=DÞagainst 1=D at given stress values. In this procedure, the apparent shear rate is assumed to be a sum of the corrected shear flow and that due to wall slip: 8u D ¼ 4 t 3 ðt 0 t 2 _g dt þ 8u s D The Rabinowitsch procedure [28] was further applied to the shear rate _g c to obtain the true shear rate: _g ¼ 3 þ d log _g c=d log t _g 4 c ð1þ The relative contribution of the wall slip in the capillary flow is evaluated as the ratio Q s Q ¼ 8u s=d _g a where Q and Q s are the total volumetric flow rate and that owing to wall slip, respectively [25]. The mechanism of the wall slip is examined further, based on the concept of apparent slip by Cohen and Metzner [32]. This treatment facilitates the thickness of the slip layer, which is assumed to consist of matrix resin, to be ð2þ estimated through [26] d s ¼ u sh s ð3þ t where the subscript s indicates the slip layer and h s is the Newtonian viscosity of the slip layer. The viscosity data of neat HDPE at corresponding shear rate were taken as the values of h s : For the neat HDPE, the Mooney analysis was found unnecessary owing to the lower shear stress level examined, which causes the negligible wall slip compared to other sources of error. Hence, the apparent flow rate data from the 2.0 mm die were used to calculate the viscosity directly through the Rabinowitsch procedure Extensional flow Extensional flow was studied with a hyperbolic die of Hencky strain of 5. The 70 MPa pressure transducer was used because high pressure is required to compress the wood filled materials before testing. The extensional viscosity is calculated from the measured pressure drop, DP; at the applied extensional flow rate _1 through DP h E ¼ ð4þ 2 _1 lnðr L =R 0 Þ where R L and R 0 are the radii at the exit and the entrance of the hyperbolic die, respectively. An estimation based on the power-law model [12] indicates that the contribution from shear for the 60% maple composite ranges from 2.5 to 3.5% when _1 changes from 0.01 to 1 s 21. At higher rate of 60 s 21, the shear contribution increases to about 9%. The Trouton ratio is defined as the ratio of extensional viscosity h E over the true, wall slip-free shear viscosity h at the same value of the second invariant of deformation tensor ð _gþ following the convention in [12]: Tr ¼ h Eð_gÞ hð _gþ 3. Results and discussion 3.1. Capillary shear flow Fig. 2 shows the overall flow curves of the wood-filled HDPE melts at difference die diameters. It can be seen that all composite melts studied show dependence on die diameter, suggesting contribution of wall slip. This dependence is strongly influenced by both the wood content and species. A comparison between Fig. 2a and b indicates that the pine formulation shows stronger dependence on die geometry than its maple counterpart at the same wood loading of 40%. For maple composites, the dependence on die diameter is greatly increased with the wood content ð5þ

4 306 T.Q. Li, M.P. Wolcott / Composites: Part A 35 (2004) Fig. 2. Overall shear flow curve of HDPE wood composites at different capillary die diameter. increasing from 40 to 60%. In contrast, the geometry dependence of the pine composite is not as significant as for its maple counterparts. Following Yilmazer and Kalyon [25], a Mooney analysis was performed by plotting the apparent shear rate against 1=D to evaluate wall slip velocity and geometry-independent shear rate (Fig. 3). As exemplified in Fig. 3a, the 40% maple formulation shows positive intercepts on the flow rate axis at all available levels of shear stress. For the other formulations, positive intercepts are observed only at higher stress levels, indicating the dominant plug flow at lower stress levels. This critical stress for positive intercepts to occur is around 100 kpa for the two pine composites (Fig. 3b and d) and is as high as 310 kpa for the 60% maple composite (Fig. 3c), indicating the existence of a yield stress. Negative ð8u=dþ intercepts, which were also reported for rubber compounds [33,34], are physically invalid [28]. Hatzikiriakos and Dealy [35] noted that uncertainties could be introduced into the Mooney analysis from experimental errors in end corrections. Regardless, it is noted that linear fits that force the relations through the origin can be made, shown as dotted lines in Fig. 3c. As compared in Fig. 4, the results on the 60% maple composites through the two different linear regression procedures are identical in the high stress region where the positive intercept values exist but deviate with decreasing shear stress. The regression that allows negative intercepts gives higher estimations of the slope of the ð8u=dþ, 1=D relationship, that is, the slip rate. The zero ð8u=dþ intercepts, however, can result from plug flow in the capillary dies while the critical stress value of 310 kpa for positive ð8u=dþ intercepts to be observed should indicate presence of a yield stress and is confirmed through viscosity data presented later. In results reported below, all linear fits were performed such that negative intercepts are avoided. The shear stress ðtþ dependent values of wall slip velocity ðu s Þ are presented for the maple and pine formulations in Fig. 5. It is worth to note that except for the 40% maple composite, the precision in estimating u s is highly limited as shown with the larger error bars in Fig. 4. More work need to be done to determine the origin of the deviation from a linear u s, t relationship on double logarithmic scale. Despite the uncertainty, it can be seen from Fig. 5 that within the experimental limitation, the two maple formulations seem to follow approximately the same u s, t relationship. These u s values of the maple composites are on the same order with those reported for neat HDPE of similar grades [35] but is lower than those for inorganic suspensions such as ammonia sulfate in poly(butadiene acrylonitrile acrylic acid) terpolymer [25], and synthetic sodium aluminum silicate in PP [23]. For these maple composites, the slip layer thickness is estimated to be approximately 0.1 mm and tends to increase with shear stress until the latter reaches a high value of about 250 kpa (Fig. 6). Such thickness values are not only thinner than those found for inorganic composites [25,36] but also much lower than the average particle size of the 40 mesh filler. The low slip layer

5 T.Q. Li, M.P. Wolcott / Composites: Part A 35 (2004) Fig. 3. Mooney plots of the wood composite for wall slip correction. thickness is hence consistent with the assumption of a matrix-dominated slip layer and validates the method of wall slip estimation used here. It is worth noting that the slip layer thickness values are similar for the two maple composites on level of shear stress from 100 to 251 kpa, the range common for both 40 and 60% composites. The similarity in slip layer thickness suggests that the surface layer responsible for the observed wall slip is mainly HDPE and has not been significantly modified through the presence of the maple filler. Unlike the maple formulations, pine composites show considerable variation in wall slip velocity with wood content. Both the u s, t relationship (Fig. 5) and the slip layer thickness (Fig. 6) are different at the different pine contents. The 40% pine composite shows much higher u s than the maple composites at the same shear stress levels (Fig. 4) and higher slip layer thickness (Fig. 5). The slip velocity and slip layer thickness of 60% pine composites, however, remain close to the values of maple composites. The change in slip velocity and the computed slip layer thickness with wood content seems to indicate that the surface layer properties rely on wood-content in pine composites, rather than merely on polyethylene behavior at the die surface. The assumption of using the HDPE viscosities for computing the slip layer thickness may not be Fig. 4. Comparison of the wall slip velocity values obtained through the two different linear regression procedures. Fig. 5. Wall slip velocity values of different HDPE wood composites.

6 308 T.Q. Li, M.P. Wolcott / Composites: Part A 35 (2004) Fig. 6. Slip layer thickness of the HDPE wood composites in capillary die. valid for the melts with pine, a wood species with a significant content of fatty acid [37]. In southern pines, ether soluble extractives are composed primarily of resin and fatty acids and account for 2 3 wt% of the total wood mass. These components could migrate to the surface, thereby causing higher viscosity and increasing the apparent slip layer. Future studies are necessary for further understanding the complex wall slip phenomena in wood composites produced with different wood species. The effects of wood species in causing different levels of wall slip are further revealed as the relative contribution of the wall slip to total flow rate. As shown in Fig. 7, the contribution of wall slip increases with decreasing die diameter for all four composites. It is also evident that the volumetric contribution of the wall slip depends significantly on wood species. At the 40% wood loading, the capillary flow of the maple composite consists of 27 48% contribution from wall slip, depending on stress level and die diameter. The pine composite, in contrast, shows 100% contribution, that is, plug flow, below about 100 kpa. The wall slip contribution in 60% maple composites is similar to the 40% pine composite while 30 70% contribution from slip is observed for the 60% pine composite, though all the flow data was observed at higher stresses at the high wood content. Besides confirming the dependence on wood species, the results in Fig. 7 also indicate the occurrence of yield stress as discussed in previously discussed Mooney plots. As shown in Fig. 7, 100% contribution of wall slip, an indication of plug flow, was observed for the composite with 60% maple and that with 40% pine below a certain stress level. A yield stress value is evident at least for these two composites as discussed in Section Shear viscosity The wall slip-corrected shear viscosity of the WPC formulations and neat HDPE resin is presented in Fig. 8.As shown in the figure, all the composites formulation tested Fig. 7. Relative contribution of the wall slip in melts with different wood content and species.

7 T.Q. Li, M.P. Wolcott / Composites: Part A 35 (2004) Table 4 Power-law constants of the wood composites Materials K (kpa s n ) n R 2 Maple 40% 97.1 ^ ^ Maple 60% ^ ^ Pine 40% ^ ^ Pine 60% ^ ^ Fig. 8. Shear viscosity of wood composites and of neat HDPE based on wall slip corrected shear rate and following Rabinowitsch procedure. Lines are fit to power-law models for composites and to Carreau model for neat HDPE. display a much higher viscosity than the neat HDPE. Among the four formulations, the 40% maple composite has the lowest viscosity at any given rate. Increasing the maple content to 60% causes considerable increase in viscosity. The two pine composites, however, exhibits similar values of the wall slip-free viscosity across the shear rates tested. It was found that the viscosity data of the 40% maple composite fit to the Carreau Yasuda equation [38] hð _gþ 2 h 1 ¼½1þðl _gþ a Š ðn21þ=a ð6þ h 0 2 h 1 with the values for zero shear viscosity h 0 ; time constant l; power-law constant n; and parameter a listed in Table 3. Over the same range of shear rates, the viscosity data of neat HDPE can be fit only by assuming a ¼ 2; that is, to the Carreau model. None of the other three composites can be fit to either Carreau model or Carreau Yasuda model. Despite the uncertainty owing to lack of low rate data, an increase in zero shear viscosity is perceived by adding 40% maple flour. The viscosity data for the three remaining formulations were modeled using a power law model: t ¼ K _g n ð7þ where K and n are power-law constants. The regression results were summarized in Table 4. As shown in Table 4,a decrease in the power-law index n with the increase in wood content is observed for both maple and pine composites. It seems that the level of shear-thinning is increased at higher wood loading and the increase is more significant with maple than with pine. Notice that the 60% maple formulation does display a slight upward tendency at low shear rates, suggesting a yield stress phenomenon. Unfortunately, the long dwell times necessary to produce lower shear rates in these wood-filled formulations cause thermal degradation and limit the shear rate range that can be reliably achieved. However, the presence of a yield stress would agree with the dominance of wall slip for the volumetric flow at shear stresses below ca. 300 kpa as shown in Fig. 7. A similar behavior is also found for the 40% pine formulation for shear stress below ca. 100 kpa. This observation, although qualitative, is informative for understanding the flow of the composites on microscopic level Extensional viscosity The extensional viscosities of the HDPE/wood formulations are compared in Fig. 8. Unlike shear viscosity, extensional viscosity appears to be dependent mainly on wood content and rather unaffected by wood species. At the lower wood loading of 40%, maple results in marginally higher extensional viscosity than pine at lower strain rates. When wood content is 60%, the two composites show nearly identical values of h E : To better characterize the extensional flow mechanism, the Trouton ratio is calculated and compared. This analysis reveals effects of both wood content and species (Fig. 9). All Table 3 Parameters for the 40% maple composite and the neat HDPE Materials Maple composite HDPE Model Carreau Yasuda Carreau h 0 (kpa s) ^ ^ 3.1 l (s) ^ ^ 0.64 n ^ ^ a ^ Fig. 9. Extensional viscosity of the wood composites and of neat HDPE.

8 310 T.Q. Li, M.P. Wolcott / Composites: Part A 35 (2004) of the wood composites studied exhibit a lower Trouton ratio than neat HDPE, which shows the Trouton ratio values that are much higher than the Newtonian limit of 3. The higher level of Trouton ratio was also observed for PP resin [12] and for glass fiber reinforced polyethylene [39] though the Trouton ratio was defined based on zero-shear viscosity in the latter case. From Fig. 9, it can be seen that the composites with 40% maple and 60% pine both show strain thinning behavior as neat HDPE does. In contrast, strain hardening was clearly evident for the other two composites. Lacroix et al. reported apparent strain hardening of blends of the strain thinning PP with the apparent hardening ethylene vinyl acetate (EVA) ethylene methyl acrylate copolymer with PP as matrices [12]. For the wood composites studied here, it seems that the strain thinning matrix polyethylene determines the extensional flow of the 40% maple composite but the inter-particle interaction is important in the melt with 40% pine. Presence of wood particles contributes considerably to the apparent hardening behavior in the composite with 60% maple. The strong interaction among the wood filler particles agrees well with the presence of yield stress in shear flow tests, as discussed in the forerunning text. The dependence on wood species is also revealed through the change of Trouton ratio with filler content. While maple composites show lower Trouton ratio at higher wood content, pine composites indicate the opposite. An increase in Trouton ratio with filler content can be predicted for suspensions of slender bodies in a Newtonian [40] and non-newtonian medium [41]. The result suggests stronger reinforcement to polyethylene melt by pine than by maple. More detailed studies on extensional flow of composites based on wood particles of well-characterized morphology and properties would be interesting to examine whether extensional viscosity can be predicted from wood filler aspect ratio based on the above-mentioned theoretical considerations (Fig. 10). Fig. 10. Trouton ratio of the wood composites compared with that of HDPE. 4. Conclusion The steady state shear flow for simple blends of HDPE and wood composites has been studied with capillary rheometry. It has been shown through the Mooney analysis that the shear flow of wood composites in capillary dies is characterized with varying level of wall slip and the viscosity depending on both wood content and species. Yield stress has been observed at both maple and pine composites at different wood contents. The different role of wood species in affecting wall slip and resulting in yield stress worth more dedicated work for selection of wood fillers and matrix resin for the wood composites. The extensional flow of the wood composites is studied with the hyperbolic die technique. It has been shown that the extensional viscosity depend strongly on wood content but less on wood species. The comparison of Trouton ratio, however, reveals also the significant role of wood species. Trouton ratio is found to increase with pine content but to decrease with maple content. Strain thinning behavior is observed for composites of 40% maple and 60% pine, respectively, while strain hardening is recorded with 40% of pine or 60% of maple. The extensional flow is hence suggested as a useful method for characterizing the wood plastics composites. Acknowledgements The authors acknowledge the Office of Naval Research (Contract N C-0488) for sponsoring in part the research reported in this series. They also thank Dr Karl Englund for help in filler particle analyses. References [1] Felix JM, Gatenholm P. The nature of adhesion in composites of modified cellulose fibers and polypropylene. J Appl Polym Sci 1991; 42: [2] Chtourou H, Riedl B, Ait-Kadi A. Reinforcement of recycled polyolefins with wood fibers. J Reinforced Plast Compos 1992;11: [3] Smith P. US Woodfiber plastic composite decking market. In: Proceedings for Sixth International Conference on Woodfiber Plastic Composites (Forest Products Society), Madison, WI; May p [4] Kokta BV, Raj RG, Deneault C. Use of wood flour as filler in polypropylene: studies on mechanical properties. Polym-Plast Technol Engng 1989;28: [5] Raj RG, Kokta BV, Daneault C. Modification of wood fiber surface by grafting and its effect on mechanical properties of polystyrene filled systems. Angew Makromol Chem 1989;173: [6] Meyers GE, Clemens C, Balatinecz JJ, Woodhams RT. Effects of composition and polypropylene melt flow on polypropylene waste newspaper composites. In: Proceeding of the Annual Technical Conference No. 50; p [7] Mahlberg R, Niemi HE-M, Denes F, Rowell RM. Effect of oxygen and hexamethyldisiloxane plasma on morphology, wettability and

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