Materials, Chemicals, and Energy from Forest Biomass - American

Chapter 11

Room Temperature Ionic Liquids as Lubricants for Wood-Polyethylene Composites

Downloaded by UNIV OF BATH on July 2, 2016 | http://pubs.acs.org Publication Date: April 16, 2007 | doi: 10.1021/bk-2007-0954.ch011

Kaichang Li and Katie Zhou Department of Wood Science and Engineering, Oregon State University, Corvallis, OR 97331

Room temperature ionic liquids (RTILs) may be used as green solvents and catalysts for various chemical reactions, as antielectrostatic agents for wood, as electrolytes for batteries, and as extraction agents for reduction of hazardous pollutants because of their unique properties such as negligible vapor pressure, high thermal stability, and high electric conductivity. In this study, we demonstrated that RTILs were excellent lubricants for making wood-plastic composites (WPCs). A lubricant is essential for making acceptable WPCs and a compatibilizer is typically added to improve their mechanical properties. However, a lubricant and a compatibilizer normally have opposite effects on the strength and stiffness of WPCs. This study revealed that RTILs had better lubrication effect than Struktol, a commonly used commercial lubricant for making WPCs. When used with M A P E , some RTILs reduced the strength-enhancing effect of M A P E (maleic anhydride-grafted polyethylene), one of the most effective compatibilizers, less than Struktol. Some RTILs were also superior to Struktol in terms of increasing or retaining the stiffness of the resulting wood-PE composites. Among five RTILs studied, RTIL F (triisobutylmethylphosphonium tosylate) was the best RTIL in terms of providing the lubrication effect and maximizing retention of the strength— enhancing effects of M A P E .

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© 2007 American Chemical Society

Argyropoulos; Materials, Chemicals, and Energy from Forest Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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Introduction Room temperature ionic liquids (RTILs) are organic salts that are liquids at room temperature or slightly above (I). As salts, RTILs are electrically conductive. Because of strong ionic bonding, RTILs also have no measurable vapor pressure even at very high temperature. Most RTILs have very high thermal stability, and some RTILs are stable even above 350 °C. Most RTILs are inert to various chemicals. Therefore, RTILs may be used as green solvents or reaction media for various chemical transformations, as solvents for dissolution of cellulose, as electrolytes for various applications such as batteries, as extraction agents for reduction of hazardous pollutants, and as antielectrostatic agents for wood (2-12). Many other new applications of RTILs are being discovered. In this study, we demonstrated that phosphonium-based RTILs were superior lubricants for the production of WPCs. WPCs consist of wood in the form of wood flour or wood fibers, a thermoplastic, and some additives for improving the product properties or assisting the production process (13-15). Commonly used thermoplastics include polypropylene, polyethylene, polyvinylchloride, and polystyrene (1315). Wood is lighter, less abrasive, and less expensive than commonly used inorganic plastic fillers such as glass fibers. Wood is hydrophilic and susceptible to biodégradation, whereas thermoplastics are hydrophobic and resistant to biodégradation. WPCs as a mixture of two materials with opposite properties not only have the good properties of wood and thermoplastics, but also have less undesirable properties of wood and thermoplastics. For example, wood is a reinforcing filler for thermoplastics, and WPCs are thus stronger than thermoplastics. Lignin and polyphenols in wood are antioxidants and are able to quench free radicals, thus alleviating the aging and photodegradation accompanied by use of thermoplastics. Wood as a hydrophilic material can absorb water and then swell, which are undesirable properties for some applications. Wood as a natural material is also susceptible to biodegredation. In WPCs, wood is surrounded by thermoplastics which greatly reduces the water uptake, swelling and biodégradation of wood. Since 1995, WPCs have been one of the fastest growing sectors in wood composites industry (16). At present, WPCs are widely used as decking materials, fencing materials, interior door panels, window moldings, automobile interior parts, and many other molded products (16). WPCs are typically produced through an extrusion process, i.e., wood, a thermoplastic, and additives are thoroughly mixed at high temperature in an extruder and then are extruded into products with different shapes (13). A compatibilizer is typically added to improve the interfacial adhesion between hydrophilic wood and hydrophobic thermoplastic (13,17). Without a



170 compatibilizer, WPCs typically have low strength and low stiffness, and cannot meet property requirements of many applications (13,17). At present, maleic anhydride-grafted polyethylene ( M A P E ) is one of the most effective compatibilizers for wood-PE (polyethylene) composites. A lubricant is an essential process additive for the production of WPCs. Without a lubricant, wood flour cannot be uniformly distributed in thermoplastic matrix, and more importantly, the surface of extruded W P C products is unacceptably rough. Struktol is one of the most commonly used lubricants. It is believed that one of the key components in Struktol is zinc stéarate, although the exact composition of Struktol is proprietary. A lubricant and a compatibilizer typically have opposite effect, i.e., a lubricant typically reduces the strengthenhancing effect of a compatibilizer (18). In this study, we found that RTILs had an excellent lubrication effect, but reduced the strength-enhancing effect of M A P E much less than Struktol in making wood-PE composites.

Materials and Methods Materials The following RTILs were provided by Cytec Industries, Inc. (West Paterson, NJ): A ) tributyltetradecylphosphonium dodecylbenzenesulfonate; B) trihexyltetradecylphosphonium dodecylbenzenesulfonate; C ) trihexyltetradecylphosphonium methanesulfonate; D) trihexyltetradecylphosphoniumbis-2,4,4-(trimethypentyl)phosphinate; E) trihexyltetradecylphosphonium dicyanamide; F) triisobutylmethylphosphonium tosylate. Pine flour (40 mesh and 2.04% moisture content) and M A P E (maleic anhydride-grafted polyethylene, A - C OptiPak 200) were provided by American Wood Fibers (Schofield, WI) and Honeywell International, Inc. (Morristown, NJ), respectively. Pine flour was re-dried at 104 °C for 20 hours prior to use, and the resulting pine flour had the moisture content of 0.50%. H D P E (high density polyethylene) (melt flow index: 0.55 g/10 min) and Struktol were provided by B P Solvay Polyethylene North America (Houston, T X ) and Louisiana Pacific Corporation (Nashville, TN), respectively. (TM)

Compositions of Wood-PE Composites The weight ratios of different components in wood-PE composites are as follows: wood/PE/MAPE/Strulrtol/Rm Where P+M+S+R=60. For the control, i.e., wood-PE composites without any additive, P=60 and M=S=R=0. For wood-PE composites containing Struktol as a sole additive,


171 P+S=60 and M=R=0. For wood-PE composites containing both Struktol and M A P E , P+S+M=60 and R=0. For wood-PE composites containing a RTIL as a sole additive, P+R=60 and M=S=0. For wood-PE composites containing both M A P E and a RTIL, P+M+R=60 and S=0. In this study, the usage of M A P E , i f used, was 2 wt% based on the total weight of the wood-PE composites, i.e., M=2 in the previous weight ratio formula. For Struktol and RTILs, the following usages (the weight percent based on the total weight of the wood-PE composites) were studied respectively: 0.5 wt%, 1.0 wt%, 2.0 wt% and 3.0 wt%, i.e., S=0.5, 1.0, 2.0, and 3.0 or R=0.5, 1.0,2.0, and 3.0.


Preparation of Wood-PE blends and Measurement of the Lubrication Effect of RTILs Re-dried wood flour, P E , and additives were mixed in a Brabender Plasticorder with a mixing bowl (60 mL) and two roller blades (C. W. Brabender Instruments, Inc., South Hackensack, NJ). The total weight of wood, P E and additives for each mixture was 44.00 g where the weight of wood flour was 17.60 g. Here is an example blending procedure for making wood-PE composites containing 2.0 wt% M A P E and 3.0 wt% RTIL A (the weight ratio wood/PE/MAPE/Struktol/RTIL = 40/55/2.0/0.0/3.0). Pine flour (17.60 g), H D P E (24.20 g), M A P E (0.88 g), and RTIL A (1.32 g) were mixed in a 400 mL beaker by mechanical stirring for 2 minutes. The bowl of the Brabender Plasticorder was preheated to 170 °C and the speed of the rotors in the bowl was set to 100 rpm. The mixture of pine flour, H D P E , M A P E , and RTIL A was added to the bowl and mixed for 15 minutes. The torque value was taken at the mixing time of 9.0 minutes. The blend was then removed from the Brabender bowl, cut into small pieces with chisel while the blend was still hot, and stored for compression molding.

Preparation of wood-PE composite board The wood-PE blend was added to a stainless steel mold with dimensions of 101.6 χ 101.6 χ 2 mm, and the blend-filled mold was placed on the lower platen of an automatic benchtop Carver press (Carver, Inc., Wabash, IN) where the platens were preheated to 185 °C. The mold was maintained on the lower platen for 10 minutes before the press was closed slowly, allowing the wood-PE blend to flow into the mold shape. The press pressure was increased from 34.5 to 344.8 kPa over two minutes and was at 344.8 kPa for an additional 10 minutes. The mold was then removed from the hot press and cooled in a cold press at 344.8 kPa under ambient conditions. The resulting wood-PE board was cut into 10 small test specimens. Each specimen was 54.50-56.50 mm long, 13.00-14.30 mm wide, and 2.10-2.40 mm thick.


172 Evaluation of strength properties of the wood-PE composites


Strength properties of wood-PE composites were evaluated in accordance with A S T M D790-02. Ten specimens were tested for each wood-PE composite. Three-point bending test with support span of 43 mm and the crosshead speed of 1.0 mm/min was performed on a Sintech machine (MTS Systems Corporation, Enumclaw, W A ) . The load-deflection curve was recorded and used for calculation of the modulus of rupture (MOR) and modulus of elasticity (MOE). The maximum force at the first point on the load-deflection curve to show a slope of zero was used to calculate the M O R . The M O E was determined from the slope in the initial elastic region of the load-deflection curve.

Results and Discussion It has been demonstrated that alkylimidazolium tetrafluoroborates can serve as promising lubricants for reducing friction between metals and between metals and ceramics (19). However, these fluoride-containing imidazolium-based RTILs are not stable at high temperature under aerobic conditions and some hazardous or corrosive chemicals such hydrogen fluoride may be released from these RTILs (20). Since WPCs are made at high temperature, halogencontaining RTILs and imidazolium-based RTILs might not be used for making WPCs. In this study, we chose halogen-free phosphonium-based RTILs, and these RTILs are not corrosive and are much more stable at high temperature than imidazolium-based RTILs (21). A torque value during the mixing of wood-PE blend is a very good indicator for the effectiveness of a lubricant. The lower the torque value the better the lubrication effect. A typical torque vs. mixing time curve is shown in Figure 1. As the P E melted, the wood-PE mixture became viscous and sticky, thus requiring higher torque for the blades to compound the mixture. This was why the torque value rapidly increased at the beginning of the mixing. As a lubricant took effect and a more uniform wood-PE blend is formed, the torque value then rapidly decreased and began to flatten out at the mixing time of around five minutes. Therefore, the torque value at the mixing time of nine min. was used to evaluate the lubrication effect of each RTIL. The effect of the usage of a RTIL or Struktol on the torque value at the mixing time of nine min. is shown in Figure 2. At 0.5 wt% and 1.0 wt%, Struktol had basically the same torque values as the control, i.e., having little lubrication effect. At 2.0 wt%, Struktol had significantly lower torque values than the control. The torque value rapidly decreased when the usage of Struktol was increased from 2.0 wt% to 3.0 wt%. Three percent is a commonly used usage for Struktol in commercial production of WPCs. For RTIL A , the torque remained statistically the same when the usage was increased from 0.5 wt% to


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The amount of RTILs and Struktol in wood-PE composites (wt%)

Figure 2. Effect of RTILs and Struktol on torque. (Control: wood-PE without any additive. Error bars show two standard errors of the means from two independent experiments)



174 1.0 wt%, and then decreased almost linearly when the usage was further increased from 1.0 wt% to 3.0 wt%. For RTILs B, C, D, E, and F, the torque value for each RTIL significantly decreased when the usage of each RTIL was increased from 0.5 wt% to 3 wt%. At 0.5 wt%, RTILs A , B, C, and D resulted in significantly lower torque values than Struktol, whereas RTILs Ε and F were as ineffective as Struktol in lowering the torque value (Figure 2). A l l RTILs at 1.0 wt% were more effective in lowering the torque value than Struktol at 1.0 wt % and even at 2.0 wt%. At 2.0 wt%, all RTILs resulted in lower torque values than Struktol. At 3.0 wt%, all RTILs except RTIL F all had lower torque values than Struktol (Figure 2). RTIL D at 1.0 wt% was as effective as Struktol at 3.0 wt%, and RTILs Β and C at 1.0 wt% were even more effective than Struktol at 3.0 wt% in terms of lowering the torque values (Figure 2). A l l these results indicated that almost all RTILs were more effective lubricants than Struktol in the same usage. Some RTILs were three times more effective than Struktol. The effect of RTILs and Struktol on the M O R of wood-PE composites is shown in Figure 3. At 0.5 wt%, Struktol had little effect on the M O R . However, increase in the usage from 0.5 wt% to 3.0 wt% resulted in linear reduction of the M O R (Figure 3). For RTIL A , the M O R remained statistically unchanged when the usage was increased from 0.5 wt% to 2.0 wt%, but significantly decreased when the usage was further increased from 2.0 wt% to 3.0 wt% (Figure 3). For RTILs Β and D, the M O R decreased when the usage was increased from 0.5 wt% to 2.0 wt%, and then remained statistically unchanged when the usage was further increased from 2.0 wt% to 3.0 wt%. For RTIL C, the M O R significantly decreased when the usage was increased from 0.5 wt% to 1.0 wt%, and then flattened out when the usage was further increased from 1.0 wt% to 3.0 wt%. For R T I L E, the M O R rapidly decreased when the usage was increased from 0.5 wt% to 3.0 wt%. Interestingly, RTIL F at 0.5 wt% resulted in significantly higher M O R than the control. For RTIL F at 1.0-3.0 wt%, the M O R was statistically the same as that of the control (Figure 3). Except RTIL F, all RTILs resulted in much lower M O R than Struktol at the same usage. The effect of RTILs and Struktol on the M O E of wood-PE composites is shown in Figure 4. Interestingly, R T I L F had higher M O E than the control at all usages studied. At 0.5 wt%, Struktol and RTIL Ε had higher M O E than the control; RTIL D had the same M O E as the control; RTILs A , B, and C had lower M O E than the control. A t 1.0 wt% and 2.0 wt%, M O E values were statistically the same for Struktol and the control. At 3.0 wt%, Struktol had lower M O E than the control. With the exception of RTIL F, all RTILs at the usage of 1 wt% to 3.0 wt% had much lower M O E than the control and Struktol (Figure 4). The effect of combinations of RTILs and M A P E and combinations of Struktol and M A P E on the torque value is shown in Figure 5. When compared with M 2 (wood-PE composites containing 2.0 wt% M A P E only), increase in the usage of Struktol from 0.5 wt% to 1.0 wt% had little effect on the torque value


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(curve Struktol-M2, Figure 5). However, further increase in the dosage of Struktol from 1.0 wt% to 3.0 wt% resulted in linear reduction of the torque value, which means that Struktol had decent lubrication effect in making woodPE composites. At 0.5 wt%, RTIL A had a significantly lower torque value than Struktol, whereas all other RTILs had the same torque values as Struktol (Figure 5). At 1.0 wt%, RTILs A and C had better lubrication effects, i.e., lower torque values, than Struktol, whereas all other RTILs had the same lubrication effect as Struktol. At 2.0 wt% and 3.0 wt%, all RTILs had better lubrication effect than Struktol. At 2.0 wt%, the lubrication effect of RTILs had the following order: Β > A > C > E = F > D . At 3.0 wt%, the lubrication effect of RTILs had the following order: E ~ A ~ C > D ~ F (Figure 5). Interestingly, the order of the lubrication effect of RTILs changed with the usage of RTILs. Therefore, it is



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The amount of RTILs and Struktol in wood-PE composites (wt%) Figure 4. Effect of RTILs and Struktol on the MOE of wood-PE composites. (Control: wood-PE without any additive. Error bars show two standard errors of the meansfromten measurements.)

difficult to speculate the relationship between molecular structures of RTILs and their lubrication effect. When compared with results in Figure 2, the lubrication effect of RTILs had different orders at the same usage, which means that M A P E affected the lubrication effect of each RTIL in a different way. The effect of combinations of RTILs and M A P E and combinations of Struktol and M A P E on the M O R is shown in Figure 6. At 0.5 wt%, Struktol had a significantly lower M O R than the control, i.e., M 2 , whereas RTIL F had a significantly higher M O R than the control. Other RTILs (A, C , D, and E) had the same M O R as the control at 0.5 wt%. At 1.0 wt%, Struktol had a lower M O R than all RTILs and the control. At 1.0 wt%, RTILs C , D, and E eac h had a lower M O R than the control, whereas RTILs A , B , and F each had the same M O R as the control (Figure 6). At 2.0 wt% of Struktol and RTILs, the order of


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The amount of RTILs and Struktol in wood-PE composites (wt%) Figure 5. Effect ofRTILs-M2 and Struktol-M2 on torque. (M2: 2 wt% MAPE. Error bars show two standard errors of the means from two independent experiments.)

the M O R for Struktol and RTILs is as follows: F > A ~ C > Ε > Struktol - D > B. In other words, four RTILs resulted in a higher M O R than Struktol. A t 3.0 wt of Struktol and RTILs, the order of the M O R for Struktol and RTILs is as follows: F > D > E = A > Struktol > C . The order of the M O R for Struktol and RTILs varied with the usage (Figure 6). The order of the M O R was not in accord with the order of the torque value at each usage (Figs. 5 and 6). Overall, RTIL F reduced the strength-enhancing effect of M A P E much less than other RTILs and Struktol at the usage of 2.0 wt% and 3.0 wt% (Figure 6). The effect of combinations of RTILs and M A P E and combinations of Struktol and M A P E on the M O E is shown in Figure 7. A t 0.5 wt%, Struktol and RTILs A , C , D , E , and F all increased the stiffness of the wood-PE composites. At 1.0 wt%, Struktol and RTILs A , B , C , and Ε each had a higher


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The amount of RTILs and Struktol in wood-PE composites (wt%) Figure 6. Effect ofRTILs-M2 andStruktol-M2 on the MOR of wood-PE composites. (M2: wt% MAPE. Error bars show two standard errors of the meansfromten measurements.)

M O E than the control, i.e., M 2 , whereas RTILs D and F had the same M O E as the control. At 2.0 wt%, RTILs F and C had a higher M O E than Struktol, whereas RTILs B , D, and Ε each had a much lower M O E than Struktol. At 3.0 wt%, Struktol had a much lower M O E than RTIL F, but had a much higher M O E than RTIL C . RTIL F was the only RTIL that resulted in a higher M O E than the control at all usages studied (Figure 7).

Conclusions In the absence of M A P E , RTILs generally had better lubrication effect than Struktol at the same usage. Except RTIL F, RTILs also resulted in lower M O R and M O E than Struktol at the same usage. RTIL F had the lubrication effect



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The amount of RTILs-M2 and Struktol-M2 in wood-PE composites (wt%) Figure 7. Effect ofRTlLs-M2 and Struktol-M2 on the MOE of wood-PE composites. (M2: wt% MAPE. Error bars show two standard errors of the means from ten measurements.)

better than or comparable to Struktol, but had higher M O R and M O E than Struktol at each usage studied. In the presence of 2.0 wt% M A P E , all RTILs at a practical usage level (2.0 wt% or 3.0 wt%) had better lubrication effect than Struktol. RTILs A , C, E, and F at 2.0 wt% usage and RTILs A , D , E , and F each reduced the strength-enhancing effect of M A P E less than Struktol. When used with 2.0 wt% M A P E , RTIL F was far superior to other RTILs and Struktol in terms of providing the lubrication effect and retaining the strength-enhancing effect of M A P E . A t 0.5 wt% or 1.0 wt% usage level, a combination of a R T I L and M A P E or a combination of Struktol and M A P E all increased the stiffness of the resulting wood-PE composites. A t 2.0 wt%, RTILs B , D , and E, when used with 2.0 wt% M A P E , significantly reduced the stiffness of the resulting woodPE composites, whereas other RTILs either increased or did not change the stiffness of the resulting composites. At 3.0 wt%, Struktol and all RTILs except


180 RTIL F significantly decreased the stiffness of the resulting wood-PE composites. Combinations of RTIL F and 2.0 wt% M A P E were superior to combinations of Struktol and 2.0 wt% M A P E in terms of increasing the stiffness of the resulting wood-PE composites.


Acknowledgments We appreciate American Wood Fibers (Schofield, WI), B P Solvay Polyethylene North America (Houston, T X ) , Honeywell International, Inc. (Morristown, NJ), and Cytec Industries, Inc. (West Paterson, NJ) for donating wood flour, high-density polyethylene, M A P E , and room temperature ionic liquids, respectively. This research was supported by a grant from the national research initiative competitive grants program of U S D A (award number: 200335103-13864).

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181 14. Clemons, C. For. Prods. J. 2002, 52, pp 10-18. 15. Sellers, Jr., T.; Miller, G D.J.; Katabian, M. For. Prods. J. 2000, 50, pp 2428. 16. Morton, J.; Quarmley, J.; Rossi, L . In The seventh international conference on woodfiber-plastic composites; Madison, Wisconsin, 2003, pp 3-6. 17. Gauthier, R.; Joly, C.; Coupas, A . C . ; Gauthier, H . ; Escoubes, M . Polym. Compos. 1998, 19, pp 287-300. 18. Harper, D.; Wolcott, M. Composites: Part A 2004, 35, pp 385-394. 19. Y e , C.; L i u , W.; Chen, Y.; Y u , L . Chem. Commun. (Cambridge, U. K.) 2001, pp 2244-2245. 20. Swatloski, R.P.; Holbrey, J.D.; Rogers, R.D. Green Chem. 2003, 5, pp 361363. 21. Bradaric, C.J.; Downard, Α.; Kennedy, C.; Robertson, A.J.; Zhou, Y . In Ionic liquids as geen solvents: progress and prospects, Rogers, R.D., Seddon, K . R . , Eds.; A C S symposium series 856; American Chemical Society: Washington, D C , 2003, pp 41-56.


Materials, Chemicals, and Energy from Forest Biomass - American

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