Morphology and Mechanical Properties of Zinc Borate- Pretreated Poplar Wood Flour/Plastic Composite

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1 Morphology nd Mechnicl Properties of Zinc Borte- Pretreted Poplr Wood Flour/Plstic Composite Alirez Bdritl, Seyyed Khlil Hosseini Hshemi,, * Behzd Kord, b Seyed Mjid Zbihzdeh, c nd Vhidrez Sfdri The effect of zinc borte (ZB) tretment on the mechnicl nd morphologicl properties of wood flour/polypropylene composites ws investigted. Wood flour ws first treted with ZB solution (1% w/w in ethnol-distilled wter), followed by 24 hours of soking on n unheted mgnetic stirrer hot plte until reltively complete sturtion ws reched. Then, composites bsed on ZB-pretreted, ZB-treted-duringmnufcturing, nd untreted wood flour, polypropylene nd coupling gent were mde by melt compounding nd then injection molding. The ZB tretment hd no significnt influence on mechnicl properties of the composite with the exception of tensile strength. The composite mde with ZB-pretreted wood flour exhibited the sme mechnicl properties s the composites mde with ZB-in-process-treted wood flour; however there were sttisticlly significnt differences between flexurl modulus nd tensile strength of ZB-pretreted composites nd ZB-in-process treted ones. Specimens contining the ZB showed lower flexurl, tensile, nd impct strength compred with the untreted specimens. However, the zinc borte tretments produced modest improvements in hrdness performnce. The SEM microgrphs reveled tht the outer surfce of the wood fibers ws coted by some crystlline deposits of zinc borte. Keywords: Zinc borte; Pretretment; In-process; Mechnicl properties; Wood-polypropylene composites Contct informtion: : Deprtment of Wood Science nd Pper Technology, Krj Brnch, Islmic Azd University, Krj, Irn; b: Assistnt Professor, Deprtment of Wood Science nd Pper Technology, Chlous Brnch, Islmic Azd University, P.O. Box: 46615/397, Chlous, Irn; c: Fculty of Nturl Resources, Sri Agriculturl Sciences nd Nturl Resources University, Sri, Mzndrn, Irn; *Corresponding uthors: hshemi@kiu.c.ir/skh_hosseini@yhoo.com INTRODUCTION Wood-bsed composite mterils re incresingly used for exteriors, where conditions re conducive to fungl, insect, nd mrine borer ttck (Eton nd Hle 1993; Grdner et l. 23). Wood-thermoplstic composites hve trditionlly been viewed s building mterils tht re not prone to biodegrdtion (Morris nd Cooper 1998), nd recently, the use of wood plstic composites (WPCs) s substitute for wood decking hs been incresing (Wolcott nd Englund 1999). Although the presence of resin nd preservtive in the wood composites my be expected to slow fungl ttck, severl studies hve shown tht the wood component sorbs wter nd eventully decys, lbeit more slowly initilly thn does solid wood of the sme species (Simonsen et l. 24). Four preservtive systems nd tretment processes tht hve been used in the preservtion of composites re: the use of pretreted wood, in-process nd post-process preservtive tretments, nd the use of recycled treted wood elements in mnufcturing Bdritl et l. (213). Zinc borte in W-P composites, BioResources 8(1),

2 or the use of wood species with high nturl resistnce ginst biodegrdtion (Grdner et l. 23). Of the four different preservtive methods, the in-process preservtive tretments, in which the preservtive tretment is incorported during the mnufcturing process, re fvored for composite mde from flkes, prticles, nd fibers, wheres the use of pretreted wood is prticulrly common in some solid lumber lmintes (Grdner et l. 23). In recent yers, bortes (disodium octborte tetrhydrte, zinc borte, nd boric cid), s composite dditives pplied in powder form, hve been used s preservtives for the in-process tretment of wood composites (Lks 1999; Grdner et l. 23). Bortes re excellent fungicides nd insecticides, but the primry borte used for wood preservtion, sodium octborte tetrhydrte, is highly wter soluble nd tends to lech from wet wood (Murphy et l. 1995). Moreover, ZB is n excellent fungicide nd insecticide, is fr less soluble, nd field testing of this mteril in Hwii under hrsh sub-tropicl conditions is ongoing, with erly inspections indicting promising results (Simonsen et l. 24; Grdner et l. 23). Zinc borte is lso commonly used s zinc flme retrdnt, good substitute for ntimony trioxide (EFRA 26). Both polymers nd wood re sensitive to fire. Thus, fire retrdnts must be employed in order to improve fire behvior of such composites. Depending on the preservtive tretment processes used, the preservtive tretment will hve either negtive impct within the mnufcturing process or on the finl product. For instnce, one criticl problem with the use of bortes s preservtives for wood composite pnels bonded with phenol-formldehyde resins is the interction between the borte nd the resin during the mnufcturing of the pnel, which could significntly decrese the resin gel time (Grdner et l. 23). Lu et l. (28) found tht ll ZB-treted wood-hdpe composites hd lower men tensile strengths thn did the control (i.e., the untreted wood-hdpe composites). However, in the concentrtion rnge between % nd 9%, the tensile strengths of ll ZBtreted wood-hdpe composites were not significntly different from tht of the control. It is well known tht zinc borte (ZB), s n in-process inorgnic biocidl dditive with very low mmmlin toxicity nd cost, brod ctivity ginst fungi nd insects, nd high leching resistnce (Schultz nd Nichols 23), is mostly used commercilly in mny wood composites, including plstic wood nd prticlebord (Lks 1999). However, there is no informtion currently vilble on the morphology nd mechnicl properties of ZB-pretreted poplr wood flour/plstic composite (ZB-PT-PWF/PC). This work investigtes the fesibility of pplying the ZB-treted wood flour to the WPCs in the preprocess (i.e., whether the ZB tretment will hve negtive impct on the mechnicl properties of the WPCs). The min objective of this study ws to determine effects of zinc borte on the mechnicl nd morphologicl properties of composites bsed on wood flour nd polypropylene. EXPERIMENTAL Mterils A polypropylene (PP) mtrix with melt flow index (MI) of 1 g/1 min nd density of.95 g/cm 3 ws supplied by the Tbriz Petrochemicl Compny of Irn. The lignocellulosic mteril used s the reinforcing filler in the composite ws fresh poplr Bdritl et l. (213). Zinc borte in W-P composites, BioResources 8(1),

3 wood (Populus deltoides), which ws obtined from the Amol frms of Irn. To obtin wood flour (WF), the wood ws cut into smll pieces nd chopped using lbortory electricl rotry mill. The WF size ws between 4 nd 6 mesh. The mleic nhydride polypropylene (MAPP) ws obtined from Estmn Chemicl Products, Inc., s Epolene G-33 TM polymer with 8% cid nhydride nd moleculr weight of It ws used s the coupling gent. Zinc borte (ZB) ws provided by U.S. Borx Inc., Tourney Rod, Vlenci, Cliforni 91355, s their product Borogrd B. Wood Flour Tretment Before the pretretment, ZB solution ws prepred by the following procedure. A required mount of ZB (25 g) ws plced into continer with 25 ml ethnol solvent nd stirred for 3 min with mgnetic stirrer. Then 25 ml suspension of ZB-ethnol solvent ws plced into continer with 225 ml distilled wter nd stirred for 3 min in similr mnner. The concentrtion level of the ZB ws designed to be 1%, bsed on the weight of oven-dried wood flour. A certin mount of oven-dried poplr wood flour (PWF) ws dded into the treting solution nd continuously stirred for 24 h. The ZBpretreted poplr wood flour (ZB-PTPWF) ws then filtered with screen nd stored t room temperture for 24 h, oven-dried t 8 ºC until it reched constnt weight, nd then stored in seled plstic bgs to wit blending with polypropylene. Before nd fter impregntion, the PWF ws kept in drying oven t 13 ± 2 ºC until constnt weight hs been chieved. After the PWF nd ZB-PTPWF were cooled in desicctors, their oven-dry weights were mesured (by 49 g nd by 547 g, respectively). The retention rtio of the ZB (R %) ws clculted s follows, R (%) = (M b M )/M 1 (1) where M nd M b (g) denote the oven-dry weights of the PWF prior to nd fter impregntion with ZB chemicl tretment, respectively. Composite Preprtion Tble 1 shows the blend design for zinc borte-pretreted wood flour/ polypropylene composites. Before preprtion of smples, poplr wood flour ws dried in n oven t (65 ± 2) C for 24 h. The mixing ws crried out with Hke internl mixer (HBI System 9, USA) t 18 C nd 6 RPM. Tble 1. Formultion of Composites Code ZB Concentrtion (wt %) Wood flour (wt %) Polypropylene (wt %) MAPP (wt %) UT-WPC ZB-T-WPC 1 in powder ZB-PT-WPC 1 in solution UT-WPC: untreted wood flour/plstic composite; ZB-T-WPC: zinc borte treted wood flour in mnufcturing process/plstic composite; ZB-PT-WPC: zinc borte pretreted wood flour/plstic composite First the polypropylene ws fed to mixing chmber. After melting of the PP, coupling gent (MAPP) ws dded. At the fifth minute, the wood flour ws fed, nd the totl mixing time ws 11 min. The compounded mterils were then ground using pilot scle grinder (Wieser, WGLS 2/2 Model). The resulting grnules were dried t 7 C for 24 h. Test specimens were injection molded into ASTM stndrd by n Bdritl et l. (213). Zinc borte in W-P composites, BioResources 8(1),

4 injection molder t 185 C nd injection pressure ws 1 MP (Emn mchine, Irn). The nominl dimensions of specimens (Fig. 1) were mm. The specimens were stored under controlled conditions (5% reltive humidity nd 23 C) for t lest 4 h prior to testing. Fig. 1. Mechnicl testing smples Mesurements The flexurl nd tensile tests were conducted ccording to ASTM D79 nd D 638, respectively, using n Instron mchine (Model 1186, Englnd); the tests were performed t crosshed speeds of 5 mm/min. A Zwick impct tester (Model SIT 2 D, Sntm Co., Irn) ws used for the Izod impct test. All the smples were notched on the center of one longitudinl side ccording to ASTM D 256. Hrdness tests were crried out ccording to ASTM D 137 specifictions by n Instron hrdness tester model 4486 nd 1 KN lod-cell. The cross-hed speed ws 5 mm/min (The mount of bll penetrtion in the specimen is 5.6 mm ccording to wood hrdness stndrd, but becuse of the rupture of specimens t this rte, it ws modified to 2 mm). The morphology of composites ws chrcterized using scnning electron microscopy (SEM, Model LEO 44i, Oxford) t 25 kv ccelerting voltge. Smples were first frozen in liquid nitrogen nd frctured to ensure tht the microstructure remined clen nd intct, nd then coted with gold lyer to provide electricl conductivity. The sttisticl nlysis ws conducted using SPSS progrmming (Version 16) method in conjunction with the nlysis of vrince (ANOVA) techniques. Duncn multiple rnge test ws used to test the sttisticl significnce t α =.5 level. For ech tretment level, four replicted smples were tested for ech property. RESULTS AND DISCUSSION The results of Duncn s test indicted tht the ZB tretment hd no significnt effect on the hrdness of wood flour/polypropylene composites. In ddition, there were no significnt differences between hrdness vlues of ZB-PT-WPC nd ZB-T-WPC smples (P<.5). As cn be seen in Fig. 2, the slight increse found in the hrdness vlues of ZB-pretreted smples my be due to the precipittion of ZB in the cell lumens nd the cell wll of wood prticles nd the subsequent increse in density. Like the Bdritl et l. (213). Zinc borte in W-P composites, BioResources 8(1),

5 Impct strength (J/m) Hrdness (Shore D) PEER-REVIEWED ARTICLE preservtive slts, fire retrdnt slts hve lso precipitted in the cell cvity nd the cell wll (Winndy nd Rowell 1984). 8 7 () 7.25 () 7.55 () Fig. 2. Effect of zinc borte tretment on hrdness of wood flour/polypropylene composites (Duncn s multiple rnge tests re given in the prentheses) The results of Duncn s test indicted tht the ZB tretment hd no significnt effect on the impct strength of the composites. There were no significnt differences between ZB-PT-WPC nd ZB-T-WPC smples (P<.5). As cn be seen in Fig. 3, the specimens contining the ZB showed lower notched impct strength compred with the untreted specimens. The impct resistnce of the ZB-PT-WPC nd ZB-T-WPC specimens decresed by 21% nd 17.9%, respectively, compred to the UT-WPC specimens. This ws minly ttributed to the poor comptibility between the wood nd polymer mtrix due to the crystlline deposits of zinc borte (Ayrilmis et l. 212). Also, the ZB-T-WPC specimens hd slightly higher verge impct resistnce thn the ZB-PT- WPC ones. It seems tht the ZB tretment is probbly ffected on dispersion nd precipittion of ZB prticles in the cvities of composites. So, we expected tht the specimens contining the ZB-PT due to the formtion of gglomertion cuse the reduction of dhesion in the composite interfce compred with the ZB-T specimens. 5 () 46 () 36 () Fig. 3. Effect of zinc borte tretment on impct strength of wood flour/polypropylene composites (Duncn s multiple rnge tests re given in the prentheses) The results of Duncn s test indicted tht the ZB tretment hd no significnt effect on the flexurl strength. However, there ws significnt differences between the flexurl modulus vlues of the ZB-PT-WPC nd ZB-T-WPC (P<.5). As cn be seen in Bdritl et l. (213). Zinc borte in W-P composites, BioResources 8(1),

6 Flexurl modulus (MP) Flexurl strength (MP) PEER-REVIEWED ARTICLE Figs. 4 nd 5, the flexurl strength nd modulus of ZB-PT-WPC nd ZB-T-WPC specimens were less thn the corresponding untreted specimens. This is probbly due to the increse in stiffness cused by formtion of ZB crystlline deposits in wood flour. This finding is consistent with those of previous studies (Ayrilmis et l. 211,b; Kurt nd Mengeloglu 211). Also, the ZB-T-WPC specimens hd higher flexurl strength nd modulus thn the ZB-PT-WPC ones. It seems tht the specimens contining the ZB-PT due to the formtion of gglomertion cuse the reduction of dhesion in the composite interfce compred with the ZB-T specimens Fig. 4. Effect of zinc borte tretment on flexurl strength of wood flour/polypropylene composites (Duncn s multiple rnge tests re given bove ech br) b b 3294 Fig. 5. Effect of zinc borte tretment on flexurl modulus of wood flour/polypropylene composites (Duncn s multiple rnge tests re given bove ech br) The results of Duncn s test indicted tht ZB tretment hd significnt effect on the tensile strength of the ZB-T-WPC specimens (P<.5). However there were no significnt differences between tensile strength nd modulus vlues of UT-WPC nd ZB- PT-WPC. As cn be seen in Figs. 6 nd 7, the tensile strength nd modulus of ZB-PT- WPC nd ZB-T-WPC specimens were less thn the untreted specimens. Also, the ZB-T- WPC specimens hd higher flexurl strength nd modulus thn the ZB-PT-WPC specimens. The reduction of tensile strength nd modulus in the zinc-borted composites cn be ttributed to the sme resons s discussed concerning flexurl strength nd modulus. Bdritl et l. (213). Zinc borte in W-P composites, BioResources 8(1),

7 Tensile modulus (MP) Tensile strength (MP) PEER-REVIEWED ARTICLE b Fig. 6. Effect of zinc borte tretment on tensile strength of wood flour/polypropylene composites (Duncn s multiple rnge tests re given bove ech br) Fig. 7. Effect of zinc borte tretment on tensile modulus of wood flour/polypropylene composites (Duncn s multiple rnge tests re given bove ech br) SEM microgrphs in Fig. 8 shows tht the outer surfce of the wood fibers ws surrounded by some crystlline deposits of ZB, which incresed the surfce re of the solids within the WPC nd reduced the bonding efficiency of the polymer. However, the interction between the PP nd WF treted with ZB cn be improved, to some extent, by incorportion of the MAPP. In ZB-treted specimens, severl holes cn be seen. These pper to result from the fiber pull out from the mtrix, indicting poor bonding between wood flour nd polymer mtrix (Fig. 8). The number of such holes in ZB-PT-WPC (Figs. 8c nd 8d) ws lrger thn ZB-T-WPC (Figs. 8 nd 8b); the comptibility between polymer nd wood flour ws probbly ffected. It is lso evident from Figs. 8e nd 8f tht there were considerbly fewer such holes nd mny broken fiber ends embedded in the polymer mtrix indicting better comptibility between wood flour nd mtrix. Bdritl et l. (213). Zinc borte in W-P composites, BioResources 8(1),

8 b c d e f Fig. 8. SEM microgrphs of the frcture surfces in the composites under different tretments: ZB-T-WPC ( - b); ZB-PT-WPC: (c - d); nd UT-WPC: (e - f) CONCLUSIONS From the reserch work herein, the following conclusions cn be drwn: 1. Zinc borte tretment hd no significnt influence on mechnicl properties of wood flour/polypropylene composites with the exception of tensile strength. 2. The specimens contining the ZB showed lower flexurl, tensile, nd impct strength compred with the untreted specimens. However, the zinc borte tretments produced modest improvements in hrdness performnce. Bdritl et l. (213). Zinc borte in W-P composites, BioResources 8(1),

9 3. The SEM microgrphs reveled tht the outer surfce of the wood fibers ws coted by some crystlline deposits of zinc borte, which incresed the surfce re of the solids within the WPC nd reduced the bonding efficiency of the polymer. ACKNOWLEDGMENTS The uthors re grteful for the support of the Deprtment of Wood Science nd Pper Technology, Krj Brnch, Islmic Azd University. This rticle is extrcted from Alirez Bdritl s M.S. disserttion. REFERENCES CITED Ayrilmis, N., Benthien, J. T., Thoemen, H., nd White, R. H. (211). Effects of fire retrdnts on physicl, mechnicl nd fire properties of flt-pressed WPCs, Eur. J. Wood Prod, in press (Doi:1.17/s ). Ayrilmis, N., Benthien, J. T., Thoemen, H., nd White, R. H. (211b). Properties of fltpressed wood plstic composites contining fire retrdnts, J. Appl. Polym. Sci. in press. Ayrilmis, N., Akbulut, T., Dundr, T., White, R. H., Mengeloglu, F., Buyuksri, U., Cndn, Z., nd Avci, E. (212). Effect of boron nd phosphte compounds on physicl, mechnicl, nd fire properties of wood-polypropylene composites, Construction nd Building Mterils 33, Eton, R. A., nd Hle, M. D. C. (1993). Wood: Decy, Pests nd Protection, Chpmn nd Hle: London, 546 pp. EFRA (26). Flme retrdnt fct sheet: Zinc borte, Europen Flme Retrdnts Assocition, Jnury ZincBorteFctsheet.pdf Grdner, D. J., Tscioglu, C., nd Wålinder, M. E. P. (23). Wood composite protection, In: Wood Deteriortion nd Preservtion, ACS Symposium Series No. 845, Goodell, B., Nichols, D., nd Schultz, T. (eds.), Americn Chemicl Society, Wshington, DC, Kurt, R., nd Mengeloglu, F. (211). Utiliztion of boron compounds s synergists with mmonium polyphosphte for flme retrdnt wood-polymer composites, Turk. J. Agric. For. In press (Doi:1.396/tr-91-58). Lks, P. E. (1999). The pst, present, nd future of preservtive contining-composites, In: 33rd Interntionl Prticlebord/Composite Mterils Symposium Proceedings; M. P. Wolcott, R. Tichy, nd D. F. Bender (eds.), Wshington Stte University, Pullmn, WA, pp Lu, J. Z., Dun, X., Wu, Q., nd Lin, K. (28). Chelting efficiency nd therml, mechnicl nd decy resistnce performnces of chitosn copper complex in woodpolymer composites, Bioresource Technology 99(13), Morris, P. I., nd Cooper, P. (1998). Recycled plstic/wood composite lumber ttcked by fungi, For. Prod. J. 48(1), Murphy, R. J., Brnes, H. M., nd Gry, S. M. (1995). Decy nd soil depletion studies with polymer/boron preservtive systems, For. Prod. J. 45(9), Bdritl et l. (213). Zinc borte in W-P composites, BioResources 8(1),

10 Schultz, T. P., nd Nichols, D. D. (23). A brief overview of non-rsenicl wood preservtive systems, In: Wood Deteriortion nd Preservtion, ACS Symposium Series No. 845, Goodell, B., Nichols, D., nd Schultz, T. (eds.), Americn Chemicl Society, Wshington, DC, Simonsen, J., Freitg, C., Silv, A., nd Morrell, J. J. (24). Wood/plstic rtio: Effect on performnce of borte biocides ginst brown rot fungus, Holzforschung 58, Winndy, J. E., nd Rowell, R. M. (1984). The chemistry of wood strength, In: Rowell, R. M. (ed.). The Chemistry of Solid Wood, Advnces in the Chemistry Series 27, Americn Chemicl Society, Wolcott, M. P., nd Englund, K. (1999). A technology review of wood-plstic composites, In: Proceedings 33rd Interntionl Prticlebord/Composite Mterils Symposium, Wshington Stte University, Pullmn, pp Article submitted: July 31, 212; Peer review completed: September 12, 212; Revised version received: December 8, 212; Accepted: Jnury 3, 213; Published: Jnury 7, 213. Bdritl et l. (213). Zinc borte in W-P composites, BioResources 8(1),