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Wu, G.-F., and Xu, M. (2014). "Effects of boron compounds on the mechanical and fire properties of wood-chitosan and high-density polyethylene composites," BioRes. 9(3), 4173-4193.

Abstract

Wood-plastic composites (WPCs) represent a growing class of durable, low-maintenance construction materials whose use can decrease dependence on petroleum. High-density polyethylene (HDPE), chitosan (CS), wood flour (WF), boric acid (BA), and borax (BX), as well as maleic anhydride grafted polyethylene (MAPE) and polyethylene wax (PE wax), were used to develop a durable wood-plastic composite (WPC) using the extrusion method. The effects of boron compounds (3%, 6%, 9%, or 12% by weight BA/BX) on the mechanical and fire properties of the WPCs were investigated. Mechanical testing indicated that as the percentage weight of boron compounds increased, the flexural modulus, flexural strength, and tensile strength significantly decreased. Cone calorimeter tests were used to characterize the fire performance of the WPCs, and these results suggested that adding BA/BX compounds to WPCs modestly improved the fire performance. As the percentage weight of BA/BX increased from 3% to 9%, the time to ignition (TTI), heat release rate (HRR), total heat release rate (HRR-Total), smoke production rate (SPR), and specific extinction area (SEA) of the WPCs were all reduced.


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Effects of Boron Compounds on the Mechanical and Fire Properties of Wood-chitosan and High-density Polyethylene Composites

Guo-Fu Wu a,b and Min Xu a,*

Wood-plastic composites (WPCs) represent a growing class of durable, low-maintenance construction materials whose use can decrease dependence on petroleum. High-density polyethylene (HDPE), chitosan (CS), wood flour (WF), boric acid (BA), and borax (BX), as well as maleic anhydride grafted polyethylene (MAPE) and polyethylene wax (PE wax), were used to develop a durable wood-plastic composite (WPC) using the extrusion method. The effects of boron compounds (3%, 6%, 9%, or 12% by weight BA/BX) on the mechanical and fire properties of the WPCs were investigated. Mechanical testing indicated that as the percentage weight of boron compounds increased, the flexural modulus, flexural strength, and tensile strength significantly decreased. Cone calorimeter tests were used to characterize the fire performance of the WPCs, and these results suggested that adding BA/BX compounds to WPCs modestly improved the fire performance. As the percentage weight of BA/BX increased from 3% to 9%, the time to ignition (TTI), heat release rate (HRR), total heat release rate (HRR-Total), smoke production rate (SPR), and specific extinction area (SEA) of the WPCs were all reduced.

Keywords: Wood flour; Chitosan; High-density polyethylene; Mechanical; Fire retardant; Boron compounds

Contact information: a: Key Laboratory of Bio-based Material Science and Technology of the Ministry of Education, Northeast Forestry University, Harbin, PR, 150040 China; b: Guangxi Eco-engineering Vocational and Technology College, Liuzhou, PR, 545004 China;

* Corresponding author: stxy002@sina.com

INTRODUCTION

Wood-plastic composites (WPCs) represent an emerging class of materials that combine the favorable performance and cost attributes of both wood and plastics and act as an environmentally friendly way to increase the use of recycled plastics (Faruk et al. 2007; Clemons 2002). However, WPCs present potential fire hazards; thus, it is necessary to improve our understanding of both the fire performance and the effects of fire retardants (FRs) on these products. Because WPCs have the potential to be widely used in products such as decking, furniture, electrical casings, industrial flooring, interior decorations, insulation, automobile parts, and architectural materials, developing safe and durable WPCs is critical.

Since the late 1970s, boron compounds such as borax (BX), boric acid (BA), and zinc borate (ZB) have been used as FRs in the plastics industry. Boron compounds have also been used as fungicides and insecticides for wood preservation and are especially effective against termites (Ayrilmis 2013). Boron-based buffers have also been used as additives in FR treatments and have been found to significantly reduce the severity of thermal degradation (Wu et al. 2010; Nagieb et al. 2011). A mixture of BA and BX has been found to effectively retard flame spread and smoldering (Chai et al. 2012). Although boron compounds reduce the spread of flame in wood, they may have diverse effects on the mechanical properties and hygroscopicity of WPCs (Nagieb 2011); therefore, more research in this area is warranted.

Chitin is the second most abundant natural polysaccharide after cellulose, and it possesses excellent mechanical and thermal properties (Martinez et al. 2010; Ou et al. 2010). Chitosan (CS), a derivative of chitin, is a linear polysaccharide that originates from the shells of crustaceans such as crab and shrimp. It is a widely available, low-cost product with high biocompatibility and anti-microbial properties (Khor and Lim 2003). Chitosan also has a low fiber/matrix adhesion ratio (Mir et al. 2011) and a large number of commercial uses. Therefore, CS was chosen as a WPC component for this study.

The fire performance of WPCs is not well understood, and there is little information regarding the effectiveness of various FRs on WPCs (Ayrilmis et al. 2012). This study used high-density polyethylene (HDPE), CS, wood flour (WF), BA/BX, maleic anhydride grafted polyethylene (MAPE), and PE wax as a lubricant to develop a durable and flame-resistant WPC with the extrusion method. The primary objective of this study was to investigate the effects of boron compounds on the mechanical and FR properties of WPCs.

EXPERIMENTAL

Materials

Poplar (Populus tremula) wood flour was obtained from the Xingrong Wood-fiber Company in Harbin, China. Particle size ranged from 20-mesh to 40-mesh. The wood-flour was dried in a laboratory oven at 103 °C ± 2 °C for 12 h to a moisture content of 3% to 4% (based on the oven-dry weight of the wood), and was then stored in a polyethylene bag. Eighty-five percent de-acetylated CS was obtained from the HuaiFang Chemical Company in Shandong, China. The high-density polyethylene (HDPE) resin (5000 S) used in this work was purchased from DaQing Petrifaction Co., Ltd. in DaQing, China. Boric acid (BA) (H3BO3) (q = 1.7 g/cm3, crystalline solid form) and borax (BX) (Na2B4O7.10H2O) (q = 1.4 g/cm3, crystalline solid form) (BA/BX, 1:1 by weight) were manufactured by the Shanghai Chemical Company of China. Maleic anhydride grafted polyethylene (MAPE) was manufactured by Fengyuan Co., Ltd. in Guangzhou, China. All additives were industrial products.

Methods

Sample preparation

Samples of WPCs were prepared according to the compound formulations shown in Table 1. All of the components were mixed in a high-speed blender (SHR-10A, Zhangjiagan, China) with a rotation speed of 80 revolutions per minute (rpm) for 5 min. Composites were then extruded by a twin-screw extruder (SJSH30/SJ45, Nanjing, China) with a rotation speed of 50 rpm. Temperature was controlled and ranged from 155 to 165 °C. The samples were then extruded by a sign-screw extruder with a rotation speed of 10 rpm. Again, temperature was controlled and ranged from 150 to 160 °C. Finally, mechanical test samples were cut for tensile strength (with dimensions of 160 mm × 13 mm × 4 mm) and flexural strength and flexural modulus (80 mm × 13 mm × 4 mm). Cone calorimeter test samples were cut with dimensions of 100 mm × 100 mm × 4 mm. Decay resistance test samples were cut with dimensions of 80 mm × 100mm × 4 mm.

Table 1. Mixing Ratios of Raw Materials in Weight Percentage and their Codes

A = control sample. a = PE wax loading content is a total of (WF+CS+HDPE+MAPE)*2%. b = BA/BX loading content is a total of (WF+CS+HDPE+MAPE)* % BA/BX (of 3%, 6%, 9%, or 12%). and total (WF%+CS%+HDPE%+MAPE%) = 100%.

Mechanical testing

Tensile strength and flexural strength and modulus were tested following the ASTM standard D790, using an Instron, model RGT-20A (Norwood, MA, USA) device. Tests were carried out at 20 °C and 40% relative humidity. Crosshead speeds were 5 mm/min for tensile tests and 2 mm/min for flexural tests using type-6 samples. Test conditions were selected from the values recommended by the International Organization for Standardization (ISO). Statistical analyses using one-way analysis of variance (ANOVA) were conducted using SPSS v10.0 (SPSS, Chicago, IL, USA).

Cone calorimeter testing

A Fire Testing Technology Limited (FTT) cone calorimeter (manufactured in the UK) was used for fire testing. According to ISO 5660-1 standards, data were taken from the FTT cone calorimeter at a heat flux of 50 kW/m2. Samples were placed on a horizontal sample holder and were protected from bending or expanding during the heating process by a stainless steel grid. In the cone calorimeter tests, the initial peak heat release rate (Peak HRR, kW/m2), heat release rate at 60 s and 300 s (HRR-60s and HRR-300 s, MJ/m2), total heat release rate (HRR-Total, MJ/m2), effective heat of combustion (EHC, MJ/m2), mass loss rate (MLR, g/s), specific extinction area (SEA, m2/kg), total smoke production rate (SPR kW/m2), and total oxygen consumed (g) were all measured as a function of time. The observation of time to ignition (TTI, s) was also recorded.

Thermogravimetric analysis (TGA)

Thermogravimetric analysis (TGA) measurements were performed on a Perkin-Elmer Pyris6 Thermal Analyzer under nitrogen flow, from 20 oC to 700 oC at a heating rate of 10 oC /min. The degradation temperature was expressed as the onset temperature of mass loss.

Decay test

Fungal decay tests were carried out according to the classification criteria proposed by Silva et al. (2007) except that the specimens were replaced by the 80 mm × 10 mm × 4 mm thick specimens, using a brown-rot fungus and a white rot fungus. Then, they were incubated at 26 oC for 12 weeks. Test results were expressed as percentage of weight losses of WPCs specimens due to fungal attacks after decay test. Three replicates were used for each decay fungus. The weight loss was calculated and expressed in percent as follows,

Weight loss%=  (1)

where the initial weight (W0) is the weight of the specimens prior to fungal exposure and the final weight (W1) is the weight of specimens after fungal exposure.

RESULTS AND DISCUSSION

Mechanical Properties

One-way analysis of variance (ANOVA) tests indicated that the tensile strength (P < 0.002), flexural strength (P < 0.0001), and flexural modulus (P < 0.0001) with the CS WPCs significantly decreased with the addition of BA/BX, while the flexural strength and flexural modulus without CS WPCs were not significantly affected except for the tensile strength (Tables 2A and 2B).

Table 2. Means with Standard Error and Results of ANOVA for Tensile Strength, Flexural Strength, and Flexural Modulus of WPCs with Different Weight Percentages of BA/BX

Table 2A. ANOVA

Table 2B. Means and Standard Error of Mechanical Properties

Fig. 1. Tensile strength of WPCs with different BA/BX loadings

Tensile strength

Figure 1 demonstrates that the tensile strength of WPC samples with CS was significantly decreased with increasing BA/BX content. Compared with the control, the tensile strengths of the 9% and 12% BA/BX WPC samples decreased by 7.9% and 9.7%, respectively. Some studies have reported deterioration of the mechanical properties of filled and unfilled plastics with the addition of FRs, as well as a decrease in the tensile strength of filled plastics (Sain et al. 2004; Chiu and Wang 1998; Horn 2000). The present study indicated that this may be attributed to poor compatibility between the wood flour and polymer matrix due to the increased surface contamination of the wood flour by the presence of loosely adhering crystalline deposits of the boron compounds. Another reason was that the boron compounds could interfere with the compatibilizer (Ayrilmis 2013; Kurt and Mengeloglu 2011; Wang et al. 2007; Ayrilmis et al. 2012). However, Fig. 1 demonstrates that the tensile strength of samples without CS WPC first increased and then decreased with increasing BA/BX content. This finding was consistent with previous studies (Ayrilmis et al. 2012). The cited study reported that tensile strength of the wood–polypropylene composites samples first increased and then decreased with increasing BA/BX content (from 4% to 12%).

The importance of coupling agent to WPCs has been demonstrated in many studies (Lu et al 2000; Wielage et al. 2003; Leu et al. 2012; Lu et al. 2005; Balsuriya et al. 2002; Matuana et al. 2001; Kazayawokoet al. 1997). Over 40 coupling agents have been used in WPCs (Lu et al 2000). Leu et al. (2012) indicated that coupling agents are organic, inorganic, or hybrid compounds that can improve the compatibility and adhesion between the hydrophilic wood fiber and the hydrophobic polymers. Wielage et al. (2003) showed that using a low-molecular weight coupling agent can dramatically increase the tensile strength of WPCs by better wetting the wood fibers, but addition of the coupling agent also caused a slight decrease of the flexural strength. Ayrilmis (2013) evaluated the tensile strength of WPC samples with BA/BX and MAPE content (from 4% to 12% and from 2% to 6%), respectively. It was estimated that boron compounds stopped most of the esterification reactions between hydroxyl on the wood-fiber surface and anhydride on the polyethylene chain, which were replaced by the reactions between the boron and MAPE. In addition, the decrease in the polymer content of the WPC, as a function of the increase in the boron content, caused a reduction in the tensile strength. When the boron content in the WPC increases, the amount of the plastic as the adhesive decreases. Lu et al. (2005), Balsuriya et al. (2002), Matuana et al. (2001), and Kazayawoko et al. (1997) observed increases in the tensile strength and stiffness, which is attributed to the improved interfacial bonding between the wood flour and the HDPE matrix as well as the modification of individual components. That was due to the formation of ester bonds between the anhydride carbonyl groups of MAPP and hydroxyl groups of the wood fibers, showing that anhydride moieties of functionalized polyolefin coupling agents entered into an esterification reaction with the surface hydroxyl groups of wood flour. Upon esterification, the exposed polyolefin chains diffuse into the HDPE matrix phase and entangle with HDPE chains during hot pressing.

Flexural strength and modulus

Figure 2 demonstrates that the flexural strength of treated samples decreased moderately with increasing BA/BX content (from 3% to 12%). The maximum percentage of reduction in flexural strength (9.65%) occurred in the 12% BA/BX sample with CS. There were no significant differences in samples without CS. Adhikary et al. (2008) observed that the flexural strength exhibits a similar trend to the tensile strength although less variation is observed in the flexural strength with different formulations than the tensile strength. And the flexural MOE of the composites increases with the wood content. With a similar trend to the tensile test results, the addition of the coupling agents significantly improves the flexural strength as well as the stiffness of the composites. Due to the similar mechanism as explained in the previous section, the flexural strength of composites depended on MAPP coupling agent. A similar trend was observed for flexural modulus values as is seen in Fig. 3. Compared with the control sample, the maximum

Fig. 2. Flexural strength of WPCs with different BA/BX loadings

Fig. 3. Flexural modulus of WPCs with different BA/BX loadings

percentage of reduction in flexural modulus of the WPC samples was 23.66%, which occurred in the 12% BA/BX sample. Kiani et al. (2011) reported that the strength of wood plastic composites depends on the properties of the constituents and the interfacial interaction. The treated samples contain BA/BX compounds; therefore, their polymer content was relatively lower than that of the control samples. The BA/BX compounds also interfered with the compatibility between the wood and polymer matrix, reducing tensile strength. These results were consistent with previous studies, which found that the use of FRs resulted in weak interfacial adhesion between the matrix and/or WF, as well as the weakening of the crystalline part of the cellulose chain in WPCs (Ayrilmis et al. 2011; Kurt and Mengeloglu 2011; Stark et al. 2010). In the present study, this is attributed to weak adhesion between the polymer matrix and WF with the addition of BA/BX compounds. Figures 2 and 3 showed that flexural strength and modulus values of samples with and without CS had less variation. Baysal et al. (2007) also found that wood specimens pretreated with BA and BX mixture generated less MOR or MOE levels compared to that of specimens without BA and BX mixture pretreatment. Yildiz et al. (2004) determined the effects of wood preservatives on MOR and MOE. There were no significant differences in MOE. Yalinkilic et al. (1999) found that boric acid pretreatment of compressed wood polymer composite (CWPC) had no significant adverse effect on the MOE and MOR. Further work will be needed to consider the combined effect of CS, MAPE, and BA/BX to the mechanical properties of WPC by IR spectroscopy and Scanning Electron Microscope (SEM).

Fire Performance of WPCs

Heat release rate (HRR)

Combustion tests were conducted to measure the heat release rate (HRR) of the BA/BX samples. This is the basic parameter for fire modeling and denotes the rate at which heat energy is released per unit area of the sample (Xiao et al. 2011). Figure 4 shows the HRR of the samples, and average values for the other parameters tested are presented in Table 3B. One-way analysis of variance (ANOVA) tests in Tables 3A.

The results shown in Fig. 4 and Table 3B indicate that the BA/BX treatment provided modest improvements in fire performance for the WPCs. The BA/BX-treated samples demonstrated peak HRR reductions of 2.45%, 4.85%, 11.68%, and 17.68%, respectively, compared with the untreated samples.

From Table 3, the initial peak heat release rate (Peak HRR), average heat release rate at 60 s and 300 s (HRR-60s and HRR-300s), total heat release rate (HRR-Total), and effective heat of combustion (EHC) of the treated WPC samples were all lower than those of the untreated WPC control, and gradually decreased with increasing BA/BX content.

Compared with the Peak HRR of 330.8 kW/m2 for the control, the lowest Peak HRR was 281.1 kW/m2 for samples treated with 12% BA/BX—a decrease of 15.0% (Table 3). The 12% BA/BX samples also had the lowest HRR-300s at 249.7 kW/m2, compared with 294.5 kW/m2 for the control (Table 3). The HRR-Total for the 12% BA/BX samples was also the lowest, at 100.5 kW/m2, compared with 116.4 kW/m2 for the control.

The control samples also demonstrated the lowest EHC values (24.85 MJ/kg), compared with 23.88 MJ/kg for the 12% BA/BX samples.

Table 3A. ANOVA

Table 3B. Cone Calorimeter Results for WPCs with Different Loadings of BA/BX

Fig. 4. Curves of heat release rates (HRR kW/m2) plotted over time for the WPC control sample and samples containing 3%, 6%, 9%, and 12% of BA/BX by weight

In general, HRR was diminished by the addition of BA/BX compounds, which can be attributed to two main factors. First, adding BA/BX diluted the volatile decomposition compounds of the WPC with non-volatile compounds. Overall, this diminished the proportion of volatiles in the WPCs and therefore decreased HRR. Second, differences in BA can promote ethanol dehydration, which can generate large amounts of carbon when the borate and cellulose undergo thermal decomposition. This difference is, again, caused by the alteration of the proportion of the BA/BX compounds within the matrix material. Reductions in Peak HRR and HRR-Total in the BA/BX samples can be attributed to the formation of a protective, glass-like covering on the WPC surface during combustion, which obstructed oxygen access and thus prevented complete combustion (Xiao et al. 2011).

Time to ignition (TTI)

In the present study, time to ignition (TTI) was a measure of the time it took for the WPC surface to reach the critical ignition temperature during combustion tests. This depended on the thermal inertia of the material (a product of heat capacity, thermal conductivity, and density), which changed as a result of incorporating fillers (Hull et al. 2011). The 9% BA/BX samples (TTI, 44 s) demonstrated an 18.9% increase in TTI compared with the control (TTI, 37 s; Fig. 5). However, BA/BX compounds only had a slight effect on TTI. Fang et al. (2013) reported that in contrast with WF–PVC, ZB only had a slight effect on TTI and av-HRR of WF–PVC but had a higher pk-HRR, which is unfavorable to fire retardancy. Babrauskas (2003) found some models correlating TTI and heat flux have also been proposed assuming no changes in the physical constants of the material prior to ignition. Thomson and Drysdale (1987) observed that generally, for non-charring polymers in radiative heating, it is assumed that TTI is independent of the imposed heat flux. This is due to the lack of considerations such as melting/decomposition of the polymer during heating and others including changes in the surface absorption/emission properties (affecting the heat flux). Thus, there is always uncertainty in predicting the TTI. Some studies have confirmed that in polymer nanocomposites, TTI exhibits completely unpredictable and variable trends relative to neat polymers (Laoutid et al. 2009; Morgan 2006; Kiliaris and Papaspyrides 2010; Levchik 2010).

Fig. 5. Time to ignition (TTI) for WPCs with different BA/BX loadings

As the BA/BX melted during combustion, it formed a vitreous outer layer along the WF fibers, which isolated them from oxygen and heat transmission. The cellulose hydroxylation and the condensed-phase boron reacted and formed BA ester, which inhibited the formation of levoglucosan and reduced the amount of combustible gas. Because BA promotes ethanol dehydration, large amounts of carbon were generated when the borate and cellulose underwent thermal decomposition. These results indicated that the addition of BA/BX compounds to WPCs can improve FR performance.

Mass loss rate (MLR)

Mass loss rate (MLR) is a measure of a sample’s mass loss per unit of time during combustion. It reflects the material’s thermal degradation, volatility, and flaming degree given a measured amount of fire strength. In the present study, as BA/BX content increased from 0% to 12%, MLR gradually decreased (Fig. 6 and Table 3). The reported thermal properties supported this (Table 4 and Fig. 7 TGA curves for WPCs with different BA/BX loadings).

The present results demonstrated that BA/BX compounds in the WPCs accelerated char formation. Therefore, the decreases in MLR seen in these tests were likely caused by the formation of a protective layer of compact black char, which prevented gas penetration into the WPC and caused incomplete combustion. Baysal et al. (2004 and 2007) found that BA and BX mixture pretreatment played an important role in reducing weight losses of wood, and weight losses of BA and BX mixture pretreated WPC were reduced than those of monomer only treated wood clarifying the protective effect of boron in combustion. Baysal et al. (2002) and Yalinkilic et al. (1998) found that the weight loss of untreated and boron–vinyl monomer combination-treated wood. This indicated that the wood became more difficult to ignite and more oxygen was needed to burn the treated materials, consistent with the effect of boron (Le et al. 1990).

Fig. 6. Mass loss rate (MLR) of WPCs with different BA/BX loadings

Smoke production rate (SPR)

Smoke production rate is defined as the rate at which smoke is produced during combustion per unit of time. During a fire, inhalation of smoke can be one of the greatest hazards to life; therefore, understanding smoke production by WPCs during combustion is critical to improving their fire safety (Anthony 1999). Table 3 shows that SPR values of the BA/BX-treated WPC samples were lower than those of the untreated controls. The 12% BA/BX samples with the lowest SPR of 10.53 kW/m2 exhibited a 9.97% decrease in SPR, compared with 11.58 kW/m2 for the untreated control samples. This can be attributed to the presence of BA/BX compounds, which altered the proportion of volatiles, increased char formation, and led to a decrease in SPR. Thus, this study supports the idea that char formation is a mechanism of smoke suppression (Starnes et al. 2003).

Thermal degradation

Thermogravimetric analysis results (curves of TGA and DTG) of samples A, B1, B2, B3, and B4 in pure nitrogen atmosphere are shown in Figs. 7 and 8, and the calculated typical data are listed in Table 4.

Table 4. Thermogravimetric Analysis Data under Pure Nitrogen

Fig. 7. TGA curves for WPCs with different BA/BX loadings

The TGA curves in Fig. 7 showed that the specimens underwent a two-step thermal degradation processes. In the first step, thermal degradation of wood flour took place, and the thermal degradation of HDPE in WF/HDPE/CS composites took place mainly at the second step. A similar phenomenon has been reported by others (Yemele et al. 2010; Bouza et al. 2009; Meng and Tjong 1999). Shen et al.(2008) found such effects for boric acid and boron oxide (B2O3). B2O3 softens at 350 oC and flows above 500 oC, leading to the formation of a protective vitreous layer. Studies by Fang et al. (2013) and Bai et al. (2011) indicated that the thermal degradation temperature scope of wood-flour is from 201 oC to 410 oC. From their DTG curves in Fig. 8, it is apparent that when the BA/BX compounds were added, the decomposition temperature of wood flour and HDPE shifted to lower and higher temperature, respectively. According to the data of Table 5, addition of the BA/BX compounds made the thermal degradation of the WF/HDPE/CS

Fig. 8. DTG curves for WPCs with different BA/BX loadings

composites system take place earlier and promoted char formation. BA/BX compounds hardly affected the initial decomposition temperature of WF/HDPE/CS composites while it reduced the Tmaxin the first stage. However the residues were increased; residue of 3%, 6%, 9%, and 12% BA/BX samples at 600 oC increased by about 35.2%, 39.7%, 62.8%, and 97.4%, respectively, more than that of 0% BA/BX samples.

Figure 6 shows the mass loss rate (MLR) of WPCs with different BA/BX loadings. These results indicate that the BA/BX compounds gave improved thermal properties at high temperature and could enhance the formation of the char. Stark et al. (2011) indicated that boron-based flame retardants are generally char producers. The presence of boron can redirect decomposition to increase the production of carbon rather than carbon monoxide or carbon dioxide, by creating a surface layer of char. In the process, the boron helps block oxygen from the surface and slows the escape of gases. Wang et al. (2004) reported that the boron residue also reacts with hydroxyl groups of the wood components (cellulose, hemicelluloses) to generate additional quantities of water and form an inorganic char, which acts as an insulator that slows down the rate of composite degradation. This has been confirmed by other work (Ning and Guo 2000; Shen et al. 2008; Yildiz et al. 2009). The thermal degradation of WF-PE-CS composites is a complex process, which can be attributed to the interaction between PE and wood/CS. Mechanisms for the effects of BA/BX compounds on thermal decomposition of WPC composites were further confirmed by FTIR–ATR.

Decay Resistance

One-way analysis of variance (ANOVA) tests indicated that the weight loss percentage of white rot fungus (P < 0.005), and the weight loss percentage of brown rot fungus (P < 0.0001) of the WPCs significantly decreased with the addition of BA/BX except 6% BA/BX (Tables 5A and 5B).

Table 5. Means with Standard Error and Results of ANOVA for the weight loss percentage of WPCs with Different Weight Percentages of BA/BX

Table 5A. ANOVA

Table 5B. Means and Standard Error of the weight loss percentage

Weight loss is an important parameter for assessing decay of solid wood (Lomelí-Ramírez et al. 2009); for this reason the discussion will only focus on the percentage of weight loss. The average weight loss percentages of samples A, B1, B2, B3, and B4 after brown and white rot fungus decay tested for 12 weeks are given in Table 5B. Figure 9 shows that the average weight loss percentages of samples with CS WPC decreased with increasing BA/BX content, except for 6% BA/BX. There were no significant differences with the brown and white rot fungus decay tested. For the white rot fungus decay tested, compared with the control, the average weight loss percentage of the 9% and 12% BA/BX WPC samples decreased by 27.3% and 36.9%, respectively, and for the brown rot fungus decay tested, the results were 48.6% and 47.4%, respectively. Usually weight losses below 10% have been reported in most studies of WPCs exposed to fungal attack. For example Clemons and Ibach (2002) reported weight losses approaching 6% for 3 mm thick WPCs made with pine and HDPE. Pendleton et al. (2002) reported weight losses from 4% to 8% in specimens made of 70% maple, 24% HDPE, and 6% processing additives. Mankowski and Morrell (2000) reported weight losses of nearly 16% for WPCs made of 70% wood and 30% HDPE exposed to P. placenta, and about 20% weight loss for those exposed to G. trabeum. Baysal et al (2007) found that monomer treatment caused higher weight losses when compared to mixture treated. Yalinkilic et al. (1999) tested decay resistance of compressed wood polymer composites (CWPC) pretreated with boric acid. Decay test results showed that boric acid pretreated CWPC has remarkably good resistance to fungal attack. But pretreatment with a boric acid and borax mixture imparted further resistance to WPC so that it became nearly totally resistant to both test fungi. The remarkably good decay resistance of WPC can be explained by its high moisture exclusion efficiency and inhibition of mycelial spread (Yalinkilic et al. 1991). The characterization of decay resistance of this family of WPC presented in this paper is the first part of a comprehensive study of these composites. In particular, scanning electron microscopy analysis and surface chemistry of decayed WPC will be presented in subsequent papers.

Fig. 9. Weight loss percentage for WPCs with different BA/BX loadings

CONCLUSIONS

1. The tensile strength, flexural strength, and flexural modulus of the WPC samples with 6% chitosan (CS) decreased when the boric acid/borax (BA/BX) content was increased from 3% to 12% by weight. The tensile strength, flexural strength, and flexural modulus of WPC samples without CS first increased and then decreased with increasing BA/BX content from 3% to 12% by weight. The mechanical properties showed no significant differences when comparing specimens without CS and specimens with 6% CS. The results showed that additive BA/BX compounds could degrade the mechanical properties of wood-plastic composites (WPC). This may likely be attributable to poor compatibility between the wood fiber and polymer matrix due to the increased surface contamination of the wood surface by the presence of loosely adhering crystalline deposits of the boron compounds. Another reason could be due to involvement of the boron compounds in the esterification reactions between wood flour and the coupling agent MAPE.

2. Cone calorimeter tests indicated that the fire-related parameters HRR, Peak HRR, HRR-60s and HRR-300s, HRR-Total, EHC, and SPR all decreased with increasing BA/BX content. The TTI increased with increasing BA/BX content, from 3% to 9%. These results suggest that adding BA/BX compounds to WPCs can modestly improve fire performance. This can be attributed to wood becoming more difficult to ignite and more oxygen being needed to burn the treated materials as consistent to the effect of BA/BX compounds, which are understood to form a protective layer of compact black char. The TGA and DTG results showed that addition of BA/BX compounds made the thermal degradation of WF/HDPE/CS composites system take place earlier and promoted char formation. Also the residues of 3%, 6%, 9%, and 12% BA/BX samples at 600 oC were increased by about 35.2%, 39.7%, 62.8%, and 97.4%, respectively, in comparison to the 0%BA/BX samples. These results indicated that the BA/BX compounds gave improved thermal properties at high temperature and could enhance the formation of the char.

3. The average weight loss percentages of samples A, B1, B2, B3, and B4 after brown and white rot fungus decay tested for 12 weeks were investigated. When white and brown rot fungus decay was tested, compared with the control, the average weight loss percentage of the 9% and 12% BA/BX WPC samples decreased by 27.3%, 36.9%, and 48.6%, 47.4%, respectively. The results showed that BA/BX compounds could enhance decay resistance of WPC. This may be attributed to its high moisture exclusion efficiency and inhibition of mycelial spreading.

ACKNOWLEDGMENTS

This work has been supported by the doctoral specialties Fund of Northeast Forestry University, Harbin, China, whose support is gratefully acknowledged. The authors would like to thank the Key Laboratory of Bio-based Material Science and Technology, Ministry of Education, China.

REFERENCES CITED

Adhikary, K. B., Pang, S. S., and Staiger, M. P. (2008). “Dimensional stability and mechanical behaviour of wood–plastic composites based on recycled and virgin high-density polyethylene (HDPE),” Composites: Part B 39(5), 807-815.

Anthony, G. M. (1999). “Kinetic and chemical studies of polymer cross-linking using thermal gravimetry and hyphenated methods: Degradation of polyvinylchloride,” Polymer Degradation and Stability 64(3), 353-357.

Ayrilmis, N. (2013). “Combined effects of boron and compatibilizer on dimensional stability and mechanical properties of wood/HDPE composites,” Composites: Part B 44(1), 745-749.

Ayrilmis, N., Akbulut, T., Dundar, T., White, R. H., Mengeloglu, F., Buyuksari, U., Candan, Z., and Avci, E. (2012). “Effect of boron and phosphate compounds on physical, mechanical and fire properties of wood-polypropylene composites,” Construction and Building Materials 33, 63-69.

Ayrilmis, N., Benthien, J. T., Thoemen, H., and White, R. H. (2011). “Properties of flat-pressed wood plastic composites containing fire retardants,” Journal of Applied Polymer Science 122(5), 3201-3210.

Ayrilmis, N., Benthien, J. T., Thoemen, H., and White, R. H. (2012). “Effects of fire-retardants on physical, mechanical, and fire properties of flat-pressed WPCs,” European Journal of Wood Products 70(1-3), 215-224.

Babrauskas, V. (2003). “Ignition handbook: principles and applications to fire safety engineering, fire investigation, risk management and forensic science,” Issaquah WA: Fire Science Publishers; 1116 pp.

Bai, X., Wang, Q., Sui, S., and Zhang, C. (2011). “The effects of wood-flour on combustion and thermal degradation behaviors of PVC in wood-flour/poly (vinyl chloride) composites,” Journal of Analytical and Applied Pyrolysis 91(1), 34-39.

Baysal, E. (2002). “Determination of oxygen index levels of Scots Pine (Pinus sylvestris L.) impregnated with melamine formaldehyde–boron combinations,”

J. Fire Sci. (20), 373-89.

Baysal, E., Peker, H., Colak, M. (2004). “Effects of borates and water repellents on physical properties of Ailanthus altissima wood,” Erciyes University, Institute of Science, 20(1-2), 55-65.

Baysal, E., Yalinkilic, M. K., Sonmez, A., Peker, H., and Colak, M. (2007). “Some physical, biological, mechanical, and fire properties of wood polymer composite (WPC) pretreated with boric acid and borax mixture,” Construction and Building Materials 21(9), 1879-1885.

Bouza, R., Pardo, S. G., Barral, L., and Abad, M. J. (2009). “Design of new polypropylene–wood flour composites: processing and physical characterization,” Polym. Compos. 30(7), 880-6.

Chai, Y. B., Liu, J. L., and Zhen, X. (2012). “Dimensional stability, mechanical properties and fire resistance of MUF-boron treated wood,” Advanced Materials Research 341-342, 80-84.

Chiu, S. H., and Wang, W. K. (1998). “The dynamic flammability and toxicity of magnesium hydroxide filled intumescent fire-retardant polypropylene,” Journal of Applied Polymer Science 67(6), 989-995.

Clemons, C. (2002). “Wood plastic composites in the United States: The interfacing of two industries,” Forest Products Journal 52(6), 10-18.

Clemons, C. M., and Ibach, R. E. (2002). “Laboratory test on fungal resistance of wood filled polyethylene composites,” in: Conference Proceedings, (ANTEC) Annual Technical Conference. San Francisco, CA: 5-9 May 2002.

Faruk, O., Bledzki, A. K., and Matuana, L. M. (2007). “Microcellular foamed wood-plastic composites by different process: A review,” Macromolecular Materials Engineering 192(2), 113-127.

Fang, Y. Q., Wang, Q. W., Guo, C. G., Song, Y. M., and Cooper, P. A. (2013). “Effect of zinc borate and wood flour on thermal degradation and fire retardancy of polyvinyl chloride (PVC) composites,” Journal of Analytical and Applied Pyrolysis 100, 230-236.

Horn, W. E. (2000). “Inorganic hydroxides and hydroxycarbonates: Their function and use as flame-retardant additives,” in: Fire Retardancy of Polymeric Materials, A. F. Grand and C. A. Wilkie (eds.), Marcel Dekker, New York, pp. 285-352.

Hull, T. R., Witkowski, A., and Hollingbery, L. (2011). “Fire retardant action of mineral fillers,” Polymer Degradation and Stability 96(8), 1462-1469.

Kazayawoko, M., Balatinecz, J. J., Woodhams, R. T. (1997). “Diffuse reflectance Fourier transform infrared spectra of wood fibers treated with maleated polypropylenes,” J. Appl. Polym. Sci. 66(6), 1163-73.

Khor, E., and Lim, L. Y. (2003). “Implantable applications of chitin and chitosan,” Journal of Biomaterials 24(13), 2339-2349.

Kiani, H., Ashori, A., and Mozaffari, S. A. (2011). “Water resistance and thermal stability of hybrid lignocellulosic filler-PVC composites,” Polymer Bulletin 66(6), 797-802.

Kiliaris, P., and Papaspyrides, C. D. (2010). “Polymer/layered silicate (clay)nano-composites: An overview of flame retardancy,” Progr. Polym. Sci. 35(7), 902-58.

Kurt, R., and Mengeloglu, F. (2011). “Utilization of boron compounds as synergists with ammonium polyphosphate for flame retardant wood-polymer composites,” Turkish Journal of Agriculture and Forestry 35(2), 155-163.

Laoutid, F., Bonnaud, L., Alexandre, M., Lopez-Cuesta, J. M., and Dubois P. (2009). “New prospects in flame retardant polymer materials: From fundamentals to nanocomposites,” Mater. Sci. Eng. R. 63(3), 100-125.

Leu, S. Y., Yang, T. H., Lo, S. F., and Yang, T. H. (2012). “Optimized material composition to improve the physical and mechanical properties of extruded wood–plastic composites (WPCs),” Construction and Building Materials 29, 120-127.

Levchik, S. V. (2007). “Introduction to flame retardancy and polymer flammability,” in: Morgan, A. B., and Wilkie, C. A. (eds.), Fire Retardant Polymer Nanocomposites, John Wiley & Sons Inc.., New Jersey, USA, pp. 1-30.

Lomelí-Ramírez, M. G., Ochoa-Ruiz, H. G., Fuentes-Talavera, F. J., García-Enriquez, S., Cerpa-Gallegos, M. A., and Silva-Guzmán, J. A. (2009). “Evaluation of accelerated decay of wood plastic composites by Xylophagus fungi,” International Biodeterioration & Biodegradation 63(8), 1030-1035.

Lu, J. Z., Wu, Q., and McNabb, Jr. HS. (2000). “Chemical coupling in wood fiber and polymer composites: A review of coupling agents and treatments,” Wood Fibre Sci. 32(1), 88-104.

Lu, J. Z., Wu, Q., and Negulescu, I. I. (2005). “Wood–fiber/high-density-polyethylene composites: Coupling agent performance,” J. Appl. Polym. Sci. 96(1), 93-102.

Mankowski, M., and Morrell, J. J. (2000). “Patterns of fungal attack in wood–plastic

composites following in a soil block test,” Wood and Fiber Science32(3), 340-345.

Martinez-Camacho, A. P., Cortez-Rocha, M. O., Ezquera-Brauer, J. M., Graciano-Verdugo, A. Z., Rodriguez-Felix, F., Castillo-Ortega, M. M., Yépiz-Gómeza, M. S., and Plascencia-Jatomeaa, M. (2010). “Chitosan composite films: Thermal, structural, mechanical and antifungal properties,” Carbohydrate Polymers 82(2), 305-315.

Matuana, L. M., Balatinecz, J. J., Sodhi, R. N. S, and Park, C. B. (2001). “Surface characterization of esterified cellulosic fibers by XPS and FTIR spectroscopy,” Wood Sci. Technol. 35(3), 191-201.

Meng, Y. Z., and Tjong, S. C. (1999). “Preparation and properties of injection-moulded blends of poly(vinyl chloride) and liquid crystal copolyester,” Polymer 40(10), 2711-8.

Mir, S., Yasin, T., Halley, P. J., Siddiqi, H. M., and Nicholson, T. (2011). “Thermal, rheological, mechanical and morphological behaviour of HDPE/chitosan blend,” Carbohydrate Polymers 83(2), 414-421.

Morgan, A. B. (2006). “Flame retarded polymer layered silicate nanocomposites: A review of commercial and open literature systems,” Polym. Adv. Technol. 17(4), 206-217.

Nagieb, Z. A., Nassar, M. A., and El-Meligy, M. G. (2011). “Effect of addition of boric acid and borax on fire-retardant and mechanical properties of urea formaldehyde saw dust composites,” International Journal of Carbohydrate Chemistry 146763.

Ning, Y., and Guo, S. Y. (2000). “Flame-retardant and smoke-suppressant properties of zinc borate and aluminum trihydrate-filled rigid PVC,” J. Appl. Polym. Sci. 77(7), 3119-27.

Ou, C. Y., Zhang, C. H., Li, S. D., Yang, L., Dong, J. J., Mo, X. L., and Zeng, M. T. (2010). “Thermal degradation kinetics of chitosan-cobalt complex as studied by thermogravimetric analysis,” Carbohydr. Polymers 82(4), 1284-1289.

Pendleton, D. E., Hoffard, T. A., Adcock, T., Woodward, B., and Wolcott, M.P. (2002).

“Durability of an extruded HDPE/ wood composite,” Forest Products Journal 52(6), 21-27.

Sain, M., Park, S. H., Suhara, F., and Law, S. (2004). “Flame retardant and mechanical properties of natural fibre-PP composites with magnesium hydroxide,” Polymer Degradation and Stability83(2), 363-367.

Shen, K. K, Kochesfahani, S., and Jouffret, F. (2008). “Zinc borates as multi-functional polymer additives,” Polym. Adv. Technol. 19(6), 469-474.

Silva, A., Gartner, B. L., and Morrell, J. J. (2007). “Towards the development of accelerated methods for assessing the durability of wood plastic composites,” Journal of Testing and Evaluation 35(2), 1-8.

Stark, N. M., White, R. H., Mueller, S. A., and Osswald, T. A. (2010). “Evaluation of various fire retardants for use in wood flour-polyethylene composites,” Polymer Degradation and Stability 95(9), 1903-1910.

Starnes, Jr., W. H., Pike, R. D., Cole, J. R., Doyal, A. S., Kimlin, E. J., Lee, J. T., Murray, P. J., Quinlan, R. A., and Zhang, J. (2003). “Cone calorimetric study of copper-promoted smoke suppression and fire retardance of poly (vinyl chloride),” Polymer Degradation and Stability 82(1), 15-24.

Thomson, H. E, and Drysdale, D. D. (1987). “Flammability of plastics. I: Ignition tem-peratures,” Fire Mater. 11(4), 163-72.

Wang, Q. W., Li, J., and Winandy, J. E. (2004). “Chemical mechanism of fire retardance of boric acid on wood,” Wood Science and Technology 38(5), 375-389.

Wang, Q. W., Shong, B., Zhao Z. J., Song, Y. M. (2007). “Effects of APP on the fire-retardant and mechanical properties of wood-flour/HDPE composite,” IUFRO D5

Conference, Taipei, Taiwan.

Wielage, B., Lampke, T., Utschick, H., and Soergel, F. (2003). “Processing of natural-fibre reinforced polymers and the resulting dynamic-mechanical properties,” J. Mater.

Wu, Z., Hu, Y., and Shu, W. J. (2010). “Effect of ultrafine zinc borate on the smoke suppression and toxicity reduction of a low-density polyethylene/intumescent flame-retardant system,” Journal of Applied Polymer Science 117(1), 443-449.

Yalinkilic, M. K., Takahashi, M., Imamura Y., Gezer E.D., Demirci Z., and Ilhan R. (1991). “Boron addition to non or low formaldehyde cross-linking reagents to enhance biological resistance and dimensional stability for wood,” Holz als Roh- und Werkstoff 57(1), 151-63.

Yalinkilic, M. K, Imamura, Y., Takahashi, M., and Demirci, Z. (1998). “Effect of boron addition to adhesive and/or surface coating on fire–retardant properties of particleboard,” Wood Fiber Sci. 30(4), 348-359.

Yalinkilic, M. K, Imamura, Y., Takahashi, M., Demirci, Z., and Yalinkilic, A. C. (1999). “Biological, mechanical, and thermal properties of compressed-wood polymer composite (CWPC) pretreated with boric acid,” Wood Fiber Sci. 31(2), 151-63.

Yemele, M. C. N, Koubaa, A., Cloutier, A., Soulounganga, P., and Wolcott, M. (2010). “Effect of bark fiber content and size on the mechanical properties of bark/HDPE composites,” Composites Part A41(1), 131-137.

Yildiz, B., Seydibeyoglu, M. O., and Guner, F. S. (2009). “Polyurethane-zinc borate composites with high oxidative stability and flame retardancy,” Polym. Degrad. Stab. 94(7), 1072-5.

Yildiz, U. C., Temiz, A., Gezer, E. D., and Yildiz, S. (2004). “Effects of wood preservatives on mechanical properties of yellow pine (Pinus sylvestris L.) wood,” Building Environ. 39(9), 1071-1075.

Article submitted: November 16, 2013; Peer review completed: December 26, 2013; Revised version received: May 14, 2014; Accepted: May 15, 2014; Published: May 22, 2014.