Degradation of a wood-plastic composite exposed under tropical conditions.
The physical and chemical changes of wood-plastic composites (WPC) were examined after 10 years in field (stake and ground proximity) tests at a tropical site near Hilo, Hawaii. The effects of biological agents (especially fungi) were examined using a Pilodyn, microscopy, and culturing. Pilodyn pin penetration increased significantly in weathered material, indicating some decline in surface hardness over time. Microscopic examination revealed that the wood particles on the exposed surfaces were degraded in both soil contact and ground proximity samples, but the damage was relatively shallow. A variety of microorganisms, but no basidiomycetous fungi, were isolated from the samples. Type 1 soft-rot cavities were also observed on wood particles. Chemical changes that occurred at the surface of WPC as a result of the combined effects of biodegradation and photo/thermal degradation were also examined. Pyrolysis-gas chromatography-mass spectroscopy analysis showed that the wood content was significantly reduced, relative to plastic content, on weathered WPC surfaces. Surface analysis by Fourier transform infrared spectroscopy showed that oxidation had occurred. The potential for developing surface treatments to arrest the shallow damage on the WPC is also discussed.
The desire for maintenance and arsenical-free materials for decking products has generated considerable interest in wood-plastic composites (WPC) (Clemens 2000, 2002; Morton et al. 2003). As a result, the markets for WPCs have seen considerable growth over the last 5 years, which is due, in part, to the perception that these materials are immune to biological attack. It has been observed, however, that wood in WPCs can be degraded, albeit at a slower rate than is found in solid wood (Morris and Cooper 1998; Mankowski and Morrell 2000; Verhey et al. 2001, 2002, 2003; Ibach and Clemens 2002; Clemens and Ibach 2002; Pendleton et al. 2002; Silva et al. 2002; Simonsen et al. 2002; Verhey and Laks 2002). Incorporation of wood flour into plastic is likely to enhance the accessibility of air, water, and microorganisms. The outdoor degradation of WPCs will likely occur via a combination of factors including photodegradation and biodegradation (Stark and Matuana 2004).
Photo-oxidation of wood and plastic increases the amount of low molecular weight compounds, surface area, and hydrophilicity, which promotes further polymer degradation (Khabbaz et al. 1998) and makes it easier for microorganisms to attack the plastic (Albertsson and Karlsson 1990). WPCs can be protected against decay and photodegradation by the addition of fungicides and stabilizers. Despite a number of laboratory studies, little is known about the resistance of WPCs (long-term field exposure) to biodeterioration agents and photodegradation, so data from field trials will be important as market share increases. One such field test was established in 1994 near Hilo, Hawaii. This paper describes the evaluation of WPC samples (stakes and blocks) exposed at this site.
Materials and methods
Solid WPC decking (Trex Winchester, VA; 37.5 mm thick by 137.5 mm wide) consisting nominally of 56 percent wood flour and 44 percent polyethylene (PE) was purchased in Charlotte, North Carolina and cut into either 19- by 25- by 450-mm stakes or 25- by 50- by 125-mm blocks. The materials were then installed at the test site near Hilo, Hawaii which receives over 3 m of rainfall per year and has an average temperature of 24[degrees] to 30[degrees]C. The site has aggressive fungal attack, particularly in wood exposed aboveground as indicated by a Scheffer Climate Index exceeding 200 (Scheffer 1971). Stakes were buried in the soil according to procedures described in American Wood-Preservers' Association (AWPA) Standard E7-04 (AWPA 2004a). Blocks were exposed out of direct soil contact in a ground proximity test according to procedures described in AWPA Standard E18-04 (AWPA 2004b). The samples were exposed in this manner for 10 years with minimal disturbance.
Sample condition was assessed by randomly removing three stakes and three blocks from the site. All samples were conditioned at 23[degrees]C and 65 percent relative humidity prior to testing. The physical condition was assessed using a 6.2 kPa Pilodyn equipped with a 5-mm-diameter pin at 20[degrees] to 23[degrees]C (Taylor 1981, Leightley 1982, Cown and Hutchison 1983). A series of four Pilodyn readings were taken on the same face of each stake 70 mm from the bottom, at groundline, and 70 mm from the top. The ground proximity samples were subjected to eight Pilodyn tests, four on the upper surface which had been exposed to sunlight, and four on the lower surface that had been in contact with the concrete. The tests were conducted a minimum of 35 mm from the end of each sample and at least 20 mm from the edge. T-tests were conducted on pin penetration data (density) from the control and exposed material ([alpha] = 0.05).
Fungal colonization was assessed by cutting three 10-mm-wide strips through the stakes immediately adjacent to the Pilodyn pin test sites or two 10-mm-wide strips from each ground proximity sample 25 mm from the ends of the ground proximity blocks. Strips were further divided into zones corresponding to 0 to 5, 5 to 10, 10 to 15, 15 to 20, and 20 to 25 mm from the surface. The specimens were wiped with 95 percent ethanol to limit surface contamination before being placed on 1 percent malt extract agar in Petri dishes and incubated at room temperature. The pieces were observed for evidence of fungal growth and any growth was subcultured onto fresh media for later identification (Barnett and Hunter 1972, Sutton 1980, Ramirez 1981, Nelson et al. 1983, Ellis and Ellis 1985).
Thin sections (3 to 5 mm) cut from the stakes and ground proximity samples near the Pilodyn testing points were examined using a dissecting scope. Next, one stake and one ground proximity sample were selected for further study. Ten-mm-thick by 10-mm-wide strips passing through the center of the material were cut 90 mm from the top, near the groundline, and 50 mm from the bottom of the stake. Similar strips were cut 15 mm from the edge and 20 mm from either side of the ground proximity sample. The strips were subdivided into 5-mm-thick zones as described in the culturing study. Two samples of control blocks were also included. Samples were sputter coated with gold-palladium, then examined under an AMR 1000A scanning electron microscope (SEM) (20 kV, tilt angle of 30[degrees], and working distance of 12 mm) for evidence of either wood or plastic degradation as well as root invasion or other evidence of damage.
The WPC samples were characterized by cutting fifty 100-[micro]m-thick slices from the surface of each stake (above and below ground), block (upper surface and the surface in contact with the concrete), or control sample using a razor blade. The slices were vacuum dried prior to analysis. Surface chemical analysis was conducted using a ThermoNicolet Avatar 370 Fourier transform infrared (FTIR) spectrometer in the attenuated total reflectance (SmartPerformer Thermo Electron Corporation Waltham MA, ZnSe crystal) mode to compare the changes in the carbonyl region of the WPC by selecting four sampling points on each slice (64 scans). Special attention was focused on the bands found at 908 [cm.sup.-1] and 1695 to 1800 [cm.sup.-1], which correspond to absorption of vinyl (C = C) and carbonyl (C = O) groups, respectively. The 2915 [cm.sup.-1] band was used as the reference band for this analysis. The carbonyl and vinyl indices were determined as the ratio of the total absorbance of the 1715 [cm.sup.-1], 1733 [cm.sup.-1], and 908 [cm.sup.-1] bands, respectively, to the reference band (Stark et al. 2004, Fabiyi et al. 2005). The degree of cellulose crystallinity was determined by the ratio of bands at 1315 [cm.sup.-1] and 1370 [cm.sup.-1] relative to the 670 [cm.sup.-1] band (Evans et al. 1995).
Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) analysis was conducted on the surface and interior of three stakes (aboveground, groundline, and below ground) and from three ground proximity blocks (upper surface and interior part) as well as from the surface and interior parts of the ground proximity block in contact with the concrete. A sample (1 g) from each location was ground (< 60 mesh) and vacuum dried. Approximately 50 [micro]g of each sample (three replicates) was then loaded into a quartz tube and pyrolyzed at 600[decrees]C in a SGE Pyrojector II (Ringwood, Australia), coupled to a ThermoFinnigan Polaris Q GC-MS (San Jose, CA). Separation of the volatile products was achieved on a ZB-1 capillary column (15 m, 0.25 mm) using a temperature ramp from 40[degrees]C (2 min) to 300[degrees]C (10 min) at 5[degrees]C/min. The wood content in the weathered WPC samples was quantified by Py-GC-MS by developing a calibration curve from the total peak areas under the wood-derived peaks, relative to the total peak areas under the PE-derived peaks, based on tests of a series of WPC formulations of known wood content (0%, 20%, 56%, and 60%).
Results and discussion
Stakes were heavily weathered a light grey color and softened above the groundline, reflecting the extreme ultraviolet (UV) exposure at the test site, and were covered with abundant moss and lichen growth. Material in soil contact had turned a dark red color, and there was evidence of root penetration for short distances in the below ground portions of the stakes. The ground proximity samples were also weathered and contained extensive lichen growth on the upper surfaces. The surfaces in contact with the concrete blocks were somewhat discolored, but did not have the surface softening evident on the material in contact with the atmosphere.
Examination of sections cut from the stakes and ground proximity samples indicated that the outer 2 to 3 mm of the aboveground zone cut from the stakes and the upper surface of the ground proximity blocks showed evidence of both plastic and wood degradation. In many cases, the wood particles were completely degraded, leaving voids in the plastic. The zone below ground had little evidence of plastic degradation, but there were numerous voids in the surface where wood particles once resided. Damage, however, was confined to a shallow zone near the sample surface. The absence of damage deeper in the samples is consistent with previous findings that moisture intrusion into WPC is limited (Schmidt 1993, Naghipour 1996, Wang and Morrell 2004). The depth of moisture intrusion is dependent on wood content and particle size. WPCs with wood content below 50 percent tend to be resistant to decay, however, moisture intrusion is adequate to initiate decay in the WPC once this benchmark is reached (Schmidt 1993, Naghipour 1996, Ibach et al. 2003, Wang and Morrell 2004). The absence of moisture sharply limits the potential for biological attack of the wood component (Ammer 1964, Zabel and Morrell 1992).
Pilodyn pin penetration increased significantly for both the upper and lower surfaces of ground proximity blocks in comparison with the control (Fig. 1a). Penetration was greatest for the lower surface of the ground proximity samples, although the differences between the upper and lower surfaces were not significant ([alpha] = 0.05, Fig. la). Pilodyn pin penetrations of stake samples were also significantly greater than the controis, but tended to be lower than those found for the ground proximity test (Fig. 1b). The highest penetration values were found below ground, suggesting that loss of relatively large wood particles, rather than shallow UV degradation of the plastic, played a greater role in altering density. The results clearly show that surface properties of the WPC have declined over the 10-year exposure period. These effects, however, were relatively shallow and are unlikely to affect its overall performance.
[FIGURE 1 OMITTED]
Fungi were isolated from virtually every sample examined. A total of six taxa were identified to genus and/or species. The remaining fungi either failed to produce spores in culture or were zygomycetes (Table 1). The most important aspect of the isolations was the inability to isolate any basidiomycetes from the WPC despite the obvious presence of decay. While basidiomycetes are clearly not the only fungi capable of decaying wood, they tend to dominate many ecosystems, including the one found at the test site, and their absence in this exposure is perplexing. It is possible that the small particles were primarily degraded by soft-rot fungi and several of the genera isolated (notably species of Trichoderma, Humicola, and Pestalotiopsis) have been reported to cause soft-rot attack (Nilsson 1974). Polarized light microscopy revealed a very limited number of Type 1 soft-rot cavities on freehand sections cut from below ground portions of the stakes.
SEM examination revealed that the surfaces of the below ground portions of the field stakes were pitted due to the complete removal of the wood particles, while samples taken 5 mm inward from the surface in the same zone showed typical plastic-coated wood particles and no evidence of microbial attack (Fig. 2a, b). The aboveground portions of stakes showed evidence of surface weathering, but wood particles were still discernable in the matrix and the damage appeared to be a combination of plastic photodegradation and wood deterioration (Fig. 2c).
[FIGURE 2 OMITTED]
SEM examination of the upper surfaces of ground proximity samples showed the presence of extensive microbial colonization and gaps between particles, while examination of a sample cut 5 mm from the surface revealed the typical wood-plastic matrix (Fig. 3). The wood particles on the undersides of the ground proximity samples were still distinguishable, but there was evidence of surface fractures (Fig. 4) and extensive microbial growth. As with the other samples, the damage was absent 5 mm from the surface. The results highlight the limited depth of damage to the WPC even after 10 years of severe decay exposure and without supplemental wood protection. This also emphasizes the importance of the plastic as an inhibitor of moisture ingress. The presence of surface degradation, while of less consequence from a structural aspect, would be a consumer concern. Developing improved methods for limiting surface degradation will be essential for the long-term performance of these materials.
[FIGURES 3-4 OMITTED]
FTIR absorption bands were assigned from the spectra of the non-exposed and exposed WPC stakes (below ground and aboveground) based upon previous reports (Fig. 5) (Faix 1992, Rodrigues et al, 2001, Fabiyi et al. 2005). The band at 1507 [cm.sup.-1] (assigned to lignin) disappeared in the above and below ground zones of the exposed WPC stakes (Fig. 5). Conversely, the absorption band 1507 [cm.sup.-1] in the ground proximity blocks exposed to sunlight was present but significantly reduced. The extent of WPC oxidation was determined by total carbonyl functionality (carboxylic acids and esters/ aldehydes) and quantified. The carbonyl region in the FTIR spectrum (between 1714 [cm.sup.-1] and 1813 [cm.sup.-1]) corresponds to: carboxylic acids (1715 [cm.sup.-1]) and esters/aldehydes (1733 [cm.sup.-1]) (Carlsson and Wiles 1969, Fabiyi et al. 2005). The presence of carbonyl groups in a degraded polymer indicates that oxidation has occurred and that the material is vulnerable to further degradation because these groups are photolabile (Feldman 2002). Surface oxidation was evident on the WPC exposed to sunlight, while the WPC ground proximity samples that were partially protected with shade cloth were less oxidized than the directly exposed material (Table 2). The esters and aldehydes (1733 [cm.sup.-1]) band decreased relative to the carboxcylic acid band in the aboveground exposure sample, suggesting that the degradation was due to photodegradation (weathering).
[FIGURE 5 OMITTED]
There were no noticeable carbonyl groups in the below ground zone of the stakes or the ground proximity surface in contact with the concrete, indicating that the degradation was attributable to fungal attack and not weathering.
Vinyl groups (908 [cm.sup.-1]), representing the plastic polymer component in a WPC, experienced no appreciable change for the upper or lower surfaces of the ground proximity samples exposed to sunlight (Table 2) compared to the unexposed samples. This indicates that little or no plastic degradation occurred in the ground proximity samples. On the other hand, the degree of unsaturation (double bonds) increased at the WPC surface on the below and aboveground portions of the stakes from 25 to 73 percent and 25 to 37 percent, respectively (Table 3). Cellulose crystallinity of the exposed WPC samples was higher than that in the unexposed samples, indicating that the amorphous wood cell wall components were removed during exposure, leaving a higher proportion of cellulose. Crystallinity tended to be higher in the ground proximity samples than in the stakes, possibly reflecting differences in the decay rate. For example, the below ground zone would likely show little change in crystallinity because all of the cellulose would be degraded. The higher crystallinity in the ground proximity test may reflect a less severe decay environment that degrades crystalline cellulose at a slower rate.
Py-GC-MS analysis was employed to assess the level of plastic in the exposed WPC. Chromatograms from the unexposed WPC sample contained molecular fragments from both wood (lignin, extractives, and carbohydrate) and PE (Fig. 6). Wood-derived peaks decreased in intensity in the aboveground stake samples relative to the PE-derived peaks. On the other hand, almost all of the peaks for PE remained, even after weathering. The PE-derived peaks were comprised of ethane ([C.sub.2]), butane ([C.sub.4]), and an alkane series up to [C.sub.34], which in eludes various isomers and unsaturated isomers. Extra peaks derived from PE were observed between 34 minutes and 40 minutes for pure PE and exposed WPC samples relative to the unexposed WPC sample. The intensity of these extra peaks was inversely dependant on wood content and suggests that the presence of wood influences the fragmentation of PE during pyrolysis. The results suggest that the wood used was mainly of softwood origin (Table 3) (Meier and Faix 1992, Fabiyi et al. 2005).
[FIGURE 6 OMITTED]
The wood to plastic ratio in the weathered WPC was determined after Py-GC-MS analysis from the calibration curve and compared with the control. Wood content decreased significantly on the surfaces of the stakes directly exposed to sunlight. The wood content that remained after 10 years of exposure at different conditions is presented using 90 percent confidence intervals with a sample size of 9 (i.e., three stakes with three replicates). The upper and lower 90 percent confidence intervals were 19 and 17 percent, 22 and 18 percent, and 24 and 21 percent for the wood that remained in the aboveground, groundline, and below ground zones, respectively. Ground proximity samples were shielded, which allowed limited UV exposure. Thus the upper surfaces still retained between 22 percent and 19 percent wood content (90% confidence interval), while the side in contact with concrete retained between 28 percent and 25 percent wood. These results support the observations made using Pilodyn pin penetration and FTIR spectroscopy, Comparisons among the wood content data from the different exposure conditions were conducted using one-way analysis of variance ([alpha] = 0.05) and Showed significant differences between the control and the other exposure conditions. There were no significant differences, however, between samples from the stake aboveground zone, groundline zone, below the ground, and the block exposed to UV radiation (shielded with cloth) (Fig. 7).
[FIGURE 7 OMITTED]
WPC samples in both soil and aboveground exposures experienced microbial and physical damage as well as substantial chemical changes. The plastic content increased at the WPC surface due to biodegradation and photodegradation over the 10-year exposure. Physical damage, however, was limited to a shallow zone near the surface. These observations suggest that process modifications to improve the performance of the outer 5 mm of the WPC may be adequate for preventing such degradation without the need to modify the entire cross section. This would limit processing costs while enhancing overall product performance.
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Christoph Schauwecker* Jeffrey J. Morrell* Armando G. McDonald* James S. Fabiyi*
The authors are, respectively, Graduate Research Assistant and Professor, Dept. of Wood Science and Engineering, Oregon State Univ., Corvallis, OR (firstname.lastname@example.org;email@example.com); and Professor and Graduate Student, Dept. of Forest Products, Univ. of Idaho, Moscow, ID (firstname.lastname@example.org; email@example.com). This paper was received for publication in May 2005. Article No. 10054.
* Forest Products Society Member.
[c]Forest Products Society 2006.
Forest Prod. J. 56(11/12):123-129.
Table 1.--Fungal taxa isolated from WPC samples exposed for 10 years near Hilo, Hawaii. Stakes Species Aboveground Groundline Surface Interior Surface Interior Fusarium oxysporum/F.solani 0 0 17 0 Penicillium spp. 0 33 0 0 Aspergillus spp. 0 0 0 0 Trichoderma harzianum 0 0 33 33 Humicola spp. 33 0 0 0 Pestalotiopsis spp. 33 0 0 8 Zygomycota 0 0 0 Non-sporulating 50 8 0 0 Unidentified 67 50 67 33 Total isolates 11 10 7 6 Total isolation attempts 6 12 6 12 Stakes Species Below ground Ground proxomity Surface Interior Surface Interior (Q/o) Fusarium oxysporum/F.solani 0 17 27 10 Penicillium spp. 17 25 0 33 Aspergillus spp. 0 0 0 5 Trichoderma harzianum 0 0 0 14 Humicola spp. 0 0 0 0 Pestalotiopsis spp. 0 0 0 0 Zygomycota 17 0 0 0 Non-sporulating 0 8 0 10 Unidentified 83 0 64 0 Total isolates 7 6 21 22 Total isolation attempts 6 12 11 21 Species Total Fusarium oxysporum/F.solani 9 Penicillium spp. 17 Aspergillus spp. 1 Trichoderma harzianum 10 Humicola spp. 2 Pestalotiopsis spp. 3 Zygomycota 1 Non-sporulating 8 Unidentified 56 Total isolates 93 Total isolation attempts 86 Table 2.--Effect of biodegradation and photodegradation on cellulose crystallinity and vinyl and oxidation indices of WPCs after 10 years of field exposure. Exposure condition Cellulose Vinyl Oxidation crystallinity index index Unexposed 0.70 25 18 Block (concrete contact) 1.10 25 0 Block (toward sunlight) 1.00 24 36 Stake below ground 0.80 73 0 Stake aboveground 0.90 37 41 Table 3.--Volatile and semi-volatile products identified in a WPC after 10 years of field exposure (Meier and Fax 1992). Relative abundance (a) Stake Retention Below time (min) Compound Unexposed Aboveground ground 0.48 Carbon dioxide ++++ +++ ++++ 1.72 Phenol ++ ++ ++ 2.34 3-Furancarboxaldehyde + ++ ++ 8.68 Guaiacol ++ + -- 10.73 Dimethyl-phenol + -- -- 11.61 4-methyl-guaiacol +++ + -- 13.95 4-ethyl-guaiacol ++ -- -- 14.49 4-allyl-phenol + -- -- 14.75 4-vinyl-guaiacol +++ -- -- 15.93 Eugenol ++ -- -- 16.25 4-propyl-guaiacol + + -- 16.51 Vanillin + + 17.17 Isoeugenol + -- -- 18.76 Acetoguaiacone ++ + -- 19.22 G-CO-CH=CHZ + -- -- 20.51 Coniferaldehyde + -- -- 20.81 Coniferyl-alcohol + -- -- 21.07 Propioguaiacone + -- -- 23.75 4-propenyl-syringol + -- -- Relative abundance (a) Ground proximity Retention Toward Concrete time (min) Compound sunlight contact 0.48 Carbon dioxide ++++ +++ 1.72 Phenol ++ + 2.34 3-Furancarboxaldehyde ++ ++ 8.68 Guaiacol + + 10.73 Dimethyl-phenol + + 11.61 4-methyl-guaiacol + ++ 13.95 4-ethyl-guaiacol -- ++ 14.49 4-allyl-phenol -- -- 14.75 4-vinyl-guaiacol ++ -- 15.93 Eugenol + ++ 16.25 4-propyl-guaiacol -- -- 16.51 Vanillin + + 17.17 Isoeugenol -- + 18.76 Acetoguaiacone + + 19.22 G-CO-CH=CHZ + -- 20.51 Coniferaldehyde + -- 20.81 Coniferyl-alcohol + -- 21.07 Propioguaiacone -- -- 23.75 4-propenyl-syringol -- -- (a) Grading: + to ++++ represent the level of relative abundance (%) of the peak. + represents > 0% to 5% of total lignin content;++ represents > 5% to 10%; +++ represents > 10% o to 20%; and represents > 20% to 70%, while--represents not detected.
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|Author:||Schauwecker, Christoph; Morrell, Jeffrey J.; McDonald, Armando G.; Fabiyi, James S.|
|Publication:||Forest Products Journal|
|Date:||Nov 1, 2006|
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