Patterns of Fungal Attack in Wood Plastic

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PATTERNS OF FUNGAL ATTACK IN WOOD-PLASTIC COMPOSITESFOLLOWING EXPOSURE IN A SOIL BLOCK TEST1

Mark MankowskiResearch Assistant

and

ProfessorDepartment of Forest ProductsOregon State UniversityCorvallis, OR 9733 1-7402(Received July 1999)

ABSTRACTThe ability of whitc and brown rot fungi to colonize wood-plastic composites was investigated bymeasuring weight loss and anatomical changes. Three composite materials were evaluated. The ma-terial containing a 70130 wood-high density polyethylene (HDPE) mixture was most susccptiblc to

fungal attack, while two different 50150 wood-HDPE composites exp erienced little or no attack. Scan-ning electron microscopic (SEM) examination of sam ples not exposed to fungus revealed the presenceof voids between the wood and HDPE in all three materials. Similar examination of decayed samplesof thc composite with a higher wood content revealed that thc fungi had thoroughly colonized theparticles, particularly near the point of initial fungal exposure. Fungal hyphae were also prevalent inthe voids deeper in the composite. The two composites containing higher HDPE levels had littleevidence of fungal attack, despite the presence of voids.Kr,ywords: Wood-plastic composite, high-density polyethylene (HDPE), white rot fungus, brown rotfungus, fungal hyphae, voids.

INTRODUCTION lead to performance problems with the boardsThe emergence of recycling programs to

collect various plastics has created a substan-tial demand for the development of practicalapplications for these materials. One commonapplication for recycled plastic has been theproduction of plastic lumber for use in deckingand picnic tables. Plastic lumber has some ex-cellent material properties, but its disadvantag-es include its weight and tendency to deformunder load, particularly when heated. Onemethod for enhancing its properties is to in-corporate wood fiber in the mixture to lightenand reinforce the resulting board (Simonsen1997). The addition of wood, however, can

I This is Paper 3356 of the Forest Research Laboratory,Oregon State University, Corvallis, OR.i ember of SWST

because the added fiber can be susceptible tomoisture absorption and degradation. The po-tential for degradation of components ofwood-plastic composites has received little at-tention. Manufacturers apparently believe thatthe plastic encapsulates the wood fiber, limit-ing access to fungal attack; however, there islittle literature to support this premise, andsome evidence indicates that wood-plasticcomposites can absorb considerable moisture,albeit at slower rates than solid wood (Schmidt1993; Naghipour 1996). A recent report notedextensive evidence of white and brown rot at-tack in a walkway in Florida exposed for 4years (Morris and Cooper 1998), but there arefew other reports on the performance of thesewood-plastic mixtures. In this report, we de-scribe a scanning electron microscopic (SEM)

Wood orld Fthrr Sc,rncr. 32(3), 2000, pp 340-3450 000 hy the Society of Wood Science and Technology


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Mankowtki und Morrrll--PATTERNS OF FUNGAL ATTACK IN COMPOSITES 341

high-density polyethylene (HPDE). The woodparticles were approximately 0.25 mm in di-ameter. Materials B and C contained 50/:50wood-HPDE, with 1- to 2-mm-diameter wood 'particles. Because the materials provided we:reproprietary, the mixture of wood species ineach was unknown. SEM examination sug-gested that the wood was primarily coniferous.An additional set of 36 ponderosa pine (Pinusponderosa Laws.) sapwood blocks were in-cluded to serve as controls.

The blocks were oven-dried at 54OC andweighed to the nearest 0.001 g. All of theblocks were soaked in water for 30 min, then

FIG. 1 . Scanning electron micrographs of non-fungal placed in plastic bags and sterilized by expo-exposed materials of all three composites showing the dis- sure to 2.5 mrad of ionizing radiation from, atribution of the wood in the plastic matrix and the poten-tial for fungal movement through intercorlnecting path- cobalt 60 source. Irradiation was used becauseways bctween individual wood particles. Figure (a) shows heat can increase the sL's-the 70130 composite, while Figures ( b) and (c) show the ceptibility of wood specimens and might havetwo sols0 composites. altered the wood-HPDE matrix. The materials

absorbed water differently, with Materials A,


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342 WOOD AND FIBER SCIENCE, JULY 2000, V . 32(3)B, and C showing weight gains of 16%, 2.4%, posed to fungus. Then the blocks exposed toand 1.9%, respectively. fungus were examined for fungal-hyphae dis-

Decay chambers were prepared in 454-ml tribution and evidence of wood or plastic deg-glass jars with either red alder (Alnus rubra radation.Bong) or western hemlock (Tsuga heterophylla[Raf.] Sarg). Alder was used for the white rot RESULTS A N D DISCUSSIONfungi, while hemlock was used for brown rotfungi. The culture of white rot fungus inocu-lated on the red alder was one of the following:Phlebia subserialis (Gourd. & Galzin) Donk(Isolate RLG10693), Trametes versicolor (L.:Fr.) Pilat (Isolate R105), or Xylobolus frustu-latus (Pers.:Fr.) Karst (Isolate FP106073-R).The culture of brown rot fungus inoculated onthe western hemlock was either Gloeoph.yllumtrabeum (Pers. ex. Fr.) Mum (Isolate Madison617), Postia placenta (Fr.) M. Larsen & Lomb.(Madison 698), or Woljiporia cocos (Wolf) Ry-varden. & Gilbertson (Isolate FP104264-SP).

The jars were then capped and incubated at28C until the alder or hemlock was thorough-ly covered by test fungus.

Once the fungus had covered the alder orhemlock water, the test blocks of Materials A,B, or C were placed on the feeder strip sur-face, and the chambers were incubated for 12weeks at 28OC. Each fungus was tested on sixblocks of each material (either Material A, B,C, or the ponderosa control). At the conclusionof the test, the blocks were removed, scrapedclean of adhering fungal mycelium or soil, andoven-dried (54C) prior to being weighed. Theblock's weight loss over the exposure periodserved as the measure of decay (i.e., the moredecay, the more weight loss).

Blocks having a range of weight losses wereselected for further microscopic study. In ad-dition, unexposed controls from each of thematerials were selected for comparison. Theblocks were oven-dried, then split in half toexpose an internal face. These blocks weresputter-coated with gold palladium and ex-amined by using an AMR lOOOA ScanningElectron Microscope (SEM, accelerating volt-age 20 kV, tilt angle 30 degrees, working dis-tance 12 rnm). The distribution of wood in theplastic was examined on the blocks not ex-

Weight loss in the blocks varied widelyamong the three materials as well as with thewhite or brown rot fungus to which the ma-terials were exposed (Table 1). Weight loss ap-proached 30% in a few blocks of compositeMaterial A. Both the wood and plastic im-mediately adjacent to the fungal myceliumwere completely degraded in many cases.

The weight losses associated with blocks ofcomposite Materials B and C were generallynegligible, in the range associated with theprocedures of wetting, oven-drying, and otherhandling, rather than with fungal damage.These low weight losses may reflect slow wa-ter uptake by samples with lower wood con-tents. We soaked additional specimens in wa-ter for 5 days and only achieved moisture con-tents (MCs) of 24%, 6.5%, and 6.2%, respec-tively, for Materials A, B, and C. If we assumethat all of the moisture was absorbed by thewood, this translates to wood MCs of 48%,21.6%, and 20.7%, respectively. Clearly, thehigher plastic composition substantially re-duced moisture uptake, creating conditionsthat were less suitable for decay.

Scanning electron microscopic examinationof the untreated control samples revealed thatthe wood particles were randomly distributedthroughout the plastic matrix of all three ma-terials (Fig. 1). Some wood particles appearedto be encapsulated in plastic, but there was anextensive series of voids around most woodparticles, suggesting that channels existed formovement of the decay fungus into the matrix(Fig. 2). Most of the blocks that experiencedlow weight losses were essentially free fromfungal attack or visible wood cell wall dam-age. Blocks of Material A that experiencedlarger weight losses tended to have more ex-tensive hyphal growth, although most of thegrowth appeared to be concentrated near the


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Munkowski und Morrell--PATTERNS OF FUNGAL ATTACK IN COMPOSITES 343

Frc. 3 . Scanning electron micrographs of blocks composed of composite Material A and exposed to brown rotfungi (a ) Gloeophyllum trabeum or (b) Postia placenta, showing voids containing extensive fungal mycelium. Theenlargement of the boxed insert is to the right in each photo.


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WOOD AND FIBER SCIENCE, JULY 2000, V . 32(3)

FIG.4. Scanning electron micrograph of a block of composite Material A-including an enlargement of the inserton the right-exposed to brown rot fungus, Postia placenta, showing thc absence of fungal mycelium or other evidenceof fungal attack in a region near the middlc of the block.

surface that was originally exposed directly tothe test fungus. Most of the mycelium tendedto be concentrated within the voids originallypresent between the wood and plastic (Fig. 3).These voids appeared to act as pathways

TABLE . Weight losses in wood-plastic compositeblocks ex po ~e d o decay fungi in u soil block test.

Weight lops (B)"Tcst Composite Composite Compositefungus Matenal A Material R Mater ~alC

White rot fungiP. subserialis 10.2 (6.1) 0.6 (0.5) 0.4 (0.3)T. versicolor 5.9 (7.6) 1.2 (0.5) 2.1 (0.7)X. frustulatus 4.4 (3.2) 1.0 (0.4) 1.2 (1.2)

Brown rot fungiG. trabeurn 20.4 (10.8) 2.7 (1.3) 3.1 (0.6)P. plucmta 15.8 (5.4) 0.3 (0.2) 1.3 (1.3)W. coco.s 9.9 (8.5) 1.8 (0.3) 2.3 (1.5)

a Values rsprcrrnt means of six rrpl~cate\par fungus/composite treatmentValuer tn parentheses reprerent onc standard dcviattan.

through which the fungus grew between thewood particles in the plastic matrix. Voidsaround particles near the center of most blockswere essentially free of fungal colonizationeven in blocks with an elevated weight loss(Fig. 4). The absence of substantial fungal at-tack in these zones implies that the fungus wasslowly growing through the block, producingnear complete dissolution as it grew, but con-fining its attack to a relatively narrow area onthe exterior of individual particles. This pat-tern of damage is somewhat abnormal for thebrown rot fungi, which typically cause sub-stantial degradation some distance away fromactual fungal growth but leave a residual lig-nin skeleton (Zabel and Morrell 1992).

Clearly, all three materials contained nu-merous voids that could serve as conduits forfungal movement through the panel. The moredecay-resistant composite Materials B and Ccontained higher percentages of HPDE, but


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Mankowski and Morrell-PATTERNS OF FUNGAL AlTACK IN COMPOSITES 345

their individual wood particles were largerthan those in Material A. Smaller particlesmight be expected to permit more uniform en-capsulation by the plastic, but higher levels ofHPDE (and lower percentage of wood) appar-ently had a greater effect on limiting wateruptake (thus enhancing resistance to fungal at-tack) than did particle size or the presence ofvoids. Because the percentage of wood wasmuch lower in Materials B and C, their voidswere less likely to interconnect and thus lesslikely to provide a conduit for fungal move-ment.

The results suggest that the wood in wood-plastic composite lumber must be protectedfrom fungal attack, or the percentage of woodmust be limited, if the product is used in ex-terior exposures where wetting and fungal at-tack are possible.

ACKNOWLEDGMENTThis research was sponsored by the U.S,.

Naval Facilities Engineering Service Centel:,Port Hueneme.

REFERENCESMORRIS,. I., AN D k? COOPER. 998. Recycled plastic/wood

composite lumber attackcd by fungi. Forest Prod. I[.48(1):86-88.NAGHIPOUR,. 1996. Effects of extreme environmental

conditions and fungus exposure on the properties ofwood-plastic composites. M.S. thesis, Faculty of For-estry, University of Toronto, Toronto, Ontario, Canada.

SCHMIDT,. L. 1993. Dccay testing and moisture changesfor a plastic-wood composite. Proc. Am. Wood-Preserv.Assoc. 89:108-109 (Abstract).

SIMONSEN,. J. 1997. Efficiency of reinforcing materialsin fillcd polymer composites. Forest Prod. J. 47(1):74-81.

ZABEL, . A,, AND J. J. MOKRELL.992. Wood microbl-ology-Decay and its prevention. Academic Press, SanDiego, CA. 474 pp.

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Patterns of Fungal Attack in Wood Plastic