International Journal of Adhesion & Adhesives 30 (2010) 754–762

Contents lists available at ScienceDirect

International Journal of Adhesion & Adhesives

0143-74

doi:10.1

n Corr

British

fax +16

E-m

journal homepage: www.elsevier.com/locate/ijadhadh

Visualization and numerical analysis of adhesive distribution in particleboardusing X-ray micro-computed tomography

Philip D. Evans a,b,n, Olivia Morrison c, Tim J. Senden b, Stephan Vollmer a, Ray J. Roberts b, Ajay Limaye d,Christoph H. Arns b, Holger Averdunk b, Adrian Lowe c, Mark A. Knackstedt b

a Centre for Advanced Wood Processing, University of British Columbia, Vancouver, V6T 1Z4, Canadab Department of Applied Mathematics, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australiac School of Engineering, College of Engineering and Computer Science, The Australian National University, Canberra, ACT 0200, Australiad Visualization Laboratory, ANU Supercomputing Facility, The Australian National University, Canberra, ACT 0200, Australia

a r t i c l e i n f o

Article history:

Accepted 2 August 2010X-ray micro-CT is used here to visualize the distribution of melamine–urea–formaldehyde adhesive in the

wood composite, particleboard, and examine changes in adhesive distribution on wood particles (flakes)

Available online 12 August 2010

Keywords:

Wood

Composites

Microscopy

X-ray micro-computed tomography

96/$ - see front matter & 2010 Elsevier Ltd. A

016/j.ijadhadh.2010.08.001

esponding author at: Centre for Advanced W

Columbia, Vancouver, Canada V6T 1Z4. Tel.:

048229159.

ail address: phil.evans@ubc.ca (P.D. Evans).

a b s t r a c t

before and after pressing. CT and scanning electron microscopy of unpressed, wax-covered, wood flakes

reveals that adhesive is evenly distributed on flakes, although it does not completely cover all surfaces.

Adhesive on these wood flakes migrates during pressing forming more continuous glue-lines in areas of the

composite where flakes are pressed closely together and more discontinuous glue-lines where inter-particle

contact is poorer. Adhesive accumulates in small, but not very large voids, and also in cracks in wood flakes

and around very small wood particles. These observations accord with previous observations of adhesive

distribution in pressed wood composites. In addition, CT reveals that resin accumulates in capillary

channels within the splintered ends of wood flakes and at the base of composite samples. The orthogonal

variation in volume of the composite occupied by adhesive, wood and void space was quantified using

maximal sphere modelling, and anisotropy in the distribution of these phases in the x, y and z directions

was quantified using chord length analysis. These numerical techniques reveal that adhesive is more closely

associated with wood flakes than void space, despite its tendency to migrate during pressing. As a result,

adhesive is mainly aligned in the same x–y direction as wood flakes forming an anisotropic discontinuous

network. We conclude that X-ray micro-CT is a powerful tool capable of providing new insights into the

distribution of adhesive in wood composites.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Particleboard is a wood composite made by compressing andbonding together small wood particles or flakes with a thermoset-ting adhesive. Particleboard is used by the building and furnitureindustries and current global production is estimated to be 69.9million cubic metres [1]. Particleboard, in common with most otherwood composites, contains relatively small quantities of adhesive(o15% w/w) compared to other composite materials and, hence, itis very difficult to produce a continuous thin film of adhesive thatcovers and bonds together all of its reinforcing elements [2]. Unlessthis can be achieved, however, the maximum bonding strength ofwood composites is unlikely to be realized [2]. Nevertheless, it was

ll rights reserved.

ood Processing, University of

+16048220517;

recognized very early on that more modest increases in the strengthof wood composites might be achieved by improving the uniformityof distribution of adhesive within the composite [2,3]. It was alsorecognized that knowledge of the factors affecting the distribution ofadhesives in wood composites and their effects on materialproperties could be used to reduce the levels of adhesive in thecomposites and the cost of manufacturing them [2,3]. The possibilitythat the performance/cost ratios of wood composites might beimproved by manufacturing boards with improved uniformity ofdistribution of adhesives led to several early studies of thedistribution of adhesives in wood composites. The distribution ofadhesives in wood composites was first studied using lightmicroscopy [4–7] and later using scanning electron microscopy[8]. These studies found that the adhesive was unevenly distributedin composite boards and occasionally accumulated in localized areaswithin boards [4–8]. However, wood composites that appeared tohave more uniformly distributed adhesive had better strengthproperties [4,6]. More uniform distribution of adhesive in woodcomposites and improved mechanical properties occurred when the

P.D. Evans et al. / International Journal of Adhesion & Adhesives 30 (2010) 754–762 755

adhesive was applied to reinforcing elements as fine rather thancoarse adhesive droplets [3,4]. This finding was important as itinfluenced commercial practice, and it also led to the developmentof image analysis methods for quantifying and imaging adhesive onflakes that can be used to ensure that atomization of adhesive in theindustrial devices (blenders) that mix wood elements and adhesiveis producing the desired fine droplets [9,10]. More recent researchon the distribution of adhesives in wood composites has shifted tonewer materials such as medium density fibreboard and wood–plastic composites and polymeric phenylmethyl diisocyanateadhesives, respectively, and has employed more advanced imagingtechnology including X-ray spectrometry and confocal laser scan-ning microscopy [11–18].

The structure of wood composites has also been examined usingX-ray micro-computed tomography [19,20]. High resolution3-dimensional images of biocomposite materials have been publishedrecently, but these studies have mainly focused on paper and wood–polymer composites rather than composites such as particleboard,oriented strand board and medium density fibreboard [16,17,21–23],or have used very costly synchrotron radiation-based CT [18,24–26].X-ray micro-CT has not been used to visualize the distribution ofadhesives in wood composites. X-ray micro-CT has the potential toaccurately image porous materials and from such images it is possibleto calculate the void and solid phase morphology and other materialproperties, for example, elasticity, thermal conductivity and perme-ability [27–30]. Quantification of the void and solid phases in woodcomposites is of interest because of their influence on physicalproperties, however, the ability to resolve the adhesive network, inaddition to the aforementioned phases, is required to develop a morecomplete understanding of composite properties and performance.

X-ray CT imaging devices now have the ability to image porousmaterials in 3-D with superb resolution thanks to the availabilityof high-brilliance X-ray sources, high resolution X-ray detectorsystems, efficient algorithms for image processing and powerfulsoftware and hardware for 3-D visualization. Accordingly, wehypothesize that X-ray CT and associated computational techni-ques will provide new insights into the microstructure of theadhesive network in particleboard. In this paper we use X-raymicro-CT to characterize the microstructure of particleboard andin particular to visualize the distribution of adhesive in pressedboard and its relationship to adhesive distribution on flakesbefore pressing. Numerical techniques are also used to quantifythe different phases (adhesive, wood and void space) and theirdistribution in particleboard.

2. Material and methods

2.1. Manufacture of particleboard and Labelling of adhesive

Radiata pine (Pinus radiata D. Don) wood flakes produced by aknife mill were obtained from Carter-Holt Harvey’s particleboardplant in Tumut, New South Wales, Australia. A melamine–urea–formaldehyde adhesive resin (5016) was obtained from ORICAPTY Ltd. This adhesive had a solids content of 65% and a melaminecontent of 14% (based on liquid resin). Copper sulphate was eitheradded to the liquid adhesive (1.7% w/w) or finely ground anddissolved directly in the adhesive itself (3.8% w/w). This increasedthe opacity of the adhesive to X-rays and allowed wood, void andadhesive to be visualized in tomograms. Copper II ions remainbound to urea-formaldehyde adhesive even at high temperaturesmaking copper sulphate a good choice as a label for the adhesive[31]. Labelling of adhesive with copper sulphate was essentialbecause unlabelled adhesive could not be differentiated fromwood by X-ray CT. Dry wood flakes with a moisture content of 2%were tumbled in a cement mixer and sprayed using a gravity-feed

spray gun with adhesive resin, hardener (Technimul 9363,ammonium sulphate) and paraffin wax (Technimul 9363). Themoisture content of the flakes was 9% after the adhesive wasapplied. Mats were manually formed from resinated wood flakesin a bottomless 300�300 mm2 wooden forming box and laid on athin plywood sheet overlaid with aluminum foil. The mats werepressed down by hand, removed from the forming box andcovered with a second aluminum-faced plywood sheet. Theresulting assembly was inserted into the opening of a hydraulicpress and pressed for 143 s using a pressing schedule thatimitated a commercial pressing cycle used for the manufactureof particleboard in Australia. The maximum temperature andsystem pressure during pressing were 205 1C and 12000 kPa,respectively. Four boards were made in total. Each one measured300�300�19 mm3 and contained 12.5% adhesive, 0.7% wax,(both expressed as a percentage of oven-dry wood used to makeboards). After pressing, boards were cooled and their edges weretrimmed using a small band-saw.

2.2. X-ray micro-CT and SEM

Particleboard samples were cut with a band-saw into specimensmeasuring 12�12�19 mm3. These specimens and small unpressedmats of resinated wood flakes were imaged using an X-ray micro-CTdevice built by the Department of Applied Mathematics within theResearch School of Physics and Engineering at The AustralianNational University. The X-ray micro-CT device is 3 m long andconsists of an X-ray source (X-Tek RTF-UF225), rotation stage(Newport RV120PP) and X-ray camera (Roper PI-SCX100:2048)mounted on a parallel optical rail [32]. These features give the devicea large field of view (20483 [8 billion] voxels [three-dimensionalpoints in the tomogram] and 55 mm cross-section), good spatialresolution (down to 2 mm depending on operating voltage) and highdynamic range for enhanced contrast (16 bits) [32].

Particleboard specimens and unpressed mats of resinated flakeswere placed on the rotation stage and probed with a polychromaticX-ray beam (breaking radiation or Bremsstrahlung, 30–225 kV/1 mAwith 2–5 mm spot size) generated by a focused electron beam (40 kVand beam current of 200 mA) and blanked with a 2.5 mm platinumshutter. The CCD camera recorded the X-ray transmission radio-graph, and a series of radiographs, collectively called the projectiondata, were collected by rotating the specimen stage through 3601.The optimum number of projections, Ny is given by Ny¼(p/2)Nw,where Nw is the number of pixels in the width of the detector.Projection data were processed with a Feldkamp reconstructionalgorithm to generate a tomogram [33], which is a three-dimen-sional representation of the variation in density within the speci-men. It took approximately 14 h to acquire the tomographic imagefor each specimen. The quality of these tomograms was optimizedusing a range of measures to overcome the limitations imposed byusing a polychromatic X-ray cone beam. These included pre-filteringthe X-ray beam to minimize beam hardening, and using a small conebeam angle (0.81). Pre-filtering typically uses a filter with a similarcomposition to the specimen and a thickness of about 37% (e�1) ofthe diameter of the specimen. The projection data must representlinear interactions between the specimen and the X-rays to generatea tomogram. For X-ray tomography, however, the interaction is notlinear because X-rays are attenuated, but Beer’s law can accuratelydescribe such attenuation and can be easily manipulated into alinear form, provided the X-ray beam is mono-energetic. Thislinearization also works with the poly-energetic X-rays employed inour instrument, but beam hardening artifacts are introduced into thetomogram. These artifacts were minimized by reducing the energydistribution of the X-rays using pre-filtering, as mentioned above.Artifacts can also be introduced into the tomogram by cosmic rays,

P.D. Evans et al. / International Journal of Adhesion & Adhesives 30 (2010) 754–762756

defects, non-linear sensitivity and spatial non-linearity in the X-raycamera, misaligned cone beam geometry and specimen rotation andvariability of the X-ray flux from the source. A full description ofthese problems and the algorithms implemented to reduce theireffects is beyond the scope of this paper.

Resinated wood flakes were attached to aluminum stubs,sputter coated with a thin layer of gold and examined using aCambridge 360 scanning electron microscope operating at 15 kV.Images formed from backscattered electrons provided the mostcontrast between adhesive and wood and these were saved intagged image file format.

2.3. Visualization and numerical analysis

Three-dimensional volumes were used to generate intensityhistograms for void, wood and labelled adhesive within specimens.Thresholding of the peaks in these intensity histograms allowedthe different phases in the tomographic image to be identified.Two-phase separation (segmentation) steps were performed;between the adhesive and wood and voids, and between the voidsand adhesive and wood. These two segmentations were combinedto delineate the wood phase. Data sets were visualized in 2 and 3-Dusing volume rendering in which a transfer function assigns eachvoxel a colour and transparency [30]. Volume rendering wasperformed using the software Drishti, which is an open sourcevolume exploration and presentation tool for visualizing tomo-graphic data and is available at http://anusf.anu.edu.au/Vizlab/drishti/ downloads.shtml. The commercial volume rendering soft-ware VGStudio was also used to construct three-dimensionalimages of the adhesive in boards from stacks of two dimensionalimages obtained from full tomographic sequences.

Numerical predictions of a range of physical properties can bederived from segmented or 3-D images including pore sizes andtopology, permeability, thermal conductivity and elastic modulus, asmentioned above [27–30]. In this study we derived numerical valuesfor the volumes of void, adhesive and wood across the thickness of arepresentative board sample. These were obtained using maximalsphere modelling, which defines locally for every point within theboard, the diameter of the largest sphere which fully lies within thevoid, adhesive or wood phase and covers that point [34]. Anothergood discriminator of structure in porous materials is based on thechord length distribution pi(r), which gives the probability that arandomly chosen chord in phase i has length r. A chord is defined asany line segment which lies entirely in phase i with endpoints at thephase interface. In this study, chords were traced through void,adhesive and wood in all three orthogonal directions (x, y and z).A frequency histogram of the lengths of the different chords wasthen constructed. For a material that is truly isotropic the histogramswill be identical in all three directions [30].

All data processing, except for volume rendering usingVGStudio, used a Compaq AlphaServer super-computer locatedat the Australian Partnership for Advanced Computing, Australia’snational supercomputing facility. Data processing with VGStudioused a personal desk-top computer.

3. Results

3.1. Distribution of adhesive in pressed board

The distribution of adhesive in pressed boards can be seen incoloured 2-D images selected from tomographic sequences(Fig. 1a–f). The full tomographic sequences consisting of 499 in-plane images and 481 orthogonal images are available at http://forstd.forestry.ubc.ca/Evans/b6reconxz.mpg (Fig. 1a–c) and http://

forstd.forestry.ubc.ca/Evans/b6reconxy.mpg (Fig. 1d–f). In theseimages the adhesive is blue or green, wood is red and voids areblack. Fig. 1a is an in-plane view of part of a particleboard sampleshowing relatively good inter-particle contact and adhesive cover-age. This part of the board mainly contained long thin flakes andrelatively few large voids. In contrast, Fig. 1b shows part of a boardwhere there were larger voids and inter-particle contact waspoorer. This part of the board contained a higher proportion ofshorter and thicker flakes (Fig. 1b). The continuity of glue-lines inthe particleboard appeared to depend on the degree of contactbetween flakes. Hence, in areas where there was good inter-particle contact the glue-lines were longer and more continuous(Fig. 1a), but where there was greater separation of flakes, the glue-lines were shorter. The glue-lines were also interrupted in placesby the presence of large voids (Fig. 1b–c). These voids were free ofadhesive and the adhesive was largely associated with wood flakes.Nevertheless, adhesive did accumulate in smaller voids creatingthe appearance of ‘spot-welds’ between wood flakes (arrowed inFig. 1d). Adhesive also accumulated around very small particles ofwood or ‘fines’ (arrowed in Fig. 1e) and in the splintered ends offlakes (arrowed in Fig. 1f).

The static images shown in Fig. 1 are not very good at showingthe spatial distribution of the adhesive in the sample. Therefore 3-Danimations showing the distribution of wood, void and adhesive inthe sample were produced. These animations are available athttp://forstd.forestry.ubc.ca/Evans/b6small.wmv and http://forstd.forestry.ubc.ca/Evans/B6_new1.wmv. Fig. 1g–i shows images fromthe latter animation. In the first part of this animation the adhesivein the particleboard is blue, the wood flakes are brown and voidsare black (Fig. 1g). Later the animation reveals the distribution ofthe adhesive wood, and void networks deep within the sample(Fig. 1h), and then subtracts the wood from the adhesive and voidsto more clearly show the spatial distribution of the adhesivenetwork and voids (Fig. 1i). The voids are coloured green (Fig. 1i) tomake it easier to differentiate them from the blue adhesive and theblack wood flakes. This animation shows that the adhesive ismainly aligned in the same plane as the wood flakes forming ananisotropic, discontinuous, network. Within this network, glue-lines vary in thickness and some are not connected with otherones, but there are relatively few discrete droplets of adhesivewithin the network.

One unexpected trend that can be observed by looking at the 3-Danimation and some of the 2-D images (Fig. 1b–c) is that thedistribution of adhesive varied across the thickness of the board.This trend can be most clearly observed in Fig. 1c, where the amountof blue-green adhesive at the bottom of the particleboard sampleappears to be greater than at the top or in the centre of the sample.To more closely examine this phenomenon, three-dimensionalimages of the adhesive network were constructed from stacks ofthirty individual tomographic images selected from the top, centreand base of the sample (Fig. 2a, b and c, respectively). These 3-Dimages show the adhesive and void in these regions of the board(without the presence of wood). The adhesive network was morecomplete at the base of the sample compared with the top andparticularly the centre of the sample (Fig. 2).

There was evidence from 2-D images that adhesive penetratedcracks or splits in the flakes (arrowed at the top of Fig. 1f). It isdifficult to visualize the penetration of adhesive in cracks in 2-D.Therefore three-dimensional images of flakes containing crackswere constructed from stacks of thirty individual tomographicimages. Fig. 2d shows part of a flake containing cracks that werefilled with lighter coloured adhesive (green). Fig. 2e and f showthe same flake after the adhesive and wood have been subtractedfrom the image, respectively. These images provide strongevidence that melamine–urea–formaldehyde adhesive migratedinto cracks in pressed particleboard flakes.

Fig. 1. Rendered tomographic data of adhesive, wood and void in particleboard; (a), in-plane view of particleboard showing adhesive (blue-green), wood (red) and void

(black). Note the long thin glue-lines where there is good interparticle contact; (b), in-plane view of particleboard showing shorter and thicker particles, larger voids and

less well developed glue-lines; (c), in-plane view of particleboard showing the accumulation of adhesive at the bottom of the sample; (d), orthogonal view of particleboard

showing large voids and adhesive ‘spot-welds’ between particles (arrowed); (e), orthogonal view of particleboard showing the accumulation of adhesive around small

particles of wood (arrowed); (f), orthogonal view of particleboard showing the accumulation of adhesive in the splintered end of flakes (arrowed right) and penetration of

flakes by adhesive (arrowed top); (g), rendered 3-D tomographic data of particleboard sample showing adhesive (blue), wood flakes (brown) and void space (black);

h, rendered 3-D tomographic data of the internal structure of the particleboard sample shown in Fig. 1g; (i), rendered 3-D tomographic data of the adhesive network (blue)

and voids (green) in the particleboard sample shown in Fig. 1h. Scale bars¼3 mm.

P.D. Evans et al. / International Journal of Adhesion & Adhesives 30 (2010) 754–762 757

Fig. 2. Rendered 3-D tomographic data constructed from stacks of thirty 2-D images: (a), 3-D image of melamine–urea–formaldehyde adhesive within the upper part of a

particleboard sample; (b), 3-D image of adhesive within the core (centre) of a particleboard sample; (c), 3-D image of adhesive within the base of a particleboard sample;

(d), part of a wood flake showing lighter coloured adhesive penetrating cracks within the flake; (e), image from Fig. 2d with the adhesive subtracted from the flake;

(f), image from Fig. 2d with the wood subtracted from the adhesive. Scale bars on Fig. 2a–c¼2 mm.

Fig. 3. SEM photomicrographs showing partial coverage of a wood flake with adhesive (a) and the accumulation of adhesive within tracheids (b).

P.D. Evans et al. / International Journal of Adhesion & Adhesives 30 (2010) 754–762758

3.2. Distribution of adhesive on unpressed flakes

X-ray micro-CT was also performed on unpressed mats ofresinated wood flakes, and the distribution of adhesive on flakeswas compared with its distribution in pressed board. A 3-Danimation of these flakes is available at http://forstd.forestry.ubc.ca/Evans/B6loose.wmv. The lighter coloured (yellow) adhesive ispresent on most flake surfaces with the exception of end-grainoriented perpendicular to longitudinal surfaces of flakes. Scanningelectron microscopy of adhesive on loose wood flakes alsorevealed that there were areas that were free of adhesive(Fig. 3). The adhesive tended to clump in certain areas on thesurface of flakes (Fig. 3a) and even within individual tracheids(Fig. 3b). Fig. 4 shows a filtered data set and segmented images ofresinated wood flakes within an unpressed mat (Fig. 4a–b) and inpressed board (Fig. 4c–d). Adhesive was evenly distributed on thelongitudinal surfaces of unpressed flakes (Fig. 4a–b), althoughthere were areas that were free of adhesive as observed in Fig. 3and in the animation of resinated flakes. The distribution ofadhesive on flakes in a section of a pressed board tended to be lessuniform than it was on the flakes before pressing because itaccumulated around small wood particles (arrowed left in Fig. 4d)and in the splintered ends of flakes (arrowed bottom right in

Fig. 4d) and it also accumulated in small voids between flakes(as noted above, Fig. 1).

3.3. Quantification of adhesive distribution in board samples

To clarify the relationship between the distribution ofadhesive, void space and wood in pressed boards we derivednumerical values for the volumes of such phases across thethickness of a board sample using maximal sphere modelling [34].These volumes of wood, adhesive and void in a cross-section of aboard are plotted in Fig. 5. This figure confirms the trendsobserved in the tomographic images (Fig. 1b, c and i) that theproportion of wood to void space was higher and lower,respectively, in the surface and core of the board. The proportionof adhesive in the board was much lower, but it tended to beslightly higher at the base of the board compared with the uppersurface, and was associated with wood to a greater extent thanvoid space (Fig. 5).

Chord length analysis was used to examine anisotropy in thespatial distribution of the three different phases [30]. Thefrequency histograms of chord lengths in adhesive, woodand void space in all three (x, y and z) directions are shown

Fig. 4. Filtered data set and segmented images of resinated wood flakes within an unpressed mat (a–b) and in pressed board (c–d). Note the adhesive which is white (a,c) or

black (b,d) is evenly distributed on unpressed flakes (a,b), but is less evenly distributed on flakes in pressed board (c,d).

0.00 201510

Thickness of pressed board (mm) 5

0.1

0.3

1.0

0.8

0.6

0.2

VoidAdhesiveWood

Vol

umet

ric c

ompo

sitio

n of

boa

rd V

V [m

m-1

]

Fig. 5. Orthogonal variation in volume of a particleboard sample occupied by

wood, adhesive and void from maximal sphere modelling. Vertical lines drawn on

the graph allow the quantities of wood, adhesive and void in the board to be

compared more easily.

P.D. Evans et al. / International Journal of Adhesion & Adhesives 30 (2010) 754–762 759

in Fig. 6a–c. These frequency histograms show that chord lengthsfor all three phases are very similar in the x and y directions, butthe chord lengths are shifted left to lower values in the z direction.

This shift indicates that glue-lines, voids and wood flakes arelonger and wider than they are thick, confirming the materialanisotropy that is readily apparent in 2-D images and 3-Dvisualizations.

4. Discussion

X-ray micro-CT is a powerful tool for visualizing the porousmicrostructure of materials [27–30] and we have demonstratedits effectiveness at visualizing the distribution of adhesive andother phases (wood and void space) in particleboard. Traditionalhistological techniques for evaluating the distribution of adhesivein particleboard are based on observations of glue-lines in small,optically transparent, sections that are cut from pieces ofcomposite embedded in a polymer to make sectioning easier[3–6]. Great skill is required to obtain these cross-sections and itis time consuming to obtain large numbers of them. In contrast,X-ray micro-CT more rapidly generates larger numbers of twodimensional images of the composite and its component phases.These two dimensional images can be easily combined to create3-dimensional images that reveal the spatial geometry of theadhesive network and other phases far better than the techniquesthat have been used previously. Furthermore, microstructures indigital images are naturally discretized and lend themselves tocomputation of numerical indices for adhesive distribution and itsrelationship to other phases in the composite, as we havedemonstrated. Our experiments were complicated by the needto label the melamine–urea–formaldehyde adhesive resin tovisualize it using X-ray micro-CT. Nevertheless, many of ourobservation of the distribution of melamine–urea–formaldehyde

Chord length (mm)

P (I

)

ADHESIVE

VOID

P (I

)

Chord length (mm)0.0 0.5 1.0 1.5 2.0 2.5 3.0

P (I

)

WOOD

Chord length (mm)0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.1

0.2

0.3

0.4

0.5

0.0

0.1

0.2

0.3

0.4

0.5

0.0

0.1

0.2

0.3

0.4

0.5

0.0

zxy

zxy

zxy

Fig. 6. Chord length distributions in the x, y and z directions for the different

phases in the particleboard; a, adhesive; b, wood; c, void.

P.D. Evans et al. / International Journal of Adhesion & Adhesives 30 (2010) 754–762760

adhesive in particleboard accord with those of previous studies ofthe distribution of other types of adhesive in wood composites.For example, we observed that adhesive tended to accumulate

around small wood particles, supporting statements in theliterature that fine particles ‘tend to absorb much of the resin’in particleboard [35]. A previous study observed that polymericphenylmethyl diisocyanate adhesive penetrated cracks in parti-cleboard flakes [36]. The melamine-urea-formaldehyde adhesiveused here was also able to penetrate cracks in wood flakes. Ourobservations of the spatial distribution of adhesive in boards alsoaccord with previous observations that adhesive in woodcomposites accumulates in some areas and not in othersfollowing pressing [3,5,8]. However, we did not observe largenumbers of discrete droplets of adhesive within the 3-dimen-sional adhesive network. Therefore, the ‘spot-welds’ that wereobserved between flakes in 2-D images, which have also beenmentioned previously [11], are mainly a product of viewing glue-lines in 2-D. When viewed in 3-D the bonds between flakesmainly appear to be elongated glue-lines rather than discretebonding sites.

X-ray CT and numerical analysis also provided new insights intothe distribution of adhesive in particleboard. It is clear from 2-Dimages and 3-D visualizations and also chord length analysis that theadhesive network is anisotropic and discontinuous. X-ray micro-CTand maximal sphere modelling also revealed that adhesive accumu-lated in the splintered ends of wood flakes, and the quantity of resinwas higher at the base than at the top of board samples. Theseobservations have received little if any comment in previous studies.

Many previous studies, however, have suggested that adhesiveapplied to particles, flakes and fibres migrates when the resinatedelements are hot pressed. For example, Lehmann [37] observedthat discrete resin droplets applied to wood flakes flowedtogether during pressing to form continuous glue-lines. Ourobservation that glue-lines were more continuous in areas ofthe composite where the degree of consolidation of flakes washigh accords with Lehmann’s observations [37]. Further evidencethat adhesive migrates during pressing is our finding that thedistribution of adhesive on flakes before pressing differs from thatof flakes in pressed boards. A similar discrepancy was observed byLoxton et al. [12]. They used confocal laser scanning microscopyto examine the distribution of a labelled UF resin on wood fibreand in pressed MDF, and found that the size of resin spots wasgreater on fibres in pressed MDF than on the fibres prior topressing. Such an effect was more pronounced on fibres thatcontained wax [12]. Our observations also suggest that wax onwood flakes may alter the distribution of the resin in the pressedcomposite, as well as on individual flakes. SEM observationsindicated that adhesive only partially wetted flake surfaces,possibly due to the presence of a coating of wax on surfaces. Asa result, adhesive did not fully coat the surfaces of flakes, eventhough quite high loadings of adhesive were applied to them. Wehypothesize that when flakes come together during pressing theyform a hinge causing adhesive resin to coalesce to minimize itssurface area, and it then migrates towards points of contact. Suchmigration of resin towards points of contact between flakes mightassist bonding, although the presence of clumps of resin in smallvoids suggests that it might also have the opposite effect. Loxtonet al. [12] suggested that the presence of wax on MDF fibreaccounted for their observation that the degree of coverage offibres with resin was greater in pressed MDF boards thatcontained wax than in boards that were free of wax. Wax isadded to wood composites to improve their dimensional stability.Our results and those of Loxton et al. [12] suggest that it may alsoinfluence the distribution of adhesive in boards. Further researchis needed to fully examine the effects of wax on the distribution ofadhesive resin on wood flakes and in pressed composites and howthis affects the properties of boards.

Adhesive can account for up to 32 percent of the cost ofmanufacturing some wood composites and the amount of

P.D. Evans et al. / International Journal of Adhesion & Adhesives 30 (2010) 754–762 761

thermosetting adhesives used in wood composites in NorthAmerica was estimated to be 1.78 million metric tons in 1998[38]. Therefore there is a strong incentive to reduce the quantityof adhesive in wood composites without reducing the propertiesof the composite. Our observations suggest that this outcomecould be achieved using two approaches. Adhesive that penetratescracks and the splintered ends of flakes is less able to bond flakestogether than adhesive that lies on smooth surfaces. Eliminationof these defects could allow for more effective use of adhesive incomposites. Adhesive that is applied to flakes that are not incontact with adjacent flakes in pressed boards is also wasted. Thiscondition was more frequently observed in areas of boards, wherethere were thick, short, irregularly shaped flakes and poor packing(low compression). These observations suggest that the use oflong thin undamaged flakes in particleboard could lead to moreeffective use of adhesive and they also help explain whycomposites composed of thinner, longer, flakes are stronger thanthose made from thicker and shorter flakes.

5. Conclusions

X-ray micro-CT was able to visualize the microstructure ofparticleboard and quantify the distribution of its different phases(adhesive, wood and void space). Visualization of adhesive in boardswas only possible if the adhesive was labelled with copper sulphateto make it more opaque to X-rays. Therefore we conclude that X-raymicro-CT is a useful technique for examining adhesive distributionin composites made in the laboratory, but it is unable to acquiresuch information from commercially manufactured wood compo-sites containing unlabelled adhesive. Nevertheless, observations ofthe distribution of labelled adhesive in particleboard using X-raymicro-CT accord with those of previous studies that have examinedthe distribution of labelled and unlabelled adhesives in compositesusing microscopy. X-ray micro-CT can rapidly generate largenumbers of two dimensional images of composites and theircomponent phases unlike microscopy. Furthermore, these twodimensional images can be easily combined to create 3-dimensionalimages that reveal the spatial geometry of the adhesive network andother phases far better than microscopy. We conclude that X-raymicro-CT is better at visualizing the microstructure of the adhesivenetwork than microscopy, although it can still only be used toexamine relatively small samples and cannot resolve featuressmaller than two microns. Nevertheless, binarized microstructuresare of sufficient resolution to calculate numerical indices foradhesive distribution and its relationship to other phases in thecomposite as we have demonstrated using maximal spheremodelling and chord length analysis. These numerical techniquesreveal that the adhesive in particleboard forms an anisotropic anddiscontinuous network as is also apparent in 2-D images and 3-Dvisualizations. Related numerical modelling techniques could beused to derive information on the elasticity, thermal conductivityand permeability of composites. In conclusion, X-ray micro-CT is apowerful tool for obtaining information on the microstructure of theadhesive network in wood composites and this may assist with thedevelopment of composites with improved performance/cost ratios.

Acknowledgements

We thank the Australian Research Council for financial support forthe development of our X-ray micro-CT system and the AustralianPartnership for Advanced Computing for providing access to theirsupercomputer. We also thank David O’Brien (Carter-Holt Harvey)and Travis McCallum (ORICA) for their expert technical assistance,and the many people at The ANU who have supported or have been

involved in the development of the X-ray micro-CT system andvisualization software (Arthur Sakellariou, Tim Sawkins, AdrianSheppard, Rob Sok and Jim Williams).

References

[1] BISShrapnel, Particleboard in the Pacific Rim and Europe 2009–2013, (accessed22nd May 2010), /http://www.bis.com.au/news/part_in_the_pacific_29-04-2010_htmlS.

[2] Meinecke E, Klauditz W. Physics and technology of bonding in particle-boardproduction. Research report of North Rhine Westfalia #1053. WestdeautscherVerlag. Cologne and Opladen (Translated from German by Israel Program forScientific Translation in 1968) 1962:81.

[3] Burrows CH. Some factors affecting resin efficiency in flakeboard. For Prod J1961;11:27–33.

[4] Lehmann WF. Improved particleboard through better resin efficiency. For ProdJ 1965;15:155–61.

[5] Lehmann WF. Resin distribution in flakeboard shown by ultraviolet lightphotography. For Prod J 1968;18:32–4.

[6] Murmanis L, Myers GC, Youngquist JA. Fluorescence microscopy of hard-boards. Wood Fiber Sci 1968;18:212–9.

[7] Youngquist JA, Myers GC, Murmanis LL. Resin distribution in hardboard:evaluated by internal bond strength and fluorescence microscopy. Wood FiberSci 1987;19:215–24.

[8] Murmanis L, Youngquist JA, Myers GC. Electron microscopy study ofhardboards. Wood Fiber Sci 1986;18:369–75.

[9] Kamke FA, Lenth CA, Saunders HG. Measurement of resin and wax distributionon wood flakes. For Prod J 1996;46:63–8.

[10] Dai C, Yu C, Groves K, Lohrasebi H. Theoretical modeling of bondingcharacteristics and performance of wood composites. Part II. Resin distribu-tion. Wood Fiber Sci 2007;39:56–70.

[11] Donaldson LA, Lomax TD. Adhesive/fibre interaction in medium densityfibreboard. Wood Sci Tech 1989;23:371–80.

[12] Loxton C, Thumm A, Grisby W, Adams T, Ede R. Resin distribution in mediumdensity fibreboard. In: Proceedings of the fifth Pacific rim bio-basedcomposite symposium, Department of Forestry, The Australian NationalUniversity, Canberra, Australia; 2000. p. 235–42.

[13] Xie Y, Feng M, Deng J. Quantification of UF and PF resins in MDF fiber with anX-ray fluorescence spectrometer. Wood Fiber Sci 2004;36:337–43.

[14] Xing C, Riedl B, Cloutier A. Measurement of urea-formaldehyde resindistribution as a function of MDF fibre size by laser scanning microscopy.Wood Sci Tech 2004;37:495–507.

[15] Xing C, Riedl B, Cloutier A, Shaler S. Characterization of urea-formaldehyderesin penetration into medium density fiberboard fibers. Wood Sci Tech2005;39:374–84.

[16] Wang Y, Muszynski L, Simonsen J. Gold as an X-ray CT scanning contrastagent: effect on the mechanical properties of wood plastic composites.Holzforschung 2007;61:723–30.

[17] Alemdar A, Zhang H, Sain M, Cescutti G, Mussig J. Determination of fiber sizedistributions of injection moulded polypropylene/natural fibers using X-raymicrotomography. Adv Eng Mater 2008;10:126–30.

[18] Badel E, Delisee C, Lux J. 3D structural characterisation, deformationmeasurements and assessment of low density wood fibreboard undercompression: the use of X-ray microtomography. Composites Sci Technol2008;68:1654–63.

[19] Wang H, Shaler SM. Computer-simulated three-dimensional microstructureof wood fiber composite materials. J Pulp Paper Sci 1998;24:313–9.

[20] Groom L, Mott L, Shaler SM. Relationship between fiber furnish propertiesand the structural performance of MDF. In: Proceedings of 33rd internatparticleboard/composite materials symposium, Washington State UniversityPress, Pullman, W.A.; 1999. p. 89–100.

[21] Faessel M, Delisee C, Bos F, Castera P. 3-D modelling of random cellulosicfibrous networks based on X-ray tomography and image analysis. Compo-sites Sci Technol 2005;65:1931–40.

[22] Holmstad R, Gregersen OW, Aaltosalmi U, Kataja M, Koponen A, Goel A, et al.Comparison of 3D structural characteristics of high and low resolutionx-ray microtomographic images of paper. Nordic Pulp Pap Res J 2005;20:283–8.

[23] Sintorn I-M, Svensson S, Axelsson M, Borgefors G. Segmentation of individualpores in 3D paper images. Nordic Pulp Pap Res J 2005;20:316–9.

[24] Shaler SM, Wang H, Landis E, Keane DT, Mott L, Holzman L. Microtomographyof cellulosic structures. In: Proceedings of Tappi process and product qualityconference, Milwaukee; 1998. p. 89–96.

[25] Lux J, Delisee C, Thibault X. 3D characterization of wood based fibrousmaterials: an application. Image Analysis Stereology 2006;25:25–35.

[26] Walther T, Thoemen H. Synchrotron X-ray microtomography and 3D imageanalysis of medium density fiberboard (MDF). Holzforschung 2009;63:581–7.

[27] Arns CH, Knackstedt MA, Pinczewski WV, Garboczi EJ. Computation of linearelastic properties from microtomographic images: methodology and agree-ment between theory and experiment. Geophysics 2002;67:1396–405.

[28] Arns CH, Knackstedt MA, Pinczewski WV, Lindquist WB. Accurate estimationof transport properties from microtomographic images. Geophys Res Lett2001;28:3361–4.

P.D. Evans et al. / International Journal of Adhesion & Adhesives 30 (2010) 754–762762

[29] Arns CH, Knackstedt MA, Pinczewski WV, Martys NS. Virtual permeametry onmicrotomographic images. J Pet Sci Eng 2004;45:41–6.

[30] Knackstedt MA, Arns CH, Saadatfar M, Senden TJ, Limaye A, Sakellariou A, et al.Elastic and transport properties of cellular solids derived from three-dimen-sional tomographic images. Proc R Soc A Math Phys Eng Sci 2006;462:2833–2862.

[31] Feng MW. X-ray measurement of resin distribution in a cellulosic materialand binder composition. US Patent 2002:2002101957. 8 pp.

[32] Sakellariou A, Sawkins T, Senden TJ, Limaye A. X-ray tomography formesoscale physics applications. Physica A 2004;339:145–51.

[33] Feldkamp LA, Davis LC, Kress JW. Practical cone-beam algorithm. J Opt SocAm A 1984;1:612–9.

[34] Thovert J-F, Yousefian F, Spanne P, Jacquin CG, Adler PM. Grain reconstruction ofporous media: application to a low-porosity Fontainebleau sandstone. Phys Rev E:Statist Nonlinear Soft Matter Phys 2001;63:1–17.

[35] Haygreen JG, Bowyer JL. Forest products and wood science: an introduction.1st ed. Ames: Iowa State Univ Press; 1989.

[36] Roll H, Troger F, Wegener G, Grosser D, Fruhwald A. Investigation on MDI-distribution in chip mixes and particleboards. Holz Roh U Werkstoff1990;48:405–8.

[37] Lehmann WF. Resin efficiency in particleboard as influenced by density,atomization and resin content. For Prod J 1970;20:48–54.

[38] Sellers T. Wood adhesive innovations and applications in North America. ForProd J 2001;51:12–22.