coatings

Article Microstructure and Mechanism of Grain Raising in

Philip D. Evans 1,2,*, Ian Cullis 1, Joseph Doh Wook Kim 1, Lukie H. Leung 1, Siti Hazneza 1,3 and Roger D. Heady 4

1 Faculty of Forestry, University of British Columbia, Vancouver BC V6T 1Z4, Canada; [email protected] (I.C.); [email protected] (J.D.W.K.); [email protected] (L.H.L); [email protected] (S.H.) 2 Centre for Wood Durability and Design Life, University of Sunshine Coast, Sunshine Coast QLD 4558, Australia 3 Bioresource, Paper and Coatings Technology, School of Industrial Technology, Universiti Sains Malaysia, Penang 11800, Malaysia 4 Centre for Advanced Microscopy, Australian National University, Canberra ACT 0200, Australia; [email protected] * Correspondence: [email protected]; Tel.: +1-604-822-0517; Fax: +1-604-822-9159

Received: 8 August 2017; Accepted: 24 August 2017; Published: 29 August 2017

Abstract: Grain raising, the lifting of fibres when water is applied to wood surfaces, is a reason why some companies are reluctant to finish wood products with water-borne coatings. However, the elements that lift-up and cause grain raising have not been identified, and the relationship between wood density and grain raising has not been clarified. Our work sought answers to both questions. We planed or sanded different using aluminum oxide paper, and characterized surfaces using profilometry and SEM. Surfaces were re-characterized after wetting and drying. Grain raising is inversely related to wood density. In particular, very low-density woods are highly susceptible to grain raising, whereas grain raising does not occur in high-density woods or planed woods. In low-density woods, sanding tears cell walls creating loosely-bonded slivers of wood that project from surfaces, particularly after wetting and drying. This mechanism for grain raising was confirmed by modelling the action of on wood cell walls using an array of hollow tubes and a serrated . Less commonly, fibres and fibre-bundles project from surfaces. We observed that grain raising was correlated with the coarseness of the abrasive and conclude that it can be reduced in severity by tailoring sanding to account for the density and surface microstructure of wood.

Keywords: wood; grain raising; sanding; water-borne coatings; fibres; slivers; confocal profilometry; scanning electron microscopy

1. Introduction Grain raising, the lifting of fibres when water is applied to sanded wood surfaces, increases the roughness of wood, and is mentioned as one of the reasons why companies in North America are reluctant to finish wood products using water-borne coatings [1]. Grain raising can be avoided by finishing wood with solvent-based finishes [2], but this approach is becoming less acceptable due to environmental legislation limiting the solvent content of finishes [3]. Grain raising can be eliminated by lightly sanding wood to remove “fibres” projecting from surfaces [2,4], or its severity can be reduced by using modified finishes containing or hydrophobes [5–8]. Alternative approaches to reducing grain raising might be developed if the mechanisms responsible for the phenomenon were fully understood. However, there are only a handful of scientific papers on grain raising [9–12]. The first paper on grain raising by Koehler in 1932 used reflectance microscopy to examine the surface of planed or sanded (Swietenia sp.) and (Quercus sp.) woods, before and after wetting [9]. Koehler’s images clearly revealed that increases in roughness of wood surfaces following

Coatings 2017, 7, 135; doi:10.3390/coatings7090135 www.mdpi.com/journal/coatings Coatings 2017, 7, 135 2 of 15 wetting were more pronounced at sanded surfaces than planed surfaces, and also greater in oak than in mahogany [9]. Koehler claimed that increases in roughness were due to “fragments of fibres and pore wall projecting over the pores” at sanded wood surfaces [9]. However, his photographs were taken at low magnification (4:1) and it is not possible to see in his photographs if the woody element projecting from wood surfaces are fibre fragments, fibres, fibre bundles, or larger woody elements [9]. Higher magnification reflectance microscopy was used by Marra in his study of the factors responsible for grain raising of oak (Quercus sp.), and (Acer sp.) [10]. His observations suggested that fragments of thin-walled vessels, which are the conducting elements in oak and other , projected from sanded oak surfaces after wetting and drying [10]. He also suggested that raised grain occurred due to swelling of the ridges between sanding scratches, and also as a result of the separation of fibres or groups of fibres that are attached at one end to sanded surfaces. Marra looked at the effects of wood structure and sanding variables on grain raising and concluded that wood structure, particularly grain angle, had a greater influence on the development of raised grain than sanding variables [10]. He also pointed out that the lack of a method of quantifying the raised grain was a problem [10]. This problem was overcome by Nakamura and Takachio in 1961 [11]. They used stylus profilometry to quantify the changes in roughness of wood following sanding and wetting. Roughness was defined as the difference in maximum peak and valley height in a 10 mm long profilometry traverse [11]. The roughness value of sanded wood surfaces after wetting minus the initial roughness was used as an index of gain raising. Nakamura and Takachios’ profilometry results showed that grain raising was positively correlated with sanding pressure and coarseness of the abrasives used to sand wood surfaces [11]. Non-contact profilometry was used recently by Landry and coworkers to examine grain raising of yellow (Betula alleghaniensis Britt.) finished with water-based and solvent-based stains [12]. Their findings accorded with previous research, including our own unpublished research [13], indicating “that sanding method has an important role in grain raising generation”. To date each of the studies on grain raising has focused on only one or two species, with the exception of the work by Nakamura and Takachio in Japan that examined grain raising of seven wood species whose density varied from 0.37 to 0.71 [11]. However, their study did not find a relationship between density and grain raising, even though earlier work by Marra suggested that lower density springwood in oak was more susceptible to grain raising than summerwood [10]. Furthermore, the woody elements that rise up from sanded wood surfaces and are responsible for grain raising have not been precisely identified. In this paper, we seek to clarify the effects of wood density on grain raising and determine the microstructural feature responsible for grain raising. We hypothesize that the use of high-resolution profilometry and electron microscopy, in combination with physical modelling, will make it possible to answer these outstanding questions. Our ultimate aim is similar to those of Koehler [9] and Marra [10] who both sought to obtain a deeper understanding of the microstructure of grain raising to help develop better methods of selecting and machining wood to avoid the problem of grain raising.

2. Materials and Methods

2.1. Effects of Sanding on the Grain Raising of Different Wood Species Preliminary research developed the experimental protocols for examining grain raising in different wood species. The species selected ranged from the lowest density wood species, such as quipo and balsa, through to the world’s densest wood species, such as and lignum vitae (Table1). One wood sample for each species was obtained from UBC’s xylarium (wood collection) and cut with a band to produce specimens measuring ~20 mm (wide) × 80 mm (long) × 20 mm (thick) with their growth rings oriented tangentially to their wide faces. Samples were selected at random and hand-sanded along the grain using 120 and 180 grit aluminium oxide abrasive papers (30 s each). Coatings 2017, 7, 135 3 of 15

Table 1. Wood species used for an experiment that examined the relationship between wood density and grain raising.

Density Growth Rings and Name Code (kg/m3)* Texture [14] Quipo (Cavanillesia platanifolia, Humb. & Bonpl.) Kunth Q 118 Diffuse porous Balsa ( pyramidale, Cav. ex Lam.) Urb. B 133 Diffuse porous Obeche (, K. Schum.) A 226 Diffuse porous Western red cedar (Thuja plicata, Donn ex D. Don) WR 263 Medium/coarse Agarwood (Aquilaria spp.) AG 265 Diffuse porous Redwood (Sequoia sempervirens, D. Don) Endl. R 329 Coarse Grand fir (Abies grandis, Dougl. ex D. Don) Lindley F 348 Medium/coarse Western hemlock (Tsuga heterophylla, Raf.) Sarg. WH 350 Medium/fine Ponderosa (Pinus ponderosa, Dougl.) ex Lawson PP 351 Medium/coarse White (Picea glauca, Moench) Voss) ex Lawson WS 372 Medium/fine Black spruce (Picea mariana, Mill.) Britt., Sterns & Poggenburg BS 376 Medium/fine Red (Alnus rubra) Bong. RA 381 Diffuse porous Black cottonwood ( trichocarpa, Torr & Gray ex Hook. F.) BC 386 Diffuse porous Huon pine (Lagarostrobos franklinii, Hook) Quinn HP 400 Fine Mahogany () King M 412 Diffuse porous Douglas fir (Pseudotsuga menziesii, Mirb.) Franco DF 449 Medium/coarse Black cherry (Prunus serotina) Ehrh. CB 461 Diffuse porous Mountain ash ( regnans) F. Muell. MA 484 Diffuse porous Black walnut ( nigra) L. WB 488 Semi-ring porous Lodgepole pine (Pinus contorta) Dougl. LP 511 Medium/fine ( grandis) L.f. TE 526 Semi-ring porous Tamarack (Larix laricina, Du Roi) K. Koch TA 553 Medium/fine American (Fagus grandifolia) Ehrh. BE 564 Diffuse porous Yellow birch (Betula alleghaniensis) Britt. YB 583 Diffuse porous (Carya spp.) H 585 Semi-ring porous Red stinkwood (Prunus africana, Hook. f.) Kalkman RS 590 Diffuse porous Red oak (Quercus rubra) L. RO 605 Ring porous Sugar maple (Acer saccharum) Marsh. SM 659 Diffuse porous (Xylia xylocarpa Roxb.) Taub. IW 694 Semi-ring porous Rasberry jam (Acacia acuminata) Benth. RJ 814 Diffuse porous Red ironbark (Eucalyptus sideroxylon,A. Cunn. ex Woolls) RI 834 Diffuse porous Banga wanga (Amblygonocarpus obtusangulus, Welw. ex Oliv.) Exell & Torre BW 896 Diffuse porous Ebony (Diospyros spp.) E 1011 Diffuse porous Lignum vitae ( officinale) L. LV 1170 Diffuse porous * N = 1. Therefore, standard deviations for density of individual samples are not included in the table.

A sanded sample was placed on the x-y stage of a non-contact surface profilometer (Cotec Altisurf 500, Altimet, Alpespace, Grenoble, France). The elapsed time between sanding and surface analysis was approximately 5 min. An area measuring 16 × 4 mm2 was scanned with a 300 µm probe using the following measurement parameters: spacing between measurement points, 7 × 7 µm2; scan speed = 1 mm/s; total scan time for each sample was 3 h 12 min. The software Papermap (Altimet, Alpespace, Grenoble, France) was used to level surfaces and remove form, calculate the roughness of sanded surfaces, and produce images of surfaces [15]. Following initial surface analysis, distilled water was applied as a droplet to the surface of each sanded sample using a micropipette (35 µL/cm2). Each droplet was drawn across the surface of the sample using a thin sheet of plastic. After 3 min the water was gently wiped off the sample using a cotton cloth. The sample was air dried in a conditioning room at 20 ± 1 ◦C and 65% ± 5% r.h. (relative humidity) for 16 h and its surface roughness was re-measured, as above. These procedures were repeated until the initial (dry) and final (dry/wet/dry) roughness of all samples were measured. Grain raising was calculated as the difference in average surface roughness (Sa) of samples before and after wetting and drying. The basic density of all wood species was assessed on separate irregularly-shaped wood blocks cut from parent samples. The wood blocks were oven-dried at 105 ◦C overnight, placed in a desiccator over silica gel for 10 min, and then weighed on an analytical balance. Samples were then vacuum-impregnated with distilled water and left to soak in water for one week. The volume of the water-saturated samples was measured using an Archimedean volume-displacement method. Basic density was calculated as oven dry weight (kg)/green volume (m3). Coatings 2017, 7, 135 4 of 15

2.2. Grain Raising of Sanded versus Planed Wood A sub-set of the 34 wood species examined above were used to further explore the grain raising of different wood species, and also the influence of surfacing methods on grain raising. The species selected were: Balsa, western red cedar, black cherry, sugar maple, and lignum vitae. Air-dried wood samples of different sizes were obtained from UBC’s xylarium or purchased commercially. Forty samples were examined (eight for each of the five species). Samples were sanded (as above) or planed using a disposable stainless steel microtome blade (Type S35, Feather Safety Razor Co., Osaka, Japan) attached to a blade holder (Type 130A, Feather Safety Razor Co., Osaka, Japan) and clamped to the stage of a microtome (Spencer Lens Co., Buffalo, NY, USA). A fresh blade was then inserted into the blade holder and each specimen was replaned to produce a clean surface. Sanded or planed samples were placed on the x-y stage of a profilometer and the roughness of the samples before and after wetting and redrying was measured as above. The software Papermap was used to calculate the roughness parameters of five regions (0.2 × 0.2 mm2) within each scanned area. Following the initial surface analysis, a droplet of distilled water was applied to the surface of each of the sanded and planed samples using a micropipette, as above. Each sample was air-dried and its surface roughness was re-measured, as above. These procedures were repeated until the dry and wet roughness of all samples was measured (5 species × 8 samples × 2 surface types × 5 replicate roughness measurements = 400 measurements in total). Analysis of variance was used to examine the effect of fixed (species and surface preparation) and random factors on grain raising, calculated as the difference in roughness of samples before and after wetting and drying. Statistical computation was performed using Genstat (version 18.2, VSN International, Hemel Hempstead, UK). Before the final analysis, diagnostic checks were performed to determine whether data conformed to the underlying assumptions of analysis of variance, i.e., normality with constant variance. These assumptions were confirmed using residual plots produced by Genstat. Significant results (p < 0.05) are plotted on graphs and error bars (p < 0.05) on graphs, which were derived from Fisher’s least significant difference (LSD) test [16], can be used to compare differences between individual means. Sanded and planed samples were also examined using scanning electron microscopy to better understand the microstructural features responsible for grain raising. Samples were attached to separate aluminium stubs using nylon polish and coated with a 10 nm layer of gold to make them electrically conductive. A scanning electron microscope fitted with a high brightness lanthanum hexaboride electron source (Cambridge Stereoscan 360, Leica Microsystems, Wetzlar, Germany) or a variable pressure scanning electron microscope (S-2600N, Hitachi Ltd, Tokyo, Japan) was used to obtain images of the tangential surfaces of samples after sanding or , and again after wetting and drying [17]. Loose material present at sanded or planed surfaces, both before and after wetting, was removed using transparent tape, and the tape was mounted on aluminium stubs and imaged using a scanning electron microscope, as above.

2.3. Physical Modelling of Grain Raising Marra commented that the “abrasive action of the grits on wood surfaces is difficult to visualize” [10]. We faced the same difficulty when trying to interpret images of sanded wood surfaces before and after wetting and drying. Furthermore, models of the ploughing action of abrasives on metals were not helpful because wood is softer than most metals, and is porous, unlike metals [18]. However, physical models of wood have been developed in the past to aid understanding of its complex three-dimensional microstructure [19]. We made a similar physical model and simulated the abrasive action of mineral grits on wood using a serrated metal tool. The model was made simply to visualize the abrasive action of mineral grits on the microstructure of wood fibres. The model was made from an 8 × 4 array plastic tubes each measuring 257.2 mm (length) × 7.1 mm (diam.) Plastic tubes were bonded together using a fast-setting epoxy adhesive. The model was fixed to a horizontal benchtop and a serrated metal tool (https://www.kuglers.com/kitchen/cuisipro-julienne-peeler) was pressed down on the surface of the model and drawn across the surface of the uppermost layer of Coatings 2017, 7, 135 5 of 15 Coatings 2017, 7, 135 plasticdown tubes. on the Photographs surface of the of model the surface and drawn of the a modelcross the were surface taken of using the uppermost a digital single-lenslayer of plastic reflex (DSLR)tubes. cameraPhotographs (Canon of 5Dthe Marksurface II-Canon of the model Inc. Tokyo, were taken Japan) using mounted a digital on asingle tripod.-lens reflex (DSLR) camera (Canon 5D Mark II-Canon Inc. Tokyo, Japan) mounted on a tripod. 2.4. Effects of Abrasive Size on Grain Raising of Maple Panels 2.4. Effects of Abrasive Size on Grain Raising of Maple Panels Twelve maple veneer faced panels purchased from a commercial retailer (P.J. White, Vancouver, BC, Canada)Twelve were maple each veneer cut intofaced four panels samples purchased measuring from a commercial 150 mm × 280 retailer mm. (P.J. Samples White, wereVancouver levelled, withBC,an Canada) 80 grit were aluminum each cut oxide into four belt samples and then measuring finish-sanded 150 mm with × 280 120, mm. 120/150, Samples or were 120/150/180 levelled gritwith aluminium an 80 grit oxidealuminum abrasive oxide belts belt usingand th aen wide-belt finish-sanded with (Unisand 120, 120/150 K 1350, or M3,120/150/180 SCM Group, grit aluminium oxide abrasive belts using a wide- (Unisand K 1350 M3, SCM Group, Rimini, Rimini, Italy) with a feed speed of 7 m/s. The roughness of sanded samples was measured using Italy) with a feed speed of 7 m/s. The roughness of sanded samples was measured using profilometry, profilometry, as above. Water was then applied to an area (as above) on the surface of the samples and as above. Water was then applied to an area (as above) on the surface of the samples and they were they were then conditioned overnight at 20 ± 1 ◦C and 65% ± 5% r.h. The roughness of the samples then conditioned overnight at 20 ± 1 °C and 65% ± 5% r.h. The roughness of the samples was re- was re-measured using profilometry. Grain raising was calculated as the difference in roughness of measured using profilometry. Grain raising was calculated as the difference in roughness of sanded sanded samples before and after wetting and drying. Analysis of variance for a randomized block samples before and after wetting and drying. Analysis of variance for a randomized block design design was used to examine the effect of grit size on grain raising of samples [20]. Wood samples was used to examine the effect of grit size on grain raising of samples [20]. Wood samples sanded sandedwith different with different abrasive abrasive papers, papers, before beforeand after and wetting after wetting and drying, and drying,were also were examined also examined as above as aboveusing using a field a fieldemission emission scanning scanning electron electron microscope microscope (Hitachi (Hitachi S-4700 S-4700,, Hitachi Hitachi Ltd, Tokyo, Ltd, Tokyo, Japan). Japan). 3. Results 3. Results TheThe difference difference in in surface surface roughness of of the the various various wood wood species species after after sanding sanding and andwetting wetting and air and airdrying, drying, hereafter hereafter referred referred to to as as grain grain raising raising,, is isshown shown in inFigure Figure 1.1 There. There are are large large differences differences in inthe the graingrain raising raising of of the the different different wood wood species.species. SomeSome of the denser woods woods show show no no grain grain raising raising,, whereas whereas graingrain raising raising is is much much more more pronounced pronounced in in lower lower densitydensity species. There is is a a significant significant (p (p << 0.001) 0.001) inverse inverse relationshiprelationship between between grain grain raisingraising andand thethe densitydensity of the different different woods, woods, but but the the proportion proportion of ofthe the 2 variancevariance in in grain grain raising raising that that is is explained explained byby densitydensity isis modestmodest (R(R2 = 0.44), 0.44), becabecauseuse there there is is significant significant deviationdeviation between between observed observed and and fitted fitted values.values. ForFor example, grain grain raising raising of of the the world’s world’s lowest lowest density density woodwood species, species quipo, quipo (Q), (Q) is, is much much higher higher thanthan predictedpredicted from linear regression. regression. There There was was a astronger stronger andand more more linear linear relationship relationship betweenbetween the logarithm (ln) (ln) of of density density and and grain grain raising. raising.

FigureFigure 1. 1.Relationship Relationship between between densitydensity ofof woodswoods and grain raising raising defined defined as as the the increase increase in in surface surface roughnessroughness after after sanding sanding and and wetting wetting and and air air drying. Wood Wood species species can can be be identified identified usi usingng the the abbreviationsabbreviations in in TableTable1 1.. Values Values for roughness roughness increases increases of ofsome some wood wood species species were weresimilar similar and their and theirabbreviations abbreviations overlapped. overlapped. In these In these cases cases the precise the precise numerical numerical values values for roughness for roughness increases increases are areindicated indicated by by a aperiod period (.). (.). In In four four cases cases an an oblique oblique line line is isdrawn drawn from from the the period period to to the the abbreviated abbreviated speciesspecies name. name. 5 Coatings 2017, 7, 135 6 of 15

Coatings 2017, 7, 135 The five species arrowed in Figure1 were examined in more detail to explore the effects of

“species”CoatingsThe and 2017 five surfacing, 7, 1species35 methodsarrowed onin Figure grain raising.1 were examined These species in more span detail almost to explore the entire the effectsnatural of range of wood“species densities” and surfacing and include methods balsa on (B), grain western raising. red These cedar species (WR), span black almost cherry the entire (BC), sugarnatural maple range (SM) and lignumof woodThe vitaedensities five species (LV). and Grain includearrowed raising balsa in Figure (B), of western the 1 werelowest red examined cedar density (WR), in woods more black detail(balsacherry to and(BC), explore westernsugar the maple effects red (SM) cedar) of is significantlyand“species lignum” (p and< vitae 0.05) surfacing (LV). greater Grainmethods than raising thoseon grain of of the higherraising. lowest densityThese density species woods wood spans (black(balsa almost cherryand the western entire and sugar naturalred cedar maple) range) is and, particularly,significantlyof wood lignumdensities (p < vitae,0.05) and includegreater which balsathan showed (B),those western no of grainhigher red raising cedardensity (WR), (Figurewoods black 2(black). cherry Analysis cherry (BC), ofand sugar variance sugar maple maple) indicated (SM) and, particularly, lignum vitae, which showed no grain raising (Figure 2). Analysis of variance that theand mainlignum effects vitae (LV). of species raising (S),of the surface lowest preparationdensity wood (SP),s (balsa and and the western interaction red cedar of) S is× SP indicatedsignificantly that ( pthe < 0.05)main greatereffects ofthan wood those species of higher (S), surface density preparation woods (black (SP) cherry, and the and interaction sugar maple) of S (F(4, 383) = 11.01, p < 0.001) were statistically significant (p < 0.001). The latter interaction occurred ×and SP ,( Fparticularly(4, 383) = 11.01,, lignum p < 0.001) vitae, were which statistically showed significant no grain ( raisingp < 0.001). (Figure The latter 2). Analysis interaction of occurred variance because there is no significant difference (p > 0.05) in the grain-raising of the higher density sanded becauseindicated there that isthe no main significant effects differenceof wood species (p > 0.05) (S), insurface the grain preparation-raising of (SP) the, andhigher the densityinteraction sanded of S woodwood× samplesSP (F samples(4, 383) and =and the11.01, the planed p planed < 0.001) wood wood were samples samplesstatistically (Figure(Figure significant 22)).. (p < 0.001). The latter interaction occurred because there is no significant difference (p > 0.05) in the grain-raising of the higher density sanded wood samples and the planed wood samples (Figure 2).

Figure 2. Grain raising of sanded and planed wood surfaces. Differences in roughness that exceed the Figure 2. Grain raising of sanded and planed wood surfaces. Differences in roughness that exceed the length of the error bar (LSD) are statistically significant (p < 0.05). length of the error bar (LSD) are statistically significant (p < 0.05). Figure 2. Grain raising of sanded and planed wood surfaces. Differences in roughness that exceed the Confocallength of theprofilometry error bar (LSD) images are statisticallyof samples significant after sanding (p < 0.05). and wetting and redrying are shown in ConfocalFigure 3. In profilometry all sanded samples images (Figure of samples 3a,c,e,g) there after are sanding ridges andof material, wetting and and in balsa redrying there are are strips shown in Figurethat3. InareConfocal all elevated sanded profilometry above samples the images ridges (Figure of and samples3a,c,e,g) orien tedafter there at sanding an are angle ridges and to wetting of the material, ridges and redrying (Figure and in 3a).are balsa shown Confocal there in are stripsprofilometryFigure that are 3. In elevated all images sanded of above samplessanded the surfaces(Figure ridges 3a,c,e,g) after and wetting oriented there andare at ridges drying an angle of are material, shown to the andin ridges Figure in balsa (Figure 3b,d,f,h. there3 area).Strips strips Confocal of profilometrymaterialthat are oriented images elevated ofat above sandedan angle the surfaces to ridges the ridges and after are orien wetting presentted at and in an all drying angle samples to are the(except shown ridges lignum in (Figure Figure vitae, 3 3a).b,d,f,h. Figure Confocal Strips3h), of although they are most obvious in balsa (Figure 3b). These strips are elevated above the surface of materialprofilometry oriented images at an angleof sanded to the surfaces ridges after are wetting present and in drying all samples are shown (except in Figure lignum 3b,d,f,h. vitae, Strips Figure of 3h), thematerial samples. oriented The at microstructure an angle to the of ridges these are strips present and in the all samples effects on (except sanding lignum and vitae, planing Figure on 3h), the although they are most obvious in balsa (Figure3b). These strips are elevated above the surface of the structurealthough ofthey wood are surfacesmost obvious was examined in balsa (Figureusing scanning 3b). These electron strips microscopy,are elevated (SEM) above (Figures the surface 4–7). of samples.the samples. The microstructure The microstructure of these of strips these and strips the and effects the on effects sanding on sanding and planing and planing on the structure on the of woodstructure surfaces of was wood examined surfaces was using examined scanning using electron scanning microscopy, electron microscopy, (SEM) (Figures (SEM)4 –(Figures7). 4–7).

Figure 3. Cont.

6 FigureFigure 3.3. Cont. .

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Figure 3. Confocal profilometry images of sanded surfaces before (a,c,e,g) and after grain raising Figure 3. Confocal profilometry images of sanded surfaces before (a,c,e,g) and after grain raising (b,d,f,h): (a,b) balsa; (c,d) western red cedar; (e,f) black cherry; (g,h) sugar maple. (b,d,f,h): (a,b) balsa; (c,d) western red cedar; (e,f) black cherry; (g,h) sugar maple. Figure 4 shows SEM images of balsa wood. Sanding of balsa created a mat of degraded cell wall materialFigure that4 shows obscured SEM the images underlying of balsa anatomical wood. Sanding features of of balsa the wood created (Figure a mat 4a). of This degraded mat consisted cell wall materialof slivers that of obscured cell wall thematerial underlying that were anatomical mainly aligned features in of the the fibre wood direction, (Figure although4a). This matsome consisted slivers of sliverswere bent of celland wall aligned material tangentially that were to fibres mainly (Figure aligned 4a). in These the fibre slivers direction, were attached although at someone end slivers to weretheir bent parent and alignedfibres and tangentially there was toa greater fibres (Figure tendency4a). for These them slivers to project were vertically attached atfrom one the end surface to their parentfollowing fibres w andetting there and was drying a greater (Figure tendency 4b). In contrast, for them it to was project possible vertically to see fromthe underlying the surface structure following wettingof balsa and in the (Figure planed4b). surface In contrast, after wetting it was and possible drying, to although see the underlyingthere is evidence structure that planing of balsa woodcaused in the tearing planed of fibre surface walls after and wetting the torn andmaterial drying, projected although from there planed is evidence balsa wood that surfaces planing (Figure caused tearing4c). Figure of fibre 4d walls shows and cell the wall torn fragments material removedprojected from from a planed sanded balsa balsa wood wood surfaces surface following (Figure4c). Figurewetting4d shows and drying. cell wall The fragments material removed removed from from a sandedthe surface balsa consisted wood surface of long following slivers of wetting cell wall and drying.material, The materialand much removed smaller fromfragments. the surface Far less consisted material of was long removed slivers of from cell wall the material,surface of and planed much balsa wood following wetting and drying. smaller fragments. Far less material was removed from the surface of planed balsa wood following wetting and drying. 7 Coatings 2017, 7, 135 8 of 15 Coatings 2017, 7, 135

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Figure 4. Balsa wood surfaces: (a) surface after sanding with 120 and 180 grit aluminium oxide abrasive Figure 4. Balsa wood surfaces: (a) surface after sanding with 120 and 180 grit aluminium oxide abrasive papers; (b) sanded surface after wetting and redrying; (c) planed surface after wetting/redrying; and (d ) papers; (b) sanded surface after wetting and redrying; (c) planed surface after wetting/redrying; looseFigure surface 4. Balsamaterial wood removed surfaces: from (a) surface (b) using after transparentsanding with tape.120 and Scale 180 barsgrit aluminium = 200 μm. oxide abrasive and (d) loose surface material removed from (b) using transparent tape. Scale bars = 200 µm. papers; (b) sanded surface after wetting and redrying; (c) planed surface after wetting/redrying; and (d) Sandingloose surfaceof western material red removed cedar from surfaces (b) using left transparent cell wall tape. material Scale bars attached = 200 μm. to the vertical walls of tracheidsSanding that ofappeared western as red ridges cedar in surfacesconfocal leftprofilometry cell wall materialimages (Figure attached 5a). to This the verticalmaterialwalls is more of Sanding of western red cedar surfaces left cell wall material attached to the vertical walls of tracheidsobvious in that the appearedsample that as was ridges wetted in confocal and then profilometry dried (Figure images 5b). In (Figure this sample,5a). This you material can see isribbons more obvioustracheids in the that sample appeared that wasas ridges wetted in confocal and then profilometry dried (Figure images5b). (Figure In this sample,5a). This youmaterial can is see more ribbons or sliversobvious of incell the wall sample material that was that wetted are andbent then-over dried and (Figure projecting 5b). In thisfrom sample, the wood you can surface. see ribbons They are or slivers of cell wall material that are bent-over and projecting from the wood surface. They are absent absentor fromslivers the of planedcell wall surface material (Figure that are 5c), bent but-over are and easily projecting seen in from the photographthe wood surface. that shows They are the cell from the planed surface (Figure5c), but are easily seen in the photograph that shows the cell wall wall absentmaterial from remov the planeded by surfacetape testing (Figure (Figure 5c), but 5d). are easily seen in the photograph that shows the cell materialwall removedmaterial remov by tapeed by testing tape testing (Figure (Figure5d). 5d).

Figure 5. Cont. Figure 5. Cont..

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Figure 5. Western red cedar wood surfaces after: (a) sanding; (b) sanding and wetting/redrying; (c) Figure 5. Western red cedar wood surfaces after: (a) sanding; (b) sanding and wetting/redrying; planing/wetting/redrying; and (d) material removed from (b) using transparent tape. Scale bars = 100 μm. (c) planing/wetting/redrying; and (d) material removed from (b) using transparent tape. ScaleSanding bars = 100 of µsugarm. maple surfaces also left slivers of cell wall material attached to the underlying wood,Figure although 5. Western they are red smaller cedar wood than surfaces those observed after: (a) sanding; on sanded (b) sandingbalsa or and cedar wetting/redrying; surfaces (Figure (c) 6a). SandingSandingplan ofing/wetting/redrying; of maple sugar surfaces maple surfacescreated and (d) materialgrooves also leftremoved at sliversthe surface from of (b) cell ofusing the wall transparent wood, material in tape.addition attached Scale barsto slivers = to 100 the μm of underlying. wood wood,attached although to cell they walls. are These smaller slivers than of those wood observed projected from on sanded wood surfaces balsa or after cedar wetting surfaces and (Figuredrying 6a). (FigureSanding 6c), as wasof sugar observed maple at surfaces balsa and also western left slivers red cedarof cell surfaces. wall material In addition, attached we to observed the underlying hollow Sandingwood, of maplealthough surfaces they are created smaller groovesthan those at observed the surface on sanded of the balsa wood, or incedar addition surfaces to (Figure slivers 6a). of wood attachedtubular to cell elements walls. projecting These slivers from of maple wood surfaces projected indicating from wood that “fibre surfaces rise” after contri wettingbuted to and grain drying raisingSanding in ofmaple. maple Maple surfaces surfaces created showed grooves little at the damage surface as of a theresult wood, of planing in addition and tovery slivers little of material wood (Figure6c), as was observed at balsa and western red cedar surfaces. In addition, we observed hollow wasattached removed to cell by walls. tape These testing. slivers In contrast, of wood the projected tape removed from wood a variety surfaces of after different wetting materials and drying from (Figure 6c), as was observed at balsa and western red cedar surfaces. In addition, we observed hollow tubularmaple elements wood that projecting had been fromsubjecting maple to the surfaces sanding indicating and the grain that raising “fibre procedure, rise” contributed including parts to grain tubular elements projecting from maple surfaces indicating that “fibre rise” contributed to grain raisingof thin in maple.-walled Maple vessel elements surfaces containing showed little helical damage windings, as a parts result of offibres planing and cell and wall very slivers, little and material raising in maple. Maple surfaces showed little damage as a result of planing and very little material was removedsmaller debris by tape(Figure testing. 6d). In contrast, the tape removed a variety of different materials from maplewas wood removed that had by tape been testing. subjecting In contrast, to the the sanding tape removed and the a variety grain of raising different procedure, materials from including maple wood that had been subjecting to the sanding and the grain raising procedure, including parts parts of thin-walled vessel elements containing helical windings, parts of fibres and cell wall slivers, of thin-walled vessel elements containing helical windings, parts of fibres and cell wall slivers, and and smallersmallerdebris debris (Figure 6d).6d).

Figure 6. Sugar maple wood surfaces after: (a) sanding; (b) sanding/wetting/redrying; (c) planing/wetting/ redrying; and (d) material removed from (b) using transparent tape. Scale bars (a,b) = 50 μm; (c,d) = 100 μm. Figure 6. Sugar maple wood surfaces after: (a) sanding; (b) sanding/wetting/redrying; (c) Figure 6. Sugar maple wood surfaces after: (a) sanding; (b) sanding/wetting/redrying; (c) planing/ planing/wetting/ redrying; and (d) material removed9 from (b) using transparent tape. Scale bars (a,b) = µ wetting/50 μm; redrying; (c,d) = 100 and μm. (d ) material removed from (b) using transparent tape. Scale bars (a,b) = 50 m; (c,d) = 100 µm. 9 Coatings 2017, 7, 135 10 of 15 Coatings 2017, 7, 135

Sanding did not not create create loosely loosely-bonded-bonded cell cell wall wall material material at atthe the surface surface of oflignum lignum vitae vitae wood, wood, in incontrast contrast to toits its effects effects on on balsa, balsa, cedar, cedar, cherry, cherry, and and maple. maple. Instead Instead sanding sanding created created a a series series of of grooves at the surface of the woodwood (Figure(Figure7 7a).a).The Thesurface surfacewas waslargely largelyunchanged unchangedafter afterthe thegrain-raising grain-raising procedureprocedure (Figure(Figure7 7b),b), andand veryvery littlelittle materialmaterial waswas removedremoved fromfrom lignumlignum vitaevitae byby tapetape testingtesting (Figure(Figure7 7d).d). Lignum Lignum vitae isis thethe world’sworld’s densestdensest wood wood and and is is very very difficult difficult to to section, section, which which accounts accounts for for why why the the surface surface of theof the planed planed lignum lignum vitae vita samplee sample is unevenis uneven (Figure (Figure7c). 7c).

Figure 7. Lignum vitae wood surfaces after: (a) sanding; (b) sanding/wetting/redrying; (c) Figure 7. Lignum vitae wood surfaces after: (a) sanding; (b) sanding/wetting/redrying; (c) planing/ planing/wetting/ redrying; and (d) material removed from (b) using transparent tape. Scale bars (a,b) = wetting/ redrying; and (d) material removed from (b) using transparent tape. Scale bars (a,b) = 50 µm; 50 μm; (c,d) = 100 μm. (c,d) = 100 µm.

Our observations of grain raising (above) on sanded wood samples were complemented by physicalOur modelling observations of the of graineffects raising of sandi (above)ng on wood on sanded surfaces. wood Arrays samples of plastic were tubes complemented modelled the by physicalporous microstructure modelling of the of effectswood and of sanding a serrated on woodtool drawn surfaces. across Arrays the tubes of plastic simulated tubes modelled the abrasive the porousaction of microstructure on of the wood microstructure and a serrated of toolwood drawn (Figure across 8). The the tubesserrated simulated tool completely the abrasive removed action ofparts sandpaper of the uppermost on the microstructure layer of plastic of woodtubes (Figurewhen it8 ).was The drawn serrated across tool the completely surface of removed the tubes. parts It also of thecreated uppermost strips of layer plastic of plastic that weretubes stillwhen attached it was drawn to the across sides the of thesurface tubes of the(Figure tubes. 8). It These also created strips stripsprojected of plastic above thatthe surface were still of the attached uppermost to the layer sides of oftubes, the tubesand were (Figure oriented8). These in the strips direction projected of the abovetubes, or the at surface an angle of to the the uppermost tubes (depending layer of on tubes, the width and were and orientedlength of in the the strips, direction Figure of 8). the tubes, or at an angle to the tubes (depending on the width and length of the strips, Figure8).

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Figure 8. Tubular plastic wood model after a serrated tool was drawn across its surface.

Our observations of grain raising of wood described above were carried out on hand-sanded solid wood samples. These observations are relevant to small craft-based industries that employ hand sanding of solid wood. However, larger industrial facilities use machine-sanding and often process composite panels with a decorative face veneer in addition to solid wood. Therefore, we examined grain raising of maple veneer-faced panels that were machine-sanded. There was a significant effect of sanding on grain raising (F(2, 22) = 5.15, p = 0.015) and a positive relationship between abrasive size and grain raising (Figure 9). In other words, sanding with finer abrasives (150 and 180 grit) Figure 8. Tubular plastic wood model after a serrated tool was drawn across its surface. resulted inFigure significant 8. Tubularly (p < plastic 0.005) wood less modelgrain raising after a serrated than sanding tool was with drawn a coarser across its abrasive surface. (120 grit). ScanningOur observations electron ofmicroscopy grain raising was ofused wood to examine described the above surface were structure carried of out maple on handveneer-sanded-faced panelssolidOur wood after observations samples. sanding Theseand of then grain observations again raising after of are wetting wood relevant described and to smalldrying. above craft Sanding- werebased carried withindustries 120 out g ritthat on abrasive hand-sandedemploy paper hand solidcreatedsanding wood groovesof solid samples. andwood. Theseridges, However, observations and loosely larger are -industrialbonded relevant material facilities to small at use craft-basedthe machine surface industries -ofsanding the veneer and that employoften(Figure process hand10a). sandingAftercomposite wetting of panels solid and wood. withdrying a However, decorativea variety largerof face woody industrialveneer material in facilitiesaddition projected useto solidfrom machine-sanding wood.the sanded Therefore, surface and we often (Figure examined process 10b) compositeincludinggrain raising slivers panels of maple of with a decorativecell wall-faced (a rrowedpanels face veneer that left), were inends addition machine of fibres to- (arrowedsanded. solid wood. There right Therefore, wasof centre) a significant we, and examined bundles effect grainof sanding fibres raising (arrowed on of grain maple extreme raising veneer-faced ( right).F(2, 22) panels Sanding = 5.15, that p surfaces = were 0.015) machine-sanded. with and abrasivea positive paper Thererelationship withwas a finer significant between abrasive abrasive effect grits of sandingcausedsize and progressively on grain grain raising raising less (Figure ( Fdamage(2, 22) 9). = to In 5.15, theother woodp = words, 0.015) surface, andsanding aand positive there with was relationshipfiner less abrasives grain between raising (150 and abrasiveafter 180 wetting grit) size andresulted grain drying in raising significant (Figure (Figure 10c,d).ly (9p). < In 0.005) These other less words,observations grain sanding raising accord with than finer withsanding abrasives measurements with a (150 coarser and of 180abrasive the grit) differences resulted(120 grit). in in significantlyroughnessScanning of ( psurfaces electron< 0.005) after microscopy less sanding grain raising was and usedwetting than to sanding examine and drying with the a (Figuresurface coarser 9).structure abrasive of (120 maple grit). veneer-faced panels after sanding and then again after wetting and drying. Sanding with 120 grit abrasive paper created grooves and ridges, and loosely-bonded material at the surface of the veneer (Figure 10a). After wetting and drying a variety of woody material projected from the sanded surface (Figure 10b) including slivers of wood cell wall (arrowed left), ends of fibres (arrowed right of centre), and bundles of fibres (arrowed extreme right). Sanding surfaces with abrasive paper with finer abrasive grits caused progressively less damage to the wood surface, and there was less grain raising after wetting and drying (Figure 10c,d). These observations accord with measurements of the differences in roughness of surfaces after sanding and wetting and drying (Figure 9).

Figure 9. 9. EffectEffect of of grit size of abrasive belts on the grain raising of maple maple veneer-faced veneer-faced panels panels sequentially sandedsanded usingusing anan industrialindustrial widewide beltbelt sander.sander.

Scanning electron microscopy was used to examine the surface structure of maple veneer-faced panels after sanding and then again after wetting and drying. Sanding with 120 grit abrasive paper created grooves and ridges, and loosely-bonded material at the surface of the veneer (Figure 10a). After wetting and drying a variety of woody material projected from the sanded surface (Figure 10b) including slivers of wood cell wall (arrowed left), ends of fibres (arrowed right of centre), and bundles of fibresFigure (arrowed 9. Effect extreme of grit right). size of Sanding abrasive surfaces belts on with the abrasivegrain raising paper of with maple finer veneer abrasive-faced grits panels caused progressivelysequentially less sanded damage using to an the industrial wood surface,wide belt11 and sander. there was less grain raising after wetting and drying (Figure 10c,d). These observations accord with measurements of the differences in roughness of surfaces after sanding and wetting and drying (Figure9).

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Figure 10. Appearance of maple veneer surfaces after composite veneer-faced panels were sanded Figure 10. Appearance of maple veneer surfaces after composite veneer-faced panels were sanded with an industrial wide belt sander: (a) surface sanded with 120 grit aluminum oxide belt; (b) surface with an industrial wide belt sander: (a) surface sanded with 120 grit aluminum oxide belt; (b) surface sanded with 120 grit aluminum oxide belt and then subjected to grain raising procedure; (c) surface sanded with 120 grit aluminum oxide belt and then subjected to grain raising procedure; (c) surface sanded with with 120/150/180 120/150/180 grit grit aluminum aluminum oxide oxide belts; andand ( (dd)) surfacesurface sanded sanded with with 120/150/180 120/150/180 grit grit aluminum oxide beltsbelts andand thenthen subjectedsubjected toto thethe graingrain raisingraising procedure.procedure. ScaleScale barsbars == 250 µμm.m.

4. Discussion 4. Discussion Our results show an inverse relationship between the density of wood species and grain raising, Our results show an inverse relationship between the density of wood species and grain raising, and suggest a mechanism to account for this relationship. In low-density wood species, such as balsa and suggest a mechanism to account for this relationship. In low-density wood species, such as and western red cedar, “fibres” are thin walled and are easily perforated and shredded during balsa and western red cedar, “fibres” are thin walled and are easily perforated and shredded during sanding resulting in the formation of slivers of cell walls that are loosely bonded to the underlying sanding resulting in the formation of slivers of cell walls that are loosely bonded to the underlying wood surface. These slivers, which were also formed when we modelled the effects of abrasives on a wood surface. These slivers, which were also formed when we modelled the effects of abrasives on tubular wood model, project from wood surfaces after sanding and also to a greater extent after a tubular wood model, project from wood surfaces after sanding and also to a greater extent after surfaces become wet and then dry. This phenomenon was observed under ambient conditions using surfaces become wet and then dry. This phenomenon was observed under ambient conditions using confocal profilometry, and also under low vacuum scanning electron microscopy. The phenomenon confocal profilometry, and also under low vacuum scanning electron microscopy. The phenomenon appears to account for grain raising in low-density wood species. In the highest density wood species, appears to account for grain raising in low-density wood species. In the highest density wood species, such as lignum vitae, sanding did not create slivers of cell wall material and grain raising did not such as lignum vitae, sanding did not create slivers of cell wall material and grain raising did not occur. Grain raising occurred in species whose density fell between those of balsa and lignum vitae, occur. Grain raising occurred in species whose density fell between those of balsa and lignum vitae, for example, sugar maple, and can be explained in part by the mechanism we have just proposed, for example, sugar maple, and can be explained in part by the mechanism we have just proposed, particularly the shredding of thin-walled vessel elements. In addition, the partial detachment of fibres particularly the shredding of thin-walled vessel elements. In addition, the partial detachment of fibres and fibre bundles also appears to contribute to grain raising in maple. However, we find little and fibre bundles also appears to contribute to grain raising in maple. However, we find little evidence evidence that swelling of ridges created by the ploughing effect of abrasive particles during sanding that swelling of ridges created by the ploughing effect of abrasive particles during sanding contributes contributes to grain raising because such ridges were prominent in higher-density species that did to grain raising because such ridges were prominent in higher-density species that did not develop not develop significant grain raising. significant grain raising. The mechanism for grain raising we have proposed helps to explain some of the results of The mechanism for grain raising we have proposed helps to explain some of the results of previous previous studies that examined grain raising. For example, the relationships between grain raising studies that examined grain raising. For example, the relationships between grain raising and abrasive and abrasive size observed by Nakamura and Takachio [11], and also here, may be explained by the size observed by Nakamura and Takachio [11], and also here, may be explained by the increased increased ability of larger abrasive grains to slice open fibre walls leaving partially-detached slivers of wood at sanded surfaces. Increased sanding pressure might produce a similar effect, as observed

12 Coatings 2017, 7, 135 13 of 15 ability of larger abrasive grains to slice open fibre walls leaving partially-detached slivers of wood at sanded surfaces. Increased sanding pressure might produce a similar effect, as observed by Nakamura and Takachio [11], because there would be more intimate contact between abrasive particles and cell walls. The observations by Marra [10] that species such as oak are susceptible to grain raising may be explained by the presence of abundant large and thin-walled earlywood vessel elements that are more easily shredded during sanding than thicker-walled fibres. Evidence here for such an effect was the presence of shredded vessel walls in the debris removed from sanded hardwoods. Sanding wood at an oblique angle to fibres results in greater grain raising according to Marra [10], possibly because it would increase the partial shredding of fibres along their length, creating more slivers of wood that are partially attached to the underlying wood. Our results confirm the observations of Koehler [9] that sanding can create a surface that is susceptible to grain raising, because grain raising was present in sanded wood surfaces and absent from carefully-matched wood samples that had been planed. However, grain raising was minimal in maple that was sanded with an industrial-scale (wide-belt) sander using 120, 150, and 180 grit abrasive belts. This finding suggests that grain raising can be minimized by using a sanding sequence that is tailored to particular wood species. Low-density wood species, or ones that possess an abundance of thin-walled cells, for example, ring porous species such as , may benefit from an additional finer sanding step. However, fine sanding reduces the ability of stains and dyes to colour wood [21] and this would need to be taken into account when deciding whether to introduce a final fine sanding step to reduce grain raising in lower-density woods. Other alternatives, as suggested by the patent literature, include the use of stains containing binders that presumably bond projecting woody slivers to the underlying wood and make them easier to remove using denibbing and inter-coat sanding steps [5–8]. Our research sheds no light on whether grain raising occurs when water is applied to wood surfaces or when it subsequently dries. Marra [10] suggested that grain raising occurred during the wetting phase as wood elements swell and presumably curl away from the underlying wood surface. This suggestion is supported by unpublished industry observations. The alternative, that curling of wood elements occurs during drying, is supported by observations of the tendency of pulp fibres to curl during drying [22]. This issue would benefit from further research to measure the moisture-induced strains that develop when sanded wood surfaces become wet and then dry. Further research is also needed on the grain raising of medium-density fibreboard (MDF), which is a problem when products made from MDF are finished with water-borne coatings. This is a fertile area for industrially-important research and the approaches used here to better understand grain raising of solid wood could provide insights into the grain raising of MDF and possibly other wood composites that are commonly finished with water-borne coatings.

5. Conclusions We have shown that there is an inverse relationship between the density of wood species and grain raising, and have suggested a mechanism to account for this relationship. This mechanism helps to explain why some wood species are more susceptible to grain raising than others, and how some sanding parameters influence grain raising. Our results suggest that grain raising can be minimized by tailoring sanding to particular wood species for example using finer sanding step with lower density species that are susceptible to grain raising. However, research is needed to fully explore the benefits and costs of developing species-specific sanding schedules to minimize the grain raising of wood finished with water-borne coatings.

Acknowledgments: We thank Natural Resources Canada (Value-to-Wood Program) and the Canadian Foundation for Innovation for their financial support. Philip D. Evans thanks Viance, Tolko, FPInnovations, Faculty of Forestry (UBC) and the Government of British Columbia for their support of the BC Leadership Chair in advanced forest products manufacturing technology at the University of British Columbia. None of the organizations acknowledged here were involved in the design of our experiments, collection, and interpretation of data, or the writing of this paper. Coatings 2017, 7, 135 14 of 15

Author Contributions: Philip D. Evans conceived and designed the experiments; Ian Cullis performed grain raising experiments and profilometry; Philip D. Evans and Roger D. Heady imaged wood surfaces using SEM; Siti Hazneza, Joseph Doh Wook Kim, Lukie H. Leung, and Philip D. Evans conceived and carried out physical modelling of grain raising; Philip D. Evans analysed all data and wrote the first draft of the paper; and all authors discussed and commented on the results and contributed to the final submitted and published manuscript. Conflicts of Interest: The authors declare that they have no affiliations with or involvement with organizations that have financial interests in the subject matter or materials discussed in this paper.

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