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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3949–3955

journa l homepage: www.e lsev ier .com/ locate / jmatprotec

ffect of sanding on surface roughness of rubberwood

. Sulaimana,∗, R. Hashima, K. Subarib, C.K. Lianga

Div. Bio-resource, Paper & Coating Technology, School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, MalaysiaUniKL MSI, Kulim Hi Tech Park, 09000 Kulim, Kedah, Malaysia

r t i c l e i n f o

rticle history:

eceived 22 March 2007

eceived in revised form

September 2008

ccepted 10 September 2008

eywords:

a b s t r a c t

The objective of this research was to study the effect of sanding on surface roughness of

rubberwood. Surface roughness was measured using stylus profilometer. Sessile water drop

technique was used to determine contact angle of the surface. The results indicated that

sanding process could improved surface roughness. Lower grit size gave rougher surface.

The radial surface of rubberwood was found to be rougher compared to tangential surface.

The roughness value across the grain was higher compared to the longitudinal section. The

contact angle on the tangential surface was higher compared to the radial surface and this

indicated that the radial surface was rougher than the tangential surface.

anding

tylus

urface roughness

ubberwood

© 2008 Published by Elsevier B.V.

ishing as mentioned by Koch (1964) and William and Morris

ontact angle

. Introduction

ubberwood (Hevea brasiliansis) is commonly used as a rawaterial for furniture industry in Malaysia. It is estimated

hat about 80% of furniture industries use rubberwood as rawaterials. Quality of rubberwood furniture product depends

n finishing quality of rubberwood. The finishing qualityepends on the quality of surface prior to applying the fin-

shing materials.Surface roughness of rubberwood prior to finishing is very

mportant in determining the quality of the finished prod-ct. Any irregularity on the surface may show through thin

ayer of the finishing materials. Surface roughness is also verymportant in other utilization such as utilization of adhesiven wood.

Surface roughness is further influenced by processing sys-

em such as cutting, peeling and planing. Rough wood surfaceould limit surface contact and result in weak glue or poornishing quality. According to Petri (1987) surface roughness

∗ Corresponding author. Tel.: +60 4 6532260; fax: +60 4 6573678.E-mail address: othman@usm.my (O. Sulaiman).

924-0136/$ – see front matter © 2008 Published by Elsevier B.V.oi:10.1016/j.jmatprotec.2008.09.009

affects adhesion of two surfaces because it increases the totalcontact area between adhesive and substrate. It could alsoprovide interlocking effect that could trap the adhesive in thecavities and act like an anchor with each other. Roughness ofwood surface could be improved to a certain extent by sand-ing.

Taylor et al. (1999) mentioned that surface roughness isinfluenced by its anatomical structure particularly the cellcavities that could cause by its non-homogeneous structure.It was also influenced by cross grain, annual ring width,rays, knots, reaction wood, ratio of earlywood and latewood.Sanding operation could reduce the inhomogeneity of woodsurfaces. Sanding could make the wood surfaces more uni-form and later could absorb the finishing material better. It isthe most common method used prior to applying wood fin-

(1998). According to Richter et al. (1995) sanding operationcould homogenize the surface that could reduce the influenceof the anatomical structure on the roughness profile.

g t e

3950 j o u r n a l o f m a t e r i a l s p r o c e s s i n

The properties and characteristics of sanded wood surfacesdiffer strongly from those obtained by planing. The Scan-ning Electron Microscopy (SEM) examination by Stewart andCrist (2001) revealed that sanding process could also producemechanical damages. The cell wall could have become com-pressed during the sanding. According to de Meijer et al. (1998)the lumens could be filled with fine dust during sanding. Thesanding action could also produce torn out to the cell wall.

The effects of different abrasive minerals on the character-istic of sanded surfaces of wood have been studied by Tayloret al. (1999). Aluminum oxide was regarded as the best abra-sive for wood sanding due to its high durability and polishingability. Juan (1992) showed that silicon carbide can producesurfaces with lower roughness; however, silicon carbide couldproduce deeper and narrower cut compared to those obtainedwith aluminum oxide.

According to Stumbo (1963) surface roughness is an impor-tant factor to understand the interaction of two adjoiningbodies and has special relevance to the study of adhesion.Several methods exist to evaluate the surface roughness ofwood. Amongst the most widely used method to determinethe surface roughness was stylus profiler. Stylus profiler couldmeasure and produce quantitative value for surface rough-ness. According to Ostman (1983) surface roughness can becalculated from actual graph of Stylus profiler obtained fromthe surface. Basic principles of stylus techniques have beendescribed by several researchers such as Hiziroglu (1996).

Besides the measurement of the surface roughness, it couldalso be estimated from the wettability of the surface or howwell the surface absorbs liquid. The wettability is normallymeasured based on the contact angle formed between a liquiddrop and solid surface. Rough surface normally absorbs moreliquid and produces high contact angle of the surface and viceversa. This measurement is to assess the surface energy ofsolid.

Even though rubberwood is a very important raw mate-rial for the making of furniture in Malaysia, the informationon roughness of sanded surface of rubberwood is relativelyscarce. Ratnasingam and Scholz (2006) worked on rubberwoodto optimize the surface roughness using sanding at variousgrit sizes for finishing purposes. In his work, the surface rough-ness was measured based on toluene absorption adopted fromEN 382-1. Surface roughness using stylus profiler was notinvestigated. Stylus profiler could measure a more direct mea-surement of surface roughness.

The objective of this study was to evaluate the effect ofsanding at various grit sizes on surface roughness of rubber-wood and to determine the effect of wettability using contactangle on the surface. In this investigation, stylus profiler wasused to determine the surface roughness. The contact angledetermination was modified using water instead of toluene.Field Emission Scanning Electron Microscope (FESEM) wasused to understand more on anatomical structure of sandedsurface at various sanding process. This is important to deter-mine the effect of sanding size on the structure of the woodsurfaces.

The combination of surface roughness using stylus profileand the wood anatomy using scanning electron microscopyallow more accurate evaluation of surface roughness of rub-berwood.

c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3949–3955

2. Experimental

Rubberwood samples were obtained from local rubberwoodindustries in Kedah, Malaysia. The samples were about 30years of age. There is no clear colour visible difference betweenthe heartwood and sapwood. The samples were air driedto about 8–10% moisture content. The samples were cutinto tangential and radial surfaces at the approximate sizeof 5 cm × 5 cm × 0.5 cm. Rubberwood were surfaced machineusing Holytek Planer with a planning speed of about 6000 rpmand with a feeding speed of 15 m/min. There were 3 knivesin the cutterhead and the cutting depth of the planer wasabout 3 mm. The samples were then sanded using sand paperthat contain aluminum oxide abrasive that was anchored withresin with different types of abrasive grit sizes: 120-, 150- and180-grit. The samples were sanded of about 30 times/strokesalong the grain of the sample surface. The surface roughnesswas scanned and properly measured using a stylus-type pro-filometer with a 90◦ cone diamond tip with a radius of 5 �m.Roughness profile was measured along and across the grainboth on radial and tangential surface. The method was alsodescribed in Hiziroglu (1996). Calibration of the device wasdone for every 100 measurements using a standard referenceplate with average roughness (Ra) values of 3.2 and 0.4 �m. Thedevice uses digital filtering system to separate the surface pro-file into two profiles: waviness and roughness. Two differentranges were used where 50 �m for the computation of Ra and250 �m for mean peak to valley height (Rz) and maximum indi-vidual peak to valley height (Rmax). Scans were repeated on6 identical samples and on each surface type 10 scans wereobtained.

The measurements obtained were Ra, Rz and Rmax. Theimage of the surface was obtained using Leo Supra, Ultra-High Resolution Field Emission Scanning Electron Microscopy(FESEM).

Determination of contact angle was carried out using ses-sile drop technique. One droplet of 10 �l of distill water wasreleased using micropipette syringe on the tangential andradial surfaces at room temperature of about 25 ◦C. The dis-tilled water droplet forms an equilibrium doom on the samplesurface. The water droplet image was saved using a digitalcamera and the contact angle was measured using image ana-lyzer from the shape of the droplet at time series of 0, 5 and10 s.

3. Results and discussion

3.1. Roughness of sanded surface

The average values of surface roughness using abrasive of dif-ferent grit sizes are shown in Tables 1 and 3. The Ra valuealong the grain on tangential surface that was sanded with120-grit size sand paper was 5.2 �m while that with 180-grit

size was 3.8 �m. The Ra is the average distance from the profileto the mean line over the length of the assessment. The resultsshowed a reduction of surface roughness of rubberwood as thegrit size become higher.

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3949–3955 3951

Table 1 – Rubberwood surface roughness along the grain using different grit sizes sand paper.

Sandinggrit size

Ra (�m) Rz (�m) Rmax (�m)

Tangential surface 120 5.2 (0.8)a 31.0 (7.0) 49 (18)150 4.0 (1.3) 27.0 (8.0) 38 (12.0)180 3.8 (0.4) 26.0 (2.0) 30 (4)

Radial surface 120 5.4 (1.1) 32.9 (5.8) 47.4 (13.1)150 5.2 (0.8) 33.7 (10.6) 39 (8.3)

4.2

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ot1st

180

a Numbers in ( ) are standard deviation.

Fig. 1 shows a representative of the measurement maden radial surface of rubberwood sanded with various gritizes. The roughness profile of the sanded radial surface (anxample) was measured either along the grain (X–X) or acrosshe grain (Y–Y). It was composed of vertical deviation of therimary profile from the mean line and it is a numerical evalu-tion using standard roughness parameter. The Rz value alonghe grain radial surface that was sanded with 120-grit sizeanded paper was 5.4 �m while that with 180-grit was 4.2 �m.s expected the roughness of 180-grit size was smoother com-ared to 120-grit size.

It could be seen that the sanded surface roughness alonghe grain of rubberwood samples decreased in the order of gritize 120, 150 and 180. Ra and Rmax showed a significant reduc-ion as the sanding grit became higher (see Table 2). However,he Rz does not differ significantly. Sanding grit of 180-gritize showed lower Ra value along the grain and across therain. Surface roughness of sanded surface across the grainll showed a significant reduction of Ra, Rmax as well as Rz

Tables 3 and 4). This is expected, as the grit size is higher,he size of the aluminum oxide on the sand paper is alsomaller. The size of aluminum oxide will be reflected on themoothness of the surface. It is obviously clear that applica-ion of higher grit of sanding reduced the surface roughnessf rubberwood.

The FESEM micrographs of the sanded surface with vari-us grit sizes on radial surface (an example) along and across

he grain are also shown in Fig. 1. The sanded surface with80-grit sizes was seen to be much smoother compared toanded surface with 120-grit sizes. The sanding action seemso be able to fill up the available pore in the cell. Fig. 2 shows

Table 2 – Analysis of Variant (ANOVA) of rubberwood surface ro

Sum ofsquare

Ra Between group 25.7Within group 39.2Total 64.8

Rz Between group 545.4Within group 2438.3Total 2983.7

Rmax Between group 3011.7Within group 6629.5Total 9641.2

(0.4) 26.9 (3.2) 32 (4.7)

some of the anatomical structure of rubberwood that con-tain cell with variable sizes. Ray parenchyma cell could beseen to have 2–3 cells wide incorporated with three typesof cells namely; procumbent, upright and square cells. Thelongitudinal parenchyma cell contain substantial amountof starch granules (see Fig. 2a, arrow). The fibre lumen isquite large. These cell structures and arrangement of thecell will determine the roughness of the surface. In order toimprove/smoothen the surface, sanding process is essential.The influence of anatomical structure on roughness has alsobeen mentioned by Gurau et al. (2005).

Rubberwood contain visible growth ring. According toOhashi et al. (2001) growth ring formation showed a periodof dominancy between January and March or May. The growthrings showed differences in cell wall thickness between ear-lywood and latewood. This results in different size of lumenespecially for the vessel. The lumen sizes influenced the sur-face roughness and also influenced the contact angle. It isexpected that the surface of rubberwood that was not sandedshowed a lower surface roughness and lower contact anglethan those that was sanded.

Sanding process on either radial or tangential surfacesmoothen the surface cells. The sanded surface breaks up andform small particles (dust) that will fill up the pore area on thesurface. The resultant of this can be seen in Fig. 1. Cell struc-ture could no longer be seen as the debris and dust have filledup the available pore in the cell.

The average value of Ra along the grain was lower comparedto the value across the grain indicating smoother surface alongthe grain compared to that across the grain. The assessmentalong the grain was measured along the longitudinal axis of

ughness along the grain.

df Meansquare

F Sig.

5 5.1 7.1 0.00054 0.759

5 109.1 2.4 0.04854 45.259

5 602.3 4.9 0.00154 122.859

3952 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3949–3955

ndedit; (c

Fig. 1 – Surface structure of rubberwood on radial surface saalong (X–X) and across (Y–Y) the grain: (a) 120-grit; (b) 150-gr

the cell. These include vessels, fibres, parenchymas and raycells. Measurements across the grain were measured in per-pendicular direction to the longitudinal axis. These produceda higher Ra as the needle of the stylus needs to measure across

with various grit sizes and surface roughness profile) 180-grit.

from one cell to the adjacent cell during the measurements.Tangential surface measurements seem lower compared

to radial surface measurements. This could be seen for bothalong the grain and across the grain. The Y–Y in Fig. 1 is the

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3949–3955 3953

Table 3 – Rubberwood surface roughness across the grain using different grit sizes sand paper.

Sanding grit size Ra (�m) Rz (�m) Rmax (�m)

Tangential surface 120 7.8 (1.1)a 76.7 (8.5) 112.9 (11.8)150 7.0 (0.7) 62.8 (12.4) 84.0 (17.8)180 6.3 (0.7) 51.3 (8.7) 75.4 (13.0)

Radial surface 120 9.2 (1.7) 80.7 (11.4) 124.7 (7.5)150 7.3 (0.9) 68.1 (10.7) 86.8 (13.7)180 6.4 (1.2) 52.0 (7.3) 76.4 (12.2)

a Numbers in ( ) are standard deviation.

Table 4 – Analysis of Variant (ANOVA) of rubberwood surface roughness across the grain.

Sum of square df Mean square F Sig.

Ra Between group 56.9 5 11.4 9.5 0.000Within group 64.6 54 1.2Total 121.6 59

Rz Between group 7475.7 5 1495.1 13.9 0.000Within group 5768.5 54 106.8Total 13244.2 59

mssspcra

Rmax Between group 21048.5Within group 9173.4Total 30221.9

easurement obtained across the grain while X–X is mea-urement obtained along the grain. The results in this casehowed that the tangential surface created much smootherurface compared to the radial surface. The radial surface

erhaps produced bigger hindrance during the measurementompared to the tangential surface and thus showed that theadial surface was much rougher than the tangential surfaces shown in Tables 1 and 3.

Table 5 – Water contact angle on rubberwood surface using diff

Surface Grit size

Tangential 120150180

Radial 120150180

a Numbers in ( ) are standard deviation.

Table 6 – Analysis of Variant (ANOVA) of water contact angle on

Sum of square

Ra Between group 2488.3Within group 178.0Total 2666.3

Rz Between group 3013.3Within group 138.0Total 3151.3

Rmax Between group 508.3Within group 66.0Total 574.3

5 4209.7 24.8 0.00054 169.959

The Rz value was calculated based on the value from thepeak to the valley within the profile. The Rz value along thegrain on the tangential surface using sanding grit of 120-grit was 31.0 �m (Table 1). The Rz value across the grain was

76.7 �m (Table 3). The samples from radial surface showed thesame trend of having higher Rz at across the grain.

The Rmax is the distance between peak and valley point ofthe profile which can be used as an indicator of the maximum

erent grit sizes.

Time (s)

0 5 10

36 (2.0)a 25 (1.6) 9 (9.0)45 (2.3) 27 (1.8) 13 (1.1)49 (1.9) 39 (1.5) 15 (1.3)

30 (1.2) 18 (0.8) 6 (0.8)38 (1.1) 20 (0.8) 10 (0.9)42 (2.1) 24 (2.5) 12 (1.5)

rubberwood.

df Mean square F Sig.

5 497.7 150.9 0.00054 3.359

5 602.7 235.8 0.00054 2.659

5 101.7 83.2 0.00054 1.259

3954 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3949–3955

rrang

r

Fig. 2 – Section of rubberwood showing structure and a

defect height within the assessed profile. The Rmax values forall samples measured across the grain were higher than thosemeasured along the grain. The value of Rmax on the tangentialsurface was higher compared to the radial surface.

3.2. Contact angle of sanded surface

Contact angle is the angle at the three-point contact betweensolid–liquid–gas interface and this give a degree of wettabil-ity of solid by liquid. A smaller contact angle means that thesurface is more wettable and more hydrophilic the surface is.Contact angle is directly influenced by the comparable sur-face roughness and also indicates an average wettability ofthe surface.

Measurement of contact angle can be done using sessiledrop technique or by toluene. In this study sessile drop tech-nique was used rather than toluene based on Fang et al. (2005)who indicated that sessile drop technique using water wasmore efficient in term of time and also accurate and insensi-tive to measurement condition.

Table 5 shows the contact angles obtained by sessile dropmethod on radial and tangential surface using various sandinggrit. The surface with lower grit sanding size showed a lowermean contact angle. Samples sanded in the radial directionwith 120-grit size having 6◦ contact angles after 10 s. Surfacethat was sanded with 180-grit size showed a contact angleof 12◦. Wood surface that was sanded with 120-grit size wasmore certainly wettable compared to those that were sandedwith 180-grit size. The contact angle of water decreased sig-nificantly over time on surface of different grit size (Table 6).

It was observed that radial surface has lower contact angle.It also meant that the radial surface is more wettable com-pared to tangential surface. This is in line with the results onsurface roughness showing rougher value on the radial sur-face compared to tangential surface. Palasantzas and Hosson(2001) indicated that as the surface roughness increased thecontact angle on that surface will also be increased.

With increasing surface roughness the absorption rate ofwater droplet decreased. This will increase the tendency ofwater to be captured by capillary forces emerging from greatersurface area exposed on the rough wood surface. Rough sur-

face can cause excessive resin usage for surface adhesion infurniture making. Roughness of rubberwood surface can beimproved significantly by sanding which is in agreement withde Moura and Hernández (2006).

ement of cells: (a) radial surface; (b) tangential surface.

4. Conclusions

1. Sanding improved the surface roughness. It was found thatlower sanding grit size gave rougher surface.

2. Rubberwood surface that was sanded with lower grit sizewas more wettable compared to those sanded with highergrit size. This means that rougher surface is more wettablecompared to smoother surface.

3. Radial surface of rubberwood was rougher than tangentialsurface.

4. Contact angle on tangential surface rubberwood washigher compared to radial surface.

5. Roughness was higher when measured along the graincompared to across the grain.

Acknowledgements

The author would like to acknowledge Prof. Salim Hiziroglufrom Oklahoma State University, USA for initial discussion ofthe project. Thanks are also due to all technical staffs in USMfor supporting to the success of the project.

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