Enhancing the mechanical properties of an epoxy coatingwith rice husk ash, a green product

Mahboobeh Azadi, Mohammad Ebrahim Bahrololoom,

Fatemeh Heidari

� ACA and OCCA 2010

Abstract In this research, rice husk ash (RHA) —acarbon neutral waste product that is an abundantsource of silica—was chosen as an innovative filler forepoxy paint. The influence of the RHA on differentmechanical properties of the cured coatings (wear,hardness, and elongation) was investigated. Wear testsin pin-on-disc mode were carried out and followed byscanning electron microscopic observations to deter-mine the wear mechanism for specimens with differentvalues of carbon and silica according to their differentcolors as fillers for epoxy paint. Pencil hardness testsand bending tests were performed according to ASTMD3363 and ASTM D522 standards, respectively. Add-ing RHA (heat treated at 700�C) at two levels—10 and20% weight fraction—to the unfilled epoxy paintincreased the wear resistance, scratch resistance, andelongation of the filled coating compared to theunfilled epoxy coating.

Keywords Epoxy paint, Rice husk ash, Wear,Pencil hardness test, Bending test, Green product

Introduction

Because steel is one of the most widely used industrialmaterials, its protection is important. In recent years,there has been much research on steel corrosioncontrol techniques.1 Although there are many waysto control steel corrosion, the best ways are the

cheapest and the safest ones.2 Using epoxy paint isone of the economical ways to protect steel againstdifferent corrosive environments.3 Epoxy resins arebeing extensively employed worldwide because of highstrength, good adhesion to different substrates, andgood resistance against industrial corrosive environ-ments.4,5 However, highly crosslinked epoxy resins arevery brittle and hydrophilic, which limits their appli-cations. Preventing the brittleness of epoxy paints isusually performed by blending them with elastomers orthermoplastics.6 There have been many studies in thelast two decades that have tried to modify epoxy paintproperties by adding different inorganic particles likesilica.7–9 Thus, it would be useful to find inexpensivesources of such materials. Rice husk ash (RHA), awaste product from rice milling, is a cheap source ofsilica. In the majority of rice-producing countries,much of the husk produced from the processing of riceis either burnt or dumped as waste.10–12 At themoment, the common fate of rice husk is uncontrolledcombustion, which creates significant environmentalproblems.13

Ash contains 92–95% silica, is highly porous andlightweight with a very high external surface area,10–12

and also contains a small proportion of impurities suchas K2O, Na2O, and Fe2O3. The cellular nature of therice husk provides the cellular porous structure of theRHA texture.14,15

Wear behavior, as one of paint’s mechanical prop-erties, is important in some applications. However,understanding of the wear behavior of inorganic-filledepoxy is still very limited.16,17 The wear resistance of acoating might be decreased or increased depending onthe type of particles, particle size and size distribution,interfacial actions between particle and matrix resin,particle content, and the state of dispersion of theparticles in the matrix.18 Because the scratch resistanceof coatings has an important role in coating lifetime inmany applications, numerous scratch test devices have

M. Azadi, M. E. Bahrololoom, F. HeidariDepartment of Material Science and Engineering, ShirazUniversity, Shiraz, Iran

M. Azadi (&)Department of Engineering Materials, Tarbiat ModaresUniversity, Tehran, Irane-mail: mahbub.azadi@gmail.com

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DOI 10.1007/s11998-010-9284-z

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been built commercially or custom built by researchersto study the scratch responses of polymers at variousscales.19

In this paper, the effects of adding differentcompositions of RHA as cheap filler, from a biologicalsource, on the mechanical behavior of epoxy paints attwo levels of filler content are investigated. Two typesof RHA with different colors were used as modelfillers. Wear tests using pin-on-disc mode were carriedout and the wear mechanisms were studied by ascanning electron microscope (SEM). Pencil hardnesstests and bending tests were performed.

Experimental details

Materials

Two types of RHA with different colors (white andblack) and properties were used in this study. The ricehusk was washed initially to separate any dirt, whichwas followed by drying completely in free air. Thesamples were then heated at 700�C for 3 h to removeany organic residual in the RHA. The ash was thenground to reach an acceptable ash size. The epoxypaint that was used in this research was a white alkydresin base with two components (hardener and resin).Because the burning process of the rice husk wasextremely dependent upon the amount of availableoxygen and the size of the crucible, the temperatureand the time of heat treatment were fixed and theamount of rice husk was changed to get different asheswith different compositions. This procedure was dif-ferent from other researchers who changed the burntemperature to get different types of ash. A stainlesssteel crucible with an internal surface area of 40 cm2

was used for burning the rice husk. Fifty grams of ricehusk was used to get black RHA, while 12.5 g of ricehusk was used to get white RHA at 700�C for 3 h. Theash color was dependent on its composition. It wasclear that white RHA meant it contained more silica

and less carbon and that more carbon and less silicawere present in a darker ash. The difference betweenRHAs with different colors (white and black) wasrelated to composition elements such as silica andcarbon. The diameter of the particles was <50 lm(Fig. 1). By comparing the SEM micrographs in Fig. 1,the average grain size of white ash is seen to be smallerthan black ash because it had more small silica content.The silica in RHA had a porous and layered structure,so they changed into small particles during milling. Thevalues of ash particle sizes are given in Table 1. As thepictures in Fig. 1 show, the surface of black RHA wassoft and round while the surface of white RHA wasrough and had acicular shape.

Preparation of painted specimens

RHA was dispersed into unfilled epoxy paint in twogiven weight percentages (10 and 20 wt%) with amechanical mixer, and then was applied on steelsubstrates in one layer (<0.5 mm). For better adhesion,the surface of the steel substrate was polished andwashed with acetone before applying the paint to it.The complete drying time of this epoxy paint was 48 hin free air.

Pin-on-disc wear test

Wear tests were carried out by using a pin-on-disc weartester. The pin was fixed in a holder on a loading leverarm. A normal load of 2 N was applied to all specimensduring the test. The rotation speed of the disc was

Fig. 1: The size and shape of rice husk ash under SEM with a resolution of 1000x: (a) Black RHA, (b) White RHA

Table 1: Average particle size

Particle Average size (lm)

White RHA 25Black RHA 35

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120 rpm and the diameter of the wear track was11 mm. The sliding velocity between the pin and therotating discs was 0.07 m s�1. The sliding distance wascalculated to be 120 m within the wear test duration of1800 s. The wear tests were performed at 23�C. Thefriction coefficient diagrams were also obtained for thetotal distance of the pin that passed on each specimenduring the wear test. This distance was fixed so allspecimens could have a better discussion of the resultsas a constant parameter.

Pencil hardness test

Pencil hardness tests were performed, according to theASTM D3363 standard, using pencil leads (with mark:creta color fine art graphite) with hardness valuesranging from 6H (very hard) to 6B (very soft). Amanual holder was used to fix the angle of pencils forall specimens to reduce the errors.

Bending test

Bending tests were performed according to the ASTMD522 standard, ‘‘Mandrel Bend Test, Method B, andCylindrical Mandrel.’’ Three mandrel diameters (1, 3/4,½ inch) were chosen for these tests.

Results

Weight loss in wear

Comparison of the weight losses for different speci-mens is shown in Fig. 2. In general, adding RHA to thepure epoxy paint reduced the weight loss in the weartest. Filler content of 20 wt% seemed to be moreeffective in reducing weight loss for the white RHA.However, adding more black filler to the paintincreased the weight loss in wear tests due to thepresence of more carbon with large particles. Inaddition, white RHA fillers in 20 wt% content seemed

to be the most effective filler in lowering the weightloss in the wear tests with 2 N applied normal load,because it had more silica content than other fillers.Small hard silica particles created small holes in thewear track during wear tests so the reduction in theweight loss was small.

Friction coefficient

Friction coefficient (l) values for the specimens wereplotted against the total distance that the pin passed onthe specimens during the wear test (Fig. 3). AddingRHA to paint caused a reduction in the frictioncoefficient. This was true for both levels of fillercontents (10 and 20 wt%) for all specimens. The blackRHA filler seemed to be more effective in lowering thefriction coefficient under 2 N applied normal load thanthe white fillers. Soft particles such as carbon couldcause less reduction in wear coefficient because theywere compressed together, and the pin was slipped onthe compressed layer of carbon. Because the sampleswith black RHA had more carbon-containing com-pounds, they showed minimum reduction in frictioncoefficient with respect to the pure epoxy paint. Incontrast, the white RHA had more silica (hard parti-cles) and it had a higher friction coefficient with respectto the black RHA. Finally, as Fig. 3 shows, thepresence of filler in pure epoxy paint could give moresmoothness to the friction coefficient diagram.

Morphology of the worn surfaces

The surface topologies for different specimens werecompared by using a SEM at magnifications of 40x(Fig. 4). The SEM photographs in Fig. 4 indicate thatadding RHA to the paint could cause some changes inthe wear mechanism of the coatings. As a result, thewidth of wear cracks was reduced but the number ofholes increased with respect to the pure epoxy paint.

Pencil hardness test

According to the ASTM D3363, two hardness numbers(scratch hardness, pencil hardness) could be reportedfor each specimen in the pencil hardness test. Thedifference between these hardness values is defined inthat method. The scratch resistance of a coating refersto the hardest pencil that will not gouge the film. Thepencil test was performed on each of the formulationsto compare their relative scratch resistance values.20

The data compiled in Table 2 reveals that the additionof RHA enhances the hardness (scratch and pencilhardness) resistance of the epoxy paint. In general, thescratch resistance of filled epoxy paint was higher thanthe scratch resistance of unfilled epoxy paint. It isevident from these results that increased silica contentsin the filler increased the scratch resistance. Therefore,

0.04

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0pure 10% black 20% black 10% white 20% white

Composition

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ght l

oss(

%)

Fig. 2: Wear results in a plot mode

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the high scratch resistance properties of white filler canbe attributed to the existence of silica in its content.These results were true about pencil hardness, too.

Bending test

According to the ASTM D522 standard, the data canbe reported either quantitatively or qualitatively. Bothof these results are reported in Table 2. In general,adding this type of filler increased paint elongation.Before doing this test, the expected result was that thespecimen that had black filler would show the maxi-mum elongation among other specimens because it hadsoft particles containing carbon. But the result inTable 2 was completely different. The specimens thathad white filler showed maximum elongation. Becausethe rice husks have two layers,21 it was possible thatapproximately all carbon-containing materials in theexternal layer of the white ash was disassociated, andthe bonding of carbon with silica had broken. How-ever, because the silica had a porous structure, a verysmall amount of free carbon (in nanometers) in theinternal layer with weak bonding was trapped in themiddle of the hard silica structure. Therefore, the whiteparticle had hard silica in the external layer and a littlesoft carbon in the internal layer. This was thought to be

the reason for these results. Consequently, the whitefiller was most beneficial filler in increasing theelongation of the filled paint among other fillers.

Discussion

The wear mechanisms of epoxy and particulate-filledepoxy composites under the pin-on-disc condition werepreviously discussed by Durand et al.18 The results ofthe present study were in agreement with the resultsreported by Durand et al.18 The SEM photographsshown in Fig. 4a indicate that the wear tracks in pureepoxy paint were wide but not deep. The wearmechanism seems to be the segregation of materiallayers with no excess force, and it was evident that theplasticity of pure epoxy paint was low and pure epoxypaint was brittle during wear tests. This type ofabrasive wear can be cutting (Fig. 5a). By adding fillerto the paint, the wear mechanism changed. Many holesappeared in the wear track and the width of wear trackreduced during the wear test. Layers of the materialwere compressed together (Fig. 4b–e) so the weightloss of specimens was reduced. This type of abrasivewear can be gouging (Fig. 5b). Thus, the plasticity ofthe epoxy composite was increased. By adding more

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Fig. 3: Friction coefficient verses sliding distance for: (a) pure epoxy; (b) epoxy-10% black filler; (c) epoxy-20% black filler;(d) epoxy-10% white filler; and (e) epoxy-20% white filler

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filler to the paint, there was more compression of thematerial in the wear track. The size of holes in the weartracks of specimens that contained white RHA wassmaller than the holes in specimens that containedblack RHA (Fig. 4b–e). By comparing the two picturesin Figs. 4b and 4c, it was concluded that when blackRHA in 20 wt% content was added to the paint,deeper holes appeared in wear tracks, and with this

mechanism the reduction of weight loss during weartests could be explained.

Conclusions

The presence of RHA in epoxy paints can enhancewear resistance, scratch resistance, and elongation. It

Fig. 4: Wear picture of specimens under SEM for: (a) pure epoxy in resolution of 40x; (b) epoxy-10% black filler inresolution of 40x; (c) epoxy-20% black filler in resolution of 40x; (d) epoxy-10% white filler in resolution of 40x; and(e) epoxy-20% white filler in resolution of 40x

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seems that this type of filler in epoxy paints increasespaint plasticity. The addition of white ash is better inimproving the wear resistance due to the presence ofmore silica. Adding 20 wt% black ash to the pureepoxy paint lowers its friction coefficient with respectto the white RHA. Finally, using this type of filler,which is cheap and abundant in nature, can modifysome mechanical properties of epoxy paints and alsoreduce air pollution from burning rice husks. Thus, a‘‘green product’’ can be produced in the paint industry.

Acknowledgment This research was financed by agrant (87-GR-ENG-62) given to Professor M.E.Bahrololoom by the Research Committee of ShirazUniversity, which is gratefully appreciated.

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Table 2: Pencil hardness test and bending test results

Composition Diameter(1 inch)

Diameter(3/4 inch)

Diameter(½ inch)

Elongation(%)

Scratchhardnessa

Pencilhardnessb

Pure epoxy Failed Failed Failed >8.55 2H BEpoxy-10% white filler Passed Passed Passed 21.95 6H 2HEpoxy-20% white filler Passed Passed Passed 21.95 >6H 3HEpoxy-10% black filler Passed Failed Failed 11.7 4H HEpoxy-20% black filler Passed Failed Failed 11.7 3H B

a Scratch hardness: a pencil that will not scratch the filmb Pencil hardness: a pencil that will not cut into or gouge the film

Direction of abrasion(a) (b) Direction of abrasion

Cutting Fatigue by repeated ploughing

Repeated deformations by subsequent grits

Fig. 5: Schematic picture for different types of abrasive wear: (a) for pure epoxy and (b) for epoxy with filler

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