Effect of Epoxy Resin Coating on Electric Resistance and Environment-Proofing Character of Woodceramics

*T.AZUMA, *M.OTSUKA,*K.HATA *J.TSUJI **T.OKABE, ***M.TAKAYAMA*Shibaura Institute of Technology, Tokyo, Japan

**Industrial Research Institute of Aomori Prefecture, Hirosaki, Japan***Three Bond Co., Ltd, Tokyo, Japan

AbstractIn order to evaluate the reliability of woodceramics

when applied to snow heater, the effect of epoxy resincoating on the electric resistance and environment-proofing characteristics has experimentally beeninvestigated by use of heat-cycle test and hightemperature-exposing test. It is found that in order toobtain a reliable coating on woodceramics, we must useresin having both small coefficient of thermal expansionand high glass transition temperature, together withsufficient time for curing.

1. INTRODUCTION

I has strongly been needed to develop environmentallyfriendly materials. A typical examle is woodceramicswhich are porous carbon material developed by Okabe[1]early in the 1990s. Woodceramics are made byimpregnating wood or woody with phenolic resin andcarbonized it at temperature ranging from 573 to 3073K.Woodceramics have the following superior characteristics.(1) used papers or woods as well as thinning tree (smalllog) can be reused as raw materials,(2) no environmental pollution is caused by waste,(3) gases whish eject during carbonizing at high tempera-ture can be collected to produce wood vinegar liquid,(4) recycled wood or woody materials can be used as rawmaterials,(5) apparent density is very low (0.6-1.1g/cm3) because ofporous structure, and

(6) manufacturing cost is not expensive.Woodceramics are superior in heat resistance abrasion

resistance and corrosion resistance and offer tremendouspotential as functional materials such as electromagneticwave shielding materials, friction materials, heatinsulating materials and structural materials. Especially,woodceramics, which are carbonized at temperaturebetween 873 and 923K, are able to generate ohmic-resistance heating under electric current, suggesting thatthey may be applied, for example, to snow-fusing heater incold region. However, due to their highly porous structure,woodceramics will easily absorb the water. Once waterinfiltrates into the pore of woodceramics, the electricresistance will change in time. As a result, the efficiency ofheat generation will decrease.

Thus we have tried to inhibit woodceramics fromwater or moisture by applying some chemically stablefilms to their surface. The purpose of this paper is toexamine the effect of epoxy resin coating on the electricresistance and the environment-proofing characteristics ofwoodceramics.

2. EXPERIMENTAL METHODS

2.1 SpecimensMain manufacturing process of woodceramics is

shown in Fig.1. Medium-density fiberboard (hereafter,denoted as MDF) which were fabricated from needle-leaved tree, pinus radiata, and then sufficiently dried, wasimpregnated with phenol resin (type PX-1600, products ofHonen Corporation, Tokyo), cured at 503K, and

carbonized at 923K, all in vacuo.

woodceramicsMDF ofradiata pine

impregnation ofphenol resinand curing

cabonizationat 923K

Fig. 1 Production process of woodceramics.

We prepared in accordance 2 patterns of treatment:coating epoxy resin and no additional treatment withepoxy resin to the woodceramics. Epoxy resin ofbisphenol-A-diglycidylether (type 2022, Three-Bond Co.Ltd.) were mixed with polyamine based curing reagent(type 2103, Three-Bond Co. Ltd.) and then coatedmanually with painting brush. They were cured at 373Kfor 1 hr. The size of test piece is 12mm x 12mm x 120mm.

In order to maintain a reliable contact of specimenwith copper lead for the measurement of electric resistance,a pair of thin holes of 10mm depth and 1.5mm diameterwere drilled on either side of the specimen beforeimpregnating phenol resin. After carbonization, copperwires were inserted into the hole and then contacted towoodceramics with silver paste.

2.2 Heat cycle testIn the case where they are applied to the snow-fusing

heater, epoxy resin coated woodceramics may inevitably

be subjected to heat cycles. This should results in thethermal fatigue failure because of large difference in the

coefficient of thermal expansion between woodceramics

and epoxy. So the effect of heat cycles on the properties of

woodceramics was examined under the heating program of

Fig.2, in which temperature changed between 273K and

373K at a ramp rate of 1.78K/min. Heating and cooling

was followed by the measurement of electric resistance

using digital-multimeter (type TR6845, Advantest Ltd.,Tokyo).

Tem

pera

ture

Time

30min

56min 56min

373K

273K

Fig. 2 Nominal temperature-time profile during thermal cycling.

2.3 High temperature exposure testIn order to clarify the reliability of coated layer, epoxy

coated woodceramics were maintained in air at 333K,373K or 413K for 3, 6, 12 and 24hrs. Specimens were theninspected by both visual and microscopic ways.

2.4 Hygroscopy test After the exposure to high temperature, the resin

coated specimens were soaked in ion-exchanged water atroom temperature for 24hrs. The amount of absorbedwater was evaluated by a parameter called waterabsorption which is defined as percent weight changebefore and after the soaking. The morphology of surfaceand cross section was microscopically observed.

3. RESULTS AND DISCUSSION

3.1 Influence of heat-cycle testFigure 3 shows the effect of heat cycles on the electric

resistivity of woodceramics with and without epoxycoating. The resistivity before heat cycling is about 1200m in both materials. It increases with increase in thenumber of heat cycles, though increasing rate is higher inepoxy coated materials than in non-coated ones.

0 50 100110

0

10

20

30

40

50

-10

Number of thermalcycles / N

Rat

e of I

ncr

ease

in

(=

1200

m

) / %

non treatment

coated with epoxy resin

Fig. 3 The rates of increase of electric resistance of woodceramics, after heat-cycle test.

No structural change was found in the epoxy-coatedspecimen before and after the heat cycling. That is coatedwith epoxy resin. However, cracks were observed justbeneath the coated zone once the specimen wasmechanically cut with high speed dicing machine and thendipped in water (Fig.4).

0.5mm

resinpenetration

layer

WCS

(a)

0.5mmcrack

resinpenetrarion

layer

WCS

(b)Fig. 4 Microcracks observed near the surface of resin

coated specimen which was subjected to heat cycletest. (a)50cycles, (b)100cycles.

As is evident in Fig.4, cracks propagate along theinterface between the resin-penetration stratum and thebulk of woodceramics, and occasionally bending towardsouter surface. Cracks after 100 heat cycles are longer thanthat after 50 cycles. Because no cracks were found in asresin coated woodceramics, above-mentioned cracks areconsidered to initiate before absorbing water. The increasein electric resistivity during heat cycling should, therefore,be attributed to the reduction in effective cross sectionalarea of specimen due to the generation of cracks. Thedriving force for crack initiation around the interfacebetween resin impregnated layer and bulk woodceramicsis thought to be strain energy caused by a large differencein coefficient of thermal expansion, i.e., 4.0 x 10-6 K-1 forwoodceramics and 4.6 x 10-5 K-1 for epoxy resin cured withpolyamine.

Cracks were observed even though the coefficient ofthermal expansion of epoxy resin is low in comparison toother common resins, suggesting that either thermally lessexpansive resin or another type of curing reagents isrequired for obtaining sound coating.

Cracking was observed even in non-coatedwoodceramics, when dipped in water after 100 heat cycles(Fig.5). The result shows that the bulk of woodceramics isdamaged during the iteration of thermal expansion andcontraction.

1mm

crack

Fig. 5 Cracks observed in non-coated woodveramics after 100 heat cycles.

3.2 High temperature exposure testFigure 6 shows the water absorption of resin coated

woodceramics as a function of duration and temperature ofexposure. We can see that woodceramics become liable to

absorb water when held above 373K for 12hr or longer,though the dependence is weaker at 333K.

Cracks appear almost without exceptions just afterpre-coated specimen is dipped into water (Fig.7). Anotable increase in water absorption described aboveshould be correlated with the infiltration of water intothese cracks.

333K

373K

413K

Duration, t / hr

Wei

ght i

ncre

ase

due t

ow

ater

abs

orpt

ion

(%)

Fig. 6 Water absorption of resin coated woodceramicsas a function of duration and temperature of exposure.

P

Fig. 7 Cracks observed in resin coated woodveramics after exposed at 373K for 12hr.

In agreement with a general trend that epoxy resinbecomes stronger when cured above its glass transitiontemperature (Tg) for 1hr, the epoxy resin employed here( Tg =363K) showed an increase in strength after curing at373K. While it is cooled after exposed to high temperature,the resin shrinks greatly, resulting in a large residual stressat the interface between resin and woodceramics whichinduces crack under the wet condition. Since thecoefficient of thermal expansion of the epoxy resin steeply

increases above Tg, high Tg resin should be recommendedfor high performance coating.

The specimens held at 373K and 413K for 3 to 6hrshowed comparatively large water absorption incomparison with that of 333K. This could be ascribed thepolyamine compound used as curing reagent. Polyamine isa hydrophilic compound commonly used for curing epoxy.In other words, polyamine compound dissolves in thewater well. So, if a small amount of curing reagentremains not reacted, it will easily combine with water,leading to a remarkable water absorption in pre-coatedwoodceramics. In the specimens exposed to 373K and413K for 3 to 6hr, curing seems to proceed perfectly.Consequently, a sufficient curing is needed in oreder toavoid water absorption. Another way is to utilize anyhydrophobic reagent, though curing reaction is not so fastas in the case of hydrophilic reagent such as polyamine.

4. CONCLUSIONS

In order to obtain a reliable coating on woodceramics,we must choose such resin as having both smallcoefficient of thermal expansion and high glass transitiontemperature, together with sufficient time for curing.

ACKNOWLEDGMENTS

Thanks are due to Mr. M.Sugioka for their kind help inthe experiment.

REFERENCES

[1] T.Okabe (Ed), New porous carbon material, Woodceramics,Uchidarokakuho, (1996)[2] K.Hata (Ed), Laser beam machining of porousWoodceramics, Journal of Porous Materials 5,65-75, (1998)