This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution

and sharing with colleagues.

Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party

websites are prohibited.

In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information

regarding Elsevier’s archiving and manuscript policies areencouraged to visit:

http://www.elsevier.com/authorsrights

Author's personal copy

Influence of activated charcoal as filler on the propertiesof wood composites

Anuj Kumar a, Arun Gupta a,n, K.V. Sharma b, Mohammed Nasir a, Tanveer Ahamed Khan a

a Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Lebhuraya Tun Razak, 26300 Gambang, Kuantan, Pahang, Malaysiab Department of Mechanical Engineering, JNTUH College of Engineering, Manthani, Karimnagar 505212, Andhra Pradesh, India

a r t i c l e i n f o

Article history:Accepted 13 May 2013Available online 5 June 2013

Keywords:Crosslink densityActivation energyActivated charcoalUrea–formaldehyde resinFormaldehyde emissionMedium density fiberboard

a b s t r a c t

In the present work a small percentage of activated charcoal was added in the thermosetting urea–formaldehyde (UF) resin and the performance of adhesives and wood composite was observed. The effectof activated charcoal on the curing kinetics and the Crosslink density of UF resin was investigated, usingdifferential scanning calorimetry. The activated charcoal has an accelerating effect on the curing of theUF resin. The Crosslink density of resin increases and the activation energy decreases. The influence ofthe activated charcoal addition was particularly noted in medium density fiberboard by the increase inthe value of modulus of rupture and internal bond strength of the panel, a direct indication of theperformance improvement with the addition of a small amount of activated charcoal. The formaldehydeemission significantly decreases with the addition of activated charcoal.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The wood-based panels largely represented by particleboard,medium density fiberboards (MDF), plywood and oriented strandboards (OSB). These panels are the major constituent for theproduction of wood furniture and other interior house construc-tions (i.e. flooring, wall paneling etc.). The global wood-basedpanel market valued over US$ 80 billion in 2011 [1]. Wood-basedpanel are typically made with a heat-curing adhesive that holdsthe wood fibers or wood-particles components together.

UF resin is one of the largely used adhesive for interior-gradewood-based panels specially in particleboard (PB), a mediumdensity fiberboard (MDF) manufacturing. It has some advantageslike fast curing, less in price and good mechanical strength of thepanels but has many drawbacks like lower water resistance, higheremission of formaldehyde. The weakness of UF resins to hydrolysisand the presence of free non-reacting formaldehyde in the panelstogether responsible for the problem of formaldehyde emissionfrom the panels during manufacturing and service life.

The molar ratio of formaldehyde to urea (F/U) is the mostessential factor for affecting the formaldehyde emission liberationfrom the panels. The formaldehyde emission can be lowered byusing the combination of UF resin and formaldehyde free adhesives[2–4]. Several methods for manufacturing the low formaldehydeemission panels have been studied, such as reducing formaldehyde

to urea molar ratios [5] and addition of formaldehyde scavengers tothe resin [6]. However, the other physical and mechanical proper-ties of wood-based panels are badly affected [7–10]. The chemicaladditives like formaldehyde scavengers are being used to reduce theformaldehyde emission from panels. The amine (primary or sec-ondary) containing compounds such as urea, ammonia, melamineare the commonly used scavengers [11]. However, the addition offormaldehyde scavengers consumes the free formaldehyde inexcessive available for the cure reaction, accordingly weakeningthe internal bonding [12].

The use of activated carbon as formaldehyde absorbent hasbeen investigated by many researchers. Rayon-based activatedcarbon as formaldehyde absorbent [13] and activated charcoalis used as bio-scavenger for decreasing the formaldehyde emissionfrom melamine formaldehyde resin [14]. The activated charcoalabsorbs the free formaldehyde from MDF panel [15]. The ligno-cellulosic materials are often used as fillers for formalde-hyde based resin like cellulose in phenol formaldehyde resin,and it increases the adhesion strength due to secondary forces,such as van der walls, H-bonding and electrostatic forces [16].Tannin powder used as filler in UF resin and effects on theactivation energies [17] and tannin from Pine wood as a filler forPF resin [18].

In the present work the effects of activated charcoal as filler onpeak curing temperature, Crosslink density, and the activationenergy of UF resin are discussed with the help of differentialscanning calorimetry. The effect of activated charcoal on theformaldehyde emission, physical and mechanical properties ofMDF was investigated.

Contents lists available at SciVerse ScienceDirect

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

International Journal of Adhesion & Adhesives

0143-7496/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.ijadhadh.2013.05.017

n Corresponding author. Tel.: +6 095492867; fax: +6 095492399.E-mail address: arun@ump.edu.my (A. Gupta).

International Journal of Adhesion & Adhesives 46 (2013) 34–39

Author's personal copy

2. Materials and methods

2.1. Materials

Urea–formaldehyde resin was provided by Dynea Malaysia Sdn.Bhd. The resin has the solid content 64.4%, the F/U molar ratio was1, pH was 8.27, and viscosity 170 CP. Mixed tropical hardwoodfibers were provided by the Robin Resources Sdn. Bhd, Malaysia.The activated charcoal granules were procured from Hamburg(HmbG) Chemicals. The granules were ground into very finepowder using RetschR, ZM 200 grinder at 18,000 RPM after thatthe very fine activated charcoal was a sieve out using 400 USmesh size.

2.2. Mechanism of mixing of UF resin and AC

To obtain a uniform dispersion of activated charcoal powder inthe resin, mechanical stirring with Heidolph model RZR 2041wasdone for 30 min at 2000 RPM. The 200 g of UF/AC resin to thedesired volume concentrations of 0.20%, 0.52% and 1.04% wasprepared.

φ¼ ½wP=ρP �ðwP=ρPÞ þ ðwR=ρRÞ� �� 100 ð1Þ

where φ is the volume concentration of innovative resin inpercent, wP ;wR are the weight of powder and resin respectivelyin kg and ρP ; ρR are the density of particle and resin respectively inkg/m3. The modified resin was named based on percent added asAC1, AC2 and AC3. The resin in the absence of carbon powder isreferred to as AC0.

2.3. Characterization techniques

(a) Differential scanning calorimetry (DSC)DSC measurements were carried out in Differential scanningcalorimetry (DSC) Q-1000 model supplied by TA Instruments,USA. The resins of 6 mg is placed on high pressure aluminumcrucibles. The samples were heated from 30 1C to 200 1C in aninert atmosphere of nitrogen maintained at 50 ml/min flowrate, with an identical empty crucible used as a reference inthe measurement process. The heating rates employed were 5,10 and 15 1C/min. The peak curing temperature (Tp) estimatedat different heating rate (i.e. 5, 10 and 15 1C/min). Ea iscalculated from the slope of a plot of the natural logarithmof heating rate versus the reciprocal of the corresponding type[in degree Kelvin (K)] using Kissinger's equation [19] Eq. (2).

lnðβ=T2pÞ ¼ −Ea=RTp þ lnðAR=EaÞ ð2Þ

where, β is the heating rate, Tp is the peak curing temperature,R is the gas constant and A is the pre-exponential factor. Thevalue of Ea and A were determined from the graph plottedbetween ½−lnðβ=T2

pÞ� and ½100=Tp�.(b) Fourier transforms infrared spectroscopy

The pre-cured resin and samples were mixed with KBr andmade into pellets to determine the bond formation usingFourier transform infrared spectroscopy (FTIR). The FTIRtransmittance spectra were obtained with Perkin-Elmer spec-trum 100 in the spectral range of 400–4000 cm−1, with aresolution of 2 cm−1 and 50 scans.

(c) Scanning electron microscopyThe morphology of the prepared samples were examinedusing Field Emission Scanning Electron Microscopy (FESEM)system (JEOL JSM 840A-Oxford ISIS 300 microscope). Thesamples were carbon coated in order to provide good con-ductivity of the electron beam. Operating conditions were

accelerating voltage 2–5 kV, probe current 45 nA, and countingtime 60 s.

2.4. Preparation of medium density fiberboard (MDF)

Table 1 The MDF panels were prepared by mixing the fiberswithin innovative resin in rotary blender equipped with resinspraying. The 10 wt% UF resin. The wood fibers were mixed withresin and formed in the form of mat using forming box. The fibermat is pre-pressed in cold molding press for 2 min at 10 kg/cm2

pressure after that it is hot-pressed to the desired thickness.Table 2 shows all the details of MDF manufacturing using innova-tive UF resin. The panels were then conditioned to relativehumidity of 6575% and a temperature of 20 1C to attain uniformmoisture content in the panels. The boards were trimmed fordetermining the modulus of rupture (MOR), internal bondstrength (IB) and estimate the formaldehyde emission frompanels. The mechanical properties of MDF panels were evaluatedas per ASTM standard D-1037. The internal bonding (IB) andmodulus of rupture (MOR) of MDF panels are estimated with theuniversal testing machine (AG-20kN, Shimadzu Precision universaltester, Shimadzu Corporation, Japan).

2.5. Formaldehyde emission testing

The formaldehyde emissions from MDF panels was evaluatedusing the EN-120 (1992) perforator method. Around 110 g samplewere put in a round bottom flask, that contain the 600 ml oftoluene. The 1000 ml of distilled water was poured into theperforator attachment. The samples were boiled with tolueneand passed through the distilled water for 2 h. In this process,

Table 1MDF manufacturing parameters with different loading of activated charcoal.

Parameters Values

Board dimensions 280 mm�280 mm�9 mmTarget density 775 kg/m3

Platen temperature 180 (1C)Pressing time 270 (s)UF resin wt% of dry wood fibers 10 wt%Activated charcoal volume concentration% of UF resin

0.0, 0.20, 0.52 and 1.04%

Number of boards for each typeof concentrations

5 boards

Table 2Shows the differential scanning calorimetry results and cure kinetics results of UFresin with different concentrations of activated charcoal.

Sample Heating rate(1C/min)

Peak curingtemperature (1C)

½−lnðβ=T2pÞ�

vs.½100=Tp�Activationenergy (kJ/mol)

AC0 5 115 y¼−14.641x+27.419R2¼0.999

121.910 12215 126

AC1 5 98 y¼−8.0991x+11.621R2¼0.993

67.410 10815 116

AC2 5 90 y¼−6.2712x+7.1197 R2¼0.991

52.210 10215 112

AC3 5 88 y¼−5.866x+6.1386R2¼0.999

48.910 10215 111

A. Kumar et al. / International Journal of Adhesion & Adhesives 46 (2013) 34–39 35

Author's personal copy

the distilled water absorbs the formaldehyde and the volatileorganic compounds stripped by the boiling toluene. Formaldehydetrapped by the water is then quantitatively determined using a UVspectrophotometer after treatment with acetyl acetone and acetylammonium.

2.6. Statistical analysis

One way of analysis of variance (ANOVA) was performed by usingMicrocal Origin statistical software and analyzed the statisticaldifference in the MDF panel properties at Po0.05 level.

3. Results and discussion

3.1. Effect of activated charcoal on the curing behaviorof UF/AC resins

The curing behavior of resin was measured by DSC in the formof activation energy (Ea), which is estimated by using Eq. (2). TheDSC curves of neat UF resin and UF resin with different concentra-tion of activated charcoal with different heating rates are shown inFig. 1. The net UF resin shows that the peak curing temperature(Tp) at 115, 122 and 126 1C for heating rate of 5, 10 and 15 1C/minrespectively. The peak curing temperature of UF resin decreaseswith the activated charcoal concentrations as given in Table 2.

The activation energy (Ea) of the curing reaction of resins isestimated from the slope of plot between ½−lnðβ=T2

pÞ� and½100=Tp�given in Table 2. Whereas the values of R2 that show the qualityof the fitting curve. The Ea of the curing reaction decreased withactivated charcoal concentration as shown in Table 3. Thereare several reasons for the decrease in activation energy as:(i) the interaction of carbonyl group present in the activatedcharcoal and possibly formed to coordinate covalent bond withUF polymer chain or a catalytic activation of the resin self-condensation induced by the charcoal derived from wood con-stituents, (ii) the mass of combined secondary forces those arebinding the molecules of oligomer to the surface of charcoal orlignocellulosic substrate is very high leading to the pronouncedweakning of the bonds most susceptible to cleavage in theadhesive molecules, these bonds need to be cleaved for thepolymerization and hardening of the resin [19]. Thus in thismanner activated charcoal accelerating adhesive curing, (iii)possibly on the charcoal separate some of the water from theresin by absorption and induces a higher resin concentration andthus a faster reaction.

3.2. Crosslink density of UF and UF/AC resins

The percentage changes in the Crosslink density of UF resinafter activation charcoal addition are estimated from the relation

Fig. 1. DSC curves of resins at different heating rate 5, 10 and 15 1C/min, (a) AC0, (b) AC1, (c) AC2 and (d) AC3 samples.

A. Kumar et al. / International Journal of Adhesion & Adhesives 46 (2013) 34–3936

Author's personal copy

given as.

%CD¼ ΔH ðUF=AC resinÞ–ΔH ðUF resinÞΔH ðUF resinÞ � 100

where % CD is the percentage change in Crosslink density of resin,ΔH ðUF=AC resinÞ is the heat evolution during curing of UF/ACresins and ΔH ðUF resinÞ is the heat evolution during curing ofUF resin.

Fig. 2 The heat evolution or enthalpy of reaction (ΔH) duringthe curing of resins are calculated from the area under theendothermic curve at 10 1C/min heating rate using DSC. In thepresent study, additional heat evolution can be attributed to extraCrosslink reactions occur due to the presence of activated charcoalin UF resin. The Crosslink density of UF resin improved by 19, 36and 49% with increasing concentration of activated charcoal asshown in Table 3. The possible reasons for enhanced in cross-linkdensity is that the charcoal contains a large amount of reactivecarbonyl group and these carbonyl groups formed covalent bond-ing with UF resin and increased the of amount of –CH2–O–CH2–

linkages in UF resin. The area under the peaks in DSC analysis(i.e. enhanced in reaction enthalpy, ΔH) is directly proportional tothe –CH2–O–CH2– linkages in UF resin [20].

3.3. FTIR analysis

The functional groups suggested in AC are carboxyl groups,phenolic hydroxyl groups, carbonyl groups and location groups[21]. The FTIR spectra of net activated charcoal is shown in Fig. 3.The C–OH stretching mode and the bending mode can be found at

3401 cm−1 and 1561 cm−1, respectively. The peak occurs at1125 cm−1 is characteristics of C–O is stretching in lactonic,alcoholic groups and carboxylate moieties [22].

The FTIR spectra of UF and UF/AC resins are shown in Fig. 4 andTable 4. The spectrum of UF/AC resins shows a strong absorptionband between at 3420–3453 cm−1 region and 3421 cm−1 for puresample. These are the charteristics absorption bands of hydrogenbonded N–H of –NH2, formed due to the methylenization reactionhappen during cross-linking [23]. The weak absorption band in allresins appear near 2962–2963 cm−1, which describes the symme-trical C–H stretching mode of CH2 of ether, CH2OH and N–CH2.

The strong absorption band is observed in the spectra, near1640, 1645,1638 and 1639 cm−1 for AC0, AC1, AC2 and AC3respectively, assigned to the stretching C¼0 (amide-I) in –CONH2

group. The very strong absorption bands around 1536, 1538 and1544 for AC0, AC1 and AC2 respectively, it may be due to –NH(amide II) are assigned. The stretching vibrations around 1350–1400 cm−1 for resin samples, represent by the C–H bending modein CH2/CH2OH/N–CH2–N. The intermediate absorption band inthe around 1121–1140 cm−1 may appear due to stretching vibra-tions of –N–CH2–N– group of the ether linkage. Bands around775–780 cm−1 due to stretching vibrations of C–O in –CH2OH,N–H stretch in 11 and 21 amines. The weak absorption bands near

Table 3Show the mechanical properties of MDF, heat evolution during crosslinking ofresins, % improvement in Crosslink density.

Sample IB (MPa) MOR(MPa)

ΔH (J/g)

% improvement in Crosslinkdensity

AC0 0.54(0.0811)

44 (4.37) 395 —

AC1 0.60(0.0789)

47 (5.33) 473 19.74

AC2 0.61(0.0892)

47 (3.88) 537 35.94

AC3 0.58(0.0933)

45 (4.93) 589 49.11

[IB and MOR are the mean values of 10 replicates and values given in brackets arethe SD (standard deviation)]

Fig. 2. FTIR spectra of net activated charcoal.

Fig. 3. FTIR spectra of UF and UF/AC resins.

Fig. 4. Formaldehyde emission from MDF boards by perforator method withdifferent concentration of activated charcoal.

A. Kumar et al. / International Journal of Adhesion & Adhesives 46 (2013) 34–39 37

Author's personal copy

600–650 cm−1 ascribed the –CH bending mode. The chemicalstructure of UF and UF/AC resins represents the same spectra,except –NH stretching of amide II is absent for AC3 resin.

3.4. Formaldehyde emission

Fig. 4 shows the formaldehyde emission testing results by theperforator method. The formaldehyde emission was estimatedfrom oven dry samples and samples having 6.5% moisture content.The value of formaldehyde emission was 9.48 and 9.82 mg/100 gfor oven dry and 6.5% M.C control MDF boards. The formaldehyde

emission value significantly decreases at Po0.005 level by theaddition of activated charcoal in UF resin. The highest decrease informaldehyde emission was from the AC3 sample as shown infigure in Fig.4.

This lowering in formaldehyde emissions was caused by thecapability of the microstructure of the activated charcoal to absorbformaldehyde in the MDF [24,25]. The activated charcoal havingchemical compound which is positively charged and polar innature, triggered the adsorption of the polar formaldehyde. Inaddition, it is considered that might be occuring of hydrogenbonding and Van Der Waals force curing. Due to the presence ofnumerous minute pores of charcoal absorb formaldehyde mole-cules, enabling charcoal to act as an absorbent [26].

3.5. SEM analysis

Fig. 5 shows the FESEM micrographs of MDF panels withdifferent loading percentage of activated charcoal; Fig. 5(a) shows the wood fiber structure along with cured UF resin. InFig. 5 (b) the activated charcoal is well dispersed between thewood fibers with resin. Fig. 5 (c) shows the FESEM image of AC2sample, the agglomerate activated charcoal appears on woodfibers embedded along with cured UF resin.

3.6. Physical and mechanical properties of MDF panels

The effect of activated charcoal on the modulus of rupture(MOR) of MDF panels are shown in Table 3. The mean values ofMOR of MDF panels are 44, 47, 47 and 45 MPa for AC0, AC1, AC2

Table 4FTIR characteristic bands observed for UF resin and UF/AC resins.

Chemical Assignment UF and UF/AC resinswavenumbers/cm−1

AC0 AC1 AC2 AC3

(NH) 21 amine and OH 3421 3420 3426 3453(C¼O) in —CONH2 (amide I) 1640 1645 1638 1639(NH) in NH—CO in 21 amine (amide II) 1536 1538 1544 —

(CH) in CH2/CH2OH/N—CH2—N) 1387 1380 1385 1384(C—C—O) 1244 1244 1255 1245(N—CH2—N), ν(C—O—C) of ether linkage 1121 1134 1134 1134(H—N—H—CH2—CH3) 11 amine 1018 1009 1013 1011(C—O) of —CH2—OH; γ(N—H) in 11 and 21 amines 778 778 770 777(—CH) 649 636 637 600

Fig. 5. FESEM monographs shows the microscopic structure of MDF panels (a) AC0 (b) AC1 and (c) AC2.

A. Kumar et al. / International Journal of Adhesion & Adhesives 46 (2013) 34–3938

Author's personal copy

and AC3 respectively. The value of MOR increases with theconcentration of activated charcoal. The increase in bendingstrength (MOR) may be due to the formation covalent bondbetween UF resin and activated charcoal.

The internal bonding strength of MDF panels along withactivated charcoal concentration are shown in Table 3. The AC0has a mean value of IB is 0.54 MPa; AC1, AC2 and AC3 have 0.60,0.61 and 0.58 MPa mean values of IB respectively. The internalbonding depends on the Crosslink density of UF resin, as the DSCresults show the decrease in activation energy and improvementin Crosslink density with activation charcoal concentrations.However, the resin cured at a faster level inside the core of themat during hot-pressing of MDF panel, which will enhance theinternal bonding of panels. The IB of samples having activatedcharcoal are enhanced statistically significant at Po0.05 levelcomparison of pure UF resin samples.

The thickness swelling and water absorption of MDF panels areshown in Fig. 6. The thickness swelling for AC0 is 15.7%, 15.8, 16.1and 15.7% for AC1, AC2 and AC3 respectively. There is not muchsignificant difference in the TS values between the UF and UF/ACresins samples. The addition of activated charcoal in UF resin didnot have any effect on the water absorption properties of MDFpanels.

4. Conclusion

In this present study, the activated charcoal was used as a fillerin UF resin and its effect on the properties of wood composite wasinvestigated. The main conclusion of this study is drawn as:

� The activation energy of UF resin decreases with the concen-tration of activated charcoal, means that the UF resin will becured at lower temperature.

� The Crosslink density of UF resin improves significantly afterthe addition of activated charcoal.

� The formaldehyde emission from MDF panels decreases withthe addition of activated charcoal.

� The internal bonding and modulus of rupture of MDF panelsincreases with the addition of activated charcoal.

� There was no effect on the thickness swelling and waterabsorption properties of MDF panels.

Acknowledgments

The authors are thankful to Universiti Malaysia Pahang fora research Grant (GRS 10308). The authors acknowledge the supportof M/s Robin Resources (M) Sdn. Bhd. for supplying wood fibers andM/s Dynea Malaysia Sdn. Bhd. (171040-P) for supplying urea for-maldehyde adhesive used for the conducting the experiments.

References

[1] Global wood-based panels—market opportunity and environment, analysesand forecast to 2016. Market Publishers Ltd. London, UK; 2012.

[2] Dunky M. Urea–formaldehyde (UF) adhesive resins for wood. Int J AdhesAdhes 1998;18:95–107.

[3] Mayer B, Andrews BAK, Reinhardt RM. Formaldehyde release fromwood products. ACS symposium series. Washington, DC: American ChemicalSociety; 2006.

[4] Roffael E. Die formaldehyd-Abagbe von spanplatten und anderen werkstoffen.Stuttgart: DRW-verlag; 1982.

[5] Dongbin F, Jianzhang L, An M. Curing characteristics of low molar ratio urea-formaldehyde resin. J Adhes Interface 2006;7:45–52.

[6] Eom Y-G, Kim J-S, Kim S, Kim J-A, Kim H-J. Reduction of formaldehydeemission from particleboards by bio-scavengers. Mokchae Konghak2006;34:29–41.

[7] Sundin B, Hanetho P. Formaldehyde emission from particleboard and otherbuilding materials. In: Maloney TM, editor. Proceedings of the 12th Interna-tional Particleboard Symposium. Pullman, WA: Washington State University;1978. p. 251–86.

[8] Meyer B, Johns WE, Woo JK. Formaldehyde release from sulfur-modified urea–formaldehyde resin systems. For Prod J 1980;30(3):24–31.

[9] Marutzky R. In: Pizzi A, editor. Wood adhesives: chemistry and technology,vol. 2. New York: Marcel Dekker; 1986. p. 307–87.

[10] Que Z, Furuno T, Katoh S, Nishino Y. Effects of urea-formaldehyde resin moleratio on the properties of particleboard. Build Environ 2007;42:1257–63.

[11] Dupre FC, Foucht ME, Freese WP, Gabrielson KD, Gapud BD, Ingram HW, et al.Cyclic urea formaldehyde prepolymer for use in phenol formaldehyde andmelamine formaldehyde resin based binders. United States Patent, patent no.:US 0,054,994; September 5, 2002.

[12] Myers GE. Effects of post-manufacture board treatments on formaldehydeemission: a literature review (1960–1984). For Prod J 1984;36:41–51.

[13] Haiqin R, Zhenyu R, Jingtang Z, Yuanli Z. Effect of air oxidation of rayon-basedactivated carbon fibers on the adsorption behavior for formaldehyde. Carbon2002;40:2291–300.

[14] Kim S, Kim HJ, Kim HE, Lee HH. Effect of bio-scavengers on the curing behaviorand bonding properties of melamine–formaldehyde resins. Macromol MaterEng 2006;291:1027–34.

[15] Darmawan S, Sofyan K, Pari G, Sugiyanto K. Effect of activated charcoaladdition on formaldehyde emission of medium density fiberboard. J For Res2010;7(2):100–11.

[16] Pizzi A, Eaton NJ. A conformational analysis approach to phenol–formaldehyderesin adhesion to wood cellulose. J Adhes Sci Technol 1987;1(3):191–6.

[17] Lee YK, Kim HJ. Relationship between curing activation energy and freeformaldehyde content in urea-formaldehyde resins. J Adhes Sci Technol2013;27(5–6):589–609.

[18] Pizzi A. Advanced wood adhesives technology. New York: Marcel Dekker; 1994213–214.

[19] Pizzi A, Mtsweni B, Parsons W. Wood-induced catalytic activation of PFadhesives autopolymerization vs. PF/wood covalent bonding. J Appl PolymSci 1994;52:1847–56.

[20] Szesztay M, László-Hedvig Z, Kovacsovics E, Tüdõs F. DSC application forcharacterization of urea/formaldehyde condensates. Holz Roh Werkst1993;51:297–300.

[21] Juang RS, Wu FC, Tseng RL. Characterization and use of activated carbonsprepared from bagasses for liquid-phase adsorption. Colloids Surf A: Physi-cochem Eng Aspect 2002;201:191–9.

[22] Deng H, Yang L, Tao GH, Dai JL. Preparation and Characterization of activatedcarbon from cotton stalk by microwave assisted chemical activation—applica-tion in methylene blue adsorption from aqueous solution. J Hazard Mater2009;166:1514–21.

[23] Edoga MO. Comparative study of synthesis procedures for urea-formaldehyderesins (Part I). Leonardo Electron J Pract Technol 2006;72:607–17.

[24] Pari G, Kurnia S, Buchari Wasrin S Tectona grandis activated charcoal ascatching agent of formaldehyde on plywood glued with urea formaldehyde.In: Proceedings of the 8th Pacific Rim Bio-Based Composites Symposium2006. Kuala Lumpur, Malaysia; 2006.

[25] Rong H, Ryu Z, Zheng J, Zhang Y. The effect of the air oxidation of rayon-basedactivated carbon fibers on the adsorption behavior for formaldehyde. Carbon2002;40:2291–300.

[26] Park SB, Kim SW, Park JY. Proceedings of the first Korean society for indoorenvironment conference. Session C; 2004. p. 154.

Fig. 6. The thickness swelling and water absorption results of MDF panels withdifferent UF resins.

A. Kumar et al. / International Journal of Adhesion & Adhesives 46 (2013) 34–39 39