Holzforschung 2014; aop

Anuj Kumar * , Arun Gupta * and Korada Viswanathan Sharma

Thermal and mechanical properties of urea-formaldehyde (UF) resin combined with multiwalled carbon nanotubes (MWCNT) as nanofiller and fiberboards prepared by UF-MWCNT

Abstract: The effect of multiwalled carbon nanotubes

(MWCNT) as reinforcement on the properties of urea-for-

maldehyde (UF) resin and medium-density fiberboards

was investigated. MWCNT was added to UF in two con-

centrations, and the effects were studied by means of dif-

ferential scanning calorimetry and dynamic mechanical

thermal analysis in terms of the curing and viscoelastic

properties of the resins. In the presence of MWCNT, the

activation energy of the resins was lowered, and their stor-

age modulus and thermal conductivity were enhanced.

The formaldehyde emission decreased and mechanical

properties increased after addition of MWCNT to UF resin.

Keywords: curing behavior, fiberboard, formaldehyde

emission, mechanical properties, multiwalled carbon

nanotubes (MWCNT), thermosetting resin, urea-formalde-

hyde resin, wood composites

DOI 10.1515/hf-2014-0038

Received February 7 , 2014 ; accepted June 12 , 2014 ; previously

published online xx

Introduction

Urea-formaldehyde (UF) adhesives are common in the pro-

duction of wood-based panels ( Dunky 1998 ) because of

their advantages such as rapid curing, good performance

of the panels, good solubility in water, and low price ( Park

et al. 2009 ). On the other hand, the formaldehyde (FA) emis-

sion of the panels is a concern, which can be reduced to a

certain degree by lowering the molar ratio of FA/U of the

resin ( Myers 1984 ). However, the panels produced with such

a resin have an inferior performance with regard to mechan-

ical strength and water resistance ( Park et al. 2006 ). Addi-

tives like melamine ( Du and Du 1995 ) and formaldehyde

catchers ( Markessini 1994 ) are also helpful with this regard.

Nanotechnology offers another alternative for

improvement of the structural performance of UF adhe-

sives. The added nanomaterials with a large surface

area modify the resin properties. Hitherto, the follow-

ing nanomaterials were tested together with UF resins:

montmorillonite nanoclay ( Lei et al. 2008 ), SiO 2 nanopar-

ticles ( Roumeli et al. 2012 ), aluminium silicate nanoclay

( Xiaolin et al. 2007, 2010 ), and modified nano-crystalline

cellulose ( Zhang et al. 2011 ). Liu and Ye (2009) tested the

performance of panel boards prepared with multiwalled

carbon nanotubes (MWCNT) together with boron phenolic

adhesive. Kumar et al. (2013a,b) has studied the effect of

aluminum oxide nanoparticles on the heat transfer prop-

erties of medium-density fiberboard (MDF) and curing

behavior of UF resin. The nanosize activated charcoal

have an accelerating effect on UF curing behavior ( Kumar

et  al. 2013c ). The modification of solid wood with poly-

mers and nanostructured inorganic fillers opens new

avenues for production of wood polymer nanocomposites

with improved properties ( Devi et al. 2012 ; Cookson et al.

2007 ; Wang et al. 2014 ). In particular, MWCNT is promis-

ing in terms of high elastic modulus and better thermal

conductivity of the panels ( Guadagno et al. 2009 ).

*Corresponding authors: Anuj Kumar, Czech Technical University

in Prague, Faculty of Civil Engineering, Department of Building

Structures, Th á kurova 7, 166 29 Praha 6, Czech Republic,

e-mail: anuj.saroha@gmail.com ; and Arun Gupta, Centre of

Excellence for Biocomposite and Innovative Materials, Faculty of

Chemical and Natural Resources Engineering, University Malaysia

Pahang, Gambang, Kuantan, 26300 Pahang, Malaysia,

Phone: + 6-095492867, e-mail: arun@ump.edu.my

Korada Viswanathan Sharma: Department of Mechanical

Engineering, Universiti Technology PETRONAS, Bandar Seri

Iskandar, 31750, Tronoh, Perak, Malaysia

Authenticated | anuj.saroha@gmail.com author's copyDownload Date | 7/10/14 1:22 PM

2      A. Kumar et al.: Properties of UF-MWCNT resins and fiberboards

In the present work, the effects of MWCNT as nanofill-

ers in UF resins are tested. The measurements by means of

differential scanning calorimetry (DSC) were focused on

the peak curing temperature and activation energy of the

UF resin. The structural properties of the resins were ana-

lyzed by dynamic mechanical thermal analysis (DMTA).

The effects of MWCNT on the FA emission and physical

and mechanical properties of the produced MDF will com-

plete the investigation.

Materials and methods The UF resin was supplied by Dynea Malaysia Sdn Bhd, the proper-

ties of which are compiled in Table 1 . The properties of the indus-

trial-grade multiwalled carbon nanotubes (MWCNT), supplied by

Chengdu Organic Chemicals Co., Ltd, Chinese Academy of Sciences,

are also presented in Table 1 . Figure 1 shows the fi eld emission scan-

ning electron microscopy (FESEM) image of the applied MWCNT.

Rubber wood fi bers ( Hevea brasiliensis ) were supplied by M/S Robin

Resources Std Bhd.

COOH functionalisation of MWCNT was performed. Some 10.0 g

of MWCNT was added into 2 mol of 68% of HNO 3 /H

2 SO

4 in 1000 ml

water solution and heated at 60 ° C for 5 h. The mixture was cooled

down to room temperature (r.t.) and washed with deionized water

until the solution became neutral, and the the solution was vacuum-

fi ltered through 0.45 μ m pp micro-porous membrane. The fi lter cake

was oven dried at 60 ° C for 24 h to yield COOH-MWCNT.

In the preparation of UF/MWCNT resin, the MWCNT was mixed

into UF resin in concentrations of 0.20% and 0.52% (by volume)

under mechanical stirring for 20 min at 2000 rpm. The samples are

designated as UF 0.20

and UF 0.52

, respectively.

Preparation of MDF was based on fresh rubber wood fi bers.

A rotary blender was applied equipped with resin spraying. The

sprayed fi ber mat prepared in a forming box was pre-pressed in a cold

molding press for 2 min at 10 kg cm -2 pressure. The details including

those of the hot pressing are presented in Table 2 .

The panels were conditioned to relative humidity (RH) of

65 ± 5% at 20 ° C to attain uniform moisture content in the panels. The

boards were trimmed to determine the modulus of rupture (MOR)

and internal bond (IB) strength and to estimate the formaldehyde

Table 1   Properties of UF resin and multiwalled carbon nanotubes

(MWCNT).

Sample   Parameters   Values

MWCNT    

  Diameter   20 – 40 nm

  Length   10 – 30 μ m

  Purity   > 90%

  Surface area   100 m 2 g -1

UF resin   

  Solid content   64.3 wt.%

  Viscosity at 30 ° C  170 cP

  pH   8.27

  Density   1.282 g cm -3

Figure 1   FESEM image of multiwalled carbon nanotubes (MWCNT).

Table 2   Details of MDF preparation with UF and UF/MWCNT resins.

Parameters   Values

Board dimensions   280 × 280 × 9 mm 3

Target density   775 kg/m 3

Platen temperature   180 ( ° C)

Pressing time   330 (s)

UF resin by oven dry wood fibers   10% (by weight)

MWCNT concentration in UF resin (by volume)   0.20 and 0.52%

Number of boards for each concentration   5

(FA) emission of the panels. The mechanical properties of MDF pan-

els were evaluated as per British Standards EN 319 (1993) and EN

310 (1993) for IB strength and MOR, respectively. For this purpose, a

universal testing machine (AG-20kN, Shimadzu Precision universal

tester, Shimadzu Corporation, Japan) was available.

The FA emission was evaluated based on British Standards

EN-120 (1993) , which is a perforator method. Around 110 g sample

was placed in a round bottom fl ask that contained 600 ml toluene.

Then 1000  ml of distilled water was poured into the perforator

attachment. The samples were boiled with toluene, and the vapor

was passed through the distilled water for 2  h (water absorbs the

FA, while the volatile organic compounds are stripped by the boiling

toluene). The trapped FA was determined by UV spectroscopy aft er

treatment of the liquid with acetyl acetone and acetyl ammonium.

DSC measurements were carried out in a DSC Q-1000 instrument

supplied by TA Instruments (USA). About 6 mg resin was placed in

the high-pressure aluminum crucibles. Heating program consisted of

the following: 30 ° C → 200 ° C in N 2 fl ow (50 ml min -1 ) with an identical

empty crucible as reference. The peak curing temperature ( T p ) was

estimated at the heating rates of 5, 10, 15, and 20 ° C min -1 ). E a was cal-

culated from the slope of the plots “ natural logarithm of heating rate

vs. the reciprocal of K ” by Eq. (1).

2ln( / ) - / ln( / )p p aT RT AR Eβ = +aE

(1)

where β is the heating rate, T p is the peak curing temperature, R is

the universal gas constant, and A is the pre-exponential factor. The

Authenticated | anuj.saroha@gmail.com author's copyDownload Date | 7/10/14 1:22 PM

A. Kumar et al.: Properties of UF-MWCNT resins and fiberboards      3

value of E a and A were determined from the graph plotted between

2ln( / )pTβ and 1000/ T p .

DMTA was applied to determine the storage modulus ( E ′ ), loss

modulus ( E ″ ), and tan ( δ ) of the UF/MWCNT resin suspensions. The

instrument used was the Q100 DMTA (TA Instruments, USA) in the

dual cantilever testing mode, from 30 ° C to 200 ° C, at a heating rate of

5 ° C min -1 . All tests were done under 0.1% deformation and 15 N normal

force, at a frequency of 1 Hz. Prior to the analysis, the same quantity

of all the samples in liquid state were sandwiched between the rubber

wood veneer. The dimensions of testing samples were 12 × 3 × 60 mm 3 .

Thermal conductivity was measured in a KD2 pro thermal proper-

ties analyzer manufactured by Decagon Devices (USA). The KS-1 sensor

is designed to measure thermal conductivity of liquid samples. A small

quantity of the resin and hybrids are fi lled in vials. The cap of the vial

is equipped with a septum allowing direct insertion of the needle into

the vial through the cap. The KS-1 sensor needle estimates the thermal

conductivity based on the principle of transient heat source.

The viscosity was measured in a Brookfi eld DV-III ULTRA Rhe-

nometer (Brookfi eld Engineering Laboratories, USA) at r.t. with the

spindle number 31.

FESEM was performed with a JEOL JSM 840A-Oxford ISIS 300

instrument. The samples were carbon coated. The accelerating volt-

age was 2 – 5 kV, the probe current was 45 nA, and the counting time

was 60 s.

The thermal stability of pre-cured UF 0.20

and UF 0.52

was deter-

mined by thermogravimetry (TGA), TAQ500 model. Samples (4 mg)

were placed in a platinum pan and heated (10 ° C min -1 ) to 600 ° C in

N 2 (50 ml min -1 ).

Wide angle X-ray diff raction was performed in a Minifl ex II,

RIGAKU, X-ray diff ractometer (XRD), Cu K α radiation, with measur-

ing scale of XG-Cu/30 kV/15 mA, duration time of 1 ° min -1 , sampling

step of 0.02 ° , and scan range of 3 – 80 ° .

Results and discussion

UF/MWCNT resin properties

Curing properties

Figure 2 shows the DSC curves of UF, UF 0.20

, and UF 0.52

resins (the subscript numbers indicate the MWCNT compo-

nent in vol.%). The peak curing temperature ( T p ) decreases

with increasing MWCNT concentration. The activation

energy ( E a ) of the resins ’ curing reaction was also calculated

according to Kissinger (1957) with Eq. (1). The correspond-

ing data are presented in Table 3 , i.e., T p  with the heating

rate ( β ) and the E a . The E

a for UF is 184 kJ mol -1 , which is

decreased by 28% in the case of UF 0.52

. According to the R 2

data ( Table 3 ) and the plots ( Figure 3 ), there is a good linear

correlation between the data. The cure enthalpy ( Δ H ) of UF

resin was enhanced as a function of MWCNT concentration

( Table 3 ). This means that the crosslink density of UF resin

increased after addition of MWCNT. The mechanism of this

effect is not clear at the moment.

5a

b

c

4

3

2

1

0

5.5

5.0

4.5

4.0

3.5

5°C/min10°C/min

20°C/min15°C/min

5°C/min10°C/min

20°C/min15°C/min

5°C/min10°C/min

20°C/min15°C/min

Hea

t flo

w (

W g

-1)

Exo 3.0

2.5

2.0

1.5

1.0

0.5

0.0

8

7

6

5

4

3

2

1

0

20 40 60 80 100 120

Temperature (°C)

140 160 180 200 220

Figure 2   DSC curves of resins (a) UF, (b) UF 0.20

, and (c) UF 0.52

.

The viscosity of UF is 172 cP, while the corresponding

data for UF 0.20

and UF 0.52

are 427 and 869 cP. The ration-

ale behind this finding is the high surface area of the

UF/MWCNT mixture, which impedes the free movement of

the liquid. This viscosity increment, however, does not have

a detrimental effect on the spraying behavior of the resins.

Structural properties

The final mechanical properties of the crosslinked

network can be characterised by DMTA. The development

of these properties during curing can be observed, and

data in terms of gelation and vitrification can be derived.

Figure 4 a permits a comparison of storage modulus ( E ′ ) of

UF, UF 0.20

, and UF 0.52

, respectively. The E ′ values decreased

down to ∼ 120 ° C till there was gel formation in the UF resin

Authenticated | anuj.saroha@gmail.com author's copyDownload Date | 7/10/14 1:22 PM

4      A. Kumar et al.: Properties of UF-MWCNT resins and fiberboards

Table 3   Various data obtained by DSC measurements of the resins investigated.

Sample   Heating rate ( ° C min-1)

  Peak curing temperature ( ° C)

  Cure enthalpy, Δ H ( j / g )

  Activation energy (kJ mol-1)

E a   R 2

UF   5  115.4  425.3  183.85  0.998

  10  120.0  444.6   

  15  122.5  384.1   

  20  124.8  416.4   

UF 0.20

  5  110.7  516.2  149.11  0.986

  10  115.3  597   

  15  120.0  223.3   

  30  121.3  450   

UF 0.52

  5  105.5  529.8  144.31  0.992

  10  110.1  854.4   

  15  114.2  918.4   

  20   116.6   431    

-8.8

-9.0

-9.2

-9.4

-9.6

-9.8

-10.0

-10.2

-10.4

UFUF0.20

UF0.52

In (

β/T

2 p)

2.52 2.54 2.56 2.58 2.60 2.62 2.64

1000/Tp (K-1)

Figure 3   The plots for estimating the activation energy according

to Kissinger (1957).

1600

a

b

1400

1200

1000

800

600

0.18

0.16

0.14

0.12

Tanδ

0.10

0.08

0.06

0.04

0.0260 80 100 120 140 160 180 200

UFUF0.20

UF0.52

UFUF0.20

UF0.52

Sto

rage

mod

ulus

(M

Pa)

Temperature (°C)

Figure 4   (a) Storage modulus and (b) tan δ of UF/carbon nanotubes

(CNTs) resins.

( Xiaolin et  al. 2007 ; Kumar et  al. 2013b ). However, the

curing reaction started in this stage of the process, and

the storage modulus gradually rose with elevated temper-

atures for all resins.

Gelation point for thermoset resins occurs at the

beginning of the formation of higher molecular weights,

when the resin viscosity is elevated exponentially. This

leads to higher values of E ′ and tan δ . This point is around

84 ° C for UF, 89 ° C for UF 0.20

, and 96 ° C for UF 0.52

. These tem-

peratures indicate the gel points of the resins. Afterwards,

tan δ ( Figure 4 b) increases up to a maximum, which is

called “ vitrification point ” , i.e., the point of the rubber-to-

glass transition ( He and Riedl 2003 ). After the mobility of

the material is reduced by vitrification, the reactions are

still continuing resulting in a further E ′ increment. Despite

the retarded speed of chemical reactions, the storage

modulus is increasing until the formation of a complete

Authenticated | anuj.saroha@gmail.com author's copyDownload Date | 7/10/14 1:22 PM

A. Kumar et al.: Properties of UF-MWCNT resins and fiberboards      5

crosslinking plateau. The decreasing tan δ can be consid-

ered as the rate of curing.

The reinforcement effect of MWCNT is evident based

on Figure 4 a as the storage modulus of the two UF/MWCNT

resins is enhanced in the whole temperature range. The

highest E ′ values are seen for UF 0.20

(1420 MPa), followed

by UF 0.52

(1385 MPa). The UF resin shows an E ′ value of

745 MPa. These observations can be explained by the high

elastic modulus of MWCNT.

Figure 5 a shows the XRD pattern of MWCNT consist-

ing of a few broad peaks located near the (0 0 2), (1 0 0),

(1 1 0), and (1 1 2) reflections of graphite. The characteristic

peak at about 2 θ = 26 ° can be attributed to the (0 0 2) reflec-

tion of carbon typically amorphous in nature. Figure 5 b

shows a strong peak at about 22.35 ° from the lattice planes

of the hardened UF ( Park and Jeong, 2012 ), 22.47 ° for UF 0.20

and 22.53 ° UF 0.52

. The characteristic peak of MWCNT disap-

peared for both UF 0.20

and UF 0.52

.

Thermal properties

The thermal conductivity of UF resin is 0.4 W mK -1 , which

increased to 0.425 and 0.475  W mK -1 in the case of UF 0.20

3500

a

b

3000

2500

2000

1500

1000

500

600

500

400

300

200

100

0

0

10 20 30 40 50 60 70 80

UF

Pristine MWCNT

UF0.20

UF0.52

Inte

nsity

(a.

u.)

2θ (°)

Figure 5   X-ray diffraction (XRD) spectra for CNT (a), UF, UF 0.20

, and

UF 0.52

(b) in the 2 θ angle range 3 – 80 ° .

100

80

60

40

20

0100 200 300 400 500 600

UF

UF0.20

UF0.52

UFCNT1CNT2

Wei

ght l

oss

(%)

Temperature (°C)

Figure 6   TGA analyses of UF and UF/MWCNT resins.

and UF 0.52

, respectively. This effect is easy to explain by the

very high thermal conductivity of MWCNT.

Figure 6 shows the weight loss (WL) of the samples

as a function of temperature. The essential degradation

steps are at 100 – 225 ° C, 225 – 300 ° C, and 300 – 600 ° C. Deg-

radation of cured adhesives begins with the release of for-

maldehyde (FA) from dimethylene ether groups, and the

maximum degradation rate occurs when the stable meth-

ylene ether linkages break down ( Siimer et al. 2003 ). It can

be concluded that the addition of MWCNT does not affect

the thermal stability of the adhesive, but, as expected,

char yield increased in the presence of MWCNT. It is

observed that WL up to 30% occurs in UF at 267.7 ° C, while

in UF 0.20

it occurs at 268.2 ° C and UF 0.52

at 267.5 ° C; 50% WL

occurs at 270.7 ° C for UF and at 290.7 ° C for UF 0.52

.

MDF properties

Physical and mechanical properties

Figure 7 a shows the effect of MWCNT on the MOR of MDF

panels. The mean values (n = 8) for samples of MOR of MDF

panels are 31.7, 34.7, and 39.2 MPa for UF, UF 0.20

, and UF 0.52

,

respectively. The value of MOR increases with the concen-

tration of nanofillers.

Figure 7 b represents the IB strength of MDF panels.

The mean value (n = 12) of IB is 0.43 MPa; UF 0.20

and UF 0.52

have 0.61 and 0.69 MPa IB data, respectively. The internal

bonding depends on the crosslink density of the UF resin.

The DSC results show the decrease in activation energy

and improvement in crosslink density with the addition of

nanofillers. However, the resin cured at a faster rate inside

Authenticated | anuj.saroha@gmail.com author's copyDownload Date | 7/10/14 1:22 PM

6      A. Kumar et al.: Properties of UF-MWCNT resins and fiberboards

the core of mat during hot-pressing of MDF panel, which

will enhance the internal bonding of panels.

The thickness swelling and water absorption of MDF

panels are shown in Figure 8 . The thickness swelling mean

of 12 samples for UF, UF 0.20

, and UF 0.52

is 15.7%, 14.8%, and

13.8%, respectively. The addition of MWCNT in UF resin

40a

b

783

780

777

774

771

768

765

773

772

771

770

769

38

36

34

32

30

0.75

0.70

0.65

0.60

0.55

0.50

0.45

0.40

MORDensity

IBDensity

UF UF0.20 UF0.52

Mod

ulus

of r

uptu

re (

MP

a)In

tern

al b

ondi

ng (

MP

a)

Den

sity

(kg

m3 )

Figure 7   MOR (a) and IB (b) of MDF prepared with UF and UF/CNTs

resins with their mean density of samples.

20

18

16

14

12

10

8 24

26

28

30

32

TSWA

UF UF0.20 UF0.52

24 h

TS

(%

)

24 h

WA

(%

)

Figure 8   Thickness swelling and water absorption results of MDF

panels.

did not have much effect on the water absorption proper-

ties of MDF panels.

Formaldehyde (FA) emission

The free formaldehyde values for the oven-dry MDF panels

with 0%, 0.20%, and 0.52% MWCNT concentrations were

12.3 mg, 9.7 mg, and 7.7 mg per100 g, respectively. Prob-

ably, the methylol groups (-CH 2 OH) of formaldehyde are

covalently attached onto the surface of MWCNT via elec-

trophilic reaction ( Yang et  al. 2007 ) and retards the FA

emission.

Conclusion In this work, multiwalled carbon nanotubes (MWCNT)

were used as nanofillers in UF resins. It was observed

that the curing kinetics and activation energy of UF resin

improved and the resin was cured at a lower temperarture.

The structural properties, i.e., storage modulus and tan δ of UF resin, increased due to loading of nanofillers. There

is no effect on the thermal stability of the UF resin. The

MOR and IB data of fiberboards prepared by means of UF

resins improved significantly in the presence of MWCNT.

The formaldehyde emission of fiberboards decreased in

the case of MWCNT-UF resins.

Acknowledgments: The authors wish to acknowledge the

University Malaysia Pahang for providing the financial

support (RDU-GRS 110308) to conduct this research. We

also thank M/S Robin Resources Std. Bhd for providing

the rubber wood fibres and adhesives for this research.

References British Standards EN 120 (1993) . Wood-based panels-determination

of formaldehyde content-extraction method called perforator

method.

British Standards EN 310 (1993) . Wood based panels, determina-

tion of modulus of elasticity in bending and bending strength.

British Standards EN 319 (1993) . Particleboards and fiberboards,

determination of tensile strength perpendicular to plane of the

board.

Cookson, L.J., Scown, D.K., McCarthy, K.J., Chew, N. (2007) The

effectiveness of silica treatments against wood-boring inverte-

brates. Holzforschung 61:326 – 332.

Devi, R.R., Mandal, M., Maji, T.K. (2012) Physical properties of simul

(red-silk cotton) wood ( Bombax ceiba L.) chemically modified

with styrene acrylonitrile co-polymer and nanoclay. Holz-

forschung 66:365 – 371.

Authenticated | anuj.saroha@gmail.com author's copyDownload Date | 7/10/14 1:22 PM

A. Kumar et al.: Properties of UF-MWCNT resins and fiberboards      7

Du, G., Du, B. (1995) Application of mineral filler of urea formalde-

hyde adhesive as plywood adhesive. China Adhes. 4:39 – 42.

Dunky, M. (1998) Urea-formaldehyde (UF) adhesive adhesives for

wood. Int. J. Adhes. Adhes. 18:95 – 107.

Guadagno, L., Vertuccio, L. Sorrentino, A., Raimondo, M. Naddeo, C.

Vittoria, V., Lannuzzo, G., Calvi, E., Russo, S. (2009) Mechanical

and barrier properties of epoxy resin filled with multi-walled

carbon nanotubes. Carbon 47:2419 – 2430.

He, G.B., Riedl, B. (2003) Phenol-urea-formaldehyde cocondensed

resol resins: their synthesis, curing kinetics, and network prop-

erties. J. Polym. Sci. Part B: Polym. Phys. 41:1929 – 1938.

Kissinger, H.E. (1957) Reaction kinetics in differential thermal analy-

sis. Anal. Chem. 28: 702 – 1706.

Kumar, A., Gupta, A., Sharma, K.V., Nasir, M. (2013a) Use of alu-

minum oxide nanoparticles in wood composites to enhance

the heat transfer during hot-pressing. Eur. J. Wood. Prod.

71:193 – 198.

Kumar, A., Gupta, A., Sharma, K.V., Gazali, S.B. (2013b) Influence of

aluminum oxide nanoparticles on the physical and mechanical

properties of wood composites. BioResources 8:6231 – 6241.

Kumar, A., Gupta, A., Sharma, K.V., Nasir, M., Khan, T.A. (2013c)

Influence of activated charcoal as filler on the properties of

wood composites. Int. J. Adhes. Adhes. 46:34 – 39.

Lei, L.H., Du, G., Pizzi, A., Celzard, A. (2008) Influence of nanoclay

on urea-formaldehyde adhesives for wood adhesives and its

model. J. Appl. Polym. Sci. 109:2442 – 2451.

Liu, L., Ye, Z. (2009) Effects of modified multi-wall carbon nanotubes

on the curing behavior and thermal stability of boron phenolic

adhesive. Polym. Degrad. Stab. 94:1972 – 1978.

Markessini, E. (1994) Formaldehyde emissions from wood-based

panels and the ways to reduce them. Monument Environ.

2:57 – 64.

Myers, G.E. (1984) How mole ratio of UF adhesive affects formalde-

hyde emission and other properties: a literature critique. For.

Prod. J. 34:35 – 41.

Park, B.D., Jeong, H.W. (2012) Hydrolytic stability and crystallinity of

cured urea – formaldehyde adhesive adhesives with different for-

maldehyde/urea mole ratios. Int. J. Adhes. Adhes. 31:524 – 529.

Park, B.D., Kang, E.C., Park, J.Y. (2006) Effects of formaldehyde to

urea mole ratio on thermal curing behavior of urea formalde-

hyde adhesive and properties of particleboard. J. Appl. Polym.

Sci. 101:1787 – 1792.

Park, B.D., Lee, S.M., Roh, J.K. (2009) Effect of formaldehyde/urea

mole ratio and melamine content on the hydrolytic stability of

cured urea-melamine-formaldehyde adhesive. Eur. J. Wood.

Prod. 67:121 – 123.

Roumeli, E., Papadopoulou, E., Pavlidou, E., Vourlias, G.,

Bikiaris, D., Paraskevopoulos, K.M., Chrissafis, K. (2012)

Synthesis, characterization and thermal analysis of urea

formaldehyde/nanoSiO 2 adhesives. Thermochim. Acta

527:33 – 39.

Siimer, K., Kaljuvee, Christjanson, T.P. (2003) Thermal behavior of

urea formaldehyde adhesives curing. J. Therm. Anal. Calorim.

7:607 – 617.

Wang, W., Zhu, Y., Cao, J., Liu, R. (2014) Improvement of dimensional

stability of wood by in situ synthesis of organo-montmoril-

lonite: preparation and properties of modified southern pine

wood. Holzforschung 68:29 – 36.

Xiaolin, C., Riedl, B., Zhang, S.Y., Wan, H. (2007) Formation and

properties of nanocomposites made up from solid aspen

wood, melamine-urea-formaldehyde, and clay. Holzforschung

61:148 – 154.

Xiaolin, C., Riedl, B., Wan, H., Zhang, S.Y., Wang, X.M. (2010) A study

on the curing and viscoelastic characteristics of melamine-

urea-formaldehyde adhesive in the presence of aluminum

silicate nanoclays. Composites, Part A 41:604 – 611.

Yang, Z., Chen, X.-H., Xia, S.-Z., Pu, Y.-X., Xu, H.-Y., Li, W.-H., Xu,

L.-S., Yi,B., Pan W.-Y. (2007) Covalent attachment of poly

(acrylic acid) onto multiwalled carbon nanotubes functional-

ized with formaldehyde via electrophilic substitution reaction.

J. Mater. Sci. 42:9447 – 9452.

Zhang, H., Zhang, J., Song, S., Wu, G., Pu, J. (2011) Modified

nanocrystalline cellulose from two kinds of modifiers

used for improving formaldehyde emission and bonding

strength of urea-formaldehyde adhesive. BioResources

6:4430 – 4438.

Authenticated | anuj.saroha@gmail.com author's copyDownload Date | 7/10/14 1:22 PM