Thermal decomposition kinetics, flammability, and mechanicalproperty study of wood polymer nanocomposite

Ankita Hazarika • Tarun Kumar Maji

Received: 22 February 2013 / Accepted: 30 August 2013 / Published online: 9 October 2013

� Akademiai Kiado, Budapest, Hungary 2013

Abstract Melamine formaldehyde-furfuryl alcohol

copolymer was impregnated into softwood in combination

with 1,3-dimethylol-4,5-dihydroxy ethyleneurea, a cross-

linking agent, nanoclay, and a renewable polymer, col-

lected as gum from a local plant (Moringa oleifera) under

vacuum condition and polymerized by catalyst heat treat-

ment. Fourier-transform infrared spectroscopy, X-ray dif-

fractometry, and scanning electron microscopy were used

to characterize the nanocomposites. Transmission electron

microscopy showed uniform distribution of nanoclay in the

composites. The mechanical properties were improved

after the addition of plant polymer. The plant polymer had

a marked influence on the flammability and thermal sta-

bility of the prepared composites. The apparent activation

energy was determined by Ozawa-Flynn-Wall’s and Vya-

zovkin methods. The activation energy of the composites

decreased up to a certain decomposed fraction thereafter it

remained constant. Higher the plant polymer content higher

was the activation energy of the prepared composites

which indicated a better interfacial adhesion and thermal

stability.

Keywords Nanocomposites � Plant polymer �Flammability � Thermal stability � Activation energy

Introduction

Wood polymer composites (WPC) have evoked consider-

able interest as one of the rapidly growing industries in

recent years. The hydrophilic nature of polymer constitu-

ents of cell wall of wood is responsible for exhibiting

hygroscopic behavior. Excessive moisture can lead to

shrinking and swelling of wood and results in fungal attack

causing changes of color and finally degradation of wood.

To enhance the long-term service life, chemical modifica-

tion of wood can be made with various monomers and

thermoset resins [1, 2].

Increased awareness of wood preservatives on environ-

mental effect has rendered special importance to furfuryl

alcohol as an eco-friendly agent for wood modification.

Furfuryl alcohol causes wood cell wall to swell and suffi-

ciently polar so that they enter wood cell walls [3].

Mantanis et al. [4, 5] have reported that low molecular

volume monomers that are capable of forming hydrogen

bonds swell the cell wall of wood permanently. Impreg-

nation of wood with furfuryl alcohol would lead to con-

siderable improvement in properties such as hardness,

density, equilibrium moisture content, dimensional stabil-

ity, and durability [6, 7]. Nevertheless, furfurylation does

not have marked influence on the bending strength and the

modulus of elasticity (MOE) of wood [8].

Modification of wood with melamine formaldehyde

resin can significantly improve the mechanical properties

of wood [9]. Besides, it contains nitrogen and as a result

it can influence thermal properties and flammability of the

prepared composites. A copolymer of melamine formal-

dehyde and furfuryl alcohol (MFFA) has been prepared

with the intend of getting overall benefits of the

properties.

There is a continuous effort to enhance thermal sta-

bility and flame retardancy of wood to expand its utility.

Flame retardancy can be achieved by the use of organo-

halogen, organophosphorus, organoantimony compounds,

various silicates and borates compounds [10, 11]. While

A. Hazarika � T. K. Maji (&)

Department of Chemical Sciences, Tezpur University,

Tezpur 784028, Assam, India

e-mail: tkm@tezu.ernet.in

123

J Therm Anal Calorim (2014) 115:1679–1691

DOI 10.1007/s10973-013-3394-7

burning most of them emit toxic fumes which are highly

health hazardous and pollute the environment. The poly-

meric flame-retarding agent can reduce the leaching

problem due to its high molecular mass and thus,

improved the service life of the polymer product. The use

of flame-retardant (FR) polymer of renewable origin is

beneficial from environmental point of view. Very few

reports are available on the use of gum from Moringa

oleifera as FR. Jana et al. [12] have studied the flam-

mability and biodegradability of starch-based biodegrad-

able film modified with gum derived from Moringa

oleifera and found an improvement in flame retardancy.

Ghosh et al. [13] have studied the adhesive performance,

flammability, and biodegradation study of various rubber/

plant polymer blends and reported an improvement in

properties of the prepared blends . There is ample scope

to do further work using Moringa oleifera gum.

Thermogravimetric analysis (TG) is one of the main

thermal analysis techniques used to study the thermal

stability, mass change, and degradation behavior of the

samples. The determination of kinetic parameters such as

activation energy from TG associated with thermal deg-

radation is an important tool in estimating the thermal

decomposition kinetics of composites and polymers

[14, 15]. Many expressions are used for the evaluation of

the non-isothermal kinetic parameters of the thermal

degradation of the samples. The activation energy of the

composites in terms of the Ozawa–Flynn–Wall (OFW)

expression is preferred as it requires less experimental

time. It is one of the integral methods for determination

of activation energy from thermal degradation reaction

without knowing the order of a reaction and is a rela-

tively simple method [16]. Zhao et al. [17] studied the

influence of fullerene on the kinetics of thermal and

thermo-oxidative degradation of high-density polyethyl-

ene by the OFW method. The Vyazovkin (V) method

[18, 19] represents an advanced non-linear isoconver-

sional method that removes the disadvantages associated

with OFW method and provides an accurate measurement

of activation energy. This method was utilized to deter-

mine the effective activation energy of non-isothermal

crystallization of the polymer melts [20].

This study will be focused on the preparation of wood

polymer composite by impregnation of MFFA copolymer,

1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU), a

crosslinking agent, plant polymer collected as a gum from

a local plant Moringa oleifera as a FR agent and nano-

clay. The use of clay in polymer composite is reported to

enhance the various properties. Keeping in view those

above, this work has been undertaken to study the effect

of plant polymer on dimensional, mechanical properties,

and kinetic parameters of the thermal decomposition of

wood.

Methods

Materials

Fig wood (Ficus hispida) and the gum from the plant

Moringa oleifera were collected locally. Melamine, furfuryl

alcohol, glyoxal, and formaldehyde were purchased from

Merck (Mumbai, India). Maleic anhydride was obtained

from G.S. Chemical Testing Lab. & Allied Industries (India).

Nanoclay (clay modified by 15–35 mass% octadecylamine

and 0.5–5 mass% aminopropyltriethoxy silane, Sigma–

Aldrich, USA) was used as such received. All other chemi-

cals used were of analytical grade.

Dispersion of nanoclay and plant polymer in MFFA

copolymer

MFFA copolymer and DMDHEU were synthesized fol-

lowing the procedure of our previous work [21]. Nanoclay

was swelled in FA-water mixture for 24 h with mechanical

stirring. FA-water mixture can swell the nanoclay and is a

good solvent for the MFFA copolymer. The dispersed

nanoclay was then sonicated for 30 min. Now MFFA was

slowly added to the dispersed nanoclay under stirring

condition. This mixture was further sonicated for 15 min.

To this mixture, plant polymer dissolved in DMF-water

was added and kept ready for use.

Preparation of WPC

All the samples were oven dried at 381 K to constant mass

before treatment. Dimensions and masses were measured.

The samples were then taken in an impregnation chamber.

Loads were applied over each sample to prevent them from

floatation during addition of impregnating mixture. Vacuum

was applied for a specific time period for removing the air

from the pores of the wood samples before addition of pre-

polymeric mixture. Now, the MFFA copolymer with the

plant polymer and maleic anhydride or MFFA copolymer

with the plant polymer, DMDHEU and maleic anhydride or

MFFA copolymer with nanoclay, DMDHEU, plant polymer

and maleic anhydride was then introduced through a drop-

ping funnel. The samples were then kept immersed in the

impregnation chamber for 6 h after attaining atmospheric

pressure. After that samples were taken out of the chamber

and excess chemicals were wiped from surfaces of the pre-

pared composites. The samples were then wrapped in alu-

minum foil and cured at 366 K for 24 h in an oven. This was

followed by drying at 381 K for another 24 h. To remove

homopolymers, if any, formed during impregnation, the

cured samples were then Soxhlet extracted using a mixture of

solvents acetone and ethanol (1:1 molar ratio) for 24 h. The

dimensions were remeasured by using slide caliper, and

1680 A. Hazarika, T. K. Maji

123

masses were taken. The abbreviations used for samples are

listed in Table 1.

Measurements

Mass percent gain (MPG) after polymer loading was cal-

culated according to the formula

MPG % ¼ M2 �M1ð Þ=M1 � 100 ð1Þ

where M1 is the oven dry mass of wood blocks before

polymer treatment and M2 is the oven dry mass of blocks

after polymer treatment.

Percentage volume increase after impregnation of wood

samples was calculated by the formula:

Volume increase % ¼ V2 � V1ð Þ=V1 � 100 ð2Þ

where V1 is the oven dry volume of the untreated wood and

V2 is the oven dry volume of the treated wood.

The hardness of the samples was measured by using a

durometer (model RR12) according to ASTM D2240

method and expressed as shore D hardness.

The treated and untreated samples were ground and FTIR

spectra were recorded by using KBr pellet in a Nicolet

(Madison, USA) FTIR Impact 410 spectrophotometer.

The crystallographic studies were done by XRD analysis

using Rigaku X-ray diffractometer (Miniflax, UK) and

employing CuKa radiation (k = 0.154 nm), at a scanning

rate of 2o min-1 with an angle ranging from 2o to 55o.

Transmission electron microscopy (TEM) was per-

formed to study the dispersion of silicate layers. The ultra-

thin sections of the samples were then mounted on grids

and examined with a JEOL JEM-2100 transmission elec-

tron microscope at an accelerating voltage of 80 kV.

The morphologies of untreated and treated wood sam-

ples were studied by using (JEOL JSM-6390LV) scanning

electron microscope at an accelerated voltage of 5–15 kV.

The fractured surface of the samples was used for the

study. These were sputtered with platinum and deposited

on brass holder.

The flexural strength of the samples was measured by

UTM-HOUNSEFIELD, England (model H100 K–S) with

a cross head speed of 3.33 9 10-5 ms-1 and by calculating

the MOE and modulus of rupture (MOR) according to

ASTM D-790 method.

MOR was calculated as follows:

MOR ¼ 3WL=2bd2 ð3Þ

Five samples of each kind were used for measuring the

flexural and tensile properties and their average values are

reported. All the data are expressed as mean ± SD. Results

were statistically analyzed using ANOVA followed by

Turkey HSD test.

Both untreated and treated wood samples were

immersed in distilled water at room temperature (306 K)

and masses were taken after 0.5, 2, 6, 24, 48, 96,120, 144,

and 168 h, expressed as

Water uptake % ¼ Mt �Mdð Þ=Md � 100 ð4Þ

where Md is the oven dry mass and Mt is the mass after

immersion in distilled water for a specified time period.

Dimensions of the oven-dried samples were measured

and conditioned at room temperature (306 K) and 30 %

relative humidity (RH). The samples were placed in dis-

tilled water and the dimensions were remeasured after 0.5,

2, 6, 24, 48, 96, 120, 144, and 168 h. Swelling was con-

sidered as a change in volume and expressed as the per-

centage of volume increase compared to oven-dried

samples.

% Swelling ¼ Vt;u � Vo

� �=Vo � 100 ð5Þ

where Vt,u is the volume of the untreated or treated wood

after water absorption and Vo is the volume of the untreated

or treated wood before water absorption.

Samples were cut into the dimensions of

(10 9 1 9 0.5) cm3 for limiting oxygen index (LOI) test

by using flammability tester (S.C. Dey Co., Kolkata)

according to ASTM D-2,863 method. The sample was

placed vertically in the sample holder of the LOI apparatus.

The ratio of nitrogen and oxygen at which the sample

continued to burn for at least 30 s was noted.

LOI ¼ Volume of O2=Volume of O2 þ N2ð Þ � 100

ð6Þ

Thermal properties of WPCs were measured in a TG

(TGA-50, Shimadzu) at four different heating rates of 3, 5,

10, and 20 K min-1 up to 876 K under nitrogen

atmosphere.

The possible kinetic mechanism can be analyzed by

finding a correlation of activation energy with conversion

in a thermal degradation process. Different isoconversional

methods like the integral method proposed by Ozawa and

Flynn Wall [22, 23] as well as advanced isoconversional

Table 1 Formulation of mixture of samples and their notations

Sample formulation Notation

Untreated wood UW

Wood treated with MFFA/plant polymer(3 phr) W/M/P3

Wood treated with MFFA/plant polymer(3 phr)/

DMDHEU

W/M/P3/D

Wood treated with MFFA/plant polymer(1 phr)/

DMDHEU/nanoclay

W/M/P1/D/N

Wood treated with MFFA/plant polymer(2 phr)/

DMDHEU/nanoclay

W/M/P2/D/N

Wood treated with MFFA/plant polymer(3 phr)/

DMDHEU/nanoclay

W/M/P3/D/N

Study of wood polymer nanocomposite 1681

123

methods [19, 20] were available in the literature. The

activation energy can be studied from the degree conver-

sion rate. The degree of conversion of decomposed fraction

(a) can be calculated using the following equation.

a ¼ Mo �Mtð Þ= Mo �Mfð Þ ð7Þ

where Mo, Mt, and Mf are initial mass at time t and final

mass of the sample, respectively.

The reaction rate in non-isothermal decomposition

kinetics is commonly described by the equation

da=dt ¼ Aexp �Ea=RTð Þ 1� að Þn ð8Þ

where A is the pre-exponential factor, Ea is the activation

energy of thermal decomposition, R is the gas constant

(kJ mol–1), b is the heating rate, T is the absolute temper-

ature (K), and n is the reaction order.

For a linear heating program with constant heating rate,

b = dT/dt, equation becomes

da= 1� að Þn¼ A=bexp �Ea=RTð Þ ð9Þ

The method of Flynn-Wall and Ozawa is an integral

method [22, 23]. Integrating the Eq. (9)

Za

0

da= 1� að Þn¼ A=bZT1

T0

exp �Ea=RTð Þ ð10Þ

If, F(a) = da/(1–a)n, y = Ea/RT then,

F að Þ ¼ AEa=bR P yð Þ ð11Þ

The values of P(y) are calculated for the normal range of

experimental values 10 \ Ea/RT \ 30.

When Ea/RT [ 20, a linear approximation is made

logP yð Þ � 2:315� 0:457y ð12Þ

Substituting Eq. (12) into Eq. (11)

logb ¼ AE=RF að Þ � 2:315� 0:457Ea=RT

The activation energy (Ea) was evaluated from the slope of

the graph between log b and 1/T for a selected fraction of

the thermal decomposition. The range of the selected

fraction was from 0.1 to 0.7 and the values for the

activation energy of each fraction were compared. Using

data obtained at several heating rates, a graph of mass loss

versus temperature is plotted for finding the activation

energy. The integral isoconversion OFW method led to

systematic errors when the value of Ea varies with a [22].

To avoid these errors, Vyazovkin developed [20] an

advanced isoconversional method which was applied to

the data obtained at arbitrary heating programs, T(t) and

was direct numerical integration of Eq. 10. In this method,

a possible variation of activation energy was taken into

consideration. A set of experiments were carried out at

different arbitrary heating programs T(t), and the activation

energy was evaluated by finding the value of Ea at any

particular value of a which minimizes the function

U Eað Þ ¼Xn

i¼1

Xn

j6¼i

J Ea; Ti tað Þ½ �=J Ea; Tj tað Þ� �

ð13Þ

In the Eq. (13), the integral

J Ea; Ti tað Þ½ � ¼Zta

ta�Da

exp �Ea=RTi tð Þ½ �dt ð14Þ

was determined numerically by using trapezoid rule. Here

i and j, denote thermal measurements with different tem-

perature programs, Da, an increase of conversion, was

varied from Da to 1 - Da typically set as 0.02 enough to

remove accumulative errors in Ea calculation. A set of

experiments were carried out according to varied temper-

ature programs to generate sufficient conversion–tempera-

ture (a-T(t)) data, and then Ea was calculated at any

particular value of a by identifying a suitable Ea value

which satisfies Eq. 13. The minimization procedure was

repeated for each value of a to find the dependence of the

activation energy on the extent of conversion.

Results and discussion

Optimum properties are obtained by varying various

parameters like vacuum, time of impregnation, monomer

concentration, initiator concentration, amount of crosslink-

ing agent, nanoclay, and plant polymer. The conditions to get

maximum improvement of properties were 500 mm Hg

vacuum, 6 h time of impregnation, 5:1 (MFFA:FA-water)

prepolymer concentration, 1 % (w/w) maleic anhydride,

3 mL DMDHEU, 3 phr nanoclay, and 1.0–3.0 % w/v plant

polymer.

Effect of variation of plant polymer Moringa oleifera

on polymer loading (MPG %), volume increase,

and hardness

It was observed from Table 2 that MPG % and volume

increase % of wood occurred due to impregnation of

polymer. MFFA prepolymer and plant polymer filled up

the void spaces in wood and hence an improvement in

properties was noticed. The properties improved further

when DMDHEU and nanoclay were added to the pre-

polymer. The addition of DMDHEU facilitated the for-

mation of crosslinked structure between wood and polymer

through its hydroxyl groups [24]. Further the mobility of

polymer chains was restricted due to intercalation of

polymer chains between the silicate layers [25]. The higher

the plant polymer, the higher was the improvement in

1682 A. Hazarika, T. K. Maji

123

properties. This might be due to the increase in interaction

among wood, polymer, DMDHEU, nanoclay, and plant

polymer. Plant polymer had hydroxyl groups and could

interact with wood/MFFA prepolymer, DMDHEU, and

nanoclay which resulted in an overall improvement in

properties.

FTIR study

The FTIR spectra of UW, MFFA, plant polymer, DMDHEU,

nanoclay are represented in Fig. 1. UW (curve a) exhibited

bands at 3,449 cm-1 (–OH stretching), 2,925 and 2,848 cm-1

(–CH2 asymmetric stretching), 1,732 cm-1 (C=O stretching),

1,642 for (–OH bending), 1,258 and 1,046 cm-1 (C–O

stretching), and 1,000–646 cm-1 (out of plane C–H bending

vibration). MFFA copolymer (curve b) was characterized by

the bands at 3,408 cm-1 (–OH stretching), 1,568 and

1,509 cm-1 (furan ring vibration), 1,346 cm-1 (N–CH2-

furan ring), 1,188 cm-1 (C–N stretching), and 812 cm-1 (out

plane trisubstitution of triazine ring) [26]. All the components

of plant polymer (L-arabinose, D-galactose, D-glucuronic acid,

L-rhamnose, D-mannose, D-xylose, and leucoanthocyanin)

have abundant hydroxyl groups. The plant polymer (curve c)

showed bands at 3,430 cm-1 (–OH stretching), 2,926 cm-1,

2,857 cm-1 (–CH2 asymmetric and symmetric stretching),

1618 cm-1 (–OH bending), and 1440 cm-1 and 1375 cm-1

(–CH bending). In the spectrum of DMDHEU, (curve d) the

appearance of absorption bands at 3,422, 1,700, 1,248,

1,021 cm-1 was –OH stretching, C=O stretching, –CHOH

stretching, –CH2OH stretching, respectively [27]. Peaks

appeared at 3,468 cm-1 for –OH stretching, 2,930 and

2,858 cm-1 for –CH stretching of modified hydrocarbon,

1,622 cm-1 for –OH bending, 1,033–456 cm-1 for oxide

bands of metals like Si, Al, Mg, etc. in the absorption spectrum

of nanoclay (curve e).

The FTIR spectra of W/M/P3, W/M/P3/D, W/M/P1/D/

N, W/M/P2/D/N, and W/M/P3/D/N are represented in

Fig. 2. The presence of the characteristic peaks of MFFA,

plant polymer, and nanoclay into wood polymer composite

indicated the successful impregnation of material into

wood (curve a-d). Further a decrease in intensity of

hydroxyl peak and shifting of the peaks occurred to

3,431 cm-1(curve a), 3,372 cm-1 (curve b), 3,292 cm-1

(curve c), 3,288 cm-1 (curve d), 3,254 cm-1 (curve e)

from 3,431 cm-1 (for UW) were observed. This indicated

the participation of hydroxyl groups of wood in bond for-

mation with MFFA, plant polymer, DMDHEU, and nano-

clay. Moreover, the peaks at 2,927, and 2,857 cm-1 (–CH2

asymmetric stretching) were more pronounced in treated

wood samples than UW suggesting an enhancement in

interaction. Similar decrease in hydroxyl peak intensity and

shifting to lower wavenumber was reported by Deka and

Maji [28] while studying the FTIR analysis WPC.

XRD study

The X-ray diffraction patterns of nanoclay, UW, and

treated wood samples are represented in Fig. 3. The

organically modified nanoclay (curve a) showed a dif-

fraction peak at 2h = 4.30o. The gallery distance was

calculated using Bragg’s equation and found to be

2.05 nm. UW samples (curve b) showed a broad diffraction

peak near 22.94o of 2h due to the (002) crystal plane of

cellulose present in wood. Appearance of small crests at

37.77o and 15.04o was assigned to (040) crystal plane of

Table 2 Effect of variation of plant polymer on mass % gain

(MPG %), volume increase, and hardness

Samples

particulars

Mass % gain

(MPG/%)

Volume

increase/%

Hardness

(Shore D)

UW – 46 (±1.07)

Samples treated with M/FA–H2O/P/D/N

100/20/3/0/0 29.43 (±0.38) 2.16 (±0.31) 59 (±1.08)

100/20/3/3/0 32.44 (±0.65) 2.23 (±0.43) 64 (±0.76)

100/20/1/3/3 43.68 (±0.28) 2.87 (±0.41) 73 (±1.04)

100/20/2/3/3 45.92 (±0.56) 2.92 (±1.12) 76 (±0.76)

100/20/3/3/3 47.44 (±0.82) 3.01 (±1.09) 77 (±0.67)

3500 3000 2500 2000 1500 1000 500

Wavenumber/cm–1

% T

rans

mitt

ance

/a.u

.

1000–646 cm–1

1033–456 cm–1

1258 cm–1

(a)

(b)

1732 cm–1

2848 cm–1

2925 cm–1

3449 cm–1

1642 cm–11188 cm–1

812 cm–1

1346 cm–1

1509 cm–11568 cm–1

1440 cm–1

1375 cm–1

1248 cm–11021 cm–1

1622 cm–12930 cm–1

2858 cm–13468

cm

–134

22 c

m–1

3408

cm

–1

1618 cm–12926 cm–12857 cm–1

3430 cm–1

(c)

(d)

(e)

Fig. 1 FTIR spectra of: a UW, b MFFA, c plant polymer, d DMD-

HEU, and e nanoclay

Study of wood polymer nanocomposite 1683

123

cellulose and amorphous region of cellulose, respectively

[29]. The crystalline peak of wood appeared at

2h = 22.94o was found to broadened slightly and shifted to

22.89o (curve c) and 22.87o (curve d) due to treatment with

polymer. W/M/P/D/N showed a further decrease in crys-

tallinity peak intensity and shifting to lower angle. With the

increase in the amount of plant polymer, the peak intensity

decreased and further shifted to 22.86o (curve e), 22.82o

(curve f), and 22.79o (curve g). The peaks at 15.04o and

37.77o for UW were become dull in the prepared com-

posites (curve c–g). The diffraction peak for the nanomer

also disappeared which might be either due to the delam-

ination of the nanomer layer or the full expansion of the

nanoclay gallery layers which was not possible to detect by

XRD [30]. Therefore it could be concluded that the crys-

tallinity in wood decreased and some nanomers, MFFA

polymer and plant polymer, were introduced into the

amorphous region of wood cellulose (curve c–g).

TEM study

The TEM micrographs of (a) W/M/P3 and (b) W/M/P3/D/

N are represented in Fig. 4. A homogenous dispersion of

clay (shown as dark slices) was observed in W/M/P3/D/N,

which was not observed in case of W/M/P3.

Morphological studies

Scanning electron micrographs of the fractured surfaces of

the UW and treated wood samples are shown in Fig. 5. The

empty pits and parenchymas present in UW (Fig. 5a) were

filled by the MFFA and plant polymer as shown in W/M/P3

(Fig. 5b). In the case of W/M/P/D/N some white patches of

nanoclay were seen indicating the successful impregnation

of nanoclay into the wood (Fig. 5c–e).

Mechanical properties

Table 3 shows the tensile and flexural values of UW and

treated wood samples. The porous structure of wood was

filled up by the copolymer due to impregnation with MFFA

and the plant polymer. W/M/P/D/N showed better proper-

ties than those of either UW or W/M/P3 or W/M/P3/D. The

hydroxyl groups of the plant polymer could interact with

the hydroxyl groups of wood and prepolymer resulting in

improved tensile and flexural values. Samples treated with

W/M/P/D/N showed better properties than the W/M/P3.

DMDHEU could further enhance the interaction among

wood, prepolymer, and plant polymer by forming a

crosslinked structure through its hydroxyl groups [24]. In

the case of nanoclay-treated samples, the mobility of the

polymer chains that were intercalated between the silicate

layers became restricted. Therefore, crosslinking along

with restriction in the mobility of the polymer chains

played a role in enhancing both the tensile and flexural

values of the nanoclay-treated wood samples. With

increase in the amount of plant polymer, further increase in

the values was observed due to enhancement in interfacial

adhesion among the wood, polymer, crosslinker, and

nanomer.

3500 3000 2500 2000 1500 1000 500

Wavenumber/cm–1

% T

rans

mitt

ance

/a.u

.

3254 cm–1

3288 cm–1

3292 cm–1

3372 cm–1

2927 cm–12857 cm–1

3431 cm–1(a)

(b)

(c)

(d)

(e)

Fig. 2 FTIR spectra of: a W/M/P3, b W/M/P3/D, c W/M/P1/D/N,

d W/M/P2/D/N, and e W/M/P3/D/N

10 20 30 402θ/°

22.79°

22.82°

22.86°

22.87°

22.89°

22.94°15.04° 37.77°

4.30°

Cou

nts/

a.u.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Fig. 3 X-ray diffraction of: a nanoclay, b UW, c W/M/P3, d W/M/

P3/D, e W/M/P1/D/N, f W/M/P2/D/N, and g W/M/P3/D/N

1684 A. Hazarika, T. K. Maji

123

Water uptake and volumetric swelling test

The water absorption capacity and volumetric swelling of

UW and treated wood samples are represented in Figs. 6

and 7. The void spaces in UW have rendered it to dem-

onstrate highest water uptake capacity and volumetric

swelling (curves 6a and 7a). With the increase in time of

immersion, an increasing trend in water uptake capacity

was found. Deposition of MFFA/plant polymer into the

capillaries and pores of wood would lessen its water uptake

capacity and volumetric swelling (curves 6b and 7b).

DMDHEU would crosslink with the wood and polymer

leading to a further reduction in water uptake capacity

(curves 6c and 7c). W/M/P/D/N showed the least water

uptake capacity (curves 6f and 7f). The silicate layers

provided a tortuous path for the transmission of water

molecules (curves 6d and 7d) [31]. With the increase in the

amount of plant polymer the absorption capacity as well as

volumetric swelling was found to decrease (curves 6d–6f

and 7d–7f). The plant polymer contains L-arabinose, D-

galactose, D-glucuronic acid, L-rhamnose, D-mannose, D-

xylose, and leucoanthocyanin. All the constituents have

hydroxyl groups capable of interacting with the wood,

polymer, crosslinker, and nanoclay thereby decreasing its

water uptake capacity.

LOI study

Table 4 shows the LOI values of treated and UW. The high

LOI value of the W/M/P3 was due to the synergistic effect

Fig. 4 Transmission electron

micrographs of: a W/M/P3 and

b W/M/P3/D/N

Fig. 5 Scanning electron micrographs of wood: a UW, b W/M/P3, c W/M/P1/D/N, d W/M/P2/D/N, and e W/M/P3/D/N

Study of wood polymer nanocomposite 1685

123

of nitrogen in melamine of the MFFA copolymer and

phosphorus present in the plant polymer. The displacement

of oxygen present on the surface of the sample by the

oxides of nitrogen and phosphorus released during com-

bustion was supposed to be the mechanism of fire control.

Addition of DMDHEU and nanoclay further enhanced its

flame retardancy. DMDHEU acted as a crosslinker and

could also provide nitrogen. Clay layers also provided a

barrier by promoting char formation. Char helps in

decreasing the rate of mass loss during thermal decompo-

sition and it shields the sample from burning thereby

improving its flame resistance property [32]. LOI value was

further improved on addition of plant polymer. The pre-

sence of phosphorus in the plant polymer and the enhanced

interaction among wood, MFFA, and DMDHEU caused by

the hydroxyl groups of plant polymer were responsible for

exhibiting improved LOI value. The fire retardancy of

biodegradable film/rubber was found to improve due to

treatment with plant gum [12, 13].

Thermal study

The TG and DTG curves of UW and treated wood samples

are represented in Figs. 8 and 9. Initial degradation

temperature (Ti), maximum pyrolysis temperature (Tm),

decomposition temperature at different mass loss (%) (Td)

and residual mass (RW %) are shown in Table 5. The mass

loss observed below 376 K in both the UW and treated

wood samples was due to moisture loss. Treated wood

samples had higher Ti and Tm values compared to the

untreated wood samples. MFFA along with the plant

polymer increased the thermal stability of wood. A further

increase in Ti values was observed in W/M/P3/D/N. This

was due to the synergistic effect of crosslinker, nanoclay,

and plant polymer. DMDHEU improved the interfacial

adhesion between wood and polymer. The silicate layers of

nanoclay provided an obstruction to the passage

of decomposed volatile products throughout the composite

[33]. The plant polymer contains phosphorus (4.34 %,

w/w) and as a result the thermal stability of the composites

enhanced further [13]. With the increase in the amount of

the plant polymer, the thermal stability of the prepared

composites improved.

Table 3 Flexural and tensile properties of untreated and treated wood

Sample Flexural properties Tensile properties

Strength/MPa Modulus/MPa Strength/MPa Modulus/MPa

UW 120.56 (±2.90) 6047.45 (±3.15) 41.50 (±0.90) 307.83 (±11.60)

W/M/P3 129.57 (±0.83) 6539.57 (±2.98) 50.50 (±0.98) 377.39 (±11.14)

W/M/P3/D 131.90 (±1.8) 6674.28 (±2.96) 53.67 (±4.27) 399.57 (±1.45)

W/M/P1/D/N 140.68 (±3.09) 7097.54 (±3.11) 67.29 (±1.51) 500.69 (±9.60)

W/M/P2/D/N 143.53 (±0.85) 7209.52 (±3.97) 69.55 (±1.55) 518.60 (±8.76)

W/M/P3/D/N 145.59 (±1.60) 7315.49 (±3.69) 70.66 (±1.62) 526.55 (±6.88)

160

140

120

100

80

60

40

20

0

0 20 40 60 80 100 120 140 160

Time/h

Wat

er u

ptak

e/%

(a)

(b)

(c)

(d)

(e)(f)

Fig. 6 Water absorption test of wood: a UW, b W/M/P3, c W/M/P3/

D, d W/M/P1/D/N, e W/M/P2/D/N, and f W/M/P3/D/N

25

20

5

0

5

00 20 40 60 80 100 120 140 160

Time/h

Vol

umet

ric

swel

ling/

%

(a)

(b)

(c)

(d)(e)(f)

Fig. 7 Volumetric swelling in water at 306 K of wood: a UW, b W/

M/P3, c W/M/P3/D, d W/M/P1/D/N, e W/M/P2/D/N, and f W/M/P3/

D/N

1686 A. Hazarika, T. K. Maji

123

Tm values for the first stage of untreated and treated

wood samples might be due to the depolymerisation of

hemicellulose, glycosidic linkage of cellulose, thermal

decomposition of cellulose [34] while the second stage was

due to the degradation of MFFA copolymer. DMDHEU,

nanoclay, and plant polymer improved the Tm values of the

composites.

UW had the highest RW value due to the high ash

content. RW value decreased when samples were treated

with MFFA and plant polymer. Addition of nanoclay

would increase its value again.

Activation energy of thermal decomposition from TG

TG was performed at four different heating rates of 3, 5,

10, and 20 K min-1 in a nitrogen atmosphere to find out

the kinetic parameters of the composites, such as the

activation energy. The higher the heating rate, the higher

was the decomposition temperature (Td). Td was deter-

mined where the mass loss started to raise. This indicated

that higher heating rate improved the thermal stability of

the prepared composites [35, 36].

At 10 % of thermal degradation region (a = 0.1), the

temperature of decomposition was determined at the four

different heating rates. Similarly, the temperatures at various

values of a were determined at different heating rates for all

samples. The temperatures at a = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6,

and 0.7, respectively, were obtained for the UW, W/M/P3,

W/M/P1/D/N, W/M/P2/D/N, and W/M/P3/D/N. The values

of a were selected between 0.1 and 0.7 and b values from 3 to

20 K min-1 to obtain the average activation energy. Log bwas calculated and was plotted as log b vs 1/T as shown in

Fig. 10a–e. The activation energy was obtained from the

slope of this plot. It was observed that linear fittings were

more closely spaced in the W/M/P/D/N compared to those of

UW and W/M/P3. Moreover the linear fittings were more

closely spaced with the increase in the amount of plant

polymer. This indicated that the thermal stability and the

decomposition temperature were controlled by the plant

polymer. Kim et al. [37] observed that thermal stability and

decomposition temperature decreased on increasing rice

husk content in rice husk flour filled thermoplastic com-

posite. They reported that linear fittings were broadly dis-

tributed with increase in the rice husk content.

The activation energy of the prepared composites is

represented in Fig. 11. In all the cases the activation energy

Table 4 Limiting oxygen indices (LOI) and flaming characteristics of treated and untreated samples

Samples LOI/% Flame description Smokes and fumes Char

UW 19 Candle-like localized flame – Little

W/M/P3 26 Small localized flame Small and black smoke Medium

W/M/P3/D 28 Small localized flame Small and black smoke Medium

W/M/P1/D/N 31 Small localized flame Small and black smoke Higher

W/M/P2/D/N 34 Small localized flame Small and black smoke Higher

W/M/P3/D/N 36 Small localized flame Small and black smoke Higher

100

80

60

40

20

0473 673 873

Temperature/K

Mas

s/%

(a)

(b)(c)(d)

(e)

Fig. 8 Thermogravimetric curves of: a W/M/P3/D/N, b W/M/P2/D/

N, c W/M/P1/D/N, d W/M/P3, and e UW

0.0 373 473 573 673 773 873

Temperature/K

(a)

(b)

(c)

(d)

(e)

DT

G/a

.u.

Fig. 9 DTG curves of: a UW, b W/M/P3, c W/M/P1/D/N, d W/M/

P2/D/N, and e W/M/P3/D/N

Study of wood polymer nanocomposite 1687

123

decreased steeply up to a = 0.3 and then it became con-

stant. W/M/P/D/N had higher activation energy than the

UW and the W/M/P3. Higher the amount of plant polymer

higher was the activation energy of the composites. The

energy barrier, which prevented the polymer chain move-

ment, was associated with the activation energy. The

interfacial adhesion determined the interaction among the

wood, polymers, crosslinker, and nanomer. The plant

Table 5 Thermal degradation of untreated and treated wood samples

Sample Ti Tma Tm

b Temperature of decomposition (Td) in K at different mass loss/% RW/% at 873 K

20 % 40 % 60 % 80 %

UW 435 578 665 537 571 602 – 26.12

W/M/P 3 505 605 700 567 594 630 679 7.2

W/M/P1/D/N 533 625 717 600 624 651 723 17.4

W/M/P2/D/N 540 630 722 607 630 656 729 18.0

W/M/P3/D/N 546 636 728 613 635 663 734 19.1

1.6

1.4

1.2

1.0

0.8

0.61.4 1.5 1.6 1.7 1.8 1.9

log

β

1.6

1.4

1.2

1.0

0.8

0.6

1.4

1.2

1.0

0.8

0.6

log

β

log

β

1.6

1.4

1.2

1.0

0.8

0.6

log

β

1.6

1.4

1.2

1.0

0.8

0.6

log

β

α0.70.60.50.40.30.20.1

1.5 1.6 1.7 1.51.4 1.6 1.7

1/T × 10–3/K–1 1/T × 10–3/K–1

1.51.4 1.6 1.7

1/T × 10–3/K–11.5 1.6 1.7 1.8 1.9

1/T × 10–3/K–1

1/T × 10–3/K–1

(a)

(b) (c)

(d) (e)

Fig. 10 Isoconversion curves

of: a UW, b W/M/P3, c W/M/

P1/D/N, d W/M/P2/D/N, and

e W/M/P3/D/N

1688 A. Hazarika, T. K. Maji

123

polymer had abundant hydroxyl groups which resulted in

an increase in the interaction among the wood, MFFA,

DMDHEU, and nanomer. Hence it showed higher activa-

tion energy.

Ea–a dependence

The variation of activation energy Ea with conversion arelates to the contributions of parallel reaction channels to

overall reaction kinetics and the change of reaction

mechanisms [38]. From the conversion values, a is plotted

as a function of T(t) for the different heating rates as shown

in Fig. 12. The conversional curves shifted to higher tem-

perature with the increase in the heating rate suggesting

that the reaction rate was a rising function of the temper-

ature. The a–T(t) values obtained from the graph were put

in the Eqs. 13 and 14. Ea was evaluated for each value of a

4.2

3.5

2.8

2.1

1.4

0.15 0.30 0.45 0.60 0.75

Conversion rate/α

Act

ivat

ion

ener

gy ×

105

/J m

ol–1

(a)

(b)(c)

(d)

(e)

Fig. 11 Activation energy of thermal decomposition according to

OFW method for wood samples treated with: a W/M/P3/D/N, b W/

M/P2/D/N, c W/M/P1/D/N, d W/M/P3, and e UW

100

80

60

40

20

0473 523 573 623 673

Temperature/K

Temperature/K Temperature/K513 553 593 633 673 713 573 593 613 633 653 673 693 713

Temperature/K

573 593 613 633 653 673 693 713Temperature/K

573 593 613 633 653 673 693 713

α/%

100

80

60

40

20

0

α/%

100

80

60

40

20

0

α/%

100

80

60

40

20

0

α/%

100

80

60

40

20

0

α/%

3 K min–1

5 K min–1

10 K min–1

20 K min–1

(a)

(b) (c)

(d) (e)

Fig. 12 Variation of

conversion a with T at heating

rates: a UW, b W/M/P3, c W/

M/P1/D/N, d W/M/P2/D/N, and

e W/M/P3/D/N

Study of wood polymer nanocomposite 1689

123

by minimizing Eqs. 13. Figure 13 shows the plot of Ea

versus a which indicated that the non-isothermal reaction

followed multi-step mechanisms as Ea varies to a great

extent with a [39]. The curves followed a similar trend as

that obtained from OFW method but Ea-dependencies

obtained evidently showed a systematic difference between

OFW and Vyazovkin methods. The treated samples

showed higher Ea values than the untreated ones. The

explanation was similar to that of described earlier.

Conclusions

Chemical modification of soft wood was done by MFFA,

plant polymer, DMDHEU, and nanomer. FTIR and XRD

study confirmed the incorporation of polymers into the

wood composite. SEM and TEM study indicated the pre-

sence of polymers and nanomer in the cell lumen or cell

wall of wood. Maximum improvement in properties was

observed in case of W/M/P3/D/N. A notable enhancement

in properties such as mass percent gain %, hardness,

dimensional stability, mechanical properties, and reduced

water uptake % were observed for the treated wood sam-

ples. The incorporation of plant polymer into the wood had

a remarkable influence on the thermal stability and flam-

mability of the composites. The activation energy of all

samples was determined by using Ozawa–Flynn–Wall’s

and Vyazovkin methods. Both the methods showed similar

trends in respect to activation energy but the results of

apparent activation energy differ significantly because of

the presence of systematic error in OFW method. With the

increase in the amount of plant polymer, there was an

increase in the interfacial interaction among the polymer,

crosslinker, and nanoclay as it could interact through its

abundant hydroxyl groups. The better dispersion and

enhanced interaction resulted in higher activation energy of

the prepared composites.

Acknowledgements University grant commission (UGC) is

acknowledged for financial support in the form of institutional fel-

lowship to one of the authors (AH).

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