pine needle/isocyanate composites: dimensional stability, biological resistance, flammability, and...

Pine Needle/Isocyanate Composites: DimensionalStability, Biological Resistance, Flammability, andThermoacoustic Characteristics

Monika Chauhan,1 M. Gupta,1 B. Singh,1 A.K. Singh,2 V.K. Gupta21Polymers, Plastics, and Composites Division, CSIR-Central Building Research Institute,Roorkee 247 667, India

2Department of Chemistry, Indian Institute of Technology, Roorkee 247667, India

The pine needle composites using isocyanate prepoly-mer were evaluated for their dimensional stability,flammability characteristics, biological resistance, andthermoacoustic properties. The thickness swelling andlinear expansion of the composite panels affected sig-nificantly under wet conditions. The optimum flamma-bility characteristics of the pine needle fibers wereobtained at the retention of 7.48 kg/m3 urea phosphateon their surfaces. During natural decay, the treatedpine needle composites exhibited 4–8% weight losscompared to 9–13% for the untreated ones. The loss ofinternal bond strength in both the treated anduntreated samples exposed to fungus culture rangedbetween 35 and 60% only. Termites caused �6% lessweight loss than the untreated samples showing theirmoderate resistance behavior. The thermal conductivityand sound transmission loss of samples were 0.136 W/m K and 26.51 dB, respectively, showing their adequateinsulation properties. It is concluded that isocyanatebonded pine needle composites can be suitably usedas panel products in buildings. POLYM. COMPOS.,33:324–335, 2012. ª 2012 Society of Plastics Engineers

INTRODUCTION

Considerable attempts [1–10] have been made on utili-

zation of nonwoody renewable raw materials such as agro

residues [1–3], leaves [4, 5] and stalks of forest plants [6,

7], and natural fibers [8–10] for making alternate building

materials in low cost housing applications. Although these

materials are gaining interest, the challenge is to replace

traditional wood materials that exhibit strength and func-

tional stability during storage and use, yet are susceptible

to environmental degradation [11–14]. Their use is

banned in the outdoor environment due to low resistance

against decay of fungi/termites, dimensionally unstable

under high humidity, and poor flammability characteris-

tics. In order to overcome some of these disadvantages,

several attempts [15–19] have been made to obtain

desired composite products through improvement in the

quality of raw materials (fibers/flakes and type of adhe-

sives), formulation chemistry, and processing parameters

(temperature and pressure). The success of various addi-

tives such as biocides [19, 20] and fire retardants [18, 21,

22] depends on their interaction with active site of fibers,

resistance to leaching in water, and rate of weight loss

through decomposition and evaporation. Product durabil-

ity and serviceability issues will be crucial to the contin-

ued growth and user acceptance of new type nonwoody

fiber-based composites.

In the present study, a systematic research work was

undertaken at the Institute on the utilization of pine nee-

dle as alternative raw material for making composite pan-

els, partitions, door inserts, etc. The pine needles are

available in huge quantity in the Western part of Himala-

yan forests and present potential hazards to forest fire and

destroy flora and fauna. The major issue with the pine

needles is their poor bondability with conventional resin

adhesives [23]. To overcome this, efforts are being made

on pretreatment of the pine needles and the use of isocya-

nate-based adhesives alternative to formaldehyde-based

resins for making dimensionally stable composite panels

[24, 25]. When isocyanate prepolymer (NCO content:

15.4%) will be used as a binder in the composites, it

would polymerize in the presence of moisture existing in

the pine needle fibers and also reacts with surface

hydroxyl groups of pine needles through its NCO groups

to form urethane bonds. Because of polymerization, the

residual NCO content is minimal and does not pose haz-

ards to human health. The physicomechanical properties

of these composites were reported with respect to various

needle treatments (alkali, steam, and alkali–steam combi-

Correspondence to: B. Singh; e-mail: [email protected]

DOI 10.1002/pc.22151

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2012 Society of Plastics Engineers

POLYMER COMPOSITES—-2012

nation), resin contents, and pine needles/wood fibers con-

tents. In earlier studies [4, 23, 26], the properties of pine

needle composites using different types of resin adhesives

such as urea-formaldehyde, phenol formaldehyde, and

polymeric isocyanate-urea formaldehyde combination

have been reported. Synergistic use of pine needle fibers

and other nonwoody fibers promotes helping in making

specification grade composite products [4]. However,

there is no report on performance and durability of the

pine needle composites under hydro/hygrothermal condi-

tions, fire, and biological attacks necessitating better

understanding to develop proven wood substitutes.

In this article, we report performance of the pine nee-

dle composites using isocyanate prepolymer adhesive.

The surface characteristics, thermal stability, and flam-

mability of pine needle fibers were studied as a function

of different urea phosphate concentrations. The proper-

ties of composite panels made with the untreated and

treated pine needle fibers were discussed with respect to

alternate wetting and drying cycles, humidity, natural

decay, termites, and fire. Thermoacoustic properties were

also measured to assess the suitability of composites in

buildings.

EXPERIMENTAL

Materials

The pine needles of 300–380 mm length were col-

lected from the Indian forests (density, 0.22 g/cm3; mois-

ture content, �20%; water absorption, �45%). The nee-

dles were comprised of cellulose (40–43%), hemicellulose

(20–24%), lignin (36–40%), and ash content (2–4%). Aro-

matic polyisocyanate prepolymer was obtained from M/s

Bayer Material Science Pvt. Ltd., India [Desmodur E

23—NCO content, (15.4 6 0.4)%; viscosity, 1800 6 250

mPa.s; density, 1.13 g/cm3]. Commercial grade sodium

hydroxide, urea phosphate, and sodium pentachlorophen-

ate (Na-PCP) were obtained from local market. Arsenic

pentoxide, copper sulfate, and potassium dichromate were

used for making chromated copper arsenate (CCA).

Sample Preparation

Processing of Pine Needles. Pine needles were cut to a

desired length (�30 mm). Thereafter, they were treated

with 2% aqueous sodium hydroxide solution (wt% of nee-

dles) as optimized in earlier work [24]. Subsequently, the

samples were dried and hammer milled to a fiber size of

�2 mm. Figure 1 shows particle size distribution of ham-

mer milled pine needle fibers. The resulting needle fibers

were treated with different concentrations of an aqueous

urea phosphate solutions (10–40 wt%). The required quan-

tity of pine needle fibers was dipped into the treating solu-

tion and stirred well for 60 min at room temperature. The

resulting mass was dried to a constant moisture content.

Preparation of Composite Samples. Composite sam-

ples were prepared with the untreated and treated pine

needle fibers and isocyanate prepolymer (3–7 wt%) on a

hydraulic press at 1408C and 10 MPa pressure for 10 min

retention. Before pressing, the resin was sprayed on nee-

dle fibers and mixed in a blender. Subsequently, the mix

was laid on a silicone paper lined mould in the form of

mats. After application of pressure, the mould was

allowed to cool at room temperature and then demolded

the samples for further work. The composite samples

were impregnated in the solution of 5 wt% CCA (compo-

sition: arsenic pentoxide 12.5%; copper sulfate 37.5%; po-

tassium dichromate 50%) and 5 wt% Na-PCP treatments.

The concentration of these preservative chemicals was

selected from the dosage range prescribed in BIS: 401-

2001 [20]. Weight gain in the sample as the grams of

treating solution absorbed was recorded.

The composite samples containing urea phosphate-

treated pine needle fibers were also prepared as per the

above described procedure.

Methods

Physical Tests. The physical tests such as density, water

absorption, thickness swelling, internal bond strength, and

flexural strength of the samples were measured as per

ASTM D 1037-2006. The density of face and core was

determined by density profiler. The flexural properties of

samples were tested at a cross-head speed of 5 mm/min

and span-to-depth ratio of 16:1. Internal bond strength

was determined by testing the samples in tensile direction

perpendicular to the surface on a Hounsfield material test-

ing machine (H 25 KS) at a cross-head speed of 0.08

mm/mm of thickness/min. The samples were prepared by

fixing 50 mm square board to 50 mm square and 25 mm

FIG. 1. Particle size distribution of the pine needle fibers.

DOI 10.1002/pc POLYMER COMPOSITES—-2012 325

thick aluminum loading blocks with an epoxy adhesive.

The thermal conductivity of composite samples (300 3300 312 mm3) was measured using a guarded hot plate

conductivity apparatus according to BIS: 3346-1990 [27].

Dimensional Stability Test. The samples were sub-

jected to various relative humidity (75% RH, 98% RH,

and 98% RH at 508C) and cold water for 60 days. In

another attempt, durability: alternate wetting/drying test

was performed by immersing the samples vertically in

water at (27 6 2)8C for 4 hr and subsequently subjected

at (38 6 2)8C for 20 hr in an air circulating oven (BIS:

2380-81) [28]. The cycle was repeated 16 times to com-

plete the exposure program. The aged samples were

examined for dimensional changes at a regular interval of

time.

Surface Examination. Surface morphology of the

untreated and treated pine needle fibers was recorded on a

field emission scanning electron microscope (FESEM,

QUANTA 200 F). Prior to examination, the samples were

vacuum coated with a thin film of Au/Pd to render them

conductive. Retention of treatment onto surface of the

pine needle fibers was estimated by an energy dispersive

spectroscopy (EDAX).

The contact angles of untreated and treated pine needle

surfaces were measured using sessile drop technique with

the help of dynamic contact angle analyzer (VCA Optima

XE, AST Products Inc.). Surface energy software (SE-

2500) was used for calculation of critical surface energy

of the untreated and treated pine needle fibers using Zis-

man plots.

Thermogravimetric Analysis. Thermogravimetric anal-

ysis (TGA) of the untreated and urea phosphate treated

samples was carried out on a simultaneous thermal ana-

lyzer (Perkin-Elmer, 6300). The samples were run from

30 to 6008C at a heating rate of 10 8C/min under nitrogen

atmosphere. The weight loss and decomposition tempera-

ture were recorded.

Flammability Tests. Cone calorimeter (FTT Ltd.) was

used to measure flammability characteristics of the

untreated and treated pine needle fibers according to ISO

5660-1: 2002. The needle fibers were consolidated into

100 3 100 3 12 mm3 size using 1 wt% isocyanate pre-

polymer as a mat forming agent. The test run was con-

ducted for 20 min at the heat flux of 50 kW/m2 and nor-

mal duct flow rate of 24 l/sec. Various parameters such as

heat release rate, total smoke release, carbon monoxide

(CO) yield, carbon dioxide (CO2) yield, mass loss rate,

heat of combustion, etc., were recorded.

The fire propagation test on the specimen of size 225

3 225 3 12 mm3 was conducted as per BS EN 476-1981

(part 6). The test run was continued for 20 min duration.

The results of propagation indices were computed from

difference between time–temperature curves of the sam-

ples and reference specimens. The surface spread of the

flame test was carried out on the samples of size 270 3900 3 15 mm3 according to BS EN 476-1981 (part 7).

Based on the extent and rate of the flame spread, categori-

zation of sample for fire class was made. The smoke den-

sity of samples was measured according to ASTM D

2843-2004 under flaming and nonflaming modes. Rate of

burning was determined as per ASTM D 635-2006 and

recorded the time taken from 30% to 70% weight loss in

the samples.

Biological Tests. The natural decay test of samples was

carried out as per ASTM D 2017-2005 using Aspergillusniger strain in the potato dextrose broth. The incubation

of samples was made at 328C for 8 weeks. The surface

morphology, weight loss, and loss of internal bond

strength of samples were measured at the termination of

exposure test.

The termite resistance of samples was tested according

to ASTM D 3345-2008 using Microcerotermus bessonitermite colony. The samples of size 50 3 50 3 9 mm3

were fully buried inside the culture media bottles along

with feeder strips for 10 weeks in the test chamber (hu-

midity: 62–70%; temperature: 288C). The quantity of

food consumed by termites in terms of weight loss was

taken as a basis of assessment.

RESULTS AND DISCUSSION

Effect of Wet Environment

The properties of pine needle composites made with 5

wt% isocyanate prepolymer are given in Table 1. Density

profiles of the samples in the thickness direction indicate

that the samples had varied density distributions because

of their nonuniform compaction. The surface layers were

more compressed than the core layer giving rise to a dif-

ference in faces and core densities during hot pressing.

The screw withdrawal strength and modulus of rupture

satisfy the requirements of commercial specification.

When immersed in water, the samples absorbed 19% and

45% water after 2 hr and 24 hr, respectively. The linear

expansions were 0.19% in the length and 0.27% in width

directions after 2 hr water immersion. The composites

had 40% more internal bond strength than those of the

commercial particle boards.

Figures 2 and 3 show dimensional stability of the com-

posite samples under alternate wetting/drying cycles and

different humidity. On visual inspection, the exposed sam-

ples did not show any sign of damage/delamination at the

end of 16 alternate wetting/drying cycles (4 hr in water

and 20 hr at 388C). As shown in Fig. 2, the thickness

swelling increases continuously upto eight cycles of wet-

ting/drying and then the samples reached the leveling-off

stage. The occurrence of spring back action besides

breaking of resin-fiber bond due to fiber swelling could

326 POLYMER COMPOSITES—-2012 DOI 10.1002/pc

be considered for higher thickness swelling in the hot

pressed composites [29]. When resin content in the sam-

ples was increased from 3 to 7%, the thickness swelling

reduced from 60% to 21.23% only. This indicates that

higher resin content encapsulates the fibers and creates a

stable adhesive bond between fiber and fiber preventing

ingress of moisture at the interface. In addition to this,

the samples were also immersed in cold water (24 hr),

hot water (708C) for 2 hr and boiling water (2 hr) and

dried at 508C upto a constant moisture content. Depend-

ency of thickness swelling was clearly seen on the adhe-

sive bonding. Higher resin content exhibited less swelling

than for the low resin content. Compression sets are seen

to be the main reason besides low density of pine needle

fibers. It is seen that composite samples were more

affected in the hot/boiling water than the cold water due

to combined action of water and temperature. It is also

essential to know the dimensional stability of samples

exposed to different humidity and immersed water for

longer periods (Fig. 3). It was found that at equilibrium

moisture content, linear expansion and thickness swelling

in the samples ranged from 0.61 to 0.93% and 14.85 to

18%, respectively, under humid conditions. Contrary to

this, in immersed water, linear expansion in the samples

was significantly high (2.5–4.5%). During exposure, linear

expansion increased rapidly at the initial level and leveled

off after 15 days toward equilibrium. At higher resin con-

tent, isocyanate groups of resin masked the hydroxyl

groups of the pine needle fibers, thereby reducing mois-

ture uptake in the samples. Consequently, dimensional

changes in the composite samples decreased. It is noted

that these changes were more in water immersion than

those of the humidity exposure.

Biological Durability

On visual inspection, the sample was swelled and

showed a discoloration along with black spots spread all

over the surface after 6 months storage at higher humid-

ity. SEM micrographs revealed that active growth was

indicated by silky white patches along with water traces

in the form of black area (Fig. 4b). The moistened sam-

ples were rough and covered with a thread-like fine

branched tubular filaments whereas, the dry samples are

smooth, clean, and free from any foreign inclusions (Fig.

4a). The fungal growth and weight loss of the samples

decrease with the increase of isocyanate resin content

TABLE 1. Properties of the pine needle/isocyanate composites (resin

content: 5 wt%).

Property Average value

Density (g/cm3)

Face 0.90 (0.06)

Core 0.84 (0.04)

Water absorption (%)

2 hr Soaking 19.11 (0.95)

24 hr Soaking 45.44 (2.20)

Linear expansion: 2 hr soaking (%)

Length 0.19 (0.02)

Width 0.27 (0.03)

Thickness swelling: 2 hr water soaking (%) 12.60 (0.60)

Modulus of rupture (MPa) 16.75 (0.84)

Tensile strength perpendicular to surface (MPa) 1.12 (0.06)

Screw withdrawal strength

Face (N) 1270 (65)

Standard deviation is given in parenthesis.

FIG. 2. Thickness swelling of composite samples containing different

resin contents after durability cycle exposure [cycle includes: 4 hr in

water and 20 hr at (38 62)8C].

FIG. 3. Linear expansion of the composite samples containing different

resin contents versus exposure time under various humidity and

immersed water conditions.


probably due to the existence of adequate resin coating

on the fiber surfaces (Table 2).

In order to improve the biological resistance, the com-

posites were treated with CCA and Na-PCP preservatives

with their retention of 23.46 kg/m3 and 8.04 kg/m3 onto

samples. When exposed to fungus culture, the treated

samples exhibited less weight loss than the untreated

samples at the end of 8 weeks supportive of reduced col-

onization of fungi as a result of chromium–lignin com-

plex formation [30] and cellulose–Na-PCP interaction

(Fig. 5). Increasing exposure of the samples in the fun-

gus culture increases their weight loss which is attribut-

able to the growth of fungal colonies possibly due to

deep penetration of cellulolytic enzymes released from

fungal hyphae into the cell wall. It is noted that there

was no fungal growth upto 15 days in the fungus culture.

Upon subsequent exposure, growth of fungus colonies in

the case of the treated samples was 7–11 only compared

to 16–28 numbers for the untreated samples at the end

of 8 weeks. This can be evidenced in the SEM micro-

graphs of the exposed samples (Fig. 6). The untreated

exposed samples were completely surrounded with the

dense fungal hyphae growth along with spores clump

(Fig. 6a). On the other hand, the treated samples showed

only scattered spores all over the surface along with few

hyphae due to retention of treatments in spite of their

leaching in the surrounding media (Fig. 6c and d). As

confirmed by EDAX, chromium and copper elements

retained 43% and 17% in the CCA-treated samples and

sodium and chlorine elements retained 99% and 60% in

the Na-PCP-treated composite samples, respectively. Ar-

senic uptake was believed to be negative in the samples.

It was observed that retention of elements in the case of

Na-PCP-treated samples was more than that of CCA-

treated samples because of their different fixation pattern

on the surfaces of lignocellulosics. As a result, CCA-

FIG. 4. FESEM micrographs of the composite samples showing fungal

infestation (a) fresh, (b) stored under high humidity for 6 months.

TABLE 2. Decay fungi test of the untreated and treated pine needle composites under Aspergillus niger culture for 8 weeks exposure (retention of

treatments: CCA: 23.46 kg/m3, Na-PCP: 8.04 kg/m3).

Sample

Average weight

loss (%)

Fungal colony

(Number)

Internal bond

strength (MPa)

Flexural strength

(MPa)

Indicated Class

(ASTM D 2017–2005)

3 wt% resin –

Untreated 12.81 (0.64) 27.67 (1.66) – 1.64 (0.08) Resistant

CCA treated 8.08 (0.40) 10.67 (0.64) – 1.41 (0.07) Highly resistant

Na-PCP treated 7.35 (0.37) 9.97 (0.59) 1.34 (0.07) Highly resistant

5 wt% resin

Untreated 11.01 (0.55) 21.67 (1.30) 0.86 (0.05) 9.67 (0.48) Resistant

CCA treated 6.58 (0.33) 10.00 (0.60) 0.56 (0.03) 9.38 (0.47) Highly resistant

Na-PCP treated 5.96 (0.29) 9.81 (0.59) 0.34 (0.02) 5.96 (0.30) Highly resistant

7 wt% resin

Untreated 9.98 (0.49) 15.67 (0.94) 1.86 (0.11) 13.49 (0.67) Highly resistant

CCA treated 4.59 (0.23) 7.67 (0.46) 1.07 (0.06) 6.72 (0.34) Highly resistant

Na-PCP treated 4.12 (0.21) 7.11 (0.43) 1.25 (0.08) 6.79 (0.34) Highly resistant


ASTM D 2017 Requirement: average weight loss 0–10%—highly resistant; 11–24%—resistant; 25–44%—moderately resistant; 45% or above—

slightly resistant or nonresistant.


treated samples exhibited more weight loss than the sam-

ples treated with Na-PCP during natural decay test. The

weight loss in the exposed samples can be well corre-

lated with their mechanical properties besides higher

thickness swelling during exposure.

Figure 7 shows split tensile stress–strain curves for

both the untreated and treated composite samples exposed

to fungus culture (5 wt% resin content). The initial slope

of the untreated samples was appreciably higher than the

preservatives-treated samples showing their superior resid-

ual fiber–fiber adhesion. This can also be evidenced in the

form of several klinks in the curve. Compared to

untreated samples, the lowering of split tensile strength

for the treated samples may be considered to the occur-

rence of higher thickness swelling at a time of preserva-

tive treatments and also their interactive involvement with

the resin adhesive. A decrease of �34.88% and 60.47%

in the internal bond strength and 3.02% and 38.36% in

the flexural strength, respectively, was noticed for both

CCA and Na-PCP-treated samples with respect to the

control at the end of 8 weeks. It was noted that CCA-

treated samples retained higher strength than Na-PCP-

treated samples probably due to the less thickness swel-

FIG. 5. Weight loss of the untreated and treated composite samples

exposed under fungus culture (resin content: 5 wt%).

FIG. 6. FESEM images of composite samples under fungus culture: (a, b) untreated, (c) CCA treated, (d) Na-PCP treated.


ling. On the other hand, the retention of internal bond

strength (67.20%) and flexural strength (50.30%) in both

preservative treatments was nearly the same at 7 wt%

resin content. It is expected that the presence of these

chemicals hydrolyzed the bond between the glucose units

and will effectively rupture the microfibrils and creating

shorter chains. In addition, unchelated/free metal ions

may interfere with the mechanical interlocking of isocya-

nate onto pine needle surfaces [30]. It is found that both

treatments showed 4–8% weight loss in the samples cate-

gorizing it under highly resistant category (0–10% weight

loss) as specified in ASTM D 2017-2005.

Termite resistance of the pine needle composite panels

exposed to Microcerotermus bessoni for 10 weeks is

given in Table 3. On visual inspection, tunneling of ter-

mites was observed on the surface and also their penetra-

tion in the samples indicating that termites were active

(Fig. 8). It was observed that termites caused less weight

loss in the CCA-treated samples (21.56%) than the

untreated samples (27.24%). The reduced weight loss

indicated that chromic acid could form a stable ester with

the aromatic rings of lignin which are ingested by the ter-

mites [31]. As a result of feeding, the treated samples

showed more termite mortalities (�25%) than the

untreated ones. When samples were laminated with the

veneer, the subsequent reduction in weight loss of

the treated samples was noticed (18.56%). Rating system

based on visual examination and weight loss showed that

samples are moderately resistant (17–30%, Class III;

ASTM D 3345-2008, Sen Sarma 1975) [32].

Effect of Fire Retardant

Pine needle fibers treated with the urea phosphate was

assessed for their surface topography, thermal stability,

and flammability characteristics. As evidenced in the

SEM micrographs, the surface of untreated needle fibers

was smooth and covered with a thick waxy coating with

obvious striations along the fiber length (Fig. 9a). Con-

trary to this, the surface of treated samples exhibited

rough surface with a thin walled cellular structure prob-

ably due to the loss of lignin and extractives (Fig. 9b).

The exposition of cellular structure and retention of addi-

tives onto surfaces increased with increasing urea phos-

phate concentration. The samples retained 3.76 kg/m3

urea phosphate at 10 wt% concentration and it increased

to 15.11 kg/m3 for 40 wt% concentration. As confirmed

by EDAX, the treated fibers had retained 5–7% nitrogen,

1–4% phosphorus, 62–74% carbon, and 18–26% oxygen

on their surfaces. Whereas, the untreated fibers showed

presence of 3% nitrogen; 0.48% phosphorus; 67% carbon,

and 29% oxygen. As shown in Table 4, higher contact

angle of the treated pine needle fibers over control may

be interpreted as a decrease of hydrophilicity due to

attachment of urea phosphate onto fiber surfaces. The po-

lar component of surface-free energy reduced upto 20

wt% urea phosphate loading and then increased with

increasing treatment concentration. This reduction sup-

ported their higher adhesion to the substrate over the

untreated ones. The critical surface energy of needle

fibers was slightly reduced showing more roughness of

their surface with respect to the control.

TGA of the untreated and treated pine needle fibers is

shown in Fig. 10. As expected, the curves showed several

degradation regions as also evidenced in their derivative

thermograms. The untreated needle fibers exhibit �10%

weight loss upto 1008C whereas the treated needle fibers

had 6% weight loss of the needle remained (Fig. 10a).

The residuals at 5008C for the treated needle fibers were

in the range of 33–38% compared to 29% for the

FIG. 7. Split tensile stress–strain curves (perpendicular to surface) of

the untreated and treated composite samples exposed to fungus culture

after 8 weeks (resin content: 5 wt%). [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]

TABLE 3. Termite resistance of the untreated and CCA treated pine needle composite panels exposed to Microcerotermus bessoni colony for 10

weeks.

Sample

Average weight loss (%) Rating system

Untreated Treated Weight lossa [32] Visualb (ASTM D 3345)

Plain composite panel 27.24 (2.18) 21.56 (1.72) Class III (Moderately resistant) 7 (Moderate attack, penetration)

Laminated composite panel 28.75 (2.30) 18.56 (1.48) Class III (Moderately resistant) 7 (Moderate attack, penetration)

Standard deviation is given in parenthesis.a Sen Sarma et al. 1975, Rating: weight loss 0–6%: Class I; 7–16% Class II; 17–30% Class III; 31–50% Class IV.b ASTM D 3345, Rating 10: sound, surface nibbles permitted; 9: light attack; 7: moderate attack, penetration; 4: heavy.


FIG. 8. Photo view of plain and laminated composite panels exposed to termite colony after 10 weeks. [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]

FIG. 9. FESEM-EDAX micrographs of the untreated and treated pine needle fibers (a) control (b) 20 wt% urea phosphate. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com.]

untreated needle fibers. However, it is noted that char

residue of the treated needle fibers at 6008C was lower

than the untreated ones mainly due to its loss of lignin

content during treatment. The untreated pine needle

fibers show only two peaks at 628C and 3358C in deriva-

tive thermogram curve (DTG) whereas the treated needle

fibers containing different percentage of urea phosphate

show peaks around 748C, 251–2668C, and 287–3208Ctemperature ranges (Fig. 10b). The lowering of decom-

position temperature peak for the treated needle fibers is

believed to be the generation of dehydrated products and

char formation as a result of reactions between the ligno-

cellulosics and urea phosphate [18]. It is expected that

cellulose component of the pine needle fibers was phos-

phorylated to form cellulose ester in the condensed phase

by the reaction between cellulose hydroxyl groups and

phosphoric acid. Subsequently, the release of ammonia

from decomposed urea phosphate reacts with these inter-

mediates to yield cellulose-phosphoramide compounds.

Formation of these compounds inhibits generation of

flammable decomposition products during thermal degra-

dation of cellulose due to synergistic action of nitrogen

and phosphorus. The proposed schematic representation

for fire retardancy of treated pine needle fibers is shown

in scheme 1.

Flammability data obtained from cone calorimeter indi-

cates that increasing fire retardant level decreases flamma-

bility of the pine needle fibers (Table 5). When compared

with the untreated needle fibers, the average heat release

rate of treated needle fibers was reduced by 59% at 20

wt% urea phosphate loading showing effectiveness of the

treatment in reducing initial contribution toward potential

fire growth. The generation of less visible smoke can be

viewed in terms of 71% decrease in specific extinction

area compared to the untreated samples. The time of igni-

tion was reduced by 60% upon treatment. The effective

heat of combustion was drastically reduced from 13.39

mJ/kg to 6.62 mJ/kg indicatives of reduced volatiles

which in turn reflect better fire performance characteris-

tics. The total mass loss rate of the treated samples was

22% lower than for the untreated samples. However, it is

noted that in both cases, the flammable part of the materi-

als was burnt out. The decrease in CO and CO2 yields in

the treated needle fibers may be attributed to the reduced

amount of flammable gases. It is believed that higher

retention level of fire retardant onto pine needles would

result in improved performance of the composite samples.

It is concluded that the treated needle fibers with 20 wt%

urea phosphate exhibited more thermal stability and fire

retardancy than the untreated samples.

The composite panels made from urea phosphate-

treated pine needle fibers (retention on needle fibers: 7.48

kg/m3) were tested for various fire tests. During surface

spread of flame test, the time of spread of flame front

along the longitudinal center-line of the specimen was

almost insignificant due to existence of reaction products

formed during flaming stage of needle fibers. However,

charring of specimen at contact point of igniting flame

TABLE 4. Surface energetic characteristics of the untreated and fire retardant treated pine needle fibers (surface tension of water: 72. 8 mN m21;

surface tension of formamide: 58.20 mN m21).

Urea phosphate

content (wt%)

Contact angle (8) probe liquidCritical surface

energy (mJ/m2)

Surface-free

energy (mJ/m2)

Polar component

(mJ/m2)

Dispersive

component (mJ/m2)Water Formamide

0 76.50 (3.06) 41.40 (1.66) 51.1 (2.04) 50.70 (2.03) 3.10 (0.12) 47.60 (1.90)

10 86.40 (3.46) 46.00 (1.84) 51.1 (2.04) 56.90 (2.28) 0.20 (0.01) 56.70 (2.27)

20 94.30 (3.77) 51.00 (2.04) 50.5 (2.02) 62.00 (2.48) 0.20 (0.01) 61.80 (2.47)

30 102.00 (4.08) 60.10 (2.40) 47.8 (1.91) 58.80 (2.35) 0.90 (0.04) 57.90 (2.32)

40 109.60 (4.38) 65.80 (2.63) 46.6 (1.86) 61.80 (2.47) 2.80 (0.11) 59.00 (2.36)


FIG. 10. Thermogravimetric analysis of the untreated and urea phos-

phate-treated pine needle fibers.


was observed along with cracks onto surfaces. As per

specified criteria of flame spread (limit: 165 6 25 mm),

the sample belongs to Class I category (BS EN 476—Part

7). Under fire propagation test (BS EN 476—Part 6), the

composite sample exhibited reduced fire propagation ini-

tial sub-indices and overall fire propagation index (17.52)

compared to kail wood (fire propagation index: 41.15). It

is mentioned that higher the fire propagation index,

greater is the influence of the product on accelerating the

growth of a fire. The smoke density of composite panels

in the flaming mode was 54.19 dm compared to 228 dm

for the kail wood whereas in nonflaming mode, smoke

density remained 221.27 dm as against to 328.70 dm for

the kail wood. The rate of burning was 2.1%/min only.

The results indicated that the urea phosphate-treated nee-

dle fibers used in composite panels exhibited satisfactory

fire performance.

Thermoacoustic Properties

Thermoacoustic properties of the composite panels are

given in Table 6. The thermal conductivity of samples was

0.136 W/m K at a density of 1.10 g/cm3 which is compara-

ble to the commercial fiber boards/panels. It is mentioned

that high density composite boards have higher thermal

conductivity than the low density composite boards

because of their low interparticle space and voids [33]. As

expected, the thermal conductivity of samples increased

with the rise of temperature. At 108C, the thermal conduc-

tivity of samples was 0.113 W/m K while at 608C, thevalue was found to be 0.158 W/m K. Based on these data,

it was found that thermal resistance of pine needle compos-

ite panels of 25 mm thickness meet commercial specifica-

tion of the insulating materials in buildings. Based on the

specified criteria of thermal resistance (0.23 m2 K/W) men-

tioned in the code [34], it is expected that the composite

Scheme 1. Schematic representation on action of urea phosphate on pine needle fibers during thermal degradation.

TABLE 5. Cone calorimetry results of the untreated and treated pine needle fibers (consolidated with 1 wt% resin).

Urea phosphate

content

Heat release

rate (kW/m2)

Total heat

release (mJ/m2)

Average mass

loss rate (g/s/m2)

Effective heat of

combustion (mJ/kg)

Average specific

extinction area (m2/kg)

CO2 yield

(kg/kg1)

CO yield

(kg/kg1)(wt%)

Retention

(kg/m3)

0 – 86.30 (4.32) 100.5 (5.0) 6.95 (0.35) 13.39 (0.67) 31.10 (1.56) 1.14 (0.06) 0.0117 (0.0006)

10 3.76 67.48 (3.37) 76.0 (3.8) 6.56 (0.33) 10.30 (0.52) 17.86 (0.89) 1.07 (0.05) 0.0083 (0.0004)

20 7.48 35.30 (1.77) 42.0(2.1) 5.44 (0.27) 6.62 (0.33) 9.15 (0.46) 0.85 (0.04) 0.0106 (0.0005)

30 11.44 16.31 (0.82) 14.2 (0.7) 5.62 (0.28) 2.76 (0.14) 66.21 (3.31) 0.24 (0.01) 0.1126 (0.0056)



panels should have a thickness of 25 mm for use at 08Cand 30 mm at 208C for various applications.

The sound transmission loss (STL) of pine needle com-

posite panels is shown in Fig. 11. It was found that STL

value of composite samples increased with sound frequency

upto 1,300 Hz and then slightly decreased before it leveled

off with a further increase of sound frequency. This trend

indicates that some of the sound waves were reflected owing

to increased boards/panels resistance to air flow and

decreased materials porosity. As seen in Figure, the STL

values of the composite panels were affected by their den-

sities. The samples are easily reflected by low frequency

sound. The average value of sound transmission loss was

26.51 dB which is in compliance with the NC-45 curve

[35]. With the appropriate variation of density, the pine nee-

dle composite panels can be suitably prepared for use as

thermoacoustic insulating materials for building uses. It was

also mentioned that pine needle composite boards exhibited

comparable thermal and acoustic properties with the com-

mercially available materials [36] (Table 7).

CONCLUSIONS

Results indicate that pine needle/isocyanate composites

can be used satisfactorily in the category of lignocellulo-

sic panel products. Prior to use, it is necessary to treat

pine needle fibers with chemical additives in composite

panel manufacturing to obtain satisfactory performance.

The panels so prepared with the treated needle fibers

exhibited adequate dimensional stability under wetting/

drying cycles and humid conditions. Fire performance of

the composite panels meets the specified criteria of

National Building Code of India. On the basis of weight

loss, the composite panels belongs to ‘‘Highly Resistant

Class’’ when exposed under natural decay and moderate

resistant toward termites attack. Laminated panels were

more resistant toward biological attacks. Thermoacoustic

behavior of these composite panels supported their suit-

ability as insulating materials for use in buildings. Further

work is underway on use of resin impregnated pine nee-

dle fibers with other lignocellulosic fibers for making lay-

ered composite boards/panels with improved performance.

ACKNOWLEDGMENTS

This article forms part of a Supra Institutional Project

of CSIR R & D programme (Govt. of India) and is pub-

lished with the permission of Director, CSIR-Central

Building Research Institute, Roorkee (India).

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