Effect of unmodified rice straw on the properties of rice straw/polycaprolactone composites

Effect of unmodified rice straw on the properties of ricestraw/polycaprolactone composites

Roshanak Khandanlou • Mansor B. Ahmad •

Kamyar Shameli • Mohd Zobir Hussein •

Norhazlin Zainuddin • Katayoon Kalantari

Received: 9 April 2014 / Accepted: 10 June 2014

� Springer Science+Business Media Dordrecht 2014

Abstract The composite materials based on rice straw (RS) fiber and polycap-

rolactone (PCL) were prepared by a solution-casting method and characterized. The

composites were prepared using different fiber loadings (1.0, 3.0, 5.0, and 7.0 wt%).

The prepared composites (Cs) were characterized by using powder X-ray diffraction

(XRD), scanning electron microscopy (SEM), electro-optical microscopy, Fourier

transform infrared spectroscopy (FT-IR), and thermogravimetric analysis, and

mechanical properties were investigated. The XRD results showed that the intensity

of the peaks decreased with the increase of RS content from 1.0 to 7.0 wt% in

comparison with PCL peaks. SEM micrographs indicated poor adhesion between

RS and PCL matrices, and FT-IR spectroscopy showed that, with the increase of

loading percentages of RS, the intensity of the peaks is decreased from 1.0 to

7.0 wt%, and the interaction between RS and PCL was a physical interaction.

Thermal stability was decreased with increasing the RS contents. Tensile mea-

surement showed an increase in tensile modulus from 1.0 to 7.0 wt% loading of RS

into the PCL, but a decrease in tensile strength and elongation at the break as the RS

contents are increased, although there is a modest increase in tensile strength of the

Cs material with 3.0 wt% loading of RS.

Keywords Polycaprolactone � Composites � Rice straw � Mechanical � Thermal

properties

R. Khandanlou (&) � M. B. Ahmad (&) � K. Shameli (&) � M. Z. Hussein � N. Zainuddin �K. Kalantari

Department of Chemistry, Faculty of Science, Universiti Putra Malaysia UPM, 43400 Serdang,

Selangor, Malaysia

e-mail: [email protected]

M. B. Ahmad


K. Shameli


123

Res Chem Intermed

DOI 10.1007/s11164-014-1746-y

Introduction

The growing of biodegradable polymers has attracted increasing attention due to the

environmental problem attributable to deposition involving waste plastic [1].

Polycaprolactone (PCL) is a biodegradable as well as biocompatible polyester

which has lots of potential applications in devices which are related to the

biomedical field and also in agricultural use. Until now, large-scale application of

PCL has been limited due to its comparatively high cost and to some lower intrinsic

properties. Mixing biodegradable polymers with other natural or synthetic materials

has proved to be a powerful and economical method in solving this problem [2].

Recently, various types of cellulose-based natural fibers, for instance sisal, jute

and hemp, have been successfully incorporated straight into biodegradable polymers

to produce the necessary materials at competitive prices. Agricultural by-products

like rice husk, wheat straw, corn stalk, etc., are alternative lignocellulosic products,

which have been produced in vast numbers each year. So far, the utilization of these

kinds of agricultural residues has not acquired sufficient consideration, and most of

them are simply flared or applied as animal feed [3].

Rice straw fiber may be considered as having great potential as a reinforcing filler

for thermoplastic composites due to its lingo-cellulosic characteristics [4].

Straw is considered a natural composite material, as it is composed mainly of

polysaccharides (cellulose and hemicellulose) and lignin. The first two components

are hydrophilic, and the second one is hydrophobic. Even so, they are hardly soluble

in water and, in organic solvents, only in part, because of the formation of hydrogen

bonds between polysaccharides along with adhesion from the lignin to the

polysaccharides. However, agricultural residues represent a plentiful, cheap and

readily accessible source of renewable lignocellulosic biomass, in particular for the

production involving chemical compounds, such as lignin and hemicellulose, for the

polymer industry with the exception of producing paper from cellulose [5].

The use of composite materials reinforced with natural fibers under diverse

environmental circumstances has become universal in recent years. A large number

of natural fibers have been used as a powerful reinforcement in polymer materials.

Fillers in fiber or particle form are processed with polymer materials to obtain the

desired thermal, mechanical and electrical properties. The properties of the fiber

composite products are highly dependent on the properties of the fiber, as well as the

microstructural parameters such as the diameter, length, distribution, orientation,

and packing arrangement of the fibers. The advantages of the natural fibers as

reinforcements in plastics are their non-abrasive characteristics, biodegradability,

and low power consumption along with low cost. Furthermore, natural fibers have

low density along with high specific properties [6].

Polymer composites are prepared by utilizing polymers and compatible natural or

synthetic fibers [7]. The common polar and hydrophilic fibers are inherently

incompatible with nonpolar matrices and hydrophobic polymers, and the incom-

patibility may cause problems in the processing of composite materials and the

material properties. Hydrogen bonds can be formed between the fibers which are

hydrophilic in nature, so that the fibers have a tendency to agglomerate into bundles

R. Khandanlou et al.

123

and distribute unevenly throughout the matrix of non-polar polymer composition

during processing [8, 9].

In this work, rice straw/polycaprolactone composites (RS/PCL-Cs) were

prepared with various percentage loadings of rice straw (1.0, 3.0, 5.0, and 7.0

wt%) in dichloromethane at room temperature, using PCL as a polymer matrix and

rice straw as a filler by a solution-casting method, and the effects of fiber loading on

the properties of composite materials were studied.

Experimental procedure

Materials

All chemicals used in this study were of analytical grade and used without further

purification. Rice straw was obtained from a local farm (Bukit Tinggi, Kedah,

Malaysia). Polycaprolactone (PCL) was from Sigma-Aldrich (USA). Dichloro-

methane (CH2Cl2) was used as a solvent (Qrec, Malaysia). All glassware used in the

experimental procedures was cleaned in a fresh solution of HNO3/HCl (3:1, v/v),

washed thoroughly with double-distilled water, and dried before use.

Preparation of PCL/RS composites

For the synthesis of RS/PCL-Cs, different ratios of RS (1.0, 3.0, 5.0, and 7.0 wt%)

were suspended in certain amounts of dichloromethane with stirring for half an

hour, and then 5 g PCL was dissolved in 50 ml dichloromethane, followed by RS

suspension being added slowly to a PCL solution with vigorous stirring. After

addition of the RS suspension, stirring was continued for an additional hour to get

the RS well dispersed in the principal composite. The suspensions were finally

poured into Petri dishes and kept for 2 days until icompletely dry. Finally, solidified

films with a thickness of approximately 0.5 mm were obtained. All the experiments

were conducted at ambient temperature.

Characterization methods and instruments

In order to investigate the morphology of RS and RS/PCL-Cs, cross-section photos

were taken with SEM (LEO1455 SEM; LEO & Leica, Cambridge, UK). Electro-

optical microscopy (EOM) was used to observe the structure of the samples. The

powder X-ray diffraction (XRD) with Cu Ka radiation was applied to measure the

crystallinity of the samples. Thermogravimetric analysis (TGA) and differential

thermal gravimetric (DTG) were used to study the thermal behavior of the samples.

Fourier transform infrared (FT-IR) in the range of 400–4,000 cm-1 was used to

study the structures of the RS, PCL and RS/PCL-Cs. The FT-IR spectra were

recorded using PerkinElmer 100 Series FT-IR 1650 spectrophotometer. Tensile

strength, Young’s modulus, and elongation at break were measured using Ian nstron

Universal Testing Machine model INSTRON 4302 at constant cross-head speed of

Properties of RS/PCL composites

123

5 mm/min and a load of 1 kN. Four samples were used for the tensile test and the

average values were calculated from five runs for each sample.

Result and discussion

The XRD pattern of RS, PCL, and RS/PCL-Cs in different ratios of RS (1.0, 3.0,

5.0, and 7.0 wt%) are shown in Fig. 1. The SEM results for RS and RS/PCL-Cs are

shown in Figs. 2 and 3. The SEM images for RS/PCL-Cs show that dispersion of RS

in the PCL is not homogenous. Figure 4 shows the EOM images of the Cs, and they

are in agreement with the SEM results. Figure 5 indicates the FT-IR spectroscopy of

Fig. 1 XRD of RS (a) PCL, and RS/PCL-Cs (b)


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RS, PCL and its Cs; it reveals that the interaction between PCL and RS is a physical

interaction. The mechanism of the interaction of PCL with RS showed that the

interaction was a physical interaction (Fig. 6).

Fig. 2 Scanning electron microscopy micrograph of RS

Fig. 3 Scanning electron microscopy micrographs of RS/PCL-Cs with a–d 1.0, 3.0, 5.0, and 7.0 wt%RS, respectively


123

TGA analysis indicated that thermal stability decreased with increasing RS

content (Fig. 7). Tensile measurement (Fig. 8) shows that with increasing RS

content the tensile strength and elongation at break gradually decreases and tensile

modulus improves with increasing RS content.

Powder XRD analysis

Figure 1a, b shows the XRD pattern of RS, and RS/PCL-Cs with different RS

concentrations, respectively. The XRD pattern of the neat RS (Fig. 1a), shows the

diffraction peak in 2h = 22.20�, Due to its hydrophilic nature of RS, only a small

amount of RS is intercalated by PCL. The XRD pattern of PCL (Fig. 1b) shows a

diffraction peak in 2h = 21.45� and 23.66�. After incorporation of RS within PCL,

the diffraction peak of all RS/PCL-Cs had a small shift to the lower value of 2hangle, corresponding to the formation of composites. It can be seen that the intensity

of the diffraction peak in RS/PCL-Cs decreases with the increasing of RS content

(Fig. 1b). As the chain of PCL was the main component of the mixture, the

crystalline peak position was almost similar to the PCL; it shows that the PCL

matrix covers the RS, therefore the peaks of RS could not appear in the XRD

pattern.

Fig. 4 EOM images of RS/PCL-Cs with a–d 1.0, 3.0, 5.0, and 7.0 wt% RS respectively


123

Fig. 5 Fourier transforms infrared spectra of RS (a), PCL, and its composites with 1.0, 3.0, 5.0, and7.0 wt% RS (b)

Fig. 6 Schematic illustration of preparation of RS/PCL-Cs


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Morphology study

Figures 2 and 3 show the surface morphology images of RS, and RS/PCL-Cs,

respectively. Figure 4 shows EOM images of the RS/PCL-Cs. Figure 2 indicates

through SEM micrographs that neat RS possesses a rough surface. The SEM

micrograph of RS/PCL-Cs (Fig. 3) shows that the RS in composites agglomerated

into bundles distributed uniformly in the matrix. This poor adhesion was due to the

formation of hydrogen bonds between the RS and the disparate hydrophilicities of

Fig. 7 TGA (a, b) and DTG (c) thermograms of PCL, RS and RS/PCL-Cs, with 1.0, 3.0, 5.0, and7.0 wt% RS


123

PCL and RS [9]. The low interaction between the two components, the

immiscibility of PCL and RS seen through the holes corresponding to unprocessed

RS under SEM, and lack of interfacial adhesion are the sources of shaping such a

rough surface. It demonstrated that the fibers were solely attached with the PCL that

relates to poor fiber wetting. After adding 3.0 wt% unmodified RS as shown in

Fig. 3b, some interactions form and the surface gets smoother, while there is still

some incompatibility between the filler and the polymer.

Furthermore, it can be seen in EOM images (Fig. 4) that the dispersion of RS in

the PCL matrix is inhomogeneous and the larger spherical shapes indicate the

agglomeration of RS in PCL, except in 3.0 wt% of RS. The structure of RS/PCL-Cs

Fig. 8 Tensile strength (a),tensile modulus (b) andelongation at break (c) of RS/PCL-Cs in different wt% of RS


123

confirmed that, in the composite with 3.0 wt% RS (Fig. 4b), the external surface of

the composite develops spherulitic morphology, and the RS is more homogeneous

compared with other percentage loadings of RS, which is also consistent with the

observation of the SEM results.

FT-IR chemical analysis

Figure 5a, b shows the FT-IR spectra of RS, PCL, and RS/PCL-Cs with different

ratios of RS. In the FT-IR spectrum of neat RS (Fig. 5a), the absorption peaks at

3,377 and 2,933 cm-1 are attributed to stretching vibrations of –OH groups and the

C–H stretching, respectively [10]. The smaller shoulder peak at 1,735 cm-1 in the

RS is attributed to the aliphatic esters in lignin or hemicelluloses. The intense band

at 1,646 cm-1 is assigned to olefinic C=C stretching vibration [11]. The peak at

1,444 cm-1 is ascribed to the aromatic C=C stretch of aromatic vibration in bound

lignin [10]. The absorbance peaks in the 1,376–1,363 cm-1 originate from C–H

bending [12]. The region of 1,200–1,000 cm-1 illustrates C–O stretching and

deformation bands in cellulose, lignin, and residual hemicelluloses [13]. The peaks

observed in the region of 890–260 cm-1 are assigned to the linkages of glycoside

deforming with ring vibration and OH bending [14].

In the FT-IR spectrum of PCL (Fig. 5b), the peaks located at 2,943, 2,863, and

1,723 cm-1 are related to the stretching vibration of –CH2 and vibration of –C=O

bonds, respectively. The peak at 1,167 cm-1 is assigned to C–O stretching [15].

The IR spectra of RS/PCL-Cs (Fig. 5b) showed that the intensity of the peaks

are decreased with the increase of loading percentages of RS from 1.0 to 7.0 wt%

in comparison with the PCL peaks. It shows the existence of the RS in the PCL

matrix and formation of RS/PCL-Cs. In addition, RS has no chemical interaction

with the PCL matrix and the interaction may be a physical interaction since no

new band or any considerable shift occurred in comparison with the IR spectrum

of PCL.

According to the above results, it can be seen in Fig. 6 which PCL can be

interacted with RS via the hydrogen bonding between the –OH groups available in

RS and the carbonyl group present in the PCL. It shows the interaction between RS

and PCL can be a physical interaction.

Thermogravimetric analysis

TGA is a quantitative way of measuring the variation of the mass of product

encountered to manipulate the temperature range program. In addition, it records the

temperature of the region of the weight loss as well as the decomposition of the

maximum temperature. TGA detects one or a number of steps associated with the

loss of weight from room temperature to 800 �C. This kind of measurement was

conducted to specify the thermal stability of the samples. The mass loss of the

sample as a result of the volatilization associated with degraded by-product is

controlled as a function of temperature.

Figure 7a shows typical TGA thermograms of the PCL, RS/PCL-Cs, RS, and

degradation temperature regarding 50.0 wt% weight loss of PCL and its composites


123

(Fig. 7b). PCL has a two-step mechanism of degradation; in the first step

(200–400 �C), there is random chain scission via pyrolysis of the ester groups,

releasing of CO2, H2O and hexanoic acid; after that, in the second step

(400–530 �C), e-caprolactone (cyclic monomer) is constituted as a product of a

decompression process of depolymerization [16].

TGA of RS shows three stages of degradation: the first stage (50–130 �C) is

related to the removal of absorbed moisture; this moisture content could play an

important role in the degradation processes owing to the OH groups available in

water being more reactive compared to the OH groups of the RS. The second step of

thermal degradation happens at 180–360 �C and is mainly assigned to the

degradation of cellulosic materials like hemicellulose and cellulose; and the third

step of the weight loss (360–480 �C) is actually related to the degradation of non-

cellulosic substances in the RS [17].

It was shown in the TGA thermograms that the RS/PCL-Cs exhibit lower onset

temperature for the thermal degradation compares to neat PCL. PCL indicates an

onset temperature of 310.97 �C, which decreased to 277.18, 281.72, 286.26, and

300.83 �C when 7.0, 5.0, 3.0, and 1.0 wt% of RS, respectively, incorporated into the

PCL. The lower onset temperature for the thermal degradation of composites

compared to PCL is due to the low thermal stability of RS [17].

It can be seen that the RS/PCL-Cs in higher loading percentages of RS have a

lower onset temperature, and shows thermal stability reduced along with increasing

amounts of RS. This result can be related to a looser structure of PCL caused by the

expansion of PCL which induced by RS [18].

The DTG curve of PCL in Fig. 7c exhibits a single peak at 409.04 �C. This kind

of decomposition refers to the entire dissolution of PCL. The DTG curve of RS

shows three peaks at 61.29, 300.21, and 336.32. The DTG curves show that the Tmax

of the composites which correspond to the maximum degradation rate is lower, that

is 390.81, 386.27, 382.91, and 369.45 �C, for PCL containing 1.0, 3.0, 5.0, and

7.0 wt% of RS, respectively, compared to the PCL. The reduction in the degradation

temperature is the result of the low thermal stability of the RS which leads to the

heat transfer and increases the diffusion of volatile substances released by the

products. Table 1 shows the degradation temperature of RS, PCL, and composites

according to TGA, and DTG.

Table 1 Degradation temperature at 5.0, 10.0, 50.0, and 80.0 % fiber degradation, obtained by the TGA

and DTG

Sample T5 % (�C) T10 % (�C) T50 % (�C) T80 % (�C) Tmax (�C) Residue at 500 �C (%)

RS 163.56 219.95 331.78 – 336.32 30.0

PCL 370.83 380.01 404.16 418.50 409.04 5.0

1.0 % 356.75 368.10 393.06 409.27 390.81 5.9

3.0 % 331.43 354.48 386.26 400.45 386.27 7.2

5.0 % 309.08 346.40 378.15 395.34 382.91 9.8

7.0 % 277.18 327.24 370.37 387.51 369.45 12.5


123

Mechanical properties

Tensile strength

The effect of fiber loading on the tensile strength of the RS/PCL-Cs is shown in

Fig. 8a. It can be seen that the addition of RS reduced the tensile strength of PCL,

even though there is a modest increase in tensile strength of the Cs material with

3.0 wt% loading of fiber. Beyond this point, the tensile strength reduced with

increasing the amount of RS. The highest tensile strength which is attributed to

3.0 wt% fiber loading is 16.95 MPa. The decrease in tensile strength with increasing

the amount of fiber may be as a result of the weak adhesion or interaction between

PCL and RS that occurs due to agglomeration. The composites of PCL at high fiber

loading tend to form agglomerates [19]. This was due to the lack of insufficient PCL

to wet the RS, caused in an incompetent transfer of stress when stress is used on the

tensile specimens. Increasing the tensile strength of 3.0 wt% RS loading demon-

strates the capability to absorb the stress which is transferred from the PCL matrix

by RS. In Cs along with higher amount of filler, there is a greater trend for filler-to-

filler interaction to happen [20].

Hence, more holes are generally formed, starting the formation of cracks and the

extension in the Cs material, in comparison with a lower amount of fiber loadings.

The decrease in tensile strength shows that there was no improvement within the

interaction between PCL and fiber.

Tensile modulus

Figure 8b shows that the tensile modulus was raised along with increasing the fiber

content. The increase of fiber content caused agglomeration, and therefore affects

the rigidity of the Cs. The increase in the tensile modulus is possibly due to the

greater stiffness of the fibers compared to the matrix [21]. It was mentioned that the

most eminent effect of the fibers is the increase of the Cs modulus. The tensile

modulus is a measure of the toughness of an elastic material and the growing

tendency of the tensile modulus to test the behavior of the rigidity of exposures

made of composite material. When these composites are subjected to tensile stress

within the elastic range, the Cs materials do not stretch a lot and will behave like a

rigid Cs product. The overall increase in modulus implies the capability of the fiber

to represent greater toughness to the Cs.

Elongation at break

Reduction in elongation at break is accompanied with an increase in RS content as

shown in Fig. 8c. It is probably caused by the presence of large agglomerates which

make the composites more fragile. The decrease in the elongation at break showed

that the ductility of the matrix gradually reduces with the increase of contents of RS.

The lignocellulose fibers such as RS are responsible for decreasing the elongation.

The PCL matrix supplies ductility while RS shows a brittle behavior with a

subsequent decrease in stiffness of the Cs. Adding RS will be interrupted by PCL


123

segment mobility and therefore converting the Cs to be more brittle as the amount of

RS increased in the Cs. Theoretically, a low percentage of elongation of the Cs

indicates that the Cs are brittle or have low ductility [22].

Conclusions

RS/PCL-Cs with different percentage loadings of RS were successfully prepared

through a solution casting of PCL and RS. The effect of different percentage

loadings of RS into the PCL was studied. The tensile strength and elongation at

break of the composites deceases by increasing the RS content, due to the poor

interaction between PCL and RS, but there is a slight increase in 3.0 wt% fiber

loading. The tensile modulus increases with the increase of loading percentages of

RS. The XRD results show, after incorporation of RS into the PCL, that the Cs make

a small shift to a lower angle and the intensity of peaks decreases with the increase

of RS content from 1.0 to 7.0 wt%; this result indicating the formation of the RS/

PCL-Cs. TGA characterization shows that the thermal stability of the RS/PCL-Cs

decreased with increasing the RS content compared to pristine PCL. The SEM

micrographs of composites illustrates the presence of holes, which would be due to

the weak adhesion between the fiber and matrix. FT-IR results exhibited a decrease

in the intensity of peaks with the increase of RS content from 1.0 to 7.0 wt%, In

addition, no chemical interaction between PCL and RS was observed, and the

interaction can be a physical interaction since there is no new band or any

considerable shift in comparison with the IR spectrum of PCL.

Acknowledgments The authors thank University Putra Malaysia (UPM) for its financial support. The

authors are also grateful to the staff of the Department of Chemistry UPM for their help in this research,

and to the Institute of Bioscience (IBS/UPM) for technical assistance.

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Documents

Effect of unmodified rice straw on the properties of rice straw/polycaprolactone composites