Influence of Cellulose Nanocrystals (CNC) from Rice Husk on the Properties of Cassava Starch Biocomposites

Ishak Ahmad* and Nurain Johar

School of Chemical Science and Food Technology Universiti Kebangsaan Malaysia

43600, Bangi, Selangor Darul Ehsan, Malaysia gading@ukm.edu.my

Abstract—A series of rice husk cellulose nanocrystals-reinforced glycerol plasticized starch composite was successfully fabricated through solution casting technique. Rice husk was undergoing alkali treatment, bleaching process and sulphuric acid hydrolysis before cellulose nanocrystals can be produced. The composition of rice husk cellulose nanocrystals in starch biocomposite was varied from 0% to 10% w/t. These materials were characterized by different techniques, namely XRD, tensile tests, TEM and water sorption assays. The tensile strength and modulus of biocomposite films revealed a significant improvement of the mechanical properties compared to unfilled starch film. Incorporating CNC into plasticized matrix also leads to a decrease in water sensitivity. Rice husk cellulose nanocrystals at the optimal 6% filler loading level exhibited the optimum reinforcing efficiency for plasticized starch bioplastic. The results obtained indicates clearly about the advantages in the use of thermoplastic starch reinforced with cellulosic fibres as it was a natural, cheap and abundant material to replace the extensive use of synthetic and petroleum based material.

Keywords—rice husk; cellulose nanocrystals; cassava, starch; solution casting

I. INTRODUCTION Natural biopolymers have an advantage over synthetic

biodegradable polymers as they are renewable raw materials. The best known materials capable of making these materials are starch and cellulose [1]. Starch-based films have potential applications in packaging industries, because of environmental appeal, low cost, flexibility and transparency. Nevertheless, the hydrophilic nature of thermoplastic starches makes them vulnerable to moisture attack which caused the deterioration in dimensional stability and mechanical properties [2,3]. Previous studies demonstrate by tailoring starch with cellulose help to overcome these problems [4,5]. This semi crystallite polysaccharide, widely used as nano filler is purposely added to increase the mechanical properties of the composites. It can be obtained through acid hydrolysis of the extracted cellulose from fibers. Although a small number of studies have been carried out to utilize plasticized starch combined with CNC to form plastic film, no report on using CNC from rice husk fiber as reinforcement in starch biocomposite.

The paper reports the investigation into the effect of the incorporation of cellulose nanocrystals extracted from rice husk on the structural, morphology, water absorption and mechanical properties of cassava starch biocomposites.

II. MATERIALS AND METHOD Materials The rice husk fibers used as raw material was obtained from Rice husk Fiber Industry Sdn. Bhd. (Malaysia). Native cassava starch was supplied by Thye Huat Chan Sdn. Bhd. (M). Sodium hydroxide (NaOH), Sodium chlorite, sodium hydroxide and sulphuric acid were used for the treatments of rice husk fibers. Sorbitol and glycerol were used as plasticizers in preparing the nanocomposites. Preparation of rice husk cellulose nanocrystals Colloidal suspensions of cellulose nanocrystals (CNC) were prepared from rice husk.6 The excess of sulfuric acid was removed by repeated centrifugation at 10,000 rpm for 10 min. After that, the resulting suspension was submitted for dialysis against distilled water using cellulose membrane. The whisker content was determined by weighting aliquots of the solution before and after drying. Preparation of cassava starch/CNC nanocomposites. Thermoplastic starch was processed by casting a mixture of cassava starch granules, glycerol/ sorbitol (50:50), and distilled water. The mixture was heated at ~ 70oC with constant stirring in order to plasticize starch. The mixtures were added with a specific amount of the aqueous dispersion of CNC and mixed for 30 min (2-10 wt %; dry starch basis). The mixture was then cast into a Petri dish and place in an oven overnight before being kept at room temperature in conditioning desiccators at 30% relative humidity before tests. Characterizations The dispersion of CNC in the starch matrix was studied using a Zeiss Supra 55VP field emission scanning electron microscopy (FESEM). The composite films were frozen in liquid nitrogen and broken into small pieces. All the samples were mounted on aluminium stub by double-faced tape, coated by gold before observation to prevent charging with thickness around 0.01–0.1 μm. (Sputter Coater model BioRAD, 2.2 kV, 2 min). Transmission electron microscopy (TEM) was conducted using a Philips CM30 microscope to investigate the morphology of the CNC. To enhance the contrast, the nanocrystals were negatively stained with 2 wt% uranyl acetate.

Water uptake experiments were conducted on circular specimens, with 15 mm diameter. The water uptake at equilibrium was computed from the gain in weight. Matrix and composite film were submitted to X-ray radiation using a diffractometer Philips, model PW 1510 with a vertical goniometer operating at Cu Kα radiation wavelength (λ= 0.154 nm) 40 kV, and 30mA. Scattered radiation was detected in the angular range 5–40o (2θ). Tensile strength and modulus of starch biocomposites were performed on universal testing machine (Instron model 5566), at room temperature.

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III. RESULT AND DISCUSSION Morphology of the cellulose nanocrystals.

Fig. 1 shows TEM micrographs of a dilute suspension of CNC from rice husk fibers. The cellulose obtained was needle-like in shape. The figure also shows separated cellulose in nanometer scale. The size of the CNC was estimated to be 12±3.04 nm in width and between 200 to 300 nm in length. This correlates with TEM analysis on CNC from other sources [7].

Films physical properties. Transparent, thin and flexible films were obtained from both matrix and nanocomposite. Visually, the films had a clear white appearance. Fig. 2 showed the comparison between matrix and 6% nanocellulose composite film. It can be seen the color of the composite still comparable with the controlled film. This indicates that CNC can be used as filler without affecting the composite transparency. Water uptake. Fig. 3 shows the water uptake of unfilled matrix and 6% nanocomposite film as a function of time. The water uptake display two well-separated zones. At shorter times, t60 h, the absorption is slow and leads to a plateau, corresponding to the water uptake at equilibrium. The unfilled matrix is found to absorb more water than the composite whereas the water uptake decreases with increasing the content of CNC. The reduction is ascribed to the formation of a cellulose nanoparticles network, which prevented the swelling of starch and therefore its water absorption [8].

Fig. 1. Transmission electron micrographs of a dilute suspension of CNC from rice husk

Fig. 2. Image of matrix (a), and 6% composite films (b)

Time (min)0 20 40 60 80 100 120

Wat

er a

bsor

ptio

n (%

)

0

20

40

60

80

100

120

MatrixComposite plasticized

Fig. 3. Water absorption of unfilled matrix and 6% composite

films Crystallinity of films. X-ray diffraction was used to observe the changes in crystallinity of the cassava starch matrix upon addition of 6 wt.% nanocrystals. The patterns obtained for both films, the matrix and the composite, are shown in Fig. 4. Peaks around 17.5o, 19.5o, 22o, 29.5o and 34o are observed. The magnitude of peaks at 2θ= 22o and 29.5o increases in composite film compared to unfilled matrix. This is probably due to the present of CNC in the composite. Mechanical properties. The tensile strength and modulus, as determined from the typical stress–strain curve, are shown in Fig. 5. The tensile strength of composites increases considerably with filler content, up until 6% CNC content. At 8 % and 10 % of CNC loadings, the value decreases slightly but still high if compared to the neat matrix value. The increment of tensile strength compared to the neat matrix indicates that the addition of cellulose did improve the mechanical properties of the composites. Additionally, the presence of CNC also caused a considerable increase in the tensile modulus but the value suddenly drops after 10% CNC loading. When the CNC loading increases above 6% w/w, there may be a strong tendency for filler-filler interaction. Hence the reinforcing effect is not very high for composites having 10% w/w filler loading.

2 theta scale10 20 30 40 50

Lin (c

ount

s)

MatrixComposite plasticized

Fig. 4. XRD patterns of matrix and 6% composite films

(a) (b)

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Fig. 5. Effect of CNC loading on (a) mechanical tensile

strength and (b) tensile modulus

IV. CONCLUSIONS

Cellulose nanocrystals from rice husk had been isolated using acid hydrolysis method. The diameter of cellulose nanocrystals recorded by transmission electron microscope (TEM) is 12±3.04 nm. Plasticized cassava starch/cellulose biocomposite films were successfully prepared via solution casting technique. Reinforcement effect of filler was investigated at the variation of CNC loadings (0-10wt.%). It was found that the addition of CNC enhance the mechanical properties of the nanocomposite films with the 6 wt% CNC showed the highest tensile strength. Incorporating cellulose

nanocrystals also leads to a decrease in water uptake, and increased film crytallinity.

ACKNOWLEDGMENT The authors would like to express their gratitude towards Universiti Kebangsaan Malaysia for allowing this research to be carried out. Financial supports were from Ministry of Higher Education under research grant number FRGS/2/2013/SG06/UKM/02/2 and Zamalah Universiti Penyelidikan from Universiti Kebangsaan Malaysia.

REFERENCES [1] M. J. John and S. Thomas, Carbohydrate Polymer (2008). [2] N. L. Garcia, L. Ribba, A. Dufresne, M. Aranguren, and S. Goyanes.

Carbohydrate Polymers, 84, 203–210 (2011). [3] A. J. Svagan, M. S. Hedenqvist, and L. Berglund, Composites Science and

Technology, 69, 500-506 (2009). [4] E.M. Teixeira, D. Pasquini, A.A.S. Curvelo, E. Corradini, M.N Belgacem,

and A. Dufresne, Carbohydrate Polymers 78, 422-431 (2009). [5] Y. Chen, C. Liu, P. R. Chang, X. Cao and D. P. Anderson, Carbohydrate

Polymer 76, 607-615 (2009). [6] H. Kargarzadeh, I. Ahmad, I. Abdullah, A. Dufresne, S. Y. Zainudin and

R. M. Sheltami, Cellulose DOI 10.1007/s10570-012-9684-6 (2012). [7] J.O. Zoppe, M.S. Peresin, Y. Habibi, R.A. Venditti and O.J. Rojas, ACS

Applied Materials and Interfaces, 1, 1996-2004 (2009). [8] M. N. Anglès, & A. Dufresne, Macromolecules, 33, 8344–8353 (2000).

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I. Introduction II. materials and methodCharacterizations The dispersion of CNC in the starch matrix was studied using a Zeiss Supra 55VP field emission scanning electron microscopy (FESEM). The composite films were frozen in liquid nitrogen and broken into small pieces. All the samples were mounted on aluminium stub by double-faced tape, coated by gold before observation to prevent charging with thickness around 0.01–0.1 μm. (Sputter Coater model BioRAD, 2.2 kV, 2 min). Transmission electron microscopy (TEM) was conducted using a Philips CM30 microscope to investigate the morphology of the CNC. To enhance the contrast, the nanocrystals were negatively stained with 2 wt% uranyl acetate. III. result and discussionIV. Conclusions