Potential of using multiscale corn husk fiber as reinforcing filler in cornstarch-based biocomposites

BIOMAC-12992; No of Pages 9 International Journal of Biological Macromolecules 139 (2019) xxx

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Potential of using multiscale corn husk fiber as reinforcing filler in cornstarch-based biocomposites M.I.J. Ibrahim a,b, S.M. Sapuan a,c,⁎, E.S. Zainudin a, M.Y.M. Zuhri a a b c

Advanced Engineering Materials and Composites Research Centre, Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Department of Mechanical and Manufacturing Engineering, Sabha University, Libya Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, UPM 43400 Serdang, Selangor, Malaysia

a r t i c l e

i n f o

Article history: Received 26 May 2019 Received in revised form 26 July 2019 Accepted 1 August 2019 Available online 2 August 2019 Keywords: Corn starch Corn husk Biocomposite film Physical properties Thermal properties Tensile properties

a b s t r a c t In this study, biodegradable composite films were prepared by using thermoplastic cornstarch matrix and corn husk fiber as a reinforcing filler. The composite films were manufactured via a casting technique using different concentrations of husk fiber (0–8%), and fructose as a plasticizer at a fixed amount of 25% for starch weight. The Physical, thermal, morphological, and tensile characteristics of composite films were investigated. The findings indicated that the incorporation of husk fiber, in general, enhanced the performance of the composite films. There was a noticeable reduction in the density and moisture content of the films, and soil burial assessment showed less resistance to biodegradation. The morphological images presented a consistent structure and excellent compatibility between matrix and reinforcement, which reflected on the improved tensile strength and young modulus as well as the crystallinity index. The thermal stability of composite films has also been enhanced, as evidenced by the increased onset decomposition temperature of the reinforced films compared to neat film. Fourier transform infrared analysis revealed increasing in intermolecular hydrogen bonding following fiber loading. The composite materials prepared using corn husk residues as reinforcement responded to community demand for agricultural and polymeric waste disposal and added more value to waste management. © 2019 Published by Elsevier B.V.

1. Introduction The increasing growth of plastic waste, which threatens the environment, has encouraged the development of materials which are from natural sources, biodegradable, and renewable. The promising materials should be environmentally friendly and do not cause polluting emissions during manufacturing as well as after disposal [1]. In an attempt to resolve the ongoing environmental crisis caused by long-term biodegradable plastics, biopolymers have been considered as potential substitutes for traditional plastics. It is known that starch is the most common polymer, which is widely used as a matrix for composite materials. In addition to being widely available, it's renewable, inexpensive, and biodegradable [2]. Corn (maize) is the main source of commercially available starch, the composition of individual corn granule contains more than 70% starch besides minor ingredients like lipids, crude protein, sugar, and ash [3]. Cornstarch is a semi-crystalline biopolymer composed of a mixture of linear polysaccharide amylose (20–28%) and highly branched amylopectin [4–6]. Several studies have been reported ⁎ Corresponding author at: Advanced Engineering Materials and Composites Research Centre, Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia. E-mail address: [email protected] (S.M. Sapuan).

on the use of thermoplastic cornstarch with various plasticizers in the production of biopolymer films. Wang et al. [7] produced TPS film using cornstarch and urea plasticizer, Isotton et al. [8] investigated the effect of different concentrations of glycerol, sorbitol, and poly(vinyl alcohol) on cornstarch-based films, as well as Ibrahim et al. [9], studied the properties of cornstarch-based films as affected by various plasticizers. However, starch-based materials showed low water barrier characteristics and poor mechanical performance compared to the nonnatural polymers, that is due to their high hydrophilicity and affinity to water, such disadvantages severely restrict their widespread application [10,11]. Recently, plant cellulosic fibers and polymers played an essential role as raw materials in many applications because of their unique environmental advantages. Also, the correlation between starch matrix and cellulose fiber was found to be high, which is important for enhancing mechanical performance and hydrous sensitivity [12]. The incorporation of natural fibers as reinforcement during the production of the starch-based composite is an effective approach to improve the functional characteristics of composite films. Therefore, many studies have been conducted by composite materials researchers to enhance the mechanical performance and the water barrier properties of thermoplastic starch-based materials without changing their biodegradability. Rabe et al. [13] studied the effect of keratin and coconut fiber on corn TPS

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biocomposite, Gazonato et al. [14] investigated the thermomechanical Properties of cornstarch-based film reinforced with coffee ground waste, Hassan et al. [15] reinforced potato TPS by PLA, Jung et al. [16] filled Tapioca TPS with Vinyl Alcohol Copolymer, and Park et al. [17] used cellulose nanofiber to reinforce corn TPS. The authors reported significant results, especially with regard to mechanical performance and water barrier properties. Agricultural harvest residues, such as corn stover (husks, stalk, and leaves), sugar palm, pineapple leaf, and oil palm are produced annually in large quantities, they could be found at low cost, abundance and can be used as a renewable source of biomaterials. Of these large quantities of remains, only a small amount is used as domestic fuel or animals' fertilizer while the bulk of the residues is usually incinerated; as a result, a negative impact on the environment due to air contamination. Therefore, the utilization of these residues as reinforcing fillers in composite materials is a vital solution [18]. The stover of corn plant usually consists of 50% stalk (stem), 35% leaves and cobs, and 15% husk. Most of the stover is disposed of as waste, while it is likely to be detected as natural fiber [19]. Corn husk is referred to the outer sheet covering the corn ear; its lignocellulosic fiber typically contains a high concentration of cellulose and a low amount of lignin and ash [20]. The unique characteristics of cornhusk fiber, such as flexibility, durability, moderate strength, low density, and high elongation will afford distinctive properties for corn fiber products [21]. To the best of our knowledge also from the available literature, Youssef et al. (2015) [22] used corn husk fiber as a reinforcing agent with recycled low-density polyethylene composites. The other applications for cornhusk fiber were limited to textiles, furniture, bio-foams, and acoustic material. Therefore, the main objective of this study is to produce and characterize biocomposite films based on cornstarch matrix and corn husk fiber as reinforcement. This work was also designed to evaluate the utilization of corn plant residues which are highly abundant and inexpensive, that may contribute to mitigate the problem of wastes and responds to the community's demand for agricultural and polymeric waste disposal, also improves the economic growth through the transfer from waste to health. The reinforcement of thermoplastic corn starch by corn husk residues adds value to the waste's products and promotes the suitability of starch-based composites as eco-friendly materials. It should be noted that the particles of corn husk used as reinforcement in the current study have not been chemically treated or thermally modified.

Fructose concentration was set to be 25% (w/w powder starch) based on our previous work [9]. Corn husk at various loadings (0, 2, 4, 6 and 8% w/w dry starch) was used as a reinforcement. The solution was heated by a thermal magnetic stirrer to 85 ± 2 °C with continuous stirring for 20 min. After that, the mixture was placed in a vacuum desiccator to eliminate the formation of air bubbles. The solution was then casted evenly in 140 mm diameter thermal plates and kept on an air circulation oven at 40 °C for 24 h with the aim of dehydration. The formed films were peeled out of the casting plates and stored in plastic bags at room temperature for a week prior to characterization processes. Films produced according to concentration were encoded as follows: CS/CHF2%, CS/CHF4%, CS/CHF6%, CS/ CHF8%, and control for 0% CHF. 2.3. Thickness and density The film thickness was measured by an electronic caliper (Mitutoyo-Co, Japan) with an accuracy of ±0.0524 mm. The actual thickness was determined from the mean of five random measurements. While the density of the films was obtained from their volume (v) and weight (m). The size of each film was calculated due to the proposed dimensions (10 mm × 30 mm) times the thickness found from the previous step. Thus, the film density (ρ) was obtained by the equation: ρ ¼ m=v ¼ g=m3

ð1Þ

2.4. Moisture content (MC) The moisture content of the material is defined as the amount of water that could be removed from the material without changing the chemical composition in relation to the base weight of the material [24]. MC of the films was measured according to the method introduced by Shogren [25]. A known weight film was kept on an oven for 24 h at 105 °C. The weight differences before (M1) and after (M2) dehydration were used to obtain the MC for each sample indicated by a percentage by the equation: MC ð%Þ ¼ ððM1−M2Þ=M1Þ  100

ð2Þ

2. Materials and methods

2.5. Water solubility (WS)

2.1. Materials

Solubility in water refers to the amount of chemicals that can melt in the water at a specific temperature. This test was carried out by the method of Shojaee et al. [26]. The samples (10 mm × 30 mm) were dehydrated in a lab oven at 105 °C for 18 h and then weighted (Wi). The dry samples were immersed in distilled water under continuous stirring for 6 h at room temperature. After that, the remains of specimens were dried at 105 °C until a fixed weight (Wf) was gained. Then the WS (%) of samples was measured by the equation:

Corn starch (CS) was extracted from grains of corn ear collected from a local market in Malaysia following the method of Ali et al. [23]. 1 kg of fresh corn granules were immersed in 4 l of distilled water for 12 h. After that, the water was emptied, and the grains were crushed in a lab blender (wet milling) to achieve the minimum possible fractions. The blended fractions were examined through a 75 μm sieve and then left to sediment. The slurry of water and starch was separated by centrifugation at 3000 rpm for 20 min. The corn starch obtained was dried in an air circulation oven for 12 h at 50 °C. Its composition was 24.64% amylose, 10.45% moisture, 7.13% lipids, and 0.62 ash. Corn husk fiber (CHF) which refers to the leaves covering the corn ear was thoroughly cleaned, dried, crushed, and screened out through 300 μm mesh sieve. Its composition was found to be cellulose 45.7%, hemicellulose 35.8%, lignin 4.03%, and ash 0.36%. Fructose was supplied by evergreen Sdn. Bhd, Malaysia was used to act as a plasticizer. 2.2. Composite films preparation A series of films were produced by the traditional solution casting method. Corn starch (5 g) was added to distilled water (100 ml).

WSð%Þ ¼ ððWi−Wf Þ=WiÞ  100

ð3Þ

2.6. Water absorption (WA) A film sample (15 mm × 15 mm) was dried on a lab oven for 3 h at 105 °C and then was cooled and immediately been weighted (Mi). The dried sample was immersed in 100 ml of distilled water at room temperature. After a specific submersing period, the sample was taken out from the water, superficially dehydrated with a smooth cloth and weighted (Mf). The evaluation of WA was determined from the mass differences between the dried and saturated states of each sample.

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The test was conducted in triplicate and was measured using the following equation: WAð%Þ ¼ ððMf −MiÞ=MiÞ  100

ð4Þ

2.7. Soil burial test Biodegradation tests were conducted by the method described by Martucci et al. [27]. Equal volumes with a known weight (Wi) of each film sample were buried in a closed environment (plastic boxes) inside moist soil at a depth of 5 cm. The assessment of weight loss (WL) was performed in triplicate by taking the samples from the soil at different times and cleaned by gently scanning using a brush. Subsequently, they were dehydrated at 105 °C for 6 h and weighed (Wf). The assay of degradation was obtained every couple of days and measured using the equation: WLð%Þ ¼ ððWi−Wf Þ=WiÞ  100

ð5Þ

2.8. Scanning electron microscopy (SEM) An electronic microscope type (Hitachi S-3400N, Japan) was employed to scan the surface morphology of the films. Each sample was surrounded by a thin golden layer (0.01–0.1 μm) and mounted on a bronze griddle before applying (20 kV) acceleration voltage. The scanning was conducted under a high vacuum atmosphere included liquid nitrogen to freeze the films. The test resulted in a high-resolution image at different magnification factors. 2.9. X-ray diffraction (XRD) The XRD diffraction patterns of biocomposite films were performed using a diffractometer type Rigaku, Tokyo, Japan. The operating current and voltage were set to be 35 mA and 40 kV, respectively. The Bragg angle 2θ scattered from 5° to 60° with an angular speed of 0.02°. The crystallinity index (Ci) was measured based on the calculus of the crystalline area (Ac) and the amorphous area (Aa) using the equation:  Ci ¼

 Ac  100 Ac þ Aa

ð6Þ

3

2.11. Thermal gravimetric analysis (TGA) The thermal inspection was carried out using an analyzer type (Q500 V20.13 Build 39). The sample size (10 mm2) was placed in platinum vessel contained Nitrogen gas and sealed tightly; then it was heated gradually from ambient temperature to 450 °C at a rate of 10 °C/min to investigate the thermal stability and calculate the weight loss as a function of temperature. 2.12. Tensile properties The mechanical behavior of the specimens was evaluated using a universal tensile machine (5KN INSTRON). The test was performed according to D882 (ASTM, 2002) standard at ambient conditions. Film strip (70 mm × 10 mm) was mounted between the tensile machine clamps and pulled with a crosshead speed of 2 mm/min, and the effective grip separation was set to be 30 mm. The results of tensile strength, elastic modulus as well as the extension at break point were calculated from the average measurements of five replicates for each specimen. 2.13. Statistical analyses The experimental data were statistically analyzed using Microsoft Excel 2016 and Tecplot 9.0 software. The average comparisons of properties were conducted using Student's t-test with about 95% confidence interval. 3. Results and discussion 3.1. Thickness and density From the data in Table 1, the thickness of CS/CHF composites recorded an insignificant increased with the addition of CHF concentration, while the density significantly decreased. The observed findings could be attributed to the intermolecular interaction between the fiber and the polymer matrix. The higher CHF content increases porosity and creates a less homogeneous structure with a lower density than thermoplastic starch, resulting in thicker and coarse films [28]. Similar results were observed about the effect of fiber loading on the composite film thickness and density when cassava starch-based film was filled with cassava bagasse [29], the production of biocomposite film from both sugar palm starch and fiber [30], and the reinforcement of sugar palm starch-based composite by seaweed fiber [31]. However, the lower density of biocomposites makes them attractive materials, especially for the applications associated with the lightweight and easy handling [32].

2.10. Fourier transform infrared spectroscopy (FTIR)

3.2. Moisture content

The existence of functional groups in the tested films was detected by spectrometer type (Bruker Vector 22). 2 mg of each sample was prepared based on a KBr disk technique. The FTIR spectrum was measured within a spectral range of 4000–400 cm−1 with 4 cm−1 resolution and sixteen scans per sample.

Despite its hydrophilic behavior, the presence of CHF, to some extent, reduces the water retention of the CS composite films, as shown in Table 1. The slight reduction in the MC of the composite films might be associated with the relatively lower moisture content of CHF than CS. Comparable findings were reported by Lopez et al. [33] for

Table 1 Physical properties of composite films. Composite

Control CS/CHF2% CS/CHF4% CS/CHF6% CS/CHF8%

Thickness (μm)

195 ± 25.4 220 ± 25.4 240 ± 25.4 255 ± 25.4 265 ± 25.4

Density (g/cm3)

1.55 ± 0.08 1.45 ± 0.07 1.43 ± 0.04 1.37 ± 0.06 1.30 ± 0.09

Moisture content (%)

11.64 ± 0.11 10.88 ± 0.12 10.25 ± 0.20 10.72 ± 0.23 10.95 ± 0.19

Water solubility (%)

22.05 ± 0.43 23.25 ± 0.52 23.15 ± 0.09 20.60 ± 0.15 20.51 ± 0.17

Water absorption (%) 30 min

180 min

82.56 ± 0.18 80.99 ± 0.43 84.97 ± 0.08 90.75 ± 0.13 90.46 ± 0.59

92.45 ± 0.07 86.73 ± 0.23 93.52 ± 0.44 99.38 ± 0.20 96.37 ± 0.06

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reinforcing wheat starch with leaf wood fiber and similar to those of Avérous et al. [34] who added fibrous residues from Pachyrhizus ahipa plant to corn starch. 3.3. Water solubility Table 1 also displays the water solubility of the biocomposites, which indicates the effect of water immersion with constant stirring on the composite films. It was noticed that the initial loading of CHF (2% and 4%) resulted in higher WS, while further increase led to a drop in WS compared to control film. This phenomenon might be attributed to the function of fiber in hindering disintegration of composite films by forming a network which firmly holds the composites together and then reduces the solubility. This result is in agreement with that obtained by Edhirej et al. [35] who studied the cassava starch/peel biocomposites and stated that films with low fiber loading were more soluble than their high-loading fiber counterparts. 3.4. Water absorption The percentage of weight gain as a function of water uptake time for the CS/CHF biocomposite is shown in Table 1. The effect of immersion time was limited to 180 min because after this period, the film samples begin to decompose. However, the composite films showed a different tendency to water uptake with increased fiber loading. The films reinforced with CS/CHF2% revealed less propensity to water uptake than none-reinforced film whereas, the films filled by 4%, 6%, and 8% showed a higher propensity to absorb water than control film. Similar behavior was observed by Salaberria et al. [36] who used chitin as filler into thermoplastic corn starch-based composites and stated that the water absorption rate of the produced biocomposites was dependent of the percentage of the filler in the TPS matrix. The biocomposites filled with low filler content showed higher resistance to water absorption than thermoplastic starch matrix, whereas those filled with high filler exhibited lower resistance than the thermoplastic starch matrix. Also, Edhirej et al. [35] concluded that the higher content of cassava peel increases the water absorption value of cassava starch-based composite films, the researchers attributed that to the hygroscopic nature for cassava peel fiber. However, this tendency is likely to be attributed to the high hydrophilic behavior of CS and CHF. The incorporation of fiber stimulates the hydrophilicity of the starch structure by increasing the porosity within the starch matrix and led to more water diffusion. Furthermore, the existence of the hydroxyl group in the film structure, as indicated in FTIR structure analysis prompt their ability to retain water [29]. 3.5. Biodegradation of biocomposites Biodegradation test is a process for measuring the weight loss of substances due to enzymatic degradation or the action of natural microorganisms. Fig. 1 shows the weight loss increment of composite films as a function of time. The monitoring of composite films biodegradation was conducted for 20 days. At the earlier burial stage, the water diffusion has caused swelling effect in the films, which in turn enhanced the bacterial attack. According to the data, On the eighth day, all film samples experienced different values for weight loss. For instance, the control film has lost 47.13% of weight while the other films have lost 55.59%, 65.13%, 69.34% and 73.22% of weight for CS/CH2F%, CS/CHF4%, CS/ CHF6%, and CH/CHF8% respectively. It has also been observed that after 18 days the control film reached the highest degradation rate, while the fiber composites lasted 14 days to decompose completely. González et al. [37] attributed this event to the availability of a close relationship between the existing moisture and microbial activity of the soil. In other words, films that absorb more water enhance the activity of microorganisms and then increase the rate of degradation. This explanation is in a good agreement with the results of water absorption

Fig. 1. Weight loss of CS/CHF composites after soil burial for 20 days.

from the current study (Table 1). Furthermore, the results suggest that the film with higher CHF contents would show better potential biodegradation, making it more susceptible to mycobacterial attack. This microorganism, in the form of fungi and bacteria, influences the composite films in the existence of an aqueous medium [38,39]. 3.6. Morphological properties SEM images of the fractured surfaces of CS/CHF biocomposites were shown in Fig. 2. The general appearance revealed coherent homogeneous surfaces and clearly coated by the starch matrix; this is due to the role of fiber in establishing strong interaction bonds with the starch matrix because both reinforcement and matrix are carbohydrates with the same polarity [40]. It should be noted that the film with CS/CHF2% showed a consistent and smooth surface, and no pores were observed in the structure. In the case of the addition of husk fiber from 4% to 6% and then to 8% appearance of voids was observed, and the fracture surfaces turned to be coarser and more rigid. Also, there was evidence of pulling out of husk fiber from the starch matrix, leaving microcracks with longitudinal gaps in the matrix when the content exceeds 8%. In films, matrix homogeneity is a useful indicator of its structural consistency [41,42]. In this regard, it can be noticed that films containing higher CHF content will appear an effective stress-transfer from fiber to the matrix and hence, offered better mechanical properties. In general, consistent dispersion and coherent polymer-fiber interactions were observed that might be attributed to the similar hydrophilic nature of both matrix and reinforcement. Similar surface fracture was found on thermoplastic cassava starch/cassava bagasse biocomposites [29]. 3.7. X-ray diffraction (XRD) The corresponding XRD pattern of the composite films is shown in Fig. 3. The XRD analysis was conducted to determine the effect of CHF loading on the crystal structure of CS-based composites. For the film without husk fiber (control) the gelatinization of starch particles led to creating sharp 2θ peaks at diffraction angles 17.57°, 20.15°, and 22.48°, which represents the typical A-type pattern of the native cereal plant starches [43,44]. The initial addition of husk fiber, especially at 2% and 4% showed a similar diffraction profile to the control and caused a slight increase in relative crystallinity. While the further addition of fiber as anticipated enhanced the intensity and then the crystallinity index of the composites (Table 3). According to Ma et al. [45], the increasing trend in the crystallinity of fiber reinforcement composites is due to the occurrence of phase separation between fiber and starch, indicating that the interior structure of composites became less amorphous and more arranged when the starch particles penetrate the pores of the fiber. A similar explanation was reported in the

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Fig. 2. Scanning electron micrograph of CS/CHF composite films with various CHF concentrations.

thermoplastic cassava starch/peel fiber composites [35], and thermoplastic corn starch/bacterial cellulose composites [46]. However, the X-ray diffractometry profiles indicated that the presence of CHF caused a remarkable impact on the relative crystallinity of the CS-based composites. The crystallinity degree of the control film was 15%, the gradual increase of the fiber resulted in an increase in the crystallinity degree reached 31% with 6% husk content. 3.8. Fourier transform infrared spectroscopy (FTIR)

Fig. 3. XRD of CS/CHF composite films with various CHF concentrations.

The FTIR technique was employed to determine the variations in the compositional structure of starch-based composites and evaluate the potential interactions between reinforcement and matrix. Since the extraction of starch and husk was from the same biological source, it was found that the FT-IR spectra of all film samples showed similar behaviors, as shown in Fig. 4. The analysis of the leading bands and functional groups of the samples were performed by dividing the FT-IR spectral curve into four regions. The first region at below 1500 cm−1, the second region between 1500 and 2800 cm−1, the third region from 2800 to

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Fig. 4. FTIR curves of CS/CHF composite films with various CHF concentrations.

3000 cm−1 (C\\H stretch region), and the last region at above 3000 cm−1 (O\\H stretch zone) [47]. In the first region, the existence of C\\O group within glucose pyranose units of starch has triggered the broad peak at 995 cm−1 and the minor peaks at less than 800 cm−1 [47]. While, the vibrational stretching of C\\O\\H group and coupling mode of C\\C and C\\O groups produced the sharp peaks at 1075 cm−1 and 1149 cm−1 respectively [48,49]. The peaks in the second region at 1665 cm−1 wavenumber were ascribed to the bending mode of water particles in the starch [38,50,51]. The sharp peaks at 2800–3000 cm−1 assigned to C\\H vibrational stretching whereas the peaks with high intensity at 3000–3500 cm−1 attributed to the elongation and oscillation of O\\H group within the matrix and reinforcement, which indicates that the composite films are water affinity due to the existence of hydroxyl groups [52,53]. However, the shifting of band positions after the fiber loading determines the interactions between the matrix and reinforcement molecules. For instance, the corresponding band at 3278 cm−1 for control film shifted to a higher position reached 3281 cm−1 following the initial fiber loading, while additional loading turns it to decrease, suggesting an increase in intermolecular hydrogen bonding between the matrix and the fiber particles. Also, the absence of new peaks indicates that no chemical reaction occurred. This result broadly supports the work of other researches in this area linking coconut fiber and cassava starch [54]. 3.9. Thermal stability The thermal stability and degradation temperatures of CS/CHF biocomposites at different fiber loading are shown in Fig. 5. TGA and DTG are proper techniques to investigate the thermal behavior of composites. The heating rate was set to 480 °C because the degradation of composites is usually between the decomposition temperatures of their main components, namely, the matrix and reinforcement filler [55]. The initial degradation temperature of the corn husk is 187 °C and reaches a maximum of decomposition at 364 °C [20], while corn starch is fully degraded at about 350 °C [56]. Base on the obtained data, the thermal decomposition and mass loss of CS/CHF biocomposites took place in three main heating events. Each event is correlated with a prominent peak in the DTG chart and is referred to a specific weight loss in TGA curve. The first thermal event occurred at temperatures around 200 °C led to the initial weight loss. This loss in weight was mainly related to volatilization of fructose fragments and evaporation of water particles [45,57]. More heating caused the second thermal event between 200 °C to slightly above 300 °C. In this stage, the maximum mass loss is occurred due to the depolymerization and degradation of Carbone chain within the starch matrix, which is the main compound

Fig. 5. Thermal analysis of CS/CHF composite films. (a) TGA, and (b) DTG.

of the biocomposite films [49]. The last thermal event was assigned to the degradation of the main components of lignocellulosic husk fiber, namely, cellulose and hemicellulose at rang of 200–270 °C and lignin at 270–370 °C [58]. Apparently, all films exhibited close onset decomposition temperatures varied between 299 °C and 303 °C, higher than the degradation temperature of the control film which reached its maximum break down at 277 °C; this indicates that the presence of husk as reinforcement improved the thermal stability of CS-based composite (up to 26 °C). In addition, the mass residues (%), determined after the final decomposition of the composite films, was slightly decreased (weight loss increased) following the addition of husk as can be seen in Table 2 evidencing that the thermal stability of the biocomposites was insignificantly affected. These findings are consistent with the previous studies [35,59,60].

Table 2 Degradation temperatures of CS/CHF composites. Film sample

Control CS/CHF2% CS/CHF4% CS/CHF6% CS/CHF8%

Degradation temperature (°C) Phase 1 (Plasticizer + water) 184 197 195 203.92 197.93

Phase 2 Phase (Starch) 3 (Fiber) 277 280.04 280.72 282.87 282.95

– 300 300.85 302.42 302.27

Mass residue (%) at 400 °C

29.63 30.09 29.16 27.67 28.41

Weight loss (%)

50.95 54.32 55.82 57.72 56.34

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M.I.J. Ibrahim et al. / International Journal of Biological Macromolecules 139 (2019) xxx Table 3 Crystallinity index of CS/CHF composite films. Film sample Control CS/CHF2% CS/CHF4% CS/CHF6% CS/CHF8%

Crystallinity index (%) 15 15.2 17.7 31.3 25.7

3.10. Tensile properties The effect of CHF loading on the mechanical performance of CSbased films was shown in Fig. 6. The tensile test was performed mainly

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to determine tensile strength (TS), tensile modulus (E), and extension at the break (EB). The incorporation of CHF and the increase in its percentage resulted in a reduction in EB and a considerable increase in E and TS, which means that the composite films become less flexible and more stiffness and turn into a more resistant material [61,62]. However, the introduction of CHF at a higher concentration of 8% revealed a higher tensile strength and tensile modulus of 12.84 MPa and 639.62 MPa respectively, which increased by 88.82% from the control film. This enhancement is a result of the compatibility and structural similarity of starch and cellulose since they are from the same biological origin. As seen in the SEM images, the needle-like of CHF covered by CS matrix and the absence of clusters and agglomeration of CHF are evidence of high compatibility, homogeneity and good dispersion of CHF in the CS matrix. This can also be illustrated by the ability of fiber to restrain the mobility of starch molecules and promote the interfacial interaction between matrix and reinforcement, which in turn provides an excellent transfer of stress [63,64]. The improvement was also associated with the crystallinity index as reported by Salaberria et al. [36], the authors announced that the increase in the relative crystallinity affects positively on the stiffness and rigidity, and therefore, the tensile strength of the system. The increase in tensile strength and modulus of starch-based composite films as fiber concentration increased were reported by numerous authors [18,30,35,49,59]. In the other hand, the influence of fiber loading on the elongation of break for CS-based composites showed an inverse behavior compared with their modulus and tensile strength. This parameter measures the ability of composite films to deform and stretch from the initial length up to the breaking point [65]. The observed decrease in film elongation with increased fiber loading is due to the fact that cellulose increases the intermolecular bonds of the starch matrix and thus, creates more hydrogen bonds between the fiber and starch molecules. Such reconstruction in the starch network induces the rigidity and reduces the flexibility of films by hindering chain mobility [66]. 4. Conclusion The effect of using multiscale corn husk fiber as reinforcing on the physical, morphological, mechanical, and thermal characteristics of cornstarch-based films was evaluated. A series of composite films were produced by the solution casting method using fructose as a plasticizer. The experimental results showed that the gradual loading in fiber concentration (2%, 4%, 6%, and 8%) enhanced the tensile properties of the composite films by 88.88% with the composite film having 8% CHF (12.84 MPa), this indicates that an efficient stress transfer was generated between matrix and reinforcement. The scanning of the fracture surfaces showed homogenous and cohesive surfaces due to the structural similarity between matrix and reinforcement. A significant increment in the thermal stability has been achieved, as evidenced by increased degradation temperature of the composite films from 277 °C to 303 °C. As well as the relative crystallinity raised from 15% to 31% of the composites, this enhancement is associated with the increasing of intermolecular hydrogen bonding as seen in the bands shift detected by FT-IR, which means strong interfacial adhesion between polymer matrix and fiber. The soil burial assay revealed that the incorporation of fiber promoted the process of biodegradation due to the high hydrophilicity of the fiber. Besides, it should be noted that the corn husk molecules used as reinforcement in the current study have not been chemically processed or modified, which leads to the development of environmentally friendly and cost-effective materials. Acknowledgments

Fig. 6. Tensile properties of CS composite films. (a) Tensile strength, (b) tensile modulus, and (c) extension at break.

The authors would like to thank Universiti Putra Malaysia for the financial support provided through Universiti Putra Malaysia Grant Scheme [grant numbers 6369107].

Please cite this article as: M.I.J. Ibrahim, S.M. Sapuan, E.S. Zainudin, et al., Potential of using multiscale corn husk fiber as reinforcing filler in cornstarch-based biocomposite..., https://doi.org/10.1016/j.ijbiomac.2019.08.015

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Please cite this article as: M.I.J. Ibrahim, S.M. Sapuan, E.S. Zainudin, et al., Potential of using multiscale corn husk fiber as reinforcing filler in cornstarch-based biocomposite..., https://doi.org/10.1016/j.ijbiomac.2019.08.015