Abstract
Nanostarch has attracted much research interest recently due to its biodegradability and biocompatibility. A type of biocomposite film based on corn nanostarch (CNS) as the matrix and cellulose nanocrystals (CNC) as the reinforcement was prepared using a solution casting method. The influences of corn nanostarch concentration (CCNS), glycerin dosage (Dg), and cellulose nanocrystals dosage (DCNC) on the tensile strength of the biocomposite film were investigated by central composite design. The results were examined by an analysis of variance (ANOVA) and response surface methodology (RSM). The optimized process conditions as follows: CCNS of 11.25%, Dg of 12.00%, and DCNC of 5.00%. The CNS/CNC biocomposite film produced under these conditions showed a high tensile strength of 12.90 MPa. The CNS/CNC biocomposite film was characterized by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), water contact angle, and scanning electron microscopy (SEM). The CNS/CNC biocomposite film has potential application prospects in the field of food and biomedical packaging.
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Response Surface Optimization of Process Conditions and Characteristics of Nanostarch-based Biocomposite Film Reinforced by Cellulose Nanocrystals
Qijie Chen,* Liling Zhou, Jiaqi Zou, and Jianhui Wang
Nanostarch has attracted much research interest recently due to its biodegradability and biocompatibility. A type of biocomposite film based on corn nanostarch (CNS) as the matrix and cellulose nanocrystals (CNC) as the reinforcement was prepared using a solution casting method. The influences of corn nanostarch concentration (CCNS), glycerin dosage (Dg), and cellulose nanocrystals dosage (DCNC) on the tensile strength of the biocomposite film were investigated by central composite design. The results were examined by an analysis of variance (ANOVA) and response surface methodology (RSM). The optimized process conditions as follows: CCNS of 11.25%, Dg of 12.00%, and DCNC of 5.00%. The CNS/CNC biocomposite film produced under these conditions showed a high tensile strength of 12.90 MPa. The CNS/CNC biocomposite film was characterized by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), water contact angle, and scanning electron microscopy (SEM). The CNS/CNC biocomposite film has potential application prospects in the field of food and biomedical packaging.
Keywords: Corn nanostarch; Cellulose nanocrystals; Biocomposite film; Response surface methodology
Contact information: Hunan Provincial Engineering Research Center for Food Processing of Aquatic Biotic Resources, School of Chemistry and Food Engineering, Changsha University of Science and Technology, Changsha, Hunan Province, 410114, People’s Republic of China;
* Corresponding author: chenqijie@126.com (Qijie Chen)
INTRODUCTION
Biocomposite film has received attention in the packaging of renewable sources due to the consumption of fossil fuel and environmental pollution caused by traditional petroleum-based plastic food packaging. Starch is a renewable, inexpensive, and biodegradable natural material that is widely used in the food industry. Starch is composed of two glycosidic macromolecules containing amylose and amylopectin. Amylose is a linear polysaccharide composed of (1-4) -D-glucopyranose with molecular weight of 2 104 to 2 106 Daltons, and amylopectin is a high-branched macromolecule composed of both (1-4) and (1-6) -D-glucopyranosyl linkages with molecular weight of 2 107 to 2 109 Daltons (Teixeira et al. 2014). Nanostarch is a research hotspot in the fields of biocomposites materials, food packaging, medicine, and cosmetics due to its advantages of small particle size, large specific surface area, high crystallinity, and biodegradability (Kim et al. 2015). Nanostarch can be prepared by various methods, including hydrolysis or enzymolysis, physical disintegration, twin screw extrusion, and self-assembly (Jiang et al. 2016; Chen et al. 2018).
Starch film has good barrier properties against oxygen, carbon dioxide, and lipids. Compared with traditional film made of synthetic polymers, starch films have poor tensile strength, which limits their industrial applications. However, much literature has shown that nanofillers can improve the mechanical properties of starch-based films, such as nanoclays (Müller et al. 2011), cellulose nanocrystals (CNC) or cellulose nanofiber (Pelissari et al. 2017), starch nanocrystals or nanoparticles (Li et al. 2015), and carbon nanotubes (Cheng et al. 2013).
Cellulose nanocrystal (CNC) is a rich natural biological polysaccharide with an ordered cellulose crystalline region, large specific surface area, and high surface energy, which make it an ideal material for biocomposite films. CNC has abundant -OH side groups, and it is easy to polymerize to achieve different mechanical properties. It can be used as reinforcement in various polymer matrices such as chitosan (Corsello et al. 2017), polylactide (Gazzotti et al. 2017), gum (Ma et al. 2017), polyvinyl alcohol (Singh et al. 2017), polyurethane (Santamaria-Echart et al. 2016), carboxymethyl cellulose (Li et al. 2016), poly(vinyl pyrrolidone) (Huang et al. 2016), and starch (Cui et al. 2017).
In this work, the corn nanostarch (CNS) was prepared by twin-screw extrusion, and it had better film forming properties than native starch (Chen et al. 2018). The biocomposite film based on corn nanostarch (CNS) as the matrix and cellulose nanocrystals (CNC) as the reinforcement was prepared using a solution casting method. And the optimal formulation of CNS/CNC biocomposite film was investigated by response surface methodology (RSM) and analysis of variance (ANOVA). The optimized CNS/CNC biocomposite films were also characterized by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), water contact angle, and scanning electron microscope (SEM).
EXPERIMENTAL
Raw Materials
Corn nanostarch (CNS) was prepared by extrusion in the laboratory according to the method described previously (Chen et al. 2018). The preparation process was as follows: corn starch was premixed with 10.0% glycerin and then mixed in a high-speed homogenizer. The mixture was fed into twin-screw extruder with twelve barrels, and the crosslinking agent of 2.0% glyoxal was injected to the extruder from the tenth barrel. The extrudate was dried and smashed to obtain the corn nanostarch. Bleached sulfate softwood pulp was provided from Yueyang paper Co. Ltd. (Hunan province, China). Glycerol, sulfuric acid, and sodium hydroxide were supplied by Sinopharm Chemical Reagents Co., Ltd (Shanghai, China). All other reagents were commercially available and of analytical grade.
Preparation of Cellulose Nanocrystals (CNC)
CNC was extracted from bleached sulfate softwood pulp as previously described (Yu et al. 2017). A total of 10.00 g of bleached sulfate softwood pulp was hydrolyzed in 80 mL of 2SO4(64% mass concentration) at 45 C for 2 h with continuously stirring, followed by successive centrifugation for 10 min at 4000 rpm until neutralization, and dialysis in distilled water.
Preparation of CNS/CNC Biocomposite Film
Different proportions of CNS, glycerol, and CNC biocomposite film was prepared by casting. First, CNS was added to the distilled water to form slurry, followed by stirring at 70 C for 30 min. Glycerol (based on the dry CNS) was added as a plasticizer and stirred at 70 C for another 20 min. Finally the CNC (based on the dry CNS) was added to the mixed solution, and the solution was treated with an ultrasonic wave (VOSHIN-501D, Voshin Instruments Manufacture Co., Ltd, Wuxi, China) at 120 W for 30 min to ensure uniform suspension. After vacuum degassing to remove air bubbles, the mixed solution was placed on Petri dishes and dried at 45 C at a relative humidity (RH) of 45% for 72 h in desiccators containing a saturated NaBr solution. The thickness of film was about 0.3 mm.
The values of corn nanostarch concentration (CCNS), glycerol dosage (Dg), and cellulose nanocrystals dosage (DCNC) were varied according to three variables-three levels of a Central Composite Design (CCD) as reported in Table 1. Preliminary tests were performed under process conditions at the CCD central point to determine the ideal moisture content of the films. For this purpose, a film sample was weighed every 10 min, until the film could be easily removed from the Petri dish without any damage.
Tensile Strength
Biocomposite film samples were stored at 45% RH for 72 h before the tensile strength was determined using a tensile testing machine (DCP-KZW300, Sichuan Changjiang Papermaking Instruments Co., Ltd, Yibin, China) according to the ASTM D882-02 (2002) standard. The tensile strength (TS/MPa) was calculated with Eq. 1,
(1)
where F is the required maximum load (N), stretching the biocomposite film to the breaking point, and A is the cross-sectional area (mm2).
Transmission Electron Microscopy (TEM)
TEM observation of CNC were carried out by JEM-2100 transmission electron microscope (Tokyo, Japan). Ultrasonic treatment was performed on the diluted CNC suspension and a drop of CNC suspension was deposited on a glow-discharged carbon-coated copper grid of the TEM and dried at room temperature before the excess liquid was absorbed by filter paper. Uranyl acetate was deposited on the grid before the observation.
FTIR Spectral Analysis
FTIR spectra of pure CNS film and CNS/CNC biocomposite film were recorded by a Bruker Vertex 70v vacuum spectrometer (Karlsruhe, Germany) in reflection mode. The range was from 4000 cm-1 to 400 cm-1, and the resolution was 2 cm-1. The pellets were prepared from powder samples by the KBr method.
Differential Scanning Calorimetry
The DSC patterns of pure CNS film and CNS/CNC biocomposite film were examined on a TA Differential scanning calorimeter (Q2000, San Diego, CA, USA). The samples were sealed in an aluminum pan and heated from 0 to 200 °C at a rate of 10 °C/min.
Water Contact Angle
The water contact angle of pure CNS film and CNS/CNC biocomposite film was measured by the SDC 350 contact angle measuring instrument (Shanghai, China). The sample film was cut into a rectangular shape (10 mm × 50 mm). Deionized water was dropped on the film surface, and the contact angle was measured automatically.
Scanning Electron Microscopy
The samples were maintained in a desiccator with silica gel for 5 d before they were fractured in liquid nitrogen and sputter-coated with gold prior to examination. The surface morphology of the CNS and nanostarch-based biocomposite film was analyzed using a Jeol scanning electron microscope (JSM-6490LV, Kyoto, Japan) at an accelerating voltage of 10 to 15 kV.
Experimental Design and Statistical Analysis
The surface response methodology was used to study the effects of corn nanostarch concentration (CCNS), glycerol dosage (Dg), and cellulose nanocrystals dosage (DCNC) on the tensile strength of biocomposite film. It was defined according to a 23 full-factorial central composite design. An analysis of variance (ANOVA), RSM, and all statistical analyses were performed by Design-Expert 8.0.6.1 software (Stat-Ease, Minneapolis, MN, USA). ANOVA was used to evaluate the statistical significance of the developed quadratic mathematical model, including P value, F value, degrees of freedom (df), sum of squares (SS), coefficient of variation (CV), coefficient of determination (R2), adjusted coefficient of determination (R2Adj), and predicted coefficient of determination (R2Pred). After fitting the data with the model, the response surface graph was constructed to predict the relationship between independent and dependent variables. The normal distribution of residuals, residuals and prediction equations, the actual and predicted values of the equations were analyzed, and the model fitted to the experimental data were measured. The polynomial of the fitting model was given in Eq. (2),
(2)
where Y is the response (tensile strength); Xi are the coded of the independent variables (X1, X2, X3); β0, βi, and βij are the model intercept coefficient, interaction coefficients of linear, quadratic and the second-order terms, respectively; K is the number of independent parameters; and ei is the error.
RESULTS AND DISCUSSION
Mathematical Model and ANOVA
The results of 23 full-factorial central composite design are summarized in Table 1. The suitable ranges for the variables CCNS, Dg, and DCNC were determined on the basis of single-factor experiment. The value of Run 16 was equivalent to the variance analysis of DCNC, which was used to analyze the experimental data and statistical significance of the established polynomial model. The P value is used to check whether the coefficients in the quadratic regression equation are significant (P < 0.05). The P value of multiple regression was very small (Prob > F < 0.0001), indicating that the model established could fully represent the real relationship among the selected parameters.
Table 1. Center Composite Design Matrix with Values of Factors and Responses to Tensile Strength of Biocomposite Film
Table 2. ANOVA of the Quadratic Modulus of the Response Surface
aR2 Adj = adjusted R2, bR2Pred = predicted R2
As shown in Table 2, the model was highly significant (Prob > F < 0.001), and the responses of process variables have obvious effects on the tensile strength of biocomposite films. According to ANOVA, the coefficients of X1, X2, X3, X1X2, X1X3, X12, X22, and X32 had statistical significance (P < 0.05), indicating that the established model was in good agreement with the real data. The P coefficients of X1 and X3 were very small (P < 0.0001), which indicates that the CNS and CNC had the greatest influences on the tensile strength of the biocomposite film. This is because CNC is a high crystalline nanoparticle, with good mechanical properties and high stiffness and elastic modulus. CNS and CNC have similar chemical properties, and the strength of biocomposite film was improved by hydrogen bonding. The “Lack of Fit F-value” of 1.76 implies the Lack of Fit is not significant compared with the pure error, and 27.47% of the probability of such a large “Lack of Fit F-value” was caused by inevitable error. The final fitting model obtained according to the variables was shown in Eq. 3:
Y=-11.16-8.032X1+9.81X2-0.618X3+0.435 X1X2+0.161 X1X3 +0.058 X2X3+0.152 X12-0.618 X22-0.175 X32 (3)
The goodness of the model fitting was checked by measuring the coefficient of determination (R2) and the adjusted coefficient of determination (R2Adj). In this model, R2 was 0.9837, indicating that the regression model can well explain 98% of the change in tensile strength process, and R2Adj was 97%, indicating that the experimental value was in good agreement with the predicted value (Yetilmezsoy et al. 2009; You et al. 2019).
Fig. 1. Diagnostic plots of the quadratic model used for biocomposite film (a) Internally studentized residuals versus Normal percentage probability; (b) Predicted versus Internally studentized residuals; (c) Actual versus Predicted
Diagnosis of Model Adequacy
In general, exploration of fitted response surface models may yield poor or misleading results, unless the models show a good fit, which makes it critical to check the adequacy of the mode. The normal distribution of the frequency graph of residuals is an intuitive test method, that is, the residuals fall within a certain range, and within the scope of the normal distribution with the corresponding probability distribution, indicating that the residuals obey the normal distribution. As shown in Fig. 1(a), the residuals and normal distributions of the experiment trial were concentrated in the range (2, 2), indicating that the model errors were normally distributed. According to the residual distribution diagram and the predicted values of the equation, the distribution of residual falling points and equations were discrete and irregular, indicating that the error of the model is small, as shown in Fig. 1(b). The corresponding relationship between the predicted graph and the measured value is shown in Fig. 1(c), the points in the graph are set on a straight line with the measured value, indicating a good correlation between predicted values and the measured values.
Interaction of Process Variables
On the basis of a quadratic ternary regression equation (Y), the three-dimensional response surface graph was drawn, which provided an intuitive view of the response system for understanding the interaction between different variables on the response. The three-dimensional response surface graph shows the top or bottom, and the highest point is the maximum value of the response surface.
While keeping one variable constant, the interactions of other variables on tensile strength were analyzed, as shown in Fig. 2. When the value of CCNS was fixed, the tensile strength of the biocomposite film increased slightly as the glycerol dosage increased from 10.0% to 11.60%. However, many studies have shown that glycerol as a plasticizer reduces the intermolecular forces between polymers, thus enhancing the molecular fluidity and flexibility of the film (Mohsin et al. 2011). The opposite results were obtained, which may have no obvious effect on weakening intermolecular forces in the range of the dosage of glycerol. When the Dg was fixed at 10.0% to 11.60%, the increasing CCNS enhanced the tensile strength of the biocomposite film. The total solid concentration of biocomposite film enhances the intermolecular forces by the formation of inter-molecular hydrogen bonds in the starch matrix. Figure 2(a) shows that the highest tensile strength resulted when lower Dg (12.05%) and high CCNS (12.25%) were used.
Figure 2(b) shows that the 11.5% of Dg was selected as the center point and 8.75% of CCNS was fixed. With the increase of DCNC from 1.0% to 5.0%, the tensile strength of biocomposite film increased by 60%. Due to the geometric shape and stiffness of CNC, the reinforcement effect obtained by CNC is attributed to the formation of a rigid network formed by hydrogen bonds and the mutual entanglement of the CNS matrix. CNC has better mechanical properties than CNS, so it can be mixed with starch matrix to increase the tensile strength of the film. When DCNC was increased from 5% to 6%, the tensile strength of the biocomposite film decreased slowly. This is due to the slight aggregation of CNC; the positive contribution of CNC to the tensile strength of the film decreased. Similarly, the interaction between CCNS and DCNC affected the tensile strength. When CCNS was more than 10.25%, the DCNC had a significant positive effect on the tensile strength of biocomposite film.
As shown in Fig. 2(c), when 11.25% of CCNS was selected as the center point, the interaction between glycerol molecules and CNC had no obvious effect on the tensile strength of the biocomposite film because the glycerol molecules only acted on the intermolecular and intramolecular hydrogen bonds in the starch matrix. The results showed that the CCNC had greater influence on the tensile strength of biocomposite film more than Dg. The reduction of intermolecular force was not obvious in biocomposite films with lower Dg. However, the increase of Dg (12%) resulted in high mobility of the chains, and the intermolecular forces between adjacent CNC chains were weaken by glycerol molecules, which reduced the tensile strength of the biocomposite film.
Fig. 2. Response surfaces for the tensile strength as a function of (a) corn nanostarch concentration (CCNS) and glycerol dosage (Dg), (b) corn nanostarch concentration (CCNS) and cellulose nanocrystal dosage (DCNC), (c) glycerol dosage (Dg), and cellulose nanocrystal dosage (DCNC).
In summary, the comparative analysis of response surface showed that CNC, interactive CNC, and CNS had positive effects on tensile strength of biocomposite films. CNC formed a stronger network structure by intra- and inter-molecular hydrogen bonds and the mutual entanglement of CNS matrix, which improved the tensile strength of CNS/CNC biocomposite films. By the analysis of each performance response surface, the optimized process conditions were defined to be CCNS of 11.25%, Dg of 12.0%, and DCNC of 5.0%. The CNS/CNC biocomposite film prepared under optimized process conditions showed the high tensile strength of 12.9 MPa.
Table 3 showed that the comparison of the tensile strength of various CNC-reinforced biocomposite film. The tensile strength of corn starch/CNC film (Miranda et al. 2015), banana starch/CNC film (Pelissari et al. 2017), and starch/CNC film (Ali et al. 2018) were reported as 6.8 MPa, 11.1 MPa, and 8.1 MPa, respectively. In the present study, the tensile strength of CNS/CNC film was 12.9 MPa. This showed that the CNS/CNC film had higher tensile strength than native starch/CNC film.
Table 3. Comparison of the Tensile Strength of Various CNC-reinforced Biocomposite Films
TEM Analysis of CNC
The TEM of CNC are shown in Fig. 3. CNC has highly ordered crystallization because its amorphous zone was eliminated by acid hydrolysis. The CNC presented needle-like nanocrystals, with an average length (L) of 150±50 nm, a diameter (D) of 30 ±10 nm. The same result was reported by Martins et al. (2011). This confirmed that the suspension contained CNC, which was mainly composed of a single nanocrystal and some aggregates due to its high specific surface area.
Fig. 3. TEM of CNC
FTIR Spectral Analysis of Biocomposite Film
The FTIR spectra of pure CNS film and optimized CNS/CNC biocomposite film are shown in Fig. 4. The peak at 3400 cm-1 corresponded to -OH stretching vibration, and the peak at 2983 cm-1 was the symmetric vibration of -CH2 groups. The bands located at 1630 cm-1 corresponded to the -OH of bound molecular water (Cao et al. 2019). The peaks of CNS/CNC biocomposite film at 1440 cm-1, 1087 cm-1, 1043 cm-1, and 879 cm-1 corresponded to -CH2 scissoring motion, C-O stretching in the cellulose anhydrous glucose, and cellulosic -glycosidic linkages, respectively (Sheng et al. 2018). These results confirmed that the CNS/CNC biocomposite film was uniformly mixed.
Fig. 4. FTIR spectra of pure CNS films and CNS/CNC biocomposite films
DSC Analysis of Biocomposite Film
The DSC thermograms of pure CNS film and optimized CNS/CNC biocomposite film are presented in Fig. 5. The DSC curves showed that the two kinds of films had similar degradation during the heating process, and the two curves had the similar broad absorption peak at 130 C. The melting peak temperature of CNS/CNC biocomposite film increased slightly, which was mainly related to the high crystallinity of CNC, indicating that CNC had good dispersibility in the CNS matrix (Ma et al. 2017).
Fig. 5. DSC curves of pure CNS films and CNS /CNC biocomposite films
Water Contact Angle of Biocomposite Film
The water contact angle measurements of pure CNS film and CNS/CNC biocomposite film are shown in Fig. 6. A higher contact angle indicates a stronger hydrophobicity of the biocomposite film. The water contact angle of optimized CNS/CNC biocomposite film was 58.5°. The water contact angle of pure CNS film was 47.7°, and it was increased by 22.5%. The results showed that CNC improved the hydrophobicity of the biocomposite film, and the CNS/CNC biocomposite film had better water barrier property than the pure CNS film.
Fig. 6. Water contact angle of pure CNS film (a) and CNS/CNC biocomposite film (b)
SEM Analysis of Biocomposite Film
The SEM images of the CNS and CNS/CNC biocomposite film are presented in Fig. 7. Figure 7(a) shows that the CNS granules were complete and smooth, and their average particle size was about 100 nm. The surface of CNS/CNC biocomposite film was smooth without any deformation (Fig. 7(b)), indicating that CNC was evenly dispersed in the CNS matrix. As shown in Fig. 7(c), the CNC formed a stronger network structure by intra- and inter-molecular hydrogen bonds and mutual entanglement with CNS matrix. This also indicated that the addition of CNC greatly improved the tensile strength of CNS matrix film.
Fig. 7. SEM of CNS(a); the surface (b) and cross section (c) of CNS/CNC biocomposite film
CONCLUSIONS
- An optimized process of nanostarch-based biocomposite film was successfully developed from corn nanostarch (CNS), glycerol, and cellulose nanocrystals (CNC) using the method of center composite design. The corn nanostarch concentration (CCNS) and dosage of cellulose nanocrystals (DCNC) had significant effects on the tensile strength of biocomposite film. By analysis of each performance response surface, the optimized process conditions were determined as CCNS of 11.25%, a glycerine dosage (Dg) of 12.0%, and DCNC of 5.0%, and the tensile strength of the optimized CNS/CNC biocomposite film was 12.9 MPa.
- The CNC was homogeneously dispersed in CNS matrix in the CNS/CNC biocomposite film, and it increased the melting peak temperature slightly. The water contact angle of optimized CNS/CNC biocomposite film was increased by 22.5% and had better water barrier property than the pure CNS film. The CNS/ CNC biocomposite films have potential packaging applications in the field of food and biomedicine.
ACKNOWLEDGMENTS
This work was financially supported by the National Natural Science Foundation of China (No. 31500495), Hunan Provincial Education Department Foundation of China (No. 18B150) and Hunan Provincial Engineering Research Center for Food Processing of Aquatic Biotic Resources Foundation of China (No. 2018KJY03, 2018CT5010).
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Article submitted: February 9, 2019; Peer review completed: March 27, 2019; Revised version received and accepted: April 6, 2019; Published: April 16, 2019.
DOI: 10.15376/biores.14.2.4344-4357