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MATERIALS AND INTERFACES Influence of Natural Biomaterials on the Absorbency and Transparency of Starch-Derived Films: An Optimization Study Weiping Ban,† Jianguo Song,† and Lucian A. Lucia*,‡ Dalian Institute of Light Industry, Dalian, China 116034, and Department of Wood and Paper Science, College of Natural Resources, North Carolina State UniVersity, 3108 Biltmore Hall, Raleigh, North Carolina 26795-8005
A D-optimization design was applied to examine the water absorbency and transparency of composite films made from starch and natural biopolymers such as lignin, gelatin, cellulosic fibers, and chitosan. The individual influence of each component and their overall interactive effects were investigated. With regard to water absorbency, it was found that lignin and gelatin can effectively enhance film water resistance, while the incorporation of cellulosic fibers had a limited positive effect on film hydrophobicity. However, increasing the levels of cellulosic fibers significantly diminished the hydrophobicity. The last component, chitosan, always contributed to reducing the hydrophobicity of starch films. The last property studied in this design, transparency, was mainly compromised by the cellulosic fibers and lignin. Cellulosic fibers, because of their bulk dispersed state in the starch films, caused higher light scattering and a concomitant film transparency reduction. Lignin also decreased film transparency because of its chromophoric nature and made the films appear darker; however, it inhibited UV radiation, even at low levels. Introduction Plastic films have played a very important role in technological advances that improve the quality of life of modern society. They have been widely applied in packaging, agricultural mulching, gas and liquor separation, and biomedical devices. In 2000, more than 40 million tons of plastic films was used worldwide in the packaging industry. The bulk of these materials, however, was for one-time use and, therefore, discarded to landfills after consumption. Given ever-increasing environmental pressures, our research efforts have been directed to developing truly biodegradable and biocompatible films.1,2 In general, there are two ways to develop biodegradable films. One way is to introduce biodegradable synthetic materials, such as polylactide, polyesters, or polycaprolactone into them; however, because of the high cost of these materials, their utilization is limited. A second, more attractive way is the utilization of natural materials, which in the current climate is a far more promising economic approach for the chemical industry and society at large. Starch is one of the most abundant and low-cost natural biopolymers available today. It has been widely used in fermenting as well as other chemical manufacturing industries. However, in the packaging industry and agricultural mulching operations, starch-based films have significant potential to replace synthetic films. In the past several decades, great efforts have been made to develop starch-based films with improved film properties. However, two inherent disadvantages of neat starch-based film have limited such utilization: relatively low
tensile strength and very high water absorbency. The low tensile strength limits its mechanical potential (handling is more difficult), whereas, more importantly, a high water absorbency limits its viability, since it will succumb to decomposition much more readily. To overcome these limitations, two methods have been usually applied: copolymerization of starch with synthetic polymers or mixing starch with other polymer components.3-7 However, the synthetic polymer components remain nonbiodegradable, even after starch has thoroughly degraded. Therefore, it still possesses potential harm for the environment and any accrued advantages from utilizing starch-based films would, thus, be impaired. Attempts to modify starch film by developing starch composite film blends have been reported.8-12 Cellulosic fiber and chitosan may be added to starch to modify its film properties, for enhancing film mechanical strength, and to improve film flexibility, in addition to other physical properties.8,10 Cellulose fibers were also found to improve starch film performance by diminishing water uptake.9 When lignin was applied as filler in starch films, it reduced the overall water affinity of the ensuing thermally molded films. A significant decrease of extensibility at higher water content was also observed.12 Our previous work investigated the fundamentals of creating starch films composed of cellulosic fibers, chitosan, gelatin, and starch.1,2 The objective of this work is to complement those recent efforts by focusing on an optimization of these variables for the manufacture of starch composite films. A D-optimum design was applied to investigate the effect of each of the above biopolymers on film water absorbency and transparency, and their interactive effects are discussed.
* To whom correspondence should be addressed. E-mail: Lucian.
[email protected]. † Dalian Institute of Light Industry. ‡ North Carolina State University.
Experimental Section Materials. The following materials were used: • Starch: Commercial corn starch used without modification.
10.1021/ie070271j CCC: $37.00 © 2007 American Chemical Society Published on Web 08/24/2007
Ind. Eng. Chem. Res., Vol. 46, No. 20, 2007 6481 Table 1. D-Optimum Design X1 (fiber)
X2 (chitosan)
X3 (lignin)
X4 (gelatin)
test no. coding value coding value coding value coding value starch 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
0 0 -1 1 -1 1 -1 1 -1 1 1.7 -1.7 0 0 0 0
15.00 15.00 6.18 23.82 6.18 23.82 6.18 23.82 6.18 23.82 30.00 0.00 15.00 15.00 15.00 15.00
0 0 -1 -1 1 1 -1 -1 1 1 0 0 1.7 -1.7 0 0
15.00 15.00 6.18 6.18 23.82 23.82 6.18 6.18 23.82 23.82 15.00 15.00 30.00 0.00 15.00 15.00
0 0 -1 -1 -1 -1 1 1 1 1 0 0 0 0 1.7 -1.7
2.50 2.50 1.03 1.03 1.03 1.03 3.97 3.97 3.97 3.97 2.50 2.50 2.50 2.50 5.00 0.00
1.8 10.00 57.50 -1.5 0.83 66.67 0.6 6.67 79.95 0.6 6.67 62.30 0.6 6.67 62.30 0.6 6.67 44.66 0.6 6.67 77.01 0.6 6.67 59.36 0.6 6.67 59.36 0.6 6.67 41.72 -0.9 2.50 50.00 -0.9 2.50 80.00 -0.9 2.50 50.00 -0.9 2.50 80.00 -0.9 2.50 62.50 -0.9 2.50 67.50
• Fiber: Softwood TMP pulp from a pulp mill was used for all experimental samples, with an original pulp brightness of 56.44. Before being used in the experiments, the pulp was washed by deionized water and air-dried. It was stored in a plastic bag before use. Pulps were dispersed in a blender and made into a pulp slurry of 10% solids concentration. • Chitosan: Chitosan used for this investigation was ordered from Sigma-Aldrich, practical grade, >85% deacetylated, viscosity > 200 cP; dissolved in 1% acetic acid to make up a 2% solution. • Gelatin: Chemistry purity grade gelatin from Sigma-Aldrich was dissolved in water to make up a 20% solution. • Lignin: Lignin used in the experiments was Kraft lignin obtained from Sigma-Aldrich, with average Mw ≈ 28 000. Film Preparation. Biopolymers were mixed with starch and a mixture was gelatinized at 80 °C for 25 min. Glycerol was used as a plasticizer, and its content was 25% based on total dry weight of composite materials. The total mixture of starch and other materials was 20 g and was kept at a concentration of 4 g/100 mL. Biopolymers were varied according to the experimental plan; see Table 1 below. After being cooled and made into a homogeneous dispersion, 50 mL of the dispersion (2 g of dry material) was taken and poured into a Petri dish and then dried at 60 °C to cast into a film. Film Water Absorbency. Film samples (80 mm × 15 mm) were conditioned at 25 °C in a desiccator containing sodium sulfate to ensure a relative humidity ratio of 95% for 24 h. They were then blotted with tissue paper under the same pressure to remove the excess water on the film surface. Water absorbency expressed by water uptake was calculated as follows:
water uptake % ) [(W - W0)/W0] × 100 where W0 and W are the weight of sample of original and after exposure to 95% RH, respectively. Film Transparency Measurements. Film opacity was measured with a model BNL-3 opacimeter manufactured by Technidyne. The standard TAPPI method was applied for opacity measurement (TAPPI T425). Experimental Design. Quadratic D-optimum design was applied in the current work and is summarized in Table 1. The experimental program used four independent variables, namely, X1, fiber (0-30%); X2, chitosan (0-30%); X3, lignin (0-5%); and X4, gelatin (0-10%). The following functions were considered as dependent variables: Y1, film water absorbency
%; and Y2, film transparency %. All experiments were duplicated and parts of them were triplicated to ensure >95% confidence level. Results and Discussion Effect of the Biopolymers on Starch Film Water Absorbency. (a) Mathematical Description of Relationship between Film Water Absorbency and Biopolymers. On the basis of a D-optimum experimental design, a statistical regression analysis was conducted to obtain specific regression equations. The contributions of each component and their influence on film water absorbency are described in eq 1. All variables could be classified into two categories: independent factors and interactive factors. The former demonstrate the individual influence of each biopolymer on film specific property, while the latter describe the interactive effects of each pair of biopolymers on film properties. Hence, each biopolymer in the films provides a contribution to film properties through their individual effects as well as their interactions with other biopolymers. In total, it was determined that it was the independent contribution of the four natural biopolymers that had the most significant contribution to film water absorbency. By eliminating the insignificant contributions, eq 1 could be simplified to eq 2. The result demonstrated that only three pairs of interactions had a significant influence on film water absorbency, i.e., cellulosic fibers with chitosan, cellulosic fibers with gelatin, and chitosan with gelatin. These interactions will be elaborated later on in this section.
Y ) 50.4301 - 2.1719X1 + 6.0614X2 - 5.5877X3 6.2634X4 + 4.6241X12 + 3.5168X22 - 1.9303X32 3.4659X42 - 1.9089X1X2 - 0.1411X1X3 - 2.7717X1X4 + 0.2861X2X3 + 1.7814X2X4 + 0.1536X3X4 (1) Y ) 50.4301 - 2.1719X1 + 6.0614X2 - 5.5877X3 6.2634X4 + 4.6241X12 + 3.5168X22 - 1.9303X32 3.4659X42 - 1.9089X1X2 - 2.7717X1X4 + 1.7814X2X4 (2) (b) Independent Contributions of Each Biopolymer. In our previous work, the effects of individual biopolymers such as cellulosic fibers, chitosan, and gelatin on water starch based film were reported.1,2 In the current work, the effects of these biopolymers on film water absorbency and their interactive impacts have been more fully investigated. The effects of cellulosic fibers on water absorption for a typical starch-based film characterized by predetermined levels of chitosan, lignin, and gelatin is presented in Figure 1. Cellulosic fibers displayed different effects on film water absorbency as a function of the change in fiber content. Initially, an increase in cellulosic fibers in the starch matrix contributed to a reduced starch film water absorption. Compared to starch, TMP cellulosic fibers are harder to swell under a high-humidity environment. One reason is the highly crystalline structure of cellulose, which hinders water molecule penetration into the bulk of the microfibers; the other is the high lignin content in the TMP fiber, which displays strong hydrophobicity. However, there was a limit to the reduction in film water absorption; beyond a threshold level, increases in the cellulosic fiber content appeared to result in an increase in film water uptake. Hence, a transition point existed; furthermore, the effect of cellulosic fiber was also influenced by the other biopolymers. The transition point depended on the existing biopolymer content. At relatively high contents, the transition
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Figure 1. Influence of cellulosic fiber on water absorbency of starch-based films.
Figure 2. Influence of chitosan on water absorbency of starch-based films.
point occurred at a higher level of cellulosic fiber contents compared to that at the lower cellulosic fiber content. The result indicates that strong interactions between cellulosic fibers and other biopolymers exist, and the effect of cellulosic fibers on reductions in film water absorbency is obvious at higher levels. The influence of chitosan on film water uptake is shown in Figure 2. The main function of chitosan on water absorbency was evidenced as an enhancement in water uptake due to its strong hydrophilicity. At all levels of co-biopolymer contents, chitosan showed the same effect. However, the influence of chitosan on water uptake was insignificant at a low level in the films. On the other hand, while not too surprising, both lignin and gelatin behaved as hydrophobes. This result is clearly observed in Figures 3 and 4. Essentially, the hydrophobic effect of lignin varied with changes in the total biopolymeric content; more specifically, its influence became more significant at a high biopolymer level, as indicated by the steep curve. Similarly to lignin, gelatin functioned as a hydrophobe; however, gelatin showed two features different compared to lignin with respect to water absorbency. First, the efficacy of gelatin’s hydrophobic effect depended on the addition level of gelatin. Below 5%, it did not show any ostensible influence on the film hydrophobicity. Only when it exceeded 5% was a significant hydrophobic effect observed. Furthermore, unlike lignin, changes of other biopolymers in the film did not show any obvious impact on hydrophobicity. In summary, increasing hydrophobicity for starch-based films may be achieved either by adding lignin or gelatin.
Figure 3. Influence of lignin on water absorbency of starch-based films.
Figure 4. Influence of gelatin on water absorbency of starch-based films.
Figure 5. Interactive effects of fiber and chitosan contents on film water absorbency.
(c) Interactive Effects of Biopolymers on Water Absorbency. In a multiple biopolymer system, each biopolymer not only contributes to the film properties alone but also engages in biopolymer-biopolymer interactions that affect the overall system properties. Occasionally, these interactions are more important than individual actions. In eq 2, three pairs of interactions are significant for film water absorbency: fiber vs chitosan; fiber vs gelatin; and chitosan vs gelatin. These interactive impacts on film water absorbency are presented in Figures 5-7. First of all, the interaction of cellulosic fiber and chitosan did not provide the highest hydrophobicity; 47.8% water absorption was attained. The reason is ascribed to the high water
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Figure 6. Interactive effects of fiber and gelatin contents on film water absorbency.
Figure 7. Interactive effects of chitosan and gelatin contents on film water absorbency.
absorption possible from chitosan and the limited hydrophobic effect available from the cellulosic fibers. Chitosan, in a like manner to its individual water absorption effect, always acted to increase film water absorbency, no matter what fiber level was used, as shown in Figure 5. On the other hand, cellulosic fibers amazingly switched their role for film water absorption from high to low by a simple variation in their content. The lowest water absorption occurred at a level of ∼15% cellulosic fiber content. Beyond that level, any increase in fiber resulted in an opposite effect on water absorbency, i.e., reductions in film hydrophobicity. Furthermore, the efficiency of a hydrophobic effect for cellulosic fibers is based on the chitosan content in these films. Cellulosic fibers were only effective at reducing water absorbency at a low chitosan level. When a high chitosan level was applied, the cellulosic fibers did not effectively reduce water absorbency, even with a high level of cellulosic fiber content in the film. This may likely be due to the ability of chitosan to readily engage in H-bonding interactions with the high surface area available on the cellulose fibers creating water cavities at the surface of the cellulosic fibers. Compared to the interaction of cellulosic fibers and chitosan, more profound interactions were observed between cellulosic fibers and gelatin. At a low gelatin level (
