Cross-Linked Potato Starch-Based Blend Films Using Ascorbic Acid

Aug 2, 2013 - The opacity and transparency of noncured and cured films with GL, XL, and AsA are listed in Table 3. The results indicate that the opaci...
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Cross-Linked Potato Starch-Based Blend Films Using Ascorbic Acid as a Plasticizer Soon-Do Yoon* Department of Chemical and Biomolecular Engineering, Chonnam National University, Yeosu, Jeonnam 550-749, South Korea ABSTRACT: The main objects of this study were to prepare the cross-linked potato starch/polyvinyl alcohol blend films with ascorbic acid (AsA) added as a plasticizer with and without heat curing and to examine their mechanical properties, elongation at break, degree of swelling, solubility, water vapor absorption, thermal properties, optical properties, and biodegradability. The specific surface area, pore volume, and topography of the films with and without heat curing were also investigated via nitrogen adsorption and desorption isotherms and atomic force microscopy analysis. The results indicate that the cured films possess mechanical, thermal, and optical properties enhanced compared to those of noncured films. The mechanical and water barrier properties of the AsA-added film were also found to be superior to those of other films with polyol plasticizers (glycerol and xylitol). The biodegradability test revealed that the prepared films are degraded by ∼35−80% after 165 days. KEYWORDS: potato starch/PVA blend films, ascorbic acid, heat curing, mechanical properties, biodegradability

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verified that the strength and flexibility of the films with added organic acid are superior to those of the films with added polyols because those with organic acid contain carboxyl groups as functional groups. In addition, N,N-bis(2-hydroxyethyl)formamide (BHF) with hydroxyl and amide groups as functional groups has been used as a new synthesized plasticizer for the starch-based films.22,23 The results showed that BHF forms more stable and stronger hydrogen bonds between the components of films than glycerol. Physical properties of the starch-based films with BHF have been improved, thus demonstrating that the type or sort of plasticizer markedly influences the properties of starch-based blend films. It is also very important to search for a cheap and nontoxic plasticizer, which gives starch-based blend films desirable properties. In this study, starch/PVA blend films were synthesized by using ascorbic acid (AsA) as an alternative plasticizer. AsA is a naturally occurring organic compound with antioxidant properties that widely exists in many biological liquids, medicines, vegetables, and fruits. It is one of the most important soluble vitamins that play a significant role in biological functions, for example, as a supplement for inadequate dietary intake, in wound healing, and in the prevention and treatment of the common cold, mental illness, and infertility.24 It is also widely used in foods and drinks as an antioxidant.25 The molecular structure of AsA consists of four hydroxyl groups, one ether group, and one ketone group as functional groups that are combined well with starch and PVA molecules. It is relatively cheap and abundant in nature. As mentioned above, various raw and synthetic materials are used as plasticizers of biodegradable polymers. Thus, AsA could be used as an alternative plasticizer in various fields such as packaging, agriculture, and edible films.

INTRODUCTION The same durable properties that make plastics ideal for applications such as packaging, coatings, building materials, and commodities as well as hygiene products can lead to waste disposal problems for traditional petroleum-derived plastics as these materials are not readily biodegradable, and because of their resistance to microbial degradation, petroleum-derived plastics accumulate in the environment. Research into the synthesis of materials from natural sources such as starch, protein, and cellulose has been undertaken with the aim of replacing their nonbiodegradable counterparts.1−3 Currently, much research is being devoted to starch, because it is very cheap and abundant. Starch has been used as a replacement for synthetic polymeric plastics in the food, textile, and paper industries following various modifications and processes.4−8 Starch-based polymers are frequently blended with highperformance synthetic polymers, i.e., starch/polyvinyl alcohol (PVA), starch/poly(lactic acid) (PLA), and starch/polyester blend polymers, to achieve specific properties required for various applications. Starch/PVA blend polymers are some of the most popular biodegradable plastics and are widely used in packaging and agriculture.9−12 Generally, when starch/PVA blend polymers are prepared, plasticizers have to be combined because of their high rigidity and low workability. The conventional plasticizers used for starch/PVA blend polymers are glycerol, water,13,14 and ethylene glycol,15 which improve the flexibility and workability, but several other chemicals like sorbitol,16,17 urea,18 malic acid, tartaric acid, citric acid,19,20 and glycerol−urea complex plasticizers21 have also been successfully employed. The type of plasticizer plays a key role in determining the physical properties of starch/PVA blend polymers. Physical properties such as mechanical properties, elongation at break, and water resistance of starch/PVA blend films with added polyols, i.e., glycerol and sorbitol, and organic acids, i.e., malic acid, tartaric acid, and citric acid, have been reported previously.16,19 The measurements of physical properties © 2013 American Chemical Society

Received: Revised: Accepted: Published: 1755

June 7, 2013 July 24, 2013 August 2, 2013 August 2, 2013 dx.doi.org/10.1021/jf4024855 | J. Agric. Food Chem. 2014, 62, 1755−1764

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“Mitutoyo”) at 15 random positions around the film. The specimen’s average thickness was ∼0.12 ± 0.003 mm. The gauge length and grip distance were both 50.0 mm. The crosshead speed was 20 mm/min, and the load cell was 250 kgf. The tests were conducted at constant values of 25 °C and 53% RH. Fourier Transform Infrared (FT-IR). The FT-IR spectra of the noncured films and cured films with and without AsA were recorded on an FT-IR spectrometer (JASCO FT/IR-430). The samples were thoroughly dried in a vacuum oven at 50 °C, and 16 scans were taken for each sample. Dynamic Thermogravimetric Measurements. Thermogravimetric measurements were taken on a Mettler-Toledo (Zurich, Switzerland) TGA/SDTA 851e thermal system from 25 to 600 °C in a N2 environment (flow rate, 20 mL/min) at a heating rate of 10 °C/min. The sample size was between 10 and 12 mg. Optical Properties. Each film specimen was cut into a rectangle piece and placed directly into a UV−visible spectrophotometer (Optizen 2120UV, Mecasys Co., Ltd.) test cell. All measurements were performed using air as the reference. A spectrum of each film was obtained at wavelengths between 200 and 800 nm. The transparency at 600 nm (T600) was calculated with eq 1:26

The object of this work was to prepare biodegradable films using potato starch, PVA, and AsA as a plasticizer and a casting method. We also evaluated the effect of AsA content and heat curing time and temperature on the optical and physical properties of the prepared films such as tensile strength, elongation at break, water resistance, and water vapor absorption, thermal properties, and biodegradability in soil.

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EXPERIMENTAL SECTION

Materials. Potato starch (PS) was purchased from Samyang Genex Co. Polyvinyl alcohol (PVA), ascorbic acid (AsA), glycerol (GL), and xylitol (XL) were purchased from Aldrich Chemical Co., Inc. (Milwaukee, WI). PVA was 99% hydrolyzed with an average molecular weight of 89000−98000. Distilled water (DW) was used in all experiments. Preparation of Cross-Linked PS/PVA Blend Films. Crosslinked PS/PVA blend films were obtained by the casting method. At first, a PVA solution was prepared by dissolving PVA in hot water (90 °C). PS and plasticizers (GL, XL, and AsA) were mixed together with water using a KitchenAid mixer (Anymix, Hyun-woo Star, Seoul, Korea) for 15 min. Formulations contained 10, 20, 30, 40, and 50 wt % GL, XL, and AsA. The PVA solution and PS/plasticizer mix were kept at 95 °C for 10 min. Then, the mixture was blended to form a homogeneously gel-like solution with a mechanical stirrer (600 rpm) at room temperature for 60 min. Bubbles, the byproduct of preparation, were removed by using an aspirator. The mixing composition is shown in Table 1. The gel-like solution thus prepared

T600 = − log T600/x or A 600 /x

where %T is percentage transmittance and x is the film thickness (millimeters). The opacity of the films was obtained using eq 2 according to the method of Gontard and Guilbert.27

opacity = absorbance at 500 nm × film thickness (mm)

Table 1. Compositions of Gel-like Solutions Used To Prepare PS/PVA Blend Films sample

PS (g)

PVA (g)

glycerol (wt %)

xylitol (wt %)

ascorbic acid (wt %)

DW (g)

PSP PSPG1 PSPG2 PSPG3 PSPG4 PSPG5 PSPX1 PSPX2 PSPX3 PSPX4 PSPX5 PSPA1 PSPA2 PSPA3 PSPA4 PSPA5

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5

− 10 20 30 40 50 − − − − − − − − − −

− − − − − − 10 20 30 40 50 − − − − −

− − − − − − − − − − − 10 20 30 40 50

100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

(1)

(2)

Degree of Swelling and Solubility of Films. The degree of swelling (DS) and solubility (S) of the films were measured by applying the following method. The dried films were immersed in distilled water at room temperature (25 °C). After equilibrium had been reached (24 h), moisture on the surface of the film was removed, and the weight of the films was measured. DS in films was calculated as

DS =

We − W0 W0

(3)

where We is the weight of the film at the adsorbing equilibrium and W0 is the first dry weight of the film. The swollen films were dried again for 24 h at 60 °C, and their solubility (S) was calculated with eq 4:

S=

W0 − Wd Wd

(4)

where W0 is the first dry weight of the film and Wd is the dry weight of the swelled film. Water Vapor Absorption. The pieces of prepared films were cut into small pieces (5 cm × 5 cm), and the weights of pieces were measured immediately. They were then dried in an oven at 60 °C overnight and weighed. The water content (k) of starch/PVA composite films was calculated with eq 5

was poured onto a prewarmed (60 °C) Teflon mold (200 mm × 200 mm × 1 mm). Water was evaporated from the molds in a ventilated oven at 50 °C for 24 h. Dried films were put in open polyethylene bags and stored at 25 °C and 53% relative humidity (RH) for 1 week. The prepared films were then cured in a vacuum oven at 95 ± 2, 120 ± 2, 140 ± 2, or 160 ± 2 °C for 30, 60, 90, 120, 180, 240, and 300 min at atmospheric pressure. After being heat cured, films were conditioned again at 53% RH and 25 °C for 1 week before the measurements were taken. The surfaces of the prepared films with and without heat curing were investigated via scanning electron microscopy (SEM) (S-4700, Hitachi, Tokyo, Japan), at an acceleration voltage of 5 kV. Mechanical Properties of Biodegradable Films. Tensile strength (TS) and elongation at break (%E) were evaluated for each film using an Instron 6012 testing machine. Five dumbbell-shaped specimens with a width of 15 mm (ASTM D-421) were cut from each film. Each piece was measured for thickness in three places along the test length using a mechanical scanner (digital thickness gauge

k=

Wf − W0 W0

(5)

where W0 is the mass of the dried sample and Wf is the mass of the sample before it had been dried. Water vapor adsorption of the films were evaluated after being stored in desiccator chambers over the salt solution of MgCl2 (54% RH) and Mg(NO3)228 for 10 days at 25 °C. The nitrogen adsorption and desorption isotherms at 77 K were measured in the relative pressure range of 10−5 to 0.99, using the volumetric adsorption analyzer (Micromeritics, ASAP 2020) to characterize the geometrical structures of the films with and without heat curing. Prior to the measurement, ∼0.2 g of the sample was outgassed at 373 K under vacuum for 24 h to remove the moisture content as well as the impurities. The textural properties such as the specific surface area [Brunauer−Emmett−Teller (BET)] and pore volume [total pore volume at P/P0 = 0.99 and Barrett−Joyner− 1756

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Halenda (BJH) for the mesopore size distribution] were calculated from the nitrogen adsorption isotherm data. AFM was also employed to assess the topography and roughness value of the surface of the films. AFM imaging was performed with a Multimode IV instrument (Veeco), and the images were scanned in tapping mode (noncontact mode) using silicon cantilevers. All measurements were taken under atmospheric conditions and at room temperature. Soil Burial Degradation Test. Soil burial degradation was conducted as described by Devi et al.29 and in our previous work30 with a slight modification. Garden pots with an approximate volume of 10 L were filled with soil taken from a culture field in Gangjin Gun, Korea. The samples were cut into 3 cm × 3 cm pieces and buried in the soil at a depth of 10 cm. The pots were placed in an uncovered gazebo. The soil was kept moist by being sprinkled with water at a regular time interval to maintain 30−50% humidity. The excess water drained through the hole at the bottom of the pot. The degradation of the specimen was determined at a regular time interval (15 days) by taking the specimen carefully from the soil and washing it gently with distilled water to remove the soil. The specimen was dried in an oven until a constant weight was obtained. The loss of weight of the specimen with time was used to indicate the degradation rate in the soil burial test.

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RESULTS AND DISCUSSION Mechanical Properties of Films. The measurement of mechanical properties such as tensile strength (TS) and elongation at break (%E) of biodegradable films plays an important role in various fields for application and modification. In addition, curing has a significant effect on mechanical properties. The methods of curing are gamma and electron beam irradiation,31,32 UV irradiation,33−35 and heat curing.36,37 The mechanical properties and water resistance for the films are improved by the use of these methods. Figure 1a shows the FT-IR spectra obtained for noncured films and cured films with and without AsA at different curing times. The strong peaks observed at 990−998, 1015−1020, and 1080 cm−1 are characteristic of the anhydroglucose ring like that found in starch. The broad band at 3265−3280 cm−1 was due to hydrogen-bonded hydroxyl groups (O−H). This band is of great importance because it indicates the presence of hydrogen bonding in the polymer. Higher-frequency shifts were observed when the heat curing time increased (∼3280 cm−1). This result is closely related to an increase in the level of hydrogen bonding among PS, PVA, and AsA hydroxyl groups, provoking an improvement in the mechanical properties and a decrease in the water solubility of the blended films.38 AsA is easily hydrolyzed by heat or light. To verify hydrolysis of AsA, the AsA was boiled in the DW at 90 °C, which was the same preparation condition used for the films. Figure 1b clearly shows the results of the FT-IR spectra before and after the AsA treatment. The results indicate that the hydrolysis of AsA did not occur under the synthesis conditions of the films. As shown in Figure 2b for the typical FT-IR spectrum of AsA, the spectra of CO, C−O−C, and O−H groups appeared at 1757, 1678, and 2920−3525 cm−1, respectively. Figure 2 shows the effect of heat curing time and temperature on the TS and %E of a nonplasticizer film and an AsA-added film. As shown in Figure 2a, TS slightly increased with increasing heat curing time whereas %E decreased until 120 min. However, when the curing time exceeded 150 min, TS increased and %E decreased rapidly because the films were discolored and oxidized. TS and %E of films with heat curing temperature are shown in Figure 2b. With an increasing curing temperature, TS slightly increased and %E drastically decreased. When heat curing was conducted at 120 °C, discoloration of

Figure 1. (a) FT-IR spectra of noncured and cured films with and without ascorbic acid (AsA). (b) FT-IR spectra of AsA and AsA treated with heat curing.

films occurred when heat curing lasted more than 150 min. When heat curing was performed for 120 min, discoloration of the film occurred after the temperature had reached ≥140 °C. Taking these results into consideration, we prepared the crosslinked PS/PVA blend films by heat curing at 120 °C for 120 min using glycerol (GL), xylitol (XL), and AsA as plasticizers. In addition, the results of the effects on heat curing revealed that TS of cured films was improved ∼1.5 times compared with that of films without heat curing. Panels a and b of Figure 3 show TS and %E for cured films with plasticizers (GL, XL, and AsA). Each plasticizer is added from 0 to 50 wt % on a mass percent ratio to total CS and PVA weight basis. As the content of GL, XL, and AsA increased, TS decreased and %E increased. A comparison of functional groups of plasticizers indicated that TS of a film with added XL with five hydroxyl groups was higher than that of a film with added GL with three hydroxyl groups. However, TS of a film with added AsA with four hydroxyl groups was highest of all the films because AsA has not only four hydroxyl groups but also one ether and one ketone. In other words, TS of an AsA-added film was improved because of the existence of the functional groups, i.e., hydroxyl, ether, and ketone groups, which can combine with starch and PVA molecules. In addition, the %E values indicated that GL-added films have higher values than 1757

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Figure 3. (a) Tensile strength (TS) and (b) elongation at break (%E) vs cured film glycerol (GL), xylitol (XL), and ascorbic acid (AsA) content.

Figure 2. Tensile strength (TS) and elongation at break (%E) for a nonplasticizer film and an AsA-added film vs heat curing time (a) and temperature (b).

with and without plasticizers. The effects of heat curing on the film without a plasticizer and the AsA-added film are shown in Figure 5a. Physical properties such as tensile properties, water resistance, and thermal stability are generally improved via the use of heat curing.32 As shown in Figure 5a, the TGA curve of the cured films with and without AsA shifted toward the hightemperature region. It means that the thermal properties of cured films are improved by heat curing, which improved the mutual reaction of inter-intramolecular combinations. In other words, heat curing treatment of the film slightly decreased the degree of thermal decomposition. This may be due to the crosslinking of the films, which increases the resistance to thermal decomposition. The thermal properties are closely related to mechanical properties. Comparison of the noncured films and cured films showed that the TS value of the film without a plasticizer increased from 55.6 to 69.2 MPa, and at the same time, the thermal stability was also improved. The TS value of the noncured AsA-added film increased from 15.6 to 20.1 MPa when the film was treated with heat curing. The degree of thermal decomposition of the cured film without AsA was higher than that of the cured AsA-added films because the strong interaction between PS and PVA was weakened by addition of AsA as the plasticizer. Because PS and plasticizers (GL, XL, and AsA) are especially sensitive to thermal

XL-added films or AsA-added films. The %E values of XLadded films were higher than those of AsA-added films when 30 wt % plasticizers were added, the %E values of AsA-added films were higher than those of XL-added films. A possible explanation of this phenomenon is that when >30 wt % AsA was added, the effect of inter-intramolecular reaction of one ether and one ketone as functional groups of AsA takes place between the components of PS, PVA, and plasticizer films. Figure 4 presents the effect of heat curing on the SEM images of the surface of films to which 40 wt % plasticizers had been added. The surface of the noncured film without plasticizers appeared to be quite homogeneous and smooth (Figure 4a). However, as shown in panels c, e, and g of Figure 4, the noncured films to which 40 wt % plasticizers (GL, XL, and AsA) had been added appeared to be somewhat rough and agglomerated because of the combination of PS, PVA, and plasticizers for the formation of blended films. SEM images of the surface of the cured films with and without added plasticizers showed no agglomeration, cracks, or pores (Figure 4b,d,f,h). Thermal Analysis of Films. Figure 5 shows the results of thermogravimetric analysis (TGA) of noncured and cured films 1758

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Figure 4. Scanning electron microscope image of the surface of PS/ PVA blend films with added 40 wt % GL, XL, and AsA with and without heat curing. (a) Noncured film without added plasticizer. (b) Cured film without added plasticizer. (c) Noncured film with added 40 wt % GL. (d) Cured film with added 40 wt % GL. (e) Noncured film with added 40 wt % XL. (f) Cured film with added 40 wt % XL. (g) Noncured film with added 40 wt % AsA. (h) Noncured film with added 40 wt % AsA. Noncured films are denoted PSP, PSPG4, PSPX4, and PSPA4, and cured films are denoted PSP-C, PSPG4-C, PSPX4-C, and PSPA4-C.

decomposition, the incorporation of plasticizers into a PS/PVA blend film intensifies its thermal decomposition. Panels b and c of Figure 5 show the TGA curves for the cured films with added 20, 40, and 50 wt % AsA and the cured films with added 40 wt % GL, XL, and AsA. As shown in Figure 5b, the degree of thermal decomposition increased but TS decreased (see Figure 3a) because the mobility between the components of the films is increased by the addition of AsA as the plasticizer. Furthermore, the flexibility of AsA-added films also increased. Figure 5c shows the TGA curves of the cured films with the same ratio of GL, XL, and AsA added (40 wt %). The changes in the rate of thermal decomposition in the cured films indicate that the thermal stability of the AsA-added film is superior to those of other GL- or XL-added films. In addition, the reversed phenomenon is observed above 350 °C because strong interactions among PS, PVA, and plasticizer are formed by the heat curing process and the addition of plasticizers. Degree of Swelling and Solubility of Films. The evaluation of the degree of swelling (DS) and solubility (S) of the prepared films plays an important role in the characterization of the degree of incorporation and cross-

Figure 5. Thermogravimetric analysis (TGA) of noncured and cured films with and without plasticizers. (a) TGA curves of noncured and cured films with and without added AsA. (b) TGA curves of cured films with 20, 40, and 50 wt % AsA. (c) TGA curves of cured films with added 40 wt % GL, XL, and AsA.

linking between the constituents of films as well as water resistance.39 Figure 6 shows the effect of heat curing time and temperature on the DS and S of the film without a plasticizer and those of the AsA-added film. As shown in Figure 6a, with an increasing heat curing time, DS values of the film without a plasticizer gradually decreased for 300 min until the end of 1759

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Figure 7. Transmittance (%) of non-cured and cured films added various plasticizers. (a) Transmittance (%) of non-cured films with/ without added 40 wt.% plasticizers. (b) Transmittance (%) of cured films with/without added 40 wt.% plasticizers.

Figure 6. Degree of swelling (DS) and solubility (S) for a film without a plasticizer and an AsA-added film vs (a) heat curing time and (b) temperature.

Table 2. Degree of Swelling and Solubility of Cured PS/PVA Blend Films in Terms of Plasticizer (GL, XL, and AsA) Content sample PSP PSPG1 PSPG2 PSPG3 PSPG4 PSPG5 PSPX1 PSPX2 PSPX3 PSPX4 PSPX5 PSPA1 PSPA2 PSPA3 PSPA4 PSPA5

degree of swelling (g/g) 2.330 1.642 1.183 0.725 0.575 0.473 1.783 1.280 0.962 0.752 0.607 1.931 1.516 1.154 0.930 0.814

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.061 0.069 0.072 0.053 0.089 0.070 0.075 0.051 0.063 0.066 0.072 0.058 0.081 0.072 0.066 0.087

Table 3. Opacity and Transparency of Noncured and Cured PS/PVA Blend Films with Various Plasticizers sample

solubility (g/g) 0.020 0.212 0.325 0.388 0.436 0.460 0.182 0.311 0.370 0.425 0.451 0.169 0.302 0.352 0.398 0.430

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

PSP PSP-C PSPG4 PSPG4-C PSPX4 PSPX4-C PSPA4 PSPA4-C

0.021 0.019 0.024 0.019 0.018 0.030 0.026 0.022 0.020 0.018 0.021 0.019 0.015 0.017 0.017 0.025

opacity 0.0185 0.0180 0.0169 0.0213 0.0180 0.0241 0.0198 0.0277

± ± ± ± ± ± ± ±

0.0055 0.0049 0.0059 0.0039 0.0051 0.0035 0.0040 0.0055

transparency 1.4289 1.3839 1.3478 1.4681 1.3755 1.6667 1.4301 1.8456

± ± ± ± ± ± ± ±

0.26 0.30 0.26 0.31 0.27 0.29 0.25 0.24

sample had been cured for 120 min, but after curing had been conducted for ≥150 min, DS values slowly decreased and S values increased. These results indicate that the inter-intramolecular interaction among PS, PVA, and plasticizer is improved until the sample is cured for 120 min, but when the sample had been heat cured for >150 min, the properties of the film markedly deteriorated, with discoloration and oxidation of the film. As for the effects of heat curing temperature, DS values gradually decreased until the final heat curing stage of 160 °C, but S values decreased rapidly up to 120 °C and then increased up to 160 °C (Figure 6b). These results suggest that these combinations between the constituents of films were

experiment but the S values drastically decreased until 120 min and increased again when the film was cured for >150 min. In the AsA-added film, DS and S values decreased rapidly until the 1760

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Table 4. Comparison of Specific Surface Areas, Pore Volumes, and Pore Diameters with and without Heat Curing

PSPA4 PSPA4-C

specific surface area (m2/g)

pore volume (cm3/g)

average pore diameter (nm)

9.01 9.89

0.0139 0.0163

5.39 4.99

destroyed by high temperatures. From the results, the changes in DS and S indicate that the water resistance was improved by heat curing. Table 2 lists DS and S values of cured films to which various plasticizers (GL, XL, and AsA) had been added. As the plasticizer content increased, DS values decreased but S values increased. DS values increased in the following order: GLadded film < XL-added film < AsA-added film. S values increased in the following order: AsA-added film < XL-added film < GL-added film. The GL-added films had the lowest DS value and the highest S value. This means that the GL-added film has a relatively weak bond strength. In contrast, the AsAadded films showed the highest DS value and the lowest S value, which indicates that the degree of combination among PS, PVA, and AsA is stronger than in the films with added GL or XL. Optical Properties. One of the demands of biodegradable polymers such as edible and packing materials is that they should protect materials from the effects of light, i.e., ultraviolet radiation. Figure 7 shows the light transmission characterization of noncured and cured films with various added plasticizers. Cured films had lower transmittance (percent) than noncured films, suggesting that cured films had improved light barrier properties as a result of heat curing. In addition, films with added AsA had the lowest transmittance (percent) compared with those of the films with added GL, XL, and AsA. It means that AsA-added films have a good barrier to ultraviolet light. The opacity and transparency of noncured and cured films with GL, XL, and AsA are listed in Table 3. The results indicate that the opacity and transparency values had a tendency to increase because of the improvement of the inter-intramolecular adhesion among PS, PVA, and plasticizers by heat curing. It could confirm that the opacity and transparency values of prepared films in this study are lower compared with the reported values of synthetic films. The results obtained seem to indicate that the AsA-added films are clear enough to be used as packaging or coating materials when their values are compared with values reported by Shiku et al.40 Water Vapor Absorption. The water absorption property of starch-based polymers is one of their notable defects. Enhancement of water resistance is also an important issue. Therefore, we investigated the water contents of the noncured films and cured films to which GL, XL, and AsA had been added as a plasticizer. The influence of humidity on the prepared films was also evaluated because PS/PVA blend films are highly sensitive to humidity. Figure 8 shows the effect of water content on noncured films and cured films to which 40 wt % plasticizers had been added. The measurements of water content for the noncured films and the cured films with and without various plasticizers were taken at the same 55% RH. The water contents are highly dependent on the plasticizers regardless of their treatment conditions. The equilibrium water contents of noncured films were in the following order: GLadded film (0.1044) > XL-added film (0.0937) > AsA-added film (0.0788) > the film without a plasticizer (0.0714) (Figure

Figure 8. Water contents of non-cured and cured PS/PVA blend films with/without added 40 wt.% GL, XL, and AsA. (a) Water contents of non-cured films with/without added 40 wt.% GL, XL, and AsA in terms of adsorption time (hr). (b) Water contents of cured films with/ without added 40 wt.% GL, XL, and AsA in terms of adsorption time (hr).

Figure 9. Nitrogen adsorption and desorption isotherms and atomic force microscopy (AFM) images of noncured and cured films with added 40 wt % AsA.

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Figure 10. Biodegradation of non-cured and cured films with/without added 40 wt.% GL, XL, and AsA. (a) Biodegradation of non-cured films with/ without added 40 wt.% GL, XL, and AsA. (b) Biodegradation of cured films with/without added 40 wt.% GL, XL, and AsA.

8a). However, as shown in Figure 8b, the water contents of the cured films, which were greater than those of the noncured films, were 0.1222 (GL-added film), 0.1147 (XL-added film), 0.0903 (AsA-added film), and 0.0990 (none plasticizer). The water content of the cured films was higher than that of noncured films. The hydrophilic properties of plasticizers are closely connected to the water resistance of starch-based blend films.20 From these results, the water content of the GL-added film and the XL-added film was higher than that of the AsAadded film, which shows that GL and XL are more hydrophilic than AsA. The results of water vapor absorption in the prepared films indicated that the water resistance of the AsA-added film was better than that of the GL-added film and XL-added film. It is important to note that the water content of the cured films is higher than that of noncured films. To confirm this phenomenon, we conducted a BET experiment and AFM analysis on the specific surface area, pore volume, and topography of both the noncured film and the cured films to which 40 wt % AsA had been added. Figure 9 shows the nitrogen adsorption and desorption isotherms and AFM images of the films. The nitrogen adsorption and desorption isotherms of the prepared films seemed to belong to type V according to

the IUPAC classification. The adsorption behaviors of the prepared films were similar to each other. However, the adsorbed amount of the cured film was larger than that of the noncured film. The detailed properties of the films such as the surface area, pore volume, and average pore size are listed in Table 4. The results of AFM analysis also confirmed that the surface roughness of the cured film is decreased by heat curing, and pores that can absorb H2O molecules are formed. Soil Burial Degradation. The biodegradability of the noncured films and the cured films to which 40 wt % GL, XL, and AsA had been added as plasticizers was estimated by using a soil burial method. As shown in panels a and b of Figure 10, the degree of biodegradation of the films was investigated by the loss of weight of the films with time. In all the films, a rapid degradation occurred in the initial 60 days, followed by a slow degradation until the end of the experiment (165 days). The degree of biodegradation of the cured films was slightly lower than that of the noncured films because of the influence of cross-linking by the treatment of heat curing. In addition, different plasticizers showed different degrees of biodegradability. The degree of biodegradation of the AsA-added films was higher than that of the GL-added films and XL-added films. 1762

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The biodegradability of the GL-added films and XL-added films is low because GL and XL as plasticizers play a role as antibacterial agents. Conclusions. Biodegradable films were successfully prepared by using potato starch, PVA, and ascorbic acid (AsA) as plasticizers and by heat curing treatment. The tensile strength (TS), elongation at break (%E), degree of swelling (DS), solubility (S), optical properties, water vapor absorption, thermal characteristics, and biodegradability of the prepared films were investigated. The results of the evaluation of the physical properties of noncured films and cured films indicated that the TS, %E, thermal properties, and water resistance of cured films were improved up to 65−295% by the use of heat curing. In addition, the optical properties of cured films were lower than those of noncared films. In particular, the optical properties of AsA-added films were superior to those of the other films. The water vapor absorption of cured films was slightly higher than that of noncured films. To verify this phenomenon, we investigated the specific surface area, pore volume, and topography of films with and without heat curing by using the BET experiment and AFM analysis. The results indicated that the nitrogen adsorption−desorption behavior of the prepared films was similar. However, the adsorbed amount of the cured film was larger than that of the noncured film. The results of AFM analysis verified that the surface roughness of the cured film was decreased by the heat curing process and that pores that can absorb H2O molecules are formed. Compared to the properties of the films with various plasticizers (GL, XL, and AsA), the mechanical, thermal, and water barrier properties of a film with added AsA with four hydroxyl groups, one ether, and ketone groups were superior to those of the other films with added GL and XL with only hydroxyl groups. The soil burial biodegradation test showed that the prepared films were degraded by ∼35−80% after 165 days. The degree of biodegradation of cured films, however, was lower than that of noncured films. In addition, the biodegradability was different with different plasticizers. The degree of biodegradation of AsAadded films was higher than that of GL-added films and XLadded films. Using the results of this study, we found that AsA could be used as an alternative plasticizer of biodegradable polymers.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +82-61-659-7297. Fax: +82-61-653-3659. E-mail: [email protected]. Funding

This study was financially supported by Chonnam National University. Notes

The authors declare no competing financial interest.

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NOTE ADDED AFTER ASAP PUBLICATION The authorship has been changed in the version of this paper published on August 20, 2013. The corrected version published August 28, 2013.

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