Carbohydrate Polymers 127 (2015) 135–144

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Water proof and strength retention properties of thermoplastic starch based biocomposites modified with glutaraldehyde Jen-taut Yeh a,b,c,∗ , Yuan-jing Hou a , Li Cheng a , Ya-Zhou Wang a , Liang Yang a , Chuen-kai Wang c a Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education, Key Laboratory for the Green Preparation and Application of Functional Materials, Faculty of Materials Science and Engineering, Hubei University, Wuhan, China b Department of Materials Engineering, Kun Shan University, Tainan, Taiwan c Graduate School of Material Science and Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan

a r t i c l e

i n f o

Article history: Received 31 December 2014 Received in revised form 13 March 2015 Accepted 15 March 2015 Available online 30 March 2015 Keywords: Water proof Strength retention Thermoplastic starch

a b s t r a c t Water proof and strength retention properties of thermoplastic starch (TPS) resins were successfully improved by reacting glutaraldehyde (GA) with starch molecules during their gelatinization processes. Tensile strength ( f ) values of initial and aged TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens improved significantly to a maximal value as GA contents approached an optimal value, while their moisture content and elongation at break values reduced to a minimal value, respectively, as GA contents approached the optimal value. The  f retention values of (TPS100 BC0.02 GA0.5 )75 PLA25 specimen aged for 56 days are more than 50 times higher than those of correspoding aged TPS and TPS100 BC0.02 specimens, respectively. New melting endotherms and diffraction peaks of VH -type starch crystals were found on DSC thermograms and WAXD patterns of aged TPS or TPS100 BC0.02 specimens, respectively, while negligible retrogradation effect was found for most aged TPS100 BC0.02 GAx and/or (TPS100 BC0.02 GAx )75 PLA25 specimens. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Thermoplastic starches (TPS) are generally prepared by the application of heat, pressure, mechanical work or by addition of plasticizers such as, glycerine, polyols or water (Biliaderis, Lazaridou, & Arvanitoyannis, 1999; Lourdin, Bizot, & Colonna, 1996; Lourdin, Coignard, Bizot, & Colonna, 1997) in native starches. Thermoplastic starch processing typically involves an irreversible order–disorder transition termed gelatinization. Starch gelatinization is the disruption of molecular organization within the starch granules and this process is affected by starch–water interactions. TPS products already have applications in the plastic market to take the place of nondegradable petrochemical based products (Bastioli, 1998; Biliaderis et al., 1999; Tatarka & Cunningham, 1998; Willett & Shogren, 2002). However, the resistance of TPS to shock or moisture is still relatively poor in

∗ Corresponding author at: Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education, Key Laboratory for the Green Preparation and Application of Functional Materials, Faculty of Materials Science and Engineering, Hubei University, Wuhan, China. Tel.: +86 13545228955. E-mail address: [email protected] (J.-t. Yeh). http://dx.doi.org/10.1016/j.carbpol.2015.03.059 0144-8617/© 2015 Elsevier Ltd. All rights reserved.

comparison with many other commodity resins (Halley, 2005). There has been an increasing research interest in thermoplastic starch reinforced with various available lignocellulosic fibers in order to improve their mechanical properties as well as to obtain the characteristic needed for actual application (Kurosumi, Sasaki, Yamashita, & Nakamura, 2009). Most recently, bacterial cellulose (BC) nanofibers were reported as an efficient reinforcing additive for preparing polymeric nanocomposites (Khaled, Richard, & Joel, 2007; Yeh et al., 2014; Yu, Wang, & Ma, 2005). The hydrophilic nature of starch causes rapid rise in moisture contents of TPS resins and hence, leads to significant reduction of their mechanical properties if the TPS resins were not modified during their preparation processes (Cova, Sandoval, Balsamo, & Müller, 2010; Zhou, Ren, Tong, Xie, & Liu, 2009). There are mainly three types of crystallinity in starch as observed in their X-ray diffraction patterns (Halley, 2005; Van Soest & Vliegenthart, 1997). “A” and “B” types of crystallinity are mainly present in cereal (e.g. maize, wheat and rice) and tuber (e.g. potato and sago) starches, respectively, while “C” type crystallinity is the intermediate between A and B type crystallinity, normally found in bean and other root starches (Van Soest & Vliegenthart, 1997). In contrast, amylose “VH , “VA or “EH types of crystallinity is processing-induced crystallinity, which is formed during thermomechanical processing (Choi, Kim,

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& Shin, 1999; Huang, Yu, & Ma, 2005; Van Soest & Vliegenthart, 1997). However, aging of starch materials in the rubbery state occurs by retrogradation, where the starch molecules reorganize in more ordered structures, for example by forming simple juncture points and entanglements, helices and crystal structures (Kainuma, 1988; Van Soest, Hulleman, de Wit, & Vliegenthart, 1996a; Van Soest & Knooren, 1997). The rate of retrogradation and crystallization is dependent on the plasticizer content and related to the glass-transition temperature of the starch molecules. Higher amounts of plasticizer cause an increase in the mobility of the starch chains and a reduction in the glass-transition temperature. Yu and coauthors (Yu et al., 2005) reported that thermoplastic starch plasticized by glycerol (GPTPS) stored at relative humidity (RH) 0 and 50% for 70 days showed VA -type crystallinity , i.e. double-helix conformation, while GPTPS stored at RH 100% for 70 days recrystallized and showed a B-type crystallinity, i.e. singlehelix conformation (Van Soest, Hulleman, de Wit, & Vliegenthart, 1996b). By contrast, as evidenced by wide angle X-ray diffraction analyses, citric acid-modified GPTPS (CATPS) effectively inhibited starch recrystallization (i.e. retrogradation) into VA -type or B-type crystallinity (Yu et al., 2005), because strong hydrogen-bond interaction was found between citric acid and starch molecules during their modification processes. Huang et al. (2005) reported that most of the crystallization in GPTPS was disrupted and only a little inconspicuous VH style crystallization occurred, which was induced in the process of plasticizing starch. The aged GPTPS showed a gradually enhanced diffraction peak originated from re-crystallization of VH style crystals, when it was stored at RH = 50% for 30, 60 and/or 90 days. As suggested by Van Soest and Knooren (1997), VH type is a single helical structure “inclusion complex”, which is made up of amylose and glycerol. Besides, cocrystallization of amylopectin with amylose probably also occurs, which has been suggested for the retrogradation of starch in starch gels (Kainuma, 1988; Van Soest et al., 1996a). In fact, re-crystallization of starch molecules restrains starch from practical use, because the starch easily becomes too weak to use during long-term storing, and loses the value in use (Van Soest et al., 1996b). Much effort has been made to improve the water proof properties of thermoplastic starches by substitution, esterification or acetylation of hydroxyl groups of starch molecules using organic acids or anhydrides (e.g. citric acid, succinic, maleic and phthalic anhydrides) (Cova et al., 2010; Van Soest et al., 1996b; Yu et al., 2005), inorganic esters (e.g. trisodium trimetaphosphate (TSTMP)), hydroxydiethers (e.g. epichlorohydrin) (Carvalho, Curvelo, & Gandini, 2005; Sagar & Merrill, 1995). Yu et al. (2005) showed that citric acid can form stable hydrogen-bond interactions with starch and improve water proof properties of glycerolplasticized thermoplastic starch at high RH values, although the tensile stress of thermoplastic starch specimen reduces significantly after modification by citric acid. It was reported that hydrophobicity of TPS improved greatly when TPS was modified by prepolymers containing NCO groups (Van Soest et al., 1996b). Carvalho et al. (2005) used several reagents, i.e., phenyl isocyanate, a phenol blocked polyisocyanate, stearoyl chloride and poly(styrene-co-glycidyl methacrylate) to react with the superficial hydroxyl groups of TPS films in the medium of methylene chloride or xylene, and found that all the treatments were effective in decreasing the hydrophilic character of the TPS surfaces. In contrast, irradiation or chemical cross-linking technologies were also used for water proof improvement of thermoplastic starches (Jane, Lim, Paetau, Spence, & Wang, 1994; Sagar & Merrill, 1995; Zhou, Zhang, Ma, & Tong, 2008). Jane et al. (1994) reported that the tensile and water proof properties of starch compounds made from starch and zein mixtures were significantly improved by crosslinking the compounds using dialdehyde. The modified TPS resins with improved water proof properties are expected

to exhibit significantly improved strength retention properties during aging processes. However, none of the above investigations (Carvalho et al., 2005; Cova et al., 2010; Jane et al., 1994; Sagar & Merrill, 1995; Van Soest et al., 1996b; Yu et al., 2005; Zhou et al., 2008) has reported the resulted strength retention properties of modified TPS resins and/or the correlation with their improved water proof properties. In this study, water proof and strength retention properties of BC reinforced TPS resins were successfully improved by reacting with glutaraldehyde (GA) in their gelatinization processes. After blending 25 wt% of poly (lactic acid) (PLA) with GA modified TPS resins, their processibility, water proof and strength retention properties were further improved. In order to understand these interesting water proof and strength retention properties found for GA and/or PLA modified TPS specimens, morphological, thermal and WAXD analyses of initial and aged GA and/or PLA modified TPS specimens were also performed in this investigation. Possible reasons accounting for the significantly improved water proof and strength retention properties of GA and/or PLA modified TPS specimens are proposed. 2. Materials and methods 2.1. Materials and sample preparation Tapioca starch powders and poly(lactic acid) (PLA) 4032D resins used in this study were purchased from Eiambeng Tapioca Starch Industry Corporation, Samutprakarn, Thailand and Nature Works Company, Blair, Nebraska, USA, respectively. The water proof and strength retention properties of tapioca starches were improved using a 25 wt% glutaraldehyde (GA) solution, which was purchased from Sinopharm Chemical Reagent Corporation, Shanghai, China. Detailed experimental procedures used for preparation of bacterial cellulose (BC) nanofibers were described in our previous investigation (Yeh et al., 2014). Thermoplastic starches (TPS) were prepared by mixing 50 g tapioca starch, 50 ml water and 20 g glycerol at 25 ◦ C using a high speed mixer for 1 h. Prior to gelatinization, BC nanofibers at 0.02 part per hundred parts of TPS resin (PHR) and 25 wt% GA solutions at various contents (i.e. 0–4 PHR) were added and mixed with the basic media prepared above, in which the BC nanofibers and GA solution were used to improve the water proof and strength retention properties of TPS. The above prepared mixtures were gelatinized in 250 ml flask at 90 ◦ C under stirring condition for 15 min, wherein the pH value of the gelatinized mixtures was adjusted to 4.0 by citric acid during all gelatinization processes. The TPS and modified TPS resins prepared above were dried in an air dry oven and then in a vacuum dry oven both at 80 ◦ C for 24 h to have the water content under 1 wt%. Small amounts (i.e. 25 wt%) of PLA were melt-blended with the dried TPS resins in a Changzhou Suyuan SU-70ML internal mixer at 180 ◦ C for 3.5 min to improve their processibility and strength retention properties. The injected specimens used for determination of moisture content, tensile and tensile retention properties of the above prepared resins were prepared in accordance with ASTM D638 type IV with a specimen thickness of 0.254 cm using a Wuhan Reiming SZ-05 mini-injection machine at 180 ◦ C and then cooled in the mold at 80 ◦ C for 30 s. Table 1 summarized the sample codes and compositions of TPS, TPS100 BC0.02 , TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens prepared in this study. After preparation, the samples were maintained or aged at 20 ◦ C/50% RH for certain amounts of time. 2.2. Fourier transform infra-red spectroscopy Fourier transform infrared (FT-IR) spectroscopic measurements of GA, TPS100 BC0.02 , TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25

J.-t. Yeh et al. / Carbohydrate Polymers 127 (2015) 135–144 Table 1 Sample codes and compositions of TPS and modified TPS specimens prepared in this study. Sample codes

Starch content (parts)

BC content (parts)

Glutaraldehyde content (parts)

PLA content (parts)

TPS TPS100 BC0.02 TPS100 BC0.02 GA0.25 TPS100 BC0.02 GA0.5 TPS100 BC0.02 GA1 TPS100 BC0.02 GA2 TPS100 BC0.02 GA4

100 100 100 100 100 100 100

0 0.02 0.02 0.02 0.02 0.02 0.02

0 0 0.25 0.5 1 2 4

0 0 0 0 0 0 0

Sample codes

Glutaraldehyde content in TPS (parts)

PLA content (parts)

(TPS100 BC0.02 )75 PLA25 (TPS100 BC0.02 GA0.25 )75 PLA25 (TPS100 BC0.02 GA0.5 )75 PLA25 (TPS100 BC0.02 GA1 )75 PLA25 (TPS100 BC0.02 GA2 )75 PLA25 (TPS100 BC0.02 GA4 )75 PLA25

0 0.25 0.5 1 2 4

25 25 25 25 25 25

specimens were recorded on a Nicolet Avatar 360 FT-IR spectrophotometer at 25 ◦ C, wherein 32 scans with a spectral resolution 1 cm−1 were collected during each spectroscopic measurement. Infrared spectra of GA, TPS100 BC0.02 , TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 film specimens were determined using the conventional KBr disk method. All the film specimens were ground and mixed with KBr disk and then dried at 60 ◦ C for 30 min except GA film specimen. A droplet of 25 wt% GA solution was drip onto KBr disk and then dried at 60 ◦ C for 30 min to prepare GA film specimen. The film specimens used in this study were prepared sufficiently thin enough to obey the Beer–Lambert law. 2.3. Morphological analyses The fracture surfaces of tapioca starch, injection molded TPS, TPS100 BC0.02 , TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 tensile specimens were observed using a Hitachi S-3000N scanning electron microscope (SEM). The fractured tensile specimens used for morphological analyses were obtained by tensile testing the injected specimens using a Hung-Ta HT-9112 tension testing machine at 25 ◦ C and a crosshead speed of 50 mm/min. Prior to morphological analyses, the fracture surfaces of the tensile specimens were gold-coated at 20 mA/15 kV for 10 s before SEM examinations. 2.4. Moisture contents Before measurement, the injected specimens prepared above were ground into powders using a grinder at 25 ◦ C for 15 s. Moisture contents of initial and aged TPS, TPS100 BC0.02 , TPS100 BC0.02 GAx (TPS100 BC0.02 GAx )75 PLA25 powder specimens were determined using a Shanghai Jingke DHS16-A infrared moisture meter at temperatures ranging from 25 ◦ C to 120 ◦ C for 30 min. 2.5. Thermal properties Thermal properties of initial and aged tapioca starch, TPS, TPS100 BC0.02 , TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 were determined at 25 ◦ C using a Du Pont 2010 differential scanning calorimetry (DSC). All scans were carried out at a heating rate of 20 ◦ C/min and under flowing nitrogen at 25 ml/min. The instrument was calibrated using pure indium. Samples weighing about 0.5 mg were placed in standard aluminum sample pans for determination of their melting temperatures.

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2.6. Wide angle X-ray diffraction analyses Wide angle X-ray diffraction (WAXD) patterns of initial and aged tapioca starch, TPS, TPS100 BC0.02 , TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens were determined at 25 ◦ C using a Shimadu XRD-6000 diffractometer equipped with a Nifiltered CuK␣ radiation operated at 40 kV and 100 mA. Each specimen with 2 mm thickness was maintained stationary and scanned in the reflection mode from 5◦ to 30◦ at a scanning rate of 5◦ min−1 .

2.7. Tensile and tensile retention properties The specimens used to determine the tensile and tensile retention properties of TPS, TPS100 BC0.02 , TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens were prepared in accordance with ASTM D638 type IV using a Wuhan Reiming SZ-05 miniinjection machine at 180 ◦ C and then cooled in the mold at 80 ◦ C for 30 s. Before injection, TPS, TPS100 BC0.02 , TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 resins were dried in an air dry oven and then in a vacuum dry oven both at 80 ◦ C for 24 h to have water content under 1 wt%. The injected specimens with a thickness of 0.254 cm were then determined using a Hung-Ta HT-9112 tension testing machine at 25 ◦ C and a crosshead speed of 50 mm/min. A 35 mm gauge length was used during each tensile experiment. The values of tensile and tensile retention properties were obtained based on the average results of at least five tensile specimens.

3. Results and discussion 3.1. Fourier transform infrared spectroscopy Figs. 1 and 2 illustrate typical Fourier transform infrared (FT-IR) spectra of GA, TPS100 BC0.02 , TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens. Two distinctive absorption bands centered at 1725 and 2751 cm−1 corresponding to the motions of C O and C H stretching vibrations of aldehyde group (Choi et al., 1999), respectively, were found on the FT-IR spectrum of the GA specimen (see Fig. 1a). The FT-IR spectrum of TPS100 BC0.02 specimen exhibited four distinctive absorption bands centered at 1384, 1640, 2928 and 3420 cm−1 , which were generally attributed to the motion of C H bending vibration, H O H, C H and O H stretching vibrations, respectively (Delval et al., 2004). In addition to the C H bending vibration, H O H, C H and gradually weaken O H stretching vibration bands, a new absorption band centered at 1157 cm−1 corresponding to ether ( C O C ) stretching vibration gradually appeared on FT-IR spectra of TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 series specimens (see Figs. 1c–g and 2c–g). However, the absorption bands centered at 1725 and 2751 cm−1 originally corresponding to the motions of C O and C H stretching vibrations of aldehyde group disappeared nearly completely in FT-IR spectra of TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 series specimens. The newly developed ether stretching bands, weaken O H stretching vibration bands and nearly disappeared C O and C H stretching bands of TPS100 BC0.02 GAx series specimens are attributed to the reaction of the hydroxyl groups of TPS100 BC0.02 specimens with the aldehyde groups of glutaraldehyde molecules during their modification processes. It is highly likely that crosslinking reaction between glutaraldehyde and starch molecules can carry out to some extent. As reported by Khaled et al. (2007), the possible crosslinking reaction mechanism of glutaraldehyde with starch molecules was occurred through nucleophilic addition of hydroxyl groups to the carbonyl groups to form semi-acetal linkages.

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Fig. 1. FT-IR spectra of (a) GA, (b) TPS100 BC0.02 , (c) TPS100 BC0.02 GA0.25 , (d) TPS100 BC0.02 GA0.5 , (e) TPS100 BC0.02 GA1 , (f) TPS100 BC0.02 GA2 and (g) TPS100 BC0.02 GA4 specimens.

Fig. 2. FT-IR spectra of (a) PLA, (b) TPS100 BC0.02 GA0.5 , (c) (TPS100 BC0.02 GA0.25 )75 PLA25 , (d) (TPS100 BC0.02 GA0.5 )75 PLA25 , (e) (TPS100 BC0.02 GA1 )75 PLA25 , (f) (TPS100 BC0.02 GA2 )75 PLA25 and (g) (TPS100 BC0.02 GA4 )75 PLA25 specimens.

As shown in Fig. 2a, PLA specimen exhibited five distinctive absorption bands centered at 1384, 1640, 1759, 2928 and 3430 cm−1 corresponding to the motions of C H bending vibration, H O H, C O, C H and O H stretching vibrations bands (Agarwal, Koelling, & Chalmers, 1998; Kister, Cassanas, & Vert, 1998; Liu, Zou, Li, Cao, & Chen, 2006), respectively. After blending 25 wt% PLA with

TPS100 BC0.02 GAx , the FT-IR spectra of (TPS100 BC0.02 GAx )75 PLA25 series specimens look nearly the same as the integration of FTIR spectra of PLA and corresponding TPS100 BC0.02 GAx specimens, in which no new vibration band but only vibration bands originally present in spectra of PLA and TPS100 BC0.02 GAx specimens were found in FT-IR spectra of (TPS100 BC0.02 GAx )75 PLA25 series

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specimens, respectively. These results suggest that no distinctive chemical reaction or molecular interactions occurred during the melt-blending processes of PLA and TPS100 BC0.02 GAx resins. 3.2. Morphology analyses Typical SEM micrographs of the fracture surfaces of TPS, TPS100 BC0.02 , TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens are summarized in Fig. 3. Granular tapioca starches with 5–10 ␮m in diameter were found on SEM micrographs of the original tapioca starches (see Fig. 3a). The granular tapioca starches were completely dismantled and gelatinized as a continuous phase after gelatinization, in which only smooth characteristic was found on the fracture surface of TPS specimen (see Fig. 3b). After modification by GA, more ductile characteristic with drawn debris was found on the fracture surfaces of TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens. As evidenced by FT-IR analyses in the previous section, this is most likely due to the reaction of the hydroxyl groups of TPS100 BC0.02 specimens with the aldehyde groups of glutaraldehyde molecules. As shown in Fig. 3h, clearly separated PLA droplets were found on (TPS100 BC0.02 )75 PLA25 specimen that is attributed to the incompatibility between TPS100 BC0.02

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and PLA molecules during their blending processes. In contrast, significantly less and smaller separated PLA droplets were found on fracture surfaces of (TPS100 BC0.02 GAx )75 PLA25 specimens than those found for (TPS100 BC0.02 )75 PLA25 specimen. These results clearly suggested that the GA modified TPS100 BC0.02 GAx molecules are much more compatible with PLA molecules. 3.3. Moisture contents The moisture contents of initial and aged TPS, TPS100 BC0.02 , TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens are summarized in Fig. 4. The initial TPS and TPS100 BC0.02 specimens exhibited relatively high moisture contents at 5.0% and 4.4%, respectively. After remaining at 20 ◦ C/50% RH for certain amounts of time, the moisture contents of aged TPS and TPS100 BC0.02 specimens increased significantly from 5.0% and 4.4% to 10.1% and 9.5%, 13.7% and 12.3% and then to 19.6% and 19.3%, respectively, as the aging time increased from 0 to 7, 28 and to 56 days. After modification with varying amounts of GA during gelatinization processes of TPS100 BC0.02 specimens, the moisture contents of initial TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens reduced significantly to 3.1% and 2.7%, respectively.

Fig. 3. SEM micrographs of fracture surfaces of (a) Tapioca starch, (b) TPS, (c) TPS100 BC0.02 , (d) TPS100 BC0.02 GA0.25 , (e) TPS100 BC0.02 GA0.5 , (f) TPS100 BC0.02 GA1 , (g) TPS100 BC0.02 GA2 , (h) (TPS100 BC0.02 )75 PLA25 , (i) (TPS100 BC0.02 GA0.25 )75 PLA25 , (j) (TPS100 BC0.02 GA0.5 )75 PLA25 , (k) (TPS100 BC0.02 GA1 )75 PLA25 and (l) (TPS100 BC0.02 GA2 )75 PLA25 specimens.

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modification processes. In addition, blending TPS100 BC0.02 GAx with inherently hydrophobic PLA can further prevent TPS100 BC0.02 GAx from absorbing moisture and hence, improve the water proof properties of the initial and aged (TPS100 BC0.02 GAx )75 PLA25 specimens. 3.4. Thermal properties

Fig. 4. The moisture contents of initial and aged TPS (), TPS100 BC0.02 (), TPS100 BC0.02 GA0.25 (), TPS100 BC0.02 GA0.5 (), TPS100 BC0.02 GA1 (), (), TPS100 BC0.02 GA4 (夽), (TPS100 BC0.02 GA0.25 )75 PLA25 TPS100 BC0.02 GA2 (), (TPS100 BC0.02 GA0.5 )75 PLA25 (), (TPS100 BC0.02 GA1 )75 PLA25 (), (TPS100 BC0.02 GA2 )75 PLA25 ( ) and (TPS100 BC0.02 GA4 )75 PLA25 () specimens.

The moisture contents of all aged TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens are significantly lower than those of corresponding aged TPS and TPS100 BC0.02 specimens maintained at 20 ◦ C/50% RH for the same amount of time. In which, aged (TPS100 BC0.02 GAx )75 PLA25 specimens exhibited even lower moisture contents than the corresponding aged TPS100 BC0.02 GAx specimens without blending with 25 wt% of PLA. Moreover, it is worth to note that aged (TPS100 BC0.02 GA0.5 )75 PLA25 specimen exhibited significantly lower moisture contents than other aged (TPS100 BC0.02 GAx )75 PLA25 specimens modified with GA contents other than 0.5 part per hundred parts of TPS resin (PHR). In fact, after aging at 20 ◦ C/50% relative humidity for 56 days, the moisture contents of aged (TPS100 BC0.02 GA0.5 )75 PLA25 specimen reached only 8.9%, which is less than half of the moisture contents of those of corresponding aged TPS and TPS100 BC0.02 specimens. As evidenced by FT-IR analyses in the previous section, significant amounts of hydroxyl groups of starch molecules were reacted with aldehyde groups of GA molecules into ether functional groups during the modification processes of TPS100 BC0.02 GAx specimens. Apparently, the significant improvement in water proof properties of the initial and aged TPS100 BC0.02 GAx and/or (TPS100 BC0.02 GAx )75 PLA25 specimens is mainly due to the efficient blocking of the moisture-absorbing hydroxyl groups of starch molecules present in TPS100 BC0.02 GAx specimens during their

Typical DSC thermograms of TPS, TPS100 BC0.02 , TPS100 BC0.02 GAx , PLA and (TPS100 BC0.02 GAx )75 PLA25 specimens are summarized in Fig. 5A–C. Smooth thermograms without any endotherm were found for the initial TPS, TPS100 BC0.02 and TPS100 BC0.02 GAx specimens (see Fig. 5A-a, A-f, B-a, B-f, B-k and B-p). A new melting endotherm with a peak temperature at about 150 ◦ C gradually appeared on the DSC thermograms of TPS and TPS100 BC0.02 specimens, respectively, after they were aged at 20 ◦ C/50% RH for 7 days or more than 7 days. The size of the new melting endotherm grew significantly, as the aging time increased. However, as shown in Fig. 5A-b–A-e and A-g–A-j, the peak melting temperatures of aged TPS and TPS100 BC0.02 specimens reduced from around 151.0 ◦ C to 150.0 ◦ C, 148.2 ◦ C and then to 143.3 ◦ C as the aging time increased from 7, 14 to 28 and 56 days, respectively. In contrast, one can barely find any endotherm on DSC thermograms of TPS100 BC0.02 GAx specimens after they were aged at 20 ◦ C/50% RH for less than 28 days (see Fig. 5B). In fact, the thermograms of TPS100 BC0.02 GA0.5 and TPS100 BC0.02 GA1 specimens remained relatively smooth without any distinguished endotherm even after aging at 20 ◦ C/50% RH for less than 56 days. Similarly, the peak melting temperatures of aged TPS100 BC0.02 GA0.25 and TPS100 BC0.02 GA2 specimens reduced from 147.1 ◦ C and 155.6 ◦ C to 143.2 ◦ C and 154.1 ◦ C, respectively, as the aging time values increased from 28 to 56 days (see Fig. 5B-d, B-e and B-s, B-t). As shown in Fig. 5C-u, a distinguished melting endotherm with a peak melting temperature 167.2 ◦ C was found on the DSC thermogram of PLA specimen. Moreover, a glass transition at 60.0 ◦ C and a recrystallization exotherm with a peak temperature at 102.6 ◦ C was found on the DSC thermogram of PLA specimen. After blending 25 wt% PLA with TPS100 BC0.02 GAx , the DSC thermograms of (TPS100 BC0.02 GAx )75 PLA25 series specimens look nearly the same as the integration of thermograms of PLA and corresponding TPS100 BC0.02 GAx specimens, respectively. It is interesting to note that thermograms of aged (TPS100 BC0.02 GAx )75 PLA25 specimens remained relatively unchanged after aging at 20 ◦ C/50% RH for varying amounts of time. In contrast to those TPS100 BC0.02 GAx specimens aged at 20 ◦ C/50% RH for 56 days, one can barely find the newly developed melting endotherm on thermograms of (TPS100 BC0.02 GAx )75 PLA25 specimens, even when they were aged at 20 ◦ C/50% RH for 56 days. 3.5. Wide angle X-ray diffraction Typical wide angle X-ray diffraction (WAXD) patterns of tapioca, initial and aged TPS, TPS100 BC0.02 , TPS100 BC0.02 GAx , (TPS100 BC0.02 GAx )75 PLA25 and PLA specimens are summarized in Fig. 6A–C. As shown in Fig. 6A-a, distinguished diffraction peaks centered at 14.9◦ , 17.4◦ , 17.7◦ and 22.6◦ were found on WAXD diffraction patterns of tapioca starches. These diffraction peaks most likely correspond to A-type starch crystals with strong reflections at 2 around 14.8, an unresolved doublet at around 17 and 22.6 (Hizukuri, 1985; Van Soest et al., 1996b; Wu & Sarko, 1978). After gelatinization, the diffraction peaks corresponding to A-type starch crystals disappeared near completely on the WAXD diffraction pattern of the initial TPS, and TPS100 BC0.02 specimens (see Fig. 6A-b and A-g). Two new diffraction peaks centered at 2 = 13.6◦ and 20.9◦ appeared gradually on WAXD patterns of TPS and TPS100 BC0.02 specimens, respectively, after they were aged at 20 ◦ C/50% RH for 7 days or more than 7 days. In fact, the sizes of

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Fig. 5. DSC thermograms of (A) TPS and TPS100 BC0.02 , (B) TPS100 BC0.02 GA0.25 , TPS100 BC0.02 GA0.5 , TPS100 BC0.02 GA1 and TPS100 BC0.02 GA2 , (C) (TPS100 BC0.02 GA0.25 )75 PLA25 , (TPS100 BC0.02 GA0.5 )75 PLA25 , (TPS100 BC0.02 GA1 )75 PLA25 , (TPS100 BC0.02 GA2 )75 PLA25 and PLA specimens aged at 20 ◦ C/50% RH for varying amounts of time.

Fig. 6. Wide-angle X-ray diffraction patterns of (A) Tapioca, TPS and TPS100 BC0.02 , (B) TPS100 BC0.02 GA0.25 , TPS100 BC0.02 GA0.5 , TPS100 BC0.02 GA1 and TPS100 BC0.02 GA2 , (C) (TPS100 BC0.02 GA0.25 )75 PLA25 , (TPS100 BC0.02 GA0.5 )75 PLA25 , (TPS100 BC0.02 GA1 )75 PLA25 , (TPS100 BC0.02 GA2 )75 PLA25 and PLA specimens aged at 20 ◦ C/50% RH for varying amounts of time.

two new diffraction peaks grew significantly, as the aging time increased to 56 days (see Fig. 6A-b–A-f and A-g–A-k). The two new diffraction peaks were reported to originate from diffraction of VH type crystallinity (Huang et al., 2005), which was induced during their plasticization processes. In contrast, one can barely find the two new diffraction peaks on WAXD patterns of TPS100 BC0.02 GAx specimens aged at 20 ◦ C/50% RH for less than 28 days (see Fig. 6B). In fact, the WAXD patterns of TPS100 BC0.02 GA0.5 and TPS100 BC0.02 GA1 specimens remained relatively smooth without any diffraction peak even after aging at 20 ◦ C/50% RH for less than 56 days. The two new diffraction peaks of TPS100 BC0.02 GA0.25 and TPS100 BC0.02 GA2

specimens reappeared and grew gradually, as the aging time were equal to or more than 28 days (see Fig. 6B-d, B-e and B-s, B-t). Distinguished diffraction peaks centered at 2 = 15◦ , 16.7◦ , 18.5◦ and 22.5◦ were found on the WAXD pattern of the PLA specimen (see Fig. 6C-u). These diffraction peaks were reported to originate from the diffraction of ˛ form PLA crystals (Ikada, Jamshidi, Tsuji, & Hyon, 1987). After blending 25 wt% PLA with TPS100 BC0.02 GAx , one can only find weak diffraction peak centered at 16.7◦ on WAXD diffraction patterns of (TPS100 BC0.02 GAx )75 PLA25 specimens (see Fig. 6C). No additional diffraction peak was found on WAXD patterns of (TPS100 BC0.02 GAx )75 PLA25 specimens, when they were

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aged at 20 ◦ C/50% RH for less than 28 days. Two new diffraction peaks centered at 2 = 13.6◦ and 20.9◦ gradually appeared on WAXD patterns of (TPS100 BC0.02 GAx )75 PLA25 specimens aged at 20 ◦ C/50% RH for 28 days or more than 28 days. In fact, the WAXD patterns of aged (TPS100 BC0.02 GA0.5 )75 PLA25 and (TPS100 BC0.02 GA1 )75 PLA25 specimens remained relatively smooth without any aditional diffraction peak even after aging at 20 ◦ C/50% RH for less than 56 days. The two new diffraction peaks of (TPS100 BC0.02 GA0.5 )75 PLA25 and (TPS100 BC0.02 GA1 )75 PLA25 specimens reappeared only as little inconspicuous diffraction peaks, as the aging time reached 56 days (see Fig. 6C-j and C-o). The magnitudes of inconspicuous diffraction peaks observed for aged (TPS100 BC0.02 GA0.5 )75 PLA25 and (TPS100 BC0.02 GA1 )75 PLA25 specimens are even smaller than those of corresponding aged TPS100 BC0.02 GA0.5 and TPS100 BC0.02 GA1 specimens without blending the hydrophobic PLA resins. WAXD analyses revealed that A-type starch crystals originally present in granular tapioca starches were completely dismantled during their gelatinization processes. The new melting endotherm and diffraction peaks of VH -type crystals found in DSC thermograms and WAXD patterns of aged TPS or TPS100 BC0.02 specimens, respectively, was attributed to the significant retrogradation of tapioca starch molecules occurred during their aging processes. During retrogradation, recrystallization of tapioca starch molecules of TPS and/or TPS100 BC0.02 specimens occurred significantly in moisture rich environment, since TPS or TPS100 BC0.02 specimens can easily absorb moisture during their aging processes. However, one can barely find any new melting endotherm or diffraction peaks on DSC thermograms or WAXD patterns of TPS100 BC0.02 GAx and/or (TPS100 BC0.02 GAx )75 PLA25 specimens, respectively, even after they were aged at 20 ◦ C/50% RH for less than 28 days. Apparently, this is due to the significant improvement in water proof properties of the TPS100 BC0.02 GAx and/or (TPS100 BC0.02 GAx )75 PLA25 specimens, since the moisture-absorbing hydroxyl groups of starch molecules were successfully reacted with aldehyde groups of GA molecules during the modification processes of TPS100 BC0.02 GAx specimens. 3.6. Tensile and tensile retention properties

Fig. 7. Tensile strength and elongation at break of initial and aged TPS (, ), (夽, ), TPS100 BC0.02 GA0.25 (, ), TPS100 BC0.02 TPS100 BC0.02 GA0.5 (♦,

), TPS100 BC0.02 GA1 (⊗,

), TPS100 BC0.02 GA4 (,

The initial and retention values of tensile strength ( f ) and elongation at break (εf ) of TPS, TPS100 BC0.02 , TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens are summarized in Fig. 7. Relatively high  f and εf values at 27.8 MPa/6.9% and 28.7 MPa/6.7% were found for the initial TPS and TPS100 BC0.02 specimens, respectively. However, after maintaining the specimens at 20 ◦ C/50% RH for certain amounts of time, the  f retention values of TPS and TPS100 BC0.02 specimens reduced rapidly from 27.8 MPa/28.7 MPa to 5.1 MPa/5.8 MPa, 1.6 MPa/1.8 MPa and then to 0.3 MPa/0.4 MPa, respectively, as the aging time increased from 0 to 7, 28 and to 56 days. In contrast, the εf retention values of TPS and TPS100 BC0.02 specimens increased significantly from 6.9%/6.7% to 8.0%/7.5%, 21.2%/21.0% and then to 34.8%/34.8%, respectively, as the aging time increased from 0 to 7, 28 and to 56 days. After modification with varying amounts of GA during TPS100 BC0.02 gelatinization processes, the initial  f values of TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens reduced significantly as GA contents increased up to 0.5 PHR. At GA contents higher than 1 PHR, the initial  f values of TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens reduced somewhat slower with the further increase in GA contents. In contrast, the initial εf values of TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens remained relatively unchanged at 5.5% and 7.5%, respectively, as GA contents increased from 0.5 to 4 PHR. However, after aging at 20 ◦ C/50% RH for certain amounts of time, the  f and εf retention values of most TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens are significantly higher but lower than those of corresponding aged TPS and TPS100 BC0.02 specimens, respectively. In

(TPS100 BC0.02 GA0.5 )75 PLA25

( ,

), TPS100 BC0.02 GA2 (,

), (TPS100 BC0.02 GA0.25 )75 PLA25 ( , ), (TPS100 BC0.02 GA1 )75 PLA25

(䊉,

) and (TPS100 BC0.02 GA4 )75 PLA25 ( , (TPS100 BC0.02 GA2 )75 PLA25 (, specimens aged at 20 ◦ C/50% RH for varying amounts of time.

), ), )

fact, after aging at 20 ◦ C/50% relative humidity for more than 7 days, all TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens exhibited significantly higher  f but lower εf retention values than those of corresponding aged TPS and TPS100 BC0.02 specimens. In which, aged (TPS100 BC0.02 GAx )75 PLA25 specimens exhibited significantly higher  f but lower εf retention values than corresponding TPS100 BC0.02 GAx specimens aged for the same amounts of time but without blending with 25 wt% of PLA (see Fig. 7). Moreover, it is worth to note that aged (TPS100 BC0.02 GA0.5 )75 PLA25 specimen exhibited the highest  f but lowest εf retention values than other (TPS100 BC0.02 GAx )75 PLA25 specimens aged for the same amounts of time but modified with GA contents other than 0.5 PHR. In fact, after aging at 20 ◦ C/50% relative humidity for 56 days, the  f and εf retention value of (TPS100 BC0.02 GA0.5 )75 PLA25 specimen remained at 20.3 MPa and 8.7%, respectively, which is equivalent to 82.5% and 1.5 times of its initial  f and εf values, respectively, and more than 50 times higher but 25% lower than those of TPS and TPS100 BC0.02 specimens aged for the same amounts of time, respectively. The rapid reduction in  f but increase in εf retention values of the aged TPS and TPS100 BC0.02 specimens is apparently due to the excessive amounts of moisture absorbed during their aging processes, because the water molecules can effectively

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plasticize, soften and recrystallize starch molecules during their aging processes. As a consequence, εf values of TPS and TPS100 BC0.02 specimens increased significantly, while their  f values reduced rapidly with the increase in aging time. In contrast, as evidenced by moisture content analyses, the water proof properties of TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens were significantly improved, because moisture-absorbing hydroxyl groups of starch molecules were successfully blocked by reacting with proper amounts of aldehyde groups of GA molecules during their modification processes. It is, therefore, reasonable to infer that TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens can exhibit significantly improved  f retention values but reduced εf retention values than those of aged TPS and/or TPS100 BC0.02 specimens.

4. Conclusions Water proof and strength retention properties of BC reinforced TPS resins were successfully improved by reacting with GA in their gelatinization processes. As evidenced by newly developed C O C stretching bands on FT-IR spectra of TPS100 BC0.02 GAx series specimens, hydroxyl groups of TPS100 BC0.02 resins were successfully reacted with the aldehyde groups of GA molecules during their modification processes. Significantly less and smaller separated PLA droplets were found on fracture surfaces of (TPS100 BC0.02 GAx )75 PLA25 specimens than those found for (TPS100 BC0.02 )75 PLA25 specimen. These results suggested that the GA modified TPS100 BC0.02 GAx molecules are much more compatible with PLA molecules. The moisture contents of all aged TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens are significantly lower than those of corresponding aged TPS and TPS100 BC0.02 specimens maintained at 20 ◦ C/50% RH for the same amount of time. In fact, for the same amounts of aging time, the moisture content values of initial and aged TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens reduced to a minimal value, as their GA contents approached an optimal value at 0.5 PHR. Apparently, the significant improvement in water proof properties of the initial and aged TPS100 BC0.02 GAx and/or (TPS100 BC0.02 GAx )75 PLA25 specimens is mainly due to the efficient blocking of the moisture-absorbing hydroxyl groups of starch molecules during their modification processes. In addition, the inherently hydrophobic PLA can further prevent TPS100 BC0.02 GAx from absorbing moisture and hence, improve the water proof properties of the initial and aged (TPS100 BC0.02 GAx )75 PLA25 specimens. The new melting endotherm and diffraction peaks of VH -type crystals found in DSC thermograms and WAXD patterns of aged TPS or TPS100 BC0.02 specimens, respectively, was attributed to the occurring of significant moisture absorption, retrogradation and/or recrystallization of tapioca starch molecules during their aging processes. However, one can barely find any new melting endotherm or diffraction peaks on DSC thermograms or WAXD patterns of TPS100 BC0.02 GAx and/or (TPS100 BC0.02 GAx )75 PLA25 specimens, respectively, even after they were aged at 20 ◦ C/50% RH for less than 28 days. This is apparently due to the significant improvement in water proof properties of the TPS100 BC0.02 GAx and/or (TPS100 BC0.02 GAx )75 PLA25 specimens. The rapid reduction in  f but increase in εf values of the aged TPS and TPS100 BC0.02 specimens is apparently due to the abundant amounts of moisture absorbed during their aging processes, because the water molecules can effectively plasticize, soften and recrystallize starch molecules during their aging processes. The  f values of initial and aged TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens improved to a maximal value, while their moisture content and εf values reduced to a minimal value, respectively, as GA contents approached 0.5 PHR. The significantly improved  f

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retention values but reduced εf values found for TPS100 BC0.02 GAx and (TPS100 BC0.02 GAx )75 PLA25 specimens is attributed to their significantly improved water proof properties by blocking and/or reacting moisture-absorbing hydroxyl groups of starch molecules with aldehyde groups of GA molecules during their modification processes. In fact, after aging at 20 ◦ C/50% relative humidity for 56 days, the  f and εf value of aged (TPS100 BC0.02 GA0.5 )75 PLA25 specimen remained at 20.3 MPa and 8.7%, respectively, which are equivalent to 82.5% and 1.5 times of their initial  f and εf values, respectively, and more than 50 times higher but 25% lower of those of corresponding aged TPS and TPS100 BC0.02 specimens, respectively. Acknowledgements The authors would like to express their appreciation to GraceBio, Nytex Composites Corporation and National Science Council (NSC 102-2221-E-168-038-MY3, NSC 102-2621-M-168-001, NSC 102-2622-E-168-012-CC2, NSC 103-2621-M-168-001 and NSC 103-2622-E-168-011-CC2) for support of this work. References Agarwal, M., Koelling, K. W., & Chalmers, J. J. (1998). Characterization of the degradation of polylactic acid polymer in a solid substrate environment. Biotechnology Progress, 14, 517–526. Bastioli, C. (1998). Properties and applications of Mater-Bi starch-based materials. Polymer Degradation and Stability, 59, 263–272. Biliaderis, C. G., Lazaridou, A., & Arvanitoyannis, I. (1999). Glass transition and physical properties of polyol-plasticised pullulan–starch blends at low moisture. Carbohydrate Polymers, 40, 29–47. Carvalho, A. J. F., Curvelo, A. A. S., & Gandini, A. (2005). Surface chemical modification of thermoplastic starch: Reactions with isocyanates, epoxy functions and stearoyl chloride. Industrial Crops and Products, 21, 331–336. Choi, H. M., Kim, J. H., & Shin, S. (1999). Characterization of cotton fabrics treated with glyoxal and glutaraldehyde. Journal of Applied Polymer Science, 73, 2691–2699. Cova, A., Sandoval, A. J., Balsamo, V., & Müller, A. J. (2010). The effect of hydrophobic modifications on the adsorption isotherms of cassava starch. Carbohydrate Polymers, 81, 660–667. Delval, F., Crini, G., Bertini, S., Morin-Crini, N., Vebrel, J., & Torri, G. (2004). Characterization of crosslinked starch materials with spectroscopic techniques. Journal of Applied Polymer Science, 93, 2650–2663. Halley, P. J. (2005). Thermoplastic starch biodegradable polymers. In R. Smith (Ed.), Biodeegradable polymers for industrial applications (pp. 140–157). England: Woodhead Publishing Ltd. Hizukuri, S. (1985). Effect of sulphated derivatives of chitosan on lipoprotein lipase activity of rabbit plasma after their intravenous injection. Carbohydrate Research, 141, 295–306. Huang, M. F., Yu, J. G., & Ma, X. F. (2005). Ethanolamine as a novel plasticiser for thermoplastic starch. Polymer Degradation and Stability, 90, 501–507. Ikada, Y., Jamshidi, K., Tsuji, H., & Hyon, S. H. (1987). Stereocomplex formation between enantiomeric poly(lactides). Macromolecules, 20, 904–906. Jane, J. L., Lim, S., Paetau, I., Spence, K., & Wang, S. (1994). Biodegradable plastics made from agricultural biopolymers. In L. F. Marshall, B. F. Robert, & J. H. Samuel (Eds.), Polymers from agricultural coproducts (pp. 92–100). America: American Chemical Society. Kainuma, K. (1988). Structure and chemistry of the starch granule. In J. Preiss (Ed.), The biochemistry of plants (pp. 141–180). San Diego: Academic Press. Khaled, E. T., Richard, A. V., & Joel, J. P. (2007). Aspects of the preparation of starch microcellular foam particles crosslinked with glutaraldehyde using a solvent exchange technique. Carbohydrate Polymers, 67, 319–331. Kister, G., Cassanas, G., & Vert, M. (1998). Effects of morphology, conformation and configuration on the IR and Raman spectra of various poly(lactic acid)s. Polymer, 39, 267–273. Kurosumi, A., Sasaki, C., Yamashita, Y., & Nakamura, Y. (2009). Utilization of various fruit juices as carbon source for production of bacterial cellulose by Acetobacter xylinum NBRC 13693. Carbohydrate Polymers, 76, 333–335. Liu, X. B., Zou, Y. B., Li, W., Cao, G. P., & Chen, W. J. (2006). Kinetics of thermo-oxidative and thermal degradation of poly(d,l-lactide) (PDLLA) at processing temperature. Polymer Degradation and Stability, 91, 3259–3265. Lourdin, D., Bizot, H., & Colonna, P. (1996). “Antiplasticization” in starch–glycerol films. Journal of Applied Polymer Science, 63, 1047–1053. Lourdin, D., Coignard, L., Bizot, H., & Colonna, P. (1997). Influence of equilibrium relative humidity and plasticizer concentration on the water content and glass transition of starch materials. Polymer, 38, 5401–5406. Sagar, A. D., & Merrill, E. W. (1995). Starch fragmentation during extrusion processing. Polymer, 36, 1883–1886. Tatarka, P. D., & Cunningham, R. L. (1998). Properties of protective loose-fill foams. Journal of Applied Polymer Science, 67, 1157–1176.

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Water proof and strength retention properties of thermoplastic starch based biocomposites modified with glutaraldehyde.

Water proof and strength retention properties of thermoplastic starch (TPS) resins were successfully improved by reacting glutaraldehyde (GA) with sta...
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