Chemical-Mechanical Hydrolysis Technique of ... - Science Direct

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ScienceDirect Procedia Chemistry 19 (2016) 638 – 645

5th International Conference on Recent Advances in Materials, Minerals and Environment (RAMM) & 2nd International Postgraduate Conference on Materials, Mineral and Polymer (MAMIP), 4-6 August 2015

Chemical-Mechanical Hydrolysis Technique of Modified Thermoplastic Starch for Better Mechanical Performance A.H. Mohd Zaina*, A.W.M. Kahara, and N.Z. Norimanb a

School of Materials Engineering, Universiti Malaysia Perlis, Perlis, Malaysia b Faculty of Engineering Technology, Universiti Malaysia Perlis, Malaysia

Abstract The effects of chemical-mechanical hydrolysis on the properties of thermoplastic starch (TPS) were studied. In the presence of citric acid (CATPS) and glycerol (GTPS) along with mechanical hydrolysis, a native starch granule was transformed into a continuous phase as shown by scanning electron microscope (SEM). As shown by thermogravimetric analysis (TGA), the improvement in thermal stability confirmed that the addition of citric acid along with mechanical hydrolysis enhanced the adhesion between glycerol, citric acid and starch. It was proven by Fourier transform infrared (FTIR) spectroscopy, that citric acid form stronger hydrogen bonding than glycerol. Strong hydrogen bond formation induced the improvement in mechanical properties of GTPS. Tensile strength of citric acid modified TPS with 2% citric acid (CATPS) higher than GTPS because citric acid improved the interaction of starch molecule and slippage of starch chain. © by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2016 2016Published The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia. Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia Keywords: Chemical-mechanical hydrolysis; Mechanical properties; Scanning Electron Microscope (SEM); Thermogravimetric Analysis (TGA)

1. Introduction The research of biodegradable polymer is growing intense as continues growing concern towards the application of green product in worldwide. Extensive studies have been performed on natural polymers such as gluten, zein, lignin, cellulose, chitin and starch1.

* Corresponding author. Tel.: +0-000-000-0000 ; fax: +0-000-000-0000 . E-mail address:[email protected]

1876-6196 © 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia doi:10.1016/j.proche.2016.03.064

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A.H. Mohd Zain et al. / Procedia Chemistry 19 (2016) 638 – 645

Among these, starch is recently being used. Starch are widely used as a viable alternative to make biodegradable plastic. It is because starch is cheap, renewable and biodegradable. Furthermore, it easily degrades when exposed to environment and bacterial. Starch usually exists in granular state, which it composes of semi-crystalline polymer structure. Starch can acts like thermoplastic polymer by incorporation of starch granule with plasticizer via aid of heat and shear mechanism2-5,14. Thus, it produce thermoplastic starch (TPS), which has properties alike conventional thermoplastic polymer. Although TPS exhibits thermoplastic-like properties, its lacks in several properties compared with conventional thermoplastics, like mechanical strength and ability to absorb moisture. To mitigate the shortcomings, several modifications are performed. Chemical modification is one of approaches that has been carried out. Starch modification achieved through derivatization, such as etherification, esterification, cross-linking and grafting of starch. These modifications alter starch gelatinization, pasting and retrogradation behavior3. Fiber is also used to reinforce TPS. Others approach performed to TPS such as chemical hydrolysis, surface treatment and additives. Carboxylic acid is usually used to perform chemical modification, where it acts as plasticizer or crosslinker agent in thermoplastic starch processing. Recent carboxylic acid has been used are ascorbic acid, malic acid, citric acid, tartaric acid and others4,6,13. Addition of carboxylic acid in TPS increases the hydrogen bonding between starch and glycerol, co-plasticized with carboxylic acid. The measurement of physical properties verified that the strength and flexibility of TPS with carboxylic acid are higher than TPS with polyols5. This due -to the existence of carboxyl groups as functional group. Generally, mechanical hydrolysis involves physical process cause substance and water molecule to split into parts. Thus, it increases the free volume and it is possible to cut down polymer chain into small or short length of chain. The objective of this preliminary research was to study the effect of chemical-mechanical hydrolysis technique onto TPS in order to improve the mechanical properties and thermal stability. The effect of citric acid (CA) content on mechanical performance and thermal performance was also evaluated.

2. Experimental detail 2.1 Materials Cassava starch (CS) was purchased from Thye Huat Chan Sdn Bhd, CA was purchased from Aldrich Chemical Co. Inc., glycerol (GL) was purchased from HmBG Co. Inc. 2.2 Plasticization The water content of cassava starch was reduced by drying in an oven at temperature 80 oC for 30 minutes. TPS was prepared by mixing 65% (w/w) cassava starch with 35% (w/w) glycerol. The mixture was sealed and stored overnight. When citric acid was used, citric acid was firstly dissolved in the additional distilled water. The abbreviations for different samples prepared and their compositions are listed in Table 1. Glycerol-plasticized TPS (GTPS) and citric acid modified TPS (CATPS) were prepared as following -: the mixtures were manually fed in a heated two-roll mill machine at temperature 150 oC for 10 minutes. Table 1 . Used symbols and corresponding sample compositions. Samples (w/w %) Symbols Cassava starch GTPS CA1 TPS CA2 TPS CA3 TPS CA4 TPS

65 65 65 65 65

Glycerol 35 35 35 35 35

Citric acid 0 1 2 3 4

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2.3 Mechanical properties The mechanical properties of the thermoplastic starch were determined on a UTM tensile tester according to ASTM D 638. Testing was done on five samples each measuring 2 mm ± 0.2 mm thickness, which were cut into dumbbell shape. Before testing, all samples were stored in dry condition at 50% relative humidity for 4 hours. 2.4 Fourier Transform Infrared (FTIR) FTIR spectra were collected from the GTPS and citric acid modified TPS with Perkin Elmer IR spectrum scanner. Typically, 64 scans were signal-averaged to reduce spectral noise. 2.5 Thermogravimetric Analysis (TGA) A thermogravimetric analyze (TGA) was used to study the thermal behavior of thermoplastic starch before and after modification. TGA curves were obtained by heating the samples at a rate 15 oC per minutes. 2.6 Scanning Electron Microscope (SEM) The surface features of TPS and CATPS were observed using a scanning electron microscopy. TPS strip samples were frozen in liquid nitrogen and then were broke. The fractured surfaces were vacuum coated with gold palladium and were observed under the SEM at a voltage 10 kV.

3. Result and Discussion 3.1. FTIR analysis The FTIR spectrum of GTPS and citric acid modified TPS with 2% of citric acid are shown in Figure 1 while Table 2 shown the wave number of GTPS and acid modified TPS with 2% citric acid. The analysis of FTIR spectra of the blends enabled the identification of hydrogen bond interaction. Lower wave number indicates stronger hydrogen bond interaction6. The strong peaks observed at 1151.93 cm-1 and 1025.85 cm-1 indicate the C-O bond stretching of C-OH group in the GTPS. The peaks 1025.85 cm -1 was shifted to 1026.20 cm-1 and 1151.93 cm-1 to 1152.22 cm-1, due to the OH group of starch involved in hydrogen bond formation. Peaks shifted due to hydrolysis in the starch backbone. The proposed hydrolysis mechanism occur in starch is shown in Figure 3 (a). Hydrolysis of starch occur at glycosidic band, which induced by moisture that available in starch. The acidity of CA induces protonation of glycosidic oxygen, and glycosyl produced is further hydrolyzed with water molecule to produce stable product7,9. The peaks at 925.89 cm-1 and 925.44 cm-1 ascribed vibrational originated from C-O-C of α-1,4glycosidic linkages from starch, which not influenced by reaction between starch and citric acid7,8. Both of the samples have similar peaks, except the presence of an additional peak in the CA2 TPS sample at 1740 cm -1. The band at 1740 cm -1 is ascribed to the carboxyl and ester carbonyl bands. The presence of carbonyl peak in CA2 TPS confirmed that chemical linkages between citric acid and starch existed. It is believed that starch that undergo hydrolysis process in the presence of carboxylic acid would reacts with CA in esterification reaction. As shown in Figure 3 (b), when CA was heated, it dehydrate yield an anhydride, which could react with starch to form a starch citrate derivative10. Further esterification on the other end of CA will lead to crosslinking reaction on other starch backbone.


GTPS

CA2 TPS

Fig.1. FTIR spectra of GTPS and citric acid modified TPS with 2% citric acid. a)

1025.85 cm-

1026.20 cm-1

b)

1740 cm-1

Fig. 2. FTIR spectra of (a) The effect of hydrolysis at peaks 1151 cm-1 (a) GTPS and (b) CA2 TPS. (b) The effect of esterification of TPS with CA at peaks 1740 cm-1 (a) GTPS and (b) CA2 TPS.

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Table 2. Vibration wave number of GTPS and citric acid modified TPS with 2% citric acid.

Sample GTPS

CA2 TPS

Vibration wave number (cm-1) 1025.85 1151.93 925.89 1026.20 1152.22 925.44 1740

a)

Type of vibration C- O bond stretching C-O-C ring vibration C-O bond stretching C-O-C ring vibration C=O bond stretching

b)

+

OH

+

OH

Fig. 3. Schematic mechanism of (a) the mechanism of starch hydrolysis with carboxylic acid; (b) the chemical reaction between citric acid and starch.

3.2. Mechanical analysis The measurement of mechanical properties such as tensile strength (TS) and elongation at break (%E) of biodegradable TPS plays an important role in various fields for application and modification. Addition of CA improved the tensile strength of the TPS, as can be seen in Figure 4. However, there are optimum amount of CA (2%) that is necessary to obtain good mechanical properties of modified TPS. Compared to GTPS (elongation, 92.1%), the elongation of CATPS increased after the chemical-mechanical hydrolysis process. CA improves the interaction of the starch molecule as the CA increases the slippage of starch molecule5,11. Acidity of CA induced to fragmentation of starch, therefore to dissolution of starch granules that deteriorates the chain entanglements12-14. With further increase of CA content (3% and more), the TS reduced due to proliferation of hydrogen bonding formation of CA with starch chains12,13. Increases of hydrogen bond formation restrict movement of chain past each other, as consequence reducing the elongation13.


Fig. 4. Effect of citric acid contents on mechanical properties of TPS via chemical-mechanical hydrolysis method.

3.3. Morphological analysis The morphology of -GTPS and CA2TPS at 300x magnification is shown in Figure 5. As seen from Figure 5 (a), continuous phase of GTPS showed rugged surface along with starch granule left, ascribed there were incomplete plasticization occur in GTPS. It also found that, the granular starch were removed from the sample surface and creating cavities in the surface, which ascribed high interfacial tension occurred between hydrophilic starch and hydrophobic glycerol. Thus, it reduced mechanical ability of GTPS. Continuous phase of CA2TPS in Figure 5 (b) showed no residual granular structure of starch left and smoother fracture surfaced than the GTPS, which ascribed complete plasticization. The acidity of citric acid induced the fragmentation and dissolution of starch granule12-14. Thus, it reduced interfacial tension between starch and citric acid and glycerol.

Fig. 5. The SEM micrograph of (a) GTPS; (b) CA2TPS at 300x magnification.

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3.4. Thermogravimetric analysis The thermal behavior of GTPS and citric acid-modified thermoplastic starch is shown in Figure 6. The mass loss below 100 ˚C ascribe to water loss in GTPS, CA2TPS and CA4TPS. The mass loss at 100 ˚C to onset of decomposition temperature was related to the evaporation of water and plasticizer 14. Thus, the difference in the decomposition onset and mass loss was mainly due to the volatility of glycerol and water in TPS with different citric acid contents. Citric acid-modified TPS gave higher onset temperature and lower mass loss than GTPS. This is due to presence of CA, which increased the binding of both water and glycerol to starch. Figure 6 also indicates that the thermal decomposition temperature varied from 300 ˚C to 360 ˚C with increase of CA contents. Addition of citric acid increased the thermal decomposition temperature due to stronger hydrogen bonds formation with starch better than glycerol. Therefore, CA -improved the thermal stability of TPS by enhancing the adhesion between glycerol, starch and citric acid. However, increase of citric acid induced to acidolysis of starch occurred. As can be seen from Figure 6, the thermal stability of CA4 TPS lowers than CA2 TPS due to acidolysis reaction. The increase of citric acid content caused increase interfacial interaction between starch granules and citric acid and glycerol. On the other hand, increasing amount of citric acid cause badly acidolysis and lost rigid structure of starch completely, which deteriorated of starch backbone15,16. Therefore, optimum citric acid content, CA2 TPS give high thermal stability than CA4 TPS.

Fig. 6. Effect of chemical-mechanical hydrolysis via citric acid modification on TGA of TPS.

4. Conclusion Biodegradable TPS films were successfully prepared by using cassava starch and glycerol as plasticizer via heat and shear process along with chemical-mechanical hydrolysis technique by using CA as chemical reagent. The tensile strength (TS), elongation at break (%E), morphological properties and thermal characteristic were investigated. The result of the evaluation of mechanical properties of citric acid-modified TPS and unmodified TPS (GTPS) indicated that the TS and %E of citric-acid-modified TPS improved. Introduction of citric acid improve the tensile properties due to modification of TPS. Citric acid promote fragmentation of starch granule, thus improved the mechanical properties and thermal stability of TPS.


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Acknowledgements The authors wish to gratefully acknowledge School of Materials Engineering and Research and Innovation Department under Universiti Malaysia Perlis for supporting this research project. References 1) 2) 3) 4) 5) 6)

7) 8) 9) 10) 11) 12) 13) 14)

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