Nonedible Starch Based “Green” Thermoset Resin Obtained via

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Research Article pubs.acs.org/journal/ascecg

Nonedible Starch Based “Green” Thermoset Resin Obtained via Esterification Using a Green Catalyst Namrata V. Patil and Anil N. Netravali* Department of Fiber Science & Apparel Design, Cornell University, 37 Forest Home Drive, Ithaca, New York 14853, United States S Supporting Information *

ABSTRACT: In this study, a biobased thermoset resin was developed from a nonedible starch source obtained from mango processing industrial waste. Mango seed starch (MSS) was extracted from defatted mango seed kernels and cross-linked using a “green” cross-linker/catalyst system, 1,2,3,4-butane tetracarboxylic acid (BTCA)/sodium propionate (NaP), to obtain the thermoset resin. The tensile properties of the cross-linked MSS were found to be adequate to replace edible starch based thermoset resins, e.g., potato or corn or proteins such as soy. The cross-linking or the esterification reaction proceeds faster and at lower temperature in the presence of a suitable catalyst. Sodium hypophosphite (SHP), a widely used catalyst for esterification using poly(carboxylic acid)s and hydroxyl groups of starch or cellulose, contains phosphorus and the effluents containing SHP, i.e., phosphorus, are toxic to humans and can adversely affect the fauna in water. Also, SHP decomposes to toxic phosphine gas when heated. The results of the present study indicate that sodium propionate (NaP), used as a nonphosphorus green catalyst, is as effective and efficient as SHP. The cross-linking of starch was confirmed directly using ATR-FTIR spectra and the degree of substitution (DS) values obtained by chemical titrations as well as indirectly through an increase in the tensile properties. Higher modulus and strength and lower degree of swelling in water of films cross-linked using NaP confirmed that NaP acts as a better catalyst than the conventional SHP. KEYWORDS: Nonedible starch, Mango seed starch, Cross-linking, Green resin, Sodium propionate, Green catalyst

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72.8% of the seed weight.11 Roughly 40% to 60% waste is generated after processing mangoes, of which 15% to 20% are kernels.12 Although the constituents of the waste kernel vary according to the variety of mangoes, about 77% of it is carbohydrate, mostly starch.9 Thus, mango seed kernel waste is a rich source of starch that can be extracted and used for nonedible applications such as packaging, adhesives and resins for green composites. None of the research in open literature so far has mentioned the use of mango seed starch (MSS) to develop a thermoset resin. A couple of major challenges associated with using starch as resin is its high hydrophilicity and weak tensile properties, a strength of around 5 MPa.13 Researchers have used starches in blends or grafted a polymer onto it to overcome the problems faced due to hydrophilicity and low thermal and mechanical properties.5,14 Cross-linking of starch is another way to reduce the hydrophilic nature of starch while simultaneously improving its tensile properties.5,15 Starch is a carbohydrate consisting of many glucose units connected by 1−4 glycosidic linkages and 1−6 glycosidic linkages.5 Although the hydroxyl groups on starch molecules (or D-glucopyranose monomers) are considered as reactive sites, their reactivity depends on their position

INTRODUCTION Starch is one of the most abundantly available plant based polymers in nature. Several researchers have worked on developing environmentally friendly thermoplastic resins based on starches including corn, potato, tapioca and waxy maize starch, because they are inexpensive, abundantly available, yearly renewable and fully biodegradable.1−6 One of the concerns of using these starches for making “green” composites is that they are edible sources. In fact, starch is one of the major ingredients in the food industry. As the world population is predicted to increase to 9 billion by 2050, there will be an increase in demand for food.7 This also means a significant increase in food processing byproducts. Many of these byproducts tend to be nonedible and are considered as waste or have low value applications.7 In the present research, starch was extracted from defatted mango seed kernels (DMSK), a byproduct that is often discarded as waste. Mango (Mangifera indica) is an important fruit crop cultivated in tropical regions and distributed worldwide.8 The mango fruit processing industry has a huge market. Oil extracted from mango seeds has been used in various applications.9,10 After industrial processing of mangoes and extracting oil from the seeds, large amount of DMSK is generated. Mango seeds are bigger as compared to most other fruits and account for 9% to 23% of the fruit weight. The seed kernel makes up 45.7% to © XXXX American Chemical Society

Received: December 18, 2015

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DOI: 10.1021/acssuschemeng.5b01740 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Scheme 1. Proposed Reaction of Crosslinking of Starch via Esterification Using BTCA as Crosslinker and NaP as Catalyst

Figure 1. Steps showing the MSS extraction process from mango seeds.

effluent contains phosphorus from the leached out SHP. This is known to cause eutrophication in rivers and lakes.25 Both the high cost and the environmentally harmful property of SHP call for a nontoxic catalyst for esterification reaction.26 In the present research, sodium propionate (NaP) was used as a nonphosphorus catalyst for starch esterification reaction between carboxylic groups of BTCA and hydroxyl groups of MSS. NaP is inexpensive compared to SHP and being completely nontoxic, it is used in food preservatives. The main goal of this study was to obtain environmentally friendly thermoset resin from MSS extracted from DMSK with properties comparable to other biobased resins obtained from edible starches. Esterification of MSS extracted from DMSK was carried out using ecofriendly, water-soluble cross-linking agent (BTCA) which has four carboxyl groups that provide increased reactive sites for esterification. Environmentally friendly catalyst, sodium propionate (NaP) was used to speed up the reaction. NaP has not been reported as a catalyst for esterification of starch earlier. In the present research we propose the cross-linking (esterification) reaction of starch via esterification using BTCA as a cross-linker and NaP as a catalyst as shown in Scheme 1. The process was completely “green” as no other toxic solvents were used. As seen in Scheme 1, the cross-linking reaction takes place in two steps. In step 1, BTCA forms a cyclic anhydride by dehydration of two adjacent carboxylic groups due to NaP. In step 2, anhydride intermediate reacts with the hydroxyl group from the starch. The results of this study confirm that NaP can efficiently cross-link starch using BTCA and replace the currently used catalyst, SHP, which is toxic in nature.

in the ring. The two secondary hydroxyl groups are less reactive than the free primary hydroxyl groups due to steric hindrance.5 Cross-linking interconnects the starch molecules through covalent bonding and increases the molecular weight, tensile strength, modulus and stiffness. Various cross-linking agents including sodium trimetaphosphate (STMP), sodium tripolyphosphate (STPP), epichlorohydrin (EPI) and phosphoryl chloride (POCl3) have been used to cross-link starch.17,18 Various nontoxic poly(carboxylic acid)s such as citric acid19 and 1,2,3,4-butane tetracarboxylic acid (BTCA)20−23 have also been used to cross-link cellulosic materials such as cotton, softwood kraft pulp fiber, etc., as well as starch. Cross-linking occurs when the carboxyl groups of the acid react with the hydroxyl groups present in starch to form an ester linkage.15 All of the above-mentioned researchers have used sodium hypophosphite monohydrate (SHP) as the catalyst to accelerate the esterification reaction between poly(carboxylic acid)s and starch. Poly(carboxylic acid)s are inexpensive and nontoxic, but need a catalyst for the esterification reaction. In 1988, Welch21 reported that esterification of cellulose requires an acid catalyst, mainly a phosphorus containing salt of an acid. SHP is commonly used as a catalyst to carry out esterification reaction with poly(carboxylic acid)s as seen from the examples cited above. Catalyst reduces the reaction temperature needed and completes the reaction by forming the cyclic anhydride intermediates.24 To date, SHP has been recognized as an efficient catalyst for esterification reaction using poly(carboxylic acid)s such as citric acid, BTCA, and others that react with hydroxyl groups from polysaccharides. Catalyst is generally completely washed out after the reaction is complete and so the B

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groups from BTCA, with and without the catalysts, SHP and NaP, separately. The ATR-FTIR spectra were collected using Thermo Nicolet Magna-IR 560 spectrometer (Madison, WI) with a split pea accessory. Each scan was an average of 150 scans from 4000 to 500 cm−1 wavenumbers. Degree of Substitution. The degree of substitution was calculated for all films cross-linked with BTCA at different temperatures and using both catalysts SHP and NaP, using the titration method, to characterize the effect of temperature and the catalyst used, on crosslinking.15 Film specimens of 10 mm × 10 mm dimensions were cut and placed individually in a sealed 100 mL vial containing 50 mL of deionized (DI) water. The sealed vial with water and MSS in one and the two films cross-linked using different catalysts in separate vials were agitated in a shaker bath at 200 rpm for 4.5 days. The excess unreacted BTCA that leached out into the water was neutralized with standard NaOH solution (1 N, 10 mL) using phenolphthalein as indicator. Excess standard NaOH solution was added and shaken on a shaker bath for 1 h at 200 rpm to achieve homogeneous mixing. The entire set up was stored at 50 °C for 3 days with occasional shaking for complete hydrolysis. At the end of 3 days the excess alkali was backtitrated with standard HCl (0.4 M) solution. A blank (control) was simultaneously titrated with MSS instead of cross-linked films. Degree of substitution was calculated using the following formula:

EXPERIMENTAL SECTION

Materials. DMSK was obtained from Manorama Industries Pvt. Ltd., Raipur, Chhattisgarh, India. Analytical grade BTCA, SHP and NaP were purchased from Sigma-Aldrich (Saint Louis, MO). Extraction of Starch. Steps showing the MSS extraction process from mango seeds are shown in Figure 1. DMSK obtained after oil extraction was converted to powder form using a blender and then passed through a 250 μm sieve to obtain fine granules/particles. The fine powder obtained after sieving was washed overnight by magnetic stirring in water using DMSK:water ratio 1:8. It was then vacuum filtered using a microfiber polyester fabric. The residue obtained after filtration was subjected to washing process again to obtain maximum amount of starch. Double washing of the residue was carried out to get rid of the water-soluble sugars. It was vacuum filtered again after washing. The filter residue thus obtained was dried in an air circulating oven at 60 °C for 24 h. The dried starch lumps were ground in a blender to obtain powder which was then passed through a 250 μm sieve to obtain fine powder with submicron particles. The MSS obtained was analyzed for the constituents by Dairy One, Ithaca, NY. The entire starch extraction process was green and used only water as the solvent. Preparation of Cross-Linked Starch Films. The method used to prepare resin from starch was a slight modification of the methods used to cross-link other starches.6,15 To gelatinize MSS, 8 g of MSS was added to 150 mL of water and was heated at 90 °C with constantly stirring for 1 h. With increase in time, the viscosity of the mixture increases as the starch gelatinizes. Theoretical calculations based on stoichiometry showed that 37.5% BTCA is required for complete cross-linking of starch. After 1 h, a predetermined amount of BTCA (40% on dry wt. basis of starch) and catalyst (50% of wt. of BTCA) were added to the gelatinized starch to give a COOH:C6-OH molar ratio of 0.28 and the mixture was maintained at 90 °C with continuous stirring at 200 rpm for 1 h. The mixture was then poured onto a Teflon coated glass plate of 150 mm × 150 mm dimensions and cooled down to room temperature. After that it was dried in an air circulated oven at 40 °C for 48 h. The dried starch film was then peeled off the plate and hot pressed (cured) for 15 min at 0.38 MPa pressure and at different temperatures (120, 130, 140 °C) in a hydraulic hot press (Carver, model 3891-4PROA00, Wabash, IN) to complete the cross-linking reaction. The cured films were then washed by soaking it in DI water to remove the unreacted BTCA as well as to wash out all of the catalyst. This washing step is important, as it removes the catalyst that is highly hygroscopic and tends to absorb a lot of water when exposed to humid conditions. Two different films were made by cross-linking MSS with BTCA, using SHP as a catalyst in one (MSS-BTCA-SHP) while using NaP as a catalyst in the other (MSS-BTCA-NaP). Both processes were identical except for the catalyst. The results were used to judge the efficacy of NaP as a catalyst as compared to SHP. Characterization of Mango Seed Starch (MSS). Chemical Analysis. The “as-received” DMSK and specimens of MSS extracted after washing, grinding, sieving and drying were characterized for their chemical composition and, in particular, to study the increase in the starch content. All specimens were analyzed by Dairy One, Forage testing laboratory, Ithaca, NY. All the analyses were performed in triplicate to ensure the exact chemical composition and reproducibility. Rheological Properties. The MSS obtained after extraction was studied for its rheological properties to obtain its gelatinization temperature. Advanced Rheometer-2000 (TA Instruments, New Castle, DE) equipped with a standard steel parallel plate with 60 mm diameter was used for this purpose. The gap between the plates was adjusted to 1000 μm and a shear rate of 20 s−1 was adjusted to study the change in the viscosity of the starch as a function of temperature. The initial temperature was set to 20 °C and the final temperature was set to 95 °C with a temperature ramp rate of 20 °C min−1. Characterization of the Cross-Linked Films. Attenuated Total Reflectance Fourier-Transform Infrared (ATR-FTIR) Analysis. ATRFTIR spectra were obtained to detect the ester groups formed due to cross-linking of the hydroxyl groups from starch and the carboxyl

%carboxylate = ([mL(blank) − mL(sample)] × normality of acid × 0.234100)/dry wt. of sample

(1)

degree of substitution (DS) = 162 × %carboxylate/[234 × 1000 − (233 × %carboxylate)] (2) Degree of Swelling. Degree of swelling of the cross-linked films was found using a slight modification of the method described by Niazi and Broekhuis.27 Washed and dried MSS-BTCA-SHP, MSS-BTCA-NaP cross-linked films and the non-cross-linked (MSS-BTCA-No catalyst) films were weighed (Wi) and immersed in distilled water at room temperature (RT) until they reached an equilibrium, after which they did not absorb any water. After 24 h, moisture on the surface was removed using Kimwipes and the films were weighed (Wf). They were immersed in distilled water again and weighed after 24 h. Readings were taken until they reached equilibrium point when the weight did not increase further. Degree of swelling was calculated using the following formula: degree of swelling = (Wf − Wi )/Wi

(3)

Tensile Properties. The cross-linked starch resin films with the two different catalysts as well as without catalyst were cut into rectangular pieces of 10 mm × 50 mm from different parts of the film to be tested for tensile properties using a laser cutter. These cut films were conditioned for 2 days at ASTM standard conditions of 21 °C ± 1 °C and 65% ± 1% relative humidity before characterization. The tensile properties were characterized using Instron, model 5566 (Instron Co., Canton, MA) according to ASTM D882-02 standards and tensile strength, tensile strain and Young’s modulus were determined. The film thickness was measured using a vernier caliper at three different locations along the length of each conditioned film specimen and the average was used to calculate the tensile properties. The thickness of the films was found to be 0.5 mm ±0.2 mm. The gauge length was adjusted to 30 mm and the strain rate was 0.6 min−1. Specimens were tested from three different films of each type, cast at different times, to ensure reproducibility of the results. These specimens were cut from different parts of the films. At least three specimens from each of the three film were used for the calculation of average and standard deviation. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) of the washed and unwashed MSS-BTCA-NaP film was studied. DSC-2920 thermal analyzer (TA Instruments, Inc., New Castle, DE) was used and accurately weighed sample was placed in hermetically sealed aluminum pans. The specimens were scanned from C

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ACS Sustainable Chemistry & Engineering 40 to 350 °C at a ramp rate of 10 °C/min, individually. MSS-BTCASHP cross-linked films were also studied using the same procedure. Thermal Degradation. Thermogravimetric analysis (TGA) of both MSS-BTCA-NaP and MSS-BTCA-SHP cross-linked films as well as the MSS (powder) was performed to characterize the thermal degradation behavior of these cross-linked films as compared to MSS. TGA-2050 (TA Instruments, Inc., New Castle DE), was used for this study. Specimens weighing 5−10 mg were scanned from 25 to 600 °C using the thermogravimetric analyzer at a heating ramp rate of 10 °C/min in nitrogen atmosphere to characterize their thermal stability and degradation behavior.

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RESULTS AND DISCUSSION Chemical Analysis. Table 1 presents the proximate analysis of the chemical constituents of DMSK (as received) and the Table 1. Proximate Chemical Analysis of Defatted Mango Seed Kernels (DMSK) and Extracted Mango Seed Starch (MSS) constituents

DMSK (%)

MSS (%)

moisture dry matter starch WSC (water-soluble carbs.) ESC (simple sugars) crude protein adjusted crude protein crude fiber crude fat ash

11.4 88.6 43.1 12.9 5.2 6.3 6.3 3.3 1.9 3.7

3.2 96.8 59.5 6.3 0.4 6.8 6.8 3.1 1.2 4.2

Figure 2. Viscosity of MSS as a function of temperature from 20 to 95 °C.

of starch molecules is known as gelatinization. The gelatinization temperature of MSS was found to be 80 °C. Above this temperature, the starch molecules open up, making the hydroxyl groups present on the glucose ring easily available for any reaction. ATR-FTIR Studies. ATR-FTIR spectra of films cross-linked with and without the two catalysts were studied to confirm the esterification reaction between the hydroxyl groups of the starch and the carboxyl groups of BTCA. The spectra of MSS, BTCA and MSS-BTCA-SHP and MSS, BTCA, MSS-BTCANaP films are presented in Figures 3a,b, respectively. As can be seen in Figure 3a, the carboxyl carbonyl group of BTCA gives a sharp absorption peak at 1689 cm−1 whereas no such peak is possible, hence not seen, in MSS. The ATR-FTIR spectrum of MSS was similar to any other starch like the waxy maize, corn and potato starch. These starches have been reported to show three peaks between 923 and 1162 cm−1 corresponding to C− O stretching.5,6,15 No shift in the acid peak at 1689 cm−1 was observed when MSS was cross-linked with BTCA without using any catalyst, confirming that no reaction had taken place. The MSS-BTCA-SHP and MSS-BTCA-NaP cross-linked films showed peaks at 1715 and 1720 cm−1, respectively, which were not observed in BTCA or in MSS (Figure 3a,b, respectively). This peak corresponds to carbonyl of the ester cross-link. This proves that catalyst is required for cross-linking MSS using BTCA. Ghosh Dastidar and Netravali15 observed an ester carbonyl peak at 1725 cm−1 formed after cross-linking pure corn and potato starches with malonic acid, whereas Reddy and Yang19 observed the peak at 1724 cm−1 for corn starch cross-linked with citric acid. Both of them used SHP as a catalyst. Welch21 reported that sodium carboxylate acts as a catalyst for the esterification reaction as it helps the formation of cyclic anhydride at lower temperatures by weakening the hydrogen bond between the carboxylic groups of BTCA.23 This anhydride intermediate then reacts with the hydroxyl group from the starch (shown earlier in Scheme 1). Cross-linking occurs when there are more than one anhydride groups created on the acid. As BTCA has four carboxylic groups, it is possible to have intermediates with more than one anhydride groups making it more efficient than citric acid which has only three carboxylic groups.22,23 Both SHP and NaP catalyze the esterification reaction as the carbonyl peak from ester was observed at 1715 cm−1 using SHP and 1720 cm−1 using NaP. The pH of the reaction using NaP as a catalyst was found to be

MSS extracted as per the procedure mentioned above. All tests were performed thrice to ensure the values listed were accurate and reproducible. As can be seen from the data in Table 1, DMSK consists of carbohydrates, proteins, fibers, fats and ash. Because DMSK was obtained after oil extraction from the mango seeds, the fat content of DMSK was very low. Carbohydrates in DMSK exist in the form of simple sugars, water-soluble carbohydrates and starch with the starch content of 43%. After repeated washing to remove water-soluble carbohydrates and simple sugars and vacuum filtration process, a starch content of 59.5% was achieved in the MSS. This is a significant and a desirable increase in starch content of about 40%. Rheological Properties. Figure 2 shows the plot of viscosity of MSS as a function of temperature from 20 to 95 °C. As can be seen in Figure 2, the viscosity of MSS suddenly increases when the temperature reaches 80 °C. Different starches have different inherent material characteristics including viscosity, gelatinization temperature and crystallinity because they have different crystalline arrangements.5 Starch that exists in the form of granules is insoluble in water at room temperature but soluble at higher temperatures. High temperature along with water disrupts the crystallinity of the starch structure as it breaks down the intermolecular forces present in the starch, causing water to penetrate in.28 Water is first absorbed in the amorphous regions and the higher temperature breaks the crystalline region into separate amorphous regions allowing diffusion of water into those regions as well. Starch granules swell as they absorb water and finally rupture. The sudden jump in the viscosity seen in Figure 2 is due to the rupture of the starch granules releasing the starch molecules in the solution. The process of rupturing of granules and releasing D

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anions. The former two peaks, ester and carboxylic group, are overlapped in the peak at 1720 cm−1 whereas the carboxylate peak appears at 1560 cm−1. The peak at 1560 cm−1 grows sharper when treated with 0.1 M NaOH because it converts the anhydride groups into sodium carboxylate (Figure S.1 in the Supporting Information).29 Thus, separating and confirming the ester carbonyl peak observed at 1720 cm−1. Degree of Substitution. Degree of substitution (DS) is the number of substituent groups attached per monomeric unit. The monomeric unit of starch consists of the D-glucopyranosyl ring with three hydroxyl groups. Thus, the DS of starch may range from zero (pure starch) to three (fully substituted). However, it is known that only the primary hydroxyl (C6) is readily available for reaction as compared to the other two hydroxyl groups (C2 and C3).15 The DS for both MSS-BTCANaP and MSS-BTCA-SHP films were calculated as per the method described earlier.15 Figure 4 shows the DS of MSS-

Figure 4. DS of MSS-BTCA-SHP and MSS-BTCA-NaP cross-linked films with 40% BTCA as a function of curing temperature.

BTCA-SHP and MSS-BTCA-NaP cross-linked films with 40% BTCA as a function of curing temperature. It is clear from Figure 4 that the DS increases significantly as the curing temperature increases. This is expected because the crosslinking occurs mostly in the hot press, as it requires high temperature. The DS increased from 0.20 to 0.65 as the reaction temperature was increased from 120 to 140 °C for the MSS-BTCA-NaP cross-linked films while it increased from 0.18 to 0.53 for MSS-BTCA-SHP cross-linked films. Overall, the DS of MSS-BTCA-NaP cross-linked film was found to be higher than that for MSS-BTCA-SHP films at 130 and 140 °C whereas no significant difference was observed at 120 °C. The DS for MSS-BTCA-NaP cross-linked film was higher than MSSBTCA-SHP cross-linked film because NaP dehydrates BTCA faster as compared to SHP. As a result, the rate of cyclic anhydride formation with NaP is higher as compared to that using SHP. These anhydrides react when they come in the vicinity of the exposed hydroxyl groups from the MSS molecules and cross-link it. This results in higher cross-link density forming a tighter network. While NaP resulted in higher DS, the DS numbers of this nonedible starch are close to edible starches found by other researchers. For example, the DS of pure potato starch cross-linked with 37.5% malonic acid at 120 and 140 °C has been reported to be 0.67 and 0.79

Figure 3. ATR-FTIR spectra of (a) BTCA, MSS, MS-BTCA-SHP cross-linked film and (b) BTCA, MSS, MSS-BTCA-NaP cross-linked film.

3.75 ± 0.2, which was higher as compared to that when SHP was used as a catalyst which was found to be 2.3 ± 0.1. NaP is a sodium salt of weaker acid, propionic acid, as compared to SHP, which is a sodium salt of hypophosphorous acid. Propionic acid has a pKa of 4.8 whereas hypophosphorous acid has a pKa of 1.2. Being a stronger base compared to SHP, NaP dehydrates BTCA faster as compared to SHP by weakening the hydrogen bond in BTCA to form cyclic anhydrides and thus provides higher cross-linking efficiency. This was further confirmed as discussed later, by higher degree of substitution (DS) and higher tensile properties as well as by reduced swelling behavior in water of the cross-linked starch when NaP is used as a catalyst. This suggests that NaP is able to produce carboxylate esters faster as compared to SHP. This carboxylate ester peak was detected at 1560 cm−1. Three types of carbonyl groups are obtained when esterification reaction occurs: (a) the carbonyl peak from the ester formed due to reaction between BTCA and starch, (b) the carbonyl peak from unreacted BTCA and (c) the carbonyl peak form carboxylate E

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ACS Sustainable Chemistry & Engineering respectively.15 Zhang et al.30 used a strong dicarbonic acid (oxalic acid) and obtained DS between 0.08 and 0.87 depending on the starch variety and the acid molar ratio used. They also reported that, in general, it is difficult to obtain high DS by just using organic acids. To obtain higher DS, fatty anhydrides are more desirable.30 Degree of Swelling. Degree of swelling was determined for MSS-BTCA-NaP, MSS-BTCA-SHP cross-linked films as well as the non-cross-linked (MSS-BTCA-No catalyst) films. Figure 5 shows the photographic images of the MSS-BTCA-

the degree of swelling is higher when the starch is cross-linked using SHP as a catalyst as compared to NaP. When the crosslinked films were immersed in water at RT, the MSS-BTCANaP films reached equilibrium in 24 h and no significant additional swelling was noticed. Whereas the MSS-BTCA-SHP films took almost 72 h to reach equilibrium and the degree of swelling was significantly higher. Lower swelling due to lower amount of water being absorbed by MSS-BTCA-NaP film as compared to MSS-BTCA-SHP film is a desirable property for the composites application as it increases its dimensional stability even in humid conditions. Tensile Properties. Figure 7 shows typical stress vs strain plots of MSS-BTCA-NaP, MSS-BTCA-SHP and control (non-

Figure 5. Photographs showing (a) MSS-BTCA-NaP cross-linked film and (b) non-cross-linked (MSS-BTCA-No catalyst) film immersed in water for 3 days at RT.

NaP cross-linked and non-cross-linked films after immersing in water for 3 days at RT. At the end of 3 days, it was observed that the cross-linked films did swell in water but remained intact (Figure 5a) whereas the non-cross-linked films disintegrated and the water became turbid as the MSS film broke into pieces and the very small pieces mixed with the water (Figure 5b). Prior to breaking, the non-cross-linked film was highly swollen, compared to the cross-linked film, as the film readily absorbed water. As discussed earlier, starch has hydroxyl groups that absorb water and causes the films to swell. Cross-linking of starch converts part of the hydroxyl groups to ester groups by covalently bonding two molecules and forms a tighter network structure which restricts the water from entering the structure and pushing the molecules apart. Also, the newly formed ester groups are hydrophobic compared to the hydrophilic hydroxyl and carboxyl groups that they replace. Both these changes contribute to lower moisture absorption and degree of swelling. Although both NaP and SHP act as catalysts, as seen from earlier results of the DS, NaP results in higher DS. This is clearly reflected in the swelling behavior of the cross-linked starch. Figure 6 shows the degree of swelling of MSS-BTCA-SHP and MSS-BTCA-NaP cross-linked films as a function of immersion time at RT. As can be seen in Figure 6,

Figure 7. Typical stress vs strain plots of MSS-BTCA-NaP, MSSBTCA-SHP and MSS-BTCA-No catalyst films.

cross-linked, i.e., MSS-BTCA-No catalyst) films. The tensile properties such as fracture stress (strength), fracture strain and Young’s modulus of the MSS-BTCA-No catalyst, MSS-BTCASHP and MSS-BTCA-NaP cross-linked films are summarized in Table 2. The plots in Figure 7 clearly indicate the differences Table 2. Tensile Properties of the MSS-BTCA-No Catalyst, MSS-BTCA-SHP, MSS-BTCA-NaP Cross-Linked Films

a

MSS-BTCA(Catalyst)

fracture stress (MPa)

fracture strain (%)

Young’s modulus (MPa)

no catalyst SHP NaP

3.52 (0.06)a 11.02 (2.13) 16.24 (1.98)

1.34 (0.05)a 1.51 (0.72) 1.54 (0.56)

298 (6.35)a 1012 (381) 1347 (324)

The numbers in parentheses are standard deviations.

due to the catalyzing efficiencies of SHP and NaP. Without any catalyst, esterification reaction takes place at an extremely slow rate and requires high temperature and much longer time for reaching equilibrium.31 This is reflected in the poor tensile properties of control films. When MSS was reacted with BTCA without any catalyst for 1 h at 90 °C, the strength was found to be very low (3.5 MPa). However, the strength of the crosslinked films formed using SHP or NaP at the same temperature and time were much higher 11.02 and 16.24 MPa, respectively. The tensile strain was found to be similar for both cross-linked films. The tensile strain and Young’s modulus values were also found to increase when a catalyst was used along with the crosslinker. This is due to the strong covalent ester bond formation through cross-linking that occurs during the curing. The crosslinking was confirmed by ATR-FTIR spectra as discussed

Figure 6. Degree of swelling of MSS-BTCA-SHP and MSS-BTCANaP cross-linked films as a function of immersion time at RT. F

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Figure 8. DSC thermograms of (a) washed and unwashed MSS-BTCA-NaP cross-linked films and (b) washed MSS-BTCA-NaP and MSS-BTCASHP cross-linked films.

any crystals in the starch films after washing perhaps because of the gelatinization of starch prior to cross-linking. Thermal Degradation. Thermogravimetric analysis (TGA) was performed to characterize the thermal stability and weight loss of MSS and cross-linked (MSS-BTCA-NaP and MSS-BTCA-SHP) films. The TGA thermograms of MSS and the two cross-linked films are shown in Figure 9. As shown in

earlier. This rigid cross-linked network also acts as a moisture barrier which further enhances the tensile properties.6,15 Although both SHP and NaP improve the tensile properties, NaP results in higher strength confirming once again that NaP is a better catalyst. The strength and Young’s modulus of the MSS-BTCA-NaP films were found to be 47% and 33% higher, respectively, as compared to the values obtained for MSSBTCA-SHP films. This is expected as higher DS values were obtained using NaP, which acts as a better catalyst as compared to SHP. The tensile properties of these cross-linked biobased MSS resins were found to be higher than other biobased resins derived from agricultural waste such as protein derived from neem or karanja seed cake.32,33 The stress vs strain plots of all specimens (Figure 7) showed no yielding and the films were brittle. It should be noted that no plasticizer was used in preparing these sheets. The efficiency of NaP over SHP as a catalyst was further confirmed by carrying out esterification reactions of pure starches with BTCA. Corn starch and potato starch were utilized for this purpose. The representative stress vs strain curves for corn starch are presented in Figure S.2 in the Supporting Information. In the case of corn starch cross-linked with BTCA using NaP as a catalyst, the tensile strength showed 54.33% increase compared to that catalyzed using SHP. Differential Scanning Calorimetry. Figure 8a shows the DSC thermograms of washed and unwashed MSS-BTCA-NaP cross-linked films. The DSC thermograms of unwashed crosslinked films show a sharp peak at 300 °C corresponding to the melting point of the catalyst (NaP). This peak disappears after washing. This suggests that almost all of the catalyst has been removed by washing the film in water. This is most likely to occur because NaP is highly water-soluble. The unwashed films also showed an endothermic peak at 190 °C, corresponding to the melting temperature of BTCA. This means that some excess unreacted BTCA was present in crystalline form, even after the reaction. That peak also disappeared after washing indicating that unreacted BTCA also can get washed out (Figure 8a). In the case of MSS-BTCA-SHP films, a similar trend was observed (Figure S.3 in the Supporting Information). Figure 8b shows the DSC thermograms of the washed MSSBTCA-NaP and washed MSS-BTCA-SHP films from 25 to 220 °C. The DSC thermograms look similar for both cross-linked starch films with no peak seen until over 220 °C, which is the degradation temperature of the cross-linked starch as confirmed by the TGA results discussed later. The absence of any endothermic peaks before degradation indicates the absence of

Figure 9. TGA of MSS, MSS-BTCA-SHP and MSS-BTCA-NaP crosslinked films.

Figure 9, MSS starts to degrade (onset degradation temperature, Td) at 250 °C and the maximum degradation temperature defined as the temperature at which the rate of weight loss reduces significantly, after the specimen has degraded, (the peak in the DTGA plot, shown in Figure S.4 in the Supporting Information) was found to be around 315 °C. This behavior is similar to the commerically available waxy maize starch.6 It was observed that the Td (220 °C) was same for both the cross-linked films (Figure 9), indicating that there was no difference between the two catalysts used, SHP and NaP. This is perhaps because after the completion of the crosslinking reaction, unreacted BTCA and the catalyst were removed by the washing process. Thus, both cross-linked films consisted of similar network structure. The Td of the cross-linked MSS was around 220 °C, which was lower than that observed for MSS (250 °C). This might be due to the presence of few partially reacted carboxylic groups of BTCA that decompose at lower temperature. A similar behavior was observed by Zhang et al.,30 who esterified corn starch using oxalic acid. In their work, gelatanized corn starch had a Td of 278 °C whereas after esterification with oxalic acid (DS of G

DOI: 10.1021/acssuschemeng.5b01740 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

ACS Sustainable Chemistry & Engineering

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0.87), the Td reduced to 204 °C.30 They also claimed that the reduction in Td was due to the low thermal stability of unreacted carboxylic groups.38 The Td values of both MSSBTCA-NaP and MSS-BTCA-SHP cross-linked films in this study were higher than that of pure corn starch cross-linked with oxalic acid. Also, cross-linking significantly reduced the percentage of degradation or the weight loss. As can be seen in Figure 9, MSS lost 67% of its weight at 350 °C, whereas after cross-linking only 41% weight loss was observed at 350 °C. DTGA plots were constructed from the TGA thermograms obtained shown in Supporting Information Figure S.4. The maximum degradation temperature (defined as the temperature at which the degradation rate decreases) of MSS was 315 °C which reduced to 295 °C after cross-linking. In this study, the films cross-linked using SHP and NaP catalysts showed same Td and maximum decomposition temperatures. This is because both of them alter the chemical structure of starch by forming ester and also result in similar network.

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CONCLUSIONS

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ASSOCIATED CONTENT

Research Article

AUTHOR INFORMATION

Corresponding Author

*Anil N. Netravali. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge NSF-CREST (Grant 1137681) for partially funding this work. The authors also acknowledge partial funding support from the Wallace Foundation.

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REFERENCES

(1) Milkowski, K.; Clark, J. H.; Doi, S. New materials based on renewable resources: Chemically modified highly porous starches and their composites with synthetic monomers. Green Chem. 2004, 6 (4), 189. (2) Ochi, S. Development of high strength biodegradable composites using manila hemp fiber and starch-based biodegradable resin. Composites, Part A 2006, 37 (11), 1879−1883. (3) Takagi, H.; Ichihara, Y. Effect of fiber length on mechanical properties of″ green″ composites using a starch-based resin and short bamboo fibers. JSME Int. J., Ser. A 2004, 47 (4), 551−555. (4) Wu, K.; Hu, Y.; Song, L.; Lu, H.; Wang, Z. Flame retardancy and thermal degradation of intumescent flame retardant starch-based biodegradable composites. Ind. Eng. Chem. Res. 2009, 48 (6), 3150− 3157. (5) Ghosh Dastidar, T.; Netravali, A. N. Improving resin and film forming properties of native starches by chemical and physical modification. J. Biobased Mater. Bioenergy 2012, 6, 1−24. (6) Ghosh Dastidar, T.; Netravali, A. N. Cross-linked waxy maize starch-based ″green″ composites. ACS Sustainable Chem. Eng. 2013, 1 (12), 1537−1544. (7) Kazmi, A, Shuttleworth, P. The economic utilisation of food coproducts; Royal Society of Chemistry: London; 2013. (8) Kaur, M.; Singh, N.; Sandhu, K. S.; Guraya, H. S. Physicochemical, morphological, thermal and rheological properties of starches separated from kernels of some indian mango cultivars (mangifera indica L.). Food Chem. 2004, 85 (1), 131−140. (9) Abdalla, A. E. M.; Darwish, S. M.; Ayad, E. H. E.; El-Hamahmy, R. M. Egyptian mango by-product 1. compositional quality of mango seed kernel. Food Chem. 2007, 103 (4), 1134−1140. (10) Abdalla, A. E. M.; Darwish, S. M.; Ayad, E. H. E.; El-Hamahmy, R. M. Egyptian mango by-product 2: Antioxidant and antimicrobial activities of extract and oil from mango seed kernel. Food Chem. 2007, 103 (4), 1141−1152. (11) Kittiphoom, S.; Sutasinee, S. Mango seed kernel oil and its physicochemical properties. Int. Food Res. J. 2013, 20 (3), 1145−1149. (12) Nilani, P.; Raveesha, P.; Duraisamy, B.; Elango, K.; Dhamodaran, P. Formulation and evaluation of polysaccharide based biopolymeran ecofriendly alternative for synthetic polymer. J. Pharm. Sci. Res. 2010, 2 (3), 178−184. (13) Dicker, M. P. M.; Duckworth, P. F.; Baker, A. B.; Francois, G.; Hazzard, M. K.; Weaver, P. M. Green composites: A review of material attributes and complementary applications. Composites, Part A 2014, 56, 280−289. (14) Chen, L.; Qiu, X.; Deng, M.; Hong, Z.; Luo, R.; Chen, X.; Jing, X. The starch grafted poly (L-lactide) and the physical properties of its blending composites. Polymer 2005, 46 (15), 5723−5729. (15) Ghosh Dastidar, T.; Netravali, A. N. ‘Green’ crosslinking of native starches with malonic acid and their properties. Carbohydr. Polym. 2012, 90 (4), 1620−1628. (16) Tester, R. F.; Karkalas, J.; Qi, X. Starchcomposition, fine structure and architecture. J. Cereal Sci. 2004, 39 (2), 151−165. (17) Hirsch, J. B.; Kokini, J. L. Understanding the mechanism of cross-linking agents (POCl3, STMP, and EPI) through swelling behavior and pasting properties of cross-linked waxy maize Starches1. Cereal Chem. 2002, 79 (1), 102−107.

A fully green biobased resin was made from the mango seed waste generated by the mango processing industry. Starch was extracted from the residue obtained after extraction of oil from the mango seed kernels using a simple, water based process. Further, it was cross-linked to improve its performance for use as a resin in composites. Cross-linking of starch was successfully carried out using a water-soluble poly(carboxylic acid), BTCA. A novel, inexpensive, ecofriendly catalyst, NaP was used for efficient cross-linking and was found to be more effective than the currently used esterification catalyst, SHP. NaP is nontoxic and has been approved as a food additive in USA, EU and Australia. The cross-linking of starch was confirmed directly using ATR-FTIR spectra and the DS values obtained by chemical titrations as well as indirectly from the tensile properties. Higher modulus and strength, higher DS values, lower degree of swelling in water of films cross-linked using NaP confirmed that NaP is a more efficient catalyst than the conventionally used SHP. Higher cross-linking reduced the moisture absorption by the films and contributed to better tensile properties. Resins such as cross-linked MSS, obtained from nonedible sources, can be further reinforced with nanoparticles, nano/microfibers or fibers to improve mechanical or thermal properties as desired to form green composites. They can easily replace the biobased resins that are obtained from edible sources, thus not depleting the food sources.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01740. ATR-FTIR spectra of the MSS-BTCA-NaP cross-linked film after treating with 0.1 M NaOH (Figure S.1), Tensile stress vs strain plot of corn starch cross-linked with BTCA as a cross-linker using different catalysts (SHP and NaP) (Figure S.2), DSC thermogram of washed and unwashed MSS-BTCA-SHP films (Figure S.3) and DTGA of MSS, MSS-BTCA-SHP and MSSBTCA-NaP cross-linked films (Figure S.4) (PDF). H

DOI: 10.1021/acssuschemeng.5b01740 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering (18) Jyothi, A. N.; Moorthy, S. N.; Rajasekharan, K. N. Effect of cross-linking with epichlorohydrin on the properties of cassava (manihot esculenta crantz) starch. Starch - Stärke 2006, 58 (6), 292−299. (19) Reddy, N.; Yang, Y. Citric acid cross-linking of starch films. Food Chem. 2010, 118 (3), 702−711. (20) Lund, K.; Brelid, H. 1,2,3,4-Butanetetracarboxylic acid crosslinked softwood kraft pulp fibers for use in fluff pulp applications. J. Eng. Fibers Fabrics 2014, 9 (3), 142−150. (21) Welch, C. M. Tetracarboxylic acids as formaldehyde-free durable press finishing agents: Part I: Catalyst, additive, and durability Studies1. Textile Res. J. 1988, 58 (8), 480−486. (22) Trask-Morrell, B. J.; Andrews, B. A. K. Thermoanalytical characteristics of polycarboxylic acids investigated as durable press agents for cotton textiles. J. Appl. Polym. Sci. 1991, 42 (2), 511−521. (23) Harifi, T.; Montazer, M. Past, present and future prospects of cotton cross-linking: New insight into nano particles. Carbohydr. Polym. 2012, 88 (4), 1125−1140. (24) Yang, C. Q. FTIR spectroscopy study of ester crosslinking of cotton cellulose catalyzed by sodium hypophosphite. Text. Res. J. 2001, 71 (3), 201−206. (25) Choi, H.; Welch, C. M.; Morris, N. Nonphosphorus catalysts for formaldehyde-free DP finishing of cotton with 1, 2, 3, 4butanetetracarboxylic acid part I: Aromatic N-heterocyclic compounds. Text. Res. J. 1993, 63 (11), 650−657. (26) Lee, E. S.; Kim, H. J. Durable press finish of cotton/polyester fabrics with 1,2,3,4-butanetetracarboxylic acid and sodium propionate. J. Appl. Polym. Sci. 2001, 81 (3), 654−661. (27) Niazi, M. B. K.; Broekhuis, A. A. Surface photo-crosslinking of plasticized thermoplastic starch films. Eur. Polym. J. 2015, 64 (0), 229− 243. (28) Ratnayake, W. S.; Jackson, D. S. Gelatinization and solubility of corn starch during heating in excess water: New insights. J. Agric. Food Chem. 2006, 54, 3712. (29) Yang, C. Q. Infrared spectroscopy studies of the cyclic anhydride as the intermediate for the ester crosslinking of cotton cellulose by polycarboxylic acids. I. identification of the cyclic anhydride intermediate. J. Polym. Sci., Part A: Polym. Chem. 1993, 31 (5), 1187−1193. (30) Zhang, S.; Zhang, Y.; Huang, H.; Yan, B.; Zhang, X.; Tang, Y. Preparation and properties of starch oxalate half-ester with different degrees of substitution. J. Polym. Res. 2010, 17 (1), 43−51. (31) Yang, C. Q.; Wang, X. Infrared spectroscopy studies of the cyclic anhydride as the intermediate for the ester crosslinking of cotton cellulose by polycarboxylic acids. II. comparison of different polycarboxylic acids. J. Polym. Sci., Part A: Polym. Chem. 1996, 34 (8), 1573−1580. (32) Rahman, M. M.; Ho, K.; Netravali, A. N. Bio-based polymeric resin from agricultural waste, neem (azadirachta indica) seed cake, for green composites. J. Appl. Polym. Sci. 2015, 132 (1), 1−11. (33) Rahman, M. M.; Netravali, A. N. Green resin from forestry waste residue “Karanja (pongamia pinnata) seed cake” for biobased composite structures. ACS Sustainable Chem. Eng. 2014, 2 (10), 2318− 2328.

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DOI: 10.1021/acssuschemeng.5b01740 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX