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Distillers dried grains with solubles is the major co-product of fuel ethanol industry. The production of DDGS is increasing dramatically in the United States, where it is estimated that 10 million tons were produced in 2006, and may increase to 25 million tons by 2012. Thus, DDGS is a rich source of protein (26.8-33.7 % dry weight basis), carbohydrates (39.2-61.9% including fibers), oils (3.5-12.8 %), and ash (2.0-9.8 %). The primary use of DDGS to date has been in animal feed applications. However, there are limits to its dietary intake by livestock based on nutritional requirements (Hanson et al., 2012; Kim et al., 2008; Li, 2010; Robinson et al., 2008; Xu et al., 2009). Simultaneously, the ethanol industry has experienced record growth, with annual production volumes increasing by an average of greater than 20% over the past four years, and greater than 10% over the past 20 years (Robinson et al., 2008). There is a need, therefore to find alternate uses and new market channels for ethanol-derived DDGS. Currently, most research focuses on the value of using protein and celllulose for developing biobased material and composites. Use of biodegradable plastics instead of traditional non-biodegradable materials can offer a solution to environmental problems caused by biostable plastic waste (Iovino et al., 2008). Several potential alternatives for using biodegradable polymer blended with low-cost organic filler have been proposed, with the prospect of extracting higher value protein and cellulose from DDGS and them for biocomposites and bioadhesives (Tatara et al., 2009; Wu, 2007; Younghui Li, 2011).
In recent years, materials called biocomposite that composed of agriculture residues and biodegradable polymers, have become very attractive materials (Chen et al., 2011; Chivrac et al., 2009; Liu et al., 2004; Reddy & Yang, 2011; Rosa et al., 2009b; Song et al., 2008; Strömberg & Karlsson, 2009; Younghui Li, 2011; Zhao et al., 2010). Biodegradable plastics, as novel materials, make claims to be environmentally friendly. Consequently, it must be proved by using scientifically based and accepted methods. The biodegradability of plastics provides these materials novel and additional properties which may also be beneficial during their use. A necessary prerequisite for extending their utilization is their biodegradability in natural environments, where they may serve as a source of carbon and energy for a variety of microorganisms (Witt U, 2001). Usually, biodegradation studies are carried out in soil and/or compost, in particular, enhanced biodegradation of these materials may occur in the presence of compost, a complex biological environment, in which microbial diversity is relatively high and an increased degradation potential for polymeric compounds may result (Chiellini et al., 2003; Corti et al., 2002; Degli-Innocenti et al., 2001). According to ISO and ASTM (Ohtaki et al., 1998; Pagga et al., 1995), biodegradation is the degradation caused by biological activity, especially by enzymatic action, leading to a significant change in the chemical structure of the exposed material and resulting in the production of carbon dioxide, methane, water, mineral salts (mineralization) and new microbial cellular constituents (biomass). It can occur under two different conditions: aerobic in the presence of oxygen and anaerobic with no oxygen available (Itavaara M, 2002). In this study we explore the feasibility of using DDGS with poly (butylenes adipate-co-terephalate) (PBAT). PBAT is an aliphatic-aromatic polyester with properties comparable to many fossil-oil-based plastics and its easily biodegradable. This material can be used for packaging, agriculture and disposable bags, but its utilization is rather limited due to high production cost. A possible problem solving strategy is the preparation of the blends of PBAT with inexpensive, readily available and biodegradable materials such as starch and polysaccharide derived from agricultural plants and therefore susceptible to biological attack (Alvarez et al., 2006; Kasuya K, 2009; Rosa et al., 2009b).
The aim of this work was to investigate the aerobic biodegradation of composites prepared using DDGS and PBAT. Biodegradability of the composites correlated with its individual constituents in the presence of compost, using standard test methods designed for biodegradable plastics (ASTM D5338). To our knowledge a very few studies have been made on the biodegradability of such novel composites. Biodegradation studies were based on the estimation of the percentage of mineralization (i.e. measuring the evolved carbon dioxide) of the material's carbon content versus with time. To monitor and describe materials' degradation, differential scanning calorimetry (DSC), Thermo gravimetric analysis (TGA) and Fourier transform spectroscopy were also employed.
The PBAT pellets were purchased from Xinfu company - China. DDGS were obtained from GreenField Ethanol Inc., Chatham, Canada.
Pretreatment of DDGS
The pretreatment of DDGS was carried out by stirring 100 g of as-received DDGS with 1.5 litre water for 30 minutes at room temperature. After pretreatment the DDGS was filtered and dried overnight at room temperature. This pretreated DDGS (wwDDGS) was used for the preparation of composites.
Processing of composite materials
PBAT and DDGS (untreated and pretreated) were dried in an oven at 80 °C for 24 hours. The composites were prepared with PBAT pellets and DDGS filler (untreated and pretreated DDGS) using a twin screw micro extruder. About 12g of PBAT and DDGS mixture was fed into a twin-screw micro extruder (DSM Research, The netharlands, model: DSM Xplore 15cc), and processed at a temperature of 160 °C for 3 min residence time, screw speed at 100 rpm. The molten mixture was collected and inject moulded to make various test specimens using a DSM xplore 12ml injection moulding machine.
2.3.1 Mechanical properties
Tensile and flexural properties of the composites were measured by a Universal testing machine, Instron 3382, according to ASTM D638 and ASTM D790 standards respectively. System control and the data analaysis were done using Blue Hill software. The notched Izod impact strength was measured with a TMI monitor Impact tester (model no. 43-02-01) according to ASTM D256 using a pendulum of 5 ft-lb.
2.3.2 Dynamic Mechanical Analysis (DMA)
A DMA Q800 from TA Instruments was used to evaluate the storage modulus of rectangular samples of 3.2 x 12.5 x 65mm3. The experiments were performed from -50 to 100°C with a ramp rate of 3 °C min-1. A dual cantilever clamp was used at the frequency of 1 Hz and oscillating amplitude of 15µm.
2.3.3 Thermal Analysis
18.104.22.168 Differential Scanning calorimeter (DSC).
Heat flow as a function of temperature was studied by a Differential Scanning calorimeter (DSC Q 200, TA Instruments Inc.) using heat-cool-heat mode. Nitrogen was used as purge gas during the experiment. The data were collected by heating the specimen from -50 to 150 â-¦C at a constant heating and cooling rate of 10 â-¦C per minute. The data was analysed through TA instruments Universal analysis software.
22.214.171.124 Thermogravimetric analysis (TGA)
Thermogravimetric analysis was carried out by a thermogravimetric analyzer (TA Instrument Inc Q500). The samples were scanned from room temperature to 600 â-¦C at heating rate of 20 â-¦C/min in a nitrogen atmosphere.
2.3.4 Fourier Transform Infrared spectroscopy (FT-IR)
Thermo Scientific Nicoletâ„¢ 6700 FTIR spectrometer in attenuated total reflection infrared (ATR-IR) mode with a resolution of 4 cmâˆ’1 and a number of 32 scans per sample was used to obtain the spectra.
The density of polymer and the composites were measured by an electronic densimeter MD-300S (Alfa Mirage, Japan). The density measurement is according to Archimedes principle.
2.3.6 Carbon and Nitorgen analysis
Total carbon and nitrogen content of the samples were evaluated through elemental analysis at Laboratory Services, University of Guelph.
2.4 Biodegradation studies
2.4.1 Stimulated aerobic composting set up
A simulated laboratory composting set up designed to satisfy the standard ASTM D 5338-98 (ASTM, 2004) performance requirements. Samples were milled to fine particles with an average size â‰¥100 µm using manual grinding under liquid nitrogen environment. Three months old organic manure compost was kindly supplied by the Sunshine environmental -Orangeville (Canada) used as the compost medium. The compost was sieved below to <0.8 cm. Its physical and chemical characteristics were determined according to the ASTM D6400 standards before to use biodegradation experiments and are shown in Table 1. Activity of the compost was measured as required by the standard method and the compost produced 65 mg of carbon dioxide per gram of volatile solids over the first 10 days of the test.
2.4.2 Biodegradation tests: apparatus and procedure
Biodegradation tests on polymeric plastics and their starting materials were carried out in a laboratory scale-compost experiment according to ASTM D 5338-98 (ASTM, 2004). Glass flasks of approximately 2 litre internal volume were used as the bioreactor. The compost and test materials in each flask were mixed in the ratio of 6:1(w:w dry mass). In each test a series of vessels (triplicates) containing compost the polymeric material and/or their constituents were prepared along with a control (compost without the polymeric materials). Carbon dioxide produced during the biodegradation process was trapped in 500 mL of 0.5 M KOH. The CO2 traps were changed in every 2-3 days depending on the degradation rate. A 10 mL aliquot of KOH from each trapping solution was titrated with 0.5 M HCl using phenolphthalein indicator. The total amount of CO2 produced was calculated with reference to the control flask. To provide aerobic conditions during the test, a continuous air-supply system was used for each composting vessel. Air was bubbled through distilled water to maintain the relative moisture content of at least 50% in compost inoculum. This setup was incubated at constant temperature of 58 ±2 °C and maintained throughout the 60 days of the experiment. The glass flasks were shaken and weighed weekly to ensure proper aeration and mixing of the bio-waste.
According to ASTM standard (ASTM, 2004), the theoretical amount of carbon dioxide produced in each flask (ThCO2 in g per vessel) which can be produced by total oxidation of incubated materials can be calculated by the following expression:
where Mtot is the total dry amount of constituent or plastic materials (g) added to the compost, Ctot is the relative amount of total organic carbon (g) in the total dry solids, 44 is the molar mass of carbon dioxide and 12 is the atomic mass of carbon.
A biodegradation curve was obtained by plotting released CO2 (%) versus exposure time. Biodegradation was calculated as the percentage of carbon in the polymer mineralized as CO2 according to the expression:
where (CO2)s and (CO2)blank are the amount of CO2 produced in the sample and in the b respectively.
Results and Discussion
3.1 Surface pretreatment of DDGS
DDGS without any pretreatment has very low thermal stability, which limits its application as filler for polymer composite processing. Since dried samples were used for the experiments, no significant weight loss was observed below 100°C in all the materials analyzed (Fig. 1). DDGS has two decomposition mechanisms initial step between 150-230°C corresponding to hemicelluloses degradation, and the second step degradation between 230°C and 360°C which corresponds to the thermal degradation of cellulose (Morey et al., 2009). However, with the water washing the decomposition pattern for DDGS was changed. This could be due to the elimination of many water soluble components and thereby shifting the weight loss curve towards higher temperature. The weight loss curves with the increase in temperature of DDGS, and water washed DDGS are shown in the Fig. 1. It can be seen that, significant difference in the thermal stability of the fillers due to their compositional differences.
3.2 Characterization of composite materials:
3.2.1 Mechanical properties
Untreated and treated DDGS (biobased filler) and PBAT based composites was investigated in detail. Fig.2 shows the stress-strain plot of neat PBAT and PBAT-DDGS composites and these results suggested that ductile behavior with the strain hardening phenomenon. However, the maximum stress values differed by the order of three compared to neat polymer. These reasons could be the very less compatibility between polymer and fillers and also it can be observed that water washed DDGS displays better resistance to applied stress when compared to unwashed DDGS. The results suggested that, water-washing treatment helps in removing the water soluble fractions from DDGS that are not compatible with polymer matrix or low molecular weight fractions Fig. 3 and Table 2 shows the mechanical properties of the PBAT and PBAT-DDGS composites. The similar trend have been previously observed by the incorporation of DDGS into PHBV bioplastic (Zarrinbakhsh et al., 2011). It can be seen from Fig. 3, with the addition of DDGS, tensile strength decreased, whereas modulus values have improved due to non-interacting filled polymer systems without any compatibilization (Rosa et al., 2009a). It is well known that tensile properties of the polymer composites depend on the strength and modulus of the filler. However, in our study only modulus values have increased with increase in DDGS loading. The decrease in the tensile properties can be attributed to the poor adhesion of DDGS with the polymer matrix leading to the absence of involuntary anchoring in the system. In other words, there was no effective stress transfer mechanism in the composites between matrix and the fillers (Zarrinbakhsh et al., 2011). Also, the improvement in modulus values is an indication of good dispersion of the DDGS particles in the PBAT matrix and thereby offering better resistance to the applied stress. It can be seen that the standard deviation of the moduli are very negligible indicating the optimization of the processing conditions. Table 3 shows the impact and percent elongation of the PBAT and its composites, which measures the energy required to break specimen completely. This impact energy value is a combination of crack initiation followed by crack propagation phenomenon. In our work, we have found that the impact strength values and percent elongation have decreased with increase in DDGS content (Table 2). It is well known that impact strength depends on various factors like filler to matrix adhesion, toughness of the matrix, the filler alone, crystalline morphology, and even distribution of fillers (Cheesbrough et al., 2008; Rosa et al., 2009a). The composites prepared in reveals that the fillers were impeding the mechanism of energy absorption rather improving the energy absorption.
The storage modulus and tan delta of the PBAT-DDGS composites, as a function of the temperature is shown in Fig. 4. It can be seen that storage modulus values decreased with increasing temperature for both polymer matrix and its composites. This could be due to the fact that the PBAT chain mobility has increased at higher temperatures leading to the softening behavior and thereby decreasing the modulus values. Also, it can be noticed that the modulus values are higher for DDGS composites than PBAT alone. These may be due to the fact that DDGS particles are stiffer than PBAT itself. The increased storage modulus with increase in DDGS quantity in the biocomposite could be due to the physicochemical interaction, intramolecular bonds, and a crystalline structure of the biocomposite, which improves the reinforcement imparted by the DDGS filler that allowed stress transfer from the matrix to the DDGS. Figure 4b shows that the height of the tan delta peak decreased with the presence of DDGS particles. It is due to that there is no restriction to the chain motion in the case of pure PBAT matrix; meanwhile, the presence of DDGS particles hinders the chain mobility which produces a reduction of sharpness and height of the tan delta peak.
3.2.2 Thermal properties
The DSC analysis of PBAT-DDGS composites showed, slightly increased glass transition temperature (Tg) of the matrix with increase of filler content, indicating that the filler did not lead to significant changes in crystalline structure (Fig. 5). Fig. 6 shows TGA and DTGA curves obtained for different composites (0, 20% and 30% pretreated DDGS). The temperature at the maximum degradation rate was shifted to lower values as the filler content increased because of the decrease in PBAT, which is much more thermally stable than pretreated DDGS. In addition, the presence of DDGS filler decreased the thermal stability compared with that of the raw matrix, because of the lower degradation temperature of pretreated DDGS fillers. The same behavior was observed with the work of (Rosa et al., 2009a) and (Younghui Li, 2011) for composite materials. In our work, we have adopted water washing technique to remove the components that are water soluble and this has helped in improving the thermal stability of the DDGS. Fig. 5 shows clearly that the initial degradation of DDGS has improved from 140°C to 240°C after water washing. This has helped us obtaining better composites with improved ductility and bonding as observed in the mechanical properties as discussed in the earlier section.
3.2.3 FT-IR characterizations
The structural characterization of neat PBAT, composite materials and DDGS were done using FT-IR (Fig. 7). The DDGS, broad centred at 3364 cm-1 was assigned to the stretching vibration of -OH and -NH from carbohydrates and protein, the peak at 2930 cm-1 to the stretching of -CH from lipids, the peak at 1654 cm-1 to the amide absorption of corn protein, and the peaks at the regions of 1250-1000cm-1 to carbohydrates (Giuntoli et al., 2009). These results confirm that DDGS is composed of carbohydrates, protein, and lipids. After thermal compounding of PBAT and pretreated DDGS the peak of -N=C=O in the composite was slightly influenced (Fig. 7) Moreover, the strong absorption peak of -OH and -NH group becomes very weak during thermal compounding. It does not influenced the filler content of pretreated DDGS 20 and 30% which is particularly overlapped with amide and -C=O absorption (ester and keto carbonyl).
3.3 Biodegradation of composite materials
The total carbon dioxide evolved during 160 days incubation was measured by manual titration and its values are reported as percentages of the theoretical CO2 in Table 4. The curves reported in Figure 8 clearly indicate a different response of the investigated materials to compost biodegradation. After 160 days of incubation, standard cellulose powder and DDGS were quite completely transformed and an extreme deterioration of the initial pieces was visually noticed. An important criterion with regard to the quality of the inoculums is the biodegradation of the positive reference of cellulose. The standard cellulose powder (microcrystalline cellulose) used as reference material and was degraded 70% within 45 days of incubation as shown in Figure 8 as prescribed in the ASTM D5338. The other natural origin material such as DDGS degraded readily, particularly the lag phase was higher compared to cellulose. The initial lag phase biodegradation of DDGS was influenced by active microbes such as thermophilic fungus and material composition contains about 26.8-33.7% protein (dry weight basis) under composting conditions at 58°C. Moreover, high-moisture conditions (50-60%) apparently provide a more suitable environment for microbial growth and proliferation by causing cells to secrete enzymes (Imam & Gordon, 2002). In this regard, environmental factors such as water, oxygen, redox potential, nutrients, pH and temperature could influence enzyme catalysis of plant polymer (Imam & Gordon, 2002).
A completely different behavior was observed for biodegradation of PBAT virgin polymer. Indeed, a microbial lag phase started at approximately 18 days were observed, and following an exponential phase after 60 days of incubation (Fig. 8). As previously studied (Kijchavengkul et al., 2010) PBAT material undergoes biodegradation under composting and reached maximum biodegradation in 180 days. In our studies, the similar degradation behavior was observed and the rate of biodegradation about 63% after 160 days. It is noteworthy that biodegradation behavior of composite material containing pretreated DDGS 20 and 30%, are very close to the natural material such as cellulose and DDGS. The pertaining information of biodegradability of composite is clearly indicate that DDGS filler highly influenced for the biodegradability of PBAT matrix.
During composting tests, the virgin PBAT particles were observed in test flask and carefully recovered in order to evaluate the effect of mineralization on the thermal properties and structural characterization of materials after 10, 45, 60 days of incubation. The recovered PBAT material particle was easily distinguishable and further washed with deionised water and dried in oven at 40°C for 4 hrs. In the case of biocomposite material was not distinguishable after 10 days of incubation. The TGA traces of virgin PBAT showed after 10, 45 and 60 days of composting tests, the T onset reduced from 368 to 353, 311 and 253 °C respectively (Figure 9). The derivative peak of TGA is clearly indicated that, 80% weight reduction of PBAT was observed after 60 days incubation. These results indicate that compost microbial systems and conditions appeared to provide a very suitable environment for the biodegradation of PBAT material. Moreover, the microbial activity, hydrolysis has a substantial influence on biodegradation and rate of degradation of biodegradable polyester such as PBAT, since hydrolysis is one of the initial processes of biodegradation (Kale G, 2007), (Kasuya K, 2009), (Lucas N, 2008). It is generally accepted that there is different enzymatic specificity of microorganism for the degradation cellulose and biodegradable polymer PBAT under composting environments.
Hydrolytic main chain scission at multiple locations in the polymer chain produces smaller molecules or oligomers, which can easily permeate out of the polymer matrix. Therefore, reduction of the carbonyl absorbance (1710 cm-1) from enzymatic hydrolysis (data not shown). There is a critical need to understand the effect of microorganism population, family of microorganisms and their enzymes specificity in different microbial environments on the ultimate biodegradation of neat PBAT and composite materials based on DDGS. Biodegradation of PBAT composite materials based on treated DDGS was strongly influenced than the neat PABT. Thermal analysis and structural characterization of PBAT matrix is more vulnerable to undergo enzymatic hydrolysis and biodegradation under composting conditions.
The effects of untreated and pretreated in DDGS/PBAT matrix on the mechanical, thermal, structural, and biodegradability properties were evaluated. The data indicated that an increase in DDGS filler content in a decrease in tensile strength (TS) and elongation (Æ), but a significant improvement in modulus (E). It was found that the pretreated DDGS has better improvement in mechanical and thermal properties than non-treated DDGS. During composting tests, DDGS was strongly influenced for the biodegradation of PBAT matrix, which was monitored by CO2 emissions. The degree of biodegradability of all composite materials was similar to the natural materials such as cellulose and DDGS. TGA and FT-IR analysis indicates the biodegradation of PBAT through enzymatic hydrolysis followed by microbial assimilation. All the results reported here indicate that the materials have short survival time in biotic environment such as compost, and therefore after their use they are suitable for disposal in landfills.