Preparation Of Starch-based Composite Films And Research On Its Reaction Mechanism

In view of the severe environmental pollution caused by plastic food packaging, the interest in edible and biodegradable films made from natural polymers and the need for environmentally friendly polymers has increased. Among natural polymers, starch is one of the most promising candidates for preparing biodegradable films because it is a renewable source and is widely available, abundant, relatively easy to handle, and inexpensive. Because of storage difficulties and inefficient processing, up to 15% of their sweet potatoes are discarded every year. Because sweet potatoes are an edible and biodegradable material, the preparation of sweet-potato-starch(SPS)-based film is a good way to reduce the waste. However, its physical properties and chemical structure restricted the application of native SPS. In addition, the poor mechanical properties of SPS-based films and they are sensitive to water and absorb water easily in a high-humidity environment which hindered their extensive use. In this article, the addition of plasticizer and fatty acids, the modification of starch were systematic studied to overcome these weaknesses. The main conclusions were listed as following:(1) Preparation of starch-based films by reactive extrusion① The glycerol significantly changed SPS pasting characteristics. Compared with the ratios of glycerol/starch of 0/1, the peak viscosity, trough viscosity, break down, final viscosity and set back values decreased 59.5%, 46.8%, 78.1%, 51.1%, 60.9%, respectively, for the ratios of glycerol/starch of 0.45/1. For the ratios of glycerol/starch of 0.25/1, the films had the highest tensile strength(5.7 MPa). In addition, when the ratio of glycerol/starch was 0.45/1, the films exhibited the highest elongation at break(16%). Compared with the films(0.25/1), the water vapor permeability values of the films(0.45/1) showed an increment of 19.9%. The light transmission values of the films were higher than 80% when the ratios of glycerol/starch between 0.3/1 ~ 0.4/1. The light transmission values of the films were lower than 70%, when the ratios of glycerol/starch were 0/1, 0.25/1 and 0.45/1. The addition of glycerol caused a decrease in relative crystallinity for the starch. The DSC and NMR measurements indicated that the hydrogen bond was formed between in glycerol and starch. The dispersion and distribution of the starch granules improved significantly, and the surface of the films became smooth for the ratios of glycerol/starch between 0.3/1 ~ 0.4/1. The films exhibited better heat endurance and the retrogradation of the films was delayed with the content of glycerol increased. These results suggested that the glycerol had a good compatibility and better plasticizing effect with the starch matrix when the ratios of glycerol/starch between 0.3/1 ~ 0.4/1.② The suitable addition amount of benzoyl peroxide was 0.7%. With the increase of maleic anhydride(MAH) content, the degree of substitution gradually increased. The most striking feature for the modified starch emerged a carbonyl stretch peak at about 1706.12 cm-1, 1718.02 cm-1 and 1723.97 cm-1 by FTIR spectra, which attested that the MAH was covalently linked to the SPS backbone. With the increase of MAH content, the intensity for the relevant band from 3200 cm-1 to 3600 cm-1 which could be attributed to the stretching of intermolecular and intramolecular bonding hydroxyl groups of SPS gradually decreased. As observed from the nuclear magnetic resonance(NMR) spectrum, the carbon atoms C1(101.3ppm), C2,3,5(72.5ppm), C4(81.7ppm), C6(61.9ppm) were readily assigned to the glucosidic unit of SPS, the resonance peak at 129ppm(C7) characteristic to the vinyl functions of MAH molecule grafted onto SPS backbone. The TGA curves exhibited that the thermal stability was increased for the SPS after the addition of MAH. The TGA curves also showed that the maximum temperature corresponding to the thermal degradation of starch backbone(270 oC) delayed to 290 oC(0%), 295 oC(1%), 290 oC(3%), 286 oC(5%) and 287 oC(7%). Scanning electronic microscope(SEM) examination showed that the surface of starch became rough and the structure of starch turned into loose after the addition of MAH. With the addition of MAH in the range of 0%~3%, the light transmission gradually increased. Compared with the control film, the elongation at break and WVP values of composite films(MAH 3%) showed an increment of 56% and 5.7%, respectively, and an increment of 25.2% in tensile strength. The tensile strength and elongation at break decreased 23% and 41.81% for the control films(30 days). On the other hand, the films containing 3% MAH, the tensile strength and elongation at break decreased 12.5% and 16.3%, respectively. The retrogradation of the films was delayed by the addition of MAH.③ Through the response surface methodology, the optimal scheme for the extrusion process was obtained as follows: which the ratio of glycerol/starch was 0.332/1, the MAH was 1%, the screw speed was 25.1r/min and the extrusion temperature was 120.5 oC.(2) Preparation of starch-based films by solution casting① Light and confocal laser scanning microscopic examination showed that the amylose-lipid complex distributed throughout the SPS granules for the SPS-SA composite which prepared by method I, while SPS-SA composite prepared by method II was mainly distributed on the surface of SPS granules. The effects from the method I on SPS pasting properties were more pronounced than those of method II. Compared with the control, for the SPS-SA composite prepared by method I, the PV and BD decreased 17.4% and 33.0%, respectively; for the SPS-SA composite prepared by method II, the PV and BD decreased 6.0% and 25.7%, respectively. In differential scanning calorimetry(DSC) thermograms, both of the composites exhibited a new transition peak, which were attributed to the melting of amylose-lipid complex endotherms(91.9- 109.1 oC, method I; 93.1- 108.4 oC, method II). The gelatinization temperatures of the SPS were 67.3 oC(To), 71.3 oC(Tp) and 82.7 oC(Tc). SEM microscopic examination indicated that the amount of intact starch granules reduced(method I), while the starch granules in the composite prepared by method II were maintained intact in morphology. XRD studies revealed that the the addition of SA caused a decrease in relative crystallinity, and emerged new peaks at around 2θ of 20.15° which were identified as corresponding to the formation of amylose-lipid complex for both of the composites. A small resonance peak characteristic to the carbon chain(-CH2, 31.6ppm) could be observed by NMR. Compared with the control film, the light transmission of the films prepared by method I showed a decrement of 10.1%, while the films prepared by method II exhibited a decrement of 5.1%. Contact angle tests showed that the contact angle of the SPS-SA composite films increased to 56.0°(method I) and 86.3°(method II). Compared with the control film, the tensile strength of SPS-SA composite films showed an increment of 5%(method I) and 10.3%(method II), a decrement of 42.7%(method I) and 34.5%(method II) in elongation at break.② Compared with the control, the starches containing AA(C20:0), SA(C18:0) and PA(C16:0) which had longer carbon chains, the PV and BD decreased 4.5%-7.0% and 8.1%-28.3%, respectively. On the other hand, for the starches containing MA(C14:0) and LA(C12:0), which had shorter carbon chains, the PV and BD decreased 13.2%-16.4% and 48.6%-58.8%, respectively. The range of Mw values of amylose by different types of SFA was from 5.10×104 to 9.72×104 compared with the control sample; which had an MW of 1.20×105. SEM images showed that the surfaces of the composite films with shorter carbon chains were smooth, whereas the films with longer carbon chains were rough. Compared with the control film, the tensile strength of films with AA(C20:0), SA(C18:0), and PA(C16:0), which had longer carbon chains, showed an increment of 5.5%-14.5%. On the other hand, the films with LA(C12:0) and MA(C14:0), which had shorter carbon chains, showed an increment of 24%-31.6% in tensile strength. A thermal study with DSC suggested that the addition of SFA increased the onset temperature(To) and peak temperature(Tp). The SPS-SFA composite films showed lower light transmission than the control. With increasing number of carbon chains(from C12:0 to C20:0), the light transmission values of the films gradually increased. Contact angle tests showed that the films with LA(C12:0) and MA(C14:0), which had shorter carbon chains, had better moisture barrier properties than their counterparts which had longer carbon chains.③ Through the response surface methodology, the optimal scheme for the casting process was obtained as follows: which the lauric acid was 2.12%, the gelatinization temperature was 95 oC, the poured amount of starch slurry was 0.219 g/cm2 and the drying temperature was 56.8 oC.