| Traditional petroleum-based plastic is difficult to degrade under natural conditions.Simultaneously,it caused packaging pollution and living waste problems,which damaged the earth's ecological environment seriously.In 1970's,a significant concept of biodegradable plastic was put forward in many developed countries such as Europe,United States and Japan.Meanwhile,based on this concept,various kinds of products were produced during that period and starch-based degradable plastics was one of them.Unfortunately,this kind of plastic was not wildely used due to its poor mechanical property and strong hydrophilicity.In the study of starch-based degradable plastics,appropriate starch and polyvinyl alcohol(PVA)were chosen as basic materials.By adjusting the proportion of starch/PVA and the pelleting and film blowing process,the starch-based nanocomposite films whose mechanical properties met the standard of the traditional plastic were prepared,and its mechanism of improvement of mechanical properties was studied in the meanwhile.Under premise of the same mechanical properties,later researches were dedicated to improve the water vapor and oxygen barrier properties and hydrophilicity.In the end,conclusion for stability and application performance of starch films was drawn.Helpfully,it could provide theoretical and technical support for the application of starch films in terms of production and life.The main results were as follows:1.Different molecular form of starch and PVA caused differences on starch films obviously.Compared with the low degree of polymerization of PVA,the composite films of starch with high degree of polymerization of PVA have better surface compactness,interface affinity and compatibility.Unplasticized starch granules were distributed on the surface of films formed by high-viscosity starch and high degree of polymerization of PVA.Fully plasticized starch granules were in favor of the formation of intercalating nano-composite structure.HP/24 showed the best performance.Its tensile strength and elongation at break were 7.84 MPa and 184.0%,respectively.Water vapor permeability was 3.18×10-10 g·m-1·s-1·Pa-1.Contact Angle was 66.1°.Light transmittance under 600 nm was 22.6%.2.Aggregation phenomenon of nanoparticles was observed on the surface of pure starch film.And a little nano intercalated layer structure was formed.When PVA was added,aggregation phenomenon of nanoparticles was disappeared,multilayer nanostructures could be easily observed in the composite film.Starch and PVA molecular chain was inserted into the OMMT layers,and it increased the gap between the two adjacent layers and formed an orderly intercalated layer structure.Making these nanoparticles dispersed homogeneously in layers-intercalating style and these nanolayers ranged by layers like brickwork in the composite films matrix,which is beneficial to improve the mechanical and barrier properties of the composite films.3.Pure starch film is a uniform continuous system,which only has the glass transition temperature,and the starch phase was continuous.When the proportion of PVA was increased to 30%,there appears a glass transition temperatures of starch and PVA,and the starch-PVA phase was continuous.Increasing the proportion of PVA up to 50%,we found that the gap of glass transition temperature between PVA and starch was minimized,while the adhesion of composite film interface and the compatibility of PVA and starch were all improved.The continuous phase of starch film matrix was changed from starch phase to PVA phase,and starch phase was changed to the dispersed phase.Transition of continuous phase is helpful to improve the mechanical properties of the composite film.4.The transverse and longitudinal tensile strength of composite films were 30.18 MPa and 25.57 MPa,respectively,while the elongation at break of the above-mentioned parameters were 196.30% and 224.40%,respectively,which are 28.0 and 1.2 times of the original datum in maximum.The results can meet the mechanical requirements of GBT 4456-2008(packing polyethylene blown film)of PE-MD film(10 MPa,150%),PE-LD/PE-LLD film(11 MPa,150%).The water vapor permeability and oxygen permeability of the composite films were reduced to 1.92×10-10 g·m-1·s-1·Pa-1,4.11×10-16 cm2·s-1·Pa-1,respectively,indicating the obvious improvement of the barrier properties.Meanwhile,absolutely different changes have taken placed in the surface morphology of the film,which turns to more smooth and continuous without any independent zones and cracks.5.1% and 2% addition of citric acid could play an important role in plasticizing modification.When more citric acid was added,starch molecules would be hydrolyzed.Boric acid can form crosslinked network with starch/PVA.Citric acid and boric acid reacted with starch/PVA system and produced the hydrophobic groups(ester).As a result,the hydrophobicity,water resistance and gel mass of composite films were increased,and the swelling degree was reduced.The maximum contact angle,minimum water vapor permeability and minimum swelling degree were 91.58°,0.87×10-10 g·m-1·s-1·Pa-1 and 4.97 cm-2,respectively.6.The addition of twelve carbon dibasic acid could improve the barrier properties of starch/PVA films.When the amount of dibasic acid was 6%,oxygen barrier properties and hydrophobic properties of starch film were the best.The oxygen permeability was 3.22 ×10-16 cm2·s-1·Pa-1,and the contact angle was 105 °.When the amount of dibasic acid was 12%,ductility of composite film was the best,and elongation at break was 239.52%.7.The compatibilizing modified starch/PVA/PLA ternary composite film could effectively improve its performance.Compatibilizing modification of PLA could reduce the crystallinity of composite film,enhance the interfacial adhesion and improve the barrier properties as well as the hydrophobic properties.The maximum water contact angle was 104°,showing hydrophobic properties.The transverse,longitudinal tensile strength and elongation at break of starch/PVA/PLA composite film were 13.82 MPa,12.04 MPa and 150.2%,182.6%,respectively.While the correspondence of starch/PVA/PLA/g-PLA composite film were 21.6 MPa,19.4 MPa and 299.7%,327.7%,respectively,which could meet the demand of more packaging.8.PVA could improve the anti-retrogradation property of starch films.If starch film containing less than 30% PVA was stored in 23 °C and 53% RH for 18 months,the tensile strength of the starch film would increase,the ductility as well as water content would also decreased.The film would become more hard and brittle.Significant retrogradation and crystallization could be detected.and retrogradation phenomenon is obvious.The starch film containing 40% or 50% PVA is relatively stable.9.Starch film could significantly extend the shelf life of apple,delayed the oxidation of high oil food such as peanut and walnut.Starch film packaging can reduce the probability of water droplets in sealed form and bacterial growth caused by the transpiration of apples.Starch film packaging can effectively prevent the oxygen entering into the sealed package and inhibit the increase of peroxide value and acid value as well as delay the oxidation of peanut and walnut. |
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