Development And Preparation Of A Novel Hydrophobically Modified Starch Film

An extensive use of traditional petroleum-based plastics has brought serious problems to the ecological environment and human health.Developing degradable and environmental friendly materials has very important social and practical significance for the development of a green and low-carbon economy in China.As a natural polymer,starch is widely distributed in nature with the advantages of low cost,safe use and excellent plasticity.Starch-based degradable materials are considered to be the most potential substitutes for traditional plastics.However,due to a large number of hydroxyl groups in starch molecules,starch-based materials show strong hygroscopic properties,and its performance is severely lost in harsh environments such as high humidity,which limits their applications.The main purpose of this study is to improve the hydrophobic properties of starch-based films,based on the esterification reaction mechanism.Using alkyl ketene dimer(AKD)which has an active lactone ring structure.And bulk blending and surface modification were used respectively to improve the hydrophobic behaviors and performances.The relevant research results are as follows:1.A new hydrophobically modified starch film was successfully prepared by direct blending casting and multiple layered casting respectively,and the related properties were characterized.(1)A hydrophobic starch film was directly casted by the blending method.Different proportions of AKD(4-20 wt%)molecules were homogeneously dispersed in the starch paste by melting and high-speed shearing,starch film was prepared by the casting method,and then the hydroxyl esterification was induced by high-temperature post-cooking treatment.The hydrophobically modified film was then prepared and characterized.Results showed that the film had a characteristic absorption peak of ester carbonyl at 1733 cm^-1,indicating that AKD reacted with the hydroxyl groups of starch molecules.The non-covalent interaction between the molecules of starch and AKD was characterized by non-equilibrium molecular dynamics.The results showed that starch molecular chains gradually moved towards the interface of AKD,and the hydrogen bond interaction at the phase interface was confirmed by RDG(reduced density gradient function)and AIM(atom in molecule)theory.Scanning electron microscopy results revealed that AKD molecules crystallized in the form of small solid particles and were uniformly dispersed in starch matrix,and with an increase of AKD's content,the particle density gradually increased,but no significant local aggregation was observed.When the amount of AKD added was about 8%:the contact angle of the film increased significantly from93.68°to 104.58°;the rate of water absorption decreased from 9.8%to 7.3%;the transmission rate of water vapor within 120 hours decreased by 50%around.It showed that AKD could effectively reduce moisture sensitivity of starch–based materials.However,the melt blending process significantly reduced the mechanical properties of the film(the elongation at break was only 83.20%).(2)A homogeneous and heterogeneous self-reinforced starch film was prepared by the multiple layered casting method to improve the mechanical properties of the film.The starch film had a three-layer structure:the outer layer was hydrophobic modified starch layer,which can regulate the hydrophobic behavior by adjusting the AKD content;the middle layer was original starch layer,which can provide mechanical force for the system.Scanning electron microscopy analysis showed that there was no obvious phase separation between the three-layer structure,proving that the compatibility between the layers was good.Compared with the pure starch film,the hydrophobic properties of the self-reinforced starch film were significantly improved.When the addition of AKD in the outer layer is about 8%:the contact angle increased from 93.68°to 103.88°;the rate of water absorption decreased from 9.8%to 7.9%;the transmission rate of water vapor decreased by about 10%;the mechanical properties were significantly improved at the same time and the elongation rate at break can be as high as146.53%.2.Regulating the hydrophobicity of starch film using surface esterification and physical coating method respectively,and then the related properties were characterized.(1)The starch film modified by surface esterification was prepared by directly dipping it in the AKD-heptane solution and then keeping at high temperature.Infrared spectrum measurement showed that the absorption peak of ester carbonyl group appeared on the starch molecules,indicating that the esterification reaction of AKD occurred.The rate of water absorption was reduced from 9.80%to 4.57%,the contact angle was increased from 93.68°to125.07°,and the transmission rate of water vapor was reduced by about 50%,indicating that the surface esterification significantly improved the water vapor barrier capacity of the film.The elongation at break of the modified film decreased from 244.73%to 225.36%,the tensile strength increased from 5.42 Mpa to 7.03 Mpa,and the Young's modulus increased from 11.56Mpa to 34.53 Mpa,indicating that the mechanical properties did not decrease significantly.(2)The native starch film was dipped and coated in AKD molecules melted at 65°C and 85°C,respectively,and then modified by ethanol etching treatment.X-ray diffraction and scanning electron microscopy results showed that low-temperature(65°C)melting promoted high-speed crystallization of AKD and induced it in a metastable state.A two-length scale(micron/nano size)hierarchical structure was formed and it induced an improve in hydrophobicity and water contact is as high as 146.55°.Such physical treatment reduced wet sensitivity of starch film,and the rate of water absorption and water vapor transmission decreased to 0.5%and 90%repetivley.The elongation at break of the modified film were between 189%?225%,Young's modulus in the range of 19.15 Mpa?28.36 Mpa,tensile strength was between 5.49 Mpa?6.06 Mpa,which were close to the native starch film(244.73%,11.56 Mpa,5.42 Mpa),indicating that the physical coating had less influence on the mechanical properties.