Studies On The Hydrophobic Modification Of Biodegradable ST/PVA Ground Films

At present,plastic film mulching technology are extensive applied to wheat, rice, corn, soybeans, sorghum, cotton, watermelons and other crops with notable improving production effect. However, the large-scale used plastic film is an undegradable biopolymer which cann not be bioerosioned by microbial corrosion, neither be able to decomposited in short time. It will bring contamination hazard to the next generation. So the study of biodegradable film is being considered. Corn starch is a renewable resource in the nature, and it is harmless and totally biodegradable. Nevertheless, the bad performance of mechanics and water proofness limited its application.Took corn starch as the raw material, the high thermoplasticity cross-link hydroxymethyl starch were made by carboxymethyl etherification, cross-link modification. After mixed the self-made cross-link carboxymethyl starch with PVA, cross-linking, plastication and hydrophobicity modified, the mechanics performance were qualified, and its absorption rate were lowered greatly. The following research were taken :First, quadratic regression orthogonal rotation design were adopted to analyze the technology parameters of carboxymethyl etherification modification, and the optimized technology parameters including : etherealization temperature at 74.44℃, molar ratio of NaOH to starch were 1.85, etherification time were 2.33h, alcohol concentration were 84.65%, the greatest degree of substitution of carboxymethylation modified starch was 0.49. In the process of cross-link modification, took epichlorohydrin as the cross-linking agent. Meanwhile, technology parameters of crossing–lingking reaction were optimized. The results of 4 factors and 3 levels orthogonal experiment showed: the amount of cross-linking agent were 0.05%, cross-linking temperature was 55℃, cross-linking time was 1.0h, the amount of NaOH was 1.5%.Brabender Amelograph, biological microscope, scanning electron microscope, and Fourier infrared spectra scanning were applied to detect the characters of hydroxymethyl starch and Crioss-linking hydroxymethyl starch. The result showed that: carboxymethyl starch granules occurred "burst" expansion without rules. Owing to the addition of carboxymethyl group, the increasing intermolecular obstacle and the effect of static electricity, raised the liquid-binding power of starch low temperature disperse system, and reduced the tendency of retrogradation and gelatinization,the cold paste of starch were stable too. Carboxymethylation only took place in the anarchic field of starch, and double-modified starch destroyed the crystal structure of starch. Granule had obviously breakage fissure and deep cave, and it broken the original starch symplex structure. The viscosity of cross-linking Hydroxymethyl Starch, solution degree, swelling strength, transmittancy and freeze thawing constancy were greatly improved than raw starch and Hydroxymethylstarch.Took double modified starch as the raw material to prepare biodegradable plastic film, The results showed that: the 0.8 sedimentation cross-linking hydroxymethyl starch mixed well with PVA, the best ratio was 7:3, the amount of compound cross linking agent was 3.3%, pH at 10,temperature was 88℃,time was 35min. the best combination of compound elasticizer was triethanolamine 0.4%, glycerine 0.4%, polyethylene glycol 0.6%, the amount of defoamer tween 80 was 0.4%, amount of defilming agent liquid paraffin was 0.8%, the amount of dewatering modifier PPE was 1.5%. Detected by scanning electron microscope, infrared spectra, DSC differential scanning, The result showed that dewatering modified film made great changed in appearance and internal structure. It had more homogeneous morphous structure and larger molecular weight. It had been greatly impoved in mechanics performance, and the absorption rate was decreased, which means the ability of water maintaining and anti- hypohydration were better, and can not be changed the character of film under different ambient humidity.Results of degradation experiment indicated that dewatering modified ST/PVA film with better biodegradable function, the lager ratio of ST to PVA was , the less time to use in totally degraded.The higher the degree of cross linking was, the speed of degraded was fast. With the increasing amount of PPE, the degradation performance was decreased. The total degradation time was from 20d to 120d, therefore, it can come ture to contral the degradation time artificially by means of controlling the amounts of these 3 agents.