Water Cast Film And Crystallization Properties Of Sugarcane Bagasse Xylans With Different Chemical Structures

Hemicellulose is an abundant plant-derived natural polymer.Xylans are the main type of hemicelluloses isolated from hardwood and graminaceous plants,and have the potential to replace petroleum-based polymers.However,the industrial utilization rate of xylan is relatively low,and a large number of xylans in paper waste or agricultural waste are directly burned as low-value energy.Xylan has good dispersibility in water,and thus can form stable colloidal solution.After drying,xylan molecules interweave with each other to form films which have a high potential as fine chemicals,especially functional coatings or packing agents.The basic essence of coatings or parking agents with water as solvent is that a dense and integral water-casting film can be formed on the coated substrate.The performance of the water-cast films is the key factor affecting the quality of coatings or packing agents.In addition,at present,there are some problems such as unclear film-forming mechanisms and poor mechanical properties of water-cast xylan films.The intrinsic reason lies on that the corresponding relationships between the physicochemical structuresof xylan and its film-forming properties as well as mechanical properties are not fully understood,which limits the applications of xylans in related fields.Based on the above situation,in this article,a representative agriculture waste,sugarcane bagasse,was selected as raw materials and the xylan fractions with different chemical structure,chemical components,molecule weight and lignin contents were prepared by different extraction,separation and modification methods.The influence of these factors of xylan on the crystallization,film forming and mechanical properties of water-cast film was systematically analyzed.The illustration of these mechanisms can provide theoretical guidance for the selective separation of xylan,which is conducive to high-valued development of xylan-based materials.(1)Xylans with different branching degrees which ranged from 0.109 to 0.261 were prepared by continuous alkali extraction,one-step alkali extraction,graded ethanol precipitation,I₂-KI precipitation and ammonium sulfate precipitation methods.The effects of branching degree of xylan on its crystallinity and film-forming properties were analyzed.The branching degree of xylan is highly related to its crystallinity.The existence of impurities such as side-chain groupd or dextran could effectively prevent the excessive aggregation or crystallization of xylan molecules,and was beneficial to the enhancement of the film-forming performance of water-cast xylan films.Xylan with high branching degree has good film-forming property.(2)Carboxymethylated xylan samples(CMH)with different degree of substitution(DS)in the range of 0.273-0.924 were made by modifying xylan with sodium chloroacetate.The effects of DS on the crystallization,film-forming and mechanical properties of CMH were analyzed.The results showed that carboxymethylation could effectively improve the film-forming properties of linear xylan.With the increase of the DS of carboxymethyl groups,the crystallinity of CMH decreased and the film-forming property was enhanced.Moderate carboxymethylation contributed to promoting the mechanical properties of CMH films,while excessive carboxymethyl modification led to poor mechanical properties of CMH films.(3)Xylan samples with Mw of 765000-1119000 were prepared via xylanase hydrolysis method.The effect of molecular weight on the xylan crystallinity was analyzed.Xylanase could cut off the molecular chain of xylan and further destroy the crystal structure of xylan.The crystalline structure of xylan partially remained after the hydrolysis of xylanase,which was detrimental to the film formation of xylan molecules.However,xylanase could not destroy the structure of the xylan molecular chain except for reducing its length.The crystal structure of xylan prepared via enzymatic hydrolysis could be formed again after the processes of redissolving in alkali solutions and precipitation.Hence,simply reducing molecule weight of xylan had no effect on its crystallinity.(4)Xylan componants from sugarcane bagasse,of which the lignin content,Mn and xylose/arabinose ratio(Xyl/Ara ratio)ranged from 1.6%to 9.5%,8200 to 26800,and 5.0 to9.1,respectively,were prepared via one-step alkaline or alkaline hydrogen peroxide solution method or two-step alkali solution extraction-delignification method.The effects of lignin content,molecular weight and Xyl/Ara ratio on the mechanical properties of the films were analyzed by statistical model.The Xyl/Ara ratio represented the proportion of linear chain structure of hemicellulose,and the higher ratio,the better the mechanical properties of films.When the molecular weight of xylan was low,the increase of lignin content was beneficial to improve the mechanical properties of films;On the contrary,when the molecular weight was high,the presence of lignin would reduce the mechanical strength of films.In a word,xylan with more linear chain structure,higher molecular weight and lower lignin content had better mechanical properties.