Extraction And Modification Of Corn Cob Hemicellulose And Preparation Of Film Materials

Biomass resource has attracted more and more attention due to its advantages of renewability and environmental protection as the global energy crisis and resource shortage become serious.Crop straws are important renewable biomass resources.Among them,corn is one of the important crop in China,and providing 45.9 million tons annually.So far,the corn cob has not been effectively utilized.The hemicellulose content in corn cob is up to 35-40 wt%.As one of the important components of lignocellulosic biomass,hemicellulose has potential market prospect and application value due to its unique structure and environmentally friendly properties.In this work,hemicellulose was extracted from corn cob using a two-step alkaline process and further modified by acetylation.Corn cob hemicellulose and its derivatives were then employed to prepare bio-based film materials.The main research contents are as follows:The soluble impurity and wax in the corn cob were removed by toluene/ethanol mixed solvent.The hemicellulose of corn cob was further extracted by two-step alkaline method.At the first step of weak alkali treatment,the yield of holocellulose at82.7%was obtained with 0.7 wt%Na Cl O₂and p H value of 4.0 at 80?for 2 h.At the second step of strong alkali treatment,holocellulose was further extracted in 6 wt%Na OH solution at 60?for 4 h.The yield and purity of corn cob hemicellulose were31.7%and 85.4 wt%,respectively.The separation and extraction of corn cob hemicellulose from holocellulose was also performed by ultrasound-assisted alkali method.The yield and purity of corn cob hemicellulose were 33.2%and 83.3 wt%,respectively after extracting at 60?for 2 h in strong alkali solution.Compared with without ultrasonic extraction,the yield was slightly increased and the extraction time was significantly shortened.The structure and morphology of corn cob hemicellulose were characterized by infrared spectroscopy(IR),thermogravimetric analysis(TGA),gel chromatography(GPC),nuclear magnetic resonance(NMR)and scanning electron microscopy(SEM).The results showed that corn cob hemicellulose extracted by alkali method was mainly composed of xylan formed by?-D-xyranose,which was attached to L-furan arabinose group at C-3 position and 4-O-methyl-glucuronic acid at C-2 position.The acetylation of hemicellulose was catalyzed by combining concentrated sulfuric acid and I₂.The effects of catalyst type and amount,acetic anhydride amount,reaction temperature and time were investigated.The yield and degree of substitution of acetylated hemicellulose were 85.7%and 1.33 when 1 g hemicellulose reacted with10 m L acetic anhydride at 60?for 6 h using 100?L concentrated sulfuric acid and0.05 g I₂as cocatalyst.The corn cob hemicellulose extracted by ultrasonic-assisted alkali method was also modified by acetylation,and the yield and degree of substitution were 80.1%and 1.09 under identical conditions.The structure of the acetylated hemicellulose was characterized by IR,NMR,TGA and GPC.The results showed that the acetyl group was introduced into hemicellulose molecule via esterified modification,and the thermal stability of the acetylated hemicellulose was improved.The preparation of bio-based film materials from hemicellulose and its modified products was then studied.The influence of reinforcing agent,plasticizer and film substrates on the properties of film materials were investigated.The structure of the film material was characterized by SEM and TGA,and the mechanical properties as well as water vapor transmittance of the film material were tested.The results showed when chitosan and sorbitol were used as strengthening agent and plasticizer,respectively,the tensile strength,elongation at break and water vapor transmittance of the film material derived from corn cob hemicellulose were 74.89 MPa,54.60%and752.47 g/m²·24h with hemicellulose:chitosan:sorbitol=1:1:0.5(mass ratio)and stirring at room temperature for 6 h.The tensile strength,elongation at break and water vapor permeability of the film prepared by acetylated corn cob hemicellulose were 78.87 MPa,62.40%and 623.65 g/m²·24h with identical conditions,respectively.The results showed that the properties of the hemicellulose-based film were improved by acetylated modification.In addition,the preservation property of the film material was measured.The results showed that the hemicellulosic film material had good fresh-keeping effect and could prolong the shelf life to a certain extent.