| Hemicelluloses, as the second largest component of lignocellulosic biomass, rank next tocellulose in abundance. The low-cost and highly-efficient conversion of hemicelluloses is thekey technology for the transformation of lignocellulosic biomass into high value-addedproducts. Recently, xylan being the main hemicelluloses in hardwood and grasses has beenattracted ever-increasing attention especially in the fields of pharmaceuticals and packagingdue to its special structure and physicochemical properties. However, xylan has the poorfilm-forming ability and its film has suffered from brittleness, low mechanical property andhumidity sensitivity, which limit its application in industries. In order to broaden itsapplication in packaging industry and solve the issues mentioned above, a series of poly vinylalcohol (PVA)/xylan composite films possessing degradability and better mechanicalproperties were prepared in this study. The influences of different additives on the propertiesof PVA/xylan composite films were investigated comparatively.(1) The water insoluble xylan citrate with carboxyl groups was prepared by theenvironmentally friendly semi-dry oven method using xylan as the raw material and citricacid as the ester reagent. The influences of reaction conditions on the product yield, thedegree of esterification and the carboxyl content of xylan citrate were discussed. Themechanism for the reaction of xylan with citric acid was proposed. It was found that catalystfacilitated the occurrence of the crosslinking reaction and the resulting products had thefilm-forming ability. However, the yield of128.2%, the carboxyl content of1174.3meq/100gand the degree of esterification of33.1%were achieved at the weight ratio of citric acid/xylanof2.4in the absence of catalyst. The product could be used as an adsorbent in the wastewatertreatment industry dut to its metal ions adsorption properties.(2) The environmentally friendly PVA/xylan composite films were prepared by castmethod using non-toxic and safe citric acid as the plasticizer and the crosslinking agent. Theeffect of citric acid on the properties of composite films was discussed. The results showedthat the citric acid content had a remarkable impact on the properties of composite films.When citric acid mainly acted as the plasticization function, the tensile strength (TS) of PVA/xylan composite films decreased from35.1MPa to11.6MPa, and the elongation atbreak (EAB) increased from15.1%to249.5%with increasing citric acid content from10%to50%. However, when citric acid mainly acted as the crosslinker function, TS decreased from49.3MPa to32.6MPa, EAB slightly increased from6.8%to9.5%with an increment in citricacid from10%to30%. Moreover, the citric acid content also had the obvious effects on thewater vapor permeability (WVP) and solubility of composite films. Fourier transformsinfrared spectroscopy (FTIR) and scanning electron microscopy (SEM) confirmed thatPVA/xylan composite films had the good compatibility. It also found that the citric acidcontent and xylan content had a great effect on the degradation of composite films.(3) In order to improve the hydrophobicity composite films, biodegradable PVA/xylancomposite films with the excellent hydrophobic property were developed by casting methodusing urea as a water resistant additive. The effect of urea on the properties of composite filmsand the reaction mechanism between urea and PVA/xylan were discussed. The results showedthat the addition of1%urea in composite films at the weight ratio of PVA/xylan at3:1without glycerol had a positive impact on improving the water resistance of composite films,the contact angle reached to114.68o, increased by43.75%, in comparison with compositefilms without urea and WVP values was up to4.11×10-11g·m-1·s-1·Pa-1. It was also found thatthe addition of glyceol in composite films improved the compatibility between PVA and xylanand decreased the thermal stability of composite films. The experiment on the enzymaticdegradation showed that the degree of degradation of composites films was achieved above90%after the enzymatic hydrolysis of48h. Urea-incorporated composite films had the lowdegree of degradation. In addition, the composite films had good fresh-keeping function ongrape, and reduced the evapotranspiration, the respiration, and the peroxidation degree of cellmembrane.(4) To improve the mechanical properties of composite films, the PVA/xylan compositefilms with excellent mechanical properties were prepared using TiO2as the inorganicreinforcing agent. The effect of crystal type and content of TiO2on the properties ofcomposite films was discussed. The resulted showed that the rutile TiO2was more beneficialthan anatase TiO2to improve the mechanical property of composite films. The TS ofPVA/xylan composite film with2%rutile TiO2reached the maximum value of30.73MPa, EAB was192.91%and the contact angle was the maximum value of115.5o, the lowest valueof WVP was achieved to3.46×10-11g·m-1·s-1·Pa-1. Thermogravimetric analysis displayed thatthe thermal stability of PVA/xylan composite films with TiO2were enhanced. Therefore, theaddition rutile TiO2in composite films contributed to improving the mechanical propertiesand hydrophobicity of PVA/xylan composite films. |
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