Effect Of Pretreatment On Preparation Of Lignin-Containing Cellulose Nanofibrils From Corncob Residue And Its Barrier Properties

In recent years,researches on the utilization of waste biomass materials such as corncob residue(CR)and the application of lignin-containing cellulose nanofibrils(LCNF)received much attention.The cellulose nanofibrils(CNF)could form a dense film structure,which makes it have enormous potential in replacing packaging materials such as petroleum-based plastics.It is hindered by its highly polarity and strong hydrophilic.Interestingly,LCNF prepared by preserving lignin in the raw material has lower polarity,improved hydrophobicity,high yield and less harmful bleaching chemicals were used.Additionally,LCNF has compatibility with non-polar polymer and endowed material with ultraviolet light resistance and increased water barrier properties.However,the properties of lignin and cellulose are changed after different pretreatments,which makes the preparation of LCNF effected and barrier properties of LCNF changed.The effects of different pretreatments on the preparation of LCNF and the barrier properties of LCNF film or coated paper deserved to be studied.This thesis used CR after industrial xylose extraction process as raw material.LCNF was prepared by combining with pretreatment and mechanical treatment.And the LCNF film was prepared by bar coating-forming method,and the carboxymethylated LCNF coated paper was prepared.The non-pretreated CR fiber(F1)and delignified CR fiber(F2)were as control samples.Four different pretreatment methods for lignin and cellulose modification were researched detailly,which including laccase oxidation(L),alkaline sodium sulfite sulfonation(S),alkaline hydrogen peroxide oxidation(P),and different degrees of carboxymethylation(C).The fiber size and lignin content of fiber material,the water retention value(WRV)of fibers and corresponding LCNF before and after different pre-treated were determined by an L&W fiber analyzer and chemical analysis method,centrifugal force method,respectively.The morphology of LCNF was observed by scanning electron microscopy(SEM).The content of acid groups of the fiber material was determined by conductivity titration.The rheology and chemical structure changes of LCNF were analyzed by a rheometer and the FT-IR Spectrometer.In addition,the surface and cross-sectional micro-morphology of the LCNF film and C-LCNF coated paper were evaluated.LCNF film's water contact angle(WCA),mechanical properties,oxygen permeability(OP),water vapor permeability(WVP)and C-LCNF coated paper's water vapor transmission rate(WVTR)were tested.The main conclusions of the study are as follows:(1)The pretreated CR fiber materail was fibrillated using a SuperMassColloider.The prepared LCNF were further used to form the film by bar coating-forming method.A comparative study of the effects of lignin pretreatment methods L,S,P on fiber material and corresponding LCNF and LCNF film properties have been done.The study found that the average length of CR fiber was 0.3-0.4mm and the lignin content of that was about 16.7wt%.The L,S,and P pretreatments had no significant effect on fiber size,but the lignin content after L,S and P pretreatment were change to 19.1wt%,12.2wt%and 8.46wt%,respectively.The presence of lignin may interfere with the hydrogen bonding between fibrils and resulting in the tensile strength reduction of the LCNF film,while pretreatment can significantly improve the performance of the LCNF film,like that pretreatment make LCNF film density increased from 1.04g/cm3 to 1.10-1.15g/cm3.Sulfonation treatment increased the WCA of the film,but compared with F1-LCNF,the S-LCNF film increased the density by 9%and the tensile strength by 113%.The S-LCNF film also had the smallest OP and WVP among all LCNF and CNF film samples.The yield of laccase pretreatment under acidic conditions was highest,the lignin content was increased to 19wt%,the density of the film was increased and the layers were tightly bonded as shown in the SEM image of cross-section of the film,but the mechanical strength and barrier properties of the L-LCNF film were not significantly enhanced.(2)Further compared the effects of alkali pretreatment,sodium sulfite pretreatment and alkaline sulfite pretreatment with different amounts of sodium sulfite on the properties of CR fiber material and corresponding LCNF and LCNF film.The WCA of pretreatment LCNF film increased by 25-148%,mechanical strength of LCNF film increased by 83-135%.The water vapor barrier property of pretreated LCNF film was obviously enhanced,because the WVP of pretreated LCNF film was reduced by 40-50%.Alkali treatment and alkaline sulfite pretreatment made the lignin in fiber material lose 44-61%,the acid group content increased by 10-30mmol/kg,and the WRV increased.When CR pretreated with sodium sulfite only,the acid group content decreased,and lignin retention ratio was about 70%.Through the film structure was relatively loose,but its LCNF film had the smallest average OP and WVP,and a smaller WCA.(3)Carboxymethylated LCNF(C-LCNF)with different carboxyl content were prepared via carboxymethylation pretreatment and high pressure homogenization.And the film was produced by a modified bar coating-forming method.C-LCNF coated paper was prepared by bar coating.The CR after processing by provider,the average length of the CR fiber material was about 0.2mm,and the lignin content was 6.2wt%.Different degrees of carboxymethylation pretreatment had no significant effect on fiber size.Controlling the dosage and proportion of chloroacetic acid and sodium hydroxide,neutralizing with acetic acid and washing with ethanol,fiber materials with carboxyl content of 0.12-1.88mmol/g and lignin content of 4-6%were obtained.The lignin retention ratio of fiber material after carboxymethylated pretreatment was as high as 80-93%.Corresponding LCNF fibrils were smaller and more uniform after homogenization,which make the C-LCNF films'density increased to 1.22g/cm3.But the density of the film was fluctuated slightly with the increased degree of carboxymethylation.Carboxymethylation reduced the WCA of the film,and low and high degree carboxymethylation increased the tensile strength and oxygen barrier properties of the C-LCNF film.SEM observation of C-LCNF coated paper showed that the surface of coated paper was smooth and compact.In the layered cross-sectional structure,as the carboxyl group content increased,the interlayer spacing decreased.Coated paper with 4-5g/m2 C-LCNF improved the water vapor barrier property by 40-70%.