Effects of Pretreatment on Bamboo Structure and Fiber and Preparation of Nanocellulose at High Yield

Author:Peng Yang Yang

Supervisor:fu shi yu

Database:Doctor

Degree Year:2019

Download:55

Pages:139

Size:5887K

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Biomass,as a renewable,environmental friendly and abundant natural energy source,is favored by researchers.Comprehensive utilization of biomass with high value is the key way to solve the problems of petrochemical resources and energy.Biomass pretreatment is the first and important step of biomass high-value utilization.In this paper,the effects of dilute acetic acid and dilute alkali pretreatment on the physical structure and chemical composition of bamboo cell wall were studied.The changes of pretreatment on the microstructure of bamboo were revealed.Also,the pretreatment with dilute acetic acid applied for the production of bamboo unbleached pulp was studied.At the same time,a method of high yield and efficient preparation of nanocellulose was proposed.The recycling of recovered waste liquid in this process were also discussed.The effects of pretreatment on the physical structure,chemical composition and cellulase hydrolysis rate of bamboo were discussed.Hemicellulose degradation mainly occurred during acid and hot water pretreatment.In the process of alkali pretreatment,hemicellulose degradation and lignin dissolution mainly occur.According to the infrared spectrogram of the pretreated samples,the ester bond or ether bond was broken in the process of acid-base pretreatment.Ion beam cutter was used to treat the surface of the pretreated samples.After SEM scanning,the surface topography of the pretreated samples showed that both acid and alkali pretreatment resulted in holes in the fiber cell area,while the parenchymal cell wall became thinner.Under acidic conditions,parenchyma cells ruptured and collapsed,while under alkaline conditions,parenchyma cells collapsed.The effect of pretreatment with acetic acid on bamboo unbleached pulp produced by sulfate process was studied.Acetic acid pretreatment can obviously improve the yield of fine pulp.It also has a promoting effect on the subsequent oxygen delignification,which can greatly improve the whiteness after oxygen delignification process,and the highest whiteness reaches 59.6%ISO.Acetic acid pretreatment can also improve the fiber quality.Bamboo and three urban cellulosic wastes in South China(mango wood,redbud,and banyan)were used as raw materials to extract cellulose by combining the environmental H2O2and HAc solution system,and the wastes were converted to nanocellulose materials.Mango wood,redbud wood and banyan are similar to hardwood with fiber lengths between hardwood and softwood of 1.55mm,1.74mm and 1.35mm,respectively.Based on the analysis of fiber components,H2O2 and HAc solution systems can effectively remove lignin components from raw materials.The study on the preparation of nanocellulose by TEMPO oxidation of the four obtained fibers showed that the redbud,banyan and bamboo fibers were easily oxidized,while the oxidation rate of mango wood fibers was slow.The order of crystallinity of four nanocelluloses was:bamboo>mango>redbud>banyan.Compared with fibrils,the thermal stability of the four nanocellulose was decreased.Nanocellulose films have good mechanical properties.Based on traditional acid hydrolysis method,an efficient and high yield method for preparation of nanocellulose was proposed.The fiber was treated with sulfuric acid with different concentrations and centrifugated to obtain WCRs.The acid in the residual WCRs was removed by alkali neutralization instead of traditional dialysis.Then AACNFs was obtained by high-pressure homogenization.At the same time,traditional dialysis treatment is eliminated,making the whole process more efficient and greatly reducing water consumption.The surface properties of AACNFs shows that due to the introduction of sulfonic acid groups on the surface,Zeta potential is low,and it can disperse stably in aqueous solution with good optical properties.Its rheological data show that when AACNFs is greater than 2 wt%,it behaves as a viscoelastic solid.The highest crystallinity of AACNFs was 81.2%.Compared with the raw material,the thermal stability of the nanocellulose decreased,but increased compared with CNC obtained by traditional method.This is because alkali neutralization eliminates the adverse effect of sulfate groups introduced on the AACNFs surface.AACNFs was combined with modified montmorillonite to prepare barrier coating,and the results showed that the coating could improve the oxygen transmittance of paper and reduce the oxygen transmittance by 74.9%at most.As AACNFs is hydrophilic,it has a limited effect on the improvement of moisture isolation,with a maximum reduction of 38.9%.The waste liquor from acid treatment was recycled and used again to prepare nanocellulose.With the increase of recycle times,the yield of WCRs increased continuously,and the higher the acid concentration,the more obvious the change trend was.The sugar components in the recovery solution were continuously enriched to a maximum of about60g/L.The concentration of sugar components affects the strength of acid treatment in the recycle process.The size of AACNFs showed a trend of increasing under the influence of recycle times.Taking 57 wt%as an example,after recycling the recovered liquid for 4 times,the sizes were 184 nm(R0),299 nm(R1),363 nm(R2),524 nm(R3),and 966 nm(R4),respectively.Both the surface properties of AACNFs and the sulfate content were affected by the time of recycle.The absolute value of Zeta potencial decreased with the increase of recycle times,and the sulfate content also showed a downward trend.The crystallinity of AACNFs decreases with the increase of recycling times,which is the result of the decrease of acid treatment extent,while the thermal stability increases slightly,which is related to the size increase of AACNFs and the decrease of sulfate content.The effects of freezing,freeze drying,drying and spray drying on the redispersing performance of AACNFs were investigated.The frozen AACNFs suspension was centrifuged to collect AACNFs,and the solid content was up to 10%.After freezing and freeze-drying,the redispersibility of the bamboo AACNFs sample was better than that of the dissolved pulp sample.The samples after drying and spray drying have poor redispersibility.The thermal stability and crystallinity of redispersed AACNFs with freeze-dried samples were lower than that of the dried samples.The water content of AACNFs is the main factor affecting its redispersed performance.In different treatment methods,the change of water bonding with fiber is different,which further affects the strength of hydrogen bonding between fibers.