Preparation,Properties And Mechanism Of Cellulose Nanocrystals/Waterborne Polyurethane Films

With the depletion of fossil resources and the aggravation of white pollution,the use of green and renewable materials to prepare environmentally friendly composite materials has received great attention.Cellulose is the world's largest natural renewable polymer material.In the past two decades,cellulose nanocrystals(CNCs)have attracted wide attention because of their high strength,excellent optical properties,and low cost.It is still the current research direction to improve the preparation method of CNCs and enhance the product quality and yield of CNCs.Waterborne polyurethane(WPU),a polymer does not contain organic solvent and answer for environmental protection requirement,is studied more and more widely.In particularly,the synthesis of WPU from green raw materials such as vegetable oil or lactic acid has become an important research direction.CNCs can be added to WPU to prepare high strength nanocomposite films,which have potential applications in biomedicine,furniture coatings,food packaging and other fields.By far,there are few studies on the preparation of nanocomposite films by adding different ionized CNCs into WPU.Based on the above considerations,this paper mainly carries out the following studies:1.The preparation conditions of CNCs were optimized based on the sulfuric acid hydrolysis method reported in the literature.The premixing stage at room temperature was adopted for the first time.The results showed that with the increase of premixing time,both the size and the yield of CNCs decreases.The cellulose type I polymorph of the prepared CNCs remained unchanged.FT-IR and TG analysis showed that cellulose was the main component of CNCs.Premixing process at 25 oC plays an important role in the production of CNCs,indicating that the temperature dependence of solubilization and depolymerization process should be more complicated.At last,the effects of different temperature combinations on sulfuric acid hydrolysis process were further revealed.2.Different from the composites with low CNCs content usually adapted in literature,WPU/CNCS nanocomposite films were prepared here in a range of 0-100% full composition by using the traditional solution-casting process.The composite film was characterized by FE-SEM,TG,and other physical and chemical methods,and its mechanical properties,light transmittance,Water vapor permeability(WVP),Oxygen permeability(OP)and other properties were characterized.The FE-SEM results showed that the CNCs were arranged in a manner parallel to the surface of the films.TG analysis showed that the nanocomposite film containing 10% CNCs had high thermal stability.The Young's modulus and tensile strength of WPU/CNCs are increased exponentially.The contact angle between the nanocomposite films and water varied between 84° and 52°,which was consistent with the FE-SEM observation.When the content of CNCs is 50 wt.%,the WVP value is the lowest,which is the balance result of the high crystallinity of CNCs and the swelling property by water.With the increase of CNCs content,from 0 to 90 wt.%,the transport path of oxygen molecules through the nanocomposite film becomes more and more tortuous,leading to a substantial decrease in OP.Because these WPU/CNCs nanocomposites have the advantages of biocompatibility and biodegradability,they may be applied in biomedical,furniture coatings,food packaging,and other fields.3.Three different ionized CNCs(SCNCs,TOCNCs,and Ca CNCs)were first added to WPU matrix to prepare their nanocomposite films.The composite films were characterized by physical and chemical test such as FE-SEM and TG,and its mechanical properties,light transmittance,WVP,and OP were characterized.The mechanical properties showed that the composite film with Ca CNCs has the optimal strength and toughness.This is consistent with the results of FE-SEM observation,which indicated that the Ca CNCs in WCC10 film are evenly distributed in the matrix and have the best interfacial compatibility with the matrix.The composite film added with Ca CNCs has the highest optical transmittance due to the best compatibility of the two components and the ensuing compact structure of the film.After the addition of three kinds of ionized CNCs,the OP of the films decreased significantly.Among these films,the OP of the film with Ca CNCs was the lowest,because this composite film was denser than others.Compared with SCNCs and TOCNCs,WPU/CNCs composite film with Ca CNCs has the best comprehensive performance.It could be concluded that the CNCs with opposite charge improved significantly the properties of waterborne polyurethane nanocomposite films.4.AWPLAU and CWPLAU,the anionized and cationized WPU,were prepared by using PLA diol as green raw material for the first time.FT-IR analysis showed that AWPLAU and CWPLAU were synthesized successfully.Furthermore,the anionic and cationic CNCs(TOCNCs and Ca CNCs)were blended with the synthesized WPU,and the obtained composite films were characterized by physical and chemical test such as FE-SEM and TG,and the mechanical properties,light transmittance,WVP,and OP of the composite films were also characterized.TG analysis showed that Ca CNCs increased significantly the thermal stability of CWPLAU.XRD showed that CNCs can promote the crystallization of WPLAU.The mechanical test showed that TOCNCs and Ca CNCs improved significantly the rigidity,strength,and toughness of CWPLAU matrix.Compared with Ca CNCs,the mechanical performance of CWPLAU composite film added with TOCNCs was better,which proved that the conclusion of the previous chapter is universal.The OP of the self-made WPU in this chapter is two orders of magnitude smaller than the commercially available WPU,so it has potential applications in food packaging and other fields.In addition,the prepared nanocomposite films in this chapter will promote the development of environmentally friendly materials because of the green nature of the raw materials.5.The CNT,WPU,and three kinds of CNCs were filtered into layered composite films with different compositions and structures.The chemical components and microstructures of the composite films were characterized.Their performance and applications potentials were explored.FE-SEM showed that CNCs can self-assembled into long-range parallel structure in the top layer.The surface hydrophilicity of the composite film was measured.It was found that the hydrophobicity of the composite film increased with the increase of the thickness of WPU layer.