Study On Supercritical CO₂ Foaming And Properties Of Polycaprolactone/Cellulose Nano-crystal Composites

Polycaprolactone(PCL)is a linear aliphatic polymer with excellent biodegradability and good biocompatibility obtained by ring-opening polymerization of monomer e-caprolactone.It is an excellent substitute for non-degradable plastics and has been widely used in agriculture and medical tissue engineering.However,the large-scale application of polycaprolactone is not ideal,with low modulus,low glass transition temperature and mostly closed-cell structure of foaming materials.The advantages of cellulose nano-crystal(CNC)lie in the regeneration of masterbatch,low cost,high strength,high modulus,good biocompatibility and recyclability of green environment.Using nano-fibre crystals as fillers to modify polycaprolactone not only improves the properties of polycaprolactone,enlarges the application fields of materials,but also maintains the excellent biodegradability and compatibility of polycaprolactone.At the same time,it realizes the complete biodegradation of the product,which is in line with the environmental strategy of green environmental protection and sustainable development implemented in China.In this paper,polycaprolactone/cellulose nano-crystal composite was prepared by melt blending with polycaprolactone as matrix and cellulose nano-crystal as filler particles,and polycaprolactone/cellulose nano-crystal composite foam was prepared by supercritical carbon dioxide fluid foaming technology.The first part of this experiment was to prepare cellulose nano-crystal by acid hydrolysis of microcrystalline cellulose(MCC).The optimal experimental process for this experiment was determined by the regulation of acid hydrolysis conditions:hydrolysis temperature 45 ~oC,sulfuric acid concentration 64%,hydrolysis bath ratio 1:8.75(g/ml),hydrolysis time 120 min.Transmission electron microscopy(TEM)showed that the cellulose nano-crystal were long needle-like crystals with a longitudinal size of about 200 nm and a transverse size of 15-20 nm.The suspension of cellulose nano-crystal did not show obvious precipitation after long time storage,and the dispersion showed blue light.FTIR results showed that compared with microcrystalline cellulose,the size and morphology of cellulose nano-crystal had changed,but the chemical functional groups had not changed.Thermal weight loss(TG)results showed that the thermal stability of cellulose nano-crystal is lower than that of microcrystalline cellulose.The second part of the experiment was to prepare polycaprolactone/cellulose nano-crystal composites by melt blending extrusion.Scanning electron microscopy(SEM)showed that cellulose nano-crystal could be uniformly dispersed in polycaprolactone matrix at low cellulose nano-crystal content,and aggregated easily at high content.FTIR results showed that the carbonyl group of polycaprolactone and hydroxyl group of nanofibers crystals formed hydrogen bonds.Thermal weight loss(TG)results showed that the addition of cellulose nano-crystal did not change the decomposition process of the polymer,and met the heat resistance requirements of the composite during processing.The results of plate rheology(MARS)showed that the storage modulus,loss modulus and apparent viscosity of the material were improved by adding cellulose nano-crystal,which limited the movement of the molecular chain of polycaprolactone and increased the viscoelasticity of the melt.Universal Tensile Test(UTM)results showed that the tensile modulus and tensile strength of cellulose nano-crystal decrease first and then increase with the increase of the crystal content of nanofibers.The third part of the experiment was the preparation of polycaprolactone/cellulose nano-crystal composite foaming material by supercritical carbon dioxide fluid foaming technology.Saturation temperature 50~oC and saturation pressure 25MPa were the optimum foaming conditions.At the same time,the addition of cellulose nano-crystal improved the cell morphology of polycaprolactone,changing from closed-cell structure to open-cell structure.At the same time,cellulose nano-crystal played a role of heterogeneous nucleation in the foaming process.With the increase of cellulose nano-crystal content,the cell diameter decreased gradually,the cell density increased gradually,and the material improved.The foaming ratio of the material improved the tensile properties of the foamed material.