Preparation And Supercritical CO₂Foaming Of Modified NCC/PLA Composites

Polylactic acid is a hot spot in the biodegradable polymer materials. It has goodbiocompatibility and degradability, but it is hard and brittle, has low melt strength lead to foamdifficulty. Nanocrystalline cellulose(NCC) is natural renewable and has large length to diameterratio, high strength. With nanocrystalline cellulose modified polylactic acid, not only canimprove the performance of polylactic acid(PLA), enlarge its application scope, but also PLAand NCC derives from biomass resources.They can be biodegradable after abandon, inaccordance with the "sustainable development" and "green packaging".First, the surface of nanocrystalline cellulose (NCC) was modificated by acetic anhydride.FT-IR analysis by infrared spectrum compared before and after the modification of NCC. It wasfound that mNCC in1750cm-1and1235cm-1had new peaks, additional hydroxyl had subsided.Part hydroxyl groups on the surface of NCC reacted after modification, forming the carboxylgroups. At the same time, the solid13c nuclear magnetic carbon spectrum proved that the successof acetylated modification. By acid-base titration and1H nuclear magnetic resonance hydrogenspectrum measured the degree of substitution(DS) of mNCC, about1.6. By thermal properties(TG), X-ray diffraction (XRD) and surface water contact Angle test, found that the internalcrystal structure of the original basic of the mNCC was retained. But the crystallinity andthermal stability reduced, and hydrophobic enhanced, polarity was abate, improving thedispersion of NCC in nonpolar polymer matrix and the interfacial adhesion, thus improved thecomprehensive performance of composite materials.Based on the matrix--PLA, the reinforced phase--modified NCC, mNCC/PLA compositewas prepared by solution casting method. By light transmittance test and field emission scanningelectron microscopy (FESEM) observation found that mNCC had better dispersion in PLA thanNCC. With mNCC, composite materials were more transparent, but with the increase of nanofiller, reunion phenomenon appeared. Thermal properties (TG), dynamic thermal mechanicalproperties (DMA) and rheological properties analysis showed that mNCC not affect thecomposite materials’ thermal weightlessness situation and almost did not affect the glasstransition temperature (Tg) of the materials. Along with the increase of the content of mNCC,the composite energy storage modulus G’, loss modulus G" and the plural viscosityη*increased. The storage modulus increased more obviously, closing to the loss modulus in the low frequencyarea. The mNCC limited the movement of PLA macromolecular chain. The melt viscoelastic ofcomposite materials improved significantly.At last, mNCC/PLA microcellular foaming material was prepared by supercritical CO2, thefoaming process parameters were optimized, and the effects of modification and conent of NCCon the foaming system were analyzed by scanning electron microscopy (SEM). The studyindicated that the best bubble structure was obtained at the condition of110℃、18MPa and2h.Because of surface chemical modification, NCC could disperse in the matrix mory evenly, thus,poly (lactic acid)(PLA) nucleation point increased, bubble hole density increased and the bubblepore structure became better. The mNCC could improve the melt strength of materials andfoaming performance, and play a role of heterogeneous nucleation in the foaming system. Withina certain range, bubble hole density increased with the increasing of mNCC. When adding6%NCC, ideal microcellular foaming material with a bubble diameter of6.97μm and a bubbledensity of6.05×109cell/cm3was obtained.