Preparation And Properties Of Polylactic Acid Nanocomposite Reinforced With Cellulose Nanospheres

Nowadays due to its biodegradable and environment friendly properties, green food packaging is gradually replacing the chemically synthesized polymer based packaging materials, causing great interests in researchers. Poly(lactic acid)(PLA) exhibits several advantages such as good transparency, degradation in biological environment, biocompatibility, and processability in relation to the petroleum-based polymers usually used for packaging. However, the intrinsic characteristics of PLA including low crystallization rate, brittleness, poor thermal stability and moderate gas barrier properties are not satisfied for packaging applications. Therefore, aiming at above PLA defects, nanocelluloses were used as nucleation agents and reinforcing fillers to physically modify the PLA to improve its tenacity, barrier, thermal and crystallization rate. The influences of surface structure designs and morphology dimensions of nanocellulose on PLA properties were investigated and the effect law of cellulose nanospheres with various contents, nanocellulose with different morphologies and dimensions and the nanosphere surface grafting with flexible long chain on mechanical, thermal, barrier and overall migration properties of PLA(I) Preparation and characterization of the spherical nanocellulose formates.functionalized cellulose nanospheres(Spherical nanocellulose formates, SCNFs) are extracted by one-step HCOOH/HCl hydrolysis of lyocell fibers with cellulose II crystal structure. The influences of fiber–to–acid ratio on the yield, microstructure and properties of the SCNFs are studied. The SCNFs show spherical nanoparticles with a narrow size distribution of 19-29 nm. X-ray photoelectron spectroscopy verifys that carboxyl groups of HCOOH are firstly grafted on the primary hydroxyl groups of SCNFs and then secondary hydroxyl groups. Moreover, it is found that SCNFs prepared at 80 oC, 8 h, and a fiber–to–acid ratio of 0.4 g/50 mL possess small diameter(27nm), high yield(85.4 %), and high thermal stability(the maximum degradation temperature(Tmax)up to 358 oC).(II) Preparation and characterization of the polylactic acid/spherical nanocellulose formates nanocomposite. Biodegradable polylactic acid(PLA) polymer is evaluated for its application as a material for food packaging and the industrial problem of slow crystallization rate of PLA is worked out by using the SCNFs as biobased nucleation agents. PLA nanocomposite films are prepared by incorporating spherical nanocellulose formates(SCNFs) into PLA. The resultant SCNFs with hydrophobic ester groups are uniformly dispersed in PLA without the aid of acompatibilizer or surfactant. Therefore, the crystallization rate of the PLA matrix were increased due to the heterogeneous nucleating effect of SCNFs. With an increase of SCNFs contents,significant improvements in mechanical performance, thermal stability, barrier and migration properties are achieved, which are ascribed to the improved interfacial interaction and increased crystallinity. Compared to neat PLA, the tensile strength, young’s modulus and maximum decomposition temperature(Tmax) of the nanocomposites all achieve their maximum values at loading levels of 10 wt.% SCNFs. Meanwhile the overall migration levels in both non-polar and polar simulants are well below the limits required by the current legislative standards for food packaging materials.(III) Property reinforcement of PLA with cellulose nanospheres and nanocellulose with high aspect ratio: A comparative study on morphology and dimension effect. Nanocelluloses including cellulose nanospheres(CNSs), cellulose nanocrystals(CNCs) and cellulose nanofibers(CNFs) were extracted from cellulosic raw materials. Therefore, their morphologies, crystalline and chemical structures, dispersion states in polylactic acid(PLA) matrix, interactions with matrix, and the resulting effects on the crystallization behavior, mechanical, thermal, barrier and overall migration properties of matrix polymer are roundly campared in this work. Through solution casting PLA/CNS, PLA/CNC and PLA/CNF nanocomposites with 10 wt % nanofillers were prepared,respectively. Due to the larger aspect ratio of CNFs and fiber entanglement, the highest tensile strength and Young’s modulus are obtained for CNFs among these three nanofillers, while due to their relatively large fiber agglomerates, lowest elongation at break for CNFs at the same nanocellulose content. The Halpin-Kardos and Ouali models are used to simulate the modulus of the composites and good agreements are found between the predicted and experimental values. This type of systematic comparative study can help to develop the criteria for selecting proper nanocellulose as a biobased nano-reinforcement material in polymer nanocomposites.(IV) Graft modification of flexible long chain on cellulose nanosphere surface and its reinforcing on PLA properties. For the case that the tensile strength and young’s modulus of the nanocomposites have been significantly improved, while toughness has not been improved in the above work, the surface of cellulose nanospheres(CNSs) are grafted with poly(ethylene glycol)(PEG) long chain with plasticization(CNS-g-PEG) and incorporated into poly(lactic acid)(PLA) as a reinforcing and toughening filler to increase the compatibility of CNS and PLA. The fracturedsurfaces of the PLA and PLA/CNS are brittle while PLA/ CNS-g-PEG are plastic, due to the plasticization of the PEG. DSC results showed that both the glass transition temperature and cold crystallization temperature decreased progressively with longer molecular chains of PEG grafted,suggesting that improved dispersion state and enhanced plasticization of CNSs in PLA are achieved by grafting PEG onto the CNSs. The tensile strength of the nanocomposite films are significantly improved with the addition of CNSs or CNS-g-PEG, indicating that the fillers tend to stay uniformly inside of the polymer matrix during the solution casting and have a strong interfacial interaction with the PLA. Thermal stability of the composite film is also effectively improved by the addition of CNSs or CNS-g-PEG. In addition, the migration and water absorption results show that PLA/CNS-g-PEG composite films exhibite improved overall migration and barrier properties,which reveals the potential application of this nanocomposite as the films in food packaging.