Studied On The Preparation, Structure&Propertiesof Blending Modified Poly (Lactic Acid) Composites

Poly(lactic acid)(PLA), synthesized from plant resource, has the characteristics ofrenewable and biodegradable and is a typical ecological environmental protection material. Asa potential alternative to petroleum-base synthetic fiber, PLA fiber can be widely used inmany fileds such as apparel, home textile and industrial textile. However, the performancedefects of PLA fiber, such as high strength, low elongation and brittleness, restrict itsapplication. Therefore, the improvement on defects of PLA has high directions on the theoryand application value to enhance its performance in practical application and tap its potentialapplication scope.In this study, PLA was modified by melt blending with nano inorganic materials SiO2,MMT and flexible polymer PCL to prepare a series of PLA blending composites. Theisotherm and non-isothermal crystallization behaviors, linear and non-linear viscoelasticbehaviors of the composites were emphatically studied, and the influence of nano SiO2, MMTand PCL on the crystallization properties and rheological properties of PLA were discussed.PLA composite melt-spun fibers were prepared with a twin screw extruder and a homemadedrafting device on this basis. The tensile properties of breaking strength and elongation of themelt-spun fibers were researched, and the degradation properties and hygroscopic propertiesof PLA composite melt-spun fibers were also evaluated.Nano inorganic materials were firstly modified before melt blending with PLA. NanoSiO2was surface modified by two methods respectively. One named L-SiO2was grafted withPLA by ring-opening polymerization of L-lactide on the surface of particles, and the othernamed M-SiO2was surface modified by silane coupling agent KH570. Nano MMT wasmodified with cetyl trimethyl ammonium bromide to prepare organic MMT (OMMT). FTIR,TGA, nanoparticle size analysis and XRD were used to evaluate the modification results.A series of PLA blending composites were prepared by melt blending PLA with L-SiO2,M-SiO2, OMMT and PCL respectively. The cross-section morphology of composites wascharacterized by SEM. It was found that nano inorganic materials and PCL were evenlydispersed in PLA matrix. The particle size of M-SiO2in the matrix was slightly smaller thanthat of L-SiO2, and the layers of OMMT in PLA matrix were partially or completelyexfoliated. Due to the incompatibility PCL spherical particles dispersed in PLA matrix to formthe―sea-island‖structure. When nano inorganic materials and PCL were melt blendingtogether with PLA, the nano materials were selectively dispersed in PLA matrix to form dualseparated phase structer together with PCL.The isothermal and nonisothermal crystallization processes were tested respectively byDSC to study the crystallization behaviors of PLA blending composites. The investigation onisothermal crystallization kinetics by Avrami equation showed that the Avrami exponent nwere similar for all composites. It suggested that the way of nucleation and crystal growthwas same. Combining with the results of isothermal and nonisothermal crystallization, it canbe concluded that the addition of L-SiO2, M-SiO2, OMMT and PCL could promote thecrystallization process and increase the crystallization rate of PLA to varying degrees. Among them, the modified nano SiO2advanced the crystallization behavior of PLA very obviously,especially M-SiO2. The crystallization rate of PLA was speeded up, the crystal structure wasperfected and the relative crystallinity was improved accordingly.The rheological properties of PLA blending composites, including shear viscosity, linearand nonlinear viscoelastic behavior were investigated by a rotational rheometer. The tests ofshear rate sweep, dynamic strain sweep and dynamic frequency sweep were performed at200oC, respectively. The results of shear rate sweep showed that all the composites based on PLAwere shear-thinning fluid in the molten state, and the shear viscosity of composites decreasedwith the increase of shear rate. The shear viscosity of PLA showed an increasing trend afteradding L-SiO2, M-SiO2, OMMT and PCL. Dynamic strain sweep was to determine the linearviscoelastic region (LVE) and critical strain eigenvalues of PLA blending composites. Theresults of dynamic frequency sweep carried out in LVE showed that the grafted PLA on thesurface of L-SiO2made the particle-particle interactions strong enough to form a networkstructure in PLA matrix; there had a steric hindrance between M-SiO2because of the silanecoupling agent coated on its surface, and the weak interactions could not lead to form anetwork structure in PLA matrix. The strong interactions between OMMT layers also madethe particle network structure form in PLA matrix. The incompatibility of PCL and PLA led tothe presence of phase interface layer, and also to the appearance of shape relaxation of PCLdispersed phase particles in PLA matrix.PLA composite fibers were obtained by melt-spinning. The fiber morphology, lineardensity, degree of orientation and crystallinity, and tensile property were all studied. Theinfluence of L-SiO2, M-SiO2, OMMT and PCL on the structure and properties of PLAmelt-spun fibers was analyzed. PLA composite melt-spun fibers had smooth surface andstraight length, observed by video microscope. The results of fiber tensile test showed thatL-SiO2could effectively improve the relative crystallinity of PLA fibers compared withM-SiO2, and the breaking strength of PLA melt-spun fibers could be improved. The lamellarstructure of OMMT was helpful to make the relative movement among the molecule chains ofPLA more easily and increase the plastic deformation of the fiber, and so the elongation atbreak of PLA melt-spun fibers could be improved. The incompatibility of PCL and PLA hadeffect on the tensile property of PLA/PCL melt-spun fibers and led to the decrease of the fiberbreaking strength. The breaking strength and elongation at break of PLA multiphasemelt-spun fibers were obviously changed under the combined effect of nanoparticles and PCL,compared with pure PLA melt-spun fibers.The moisture regain of PLA composite melt-spun fibers was tested to investigate itshydroscopic property. It was found that the moisture regain of all PLA composite melt-spunfibers were less than pure PLA melt-spun fiber, but they still kept in the range of0.60%to0.70%. The hydroscopic property of fibers was hardly affected by L-SiO2, M-SiO2, OMMTand PCL.Soil suspension liquid was applied to simulate the environment on the biodegradationexperiments of PLA composite melt-spun fibers. The effects of nano inorganic materialsL-SiO2, M-SiO2and OMMT as well as PCL on the biodegradability of PLA fibers wereanalyzed and researched based on the changes of fiber quality and strength before and after biodegradation experiments. The results showed that quality and strength of fiber samplesdecreased gradually as the increase of degradation time, and the fall of fiber strength was bigand the weight loss was small. The addition of L-SiO2, M-SiO2, OMMT and PCL was helpfulto make the fiber not easy to degrade and the service life longer.