| Polylactic acid(PLA)is a kind of biodegradable aliphatic polyester,which has the advantages of high strength,good biocompatibility and good machinability.It is considered as one of the promising biodegradable materials.However,the poor toughness of PLA limits its large-scale commercial applications.To solve this problem,blending rubber or flexible polymers with PLA can significantly improve the toughness of PLA.However,this will lead to a significant decrease in the strength and modulus of PLA,and thus it is difficult to combine the strength,rigidity and toughness of PLA with this method.In order to obtain PLA blends with good rigidity and toughness balance,in this thesis,four different types of starch-based core-shell particle tougheners were synthesized by soap-free emulsion polymerization using natural macromolecular starch as hard core:starch-based core shell particles(GMA-CSS)with epoxy functional groups modified by glycidyl methacrylate(GMA),three layers of starch-based core shell particles(T-CSS)with hard core,soft shell and hard shell,fully degradable starch-based core shell particles(D-CSS)and phosphorus-containing starch-based core-shell particles(FR-CSS).Then PLA blends with excellent comprehensive properties was prepared by melt-blending starch-based core-shell particles with PLA.The effects of starch-based core-shell particle contents on the microstructure,physical and mechanical properties and flame retardancy of PLA blends were studied,and the toughening mechanism of starch-based core-shell particles on PLA was further discussed.The main research contents of this paper include:1.In order to further improve the toughness of PLA,in this chapter,GMA was used to modify starch-based core-shell particles and a starch-based core-shell particle with epoxy functional group on the surface was prepared for toughening PLA.Firstly,the core-shell starch-based particles(CSS)were fabricated via soap-free emulsion copolymerization of ethyl acrylate to form poly(ethyl acrylate)(PEA)layer on the surface of starch.Then,GMA was grafted onto the surface of CSS to synthesize a starch-based core-shell particle with epoxy functional group(GMA-CSS).Finally,PLA blends were prepared by melt blending.The results showed that the toughness of PLA was improved by the addition of GMA-CSS.When the addition amount of GMA-CSS was 10 wt%,the tensile elongation at break of the blends was 449%and the tensile strength reached 40.4 MPa.It can be seen that GMA-CSS can greatly improve the toughness of PLA while maintaining high tensile strength of PLA.In addition,the notched impact strength of PLA increased significantly with the increase of GMA-CSS content.When the content of GMA-CSS was 20 wt%,the notched impact strength of the blend reached 68.8 k J/m~2,which was 22 times higher than that of PLA.Further studies showed that the toughening effect of GMA-CSS was obviously better than that of unmodified starch-based core-shell particles,and the matrix was easier to achieve brittle and toughening transformation.This was due to the interfacial compatibilization reaction between GMA-CSS with epoxy functional groups on the surface and PLA chain in the melting blending process,which improved the compatibility between GMA-CSS and PLA matrix and made GMA-CSS disperse more evenly in the matrix.Therefore,the starch-based core-shell particles modified by GMA can toughen PLA more efficiently.2.In order to improve the tensile strength of PLA blends,starch-based three-layer core-shell particles(T-CSS)were synthesized in this chapter.Its core was consisted of starch,the middle soft layer was poly(ethyl acrylate)(PEA),the hard outer layer was poly(methyl methacrylate)(PMMA).The results show that the introduction of PMMA hard shell has better toughening and strengthening effect on PLA blends.When the content of T-CSS was 15 wt%,The tensile elongation at break and tensile strength of PLA/15CSS modified by two-layer core-shell particles were 296%and 36.2 MPa respectively,while that of PLA/15T-CSS modified by three-layer core-shell particles were 361%and 45.0 MPa respectively.It can be seen that at the same content,the toughening and strengthening effect of three-layer core-shell particles were better than two-layer core-shell particles.This is because the introduction of hard shell can improve the modulus of core-shell particles,on the other hand,the shell with high glass transition temperature can reduce the agglomeration of core-shell particles in the blending process and improve the dispersion.Therefore,it is easier to achieve the balance of rigidity and toughness.3.Although starch can be completely degraded,the polyacrylate shell of core-shell particles is degradable difficultly.In order to achieve full biodegradability of starch-based core-shell particles,in this chapter,the ring-opening polymerization of methyl acrylate(MA)with2-methylene-1,3-dioxepane(MDO)was adopted to solve the problem of shell degradation,and the fully degradable starch-based core-shell particles(D-CSS)were prepared.The results showed that the mass loss of D-CSS reached 100%after hydrolysis for 24 h,indicating that the degradation of the toughening agent D-CSS shell was achieved by introducing a degradable ester group into the main chain of the polyacrylate.The results of mechanical properties showed that with the addition of D-CSS,the tensile elongation at break and tensile strength of the blends increased significantly.When the content of D-CSS was 15 wt%,the tensile elongation at break of the blends reached 310%,while the tensile strength reached 40.1 MPa,maintaining a high level.Therefore,by introducing fully degraded starch-based core-shell particles(D-CSS),a fully degradable PLA blend with excellent rigidity and toughness balance was prepared.4.In practice,the flame retardancy of materials is becoming more and more important.In order to realize the flame retardancy and toughness of PLA at the same time,the starch-based core-shell particles were further designed.A phosphorus-containing starch-based core-shell particle(FR-CSS)was prepared by free radical copolymerization of 2-hydroxyethyl methacrylate phosphate(PAM)and ethyl acrylate(EA)on the surface of starch.The flame retardancy of PLA was improved by melting blending.The flame retardant results showed that the limit oxygen index(LOI)of PLA increased from 19.2 to 24.5 with the addition of only 10wt%FR-CSS,indicating that PLA has changed from flammable material(LOI<22)converted to combustible materials(22<LOI<27);In addition,vertical combustion test results showed that the PLA/10FR-CSS blend achieved V-2 rating and was capable of self-extinguishing within 10seconds after the igniter was removed.The mechanical properties showed that the tensile elongation at break of the blend reached 308%and its tensile strength reached 39.4 MPa with only 10 wt%FR-CSS content.Therefore,by blending FR-CSS with PLA,a blend with both flame retardancy and high toughness was prepared. |
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