Preparation And Properties Of Polylactic Acid-based Barrier Materials

Polylactic acid(PLA)is a type of bio-based degradable material with the advantages of high strength,good biocompatibility,transparency,and processing performance,which make it have broad application prospects in the fields of plastic products,fast food lunch boxes,non-woven fabrics,disposable infusion equipment,disassembly-free surgical sutures and so on.However,the disadvantages of poor gas barrier properties and insufficient toughness limit its widespread use.Therefore,the main goal of this project is to improve the barrier properties of PLA while obtaining good mechanical properties.A series of barrier PLA-based blends were prepared by means of designing melt blending,adding inorganic nucleating agent and biaxial stretching.The main research contents are as follows:(1)Poly-3-hydroxybutyrate-co-poly-3-hydroxyhexanoate(PHBH)has excellent barrier properties,biocompatibility,and biodegradability.PLA/PHBH blends were prepared by melt blending,the effects of PHBH content and crystallinity on the barrier,mechanical and light transmittance properties of PLA were studied.It was found that when the content of PHBH was 30 wt%,the water vapor permeability of PLA/PHBH blends decreased by about 10%,the elongation at break increased from 8% to 150%,at the same time,it had good transparency(visible light transmittance was about 80%).On this basis,the slow crystallization behavior of the dispersed phase PHBH during storage and its influence on the barrier,mechanical and light transmittance properties of the PLA matrix were studied.The crystallinity of PHBH in PLAH-10 did not change significantly during storage and it was amorphous;with the increase of PHBH content,the crystallinity of PHBH gradually increased with time;the crystallization of PHBH in PLAH-30 was relatively complete in a short time,and the crystallinity showed a trend of rapid increase at first and then gradual increase,when the content of PHBH was 40%,the initial crystallinity of PHBH was relatively high,and the change with time was not obvious.When the PHBH content was 30 wt%,after 20 days of storage,the water vapor permeability of PLA/PHBH blends decreased by 27%,and also decreased by about 30%compared with pure PLA,the toughness of PLA/PHBH blends deteriorated during storage,but the strength remained unchanged.(2)On the basis of PHBH,the nucleating agent boron nitride(BN)was added to further improve the barrier properties of PLA.It was found that the water vapor permeability of PLA/PHBH/BN composites decreased by 27%;and the tensile strength of PLA/PHBH/BN composites increased to 43.3 MPa,an increase of about 55% after adding 2 wt% BN.Further annealing of PLA/PHBH/BN composites was carried out to study the effects of two annealing processes on the crystalline properties,barrier properties,microscopic morphology and mechanical properties of PLA/PHBH/BN composites.After annealing,PLA was completely crystallized,and the crystallinity of PHBH was further improved,PHBH layered structure which was conducive to prolonging the permeation path of small molecules.was also formed in PLA/PHBH/BN composites.Compared with pure PLA,the oxygen permeability coefficient and water vapor permeability of PLA/PHBH/BN composites decreased by about85% and 75%,respectively.The barrier properties of the PLA/PHBH/BN composites prepared in this chapter were comparable to those of commercial PET.(3)Using commercial high-barrier PVOH(ethylene-vinyl alcohol copolymer)as modifier and EGMA as reactive compatibilizer,high-barrier PLA/PVOH/EGMA biaxial stretching films were prepared by melt blending-biaxial stretching process,the effects of PVOH content and biaxial stretching process(stretching ratio,stretching rate and stretching temperature)on the crystalline properties,mechanical properties,water vapor permeability properties and microscopic morphology of PLA were studied.When the stretching rate was 4mm/s,the stretching temperature was 90 ℃,and the stretching ratio λ=3,the water vapor permeability of PLA/PVOH/EGMA decreased by about 20%,it also had good mechanical properties in MD and TD directions,the tensile strength in both directions were 57 MPa,which was about 12 MPa higher than that when it was not stretched,the crystallinity of PLA after stretching was 39%.The water vapor permeability of the PLA/PVOH/EGMA composite films gradually decreased with the increase of the stretching rate,when the stretching rate was8 mm/s,the water vapor permeability decreased by 40%.