Research On The Flame Retarded And Toughened Poly(L-lactic Acid) Composites Prepared Via Reactive Blending

With the rapid development of the biodegradable plastics industry,biodegradable materials have attracted extensive attention.There is no doubt that poly(L-lactic acid)(PLLA)is one of the most promising bio-based polymers due to its good biodegradability,biocompatibility and renewability as well as high modulus and high strength.However,the inherent flammability and brittleness have restricted its application in many fields.Therefore,it is significant to develop PLLA composites with high toughness and excellent flame retardancy to expand its application.In this paper,the reactive blending was utilized to prepare PLLA composites.Based on the active hydroxyl groups on the polyester diol and PLLA main chain,the elastomers were generated through in-situ reaction,which improved the toughness of PLLA composites.At the same time,the reactive flame retardant was introduced into the PLLA composites and increased the compatability through interface reaction combining with the formula design of the composites.By this way,the polymer molecular chain was able to bond on the surface of the flame retardant,which improved the interfacial adhesion between flame retardant and PLLA matrix effectively.Thus,PLLA composites with both flame retardancy and toughness were prepared.The specific results and conclusions were obtained as follow:(1)The blends of PLLA and polycaprolactone(PCL)were selected as the research object,and the ammonium polyphosphate(APP)was used as the halogen-free flame retardant in the system.Besides,the in-situ reaction was initiated after the addition of 4,4'-diphenylmethane diisocyanate(MDI)during the melt blending.The results indicate that the PLLA/polyurethane(PU)/APP composite with 5 wt%APP,which was able to pass the V-2 grade of the vertical burning test,owned a 18-fold increase in elongation at break(up to 73%)and a 9-fold increase in impact strength(up to 13.5 k J/m2)compared to neat PLLA.The PU elastomer formed in the PLLA matrix by in-situ reaction effectively improves the toughness of the PLLA/PU/APP composites.At the same time,the interfacial reaction compatibilization of APP and PLLA matrix was realized via reactive blending,which increased the interfacial bonding strength between APP and matrix interface and reduced the negative influence of the addition of flame retardant on the mechanical properties of the material.In addition,the addition of APP was able to promote the formation of carbon layer during the burning process,and improve the flame retardancy of the material by condensed phase flame retardant.(2)Based on the previous chapter,glycerol was selected as a cross-linking agent in the system.A series of toughened and flame retardancy PLLA composites with cross-linked polyurethane elastomer(CPU)were prepared by adjusting the content of glycerol to control the cross-linking degree of CPU.The influence the cross-linking degree of CPU on mechanical properties and flame retardancy of the material was studied in detail.The results show that CPU with cross-linked structure was formed through the reaction between the polyhydroxy glycerol and the terminal hydroxyl group of PCL,which is beneficial to improve the toughness of PLLA/CPU/APP composites.The PLLA/CPU/APP composite with 10 wt% APP and the degree of crosslinking of 10 %,had an elongation at break of 207.7 % and a tensile toughness of 56.8 MJ/m3.Moreover,due to the existence of the CPU cross-linking structure,the melt strength of the PLLA/CPU/APP composite material was improved,and the droplet drop phenomenon was alleviated.(3)To further improve the flame retardancy of the PLLA/CPU/APP composites,the reactive biomass chitosan(CS)was chosen as the carbon source compounded with APP to construct the intumescent flame retardant(IFR).In addition,a series of PLLA/CPU/APP/CS composites with various APP/CS ratios were prepared via reactive blending.The results indicated that the toughness of PLLA/CPU/APP/CS composites reduced with the increase contents of the large size CS.The synergization between CS and with APP during combustion that forms a dense carbon layer by esterification,endowed the matrial with excellent flame retardancy.When the ratio of APP/CS was 1:1,the vertical burning test of the PLLA/CPU/APP/CS composite passed the V-0 grade and the oxygen index reached 28.8.The use of natural polymer CS not only synergizes with APP to flame-retard the PLLA composite,but also ensures the degradability of the material.