Research On Flame Retardancy And Mechanical Properties Of Hemp Stalk Powder/polylactic Acid Wood-plastic Composites

A fully degradable and environment friendly wood plastic composite(WPC),hemp stalk powder/polylactic acid wood plastic composite(HPWPC)has a broad application prospect.HPWPC is easy to burn under high temperature radiation and has the droplet phenomenon,so it needs to be treated by flame retardant.In this paper,HPWPC was prepared by molding process with hemp stalk powder as reinforcement and polylactic acid powder as matrix.The flame resistant modification of HPWPC was modified by single flame retardant and compound flame retardant.After modification the problem of deceasing mechanical properties of HPWPC was solved.The main research contents are as follows:(1)For dealing with the combustion problem of HPWPC,ammonium polyphosphate(APP),melamine(MEL)and aluminum hydroxide(ATH)were used as flame retardant to modify HPWPC.The results showed that when the content of APP,MEL and ATH were 20 wt%,30 wt%and 40 wt%,respectively,the flame retarding effect of HPWPC was improved.The HPWPC with APP showed the best flame retarding effect,which was no drop in the vertical combustion test,the 4S continuous combustion time,the 8mm damage length,the 28.2%limiting oxygen index(LOI)and reaching the level of flame-retardant.Morever,the thermal stability of the composites was improved.The mechanism of thermal degradation of flame retardant was studied by Coats-Redfern thermokinetic model.The apparent activation energy of the three flame-retardant modified composites was 370.74KJ·mol^-1,213.03 KJ·mol^-1 and 404.25KJ·mol^-1,respectively,which is 100.77%,118.92%and 15.36%higher than that of the untreated composites.(2)In order to reduce the consumption of flame retardant and improve the flame retarding performance,APP and MEL were used as compound flame retardant to modify the HPWPC,and intumescent flame retarding system of APP-MEL-stalk powder was constructed.It was found that when the ratio of APP/MEL was 7:3,the flame retarding performance of the composite was the best.In the vertical combustion test,the continuous combustion time was 0s,the damage length was 9mm,and there was no drop phenomenon.The LOI was 29%,and the apparent activation energy of the composites was 677.71 KJ·mol^-1,which was 2.67 times of the apparent activation energy of the untreated composite(184.66KJ·mol^-1).The tensile strength and tensile modulus of composites treated by APP-MEL were 6.91MPa and 1.63GPa,respectively.The flexural strength and flexural modulus were 14.62MPa and 3.5GPa.(3)In view of solving the problem of mechanical property degradation after flame retardant,the HPWPC was modified by alkali+coupling agent+nano montmorillonite treatment.The flexural strength of the joint modified composite was 22.2MPa,and the flexural modulus was 5.84GPa,which was 50.71%and 64.04%higher than that of the untreated composite,respectively.The composite had no drop in the vertical combustion test,the continuous combustion time was 0s,the damage length was only2mm,and the LOI was 30.4%.Besides,after joint modification the water absorption of the composites decreased significantly,which was 1.98%.It is 65.66%less than that of untreated composite.The density,moisture content,water absorption,water absorption thickness expansion,bending strength,bending modulus and fire resistance of the composite prepared by the combined modification process can meet the requirements of relevant standards.