Molecular Construction Of Intumescent Flame Retardant And Study On Flame-retarded Polylactic Acid

Despite extraordinary mechanical properties and excellent biodegradability,poly(lactic acid)(PLA)still suffers from a highly inherent flammability,with the value of LOI is 19%,restricting its wide applications in the electric and automobile fields.In recent years,researchers have developed a variety of flame retardants to reduce the flammability of PLA,but they usually sacrifice the mechanical strength of PLA.In this article,four types of intumescent flame retardants were designed and prepared to improve the flame retardancy of polylactic acid and reduce the loss of mechanical properties.Mannich reaction was used to graft melamine onto alkali lignin to obtain melamine modified lignin(PN),which was mixed with ammonium polyphosphate(APP)in different proportions and added to PLA resin to study the flame retardancy of the material and Mechanical properties.The results showed that when the total amount of flame retardant was 20wt%,APP was 15wt%,PN was 5wt%,PLA/PN/APP material could reach UL-94 V-0 level,and LOI was 29.9%.The mechanical properties of the material are greatly lost.In order to improve the compatibility between flame retardant and PLA,the mechanical properties of fire-retardant materials were improved by using melamine-modified lignin compounded with phytic acid to form a salt.When the PN-phytic acid addition amount is 20wt%,the PLA material achieves the best flame-retardant effect,can pass UL-94 V-0 level,and the LOI value reaches 25.4%.However,the mechanical properties are still poor.In order to further improve the compatibility,dimethyl phosphite(DMP)was grafted onto PN to obtain PN-DMP.When the amount of PN-DMP added was 15wt%,the PLA material passed UL-94 V-2 grade and the LOI value reached 26.7%.The addition of PN-DMP makes the material flow easily after combustion,resulting in a decrease in flame retardancy.With the use of strong mechanical properties of nanocellulose(CNF),APP was grafted onto CNF to prepare a new flame retardant.When the amount of APP@CNF added was 10% by weight,the flame-retardant polylactic acid material achieved the best flame retardant effect and could pass the UL-94 V-0 level with a LOI value of 30.0%.Under the atmosphere of nitrogen and air,the addition of APP@CNF significantly decreased the initial thermal decomposition temperature and the maximum thermal degradation rate of the PLA.At the same time,the THR and HRR of the flame-retardant material decreased significantly,and the carbon layer formed after combustion of the material was dense.Continuously,there is almost no hole on the surface,and the thickness of the carbon layer is also higher than that of the previous flame-retardant material.In terms of mechanical properties,the addition of 5 wt% APP @ CNF allows the PLA to maintain a tensile strength of up to 50.3 MPa.When 10 wt% of APP @ CNF was added to the PLA matrix,a high tensile strength of 48.6 MPa was still obtained,and the flame retardant had an enhanced effect on the mechanical properties of the PLA material.