| Polylactic acid(PLA)is a green and environmentally friendly biodegradable material.It has many advantages,such as abundant raw materials,biocompatibility,good processability and mechanical properties.It is widely used in biomedical,textile,home textile,packaging materials and other fields.So far,PLA is one of the most promising biodegradable materials.However,like other general polymer materials,PLA is highly flammable with a droplet phenomenon and can easily ignite fire causing serious life threats and loss of property.As a result,the application of PLA in the fields with high flame retardant requirements such as electronic products and building decoration is limited.Therefore,the flame retardant modification of PLA has become the focus of researchers.At present,physical blending is the most commonly used method for flame retardant modification of PLA.However,conventional additive flame retardants have problems such as low flame retardant efficiency,high addition and poor compatibility,resulting in a sharp decline in the spinnability of flame retardant PLA resin and fiber-fabric properties.The existing flame retardant PLA is mostly plastic products and less applicable in the textile fiber field.Such PLA fibers have poor performances and difficult to meet requirements for many applications.Therefore,it is still a challenge to design and prepare flame retardant PLA fibers with excellent processability,mechanical properties and flame retardancy.In view of the above problems,from the point of view of flame retardant system design and flame retardant fiber forming process optimization,a series of flame retardant PLA fibers were successfully prepared by adjusting the melt spinning fiber forming process,using organic phosphorus flame retardant inorganic layered nano materials as a binary synergistic hybrid system.The influence of the hybrid flame retardant system on the flame retardant properties of PLA was studied.The flame retardant mechanism of the hybrid system,the internal relationship between the material components,the forming process and the structural properties of the flame retardant fiber were clarified.The specific research contents and conclusions are as follows:(1)Study on preparation and flame retardant mechanism of organophosphorus flame retardant/polylactic acid Flame Retardant Composites:according to the requirements of PLA molecular structure characteristics and environmental protection,this topic selected high-efficiency green aliphatic organophosphorus flame retardant as the flame retardant system(OP),and prepared PLA-OP composite resin by melt blending process.The effects of OP on the flame retardant performance,thermal stability,flame retardant mechanism and mechanical properties of PLA-OP composite resin were comprehensively analyzed.The limiting oxygen index(LOI)and UL-94vertical combustion results showed that with the increase of OP content,the flame retardant performance of the material significantly improved.4 wt.%OP can improve the UL-94 grade of the composite(PLA-4OP)from the NR grade of pure PLA to the V-1 grade,and the LOI from 22.6%to 29.1%of pure PLA.In addition,the test results of mechanical properties of PLA-OP composite resin showed that the tensile strength of pure PLA was 66.53 MPa,that of PLA-4OP composite resin was 64.41 MPa,and the retention rate of tensile strength of the resin exceeded 95%,which has a good practical prospect.The results of TG-IR and Raman spectra showed that the flame retardant mechanisms of PLA-OP composites were gas phase flame retardant,condensed phase flame retardant and interrupted heat exchange mechanism.The mechanism of gas phase flame retardance is reflected in the degradation process of PLA,which produces H·and O·and other active free radicals.OP decomposes by heating to generate PO·and PO2·and other free radicals,which are effectively captured to generate HPO and HPO2 with low activity,so as to achieve the flame retardancy effect.For the condensation mechanism OP catalyzes further dehydration of PLA into carbon,forming a barrier carbon layer attached to the surface of PLA matrix;In addition,PLA generates low molecular oligomers with good fluidity during degradation,which form droplets during combustion,thus taking away heat to realize flame retardance.(2)Preparation and flame retardant mechanism of organophosphorus flame retardant inorganic zirconium phosphate/polylactic acid flame retardant composites:In order to further improve the flame retardant performance of PLA,inorganic two-dimensional layered zirconium phosphate was used[?-Zr(HPO4)2·H2O,ZrP]as a binary synergistic flame retardant system and OP were used to prepare organic-inorganic hybrid flame retardant PLA composite resin(PLA-OP-ZrP)viamelt blending.The comprehensive properties and flame retardancy of PLA-OP-ZrP hybrid composite resin were comprehensively analyzed.The results showed that when the binary synergistic flame retardant system was 0.5 wt.%ZrP and 3.5 wt.%OP,the UL-94 grade of PLA-OP-ZrP increased to V-0 grade,while the LOI increased to 31.6%.At the same time,the heat release rate(HRR)and total heat release rate(THR)in the cone calorimetric test decreased.When the total amount of flame retardants was the same,the flame retardancy of the binary synergistic system PLA-OP-ZrP was better than that of the single component system PLA-OP,indicating that there was a good synergistic flame retardancy between ZrP and OP.The flame retardant mechanism of PLA-OP-ZrP material was inferred from TG,TG-IR,Raman and other tests:inorganic ZrP has a layered structure and catalyzes the high-temperature carbonization of the matrix.ZrP and the carbon layer are attached to the surface of PLA material,which blocks the heat and oxygen contact.This reflects well the flame retardant mechanism of condensed phase,and it can be dehydrated at high temperatures to form water vapor to dilute the combustible gas.The gas-phase flame retardant effect of OP effectively captures the active free radicals generated during the degradation of PLA,and also reduce the gas-phase pyrolysis products.The synergistic flame retardant effect of the two is verified again.(3)Study on melt spinning forming and structural properties of flame retardant PLA blend fiber.Considering the flame retardant and mechanical properties of flame retardant PLA composite resin,PLA,PLA-4OP,and PLA-3.5OP-0.5MZrP samples were selected to study the melt spinnability of flame retardant PLA fiber.Firstly,the rheological behavior and spinnability of the composite resin were studied.The apparent viscosity,non-Newtonian index,viscous flow activation energy and structural viscosity parameters of the composite resin were characterized by capillary rheometer.The results showed that the three composites had good flow properties and processability.By comparing the rheological properties and spinnability parameters,the spinning temperature was finally determined as PLA(230?),PLA-4OP(225?)PLA-3.5OP-0.5MZrP(225?).In addition,PLA flame retardant fibers were prepared by melt spinning,and their mechanical properties and fiber condensed structure were characterized.The results showed that the mechanical strength of the three fibers reached 3.0 cN/dtex under appropriate drafting ratio,the condensed structure of the fibers corresponded to the changing trend of their mechanical properties,and the composite fibers had high crystallinity and orientation and could meet the performance requirements for several end products. |
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