| Polyethylene terephthalate(PET)is the most productive and widely used polymer material.However,the flammability of PET products,especially PET fibers,have brought great fire hazards to human life and property safety.Therefore,it is of great significance to improve the flame retardancy of PET.Currently,the traditional flame retardants that commonly used in PET have exposed many defects,and the development of new flame retardants with the characteristics of"halogen-free,environmental protection,low addition,smoke suppression,and spinnability"has become a research hotspot of flame retardant modification for PET.As an emerging flame retardant,carbon materials have shown the advantages of"halogen-free,environmental protection and high efficiency"in the flame retardant modification of polymer.Previous studies have found that carbon microspheres(CMSs)have high flame retardant efficiency for PET,but the research on carbon material-flame retardanted PET is still in its infancy,and the flame retardant mechanism of CMSs/PET system is still unclear.Furthermore,the original CMSs cannot meet the requirement of"smoke suppression"when used directly as a flame retardant.Magnesium hydroxide(MH)is a green flame retardant with outstanding smoke suppressing effect,but its flame retardant efficiency and compatibility with polymers are poor.Therefore,the combination of CMSs and MH to build a new composite flame retardant for the flame retardant modification of PET and PET fiber has significant research value and broad development prospects.In this study,a microcapsulated carbon microsphere flame retardant with double-shell structure was constructed for the purpose of"low addition +flame retardant + smoke suppression + spinnability",which was used for the flame retardant modification of PET.Firstly,flame retardant properties and flame retardant mechanisms of CMSs,carbon microsphere coated by magnesium hydroxide core-shell flame retardant(MH@CMSs)and double-shell microcapsulated flame retardant(MMH@CMSs)in PET composites were studied in turn.After that,the crystallization and rheological properties of MMH@CMSs/PET flame retardant masterbatch were studied.Finally,flame retardant polyester fibers were prepared by melt spinning method,and the flame retardant fabrics were woven with those fibers.The properties of the flame retardant fibers and fabrics were studied.The main research contents and results are summarized as follows:(1)CMSs and CMSs/PET flame retardant composites were prepared,the flame retardant properties of CMSs/PET were characterized,and the flame retardant mechanism of CMSs on PET was mainly studied.The results showed that the flame retardant mechanism of CMSs/PET was cross-linking and char formation mechanism.In the initial stage of CMSs/PET thermal decomposition,the large specific surface area of CMSs allowed them to migrate to the surface of PET matrix when heated.The free radicals on the surface of CMSs coupled with macromolecular free radicals generated by PET degradation to form a cross-linking network in situ.This reaction inhibited the breakage of molecular chains and isolated heat,which enhanced the flame retardancy and thermal stability of PET.Meanwhile,CMSs could capture highly reactive hydroxyl radicals(HO-)to restrain free radical chain reactions.In addition,CMSs promoted PET to decompose into carbonaceous particles,which deposited on the surface of PET under the action of cross-linking net,and thus the thermal stability and compactness of the char layer were improved.Consequently,CMSs promoted PET to form a dense and continuous three-dimensional char layer during the combustion,which firmly covered on the surface of PET to insulate the transfer of heat and mass,thereby providing a flame retardant effect.(2)In order to improve smoke suppression of the CMSs/PET system,the smoke suppressant of MH was introduced.The results showed that CMSs and MH had a synergistic flame retardant effect on PET.When CMSs and MH were directly compounded,the flame retardant and smoke suppression effects were optimized at the mass ratio of 5:5(CMSs:MH).In this case,the total smoke release(TSR)of CMSs/MH/PET flame retardant composite was 12.2%lower than that of CMSs/PET.However,the flame retardants agglomerated severely in this method,which resulted in poor mechanical properties of CMSs/MH/PET.Therefore,the flame retardant with core-shell structure(MH@CMSs)was synthesized by coating MH on the surface of CMSs.When the dosage of MH@CMSs was 1.0 wt.%,the limiting oxygen index(LOI)of MH@CMSs/PET was 27.5%with a UL 94 V-0 grade,and the TSR of MH@CMSs/PET reduced by 31.9%compared with that of CMSs/PET.Flame retardant mechanism exploration showed that the condensed phase flame retardant effect was further enhanced by coating MH on CMSs.Compared with CMSs/PET,the compactness,continuity and thermal stability of the char layer produced by the MH@CMS/PET were significantly improved.During the combustion,the core-shell type flame retardant MH@CMSs decomposed in a gradient from the shell to the core.Role of the CMSs core was to form cross-linking net and promote PET carbonization.And the MH shell played a supporting role in four aspects:protecting CMSs,absorbing heat from the burning zone,diluting the concentrations of combustible gas,and consolidating the structure of the char layer.(3)In order to improve the compatibility and interfacial bonding between flame retardant and the PET matrix,microcapsule technology was used to coat the surface of the core-shell flame retardant MH@CMSs with a PET capsule wall to make make a microcapsuled flame retardant with double-shell structure(MMH@CMSs),which combined the microcapsule technology with the theory of"similarity and compatibility".This strategy effectively improvedthe mechanical properties of the flame retardant composites.When the dosage of flame retardant was 1.0 wt.%,the tensile strength and elongation at break of MMH@CMS s/PET were 57.3%and 42.1%higher than those of MH@CMSs/PET,respectively.The LOI of the MMH@CMSs/PET was 27.4%.Compared with pure PET,the time to ignition(TTI)of MMH@CMSs/PET was extended by 20 s,and the heat release rate peak(pk-HRR),total heat release(THR),and TSR were reduced by 36.6%,22.0%and 40.8%,respectively.The encapsulation treatment did not have a substantial effect on the flame retardant mechanism of the original fname retardant of MH@CMSs.However,fname retardant efficiency of MMH@CMSs was slightly lower than that of MH@CMSs.(4)In order to provide a theoretical basis for spinning,the crystallinity and rheology of MMH@CMS s/PET fname retardant masterbatch were studied.The results showed that the introduction of MMH@CMSs wouldn't change the spherulite growth pattern of PET.MMH@CMSs had a heterogeneous nucleating agent effect on PET,which significantly accelerated the crystallization rate of PET and increased the crystallization activation energy of PET.Both pure PET and MMH@CMSs/PET melts were typical pseudoplastic fluids.At the same temperature,the non-Newtonian index and structural viscosity index of MMH@CMSs/PET were close to those of pure PET.At the same shear rate,the viscous flow activation energy of MMH@CMSs/PET was slightly higher than that of pure PET,which means that the sensitivity to temperature of MMH@CMS s/PET was increased.(5)Flame retardant fibers(FR-PET fibers)were prepared by melt spinning method.The results indicated that the optimum dosage of MMH@CMSs in PET fiber was 0.6 wt.%.In this case,the LOI of FR-PET fiber was 25.8%,and the pk-HRR,THR and TSR were reduced by 27.3%,23.6%,and 13.6%,respectively,compared with pure PET fiber.To weave flame retardant polyester fabrics(FR-PET fabrics),pure polyester yarn and prepared FR-PET filaments yarn was adopted as warp and weft yarn,respectively.When warp density was 180/10 cm and weft density was 200/10 cm,the fabric reached B1 grade of the flame retardant textile.In summary,by studying the flame retardant properties of CMSs and MH@CMSs in PET composites,the cross-linking and char formation mechanism of CMSs and the synergistic flame retardant mechanism of MH@CMSs were proposed.A double-shell microcapsuled flame retardant MMH@CMSs was constructed for improving the interface bonding between flame retardants and PET.MMH@CMSs exhibited good flame retardant and smoke suppression effects on PET,and was suitable for the melt spinning of polyester fiber,which achieved the research purpose of "low addition + flame retardant+smoke suppression+spinnability". |
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