The Effects Of Flame Retardants On Anti-aging Properties Of Polypropylene Composites

Polypropylene（PP）as one of the lightweight thermoplastic plastics has been widely used in many fields due to its corrosion resistance,high electrical insulation,high mechanical strength,good wear resistance,and simple processing process.However,its high flammability companied with a large amount of heat may cause fire hazards.To improve the flame retardancy of PP,many kinds of flame retardants are introduced into PP.In addition,the methyl groups in the molecular chain structure of PP are easy to be broken down to form short chains,when it is exposed to natural environment.In order to delay the aging process of PP,the stabilizers including UV absorbers,antioxidants are added.Unfortunately,the flame retardants would inactivate,migrate and precipitate after UV irradiation,in addition,the light stabilizer would interact with the flame retardants,which decreases the ability of delaying the aging process,and hence reduces the lifetime of PP products and decreases the mechanical properties of PP composites.This work aimed to investigate the effects of often used flame retardants including bromine、inorganic metal and phosphorus flame retardants on the aging process of PP,discusses the mechanism between structure of flame retardants and photoaging properties.Anti-photoaging and flame retardant PP composites were prepared based on the above investigation.The main contents and outcomes are summarized as following:（1）The effect of bromine flame retardants Tetrabromophthalicanhydride（TBPA）and Tetrabromobisphenol A（TBBA）on the photoaging process of PP was explored.The presence of TBBA significantly delayed the aging process and improved the thermal stability of PP due to the presence of phenol in TBBA structure.For the PP/TBBA sample,the elongation at break decreased by 14.9%,which was much lower than that of the control PP sample after 120 h UV irradiation.While TBPA has cross linked with the main chains of PP under UV irradiation because of the unstable anhydride groups.The ignite time(t_ign)and time to peak heat release rate(t_{p HRR})were both reduced by nearly 50%,resulting in the increase of fire hazard potential due to the presence of short chains after UV irradiation.In addition,the bromine flame retardants would migrate along the direction of crack extensions from the surface face to UV light to the back surface.（2）The effect of inorganic metal flame retardants nano aluminum hydroxide（ATH）and inorganic aluminum hypophosphate（AHP）on the photoaging process and flame retardancy of PP was investigated.The presence of ATH and AHP can delay the photo-aging process of PP,the structure of ATH and AHP was stable and not easy to be broken due to the high bond energy of metal-oxygen.For the PP/ATH and PP/AHP samples,the tensile strength was decreased by 14.1 and 13.0%,respectively,which were significantly lower than that of control PP sample after 120 h UV irradiation.The decrease rate of the elongation at break of PP/ATH and PP/AHP sample was also lower than that of the control PP.The p HRR was decreased after 120 h UV irradiation,however,the t_ign and t_{p HRR} were also reduced after UV irradiation.For the PP/ATH and PP/AHP sample,the fire growth index（FGI）was increased from 7.2 m²·s/k W to 8.4 m²·s/k W and from 9.9 m²·s/k W to 12.6 m²·s/k W,respectively.It was demonstrated that the ATH migrated from the lower surface to the upper surface after receiving UV light.AHP showed the opposite migration direction because the micron particle size restricted its migration cross cracks to the surface face to light.（3）The effect of melamine phosphate（MPP）and ammonium polyphosphate（APP）on the photoaging process of PP and its fire performance after UV aging were explored.The chain of APP was broken to produce phosphoric acids,which accelerated the aging process of PP.The presence of triazine structure in MPP significantly absorbed UV lights,delayed the aging process and improved the thermal stability of PP.After being irradiated for 120h,the decrease rate of the tensile strength and elongation at break was lower than that of the control PP sample.The flammability characterization revealed that the aging process reduced the t_ign and t_{p HRR}.After introducing triazine structure,the accelerate effect of APP was suppressed.（4）In order to eliminate the accelerating effect of APP on the photo-aging and the antagonistic effect between APP and HALS770,microencapsulated APP（Si-APP）was prepared by coating ammonium polyphosphate（APP）with polysiloxane,and then was introduced into polypropylene（PP）in association with a light stabilizer Bis（2,2,6,6-tetramethyl-4-piperidyl）sebacate（HALS770）to synchronously improve both the photo-aging resistance.By adding 0.5wt%HALS770,the limit oxygen index（LOI）of PP/Si-APP sample was improved to25.4,and the heat and smoke release of PP composites containing Si-APP and HALS770 were decreased compared with that of the control PP sample.It was proved that the piperidine structure in HALS770 could combine with the polysiloxane to form the hydrogen bonds,which promoted char formation in the condensed phase and suppressed the release of heat and smoke in the gas phase.Moreover,the flame retardancy after UV irradiation was also maintained.（5）Microencapsulated APP（PTA@APP）was prepared by in situ polymerizations of the tannic acid on the surface of ammonium polyphosphate（APP）,which was then introduced into PP in association with a triazine charring formation agent（CFA）to synchronously improve both the flame retardancy and photo-aging resistance of PP.The fire behavior evaluation indicated that the p HRR and the peak smoke release rate（p SPR）of the PP sample with a loading of 23%PTA@APP/CFA were reduced by 63.4%and 64.4%respectively compared with that of the control PP.For PP/23%PTA@APP/CFA sample,the LOI increased from 18.7 of pure PP to 34.2,and the sever dripping was suppressed.It was proved that the polyaromatic ring in of PTA formed the cross-linking structure in the condensed structure,improved the quality of char residue.Moreover,the presence of PTA not only eliminated the acceleration effect of APP on PP aging,but also improved the ultraviolet（UV）resistance by absorbing UV light and capturing free radicals,and hence significantly reduced the loss of mechanical property after UV irradiation.Importantly,the flame retardancy of the PP composite was well remained.The LOI reached to 32.3and achieved V-0 grade in the UL-94 test after aging.