Preparation And Fire Retardant Mechanism Of Waterborne Ultra-thin Fire Retardant Coatings For Steel Structure

Solventborne ultra-thin fire retardant coatings for steel structure has been limited, evenforbidden in many developed countries owing to its high content of volatile organiccompounds （VOC）. In reverse, waterborne ultra-thin fire retardant coatings for steel structurewith polymer latex as binder have become the emphasis of modern research works, as it cutsdown the emissions of VOC, reduces the health hazards and environmental pollution duringthe production, construction and application, being in accordance with the trend of energyconservation and environmental protection. In order to improve the fire retardant property ofcoatings, the intumescent ratio and the structural strength of char layer, and the water-resistantof the coating film, etc., in this dissertation, studies about the preparation and performance ofwaterborne ultra-thin steel structure fire retardant coatings are carried out, and the thermaldegradation and char formation mechanism of vinyl acetate-vinyl ester of versatic acid（VAc-VeoVa） polymer, the influences of ammonium polyphosphate （APP） on the thermaldegradation of VAc-VeoVa are investigated. Moreover, the application of nanosized zirconiaand waterborne phosphorus-containing polymer in fire retardant coatings are also exploratorystudied. The main research works are as follows:In part one, the effects of binder on the intumescent and fire retardant performance offire retardant coatings were intensively studied in terms of thermal degradation characteristic,dynamic rheology and composite viscosity. The results revealed that, for acrylic,styrene-acrylic molten polymers, the elastic modulus （G＇） were larger than the viscousmodulus （G＂）, which meant the elastic behavior was predominant in these melts, and thecomplex viscosity was high up to1000Pa·s. While for VAc-VeoVa molten polymer, G＂waslarger than G＇, which meant the viscous behavior was predominant in this melt, and thecomplex viscosity was low as33Pa·s. Polymer with good thermal stability, strongcarbonization capacity, low complex viscosity, and significant viscous flow behavior, wassuitable as the binder for improving the fire retardancy of coatings.In part two, when VAc-VeoVa latexes was used as the binder, and ammoniumpolyphosphate/pentaerythritol/melamine/expandable graphite （APP/PER/MEL/EG） were usedas the intumescent flame retardant （IFR） system, the effects of IFR and its proportion with VAc-VeoVa latexes on fire retardant performance, intumescent ratio, structure and strength ofswollen layers were studied. The orthogonal direct analysis showed that the effect order ofIFR on fire retardant coatings was APP＞EG＞PER＞MEL, and the optimal ratio wasAPP:PER:MEL:EG=22:8:8:4.5. When the weight content of VAc-VeoVa latexes and the IFRsystem was30%and40%, respectively, the coatings films presented excellent fire retardantproperty （the temperature of steel backside was262℃after exposing to fire for30min）.In part three, the effects of the component, size and morphology of inorganic fillers onthe fire retardant coatings were investigated. The results of thermogravimetry （TG）, capillaryrheometer, fire retardant test, etc., demonstrated that, the coatings using TiO₂/SAp/HNTs（8/1/1） as multiple fillers presented outstanding fire retardant performance （3327s）, and highintumescent ratio （28.14）. The structure and strength of the intumescent layer was alsoexcellent. The inorganic fillers immobilized the relaxation and rotation of polymer chain, andreduced the catalytic dehydration efficiency of APP with PER and VAc-VeoVa, which, as aresult, affected the intumescent ratio and fire retardant performance of coatings. Moreover,under high melt viscosity, it was not easy for the filler to transfer to the surface to form aninorganic layer, which reduced the fire retardant performance of the coatings. Theperformance of home-made waterborne fire retardant coatings with optimized recipe wasbetter than that of the world famous brand one.In part four, the effects of nano-ZrO₂on the performance of fire retardant coatings wereinvestigated. The results demonstrated that, nano-ZrO₂had excellent dispersion property inVAc-VeoVa polymer emulsion after surface modified by silicate coupling agent γ-methacryl-oxypropyltrimethoxysilane （Z-6030）. In halogen-free fire retardant coatings and withoutexpandable graphite （EG）, the addition of nano-ZrO₂resulted in the formed intumescent layerwas not compact, and the structural strength was decreased, so the fire retardant performancewas decreased as the content of nano-ZrO₂increased. But in the halogen fire retardantcoatings containing EG, EG penetrated in the intumescent char layer and supported itsstructural strength, nano-ZrO₂stayed in the intumescent char layer, prevented itsthermal-oxidative degradation and insulated heat, and the fire retardant time of the coatingswas3837s. XRD results demonstrated that titanium pyrophosphate （TiP2O7）, zirconiumpyrophosphate （ZrP2O7）, zirconate titanate pyrophosphate solid solution （Ti0.8Zr0.2P2O7） were formed in high temperature during fire.In part five, the thermal degradation and the effect of APP on the thermal degradation ofVAc-VeoVa polymer were studied. The results revealed that, at air atmosphere, thethermal-oxidative degradation of VAc-VeoVa had three steps,（1） elimination of acetic acidand versatic acid to form unsaturated olefin carbon chain;（2） oxidative dehydrogenation,cyclization and aromatization of unsaturated olefin carbon chain to form hexagonal systemcarbon;（3） thermal oxidative of hexagonal system carbon. At nitrogen atmosphere, its thermaldegradation contained two steps,（1） elimination of acetic acid and versatic acid to formunsaturated olefin carbon chain;（2） scission or cyclization and aromatization of unsaturatedolefin carbon chain to form hexagonal system carbon. The results of density functional theory（DFT） revealed that the elimination of versatic acid was competing with acetate acid. Theresults of thermogravimetry-fourier transform infrared （TG-FTIR） analysis of VAc-VeoVa/APP at nitrogen atmosphere indicated that APP has reduced the thermal degradationtemperature and accelerated the side group elimination rate of VAc-VeoVa polymer. Moreover,the polyphosphate acid, formed by APP thermal degradation, reacted with the sidechainhydroxyl of VAc-VeoVa which was formed by eliminating ketene, to form P-O-C bond. ThisP-O-C bond could connect the aromatic product, formed by unsaturated olefin aromatizationreaction, into a crosslinked network structure, therefore improving the thermal stability ofcarbonate redidue.In part six, waterborne phosphorus-containing styrene-acrylic polymer P（MMA/BA/MAA/PAM100）, which was synthesized by free radical emulsion polymerization, was used asbinder in fire retardant coatings and the performance was evaluated. It was found that thethermal stability of the polymer was improved by phosphate esters of polyethylene glycolmonomethacrylate （PAM100）, through condensed phase mechanism. The results of fireretardant test showed that the fire retardant coatings with StA-P1.5as binder processed the bestfire retardant performance. The calculation results from Kissinger-Akahira-Sunose （KAS）method and Flynm-Wall-Ozawa （FWO） method revealed that the thermal degradationactivation energy （Ea） of polymer was remarkably improved by PAM100, but Eawasdecreased as the weight content of PAM100increased. The thermal degradation mechanismfunctions of polymer were g（α）=-ln（1-α），f（α）=1-α, respectively, the order of reaction was n=1, the thermal degradation rate expression wasdt|da=A·exp（-E/RT）.（1-a）.