Preparation And Flame Retardant Application Of Boric Acid Modified Expandable Graphite And Oxide Graphite

Graphite-based materials can effectively improve the conductivity,flame retardant properties and the gas or heat blocking effect of the polymer.Expandable graphite?EG?can decompose at high temperature and release SO₂,CO₂,H₂O,which are non-flammable gases as coolants and free-radical diluents to terminate the chain reaction.At the same time,it can rapidly expand to form porous expanded graphite which can impede mass and heat transmission from solid phase to gas phase.In addition,the fluffy residues cover on the molten area of the burning polymer to prevent the melt dripping and fire spread.Graphene also named"monolithic graphite"is widely used in physics and mechanics,but its application in chemistry and related field is relatively less.Therefore,a preliminary study is carried out in this paper about its influence on polymer combustion performance and thermostability.Ammonium polyphosphate?APP?can decompose at high temperature to release non-flammable gases NH₃ and H₂O,and then produce phosphoric acid,polyphosphoric acid or polymetaphosphoric acid.Dimethyl methylphosphonate?DMMP?is an internally-added flame retardant with a high phosphorus content.It can achieve the same fireproofing effect with half or less dose than the usual flame retardants,and the modified composites normally hold good compatibility and excellent flame retardancy.Boric acid?H₃BO₃?can dehydrate to form a glassy material,which is helpful for the improvement the flame retardancy.At first,EG modified by H₃BO₃ was prepared with silane coupling grafting,and then applied to the flame retardance of linear low density polyethylene?LLDPE?and rigid polyurethane foam?RPUF?respectively.In addition,graphite oxide?GO?was prepared by a closed oxidation method,and then modified with H₃BO₃ by physical impregnation to get the modified GO?MO?;GO was redused by ammonia-hydrahydrate hydrazine to obtain graphene.The influence of GO,MO and graphene on polylactic acid?PLA?combustibility,thermal stability and mechanical properties was investgiated.The completed four part works were described as following:First,the H₃BO₃ modified expandable graphite?MEG?was prepared by silane coupling grafting method.With the maximum expansion volume?EV?as a standard,the MEG preparation conditions were optimized.It presented a initial expansion temperature of 155?and a EV of 367 m L·g^-1 at 800?.Results showed that H₃BO₃ had been successfully bonded to the surface of EG by silane coupling grafting method.Second,MEG and APP?II?were used as additives in order to improve LLDPE flame retardancy.For the purposes of flame-retarding and ensuring material mechanical properties at the same time,the composition of the composite was determined as87.0LLDPE/8.7MEG/4.3APP.Characterized results indicated that the limiting oxygen index?LOI?value of 87.0LLDPE/8.7MEG/4.3APP reached 27.1%,and the UL-94 level reached V-0.The cooperation of APP and MEG could effectively increase the cross-linking degree and tightness of residue,which was the key to the improvement of flame-retardant properties.Aothough addition of flame retardant reduced the low temperature thermal stability of LLDPE.It could reduse the maximum heat release rate and improve its high temperature thermal stability at the same time.MEG presented the abilities of inhibiting the heat and smoke release rate,and reducing the release of CO and CO₂ of flame retardant LLDPE,which was superior to that of ordinary EG and MEG/APP cooperating system.However,the influence of APP,H₃BO₃,EG and MEG on mechanical properties was negative.Third,MEG cooperating with APP or DMMP as a flame retardant was applied to RPUF,and the influence on combustibility,thermal stability and mechanical properties was investgiated.?1?When the mass ratio of RPUF to MEG was set as 8:1,it was found the LOI could reach 26.2%and the UL-94 rating get V-0,which could meet industrial flame retardant standard.?2?The mechanical properties and morphology analysis showed that the average cell size of the flame retarded RPUF gradually decreased with the increasing MEG dose,the bubble collapse became more serious;When the mass ratio of DMMP/MEG or APP/MEG equaled 1:2,DMMP or APP with MEG produced a synergistic effect and showed the highest compressive strength.?3?Combustion tests showed that when MEG/APP was added in a ratio of 1:1 and MEG/DMMP was added in a ratio of 2:1,the flame retardant performance was the best.?4?The thermal analysis showed that the addition of flame retardant could not change RPUF thermal decomposition tendency,but it could increase the residue amount at high-temperature,and the residue amount of MEG/APP flame retarded RPUF was higher than that of only APP or MEG system.Moreover,addition of flame retardant could reduce the maximum heat release rate,single APP or DMMP showed poor flame retardancy and smoke suppression effect on RPUF.On the contrary,the addition of MEG and APP or MEG and DMMP could reduce the CO,CO₂ yield.Forth,with 50 mesh natural flake graphite as raw material,GO and H₃BO₃ modified graphite oxide?MO?were prepared by closed oxidation method.Graphene was prepared by ammonia-hydrazine hydrate complex reduction method.Modified graphite oxide?MO?was prepared by physical immersion method.Characterized results showed that boric acid presented in MO.Graphene was highly stripped and the long-range order lost in the c-axis direction.A masterbatch-melt blending method was used to prepare PLA composites,and the influence on matrix mechanical properties,thermal stability and combustion behavior was investigate.Results showed that the elongation at break and the tensile strength of the composites were slightly higher than that of pure PLA.The PLA/GO and PLA/MO systems held the highest tensile strength at the adding dose of 0.08%.While,that of the PLA/Graphene was gradually decreased with the increasing additive amount,but a reverse relationship was observed for hardness.The modified PLA composites showed the similar thermal decomposition trend as pure PLA,which showed those additives could not change PLA thermal decomposition tendency.When graphene dose was set as 0.08%,the thermal conductive effect of graphene was greater than that of lamellar barrier effect.As for GO,the greater amount added,the more obvious effect was.Except the maximum heat release rate of99.92PLA/0.08Graphenen,the maximum heat release rate,total heat release,CO yield and CO₂ yield of the other composites were all lower than that of PLA,which indicated that the tested three additives could basically improve the combustion properties of PLA.The total heat release of 99.92PLA/0.08Graphenen was lower than that of 97.0PLA/3.0GO.