The Mechanism Of Carboxyl Cellulose/Sucrose Char-forming Agent Used In Intumescent Flame Retardant And Smoke Suppressing Epoxy Resin

Epoxy resin（EP）is one of the typical thermoset polymers in the manufacturing industry,such as casting materials,adhesives,coating,and electronic circuit boards due to its excellent mechanical performance,good adhesive properties,stable chemical resistance,and superb electrical properties.Nevertheless,the drawbacks during the combustion of EP（e.g.,high flammability,large smoke generation,and toxic gases）severely restrict its further potential applications in composites.Therefore,many efforts have been made to improve the flame retardant performance of EP,such as the functionalized nano-particles,the selection of blowing agent and curing agent etc.However,for most IFR,the char layer formed during combustion is easy to break due to the low char strength and complex burning condition,thus limiting the generation of a compact and high intumescent structure.The flame retardancy performances will be decreased and the formed fragile char layer cannot prevent the heat and mass transfer between the gas and condensed phases.Additionally,the mechanism of the expansion process in IFR has not been well addressed.It remains a challenge to fabricate a high-efficiency carbonization agent and develop cost-effective,high-performance,and IFR systems.Epoxy resin（EP）is widely used in electronic and electrical,aerospace and chemical industries because of its excellent adhesion,mechanical properties,electrical insulation and good chemical stability.However,EP is flammable and releases toxic gases such as CO and dense smoke at the same time,which is a serious hazard.Intumescent flame retardants can effectively improve the EP flame retardant grade,reduce combustion heat release and smoke density.However,in actual combustion,due to the low strength and poor plasticity of the expanded carbon layer,the carbon layer ruptures during the thermal expansion process,resulting in a decrease in expansion rate,heat and mass barrier effect,and flame retardant effect and smoke suppression.When using hydrogen peroxide to prepare carboxyl cellulose（ORC）and polyoxymethylene coated APP（MFAPP）synergistically flame-retardant EP,compared with pentaerythritol（PER）and regenerated cellulose as the char-forming agent system,the new intumescent flame-retardant system,the expansion rate,the plasticity of the carbon layer and the melt strength are increased,and the smoke and heat release during the combustion process are reduced.It is urgent to explore the mechanism of the change caused only by the change of the char-forming agent.Under the background of environmental protection and sustainable development as the theme of the times,research the mechanism of intumescent flame retardant IFR-EP of new type char-forming agents such as ORC,explore the morphological evolution of the intumescent carbon layer,deepen the understanding of the intumescent flame retardant mechanism and realize the expansion behavior.The new approach is of great significance.oxidized regenerated celluloses has exhibited superior performance as a carbonization agent for polymers.However,it is a major challenge to fabricate fire safety and intumescent flame-retardant polymer composites with high-efficiency,and“heterogeneous char-forming agent（HCA）”is worth pursuing.Herein,regenerated cellulose（RC）was oxidized by H₂O₂ to achieve oxidized RC（ORC）with high carboxyl content.Interestingly,ORC,the potential HCA,achieved higher catalytic charring effect and lower content of the toxic gas release compared with the presented carbonization agent.5 wt.%ORC and 3.75 wt.%microencapsulated ammonium polyphosphate（MFAPP）as intumescent flame retardants（IFR）were utilized to fabricate IFR epoxy resin（EP）（noted as EP/MFAPP/ORC）.Amazingly,a super expansion ratio（41.5-fold）intumescent char layer was formed in EP/MFAPP/ORC27（ORC with 27%carboxyl content）after the cone calorimetry test.Moreover,the peak heat release rate,total heat release and total smoke production of EP/MFAPP/ORC27 largely decreased by 55.6%,61.8%and62.2%,respectively.Furthermore,the residual char yield（41.8%）significantly enhanced by9.7-fold,compared with those of EP.Meanwhile,it achieved 30.3%limiting oxygen index and V-0 burning test,and the flame retardant index（FRI）showed"good"flame retardant performances.The concept of HCA and flame-retardant mechanism were elaborated by analyzing the evolution process of IFR-EP.These results demonstrate that ORC27,acts as HCA,is a novel and efficient strategy for fabricating fire safety EP with excellent flame-retardant efficiency.The fourth chapter of the thesis uses hydrogen peroxide to treat microcrystalline cellulose（MCC）to obtain oxidized microcrystalline cellulose（OMCC）.The FTIR results confirmed that the main hydroxyl group at the c-6 position in the MCC glucose unit was selectively oxidized to a carboxyl group by H₂O₂.SEM characterizes the apparent structure of OMCC and counts the aspect ratio of OMCC.Add MFAPP（3.75wt.%）and char-forming agent OMCC（5wt.%）to the epoxy resin.The flame retardancy of IFR-EP composites were evaluated by the UL-94test,LOI and cone calorimeter test（CCT）.EP/MFAPP/OMCC（3.75/5wt.%）is classified as V-0,with a LOI of 28.6%.This study proposes a new green method for manufacturing OMCC with H₂O₂,and proves that OMCC is an excellent char-forming agent for IRF-EP composites.At the same time,the evolution of the carbon layer was studied in detail through the muffle furnace.A new way to characterize the thermal insulation of the carbon layer by an infrared thermometer is used to obtain the thermal insulation performance of the carbon layer at different temperature stages.Using SEM-EDS to perform detailed characterization of the samples placed in a 550℃environment for 1,3,and 5 minutes,it is found that the IFR-EP composite material undergoes complex physical and chemical changes within 3-5 minutes,revealing that the carbon layer is in a high-temperature environment.The law of evolution under.In the fifth chapter of the thesis,in order to understand the mechanism of polysaccharide char-forming agents more deeply,the flame retardant properties of IFR-EP composites with monosaccharides,disaccharides and oligosaccharides as char-forming agents are analyzed.The limiting oxygen index（LOI）test and vertical burning test（UL-94）of the above-mentioned composite materials were carried out.In this study,white granulated sugar（WS）was used as an environmentally friendly additive and was cleverly added to the intumescent flame-retardant epoxy resin to improve the flame-retardant efficiency.As expected,the synergistic effect of WS and MFAPP can further reduce the heat release of epoxy thermosetting resin,including peak heat release rate（PHRR）,smoke production rate（SPR）and total smoke production（TSP）.Under normal circumstances,EP2 using 10 wt.%flame retardant（the mass ratio of MFAPP and WS is 1:1）easily passed the UL-94 V-0 rating and obtained the highest LOI value of 27.8%;in addition,During the combustion process,its TSP value is reduced by 76%compared with pure EP.According to the analysis of the carbon residue,it is shown that the existence of MFAPP-WS is beneficial to the formation of a more continuous and denser carbon from EP,which can more effectively isolate the transfer of heat and the diffusion of fuel.In addition,with the introduction of WS,the notched impact strength of IFR-EP composites increased by32.2%.