Synthesis And Properties Of A Novel Flame Retardant Epoxy Curing Agent

Epoxy resin,phenolic resin and polyester resin are called the three thermosetting materials,because of its excellent physical and mechanical properties,chemical resistance,electrical insulation,processing properties and good adhesion properties.They are widely used in coatings,adhesives,electrical and electronic components and other fields,and epoxy resin has become an indispensable national economic development important material.However,the limiting oxygen index of epoxy resin is only 19.8%,which belongs to flammable material,so that its use range is greatly limited.Therefore,it is necessary to improve the flame retardancy of epoxy resin.Flame retardant epoxy resin can be divided into additive type and reaction type,which add type is simple,convenient and widely used,but also more obvious shortcomings,while the reaction type is complex and the use of higher prices,but is not easy to transfer,easy leakage,has excellent flame retardancy,and permanent polymer materials with little effect on performance.At the same time,halogen flame retardants are gradually banned by the countries all over the world,and the use of phosphorus containing flame retardants has been gradually favored by researchers.Therefore,combining flame retardant of epoxy resin and curing agent,it is of study and practical value to develop a halogen-free flame retardant epoxy resin curing agent.In this dissertation,DOPO was used as a phosphorus-containing substance to synthesize a DOPO-reactive epoxy resin curing agent and dicyandiamide?DICY?as a complexing agent to prepare epoxy resin curing systems with different phosphorus contents.The isothermal differential scanning calorimetry?DSC?was used to study the curing process,which was used in the epoxy resin E51 system.The flame retardancy,heat resistance and mechanical properties of the cured system were systematically studied.The main research contents and the result is as follows:?1?DOPO and methacrylamide were used as main raw materials to synthesize the curing agent DOPO-MAC.The structure of the compound was characterized by FT-IR and ¹H-NMR.The FT-IR spectrum showed the presence of characteristic peaks at 3439cm^-1,1665 cm^-1,1204 cm^-1 and 914 cm^-1 and the disappearance of the characteristic peak at 2400 cm^-1 around indicate the presence of N-H,C=O and O=P-C.The ¹H-NMR spectrum showed?=2.5,1.17,2.65,2.90 and 7-8 chemical shifts and the peak area exactly match the structure of DOPO-MAC,indicating DOPO and carbon-carbon double bond in methacrylamide,and the product is DOPO-MAC.The experimental scheme was designed by orthogonal test.The reaction yield was taken as the index.The reaction conditions were as follows:reaction time,reaction temperature and molar ratio of reaction were the three factors.The optimal reaction conditions were as follows:the reaction temperature was 140-150°C,Reaction 4-8 h,the molar ratio of methacrylamide to DOPO reaction is 1.3:1.?2?The non-isothermal kinetics of epoxy resin/DOPO-MAC system with 1%phosphorus content was studied systematically by non-isothermal DSC method,and the most suitable curing process of the curing system was determined:100°C pre-curing 1.5h,115°C curing 5 h,135°C after curing 2 h.The activation energy Ea was calculated by Kissinger and Ozawa method,and the activation energy was about 46 k J/mol,indicating that the reaction was easy to proceed.?3?The flame retardancy of the cured system was characterized by UL-94 and LOI.The results showed that the flame retardant grade and LOI of the curing system are gradually increased with the increase of phosphorus content,and when the P=1.0%,the epoxy resin system can reach UL94 V-0 level.The UL-94 of the curing system turned from V-2grade to V-0 grade,and the LOI increased from 22.8%to 31.7%.It was proved that the introduction of the flame retardant curing agent DOPO-MAC can effectively improve the flame retardancy of the epoxy resin curing system.At the same time,the thermodynamic properties of the epoxy resin curing system were studied.The results show that the carbonization rate of the epoxy resin system TG test at 700°C significantly increased from 7.6%to 22.1%with the increase of phosphorus content in the epoxy resin system.However,both the initial decomposition temperature and the maximum decomposition temperature of the epoxy resin system are reduced to a certain extent,so that the phosphorus-containing groups are easily decomposed under the relative decrease of the temperature to initiate the epoxy resin system initial decomposition.The phosphorus-containing components are thermally decomposed to form phosphoric acid dehydrating agents to promote the dehydration of the polymer into char and reduce the mass loss of the materials.On the other hand,most of the phosphorus components are enriched in the carbon layer,and the dense carbon layer can be isolated the protective layer,and the suppression of flammable gas spillover and isolated oxygen and heat source,thereby preventing thermal oxidation of the polymer,thereby reducing the mass loss of the material,so that the carbonization rate can be significantly increased,but also that the introduction of the flame retardant curing agent DOPO-MAC can effectively improve the flame retardancy of the epoxy resin curing system.The mechanical properties of epoxy resin cured by tensile,flexural and impact strength tests were analyzed.With the increase of phosphorus content in the epoxy resin system,the tensile,flexural and impact strength of the epoxy resin cured system reduced.The reason is the introduction of DOPO,which leads to the increase of the proportion of rigid structure and the increase of steric hindrance,which leads to the decrease of crosslink density,the increase of the rigidity of cured material and the decrease of strength and toughness.