Curing Kinetics Of Flame Retardant Epoxy Resins With DOPO

An interest in halogen-free flame retardant epoxies has been around for recent years both at home and abroad. The flame retardant epoxy resins, the flame retardant curing agents and the nano-flame retardants are the focus, also the flame retardant curing performance (limiting oxygen index, vertical combustion performance, heat release rate and total amount etc.) and the thermal decomposition process (thermal decomposition temperature, residue rate and mass spectrum analysis of decomposition productions) are cared about in order to reveal the possible mechanism of flame resistance. It is seldom that the kinetics of the flame retardant epoxy curing process be described. And the performances of the flame retardant epoxy resins depends on their cure conditions. So it is necessary to understand and apply the curing kinetics to get high-performance flame retardant epoxy resins.Curing kinetics is studied by the change of enthalpy through DSC. It is convenient to describe cure behaviors and can calculate the conversion rate quantitatively. The response of the change of epoxy structure could be tracked by rheology.Therefore, the kinetics of the flame retardant epoxies containing DOPO was studied. The relations between the cure degree and the temperatures were obtained through the heat enthalpy of the cure process and the change of the structure. The effects of different cure catalyst on curing kinetics were studied. Also the influence of vitrification and gelation to the cure system have been stated. Our work provide guidance for the technology and the production performance.1、In order to increase thermal stability and decrease water uptake of conventional epoxy resins, the biphenyl structure was introduced through the curing agent. A novel biphenyl-containing amine (BPDP) was synthesized by one-pot method and used as the curing agent for bisphenol-A epoxy resin EP828. Thermal property, water uptake and curing kinetics of the co-cured epoxy with diaminodiphenyl methane (DDM) were studied. The introduction of BPDP obviously improved the thermal stability, char yield, and water-uptake of the cured epoxy resins. The glass transition temperature Tg and weight-loss temperature increased with increasing BPDP content in the co-curing agent of BPDP and DDM. BPDP showed lower reactivity towards epoxy DGEB A than DDM with higher apparent activation energy Ea. Curing reaction of the epoxy resins co-cured with BPDP and DDM was investigated by non-isothermal DSC and the curing kinetics was described by a truncated two-parameter autocatalytic equation of the Sestak-Berggren model with experimentally determined parameters, which fitted the observed non-isothermal curing reaction rate quantitatively. The cured samples of DOPOER BPDP/DDM with 1wt% phosphorous content achieved UL-94 V-0 grade, so the flame retardant performance was improved by BPDP.2、A halogen-free flame retardant epoxy resin was prepared consisting of DOPO-based glycidyl ether of cresol formaldehyde novolac and diglycidyl ether of bisphenol A cured by micronized dicyandiamide with accelerator U-52. The optimized epoxy resin formula contained 1.5 wt% of phosphorus and achieved UL-94 V-0 grade and LOI of 32%, with high tensile strength and impact strength of 48 MPa and 14.5 kJ/m2, respectively. Curing reaction kinetics for the DOPO-containing epoxy with 1.5 wt% of P was investigated by DSC through both non-isothermal and isothermal scanning. A two-parameter autocatalytic equation of the Sestak-Berggren model was constructed with experimentally determined kinetics parameters, which described the non-isothermal curing reaction rate quantitatively. For the isothermal curing kinetics, the Kamal-Sourour model was adopted to express the autocatalytic effect. The parameters of the reaction rate constant and reaction order were evaluated by curve fitting, and the calculated curve described the observed reaction rate fairly well. The determined kinetics parameters reflected the characteristics of the autocatalytic reaction for the curing of the phosphorus-containing epoxy system.3、The 1.5P-D-U-EMI system was changed from the optimum flame retardant epoxy formula by using different catalyst. The curing process of the 1.5P-D-U-EMI system was compared. The relationship between the glass transition temperature and conversion was established by DSC. The phenomenon of diffusion control was observed at the later stage at the isothermal conditions below the completely cured glass transition temperature Tg∞. The Kamal-Sourour model with diffusion was adopted to calculated curve described the observed reaction rate fairly well. The isothermal rheological test was under continuous frequency sweep. Vitrification phenomenon was observed in the process of isothermal rheological test. The modulus of the epoxy system increased continuously after vitrification. The G’ value of the epoxy system finally reach the same platform finally because the epoxy system achieves the same cure degree in the end although the isothermal conditions or the cure time are different. The conversion of the liquid-solid transition point at different isothermal conditions is the same. And the activation energy by rheology is 83.28kJ/mol, which is very close to the activation energy 85.46 kJ/mol by Kissinger method. It is demonstrated that the cure essence is the same under the same condition by DSC and the rheological test.4、The isothermal cure mechanism of 1.5P-D-U-EMI was not changed by adding lphr GO-IPDI-MZ. Tonset and Tp of 1.5P-D-GOM-1 are reduced compared to that of 1.5P-D-U-EMI, but the total heat ΔH of 1.5P-D-GOM-1 was higher than that of 1.5P-D-U-EMI. So lphr GO-IPDI-MZ plays a catalytic and co-curing action on the epoxy system. The phenomenon of diffusion control was observed at the later stage at the isothermal conditions. The Kamal-Sourour model with diffusion was adopted to calculated curve described the observed reaction rate fairly well. The isothermal rheological test was under multiple frequency sweep. The Winter gel point can be observed in both rheological non-isothermal or rheological isothermal tests. And the activation energy by rheology is 79.42kJ/mol, which is lower than the activation energy 83.28 kJ/mol of 1.5P-D-U-EMI. Also the results of the rheological tests show that the GO-IPDI-MZ plays a catalytic action on the epoxy system.5、The catalyst action of GO-IPDI-EMI for the cure reaction of the epoxy-dicyandiamide system. Tonset and Tp of the system move to lower temperatures with the increase of the GO-IPDI-EMI content. The Friedman method showed that a catalytic action occured at the begin since the activation energy decreased. The curing rate constant k1 and k2 determined by Kamal model became lower and the activation energy was reduced when GO-IPDI-EMI was added. The Winter gel points moved to lower temperatures in rheological non-isothermal tests with the content of GO-IPDI-EMI increased. Also the activation energy was reduced by rheological isothermal tests with the content of GO-IPDI-EMI increased. It showed that GO-IPDI-EMI catalyze the flame retardant epoxy systems by calorimetry and rheological tests. In other words, GO-IPDI-EMI can be used as a catalyst for epoxy resins.