Halogen-free Flame-retarded Epoxy Resin With High Glass Transition Temperature

Epoxy resins are widely used as microelectronics packaging material and adhesive due to its excellent adhesion, insulation performance, thermal stability and mechanical properties,. However, both very poor resistances to fire and high smoke densities associated during burning limit its application in some areas. Therefore, it is necessary to improve the flame retardancy of epoxy resins in order to reach their full potential. The research of organic phosphorus-containing halogen-free flame retarded epoxy resins have received a lot of attention in recent years, owing to the environmental friendliness and efficacy in flame retardancy. Nevertheless, glass transition temperature(Tg), initial degradation temperature(T5%) and mechanical properties of cured epoxy product may be dramatically deteriorated due to the incorporation of weak chemical bonds of P-O and P-C, then it is hard to satisfy the requirements of electronic packaging processing with lead-free and halogen-free. Therefore, new kinds of epoxy resins with high Tg, low toxicity and good mechanical properties are urgent to be investigated.It is an effective way to improve the phosphorus-containing flame retarded epoxy resins by means of introducing rigid group into curing systems or increasing its crosslinking density. As a new type of phenolic-formaldehyde resin,polybenzoxazine not only has the advantages of traditional phenolic resins, such as good thermal stabilitiy, excellent insulation and mechanical properties, and outstanding flame retardance, but also provides unique characteristics like molecular design flexibility. The incorporation of 9, 10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide(DOPO) and benzoxazine is expected to endow cured epoxy resin outstanding flame retardancy and improve its thermal and mechanical properties. Based on the above ideas, DOPO-based benzoxazine and bismaleimide compounds were synthesized and introduced into epoxy resins to prepare a series of intrinsic halogen-free flame-retardant cured epoxy resins. The performance of cured products, including tensile properties, thermal stability and flammability behaviors was investigated, and the flame retardant mechanism was clarified.(1) A novel flame retardant DOPO-based benzoxazine named P-Bz was synthesized from DOPO, 4-aminophenol and 2,4-dihydroxybenzaldehyde. The structure of P-Bz was characterized and confirmed by ~1H NMR, 13 C NMR, 31 P NMR, Fourier transform infrared spectroscopy(FTIR) and elemental analysis. And a series of intrinsic halogen-free flame-retardant cured epoxy resins were prepared by introducing P-Bz into epoxy matrix. The flammability and thermal stability of samples were evaluated by thermogravimetric analysis(TGA), differential scanning calorimetry(DSC), dynamic mechanical analysis(DMA) and limiting oxygen index(LOI), vertical flame test, cone calorimetry test. The results showed that the introduction of P-Bz obviously enhanced char yield, and endowed the cured products with excellent flame retardancy and smoke suppression performance. When the content of P-Bz was 10wt%, where the phosphoruscured content was 0.68wt%, cured epoxy resins of 10-P-Bz with LOI 34.8% passed UL-94 V-0 @3.2 mm rating. T5% and Tg of 10-P-Bz were 355.7 ℃, 202.5 ℃, respectively. Compared with cured epoxy resin of DDS-pure, the peak heat release rate(PHRR), total heat release(THR) and smoke production release(SPR) of 10-P-Bz decreased by 49.6%, 28.0%, 42.4%, respectively. In addition, results from tensile testing demonstrated there existed a little negative effect on the mechanical properties of the samples by the incorporation of P-Bz. The tensible strength of 10-P-Bz was 76.8 MPa which only decreased 5.5% compared with DDS-pure. Furthermore, TGA-FTIR, scanning electron microscopy(SEM) and FTIR were employed to analyze the gas and condensed phase products from thermal degradation, providing insight into the degradation mechanism. The results indicated that P-Bz acted in the gas phase through flame inhibition and in the condensed phase through formation of protective residual char.(2) To improve Tg of flame retarded cured epoxy resin, another flame retardant containing bis-benzoxazine group named P-DDS-Bz was synthesized from DOPO, 2,4-dihydroxybenzaldehyde, and 4,4’-diaminodiphenylsulfone(DDS). Corresponding flame retarded cured epoxy resins with various amounts of P-DDS-Bz were prepared. The introduction of P-DDS-Bz enhanced flame retardancy of cured products. When the content of P-DDS-Bz was 10 wt%, where the phosphoruscured content was 0.66 wt%, cured epoxy resins of 10-P-Bz with LOI 35.3% passed UL-94 V-0@3.2 mm, and performed lower PHRR, THR, SPR, decreased by 52.6%, 28.0%, 51.3%, respectively, compared with DDS-pure. T5% and Tg of 10-P-Bz were 361.5 ℃, 222.2 ℃(dynamic thermomechanical analysis, DMA), increased by 5.8 ℃, 19.7 ℃(DMA)respectively, compared with 10-P-Bz. Based on the SEM results, this is believed to be attributed to that P-DDS-Bz catalyzed the degradation of epoxy resin to form the intumescent protective char layer, which inhibited the heat and mass transfer between epoxy resin matrix and gas phase. Furthermore, P-DDS-Bz had no serious negative impact on the tensile properties of cured epoxy resins. The tensible strength of 10-P-DDS-Bz was 74.5 MPa which only decreased by 6.8 MPa.(3) A DOPO derivative containing bismaleimide named P-BMI was synthesized from 4-aminoacetophenone, aniline, DOPO and maleic anhydride, and incorporated into epoxy resins via Michael addition reaction to prepare a series of intrinsic halogen-free flame-retardant cured products. The introduction of P-BMI improved flame retardancy of cured epoxy resins. LOI value of the products with 12.5 wt% P-BMI contents(0.66 wt% phosphorus contents in cured products) could reach to 35.1% and passed the UL-94 V-0@3.2 mm rating. The thermal properties results revealed that T5% and Tg were improved due to the rigid maleimide group. T5% of 12.5-P-BMI was 378.6 ℃, increased by 22.9 ℃, 17.1 ℃(DMA) and Tg was 233.4 ℃(DMA) increased by 30.9 ℃(DMA) 、 11.2 ℃(DMA) respectively, compared with cured epoxy resins 10-P-Bz and 10-P-DDS-Bz, which overcome the shortcoming of decreased initial thermal stability caused by phosphorus-containing flame retardant. The tensible strength of 12.5-P-BMI was 74.2 MPa which only decreased by 8.6% compared with DDS-pure. Moreover, SEM, FTIR and Electron Probe Microanalysis Energy Disperse Spectroscopy(EPMA-EDS) were employed to study the microstructure and composition of char residue, and the results revealed that the introduction of P-BMI promoted the formation of protective residual char through the condensed phase mechanism, which can shield the underlying polymeric substrate from further burning.