Study On Preparation And Properties Of New Benzoxazine Resins

Benzoxazine can be synthesized from amine, phenolic and aldehyde compound via Mannich reaction. Benzoxazine monomers have a six-membered heterocyclic ring structure which can be easily cured through ring-opening polymerization by heat treatment. Benzoxazine resins have some excellent properties such as thermal stability, flame retardance, near-zero shrinkage or volumetric expansion, electronic properties, low moisture absorption and mechanical properties. Benzoxazine resins have been widely used as matrix for high-performance composites in the fields of electronic/microelectronic and aerospace industries.Benzoxazine resins have some shortcomings such as high curing temperature, relatively poor thermal and flame resistance and so on. Reducing the benzoxazine ring-opening curing temperature and improving the thermal properties of resins are the main research directions. By taking advantage of the molecular design flexibility of benzoxazine, methoxy as electron donating group is introduced into the benzoxazine, which is beneficial to reduce the ring-opening curing temperature and increase the char yield. A new ether-based benzoxazine monomer with a benzoyl moiety in the side chain has been synthesized. The resin has better thermostability and mechanical properties than the comparative benzoxazine without a benzoyl moiety in the side chain.Three aromatic diamine-based benzoxazine monomers were synthesized from the reaction of 4,4’-diaminodiphenyl methane, paraformaldehyde and phenol, p-methoxyphenol and 3,5-dimethoxyphenol, which marked as B-1, B-2 and B-3, respectively. The structures of these monomers were confirmed by 1H and 13 C nuclear magnetic resonance spectroscopy(NMR), fourier transform infrared(FT-IR) and elemental analysis. The curing behaviors of the monomers were monitored by FT-IR and differential scanning calorimetry(DSC), and the thermal properties of the cured resins were studied by thermogravimetric analysis(TGA). The non-isothermal DSC curing method was used to research the curing kinetics of benzoxazine resins. The curing activation energy were 81.49, 90.14 and 97.15 kJ/mol, respectively. The methoxy group can significantly reduce the curing temperature of the benzoxazine monomer and curing enthalpy, increase the char yield of cured resins while reduce the thermostability.In order to improve the thermostability of benzoxazine, a new ether-based benzoxazine monomer(B-4) with a benzoyl moiety in the side chain was synthesized from the reaction of aromatic diamine, paraformaldehyde and phenol. The curing behaviors of the monomers were monitored by FT-IR and DSC, and the thermal properties of the cured resins were studied by TGA. The cured B-4 had better thermostability with a 5-wt% weight loss temperature of 380 oC and a char yield of 66% at 800 oC than cured B-5. The addhesion ability of the resins were investigated and the shear strength were 6.5 and 4.7 MPa, respectively. The curing activation energy were 84.95 and 88.54 kJ/mol, respectively, which obtained from the non-isothermal DSC curing method. These results indicated that the introduction of a benzoyl moiety into the ether-based benzoxazine as the side chain can efficiently improve the thermostability and mechanical properties of the benzoxazine.