Formation of new thermosets by the reaction of cyanates with thiophenols
Sulfur-containing aromatic thermosets, mainly consisting of aryloxy- and arylthio-substituted 1,3,5-triazines, have been prepared through the reaction of difunctional cyanates with mono- and difunctional aromatic thiols. A straightforward three-step reaction scheme is proposed and verified by the identification of key substances: (1) the addition of thiol and cyanate groups, (2) the stepwise addition of (thio)imino carbonic esters with one another, and (3) the ring closure of chain-extended (thio)imino carbonic esters to form 1,3,5-triazines. Reactions of types 2 and 3 are associated with an abstraction of phenol or thiophenol, which can enter into reaction 1 again. Characterization Of the curing behavior of dicyanate of bisphenol A with thiophenol as well as dimercaptodiphenyl sulfide by differential scanning calorimetry shows that the reaction rates are significantly enhanced by the admixture of thiols to the cyanate. Dynamic mechanical analysis of resulting thermosets showed that large amounts of comonomers can be incorporated into the network resulting in a decrease of glass temperature but increase of fracture toughness. Finally, the fully cured thermosets resulting front the reaction of dicyanate of bisphenol A with different admixtures of dimercaptodiphenyl sulfide were characterized by cone calorimetry to get information about flame retardancy. The flame retardancy is influenced by incorporation of dimercaptodiphenyl sulfide into the triazine network only slightly. .