Synthesis and characterization of trifluoromethyl-modified Schiff base flame-retardant PET copolyesters

A novel Schiff Base as a flame-retardant monomer (FSB, 5-((3,5-bis(trifluoromethyl) benzylidene) amino) isophthalic acid) with -CF 3 was designed and used to synthesize FSB n PETs (PET copolyester) by copolymerization, which shows good flame retardancy and anti-dripping. The introduction of the –CF₃ group increases the cyclization temperature of the FSB Schiff base unit by suppressing the cyclization reaction through its strong electron-withdrawing effect, which is 350–398 °C, higher than 250–348 °C from the PET copolyesters modified by Schiff Base without -CF 3 , indicating that FSB n PETs exhibits a broadened processing temperature window. And FSB also increased the glass transition temperature (Tg), reduced the melting temperature (Tm) and the crystallizability of FSB n PETs. FSB n PETs possessed good flame retardancy and anti-dripping properties, FSB 10 PET (7.4 mol% of FSB) owned V-0 level of UL-94 and 31% value of LOI without dripping phenomenon. The bond of -C-F had not broken into the air phase to make an obvious influence on the flame retardancy property of FSB n PETs, whose flame retardancy mechanism is still type of the condense flame retardancy. Additionally, in the new preparation process for FSB n PETs, the dicarboxylic acid structure of FSB could speed up the condensation polymerization between the BHET precursors, which could save 50% of the preparation time, showing an exciting economic advantage. The -CF₃ modified Schiff base (FSB) is an effective modifier for PET copolyesters, offering improved processing performance and good flame-retardant properties.