Zinc Borate (Flame Retardant, Smoke Suppressor)

Zinc borate (chemical formula 2ZnO·3B₂O₃·3.5H₂O, CAS 138265-88-0) is an inorganic flame retardant and smoke suppressant that functions synergistically with ATH, Mg(OH)₂, and halogenated flame retardants to significantly enhance fire performance beyond what any single additive can achieve alone.

Zinc borate acts through multiple mechanisms: at elevated temperatures it releases water (endothermic effect similar to ATH), produces a zinc oxide-borate glass layer on the burning surface that physically prevents oxygen access and inhibits combustion (char barrier effect), and zinc boron oxide in the gas phase inhibits radical chain reactions. The combination of these mechanisms makes zinc borate a highly efficient multifunctional additive that synergizes with halogen-containing flame retardants in the vapor phase and with mineral hydrates in the condensed phase.

In UPR, epoxy, and VER composite systems, zinc borate is typically used at 3–15 phr as a synergist alongside ATH (100–150 phr), enabling formulators to achieve passing UL 94 V-0, BS 476 Part 7 Class 1, or IMO FTP Code performance at lower total ATH loading. This reduces compound viscosity, improves mechanical properties, and lowers cost compared to ATH-alone formulations at equivalent fire performance. Zinc borate also effectively suppresses smoke density (ASTM E662) and afterglow, reducing toxic hazard in fire scenarios.