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Zinc borate

Zinc borate
Identifiers
UNII
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Zinc borate refers to a family of inorganic compounds consisting of borate of zinc. They are white solids with the formulas 4ZnO·B2O3·H2O, ZnO·B2O3·1.12H2O, ZnO·B2O3·∼2H2O, 6ZnO·5B2O3·3H2O, 2ZnO·3B2O3·7H2O, 2ZnO·3B2O3·3H2O, 3ZnO·5B2O3·14H2O, and ZnO·5B2O3·4.5H2O. They are coordination polymers consisting of zinc(II) centers bonded to cyclic boron oxide rings.[1]

Part of one zinc borate showing the chain of boron-oxide rings that bind to the zinc ion[1]

Variants

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Several variants of zinc borate exist, differing by the zinc/boron ratio and the water content:[2]

The hydrated variants lose water between 290–415 °C.

Uses

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Zinc borate is primarily used as a flame retardant in plastics and cellulose fibers, paper, rubbers and textiles. It is also used in paints, adhesives, and pigments. As a flame retardant, it can replace antimony(III) oxide as a synergist in both halogen-based and halogen-free systems.[3] It is an anti-dripping and char-promoting agent, and suppresses the afterglow. In electrical insulator plastics it suppresses arcing and tracking.

In halogen-containing systems, zinc borate is used together with antimony trioxide and alumina trihydrate. It catalyzes formation of char and creates a protective layer of glass. Zinc catalyzes the release of halogens by forming zinc halides and zinc oxyhalides.

In halogen-free system, zinc borate can be used together with alumina trihydrate, magnesium hydroxide, red phosphorus, or ammonium polyphosphate. During burning the plastics, a porous borate ceramics is formed that protects the underlying layers. In presence of silica, borosilicate glass can be formed at plastic burning temperatures.[4]

Zinc borate is used in polyvinyl chloride, polyolefins, polyamides, epoxy resins, polyesters, thermoplastic elastomers, rubbers, etc. It is also used in some intumescent systems.[5]

Zinc borate has synergistic effect with zinc phosphate or barium borate as a corrosion inhibitor pigment.

Zinc borate acts as a broad-spectrum fungicide in plastics and wood products.[6]

Zinc borate can be used as a flux in some ceramics. In electrical insulators it improves the ceramics properties.

References

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  1. ^ a b Schubert, David M.; Alam, Fazlul; Visi, Mandana Z.; Knobler, Carolyn B. (2003). "Structural Characterization and Chemistry of the Industrially Important Zinc Borate, Zn[B3O4(OH)3]". Chemistry of Materials. 15 (4): 866–871. doi:10.1021/cm020791z.
  2. ^ "Flame Retardants Fact Sheet, Zinc borate" (PDF). Cefic. January 2006. Archived from the original (PDF) on 9 August 2011. Retrieved 6 February 2010.
  3. ^ "Zinc Borate (Boric Acid, Zinc Salt)". Chemical Land21. Retrieved 6 February 2010.
  4. ^ "Firebrake zinc borates for plastics". Luzenac Group. Retrieved 6 February 2010.
  5. ^ Bellingham, F. (25 November 1992). "Zinc borate in intumescent paints". European Polymers Paint Colour Journal. Retrieved 6 February 2010.
  6. ^ "zinc borate (Firebreak ZB) EPA Pesticide Fact Sheet 10/91". Pesticide Management Education Program. 1 October 1991. Retrieved 6 February 2010.
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