Zinc Borates: 30 Years of Successful Development as Multifunctional

Chapter 18

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Zinc Borates: 30 Years of Successful Development as Multifunctional Fire Retardants Kelvin K . Shen U.S. Borax Inc., 26877 Tourney Road, Valencia, CA 91355

Zinc borates can function as flame retardant, smoke suppressant, afterglow suppressant, and anti-tracking agent in both halogen-containing and halogen-free polymers. This paper will review the development and recent advances in the use of these multifunctional fire retardants in electrical/electronic, transportation, and building product applications. The mode of action of these fire retardants in both halogen-containing and halogen-free polymers will also be reviewed.

Introduction The use of an organic chlorine or bromine source to impart fire retardancy to polymers is well known in the plastics and rubber industries. To further enhance the fire test performance of halogen-containing polymers, antimony oxide is usually used as a synergist. In recent years, however, much effort has been expended to find either partial or complete substitutes for antimony oxide. This effort has been spurred by the desire to achieve better smoke suppression, better cost/performance balance, and by environmental concerns. One of the most commercially successful substitutes is zinc borate (1-7). Depending on the reaction conditions, a host of zinc borates with different mole ratios of Z n O : B 0 : H 0 can be produced. In the 1970s, a unique form of zinc borate 2

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© 2001 American Chemical Society

In Fire and Polymers; Nelson, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

229 with a molecular formula of 2 Ζ η Ο 3 Β 0 · 3 . 5 Η 0 was discovered (5). In contrast to previously known zinc borates, this material (known in the trade as Firebrake®ZB), is stable to 290°C (Figure 1). During the past thirty years, it was developed into a major commercial fire retardant used in both halogencontaining and halogen-free polymer systems. In recent years, due to the demand of high production throughput and thin-walled electrical parts, engineering plastics are being processed at increasingly higher temperatures. To meet the market demand of engineering plastics, U.S. Borax recently developed an anhydrous zinc borate, Firebrake®500 ( 2 Z n O 3 B 0 ) , that is stable to at least 450°C and 4 Ζ η Ο · Β 0 · Η 0 , Firebrake® 415, that is stable to about 415°C. This paper will review recent advances in the use of zinc borates as multifunctional fire retardants in polymers. Special emphasis will be on electrical/electronic, transportation, and building material applications.


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Figure 1. Thermogravimetric analysis of zinc borates.

Results and Discussion Zinc borates are multifunctional fire retardants. They can provide the following benefits in fire retardant polymer systems. • Synergist of most halogen sources - They are synergists of both chlorine and bromine-containing flame retardants. Their efficacy depends on the type of halogen source (aliphatic vs. aromatic) and the base polymer used.


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Synergist of antimony oxide - In certain polymers and in the presence of a certain halogen source, zinc borates can display synergistic effects with antimony oxide in fire retardancy. Partial or complete replacement of antimony oxide - Their efficacy depends on both the polymer used and the halogen source used. Smoke suppressant - In contrast to antimony oxide, zinc borates are predominately a condensed phase flame retardant and can function as smoke suppressants. Afterglow suppressant - The B 0 moiety in zinc borates are responsible for its afterglow suppression effect. Fire retardancy synergism with alumina trihydrate (ΑΤΗ) in many halogencontaining systems. Promoter of char formation and prevent dripping - The zinc oxide moiety of zinc borates can promote char formation with halogen sources; the B 0 moiety can stabilize the char. Improvement of the Comparative Tracking Index (CTI) - The B 0 moiety of zinc borates is believed to play a major role for this unique property. Formation of porous ceramic with ΑΤΗ that act as an insulator in protecting the underlying polymers. 2

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are • • • • • •

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Some of the recently discovered benefits from U.S. Borax and the literature as follows. Reduce rate of heat release Improve aged elongation properties of polyolefins Improve thermal stability of halogen/antimony oxide system Improve corrosion resistance by replacing antimony oxide Improve laser marking quality of polymers Function as flame retardants in certain halogen-free systems

Zinc borate ( 2 Ζ η Ο · 3 Β 0 · 3 . 5 Η 0 ) 2

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This zinc borate is of special commercial importance (Figure 2) (5). It is the most widely used among all of the known zinc borates; it has been used extensively in P V C , nylon* epoxy, polyolefin, and elastomers (1-4). It can function as a flame retardant, smoke suppressant, afterglow suppressant, and anti-tracking agent. For example, the use of this zinc borate as a smoke suppressant in flexible P V C is illustrated in Figure 3. In a recent Cone Calorimeter study in flexible P V C , partial replacement of antimony oxide with the zinc borate can not only reduce the rate of heat release, but also carbon monoxide production at a heat flux of 35 k W / m (Figure 4) (8). 2


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"415°C. It is recommended for use in engineering plastics as a synergist of halogen sources. It can also be used as a smoke suppressant in flexible P V C . Interestingly enough, a recent study showed that partial replacement of magnesium hydroxide with this zinc borate in halogenfree E V A results in the formation of a rigid residue. Cone calorimeter test showed that the zinc borate can also decrease Rate of Heat Release and delay the second heat release peak (26).

Mode of action Zinc borate in halogen-containing systems such as flexible P V C , is known to markedly increase the amount of char formed during polymer combustion; whereas the addition of antimony oxide, a vapor phase flame retardant, has little effect on char formation (1). Zinc borate can react with hydrogen halide, released from thermal degradation of a halogen source, to form zinc chloride, zinc hydroxychloride, boric oxide, water and possibly a small amount of boron trichloride. The zinc species remaining in the condensed phase can alter the pyrolysis chemistry by catalyzing dehydro-halogenation and promoting crosslinking between polymer chains, resulting in increased char formation and decrease in both smoke production and flaming combustion. The released boric oxide, a low melting glass, can stabilize the char and inhibit afterglow through glass formation. The water released between 290-450°C (accompanied by an endothermicity of 500 Joules/gram) can promote the formation of a foamy char. The mode of action between zinc borate and an aromatic halogen source is, however, not clearly understood. In halogen-free systems, previous study showed that partial replacement of ΑΤΗ with zinc borate ( 2 Z n O 3 B 0 - 3 . 5 H 0 ) in E V A can favorably alter the oxidative-pyrolysis chemistry of the base polymer, as shown by the decrease in the exothermicity in the oxidative pyrolysis range and the delay of the peak oxidative-pyrolysis weight loss rate (17). The combination of ΑΤΗ and zinc borate can form a porous and ceramic-like residue at temperatures above 550°C (probably in the range of 700-800°C). This residue is an important thermal insulator for the unburned, underlying polymer (Figure 5). Figure 6 shows the use of zinc borate alone in E V A can result in the formation of a glassy residue. It should be noted that, due to the flaming combustion at the early stage of the air pyrolysis, the sample is actually being exposed to temperature much higher than 550°C. 2

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Figure 5. Scanning electron micrographs (6000X) of residues of air-pyrolyses of EVA containing the zinc borate (2ZnO'3B 0 '3.5H 0) (33%) and ΑΤΗ (33%). At 500°C (top picture), a powdery residue was obtained; no sintering took place between the zinc borate (small particles) and ΑΤΗ (large particles). At 550°C (bottom picture), a hard and porous residue was obtained. The zinc borate acted as a sintering aid. 2

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Figure 6. Scanning electron micrographs (6000X) of residues of air-pyrolysis of EVA containing the zinc borate (2ZnO -3B 0 -3.5H 0) (50%) but no ATH. 2

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Conclusions •

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Zinc borates are multifunctional fire retardants. They can function as a flame retardant, smoke suppressant, afterglow suppressant, and anti-tracking agent. They can display synergistic/beneficial effect with other synergists or additives such as antimony oxide, ΑΤΗ, magnesium hydroxide, red phosphorus, and ammonium polyphosphate in both halogen-containing and halogen-free polymers. The choice of optimal synergistic combination depends on the base polymer and halogen source used (i.e. additive vs. reactive approach, aromatic vs. aliphatic halogen source). Zinc borates alone can function as fire retardants in some polymers.

References 1. 2. 3. 4. 5. 6. 7.

Shen, K.K. Plastics Compounding, 1985,8(5), 66-78. Shen, K.K.; O’Connor, R. in “Plastics Additives”, Pritchard, G., Ed.; Chapman & Hall, London, 1998, pp 268-286. Shen, K.K.; Griffin, T.S. in “Fire and Polymers”-ACS Symposium Series 425; Nelson, G.L., Ed.; American Chemical Society, 1990, pp. 157-177. Shen, K.K.; Schultz, D , “Rubber Processing and Performance,”Hanser Publication, 2000 (in press). Nies, Ν.; Hulbert P. U.S. Patent 3,549,316 and 3,649,172. Shen, K.K.; Ferm D.J. Sixth BCC Conference on Fire Retardancy, Stamford, C T ; 1995. Ferm, D.J.; Shen K.K. Proceedings International Conference on Fire Safety, San Francisco, 1992.



239 8. Ferm, D.J.; Shen, K.K. Plastics Compounding, 1994, 17(6)40-44. 9. Research Disclosure, June 1998/737. (RD 0410059). 10. Shen, K.K.; Ferm D.J. Eighth BCC Conference on Fire Retardancy, Stamford, Conn., 1997. 11. Markezieh, R.; Ilardo, C. SPE ANTEC Proceedings, 1988, 1412. 12. Miyabo, Α.; Koshida, R. PCT Int. Appl. WO 9,814,510, 1998. 13. Martens, M.M; Koshido, R; Tobin, W ; Willis, J. PCT Int. Appl. WO 9,947,597, 1999. 14. Martens, M.M.; Kasowski, R; Cosstick, K.B; Penn, R.E. PCT Int. Appl. WO 9,723,565, 1997. 15. Yadoshima, S Japan Kokai Tokkyo Koho JP 03,259,938, 1991. 16. Japan Kokai Tokkyo Koho JP 11,189,711-A, 1999. 17. Shen, K.K. Plastics Compounding, 1988, 11(7), 26-32. 18. Bourbigot, S; LeBras, M.; Leeuwendal, R.M; Shen, K.K.; Schubert, D.M. Polym. Degrad. Stab., 1999, 64, 419-425. 19. Japan Kokai Tokkyo Koho JP 58,109,546, 1983. 20. Kamikooryma, Japan Kokai Tokkyo Koho JP 06,107,914, 1994. 21. Istvan, B . ; Marosi, G.; Peter, Α.; Istvan.; Andras, T.; M. Maatoug.; Karoly, S. Muanyag Gumi 1997, 34, 237-243. 22. Chandrasekaran, S.; Kundel, N . K . ; Garg, B . ; Chin, H . B . U.S. Patent 4,957,961 1990. 23. Kelley, W.; Matzner, M.; Patel, S. PCT Int. Appl. WO 91 15,539, 1991. 24. Kato, H. European Patent Application 0 675 001 A1 1995. 25. Schubert, D . U.S. Patent 5,342,553 and 5,472,644. 26. Carpentier, F.; Bourbigot, S.; LeBras, M . ; Delobel, R. private communication.


Zinc Borates: 30 Years of Successful Development as Multifunctional

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