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Antimony oxide price spiral draws attention to alternatives.

Driven by an unbalanced supply-demand situation, prices of antimony oxide have climbed steeply this year with no end in sight. The result has been a scramble to reformulate where possible. Compounders that haven't actively considered alternative flame-retardant synergists since the last time antimony experienced unstable market conditions are reacquainting themselves with the likes of zinc borate and other zinc compounds, as well as compounds of molybdenum and tin.


Since late last year, prices of antimony metal have more than tripled while prices of the oxide have more than doubled. Through most of 1993, antimony oxide sold for $1/lb or a bit less. Prices now are about $3/lb and could go higher this month. Antimony oxide suppliers, consultants and other industry sources do not expect any relief soon.

But as in past instances (in the late- 1970s and mid-'80s) where antimony went through similar dramatic price/supply fluctuations, this situation will stabilize eventually. When that happens, one FR synergist supplier expects antimony oxide to hover around $2/lb. "I doubt we'll see the former lower levels," he says.

The main cause of the price increases is price setting by central planners in the People's Republic of China, the largest producer of antimony metal.

However, China and another producer, Bolivia, are suffering from depletion of reserves, while mine closings in places like Canada, Mexico, Bolivia, and Turkey have contributed to shortages in antimony metal. At the same time, demand is strong in the U.S.- up 10% so far this year - and in the Far East and developing countries.

Industry observers expect that old mines, particularly in Central and South America, will be reopened. But the time required to do so ranges from six months to a more likely 12-18 months.


Despite the runup in prices, no one expects to see large-scale replacement of antimony oxide in the near future. One industry source guesses that serious efforts at substitution won't come until and unless the oxide reaches $5.50/lb. For now, most of the reformulation activity is aimed at lowering the amount of antimony used - either by increasing the halogen content in the case of brominated FR additives or by partially replacing the synergist with another additive.

According to one supplier of brominated FR compounds, formulators have traditionally used bromine and antimony oxide at a 3:1 ratio, respectively. But with the rising cost of antimony oxide most are now going to a 4:1 ratio. "Three to one is believed to be the ideal ratio. An increase of the halogen fraction is likely to decrease the flame-retardant efficiency of the additive package but it does allow for cost containment," notes Dr. Marcelo Hirschler, a consultant on flame retardants based in Rocky River, Ohio.

According to consultant James Innes of Flame Retardants Associates, Naperville, Ill., the first major target for partial antimony replacement is PVC, as it is the most cost-sensitive application for the synergist. Other targets include polyolefins, PS, ABS, and engineering resins such as nylon and PBT. However, far less research has been done on synergism with bromine flame retardants than with PVC.

The consensus is that no alternative synergist is a true drop-in substitute for antimony oxide, making combination systems with antimony more likely. The more stringent the fire test that an end product must pass, the lower the likelihood that any alternative compound alone will suffice, notes Hirschler.


"When people are looking to reformulate, we always start with zinc borate because it is still selling at less than half the price of antimony oxide. In flexible PVC, you can replace up to 50% of the antimony, depending on the applications," says Larry Musselman, technical director of the Polymer Additives Group of minerals supplier R.J. Marshall, Southfield, Mich. (The company supplies magnesium hydroxide, alumina trihydrate, molybdenum compounds, and a family of low-smoke inorganic complexes.)

Based on current market prices for zinc borate at $1.00-1.29/lb, partial replacement of antimony oxide with zinc borate can result in a cost reduction of 25-40% of the total synergist package.

Zinc borate can replace antimony oxide in whole or in part, depending on the resin and the flame-resistance specification, says Dr. Kelvin Shen, market development manager at leading zinc borate producer U.S. Borax, Inc., Valencia, Calif. "For example, in flexible PVC you want to use a high ratio of zinc borate to antimony oxide to achieve low smoke and good flammability performance."

To address the supply shortage and help users cut costs, Anzon Div. of Cookson Specialty Additives, Philadelphia, is working on synergistic systems of antimony oxide with zinc borate for flexible PVC applications such as wire and cable. Anzon, a long-time antimony oxide supplier, recently acquired the zinc borate business of Climax Metals Co., Englewood, Colo. (CIRCLE 26)

Beyond PVC, there is potential for replacing antimony with zinc borate in most halogenated systems, including polyolefins and styrenics, at typical use levels of 3-15 phr, says Shen. "In HIPS and ABS, you can replace up to 50% of the antimony and in PVC, nylon, or epoxy you can completely replace it with zinc borate."

Unlike antimony oxide, zinc borate also functions as a smoke and afterglow suppressant, and in most systems, it works synergistically with antimony oxide. In other words, flame resitance may be improved by partial substitution of antimony with equal parts of zinc borate. HIPS and ABS are exceptions in that synergism has not been observed, but fire-test performance is not reduced with zinc borate/antimony combinations, Shen says.

Besides its standard Firebrake ZB grade, U.S. Borax has experimental Firebrake 415 and 500 grades, both with higher thermal stability (over 770 F) for use in engineering resins. (CIRCLE 30)


Molybdenum compounds are other synergists that can be used to partially or totally replace antimony oxide, says consultant Innes. Like zinc borate, molybdenum compounds function synergistically with halogen flame retardants and suppress smoke as well as flame. And as with zinc borate, most experience with molybdenum has been in PVC, particularly flexibles. Innes says it's "easy" to meet some less stringent flame specifications with molybdenum compounds; but in "applications where fire tests are very tough, such as plenum wire and cable jacketing, you cannot totally replace antimony."

Pure molybdenum compounds that can be used as flame and smoke suppressants are supplied by Climax Molybdenum Co., Pittsburgh, and R.J. Marshall's Polymer Additives Group. They typically cost $4-5/lb. (CIRCLE 43)

Pure molybdenum compounds from Climax Molybdenum include molybdenum trioxide, which sells for a bit under $4/lb, and ammonium octamolybdate, priced at $4-5/lb. Senior technical sales representative Bill Kennelly says these compounds can replace up to 50% of antimony oxide without a reduction in LOI (oxygen index). Kennelly says a side advantage is a drastic reduction in smoke. Besides flexible and rigid PVC, pure molybdenum compounds have been shown to work as total replacements for antimony oxide in brominated epoxies and as partial replacements in thermoset polyesters, Kennelly says. "Generally, these compounds work best where the halogen is part of the backbone of the polymer rather than applied as an additive," he adds.

Sherwin-Williams Chemicals, Coffeyville, Kans. (for which Innes consults), supplies Kemgard complex inorganic compounds of molybdenum with calcium and/or zinc. These molybdenum compounds are extended on an inert pigment core to lower their cost (92cents-$1.29/lb), according to technical services manager Charles Simpson. These products are said to provide smoke-suppressant performance similar to pure molybdenum compounds but require higher use levels - typically 2-10 phr vs. 1-5 phr. At current market prices, partial replacement of antimony oxide with Kemgard can reduce flame-retardant costs by 20-50%, Simpson says.

The Kemgard family includes grades 911A and 425 molybdenum/calcium/zinc complexes for PVC plastisols, wire/cable coatings, wall coverings, and calendered films. Kemgard 911C molybdenum/zinc complex is for flexible and rigid PVC where increased thermal stability is required. Kemgard 981 is a zinc-phosphate/molybdenum additive for calendered PVC upholstery and polyolefins. (CIRCLE 44)


Tin compounds such as zinc stannate are the alternative that most closely approaches a direct replacement for antimony oxide in halogen flame-retardant systems, according to James Innes and other industry sources. As is the case with pure molybdenum compounds, however, they come at a cost premium.

Notes consultant Hirschler, "The key advantage of tin compounds over antimony oxide is that they also act as very effective smoke suppressants, while antimony may actually increase smoke production per unit of mass burnt. And there's no need for any significant change in use levels."

Cleveland-based Alcan Chemicals has been offering Flamtard S zinc stannate (51% tin content) since 1989 as a drop-in replacement for antimony oxide, according to marketing manager Ray Shaw. The product sells for $5.50/lb. A lower-cost version with lower temperature stability is also now offered. This Flamtard H is a zinc hydroxy stannate designed for PVC that is processed below 400 F. It sells for $4.50/lb. Typical use range of these products is 1-5 phr.

Alcan has established that Flamtard S works well in both flexible and rigid PVC, chlorinated PE (used in foams and some wire/cable compounds), brominated unsaturated polyester resins for composites, nylon, and PE. Regarding the latter, Shaw recommends Flamtard S for use with chlorinated paraffins instead of bromine compounds. (CIRCLE 45)


For the last year, R.J. Marshall's Polymer Additives Group has been marketing a family of proprietary flame-retardant complexes as partial or total replacements for antimony oxide in many halogen-containing systems, including urethanes, polyolefins, and nylon. "Most of our work has involved flexible and rigid PVC, but we have shown it to work successfully in other systems," he says.

The Charmax LS family includes four grades that have both an endothermic (heat-absorbing) mechanism, like ATH and magnesium hydroxide, and a char-forming mechanism, typical of zinc borate or molybdenum compounds. Says Musselman, "By using a range of elements to match an endothermic with a char-forming mechanism, we can address specific requirements for each polymer system. We're trying to deal with both flame retardancy and smoke suppression. We can get the same results with Charmax LS at lower cost as we can with our straight molybdenum."

Charmax LS complexes may include different combinations of molybdenum, zinc borate, ATH, magnesium hydroxide, or tin compounds, such as zinc stannate. These complexes are said to avoid the problems of using their individual components singly, such as degradation of physical properties and loss of uv or thermal stability. Charmax LS compounds sell for $1.00-1.75/lb and are typically used at 2-10 phr. (CIRCLE 46)
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Title Annotation:Technology News
Author:Sherman, Lilli Manolis
Publication:Plastics Technology
Date:Sep 1, 1994
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