Flame retardants: a newcomer finds a niche.

Desire for halogen alternatives has brought Mg|(OH).sub.2~ from relative obscurity to a new popularity.

Over the last few years, there has been a growing interest in magnesium hydroxide as a flame retardant for plastics. Gradually spreading desire in some quarters for "halogen-free" solutions to flame resistance has been driving this interest in magnesium hydroxide, which was rarely mentioned as recently as five years ago.

One sign of broadening market interest is the growing roster of suppliers. Established producers like AluChem, Morton International and Solem were recently joined by Climax Performance Materials and Lonza, with Alcan Aluminum waiting in the wings.

Major applications so far have been in wire and cable (PVC, XLPE, EVA and EPDM), though suppliers see potential in injection molded PP, PET and PBT.

COMPLEMENTS ATH

Magnesium hydroxide is a nontoxic inorganic product (either natural or synthetically precipitated), with low corrosiveness, smoke-suppressant properties, and relatively low cost. It retards combustion by absorbing heat endothermically, cooling the substrate, promoting char formation, and ultimately decomposing to release its water of hydration, similar to alumina trihydrate (ATH). But magnesium hydroxide decomposes at a higher temperature--around 540 F, vs. around 392 F for ATH. That permits it to be used in higher temperature processing of thermoplastics. Mildred Hastbacka, senior marketing manager for Morton International, adds that magnesium compounds generally suppress smoke better than ATH.

Both Mg|(OH).sub.2~ and ATH are fillers that require significantly higher loadings (typically 30-60% by weight) than other types of flame retardants to be effective. Morton's Hastbacka says minimum effective loadings of magnesium hydroxide are 8-15 phr. "We see noticeable reduction of smoke evolution at this level, but we recommend 30 phr for obvious effect on flame suppression and rate of flame spread."

Suppliers caution that magnesium hydroxide should not be viewed as an alternative to ATH. Since Mg|(OH).sub.2~ can cost two to three times more, there would be no reason to prefer it in any application where ATH functions satisfactorily. Rather, they say, magnesium hydroxide may be serve as an alternative to halogenated additives in some applications.

Kim Crane, market manager for Solem Div. of J.M. Huber, which supplies both Mg|(OH).sub.2~ and ATH, stresses that "people will continue to use ATH wherever they can," due to its lower cost and greater ease of processing. "Generally, we feel that the price of magnesium hydroxide and the fact that it tends to have some processing difficulties makes people stay with ATH. However, in situations where one wants to run an extruder faster; where a polymer with higher processing temperature is involved; or, if someone is having problems with smoke suppression, then they will want to look at magnesium hydroxide."

NOT ALL CREATED EQUAL

Magnesium hydroxides are not all "created equal," suppliers point out. There are two major classes--natural grades derived from brucite mineral, and synthetically precipitated grades. Both classes come with various purity levels, particle sizes, and surface coatings.

Says Morton's Hastbacka, "Magnesium hydroxides can vary terrifically in how they react with a polymer." In general, she says, the synthetic grades have fewer contaminants than the natural grades. Synthetic grades are also generally more expensive--around $1/lb, vs. 50-60|cents~/lb for natural grades.

Kim Crane of Solem notes, "You have levels of contaminants in the natural grades that some polymers cannot take. Sometimes, if the polymer system's smoke and flame retardancy requirements are fairly low, then we might suggest a natural grade on the basis of cost." Solem's Zerogen product line based on magnesium hydroxide is, like Morton's, primarily synthetic, although Crane says his company is been looking into natural grades as well.

Tony Barnhorst, sales manager for AluChem, maintains that the company's natural brucite has 93-94% purity, "And we are now moving to even higher purity as well as lower particle size."

Lonza, a relatively new player, offers synthetic Mg|(OH).sub.2~ flame retardants, which boast 97-98% purity with few or no calcium ions on the surface of the material. According to plastics market manager Steve Johnson, "This is because we do not use sea water as a starting material when we make our grades. Synthetic grades that are derived from sea water have a lot of calcium and sodium ions that come out to the surface, which make it difficult to process. Our grades are easier to coat, and as a result, easier to process."

Ray Shaw, marketing manager for Alcan Chemicals, which will make its official entry into the business within the next year, says the company will be offering both natural ore-derived grades and precipitated types. "We have developmental grades now of 1-micron particle size. Our highest quality products will be priced up to $1.20/lb."

Climax, which recently entered the business, offers both natural and synthetic grades in a price range of 50|cents~ to $1.50/lb.

FUTURE DIRECTIONS

Suppliers indicate they are all working on new Mg|(OH).sub.2~ product developments, aiming to address processing problems by making improvements in particle sizes and/or surface coatings. One area that is already attracting considerable R&D activity is combining Mg|(OH).sub.2~ with other flame retardants. Lonza's Johnson says, "We have been working to reduce loadings by combining Mg|(OH).sub.2~ with red phosphorous, for example."

Says Shaw from Alcan, "There can be beneficial effects of combining magnesium hydroxide with another flame retardant in order to improve that flame retardant's smoke characteristics."

Although relatively little research data is available, suppliers of both Mg|(OH).sub.2~ and halogenated flame retardants are intrigued by suggestions that the two can work together to reduce smoke generation and toxic acid-gas emissions. Sources such as Morton's Hastbacka think that this acid-scavenging effect has been established with PVC, but there is little evidence yet as to whether such benefits occur with brominated flame retardants in other polymers. The subject has aroused intense interest, however, and is likely to attract further experimental work.

More New Products

Zinc borate: Whereas U.S. Borax's standard Firebrake ZB grade releases its bound water at around 550 F, a brand-new version, Firebrake 415, is stable to almost 780 F. That's said to suit it to specialty engineering thermoplastics. Also new is Firebrake 500, a dehydrated version that is stable to 930 F and reportedly has proved effective in halogen-free nylon 46, polyetherketones, and polyethersulfones.

In addition, U.S. Borax has a new experimental extra-fine grade of Firebrake ZB, designated XPI-228, which has an average particle size of 2 microns and virtually all particles below 6 microns.

Buckman Laboratories has a new hydrated borate called Bulab Flamebloc that's said to act as a synergist in formulations containing phosphorus and/or halogen. In many cases, it reportedly can replace antimony trioxide on an equal weight basis. It has a decomposition temperature of about 392 F and costs 87|cents~ to $1.50/lb, depending on the quantity.

ATH: Aimed at use in cast onyx is KC-31 (38-micron average size), the first in a planned line of white ATH from Georgia Marble, which will eventually span an average size range of 3-40 microns.

For PVC: Anzon Inc. recently added a sixth grade to its Trutint line of low-tinting antimony trioxides. New Trutint 40 has the lowest tinting strength in the line and 2.6-micron average particle size. It's aimed at PVC film and sheeting for automobiles, upholstery and wall coverings.