Flame retardant additives are a very important business sector,
with annual world sales estimated at over 1.5m. tonnes and a
value of about 2bn. The importance of some minerals as
flame retardants - such as aluminium hydroxide - is
well known and has been discussed in some recent
IM articles (see
IM August 2007). What is not so well recognised is
that nearly all of the main commercial products are derived, at
least in part, from mineral resources, a link which is
discussed in this article.
Two types of flame retardant (FR)
additive can be recognised (primary and secondary) and both
types are included in the discussion.
Primary FR additives are those that
are capable of significantly reducing the flammability of
polymers in their own right. The main commercial products of
this type are:
-
Metal hydroxides and related products, principally
aluminium hydroxide;
-
Phosphorus and organic and inorganic phosphates;
-
Halogen compounds (brominated and chlorinated organics);
-
Expandable graphite.
There is also significant use of
chloro-phosphates, which confuses the statistics, as they
sometimes get counted with phosphates and sometimes as separate
entities.
Secondary FR additives have little
or no effect on their own, but significantly improve the
performance of primary FR types. The main products of this type
are:
The estimated share of the
principal products in the world market is given in Figures 1
and 2. The metal hydroxides are used at significantly higher
loadings than most of the other types, and this accounts for
their relatively high tonnages. Another feature worthy of
comment is the large change in ranking for brominated compounds
and antimony trioxide compared to metal hydroxides when looking
at tonnage and value. This reflects the relatively high price
for the former additives.
The European market differs from
the world one in having less use of halogens and antimony and
greater use of metal hydroxides, as concern about the smoke and
related issues associated with halogenated products is more
prevalent.
While increasing safety, the use of
flame retardants generally increases product costs and
compromises end use properties. Because of this, their use is
more prevalent in the developed world economies, especially
NAFTA, the European Union and Japan. Their use in the emerging
markets is now expected to increase rapidly. Indeed some
analysts predict double digit growth figures in China and
possibly India.
Figure
1: Estimated world consumption of main FR types (by
tonnage)
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Halogenated organic compounds
Chlorinated and especially brominated organics are the major
flame retardant products in use today. They largely function by
releasing halogen compounds into the vapour phase during a
fire. These interfere with the free radical processes in the
flame and this makes them very effective additives.
On the downside, the mechanism
results in high levels of smoke and corrosive gases, which has
led to concerns about the generation of toxic products. Despite
these limitations they continue to dominate much of the
market.
The chlorine used is ultimately
derived from deposits of salt (halite), which is usually
solution mined and then electrolysed to produce the chlorine.
Bromine is generally produced from bromide-rich brines by
treatment with chlorine gas. Various chemical processes are
used to turn the elemental chlorine and bromine into suitable
organic compounds.
Metal hydroxides
The main products of this type are aluminium and magnesium
hydroxides and basic magnesium carbonates. These function by
decomposing endothermically, with the release of water and/or
carbon dioxide, at temperatures close to where the host polymer
itself degrades. This both cools the article and dilutes the
combustible fuel.
This type of product is regarded as
environmentally friendly compared to most other technologies,
as it gives low levels of smoke and corrosive or toxic gases
under fire conditions. Already well established, these products
are growing faster than the overall market due to their green
credentials.
Currently the European market for
metal hydroxides is the most developed, due to greater concerns
over halogenated alternatives compared to other regions.
Figure 2: Estimated world consumption of main FR
types (by value) |
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Aluminium hydroxide
Aluminium hydroxide (AlOH3) with an estimated world
use of about 700,000 tpa has over 90% of the market for metal
hydroxide flame retardants and exists as the mineral gibbsite.
The main source of gibbsite is the ore bauxite, but this is not
pure enough to be used directly and virtually the entire
aluminium hydroxide product used is made synthetically to both
improve purity and to control particle size and shape.
The production method used is
essentially the same as that operated to make Bayer alumina;
indeed fire retardant grades were originally coproduced on the
same plant and significant amounts are still made this way
today.
In the process bauxite is dissolved
in sodium hydroxide to produce sodium aluminate. This solution
is then purified (the origin of the notorious red mud often
associated with alumina production). After purification, the
solution is seeded and cooled to precipitate the aluminium
hydroxide. This gives a relatively large sized particle which
can be milled down to more suitable sizes. A significant part
of the flame retardant grades is now made by re-dissolving
Bayer product, followed by further purification and
precipitation. This gives greater purity and better control of
particle size and shape.
There is also a niche market for
the monohydrate boehmite [AlO(OH)]. Again, while this occurs as
a natural mineral, the flame retardant product is made
synthetically with the ultimate mineral source usually being
bauxite.
The main limitation of aluminium
hydroxide is its decomposition temperature, which is about
200ºC. This is uncomfortably close to the processing
temperature of some polymers and this is where magnesium
hydroxide comes in.
Magnesium hydroxide
Magnesium hydroxide has a very similar fire retardant effect to
aluminium hydroxide but a significantly higher decomposition
temperature (about 300ºC). It is generally more expensive
to produce than equivalent grades of aluminium hydroxide and
this has restricted its market penetration.
Just as with aluminium hydroxide,
there is a direct mineral equivalent, known as brucite. In this
case some deposits of brucite are pure enough to enable
effective FR products to be produced by grinding, beneficiation
and drying. Such deposits are mainly found in China, but are
exported to most world markets. In some cases the brucite is
mixed with significant amounts of calcite, but such mixtures
still have some utility.
The natural brucite products have
limitations, however, and a significant amount of synthetic
product is also produced. Strangely, given the much smaller
size of the market, these synthetic methods are much more
varied than for aluminium hydroxide. They use a variety of raw
materials and also result in a wide variety of product
qualities.
The highest grade products are made
from concentrated and purified solutions of magnesium chloride.
This magnesium chloride solution can come from many sources. It
can be a natural brine or produced by solution mining (eg. such
as the method used by Kyowa in Veendam, Netherlands, ICL in
Israel and in the planned production facility of Nikochem in
the USSR), from sea water desalination (as at least one plant
in Japan), or by dissolving a magnesium mineral in hydrochloric
acid.
The main mineral in use today is
serpentine (magnesium iron silicate hydroxide). This is used by
Albemarle at a plant in Austria, where the serpentine is
leached with hydrochloric acid to produce a magnesium chloride
solution, which is then purified and subjected to
hydropyrolysis to give magnesium hydroxide and hydrochloric
acid. The acid is concentrated and recycled to the leach stage.
The leach co-produces silica gel which is also marketed. Other
magnesium silicates could probably be used in this process.
A number of other processes have
been seriously examined. Most use precipitation by adding lime
or dolomitic lime to sea water, a variant of a long established
process used to make magnesia for refractories. Another option
is to hydrate magnesium oxide produced by calcination of
magnesite (magnesium carbonate).
Worldwide consumption of all
magnesium hydroxide types is estimated at 50-60,000 tonnes,
with about half of this being used in Western Europe. The
market is about evenly split between the synthetic and natural
types. Use of natural brucite is expected to significantly
increase in China as its market develops, due to the
countrys indigenous resources. Use of synthetic products
in Eastern Europe is also expected to increase significantly if
Nikochems planned production facility in Russia is
realised (see IM March 2010:
Russia to start magnesium hydroxide production).
Basic magnesium carbonate
There are several forms of basic magnesium carbonate and one,
known as the mineral hydromagnesite
[Mg5(CO3)4(OH)2.4H
2O], has particularly useful properties for use as an FR
additive - it has been commercially developed by Minelco
Group. Hydromagnesite releases both water and carbon dioxide on
heating, with a decomposition temperature a little higher than
aluminium hydroxide.
As with brucite, there are good
deposits suitable for use after grinding, beneficiation and
drying. These almost invariably co-exist with the mineral
huntite [Mg3Ca(CO3)4] which is
difficult to remove but does not appear to seriously compromise
the fire retardant performance, even if present at up to 50%
w/w.
The main deposits being exploited
are in Greece and Turkey. There is also a small amount of
synthetic product made, but the present precipitation processes
do not give a totally satisfactory particle form.
Phosphorus and compounds
A number of phosphorus-containing products are used as fire
retardant additives. The simplest is phosphorus itself, in the
elemental form known as red phosphorus. This is quite
surprising at first sight, given the flammable nature of this
element, but is has proven to be a very effective product for
use in oxygen or nitrogen-containing polymers, especially
polyamides. In other polymers it has to be used with a
synergist.
Ammonium polyphosphate is also a
very effective flame retardant under the right conditions,
giving useful intumescent effects; again it is often used with
synergists.
The other large class of
phosphorus-containing flame retardants is the phosphate esters,
widely used in plasticised PVC, when high levels of flame
retardance are required. In some cases chlorophosphate esters
are used.
All of the above products are
ultimately derived from phosphorus-containing ores. The main
deposits of these ores are to be found in North Africa and the
USA, although China is the main producer today, followed by the
USA (IM December 2009:
Phosphate face-off).
Ammonium polyphosphate and the
phosphate esters are produced from phosphoric acid, which can
be derived from the rocks either by leaching them with
sulphuric acid or by heating with sand and coke to produce
white phosphorus and then hydrolysing this in water.
The white phosphorus route has the
advantage that the phosphorus is removed in the vapour phase
and so is obtained in a pure form, thus allowing quite low
grade ores to be used. Red phosphorus is produced from the
white phosphorus.
Expandable graphite
Expandable graphite (EG) is a very effective flame retardant in
some polymers, especially polyurethane foam. It is made from
natural graphite, which is converted to flake graphite by
grinding and flotation followed by chemical purification. The
flake graphite is then treated to intercalate an acid between
the graphite layers (sulphuric, nitric or acetic acid are
commonly used). This acid provides the mechanism for expansion
and the resulting flame retardancy. China and India have large
reserves of natural graphite.
Antimony trioxide
This is the main secondary flame retardant in commercial use.
It has little effect alone, but as a synergist it very
significantly increases the efficiency of halogen-containing
systems.
The main source of antimony
trioxide is the mineral stibnite (Sb2S3)
and this can be directly converted to the trioxide by reaction
with air or oxygen. Sublimation is used to purify the oxide,
especially to remove the toxic arsenic oxide (most stibnite
ores contain arsenic impurity). Some companies prefer to
produce the metallic element first and then oxidise it.
Most of the world production of
antimony metal and the oxide is carried out in China today, but
some metal is exported and converted elsewhere.
Borates
Borates, especially zinc borate, are another important class of
secondary flame retardants, most often used in conjunction with
halogenated systems. They are derived from various borate ores
of which there are four main ones; the sodium borates, tincal
and kernite, the calcium ore, colemanite, and the mixed sodium
calcium ore, ulexite. The largest deposits are in Turkey, the
USA and Russia. Turkey has about two thirds of the worlds
known reserves at over 1bn tonnes.
Others
There are a number of minor additives, largely of the secondary
type. The main ones are organo-clays (mainly based on
montmorillonite), molybdenum compounds, ferrocenes and zinc
stannates. Many of these are mainly seen as alternatives to
antimony trioxide which has had a very volatile price history
(see Mineral Pricing 101).
The organo-clays are mostly used as
synergists with metal hydroxides and some premixed versions of
aluminium or magnesium hydroxide are commercially available
from Albemarle.
Supply
Many of the flame retardants mentioned above account for quite
small tonnages compared to their mineral resources and are also
used in other, much larger, applications; so supply should not
be an issue in the medium or even long term.
The high volume FR products are the
halogenated ones, aluminium hydroxide, phosphorus-based grades
and the borates, but even here supply does not seem to be an
issue. Halogen-based flame retardants are derived from salt
deposits or brines and these are in abundant supply. Bauxite
reserves are plentiful and well spread, so supplies for
aluminium hydroxide production should also be adequate, despite
the high tonnages consumed. Both borate and phosphate reserves
are also plentiful, although of more limited geographical
distribution.
Table 1: Selected leading producers of flame retardant
additives
Company |
Location |
Products |
Albemarle Corp. |
Baton Rouge, Louisiana, USA |
Metal hydroxides, bromine and phosphorus
compounds |
Campine NV |
Beerse, Belgium |
Antimony trioxide |
Chemtura Corp. |
Philadelphia, Pennsylvania, US |
Brominated compounds |
Clariant International Ltd |
Muttenz, Switzerland |
Phosphorus compounds |
Dover Chemical Corp. |
Dover, Ohio, USA |
Chlorinated and brominated compounds |
Israel Chemicals Ltd |
Dead Sea, Israel |
Bromine, magnesium hydroxide, phosphorus
compounds |
Kyowa Chemical Industry Co. Ltd |
Kagawa, Japan |
Metal hydroxides |
Minelco Group |
Derby, UK |
Aluminium hydroxide, hydromagnesite, huntite |
Nabaltec AG |
Schwandorf, Germany |
Metal hydroxides |
US Borax (Rio Tinto Minerals) |
Boron, California, USA |
Borates |
Contributor: Professor Roger Rothon, Rothon
Consultants and Manchester Metropolitan University, UK.