Page 364 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

5.44 CHAPTER 5

and polyurethanes. It is fairly expensive but reduces flammability very efficiently, without
creating smoke or toxic corrosive gases. (Elemental red phosphorus may be used as a
flame retardant, but its inherent flammability makes it tricky.)
5.7.3.2 Organic Bromine. This is an efficient flame retardant that can be greatly syner-
gized by addition of antimony trioxide. Since aliphatic bromine is too unstable for plastic
processing, preferred compounds are polybrominated diphenyl ethers for thermoplastics,
and tetrabromo bisphenol A and tetrabromophthalic anhydride for epoxies and polyesters.
In a fire, it does produce smoke and toxic corrosive gases, so this must be considered in
specific applications. In Europe, environmental concerns may limit the use of bromine.
5.7.3.3 Organic Chlorine. This is a good flame retardant, less expensive but less effi-
cient than bromine. It too is greatly synergized by antimony trioxide. A particularly popu-
lar compound is Dechlorane Plus, the adduct of hexachlorocyclopentadiene with
cyclooctadiene. Chlorinated paraffins are also widely used. Like bromine, chlorine may be
limited by problems with smoke, toxic corrosive gases, and environmental concerns.

5.7.3.4 Water of Hydration. A fireman uses water to put out a fire. A plastics chemist
can do the same. Alumina trihydrate Al(OH) is a low-cost filler that contains 35 percent
3
water, which it loses above 205°C. In a fire, evaporation of water removes so much heat
that it cools the plastic below the temperature at which it can burn and then surrounds it
with water vapor, which excludes atmospheric oxygen and thus chokes out the fire without
creating any smoke or toxic corrosive gases. Its main problem is that it loses water during
high-temperature plastic processing, so it is useful mainly in low-temperature processes
such as polyethylene, vinyl plastisols, epoxies, and thermosetting polyesters.
Magnesium hydroxide Mg(OH) contains 31 percent water, which it loses at above
2
320°C. This is stable enough for many plastic processes and provides just as good flame
retardance as alumina trihydrate, so it has become very popular in applications such as
polypropylene.
Both of these compounds must be used at very high concentrations to provide enough
water to put out a fire (typically 30 to 75 percent), at which they cause brittleness, so com-
pounders often add rubber to counteract the brittleness and then add more flame retardant
to protect the rubber.
5.7.3.5 Other Flame Retardants. Other retardants frequently mentioned in the litera-
ture include zinc borate, ammonium molybdate, other metal oxides, and melamine deriva-
tives. One particularly intriguing approach is intumescence, a mixture of a “carbonific”
such as pentaerythritol, an acid catalyst such as an amine phosphate, and a “spumific”
such as an amine or melamine, which react to form a foamed char that insulates the plastic
against the heat of the fire and also acts as a barrier to exclude atmospheric oxygen. Recent
research interest also includes nanoclay fillers, which are believed to trap polymer mole-
cules between their platelets, preventing degradation and thus inhibiting burning.
5.7.3.6 Method of Use. Compounders add flame retardants to the finished polymer;
these “additive” flame-retardants are 87 percent of the market. The other 13 percent of “re-
active” flame-retardants are built into the polymer molecule, either during polymerization
or in the curing agents added to thermosetting plastics.

5.7.4 Markets
The total world consumption of flame retardants is over 2 billion pounds per year. Half is
alumina trihydrate, one-quarter is organic bromine, with organic phosphorus, organic
chlorine, and antimony oxide in smaller amounts.

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