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Types of Mineral Filler Fire Retardants

Table 2 shows the main physical properties and chemical formulae of the main mineral filler fire retardants, using data derived from thermogravimetric analysis and differential scanning calorimetry (see section “Thermal Analysis Techniques”).

Metal Hydroxides

Aluminum hydroxide (ATH, Al(OH)3, often incorrectly called alumina trihydrate or aluminum trihydroxide) is the highest-tonnage fire retardant on the market as a combined result of its low cost and the need for high loadings (usually above 50%) to achieve satisfactory fire retardancy in a polymer composite (Beard 2007). Production methods for ATH involve the use of mineral bauxite, followed by a chemical process involving sodium hydroxide, which results in storage and disposal problems of an equal volume of a caustic red sludge by-product.

Magnesium hydroxide (MDH, Mg(OH)2), while less widely used than ATH (less than 5% by mass of total fire retardant market consumption) works in the same way as ATH, only it decomposes at temperatures approximately 100 °C higher (see Table 2).

The higher decomposition temperature of calcium hydroxide (~430 °C) suggests that it may perform well with polymers of higher thermal stability. In practice, however,

Table 2 Mineral fillers with potential fire retardant applications and corresponding decomposition data

Mineral filler

Decomposition

onset/°C

Enthalpy of

decomposition/

kJkg-1

Volatile content/% w/w

H2O

О

О

to

Total

Aluminum hydroxide, (ATH) Al(OH)3

180-200

1300

34.5

0

34.5

Magnesium hydroxide, (MDH) Mg(OH)2)

300-320

1450

31

0

31

Calcium hydroxide, (CDH) Ca(OH)2

430-450

1150

24

0

24

Hydromagnesite,

Mg5(CO3)4(OH)2.4H2O

220-240

1300

20

36

56

Huntite, Mg3Ca(CO3)4

400-420

980

0

50

50

Hydromagnesite and huntite as 60:40 mixtures, (HMH)

220-240a

990

11

43

51

Nesquehonite MgCO3.3H2O

70-100

1750

39

22

61

Magnesium carbonate subhydrate,

Mg°.CO2 (0.96)H2O(0.30)

340-350

Not available

9

47

56

Boehmite (AlO(OH))

340-350

560

15

0

15

decomposition begins as the hydromagnesite decomposes but the decomposition occurs over a temperature range from 220 to 750 °C (Hollingbery and Hull 2010)

calcium hydroxide does not perform well, due to the high thermal stability of the carbonate, which forms exothermically, in preference to the oxide (Rothon 2003).

 
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