Thermal properties magnesium hydroxides

Reasons for use abrasion resistance, cost reduction, electric conductivity (metal fibers, carbon fibers, carbon black), EMI shielding (metal and carbon fibers), electric resistivity (mica), flame retarding properties (aluminum hydroxide, antimony trioxide, magnesium hydroxide), impact resistance improvement (small particle size calcium carbonate), improvement of radiation stability (zeolite), increase of density, increase of flexural modulus, impact strength, and stiffness (talc), nucleating agent for bubble formation, permeability (mica), smoke suppression (magnesium hydroxide), thermal stabilization (calcium carbonate), wear resistance (aluminum oxide, silica carbide, wollastonite)... 

Properties. The physical properties of magnesium hydroxide are Hsted in Table 8. The crystalline form of magnesium hydroxide is uniaxial hexagonal platelets (Fig. 4). Magnesium hydroxide begins to decompose thermally above 350°C, and the last traces of water are driven off at higher temperatures to yield magnesia. 

Hydroxides, hydroxy carbonates, and hydrates of aluminum, calcium, and magnesium that potentially meet these requirements are shown in Table 7.1, together with relevant thermal properties and gaseous products evolved on decomposition. However, of those in commercial use, aluminum hydroxide makes up about 90% of the market by tonnage, with magnesium hydroxide and basic magnesium carbonate products being used in niche applications. 

In the composite, the magnesium hydroxide is selectively dispersed in the PPE domains with an average domain size of about 1.5 p,m. The surface treatment of magnesium hydroxide by dodecanoic acid with polystyrene-fetoci -poly(ethylene-co-butylene)-i /ocA -polystyrene as a compafibilizer significantly improves the macroscopic mechanical and thermal properties of the composites in a synergetic manner. 

Oyama HT, Sekikawa M, Shida S. Effect of the interface structure on the morphology and the mechanical, thermal, and flammability properties of polypropylene/poly(phenylene ether)/magnesium hydroxide composites. Polym Degrad Stabil 2012 97(5) 755-65. 

The main sectors for magnesium hydroxide use are in elastomers and thermoplastics, since they cause excessive thickening in the main thermoset application of unsaturated polyester systems. It is also much more expensive than ATH with which it shares comparable properties and flame retardancy, and so ATH will be preferred by processors wherever possible. As a result the principle opportunities are in applications where the extra thermal stability is essential, mainly in PP and polyamides. 

It has been observed from the above discussion that mechanical, physico-chemical and fire retardancy properties of UPE matrix increases considerably on reinforcement with surface-modified natural cellulosic fibers. The benzoylated fibers-reinforced composite materials have been found to have the best mechanical and physico-chemical properties, followed by mercerized and raw Grewia optiva fibers-reinforced composites. From the above data it is also clear that polymer composites reinforced with 30% fibers loading showed the best mechanical properties. Further, benzoylated fibers-reinforced composites were also found to have better fire retardancy properties than mercerized and raw fibers-reinforced polymer composites. Fire retardancy behavior of raw and surface-modified Grewia optiva/GPE composites have been found to increase when fire retardants were used in combination with fibers. This increase in fire retardancy behavior of resulted composites was attributed to the higher thermal stability of magnesium hydroxide/zinc borate. 

Figure 17.4 Comparison of the thermal degradation properties of magnesium hydroxide and...

Magnesium hydroxide is a white crystalline material, with similar flame-retardant properties to ATH but with superior thermal stability. Until recently, commercial interest in its use as a flame-retardant filler was minimal outside Japan. This is because the only products generally available were either of poor quality or expensive. 

Although the thermal decomposition of magnesium hydroxide has been extensively studied in the literature the precise mechanism of decomposition is still not fully understood. Furthermore, there remains considerable controversy over the structure of the decomposition products. The bulk of the published work has Involved either crystallographic studies of the structural transformations involved (reviewed in [1,2]) or spectroscopic Investigations of the nature of the active sites generated (reviewed in [3,4)). Rather less use has been made of detailed adsorption isotherm measurements to elucidate the textural and surface chemical properties of either Mg(OH) itself, the products of its thermal decomposition or their rehydratlon products (a guide to recent work can be found in [5-9)). 

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