COMPOSITE MATERIAL OF IMPROVED FIRE RESISTANCE

Abstract: A nanostructured alumina is described which, due to its characteristics, imparts excellent fire resistant properties to composite polymer systems.

Description: Composite Materials of Improved Fire Resistance The present invention relates to new nanostructured particulate alumina and its use for improving the fire resistance in composite polymer systems. Fire safety of composite materials based on organic polymers is increasing in importance as structural plastics are used more and more to replace traditional materials such as metals which do not burn under normal fire conditions. In this regard, the outstanding Fire, Smoke and Toxicity (FST) properties of aluminium hydroxide have been widely documented. The overall concept of fire safety however also comprises the concept of fire resistance, i. e. the property whereby a composite material retains its physical integrity both during and as a consequence of a fire situation. Here, aluminium hydroxide is only partially successful. When heated above 200 °C, it releases ca. 34.5% of its weight as water vapour. Despite this loss of weight, the particles retain their external form and shape. In a clean burning system (highly desirable in terms of low smoke and toxicity of gases created) the organic phase will disappear leaving no residue which might otherwise bind the now aluminium oxide particles in situ. Composite materials based on systems which promote pronounced charring of the organic polymer will promote fire resistance. Such systems may involve the synthetic resin itself (e. g. phenolic resins where the fire load is relatively low) or additives based on phosphorus or phosphorus in combination with nitrogen containing compounds to create an intumescent effect. These macro charring systems have a major disadvantage however, they raise substantially the toxicity of the gases released in a fire situation. On a basic level, aluminium hydroxide is not compatible with such char forming systems for two reasons. Firstly, the release of fire fighting water vapour interferes with the ability of the organic system to build a stable char. Secondly, aluminium hydroxide in converting to the aluminium oxide goes through a pronounced 'active' phase which catalyses the oxidation of carbonaceous material to gaseous carbon dioxide with minimum little or no formation of the highly toxic intermediate, carbon monoxide. What is needed in the art is a means of imparting fire resistance to aluminium hydroxide containing composite materials without recourse to the use of inherently toxic char promoting additives. Object of the present invention therefore is to provide a material which does not comprise the drawbacks of the systems known in the art and which imparts excellent fire resistance properties to composite polymer systems. It was found that a nanostructured alumina as defined in claim 1 surprisingly meets these demanding requirements for fire resistance. The nanostructured alumina is characterised by an average particle size in the 50% range (d 50 ) of 1 μm to 5 μm, a specific surface area according to BET of 10 m 2 /g to 350 m 2 /g, preferably 10 m 2 /g to 200 m 2 /g, and by structural channels having a width of 1 to 100 nm, preferably of 10 to 50 nm. The nanostructured alumina can further be characterised by a particle size in the 10% range (d \ o) of 0.1 μm to 2 μm and in the 90% range (<i 0 ) of 2 μm to 10 μm and by a loss on ignition at 1000 °C (LOI) of 1 to 15%. The main mineralogical form of aluminium hydroxide is gibbsite, with the chemical formula Al(OH) 3 . The crystal habit of naturally occurring gibbsite is usually pseudohexagonal, tabular, while that of synthetic gibbsite (produced by the Bayer process) is determined by the conditions of crystallisation. Despite the size of the aluminium hydroxide crystals being of micron dimensions, the material is in fact highly polycrystalline and composed of crystallites of a significantly smaller size. Even so, it is at the atomic level that the key features of aluminium hydroxide as a base for creating a nanostructured material become evident. Millions (per cm 2 ) of structural channels within the crystal lattice both parallel and perpendicular to the c-axis provide the preferred routes for the removal of water which forms at temperatures above 200 °C. It was found that the structural channels already present in the aluminium hydroxide open up when the system is heated with the progressive loss of water causing the structure to shrink since the loss of mass is not accompanied by a decrease in the external dimensions of the particles. When the specific surface area of nanostructured alumina has dropped, the structural channels have opened up to such an extent that the channels become accessible to liquid molecules, with the widest channels having opened up perpendicular to the c-axis. Moreover, at higher temperatures any boehmite (aluminium monohydroxide, A1OOH) which may have formed during the heating process will have decomposed endothermically thereby releasing its associated water of crystallisation and stabilising the nanostructure. Preparation of the nanostructured alumina of the present invention may accordingly be accomplished by a heat treatment at a temperature between 100 °C and 1000 °C, preferably between 300 °C and 750°C, of an aluminium hydroxide having an average particle size in the 50% range ( 50 ) of 1 μm to 5 μm and a specific surface area according to BET of 1 m 2 /g to 5 m 2 /g. The aluminium hydroxide preferably used as starting material can further be characterised by a particle size in the 10% range (dio) of 0.1 μm to 2 μm and in the 90% range (d 90 ) of 2 μm to 10 μm. Good results have been achieved with standard aluminium hydroxides obtained from the Bayer process, e. g. the MARTINAL types of Alusuisse Martinswerk, Bergheim, Germany. Heat treatment as a rule takes place in such a manner that the starting aluminium hydroxide is slowly heated, e. g. at a controlled rate of 1 to 20 K/min, from ambient temperature to a maximum temperature of 100 °C to 1000 °C, preferably 300 °C to 750 °C, most preferably 400 °C to 700 °C, and then kept at this temperature for 10 to 60 min. The heating usually takes place in air at normal pressure or under reduced pressure. In a preferred embodiment, the heat treatment is carried out under reduced pressure, the reduced pressure more preferably being 100 mbar or less, most preferably 50 mbar or less. The nanostructured alumina according to the invention can be filled into synthetic polymer systems using methods known to the person skilled in the art and in amounts sufficient to impart fire resistance to the polymer system. In general both thermoplastic and thermosetting polymer systems can be filled with the nanostructured alumina of the invention. Suitable thermoplastic polymer systems are based on polyacrylates, polymethacrylates, polyesters or polyolefins like polyethylene and polypropylene. Suitable thermosetting polymer systems are epoxides, polyurethanes, unsaturated polyesters, vinyl esters or acrylic resins. In the preparation of fire resistant composite polymer systems based on thermoplastic polymers, the nanostructured alumina and the additional flame retardant(s) (if any) may be mixed either with the thermoplastic polymer or with an appropriate polymer precursor (mono- or oligomer) which is subsequently polymerised (cf. examples). In the preparation of fire resistant composite polymer systems based on thermosetting polymers, the nanostructured alumina and the additional flame retardant(s) (if any) have to be mixed with an appropriate polymer precursor before curing. Usually, however depending on the polymer system, the filling level is in the range of 1 wt.% to 50 wt.%, preferably 2 to 40 wt.%, most preferably 5 to 15 wt.%. It is possible to admix the nanostructured alumina with other flame retardants known in the art. Most preferred additional flame retardant is aluminium hydroxide and/or magnesium hydroxide. Excellent results have been obtained by mixing the polymer or polymer precursor with the nanostructured alumina under high shear conditions. Under such conditions the structure of the nanostructured particles will possibly undergo some modification, thus further improving the properties of the composite. It appears that in particular the aluminas produced by heat treatment under reduced pressure will exhibit this effect. It has been found that excellent fire resistance properties may be achieved with a polymer system based on a polymethyl methacrylate containing nanostructured alumina and aluminium hydroxide. It could be shown that polymethyl methacrylate trapped and more or less immobilised within the structure of the nanostructured alumina particles was prevented from oxidising, thereby decomposing to a char. This char at the surface of the particles acted as a "cement", to hold together the entire solid content within the partially decomposed organic matrix. Examples: Production of a nanostructured alumina: 500 g of a fine grade of standard aluminium hydroxide, MARTINAL ® ON 901 (Alusuisse Martinswerk GmbH, Bergheim, Germany) having a platy habit with average particle size 50 of 2 μm, d \ o of 0.5 μm , a <i 0 of 5μm and a specific surface area (BET) of 3 m 2 /g was slowly heated at a controlled rate of 10 K/min from ambient temperature to a temperature of 600 °C. The temperature was held at 600 °C for 30 min and then cooled back to room temperature in a desiccator. 335 g of a nanostructured alumina was obtained with a specific surface area (BET) of 48 m 2 /g and a loss on ignition (at 1000 °C/2 h) of 2.5 wt.%. The particle size distribution of the nanostructured alumina remained unchanged from the starting aluminium hydroxide according to laser scattering measurement with Cilas ® 850. X-ray diffraction indicated the absence of boehmite. Scanning electron microscopy revealed the existence of the nanostructure with delaminations up to 40 nm parallel to the (001) crystal faces observed and a random pattern of channels up to 10 nm in width on the (001) faces and hence running parallel to the prismatic side faces of the pseudohexagonal crystals. Preparation of a fire resistant polymethylmethacrylate (PMMA) a) Filled with nanostructured alumina and aluminium hydroxide To 100 g of methyl methacrylate monomer 20 g of the nanostructured alumina prepared as described above was added and fully dispersed in a dispersing unit (Dispermat ® CA from VMA) at 500 rpm at ambient temperature for 1 to 2 minutes. Then 80 g of MARTINAL ® ON 901 (Alusuisse Martinswerk GmbH, Bergheim, Germany) was added to this mixture and the mixture was further dispersed for 2 minutes. Curing was effected by the addition of 0.5 wt.% tcrt-butylcyclohexyl peroxydicarbonate at a temperature of 60 °C. The solidified, translucent composite was cooled to room temperature. A specimen of 10 cm 2 with a thickness of 3 mm was cut off for the flame resistance test (Example 1). b) Filled with nanostructured alumina only The example above was repeated with 100 g of the nanostructured alumina and without the addition of aluminium hydroxide. The solidified, opaque composite was cooled to room temperature. A specimen of 10 cm with a thickness of 3 mm was cut off for the flame resistance test (Example 2). Flame resistance tests: For comparison, the experiments were also carried out with methyl methacrylate only. Fire resistance testing was accomplished according to BS 6853 small-scale alcohol test whereby the specimen was held 15 cm above a methanol flame for 5 minutes. A specimen surviving the test without any changes in structure and shape was qualified as "passed". Slight deformations of structure and shape were qualified as "partially passed". Total deformation, loss of structure or combustion was qualified as "failed".

Claims: Claims 1. Nanostructured alumina characterised by an average particle size in the 50% range (dso) of 1 μm to 5 μm, a specific surface area according to BET of 10 m 2 /g to 350 m /g, preferably 10 m 2 /g to 200 m 2 /g, and structural channels having a width of 1 to 100 nm. 2. Nanostructured alumina according to claim 1, characterised by a particle size in the 10% range (dio) of 0.1 μm to 2 μm and in the 90% range (t/ 0 ) of 2 μm to 10 μm and a loss on ignition at 1000 °C of 1 to 15%. 3. Process for the preparation of a nanostructured alumina according to claim 1 or 2, characterised in that an aluminium hydroxide defined by an average particle size in the 50% range (<i 50 ) of 1 μm to 5 μm and a specific surface area according to BET of 1 m 2 /g to 5 m 2 /g is heat treated at a temperature between 100 °C and 1000 °C, preferably between 300 °C and 750 °C. 4. Process according to claim 3, characterised in that the heat treatment is carried out under reduced pressure. 5. Process according to claim 4, characterised in that the reduced pressure is 100 mbar or less, preferably 50 mbar or less. 6. Fire resistant composite polymer system comprising a synthetic polymer and a fire resistance-imparting filler, characterised in that it contains a nanostructured alumina according to claims 1 or 2 in an amount sufficient to impart fire resistance to the polymer system. 7. Fire resistant composite polymer system according to claim 6, characterised in that the content of the nanostructured alumina is in the range of 1% by weight to 50% by weight. 8. Fire resistant composite polymer system according to claims 6 or 7, characterised in that it additionally contains aluminium hydroxide and/or magnesium hydroxide. 9. Fire resistant composite polymer system, obtainable by mixing a polymer and/or a polymer precursor and a nanostructured alumina according to claim 1 or 2 under high shear conditions. 10. Fire resistant composite polymer system according to claim 9, characterised in that the nanostructured alimina has been prepared by the process of any of claims 3 to 7.

Inventors: BROWN NEIL, BROWN, NEIL
Applicants: ALUSUISSE MARTINSWERK GMBH, BROWN NEIL, BROWN, NEIL
Application year: 2000
Main classification: C01F7
IPC codes: C01F7, C08K3, C08L101