Note: Descriptions are shown in the official language in which they were submitted.
CA 02383079 2002-03-13
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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.
CONFIRMATION COPY
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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 (dso)
of 1 ~m to 5 q.m, a specific surface area according to BET of 10 m2/g to 350
m2/g, preferably
10 m2/g to 200 m2/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
(dlo) of 0.1 ~.m to 2 ~m and in the 90% range (d9o) of 2 ~,m to 10 ~m 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 cm2) 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
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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, AIOOH) 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 (d5o) of 1 pm to 5 ~m and a specific surface area according to BET
of 1 m2/g to
5 m2/g.
The aluminium hydroxide preferably used as starting material can further be
characterised by
a particle size in the 10% range (dlo) of 0.1 p,m to 2 p,m and in the 90%
range (d9o) of 2 pm to
10 Vim. 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.
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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 (c~ 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.
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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 dso
of 2 Vim, dlo of 0.5 ~m , a d9o of Sp.m and a specific surface area (BET) of 3
m2/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 m2/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 S00 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.%
tort-butylcyclohexyl peroxydicarbonate at a temperature of 60 °C.
The solidified, translucent composite was cooled to room temperature.
A specimen of 10 cm2 with a thickness of 3 mm was cut off for the flame
resistance test
(Example 1 ).
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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 cm2 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".
Specimen Result
Example 1 ("nano-alumina" and Passed
Al(OH)3)
Example 2 ("nano alumina") Partially passed
Comparison example (no filler, Failed
PMMA only )
