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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for producing hollow microspheres and
to the hollow microspheres so produced. More particularly, the invention
relates to the production of hollow microspheres containing or consisting
entirely of carbon, and to the production of hollow microspheres which are
precursors of such carbon-containing hollow microspheres.
2. Discussion of the Prior Art
Microspheres made of carbon and other materials have numerous uses in
industry. For example, they can be used for the preparation of metal foams
and syntactic foams (hollow carbon spheres in a polymer matrix), for the
formation of filter beds and for the production of lightweight carbon
composites. One known method of producing carbon microspheres involves the
carbonization of pellets made from pitch (Y. Amagi et. al. "Hollow Carbon
Microspheres from Pitch Material and their Applications," SAMPLE 10th
National Symposium 71), but pitch pellets can fuse together during
carbonization unless steps are taken to avoid this by a time-consuming and
expensive pretreatment.
Another method of forming hollow microspheres is disclosed in U.S. Pat. No.
2,797,201 to Veatch et. al. issued on June 25, 1957. This method involves
forming droplets of a solution in a volatile solvent of a gas-generating
material and a film-forming polyvinyl alcohol or phenolformaldehyde resin,
and heating the droplets by a spray drying technique to form hollow
microspheres of 1-500.mu. in size. However, this process does not result
in particles of a uniform size and, indeed, is not effective at all for
producing microspheres larger than about 0.5 mm (500.mu.) in diameter.
This is because almost 70-85% of each droplet consists of solvent, so
that, during the spray drying step, a large amount of heat must be
transferred to the droplet in a short period of time in order to vaporize
the solvent completely. This must take place while gas is being generated
within the droplets and during the short time the droplets remain out of
contact with each other, otherwise agglomeration will take place. All of
this is extremely difficult in a spray drying system when the droplets
exceed a certain maximum size.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an
improved process for forming hollow microspheres, and particularly those
having a diameter exceeding about 0.5 mm.
According to one aspect of the invention there is provided a process of
forming hollow microspheres, which comprises: forming a solution in a
liquid solvent of a polymer having the following properties: (a) a
molecular weight of at least 10,000 and a long chain structure of at least
200 monomer units; (b) an ability to be coagulated or precipitated from
the solution upon contact of the solution with a non-solvent for the
polymer; (c) an ability to form a continuous stretchable film when
coagulated or precipitated from solution; (d) a chemical structure which
is infusible or which is capable of being rendered infusible; and (e) a
high carbon yield of at least 30% by weight upon being carbonized in a
non-reactive atmosphere; incorporating into said solution an insoluble
solid particulate blowing agent which is decomposable by heat to generate
a gas; dividing the solution into droplets and introducing the droplets
into a liquid bath containing a non-solvent for the polymer, said
non-solvent being such that the polymer is rapidly coagulated or
precipitated from said solution, and said bath having a temperature high
enough to cause decomposition of the blowing agent; and removing the
resulting hollow microspheres from the bath.
The hollow microspheres produced in this way are so-called "green"
microspheres because they contain a polymer which must be converted to
carbon by a subsequent step if carbon-containing microspheres are
required. However, the green microspheres may themselves be a useful
product and consequently, in some cases carbonization of the green
microspheres may not be required.
If carbon-containing microspheres are required, they can be prepared by
heating the green microspheres in a non-reactive atmosphere to a
temperature usually in excess of about 500.degree. C. As will be explained
later, however, the green microspheres may have to undergo a treatment to
render the polymer infusible prior to the carbonization treatment.
The invention is capable of producing hollow green or carbon-containing
microspheres of a uniform size of about 0.5 mm (500.mu.) or larger having
a high degree of sphericity.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 to 4 are photomicrographs of sectioned microspheres produced by the
present invention, as indicated in the Examples;
FIG. 5 is a photomicrograph of whole green microspheres produced according
to the present invention; and
FIG. 6 is a photomicrograph of green microspheres produced by a process
different from the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The polymer selected for use in the present invention must have certain
characteristics as described above. These requirements and their
importance are explained in more detail below.
The polymer should have a molecular weight of at least 10,000, and
preferably about 50,000 to 100,000 or more, and a long chain structure
which may be branched or straight, although substantially straight chain
structures are preferred. By the term "long chain structure" we mean a
polymer comprising, on average, at least about 200 monomer units and
preferably about 1000 monomer units or more. High molecular weight long
chain polymers are capable of forming stable and uniform suspensions when
the solid particulate blowing agent or other solid material (if required)
is introduced into the polymer solution.
It is theorized that the long polymer chains encircle the individual
particles and overcome any tendency of the particles to settle or
agglomerate. The reduced tendency of the particles to settle or segregate
is important because identically-sized droplets produced by dividing the
polymer solution then contain equal amounts of the blowing agent and other
solid (if present). Uniformly sized hollow microspheres of the same
composition can consequently be formed.
The polymer must be capable of being rapidly coagulated or precipitated
from its solution when the solution is contacted with a suitable
non-solvent for the polymer.
Coagulation or precipitation must be quite rapid because an impermeable
polymer skin must form at the surface of the droplet before (or
simultaneously with) the decomposition of the blowing agent so that hollow
microspheres can be formed. The choice of a suitable solvent/non-solvent
system for the polymer is important and is described in more detail later.
The polymer must be capable of forming a continuous stretchable film when
coagulated or precipitated from its solution. This is because the
impermeable polymer skin formed at the surface of each droplet is
stretched and inflated by the gases generated by the blowing agent.
Generally, high molecular weight long chain polymers have this ability.
The polymer should be of a kind which can be carbonized without melting,
i.e. the gases produced by heating the polymer in a non-oxidizing
atmosphere should be evolved from a solid rather than a liquid. This is
important because microspheres which tend to melt may fuse together during
carbonization or their surfaces may become misshapen. Furthermore, if the
coagulated or precipitated polymer contains an added particulate material
uniformly dispersed throughout the polymer, the particles of this material
may be undesirably displaced by the evolving gases if the polymer melts
while being carbonized. When the polymer remains solid, any added solid
particles remain fixed in their original locations.
Polymers which tend to fuse when heated may be used in the present
invention if they can be treated prior to the carbonization step in a way
which renders the polymers infusible. For example, some polymers become
infusible when cross-linked or cyclized, e.g. by being heated at
non-oxidizing temperatures in an oxygen-containing atmosphere or by being
subjected to the action of a chemical oxidizing agent (e.g. an
oxygen-containing compound of a metallic transition element). This is
referred to hereinafter as a stabilization treatment and, when required,
is carried out after the hollow microspheres have been removed from the
bath but prior to the carbonization step.
Furthermore, the polymers should have a high carbon yield of at least 30%
by weight, and more preferably at least 40% by weight, upon being
carbonized in a nonreactive atmosphere. This is to ensure that, following
carbonization of the microspheres, they contain a suitably large amount of
carbon. If the carbon yield is too low, the carbonized microspheres may be
too porous and fragile. Polymers having a lower carbon yield than 30% by
weight may be used in the invention if they can be modified to increase
the carbon yield to the stated minimum or more.
Generally speaking, treatments which render a polymer infusible also
increase its carbon yield. For example, cross-linking and cyclization
makes it less likely that low molecular weight carbon-containing
components will separate from the polymer mass and volatilize when the
polymer is undergoing the carbonization step. Consequently, polymers of
low carbon yield which can undergo a stabilization step may be suitable
for the present invention.
Additionally, as a practical matter, the polymer must be sufficiently
soluble in the solvent to produce a solution which contains a suitably
high polymer content and which can be readily divided into droplets. For
ease of droplet formation, the polymer solution (after additional
materials have been incorporated therein, if required) preferably has a
viscosity of 200-5000 cp at 25.degree. C., and more preferably 500-2000 cp
at 25.degree. C. Very high viscosities make division of the solution quite
difficult and result in droplets having a "tail" and thus in the
production of non-spherical hollow particles. Very low viscosities usually
means that there is insufficient polymer in the solution. The amount of
polymer dissolved in the solution should be sufficient to enable hollow
microspheres to be formed. That is, if the polymer content is too low, the
walls of the spheres will be too thin and too permeable to contain the gas
generated by the blowing agent. The minimum polymer content depends on the
polymer employed and on other conditions, but it is usually about 5% by
weight based on the total weight of the solution.
The preferred polymers for use in the present invention are
polyacrylonitrile and its copolymers and terpolymers (collectively
referred to hereinafter as PAN), cellulose and its derivatives, polyvinyl
alcohol and its copolymers and terpolymers, polyarylether,
polyacenaphthylene, polyacetylenes, and the like. Suitable materials are
also disclosed in "Precursors for Carbon and Graphite Fibers" by Daniel J.
O'Neil, Intern. J. Polymeric Meter Vol. 7 (1979), p. 203.
PAN is the most preferred material for use in the present invention. PAN is
a known polymer widely used for textiles, for the production of carbon
fibres and for other purposes. For example, it is sold under the trade
mark ORLON by E. I. DuPont de Nemours and Company, and the structure of
this particular product is disclosed in an article by R. C. Houtz, Textile
Research Journal, 1950, p. 786. Textile grade PAN is commonly a copolymer
of polyacrylonitrile and up to 25% by weight (more commonly up to 10% by
weight and usually about 6% by weight) of methacrylate or
methylmethacrylate. Textile grade PAN copolymers can be used in the
present invention and are in fact preferred to PAN homopolymer because the
additional units in the copolymer assist in the cyclization of the polymer
when heat stabilization is carried out to make the polymer infusible.
Inexpensive waste PAN from the textile industry, such as the so-called
"dryer fines", are particularly useful in the invention.
Suitable solvents for PAN include dimethylformamide (DMF),
dimethylsulfoxide (DMSO) and dimethylacetamide (DMAC). DMF is the
preferred solvent and solutions of the required viscosity can be made by
dissolving a sufficient amount of PAN in DMF to give a solution containing
5-20% by weight, more preferably 8-16% by weight, and most preferably
12-15% by weight of PAN.
When cellulose or a cellulose derivative (e.g. the textile material sold
under the trademark RAYON) is used as the polymer, a mixture of about 10%
by weight of LiCl in DMF may be used as a solvent. It is found that the
LiCl acts as a solubilizing aid which increase the solubility of cellulose
in DMF. When polyvinylalcohol is used as the polymer, DMF is a suitable
solvent. Suitable solvents are also available for the other polymers
mentioned above.
When a solution of the polymer in the solvent has been formed, a
heat-decomposable blowing agent is incorporated into the solution before
the solution is contacted with the non-solvent. The blowing agent is in
the form of a finely divided solid which is insoluble in the polymer
solution. As stated above, the nature of the polymer is such that the
particles of the blowing agent are held in a uniform suspension in the
polymer solution, so that droplets of equal size contain the same amount
of blowing agent and thus produce microspheres of substantially identical
size. Preferably, the solid blowing agent is used in the form of particles
of less than 100 Tyler mesh in size. However the size of the particles is
less important than the requirement that they be uniformly dispersed so
that, upon division of the solution, each droplet of solution contains the
same amount of blowing agent as all of the other droplets.
Examples of solid blowing agents which may be employed in the present
invention are (NH.sub.4).sub.2 CO.sub.3, NH.sub.4 HCO.sub.3 and ammonium
carbamate.
The amount of blowing agent employed depends on the polymer, the
concentration of the solution etc., but is usually in the range of 1-5% by
weight of the polymer solution.
As well as the blowing agent, additional solid particles which are
non-reactive with the polymer and solvent may be incorporated into the
polymer solution. For example, it may be desirable to produce microspheres
which contain fine coke dust, metals, metal oxides, metal fluorides (e.g.
AlF.sub.3), activated carbon and the like. These materials may be added to
the solution in any quantities which do not affect the ability of the
polymer solution to form hollow microspheres.
Materials which are soluble in the polymer solution may also be added, if
desired. For example, tar, pitch or phenolic resins may be incorporated
into the polymer solution. This may be desirable because such materials
are inexpensive and their presence is not harmful if the quantities are
kept low enough not to adversely affect the desired characteristics of the
polymer.
Since PAN is a good film-former, it may incorporate a large proportion of
additional solids, e.g. up to 10 parts by weight of additional solids per
part by weight of PAN. PAN may also accommodate up to 1 part by weight of
tar or pitch per part by weight of PAN. It is found that the presence of
the tar or pitch in such amounts does not make the particles fusible owing
to the presence of the PAN.
The polymer solution containing the blowing agent and additional materials
(if any) is divided into droplets of equal size which are then introduced
into a non-solvent bath. The droplet formation can be carried out, for
example, by feeding the solution through a hollow tube (e.g. 1-3 mm in
diameter) and allowing droplets of solution to fall from the end of the
tube into the bath. Alternatively, a vibrating rod may be used to form the
droplets, e.g. by allowing a stream of the polymer solution to run down
the rod as it vibrates.
The choice of an appropriate non-solvent for use in the bath is important.
The non-solvent should be readily miscible with the solvent, but should be
capable of precipitating or coagulating the polymer virtually
instantaneously. This is necessary to permit the polymer to form a
stretchable film at the surface of the droplet at the same time that the
blowing agent is decomposed. The resulting droplet is then inflated to
form a hollow microsphere. If the precipitation or coagulation takes place
too slowly, the gases will escape and the droplet will remain uninflated.
Generally, it has been found that non-solvents which do not tend to wet
the polymer solution (i.e. those forming a low contact angle with the
polymer solution) allow the droplets of polymer solution to remain
spherical and thus permit the formation of hollow microspheres having good
sphericity. However, it has also been found that the identity of the
solvent can also affect the choice of a suitable non-solvent. Thus, for
every polymer solution used in the present invention, a suitable
non-solvent must be located and this can be done by simple trial and
experimentation.
When PAN is used as the polymer and DMF is used as the solvent, the
non-solvent may be water or methanol. Suitability as a non-solvent for the
PAN/DMF system appears to be associated with a high polarity and the
presence of --OH groups. Acetone, for example, is not suitable as a
non-solvent for the PAN/DMF system because the coagulation or
precipitation of the polymer is not sufficiently rapid.
Since water is inexpensive, it is the preferred non-solvent, but the bath
preferably comprises 0-80% by weight of the solvent (DMF) in water, more
preferably 25-60% by weight and usually about 40% by weight of DMF when
the method commences.
When the polymer is cellulose or a derivative thereof in a DMF solution
containing 10% LiCl, the non-solvent may be water.
Polyvinyl alcohol in particular illustrates the point that the choice of
the solvent and non-solvent is extremely important for the production of
suitable hollow microspheres. Polyvinyl alcohol can be dissolved in either
water or DMF, and methyl ethyl ketone can be used as a non-solvent.
However, when water is used as the solvent, spherical microspheres are not
obtained. On the other hand, when DMF is used as a solvent, spherical
microspheres are obtained, showing that it is important to select the
right solvent/non-solvent combination.
The temperature of the bath should be above the decomposition temperature
of the blowing agent and below the boiling temperature of the bath
(boiling of the bath causes deformation of the microspheres). Preferably,
the maximum temperature of the bath should be 10.degree.-20.degree. C.
below its boiling temperature. Incidentally, by choosing a blowing agent
having a decomposition temperature of at least 25.degree. C., the polymer
solution can be prepared and delivered to the bath at room temperature,
which is a considerable convenience.
The bath temperature is normally in the range of 50.degree.-70.degree. C.
when the polymer solution is PAN dissolved in DMF, the bath comprises DMF
and water and ammonium bicarbonate is used as the blowing agent.
Although the identity of the non-solvent is primarily responsible for
determining the rate of coagulation or precipitation of the polymer from
solution, the conditions of the bath, i.e. its composition and its
temperature, also have some effect. The bath conditions also affect the
time and rate of decomposition of the blowing agent and the stretchability
of the coagulated or precipitated polymer. The ideal conditions for each
system can be found by simple trial, but the following guidelines are
provided.
The rate of coagulation or precipitation of the polymer from the solution
can be varied by changing the ratio of non-solvent to solvent in the bath.
When the ratio is increased, the speed of coagulation or precipitation is
increased. However, the bath preferably contains at least 25% by weight of
the solvent at the start of the procedure so that the solvent extracted
from the droplets as the coagulation or precipitation step proceeds does
not cause a large percentage changes in the solvent concentration in the
bath, which can affect the rate of polymer coagulation or precipitation.
Alternatively, the concentration of the solvent in the bath can be kept
constant by adding non-solvent to the bath at a suitable rate. The rate
and amount of gas generated by the blowing agent can be controlled by
adjusting the bath temperature and the amount of blowing agent used in the
solution. The viscosity of the polymeric solution can be varied by
changing the concentration of the polymer in the solution. The droplet
size can be varied quite easily according to the method employed for
dividing the solvent. For example, the size of droplets formed at the end
of a hollow tube depends on the diameter of the tube and to some extent on
the viscosity, temperature and composition of the solution. By suitably
adjusting the above factors, the size and wall thickness of the
microspheres can be varied.
Since the droplets of polymer solution can be made of uniform size and each
contains a substantially identical amount of blowing agent, microspheres
of very uniform size can be produced (e.g. microspheres having a
uniformity in terms of the sizes of their diameters of 5-10%). Moreover,
since the droplets are inflated from within by the blowing agent gases to
form hollow microspheres, a product having a high degree of sphericity can
be obtained, e.g. the microspheres may have a sphericity of 0.95 or more.
The use of a non-solvent bath to cause simultaneous coagulation and blowing
of the droplets enables large sized hollow microspheres to be produced,
which is difficult or impossible by other techniques. Generally, the
particles produced by the present invention have diameters of 0.5 mm and
larger. Microspheres having diameters smaller than 0.5 mm are difficult to
obtain by this technique because very small droplets may tend to float on
the bath surface and become deformed. The practical upper size limit is
about 10 mm, although theoretically larger particles could be obtained.
The most common size range of the microspheres is 0.5-5 or 6 mm
(diameter).
The use of a non-solvent bath to form the microspheres also has the
advantage that the temperatures employed are quite low, so no degradation
of the polymer takes place.
Once the microspheres have been formed they can be removed from the bath
and have no tendency to agglomerate since the polymer has been
precipitated or coagulated to form a non-tacky solid. The microspheres are
then preferably dried under gentle heating, e.g. at about 100.degree. C.
in air.
The resulting polymer microspheres (the so-called "green" microspheres) may
in themselves be a useful product, in which case no further treatment may
be required. More usually, however, the green microspheres are subjected
to a further treatment which includes a carbonization step to convert the
polymer to carbon.
The exact nature of the subsequent treatment of the microspheres when
carbonization is required depends on the type polymer present. If the
polymer is already in a non heat-fusible form, the microspheres may be
subjected directly to the carbonization treatment. However, the polymer
may first require heat stabilization, i.e. cross-linking or cyclisation,
to make it infusible.
PAN, for example, requires a heat stabilization treatment prior to the
carbonization step in order to make the polymer infusible. The heat
stabilization step causes the PAN polymer to cyclize, as follows:
##STR1##
The heat stabilization also increases the oxygen content of the polymer,
which improves the carbon yield by increasing the extent of aromatization
and cross-linking codal aromatization. The heat stabilization of PAN is
carried out by heating the polymer in air or oxygen at a temperature of
about 200.degree.-210.degree. C. for several hours, e.g. 8-16 hours.
The carbonization step can then be carried out. This involves heating the
microspheres in a non-reactive atmosphere (e.g. under argon or nitrogen)
for a period of up to several hours at a temperature in the range of
500.degree.-700.degree. C., preferably at a heating rate of 100.degree. C.
per hour or more. This heating step converts the polymer to carbon and to
volatile gases, which are driven off.
The following Examples and Comparative Examples provide further explanation
of the present invention. In the Examples and Comparative Examples,
percentages are by weight unless otherwise specified.
EXAMPLE 1
Polyacrylonitrile (PAN) copolymer sold under the trade mark ORLON was
dissolved in DMF (dimethyl formamide) to make a 14% (w/w) solution having
a viscosity of 1300 cps at 25.degree. C. Approximately 2% (by weight of
solution) of finely ground (-100 mesh) (NH.sub.4).sub.2 CO.sub.3 was
uniformly suspended in this by stirring. This suspension was pumped
through an orifice of 2 mm diameter to produce droplets at the rate of
about 40 drops per minute. These were allowed to fall from a height of 30
cms into a bath containing 40% DMF in water maintained at about 60.degree.
C. The (NH.sub.4).sub.2 CO.sub.3 decomposed at this temperature to produce
NH.sub.3 and CO | | |
