Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Thermoplastic microspheres, process for their preparation
and use of the microspheres
The present invention relates to thermoplastic
microspheres and more particularly to such microspheres
containing special blowing agents. The invention also
relates to preparation of the microspheres and the use of
these.
Expandable and expanded thermoplastic microspheres
are used in a great number of fields, for example as
fillers in polymers, paints, plastisols, paper, cable
insulation etc, and have been produced on a commercial
scale for several years. Expandable thermoplastic micro-
spheres are principally prepared according to the process
disclosed in the US patent 3615972. The microspheres are
thus conventionally prepared by suspension polymerisation
of a liquid monomer or monomer mixture containing a con-
densed blowing agent which is dispersed in an aqueous~ phase
containing a suspending agent and polymerisation i~itiator.
The microspheres obtained after the polymerisation consist
of a polymer shell which encapsulates the li~uid, volatile
blowing agent. The spheres expand by heatin~ to a -tempera-
ture above the boiling point of the blowing agent and above
the softening point of the polymer.
At the production of expandable thermoplastic micro-
spheres hydrocarbons such as n-butane, isobutane, iso-
pentane and neopentane are conventionally used, and e-
specially isobutane and isopentane which give microspheres
with very good expansion capability. The commercially
available microsphere product Expancel(R) contains iso-
butane as blowing agent and has a polymer shell of a co-
polymer of vinylidene chloride and acrylonitrile. Other
blowing agents than pure hydrocarbons have been suggested
for use in the preparation of microspheres. In the above
mentioned ~S patent 3615972 it is, for example, mentioned
that certain chlorofluorocarbons can be used, but these
have, however, not been used commercially. Chlorofluoro-
carbons do not give the microspheres satisfactory expansion
properties and they also have other disadvantages.
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According to the present invention it has been found
that aliphatic fluorocarbons and fluorohydrocarbons are
excellent blowing agents for microspheres and give spheres
with very good expansion properties. Expanded microspheres
containing aliphatic fluorocarbons and fluorohydrocarbons
have densities of corresponding magnitude as those contain-
ing isobutane and isopentane. The use o~ the specific
blowing agents also lead to other advantages, particularly
with regard to fire properties and insulation properties.
The present invention thus relates to thermoplastic
microspheres containing chlorine-free aliphatic fluoro-
carbons and fluorohydrocarbons as further defined in the
claims. The invention also relates to a method for the
production of the microspheres and to the use of these as
additives/fillers in products for which heat insulation
capacity and/or fire resistance are of importance.
The basis for the present invention is thus the use
of chlorine-free aliphatic fluorocarbons and Eluorohydro-
carbons as blowing agent in the production of expandable
thermoplastic microspheres. The expandable thermoplastic
microspheres encapsulates the volatile chlorine-free
aliphatic fluorocarbons and fluorohydrocarbons in liquid
form in a shell of polymerized ethylenically unsaturated
monomer or mixture of ethylenically unsaturated monomers.
When the expandable microspheres are heated to temperatures
above the boiling point of the blowing agent and above the
softening point of the polymer the propellant is volatil-
ized and the microspheres expand which results in micro-
spheres having a substantially increased diameter and which
contain the blowing agent in gas form.
The microspheres according to the present invention
contains chlorine-free aliphatic fluorocarbons and/or
fluorohydrocarbons. These can make up the whole amount of
blowing agent but it is also within the scope of the
invention that the microspheres contain these blowing
agents in combination with other per se conventionally used
blowing agents such as n-butane, lsobutane, isopentane and
neopentane and preferably in combination with isobutane or
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isopentane. The amount of other blowing agent than fluoro-
carbons or fluorohydrocarbons should suitably not exceed
50~ by weight and preferably not exceed 25% in order to
utili~e the advantages obtained with fluorocarbons and
fluorohydrocarbons. The greatest advantages with regard to
fire and insulation are of course obtained when the entire
amount of blowing agent is made up from fluorocarbons or
fluorohydrocarbons. However, mixtures with other blowing
agents can be advantageous for example to compensate for
high pressures during polymerisation when the utilized
fluorocarbon or fluorohydrocarbon has a lower boiling
point.
The -thermoplastic shell of the microspheres is made
up from polymers or copolymers of ethylenically unsaturated
monomers. Examples of suitable monomers are vinyl chloride,
vinylidene chloride, acrylonitrile, methacrylonitrile,
acrylic esters, methacrylic esters, styrene etc, and
mixtures of two or more of these. Preferred microspheres
have a shell based on a copolymer containing acrylonitrile
and then particularly copolymers of acrylonitrile and
vinylidene chloride and/or methyl methacrylate and/or
methacrylonitrile. These copolymers can for example contain
30 to 80% by weight o~ acrylonitrile, 0 to 70% by weight of
vinylidene chloride and/or 0 to 50% by weight of methyl
methacrylate and/or methacrylonitrile. The shell of the
microspheres can be cross-linked or not cross-linked.
The expandable microspheres can be prepared in per se
known manner by suspension polymerisation of the monomers
using conventional polymerisation initiators such as
dialkyl peroxides, diacyl peroxides, peroxy esters, peroxy
dicarbonates and azo compounds. The polymerisable monomer
or monomer mixture, the condensed blowing agent, optional
cross-linking agent and the initiator are suspended in an
aqueous medium containing suspending agent in a reaction
vessel. As cross-linking agents divinylbenzene, ethylene
glycol dimethacrylate, triethylene glycol d`imethacrylate,
triallyl isocyanate etc, can for example be used. Usually a
powder stabilizer, such as colloidal silicic acid, is used
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as suspending agent. The powder stabilizer is usually used
in combination with a so-called co-stabilizer, such as for
example polyethylene oxide, polyethyleneimine, tetramethyl-
ammonium hydroxide, condensation products of diethanolamine
and adipic acid, condensation products of urea and form-
aldehyde. The powder stabilizer is usually used in an
amount of from about 3 to about 10% by weight, based on the
monomer, while the co-stabilizer usually is used in an
amount of some tenth %.
The blowing agent, the chlorine-free aliphatic
fluorocarbons and fluorohydrocarbons and optional other
blowing agents, as stated earlier, is usually used in
amounts of from about 10 to about 70~ by weight, based on
total monomer weight, so that they make up from about 10 to
about 45% of the weight of the produced expandable micro-
spheres. The process e~uipment used for the polymerisation
is decisive for the choice of the chlorine-free aliphatic
fluorocarbons and fluorohydrocarbons since a too low
boiling point for these gives rise to too high pressures
during the polymeri.sation process. The boiling point should
usually not be lower than -40C and preferably not lower
than -20C. The upper limit for the boiling point of the
compounds is in the first hand dependent on the monomer
composition, since the boiling point of the compounds shall
be below the softening point for the polymer in question in
order to carry out expansion of the microspheres. Generally
the boiling point of the compounds should thus not exceed
60C and as a rule not exceed 40C. As examples of fluoro-
carbons and fluorohydrocarbons which can be used can be
mentioned CH2FCF3, CH3CHF2, CH3CH2F, CF3CF2cF3~
CF3CF2CF2CF3~ CYC1-C4F8, CF3(CF2)3CF3 and CF3CHFCF3. 2H-
heptafluoropropane ~ CF3-CHF-CF3) iS especially suitable.
The polymerisation can be carried out in per se known
manner and usually polymerisation temperatures of from
about 40C to about 70C are used and the polymerisate is
normally post-treated by filtration, washing and dewater-
ing. The particle size for the unexpanded spheres, and thus
also for the expanded spheres, can vary within wide limits
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and is selected with regard to the desired properties of
the finished product. At the polymarisation the particle
size is controlled mainly by tne composition of the poly-
merisation mixture and the degree of stirring. 1 ~m to 1
mm, preferably 2 ~m to 0.5 mm and especially 5 ~m to 50 ~m
can be mentioned as examples of particles sizes of un-
expanded spheres.
The expandable spheres are expanded by heating to a
temperature which gives rise to softening of the polymer
shell and volatilization of the propellant whereby the
particles expand to a diameter substantially larger than
the diameter for the unexpanded particles and the expansion
can be carried out in per se known manners. The spheres can
for example be dried and expanded by dispersing the un-
expanded spheres in an inert liquid, atomization of the
dispersion and bringing this in contact with a warm inert
gas stream. Another suitable manner for expansion is
disclosed in the European patent applicatlon 03~8372.
According to this process the expandable microspheres are
first dried to a certain dry content and then expanded by
heating, eg by IR-heating. The expansion temperature is set
by the boiling point of the blowing agent and the softening
point of the shell-polymer and is usually within the range
of from about 70C to about 140C. It has been found that
the blowing agents used according to the present invention
give at least as good expansion as the commercially used
blowing agents isobutane and isopentane and thereby give
expanded microspheres with about the same densities as
those which are commercially acceptable. ThiS is in con-
trast to chlorofluorocarbons and chlorofluorohydrocarbonswhich give substantially inferior expansion, which is
believed to be due to their interference at the polymer-
isation, and expanded microspheres which have densities
about 3 to about 7 times higher than when isobutane or
isopentane is used. Compared with isobutane and isopentane
2H-heptafluoropropane, for example, is also advantageous
since it is non-combustible which is of importance both at
the production and the use of the microspheres. Since the
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compounds are non-combustible problems with dust explosions
in connection with drying of expandable as well as o~
expanded spheres are reduced. ~nother advantage in com-
parison with isobutane and isopentane is that for example
2H-heptafluoropropane has substantially lower thermal
conductivity which broadens the fields of application for
the microspheres. Compared with chlorofluorocarbons the
present fluorocarbons and fluorohydrocarbons are also
advantageous since they do not have any ozone degrading
effect.
Expandable and expanded thermoplastic microspheres
have a large number of applications. The use of unexpanded
microspheres is based on the expansion capability of the
spheres and they are then expanded in situ in the mate-
rials in which they are incorporated when these materialsare heated. As some examples of such use can be mentioned
printing inks for the production of relief print on paper
and textiles, fillers in paper and board and Eoaming of Eor
example PVC-plastisol. At the use of expanded microspheres
the low densit~ and filling effect of the spheres are
utilized and they can for example be used as fillers in
paints, putty, polymers and resins such as polyester,
polyurethane, epoxy resins, composite materials based on
polymers, paper, insulation materials etc.. The micro-
spheres of the present invention can be used for the samepurposes for which microspheres are generally used. The
present microspheres are particularly suitable for use in
products for which fire resistance and thermal insulating
capacity are of importance since the microspheres are
advantageous in such applications since they contain
chlorine-free fluorocarbons and fluorohydrocarbons, such as
2H-heptafluoropropane, which are non-combustible and which
have low thermal conductivity. The present invention thus
also relates to use of the microspheres in fire-resistant
paints and in insulation materials. In fire-resistant
paints the microspheres are used as fillers/`additives. The
unexpanded microspheres are advantageously used in fire-
resistant paints since they expand at heating and thereby
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give an insulating layer which protects the substrate. When
the temperature becomes so high that the microspheres
break the non-combustible blowing agent is released and the
fire is retarded. As concerns insulation materials the
entire product can be made up from microspheres, for
economical reasons the microspheres are, however, also in
this application as a rule used as ~illers/additives. As
examples of insulation materials wherein the microspheres
can be used are jointing compounds for, among other things,
cable entries, where good insulation and fire-resistant
properties are desired.
The invention is further illustrated in the following
examples which, however, are not intended to limit the
same. Parts and per cent relate to parts by weight and per
cent by weight respectively, unless otherwise stated~
xample 1
125 parts of water were mixed with 5.5 parts of 1
molar NaOH-solution and 10 parts oE 10~ acetic aclcl solu-
tion, 6 parts of 40% colloidal sillclc acld, 0.5 parts of a
condensation product of diethanolamlne and adipic acid and
0.5 parts of dicetyl peroxydicarbo~ate and charged to a 15
1 reactor e~uipped with stirrer. The reactor was sealed and
evacuated. A mixture of 0.3 parts of divinylbenzene, 7
parts of methylmethacrylate, 32 parts of acrylonitrile, 32
parts of vinylidene chloride and 29 parts of 2H-hepta-
fluoropropane were then charged. The polymerisation mixture
was stirred at 850 rpm during 60 minutes. After homogeniz-
ation the rotation speed was lowered to 400 rpm and the
mixture was then heated to 55C and polymerized at this
temperature for 8 hours. The obtained polymerisate was
washed and dewatered. The unexpanded microspheres had an
average particle size, by weight, of 16 ~m. The micro-
spheres were dried and their expansion capacity was invest-
igated by means of thermomechanical analysis. They were
found to have the same expansion capacity and temperature
resistance as if isobutane or isopentane had been used, ie
a density of about 17 kg/m3 was reached.
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Comparative Exam~les 2a) - 2c)
The process according to ~xample 1 was repeated with
other blowing agents.
2a) Instead of 2H-heptafluoropropane 12 parts of isopentane
were used. The dried microspheres had a density below 20
kg/m3.
2b) Instead of 2~-heptafluoropropane 30 parts of trichloro-
fluoromethane were used. The dried microspheres had a dens-
ity of about 60 kg/m3.
2c) Instead of 2H-heptafluoropropane 30 parts of l,1-di-
chloro-2,2,2-trifluoroetha~e were used. The dried micro-
spheres had a density of about 100 kg/m3.
Example 3
For evaluation of the heat insulation capacity of
products produced from microspheres according to the
invention a plate having the dimenslons 300x~00x20 rnm was
produced. This was produced by spreading dry unexpanded
microspheres, prepared according to Example 1, in a mould
which was then sealed and placed in an oven where it was
allowed to stand during ~5 minutes at a temperature of
135C. The obtained plate had a density o~ 40 kg/m3. The
heat conductivity was measured to 0.0235 ~/mC. For a plate
produced in the same manner from microspheres containing
isobutane as the blowing agent the measured heat conduct-
ivi-ty was 0.0275 W/mC. A clearly improved insulation
capability was thus obtained with microspheres according to
the invention containing 2H-heptafluoropropane as blowing
agent.
