Polyoxymethylene fibrids, a process for their production and their use

The invention relates to polyoxymethylene fibrids with a reduced specific viscosity of 0.4 to 2.0 dl/g, a specific surface area of 50 to 200 m.sup.2 /g and a freeness of 30.degree. to 80.degree. SR. The fibrids are produced by flash-evaporation of a superheated solution of the polymer, a mixture of 50-95% by weight of a lower alcohol with 1-4 C atoms and 5-50% by weight of water being used as the solvent and are suitable for the production of paper.

By fibrids there are understood small fibers which are orientated in the 
longitudinal direction and have a cellulose-like structure, that is to say 
a finite but non-uniform length, an irregular density, a fissured surface 
and a high degree of branching. As a result of their structure, they are 
particularly suitable, inter alia, for the production of paper. 
The production of such fibrids is described, for example, in German Patent 
Specification No. 1,290,040. In this procedure, plexus filaments are first 
produced and are then cut into staple lengths, the staple fiber particles 
are suspended in a liquid and the particles in suspension are shredded in 
a manner which is in itself known. In this context, the term plexus 
filaments relates to a filament-like product of a crystalline plastic with 
a three-dimensional network, which is virtually free from tunnel-like 
channels and hollow spaces, of numerous molecular orientated film-like or 
silver-like fibrids which are less than 2 .mu. thick, are combined with 
one another and separate from one another along their length at irregular 
intervals and are preferably orientated in the direction of the 
longitudinal axis. 
These plexus filaments and their production are described in more detail in 
Belgian Patent Specification No. 568,524. To produce the plexus filaments, 
a solution of a synthetic polymer under its autogenous pressure or under a 
higher pressure is extruded, at a temperature above the boiling point of 
the solvent under normal pressure, through an orifice into a chamber which 
is under a lower pressure. This production of plexus filaments is also 
called flash-spinning or expansion-spinning. 
Plexus filaments can also be prepared from polyoxymethylene by this route, 
and then be shredded to fibrids. In Belgian Patent Specification No. 
568,524, aprotic solvents, and in particular methylene chloride, ethylene 
chloride, acetonitrile and methyl ethyl ketone, are used as solvents for 
the production of the plexus filaments. As experiments have shown, no 
fibrids, but exclusively plexus filaments, are formed even at extremely 
low polymer concentrations, for example of 1 % by weight in methyl ethyl 
ketone. Since plexus filaments are unsuitable for the production of paper, 
according to the abovementioned German Patent Specification No. 1,290,040 
they must be processed to fibrids in a second process step, and as a 
result the process becomes expensive. The polyoxymethylene fibrids thus 
obtained have, in particular, a relatively low specific surface area and a 
low freeness and the paper produced therefrom is of relatively low 
strength. 
Fibrous products are likewise obtained on precipitation of polyoxymethylene 
from its solutions by supercooling the solution or by discharging into a 
precipitating agent (compare German Auslegeschrift No. 1,241,116). 
However, paper with satisfactory properties cannot be prepared therefrom, 
since these fibers are very short and thick and are highly contaminated by 
two-dimensional film-like structures. 
Finally, it is also known, from Japanese Patent Application No. 71 41,110, 
that fibers are obtained by thorough stirring of a supercooled 
polyoxymethylene solution. However, this method is too protracted, and is 
also unsuitable for an industrial process because of its low yield. 
The object of the present invention is to provide polyoxymethylene fibrids 
which are free or at least substantially free from the disadvantages of 
the state of the art, and in particular of the abovementioned 
disadvantages and to provide a process for preparing the polyoxymethylene 
fibrids. 
The invention thus relates to polyoxymethylene fibrids with a reduced 
specific viscosity of 0.4 to 2.0 dl .times.g.sup.-1, preferably 0.6 to 
1.20 d1 .times.g.sup.-1 (measured at 140.degree. C. in butyrolactone, 
which contains 2 % by weight of diphenylamine, in a concentration of 0.5 
g/100 ml), a specific surface area (measured by the BET method, using 
argon) of 50 to 200 m.sup.2 /g, preferably 70 to 110 m.sup.2 /g, and a 
freeness (measured by the Schopper-Riegler method) of 
30.degree.-80.degree.SR, preferably 40.degree. to 70.degree.SR. 
The present invention also relates to a process for the production of 
polyoxymethylene fibrids by flashevaporation of a superheated solution of 
the polymer through a nozzle into a low-pressure zone, which comprises 
using a mixture of 50-95% by weight of a lower alcohol with 1-4 C atoms 
and 5-50% by weight of water as the solvent. 
Suitable materials for the production of the fibrids according to the 
invention are the known polyoxymethylenes. By these products there may be 
understood homopolymers of formaldehyde or of a cyclic oligomer of 
formaldehyde, for example trioxane, the hydroxyl end groups of which have 
been stabilized chemically, for example by etherification or 
esterification, against degradation.

According to the invention, the term polyoxymethylene also includes 
copolymers of formaldehyde or of a cyclic oligomer of formaldehyde, 
preferably trioxane, in which, in addition to oxymethylene units, the 
copolymers have, in the main valency chain, oxyalkylene units with at 
least two, preferably two to eight and in particular two to four, adjacent 
carbon atoms, and primary alcohol end groups. The comonomer content of the 
copolymers is appropriately 0.1 to 20% by weight, preferably 0.5 to 10 and 
in particular 0.7-5 % by weight. 
As compounds which are suitable for copolymerization with formaldehyde or 
cyclic oligomers of formaldehyde, preferably trioxane, there are used, 
above all, cyclic ethers, preferably with 3, 4 or 5 ring members, and/or 
cyclic acetals other than trioxane, preferably formals with 5 to 11, 
preferably 5, 6 or 7, ring members and/or linear polyacetals, preferably 
polyformals. 
Suitable comonomers for trioxane are, in particular, compounds of the 
formular 
##STR1## 
in which (A) R.sup.1 and R.sup.2 are identical or different and each 
denote a hydrogen atom, an aliphatic alkyl radical with 1-6, preferably 1, 
2, 3 or 4, carbon atoms or a phenyl radical and (a) x is 1, 2 or 3 and y 
is zero, or (b) x is zero, y is 1, 2 or 3 and z is 2, or (c) x is zero, y 
is 1 and z is 3, 4, 5 or 6, or (B) R.sup.1 denotes an alkoxymethyl radical 
with 2-6, preferably 2, 3 or 4, carbon atoms or phenoxymethyl radical, and 
wherein x is 1 and y is zero or y and z are 1, and R.sup.2 has the 
abovementioned meaning. 
Cyclic ethers which are employed are, above all, epoxides, for example 
ethylene oxide, propylene oxide, styrene oxide, cyclohexene oxide, 
oxacyclobutane and phenyl glycidyl ether. 
Suitable cyclic acetals are, above all, cyclic formals of aliphatic or 
cycloaliphatic .alpha.,.omega.-diols with 2 to 8, preferably 2, 3 or 4, 
carbon atoms, the carbon chain of which can be interrupted by an oxygen 
atom at intervals of 2 carbon atoms, for example glycol formal 
(1,3-dioxolane), propanediol formal (1,3-dioxane), butane-diol formal 
(1,3-dioxepane) and diglycol formal (1,3,6-trioxocane), as well as 
4-chloromethyl-1,3-dioxolane and hexanediol formal (1,3-dioxoane). 
Unsaturated formals, such as butenediol formal (1,3-dioxacyclohept-5-ene) 
can also be used. 
Suitable linear polyacetals are both homopolymers and copolymers of the 
cyclic acetals defined above, and linear condensates of aliphatic or 
cycloaliphatic .alpha.,.omega.-diols with aliphatic aldehydes or 
thioaldehydes, preferably formaldehyde. Homopolymers of linear formals of 
aliphatic .alpha.,.omega.-diols with 2-8, preferably 2-4, carbon atoms, 
for example poly-(1,3-dioxolane), poly-(1,3-dioxane) and 
poly-(1,3-dioxepane), are used in particular. 
Compounds with several polymerizable groups in the molecule, for example 
alkylglycidyl formals, polyglycol diglycidyl ethers, alkanediol diglycidyl 
ethers or bis-(alkanetriol) triformals, are optionally also used as 
additional comonomers for trioxane, and in particular in an amount of 0.05 
to 5 % by weight, preferably 0.1 to 2 % by weight, relative to the total 
amout of monomer. Such additional comonomers are described, for example, 
in German Auslegeschrift No. 2,101,817. 
The values of the reduced specific viscosity (RSV values) of the 
polyoxymethylenes employed according to the invention and hence also of 
the fibrids obtained therefrom are in general between 0.4 and 2.0 
d1.multidot.g.sup.-1, preferably between 0.6 and 1.20 d1.multidot.g.sup.-1 
(measured at 140.degree. C. in butyrolactone, which contains 2 % by weight 
of diphenylamine, in a concentration of 0.5 g/11 ml). 
The crystallite melting points of the polyoxymethylenes are in the range 
from 140.degree. to 180.degree. C., preferably 150.degree. C. to 
170.degree. C., and the densities of the polyoxymethylenes are between 
1.38 and 1.45 g.multidot.ml.sup.-1, preferably 1.40 and 1.43 
g.multidot.ml.sup.-1 (measured in accordance with the method of DIN 
53,479). 
If polymers with a lower RSV value than that given above are used, fibrids 
are indeed also formed; nevertheless, they become relatively short and are 
mixed with an increasing amount of non-fibrous constituents. The length of 
fiber and the degree of fineness and of branching of the fibrids can thus 
be controlled by the RSV value of the polymer, so that the preferred 
ranges depend on the field of use of the fibrids. At higher RSV values 
than that given above, the danger of the formation of plexus filaments or 
of predominantly two-dimensional, film-like structures increases. 
The oxymethylene copolymers, which are preferably binary or ternary, used 
according to the invention are prepared in a known manner by polymerizing 
the monomers in the presence of cationically active catalysts at 
temperatures between 0.degree. and 100.degree. C., preferably between 
50.degree. and 90.degree. C. (compare, for example, U.S. Pat. 
specification No. 3,027,352). Catalysts which are used in this preparation 
are, for example, Lewis acids, for example boron trifluoride and antimony 
pentafluoride, and complex compounds of Lewis acids, preferably etherates, 
for example boron trifluoride diethyl etherate and boron trifluoride 
di-tert.-butyl etherate. Proton acids, for example perchloric acid, and 
salt-containing compounds, for example triphenylmethyl 
hexafluorophosphate, triethyloxonium tetrafluoborate or acetyl 
perchlorate, are also suitable. The polymerization can be carried out in 
bulk, in suspension or in solution. In order to remove unstable 
constituents, it is expedient to subject the copolymers to controlled 
partial thermal or hydrolytic degradation until primary alcohol end groups 
are obtained (compare U.S. Pat. specification Nos. 3,103,499 and 
3,219,623). 
The homopolymers of formaldehyde or of trioxane which are used according to 
the invention are likewise prepared in a known manner by catalytic 
polymerization of the monomer (compare, for example, U.S. Pat. 
specifications Nos. 2,768,994 and 2,989,505). 
The fibrids according to the invention are irregular in length, the length 
in most cases being about 0.1 to 5 mm, preferably 0.2 to 2 mm. The 
cross-section is likewise irregular in shape and size; the apparent 
diameter is predominantly about 1 to 200 .mu.m, preferably 2 to 50 .mu.m. 
Since the fibrids according to the invention are highly branched, they also 
have a high specific surface area (measured by the BET method, using 
argon) of 50 to 200 m.sup.2 /g, preferably 70 to 110 m.sup.2 /g, the 
samples being dried beforehand by freeze-drying. Accordingly, the freeness 
is also high and is 30.degree. to 80.degree. SR, preferably 40.degree. to 
70.degree.SR. The freeness is determined as the Schopper-Riegler value in 
accordance with the method in Leaflet V/7/61 (old version 107) of the 
Association of Cellulose and Paper Chemists and Engineers (published on 
July 1, 1961). 
The polyoxmethylene fibrids according to the invention have hydrophilic 
surface properties and are therefore readily dispersible in water, in most 
cases even without wetting agents. Filters produced therefrom have 
improved adsorptive properties. In special cases it may also be expedient 
to produce hydrophobic surface properties by adding suitable agents which 
impart hydrophobic properties. 
On the basis of their branched morphology, the fibrids according to the 
invention can very readily be processed to paper in a known manner, for 
example as described in German Pat. specification No. 1,290,040. The 
strength properties of these pure polyoxymethylene papers (tensile 
strength, initial wet strength, surface strength, Z-tensile strength and 
folding strength) are superior to those of corresponding papers according 
to the state of the art. Thus, the tensile strength of sheets of paper 
produced from the fibrids according to the invention on a Rapid-Kothen 
sheet-forming apparatus is 2 to 25 N/mm.sup.2, preferably 4 to 20 
N/mm.sup.2 and in particular 7 to 17 N/mm.sup.2, measured with the Instron 
testing unit with a sheet weight of 160 g/m.sup.2, a sample width of 15 
mm, an elongation rate of 10 mm/minute and a measuring length of 100 mm. 
The measurements are carried out at 23.degree. C. and at a relative 
humidity of 50%. 
Excellent papers can also be produced with mixtures with other fibrous 
substances, such as cellulose, cellulose fibers and synthetic fibers, and 
these papers can be glazed, coated, laminated and printed in the customary 
manner. The strength properties of these mixed papers are also 
considerably better than those of comparable papers produced from known 
polyoxymethylene fibrids. 
The POM fibrids according to the invention can be employed, for example, 
for wallpapers, filters, labels, graph paper and other special papers and 
the like. The polyoxymethylene fibrids can also be processed on 
cardboard-making machines, the resulting cardboard having an excellent 
resistance to water. The polyoxymethylene fibrids according to the 
invention can furthermore be employed in "nonwovens" and can be used as 
thickeners in rapid curing cutback and in dyes, plaster, adhesives, 
sealing compositions and coating materials based on unsaturated 
polyesters, epoxide resins, bitumen pastes and PVC plastisols. 
The known flash-evaporation of a superheated polymer solution under 
pressure into a low-pressure zone, such as is described, for example, in 
Belgian Patent specification No. 568,524, is carried out in the process, 
according to the invention, for the production of the fibrids. 
In this procedure, a solution, which is preferably homogeneous, of the 
polymer is first prepared, it being possible to use dry or solvent-moist 
powder or granules, depending on the manufacturing process, and the 
polymer being mixed with the solvent and, for example, being heated in 
pressure autoclaves, while stirring, for example by steam jacket heating 
or by blowing in steam. If, during polymerization or the subsequent 
stabilizing and working-up process, the polyoxymethylenes are obtained as 
a solution or suspension in an alcohol/water mixture of the composition 
according to the invention, it is also possible to employ this solution or 
suspension directly in the process according to the invention. 
According to the invention, as already disclosed, a mixture of 50-95% by 
weight of a lower alcohol with 1-4 C atoms and 5-50% by weight of water, 
in each case relative to the total solvent mixture, is employed as the 
solvent. Possible lower alcohols for this are for example, methanol, 
ethanol, isopropanol, n-propanol, n-butanol, i-butanol or t-butanol, or 
mixtures of these alcohols. If higher alcohols with more than 4 C atoms, 
for example n-hexanol, are used, fibrids are indeed also formed, but the 
temperature required for the preparation of the solution is then 
relatively high. Methanol and isopropanol are preferably employed. 
The ratio in the mixture of alcohol and water is of considerable importance 
for the production of fibrids. For example, if less than 50% by weight of 
the lower alcohol and more than 50% by weight of water are used, plexus 
filaments are readily formed. Nevertheless, this dividing line between the 
formation of fibrids and plexus filaments is not sharp and can also be 
influenced to a certain extent by the choice of the temperature and of the 
concentration of the solution and by the choice of the molecular weight 
and of the dimensions of the nozzle and also of the level of the pressure 
in the expansion zone. With increasing temperature, decreasing polymer 
concentration and decreasing molecular weight and with an increasing 
length/diameter ratio of the nozzle and decreasing pressure in the 
expansion zone, this dividing line shifts somewhat in the direction of the 
higher content of water. If more than 95% of the lower alcohol and less 
than 5% of water are used, the dissolving temperature required is in most 
cases uneconomically high. A mixture of 45-10% by weight of water and 
55-90% by weight of lower alcohol is preferred. 
The concentration of the polymer in the solvent mixture is as a rule 
between 10 and 300 g per liter of solution, preferably between 50 and 200 
g per liter. Lower concentrations are as a rule uneconomical since they 
require a high circulation of solvent; higher concentrations frequently 
involve the danger of the formation of plexus filaments. The upper limit 
of the polymer concentration depends to a certain extent on the molecular 
weight; the lower the molecular weight, the higher is the permissible 
concentration. 
The temperature of the solution of the polyoxymethylene depends on the 
molecular weight of the polymer, on the nature and amount of comonomer and 
on the composition of the solvent. If homogeneous solutions are used, the 
lower limit of the temperature is to be regarded as the dissolving 
temperature which is required, while the upper limit of the temperature is 
essentially imposed only by economic considerations. The dissolving 
temperatures is known for many examples, and can otherwise easily be 
interpolated from known data or experimentally determined by the expert. 
In any case, the lower limit of the temperature must be such that 
evaporation sufficient for the formation of fibrids takes place in the 
expansion zone under the chosen pressure conditions. This is as a rule the 
case if it is about 30.degree. C. above the boiling point under normal 
pressure and at the same time the solidification point of the polymer is 
reached. For the preferred alcohols in their preferred ratios in the 
mixture, the preferred temperature range is between 150.degree. and 
180.degree. C. 
The solution is as a rule under the autogenous vapor pressure of the 
solvent mixture at this temperature, but the pressure can be increased 
considerably by an inert gas pressure or by a pump. In general, the 
pressure is between 15 and 60 bars, preferably between 15 and 30 bars. 
In addition to the polymers, the solution can also contain auxiliaries from 
the polymerization, for example decomposition products of the catalysts 
for cationic polymerization, which are described in British Pat. 
specification No. 1,146,649, in German Offenlegungsschriften Nos. 
1,595,705 and 1,595,668 and in German Auslegeschriften Nos. 1,199,504 and 
1,175,882, or basic compounds in order to remove unstable constituents 
until the primary alcohol end group is obtained (for example lower 
tertiary aliphatic amines, such as triethylamine or triethanolamine, or a 
secondary alkali metal phosphate, such as disodium hydrogen phosphate 
(compare U.S. Pat. Nos. 3,174,948, 3,219,623 and 3,666,714)), and the 
resulting reaction products, for example methylal, trioxane, tetroxane, 
formic acid and methyl formate. 
The polymer solution can also contain the most diverse known additives. 
Possible additives are, for example, the customary nucleating agents which 
accelerate crystallization and with the aid of which the morphology of the 
fibrids can be influenced, such as, for example, branched or crosslinked 
polyoxymethylenes, talc or boron nitride (compare German Pat. 
specification No. 2,101,817 and German Offenlegungsschrift No. 1,940,132). 
There may also be mentioned in this context the known stabilizers against 
the influence of heat, oxygen and/or light, such as are described, for 
example, in German Offenlegungsschrift No. 2,043,498. Bisphenol compounds, 
alkaline earth metal salts of carboxylic acids and guanidine compounds are 
particularly suitable for this. Bisphenol compounds which are used are 
chiefly esters of monobasic 4-hydroxyphenylalkanoic acids which contain 
7-13, preferably 7, 8 or 9, carbon atoms and are monosubstituted or 
disubstituted on the nucleus by an alkyl radical containing 1-4 carbon 
atoms, with aliphatic dihydric, trihydric or tetrahydric alcohols which 
contain 2-6, preferably 2, 3 or 4, carbon atoms, for example esters of 
.omega.-(3-tert.-butyl-4-hydroxy-phenyl)-pentanoic acid, 
.beta.-(3-methyl-5-tert.-butyl-4-hydroxy-phenyl)-propionic acid, 
(3,5-di-tert.-butyl-4-hydroxy-phenyl)-acetic acid, 
.beta.-(3,5-di-tert.-butyl-4-hydroxy-phenyl)-propionic acid or 
(3,5-di-isopropyl-4-hydroxy-phenyl)-acetic acid with ethylene glycol, 
propane-1,2-diol, propane-1,3-diol, butane-1,4-diol, hexane-1,6-diol, 
1,1,1-trimethylolethane or pentaerythritol. 
Alkaline earth metal salts of carboxylic acids which are used are, in 
particular, alkaline earth metal salts of aliphatic monobasic, dibasic or 
tribasic carboxylic acids which preferably contain hydroxyl groups and 
have 2-20, preferably 3-9, carbon atoms, for example the calcium or 
magnesium salts of stearic acid, ricinoleic acid, lactic acid, mandelic 
acid, malic acid or citric acid. 
Possible guanidine compounds are compounds of the formula 
##STR2## 
in which R denotes a hydrogen atom, a cyano group or an alkyl radical with 
1-6 carbon atoms, for example cyanoguanidine, N-cyano-N'-methylguanidine, 
N-cyano-N'-ethylguanidine, N-cyano-N'-isopropylguanidine, N-cyano-N'- 
tert.-butylguanidine or N,N'-dicyanoguanidine. The guanidine compound is 
employed, if appropriate, in an amount of 0.01-1% by weight, preferably 
0.02-0.5% by weight, relative to the total weight. 
Furthermore, the solution can additionally also contain other additives, 
such as known antistatic agents, flameproofing agents or slip agents or 
lubricants and the like. 
Filled fibrids can be obtained according to the invention by uniformly 
suspending mineral fillers in the polymer solution and then proceeding as 
described. Suitable fillers are titanium dioxide, calcium carbonate, talc, 
woolastonite, dolomite, silicon dioxide and the like. 
Dyed polyoxymethylene fibrids can be obtained by dissolving or dispersing 
dyestuffs in the polymer solution. For some applications, the addition of 
optical brighteners is also of interest. 
Surface-active agents, such as oxyethylated alcohols, carboxylic acids or 
amines, alkanesulfonates or polymers carrying hydroxyl groups, such as 
polyvinyl alcohol or carboxymethylcellulose, can also be added to the 
solution in order to improve the dispersibility of the polyoxymethylene 
fibrids. 
The polymer solution is then forced through one or more nozzles, the design 
(size, shape and length) of which can indeed influence the dimensions of 
the fibrids formed and change somewhat the concentration limits given for 
the solvent mixture, but is not essential to the invention. Suitable 
nozzles are described, for example, in Belgian Pat. specification No. 
568,524. In this context there may be mentioned simple nozzles with a 
diameter of, for example, 0.5-5 mm and a length of 0.1 to 1,000 cm, 
conical nozzles with a comparable annular gap and two-material nozzles, it 
being possible to use inert gases, such as hydrogen, steam and the like, 
or liquids, for example superheated water, as the propelling medium. 
The polymer solution passes through the nozzle into a zone of lower 
pressure, in which the solvent is completely or partly evaporated 
spontaneously. It is also possible to subject the solution to a controlled 
pressure release, before its exit from the nozzle, for example by passing 
it through a chamber or a tube which has a greater diameter than the 
outlet opening of the nozzle. Any residues of the lower alcohol present 
can be removed, for example, by steam. As a rule, the low-pressure zone is 
a closed container, from which the solvent vapors are removed with a pump. 
These vapors can be recycled back, after condensation, into the process. 
The pressure in the low-pressure zone is between 0.02 bar and 2.0 bars, 
preferably between 0.1 and 1 bar. 
The fibrids are then freed from most of the solvent which has not 
evaporated off, using known mechanical methods, for example by filtration, 
centrifugation and the like, and, if necessary, are washed with water and 
then either put to use in moist form or loosened mechanically to a density 
of 10 to 200 g/l, preferably 30 to 100 g/l, and then dried in a steam of 
hot gas. 
A considerable advantage of the process according to the invention is that 
the fibrids are formed directly during atomization and the troublesome and 
energy-intensive two-stage process, which consists of the production of 
plexus filaments of infinite length and subsequent mechanical comminution, 
is thereby avoided. Since the plexus filaments have a high extensibility, 
this mechanical comminution can be carried out only with a high energy 
consumption. 
This process advantage and the substance advantages of the polyoxymethylene 
fibrids according to the invention could not be predicted and are 
therefore to be regarded as surprising. 
The examples which follow are intended to illustrate the invention in more 
detail. 
EXAMPLE 1 
2 kg of a copolymer which is obtained from 98% by weight of trioxane and 2% 
by weight of ethylene oxide, and has a RSV value of 0.82 d1/g, and 20 ml 
of triethylamine are dissolved in a mixture of 13 l of methanol and 7 l of 
water at 160.degree. C., while stirring, in a pressure vessel which has a 
volume of 70 l and is provided with a fiveblade multi-stage impeller 
countercurrent stirrer. An overall pressure of 30 bars is established with 
the aid of nitrogen. After opening the bottom valve, the solution flows 
through a single-hole nozzle with a diameter of 1 mm and a length of about 
0.5 mm into the gas spaced of a closed collecting tank which has a 
capacity of 200 l and is filled with 40 l of water and in which a vacuum 
pump ensures a pressure of 0.8 bar. When atomization has ended, methanol 
is driven off for 30 minutes by sweeping with steam, the pressure being 
maintained at 0.8 bar. The fibrid suspension formed is removed through a 
bottom flap and centrifuged down to a solids content of about 20%. 
In order to determine the specific surface area of the resulting fibrids, a 
sample was freeze-dried and measurements were carried out by means of the 
BET method, using argon. The specific surface area was 73 m.sup.2 /g and 
the freeness was 50.degree.SR. 
Fiber classification in a Brecht Holl fiber classification unit serves as 
an indirect measure of the fiber length distribution. In this unit, 2 g of 
fibers are washed successively through sieves of different sizes for 10 
minutes with the aid of water jets under a water pressure of 0.5 bar and 
using a pulsating suction membrane (with as large as possible a stroke). 
The residue on the sieves with mesh widths of 1.2 mm, 0.4 mm and 0.12 mm 
and the amount which passes through are given in percent. 
The following fractions were determined in the fiber classification 
described above: 
______________________________________ 
Residue 1.2 mm: 0% 
Residue 0.4 mm: 17% 
Residue 0.12 mm: 54% 
Amount passing 0.12 mm: 29% 
through 
______________________________________ 
Sheet of paper of 160 g/m.sup.2 are produced with these fibrids on a 
Rapid-Kothen sheet-forming apparatus. The tensile strength measured for 
these sheets was 14.3 N/mm.sup.2. 
COMISON EXPERMIMENT 1 
As described in Example 1, 2 kg of the same copolymer were dissolved in 20 
l of methyl ethyl ketone under the same temperature and pressure 
conditions and the solution was atomized into the same tank through the 
same nozzle, a pressure of 0.8 bar likewise being maintained in the tank. 
The product formed consisted exclusively of continuous plexus filaments 
which, in this form, were unsuitable for the production of paper. A sample 
was therefore comminuted by means of a disk refiner, in 6 passes. The 
following values were found on the fibrids thus produced: 
______________________________________ 
Specific surface area 39 m.sup.2 /g 
Freeness 27.degree. SR 
Fiber classification: 
Residue 1.2 mm 3% 
Residue 0.4 mm 4% 
Residue 0.12 mm 26% 
Amount passing 67% 
through 0.12 mm 
Tensile strength of the 
0.37 N/mm.sup.2 
sheet (160 g/m.sup.2) 
______________________________________ 
Plexus filaments which are unsuitable for the production of paper were also 
obtained when the polymer concentration was reduced to 0.01 kg/1. 
COMISON EXPERIMENT 2 
The procedure followed was as in Example 1. However, the solvent 
composition was 5 l of methanol and 15 l of water. Plexus filaments which, 
in this form, were unsuitable for the production of paper were exclusively 
formed. 
EXAMPLE 2 
The procedure followed was as in Example 1. However, the RSV value of the 
polymer used was 1.0 dl/g and the solvent composition was 15 l of methanol 
and 5 l of water. The following values were measured on the resulting 
fibrids: 
______________________________________ 
Specific surface area 98 m.sup.2 /g 
Freeness 61.degree. SR 
Fiber classification: 
Residue 1.2 mm 0% 
Residue 0.4 mm 5% 
Residue 0.12 mm 55% 
Amount passing 40% 
through 0.12 mm 
Tensile strength of paper 
13.2 N/mm.sup.2 
______________________________________ 
EXAMPLE 3 
The procedure followed was in Example 1, but isopropanol was used instead 
of methanol. The following values were measured on the resulting fibrids: 
______________________________________ 
Specific surface area 108 m.sup.2 /g 
Freeness 68.degree. SR 
Fiber classification: 
Residue 1.2 mm 6% 
Residue 0.4 mm 4% 
Residue 0.12 mm 24% 
Amount passing 66% 
through 0.12 mm 
Tensile strength of paper 
11.7 N/mm.sup.2 
______________________________________