Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR MAKING POLYMER FILAMENTS
WHICH HAVE A HIGH TENSILE STRENGTH
AND A HIGH MODULUS
BACKGROUND OF THE INVENTION
S The present invention relates to polymer
filaments which have a high tensile strength and a
high modulus, and to a process for making such
filaments.
Filaments are usually made by spinning
linear polymers. A polymer is first made into a
liquid such as a melt or a solution and is then
spun forming a filament. Although other
substances are capable of being spun the chain
formatîon of the macromolecules is an important
consideration as side branches have an adverse
effèct on filament formation and mechanical
properties. Therefore, the production of
filaments in accordance with the process of the
present invention is premised on th~ use of linear
polymers although a limited degree of branching is
usually unavoidable and ~illhaveto be.accepted.
The rando,nly oriented chains of molecules
in this filament must next be oriented lengthwise
in the filament which is accomplished by
stretching.
Stretching of the filament results in
orienting the chain macromolecules lengthwise
which also increases the strength of the
filaments. However, in many cases the strength of
the stretched filaments is still far below the
value that is theoretically expected. Many
attempts have already been made at producing
filam~ents with a tensile strength and a modulus
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closer to what are theoretically possible. These attempts, of which a survey
is given in publications by Juyn in Pla5tica 31 ~1978~ 262-270 and Bigg in
Polymer Eng. Sci. 16 ~1976) 725~734, have ~ailed to yield satisfac~ory results.
In a number of cases the modulus was improved sufficiently, but not the
tensile strength. There was an additional drawback in tha~ the filament
formation was so slow that economic production would be impossible.
It has now been found that polymer filaments which have a high
tensile strength and a high modulus can be made by stretching a polymer fila-
ment containing an appreciable amount of polymer solvent at a temperature
between the swelling point and the melting point of the polymer. Preferably
a spinnable solution is spun by any of the known methods, the resultant fila-
ment is cooled to below the solution temperature of the polymer, the tempera~
ture of the filament is then adjusted at a value between the swelling point
of the polymer in the solvent and the melting point of the polymerJ and the
filament is then stretched.
According to one aspect of the present invention there is provided
a process for making polymer filaments which have a high tensile strength and
a high modulus comprising the steps of:
a) spinning a solution of a linear polymer, ranging from about
1% to about 5% by weight of polymer to solvent, through a spinning aperture
to form a filament,
b) cooling said filament of step (a) to below the solution tempera-
ture of the polymer, either in a zone containing a gas without promoting the
evaporation of the solvent or with a cooling liquid that does not dilute or
extract the solvent in the filament,
c) bringing said filament to a temperature between the swelling
point of the polymer in the solvent and the melting point of the polymer, and
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d) stretching the filament, that still contains at least 25% by
ueight of solvent relative to the polymer, at a stretching ratio of at
least 5.
According to another aspect of the present invention there is
provided a process for making polymer filaments which have a high tensile
strength and a high modulus comprising the steps of:
a) spinning a solution of a linear polymer, ranging from about
1% to about 5% by weight of polymer to solvent, through a spinning aperture
to form a filament,
b) cooling said filament of step (a) to below the solution tempera-
ture of the polymer to form a polymer gel filament either in a zone containing
a gas without promoting the evaporation of the solvent or with a cooling liquid
that does not dilute or extract the sdlvent in the filament,
c~ adjusting the temperature of said polymer gel filament at a
value between the swelling point of the polymer in the solvent and the melting
point of the polymer,
d) stretching the filament, that still contains at least 25% by
weight of solvent relative to the polymer, at a stretching ratio of at least 5,
wherein there is at least a partial evaporation of the solvent, and
e) recovering a substantially solvent-free filament.
In the dry spinning process which is widely applied on a technical
scale, a solution of a spinnable polymer is spun in a shaft through which air
is blown to evaporate all or most of all of the solvent to form the filament.
While the air is usually heated the temperature in the shaft is kept below the
melting point of the polymer in order to increase the mechanical strength of
the filament, which is veTy low as it exits the spinning aperture.
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The mechanical strength can be further
increased in the subsequent stretching step at
temperatures below the melting point of the
polymer.
DESCRIPTION OF THE INVENTION
According to the present invention, the
evaporation of the solvent from the filament
immediately after spinning is not promoted during
the cooling phase. The filament can be cooled to
below the solution temperature, and particularly
to below the swelling point of the polymer in the
solvent by any suitable way, inclùding for
e~ample, by passing the filament through a water
bath, or through a shaft without any air or only
minimal amounts of air being blown through the
shaft. Some evaporation of the solvent from the
filament will often take place spontaneously and
cannot be prevented. This is acceptable as long
as the evaporation is not actively promoted and
the amount of solvent in the filament is not
reduced~to a low value, e.g., to less than 25% by
weight of solvent relative to the polymer.
Preferably the amount of solvent will not be less
than equal amounts by weight of solvent and
polymer. If desired, the evaporation of the
solvent may be reduced or suppressed by carrying
out the spinning in an atmosphere containing
solvent vapor.
In cooling to below the solution
temperature, in particular to below the swelling
temperature of the polymer in the solvent, the
polymer precipitates from the solution, and a gel
is formed. A filament consisting of this polymer
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gel has sufficient mechanical strength for further
processing, for example, by means of the guides,
rolls, and the like customarily used in spinning
techniques. A filament of this kind is heated to
a temperature between the swelling point of the
filament in the solvent and the melting point of
the polymer, and is then stretched at that
temperature. This can be effected by passing the
filament into a zone containing a gaseous or
liquid medium kept at the required temperature. A
tubular oven with air for the gaseous medium is
very suitable, but it is also possible to use a
liquid bath or any other suitable device. A
gaseous medium is easier to handle, and is
therefore preferable.
When the filament is being stretched in a
gaseous medium, solvent evaporation will occur.
If a liquid medium is used, there will be
dissolution of the solvent in the liquid.
Preferably, the evaporation is promoted by
suitable means such as removal of the solvent
vapor, by for instance leading a gas or air
current past the filament in the stretching
zone. At least part of the solvent should be
evaporated, but preferably at least the greater
part of the solvent is evaporated, so that by the
end of the stretching zone there will be at most
only a small amont, e.g., not more than a few
percent, calcu].ated on a solid-substance basis of
solvent contained in the fi~lament. The filament
5J~ould PrQ ~: e ra bJ
B which is eventually obtained must be free of
solvent, and it is advantageous to apply such
conditions that it is free, or virtually free, of
solvent by the time it exits from the stretching
zone.
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Surprisingly, the process in accordance
with the present invention can produce filaments
- that are considerably stronger, i.e. are filaments
with considerably higher tensile~strength and
s higher modulus than filaments of the same material
made by any of the usual dry spinning processes.
By means of the methods described in the above-
mentioned publications by Juyn and Bigg filaments
of higher modulus have been obtained, but the
tensile strength is still unacceptable. Moreover,
the productivity of these methods is low.
The process in accordance with the
present invention differs from the usual dry
spinning processes in that a filament containing
an appreciable amount of solvent is stretched with
removal of solvent at a temperature at which the
spinnable material will at least swell in the
solvent, whereas in the usually applied spinning
processes solvent free filaments are subjected to
stretching.
One requirement of dry spinning is that
the linear polymer be soluble in a suitable
solvent. For any given soluble polymer a number
of different solvents are available. A suitable
solvent is one with a boiling point not too high
so that it will not be difficult for the solvent
to be evaporated from the filament, and not too
low so that it is not too volatile and thus hinder
filament formation because of rapid evaporation~
Also, if it is too volatile, it may have to be
processed under pressure to prevent rapid
evaporation.
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Dissolution of a polymer in a suitable
solvent involves swelling. While the solvent is
being absorbed and the volume increases, a swollen
gel is formed, which, however, because of its
consistency and stability of shape is still
regarded as a kind of solid substance. It is
generally assumed that the polymer is composed of
ordered or crystalline areas and less ordered or
amorphous areas. The ordered areas are believed
to act as anchoring points and thus lend stability
of shape to the gel. The formation of the gel and
the dissolution are time dependent. A given
polymer can be dissolved in a given solvent only
above a given temeprature. Below this solution
temperature only swelling takes place, and
according as the temperature is lowered, the
swelling becomes less, until at a certain
temperature the swelling will be negligible. The
swelling point or swelling temperature is
considered to be that temperature at which a
distinct increase in volume and a distinct
absorption of solvent in an amount of about 5 to
10% of the polymer weight occur. A simple rule of
thumb is that the swelling temperature above which
the stretching is to be effected is the
temperature at which 10% of solvent is
unquestionably absorbed into the swelling polymer.
In dry spinning processes usually 5-30~
wt. solutions are used ~or technical and economic
reasons. Such solutions are also suitable for the
process of the present invention, although
generally solutions of lower concentration may be
used. Solutions in the range of about 1 to 5% by
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weight of polymer to solvent can be advantageously
used while still lower concentrations can be used
at times but these do not present advantages and
are economically disadvantageous.
Suitable stretching ratios can be easily
determined by experiment. Tensile strength and
modulus of the ~ilaments are, within certain
limits, about proportional to the stretching
ratio. Accordingly as the filaments are to be
stronger, a greater stretching ratio will have to
be selected.
The stretching ratio is at least 5, by
preferance at least 10, and more in particular at
least 20. High stretching ratios such as 30 to 40
and even higher can be applied without objection,
and will result in filaments whose tensile
strength and modulus are appreciably higher than
those of filaments made by the usual dry spinning
processes.
In dry spinning proce~ses the diameters
of the spinning apertures in the spinning nozzles
are usually small. In general, these diameters
range from about 0.02 mm to about 1.0 mm. When
small spinning apertures of les than about 0.2 mm
are used, the spinning process is highly sensitive
to the presence of impurities in the spinning
solution. Therefore the spinning solution shouid
be carefully freed and kept free of solid
impurities. In most cases, filters are placed on
the spinning nozzles, but regardless the spinning
nozzles need to be cleaned after a short time, and
blockage still occurs frequently. One significant
advantage of the process of the present invention
is that larger spinning apertures can be used.
Spinning apertures of for example about 0.5 mm to
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2.0 mm or larger can be used because considerably
higher stretching ratios can be applied and,
additionally, generally lower polymer
concentrations in the spinning solution are used.
The process of the present invention has
a wider application and can be used in general for
any materials that can be dry spun to filaments.
Polymers that can be spun by the process
according to the present invention are, for
instance, polyolefins such as polyethylene,
polypropylene, ethylene-propylene copolymers,
polyoxymethylene, polyethylene oxide; polyamides,
` ~ such as the various types of nylon; polyesters,
such as polyethyleneterephtalate, poly-
acrylonitrile: vinyl polymers such as polyvinyl-
alcohol, polyvinylidenefluoride.
Polyolefins such as polyethylene,
polypropylene, ethylene-propylene copolymers and
higher polyolefins can without objection be
dissolved in hydrocarbons such as saturated
aliphatic and cyclic hydrocarbons as well as
aromatic hydrocarbons, or mixtures thereof such as
mineral oil fractions. Very suitable are
aliphatic or cyclic hydrocarbons such as nonane,
decane, undecane, dodecane, tetralin, decalin,
etc., or mineral oil fractions corresponding in
boiling range. Polyethyelene or polypropylene is
preferably dissolved in decalin or dodecane. The
present method is particularly suitable for the
preparation of filaments of polyolefins,
preferably polyethylene.
It is also possible to make filaments of
solutions of two or more polymers in a common
solvent by the present process. For this purpose
the polymers need not be miscible with each
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other. For instance, it is possible to dissolve together in decalin or
dodecane polyethylene and polypropylene whose melts are i~miscible, and to
spin the solutions thus obtained.
The fila~ents according to the invention can be used for many
purposes. They can be applied as reinforcement in many kinds of materials
for which reinforcement with fibers or filaments is known, such as tire yarnsJ
and for all possible applications in which low weight combined with high
strength is a desirable feature.
The invention will be further described with reference to the
accompanying drawings showing by way of example a preferred embodiment of
the:invention in which:
Figure 1 is a diagram showing the process for making polymer
filaments according to the invention, wherein
A refers to the polymer solution
B refers to a cooling bath
C refers to a wet filament
D refers to a feed reel
E refers to an oven
F refers to a stretching reel
Figure 2 is a diagram showing the dependency of the tensile
strength (a) in GPa versus the stretching ratio ~b).
Figure 3 is a diagrAm showing the dependency of the modulus (b~
in GPa versus the stretching ratio (b).
The invention will be elucidated by means of the following examples,
without being restricted thereby.
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EXA~PLES
Example 1
A polyethylene of high molecular weight, with Mw Y 1.5 x 106, was
dissolved in decalin at 145C to fo~m a 2% wt. solution. This solution was
spun through a spinning nozzle with a spinning aperature of 0.5 mm dia., at
130C. The filament was passed into a water bath kept at room temperature,
where it was cooled. The cooled, 0.? mm thick filament, which was gel-like in
appearance and still contained about 98% solvent, was next passed through
a tubular oven heated at 120C, and stretched, with the use of various stretch-
ing ratios. This process is shown in diagram in Figure 1.
Figures 2 and 3 show, respectively, the tensile strength and themodulus plotted against the stretching ratio. A modulus of more than 60
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GPa and tensile strength of almost 3 can be
attained using the process of the present
invention whereas the modulus of polyethylene
filaments madè in the conventional way is-2-3 GPa,
and the tensile strength about 0.1 GPa.
The values of modulus and tensile
strength of polyethylene filaments made with
different stretching ratios as polotted in Figures
2 and 3 are given in Table 1.
Polyethylene filaments having a tensile
strength of over 1.2 GPa can easily be produced by
means of the present process.
.
Table 1
Expt. Stretching Modulus, Tensile strength
ratio GPa GPa
1 1 2.4 0.09
2 3 5.4 0.27
3 7 17.0 0.73
4 8 17.6 0.81
11 23.9 1.32
6 12 37.5 1.65
7 13 40.9 1.72
8 15 41.0 1.72
9 17 43.1 2.11
69.0 2.90
11 32 90.2 3.02
... . . . . . . .. _ _ _ _ _ _
Example 2
In accordance with the process described
in Example 1, a 2% wt. solution of a mixture of
equal parts of high-molecular-weight polyethylene,
with Mw ~ 1.5 x 106, and a high-molecular-weight
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polypropylene, with Mw ~ 3.0 x 106, was spun at
140C and stretched at 130C, using a stretching
ratio of 20. The filaments had a tensile strength
of 1.5 GPa.
S Example 3
In accordance with the process described
in Example 1, a 2% wt. solution of isotactic
polypropylene, with Mw - 3.0 x 106, was spun at
140C and stretched at 130C, using a stretching
ratio of 20. The tensile strength of the
resulting filaments was 1 GPa.
