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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improved solvent having a low toxicity,
a low combustibility and a low capability of depleting an ozonosphere and
a polyolefin solution using said solvent. Further, the present invention
provides a process for producing an improved process for producing a
three-dimensional fiber of a polyolefin having an excellent strength and
spreadability for use in an nonwoven fabric sheet through the use of said
solvent and said solution.
Namely, the present invention relates to an improved solvent which is
non-toxic and non-combustible and very safe when used, and a polyolefin
solution and an improved flash spinning process using said solvent. The
solvent according to the present invention can be applied not only to a
polyolefin solution for flash spinning but also to a cleaning agent, a
foaming material, a gas for the production of a hollow filament, and a
reaction solvent, etc.
2. Description of the Related Art
A process for producing a polyolefin fiber having a plexifilamentary
structure is known as a flash spinning process. The flash spinning process
is a well known spinning process which comprises adding a polyolefin to an
organic solvent also known as a liquefied gas, preparing a polyolefin
solution under high temperature and high pressure conditions, passing the
solution through a pressure let-down orifice to once lower the pressure of
the solution to bring about a phase separation, and passing the opaque
solution through a spinneret to inject the solution into an atmosphere
where the temperature and pressure are room temperature and normal
pressure, respectively, to thereby form a fiber having a three-dimensional
structure.
This spinning process is described in, for example, U.S. Pat. No. 3081519,
U.S. Pat. No. 3227794, U.S. Pat. No. 3227784, U.S. Pat. No. 3467744, U.S.
Pat. No. 3564088 and U.S. Pat. No. 3756411, EP 285670 Al, EP 321567 Al, EP
357364 A2, Japanese Examined Patent Publication (Kokoku) No. 40-28125,
Japanese Examined Patent Publication (Kokoku) No. 42-19520, Japanese
Unexamined Patent Publication (Kokai) No. 62-33816 and Japanese Unexamined
Patent Publication (Kokai) No. 63-50512.
The fiber prepared by this flash spinning process is classified into a
short fiber-like material and a three-dimensional fiber material. The
former is used as a synthetic pulp, and the latter is used as a nonwoven
fabric sheet. The nonwoven fabric sheet is generally known as a synthetic
paper. The largest feature of this product resides in a good water
resistance, high strength and light weight, and freedom from fuzzing. This
is highly appreciated in the art, and this nonwoven fabric is applied to
envelopes for air mail, sleeves for floppy disks, bags for deoxidizers,
bags for desiccants, medical sterilizing bags, clothes for preventing
dewing caused by thermal insulation of buildings, working wear for working
in nuclear power generation, working wear for asbestos, and working wear
for safety and protection, etc. A three-dimensional fiber which has a high
strength and is highly spreadable is indispensable to the production of
products having these features. This is because a uniform sheet which is
dense and permeable to air can not be produced without the use of such a
fiber.
The above-described nonwoven fabric sheets based on a plexifilamentary
polyolefin fiber are already commercially available as TYVEK.RTM. from
DuPont in U.S.A., and LUXER.RTM. from the applicant of the present
invention.
The solvent for the polymer used in the flash spinning process should have
the following properties. This is also shown in U.S. Pat. No. 3081519. (1)
The boiling point of the solvent is at least 25.degree. C. below the
melting point of the polymer used; (2) the solvent is inert to the polymer
under spinning conditions; (3) the solvent is a good solvent for the
polymer under temperature and pressure conditions suitable for the
preparation of a polymer solution; (4) the solvent dissolves only 1% of
the polymer when the temperature is below the boiling point of the
solvent; and (5) the solvent can immediately give rise to a phase
separation at the time of spinning to form a phase consisting essentially
of a polymer, and the separated polymer phase is substantially free from
the solvent.
Specific known examples of the solvent include aromatic hydrocarbons such
as benzene, toluene, aliphatic hydrocarbons such as butane, pentane,
hexane, heptane and octane and their isomers and homologues, alicyclic
hydrocarbons such as cyclohexane, unsaturated hydrocarbons, halogenated
hydrocarbons such as methylene chloride, carbon tetrachloride, chloroform,
ethyl chloride and methyl chloride, alcohols such as ethanol, methanol and
hexafluoroisopropanol, esters, ethers, ketones, nitriles, amides,
fluorochlorinated aliphatic hydrocarbons such as trichlorofluoromethane,
1,1,2-trichloro-1,2,2-trifluoromethane, sulfur dioxide, carbon disulfide,
nitromethane, water and various liquid mixtures of the above-described
solvents.
An optimal solvent is properly selected from these solvents after various
conditions for the spinning process used and the kind of polymers used are
taken into consideration. Trichlorofluoromethane and
1,1,2-trichloro-1,2,2-trifluoroethane which has a high capability of
dissolving the polymer and an excellent spinnability and are
noncombustible and nontoxic are favorable as the solvent for the flash
spinning process of a polyolefin. Among others, trichlorofluoromethane is
the best solvent.
In the flash spinning, to inject a polymer solution under high temperature
and high pressure into the air to gasify the solution, the solvent should
have a low boiling point, remain undecomposed even at a high temperature,
have a lipophilic property sufficient for dissolving the polyolefin, and
have at least a low toxicity and be fire-resistant. Specifically, in the
flash spinning, the solvent is gasified to separate the solvent from the
polymer, and the gasified solvent is recovered and liquefied by
compression with cooling. Therefore, the flash spinning is conducted in an
extensive, sealed space. This is because a gasified solvent can not be
recovered when the flash spinning is not conducted in such an extensive
space. The size of the sealed space is, for example, as large as 2000
M.sup.3. The filling of such an extensive space with a combustible gas
increases the possibility of fire and explosion, and therefore is, very
dangerous, which makes it substantially impossible to use a combustible
gas as a solvent.
In general, a corona discharge device or a high-voltage destaticizer is
contained in the sealed space and can be an ignition source of the
combustible gas. This further makes the use of a combustible gas
unfavorable. Further, various facilities such as a metallic conveyor for
the formation of a nonwoven fabric, a corona discharge device and a
spinning head are provided in the sealed space, and this makes it
unavordalbe that workers must enter the sealed space for repair and
maintenance work. Further, the outlet port for the formed nonwoven fabric
sheet is non-contact sealed, and the gas within the sealed space always
leaks into the work section. Therefore, when the solvent is toxic, it
cannot be used as a solvent for flash spinning. For this reason,
trichlorofluoromethane, which is noncombustible and nontoxic, has been
regarded as the only solvent useable for the flash spinning process.
In recent years, however, it has been found that a wholly halogenated
hydrocarbon wherein all the hydrogen atoms are substituted with chlorine
and fluorine is a particular flon (chlorofluorocarbon also known as "CFC")
having a very high capability of depleting ozonosphere. It has been
decided that the production of a CFC be prohibited by A.D. 2000, from the
viewpoint of a protection of the environment. It is a matter of course
that the production of trichlorofluoromethane,
1,1,2-trichloro-1,2,2-trifluoroethane, etc. as particular flons also will
be prohibited, and these flons become commercially unavailable. Therefore,
trichlorofluoromethane will become unable to be utilized as a solvent in
the flash spinning process of a polyolefin.
Under these circumstances, a flash spinning process wherein a new solvent
is used without the use of trichlorofluoromethane as a particular flon has
been already proposed.
Specifically, U.S. Pat. No. 5032326, EP 0357381A2 and Japanese Unexamined
Patent Publication (Kokai) No. 2-139408 disclose a flash spinning process
wherein use is made of a mixed solvent comprising methylene chloride and
an alternative flon, for example, chlorofluoromethane,
1,1,1,2-tetrafluoroethane, 1,1-difluoroethane,
1,1,1,2-tetrafluoro-2-chloroethane or 1-chloro-1,1-difluoroethane.
Further, U.S. Pat. No. 5081177, U.S. Pat. No. 5023025, EP 0 361684A1,
Japanese Unexamined Patent Publication (Kokai) No. 2-160909 disclose a
spinning process wherein use is made of
1,1-dichloro-2,2,2-trifluoroethane, 1,2-dichloro-1,2,2-trifluoroethane,
1,1-dichloro-2,2-difluoroethane, 1,2-dichloro-1,1-difluoroethane or
1,1-dichloro-1-fluoroethane. Further, EP 0407953A2 discloses a spinning
process wherein 1,1-dichloro-2,2,2-trifluoroethane,
1,2-dichloro-1,2,2-trifluoroethane or the like is used as a solvent for
polypropylene. Further, EP 357364A4 and Japanese Unexamined Patent
Publication (Kokai) No. 3-76809 disclose a process wherein spinning is
conducted through the use of methylene chloride and carbon dioxide.
Further, EP 0414498A2 and Japanese Unexamined Patent Publication (Kokai)
No. 3-152209 discloses a process wherein use is made of a mixed solvent
comprising a water-containing organic solvent. Further, EP 431801
discloses a process wherein spinning is conducted through the use of
carbon dioxide and water.
Japanese Unexamined Patent Publication (Kokai) No. 4-185708 discloses a
spinning process using 1,1-dichloro-2,2,3,3,3-pentafluoropropane and/or
1,3-dichloro-1,2,2,3,3-pentafluoropropane mixed with hydrocarbon
derivative.
All of the above-described proposed flash spinning processes, however, give
rise to drawbacks when spinning is conducted through the use of a
polyolefin.
For example, the mixed solvent comprising methylene chloride and an
alternative flon proposed in U.S. Pat. No. 5032326 may be tentatively used
as a solvent for use in a laboratory, but, it cannot be used as an
industrial solvent. This is because methylene chloride is highly toxic and
carcinogenic. TLV (threshold limit values of airbone contaminants)
established by ACGIH (American Conference of Governmental Industrial
Hygienists) is known as an index for indicating the degree of toxicity.
The TLV of trichlorofluoromethane is 1000 ppm, but the TLV of methylene
chloride is as low as 50 ppm. Therefore, it is apparent that methylene
chloride has a higher toxicity than trichlorofluoromethane. Further,
methylene chloride is registered as a carcinogenic substance. This clearly
suggests that methylene chloride and its mixed solvent cannot be used on a
commercial scale as a solvent for flash spinning. Further, the use of
methylene chloride involves a problem from the viewpoint of properties
required at the time of spinning. The reason for this is that the heat of
evaporation of methylene chloride is 78.7 cal/g which is much larger than
that of trichlorofluoromethane, i.e., 43.5 cal/g. This means that the use
of methylene chloride as a solvent causes a spun yarn to be liable to be
damped by the residual solvent. As described also in Japanese Unexamined
Patent Publication (Kokai) No. 3-76809, the damped yarn is liable to
adhere to and wind around a roller used for pressing the yarn into a sheet
structure, so that a no nonwoven fabric sheet having a commercial value
cannot be produced, which makes it impossible to produce the nonwoven
fabric sheet on a commercial scale. For this reason, when methylene
chloride is used as a solvent, it is necessary to allow a gas having a low
boiling point to exist together with the methylene chloride solvent, for
accelerating the evaporation of methylene chloride remaining in a fiber as
spun, and at the same time, to previously enhance the polymer
concentration of a spinning solution. The enhancement of the polymer
concentration is effective for increasing the amount of occurrence of heat
of solidification at the time of flashing of the polymer solution to
accelerate the drying of a spun yarn through the utilization of the heat.
In this case, the yarn becomes damped when either the use of a gas having
a low boiling point is omitted or the enhancement in the polymer
concentration is low. However, when the polymer concentration of the
solution is enhanced, the spread state of the spun yarn becomes so poor
that the quality of the sheet products lowered. Therefore, also from the
viewpoint of spinnability, it is understood that methylene chloride cannot
be used.
The proposal in U.S. Pat. No. 5081177 wherein use is made of an alternative
flon has a problem. Specifically, studies in recent years have revealed
that 1,1-dichloro-2,2,2-trifluoroethane and its isomers give rise to a
tumor in rats, although it is benign. Further, since these alternative
flons are a poor solvent for a high-density polyethylene, which is a
typical example of a polyolefin, they cannot dissolve the high-density
polyethylene when used alone. For this reason, to improve the solubility,
a technique wherein a hydrocarbon, methylene chloride, etc. are used as a
co-solvent is simultaneously disclosed. However, even when the alternative
flon is allowed to exist together with the hydrocarbon, methylene chloride
or the like, since the proportion of the alternative flon in the solvent
is high and 50% or more, the property that the alternative flon is a poor
solvent for the high-density polyethylene strongly remains, so that it is
difficult to dissolve the high-density polyethylene. Therefore, a problem
that the dissolution rate is low remains unsolved. Thus,
1,1dichloro-2,2,2-trichloroethane and its isomers have problems of the
toxicity and the dissolution of the high-density polyethylene. Similarly,
1,1-dichloro-2,2-difluoroethane and its isomers have problems of the
toxicity (toxicity against genital organs) and the dissolution of the
high-density polyethylene. On the other hand, 1,1-dichloro-1-fluoroethane
and its isomers dissolve the high-density polyethylene and provide a good
yarn even when used alone. These solvents, however, are highly liable to
thermal decomposition. Even when dissolution is conducted by means of an
extruder, they are liable to thermally decompose with evolution of a large
amount of hydrogen chloride or hydrogen fluoride to give a halogenated
oligomer. These decomposition products give rise to serious problems such
as coloring of a product and corrosion of a spinning apparatus. Further,
1,1-dichloro-1-fluoroethane as the solvent has an ozone depletion
potential (ODP) exceeding 0.1, i.e., unfavorably has a high capability of
depleting ozone. Therefore, 1,1-dichloro-1-fluoroethane and its isomers
cannot be used as a solvent for flash spinning because they are highly
liable to thermal decomposition and have a high capability of depleting
ozone.
The technique proposed in EP 357364A4 cannot be used as a solvent for flash
spinning for the reasons set out above because methylene chloride is used
as the solvent.
The spinning process proposed in EP 431801 wherein flash spinning is
conducted through the use of a solvent comprising carbon dioxide and water
can be applied to a particular polyolefin having a high hydrophilicity as
a comonomer component. Since, however, this solvent has a poor capability
of dissolving general polyolefins, i.e., polyethylene and polypropylene,
it is substantially impossible to prepare a fiber having favorable
properties. Further, in this method, it is practically necessary to use a
surfactant in combination with the solvent. This not only renders the
process complicated but also causes the surfactant to remain in the
resultant fiber to lower the practical properties.
Further, in a technique proposed in EP 0414498A2 wherein use is made of a
mixed system comprising a water-containing organic solvent, since the
solvent used is highly combustible, this technique cannot be used.
The spinning process using 1,1-dichloro-2,2,3,3,3-pentafluoropropane and/or
1,3-dichloro-1,2,2,3,3-pentafluoropropane mixed with hydrocarbon
derivative proposed in Japanese Unexamined Patent Publication (Kokai) No.
4-185708 has a drawback due to use of the hydrocarbon derivative having a
flammability and a high toxicity. Accordingly, this process cannot be
applied to a practical production.
Thus, all the solvents alternative to trichlorofluoromethane for flash
spinning proposed up to now apparently have an unsolved problem, and no
satisfactory solvent which can be used instead of trichlorofluoromethane
has been proposed in the art.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a solvent which can be
used as the alternative to trichlorofluoromethane or is superior to
trichlorofluoromethane. More specifically, an object of the present
invention is to provide a solvent which is excellent as a solvent for
flash spinning of a polyolefin and less combustible and has a low toxicity
and a capability of depleting ozone.
Another object of the present invention is to provide a polymer solution
using said solvent.
A further object of the present invention is to provide an improved flash
spinning process for a polyolefin which enables a three-dimensional fiber
having a high strength and an excellent spreadability to be prepared
through the use of said solvent. It is a matter of course that the solvent
and solution according to the present invention can be utilized also as,
for example, a reaction solvent, a foaming agent and a cleaning agent
which are used as an alternative flon in other technical regions where
their properties can be utilized.
The present inventors have made extensive studies with a view to attaining
the above-described objects through trial-and-error experiments on how to
prepare a fiber having properties comparable or superior to those of a
three-dimensional polyolefin fiber formed by the conventional flash
spinning process, which has led to the completion of the present
invention.
Specifically, the first invention is directed to a halogen solvent
comprising a mixed solvent consisting essentially of at lease one solvent
selected from the group consisting of bromochloromethane and
1,2-dichloroethylene, and a co-solvent, wherein said co-solvent is at
least one member selected from the group consisting of carbon dioxide,
sulfur hexafluoride, difluorochloromethane, 1,1,1,2-tetrafluoroethane,
1-chloro-1,2,2,2-tetrafluoroethane, 1-chloro-1,1-difluoroethane,
1,1-dichloro-2,2,3,3,3-pentafluoropropane,
1,3-dichloro-1,2,2,3,3-pentafluoropropane, dodecafluoropentane and
tetradecafluorohexane, the content of said co-solvent in the mixed solvent
being 3 to 65% by weight.
The bromochloromethane or the 1,2-dichloroethylene can be used as the
solvent selected from the group. The mixed solvent is essentially a
two-component solvent comprised of bromochloromethane and
1,2-dichloroethylene and a co-solvent, and the content of
bromochloromethane in the two-component solvent is preferably 40 to 75% by
weight, and a proportion of the co-solvent in the mixed solvent is
preferably 10 to 30% by weight.
A 0.001 to 5% by weight, based on the mixed solvent, of at least one
stabilizer selected from the group consisting of propylene oxide,
1,2-butylene oxide, nitromethane, a phosphite represented by the following
structural formula (1), a diphosphite represented by the following
structural formula (2) and a diphosphite represented by the following
structural formula (3) may be contained in said mixed solvent:
##STR1##
wherein R.sup.1, R.sup.2 and R.sup.3 which may the same or different each
stand for a monovalent hydrocarbon group having 1 to 30 carbon atoms;
##STR2##
wherein R.sup.4 stands for a monovalent hydrocarbon group having 8 to 30
carbon atoms; and
##STR3##
wherein R.sup.5 stands for a monovalent hydrocarbon group having 8 to 30
carbon atoms.
The second invention is directed to a polyolefin solution prepared under
high temperature and high pressure conditions, wherein a halogen solvent
is used as a solvent, said halogen solvent comprising a mixed solvent
consisting essentially of at least one solvent selected from the group of
bromochloromethane and 1,2-dichloroethylene, and a co-solvent is used as a
solvent, said co-solvent is at least one member selected from the group
consisting of carbon dioxide, sulfur hexafluoride, difluorochloromethane,
1,1,1,2-tetrafluoroethane, 1-chloro-1,1-difluoroethane,
1-chloro-1,2,2,2-tetrafluoroethane,
1,1-dichloro-2,2,3,3,3-pentafluoropropane,
1,3-dichloro-1,2,2,3,3-pentafluoropropane, dodecafluoropentane and
tetradecafluorohexane, the content of said co-solvent in the mixed solvent
being 3 to 65% by weight, and the concentration of a polyolefin in said
polyolefin solution is 5 to 25% by weight.
The third invention is directed to a process for producing a
three-dimensional polyolefin fiber, comprising passing a polyolefin
solution prepared under high temperature and high pressure conditions
through a pressure let-down orifice, a pressure let-down chamber and a
spinneret into a region where the temperature and pressure are room
temperature and atmospheric pressure, respectively, to prepare a
fibrilated, three-dimensional polyolefin fiber, wherein a mixed solvent
consisting essentially of at least one solvent selected from the group
consisting of bromochloromethane and 1,2-dichloroethylene and a co-solvent
is used as a solvent, said co-solvent being at least one member selected
from the group consisting of carbon dioxide, sulfur hexafluoride,
difluorochloromethane, 1,1,1,2-tetrafluoroethane,
1-chloro-1,2,2,2-tetrafluoroethane, 1-chloro-1,1-difluoroethane,
1,1-dichloro-2,2,3,3,3-pentafluoropropane,
1,3-dichloro-1,2,2,3,3-pentafluoropropane, dodecafluoropentane and
tetradecafluorohexane, the content of said co-solvent in the mixed solvent
being 3 to 65% by weight, and the concentration of a polyolefin in said
polyolefin solution is 5 to 25% by weight.
The mixed solvent consisting essentially of a two-component solvent
comprised of bromochloromethane and 1,2-dichloroethylene and a co-solvent
is preferably used and, the content of bromochloromethane in the
two-component solvent may be 40 to 75% by weight.
As opposed to the conventional process, the present inventors can prepare a
three-dimensional fiber of a polyolefin having a very high strength and a
good spreadability despite the use of a solvent having a small capability
of depleting ozone.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a measuring device wherein use is made of
an optical cell vessel for measuring the cloud point of a polymer
solution;
FIG. 2 is a schematic diagram of an optical cell vessel for measuring the
cloud point;
FIG. 3 is a graph showing an example of a cloud point curve of the polymer
solution according to the present invention, that is, a graph showing
cloud point curves of solvents respectively having compositions of A)
bromochloromethane/carbon dioxide (85/15% by weight) and B)
bromochloromethane/HFC-134a (80/20% by weight) and (75/25% by weight); and
FIG. 4 is a graph showing cloud point curves of solvents respectively
having compositions of
trans-1,2-dichloroethylene/bromochloromethane/carbon dioxide (45/40/15% by
weight), (50/35/15% by weight) and (50/40/10% by weight).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As described above, the solvent used for flash spinning should be an
organic solvent which can be gasified under room temperature and
atmospheric pressure conditions. Specifically, a polyolefin is dissolved
under high temperature and high pressure conditions, once subjected to a
reduction in the pressure to give rise to a change from a transparent
solution to an opaque solution and passed through a spinneret to inject
the opaque polymer solution into an atmosphere where the temperature and
the pressure are room temperature and atmospheric pressure, respectively.
At that time, the organic solvent is gasified to form a supersonic gas
jet. The gas jet causes the polymer to be solidified and, at the same
time, to be drawn, thereby forming a three-dimensional fiber having a high
strength.
As described above, the properties which the solvent for flash spinning
should have are widely known in the art, and will now be described in more
detail.
(1) The solvent does not dissolve the polymer at all under room temperature
and atmospheric conditions and dissolves the polymer at a temperature
above the melting point of the polymer and a pressure much higher than the
atmospheric pressure.
(2) A phase change from a transparent solution to an opaque solution occurs
at a temperature in the range of from a temperature above the melting
point of the polymer to a temperature at which no thermal deterioration
occurs. In particular, in the case of flash spinning, it is preferred that
the polymer solution have a phase diagram known as a LCST (lower critical
solution temperature) phase diagram in the theory of a polymer solution.
In this case, it is preferred for the solvent for flash spinning as well
to have an LCST phase diagram. It is still preferred that the polymer
solution has an LCST phase diagram and gives rise to a phase change in a
moment. This property is important because in the flash spinning, the
phase change from a transparent solution to an opaque solution is
conducted by varying the pressure.
(3) The solvent should be gasified immediately after ejection from the
spinneret. This means that the solvent should have a boiling point near
room temperature under atmospheric pressure. That is, the solvent should
be an organic solvent having a low boiling point.
(4) The change between before and after the spinneret is a substantially
isoentropic change. Therefore, a liquid/gas mixture spontaneously occurs
at the outlet of the spinneret. This mixture, as such, cannot be used
because it provides a wet three-dimensional fiber. However, since the
polymer has heat, the heat gasifies the liquid to form a dried
three-dimensional fiber. This means that the heat of vaporization of the
organic solvent should be proper.
(5) The solvent should have an excellent thermal stability because it is
exposed to a temperature above the melting point of the polymer. In the
present invention, the term "thermal stability" used in the present
invention is intended to mean that the solvent is difficult to thermally
decompose under a temperature at which the polymer is dissolved.
(6) The solvent should be noncombustible or flame-retardant because a
sealed space having a large volume is filled with a gas and electrical
facilities which can become an ignition source are provided within the
sealed space.
(7) The solvent should be nontoxic because the gas filled into the sealed
space often comes into contact with men.
(8) The corrosivity of the solvent should be low because the whole flash
spinning device is a high pressure equipment.
(9) The ODP should be low, preferably less than 0.01.
The satisfaction of the requirements of low boiling point, LCST polymer
solution, thermal stability, low combustibility, nontoxicity and low ODP
are particularly important to the solvent.
The present inventors have conducted many experiments with a view to
finding a solvent for flash spinning capable of satisfying the six
requirements, that is, low boiling point, LCST polymer solution, thermal
stability, low combustibility, nontoxicity and low ODP.
As a result, they have found that a mixed solvent consisting essentially of
bromochloromethane and/or 1,2-dichloroethylene and a co-solvent comprising
at least one member selected from the group consisting of carbon dioxide,
sulfur hexafluoride, difluorochloromethane, 1,1,1,2-tetrafluoroethane,
1-chloro-1,1-difluoroethane, 1-chloro-1,2,2,2-tetrafluoroethane,
1,1-dichloro-2,2,3,3,3-pentafluoropropane,
1,3-dichloro-1,2,2,3,3-pentafluoropropane, dodecafluoropentane and
tetradecafluorohexane can completely or substantially completely satisfy
the above-described six requirements.
The reason why the solvent and solution according to the present invention
can satisfy the six requirements necessary particularly for flash spinning
will now be described. Further, features of the solvent and a preferred
composition range thereof will be described.
For simplification, in the following description, difluorochloromethane
will be referred to as "HCFC-22", 1,1,1,2-tetrafluoroethane as "HFC-134a",
1-chloro-1,1-difluoroethane as "HCFC-142b",
1-chloro-1,2,2,2-tetrafluoroethane as "HCFC-124",
1,1-dichloro-2,2,3,3,3-pentafluoropropane as "HCFC-225ca",
1,3-dichloro-1,2,2,3,3-pentafluoropropane as "HCFC-225cb",
dodecafluoropentane as "FC-6112", and tetradecafluorohexane as "FC-7114".
(1) Toxicity
Bromochloromethane and 1,2-dichloroethylene each have a TLV value
established by ACGIH of 200 ppm which is a high value (that is, has a low
toxicity) for chloro compounds. The TLV value of the co-solvent is, for
example, 5000 ppm for carbon dioxide and 1000 ppm for sulfur hexafluoride,
and these co-solvents are known to have a very low toxicity. With respect
to other co-solvents, although no TLV is specified, the toxicity is
considered to be very small. Further, there is no report on all of these
solvents that they cause carcinogenicity in human beings. Therefore,
although the solvent for flash spinning of the present invention
comprising these solvents is not completely nontoxic, the toxicity is
considerably low. It does not injure health of human beings as long as
attention is given to the leakage of gas and the ventilation of the
working space, the control of gas concentration is maintained in the
working space and a protector such as an air line mask is worn when human
beings come into contact with the gas.
(2) Combustibility and Thermal Stability
Bromochloromethane and 1,2-dichloroethylene are often thermally decomposed
upon being exposed to a high temperature, so that it is necessary to use a
stabilizer or the like according to need. Although many stabilizers are
nowadays developed, only a few stabilizers can exhibit the effect under
high temperature and high pressure conditions (typical temperature and
pressure are about 200.degree. C. and 200 kg/cm.sup.2, respectively)
necessary for flash spinning. The reason for this is that the service
condition of the solvent is very severe and since the stabilizer used
herein is a stabilizer for the solvent, the use of the stabilizer in an
excessively large amount causes the stabilizer to be concentrated in a
yarn after spinning, so that the solvent blooms or bleeds from the yarn.
Therefore, a stabilizer which can exhibit a high effect in a small amount
under high temperature and high pressure conditions is necessary. As a
result of investigations and studies on many stabilizers, it has been
found that epoxy compounds, nitro compounds, diphosphites and phosphites
are useful as the stabilizer. In particular, the diphosphite exhibited a
high thermal stabilization effect. Further, as a result of detailed
studies on the structure | | |
