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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to flash-spinning polymeric film-fibril strands.
More particularly, the invention concerns an improvement in such a process
which permits flash-spinning of polyethylene strands from a liquid medium
which, if released to the atmosphere, Will contain a reduced amount of the
trichlorofluoromethane (or "F-11") which has been implicated as a source
of depletion of the earth's ozone.
2 Description of the Prior Art
Blades and White, U.S. Pat. No. 3,081,519, describes a flash-spinning
process for producing plexifilamentary film-fibril strands from
fiber-forming polymers. A solution of the polymer in a liquid, which is a
non-solvent for the polymer at or below its normal boiling point, is
extruded at a temperature above the normal boiling point of the liquid and
at autogenous or higher pressure into a medium of lower temperature and
substantially lower pressure. This flash-spinning causes the liquid to
vaporize and thereby cool the exudate which forms a plexifilamentary
film-fibril strand of the polymer. Preferred polymers include crystalline
polyhydrocarbons such as polyethylene and preferred spinning liquids
include halocarbons such as F-11
Although F-11 has been a very useful solvent for flash-spinning
plexifilamentary film-fibril strands of polyethylene, and has been the
solvent used in commercial manufacture of polyethylene plexifilamentary
strands, the escape of such a halocarbon into the atmosphere has been
implicated as a source of depletion of the earth's ozone. A general
discussion of the ozone-depletion problem is presented, for example, by P.
S. Zurer, "Search Intensifies for Alternatives to Ozone-Depleting
Halocarbons", Chemical & Engineering News, pages 17-20 (Feb. 8, 1988).
An object of this invention is to provide a process for flash-spinning
plexifilamentary film-fibril strands of fiber-forming polyethylene wherein
the solvent contains a reduced amount of F-11 and the process can be
operated without major apparatus modifications to a facility which was
constructed for flash-spinning from F-11 alone.
SUMMARY OF THE INVENTION
The present invention provides an improved process for flash-spinning
plexifilamentary film-fibril strands wherein polyethylene is dissolved in
a halocarbon spin liquid to form a spin solution containing 10 to 20
percent of polyethylene by weight of the solution at a temperature in the
range of 130 to 210.degree. C. and a pressure that is greater than 1000
psi which solution is flash-spun into a region of substantially lower
temperature and pressure, the improvement wherein the halocarbon spin
liquid contains about 20 to about 90 weight percent of at least one isomer
of dichlorotrifluoroethane, preferably 1,1-dichloro-2,2,2-trifluoroethane,
and about 10 to about 90 weight percent trichlorofluoromethane.
In one preferred embodiment of the invention, the halocarbon spin liquid is
an azeotrope of about 22 weight percent 1,1-dichloro-2,2,2-trifluoroethane
and about 78 weight percent trichlorofluoromethane.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The term "polyolefin" as used herein, is intended to embrace not only
homopolymers of ethylene, but also copolymers wherein at least 85% of the
recurring units are ethylene units. One preferred polyethylene is a linear
polyethylene which has an upper limit of melting range of about 130 to
135.degree. C., a density in the range of 0.94 to 0.98 g/cm.sup.3 and a
melt index (as defined by ASTM D-1238-57T, Condition E) of 0.1 to 6.0.
However, other polyethylenes having densities as low as 0.92 and melt
index values of up to about 100 can also be used.
The term "plexifilamentary film-fibril strands" as used herein, means a
strand which is characterized as a three-dimensional integral network of a
multitude of thin, ribbon-like, film-fibril elements of random length and
of less than about 4 microns average thickness, generally coextensively
aligned with the longitudinal axis of the strand. The film-fibril elements
intermittently unite and separate at irregular intervals in various places
throughout the length, width and thickness of the strand to form the
three-dimensional network. Such strands are described in further detail by
Blades and White, U.S. Pat. No. 3,081,519 and by Anderson and Romano, U.S.
Pat. No. 3,227,794, the disclosures of which are incorporated herein by
reference.
The present invention provides an improvement in the known process for
producing plexifilamentary film-fibril strands of fiber-forming
polyethylene from a halocarbon spin liquid that contains 10 to 20 weight
percent of the fiber-forming polyethylene. A fiber-forming polyethylene is
dissolved in the spin liquid to form a spin solution containing 10 to 20
percent of the linear polyethylene by weight of the solution and then is
flash-spun at a temperature in the range of 130 to 210.degree. C. and a
pressure that is greater than the autogenous pressure of the spin liquid
into a region of substantially lower temperature and pressure.
The key improvement of the present invention involves the replacement of a
portion of the conventionally used F-11 with at least one isomer of
dichlorotrifluoroethane, preferably 1,1-dichloro-2,2,2-trifluoroethane
("HC-123"). The other two isomers are 1,2-dichloro-1,2,2-trifluoroethane
("HC-123a") and 1,1-dichloro-1,2,2-trifluoroethane ("HC-123b").
The following table lists the known normal atmospheric boiling points
(Tbp), critical temperatures (Tcr) and critical pressures (Pcr) for the
individual halocarbons and for some prior art solvents.
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Tbp, .degree.C.
Tcr, .degree.C.
Pcr, psia
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HC-123 28.7 185 550
HC-123a 28
HC-123b 30.2
Trichloro- 23.8 198.0 639.5
fluoromethane
Methylene- 39.9 237.0 894.7
chloride
Hexane 68.9 234.4 436.5
Cyclohexane 80.7 280.4 590.2
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The HC-123 and its isomers appear to have only a minimal effect upon ozone
in the earth's atmosphere. Also it does not appear to suffer from undue
toxicity characteristics and it is stable under a wide variety of
processing conditions. By replacing a portion of the conventionally used
F-11 with HC-123 or its isomers, it is possible to operate a commercial
flash-spinning facility without substantial modification and in such a way
that when any traces of spin liquid escape to the atmosphere, they will
contain a reduced proportion of the F-11 constituent.
A very convenient composition of HC-123 and F-11 is one composed of about
22 and 78 weight percent, respectively, as this is an azeotrope which is
easily used and conveniently recovered as such for recycling. However,
increasing amounts of the HC-123 or its isomers can also be used, and
indeed it is preferred to replace as much of the F-11 as is possible.
The concentration of fiber-forming polyethylene in the spin liquid usually
is in the range of 10-20 percent, based on the total weight of the liquid
and the fiber-forming polyethylene.
The spin solution preferably consists of halocarbon liquid and
fiber-forming polyethylene, but conventional flash-spinning additives can
be incorporated by known techniques. These additives can function as
ultraviolet-light stabilizers, antioxidants, fillers, dyes, and the like.
The various characteristics and properties mentioned in the preceding
discussion and in the examples below were determined by the following
procedures.
TEST METHODS
The quality of the plexifilamentary film-fibril strands produced in the
examples were rated subjectively. A rating of "5" indicates that the
strand had better fibrillation than is usually achieved in the commercial
production of spunbonded sheet made from such flash-spun polyethylene
strands. A rating of 4"indicates that the product was as good as
commercially flash-spun strands. A rating of "3" indicates that the
strands were not quite as good as the commercially flash-spun strands. A
"2"indicates a very poorly fibrillated, inadequate strand. A "1" indicates
no strand formation. A rating of "3" is the minimum considered
satisfactory for use in the process of the present invention. The
commercial strand product is produced from solutions of about 12.5% linear
polyethylene in tichlorofluoromethane substantially as set forth in Lee,
U.S. Pat. No. 4,554,207, column 4, line 63, through column 5, line 10,
which disclosure is hereby incorporated by reference.
The surface area of the plexifilamentary film-fibril strand product is
another measure of the degree and fineness of fibrillation of the
flash-spun product. Surface area is measured by the BET nitrogen
absorption method of S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem
Soc., V. 60 p 309-319 (1938) and is reported as m.sup.2 /g.
Tenacity of the flash-spun strand is determined with an Instron tensile
testing machine. The strands are conditioned and tested at 70.degree. F.
and 65% relative humidity.
The denier of the strand is determined from the weight of a 15 cm sample
length of strand. The sample is then twisted to 10 turns per inch and
mounted in the jaws of the Instron Tester. A 1-inch gauge length and an
elongation rate of 60% per minute are used. The tenacity at break is
recorded in grams per denier (gpd).
The invention is illustrated in the Examples which follow with a batch
process in equipment of relatively small size. Such batch processes can be
scaled-up and converted to continuous flash-spinning processes that can be
performed, for example, in the type of equipment disclosed by Anderson and
Romano, U.S. Pat. No. 3,227,794. Parts and percentages are by weight
unless otherwise indicated.
EXAMPLES
For each of of the Examples, a high density linear polyethylene of 0.76
Melt Index was flash-spun into satisfactory plexifilamentary film-fibril
strand in accordance with the invention.
The apparatus used consists of two high pressure cylindrical chambers, each
equipped with a piston which is adapted to apply pressure to the contents
of the vessel. The cylinders have an inside diameter of 1.0 inch
(2.54.times.10.sup.-2 m) and each has an internal capacity of 50 cubic
centimeters. The cylinders are connected to each other at one end through
a 3/32 inch (2.3.times.10.sup.-3 m) diameter channel and a mixing chamber
containing a series of fine mesh screens is used as a static mixer. Mixing
it accomplished by forcing the contents of the vessel back and forth
between the two cylinders through the static mixer. A spinneret assembly
with a quick-acting means for opening the orifice then attached to the
channel through a tee. The spinneret assembly consists of a pressure
letdown orifice of 0.03375 inch (8.5.times.10.sup.-4 m) diameter and 0.030
inch length (7.62.times.10.sup.-4 m), a letdown chamber of 0.25 inch
(6.3.times.10.sup.-3 m) diameter and 1.92 inch length, and a spinneret
orifice of 0.030 inch (7.62.times.10.sup.-4 m) diameter and either 0.020
or 0.030 inches in length. The pistons are driven by high pressure water
supplied by a hydraulic system. Pressure transducers are used to measure
the pressure before and after the letdown orifice.
In operation, the apparatus is charged with polyethylene pellets and
solvents, and high pressure water, e.g. 1800 psi (12410 kPa) is introduced
to drive the piston to compress the charge. The contents then are heated
to mixing temperature and held at that temperature for about an hour or
longer during which time a differential pressure of about 50 psi (345 kPa)
is alternatively established between the two cylinders to repeatedly force
the contents through the mixing channel from on cylinder to the other to
provide mixing and effect formation of a solution. The solution
temperature is then raised to the final spin temperature, and held there
for about 15 minutes to equilibrate the temperature. Mixing is continued
throughout this period. Finally, the spinneret orifice is opened, and the
resultant flash-spun product is collected. The pressure inside the letdown
chamber recorded during spinning using a computer is entered as spin
pressure. For Examples I, II, and III, the letdown chamber was omitted,
the pressure was reduced manually to the desired spinning pressure, and
the pressure measured just before the spinneret during spinning was
entered as the spin pressure.
TABLE
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Example No. I II III IV V
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Solvent Composition
HC-123, % 50 66.7 85 50 22
F-11, % 50 33.3 15 50 78
Mixing
Temp., .degree.C.
140 140 140 170 150
Press., psig 2800 2800 5500 2000 1800
Spinning
Temp., .degree.C.
170 170 160 170 170
Press., psig 2900 2900 .sup..about. 5000
1200 .sup..about. 900
Spinneret,
DxL, in. 0.030 0.030 0.030 0.030 0.030
.times. .times. .times.
.times.
.times.
0.020 0.020 0.030 0.030 0.030
Strand Product
Denier -- -- 639 378 396
Tenacity, gpd
-- -- 2.7 2.46 2.52
Quality 5 4 4.5 4.5 5
Surface Area,
-- -- 79.2 -- --
m.sup.2 /g
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