Note: Descriptions are shown in the official language in which they were submitted.
WO 91/13193 ' PGT/~J590/a0875
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TITLE
Halocarbons for Flash-Spinning
Polyethylene Plexifilaments
BACRGROUIND OF THE INVEIdTIOTd
Field of the Invention
This invention relates to flash=spinning
polyethylene film-fibril strands. More particularly,
the invention concerns an improvement in such a process
which permits flash-spinning of the strands from a
liquid which, if released to the atmosphere, would not
detrimentally affect the earth's oaone.
Description of the Prior Art
Blades and White, United States Patent
3,081,519, describes a flash-spinning process for
producing plexifilamentary film-fibril strands from
fiber-forming polymers such as polyethylene. 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
plexifil~mentary film--fibril strand of the polymer.
Anderson and Romano, United States F~atent
3,227,794, discloses technology for selecting conditions
for spinning plexifilamentary strands.. A graph is
presented of spinning temperature versus spinning
pressure for solutions of 10 to 16 weight percent of
linear polyethylene in trichlorofluoromethane (or .
"F-11"). This patent also descrih~~ in detail the
preparation of a solution of 14 wei.gh+;, percent high w
density linear polyethylene in trs.c:hlorofluoromethane at
a temperature of about 185°C and a pressure of about .,
1640 psig which is then flash-spun from a let-down .
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chamber at a temperature of 185°C and a pressure of 1050
psig. Very similar temperatures, pressures and
concentrations have been employed in commercial
flash-spinning of polyethylene into plexifilamentary ;
film-fibril strands, which were then converted into
sheet structures.
Although trichlorofluoromethane 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 Halocz~rbons", Chemical ~
Engineering News, pages 17-20 (February 8, 1988).
An object of this invention is to provide an
improved process for flash-spinniing plexifilamentary
film-fibril strands of fiber-forming polyethylene,
wherein the solvent should not be a depletion hazard to
the earth's ozone.
SUMRiARY O~' THE IN'lEPITIOTd
The present invention provides an. improved
process for flash-spinning plexifilamentary film-fibril
strands wherein polyethylene having a melt index of at
least 4 and a density of about 0.92-0.98 is dissolved in
at least one isomer of dichlorotrifluoroethane,
preferably~l,l-dichloro-2,2,2-trifluoroethane, to form a
spin solution containing 10 to 20 percent of the
polyethylene by weight of the solution at a temperature
in the range of 130 to 210°C and a pressure that is
greater than 2400 psi followed by LJ.ash-spinning the
solution into a region of substantially lower
temperature and pressure.
The present invention provides a novel
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solution consisting essenti:..ly of 10 to 20 weight
percent of polyethylene having a melt index of at least
4 and a density of about 0.92-0.98 and 90 to 80 weight
percent of at least one isomer of
dichlorotrifluoroethane, preferably
1,1-dichloro-2,2,2-trifluoroethane.
DETAILED DESCRIPTION OF PREFERRED EI~IBODIMEI~ITS
"Polyethylene" as used herein is intended to w
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 high density polyethylene which has an upper
limit of melting range of about 130 to 135°C, a. density
in the range of 0.94 to 0.98 g/cm' and a melt index (as
defined :v:y ASTM D-1238-57T, Condition E) of greater than
4, and preferably below 100. Another preferred
polyethylene is a linear ,low density polyethylene having
a density of about 0.92-0.94 and a melt index of at
least 4, preferably also below 100.
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 coextensi~rely 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, United States
Patent 3,081,519 and by Anderson and Romano, United
States Patent 3,227,794.
A convenie~.°;' test to determine whether a given
solvent would be su~:able for flash-spinning a given
polymer is disclosed by Woodell, United States Patent
WO 91/13193 ~ ~ ~ ~ ~ ~ ~ -4- pCf/1~~9~D968f
3,655,998. This test has been used extensively to
determine the suitability of alternatives to the
trichlorofluoromethane solvent for preparing
plexifilamentary strands. In the test, a mixture of the
polymer plus the amount of solvent calculated to give
about a 10 weight percent solution, is sealed in a '
thick-walled glass tube (the mixture occupies about
one-third to one-half the tube volume) and the mixture
is heated at autogenous pressure. Test temperatures
usually range from about 100°C to just below the
critical temperature of. the liquid being tested.
Woodell states that if a singled-phase, flowable
solution is not formed in the tube at any temperature
below the solvent critical temperature, T~, (or the
polymer degradation temperature, which is lower) the
solvent power is too low.
It has been found that, contrary to the prior
art of Woodell, when an isomer of
dichlorotrifluoroethane such as
1,1-dichloro-2,2,2-trifluoroethane ("HC-123") is the
solvent it is entirely practical to produce a solution
of 10 to 20 weight percent of polyethylene having a melt
index of at least 4 and a density of about 0.92-0.98 arad
then to flash-spin the solution at temperatures of 130
to 210°C and comparatively low pressures to produce high
quality products. For this combination it is not
necessary that the solution be formed into a single
phase, it is sufficient that a homogeneous two phase
solution be formed and spun as such. Indeed at
pressures below about 5000-8000 psi such solutions will
usually be of two-phases but high quality products can
nonetheless be produced.
In accordance with the F!:esent invention the
-t ri. f luoroethane
halocarbon is 1,1-dichloro-2,2,2
'35 ("HC-123"), 1,2-dichloro-1,2,2-trifluoroethane
("HC-123a"), or 1,1-dichloro-1,2,2-trifluoroethane '
WO 91/13193 PC'~'/~JS9~/gyG~~7a
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("HC-123b"). The parenthetic designation is used herein
as an abbreviation for the chemical formula of the
halocarbon. The following table lists the known normal
atmospheric boiling points (Tbp), critical temperatures
(Tcr) and critical pressures (Pcr) for thesehalocarbons
and far some prior art solvents. Tn the column labeled
"Solubility", the Table also lists whether a 10~
polyethylene solution can be formed as a single phase in
the halocarbon or hydrocarbon at temperatures between
100 and about 225°C under autogenous pressures.
Tbp,°C Tcr,°C Pcr, psia Solubility
HC-123 28.7 185 550 no
HC-123a 28
HC-123b 30.2
Trichloro-
fluoromethane 23.B 198.0 639.5 yes
Methylene-
chloride 39.9 237.0 894.7 yes
Hexane 68.9 234.4 436.5 yes
Cyclohexane 80.7 280.4 590.2 yes
It is to be noted that the halocarbons of the
present invention do not dissolve the polyethylene at
autogenous pressures, in contrast to the prior art
solvents shown above. In contrast to the flash spi~aa~i~ag
fluids of the past, they do not form a single phase
solution with polyethylene at the rewired
concentrations and temperatures at a pressure of less
than 5,000 Asia. Indeed it is not necessary that these
halocarbons form a single phase solution even at the
mixing temperature. Thus the polyethylenes of this
invention can be dissolved in the various HC-123 isomers
to form a uniform two phase solution which can be spun
directly.
In forming a solution of fiher-forming
polyethylene and HC-123 or one of _ts isomers, a mixture
of the components is raised to a temperature in the ;
WO 91/13193 P~d'/iU~9~/~~~B'7L~
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range of 130 to 210°C. Below pressures of about
5000-8000 psi the solution will usually be of two
phases, whereas above that range there will usually be
only one phase. The mixtures described above are held '
under the required pressure until a homogeneous one
phase or two phase solution is formed. Usually, maximum
pressures of less than 10,000 psi are satisfactory. The
pressure may optionally be reduced somewhat and the
mixture then flash spun to form the desired high qua:~aty
plexifilamentary strand structure.
The spin solution preferably consists of
HC-123 or its isomers and fiber-forming polyethylene.
However conventional flash-spinning additives can be
incorporated into the spin mixture 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 was 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 sating 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. r "J." indicates no
strand formation. A rating of "3" i.s the minimum
considered satisfactory for use in the process of the
present invention. The commercial strand product is
s
WO 91/13193 1PC'f/~.JS90d~f~lS'9S
groduced from solutions of abaut 12.5 linear
polyethylene in trichlorofluoromethane substantially as
set forth in Lee, United States patent 4,559,207, column
4, line 63, through column 5, line L0.
The surface area of the plexifilamentazy
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. Hzunauer, P.H. Emmett and E.
Teller, J. Am. Chem Soc., V. 60 p 309-319 (1939) and is
reported as m2 /g.
Tenacity of the flash-spun strand is
determined with an Instron tensile-testing machine. The
strands are conditioned and tested at 70°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 Instzon Tester. A 1-inch gauge
length and an elongation rate of 60% per minute are
used. the tenacity at break is recozded 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 convested to continuous flash-spinning
processes that can be performed, for example, in the
type of equipment disclosed by Anderson and ~tomano,
United States Patent 3,227,799. Parts and percentages
are by weight unless otherwise indicated.
EXAPqPLES
For each of Examples 1-?.?. a.solution of HC-123
and polyethylene was flash-spun into, satisfactory
plexifilamentary film-fibril strands in accordance with
the invention. Five different polyethylenes were used,
differing in melt index (molecular weigh.t). LLDPE
2 ~cri~»~~~r~~~~a
WO (1/13193
8
stands for linear low density polyethylene, HDPE for
high density polyethylene.
The apparatus employed comprises a pair of
high pressure cylindrical vessels, each fitted with a
piston for applying pressure. The vessels are
cylindrical and are connected to each other with a '
transfer line. The transfer line contains a series of
fine mesh screens intended for mixing the contents of
the apparatus by forcing the contents through the
transfer line from one cylinder to the other. A
spinneret assembly having an orifice of 0.030-inch
diameter is connected to the transfer lines with quick
acting means for opening and closing the orifice. Means
are included for measuring the pressure and temperature
inside the vessel. For Exmple 1 the spinneret assembly
consists of a pressure letdown orifice of 0.03375 inch
(8.5X10-°m) diameter and a 0.030 inch length
(7.62X10-°m), a letdown chamber of 0.25 inch (6.3X10-3m)
diameter and 1.92 inch length, and a spinneret orifice
of 0.30 inch (7.62X10-°m) diameter. In operation, the
apparatus is charged with polymer and HC-123 and a high
pressure is applied to the charge. The contents then
are heated at the desired temperature for about an hour
and a half during which.time a differential.pressure ok
about 50 psi is alternately established between the two
cylinders to repeatedly force the contents. through the,
transfer line from one cylinder to the other to provide
mixing and effect formation of a solution. The pressure
desired for spinning is then set and the spinneret
orifice opened. The .resultant flash-spun product is
then collected.
All Examples were performed in a similar
fashion under the specific conditi.c~n: and with the
particular ingredients shown in the following summary '
table. The table also records characteristics of the
strands produced by the flash-spinning.
WO PC('/tJ~96~AQ~08'7~9
91/13193
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In all of the Examples solution
the which
is
spun is composedof two phases.
Table I
Example too. 1 2 3 4
Polyethylene LLDPE LLDPE HDPE HDPE
Melt Index 12 12 55 33
Density, g/cm3 0.933 0.933 0.955 0.955
Conc, wt ~ 15.4 15 15 15
Mixing
Temp, C 140 180 180 180
Press, psig 2400- 2550 3500 3500 3500
Spinning
Temp, C 160 180 180 180 (.
Press, psig 1950 3500 3500 3500
Strand Product
Denier 554 570 457 525
- Tenacity, gpd 1.15 1.3 1.05 1.6
Quality 4 4 4 4
30
~O 91113193 1PCT/U~9~!/8~& 7e~
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Table I (cont.)
Example No. 5 6 7 8 9 10 11
Polyethylene HDPE HDPE HDPE HDPE HDPE HDPE HDPE
Melt Index 17.5 6 6 6 6 6 6
Density, g/cm3 0.948 0.96 0.96 0.96 0.96 0.96 0<.94
Conc, wt ~ 15 15 16 12 16 16 16
Mixing
Temp, °C 180 180 160 180 140 380 140
Press, psig 3500 3500 3000 3500 3500 3500 2500
Spinning
Temp, °C 180 180 160 180 140 180 140
Press, psig 3500 3500 3000 3500 3500 3500 2500
Strand Product
Denier 561 624 853 686 852 607 968
Tenacity, gpd 1.8 2.5 2.3 2.3 2.1 2.5 2.2
Quality 4 4 4 4 4 4 4
30
