Note: Descriptions are shown in the official language in which they were submitted.
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FLASH-SPUN PRODUC~'S
Field of the Invention
This invention relates to flash-spun plexifilaments and
particularly to nonwoven ~lash-spun sheets or fabrics made with flash-spun
S plexifilaments.
Back~round of the Invention
E. I. du Pont de Nemours and Company (DuPont) has been
making Tyvek~ spunbonded olefin for a number of years. Tyvek(~) spun
bonded olefin is used as a fabric for garments, especially for use in
10 protective apparel for chemical or hazardous exposure, as an air infiltrationbarrier for construction applications, as medical packaging, and also for
envelopes such as overnight express envelopes. New applications for
Tyvek(~) spunbonded olefin are always being considered and developed.
The properties of Tyvek(~) spunbonded olefin, such as high
15 strength, low basis weight, high barrier, low cost, high opacity, porosity, the
ability to accept printing with vivid results and many other qualities, make it
quite unique. No other product has been commercially available with a
combination of properties comparable to Tyvek(~) spunbonded olefin.
However, DuPont is always looking to improve its product offerings and it
20 is quite desirable to push the properties of Tyvek(~ spunbonded olefin
beyond its current limits.
One particular property that would be desirable to improve is
elongation to break or "break elongation". Break elongation is the
percentage the sheet material stretches before it breaks. It is desirable to
25 increase break elongation to provide the nonwoven sheets with some give
prior to breaking. For example, as a garment for protective apparel, the
wearer may stretch his arm outwards from the body and then bend it at the
elbow. If the garment is at all tight fitting, the fabric of sleeve, under this
circumstance, would be stretched. However, it is preferred that the fabric
30 give or yield rather than rip or break. High break elongation also tends to
increase another related property called toughness. In general toughness is a
measure of a combination of tensile strength and break elongation.
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Materials that have high toughness tend to have substantial tensile strength
with the ability to stretch before failure.
Thus, it is an object of the present invention to improve the
elongation of flash-spun nonwoven fabrics while maintaining its other
properties.
Summary of the Invention
The above and other properties of the present invention are
achieved by a sheet material having an opacity greater than 85%, a basis
weight greater than 30 gjm2 but less than 1 OOg/m2, a Spencer puncture
greater than 20 in-lb/in2 and an average break elongation of greater than
about 30%.
The invention further relates to a process for flash spinning
polymer and forming sheet material therefrom, the improvement comprising
mixing the polymer in a hydrocarbon spin agent at a ratio of less than about
l S 16% polymer, and emitting the polymer solution through a spin orifice at atemperature of at least about 1 80~C, wherein the spin orifice has a length to
diameter ratio of at least 2Ø
The invention further relates to an improvement to flash-spun
fabrics by spinning a polymer solution through a spin orifice having a length
to diameter ratio of at least 2.0 and including an inline mixer in a letdown
process prior to the spinning orifice.
Brief Description of the Drawin~s
The invention will be more easily understood by a detailed
explanation of the invention including drawings. Accordingly, drawings
which are particularly suited for explaining the invention are attached
herewith; however, it should be understood that such drawings are for
explanation only and are not necessarily to scale.
Figure 1 a schematic cross sectional view of a spin cell
illustrating the basic process for making flash-spun nonwoven products, and
Figure 2 is an enlarged cross sectional view of the spinning
e~uipment for flash spinning fiber.
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Detailed Description of the Preferred Embodiment
The basic flash spinning process for making flash-spun
nonwoven products, and specifically Tyvek~) spunbonded olefin, was first
developed more than twenty-five years ago and put into commercial use by
5 DuPont. The basic process is illustrated in Figure 1 and is similar to that
disclosed in U.S. Patent 3,860,369 to Brethauer et al., which is hereby
incorporated by reference. The flash-spinning process is normally
conducted in a chamber 10, sometimes referred to as a spin cell, which has
an exhaust port 11 for exhausting the spin cell atmosphere to a spin agent
10 recovery system and an opening 12 through which non-woven sheet material
produced in the process is removed.
A solution of polymer and spin agent is provided through a
pressurized supply conduit 13 to a letdown orifice 15 and into a letdown
chamber 16. The pressure reduction in the letdown chamber 16 precipitates
15 the nucleation of polymer from a polymer solution, as is disclosed in U.S.
Patent 3,227,794 to Anderson et al. One option for the process is to include
an inline static mixer 36 (see Figure 2) in the letdown chamber 16. A
suitable mixer is available from Koch Engineering Company of Wichita
Kansas as Model SMX. A pressure sensor 22 may be provided for
20 monitoring the pressure in the chamber 16. The polymer mixture in
chamber 16 next passes through spin orifice 14. It is believed that passage
of the pressurized polymer and spin agent from the letdown chamber 16 into
the spin orifice 14 generates an extensional flow near the approach of the
orifice that helps to orient the polymer into long polymer molecules. As the
25 polymer passes through the spin orifice, the polymer molecules are further
stretched and aligned. When polymer and spin agent discharge from the
spin orifice 14, the spin agent rapidly expands as a gas and leaves behind
fibrillated plexifilamentary film-fibrils. The gas exits the chamber 10
through the exhaust port 11. The spin agent's expansion during fl~hing
30 accelerates the polymer so as to further stretch the polymer molecules just as
~ the film-fibrils are being formed and the polymer is being cooled by the
adiabatic expansion. The quenching of the polymer freezes the linear
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orientation of the polymer molecule chains in place, which contributes to the
strength of the resulting flash-spun plexifilamentary polymer structure.
The polymer strand 20 discharged from the spin orifice 14 is
conventionally directed against a rotating lobed deflector baffle 26. The
5 rotating baffle 26 spreads the strand 20 into a more planar web structure 24
that the baffle alternately directs to the left and right. As the spread web
descends from the baffle, the web is passed through an electric corona
generated between an ion gun 28 and a target plate 30. The corona charges
the web so as to hold it in a spread open configuration as the web 24
10 descends to a moving belt 32 where the web forms a batt 34. The belt is
grounded to help insure proper pinning of the charged web 24 on the belt.
The fibrous batt 34 is passed under a roller 31 that compresses the batt into a
sheet 35 formed with plexifilamentary film-fibril networks oriented in an
overlapping multi-directional configuration. The sheet 35 exits the spin
15 chamber l O through the outlet 12 before being collected on a sheet
collection roll 29.
The sheet 35 is subsequently run through a f1nishing line which
treats and bonds the material appropriate for its end use. For example, a
significant part of the Tyvek product line is hard product which is pressed on
20 a smooth heated bonder roll. The hard product has the feel of slick paper
and is used commonly in overnight mailing envelopes and for air infiltration
barriers in construction applications. By this bonding process, both sides of
the sheet are subjected to generally uniform, full surface contact thermal
bonding. For apparel, the sheet 35 is typically point bonded to have a softer,
25 fabric like feel. The intent is to provide closely spaced bonding points withunbonded fiber therebetween in an aesthetically pleasing pattern. DuPont
uses one particular point bonding pattern where one side of the sheet is
contacted by a quite undulated surface thermal bonder providing portions
having very slight thermal bonding while other portions are more clearly
30 subjected to the bonding. After the sheet is bonded, it is often subjected to mechanical softening to remove some harshness that may have been
introduced during the bonding.
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Referring again to Figure 2, one aspect of the present invention
relates to the size and shape of the spin orifice 14. The spin orifice 14 may
be characterized as having a length to diameter ratio. The diameter of the
spin orifice 14 is indicated by the letter "d". The length of the spin orifice 14
5 is indicated in the figure by the letter "1" and relates to the length of the spin
orifice which has the diameter "d". The conventional spin orifice has a
length to diameter ratio of 0.9. Thus the length of the orifice is slightly lessthan its diameter. It has been found that a spin orifice that is much longer
than its diameter creates webs that when laid down into fabric sheets have
10 much higher elongation properties. This will be further discussed in relation to examples below.
The foregoing described process for flash spinning and finishing
has been in commercial use for a number of years. Until recently, the only
commercial facilities for flash spinning were based on the use of a
15 chlorofluorocarbon (CFC) spin agent, trichlorofluoromethane
(FREON(~)- 1 1). Considering the complexity of a flash spinning
manufacturing facility and the multitude of considerations for operating such
a facility, Freon- 1 1 would, until recently, have been the only logical choice
for a spin agent because DuPont has proved that it will work. However,
20 according to present law, it CFC's must be phased out of industrial use to
protect the ozone layer.
With the present need to elimin~te CFC's from industrial use,
DuPont has been working extensively on revising the process for making
Tyvek~ spunbonded olefin to use a non-CFC, non-ozone depleting spin
25 agent. After much testing and consideration, the process has necessarily
been redeveloped around a hydrocarbon spin agent, namely pentane. The
transition has required numerous and extensive changes to the process and
has required that a completely new facility be built to implement the new
spin agent. Many of the developments in the project have been the sub3ect
30 of many patents and patent applications. As part of the development and
transition process (which is still ongoing), full capability test facilities were
built to find optimal operating regimes for the numerous aspects and
parameters of flash spinning.
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Initially, the operating ranges for the letdown pressure, solution
temperature, and polymer ratio as well as other operating parameters were
developed in the lab based on web properties alone. With an eye to seek
improved manufacturing and product performance, broad testing vvas
S permitted in the test facilities. Previous tests in the commercial facilities had
proven that the system is prone to significant problems with large scale
coating of the equipment when operating parameters are varied even
slightly. When the equipment becomes coated, it must be disassembled,
aggressively cleaned and reassembled. In a commercial facility, this would
cause prolonged downtime which is unaffordable.
Eventually, it was discovered that by substantially lowering the
polymer concentration in the solution mixture and by increasing the solution
temperature, that stronger fabrics were being made that had better barrier
properties while also having better comfort qualities. A particularly
interesting discovery during this development process was that lower
concentration does not appear to increase elongation until the spin orifice is
reconfigured to have a long length to diameter ratio (L/D). At a
conventional L/D ratio of about 0.9, virtually no difference in elongation
was found. However, when a replacement spin orifice was installed having
a longer L/D ratio, the elongation substantially improved with reductions in
polymer concentration.
There are a number of properties of Tyvek(~) fabric and sheet that
are measured by DuPont. For purposes of explaining the instant invention,
the following tests are presented:
Gurley Hill Porosity is a measure of the barrier strength of the
sheet material for gaseous materials. In particular, it is a measure of how
long it takes for a volume of gas to pass through an area of material wherein
a certain pressure gradient exists.
Gurley-Hill porosity is measured in accordance with
TAPPI T-460 om-88, which is hereby incorporated by reference, using a
Lorentzen & Wettre Model 121D Densometer. This test measures the time
of which 100 cubic centimeters of air is pushed through a one inch diameter
sample under a pressure of approximately 4.9 inches of water. The result is
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expressed in seconds and is usually referred to as Gurley Seconds. ASTM
refers to the American Society of Testing Materials and TAPPI refers to the
Technical Association of Pulp and Paper Industry.
Elon~ation to Break of a sheet is a measure of the amount a sheet
5 stretches prior to failure (breaking) in a strip tensile test. A 1.0 inch (2.54
cm) wide sample is mounted in the clamps - set 5.0 inches (12.7 cm) apart -
of a constant rate of extension tensile testing machine such as an Instron
table model tester. A continuously increasing load is applied to the sample
at a crosshead speed of 2.0 in/min (5.08 cm/min) until failure. The
10 measurement is given in percentage of stretch prior to failure. The test
generally follows ASTM D1682-64, which is hereby incorporated by
reference. Average elongation to break or average break elongation is the
average of the cross directional break elongation and the machine direction
break elongation.
Opacity relates to how much light is permitted to pass through a
sheet. One of the qualities of Tyvek~) sheet is that it is opaque and one
cannot see through it. Opacity is the measure of how much light is reflected
or the inverse of how much light is permitted to pass through a material. It
is measured as a percentage of light reflected. Although opacity
20 measurements are not given in the following data tables, all of the examples
have opacity measurements above 90 percent and it is believed that an
opacity of at least about 85 is minim~lly acceptable for almost all end uses.
Hydrostatic Head is a measure of the resistance of the sheet to
penetration by liquid water under a static load. A 7x7 in (17.78x17.78 cm)
25 sample is mounted in a SDL 18 Shirley Hydrostatic Head Tester
(manufactured by Shirley Developments Limited, Stockport, England).
Water is pumped into the piping above the sample at 60 +/- 3 cm/min until
three areas of the sample is penetrated by the water. The measured
hydrostatic pressure is measured in inches, converted to SI units and given in
30 centimeters of water. The test generally follows ASTM D 583 (withdrawn
from publication November, 1976).
Spencer puncture is measured according to ASTM D-3420-91
Procedure B, which is hereby incorporated by reference, with the exception
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that an impact head with contact area of 0.35 square inches was used on a
modified Elmendorf tester having a capacity of 6400 gram-force. Results
are normalized by dividing the measured energy to rupture by the area of the
impact head and are reported in in-lbs/in2 (J/cm2). The results below are
5 each based on an average of at least six measurements on the sheet.
Examples 1-7
Examples 1-7, Tables I and II were formed in the hydrocarbon
spin agent system with high density polyethylene, a spin orifice L/D ratio of
5.1 and point bonded with a linen and "P" point pattern at 5515 kPascals
10 (800 psi) on a 34" bonding calendar with steam pressure at 483
kPascals-gauge (70 psig) without mechanical softening.
TABLE I
Ex. l Ex.2 Ex. 3 Ex. 4
Spinning Conditions
Concentration (%) 22 18 16 16
SolutionTemp. (~C) 175 189 175 185
Physical Properties
Basis Weight (g/m2) 40.5 40.5 40.5 40.5
Del~min~ion (N/m) 24.5 10.5 24.5 26.5
Hydrostatic Head (cm) 79 163 203 201
Tensile Strength MD (N/m)1600 1950 2300 1750
Tensile Strength XD (N/m)1950 2100 2650 1600
Elongation MD (%) 14 16 15 17
ElongationXD (%) 23 22 20 25
Work to Break MD (N-m) 0.6 0.7 0.8 0.7
Work to Break ~) (N-m) 0.9 0.9 1.0 0.8
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TABLE II
Ex. S Ex. 6 Ex. 7
Spinnin~ Conditions
Concentration (%) 14 14 12
Solution Temp. (~C) 175 184 175
Physical Properties
Basis Weight (g/m2) 44 40.5 40.5
Del~rnin~tion (N/m) 23 24.5 61.5
Hydrostatic Head (cm) 175 231 196
Tensile Strength MD (N/m) 1750 1950 1950
Tensile Strength XD (N/m) 1950 2300 2300
Elongation MD (%) 27 23 29
Elongation XD (%) 39 37 49
Work to Break MD (N-m) 1.0 1.0 1.2
Work to Break XD (N-m) 1.5 1.2 1.5
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Examples 8- 14
Examples 8-14, Tables III and IV were formed in the
hydrocarbon spin agent system with high density polyethylene, a spin ori~lce
L/D ratio of 5.1 and point bonded with a rib and bar pattern at 5515 kPascals
(800 psi) on a 34" bonding calendar with steam pressure at 483
kPascals-gauge (70 psig) without mechanical softening.
TABLE III
Ex. 8 Ex. 9 Ex. 10 Ex. 11
Spinnin~ Conditions
Concentration (%) 22 18 16 16
Solution Temp. (~C) 175 189 175 185
Physical Properties
Basis Weight(g/m2) 40 5 40 5 40 5
Del~lnin~tion (N/m) 23 16 19 24.5
Hydrostatic Head (cm) 124 180 229 234
Tensile Strength MD (N/m)1600 1600 2100 2100
Tensile Strength XD (N/m)1750 1950 2650 1950
Elongation MD (%) 13 15 12 18
Elongation XD (%) 24 24 19 26
Work to Break MD (N-m) 0.35 0.45 0.6 0.8
Work to Break XD (N-m) 0.9 0.9 1.0 1.0
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TABLE IV
Ex. 12Ex. 13Ex.14
Spinnin~ Conditions
Concentration (%) 14 14 12
Solution Temp. (~C) 175 184 175
Physical Properties
Basis Weight (g/m2) 44 40.5 40.5
Del~rnin~tion (N/m) 37 19.5 42
Hydrostatic Head (cm)175 178 229
Tensile Strength MD (N/m) 1950 1950 1750
Tensile Strength XD (N/m) 2300 2300 2100
Elongation MD (%) 28 22 29
Elongation XD (%) 40 36 52
Work to Break MD (N-m)1.2 0.8 1.1
Work to Break XD (N-m)1.8 1.2 2.1
All of the examples above have Opacity measurements above 90
and it is believed that an opacity of at least about 85 is minim~lly acceptable
5 for almost all end uses.
One particular property to note in the above examples is the
elongation of the fabric. Elongation of nearly 50% is quite substantial as
indicated in Example 15. Clearly, it is desirable to have substantial
elongation percentages so that the fabrics stretch and give before they break
10 or rip. This improvement was obtained by providing the system with an
elongated spin orifice 14 in combination with an inline static mixer in the
letdown chamber.
The data thus far has been focused on soft structure "point
bonded" material. The benefits of the present invention also translate to the
15 hard structure which is fully bonded on both sides of the sheet. Hard
structure is unlikely to be used in apparel applications but improvements in
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elongation and toughness would be appreciated in applications suitable for
area bonded flash-spun nonwovens.
Examples 15-22
Examples 15-22, Tables V and IV were formed in the
5 hydrocarbon spin agent system with high density polyethylene, a spin orifice
L/D ration of 5.1 and area bonded using a thermal bonder.
TABLE III
Ex. 15 Ex. 16 Ex. 17 Ex. 18
Spinning Conditions
Concentration (%) 24 18 18 16
SolutionTemp. (~C) 175 175 189 175
Physical Properties
Basis Weight (g/m2) 57.5 57.5 57.5 61
Delz~min~tion (N/m) 63 54.5 63 70
Hydrostatic Head (cm) 102 150 147 216
Tensile Strength (N/m) 32S0 4150 5050 4400
Elongation (%) 16 22 26 28
Spencer Puncture (in-lb/in2) 20 26 28 31
Opacity(%) 96 97 92 97
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TABLE VI
Ex. 19 Ex. 20 Ex. 21 Ex. 22
Spinning Conditions
Concentration (%) 16 14 14 12
Solution Temp. (~C) 185 175 184 175
Physical Properties
Basis Weight (g/m2) 57.5 61 57.5 57.5
Del~min~tion (N/m) 63 71.8 66.5 64.8
Hydrostatic Head (cm) 173 218 257 264
Tensile Strength (N-m/g) 4750 4750 4750 4750
Elongation (%) 28 35 33 49
Spencer Puncture (in-lb/in2) 33 28 33 26
Opacity(%) 95 97 96 95
Conclusion
To summarize the foregoing described invention and put it into
5 perspective, the developments described herein will lead to substantially
improved products
The foregoing description and drawings were intended to explain
and describe the invention so as to contribute to the public base of
knowledge. In exchange for this contribution of knowledge and
10 understanding, exclusive rights are sought and should be respected. The
scope of such exclusive rights should not be limited or narrowed in any way
by the particular details and preferred arrangements that may have been
shown. Clearly, the scope of any patent rights granted on this application
should be measured and determined by the claims that follow.
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