U.S. patent number 3,593,074 [Application Number 04/886,967] was granted by the patent office on 1971-07-13 for apparatus and process.
This patent grant is currently assigned to E. I. du Pont de Nemours and Company. Invention is credited to Lawrence Isakoff.
United States Patent |
3,593,074 |
Isakoff |
July 13, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
APPARATUS AND PROCESS
Abstract
A process for preparing fibrous sheets of organic synthetic
polymers in which a filamentary web is entrained in a gaseous
stream flowing in a path toward a receiving surface and the web is
electrostatically charged before being collected, the improvement
being confining the gaseous flow before charging by directing it
through a critically dimensioned passage which converges in the
direction of flow. The apparatus includes an element which together
with the target plate of the charging device for electrostatically
charging the fibers structurally defines the passage.
Inventors: |
Isakoff; Lawrence (Wilmington,
DE) |
Assignee: |
E. I. du Pont de Nemours and
Company (Wilmington, DE)
|
Family
ID: |
25390171 |
Appl.
No.: |
04/886,967 |
Filed: |
December 22, 1969 |
Current U.S.
Class: |
361/230; 19/299;
156/167; 156/273.1; 425/72.2; 425/86; 425/169; 425/174; 425/327;
264/441; 264/465 |
Current CPC
Class: |
D04H
3/16 (20130101) |
Current International
Class: |
D04H
3/16 (20060101); D04h 005/00 () |
Field of
Search: |
;317/3,4,262R,262A
;264/24 ;156/167 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Hix; Lee T.
Claims
I claim:
1. In an apparatus for forming a fibrous web that includes a means
for forming a filamentary strand entrained in a gaseous stream,
means for spreading the strand into a web and directing it in a
path and an opposed ion gun and a target plate positioned on
opposite sides of said path below said spreading and directing
means, a device for confining the flow of the gaseous stream in the
direction of said path comprising an element positioned on the same
side of said path as but above said ion gun, said element having a
surface disposed toward said plate, said surface converging toward
said target plate and terminating in downstream edge spaced from
said target plate, said surface and said plate forming a passage
having an outlet defined by the distribution of distance between
said edge and said plate, said distribution being symmetrical about
the midpoint of said edge, the distance between said midpoint and
said plate being in the range of from 0.03 inch to about 0.05
inch.
2. The apparatus as defined in claim 1, the ratio of the distance
from a point along said edge having a straight line distance one
inch from said midpoint to said plate and the midpoint distance to
said plate being in the range of from 1.0 to 3.25.
3. The apparatus as defined in claim 2, where said midpoint
distance is in the range from 0.03 inch to 0.25 inch. 4The
apparatus as defined in claim 3, including means for mounting said
element for swinging movement toward and away from said plate; and
means for biasing said element toward said
plate. 5. A process for forming fibrous sheets that includes the
steps of entraining a web in a gaseous stream flowing downwardly
toward a collecting means, electrostatically charging said web, and
collecting said web in a tensionless state wherein the improvement
comprises confining the gaseous stream flow prior to the charging
step by directing said flow through a passage converging in the
direction of flow to an outlet having
a minimum width dimension of from 0.03 to about 0.05 inch. 6. The
process of claim 5 said width dimension being in the range of from
about 0.03 to 0.25 inch.
Description
BACKGROUND OF THE INVENTION
This invention relates to a process and apparatus used in the
preparation of nonwoven fibrous sheets of synthetic organic
polymers. More particularly, it is directed to a process and
apparatus for spreading a plexifilamentary strand into a planar
web, directing the web towards a surface, charging the web and
collecting the web in the form of a nonwoven fibrous sheet having
improved uniformity over that obtained by prior art methods.
In the preparation of fibrous nonwoven sheets, various methods and
apparatus have been developed for dispersing the filaments from a
bundle into a wide band and for directing the web by oscillating
means in a programmed manner to various locations across the width
of a moving collecting surface. For example, the Steuber patent
U.S. Pat. No. 3,169,899 describes a process for making a nonwoven
sheet from flash-spun fibrous materials. In the flash-spinning
technique a solution of an organic polymer which is under pressure
and at a temperature far above the boiling point of the solvent is
extruded into an area of substantially atmospheric pressure. As the
material issues from the orifice, the solvent expands rapidly and a
plexifilamentary strand is formed. The plexifilamentary strand is
composed of very thin film-fibril elements which are interconnected
in a three-dimensional network as described in detail in Blades
& White U.S. Pat. No. 3,081,519. The three-dimensional network
is spread into a wide web by causing it to be swept along a smooth
path past a curved surface whereupon the expanding solvent gas
spreads the material. The oscillating motion serves to direct the
web to various areas across the width of a moving collecting belt
where it is deposited in the form of swaths. The web can be
electrostatically charged to increase its width and increase the
separation between the fibrils. A fibrous nonwoven sheet is thereby
obtained.
In an alternate process described in copending U.S. Pat.
application Ser. No. 628,871 filed Apr. 6, 67, now U.S. Pat. No.
3,497,918 the oscillating baffle can be replaced by a rotating
baffle, having specially contoured surfaces, which simultaneously
spreads and oscillates the web as it is directed through an
electrostatic device to apply uniform electrostatic charge on the
web and promote uniform deposition of the web on a moving
collecting surface. A particularly advantageous charging apparatus
is described in Kilby and Smith, U.S. Pat. No. 3,456,156. The
apparatus consists of an annular disc target electrode which is
concentric with the rotating baffle and rotates independently of
said baffle. A multineedle ion gun is positioned opposite the
target plate, the needles being aimed at a portion of the target
electrode to provide a corona discharge zone. The fibrous material
moving in a planar path between the target electrode and the ion
gun needles is electrostatically charged before being deposited on
the moving collecting surface.
A number of requirements must be satisfied in order to obtain wide,
fibrous, nonwoven sheets having a uniform appearance and a uniform
fabric weight. In general, wide nonwovens are obtained by blending
and overlapping the output from several spinning positions.
Copending U.S. Pat. application Ser. No. 628,872 filed Apr. 6, 67,
describes a mechanism for making fine adjustments and varying the
weight distribution of the webs deposited on the collection
surface. Tests have shown that optimum fabric weight uniformity in
the cross-machine direction (i.e., the direction at right angles to
the direction of movement of the receiving surface) is obtained
when the width of the swath at this surface is within certain
limits which depend on the shape of the cross-machine direction
fabric weight profile produced by each spinning position. This
width is a function of the amplitude of the oscillation imparted to
the web by the baffle, the amount of electrostatic charge on the
web and the distance between the baffle and the receiving surface.
Within certain limits, the wider the swath at the receiving surface
the easier it is to obtain a sheet having uniform fabric weight in
the cross-machine direction.
Uniformity of fabric weight in the machine direction is also
important and it has been found to be deleteriously affected by an
uncontrolled random oscillation of the web in a direction at right
angles to that imposed by the baffle, i.e., in the machine
direction. This uncontrolled oscillation, presumably due to gaseous
turbulence around the web, also contributes to a surface defect on
the sheet termed "fiber swirl" which detracts from its visual
appearance. This random oscillation also increases the number of
other defects such as folds, twists, or pleats in the sheet which
produces localized areas of high fabric weight. This reduction in
sheet uniformity becomes more severe as the amplitude of the
uncontrolled oscillation increases and hence as the distance from
the baffle to the receiving surface increases. Decreasing the
distance between the baffle and the receiving surface improves the
machine direction uniformity, however, this simultaneously
decreases the width of the swath (in the cross-machine direction)
at the receiving surface which reduces the blendability of the
individual swaths and worsens the fabric weight uniformity of the
sheet in the cross-machine direction.
It has been found that as spinning throughputs are increased the
larger volumes and velocities of gas produced in the flash-spinning
operation create an undesirable increase in turbulence. This
increases the random oscillation of the web producing a nonwoven
sheet having less than the desired uniformity.
The object of the present invention is therefore to provide a
process and apparatus for the production of nonwoven sheets of
plexifilamentary materials having improved uniformity. In
particular, the process and apparatus provides such sheets having
heretofore unattainable uniformity at high spinning
throughputs.
SUMMARY OF THE INVENTION
The apparatus of the invention comprises means for forming a
filamentary strand entrained in a gaseous stream, means for
spreading the strand into a web and directing it in a plane towards
a receiving surface, a short critically dimensioned confining
passage which converges in the direction of movement of the web and
which is formed between two surfaces having no direct contact with
one another, means for applying an electrostatic charge to the web
and a receiving surface for collecting the web in the form of a
nonwoven sheet. The confining passage provides a spreading action
on the gaseous flow by decreasing the velocity component in the
direction of web movement and increasing the velocity component in
the directions perpendicular to the web and parallel to the width
direction of the passage. This spreads the web further and
decreases gas turbulence in the direction of web movement to
produce a nonwoven sheet having improved uniformity.
The apparatus is used in conjunction with a process for the
preparation of nonwoven fibrous sheets which comprises flash
spinning a plexifilamentary strand in a generally horizontal
direction, deflecting the strand into a generally vertical plane
downwards towards a receiving surface by passage along the curved
surface of a baffle member, spreading the strand into a web and
oscillating said web in the generally vertical plane, passing the
web through the converging passage and then between an ion gun and
a grounded target plate to impart an electrostatic charge on the
web and collecting the web on the receiving surface in the form of
a nonwoven sheet.
The converging passage is formed by positioning a "scoop" element
opposite the target plate above the point of application of the
charge. The scoop element comprises a surface having an upstream
edge and a downstream edge which, together with the target plate,
define the inlet and outlet to the passage. Looking downwards into
the passage, the shape of the outlet is defined by the distribution
of distances between the downstream edge of the scoop and the
target plate. The passage has a length dimension measured in the
direction of movement of the web and a width measured in the plane
of the target plate in a direction at 90.degree. to the direction
of movement. This width should be greater than that covered by the
oscillation of the web at the exit of the passage.
The dimensions of the outlet to the passage are critical. When the
distance between the downstream edge of the scoop and the target
plate is too small, the web will plug the passage. If the distance
is too large, there is no confining effect and no accompanying
improvement in the uniformity of the product. Considering the
projection of the outlet to the passage onto a horizontal plane,
the minimum distance D.sub.1 between the target plate and the
downstream edge of the scoop should be at about the midpoint of
said edge and should be equal to or greater than 0.03 inch and in
the range 0.03--0.25 inch. When this distance exceeds about 0.5
inch, the sheet uniformity is comparable to or may be worse than
with no scoop present.
The other critical dimension is D.sub.2 , measured between the
downstream edge of the scoop and the target plate at points
symmetrically disposed about the midpoint of said edge. The points
on the edge at which D.sub. is measured are located, with reference
to the projection of the outlet on the horizontal plane, by drawing
a circle having a radius of 1 inch and its center on the midpoint
of the projection of the downstream edge on the plane and
determining the points at which this circle intersects the
projection of the downstream edge. D.sub.2 is chosen such that
D.sub.2 /D.sub.1 is in the range 1--3.25.
The angle at which the passage converges in the direction of
movement of the web is preferably between about 10.degree. and
60.degree. and the length of the passage in this direction is
preferably between about 0.5 and 3 inches. The scoop element may
have various shapes which depend on the shape of the target plate.
Preferably, the scoop has a wedge-shaped cross section with the
downstream edge being thinner than the upstream edge to provide
uniform and minimum aerodynamic turbulence during operation. In a
more preferred embodiment, the scoop element is pivotably mounted
and spring-loaded, as hereinafter described, to provide automatic
relief of partial plugs.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional elevation indicating schematically the
arrangement of various elements of an apparatus which can be used
in the practice of the invention.
FIG. 2 is a view seen from the top, of a confining, converging
passage used in the process of the present invention.
FIG. 3 is a cross section taken along the line 3-3 of FIG. 2.
FIG. 4 is a view of another converging passage used in the process
of the present invention.
FIG. 5 is a cross section along line 5-5 of FIG. 4.
FIG. 6 is a front elevation of a semicircular scoop which can be
used with a circular target plate.
FIG. 7 is a plan view of the scoop in FIG. 6.
FIG. 8 is a cross section along the line 8-8 of FIG. 6.
FIG. 9 is a front elevation showing the scoop of fig. 6 positioned
opposite a circular target plate and a rotary baffle.
FIG. 10 is a cross section along the line 10-10 of FIG. 9 with some
of the elements removed for clarity.
FIG. 11 shows the operating principle of a scoop designed to open
quickly in the event of a plug.
FIGS. 12 and 13 are a front and side elevation, respectively, of an
assembly drawing of a scoop incorporating the quick-release
principle.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, a spinneret device 1, is shown connected to a
polymer solution supply source. Polymer solution 2 under pressure
is fed through an orifice 3 into intermediate pressure or letdown
pressure zone 4 and then through spinning orifice 5 into web
forming chamber 6. The extrudate from spinning orifice 5 is a
plexifilament 7. Due to the pressure drop st spinning orifice 5 and
the high temperature of the spinning solution, vaporization of
solvent creates a vapor blast which, by passage along the surface
of baffle 8 concomitantly with plexifilament 7, generally follows
the path of advance of the plexifilament 7 from spinning orifice 5
to collecting surface 9, thereby creating a flow pattern within
chamber 6. Baffle 8 is mounted on shaft 10 which in turn is
oscillatably mounted in bearing 11 and is powered to oscillate by
means not shown. While oscillating of the baffle is not essential,
it is preferred for the preparation of wide sheets.
Target plate 13 and an ion gun 14 are disposed on opposite sides of
the path of advance of the plexifilament web downstream from the
baffle 8 and scoop element 45 is positioned above the charging zone
on the same side of the path of advance as gun 14. Surface 48 of
scoop element 45 in cooperation with plate 13 forms a confining
passage which will be described in more detail hereinafter.
Target plate 13 is so disposed that the vapor blast originating at
5 and the airflow pattern in chamber 6 carry plexifilament 7 along
its charging surface. Target plate 13 is connected to ground by
wire 15 and microameter 16 which indicates target plate current.
The location of ion gun 14 is schematically illustrated along the
path of advance of plexifilament 7 during operation. The ion gun
may be suspended from the ceiling of chamber 6 or from spinneret
device 1 or mounted on brackets to the wall of chamber 6.
After passing through the charging zone, plexifilament 7 is
deposited upon a collecting surface 9. The surface illustrated is a
continuous belt forwarded by drive roll 36. The belt is grounded,
or power source 37 is used to give it an opposite electric charge
to that imposed on plexifilament 7 in the charging zone. Due to
differences in their electrostatic charge, the plexifilament 7 is
attracted to surface 9 and clings to it in its arranged condition
as a sheet 38 with sufficient force to overcome the disruptive
influences of whatever vapor blast may reach this area. Surface 9
carries sheet 38 out of chamber 6 through port 39. Flexible
elements 40 across port 39 and also across port 41 which permit
re-entry of the unloaded continuous belt, assist in retention of
vapor within chamber 6. The sheet is then lightly compacted by
compacting roll 44 and is collected on windup roll 42. A
conventional solvent recovery unit 43 may be beneficially employed
to improve economic operation. Wide sheets are produced by blending
and overlapping the output from several spinning positions placed
in an appropriate manner across the width of a receiving surface
such as the belt 9.
FIGS. 2 and 3 represent one embodiment of the confining passage of
this invention. FIG. 2 is a view from above showing the target
plate 13 and the scoop element 45, having an upstream edge 46 and a
downstream edge 47 as boundaries for surface 48, in face-to-face
relationship. FIG. 3 is a cross section taken along the line 3-3 of
FIG. 2 showing the scoop element positioned at an angle relative to
the target plate 13 thereby forming the converging and confining
passage between plate 13 and surface 48 of element 45. In this
particular embodiment the scoop is a flat plate having a
wedge-shaped cross section and the minimum distance D.sub.1 is
measured at the midpoint of the downstream edge 47 while the
distance D.sub.2 is measured along this edge at points 1 inch on
either side of the midpoint. With this arrangement, D.sub.1 must
equal D.sub.2. Forcing the web and its enveloping gaseous stream to
pass through this confining passage diverts a portion of the gas
from its mainly downward direction to the horizontal direction as
shown by arrows in FIG. 2. This slows down the velocity of the web
in the direction of the receiving surface and dampens down
vibrations while simultaneously increasing the width of the web as
it emerges from the passage.
FIGS. 4 and 5 represent a different embodiment of the confining
passage of this invention. In this embodiment, the flat plate 45 is
replaced by a curved plate 45', thus forming a passage having its
narrowest point at the center and widest point at the edges of the
plate 45'. Again D.sub.1 is measured at the midpoint of edge 47'
and D.sub.2 (greater than D.sub.1 in this case) is measured at
points on edge 47' symmetrically disposed about the midpoint and
having a straight line distance of 1 inch from the midpoint.
FIGS. 6, 7 and 8 are different views of a scoop which can be used
in a more preferred embodiment of the apparatus and process of this
invention, the assembly being shown in FIGS. 9 and 10. In this
embodiment, the oscillating baffle 8 in FIG. 1 is replaced by a
multilobed, rotating baffle 8' of the type described in copending
U.S. Pat. application Ser. No. 628,871 filed Apr. 6, 67. Associated
with the rotating baffle is a rotating annular target plate 13'. An
ion gun (not shown) is located in proximity to the path of the
fibrous material and has a multiplicity of needles disposed across
the width of the path and pointing towards the path, the needles
being located in a plane generally parallel to the path to form a
charging zone 49. Details of this are given in copending U.S. Pat.
application Ser. No. 628,868 filed Apr. 6, 67. The outlet to the
confining passage has its narrowest point (D.sub.1 ) at the
midpoint of the downstream edge 47" of the scoop element 45".
D.sub.2 is again measured at a straight line distance of 1 inch
from this midpoint.
A quick release scoop is one which automatically moves away from
the target plate to allow a mass of material which is beginning to
plug the passage to pass through the scoop gap and then returns to
its original position. One method of accomplishing this is
illustrated in FIG. 11 which shows a spring-loaded, pivoted scoop.
In operation, the main flow of gas downward 50 provides a component
of force 51 which tends to move the scoop element 45 away from the
target plate 13. The counterbalancing force 52 provided by spring
53 keeps the scoop element in place against the stop 54. If
plugging begins, the gas force increases and overcomes the
counterbalancing force to move the scoop. The mass of plugging
material passes through the increased gap, the gas force decreases
and the counterbalance force moves the scoop back to its normal
position. To insure stable scoop clearances throughout minor gas
force variations, the scoop is slightly overloaded against the
fixed stop 54.
The counterbalance force to be provided by the spring is a function
of the scoop gap and the amount of gas generated in the spinning
operation and can be determined by experimentation. "Neg'ator"
constant tension springs (e.g., of the type manufactured by Ametek
Corp. of Hatfield, Pa. ) have been found particularly suitable for
applying this force. Under the conditions of Example 4, the use of
springs which provide a counterbalancing force of about 1.3 lbs. at
the midpoint of the scoop has been found satisfactory.
FIGS. 12 and 13 are a front and side elevation of a scoop assembly
incorporating the quick-release features described above and which
is conveniently attached to the bracket or other device holding ion
gun 14 (FIG. 1). Two springs 53 are used to provide the
counterbalance force one on either side of the scoop. As shown in
FIG. 13, one end of the spring is attached to the bracket at 56 and
the other end is attached to the scoop at 57. Screws are used as
the stops 54 to permit adjustment of the scoop clearances. The
pivot is provided by a cylindrical pin 55, one end of which is
cemented to the vertical extension 58 of the scoop element and the
other end being carried in a bearing 59 fixed in support 60.
In the following examples which illustrate the invention, the
improvements in fabric weight uniformity and reduction of sheet
defects are measured in the following manner:
PERCENT CV OF FABRIC WEIGHT UNIFORMITY
A sheet of material about 500 inches long and at least 8 inches
wide is used. Eighty 1-inch diameter circles are cut from the sheet
along three rows, the center-center distances of these circles
being about 3 inches in the width direction of the sheet and 6
inches in the length direction. The coefficient of variation
(percent CV) of the weights of these 1 inch circles in each row is
calculated and the average percent CV for the three rows is used as
a measure of the sheet uniformity.
In order to make comparisons between materials having different
fabric weights, a corrected percent CV is calculated which
expresses the results in terms of a 2 oz./yd..sup.2 sheet. This is
calculated from the measured percent CV in the following
manner:
Low values of the percent CV (corrected) indicate better sheet
uniformity.
WEB DEFECTS PER CYCLE
During the flash spinning laydown process, the web decelerates and
undergoes various types of folding and twisting as it lands onto
the moving laydown receiver and this shows up as visual defects in
the sheet.
The measurements are made on a lightly consolidated sheet of
material by locating one end of the plexifilamentary web and
carefully pulling it out of the sheet. The number of defects (i.e.,
folds, twists, pleats or other visual defects) for 12 complete
cycles of web oscillation are counted and this number divided by 12
is recorded as the average defects/cycle.
MACHINE DIRECTION (MD) MOTION
This is the maximum distance covered by the web in the machine
direction during a web cycle. The average value is obtained by
making this measurement over 12 web cycles and dividing the sum by
12. The visual aesthetics of the sheet are improved the lower the
MD motion.
EXAMPLE 1
A series of sheets from a plexifilamentary material were prepared
using an apparatus similar to that in FIG. 1, having a single flash
spinning position and modified as in FIG. 9 to use a circular
target plate and a trilobal rotating baffle. In some of these
experiments, a scoop of the type shown in FIGS. 6--8 was used at
various settings of the distances D.sub.1 and D.sub.2 /D.sub.1.
Linear polyethylene having a density of 0.95 g./cc. and a melt flow
rate of 0.9 gram/10 minutes as determined by ASTM method
D-1238-57T, condition E, is flash spun from a 12.6 percent solution
in trichlorofluoromethane. The solution is continuously pumped to
the spinneret assembly at a temperature of 186.degree. C. and a
pressure of about 1600 p.s.i., at a polymer throughput of 35
lbs./hr. The solution is passed through a first orifice (L/D
0.025/0.035 inches) to a pressure letdown zone where the pressure
is reduced to 1050 p.s.i. and finally into the surrounding
atmosphere through a second orifice (L/D: 0.025/0.030 inches). The
resulting plexifilamentary strand passes along the surface of a
rotating baffle which simultaneously spreads it, imparts an
oscillation of 50 c.p.s. and directs it vertically downwards
through a corona charging zone between a multiple point corona
discharge electrode and a grounded target plate towards a moving
belt, located a distance of 10 inches from the baffle, where it is
collected in overlapping layers. The sheet is then lightly
consolidated by passage between a pair of rolls under a pressure of
about 10 lb./lineal inch. The speed of the laydown receiver is
adjusted to obtain a fabric weight of about 2 oz./yd..sup.2.
When a scoop element is used, it is positioned as shown in FIGS. 9
and 10 at a distances D.sub.1 and D.sub.2 /D.sub.1 given in Table
I. The results listed in this table show that use of the scoop
provides a 13--21 percent reduction in the number of defects/cycle,
a 10--.ltoreq.percent improvement in the percent CV of fabric
weight and about 10 percent reduction in MD motion. ##SPC1##
EXAMPLE 2
The procedure of Example 1 is repeated except that the baffle
frequency is increased to 70 c.p.s. and the distance between the
baffle and the laydown belt is increased to 13 inches. The results,
shown in Table II indicate a 19 percent improvement in the percent
CV and about 10 percent reduction in the web defects per cycle and
in the MD motion. ##SPC2##
EXAMPLE 3
The procedure of Example 1 was repeated except the polymer
throughput was increased to 75 lb./hr., the first orifice of the
spinneret assembly had an L/D of 0.025/0.049 inch and the second
orifice an L/D of 0.025/0.044 inch. Sheets were collected at
various scoop settings as well as without a scoop as a control.
Attempts to place the scoop at a minimum distance D.sub.1 less than
about 0.03 inch resulted in frequent plugging and an inoperable
process. The results obtained with various scoop settings are given
in Table III.
It is seen that at the higher throughput, the general level of
uniformity was somewhat poorer than at the lower throughputs used
in the previous examples.
Run 7 shows that placing the scoop at a distance D.sub.1 from the
target plate which is outside the critical range may give a product
with poorer uniformity than the control. The remaining runs, all
carried out at scoop settings within the critical limits provide
improvements of from 6--22 percent in the percent CV compared to
the control. Corresponding improvements are observed in the web
defects and in the MD motion. ##SPC3##
EXAMPLE 4
The procedure of Example 3 was repeated except that a
self-relieving scoop of the type shown in FIGS. 12 and 13 was used
with a baffle frequency of 60 c.p.s. and a baffle to laydown belt
distance of 11 inches. Two 1.48 lbs. constant tension "Neg'ator"
springs (Ametek Corp., Hatfield, Pa. Spring - SH6F21) were used to
provide the counterbalance force which gave an equivalent 1.34 lb.
load at the midpoint of the scoops. The scoop was positioned so
that D.sub.1 =0.063 inch and D.sub.1 1 =1.83. The nonwoven material
collected was found to have a percent CV corrected in the range
7.4--7.7 and the swath width was 23 inches. When the experiment was
repeated without the scoop, the percent CV corrected was 9.3 and
the swath width 22 inches.
EXAMPLE 5
A series of plexifilamentary sheets are prepared at a polymer
throughput of about 140 lbs./hr. with an apparatus modified from
that given in FIG. I. Instead of forwarding the flash spun web in a
horizontal direction, advancing it through a 90.degree. turn to
spread the web and then oscillating it across the receiving
surface, the plexifilamentary web is formed and spread at the exit
to the flash-spinning nozzle, forwarded without change of direction
or oscillation parallel to a large, flat rectangular target plate
towards the receiving surface. Opposite the target plate about one
inch above its lower edge and less than two inches away from the
surface is a line of sharp needles spaced about one-half inch apart
and projecting from a 36 inch long ion gun. A high voltage drop
across the gap between the needles and the target plate provides a
corona discharge field in which the plexifilamentary web is
charged. Between the charging zone and the top of the target plate
is a converging passage defined by the target plate and a scoop.
The target plate measures about 36 inches in width, 8 inches in
length (the direction of web flow) and 1 inch in thickness. The
scoop, which is a flat rectangular bar, measures about 36 inches in
width and 3 inches in length and is inclined toward the target
plate so that the dimensions of the entrance and exit to the
converging passage can be set and varied from test to test. In this
series of tests the entrance to the converging passage, which is
approximately uniform along its width (the 36-inch dimension) is
varied between about 0.3 and 0.5 inch. The exit of the passage,
also uniform along its width, is varied between about 0.045 inch
and 0.020 inch. The width of the spread web is controlled by
varying the exit dimensions of the passageway; the narrower the
passageway the wider the web. Maximum web widths of about 3 feet
are obtained with the scoop in place. Without the scoop, the
corresponding web width is less than 2 feet. When the web width is
controlled with a scoop to give about the same width as is obtained
without the scoop, an improvement in the local uniformity of the
deposited sheet of up to about 24 percent is obtained.
* * * * *