Note: Descriptions are shown in the official language in which they were submitted.
1284~
EXTRUSION PROCESS
AND AN EXTRUSION DIE
WIT~ A CENTRAL AIR JET
Technical Field:
The present invention relates to an extrusion
proce~s for producing fibers and nonwoven mats there-
from and to an apparatus used therefor. More parti-
cularly, the present invention relates to melt-blow-
~ ing proce~ses in which a thermoplastic material in
- molten form iæ extruded from outlet nozzles such that
the molten extrudate merges with the shear layers of
a gas jet emanating from a high velocity gas delivery
nozzle.
Backaround Art:
~? ~ 25
Various known melt blowing processes have been
described in "Superfine Thermoplastic Fibers" by
Wente, Industrial and Enqineerinq ChemistrY, Volume
48, Number 8, Pages 1342-1346, August 1956, "Manufac-
ture Of Superfine Organic Fibers", Naval Research
Laboratory RePort, Number 111437, 1954, and U. S.
Patent 3,676,242 to Prentice. Apparatuses suitable
~: for use in such processes are described in "An Im-
proved Device For The Formation Of Superfine, Thermo-
plastic Fibers~, by K. D. Lawrence et al, Naval Re-
.,
.
~: . , ,
, . ..
~28~
search Laboratory Report. ~umber ~265. February 11. 1959. and in.S. Patent 3.981.650 to Page.
~ onwoven mats produced by these and other currently known
melt blowing processes and the apparatuses used therefor employ
an extruder to force a hot melt of thermoplastic material through
a row of fine orifices and directly into converging high velocity
streams of heated gas. usually air arranged on alternate sides of
the extrusion orifices. Pibers of the thermoplastic material are
~ 10 attenuated within the gas stream. the fibers solidifying at a
; point where the temperature is low enough.
The present invention provide~ the potential to at least
double the throughput rate realized by currently used melt
blowing processes and apparatuses used therefor.
The apparatus and method of the present invention also
permit the formation of composite webs of two or more different
polymers.
~` The present invention fùrther provides enhancement of
quenching of fibers or filaments formed by the method of the
present invention due to the closer proximity of the fibers to
the quenching air or water vapor used in the process.
The present invention additionally provides more quiescent
exit conditions for extruded thermoplastic material. resulting in
less flow disturbance in the downstream region.
The present invention also permits the entanglement of
filaments or fibers in the initial shear region in which tur-
bulence scales are smaller.
.~ ,.~
,
128~4~1
-- 3
According to one aspect of the present invention there is
provided a method of producing fibers of a thermoplastic material
which includes the steps of forming at least one centrally
positioned high velocity gas jet having initial jet shear layers
of a small scale turbulence adjacent the outlet of thermoplastic
material delivery means and extruding at least one stream of
molten thermoplastic material from the thermoplastic material
delivery means. The thermoplastic material delivery means is
arranged at least partially surrounding the at least one high
velocity gas jet. The at least one molten thermoplastic material
stream is merged with the shear layers of the at least one high
velocity gas jet to attenuate the thermoplastic material into
fiber~ within the shear layers forming thereby reduced diameter
~Iber ~treams of the thermoplastlc material.
Another aspect of the invention resides in a thermoplastic
material extrusion mechanism including a die head including
therein a centrally disposed high velocity gas delivery means
adapted to continuously emit at least one jet of gas having shear
layers. At least one chamber is provided for the thermoplastic
material. and thermoplastic material delivery means is arranged
adjacent and at least partially surrounding the centrally
disposed high velocity gas delivery means for directing molten
extruded thermoplastic material emitted from the thermoplastic
material delivery means towards the gas jet. causing the extruded
thermoplastic material to be introduced into the shear layerQ of
the gas jet. Thermoplastic material conduit means communicates
the at least one chamber with the thermoplastic material delivery
means.
The centrally disposed high velocity gas delivery means may
; be placed between or surrounded by the one or more thermoplastlc
.,
~284~11
extrusion openings. When more than one thermoplastic extrusion
opening is used. more than one thermoplastic material may be
supplied to individual extrusion openings from separate chambers.
A means for supplying the thermoplastic material to the
chamber or chambers may also be provided. The thermoplastic
material extrusion openings are arranged to direct the extruded
thermoplastic material toward the gas jet such that the extruded
thermoplastic material is introduced into the shear layers of the
gas jet. A depositing surface may be provided for collection of
streams oS attenuated fibers which are formed by the extruded
thermoplastic material after contact with the jet of gas.
Unlike the method of the present lnvention, melt blowlng
processes for produclng nonwoven mats known heretofore have
extruded fiber-forming thermoplastic polymer resin in molten form
through orifices of a heated nozzle into generally two streams of
a hot inert gas supplied by ~ets which at least partially
surround the extrusion orifices to attenuate the molten resin as
a single stream or row of fibers which are thereafter collected
on a receiver to form a nonwoven mat.
Brief Description of Dpawings.
- Figure 1 is a somewhat schematic side elevational view of a
thermoplastic flow diagram showing a die head having a structure
~- and operation according to the principles of the present inven-
tion:
Figure 2 is a side elevational view. in section. of an
~; embodiment of the die tip of the present inventlon:
Figures 3a-f illustrate bottom views of die tip~ of the
present invention including thermoplastic material extrusion
openings and centrally disposed high velocity gas delivery means:
Figure 4 is a schematic representation of the
`~
'; ,,
~28~
formation of filament streams in the shear layers of
a gaseous jet;
Figure 5 is a side elevational view, in section,
of an alternative embodiment of a die tip according
to the present invention;
Figure 6 shows an elevational view, in section,
of an embodiment of a die tip according to the pre-
sent invention provided with an auxiliary duct;
Figure 7 illustrates in section a side eleva-
tional view of an embodiment of a die tip accordingto the present invention provided with a means for
adju~ting the slots; and
Pigure 8 i5 a somewhat schematic side elevation-
al view of an embodiment of a die head provided with
lS two thermoplastic material chamber~.
9e~t Modes For CarrYina Out The Invention:
; While the invention will be described in connec-
tion with certain preferred embodiments, it is to be
: understood that the invention is not to be limited to
: those embodiments. On the contrary, it is intended
to cover all alternativeq, modifications, and equiva-
lents as can be included within the spirit and scope
of the invention as defined in the appended claims.
One embodiment of the present invention is il-
; lustrated in Figure 1 in which a die head or extru-
sion head 10 is provided with a chamber 12 for con-
taining a polymeric, generally a thermoplastic mater-
ial. The thermoplastic material may be supplied to
chamber 12, generally under pressure, by delivery
means or devices 36 such as a supply hopper and an
extruder screw or the like. The thermoplastic mater-
~: ial may be rendered fluid or molten by one or more
heaters 39 placed appropriotely, such as surrounding
:
.
~L2S9~
the chamber 12, surrounding the hopper and/or betweenthe hopper and the chamber. As shown in Figures 1
and 3a-f, chamber 12 is provided with outlet passages
14 and 16 which permit the flow of molten
thermoplastic material from the chamber to a
plurality of thermoplastic extrusion outlets,
openings or orifices 18 and 20 or a single such
opening 19 located in a preferably circular die tip
and arranged surrounding a centrally placed means for
delivering a generally inert gas as, for example,
air, at a high velocity, with an opening such as a
nozzle 22 or the like from a source of inert gas
23. Like the thermoplastic material, the air
emanatinq from the high velocity nozzle may be heated
by a heater (not shown), appropriately placed, such
as ~n oc surrounding the source of inert gas 23 or
nozzle 22 itself. Alternatively, chamber 12 may be
prov~ded w~th a ~ingle outlet ~shown in phantom in
Figure 1) wh~ch branches or forks ~nto two or more
; 20 passages. As used herein in referring to the inert
gas or a jet of inert gas, ~high velocity" generally
describes jets having velocities of about 300 to over
; 2,000 feet/second. Also as used to describe the pre-
sent invéntion, the terms "central~ or "centrally",
as applied to the gas delivery means or jets, gener-
ally includes all situations in which the gas de-
livery means is surrounded by or arranged between
thermoplastic extrusion openings or a portion there-
of.
According to the present invention, there may be
as few as a single thermoplastic extrusion opening 19
surrounding or at least two thermoplastic extrusion
openings 18 and 20 placed around an opening
comprising the high velocity gas delivery means or
air nozzle 22. However, as is more common among melt
:
. . .
.
...
~284~1~
blown die tips, the high velocity gas delivery means
22 has the form of an elongated opening or slot and a
series or individual thermoplastic extrusion openings
or slits 18 and 20 are arranged in rows on opposite
sides of the gas delivery means 22 as in Figures 3a
and 3b. The openings 18 and 20 are arranged such
that their longitudinal axes form an included angle
with the longitudinal axis of the high velocity gas
delivery nozzle of about 30 degrees to less than
about 90 degrees. As indicated by the embodiment
shown in Figure 2, typically this angle is about 60
degrees .
Some of the arrangements of the centrally placed
gas jet and thermoplastic extrusion openings of the
present invention, as viewed from the bottom, are
shown ln Pigures 3a-f. One preferred arrangement is
shown in Pigure 3a in which two series of holes 18
and 20 are arranged in rows substantially parallel to
and on opposite sides of nozzle 22, formed as a lin-
ear, elongated opening or slot. Each of the openingsin series 18 may be arranged opposite to a corre-
sponding hole in series 20. Alternatively, the holes
in the two series may have a staggered or skewed
relationship with respect to one another. Figure 3b
depicts an arrangement in which two thermoplastic
extrusion openings 18 and 20 take the form of elon-
gated linear openings or slits placed parallel to and
on opposite sides of the elongated linear gas nozzle
or slot 22. The arrangement shown in Figure 3c pro-
vides for the inert gas to be emitted from capillarygas nozzles 22 arranged within an elongated slit 19
from which the polymeric material flows. Although
nozzles 22 are arranged here linearly along a plane
; passing through the center and parallel to the elon-
gated edges of the slit, other arrangements, such as
~Z~
an alternating or zigzag arrangement of the air noz-
zles, are also possible.
Figure 3d illustrates an extrusion arrangement
in which an inert gas nozzle 22, having a circular
cross section, is arranged concentrically within a
cylindrical opening so that the inner surface of the
cylindrical opening and the outer surface of the
inert gas nozzle form an annular extrusion opening
19. In this embodiment and the arrangement shown in
Figure 3e, the central air nozzle 22 may have a dia-
meter of up to about two inches. The embodiment
shown in Figure 3e includes a plurality of thermo-
plastic polymer extrusion openings 18 and 20 arranged
in spaced relationship to one another and to the
inert gas nozzle around the circumference of the
inert ga~ nozzle. Finally, Pigure 3f illustrates a
plurality of capillary gas nozzles 22 arranged cen-
trally within a thermoplastic extrusion opening 19
having a circular cross section.
The die head arrangement of the present inven-
tion permits molten thermoplastic material to be
transferred from chamber 12 through the passages or
conduits 14 and 16 to the extrusion openings 19 or 18
and 20, whereupon, as shown in Figure 4, the molten
extrudate emerges and contacts the shear layers of
the at least one jet of high velocity gas which is
being continuously emitted in a stream from the one
or more centrally placed nozzles 22. As used herein,
the shear layers are considered to be those layers or
portions of the inert gas jet located in the peri-
pheral regions of the jet. This arrangement results
in a plurality of streams, preferably two streams, in
the preferred embodiments shown in Figures 3a and 3b
of molten extrudate being first attenuated in the
peripheral portions or shear layers of the jet or
, . ,
~28~
jets, thereby forming filaments or fibers which are
mixed and directed to a forming or collecting
foraminous surface 37, such as a roll, (shown in
Pigure 8) or a moving wire placed in the vicinity of
S the die heads, where the fibers form a matrix or mat
38.
Since, with the exception of the embodiment
shown in Figure 3d in which the annular extrusion
opening 19 extends around the circumference of the
nozzle opening 22, at least two streams of thermo-
plastic material extrudate are formed by the extru-
sion head of the present invention, which streams may
be ultimately attentuated to form fine filaments or
fibers in the nonwoven mat, the present invention
lS provides the potent~al to more than double the
throughput rate of fiber formatlon compared to exist-
ing processes and apparatus used therefor. In addi-
tion, since the filaments formed by the die head of
the present invention are attenuated in the shear
layers of the high velocity gas stream, these fila-
ments are closer to the air entrained from the atmos-
phere surrounding the apparatus and quenching becomes
` much more effective than conventional apparatus in
which air jets converge on a centrally emitted stream
of thermoplastic material.
Figures 2 and 5 illustrate in section several
configurations of the exit portion of the high velo-
city gas delivery nozzle 22. Thus, the wall sections
24 of the outlet portion of the nozzle 22 may be
straight and may be arranged substantially parallel
to one another, as shown in Figures 5 to 7 or may be
arranged to form an included angle with respect to
each other, as is shown in Figure 2. Typically, with
this latter arrangement, the included angle Çormed by
the wall sections of the tip of the high velocity gas
^` ~.2~
outlet nozzle is about 60 degrees. With the other
preferred wall configurations in which the wall sec-
tions 24 are substantially parallel, the tip of the
nozzle has a slightly different configuration. As
illustrated, the tip of the nozzle has a contoured or
gradually curving and tapering configuration in which
the outlet nozzle walls 26, which are arranged in
approximately parallel relationship, taper through a
gradual S-shaped configuration 27 to a more con-
stricted nozzle tip 28 in which the walls are approx-
imately parallel or arranged at a slight angle to one
another.
Another embodiment of the present invention
provides a means for introducing an additive to the
. 15 air stream or jet which merges with the streams of
molten extrudate. ~hus, a~ illu5trated in Figure 6,
a conduit, such as a tube or duct 30, may be placed
concentrically w~thin and spaced from the walls 24 of
the high velocity gas delivery nozzle. As is illus-
trated in Pigure 6, the additive delivery conduit may
take the form of a duct 30, the outlet end of which
is recessed from the outer portion or exit plane 32
formed by the outer surfaces of the high velocity gas
delivery nozzle. Alternatively, as is shown in phan-
tom in Figure 6, the additive delivery conduit maytake the form of a duct 34, the outlet end of which
extends from the outer portion or beyond the exit
plane of the high velocity gas delivery nozzle. The
end of the duct may also be arranged with the outlet
end having a position between those shown in solid
line or in phantom in Figure 6, particularly one in
which the outlet end of the duct is flush with plane
32. A means may also be provided to move the duct
between the two positions illustrated.
The additive which is introduced into the air
~l2844~1
11
stream through the duct may be any gaseous, liquid
(such as surfactants or encapsulated liquids), or
particulate material (such as a superabsorbent mater-
ial, i.e., a material capable of absorbing many times
its weight of liquid, preferred being materials such
as carboxymethyl cellulose and the sodium salt of a
cross linked polyacrylate; wood pulp or staple fi-
bers, as, for example, cotton, flax, silk or jute),
which is intended to form part of the fibers or the
finished web. The additive material may be fed from
a source located within the extrusion head or remote
therefrom. Although the velocities of the inert gas
flowing through the high velocity gas delivery nozzle
22 and the ~ixture of gas and particles flowing
through the duct 30 or 34 should be optimized, there
ig no need that they be the ~ame. The material may
be fed to the duct by any conventional means using
gas as a conveying medium. Alternatively, the addi-
tive and a suitable fluidizing gas may be mixed and,
in some instance~, supplied to the duct 22 directly,
thus eliminating the use of a duct.
In accordance with another aspect of the present
invention, composite webs of two or more different
thermoplastic materials may be formed. Thus, the
present invention provides for the introduction of
molten extruded thermoplastic material to the shear
layers of at least one rapidly moving stream or jet
of an inert gas from, with the exception noted above,
two or more extrusion openings or sets of openings,
such as 18 and 20, placed surrounding or on alternate
or opposite sides of the high velocity gas delivery
nozzle 22. The thermoplastic material which is ex-
truded from these openings may be the same material
or, alternatively, materials which differ from one
another in their chemical and/or physical proper-
;~
,
. .
,
.,
128~4~1
- 12 -
t:les. Designated as first. second. ...n thermoplastic materials,
wherein n represents a plurality, the materials may be of the
same or different chemical composition or molecular structure
and, when of the same molecular structure, may diffe.r in
molecular weight or other characteristics which results in
differing physical properties. In those situations in which
thermoplastic materials are used which differ from one another in
some respect, such as in physical properties, the extrusion or
die head will be provided with multiple chambers, one for each of
the thermoplastic materials, such as first, second, ...n ther-
moplastic materials, wherein n represents a plurality. That is.
as illustrated in Figure 8, the die head is provided with a first
chamber 12a for t~ls first thermoplastic material and a second
chamber 12b ~or the second thermoplastic material, etcetera. In
contrast to the arrangement lllustrated ln Pigure 1. wherein a
20 slngle chamber 12 is provided with conduits or passages 14 and 16
~qhich provide communication between the single chamber and each
of the first and the second thermoplastic extrusion outlet
openings 18 and 20. when a first chamber 12a and a second chamber
12b are employed for first and second thermoplastic materials.
; respectively, each chamber is provided with passages to only one
extrusion outlet opening or set of openings. Thus, the first
thermoplastic material chamber 12a communicates with the first
extrusion outlet openinp 18 by means of the first thermoplastic
material passage 14a. while the second thermoplastic material
chamber 12b communicates with the second thermoplastic extrusionopening 20 through the second thermoplastic material passage 16b.
The extrusion head may be cast either as a single piece or
may be formed in multiple component
~'
- ~284~1
parts, preferably in two generally symmetrical por-
tions 42 and 44 which are suitably clamped, bolted or
welded together. Each of these portions may also be
formed from separate parts which may also be suitably
clamped,-bolted or welded together. Depending upon
the particular arrangement of the component elements
of the system, when two or more chambers for thermo-
plastic material are employed, the die head may be
provided with a suitable insulating material placed
so as to reduce the thermal influences of air
surrounding the apparatus or regions of the
apparatus. Accordingly, insulation may, for example,
be placed between the chambers and, perhaps, the
thermoplastic material conduit means 14a and 16b.
15 This permit5, when suitable means are provided
therefor, separate and independent control of
appropriately placed heaters, such as 39a and 39b
(Pigure 81 and, as a result, the temperatures of the
thermoplastic materials supplied separately to the
orifice~ 18 and 20. Thus, the first thermoplastic
material having one set of properties may be main-
tained at a first temperature and the second thermo-
plastic material with a different set of properties
may be maintained at a second temperature,
etcetera. Similarly, the temperature of the gas and
the polymers may be different. In addition, the
heaters themselves and, perhaps, the means of
delivering or supplying the thermoplastic material,
may also be insulated. There may also be provided
multiple (such as first and second) thermoplastic
supply or delivery means for the first and second
thermoplastic materials, unlike the apparatus shown
; in Figure 1 in which a single thermoplastic material
supply means and chamber are used. Like the
apparatus containing a single thermoplastic material
. ,
~ ~84~
chamber, however, the apparatus of the present
invention which uses two thermoplastic material
chambers, includes delivery means which delivers
thermoplastic material from a source thereof to the
chambers under pressure. In the embodiment with
multiple (first and second) thermoplastic material
chambers, separate controls may be provided for sup-
plying the thermoplastic material at different pres-
sures.
In both the single piece and multiple part em-
bodiments of the die head, the thermoplastic chambers
may be formed by any suitable means, such as by ap-
propriately coring or drilling the die head, and tbe
open~ngs and passages or conduits may be drilled.
It should also be noted that, although the dis-
cussion herein of the present invention has been
directed to a common extrusion or die head containing
all or most of the enumerated elements, most of these
elements may be located remote from the die head
emp}oy$ng 5uitable communicating means. Such struc-
tures may also include separate thermoplastic extru-
sion openings and centrally placed high velocity gas
delivery nozzle(s), all with associated conduit
means. The openings and outlets are arranged with
the orientations and configurations previously de-
scribed and shown in the drawings.
Both the high velocity gas delivery nozzle 22
and the extrusion openings 18 and 20 may have dimen-
sions which vary widely depending upon the material
being extruded and the concomitant parameters employ-
! ed, as well as the arrangement of the component parts
of the die head. Preferred widths of the air nozzle
22 at its effluent end contiguous to the extrusion
surface, however, lie in the range of about 0.01 inch
to about 1/8 inch but may be larger to permit unim-
21!3~
peded flow of a particulate additive, such as wherean additive introduction duct 30, 34 or the like is
employed. The preferred width of the polymer extru-
sion openings is about 0.005 inch to about 0.05 inch
at their effluent ends contiguous to the polymer
- extrusion surface. The latter dimension is most
preferably about 0.015 inch. The dimensions of the
thermoplastic extrusion openings may also be made
somewhat larger, however, to accommodate the central-
ly arranged high velocity gas delivery nozzles 22, as
shown in Figures 3c, 3d and 3f.
The present invention also contemplates an em-
bodiment in which the size of each of the first and
second thermoplastic material slot openings is ad-
justable. Thia may be accomplished by suitable ad-
justment means as, for example, 810t adjustment
struts 46 as shown in Figure 7.
As discussed above, a nonwoven mat formed from
fibers of a polymeric or thermoplastic material may
be formed according to the present invention by ex-
truding and collecting multiple streams of thermo-
plastic material, that is, extruding a first stream
of a molten thermoplastic material from one or more
first thermoplastic material extrusion openings and
concurrently extruding the same or a different molten
thermoplastic material from one or more second ther-
moplastic extrusion openings, which first and second
thermoplastic extrusion openings are arranged at
least partially surrounding or on opposite sides of
the high velocity gas nozzle. The extruded thermo-
plastic material is attenuated to fibers or filaments
by a jet or stream of high velocity inert gas passing
between the first and second streams of extruded
thermoplastic material. The fibers form as the first
and second thermoplastic material-containing streams
_,
. . .
128441~
merging with the shear layers of the inert gas stream,
as shown in Figure 4. The fibers are then directed
onto a collecting surface, such as a hollow
foraminous forming roll or a moving wire belt 37
located about l to about 16 inches from the die
head. The fibrous web or mat 38 is formed largely
when the fibers are deposited on the collecting
surface. Accord~ng to the method and apparatus of
the present invention, some entanglement of the
fibers may occur in the initial shear region where
the streams of thermoplatic material merge with the
inert gas stream and where the turbulence scales are
generally smaller as well as further downstream at
the confluence of the two streams of fibers.
The materials suitable for use in the present
invention as polymeric or thermoplastic materials
include any materials which are capable of forming
fiber~ after pa~sing through a heated die head and
sustaining the elevated temperatures of the die head
and of the attenuating air stream for brief periods
of time. This would include thermoplastic materials
such as the polyolefins, particularly polyethylene
and polypropylene, polyamides, such as polyhexamethy-
lene adipamide, polyomega-caproamide and polyhexame-
thylene sebacamide, polyesters, such as the methyl
and ethyl esters of polyacrylates and the polymeth-
acrylates and polyethylene terephthalate, cellulose
- esters, polyvinyl polymers, such as polystyrene,
polyacrylonitrile and polytrifluorochloroethylene.
Any gas which does not react with the thermo-
plastic material under the temperature and pressure
conditions of the melt blowing process is suitable
for use as the inert gas used in the high velocity
gas stream which attenuates the thermoplastic mater-
ials into fibers or microfibers. Air has been found
mc/ch
~284~
to be quite suitable for such purposes.
The fibers may generally be formed in any configuration
and diameter commensurate with the shape of the extrusion
orifices.
The process of the present invention is capable of
forming coarse fibers, that is, fibers having diameters
generally up to about 100 microns and, in some instances,
higher, but is generally directed to the formation of fine
fibers, known also as microfibers or microfilaments. The
microfibers produced by the present invention frequently have
diameters in the range of about 1 to about 20 microns
however, microfibers may be formed having diameters down to
as fine as 0.1 micron. Among the limiting factors which
determine the ability of a given thermoplastic material, such
as a polymer, to be attenuated to a fine fiber are the
parameters of the extrusion system, the nature of the
polymeric material, such as the material's molecular weight,
melting point, surface tension and viscosity-temperatùre
characteristics, and the pressures and flow rates of air.
Optimum conditions for any particular thermoplastic material
may be achieved by varying such operating parameters as air
temperature, nozzle temperature, air velocity or pressure,
and the polymer feed rate or ram pressure. These and other
variables may be easily determined by one familiar with melt
; 25 blowing processes. Ample guidance, however, is provided by
Wente in "Superfine Thermoplastic Fibers", Industrial And
Enqineerinq ChemistrY, Volume 48, Number 8, Pages 1342-1346
(1956); "Manufacture Of Superfine Organic Fibers", Naval
Research Laboratorv Report Number 11,437 (1954); Lawrence et
al, "An Improved Device For The Formation Of Superfine
Thermoplastic Fibers", Naval Research Laboratory Report
Number 5265 (1959); and U.S. Patents
.
34~1
18
4,041,203; 4,100,324; 3,959,421; 3,715,251;
3,704,198; 3,692,618; 3,676,242; 3,595,245;
3,542,615; 3,509,009; 3,502,763; 3,502,538;
3,341,394: 3,338,992; and 3,276,944; British Specifi-
cation 1,217,892; and Canadian Patent 803,714.
Generally, the operating conditions may be sum-
marized as follows. The air temperature suitable for
attentuating microfibers may be as low as ambient
temperature. ~owever, it is ordinarily on the order
of at least 200 degrees F above the melting point of
the thermoplastic material, although under certain
conditions some materials, such as the polyolefins,
particularly polyethylene, and polystyrene, require
air temperatures on the order of 300 degrees F above
the melting or softening points of the thermoplastic
; materials. When polypropylene is chosen as the poly-
meric material, a temperature in the range of about
400 to about 700 degrees F is generally used.
The time during which the thermoplastic material
remains and becomes attentuated in the heated, high
velocity inert gas stream is relatively short and
there i8, therefore, relatively little chance of
degradation of the thermoplastic material occurring
when elevated temperatures are employed. However,
generally the thermopla-~tic material remains in a
heated portion of the die head for a longer period of
time than when it is in the high velocity inert gas
stream and the susceptibility to degradation
increases with both the residence time in the die
head and the temperature at which the thermoplastic
material is maintained. Therefore, when polymer
degradation is being sought, this may be achieved by
control of the residence time of the polymer in the
die head and the delivery system upstream.
3s Generally, a thermoplastic material extrusion opening
~2~441~
or polymer nozzle temperature may be used which is
about equal to or as much as 200 degrees Fahrenheit
above the air temperature, depending upon the
~esidence time within the heated portion of the die
head. The temperature of the polymer nozzle is not
normally controlled, however, to achieve or maintain
a particular temperature. Rather, the temperature of
the thermoplastic material extrusion openings is
determined in large part from the heat given up by
the thermoplastic material passing through the open-
ings and the surrounding air, both that passing
through the high velocity gas delivery nozzle and
ambient air. In some instances, in order to maintain
the polymer nozzles within a certain temperature
range, insulation may be placed around the polymer
nozzles, the high ~elocity gas delivery nozzle, or
both.
The velocity of the heated inert gas stream,
which depends at least in part on the gas pressure,
also varies considerably depending upon the nature of
the thermoplastic material. Thus, with some thermo-
plastic materials, such as the polyolefins, particu-
larly polyethylene, air pressures on the order of 1
to 25 psi may be suitable whereas other thermoplastic
materials may require 50 psi for fibers of the same
diameter and length. Consistent with such variables,
the air pressure generally is in the range of 1 to
about 60 psig.
As suggested above, one of the advantages re-
alized with the present invention, as compared toknown melt-blowing apparatus and methods which employ
a single thermoplastic extrusion material opening or
; set o~ openings, is the increase in throughput
rates. Whereas a standard single row or set of open-
ings will frequently be operated at a rate of 3
mc/ch
~ 284~
pounds/inch/hour with a maximum rate on the order of
25 pounds/inch/hour, the present invention permits a
comparable operating rate of 6 pounds/inch/hour up to
a rate of about 50 pounds/inch/hour.
It should be clearly understood by those skilled
in the art that certain changes may be made in the
foregoinq apparatus and method without departing from
the spirit and scope of the invention described here-
in.
. ,
,
. .
