U.S. patent number 3,716,449 [Application Number 05/091,104] was granted by the patent office on 1973-02-13 for method and apparatus for forming a non-woven fibrous web from a foamed fiber furnish.
This patent grant is currently assigned to Wiggins Teape Research & Development Limited. Invention is credited to Albert Peter John Gatward, Bronislaw Radvan.
United States Patent |
3,716,449 |
Gatward , et al. |
February 13, 1973 |
METHOD AND APPARATUS FOR FORMING A NON-WOVEN FIBROUS WEB FROM A
FOAMED FIBER FURNISH
Abstract
Improvements are afforded in method and apparatus for making
non-woven fibrous webs including paper from a foamed aqueous
furnish containing a surfactant in that the foam acquires and has
imparted thereto physical characteristics as regards attainment of
such a highly emulsified state that the bubbles are scarcely
visible to the naked eye, as regards viscosity build-up of the foam
to at least 22 seconds as measured with a Ford Cup Type B-4 and as
regards the specific gravity being no greater than 0.35. Other
improvements pertain to vortical mixing in the production of the
foam, to combining the method and apparatus with conventional
methods and apparatus and to handling the surfactant-containing
water when in the foamed furnish and likewise as drained from the
formed web.
Inventors: |
Gatward; Albert Peter John
(Beaconsfield, EN), Radvan; Bronislaw (Flackwell
Heath, EN) |
Assignee: |
Wiggins Teape Research &
Development Limited (London, EN)
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Family
ID: |
10206801 |
Appl.
No.: |
05/091,104 |
Filed: |
November 19, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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854246 |
Aug 29, 1969 |
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640411 |
May 22, 1967 |
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Foreign Application Priority Data
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Mar 31, 1966 [GB] |
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24125/66 |
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Current U.S.
Class: |
162/101; 162/183;
209/168; 162/190; 162/315 |
Current CPC
Class: |
D21F
11/002 (20130101) |
Current International
Class: |
D21F
11/00 (20060101); D21d 003/00 (); D21d
005/28 () |
Field of
Search: |
;162/101,166,183,190,191,315 ;103/113J,117M ;209/168,196
;252/315,316 ;259/7,8 ;261/93 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bashore; S. Leon
Assistant Examiner: D'Andrea, Jr.; Alfred
Parent Case Text
This application is a continuation-in-part of application Ser. No.
854,246, filed Aug. 29, 1969, which is a continuation in part of
application Ser. No. 640,411 filed May 22, 1967, both of which are
now abandoned.
Claims
We claim:
1. Method of producing a non-woven fibrous web which comprises
a. forming by a vigorous shearing action an aqueous foam consisting
essentially of air dispersed in the form of very small bubbles in
an aqueous solution containing a surface active agent, the number
average diameter of said bubbles being not more than substantially
0.2mm, said foam having a viscosity of at least 22 seconds as
measured by Ford Cup Type B-4 at 20.degree. C and having an air
content by volume of at least about 65 percent;
b. forming a dispersion of fibers in said aqueous foam to produce a
mixture possessing thixotropic properties with low viscosity and
rapid rate of release of said fibers when subjected to draining
action; and
c. depositing said fiber dispersion on a foraminous support with
concomitant collapse of the foam and formation of a non-woven web
on said support accompanied by drainage of residual water from the
web.
2. The method of claim 1 in which the dispersion of fibers in said
aqueous foam is formed by subjecting surfactant-containing water in
which the fibers are dispersed to the vortical foaming action of an
impeller which creates a vortex of liquid drawn towards the
impeller at the base of the vortex.
3. A method according to claim 2, wherein surfactant-containing
water and fibers to be dispersed therein are continuously supplied
so as to be subjected to said vortical mixing action and wherein
foam having the defined properties and characteristics is
continuously removed from the region adjacent the base of the
vortex.
4. Method of producing a non-woven fibrous web which comprises
a. forming a foam of surfactant-containing water, said foam being
formed by a vigorous shearing action and consisting essentially of
air dispersed in the form of very small bubbles, the average
diameter of said bubbles being not more than substantially 0.2 mm,
said foam having a viscosity of at least 22 seconds is measured by
Ford Cup Type B-4 at 20.degree. C and having an air content by
volume of at least about 65 percent, by subjecting water containing
an effective amount of a foam-producing surfactant to the vortical
foaming action of an impeller which creates a vortex of liquid
drawn towards the impeller at the base of the vortex together with
some air that is emulsified by the action of the impeller with
return flow of foam to the mouth of the vortex carrying with it
coarse bubbles which at least in part are drawn into the
vortex;
b. removing foam from the region adjacent the base of the vortex
while it is in the condition produced in that region by the action
of the impeller;
c. dispersing fibers in said removed foam; and
d. depositing the dispersion of fibers in the removed foam on a
foraminous support with concomitant collapse of the foam and
formation of a non-woven web on said support accompanied by
drainage of residual water from the web.
5. A method according to claim 4, wherein the fibers to be formed
into the web are dispersed in the foam as it is formed by said
vortical foaming action.
6. A method according to claim 4, wherein fibers are dispersed in
the foam after removal of the foam from adjacent the base of the
vortex and the foam with the fibers contained therein is subjected
to a second vortical foaming step from which foam containing fibers
therein is removed at the region adjacent the base of the
vortex.
7. A method according to claim 4, wherein surfactant-containing
water having fibers dispersed therein is subjected to a succession
of vortical foaming steps with effluent from one flowing into the
next, foam of emulsified air with the fibers dispersed therein
being removed from adjacent the base of the vortex of the last
vortical mixing step and being directed for deposit on the
foraminous support of the web-forming machine.
8. A method according to claim 7, wherein at a point between the
first vortical mixing step and the second any unfoamed
surfactant-containing water is separated and is returned to the
vortex maintained during the first vortical foaming step.
9. The method of claim 4, in which there are concurrently
introduced from separate sources fibers and surfactant-containing
water into a foam-producing zone in which said foam is produced by
a vigorous shearing action to form a fiber dispersion possessing
thixotropic properties with low viscosity and a rapid rate of
release of said fibers when subjected to draining action.
10. A method according to claim 9, wherein fibers are introduced
while dry.
11. A method according to claim 9, wherein fibers are introduced in
the form of a thick stock of fibers dispersed in water that is
substantially free of surfactant.
12. A method according to claim 9, wherein surfactant-containing
water is drained from the web during its formation on the
foraminous support, and wherein said surfactant-containing water
drained from the web is introduced into the foam-producing zone
simultaneously with the introduction of fibers into said zone.
13. A method according to claim 9, wherein the foam having fibers
dispersed therein is continuously removed from the foam-producing
zone and the rate of introduction into the foam-producing zone of
surfactant-containing water drained from the web during its
formation is controlled responsive to the level of the liquid head
in the foam-producing zone.
14. Method of producing a non-woven fibrous web which comprises
a. forming a suspension of fibers in water;
b. thickening the suspension of fibers to form a thickened fibrous
stock;
c. mixing the thickened fibrous stock with surfactant-containing
water to form a dilute fiber furnish of web-forming
consistency;
d. foaming the dilute furnish by subjecting the furnish to vigorous
shearing action to produce therein an aqueous foam consisting
essentially of air in the form of very small bubbles in an aqueous
solution containing a surfactant, the number average diameter of
said bubbles being not more than substantially 0.2 mm, said foam
having a viscosity of at least 22 seconds as measured by Ford Cup
Type B-4 at 20.degree. C and having an air content by volume of at
least about 65 percent;
e. depositing said foamed furnish on a foraminous support to form a
web with concomitant drainage of surfactant-containing water so as
to partially dry the web;
f. removing partially dried web;
g. returning surfactant-containing water to the mixing step (c);
and
h. substantially balancing the water introduced into said mixing
step with the thickened stock, with the amount of
surfactant-containing water contained in the partially dried web
removed from web-forming step (e).
15. The method of claim 14, in which a portion of the
surfactant-containing water drained from the web during its
formation in step (e) is added to the surfactant-containing water
that is introduced substantially free of fibers into the foam
producing step (d); another portion of surfactant-containing water
drained from the web is admixed with waste portions of the formed
web to form a dispersion of fibers in surfactant-containing water;
and said fiber dispersion is introduced into the foam-producing
step.
16. A method according to claim 14, wherein the fibers are
introduced into the foam-producing zone at a substantially constant
rate and the rate of introduction of said surfactant-containing
water that is substantially free of fibers is controlled responsive
to the level of liquid in the foam-producing zone.
17. Apparatus for making a non-woven fibrous web which comprises in
combination:
a. fiber stock supply means;
b. web-forming means provided with a foraminous support on which an
aqueous fiber furnish is deposited with formation of fibrous web
and drainage of aqueous liquid therefrom;
c. foam-producing means including a casing having means for
admission of air thereto, a blade impeller mounted within said
casing so as to be rotatable about an axis at right angles to the
upper surface of the casing, the upper edge of said impeller being
situated closely adjacent to the upper inner surface of said casing
so as to produce a vortex in the aqueous liquid supplied thereto,
the distance between said upper edge of the impeller and the
surface of the casing being in the range 0.010 inch to 0.125 inch
and such as to form a foam wherein the number average diameter of
the bubbles is not more than substantially 0.2 mm, by vigorous
shearing action on said aqueous liquid;
d. means for mixing said fiber furnish with said foam;
e. means for directing fiber stock from said stock supply means
selectively to said mixing means and to said foam-producing
means;
f. means for directing fiber furnish to said web-forming means
selectively from said foam-producing means or from said mixing
means; and
g. means for directing aqueous liquid drained from said web-forming
means selectively to said mixing means and to said foam-producing
means.
18. Apparatus according to claim 17, wherein fiber stock
selectively directed to said foam-producing means from said fiber
stock supply means is directed from said fiber stock supply means
to stock thickening means and from said stock thickening means to
said foam-producing means.
19. Apparatus for producing a non-woven fibrous web from a foamed
fibrous furnish which comprises in combination:
a. foam-producing means comprising a mixing compartment, a rotary
impeller adjacent the bottom of said compartment, and means for
rotating said impeller at high speed, said foam-producing means
being capable of forming a foam by vigorous shearing action on
aqueous liquid supplied thereto the distance between the adjacent
edge of said impeller and the bottom of said compartment being in
the range 0.010 inch and 0.125 inch;
b. a web-forming apparatus including a travelling foraminous
support on which an aqueous fiber furnish is deposited with
formation of a fibrous web; and
c. means for removing foam-containing dispersed fibers from
adjacent said impeller near the bottom of said mixing compartment
and directing them onto said travelling foraminous support.
20. Apparatus according to claim 19, which comprises a reservoir,
means for directing surfactant-containing liquid drained from said
web during its formation on said foraminous support into said
reservoir, and means for admitting surfactant-containing water from
said reservoir into said compartment responsive to the level of
liquid in said compartment.
21. The apparatus of claim 17, which further includes a reservoir
for supplying surfactant-containing water to said foam-producing
means, means for directing surfactant-containing water from said
reservoir into said foam-producing means, means for collecting
surfactant-containing water drained from said web-forming means,
means for directing surfactant-containing water from said
collecting means into said reservoir, and means for depositing foam
produced by said foam-producing means having fibers disposed
therein onto said foraminous support.
22. The apparatus of claim 21 wherein said collecting means
comprises a first collecting means for collecting a first portion
of said surfactant-containing water drained from said web-forming
means, and a second collecting means for collecting a second
portion of surfactant-containing water from said web-forming means,
and wherein said means for directing water from said collecting
means into said reservoir comprises a mixing reservoir, means for
directing surfactant-containing water into said mixing reservoir,
means for introducing fibrous stock into said mixing reservoir, and
means for directing fibrous stock admixed with
surfactant-containing water from said mixing reservoir into said
foam-producing means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the forming of non-woven fibrous webs and
in particular to the production of such webs utilizing a liquid
suspending medium in the form of an aqueous foam in which the
fibers are dispersed and suspended and from which they are formed
into a layer on the wire of a paper-making machine, which layer is
then collapsed and drained to form the web.
2. Description of the Prior Art
It is known to form paper and non-woven fibrous materials by
depositing a suspension of fibers in a liquid, usually water, onto
a foraminous support, called the wire, which allows the liquid to
drain through while retaining most of the fibers in the form of a
web, or felt, in which the fibers lie intermeshed, all
substantially in the plane of the web. Due to the random nature of
the process of deposition, and also because of the natural tendency
of most fibers to form flocs, or clumps, the web is usually not
uniform but contains areas which are particularly thin or light, or
which are particularly thick or heavy. The degree of uniformity, or
the lack of uniformity, may be controlled to some extent by the
exercise of the operator's skill and by the design of the machine.
In particular, the formation of acceptably uniform webs from fibers
which have an excessive tendency to flocculate, or clump together,
such as long synthetic fibers, or long, lightly beaten cotton or
wood fibers, or other natural fibers, requires that the fibers be
dispersed in very large volumes of water. The subsequent drainage
of such large volumes of water cannot be accomplished on the usual
paper-making machinery but requires costly modifications thereto,
or the use of machinery specially adapted for the rapid drainage of
large volumes of water.
A less common known method of producing relatively uniform
dispersions of fibers in a liquid medium, and hence substantially
uniform fibrous webs, is that by which the fibers are dispersed in
a liquid medium of high viscosity, such as aqueous solutions of
sugar, or of natural gums. With this method, however, it is found
that when the dispersing action ceases the fibers very quickly
cease their motion, and become immobile before becoming
reflocculated and clumped together to an appreciable extent. Thus
the suspension of fibers in such a liquid medium remains well
dispersed until the liquid drains and the web is formed. However,
due to the high viscosity of the suspending medium its drainage
through the forming web and through the supporting wire is slow and
difficult so that this method is not well suited to the large-scale
continuous manufacture of fibrous webs.
It likewise has been proposed to add a surfactant to the water
conventionally used in the water-laying of non-woven webs and by
agitation produce a foam for the purpose of assisting in the
formation of non-woven webs and, more especially, non-woven webs
comprising fibers longer than those conventionally employed in the
manufacture of such webs. However, so far as is known, such
proposals have not provided the correct conditions that are
critically essential and, accordingly, have not been such as to
warrant commercial exploitation.
SUMMARY
According to this invention, fibers to be formed into a web are
dispersed in foamed surfactant-containing water which has been
brought to such a state of extreme emulsification of air therein
that the inadequacies of prior proposals for the employment of foam
in a web-forming operation have been successfully overcome. A
thick, viscous emulsion is formed by dispersing air in an aqueous
solution of an emulsifying agent in the form of bubbles, so small
that even the largest is barely visible to the naked eye. The
number average bubble diameter, as determined in the manner
hereinafter described, should be no greater than about 0.2 mm. The
foam should contain at least 65 percent of air or, in other words,
the specific gravity of the foam should not be greater than 0.35.
Moreover, the viscosity of the foam emulsion should not be less
than 22 seconds as measured at 20.degree. C using Ford Cup Type B-4
in the manner called for by British Standard 1733.
Other features of this invention which are novel and useful per se
but which preferably are employed in combination relate to vortical
foam production so as to cause maximum emulsification as minute
bubbles to occur at the base of a vortex while coarser bubbles tend
to rise and accumulate at the surface in equilibrium with the
vortical forces which tend to suck them down into the vortex with
air so as to become further emulsified into minute bubbles. The
vortical foaming action is continued until the desired
emulsification of air is attained in the region adjacent the base
of the vortex from which the foam is taken for deposit on the
web-forming foraminous surface of the web-forming machine. During
the formation of the foam the fibers to be formed into the web are
dispersed in the foam so as to be present in the highly emulsified
foam taken to the headbox of the web-forming machine.
Other features of the invention relate to the sources of fibers and
of surfactant-containing water and their introduction, preferably
concurrently from separate sources, into the foam-producing means.
Still other features relate to the manner of reutilizing the
surfactant-containing water that drains from the foam when the foam
collapses to form a fibrous web on the wire or other foraminous
support of the web-forming machine and its reintroduction directly
into the foam-producing means, said reintroduction preferably being
controlled responsive to the liquid level occurring in the
foam-producing means. It is a further feature that
surfactant-containing liquid drained from the web during its
formation is returned to the foam-producing means partly
substantially fiber-free and partly with fibers dispersed therein.
Another feature of the invention relates to balancing introduction
of fresh water in a thickened fiber dispersion in relation to the
dryness of the web that is removed from the web-forming machine
whereby maximum conservation of surfactant may be afforded.
Still other feature relate to apparatus. One such feature is
concerned with apparatus which comprises vortex type foam-producing
means so combined with web-forming means that foamed fiber furnish
deposited on the wire of the web-forming machine is taken from
adjacent the base of the vortex that occurs in vortical type
foam-producing means. Other features relate to the apparatus that
is employed in utilizing and pumping surfactant-containing water
drained from the web.
Further features of the method and apparatus of this invention are
concerned with the overall system whereby the employment of a
foamed fibrous furnish may be carried out in the commercial
production of non-woven webs and in affording means in combination
with conventional paper-making equipment whereby responsive to
simple selective manipulation a non-woven web may be produced
either in the conventional way or when employing a foamed
furnish.
DESCRIPTION OF THE PRACTICE OF THE INVENTION
It has been found that when fibers are dispersed in a liquid medium
of the kind specified in the above summary the medium possesses
thixotropic-pseudoplastic properties in that it combines the
advantages of high viscosity under low stresses such as occur when
the fibers cease the motion imparted thereby by the dispersing
machinery, with low viscosity and an acceptably fast rate of
drainage when subject to pressure against a supporting wire during
the drainage and formation of a fibrous web on a usual paper-making
machine. It is also found that fibers dispersed in such a liquid
medium quickly become immobile when agitation ceases and remain
well dispersed to form a substantially uniform web. Materials
formed in this manner have the advantage of very uniform
distribution of fibers over their area, as compared with those
obtained by methods known heretofore, especially if the fibers are
long or liable to clump and flocculate together. Another advantage
is that the suspension of fibers in an emulsion of air can be
obtained at a consistency, that is ratio of fiber to liquid,
substantially higher than is practicable with the previously known
methods. Yet another advantage is that the deposition and drainage
of the suspensions in order to form a fibrous web may be
accomplished using currently available papermaking machinery with
only slight modifications. Alternatively, new machinery can be
constructed to form fibrous webs from the suspension, which
machinery is substantially more simple in design and operation than
is conventional paper-making machinery.
When preparing the thixotropic-pseudoplastic liquid medium the
chemical nature of the emulsifying agent used is found not to be
critical provided that it has an emulsifying power sufficient to
produce a thick, viscous emulsion of very small air bubbles. The
emulsifying agent may be anionic, cationic, or non-ionic and it has
been found that proprietary surfaceactive agents such as that sold
under the name "ACE" liquid, this being an anionic substance, by
Industrial Soaps Ltd., that sold as "TEXOFOR FN 15," a non-ionic
substance, by Glover Chemicals Ltd., and that sold as "AMINE Fb
19," a cationic substance, by Float-Ore Ltd., are all suitable for
making the required viscous emulsion. Another non-ionic emulsifying
agent that has been used is octylphenoxypolyethoxy ethanol. One of
the most useful emulsifying agents is commercial grade dodecyl
benzene sulfonate. Soap flakes may also be used. It is, however, to
be understood that the emulsifying agent used may depend on the
properties which it may impart to a finished fibrous material. The
emulsifying agent is mixed with water and the use of hot water may
be an advantage in preparing the emulsion but hot water is not
essential.
The emulsion may be prepared in any suitable kind of emulsifying
apparatus subject to the proviso that the apparatus is capable of
emulsifying air in the form of very small bubbles, the largest of
which is barely visible to the naked eye, in the liquid and of
maintaining this dispersion. In general, simply stirring the liquid
does not produce a sufficiently vigorous emulsifying action. Thus,
while it was possible to incorporate 65 percent or more of air in a
dilute dispersion of fibers in surfactant-containing water when
using a conventional blender type mixer, the resulting foam fell
considerably short of attaining the aforesaid minute bubble size or
a viscosity of at least 22 seconds or failed to attain either and
the web formed from the foamed furnish was inferior to a web formed
from a corresponding fiber dispersion when the aforesaid properties
were attained. Machines which provide high rates of shear are
required. It has been found that a modified froth flotation cell of
the type made by Denver Equipment Co., Denver, Colorado, provides a
suitable emulsifying action. Effective vortical mixing utilizing
such foam-producing means is described more in detail
hereinbelow.
It is essential that the concentration of the emulsifying agent be
sufficiently high and that the action of the emulsifying apparatus
be sufficiently vigorous to produce a finely dispersed emulsion of
air in water. It will be understood that if the concentration of
the emulsifying agent is too low only a thin fluid emulsion will be
obtained which will soon separate into a liquid layer and a coarse
froth, and that if the concentration is sufficiently high but the
emulsifying action of the apparatus is insufficiently vigorous a
thick, viscous emulsion may not form at all. It will further be
understood that it is necessary to control the amount of air
dispersed in the emulsion because if too much air is folded in the
emulsion will turn into a dry, coarse froth with large bubbles and
will not be suitable for making a uniform dispersion of fibers. A
satisfactory emulsion is one which has a viscosity greater than 22
seconds, when measured by Ford Cup Type B-4 according to British
Standard 1733, and the air content of the foam is not less than 65
percent by volume, that is, the foam has a specific gravity not
greater than 0.35, provided that the largest bubble is barely
visible to the naked eye. When reference is made herein and in the
claims to the viscosity being at least 22 seconds (measured in the
manner aforesaid) the reference is to the viscosity of foam
produced in the absence of the fibers, but under conditions
corresponding precisely with the conditions that prevail when
producing the foamed furnish, the reason being that the presence of
the fibers interferes with obtaining an accurate measurement of
viscosity. The extent to which the largest bubble may be visible to
the naked eye may be determined by examination of the foamed
furnish. The number average diameter of the bubbles in a foam is
determined by placing a drop of foam produced in the absence of
fibers but produced under conditions precisely corresponding with
those that prevail during the formation of the foamed furnish on a
microscope slide. Photomicrographs are taken at 100 magnification
at successive intervals of 1 minute. The number of bubbles visible
in the field of view on each photomicrograph is counted. It is
found that the number of visible bubbles appears to decrease
exponentially with time to a constant value, that is:
log (N-X) = No - k T
where N = the number of bubbles visible at time T
x = a number to be determined by trial and error
No = the (supposed) number of bubbles visible at time T = 0, i.e.,
when the sample is taken
k = a constant
T = time elapsed from the time the sample was taken.
The value of No is determined graphically by plotting log (N-X)
against T, value of X being adjusted until the graph forms the best
possible straight line. Extrapolating the graph to T = 0, namely,
the value of No at the intersection of the line with the log (N-X)
axis, one may obtain No, the "supposed" number of bubbles at the
time when the sample was taken. Dividing the area of the field of
view by No, one obtains the number average area per bubble, and
hence the number average bubble diameter.
It is possible to disperse fibers in the liquid medium concurrently
with the formation of the emulsion, or alternatively to insert the
fibers, either dry or as a suspension of fibers in water, into the
prepared, i.e., foamed, liquid medium and effect dispersion in the
same apparatus as is used to prepare the liquid medium. The
preferred method of effecting dispersion of fibers in the liquid
emulsion is to transfer the prepared liquid medium from the
apparatus in which it is prepared to another apparatus and there to
add to it and to disperse in it a suspension of fibers in a small
quantity of water.
The prepared, substantially uniform, suspension of fibers flows or
is pumped to and deposited as a layer on the moving wire of a
Fourdriner type paper-making machine, or the surface of a cylinder
type board machine, or the web-forming area of other suitable forms
of apparatus known in the art of paper making. Upon the application
of suction to the other side of the wire the layer of emulsion
collapses and drains, leaving on the surface of the wire a web of
fibers of a superior degree of uniformity. While spreading of the
suspension of fibers on the wire and the collapsing and draining of
the resulting layer of emulsion can be effected by any of the
methods known in the art, the spreading is preferably effected by
the use of an enclosed flow spreader of relatively low volume, for
example, of the kind described in British Pat. No. 1,075,103, and
is then collapsed by means of suction applied to the underside of
the wire, after which the formed web is handled as in a usual
paper-making process.
Surfactant-containing water drained from the foam during formation
on the foraminous support of the web-forming machine is effectively
reutilized and at least a part thereof is returned directly to the
foam-producing means as, for example, so as to be introduced
adjacent the mouth of the vortex of a vortical foam-producing unit,
and in order to control the consistency of the foamed fibers
furnish its introduction may be permitted by a sensing device that
is responsive to liquid level in the foam-producing means. Fiber
may be introduced separately either dry or in the form of a
thickened slurry. When introduced in the form of a thickened
slurry, the fresh water content of the slurry is controlled in
balanced relation to the water content of the web as it is removed
from the web-forming machine. Part of the drained water may be
mixed with edge trim or other fibers and separately introduced into
the foam-producing means. Erratic pumping action occasioned by the
presence of foam in the surfactant-containing water is successfully
overcome preferably by the employment of a positive displacement
pump whose capacity preferably is greatly increased in a novel way
by sucking additional liquid into a jet stream of the output of
said pump.
An advantage of the process according to the invention is that
multilayer sheets may be formed without difficulty. Several layers
of fibrous suspension may be collapsed and drained consecutively
upon an already formed mat, that is, by using several headboxes
suspended above and along part of the length of the wire. The
position of each headbox would coincide with a bank of suction
boxes positioned on the underside of the wire. In the normal
paper-making process, where the fibers are dispersed in water this
would necessitate large quantities of water from the second and
successive headboxes being drained through the previously formed
mats, resulting in poor formation. In the process according to the
invention little liquid has to be drained through the previously
formed mats and collapsing and draining of the emulsion does not
unduly disturb the formation. Because of the high viscosity of the
foamed suspension its drainage may be accelerated at a more
controlled rate, namely, on a larger number of suction boxes,
without the disadvantage of the stock flocculating.
DETAILED DESCRIPTION OF THE INVENTION
Further objects, features and advantages of the invention will
become apparent in connection with the following description for
purposes of illustration and exemplification of typical practice of
this invention in connection with the accompanying drawings,
wherein:
FIG. 1 is a schematic side elevation of one type of system
embodying features of this invention wherein a foam emulsion of the
character aforesaid is produced and utilized according to this
invention in the production of a non-woven web;
FIG. 2 is a side elevation of a preferred foam-producing means that
is employed according to this invention for producing a fibrous
foam suspension, the foam-producing means being shown on a larger
scale as compared with the showing in FIG. 1;
FIG. 3 is a plan view of the foam-producing means shown in FIG.
2;
FIG. 4 is a detail sectional elevation through a foam-producing
means of the type shown in FIGS. 2 and 3;
FIG. 5 is a side elevation, largely schematic, of the web-forming
means wherein the foamed fiber furnish is formed into a web on the
wet end of a machine of the Fourdrinier type, this portion of the
system being shown in greater detail than the showing in FIG.
1;
FIG. 6 is a detail view on a larger scale of the headbox end of the
Fourdrinier machine shown in FIG. 5;
FIG. 7 is an enlarged view of the positive displacement type pump
which preferably is used at different locations in the system for
pumping surfactant-containing water; and
FIG. 8 is a block flow diagram showing an overall system which may
be selectively operated in conventional formation of a non-woven
web or utilizing aqueous foam.
In the embodiment illustrated in connection with the drawings the
fiber to be formed into the non-woven web is first beaten with
water and water-containing stock after thickening is mixed with
water containing surfactant to form a dilute fiber suspension
wherein the fibers are entrained in a foam emulsion having the
properties and characteristics hereinabove mentioned. To this end,
the fiber which is introduced into the system is beaten or
otherwise refined in a conventional beater 10, such as a Hollander
beater, to form a stock consistency at about 4 percent by weight
based on bone dry fiber. The stock is dropped by the line 11
controlled by the valve 12 to the machine chest 13 where it is held
so as to provide bulk storage for the system, the fiber being kept
uniformly in suspension as by the use of a conventional agitator
14. From the chest 13 the stock is directed by the line 15
containing the pump 16 to the constant headbox 17 which maintains
its desired head by an overflow over a weir back into the chest 13
via line 18. Preferably the stock in the headbox 17 is passed
through a conventional refiner 19 and back into the headbox from
which it finally flows at a constant head by line 20 into the
thickener 21. The thickener serves to de-water the stock to an
extent which facilitates reuse of surfactant-containing liquid by
balancing the amount of liquid entering the system with that
removed from the system whereby surfactant containing water drained
from the web during its formation on the web-forming machine may be
conserved and reused. The consistency of the stock as it leaves the
thickener is usually between 12 and 20 percent by weight based on
bone dry fiber, but should be sufficiently high to minimize the
input of fresh water into the system, depending upon the dryness of
the web that is removed from the web-forming step prior to drying.
Water extracted from the stock in the thickener 21 is removed
through the drain line 22 and may either go to drain or re-directed
into mill back-water for reuse in the initial beating step.
The thickened stock is next combined with surfactant-containing
water in the foam-producing means, which foam-producing means is
indicated generally in FIG. 1 by the reference character 23 and
which is shown in detail in FIGS. 2, 3 and 4. In preferred practice
of this invention the foam-producing means comprises a plurality of
vortical mixing units arranged for sequential foam production and
withdrawal of foamed furnish in the manner to be described. In the
embodiment shown the foam-producing means comprises a succession of
open top compartments A, B, C and D. Each of compartments A, C and
D contains two vortical foam-producing units. Two are shown in each
compartment and, while there are four compartments, this particular
arrangement is not essential in the practice of this invention. In
the system as illustrated in FIG. 1, the thickened stock from
thickener 21 and the supply of surfactant-containing water from
sources to be described are shown as being introduced into
compartment A so that the foaming and the fiber dispersion occur
simultaneously in this compartment. Alternatively, the
surfactant-containing water may be introduced into compartment A
and the thickened stock may be introduced into compartment C
whereby the foam is initially produced to a substantial extent in
the absence of the fiber and the production of the foam is
completed while being blended with the fiber stock.
The vortical foaming unit which is preferably employed in the
practice of this invention is, as aforesaid, a modification of
foaming equipment designed for use in froth-type mineral separation
by the Denver Equipment Co. However, instead of producing bubbles
for the objective of producing a coarse froth which carries mineral
particles to the surface where they overflow into a so-called
launder, the modified vortical unit which is preferably employed in
the practice of this invention effects vortical mixing under
conditions such that the coarse bubbles rise to the surface without
overflowing and with reintroduction into the vortex along with such
air as may be necessary to provide the desired specific gravity so
as to be further emulsified at the foot of the vortex; and when
foam is taken from the foam-producing means it is taken from a
region adjacent the base of the vortex of one of the vortical
foaming units immediately after the emulsifying action of the
impeller means at the base of the vortex by which the air is caused
to be emulsified in the form of minute bubbles.
Each of the vortical foaming units comprises a bladed impeller 24
which is rotated at a suitable speed such as from 300 to 500 r.p.m.
by means of the driven shaft 25 which rotates in bearings mounted
within the housing 26. The impeller or rotor 24 is interposed
between the floor 27 of the foaming unit and the stator 28 and
there is upstanding from the stator 28 a collar 29 on the first two
units only. The distance between the impeller 24 and stator 28 is
chosen so that the fluid passing therebetween is subjected to a
high rate of shear. In practice this distance is in the range 0.010
to 0.125 inch. During foaming in an operation wherein the liquids
and fiber stock are all supplied to compartment A the fiber to
water consistency in ordinary practice is about 1 to 2 percent and
the liquid level is substantially above the upper extremity of the
collar 29. When the vortical foaming units are in operation the
aqueous furnish is acted upon with powerful shearing action by the
impeller 24 and is projected outwardly centrifugally at the base of
the vortex and at the same time the furnish is vortically drawn
into the collar 29. Because of the vortex action air is drawn into
the dilute furnish and caused to be produced in the form of bubbles
which are effectively emulsified by repeated vortical circulation.
While the mixing is being carried on coarse bubbles tend to rise to
the surface but tend to be sucked into the vortex for further
emulsification so that the amount of coarse bubbles in the region
of the mouth of the vortex becomes relatively stable during flow of
the furnish through the foam-producing means.
In the embodiment shown the foam produced in compartment A flows
over the top of weir 30 which controls the liquid level in
compartment A into compartment B, the principal function of which
is to drain off any unfoamed surfactant-containing water through
line 31 and return it into the vortex of the vortical foaming unit
at the inlet 32 of each unit in compartment A to be further acted
upon to emulsify air therein.
The foamed furnish flows under baffle 33 into compartment C in
which there are two vortical foaming units, indicated generally by
the reference character 34, whose construction is the same as that
of the units in compartment A except for the omission of the collar
29 and inlet 32. In compartment C the vortical units serve to
further emulsify the air so as to reduce the bubble size and,
again, such coarse bubbles as there may be tend to rise to the
surface and become sucked into the vortex again for further
emulsification. In flowing from compartment C to compartment D the
foamed fiber furnish passes through apertures 35 which are disposed
below the level maintained in compartment C, with the result that
coarse bubbles in compartment C at the surface are not transferred
to compartment D and only the more minutely emulsified bubbles pass
into compartment D through the apertures 35.
In compartment D there is another pair of vortical foaming units,
indicated generally by the reference character 36, which are
identical in construction and operation as compared with the units
contained in compartment C. In compartment D such coarse bubbles as
there may be again rise to the surface where they are recycled into
the vortex for further emulsification and the foam is taken from
the foam-producing means by the outlet conduit 37. Baffles 38
assist in maximizing the emulsified characteristics of the foamed
furnish as it is withdrawn through the outlet conduit 37.
Depending somewhat on the amount and type of emulsifying surfactant
which is used, the fineness of the bubble size of the emulsified
air may vary somewhat responsive to different rates of rotation of
the impeller in the vortical foaming units. It is well, therefore,
before instituting a commercial run to make adjustments in the
rotational speed of the impellers in the respective vortical
foaming units in order to determine the most effective rotational
speed for the impeller conditions otherwise prevailing during the
operation in question. As aforesaid, when reference is made to
bubble size and viscosity the reference is to the bubble size and
viscosity of foam produced in the absence of fiber in the solution
of the surfactant and water inasmuch as the presence of the fiber
interferes with examination as regards pore size and also prevents
the making of a viscosity determination of the kind referred to
hereinabove.
When the dilute fiber suspension leaves the foam-producing means
through the outlet conduit 37, the foam is of the highly viscous
character hereinabove mentioned wherein the individual bubbles are
scarcely visible to the naked eye. In this condition the foam is
taken by the conduit 37, controlled by the valve 39 to the headbox
40' of the Fourdrinier type web-forming machine, as shown in FIG. 1
and in greater detail in FIGS. 5 and 6. For feeding the foam onto
the wire 91 the headbox 40 may be provided with a simple open type
Vee box and may be provided with a slice of known design.
Some drainage of the liquid phase water occurs at the table rolls
41 immediately after the breast roll 42. During travel of the wire
over the wet vacuum boxes 43, the foam partially collapses. Some
drainage of liquid continues to occur during the travel of the wire
over the larger area in the region of table rolls 44 and further
collapse and further drainage occurs at the dry vacuum boxes 45.
After passing over the vacuum couch roll 46 the web is then carried
on through the conventional press sections of a Fourdrinier-type
machine and from there is passed on to conventional dryers. The
final consolidation of the web occurs when the web is taken to the
first press-felt. When the web leaves the vacuum couch roll the
water content will be less than in the normal paper-making process,
typically 75 percent instead of 82 percent (that is 25 percent
instead of 18 percent by weight based on bone dry fiber),
corresponding with the water content of the thickened pulp which is
introduced into the foam-producing means and commingled with the
surfactant-containing water to make the foam. The thickened pulp
introduced into the foam-producing means should be de-watered at
least to such an extent that nearly all of the
surfactant-containing backwater may be returned to be commingled
therewith, and only very little of the said back-water needs to be
disposed of. If the pulp is de-watered to a sufficient extent, no
back-water needs to be disposed of, and the system is then
balanced. To the extent that surfactant contained in
surfactant-containing water retained in the web leaving the vacuum
couch roll or in the small quantity of back-water which is disposed
of is removed from the system, a corresponding amount is introduced
into the system as by metering it into the foam-producing
means.
The vacuum maintained in the vacuum boxes 43 and 45 is provided by
a conventional vacuum pump 47. Surfactant-containing liquid from
the wet vacuum box 43 disposed closest to the breast roll 42 is
removed therefrom by the line 48 under the influence of the applied
vacuum to a separator tank 49 wherein the liquid becomes separated
from air and the air is taken into the vacuum pump 47 by the line
50 from which it is discharged into the atmosphere.
Surfactant-containing water is directed from the bottom of the tank
49 to the pump 51 by which it is pumped by the line 52 to the
back-water tank 53.
While specific web-forming means has been shown and described
whereby a fibrous web is formed with concomitant drainage of
surfactant liquid, it is to be understood that this has been done
for purposes of illustration and that other web-forming means may
be employed since the particular means whereby the fibers are
formed into a web is not critical.
The pumping of water containing surfactant in a quantity adapted to
produce a viscous foam presents a special problem inasmuch as a
conventional centrifugal pump is not adapted to handle liquid that
contains air. Accordingly, in order that the surfactant-containing
water may be effectively pumped a special type of positive
displacement pump is employed. A pump which is commercially known
as type F80 Mono pump has been found to be suitable. The pump 51 is
a pump of the positive displacement type wherein, as shown in
detail in FIG. 7, the rotation of the rotatable sinusoidal part 54
effects positive displacement pumping to produce a continuous
unfluctuating stream. It is one of the novel features of this
invention that the exit of the pump 51 is provided with a jet 55
which creates in the surrounding chamber 55A a suction imposed on
line 56 which communicates with the other two wet vacuum boxes 43.
In this manner the relatively small capacity positive displacement
pump is used so as to greatly increase its capacity for withdrawing
surfactant-containing water from the wet boxes 43 and directing the
water to the back-water tank 53.
The drained water which during travel of the wire drains therefrom
in the region of the table rolls 41 and likewise in the region of
the table rolls 44 is collected by the collector 57 from which it
drains to pump 58 which pumps the surfactant-containing water by
the line 59 to the back-water tank 53.
Surfactant-containing water is directed from tank 53 to compartment
A of the foam-producing means by line 88, the amount being
controlled by passage through the pump 89. A sensing device which
is responsive to liquid level in compartment D of the
foam-producing means 23 regulates the speed of operation of the
pump 89 so as to maintain the level in compartment D substantially
constant. When make-up surfactant is needed it is metered into
compartment A of the foam-producing means 23.
Vacuum provided by the vacuum pump 47 also communicates with the
separator tank 60 into which surfactant-containing water is taken
from dry vacuum boxes 45 by the line 61. The surfactant-containing
water from the bottom of the tank 60 is pumped by the pump 61,
which is of the same type as the pump 51 which has previously been
described. Part of the water is pumped directly through the pump 61
and thence to a jet in the jet region 62 which serves to suck the
balance of the water from bank 60 into the stream being pumped. The
surfactant-containing water so propelled is directed by the line 63
so as to go into the hogpit 64.
The hogpit also normally receives the surplus trim width of the web
and during any wet end break any such web material also falls into
the hogpit wherein it is slushed by means of conventional agitating
means such as the agitator 65. The slushed fiber in the hogpit is
maintained at a consistency which is desired for being pumped by
pump 66 and line 67 into the foam-producing means 23. The pump 66
again is of the positive displacement type in order to effectively
pump the fibers slushed with surfactant-containing water back into
the foam-producing means. Since some surfactant-containing liquid
may be retained on the wire after it leaves the couch roll 46, a
vacuum box 68 preferably is installed to extract the moisture from
the interstices of the wire and any such collected
surfactant-containing water is directed by line 69, which
communicates with the line 61 leading to the separator tank 60, and
any such liquid eventually goes into the hogpit.
In typical practice approximately 80 percent of the total recovered
surfactant-containing liquid is returned to the back-water tank 53
and about 20 percent to the hogpit 64. However, the relative
amounts thereof are variable.
On the underside of the wire where it is returned to the breast
roll water sprays may be used for washing the wire and, in order
that fresh water may not be introduced into the system via the
wire, a further vacuum box 70 is applied to the wire immediately in
front of the breast roll and fresh water discharged therefrom is
allowed to go to drain along with the wash water from water sprays
71 which is collected by the collector 72 and directed to the drain
73.
Examples of the manufacture of fibrous web materials using the
method according to the invention are as follows:
EXAMPLE 1
A web of filter paper grade was produced as above described. The
furnish consisted of 50 percent bleached kraft, 25 percent sulfite
and 25 percent cotton linters. The fibers were beaten and stored
and after thickening as aforesaid were supplied to the
foam-producing means at a consistency level of about 1 percent. The
viscosity of the foam was 23 seconds. Its specific gravity was 0.3
and the number average diameter of the bubbles was less than 0.2
mm. The take-off moisture for the web was 73 percent, or 27 percent
dry. The suction maintained in the suction boxes was about 10
inches Hg and that in the suction couch roll about 14 inches Hg.
The surfactant solution fed into the mixing means contained about
0.2 percent of octylphenoxypolyethoxy ethanol as the emulsifying
surfactant. The weight of the finished paper was approximately 120
g/sq.m. and it was formed into a web about 84 inches wide at the
rate of about 100 ft. per minute. The mean residence time in
association with each pair of vortical forming units was about 60
seconds. The fiber formation was excellent.
While the same fiber stock was being run long fibers were fed into
the foam-producing means in sufficient quantity to constitute about
10 percent by weight of the total fiber. The long fibers that were
added consisted of cut, staple nylon fibers about 1/2 inch in
length and 2 denier per fiber. Similar fibers, but 3/4 inch in
length, were added in another run. In each run the fiber formation
in the web was excellent with good lateral fiber disposition in all
directions.
EXAMPLE 2
Paper-making stock was prepared by known methods by disintegrating
cotton linter fibers in water and beating to 18.degree.
Schopper-Riegler. The stock was treated in the normal way with a
wet strength resin, that used being 2 percent melamine formaldehyde
by weight of fiber.
An emulsion of air in water was prepared by continuously pumping a
0.2 percent by weight solution of "TEXOFOR FN 15" (a
polyoxyalkylene condensate) in water through a modified Denver
flotation cell of the kind hereinabove described at such a rate
that the mean residence time in the cell was 15 seconds. The
admission of air to the impeller of the cell was restricted so that
little coarse froth formed at the surface of the emulsion and was
not allowed to overflow into the launder of the cell in the manner
used in the froth flotation process.
The emulsion and the prepared stock were taken pumped into another
Denver cell at such rates of flow that the mean residence time was
45 seconds and the proportion of fiber to water in the suspension
was 2 parts by weight of fiber to 100 parts of water. The admission
of air to the cell was restricted so that little of the coarse top
froth was formed and the suspension did not overflow into the
launder of the cell. The specific gravity of the suspension was 0.3
and the viscosity and bubble size were substantially as in Example
1.
The suspension was pumped from the bottom of the second cell
adjacent the base of the vortex maintained in the cell and spread
and deposited in a layer on the wire of a Fourdrinier type
paper-making machine through a V-shaped nozzle of the kind
described in British Pat. No. 1,075,103, the thickness of the layer
of suspension being approximately 11/2 cm. The V-shaped nozzle was
suspended over the wire at a position such that the suspension was
deposited on the wire directly above the suction boxes and on
passage over the surfaces of the suction boxes the layer collapsed
and drained to form a web of fiber of a substance of 100 g/sq. cm.
The suction in the boxes was approximately 11/2 to 3 inches Hg and
that in the suction couch was 12 to 14 inches Hg.
The fibrous web was then transferred from the wire to the
succeeding sections of the paper-making machine in the normal
manner. The liquid and froth collected from the suction boxes was
largely separated from the air in a cyclone type separator of known
kind. The air was exhausted by a wet vacuum pump and the separated
liquid and froth were returned to the Denver cell.
The uniformity of the resulting material was much greater than that
of the usual material made from similar fibrous stock with the
resulting advantage of more uniform distribution of the size of
open pores in the material, and a superior filtering
performance.
EXAMPLE 3
The process followed was that of Example 2 but the stock consisted
of 67 percent of free beaten sulfite pulp and 33 percent of an
artificial latex such as is referred to in British Pat.
specification No. 920.848. The latex was added to the suspension in
the second Denver type cell. The web formed had a substance of 40
g/sq.m. and the proportion of latex retained in the web was 85
percent to 90 percent as compared with 70 percent retained when a
web is made by the usual known paper-making processes. Thus it will
be understood that by the use of a suspension prepared in
accordance with the invention there can be obtained an increased
retention of loadings and other additives within the fibrous
web.
EXAMPLE 4
Cotton linter stock as in Example 2 had added to it in the beater a
proportion of 10 percent Terylene fibers 1 inch long and 15 denier.
The stock was then pumped continuously through a thickener into a
modified hydrapulper dispersing machine having a vortical mixing
action very similar to that hereinabove described as produced when
using the modified Denver cell, the hydrapulper screen having been
removed. In the hydrapulper the furnish was diluted to a
consistency of 1 percent and it had a proportion of 0.2 percent by
weight of liquid of TEXOFOR FN 15 added to it. The mean residence
time in the hydrapulper was approximately 5 minutes which was found
to produce a sufficiently viscous suspension of fibers in
emulsified foam meeting the aforesaid requirements as regards
specific gravity and bubble size. This suspension was pumped from
adjacent the base of the vortex onto the wire of a Fourdrinier type
machine and collapsed there as described in the preceding examples.
The resulting product was a filter paper of superior uniformity and
heat resistance.
A further feature of this invention resides in using in combination
with a Fourdrinier type machine or other web-forming machine
apparatus which may be selectively employed either for the
production of paper or other non-woven web material in the
conventional manner or for utilizing fibers which have been
dispersed in foam that is fed to the headbox of the web-forming
machine. Such apparatus combination is illustrated in the block
flow diagram of FIG. 8. In this flow diagram the operating
components have been indicated generally. Means for preparing a
fiber stock suspension is indicated at 74 and prepared stock is
transferred to the thick stock storage means 75. The wet end of a
paper- or other web-forming machine is indicated at 76. When
employing conventional equipment the thick stock from storage means
75 is directed through the mixing means 77 wherein the furnish of
desired dilution is produced that is fed into the headbox of the
web-forming machine. Transfer of the thick stock to the mixing
stage is controlled by valve 78 and transfer of the furnish to the
headbox of the web-forming machine is controlled by valve 79. Water
drained from the web during its formation may be directed to the
back-water tank 80 so that it may be fed back into the mixing stage
77. The flow of water into the back-water tank is controlled by the
valve 81. Of course, the web formed on the wet end of the
web-forming machine is removed therefrom and processed in the usual
way. When the apparatus is being employed in the conventional
manner the valves 78, 79 and 81 are open and the valves 82, 83 and
84 are closed.
When it is desired to form the web from a foamed fiber furnish the
valves 82, 83 and 84 are opened and the valves 78, 79 and 81 are
closed. When the valve 82 is open, thick stock from the storage
means 75 is transferred to the stock thickener 85 which corresponds
with the stock thickener 21 shown in FIG. 1. Thick stock from the
stock thickener 85 is directed to the foam-producing means 86 which
in the flow diagram corresponds with the foam-producing means 23
shown in FIG. 1. The foamed fiber furnish may then be directed
through open valve 83 to the headbox of the web-forming machine. In
FIG. 8 all water separated from the foam after it is deposited on
the foraminous surface of the web-forming machine is shown
generally as being directed into the container or back-water tank
87 from which it is fed, as desired, into the foam-producing means
86. No attempt in FIG. 8 has been made to illustrate the different
means shown in connection with FIG. 1 for returning the
surfactant-containing water to the foam-producing means. Obviously,
however, the expedients shown and described in connection with FIG.
1 may be employed in the combination shown as a flow diagram in
FIG. 8.
* * * * *