Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates generally to wet-laid
inorganic fibrous sheet material and is more particularly con-
cerned with a new and improved inorganic fibrous web of light
weight produced on production size papermaking machines.
Inorganic fibrous web materials, such as glass fiber
papers, have been manufactured for a considerable period of
time but have constantly presented the papermaker with special
uniform fiber distribution problems. In this connection the
art has recognized that uniformity of fiber dispersion pTior
to sheet formation is ine~orably tied to uniform fiber forma-
tion ~ithin the resultant web material. Due to the difficul-
ties associated with achieving the necessary uniform fibersuspension, the resultant inorganic webs of fine diameter fi-
bers were of a heavy basis weight, i.e., about 50 gra~.s/square
meter and heavier, since the heavier weight materials were
sufficiently thick to mask the non-uniform characteristics of
the resultant fiber array. In the typical wet-laid paper-
making process, the fibers are micron diameter glass fibers
and are supplied to the dispersing medium in the form of bun-
dles chopped from continuous multiple strand glass rovings.
The dispersing medium is usually an acidic aqueous solution
and may be slightly viscous in order to promote and maintain
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the dispersion and isolation of the individual fibers within
the multiple strand bundles. The fibers within the dispers-
ing medium are agitated within a beater to affect bundle
separation and then the stock is conveyed to holding tanks -
containing con~entional mixing units to maintain the fibers
within their desired suspended condition. As can be appre-
ciated, failure to provide sufficient agitation during the
initial dispersion sf the fibers causes incomplete separa-
tion of the glass fibers and fiber bundles are visible
within the resultant continuous sheet material.
In recent years glass fibers longer than conven-
tional papermaking length, namely, fibers having a length of
between a~out 1/4 inch to one inch and more have been used.
However, when these fibers have been dispersed in accordance
with the prior known technique, it was found that the indi-
~idual fibers tended to snag within the beater and holding
tanks and cou~d not easily be redispersed, resulting in
clumps or other irregularities within the sheet product. It
; was also found that the long glass fibers reaccumulated in
such a manner as to form fiber bundles exhibiting the con-
figuration of a haystack or spider. Although these "hay-
, .
stacks" can be tolerated in the heavy weight materials and
for certain applications where the aesthetic appearance of
the sheet material is not of concern, they are considered
major defects in light weight materials and for those appli-
cations where the glass sheet provides a surface veil or is
intended to provide a smooth surface of a reinforced plastic
structure.
The thicker, heavy weight sheets have been used in
vinyl flooring tile and the like to provide dimensional
~06~
stability. However, the heavy weight glass material has
; poor resin penetration characteristics and therefore poor
lamination resulting in a tendency of the tiles to delami-
nate. Thin, light weight, hand sheets having good fiber
distribution can be individually formed when appropriate
care is taken. However, the uniform fiber distribution ne-
cessary to provide for elimination of the visually percepti-
ble, overall density variation referred to as the "cloud ef-
fect" coupled with substantial minimization of isolated fi-
ber bundles or "haystacks" has not been achieved on continu-
ous papermaking machines when producing light weight glass
fiber web material.
In a continuous papermaking operation on a produc-
tion basis, long fiber sheet material is typically produced
from very dilute fiber suspensions using an inclined wire or
; similar type of papermaking machine. In such machinery there
is used a conventional open type headbox of sufficient vol-
ume to establish a calm and relatively placid fluid approach
to the web forming zone. The advantage of such a headbox is
that sufficient time is provided in the headbox for the re-
lease of air bubbles from the fiber suspension prior to web
formation. However, the desired calm and placid fluid ap-
proach has a distinct disadvantage for long glass fiber sus-
pensions. It has been found that as the air bubbles are re-
.
leased at the headbox they tend to permit and even encourage
the formation of fiber "haystacks". The bubbles carry these
fiber bundles to the surface causing them to be deposited on
the surface of the web material as it is being formed. This
provides not only an un~cceptable sheet materi~l from a vis-
ual appearance standpoint, but also produces an irregular or
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roughened surface feel that is readily detected by simply
passing a hand across the surface of the sheet material.
Accordingly it is a primary object of the present
invention to provide a new and improved long fiber glass web
material of extremely light weight yet of uniform fiber for-
mation that is produced on production size papermaking ma-
chinery.
Another object of the present invention is to pro-
vide a new and improved glass fiber web material of the type
described that exhibits a visually perceptible, overall uni-
form fiber distribution and a minimum of isolated fiber bun-
dle defects. Included in this object iscthe provision for a
light weight glass sheet material of continuous length that
is essentially devoid of visible "cloud effect" fiber den-
k sity vaTiations.
Still another object of the present invention is
to provide a light weight glass fiber material that exhibits
improved aesthetic and physical properties and renders the
material well sui~ed for use in reinforced plastic films,
tiles and the like.
Other objects will be in part obvious and in part
pointed out more in detail hereinafter.
These and related objects will be achieved in ac-
cordance with the present invention by providing a continu-
ous machine-made light weight inorganic fiber web material
comprised o~ micron diameter inorganic fibers having a fiber
length of about 1/4 inch or more and a minor amount of a
binder for the inorganic fibers. The web material has a
basis weight of about 5-30 grams/square meter and exhibits
an isolated fiber bundle defect count of less than 10 per
~ -4-
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106131~
100 square feet. Further, the web exhibits a visually per- -
ceptible overall uniform fiber distribution essentially de-
void of "cloud effect" fiber density variations.
A better understanding of this invention will be
obtai~ed from the following detailed description and the ac-
companying drawin~ wherein the article of manufacture pos-
;~ sesses the features, properties and relation of elements de- -
scribed and exempli~ied herein.
The single sheet of drawing shows a block diagram
: 10 of a preferred technique used in forming the light weight
;~ web material of the present invention.
As mentioned hereinbefore, a major factor in ob-
taining the desired uniform fiber distribution within the re-
sultant sheet product is the achievement of a complete and
uniform fiber suspension of the glass fibers within the dis-
persing medium and the conveyance of that dispersion intact
to the forming area. Thus, for clarity of description and
ease of understanding, the glass web material of the present
invention will be described in connection with the preferred
technique OT method used for its manufacture.
Numerous factors affect the quality of an aqueous
fiber dispersion and its ability to be fed to the forming
area of a papermaking machine. Among these are the type of
fiber, including the fiber finish and the condition of the
strand rovings used to supply the fibers, the chopping or
cutting performance, the composition and characteristics of
the dispersing medium, the performance of the mixing or dis-
persing apparatus and the treatment of the fiber stock mate-
rial after it lea~es the disperser. Although each of these -
factors is important, it has been found in accordance with
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the present invention that a substantial and significant
factor is the dwell time of the fibers within the system be-
tween the point at which they enter the disperser and the
point at which they are removed from the dispersion at the
web forming zone of the papermaking machine. Thus, in ac-
cordance with the present invention, it has been determined
that best results are achieved by completely eliminating the
holding tanks utilized heretofore and by using an in-line
disperser rather than the batch mixers utilized in the past.
,~,
In conjunction with the elimination of the holding tanks is
the immediate conveyance of the dispersed fibers to a dilu-
tion station and the utilization of a smooth, low volume
headbox characterized by high turbulence and high stock ve-
locity. In such a system the flow of the fiber suspension
from the disperser to the forming area of the papermaking
machine occurs within a matter of a few seconds and the
dwell time within the disperser is a major time-controlling
factor for the passage of the glass fibers through the sys-
tem~ Such time control is important since it has been found
that optimum dispersion of long glass fibers is reached rel-
atively quickly, that is, within about one to two minutes,
and is maintained in its most uniformly dispersed condition
for a period of only four to five minutes. Thereafter, the
glass fibers tend to accumulate, cling to each o~her or form
the undesirable "haystacks" or multi-fiber bunches mentioned
hereinbefore. It will of course be appreciated that the wet
papermaking process is a dynamic system which is affected by
numerous other conditions or factors within the system, such
as the viscosity of the dispersing mediumJ the fiber
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~068
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consistency, the rate at which the fibers are metered into
the disperser and numerous other process variables. ~onse-
quently, the exact dwell time will vary depending on these
various conditions or factors. However, best results have
been achieved with controlled dwell times within the disperser
of less than ten minutes and generally from about one to
seven minutes. An acceptable operating range falls between
approximately two to six minutes while the preferred dwell
time is about two and one half to five minutes.
Although the inorganic fibers that may be used in
the present invention includes substantially all of the con-
ventional inorganic materials commercially available in fi-
ber form, such as aesbestos, mineral wool and the like,
glass fibers are generally preferred. The fibers will vary
substantially in thickness although in the preferred embodi-
ment the fiber diameters are within the coarser fiber range
such as between about 5 microns to 15 microns. It will of
course be appreciated that somewhat finer or coarser diame-
ter fibers may be used for particular applications. The
glass fibers constitute the major portion of the fiber con-
tent and preferably account for as much of the fiber content
as possible. Thus, about 85-90 percent or more of the fi-
bers within the sheet structure are inorganic, and prefera-
bly glass fibers. As exemplified herein mixtures of differ-
ent types and sizes of glass fibers may be employed or the
sheet can be forme~ from only a single type and size of
glass fiber.
Due to the type of preferred glass fibers utilized,
; it is generally desirable to provide a binder in the inor-
ganic sheet material. Although a binder can be applied as a
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- dilute solution after the web is formed or can be incorpo-
rated within the fiber furnish as a portion of the dispers-
- ing medium, it is generally preferred to provide binder fi-
bers which cons~itute up to about 10-15 percent of the total
fiber content and preferably about 5 to 10 percent thereof.
Various binder fibers can be used with good results, among
these, polyvinyl alcohol fibers have been found to produce
superior results relative to post formation spraying with
adhesives and the like. The binder fibers also enhance the
handling characteristics of the web through the papermaking
machine. Preferably the fibers are activated or at least
softened in the dTier section of the machine to provide the
sheet material with its desired structural integrity.
The binder fibers are preferably added to the fi-
; ber suspension during or after dilution of the fiber consist-
ency and prior to the flow of the suspension to the headbox
of the papermaking machine. Thus the polyvinyl alcohol fi-
bers which act as the binder component of the fiber web can
be added conveniently at an adjustable speed fan pump down-
stream of the dilution operation without interfering with
the dispersion of the glass fibers within the uniformly dis-
persed fiber stock material. If desired subsequent size
press treatment or other binder treatments can be utilized
depending upon the particular end use for which the sheet
material is intended.
Referring now specifically to the drawing, it has
been found desirable in the preferred technique to provide a
controlled or metered feed of long glass fibers in order to
achieve the best fiber dispersion characteristics. The fi-
bers are preferably metered at a selected rate into a
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1~681~
continuous in-line disperser and from the disperser are fed ,j~'
directly to the dilution and forming area of the conventional
papermaking machine. This arrangement obviates the need for
retaining the dispersed fibers within a stock chest or other
holding tank and the resultant deterioratiGn of the quality
of the dispersion. Additionally, it is an advantage of the ~
present invention that the continuous dispersing equipment is ',
of relatively simple construction and inexpensive compared to
conventional stock preparation equipment. If desired the fi-
bers can be precut and fed by a dry fiber meter or can be fed ''~
' as continuous strands and cut or chopped as they are delivered
to the in-line disperser.
, In the preferred embodiment it has been found advan-
tageous to provide a cutter at the inlet to the disperser so
that continuous lengths of glass rovings can be fed from
, spools and cut for immediate delivery to the disperser. This
' delivery of the continuous filaments provides excellent con-
trol over both the fiber length and the rate at which the fi-
bers are fed to the disperser. Additionally, it provides
flexibility by permitting the utilization of different fiber
lengths and adjustable control over the fiber lengths.
, Where prechopped or precut fibers are employed it
is possible to provide control over the fiber feed rate to the
disperser by employing a weigh belt or the like between the
- dry fiber meter and fiber dîsperser, in which event the dry
fiber meter functions as a pre-feeder with its speed modulated
and controlled by a signal from the weigh belt, in order to
' achieve the desired feed rate for the fibers.
, The fluid used as the dispersing medium is also fed
to the inlet of the disperser to provide the desired fiber
~.
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consistency therein. This fluid is an acidic aqueous solution
that may contain a suitable agent for controlling the viscosity
of the dispersing medium. Thus, in accordance with the pre-
ferred embodiment an aqueous solution of dilute sulphuric acid
having a pH of between 2 and 4 and containing a sufficient
amount of a viscosity forming agent is employed. Typically the
solution exhibits a viscosity between about 5 and 20 centi-
poise. The viscosity producing agent may be a natural or syn-
thetic material or blends or combinations thereof. The agents
are preferably water soluble materials such as resins or natu-
ral gums which can be used along or in combination with other
materials to provide the desired viscosity. Examples of natu-
ral gum materials are locust bean gum and guar gum derivitives.
Among these, the guar gum derivatives are preferred and excel-
; lent results have been obtained with an aqueous solution of a
guar gum derivative sold by General Mills Company under the
A tradename "Gendriv"~ In addition to the natural viscosity pro-
ducing agents it is also possible to utilize synthetic materi-
als such as high molecular weight resins, dispersants, surfac-
tants and the like to control the properties of the dispersing
medium. These synthetic materials are preferably water soluble
and are stable within the acidic environment utilized for the
- glass fibers. Among the synthetic viscosity producing materi-
als, the preferred resins are polyacrylamide polymers which can
be used in dilute aqueous solutions at low concentration (e.g.,
; 0.025-0.2 percent) to provide the desired control over the vis-
cosity. Typical of such materials is the polyacrylamide resin
; sold by Dow Chemical Company under the tradename "Separan ~P-
30" and by American Cyanamide Company under the tradename
"Cytame 5".
/~e~;s~e~eq/ ;~r~ct/e~ark -10-
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6814~
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The viscous dispersing medium is utilized since it
prevents fiber entanglement during the dispersing operation
and assists to maintain the fibers in their dispersed state
during passage of the suspe~sion through the disperser. As
will be appreciated the viscosity of the solution will affect
the dwell time required and must be adjusted for the particu-
lar fiber and fiber consistency utilized. A high viscosity
medium and a short dwcll ~ime might lead to an under-dispersed
fiber stock while a low viscosity and a long dwell time could
lead to over-dispersion and the formation of "haystacks" and
other major defects. A viscosity in the range of about 5-10
centipoises and a dwell time of about 2.5-5.0 min. has been
;~ found to produce good dispersion results. As will be appreci-
ated other additives, such as dispersing aids, e.g., surfac-
tants such as sodium hexametaphosphate sold under the trade-
A name "Calgon",~ may be added to the dispersing medium i~ order
to achieve the desired control oveT the dispersed fibers and
to assist in preventing the recombination of fibers into the
undesirable haystack configurations.
As mentioned, it has been found that the fibers are
dispersed quite rapidly within the dispersing medium and reach
a peak of percent fibers dispersed within a relatively short
time following which the fibers tend to cling or bind together
~ slightly to form the undesirable "haystacks". Thus upon reach-
; ing optimum dispersion, it is desirable to maintain the agita-
tion for a limited period of time and control the dwell time of
the fibers within the disperser so that prolonged agitation is
avoided. In this connection it has also been found that even
ater the optimum dispersion has been reached at the desired
. . .
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dwell time, the agitators within the disperser cannot be shut
off without damage to the quality of the dispersion. Of
course as will be appreciated, surface treatment of the fibers
will substan*ially affect the ability of the fibers to toler-
ate a prolonged dwell time. However, for most glass fibers
presently available on a commercial basis, it has been found
that the optimum dwell time is between 2 1/2 and 5 minutes
when operating with a dispersing medium having a viscosity of
about 5-10 centipoises and a pH of about 2-3 at a solution
temperature of approximately 80-100F and a fiber consistency
of about 0.3-1.0 percent.
Preferably the disperser should be of the type that
exhibits a relatively smooth interior surface and is free of
any edges or surfaces on which the long glass fibers can snag
or drape. However the disperser may consist of a plurality
of mixing or dispersing stations or compartments with continu-
ous flow directly from station to station in order to provide
the desired dwell time characteristics.
As will be appreciated the specific design of the
disperser can vary so long as it achieves the desired function
of separating the individual fibers from the fiber bundles fed
~` to the disperser and produces a uniform dispersion of the in-
dividual fibers while conveying the fiber dispersion through ;
the disperser within the required dwell time. As will be ap-
preciated the fibers are metered into the dispersing medium
flowing through the disperser to provide the desired fiber
consistency. Usually the consistency is substantially higher
than the fiber consistency within the headbox by a factor of
from as much as 10-100 times. In accordance with the pre-
; 30 ferred embodiment the fiber consistency is less than two
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10681~
percent and generally is in the range of about 0.3-1.3 per-
cent with a preferred range of about 0.5-0.9 percent.
As mentioned hereinbefore the fiber dispersion
moves rapidly from the disperser to the forming portion of
the papermaking machine and in fact reaches the forming wire
within a few seconds after leaving the disperser. However,
during that period the fiber consistency of the dispersion is
adjusted so as to more fully dilute the fiber stock. This
can be achieved by feeding the dispersion to a separate flow-
through mix tank where it is mixed with the main white water
flow from the web forming operation~ The fiber c4nsistency
is diluted from a value of 0.3-1.2 percent to a value of
about 0.005-O.OS percent. Thus, as can be seen, the dilution
is greater than 10 to 1 and usually 15-25 to 1 in order to
provide the highly dilute fiber suspension fed to the headbox
of the papermaking machine.
As indicated in the drawing, the headbox utilized
in accordance with the present invention is unlike the open
headbox of the conventional inclined-wire papermaking ma-
" 20 chines and is provided with a smooth contour and a reduced -
volume so that the highly dilute fiber suspension flows rap-
; idly through the headbox toward the web forming area. The ;
reduced volume headbox with its smooth contour not only in-
creases the velocity of the fiber suspension traYeling there-
through but also increases the level of random turbulence im
mediately over the forming zone. The increased level of tur-
bulence prohibits the accumulation of foam and fiber masses
that would otherwise float to the surface and form "hay- -
stacks" or other fibeT defects. As will be appreciated flow
control of the dilute fiber dispersion can be achieved by a
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suitable flow control mechanism such as a variable speed fan
pump, provided however that the pump is of smooth configura-
tion and free of elements that would produce eddies in the
flow or otherwise cause fiber entanglement. Thus the head-
box utilized in accordance with the present invention pre-
vents holding of the fiber dispersion for a prolonged period
of time, thereby preventing the dispersed fibers from recom-
bining to form defects in the sheet structure.
The following examples are given in order that the
- 10 effectiveness of the present invention may be more fully un-
deTstood. These examples are set forth for the purpose of
illustration only and are not intended to in any way limit
the practice of the invention. Unless otherwise specified
all parts are given by weight.
EXAMPLE I:
A light weight glass fiber web material was pro-
; duced using production size, papermaking machinery. Glass
fibers having a fiber diameter of 9 microns were cut to a
length of 1/2 inch from strands of glass rovings fed from
bobbins. The cut fibers were delivered directly to an in-
line disperser at a rate of one pound per minute. The in-
line disperser has a capacity of 100 gallons and was operated
at a throughrate of 30 gallons per minute, thus providing a
dwell time of slightly more than 3 minutes. The dispersing
media used was a dilute sulphuric acid solution containing a
::~ A guar gum derivative (Gendriv ~ 92 SR) in amounts sufficient to
provide a solution viscosity of about 5 cps at a pH of 2.3
and a temperature of 88F. The fiber dispersion at a fiber
consistency of 0.4 percent was fed from the disperser to a
mix tank where the fiber consistency was diluted at a ratio
-~ ~e9is~e/~e~ ~a,~k 14-
10~
of approximately 24:1. Polyvinyl alcohol fibers were added
to the dilute suspension in amounts sufficient to provide a
polyvinyl alcohol fiber concentration of 8 percent based
upon the weight of the glass fibers. The fiber dispersion
was then fed to a low volume high velocity h~adbox at a con-
sistency of 0.017 percent and a glass fiber web was formed
at a medium speed production rate.
The resultant web material had a basis weight of
13.6 grams/square meter, a thickness of 84 mîcrons and an
air porosity of 8263 liters per minute per 100 cm2 at 12.7
mm H2O pressure. The light weight web had a dry tensile
strength of 507 gm/25 mm. in the machine direction and 333
gm/25 mm. in the cross direction. It exhibited tongue tear
of 34 gms in the machine direction and 44 gms in the cross
direction.
Samples taken from various portions of the sheet
material exhibited a major defect count of 0-2 and a minor
defect count of 0-5 per 100 square feet corrected to a basis
weight of 17 grams/square meter. A maior defect is catego-
; 20 rized as a fiber bundle either of an undispersed or partially
dispersed nature or of a haystack configuration while a minor
defect is categorized as two or three fibers which have re-
.,. ;
, mained undispersed or been drawn together. Commercially ac-
- ceptable light weight materials are considered those which
have about 10 or less and preferably 5 or less major defects
, :- .
per 100 square feet of web material. The minor defects are
not considered significant. The sheet material also exhib-
ited a uniform fiber distribution substantially free of any -
density variation upon a visual examination.
EXAMPLES II - VI
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The procedure of Example I was repeated on the same
papermaking machine except for variations in the process op-
erating conditions, the fiber furnish and the basis weight ofthe material produced. The results are tabulated below:
TABLE
Ex. II Bx. IIIEx. IV Ex. VEx. VI
Fiber
9 micron (%) 70 46 90 70 22
13 micron (%) 22 46 -- 22 70
Binder ~%) 8 8 10 8 8
Basis weight19~8 18.3 22.0 22.4 23.1
~, (gmlm2)
Thickness 123 115 133 138 115
, (microns)
Air porosity5648 6552 4742 5512 6149
(l/min.)
Dry tensile
(gm/25 mm)
MD 1109 609 1828 1456 1121
CD gl5 7~5 1034 1362 1037
Ton~ tear
(gms)
` MD 51 60 40 62 89
CD 51 44 60 63 99
Defect Coun~
per 100 ft.
Major 0-3 0-4 0-3 0-1 0
Minor 3-4 0-5 7-13 1-4 2-4
********
EX~LES VII - IX
The procedure of the preceding examples was repeated
on a small size production machine using finer diameter glass
30 fibers and no binder fiber. In each instance, the glass
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fibers constituted lOO percent of the fiber component and
were 1/2 inch in length and 6 microns in diameter. The basis
weight and defect count per 100 square feet are given below.
The high minor defect count reflects the very fine fiber di-
ameter and the subjective determination of the analyst but in
each instance is considered a perfect sheet material from a
commercial standpoint.
Defects
Basis WRight
Ex. #tgm/mG) Major Minor
VII 15.8 1 222
VIII 16.6 0 356
IX 17.6 0 198
. ********
As will be apparent to persons skilled in the art,
various modifications, rariations, and adaptations can be
made from the foregoing specific disclosure without departing
`. from the teachings of the present invention.
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