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Description  |
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FIELD OF THE INVENTION
This invention relates to improvements in paper-making and non-woven web
manufacturing operations and particularly to the provision of a
low-density, soft, bulky and absorbent paper sheet characterized by having
significantly greater cross-directional stretch, as well as improved
softness, surfaces feel and drape when compared to paper sheets produced
by prior art papermaking and non-woven web manufacturing methods.
More particularly, in one important embodiment, the present invention
consists of a monofilament, polymeric fiber, semi-twill fabric which when
used to imprint an uncompacted paper web at selected fiber consistencies,
induced by thermal pre-drying, will produce a dot-dash pattern wherein the
long axis of the dash impressions is aligned parallel to the machine
direction of papermaking and the long axis of the dot impressions is
aligned parallel to the cross-machine direction. The aforesaid imprinting
fabric is especially suitable for use in papermaking and non-woven web
manufacturing operations, such as the papermaking operation disclosed in
U.S. Pat. No. 3,301,746, issued to Sanford et al. on Jan. 31, 1967, said
patent being incorporated herein by reference, wherein the surface
characteristics of such fabrics are of operational and product
characteristic importance.
In one preferred embodiment, the present invention consists of a
monofilament, polymeric fiber, semi-twill fabric of the type normally used
for transporting a moist web through the forming, pressing and drying
sections of a papermaking machine, which monofilament, polymeric fiber,
semi-twill fabric is woven and thereafter shrunk by heat treatment to
result in a dimensionally heat stable fabric having uniform knuckle
heights in conjunction with minimum "free" or interstitial area on the
surface of the fabric which will contact the uncompacted paper web, said
fabric having been further improved by abrading its web contacting surface
with a fine abrasive medium to increase its knuckle imprint area.
BACKGROUND OF THE INVENTION
In a Fourdrinier paper machine, paper stock is fed onto a traveling endless
belt that is supported and driven by rolls associated with the machine and
which serves as the papermaking surface of the machine. Fourdrinier belts
are commonly formed from a length of woven Fourdrinier fabric with its
ends joined together in a seam to provide an endless belt. Fourdrinier
fabrics of this type generally comprise a plurality of spaced longitudinal
warp filaments and a plurality of spaced transverse woof or weft filaments
which have been woven together on a suitable loom. It should be noted that
the warp filaments of the fabric are, for purposes of this specification,
defined as those which run parallel to the machine direction of
papermaking and non-woven web manufacturing machines to form a continuous
carrier belt; woof or weft filaments are, for purposes of this
specification, defined as those which run in the cross-machine direction.
Although the weaving and fabric treatment criteria of the present invention
are applicable in other areas of monofilament, polymeric fiber fabric use,
the instant features will be most readily understood in respect to the use
of such fabrics for imprinting purposes in web formation operations. In
these operations, for example in the operation of a paper machine
according to the teachings of U.S. Pat. No. 3,301,746, improved web
transferability and dryer surface contact are desirable in an imprinting
fabric, and the monofilament, polymeric fiber fabric used should not
contribute factors to the final paper product other than those desired by
the papermaker and designed into the paper product.
In referring to monofilament, polymeric fiber fabrics herein, applicant
intends reference to moist web carrier fabrics woven, for example, from
the polyamide fibers, vinyl fibers, acrylic fibers and polyester fibers
sold under the respective trade names of "nylon," "Saran," "Orlon,"
"Dacron," and "Treviera." While both wrap and woof filaments in fabrics
can be made up of a multiplicity of fibers, the present invention is
concerned with warp and woof filaments comprised of one fiber, i.e.,
monofilaments.
While a number of different weaves have been proposed for Fourdrinier
fabrics, two such weaves which find extensive use today are the so-called
"plain" weave and the "semi-twill" (sometimes also called "long crimp")
weave. In the plain weave, each weft filament passes successively under
one warp filament and then over the next warp filament, whereas in the
semi-twill weave each weft filament passes over two warp filaments, under
the next warp filament, and then over the next two warp filaments in a
repeated pattern. Of these two weaves, the semi-twill weave is the most
widely used.
The imprinting fabric suggested for use in U.S. Pat. No. 3,301,746, to
which the present invention has particular relevance, may be of square or
diagonal weave, and can be of any specific construction including, for
example, plain or semi-twill weave. A preferred imprinting fabric,
according to the teachings of the aforesaid Sanford et al. patent, has
about 20 to about 60 meshes per inch and is formed from filaments having a
diameter of from about 0.008 to about 0.02 inches.
Paper sheets produced in accordance with the teachings of U.S. Pat. No.
3,301,746 utilizing a monofilament, polymeric fiber, semi-twill imprinting
fabric exhibit properties similar in most respects to paper sheets
produced utilizing a plain weave imprinting fabric having filaments of
approximately the same diameter when the semi-twill fabric is installed so
that its conventional "face" side is used to imprint the uncompacted
paper web. This is due to the fact that the conventional face side of the
semi-twill fabric, assuming the fabric has uniform knuckle heights on its
web contacting side, will produce a dot-dash pattern wherein the long axis
of the dash impressions is aligned parallel to the cross-machine direction
and the long axis of the dot impressions is aligned parallel to the
machine direction. The dash impressions result from each weft filament
passing in a repeated pattern under one warp filament and then over the
next two warp filaments, while the dot impressions result from each warp
filament passing in a repeated pattern over one weft filament and then
under the next two weft filaments on the conventional face side of the
fabric.
When paper sheets imprinted by the conventional face side of a semi-twill
fabric, as described above, are doctored from the drying drum, the
dot-dash knuckle impressions are aligned essentially between the creping
folds. The resulting creping folds are, therefore, substantially
uninterrupted across the sheet's surface. Thus paper sheets produced
utilizing the conventional face side of a semi-twill imprinting fabric
exhibit properties substantially similar to paper sheets produced
utilizing a plain weave imprinting fabric, i.e., a low-density, soft,
bulky and absorbent paper sheet characterized by having uniform creping
folds which extend substantially uninterrupted across the width of the
sheet.
On the other hand, utilization of the "back" side of a monofilament
polymeric fiber, semi-twill fabric to imprint an uncompacted paper web in
accordance with the teachings of U.S. Pat. No. 3,301,746 will, assuming
the fabric has uniform knuckle heights on its web contacting side, produce
a dot-dash pattern wherein the long axis of the dash impressions is
aligned parallel to the machine direction of the paper machine and the
long axis of the dot impressions is aligned parallel to the cross-machine
direction. The dash impressions result from each warp filament passing in
a repeated pattern under one weft filament and then over the next two weft
filaments, while the dot impressions result from each weft filament
passing in a repeated pattern over one warp filament and then under the
next two warp filaments on the back side of the fabric.
Paper sheets imprinted with the back side of a conventional semi-twill,
monofilament, polymeric fiber fabric, unlike paper sheets imprinted with
either a plain weave fabric or the conventional face side of a semi-twill
fabric, exhibit a diamond-shaped pattern after creping.
Applicant has discovered that by increasing the knuckle imprint area on the
back side of a conventional semi-twill, monofilament, polymeric fiber
fabric in accordance with the teachings of U.S. Pat. No. 3,573,164 issued
to Friedberg et al. on Mar. 30, 1971, said patent being incorporated
herein by reference, unexpected improvements in paper sheet
characteristics can be realized. These unexpected advantages take the form
of improved cross-directional stretch, softness, surface feel and drape.
The improvements become more pronounced as the knuckle imprint area on the
back side of the semi-twill fabric is increased.
Although improved web transfer characteristics and improved drying of the
web are realized when the web contacting knuckle surfaces of nearly any
monofilament, polymeric fiber fabric are abraded in accordance with the
teachings of U.S. Pat. No. 3,573,164, applicant has learned that the
aforementioned improvements in cross-directional stretch, softness,
surface feel and drape are realized only with respect to the back side of
a semi-twill imprinting fabric, such as is described above.
In order to maximize the beneficial effects of abrading the knuckle
surfaces on the back side of a semi-twill imprinting fabric, applicant has
found it desirable to obtain a semi-twill fabric having uniform knuckle
heights and minimum free area on its back side prior to initiating any
abrading process. Uniform knuckle heights permit a greater increase in
knuckle imprint area while minimizing the danger of abrading completely
through any particular filament. In addition, if knuckle heights are
uniform prior to initiating any abrading process, the resulting imprint
pattern after abrading will be more uniformly consistent.
Because a fabric such as is utilized for imprinting purposes in U.S. Pat.
No. 3,301,746 is subjected to elevated temperatures during use, it is
desirable to dimensionally heat stabilize the fabric prior to subjecting
it to an abrading process to increase its knuckle imprint area. If this is
not done, the uniform imprinting surface produced by carefully weaving the
fabric and abrading the web contacting surface of the fabric prior to use
will tend to warp as the temperature of the fabric becomes elevated,
thereby losing most of the benefits to be obtained by such careful
pre-treatment.
A means of preparing a dimensionally heat stable, plain weave,
monofilament, polymeric fiber fabric having uniform knuckle heights and
minimum free area on each side of the fabric is disclosed in U.S. Pat. No.
3,473,576 issued to Amneus on Oct. 21, 1969, said patent being
incorporated herein by reference. A plain weave fabric is prepared by
selecting polymeric warp monofilaments having a relatively high
heat-induced shrinkage potential and further selecting an initial warp
monofilament spacing in the loom according to a mathematical equation
disclosed in the aforementioned Amneus patent. Polymeric woof
monofilaments are then selected which have a relatively low heat induced
shrinkage potential, and these woof monofilaments are woven and beaten in
the weaving process into a plain weave fabric having an initial caliper
calculated according to yet another mathematical equation disclosed in the
aforementioned Amneus patent. After the initial weaving process, the
fabric knuckles are brought to uniform heights on both sides of the fabric
and the minimum free area of the fabric is set by a heat shrinkage
treatment which maintains the fabric in warp tension while allowing it to
shrink in the woof direction. Successive heat treatments are repeated
until the monofilament, polymeric fiber, plain weave fabric does not
shrink further at the treating temperature, at which point it is said to
be "locked-up", i.e., no further shrinkage will occur if the fabric is
later subjected, in use, to elevated temperatures equivalent to the
treating temperature.
It is important to note that due to the symmetry of the plain weave,
uniform knuckle heights and minimum free area are achieved simultaneously
on both sides of the fabric when the weaving and heat treatment processes
described in the aforementioned Amneus Patent are utilized. This is not
the case with a semi-twill weave fabric. If a monofilament, polymeric
fiber, semi-twill fabric is subjected to a heat treatment process similar
to that disclosed in the Amneus Patent, the knuckles on the conventional
face side of the fabric will become coplanar before the knuckles on the
back side of the fabric have reached a uniform height. Thus, in order for
the knuckles on the back side of the fabric to become coplanar, the fabric
must be subjected to further heat treatment. The additional heat treatment
required to make the knuckle heights on the back side of the fabric
uniform causes the knuckle heights on the conventional face side of the
fabric to again become non-uniform.
Therefore, the initial warp filament spacing and caliper of a semi-twill
fabric necessary to produce minimum free area and uniform knuckle heights
on the back side of the fabric after heat treatment are determined
experimentally by trial and error.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a low-density, bulky
and absorbent creped paper structure exhibiting a diamond-shaped pattern
in its surface, said paper structure having significantly improved
softness, surface feel and drape, as well as significantly improved
cross-directional stretch.
It is a further object of the present invention, in a preferred embodiment,
to produce the above mentioned paper structure in accordance with the
teachings of U.S. Pat. No. 3,301,746 by utilizing the back side of a
conventional, monofilament, polymeric fiber, semi-twill fabric which has
been abraded in accordance with the teachings of U.S. Pat. No. 3,573,164
to imprint the uncompacted paper web prior to creping.
It is a further object of the present invention, in a preferred embodiment,
to produce a paper structure in accordance with the teachings of U.S. Pat.
No. 3,301,746 wherein a dot-dash pattern is imprinted on the uncompacted
paper web, prior to creping, such that the long axis of the dash
impressions is aligned parallel to the machine direction and the long axis
of the dot impressions is aligned parallel to the cross-machine direction.
It is another object of the present invention, in a preferred embodiment,
to provide dimensionally heat stable, monofilament, polymeric fiber,
semi-twill fabrics for use in fibrous web carrying, imprinting, and other
fabric using operations, which monofilament, polymeric fiber, semi-twill
fabrics are characterized by having uniform knuckle heights and minimum
free area on their back side, thus contributing materially to the
avoidance of transfer and contact problems in papermaking and web
formation operations.
It is a further object of the present invention, in a preferred embodiment,
to provide a process for the production of dimensionally heat stable,
monofilament, polymeric fiber, semi-twill fabrics, which process sets
criteria for the weaving and heat treating operations necessary to achieve
uniform knuckle heights and minimum free area on the back side of said
fabrics.
Another object of the present invention, in a preferred embodiment, is to
provide a monofilament, polymeric fiber, semi-twill fabric for use in
papermaking and non-woven web manufacturing operations, the back surface
of which fabric has a total knuckle imprint area of from about 20 to about
50 percent of the total fabric surface area, as measured in the plane of
the knuckles, and which knuckle imprint area has a surface finish at least
equal in smoothness to the surface finish induced by abrasion with an
abrasive medium having an effective abrasive grain size of less than about
300 mesh.
It is yet another object of the present invention, in a preferred
embodiment, to provide a monofilament, polymeric fiber, semi-twill fabric
for use in the imprinting and drying sections of a papermaking machine,
the back side of which fabric presents an increased knuckle area to the
moist paper web for use in pressing the web onto the surface of a dryer
while it contributes materially to the final tensile strength of the dried
paper product by avoiding the rupture of fiber bonds.
SUMMARY OF THE INVENTION
In a preferred embodiment of the present invention, a low density, soft,
bulky and absorbent paper sheet is provided, said paper sheet exhibiting a
diamond-shaped pattern in its surface after creping, said paper sheet
being characterized by having a cross-directional stretch of from about 2
to about 6 percent, as well as improved softness, surface feel and drape,
said paper sheet being particularly suitable for use in tissue, toweling,
and sanitary products.
The soft, bulky and absorbent paper sheets of the present invention are
produced, in a preferred embodiment, generally in accordance with the
teachings of U.S. Pat. No. 3,301,746 by forming an uncompacted paper web,
supporting said uncompacted paper web on the back side of a monofilament,
polymeric fiber, semi-twill imprinting fabric having about 20 to about 60
meshes per inch, said imprinting fabric having been formed from filaments
having a diameter of from about 0.008 to about 0.025 inches, the back side
of said fabric having had its knuckle imprint area increased in accordance
with the teachings of U.S. Pat. No. 3,573,164, thermally pre-drying said
uncompacted paper web to a fiber consistency of about 30 to about 98
percent, imprinting a dot-dash knuckle pattern with the back side of said
semi-twill imprinting fabric such that the long axis of the dash
impressions in said pattern is aligned parallel to the machine direction
and the long axis of the dot impressions is aligned parallel to the
cross-machine direction of the pre-dried uncompacted paper web, and final
drying and creping the paper sheet so formed.
In a preferred embodiment of the present invention, the back side of the
monofilament, polymeric fiber, semi-twill imprinting fabric is prepared in
accordance with the teachings of U.S. Pat. No. 3,573,164 by abrading the
knuckle surfaces to increase the knuckle imprint area to between about 20
and about 50 percent of the total fabric surface area, as measured in the
plane of the knuckles, as well as to polish the knuckle surfaces.
In yet another preferred embodiment of the present invention, the
monofilament, polymeric fiber, semi-twill fabric is woven and heat treated
so as to produce a dimensionally heat stable fabric having uniform knuckle
heights and minimum free area on its back side prior to abrading the
knuckle surfaces on the back side of the fabric.
BRIEF DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims particularly pointing out and
distinctly claiming the subject matter which is regarded as the present
invention, it is believed that the invention will be better understood
from the following description taken in connection with the accompanying
drawings in which:
FIG. 1 is a plan view of an enlarged portion of a conventional right-hand
semi-twill, monofilament, polymeric fiber fabric as viewed from the back
side, i.e., that side of the fabric which according to the teachings of
the prior art does not normally contact the web. The monofilament,
polymeric fiber, semi-twill fabric is shown prior to any abrasion
treatment and prior to use as an endless or continuous fabric belt in
papermaking or non-woven web manufacturing operations.
FIG. 2 is an enlarged cross-sectional view of the semi-twill fabric
illustrated in FIG. 1, taken looking in the cross-machine direction (CD)
along line 2--2 in FIG. 1, which cross-sectional view illustrates the
higher relative elevation and the smooth knuckle surfaces of the warp
filaments on each side of the fabric.
FIG. 3 is an enlarged cross-sectional view of the semi-twill fabric
illustrated in FIGS. 1 and 2, taken looking in the machine direction (MD)
along line 3--3 in FIG. 1, which cross-sectional view illustrates the
lower relative elevation and the smooth knuckle surfaces of the woof or
weft filaments.
FIG. 4 is a simplified illustration of an enlarged partial plan view of an
uncreped paper sheet which has been imprinted utilizing the conventional
face side of a semi-twill fabric such as is shown in FIGS. 1 through 3.
The long axis of the dot impressions formed by the warp filaments is
aligned parallel to the machine direction.
FIG. 5 is a simplified illustration of an enlarged partial plan view of an
uncreped paper sheet which has been imprinted utilizing the back side of a
semi-twill fabric such as is shown in FIGS. 1 through 3. The long axis of
the dash impressions formed by the warp filaments is aligned parallel to
the machine direction.
FIG. 6 is an enlarged cross-sectional view of a monofilament, polymeric
fiber, semi-twill fabric such as is illustrated in FIGS. 1 through 3,
taken looking in the cross-machine direction at a point corresponding to
line 2--2 in FIG. 1, after the fabric has been subjected to a heat
treatment process sufficient to produce uniform knuckle heights on the
conventional face side of the fabric.
FIG. 7 is an enlarged cross-sectional view of the semi-twill fabric
illustrated in FIG. 6, taken looking in the machine direction at a point
corresponding to line 3--3 in FIG. 1.
FIG. 8 is a simplified illustration of an enlarged partial plan view of an
uncreped paper sheet which has been imprinted utilizing the conventional
face side of a monofilament, polymeric fiber, semi-twill fabric such as is
illustrated in FIGS. 6 and 7. The long axis of the dash impressions formed
by the woof or weft filaments is aligned parallel to the cross-machine
direction, while the long axis of the dot impressions formed by the warp
filaments is aligned parallel to the machine direction.
FIG. 9 is a simplified illustration of an enlarged partial plan view of an
uncreped paper sheet which has been imprinted utilizing the back side of a
semi-twill fabric such as is illustrated in FIGS. 6 and 7. The long axis
of the dash impressions formed by the warp filaments is aligned parallel
to the machine direction.
FIG. 10 is an enlarged cross-sectional view of a monofilament, polymeric
fiber, semi-twill fabric such as is illustrated in FIGS. 1 through 3 and 6
and 7, taken looking in the cross-machine direction at a point
corresponding to line 2--2 in FIG. 1, after the fabric has been subjected
to a heat treatment process sufficient to produce uniform knuckle heights
and minimum free area on the back side of the fabric. It should be noted
that at this point, the knuckle heights on the conventional face side of
the fabric are no longer uniform.
FIG. 11 is an enlarged cross-sectional view of the semi-twill fabric
illustrated in FIG. 10, taken looking in the machine direction at a point
corresponding to line 3--3 in FIG. 1.
FIG. 12 is a simplified illustration of an enlarged partial plan view of an
uncreped paper sheet which has been imprinted utilizing the conventional
face side of a semi-twill fabric such as is illustrated in FIGS. 10 and
11. The long axis of the dash impressions formed by the woof or weft
filaments is aligned parallel to the cross-machine direction.
FIG. 13 is a simplified illustration of an enlarged partial plan view of an
uncreped paper sheet which has been imprinted utilizing the back side of a
semi-twill fabric such as is illustrated in FIGS. 10 and 11. The long axis
of the dash impressions formed by the warp filaments is aligned parallel
to the machine direction, while the long axis of the dot impressions
formed by the woof or weft filaments is aligned parallel to the
cross-machine direction. The dot impressions are present at this stage due
to the fact that the knuckles on the back side of the fabric are of
uniform height.
FIG. 14 is an enlarged cross-sectional view of a monofilament, polymeric
fiber, semi-twill fabric such as is illustrated in FIGS. 10 and 11, taken
looking in the cross-machine direction at a point corresponding to line
2--2 in FIG. 1, after the back side of the fabric has been abraded to
increase its knuckle imprint area.
FIG. 15 is an enlarged cross-sectional view of the semi-twill fabric
illustrated in FIG. 14, taken looking in the machine direction at a point
corresponding to line 3--3 in FIG. 1.
FIG. 16 is a plan view of an enlarged portion of the monofilament,
polymeric fiber, semi-twill fabric illustrated in FIGS. 14 and 15, as
viewed from the back side of the fabric.
FIG. 17 is a plan view photograph, enlarged about 12 times actual size, of
an uncreped paper sheet which has been imprinted utilizing the back side
of a semi-twill fabric such as is shown in FIGS. 14, 15, and 16. The
pattern produced is similar to that shown in FIG. 13, but the dot-dash
impressions constitute a greater percentage of the surface area of the
paper due to the increased knuckle imprint area of the fabric.
FIG. 18 is an illustration of an enlarged cross-sectional view of the
uncreped paper sheet of FIG. 17, taken looking in the cross-machine
direction along line 18--18 in FIG. 17.
FIG. 19 is a plan view photograph, enlarged about 6 times actual size, of a
paper sheet such as is shown in FIGS. 17 and 18 after creping. The long
axis of the impressions visible after creping is oriented generally in the
cross-machine direction, while the overall surface of the paper exhibits a
diamond-shaped pattern characteristic of paper sheets made in accordance
with the present invention.
FIG. 20 is an illustration of an enlarged cross-sectional view of the
creped paper sheet of FIG. 19, taken looking in the cross-machine
direction along line 20--20 in FIG. 19.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In describing preferred embodiments of the invention disclosed herein,
specific terminology will be adhered to for the sake of clarity in
referring to the features of the monofilament, polymeric fiber fabrics for
use in papermaking and non-woven web manufacturing processes. The
conventional face side of the semi-twill fabrics referred to herein refers
to that side of the fabric which, according to the teachings of the prior
art, would normally come in contact with the paper web, i.e., the side of
the semi-twill fabric which would, depending upon its particular
condition, produce one of the imprint patterns illustrated in FIGS. 4, 8
or 12 (assuming it is a right-hand semi-twill fabric). In the
aforementioned Figures, the long axis of the dash impressions 9, where
present, is aligned parallel to the cross-machine direction, while the
long axis of the dot impressions 3, where present, is aligned parallel to
the machine direction. The back side of the semi-twill fabrics referred to
herein shall be defined as that side which would not normally contact the
paper web according to the teachings of the prior art, i.e., the side of
the semi-twill fabric which would, depending upon its particular
condition, produce one of the imprint patterns illustrated in FIGS. 5, 9,
13, or 17 (assuming it is a right-hand semi-twill fabric). In the
aforementioned figures, the long axis of the dash impressions 8 is aligned
parallel to the machine direction, while the long axis of the dot
impressions 10, where present, is aligned parallel to the cross-machine
direction.
It should be noted that although a right-hand semi-twill fabric is utilized
for purposes of illustration throughout this specification, the benefits
disclosed can also be obtained utilizing a left-hand semi-twill fabric,
which is woven as a mirror image of a right-hand semi-twill fabric.
FIG. 1 represents an enlarged plan view of a portion of a conventional
right-hand, monofilament, polymeric fiber, semi-twill fabric as viewed
from the back side. The semi-twill fabric illustrated in FIG. 1 has not
been used on a paper machine nor has it been accorded any special abrading
treatment. The warp monofilaments 1 are aligned parallel to the machine
direction, while the woof or weft monofilaments 2 are aligned parallel to
the cross-machine direction. In a preferred embodiment of the present
invention, the imprinting fabric illustrated in FIG. 1 has about 20 to
about 60 meshes per inch and is formed from monofilament polymeric fibers
having diameters ranging from about 0.008 to about 0.025 inches. Both warp
and woof monofilaments may, but need not necessarily be of the same
diameter. FIGS. 2 and 3 are cross-sectional views of the semi-twill fabric
illustrated in FIG. 1, taken looking respectively in the cross-machine and
machine directions. The knuckles formed at the cross-over points of the
warp filaments 1 and the woof filaments 2 are not coplanar on either the
face or the back side of the fabric. As can be seen in FIGS. 2 and 3, the
warp filaments 1 are at a higher relative elevation than the woof
filaments 2 on both sides of the fabric. This is termed, for purposes of
this specification, a "warp-high" condition of the fabric.
FIG. 4 is a simplified illustration of the knuckle imprint pattern which
would result if a semi-twill fabric such as is illustrated in FIGS. 1
through 3 were installed so that the conventional face side of the fabric
were utilized to imprint an uncreped paper web produced in accordance with
the teachings of U.S. Pat. No. 3,301,746 issued to Sanford et al. on Jan.
31, 1967, said patent being incorporated herein by reference. The dot
impressions 3 visible on the surface of such an uncreped paper sheet after
imprinting form a pattern corresponding to the knuckles 4 of the warp
filaments 1 on the conventional face side of the fabric. Since the dot
impressions 3 are formed by the warp filaments 1, the long axis of the dot
impressions is aligned parallel to the machine direction. The knuckles 7
formed by the woof filaments 2 on the conventional face side of the fabric
do not form a corresponding impression in the uncompacted paper web due to
the fact that they are at a lower relative elevation than the warp
filament knuckles 4.
FIG. 5 illustrates the knuckle imprint pattern which would result if an
uncompacted paper web produced in accordance with the teachings of U.S.
Pat. No. 3,301,746 were imprinted utilizing the back side of an
imprinting fabric such as is illustrated in FIGS. 1 through 3. Because the
warp filaments 1 are at a higher relative elevation than the woof
filaments 2 on the back side of the fabric, only the peaks of the knuckles
5 formed by the warp filaments are impressed into the paper web during the
imprinting process. Since the warp filaments 1 run in the machine
direction, the resulting pattern consists of a series of relatively long
dash impressions 8, wherein the long axis of the impressions is aligned
parallel to the machine direction.
The imprint pattern illustrated in FIG. 5 differs from the imprint pattern
illustrated in FIG. 4 in two important respects. First, since each warp
filament 1 passes over two woof filaments 2 on the back side of the
semi-twill fabric as compared to only one woof filament 2 on the face side
of the fabric, the length of the impressions is approximately twice as
great when the web is imprinted with the back side of the fabric.
Secondly, when a paper web imprinted with the pattern illustrated in FIG.
5 is removed from the drying drum by means of a conventional doctor blade,
a diamond-shaped pattern is imparted to the surface of the paper, whereas
when a paper web imprinted with the pattern illustrated in FIG. 4 is
removed from the drying drum by means of a conventional doctor blade, a
regulated creping pattern results in which the crepe ridges are
substantially unbroken across the width of the sheet. This characteristic
difference in finished product appears to be due to the fact that the web
illustrated in FIG. 4 is adhered to the dryer drum only at interrupted
intervals, i.e., by the dot impressions 3, which impressions are not
sufficiently long to overlap each other in the machine direction. The
paper web illustrated in FIG. 5, on the other hand, is adhered to the
dryer drum in a continuous fashion, i.e., by the dash impressions 8, which
impressions are sufficiently long to overlap each other in the machine
direction.
Based on the teachings of the prior art, and particularly on U.S. Pat. No.
3,473,576 issued to Amneus on Oct. 21, 1969, said patent being
incorporated herein by reference, it is recognized that smooth web
transfers and maximum drying effectiveness are not realized with fabrics
having rough or inconsistent web contacting surfaces. Smooth web transfers
are particularly desirable where, as in the case of the papermaking
process disclosed in U.S. Pat. No. 3,301,746, the imprinting fabric is of
product characteristic importance. It has, therefore, been found desirable
to utilize imprinting fabrics having uniform knuckle heights and minimum
free or interstitial area on the side of the fabric contacting the
uncompacted paper web. Because such imprinting fabrics are subjected to
elevated temperatures during use, it has also been found desirable to
dimensionally heat stabilize such fabrics prior to use to prevent warpage.
It is important to note that due to the symmetry of a plain weave fabric,
uniform knuckle heights and minimum free area are achieved simultaneously
on both sides of the fabric when the fabric is subjected to a heat
treatment process such as that disclosed in U.S. Pat. No. 3,473,576. This
is not the case with a semi-twill weave fabric. If a monofilament,
polymeric fiber, semi-twill fabric is subjected to a heat treatment
process such as that disclosed in U.S. Pat. No. 3,473,576, the knuckles 4
and 7 on the conventional face side of the fabric will become coplanar
before the knuckles 5 and 6 on the back side of the fabric. In order for
the knuckles 5 and 6 on the back side of the fabric to reach uniform
heights, the fabric must be subjected to further heat treatment. The
additional heat treatment in turn causes the heights of the knuckles 4 and
7 on the conventional face side of the semi-twill fabric to again become
non-uniform.
Therefore, the initial warp filament spacing and caliper of a semi-twill
fabric necessary to produce minimum free area and uniform knuckle heights
on the back side of the fabric after heat treatment is determined
experimentally by trial and error.
In a preferred embodiment of the present invention, a monofilament,
polymeric fiber, semi-twill fabric is prepared by selecting warp
monofilaments having a relatively high heat-induced shrinkage potential in
the range of about 10 to about 30 percent, preferably about 16 percent.
After selecting and spacing the warp monofilaments, polymeric woof
monofilaments are selected which have a relatively low heat-induced
shrinkage potential in the range of about 2 to about 8 percent, preferably
about 4 percent. The heat shrinkage treatment takes advantage of the
aforementioned shrinkage characteristics of the warp and woof
monofilaments. The heat shrinkage treatment comprises subjecting the
initially woven fabric to a series of heat applications as it is stretched
and secured at its ends in the lengthwise or warp direction, while it is
free to shrink in the woof direction.
The heat shrinkage treatment is conveniently applied to the initially woven
semi-twill fabric while the fabric is mounted as an endless belt on a
finishing table such as those conventionally used in finishing metal
Fourdrinier wires. A conventional wire finishing table consists of two
adjustable rolls for supporting, tensioning and driving the wire or fabric
to be finished as an endless belt. The heat shrinkage can be induced
conveniently by an infrared source mounted as a bank above and across the
initially woven fabric. The infrared source heats areas of the initially
woven fabric as the fabric slowly revolves on the rolls of the wire
finishing table. Heat is applied to the fabric in successive treatments of
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