Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2096978 PATENT
METHOD FOR MAKING PAPER SHEETS HAVING HIGH BULK AND ABSORBENCY
Backqround of the Invention
5In the manufacture of paper products such as paper towels,
dinner napkins, tissue and the like, there are generally two
different methods of making basesheets for these various products.
One method is commonly referred to as wet-pressing and the other is
referred to as throughdrying. While the two methods may be the same
at the front end and the back end of the process, they differ
primarily in the manner in which water is removed from the wet web
after its initial formation.
For example, in the wet-pressing method, which is the older and
more conventional method of making paper towels, the newly-formed wet
web is typically transferred onto a papermaking felt and thereafter
pressed against the surface of a steam-heated Yankee dryer while it
is still supported by the felt. As the web is transferred to the
surface of the Yankee, water is expressed from the web and is
absorbed by the felt. The dewatered web, typ1cally having a
cons;stency of about 40 percent, is then dried while on the hot
sur~ace of the Yankee. The web is then creped to soften it and
provide stretch to the resulting sheet. A disadvantage of wet
pressing is that the pressing step densifies the web, thereby
decreasing the bulk and absorbency of the sheet, which must be
restored by the subsequent creping step.
In the throughdrying method, which has become more common in
recent years, the newly-formed web is transferred to a relatively
porous fabric and non-compressively dried by passing hot air through
the web. The resulting web can then be transferred to a Yankee dryer
for creping. Because the web is substantially dry when transferred
to the Yankee, the density of the web is not significantly reduced by
the transfer. Also, by drying the web while supported on the
throughdrying fabric, a less dense sheet is produ~ed in the first
place. This results in a more bulky and absorbent sheet. However, a
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disadvantage of throughdrying is the operational energy costs and the
capital costs associated with the throughdryers.
Because there are many existing wet-pressing paper machines
making paper towels and the like, and because there is a continuing
desire to improve the bulk and absorbency of such products, there is
a need for a means of producing paper towels having throughdried
characteristics using existing wet-pressing paper machines without
the expense of adding new throughdryers.
SummarY of the Invention
It has now been discovered that a wet-pressed product can be
made having bulk and absorbency properties equivalent to those of
comparable throughdried products. More particularly, wet-pressed
paper towels can be made by incorporating a wet strength resin
into the furnish and substituting a "molding" fabric for the
conventional wet-pressing felt in order to impart more contour or 3-
dimensionality to the wet web. The wet web is preferably thereafter
pressed against the Yankee dryer while supported by the molding
fabric and dried. The resulting product has exceptional wet bulk and
absorbency exceeding that of conventiona1 wet-pressed towels and
equal to that of throughdried towels currently on the market.
Hence, in one aspect the invention resides in a method for
making an absorbent paper sheet comprising: ~a) depositing an aqueous
suspension of papermaking fibers containing a wet strength resin onto
a forming Pabric which allows water to pass through while retaining
fibers thereon to for~ a wet web; (b) dewatering the wet web to a
consistency (dry weight percent fiber) of from about lQ to about 30
percent; (c) transferring the wet web to a molding fabric
(here~nafter described3 and substantially conforming the wet web to
the surface of the molding fabric; (d) pressing the web aga;nst the
surface of a heated drying cylinder, such as a Yankee dryer, to at
least partially dry the web while preserving its molded structure;
and (e) drying the web. The web can be partially dried on the heated
drying cylinder and wet creped at a consistency of from about 25 to
about 80 percent and thereafter dried (after-dried) to a consistency
of about 95 percent or greater. Suitable means for after-drying
include one or more cylinder dryers, such as Yankee dryers and can
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dryers, throughdryers, or any other commercially effective drying
means. Alternatively, the molded web can be completely dried on the
heated drying cylinder and dry creped. The amount of drying on the
heated drying cylinder will depend on such factors as the speed of
the web, the size of the dryer, the amount of moisture in the web,
and the like.
In another aspect the invention resides in ,a wet-molded paper
sheet, such as a single-ply paper kitchen towel, containing from
about 1 to about 40 pounds of wet strength resin per ton of fiber,
said sheet having an Absorbent Capacity of about 10 grams per gram or
greater; an Absorbent Rate (hereinafter defined) of about 2.5 seconds
or less; a Wipe Dry Area (hereinafter defin~d) of about 2500 square
millimeters or less, preferably about 2300 sguare millimeters or
less, more preferably about 2000 square millimeters or less, and
suitably from about 2000 to about 2300 square milllmeters or,
alternatively, to about 2500 square millimeters; and a Wipe Dry Mass
(hereinafter defined) of about 40 or less, preferably about 30 or
less, and suitably from about 30 to about 40. As used herein, "wet-
molded" paper sheets are those which are conformed to the surface
contour of a molding fabric while at a consistency of from about 10
to about 30 percent and initially thermally dried by thermal
conductive drying means, such as a heated drying cy1inder, as opposed
to other drying means such as a throughdryer.
Suitable fibers useful for making products of this invention
include any papermaking fibers, such as hardwood and softwood fibers,
nonwoody fibers, synthetic fibers, and the like.
NMolding fabrics" suitable for purposes of this invention
include, without limitation, those papermaking fabrics which exhibit
significant open area or surface contour sufficient to impart greater
z-directional deflection of the web. Such fabrics include single-
layer, multi-layer, or composite permeable structures. Preferred
fabrics have at least some of the following characteristics: (1) On
the side of the molding fabric ~hat is in contact with the wet web
(the top side), the number of machine direction (MD) strands per inch
(mesh) is from 10 to 200 and the number of cross-machinP direction
(CD) strands per inch (count) is also from 10 to 200. The strand
diameter is typically smaller than 0.050 inch; (23 On the top side,
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the d;stance between the highest point of the MD knuckle and the
highest point of the CD knuckle is from about 0.001 to about 0.02 or
0.03 inch. In between these two levels, there can be knuckles formed
either by MD or CD strands that give the topography a 3-dimensional
hill/valley appearance which is imparted to the sheet during the wet
molding step; (3) On the top s;de, the length of the MD knuckles is
equal to or longer than the length of the CD knuckles; (4) If the
fabric is made in a multi-layer ccnstruction, it is preferred that
the bottom layer is of a finer mesh than the top layer so as to
control the depth of web penetration and to maximize fiber retention;
and (5) The fabric may be made to show certain geometric patterns
that are pleasing to the eye, which typically repeat between every 2
to 50 warp yarns.
The wet strength resins that are preferred for use in connection
with the present invention include those polymers that are usually
used in the paper industry to provide strength to paper products when
they are wetted in use. Paper products that do not contain these
types of res;ns will quickly fall apart or lose ;ntegrity when they
are wet w;th water. Presently, the most commonly used wet strength
resins belong to the class of polymers termed polyamide-polyamine
epichlorohydrin res;ns. There are many commerc;al suppliers of these
types of resins including Hercules, Inc. (Kymene0~, Henkel Corp.
(Fibrabond~), Borden Chemical (Cascamide~), Georgia-Pacific Corp. and
others. These polymers are character;zed by having a polyamide
backbone containing reactive crosslinking groups distributed along
the backbone. Other agents that have been found useful in the
present invention include wet strength agents based on formaldehyde
crosslinking o~ polymeric resins. These are typified by the urea-
formaldehyde and melamlne formàldehyde-type wet strength resins.
Wh;le not used as commonly as the polyam;de-polyamine ep;chlorohydr;n
type resins, they are st;ll useful in the present invention. Another
class of wet strength resins found to be useful in the invention are
those classed as aldehyde derivatives of polyamide resins. These are
exemplified by materials marketed by American Cyanamid under the
Parez~ tradename as well as materials described in U.S. Patents
5,085,736; 5,088,344 and 4,981,557 issued to Procter & Gamble, which
are herein incorporated by reference.
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Although there are different chemical structures embodied by all
of these resins, the mechanism by which they provide the effects in
the present invention is essentially the same. All of the wet
strength resins provide wet strength through a crosslinking reaction.
This crosslinking either occurs between different portions of the
resin itself, or through crosslinks with the surface of the fibers in
the tissue, towel, paper or nonwoven product. The crosslinking of
the resin is generally believed to prevent the water-induced
disruption of the hydrogen bonds that hold the substrate together in
the dry state. For a discussion of the mechanism of wet strength,
see Pulp a~nd_PaDer, ChemistrY and Chemical TechnoloqY. Third Edition,
Volume IiI, pages 1609-1624, James P. Casey, Editor, John Wiley &
Sons, New York, 1981.
In the present invention, advantage is taken of the ability to
induce the crosslinking of these resins to lock the substrate into a
molded structure. In this instance it is somewhat analogous to fiber
reinforced composites like fiberglass or carbon fiber composites,
except that in this instance the amounts of bonding material relative
to the fibrous portion of the composite are much lower. In the
present invention, the effective amounts of added resin can range
from about 1 pound of resin (dry solids) per ton of fiber, up to
about 40 pounds of resin (dry solids) per ton of fiber. The exact
amount of material will depend on the specific type of resin used,
the type of fiber used, and the type of forming apparatus used. The
preferred amounts of resin to be used are in the range of from about
5 to about 15 pounds of resin per ton of fiber, with a particularly
preferred range of from about 8 to about 12 pounds per ton of fiber.
These materials are typically added to the wet end of the paper
machine and are absorbed onto the surface of the fiber and the fines
prior to the formation of the sheet. Differences in the amounts of
resin necessary to bring about the desired effects result from
different resin efficiencies, differences in the fibers and the types
of contaminants that might be contained in or with the fibers
(particularly important when using recycled fibers~ and the ability
to dry the sheet while in the molded state. It is important that the
crosslinking of the resin is made to occur after the product has been
molded in the wet state and before significant amounts of subsequent
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processing occur that might remove or reverse the molding. It has
been found that once the molded structure has been formed and the
resin has been fully cured to provide the final level crosslinking,
the molded sheet can be deformed (either wet or dry) into another
shape (flat or other pattern, such as embossments) and when the sheet
is rewet, the original molded shape is reformed. This results ~rom
the reinforcement of the resin on the fibers and on the fiber/fiber
bonds and provides increased bulk and Absorbent Capacity. It also
enables the sheet to retain or at least substantlally retain its bulk
or thickness after wetting, as measured Rpeak-to-peak" from one side
of the molded sheet to the other, even after being pressed in the wet
state using finger pressure equivalent to that experienced during
ordinary use. In general, the sheets of this invention will retain
at least about 50YO of their original dry bulk, preferably about 80
percent or more, and more preferably 90% or more, depending upon the
amount of wet strength resin incorporated into the sheet. Dry bulk
increases to the sheet as a result of the use of a molding fabric in
accordance with this invention can be from about lO to about 300
percent, more often from about 20 to about 100 percent relative to
the bulk of a comparable unembossed wet-pressed sheet.
As used herein, "Absorbent Capacity" is the max1mum amount of
distilled water which a sheet can absorb, expressed as grams of water
per gram of sheet. More specifically, the Absorbent Capacity of a
sample sheet can be measured by cutting a 4 inches x 4 inches sample
of the dry sheet and weighing it to the nearest 0.01 gram. The
sample is dropped onto the surface of a room temperature distilled
water bath and left in the bath for 3 minutes. The sample is then
removed using tongs or tweezers and suspended vertically uslng a -
spring clamp to drain excess water. Each sample is allowed to drain
for 1 minute. The sample is then placed in a weighing dish by
holding the weighing dtsh under the sample and releasing the clamp.
The wet sample is weighed to the nearest 0.01 gram. The Absorbent
Capacity is the wet weight of the sample minus the dry weight (the
amount of water absorbed), divided by the dry weight of the sample.
Five representative samples of each product should be tested and the
results averaged.
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"Absorbent Rate" is the time it takes for a product to become
thoroughly wetted out in distilled water. It is determined by
dropping a single, 4 in. x 4 in. sa~ple of the product onto the
surface of a distilled water bath having a temperature of 30C. The
elapsed time fro~ the moment the sample hits the water until it is
completely wetted (as determined visually) is the Absorbent Rate.
The "Wipe Dry Area" and "Wipe Dry Mass" are determined by image
analysis and will be fully described below. Generally, "Wipe Dry
Mass" is a number approxi~ately proportional to the mass oF an
aqueous residue left after a sample has been insulted with an aqueous
solution. As will be hereinafter described, it is simply the product
of mean optical density and area of the residue. "Wipe Dry Area" is
the area coverage in square millimeters of the residue left by the
sample.
Brief Description of the Drawinq
Figure 1 is a schematic flow diagram of a method in accordance
with this invention.
Figure 2 is a schematic diagram of the equipment used to
determine the Wipe Dry Area and the ~ipe Dry Mass.
Detailed DescriDtion of the Drawinq
Referring to Figure 1, the invention will be described in
greater de$ail. Shown is a papermaking headbox 10 which deposits a
papermaking furnish comprising an aqueous slurry or suspension of
papermaking fibers and wet strength resin onto a forming fabric 11 to
form a wet web 12. The forming section of the papermaking ~achine
can include any forming configuration suitable for making towels and
tissue products, including Fourdrinier formers, twin wire formers,
crescent formers, and the like. While the wet strength resin is
preferably added to the furnish prior to web formatlon, it can also
be sprayed onto the wet web during or aFter formation. Optionally,
the newly-formed web can be hydroneedled as described in U.S Patent
No. 5,137,600 to Barnes et al. (1992) entitled "Hydraulically Needled
Nonwoven Pulp Fiber Web", which is herein incorporated by reference.
Such hydroneedling involves impingement of the newly-formed wet web
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with a large number of small, pressurized water jets to alter the
structure of the web.
Dewatering of the web is suitably accomplished using vacuum
suction by pulling a vacuum from beneath the forming fabric, or by
using an optional consolidation press 13 comprising a felt 14 which
is pressed against the wet web to absorb some of the moisture.
Vacuum box 15 is used to maintain the web on the forming fabric and
keep it from following the felt. The consistency of the wet web
should be from about 10 to about 30 percent before being transferred
from the forming fabric to the molding fabric 16.
Transfer of the web from the forming fabric to the molding
fabric is easily achieved using a vacuum suction box 17, which pulls
the web onto the surface of the molding fabric and causes the wet web
to conform to the surface of the mold;ng fabric. Conformation and
further dewatering of the web can be further augmented by the use of
additional vacuum boxes and/or an air press 18.
While supported by the molding fabric, the web is transferred to
the surface of a heated drying cylinder such as a hooded Yankee dryer
19 with pressure roll 20. By transferrlng the web directly from the
Z0 molding fabric to the Yankee, compression of the web is confined to
the areas of the web corresponding to the knuckle points of the
molding fabric. This preserves the 3-dimensional shape of the web
and also minimizes the extent to which the web is pressed or
compressed.
After the web is dried, it is dislodged from the Yankee by
contact with a doctor blade 21 to yield an absorbent web 22 having a
high degree of wet bulk.
Although not shown in Figure 1, further drying of the web using
a suitable partial drying ~eans between the suction box 17 and the
pressure roll 20 can also be utilized to lessen the drying burden on
the Yankee dryer 19 if desirable. Such additional drying can be
achieved using a variety of drying means well known in the
papermaking art, including heated cylinders, such as can dryers or a
Yankee dryer, flat bed throughdryers, cylindrical throughdryers,
infra-red or microwave dryers, or the like. It is preferable that
the consistency of the web after the pressure roll 20 be about 40
percent or greater for ease of drying at high speeds.
2~9~978
Referring to Figure 2, the apparatus set-up for determining the
Wipe Dry Area and the Wipe Dry Mass is illustrated. Shown is a
Kreonite Macroviewer 30 (Kreonite, Inc., Wichita, Kansas) which
supports two hooded flood lamps 31 and 32 ~Polaroid~ 4, 150 watt). A
white posterboard background 33 is provided beneath the glass platPs
34 which contain the residue to be quantified as described below.
Also shown is a Leica/Cambridge Newvicon scanner 35 with a 35 mm.
Nikon lens 36, mounted on a Polaroid ruled pole 37 with scanner fork
attachment.
To measure the Wipe Dry Mass and Wipe Dry Area of a particular
sheet sample, such as a paper towel, a dye solution of Marker-Blue NS
dye (Keystone Aniline Corp., Chicago, Illinois) at 10.5 weight
percent solids is diluted with distilled water to a solids
concentration of 0.5 weight percent. The surface tension of the
resulting solution is 64 dynes/cm. Five 10 x 12 x 1/8 inch glass
plates are cleaned with "Alcojet" (Alconox, New York, New York~
detergent powder and then conditioned with "Glass Plus" (Dow Brands,
Indlanapolis, Indiana) spray glass cleaner. An 8 x 8 x 1/2 inch
aluminum plate, cut and drilled with a 1/2 inch diameter hole in its
center, is placed on top of the sample to be tested. A "BD" syr~nge
(Becton-Dickinson, Rutherford, New Jersey), without needle, is used
to evenly apply the dye solution to the area of the sample exposed by
the hole in the aluminum plate during a period of 2 seconds. The
amount of dye applied to the sample is 3 cubic centimeters. Dye
insults are made in the center of a single towel sheet rather than
near an edge. After a 10 second wait, the aluminum plate and the
sample are removed vertically from the glass. The wet deposit
remaining on the glass is allowed to dry, which may take from about
10 to 60 minutes, depending on whether or not a drying oven is used.
The drled residue on the glass is then subjected to image
analysis to determine the characteristics of the deposit and hence
the absorbency effectiveness of the sample tested. The equipment
set-up using a standard macroviewer with flood lamps, a white paper
background and a 35 mm. Nikon lens is shown in Figure 2 and described
above. A Quanti~et (Leica/Cambridge, Deerfield, Illinois) image
analysis program is used to interpret the residue image and calculate
the Wipe Dry Mass and the Wipe Dry Area.
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The image analysis program is shown below:
Cambridge Ir~trumcnts CUA~TI~ET 900 ~UIPS/MX V03.02
OO~D = 35 mm. lens; Pole ~ 78 cm. f/2.8; 4 f~oods ir~ident
Enter specimen idsntity
Scanner (~lo. 2 ~e~vicon LV = 2.00 SE~IS = 1.65 PAUSE)
Lo~d Shodir~ Corrector ~pattern - SCLIN1)
Calibr~te User Specified ~Calibration Value = 0.1425 1m. per pixel)
CALL STAhDA~D
TOTFIELDS : = O
TOTM EA : = O
ToTAvEB2r : = 0
TOTDAR~ : - O
TOT~USS : = O
For FIELD
Det Yt 2D ~Darker than 55 PAUSE)
A ~ d ~OPEN by 1)
Pause Hess3ge
EDIT W T ANY APTIFACTS
Edit ~PQuse)
~easure field - P~ram2ter3 into array FIELD
~easure field - Integrared 8rightne~s wu~ked by Binary into array FIELD
AREA : ~ FIELD AREA
AVEBRIGHT = ~FIELD TOTBRIG~T/~AREA/~CAL.CO~ST. ~ CAL.CO~ST)))
DARK~ESS ~ = 64 ~ AVE8RIGHT
~ASSFAOT : - AREA D DARKNESS / 1000
TOTAREA : = TOTAREA ~ A~EA
TOTAVE~RT s TOTAVEBRT ~ AVEBRIGHT
TOTDARK ; ~ TOTDARK ~ DA2KNESS
TOTHASS : = TOT~ASS ~ ~ASSFACT
TOTFIELDS : ~ TOTFIELDS
Pause uessa~e
PLEASE CHOOSE A~IOTHER FIELD, or 'FINISH'
P~
Next FIELD
Print ~ u
Print "AVE TOTAL AREA ~q m~) = " , TOTAREA/TOTFIELDS t~E~ PE DRY AREA~
Print ~ "
Print UAVE nASS FACTOR 5 ~1, TOTYASS/TOTFIELDS ~lM: YIPE DRY ~USS]
Pc~nt ~ ~
Print ~TOTAL ~ E2 OF FIELDS 5 )~, TOTFIELDS
For LOOPC0U~T ~ 1 to 5
Print
~ext
Er~ of Progr&~
ExamPles
Example 1: (Absorbency). In order to further ;llustrate the
invention, different kitchen towel samples were made and compared to
a commercially available throughdried kitchen towel (BOUNTY0) for a
variety of properties, including absorbency and bulk.
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2~96~8
Two-ply products of this invention were made in accordance with
the method described in Figure 1. Specifically, a 50/50
softwood/hardwood blend was dispersed in a hydropulper and pumped to
a stock chest where 20 pounds of Kymene~ wet strength resin per ton
of dry fiber was added to the furnish. The stock was deposited on a
94 mesh forming fabric at a consistency of about 0.1 percent and
exposed to a slight vacuum ~o begin drying the sheet. The sheet was
transferred to an Albany 31A fabric (molding fabric) at a consistency
of about 15 percent with the use of a vacuum box and molded into this
fabric using a vacuum of 17 inches of mercury. (In the case of
Sample 3, the amount of Kymene~ added was 15 pounds per ton of fiber
and the speed of the 31A fabric to which the web was transferred was
5% slower than the speed of the forming fabric from which the web was
transferred. For Sample 2, the fabric speeds were the same.) While
supported by the 31A fabric, the molded sheet was transferred to thP
surface of a Yankee dryer where the sheet was dried to a 95%
consistency and creped and thereafter plied together with a like
sheet using a glued-nested embossing techn;que to form a two-ply
kitchen paper towel.
The control product (Sample 1) was a conventional wet-pressed
product made under the same conditions, except the molding fabric
illustrated in Figure 1 was replaced with a conventional papermaking
felt (Albany ~bottom~ felt) as is typically used for making wet-
pressed paper towels.
A comparison of product properties is set forth in Table 1
below. The various properties listed in the table are expressed in
the following units: Basis Weight, grams per sguare meter; Absorbent
Capacity, grams of water per gram of fiber; Absorbent Rate, seconds;
Wip~ Dry Area, square millimeters; and Wipe Dry Mass, d;mensionless.
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TABLE 1
(Absorbencv)
5 PropertY 1 SaMple 3 4
(Control3(Invention)(Invention) BOUNTY~
Basis Weight 38.4 36.8 35.2 44
Absorbent
Capacity 6.9 10.4 11.5 11.1
Absorbent
Rate 2.9 1.9 2.3 2.2
Wipe Dry
Area 2607 2054 2058 2608
Wipe Dry
Mass 43.8 32.7 32.3 48.0
The results clearly show that the paper towels of this invention
exhibit absorbency character;stics greater than conventional wet-
pressed paper towels and equlvalent to or exceeding those of acommercially available throughdried paper towel. The Absorbent
Capacity exceeded 10 grams per gram for both samples of the
invention. The Absorbent Rate was greatly improved over the wet-
pressed control and was equal to that o~ the throughdried product,
BOUNTY. The ~ipe Dry Area and Wipe Dry Mass were significantly lower
than either the wet-pressed control or the throughdried product,
illustrating that less of a residue was left behind. This is
obviously a desirable attribute for a paper towel.
Example 2: ~Wet Bulk Retention~. In order to illustrate the wet bulk
or caliper retent10n of the products of this invention, round
handsheets having a 4 inch diameter were formed on a 94 mesh forming
fabric using standard handsheet formation techniques. The nominal
weight was 0.3 grams per sheet and otherwise identical sheets were
made with and without KYMENE wet strength resin. The sheets were
hand-couched using a blotter and transferred to a 14 x 14
(mesh/count) metal wire ~molding fabric). The sheets were then
molded into the metal wire using a rubber-coated brass couching roll
with light pressure. The wire containing the molded sheet was then
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~996~78
placed on a steam-heated dryer to dry the sheet in the molded state.
The KYMENE was allowed to cure for at least 8 hours before testing
the sheets for wet caliper.
The dry caliper of the sheets was determined using a ~MI model
49-70 caliper tester with a 2 inch foot and 0.176 pounds per square
inch pressure. As used herein, dry caliper refers to the caliper of
a dry sheet as made, prior to any wetting in-use or to simulate in-
use conditions. To test the sheets for wet caliper, each sheet was
thoroughly wetted to simulate the water absorption associated w;th
cleaning up a large spill. The sheet was then pressed with a finger
to simulate the compressive actions common to normal usage. The
sheets were then air dried and the caliper again measured with the
same instrument. For purposes herein, this "simulated" wet caliper
is referred to as the wet caliper of the sheet.
The results are summarized in Table 2 below. "Weight" is
expressed in grams; "Initial Caliper" is the dry caliper prior to
wetting, expressed in inches; and "Final Caliper" is the caliper
after wetting, compression, and drying, expressed in inches.
TABLE 2
(Wet Bulk Retention)
Sample 1 SamDle 2 Samp~ Sample 4
KYMENE NO YES NO Yes
Weight 0.290 0.304 0.313 0.300
Initial
Caliper 0.0152 0.0157 0.0161 0.0173
Final
Caliper 0.0095 0.0156 0.0122 0.0165
Percent
35 Caliper
Retained 62.5 99.4 75.8 95.4
The results clearly show the substantially improved bulk or
caliper rçtention exhib1ted by Samples 2 and 4 of this invention,
which contained a wet strength resin, compared to that of Samples 1
and 3, which did not contain a wet strength resin. Wet bulk
retention is complimentary to the absorbent properties also exhibited
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2~96~78
by the products of this invention, providing very desirable basesheet
properties for paper towels and other absorbent sheet products.
In addition, for purposes of further comparison, an unmolded
handsheet sample weighing 0.295 grams was preparled in the same manner
as described above. The sample had an initial dry caliper of 0.0092
inch and, after wetting, pressing and drying again, a final caliper
of 0.0092 inch. This illustrates the substantial increase in dry
caliper imparted to the sheet by wet molding when compared to an
ordinary wet-pressed sheet.
Example 3: (Wet Bulk Retention- Conventional Embossed Towels). In
order to illustrate the benefits of this invention relative to
conventional wet-pressed sheets which have been embossed, four inch
diameter circular samples were cut out of a com~ercially available
two-ply paper towel which had been made with a conventional wet-
pressing process and thereafter embossed with an overall "random dot"embossing pattern. The furnish contained KYMENE wet strength resin.
The circular samples were separated into single plies and tested as
described above. The results are summarized in TABLE 3 below:
TABLE 3
(Wet Bulk Retention: Conventional Wet-Pressed/Embossed)
Sample 1 Sample 2 Sample 3 Sample 4
KYMENE Yes Yes Yes Yes
We~ght 0.132 0.133 0.139 0.137
Initial
Caliper 0.0148 0.0189 0.0169 0.0175
Final
Caliper 0.0075 0.0071 0.0075 O.OG68
Percent
Caliper
Retained 51 38 44 39
The samples of this example retained so little bulk relative to
Samples 2 and 4 of TABLE 2 because the wet-pressed samples of this
example were not wet-molded. Although the caliper was increased by
embossing, the increased caliper was not retained when the sheet was
wetted and subjected to slight finger pressure.
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Example 4: (Conventional/Embossed vs. Invention/Embossed)
A standard wet-pressed basesheet (Control) and a wet-molded basesheet
in accordance with this invention were made as described in Example
1, except the basesheets were combined into a two-ply sheet and
embossed as described in E~ample 3. Both samples were tested as a
two-ply product for wet bulk retention as described in Example 3.
The results are summarized in TABLE 4 below:
TABLE 4
(Conventional/Embossed vs Invention/Embossed)
Control Invention
KYMENE Yes Yes
Weight 0.320 0.272
Initial
Caliper 0. 0173 0 . 0202
20 Fi nal
Caliper 0.0090 0. 0147
Percent
Caliper
25 Retained 52 73
The Control sample was a two-ply analog of the samples of
Example 3 and showed similar results in that, in both cases, the high
in;tial dry caliper was due to the embossing pattern developed during
converting of the basesheet. The wet-molded sample of this invention
was similarly converted, but retained more caliper after
wetting/pressing because the portion of the caliper due to the wet
molding was not lost when wetted/pressed. In both cases the caliper
due to embossing was lost.
ExamDle ~: (Calendared Sheets). A handsheet weighing 0.279 grams and
containing KYMENE was made and molded as described in Example 2. The
sheet was calendared between two steel rolls and the resulting
caliper (Initial Caliper) was measured to be 0.0084 inch. The wet
caliper (Final Caliper) of the calendared sheet was also determined
as described in Example 2, which was measured to be 0.0124 inch. The
percent increase in caliper resulting from wetting was 48 percent.
Hence the wet-molded sheet retains its memory during calendaring,
resulting in a wet caliper: dry caliper ratio of greater than 1. The
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2~9697~
extent to which the ratio exceeds 1 depends upon the 3-dimensionality
of the molding fabric and the severity of the calendaring. Molding
fabrics with high hills and low valleys will produce sheets with very
high dry bulks. Combined with heavy calendaring, such sheets can
have wet caliper:dry caliper ratios of about 2 or greater.
Typically, the wet caliper:dry caliper ratio will be from about 1.2
to about 2, more specifically from about 1.5 to about 2. Such sheet
behavior is advantageous for use in paper towels, for which low dry
bulk and high wet bulk can be a very desirable combination.
It will be appreciated that the foregoiny examples, given for
purposes of illustration, are not to be construed as limiting the
scope of this invention, which is defined by the following claims and
which includes all equivalents thereto.
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