-
The present invention relates to multi-ply tissue, and in particular to facial
tissue, and disposable handkerchiefs.
-
Paper webs or sheets, sometimes called tissue or paper tissue webs or
sheets, find extensive use in modern society. Such items as facial and toilet
tissues are staple items of commerce. It has long been recognised that four
important physical attributes of these products are their strength, their softness,
their absorbency, including their absorbency for aqueous systems; and their lint
resistance. Research and development efforts have been directed to the
improvement of each of these attributes without seriously affecting the others as
well as to the improvement of two or three attributes simultaneously.
-
Softness is the tactile sensation perceived by the consumer as he/she
holds a particular product, rubs it across his/her skin, or crumples it within his/her
hand. This tactile sensation is a combination of several physical properties. One
of the more important physical properties related to the softness is generally
considered by those skilled in the art to be the stiffness of the paper tissue from
which the product is made. Stiffness, in turn, is usually considered to be directly
dependent on the dry tensile strength of the web.
-
Strength is the ability of the product to maintain physical integrity and to
resist tearing, bursting, and shredding under use conditions.
-
Absorbency is the measure of the ability of a product to absorb quantities
of liquid, particularly aqueous solutions or dispersions. Overall absorbency as
perceived by the human consumer is generally considered to be a combination of
the total quantity of a liquid a given mass of tissue paper will absorb at saturation
as well as the rate at which the mass absorbs the liquid.
-
Lint resistance is the ability of the fibrous product, and its constituent webs,
to bind together under use conditions, including when wet. In other words, the
higher the lint resistance is, the lower the propensity of the web to lint will be.
-
WO95/11343, published on 27th April 1995, discloses a process for making
layered paper tissues. Example 3 discloses a two-ply facial tissue having a basis
weight of about 32 g/m2 (20 lbs/3000 sq. ft.) The tissue of this example comprises
0.475% of a wet strength resin.
-
US-A-4 481 243, issued on 6th November 1984 discloses facial tissues
which comprise multiple plies secured together by embossing only along the
margins of the tissue.
-
Disposable paper products having high wet burst strength are known, for
example Bounty™, sold by The Procter & Gamble Company, has a wet burst
strength which is greater than 200 g. However such kitchen towels are embossed
over the whole surface which results in a surface texture which is rough and does
not provide a suitably smooth wiping surface for blowing the nose.
-
Facial tissues are commercially available comprising at least two plies, the
tissue having a surface area in one plane, and a thickness orthogonal to the
plane, wherein the thickness is a caliper of at least 0.35mm, and wherein the
tissue has an unembossed wiping surface over a major part of the surface area of
the tissue. However the rather low wet burst strength of today's facial tissues
often results in tearing or bursting which in turn results in contamination of the
user's hand with mucus or other bodily fluids.
-
The object of the present invention is to provide a multi-ply facial tissue
having the at least the desired softness and absorbency of known facial tissues,
but also providing enhanced protection against tearing or bursting when used, in
particular when used for blowing the nose.
Summary of the Invention
-
The object of the invention is achieved by a multi-ply tissue having an
average basis weight of at least 40 g/m2, and preferably at least 45 g/m2, and
having a wet burst strength of at least 150g, preferably at least 200g. It is also
preferred that the multi-ply tissue has a caliper of at least 0.4 mm. In one
embodiment of the invention at least one of the plies comprises first and second
regions, the first region comprising a macroscopically monoplanar, continuous
network having a high density and low basis weight relative to the second region,
the second region being composed of discrete domes having low densities
relative to the first region, essentially all of the domes being dispersed throughout,
encompassed by, and isolated one from another by said network region.
Furthermore it is preferred that the ratio of the average basis weight of the first
network region to the average basis weight of the domes of the second region is
greater than 0.8, and less than 1.0.
-
The invention also relates to a process for the manufacture of the multi-ply
tissue, wherein the process comprises the steps of :
- mechanically refining a first slurry of fibres wherein the fibres have an
average length of at least 2mm, preferably the first slurry comprises a
substantial proportion of softwood fibres, such as Nothern Softwood
Kraft fibres;
- mixing the refined slurry with a second slurry of fibres, the average
length of the fibres of the second slurry being shorter than the average
length of the fibres of the first slurry, preferably the second slurry
comprises a substantial proportion of hardwood fibres, such as
eucalyptus fibres;
- providing a embryonic web upon a foraminous surface, the composition
of fibres in the embryonic web being substantially homogeneous
throughout the thickness of the web;
- removing water from the embryonic web to form a ply; and
- juxtaposing at least two plies to form the multi-ply tissue.
Most preferably, the ratio of long softwood fibres to shorter hardwood fibres
is greater than 60:40, and preferably about 70:30.
Detailed Description of the Invention
-
The present invention may contain, as a highly preferred component, up to
about 3.0%, preferably at least 0.5%, and more preferably at least 0.8% by
weight, on a dry fiber weight basis, of wet strength chemical agent, such as water-soluble
permanent and temporary wet strength resin.
-
Wet strength resins useful herein can be of several types. For example,
Westfelt described a number of such materials and discussed their chemistry in
Cellulose Chemistry and Technology, Volume 13, at pages 813-825 (1979).
-
Usually, the wet strength resins are water-soluble, cationic materials. That
is to say, the resins are water-soluble at the time they are added to the
papermaking furnish. It is quite possible, and even to be expected, that
subsequent events such as cross-linking will render the resins insoluble in water.
Further some resins are soluble only under specific conditions, such as over a
limited pH range. Wet strength resins are generally believed to undergo a cross-linking
or other curing reactions after they have been deposited on, within, or
among the papermaking fibres. Cross-linking or curing does not normally occur so
long as substantial amounts of water are present.
-
Of particular utility are the various polyamide-epichlorohydrin resins. These
materials are low molecular weight polymers provided with reactive functional
groups such as amino, epoxy, and azetidinium groups. The patent literature is
replete with descriptions of processes for making such materials,including US-A-3
700 623, issued to Keim on October 24th 1972, and US-A-3 772 076, issued to
Keim on November 13th 1973.
-
Polyamide-epihydrochlorin resins sold under the trademarks Kymene 557H
and Kymene LX by Hercules Inc. of Wilmington, Delaware, are particularly useful
in this invention. These resins are generally described in the aforementioned
patents to Keim.
-
Base-activated polyamide-epichlorohydrin resins useful in the present
invention are sold under the Santo Res trademark, such as Santo Re 31, by
Monsanto Company of St. Louis, Missouri. These types of materials are generally
described in US-A-3 855 158 issued to Petrovich on December 17th 1974; US-A-3
899 388 issued to Petrovich on August 12th 1975; US-A-4 129 528 issued to
Petrovich on December 12 1978; US-A-4 147 586 issued to Petrovich on April 3rd
1979; and US-A-4 222 921 issued to Van Eenam on September 16th 1980.
-
Other water-soluble cationic resins useful hererin are the polyacrylamide
resins such as those sold under the Parez trademark, such as Parez 631NC, by
American Cyanamid Company of Sandford, Connecticut. These materials are
generally described in US-A-3 556 932 issued to Coscia et al on January 19th
1971; and US-A3 556 933 issued to Williams et al on January 19th 1971.
-
Other types of water-soluble resins useful in the present invention include
acrylic emulsions and anionic styrene-butadiene latexes. Numerous examples of
these types of resins are provided in US-A3 844 880. Meisel Jr et al, issued
October 29th 1974. Still other water-soluble cationic resins finding utility in this
invention are the urea formaldehyde and melamine formaldehyde resins. These
polyfunctional, reactive polymers have molecular weights on the order of a few
thousand. The more common functional groups include nitrogen containing
groups such as amino groups and methylol groups attached to the nitrogen.
Although less preferred, polyethylenimine type resins find utility in the present
invention.
-
More complete descriptions of the aforementioned water-soluble resins,
including their manufacture, can be found in TAPPI Monograph Series No. 29,
"Wet Strength in paper and Paperboard, Technical Association of the Pulp and
Paper Industry (New York; 1965).
-
Temporary wet strength agents, such as modified starch may also,
optionally, be used. Combinations
of permanent and temporary wet strength agents may be used.
-
The present invention may contain dry strength chemical agents, preferably
at levels up to 3% by weight, more preferably at least 0.1% by weight, on a dry
fiber weight basis. A highly preferred dry strength chemical agent is
carboxymethyl cellulose. Other suitable dry strength chemical agents include
polyacrylamide (such as combinations of Cypro™ 514 and Accostrength™ 711
produced by American Cyanamid of Wayne, N.J.); starch (such as corn starch or
potato starch); polyvinyl alcohol (such as Airvol™ 540 produced by Air Products
Inc. of Allentown, PA); guar or locust bean gums; and polyacrylate latexes.
Suitable starch materials may also include modified cationic starches such as
those modified to have nitrogen containing groups such as amino groups and
methylol groups attached to nitrogen, available from National Starch and
Chemical Company (Bridgewater, NJ).
-
Chemical softening compositions, comprising chemical debonding agents
are optional components of the present invention. US-A-3 821 068, issued June
28th, 1974 teaches that chemical debonding agents can be used to reduce the
stiffness, and thus enhance the softness, of a tissue paper web. US-A-3 554 862,
issued on January 12th 1971 discloses suitable chemical debonding agents.
These chemical debonding agents include quaternary ammonium salts.
-
Preferred chemical softening compositions comprise from about 0.01% to
about 3.0% of a quaternary ammonium compound, preferably a biodegradable
quaternary ammonium compound; and from about 0.01% to about 3.0% of a
polyhydroxy compound; preferably selected from the group consisting of glycerol,
sorbitols, polyglycerols having an average molecular weight of from about 150 to
about 800 and polyoxyethylene glycols and polyoxypropylene glycols having a
weight average molecular weight from about 200 to 4000. Preferably the weight
ratio of the quaternary ammonium compound to the polyhydroxy compound
ranges from about 1.0:0.1 to 0.1:1.0. It has been discovered that the chemical
softening composition is more effective when the polyhydroxy compound and the
quaternary ammonium compound are first premixed together, preferably at a
temperature of at least 40°C, before being added to the papermaking furnish.
Either additionally, or alternatively, chemical softening compositions may be
applied to the substantially dry tissue paper web, for example by means of a
printing process (N.B. all percentages herein are by weight of dry fibres, unless
otherwise specified).
-
Examples of quaternary ammonium compounds suitable for use in the
present invention include either unmodified, or mono- or di- ester variations of :
well-known dialkyldimethylammonium salts and alkyltrimethyl ammonium salts.
Examples include the di-ester variations of di(hydrogenated tallow)dimethyl
ammonium methylsulphate and di-ester variations of di(hydrogenated
tallow)dimethyl ammonium chloride. Without wishing to be bound by theory, it is
believed that the ester moity(ies) lends biodegradability to these compounds.
Commercially available materials are available from Witco Chemical Company
Inc. of Dublin, Ohio, under the tradename "Rewoquat V3512". Details of analytical
and testing procedures are given in WO95/11343, published on 27th April, 1995.
-
Examples of polyhydroxy compounds useful in the present invention
include polyoxyethylene glycols having a weight average molecular weight of from
about 200 to about 600, especially preferred is "PEG-400".
-
The tissue paper of the present invention may be made by common
methods well-known to the person skilled in the art, such as by dewatering
suitable pulp using, for example, one or more papermakers felts and/or belts.
-
In one embodiment of the present invention, at least one ply of the tissue
paper has two primary regions. The first region comprises an imprinted region
which is imprinted against the framework of the papermaking belt. The imprinted
region preferably comprises an essentially continuous network. The continuous
network of the first region of the paper is made on the essentially continuous
framework of the belt and will generally correspond thereto in geometry and be
disposed very closely thereto in position during papermaking.
-
The second region of the paper comprises a plurality of domes dispersed
throughout the imprinted network region. The domes generally correspond in
geometry, and during papermaking in position, to the deflection conduits in the
belt. The domes protrude outwardly from the essentially continuous network
region of the paper, by conforming to the deflection conduits during the
papermaking process. By conforming to the deflection conduits during the
papermaking process, the fibers in the domes are deflected in the Z-direction
between the paper facing surface of the framework and the paper facing surface
of the reinforcing structure. Preferably the domes are discrete.
-
Without being bound by theory, it is believed the domes and essentially
continuous network regions of the paper may have generally equivalent basis
weights. By deflecting the domes into the deflection conduits, the density of the
domes is decreased relative to the density of the essentially continuous network
region. Moreover, the essentially continuous network region (or other pattern as
may be selected) may later be imprinted as, for example, against a Yankee drying
drum. Such imprinting increases the density of the essentially continuous network
region relative to that of the domes.
-
The paper according to the present invention may be made according to any
of commonly assigned U.S. Patents: 4,529,480, issued July 16, 1985 to Trokhan;
4,637,859, issued Jan. 20, 1987 to Trokhan; 5,364,504, issued Nov. 15, 1994 to
Smurkoski et al.; and 5,529,664, issued June 25, 1996 to Trokhan et al. and
5,679,222 issued Oct. 21, 1997 to Rasch et al., the disclosures of which are
incorporated herein by reference.
-
If desired, the paper may be dried and made on a through-air drying belt not
having a patterned framework. Such paper will have discrete, high density regions
and an essentially continuous low density network. During or after drying, the
paper may be subjected to a differential vacuum to increase its caliper and
dedensify selected regions. Such paper, and the associated belt, may be made
according to the following patents: 3,301,746, issued Jan. 31, 1967 to Sanford et
al.; 3,905,863, issued Sept. 16, 1975 to Ayers; 3,974,025, issued Aug. 10, 1976 to
Ayers; 4,191,609, issued March 4, 1980 to Trokhan; 4,239,065, issued Dec. 16,
1980 to Trokhan; 5,366,785 issued Nov. 22, 1994 to Sawdai; and 5,520,778,
issued May 28, 1996 to Sawdai, the disclosures of which are incorporated herein
by reference.
-
In yet another embodiment, the reinforcing structure may be a felt, also
referred to as a press felt as is used in conventional papermaking without
through-air drying. The framework may be applied to the felt reinforcing structure
as taught by commonly assigned U.S. Patents 5,549,790, issued Aug. 27, 1996 to
Phan; 5,556,509, issued Sept. 17, 1996 to Trokhan et al.; 5,580,423, issued Dec.
3, 1996 to Ampulski et al.; 5,609,725, issued Mar. 11, 1997 to Phan; 5,629,052
issued May 13, 1997 to Trokhan et al.; 5,637,194, issued June 10, 1997 to
Ampulski et al.; 5,674,663, issued Oct. 7, 1997 to McFarland et al.; 5,693,187
issued Dec. 2, 1997 to Ampulski et al.; 5,709,775 issued Jan. 20, 1998 to Trokhan
et al., 5,814,190 issued Sept. 29, 1998 to Van Phan; and 5,817,377 issued
October 6, 1998 to Trokhan et al. the disclosures of which are incorporated herein
by reference.
-
If desired, in place of a belt having the patterned framework described
above, a belt having a jacquard weave may be utilized. Such a belt may be
utilized as a forming wire, drying fabric, imprinting fabric, transfer clothing etc. A
jacquard weave is reported in the literature to be particularly useful where one
does not wish to compress or imprint the paper in a nip, such as typically occurs
upon transfer to a Yankee drying drum. Illustrative belts having a jacquard weave
are found in U.S. Pat. Nos. 5,429,686 issued July 4, 1995 to Chiu et al. and
5,672,248 issued Sept. 30, 1997 to Wendt et al.
-
Two or more plies of tissue paper are combined to form the multi-ply tissue.
The plies may, optionally, be attached together by means, for example, of gluing
or embossing. Gluing is less preferred because it tends to result in a stiffer, less
soft product. Indeed it is preferred that no glue is used to attach the plies.
Embossing may be used to attach the plies together, for example, as disclosed in
EP-A-0 755 212, published on 29th January 1997. According to the present
invention the tissue has an unembossed wiping surface over a major part of the
surface area of the tissue. As used herein, this means that the tissue has one or
more unembossed regions and, optionally, one or more embossed regions, and
that the unembossed region is at least 50%, and as much as 100%, of the surface
area of the tissue. As used herein an embossed region is a region of the tissue
having a plurality of embossed points. Most commonly the embossed regions lie
close to the edge of the tissue (for example along two or four edges); and
embossed regions may also be used for decorative purposes (for example to
create a pattern or to spell out a logo or brand name). The unembossed region is
the continuous region between and/or around the embossed regions.
-
One or both surfaces of the tissue may, optionally, be further treated with a
lotion. The lotion may comprise softening/debonding agents, emollients,
immobilizing agents and mixtures thereof. Suitable softening/debonding agents
include quaternary ammonium compounds, polysiloxanes, and mixtures thereof.
Suitable emollients include propylene glycol, glycerine, triethylene glycol,
spermaceti or other waxes, petrolatum, fatty acids, fatty alcohols and fatty alcohol
ethers having from 12 to 28 carbon atoms in their fatty acid chain, and mixtures
thereof. Suitable immobilizing agents include polyhydroxy fatty acid esters,
polyhydroxy fatty acid amides and mixtures thereof. Other optional components
include perfumes, antibacterial actives, antiviral actives, disinfectants,
pharmaceutical actives, film formers, deodorants, opacifiers, astringents, solvents
and the like. Particular examples of lotion components include camphor, thymol
and menthol.
-
"Long fibres" as defined herein are considered to be of an average fibre
length of at least 2.0 mm. These long paper making fibres are typically softwood
fibres, preferably Northern Softwood Kraft.
-
"Short fibres" as defined herein are considered to have an average fibre
length of less than 2.0 mm, preferably from 0.2mm to 1.5mm. These short
papermaking fibres are typically hardwood fibres, preferably Eucalyptus fibres.
Alternatively low cost sources of short fibres such as sulfite fibres,
thermomechanical pulp, Chemi-ThermoMechanical Pulp (CTMP) fibres, recycled
fibres, and mixtures thereof can also be used.
Test Methods
-
The wet burst strength is measured using an electronic burst tester and the
following test conditions. The burst tester is a Thwing-Albert Burst Tester Cat. No.
177 equipped with a 2000 g load cell. The burst tester is supplied by Thwing-Albert
Instrument Company, Philadelphia, PA 19154, USA.
-
Take eight paper tissues and stack them in pairs of two. Using scissors, cut
the samples so that they are approximately 228 mm in the machine direction and
approximately 114 mm in the cross-machine direction, each two finished product
units thick.
-
First age the samples for one to two hours by attaching the sample stack
together with a small paper clip and "fan" the other end of the sample stack to
separate the sheets, this allows circulation of air between them. Suspend each
sample stack by a clamp in a 107°C (± 3°C) forced draft oven for 5 minutes (± 10
seconds). After the heating period, remove the sample stack from the oven and
cool for a minimum of three minutes before testing.
-
Take one sample strip, holding the sample by the narrow cross direction
edges, dipping the centre of the sample into a pan filled with about 25mm of
distilled water. Leave the sample in the water four (4.0 ± 0.5) seconds. Remove
and drain for three (3.0 ± 0.5) seconds holding the sample so the water runs off in
the cross direction. Proceed with the test immediately after the drain step. Place
the wet sample on the lower ring of the sample holding device with the outer
surface of the product facing up, so that the wet part of the sample completely
covers the open surface of the sample holding ring. If wrinkles are present,
discard the sample and repeat with a new sample. After the sample is properly in
place on the lower ring, turn the switch that lowers the upper ring. The sample to
be tested is now securely gripped in the sample holding unit. Start the burst test
immediately at this point by pressing the start button. The plunger will begin to
rise. At the point when the sample tears or ruptures, report the maximum reading.
The plunger will automatically reverse and return to its original starting position.
Repeat this procedure on three more samples for a total of four tests, i.e., 4
replicates. Report the results, as an average of the four replicates, to the nearest
gram.
-
Caliper of the multi-ply tissue paper, as used herein, is the thickness of the
paper when subjected to a compressive load of 14.7 g/m2. Preferably, caliper is
measured using a low load Thwing-Albert micrometer, Model 89-11, available
from the Thwing-Albert Instrument Company of Philadelphia, Pa.
Example
-
An aqueous slurry comprising 3% by weight of Nothern Softwood Kraft
(NSK) fibres was prepared in a conventional re-pulper. The NSK slurry was
refined gently and a 2% solution of the permanent wet strength resin (Kymene™
557H) was added to the NSK stock pipe at a rate of 1% by weight of the dry
fibres. The absorption of the permanent wet strength resin onto the NSK fibres is
enhanced by an in-line mixer. A 1% solution of the dry strength resin
(carboxymethyl cellulose) is added to the NSK stock before the fan pump at a rate
of 0.15% by weight of the dry fibres. The NSK slurry was diluted to about 0.2%
consistency at the fan pump.
-
A chemical softening composition was prepared comprising di-hard tallow
diethyl ester dimethyl quaternary ammonium chloride and polyoxyethylene glycol,
having an average molecular weight of 400 (PEG-400). The PEG-400 was heated
to about 66°C, and the quat was dissolved into the molten PEG-400 so that a
homogeneous mixture was formed.
-
An aqueous slurry comprising 3% by weight of eucalyptus fibres was
prepared in a conventional re-pulper. A 1% solution of the chemical softening
composition was added to the Eucalyptus stock pipe at a rate of 0.15% by weight
of the dry fibres. The Eucalyptus slurry was diluted to about 0.2% consistency at
the fan pump.
-
The two slurries were combined so that the ratio of NSK to eucalyptus
fibres was 70:30 and the resulting slurry was deposited, by means of a single
layer headbox onto a Fourdrinier wire to form an embryonic web. Dewatering
occured through the Fourdrinier wire and was assisted by a deflector and vacuum
boxes. The Fourdrinier wire was a 5-shed, satin weave configuration having 3.3
machine-direction and 3.0 cross-machine direction monofilaments per millimeter
respectively.
-
The embryonic web was transferred from the Fourdrinier wire, at a fibre
consistency of about 20% at the point of transfer, to a photo-polymer fabric having
0.87 Linear Idaho cells per square millimeter (562 cells per square inch), 40%
knuckle area, and 0.2 mm of photo-polymer depth. Further dewatering was
accomplished by vacuum assisted drainage until the web has a fiber consistency
of about 28%. The patterned web is predried by air blow-through to a fibre
consistency of about 65% by weight. The web was then adhered to the surface of
a Yankee dryer with a sprayed creping adhesive comprising 0.25% aqueous
solution of Polyvinyl Alcohol (PVA). The fibre consistency was increased to an
estimated 96% before dry creping the web with a doctor blade. The doctor blade
had a bevel angle of about 25° and is positioned with respect to the Yankee dryer
to provide an impact angle of about 81°. The Yankee dryer was operated at about
4 m/s and the dried paper was formed into a roll at a reel.
-
The dry web comprised Kymene™ at a level of 0.7%, carboxymethyl
cellulose at a level of 0.11%, chemical softening composition at a level of 0.05%,
all by weight of dry fibre.
-
The web is converted into a two ply tissue paper product, having overall
dimension of 210 mm square. The tissue paper product was folded and packaged
without embossing.
-
In a second example the same two-ply tissue paper product was subjected
to an embossing step before folding. The margin of the tissue paper product,
extending about 15mm in from the edge was embossed following the process
described in WO95/27429, published on 19th October 1995. The major part of the
surface area of the tissue paper product (i.e. all of the surface area within the
15mm margin) was unembossed.
-
In a third example the product of the previous example was taken and
decorated by embossing the brand name over a small area of the previously
unembossed area. Alternatively four decorative leaf patterns where embossed in
the previously unembossed area. Each decorative pattern being about 30mm
square.
-
The process of the previous examples was repeated and the paper was
calandared either at the reel; or during combining of the plies; or during
converting; or calandared two or three times by combination of these steps.
-
The two-ply tissue paper product of these examples has a caliper of
0.45mm, an average basis weight of 50g/m2 and a wet burst strength of 250 g.