U.S. patent application number 11/538219 was filed with the patent office on 2007-05-24 for paper product and method of making.
This patent application is currently assigned to Weyerhaeuser Co.. Invention is credited to David W. Park.
Application Number | 20070113995 11/538219 |
Document ID | / |
Family ID | 38052337 |
Filed Date | 2007-05-24 |
United States Patent
Application |
20070113995 |
Kind Code |
A1 |
Park; David W. |
May 24, 2007 |
Paper product and method of making
Abstract
Paper having improved curl and cockle properties for water
based, ink jet printing applications and a method of producing
them. The paper contains at least 50 pounds per ton of a mono
alkali metal salt of citric acid. The paper has a maximum Cockle
Value of 0.25. It also contain a hexasulfonated optical brightener.
The paper is also water fast and can be preprinted with offset
printing inks. A method for making the paper.
Inventors: |
Park; David W.; (Puyallup,
WA) |
Correspondence
Address: |
WEYERHAEUSER COMPANY;INTELLECTUAL PROPERTY DEPT., CH 1J27
P.O. BOX 9777
FEDERAL WAY
WA
98063
US
|
Assignee: |
Weyerhaeuser Co.
Federal Way
WA
|
Family ID: |
38052337 |
Appl. No.: |
11/538219 |
Filed: |
October 3, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10743846 |
Dec 22, 2003 |
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11538219 |
Oct 3, 2006 |
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10744856 |
Dec 22, 2003 |
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11538219 |
Oct 3, 2006 |
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Current U.S.
Class: |
162/158 ;
162/162; 162/175 |
Current CPC
Class: |
D21H 21/16 20130101;
D21H 21/30 20130101; D21H 17/15 20130101 |
Class at
Publication: |
162/158 ;
162/162; 162/175 |
International
Class: |
D21H 21/30 20060101
D21H021/30; D21H 23/00 20060101 D21H023/00 |
Claims
1. Uncoated printing paper comprising cellulosic fibers,
hexasulfonate optical brightener and at least 50 pounds per ton of
said papermaking fibers of a mono alkali metal salt of citric
acid.
2. The printing paper of claim 1 in which the mono alkali metal
salt of citric acid is a mono sodium citrate.
3. The printing paper of claim 1 in which the mono alkali metal
salt of citric acid is a mono potassium citrate.
4. The printing paper of claim 1 in which the salt of citric acid
is present in the range of 50 to 300 pounds per ton of paper.
5. The printing paper of claim 4 in which the mono alkali metal
salt of citric acid is a mono sodium citrate.
6. The printing paper of claim 4 in which the mono alkali metal
salt of citric acid is a mono potassium citrate.
7. The printing paper of claim 1 in which the mono alkali metal
salt of citric acid is present in the range of 75 to 250 pounds per
ton of paper.
8. The printing paper of claim 7 in which the mono alkali metal
salt of citric acid is a mono sodium citrate.
9. The printing paper of claim 7 in which the mono alkali metal
salt of citric acid is a mono potassium citrate.
10. The printing paper of claim 1 in which said paper has a maximum
Cockle Value of 0.25.
11. The printing paper of claim 1 in which said paper is water
fast.
12. The printing paper of claim 1 further comprising starch,
ethylated starch or polyvinyl alcohol.
13. Uncoated printing paper comprising cellulosic fibers, optical
brightener and at least 50 pounds per ton of said papermaking
fibers of a mono potassium salt of citric acid, the paper having a
maximum Cockle Value of 0.25
Description
RELATION TO OTHER APPLICATIONS
[0001] This application is a continuation-in-part of application
Ser. No. 10/743,846 and 10/744,856, both filed Dec. 22, 2003.
FIELD
[0002] The present invention is directed to printing paper, and the
method of making this paper.
BACKGROUND
[0003] Hardwood and softwood wood pulp fibers are used in the
manufacture of printing paper and newsprint. These fibers are
produced in a chemical pulping process, either sulfate or sulfite,
or in a mechanical pulping process. Mechanical processes would
include thermomechanical and chemithermomechanical. To form the
printing paper or newsprint, these hardwood or softwood pulp fibers
and wet end chemicals are mixed with water in the headbox of the
paper machine to form a suspension of fibers and chemicals. The wet
end chemicals may include fillers such as calcium carbonate and
clay. The suspension of fibers and chemicals flow from the headbox
onto a wire. The water is removed from the fibers and chemicals by
both gravity and vacuum to form a wet web of pulp fibers into which
the chemicals are incorporated. The chemicals are throughout the
sheet. The sheet may be pressed and dried to remove more water.
[0004] Starch, optical brightener additives and surface size may be
placed on surface of the sheet in a surface sizing step at the size
press Some of the materials may enter into the web if the pressure
of the nip at the press is great enough.
[0005] Thereafter the web of fiber, wet end chemicals and other
materials is dried by heat, calendered and rolled into rolls. The
resulting product is referred to as an uncoated or lightly coated
paper sheet or web.
[0006] The uncoated sheet may be coated in another application of
one or more coating layers placed on the sheet in an off-line
coating operation. The uncoated sheet passes through a coating
station and a second drying station. It may pass through a second
calendering operation. The resulting product is referred to as a
coated paper sheet or web.
[0007] Uncoated or coated printing paper has a basis weight of from
16 to 180 pounds per 3300 square feet.
[0008] The application of high speed, variable printing is
experiencing tremendous growth in the printing industry, displacing
conventional offset printing for many applications. A digital
printing technology such as web-fed ink jet printing presents new
and different challenges for the paper maker as the optimum surface
physics and chemistry of paper for these printers are very
different than those required for conventional offset inks.
[0009] High speed, ink jet printing is exceptionally challenging
because it employs aqueous inks and a great deal of water is placed
on the paper surface during the printing process. In the process,
these water based inks may be applied at high coverage at paper
speeds of 500-1,000 ft. per minute. It is difficult to completely
dry the paper before the paper leaves the printer. If uncoated
paper is used, the water from the ink penetrates the sheet and
disrupts the bonding between the paper fibers. This creates a
deformation of the paper surface, which results in unacceptable
curling, cockling, or puckering of the printed paper.
[0010] Standard desk top ink jet printers are increasing in speed
and some of the same challenges are found when printing with these
printers because of the water placed on the paper and the
difficulty of completely drying the paper before it leaves the
printer.
[0011] Wide printers have similar challenges because of the amount
of ink and water placed on the paper.
[0012] Because of these factors, special papers are used when the
print job requires high levels of ink coverage. These special
papers are coated with water-absorbent silica or swellable gel
materials such as polyvinyl pyrilodone, or combinations of these
materials. Typically, these materials are applied by an off-line
coating operation. The price of these materials and the off-line
application significantly increases the cost of paper for these
applications.
[0013] The optical density of the printed image is also of primary
concern for many print jobs as high levels of ink are required to
provide vivid, robust colors. This is known as high optical
density. Uncoated papers are limited in the amount of ink they can
tolerate because of their tendency to curl and cockle. Thus more
expensive coated papers are generally required when high optical
densities are needed.
[0014] The inks are anionic. Highly cationic chemicals are usually
added to the paper in order to precipitate the ink and cause the
ink to be water fast.
[0015] Hexasulfonate optical brighteners are used to enhance the
brightness of the paper and make it appear better visually. These
optical brighteners are also anionic and the highly cationic
chemicals will react with the optical brightener also. This reduces
the ability of the brightener to brighten the paper.
[0016] Many of these papers are preprinted in an offset press
before being printed in an ink jet system. An example would be
placing a watermark or logo on the paper. These inks are also
anionic and a highly cationic chemical reacts with these inks also.
This creates a problem in cleanup of the offset presses.
[0017] It is desirable to find a material that may be placed on an
uncoated paper in a size press operation, that will reduce the
cockle in paper so that it may be used in high speed ink jet
printing, make the ink water fast so that it does not bleed when
sprayed with water, and may allow the paper to be pre-printed in an
offset press.
SUMMARY
[0018] The present invention is directed to an uncoated paper
usable with ink having a water content and which has a maximum
Cockle Value of 0.25. The Cockle Value is used to determine the
amount of cockle or water induced curl in the paper. An embodiment
of the invention is an uncoated paper having a paper basis weight
of 16 to 180 pounds per 3300 square feet and a maximum Cockle Value
of 0.25.
[0019] An embodiment of the invention is a paper that has been
treated with a at least 50 pounds per ton of paper with a mono
alkali metal salt of citric acid, a mono sodium or potassium
citrate, that is capable of being added at the size press, blade
coater or by a spray before the heated drying section. Another
embodiment is a paper that has been treated with at least 75 pounds
of the citrate per ton of paper. Another embodiment is a printing
paper that has been treated with up to 250 pounds of the citrate
per ton of paper. Another embodiment is a paper that has been
treated with up to 300 pounds of the citrate per ton of paper.
[0020] It has been found that a mono alkali metal salt of citric
acid such as mono sodium or potassium citrate has enough
cationicity to react with the ink jet inks to cause them to be
water fast. The mono alkali metal salt of citric acid also causes
the paper to have a Cockle Value of 0.25 or less. The mono alkali
metal salt of citric acid does not have enough cationicity to react
with an optical brightener, including hexasulfonate optical
brighters, or to react with offset printing inks. It provides a
paper that is water fast,that has a reduced cockle and can be
preprinted in an offset press.
[0021] In another embodiment of the invention the paper also
contains a binder such as starch, ethylated starch, latex,
polyvinyl alcohol, styrene acrylic acid or an ester in addition to
the mono alkali metal salt of citric acid and the low viscosity of
the additive can be maintained.
[0022] In another embodiment of the invention the paper also
contains a florescent whitening agent, an optical brightener or a
hexasulfonated optical brightener in addition to the mono alkali
metal salt of citric acid.
BRIEF SUMMARY OF THE DRAWINGS
[0023] FIG. 1 is a drawing of the work station for carrying out the
second side cockle test method.
[0024] FIG. 2 is a schematic diagram of a paper machine.
DETAILED DESCRIPTION
[0025] One embodiment of the present invention is directed to an
uncoated or lightly coated paper having a mono alkali metal salt of
citric acid and which may be used for printing on ink jet printers
and which has a maximum Cockle Value of 0.25 after such printing.
Another embodiment is directed to an uncoated or lightly coated
printing paper having a mono alkali metal salt of citric acid and
which is water fast.
[0026] It has been found that a mono alkali metal salt of citric
acid such as mono sodium or potassium citrate has enough
cationicity to react with the ink jet inks to cause them to be
water fast. The mono alkali metal salt of citric acid also causes
the paper to have a Cockle Value of 0.25 or less. The mono alkali
metal salt of citric acid does not have enough cationicity to react
with an optical brightener, including hexasulfonate optical
brighters, or to react with offset printing inks. It provides a
paper that is water fast, that has a reduced cockle and can be
preprinted in an offset
[0027] In another embodiment of the invention the paper also
contains a binder such as starch, ethylated starch, latex,
polyvinyl alcohol, styrene acrylic acid or an ester in addition to
the mono alkali metal salt of citric acid and the low viscosity of
the additive can be maintained.
[0028] In another embodiment of the invention the paper also
contains a florescent whitening agent, an optical brightener or a
hexasulfonated optical brightener in addition to the mono alkali
metal salt of citric acid.
[0029] A quantitative test has been developed to determine the curl
and cockle of paper. It replaces the subjective test of viewing the
paper to determine whether there was curl and cockle and the amount
of curl and cockle. This prior subjective test also determined
whether a sheet of paper had sufficient treatment. The quantitative
test is the second side cockle test method.
[0030] The second side cockle test method is used to evaluate the
amount of cockle that an inkjet print, at an ink application level
of 5.9 grams/square meter, produces in the unprinted or second side
of a paper printed with a block print. The present test used a
Scitex Test Cockle Form Print. The unprinted side of the inkjet
print is illuminated using low angle (15.degree.) lighting. A
digital image is made of the cockled area on the unprinted side
associated with a 3.5 by 3.5 inch half-tone printed square on the
printed side of the sample. The image is then evaluated to
determine the amount of second side cockle.
[0031] The apparatus used for the second side cockle test method is
shown in to FIG. 1. It includes a test platform 10, a Kodak.RTM.
megaplus 8-bit digital camera 12, and a Dedolight.RTM. light 14.
The camera 12 is mounted above surface 16 of the test platform 10
and at 90.degree. to the surface 16 of the test platform 10. The
camera is aimed directly at the center of the surface 16 of the
platform. The Dedolight light 14 is mounted at an angle of
15.degree. to the surface 16 and also aimed at the center of the
surface 16. Mathworks, Inc. Matlab.RTM. computer software is used
to analyze the images.
[0032] The samples of paper to be tested are printed on one side
with a Scitex Test Cockle Form using an inkjet printer and inkjet
ink. In the following tests a Hewlett Packard ink jet printer
HP560C was used. The ink used was Scitex Ink 2002 and the ink
application level was 5.9 g/square meter. The ink should be a water
based ink. The paper was handled carefully so as not to crease or
wrinkle the paper because creases or wrinkles would be analyzed as
cockle.
[0033] The settings of the camera 12 were adjusted to a pixel
resolution of 100 microns/pixel and an f-stop of F8. The camera
control was on Fixed and the image centering was at 127. The
Dedolight light 14 was adjusted for uniform low angle lighting. All
lighting was from the Delolight light 14. Other room lights were
turned off.
[0034] The paper sample 18 was placed on the surface 16 of the test
stand 10 with the unprinted side of the paper turned to the camera
and facing up. The 3.5 by 3.5 inch cockle area was centered in the
camera field of view with the light aimed at the center of the
cockle area. The camera's exposure was adjusted until the average
image pixel value was 127. The image was collected and saved to a
disk.
[0035] This process was repeated for each sample.
[0036] The images were analyzed using the Mathworks, Inc.
Matlab.RTM. computer software. Version 6, release 13 was used. The
image is read into the program and smoothed with a 5.times.5 median
filter to remove high frequency noise. The mean, standard deviation
and coefficient of variation were calculated for each row and
column. The larger of the maximum row coefficient of variability
and maximum column coefficient of variability is taken as the
sample Cockle Value. The program is evaluating the differences
between the light and dark areas of the image and determining the
variability.
[0037] Cockle Value means the cockle value determined by this
test.
[0038] An embodiment is an uncoated paper that has been treated
with a hexasulfonated optical brightener and at least 50 pounds per
ton of paper with a mono alkali metal salt of citric acid, a mono
sodium or mono potassium citrate. The maximum Cockle Value of the
treated printing paper is 0.25. A ton is defined here as 2000
pounds. The salt of citric acid would be applied in a solution at a
concentration of 20-50% of the total weight of the solution at room
temperature or at temperatures of 50.degree. C. or less.
[0039] In another embodiment of the invention at least 75 pounds of
citrate per ton of paper is used. In another embodiment of the
invention as much as 300 pounds of citrate per ton of paper may be
used. In another embodiment as much as 250 pounds of citrate per
ton of paper may be used.
[0040] The citrate is applied at the size press or the blade
coater. It may be applied using a puddle, gate roll or metered size
press, or a knife or blade coater. In one embodiment the citrate
may be applied in a solution containing at least 20% by weight of
material. In another embodiment the citrate may be applied in a
solution containing 20 to 50% by weight of the material.
[0041] 60 gm./m.sup.2 unsized paper was used as the base paper for
the sheets in this example.
[0042] The percentages in this example are weight percentages.
[0043] A control sample of paper was coated in a laboratory size
press with ethylated starch at 12% concentration. Both side of the
paper were coated to a coat weight of 40 pounds of starch per ton
of paper per side. This is typical of most uncoated paper grades
(Formula I).
[0044] Each of the sheets were then dried and conditioned at 50%
R.H.
[0045] Two commercial paper products A and B, printed with a HP 560
printer using Scitex high speed ink jet ink were digitally recorded
with a SONY Mavica digital camera, under low angle light.
[0046] A sample of paper was treated in a lab size press with a
solution containing a concentration a 25% of the monosodium salt of
citric acid (monosodium citrate), heated to 50 degrees C. Both side
of the paper were coated to a coat weight of 37.5 pounds of
material per ton of paper per side (Formula II).
[0047] The sheets were then dried and conditioned at 50% R.H.
[0048] One set of the sheets was printed using an HP 560 printer
and Scitex High Speed ink jet ink. The image was a 3''.times.3''
square, printed at 60% density, using Corel Draw, Version 10.
[0049] The printed sheets were then placed in a darkroom, face down
and viewed under a LANDSCO triple-bulb, low angle light. The degree
of curl and cockle were then visually estimated. The sheets were
judged against the starch control. The results are shown in Table
1. 100% is the base case for a starch control.
[0050] The sheets were also tested for water fastness via
submersion in water for 60 seconds and the ink dye was completely
immobilized by the salt. The results are also shown in Table 1.
TABLE-US-00001 TABLE 1 Degree of Curl/ Cockle: Water Formula Scitex
ink Fastness Formula I 100% Poor Formula II .sup. <5%
Excellent
[0051] Water fast means the ability of ink to remain intact when
exposed to water or moisture. Water fast inks do not bleed. Water
based inks must be treated to be water fast. The inks are anionic.
It has been found that the mono alkali metal salt of citric acid
has enough cationicity to react with the anionic ink and make it
water fast. It does not, however, have enough cationicity to react
with offset printing inks.
[0052] Water fastness is typically obtained with a
nitrogen-containing organic compound of a cationic nature and
functions by precipitating the dye in the ink, rendering it
immobile, when exposed to moisture after printing. Unfortunately,
these types of materials are incompatible with anionic fluorescent
whitening agents, optical brighteners, or hexasulfonated optical
brighteners which are typically applied at the size press to
brighten paper. As such, these types of additives reduce the
overall paper brightness, often times to levels below customer
acceptance. Again, the mono alkali metal salts of citric acid, such
as sodium and potassium citrate, do not have enough cationicity to
react with the optical brighteners, the flourescent whitening
agents or the hexasulfonated optical brighteners.
[0053] The pH of the mono alkali metal salts of citric acid is acid
enough to react with the ink jet inks but not acid enough to react
with the optical brighteners or the offset printing inks.
[0054] Monosodium citrate maintains the brightness of the paper
with fluorescent whitening agents, optical brighteners or
hexasulfonated optical brighteners while providing water fastness
and a low degree of curl and cockle.
[0055] Water based inks, those found in ink jet printing, are
anionic. Water based inks will bleed when placed in water. The inks
must be fixed by precipitating the dye in the ink. This is usually
done with highly cationic fixatives. The mono alkali metal salt of
citric acid is acid enough to react with the ink jet inks but not
acid enough to react with the optical brighteners or offset
printing inks.
[0056] Samples were also evaluated using the second side cockle
test method.
[0057] The percentage shown are weight percentages.
[0058] 60 gm./m2 unsized paper was used as the base paper for the
sheets in this example.
[0059] The Formula 1 control samples were also used as the control
samples.
[0060] A second sample of paper was treated in a lab size press
with a solution containing a concentration a 25% of the monosodium
salt of citric acid, heated to 50 degrees C. This percentage is a
weight percentage. Both side of the paper were coated to a net coat
weight of 37.5 pounds of material per ton of paper per side. The
amount of citrate was 37.5 pounds per ton of paper per side.
[0061] Two commercial papers were added to the study.
[0062] The samples were evaluated both visually and using the
second side cockle test method. The image evaluation test
correlated well with the visual observation. The results are given
in Table 2. TABLE-US-00002 TABLE 2 Formula Subjective Value Cockle
Value 25% mono sodium Acceptable 0.2296 salt of citric acid 12%
Starch control Unacceptable 0.2658 First Choice .TM. Unacceptable
0.2851 CI-2000 .TM. Unacceptable 0.3211
[0063] It was determined that paper sheets having Cockle values of
0.25 or less were acceptable.
[0064] FIG. 2 is a schematic drawing of a paper machine. Wood pulp
fiber furnish and wet end chemicals are mixed with water in a
headbox 20 to form a slurry. The slurry exits the headbox through a
slice 22 onto a wire 24. The water in the slurry drains from the
wire. A vacuum chest 26 is also used to draw water from the slurry
to form a wet paper web. The web is carried through press rolls 28
and a drier 30 that remove additional water.
[0065] Additional size press chemicals or materials are placed on
the wet paper web at the size press 32. The size press may be a
horizontal type with the rolls horizontally aligned, a vertical
type with the rolls vertically aligned. The materials may be placed
on the web from the rolls or from a puddle between the rolls. The
web may, in some instances, be coated with material by the spraying
apparatus 34. The materials described in the various embodiments in
the present application would also be applied at the size press 32
or the spraying apparatus 34.
[0066] The paper web then passes through a drying section 36. The
drying is usually done by steam heated drier cans through which the
paper web is threaded. The paper is then calendered by calender
rolls 38 and rolled into paper rolls at the winder 40. The
resulting product is known as uncoated paper.
[0067] This is the product of the present invention. Additional
expensive off-machine coatings would not be required to provide a
paper that has a maximum Cockle Value of 0.25.
[0068] Those skilled in the art will note that various changes may
be made in the embodiments described herein without departing from
the spirit and scope of the present invention.
* * * * *