U.S. patent application number 10/221772 was filed with the patent office on 2003-02-27 for electrophotographic paper and method for manufacturing thereof.
Invention is credited to Niskanen, Kaarlo, Simula, Sami, Tamminen, Tarja.
Application Number | 20030037893 10/221772 |
Document ID | / |
Family ID | 26160983 |
Filed Date | 2003-02-27 |
United States Patent
Application |
20030037893 |
Kind Code |
A1 |
Simula, Sami ; et
al. |
February 27, 2003 |
Electrophotographic paper and method for manufacturing thereof
Abstract
Electrophotographic paper and a method for its production,
according to which method a cellulose pulp is slushed, a filler is
mixed with the slush, and the slush is formed in the paper machine
into a web, which is dried. According to the invention, bivalent or
trivalent cations in an amount of at least 0.1% of the dry weight
of the pulp are attached to the acid groups of the cellulose pulp
before the forming of the web. Thereby a paper web is obtained the
surface charge of which is at least approximately 600 V when
relative humidity is 40%. The surface charge of the paper according
to the invention remains stable within a wide humidity range. For
this reason, when the paper according to the invention is used, the
need for drying the paper in the printing press is significantly
reduced.
Inventors: |
Simula, Sami; (Espoo,
FI) ; Tamminen, Tarja; (Espoo, FI) ; Niskanen,
Kaarlo; (Porvoo, FI) |
Correspondence
Address: |
KUBOVCIK & KUBOVCIK
SUITE 710
900 17TH STREET NW
WASHINGTON
DC
20006
|
Family ID: |
26160983 |
Appl. No.: |
10/221772 |
Filed: |
September 17, 2002 |
PCT Filed: |
April 4, 2001 |
PCT NO: |
PCT/FI01/00329 |
Current U.S.
Class: |
162/158 ;
162/135; 162/175; 162/181.2; 162/181.8; 162/189 |
Current CPC
Class: |
G03G 7/0066 20130101;
D21H 11/04 20130101; D21H 11/22 20130101; G03G 7/0073 20130101 |
Class at
Publication: |
162/158 ;
162/135; 162/181.2; 162/181.8; 162/175; 162/189 |
International
Class: |
D21H 017/00; D21H
017/28; D21H 017/66 |
Claims
1. An electrophotographic paper which comprises a fibrous paper web
formed from a cellulose pulp, characterized in that it contains, in
an amount of at least 0.1% of the dry weight of the pulp, bivalent
or trivalent ions bonded to the acid groups of the cellulose pulp,
in which case the surface charge of the paper web is at least
approximately 600 V when relative humidity is 40%.
2. The paper according to claim 1, characterized in that the ions
bonded to the acid groups are calcium, magnesium or aluminum ions
or a mixture thereof.
3. The paper according to claim 1 or 2, characterized in that the
amount of the calcium, magnesium or aluminum bonded to the acid
groups is approximately 0.15-0.5% of the dry weight of the
fibers.
4. The paper according to any of claims 1-3, characterized in that
the surface charge of the paper is at least 610 V when relative
humidity is 40%.
5. The paper according to any of the preceding claims, which paper
contains calcium as the counter-ion for acid groups, characterized
in that the change in the surface charge of the paper is at maximum
.+-.25% when relative humidity is 20-50%.
6. The paper according to any of the preceding claims,
characterized in that the cellulose pulp is a chemical hardwood
pulp that contains hemicellulose compounds over 10% of the dry
weight of the pulp.
7. The paper according to claim 6, characterized in that the
cellulose pulp is birch sulfate pulp.
8. The paper according to any of the preceding claims,
characterized in that the bivalent or trivalent ions are mainly
bonded to methylglucuronic acid or hexenuronic acid groups.
9. The paper according to any of the preceding claims,
characterized in that the cellulose pulp contains, in an amount of
approximately 0.1-20% of the weight of the pulp, a cellulose
derivative having a bivalent or trivalent counter-ion.
10. The paper according to claim 9, characterized in that the
cellulose pulp contains a calcium-form carboxymethyl cellulose.
11. The paper according to any of the preceding claims,
characterized in that it contains a filler 1-40%, preferably
approximately 5-30%, of the weight of the cellulose pulp.
12. The paper according to claim 11, characterized in that the
filler is kaolin or precipitated or ground calcium carbonate.
13. The paper according to any of the preceding claims,
characterized in that on the surface of the paper web there is a
coating pigment layer.
14. The paper according to claim 13, characterized in that the
pigment is calcium carbonate, kaolin, gypsum, talc, chalk and/or a
mixture thereof.
15. The paper according to any of the preceding claims,
characterized in that the grammage of the paper is 50-180 g/m.sup.2
and it contains coating approximately 0-30 g/m.sup.2 of
surface.
16. A method for the production of an electrophotographic paper
from a cellulose pulp, according to which process the cellulose
pulp is slushed, a filler is mixed with the slush, and the slush is
formed in the paper machine into a web, which is dried,
characterized in that bivalent or trivalent cations in an amount of
at least 0.1% of the dry weight of the pulp are attached to the
acid groups of the cellulose pulp before the forming of the
web.
17. The method according to claim 16, characterized in that the
cellulose pulp used is a chemical pulp made from hardwood.
18. The method according to claim 16 or 17, characterized in that
the cellulose pulp used is birch sulfate pulp.
19. The method according to any of claims 16-18, characterized in
that the bivalent or trivalent cations are attached to the acid
groups of the cellulose pulp by ion exchange.
20. The method according to claim 19, characterized in that the
bivalent or trivalent cations are attached to the acid groups by
contacting the cellulose pulp with an aqueous solution of a salt
comprising bivalent or trivalent cations, in conditions wherein the
salt is at least in part in a dissociated form.
21. The method according to claim 20, characterized in that calcium
ions are attached to the acid groups by adding to an aqueous
suspension of the cellulose pulp a calcium salt so that the
hardness of the water rises to 10 dH.degree..
22. The method according to any of claims 16-21, characterized in
that the bivalent or trivalent ions are attached to the acid groups
of the cellulose pulp before retention chemicals, cationic polymers
and/or fillers are added to the cellulose pulp.
23. The method according to any of claims 16-22, characterized in
that the cellulose pulp used is a recycled chemical pulp.
24. The method according to any of claims 16-23, characterized in
that a cellulose derivative having a bivalent or trivalent
counter-ion is applied to the paper web in an amount of 0.1-20% of
the weight of the cellulose pulp.
25. The method according to claim 24, characterized in that the
cellulose derivative comprises a calcium-form carboxymethyl
cellulose.
26. The method according to any of claims 16-25, characterized in
that a filler is added to the slush in an amount of approximately
1-40%, preferably approximately 5-30%, of the weight of the
cellulose pulp.
27. The method according to claim 26, characterized in that the
filler used is kaolin or precipitated or ground calcium
carbonate.
28. The use, as the print medium for printed matter to be produced
by electrophotographic, a paper web made up of a cellulose pulp
that contains, in an amount of at least 0.1% of the dry weight of
the fibers of the cellulose pulp, bivalent or trivalent ions bonded
to the acid groups of the cellulose pulp, in which case the surface
charge of the paper web is at least approximately 600 V when
relative humidity is 40%.
29. The use according to claim 28, characterized in that the
bivalent or trivalent ions are calcium, magnesium or aluminum, or a
mixture thereof.
Description
[0001] The present invention relates to the production of paper and
board. The invention relates in particular to the
electrophotographic paper according to the preamble of claim 1,
which comprises a fibrous paper web formed from cellulose pulp.
[0002] The invention also relates to a method according to claim 16
for the production of electrophotographic paper from cellulose
pulp. According to such a method, the cellulose pulp is slushed, a
filler is added to the slush, and the slush is formed in a paper
machine into a web, which is dried.
[0003] The invention also relates to the use according to claim
28.
[0004] The importance of the electrophotographic technique in the
production of printed matter is increasing. Said technique is
particularly well suited for the production of small quantities of
printed matter to a customer's order. This is possible because the
image to be printed is formed on the photoconductor drum separately
for each revolution of the drum. Successive pages may have
completely different contents, for which reason it is possible, for
example, to print a book to completion in such a manner that the
pages arrive on the delivery table of the printing machine in the
correct order of pages. Electrophotographic presses and printers
are available for both black-and-white printing and four-color
printing.
[0005] Electrophotography has long been used as a technique in
office copiers and laser printers. The papers used have in this
case been uncoated papers, with which there has been obtained a
sufficiently high image quality for black-and-white material
containing text. For example, in printed advertising material there
are a large number of four-color images, and therefore the quality
of color images has become an important issue. This, as well as the
increased printing speed, increases the demands set on paper.
[0006] In color printing based on electrophotography, the toner
particles are transferred from a photoconductor drum to the paper
by means of an electric field, negatively charged toner particles
transferring onto positively charged paper. The charge is created
on the paper surface by means of a first corona. This corona is
installed in the machine so that the paper runs between the corona
wire and the photoconductor drum.
[0007] The conductivity of paper is a factor important in terms of
the success of this printing process. Thus, if the paper is too
conductive, the charge discharges from the paper and the toner
particles may return to the surface of the photoconductor drum. On
the other hand, if the paper is too insulating, a sufficiently
strong electric charge does not develop on the paper surface.
Furthermore, if the paper is too insulating, static electricity may
cause paper sheets to cling to the rolls of the printing machine or
cause electric breakdown. To prevent this, the color-printing press
may have, after the toner transfer unit, a second corona, which
discharges the surface charge of the paper.
[0008] The conductivity of paper varies strongly according to the
moisture content of the paper. As is shown in accompanying FIG. 1,
the surface charge of commercial electrophotographic papers is in
general at its highest (approximately 600-800 V) when relative
humidity is approximately 10-20%. When humidity increases to 40%,
the surface charge is typically halved or drops to as low as
one-tenth of its highest value. Advanced printing machines
therefore have pretreatment units wherein the paper is heated in
order to adjust the charging capacity of the paper to the correct
level. Heating dries the paper, from which there follow warping and
waviness. Hot paper is also brittle, which increases breaks in
web-fed printing. Preheating of paper is thus not problem-free.
[0009] In advanced presses it is often also possible to adjust the
corona wire control currents. However, the finding of the correct
settings takes a great deal of time, which reduces the printing
press time usable for printing. The finding of the correct settings
also causes extra materials costs as toners and papers are wasted.
On the other hand, there are on the market also electrophotographic
presses in which it is not possible significantly to adjust the
parameters of copying.
[0010] It is an object of the present invention to eliminate the
disadvantages associated with the current technology and to provide
an entirely novel paper or similar fiber product suitable for use
as a print medium for the electrophotographic technique.
[0011] The invention is based on the unexpected observation that by
attaching as a counter-ion for the carboxylic acid groups in the
paper pulp a bivalent or trivalent metal ion in the place of
univalent ions it is possible to adjust the resistivity and
dielectric properties of the paper into a range especially
advantageous for electrophotographic printing. The paper of a novel
type according to the invention contains bivalent or trivalent
cations in an amount of at least 0.1% of the dry weight of the
pulp. Said cation is attached to the acid groups of the cellulose
pulp before the forming of the web. The surface charge of the paper
web produced is at least approximately 600 V when relative humidity
is 40%.
[0012] With the help of the invention there is provided a method
for producing printed matter, according to which method the desired
image is printed on the surface of a paper or board sheet or web by
the electrophotographic technique.
[0013] More specifically, the electrophotographic paper according
to the invention is mainly characterized by what is stated in the
characterizing part of claim 1.
[0014] The method according to the invention for the manufacture of
electrophotography paper is, for its part, characterized by what is
stated in the characterizing part of claim 16.
[0015] The use according to the invention is characterized by what
is stated in claim 28.
[0016] The invention provides considerable advantages. Thus, by
binding calcium ions or corresponding multivalent metal ions to
carboxylic acid groups as counter-ions for said carboxylic acid
groups, a change of two orders can be obtained in the resistivity
of the paper without a need to change the structure or the paper
(e.g. its coating). It is particularly significant that the surface
charge of the paper according to the invention remains quite stable
within a wide humidity range. When calcium is used as the
counter-ion, the change in the surface charge is less than 25% when
relative humidity varies between 20 and 50%. For this reason, when
the paper according to the invention is used the need for drying
the paper in a press is significantly reduced.
[0017] The invention will be discussed below in greater detail with
the help of a detailed description and a number of working
examples.
[0018] FIG. 1 shows graphically the surface charges of 5 commercial
copier papers and a PET film, as a function of relative humidity;
and
[0019] FIG. 2 is a corresponding graphic representation of the
change in the surface charge of papers made of two different birch
pulps, as a function of relative humidity, the black squares
indicating the surface charge of the reference paper containing
pulp in the hydrogen form and the circles indicating the surface
charge of the paper according to the invention.
[0020] According to the invention, the electrophotographic paper
used is a web of material formed from a cellulose pulp wherein at
least some of the acid groups contain as the counter-ion a bivalent
or trivalent metal ion in order that the surface charge of the
paper can be adjusted to a value higher than 600 V when relative
humidity is 40%.
[0021] By "web of material" is meant in the present invention a
paper or board or a corresponding cellulose-containing material
derived from a lignocellulose-containing raw material.
"Electrophotographic paper" means the above-mentioned web of
material, which contains a filler and which is possibly coated with
a pigment layer.
[0022] The wood material in the web of material may be derived from
softwood or hardwood. Examples of suitable Finnish species of wood
include pine, spruce, birch, aspen and alder. Other species of wood
usable as the raw material include eucalyptus, acacia, maple, and
mixed tropical hardwood. The invention can also be applied to
annual plants, such as grain straw, reed canary grass, reed, and
bagasse.
[0023] The web of material may be wood-containing or wood-free, and
it is produced in particular from chemical pulp. The pulp may be
bleached or unbleached. The material may also contain recycled
fibers, in particular recycled office paper or printing paper. The
proportion of recycled fibers may be 0-100%.
[0024] It is particularly preferred to use a chemical hardwood pulp
having a high hemicellulose content, such as birch pulp. Such a
cellulose pulp contains hemicellulose compounds over 10% of the dry
weight of the pulp. The chemical pulp may be produced by cooking
processes known per se, in alkaline, acid or neutral conditions.
Examples to be mentioned include sulfate, sulfite, neutral sulfite
and various organosolv processes. The sulfate process (as well as
its modified forms, such as extended cooking processes) is regarded
as particularly advantageous.
[0025] It is known that chemical cellulose pulp contains carboxylic
acid groups, primarily methyl glucuronic groups and, according to
the conditions of the cooking and bleaching, also hexenuronic acid
groups. According to the invention, the counter-ions of at least
these acid groups are replaced by bivalent or trivalent metal ions.
Preferably, in papers according to the invention, bivalent or
trivalent metal ions constitute most, i.e. at least 50%, preferably
at least 70%, especially preferably at least 80%, of the
counter-ions of the acid groups.
[0026] The bivalent or trivalent cations are attached to the acid
groups of the cellulose pulp by ion exchange. Preferably this is
carried out by contacting the cellulose pulp with an aqueous
solution of a salt comprising bivalent or trivalent cations, in
conditions wherein the salt is at least in part in a dissociated
form. The aqueous phase preferably contains salt approximately
0.001-10% by weight, preferably approximately 0.01-5% by weight.
The pH value of the ion exchange is approximately 6-10, preferably
approximately 6.5-9, and the consistency of the pulp is
approximately 0.1-10%. The ion exchange time is approximately 1
min-24 hours and temperature approximately 10-60.degree. C.,
preferably approximately 15-50.degree. C.
[0027] Most commonly, sodium-containing chemicals, such as sodium
hydroxide, are used for the preparation of sulfate pulp, both in
the cooking and in the bleaching. For this reason the acid groups
in the pulp obtained are mainly in the sodium form. According to a
preferred embodiment of the invention, the univalent metals of the
acid groups are first removed from the pulp used for the making of
the material web, whereafter the pulp is optionally washed and
mixed with a salt of a bivalent or trivalent metal in order to
replace the hydrogen atoms of the carboxyl groups with bivalent or
trivalent metals. The ion exchange may also be carried out by
washing the pulp obtained from the pulp making with water that
contains bivalent or trivalent metal ions.
[0028] The ion exchange can be implemented as a batch process or a
continuous-working process. In practice the ion exchange can be
carried out in a separate mixing container or a mixing
container/washer combination fitted at a point before the paper
machine headbox.
[0029] Cellulose pulp ion exchange is also described in an article
by Scallan and Grignon [Scallan, A. M. and Grignon, J., Svensk
Papperstidn. 82 (1979) 40-47]. According to the method of Scallan
and Grignon, the pulp is first converted to the hydrogen form by
soaking the pulp alternately in a dilute acid solution and in a
sodium chloride solution, the soaking being concluded in acid
conditions. The pulp is thereafter washed with ion-exchanged water,
and the pH of the pulp is set at an acid level. The pulp in the
hydrogen form is thereafter contacted with a metal salt
solution.
[0030] According to a preferred embodiment of the invention,
calcium ions are attached to the acid groups by adding to an
aqueous suspension of the cellulose pulp a calcium salt so that the
hardness of the water rises at least to a level of 10 dH.degree.
(which corresponds to a calcium ion concentration of 70 mg/l),
whereafter the pH of the suspension is raised to a neutral or
mildly alkaline level.
[0031] Preferably the ion exchange is carried out before other
paper machine chemicals, such as cationic polymers, retention
chemicals and/or fillers, are added to the pulp.
[0032] The water-soluble salts of bivalent or trivalent metals used
are their halides, carbonates, sulfates or organic salts. It is
also possible to use hydroxides of the metals. Examples of suitable
calcium compounds include calcium chloride, calcium hydroxide,
calcium ascorbate, and calcium gluconate. Suitable salts of
magnesium include magnesium chloride and magnesium sulfate, and
suitable salts of aluminum include aluminum sulfate. It is also
possible to use mixed salts, such as alum. By ion exchange, there
are bound to the acid groups bivalent or trivalent ions in an
amount of at least 0.1%, especially preferably 0.15-0.5%, in which
case the surface charge of the paper web will be at least
approximately 600 V, preferably at least approximately 610 V, and
especially preferably at least 620 V, when relative humidity is
40%. The ions bound to the acid groups are preferably calcium,
magnesium or aluminum ions or a mixture thereof.
[0033] The ion-exchanged paper pulp is slushed in a manner known
per se to a suitable consistency (typically to a solids content of
approximately 0.1-1%) and is spread onto the wire, where it is
formed into a paper or board web. For the production of
electrophotographic paper, before the paper machine there is added
to the fiber slush a filler, such as kaolin or precipitated or
ground calcium carbonate, in general approximately 1-50%,
preferably approximately 5-30% of the weight of the fibers. It is
also possible to add to the slush various retention chemicals and
cationic polymers, including starch and modified starches.
[0034] The paper web may be uncoated or coated. The grammage of an
uncoated web of material typically varies within the range of
30-180 g/m.sup.2. A coated paper web has in this case preferably a
coating pigment layer, wherein the coating pigment is preferably
calcium carbonate, kaolin, gypsum, talc, chalk and/or a mixture
thereof. The grammage of the web to be coated is 50-250 g/m.sup.2,
and it contains coating approximately 8-35 g/m.sup.2 of
surface.
[0035] According to a preferred embodiment of the invention, the
surface charge of the surface can be modified, in particular it can
be raised, by applying to the paper surface, for example, by
surface sizing or in connection with the coating, a cellulose
derivative in an amount of approximately 0.1-20% of the pulp
weight, the derivative having a bivalent or trivalent counter-ion.
For example, a calcium-form carboxymethyl cellulose can be used as
such a cellulose derivative.
[0036] After the drying of the paper web there is obtained a paper
which contains a bivalent or trivalent metal ion approximately
0.1-1% of the weight of the fibers (indicated as metal), and the
surface charge of which is 600-800 V when relative humidity is
approximately 20-40%. The paper typically contains a filler
approximately 10-30% by weight of the total weight of the
paper.
[0037] The paper according to the invention is suitable for use as
a print medium in the production of printed matter by the
electrophotographic method. It can be used, for example, in Xeikon
four-color electrophotography printing presses [Van Daele, J.,
Verluyten, L., and Soulliaert, E., Print Media for Xeikon's DCP/32D
Digital Color Press, IS&T's NIP12: International Conference on
Digital Printing Technologies pp. 382-386].
[0038] The following examples illustrate the invention.
EXAMPLE 1
[0039] A sulfate pulp was prepared in laboratory conditions from
pure birch chips. The pulp was oxygen delignified, was chelated
with EDTA, and was bleached with peroxide in two steps. The kappa
number of the pulp was 7.2, its viscosity 1020 ml/g and its ISO
brightness 85.3%.
[0040] The carboxylic acid concentration was assayed by
conductometric titration and the uronic acid concentration was
assayed with HPLC after a complete hydrolysis performed with
enzymes. The methylglucuronic acid concentration was 34 mmol/kg and
the hexenuronic acid concentration 72 mmol/kg. Uronic acids
constituted 89% of the acid groups in the pulp.
[0041] The pulp was converted to the hydrogen form and thereafter
to a sodium, calcium or aluminum form. The ion exchange was carried
out by treating the acid-form pulp in aqueous solutions of salts of
metal ions. The sodium and calcium ions were introduced in alkaline
conditions; in the case of aluminum ions the pH value of the
solution was not controlled.
[0042] The metal concentrations in the pulps were: Na pulp: 2672 mg
Na/kg; Ca pulp: 2219 mg Ca/kg; and Al pulp: 944 mg Al/kg. Of the
available acid groups, at least 75% had been substituted for by
metal ions. In general the conversion of the pulp was at least
approximately 95% (i.e. 95% of the bonded counter-ions were of the
desired ion type).
[0043] The surface charge was determined from the paper samples
made from the pulps by using a JCI 155 Charge Decay Test Unit. The
charge voltage was 6 kV and the charging time was 7.0 s.
[0044] FIG. 2 shows a comparison between the surface charges of
paper sheets in the sodium and respectively the calcium form as a
function of relative humidity. As is evident from the said figure,
the surface potential of the Ca-form paper is quite uniform within
a wide humidity range, whereas the surface potential of the
sodium-containing paper drops sharply when relative humidity rises
above 40%.
[0045] FIG. 1 presents the surface charge of five commercial
electrophotography papers, as a function of relative humidity (rh).
The commercial paper grades contain calcium carbonate as a filler,
the concentrations being approximately 15-23% by weight. A fact
noteworthy in a comparison between the paper according to the
invention and these commercial papers is in particular that the
surface charge of not a single commercial paper exceeds 400 V when
relative humidity is 40%, whereas the surface charge of the paper
according to the invention is over 500 V even at an rh value of
50%. This shows that in terms of the surface charge formation it is
important that the calcium ions are bonded to the acid groups. A
salt of a bivalent or trivalent metal as a filler does not affect
the charging of the paper. We have observed this also in laboratory
sheet experiments in which calcium carbonate was added to pulp
sheets the pulp of which had not been ion exchanged.
EXAMPLE 2
[0046] The acid groups in a chemical cellulose pulp were converted
to the calcium form in an industrial process, through the following
steps:
[0047] 1) the pulp was washed at a consistency of approximately 1%
at a temperature of 45.degree. C. (added NaOH 1% of the dry weight
of the pulp)
[0048] 2) CaCl2 was added so that the hardness of the water rose to
10 dH.degree. (70 mg/l Ca.sup.2+)
[0049] 3) the pulp was washed with tap water
[0050] 4) the pH was adjusted to 5, the pulp was mixed for 10
min
[0051] 5) the pulp was washed carefully to remove the acid and to
raise the pH value to 7.
[0052] The bleached pine sulfate initially contained sodium 45
meq/kg and calcium 2 meq/kg. After the treatment the figures were
sodium 1 meq/kg and calcium 48 meq/kg. In the bleached birch
sulfate the initial situation was sodium 29, calcium 7 and
magnesium 35 meq/kg, the final situation was sodium 2, calcium 21
and magnesium 11 meq/kg.
* * * * *