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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a paper coating agent, and more
particularly, it relates to a paper coating agent that imparts to paper
water resistance, printability, surface strength, and barrier properties
that make paper less permeable to air, oil, and organic solvents.
2. Description of the Prior Art
Heretofore, polyvinyl alcohol (abbreviated as PVA hereinafter) has been
widely used as a clear coating agent to improve the surface strength,
smoothness, gloss, and barrier properties of paper and as a binder for
pigment coating compounds. It is also known that PVA is far superior to
any other sizing agents in the film forming property and strength.
Recently, the surface strength of paper has tended to decrease as Asian
tropical wood has come to account for a greater percentage in pulpwood and
the printing speed has tended to increase. Under these conditions, there
has arisen a demand for PVA that improves the surface strength and other
properties of paper. In addition, in view of the remarkable development of
barrier paper, there is a demand for PVA that imparts to paper improved
barrier properties. Conventional PVA, however, does not meet completely
these requirements. In order to overcome the disadvantage of conventional
PVA, there was proposed in Japanese Patent Publication No. 39,442/1977 and
Japanese Patent Laid-Open No. 20,698/1981 the use of PVA with a lactone
ring or a carboxyl group introduced into the molecule. The proposed PVA is
not effective in practical use. Thus the development of new PVA that meets
the above-mentioned requirements has been expected.
New PVA is also required for making improved offset printing plates. Offset
printing plates commonly used for office printing are made up of a
substrate of wet-strength paper, an image forming layer, and an
intermediate layer placed between them. The intermediate layer is provided
to improve the durability of the plate. The image forming layer and
intermediate layer should have a sufficient affinity for water to repel
ink during printing as well as a sufficient resistance to damping water
which is supplied repeatedly during printing. In order to provide both
affinity for water and resistance to water, the conventional image forming
layer and intermediate layer have been incorporated with a water-soluble
polymeric substance such as PVA, casein, and starch, or a crosslink
reaction product of a water-soluble polymer and an amino resin condensate.
The crosslinking reaction, however, has a disadvantage. If the
crosslinking reaction is allowed to take place before the compound is
applied to the substrate, the compound becomes excessively viscous due to
gellation, making coating impossible. On the other hand, the crosslinking
reaction during or after the coating process requires such a high
temperature and long time that the productivity is reduced to a great
extent.
The above-mentioned system composed of water-soluble resin and crosslinking
agent loses the balance of water affinity and water resistance when the
mixing ratio of the two components varies or the crosslinking conditions
vary. Thus, the offset printing plates prepared from it tend to vary in
uniformity.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a paper coating agent composed
of modified polyvinyl alcohol containing silicon in the molecule.
On contact with paper, the coating agent of this invention becomes viscous
and gelled, forming a water-resistant film on the surface of paper. The
film thus formed minimizes the penetration of the coating compound into
paper and improves the surface strength and printability of paper.
The coating agent of this invention can also be applied to wet-strength
paper to prepare offset printing plates which are superior in both water
affinity and water resistance under normal conditions without using
special curing that requires a high temperature and a long time.
DESCRIPTION OF SPECIFIC EMBODIMENTS
The silicon-containing modified PVA used in this invention may be any one
which contains silicon in the molecule. Preferably, it should have such a
structure that the substituent groups connecting to silicon are partly a
hydroxyl group or an alkali salt of hydroxyl group.
Such modified PVA may be produced by (1) introducing silicon into the
molecule by post-modification with a silylating agent or introducing
silicon into modified polyvinyl acetate having functional groups such as
carboxyl group or hydroxyl group that react with a silylating agent by
post-modification with a silylating agent and subsequently saponifying the
modified polyvinyl acetate; or (2) saponifying a copolymer of vinyl ester
and silicon-containing olefinic unsaturated monomer.
The post-modification of PVA with a silylating agent may be accomplished by
dissolving a silylating agent in an organic solvent such as benzene,
toluene, xylene, hexane, heptane, ether, and acetone which is inert to the
silylating agent, and suspending PVA powder or modified polyvinyl acetate
powder in the solution with stirring. Silylation is performed at a
temperature from normal temperature to the boiling point of the silylating
agent, and the saponification of vinyl acetate units is accomplished by
the use of an alkali catalyst.
The silylating agent for post-modification should preferably be a compound
having two or more hydrolyzable functional groups. Examples of such a
compound include organohalogenosilane such as dimethyldichlorosilane,
methyltrichlorosilane, vinyl trichlorosilane, and diphenyldichlorosilane;
organosilicon ester such as dimethylacetoxysilane; organoalkoxysilane such
as dimethyldimethoxysilane; organosilanol such as diethylsilanediol; and
aminoalkylsilane such as N-aminoethylaminopropyltrimethoxysilane.
The ratio of introduction of silylating agent, i.e. the ratio of
modification, may be adjusted properly by the quantity of the silylating
agent and the reaction time. Usually, it is adjusted in the range of 0.01
to 10 mol%.
The degree of polymerization and saponification of the silicon-containing
modified PVA to be produced may be properly adjusted by selecting the
degree of polymerization and saponification reaction of PVA or modified
polyvinyl acetate.
According to the process for saponifying a copolymer of vinyl ester and
silicon-containing olefinic unsaturated monomer, the silicon-containing
modified PVA is produced as follows: At first, a vinyl ester and
silicon-containing olefinic unsaturated monomer are copolymerized by the
use of a radical initiator in the presence or absence of solvent such as
alcohol, and then the resulting copolymer is saponified by adding an
alkali or acid catalyst to the alcohol solution of the copolymer.
The vinyl ester used for this process includes, for example, vinyl acetate,
vinyl propionate, and vinyl formate. Preferable among them from an
economical standpoint is vinyl acetate.
The silicon-containing olefinic unsaturated monomer used in this process
includes vinylsilanes represented by the formula (I) and
(meth)acrylamide-alkylsilanes represented by the formula (II).
##STR1##
(where n is 0 to 4; m is 0 to 2; R.sup.1 is a lower alkyl group such as
methyl and ethyl having 1 to 5 carbon atoms, an aryl group having 6 to 18
carbon atoms, or a lower alkyl group of 1 to 5 carbon atoms having an aryl
group of 6 to 18 carbon atoms; R.sup.2 is an alkoxyl group having 1 to 40,
preferably 1 to 18, carbon atoms, an acyloxy group having 2 to 40,
preferably 2 to 18, carbon atoms; (the alkoxyl group and acyloxyl group
may have a substituent group containing an oxygen or nitrogen atom);
R.sup.3 is hydrogen or a methyl group; R.sup.4 is hydrogen or a lower
alkyl group having 1 to 5 carbon atoms; and R.sup.5 is an alkylene group
having 1 to 5 carbon atoms or a divalent organic residue in which carbon
atoms are connected with each other by an oxygen or nitrogen atom. Where
two groups represented by R.sup.1 are present in the same monomer, they
may be the same or different; and where two groups represented by R.sup.2
are present in the same monomer, they may be the same or different.)
Examples of the vinylsilanes represented by the formula (I) include
vinyltrimethoxysilane, vinyltriethoxysilane,
vinyltris(.beta.-methoxyethoxy)silane, vinyltriacetoxysilane,
allyltrimethoxysilane, allyltriacetoxysilane, vinylmethyldimethoxysilane,
vinyldimethylmethoxysilane, vinylmethyldiethoxysilane,
vinyldimethoxyethoxysilane, vinylmethyldiacetoxysilane,
vinyldimethylacetoxysilane, vinylisobutyldimethoxysilane,
vinyltriisopropoxysilane, vinyltri-n-butoxysilane,
vinyltri-secbutoxysilane, vinyltrihexyloxysilane,
vinylmethoxydihexyloxysilane, vinyldimethoxyoctyloxysilane,
vinylmethoxydioctyloxysilane, vinyltrioctyloxysilane,
vinylmethoxydilauryloxysilane, vinyldimethoxylauryloxysilane,
vinylmethoxydioleyoxysilane, vinyldimethoxyoleyloxysilane, and
polyethyleneglycol-modified vinylsilane represented by the formula
##STR2##
(where R.sup.1 and m are as defined above, and x is 1 to 20.)
Examples of the (meth)acrylamide-alkylsilane represented by the formula
(II) include:
(meth)acrylamide-linear or branched alkyltrialkoxysilanes (where R is
hydrogen or a methyl group) such as
3-(meth)acrylamide-propyltrimethoxysilane,
##STR3##
3-(meth)acrylamide-propyltriethoxysilane,
##STR4##
3-(meth)acrylamide-propyltri(.beta.-methoxyethoxy)silane,
##STR5##
3-(meth)acrylamide-propyltri(N-methylaminoethoxy)silane,
##STR6##
2-(meth)acrylamide-ethyltrimethoxysilane,
##STR7##
1-(meth)acrylamide-methyltrimethoxysilane,
##STR8##
2-(meth)acrylamide-2-methylpropyltrimethoxysilane, and
##STR9##
2-(meth)acrylamide-isopropyltrimethoxysilane;
##STR10##
(meth)acrylamide-nitrogen-containing or oxygen-containing
alkyltrialkoxysilanes (where R is hydrogen or a methyl group) such as
N-(2-(meth)acrylamide-ethyl)-aminopropyltrimethoxysilane, and
CH.sub.2 .dbd.CR--CONH--CH.sub.2 CH.sub.2 NH(CH.sub.2).sub.3
Si(OCH.sub.3).sub.3
(3-(meth)acrylamide-propyl)-oxypropyltrimethoxysilane;
CH.sub.2 .dbd.CR--CONH--(CH.sub.2).sub.3 --O--(CH.sub.2).sub.3
Si(OCH.sub.3).sub.3
acrylamide-alkyltriacyloxysilane (where R is hydrogen or a methyl group)
such as
3-(meth)acrylamide-propyltriacetoxysilane,
CH.sub.2 .dbd.CR--CONH(CH.sub.2).sub.3 --Si(OCOCH.sub.3).sub.3
2-(meth)acrylamide-ethyltriacetoxysilane,
CH.sub.2 .dbd.CR--CONH(CH.sub.2).sub.2 --Si(OCOCH.sub.3).sub.3
4-(meth)acrylamide-butyltriacetoxysilane,
CH.sub.2 .dbd.CR--CONH(CH.sub.2).sub.4 --Si(OCOCH.sub.3).sub.3
3-(meth)acrylamide-propyltripropionyloxysilane,
CH.sub.2 .dbd.CR--CONH(CH.sub.2).sub.3 --Si(OCOCH.sub.2 CH.sub.3).sub.3
2-(meth)acrylamide-2-methylpropyltriacetoxysilane, and
##STR11##
N-(2-(meth)acrylamide-ethyl)-aminopropyltriacetoxysilane;
CH.sub.2 .dbd.CR--CONH--CH.sub.2 CH.sub.2 NH(CH.sub.2).sub.3
Si(OCOCH.sub.3).sub.3
(meth)acrylamide-alkyl di- or mono-alkoxy- or di- or mono-acyloxysilane
(where R is hydrogen or a methyl group) such as
3-(meth)acrylamide-propylisobutyldimethoxysilane,
##STR12##
2-(meth)acrylamide-ethyldimethylmethoxysilane,
##STR13##
3-(meth)acrylamide-propyloctyldiacetoxysilane,
##STR14##
1-(meth)acrylamide-methylphenyldiacetoxysilane, and
##STR15##
3-(meth)acrylamide-propylbenzyldiethoxysilane; and
##STR16##
(N-alkyl(meth)acrylamide)-alkyltrialkoxy- or triacyloxy-silanes (where R
is hydrogen or a methyl group) such as
3-(N-methyl-(meth)acrylamide)-propyltrimethoxysilane, and
##STR17##
2-(N-ethyl-(meth)acrylamide)-ethyltriacetoxysilane.
##STR18##
The above-mentioned silicon-containing olefinic unsaturated monomers (I)
and (II) should preferably have as few non-hydrolyzable groups
(R.sup.1.sub.m) as possible and as many hydrolyzable groups
(R.sup.2.sub.3-m) as possible, so that the resulting modified PVA has the
maximum reactivity with paper. Thus, most preferably, m should be 0.
In the case where polymerization is accomplished in a solvent, preferable
solvents are a lower alcohol such as methanol and ethanol.
Polymerization may be performed batchwise or continuously. In batchwise
polymerization, the composition of copolymer change with conversion of
polymerization according to the ratio of copolymerization reactivity
(r.sub.1, r.sub.2). In order to obtain a copolymer of uniform composition,
it is desirable to employ the semi-batch process in which either or both
of the monomers are added in such a way that the ratio of concentrations
(by weight) of the monomers is kept substantially constant during
polymerization. The quantities to be added may be calculated according to
the formula proposed by R. J. Hanna in Industrial and Engineering
Chemistry, Vol. 49, No. 2, 208-209 (1957).
In the case of multicolumn continuous polymerization, the monomer should
preferably be added to the second and subsequent columns so that the
monomer composition in each column is constant for the same reasons
mentioned above.
The polymerization initiator is 2,2'-azobisisobutyronitrile, benzoyl
peroxide, lauroyl peroxide, acetyl peroxide, and other known radical
polymerization initiators. The polymerization temperature is usually in
the range from 50.degree. C. to the boiling point of the system.
That the ratio of concentrations of monomers is substantially constant
means that the actual ratio of concentrations is in the range of 80 to
120% (variation .+-.20%), preferably 90 to 110% (variation .+-.10%), most
preferably 95 to 105% (variation .+-.5%), of the set ratio of
concentrations during polymerization.
The above-mentioned polymerization may be carried out in the presence of a
small quantity of copolymerizable unsaturated monomers. Examples of such
monomers include olefins such as styrene, alkylvinyl ether, vinyl ester of
Versatic acid, (meth)acrylamide, ethylene, propylene, .alpha.-hexene, and
.alpha.-octene; unsaturated acids such as (meth)acrylic acid, crotonic
acid, maleic acid (anhydride), fumaric acid, and itaconic acid, and
alkylesters and alkali salts thereof; sulfonic acid-containing monomer
such as 2-methacrylamide-2-methylpropane-sulfonic acid and alkali salts
thereof; and cationic monomers such as
trimethyl-3-(1-(meth)acrylamide-1,1-dimethylpropyl)ammonium chloride,
trimethyl-3-(1-(meth)acrylamidepropyl)ammonium chloride,
1-vinyl-2-methylimidazole, and quaternary compounds thereof.
The silicon-containing modified PVA used in this invention should
preferably be produced by saponifying a copolymer of a vinyl ester and a
silicon-containing olefinic unsaturated monomer. This process is easy to
perform on an industrial scale and provides modified PVA of uniform
quality.
When a copolymer of a vinyl ester and a silicon-containing olefinic
unsaturated monomer as represented by the formula (I) is saponified, the
saponified product is stable in viscosity and is also stable in an
alkaline aqueous solution. On the other hand, when a copolymer of a vinyl
ester and a monomer as represented by the formula (II) is saponified, the
saponified product is inferior in stability in alkali, but superior in
that the aqueous solution foams very little and provides a coating film,
after drying, which is resistant to organic solvents and other agents.
Thus, which monomer to select is dependent on the object and conditions of
use.
The copolymer may be properly adjusted for the degree of polymerization by
selecting the type and quantity of alcohol used as the solvent. After
copolymerization, residual vinyl ester in the reaction solution should be
distilled away. The silicon-containing olefinic unsaturated monomer
remaining unreacted may be removed, or may be left unremoved without any
trouble if the quantity is small.
The compolymer thus prepared is subsequently saponified. The saponified
reaction is usually performed in an alcohol solution of the copolymer so
that the reaction proceeds through alcoholysis. For this purpose, it is
possible to use absolute alcohol or alcohol containing a small quantity of
water. The alcohol may contain organic solvents such as methyl acetate and
ethyl acetate as desired. The catalyst for saponification is an alkali
metal hydroxide such as sodium hydroxide and potassium hydroxide; an
alcoholate such as sodium methylate and potassium methylate; and an alkali
such as ammonia. An acid catalyst such as hydrochloric acid and sulfuric
acid may also be used. An alkali catalyst is advantageous from the
standpoint of saponification rate, and sodium hydroxide is advantageous
economically and industrially.
The saponification temperature is usually in the range from 10.degree. to
50.degree. C. As the result of saponification reaction, the vinyl ester
units are saponified partly or to a great extent, and converted into the
vinyl alcohol units. The ratio of conversion or the degree of
saponification may be any value according to the intended object of use.
During the saponification reaction, alkoxyl group, or acyloxyl group
connected to the silicon in the silicon-containing olefinic unsaturated
monomer units is also saponified partly or to a great extent into a
silanol group or an alkali salt of silanol. These groups may partly form
siloxane bonds when the modified PVA after saponification is dried. This
is within the scope of this invention.
There are no specific limitations with respect to the content of silicon in
the silicon-containing modified PVA, the degree of saponification, and the
degree of polymerization. They are properly selected according to the
intended object of use. The content of silicon is usually from 0.01 to 10
mol%, preferably 0.1 to 3 mol%. The degree of saponification is usually 70
to 100 mol%, preferably 80 to 100 mol%. The degree of polymerization is
usually 100 to 3000, preferably 300 to 3000.
The silicon-containing modified PVA may be dissolved in water uniformly
with stirring and heating. Where the silicon content is high and the
siloxane bond exists in a large quantity, a uniform aqueous solution of
the modified PVA may be obtained by dispersing it in water and then adding
a small quantity of alkali metal hydroxide such as sodium hydroxide and
potassium hydroxide, alkaline earth metal hydroxide such as magnesium
hydroxide and calcium hydroxide; or an alkali such as ammonia water and
amine, and heating with stirring. If necessary, the solution may be
adjusted to a proper pH with an acid.
The coating agent of this invention is prepared by dissolving the modified
PVA in water and is used in the form of aqueous solution as mentioned
above. The concentration of the modified PVA is usually 0.2 to 30 wt%,
preferably 2 to 10 wt%. An alkaline aqueous solution is preferable from
the standpoint of stability of solution viscosity.
The coating agent of this invention may be incorporated, as required, with
a tin compound or zinc compound which catalyzes the hydrolysis of the
alkoxyl group or acyloxyl group connected to the silicon in the modified
PVA; water resistant agent such as glyoxal and urea resin; a plasticizer
such as glycol and glycerin; a pH adjusting agent such as ammonia, sodium
hydroxide, sodium carbonate, and phosphoric acid; and other known
additives such as antifoam, release agent, and surface active agent.
In addition, the coating agent of this invention may be incorporated with
other coating compounds such as PVA, modified PVA (e.g., carboxyl-modified
PVA, sulfonic-modified PVA, acrylamide-modified PVA, cation modified PVA,
long chain alkyl-modified PVA), starch, modified starch, casein, CMC, or
synthetic resin emulsion (e.g., styrene-butadiene latex, polyacrylate
ester emulsion, polyvinyl acetate emulsion, vinyl acetate-acrylate ester
copolymer emulsion, and vinyl acetate-ethylene copolymer emulsion).
Further, the coating agent of this invention may be incorporated, as
required, with pigments such as clay, calcium carbonate, titanium dioxide,
satin white, zinc oxide, silica, aluminum oxide, and cadmium sulfide.
The coating agent of this invention is used for paper coating in order to
improve the paper surface properties such as surface strength,
printability, and barrier properties. It is particularly suitable for
clear coating, but it may be incorporated with pigments such as clay,
calcium carbonate, titanium dioxide, and satin white. For clear coating,
the silicon-containing modified PVA is applied in an amount of 0.1 to 20
g/m.sup.2, preferably 0.5 to 5 g/m.sup.2. In the case of pigmented
coating, the silicon-containing modified PVA is used in an amount of 3 to
10 parts for 100 parts of pigment. The solid content in the coating
solution is 30 to 63%.
The coating agent of this invention may be applied to any kind of paper to
achieve the above-mentioned objects. It may be applied preferably to paper
board such as manila board, white board, and liners; and printing paper
such as wood free paper, paper containing wood, gravure paper, and
wet-strength paper, which papers are cellulosic ones made from pulp. It
may also be applied to synthetic paper, synthetic fiber paper, and paper
of a mixture of synthetic fibers and cellulose.
The above-mentioned coating solution may be applied to paper by means of
the known size press coater, roll coater, air knife coater, or blade
coater. The quantity of coating is not specifically restricted, but is
usually 1 to 30 g/m.sup.2 as solids.
When applied to paper, the paper coating agent of this invention greatly
improves the surface properties of paper such as surface strength,
printability, and barrier properties as mentioned above. No complete
elucidation has been made yet as to the mechanism for such performance,
but the following presumption is made. The silicon atom to which alkoxyl
groups or acyloxyl groups connect in the silicon-containing modified PVA
or the silanol group or salt thereof which is their hydrolyzate, is
reactive with cellulose fibers of paper and pigments and aluminum
compounds in paper, and also with the hydroxyl group or silanol group in
the modified PVA. Therefore, the reaction takes place on the surface layer
of paper as the solution of the silicon-containing modified PVA comes in
contact with paper, and the solution becomes viscous and gelled
immediately. Thus the agent does not penetrate into paper but forms a firm
uniform film on the surface layer of paper.
The coating agent of this invention may also be used to prepare offset
printing plates by coating it on wet-strength paper. The wet-strength
paper means paper prepared by adding or applying melamine resin, urea
resin, polyamide resin, or polyethyleneimine resin.
The offset printing plate is composed of a substrate of wet-strength paper
and an image receiving layer or photoconductive layer formed on the
substrate, with or without a water resistant intermediate layer between
the substrate and the image receiving layer or photoconductive layer. The
substrate is backed by a back coat layer. The coating agent of this
invention may be used to form at least one of the above-mentioned layers.
It is particularly useful to form either or both of the image receiving
layer and the water resistant intermediate layer. For the image receiving
layer and water resistant layer, the above-mentioned modified PVA or vinyl
resin emulsion containing the modified PVA is used as the coating agent.
The coating agent is usually incorporated with pigment such as clay,
calcium carbonate, zinc oxide, silica, aluminum oxide, barium sulfate, and
titanium oxide, pigment dispersant, antifoam, and water-soluble resin such
as PVA.
The image receiving layer or water resistant intermediate layer may be
formed by applying the coating compound onto the substrate and drying it.
The quantity of application is 1 to 30 g/m.sup.2 as dry solids. The
application should be made so that the quantity of the silicon-containing
modified PVA is 0.1 to 20 g/m.sup.2, preferably 1 to 15 g/m.sup.2.
To form the photoconductive layer, the modified PVA or the vinyl resin
emulsion containing the modified PVA is incorporated with pigment such as
zinc oxide, cadmium sulfide, and titanium oxide having photoconductivity.
The photoconductive layer is formed by applying the above-mentioned
coating agent onto the above-mentioned water resistant intermediate layer.
The offset printing plate is sometimes provided on its back side with a
back coat layer containing a curl preventing agent, conductive agent, or
humectant. And the coating agent of this invention may be used to form
such a back coat layer.
The image receiving layer, water resistant intermediate layer, or back coat
layer formed by the coating agent of this invention is hydrophilic and
superior in water resistance. This remarkable performance is attributable
to the hydroxyl groups derived from PVA and water-insoluble film derived
from silicon. In other words, the alkoxyl groups or acyloxyl groups
connected to silicon in the modified PVA, or the silanol groups of salt
thereof which are their hydrolyzate, self-condensate, crosslink, and react
with the pigment of substrate to form a water-insoluble film when the PVA
solution is dried.
Planographic printing (or offset printing) is described below. In the case
of direct writing type offset printing, letters are written on the image
receiving layer by a typewriter or by hand with oil ink. The image is also
formed by transferring a xerograph image onto the image receiving layer,
followed by thermal fixing.
In the case of planographic printing plates based on electrophotography,
the photoconductive layer is exposed to uniform corona discharge and then
an image is formed by dry or wet development.
The plate with an image formed thereon is then treated with an etching
solution so that the non-image area is desensitized and is made
hydrophilic. The image area remains lipophilic because it is covered with
a lipophilic ink or toner which keeps out the etching solution. In the
desensitizing process, the plate is treated with an aqueous solution
containing sodium ferrocyanide, ammonium phosphate, citric acid, or phytic
acid.
The desensitized area dampened with damping water repels oil ink, and the
lipophilic image area attracts oil ink. The oil ink on the image area is
transferred to paper. Thus, the offset printing plate desensitized as
above can perform printing.
The coating agent of this invention may be used to produce converting
paper. Converting paper is a stock from which are produced release paper
by applying a release agent such as silicone resin, paper having low
abrasion resistance by applying a slip agent such as silicone resin,
masking paper, and transfer paper.
In the production of converting paper, the coating agent of this invention
may be applied efficiently in the same way as used for the conventional
water-soluble resin. The resulting converting paper prevents an organic
solvent or coating material from penetrating into paper and increases the
retention of a coating material on the surface of paper. No complete
elucidation has been made yet as to the mechanism for such performance,
but the following presumption is made. The silicon atom to which alkoxyl
groups or acyloxyl groups connect in the silicon-containing modified PVA
or the silanol group or salt thereof which is their hydrolyzate, is
reactive with cellulose fibers of paper and pigments and aluminum
compounds in paper, and also with the hydroxyl group or silanol group in
the modified PVA. Therefore, the reaction takes place on the surface layer
of paper. Thus the compound does not penetrate into paper but forms a firm
uniform film on the surface layer of paper. In the particular case where
the coating material is silicone, the silicon atoms in the modified PVA
connect firmly to the silicone. Thus the silicone film stay firmly on the
surface of paper and does not migrate to the adhesive layer in the case of
tackified paper. The silicone is usually a silicone resin.
The coating composition of this invention may also be used to produce
thermosensitive paper. Thermosensitive paper is composed of a paper
substrate and a thermosensitive coating layer formed thereon. The coating
layer is formed by applying a thermosensitive coating liquid composed of a
color-developing lactone compound, an acid compound such as phenol
compound, and a watersoluble binder. The color developing lactone compound
and the phenol compound are pulverized into fine powder separately. The
coating liquid is incorporated with a filler such as clay, wax, and
surface active agent, as required. If the coating agent of this invention
is used as the above-mentioned water-soluble binder, the resulting
thermosensitive layer is superior in barrier properties for water and
organic solvents. Therefore, the image formed on the thermosensitive layer
is well conserved even when the thermosensitive paper comes in contact
with water or organic solvents.
In the production of thermosensitive paper, the coating agent of this
invention may be applied to the substrate prior to the application of the
thermosensitive coating liquid, so that the layer of the
silicon-containing modified PVA is formed on the substrate. In the other
way, it is also possible to apply the thermosensitive coating liquid
(which does not contain the coating agent of this invention) to the
substrate to form the thermosensitive layer, and then to apply the coating
agent of this invention on the thermosensitive layer to form the layer of
the silicon-containing modified PVA.
In case where the coating agent of this invention is used as the
water-soluble binder of the thermosensitive coating liquid, the
silicon-containing modified PVA should be used in an amount of 1 to 500
parts by weight, preferably 2 to 200 parts by weight, for 100 parts by
weight of the color developing component.
The thermosensitive paper produced by applying the coating agent of this
invention is characterized by that the image is well conserved even when
the paper comes in contact with water or organic solvents. No complete
elucidation has been made yet as to the mechanism for such performance,
but the following presumption is made. The silicon atom to which alkoxyl
groups or acyloxyl groups connect in the silicon-containing modified PVA
or the silanol group or salt thereof which is their hydrolyzate, readily
self-condensate, crosslink; and react with the filler and substrate, to
form a water-insoluble film which is superior in barrier properties for
organic solvents. This film firmly fastens the developed image and
protects it from water and organic solvents.
The invention is now described in detail with reference to the following
examples, in which "parts" and "%" are "parts by weight" and "wt%" unless
otherwise specified.
EXAMPLE 1
In a reactor equipped with a stirrer, thermometer, and reflux condenser
were placed 2800 parts of vinyl acetate, 500 parts of methanol, and 16.8
parts of vinyltrimethoxysilane. The atmosphere in the reaction system was
replaced with nitrogen while stirring. The temperature was raised to
60.degree. C. To this system was added 200 parts of methanol solution
containing 1.96 parts of 2,2'-azobisisobutyronitrile. Polymerization was
continued for 3.5 hours. When polymerization was terminated, the solid
content in the system was 40%. Vinyl acetate remaining unreacted was
expelled by introducing methanol vapor. Thus, 35% methanol solution of
copolymer was obtained. For saponification reaction, a methanol solution
containing sodium hydroxide in an amount of 3 mol% for the vinyl acetate
unit was added with stirring to the methanol solution of copolymer at
40.degree. C. The resulting white gel was crushed and washed with methanol
completely to remove unreacted vinyltrimethoxysilane. After drying,
silicon-containing modified PVA was obtained. The degree of polymerization
was 1700. According to NMR spectroscopy, IR spectroscopy, and
atomic-absorption spectroscopy, the copolymer was found to contain 0.5
mol% of vinyltrimethoxysilane unit and the degree of saponification of
vinyl acetate unit was found to be 99.9 mol%.
A coating solution containing 4.5% of PVA was prepared by heating 4.5 parts
of the modified PVA, 95.5 parts of water, and 0.045 parts of sodium
hydroxide. This coating solution was applied to wood-free paper (basis
weight 64 g/m.sup.2) at 50.degree. C. using a laboratory size press (made
by Kumagai Riki Kogyo K.K.) at a nip pressure of 18 kg/cm and at a rate of
60 m/min. The quantity of sizepress coating was 1.0 g/m.sup.2 (both sides)
in terms of solids of modified PVA.
In Comparative Example 1, coated paper was prepared as in Example 1 using
PVA having the degree of polymerization of 1700 and the degree of
saponification of 98.5% (except that sodium hydroxide was not used).
In Comparative Example 2, coated paper was prepared as in Example 1 using
modified PVA containing 2.1 mol% of lactone ring in the molecule and
having the degree of saponification of 98.5% for vinyl acetate units
(except that sodium hydroxide was not used).
In Comparative Example 3, coated paper was prepared as in Example 1 using
modified PVA obtained by saponification of a copolymer of itaconic acid
and vinyl acetate, said modified PVA containing 1.0 mol% of carboxyl group
and having the degree of saponification of 88.0% for vinyl acetate units
(except that sodium hydroxide was not used).
The resulting coated paper was measured for properties after conditioning
in a constant-temperature room for 72 hours at 20.degree. C. and 60% RH.
The results are shown in Table 1.
TABLE 1
______________________________________
*.sup.1 Coat- *.sup.3
Viscosity ing *.sup.2 Air
of coating quan- Surface perme- *.sup.4
solution tity strength ability
Print-
(cp) (g/m.sup.2)
(cm/sec) (sec) ability
______________________________________
Example 1
17 1.0 220 1500 Good
Compara-
17 1.0 148 25 Fair
tive
Example 1
Compara-
17 1.0 160 30 Fair
tive
Example 2
Compara-
17 1.0 155 700 Fair
tive
Example 3
______________________________________
Table 1 indicates that the coating agent of this invention imparts
imporved surface strength, air permeability, and printability to the
coated paper.
Note:
*.sup.1 Measured with a Brookfield viscometer at 60 rpm and 50.degree. C
*.sup.2 Measured with a IGT printability testing machine and IGT pick oi
M ink (made by Dainippon Ink & Chemicals, Inc.) at a printing pressure of
35 kg/cm, spring drive B. The greater the values, the higher the surface
strength.
*.sup.3 Measured with an Okentype air permeability tester. The greater
the values, the higher the barrier properties.
*.sup.4 Snap dry ink (made by Dainippoin Ink & Chemicals, Inc.) was
applied in a thickness of 5.mu. using an RI printability testing machine.
The results were ranked in three grades of good, fair, and poor according
to the overall evaluation as to ink receptivity and printing gloss and
others.
Example 2
In the same reactor as in Example 1 were placed 2800 parts of vinyl
acetate, 44 parts of vinyltriacetoxysilane, and 3.92 parts of
2,2'-azobisisobutyronitrile. The atmosphere in the reaction system was
replaced with nitrogen while stirring. The temperature was raised to
60.degree. C. Polymerization was continued for 3.5 hours. When
polymerization was terminated, the solid content in the system was 49%.
Unreacted vinyl acetate was expelled by introducing methanol vapor. Thus,
35% methanol solution of copolymer was obtained. For saponification
reaction, a methanol solution containing sodium hydroxide in an amount of
5 mol% for the vinyl acetate unit was added with stirring to the methanol
solution of copolymer at 40.degree. C. The resulting white gel was crushed
and washed with methanol completely to remove unreacted
vinyltriacetoxysilane. After drying, silicon-containing modified PVA was
obtained. The degree of polymerization was 1500. According to
atomic-absorption spectroscopy and chemical analysis, the copolymer was
found to contain 1.1 mol% of vinylsilane unit and the degree of
sapanification of vinyl acetate units was found to be 99.9 mol%.
A coating solution containing 4.5% of PVA was prepared by heating 4.5 parts
of the modified PVA, 95.5 parts of water, and 0.225 part of sodium
hydroxide. Using this coating solution, coated paper was prepared as in
Example 1. The properties of the resulting coated paper are shown in Table
2.
EXAMPLE 3
In a reactor equipped with a stirrer, thermometer, dropping funnel, and
reflux condenser were placed 2800 parts of vinyl acetate, 11 parts of
triacetoxysilane, and 3.92 parts of 2,2'-azobisisobutyronitrile. The
atmosphere in the reaction system was replaced with nitrogen while
stirring. The temperature was raised to 60.degree. C. Polymerization was
continued for 3 hours while adding dropwise 33 parts of
vinyltriacetoxysilane so that the concentration ratio of
vinylacetoxysilane to vinyl acetate is kept constant in the system. When
polymerization was terminated, the solid content in the system was 49%.
Unreacted vinyl acetate was expelled by introducing methanol vapor. Thus,
35% methanol solution of copolymer was obtained. This copolymer was
saponified as in Example 2. Thus, silicon-containing modified PVA was
obtained which has the degree of polymerization of 1550 and the degree of
saponification of 99.9 mol% for vinyl acetate unit and contains 1.0 mol%
of vinylsilane unit. Using a coating solution of this modified PVA, coated
paper was prepared as in Example 2. The properties of the resulting coated
paper are shown in Table 2.
TABLE 2
______________________________________
Viscosity
of coating Coating Surface Air per-
solution quantity strength meability
Print-
(cp) (g/m.sup.2)
(cm/sec) (sec) ability
______________________________________
Example 2
17 1.0 230 3500 Good
Example 3
15 1.0 265 6500 Good
______________________________________
It is to be noted from Table 2 that the product in Example 3 in which
vinyltriacetoxysilane was added continuously so that the concentration
ratio of monomer is kept constant in the polymerization system is superior
in performance to the product in Example 2 in which vinyltriacetoxysilane
was added all at once at the beginning of polymerization.
EXAMPLES 4 TO 11
Sizepress coating was carried out as in Example 1 except that the modified
PVA was replaced by various kinds of silicon-containing modified PVA. The
results are shown in Table 3.
Polymerization was carried out by adding continuously silicon-containing
olefinic unsaturated monomer so that the concentration ratio of monomers
(vinyl acetate to silicon-containing olefinic unsaturated monomer) is kept
constant in the polymerization system.
TABLE 3
__________________________________________________________________________
Modified PVA Coating
Silicon-
Degree of solution
containing
saponification
Degree
NaOH Properties of coated paper
Ex-
Silicon-containing
monomer
of vinyl
of %/ Visco-
Coating
Surface
Air per-
am-
olefinic unsaturated
unit acetate unit
polymer-
(modified
sity quantity
strength
meability
Printa-
ple
monomer (mol %)
(mol %)
ization
PVA) (cp) (g/m.sup.2)
(cm/sec)
(sec)
bility
__________________________________________________________________________
4 Vinyltriacetoxysilane
3.0 99.9 1600 2 15 1.0 300 10000
Good
5 Vinyltriethoxysilane
1.0 99.9 1650 1 17 1.0 250 5000 Good
6 Vinylisobutyldimeth-
0.5 99.8 1700 0.5 17 1.0 220 2500 Good
oxysilane
7 3-Acrylamide-propyl-
0.2 87.8 1750 0 16 1.0 190 5000 Good
trimethoxysilane
8 3-Acrylamide-propyl-
0.3 99.8 1700 0 18 1.0 200 800 Good
triethoxysilane
9 2-Acrylamide-2-methyl-
1.0 99.5 1650 0.5 17 1.0 250 5000 Good
propyltrimethoxysilane
10 2-Acrylamide-isopro-
0.5 99.9 1700 0 17 1.0 210 1500 Good
pyltrimethoxysilane
11 N--(2-acrylamide-ethyl)-
0.2 87.6 1700 0 16 1.0 200 5000 Good
aminopropyltrimethoxy-
silane
__________________________________________________________________________
EXAMPLE 12
Three parts of modified PVA prepared in Example 1 and 0.03 part of sodium
hydroxide were dissolved in 97 parts of water, whereby a coating solution
containing 3.0% of PVA was prepared. Using a Dixon coater, this coating
solution was applied (0.4 g/m.sup.2) to white paper board (basis weight of
300 g/m.sup.2) at a | | |
