Whitened polymers and the use thereof in coating slips for...

Details Whitened polymers and the use thereof in coating slips for... Whitened polymers and the use thereof in coating slips for...

2000-02-28

2002-01-15

Szekely, Peter (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C524S087000, C524S089000, C524S090000, C524S091000, C524S092000, C524S094000, C524S820000, C524S827000, C524S828000, C524S831000, C427S385500, C427S389900, C427S391000, C427S393000, C427S393500, C427S393600, C427S394000, C427S395000, C427S396000, C442S130000, C442S154000, C442S155000, C442S164000, C428S512000, C428S513000, C428S514000, C428S522000

Reexamination Certificate

active

06339117

ABSTRACT:

The invention relates to whitened polyacrylonitriles and acrylonitrile copolymers, especially as aqueous dispersions, to coating slips comprising them, and to their use for coating a wide variety of substrates.
In addition to the customary white pigments, especially china clay and calcium carbonate, the coating slips generally include anionic whiteners for the purpose of optical brightening, examples being those of the bis-triazinyl-aminostilbene-disulfonic acid type. These whiteners, however, lead only to highly unsatisfactory white effects and to very low grayness limits (=whitener concentration at which further addition of whitener produces no increase, or even a decrease, in whiteness). Furthermore, the whitener types referred to possess inadequate lightfastness in the coating slips.
DE-A-195 00 195 describes how whitened, finely divided polymer materials affixed with brighteners suitable for the polymer material can be used as powders or as dispersions for whitening paper coating slips based on synthetic binders. This method is still in need of improvement, however, in terms of the rheological properties when the whitened polymer powder is used, or in terms of the solids contents of their dispersions.
The preparation of whitened coatings on various substrates, especially on coated papers and cards with high whiteness, a high grayness limit and good lightfastness, is a difficult problem which has not been solved satisfactorily to date.
It has now been found, surprisingly, that when specific whitened polymers are used in coating slips, they give the coatings produced therewith on a wide variety of substrates very high whitenesses, high grayness limits, high lightfastnesses, and high bleedfastnesses.
The invention therefore provides polyacrylonitriles or acrylonitrile copolymers, referred to below simply as polymers, which are whitened by means of optical brighteners and are obtainable by emulsion polymerization of acrylonitrile with or without comonomers, in the presence or absence of graftable substrates, with the addition of an optical brightener before, during or after the emulsion polymerization.
In one preferred embodiment the polymers of the invention are in the form of their aqueous dispersions.
The aqueous polymer dispersion contains preferably:
a) 1-65% by weight, in particular 10-55% by weight, of polymer of the invention, based on the dispersion, and
b) 1-15% by weight, in particular 5-10% by weight, of surface-active substances, based on polymer of a).
The aqueous polymer dispersion may additionally include customary additives such as preservatives, preferably from 0 to 15% by weight, based on the polymer of component a).
Suitable surface-active substances are preferably anionic and/or nonionic surfactants.
Preferred surface-active substances are the graftable substrates of types I to III below, preferably those of types I and II, and, in particular, those of type I.
Examples of suitable preservatives are methyl- and chloromethyl-isothiazolin-3-one, benzisothiazolin-3-one, or mixtures thereof.
The polymers of the invention are preferably finely divided and in particular have an average particle size (measured by the method of laser correlation spectroscopy) of <1 &mgr;m, preferably 50-800 nm, in particular 50-500 nm, and, with particular preference, 50-400 nm.
The polymers preferably contain 50 or more mol-% of acrylonitrile units. Examples of suitable comonomers are nonionic and/or ionic-ethylenically unsaturated comonomers.
Preferred nonionic comonomers are alkyl esters of acrylic and methacrylic acid, preferably C
1
-C
8
alkyl esters, and also ethoxylated and propoxylated esters of acrylic and methacrylic acid, styrene and &agr;-methylstyrene and their derivatives, vinyl carboxylates, preferably vinyl acetate, conjugated dienes such as, for example, butadiene or isoprene, methacrylonitrile, vinyl chloride and vinylidene chloride, N-alkyl and N-aryl-substituted maleimides, and also hydrophilicizing comonomers such as acrylamides and methacrylamides, acrylic and methacrylic acid, N-vinylamides, such as, for example, 1-vinyl-2-pyrrolidone.
Examples of suitable ionic comonomers are vinylsulfonic acid and its salts, methallylsulfonic acid and also allylsulfonic acid and salts thereof, styrenesulfonic acid and its salts, (2-acrylamido-2-methyl)-propanesulfonic acid, its esters and its salts, (3-sulfopropyl)-acrylic acid and its salts, (3-sulfopropyl)-acrylic esters, and also the alkyl esters and oxyalkyl esters of said vinylic sulfonic acids.
Particularly preferred comonomers are alkyl acrylates, styrene, and butadiene/styrene mixtures. Preferably, however, it is also possible to operate with acrylonitrile as sole monomer.
In one preferred embodiment the whitened polymers of the invention are obtainable by emulsion polymerization in the presence of graftable substrates. These graftable substrates are preferably polymeric emulsifiers containing sulfonate groups. Particularly suitable are polymers of types I and II, which preferably have average molecular weights of M
w
>5000 g/mol. Likewise preferred are poly- and oligourethanes of type III, which preferably have an average molecular weight of >1000 g/mol.
The emulsifier of type I corresponds to the formula (I) and that of type II to the formula (II)
in which
M denotes a cation-forming radical, preferably H, Na, K, NH
4
or R
3
NH
3
,
n denotes 1 or 2,
R denotes a hydrocarbon chain having 1 to 10 carbon atoms or an aromatic radical having 6 to 10 carbon atoms, or preferably —(CH
2
)
2
NH(CH
2
)
2
— or a radical derived from C
1
-C
4
alkane or from benzene, methylbenzene or naphthalene,
R
1
denotes H or —CH
3
,
R
2
denotes H, C
1
-C
16
alkyl, preferably —CH
3
, —C
2
H
5
, —C
4
H
9
, —CH
2
—C(CH
3
)
3
, —C
14
H
29
, —C
16
H
33
or phenyl,
R3 denotes a hydrocarbon radical of an amine,
x, y are chosen such that the average molecular weight M
w
of the polymers (I) and (II) is from 2000 to 500,000, preferably from 5000 to 100,000, and the ratio x:y=from 1:4 to 1:1, preferably from 1:3 to 1:1.
Particularly preferred emulsifiers are those of the formulae (I) and (II), in which
1A)
R
1
=CH
3
and
R
2
=—CH
2
—C(CH
3
)
3
or
2A)
R
1
=CH
3
and
R
2
=CH
3
or
3A)
R
1
=H and
R
2
=C
6
H
5
,
where in each case
R=—CH
2
CH
2
—,
M=Na,
n=1 and
x:y=from 1:1.0 to 1:1.1.
Very particular preference is given to the emulsifier of the type 1A).
Emulsifiers of the formulae I and II are known, for example, from EP-A 590 460.
Emulsifiers of the type III are the oligourethanes known from EP-A 400 410 which are used in connection with the free-radically initiated emulsion polymerization for preparing surface sizing agents for paper.
Particular preference is given as emulsifiers of the type III to the oligourethanes specified on p. 7 of EP-A 400 410, especially those obtainable by reacting a macrodiol and an oligoisocyanate having an NCO functionality of >2.
A macrodiol is, for example, a polyester of adipic acid, hexanediol, neopentyl glycol having an average molecular weight M
w
=1700 g/mol. The oligoisocyanate is, for example, a hexamethylene diisocyanate (HDI) based trimer of the isocyanate type (f=3) and higher molecular mass oligomers (f=3) derived therefrom.
At medium and high molecular weights, suitable linear and branched polyacrylonitriles and acrylonitrile copolymers exhibit intrinsic viscosities (measured in dimethylformamide at 20° C.) of [&eegr;]=from 0.5 to 10.0 dl/g, preferably of [&eegr;]=from 1.0 to 5.0 dl/g. High molecular mass products exhibit heightened long-chain branching.
Preferred polymers are those modified by strongly acidic groups. Preferred strongly acidic groups are sulfonate groups, the amount of strongly acidic groups being preferably 50-500, particularly 100-500, milliequivalents/kg of polymer.
The incorporation of strongly acidic groups into the copolymer can take place, for example:
a) by using the above-described graftable emulsifiers containing sulfo group

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