Fine-particle polymer dispersions for paper sizing

Details Fine-particle polymer dispersions for paper sizing Fine-particle polymer dispersions for paper sizing

2000-08-11

2002-07-30

Wu, David W. (Department: 1713)

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

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C526S238220, C526S319000, C524S832000, C524S833000

Reexamination Certificate

active

06426381

ABSTRACT:

The invention relates to fine-particled, aqueous polymer dispersions based on styrene/(meth)acrylate copolymers, processes for their preparation and their use as sizes for paper, cardboard and board.
The polymer dispersions according to the invention are particularly suitable as sizes for the production of graphic papers which are used for modern printing processes since they both produce a good printed image by inkjet printing and have good toner adhesion, as required, for example, for use in laser printers or copiers.
Sizes for paper which are based on styrene/acrylate dispersions are known.
Thus, Japanese Application JP 58/115196 describes aqueous dispersions based on styrene/acrylate copolymers, grafted onto water-soluble high molecular weight polyhydroxy compounds including starch, as paper strength agents having a sizing effect. These graft copolymers are obtained by polymerizing styrene and an acrylate, such as n-butyl acrylate, in the presence of an aqueous solution of starch with the formation of an aqueous dispersion. In the process described, starch is used in high molecular weight form and is not further degraded before the polymerization. The initiators used, such as potassium peroxodisulphate, ammonium peroxodisulphate or 2,2′-azobis(amidinopropane) dihydrochloride, moreover have unsatisfactory grafting activity, so that only coarse-particled dispersions having a low grafting yield are obtained, which dispersions can be used for increasing the strength, but whose sizing effect is unsatisfactory. In particular, the sizing effect of these products declines on papers which have been engine-sized beforehand with alkyldiketene (AKD) or alkenylsuccinic anhydride (ASA), as are usually used for the production of graphic papers, and in the case of acidic inks as are used, for example, in the Hercules sizing test for testing the sizing effect.
European Patent Application EP-A 257 412 and EP-A 276 770 claim graft copolymers of acrylonitrile and acrylates on starch, which are likewise used in the form of fine-particled aqueous dispersions for paper sizing.
Fine-particled size dispersions which are obtained by emulsion polymerization of monomers, such as, for example, acrylonitrile, butyl acrylate or styrene, in the presence of polymeric anionic emulsifiers containing sulpho groups are furthermore known (cf. EP-A 331 066 and EP-A 400 410).
These dispersions known from the prior art have excellent performance characteristics in particular on neutral and chalk-containing papers. However, they have low stability to divalent and trivalent cations, such as, for example, Ca
2+
or Al
3+
. Under unfavourable conditions in practice, this can lead to precipitates in the size press and hence to impairment of its sizing effect.
Fine-particled polymer dispersions of starch graft copolymers based on styrene/(meth)acrylates having improved performance characteristics have now been found.
The present invention relates to aqueous dispersions obtainable by free radical emulsion copolymerization of ethylenically unsaturated monomers in the presence of starch, characterized in that
(a) 30 to 60% by weight of at least one optionally substituted styrene,
(b) 60 to 30% by weight of at least one C
1
-C
4
-alkyl (meth)acrylate,
(c) 0 to 10% by weight of other ethylenically unsaturated copolymerizable monomers
are used as ethylenically unsaturated monomers,
(d) 10 to 40% by weight of degraded starch having a molecular weight M
n
=500 to 10,000 are used as starch, the sum (a)+(b)+(c)+(d) being 100%,
and a graft-linking, water-soluble redox system is used as free radical initiator for the free radical emulsion polymerization.
Suitable monomers of group (a) are styrene and substituted styrenes, such as &agr;-methylstyrene or vinyltoluene or mixtures thereof.
Suitable monomers of group (b) are C
1
-C
4
-alkyl acrylates, C
1
-C
4
-alkyl methacrylates or mixtures thereof, such as, for example, n-butyl, iso-butyl, tert-butyl or 2-butyl acrylate and the corresponding butyl methacrylates, and furthermore methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate or propyl methacrylate. A mixture of at least two isomeric butyl acrylates is preferred, it being possible for the mixing ratio to be 10:90 to 90:10. Mixtures of n-butyl acrylate and tert-butyl acrylate and mixtures of n-butyl acrylate and methyl methacrylate are particularly preferred.
Suitable monomers of the group (c) are further ethylenically unsaturated monomers, such as ethylhexyl acrylate, stearyl acrylate, stearyl methacrylate and further esters of acrylic and methacrylic acid with alcohols which have more than four C atoms, and furthermore acrylonitrile, methacrylonitrile, acrylamide, vinyl acetate or anionic comonomers, such as acrylic acid, methacrylic acid, styrenesulphonic acid. Particularly preferred monomers of group (c) are acrylic acid and styrenesulphonic acid.
The % by weight of components (a) to (d) relate to the total solids content of the dispersion, i.e. the sum of the amounts by weight of components (a) to (d).
Natural starches, such as potato, wheat, maize, rice or tapioca starch, are suitable as starch, potato starch being preferred. Starch types having a high amylopectin content of 80% or higher are preferably used. Potato starch having an amylopectin content >95% is particularly preferred.
It is also possible to use chemically modified starches, such as hydroxyethyl- or hydroxypropyl-starches, or starches which contain anionic groups, such as, for example, phosphate starch, or cationic starches which contain quaternized ammonium groups, a degree of substitution DS=0.01-0.2 being preferred. The degree of substitution DS indicates the number of cationic groups which are contained in the starch on average per glucose unit. Amphoteric starches which contain both quaternary ammonium groups and anionic groups, such as carboxylate and/or phosphate groups, and which optionally can also be chemically modified, for example hydroxyalkylated or alkyl-esterified, are particularly preferred.
The starch (d) to be used according to the invention is obtained by subjecting said starch types to oxidative, thermal, acidic or enzymatic degradation. Oxidative degradation of the starch is preferred. Oxidizing agents, such as hypochlorite, peroxodisulphate or hydrogen peroxide, or combinations thereof, which are preferably used in succession to establish the desired molecular weight of the starch, are suitable for the degradation. Starch degradation with hypochlorite, as usually carried out for improving the dissolution properties of the starch, and a further degradation, for example with hydrogen peroxide, which can be carried out, for example, shortly before the subsequent graft copolymerization, is particularly preferred. In this case, hydrogen peroxide (calculated as 100%) is used in concentrations of 0.3 to 5.0% by weight, based on starch employed. The amount of hydrogen peroxide depends on the molecular weight to which the starch is to be degraded.
The starches (d) degraded in this manner preferably have an average molecular weight M
n
of 500 to 10,000, with the result that, on the one hand, good dispersing of the emulsion polymers is ensured and, on the other hand, premature crosslinking and precipitation of the polymerization batch is avoided. The average molecular weight of the degraded starch can readily be determined by gel chromatographic analysis processes after calibration, for example with dextran standards, by known methods. Viscosimetric methods, as described, for example, in “Methods in Carbohydrate Chemistry”; Volume IV, Academic Press New York and Frankfurt, 1964, page 127”, are also suitable for the characterization. The intrinsic viscosity thus determined is preferably 0.05 to 0.12 dl/g.
The polymerization is carried out as a rule by a procedure in which both the monomers, either individually or as a mixture, and the free radical initiators suitable for initiating the polymerization are added to the aqueous solution of the degraded starch.
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