Polymer dispersion and method to produce the same

Details Polymer dispersion and method to produce the same Polymer dispersion and method to produce the same

2001-08-06

2004-06-22

Wyrozebski, Katarzyna (Department: 1714)

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...

C524S827000, C524S831000, C524S832000, C524S047000

Reexamination Certificate

active

06753377

ABSTRACT:

The invention relates to starch-based polymer dispersion, which contains a co-polymer of starch with monomers. The invention also relates to a process for the preparation of said dispersion and its use in paper manufacture, particularly as a surface sizing additive for paper and as a wet- and dry-strengthener for paper which is added to the wet end of a paper machine. The term paper as used in this invention refers to both paper and paperboard, which are prepared by using either a wood-containing or a chemically processed fibre or a recycled fibre or a mixture thereof. These papers include fine papers, LWC papers, SC papers and newsprint papers, packing boards and folding boxboards.
U.S. Pat. No. 4,301,017 (Standard Brands Inc.) discloses an aqueous polymeric dispersion, which is produced by co-polymerization of at least one vinyl monomer in an aqueous solution of a thinned/degraded starch derivative. The starch derivatives used were diethylaminoethyl, acetyl, cyanoethyl or carbamoethyl derivatives. Such derivatives are just slightly ionic at neutral pH values, and in papermaking processes, best functioning is achieved only at low pH values (pH<6) in which they are partly ionized. Nowadays, most papermaking processes are neutral, and, consequently, polymers should also act at pH values in the range from 6 to 9, preferably from 7 to 8.5. The degree of substitution of the starch used in the above mentioned patent and having the above mentioned substituents should be at least 0.05. Before polymerization, starch is degraded/thinned enzymatically to a preferable intrinsic viscosity value ranging from 0.12 to 0.28 dl/g. Ce(IV) salts have been used as a polymerization catalyst (initiator).
Patent DE 37 02 712 (U.S. Pat. No. 4,835,212; BASF AG) describes the preparation of polymer dispersions from starches having an intrinsic viscosity of from 0.04 to 0.12 dl/g. Also the starches used in this patent are enzymatically highly degraded before the polymerization. In the examples, slightly cationic commercial starches have been used which have a maximum degree of substitution (DS) of 0.07. As monomers, in patent DE 3,702,712 it was used from 40 to 140 parts by weight, based on the total polymer, of a monomer mixture containing from 20 to 65% by weight of acrytonitrile or methacrylonitrile, from 80 to 35% by weight of acrylic acid esters and from 0 to 10% by weight of other co-polymerizable monomers. Peroxides were used as catalysts.
It is an object of this invention to provide dispersions which have a novel composition and an improved reactivity in comparison with the prior known dispersions, and a good retention and adherence to paper fibres. The dispersions according to the invention can be used as an additive in paper surface sizing and as a wet- and dry-strengthener for paper which is added to the wet end of a paper machine. The invention also relates to a process for the preparation of said dispersions.
In surface sizing, different kinds of polymers are generally added to the surface-sized starch in order to improve printability. These polymers can be copolymers of styrene with maleic acid, various acrylates etc. In these cases, the effect of the expensive polymers added will not be totally utilized, while the polymers are not completely inter-miscible with starch (thermodynamic solubility coefficients differ too much from each other), and thus they do not form a completely inter-miscible polymeric network (interpenetrating network) while drying.
It is one object of the present invention to produce polymer particles which have a surface structure similar to that of the used surface sizing starch, and, consequently, these two achieve a completely interpenetrating network. This is achieved by using as a polymer intermediate and a starting batch the same starch as or a starch chemically similar to the starch in the surface sizing agent itself. Then the particles formed during the polymerization remain in the starch film and are not separated during drying. If the starch is enzymatically degraded before the polymerization, other means are needed than only the starch itself for stabilizing the polymer, for example an emulsifier or water soluble monomers. In that case, the surface structure of the polymer is no longer similar to that of the surface sizing starch.
A copolymer suitable to be added into the pulp on a paper machine can be prepared by customizing for each paper machine a starch derivative having a cationic value and other properties suitable for the wet end system of the machine. Also herein, the guiding principle is that the surface structure of the polymer particles is as similar as possible as the molecular structure (molecular weight, cationic value, branching etc.) of the starch or the starch-based strengthener used at the wet end. Then the polymers are not separated from the starch matrix while drying but form a completely interpenetrating network.
Unlike in the two processes mentioned above, in the present invention it is possible to produce copolymers by using non-degraded or only slightly oxidized (e.g. peroxide- or hypochlorite-oxidized) starch. For example, the starch can be potato starch, corn starch, barley starch, wheat starch or tapioca starch, of which the potato starch and corn starch are preferable. The starch is not enzymatically degraded before the polymerization, and thus it has an intrinsic viscosity of at least 1.0 dl/g, preferably from 1.5 to 15 dl/g, and most preferably from 3 to 15 dl/g.
The polymer dispersion according to the invention is stable, and its viscosity does not increase too much during any preparation step. By using the oxidized starch, the carboxylic groups formed in the oxidation can also be utilized in the stabilization of the polymer. Because of the higher molecular weight of starch, it stabilizes the particle also sterically. Furthermore, the ionic groups formed in the oxidation and cationization stabilize the particle, and thus the water-soluble monomers are not necessary for the stabilization of the particle in the polymerization. In the enzymatic treatment of the starch, such stabilizing groups are not formed and the steric stabilization is disappeared as well, and then it is necessary to use emulsifiers, which are generally known to impair sizing. At the same time, the strength properties of the paper are drastically impaired.
During the preparation of the polymer dispersions according to the invention, the starch is preferably cationized by a suitable cationizing chemical depending on the intended use so that in surface sizing applications the DS values are in the range from 0.01 to 1.0, preferably from 0.01 to 0.08, and in pulp sizing applications in the range from 0.05 to 1.0, preferably from 0.08 to 1.0, and more preferably from 0.1 to 0.5. However, a native or anionized starch can be used as well.
If the starch is cationized, suitable cationizing chemicals include cationizing chemicals containing quaternary nitrogen, for example 1,3-epoxy or 1,3-hydrochloride derivatives. 2,3-Epoxypropyltrimethyl ammonium chloride is a preferable cationizing chemical. Cationizing is performed when the starch is in soluble form in an alkaline aqueous solution (pH>7, preferably from 9 to 11). Then the aqueous starch solution has a solids content of under 50%, preferably from 10 to 35%, and a temperature of over 60° C., preferably from 75 to 90° C. The oxidation and cationization of the natural starch can also be performed simultaneously, but it is preferred that the oxidation is performed before the cationization, because the cationization preferably occurs at higher temperatures than at which the oxidation has to be performed at first because of the gelatinization of native starch.
We have discovered that the best results in co-polymerization are achieved when, unlike in the prior known processes, a starch having a higher molecular weight (i.e. a higher intrinsic viscosity) is used together with a suitable degree of substitution achieved by cationic and/or anionic substituents, and a suitable monomer composition. In the present invention

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