Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2000-12-07
2003-11-11
Beck, Shrive P. (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S385500, C427S508000, C427S496000, C427S407100
Reexamination Certificate
active
06645563
ABSTRACT:
The present invention relates to a method of coating a shaped mineral article using aqueous formulations comprising as film-forming constituent at least one copolymer P, synthesized from ethylenically unsaturated monomers M.
By shaped mineral articles are meant hereinbelow, in particular, shaped articles which comprise a mineral binder. Specifically, a shaped article is understood here as being an article produced, after shaping, from a mixture (a mortar) comprising a mineral binder, water, aggregates and, if used, auxiliaries by virtue of the fact that the mineral binder/water mixture hardens as a function of time with or without the action of elevated temperature. Mineral binders are common knowledge. They comprise finely divided inorganic substances, such as lime, gypsum, clay and/or cement, which are stirred up with water to convert them to their ready-to-use form which, when left to itself, hardens over time both in air and underwater, with or without the action of elevated temperature, to give a stonelike mass.
The aggregates consist in general of granular or fibrous, natural or synthetic stone (gravel, sand, mineral fibers), including in special cases metals as well, or organic aggregates or mixtures of said aggregates, with grain sizes or fiber lengths that are adapted in a conventional manner to the specific application intended. In many cases, color pigments are also used as aggregates, for the purpose of coloring.
Particularly suitable auxiliaries are those substances which accelerate or retard hardening or which influence the elasticity or porosity of the solidified, shaped mineral article. In particular the auxiliaries comprise polymers, as are known, for example, from U.S. Pat. No. 4,340,510, GB-PS 15 05 558, U.S. Pat. No. 3,196,122, U.S. Pat. No. 3,043,790, U.S. Pat. No. 3,239,479, DE-A 43 17 035, DE-A 43 17 036, JP-A 91/131 533 and other documents.
Examples of shaped mineral articles to be coated in accordance with the invention are concrete pipes, as used to transport waste water (cf. for example JP-A 55/44 273), concrete roof tiles (cf. for example DE-A 21 64 256 or DE-A 39 01 073) or curb stones, steps, base slabs, and plinth tiles based on mineral binders, and also fiber-reinforced concrete panels, ie. flat shaped mineral articles filled with inorganic or organic fibers such as polyester fibers or nylon fibers, for example.
A disadvantage of shaped mineral articles is that under the influence of weathering (especially exposure to water) the cationic constituents, such as Ca
2+
, are leached out over the course of time, reducing the strength of the articles. Another disadvantageous property of shaped mineral articles is the occurrence of efflorescence phenomena. These are probably attributable to the fact that the mineral binders contain cations with a valency of two or more, such as Ca
2+
, in an alkaline environment. Reaction with the carbon dioxide from the air can therefore cause the formation, on the surface of the shaped mineral articles, of white spots of lime which are unsightly and relatively insoluble in water. The phenomenon of efflorescence may appear either during the hardening of freshly prepared shaped mineral articles or on exposure to weathering of shaped mineral articles which have already hardened.
In order to avoid these disadvantageous properties, the shaped mineral articles are often provided with a coating. For this purpose use is generally made of aqueous coating systems whose film-forming constituent comprises an aqueous polymer dispersion. Customary binders include styrene-acrylate copolymers, vinyl acetate homo- and copolymers, pure acrylates and the like (cf. for example DE 21 64 256). However, the coatings obtainable with these binders are unable to prevent satisfactorily the emergence of the cationic constituents (efflorescence). Furthermore, such coatings are easily soiled.
DE-A-38 27 975 and DE-A-40 03 909 disclose coatings for concrete slabs that are based on aqueous polymer dispersions comprising at least one aromatic ketone as photosensitizer. The photosensitizer causes superficial crosslinking of the coating. Although this largely prevents the unwanted efflorescence, the coatings are not stable under long-term weathering.
It has proven possible to improve the protection of shaped mineral articles from the efflorescence described above by using the coating compositions based on styrene/acrylate dispersions or pure acrylate dispersions of EP-A-469 295 and DE-A-195 14 266. EP-A-469 295 recommends for this purpose the use of a specific anionic emulsifier, and DE-A-195 14 266 the use of polymers whose incorporated monomers include specific, sulfonate-functional monomers.
The prior art coatings all have the disadvantage of comparatively high water absorption. This water absorption carries with it the risk that low molecular mass constituents in the coating, such as surfactants, which are frequently employed to prepare the aqueous formulations, will be leached out in the course of prolonged weathering. Such leaching is manifested, for example, in intensified blushing or in embrittlement of the coating. The uptake of water, moreover, owing to the polymer swelling which accompanies it, causes a drop in the mechanical stability of the coating and an increase in the slipperiness of the surface, which is likewise undesirable on safety grounds. With pigmented coatings, moreover, there is the danger that the perceived color will be adversely altered should the coating take up water.
It is an object of the present invention to provide coatings for shaped mineral articles which feature low water absorption and at the same time do not suffer any adverse alteration in their strength and visual appearance even on prolonged exposure to moisture.
We have found that this object is achieved and that the water uptake of coatings based on copolymers of ethylenically unsaturated monomers can be reduced if the monomers copoymerized include from more than 0.2% by weight up to 5% by weight of itaconic acid, based on the overall amount of monomer to be polymerized.
Copolymers comprising itaconic acid are known in principle from EP-A-264 903, where the itaconic acid is used to enhance the mechanical stability of elastomeric polymers based on alkyl acrylate. As far as references to the weathering stability and water uptake of such polymers on weathering are concerned, EP-A-264 903 gives none.
The present invention therefore provides for the use of aqueous formulations comprising as film-forming constituent at least one copolymer P, synthesized from ethylenically unsaturated monomers M, for coating shaped mineral articles, the copolymer P having a glass transition temperature T
g
in the range from −25 to +80° C. and the monomers M comprising more than 0.2% by weight and up to 5% by weight, based on their overall weight, of itaconic acid as monomer M1.
The monomers M from which the copolymer P is synthesized preferably comprise from 0.3 to 4% by weight and, in particular, from 0.5 to 3% by weight of itaconic acid. Although itaconic anhydride can be used for the preparation, instead of the acid, it is preferred to employ itaconic acid itself as monomer M1.
The polymers containing itaconic acid are usually prepared by free-radical polymerization of ethylenically unsaturated monomers M comprising itaconic acid plus at least one further comonomer. Appropriate comonomers are generally selected from vinylaromatic monomers, such as styrene, &agr;-methylstyrene, o-chlorostyrene or vinyltoluenes, from the vinyl esters of aliphatic C
1
-C
18
monocarboxylic acids, such as vinyl acetate, propionate, butyrate, valerate, hexanoate, 2-ethylhexanoate, decanoate, pivalate, laurate and stearate, and also commercial monomers VEOVA® 5-11 (VEOVA® X is a trade name of Shell and stands for vinyl esters of versatic® X acids—&agr;-branched aliphatic carboxylic acids of X carbons), and from the esters of ethylenically unsaturated C
3
-C
8
mono- or dicarboxylic acids with C
1
-C
18
-, preferably C
1
-C
12
- and, in particular, C
1
-C
8
-alkanols or C
5
-C
8
Bechert Bertold
Hümmer Wolfgang
Schwartz Manfred
Wiese Harm
Zhao Cheng-Le
BASF - Aktiengesellschaft
Beck Shrive P.
Cleveland Michael
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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