The present invention relates to shelf-stable, pigmented monomeric (meth)acrylate-based compositions comprising a stabilizer selected from sterically hindered aminoxyl radical compounds, and a urea-aldehyde resin. Moreover, the present invention relates to a process for the stabilization of pigmented monomeric (meth)acrylate-based compositions and to the use of sterically hindered aminoxyl radical compounds for the stabilization of pigmented monomeric (meth)acrylate-based compositions in the presence of a urea-aldehyde resin.

Construction systems based on reactive monomers, such as (meth)acrylates, are used in a broad range of different flooring and waterproofing applications. Materials need to be colored, preferably by using color pigments. Some color pigments are especially difficult to stabilize in a (meth)acry- late carrier system. Especially in the case of pigmented preparations based on iron oxide pigments (black or red) and Naphthol AS, those preparations quickly turn solid already at room temperature. There existed a need for sufficient stabilization to obtain a shelf-stable, pigmented (meth)acrylate-based composition.

EP 3 053 974 A1 discloses (Abstract) a pigment dispersion composition containing a pigment, a polymerizable compound, and a compound having a structural unit represented by Formula (A), a structural unit represented by Formula (B), a structural unit represented by Formula (C) derived from polyalkylene oxide having a number average molecular weight of equal to or greater than 300 and less than 5,000, and a structural unit represented by Formula (D), in which a mass ratio [(B)/(C)] is 20/80 to 60/40.

Since Q represents a group derived from poly(meth)acrylate having a glass transition temperature of equal to or higher than 50°C, said compound represents a polymeric (meth) acrylate compound.

WO 01/42312 A1 discloses (Abstract) a composition comprising: a1) at least one oligomer, cooligomer, polymer or copolymer or a mixture thereof having at least one ethylenically unsaturated bond; or a2) a mixture of at least one ethylenically unsaturated monomer with at least one oligomer, co-oligomer, polymer or copolymer having at least one ethylenically unsaturated bond; and b) at least one stable sterically hindered nitroxyl free-radical or at least one compound of the benzofuran-2-one type or a mixture of the two compounds. The invention relates also to a method of stabilizing ethylenically unsaturated monomers, oligomers, co-oligomers, polymers or copolymers, and to the use of a stable sterically hindered nitroxyl free-radical or a compound of the benzofuran-2-one type in preventing polymerization or crosslinking. The composition may also contain as additional additives dyes and/or white or colored pigments.

In WO 01/42312 A1 , a composition of at least one oligomer, co-oligomer, polymer or co-polymer or a mixture thereof is described with at least one ethylenically unsaturated bond. Stabilization of monomeric (meth)acrylates alone is not described.

Color pigments are advantageously formulated together with urea-aldehyde resins, especially with urea-formaldehyde resins. Such resins improve the wetting of the pigments as well as their dispersibility, resulting in better end properties such as opacity, gloss, coverage, etc. Moreover, the settling stability of the components in the color paste is also improved.

Initial trials based on a prototype liquid carrier (a mixture of a urea-aldehyde resin with triethylene glycol dimethacrylate, TEGDMA) showed that ordinary (meth)acrylate stabilization was not sufficient. Especially in the case of pigment preparations based on "Oxide Black 318" (Fe2Os), Naphthol AS (an organic azo dye) as well as "Oxide Red" (Fe2Os), those preparations turned solid already at room temperature (23°C). Therefore, storage stability without special stabilization is poor.

There existed a need for the stabilization of monomeric (meth)acrylates in the presence of pigments (black, white, colored), even at elevated temperatures, to obtain shelf-stable, pigmented (meth)acrylate-based compositions. The stabilization of purely monomeric (meth)acrylates in the presence of pigments is more difficult than the stabilization of oligomer- or polymer-diluted (methacrylates due to the much higher concentration of polymerizable double bonds.

The before-mentioned technical problem is solved by the features of the independent claims. The dependent claims relate to preferred embodiments.

As will be shown in the experimental section, (meth)acrylates with color pigments have low viscosities but polymerize fast, especially at elevated temperatures. Urea-aldehyde resins increase the viscosities of such formulations and accelerate polymerization, especially at elevated temperatures. It was surprisingly found that the stabilization of (meth)acrylates with color pigments and urea-aldehyde resins could be achieved by the use of sterically hindered aminoxyl radical compounds alone.

In trials with a solution of 4-Hydroxy-TEMPO in methacrylic monomers such as TEGDMA added to the pigment paste containing the pigment and the urea-aldehyde resin, it was found that these preparations remained liquid and stable in viscosity. Stability was proven for storage at room temperature as well as storage at elevated temperatures over longer periods of time (at least 6 weeks, preferably at least 7 weeks, more preferably at least 8 weeks, e.g. at least 3 months, at least 6 months, and in particular at least 12 months). Thus, the term "shelf-stable" within the meaning of the present invention denotes a stability of the compositions or preparations of the invention which allows for a shelf life or storage time of at least 6 weeks, preferably at least 7 weeks, more preferably at least 8 weeks, e.g. at least 3 months, at least 6 months, and in particular at least 12 months. A stabilized liquid carrier system based on a urea-aldehyde resin mixed with TEGDMA was made. The stabilizer was present at different concentrations. There was no need for further addition of any other stabilizer.

According to a first aspect of the invention, the present invention provides a shelf-stable, pigmented (meth)acrylate-based composition comprising at least one polymerizable monomeric (methacrylate coumpound, at least one pigment, and at least one stabilizer selected from sterically hindered aminoxyl radical compounds. The composition further contains at least one urea- aldehyde resin. More preferably, the urea-aldehyde resin is a urea-formaldehyde polycondensation product.

For the purposes of the present invention, the term "comprising" is intended to include the narrower term "consisting of", but not to be synonymous therewith. It is moreover intended that in each actual case the sum of all of the percentages of the specified and unspecified constituents of the formulation of the invention is always 100%. Moreover, process steps may be conducted in any order or sequence. Preferably, however, the steps are to be carried out in the order indicated.

The term "polymerizable monomeric (meth)acrylate compound" excludes oligomeric and/or polymeric (meth)acrylate compounds regardless of whether they still contain polymerizable double bonds. Such compounds should not be present in the composition of the invention. The polymerizable monomeric (meth)acrylate contains at least one ethylenically unsaturated double bond. It may however contain more than one ethylenically polymerizable double bonds. It is preferred that the (meth)acrylate contains one to four (meth)acrylate moieties in the molecule, for example one (meth)acrylate moiety in the molecule, two (meth)acrylate moieties in the molecule, three (meth)acrylate moieties in the molecule or four (meth)acrylate moieties in the molecule, wherein two to four (meth)acrylate moieties are particularly suitable for a better crosslinking. These moieties can be bound, for instance, to a (straight chain or branched) polyether polyol or polyester polyol backbone via an ester bond between the carboxylic group of the (meth)acrylate and an OH-group of the respective alcohol.

A preferred (meth)acrylate is the triethylene glycol dimethacrylate (TEGDMA) due to several reasons. It is non-toxic, relatively cheap, not highly volatile and thus not excessively smelly and highly compatible with the other constituents of the composition of the invention. Moreover, it exhibits low viscosity.

The term "(meth)acrylate" as used throughout the present specification is meant to include both acrylates and methacrylates.

The composition of the invention should contain pigments which are selected from inorganic pigments, organic pigments and mixtures thereof, namely baryte, titanium dioxide, chromium (III) oxide, iron (III) oxide, copper (II) phthalocyanine, carbon black, bismuth vanadate, Naphthol AS- pigments, and mixtures thereof.

The stabilizer according to the invention should be selected from compounds having the following structural features: wherein R is alkyl and T is a group required to complete a 5- or 6-membered ring. R is preferably a Ci-4 alkyl, more preferably methyl, when the stabilizer is a cyclic nitroxyl radical. The most preferred choices are 1-oxy-2,2,6,6-tetramethylpiperidine (TEMPO) and 4-hydroxy- 1- oxy-2, 2,6,6- tetramethylpiperidine (4-OH-TEMPO), and mixtures thereof. The stabilizer should generally be used in amounts of 0.01 to 5.0% by weight. 4-OH-TEMPO has the advantage that it smells less than TEMPO. Generally the composition of the invention should comprise about 10 - 35% by weight of the polymerizable monomeric (meth)acrylate, 0.01 - 5.0%, preferably 0.05 - 2.0%, and in particular 0.075 - 1.0% by weight of the sterically hindered aminoxyl radical compound, about 10 - 35% by weight of the urea-aldehyde resin, and about 4 - 80% by weight, preferably 10 - 50% by weight of the pigment. In the case of carbon black, about 4% by weight of pigment is sufficient while in the case of TiC>2, up to 80% by weight of pigment are possible.

According to a second aspect of the invention, the present invention provides a process for the stabilization of pigmented (meth)acrylate-based compositions, comprising the steps of providing at least one polymerizable monomeric (meth)acrylate, adding a urea-aldehyde resin, adding from 0.05 to 5.0% by weight of a sterically hindered aminoxyl radical compound, dispersing in the obtained mixture a pigment, and optionally adding and/or dispersing further additives.

According to a third aspect of the invention, the present invention provides the use of sterically hindered aminoxyl radical compounds for the stabilization of pigmented monomeric (methacrylate-based compositions in the presence of a urea-aldehyde resin.

The present invention will now be illustrated in further detail by means of the following non-limiting examples.

EXAMPLES

(Basic Mix Design)

Preparation of a Laropal® A 81 solution: 49.5 g of triethylene glycol dimethacrylate (TEGDMA - Sartomer® SR205H, Arkema Group) was provided, 0.5 g of 4-hydroxy-1 -oxy-2, 2,6, 6- tetramethylpiperidine (4-OH-TEMPO, Lignostab® 1198, BASF SE) was added, and 50.0 g of Laropal® A 81 (urea-aldehyde resin, BASF SE) was added. The mixture was stirred for 4-6 hours until completely mixed. A stabilized monomer solution was obtained.

25 g of the Laropal® A 81 solution was taken, 5 g of Disperbyk® 110 (a phosphoric acidcontaining dispersing and wetting additive, BYK-Chemie GmbH) was added under stirring and 70 g of titanium dioxide pigment (Tioxide TR 92, HUNTSMAN) was dispersed in the mixture. This pigmented paste exhibited satisfactory stability, even for prolonged times at elevated temperatures.

Example 2:

Example 1 was repeated with 50.0 g of triethylene glycol dimethacrylate (TEGDMA - Sartomer® SR205H, Arkema Group) and 50.0 g of Laropal® A 81 (aldehyde resin, BASF SE). An unstabilized monomer solution was obtained.

54 g of this solution was taken, 5 g of Disperbyk® 110 (a phosphoric acid-containing dispersing and wetting additive, BYK-Chemie GmbH) was added under stirring and 20 g of Novoperm Red® F2RK70 (Naphthol AS-pigment, red, Clariant) and 21 g of Portaryte® B15 (BaSO4, Sibelco) were dispersed in the mixture.

In four different batches, 0 g, 0.75 g, 0.38 g and 0.075 g of 4-hydroxy-1-oxy-2,2,6,6-tetramethyl- piperidine (4-OH-TEMPO, Lignostab® 1198, BASF SE) was added (as a solution). Viscosities of the mixtures obtained were measured after storage of the mixtures for the periods of time and at the temperatures as indicated in Table 1 below in regular intervals with a "Modular Compact Rheometer MCR 302" (Anton Paar). The results are given in Table 1 below. Table 1

In all tables, "RT" means room temperature, "h" means hours(s) and "d" means day(s). Table 1 indicates that the unstabilized mixture with color pigment solidified at 50°C during only 24 hours while stabilized mixtures kept liquid at 50°C for 56 days with as little as 0.075% of stabilizer.

Example 3:

Example 2 was repeated with 48.50 g of the unstabilized solution, 5 g of Disperbyk® 110 (a phosphoric acid-containing dispersing and wetting additive, BYK-Chemie GmbH) and 50.2 g of

Bayferrox® 3920 (Fe2Os pigment, yellow, LANXESS Deutschland GmbH). In four different batches, 0 g, 0.75 g, 0.38 g and 0.075 g of 4-hydroxy-1-oxy-2,2,6,6-tetramethylpiperidine (4-OH- TEMPO, Lignostab® 1198, BASF SE) was added (as a solution). Viscosities were measured in regular intervals as mentioned above. The results are given in Table 2 below. Table 2

Table 2 indicates that the unstabilized mixture with color pigment solidified at 50°C during 7 days while stabilized mixtures kept liquid at 50°C for 56 days with as little as 0.075% of stabilizer.

Example 4:

Example 2 was repeated with varying color pigments with and without stabilizer (4-OH-TEMPO). Viscosities were measured in regular intervals as mentioned above. The results are given in Table 3 below.

Table 3 hereinbelow indicates that the stabilized mixtures with the individual color pigments exhibited much better long-term stabilities than the unstabilized mixtures - even at elevated temperatures. Moreover, the stabilized mixtures exhibited very satisfactory overall stabilities in absolute terms. TABLE 3

"Thixo." means thixotropic.

Example 5:

Example 1 was repeated with 50.0 g of triethylene glycol dimethacrylate (TEGDMA - Sartomer®

SR205H, Arkema Group) and 50.0 g of Bayferrox® 130 M (micronized Fe2Os pigment, red, LANXESS Deutschland GmbH) without other additives. The results are given in Table 4 (Run V1).

Run V-1 was repeated with 25.0 g of TEGDMA, 50.0 g of Bayferrox® 130 M, and 25.0 g of Laropal® A 81 (urea-aldehyde resin, BASF SE) without stabilizer. The results are given in Table 4 (Run V2).

Run V-2 was repeated with 24.75 g of TEGDMA, 50.0 g of Bayferrox® 130 M, 24.75 g of Laropal®

A 81 and 0.50 g of 4-hydroxy-1 -oxy-2, 2,6, 6-tetramethylpiperidine (4-OH-TEMPO, Lignostab® 1198, BASF SE). The results are given in Table 4 (Run V3).

Table 4

Table 4 indicates that the unstabilized mixture without urea-aldehyde resin solidified at 50°C after 14 days. The unstabilized mixture with urea-aldehyde resin was more viscous and solidified already after 24 hours, while the stabilized mixture with urea-aldehyde resin did not solidify at all but remained stable at room temperature and 50°C for at least 49 days.

Example 6:

Example 4 was repeated with different color pigments. The results are shown in Table 5 below.

Table 5

*) Prepared according to Example 1

With all color pigments satisfactory viscosities and long term stabilities at elevated temperatures could be achieved.

Example 7: (Different Methacrylates, Different Stabilizer)

A Laropal® A 81 solution was produced according to Example 1 (Table 5, Run V1).

An analogous solution was prepared with trimethylolpropane trimethacrylate (TMPTMA - Sartomer® 350D, Arkema Group) instead of TEGDMA (Table 5, Run V2).

Another analogous solution was prepared with methoxy polyethylene glycol 350 methacrylate (MPEG350MA - Sartomer® SR 550, Arkema Group) instead of TEGDMA (Table 5, Run V3).

Yet another analogous solution was prepared according to Example 1 with TEMPO instead of 4-OH- TEMPO (Table 5, Run V4)

For each run, a color paste was prepared from 45.5 g of the Laropal® A 81 solution or the other analogous solutions, 2.50 g of Disperbyk® 110 (a phosphoric acid-containing dispersing and wetting additive, BYK-Chemie GmbH) and 50.5 g of Bayferrox® 130 M (micronized Fe ₂ Os pigment, red, LANXESS Deutschland GmbH) plus 1.00 g of Sartomer SR 205H (TEGDMA, diluent). Viscosities of the mixtures obtained were measured after storage of the mixtures for the periods of time and at the temperatures as indicated in Table 7 below in regular intervals with a "Modular Compact Rheometer MCR 302" (Anton Paar). The results are given in Table 7 below.

Table 7

T able 7 indicates that viscosities were essentially maintained in the case of TEGDMA and TMPTMA, and even decreased in the case of MPEG350MA. TEMPO is about as effective as 4-OH-TEMPO. The individual runs were repeated with Bayferrox® 306 (Fe2Os pigment, yellow, LANXESS Deutschland GmbH) with essentially identical results.