The invention relates to an aqueous polymer dispersion containing nano-particles which aqueous dispersion has a high stability without the use of external surfactants. The aqueous dispersion can be used in formulations to coat substrates such as paper, plastics, wood, glass, textile and leather. It is the intention of this invention that the aqueous dispersion of nano-particles can be used in combination with other components. Such combinations may contain elements that destabilize the dispersion of the nano-particles. Furthermore, such combination may need a long shelf life, which is the case for instance for paints.

Usually, an increased dispersion stability is obtained by adding external surfactants, such as non-ionic or anionic surfactants. However, such surfactants have a negative effect on the properties of the resulting coatings. Surfactants facilitate the migration of water through the coating and furthermore they disturb the surface properties of such a coating. The nano-particles of this invention will be used amongst others to increase the hydrophobic character of the surface of the coating. It is clear that the use of surfactants will have a negative effect on the properties, such as the hydrophobic character, water permeability etc.

The preparation of aqueous dispersed nano-particles from a copolymer of anhydride and vinyl monomer units, which co-polymer has been subjected to an imidization reaction, has been described in WO-A-2004/031249. However, according to this patent application the co-polymers have an imidization degree of at least 90%. It has been found that such dispersions have a limited shelf stability and furthermore they often coagulate when other dispersions are added.

In patent application WO-A-99/45039 an aqueous polymer dispersion has been described whereby the imidization degree of the co-polymer containing maleic anhydride and vinyl aromatic monomer units is at most 75%. However, the obtained dispersions do not contain distinct nano-particles. The resultant imidized co-polymer has film forming properties and the solids content is according to the described examples not higher than 20% because of its high viscosity. The aqueous dispersions of this invention have a low viscosity and do not have film forming properties at all. All examples described in WO-A-99/45039 have a ratio of amine to anhydride of more than 2. It has been found that with a ratio amine to anhydride of above 1.5 it is not possible to make an aqueous dispersion of distinct nano-particles. An alternative manufacturing process is provided in WO-A- 00/34362, and requires the presence of a caustic solution. In said application it is shown that in the absence of a caustic solution and in the presence of a ratio of amine to anhydride of below 1.5, a very viscous polymer mass is obtained but no aqueous polymer dispersion containing nano-particles as in the present case.

The invention relates to an alternative method for making such an aqueous dispersion.

There is thus a need for a new polymer dispersion composition which can be combined with other dispersions and emulsions to formulations with a long shelf life.

There is further a need for a process of producing such a polymer dispersion composition.

This aim is achieved with the present invention, with the technical features of the characterising part of the independent claims.

Similar to the previously described aqueous polymer dispersions, the aqueous polymer dispersion of this invention comprises a copolymer of anhydride monomer units and vinyl monomer units, of which copolymer less than 90 % of the moles of the anhydride monomer units are imidized, preferably less than 85%. This polymer dispersion contains polystyrene maleimide (SMI) in the form of discrete nano- particles.

Suitable anhydride monomers for use in the copolymer are, for example, α-β- unsaturated dicarboxylic anhydrides such as maleic anhydride, alkyl or alkenyl maleic anhydrides, citraconic anhydride, itaconic anhydride and mixtures thereof. Preferably the copolymer contains maleic anhydride monomer units. Suitable vinyl monomers for use in the copolymer include vinyl aromatic monomers (such as styrene, a-methyl styrene, vinyl toluene and indene), mono- olefinic unsaturated hydrocarbons (such as ethylene, propylene and isobutylene), α-β-unsaturated carboxylic esters (such as acrylate esters (like ethylacrylate, butylacrylate and 2-ethylhexylacrylate), methacrylate esters (like methylmethacrylate, ethylmethacrylate and 2-hydroxyethylmethacrylate) and maleate diesters (like dioctylmaleate)), halogenated olefins (such as vinyl chloride and vinylidene chloride) and mixtures thereof. Preferably the copolymer contains readily commercially available styrene or a-methyl styrene, although the presence of styrene monomer units is most preferred.

Preferably use is made of a copolymer in which the anhydride monomer content ranges between 15-50 mole %, more preferably between 15 and 43 mole %, even more preferably between 20-36 mole % and most preferably 22-32 mole %, because of the end product properties.

The anhydride monomer content of 22-32 mole % is particularly preferred, as in this range the copolymer shows suitable water solubility, giving optimum imidization yield and high solid content of the final dispersion.

The vinyl monomer content of the copolymer ranges between 85-50 mole %, preferably between 80-67 mole %.

The aqueous polymer dispersion of this invention preferably has a solid content of more than 20 wt. %, more than 30 wt. % or even more than 40 wt. %. The dispersion comprises discrete particles having a particle diameter above 30 nm, sometimes above 40 or 50 nm, but smaller than 400 nm, often smaller than 250 or even 120 nm, the particle size distribution being narrow. As the diameter of the particles is smaller than the wavelength of visible light, a smooth, high gloss and transparent coating may be obtained. By controlling the particle size, preference may be given to a coating with a higher or lower gloss, being more transparent or showing some opaqueness. The formation of small particles further entails the advantage that stabilisation of the dispersion is higher. This is in contrast to a dispersion containing larger particles which needs the presence of an emulsifier to attain a stable dispersion. The aqueous polymer dispersion of this invention has no film forming properties. Upon drying at temperatures below the glass transition temperature of the imidized copolymer of cyclic anhydride and vinyl-monomer units a coating will be obtained consisting of distinct nano-particles. This means that pores are present between the particles and that a gas or sometimes even a liquid can easily migrate through the coating.

The present invention also relates to a process for the production of the above described aqueous polymer dispersion. According to this process an aqueous polymer dispersion is prepared by

1 ) reacting a starting copolymer of anhydride monomer units (preferably maleic anhydride) and vinyl monomer units (preferably styrene) with an aqueous solution of NH ₃ or an amine (RNH ₂ ) in the absence of a caustic solution, i.e. in the absence of OH ^" ions at the beginning of the reaction, i.e. in the absence of between 0.01 and 2.0 mol OH-ions per mol of alpha- beta- unsaturated dicarboxylic acid monomer units present at the beginning of the preparation. 2) subjecting the thus obtained reaction mixture to an imidization reaction until less than 90 mole % of the anhydride monomer units have been imidized, preferably no more than 85% of the anhydride monomer units.

A copolymer containing vinyl monomer units and anhydride monomer units may be synthesised according to processes well known to the man skilled in the art, such as for example the process described in Hanson and Zimmerman, Ind. Eng. Chem. Vol. 49, nr. 1 1 (1957), p. 1803-1807.

A caustic solution'is understood to be a solution of LiOH, NaOH or KOH or mixtures thereof in an aqueous medium. Typically use is made of KOH or NaOH, wherein the amount is typically chosen so that between 0.01 and 2.0 mol OH-ions per mol of alpha- beta- unsaturated dicarboxylic acid monomer units is present at the beginning of the preparation. In the method of this invention, the copolymer is reacted in water, an emulsifier optionally being present. To this mixture an aqueous solution of NH ₃ or an alkylamine(RNH ₂ ) is added, in which R may be an alkyl group having between 1 - 18 carbon atoms or an aryl group. It is however preferred to use NH ₃ .

In order to get distinct nano-particles it is preferred that the molar ratio between the amine or NH ₃ and the anhydride monomer in the copolymer to be imidized ranges between 0.7:1 and 1.4:1 , more preferably between 0.8:1 and 1.2: 1. When a ratio of more than 1.5:1 is used the resultant reaction product has a high viscosity and shows film forming properties.

The anhydride monomer/vinyl monomer copolymer has a molecular weight which preferably is not too high and neither too low so as to allow obtaining a dispersion with a sufficiently high solid content. In the present invention, the anhydride monomer/vinyl monomer copolymer has a molecular weight (Mw) which is at least 5000 g/mole, preferably at least 25000 g/mole, more preferably at least 40000 g/mole. The molecular weight of this copolymer is preferably less than 500000, more preferably less than 200000 g/mole or less than 100000 g/mole. Ideally, the molecular weight of the starting copolymer is between approximately 50000 and 80000 g/mole as it allows obtaining so-called monodisperse dispersion with a narrow particle size distribution of between 50 and 100 nm, the mean particle diameter being approximately 70 nm. Ultimately such dispersion allows obtaining a coating with an optimum gloss. A too high molecular weight of the copolymer involves the risk that the viscosity of the dispersion becomes too high and the solid content too low.

In the method of this invention, the imidization reaction will mostly be carried out at a temperature above 100°C, preferably between 1 10-185°C, more preferably at a temperature between 120-170°C, or even 130-165°C. Below 100°C insufficient imidization has been observed. At a temperature above 170°C and in particular above 195°C, there is an increasing risk to agglomeration of the polymer, as a consequence of which particle formation in the dispersion is counteracted, giving particles with a too large size which are visible when applied as a coating and easily involve film formation. The temperature range of 130-180°C is preferred as within this range a well-defined dispersion with respect to Tg and mechanical properties and composition is obtained and the process showing good reproducibility.

The degree of imidization can be adapted by different means. A lower equivalent amount of ammonia or alkylamine compared to the anhydride group will lead to lower imidization degrees. With a higher than equivalent amount of ammonia or alkylamines compared to the anhydride groups also lower imdization degrees can be obtained because the excess of ammonia or alkylamine will react with the cyclic imide to form di-amides. Usually, this ring-opening reaction takes place at temperatures above 140 C. Another way of having lower imidization degree is by ending the reaction at a desired imidization degree, which is lower than 90%. This can be done by cooling down the reaction mixture. A man skilled in the art will be able to modify the reaction parameters so that an imidization degree lower than 90% can be obtained. Sometimes, some trial and error work is needed to get to the desired parameters. Dispersions with an imidization degree of higher than 90% usually have a pH around 7. Dispersions of this invention usually have a pH between 5 and 6 or between 8 and 10.

The imidization degree should not be lower than 50%, preferably not be lower than 60% to prevent film forming properties of the SMI nano-particles.

If so required, the imidization reaction may be carried out in the presence of an anti-foaming agent and/or an emulsifier. Suitable emulsifiers may be anionic or nonionic surfactants.

With the above described production process, an aqueous dispersion of the imidized organic pigment may be obtained with the above-described solid content and particle size.

If desired, the solid content of the dispersion may be increased by methods known by man skilled in the art, especially suitable are evaporation and ultra filtration. The formation of small pigment particles has the advantage that inter-particle attraction is governed by Van der Waals forces, giving strong inter-particle adhesion and good adhesion to the surface to be coated. The present invention further relates to a coating composition for a surface to be coated, the coating composition comprising an amount of the aqueous dispersion of this invention. The amount of organic pigment incorporated may vary within wide ranges and will mostly be determined by the application. For example, the aqueous dispersion of this invention appears suitable for coating paper, paperboard, cardboard, an organic film (for example a polyethylene film), a metal foil, a textile sheet, etc.

The dispersion obtained according to this invention may be blended with other dispersed or water soluble materials. Such a formulation may contain pigments, fillers, binders, crosslinkers, UV absorbers, rheology modifiers, water retention aids, biocides and so on. The bulk of the formulation may contain fillers such as dispersion of clay, talcum, kaoline and CaC03. Such products may have a large effect on the stability of the dispersion according to this invention. Principally, the stability can be much improved by adding external surfactants, however such additives have a negative effect on the resultant coating. External surfactants facilitate the penetration and the migration of water through the coating, whereby the coated substrate gets affected by the water.

Examples

Characterisation methods: PCS measurements

The average hydrodynamic radius of the particles of the dispersion after imidization was determined using Photon Correlation Spectroscopy. Measurements were carried out using an ALV Laser of the Vertriebsgesellschaft mbH, Langen, Germany.

Solid content.

The solid content was determined using an infrared instrument, type Mettler LP16/PM600. pH measurements.

The pH value of each sample was measured with a Knick 752 CI, nr. 051489 pH measurement instrument.

Determining the degree of imidization.

The degree of imidization may for example be determined with Raman FTI R spectroscopy, by correlating the absorption intensity to the intensity of the absorption at the same wavelength of a completely imidized and a non-imidized reference sample. Before carrying out any calculations, the Raman-FTI R signals were normalised based on the absorption signals originating from the aromatic rings in the polymer chains. The calculations were based on the following absorptions:

C=0 imide absorption band, relatively intense signal at approximately 1768 cm ^"1 C=0 anhydride absorption band, at approximately 1860 cm ^"1

C=0 relatively weak absorption band of carboxylic acid groups, at approximately 1715 cm ^"1

As a reference use was made of (1 ) an aqueous ammonia solution of an imide free polymer, prepared starting from 26 mole % of maleic anhydride (MA) and 74 mole % of styrene, a NH ₃ :maleic anhydride ratio of 3:1 , at 50°C; (2) a SMA powder that had been subjected to an imidization reaction by mixing 2 g of SMA (28 wt. % of MA, 72 wt. % of styrene; molecular weight (Mw) 1 10000 g/mole) with 0.50 g of ureum in a double vice mini extruder, at 240°C for 5 minutes at a rotation speed of 100 rpm.

Contact angle measurements

Contact angles were measured with a contact angle meter type Digidrop, GBX, Roman, France.

Comparative Example

340 g ground SMA and water were charged into a double walled, oil heated reactor of 1 I, which contained a stirrer. The SMA had a MA content of 26 mole % and a molecular weight (Mw) of 80000 g/mole. To this solution a 25 % NH ₃ solution was added, so that the MA:NH ₃ ratio was 1 :1. Water was added until a total volume of 700 ml was obtained. The pressure was adjusted to 0.2 MPa with nitrogen. After increasing the temperature to 160°C the reaction was proceeded for 4 hours and the reaction mixture was cooled down over 1 hour. A polymer dispersion was obtained having a solid content of approximately 33 wt. %, the particle size being between 80 and 120 nm. The MA had been completely converted to imide. The Tg of the polymer after completion of the imidization was found to be between 190 and 200°C. The dispersion had a pH of 7.0. The dispersion shows an initial good stability, but after 6 months the dispersion starts to separate in two layers. This instability is more pronounced when the dispersion is first diluted to for instance 10%. Phase separation occurs already after 2 months.

Example 1 The experiment described in the comparative example was repeated, except that the MA:NH ₃ ratio was now 1 : 0.85. The reaction was performed under the same reaction conditions. The degree of imidisation was 82%. The particle size was similar as the one described in the comparative example. The Tg of the polymer was somewhat lower, 185°C, and the pH of the dispersion was 5.7. The dispersion showed a good stability, even after standing for 1 year. A 10% diluted form of the dispersion had a shelf life for at least 3 months.

Example 2

The experiment described in the comparative example was repeated, except that the MA:NH ₃ ratio was now 1 : 1.15. The reaction was performed under the same reaction conditions. The particle size was similar as the one described in the comparative example. The degree of imidisation was 87%. The Tg of the polymer was 190°C and the pH of the dispersion was 8.1.

The dispersion showed a good stability, even after standing for 1 year. A 10% diluted form of the dispersion had a shelf life for at least 3 months.