WO2011028125A1

WO2011028125A1 - Coating compositions comprising micro silica

Info

Publication number: WO2011028125A1
Application number: PCT/NO2010/000314
Authority: WO
Inventors: Raphaél LAMY
Original assignee: Elkem As
Priority date: 2009-09-03
Filing date: 2010-08-25
Publication date: 2011-03-10
Also published as: EP2473571A4; NO20092956A1; EP2473571A1

Abstract

The invention relates to a coating formulation in the form of a solid/liquid dispersion, comprising a liquid solvent, a resin and up to 25wt% microsilica, based on the total weight of the formulation. The solvent may be water or an organic solvent.

Description

Coating compositions comprising micro silica

Field of Invention

The present invention is concerned with coating formulations. These may be aqueous or solvent based and may be UV-cured. In particular, the invention is applicable to water-based primers, solvent-based top coats, water-borne clear coats and UV-cured coatings. Background art

Coating formulations in the form of dispersions which are dried and cured to form a final coating are extensively used in many fields. A significant factor which is relevant to the coating formulations is the amount of solvent that has to be recovered and either re-cycled or disposed of. Since this is a costly aspect of the coating operation and represent serious HSE issues, it is desirable that the solvent duty is minimised, and this represents a further aspect of the invention. As the solvent is mainly used for lowering the viscosity of the formulation any non-volatile compound that decreases the viscosity will allow the formulation to save time and avoid strict regulation constrains.

Another factor in all these formulations is the energy required to remove the solvent, whether or not the solvent is water. The amount of solvent required will be determined up to a point by the viscosity of the ultimate formulation which is to be applied.

Also the surface properties of the coating, like abrasion resistance and scratch resistance are important factors for coatings. It is therefore an object of the present invention to modify a coating formulation in such a way that the solvent proportion is reduced while maintaining a suitable viscosity and to improve wear resistance of the coatings. In addition to these generally applicable factors, individual formulation systems have respective issues such as adhesion, gloss, waviness, distinction of image (DOI), hardness, and matting. It is important that these properties do not deteriorate significantly and it would be desirable is they could be maintained or even improved.

It is known in the art to incorporate fillers in coating formulations. Known fillers include talc, TiO₂, natural silicates, fumed silica and precipitated silica. Fumed silica is produced by burning silane gas and has a very small particle size and a very high surface area of 100 to 200 m /g. Precipitated silica is produced by precipitating silica particles from aqueous solutions.

Typically, talc is a common filler used for primers and for topcoats. It has a lamellar structure and can be used to adjust rheology. Since it has some hydroxyl groups on the surface it improves adhesion. Due to its low hardness it leads to advantages in sanding and flexibility. Silicates, such as SILLITHIN Z86 may be used as a filler in water-based coatings to improve abrasion resistance.

Fumed Silica may be incorporated for rheology and levelling control. Precipitated silica may be employed for cost reduction. Both these forms of silica may be used as a matting agent, to decrease gloss. Both fumed silica and precipitated silica do exhibit a high water and oil absorption and will thus increase the solvent demand for coatings in order to have a suitable viscosity. In addition, fumed silica has other disadvantages like strong hazing. Finally fumed silica is, due to its very low particle size, a very fluffy material resulting in handling problems and also negative health issues.

Description of the Invention

According to the invention, there is provided a coating formulation in the form of a solid/liquid dispersion, comprising a liquid solvent, a resin and up to 25 wt% microsilica, based on the total weight of the formulation.

The resin may be curable or not curable.

The term "microsilica" used in the specification and claims of this application refers to particulate, amorphous SiO₂ obtained from a process in which silica (quartz) is reduced to SiO-gas and the reduction product is oxidised in the vapour phase to form amorphous silica. Microsilica may contain at least 70% by weight silica (SiO₂), and preferably >97% and has a specific density of 2.1-

2.3g.cm 3 and a surface area of 15-40m 2 /g, typically 20m 2 /g. The primary particles are substantially spherical and may have an average size of about 0.15μπι. Microsilica particles further have a solid surface and do not absorb water. Microsilica is preferably obtained as a co-product in the production of silicon alloys in electric reduction furnaces.

Preferably, the formulation includes from 1 to 15 wt% microsilica.

The solvent may be water. The resin may be a polyester and preferably the formulation comprises from 2 to 15% microsilica. Preferably, the resin comprises up to 37wt% of the formulation, more preferably, 25 to 37 wt% and the formulation optionally from 10 to 22 wt% of an additional filler. Preferably, the formulation additionally comprises additives selected from pigment and eventually a curing agent or polymerisation initiator. A preferred formulation for water based solvent comprises, by weight, 37% polyester resin, 35% water, 10% TiO₂, 2% additives and 15.5% microsilica. Another preferred formulation comprises, by weight, 36% polyester resin, 34% water, 10% TiO₂, 2% additive, 12% silicate filler and 5% microsilica.

The water-borne resin may alternatively be a polyurethane resin. Preferably, the formulation then comprises 2 to 5 wt% microsilica, and preferably comprises up to 32 wt% resin and optionally up to 13 wt% of an additional filler Preferably the formulation comprises 5 to 30 wt% resin. Such a formulation may provide a clear coating.

A preferred formulation comprises; by weight 32% polyurethane resin, 53% water, 2% microsilica and 13% other additives. Another preferred formulation comprises by weight; 32% polyurethane resin, 53% water, 5% microsilica and 10% other additives. Formulations with contents of microsilica and other additives between these two formulations are also contemplated. The solvent may be non-aqueous. The formulation may then be a (nonaqueous) solvent-based top coat and the resin may compromise a polyurethane (PU) resin. The solvent may be a hydrocarbon resin, xylene, ethylacetate or another such solvent. The formulation preferably comprises from 1 to 2.5 wt% microsilica.

One preferred formulation includes as a filler, 1.9 wt% talc and 1.0 wt% microsilica. Alternatively, the talc may be replaced entirely and the formulation may include 2.5 wt% microsilica. Formulations with contents of talc and microsilica between these two formulations are also contemplated. A preferred formulation comprises up to 55 wt% resin, up to 19 wt% solvent, up to 9 wt% pigment and up to 12 wt% TiO₂ as a filler. When producing the formulation, the microsilica can be incorporated in a solvent or in water to form a concentrated paste, by using specific additives and mixing operations.

In another non-aqueous solvent variant, the formulation may comprise an aery late resin, much as an epoxy aery late oligomer. The solution monomer (solvent) may be tripropylene glycol diacrylate (TPGDA). The formulation may then contain from 2 to 10 wt% microsilica. Preferably, the formulation additionally includes one or more photo-initiators for the resin and the solvent and optionally one or more adhesion promoters.

One preferred formulation comprises up to 64 wt% resin, up to 30 wt% solvent and 2 wt% microsilica. A second preferred formulation comprises up to 60 wt% resin, up to 30.4 wt% solvent and 5 wt% microsilica. A third preferred formulation comprises up to 54 wt% resin, up to 31.2 wt% solvent and 10 wt% microsilica. Formulations with contents of resin, solvent and microsilica between the first and third of these formulations are also contemplated.

The reduction of the viscosity is due to the shape and size of the microsilica. Its spherical, non-porous morphology promotes a so-called "ball bearing" effect. Hence, when it is formulated with an uneven, angular filler of greater size, the flow of the liquid formulation is facilitated. It should be noted that the "ball bearing effect" is great enough to overcome the higher specific surface area of the microsilica (20g/m , compared to 14 g/m for the filler), which should act as a countereffect in the viscosity reduction. By replacing 5% of a conventional filler (e.g. silicate), it may therefore be possible to decrease the water content and thereby achieve a higher solid content. It is assumed that a higher solid content is an advantage because it will allow a dilution of the binder at constant viscosity, and/or lower the energy consumption when evaporating the water.

Simultaneously, the coatings may show a drastic improvement in surface properties. The overall surface properties, e.g. dullness and waviness can be lowered, and the gloss was increased. As a consequence, typical inconveniences such as "orange peel" can be overcome without costly efforts of re-formulation.

The enhancement of surface properties can be explained by the better flow during paint application, due to the above-mentioned positive effect of the spherical shape of the microsilica particle. Additionally, the small size of the microsilica allows the particles to fill the filler's interparticle voids, allowing more water to be available. Similar benefits can be observed when replacing the conventional filler (e.g. talc) with microsilica in a solvent borne formulation. As previously mentioned, the spherical shape of the particles allows a "ball bearing effect" to take place, hence allowing a formulation of higher solid content, and/or enhancing the flow. The greater oil demand for microsilica as compared to talc (60 and 40g linseed oil/lOOg resp.) is compensated by the benefits imparted by the shape of the particles. Hence, coatings with lower VOC can be achieved.

The invention also extends to a method of forming a coating on a substrate by applying a formulation as described to the substrate, drying the formulation to drive off the solvent, and curing the resin. The curing may be effected by any convenient mechanism, such as heating and/or applying a source of UV radiation. The invention also extends to a coating formed in this way and to the coated substrate.

Detailed Description of the Invention

The invention may be carried into practice in various ways and some embodiments will now be illustrated in the following non-limiting examples.

Example 1

Water-based primer/surfacer.

This is intended as an OEM application.

The composition of a prior art coating formulation used in this Example is set out in Table 1. All percentages are by weight. The formulation is an aqueous polyester dispersion.

Table 1

This example examines the effect of partially or wholly replacing the conventional filler (SILLITHIN Z 86) in the formulation shown in Table 1 with microsilica. Details of the fillers are set out in Table 2. The conventional filler, SILLITHIN Z 86 is a quartz/kaolinite silicate. The SIDISHIELD C25 filler is microsilica from Elkem AS having a specific surface area of about 25 m²/g and the SIDISHIELD C30 filler is microsilica having a specific surface area at about 30 m²/g. Table 2

lg linseed oil absorbed per lOOg filler measured according to DIN EN ISO787/5

In this example, the TiO₂ also acts as a filler material. The formulation was formed into an aqueous dispersion using a pearl-mill.

Table 3 shows the effect of partially and wholly replacing the conventional filler by microsilica. Table 3

² % by weight

³ to fix the spray viscosity at 30-35s (consistency cup)

⁴ related to surface smoothness (flatting/levelling ability): the lower the waviness value the better

⁵ Distinction of image, from 0 (worse) to 100 (best)

⁶ the lower the value the higher the flexibility

⁷ Buchholtz

⁸ Cross cut test (GTO=best)

⁹ Erichsen equipment

The formulations were applied to a pre-treated steel substrate by means of a compressed air spray gun, operating at a pressure of 2.5 bar, at 22 ^°C and 60% relative humidity. The primer coatings were dried and cured by ventilating for

10 minutes at 23°C, drying for 10 minutes at 45°C, then baking for 20 minutes at 165^°C. As can be seen from the results in Table 3, less water is required to provide the desired viscosity when the known filler is replaced by microsilica, either in part or completely. Therefore, less energy is needed to evaporate the water, leading to a shorter drying time or a higher solids content.

Example 2

Solvent-based topcoat.

This is intended as an OEM application.

The composition of a prior art two-component coating formulation is set out in Table 4. All percentages are by weight. The formulation is a solvent-borne polyurethane.

Table 4

This example examines the effect of partially and wholly replacing the conventional talc filler in this formulation with microsilica (SIDISHIELD C25). Details of the fillers are set out in Table 5.

Table 5

The formulations were prepared as follows: The component A was prepared by mixing the two resin binders before adding the Bentone paste with stirring. It was then mixed for 5 minutes. The two additives were then added with stirring and mixed for 5 minutes. The pigments, TiO₂, extender and filler were added to the formulation and mixed for 10 minutes. The formulation was further processed through a pearl mill to a finenesss of <10μιη. The remaining solvent was then added under stirring, and mixed for 10 minutes.

The component B was prepared by stirring the solvent slowly and adding the curing agent before a ten minute mixing. It was then put in a container and stored under nitrogen. The replacement of the filler with SIDISHEILD was made on a volume basis in order to compare the viscosity.

Table 6 shows the effect of partially and wholly replacing the talc by microsilica. Table 6

to fix the spray viscosity at 30-35s (consistency cup)

^{1 1} the lower the better

¹² the lower the value the higher the flexibility

¹³ Taber test, 400 rotations and lOOOg weight using a CS17 abrader.

¹⁴ The scratch resistance test was carried out on an Erichsen equipment. The weight at which the first damage to the paint film was observed was recorded. Adhesion was evaluated though the cross-cut test. The formulations were applied to a pre-treated steel substrate by means of a compressed air gun with a pressure of 2.5 bars, at 23°C and 65% relative air humidity. The drying lasted 7 days at ambient temperature.

As can be seen from the results in Table 6, less solvent is required to provide the desired viscosity when the talc is replaced by microsilica, either in part or completely. Therefore, less energy is required when evaporating the solvent and/or drying can be achieved in a shorter time. In addition, where the talc is partially replaced (Formulations 7 and 9), the wear resistance is improved.

Example 3

Water-based clear coat for parquet flooring.

The composition of a prior art (reference) coating formation is set out in Table 7. All percentages are by weight. The formulation is an aqueous polyurethane dispersion.

Table 7

Formulation 1 1

This example examines the effect of adding microsilica to an aqueous polyurethane dispersion which is used to provide a clear coat on parquet flooring. The dispersion was formed using a pearl-mill and the formulation was applied to a parquet tile to a thickness of 150μιη in two layers using a doctor blade. The coatings were then allowed to dry for three weeks at ambient temperature. The effect on gloss of using microsilica as a matting agent in place of the Acematt TS 100 in formulation 1 1 is shown in Table 8. Table 8

SIDISHIELD C25 acts as a matting agent with no "polishing" effect. Polishing is a decrease of matting when low shear frictions are applied. Typical matting agents, e.g. wax, are subject to the polishing effect.

Example 4

Non-aqueous UV-cured coating

This example examines the effect of adding microsilica to a non-aqueous UV- cured coating formulation. The microsilica is added as a paste or dispersion whose components are set out in Table 9.

Table 9

TPGDA is tripropylene glycol diacrylate

Disperbyk 2009 is an acrylic copolymer dispersing agent. The three components are formed into a dispersion using a pearl-mill. Table 10 shows a conventional UV coating formulation and similar formulations with various proportions and microsilica (SIDISHIELD C25) added.

Table 10

The formulations were formed into dispersions and applied to an aluminium substrate and UV-cured. All coatings were applied with a doctor blade in 3 strokes with a thickness of 25μιη. The distance between substrate and UV- lamp was 8cm and the curing time in the UV-tunnel was 30 seconds. A mercury vapour lamp was used with an approximate power of 150m/cm². The resulting coatings were tested for abrasion resistance using a Taber Abrasion Wheel CS-10 with a 500g load. The results after 3000 rotations are set out in Table 1 1. All percentages are by weight. Table 1 1

As can be seen, the presence of microsilica in the coating significantly improves abrasion resistance, even when the content is as low as 2% by weight.

Thus, it will be understood, that replacing conventional fillers in water based and solvent based coatings according to the invention by microsilica in typical OEM formulations allows the following advantages to be achieved:

• Reduced viscosity and consequently water/solvent allowance, resulting in faster application and lower volatile organic compound content;

• Enhanced optical properties: higher gloss, lower waviness;

• Better abrasion resistance (except when the filler was totally replaced in solvent based topcoat, probably due to self abrasion);

• Increased flexibility (for high amount of added microsilica);

• Other properties such as scratch resistance, corrosion, adhesion and weathering) were not adversely affected.

In UV cured coatings:

• Improved wear resistance (Taber abrasion, scratch resistance). In PU-Acryl clear-coat (parquet flooring application)

• Improved reflow (recovery of gloss upon moderate heating); • Improved matting agent (avoid so-called "polishing"

Claims

1. A coating formulation in the form of a solid/liquid dispersion, comprising a liquid solvent, a resin and up to 25 wt% microsilica, based on the total weight of the formulation.

2. A formulation as claimed in Claim 1, in which the resin is a curable resin.

3. A formulation as claimed in Claim 1 or Claim 2, including from 1 to 15wt% microsilica.

4. A formulation as claimed in any preceding Claim, in which the solvent is water.

5. A formulation as claimed in Claim 4, in which the resin is a polyester and the formulation comprises from 2 to 15 wt% microsilica.

6. A formulation as claimed in Claim 5, which comprises up to 37 wt% of the resin and optionally from 10 to 22 wt% of an additional filler.

7. A formulation as claimed in Claim 6, which additionally comprises additives selected from pigment and a curing agent.

8. A formulation as claimed in any of Claims 1 to 4, in which the resin is a polyurethane resin and the formulation comprises 2 to 5 wt% microsilica.

9. A formulation as claimed in Claim 8, which comprises up to 32 wt% of the resin and optionally up to 13 wt% of an additional filler.

10. A formulation as claimed in any of Claims 1 to 3, in which the solvent is non- aqueous.

1 1. A formulation as claimed in Claim 10, in which the resin is a polyurethane and the formulation comprises from 1 to 2.5 wt% microsilica.

12. A formulation as claimed in Claim 1 1, in which the solvent is selected from a hydrocarbon resin, xylene, and ethylacetate.

13. A formulation as claimed in any of Claims 10 to 12 which comprises, by weight 1.9% talc and 1.0% microsilica, as a filler.

14. A formulation as claimed in any of Claims 10 to 13, which comprises 2.5 wt% microsilica as a filler.

15. A formulation as claimed in any of Claims 10 to 14, comprising up to 55 wt% resin, up to 19 wt% solvent, up to 9 wt% pigment and up to 12 wt% TiO₂ as a filler.

16. A formulation as claimed in Claim 10, in which the resin is an epoxy resin and the formulation comprises from 2 to 10 wt% microsilica.

17. A formulation as claimed in Claim 16, in which the solvent is tripropylene glycol diacrylate.

18. A formulation as claimed in Claim 16 or Claim 17, which additionally includes one or more photo-initiators for the resin and optionally an adhesion promoter.

19. A formulation as claimed in Claim 18, which comprises up to 64 wt% resin, up to 30 wt% solvent and 2 wt% microsilica.

20. A formulation as claimed in Claim 18, which comprises up to 60 wt% resin, up to 30.4 wt% solvent and 5 wt% microsilica.

21. A formulation as claimed in Claim 18, which comprises up to 54 wt% resin, up to 31.2 wt% solvent and 10 wt% microsilica.

22. A method of forming a coating on a substrate which comprises applying a formulation as claimed in any preceding claim to the substrate, and curing the resin.

23. A method as claimed in Claim 22, in which the curing comprises heating and/or applying a source of UV radiation.