TECHNICAL FIELD;

This invention relates to a magnetic paint for application to walls, partitions, building materials and the like and to walls coated with the magnetic paint. The invention also relates to magnetic building materials. BACKGROUND ART;

It has hitherto been practiced to removably attach magnets and magnetized symbols to metal white goods. The use of "refrigerator magnets" both to decorate the white goods surface or to clamp notes and the like thereto is so well-known as to require no detailed description.

It has also been practiced to coat chalkboards ("blackboards") with a magnetic paint so as to enable magnets and magnetized symbols to be removably attached to the blackboard for use in teaching and play.

As herein used the term "magnetic" means capable of attracting a magnet.

The present invention is based on the novel concept that if one or more walls of a room could be painted with a magnetic paint it would be possible to removably attach posters and the like by means of magnets without the use of tacky substances, nails or other devices which impair the surface finish of the wall. Moreover, magnetic walls would also afford opportunities for creative expression and play by facilitating the

movement, attachment or detachment of magnetized shapes on to or from the magnetic wall surface without damage to the surface.

Magnetic blackboards have been prepared by application of a resin to the surface and then dusting with an iron powder prior to curing of the resin. This system is messy to apply and results in a rough surface texture. Magnetic paint comprising magnetite incorporated into an oil base admixed with drying oils and solvent thinners is also known. Such paint has not been considered suitable for use on walls for a number of reasons. Generally, in order to achieve a sufficiently-high magnetic attraction, a heavy loading of a magnetic material is required. It is difficult to maintain stable suspensions at a satisfactory loading without grinding the pigment to a very small particle size, preferably less than one micron. Magnetite is a jet black pigment and while paints with a high loading of magnetite are suitable for coating blackboards they are unsuitable for use in coating large areas of wall, for example in a child's nursery, or even for use as an undercoat to a light-coloured paint. This problem is exacerbated by grinding the magnetite to improve its suspension stability. Furthermore, the use of organic solvents and thinners renders the application and clean-up of such paints inconvenient and environmentally unpleasant. In contrast to use for a blackboard, a surface coating suitable for use in interior decorating must combine an acceptably light colour and smooth surface texture with sufficient durability, rub resistance, colour stability, shelf stability, and ease of application. The coating should be rust and stain resistant and should have a rheology which facilitates application by brush or roller, among other desiderata. Hitherto, surface coatings which provide sufficient magnetic attraction to retain magnets have had properties incompatible with one or more of these requirements of an interior decorative coating.

It is an object of the present invention to provide a surface coating which avoids or at least ameliorates the above discussed disadvantages of prior art. It is a further object of the present invention to provide a paint suitable for use in interior decoration, for example as an undercoat to a light coloured top coat, or as a top coat, and which will provide a surface to which magnets or magnetizable symbols will magnetically adhere.

DISCLOSURE OF THE INVENTION:

According to a first aspect the invention consists in a coating composition comprising a film forming resin emulsion, and a magnetic material dispersed in the emulsion in a proportion such that a dry cured coating of the composition on a non¬ magnetic substrate is capable of holding a magnet by magnetic attraction when vertical.

In preferred embodiments of the invention the magnetic material is magnetite and is dispersed in the aqueous phase of the emulsion. Desirably, the magnetite is present in a quantity such that a 50 x 50 mm flexible magnet will not only remain attached to a vertical or inverted dried film of the composition, but will also support in excess of 25 g, more preferably in excess of 30 g, against gravity.

Compositions according to the invention typically have in excess of 40% by weight, and often in excess of 50% by weight of a selected magnetite.

According to a second aspect, the invention consists in a coating composition comprising a film forming resin emulsion, a white or light coloured pigment, and magnetite dispersed in the emulsion in a proportion such that a dry cured coating of the composition on a non-magnetic substrate is capable of holding a magnet by magnetic attraction.

For preference, the pigment is titanium dioxide and is present in a weight ratio of from 10:1 to 2:1 (magnetite: titanium dioxide) and more preferably about 3.5:1 to 7:1.

The invention will now be more particularly described by way of example only with reference to various formulations.

Coating compositions according to the invention comprise a film forming resin binder for example an acrylic or vinyl paint resin emulsified in water. For this purpose, an acrylic or vinyl paint, and preferably an acrylic or vinyl undercoat paint of the type readily available commercially, may be used. Preferably, the coating is formulated based upon resin emulsions such as water-borne urethane; water-borne epoxy; vinyl acetate/dibutyl maleate co-polymer and polymers of other vinyl esters; water-borne alkyd; styrene (meth) acrylic copolymers; polymers of fumaric and maleic esters; styrene maleic anhydride resins; polymers of hydrocarbons e.g. styrene, vinyl toluene, butadiene, ethylene and copolymers thereof; polymers of acrylonitrile, vinyl chloride and substituted acrylamides.

The preferred magnetic material to be dispersed in the emulsion is a crushed and sieved naturally occurring magnetite which generally comprises at least 95% of Fe3θ4 and having a magnetic susceptibility at 800 OERSTEDS of not less than 0.05 e.m.u./oe/g. However, materials such as pyrrhotite, hematite, garnet, rutile, omphacite, ilmenite, and gamma iron oxide may also be used. Powdered iron, iron alloys, steels and alloy steels may also be used if suitably protected against oxidation. Magnetite having very low water absoφtion properties is highly preferred. It has been found (Table 1) that some commercially available iron oxide powders have a water uptake as great as 25 gms per 100 gm of oxide while the preferred form of magnetite for use in the invention has a water uptake of from 4 gms to 14 gms per 100 gms of oxide depending upon particle size. It is also preferred to employ a form of magnetite having Specific Gravity of above 4.6, for example Specific Gravity = 4.9.

TABLE 1 WATER REQUIRED PER 100 g OF PIGMENT/FILLER

Pigment/filler Wt of Water ("g

Magnetite (Medium) 4.56

Magnetite (Fine) 5.75

Magnetite (Super fine) 9.66

Magnetite (Ultra fine) 13.33

Fine micro bubbles 20.16

TiO ₂ 22.91

Synthetic Iron Oxide (4.6 g/cc) 25.53

CaCO ₃ 65.75

Al Silicate 79.25

Magnetic Attraction Test

The following test has been used as a measure of magnetic attraction of dried coatings according to the invention. "Gyprock" Gypsum tiles 150 x 100 mm in dimension were wiped prior to use. A tile is then weighed by means of a digital top

loading balance. A coating of a composition according to the invention is then applied using a 50 mm paint brush to a wet coating weight of 4 gms (267 gms/sq. m wet). The coating is allowed to dry and the tile re-weighed in order to calculate the dry coating weight.

The magnetic holding weight of a tile is determined by means of a flexible fridge magnet (50 x 50 mm) which is magnetically attracted to the surface and to which a chain and pan apparatus is attached. The tile is clamped with the coated surface in a peφendicular plane and weights are added to the pan suspended from the magnet. A successful "lift" is recorded if the weight does not move for a minimum of 30 seconds. The maximum weight which can be supported in this way per unit weight of coating is reported as "magnetic attraction" ("MA").

With a magnetite to emulsion weight ratio of 2:1, it was found that magnetic attraction varies linearly with coating weight.

Particle size of the magnetite was found to have no significant effect on magnetic attraction. However, particle size of the magnetite had a noticeable effect on colour, the colour being lightest at a magnetite particle size such that 65% - 75% is less than 53 microns ("medium"), becoming darker with grinding to 85% - 90% less than 53 microns ("Fine"), darker still at 90% to 95% less than 53 microns ("super fine") and being very dark at 95% - 99% less than 53 microns (average less than 45 microns "ultra fine"). Magnetite having 99% - 100% less than 53 microns (average approximately 0.2 microns "Ultra ultra fine") was black with very high colour strength. However, while magnetite having 65% - 75% less than 53 micron is lightest in colour, the surface finish of paints incoφorating that particle size is too rough and finer grades are preferred for surface finish and for suspension stability. BEST MODE OF CARRYING OUT THE INVENTION:

A formulation for a water-based emulsion paint according to the first aspect of

Example 1

Component Weight %

A Water 38.91

B Thickener 0.43

C Fungicide 0.34

D Dispersing Agent 1.73

E Wetting Agent 0.13

F Magnetite 52.31

G Antifoam 0.13

H Resin Emulsion 5.06

I Ammonia (5M) __ . 100.00

The water is charged to a tank. D, G, E and B are added with slow stirring. Preground F is then dispersed in the water using high shear dispersion. Subsequently, ammonia is added and the resin emulsion H together with stabilizers and fungicide are added as required. The resulting composition has 56.06% solids and an S.G. of 1.71 and PVC 80.8.

The coating composition of Example 1 when applied to a surface as a paint and allowed to dry gives satisfactory magnetic attraction. Typical Magnetic Attraction values in excess of 20 gms were obtainable at normal coating thickness (260 g/sq.m wet film, one coat). It was found to be desirable that the emulsion composition has a high specific gravity since that allows a substantially higher weight of the coating to be applied to a surface and therefore improves the magnetic attraction.

Siuprisingly, the water miscible composition is stable, dries to provide a coating which is usefully magnetic without impurities of the ferric oxide exhibiting rust-staining, and is suitable for use as an undercoat for a conventional wall interior decorating paint.

The coating of Example 1 however, is of a dark colour and thus is not generally suitable for use as a top coat. Moreover it is darker than desirable for use as an undercoat.

Initial attempts to lighten the colour by adding TiO ₂ white pigment resulted in a loss of magnetic attraction, instability and a deterioration in rub resistance. Likewise, when magnetite was blended with commercially available acrylic white paint in a weight ratio of 2:1 (magnetite:paint), a dark paint of high pigment volume concentration and low rub resistance resulted. However, it was found that as the weight ratio of magnetite to titanium dioxide changes from 1 :0 to 0.4:1 the colour becomes lighter up to a ratio of about 3.7: 1. Beyond that ratio, there is little significant lightening of colour while the deterioration in magnetic attraction becomes substantial. A weight ratio of between 3.7:1 and 6.5:1 gives a colour acceptable as an undercoat and effectively magnetic.

Example 1

Samples A-X were formulated as shown in Table 2 and prepared in a manner similar to formulation 1 , the titanium dioxide being preground and added with or after the magnetite. — — —

Sample G is a blend of two parts by weight magnetite to 1 part by weight of the commercially available acrylic white paint X.

The properties of these compositions is shown in Table 3.

TABLE 3 EFFECT OF MAG/Ti0 ₂ RATIO ON RELATIVE COLOUR OF FINAL PAINT

Samples C and D have an acceptable magnetic attraction and colour while samples E and F are of poor magnetic quality. Coating G, while magnetic, is very dark, presumably because the commercial paint base contains non-opacifying fillers rather than or in addition to titanium dioxide. At least 30% of magnetite appears to be required for sufficient magnetic attraction.

The effect of magnetite :titanium dioxide ratio on magnetic attraction and scrub resistance is shown in Table 4 for formulations similar to samples L & M but of various magnetite :titanium dioxide ratios. Scrub resistance was measured by rubbing two coated squares face to face for 20 rubs and measuring coating weight loss.

TABLE 4

EFFECT OF MAG/Ti0 ₂ RATIO ON MAGNETIC ATTRACTION (M.A.)

AND SCRUB RESISTANCE OF A FINAL PAINT

__&π_i_ ₂ S.G. Solid Solids PVC M.AJg ____

(W%) (Y%) % Resistance %Wt.loss

4.8 2.05 69.2 36.9 76.8 28 20.8

5.3 2.08 70.2 38.2 75.2 32 15.8

5.8 2.08 70.3 38.2 75.2 33 21.0

6.3 2.09 70.9 38.2 75.3 38 19.5

7.0 2.09 70.5 38.3 75.1 30 20.4

As the ratio of magnetite/titanium dioxide increases from 4.8:1 to 7.0: 1 the pigment volume concentration (PVC) was maintained around 75%. As expected, the magnetic attraction increases as the magnetite content increases. However, suφrisingly, the rub test (percentage weight loss) indicated no significant variation between titanium dioxide as the principal pigment and magnetite as the principal pigment. This was contrary to the expectation that titanium dioxide would require higher binder levels (i.e. magnetite would have a higher critical PVC).

Table 5 shows the effect of increasing PVC on rub resistance for a paint according to the invention having a magnetite to titanium dioxide ratio of 4.8:1. Dry rub resistance was measured by rubbing two squares face to face and recording the number of rubs when the substrate shows through. Critical PVC is defined as the pigment volume concentration at which weight loss with rubbing exceeds 30%.

TABLE 5

EFFECT OF PVC ON SCRUB RESISTANCE OF FINAL PAINT MAGNETITE:TITANIUM DIOXIDE WEIGHT RATIO 4.8: 1

Table 5 shows that at a PVC of above 81.5% there is a large and sudden failure in wet and dry rub resistance. This was unexpected as samples using magnetite without titanium dioxide can be made without apparent binding problems at 88% PVC. Likewise comparative formulation G using an acrylic emulsion (e.g. Rohm and Haas E 2300) at various solids % but without magnetite exhibit excellent rub resistance at PVCs above 80% (Table 6). Paint Y & Z in Table 6 are commercially available white paints.

IΔBLE__

PURE TiO ₂ FORMULATION: EFFECT OF PVC ON SCRUB RESISTANCE

OF PAINT

EY£ S.G. S_lid(W%) Solids(Y%) Wt.Loss(% Wet Ru

41.4 1.49 59.1 38.9 5.0

52.1 1.59 60.2 36.8 5.1

62.9 1.67 61.1 34.9 7.2

79.1 1.75 61.8 33.2 3.0

81.6 1.80 62.2 32.0 2.4

86.3 1.83 62.5 31.4 4.6

Paint Y 1.44 N.A. N.A. 0.85 >18

Paint Z 1.43 N.A. N.A, 0.0 >18 .

Paints according to the second aspect of the invention are especially suitable for use as an undercoat by virtue of their relatively light colour. Paints according to the invention are desirably formulated with a high viscosity and S.G. in comparison with commercially available undercoat and top coat paints intended for interior decoration. This facilitates liberal application and results in an optimum coating weight in one coating. Formulations such as L & M (Table 2) provide an exceptionally good undercoat on unsealed surfaces such as plasterboard, provide high surface loadings, and are easily applied at an effective coating weight.

It would be expected that, by virtue of its comparatively poor rub resistance, the undercoat would give poor top coat adhesion and durability. Suφrisingly that expectation is incorrect. When paints according to the invention are applied to a surface such as plasterboard, it appears that the surface area of the substrate is greatly increased resulting in an uptake of about 50% more top coat per application than usual for that substrate. Consequently, a top coat of greater opacity and integrity is obtained than would have been expected. Commercially available top coat interior wall paints thus have a satisfactory rub resistant surface when applied over the undercoat and the combination provides an excellent white or light coloured decorative wall coating with magnetic properties. A wide range of thickeners are suitable for use as rheology modifiers for the paint. These include polyacrylates, polyvinyl pyrrolidone, modified polyvinyl ether-maleic anhydride, other hydrophobic alkali soluble emulsions (HASE), hydrophobic ethoxylated urethane resins (HEUR), gelatin, natural gums, casein, alginates, starch and starch derivatives such as hydroxyethyl cellulosics, carboxymethyl cellulose and quaternised ammonium substituted cellulosics.

In addition, inorganic thickeners such as fumed silicas, sodium magnesium silicates, etc., may be incoφorated.

It is understood that combinations of the above types of thickeners may be used to achieve the necessary Theological properties in regard to flow, levelling, film build, etc.

In yet other embodiments of the invention, the magnetite is incoφorated into a building panel for example a plaster board wall panel.

Example 3 - Gypsum (piaster . Board A gypsum/magnetite slurry was prepared in water (400g plaster, 1 OOOg water, 700g magnetite).

This slurry was applied in various film weights to a pre-cast mould containing formed plaster.

Results were as follows:-

Wet film coating weight ( sm . Magnetic Attraction (ζ.

3160 17 1050 27

The plaster is reduced in strength by using this loading level (64% of magnetite in final dry plaster). The results suggest that a lkg/sq.mtr application to a gypsum board base is the minimum necessary to achieve reasonable magnetic strength and that a magnetic plasterboard may be manufactured with suitable magnetic attraction properties.

Example 4 - Incoφoration into Cement Render Various loadings of magnetite in a cement render base (sand/cement 340g, lime 200g, water lOOg) were assessed for magnetic strength by casting lOOg of the above mix containing magnetite into a 11cm diameter mould.

The results indicated that a cement render containing 5% by mass magnetite will have sufficient magnetic strength to hold up a magnetic strip. Higher loadings will give better results but do give a slower to dry and lower strength cement render. Magnetic walls and walls coated with magnetic paint as herein described facilitate attachment of sheets of paper, for example maps, posters and the like which may be clamped to the wall by means of magnets, for example fridge magnets, ferrite magnets or bar magnets. Alternatively, magnets of various shapes, colours and sizes may be attached as decorations of the wall. The invention extends to the use of magnetized paper or magnetized film which may be attached by magnetic attraction directly to the magnetic wall or magnetic paint. The magnetized paper or film may be in

roll form, or sheet form, or may be in cut shapes, for example die-cut patterns for use as decoration or forming puzzles or games on the wall. For example die-cut magnetized farmyard animal shapes may be attached to a wall coated with the magnetic paint herein described or "Naughts and Crosses" may be magnetically attached. Magnetized wall- paper or friezes may be attached to a magnetic wall according to the invention. The magnetized film may for example be manufactured from a plastics material incoφorating magnetized and aligned ferrite or from a film or paper coated with a ferrite-in-resin coating in which the ferrite is aligned in a magnetic field as the coating is cured. In another embodiment a magnet is coated on one side with a low tack high cohesive film which produces a "peel and stick" surface. By this means, a magnetized backing may be clamped to the magnetic wall and the exposed "peel and stick" surface may be used removably to hold sheets of paper or the like to the wall in a manner which does not obscure any part of the exposed surface of the sheet.

As will be apparent from the disclosure hereof pigments other than titanium dioxide can be incoφorated in paints embodying the inventive concept with suitable adjustment to the formulation. Various paint additives of the kind known in the art may be added to improve mould resistance properties in a conventional manner for example, flow surfactants, mould inhibitors and the like. The formulations may be varied to an extent without departing from the scope hereof.