Opacification of paint.

Technical Field

0001This invention relates to the pigmentation of paint using a plurality of opacifying agents in order to enhance the hiding quality of the paint while minimizing the proportion of titanium dioxide which is present as well as the undesired softening of the paint.

Background Art

0002A conventional paint comprises a resinous binder. which serves to hold together particles of pigment. These particles of pigment obscure the imperfections in the surface of the substrate which is painted, and this obscuring capacity is known as "hide". In modern paints, heavy reliance is placed upon the use of titanium dioxide, rutile, as the pigment because it contributes excellent hide. Unfortunately, this pigment is expensive.

0003Because of the expense of the titanium pigment, other pigments have been developed, including pigmented vesiculated beads and opaque polymer particles. Both of these are known articles of commerce for use in the paint industry and are less costly than titanium dioxide. However, the pigmented vesiculated beads do not entirely maintain the hide which is reduced in proportion to the amount of titanium which is replaced. The opaque polymer particles do enhance the hide, but these are soft and, when used in large amount, degrade the burnish resistance and scrub resistance of the final paint film.

Disclosure of Invention

0004In accordance with this invention, an aqueous paint having improved hide at lowered titanium dioxide content comprises water, a resin binder, and pigment dispersed in the water, this pigment being present in a pigment volume concentration (PVC) of from 20% to 80%, and the pigment contains from 5% to 65% of titanium dioxide, rutile, with the balance of the pigment consisting essentially of pigmented vesiculated beads and opaque polymer particles in a ratio of 30/70 to 90/10.

0005By limiting the proportion of opaque polymer particles and by using smaller proportions thereof at higlier pigment volume concentrations, very little opaque polymer is present in the final paint, and the burnish resistance and the scrub resistance remain high. At the same time, if the replacement of the titanium dioxide is entirely by pigmented vesiculated beads, the improvement in hide is much smaller than that obtainable if it were the sole replacement for the titanium dioxide, and the pigmented vesiculated beads allows this to be achieved without undue loss in burnish and scrub resistance.

0006All proportions and ratios herein are by volume based on dry solids, unless otherwise specified.

0007This invention particularly contemplates paints having combinations of properties which are commonly used in the paint industry, these being designated flat paints, satin paints and gloss paints. The term "flat" is well known in the latex paint industry and denotes a paint which deposits a film which, when dry, exhibits a gloss not in excess of 6 regardless of whether a 60° or 85° gloss meter is used to determine the proportion of light which is reflected. The term "satin" is well known in the latex paint industry and denotes a paint which deposits a film which, when dry, exhibits a 60° gloss meter reading in the range of 7-20 and an 85° gloss meter reading in the range of 15-70. It will be understood that gloss meters report the proportion of light which is reflected, and that gloss paints _: reflect more light than do satin paints.

0008The aqueous flat paints under consideration comprise water, a resin binder, and pigment dispersed in the water, this pigment being present in a pigment volume concentration (PVC) of from 60% to 80%, and the pigment contains from 5% to 25% of titanium oxide, rutile, with the balance of the pigment consisting essentially of pigmented vesiculated beads and opaque polymer particles in a ratio of 80/20 to 90/10. By blending a large proportion of pigmented vesiculated beads with a small proportion of opaque polymer particles in the described combination, very little opaque polymer is present in the paint, and the burnish resistance and the scrub resistance remain high. At the same time, if the replacement of the titanium dioxide is entirely by pigmented vesiculated beads, the improvement in hide would be less than about 20% of that obtainable using opaque polymer particles as the sole replacement of titanium dioxide. However, the blends of this invention in which only 10% to 20% of the volume of the mixture is opaque polymer particles provides about 60% or more of the hide improvement obtainable using opaque polymer particles as the sole replacement for the titanium dioxide.

0009The aqueous satin paints under consideration comprise water, a resin binder, and pigment dispersed in the water, this pigment being present in a pigment volume concentration (PVC) of from 25% to 50%, and the pigment contains from 30% to 60% of titanium dioxide, rutlile, with the balance of the pigment consisting essentially of pigmented vesiculated beads and opaque polymer particles in a ratio of 60/40 to 70/30. By using such blends containing a minor proportion of opaque polymer particles with 30% to 60% of titanium dioxide (preferably at least 40%), _: the proportion of opaque polymer is too small to significantly impair the burnish resistance and the scrub resistance. At the same time, hide is improved more than the respective proportions of pigmented vesiculated beads and opaque polymer particles would suggest. This can be seen by graphing the hide at constant total pigment and constant titanium dioxide content with only one of the two replacements being used and then connecting the hide values so obtained by a straight line to show what blends should provide. The hide obtained by the claimed blends are significantly better than that straight line would suggest.

0010In the gloss paints under consideration, the PVC is from 20% to 40%, the pigment contains from 35% to 65% of titanium dioxide, and the balance of the pigment consists essentially of pigmented vesiculated beads and opaque polymer particles in a ratio of 30/70 to 50/50.

0011This invention is applicable to paints in which the resin is dissolved or colloidally dispersed in the aqueous phase of the paint, but the invention is primarily adapted to latex paints in which the resin binder is constituted by particles of solid resin suspended (emulsified) in the aqueous continuum with the aid of a surfactant.

0012The opaque polymer particles employed herein are emulsion polymer particles of generally transparent water insoluble polymers having a sufficiently high glass transition temperature to avoid coalescence in the aqueous paints which are involved. These polymer particles have an average particle size in the range of 0.1-2.5 microns and contain microvoids which scatter incident light and thereby produce opacity. These micro-voids can be provided in various ways, conveniently by first polymerizing in aqueous emulsion a swellable core polymer containing carboxylic acid groups, typically provided by from 5% to 50% by weight of acrylic acid or methacrylic acid, which enable the core to be swollen with a volatile amine or ammonia. A sheath of polymeric material is then emulsion polymerized around the acid-containing core. The sheathed particles are subjected to the volatile amine or ammonia in aqueous dispersion to swell the core, and the dispersion is dried to remove the swellant and proviae a microvoid within the sheathed polymer particle. Typical opaque polymer particles and their production are disclosed in European patent application having Publication No. 0022633 A2 of Rohm and Haas Company. The microvoid-containing product of Example 1C of that disclosure is identified herein as opaque polymer A and will be used in the Examples of this application. In opaque polymer A, the core is a copolymer of 69.65% methyl methacrylate, 29.85% methacrylic acid, and 0.5% ethylene glycol diacrylate having an average particle diameter of 0.35 micron. This core has a swell ratio at pH 10 with ammonia of around 12, by volume. The described core is sheathed with polymerized methyl methacrylate to an average diameter of 1.0 micron and the sheathed particle is neutralized with ammonium hydroxide and heated to 95°C. to swell the particles to 1.15 micron before the ammonia is evaporated to provide an internal void.

0013The pigmented vesiculated beads which are used herein are generally larger than the opaque polymer particles and contain a plurality of small cells or vesicles whose walls are provided by the polymer of the beads. These beads have an average particles diameter of from 3 to 30 microns and contain from 10% to 55%, by weight (dry) of finely divided pigment, usually titanium dioxide, rutile. These vesiculated beads are available in commerce and are described in Kershaw et al. U. S. Patent No. 3,933,579 dated January 20, 1976. They are generally spheroidal with a ratio of bead diameter to mean vesicle diameter of at least 5:1.

0014The pigment volume concentration is important to the provision of a flat, satin or gloss paint. The narrow ranges of PVC described previously is thus a necessary aspect of these paints.

0015The proportion of titanium dioxide used herein is also important. In preferred practice, the proportion of titanium dioxide, rutile, is from 7% to 15% for flat paints, at least 40% for satin paints, and from 40% to 50% for gloss paints. This minimizes the cost of the paint at the same time that it maximizes the improvement in hide.

0016The intended use of the paint, such as for interior or exterior application, does not change the parameters of this invention. However, air dry paints are primarily contemplated herein because elevated curing temperatures which destroy the voids in the opaque polymer particles which help to create the desired opacity are counterproductive to this invention.

0017It should be appreciated that it is common in latex paints to include a proportion of extender particles which contribute little to the hide. Such extender particles, illustrated herein by silica or calcium carbonate, are commonly present in amounts up to about 30% of the total particulate content of the paint (up to about 20% for satin paints), but these do not alter the synergism of this invention.

0018It will also be evident that while the three pigments noted herein are to provide the desired opacity, one may still wish to add small amounts of other pigments or colorants for special purposes, such as to provide desired coloration or to confer other optical effects. Thus, the pigmentation consists essentially of the three pigments which have been discussed since these determine the hiding capacity of the paint.

0019The invention will be illustrated in the examples which follow.

Example 1 - Gloss Paint

0020<tables id="tabl0001" num="0001"><img file="EP0113435A2_D0001.tif" /></tables>

0021This paint has a PVC of 32%, titanium dioxide constitutes 44% of the prime pigmentation and the ratio of pigmented vesiculated beads to opaque polymer particles is 44.4/55.6. The burnish resistance and scrub resistance are essentially the same as the same paint pigmented with only titanium dioxide, but the hide is almost as good despite the small amount of opaque polymer present.

Example 2 - Flat Paint (non-extended)

0022<tables id="tabl0002" num="0002"><img file="EP0113435A2_D0002.tif" /></tables>

0023This paint has a PVC of 75%, titanium dioxide constitutes 13% of the prime pigmentation and the ratio of pignmented vesiculated beads to opaque polymer particles is 85/15. The burnish resistance and scrub resistance are essentially the same as the same paint pigmented with only titanium dioxide, but the hide is almost as good despite the small amount of opaque polymer present.

Examnle 3 - Flat Paint (extended)

0024<tables id="tabl0003" num="0003"><img file="EP0113435A2_D0003.tif" /></tables>

0025This paint has a PVC of 75%, titanium dioxide constitutes 8% of the prime pigmentation and the ratio of pigmented vesiculated beads to opaque polymer particles is 84/16. The extender is 19% of the total pigmentation. Extender may be added up to 30% of pigment volume to improve film properties, as long as the prime pigment ratios of titanium dioxide, opaque polymer particles and pigmented vesiculated beads are maintained. Again, the burnish resistance and scrub resistance are essentially the same as the same paint pigmented with only titanium dioxide, but the hide is almost as good despite the small amount of opaque polymer present.

Example 4 - Satin Paint

0026<tables id="tabl0004" num="0004"><img file="EP0113435A2_D0004.tif" /></tables>

0027This paint has a PVC of 34%, titanium dioxide constitutes 53% of the prime pigmentation and the ratio of pigmented vesiculated beads to opaque polymer particles is 62/38. The burnish resistance and scrub resistance are essentially the same as the same paint pigmented with only titanium dioxide, but the hide is almost as good despite the small amount of opaque polymer present.

Note 1

0028In all examples, the opaque polymer particles were provided by a commercial product called Phopaque OP-42 marketed by Rohm and Haas of Philadelphia, Pennsylvania.

Note 2

0029In all examples, the pigmented vesiculated beads were provided by a commercial product called Spindrift PVB made by The Enterprise Companies, Wheeling, Illinois.

Note 3

0030The vinyl acrylic latex refereed to in Examples 1 and 4 was an aqueous emulsion copolymer of 84% by weight vinyl acetate, 14% by weight n-butyl acrylate and 2% by weight of the 2-hydroxypropyl maleate ester of 2-hydroxyethyl ethylene urea. A suitable commercially available latex is UCAR 367 available from Union Carbide Corporation of New York.

0031The acrylic latex in both Examples 2 and 3 is AC-388 made by Rohm and Haas.

Note 4

0032The additives are those normally used in the production of paints, and include surfactants, thickeners, preservatives, fungicides, coalescing solvents, materials for the adjustment of pH, defoamers and driers. These are of no consequence in this invention.