GB2058110A - Aqueous correction fluid and method for making same - Google Patents

Abstract

Novel aqueous correction fluids comprise a stable emulsion of a water- dispersed, water-insoluble acrylic resin as the binder for the masking pigment. The compositions form smooth, receptive, highly opaque, non-brittle masking coatings and have an acceptable drying time due to the fact that the water content is as low as 25 to 50% of the total weight of the composition. The acrylic binder constitutes more than 10% by weight of the correction fluid and has such a high pigment binding power that it provides, on drying, pigment- air interfaces (without loss of film integrity) which, due to the large differences in refractive index between air and pigment, produce a much more opaque coating than is possible with other resin binder systems. As a result, up to half of the masking pigment content can be replaced by less expensive fillers while still providing acceptable opacity.

Description

SPECIFICATION Aqueous correction fluid and method for making same Background of the invention A variety of correction compositions have been commercially-available over the years. Such compositions comprise a masking pigment such as titanium dioxide, a resinous binder material such as a cellulose ether, vinyl resin, acrylic resin, or the like, and a volatile organic solvent forth binder material such as benzene, toluene, ethyl acetate, ethanol, trichlorethylene, or the like. Reference is made to U.S. Patents 3,276,870 and 3,997,498.

Such compositions have one or more disadvantages which render them unsafe and/or unsatisfactory for their intended function. The low boiling point hydrocarbons, ketones, esters and alcohols are flammable and represent a danger when used in the presence of an open flame, such as a match lit by a typist. Moreover, such volatile solvents have rapid evaporation rates so that they evaporate from the applicator brush and from the mouth of the bottle during use, causing caking of the composition on the brush, narrowing of the inner neck of the bottle and the deposit of particles of solidified resin in the composition, which particles form an uneven masking coating over the images being covered during the subsequent use.

Another disadvantage encountered with liquid masking compositions based upon volatile organic liquids is due to the solvent power which said liquids have for a variety of dyes used in typewriter ribbon inks and carbon transfer compositions. Even a minimal dissolution of such dyes discolors the white masking composition and makes the corrected sheet unsightly.

Yet another disadvantage of known correction liquids is the high percentage of relatively expensive white pigments, such as rutile titanium dioxide, required to provide the composition with sufficient covering power or opacity that it can accomplish its required result in a single application.

It has been proposed to produce liquid correction compositions based upon water solutions of the resinous binder material. While such compositions overcome many of the problems caused by the presence of volatile organic solvents, they retain many of the other problems and give rise to new problems. Thus the known aqueous solution compositions are unstable and settle or cake in the container and/or on the brush.

They do not form smooth masking coatings and are not receptive to correct images typed thereover. They are not sufficiently opaque or resistant to flaking, nor do they have sufficient covering power or opacity that they can accomplish the required results in a single application.

These deficiencies are due to the fact that compositions based upon soluble resinous binder materials can only contain a relatively small percentage by weight, based upon the total weight of the composition, of the dissolved resinous binder material, i.e., less than about 10% and generally about 5% to 6% of the total composition. Otherwise, the viscosity of the composition is so high that the composition cannot be applied over erroneous images andior does not have any levelling ability, i.e., ability to flow or settle to form a smooth receptive masking coating over the erroneous image.The inability to use high amounts of resinous binder materials, necessary to bind the high percentage of masking pigments required for proper opacity, results in compositions in which the pigment particles are inadequately bound together and/or bound to the copy sheet whereby they tend to crack and/or flake and become powdery when they dry.

Attempts to overcome these problems by the use of aqueous dispersions in place of aqueous solutions of resinous binder materials have met with limited success. Since flowable aqueous dispersions can contain substantially larger percentages of resinous binder materials than aqueous solutions can contain, without becoming unduly thick or viscous, correction fluids based upon such aqueous dispersions have a lower water content and a faster drying time. However, many aqueous dispersions are not stable and tend to settle or cake, and/or they do not form smooth, receptive coatings, and/or they require large amounts of expensive masking pigments such as titanium dioxide in order to have adequate masking or hiding properties in a single application.Such large amounts of masking pigment must exceed the critical pigment volume concentration in order to provide sufficient opacity and in such case, the tensile strength of the masking layer is deteriorated because the pigment is underbound. Thus the dried masking layer is susceptible to cracking and flaking from the copy sheet. Also, the use of large amounts of masking pigment with conventional water-dispersible binder materials unduly thickens the composition and requires the addition of more water to render the composition coatable, again increasing the drying time to unacceptable limits and reducing the content of the resinous binder material to about 5% or 6%, based upon the total weight of the correction fluid.

Objects ofthe invention It is the principal object of the present invention to provide aqueous dispersion correction fluid compositions which contain a relatively high percentage of resinous binder material, above about 10% by weight of the total composition, such binder material having sufficient cohesive strength to bind masking pigment(s) and, optionally, filler(s) in amounts in excess of the critical pigment volume concentration of such resinous binder material, a relatively low percentage of water, below about 50% by weight of the total composition, and which have excellent coatability and masking properties in a single application.

It is another object of the present invention to provide compositions of the foregoing type which have excellent flow characteristics and levelling properties, acceptable drying rates, strong bonding powers for a copy paper, resistance to cracking or flaking or drying, and good receptivity for images typed thereover.

It is yet another object of the present invention, according to one embodiment thereof, to provide liquid masking compositions which contain a relatively high percentage by weight of relatively inexpensive filler in place of an amount of the relatively expensive masking pigment and yet which retain the necessary opacity and covering power to accomplish their required function in a single application.

These and other objects and advantages of the present invention will be apparent to those skilled in the art in the light of the present disclosure.

Summary ofthe invention The present invention is based upon the discovery that stable aqueous correction compositions may be formulated from water-dispersible, water-insoluble acrylic resinous binder materials, opaque pigments and, if desired, extenderfillers and that such compositions overcome the disadvantages of prior-known correction fluids and also provide advantages not found in such prior-known correction fluids.

The discovery that aqueous acrylic emulsions can be used in relatively high percentages, compared to other aqueous resin emulsions such as vinyl acetate emulsions or dispersions, while retaining good flowability, stability, opacity and levelling properties, enables the formulation of correction fluids which contain less than about 40% by weight of water vehicle, i.e., more than about 60% by weight of solids, and which therefore have acceptable drying times, substantially faster than aqueous solutions or emulsions containing 50%, 60% or more of water vehicle.

Also, the discovery that acrylic resin emulsions have such high binding power for piments and fillers that a greater degree of pigment filler loading is possible before loss of film integrity occurs, enables the formulation of exceptionally opaque correction compositions in which the pigment/filler content exceeds the critical pigment volume concentration. Thus pigment-and filler-air interfaces are produced with subsequent enhancement of dried film opacity. This makes possible the use of a substantial percentage of less expensive filler materials, such as talc and kaolin, in place of the more usual and more expensive masking pigments such as titanium dioxide, zinc oxide, and the like, to provide a correction fluid which still retains sufficient opacity or covering power to be effective in a single application.This is not possible with other aqueous dispersion resins such as vinyl acetate polymers, since they cannot carry a sufficiently high pigment/filler concentration, in excess of the critical pigment volume concentration, to produce the pigment-air and/or filler-air interfaces necessary for opacity purposes, without such loss of cohesive strength and particle binding as results in a dried masking coating which cracks and flakes from the copy sheet.

Relatively high resin content is important for a correction fluid because of the large amount of masking pigment present therein. Normally, if the critical pigment volume concentration is exceeded, the pigment is inadequately bonded together in the dried correction composition and the composition is brittle or powdery and can crack, particularly if the corrected area of the sheet is folded. Portions of the masking pigment can become detached and separate from the sheet. However, the correction fluids of th present invention contain relatively large amounts of acrylic resinous binder material in the form of particles stably dispersed or emulsified in the aqueous vehicle.Such acrylic resins have been found to have such strong bonding properties for masking pigments and fillers that these additives can be present in amounts in excess of the critical pigment volume concentration (CPVC) without producing a masking composition which becomes brittle and flaky on drying. Thus, as the pigment filler content is increased beyond the CPVC, there is insufficient resin present to fill the interstices between all of the pigment and filler particles. Thus, air is drawn into the masking composition, during drying, to produce pigment-air and filler-air interfaces. Such interfaces produce a greater opacity or hiding power than is the case when the CPVC is not exceeded, because of the difference between the refractive index of air (1) and that of the resin (about 1.5), and filler (about 1.49 for talc).Thus, unless the CPVC is exceeded, the filler cannot be used to produce hiding power or opacity since the refractive index of the resin and the filler is about the same.

Aqueous acrylic emulsions are commercially-available which contain up to about 60% by weight of the acrylic resinous binder material and which have low viscosities, good flow properties and excellent pigment binding properties, whereby substantial amounts of masking pigment and extender filler, if desired, can be added to provide a correction fluid which has a resin binder content higher than 10% and up to about 20%, based upon the total weight of the correction fluid, and yet has a relatively low viscosity and good flow and levelling characteristics. Such high binder compositions provide smooth, opaque, dried coatings which are tough and flexible and which resist cracking and flaking, even if folded.

The preferred acrylic resin binder materials for use, according to the present invention, are polymers comprising acrylic acids, esters, and amides which are dispersed in particulate form in an aqueous vehicle.

Such aqueous dispersons are commercially-available under the trademarks Vinacryl, Ucar, Primal, and others.

Such acrylic emulsions are preferred because they have high binding power for even excessive amounts of conventional pigments, extenders and fillers so that the combination of such acrylics and such additives provides a dried coating which is cohesive, non-brittle, flexible and which is quite opaque. Thus, it is possible to use inexpensive fillers in place of up to about 50% by weight of the more expensive masking pigments, such as titanium dioxide, which would normally be present in the correction fluid.

The masking pigments useful according to the present invention are the conventional opaque masking pigments such as titanium dioxides, zinc sulfide, zinc oxide, and the like. Such pigments generally comprise the major amount by weight of th solids content of the correction fluids, equal to from about 40% to 90% by weight of such solids content. However, up to about one-half of such pigment content can be replaced with suitable fillers, as discussed hereinbefore.

The following table illustrates the ingredients of the compositions of the present invention and their effective ranges. It should be understood that small amounts of other ingredients, such as tinting colors, wetting agents, grinding aids, plasticizers and/or additives, may be included.

Ingredient % by weight total % by weight solids Water-dispersible resinous binder 10-20 . 20-40 Opaque pigment 20 - 60 40 - 90 Filler 0-30 0-45 Water 25 - 50 It should be understood that the viscosity of the compositions of the present invention is important. The viscosity must be sufficiently low that the composition is not unduly thick, i.e., is a smooth, coatable fluid having good levelling properties whereby it has the ability to flow or settle during drying to form a smooth, level, receptive masking coating over the erroneous image. Otherwise, the deposit is not uniformly opaque, inconspicuous or perfectly receptive to the correct image typed thereover. However, the viscosity must not be so low that the composition is unduly thin or watery, whereby the composition spreads over the unintended areas and its opacity is diminished.In general, the present compositions must have a Zahn viscosity of between about 55 seconds and 80 seconds (measured by a number two Zahn cup), and most preferably between about 60 seconds and 75 seconds.

The following example is given as illustrative of a preferred composition coming within the scope of the present invention. However, such example should not be considered as limitative with respect to all of the ingredients included and their respective proportions.

Example Ingredients Parts by weight % Solids Vinacryl 7172 (50% aqueous) 24.8 18.4 Titanium dioxide 24.0 39.0 Kaolin clay filler 24.0 39.0 Sodium hydroxide (4% aqueous) 6.5 .5 Anionic surfactant 0.4 .7 Grinding aid 0.4 .4 Defoamingagent 1.2 2.0 Water 18.7 100.0 100.0 Vinacryl 7172 is a registered trademark of Vinyl Products, Ltd. for a 50% aqueous emulsion of a styrene-acrylic copolymer.

The composition is prepared by mixing all of the above ingredients, except for the Vinacryl and defoaming agent, in a ball mill for about two hours, thereafter adding the defoaming agent and milling for an additional hour and removing the ground mixture from the ball mill. Then the Vinacryl resin is added to the ground mixture to produce the final composition and, if desired, a small amount of dilute black dye may be added to tint the composition and reduce its whiteners to match the color of conventional white typing paper.

The composition of the Example is a preferred composition from the standpoint of cost, since it contains a substantial amount of inèxpensive white clay filler in place of the more expensive titanium dioxide white pigment, and yet has satisfactory opacity or covering power to mask erroneous images in a single application. It appears that the styrene-acrylic copolymer is a preferred binder for use with such fillers, since it has a very high binding power for the pigment and filler materials.

Variation and modifications may be made with respect to the present compositions to adapt them for various uses. For instance, different colored tints may be used to adapt the correction fluids for use on copy sheets of different colors. Also lamelliform reinforcing fillers, such as mica, may be included to improve opacity, levelling and receptivity for the correct image typed thereon.

Claims (10)

1. Aqueous correction composition adapted to be applied over an erroneous image to form an opaque masking layer thereover which is resistant to cracking and flaking and which is receptive to the application of a correct image, said composition being a stable emulsion comprising a water-dispersible, water-insoluble acrylic resin binder material, an opaque masking pigment and water and having a Zahn viscosity of between about 55 seconds and 80 seconds as measured with a number two Zahn cup.

2. An aqueous correction composition according to claim 1 in which said acrylic polmer comprises a styrene-acrylic copolymer.

3. An aqueous correction composition according to claims 1 or 2 which further comprises a particulate filler material.

4. An aqueous correction composition according to claims 1 or 2 which comprises more than 10% by weight of said acrylic resinous binder material.

5. An aqueous correction composition according to claim 1 having a Zahn viscosity of between about 60 seconds and 75 seconds, measured by a number 2 Zahn cup, comprising more than 10% and up to 20% by weight of the acrylic resinous binder material, from 20% to 60% by weight of the masking pigment, from 10% up to a weight equal to one-half the weight of said masking pigment of a particulate filler material and from 25% to less than 50% by weight of water.

6. Method for producing an aqueous correction composition which comprises the steps of (a) mixing an opaque masking pigment and water and milling to form a finely-divided slurry of the pigment in the water and (b) mxiing with said slurry an aqueous emulsion of a water-dispersible, water-insoluble acrylic resinous binder material to form a composition having a Zahn viscosity of between about 55 seconds and 80 seconds as measured with anumbertwo Zahn cup.

7. A method according to claim 6 in which the resinous binder material of step (b) comprises a styrene-acrylic copolymer.

8. Method according to claims 6 or 7 in which a particulate filler material is present in the mixture of step (a).

9. Method according to claim 6 in which a defoaming agent is present in the mixture of step (a).

10. Method according to claims 6 to 7 in which the aqueous correction composition comprises from more than 10% up to 20% by weight of the acrylic resinous binder material and from about 25% up to less than 50% by weight of water.