Symposium on Microvoid Coatings Sponsored by the Organic Coatings and Plastics Division of the American Chemical Society, Chicago, lllinois August 27, 7973
Introduction Organic coatings containing microvoid air interfaces have been known and used for many years as reviewed in the first paper by J . J. Clancy. Interest has recently increased because of economies permitted by at least partial replacement of titanium dioxide. As explained by B. N. McBane and R. Dowbenko in the second and P. E. Pierce, S. Babil, and J. Blasko in the third paper, combinations of microvoids and Ti02 can be synergistic. The lack of satisfactory theoretical explanation for such augmented hiding was pointed out by J. A. Seiner and H. L. Gerhart at the Xlth FATIPEC Congress. This prompted the Paint Research Institute to sponsor E. Allen's project at Lehigh University (reported separately; see comment by the Editor+). Some theoretical aspects are discussed
by W. D. Ross particularly with regard to void concentration and shape. By reconsidering previously known data on dry-hiding, F. B. Stieg shows in his paper that correct application of the Fresnel equation explains the mysterious synergism. The combined papers of this symposium contain a reasonably complete bibliography of the subject and provide both theoretical as well as practical insight to chemists attempting to formulate microvoid coatings.
H. BURRELL lnmont Corp. Clifton, N. J.
'
A sixth paper from the Symposium on Microvoid Coatings by Dr. Eugene Allen, Lehigh University. was published in J. Paint Techno/., 45, No. 584, 65-72 (1973),under the title "Prediction of Optical Properties of Paints from Theory."
Microvoid Coatings in Graphic Arts Applications.
A Patent Survey
J. J. Clancy Arthur D. Littie, Inc., Cambridge. Massachusetfs 02740
Traditionally we think of opacity and brightness in terms of high refractive index pigments but i n reality pigments are only one means of imparting light-scattering properties to a coating system. Another means that is assuming increasing importance is to make use of the interfacial structures defined by microvoids having dimensions approximating the average wavelengths of visible light. Who among us has not wondered about the extreme whiteness of an Easter lily or marvelled at the brilliance and opacity of cumulus clouds on a sunny summer's day-and no trace of titanium dioxide. There are many other examples of such systems in nature. Lately, we have been seeing a growing number of industrial products whose opacity and brightness also depend not upon titanium dioxide but upon the light-scattering efficiency of microvoid structures. This paper discusses some of these industrial products and the coatings upon which they are based. Most of the examples are taken from the patent literature but a complete survey was not attempted. Rather, we selected representative product applications and patents which depict the evolution of microvoid technology. Accordingly, the patent examples are presented in chronological order. In those cases where the author did not know or was uncertain whether a product had been produced commercially under the teachings of a specific patent, the question was addressed to the owner of the patent and the information was readily supplied. Microvoid structures have been produced in a number of ways, e.g.,by freeze-drying, extraction, phase incompatibility, imperfect packing, etc., but the two most popular methods involve the use of solvent and nonsolvent combinations for a polymer in wet coating formulations. The coatings made by these two techniques are generally referred to as "blush" lacquers or coatings and "bubble" coatings.
Blush coatings are formulated by dissolving a polymer in a mixture of compatible liquids, one being a solvent for the polymer and the other a nonsolvent. The solvent should have a higher vapor pressure than the nonsolvent. In the initial stages of drying the wet coating, the exhaust gas will be rich in the solvent for the polymer by virtue of 30
Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No. l , 1974
its higher vapor pressure. Consequently, it will become relatively less concentrated in the wet coating and, if the composition has beep formulated properly, a point will be reached where the polymer is no longer soluble in the vehicle and the polymer will precipitate from solution. The structure of the resulting precipitate can vary widely, de-
pending upon such factors as the nature of the polymer, the formulation, the solvent characteristics of the liquids, the method of drying, etc. Solvent and nonsolvent liquids are also used in formulating bubble coatings, but in this case the two solvents are incompatible and an emulsion technique is required. In preparing the wet coating the polymer is usually dispersed in the solvent, the nonsolvent added, and the mix processed to produce a fine-particle-size emulsion. As in the case of blush coatings the solvent should have a higher vapor pressure than the nonsolvent liquid. Two distinct phases occur during drying although the total elapsed time can be as little as 1 or 2 sec. First the continuous-phase solvent is evaporated, leaving the polymer in solid or gelled form. The nonsolvent liquid remains dispersed uniformly in the binder as fine droplets. Continued drying volatilizes the dispersed phase liquid to produce a film that has voids distributed throughout its volume. The size of these voids is substantially equivalent to that of the droplets prior to their evaporation. Both methods can produce coatings that are highly efficient lightscattering systems and whose characteristics can be varied widely. The following patents are concerned with microvoid coatings. They either teach the method of producing the microvoid structure or use a microvoid coating in a particular product application. Most of the applications depend upon the functional properties of the microvoid coatings. Such functional properties include the ability to collapse the coating with heat or pressure, make it transparent with a liquid of the same index of refraction, print on it, etc. Willkie (1923) probably disclosed the first blush coating. He described a n enameling composition which, in a single application, produces an opaque coating that has a high-gloss surface. The coating is based on a cellulose ester, a limited amount of a solvent plasticizer, and solvents for the resin. His solvents are selected to give constant-boiling mixtures with the water normally present in such a system. Initial drying eliminates the water near the surface of the coating and causes a transparent skin to form, which retards further drying. The solvent in the body of the coating must then dry by diffusion. This drying leaves a water-rich vehicle behind that causes the resin to precipitate and produces the dense opaque structure claimed. Kallock (1942), referred to the Willkie disclosure of blushing lacquers and applied that technology to the production of pressure-sensitive chart papers. Kallock’s product consists of a blush coating applied to a colored paper base or to a white paper base precoated with a dark formulation. In use, a stylus or other mechanically or manually applied pressure collapses the coating and makes it transparent, exposing the contrasting substrate to produce an image. Kallock coatings are based on cellulose derivatives. The invention was made while Kallock was at McLaurin-Jones in Brookfield, Mass. This company was later acquired by Ludlow Corp. and the technology has continued to be used to produce various types of chart papers. James (1950) described another recording material based on a blush coating but one which was activated by heat instead of pressure. James disclosed the use of cellulose derivatives and other thermoplastic materials including methacrylates and polystyrene. James’s work was done at Arthur D. Little, Inc., for the Sanborn Co., now a part of Hewlett-Packard. The technology has been used for a number of years in manufacturing charts for electrocardiogram recordings. Rosenthal (1956) disclosed still another recording paper.
The basic product is similar to earlier developments in that it uses an opaque coating over a contrasting substrate, with the coating being collapsed selectively by a stylus a t the point of use to produce an image. Rosenthal’s system is interesting because it is the first to teach the use of an emulsion to obtain the desired structure. His work is based on water-in-oil emulsions; the continuousphase resin includes cellulose derivatives, polystyrenes, vinyl copolymers, acrylates, etc. A variety of products has been produced under this patent by the Nashua Corporation for various recording applications. Newman, et al. (1959), disclosed pressure-transfer elements that serve the function of and represent improvements over conventional carbon papers. The patent teaches blush lacquer structures comprising coloring matter of the desired type. These structures have unique pressurereleasing properties and are free from the smearing associated with conventional carbon paper. These products are particularly useful in meeting the requirements of OCR and MCR systems. They have been produced commercially by Columbia Ribbon & Carbon Manufacturing Co., Inc. Newman, et al. (1961), described an ink-receptive coating based on a blush lacquer composition. In some applications one desires to image a transparent film but the surface of the film is substantially impermeable and has poor ink-retentive properties. The application of the blush coating to the plastic film provides a receiving surface to which the pressure-inscribed ink composition will adhere. Such products have been produced commercially by Columbia Ribbon & Carbon Manufacturing co.,Inc. Larsen (1962) described an improved blush lacquer coating for use in stylus recording materials. The coatings continue to be used over a contrasting base with one objective of the invention being to improve the water resistance of the products. This patent is being used by Ludlow Corp. in the manufacture of recording chart products. Clancy, et al. (1963), disclosed a bubble-coating system based on the use of an oil-in-water emulsion. In effect this system is the reverse of that taught by Rosenthal, who used a water-in-oil system and quite different resins. This patent discloses the use of proteinaceous binders, such as casein, as well as elastomeric latices. The products differ from the microvoid systems disclosed heretofore in that the cost of the coating to achieve the same level of brightness is approximately one-tenth that of the previous systems. The lower cost, together with the type of binders used, made the coatings of interest in the manufacture of printing papers and paperboard. Coatings described in the patent have been used commercially in the manufacture of boxboard. This work has included coatings for the topside of the board (normally printed) and for the back of dark news boards. The work covered by U. S. Patent 3,108,009 was carried out at Arthur D. Little, Inc., on behalf of the Boxboard Research & Development Association. Newman (1963) in still another patent described a thermographic reproduction copy sheet that utilizes a microporous calcium carbonate layer. The system comprises a donor sheet carrying a heat-meltable layer of a wax composition and a copy sheet having a colored base and an opaque top coating which can be made transparent without the use of heat. In use, the original to be opaque is placed in contact with a manifold of the donor and receptor sheets. The assembly is exposed with infrared radiation which causes the wax to melt in those areas corresponding to the images on the original. The melted wax transfers to the opaqued copy sheet with which it is in contact, selectively impregnating the porous coating causing it to become transparent, and thus imaging by exposing the colored substrate. Ind. Eng. Chern., Prod. Res. Develop., Vol. 13,No. 1. 1974
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This technique is a variation of the Thermo-Fax process and is said to represent an improvement in the copy paper by virtue of its not being pressure sensitive. The product has been in limited production by Columbia Ribbon & Carbon Manufacturing Co., Inc. Clark (1964) disclosed an improved heat-sensitive copy sheet. The product consists of a backing material, ( e . g . , paper), a visibly reactive heat-sensitive layer, and a top layer of an opaque heat-transparentizable blush coating. In use, the differentially radiation-absorptive graphic original receives a brief intense irradiation while in heat-conductive contact with the copy sheet. Where an image occurs in the original, heat is transferred to the copy sheet selectively, making the opaque surface layer transparent and simultaneously causing a color-producing reaction to occur in the heat-sensitive layer. In one variation of the Clark system, both the opaque quality and the reactive components occur in a single layer. In another variation the opaque layer is applied first to a transparent backing and then overcoated with the reactive layer. In the latter case imagi.ng would be through the transparent base. Products based on these concepts have been used commercially in cardiographic chart paper where the two-phase reaction system is said to have contributed improved resolution. It has also been used in early Thermo-Fax applications by 3M Co. Clancy, et al. (1964), described a printing paper coating. This is a bubble-coating composition that contains a substantial amount of starch and particulate matter, the latter being added for the purpose of controlling printing properties and not to achieve opacity. The composition was specifically developed to meet the higher shear viscosity requirements in the high-speed coating of publication paper with a trailing blade coater. These coatings, when used at a level of 2 lb/ream (3300 ft2), duplicate almost all of the properties of a conventional publication paper coated a t a 10-lb level. The single deficiency was in the level of gloss ink holdout which could be achieved; because of this deficiency the coatings were used commercially to a limited extent. Hoge, e t al. (1966), taught the use of a microvoid coating in the construction of a pressure-sensitive, heat-resistant record sheet. The coatings are based on an emulsion system comprising a thermoplastic resin, a plasticizer, and a pigment. The formation of the voids in the coating is said to be primarily a function of the pigment. The coating is applied over a colored base to yield a product that appears white and opaque. The coating becomes relatively transparent under writing pressure, thus exposing the colored substrate of contrasting color. A carbonless copy paper product has been produced by the Oxford Paper Co. under the teachings of this patent for use in manifold forms but its use has been discontinued because it was not economical to manufacture. Ploetz (1970) disclosed an opaque printing paper produced by impregnating a light, porous, fibrous web with a blush lacquer. The resin used in the blush coating is precipitated during the drying step to produce the light-scattering elements. Self-supporting opaque films are also claimed. It is not known whether Feldmuhle Aktiengesellschaft has produced products under this patent. The patent is interesting, however, because of the proposed use of the blush lacquer as an impregnant for a fibrous sheet and in the form of a self-supported film. Latent-image printing is being used increasingly in instructional materials to provide feedback and reinforcement t o the student. This feedback may take the form of an invisible correct answer printed in a specific area with latent image ink. The student can develop the image to 32
Ind. Eng. Chem., Prod. Res. Develop., Vol. 13,No. 1, 1974
make it visible instantaneously by means of a felt tip pen or other instrument. Most such processes are irreversible and can be used only once. Thomas (1970) described a multiple-use, concealed-image device. In Thomas’ product the latent image is in fact a visible image printed on a transparent base but overcoated with an opaque blush coating. The student applies a volatile liquid to the latent-image area filling the voids in the blush coating and rendering the coating transparent and the latent-image visible. When the volatile liquid evaporates, the image is again obscured and the sheet can be reused. Although this product has not been used commercially by A. B. Dick Co., it is another interesting potential application of a microvoid coating. Brenneman, et al. (1972), disclosed a series of bubble coatings and products made therefrom based on structural and film-forming resin components, the suitability of which is determined by the physical properties of a nonbubble film of the resins a t room temperature. This is the latest bubble coating patent to be issued to Arthur D. Little, Inc. The patent is licensed for paint applications. Fadner, et al. (1972), disclosed a novel process for producing a microvoid coating on paper. This invention relates to the freeze-drying of coated paper webs to yield products that exhibit high opacity, low density, smoothness, and gloss. The opacity is a function of the microvoid structure produced during the drying step. The process involves coating the web of paper, freezing the wet coating, and removing the water under vacuum and radiant heat: The residue that remains has a void structure equivalent to that occupied by the water in the coating. The process has been demonstrated on pilot scale equipment, but it has not been used commercially by Oxford Paper Co. The patent is believed to be available for licensing. Crown Zellerbach has recently developed a new papermaking fiber, SWP, based on polyolefin fibrils. It can be used in conventional papermaking operations, with or without conventional cellulose fibers, and imparts new and interesting properties to paper. One of the first commercial applications is in Japan where it is being used to manufacture expensive embossed wallpapers. Products made from SWP fibers are generally opaque as a result of air voids. Because the fibers are thermoplastic, hot embossing causes the embossed areas to become transparent and produces attractive decorative effects. Other applications of microvoid structures, perhaps familiar to most of us, include the Dymo Tape Writer, which produces an opaque effect by phase separation, and Labelon tape, that combines the collapse of a blush coating and the magic slate principle with an overlaying transparent film. Perhaps no family of coatings is used in greater volume than conventional clay coatings on printing papers and paperboard. These coatings depend upon their void volume for a large part of their opacity. Because the clay and binders normally used have nearly the same index of refraction, the coatings would be essentially transparent without a substantial void volume, which usually approximates 25% of the coating.
Literature Cited Brenneman, R . S., Ciancy, J. J.. MacLeish, W. T.. Wells, R. C.. U. S. Patent 3,637.431 (1972) Clancy, J. J., Lovering, D. W . , Brenneman, R . S., Wells, R . C . , U . S. Patent 3,157,533 (1964). Clancy, J . J . , Lovering, D . W . , Wells, R . C . , U . S. Patent3,108.009 (1963). Clark, B. L., U . S . PaCent3,147,134 (1964). Fadner, T. A., Kraske, K. V.. U. S. Patent 3,702,779 (1972) Hoge. W . H . , Barbour, M . S . , U. S. Patent3,247,006 (1966). James, R . W . , U . S. Patent 2,519,660 (1950). Kallock, W. F . , U . S. Patent 2,299,991 ( 1 9 4 2 ) . Larsen, G. H . , U . S. Patent 3,031,328 ( 1 9 6 2 ) .
Newman, D. A., U. S. Patent 3,109,748 (1963). Newman. D.A., Schlotzhauer, A. T., U. S. Patent 2,872,340 (1959). Newman, D. A . . Schlotzhauer, A. T., U. S. Patent 3,002,858 (1961). Ploetz, T., U. S.Patent 3,504,072 (1970). Rosenthal, F.,U. S. Patent 2,739,909 (1956). Thomas, R. E., U. S. Patent 3,508,344 (1970). Willkie, H. F., U. S. Patent 1,449,157 (1923).
Received for review August 27, 1973 Accepted November 29,1973 Presented a t t h e D i v i s i o n of Organic Coatings a n d Plastics Chemistry, 166th N a t i o n a l M e e t i n g of the American Chemical Society, Chicago, Ill., A u g 1973.
Pittmentized Coatings Based on Microporous Films Obtained by Solvent Extraction Bruce N. McBane* and Rostyslaw Dowbenko PPG Industries, Inc., Research & Development Center, Coatings & Resins Division, Springdale, Pennsylvania 15144
Nonpigmented, opaque coatings are based on microporous films for which the term “pittmentized” is proposed. A method of preparation and the type of compositions used is discussed in detail. The system consists of a thermosetting po!ymer, based on a hydroxyl-containing acrylic and a butylated melamine-formaldehyde resin, cured in the presence of a thermoplastic polymer. Though completely compatible in solution, the system, after curing, gives rise to a two-phase composition consisting of a cross-linked matrix and the thermoplastic polymer dispersed throughout the matrix in the form of microglobules or channels. The thermoplastic polymer is then extracted from the matrix and the film is dried to give an opaque pittmentized coating. Such coatings are discussed in terms of property dependence on composition, microvoid volume concentration, reinforcement with white and colored pigments, and other variables. Other functionalities for pittmentized coatings are described.
Introduction It is a common experience to encounter in the natural environment transparent materials interacting with light in such a manner that they appear opaque. Examples are water in the form of snow or mist, ground glass, or even titanium dioxide, which as a large crystal is nearly transparent while as pigment powder it performs efficiently as a n opacifying agent. There are identifiable factors that are useful in explaining the opacifying function that operates in the examples cited above as well as in the organic polymer coatings with which this paper is concerned. These factors are reflection, diffraction, and refraction of incident light, and when observed acting in concert, the total event is called “light scattering.” Conventionally, opacity has been achieved in coatings through light scattering induced by dispersed pigment particles in a manner and concentration that takes into account their particle size and probable spacing or packing in the ultimate film. In this paper we deal with one of several methods by which microvoids (useful as light-scattering sites) may be formed in a film in order that pigment loading for opacity might be minimized. Anticipated advantages occurring to the coatings chemist by the replacement of pigment with microvoids are increased efficiency of light scattering, improved mechanical properties of films, release from constraints of raw material costs of pigment and the costs of pigment dispersion, and finally lower product density for easier shipping and storage. For convenience it is suggested that the process employing light scattering from microvoids in coatings be termed “pittmentation” and the product so specified be called “pittment .” Discussion Principles of Hiding Power. Surfaces appear black and opaque when they absorb the light incident on them,
while white surfaces owe their characteristic appearance and opacity to the fact that the component wavelengths of incident light are reflected rather uniformly (Judd, 1952; Judd and Wyszecki, 1963). Paint films, however, usually have very little of the optical nature of a mirror where most of the reflectance of light occurs a t the surface. Rather, the incident wave penetrates the film undergoing refraction a t the air-polymer interface of the film surface and further refraction as it encounters and penetrates the interface between pigment particle or a microvoid and the polymer. Many such successive encounters and refractions ultimately return the wave to the viewer’s eye, hence the use of the general term “reflectance” to describe the light path. The relationship between absorption ( K ) and the scattering ( S ) of light incident onto a totally opaque film is given by Kubelka and Munk (Judd, 1952) as
K -- (1 - RJ -
S
2R,
where R, is a reflectance by a film so thick that an increase in thickness does not change the reflectance. The back-scattering of films opacified with microvoids or white pigment involves very little absorption, and accordingly there will be greater interest in reflectivity due to refraction. Fresnel has formalized the observation that the greater‘the difference in refractive indices of two adjacent media, the more the incident wave is scattered. If F is the Fresnel reflectance coefficient, N1 the refractive index of the more refractive material, and N2 the refractive index of the less refractive material a t an interface
Inspection will show that as the quantity N I - NZ becomes greater, the value of the ratio (reflectance) described in eq 2 also increases. When the front of a light Ind. Eng. Chem., Prod. Res. Develop., Vol. 13,No. 1, 1974
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