Annual Review
V. J. BUTTIGNOL R. E. CUTFORTH D. E. ESKRA N. H. FRICK H. L. GERHART
Protective Coatings More stringent environmental requirements, created ly advances in other technologies, stimulate development of new coatings very area of art and science has its vanguard of
E exploratory work. In the protective coatings field, one gage of this effort is the prcducts developed annually
which either provide a new solution to an old problem or combine existing technology in a novel way to solve a new problem. Almost by definition, many of these avant-garde coatings will survive only briefly as technical curiosities. Yet the few which endure will determine the future course of the industry.
FORMULATION CONCEPTS At times new coatings may be specialized to the point of being amusing. One must doubt for example that a luminous paint designed to facilitate following the migratory patterns of elephants ( 4 4 will have any great impact on the paint industry. We have resisted the considerable temptation to include additional examples of thii sort and instead have selected examples which give some intimation of future broad utility. New coatings frequently are developed in response to new environmental requirements created by advances in other technologies. Coatings developed for new aircraft and missiles offer a conspicuous example of this pattern. Dalton (9A) reviewed the current status of aircraft coatings and listed the following new product trends: general aircraft finishing, urethane enamel to replace present acrylic lacquers -for high temperature performance, polyimide and polyphenylene oxide and sulfide lacquers for dark colors, polyphenylsilsequioxane lacquer for light colors -for aircraft integral fuel tanks, one-package, rapid curing urethane to replace present two-component urethane -for fastener sealants, encapsulated zinc chromate primer -for
44
INDUSTRIAL A N D ENGINEERING CHEMISTRY
The last coating is quite novel, and its completed development was described in a later article (IOA). A conventional zinc chromate primer was encapsulated by coacervation in a gelatin-carrageenin system, and a fast drying alkyd was used as a vehicle to apply the microcapsules to the metal fasteners. The capsules were designed to mpture under the pressure of fabrication and to coat the critical area with primer. This technique eliminated a serious exfoliation corrosion problem, and, according to a recent report (64,the capsule coated fasteners already are being produced at a rate of 25 million annually. As discussed in an earlier review (134,microencapsulation offers potential solutions to a host of coatings development problems, and increased commercial exploitation can be anticipated.
In paints designed for missiles and space vehicles, emphasis is on thermal control. Briefly stated the general requirements of these coatings are high temperature stability, good durability in a space environment, and usually a low CY/E value (ratio of solar radiation absorptivity to emittance). Most of the coatings actually in use employ a water thinned alkali metal silicate binder in combination with a refractory silicaceous pigment. Zirconium silicate is used for white coatings and carbon precipitated on lithium-aluminum silicate for blacks and grays where a higher absorptive value is desired. Illinois Institute of Technology Research Institute has developed another class of coating based on a General Electric silicone resin and pigmented with pure zinc oxide. Bennett (7.4) pointed out potential commercial applications for the alkali silicate coatings in nontoxic paints for submarine interiors and “clean rooms,” thermally stable coatings for high temperature equipment, and anticorrosive paints for magnesium, aluminum, beryllium, and steel. I n addition to developing the silicone based satellite coating already mentioned, I I T R I also is reported to be working on microporous polymer films in which entrapped air bubbles will reflect the bulk of the solar spectrum (7A). The prospect of using air rather than prime pigments for opacity warrants separate discussion. Arthur D. Little, Inc., crediting a suggestion of Rosenthal (U. S. Patent 2,739,909),reported (8.4) on a coating for boxboard wherein the light scattering characteristics are derived from small bubbles in a transparent binder. The air filled or vapor saturated voids, having diameters in the range of the wavelength af light, are trapped in a water soluble medium such as soya protein. This type of coating is suggested for its low specific weight and high brightness as a general purpose decoration and protective film on paper and paperboard. A definition is proposed under the term “bubble coatings.” Such formulations are valued for their opacity as well as brightness coefficient, and in a collateral application, / there was developed a “white carbon paper” in which the bubble structure collapses under localized pressure of a type face to expose a contrasting undercoat. Formulation details for nonpigmented and low pigmented opaque coatings are given by Nadelman (74A) in a communication which also provides pertinent patent and trade references to this concept. Radioactive areas are another example of a man made environment which puts unusual demands on a protective coating. The most frequent requirement is maintenance of a surface which can be easily decontaminated. The known mechanisms of contamination are: adsorption and ion exchange from solution, chemical reaction between radio elements and paint constituents, activated V . J . Buttignol, R. E. Cutforth, D. P. Eskra, and N . H. Frick are members of the staf of the Research and Development Department, Coatings and Resins Division, Pittsburgh Plate Glass Co., Pittsburgh, Pa., of which H. L. Gerhart is the director. Each of these authors is expert in one or more of the various technologies that together constitute Protective Coatings. AUTHORS
diffusion intQ pores, solvent extraction by plasticizer and retained solvent. In recent years a number of ad hoc examinations have led to certain generalizations concerning formulation of easily decontaminated paints. Walker (774,in a thorough and carefully controlled program, confirmed the validity of most of these principles. Test specimens were contaminated with admixtures of cerium-144 and plutonium-239, one mixture treated to stimulate ion exchange, another to stimulate diffusion. The specimens were then decontaminated by means of a standard, two-step detergent wash. Following each step of contamination and decontamination, the radiation levels of the specimens were measured with a scintillation counter for CY radiation and an end window Geiger tube for @ and y. Walker concluded that polymer choice was the most significant variable, chlorinated rubber giving best results. The generally recognized but surprisingly poor performance of thermosetting enamels appeared to be related to the concentration of amino crosslinking resin present. In plasticized systems chlorinated paraffin? and diphenyls were best and phosphate and phthalate types poorest. I t was essential that pigment volume concentration be kept below 20% and desirable that inert pigments of negligible porosity be used. Anatase titanium dioxide and antimony oxide were listed as good choices, zinc oxide and rutile titanium dioxide as poor ones. I n some cases, ease of decontamination is secondary to the ability of the coating to withstand high radiation dosages without deterioration. Wells (78A) reported that urea or melamine formaldehyde modified phenolic enamels were best for this purpose while cellulose esters were poorest. Several recently developed naval coatings employ glass extenders to achieve desired performance properties. Berger and Cizek (2-4) were faced with the problem that the increased speed and weight of carrier based aircraft had rendered conventional flight deck resurfacers obsolete. No naturally occurring abrasive could be found which combined the required features of durability, slip resistance, and minimal wear of arresting gear cable. The problem was solved by using angular glass particles as abrasive in a moisture curing urethane binder. The new coating proved to be about seven times better than conventional ones in terms of cable wear, yet it cost only about one third as much per unit area covered. A new protective coating for ship bottoms was developed consisting of glass flake in a polyester binder (774. Service tests showed that this coating was substantially better than the conventional fivecoat vinyl system in resistance to both corrosion and abrasion. The increased revenue from keeping the ship in operation for longer periods between drydock repairs was claimed to greatly outweigh the slightly higher cost of the new coating. The novel method of applying thin polymeric films consists of introducing monomer vapors to a chamber under partial vacuum and applying an a x . voltage to VOL. 5 8
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initiate and maintain a glow discharge between two parallel electrodes. Formation of free radicals, ions, and excited species causes polymerization of a thin film on the electrodes. Rate of film formation and film properties are controlled by varying current density, pressure, electrode spacing, electrode temperature, monomer flow rate, air concentration, and current frequency. The method is economically attractive because resin synthesis is achieved during the application process rather than in a separate manufacturing step. O n the other hand, the potential uses of the method are rather limited. Williams (79A) discussed a glow discharge technique for high speed strip coating of steel and listed lacquering of tinplate, coating of nonmetallic surfaces, and deposition of dielectric films as other potential uses. The joint effort of Continental Can Co. and Radiation Research Corp. to develop a commercial process specifically for can coatings was disclosed (3A). Electrodeposition of paint from a dilute aqueous bath onto a metal anode was described in last year’s review (73A). Since then a substantial number of additional papers have been published on the subject. Several of the more valuable ones are those by Tawn and Berry ( 7 6 4 ) and by Finn and Hasnip (72A) on theory and mechanisms and an article by Burnside and coworkers ( 5 A ) dealing with the prediction of current requirements for production installations. Despite the fact that the first significant commercial use of the process was in American automotive priming, there has been less expansion in the Tj‘. S. A. than predicted of existing facilities in this market. This lag is due not only to stringent product requirements, but also to the massive logistical problems associated with a major process change. The automotive manufacturers of Western Europe and Japan, less encumbered by size and production decentralization than their American counterparts, have made substantial commercial progress. I n the industrial coatings area, the sheer multiplicity of potential end uses for the process should ensure an expanding market for the next several years. Many manufacturers, among them General Electric, AllisChalmers, Westinghouse, and Western Electric, have pilot studies in progress; and at least several are expected to be in production by the end of 1966. The greatest demand for electrodeposition is for applying a primer or one-coat finish to mass produced, complex metalware. I t is safe to assume, however, that many laboratories are working to take advantage of the unique capabilities of the process to solve more specialized metal finishing problems. As a possible glimpse of the future, one might consider work in progress on photochromic additives (75A). These materials darken and retard light transmission when exposed to UV or visible light, but revert to their original state when light intensity is reduced. So far, such additives have been used only in plastics for items as sunglasses. Yet how many applications there must be for a paint which would be free from glare in bright light but which ~7ouldreflect efficiently in subdued light. 46
INDUSTRIAL AND ENGINEERING CHEMISTRY
AVANT-GARDE COATINGS The availability of flamboyant finishes has created an interest in unique display models or in custom-constructed sports cars. Current finishes for this application are of several types including two packaged systems. Also available are formulations based on coarse (60-200 mesh) polished nonleafing aluminum plates dispersed in either a conventional thermosetting or thermoplastic system (5B). This provides a single package having the aluminum and colored pigmentation combined. After application, the highly reflective dramatic surface is covered with a clear layer of a corresponding vehicle to obtain maximum durability and a “candy apple” effect. Unsaturated polyester coatings have desirable properties for certain applications because of their durability and hardness. A present shortcoming to their use is poor adhesion to metal substrates and few primers have sufficient adhesion to both metal and polyester to perform with satisfaction. An epoxy-polyamid type primer cured to a Sward hardness of 38-56 is recommended (6B) The advantages of both the thermosetting and the thermoplastic acrylic automotive topcoats are well documented. Through the use of methacrylate polymers containing pendant modified carbamyl groups, coatings may be applied which have the potential properties of thermosetting enamels but can be arrested at a point where excellent repair lacquer adhesion may be provided (4B). Because of the national concern with air pollution and the proposals of Los Angeles County Rule 66, the paint industry is confronted with a potential obligation extensively to reformulate its products to comply with present and subsequent regulations. A recent analysis puts this problem into perspective (3B). The problem which confronts the formulators of polymer coatings is the requirement that the thinners used be chosen from a concise list of “less reactive solvents.” This list of permissible “A” compounds includes paraffinic or aromatic hydrocarbons as well as ketones “having no branched structures or substituent groups other than halogens” ; also alcohols, esters, or ethers. The choice of straight chain hydrocarbons is restrictive and poses questions beyond the redesign of polymers to accommodate the objective. Obvious choices of other directions are to develop water-borne counterparts of present solvent polymeric systems, high solids coatings, and polymeric powders in coatings applications. The search is on for resins having low viscosity in formulations of diminished solvent ratios. One approach is to synthesize allyl ethers of polyols to be considered as reactive solvents in conjunction with alkyds in substantially 100% solids systems. Certain goals were accomplished operationally at the British Paint Research Station and are detailed for air dry, baked, or water soluble types (2B). Another review updates the considerations in favor of utilizing powdered polymers applied by spray or fluidized bed techniques (7B).
The chemical industry is responding to the increasing prices of antifungicidal mercury derivatives by providing new organic compounds which offer protection to mold growth equivalent to the traditional products. These chemicals, not yet publicly identified as to precise composition, are available at reasonable prices and, being free of metal ions, are, furthermore, preferred for their lack of staining in sulfide atmospheres. Pigments
Derivatives of tetrachlorophthalimide designated by the trade name Irgazin pigments are a promising new series. The synthesis and colorant characteristics of isoindolinone derivatives are detailed as a review of a 1964 release to the VIIth FATIPEC Congress (7C). A dramatic development in the technology of titanium dioxide is a pigment uniformly encapsulated in a heavy coat of alumina and silica to provide extra chalk resistance. Two new shades, orange and maroon, have been announced in the quinacridone series. An antimony-nickel-titanium yellow pigment for paint use is available, superior in color strength, opacity, and dispersibility to previous titanium yellows.
and interior trade sales paints are vinyl acetate-ethylene copolymers sometimes designated acetoxylated polyethylene. The facile fusibility which demands minimal ratios of external coalescing agent (responsible for pigment flocculation, in some cases) is one initial observation. This year witnessed the introduction of the first successful latex interior semigloss trade sales trim enamels with flow and leveling properties expected and experienced from traditional alkyd (solvent-type) formulations. This difficult result is the consummation of basic studies by a score of investigators concerned with latex dispersion, particle size, size distribution, and coalescence. T o those who recall the difficulties and have struggled so long with this objective, the accomplishment is a substantial victory. Acrylic copolymers are further pushing aside theonce honored position of “oil based” consumer products. High molecular weight urethane latices as nonionic and anionic types now available are especially suggested for textiles, foam coatings, upholstery fabrics, binders for nonwoven fibers and as saturants and finishes for synthetic and natural leathers, as well as adhesives ( 7 0 ) . Thermosetting Acrylic Resins (TSA)
SYNTHETIC RESINS An “abbreviated textbook” (50)correlates the established characteristics of selected polymers to actual and predictable properties of derived coatings. This is an excellent minimum but adequate refresher base for technologists interested in resin synthesis, coating formulation, or paint evaluation. Each paragraph contains a base principle which can serve to join high polymer science to the coatings art. latices
Latex paints have in 15 years successfully moved against the lay opinion of practitioners that painting wood with water causes rot. Now the contest is on to see if new latex maintenance paints can overcome the likely suspicion that painting steel with water paints causes rust. The advantages of latex maintenance paints are as well cataloged as the intuitive negative feeling: they do protect against rust and are easier to apply than solvent types; expensive, flammable, and toxic solvents are avoided; short dry and recoat times are typical ; admirable corrosion resistance coupled with the fundamentally high level of exterior durability had been proved ; self-priming systems are available. Practical thermosetting acrylic emulsions utilizing hexakis(methoxymethy1)melamine to cross-link a variety of reactive groups, especially hydroxyl and carboxyl, have entered the market (60). The same cross-linking chemical is utilized to prepare films from polyspiroacetal dispersions such as the copolymer of pentaerythritol and dipentaerythritol with glutaraldehyde. Films are tough and chemical resistant whencured at 200’ C. (40). New latices on deck with every formulator of exterior
Having now become firmly established in the preparation of baking enamels for automotive topcoats, this series has shown remarkable growth in recent years. Home appliances, metal decorating as enclosures and containers for food and beverages, and industrial baking enamels for metal siding are other important areas. As a class, acrylic polymers are chosen because of the unusual combination of physical and chemical properties tailored into these series: resistance to discoloration on exposure ; outstanding exterior durability ; water clear color; serviceability in the presence of oils, greases, kitchen chemicals, and the common substances encountered in industrial use, such as water, alcohol, alkali, acids, and solvents ; low pigment reactivity; and good electrical properties and color holding at elevated temperatures. Three mechanisms that are identified in commercial film formation on the basis of co-reaction with other species are shown in Figure 1. Typical commercial products under the first two types above are described (9D).Polymers of the third type are reviewed ( 7 7 0 ) with emphasis upon the properties of unsaturated polyester modified “acrylamide” TSA resins. A preference is expressed for use of alkaline catalyst in a desire to preserve the reactivity of the additional functional groups (introduced by the polyester) for subsequent curing of the deposited film. Curing mechanisms are postulated by a study of infrared spectroscopic identification of substituent groups during the film forming process. Nonaqueous Dispersions
New technology has created extensive activity with these products. Basic enlightenment is provided by VOL. 5 8
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Fitch (120) in a scholarly treatment of the thermodynamic stability of organisols in comparison with hydrosols. This article conveys an especially fine understanding of the polymer structure requirement and the complicated interactions requisite to theoretical and applied practices. Hoy (30) selected poly(viny1 chloride) for a fundamental report on pourable dispersions of fine particle polymers in nonvolatile liquid plasticizers. He presents an interpretation of viscosity changes during temperature increases through the rapid swelling stage into the opaque gel stage and finally, the particle fusion and toughening semisolid condition. A sensible goal to favorable economy in industrial coating processes is the use of resins which can be applied at relatively high solids at sprayable viscosity. This applies particularly to industrial color coats where a rule-of-thumb solids a t application conditions is indicated for commercially utilizable types of the past decade.
The goal of 100% solids is not readily attainable in practical coatings. The substantial advances with high solids vinyl organisol and plastisol types have created the incentive for equivalent translation to the acrylic field. Unfortunately, the solubility characteristics of acrylic resins create a circumstance which has made more difficult the preparation of fine particle polymers which can be practically formulated in the balanced solventnonsolvent technique. I t now seems predictable that coatings based on acrylic dispersions will soon be applied as low-bake systems at a spray viscosity in the 50-6OyG solids or higher range. The apparent breakthrough at two locations provides for the grafting of methyl methacrylate along with acrylic esters into polymeric dispersing agents. I t is requisite that the graft base or precursor contain two moieties, one portion being preferentially soluble in the hydrocarbon diluent for the polymerization reaction, and another insoluble in the organic liquid but compatible with the polymer to be formed. The organic block or graft copolymer may be preformed and utilized as a kind of philo-phobic ambivalent stabilizer during the polymerization of the acrylic monomers. The detailed mechanism seems not yet to have been described in the technical literature, but the result is the formation of a dispersion of discrete particles having colloid dimensions which are unchanged in the diluent liquid. I n one instance (lOD),heat bodied blown linseed oil of
Film Forming Solids, %
Trpe
15-18 34-36 40-42 18-22 50-60 90-1 00 40-42 20-22
Nitrocellulose lacquers Oxidizing alkyd-melamine Nonoxidizing alkyd-melamine Vinyl solution Vinyl organisol Vinyl plastisol Thermosetting acrylic Acrylic lacquer
FIGURE 1
.
Hydroxy functional acrylic polymers reacted with melamine (or urea) formaldehyde resins
___________-----
’ H R O ~ H ~ /B~O~CH~N-C ‘
:--.---.’
+
L-----J
N
--c_
r - - - --! C-NCH~/OB~ - - - -/- - - , 2
butylated melamine-formaldehyde condensate
hydroxyl-containing acrylic polymer . .
+
/H~OR hydroxyl-containing acrylic polymer
H ROCHIN-C 2BuOH acid catalyst
\*/
Carboxy functional acrylic resins reacted with epoxy types 0 tertiary amine / \ or CHz-CHquaternary epoxy group ammonium complex
IT C-OH
OH
l
+
carboxyl group on acrylic polymer
N-( alkoxymethy1)carbamyl
functional resins co-reacted with alkyds, epoxy, phenolic, polyester, and “nitrogen” resin species
I
c=o I I CHz I
NI-I
OR 48
I
Condensation uolvmerization with : Melamine formaldehyde I Urea formaldehyde Alkyd Polyester Epoxy species Phenolics
INDUSTRIAL A N D ENGINEERING CHEMISTRY
o=c
I I HzC I
HN
RO
2-3 viscosity is a possibility as a graft base. Nine parts of acrylic monomer are polymerized by free radical mechanism in the presence of the oil dissolved in octane. A milky latex of low viscosity at 40% total solids consists of 85% acrylic polymer as suspended particles in the range of 0.1-5 p and i5Y0 dissolved grafted oilacrylate copolymer. Expert formulation with a balanced high boiling solvent and relatively lower boiling nonsolvent mixture provides a stable vehicle for pigmentation. I n an earlier instance (80),crepe rubber degraded to a molecular weight of 30,000 was utilized as the precursor graft base dissolved in white spirit. Twenty times its weight of methyl methacrylate was reacted with benzoyl peroxide catalyst. A creamy dispersion of polymethacrylate particles resulted with a molecular weight of approximately 350,000 in a particle size range of 0.10.5 p . I t is recalled that in an otherwise identical attempt without the rubber or other graft base, the acrylic polymer formed in white spirit is a gummy deposit impossible to disperse as colloid particles. The technique is noteworthy for its latitude in the choice of dispersant polymer as well as the monomer. For the durable acrylic series, the stabilizer might be a polymer of lauryl methacrylate and numerous acrylic copolymers. The monomer choices for the bulk colloidal fraction are potentially those now well known which are susceptible to homo- and copolymerization by azo or peroxy free-radical catalysis. Additionally, the nonaqueous feature permits polymerization by anionic and cationic initiators normally not possible in aqueous systems. The particles in the ideal case are discrete and not susceptible to reagglomeration by slow swelling. The absence of hydrophilic emulsifier or dispersing agent is an advantage later in providing insensitivity to moisture in the applied film. Yet, it seems possible also to prepare certain water soluble particles in anhydrous medium. The magnitude of the thermodynamic problems involved here have relevance to a symposium introduced by Brown on the nature of energy barrier concepts to particle coalescence in storage which barriers must be nullified during the application process at elevated temperatures in order to have use value. The papers about to be published are noteworthy (20). Silicones
While not “new,” it is appropriate to recite the current impact of chemicals such as the methoxylated hydrolyzates of phenyl silanes as components of synthetic paint vehicles. The essential technique calls for reacting with resins especially prepared to have optimum available hydroxyl groups followed by further reaction with carboxylic acid. The industry is engaged in a fever of introducing this and other silicone intermediates as reactants with alkyds, epoxies, phenolics, cellulosics, polyesters, hydroxy alkyl acrylics, and other hydroxy sources. The attraction is the fact that silicones, which initially were prized
for their high temperature and chemical resistance properties, have taken a firm position in the important category of enabling the enhancement of exterior durability as air dry maintenance finishes and industrial coatings generally. The subject is updated with original results and an analysis by Graziano and Glaser (ID). Vinyl Chloride Exterior House Paint
The original use of acrylic and also vinyl acetate latices types during the past decade represented a dramatic advance in the technology of exterior wood coatings. The popularity and usefulness of latex paint for wood and masonry are firmly established and growing. This experience led to the adoption of vinyl acetate latices, often modified with oil based resins as well as alkyd-acrylic systems for exterior wood-a practice which later enabled the recoating of chalked previously painted surfaces without need for repriming. I t must be recognized that the industry practices are by no means uniform with respect to the resin raw materials, but replacement of linseed oil by synthetic latices types has been progressive. Presently another candidate, a carboxy modified vinyl chloride-acrylic copolymer latex, is offered with claims for superior performance on difficult weathered surfaces ( I E , 2E). While not yet widely used, the performance tests are well documented and at least one company is offering a first line house paint in national distribution. The obvious motivation is the high regard for the substantial vinyl chloride capacity at low cost and the inherent distensibility characteristics of these formulations. The similarity of alkyds to conventional polyester plasticizers of PVC is judged to be t& reason that alkyd fortified vinyl chloride copolymers provide the high performance. Fluorocarbon Polymers
Fluorocarbon polymers as currently available contain polyvinyl fluoride, polyvinylidene fluoride, or a COpolymer of vinyl fluoride and chlorotrifluoroethylene and can be utilized as laminates or protective-decorative coatings. Coatings based on fluorocarbon polymers have exceptional chemical, heat, and weathering resistance with good postformable properties. Due to their unique chemical nature, fluorocarbon coatings need highly polar organic solvents such as dimethyl acetamide or butryollactone to facilitate dispersion and fusion a t temperatures of 450’ to 500” F. Application by conventional spray or roll coating techniques Over aluminum, cold rolled or galvanized steel costs seven to ten times more per surface area than corresponding acrylic or vinyl finishing systems. Polyureas
An account of the evolution of the diisocyanate from dimer acids as an intermediate for coatings is available ( I F ) . This chemical, less hazardous than other diisocyanates, has controlled reactivity and has been utilized to prepare polyureas by reaction with polyamines. These are under test as coatings for wood where the VOL. 5 8
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accommodation to dimensional change is desirable. Contrasted to polyurethanes, the long chain polyureas are flexible at extreme low temperatures and qualify as sealants. The slow reactivity of dimer diisocyanate permits the preparation of stable water emulsions for textile and paper surfacing. Bitumens
I n these days of emphasis on synthetic media, the bitumens are subconsciously relegated to the status of second-class citizens. Bituminous formulations nonetheless, in total, represent an enormously large, important component of the coatings vehicle list. They are also dramatically durable in their proper application-e.g., gates, locks, and penstocks of the Panama Canal since 1913, steel pipe for potable water in New York since 1914. Normally creeping technological changes were accelerated a decade ago by epoxy modifications, an important upgrading for industrial uses. A recent symposium presents the status of attempts whereby almost every high molecular weight resin, polymer or elastomer, has been courted by bitumens with a few successful and legitimate marriages of record (1G). Despite the humble parentage, one can expect the matchmaking to continue.
PHYSICAL AND MECHANICAL PROPERTIES The concepts of the physicist and physical chemist are now actively employed in the interpretation of coatings knowledge and in its further development. A high ratio of investigations on polymer structure was once directed toward the understanding of the crystalline and amorphous states as found in fibers, elastomers, and shaped plastics. Happily, sophisticated studies on the amorphous state of coatings have come into their own. A review of the papers published in the Division of Organic Coatings and Plastics Chemistry Preprints, American Chemical Society, and in OfficialDigest (now Journal of Paint Technology) points out the trend. In this context a prize article (26H) in the Roon Award series, “Long Range Effects of Polymer Pigment Interaction in the Solid State,” demonstrates this sophisticated activity in the coatings field. The coatings technologist will continue to draw from the extensive studies of observations on unsupported structures in his analysis of observations on films attached to substrata. However, we have now well begun a generation of investigation which stresses a knowledge about the fine structure of coatings presently available without necessarily spawning new resin series or endless permutations. A purpose of this section is to assess the impact which physical measurements have had in the role of total technological improvements of supported films. Color Measurement and Control
There is a double significance to the 1965 presentation of the Matiello Memorial Lecture of the Federation of Societies of Paint Technology. First, the recipient, Dr. D. L. MacAdam, is a world authority on understancl50
INDUSTRIAL A N D ENGINEERING CHEMISTRY
ing color, its control, and research associated with the scientific and subjective nature of using instrumentation to assist and duplicate the acuity of normal vision. Second, the paper is the dernier mot-the fullest account of the assets and liabilities in color determination and matching in the paint and fabric industry (28H). The conclusion: “. . . . . .even today the best instruments fail when they are not used with utmost care. After attempting many shortcuts and trying many ‘practical’ instruments, we are forced to conclude that color can be measured accurately enough for the paint industry-only by careful use of the best reflection spectrophotometers. T o meet the needs of the paint industry for a fast and positive method of color analysis, systems are available which quantiiji color difference in MacAdam units. One of these stresses the ease with which tolerances based on preferences rather than differences can be programmed (2OH). Presently the progressive laboratories have spectrophotometers, calculators, and computers. Private communications indicate that in many instances, the difficult translation of methods to production is well under way and is in active utilization in the case of consumer products. New systems of color measurem,ent and calculation are under development. These will in the fullness of time be welcome, but one can conclude that currently the lack is in a sufficient number of trained color technologists, especially those with sound understanding of instrumentation and receptivity in paint production to make the cordial translation from the laboratory to the factory. ))
Glass Transition Temperature
I t is generally accepted that the most important physical measurement of a coating is its glass transition temperature ( T o ) . I t may not be explicitly stated as such, but when hardness, mar resistance, flexibility, abrasion resistance, and film coalescence are investigated, the T , of the system is involved. The effect on T , of mixtures of compatible polymers has been investigated. Blends of syndiotactic with isotactic poly(methy1 methacrylate) and isopropyl methacrylate with isopropyl acrylate have shown a gradual increase in T , with increasing content of the first species. Evidence was also presented that T , is proportional to molecular weight. I n poly(methy1 methacrylate) this dependence holds up to a number average molecular weight of 44,000. Polystyrene has a T odependence on molecular weight up to 22,000 (25H). Some thermoplastic coatings are close to this range and care must be taken in using literature values for T o in their design. Thermoset coatings are easily below these molecular weights, but the Towill increase after the coating has been applied and cured. Plasticizers are used in coatings to lower To’s of the polymers to useful ranges. A free volume interpretation of plasticizer effectiveness has been published. A torsion pendulum was used to measure the plasticizer effectiveness in coatings attached to foil substrates
(77H). Birefringence has also demonstrated its effectiveness to rate the plasticizing efficiency of tricresyl phosphate, dibutyl phthalate, and dioctyl sebacate in poly(methy1 methacrylate). The order of effectiveness was T C P > DBP > DOS (4H). The diffusion of dioctyl phthalate into poly(viny1 chloride) has been studied, and an inverse dependence of diffusion rates on molecular weight was found (14H). This work leads to speculation that the permanence of plasticizers is dependent on the molecular weight of coatings polymers, as well as the recognized relationship with compatibility. An interesting effect of internal plasticization of poly(viny1idene chloride) with ethyl acrylate and with methyl acrylate was reported by Woodford (48H) and Illers (79H).They found a maximum in T , for about 60T0 of the first monomer. Woodford states that minimum film forming temperatures of pdy(viny1idene chloride) copolymers generally show this behavior. This leads to tailoring copolymers with widely different physical properties. Molecular Weight Distribution
The effect of molecular weight distribution (MWD) on polymer properties has been discussed to a large extent and investigated much less because of the tedious procedures necessary to determine the distributions. A rapid analytical technique has recently been developed to determine these distributions (37H). It is called Gel Permeation Chromatography (GPC). A few laboratories have used GPC to determine the sprayability and thermocycle behavior dependence on molecular weight distribution and to develop methods for producing narrow distribution polymers. The influence of MWD on the properties of acrylic and alkyd polymers has been investigated (4OH). For acrylic polymers, the high fraction of a broad distribution lowers sprayability, and the low fraction decreases film strength. For alkyds, narrowing the MWD decreased the viscosity. The distribution of comonomer species has been correlated with molecular weight of three different alkyds. The alkyds were fractionated and acid number, iodine number, hydroxyl number, and Sward hardness of the fractions were determined (27H). The use of GPC has been discussed in a variety of references (ZH, SH, 78H,39H). The effect of molecular weight in melting temperature and fusion of polyethylene was determined (77H).I t was shown that crystallinity is affected by molecular weight. This technique can be applied to determine the application properties of crystalline coatings now under investigation. Chain Branching
The investigation of properties of branching in dispersion-type polymers is useful. Myers and Dagan (32H) studied the solution properties of bisphenol Aepichlorohydrin polymers and the effect of branching in high molecular weight fractions. The effect of long chain branching on polybutadiene properties has been investigated. The addition of one or two branches on a
low molecular weight polymer decreased the Newtonian viscosity in relation to a linear polymer of the same molecular weight (24H). Dilute solution viscosities of star- and comb-shaped polymers are less than those of linear polymers of the same molecular weight (3H). Filler Effects
Another area of study common to coatings and polymeric systems generally is the properties of filled systems. Park (34H) has investigated the effect of inert fillers on vinyl systems. Fillers generally increase the hardness of polymeric systems. Mineral fillers in polyethylene, styrene-butadiene, and natural rubber increase the modulus with decreasing particle size and increased loading. Relative elongations and ultimate elongations decrease with decreasing particle size and increased loading (7H). A polyurethane system filled with sodium chloride gave evidence that T,does not depend on filler particle size or extent of loading while bulk moduli and thermal expansion coefficients do depend on amount of filler but not on particle size (46H). Tolstaya et al. (44H) have studied the adsorption activity and reinforcing dependence of mineral fillers on polymers. The moduli of rubber at high extensions were correlated with surface bonding of the rubber to carbon black. But, at low extensions, surface activity doesn’t affect the moduli. At low extensions, structure and surface area are important (6H). The flex-brittleness temperature of polyethylene filled with carbon black decreases with both increased particle size and increased loading. The surface chemistry of the carbon blacks did not seem to affect the yield stress while the particle size and extent of loading did (47H). Stress Crazing and Fracture
There has been a large amount of work published in the areas of fracture and/or stress crazing of glassy polymers (7H, 76H, 22H) 23H, 37H, 38H, 45H). This area of theory could well be applied to the testing of coatings that must have certain thermocycling properties or other stress resistant properties. I t might also be applied to explain certain stress failures of coatings that develop upon exterior exposure. A mechanism for fracture in glassy polymers was developed by observing the colors appearing on the fracture surfaces of poly(methy1 methacrylate). The effect of temperature, cross-linking, and preorientation on fracture surface work measurements using cleavage techniques on glassy polymer has been determined. The ultimate strength data of polymers has the same type of time-temperature superposition as modulus behavior (43H). The same type of correlation for time-to-break measurements of amorphous polymers has also been demonstrated. Tung postulated the formation of new flaws under stress for the brittle failure of polyethylene in the presence of surface active agents. He presented data that show the time-to-break at the same stress is decreased when the sample is exposed to a surfactant. VOL. 5 0
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Permeability
Permeability dependence of water on filler content in coatings was investigated by Michaels (30H). He found that fillers can either decrease or increase water diffusion. The chemical nature of the filler surface was the dominant factor. Platelet shaped fillers generally imparted less permeability than spherical ones. Permeability theory has been presented by Rogers (35H). McSweeney (29H) has shown that contaminants (salts) on a substrate can lead to very early failure, since they act as a driving force for the osmotic flow of water through the film. The effect of different polarities of functional groups in polymers is to increase water permeation with increasing polarity (4727). The establishment of compositional gradients across the permeation direction of a film has shown some interesting results. A film of poly(ethylene-vinyl acetate), with a compositional gradient, has shown an enhanced water permeability in the direction of increasing vinyl acetate concentration and a much-below normal permeability in the opposite direction (35H). Solubility Parameter
The approach to the solubility parameter concept has recently been modified, in two papers, to include a polarity term. The first paper (12H)modified the previous concept of Burrell ( S H ) and Lieberman (27H) by the separation of a polarity term from the hydrogen bonding effect. The second paper refined the hydrogen bonding and polarity concepts even further. Instead of using relative values for the hydrogen bonding, the authors used wave number shift caused by solvent hydrogen bonding with deuterated methanol (IOH). They made three dimensional plots of solubility parameter us. dipole moment us. hydrogen bonding number of the solvents and solvent mixtures. Cellulosic polymers were used in this study. The solubility envelope was described by plotting the borderline solubility points of the polymers. Predicting Maximum Durability
The correlation of accelerated exposure results with those of Florida exposures, using a “dew-cycle)’ XW-R Weather-0-meter, was shown to be surprisingly good. The author points out that Florida exposure ratings depend on the time of year the panels were started and that conditions vary considerably from year to year (42H). A series of four house paints, three auto enamels, and three auto lacquers was studied using three years exterior exposure in Florida, Delaware, North Dakota, Arizona, California, and Minnesota, and fourteen weeks on the Emmaqua accelerated weathering device in Phoenix, Ariz. A good correlation was obtained using the Arizona-Emmaqua comparisons ( 9 H ) . The Cincinnati-Dayton-Indianapolis-Columbus Society for Paint Technology evaluated the Xenon Arc, 52
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Twin Arc, and Sunshine Weather-0-meters as compared to exterior exposure (in Columbus, Ohio) using 12 different coating systems. These systems consisted of alkyds, acrylics, poly(viny1 acetate), poly(viny1 chloride), polyurethanes, and spar varnish. ASTM yellow acrylic chips were exposed along with the coatings. Their conclusions were that the physical signs of deterioration were the same using accelerated or exterior exposure but the correlation of the rate of change of these properties between accelerated and exterior exposure was not good (33H). Prediction of the durability of copolymer systems is a very desirable and sought after technique. Recently, work was done in this area using three-, four-, and fivecomponent copolymers of acrylamide (thermoset systems). Loss in gloss on exterior and accelerated exposure was measured. Atlas XIA Sunshine Twin Arc Weather-0-meters with continuous water spray were used in the accelerated tests. The authors presented evidence that an optimum composition based on hard monomer content was obtained in both copolymers and copolymer blends. The accelerated and exterior exposure tests correlated fairly well when aluminum pigmentation was used, and the same trends were indicated using rutile titanium dioxide (13H).
P I G M E N T DISPERSION Dispersion Equipment Design
As in other areas of paint technology, pigment dispersion is progressing from an art to a scientific discipline. The demand for improved and more economical means of dispersion has brought about the development and use of the sand mill, Attritor, and high speed dispersers with a marked reduction in the use of ball mills and roll mills. Although other devices have appeared, their impact has been less significant (441, 561, 591). Progress has actually been slow, as the dispersion process has not yet advanced to a unit operation. Paint dispersion lacks sound underlying theory to explain the mechanisms of pigment agglomerate breakdown, the energetics, and the requirements for equipment design. In a related field, solids grinding, inroads are being made into understanding the fundamentals through a mechanistic approach and computer technique (51). Any real further progress in dispersion equipment design appears highly unlikely without a much improved basic understanding of the dispersion equipment. Rahacek has presented a theoretical model of the dispersion process in a ball mill by characterizing the operating functions (501). The fact that laboratory tests at least partially confirm his model is an indication that conclusions may be coming (571). Present equipment, when properly operated, usually provides the desired product. The inadequacies are concerned with efficiency and reproducibility. I n the past few years the pigment industry has done much tG improve the dispersibility of pigments, through modifica-
tions in the pigment process and in the pigment itself. Studies indicate that the major structure of most pigments is rapidly dispersed. Greatly prolonged milling is then required to reduce the last 1-10% of the pigment (31, 791, 631) to an acceptable degree. Mill Base Formulation Guides
Despite the poor understanding of the dispersion mechanism, empirical studies have provided a number of guides for optimum mill utilization. A survey of the advantages of the various mills was made (261). Patton (491) has compiled a thorough equipment review while specific references to the Attritor (66Z), sand mill (121), and high speed dispersers (751, 391) are also available. The advantages and practical application of the high speed dispersion were also a topic of discussion at a recent Paint Society meeting (731, 141). Requisite to their optimum utilization is the proper formulation of the mill base composition. Daniel early devised a workable procedure for ball mill formulation (201). This procedure has found only limited use, as many organic pigments do not respond well to this technique. Another recently devised method, employing a jiffy mill technique, is judged faster and does not have the drawbacks of the Daniel method (171). I t evaluates organic pigments and gives an indication of rate of dispersion and the paste flow characteristics. Sheppard (591) has devised a method for the ball mill which takes into account the factors related to the mill size and type of ball mill. Similar methods have been devised for the other mills (61, 401). I t is generally agreed that shear is the dominant dispersive force in most mills. None of the optimizing procedures, however, specifically evaluates this factor. Several experimenters have attempted to devise a method based on the premise that there is an optimum rheological condition for dispersion (781). The changing rheological conditions during milling seem to have hindered their progress. Unfortunately, no treatment of the modern high shear equipment, like the sand mill and high speed disperser, was indicated. Weisberg also experienced changing rheology during dispersion when studying the high speed disperser process (691). I n spite of the system’s complexity it was characterized, and Weisberg has since shown that optimum conditions can be selected (701). Noteworthy is his selection of a dilatant system for optimum dispersion. This rheological approach not only provided further insight into the dispersion mechanism but also shows promise in reducing costs through improved dispersion and more efficient power utilization. Further exploration into the rheological conditions during dispersion is called for. Pigment-Medium Relationships
Aside from the actual mechanical processes, an important aspect of dispersion is the relationship between pigment surface and vehicle system. These relationships are controlling factors in the rate and ease of pig-
ment wetting and in the dispersion stability once it has been obtained. Smith has prepared a very extensive review of these factors (601). Although the review was concerned with inks, factors related to the pigment surface and to the vehicle system are equally relevant to paints. The last few years have seen a number of studies with the broad goal of producing good wetting vehicles and easily wet pigment surfaces. These studies have taken either a basic approach (characterization of pigment surfaces and their reaction with various organic species) or a practical one. Titanium dioxide, the industry’s most common pigment, has received the major part of the effort. I n these studies, various simple functional groups, such as carboxyl and hydroxyl groups on short chain molecules, have been evaluated to determine the mechanisms of adsorption (71, 221, 471, 551, 651, 721). At first, this might seem like a simple request. However, variables such as crystalline form of titanium dioxide, amount of surface water present (either physically or chemically adsorbed), and treatments given to commercial grades are quite significant and have made this no simple task. Moisture has been found to interfere with the adsorption of certain species but not of others. Rigorously dried pigments, however, are found to be difficult to disperse (241, 671). Excess moisture can adversely affect product quality. Organic molecules apparently adsorb onto pigment surfaces giving Langmuir type isotherms which indicates monolayer coverage. I n some cases monolayer coverage is based on the molecules adsorbing with their major axis parallel with the surface while in others perpendicular orientat’on is claimed. Adsorption onto rutile surfaces of carboxyl groups has been found to be by a combination of hydrogen bonding and chemisorption. Every titanium dioxide appears to have a characteristic ratio of these two mechanisms (651). Bobalek (91) reports that on pure rutile surfaces, adsorption is onto active sites and does not involve monolayers. Silica pigments with hydrated surfaces show definite adsorption by hydrogen bonding (371). The amount of adsorption varies as the degree of surface hydroxylation and is a function of the manufacturing process. Much more work in this area is needed if a satisfactory characterization of these surfaces is desired. Adsorption of Resinous Materials
I n the adsorption of polymeric materials, as would be the case in paint systems, recent studies leave little doubt that polymers adsorb from solution onto surfaces in the form of monolayers of molecular coils (251). By measuring the intrinsic viscosity of various systems, Rothstein (541) determined the thickness of the adsorbed layer, its density, and its macromolecular configuration. The stability of dispersed particles is dependent upon the condition of the adsorbed layer-Le., the more complete the coverage and the farther out the polymer chains extend away from the surface, the higher the stability. Unstable dispersions would be expected where surface coverage is slight or very compact. The conclusions VOL. 5 8
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from this work rely heavily on the surface area measurements of the pigments. Schaeffer (571) recalculated some of Rothstein’s data using surface areas determined by inert gas adsorption instead of microscopic determinations and presented some different conclusions. T o illustrate, the revised data show the amount of polymer adsorbed per unit area of titanium dioxide pigment is comparable to that on silica and phthalocyanine pigment surfaces instead of an order of magnitude lower. Schaeffer suggests that low molecular weight acid modified polymers are adsorbed through the functional group. The modifiers, however, do not contribute appreciably to the adsorption characteristics when the molecular weights are high. Other experimenters have suggested coiled molecule adsorption and also the existence of adsorption of molecular bunches (711). The above studies and others (641, 681) have shown the important role of the solvent in the system. Solvents appear to control dispersion stability on their own and, in polymer systems, they affect the structural characteristics of the polymer and the polymer coil dimensions. I n nonpolar polymer systems dissolved in polar solvents, the solvent may be preferentially adsorbed instead of the polymer (711). Dispersion Instability
One of the major concerns of the dispersion technologists is the result of poor stability evidenced by pigment flocculation. Flocculation is predominantly caused by forces resulting from high free surface energy (291) and is most common in very fine particle systems. Organic pigments, such as carbon blacks, with particle diameters of 0.01 to 0.5 p and surface areas up to 1000 sq. meters per gram, are particularly prone to flocculation. The adsorbed resin layer, by satisfying surface energy demands and by mechanical separation of particles, minimizes flocculation (291, 321, 461). Solvents can play a major role in stability. Cases have been reported where the solvent in the let-down vehicle will strip the adsorbed vehicle from the pigment and cause flocculation (271). Some polymer molecules, however, can have an antagonistic influence (341). It has been suggested that long chain molecules can bridge between solid particles to produce a state of flocculation (241, 281). Daniel (271)has illustrated another type of antagonism between ingredients. O n mixing, two different unflocculated dispersions may coflocculate or only one may flocculate. No real expalanation has been given since coflocculation is still highly unpredictable. I n aqueous systems, this coflocculation may be caused by opposite surface changes. Although Daniel does not attribute this phenomenon to electrical charges, there is still some disagreement as to the role of the Overbeck-Verwey electrical double layer theory in nonaqueous systems ( 161). I n an extensive review, La Mer has discussed the flocculating effects of macromolecules on colloidal dispersions (381). Although concerned with aqueous noncoatings, the principles of polymer adsorption and colloid flocculation discussed in this paper are very basic to 54
INDUSTRIAL A N D ENGINEERING CHEMISTRY
the behavior of paint systems. Further work is needed to apply colloid theory to paint systems. Along practical lines, surfactants play an important role in obtaining faster and more stable dispersions. A list of these materials is almost endless and their use is highly specific. Further, dispersants are very concentration dependent; excess dispersant can be worse than none at all (271). To date, only one system has been devised for the systematic approach to surfactant selection: the HLB System (481, 671). This system is not the full answer as it mainly treats nonionic surfactants and doesn’t take into account the chemical nature of the surfactant. Although the above literature indicates progress, we have only scratched the surface and now have more unanswered questions than before. Hopefully, further studies, like those being conducted a t Lehigh University and under the Paint Research Institute sponsorship (451),will provide new and more adequate relationships between adsorption studies and dispersion. Effect of Degree of Dispersion
Frequently it has been shown that no two energy mechanisms will produce exactly the same dispersion (Pittsburgh Plate Glass Co., unpublished data). These differences are usually of little practical significance but at times cannot be ignored. Ason (21) suggests that in the new high speed equipment, processing times are too short to reach equilibrium. The reliance on inadequate tools for dispersion assessment has been the major hindrance to the industry’s appreciation of these variations. The Hegman gage, presently the most common tool, only gives a measure of the coarsest material present. Little indication is obtained of the state of dispersion of the major portion of the pigment. Having a measure of the total agglomerate distribution would be more meaningful. Unfortunately no technique is presently available for this determination as both the pigment concentration and the paste viscosity are usually high. Methods have been proposed to measure the dispersion of the concentrated paste that are more meaningful than the Hegman gage (581). Although not giving a measure of particle size distribution, rheological measurements, in particular, show promise in being a very sensitive measure of dispersion. Dilution of the paste allows the use of established centrifugal sedimentation methods. I t has been argued that the dispersion is disrupted by this extreme dilution, that the method is too slow, and that present sedimentation theory is inadequate for the assessment of nonspherical pigment particles. The use of this method, however, has been reported by the textile dyeing industry, particularly since the development of the disk centrifuge (31, 41). I n spite of the counterarguments, dye manufacturers have determined that effective distributions correlate with vat dye quality. SomL limited work with this instrument has been reported in the paint field (631). With phthalocyanine pigments it was found that very long milling times are required in the ball mill to reduce
all agglomerates below 1 EL and that the distribution appears bimodal. Flocculation resistance of the dispersion increased with increased dispersion. O n the other hand, flocculation resistance may have been a function of the increased milling time, as Ason has suggested. As information of the effect of dispersion on coatings quality is so often of a proprietary nature, very little has appeared in the literature on this subject. In the area of electrodeposition coatings, the effect of dispersion has been partially shown (621). As the various pigments are more fully dispersed, the electrical resistance of the coated film increases and in turn, the “throwing power” of the coating is effected. Effect of Dispersion on Color
Probably the area where dispersion has its greatest effect is in the color of pigmented coatings. The coloration properties of a pigment are determined by its absorptive and scattering characteristics over the visible light spectrum. The scattering coefficient of a pigment shows a maximum when its diameter is about half the wavelength of light (231). The absorptive properties of colored pigments increase with decreased particle size (301, 371). As these two properties change at different rates with particle size, the color can be effected by changes in dispersion. I n red oxide pigments, the color goes from a yellow shade to a maroon shade as the particle size is increased from 0.1 to 1.0 p (371). The high impact mills, such as the ball mill, can adversely affect the crystalline shape of pigments and change the basic pigment color (331). I n contrast to white pigments, the theoretical relationships of the optical properties of colored pigments and their size have not been well developed. This is somewhat understandable as the calculations are very long without computer methods, and colored pigments often have complex refractive indexes. Brockes has made definite inroads into this problem by using the Mie-light scattering theory and the Kubelka-Munk relationship ( 7 71). He found that color intensity is independent of particle size for very small particles. Further dispersion does not result in increased absorption since the particle interior is just as active as the surface. With particles larger than the critical diameter, the absorption is only a function of specific surface and will be a function of degree of dispersion. These results leave little doubt that the particle size distribution of a dispersion will have an effect on the color. Thus, dispersions from different mills would not be expected to have identical distributions. Although some comparative evaluations have been performed between milling operations (81,521), very little has yet to appear in the literature. Pigment Flotation
A long standing color problem is that produced by pigment flotation. Lynch, in reviewing this problem, has described a number of factors not entirely related to dispersion (421). Dispersion has been given as a factor and, in an actual example, shown to affect the type of
flotation experienced (431). Kresse has prepared a detailed breakdown of some factors concerned with dispersion, primarily the particle size and shape of multipigment systems (371). Flooding was related to differential settling rates of the pigments as defined by the Stokes settling relation. Dispersion, then, can influence flooding to the extent that complete or incomplete dispersion can accentuate these settling effects.
Dispersion and Coatings Performance
The critical pigment volume concentration (CPVC) of a system is usually assumed to be constant. Often ignored, however, is that CPVC is defined as the densest degree of packing of the pigment, commensurate with the degree of dispersion ( 7 1 ) . Thus, the CPVC will actually change with degree of dispersion. Paint properties, such as gloss and enamel holdout of coating formulated near the CPVC, could be significantly affected by variations in dispersion. Variations in the bronzing of dark blue coatings could also be attributed to variations in wetting and dispersion (701). I t is known that dispersion can affect the gloss of paint films (531). From Jacobsen’s proposed explanations for the mechanisms of chalking of paint films (351), it would also appear that degree of dispersion should also affect the rate and degree of chalk formation.
Dispersion State in Cured Film
A yet grossly neglected area is the degree of dispersion and its effect on the particle size distribution in the cured paint film. It has been said that even in our best dispersions we do not obtain complete dispersion (471). Electron photomicrographs of film cross-sections show much pigment aggregation (361). I t is suggested here, however, that nearly complete dispersion can and is obtained. But to expect this fine dispersion to remain through the subsequent paint manufacturing stages and through the curing of the paint film is probably unrealistic. REFERENCES Formulation Concepts (1A) Bennett, J. L., Can. Paint Varnish Mug. 39 (11, 161-73 (1965). . Federation Soc. Paint Technol. 36 (477), (2A) Berger, A. J., Cizek, A . W., Jr., O ~ CDig. 1145-54 (1964). (3A) Brick, R. M., Knox, J . R., Mod. Packaging 38 (5), 123-128 (1965). (4A) Brushwell, William, Am. Paint J . 50 (5), 57-61 (1965). (5A) Burnside, G . L., J . Paint Tecknol. 38 (493), 101-04 (1966). (6A) Chem. Eng. News43 (45), 56 (Nov.8,1965). (7A) C h m . Week 96 (7), 61-4 (Feb. 13, 1965). (8A) Clancy, J. J., Wells, R. C.,Tappi 48 (lo), 51A-3A (1965). (3A) Dalton, A. Stanley, Am. Paint J . 49 (25), 32-9 (1964). (10A) Dalton, A. Stanley, Oflc. Dig. Federation Soc. Paint Technol. 97 (491), 1593-1622 (1965). (11A) Devoluy, R. P., Mater. Protect. 4 (4), 10-14 (1965). (12A) Finn, S. R., Hasnip, J. A , , JOCCA 48,1121-35 (1965). (13A) Gerhart, H . L., IND.ENO.CHEM.57 (E), 57 (1965). (14A) Nadelman, A. H. Paper Trade J . 150 (17), 70-4 (1965). (15A) Plastics Technol. 11 (6), 15, 125 (1965). (16A) Tawn, A. R. H., Berry, J. R., JOCCA 48, 780-836 (1965). (17A) Walker, P., Zbid. 49, 117-36 ( 1966). (18A) Wells, H., Zbid. 48,76-86 (1965). (19A) Williams, T., Zbid. 48 936-55 (1965).
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Avant-Garde Coatings
(1B) Kennett, J., Znd. Finishing 4 1 (12), 64, 66, 68 (1965). (2B) O’Neill, L. A,, Brett, R . A , , JOCCA 48, 1025-42 (1965). (3B) McCabe, L. C., Lagarias, J. S.,J. Paint Techn. 38, (495): 210-16 (1966). (4B) Porter, S. (to Pittsburgh Plate Glass Co.), U. S. Patent 3,198,654 (Aug. 3, 1965). (5B) Stephens, D. T. (to Pitrsburgh Plate Glass Co.), Zhid. 3,234,038 (Feb. 8 , 1966). (6B) Workman, C. E. (to General Mills, Inc.), I6id. 3,164,488 (Jan. 5, 1966). Pigments (IC) Pugin, A,, Van der Crone, J., O.@. (488) 1071-94 (1965).
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Synthetic Resins ( I D ) ACS Div. Org. Coatings and Plastics Chemistry, Graziano, F . D., Glaser, M. A , , Preprints 25 (l), 68-75 (1965). (2D) Brown, G. L., Zhid., pages 313-18. (3D) Hoy, K . L., Zhid., pages 375-90. (4D) Cohen, S . M., Kellert, M. D., Snelgrove, J. A , , Ibid., pages 39-50. (5D) Gaylord, N. G., J.Polqmer Sci., P a r t C (12) 151-67 (1966). (6D) Koral, J. N., Petropoulas, J. C., ACS Div. Org. Coatings and Plastics ChemIstry, Preprints 25 (l), 34-8 (1965). (7D) Mater. Design Eng. 63 (4), 21, 23 (1966). (8D) Osmond D. W Thompson H. H . (Imperial Chemical Industries), U. S. Paten; 3,095,386 (June 25, lb63). (9D) Resin Reuiew XV (3), 24-35, Rohm & Haas Co., Philadelphia, Pa. (1965). (10D) Schmidle, C. J., Brown, G. L. (Rohm & Haas Co.), C. S. Patent 3,232,903 (Feb. 1, 1966). (1lD) Sekmakas, K., Stancl, R . F., J . Point Tech. 38, (495) 217-25 (1966). (12D) Fitch, R . M., 0 5 c . Dig. Federation SOC.Paint Techtml. 37, (482), 243-258 (1965). Vinyl Chloride Exterior House Paint (1E) Altfinger, G. J., SOC. Plastics Engrs., Philadelphia Sect., Regional Tech. Conf., Cherry Hill, N. J., pp. 89-103 (April 1966). ( 2 E ) Saroyan, John R . , SOC.Plastics Trans., Philadelphia Sect., Regional Tech. Conf., Cherry Hill, K. J., 37-62, Aprll 1966. Polyureas (IF) Kamal, hi,, Progress Thru Research (General Mills, I n c . ) 20 ( l ) , 12-16 (1966 ). Bitumens
ENC.CHEhf. 5 8 (4), 33-6 (1966). (1G) Whittier, F., IND. Physical a n d Mechanical Properties (1H) Alter, H., J . Appl. Polymer Sci. 9, 1525 (1965). (2H) Altgelt, K. H., Mokromol. Chem. 88, 75 (1965). (3H) Attares, T., Vayman, D. P., Allen, V. R., Meyerson, K., J . Polymer Sci. A3, 4131 (1965). (4H) Bedernivona, N. F., et al., Kolloidn. Zh. 27, 3 (1965). (5H) Berger, H. L., Schultz, A . R . , J . Polymer Sci. A3, 3643 (1965). (6H) Brennan, J . J., Jermyn, T. E., J . Appl. PolymerSci. 9, 2749 (1965). (7H) Broutman, L . J., McGarry, F. J., Zbid., pp. 589, 604. (8H) Burrell, H., Znferchem. Reu. 14 (3), 31 (1955). (9H) Caryl, C. R., Rhunick, A. E., Ofic. Dig. FederotionSoc. Paint Technol. 37 (481), 129 (1965). (10H) Crowley, J. D.,Teague, G. S . , Lowe, J. W., J.Paint Technol. 38.(496), 269-80 (1966). (11H) Fatou, J. G.,Mandelkern, L . , J . Phys. Chem. 69,417 (1965). (12H) Gardon, J. L., J . Paint Technol. 38 (492), 43-57 (1966). (13H) Graham, N. B Crowne, F. R., MacAlpine, D. E., ACS Div. Org. Coatings and Plastics Chemisiry, Preprints 25 (l), 51-67 (1965). (14H) Grotz, L. C., J . &pi. Polymer Sei. 9,207 (1965). (15H) Halpin, J. C., Rubber Chem. Technol. 38, 263 (1965). (16H) Halpin, J. C., Bueche, F., Rubber Cliem. Technoi. 38, 278 (1965). (17H) Hansen, C. M., 0 5 6 . Dig. Federation Soc. Paint Technol. 37, 57 (1965). (18H) Heufer, G., Braun, D., J . Polymer Sci. B3, 495 (1965). (19H) Illers, K, H., Kolloid-Z. 190, 16-34 (1963). (20H) IND.ENG. CHEM.5 8 (5), 63 (1966). (21H) Ivanfi, J., Paint Manuf. 35, 37 (Oct. 1965). (22H) Kambour, R. P., J . Polymer Sci. A3, 1713 (1965). (23H) Knight, A. C., Zhid., p. 1845. (24H) Kraus, G., Gruver, J. T., Zbid., p. 105. (25H) Krause, S., Roman, N., Ibid., p. 1631. (26H) Kumins, C. A,, Ofic. Dig. Federotion SOC. Point Technol. 37 (490), 1313-36 (1 965). (27H) Lieberman, E. D., Zhid., 34 (444), 30 (1962). (28H) MacAdam, D. L., Ibid., 37 (491), 1487-1531 (1965). (29H) McSweeney, E. E., Zhid., p. 670. (30H) Michaels, A. S., Zbid., p. 639. (31H) Moore, J., J . PolymerSci. A2, 835 (1964). (32H) Myers, G..E., Dagan, J. R., Zbid., p. 2631. (33H) Kowacki, L. J., et al., O$c. Dig. Federation Soc. Paint Technol. 37 (490), 137191 (1965). (34H) Park, R . A , , Paint Varnish Prod. 5 5 (6), 51 (1965). (35H) Rogers, C. E., J . Polymer Sci., c 10, 93 (1965). (36H) Rogers, C. E “Solubility and Diffusivity,” D. Fox, M . Labes, and A. Weissberger, Eds., ’interscience, New York, 1965. (37H) Ronay, M., Rubber Chem. Technol. 38, 248 (1965). (38H) Smith, T. L., Polymer Sci. ( U S S R ) 5 , 270 (1965). (39H) Smith, W. B., Kollmansberger, A., J . Phys. Chem. 69, 4157 (1965). ( 40H) Soloman, D. H., J . Oil Colour Chemists’ Assoc. 48, 282 (1965).
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(41H) Spencer, H. G., Ojic. Federation SOC.Paint Technol. 37 (486), 757 (1965). (42H) Stieg, F. B.,FJ. Paint Technol. 38, (492), 29-36 (1966). (43H) Tobolsky, A. V., “Properties and Structure of Polymers,” Wiley, New York, 1960. (44H) Tolstaya, S. N., et al., Kolloidn. Zh. 2 7 (31, 1446 (1965). (45H) Tung, L . H., J. PolymerSn‘. A3, 1045 (1965). (46H) Vanderval, C. W., Bree, H. W., Schwartz, R. F., J . Appl. PolymerSci. 9,2143 (1965). (47H) Williams, F. R., Jordan, M. E., Dannenberg, E. M., Zhid., p. 861. (48H) Woodford, D. M., Chem. 2nd. (London), 1966, p. 316.
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