protective coatings - ACS Publications

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annual review

H. L. GERHART

Protective Coatings

Spray process is used to appb Selectrofoam coating to liquid ammonia storage sphere in order to minimize heat loss to atmoskhere 52

INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY

Systematic development in coating formulation coupled with pioneering polymer research are key elements in this technology’s transitionfrom craft

to sczence ecent progress in protective coating research and

R development indicates a transition of this field of

activity from a n art or craft to a science. The usual evolution of hundreds of new coating formulations and application techniques which takes place every year, (7-3) has recently taken direction toward scientific approaches to materials protection. The art of coating formulation requires combined skills of scientist, artist, and mechanic to select resins, pigments, solvents, and additives; it requires a thorough knowledge of hundreds of materials and their interactions in many solvents. The formulator, long-reputed as the foot soldier of the paint industry, is maturing to become an innovator utilizing backup of fundamental research to direct his formulating function. Patentable novelty has resulted with advances including vinyl phenolic systems for beverage-can linings, epoxy bituminous systems, thermosetting acrylic, epoxy enamels; spatter lacquers, and wrinkle finishes. Large industrial firms are focusing research attention on vehicle development and modification-i.e., development of new synthetic polymers-while their formulators are finding ways to put the new products to work. Industrial paint research is using the most sophisticated of tools including spectrophotometric equipment coupled with computers to indicate color deficiency and overage, as well as computer programs to predict characteristics

AUTHORS H. L. Gerhart, Director, Research and Development Department, Coatings and Resins Division, Pittsburgh Plate Glass Co., coordinated the collaborative preparation of this review. Members of his staf who contributed sections were R. E. Cutforth, J . A . Erikson, N . H . Frick, R. S. Hartzell, Ruth Johnston, L. R. LeBras, J . A . Seiner, H. A . Vogel, and M . Wismer.

of copolymerized forms of mixed monomers and to select monomers for applications. This review surveys the key areas of protective coating research where the greater strides have been made. Pioneering research with monomers of acrylic and methacrylic esters in the past decade has been especially fruitful. Acting on the knowledge that homo- and copolymers of these monomers exhibit excellent color and retention thereof on exterior exposure, investigators have added what was lacking-namely, the elements of distensibility (post forming) hardness, thermosetting, and adaptability to diverse methods of application. One result has been appearance and color effects not possible with previous coatings, and durability demonstratably superior to anything available 10 years ago. Another result: The high speed operation of coil-coating machinery provides a surface with excellent post formability and a durability expectation said to approach 20 years. Tile-like Masonry Coatings

There are probably few other end-use categories for coatings in which formulators have used their imaginations as much and, conversely, in which there is so little general agreement on the composition of most suitable systems. Variation is due to economics rather than to lack of technical know-how. O n the one extreme are the cost and performance of glazed or ceramic tile, while on the other extreme is the type of low cost construction using concrete or cinder block finished with conventional latex or alkyd-based paints. A coating system for laid-up concrete block interior walls, approaching the general inertness and ability to withstand abuse exhibited by ceramic tile, has been a formulating goal for a long time. The market for such material is in public buildings, schools, nursing homes, public housing installations, factories, and numerous other places. Extension of such a coating system to other surfaces such as plaster and dry wall construction is logical, since there are many conditions of hard use where traditional alkyd and latex paints are not adequate and initial lower cost is exchanged for higher long term maintenance and repaint. Among systems currently being suggested for this use are : cementitious or vitreous compositions, epoxy mastics plus epoxy enamel finish VOL. 5 7

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coats, high solids epoxies-generally the liquid epoxy resins using an amine catalyst cure, vinyl mastic plus glaze coats of several types, unsaturated polyester-styrene compositions, urethanes (either as prepolymers or as moisture-cured enamels), and polyester-epoxy formulations. If coating systems on concrete or cinder block are considered, the first applications are expected to be heavy consistency fillers, based upon latex, epoxy, epoxy ester, sand-cement-latex grouts, or even older oil-based compositions. Finish coats have been based upon epoxy, urethane, vinyl, acrylic, polyester-styrene, or polyesterepoxy resin combinations. Cementitious or vitreous systems in the past ha\-e been somewhat out of the field of composition normally supplied by organic coating manufacturers. Recently, however, many of these systems include a glaze finishing coat which is often a solvent acrylic lacquer. These glaze coats are applied to give a higher uniform gloss than can readily be obtained with inorganic compositions. Performance of these systems is very good, while drawbacks are high applied cost and limitation of application to franchised applicators. The unsaturated polyester-styrene coinpositions are chemically akin to the resins used in the plastic industry (often misnamed “fiber glass plastic” because of the reinforced-with-fiber-glass uses). Their performance is excellent, their cost reasonable, but the drawbacks are difficult application, short pot-life when catalyzed, and a t times uncertain cure. The various epoxy compositions, whether high solids or more conventional epoxy enamel-type, are based upon Bisphenol A structures. Performance in terms of chemical resistance is good, but these resins have a yellowing tendency, and delicate pastel shades may show color change and lose decorative value. Urethane compositions give performances similar to epoxies and have a more severe handicap in yellowing resistance. However, recently developed urethane intermediates promise relief from yellowing, but at extremely high cost. A recent new cross-linking mechanism is based on polyesters prepared from chlorinated dibasic acids and epoxide oxygen containing components not related to Bisphenol A structures. These systems formulated for application to the masonry coating problem offer all the advantages of regular epoxy finishes, with none of the drawbacks of poor yellowing resistance. They use less hazardous solvents than those used for epoxy systems and give application properties similar to conventional alkydbased enamels. I t is certain that high construction costs coupled with realistic analysis of costs of maintenance will continue to create a market for organic coatings of newer types. The “specialty” coatings for heavy-duty use today are expected to become the “conventional” coating for the professional painter in many public buildings. Coatings for Prefinished Wood Products

The factory application of durable weather resistant compositions to metal substrates such as aluminum and galvanized iron strip is an accomplished fact ( 7 A ) . The 54

INDUSTRIAL A N D ENGINEERING CHEMISTRY

use of alkyd melamine, plasticized vinyl, thermosetting acrylic, and, more recently, polymers containing fluorine is customary. The expectation of long exterior service has brought a dramatic change in residential and industrial construction. The wood and wood-product industry also continues to press for satisfactory coatings for factory prefinishing of its products. Market demands for prefinished products continue strong and are expected to grow in intensity. Several new installations for prefinishing of interior paneling have been built, and production of factory-primed exterior building components is steadily increasing. Interior Paneling. Materials used for prefinishing interior paneling must meet the fast cure cycles imposed by high-speed coaters and fast automated lines. Fillers and ground coats are primarily of alkyd quality with some interest in epoxies and modified lacquers. Polyester fillers have shown some promise in experimental applications and may overcome the disadvantage of multiple fills needed with alkyd qualities. Yitrocellulose lacquers and alkyd conversion varnishes are most popular for clear topcoats, with considerable butyrate lacquers also being used. The stringent concurrent requirements for hardness, mar-resistance, and strain resistance are controlling factors as the competition with plastic laminants continues to be felt. We can expect increased interest and activity in synthetic type clears that demonstrate the exceptional physical properties and resistance characteristics demanded by this industry. Exterior Coatings. Production of factory-primed siding and other building components is showing steady growth, Coating types used are primarily alkyd and acrylic quality, with considerable experimental work with other type compositions. The lumber industry is pressing hard for complete finish systems, and there are a few products getting actual field testing. Vinyl and polyester systems have shown considerable promise. The basic problem of a relatively nonuniform and unstable substrate reduces confidence in long-range durability. The increased use of overlays, usually paper, promises to overcome many of the problems of dimensional stability and variations in substrate ; this development could be a deciding one in advancing the acceptance of completely prefinished exterior siding. Thermal Insulation Coatings

There has been a n urgent need in industry for an organic coating which not only protects the substrate from corrosion but also has thermal insulating properties ( I B , 2B). Such coating systems consist of a sprayed urethane foam having the surface sealed with a two-part elastomeric protective finish. The combination of urethane foam and top coat presents an insulation with utility applied at low cost and which has, in addition, protective and decorative value. These insulating coating systems are nonburning and have a low water vapor transmission and water absorption and a low thermal conductivity. Application is simple and is achieved using spray guns.

The paint industry and equipment designers cooperated in the development of inexpensive and efficient spray guns for urethane foams. As two components of the foamable coating are sprayed, the operator can immediately see imbalances in the ratio due to a unique color indicator incorporated into the coating systems. A properly proportioned coating has a characteristic blue-gray color. Foaming is complete in 10 minutes; an example of application is a liquid ammonia storage sphere coated with a foamed paint system. This sphere, which is 60 ft. in diameter, required 12,000 sq. ft. of insulation applied in less than 3 weeks. Raw material basis for these insulating coatings includes such materials as sugar, propylene oxide, chlorofluoro hydrocarbons, and isocyanates. A current objective of basic research in coatings is to understand the behavior of coatings at elevated temperatures. Of particular interest is the strength of polymeric material in its carbonized form. This is a n attempt to design polymers having flame retardancy because of strong and stable char formation during the exposure of a polymer or coating to a conflagration. The possibility of strengthening the fire retardancy of organic coatings by the incorporation of inorganic materials was explored. Other work led into the field of inorganic insulating coatings and refractory materials. As a result of studying numerous polymers as carriers for inorganic fillers, a unique organic material was found which can be converted into an inorganic insulating foam when subjected to temperatures up to 3000' F. Such products show promise as inorganic cellular materials with good thermal insulation properties at temperatures between 2000' to 3000' F.

Fire-Retardant Coatings

The need for coatings to retard the spread of flame and to minimize the extent of structural damage which a fire causes has existed for a long time (IC-5C). There has been considerable confusion on a t least two points: (1) Where and on what substrates are fire-retardant coatings necessary? Conversely, on what substrates and at what thicknesses have conventional paints materially contributed to the spread of a fire? (2) What are reliable and meaningful laboratory tests for evaluation of fireretardant coatings? Divergent viewpoints on the first question are presented. There is agreement that conventional paints on a noncombustible substrate such as cement-asbestos or metal contribute little to flame-spread if the paint layer does not exceed 0.05 in. Similarly, the Underwriters Laboratory, generally considered the most critical testing agency, has found that conventional alkyd semigloss finishes on cement-asbestos board add no significant fire hazard at thicknesses below 0.05 in. By the Tunnel Test they show uncoated red oak wood a t a standard value of 100 on flame-spread, and a flame-spread of 15 for a '/do-in. coating of alkyd semigloss on the cementasbestos. Flame-spread increased to a value of 54 when coating thickness was increased to I / ~ oin. The thickness

a t I / ~ oin. represents probably an average of 25-30 coats of paint at normal application. O n the other hand, many public safety officials, fire marshals, and U.S. Navy personnel feel that conventional paints may contribute to fire hazard and should be tested for their effect even on noncombustible surfaces. Therefore many codes and specifications of various states require some evidence of such tests. Recently one state fire marshal, in testing for compliance to his state's code for institutional buildings, subjected various conventional paints on a noncombustible panel, to a 1200" F. furnace test and found many to be satisfactory for his purpose. Included were latex flat, alkyd flat, alkyd semigloss, alkyd gloss enamels, as well as a number of floor and trim varnishes. I n considering finishes for combustible substrates such as wood or cellulosic ceiling tile, there is little question about the desirability of coatings designed to decrease flame spread. Conventional finishes do not greatly add to fire hazard here, unless they are clear nitrocellulose lacquers, and some evidence exists that high pigment-low resin paints such as alkyd or latex flat finishes show lower flame spread than the uncoated wood. Nevertheless, to decrease flame-spread to an effectively low value, coatings of special design are required. The area of methods of test and their reliability has been the subject of numerous reports and subsequent debate. I n great part, this situation has arisen from the fact that the most widely recognized test method as described under ASTM-E-84-61 (the above reference Tunnel Test) requires an expensive installation, is difficult to run, and results of recognized reliability are available from only several testing agencies in the country. This has led to innumerable test methods ranging from simple Bunsen burner exposure to more ambitious types using controlled propane torches or high-temperature ovens. The present position is generally felt to be that the ASTM Tunnel Test data are most meaningful. The Underwriters Laboratories, Inc., as a testing agency of the National Board of Fire Underwriters, offers a program which includes conducting such tests and certifying that the product tested will meet flame-spread ratings when properly manufactured and applied. Relatively little technical information appears in published literature on the formulation of fire-retardant paints. Two general types differing in the physical principle a t work are recognized. The first are paints formulated with large amounts of nonflammable ingredients which will contribute little fuel to a flame or which emit gaseous products to lower the effective oxygen content of gases surrounding the flame. Paints of this class have been formulated containing one or several compounds such as sodium silicate, borax, chlorinated rubber resin, antimony oxide, zinc borate, chlorinated alkyd, chlorinated paraffin, and antimony silico oxide. A number of military specifications, notably some under the U. S. Navy Bureau of Ships, can be met by proper use of the above ingredients. These paints more properly may be classified as noncombustible, but in themselves do not act to set u p a heat-insulating layer to proVOL. 5 7

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vide greater protection to an underlying wood substrate. For this reason most of the types find uses on metal surfaces on ships. The second general type are the intuniescent paints which, on exposure to flame, M-ill char to form an expanded spongy charred foam. This foain contains entrapped gaseous reaction products and acts as a heatinsulating material. Intumescent paints may be formulated using solvent-dispersed resins or, more recentl)-, water-dispersed latex polymers. In each case, key ingredients used will include nitrogen and 'or phosphorus derivatives. Coinnion ingredients at times include aininonium phosphates, dicyandiamide, urea-forrnaldehyde resin, pentaerythritol, melamine, along with additives to the resin system such as chlorinated hydrocarbons, chlorinated plasticizers, or chlorinated resins. Intumescent paints based upon solvent systems to carry all ingredients offer excellent flame-spread control, but often have severe shortcomings with regard to paint properties including poor application characteristics, poor decorative appeal, and fire hazard during their application until all solvent evaporates. Considerable recent technology has resulted in improvement of intumescent latex paints to the point where the newer products of this type have application properties of normal latex paint and possess adequately low flame-spread ratings. Improvement in water insensitivity is such that these paints show retention of foaming ingredients after washing and in this way retain a good measure of their original ability to form the insulating char foam layer. Specific formulation details of such newer products are generally proprietary and do not appear in published industry literature. I n summary, paint formulators are finally making significant strides toward the goal of providing coatings which will decrease fire hazard on combustible substrates and at the same time provide ease of application and decorative effects approaching conventional paints. Water-Dispersed Gloss Finishes

The advent of excellent quality latex paints in the late 1940's has generally been considered one of the more significant advances in paints designed for use by the consumer (70--50).Ease of application and clean-up with soapy water almost immediately created a demand for similar paints in the gloss or semigloss range for use on wood trim, in kitchens, bathrooms, and similar areas. For these end uses the currently recognized best coatings are those formulated with air-drying alkyd resins, often dissolved in low-odor mineral spirits, which give hard, stain-resistant, easily washable surfaces. Their application by brush or roller is generally very good, and flow to permit brush marks to level out is satisfactory. Miaterdispersed gloss or semigloss finishes of equally good application properties have been difficult to formulate. The basic reasons are that in latex polymers it is considerably harder to match the flow, leveling, and smooth knitting of a film which is deposited from a solution such as alkyd resin-mineral spirits. O n the other hand, many resin modifications to arrive at a water-soluble polymer 56

INDUSTRIAL A N D ENGINEERING CHEMISTRY

have been made. These, in general, h a x pro\-ed difficult to convert into filrris of sufficient hardness and full water resistance. Assessing the present state of technology in this area is not possible on the basis of published literature references. There are periodic articles discussing specific formulation approaches, or presenting data on proprietary resins claimed to be suitable for these applications. A number of commercial paints have been available in the past, although none had outstanding commercial success. Ne\-ertheless, advances in the art have been and continue to be made, and better semigloss or gloss finishes dispersed in water will become available. There are a number of formulating approaches. Latex polymers of the vinyl acetate, acrylic, and vinylidene chloride type have been recommended. Particle size of the latex polymer has been felt to be of critical importance. Commercial latices varying from extremely small particle size of less than 0.2 p up through large particle emulsions in the range of 2-3 1.1 have been suggested. Hence, there is no full agreement that any range of particle size is most successful. Detailed study has been done on surfactant balance and addition of solvents to modify the coalescence of the latex particles during film formation. Here again, no general conclusions are available since each proprietary latex will respond best to specific combinations of these formulating additives. It is also possible to use emulsions prepared from resins originally prepared in solvent solution. These may include emulsions of alkyd resins or unsaturated fatty acid esters of various polyols. Emulsions of these vehicles may be prepared from either a totally solvent-free liquid resin or from high-solids solutions of these resins in hydrocarbon solvents. As compared to latex polymers, the emulsion stability and proper control of particle size in these emulsions generally present more difficulty. A third general approach has been the use of watersoluble vehicles. The solubilizing group in such resins is generally excess carboxylic groups derived from various dibasic acids and solubilized through the use of ammonia or organic amines to form soaps. I n this approach, resins which are truly water-soluble require such extensive modification that final film properties of hardness and water resistance are sacrificed at ambient temperature cure. Water-soluble alkyds have shown promise in applications wherein they can be modified with watersoluble coreactant resins of the urea- or melamineformaldehyde class, and a high-temperature baking forces elimination of amine groups to give hard crosslinked water-resistant films. For air-drying applications, the water-soluble approach has generally been modified to give resins which are soluble through the aid of COUpling solvents. The use of coupling solvents, such as butyl cellosolve, butyl carbitol, or other solvent glycols, often results in films with insufficient solvent resistance for application of a second coat of the same type of paint. More recently there have appeared several latex compositions which appear to lend themselves well to formulating gloss or semigloss finishes with improved applica-

tion properties. Details and evidence for this can be found only in proprietary trade literature. Blends of several latex emulsions varying in particle size as well as in hardness of the individual latex polymers promise more freedom in formulating a balance between good coalescence and working properties, on the one hand, and satisfactory hardness on the other. Since the nonprofessional painter greatly prefers using a water-dispersed paint, it is highly possible that products which sacrifice only slightly such alkyd enamel properties as flow, leveling, and appearance at lap-marks when brush applied will gain acceptance in the near future. Nonaqueous Dispersions

Concurrent with the search for product changes and advances through the synthesis and modification of resins, there is an effort to use available resins in new physical form. Solid resins suspended as fine powders are one way to afford a high concentration of film-forming components with a reduced number of applications of successive coats. Subsequent heat fusion produces relatively thick films having the properties expected of those derived from true solutions or aqueous dispersions. Historically, it has been difficult to produce industrially acceptable coatings for metals using resins dispersed in aqueous media. Rusting of sheet metal during prolonged exposure to moisture and poor wetting of metals by aqueous systems have retarded the use of waterdispersed coatings. Recent disclosures on nonaqueous dispersions ( I E , 2E) indicate that appropriate research approaches to satisfactory vehicles are available. The combination of high molecular weights inherent from emulsion polymerization techniques and the excellent metal wetting, nonrusting characteristics of organic solvents provides a foundation for a new family of vehicles. Spray application at 50% resin solids is possible us. 20-3OyG solids for the same resins in lacquer solvents. Oil-Free Alkyds

As thermoset finishes become increasingly important industrially, the trend in alkyds is away from unsaturated oil-modified alkyds toward the types that have improved color retention and durability. I n fact, oil-free alkyds cross-linked with amino resins are recapturing some of the markets lost by oil-modified alkyds to the modern acrylic and vinyl copolymer vehicles ( I F ) . Their success depends on the chemical characteristics derived from esters of isophthalic acid, which does not form a monomeric cyclic anhydride, and the neopentyl structure of trimethylol propane and neopentyl glycol used in some of these formulations. Primary alcohols cross-link more readily with amino resins, and these ester linkages are more stable than those in the classical oil-free alkyd glyceryl phthalate. Materials Encapsulation

Encapsulation is a new technique now being investigated in the coatings industry. I t is a process in which material or particles are coated or packaged in a n

impervious film or foam. Although the process has no known commercial uses to date in the coatings industry, it offers some interesting new approaches to materials protection. For example, by encapsulating a material in an impervious wall it is possible to change the physical form of a material from a liquid to a solid, to isolate a material from a reactive or otherwise undesirable environment, and to retain the capability of releasing the encapsulated material when it is needed. I t is conceivable to make two package paint systems into one package by encapsulating the catalyst. O r it might be desirable to encapsulate a corrosion inhibitive reactive pigment within an impervious capsule for use with a paint vehicle with which it normally could not be mixed. The techniques used to encapsulate particles or liquids at present are phase separation from both aqueous and organicsolvent solutions, solvent exchange in preformed capsules, interfacial polymerization, melt techniques, vacuum metallizing, fluidized bed coating, and centrifugal casting. Generally, these techniques were developed for specific applications, and it is highly probable that new encapsulating techniques will soon be worked out for coatings materials (IG-3G). Organic Coatings for Space Environment

Possibly the most esoteric area of coatings research deals with performance in the space environment. The coatings, chosen for their electrical insulating or optical properties, must be durable under conditions only remotely related to those on earth. Aside from the frictional heat and possible oxidation encountered during the ascent of the space vehicle, the coating must with stand as nearly as possible the following conditions : VACUUM:

to

mm. Hg THERMAL RADIATION: low quantum energy of visible or longer wavelength; vehicle surface temperature should be controlled a t 50' C. maximum but may reach 100' C. minimum RADIATION : 1000-3800 A. range ULTRAVIOLET HIGH-ENERGY RADIATION : from x-rays, gamma rays, electrons, beta rays, neutrons, protons, alpha particles, and fission products SOLIDPARTICLES Meteoroids (>1 mm. radius) V = 15 miles/sec. d = 0.1 g./cc. Flux = 2 . 5 X particle/ sq. cm.-sec. Micrometeorites (