Document zz750r330BO8oZ4O5wKjk3D40

"-Mt TAL. Pf?oC~ ?s~ At C C y- This International Harvester truck (left) was painted with an au tomotive super enamel. The coating is being cured here. Systems Approach to To assure a protective finish on your product that is bpth effective and economical, you must have a combina tion of factors working for you. The coatings you select must fit your application method, your curing method, and the job you want it to do for your product. Primers and topcoats must form a strong adhesive bond, and jt is generally desirable that these prod ucts be procured from the same manufacturer. In other words, a systems approach is required. And you will find that this point is emphasized in each of the three articles that follow. Dissemination of information of this type is par ticularly important for two reasons. First, many product finishes are special systems formulated for a single user. Even though they have broad poten tial, you may not be aware of their availability. Second, these paints, which we call chemical coat ings, differ greatly from conventional paints and from each other. They differ in composition, in the ways they can be applied, and in the ways they can be cured or converted to a protective film. Royal A. Rrowrt, Technical Director National Paint, Varnish & Lacquer Assn. Washington How to Select the Correct Coating By T. R. MATTHEWS and M. M. COBURN A variety of coatings are available. Each has its awn protective, decorative, application, and performance characteristics. IVlANY p a in t f o r mu l a t io n s are available. Each one has distinct advantages and limitations. The pro's and con's of the more impor tant topcoat systems and their rela tive durability are listed in Table I. You must evaluate each system for its suitability to a given application. Nitrocellulose Lacquers These quick-drying coatings elimi nated a production bottleneck in the auto industry following their intro duction in 1924. Today, their usage is deterred by their limited durabil ity and relatively high applied cost. But their ability to air dry by solvent evaporation and their attractive ap pearance (obtainable by rubbing and polishing) are still important factors. Alkyd Systems The alkyds were developed back in the early 1930's, but they are still competitive because of their price, performance, and versatility. These resins may be used alone, or in com bination with other modifiers -- com monly nitrogen, such as urea, and amino resins, such as the melamine formaldehyde types, Baking promotes a chemical re action between the different mate rials to form a crosslink or thermoset polymer with a generally attrac tive balance of hardness, flexibility, and adhesion. It is important to note that the alkyd-amine systems are thermoset in comparison with nitrocellulose lacquer, which is thermoplastic and resoluble in the solvent from which Mr. Matthews is assistant manager and Mr. Cobum is a supervisor, Marshall Laboratory, E. I. du Pont de Nemours & Co. Inc., Philadelphia. N42563 ': iI it is applied. Alkyds are highly compatible with a variety of materials and can be blended with additives to develop desirable properties. Their compati bility also reduces the possibility of contamination in the finishing op eration. Alkyd paints can be applied by most of the common methods such as spray, roller, dip, and brush. They can be formulated with a wide degree of cure response from air dry, through force dry, to baking. Even the baking cycle can have a relatively wide range of time-tem perature combinations. Alkyds may be considered jackof-all-trade paints, but generally they are not regarded as high-per formance finishes. Weaknesses in clude water resistance, chemical resistance, adhesion, hardness, and durability. Yet alkyd-based enamels are still the most commonly used coatings for trucks, buses, and offhighway equipment. Acrylic Lacquers These thermoplastic finishes be came commercially important when they replaced nitrocellulose lacquer oil passenger cars and on railway equipment. They meet demands for higher performance finishes. Solvent solutions are the most common form for industrial usage, but water and solvent dispersions are getting a lot of attention on an experimental basis. j The acrylic lacquers provide high aesthetic appeal as topcoats. They offer significantly better durability than their nitrocellulose predeces sors. They have the ability to reflow at temperatures of around 275 to 300 F into smooth, mirrorlike fin ishes, yet they can be repaired at baking schedules as low as 30 min at 120 F. Because of their lack of cure or crosslinking, these lacquers tend to be sensitive to solvents and other chemicals. Under adverse conditions, they also show some water spot dis tortion. Their thermoplastic nature makes them unsuitable for applica tions requiring high hardness at elevated temperatures. They often require special undercoats for op timum performance. 68 Acrylic Enamel This thermosetting paint is find ing increasing acceptance in the transportation industry as a replace ment for the alkyd-amine enamels. Durability, color retention, and toughness (reduced repair) are con tributing factors. They offer good flexibility and can be used for coil coatings where postformability is an important requirement. They are limited by application parameters which are more critical than the alkyd-amine systems. The formula is likely to require tailoring to the application. Baking tempera tures and times fall into a narrower range. Failure to meet baking re quirements can result in an undercuring with a loss in properties. Vinyl Systems Flexibility and resistance to chem ical attack account for the popularity of vinyl finishes. Vinyl coatings are available as solutions, organosols, plastisols, and emulsions. The solu tion vinyls are generally of more interest to the transportation indus try. Besides flexibility and chemical resistance, they offer toughness, good exterior durability, nonflammability, and high electrical resistance. Most vinyls are thermoplastic. As such, they are sensitive to solvents and heat. Like the acrylics, vinyls must be more critically fitted to the application than nitrocellulose lac quer or the alkyds. Vinyls should be used where their benefits outweigh their relatively high cost. Epoxy and Epoxy Esters Epoxies are generally two-pack age products. Epoxy esters are a one-package product. They are based on resins formed by the reaction of the epoxy with fatty or rosin acids giv ing the ester. Properties can be tail ored to the application by control ling (the amount and type of fatty acid. Some latitude is available through selection of curing agents but not to the same extent as with epoxy esters. Epoxy topcoats have good chem ical resistance to all but severe acid environments. They also have good solvent resistance, good flexibility and hardness, plus excellent adhe sion to most substrates. Curing takes place at ambient temperatures, or it can be accelerated by baking. Epoxy ester topcoats have excel lent adhesion and can be formulated to give the degree of flexibility and hardness required. Their resistance to chemical attack is better than that of the alkyds but inferior to that of the epoxies. They are less expen sive than the epoxies but do not resist solvents as well. The epoxy ester can be formulated as air-dry coatings or for baking. Urethanes These coatings fall into two cate gories ~ aliphatic isocyanates where exterior durability is essential, and aromatic isocyanates, where reduced cost is desired. They are most gen erally available as two component systems with limited potlife. Curing is promoted by absorption of atmos pheric moisture. The advantages of urethane top coats include exceptional toughness, abrasion resistance, adhesion, and resistance to chemical attack. The aliphatic urethanes provide superior resistance to mineral acids. Both afford good resistance to alkali. The added cost of the aliphatic urethanes (2.5 times higher than the aromatic versions) can be justified where appearance retention on ex terior exposure is a must. These finishes are applied on aircraft and diesel locomotives in this country and on buses and trolleys in Europe. The less durable, less costly aro matic urethanes should be consid ered for interior coatings requiring great abrasion resistance. They are used as linings for hopper cars and dry cargo bins. Silicone Alkyds and Acrylics These coatings are thermosetting. They can be chemically modified with silicone-forming resins to pro vide systems which offer significantly greater weathering resistance and thermal stability than the parent coatings. They require compara tively high bakes to achieve opti mum properties. Siliconized systems fill the gap between the normal service tempera ture range of organic coatings and METAL PROGRESS DUP030025029 Table I -- Topcoat System Characteristics Topcoat System Durability Rating* Advantages Disadvantages Relative Costt Nitrocellulose lacquers Alkyd-amine Acrylic lacquer Acrylic enamel Vinyl solution Epoxy Epoxy esters Urethanes (aliphatic) Urethanes (aromatic) Silicone alkyds and acrylics Fluoropolymers 2 years 5 8 8 8 5 2 10 2 8 to 15 20 Fast drying Versatility, price Color rentention Color retention, toughness Flexibility, chemical resistance Corrosion and chemical resistance Corrosion and chemical resistance Poor durability, low solids content Adhesion, hardness Adhesion High baking required Low solids Short pot life Durability Toughness, chemical resistance Cost Toughness, chemical resistance Durability Gloss retention Superior durability, chem ical resistance, mechanical properties Flexibility, postformability, cost Cost, appiication, and curing requirements 0.9 to 1.1 0.5 to 0.6 0.8 to 0.9 0.6 to 0.7 0.9 to 1.1 2.0 1-0 2.5 1.0 2.0 to 3.0 4,5 to 5.0 * Time you might expect a quality white in this vehicle to stand up in service, t Figures are cents per mil thickness per square foot of coverage. They include only the cost of the coating. Table II -- Considerations in Selecting a Total Coating System Factor Checkpoints Material to be coated Coating quality 1 Processing equipment Intangible, and miscellaneous items Sire, shape, type of substrate and substrate mix, quantity of units. Exterior durability, corrosion resistance, humidity resistance, finish adhesion, chemical resistance (type and degree), heat resistance, hardness (mar resistance, abrasion resistance, damage resistance), visual esthetics (color, appearance, glamour), special requirements (detergent resistance). Part handling (monorail, strip line, body line), application method, production rate, available beat, type of heating, sludge handling equipment, construction materials (mild steel, stainless, lined), space available (pretreatment, processing, storage). Nature of metal treatment, competitive situation, customer preference or prejudice, cost, personnel (operating, control, maintenance), legal responsibilities (pollution, waste disposal). that of pure silicones, or for that matter, porcelain enamels. Adequate performance of the coating system at 500 F can be obtained by an ad dition of 25% silicone in dark colors and 75% silicone in pastel shades. Weathering resistance is markedly improved by similar modification. For example, a conventional white alkyd system on an accelerated weathering machine retained 70% of its gloss after 300 hr exposure. A 25% silicone modification retains this same gloss level after 600 hr. A 50% silicone addition extended this level of gloss retention to 900 hr. At that point, the unmodified alkyd had lost all of its gloss. Fluoropolymers If you are loking for the best in thermal stability, exterior durability, resistance to ultraviolet radiation, nonstick properties, and fire re- tardancy, evaluate fluoropolymer systems. They are expensive but can often be justified on a value-in-use basis because of their superlative balance of properties. Dulite fluoropolymer coatings are used on siding for industrial build ings. They are warranted mainte nance-free for 20 years. Teflon has been a highly successful coating for nonstick cookware. Teflon S is used on saws and snow shovels to lubri cate and prevent sticking. These coatings are also used on long-wear ing bearing surfaces. System Economics The cost factors in a coating sys tem include (in random order): top coat quality, color, primer, metal treatment, application method, quan tity of units, size and shape of units, and type of substrate and substrate mix. Some typical cost figures are included in Table I. They compare a white pigmentation at constant hiding in all topcoat systems. Uni form film thickness and 100% ap plication efficiency are assumed. Preparation, priming, application, and curing costs are excluded. Natu rally, all of these factors are im portant in selecting a system, but the figures given are useful in com paring the cost of the various top coat materials in terms of desired properties. Substrate Preparation The ultimate life of any paint system depends upon two major factors: the chemistry of the organic coating and surface preparation. Different metals require different pretreatroents to produce an opti mum cost vs performance relation ship, Qualified suppliers can pro vide accurate cost information on JUNE 1969 69 DUP030025030 pretreatment systems designed to process the type of part in which you are interested. Selecting a Total System A plant painting five buses a day is designed quite differently from a passenger car assembly line turning out one unit every minute. Some of the many factors which must be considered appear in Table II. We have already considered how the performance requirements of the coating can influence the choice of quality and application conditions. Even these must be reconciled with available space and capital appro priations. In the final analysis, a bal ance must be obtained between quality and cost. Your chances of selecting the optimum systems will be greatly enhanced by open con sultations with qualified paint and equipment suppliers. How to Apply the Coatings By L. R. LeBRAS A change in method can improve product finish or reduce painting costs. Lower costs can be the result of less manpower input, faster application, or higher paint utilization. T HEBE ABE MANY WAYS to apply organic coatings. The more common industrial techniques in clude: compressed air spray, airless spray, electrostatic spray, dipping, electrocoating, flow coating, curtain coating, direct roll coating, reverse roll coating, fluidized bed,' tumbling, centrifugal, autoclaving, silk screen, aerosol spray, brush, and roller. A look at the advantages and limita tions of each one will help you se lect the best procedure for your needs. Compressed Air Spray This method is still used to coat an extremely wide variety of parts and substrates. It is inexpensive to install, but labor costs are high, and appearance of the finish depends upon the skill of the Operator. Other features: all exposed areas are coated; color changes are fast and easy; a wide range Of coatings can be applied; and practically any substrate can be coated. Some limi tations: high paint overspray, orange peel or sagging, difficulties in film thickness control, and difficulties in coating recessed areas. Airless Spray The paint may be sprayed hot or at ambient temperature. In either case, it is under high pressure. There Mr. LeBrass is director. Industrial Coatings Development, PPG Industries Inc., Springdale, Pa. 70 Line at General Electric Co. plant at Salem, Va., electrocoats small parts. is less overspray in comparison with conventional air spraying, but the applied film is more prone to sag ging. Electrostatic Spray Air, airless, and disc application methods work well. Electrostatic spraying reduces overspray and pro duces a wraparound of the film which results from atomizing the paint into a high voltage field. Spraying may be done manually or by completely automatic equipment. Disc spraying is used exclusively with automatic control. Colors can be changed easily. The limitations of electrostatic coating include the orange peel effect, difficulty in coating recessed areas, relatively high installation costs, and difficulty in coating mixed shapes. Dipping This is probably the simplest method. It has been used success fully on many conveyorized finish ing systems. The equipment is quite simple. Labor costs are low, and the paint is used efficiently. Other ad vantages are: ability to coat non- METAL PROGRESS DUP030025031  ....... ........ Du Pont de Nemours, is, & Co. F Dept, - Mar shall Development -La t. Publication. How to select the correct coating, by T. R. Matthews and M. M. Coburn. DATE June 19 69. -*______ 6ISSUED TO j L ST~ i/// &.// FA/S/Ss/iS aRi jn Vf/t* ' GAVi.OD *0 DUP030025032 DATE is s u ed t o ... ' > f > p- s\; , DUP030025033