37
Appliance Coatings THOMAS J. MIRANDA Whirlpool Corporation, Benton Harbor, MI 49022 History Appliance Markets Coating Processes Cleaning Phosphating Flow Coating Electrocoating Topcoats Application of Topcoats Appliance Coating Specifications Recent Developments Flow Coat Conversion Electrocoating High Solids Powder Coatings Low Energy Cure Coatings Coatings to Replace Porcelain Future
Appliances may be classified into three major types: white goods, brown goods, and traffic appliances. White goods are primarily those products associated with laundry and refrigeration such as washers, dryers, dishwashers, ranges, refrigerators, and freezers. The name was due to the primary use of white for these products. Brown goods include radios, television sets, furnaces, air conditioners, and home entertainment products, while traffic appliances include toasters, mixers, electric knives, fans, blenders, and other more portable items. The larger appliances are generally prepared from metals, while the traffic appliances employ large amounts of plastics because of light weight, durability, design flexibility, productivity, and cost. 0097-6156/85/0285-O883$06.75/0 © 1985 American Chemical Society
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APPLIED POLYMER SCIENCE
Major appliances consume large quantities of steel and aluminum that must be coated to prevent corrosion. Cold roll steel and enameling iron (for porcelain) are the predominant metals used in large appliances. Because of the environment to which appliances are exposed, coatings vary from the inert glass of porcelain to hard flexible organic finishes. Appliance coatings are exposed to detergents, water, cleaning agents, abrasion, impact, and food stains. For severe applications such as the automatic washer tub, basket, top, and l i d , porcelain has been the preferred coating. In some refrigerator and freezer liners, porcelain had been used extensively but is being replaced by powder coatings and precoated steel or aluminum. The high temperatures used in ranges require porcelain enamels, and i t is doubtful that organic coatings can replace porcelain for these applications. In addition to providing protection for the substrate, the coating must above a l l present an attractive finish for showroom appeal and compliment the decorative scheme of the homeowner, meet rigid test specifications, and represent a favorable cost/benefit relationship for the manufacturer. History The history of major appliances goes back to the beginning of the twentieth century involving hand-operated washing machines, ice boxes, and wood-, coal-, gas-, and kerosene-fueled ranges. Less than a million homes were wired for electricity. The turning point came with the development of the universal electric motor, which led to the introduction of the vacuum cleaner. By 1909 vacuum cleaners and washers were being powered by a fractional horsepower motor; by 1920 the first million washers and vacuum cleaners had been sold (I). With the advent of rural electrification, the major appliance industry took hold. From 10,000 refrigerators in 1920, the appliance industry now sells about 6 million refrigerators per year in the United States. Early coatings for appliances were slow-drying varnishes based on shellac and later on natural oils and gums. These were replaced with faster drying phenolics modified with tung o i l with excellent acid-alkali and corrosion resistance (2). With the advent of the nitrocellulose lacquers following World War I, rapid-drying topcoats became available that revolutionized the industry. The development of the alkyd resin by Kienle (_3) provided a system that lasted from the thirties until they were largely supplanted by the thermosetting acrylic. Although alkyds developed more build, they required long baking times and high temperatures, 200 °C or higher, compared to the nitrocellulose lacquers. Other resin types entered the picture such a vinyls, melamines, epoxies, and silicones. The alkyd resin became the major appliance topcoat as improvements in alkyd chemistry advanced. Melamine-modified alkyds improved the properties as well as lowered the bake schedule. Since the alkyds are based on natural oils, long-term yellowing and food staining were an ongoing problem.
37.
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Appliance Coatings
885
A major revolution in appliance coatings came about with the discovery by Strain (4) of the thermosetting acrylic. This was made practical by Christenson (5), Sekmakas (6, 7), and others (8). Thermosetting acrylics became the hallmark of appliance finishes for over a decade but became a victim of the environmental movement, which required lower emissions. To achieve lower emissions, the volume solids content had to be increased. However, increasing the s o l i d s content increased the viscosity beyond the capability of existing application equipment. This led to the development of alternative coatings designed to meet environmental regulations (9). The new approach to compliance included high s o l i d s based on polyester, electrocoating, and powder coatings (10). Concurrent with the development of these technologies was the emergence of high-speed turbines to deposit the higher viscosity l i q u i d s , cationic electrocoating for single-coat applications, and powder coatings capable of being deposited in thin films. Appliance Markets The appliance market consumes a large volume of coatings, adhesives, and p l a s t i c s . In 1980 over 383 m i l l i o n units were sold (11). Of these, major appliances represented some 35 million units. Table I l i s t s some major appliance shipments for 1980. The quantity of paint per unit for several appliances is shown in Table II. While the total gallonage varies, the per unit value provides a means for estimating the yearly consumption of coatings in this industry. Although several appliances such as the refrigerator and washer are mature products, the ongoing replacement market opportunity prevents these products from becoming obsolete. New major appliances are d i f f i c u l t to develop. The last new major appliance to be marketed was the trash compactor, which followed the dryer introduced 35 years e a r l i e r . The appliance market exceeded the 10 b i l l i o n dollar mark in 1980. The number of major producers has decreased in recent years through acquisitions and mergers, so that today there are f i v e major producers who account for the biggest market share. For example, i n refrigerators, four suppliers share 93% of the market. Coating Processes Coating of appliances involves several key steps on conveyorized l i n e s . These are cleaning, metal preparation, priming, baking, topcoating, and baking. Appliance coating l i n e s are highly automated, consisting of overhead chain conveyors that carry parts throughout the various stages of processing; even part storage i s accomplished on conveyors. Cleaning. Cleaning i s generally carried out in a multiple-stage operation, which includes treatment with zinc or iron phosphate. The objective i s to remove o i l s used in forming parts as well as m i l l o i l s added during the manufacture of steel. Alkaline cleaners and detergents are used in spray washers followed by hot water rinses. Water temperatures of 74 °C are required to melt fats and o i l s and to facilitate removal of smut or
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Table I.
Major Appliance Shipments 1980
Appliance
Unit (Millions)
Automatic washer
4.5
Dryers (gas and electric)
3.5
Dishwashers
3.5
Disposers
3.3
Trash compactors
0.29
Ranges (gas, electric, and microwave ovens)
7.8
Refrigerators
5.1
Freezers (chest and upright)
1.7
Air conditioners
6.0
Furnaces
2.3
Water heaters
5.3
Television sets Source:
Appliance, March 1981.
16.4
Appliance Coatings
37. M I R A N D A
Table II.
887
Quantity of Paint Used per Appliance gal./unit
Appliance
Total (millions of gal.)
Room air conditioner
0.6
1.8
Gas dryer
0.5
0.385
Electric dryer
0.5
1.4
Automatic washer
0.4
1.96
Electric range
0.3
0.81
Refrigerator
0.6
3.48
Central air conditioner
0.2
0.42
Color TV
0.02
0.164 10.419
Total Source:
Appliance Manufacturer, 1976.
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A P P L I E D POLYMER SCIENCE
other d i r t on s t e e l . Since the energy c r i s i s of 1974, there has been a significant effort to reduce the temperature of the cleaning process. Some systems operate at temperatures as low as 40-45 °C (13, 14). One of the d i f f i c u l t i e s of lower wash temperatures i s that certain higher melting o i l s cannot be effectively removed in the washing cycle. The cleaning and phosphating process is very energy intensive and consumes up to 50% of the process energy in an appliance plant. On the other hand, the cleaning process i s the most c r i t i c a l step in the coating process and u n t i l recently has not received the proper attention by process engineers. Methods for determining surface cleanliness are too involved for online evaluation, so that simple tests such as water break are employed. Recently, Buser reported a rapid method based on surface tension (15). Evaporative rate analysis (16) has also been used to determine surface cleanliness as well as more sophisticated methods employing scanning auger spectroscopy (17, 18). Phosphating. Two types of phosphate treatment are used for treating steel, depending upon the degree of corrosion resistance required. Phosphate treatment imparts corrosion s t a b i l i t y , prevents rust creep, and imparts the necessary tooth to promote adhesion of primers. For lower corrosion requirements such as for a furnace, dryer, or freezer, iron phosphate is used. Iron phosphate provides a smooth dense f i l m and i s deposited at a film weight of about 70100 mg/ft . Being smooth, the amorphous iron phosphate coating is preferred for single-coat applications. When greater corrosion protection is required, especially for laundry products or air conditioners, zinc phosphate treatment i s preferred. Zinc phosphate is more coarse than iron and is used with a primer. With microcrystalline zinc phosphates, direct topcoating can be achieved. Film weights of 180-210 mg/ft are used to ensure adequate protection. Recently, some suppliers have converted zinc phosphate back to iron phosphate because of the reduced amount of sludge generated by iron phosphate. However, detergent resistance and corrosion resistance are lower for iron. Following the phosphate treatment, the substrate i s given a chrome rinse for passivating the surface. Because of environmental problems there has been a trend away from chrome to chrome-free rinses. In some cases where chrome rinses are used, Cr is reduced to Cr + before discharging to waste treatment. A typical system for cleaning and phosphating of refrigerators may involve the following steps: tank 1, cleaner at 160-170 °F (7075 ° C ) ; tank 2, rinse at 145 ° F (62-68 ° C ) ; tank 3, cleaner at 145155 ° F (62-68 ° C ) ; tank 4, rinse at 145 ° F (62 ° C ) ; tank 5, zinc phosphate at 150-160 °C (65-70 ° C ) ; tank 6, cold water rinse; tank 7, chromic acid rinse; tank 8, deionized water rinse (ambient). Flow Coating. The principal priming methods are dip or flow coating. For small parts and nonappearance parts, such as heat exchangers on freezers, dipping is the preferred method. For faster line speed, flow coating i s the preferred method (Figure 1). Before the introduction of electrocoating (a dip process), flow coating was the
MIRANDA
Appliance Coatings
Figure 1.
Flow coating dryer parts.
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preferred priming method. In many cases the prime coat is actually the only coat that the metal w i l l receive. Examples include toe plates, gussets, the supporting member, dryer drums, and bulkheads. Flow coaters permit fast line speed, use less coating than dip coats, but require a solvent flash off. The process is basically a pumping system that floods the parts with paint from spray nozzles. The moving ware then moves to a vapor chamber where bubbles can break and the f i l m can flow out. Flow coating has several disadvantages. The film tends to wedge with fat edges on the bottom of the part and heavy accumulations that can solvent pop in the oven. The presence of a vapor chamber for organic solvent presents an explosion or fire hazard. Finally, recessed edges are difficult to cover, and puddling can occur in crevices. The flammability problem was resolved by converting to waterborne flow coat primers. The first waterborne appliance flow coat primer was i n s t a l l e d on a dryer drum l i n e in 1972 (9). The primer was a water-soluble epoxy-modified a c r y l i c that had excellent hardness, abrasion resistance, and chemical resistance. Epoxy resins have provided excellent corrosion resistant primers for appliances. These can be made from epoxy resins esterified with dehydrated castor o i l f a t t y acids and cured with melamine. Excellent detergent resistance and corrosion resistance are obtained from this type of resin system. Another type of primer i s based on a s t y r e n e / a l l y l alcohol copolymer (Monsanto RJ-100). This polymer can be modified with fatty acids and cured with melamine resin (19). Reaction schemes for primers are shown as follows: EPOXY RESIN 0
0 / \ -0-CH -CH-CH
/ \ CH -CH-CH -0 2
2
2
CH
2
+ 2RC00H — )
3
0
OH
0
ti
ι
II
R-C-0-CH -CH-CH -0 2
-0-C-R
2
RJ100 RESIN -CH -CH-CH -CH 2
2
+ RCOOH — > I
-CHo-CH-CHoCHh ^
n
CH 0C R 2
Electrocoating. Major disadvantages of the flow coat system is the inability to coat recesses and edges and the presence of thin spots due to the solvent wash of deposited primer. This leads to lower
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detergent and s a l t spray resistance. A major improvement in appliance priming occurred with the introduction of electrocoating in 1962 on automatic washer cabinets. Electrocoating was introduced to the automotive industry by Brewer (20) to reduce corrosion. This highly automated system provides excellent coverage of edges and recesses and i s not affected by solvent wash since the deposited coating is not soluble in rinse water. Electrocoating vehicles were i n i t i a l l y prepared from natural o i l s and maleic anhydride (21). Those adducts can then be solubilized by ammonia or other bases. Electrocoating resin systems were i n i t i a l l y anodic because of ease of preparation and a v a i l a b i l i t y of acidic monomers. About 10 years after the introduction of anodic electrocoating, cathodic electrocoating was developed by Bosso and first successfully applied to air conditioner compressors (22, 23). The results obtained from electrocoating were so good that flow coating's domination as a priming process was replaced for laundry cabinets and small parts. The remaining flow coat systems were either phased out or converted to waterborne types. In many instances the flow coat or electrocoat primer serves as the f i n i s h coat as in dryer drums and bulkheads, a i r conditioner cabinets and trash compactor boxes, back plates, and other small parts. Electrocoating requires a number of controls. F i r s t , conduct i v i t y must be controlled since the solubilizing base (or acid with cathodic types) accumulates in the tank, thereby increasing conductivity and affecting deposition. This can be controlled by adding makeup primer deficient in amine (or acid) or by using a process c a l l e d u l t r a f i l t r a t i o n (24). As solids are depleted, a makeup charge consisting of resin, pigment, and additives is fed to the electrocoat tank. Rinsing of ware provides an economy as electrocoating solids can be returned to the tank, providing high utilization of paint. The pH must also be carefully controlled to avoid kick out of the resin. Cooling must also be provided to control bath temperature. U l t r a f i l t r a t i o n not only provides high u t i l i t y of coating material but also contributes to lower pollution. Baking temperatures for electrocoating vehicles vary depending upon the polymer used and the degree of corrosion resistance required. Anodic primers are baked from 375 to 450 ° F , while cathodic primers are baked as high as 4 5 0 ° F . Current research by manufacturers of coatings is directed toward lower bake temperatures without compromising physical properties. Typical properties of a cationic appliance primer are shown in Table III, and an electrocoat tank is shown in Figure 2. Topcoats. For some applications such as chest freezers or vacuum cleaners, alkyds are s t i l l employed. These evolved from o i l modified types to the o i l - f r e e polyesters that have good stain resistance, high gloss, and excellent f l e x i b i l i t y . But u n t i l recently, the thermosetting acrylic was the hallmark of appliance coatings. Acrylics exhibit excellent hardness, adhesion, chemical
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resistance, light fastness, and resistance to stain, detergents, and salt. In addition, acrylics provide high gloss for refrigerators but are formulated at lower gloss for laundry products. Prior to environmental regulations, the acrylic topcoats were applied at 34% volume solids and provided high build and good flow and leveling. With regulations, volume solids were pushed up to 62% volume solids to comply with EPA guidelines (25,26). Thermosetting a c r y l i c s can be produced from a variety of monomers in varying percentage compositions; the systems u t i l i z e a variety of cross-linking mechanisms (Table IV). A typical thermosetting a c r y l i c can be prepared from 15-80% styrene, 15-18% alkyl acrylate, and 5-10% acrylic acid (27). Acrylic acid provides the functionality for cross-linking with epoxy resins. Acrylic acid copolymers can be modified with 1,2-butylene oxide to provide hydroxyl sites for cross-linking (28). The copolymer can then be cross-linked with amino resins. Another approach is to prepolymerize an epoxy methacrylate or acrylate to provide an epoxy function that can be cross-linked by acids (29, 30). More elaborate cross-linking mechanisms include oxazolines (31), which can be formed in situ. The methylolated acrylamide type has been widely used in appliance finishes. These are cross-linked with melamine or epoxy modifiers and form hard, chemically resistant finishes. Detailed discussion of thermosetting a c r y l i c s can be found in References 2 and 8. Recently, topcoats based upon high-solids liquid polyesters have replaced thermosetting acrylics in response to environmental
Table III.
Properties of an Electrocoat Primer
Properties
Values
Coating Type pH Solids (w/w) Solvent
epoxy cationic/electrocoat 6.2-6.5 10-14% water, coupling solvents
Film Bake Color Adhesion Detergent, 1.5% at 165 F Impact, direct Thickness Chlorox, neat 72 h at 140 F Hardness, pencil
20 min at 410 F gray pass 1000 h 40 i n . l b 0.5 mils pass 4H
e
e
e
MIRANDA
Appliance Coatings
Figure 2.
Electrocoating of washer cabinets.
894
Table IV.
A P P L I E D P O L Y M E R SCIENCE
Types of Thermosetting Acrylics
Type Acid
Functional Monomer CH2=CH-COOH
Cross-linker Epoxy resin
Acrylic acid Hydroxyl
CH9=CH-C0
Melamine
\
2
0-CH2-CH2OH 2-Hydroxyethyl acrylate Epoxy
CH2=CH
Acid 0
/\ COOCH -CH-CH 2
2
Glycidyl acrylate Methylol
CH2=CH
Epoxy, melamine
CONHCH2OH Methylolated acrylamide Oxazoline
A
Melamine
-CHo-CH-
0
CHn
Ν (CH2OH)2
37.
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Appliance Coatings
895
constraints. Powder coatings are also making inroads into the a c r y l i c market. These polyesters are based on neopentyl g l y c o l , isophthalic acid, and trimethy l o i propane and are cross-linked with epoxy resins (32). They possess excellent gloss, stain resistance, f l e x i b i l i t y , and adhesion and have a pencil hardness of 2H or higher. Application of Topcoats. Appliance topcoats are applied by both hand and automatic electrostatic spray. For high-volume production on washer and dryer cabinets, a Ransburg No. 2 e l e c t r o s t a t i c disk typically is used. The paint is supplied to a rotating disk that is at a 90,000-110,000-V potential to the part being coated. As the paint is spun off the rotating disk (900-1800 rpm), i t is broken up into droplets and charged. This provides excellent coverage, good wraparound, and an efficiency of application of over 90% utilization (Figure 3). Another method for applying topcoats is the Ransburg No. 2 b e l l . This unit consists of a small rotating b e l l that breaks up and charges the paint as i t leaves the periphery of the b e l l (Figure 4). Older b e l l s had lower speeds, but newer high-speed Mini B e l l s operate at higher speeds and permit the use of newer high-solid coatings. B e l l s arranged on stands are an effective means for coating long parts such as refrigerator cabinets or doors. Unlike the disk, which is mounted vertically, the b e l l can operate from a number of positions and is more versatile. Hand guns are used to touch up areas that are not adequately covered by the disk or bells and are also used to reinforce c r i t i c a l areas where more coating is desired. Film thicknesses vary with the product. For primers, 0.4-0.5 mil i s t y p i c a l while topcoats are 0.7-1.5 mils applied over a primer. A t o t a l system of 1.2-2.5 mils i s suitable for laundry products. On refrigeration products either a wet-on-wet topcoat or a primer and a thinner topcoat system i s used, depending upon the condition of the metal. If there are scuff marks on the metal, the latter system is employed. Porcelain coatings are much thicker, for example, up to 10 mils. Appliance Coating Specifications Table V l i s t s some specifications for a laundry appliance finish, which vary according to model as well as type of laundry appliance. For example, an automatic washer has a more severe detergent resistance requirement than a dryer. On the other hand, a refrigerator has no detergent requirement but must be stain resistant and chemical resistant to a variety of cleaners or foods that this product encounters. For washers and dishwasher tubs, porcelain is required that has an entirely different specification. Some manufacturers use vinyl plastisols for dishwasher liners with good success. A l i s t of specifications for refrigerators is shown in Table VI. Note the emphasis on s t a i n r e s i s t a n c e due to the exposure requirements of the kitchen.
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Figure 3.
Electrostatic disk.
MIRANDA
Appliance Coatings
igure 4.
Electrostatic bell coating refrigerator doors.
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Table V.
A P P L I E D P O L Y M E R SCIENCE
Laundry Specifications:
Washer
Property
Value
Pencil hardness
2H
Adhesion, 1/16-in. Crosshatch
No loss
Stain resistance, 24 h
None
Detergent immersion
250 h
Abrasion, taber, CS10, 1000g
15 mg/100 cycles
Humidity, 100%
1000 h
Salt spray, 5%
500 h
Flexibility
1 i n . from small diameter of mandrel
Heat, 200 h at 200 °F
1.5 RD units
Impact, direct
40 i n . lb
Grease, 50/50 oleic cottonseed o i l
168 h
Gloss, 20°
50-75
37.
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Table VI.
Appliance Coatings
899
Refrigerator Finish Specification
Property Pencil hardness minimum Abrasion, taber, CS10, 1000 g Intercoat resistance (Hoffman scratch) Flexibility, 180° bend, 1/4-in. mandrel Impact, direct/reverse Adhesion C r o s s h a t c h , 1/32 i n . Salt spray, 5% Warm water soak, 120 °F Humidity, 100% Gloss, 60° minimum (%) Mold resistance Weatherometer Grease, 50/50 oleic acid/cottonseed o i l grease fume, 60 min Stain resistance (24-h exposure) Anthraquinone Violet R Butter Cigarette burn Cigarette smoke Citric acid, 1% Ethyl alcohol Grease pencil Lipstick Mustard Sodium hydroxide, 0.5% Tape Vinyl gasket
Value 2H 60-100 mg 2500 g No flaking 20/10 Pass 500 h 750 h 1000 h 85 No growth 300 h 5 weeks no change in gloss or hardness No color change None None Slight None None None None None None None None None
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Recent Developments Beginning in 1970 the appliance finish was subjected to a complete revolution brought about by government r e g u l a t i o n s of the Environmental Protection Agency (EPA) and by the energy c r i s i s of 1974. The first attempt by EPA was to convert appliance coatings to nonexempt solvents based upon Rule 66 as practiced in California. However, this was not a reasonable approach since hydrocarbon emissions were s t i l l present. Nevertheless, coatings suppliers reformulated to exempt solvents at a great cost. The advent of EPA did bring about a number of significant changes in appliance coatings. These are conversion of flow coating from solventborne to waterborne systems, replacement of flow coating with electrocoating, introduction of cationic electrocoating, introduction of high-solids coatings, introduction of powder coating, development of low-temperature baking systems, and development of porcelain replacement coatings. Flow Coat Conversion. Solventborne flow coaters are a major source of hydrocarbon emissions. This source became an early target for a replacement system. An alternative to reduce emission was the gas incinerator, but high c a p i t a l costs and gas consumption precluded i t s use. The f i r s t successful waterborne flow coat for appliances was developed by the Glidden Division of SCM and applied to dryer drums and bulkheads. Emissions were reduced by 80%, and f i r e hazards associated with the solvent system were reduced significantly. Electrocoating. Electrocoating is a significant improvement over flow coating. Recessed edges, corners, and holes are uniformly coated, and the system is highly automated. Anodic electrocoating i s used as a primer for washer cabinets, small parts of dryer cabinets, and trash compactor components. A major innovation in electrocoating was the successful i n t r o d u c t i o n of c a t i o n i c electrocoating. The f i r s t application was for a i r conditioner compressors (23). This was followed by a i r conditioner cabinets replacing a two-coat system consisting of a flow coat primer and an acrylic topcoat. Corrosion resistance of 1000 h was achieved with a cationic epoxy electrocoating system. Cationic electrocoating i s now used on dryer drums, washer cabinets, and areas where superior corrosion resistance is required. Although cationic electrocoating i s somewhat more expensive (10% higher than anodic), i t s advantages for certain applications justifies the higher cost. After the successful introduction to appliances, the automotive industry began a major conversion to cationic electrocoating. Advantages of the system are good chemical resistance, excellent s a l t spray resistance, hardness of 4H+, f l e x i b i l i t y , and low dissolution of phosphate at the anode (33). Some problems were encountered with u l t r a f i l t r a t i o n , but they have since been resolved. Cationic epoxy resin based systems can be prepared by reacting quaternary amine salts with epoxy resins to introduce the cationic functions. The polymers can then be cross-linked with blocked isocyanates at reasonable baking temperatures (34, 35).
37.
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Appliance Coatings
R3
901 R3 •> R 1 -N + -R 2
X"
X"
CH2 CH-OH Ζ
A survey of polymer compositions for cationic electrocoating was prepared by Kordomenos and Nordstrom (36). Recently, cationic acrylic white enamels have been developed that have high gloss, hardness and abrasion resistance and provide a means for developing single coat applications. A recent application for this system is on freezer baskets. For primers not requiring high corrosion resistance, anodic electrocoating w i l l s t i l l be a viable choice. High Solids. In an effort to reduce emissions, higher s o l i d s coatings were developed for the appliance industry. EPA regulations require no more than 2.8 lb of s o l v e n t / g a l . of paint (0.34 kg of solvent/L of paint) by 1982. For the conventional thermosetting a c r y l i c , raising volume s o l i d s increases the viscosity such that conventional application equipment is not capable of applying the coating. Two major developments occurred that overcame the problem. Coating research laboratories developed high-solids low-viscosity coatings based on polyester intermediates or oligomers. Those lower molecular weight polymers have higher functionality. In addition, new melamines were developed to cross-link the more reactive systems (37). Manufacturers have also recognized the need to develop appli cation equipment to handle higher s o l i d s coatings. As a r e s u l t , high-speed turbines have been developed by a number of firms. Depending upon the system, coatings having up to 80% solids can be applied with or without heaters (38). Turbines with speeds up to 60,000 rpm were developed in contrast to conventional disks operating at 1500 rpm. At 20,000 rpm these turbines can effectively atomize coatings having Ford No. 4 viscosities of 100 s. Recently, oligomeric acrylics cross-linked with isocyanates or epoxy resins have become a v a i l a b l e for appliance finishes (39). High-solids coatings should be a major factor in the future of appliance finishes in that they permit the use of existing paint lines with l i t t l e modification. At 62% volume solids, minibells can be used. Because of the bubble concept, an appliance coating employing a waterborne flow coat or electrocoat system can be used to offset a topcoat with slightly lower volume solids that can be applied on lower speed disks.
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Powder Coatings. Powder coatings have made inroads into the appliance market (40). Their p r i n c i p a l value i s l i t t l e or no solvent emission and high u t i l i z a t i o n of coating. Fluidized bed v i n y l systems are used on wire goods such as dishwasher racks. Electrostatic powder spray using guns or disks are used on freezer and refrigerator liners, dryer drums, doors, and range cabinets. An interesting application of powder coatings is in replacement of plating. Refrigerator shelves are coated with an epoxy powder replacing zinc plating, thereby eliminating wastewater disposal and providing a quality f i n i s h . On chest freezer l i n e r s , powder coatings have replaced porcelain, thus greatly reducing the high energy consumption of porcelain furnaces. Acrylic, epoxy, and polyester powders are a l l used in appliance finishes. Properties of a t y p i c a l a c r y l i c as shown in Table VII (41, 42). For exterior exposure, the a c r y l i c or polyester i s preferred. Low Energy Cure Coatings. With strong emphasis on energy conservation, appliance manufacturers sought to lower the energy required in the baking or curing process. One approach to energy-independent coatings is the urethane coating. Urethanes can be formulated to a v a r i e t y of curing schedules, e s p e c i a l l y the blocked and multicomponent types. Appliance finishes have been formulated and tested by using acrylic or polyester modified urethanes that can cure to a suitable hardness in 10 min at 150 °F and cure to an ultimate hardness of 2H in 168 h (43). The major barriers to the application of urethanes in appliances i s cost and perceived t o x i c i t y . However, the outstanding performance of urethanes may yet overcome the problems cited above. Another approach to low energy cure coatings i s the a l i p h a t i c epoxy systems (44) and the low fusing powders that cure at 250-275 °F. Self-cross-linking emulsions suggest another way of achieving low energy cure coatings. Coating To Replace Porcelain. Porcelain is s t i l l widely used in appliances and is accepted as a quality finish. Porcelain frits are generally ground on site in large b a l l m i l l s and let down to spray viscosity. Porcelain also requires zero carbon steel and a pickling process for proper adhesion. Recent developments in porcelain include powdered f r i t and the use of cold rolled steel instead of enameling iron. Porcelain steel requires a high fusion temperature, 1500 °F. This has led appliance manufacturers to seek alternatives to porcelain. The first successful application was on a range door, replacing porcelain with an acrylic system. Because of federal standards, the outer door temperature must be below 160 ° F , which permits the use of organic coatings. Chest freezer and refrigerator l i n e r s have been successfully coated with powders or precoated metals to eliminate porcelain with good success. The improved impact resistance results in significant savings from shipping damage.
37. M I R A N D A
Appliance Coatings
Table VII (41).
903
Properties of an Acrylic Powder
Properties
Values
Powder properties Specific gravity
1.2-1.8
Storage stability
1 year at 80 °F
Particle size
20-40 ym average
Application data Voltage
50-100 kV
Typical cure
15 min at 375 °F metal temperature
Minimum cure
330 °F metal temperature
Overbake color stability
100%—time at temperature
Film thickness range
0.7-3.0
mils
Film properties Gardner impact
20-100 i n . lb
Pencil hardness
2H-3H
Flexibility
Pass 1/8-in. mandrel
Salt spray
1/16-in. creepage at 1000 h
Adhesion
No failure with 1/16-in. squares
Weathering
Minimum change 500 h—Atlas Weatherometer
Note:
A l l tests were on the B-1000 test panel at 1.5 mils.
904
APPLIED POLYMER SCIENCE
For laundry applications, organic coatings are replacing porcelain on tops and lids and can conceivably be used on tubs and baskets since the technical feasibility of organic coatings in these applications has been demonstrated (10). Future Future coating trends for appliance coatings will be high-solids, waterborne including electrodeposition, and powder coating in that order. High-solids coatings permit the use of existing equipment whereas powder requires new capital equipment (45). Electrocoating and waterborne flow coats will be used for priming and in some cases as the topcoat. Cationic electrocoating will find favor in highcorrosion environments and for porcelain replacement. Porcelain usage w i l l continue to be eroded in favor of high-performance coatings. Coil coatings w i l l become an increasing factor in appliances. The use of coil reduces pollution, cleaning, and phosphating, and bake ovens. Higher cost of scrap and raw edges are major concerns. However, coil coatings are being used successfully on refrigerator liners, range shells, and dehumidifier cabinets. For large cabinets such as the refrigerator, special joining techniques are required to reduce damage to the finish since weld burns damage the finish. An interesting approach to coil coating is glass transition forming in which the coated coil is heated above its glass temperature for forming. This prevents cracking and permits the use of a high-hardness coating with good scuff resistance. Dehumidifier covers are prepared in this manner (46). Robots should play an increasing role in the coating of appliances such as microwave oven cavities and roundware where bounceback is a problem. Greater emphasis w i l l be placed upon the quality of metal surfaces. Methods for determining the quality of cleaning and phosphating on line w i l l be implemented. A good example is the development of an infrared analysis technique for zinc phosphate by Cheever (47), which permits rapid determination of coating weight in a nondestructive manner. An instrument for this purpose was developed by Foxboro (48). Literature Cited 1. Franklin, J. F. "Full-line Development, Related Mergers and Competition in the Major Appliance Industry During the 1980s"; University Microfilms: Ann Arbor, Mich., 1963; 64-6679. 2. Shur, E. G. "Treatise On Coatings"; Myers, R. R.; Long, J. S., Eds.; Marcel Dekker: New York, 1975; Vol. 4, Chap. 2. 3. Kienle, R. H. Ind. Eng. Chem. 1949, 41, 726. 4. Strain, D. E. U.S. Patent 2 173 005, Sept. 12, 1939, to Ε. I. du Pont de Nemours & Co. 5. Christenson, R. M. U.S. Patent 3 037 963, June 5, 1962, to PPG. 6. Sekmakas, K. U.S. Patent 3 163 615, 1964, to DeSoto. 7. Sekmakas, K.; Ansel, R.E.; Drunga, K. U.S. Patent 3 163 623, 1964, to DeSoto. 8. Brown, W. H.; Miranda, T. J. Off. Dig. 1964, 36(475), 92.
37. MIRANDA
Appliance Coatings
905
9. Miranda, T. J. "Water Soluble Polymers"; Bikales, N. M., Ed.; Plenum: New York, 1973; p. 187. 10. Miranda, T. J. J. Coat. Technol. 1977, 49, 66. 11. Appliance 1981, March, 22. 12. Appliance Manufact. 1976, Jan. 92. 13. Varga, C. Ind. Finish. 1974, 50, 62. 14. Obrzut, J. J. Iron Age 1981, June 1, 1981, 43. 15. Buser, K. R. Org. Coat. Plast. Chem. Preprints 1980, 43, 498. 16. Anderson, J.; Root, D.; Greene, G. J. Paint Technol. 1968, 40(523), 320. 17. Wojtkowiak, J.; Bender, H. S. Org. Coat. Plast. Chem. Preprints 1980, 43, 513. 18. Zurilla, R. W.; Hospadaruk, V. SAE Technical Paper 780187, March 1978. 19. Monsanto Technical Bulletin 5035. 20. Brewer, G. E. F. J. Paint Technol. 1973, 45(587), 36. 21. Miranda, T. J. Off. Dig. 1965, 37(489), 62. 22. Wismer, M.; Bosso, J. F. Chem. Eng. 1971, 78(13), 114. 23. Miranda, T. J. Org. Coat. Plast. Chem. 1981, 45, 109. 24. LeBras, L. R.; Christenson, R. M. J. Paint Technol. 1972, 44(566), 63. 25. Obrzut, J. J. Iron Age, 1981, Sept. 16, 51. 26. Schrantz, J. Ind. Finish. 1978, June, 24. 27. Segall, C. H.; Cameron, J. L. U.S. Patent 2 798 861, 1956, to Canadian Industries, Ltd. 28. Vasta, J. Belgian Patent 634 310, 1963, to Du Pont. 29. Simms, J. A. J. Appl. Polym. Sci. 1961, 5, 58. 30. Ravve, Α.; Khamis, J. T. U.S. Patent 3 306 883, 1967, to Continental Can. 31. Miranda, T. J. J. Paint Technol. 1967, 39, 40. 32. Eastman Chemica l Products Bulletin N-277, August 1981. 33. Anderson, D. G.; Murphy, E. J.; Tucci, J. J. Coat. Technol. 1978, 50(646), 38. 34. Jerabek, R. D. U.S. Patent 3 799 854, 1974, to PPG Industries, Inc. 35. Bosso, J. F.; Sturni, L. C. U.S. Patent 4 101 486, 1978, to PPG Industries, Inc. 36. Kordomenos, P.; Nordstrom, J. D. Org. Coat. Plast. Chem. 1980, 43, 154. 37. Blank, W. J. Coat. Technol. 1982, 54(687), 26. 38. Tholome, R.; Sorcinelli, G. Ind. Finish. 1977, 53(11), 30. 39. Brushwell, W. Am. Paint Coat. J. 1981, Nov. 23, 47. 40. Gribble, P. R. N.P.C.A. Chemical Coating Conference, Cincinnati, Ohio, May 10 and 11, 1978, Powder Coating Session, p. 47-61. 41. Glidden Chemical Coatings and Resins. Pulvalure 154. 42. Harris, S. T. "The Technology of Powder Coating"; Portcullis: London, 1976. 43. Consdorf, A. P. Appliance Manufact. 1977, May, 64. 44. Bauer, R. S. CHEMTECH 1980, 10(11), 692. 45. Petrovich, P. SME Technical Paper FC79-701, 1979. 46. "Glass Transition Forming Saves Painting Costs"; Precis. Met. 1978, Nov., 69. 47. Cheever, G. D. J. Coat. Technol. 1978, 50(640), 78. 48. Infracoat 450 Analyzer, Foxboro Analytical, South Norwalk, Conn.
