Handbook of Materials for Product Design Part 12 ppt

For this reason surfaces mustfirst be cleaned to remove oily soils, corrosion products, and particu-lates, and then pretreated before applying any coatings and finishes.After coatings ar

Coatings and Finishes 9.3

primers Powder coating material and equipment suppliers worked verishly to solve the problems associated with automotive topcoats.Other powder coating applications saw rapid growth Radiation curablepigmented coatings for three-dimensional products were developed

fe-In the 1990s, the decade of compliance, resin and coating suppliersdeveloped compliance coatings—electrocoating, high-solids, powder,radiation curable, and waterborne Equipment suppliers developed de-vices to apply and cure these new coatings These developments were

in response to the 1990 amendments to the Clean Air Act of 1970 Theamendments established a national permit program that made the lawmore enforceable, ensuring better compliance and calling for nation-wide regulation of VOC emissions from all organic finishing opera-tions The amendments also established Control Technique Guidelines

to allow state and local governments to develop Attainment Rules.Electrocoating was the process of choice for priming many industrialand consumer products Powder coatings were used in a host of appli-cations where durability was essential UV curable coatings were ap-plied to three-dimensional objects Equipment suppliers developedmore efficient application equipment

In the 2000s, the beginning of the green millennium, coatings andequipment suppliers’ investments in research and development willpay dividends Improvements in coating materials and applicationequipment have enabled end users to comply with air quality regula-tions Primers are applied by electrocoating One-coat finishes are re-placing two coats in many cases High-solids and waterborne liquidcoatings are replacing conventional solvent-thinned coatings Powdercoatings usage has increased dramatically Radiation-cured coatingsare finding more applications Coatings and solvent usage, as well asapplication costs, are being reduced Air quality standards are beingmet Coatings and equipment suppliers, as well as end users, are rec-ognizing the cost savings bonus associated with attainment Compli-ance coatings applied by more efficient painting methods will reducecoatings and solvent usage, thereby effecting cost savings

Since coatings today are considered to be engineering materials,their performance characteristics not only must match service re-quirements, they must also meet governmental regulations and pro-duction cost considerations In the past, the selection of a coatingdepended mainly on the service requirements and application method.Now, more than ever before, worker safety, environmental impact, andeconomics must be considered For this reason, compliance coatings-electrocoating, high-solids, powders, radiation-cured, and waterbornecoatings are the most sensible choices

Coatings are applied to most industrial products by spraying ure 9.1 shows a typical industrial spray booth In 1890, Joseph Binks09Izzo Page 3 Wednesday, May 23, 2001 10:27 AM

Fig-9.4 Chapter 9

invented the cold-water paint-spraying machine, the first airlesssprayer, which was used to apply whitewash to barns and other build-ing interiors In 1924, Thomas DeVilbiss used a modified medical at-omizer, the first air-atomizing sprayer, to apply a nitrocelluloselacquer on the Oakland automobile Since then, these tools have re-mained virtually unchanged and, until the enactment of the air qual-ity standards, they were used to apply coatings at 25 to 50% volumesolids at transfer efficiencies of 30 to 50% Using this equipment, theremainder of the nonvolatile material, the overspray, coated the floorand walls of spray booths and became hazardous or nonhazardouswaste, while the solvents—the VOCs—evaporated from the coatingduring application and cure to become air pollutants Today, finishesare applied by highly transfer-efficient application equipment Thechoice of application equipment must be optimized

Even the best coatings will not perform their function if they are notapplied on properly prepared substrates For this reason surfaces mustfirst be cleaned to remove oily soils, corrosion products, and particu-lates, and then pretreated before applying any coatings and finishes.After coatings are applied, they form films and cure Curing mecha-nisms can be as simple as solvent evaporation or as complicated asfree-radical polymerization Basically, coatings can be classified as

Figure 9.1 A typical industrial spray booth used for applying industrial coatings

(Cour-tesy of George Koch Sons, LLC.)

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Coatings and Finishes 9.5

The curing method and times are important in coating selection, cause they must be considered for optimizing equipment and produc-tion schedules choices

be-The purposes of this chapter are threefold: (1) to aid the designer inoptimizing the selection of coating materials and application equip-ment; (2) to acquaint the reader with surface preparation, coating ma-terials, application equipment, and curing methods; and (3) to stressthe importance of environmental compliance in coating operations

In the past, changes in coating materials and coating application lineswere discussed only when lower prices, novel products, new coatinglines, or new plants were considered Today, with rising materialcosts, rising energy costs, and more restrictive governmental regula-tions, they are the subject of frequent discussions During these dis-cussions, both in-house and with suppliers, choices of coatingmaterials and processing equipment are optimized Coating materialand solvent costs, which are tied to the price of crude oil, have risensince the 1970s, as has the cost of natural gas, which is the most fre-quently used fuel for coating bake ovens The EPA has imposed re-strictive air quality standards The Occupational Safety and HealthAct (OSHA) and the Toxic Substances Control Act (TSCA) regulatethe environment in the workplace and limit workers’ contact withhazardous materials These factors have increased coating costs andthe awareness of product finishers To meet the challenge, they mustinvestigate and use alternative coating materials and processes forcompliance and cost effectiveness

Initial attempts to control air pollution in the late 1940s resulted insmoke-control laws to reduce airborne particulates The increased use

of the automobile and industrial expansion during that period caused

a condition called photochemical smog (smog created by the reaction ofchemicals exposed to sunlight in the atmosphere) in major citiesthroughout the United States Los Angeles County officials recognizedthat automobile exhaust and VOC emissions were major sources ofsmog, and they enacted an air pollution regulation called Rule 66.Rule 66 forbade the use of specific solvents that produced photochemi-cal smog and published a list of exempt solvents for use in coatings.Further study by the EPA has shown that, if given enough time, eventhe Rule 66 exempt solvents will produce photochemical smog in theatmosphere

The Clean Air Act of 1970, and its 1990 amendments, formulated

by the EPA, established national air quality standards that regulate09Izzo Page 5 Wednesday, May 23, 2001 10:27 AM

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the amount of solvents emitted The EPA divided the 50 states into

250 air quality regions, each of which is responsible for the mentation of the national air quality standards It is important torecognize that many of the local standards are more stringent thanthe national ones For this reason, specific coatings that comply withthe air quality standards of one district may not comply with an-other’s Waterborne, high-solids, powder, electrophoretic, and radia-tion-cured coatings will comply The use of precoated metal caneliminate all the compliance problems

imple-Not only because the EPA mandates the reduction of VOC sions, but also because of economic advantages, spray painting, which

emis-is the most used application method, must be done more efficiently.The increased efficiency will reduce the amount of expensive coatingsand solvents used, thereby reducing production costs

The most important step in any coating operation is surface tion, which includes cleaning and pretreatment For coatings to ad-here, surfaces must be free from oily soils, corrosion products, andloose particulates New wood surfaces are often coated without clean-ing Old wood and coated wood must be cleaned to remove oily soilsand loose, flaky coatings Plastics are cleaned by using solvents andchemicals to remove mold release Metals are cleaned by media blast-ing, sanding, brushing, and by solvents or aqueous chemicals Thechoice of a cleaning method depends on the substrate and the size andshape of the object

prepara-After cleaning, pretreatments are applied to enhance coating sion and, in the case of metals, corrosion resistance Some wood sur-faces require no pretreatment, while others require priming of knotsand filling of nail holes Cementitious and masonry substrates are pre-treated using acids to remove loosely adhering contaminants and topassivate the surfaces Metals, still the most common industrial sub-strates, are generally pretreated using phosphates, chromates, andoxides to passivate their surfaces and provide corrosion resistance.Plastics, second only to steel, are gaining rapidly in use as industrialsubstrates Some are paintable after cleaning to remove mold releaseand other contaminants, while others require priming, physical treat-ments, or chemical etching to ensure coating adhesion Since most ofthe industrial substrates coated are metals and plastics, their clean-ing and pretreatment are described in the next sections Because oftheir complexities, detailed descriptions of cleaning and pretreatmentprocesses are beyond the scope of this chapter Enough detail will begiven to allow the reader to make a choice As with the choice of a09Izzo Page 6 Wednesday, May 23, 2001 10:27 AM

adhe-Coatings and Finishes 9.7

cleaning method, the choice of a pretreatment method depends on thecomposition, size, and shape of the product

9.3.1 Metal Surface Cleaning

Oily soils must be removed before any other surface preparation is tempted Otherwise these soils may be spread over the surface Thesesoils can also contaminate abrasive cleaning media and tools Oilysoils can be removed faster using liquid cleaners that impinge on thesurface or in agitated immersion baths It is often necessary to heatliquid cleaners to facilitate soil removal

at-9.3.1.1 Abrasive cleaning. After removal of the oily soils, surfaces areabrasive cleaned to remove rust and corrosion by media blasting, hand

or power sanding, and hand or power brushing Media blasting sists of propelling materials, such as sand, metallic shot, nut shells,plastic pellets, and dry ice crystals, by gases under pressure, so thatthey impinge on the surfaces to be cleaned High-pressure water-jetcleaning is similar to media blasting

con-9.3.1.2 Alkaline cleaning. To remove oily soils, aqueous solutions of kaline phosphates, borates, and hydroxides are applied to metals byimmersion or spray After cleaning, the surfaces are rinsed with clearwater to remove the alkali These materials are not effective for re-moving rust and corrosion

al-9.3.1.3 Detergent cleaning. Aqueous solutions of detergents are used

to remove oily soils in much the same way as alkaline cleaners Thenthey are rinsed with cold water to flush away the soils

9.3.1.4 Emulsion cleaning. Heavy oily soils and greases are removed

by aqueous emulsions of organic solvents such as mineral spirits andkerosene After the emulsified solvent has dissolved the oily soils, theyare flushed away using a hot-water rinse Any remaining oily residuemust be removed using clean solvent, alkaline, or detergent cleaners

9.3.1.5 Solvent cleaning. Immersion, hand wiping, and spraying ing organic solvents are effective methods for removing oily soils.Since these soils will contaminate solvents and wipers, it is important

us-to change them frequently Otherwise, oily residues will remain onsubstrates Safe handling practices must be followed because of thehazardous nature of most organic solvents

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9.3.1.6 Manual spray cleaning. For large products, detergent and kaline cleaners applied using steam cleaners are a well-known de-greasing method In addition to oily soils, the impingement of thesteam and the action of the chemicals will dissolve and flush awayheavy greases and waxes Hot-water spray cleaning using chemicals isnearly as effective as steam cleaning

al-9.3.1.7 Vapor degreasing. Vapor degreasing has been a very popularcleaning method for removing oily soils Boiling solvent condenses onthe cool surface of the product and flushes away oily soils, but does notremove particulates Since this process uses chlorinated solvents,which are under regulatory scrutiny by government agencies, its pop-ularity is declining However closed-loop systems are still available

9.3.2 Metal Surface Pretreatment

Cleaning metals will remove oily soils but generally will not removerust and corrosion from substrates to be coated Abrasive cleaning willremove corrosion products, and for this reason it is also considered apretreatment, because the impingement of blasting media and the ac-tion of abrasive pads and brushes roughen the substrate and thereforeenhance adhesion The other pretreatments use aqueous chemical so-lutions, which are applied by immersion or spray techniques Pre-treatments for metallic substrates used on industrial products arediscussed in this section Because they provide corrosion protection toferrous and nonferrous metals, chromates are used in pretreatmentstages and as conversion coatings They are being replaced by non-chromate chemicals

9.3.2.1 Aluminum. Aluminum is cleaned by solvents and chemical lutions to remove oily soils and corrosion products Cleaned aluminum

so-is pretreated using chromate conversion coating and anodizing phoric acid-activated vinyl wash primers, which are also consideredpretreatments, must be applied directly to metal and not over otherpretreatments

Phos-9.3.2.2 Copper. Copper is cleaned by solvents and chemicals andthen abraded to remove corrosion Bright dipping in acids will also re-move corrosion Cleaned surfaces are often pretreated using chro-mates and vinyl wash primers

9.3.2.3 Galvanized steel. Galvanized steel must be cleaned to removethe oil or wax that is applied at the mill to prevent white corrosion Af-09Izzo Page 8 Wednesday, May 23, 2001 10:27 AM

Coatings and Finishes 9.9

ter cleaning, the surfaces are pretreated using chromates and phates Vinyl wash primer pretreatments can also be applied ongalvanized steel surfaces having no other pretreatments

phos-9.3.2.4 Steel. Steel surfaces are cleaned to remove oily soils and, ifnecessary, pickled in acid to remove rust Clean steel is generally pre-treated with phosphates to provide corrosion resistance Other pre-treatments for steel are chromates and wash primers

9.3.2.5 Stainless steel. Owing to its corrosion resistance, stainlesssteel usually is not coated Otherwise, the substrate must be cleaned

to remove oily soils and then abraded to roughen the surface Washprimers will enhance adhesion

9.3.2.6 Titanium. Cleaned titanium is pretreated like stainless steel

9.3.2.7 Zinc and cadmium. Zinc and cadmium substrates are treated like galvanized steel

pre-9.3.3 Plastic Surface Cleaning

9.3.3.1 Alkaline cleaning. Aqueous solutions of alkaline phosphates,borates, and hydroxides are applied to plastics by immersion or spray

to remove oily soils and mold release agents After cleaning, the faces are rinsed with clear water to remove the alkali

sur-9.3.3.2 Detergent cleaning. Aqueous solutions of detergents are used

to remove oily soils and mold release agents in much the same way aswith alkaline cleaners Then they are rinsed with cold water to flushaway the soils

9.3.3.3 Emulsion cleaning. Heavy, oily soils, greases, and mold releaseagents are removed by aqueous emulsions of organic solvents such asmineral spirits and kerosene After the emulsified solvent has dis-solved the oily soils, they are flushed away using a hot-water rinse.The remaining oily residue must be removed using clean solvent, alka-line, or detergent cleaners

9.3.3.4 Solvent cleaning. Immersion, hand wiping, and spraying, ing organic solvents, are effective methods for removing oily soils and09Izzo Page 9 Wednesday, May 23, 2001 10:27 AM

us-9.10 Chapter 9

mold release agents Since these soils will contaminate solvents andwipers, it is important to change them frequently Otherwise, oily res-idues will remain on substrates Compatibility of cleaning solventswith the plastic substrates is extremely important Solvents that af-fect plastics are shown in Table 9.1 Suppliers of mold release agentsare the best source for information on solvents that will remove theirmaterials Safe handling practices must be followed because of thehazardous nature of most organic solvents

9.3.3.5 Manual spray cleaning. Detergent and alkaline cleaners plied using steam and hot-water spray cleaners are a well known de-greasing method The method can also be used for removing moldrelease agents The impingement of the steam and hot water and theaction of the chemicals will dissolve and flush away the contaminants.Manual spray cleaning is used for large products

ap-9.3.4 Plastic Surface Pretreatment

Cleaning will remove oily soils and mold release agents, but additionalpretreatment may be needed on certain plastic surfaces to ensure ad-hesion Many of the plastic substrates are chemically inert and will notaccept coatings because of their poor wettability Depending on their09Izzo Page 10 Wednesday, May 23, 2001 10:27 AM

Coatings and Finishes 9.11

chemical composition, they will require mechanical, chemical, andphysical pretreatment or priming to enhance coating adhesion Sincemechanical pretreatment consists of abrasion, its effect on the sub-strate must be considered Chemical pretreatments involve corrosivematerials that etch the substrates and can be hazardous Therefore,handling and disposal must be considered Physical pretreatmentsconsist of plasma, corona discharge, and flame impingement Processcontrol must be considered

9.3.4.1 Abrasive cleaning. After removal of the oily soils, surfaces areabrasive pretreated to roughen the substrate by media blasting, hand

or power sanding, and hand or power brushing Media blasting sists of propelling materials such as sand, metallic shot, nut shells,plastic pellets, and dry ice crystals by gases under pressure so thatthey impinge on the surfaces to be pretreated

con-9.3.4.2 Chemical etching. Chemical pretreatments use solutions ofcorrosive chemicals, which are applied by immersion or spray tech-niques, to etch the substrate

9.3.4.3 Corona discharge. During corona discharge pretreatment, theplastic is bombarded by gases directed toward its surface

9.3.4.4 Flame treating. During the flame pretreatment, an open flameimpinging on the surface of the plastic product causes alterations inthe surface chemistry

9.3.4.5 Plasma pretreatment. Low-pressure plasma pretreatment isconducted in a chamber, while atmospheric plasma pretreatment isdone in the open In both cases, ablation alters the surface chemistryand causes changes in surface roughness

9.3.4.6 Laser pretreatment. Laser pretreatment ablates the plasticsubstrate causing increased surface roughness and changes in the sur-face chemistry

9.3.5 Priming

Priming involves the application of a coating on the surface of theplastic product to promote adhesion or to prevent attack by the sol-09Izzo Page 11 Wednesday, May 23, 2001 10:27 AM

9.12 Chapter 9

vents in a subsequent protective or decorative coating In some cases,priming can be done after cleaning In others, it must be done afterpretreatment

To aid in their selection, coatings will be classified by their use in ish systems, physical state, and resin type Coatings are also classified

fin-by their use as electrical insulation It is not the intent of this chapter

to instruct the reader in the chemistry of organic coating but rather toaid in selection of coatings for specific applications Therefore, thecoating resin’s raw material feed stock and polymerization reactionswill not be discussed On the other hand, generic resin types, curing,physical states, and application methods are discussed

9.4.1 Selection by Finish Systems

Finish systems can be one-coat or multicoat schemes that use primers,intermediate coats, and topcoats Primers provide adhesion, corrosionprotection, passivation, and solvent resistance to substrates Topcoatsprovide weather, chemical, and physical resistance and generally de-termine the performance characteristics of finish systems Perfor-mance properties for coatings, formulated with the most commonlyused resins, are shown in Table 9.2

In coating selection, intended service conditions must be considered

To illustrate this point, consider the differences between service tions for toy boats and for battleships Table 9.3 shows the use of in-dustrial finish systems in various service conditions

condi-9.4.2 Selection by Physical State

A resin’s physical state can help determine the application equipmentrequired Solid materials can be applied by powder coating methods.09Izzo Page 12 Wednesday, May 23, 2001 10:27 AM

Coatings and Finishes 9.13

Table 9.4 lists resins applied as powder coatings Liquids can be plied by most of the other methods, which are discussed later Many ofthe coating resins exist in several physical states Table 9.5 lists thephysical states of common coating resins

ap-9.4.3 Selection by Resin Type

Since resin type determines the performance properties of a coating,

it is used most often Table 9.6 shows the physical, environmental,and film-forming characteristics of coatings by polymer (resin) type It09Izzo Page 13 Wednesday, May 23, 2001 10:27 AM

9.14

9.15

9.17

9.18 Chapter 9

is important to realize that in, selecting coatings, tables of mance properties of generic resins must be used only as guides, be-cause coatings of one generic type, such as acrylic, epoxy, orpolyurethane, are often modified using one or more of the other ge-neric types Notable examples are acrylic alkyds, acrylic urethanes,acrylic melamines, epoxy esters, epoxy polyamides, silicone alkyds,silicone epoxies, silicone polyesters, vinyl acrylics, and vinyl alkyds.While predicting specific coating performance properties of unmodi-fied resins is simple, predicting the properties of modified resins is dif-ficult, if not impossible Parameters causing these difficulties areresin modification percentages and modifying methods such as simpleblending or copolymerization The performance of a 30% copolymer-ized silicone alkyd is not necessarily the same as one which was mod-ified by blending These modifications can change the performanceproperties subtly or dramatically.3

perfor-There are more than 1200 coating manufacturers in the UnitedStates, each having various formulations that could number in thehundreds Further complicating the coatings selection difficulty is thewell known practice of a few coating manufacturers who add smallamounts of a more expensive, better performing resin to a less expen-sive, poorer performing resin and call the product by the name of theformer An unsuspecting person, whose choice of such a coating isbased on properties of the generic resin, can be greatly disappointed.Instead, selections must be made on the basis of performance data forspecific coatings or finish systems Performance data are generated bythe paint and product manufacturing industries when conductingstandard paint evaluation tests Test methods for coating materialevaluation are listed in Table 9.7

9.4.4 Selection by Electrical Properties

Electrical properties of organic coatings vary by resin (also referred to

enamels, and magnet wire enamels, the electrical properties andphysical properties determine the choice

Table 9.8 shows electric strengths, Table 9.9 shows volume ties, Table 9.10 shows dielectric constants, and Table 9.11 shows dissi-pation factors for coatings using most of the available resins Magnetwire insulation is an important use for organic coatings National Elec-trical Manufacturer’s Association (NEMA) standards and manufactur-ers’ trade names for various wire enamels are shown in Table 9.12 Thisinformation can be used as a guide in the selection of coatings How-ever, it is important to remember the aforementioned warnings aboutblends of various resins and the effects on performance properties.09Izzo Page 18 Wednesday, May 23, 2001 10:27 AM

resistivi-TABLE 9.7 Specific Test Methods for Coatings *

resistance

D229 D257

6161.1 (Outdoor rack)

D1310 (Tag open cup)

4291 (Tag Closed Cup)

4294 (Cleveland Open Cup)

MIL-E-5272, proc 1 Fed Std 810, 507

MIL-E-5272, 4.6 Fed Std 151, 811.1 Fed Std 810, 509

MIL-T-5422, 4.1 MIL-STD-810, 504.1

C177 (Guarded hot plate)

Fed Std 151, 520, 521.1

D562 D1200 D88

Bethesda, MD This has gone through many editions.

TABLE 9.7 Specific Test Methods for Coatings * (Continued)

Coatings and Finishes 9.21

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Since it is the resin in the coating’s vehicle that determines its mance properties, coatings can be classified by their resin types Themost widely used resins for manufacturing modern coatings are acryl-ics, alkyds, epoxies, polyesters, polyurethanes, and vinyls 3 In the fol-lowing section, the resins used in coatings are described

perfor-9.5.1 Common Coating Resins

9.5.1.1 Acrylics. Acrylics are noted for color and gloss retention inoutdoor exposure Acrylics are supplied as solvent-containing, high-solids, waterborne, and powder coatings They are formulated as lac-quers, enamels, and emulsions Lacquers and baking enamels areused as automotive and appliance finishes Both these industries useacrylics as topcoats for multicoat finish systems Thermosetting acryl-ics have replaced alkyds in applications requiring greater mar resis-tance, such as appliance finishes Acrylic lacquers are brittle andtherefore have poor impact resistance, but their outstanding weatherresistance allowed them to replace nitrocellulose lacquers in automo-tive finishes for many years Acrylic and modified acrylic emulsionshave been used as architectural coatings and also on industrial prod-ucts These medium-priced resins can be formulated to have excellenthardness, adhesion, abrasion, chemical, and mar resistance Whenacrylic resins are used to modify other resins, their properties are of-ten imparted to the resultant resin system

Uses. Acrylics, both lacquers and enamels, were the topcoats of choicefor the automotive industry from the early 1960s to the middle 1980s.09Izzo Page 22 Wednesday, May 23, 2001 10:27 AM

Coatings and Finishes 9.23

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9.28 Chapter 9

Thermosetting acrylics are still used by the major appliance industry

Acrylics are used in electrodeposition and have largely replaced

alkyds The chemistry of acrylic-based resins allows them to be used

in radiation-curing applications alone or as monomeric modifiers for

other resins Acrylic-modified polyurethane coatings have excellent

ex-terior durability

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Coatings and Finishes 9.29

9.5.1.2 Alkyds. Alkyd resin-based coatings were introduced in the 1930s

as replacements for nitrocellulose lacquers and oleoresinous coatings

They offer the advantage of good durability at relatively low cost These

low- to medium-priced coatings are still used for finishing a wide variety

of products, either alone or modified with oils or other resins The degree

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and type of modification determine their performance properties They

were used extensively by the automotive and appliance industries

through the 1960s Although alkyds are used in outdoor applications,

they are not as durable in long-term exposure, and their color and

gloss retention is inferior to that of acrylics

Uses. Once the mainstay of organic coatings, alkyds are still used for

finishing metal and wood products Their durability in interior

expo-sures is generally good, but their exterior durability is only fair Alkyd

resins are used in fillers, sealers, and caulks for wood finishing

be-cause of their formulating flexibility Alkyds have also been used in

electrodeposition as replacements for the oleoresinous vehicles They

are still used for finishing by the machine tool and other industries

Alkyds have also been widely used in architectural and trade sales

coatings Alkyd-modified acrylic latex paints are excellent

architec-tural finishes

9.5.1.3 Epoxies. Epoxy resins can be formulated with a wide range of

properties These medium- to high-priced resins are noted for their

ad-hesion, make excellent primers, and are used widely in the appliance

and automotive industries Their heat resistance permits them to be

used for electrical insulation When epoxy topcoats are used outdoors,

they tend to chalk and discolor because of inherently poor ultraviolet

light resistance Other resins modified with epoxies are used for

out-door exposure as topcoats, and properties of many other resins can be

improved by their addition Two-component epoxy coatings are used in

environments with extreme corrosion and chemical conditions

Flexi-bility in formulating two-component epoxy resin-based coatings

re-sults in a wide range of physical properties

Uses. Owing to their excellent adhesion, they are used extensively as

primers for most coatings over most substrates Epoxy coatings provide

excellent chemical and corrosion resistance They are used as electrical

insulating coatings because of their high electric strength at elevated

temperatures Some of the original work with powder coating was done

using epoxy resins, and they are still applied using this method Many

of the primers used for coil coating are epoxy resin-based

9.5.1.4 Polyesters. Polyesters are used alone or modified with other

resins to formulate coatings ranging from clear furniture finishes

(re-placing lacquers) to industrial finishes (re(re-placing alkyds) These

mod-erately priced finishes permit the same formulating flexibility as

alkyds but are tougher and more weather resistant There are

basi-cally two types of polyesters: two-component and single-package

Two-09Izzo Page 30 Wednesday, May 23, 2001 10:27 AM

Coatings and Finishes 9.31

component polyesters are cured using peroxides that initiate

free-rad-ical polymerization, while single-package polyesters, sometimes called

important to realize that, in both cases, the resin formulator can

ad-just properties to meet most exposure conditions Polyesters are also

applied as powder coatings

Uses. Two-component polyesters are well known as gel coats for

glass-reinforced plastic bathtubs, lavatories, boats, and automobiles Figure

9.2 shows tub and shower units using a polyester gel coat

High-qual-ity one-package polyester finishes are used on furniture, appliances,

automobiles, magnet wire, and industrial products Polyester powder

coatings are used as high-quality finishes in indoor and outdoor

appli-cations for anything from tables to trucks They are also used as coil

coatings

9.5.1.5 Polyurethanes. Polyurethane resin-based coatings are

ex-tremely versatile They are higher in price than alkyds but lower than

epoxies Polyurethane resins are available as oil-modified,

moisture-curing, blocked, two-component, and lacquers Table 9.13 is a selection

Figure 9.2 Polyester gel coats are used to give a decorative and protective surface

to tub shower units that are made out of glass fiber-reinforced plastics (Courtesy

of Owens-Corning Fiberglas Corporation.)

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Coatings and Finishes 9.33

guide for polyurethane coatings Two-component polyurethanes can be

formulated in a wide range of hardnesses They can be abrasion

tant, flexible, resilient, tough, chemical resistant, and weather

resis-tant Abrasion resistance of organic coatings is shown in Table 9.14

Polyurethanes can be combined with other resins to reinforce or adopt

their properties Urethane-modified acrylics have excellent outdoor

weathering properties They can also be applied as air-drying,

forced-dried, and baking liquid finishes as well as powder coatings

Uses. Polyurethanes have become very important finishes in the

transportation industry, which includes aircraft, automobiles,

rail-roads, trucks, and ships Owing to their chemical resistance and ease

of decontamination from chemical, biological, and radiological warfare

agents, they are widely used for painting military land vehicles, ships,

and aircraft They are used on automobiles as coatings for plastic

parts and as clear topcoats in the basecoat-clearcoat finish systems

Low-temperature baking polyurethanes are used as mar-resistant

fin-ishes for products that must be packaged while still warm

Polyure-thanes are used in an increasing number of applications They are

also used in radiation curable coatings

9.5.1.6 Polyvinyl chloride. Polyvinyl chloride (PVC) coatings,

com-monly called vinyls, are noted for their toughness, chemical

resis-tance, and durability They are available as solutions, dispersions, and

lattices Properties of vinyl coatings are listed in Table 9.15 They are

applied as lacquers, plastisols, organisols, and lattices PVC coating

powders have essentially the same properties as liquids PVC

or-ganisol, plastisol, and powder coatings have limited adhesion and

re-quire primers

Uses. Vinyls have been used in various applications, including

bever-age and other can linings, automobile interiors, and office machine

ex-teriors They are also used as thick film liquids and as powder

coatings for electrical insulation Owing to their excellent chemical

re-sistance, they are used as tank linings and as rack coatings in

electro-plating shops Typical applications for vinyl coatings are shown in Fig

9.3 Vinyl-modified acrylic latex trade sale paints are used as trim

enamels for exterior applications and as semigloss wall enamels for

in-terior applications

9.5.2 Other Coating Resins

In addition to the aforementioned materials, there are a number of

other important resins used in formulating coatings These materials,

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Figure 9.3 Vinyl plastisols and organisols are used extensively for dip coating of wire

products The coatings can be varied from very hard to very soft (Courtesy of M & T

Chemicals.)

9.35

in automotive and appliance finishes.

Uses. Melamine and urea formaldehyde resins are used as modifiersfor alkyds and other resins to increase hardness and accelerate cure

9.5.2.2 Cellulosics. Nitrocellulose lacquers are the most important ofthe cellulosics They were introduced in the 1920s and used as fast-drying finishes for a number of manufactured products Applied at lowsolids using expensive solvents, they will not meet air quality stan-dards By modifying nitrocellulose with other resins such as alkydsand ureas, the VOC content can be lowered, and performance proper-ties can be increased Other important cellulosic resins are celluloseacetate butyrate and ethyl cellulose

Uses. Although no longer used extensively by the automotive try, nitrocellulose lacquers are still used by the furniture industry be-cause of their fast-drying and hand-rubbing properties Celluloseacetate butyrate has been used for coating metal in numerous applica-tions In 1959, one of the first conveyorized powder coating lines in theUnited States coated distribution transformer lids and hand-hole cov-ers with a cellulose acetate butyrate powder coating

indus-9.5.2.3 Chlorinated rubber. Chlorinated rubber coatings are used asswimming pool paints and traffic paints

9.5.2.4 Fluorocarbons. These high-priced coatings require high cessing temperatures and therefore are limited in their usage Theyare noted for their lubricity or nonstick properties due to low coeffi-cients of friction, and also for weatherability Table 9.16 gives the coef-ficients of friction of typical coatings

pro-Uses. Fluorocarbons are used as chemical-resistant coatings for cessing equipment They are also used as nonstick coatings for cook-ware, friction-reducing coatings for tools, and as dry lubricatedsurfaces in many other consumer and industrial products, as shown inFig 9.4 Table 9.17 compares the properties of four fluorocarbons

pro-Coatings and Finishes 9.37

Figure 9.4 Nonstick feature of fluorocarbon finishes

makes them useful for products such as saws, fan and

blower blades, door-lock parts, sliding- and

folding-door hardware, skis, and snow shovels (Courtesy of

E.I DuPont de Nemours & Company.)

9.38

Coatings and Finishes 9.39

9.5.2.5 Oleoresinous. Oleoresinous coatings, based on drying oilssuch as soybean and linseed, are slow curing For many years prior tothe introduction of synthetic resins, they were used as the vehicles inmost coatings They still find application alone or as modifiers to otherresins

Uses. Oleoresinous vehicles are used in low-cost primers and enamelsfor structural, marine, architectural, and, to a limited extent, indus-trial product finishing

9.5.2.6 Phenolics. Introduced in the early 1900s, phenolics were thefirst commercial synthetic resins They are available as 100% phenolicbaking resins, oil-modified, and phenolic dispersions Phenolic resins,used as modifiers, will improve the heat and chemical resistance ofother resins Baked phenolic resin-based coatings are well known fortheir corrosion, chemical, moisture, and heat resistance

Uses. Phenolic coatings are used on heavy-duty air-handling ment, chemical equipment, and as insulating varnishes Phenolicresins are also used as binders for electrical and decorative lami-nated plastics

equip-9.5.2.7 Polyamides. One of the more notable polyamide resins is lon, which is tough and wear resistant and has a relatively low coeffi-cient of friction It can be applied as a powder coating by fluidized bed,electrostatic spray, or flame spray Table 9.18 compares the properties

ny-of three types ny-of nylon polymers used in coatings Nylon coatings erally require a primer Polyamide resins are also used as curingagents for two-component epoxy resin coatings Film properties can bevaried widely by polyamide selection

gen-Uses. Applied as a powder coating, nylon provides a high degree oftoughness and mechanical durability to office furniture Other polya-mide resins are used as curing agents in two-component epoxy resin-based primers and topcoats, adhesives, and sealants

9.5.2.8 Polyolefins. These coatings, which can be applied by flamespraying, hot melt, or powder coating methods, have limited usage

Uses. Polyethylene is used for impregnating or coating packaging terials such as paper and aluminum foil Certain polyethylene-coatedcomposite packaging materials are virtually moisture-proof Table9.19 compares the moisture vapor transmission rates of various coat-

ther-Uses. Polyimide coatings are used in electrical applications as lating varnishes and magnet wire enamels in high-temperature, high-reliability applications They are also used as alternatives to fluoro-carbon coatings on cookware, as shown in Fig 9.5.

insu-9.5.2.10 Silicones. Silicone resins are high in price and are usedalone or as modifiers to upgrade other resins They are noted for theirhigh temperature resistance, moisture resistance, and weatherability.They can be hard or elastomeric, baking or room temperature curing

Uses. Silicones are used in high-temperature coatings for exhauststacks, ovens, and space heaters Figure 9.6 shows silicone coatings onfireplace equipment They are also used as conformal coatings forprinted wiring boards, moisture repellents for masonry, weather-resis-tant finishes for outdoors, and thermal control coatings for space vehi-cles The thermal conductivities of coatings are listed in Table 9.20

Coatings and Finishes 9.41

The selection of an application method is as important as the selection

of the coating itself Basically, the application methods for industrialliquid coatings and finishes are dipping, flow coating, and spraying, al-though some coatings are applied by brushing, rolling, printing, andsilk screening The application methods for powder coatings and fin-ishes are fluidized beds, electrostatic fluidized beds, and electrostaticspray outfits In these times of environmental awareness, regulation,and compliance, it is mandatory that coatings be applied in the most

9.42 Chapter 9

Figure 9.5 Polyimide coating is used as a protective finish on the

inside of aluminum, stainless steel, and other cookware (Courtesy

of Mirro Aluminum Company.)

Figure 9.6 Silicone coatings are used as heat-stable

finishes for severe high-temperature applications

such as fireplace equipment, exhaust stacks, thermal

control coatings for spacecraft, and wall and space

heaters (Courtesy of Copper Development Assn.)