Handbook of Plastics, Elastomers and Composites Part 10 pot

The most widely used resins formanufacturing modern coatings and finishes are acrylics, alkyds, epoxies, polyesters,polyurethanes, and vinyls.3 In the following section, the resins used i

whose choice of such a coating is based on properties of the generic resin, can be greatlydisappointed Instead, selections must be made on the basis of performance data for spe-cific coatings or finish systems Performance data are generated by the paint and productmanufacturing industries when conducting standard paint evaluation tests Test methods

for coating material evaluation are listed in Table 6.7.

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

polymer) type When selecting insulating varnishes, insulating enamels, and magnet wire

enamels, the electrical properties and physical properties determine the choice

Table 6.8 shows electric strengths, Table 6.9 shows volume resistivities, Table 6.10shows dielectric constants, and Table 6.11 shows dissipation factors for coatings usingmost of the available resins Magnet wire insulation is an important use for organic coat-ings National Electrical Manufacturer’s Association (NEMA) standards and manufactur-ers’ trade names for various wire enamels are shown in Table 6.12 This information can

be used to guide the selection of coatings However, it is important to remember the mentioned warnings about blends of various resins and the effects on performance proper-ties

afore-6.5 Coating and Finishing Materials

Since it is the resin in the coating’s vehicle that determines its performance properties,coatings and finishes can be classified by their resin types The most widely used resins formanufacturing modern coatings and finishes are acrylics, alkyds, epoxies, polyesters,polyurethanes, and vinyls.3 In the following section, the resins used in coatings and fin-ishes are described

Acrylics are supplied as solvent-containing, high-solids, waterborne, and powder coatings.They are formulated as lacquers, enamels, and emulsions Lacquers and baking enamelsare used as automotive and appliance finishes Both these industries use acrylics as top-coats for multicoat finish systems Thermosetting acrylics have replaced alkyds in applica-tions requiring greater mar resistance such as appliance finishes Acrylic lacquers arebrittle and therefore have poor impact resistance, but their outstanding weather resistanceallowed them to replace nitrocellulose lacquers in automotive finishes for many years.Acrylic and modified acrylic emulsions have been used as architectural coatings and fin-ishes and also on industrial products These medium-priced resins can be formulated tohave excellent hardness, adhesion, abrasion, chemical, and mar resistance When acrylicresins are used to modify other resins, their properties are often imparted to the resultantresin system

Uses. Acrylics, both lacquers and enamels, were the topcoats of choice for the motive industry from the early 1960s to the mid 1980s Thermosetting acrylics are stillused by the major appliance industry Acrylics are used in electrodeposition and havelargely replaced alkyds The chemistry of acrylic-based resins allows them to be used inradiation curing applications alone or as monomeric modifiers for other resins Acrylic-modified polyurethane coatings and finishes have excellent exterior durability

auto-Plastics in Coatings and Finishes 363

6.5.1.2 Alkyds. Alkyd resin-based coatings and finishes were introduced in the 1930s

as replacements for nitrocellulose lacquers and oleoresinous coatings They offer the vantage of good durability at relatively low cost These low- to medium-priced coatingsand finishes are still used for finishing a wide variety of products, either alone or modifiedwith oils or other resins The degree and type of modification determine their performanceproperties They were used extensively by the automotive and appliance industries throughthe 1960s Although alkyds are used in outdoor applications, they are not as durable inlong-term exposure, and their color and gloss retention is inferior to that of acrylics

ad-Uses. Once the mainstay of organic coatings and finishes, alkyds are still used for ishing metal and wood products Their durability in interior exposures is generally good,but their exterior durability is only fair Alkyd resins are used in fillers, sealers, and caulksfor wood finishing because of their formulating flexibility Alkyds have also been used inelectrodeposition as replacements for the oleoresinous vehicles They are still used for fin-ishing by the machine tool and other industries Alkyds have also been widely used in ar-chitectural and trade sales coatings Alkyd-modified acrylic latex paints are excellentarchitectural finishes

These medium- to high-priced resins are noted for their adhesion, make excellent primers,and are used widely in the appliance and automotive industries Their heat resistance per-mits 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 outdoor exposure as topcoats, and proper-ties of many other resins can be improved by their addition Two-component epoxy coat-ings and finishes are used in environments with extreme corrosion and chemicalconditions Flexibility in formulating two-component epoxy resin-based coatings and fin-ishes results in a wide range of physical properties

Uses. Owing to their excellent adhesion, they are used extensively as primers formost coatings and finishes over most substrates Epoxy coatings and finishes provide ex-cellent chemical and corrosion resistance They are used as electrical insulating coatingsand finishes because of their high electric strength at elevated temperatures Some of the

Plastics in Coatings and Finishes 365

original work with powder coating was done using epoxy resins, and they are still appliedusing this method Many of the primers used for coil coating are epoxy resin-based.

formu-late coatings and finishes ranging from clear furniture finishes (replacing lacquers) to trial finishes (replacing alkyds) These moderately priced finishes permit the sameformulating flexibility as alkyds but are tougher and more weather resistant There are basi-

indus-Plastics in Coatings and Finishes 371

cally two types of polyesters: two-component and single-package Two-component ters are cured using peroxides, which initiate free-radical polymerization, while single-

polyes-package polyesters, sometimes called oil-free alkyds, are self-curing, usually at elevated

temperatures It is important to realize that, in both cases, the resin formulator can adjustproperties to meet most exposure conditions Polyesters are also applied as powder coatings

Uses. Two-component polyesters are well known as gel coats for glass-reinforcedplastic bathtubs, lavatories, boats, and automobiles Figure 6.2 shows tub and shower units

using a polyester gel coat High-quality one-package polyester finishes are used on ture, appliances, automobiles, magnet wire, and industrial products Polyester powdercoatings are used as high-quality finishes in indoor and outdoor applications for anythingfrom tables to trucks They are also used as coil coatings.

versatile They are higher in price than alkyds but lower than epoxies Polyurethane resinsare available as oil-modified, moisture-curing, blocked, two-component, and lacquers Ta-ble 6.13 is a selection guide for polyurethane coatings Two-component polyurethanescan be formulated in a wide range of hardnesses They can be abrasion resistant, flexible,resilient, tough, chemical resistant, and weather resistant Abrasion resistance of organiccoatings is shown in Table 6.14 Polyurethanes can be combined with other resins to rein-force or adopt their properties Urethane-modified acrylics have excellent outdoor weath-ering properties They can also be applied as air-drying, forced-dried, and baking liquidfinishes as well as powder coatings

Uses. Polyurethanes have become very important finishes in the transportation try, which includes aircraft, automobiles, railroads, trucks, and ships Owing to theirchemical resistance and ease of decontamination from chemical, biological, and radiolog-ical warfare agents, they are widely used for painting military land vehicles, ships, and air-craft They are used on automobiles as coatings and finishes for plastic parts and as cleartopcoats in the basecoat-clearcoat finish systems Low-temperature baking polyurethanesare used as mar-resistant finishes for products that must be packaged while still warm

indus-Figure 6.2 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)

Polyurethanes are used in an increasing number of applications They are also used in diation curable coatings

vi-nyls, are noted for their toughness, chemical resistance, and durability They are available

as solutions, dispersions, and lattices Properties of vinyl coatings are listed in Table 6.15.They are applied as lacquers, plastisols, organisols, and lattices PVC coating powdershave essentially the same properties as liquids PVC organisol, plastisol, and powder coat-ings have limited adhesion and require primers

Uses. Vinyls have been used in various applications, including beverage and othercan linings, automobile interiors, and office machine exteriors They are also used as thickfilm liquids and as powder coatings for electrical insulation Owing to their excellentchemical resistance, they are used as tank linings and as rack coatings in electroplatingshops Typical applications for vinyl coatings are shown in Fig 6.3 Vinyl-modified acrylic

TABLE 6.14 Abrasion Resistance of Coatings

Coating

Taber ware index, mg/1000 rev.

Polyurethane type 1

Polyurethane type 2 (clear)

Polyurethane type 2 (pigmented)

Phenolic spar varnish

Clear nitrocellulose lacquer

Chlorinated rubber

Silicone, white enamel

Catalyzed epoxy, air-cured (PT-401)

Catalyzed epoxy, Teflon-filled (PT-401)

Catalyzed epoxy, bake-Teflon-filled (PT-201)

Alkyd TT-E-508 enamel (cured for 45 min at 250°F)

Alkyd TTLE-508 (cured for 24 hr at room temperature)

55–67 8–24 31–35 60 155 147 85–106 38 95 196 50 172 96 200–220 113 208 122 136 9.7 44 305 290–310 360 51 70

latex trade sale paints are used as trim enamels for exterior applications and as semiglosswall enamels for interior applications.

In addition to the aforementioned materials, there are a number of other important resinsused in formulating coatings These materials, used alone or as modifiers for other resins,provide coating vehicles with diverse properties

used in modifying other resins to increase their durability Notable among these modifiedresins are the super alkyds used in automotive and appliance finishes

Uses. Melamine and urea formaldehyde resins are used as modifiers for alkyds andother resins to increase hardness and accelerate cure

They were introduced in the 1920s and used as fast-drying finishes for a number of factured products Applied at low solids using expensive solvents, they will not meet air-quality standards By modifying nitrocellulose with other resins such as alkyds and ureas,the VOC content can be lowered, and performance properties can be increased Other im-portant cellulosic resins are cellulose acetate butyrate and ethyl cellulose

manu-Uses Although no longer used extensively by the automotive industry, nitrocellulose

lacquers are still used by the furniture industry because of their fast-drying and bing properties Cellulose acetate butyrate has been used for coating metal in numerousapplications In 1959, one of the first conveyorized powder coating lines in the UnitedStates coated distribution transformer lids and hand-hole covers with a cellulose acetatebutyrate powder coating

hand-rub-Figure 6.3 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)

Plastics in Coatings and Finishes 377

swimming pool paints and traffic paints

pro-cessing temperatures and therefore are limited in their usage They are noted for their bricity or nonstick properties due to low coefficients of friction, and also forweatherability Table 6.16 gives the coefficients of friction of typical coatings

lu-Uses. Fluorocarbons are used as chemical-resistant coatings and finishes for ing equipment They are also used as nonstick coatings and finishes for cookware, friction-reducing coatings and finishes for tools, and as dry lubricated surfaces in many other con-sumer and industrial products, as shown in Fig 6.4 Table 6.17 compares the properties offour fluorocarbons

and linseed, are slow curing For many years prior to the introduction of synthetic resins,

TABLE 6.16 Coefficients of Friction of Typical Coatings

Coating

Coefficient of friction, µ Information source Polyvinyl chloride

Steel on steel with SAE no 20 oil

Polymethyl methacrylate to self

Polymethyl methacrylate to steel

0.4–0.5 0.4–0.5 0.4–0.5 0.3 0.6–0.8 0.05–0.1 0.15 0.25 0.29 0.31–0.33 0.17 0.18 0.02–0.06 0.45–0.60 0.44 0.33 0.4 0.044 0.8 (static) 0.4–0.5 (static)

a a a a a a b c c c d d e e e e e e e e a

R.P Bowder, Endeavor, Vol 16, No 61, l957, p 5.

Plastics in Coatings and Finishes 379

they were used as the vehicles in most coatings They still find application alone or asmodifiers to other resins

Uses. Oleoresinous vehicles are used in low-cost primers and enamels for structural,marine, architectural, and, to a limited extent, industrial product finishing

commer-cial synthetic resins They are available as 100 percent phenolic baking resins, fied, and phenolic dispersions Phenolic resins, used as modifiers, will improve the heatand chemical resistance of other resins Baked phenolic resin-based coatings and finishesare well known for their corrosion, chemical, moisture, and heat resistance

oil-modi-Uses. Phenolic coatings and finishes are used on heavy-duty air-handling equipment,

on chemical equipment, and as insulating varnishes Phenolic resins are also used as ers for electrical and decorative laminated plastics

tough, wear resistant, and has a relatively low coefficient of friction It can be applied as apowder coating by fluidized bed, electrostatic spray, or flame spray Table 6.18 comparesthe properties of three types of nylon polymers used in coatings Nylon coatings and fin-ishes generally require a primer Polyamide resins are also used as curing agents for two-component epoxy resin coatings Film properties can be varied widely by polyamide se-lection

Uses. Applied as a powder coating, nylon provides a high degree of toughness andmechanical durability to office furniture Other polyamide resins are used as curing agents

in two-component epoxy resin-based primers and topcoats, adhesives, and sealants

Figure 6.4 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 &

Com-pany)