Overview of Industrial Coating Materials.

Industrial coating materials must now be applied to manufactured products in compliance with a host of governmental regulations that control air and water quality, toxicity in the work place and the disposal of hazardous wastes. Coatings and resin suppliers recognized that their materials contained

These changes are not only evident in the alterations to the form and substance of conventional coatings but also in the development of other methods to apply and cure coating materials. Some of these developments are in the areas of electrocoating, vapor deposition, vapor curing, radiation curing and powder coating. These changes led to many advances in state-of-the-art of resin technology.

As a result of these advances, today's coatings are not only designed to have performance characteristics matching service requirements but are also designed to meet governmental regulations. Consequently, the finisher is faced with an ever-increasing number of choices, not only on coating quality, but on environmental impact, worker safety and economics.

In the past, the selection of a finish system depended on the answer to two questions: what are the service requirements; and what is the best application method? There is now a third question: will the selected coating comply with governmental regulations? The answer to the first question will determine the resin type of the coating, the number of coats required and to some extent its physical state. The answer to the second will determine the physical state of the coating and the equipment needed for its application. Since the answer to the third question is so complex, it will not be addressed here but will be discussed in other sections of this Directory. Instead, this section will give an overview of performance properties, resin types and physical states of industrial coating materials applied by the more conventional methods.

To aid in this discussion, coatings will be classified by their use in finish systems, resin types and physical state.

The first classification of coatings is by their use in finish systems--primers, one-coat enamels, intermediate coats and topcoats. The term "finish system" includes single- and multi-coat paint finishes.

Primers are the foundation of the finish system. They provide corrosion protection to the substrate and act as a suitable base for subsequent coats.

One-coat enamels not only act as primers but also can meet light-duty service requirements.

Intermediate coats have many functions, including acting as tie coats and insuring adequate film thickness.

Topcoats provide overall protection against environmental exposure. To help understand this concept, the uses of typical industrial finish systems in various service conditions are shown in Table I.

TABLE I--Typical Industrial Finish Systems

Service                   One-Coat   Intermediate   Top
Conditions       Primer    Enamel        Coat       Coat

Interim
  Light Duty                 X
  Heavy Duty       X                                 X

Exterior
  Light Duty                 X                       X
  Heavy Duty       X                                 X
  Extreme Duty     X                      X          X

A second way to classify industrial coatings is by resin types--alkyds, epoxies, polyesters, polyurethanes and vinyls. Resin types determine, to a great extent, the performance characteristics the coating will exhibit. Some of the critical properties of common industrial coatings are shown in Table II. This table is offered only as a guide. Keep in mind that specific coating properties will depend not only on the resin but also on the pigments and additives used in a specific formulation.

TABLE II--Uses of Typical Industrial Finish Systems

Resin           Humidity    Corrosion     Exterior
Type           Resistance   Resistance   Durability

Acrylic            E            E            E
Alkyd              F            F            P
Epoxy              E            E            G
Polyester          E            G            G
Polyurethane       E            G            E
Vinyl              E            G            G

Resin           Chemical       Mar
Type           Resistance   Resistance

Acrylic            G            E
Alkyd              G            G
Epoxy              E            E
Polyester          G            G
Polyurethane       G            E
Vinyl              G            G

E = Excellent, G = Good, F = Fair, P = Poor

A third classification of industrial coatings is by their physical states--solvent-borne, waterborne, high-solids, 100% solids liquid, two-component liquid and powder coatings. The physical state is sometimes helpful in determining the type of coating to be used for a particular product, but more often it determines the coating equipment used for application. Table III, which shows the physical states of common industrial coatings, is only intended as a selection guide.

TABLE III--Physical States of Common Industrial Coatings

                         Physical State

Resin           Conv.
Type           solvent   Water-borne   High-Solids

Acrylic           X           X             X
Alkyd             X           X             X
Epoxy             X           X             X
Polyester         X                         X
Polyurethane      X           X             X
Vinyl             X           X             X

                     Physical State

Resin          Powder     100%        Two
Type           Coating   solvent   Component

Acrylic           X
Alkyd
Epoxy             X         X          X
Polyester         X         X          X
Polyurethane      X         X          X
Vinyl             X

As mentioned above, the performance characteristics of industrial coatings are determined by their resin types. Listed below are descriptive summaries of the most commonly used resins as well as others also used in industrial coatings. These descriptions are intended to help the reader choose an appropriate product finish system.

Acrylics

Still used in the automotive and appliance industries, moderately priced coatings based on these resins are being supplied in baking and air-drying versions of conventional solventborne, high-solids, waterborne and powder coatings. Acrylics are known for color and gloss retention in outdoor exposure and have excellent resistance to overbake. When properly formulated, using other resins and modifiers, they have excellent adhesion, abrasion resistance and hardness. They also resist chemicals, corrosion and impact. Acrylic powder coatings have properties similar to liquid acrylics.

Alkyds

Owing to their formulating versatility, these low- to medium-priced resins, when used alone or modified with drying oils and other resins, enjoy use in finishing a wide spectrum of manufactured products from machine tools to toys. Fast air-drying, low-temperature curing or forced-drying alkyds can be used for many outdoor applications requiring short-term durability. These finishes, which replaced cellulose esters and oleoresinous paints, were once the mainstay of the industrial coatings market and still enjoy a fair share.

Epoxies

Paints based on these medium- to high-priced resins, which are noted for their excellent adhesion, are widely used as primers. However, owing to their versatility through modification with other resins, they are also used as topcoats and one-coat enamels. Two-component epoxy finishes are used in applications requiring extreme corrosion resistance. Their main disadvantage is poor resistance to ultraviolet radiation, which results in poor color and gloss retention. Since they are heat-resistant, epoxies find many uses in electrical apparatus as decorative coatings as well as electrical insulation. Epoxy coating powders are widely used.

Polyesters

They are used alone or with other resins to formulate coatings ranging from clear wood finishes to paints used for precoated metal siding. Moderate in cost, polyesters have replaced other coating materials because of their weatherability, toughness and faster drying times. Many interior and exterior products are finished using polyester powder coatings.

Polyurethanes

Polyurethane paints are used in the transportation industry for painting everything from trucks to trains. They are also widely used for clearcoats by the automotive industry. Although higher in price than polyesters and lower than epoxies, their use has increased because of their excellent performance properties, including weatherability, flexibility, wear resistance and chemical resistance. They are applied as powder coatings and single- or two-component liquid coatings and formulated to be air-dried, force-dried or baked. By the proper selection of modifiers and catalysts, they significantly reduce the amount of energy used for curing.

Vinyls

Noted for their toughness and durability, vinyl coatings are used extensively as interior coatings for beverage cans as well as other applications from automobile interiors to business machine exteriors. In addition to their use as thin-film lacquers, vinyls can also be applied as organosols and plastisols where heavy film thicknesses are required. Perhaps the most recognizable of the vinyl thick film applications is tool handle coatings. Vinyl powder coatings are often used for these heavy film applications.

Aminos

Notable resins of this type, melamine and urea, are used to modify other resins to increase their durability.

Cellulose Esters

The most important of this type is the nitrocellulose vehicle used in fast-drying lacquers that are generally applied as low volume solids, using expensive solvents. Although they were used for automotive finishes for many years, these materials are not noted for their color and gloss retention. Originally developed in the 1920s, cellulose esters are still used today.

Fluorocarbons

These coatings are noted for their lubricity, anti-stick properties and weatherability. However, because of their higher costs and baking temperatures, they have experienced limited use.

Oleoresinous

Although once used extensively as maintenance finishes, oleoresi-nous coatings, which are based on drying oils such as linseed, have limited use for product finishing. They find application as low-cost primers and air-dried and force-dried topcoats.

Phenolics

These moderately-priced, baked coatings have excellent corrosion and chemical resistance. In addition to primers for metal, they are also used where chemical and heat resistance are required.

Polyamides

One example of this type is used as a hardener for two-component, epoxy-resin-based coatings. Another, nylon, is applied as a specialty powder coating where a high degree of durability is required.

Polyimides

These high-priced coatings, noted for high temperature resistance and long term thermal stability, are used as insulating varnishes and wire enamels. They are also used as anti-stick coatings for cookware.

Polyolefins

These coatings are applied by flame spraying, hot melting or powder coating. Their use is somewhat limited. However, polyethylene coatings are used extensively on composite food packaging materials.

Silicones

These high-priced resins are used as heat-resistant coatings for engines, exhaust stacks and, because of their ultraviolet light resistance, aerospace applications.

Synthetic Rubbers

These elastomeric coatings, which are generally water- and chemical-resistant, have limited uses for finishing products.

Modified Resins

There are many coatings that are based on modifications and combinations of these and other resins. Notable among them are acrylic alkyds, acrylic urethanes, alkyd melamines, epoxy esters, epoxy polyamides, silicone alkyds, silicone polyesters and vinyl alkyds. It is often difficult to predict the performance of a given coating based on resin modifications. For example, the performance of a 30% silicone alkyd, modified by co-polymerization, is not the same as one modified by blending.

This overview is intended to give the reader a broader understanding of the formulating versatility of industrial coating materials. This versatility also contributes to their principal fault, which is the decrease in one property when another is enhanced. But when a particular product is to be coated, the choice of finish system must be based on specific performance requirements and not on generalizations. This choice is further complicated by the need to comply with governmental regulations. To accommodate these specific requirements, coatings are formulated using well-known combinations and modifications of available resins.

Today, finishers have all the tools at their disposal. They can choose compliance coatings that apply easily, coat uniformly, cure rapidly and efficiently, have characteristics meeting all performance requirements, comply with governmental regulations and reduce costs.

By Carl Izzo Consultant Export, PA