COATINGS

Coatings is an important and rapidly growing value-added application of PU chemistry [36, 37]. All the material aspects that make PU into successful elastomers, adhesives, and thermoplastics translate to making high-performance coatings as well. Thus, PU coatings are valued particularly for durability, abrasion resistance, aesthetics, and formulation flexibility. In addition, like PU adhesives, PU coatings can be delivered in numerous formats to meet the process requirements of almost any coating operation.

Quantifying market consumption based on type and application is complicated by the varying classification of a coating. For example, should urethane modification of an acrylic or epoxy coating be counted as urethane? Should a sprayed truck bed liner be grouped with a sprayed clear furniture coating? Is the common means of application a sufficient qualification to group these very different products within the same category? Some analyses of usage do, and others do not. Variance in market data reflect simply an individual perception of what a coating is. Another quantitative variable is counting volumes of formulated (and diluted) coatings versus just the volume of PU in that coating.

Another distinction of coating classifications are those that are defined as "architectural" and those defined as "industrial" [38]. Industrial coatings are all of those coatings applied at a manufacturing facility. Architectural coatings are those meant to be applied in or onto buildings. There is obviously overlap in the coatings themselves since wood coatings can be used in both places as can paints and alkyd coatings. The distinction is merely the location, and in some cases formulation specification, consistent with use by unlicensed individuals in unventilated environments. Thus, it is reasonable to approach coatings market data with the perspective of viewing a snapshot that may reflect a set of definitional biases along with the usual lack of precision accounting for volumes consumed by many thousands of manufacturers and billions of consumers.

While absolute volumes are potentially imprecise, the ratio of volumes has been relatively constant and can perhaps represent useful data for comparison. Figure 10.17 shows the proportional amounts of polyols and isocyanates used in coatings. For specialty performance products such as coatings, the choice of building block is anything but arbitrary. The use of specific polyols and isocyanates are designed to meet stringent requirements for the application; and while price is a consideration in every real function, performance will often be the driver of what particular PU

(a) Distribution of polyol consumption in polyurethane coatings based on chemistry and (b) distribution of isocyanate consumption in polyurethane coatings based on chemistry.

FIGURE 10.17 (a) Distribution of polyol consumption in polyurethane coatings based on chemistry and (b) distribution of isocyanate consumption in polyurethane coatings based on chemistry.

Distribution of industrial applications employing polyurethane coatings. The importance of

FIGURE 10.18 Distribution of industrial applications employing polyurethane coatings. The importance of "other" is consistent with the high service requirements for participation in this market, providing custom solutions for smaller, customer-specific applications.

chemistry and product format will be used [39]. In this regard, the applications that use PU-based coatings and their approximate ratio is shown in Figure 10.18. Architectural coatings use little PU resin and represents about 10% of total PU resin used for coatings and is not included in the figure. Most of this PU is in the form of PU-modified alkyd coatings for wood coverage. Additional architectural volume is PU waterborne dispersions for wood coatings.

Table 10.7 is a list of PU coating technologies as defined by ASTM conventions including a list of applications for which those technologies are found useful. The technology choice (i.e., solvent-borne versus waterborne) reflects the demands and the conditions of application.

As a rule, almost all coatings that are applied to impart an aesthetic appeal utilize aliphatic isocyanates since their tendency to yellow is much reduced relative to aromatic polyisocyanates (see Chapter 2). However, aromatic polyisocyanates are used for coatings that are nondecorative, are part of an interior lining, have an underground function, or are part of a formulation that does not use a solvent.

Much as any technology, coatings are tested for their performance attributes. Given the range of environments and functions for coatings, the number of standard tests is quite large. Table 10.8 is an abridged list of tests commonly applied for coatings testing. The Standard Guide tests should be consulted initially to provide a complete list of standards that apply to numerous applications for the specified technology. Since testing of coatings is largely a pursuit of industry, many individual tests are automated or have commercially available equipment that are designed and sold for specifically testing for an ASTM standard, and provide a modicum of standardization across different laboratories. Regardless, it is not uncommon for separate labs

TABLE 10.7 ASTM classification of polyurethane coating based on format and method of cure and their predominant application areas

ASTM convention

Description of technology

Applications

Type I

One-part nonreactive, unsaturated aliphatic esters; no free isocyanate groups in coating, cure by oxidative cross-linking of unsaturated groups and solvent evaporation. Uses soybean or linseed oil.

Architectural floor and maintenance, topcoats. Better performance than unmodified alkyds

Type II

One-part moisture curing, contains free

isocyanates, reacts with environmental water and isocyanate reactive groups on the substrate surface. Polyether or polyester soft segments. Can use various blocking techniques to preserve isocyanate reactivity for extended shelf life.

Leather, concrete, maintenance

Type III

One-part heat cure. Uses blocked isocyanates that are liberated upon heating to react with isocyanate-reactive components in the formulation. Polyether or polyester, often phenol, caprolactam, or cresol-blocked aliphatic polyisocyanates or TDI.

Coil and electric wire

Type IV

Two-part solvent-borne. One part is the

prepolymer polyisocyanate, whereas the other is the polyol(s), catalyst, and all other formulation components such as solvents, pigments, and other additives. Acrylic and polyester polyols are common components. Also can use hydroxyl functional alkyds. Ambient or heat cure. Pot life can be an issue and can be influenced by using aprotic complexing solvents.

Plastics, wood furniture, marine exteriors

Type V

Two-part high solid (>50%) coatings. One part is a prepolymer and the other is polyol, usually acrylic, polyesters, or polyethers. Considered high-performance coatings can have limited pot life complicating application.

Leather, wood, automotive clear coats, refinishes, aircraft, bus, truck, industrial structure maintenance coatings

Type VI

One-part nonreactive low solid (<20%) solvent borne. Referred to as a lacquer. High gloss film forms upon solvent evaporation

Textiles

Powder coatings

One-part reactive system using caprolactam or 1,2,4 tnazole blocked aliphatic isocyanates especially IPDI and H12MDI, polyester, and epoxy-modified soft segments. Viscosity must be controlled and kept low to ensure powder coalescence during application and bake.

Automotive exterior panels and parts, wire, electrical transmission equipment (i.e., transformers) reflective surfaces, metal surfaces, outdoor lawn furniture.

Radiation

High solid coatings, rapid cure, high gloss, not practical for home use or complex shapes. Made by reacting isocyanate-capped prepolymer with hydroxyl functionahzed acrylate or methacrylate. Requires use of photoimtiators in formulation. Any isocyanate can function but usually aliphatic.

Manufactured wood flooring, cabinets, metal surfaces, plastic.

Waterborne

Broadly applied to one- and two-part systems using aliphatic or aromatic isocyanates. Reduces VOC exposure, high growth of application in all geographies, can be used in hybrid technologies like uralkyds and PU acrylates. May be formulated with water-soluble organic solvents for better solubilization of components. May also use defoamer. Cure speed/drying is less of an issue as percentage of solids have increased.

Wood coatings,

decorative coatings, artificial leathers, textiles, plastics, inks, architectural, automotive

Powder, radiation cure, and waterborne coatings do not yet fit into the ASTM conventions but are included here as well.

TABLE 10.8 Abridged list of standardized tests and methods for characterizing coatings

Test

Standard description

D5324

Standard Guide for Testing Water-Borne Architectural Coatings includes list of tests for flat interior and exterior latex paints, waterborne floor paints, and semi- and high gloss latex paints. References 80 standards for characterization of architectural coatings

D6577

Standard Guide for Testing Industrial Protective Coatings is specifically intended for paints applied to substrates on-site of structures and buildings, especially where subject to corrosive environments, as industrial, urban, and marine environments. References 129 standards for characterization of industrial protective coatings.

D5146

Standard Guide to Testing Solvent-Borne Architectural Coatings includes interior low-gloss wall finish, interior gloss and semi-gloss wall and trim enamels, exterior house and trim coatings, and exterior and interior floor enamels. References 75 standards for characterization of solvent-borne architectural coatings.

D3451

Standard Guide for Testing Coating Powders and Powder Coatings, particularly those applied by electrostatic, fluidized bed, or alternative method. References 85 standards for characterization of powder coatings and coating characteristics.

D3730

Standard Guide for Testing High-Performance Interior Architectural Wall

Coatings. High performance denoting coating that are tougher, more abrasion, and more stain resistant than usual coatings. References 49 standards for characterizing this class of architectural coatings.

D2832

Standard Guide for Determining Volatile and Nonvolatile Content of Paint and Related Coatings. Applicable to all ASTM coating types as well as powder coatings, and water-based coatings. References 15 other standards for specific coating types that apply in conjunction with the present standard.

D5178

Standard Test Method for Mar Resistance of Organic Coatings. Coating is applied to a flat surface and progressively challenged with a weighted stylus that is incrementally loaded until coating failure.

D3732

Standard Practice for Reporting Cure Times of Ultraviolet-Cured Coatings. Specifies UV cure but is generally applicable. Involves application of the coating to a desired substrate at a desired thickness and curing for incrementally longer exposure. Cure is measured by property measurement including impact resistance, hardness, solvent rub test, abrasion resistance, stain resistance. The shortest time that yields a cured film is the cure time.

D7394

Standard Practice for Rheological Characterization of Architectural Coatings using Three Rotational Bench Viscometers. Applicable to waterborne architectural coatings. Instructions for using a Brookfield viscometer, a paddle viscometer, and a cone-and-plate-type viscometer for sampling behavior in low-, medium- and high-shear rates.

D4708

Standard Practice for Preparation of Uniform Free Films of Organic Coatings. Useful procedure for direct determination of coating physical properties including permeability independent of substrate. Involves casting of film on to either fluorinated sheet, sihcone-coated sheet, or halosilane-coated glass followed by cure and sample removal.

B499

Standard Test Method for Measurement of Coating Thicknesses by the Magnetic Method: Nonmagnetic Coatings on Magnetic Basis Metals. Nondestructive test method for measuring nonmagnetic coating thickness using commercially available device. Device monitors the magnetic flex density. Method cannot distinguish thickness of individual layers, only cumulative thickness.

D2134

Standard Test Method for Determining the Hardness of Organic

D4366

Coatings with a Sward-Type Hardness Rocker. Common method in coatings industry for determining hardness and cure. Monitors hardness and cure by time the amplitude of sinusoidal oscillation of a specific tester. Linear relationship between this technique and other techniques such as the Koemg pendulum hardness tester, also commonly used (D4366 withdrawn).

D6577

Standard Guide for Testing Industrial Protective Coatings. Includes industrial and architectural coatings with reference to 126 related standards including salt spray testing, graffiti resistance, MEK double rub, and so on.

D6943

Standard Practice for Immersion Testing of Industrial Protective Coatings Covers testing of samples at standard temperature and pressure, with a temperature gradient, and at constant temperature and increased pressure. Many applications, including can coatings.

Simplified supply chain and commercial relationships between polyure¬thane coatings market participants.

FIGURE 10.19 Simplified supply chain and commercial relationships between polyurethane coatings market participants.

to achieve different results due to differences in surface preparation and differences in coating application, which every standard warns, is critical to results [40, 41].

The PU coating industry has much in common with the PU adhesive industry making it convenient to group them together. In both cases, the application is a film that must adhere to a surface and cure within a predetermined time. The forms, solvent-borne, waterborne dispersions, moisture cured, and one part and two part, are all in common with the adhesives industry. Also in common is the general similarity in industry structure depicted in Figure 10.19. This figure (simplified as it is) depicts the inter-relationships along the supply chain in situations where participants can be simultaneously customers, suppliers, and competitors. This entangled system creates market opportunities and vulnerabilities that are very different from a landscape of limited and well-known competitors and customers [42]. For instance, the coating formulator position can be exposed to unusual pressures as they can be simultaneously squeezed by high raw material costs that can coexist with depressed consumer demand depressing final product prices. While the coatings producer can purchase from alternative suppliers to create price competition, these suppliers are relatively few in number, and their commodity feedstocks tend to have well-characterized supply, demand, and pricing. Like other products that may only have subtle differences (think gasoline), their prices tend to move quickly and in unison up and down. Price differences, such as exist, are related to producer efficiency and spot supply conditions, and tend to be relatively small. Even noncommodity raw materials such as aliphatic isocyanates are less price elastic due to limited volumes and limited producers resulting from exigencies of phosgenation (see Chapter 2) [43].

 
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