Binders in Waterborne Coatings

Waterborne coatings require some strategy to reverse binder-water interactions before and after film formation. Water must dissolve or disperse the binder before and during paint application, but after the film is cured it must be resistant to dissolution or swelling by water. In latex paints the binder is dispersed in the form of particles. The interior portion of each particle is hydrophobic, whereas the surface of each particle is made hydrophilic to aid dispersion and to prevent coagulation during storage. The polymer molecules inside the particles can have high molecular weight without contributing to high viscosity because the molecules remain inside the particles. After the film is applied, water is lost by evaporation, the particles move closer together, and eventually come into contact. Then the polymer molecules diffuse across the former boundaries of the particles in a process called coalescence. Many latex paints contain coalescing agents, which are oxygenated organic solvents that are selected to partition in an optimum way to promote interparticle diffusion during coalescence.

A different strategy for reversing binder-water interaction is used in the so-called water-soluble or water-dispersible systems. The same thermoset binder components used in solvent-borne systems are modified by introducing hydro-philic groups such as carboxylic acid groups. The acid groups are neutralized by amine groups from aminoalcohols such as dimethylaminoethanol (DMAE). The ionic sites contribute more to solubilization and/or dispersion than do the —COOH groups alone. During heat cure, part of the amine is driven off and often some of the —COOH groups react with the crosslinker. Both of these changes help convert the system from a slightly hydrophilic dispersion to a hydrophobic cure film. Latex systems are pure white like whole milk, but water-dispersible systems are often completely clear like a true solution. They look like solutions (thus the term "water soluble"), but rheology and other studies indicate that they actually consist of very small particle dispersions [12, 13]. The particles are too small to scatter visible light. Usually there are too few ionic or other solubilizing groups for dispersion in water alone, and coupling solvents are used along with water. The term "coupling solvent" is applied to solvents that tend to prevent phase separation in systems that contain both hydrophilic and hydrophobic components. Most coupling solvents are alcohol ethers such as propylene glycol monomethylether (I). As the structure suggests

(I) HOCH2CH(CHB)OCH3

this solvent is made by reacting propylene oxide with methanol. All of the alcohol ethers used in coatings are obtained by reacting various alcohols with either propylene oxide (P) or with ethylene oxide (E). In a commonly used two-letter designation, the oxide is identified first (P or E) and the alcohol next by the first letter in its name: (M) methanol, (E) ethanol, (P) propanol, or (B) butanol. Structure (I) is PM, a considerably shorter name than propylene gly-col monomethylether. EB (aka butoxyethanol or ethylene glycol monobutylether or Butyl Cellosolve) was used very extensively in early water-reducible coatings, but toxicity concerns for all members of the ethylene glycol series have resulted in a switch to coupling solvents based on propylene glycol [10]. A remarkable property of many alcohol ethers is their tolerance for mixing with other compounds covering a wide range of polarity. For example, several of these coupling solvents form one phase solution in all proportions with both water (very polar) and heptane (very nonpolar).

Binders used in waterborne latex paints are prepared by emulsion polymerization of mixtures of monomers selected to give the optimum glass transition temperature, Tg [4(e)]. Low Tg contributes to good film formation so that the paint can be applied on cool days. Low-temperature application also depends on the effectiveness of coalescing agents included in the formulation. Low-temperature limits printed on paint can labels vary from about 35 to 50°F. High Tg contributes to increased hardness and toughness of the dry film. Slow volatilization of coalescing agent causes an increase in hardness over an extended period. In the United States most exterior latex paints are prepared from acrylic monomers. The high Tg monomer is usually methylmethacrylate and the low Tg monomers are butyl acrylate and/or ethyl acrylate. Occasionally, some styrene is also included along with the acrylic monomers. Interior latex paints are often prepared from a mixture of vinyl acetate and a low Tg monomer such as butyl acrylate.

Table 6.1 contains data from the U.S. Census Bureau's Current Industrial Report on Paint and Allied Products [1]. Manufacturer's estimated shipments are reported in four categories: architectural coating, product finishes for original equipment manufacturer (OEM), special-purpose coatings, and miscellaneous allied paint products. Shipment volume in gallons and the dollar values for each category for 1995-2000 are given in Table 6.1. Architectural coatings include interior and exterior, primer and topcoat house, barn, and roof paints; floor paints; tinting bases; trim paints; and stains and lacquers. Product finishes for OEM include assembly-plant-applied auto, truck, and SUV finishes (excludes refinish paint), appliance finishes, container and closure coating (beverage and food cans, bottle caps), factory-finished or primed wood or composition board (house siding), aluminum siding and extrusions, machinery and equipment and finishes, and wood furniture and cabinet finishes. The powder coating portions of these OEM finishes were reported separately in pounds and were converted by the Census Bureau to gallons by using a conversion factor of 5 (5 lb = 1 gal) [1]. The powder coating data will be discusses in detail in Section 6.7.3. Specialpurpose coatings include traffic marking paints, automotive refinish (body shop use), marine coating for ships, and yachts and pleasure craft, and industrial new construction and maintenance and aerosol paint concentrates for packaging in aerosol containers (spray cans). Allied paint products include paint and varnish removers, pigment dispersions, thinners for lacquers and solvent-borne paints, brush cleaners, some (but probably not all) ink vehicles, and putty and glazing compounds. The two largest categories, architectural coatings and product finishes for OEM, accounted for 43.8 and 30.7% of the shipment volume, respectively.

These two categories were nearly equal in dollar value, representing 36.0 and 34.5% of the total in 2000. The average selling prices calculated simply as dollars/gallon in 2000 for each category were: architectural $9.94, OEM $13.56, special purpose $20.00, and allied paint products $8.41. If the miscellaneous and allied paint products category is subtracted from the total quantity for 2000, the result is 1280.4 million gallons or close to 1.3 billion gallons, which is the number often quoted for 2000.

The changes in quantities shipped from 1995 to 2000 are small, perhaps reflecting a mature industry with slow or no growth. These volumes will reflect changes made in response to regulations. Changes from conventional solvent borne to HSSB will reduce volume shipped, but the dollar value of paint shipped

Table 6.1 Quantity and Value of a Paint and Allied Products [1]

Quantity Value

Year (millions of gal) (millions of dollars)

Table 6.1 Quantity and Value of a Paint and Allied Products [1]

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