Formulating Aids for Polyurethane Coatings

Tips on Modifiers, Solvents, Pigments, Extenders and More

[You can find this and more information and accompanying illustrations in the Bayer publication, “The Chemistry of Polyurethane Coatings” by clicking here.]

Modifiers

In addition to the main resin components, one- and two-component polyurethane coating systems frequently contain other modifying ingredients. These are added to improve specific application properties (leveling agents or thickeners, for example), and are normally incorporated as a small weight percentage of the totalbinder. For certain applications, it may be best to formulate coatings with a high percentage of a modifying binder or resin component. Products such as a cellulose acetate butyrate, low molecular weight acrylic resins, and polyvinyl chloride/polyvinyl acetate (PVC/PVAC) copolymers are suitable modifiers.

Solvents

Suitable solvents for one- and two-component coating systems include esters, ketones, and ether esters. Possible diluents are aromatic hydrocarbons such as toluene, xylene, or higher boiling fractions. Desmodur® Z polyisocyanates are compatible with paint systems containing aliphatic hydrocarbons such as VM&P naphtha. Chlorinated hydrocarbons may be used only if there are no finely divided metal additives in the coating. The choice of solvents and diluents should be evaluated for each coating system.

Should it be necessary to dilute polyisocyanates, special care should be taken when selecting the solvents. Any solvent chosen must not contain hydroxyl groups. This means that no alcohols or solvents contaminated with water should be used. It is not recommended to dilute polyisocyanates below 35% solids since precipitation may occur.

The water content of the solvents or solvent blends used for diluting polyisocyanates should not exceed 0.05% (500 ppm). Urethane grade solvents are generally suitable for polyols and polyisocyanates.

Solvents which contain reactive groups such as amines should not be used since they react with isocyanate groups.

Regulations and safe handling procedures governing flammable and combustible liquids must be followed.

Pigments and Extenders

The following inorganic pigments are suitable for most two-component polyurethane coatings:

• White: Titanium dioxide
• Yellow: Iron oxide yellow, nickel and chrome titanates, chrome and cadmium yellows
• Brown: Iron oxide brown
• Red: Iron oxide red, cadmium red
• Black: Iron/manganese mixed metal oxide black, iron oxide black, some carbon blacks
• Blue: Mixed metal oxide blue
• Green: Chrome oxide green

A marked reduction in potlife may be expected when using the following pigments: zinc oxide, red lead, and some carbon blacks. Zinc phosphate has gained special importance as a corrosion inhibitor, but may shorten the potlife also. Zinc dust and non-leafing aluminum may be used for anti-corrosion primers and nonleafing aluminum types in barrier type primers.

The following organic pigments are suitable:

• Blue: Phthalocyanine blue
• Green: Phthalocyanine green
• Red: Perylene and quinacridone red
• Yellow: Monoazo, isoindoline, monoarylide yellow

Other classes of organic pigments may be suitable. Some organic pigments may catalytically accelerate the curing reaction of polyurethanes because of their metal content. Potlife studies should be done in systems containing organic pigments. Because of their transparency, some organic pigments will not give sufficient coverage in single-coat applications.

In addition to transparent pigments, soluble dyestuffs can be also used to obtain transparent shades. Soluble dyestuffs generally do not have the same lightfastness as suitable organic pigments. Metal complex dyestuffs can also be used.

Conventional extenders for one- and two-component polyurethane coating systems are typical of those used in other types of coatings and include barytes, calcium carbonate, talc, kaolin, mica, precipitated and amorphous silica, and various other silicate types.

Flattening Agents

The incorporation of conventional flattening agents based on silica allows any desired level of gloss to be obtained, either with clear or pigmented two-component polyurethane coating systems. Incorporating polyolefin wax is also advantageous. Micronized polypropylene waxes can be incorporated without difficulty and improve the appearance of the matted surface. In the case of readily dispersible products, it is sufficient to mix in the flattening agent by simple high-speed stirring.

To obtain a matte or eggshell gloss effect, 4-15% flattening agent, calculated on binder, is generally required. This amount will vary depending on the choice of polyol and the mill base composition.

Leveling Agents

Apart from the choice of solvents, adding suitable leveling agents can improve the flow properties when needed.

Cellulose acetate butyrate or low molecular weight acrylic resins are used at levels of 0.2 - 2.0% calculated on solid binder. The level depends on compatibility with the binder.

Polyvinyl acetate, copolymers of PVC/PVAC, and some urea resins may improve leveling properties when used at levels of  0.5 - 3.0% calculated on solid binder. Silicone and polymeric fluids and fluorochemical additives can improve flow by lowering the surface tension of the coating material.

Thickening Agents

Certain application methods such as curtain coating require increased viscosity of the polyurethane coating system. Suitable thickening agents are copolymers of vinyl chloride and vinyl acetate, precipitated silicas, and bentonite clay.

If an increase in viscosity is desired, copolymers of vinyl chloride and vinyl acetate can be added to the polyol solution in quantities of 5 - 10% based on solid binder.

Precipitated silicas increase the viscosity of polyurethane coatings and also provide thixotropy. An addition of 3% is often adequate and has only a minor influence on the gloss level. These products are best suspended with solvents in a dissolver before use.

Bentonite clay thickeners are used in additions of up to 1.5% on solid binder to prevent the settling of pigments and extenders. These agents are best suspended with solvents into a gel before use.

When selecting thickeners and pigment suspending agents, their compatibility with polyols must be considered. Additions that adversely affect the characteristic properties of the polyurethane coatings should be avoided.

Air Release Agents

Air release agents are particularly useful for the prevention of blistering during application by brush or roller coating. Modified polysiloxanes and other types of polymeric additives are commonly used as defoamers and air release agents. Very careful screening should be done to avoid gloss loss problems which may occur when using too high levels of these additives.

Catalysts

Catalysts are used in one- and two-component polyurethane coatings to shorten the curing time, especially in those containing Desmodur® N aliphatic polyisocyanates. Various metal compounds such as dibutyl tin dilaurate and zinc octoate are commonly used catalysts in both two-component coatings and one-component moisture-curing types. Desmorapid PP catalyst, a long chain tertiary amine, is also used in two-component systems. While accelerating the cure of two-component poyurethanes, the use of a catalyst will also shorten the potlife. Excessive catalyst levels can also have a detrimental effect on film appearance, property development, and on exterior durability.

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