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varnishes - or the oil on urethanes

Architect's memo 33a: December 1983

The market of interior clear varnishes is often dominated by the polymers known as polyurethanes - but just what are these products?

The name polyurethane is a generic name covering five major categories of coating in addition to many minor classes; a range of sealants; and a variety of plastics, rubbers, and elastomers. Only three types, the most commonly encountered, will be dealt with in this memo. In 1983 they all involve the use of a reactive material known as an isocyanate.

Isocyanates can react either with themselves or with other polymers containing hydroxyl groups. The reaction with hydroxyl groups is very fast and positive and the two components must be kept separate until just prior to use. Because of this they are known as two-pot urethanes or reaction lacquers. The range of isocyanate containing polymers and hydroxyl containing polymers is vast and between these two ranges a wide variety of coatings can be obtained varying from hard, glass-like materials, to soft, rubber-like coatings. Because of the inconvenience of two component mixtures they are normally only used where their specialist properties are required, such as. chemical resistant clears for laboratory benches or bar tops.

As mentioned earlier isocyanate bearing polymers can react with themselves particularly if water is present as a catalyst. This presents the opportunity of preparing and storing an anhydrous polymer that will react when exposed to atmospheric moisture. These are widely used and known as moisture-cured or one-pot polyurethanes. They have unique abrasion resistance characteristics and have virtually captured the flooring market completely. Their excellent chemical and stain resistance, whilst not as good as some two-pot types are also satisfactory for bar and bench tops.

The history of oil-based varnishes has been one of blending hard resinous components (such as rosin, copals, kauri gum etc.) with oils to achieve a blend of toughness combined with the air-curing ability of the oil.

Alkyd resins are simply a way of combining another hard resin (glyptal) with oil to achieve a range of properties. In a similar manner isocyanates can be combined with oils to give another type of hard resin/oil mixture showing excellent speed of dry and toughness. In this type of product the isocyanate is fully reacted with the oils and further curing occurs only by the standard oil drying mechanisms. These products are referred to as urethane oils, urethane alkyds and unfortunately also as one-pot polyurethanes.

This conflicting nomenclature is unfortunate as these products are very different; the one pot moisture cures being derived from and resembling the two-pot urethanes; the one-pot urethane oils being derived from and resembling traditional varnish technology.

Pointers to enable recognition from data sheets are as follows:


  1. Moisture cured Generally highly aromatic. Must be free from moisture.
  2. Oil or alkyd modified Normal paint thinners (turps).


  1. Moisture cured Generally always glossy as moisture content of flatting agents is too high for stability.
  2. Oil or alkyd modified Available in series of finishes.


  1. Moisture cured Requires only presence of moisture.
  2. Oil or alkyd modified Requires air supply and dries.

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