The group contains three types of film former:

One Pack Urethane Alkyds

Prepared by using the glycerides from a drying oil and reacting (at 60-80°C) the free hydroxyl groups with Toluene Di-Isocyanate (TDI).

They do not contain any unreacted insocyanate groups or volatile insocyanate residues and are therefore safe to handle.

Soluble in White Spirit and dry by the normal alkyd oxidation process. The urethane links give an improvement in abrasion resistance, and chemical resistance, plus faster through dry than conventional alkyds. Easily pigmented, they are used extensively in brush applied protective and decorative paints where hard wearing is required.

Moisture Curing Urethanes

These are one-pack and are formed when polyethers containing secondary hydroxyl groups are mixed with an excess of di-isocyanate to form a pre-polymer.

When applied as a coating, the isocyanate reacts with the moisture in the air to form a hard, tough and glossy film.

Generally used as clearcoats, as any pigmentation (with its moisture content) would cause cross-linking and gelation in the can.

Two Pack Polyurethanes

The most important type as far as the transport and refinish industries are concerned.

The coating is formed by the cross-linking mechanism between the nitrogen groups of the isocyanate, when mixed with a resin containing hydroxyl groups.

Isocyanate

The most widely used is Toluene di-isocyanate (TDI), an aliphatic form (free from the yellowing characteristics displayed by the aromatic type).

Isocyanates are usually brought in as 75% solutions.

Resins

These are either saturated polyesters or acrylic resins, both high in hydroxyl value, and designed to suit the end uses of the product, which can range from soft flexible coatings to hard, resistant coatings.

The density of the cross-linking has a marked influence on hardness, flexibility, chemical and water-resistance.

Being low molecular weight, the resins require relatively little thinner after mixing with the isocyanate.

The ratio of the polyester or acrylic res1n to the isocyanate is important.

The resin content is calculated on the basis of a value known as “isocyanate equivalent" i.e. the weight in grams of the resin which reacts with one gram equivalent of isocyanate. The ideal is 1:1, but in practice it is usual to have NCO: OH of 1.1:1.0. The excess NCO is allowable for reaction with moisture.

Pigments are incorporated in the hydroxy resin part. Most can be used, with the exception of some basic pigments derived from metals.

Additives for flow, anti-cratering, etc., also catalysts are carried in this part.

Solvents

The di-isocyanates are soluble in esters. The polyester and acrylic resins are soluble in esters and ketones, and a balance of low, medium and high boilers is used. Aromatic hydrocarbons may be used as diluents, but alcohols (containing OH groups)_must be avoided to prevent pre-reaction. All solvents are moisture free to prevent the formation of carbon dioxide in the mix and in the applied film.

General

The polyester resins are used in the highly durable and organic solvent resistant finishes required for road tankers and aircraft. They are not suitable for metal flake, and are used mainly for solid colours.

The acrylic resins, whilst not having the chemical resistance of the polyesters, however, form hard, flexible, strongly adherent and durable coatings. They are suitable for solid colours, metallics and clearcoats.

The acrylic resin also imparts good polishability.

Regulation of the solvent evaporation rate and catalyst content give good control over the application and drying features.

Two pack polyurethanes are characterised as follows:

Di isocyanates will cross-link hydroxy acrylic resins in a similar way to the amino cross-linking of thermo set resins and is the system used for most warrantee repair paints for T.S.A.

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Isocyanates will also react with water which may lead to the consumption of isocyanate groups and linking of isocyanate molecules by urea linkage.

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Further water will polymerise the isocyanate further with the gassing of Carbon Dioxide.

2 PACK CAR REPAIR PAINT

Spray at 18 sec B4 at 35% solids.

Macronal slightly branched Hydroxy Acrylic Resin 60%.

Desmodure ‘N‘Bexamethyane Di-iso-cyanate at 40% solids.