A typical polyurea consists of a multi-ingredient chemical formulation commonly shipped in 55-gallon drum sets or 5-gallon pales. Part A is a dark colored viscous liquid called isocyanate. Part B is commonly called the amine resin blend. It is often colored, or pigmented, and normally requires agitation or stirring before use.
When the liquids are thoroughly mixed, they result in an immediate chemical reaction that becomes viscous and ultimately solid. This reaction is very fast and typically sets up dry to the touch within seconds. Ultimate cure usually takes up to 24-48 hours, however longer and shorter cure time durations are quite possible depending on specific formulation and characteristics of mix.
The use of different types, and/or different volumes, of isocyanates, polyether, or polyester amines, chain extenders and other additives can have a significant effect on the ultimate physical properties and final characteristics of the polyurea coating/lining system.
Technical Aspect of Polyurea Formulations
Polyureas have been described as the resin from a polyurethane reacted with the curative of an epoxy. This is a good description, as polyurea coatings do seem to take the best from both of these polymer technologies. They have improved chemical and solvent resistance, and higher temperature resistance compared with the polyurethanes. They also have better impact resistance and higher elongation vis-à-vis the epoxy.
A polyurea is formed when amines react (cure) with the isocyanate. This reaction is fast, auto-catalytic (that means it does not need a catalyst to react – even at cold temperatures) and leads to many of the special properties that allow polyureas to distinguish themselves from the other polymers.
There are three main properties:
- Polyurea reactivity is independent of the ambient temperature. Polyurea reacts fast – and it will react at the same speed regardless of the temperature. It can be 100 F or -25 F and the reactivity is almost the same. Polyurethanes can be catalyzed to also react very fast, but a system designed for 70 F, will take forever to cure at -20 F. A polyurethane system that will cure properly (fast) at -20 F will be too fast to handle at 70 F. Epoxy cannot cure at these very low temperatures.
- Polyurea reactivity is independent of the ambient humidity. It can be 98% RH and the polyurea coating will spray bubble-free. (Be careful of the dew point – that’s a different story) A polyurethane must have catalysts to complete the reaction. These catalysts are designed to catalyze either the polymer reaction (gelation) or the “blowing” reaction (many polyurethanes use this well known use of water to react with the isocyanate to release CO2 to use as the blowing agent in polyurethane foams). Unfortunately, either types of catalyst will catalyze BOTH reactions – to some degree. The choice of a good gelation catalyst will still catalyze the ISO/water (from the humid air) reaction – and when it does you WILL have bubbles.
- Polyureas develop their physical properties FAST. This gives the polyurea “FAST RETURN TO SERVICE” — meaning you can drive on the coated floor and/or use the coated product within 12 hours of application. Polyurethanes take up to 14 days to fully develop their physical properties. Epoxy require several days.
As mentioned before, polyureas have improved chemical and solvent resistance and higher temperature resistance compared with polyurethanes. They have improved impact resistance and elongation compared with epoxies. All of the properties can be impacted by the formulating chemist.
The formulating chemist will always want to know the application for which his polyurea system is intended. The proper selection and amount of raw material components will effect the performance – both processing and physical properties.