Self-Leveling Coatings: Polyols for Urethanes

In previous articles we have covered self-leveling coatings, urethanes and isocyanates. In this month’s column, I will cover the isocyanate-reactive components: polyols and amines. 

An isocyanate can react with a hydroxyl group, water or an amine. Figure 1 shows these reactions. If an isocyanate reacts with a hydroxyl group, a urethane is created (a). If it reacts with water, an amine and CO₂ evolve (b). If an isocyanate reacts with an amine, then a urea results (a). 

However, functional groups have different reactivities. They are detailed in Table 1. A couple of things to notice from Table 1: water is much more reactive than secondary alcohols, and the amine groups are incredibly fast compared to polyol groups. I will cover how this affects formulation in the next column.

For most self-leveling coatings, 100% solids are needed. This precludes polyols like acrylic-based ones. Also, since water pools on self-leveling coatings, polyester polyols have limited use due to the hydrolysis of the backbone. This leaves polyether polyols and castor oil as the most used polyols.

Due to the high reactivity of amines with methyl diphenyl diisocyanate (MDI), the pot life is too short, and they are used with aliphatic isocyanates and UV-resistant topcoats. It is rare for monomeric MDI to be used; generally, ~200 centipoise/~2.7-functional polymeric MDI (pMDI) is used due to increased crosslinking.

Castor oil is used in large quantities in self-leveling coatings but needs some polyether polyols to balance properties. Triols (usually alkoxylated glycerin) are used to increase toughness (compression strength and hardness), while diols are used to increase flexibility and crack bridging. It is rare to use only castor oil with polymeric MDI. Castor oil is unique; it contains ~92% ricinoleic acid, so it is very consistent in structure and properties across the world (Figure 2).

If higher hydroxyl content is needed, you can hydrogenate the double bonds in the castor backbone. Functionality can be increased up to six from three. This will significantly increase crosslink density and toughness of the coating. However, nothing comes without cost: hydrogenated castor oil has a high viscosity and can reduce the flowability of the coating.

Polyether polyols are based on the alkoxylation of the hydroxyl groups on a starter. Since the hydroxyl group is extended out during the alkoxylation, if you start with a diol, you end up with a diol — the same for triols (Figure 3 shows the reaction). Normally, for self-leveling coatings, most starter alcohols used are alkoxylated with propylene oxide (proproxylated). While ethoxylation with ethylene oxide is sometimes used, it increases the hydrophobicity and viscosity of the polyol when compared to proproxylated polyethers.

If using a base catalyst like KOH, as proproxylation increases, side reactions and backbiting lead to unsaturation. This lowers the crosslink density of the polyurethane, and monols cap chain growth and decrease physical properties. So, it is common to put an ethoxylated cap on the end of the proproxylated polyol since the side reactions are reduced. Another advantage of an EO cap is similar reactivity as water, lessening the water side reactions. Newer catalysts like double metal cyanide (DMC) catalysts eliminate most of the side reactions compared to base catalysts, yielding a “purer” polyol.

In next month’s edition of Formulating with Mike, we will complete self-leveling coatings with formulation tips.

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