Urethane coatings--part 2.
EW=%functional group/mol weight of the functional group
The stoichiometry calculation is:
Weight of Isocyanate Needed = Weight of polyol/HEW x IEW x Stoichiometry Factor
Since there will always be some polyurea side reactions with the isocyanate, the isocyanate should be over indexed. The source of the side reaction can be atmospheric moisture, carboxylic groups, or residual water in other raw materials.
For solventborne or 100% solids applications the factor is between 1.05 and 1.1.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
For waterborne applications, the factor is between 1.5 and 3. This is due to the high amount of polyurea formed by water and amines in water-based systems. Most water-based additives in either the emulsion or the formulation (surfactants, flow additives, defoamers, etc.) are carboxyl functional and will react with the isocyanate. It is also important to use non-hydroxyl functional co-solvents in water-based two-component urethane systems, since the primary hydroxyl of the solvent will react much faster than the polyol (Figure 1).
Most urethane systems also need some form of catalyst (usually organometallics or amines) to speed up the reaction. Issues arise when you speed up the reaction too much and end up with too short a potlife (the time it takes for the viscosity of the system to double). Some of the conventional organometallic driers are based on tin, zinc, bismuth, and zirconium, with dibutyl tin dilaurate being the most frequently used.
Potlife extenders work by tying up the catalyst until they evaporate. 2, 4-Pentanedione works well but increases the volatile organic content of the coatings.
The last class of urethanes are polyurethane dispersions (PUD). These are difunctional isocyanates reacted with difunctional polyols to form linear polyure-thanes. These urethanes are then dispersed in water (using surfactants or neutralized carboxylic groups in the backbone). They have the balance of flexibility and hardness of a urethane system but, due to very little or no three-dimensional crosslinking, chemical resistance is worse than standard urethane systems. Most PUDs have the potential for further crosslinking and using a polyisocyanate, aziridine, or carbodiimide to crosslink the linear chains vastly improves the properties of the coating. Figure 2 demonstrates the resistance to methyl ethyl ketone (MEK) rubs for a PUD and the same PUD crosslinked with different chemistries.
The final properties of a given urethane system will depend on the stoichiometry ratio used. The cure time versus potlife balance can also be changed with catalysts and potlife extenders.
Mike Praw is group leader, technical marketing for resins at BASF Corporation; michael.prawAtbasf.com.