Solvent
In water-based polyurethane coatings, although eventually dissolved in water, when the Polyurethane Resin is synthesized, the system is absolutely water-free. This has a higher requirement for the choice of solvent, in addition to considering the solvent commonness such as solubility, volatility, etc. Consider the characteristics of the isocyanate groups in the coating.
1 The solvent can not contain substances that react with isocyanate
1. Influence of water The water contained in the solvent reacts with isocyanate to generate bubbles or pinholes, and may also cause cross-linking.
2RNCO + H2O RNHConHR + CO2
It can be seen that not only the performance change but also the product ratio is affected, because 1 mol of water consumes 2 mol of isocyanate. For TDI, 1 mol (18 g) of water consumes 1 mol (174 g) of TDI, which causes a mismatch in the feed ratio and eventually produces waste products. .
Therefore, in polyurethane coatings, both the solvent used for making and diluting must be anhydrous.
The solubility of water in solvents is shown in the following table:
Solvents Solubility per 100 g Water Solvents Solubility per 100 g Solvent Water Dissolved Acetone Total Methyl Isobutyl Ketone 1.90 g (25°C)
Cyclohexanone 8.70g (20°C) Butyl acetate 1.37g (20°C)
Acetate Cellosolve 6.50 g (20°C) Benzene 0.06 g (23°C)
Ethyl acetate 3.01 g (20°C)
2, urethane grade solvent
The urethane-grade solvent means that the impurity content (especially water, alcohol, and acid) is very small, and the solvent that can be used for polyurethane coatings has higher purity than general industrial products.
The greater the isocyanate equivalent, the better the stability. In general, less than 2500 are unqualified solvents. "Isocyanate equivalents": The mass (g) of solute required to consume 1 mole of isocyanate groups is the isocyanate equivalent weight.
Urethane ester solvent isocyanate equivalent ester solvent Purity % Boiling range °C Isocyanate equivalent ethyl acetate 99.5 76.0-78.0 5600
Butyl acetate 99.5 122.5-128.0 3000
Acetic acid cellosolve 99.0 150.0-160.0 5000
2. Effect of Solvent on Reaction Rate of Isocyanate Group
The more polar the solvent is, the easier it is to form a hydrogen bond with the hydroxyl group and the association occurs, which slows down the reaction of the isocyanate with the hydroxyl group. For example, the difference between toluene and butanone is 24 times.
In the preparation of polyurethane coatings, the reaction rate with hydrocarbon solvents (such as xylene) is faster than with esters and ketones; but in two-component polyurethane coatings, esters and ketones are used because the reaction rate is reduced and the construction period is reduced. increase.
3 Influence of the surface tension of the solvent In the polyurethane coating, poorly arranged or improperly applied, the film tends to generate air bubbles, which is especially true in the case of moisture curing.
When the surface tension of the solvent exceeds 0.035 N/m, blistering is less likely to occur.
Solvent surface tension Solvent surface tension, N/m Solvent surface tension, N/m
Cyclohexanone 0.0381 Methyl isobutyl ketone 0.0254
Xylene 0.0328 Butyl Acetate 0.0276-0.0289
Acetic Acid Cellosolve 0.0318-0.0327 Ethyl Acetate 0.0239-0.0243
Toluene 0.0300
Solvent and polyurethane coating surface tension relationship (N/m)
Solvent involatile content mass fraction cyclohexanone xylene acetate cellosolve butyl acetate
60% 0.0404 0.0379 0.0379 0.0333
50% 0.0423 0.0347 0.0379 0.0334
40% 0.0420 0.0355 0.0359 0.0321
Source: King of PU
