Ester solvents are not allowed to contain water, free acids and alcohols, and will react with NCO groups. Ester solvents used in polyurethane should be high-purity "polyurethane-grade solvents". That is, the solvent reacts with excess isocyanate, and then the unreacted isocyanate is measured with dibutylamine to check whether it is suitable for use. The principle is that the consumption of polyisocyanate is not applicable, because the water, alcohol and acid contained in the ester will consume the total value of isocyanate. If expressed in grams of solvent required to consume leqNCO groups, the larger the value, the better the stability, as shown in Table 2-6.
Isocyanate equivalent less than 2500 shall not be used as polyurethane solvent.
The polarity of solvent has a great influence on the reaction of resin formation. The greater the polarity, the slower the reaction. For example, the difference between toluene and methyl ethyl ketone is 24 times. This solvent is polar and can form hydrogen bonds with hydroxyl groups of alcohol, which slows down the reaction.
Aromatic hydrocarbon is the best solvent for polychloroprene, and their reaction speed is faster than that of esters and ketones, such as xylene. In the construction of double-buckle polyurethane, the use of ester and ketone solvents can prolong its service life. In the production of coatings, the "urethane solvent" mentioned above is beneficial to the storage of stable parts.
Ester solvents have strong solubility, moderate volatilization speed, low toxicity and are widely used. Cyclohexanone is also used more. Hydrocarbon solvents have low solid solubility and are rarely used alone, but are often used together with other solvents.