Лако-красочные материалы — производство

There are a number of complex reactions that can occur besides the desired reaction of the polyol hydroxyl group with the isocyanate group to form a urethane, as shown in Fig. 4. Isocyanates can continue to react with undesirable consequences under conditions of high heat or strong bases. Basic impurities and excess heat catalyze branching reactions, leading to variations in prepolymer viscosity, gelation, and exotherms. Most basic impurities arise from the polyol, since polyols are typically produced under basic condition. As such, the net acidity of the overall system (contribution of acidic or basic components from the reactants) plays a critical role in determining the final viscosity achieved [21,22].

The presence of water will lead to the formation of ureas and evolve CO2 as shown in Fig. 5. This mechanism is thought to proceed through the formation of an unstable intermediate, carbamic acid, which then decomposes to give CO2 and an aromatic amine. The amine will then react further with another isocyanate to give a urea linkage. All common moisture-cured urethanes give off CO2 upon curing, which can pose problems

Urethane Prepolymer

Figure 4 Reaction of polyol hydroxyl group with isocyanate group to form a urethane.

Isocyanate

V F

OCN — R — NCN — R — NCO + CO,

II

О

Urea Carbon dioxide gas

Figure 5 Reaction of isocyanate with water.

if not properly controlled. Urea groups are known to cause high prepolymer viscosity because of increased hydrogen bonding and because of their ability to react further with excess isocyanate groups to form a biuret, as shown in Fig. 6.

At room temperature the biuret reaction proceeds very slowly; however, elevated temperatures and the presence of trace amounts of basicity will catalyze the biuret reaction as well as other branching reactions. These would include the formation of allophanate groups, as shown in Fig. 7 (due to the reaction of urethane groups with excess isocyanate groups), or trimerization of the terminal NCO group (to form an isocyanurate), as shown in Fig. 8. Biurets and allophanates are not as stable thermally or hydrolytically as branch points achieved through multifunctional polyols and isocyanates. The allophanates shown in Fig. 7 can continue to react with excess isocyanates to form isocyanurates (as shown in Fig. 8), a trimerization reaction that will liberate considerable heat. In most cases the desired reaction product is the simple unbranched urethane or a urea formed by direct reaction of an isocyanate with an amine. Ureas are an important class because they typically have better heat resistance, higher strength, and better adhesion. By controlling the reaction temperature (typically less than 80°C) and stoichiometry, and using a weakly

о о

Urethane

V?

OCN-R-NCO——— OCN-R-NCO

О (j=o

NH

I

R-NCO

Allophanate

Figure 7 Reaction of urethane with isocyanate.

basic catalyst (or none at all), the reaction will stop at the urethane or urea product. Increasing the functionality of the polyol or the isocyanate will achieve branching or cross-linking in a more controlled fashion.