1. Effects of Catalysts on Isocyanate Reactivity
Catalysts can reduce the reaction activation energy, speed up the reaction rate, shorten the reaction time, and control side reactions. Therefore, catalysts are often used in the preparation of polyurethane.
The catalysts used in polyurethane synthesis mainly include organic tertiary amines and organic metal compounds, both of which can catalyze the reaction with hydroxyl groups and the reaction with water. However, all catalysts vary in their catalytic activity for these two reactions. In general, the catalytic efficiency of tertiary amine catalysts for the reaction between isocyanate and water (the so-called “foaming reaction”) is greater than that for the reaction between isocyanate and hydroxyl groups (the so-called “gel reaction”) The catalytic efficiency of organometallic catalysts for gel reactions is more significant, that is, each catalyst has its selectivity.
1、isocyanate reaction Catalytic mechanism
It is generally believed that the catalytic mechanism of the reaction between isocyanate and hydroxyl compound is that isocyanate or hydroxyl compound first reacts with the catalyst to form an insulator. The stable complex then reacts to form polyurethane. However, there are several theories on this complex catalytic reaction theory, which are still not very clear yet.
A recognized catalytic mechanism is based on the attack of isocyanate by a nucleophilic catalyst to form an intermediate complex, which then reacts with a hydroxyl compound .
In addition, some people believe that the catalytic mechanism of metal-organic compounds is different from that of tertiary amines, and it forms a ternary activation complex. It was proposed that the hydroxyl compound forms a four-membered ring activation complex with the catalyst, and then reacts with isocyanate to form urethane.
2 , The effect of acidity and alkalinity of tertiary amine catalysts on reaction activity
In polyurethane preparation reactions, acids are generally rarely used Catalysts, acidic catalysts (such as benzoyl chloride, inorganic acids and organic acids) have a low catalytic effect on the formation reaction of carbamate and allophanate, but the important thing is that they can inhibit the formation reaction of biuret. , thus inhibiting the cross-linking reaction. If there is still a trace amount of alkali (KOH used in ring-opening polymerization) in the polyether that has not been removed, the alkali metal compound will catalyze the cross-linking side reaction when reacting with diisocyanate, resulting in gelation. Therefore, acid can be added to neutralize, and if the acid is slightly excessive, the cross-linking reaction will be inhibited, allowing the prepolymer to be stored for a long time.
The influence of tertiary amine catalysts on the reaction of isocyanate and hydroxyl compounds depends on factors such as its alkalinity and steric hindrance effect. Generally speaking, greater alkalinity and less steric hindrance means greater catalytic ability. Tertiary amines have a greater impact on the catalytic activity of the reaction between water and isocyanate than the catalytic activity of the reaction between hydroxyl groups and isocyanate, and are mostly used in the preparation of polyurethane foam. Among all tertiary amine catalysts, triethylenediamine is a catalyst with a special structure. Because it is a heterocyclic compound and there is no steric hindrance on the N atom of the tertiary amine, it is not suitable for foaming reactions and gelation. The reaction has strong catalytic performance and is one of the commonly used catalysts for polyurethane foam. It can also be used in the preparation of polyurethane adhesives, elastomers, etc. It is estimated that in a water/alcohol mixed system, its catalytic ability for hydroxyl groups accounts for 80% and water accounts for 20%, and it has catalytic properties similar to those of organometallic compounds. Different tertiary amine catalysts have different catalytic activities for various reactions of isocyanates.
3、organicEffect of genus compounds on isocyanate reaction
Metal salts catalyze the reaction between isocyanate and active hydrogen compounds. Generally speaking, organometallic compound catalysts are suitable for NCO and The catalytic activity of the reaction of OH is stronger than the reaction of NCO and water. It can be seen from Table 2-7 that the same catalyst has different activities for different diisocyanates. Organotin has good catalytic performance for the reaction of aromatic isocyanates and aliphatic isocyanates with hydroxyl groups. The lead octoate catalytic system has the fastest gelling rate because it has a strong catalytic effect on the reaction between isocyanate and urethane. The formation of allophanate makes the resin rapidly cross-linked.
4<font face= Synergy of catalysts Effect
People know that different catalysts have different activities for NCO, and the catalytic activity is also related to different reactions It is related to conditions such as substance concentration and reaction temperature. It can be seen from the data in Table 2-7 that different catalysts have greatly different reaction chemistry between diisocyanate and polyether polyol. For example, the catalytic effect of triethylenediamine on the reaction between aromatic isocyanates and hydroxyl groups is much higher than that of aliphatic HDI and araliphatic isocyanate XDI. Research has found that as the concentration of the catalyst increases, the reaction rate accelerates; when two different catalysts are combined, the catalytic activity is much stronger than that of a single catalyst.
Tertiary amine catalysts also have greater catalytic activity for the reaction betweenNCO and OH, but organotin catalysts have stronger catalytic activity. The combined use of two catalysts can double the catalytic capacity, which is the synergistic effect of the catalysts. In the formula design of polyurethane foam, it is common to use two or two catalysts together, so as to control the balance of foaming reaction and gel reaction and obtain good process performance and foam physical properties.
In a specific reaction system, the appropriate catalytic system should be selected based on the type of reaction and product, the relative activity of different catalysts in relevant data, and practical experience.
II. The effect of temperature on reaction rate
Reaction temperature is an important control factor in the preparation of polyurethane resin. Generally speaking, as the reaction temperature increases, isocyanate interacts with various types of The reaction rate of active hydrogen compounds is accelerated. Under the action of special catalysts, the rate of isocyanate self-polymerization is also accelerated. However, when the reaction temperature is between 130 and 150°C, the rate constants of each reaction are similar.
But it is not that the higher the reaction temperature, the better. When 130℃ or above, the isocyanate group reacts with the urethane or urea bond to produce a cross-linking bond, and Above this temperature, the resulting carbamates, allophanates or biurets are unstable and may decompose. The reaction temperature between hydroxyl compounds and diisocyanates is generally 60 to 100°C.
3. The influence of solvent on reaction rate
To prepare polyurethane synthetic leather resins, adhesives, coatings and other products, solution polymerization is often used, and the type of solvent has a greater impact on the reaction rate.
The greater the polarity of the solvent, the slower the reaction between isocyanate and hydroxyl groups. This is because the solvent molecules are highly polar and can form hydrogen bonds with hydroxyl groups to associate, making the reaction slow. Therefore, in the preparation of solvent-based polyurethane products, hydrocarbon solvents such as toluene are used, and the reaction rate is faster than ester and ketone solvents. Generally, diisocyanate and oligomer diol liquids are bulk polymerized under heating. When the viscosity increases to To a certain extent, when stirring becomes difficult, add an appropriate amount of urethane-grade solvent to dilute it and lower the viscosity so that the reaction can continue evenly. To make the resin have a higher molecular weight, this method should generally be used. Moreover, compared with the solution polymerization method, this method can shorten the reaction time and minimize the impact of the solvent on the reaction. The impact of the solvent on the reaction rate indirectly affects the increase in molecular weight and increases the chance of side reactions.
When the viscosity increases to a certain level and stirring is difficult, add an appropriate amount of urethane grade solvent to dilute it to reduce the viscosity so that the reaction can continue evenly. To make the resin have a higher molecular weight, this method should generally be used. Moreover, compared with the solution polymerization method, this method can shorten the reaction time and minimize the impact of the solvent on the reaction. The impact of the solvent on the reaction rate indirectly affects the increase in molecular weight and increases the chance of side reactions.
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