Amino resin production consists of two stages: hydroxymethylation and condensation.
Hydroxymethylation
Hydroxymethylation can be carried out using either alkaline or acid catalysis, but the former is faster. The hydroxymethylation of urea is slightly exothermic. Industrial processes produce variable mixtures of oligomers with mixed repeat units, so the overall reaction rate is also variable and cannot be defined. Moreover, the important resin properties have not been correlated with the individual reaction rates. In general, the hydroxymethylation rate increases with increased concentration, temperature, and pH. Substituents also have an effect; adjacent electron-withdrawing groups reduce the reaction rate, as do bulky substituents.
Condensation
The condensation is catalysed by acids since the methylene bonds between two urea molecules are very stable and can be hydrolysed only by strong acids at elevated temperatures. The condensation reaction is slightly exothermic. It is impossible to give a single reaction rate for the condensation process because there are many different starting materials (hydroxymethyl compounds) and many end products (methylene compounds). The type of solvent also has an effect on the condensation rate.
Condensation of the resin by means of acid catalysis, i.e., cross-linking to give a substantially infusible product, is not carried out until the resin is put to use. This process is called curing. However, free hydroxymethyl groups are present even in this state. Condensations carried out in an excess of an alcohol, with the addition of acid and removal of water as necessary, produce curable etherified amino resins. These are soluble in nonaqueous solvents and can be mixed with alkyd resins, epoxy resins, etc. They are used as starting materials in the production of surface coatings. Resin glues based on urea have been subjected to partial etherification to stabilise them against further condensation.
Amino resins usually are prepared in 5 – 20 m3 batches. However, continuous processes are employed also, particularly for the production of resin glues. The patent literature describes processes where a particular stage is operated continuously while other stages are operated batchwise. In general, and because the reaction rate and the composition of the end product are influenced by a large number of parameters, the reaction must be controlled carefully. Only the most important processes are described here since there is a large number of reaction parameters and resultant adaptations of the process.
Batchwise Production
The batchwise procedure is the most widely used method for the industrial production of amino resins. Its disadvantage is its relatively small production capacity but on the other hand, this process permits an extensive variety of products and frequent changes of product.
The reactions are carried out in stirred kettles, in two or more stages, at 70 – 100 °C. In the first stage, which is carried out in a slightly acidic to alkaline solution, the principal reaction is hydroxymethylation. In subsequent stages, the condensation is carried out, water is split off, and higher molecular weight products of increasing viscosity are obtained. When the condensation is complete, the product is evaporated in the stirred kettle, or more economically in a tubular vaporiser, with the pressure reduced to protect the resin. The quality of the resin is then compared to its specifications and it is discharged into a storage container.
Continuous Production
The disadvantages of continuous production are that the amount produced per unit time in a particular plant can vary only within relatively narrow limits because prolonged residence under otherwise constant conditions increases the proportion of molecules with high degrees of condensation. Changing product is not simple either. Thus, the continuous production process is employed where the variety of products is restricted to a relatively narrow range. On the other hand, continuous procedures give a very uniform product quality. There are a large number of continuous processes that differ only in process technology for the product flow. In general, the differences include variations in the temperature, pH, concentration, or modifiers. In a typical example, urea and formaldehyde are combined and the solution pumped through a multistage unit. The product is then concentrated in a continuous evaporator to about 60–65% solids.
Reference:
Althaus H.-J., Chudacoff M., Hischier R., Jungbluth N., Osses M. and Primas A. (2007) Life Cycle Inventories of Chemicals. ecoinvent report No. 8, v2.0. EMPA Dübendorf, Swiss Centre for Life Cycle Inventories, Dübendorf, CH.