The most common method of preparation for commercial UF resin adhesives is the addition of a second amount of urea during the preparation reaction. This consists of reacting urea and formaldehyde in more than equivalent proportions. Generally, an initial urea/formaldehyde molar ratio of 1:2.0 to 1:2.2 is used. Methylolation can in this case be carried out in a much shorter time, by using temperatures of up to 90 to 95 C. The mixture is then maintained under reflux. When the exotherm subsides (usually after 10 to 30 min), the methylol compounds have formed, and the reaction is completed under reflux by adding a trace of an acid to decrease the pH to the UF polymer-building stage (pH 5.0 to 5.3). As soon as the right viscosity is reached, the pH is increased to stop polymers building and the resin solution is cooled to about 25 to 30 C. More urea (called second urea) is added to consume the excess of formaldehyde, until the molar ratio of urea to formaldehyde is in the range 1:1.1 to 1:1.7. After this addition of urea, the resin is left to react at 25 to 30 C for as long as 24 h. The excess water is eliminated by vacuum distilla- tion until a resin solids concentration of 64 to 65% is reached, and the pH adjusted to achieve suitable shelf life or storage life.