Лако-красочные материалы — производство

1. Post-Addition of Urea

The addition of urea to a phenolic resin causes several effects:

decrease of the content of free formaldehyde decrease of the viscosity of the adhesive resin

acceleration of the hardening reaction via the possible higher degree of condensation of the resin

reduction of the costs of the resin.

The urea can be added to the finished PF resin or during its manufacture. The distinct decrease of viscosity observed when urea is added to the finished PF resin is caused by the cleavage of hydrogen bonds [162] and by the dilution effect. There is obviously no cocondensation of this postadded urea with the phenolic resin. Urea reacts only with the free formaldehyde of the resin to form methylols which, however, do not react further due to the high pH [163]. Only at high temperatures did Scopelitis and Pizzi [164] suppose some phenol-urea cocondensation occurs, but in their case the phenol used was the much more reactive resorcinol.

The higher the amount of postadded urea, the worse the properties of the boards. A reason for this might be urea’s diluting effect on the PF resin. Surprisingly Oldoerp and

Marutzky [165] found enhanced board properties at higher degrees of addition of uncon­densed urea. Since, however, in these experiments the postadded urea could be extracted completely from the boards, no significant cocondensation between the urea and the phenolic resin could have occurred. Using such PUF resins, the adhesive solids content should be calculated based only on the PF resin solids content in the PUF resin.