Theories of resinification

If the product of the first stage, which in practice usually also contains unreacted urea and formaldehyde, is then subjected to acid conditions at elevated temperatures the following sequence of events is observed during the second stage  

For technical purposes it is often convenient to both arrest the reaction prior to gelation by changing to a slightly alkaline pH and to remove some if not all of the water. The first hardening reaction may then be carried out when desired by changing once again to an acid pH. 

The precise mechanisms involved during the second stage are not fully understood although a number of theories have been proposed, for example by Brookes, Kadowaki, Marvel, Redfam, ° Thurston, de Jong and de Jonge, and Zigeuner.  

The subject has been extensively reviewed by Vale and Taylor , who place particular emphasis on the work described in references 12 and 13. 

Since groups (as found in dimethylol urea) are apparently less reactive 

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