Force Formaldehyde resin

The same chemical mechanisms and driving forces presented for phenol-formaldehyde resins apply to resorcinol resins. Resorcinol reacts readily with formaldehyde to produce resins (Fig. 2) which harden at ambient temperatures if formaldehyde is added. The initial condensation reaction, in which A-stage liquid resins are formed, leads to the formation of linear condensates only when the resorcinol/formaldehyde molar ratio is approximately 1 1 [119]. This reflects the reactivity of the two main reactive sites (positions 4 and 6) of resorcinol [120]. However, reaction with the remaining reactive but sterically hindered site (2-positiori) between the hydroxyl functions also occurs [119]. In relation to the weights of resorcinol-formaldehyde condensates which are isolated and on a molar basis, the proportion of 4- plus 6-linkages relative to 2-linkages is 10.5 1. However, it must be noted that the first-mentioned pair represents two condensa-... 

The same chemical mechanisms and driving forces presented for phenol-formaldehyde resins apply to resorcinol resins. As regards application and hardening, PRF adhesives are always delivered as a liquid and must be blended with a liquid or powder hardener (generally paraformaldehyde and wood flour fillers) before use. 

Forced by the necessity to limit the subsequent formaldehyde emission, the UF-resin molar ratio, F/U, has been progressively decreased to very low values. The main differences between UF-resins with high and with low content of formaldehyde, are (1) the reactivity of the resin due to the different content of free formaldehyde, and (2) the degree of crosslinking in the cured network. 

The driving force in the use of these salts as hardeners is their capacity to release acid, which decreases the pH of the resin and thereby accelerates curing. The speed of the reaction between the ammonium salt and formaldehyde (or ammonia and formaldehyde when this is present) also determines, together with the amount of heat supplied, the rate of acid release and therefore the rate of curing ... 

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