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title(En) :Studies on the Powdered-Dry Bonding Process in the Manufacture of Plywood. III. On the Gelation Behaviours of Urea-Resin Adhesive
author(En) :Atsuo Tanaka, Kiyomine Takashima
information :Mokuzai Gakkaishi 12(6), 272-276 (1966)
assort :Original Article
summary(En) :   When urea-resin adhesive is used for bonding of wood, the heat of wetting for the wood in the spreading and the reaction heat of, urea resin in the curing process have effects on the curing rate of urea-resin in the glue line. Therefore, the actual curing time of urea-resin adhesive in the glue line does not always coincide with the gelation time measured in the test tube in the thermostat.
   This paper is the first of a series in which the studies of the exothermic behaviours of urea-resin adhesive in the curing process will be presented. This paper outlines the gelation behaviours of urea-resin adhesive after mixing NH4Cl hardner by measuring the variation of pH, viscosity, composition and reactant temperature.
   The results obtained are as follows;
   1) In the presence of ammonium chloride hardner, the pH of urea-resin adhesive drops as time goes on and the gelatinization occurs around the same value of pH. Therefore, the larger the rate of pH descent is, the shorter the gelation time of urea-resin is.
   2) As the condensation degree of urea-resin progresses, the effect of an added quantity of ammonium chloride hardner on the gelation time is decreased. It seems that this tendency is based on the formaldehyde quantity produced in the curing process.
   3) In the case of addition of ammonium chloride hardner, the rate of cure of the urea-resin adhesive is relatively slow in the initial stage before gelatinigation, but in the following stage a very rapid rate of change of cure commences as the pH come near to a critical stage. Here, the formation of the high molecular weight products and the cross-linking of molecules in three dimensions take place.
   4) As will be seen from Fig. 7, the general form of the reactant temperature-time curve in the curing process can be divided into three exothermic stages differing in the rate of change of reactant temperature. It seems that this behaviour is mainly attributable to the rate of change of pH and the curing reaction velocity of the urea-resin.