Impact of curing condition on pH and alkalinity/acidity of structural wood adhesives

摘要

Nine formulations were selected for evaluating the effect of different curing methods on pH and alkalinity or acidity of various structural wood adhesives. These included four phenol-formaldehyde (PF) resins with high pH, one phenol-resorcinol-formaldehyde (PRF) resin with intermediate pH, two melamine-urea-formaldehyde (MUF) resins, and two melamine-formaldehyde (MF) resins with low pH. The four curing methods used in the study were: (1) curing at 102-105°C for 1 h (based on CSA O112.6-1977), (2) four-hour curing at 66°C followed by 1-hour curing at 150°C (based on ASTM D1583-01), (3) curing at room temperature overnight (based on ASTM D 1583-01), and (4) cured adhesive squeezed out from glue lines of bonded shear block samples. The effect of the different methods on pH and alkalinity/acidity of the cured adhesive depended strongly on the individual adhesives. For the PF, the alkalinity was different for the different formulations in the liquid form, while in the cured form, the difference in the alkalinity depended on the curing method used. The MF and the MUF were the adhesives most affected by the method used. In particular, the MUF showed much higher cured film pH values when cured by method 2 compared to the other three methods, while both the cured MF and MUF exhibited quite variable acidity values when cured with the different methods. The PRF showed reasonably uniform cured film pH but varying acidity values when cured with the different methods.