Nine structural adhesive formulations were selected to evaluate the effect of different curing methods on pH and alkalinity and/or acidity of adhesives. These included four phenol-formaldehyde (PF) resins with high pH, one phenol-resorcinol-formaldehyde (PRF) resin with intermediate pH, two melamine-urea-formaldehyde resins (MUF) with low pH and two melamine-formaldehyde (MF) resins with low pH. Four curing methods were used to prepare cured resin samples for the study: 1) curing at 102-105oC for one hour based on the CSA O112.6-1977 Standard; 2) curing for four hours at 66oC, followed by one hour at 150oC based on the ASTM D 1583-01 Standard; 3) curing at room temperature overnight based on the ASTM D 1583-01 Standard; and 4) cured adhesive collected from glue line squeezed-out from block shear assembly.
The effect of the curing method on pH of the cured adhesive strongly depended on the individual adhesives. For the PF, the alkalinity observed was different for each resin tested in the liquid form, while in the cured form, the difference in the alkalinity depended on the curing method. The MUF and MF were the most affected by the curing method, particularly the MUF, which showed much higher cured film pH values when tested by method 2 compared to the other three methods, while both the cured MF and MUF exhibited quite variable acidity values when measured with the different methods. The PRF showed reasonably uniform cured film pH but varying acidity values when measured with the different methods.
A reasonable relationship was observed between pH and alkalinity and between pH and acidity when the adhesives were considered as a group (i.e., adhesives of high pH as one group and adhesives of low pH as another group). Such a relationship was weaker when the adhesives were considered individually.
The use of a liquid sample in the determination of alkalinity/acidity of adhesives by titration was more convenient than using a cured film sample.
Full title: Impact of extreme pH of structural adhesives on bond durability as related to development and modification of CSA O112 wood adhesive standards. Part I. Investigation of different test methods for measuring pH, alkalinity and/or acidity of cured adhesive films and cured adhesives