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
This is a continuation of the short-term testing performed in Phase II of this study to determine the effects of adhesive pH on wood-adhesive bond durability. In this phase, the Douglas-fir block shear specimens prepared in Phase II using the nine structural adhesives, viz. four high pH phenol-formaldehyde (PF), one intermediate pH phenol-resorcinol-formaldehyde (PRF), two low pH melamine-urea-formaldehyde (MUF), and two low pH melamine-formaldehyde (MF), were tested periodically for up to 12 months under long-term vacuum-pressure / re-dry (VPD) condition. The VPD consisted of vacuum-pressure treatment followed by 0, 4, 8, and 12-month exposure durations at 50°C. The specimens were dried, in each exposure period, to their original moisture content prior to testing for shear strength and wood failure.
Indications of the extent of degradation of the wood layer, adjacent to the glue line due to pH during the long-term exposure, were also examined by the 1 % sodium hydroxide solubility test. The results indicated that the wood-layer samples closest to the glue line, which contained included-glue, showed higher solubility compared to those farther from the glue line. This suggests that wood degradation and/or potential glue decomposition occurred and is considered to be induced by the adhesive alkalinity or acidity under the long-term exposure conditions.
The PF showed the best durability performance followed by PRF and MF/MUF. The MF/MUF degraded completely after the 12-month exposure period.
For the PF, there are indications that some degree of degradation occurred in the wood layer adjacent to the bond line during the 12-month exposure period, which could be attributed to the high pH of the adhesives. This observation was not apparent for the PRF, and is considered inconclusive for the MF/MUF since they degraded during the exposure period.
Full title: Impact of extreme pH of structural adhesives on bond durability as related to development and modification of CSA O112 wood adhesive CSA standards. Part III. Evaluation of block shear properties of selected wood adhesives by long term exposure test
Nine structural adhesives with varying pH were selected to examine the effect of pH on wood-adhesive bond quality. These included four high pH phenol-formaldehyde (PF), one intermediate pH phenol-resorcinol-formaldehyde (PRF), two low pH melamine-urea-formaldehyde (MUF), and two low pH melamine-formaldehyde (MF) adhesives. Block shear specimens were prepared with these adhesives using Douglas fir and black spruce. The adhesive performance was evaluated by measuring the shear properties (strength and wood failure) of the specimens tested at the dry and vacuum-pressure / re-dry (VPD) conditions.
Adhesive pH, test condition, and wood species showed significant effects on the shear properties. Douglas fir yielded about 40% higher shear strength at the dry condition compared to the VPD condition. Black spruce showed smaller difference in shear strength between the dry and the VPD conditions, the difference being only about 6%.
The different adhesives performed differently at the dry and VPD conditions. The high pH adhesives showed similar wood failures at both test conditions. On the other hand, the low pH adhesives showed high wood failure at the dry condition, but dropped significantly at the VPD condition for both species. This indicates that the low pH adhesives were less durable than the high pH adhesives.
Some correlation was observed between shear properties (strength and wood failure) and cured adhesive pH in the VPD condition, but not in the dry condition. Such a correlation was stronger in Douglas-fir than in black spruce.
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 II. Evaluation of block shear properties of selected wood adhesives by short term exposure test
A round robin study using the CSA O112.9 standard was conducted with six participating test laboratories, three in Canada and three in the U.S., with the support of the CSA Wood Adhesive Sub-Committee. The tests included block shear (dry, vacuum-pressure, and boil-dry-freeze), delamination, and condition B1 creep. Test specimens were prepared using Douglas-fir as substrate and four adhesives (phenol-resorcinol formaldehyde, melamine-urea formaldehyde with 80% melamine, melamine-urea formaldehyde with 40% melamine, and catalyzed polyvinyl acetate).
Precision and bias statements (repeatability and reproducibility limits) were developed for the block shear test (strength and wood failure) and delamination test for O112.9. The statistical procedures of ASTM E 691 were found to be inadequate for the block shear test, so appropriate statistical procedures were identified and applied. However, a refinement of the E691 method was used to analyze the delamination test. The creep data was not analyzed because it showed huge variability across the laboratories. The rest of the data will be analyzed in the future.
In the block shear test, the repeatability estimates obtained from the round robin test tend to overestimate those of O112.9, that is, the outcomes from using the O112.9 test procedures would actually vary less than those indicated by the limits derived from the study.
The performance of four structural wood adhesives was evaluated in round-robin testing according to Canadian wood adhesive standard CSA O112.9, and the repeatability and reproducibility of selected procedures in that standard were assessed [W-2622]. This is a report of the data collected in the project, with file references (it does not include the data).