Effect of Extreme pH on Bond Durability of Selected Structural Wood Adhesives

Authors

  • Xiang-Ming Wang FPInnovations 391, rue Franquet Quebec, QC, Canada G1P 4R4
  • Romulo Casilla FPInnovations 2665 East Mall Vancouver, BC, Canada V6T 1W5
  • Yaolin Zhang FPInnovations 391, rue Franquet Quebec, QC, Canada G1P 4R4
  • Paul Cooper University of Toronto 33 Willcocks Street Toronto, Ontario, Canada M5S 3B3
  • Zeen Huang FPInnovations 2665 East Mall Vancouver, BC, Canada V6T 1W5
  • Xiaodong (Alice) Wang Luleå University of Technology

Keywords:

structural wood adhesives, adhesive pH, long-term exposure, block shear test, bond durability

Abstract

This is the second part of a two-part study aimed at examining the effect of extreme adhesive pH on bond durability. The first part dealt with short-term exposure and this second part dealt with long-term exposure. This part also included an examination of wood degradation by adhesive pH.

Nine structural wood adhesives [four high pH phenol formaldehyde (PF), one intermediate pH phenol-resorcinol formaldehyde (PRF), two low pH melamine formaldehyde (MF), and two low pH melamine-urea formaldehyde (MUF)] were studied in terms of their pH effect on wood-adhesive bond durability using Douglas-fir wood substrate with specimens tested in block shear. The block shear specimens were initially subjected to vacuum-pressure treatment under water, followed by exposure, while wet, at 50°C for 0, 4, 8, 12, and 17 months. At each exposure period, the specimens were dried to their original moisture content prior to testing for shear strength and evaluation of wood failure.

Indications of the extent of degradation of the wood layer adjacent to the bond line due to adhesive pH during the long-term exposure were also examined by the 1% sodium hydroxide solubility test. There were indications that the wood layer closest to the bond line, which contained included glue, had higher solubility compared to those farther from the bond line. This suggests that wood degradation and/or adhesive decomposition occurred and was considered to be induced by the adhesive alkalinity or acidity under the long-term exposure conditions.

The PF showed the best durability performance followed, in decreasing order, by PRF and MF/MUF. The latter adhesives degraded completely after an exposure period of 8 to 17 months.

The four PF adhesives passed the shear strength and wood failure requirements of the well-known North American structural wood adhesive standards indicating that their high pH had no significant detrimental effect on the wood-adhesive bond durability after the 17-month exposure period despite their being subjected to multiple cyclic tests. This observation was not apparent for the PRF, and the pH effect was considered inconclusive for the MF/MUF since they degraded during the exposure period.

The results of this study provide support to wood adhesive standards that do not impose restriction on the upper spectrum of the pH range, and would be useful to adhesive standard developers. These results also serve as background information for adhesive companies in their formulation of wood adhesives as well as for bonded wood product manufacturers in their use of adhesives and for builders in their use of bonded wood products.

Author Biographies

Xiang-Ming Wang, FPInnovations 391, rue Franquet Quebec, QC, Canada G1P 4R4

Principal Scientist/Associate Research Leader
Engineered Wood Products Manufacturing

Romulo Casilla, FPInnovations 2665 East Mall Vancouver, BC, Canada V6T 1W5

Research Consultant
Advanced Building Systems

Yaolin Zhang, FPInnovations 391, rue Franquet Quebec, QC, Canada G1P 4R4


Senior Research Scientist
Wood Biomaterials

Paul Cooper, University of Toronto 33 Willcocks Street Toronto, Ontario, Canada M5S 3B3

Professor Emeritus
Faculty of Forestry

Zeen Huang, FPInnovations 2665 East Mall Vancouver, BC, Canada V6T 1W5

Research Scientist
Engineered Wood Products Manufacturing

Xiaodong (Alice) Wang, Luleå University of Technology

Assistant Professor

Division of Wood technology and Engineering

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Published

2016-11-21

Issue

Section

Research Contributions