PARALOID B-72 AS A STRUCTURAL ADHESIVE AND AS A BARRIER WITHIN STRUCTURAL ADHESIVE BONDS: EVALUATIONS OF STRENGTH AND REVERSIBILITY
JERRY PODANY, KATHLEEN M. GARLAND, WILLIAM R. FREEMAN, & JOE ROGERS
These studies were prompted by reported uses of B-72 solutions in either acetone or toluene as barriers applied to substrates of a joint prior to the introduction of Araldite AY 103 epoxy or Akemi polyester structural adhesives. The evident purpose of this combination was to lend reversibility to the joint. As noted previously there is little information in the literature regarding the strength of B-72 as an adhesive. Since the strength of a bond in which B-72 might be used as a barrier directly depends upon the physical adhesive characteristics of the copolymer, it was decided to evaluate both B-72 and the commonly used structural adhesives, Araldite AY 103 epoxy and Akemi polyester, using standard tensile and shear tests.
The test results suggest that the application of a barrier, if relatively thin, conformal, fully continuous, and fully dried (solvent free) before the introduction of a secondary adhesive, provides a considerable degree of reversibility to a joint without decreasing the needed strength required to sustain either shear or tensile loads imposed on most structural bonds made for the reassembly of large stone sculpture. It appears that the B-72 is sufficiently strong to safely be part of a structural joint and to provide a reversible barrier between the substrates and the less reversible structural adhesive. However, the B-72 barrier must be fully dry. This state can be achieved by applying a thin barrier on both faces of the joint when both substrates are fully exposed and can easily lose solvent prior to the application of the structural adhesive. Because of the critical function of the barrier coating, the conservator must be diligent in assuring a consistent and thorough coating of the substrate. Any gaps or voids in the coating might provide an opportunity for the secondary adhesive (epoxy or polyester) to bond with the substrate and reduce the reversibility of the joint. These imperfections may also weaken the adhesive and cohesive strength of the barrier layers.
It must be kept in mind that these are preliminary results and the number of variables that must be considered and tested in the future are significant. The two types of marble used in these tests have a relatively dense structure, and results may change significantly if a more porous substrate were used. Perhaps a more important issue is that to date little has been reported on the cold flow or creep of B-72 (Griffin et al. 1983; Selwitz n.d.), particularly when prepared using a variety of solvents, under either continual tensile or shear loads over a range of temperatures or over extended periods of time. Additionally, solvent type may affect the thickness and consistency of the barrier (Selwitz n.d.), which in turn may have a significant effect upon the cohesive and adhesive strength of the barrier layers and on the strength of the overall bond. Retention of solvent will also affect the overall bond strength when practical drying times are used. Some solvents in B-72 mixtures require considerably greater time to leave the polymer, as in the case of toluene, which can take up to two months to evaporate thoroughly (Horie 1987, 104). The barriers and solvent-based adhesive bonds in this study were allowed to dry for several weeks at room temperature. Additionally, these evaluations were done by considering objects that are meant to be protected within a relatively controlled environment. Environmental variables, particularly in the outdoors, might have significant effects on the characteristics of the adhesive bond and also need further study.
As in any effort within conservation, progress toward a more reversible and yet sufficiently strong adhesive bonding system must be made cautiously and slowly, as each of these variables is identified and evaluated. The authors have presented this study as an initial step to bring this effort under way.