Paraloid B-72 as a Structural Adhesive and as a Barrier within Structural Adhesive Bonds: Evaluations of Strength and Reversibility

Abstract

Acrylic copolymers are now well known in conservation practice. Based upon relative stability and reversibility, Paraloid B-72 has found a particularly wide range of uses as a coating, consolidant, and adhesive. Increasingly, B-72 is being used as an adhesive for reassembly of large stone sculpture. It has also found use as an interface barrier that, in theory, lends greater reversibility to joints made with less reversible adhesives such as epoxies or polyesters and limits the penetration of these materials into the adherend substrate. The following two studies were undertaken as an initial evaluation of these systems and to make a direct comparison between B-72 and the more commonly used structural adhesives, epoxy and polyester, from the standpoint of shear and tensile strength, as well as reversibility.

Modified ASTM standard tests were used to determine the strength of marble-to-marble bonds formed with epoxy, polyester, and high-concentration solutions of B-72 in two commonly used solvents. Bonds made with epoxy or polyester adhesives including additional barriers of B-72 on both substrates of the adherends were also tested. The results indicate that in some solutions B-72 alone may be sufficiently strong under tensile load for use as a structural adhesive and that B-72 used as an interface barrier is sufficiently strong for structural joints as well as able to increase reversibility of bonds where polyester or epoxy is used as the adhesive. However, solvent loss from the B-72 adhesive or B-72 barrier must be sufficient to assure full bond strength. Further study is needed to evaluate the adhesive qualities of B-72, since characteristics such as the porosity and absorbency of the substrate and the possible long-term reordering and creep of the copolymer under constant long-term stress could affect the adhesive bond.