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
1. In 1995 the Department of Antiquities Conservation of the Getty Museum undertook an evaluation of the swelling characteristics of seven common epoxies and two polyester formulations. It was found that after sufficient exposure to dichloromethane (methylene chloride) fumes in order to destroy the cohesive and adhesive strengths of the material, unrestrained samples of the adhesives swelled between 8% and 14%. This increase in volume, when experienced by a restrained sample (adhesive in a closed joint or within a hole under confined conditions), may induce sufficient pressure to fracture marble and disrupt fragile surfaces.
2. For calculation A: The stress (S) is arrived at by multiplying the moment, M (which is arrived at by multiplying the distance from the joint to the center of gravity, L), by the distance to the extreme fiber (C), which is measured outward from the axial center of the cross section of the segment to the farthest edge of that cross section. This product is then divided by the section moment of inertia (I), which, for a round cross section, is p multiplied by the diameter to the fourth power. This is then divided by 64. By necessity some measurements are generalized or idealized, such as the average diameter. The shear load on the joint is calculated by dividing the weight of the arm (force) by the area of the joint surface, which for a 15.24 cm (6 in.) surface is A = pr2 or 182 cm2(28 in. 2).For calculation B: Center of gravity is that point in the mass of the segment where all forces act upon the mass equally. It is a point at which the arm could be supported or suspended in perfect balance. The diameter used in this stress calculation is the diameter of the joint, since that is where the stress we are interested in is. The diameter used to calculate the shear load discussed above is the diameter of the arm segment at the joint interface. Fy is equal to the weight of the arm or force. In this calculation a/(maximum tension) and b/ (maximum compression) are equal.
3. A 17% solution was chosen for samples 6-9, rather than the 22% solution used for the shear test specimens, since this has been commonly used in the Getty laboratory as a barrier coating. The authors are confident that this 5% concentration difference had no substantial effect upon the results reported.
4. Stress is computed by dividing the load at failure by the area that is resisting the load. This is the average stress. If the stress distribution is perfectly uniform, the average stress is the same as the stress at all locations in the joint. Failure occurs when the stress exceeds the maximum that the material can withstand. If the stress distribution is nonuniform, failure still occurs when the highest stress anywhere in the joint reaches the failure stress limit. But in the case of nonuniform stress distribution, the average stress that is computed will be lower than the actual maximum stress that precipitated the failure. Therefore, in most cases, the reported failure stress (average stress) will be lower than the actual failure stress (local stress), due to nonuniform distribution of the stress.
5. Horie (1987) points out that the value of the measured glass transition temperature can be lowered considerably by increasing the time during which the force is applied. He gives an example of a polymer with a 30°C (86°F) Tg, but when measured over a period of 1.2 years with continual stress, the Tg lowers to 12°C (53.6°F) (Horie 1987). This finding has significant implications for structural joints that are meant to be serviceable for long terms.
The authors would like to thank the following people for their valuable assistance and involvement: Martha Simpson for her initial work on the study of B-72 barriers during her internship in the Department of Antiquities Conservation at the J. Paul Getty Museum; Will Thornton, mount maker in the Department of Antiquities Conservation, J. Paul Getty Museum, for preparing the aluminum holding fixtures used in the tensile tests; Bill Ginell and Michael Schilling of the Getty Conservation Institute, and Charles Selwitz, consultant and chemist, for their guidance, review of the manuscript and many helpful insights; and Narayan Khandekar for his invaluable advice regarding UV fluorescence microscopy while an associate scientist at the Getty Conservation Institute. We are also grateful to our respective spouses for giving up numerous weekends so that we might play with marble blocks.
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SOURCES OF MATERIALSAkemi Marmokitt 1000
(an unsaturated polyester containing styrene monomer and fumed silica, catalyzed using High Speed Hardening Paste, a benzoyl peroxidebased catalyst) Akemi Plastics Inc. P.O. Box 40 Eaton Rapids, Mich. 48829Sivic marble
Walker Zanger Inc. 8901 Bradley Ave. Sun Valley, Calif. 91352B-72 adhesive
HMG (H. Marcel Guest Ltd.) Riverside Works Manchester M10 7RU UKAraldite AY 103
(a modified epoxy used in this case with HY 991, a polyamine hardener) Ciba Polymers Duxford Cambridge UKParaloid B-72
Rohm and Haas Co. Philadelphia, Pa. 19105
JERRY PODANY received his M.F.A. from Claremont Colleges, Claremont, California, in 1978 and a certificate in archaeological conservation from the Institute of Archaeology, University of London, in 1982. He has been employed in the Department of Antiquities Conservation at the J. Paul Getty Museum since 1978 and has served as the head of that department since 1985. He is adjunct professor in the Museum Studies Program at the University of Southern California. He has a particular interest in the reassembly methodologies for large-scale sculpture, on-site conservation, and protection of collections and monuments from seismic risks by the application of engineering principles to conservation practice. He has been an AIC Fellow since 1998. Address: J. Paul Getty Museum, Department of Antiquities Conservation, 1200 Getty Center Drive, Suite 1000, Los Angeles, Calif. 90049-1687. E-mail: JPodany@getty.edu
KATHLEEN M. GARLAND has a B.A. in art history from Brown University (1981) and an M.A. in conservation from the State University of New York, Cooperstown (1985). She had an internship in sculpture conservation at the Victoria and Albert Museum, London, and served as senior sculpture conservator for the National Trust for Great Britain, 1986-89. Currently, she is conservator of objects, Nelson-Atkins Museum of Art, Kansas City, Missouri. She is an AIC Fellow. Address: Conservation Department, Nelson-Atkins Museum of Art, 4525 Oak St., Kansas City, Mo. 64111. E-mail: Kgarland@nelson-atkins.org
WILLIAM R. FREEMAN has a B.S. in mechanical engineering, University of Florida (1973), and a master of engineering in mechanical engineering, University of Florida (1975). He was a machinery engineer at Union Carbide Technical Center, 1975-78, and since 1979, he has been staff engineer at Allied Signal Federal Manufacturing and Technologies, Kansas City, Missouri. The simulation of physical phenomena by computer has been a specialty since his graduate research. He currently utilizes a world-class dedicated SGI/Cray supercomputer to simulate a wide range of mechanical phenomena for the U.S. Department of Energy utilizing both implicit and explicit finite element methods. He is author of several papers on computer modeling, advanced simulation techniques, and bioengineering topics. He provides engineering assistance for a number of small businesses and nonprofit organizations (such as the Nelson-Atkins Museum of Art and J. Paul Getty Museum) supported under DOE's Technical Assistance Program. Address: Allied Signal FM&T, D/ME1, MC 1C41, 2000 E. 95th St., Kansas City, Mo. 64131. E-mail: firstname.lastname@example.org
JOE ROGERS has been a staff member of the Nelson-Atkins Museum of Art since 1983. He was the conservation assistant in the Paintings Conservation Department until 1988 and is currently the objects conservation associate. He is interested in the treatment of furniture and large sculpture. Address as for Garland. E-mail: email@example.com Received for review January 27, 2000. Revised manuscript received July 25, 2000. Accepted for publication September 11, 2000.