Pitfalls of transparency. Investigating bonding defects and failures in transparent poly (methyl methacrylate) objects.

Anna Laganà, Melissa David, Suzan de Groot, Henk van Keulen, Odile Madden, Michael Schilling, Michael Doutre, Maarten van Bommel
Getty Conservation Institute (GCI), Los Angeles, USA
Cultural Heritage Agency of the Netherlands (RCE) Amsterdam, The Netherlands,
University of Amsterdam (UVA), The Netherlands

Bonding transparent poly (methyl methacrylate) (PMMA) plastics is challenging due to the difficulty of obtaining joints that closely approximate PMMA in strength and transparency. PMMA is particularly susceptible to crazing and other joint defects that can occur during fabrication or appear later. As part of a broader project exploring the conservation of transparent plastics used in art and design, an international collaborative study was undertaken to investigate methods that have been used to bond PMMA plastics throughout its 90-year history and the related defects that can be observed in museum objects today.

PMMA has been bonded to itself with range of solvents, swelling agents, and adhesives. The most common and best known problem in the PMMA near these bonds is stress-crazing.
High localized stresses in the PMMA, introduced during manufacturing, can be released upon exposure to certain liquids or solvents found in cleaning agents or bonding materials. These liquids and solvents may attack the PMMA releasing some of the internal stresses unequally. This results in the development of fine fissures in the PMMA, which reduce transparency and are a warning sign that that the plastic is weakening.

However, crazing is not the only problem that can compromise transparency and strength of PMMA joints; objects in museum collections also show a variety of phenomena in the joints themselves. Particular types of defects and failures seem to recur, which suggests they may relate to specific bonding techniques. Hazy joints, bubbles in various patterns, signs of shrinkage, and various types of cracks are only some of the issues that can be observed at the joint interface.

The research sought to observe and classify these conditions, to understand their causes, and to assess their potential influence on the long-term stability of PMMA objects. Questions included whether the observed defects occurred during fabrication or as a result of material degradation processes. Do particular defects continue to develop over time, and what effect, if any, do these conditions have on the long-term stability of the PMMA object?

To investigate these questions, PMMA art and design objects in museums were surveyed (Stedelijk Museum Amsterdam, Cultural Heritage Agency of the Netherlands Art Collection,
Museo del Novecento), and several types of plastic defects were described. Trade literature and craft manuals from the 1940s to today were reviewed to learn how PMMA has been bonded over time. A selection of these bonding materials and methods was recreated in the laboratory, and several types of defects that had been observed in the survey were successfully reproduced. These mock-ups were compared visually and analytically to selected surveyed objects using strain viewers, UV light, Py-GCMS, FTIR spectroscopy, and SEM.

Understanding the nature of bonding and related defects in PMMA art and design objects is valuable knowledge on several fronts. This presentation will discuss the study and results in order to introduce conservators to the kinds of defects one might encounter, their causes, and their potential to affect bond stability over time.