Synthetic Materials in Art Conservation Anthony Werner Pacific Regional Conservation Center. 1355 Kalihi Street, P.O. Box 19000-A, Honolulu, HA 96819 One of the exciting developments that has heen taking place in recent decades in the conservation of antiquities and works of art has resulted from the annlication of the chemist's sne.. cialized knowledge to the prohlem of devising new methods of treatment based on the studv of svnthetic materials. In the past the methods used for the remedial treatment of antiouities and works of art that had suffered damage in museums or had undergone corrosion while buried in the ground had been limited to the use of natural materials such as beeswax, glue of vegetable or animal origin, and natural resins. This meant that the actual methods of conservation were to a large extent dictated by the properties of whatever naturally occurring material seemed most suitable for the particular work a t hand. Proeress was therefore constrained within narrow limits. However, during the past three decades or so there have been remarkable advances in the domain of high polymer chemistry-the so-called plastics industry. These have led to the production of a wide range of new synthetic materials. Conservation scientists have found that many of these synthetics are of ootential value in the field of conservation and restoration of antiquities and works of art. Many of the synthetic materials have been found to possess a combination of physical and chemical properties not found in materials of natural origin. They are, in fact, quite distinctive materials. For examole there are svnthetic wax-like materials which exhibit a combination of properties not found in any animal or vegetable wax. It is for this reason that the use of these synthetic materials has afforded the opportunity of develooinz imoroved and more reliable methods of conservation:It s ndw often possible to carry out remedial treatments which were very difficult, if not virtually impossible, to do solely with materials of natural origin. The purpose of this article is to demonstrate how the chemist working in museums with conservators has succeeded in developing new techniques for the conservation of antiouities and works of art. An overview of some of the research aimed a t incorporating synthetic materials which are specifically designed to meet the requirements of the problems involved will he presented also. It must, however, be emphasized a t the outset that these svnthetic materials must not he regarded as mere substitutes for the naturally occurring materials previously used.
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Structure of Synthetic Materials In general, synthetic resins have a macromolecular structure Thev are comand are often referred to as high - oolvmers. . . posed either of long chains or networks of simple repeating units, known as monomers. Polyvinyl acetate, a synthetic resin is, for example, produced by the polymerization of the monomeric vinyl acetate according to the following equation
T h e average number (n)of monomeric units incorporated in the polymer is known as the degree of polymerization (D.P.) The decree and thus the leueth of the final - of .nolvmerization . " polymer chain may vary depending on the conditions under which it is produced. Thus a series of polyvinyl acetates can
he made of long, short, or intermediate length. The mechanical properties of these polymers will depend upon the D.P. In general, with increasing D.P., the resins will he tougher, harder, and a t a given concentration produce solutions of higher viscosity. If a single monomer is polymerized, the resulting resin is referred to as homooolvmer. It is. however.. nossihle to . polymerize a mixturehf two different monomers, thus producing a co-polymer in which units of both monomers are incorporated-within the same chain. Such a co-polymer is a distinct kind of macromolecule with its own special properties. Synthetic resins may be divided into two main groups which are fundamentallv different in structure. namelv thermoplastic and thermosetting. ~ h e r m o ~ l a i t resins ic have a structure in which the monomeric units form indenendent. two-dimensional, linear chains which are not connected by covalent bonds. Thev can, therefore, be dissolved in an anpropriate solvent andare permanently fusihle, i.e., they soften when heated and harden again on cooling. On the other hand, the thermosetting resins have a more complex structure in which the constituent monomeric units are condensed together to form a three-dimensional network in which all the monomeric units are united by covalent bonds; consequently, they are insoluhle and infusible. I t should, incidentally, be noted that, under certain conditions, linear thermoplastic resins mav he converted into the insoluble networks. This
