1 INTRODUCTION

The emphasis of most fiber research has been on forming new fibers and understanding their structures and properties as opposed to examining aged fibers. Some work concerning the structure and properties of fibers during or after use has been done in an effort to predict performance during long-term use, but the fiber science research literature contains little information pertinent to aged fibers. The aim of this paper is to discuss five types of fiber ageing and some general concepts that are useful to textile conservators. In this paper, ageing is intended to include any process that alters the original structure and/or properties of materials.

Nearly all textile fibers are organic, high molecular weight polymeric materials. For example, cotton, flax and many other natural fibers are composed mostly of a common high molecular weight carbohydrate polymer, cellulose, whereas many other natural fibers such as silk and wool are composed mostly of high molecular weight protein polymers. Most man-made fibers also are composed of high molecular weight organic polymers. Textile fibers can be considered to be three-dimensional rodshaped materials having enormous surface-to-volume ratios compared to most materials, to be semicrystalline (partially crystalline and partially noncrystalline), and to posess some amount of net polymer orientation (the alignment of polymer molecules is not random). Textile fibers are physically complex materials. Chemically, they may be either rather simple or quite complex when not aged, but all fibers can be expected to become chemically complex when aged.

The major structural changes expected to occur during ageing can be ascertained from a structural view of fibers. An ageing reaction may occur homogeneously throughout a fiber or may proceed heterogeneously, such as beginning at the fiber surface and subsequently proceeding inward. In addition, ageing may occur with morphological specificity, such as only in noncrystalline regions of a fiber. An ageing reaction may alter the molecular weight, crystallinity, or orientation of fibers. The gross size or shape of fibers may change during ageing. Finally, the chemical composition of fibers may be altered, such as by altering the chemical structure of the polymer or by the addition of soil.

There are many ways to classify the types of ageing in polymer materials. For this review, five types are identified. In physical ageing, strictly physical structural changes occur over time, and no additional energy needs to be supplied for physical ageing to occur. Photochemical degradation results from chemical changes when additional energy is supplied to materials through the absorption of electromagnetic radiation (photons), such as visible or ultraviolet light. Thermal degradation occurs when structural changes result from the absorption of thermal energy (heat). Chemical attack may result in ageing when energy is supplied to materials through attack by external chemical species, such as when oxidation occurs from peroxide bleach. Finally, ageing may occur through mechanical stress, such as when the gross shape of textiles changes as a result of sagging during display or storage.

Accelerated ageing conducted under laboratory conditions usually attempts to control most of these means of ageing while varying exposure to a limited number of these ageing types (usually one). Natural ageing, on the other hand, results from complex mixtures of many or all of these types of ageing. Consequently, the structures and properties of naturally aged textiles would be expected to be considerably more complex than those from simple laboratory ageing. However, one or more means of ageing might be expected to be eliminated in some cases during natural ageing so changes in structure or properties may be dominated by a limited number of ageing types. It is thus useful to examine the general effects of each ageing type on textile structures and properties to provide general guidelines when working with textiles during conservation.