THE DEVELOPMENT OF A RESEARCH APPROACH TO THE SCIENTIFIC STUDY OF CELLULOSIC AND LIGNEOUS MATERIALS
HELEN D. BURGESS, & NANCY E. BINNIE
2 CHEMICAL CONSIDERATIONS
AN IMPORTANT starting point for this project was to survey the chemical reactions of sulphuryl fluoride and to relate this behavior to the known chemical and physical reactions of cellulose and lignin. An attempt was made to look specifically at the interactions most likely to occur under the conditions (e.g., temperature, RH, and Vikane concentration) used in the fumigation of museum collections. The most important aspects of the chemical behavior of the system under study are given below, along with our predictions of how the major components of paper and cellulosic textiles may be affected.
The solubility and reactions of sulphuryl fluoride with water have been found to be strongly pH dependent (Cady and Misra 1975). Rapid hydrolysis in basic solution and slow hydrolysis in more neutral media result in increased acid and fluoride (F−) content (Cady and Misra 1975). Buffering salts and nucleophiles such as amines are subject to attack by sulphuryl fluoride (Cady and Misra 1975; Padma et al. 1982). Consequently, any alkaline reserve in materials (e.g. deacidified paper) may be reduced during fumigation, and acidic residues may be deposited on the fibers. It is unclear whether any observed increase in the rate of degradation of deacidified paper by Vikane fumigation will be due mainly to the loss of buffer reserve or if there are other reactions that may lead to deterioration of the substrate.
An examination of the literature concerning the manufacture of sulphuryl fluoride indicates that hydrofluoric and hydrochloric acids may also be present as impurities in the commercial-grade of Vikane (Gustafson and Skinner 1976; Cook and Gustafson 1978). If present in any significant quantities, these acids will also contribute to depletion of buffer reserve and/or increase in the acid content of the fibers being fumigated. The manufacturer of Vikane is also investigating preparing this fumigant in a process that will reduce the final impurity concentrations (experimental-grade Vikane). Both the commercial-grade and experimental-grade Vikane were included in the GCI/CCI/CAL project.
The consequences of acidic impurities in the fibers can be considerable. In particular, cellulose is sensitive to acid in the presence of moisture. The 4%–9% moisture commonly found in fibers under common ambient conditions (30%–70% RH) is ample to allow acid-catalyzed hydrolysis to proceed, both during the fumigation itself and later, as acidic residues remain in contact with the fibers. Hydrolysis of cellulose most frequently occurs at the 1,4-glycosidic linkage. As hydrolysis proceeds over time, oxidation of the fiber (visualized by yellowing or darkening) and physical embrittlement of the substrate will be observed. Hydrolysis is dependent on time, temperature, and the quantity of water and acid available. Therefore, it was decided that the emphasis in this project should lie upon analytical methods that can detect acidic materials as well as follow any changes that may result from acid attack.
The chain breakage due to hydrolysis leads to a decrease in the average polymer length (also referred to as degree of polymerization, DP¯). Therefore, some procedure that can monitor any changes in polymer length should be included in the experimental protocol.
Acid-catalyzed reactions of lignin have also been described as being of paramount importance in many aspects of lignin chemistry (Sarkanen and Ludwig 1971, 345). Therefore, one would expect that the lignin component of cellulosic fibers would also experience significant degradation, providing the acid content of the fibers increases considerably. The chemistry of the reactions between sulphuryl fluoride and lignin has not been well studied. However, numerous references exist in the literature concerning the chemical and physical interaction of sulphuryl chloride and lignin. It is likely that this information has considerable application to the fluoride derivative. As shown by the chemistry of sulphuryl chloride, the areas of potential reaction of fumigant with lignin are as follows:
- halogenation of the aromatic nucleus (Telysheva et al. 1966). The hydrochloric acid which is present as an impurity in Vikane has been cited as a potential halogenation agent of lignin. (Rassow and Zickmann 1929);
- esterification of side chains involving halogenation, demethylation, and degradation (Paschke 1922; Aparicio and Sastre 1969; Telysheva et al. 1966).
These reactions can degrade lignin by introducing chromophoric groups that may contribute to color changes and lead to reduction in polymeric length and physical strength.
It is unclear whether the fumigation process can have any effect on the state of oxidation of the cellulose and lignin. However, any study that seeks to follow fiber breakdown should include some method capable of determining the state of oxidation of the material. Similarly, it was considered essential to do some type of physical testing, especially since loss of physical strength is a parameter frequently used in scientific projects of this type. An added advantage of physical testing methods is that they give data of value in determining changes in both the cellulose and the lignin portion of fibers. As shown below, there are significant problems in monitoring changes in lignin by the chemical methods currently available.
The available literature suggests that cellulosic and ligneous fibers are not greatly affected by sulphuryl fluoride fumigation (Kenaga 1957; Gray 1960). However, these studies did not use analytical methods capable of detecting very small changes; nor did they include accelerated aging of treated samples or monitoring of materials over a long period of time. Accelerated aging is essential if we are to draw any conclusions about the long-term effects of fumigation with Vikane.
Much of the controversy surrounding fumigation includes the possibility of residues of the fumigant remaining with the fibers and slowly releasing over time (Meikle and Stewart 1962; Scudamore and Heuser 1971). Ethylene oxide has been especially criticized for this problem (Scudamore and Heuser 1971; Green and Daniels 1987; McGiffin 1985). Therefore, it was thought to be essential to investigate this question, particularly since it is such an important health and safety concern. It was also considered desirable to try and include some way of determining if there are residues of reaction products of sulphuryl fluoride associated with the fumigated samples. Detection of chemicals such as these would provide circumstantial evidence of a reaction between the fumigant and the samples and would support any finding of fiber degradation.