EFFECTS OF AGING AND SOLVENT TREATMENTS ON SOME PROPERTIES OF CONTEMPORARY TRACING PAPERS
DIANNE VAN DER REYDEN, CHRISTA HOFMANN, & MARY BAKER
5 PROJECT 3: EFFECTS OF SOLVENTS AND APPLICATION TECHNIQUES ON SELECTED CONTEMPORARY TRACING PAPERS
5.1 RESEARCH PROCEDURES
5.1.1 Solvent Treatment Methods
As shown in table 1, four solvents (water, ethanol, acetone, and toluene) were applied by three techniques (immersion, poultice, and suction disk) to each sample of paper using the following techniques.
IMMERSION TECHNIQUE: For the immersion technique, a 150 square mm section of each sample was dipped into a 3 ml solution of solvent and held there for 3 seconds. The sample was then allowed to air-dry.
POULTICE TECHNIQUE: The poultice technique consisted of placing diatomaceous earth saturated by solvent (0.3 gm diatomaceous earth to approximately 1–2 ml solvent, depending on solvent) on the front of each paper sample lying on a nonabsorbent support.2 The poultice covered an area approximately 5mm in diameter, and, contrary to normal practice, the wet poultice was not surrounded by dry poultice to diffuse the transition from wet to dry areas. The poultice was allowed to air-dry before being removed by an air bulb and brushing.
2 Diatomaceous earth (hydrated silica from diatom plant skeletons) was selected as a poultice for its working properties since, unlike gel poultices (methylcellulose, agarose, starch paste, or hydroxypropylethylcellulose) it can be mixed with aqueous or nonaqueous solvents to form a plaster or paste that absorbs solutes as it dries to a powder, which can then be brushed off. It is more cohesive than fused silica. It is whiter than Fuller's earth, which is formed from hydrated silicates of magnesium, calcium, aluminum, or other metals. It is more controllable than organic solid poultices such as powdered cellulose, paper, or cotton.
SUCTION DISK TECHNIQUE: The suction disk technique consisted of applying three drops of each solvent locally by dropper on a 15 cm fritted glass bead disk (masked off with polyester film), reaching an optimum pressure of ca. 25 in Hg.
5.1.2 Measurement of Properties
Evaluation of the effects of solvents and application techniques on tracing papers included measurement of properties (table 3), as outlined in section 3.1.3, as well as subjective observation of overall appearance (color, opacity, and gloss) in visible and ultraviolet light, and tracking dislocation of furnish materials by SEM and UV microscopy.
5.2 FINDINGS AND DISCUSSION FOR PROJECT 3
Aqueous and nonaqueous solvents are used in conservation treatments of tracing papers to aid in the removal of adhesives and stains, and in humidification prior to flattening. Solvents may also interact, however, with the special additives and morphologies characteristic of tracing papers. For example, water, ethanol, toluene, or acetone may affect the morphology of coatings and impregnants used to transparentize tracings by causing crazing or dissolution of the polymeric films. This effect in turn increases permeability, enabling the solvents to affect the morphology of the paper structure itself by debonding, swelling, and altering the porosity (and thereby the translucency) of the paper. The nature of the interaction of the solvent with the paper substrate is determined by several factors, and a review of a few of these might aid in interpreting the findings of this project. For example, the degree of interaction is affected by the solvent solubility parameters. On the other hand, the speed of interaction may be dictated by structure and evaporation rate of the solvents. Some solubility and evaporation rate parameters for the solvents used in this project are summarized in table 5.
TABLE 5 SOME PROPERTIES OF SELECTED SOLVENTS
The effect of solvents on paper composition may also be influenced by how long a solvent is retained in a paper. Solvents retained in coating films may change the dimensional or chemical stability of a film. The retention time may be influenced not only by the evaporation rate but also by the solvent structure, concentration, temperature, ambient RH, and paper porosity. Depending on solvent molecular structure and shape, for example, water, ethanol, acetone, and toluene would have respectively increasing solvent retention times, since small solvent molecules that are linear, unbranched, and symmetrical (like water) can pass more easily between polymer molecules than solvents that are larger or branched. However, the speed of evaporation can reduce the normal retention time, so that, acetone for example may be removed from paper faster than water and ethanol, even though it is a larger molecule. Likewise, toluene is removed faster than water, due to its higher evaporation rate, even though it is several times larger.
On the other hand, the degree and speed of solvent-substrate interaction and retention time may be manipulated by conservators. Conservators can alter this interaction by selecting and controlling any of various solvent application techniques, using immersion, poultice, or suction systems. Such manipulations alter the conditions of solvent concentration; of the direction of penetration, evacuation, and evaporation of solvent; and of time and rate of solvent exposure. During conservation treatments, for example, as a solvent volatizes or evaporates, the resultant removal of heat may lower the temperature of the surrounding area to below the dew point, causing condensation of water from the atmosphere. Water, whether introduced as condensation or as a solvent, may be absorbed into a polymer film coating and become trapped there as chemically bound water. Such alteration in the hydration state of a film may change its refractive index, and an increase in light scatter causes the film to appear lighter and consequently more opaque, a phenomenon referred to as “bloom,” sometimes seen in coatings on furniture and paintings. For instance, acetone, a hydrophilic ketone that can dissolve cellulose derivatives as well as certain resins and waxes, has a high vapor pressure, and its high evaporation rate can cool coating surfaces, causing moisture condensation leading to bloom (Hess 1965; Horie 1987). Under normal circumstances, pure ethanol is anhydrous and not likely to cause bloom unless it absorbs moisture from the air. Toluene, with an aromatic benzene ring affecting resins and cellulose derivatives, is hydrophobic and a slower evaporator than acetone and so should be able to dry without bloom. However, if the evaporation rate of a solvent is speeded up at room temperature, for example as a result of application and evacuation of solvent by suction disk, the drop in temperature might reach dew point, causing condensation and bloom. Bloom may be confused with physical changes in appearance caused by crazing, leaching and precipitation or redeposition of coatings or additives, and fiber debonding and swelling. Crazing is a network of fine cracks or microfissures within a coating. Leaching can be defined as the dissolution, movement, and redeposition or precipitation of one or more components of a coating, leaving a less compact, often porous or uneven surface. Fiber debonding causes the formation or expansion of interfiber interstices, increasing refractive surfaces and, hence, light scatter. These variables may be responsible for some of the effects solvents and application techniques have on the properties of tracing papers (van der Reyden et al. 1992a, 1992b).
Under the conditions of this study, color was not significantly affected by the application of solvents. In addition, although solvents caused the opacity in most papers to increase to a degree that conservators might find visually unacceptable, the measurable percentage point change in opacity was generally less than the 3–7 percentage points for rag or the 9 percentage points for chemical wood pulp paper considered unacceptable (following aging) by the U.S. Federal Specifications for Tracing Papers (UU-P-561H 1972) (fig. 4). The major exception occurred with the prepared tracing paper, which underwent significant change in opacity (greater than 7 percentage points) following suction application of all solvents except ethanol (figs. 3b–d, 4). Water applied by suction disk caused a significant change in opacity for the chemical pulp imitation parchment. Water in fact generally caused the greatest net change in properties regardless of application technique or type of paper, resulting in appreciable planar distortions and a decrease in gloss. Ethanol decreased gloss in all but the natural tracing paper, regardless of application technique. Toluene generally appeared to have the least effect on the most properties of most of the papers tested in this study, other than its effect on the opacity of prepared tracing paper when applied by suction disk. Suction disk application generally caused the greatest changes in opacity, while poultice most frequently caused the greatest changes in gloss. The visual change induced by poultice may result in part from residual poultice material. Suction application appears to cause the greatest change to cross-section morphology, as seen in SEM images (van der Reyden et al. 1992a, 1992b).
Effects of solvent type and application technique on opacity
SEM photomicrographs showing showing the top surface and cross-sectioned edge of prepared tracing paper after treatment with water (applied with suction disk), showing area characteristic of cracking or fissuring of coating
SEM photomicrographs showing showing the top surface and cross-sectioned edge of prepared tracing paper after treatment with acetone (applied with suction disk) showing increased porosity, possibly from fiber debonding by acetone
SEM photomicrographs showing showing the top surface and cross-sectioned edge of prepared tracing paper after treatment with toluene (applied with suction disk) showing possible dissolution and loss of impregnating material
The increase in opacity measured in this study (fig. 4) could be caused by different reactions (i.e., chemically bound water, crazing, leaching of coating material or additives, or fiber debonding), depending on the solvent (water, ethanol, acetone, or toluene) and application technique (immersion, poultice, or suction). For example, treatment with ethanol applied by immersion or suction disk on the heavily coated prepared paper resulted in a concentration of material on the surface that was identified by SEM/EDS dot mapping as silica (van der Reyden et al. 1992b). A similar phenomenon occurred when the coated vellum paper was poulticed with water, as seen in cross-section SEM photomicrographs (figs. 1e–f). Increased concentration may be due to leaching of the medium, leaving the silica particles uncovered, rather than reprecipitation of material. When the heavily coated prepared tracing sample was treated with water, acetone, and toluene on the suction disk, the change in opacity increased substantially (fig. 4). However, in each case the increase in opacity may be the result of different mechanisms relating to the nature of the solvent-substrate interaction. Comparison of the opacity data (fig. 4) for the prepared tracing paper with the SEM details (fig. 3) of the same paper suggests that the change in opacity following water application on the suction disk may result from the scatter of light caused by crazing or microfissures in the coating (fig. 3b). The change in opacity following acetone application may be compounded by light scatter from fiber debonding (fig. 3c). Finally, the change in opacity following toluene application on the suction disk may be primarily attributable to light scatter from increased porosity following leaching of aromatic soluble resinous material (fig. 3d). The more pronounced change in appearance resulting from suction disk application may relate to the increase evaporation rate, a drop in temperature to dew point, and subsequent condensation of water on or within the polymer coating and paper substrate, causing crazing, bond breaking, swelling, and bloom.
SEM photomicrographs showing the top surface and cross-sectioned edge of prepared tracing paper, untreated
SEM/EDS dot map of cross and surface sections of sample 1e showing location of silica (concentrated in treated area)
The paper most affected by all solvents was the heavily coated prepared tracing sample. Only immersion in toluene had no readily apparent effect, whereas when applied by suction disk, toluene caused an increase in opacity readily apparent in visible light. This finding suggests that the effect of toluene on this paper during a treatment, such as the removal of pressure sensitive tape or dry mount adhesive, might vary according to application technique. However, ultraviolet illumination revealed that areas treated by toluene applied by immersion and poulticing underwent increased absorption (reduced fluorescence), possibly indicative of some minor breakup of surface coating. As already noted, this paper may be prepared with a resin dispersed in toluene, as suggested by the manufacturer. The natural tracing sample (fig. 1a) was the least affected by all treatment conditions and had only a barely perceptible decrease in gloss and increase in planar distortion with water. The highly calendered imitation parchment sample (fig. 1d) and the lightly coated vellum paper sample (figs. 1e–f) were most affected by water.
5.3 CONCLUSIONS FOR PROJECT 3
The effects of solvents on the surface of tracing papers vary a great deal and seem to depend on the composition of the paper, the type of solvent, and the application technique. Of the papers studied here, the natural tracing paper was the least affected and the heavily coated prepared tracing paper was the most affected by the various solvents and application techniques. Water effected the greatest changes (increase in surface distortion and opacity and decrease in gloss) and toluene the least. The property most severely affected was opacity, which increased in most cases. Dimensional stability was most affected by water.