2.7 Evaluation of Parylene for Treatment of Ethnographic Objects
The Royal British Columbia Museum
The Getty Conservation Institute
Mary Lou Florian
Period of Activity: 9/88 to 9/89
The vapor-deposited parylene polymer treatment produces a conformal coating on materials. Because of the parylene film's high strength, pliability, transparency, and water resistance, it has been under investigation for treatment of museum artifact materials that have little inherent strength.
An evaluation was made of the effects of this treatment on various types of materials found in ethnographic objects and natural history specimens (i.e., cedar barks, textiles, leather, bone, wood, baleen, paper, sinew, and insects). It was found that the vacuum and temperature conditions existing in the parylene treatment chamber during treatment caused materials temperature increases from 5 °C to 15 °C and weight loss of between 50% and 90% of their equilibrium moisture content. After treatment, near complete weight regain was observed for most materials over time, but thermogravimetric analysis of vacuum-treated samples showed some bound water loss.
Parylene film thickness variations occurred up to 50% as a function of location in the deposition chamber. With porous materials the actual film thickness at a given location could not be controlled. Scanning electron microscopy showed irregular deposition on insect parts and the distribution of parylene as a function of coating thickness.
Florian, M-L., and G. Field, "Evaluation of Parylene Conformal Coating for Treatment of Ethnographic Objects," Final Report to the Getty Conservation Institute, June, 1990.
ABSTRACT-see Project Abstract.
Florian, M-L., "Parameters of the Parylene Conformal Deposition Treatment and Their Effects on Ethnographic Materials," The American Institute for Conservation of Historic and Artistic Works, Paper presented at the 18th annual meeting, May 29-June 3, 1990, Richmond, Virginia, p. 48.
ABSTRACT-Parylene is a clear plastic which is used in industry to conformally coat microcircuitry and metals to prevent corrosion. The plastic film is water repellant but permeable to water vapour. Because of its conformal application, strength, pliability, transparency, and permeability, it has been suggested for treatment of artifact materials which have little inherent strength.
The purpose of this project was to evaluate this treatment for ethnographic objects. Materials (cedar barks, textiles, leather, bone, wood, baleen, paper, sinew) used in ethnographic artifacts were tested.
The coating is done in a deposition unit which contains furnaces for vaporization and pyrolysis, and a vacuum chamber. In the furnaces the solid plastic parylene dimer is converted to a monomer gas. The gas moves into the vacuum chamber in which the material to be treated has been placed. On contact with a surface, it is absorbed and polymerized without going through the liquid phase to form a conformal film packaging the material.
The parameters of treatment and the changes of the materials were measured. The results showed an increase in the temperature in the vacuum chamber from 19-22 ºC to 27.5-35 ºC. Times from 62-240 minutes were required for evacuation and deposition. Materials lost from 50% to 90% of their equilibrium moisture content (EMC). This suggests an adverse effect of the vacuum treatment. Dimensional changes of material due to loss of EMC could not be measured, but must have occurred. Textural changes of vacuum-treated textiles were observed.
In the vacuum chamber, on releasing the vacuum, a change from 0% RH (theoretical) to ambient room 50% RH occurs. On initial regain of materials the temperature of condensation or heat of wetting must occur, increasing the temperature of materials.
Weight regain showed complete "weight" regain in most materials. But thermalgravimetric analysis of the water in vacuum-treated materials showed permanent loss of bound water in baleen and a suggestion of water alteration in other materials. At extreme reduction of EMC, a permanent loss of molecularly bound water is likely to occur.
Stratification of parylene gas in the vacuum chamber, in a specific run, resulted in films from 0.8 microns to 1.9 microns in thickness. Because of this stratification and the variable porosity of materials, it is difficult to predetermine a film thickness.