Parameters of the parylene conformal deposition treatment and their effects on ethnographic materials

Mary-Lou Florian

Abstract

Parylene is a clear plastic which in industry is used 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 adsorbed 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 19-22ºC to 27.5 – 35ºC. 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 material 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.

Scanning electron micrographs illustrate the dynamics of deposition of films of different thicknesses. Empirical visual analysis of a variety of materials is presented. The pros and cons of this treatment will be discussed.