8 MOISTURE CONTENT AND DEPOSITION EFFICIENCY

STUDIES WERE CONDUCTED to determine the approximate amount of parylene (by weight) deposited on a book during the coating process. The material adsorbs and polymerizes on any cool surface, so it naturally coats the chamber walls and some is lost down the cold trap. Some of the books treated during the course of the study were subjected to the following analysis.

The book was weighed and then placed in the vacuum chamber and evacuated to a point where parylene deposition would normally be initiated (4 – 10 microns). At this point the book was brought back to atmosphere by bleeding of dry nitrogen gas into the chamber. The book was quickly removed from the chamber and weighed. It was then returned to the chamber to which had been added a charge of parylene. The system was then evacuated to the previous level and the parylene was deposited. The chamber was then returned to atmospheric pressure, again using dry nitrogen gas. The book was removed and quickly weighed. The dry nitrogen gas was used to prevent moisture from the air entering through the bleed valve.

The difference obtained between the first and second weighing is the amount of water that is lost during the process of evacuating to deposition pressures. The difference between the second and third weighing is a close approximation of the weight gain caused by deposition. The efficiency factor is obtained by dividing the weight gain by the amount of parylene initially placed in the chamber. Some sample results are as follows:

Example 1 -

• Book weight prior to evacuation = 245.8 grams
• Book weight after evacuation = 234.9 grams
• Weight Loss = 4.4% of total weight
• Book weight after treatment = 250.3 grams
• 15.4 grams parylene deposited on book (47.5% of the original parylene charge used).

Example 2 -

• Book weight prior to evacuation = 307.7 grams
• Book weight after evacuation = 296.5 grams
• Weight loss = 3.6% of total weight
• Book weight after treatment = 321.8 grams
• 25.3 grams parylene deposited on book (44% of the original parylene charge used).

Tests on several other books showed deposition efficiencies ranging from 45 to 50%. This is a higher rate than is obtained for the usual commercial substrates. It is suspected that this higher efficiency is due to the greater amounts of surface area found in books. The moisture loss incurred during the process averages 3 to 5%. The book will regain this lost moisture, usually in three to five days after removal from the vacuum system, depending on the ambient humidity in the room. The process can be hastened by placing the book in a high humidity environment. All books treated by vacuum processes must have their normal moisture content restored. Some types of paper lose flexibility when subjected to high vacuum and must be handled with care until the moisture content returns to normal.