Printer Friendly

Parylene at the Canadian Conservation Institute.

Parylene at the Canadian Conservation Institute

The Parylene method is a Union Carbide process in which thin films of poly-para-xylylene are created on surfaces. Films are slowly deposited as monomer, molecule by molecule, in a highly controlled manner. Parylene forms a completely even and transparent film over all surfaces, including internal ones. It conforms to all surface irregularities and thus a coated surface maintains morphology, hence appearance is little affected.

Films have useful consolidant properties in the range 500 to 20,000 nM and these are beginning to be studied in conservation laboratories.

The deposition process, which takes place under vacuum, consists of several phases:

1. Parylene dimer (I) is vaporized.

2. Dimer vapour passes through a pyrolysis tube at 670 20 C, where dimer molecules split to create two active monomer units (II).

3. Active monomer diffuses into the sample chamber at room temperature. Upon collision with a surface the active monomer condenses and then polymerizes to form a coherent polymer film.

The diffusing molecules penetrate porous substrates, coating the inner surfaces. Film thickness can be controlled to within 1,000 nM.

The process envelopes specimens within a perfectly conformal film. Two versions of Parylene are commonly used, `N' is unsubstituted, while `C' has one chloride on each aromatic ring. Parylene N is used in applications where better diffusion is required.

CCI and the Fossil Forest Interest in Parylene at CCI was generated when the institute was asked to assist the National Museum of Natural Sciences in the conservation of unique plant macrofossils. These specimens came from recently discovered sites in the Geodetic Hills region of Axel Heiberg Island, NWT. Here, preserved in poorly consolidated Tertiary deposits, lie the remains of several ancient subtropical forests.

The `Forest' Strata and Specimens within the `Leaf Mat'

The forests are preserved in stratified deposits as intact `floors'. Within strata are stumps, fallen trees and leaf mat containing leaves, ferns, cones, seeds of a wide range of species. Despite their great age, the plant fossils are non-mineralised and sometimes non-compressed. The preservation is astonishing. Unfortunately, the specimens become extremely delicate on drying and some have the consistency of cigarette ash.

The National Museum of Natural Sciences recovered a large number of specimens for display and study. The whole collection, however, was in imminent danger of destruction, making radical action necessary.

Consolidation of the Delicate Specimens

Normal consolidation procedures were considered but were ruled out because of the extreme delicacy of the specimens. It was necessary to consider unconventional approaches, and that is why Parylene was investigated. The great virtues of this method are that it is so gentle, and that consolidation can be accomplished in stages until just sufficient strengthening has been achieved.

To test the process, four very deteriorated spruce, four flattened dawn-redwood, and four spruce cone fossils were taken to Union Carbide's Nova Tran laboratories in Clear Lake, WI. Specimens were placed carefully in the vacuum deposition chamber. The most crucial part of the process is the outgassing, when loss of moisture occurs. Outgassing interferes with deposition and must be complete before the coating can start. During this phase the cones were watched closely. While there was some opening of the scales this enhanced rather than diminished the cones' appearance. During the deposition no change was seen. On completion, the results were remarkable. Parylene had strengthened them effectively without any noticeable effect on appearance.

Examination under the stereo microscope at x 100 magnification showed little evidence of film on the surface. This `invisibility' arises from the conformal nature of this coating. At higher magnification, in thin section, however, the Parylene is clearly visible as a very even birefringent layer.

The Study The initial success of the method was very convincing and led to a wider study of Parylene. Various museums and galleries in the Ottawa area have been contacted and a wide range of applications are being tested in a joint study. A number of techniques are being used to study the treated items (where appropriate) including colour change by Minolta chromameter, surface gloss, response to RH change and various aging methods.

Possible applications for the Parylene process include:

Textile conservation: the consolidation of disintegrating silk;

Archaeological conservation: the consolidation of fragmenting textile fragments;

Ethnographic conservation: the consolidation of shredded cedar bark, the prevention of hair slippage in furs, the consolidation of dry pigment and the consolidation of wood badly damaged by insect activity;

Objects conservation: the stabilization of deteriorated synthetic plastic objects;

Book, paper and library conservation: The strengthening of fragile and brittle paper and whole books;

Natural history conservation: the consolidation of various types of fossil, the preservation of plant and insect, crustacean and other marine invertebrate specimens;

Museum display and didactic specimen preparation: The preparation of a wide range of plant and animal specimens to allow display or withstand frequent handling;

Other applications: RCMP Forensic Laboratory, preservation of charred paper, Canadian Aviation Safety Board strengthening of deteriorated water- or fire-damaged documents.

Reversibility of Treatment As used in conservation the term reversibility refers to the ability to reverse a treatment and for Parylene it would thus mean removability of the Parylene film. There are three aspects of this ethical question to consider: reversibility and how it applies to different categories of specimens or artifacts; the action of Parylene relative to other consolidants and the interaction with substrates; and the deterioration of Parylene.


Ideally, materials for conservation should be those which can be removed easily and completely, so that if treatment fails the potential for re-treatment exists. Parylene, obviously, does not meet this important criterion, (although the lack of removability may not prevent further treatment). Failure to meet this criterion is, by itself, not a reason for not considering Parylene because there are many other important criteria. Parylene strengthens delicate materials in a uniquely effective way and it thus has the potential to save materials for which there is no other hope, and also conserve materials in a way superior to normal procedures.

Another point to consider is that all artifacts, works of art or specimens are not created equal. Library books, for example, may well benefit from application of Parylene to increase their durability. It may be a much more attractive solution than microfilming and destruction! It is also becoming cost competitive. It is helpful to categorise items based on condition, uniqueness and condition to establish which could benefit from Parylene treatment.


How does Parylene compare with other consolidation processes? Comparisons are difficult because Parylene has unique abilities. For example, what other methods could be used to preserve a spider's web?

Compared to other methods Parylene has three considerable advantages. Unlike nearly all other consolidants, it consolidates without altering appearance. In most instances its presence is hardly evident.

It does not interact with the substrate, and accords well with the conservation ethical of minimal intervention. By contrast, many conventional consolidants interact chemically with the substrate. For example, in all other paper strengthening processes the strengthening agent bonds chemically with the cellulose. Chemical bonding also takes place for siliceous consolidation of stone and with epoxy resin consolidation of wood.

Parylene is used in far lower quantities by comparison with other materials, again effectively meeting the requirement of minimal treatment.

Although few impregnants are truly removable, some do have short term solubility. With Parylene the problem can be avoided by ability to conduct the process in a series of small steps.

On the stability of Parylene, studies show it is thermally stable and if antioxidants are added it becomes even more stable. Arhenius plots suggest that Parylene should survive in darkness at 25C for between 10,000 and 100,000 years before 50% of the strength is lost, making it more stable than most substrates. Stability to light is less well studied. Some data shows that it is sensitive to ultraviolet light but less so to visible. It is noteworthy that Parylene films as normally employed are so thin compared to other consolidants or varnishes are so thin that even if they yellow the effect may be imperceptible. At CCI and elsewhere this is being actively investigated.

PHOTO : Scanning electron microscope image of an iridescent butterfly wing shown here before

PHOTO : coating with Parylene N. Photograph recorded by Tom Strang, senior assistant conservation

PHOTO : scientist at the Canadian Conservation Institute.

PHOTO : Scanning electron microscope image of an iridescent butterfly wing photographed by Tom

PHOTO : Strang after coating with 0.25 m of Parylene.

DAVID W. GRATTAN, MCIC Canadian Conservation Institute, Department of Communications, Ottawa
COPYRIGHT 1989 Chemical Institute of Canada
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Author:Grattan, David W.
Publication:Canadian Chemical News
Date:Oct 1, 1989
Previous Article:Telling images: the Bader gift of European paintings to Queen's University.
Next Article:Preliminary study for the production of propylene and ethylene in Nova Scotia.

Related Articles
Parylene conformal coatings boost elastomer seal performance in alternative fuels.
Transparent film adds value to elastomers.
Sport Fishing Institute of BC Applauds Salmon Treaty Renewal.
Canadian Regulators Approve New Coating to Strengthen Penile Implants By American Medical Systems.
Elastomer coating. (Materials).
Conformal coatings enhance product surface.
Specialty Coating Systems acquires Parylene Japan K.K.
SCS acquires Parylene Japan K.K.
Specialty Coating Systems' new adhesion promotion technology.

Terms of use | Copyright © 2017 Farlex, Inc. | Feedback | For webmasters