2 AGING

UNTIL RECENTLY, the tapes most frequently encountered on archival materials and art on paper were masking tape and cellophane tape. Both have rubber-based adhesive. As these adhesives oxidize, they pass through distinct stages of deterioration, the exact nature of which has been documented in studies conducted by Robert Feller.3 Initially, there is a period of little alteration, an induction time. During the induction time, removal is relatively easy. As oxidation progresses, this stage is followed by a fairly abrupt change in adhesive consistency and color. The adhesive mass gets very sticky and oily, perhaps, in part, because of the breaking up of the rubber polymer. It also starts to yellow. Apparently during this stage various components of the adhesive soak into the paper, rendering it translucent. Some components probably remain, at least temporarily, on the surface. In this oily condition the adhesive mass can penetrate the paper entirely and move into adjacent sheets. Its components can also begin to affect certain media—particularly printing, typing, and ballpoint pen inks—causing them to bleed. Removal of tape during this very sticky period is usually still possible, but is more difficult.

The adhesive, having permeated the paper, continues to oxidize, and gradually loses its adhesive properties. The carrier may fall off, and the adhesive residues crosslink, becoming hard, brittle, and highly discolored. Once it has reached this condition, the adhesive residue and the stain it has created are very difficult, sometimes impossible, to remove.

The aging process differs in the new acrylic adhesive tapes. They don't discolor appreciably. The adhesive mass does not typically soak into the paper as rubber-based adhesives do. The acrylic adhesive is, however, subject to cold flow and will penetrate to the degree that paper porosity allows. According to 3M literature, this difference in aging behavior may occur because each acrylic adhesive is one homogeneous polymer which, once coated to the backing in its final form, is pre-crosslinked. Because of this pre-crosslinking the acrylic pressure-sensitive adhesives (with the possible exception of those used in the so-called “archival” tapes) are not soluble in any of the solvents used in paper conservation. They can only be swollen and scraped or brushed off mechanically.

The initial reaction to the advent of Scotch Brand #810 Magic Mending Tape was one of great excitement. In 1961 3M summarized the tape's characteristics as follows:

No. 810 is very stable, in that both adhesive and backing are, for all practical purposes, inert… The backing, being acetate film, is not affected by changes in temperature and/or humidity. To the best of our knowledge, it will not shrink or discolor under normal conditions of natural aging… The adhesive… will not “set up” and become hard, it will not discolor into the paper to which it is applied. Although we do not have sufficient natural aging experience to guarantee the tape indefinitely, we do have accelerated aging data which we feel is very significant. In our tests, No. 810 was applied to various types of papers and subjected to severe conditions of temperature and relative humidity, and to artificial sunlight… Upon completion of the test, there was no indication of discoloration, oozing, lifting, nor penetration of the adhesive into the paper.4

It is worth remembering that the composition and formulae of proprietary products may be altered by manufacturers, with no corresponding change in either trade name or packaging. Magic Mending Tape is no exception. A newly purchased roll of the tape in the Library of Congress Conservation Office was lighter in color than an older roll, which was noticeably more yellow. In the absence of contrary information, one might think that the yellower roll had deteriorated. In fact, according to 3M, the formulation of the tape had been changed. Also, optical changes may have occurred due to shrinkage in the roll resulting from storage conditions. Because of the possibility of formula changes in manufactured products, it is unwise to automatically apply testing data from a roll of tape manufactured in 1983, for example, to one manufactured in 1984.

Nevertheless, we have now had an opportunity to observe the behavior of some acrylic tapes as they age naturally. We have seen slight penetration in some papers. We have also observed some discoloration of tape and substrate on groundwood paper. A piece of acrylic frosty tape has been observed to cause bleeding of red ballpoint pen ink in the course of natural aging. It is possible that the newer tapes (Filmoplast P, Filmoplast P90, and Document Repair Tape) will behave better and be more easily removed than other currently available pressure-sensitive tapes. However, experiments conducted at the Library of Congress in 1982 that artificially aged various tapes by standard paper-aging tests, show that bleeding of ballpoint pen and carbon-copy inks is an issue in these three tapes as well.