Until recently, the tapes most frequently used to mend torn paper were masking tape and cellophane tape. Both have rubber based adhesive. As these adhesives oxidize, they pass through distinct stages of deterioration. 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 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 “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.

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Old World Restorations, Inc.  5729 Dragon Way Cincinnati, Ohio 45227  USA   (513) 271-5459   E-mail: info@oldworldrestorations.com

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