Pressure sensitive tapes: Degradation mechanisms and removal
Sellotapes, Scotch tapes, these pressure-sensitive adhesives to “repair” paper, are part of numerous artefacts from documentary heritage in the need to be preserved. We owe Mr. Richard Drew this fabulous invention in 1925 (or 1930?).
The fatty adhesive accompanying the plastic strip oxidizes and stains the sticked paper, causing with time more damages than benefits. A solid knowledge of writing techniques and solvents will enable a quite satisfactory removal of this greasy glue.
Damages caused by tapes at medium and long term are:
Yellowing (and oxidation) of the paper affected.
¿Could it be that tapes created a sort of sickly addiction that pushed to put it one and again on the same document? Detail of a large scale printed plan, of the Eixample district in Barcelona, 1900. It belongs to Col·legi d’Arquitectes de Catalunya (CoAC). Click here to see it restored.
Fat is not evaporated by a physical phenomenon (like water) but a chemical reaction (oxidation). The dark tone of paper is not only the effect of the fat still present, but also the oxidation of paper fibres. Chemical structure has changed, and so fibres are darker now.
Yellowing (and oxidation) of the neighbouring papers.
Weakening of the perimeter paper due to differential thickness, that form at the end tears next to the plastic strip, specially when paper is really thin -like this one, an onion skin paper-.
Yellowing and oxidation are evident, both at the side repaired with tape (left) and at the unfolded area next to it (right). The different mechanical strength between paper and the plastic tape causes tears, as it is shown just under the tape which is top left on the image (click to enlarge the image). Architectural drawing by the architect Barba Corsini (C. 1960). Pencil manuscript on onion skin paper. Property of Col·legi d’Arquitectes de Catalunya (CoAC).
Slipping of plastic supports and dirt adhesion. Moderatelyhigh temperatures can make the adhesive become labile, shifting the plastic strip from its original position. And as it issticky, areas with no plastic are easily filled with dirt.
In this tracing paper dirt was already there before putting the adhesive… No need for CSI special labs to see that! And yet it is so common that those ones who use these tapes are not the kind of very clean and meticulous ones. Look at the plastic strips, which has slipped when heat melted fat. Macro detail of an architectural plan of Meridiana canodrom (Barcelona), by the architect Bonet Castellana (c. 1960). Ink manuscript on tracing paper. It belongs to Col·legi d’Arquitectes de Catalunya (CoAC).
And you might be wondering… why isn’t she sowing the “after” images for these plans? Wasn’t she able to remove tapes from them? Well… these particular plans were an absolute poltergeist case: Slipping supports, changing scale of the drawing in some areas, written lines ABOVE the plastic strip (should it be removed? Or left?)… I leave these cases for a single post just about them… (“The sellotape is back“!).
Unwanted adhesion of the greasy glue beyond the initially intended area, provoking at worst tears, as is this case. When the folded paper gets sticked with itself, the unfolding action implies breakage. Or, much general case, when the paper gets sticked to another, simple handling involves breakage.
Notice at right a new tear caused by the adhesive remnants coming from the opposite side (left), where a former tear was repaired with tape. Time and temperature (moderate-high) can soften fatty glues and extend beyond the initially intended area to repair. A probable hazard, besides dirt and oxidation, is formation of new tears when unfolding or handling these documents. Detail of the same plan before.
Wrinkles around the tape repair, due to different hygroscopicity and dimensional stability between plastic and paper.
Before restoration the central scotch tape tensions the document impeding its natural hydration and forming wrinkles al along. After removing sellotapes the paper expands and contracts in a homogeneous manner, without wrinkles. Brochure printed on coated paper from the Joan Ballester fund, property of the Youth Documentation Centre of the Youth General Directorate of Generalitat (catalan government). Digital photomontage (before / after).
Can scotch tapes be removed?
In order to remove adhesive tapes and the stain they cause, several issues need do be considered, such as the kind of paper, but mainly the writing technique (graphite pencil, print, watercolour…) which will determine which solvents can be used. Detail of diazotype property of Regional Archive of Valles Occidental (Terrassa, Barcelona)
Not all damages are irreversible. Both the plastic strip and the fatty remnant can be removed, and it is recommended to do so, because then paper oxidation is stopped.
Greasy stains can be softened as long as the fatty adhesive can be extracted, applying the adequate solvents (which will vary depending on the pictorical technique and the paper).
But when darkening is effect of the chemical degradation of paper, the stain can no longer be removed by physical methods (dragging it with the solvent): we can only resort to chemical treatments, such as deacidification.
Not all fatty adhesives stain the same, neither all kind of papers react the same way at equivalent treatments.
This ink manuscripted and hand-coloured diazotype, had all kind of tapes. The vertical strip on the left is strongly oxidized by an adhesive plaster tape that was in the back. Shorter plastic tapes on the central area did not yellow the paper that much. Left: during the extraction and processing of sellotape grease stains. Right: after tapes removal and fatty stains treatment. Gaps have been inpainted too (neutral tone). It belongs to large scale album (74 x 54 cm) from 1940. It has folded plans, hand-coloured photo-reproductions (diazotypes). The album is property of the Regional Archive of Vallès Occidental (Terrassa, Barcelona).