Pyrolysis gas chromatography - mass spectrometry of pressure sensitive adhesive tapes

Conservators and restorers often encounter pressure sensitive adhesive tapes used for conservation reasons. This type of tape has been used to repair documents or pages of books or on torn documents. The adhesives frequently found are composed of either natural or synthetic rubber or of acrylics. The pressure sensitive adhesive tapes (PSTs) consist of four layers: the release coating, the backing, the primer and the adhesive. The restoration often requires the removal of the adhesive or backing or any other of the PST layers affecting paper artworks since degradation products could originate from one or more of these layers. In this work, six pressure sensitive adhesive tapes often found on paper works of art were analysed with Py-GC-MS in order to obtain a chemical classification of the PSTs as a whole, as well as, for the first time, of the single layers. The pyrolytic products detected allowed to recognize two main classes of adhesives: acrylic and styrene block copolymers. Among acrylic types, different copolymers, such as poly(n-butyl acrylate - methyl methacrylate - 2-ethylhexyl acrylate), poly(n-butyl acrylate - methyl methacrylate) and poly(n-butyl acrylate - ethyl acrylate) have been used often together with phthalate and adipate plasticizers. On the other hand, the styrene-isoprene-styrene and the styrene-butadiene-styrene copolymers were identified for styrene block type. In these adhesives, more terpenoid tackifiers have been revealed. In the backing layers the pyrolyzates suggested the use of polypropylene, cellulose acetate, paper or polyvinyl chloride. Only for one sample, the insulating tape, it was possible to detach the release coating from the backing layer and so to identify its acrylic nature. The exact knowledge of different layers of PSTs could help restorers to select an optimal removal procedure.