Application of confocal Raman spectroscopy to thin polymer layers on highly scattering substrates: a case study of synthetic adhesives on historic textiles

Abstract

This study considers confocal Raman spectroscopy as a means of identifying a range of synthetic polymeric adhesives used in textile conservation. Many of the synthetic adhesives applied to support fragile textiles in the 1970s are now showing signs of ageing and the textiles are therefore being presented for retreatment. With no record of the adhesive used, conservators are unsure of the appropriate protocol to remove the original adhesive prior to retreatment. We have shown that Raman spectroscopy lends itself to the analysis of these thin polymer layers as it is inherently non-destructive and can be applied in situ. We also found that, for particularly thin polymer layers (∼20 µm), the underlying textile often scatters far more efficiently than the overlying polymer and therefore the polymer cannot always be identified unambiguously in situ. Although thick, transparent polymer samples are known to produce maximum Raman peak intensity when the laser is focused a few microns below the sample surface, focusing on or just above (0-5 µm) the polymer layer is shown to maximize the ratio of polymer peak intensity to textile substrate peak intensity, thus facilitating identification of the polymer. Defocusing further to a point 20 µm above the upper surface is shown to reintroduce interference from the substrate spectrum to a level comparable to that acquired when focused within the thin polymer layer, closer to the textile substrate.

Confocal Raman Microscopy: Performance, Pitfalls, and Best Practice

“…Even more surprisingly, when focusing 5 lm above the PE, the PET bands were much stronger than the PE, as though the coating had almost disappeared! This effect is reproducible for a variety of systems; we have observed identical effects with other coated substrates, 24,43 and Macdonald et al 44 reported the same effect when focusing above a polymer-adhesive coated onto a textile substrate; they found that the substrate contribution increased (relative to the adhesive) when focusing well above the adhesive surface. We incorrectly interpreted this as being solely a consequence of the unfocused paraxial rays, which sample substrate and coating in direct proportion to their relative thickness, 43 but as was pointed out by Macdonald and Vaughan, 45 this is an incomplete explanation.…”

The role of beam dispersion in Raman and photo-stimulated luminescence piezo-spectroscopy of yttria-stabilized zirconia in multi-layered coatings

“…The difference can be accounted for by the complex microstructure of the two YSZ coatings used in this study which has the potential to reduce the absorption coefficient. The total absorption is proportional to the cross-section of light absorption by a single particle and the density (number per unit volume) of absorbing particles [46]. The smaller value obtained in this set of experiments may indicate that the existence of the pores (15-20% in APS-YSZ [47] and [48]), defects, etc., reduces the density of the equivalent absorptive material.…”

“…For the application of Raman spectroscopy to thermally sprayed YSZ layers, the illuminated volume below the focus of the incident laser beam is cone shaped [46]. The Raman signal collected from the surface extends to about one-fifth of the YSZ layer thickness.…”

Breaking down banners: analytical approaches to determining the materials of painted banners

“…The use of confocal Raman microscopy has been reported as a way to prevent this [29] and Lorenzetti et al [30] reported on its usefulness to determine the dyes on historic dyed cotton yarns by using a confocal condition 2-3 µm lower than the fibre surface. However Macdonald et al [31] reported that the results often included output from both above and below the point of focus and so these contributed to the Raman response albeit to varying degrees. Thus the use of confocal microscopy may not be ideal as it is not always certain which area in the sample yields the results.…”