Hierarchical cluster analysis as a tool for preliminary discrimination of ATR-FT-IR spectra of OPP acrylic and rubber-based adhesives

It is difficult to discriminate colorless and transparent oriented poly(propylene) (OPP) packaging tapes by the physical measurements of the backings. They are normally discriminated by the analysis of the adhesives. In the present study acrylic and rubber-based adhesives of colorless and transparent OPP packaging tapes were analyzed by attenuated total reflection (ATR) FT-IR. The performance of hierarchical cluster analysis (HCA) as a tool for discriminating ATR-FT-IR spectra was studied. The spectra were collected over the range from 4000 to 650 cm−1 and the spectral resolution was 4 cm−1; the number of the data per spectrum was ca. 1700. The spectrum data was analyzed by a built-in function, “agnes”, in R language. The HCA classification of the raw spectra of acrylic adhesives was almost consistent with the visual classification. HCA excluded the importance of the characteristic absorption peak in discriminating spectra. The classification of the spectra of rubber-based adhesives by HCA and the visual classification were same. HCA can reduce the forensic examiner's workload to classify the adhesives.