Quality control of paints: Pyrolysis-mass spectrometry and chemometrics

The thermal degradation of a standard (sample 1) and five resin-modified paints has been investigated by pyrolysis-field ionization mass spectrometry (Py-FIMS). The modifications simulate possible industrial disturbances: cross-linker modification (omission of methyl melamine: sample 2, replacement by benzoguanamine: sample 3), resin-reduction (acrylic resin: sample 4, polyester: sample 5) and resin-exchange (epoxy resin: sample 6). For the investigated resins, the following marker signals have been assigned: melamine (mz 126) and methyl melamine (mz 140); benzoguanamine (mz 187); propenal (mz 57) and styrene (mz 104) for the acrylic resin; adipic (mz 129) and phthalic acid (mz 149) for the polyester; bisphenol A diglycidyl ether monomer (mz 340) and dimer (mz 624) for the epoxy resin. Apart from these resin marker signals, major pyrolysis products of the polymer backbone, like esters of trimethylolpropane units with phthalic (mz 413, 561, 807) and adipic acid (mz 541, 727 and homologues), and cross-linking products, e.g. between methyl melamine and trimethylolpropane ester fragments (mz 785, 805, 825), have been identified. Using these signals, all six samples were differentiated and the origins of the modifications specified. The data were interpreted by principal component (PCA) and discriminant analysis (DA). Chemometric evolution as a means of visualizing the data enabled the reproducibilty of the measurements to be estimated and the samples to be differentiated according to the influence of their modifications. The most intense effects were assigned to the cross-linker modifications. They affected several resin signals, while other modifications showed minor effects mainly on their own marker peaks.