Analysing for 4,4′-diaminodiphenylmethane in heritage collections containing solid and medium density flexible linear polyester polyurethanes using liquid chromatography/mass spectrometry

Abstract

Polyurethane (PUR) shoe soles from collections held by the Alfred Gillett Trust, stored for up to 50 years, were analysed for the presence of 4,4′-diaminodiphenylmethane (4,4′-MDA)—a substance of very high concern in Europe and classified as a carcinogen in USA. A review of the literature revealed no papers on long term room-temperature hydrolysis of urethane or urea linkages leading to the spontaneous formation of 4,4′-MDA in polyester or polyether polyurethanes made from 4,4′-diisocyanatodiphenylmethane (MDI). However, evidence emerged of its potential formation and a possible handling hazard was consequently identified in the heritage collection. By chemical analysis, shortcomings in the current literature could be addressed. Calibration of liquid chromatography–mass spectrometry equipment showed analysis of 4,4′-MDA was possible down to 1 ppm with an error of 2.5 ppm. No 4,4′-MDA was found in the PUR analyte solutions at a concentration > 1 ppm. Under these experimental parameters the samples were shown to comply with the industrially accepted CertiPUR 2017 standard for commercial slab-stock foams. Furthermore, no 4,4′-MDA was found in solution after an ‘accelerated anaerobic hydrolysis test’ on the sole materials. This test was designed and developed to assess the likelihood of future formation of the aromatic amine via a hydrolysis-only mechanism/s. Finally, 4,4′-MDA itself was heated in air at 70 °C under ‘humid’ conditions to examine its stability. In this experiment the 4,4′-MDA altered in appearance and was reduced to ca 30% of its original weight. Subject to more work, it is conceivable 4,4′-MDA could be formed by hydrolysis, but degraded over time, and not detected in these tests.