Abstract
Laser desorption mass spectrometry (LDMS) was used to directly characterize the surfaces of vulcanized natural-rubber compounds with or without an antiozonant (AOz) and an antioxidant mixture. Molecular ions of five intact molecules were observed on the rubber surface using a single laser pulse. The molecular ions represent the AOz, the three components comprising the antioxidant mixture, and a production impurity in the AOz. The surface chemistry of the AOz, N-(1,3-dimethylbutyl)-.N′-phenyl-para-phenylenediamine (HPPD), was explored in order to investigate the mechanism of rubber-surface aging and protection. Two sets of samples were prepared: ozone-aged HPPD films that had been deposited on a glass plate and ozone-aged cured rubber strips containing HPPD. Rubber samples were continually stretched to 25% elongation and relaxed during exposure to ozone. The mechanisms of ozone reaction with HPPD on glass and on stretched rubber are similar, since many reaction products were identical based on the presence of the same LDMS peaks; however, the reaction on rubber was considerably more complicated, since numerous additional high-mass peaks were observed. Based on the present LDMS data and in agreement with the conclusions of Lattimer, Rhee and coworkers, the mechanism of antiozonant protection of rubber is consistent with both scavenger and protective barrier models being operative. Sections from a truck tire that displayed premature sidewall cracking and a passenger tire that displayed sidewall discoloration were analyzed using LDMS. The specific modes of sidewall failure were determined.
High-resolution laser desorption mass spectrometry of peptides and small proteins.