The ozonation products of a common rubber antiozonant, N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (HPPD), have been separated by liquid chromatography and identified by mass spectrometry. Three principal mechanisms appear to govern the ozonation of HPPD. Amine oxide formation leads to observed nitrosoaryl and nitroaryl products. Side-chain oxidation leads to several low molecular weight products, including some that contain an amide moiety. Nitroxide radical formation leads to a nitrone that is the most abundant ozonation product; a dinitrone is also formed. Ozonation of HPPD occurs mainly with degradation of the alkyl portion of the molecule. The results of this study are consistent with a combined “scavenger-protective film” theory of antiozonant protection of rubber compounds.
The ozonation products of a common rubber antiozonant, N,N′-di-(l-methylheptyl)-p-phenylenediamine (DOPPD), have been separated by liquid or gas chromatography. Molecular weights of about thirty LC separated components have been measured by field desorption mass spectroscopy. Elemental formulae have been determined by atomic composition mass spectroscopy. Other structural details have been elucidated by electron impact mass spectroscopy and attenuated total reflectance infrared spectroscopy. Two principal mechanisms appear to govern the ozonation of DOPPD. Amine oxide formation leads to observed nitrosoaryl and nitroaryl products. The second major mechanistic pathway is side chain oxidation. This leads to a number of low molecular weight components, including some that contain an amide moiety. A third (minor) mechanistic pathway involves a nitroxide radical intermediate and leads to the formation of a stable dinitrone species. The surface film formed on ozonation of a black loaded natural rubber sheet containing DOPPD has also been examined. The film contains appreciable quantities of unreacted DOPPD and many of the same low molecular weight components as observed in the ozonized liquid antiozonant. It is clear that DOPPD blooms to the rubber surface and acts as a scavenger for ozone. The results are consistent with a combined “scavenger-protective film” mechanism for antiozonant protection.
Liquid chromatography has proven to be an extremely valuable tool for many separations involving polymers and polymer chemicals, especially when used in conjunction with mass spectroscopy for chemical identification. The complex products of the condensation reaction of aniline with acetone have been characterized by liquid chromatography and mass spectroscopy. The LC separation procedure utilized a C18 Micro Bondapak (reverse phase) column with a water/tetrahydrofuran gradient system. Eluted peaks were trapped individually for field desorption mass spectral identification. Syringe deposition was used to transfer the isolated components to the field emitter. The major components found in the reaction mixture were oligomers of 2,2,4-trimethyl-l,2-dihydroquinoline (TMDQ), although several other monomeric and oligomeric side-products were observed and characterized. Forty-two components were identified in the product resin. These can be classified into eight major series of oligomers; all series have oligomers separated by the TMDQ monomer unit (173 amu).
Phenolic resins are added to rubber compounds to improve the tack (or autohesion) characteristics. These resins are generally oligomers of alkylated phenols and formaldehyde. Analytical characterizations of these resins are very limited and incomplete in the open literature. We have examined several types of resins using mass spectroscopy (principally field desorption, FD-MS), liquid chromatography (LC), and infrared spectroscopy (ATR-IR). Several oligomeric series were identified by FD-MS in t-octylphenol and t-dodecylphenol resins. ATR-IR analysis showed that the t-octyl groups are essentially completely p-substituted on the phenolic ring. LC was only of limited utility in obtaining analytical separations of resin oligomers. Higher molecular weight oligomers (≿1500) showed little if any LC resolution. Resins prepared under different manufacturing conditions showed additional series of oligomers. These were either “cyclic ethers” and/or amine-containing compounds, depending on the synthetic procedures used.
Ozone attack on rubber compounds causes characteristic cracking perpendicular to the direction of applied stress. This degradation is caused by reaction of ozone with the double bonds in the rubber molecules. This causes chain scission and the formation of various decomposition products. The general subject of protection of rubber against ozone attack has been reviewed by a number of authors. In order to control the effects of rubber ozonation, either paraffin waxes or chemical antiozonants are added to unsaturated rubbers. The most effective antiozonants are N,N′-disubstituted-p-phenylenediamines (PPDAs), in which at least one of the side groups is alkyl (preferably sec-alkyl). Several theories have appeared in the literature regarding the mechanism of antiozonant protection. The “scavenger” model states that the antiozonant blooms to the surface and preferentially reacts with ozone so that the rubber is not attacked until the antiozonant is exhausted. The “protective film” theory is similar, except that the ozone-antiozonant reaction products form a film on the rubber surface that prevents (physically and perhaps chemically as well) ozone attack on the rubber. A third “relinking” theory states that the antiozonant prevents scission of the ozonized rubber or else recombines severed double bonds. A final theory states that the antiozonant reacts with the ozonized rubber or Criegee zwitterion (carbonyl oxide) to give a low-molecular-weight, inert, “self-healing” film on the rubber surface. Currently, the most accepted mechanism of antiozonant action is a combination of the scavenger and protective film theories.
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