No abstract

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.

Vulcanizates, to which a curative is added by swelling, can be recured to easily study a variety of aspects of the vulcanization process, such as maturation, reversion, and even how much accelerator remains active as its zinc salt at the end of the cure. In effect, vulcanizates can be viewed as high-molecular-weight model compounds. In this study, we find that recuring SBR/BR vulcanizates, to which sulfur or the sulfur donor, N,N′ -dithiodimorpholine, is added, develops the same state of cure as the same amount of sulfur (or sulfur donor) added for the initial cure. This suggests that exchange reactions occur between crosslinks and the zinc-sulfur-accelerator complex during the cure and that all of the accelerator remains as its zinc salt at the end of the cure. This last result is interesting, since it is not consistent with the current view that most of the accelerator becomes irreversibly bound to the rubber and lost during the vulcanization of polybutadiene rubbers.

It is well known that during the sulfur curing of unsaturated rubbers, two competing reactions occur: (a) crosslinking or vulcanization, and (b) reversion or devulcanization. In the case of butyl rubber, these two competing reactions have been summarized in earlier reports. Tire curing bag (bladder) compounds are usually made of butyl rubber (IIR), a copolymer of isobutene and isoprene, with typically 1–5% of the diene monomer. Curing bags were originally manufactured using sulfur cures. The high temperatures (140–180°C) employed in tire curing caused reversion, however, and these bladders had very short service lives. The deterioration of the IIR bladders was evidenced by a gradual softening of the surface. A major technical advancement for increasing the service life of curing bladders was the development of phenol/formaldehyde (resole) resins for vulcanizing IIR. These resins can give IIR cures with very thermally stable crosslinks. The vulcanizates are essentially immune to reversion, even at the high use temperatures of tire curing operations. The basic curing resins used are generally 2,6-dihydroxymethyl-4-alkylphenols 1 or their condensation polymers 2 (Scheme 1). These materials are produced via the base-catalyzed reaction of the p-substituted phenol with formaldehyde. R is typically methyl, t-butyl, or t-octyl in commercial resins. The use of a blocking substituent in the para position maximizes the formation of o-hydroxymethyl groups. R′ is either methylene (—CH2—) or dibenzylether (—CH2—O—CH2—), depending on the conditions of the resin synthesis or the cure.

Currently, chemical antiozonants can be considered to function by scavenging ozone and, in the process, forming a protective barrier to prevent further ozone attack. Therefore, antiozonants must do two things: 1. react more rapidly with ozone than the rubber and 2. in the process, form products on the surface which prevent ozone from reaching both the underlying antiozonant and the rubber. Presently, more work is needed to define exactly what characteristics are required for the ozonized products to serve as an effective barrier. However, indications are that they must have a higher molecular weight than the antiozonant.

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.

An improved method for measuring the relative rates of reaction of materials with ozone is given. This method is used to evaluate the reactivities of new antiozonants. The similar reactivities of isomeric N-alkyl-p-phenylenediamines indicate that antiozonants may in large measure be active as a result of the physical nature of the ozone and antiozonant (and possibly rubber) reaction product. Proton magnetic resonance spectroscopy was used to evaluate the types of products formed on the ozonation of p-phenylenediamines. These results are compared with previously reported infrared work. Evidence for a one electron oxidation of p-phenylenediamines with ozone is also presented. This evidence is not conclusive. A correlation of the HOMO energies of N-substituted p-phenylenediamines with the reactivities of p-phenylenediamines towards ozone is attempted. Although a rather broad correlation is noted, an exact correlation is not found. On the basis of these results, it is felt that the reaction between phenylenediamines and ozone may be free radical in nature. But the general type of substitution on the phenylenediamine alters the products that are formed and possibly changes the rate-determining step of the reaction.

Vulcanizates, to which more accelerator has been added by swelling, were recured to study the interaction between the accelerator and the sulfur crosslinks. The addition of benzothiazole, or dithiocarbamyl sulfenamides, to SBR, SBR/ BR, or NR vulcanizates caused more crosslinks to form on recuring. A typical induction period was also noted. The results were essentially the same in all rubbers. Crosslink formation, as measured by modulus development or rheometer torque, increased with increasing loadings of accelerator. Similarly, at a given accelerator loading, crosslink formation increased with increasing length of the polysulfide crosslink. These results indicate that an equilibrium exists between the zinc salt of the accelerator and the polysulfide crosslinks.