6-QDI - a review of a multifunctional chemical for the rubber industry.

The quinone diimines are prepared by the oxidation of the commonly used p-phenylene diamine antidegradants. Quinone diimines show multiple functional activities useful to the rubber industry. Figure 1 shows how N-1,3-dimethyl-butyl-N'-phenyl-p-quinone diimine or 6-QDI is derived.

[FIGURE 1 ILLUSTRATION OMITTED]

Activities demonstrated useful to the rubber and polymer industry include:

* Bound antioxidant;

* diffusable antiozonant;

* PVI -- process safety -- delays scorch in new and reprocessed rubber;

* improved productivity -- viscosity reduction increases processing rates;

* modifies dynamic mechanical properties;

* antioxidant for polymers and hydrocarbon liquids;

* polymerization inhibitor for vinyl monomers.

Quinone diimines have been studied for use in rubber for more than thirty years[ref. 1]. In early studies, it was shown that after vulcanization of natural rubber or general-purpose elastomers in the presence of quinone diimines, a portion of the antidegradant is no longer extractable from the rubber. In addition, the remainder of the quinone diimine antidegradant is reduced to the commonly used p-phenylene diamine antiozonant (PPD)[ref. 2]. Corresponding experiments conducted with p-phenylene diamine antidegradants return nearly all of the antidegradant in its original form. Table 1 summarizes independent work conducted at the Natural Rubber Producers Association (NRPA) laboratories in England and at the Monsanto Rubber Chemicals laboratories.

[TABULAR DATA 1 NOT REPRODUCIBLE IN ASCII]

That the antioxidant is polymer bound has been demonstrated independently in experiments at the NRPA and at Voronezh Subsidiary of the All-Union Scientific Research Institute of Synthetic Rubber[ref. 3]. Raevsky et al. showed by ESR experiments that at least a portion of the antidegradant becomes polymer bound and forms somewhat stable radicals. Synthetic poly(isoprene) was oxidized in the presence of quinone diimines. ESR signals were observed which correspond to those expected for the quinone diimine antidegradant. Even after re-precipitation in alcohol three times from solution in benzene, a reduced but persistent ESR signal was still observed. Since the polymer was re-precipitated, the authors concluded that the radicals must be chemically bonded to the polymer.

Perhaps Cain et al. gave better proof of the polymer bound nature of the antidegradant by demonstrating antioxidant activity that persisted even after solvent extraction. Cain tested vulcanizates protected with quinone imines or quinone diimines before and after hot methanol-acetone-chloroform azeotropic extraction. These vulcanizates showed antioxidant activity 2-30 times that of a control compound (unprotected azeotrope extracted vulcanizate). The results are summarized in table 2.

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Cain et al. compared the vulcanizate extraction experiments to monoalkene model studies. The amount of reduced antidegradant obtained in the model studies was in agreement with the vulcanizate results above. In addition, the results indicated that the additional products formed would indeed be bound to the polymer[ref. 4]. Rubber parts are generally exposed to environmental factors such as heat, water, oils and detergents causing losses of antidegradant through volatilization, and extraction or leaching Mechanisms. Polymer-bound antidegradants will not be susceptible to these loss mechanisms. This persistent antioxidant activity demonstrated above should readily translate into long lasting antioxidant performance in rubber articles.

In order to observe the same level of antiozonant protection, the quinone diimine antidegradants require a higher loading than that of PPD antidegradants. This is to be expected when a portion of the antidegradant becomes bound to the polymer and is no longer diffusionally mobile. Ozone degradation occurs at the surface of the rubber. The antiozonant must be capable of migrating to the surface of the rubber in order to provide chemical or barrier protection.

Vulcanization characteristics and PVI effect

Quinone diimines are unlike phenylene diamines in their effects on vulcanization characteristics. While the aryl-alkyl phenylene diamines such as 6-PPD and 3-PPD (N-i-propyl-N'-phenyl-p-phenylene diamine) are only slightly scorch reducing, the dialkyl phenylene diamines are notoriously scorch reducing. Quinone diimines, on the other hand, exhibit prevulcanization inhibitor effects[ref. 5]. Table 3 compares the effects of quinone diimines on the scorch safety of sulfenamide accelerated natural rubber compounds. The well-known prevulcanization inhibitor N-cyclohexylthiophthalimide (CTP) is included for comparison. In addition, a quinone imine sample is also compared. The quinone diimines show good prevulcanization inhibition behavior. However, the efficiency is not as high as CTP. This is desirable since one would like to use two to five phr of antidegradant in typical compounds. At usage levels of about 1-2 phr of quinone diimine, scorch delay is comparable to typical loading of CTP.

The activity of the N,N'-diphenyl quinone diimine or the quinone imine is not that of a prevulcanization inhibitor but that of a retarder. In table 3, the time for a 30 Mooney unit rise, t[Delta.sub.30], (sometimes referred to as the Mooney cure rate) is given along with the scorch delay [t.sub.5]. Inhibitors show an increase in [t.sub.5] with little or no change in [t.sub.30]-[t.sub.5]; retarders, on the other hand, will have a noticeable increase in [t.sub.30]-[t.sub.5]. The di-alkyl and aryl-alkyl diimines show a slight increase in t[Delta.sub.30]. However, the diphenyl quinone diimine and the quinone imine show significant increases in t[Delta.sub.30]. This increase is associated with retarders; inhibitors delay the onset of scorch without affecting t[delta.sub.30].

[TABULAR DATA 3 NOT REPRODUCIBLE IN ASCII]

A mechanism for prevulcanization inhibition was proposed to account for the scorch delay characteristics. This mechanism is similar to that of CTP in that the main reaction involves the removal of 2-mercaptobenzothiazole (MBT) from the reaction medium. This intermediate can undergo several reactions including the Retro-Michael reaction to liberate the starting materials and also a substitution-elimination reaction yielding 2,2'-dithiobis-benzothiazole (MBTS) and the corresponding PPD. Both reactions provide for longer scorch delay by chemically `trapping' accelerator and later releasing it along with the starting quinone diimine or PPD (figure 2).

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Processing aid characteristics

A previously unreported and unique feature of quinone diimines relates to their ability to reduce the viscosity of natural rubber mixes. Mixing carbon black and natural rubber in the presence of quinone diimines at high temperatures results in compounds having lower viscosity than compounds mixed in the presence of PPDs or without antidegradants[ref. 7]. Natural rubber (60CV) was mixed with 50 phr of N-326 carbon black. 5.0 phr oil, 5.0 phr ZnO. and 2.0 phr of stearic acid without antidegradant, with three and five phr of 6-QDI and 6-PPD for comparison. The compounds were mixed at high speeds (in order to reach high temperatures) for 8.0 minutes and discharged at 160-170 [degrees] C. Mooney viscosity, ML(1+4) for the compounds is reported in table 4.

Table 4

Additive (phr)             Mooney viscosity ML (1+4)
Control (0.0)                         44.4
6-QDI (3.0)                           30.5
6-PPD (5.0)                           45.6
6-QDI (3.0)                           31.62
6-PPD (5.0)                           46.22

When quinone diimines are added to natural rubber and carbon black in an internal mixer, and the mix is maintained at high temperatures for several minutes, the natural rubber experiences a softening or peptizing behavior. The softening effect of the quinone diimines is about as efficient as that observed by several common softening agents. Figure 3 compares the softening effect of a paraffinic oil, two fatty acid modifiers and a common peptizing agent for natural rubber to the softening effect observed with 6-QDI. The Mooney viscosities of the softened materials are similar, indicating that the efficiency of softening for each compound is similar. The efficiency of softening imparted to the natural rubber by the 6-QDI material is comparable in magnitude to those common modifiers. In addition, the 6-QDI provides the compound with antioxidant and antiozonant protection.

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Summary

Quinone diimines are chemicals that exhibit multiple functionality in rubber. These chemicals show activity as:

* Bound antioxidant;

* diffusable antiozonant;

* PVI -- process safety -- delays scorch in new and reprocessed rubber;

* improved productivity -- viscosity reduction increases processing rates;

* modifies dynamic mechanical properties;

* antioxidant for polymers and hydrocarbon liquids;

* polymerization inhibitor for vinyl monomers.

These activities should show benefits to the industry by providing long term antioxidant activity, along with antiozonant capacity. The processing of natural rubber compounds can be improved through both the PVI effect (longer scorch delay) and increased productivity (through the viscosity reduction).

References

[1.] Campbell, Monsanto Technical Summary, 1966.

[2.] Campbell, Monsanto Technical Summary, 1966, M.E. Cain, I.R. Gelling, G.T. Knight and P.M. Lewis, Rubber Industry, 216-226, 1975.

[3.] A.B. Raevsky, L.F. Kovrizhko, A.B. Romanova, T.I. Yesina, V.V. Shishkina and I.F. Gaynulin, Kauch. Rezina, 29 (3) 9-10 (1970).

[4.] M.E. Cain, I.R. Gelling, G.T. Knight and P.M. Lewis, Rubber Industry, 216-226, 1975.

[5.] M.E. Cain. G.T. Knight, P.M. Lewis and I.R. Gelling, The Natural Rubber Producers Association, Ger. Pat. 2342453, 1974, and Fr. Pat. 2331247, 1974.

[6.] M.E. Cain, I.R. Gelling, G.T. Knight and P.M. Lewis, Rubber Industry, 216-226, 1975.

[7.] Ignatz-Hoover, Fred, U.S. Patent Application.