QDI antidegradant for cost saving, productivity in various grades of NR.

Improvements in mixing efficiency can offer significant savings in natural rubber compounds. Mixing fine particle carbon black into tough grades of natural rubber can be a difficult and time consuming process. Often long mixing times or multistage ,nixes are required to produce compounds which can be handled adequately on downstream processing equipment. In some cases, pre-mastication of the natural rubber is helpful, but the penalty to pre-masticate the natural rubber is lost capacity in the mixer, as time in the mixer is dedicated to pre-mastication and not to productive mixing. Other processing aids such as soaps and oils may provide some lubricity and softening activity, while peptizers enhance the oxidative degradation of natural rubber during the mixing process. Processing aids and peptizers result in compounds with lower Mooney viscosity and processing improvements, but the final vulcanizates often suffer from increased heat build-up during dynamic applications. Quinonediimine antidegradant, QDI, protects natural rubber from oxidative degradation during the ,nixing process. Viscosity reduction is provided because QDI is extremely efficient at capturing chains broken during the shearing action of the mixing process. Viscosity reduction benefits of QDI will be shown to be greatest in the toughest grades of natural rubber. In addition, detailed mixing cost analysis will be presented demonstrating the cost saving potential of QDI.

With today's steadily increasing demand for increased productivity, capacity improvements in mixing are becoming more highly valued. In addition, increasing demands for difficult to mix compounds require further increases in mixing capacity. These difficult to mix compounds are generally composed of 100% natural rubber or high natural rubber content in blends with polybutadiene or styrene butadiene rubber. These polymers are filled with highly reinforcing, small particle carbon black. The finer grade carbon blacks are notoriously difficult to mix. Trends toward higher mileage truck tires shift the demand in tread grade blacks to finer particle, harder to mix carbon blacks, further exacerbating the problem.

In the production environment, the Mooney viscosity of the compound gauges the quality of the mix. Only upon achieving a pre-determined Mooney viscosity does further downstream processing continue (calendering and extrusion). Often these difficult to mix compounds require several passes through a mixer in order to achieve the desired viscosity. While pre-mastication techniques and additions of oils or soaps may reduce compound viscosities, physical and dynamic mechanical properties often suffer.

N-(1,3-dimethylbutylJ-N'-phenyl-p-quinonediimine, QDI, enhances mixing performance and viscosity reductions in natural rubber compounds, with little or no loss in vulcanizate performance. Work in large-scale mixers (ref. 1) has shown that capacities for difficult to mix tread black compounds can be increased significantly when QDI is employed. In addition, performance characteristics of the mixes are favorably improved.

QDI is a multi-functional chemical that functions primarily as a long-lasting antidegradant. It provides both bound antioxidant and diffusible antiozonant activity. It also functions as a peptizer for natural rubber compounds (refs. 1 and 2) without negatively affecting compound properties.

This article summarizes work done to date with QDI as a productivity enhancer, and reports results from a recent study on the effects of QDI in viscosity modification in various grades of natural rubber. A mixing cost analysis will be presented demonstrating the cost saving potential with QDI.

Experimental

All rubber chemicals, including N-(1,3-dimethylbutyl)-N'-phenyl-p-quinonediimine, QDI, were used without further purification. Compounds were characterized for processing and vulcanization properties in a Mooney viscometer and a moving die rheometer, respectively. The natural rubber compounds were cured at 150[degrees]C to a state equivalent to t90 times, and samples were prepared from those vulcanizates for mechanical and dynamic mechanical properties.

Mooney scorch, viscosity and rheometer work were done in accordance with D6164-90 and D5289-92 accordingly. Tensile testing was done in accordance with D412-92. Fatigue-to-failure testing was done in accordance with D4482-85. Aging was done in accordance with ASTM D865-88. Other physical testing procedures used were trouser tear and Pico abrasion (ASTM D2228).

Background information

Ignatz-Hoover (ref. 1) first reported that QDI would enhance mixing performance and viscosity reductions in natural rubber compounds with no loss in performance. Four types of carbon black were mixed into ribbed smoke sheet rubber in a multistage mix. All four types of carbon blacks gave similar results. The result for the N-234 mixing experiment conducted in an 80 liter mixer is presented in figure 1.

[FIGURE 1 OMITTED]

Viscosity reduction for QDI containing stock was substantial as compared to the control. After three or four passes through the mixer, it could readily be seen that one mixing step could be eliminated when QDI was employed in the mix. On the average, the QDI containing compound mixed 18-20% faster, as compared to the control, when the elimination of a mixing step was considered. This will directly relate to higher plant capacities and lower specific energy costs to produce a given quantity of product.

Dispersion characteristics were shown for the third and fourth pass mixes as indicated in figures 2A and 2B. The QDI containing compounds, in spite of having lower Mooney viscosities, were showing significantly better carbon black dispersion.

[FIGURE 2 OMITTED]

Comparison with peptizers

Comparisons in this study (ref. 1) were made with the peptizer bis(benzamido)phenyl disulfide. Viscosity reduction as a function of mixing step is provided in figure 3. It is clear that stocks containing QDI performed equal to or better than the control or the compound containing peptizer. The total mixing time (the sum of the time per step) is given in figure 4. The compound using peptizer mixed 15% faster than the control compound, while stocks containing 1 phr or more of QDI mixed about 33% faster than the control, or about 16% faster than the peptized compound. Other key properties are shown in table 1. Tensile characteristics were very similar.

[FIGURE 3 OMITTED]

The softer compounds (or lower Mooney viscosity) showed slightly higher reinforcement, probably attributable to better carbon black dispersion. Fatigue results support this conclusion in that the control compound had about 10-15% lower fatigue life than all the other compounds. This result supports the improved carbon black dispersion in the softened compounds. Pico abrasion results suggest that increasing QDI loading may lead to increases in abrasion resistance. High loadings of QDI showed about 10-20% improvement in laboratory abrasion resistance, while the peptized compound showed about a 10% reduction in abrasion resistance.

More recent work on the comparison with peptizers was reported by Ignatz-Hoover (ref. 2), who concluded that mild mixing conditions did not discriminate between disulfidic peptizers and the control compound, while QDI containing compounds showed significant viscosity reduction. However, under oxidative mixing conditions, peptizers were more effective and QDI containing compounds showed large viscosity reductions.

The change in Mooney viscosity is plotted as a function of turns of the mixer (i.e., rpm x cumulative minutes of mixing), with the results presented in figures 5 and 6. Under mild mixing conditions (low shear and low oxidation condition--LTM), little discrimination was observed between disulfidic peptizers (Pepton 44 and Renacit 11) and the control compound, while QDI containing compounds showed significant viscosity reduction.

[FIGURES 5-6 OMITTED]

Under high shear and high oxidative mix conditions (HTM), peptizers begin to demonstrate their chemical effectiveness in promoting oxidatively induced radical degradation, while QDI containing compounds showed large viscosity reductions in the early stages of mixing.

Goodrich heat build-up data showed some discrimination between the QDI containing compounds and the others under both mild and oxidative mixing conditions. Figure 7 shows the delta temperature observed during flexometer testing as a function of mix viscosity. The compounds containing the disulfidic peptizers and the control showed increased heat build-up as a function of decreasing Mooney viscosity. In QDI containing compounds, a trend to higher heat build-up with lower viscosity was also observed. However, the slope and intercept values of the regression line for QDI are lower than those for the control or peptized compounds. Figure 7 shows a general trend toward higher heat build-up as a function of lower Mooney viscosity. There was about a 5 to 7% reduction in heat build-up when QDI was used.

[FIGURE 7 OMITTED]

The Pico abrasion results shown in figure 8 suggest that the QDI containing materials gave better abrasion resistance than the other compounds. The oxidatively mixed compounds (HTM) tended to give a lower average than the mild mixed conditions (LTM), however, the confidence intervals on the mean suggest there is no significant difference.

[FIGURE 8 OMITTED]

Summary from earlier studies

From these earlier studies, it was concluded that QDI is highly efficient at capturing radical chain ends produced either by shear-induced or oxygen-induced cleavage reactions. QDI can be used effectively for viscosity reduction without any pre-mastication steps or breakdown time in the mixer before the carbon black is added (no time penalty for pre-mastication or breakdown). The antidegradant character of QDI reduces the deleterious effects of random scission that are observed in the oxidative breakdown of natural rubber. The viscosity reduction observed in mixes employing QDI is, in part, a result of improved carbon black dispersion.

Thus, QDI, which functions as the ideal dispersing agent, lowers the viscosity through capture of polymeric radicals. It is expected that the newly formed polymer-bound antidegradant end group will have better interaction with the carbon black and thus promote better and/or faster dispersion. Limitation of oxidative scission allows for relatively better dynamic mechanical properties. Lower viscosities are reached in shorter times with comparable or improved carbon black dispersion. The result is faster processing compounds that produce equal or improved dynamic mechanical properties.

Theoretical considerations

Studies characterizing the reactions of alkenyl radicals with quinone-diimines were published in the late 1970s (ref. 3). Quinonediimines react with allylic radicals yielding both the reduced PPD and the alkylated product. In these experiments, 2-methyl-2-pentene served as a model olefin (model for natural rubber). Samples of the olefin and quinone imines or quinonediimine were heated to 140[degrees]C. Isolation and analysis of products demonstrated that 40-70% of the imine or diimine was reduced to the corresponding PPD, while 2050% was isolated as the alkylated product. This alkylation reaction (via an allylic radical) represents the pathway to the lormation of rubber bound antidegradant. In the case of the diimines, the alkylation was nearly exclusively N-alkylation.

Pospisil et al (ref. 4) demonstrated that QDIs are also susceptible to alkylation by radicals. In that work, the radicals were generated by decomposition of 2,2"-azobis (iso-butyronitrile), and alkylation efficiency was maximized when the reaction contained a mixture of the corresponding PPD and QDI.

The addition of a 2-methyl-2-penten-4-yl radical to the QDI (based on p-phenylene diamines thus producing the corresponding PPD radical) is highly exothermic. The reaction not only stabilizes the relatively unstable alkenyl radical, but also allows for the aromatization of the diimino-cylcohexadienyl ring. The enthalpy of reaction for this reaction is calculated (using MOPAC/AM 1 Hamiltonian [ref. 5]) to be -39.26 kcal/mol.

Ignatz-Hoover et al (ref. 6) developed a model using QSAR techniques to estimate the reactivity of a molecule with the styryl radical based on a series of chain transfer constants found in the literature. Using this model as a predictive tool, the relative reactivity of 6PPD, QDI and bis(benzamido) phenyl disulfide towards styryl radical were determined. Results indicate that bis(benzamido) phenyl disulfide is the most reactive, followed by QDI and then followed by 6PPD. Within the experimental error of the estimates, the relative reactivities are on the order of 1,000:500:1 for bis(benzamido) phenyl disulfide:QDI:6PPD. This discrepancy in reactivities explains why the use of conventional PPD-type antidegradants as peptizing agents has never proven successful. It additionally supports that QDI is sufficiently reactive, with radical chain ends formed in the mixer, such that it can show softening behaviors similar to commercially available peptizers. The model studies cited above suggest that these reactions result in the formation of polymer bound antidegradant in the form of a polymer bound PPD moiety. This addition of a somewhat polar 'PPD functional' end group enhances the interaction of the polymer with carbon black, thereby enhancing carbon black dispersion and providing slight improvements to some other physical properties such as abrasion resistance, fatigue and tensile properties (as shown above).

Thus, mastication of isoprene-based rubber (with or without fillers) produces shear-induced radicals that are captured, to a certain extent, by the quinine diimine molecule. This results in a reduction in molecular weight, as shown in many previous studies on the mastication of natural rubber. Benefits of this reaction include increased rates of mixing and dispersion, higher capacity throughputs in mixing equipment, and lower manufacturing costs with equal or better physical properties compared to control compounds. Volume capacity increases can be realized on the order of 15-30% over conventionally mixed compounds.

Effects in various grades of natural rubber

To further confirm the attribute of QDI in providing either mix stage or mix time reduction in natural rubber compounds, the following five grades of natural rubber were evaluated with and without QDI: SMR CV 60; SIR 10; pale crepe; RSS#2; and synthetic natural rubber.

The rubber was pre-masticated with 5 phr of N-234 in a 'OO' internal mixer. The batches were mixed until the thermocouple probe in the mixer registered 160[degrees]C. The average batch temperature measured by direct measurement of the batch after mixing was 160[degrees]C. The formulation and mixing procedure is presented in table 2.

The change in Mooney viscosity is plotted as a function of mix pass. The results are presented in figures 9 through 13. QDI containing compounds show significant viscosity reduction as a function of mix pass.

[FIGURES 9-13 OMITTED]

Viscosity reductions were substantial with QDI for each grade of natural rubber, as compared to the controls without QD1. After three or four passes through the mixer, it can readily be seen that one mixing step can be eliminated when QDI is employed in the mix. On the average, compounds containing QDI mixed 25% faster, as compared to the control, when the elimination of a mixing step was considered. This will directly relate to higher plant capacities and lower specific energy costs to produce a given quantity of product. There were no significant effects on key properties such as fatigue, tear and modulus, as shown in figures 14-16.

[FIGURES 14-16 OMITTED]

Cost benefits analysis of QDI application

As pointed out in the above discussions, the use of quinonediimine antidegradant, such as QDI, would allow compounders to obtain shorter mixing cycles, either by shortening the mixing time for each mixing stage or by the elimination of a mixing step. A shorter mix cycle will result in increased productivity, which could lead to lower cost.

The model calculation in table 3 illustrates the potential cost benefits of this revolutionary mixing concept with QDI. This model calculation shows that the net cost of QDI per kilogram of compound is $0.0559. The estimated savings per kilogram of compound as the result of eliminating one mix step is $0.291. Therefore, there is a net savings of $0.23497 per kilogram.

Figure 18 shows the projected annual savings per mixer resulting from eliminating a mixing step by the use of QDI. This example demonstrates that the added cost of using QDI is more than offset by the savings in the total cost to produce the compound. Similar calculations can be obtained if only mix time instead of mix step reduction can be achieved.

[FIGURE 18 OMITTED]

Conclusion

In conclusion, the use of QDI to control the viscosity of natural rubber compounds during mixing would result in productivity increases through shortening of the mix cycle. Mix time reductions up to 25% have been demonstrated under factory mixing conditions without penalty to physical properties. The added raw material cost of using QDI as a productivity enhancer is generally offset by the potential savings from mix time reduction.

[FIGURE 17 OMITTED]

Table 1--tensile, fatigue and abrasion properties

                      Control    0.5    1.0    2.0    4.0        0.2
                                 QDI    QDI    QDI    QDI   Peptizer

Hardness                 62.7   62.0   62.3   63.3   60.0       57.7
Peak Stress (MPa)        29.6   30.6   30.7   29.9   29.7       30.2
M100 (MPa)                2.2    2.4    2.3    2.5    2.7        2.5
M300 (MPa)               13.1   14.7   14.2   15.1   15.4       14.9
Peak strain (%)           570    547    556    534    533        539
Fatigue, Kc to fail        92    103    106    110    105        129
Pico abrasion index       156    168    167    182    188        149

Table 2
Formulation (phr)

                 Control   Control + QDI

Natural rubber     100          100
N 234              30           30
QDI                --            2
Stearic acid        2            2
Zinc oxide         3.5          3.5
6PPD                2            1
TMQ                 1           --
TBBS                1            1
Sulfur              1            1

6PPD - N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine

TBBS - N-tert-butyl-2-benzothiazolesulfenamide

TMQ - Polymerized 1,2-dihydro-2,2,4-trimethylquinoline

Mixing procedure:

First pass Speed 3 (155 rpm), full cooling
 0' 1/2 rubber
15" Rest of rubber
30" (QDI), 1/2 black, 1/2 chemicals
45" Rest of black + chemicals
75" Add any loose black in tray; sweep
Dump @ 160[degrees]C

Second pass Speed 3 (155 rpm), full cooling
 0' 1/2 rubber
15" Black, rest of rubber
75" Add any loose black in tray; sweep
Dump @ 160[degrees]C

Third pass Speed 3 (155 rpm), full cooling
 0' 1/2 rubber
15" Rest of rubber
Dump @ 160[degrees]C

Fourth pass Speed 3 (155 rpm), full cooling
 0' 1/2 rubber
15" Rest of rubber
Dump @ 160[degrees]C

Final pass Speed 1 (77 RPM), full cooling
0' 1/2 rubber, curatives, rest of rubber
Dump @ 110[degrees]C

Table 3--cost analysis model calculation

QDI in formulation (phr)                                  1.0
QDI Price ($/kg.)                                         11.0
6PPD Price ($/kg.)                                        4.1
Mixer volume (liter)                                      650
Batch size (kg.)                                          488
Natural rubber (kg.)                                      305
Kg. of QDI @ x phr                                        3.1
Cost of QDI ($)                                           33.6
Added cost to the compound per kilogram ($)              0.0688
Savings in PPD ($)                                       6.2525
6PPD credit per kilogram ($)                             0.0128
Net cost per kilogram ($)                                0.0559
Estimated savings in elimination of 1 pass per kg. ($)   0.291
Net savings ($)                                          0.235

References

(1.) Frederick Ignaz-Hoover, "Productivity enhancements in difficult to mix NR compounds," paper no. 141, presented at the Rubber Division, ACS, meeting in Pittsburgh, PA, Oct-ober; 2002.

(2.) Frederick Ignatz-Hoover and Byron H. To "Softening NR compounds: A comparison of methods used by the industry to increase productivity of NR compounds by various methods of softening," presented at the Rubber Division, ACS, meeting in San Francisco, CA, April, 2003.

(3.) I.R. Gelling, G.T. Knight, Plastics and Rubber: Process-ing, p. 83, September 1977.

(4.) L. Taimr, M. Prusikova, J. Pospisil, Die Angewandte Makromolekulare Chemie, 175, pp. 169-180 (1990).

(5.) Frederick Ignatz-Hoover unpublished results.

(6.) Ignatz-Hoover, Katritzky, Karlson, Petrukin, Journal Chem. Inf. Comp. Sci., 41, 2, pp. 295-299 (2001).