Process promoters to maximize silica reinforcement performance in tires.

$5.00 a gallon gas! It is a reality in Europe and now showing up in North America. Soon the movement to silica reinforced tire tread compounds will become as prevalent here as it now is in Europe. These compounds have been inherently difficult to process, due to the incompatibility of silica reinforcement with normal tread polymers.

Since the introduction of silica as a performance enhancing reinforcement for tires, many changes have occurred in tire compound mixing and processing. The significant challenges created by exchanging traditional carbon black reinforcement with silica technology have given rise to many innovative changes in equipment, compounding strategy and mixing techniques. The goal is to achieve the anticipated performance improvements of silica reinforced tire tread compounds, while mitigating the inherent problems associated with the incorporation of this extremely difficult to disperse reinforcement into a tread rubber matrix. Yet one approach that has been overlooked until recently is potentially one of the most cost-effective methods.

Rhein Chemie has designed a uniquely formulated process promoter, specifically created to eliminate many of the costly negative aspects of achieving proper silica dispersion, while helping to maximize the positive effects which silica provides in relationship to tire performance. This article will detail the processing performance enhancements achieved by using this unique product, GE 1872, to improve silica incorporation and dispersion.

Background

Several years ago, silica reinforcement was found to give some significant advantages over conventional carbon black reinforced tire tread compounds (ref. 1). It was also found that adding a coupling agent such as TESPT further enhanced the performance by chemically attaching the silica to the polymer network (ref. 2). In order to achieve these successes, special mixing techniques had to be used to maximize the dispersion for optimum performance improvements (refs. 3 and 4). The silica reinforcements, along with the new mixing techniques, resulted in compounds that could not be easily processed through the extrusion and molding phase of the tire production. Early work indicated that zinc soaps could significantly reduce Mooney viscosity, improving both extrusion and mold flow of tire tread compounds containing silica as part or all of their reinforcement (ref. 5). Some manufacturers try to achieve the best aspects of both carbon black and silica reinforcement types by blending them together in the compound. Zinc soaps have shown to be an acceptable way of improving the processing performance for those compounds whose reinforcement combinations are dominated by carbon black over the silica. The increasing demand for low-er rolling resistance will change the strategy of tread compounds to silica-dominated reinforcement combinations. Today in Europe, some tire manufacturers are moving toward 100% silica reinforcement, but find that, even though zinc soaps still give favorable viscosity reduction, in silica-dominated compounds they do not achieve the desired performance properties.

In order to develop a better processing promoter, Rhein Chemie began with its standard working definition of a process promoter: "a tailor made additive, which enables a homogeneous mixing and subsequent processing of a rubber compound. This additive is designed to improve the mixing, extrusion and/or molding without negatively influencing the vulcanization properties of the compound."

Using this definition, questions could be raised whether or not a true process promoter has ever been designed for silica reinforced tire tread compounds. Certainly several products designed for other applications have been adapted and promoted for silica tires, but they usually demonstrate disappointing vulcanization properties when compared to the control compound without their use. Some of these recommendations even failed to provide adequate reduction in Mooney viscosity to significantly improve the extrusion or molding of the silica compound. Generally speaking, process promoters are divided into internal lubricants (homogenizers and viscosity reducers) and external lubricants (anti-friction agents). Previous work seemed to indicate drawbacks in trying to design a process promoter for silica reinforced compounds using 100% of either an internal or external lubricant. Internal lubricants appeared to interfere with the filler/polymer interaction, and external lubricants did not lower the viscosity enough to achieve any processing benefits. A new product, GE1872 is specifically targeted at silica filled tire tread compounds. GE 1872 contains a proprietary blend of both internal and external lubricants that can give the desired processing improvements without interfering with the filler/ polymer interaction. This one product will both maintain the extrusion and molding of the silica reinforced compounds and improve the performance properties of the vulcanizate.

Objective

The objective is to develop a process promoter specifically designed for the improvement of extrusion and molding of silica loaded tire tread compounds, while maintaining the physical properties. This article will show the basic design of a good process promoter, discuss the conventional choices available, and compare them against the newly developed GE 1872. This article will also show that GE 1872 will significantly improve the processability of silica tread compounds while maintaining the desired performance properties.

Experimental data

Compound development

The first consideration for this project was to choose a compound that would demonstrate the best process promoter type for silica reinforcement. The polymers chosen were a combination of 75 phr SSBR and 25 phr BR in combination with 100% precipitated silica reinforcement at 80 phr, 37.5 phr TDAE oil and 6.4 phr of TESPT coupling agent. A 4 phr antioxidant/antiozonant package was used, along with 2.5 phr zinc oxide and 1 phr stearic acid. The cure system was kept simple with 1.7 phr of TBBS, 1.4 phr sulfur and 2 phr DPG. The zinc oxide was deliberately kept low, and carbon black was eliminated to keep them from interfering with the action of the various process promoters being tested. The process promoters were added in the first pass to allow each of them to show its best effect.

The process promoters selected for the trials were a fatty acid combination, a zinc soap combination and newly de-veloped GE 1872. Each variable was tested at 3 phr loading to determine their differences in improvement. Mixing was done in the recommended three-pass system, with the silica being split 50/50 in the first two passes and the cure added in the third pass. The resulting compounds were tested for both processing and performance properties. Table 1 shows the formulations in detail.

Rheology 1 (rheometer)

Figure 1 shows the rheometer plots of the four compounds. All of the selected process promoters show reduction in the marching modulus sometimes seen with silica compounds. The GE 1872 maintains a higher maximum torque and a better defined cure curve than the other process promoters tested. All of the rheometers were tested at 160[degrees]C on a 60 minute chart.

[FIGURE 1 OMITTED]

Rheology II (Mooney scorch) In figure 2, both the fatty acid process promoter and the zinc soap show lower viscosity, but significantly longer scorch times, making both of these products act as retarders. The GE 1872 demonstrates that it can achieve both a reduction in viscosity and a minimal change in scorch, allowing for improved extrusion speeds over the control without sacrificing cure rate.

[FIGURE 2 OMITTED]

Rheology III (Rheovulkameter) In figure 3, it can be seen that the flow improvement of the GE 1872 in the Rheovulkameter (spider mold) over the control is almost equal to the zinc soap product, and much better than the fatty acid combo. This test again shows that the processing improvements of the fatty acid combo and zinc soap products can be obtained with the GE 1872 without the negative effects on the cure rate.

[FIGURE 3 OMITTED]

Physical properties

The physical properties (table 2) were taken from standard tensile sheets and other test specimens cured at 160[degrees]C for the t90% time of each sample. Properties were also tested alter hot air aging for 72 hours at 100[degrees]C. This way, the longer cure rates of the fatty acid and zinc soap products did not bias the values. The results show that the GE 1872 has no loss in physical properties, while both the fatty acid and zinc soap products were not able to achieve the values of the control compound. The GE 1872 also showed improved aged properties compared to the control, showing that this new product can maintain and may actually improve the performance of the control compound, while also enhancing the extrusion processing and molding.

100% modulus

One of the major differences between the two conventional process promoters and the newly designed GE 1872 is the effect on 100% modulus (figure 4). The GE 1872 showed no change in modulus, while the fatty acid and zinc soap products showed significant reductions in 100% modulus.

[FIGURE 4 OMITTED]

Dynamic: Tan delta effect

In figure 5, we see the detail of the dynamic effect of the various process promoters on the dampening properties of the vulcanizates. The testing was done in a normal temperature sweep, preformed on an RPA tester with the controls set at 10% strain and 10 Hz. The results show that the zinc soap increases the tan delta at high temperature, which could take away some of the rolling resistance improvement of normal silica reinforced tread compounds. The fatty acid tracks the control, showing no difference at either high or low temperatures. Unlike the other two process promoters, the GE 1872 shows a slightly lower tan delta in the 70-90[degrees]C range, and a noticeably higher tan delta from the control as the temperature decreases. These readings were taken with only 3 phr loadings, and further work needs to be done to determine if this trend can be exploited to give even greater differences. These new data could lead to a novel way to improve the performance properties of silica reinforced tread compounds by strengthening the wet traction, while maintaining the traditional low rolling resistance (ref. 6).

[FIGURE 5 OMITTED]

Conclusion

GE 1872, with its optimum balance between internal and external lubrication, demonstrates significant improvement over traditional fatty acid and zinc soap process promoters in improving processing properties, while maintaining the performance and dynamic properties of the silica reinforced compounds. This new product matches the lower viscosity and improved mold flow of the fatty acids and zinc soaps, without negative retarder effects, as shown in the rheology charts. In vulcanized performance properties, it maintains the positive effects of the silica reinforcement without the loss in physical properties usually observed with conventional process aids. The dynamic testing also indicates that further investigation needs to be done with this new product. Benefits could be realized in reducing the traditional tradeoffs of sacrificing wet traction and abrasion resistance for lower rolling resistance. Modification in coupling agent loading or better reinforcement designs in combination with GE 1872 could further enhance the performance of silica reinforced tires.

References

(1.) R. Rauline, U.S. patent 5,227,425 (Michelin).

(2.) F. Thurn, et.al, U.S. patent 3,873,489 (Degussa).

(3.) J. ten Brinke, P. van Swaaij, L. Reuvekamp and J. Noordermeer, Rubber Chem. and Tech., vol. 76-1, p.12, 2003.

(4.) C. Stone. K. Menting and M. Hensel, paper no. 59, presented at the Rubber Division, ACS, meeting, Oct. 2000.

(5.) L. Steger, H. Schulz and A. Johansson, Rhein Chemie Technical Report No. 60, "Processing promoters in silica tread compounds," Sept. 1996.

(6.) M-J. Wang, Rubber Chem. and Tech., vol. 71-3, p. 520, 1998.

by Steven F. Monthev and Dr. Hans-Martin Issel, Rhein Chemie

Table 1

Compound                    1      2      3      4

Buna VSL 5025-OHM          75     75     75     75
Buna CB 24                 25     25     25     25
Precipitated silica        80     80     80     80
TESTP                     6.4    6.4    6.4    6.4
TDAE (oil)               37.5   37.5   37.5   37.5
Antiozonant wax             2      2      2      2
TMQ                         1      1      1      1
6PPD                        1      1      1      1
ZnO                       2.5    2.5    2.5    2.5
Stearic acid                1      1      1      1
Sulfur                    1.4    1.4    1.4    1.4
TBBS                      1.7    1.7    1.7    1.7
DPG                         2      2      2      2
Fatty acid combination      0      3
GE 1872                     0             3
Zinc soap                   0                    3

Table 2

                        Control   Fatty acid   Zinc   GE 1872
Mooney                                 combo   soap       120

Initial                     159          118    110        99
ML(1+4) 100[degrees]C       119           97     92        76
Durometer A                  77           75     75        84
3 d/100[degrees]C            86           83     82        29
Rebound                      29           29     29        31
3 d/100[degrees]C            32           32     32       112
Abrasion lost               109          114    113
Stress-strain                                            17.5
Tensile                    18.0         17.3   16.5       305
Elongation                  315          341    352      1.47
Mod. 20                    1.38         1.16   1.16      2.25
Mod. 50                    2.22         1.84    1.8         4
Mod. 100                    4.0          3.4    3.1      10.0
Mod. 200                    9.8          8.4    7.5      17.2
Mod. 300                   16.9         14.7   13.4
3 d/100[degrees]C                                        15.4
Tensile                    13.6         15.0   13.8       218
Elongation                  194          230    244      3.13
Mod. 50                    3.11         2.78   2.55       5.9
Mod. 100                    5.9          5.2    4.5      13.9
Mod. 200                   14.2         12.7   10.7