A dispersion aid for silica filler in rubber compounds.

The use of silica fillers in combination with coupling agents in tread compounds to improve tire performance, i.e., rolling resistance, wear and traction, is well known. Also well known are difficulties in processing these compounds. These difficulties are due in part to the strong filler to filler interaction of silica particles (ref. 1). Silane coupling agents have been effective in reducing filler to filler interaction as well as promoting polymer to filler interaction (ref. 2).

PPT-HDI is an effective low cost dispersion aid that will significantly aid in mixing and may allow a reduction in the level of coupling agent required relative to its role in reducing filler to filler interaction.

Experimental

A typical silica filled passenger tire tread compound (table 1) was used in this study. Mixing was accomplished using a conventional two-pass mix.

Table 1

Ingredient     Parts

Duradene 706   70.0
Budene 1207    30.0
Hi Sil 233     65.0
Degussa X50S   12.5
N110           10.0
Zinc oxide      3.0
Stearic acid    2.0
6PPD            1.5
MC wax          1.0
TMQ             1.0
Aromatic oil   25.0
CBTS            1.6
DPG             2.0
Sulfur          1.6

Test compounds were cured to an optimum state of cure (t90) as determined by rheometer measurement. Properties of the vulcanizates were determined through methods commonly used in the industry.

Mixing

An effective dispersion aid should lower energy requirements during mixing and provide a more rapid incorporation of the filler. Power consumption measurements (table 2) show that a reduction in power consumption and lower batch temperature resulted from the addition of the dispersion aid.

Table 2

                         Parts
                    dispersion aid

                        0       3

KW                   3.38    3.27
KWH                  .608   0.573
Temp. [degrees] F     292     287
Probe [degrees] F     329     317

A comparison of the power curves (figures 1 and 2) shows a sharp increase in power consumption immediately following the addition of silica, coupling agent and dispersion aid followed by a rapid decrease. This sharp peak is not seen following the addition of silica and coupling agent alone.

[Figures 1-2 ILLUSTRATION OMITTED]

This difference is believed to be the result of rapid breakdown of silica particles by the dispersion aid. A comparison of the peaks also indicates faster incorporation of the silica filler.

Rheology

Rheometer data (table 3) show lower torque values for a compound containing the dispersion aid. Lower torque values are generally accepted as an indication of lower filler to filler interaction.

Table 3 - rheometer data

                      Control                 With HDI

                  Time      Torque        Time      Torque
                 (min.)   (in.(*)lbs.)   (min.)   (in.(*)lbs.)

Min.              1.58        9.8         1.27        8.8
ts 1 in.*lb.      2.85       10.8         3.07        9.8
ts 2 in.*lb.      3.4        11.8         3.83       10.8
tc 30%            4.82       21.4         5.38       20.0
tc 50%            5.29       29.2         5.87       27.4
tc 90%            9.72       44.3        10.88       42.1
tc 95%           13.57       46.2        15.19       43.9
Max.             29.41       48.1        29.64       45.8

Cure characteristics

The dispersion aid has a slight retarding effect on cure rate (table 3) that results in a lowering of physical properties (table 4). No attempt to adjust cure rate or state of cure was made for the purposes of this study. That will be left to the compounder. It should be noted that the ratio of 300% modulus to 100% modulus, an accepted measurement of polymer to filler interaction, was unchanged. The retention of the reinforcing index suggests that physical properties can be restored at an acceptable cure rate by an increase in the level of accelerator and/or sulfur.

Table 4

                         Parts
                    dispersion aid

                         0       3
Shore A               72.2    69.6
100% modulus           501     443
300% modulus         2,070   1,835
Reinforcing index     4.13    4.14
Tensile              2,750   2,470
% elongation           366     379

Dynamic properties

Elastic modulus measurements for compounds with and without the dispersion aid (figure 3) show lower values with the addition of the dispersion aid. Lower values are considered to be an indication of lower filler to filler interaction, i.e., better dispersion of filler particles. Lower values may also translate to easier processing and improved dimensional stability of extruded profiles.

[Figure 3 ILLUSTRATION OMITTED]

Loss modulus values were also lower with the addition of the dispersion aid. Loss modulus is a measure of the breakdown and reformation of filler networking. Filler reagglomeration is most likely to occur during vulcanization as the result of lower compound viscosities and higher diffusion rates of silica particles (ref. 3). The lower loss modulus for the compound with dispersion aid may be attributed to its ability to reduce the diffusion rate and reagglomeration of silica particles during vulcanization.

Static discharge

Static build-up in silica filled compounds is known to occur. This build-up of static charge is due in part to the insulating characteristics of silica. The presence of coupling agents on the surface of the silica particles makes them even less conductive.

Understanding the nature of the dispersion aid, it seemed possible that the dispersion aid might be effective in reducing static build up by improving the conductivity of rubber compounds. To test this possibility, vulcanizates with and without dispersion aid were tested using a static decay meter, as described under Federal Test Method 4046 for determining antistatic properties of materials. Static charges were more rapidly dissipated from specimens containing the dispersion aid. The times to dissipate charges induced on the surface of the specimens are shown in table 5.

Table 5

Time (seconds) to dissipate a charge of +5,000V and
-5,000V induced on the surface of samples with 0% HDI
(A) and 3% HDI(B) at a relative humidity of 11.7%

Sample A            Sample B

+ charge - charge   + charge - charge
210 sec. 136 sec.   131 sec. 102 sec.

Any connection between the ability of the dispersion aid to improve the dissipation of static charges under the conditions of this test and those generated in tires is yet to be determined.

Summary

The effect of a polar material on the dispersion of silica fillers in rubber compounds was investigated. The positive results on filler dispersion and improvements in processing characteristics suggest that it is worthy of further evaluation in tire testing. In highly loaded silica compounds, it also has been shown to dissipate static charges very effectively.

References

(1.) S. Wolff and M.J. Wang, Rubber Chem. Technol. 65, 329 (1992).

(2.) M.J. Wang and S. Wolff, Rubber Chem. Technol. 65, 715 (1992).

(3.) G.G.A, Bohm and M.N. Nguyen, J. Appl. Polymer Sci., 55, 1041 (1995).