Viscosity modifier for HP elastomers.

One of the largest problems in the development and manufacturing of rubber articles is the relation between the physical properties of the vulcanizate and the processing of the compound. The performance requirements of the final article, for example, tensile and tear strength, fire retardancy, dynamic properties or maximum service temperature, often require high filler loadings. However, the manufacturing and processing of these compounds are very difficult due to their high viscosity. Traditional process aids or plasticizers cannot be used, or can only be used in low concentrations due to their detrimental effect on the physical properties of the vulcanizate. The compound recipe is therefore always a compromise between the desired properties and the processing requirements.

It is therefore highly desirable to develop an additive that allows high filler loadings at reduced viscosity, but has no negative influence on the performance criteria of the rubber article.

The new viscosity modifier, Sidistar, offers the opportunity to increase the dosage level of reinforcing fillers, like precipitated silica or carbon black, without sacrificing the compound processability and the physical properties of the finished elastomeric article.

This inorganic additive, with spherical primary particles of an average diameter of 150 rim, lowers the compound viscosity but improves the mechanical performance of the vulcanizate at the same time. This can be explained by its ability to be dispersed as primary particles in the elastomeric matrix, thus reducing internal friction and filler-filler interaction. Properties such as heat build-up and abrasion resistance are improved. The required mixing energy is significantly reduced.

The examples presented in this article illustrate the effect of the additive in compounds for high performance applications, for instance tires and cables, based on elastomers like solution SBR, chloroprene and HNBR/EVA, respectively. New combinations of compound properties become available, opening new or extending existing areas for the use of rubber articles.

Characteristics of Sidistar R

Sidistar R (S-R) is an amorphous silicon dioxide with an average particle size of 150 nm and a surface area of about 20 [m.sup.2]/g (BET). The primary particles form agglomerates with

excellent dispersibility, due to small contact areas and low interaction between them. The material is manufactured by a gas phase process at high temperatures. Its low chemical reactivity is advantageous, since it reduces the interaction with other compound ingredients. Figure 1 is a TEM of Sidistar R at a magnification of 55,000x.

[FIGURE 1 OMITTED]

Comparison to fillers When comparing the main characteristics of Sidistar R to conventional silica (precipitated and fumed) and carbon blacks (active N 330 and non-active N 990) used as traditional fillers in the rubber industry, the differences are obvious. It consists of rather large primary particles with a very low structure (table 1).

These differences in physical properties, together with the neutral pH-value, low water content and low chemical surface reactivity, make it a unique type of compound ingredient.

Experimental

All compounds were mixed in laboratory scale using conventional technology and equipment, e.g., internal mixer and two-roll mill. The vulcanization time was based on t90, determined by a MDR 2000EA-1 at 160 [degrees] C, 177 [degrees] C and 180 [degrees] C, respectively, and varied depending on the thickness of the specimen.

Results and discussion

Passenger car tire tread--SBR silica filled--"green tire" In this formulation (table 2), Sidistar R was added at 20 and 30 phr, respectively, to demonstrate its effect. In the case of the 30 phr addition, the precipitated silica concentration was reduced to 70 phr. Control 2 shows the effect of a 100 phr loading with precipitated silica.

S-R lowers the compound viscosity and improves, at the same time, the mechanical performance of the vulcanizate due to its compatibility with the precipitated silica (figures 2 and 3). No additional silane coupling agent is required for the added amount of S-R. Heat built-up and abrasion resistance are especially improved in comparison to control 2 with equal filler concentration. The required mixing energy is significantly reduced.

The dispersion of S-R in the silica system was investigated by transmission electron microscopy (TEM), as shown in figures 4 and 5. It is obvious that Sidistar is dispersed as primary particles between the agglomerates of the precipitated silica.

[FIGURES 4-5 OMITTED]

Cables Besides tires and driving (power) belts, cables are probably the rubber articles with the most stringent requirements. Not only do they have to meet specifications for electrical and mechanical performance, fire retardancy, oil and aging resistance, they are also processed at very high extrusion speed to be manufactured cost effective. The following examples provide initial data on the improvement of both performance and processing, available by the application of Sidistar.

The reference formulations used are not optimized and therefore not suitable to produce cables, but can be considered as a good starting point for compound development.

Railway energy cable--chloroprene, silica filled Table 3 shows the formulation of a chloroprene based energy cable compound. In this attempt to improve processing without negatively affecting compound properties, a blend of 30 phr of precipitated silica and 30 phr of Sidistar was chosen as the first approach. The results are compared to the original formulation (control 1) and a compound containing 60 phr of precipitated silica (control 2).

The reduction in compound viscosity and therefore the expected improvement in processing are significant (figure 6). Looking at the mechanical properties (figure 7), it becomes obvious that the addition of Sidistar had no influence on tensile strength and only a marginal one on tear strength, which probably could be optimized by choosing a different blend ratio.

Flame retardant cable--HNBR/EVA filled with ATH A challenging task in cable compound development is the requirement for high flame retardancy and high oil resistance at the same time. The following example shows a solution to this based on a blend of HNBR and EVA (table 4). Again the addition of Sidistar provides a reduction of compound viscosity (figure 8), and even improves the mechanical properties of the material (figures 9 and 10), without negative effects on oil resistance (figure 11) and flame retardancy or other required properties.

The effect of S-R addition was also investigated in elastomeric compounds based on EPDM, NBR/PVC blends and other polymers. In all cases, it was found that the processing could be improved significantly due to reduced compound viscosity. The physical properties were maintained or even improved. In some cases, a reduction of the loading of reinforcing fillers was possible. Further valuable advantages include improved compression set and flame retardancy. Depending on the application areas of these compounds, properties like water uptake, electrical resistivity, oil resistance and thermal aging have been tested. No negative influence of S-R addition has been observed.

Conclusions

The addition of S-R to highly filled elastomeric compounds reduces the viscosity, but maintains or even improves the physical properties. This can be explained as a result of the function of S-R primary particles as an internal physical lubricant that reduces the friction and the interaction between the filler particles (due to their spherical shape and their low chemical reactivity).

Table 1--main characteristics of Sidistar R in comparison to fillers

                         Sidistar   Ultrasil     Aerosil
                          R300      7000 GR    OX 50   200

Particle size (nm)         150       ca 10      40     12
BET ([m.sup.2]/g)           20        175       50     200
CTAB ([m.sup.2]/g)          30        160        -      -
DBP ([cm.sup.3]/100 g)      81        240        -

                            Carbon black
                          N 330      N 990

Particle size (nm)          28        250
BET ([m.sup.2]/g)           75         9
CTAB ([m.sup.2]/g)          83         9
DBP ([cm.sup.3]/100 g)     102

Table 2--formulation of green tire compound

                      Control   Control    Mix     Mix     Mix
Materials                1         2        2       5       7

Buna VSL 5025-1         103.1     103.1   103.1   103.1   103.1
Buna CB 10                 25        25      25      25      25
Ultrasil 7000GR            80       100      80      70      70
Silane X50S              12.5      12.5    12.5    12.5    12.5
S-R 300                    -         -       20      30      -
S-R 300 densified                    -       -       -       30

Table 3--formulation of chloroprene based energy cable compound

Materials             Control 1   Control 2   S-R Mix

Chloroprene              100         100        100
  (mercaptane type)
Ultrasil VN2             50          60         30
Silane                    3           3          3
S-R 300                   -           -         30

Table 4--formulation of flame retardant HNBR/EVA cable compound

Materials                       Control   Mix 1   Mix 2

EVA (70% VA)                       50       50      50
HNBR (34% ACN)                     50       50      50
ATH                               148      128     148
Silane                           0.35     0.35    0.35
Perkadox 14/40 (40% peroxide)      5         5       5
S-R 300                                     40

Figure 2-compound properties of tire tread, SBR based

Control 1 80 Si [O.sub.2]             16        82
Control 2 100 Si[O.sub.2]             18        117
Mix 2 80 Si[O.sub.2] 20 S-R und.      15        84
Mix 5 70 Si[O.sub.2] 30 S-R und.      15        75
Mix 7 70 Si[O.sub.2] 30 S-R den.      15        75

Note: table made from bar graph.

Figure 3-vulcanizate properties of tire tread, SBR based

                                     HBU         Abrassion   Tear
                                ([degrees] C)     loss       strength
                                                             (N/mm)
                                                           ([mm.sup.3])

Control 1 80 Si[O.sup.2]              17           35
Control 2 100 Si[O.sup.2]             18           49          175
Mix 2 80 Si[O.sub.2] 20 S-R und.      17           36          152
Mix 5 70 Si[O.sub.2] 30 S-r und.      14           34          159
Mix 7 70 Si[O.sub.2] 30 S-R den.      15           33          145

Note: table made from bar graph.

Figure 8-Mooney viscosity of HNBR/EVA cable compound

Mooney viscosity ML (1+4) 00 [degrees] C

1     Control
2     Mx 1
3     Mx 2

Note: table made from bar graph.