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Precipitated silica in tires.

The major application of precipitated silica is probably in tires. However, before the 1970s, it was not so popular. The hydrophilic nature of silica was responsible for this. Precipitated silicas have been used in tire compounds as a minor portion of the filler in combination with carbon black as the major filler. The substitution of low levels (5 to 10 phr) of silica for carbon black can enhance tear resistance, cut growth and adhesion properties (ref. 1). In the tread area, the primary usage of silica has been in large off-the-road tires and in on- and off-road truck tires that are subjected to severe cutting and tearing during their service life. Carbon blacks have been the dominant filler in tread compounds for various types of tires. A wide range of choice among available carbon black grades has enabled optimization of tread properties for a broad range of service conditions and performance requirements. In contrast to carbon blacks, silicas, which are less compatible with the polymers used for tire treads, are more difficult to disperse and can reduce the effectiveness of some components of the vulcanization system. The overall result is reduced reinforcement and poorer performance than that of a carbon black compound. Systematic studies undertaken to correlate the physical properties of precipitated silica with compound performance were carried out. OTR tire treads were compounded with NR, carbon black and silica to reduce heat buildup and chipping/ chunking (refs. 2-5). Rubber coats for brass-coated wire use NR, carbon black and adhesion promoters such as organocobalt, resorcinol-formaldehyde resin-silica, or both systems as reviewed by Van Ooij (refs. 6 and 7). In fact, these are not only in literature; rather, these are being used by the tire manufacturers. Silica is also reported to be used in tire sidewall (refs. 8 and 9). The use of precipitated silica in sidewall increases the tear strength, cut growth resistance and resistance to ozone aging (ref. 10). The use of precipitated silica to extend the effectiveness of antiozonant has been evaluated (ref. 11). Table 1 highlights the use of precipitated silica in black tire sidewall.

The polar nature of silica prevents the excessive blooming of the antiozonant, and thus the appearance of the sidewall remains good. The authors have used 5 phr of silica in a typical tire tread compound and found that the properties were comparable (table 2). This approach is also being used by tire manufacturers to reduce the cost of the compound by replacing part of the carbon black with precipitated silica.

The latest major breakthrough in silica technology came with the discovery of silane coupling agents such as 3-mercapto propyltrimethoxyl silane. It was applied in silica-filled rubber to improve the reinforcing properties (refs. 12-14). This silane had a scorch problem. Therefore, a new silane bis-(3-triethoxysilylpropyl) tetrasulfide (TESPT) was introduced by Degussa in 1972 (refs. 12-14). This new silane system achieved a better performance of the winter tire in 1974. In the early 1990s, "green tire technology" was introduced by Michelin (ref. 15). This technology contributes vehicle fuel savings of 3-4% as compared to tread compounds with carbon black, corresponding to a reduction of the rolling resistance of the tire of approximately 20%. The environmental and economical advantage of the silica technology is most important, even though it encompasses many problems, such as higher production costs and difficulties in processing.

The reason why this technology can be considered so revolutionary is best described as follows: Grip is affected by the degree to which a tire is distorted at high frequencies; in other words, the degree to which it complies to small stones and unevenness in the road surface. Grip is best served by rubber compounds, which absorb high levels of energy (high hysteresis compounds). Rolling resistance, on the other hand, is affected by low frequency distortion; the deflection of the tire as it revolves. Low rolling resistance requires compounds which absorb low quantities of energy (low hysteresis compounds). The contrast with grip is why it has been impossible in the past to provide tires with both reduced rolling resistance and increased wet grip. With the addition of silica, however, tire engineers have been able to produce compounds which provide higher hysteresis at high frequencies, but lower hysteresis at lower frequencies than achievable with carbon black (ref. 16). The use of silica can result in a reduction in rolling resistance of 20% and is more relative to carbon black. The use of silica can also improve wet skid performance. By incorporating silica in their winter tire range, Vredestein claims to have improved wet skid performance by as much as 15%, substantially improving braking distances at the same time (ref. 17). Silica also provides substantial benefits in winter tires and all-season tires. Compounds using silica are more elastic and flexible at lower temperatures, allowing better grip and braking during wintry weather.

For tire performance, several parameters are required, including high (wet and dry) traction, high wear resistance, low rolling resistance and high steering performance. These performances depend on the physical properties of tire tread compounds, tire construction, including structure and tread profile, and the road condition. In particular, three important properties, such as wet traction, wear resistance and rolling resistance, are described as the "magic triangle" of properties, and they need to be well balanced, in general, improvement of any one property will cause a sacrifice of the other two. Judicial compounding with silica can improve the rolling resistance and wet grip, while keeping the wear resistance constant.

Another improvement of the precipitated silica came with the development of highly dispersible silica (HDS or HD silica). Rhodia scientists were the first to create highly dispersible silica (HDS), an innovation that has proven to be a breakthrough for the tire industry. Highly dispersible silica has become the worldwide benchmark, enhancing low rolling resistance and high wet-grip properties when compared to carbon black compounds. HDS, compared to conventional silicas, provides substantially better dispersibility in robber compounds due to its specific structure. Use of HDS has further enhanced the magic triangle for silica loaded compounds. The loading of HDS silica can easily be higher than the conventional silica (CV silica).

Proper understanding of silica morphology (structure, surface area, pore volume, pore size distribution, silanol group density) is necessary to understand the behavior of silica in rubber. During processing of silica, many primary particles remain condensed into aggregates of typical dimensions of 100-200 nm, which are the real reinforcing species in rubber compounds. When the particles are close together, an interaction between the primary particles can take place. The degree of condensation in aggregates, commonly designated by structure, determines the inter-particle void volume and pore diameter within the aggregates. The measurement of this "structure" is based on the adsorption of dibutylphthalate, so called DBP. Conventional silica has a DBP value of typically 175 g/100 g; especially for the HD silicas, the DBP value is typically 200 g/100 g or above. It was found by the so-called crushed DBP measurements that the HD silicas show a high structural level and are less fragile compared to the CV silicas. In addition, aggregates of the HD silicas have a more branched structure, with 3-4 major branches on average. This means that the HD silica is highly capable of dispersing due to shear forces during the mixing process. The structure, the strength of the structure, pore size and size distribution of HDS, is maintained in such a way that at the initial stage of mixing, the silica structure withstands the force and the rubber gets more time to penetrate inside the pores of the silica. At the later stage of mixing, the high shearing force of the mixing and the elongation force of the penetrated rubber distributes the silica evenly throughout the matrix. Its highly porous structure provides a significant improvement in dispersion properties, without having a negative impact on the mixing and processing properties. Furthermore, the pore structure provides optimal accessibility for rubber to have better interaction with the silica, resulting in an improved reinforcement index and good tread wear properties. Table 3 shows the differences between the HDS and the conventional silica. Table 4 indicates some of the rubber grade silicas available from Madhu Silica.

At a given surface area, the higher the silanol group densities, the higher will be the unreacted hydroxyl groups after silanization. This will lead to higher filler-filler interaction (popularly known as the Payne effect). The effect of silanol groups on the dynamic modulus (E*) has been well demonstrated by Blume (ref. 19). Dynamic modulus is directly related to the dynamic stiffness of the compound, which is a very important parameter for high speed tread compound in terms of cornering response of the tire. The dynamic modulus E* at low strain depends on crosslinking density, as well as on filler-filler interaction. The variation of the specific silanol group density (number of -OH/[m.sup.2]) has an influence on the dynamic stiffness E* at 60[degrees]C. The silanol groups fulfill two important functions, including the formation of the filler network and reaction with the silane. It has been demonstrated that the lower the specific silanol group density, the higher the dynamic stiffness at 60[degrees]C. This result is somewhat unexpected. At a higher silanol group density, a higher filler-filler interaction could also be expected. Consequently, the E* at 60[degrees]C should be higher, too. But this is not the case. To understand this relationship, two factors need to be taken into consideration: First, the reaction between the silane and the silanol groups, and second, the correlation of the Mooney viscosity with the degree of hydrophobation. If the amount of silica and silane is kept the same, the higher the specific silanol group density, the lower the Mooney viscosity will be (ref. 19). This means that the degree of hydrophobation is higher at a higher specific silanol group density. If the silica is more hydrophobic, the filler-filler interaction decreases and thus results in low Mooney viscosity. The lower the Mooney viscosity, the higher the reinforcement and the lower the filler-filler interaction (Payne effect). Consequently, the dynamic stiffness E* at 60[degrees]C will be higher at a lower specific silanol group density with the obvious disadvantage of a lower reinforcing index. On the one hand, decreasing the specific silanol group density is a good way to increase the dynamic stiffness E* at 60[degrees]C; but on the other hand, it has a negative influence on the reinforcing index. Therefore, the specific silanol group density needs to be finely adjusted to achieve an optimum balance between these two important properties. In general, the common grades of HDS (Ultrasil 7000GR, MFIL-200 [HDS-G]) have the sears number (indicator of silanol groups) of 9-11 ml/1.5 g. It has been demonstrated by Fumito et al. (ref. 20) by TEM that the average size of agglomerates that formed without a rubber mixture increased with an increase in silanol number per unit area. Further, the silica with a low silanol number could not form the aggregates or agglomerates.

Another important parameter is the surface area of the silica. It has been well demonstrated (ref. 21) that with the increase of surface area (measured by BET), mechanical properties like tensile, modulus and abrasion resistance increase. However, higher surface area also means higher silanol groups, which lead to higher filler-filler interaction. Higher filler-filler interaction leads to higher heat build-up, and higher rolling resistance resulted from higher hysteresis. However, the total surface area measured by the BET principle is not the true surface available for the rubber to interact with silica. All types of silica have micro pores (<2 nm), meso pores (2-50 nm) and macro pores (>50 nm). Polymer/rubber molecules, being large enough, cannot penetrate the micro pores. However, the small compounding molecules can sit inside the micro pore and thus are unavailable for curing reaction. This leads to a slow cure rate and low extent of curing. The BET values of conventional silica may vary in the range of 50-350 [m.sup.2]/g. Especially for highly dispersible silica (HD silica), the BET value is 170 [m.sup.2]/g or above. The CTAB measurement is commonly used in the determination of the specific surface area of silica. CTAB molecules are so large that they cannot penetrate into the micro pores, as shown in figure 1. Therefore, it is possible to measure the external surface area of the particle. The standard values of CTAB may vary in the range of 100-200 [m.sup.2]/g; especially for HD silica, the standard value of CTAB is around 160 [m.sup.2]/g or above. CTAB values show a good correlation with the primary particle size. Therefore, the physical properties of the filled rubbers strongly correlate with the CTAB surface area; better than with the BET surface area. The difference between the BET and CTAB surface area is called the microporosity. In HD silica, the main aim is to make the BET to CTAB ratio close to unity. Thus, the optimization is achieved in HD silica with respect to filler-filler interaction and rubber-filler interaction.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Pore volume and pore size distribution represent another important parameter to understand. It has been found that the micro pores are not suitable, as described above, for rubber filler interaction. Meso porous silica is important in temps of better rubber to filler interaction. Also, total pore volume plays an important role for rubber penetration in the early stage of mixing. In the HD silica, the total pore volume is kept around 1.5-2 ml/g of Hg to get the optimized property. The key to the development of highly dispersible silica is a carefully designed precipitation process in combination with improved drying technology, followed by a controlled granulation process to achieve the required pore structure.

It has been found that the silica used in automobile tires is not suitable for truck tires, motorbike tires and very high speed tires. Thus, different types of high performance silica have been developed to meet this requirement (Ultrasil 9000 GR, Zeosil 195HR, MFIL 200(G)-HG). The use of carbon blacks with high surface areas (CTAB area >130 [m.sup.2]/g) are limited in such applications, owing to the sharply increasing heat buildup due to higher hysteresis. Owing to the greatly reduced hysteresis when silica is used as a filler, therefore, it is possible to realize surfaces which are prohibited in the case of carbon black. This leads to the development of the ultra high performance silica (highly dispersible high surface area) with very high surface area (BET surface area 200-230 [m.sup.2]/g and CTAB area 180-200 [m.sup.2]/g) (refs. 22-24). However, there is some sacrifice in the processing front.

Rhodia has introduced a new dust-free silica called micro pearls or dust free silica. At the end of the precipitation process, after drying and washing, the precipitated amorphous silica is mechanically processed into micro pearls or granules (dimension of 1/10 mm to a few mm) to ease shipping, handling and use. It is this form that is used by the tire industry. The formation of these micro pearls and granules reduces the dustiness of the precipitated silica, and therefore reduces the potential for worker exposure to particles of amorphous silica during handling. During rubber compounding, due to the high energy involved, the granules or micro pearls of the precipitated silica are broken down, transforming back to the agglomerates with dimensions between 1 and 40 microns. Because of the strong mechanical energy applied to the rubber, agglomerates may be broken down and transformed into aggregates, some of which are of nanometric size (their dimensions may range between 100 to 500 nanometers). Figure 2 shows the typical SEM micrograph of micro pearl and conventional non-pearl silica.

[FIGURE 3 OMITTED]

Although in pearl shape, this silica has excellent dispersion capability due to its special structural characteristic. This has been demonstrated by very low wK coefficient (refs. 19, 23 and 24). The wK coefficient is the ratio of the peak height of the non-degradable particle, the maximum of which lies in the range of 1.0-100 microns, to the peak height of the degraded particles, the maximum of which lies in the range of <1 micron. The wK coefficient is therefore a measure of the "degradability" (dispersibility) of the precipitated silica. It has been reported that the wK coefficient has very good correlation with the dispersion coefficient (ref. 19). A typical particle size distribution graph of micropearl (after and before ultrasonication) is given in figure 3.

The main peak at approximately 10 microns is attributed to the initial structure with large agglomerates of silica. This higher structure is partly destroyed during the ultrasonic treatment. The extent of this breaking up of the higher structure can be correlated to the size of the first peak at 0.5 micron. The ratio of the peak height of the original agglomerates to the peak height of the decomposed agglomerates is defined as the wK-coefficient. The energy input of the ultrasonicator is around 100 W for 270 seconds. This energy is correlated with shearing energy of the rubber mixing (ref. 18).

Conclusions

Precipitated silica has many applications, especially in the polymer and rubber field. The technical requirements are also different for different applications. Proper understanding of the silica morphology and the silica structure-property relationship is required to fulfill the requirements. Among all the applications, probably the most changing and stringent field is rubber, and more specifically tires; rapid development in the automotive industry forcing the tire manufacturers to develop new tires. Silica, being a major raw material for tires, is also technologically changing rapidly. Tire and silica manufacturers are joining hands to develop new kinds of silica to address the contradicting requirements of the "magic triangle."

References

(1.) Compounding Precipitated Silica in Elastomers, Ed: Norman Hewitt, William Andrew Publishing, ISBN-13: 978-08155-1528-9 (978-0-8155), New York (2007).

(2.) K.M. Davies and R. Lionnet, Rubbercon '81, G4-1 (1981).

(3.) S. Wolff Rubber Chem. Technol., 55, 967 (1982).

(4.) L.A. Walker, Kautsch. Gummi Kunstst. 38, 494 (1985).

(5.) W.H. Waddell and J.R. Parker, Rubber World 207, 29 (1992).

(6.) W.J. van Ooij, Rubber Chem. Technol., 52, 605 (1979).

(7.) W.J. van Ooij, Rubber Chem. Technol., 57, 421 (1984).

(8.) C. Banchieri (to Pirelli S.p.A.), U.S. Patent US4210188 (July 1, 1980).

(9.) T.J. Segatta, P.H. Sandstrom and Z. Ronen (Goodyear Tire and Rubber Co.), U.S. Patent US5244028 (September 14, 1993).

(10.) W.H. Waddell, J.B. Douglas, T.A. Okel and L.J. Snodgrass, Rubber World 208 (3),21 (1993).

(11.) W.H. Waddel, R.S. Bhakuni, W.W. Barbin and P.H. Sandstrom, "Pneumatic Tire Compounding," in "'The Vanderbilt Rubber Handbook," R.F. Ohm, Editor, R.T. Vanderbilt Company, Inc., Norwalk, CT, p. 605, (1990).

(12.) F. Thum and S. Wolff, Kautsch. Gummi Kunstst. 28, 733 (1975).

(13.) S. Wolff, Kautsch. Gummi Kunstst. 34, 280 (1981).

(14.) S. Wolff, Rubber Chem. Technol. 69, 325 (1996).

(15.) R. Rauline (to Michelin & Cie), EP 0501227A1 (February 12, 1992).

(16.) Internet Page, www-tyres-online.co.uk/technology/silica.asp.

(17.) Internet Page, http://www.tyresavings.com/articles/helpadvice/technology-of-tyres/silica-technology-explained.

(18.) S. Uhrlandt, M. Siray, A. Blume and B. Freund (to Degussa AG), US6180076B1 (January 30, 2001).

(19.) A. Blume, Kautsch. Gummi Kunstst. 53, 338 (2000).

(20.) Fumito Yatsuyanagi, Nozomu Suzuki, Masayoshi Ito and Hiroyuki Kaidou, Polymer 42, 9,523 (2001).

(21.) T.A. Okel and W.H. Waddell, Rubber Chem. Technol. 67, 217 (1994).

(22.) O. Stenzel, S. Uhrlandt, H. Luginsland and A. Wehmeier (to Degussa AG), US007628971B2 (December 8, 2009).

(23.) Y. Chevallier (to Rhodia Chimie), US6335396B1 (January 1, 2002).

(24.) A. Blume, B. Freund, B. Schwaiger, M. Siray and S. Uhrlandt (to Degussa AG), EP 0983966B1 (November 7, 2001).

by Sugata Chakraborty and Darshak Shah, Madhu Silica
Table 1--partial replacement of carbon
black with precipitated silica in tire sidewall

 Silica/CB
Property 0/50 phr 8/50 phr 8/46 phr 8/42 phr

Scorch (Ts2, min.) 4.4 4.1 4.2 4.3
Cure (tc90, min.) 10.7 10.4 11.0 10.4
Max. torque (dNm) 16.4 16.0 15.8 14.8
Hardness (durometer A) 56 55 54 52
Tensile (MPa) 23.0 21.9 21.1 21.9
Elongation (%) 658 682 681 714
Tear (N/mm) 13.5 15.3 13.2 16.4
Cut growth, mm 25.0 25.0 16.6 16.1
@ 100 kc
Ozone rating 14.5 14 15 19

Formula- NR/BR-50:50, N330-50, oil-10, stearic acid-2,
wax-1, 6PPD-4, DTPD-1, ZnO-3, sulfur-1.8, MBS-1

Table 2--partial replacement of carbon
black by silica in tread compound

Compound Regular Experimental

Natural rubber 95 95
Devulcanized rubber compound (DRC) 10 10
Aromatic oil 9 9
N330 HAF carbon black 50 48
Silica, MFIL 200 G from Madhu Silica -- 5

Parameters Regular Exp.

M100 (MPa) 2.13 2.17
M200 (MPa) 5.69 5.71
M300 (MPa) 10.65 10.14
TS (MPa) 23.9 22.57
EB (%) 541 521
Hardness (durometer A) 62 63
Tear strength (N/mm) 88.2 92.1
HBU (delta T at base) ([degrees]C) 18.3 19.1
HBU (delta T at center) ([degrees]C) 32.8 33.1
Abrasion 120 122

Dynamic properties at 60[degrees]C/0.25% strain

Parameters Regular Exp.

E' (MPa) 13 13.9
E" (MPa) 1.53 1.61
Tan delta 0.118 0.116
Loss compliance (MPa-1) 0.0896 0.0822

DeMattia cut growth properties (cut length in mm)

 Regular Exp.

5.0 Kc 3.56 3.89
10.0 Kc 3.80 4.12
30.0 Kc 4.52 5.11
50.0 Kc 6.20 6.79
75.0 Kc 7.50 8.25

Table 3--comparison of HD silica with.
conventional silica (ref. 18)

Parameter HDS silica Conventional Silica

Mercury porosimeter Typical pore area Typical pore area
 is in the range of varies from 20-350
 175-195 [m.sup.2]/g [m.sup.2]/g

Hg intrusion volume Typical intrusion Not specified
 volume is in the
 range of 1.7-2.0
 ml/g

Ratio of BET/CTAB ~1.0, indicates less Typically more than
surface area microporosity 1.2, indicates more
 microporosity

Dispersion behavior Small amount of Undispersed silica
in a tire tread undispersed particles particles are
compound are observed or the observed or the
 size of the size of the
 undispersed particles undispersed
 is small particles is large

Advantages in tire HDS offers good grip, Properties like HDS
tread good tread wear, low not achieved by
 rolling resistance and normal precipitated
 longer life silica

Table 4--different Madhu Silica grades with
properties

 BET pH (5%
 surface area suspension
Silica grade ([m.sup.2]/g) in water)

MFIL-150 (G) 125 6.9
MFIL-200 (G)-LG type 160 6.5
MFIL-200 (G)-RG type 180 6.5
MFIL-200 (G)-HG type 220 6.5
MFIL-125 (G) 125 6.9
MFIL-100 (G) 115 6.5
MFIL-200 (HDS-G) 175 6.5

 Loss on Bulk
 drying density
Silica grade (%) (91110 Type

MFIL-150 (G) 5.5 300 C
MFIL-200 (G)-LG type 5.5 330 C
MFIL-200 (G)-RG type 5.5 330 C
MFIL-200 (G)-HG type 5.5 330 C
MFIL-125 (G) 5.5 300 SD
MFIL-100 (G) 5.5 300 HD
MFIL-200 (HDS-G) 6.0 280 HD

C--conventional, SD--semi-dispersible, HD--highly dispersible
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Author:Chakraborty, Sugata; Shah, Darshak
Publication:Rubber World
Date:Sep 1, 2013
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