2.3.4. Linear Friction Tester

Experiments on the friction properties of the SSBR-silica composites were carried out, using a tribometer-linear friction tester, located at the Deutsches Institut für Kautschuktechnologie (DIK), Hannover, Germany [22]. The instrument is shown schematically in Figure 1a. The test samples 20 mm × 20 mm × 10 mm by size were prepared in the compression molding machine (Fontijne TP1000, Delft, The Netherlands) at 160 ◦C at a force of 150 kN. The edges of the specimen are chamfered to avoid kinking of the elastomer specimen on contact with the asphalt surface. An example of the test specimen is shown in Figure 1b. Measuring temperatures from 2 ◦C to 100 ◦C, sliding velocities from 0.1 mm/s to 300 mm/s, and a variation of the load from 1 bar to 7 bar are possible at this experimental setup. Current experiments were performed at 15 ◦C with a load of 2 bars at velocities 0.1 mm/s to 300 mm/s. Measurements were performed on fine asphalt surface wetted with water to mimic wet friction properties of rubber composites similar like wet skid behavior of tires under ABS-braking conditions. It is commonly assumed that the ABS wet braking temperature for tire tread compound is ca. 40 ◦C (i.e., usually somewhat larger as the ambient temperature in mid-Europe) and the slipping velocity is around 1 m/s. It should be noted that, for the vehicle friction, the tread block slip velocity depends on time. In other papers, an effective (say time-averaged) slip velocity *v* = 0.3 m/s in water and *v* = 0.1 m/s in the dry state [23] was presumed. In reality, however, the tread blocks experience an inhomogeneous slip dynamic, where the slip velocity is near zero when a tread block enters the tire road footprint, while the slip velocity is typically in the order of a few m/s when the tread block is retracted from the footprint. The friction tester in our experiments is limited to velocities lower than 1 m/s. For this, we used

the time-temperature superposition principle (Williams–Landel–Ferry (WLF) equation) to calculate the correct temperature for 10 mm/s (the measurable velocity of the machine). Therefore, the test temperature was determined using the universal WLF constants (*C*1 = 17.44 and *C*2 = 51.6 K) with help of the glass transition temperature of composites (*Tg*). The *Tg* values of the unfilled rubber as well as the silica-filled composites were determined by dynamic mechanical measurements (temperature sweep mode at a frequency of 10 Hz) [20].

**Figure 1.** (**a**) Setup of the friction tester (schematically) reprinted from Reference [22] with permission from Elsevier and (**b**) an example test specimen.

Derived from the *Tg* of the elastomer compounds and the WLF principle, the temperature *Tcalc* was calculated to be ca. 15 ◦C for a velocity of 10 mm/s (see Table 1). This temperature *Tcalc* was further used in our experimental measurements.


**Table 1.** Glass transition temperatures of elastomer compounds and the calculated temperatures for the friction measurement by using the WLF principle.
