Mixer and mixing conditions

The mixer was Moriyama internal mixer, model D3-75, with a 3 litre capacity. The fill factor was 0.7, and the rotor speeds were 72.2 rpm for the front rotor and 52.1 rpm for the back rotor. Also, a laboratory size Banbury mixer was used to assure the similarity of mixing performance. 

The mixing procedure follows general industrial practice. In this case the three stage mixing given in Table 12.3 was followed. 

The order of addition was rubber, carbon black, silica, coupling agent and a mixture of zinc oxide and stearic acid. The filler and other additives were charged very quickly in succession. 

The compound temperatures at dump after both first and second stage were specified to be 155 °C. This was to assure the completion of reaction between the silica and the coupling agent [10]. However, the dump temperature, i.e., the highest temperature experienced by the compound, became an important issue and will be discussed later. 

Because the Mooney index [11] of SIR-10 was about 100, it was necessary to masticate the compound for several minutes to bring it down to 70 to 75. This may be called the zeroth stage. 

---

The most commonly used technique to qualify filler dispersibility is to study light reflectivity of clean-cut mixes. Some apparatuses have been developed to evaluate filler dispersion using a calibrated set of reference mixes (Dispergrader). However, such characterization mainly detects dispersion defects of a few tens of microns, and direct comparison of carbon black and silica mixes has to be done cautiously. In any case, it is necessary to make a mix, which means choosing a formula, a mixer, and mixing conditions thus the result cannot be considered as an intrinsic dispersibility measurement of the filler, but just reflects the dispersibility of the filler in one mix with a set of mixing conditions. 

---