Reinforcing Fillers and Liquid Additives

9.1 Reinforcing fillers
9.1.1 Introduction
In this chapter, reinforcing fillers, carbon black and precipitated silica, will be discussed.
Because a reference book on carbon black [1] and a review article on silica [2] are available,
the discussion will be limited to the aspects that are pertinent to mixing.
Major properties characterising these fillers are surface area (particle size) and structure
(bulkiness). The commercial grades of carbon black and their characteristics are given in
Table 9.1 [3]. When discussing mixing one more property must be considered, the mixing
ease, which will be part of the discussion that follows.
9.1.2 Mixing ease of fillers
The ease of mixing has two aspects, one is ease of incorporation and the other is dispersion.
How can the ease of mixing be quantified and how can it be related to the appropriate
properties of a filler? These questions concern characterisation of fillers and may be
called the property-processability relationship. Whereas the effect of fillers on the
performance of vulcanisates has been quite extensively researched and the reinforcing
mechanisms have been examined, the ease of mixing has not been investigated as much.
This chapter is also limited to primarily qualitative discussions. Scientific investigation is
very much in order.
Consider first the phenomenon, which is called rejection of a filler by rubber. When a
silica filler is charged onto premasticated natural rubber, fine particles of the filler are
blown out from the charge hole of the internal mixer like smoke. It is interpreted as a
repulsion by the electric charge built in the rubber during mastication. When silica is
premixed with carbon black or fine powdered metal carbonates, the repulsion is
significantly reduced [9].
The second aspect is the affinity between filler and rubber which, affects incorporation.
The stronger this affinity the easier that incorporation appears. Concerning the filler