Page 288 - Handbook of Plastics Technologies

ELASTOMERS

4.80 CHAPTER 4

High-structure fillers give rise to reduced elasticity in the uncured state. Unfilled elas-
tomers, when extruded in the uncured state (e.g., during processing) expand or swell when
they leave the extruder die (have memory or nerve). Along with this die swell is a shorten-
ing of the extruded profile. It is called extrusion shrinkage. Extrusion shrinkage is greatly
reduced by fillers, especially those of high structure. Also, as the structure of the filler in-
creases, the viscosity of the uncured composition or the stiffness of the vulcanizate in-
creases. This is because the higher-structure fillers immobilize more of the elastomer
during its straining in either the cured or uncured state.
The amount of structure is measured by using the dibutyl phthalate (DBP) absorption
method. Small amounts of DBP (a nonvolatile liquid) are added to dry filler until a non-
crumbling paste is obtained. The DBP absorption is expressed in ml of DBP per 100 g
filler.
Filler Surface Activity. A filler can have high surface area and high structure and still
give poor reinforcement if its surface does not interact at all with the elastomeric matrix.
For example, carbon black, which is a highly effective reinforcing filler, loses much of its
reinforcing effect if it is graphitized. During the graphitization processing (high-tempera-
ture heating in the absence of reactive gases such as air), most of the reactive chemical
functional groups are removed from the particulate surfaces.
A way to infer the activity of a filler toward an elastomer is to measure so called
“bound rubber.” When an uncured elastomer-filler mixture is extracted with a solvent (e.g.,
toluene), then the gel-like elastomer, which is bound to filler, cannot be dissolved, whereas
the rest of the elastomer is soluble and is extracted away from the gel-like mixture. The
more the bound rubber, the more active the filler is assumed to be.
In the case of carbon black, chemical functional groups on the filler that may have
some relation to reinforcement include carboxyl, lactone, quinone, hydroxyl, and so forth.
These are located at the edges of graphitic planes.
4.5.4.2 Carbon Black. Carbon black has been used in rubber compounds for well over a
100 years. First, there was lamp black, produced by the deposition from oil flames onto
china plates. It was used as a black pigment. Then, channel blacks (formed by exposing an
iron plate to a natural gas flame and collecting the deposited soot) were used as reinforcing
fillers in 1910. More recently, furnace black (produced industrially from petroleum oil in a
furnace by incomplete combustion in an adjustable and controllable process) was intro-
duced. Thermal carbon blacks are generally produced from natural gas in preheated cham-
bers without air. They are essentially nonreinforcing fillers that improve tensile strength
only slightly. However, they give only moderate hardness, even at high loadings, and their
compounds are easily processed. Furnace blacks are the main types used today. ASTM
designations, the older nomenclature, particle size, surface area, and structure of some
blacks are given in Table 4.13.
The first letter of the ASTM classification indicates the expected type of cure rate for
the compound as below:
• N for normal cure rate (indicates that the compounds will cure at a normal rate)
• S for slow cure rate
The letters N and S correspond, respectively, to the furnace blacks and channel types.
The first digit indicates particle size ranges as follows:

• 1 for 10 to 19 nm
• 2 for 20 to 25 nm
• 3 for 26 to 30 nm

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