4.3 Carbon Black Dispersion as Revealed by 3D-TEM
71
4.3.2 3D-Imaging of Carbon Black Dispersion and Its
Analysis
Table 4.4 shows the compounding recipes of CB-loaded NR vulcanizates [19–21],
which are supposed somewhat standardized ones using a delayed action accelerator,
CBS, with an activator system consisting of stearic acid and zinc oxide for vulcanization of NR by sulfur. As the reinforcing filler, HAF grade CB is employed which is a
popular grade for tire rubbers. Nine compounds of the varied amount of HAF were
press cured at 140 °C for 5 min. As described in 4.2.1, the vulcanizates were subject
to the pretreatment in order to remove the zinc compounds in the rubber sample.
In Fig. 4.14 is shown the 3D-TEM images of four specimens from the nine vulcanizates (cured at 140 °C for 15 min) from the compounds shown in Table 4.4 [21,
22]. In these images, CB is represented by the white. An aggregate separated more
than 1 nm from the adjacent is differentiated by the degree of whiteness or color as
shown in Fig. 4.15 [21, 22]. From Figs. 4.14 and 4.15, dispersion of the aggregates
is not homogeneous and the presence of them is much localized at the lower CB
loading. With the increase of CB-loading amount, clustering of the aggregates is
more and more observed up to the apparently full packing at 80 phr.
Those 3D images enabled us to calculate d p and STD (d p ), which are plotted in
Fig. 4.16 against the CB-loading amount [22–24]. As observed in the case of silica
(see Fig. 4.9), increase of CB amount resulted in a drastic decreasing of d p , and more
than 40 phr gave an almost constant value. The convergent value is approximately
3 nm, which suggests the presence of the rubber layer inhibiting the direct contact
of CB aggregates. In other words, bound rubber on the surface of CB aggregates is
ultimately compressed to 3 nm thickness by the increasing CB amount up to 80 phr,
where presumably the maximum packing of CB is observed, exactly as observed in
particulate silica. This estimation is somewhat in conformity with an experimental
observation that mixing of more CB than 100 phr into rubber is often difficult in
operating to get a good-looking rubber compound.
In addition, the volume resistivity (ρ v ) of the CB-loaded NR vulcanizates is plotted against the CB amount in Fig. 4.17. Since CB is electron conductive, addition
of CB lowered the resistivity, and after 40 phr, it tends to show an asymptomatic
value. This trend is in accord with the behavior of d p shown in Fig. 4.16, and is
suggesting an electrical percolation. These behaviors are exactly corresponding to
those of particulate silica shown in Figs. 4.9 and 4.10. The two nanofillers for rubber,
CB and particulate silica, show percolation behavior, and both seem ultimately to
form the network structure in rubber matrix.
The thickness value of 3 nm estimated from Fig. 4.17 is the distance which is
most probably minimal between the CB aggregates observed at higher CB loading
than 40 phr. This rubber layer is due to the bound rubber or the immobilized rubber
layer (see 2.5.2). A number of papers have reported various values of the thickness
of the bound rubber. Among them, most cited and hence dependable ones have
been evaluated by NMR technique, and they are between 5 and 20 nm [25–28].
The value 3 nm obtained here is considered to be minimal, reasonably the lowest
71
4.3.2 3D-Imaging of Carbon Black Dispersion and Its
Analysis
Table 4.4 shows the compounding recipes of CB-loaded NR vulcanizates [19–21],
which are supposed somewhat standardized ones using a delayed action accelerator,
CBS, with an activator system consisting of stearic acid and zinc oxide for vulcanization of NR by sulfur. As the reinforcing filler, HAF grade CB is employed which is a
popular grade for tire rubbers. Nine compounds of the varied amount of HAF were
press cured at 140 °C for 5 min. As described in 4.2.1, the vulcanizates were subject
to the pretreatment in order to remove the zinc compounds in the rubber sample.
In Fig. 4.14 is shown the 3D-TEM images of four specimens from the nine vulcanizates (cured at 140 °C for 15 min) from the compounds shown in Table 4.4 [21,
22]. In these images, CB is represented by the white. An aggregate separated more
than 1 nm from the adjacent is differentiated by the degree of whiteness or color as
shown in Fig. 4.15 [21, 22]. From Figs. 4.14 and 4.15, dispersion of the aggregates
is not homogeneous and the presence of them is much localized at the lower CB
loading. With the increase of CB-loading amount, clustering of the aggregates is
more and more observed up to the apparently full packing at 80 phr.
Those 3D images enabled us to calculate d p and STD (d p ), which are plotted in
Fig. 4.16 against the CB-loading amount [22–24]. As observed in the case of silica
(see Fig. 4.9), increase of CB amount resulted in a drastic decreasing of d p , and more
than 40 phr gave an almost constant value. The convergent value is approximately
3 nm, which suggests the presence of the rubber layer inhibiting the direct contact
of CB aggregates. In other words, bound rubber on the surface of CB aggregates is
ultimately compressed to 3 nm thickness by the increasing CB amount up to 80 phr,
where presumably the maximum packing of CB is observed, exactly as observed in
particulate silica. This estimation is somewhat in conformity with an experimental
observation that mixing of more CB than 100 phr into rubber is often difficult in
operating to get a good-looking rubber compound.
In addition, the volume resistivity (ρ v ) of the CB-loaded NR vulcanizates is plotted against the CB amount in Fig. 4.17. Since CB is electron conductive, addition
of CB lowered the resistivity, and after 40 phr, it tends to show an asymptomatic
value. This trend is in accord with the behavior of d p shown in Fig. 4.16, and is
suggesting an electrical percolation. These behaviors are exactly corresponding to
those of particulate silica shown in Figs. 4.9 and 4.10. The two nanofillers for rubber,
CB and particulate silica, show percolation behavior, and both seem ultimately to
form the network structure in rubber matrix.
The thickness value of 3 nm estimated from Fig. 4.17 is the distance which is
most probably minimal between the CB aggregates observed at higher CB loading
than 40 phr. This rubber layer is due to the bound rubber or the immobilized rubber
layer (see 2.5.2). A number of papers have reported various values of the thickness
of the bound rubber. Among them, most cited and hence dependable ones have
been evaluated by NMR technique, and they are between 5 and 20 nm [25–28].
The value 3 nm obtained here is considered to be minimal, reasonably the lowest