Nippon Gomu Kyokaishi, No. 1, 2012, pp. 18–xx
Ultra fine particle calcium carbonate for functional
additives
S. Tsutsui, T. Sugita
Shiraishi Central Laboratories Co. Ltd. (78, 4-chome, Motohama-cho, Amagasaki 660-0085 Japan
Selected from International Polymer Science and Technology, 39, No. 3, 2012, reference NG 12/01/18; transl. serial no. 16410
Translated by K. Halpin
Introduction
The rubber industry has recently been making greater use
of wet process silica compounds, mainly for application
in automotive tyres. Compared with other white inorganic
fillers, wet process silica has an extremely fine primary
particle size, and when used in conjunction with a silane
coupling agent, can provide reinforcement rivalling that
of carbon black. At the same time, however, it presents
certain problems, notably the difficulty of dispersion
in the rubber mixing step, and the limited range of
conditions for reaction between wet process silica and
silane coupling agents.
electron photomicrograph and Table 1 lists the powder
properties.
Its action and effect on the rubber compound differ
from conventional calcium carbonate in that addition in
small quantities to a reinforcing agent like wet process
silica improves dispersibility, and the interfacial adhesion
due to the silane coupling agent used for surface treatment
affords better reinforcement and enhanced elasticity.
Moreover, since UFPCC increases the tack of the uncured
rubber, an improvement in roll processability (tackifier
While attempts have long been made to improve
the dispersibility of wet process silica by optimising the
mixing procedure or using organic additives (dispersion
aids such as the metal salts of fatty acids), it has been
found that dispersion can also be improved with calcium
carbonate microparticles (surface-treated synthetic
calcium carbonate) [1, 2].
We review here ultra-fine precipitated calcium
carbonate (UFPCC), which both acts as a dispersion aid
and can invest rubber with various interesting functions
arising from the dispersant action.
Figure 1. Electron photomicrograph of UFPCC
Table 1. Typical powder properties of UFPCC
UFPCC
Overview of UFPCC
UFPCC has already been reviewed in this journal [2].
It is unique among the surface-treated synthetic calcium
carbonates currently on the market in having the smallest
primary particle size (20 nm) and having been surfacetreated with silane coupling agent. Figure 1 shows an
© 2012 Smithers Rapra Technology
Crystal form
Particle geometry
Whiteness [%]
pH
BET specific surface area [m2/g]
Mean particle size [nm]
(electron microscope observations)
Calcite
Cubic
90
8.8
70
20
T/57
substitution) and improvement in hot reinforcement can
be achieved.
Effect of UFPCC on processability in
compounding
Shorter mixing time
Figure 2. Mixing chart for wet process silica compound.
Compounding recipe (phr) S-SBR = 60, NR = 40, ZnO = 4,
stearic acid = 2, age resistor #224 = 1, process oil = 25,
wet process silica = 50, silane coupling agent (Si69) = 5,
UFPCC = 0 or 3.
(a)
Figure 2 shows the mixing chart when a compound
with 50 phr of wet process silica in a rubber blend of
solution-polymerised styrene-butadiene rubber (S-SBR)
and natural rubber (NR) was mixed in a 3L pressure
kneader with and without the addition of 3 phr of UFPCC.
It will be seen from Figure 2 that, compared with
the compound incorporating wet process silica alone,
the time for integration of the rubber stock and rubber
chemicals is shortened by addition of UFPCC, affording
a great reduction in net mixing time.
State of dispersion of silica (TEM
microphotographs)
(b)
Figure 3. TEM observations of rubber compounded with
UFPCC, TEM conditions: ultrathin section, thickness 100
nm. (a) Wet process silica alone incorporated (wet process
silica = 50 phr); (b) UFPCC also added (wet process silica
= 50 phr, UFPCC = 3 phr)
Figures 3a, b show the TEM microphotographs from
examination of the state of dispersion of wet process
silica in vulcanisates of the rubber compounds in Figure 2
mixed on 8 inch rolls with 1 phr each of vulcanisation
accelerators (Noccelar D and CZ) and 2 phr of sulphur.
Compared with the compound (A) mixed with wet
process silica alone, the compound (B) additionally
containing UFPCC clearly has improved silica dispersion.
Although a number of mechanisms may be suggested
for the improvement in dispersibility of wet process silica
by UFPCC, the key factors appear to be the tackiness
imparted to uncured rubber by UFPCC and the effect
of enhanced green elongation.
Mechanism of dispersion by UFPCC
Figure 4. Effect of adding UFPCC on the tackiness of
uncured rubber
Figure 5. Effect of adding UFPCC on the elongation of
uncured rubber
T/58
Figures 4 and 5 show the results for tackiness and
elongation of the uncured rubber when UFPCC alone
was added to a blend of S-SBR and NR. Clearly, the
tackiness and elongation of the uncured compound
improve the greater the proportion of UFPCC added.
It may hence be inferred that UFPCC causes
compounding chemicals such as wet process silica to
adhere to the surface of the uncured rubber in the early
stages of mixing, promoting uptake and integration into
the rubber, and that owing to the improved elongation of
the uncured rubber, the fillers taken up into the rubber are
dispersed while being stretched out (stretch dispersion).
Dispersion would thus appear to pass through the
following steps (Figure 6).
International Polymer Science and Technology, Vol. 39, No. 4, 2012
Step 1: UFPCC, which has good affinity
for rubber, is first taken up into
the rubber stock (increasing the
tack of the rubber surface and
green elongation).
Step 2: The rubber phase with the tack
imparted in Step 1 facilitates
uptake of wet process silica,
integrating the materials at an
early stage.
Step 3: In keeping with the improved
elongation of the uncured
compound, the wet process
silica and other compounding
chemicals taken up into the
rubber undergo extension (stretch
dispersion proceeds).
Figure 7 shows the motion of the rubber
in the internal mixer, indicating that
dispersion in rubber compounded with
UFPCC proceeds as the rubber is stretched
out by the rotor.
On the basis of the above mechanism,
stretch dispersion with UFPCC should be
more effective the lower the rotor speed.
Thus, the uncured rubber is fully stretched
when the rotor turns at low speed, whereas
at high speed the uncured rubber breaks
up before being stretched, making stretch
dispersion ineffective. Hence, the preferred
mixer type would have a relatively low rotor
speed like a roll mill, kneader or intermixer,
the effect being a little more difficult to
achieve with mixer types like the Banbury
that have a high rotor speed. Even with a
Banbury mixer, however, the desired effect
should still be obtained if the mixer runs
at low speed, in which case additional
benefits such as a decrease in heating can
be expected.
As illustrated in Figure 8, an effect
similar to the above is seen in mixing
and compounding with an actual internal
mixer; tackiness and elongation in the
uncured compound improve, and the
filler dispersibility improves as shown in
Figure 9. As a result, the improvements
in rubber properties illustrated in Table 2
can be expected.
Effect of UFPCC on roll processability
As we have seen, uncured rubber
incorporating UFPCC is invested with tack.
© 2012 Smithers Rapra Technology
Figure 6. Mechanism of filler dispersion by UFPCC
Figure 7. Motion of rubber in an internal mixer
Processability on the roll is therefore improved. In particular, although rubber
with a high filler loading of wet process silica is susceptible to bagging,
this can be remedied by making a small further addition of UFPCC (see
Figure 10). Since the rubber also has good surface smoothness when
sheeted, it is ideally suited to doubling and allied processing.
T/59
Table 2. Test results for properties of UFPCC compounds
UFPCC added
[phr]
Curelastometer (160°C)
tc (10)
[min]
tc(90)
[min]
Press cure (160°C)
300% modulus
[MPa]
500% modulus
[MPa]
Tensile strength
Elongation
[MPa]
[%]
Hardness
Other effects of UFPCC
Figure 8. Effect of UFPCC on tackiness and elongation of an actual
uncured rubber compound. [Mix A] [Recipe] S-SBR (SL-552): 60
(phr), NR (SMR-L): 40, wet process silica (Nipsil AQ): 50, oil:
25, stearic acid: 2. [Mixing conditions] Mixer: Banbury (1.7 L),
loading: 60%, ram pressure: 0.3 Mpa, rotor speed: 70 rpm, start
temperature: 70°C, dump temperature: 130°C, mixing time: 5 min
Since UFPCC confers tackiness on the uncured
rubber, it is a potential tackifier substitute, with
the further characteristic that, being an inorganic
additive, it can maintain or improve the flow of
the uncured rubber and improve the hot wear
resistance of the vulcanisate.
Tackifying effect of UFPCC
Figure 11 shows the results of tack tests on a blend
of NR and butadiene rubber (BR) compounded
with microparticulate carbon black as reinforcing
agent and coumarone-indene resin as tackifier.
The tackiness of the uncured rubber was reexamined when half or all of the tackifier had been
replaced with various kinds of calcium carbonate.
Tackiness was maintained despite substitution of
half the amount of tackifier with UFPCC of mean
particle size 20 nm but diminished greatly on
substitution with calcium carbonate of not less
than 30 nm. Moreover, tackiness decreased to
a greater extent than when UFPCC was used for
the whole of the tackifier, showing just how great
a tackifying effect UFPCC has.
Figure 9. Electron photomicrographs of UFPCC compound
Improvement in wear resistance by UFPCC
in tackifier compounding system
Figure 10. Inhibitory effect of UFPCC additive on bagging
T/60
Figure 12 shows the results for hot wear resistance
in a cured rubber of the same composition as 4.1.
Compared with a tackifier-only compound, there is
no notable improvement in wear resistance when
hot (80°C environment) when half the tackifier is
substituted with calcium carbonate of mean particle
size not less than 50 nm, but wear resistance is
found to be greatly enhanced when half or all the
amount is substituted with 20 nm UFPCC. The effect
may for the most part be attributed to improvement
in the dispersibility of microparticulate carbon
International Polymer Science and Technology, Vol. 39, No. 4, 2012
black by UFPCC, though reduction in the amount of
tackifier also has some effect. Thus, the difference in
hot wear is thought to arise because the tackifier is an
organic material and softens when heat is applied,
resulting in lower resistance to wear when the rubber
is hot, whereas UFPCC is inorganic and does not soften
despite application of heat.
Baseline
UFPCC
Oridnry colloidal
calcium carbonate
Mean size, nm
-
20
30
50
Tackifier, phr
5
2.5
-
2.5
2.5
UFPCC, phr
-
2.5
5
-
-
Colloidal CaCO3 1, phr
-
-
-
2.5
-
Colloidal CaCO3 2, phr
-
-
-
-
2.5
Recipe: NR+BR: 100 (phr), C/B: 65, oil: 10.5, others: 14.
[Mixing conditions] Mixer: Pressure kneader (3 L), start temperature: 60°C,
dump temperature: 30°C, load: 80%, ram pressure: 0.5 MPa, rotor speed:
40 rpm.
Figure 11. Tackifier substitution effect of UFPCC
Conclusions
Although calcium carbonate has long been incorporated
in large amounts in rubber compounds for a variety
of purposes, we have found that, as reviewed here, a
number of interesting properties can be imparted by
incorporating just a small amount in the form of UFPCC.
The presence of UFPCC in rubber milling enables the
milling time to be shortened, improves the dispersibility
of wet process silica and other rubber chemicals, and
improves roll processability (bagging), effects which are
attributable to the action of UFPCC in conferring tack
on the rubber and enhancing elongation in the uncured
compound.
Future developments
Baseline
UFPCC
Oridnry colloidal
Mean size, nm
-
20
Tackifier, phr
5
2.5
UFPCC, phr
-
2.5
Colloidal CaCO3 1, phr
-
-
-
2.5
-
Colloidal CaCO3 2, phr
-
-
-
-
2.5
calcium carbonate
30
50
-
2.5
2.5
5
-
-
[Recipe] NR+BR: 100 (phr), C/B: 65, oil: 10.5, others: 14.
[Mixing conditions] Mixer: Pressure kneader (3 L), start temperature: 60°C,
dump temperature: 130°C, load: 80%, ram pressure: 0.5 MPa, rotor speed:
40 rpm
The UFPCC product reviewed here has an extremely fine
particle size and has been surface treated with silane
coupling agent. As such, it can confer characteristics
on the mixing processability of rubber and properties of
the uncured rubber and vulcanisate hitherto unavailable
from calcium carbonate products.
More recently, we have been investigating UFPCC as
an additive for EVA sponge, etc, and have discovered
effects including shrinkage control, improvement in set,
and increase in strength, and new effects may also be
anticipated in other areas.
Figure 12. Effect of UFPCC in improving hot wear resistance
References
© 2012 Smithers Rapra Technology
1.
Shiraishi Kogyo Kaisha, Ltd., Polyfile, 6, 45-49
(2009).
2.
Tsutsui, S., Nippon Gomu Kyokaishi, 78, 218223 (2005).
T/61