Dispersibility measurements of prec. silicas' influence of dispersion on mech. properties.

Amorphous precipitated silicas are commonly used as reinforcing filler in rubber applications. The morphology of the precipitated silica used is an important parameter (refs. 1 and 5). Key reinforcement parameters are the total filler contact area within the rubber and low particle-particle interactions (ref. 6). It does mean a high dispersibility of the filler. Therefore, we made a study in order to state "dispersibility" effects on the properties of vulcanizates.

Experimental

Silicas evaluation

Eleven hydrated precipitated silicas, with two different BET specific surface areas ([S.sub.sp]): 125 [m.sup.2]/g and 170 [m.sup.2]/g were used (table 1). In this sample of silicas there are classical precipitated silicas A, B, D and E and for each surface range a new generation precipitated silica C for the 170 [m.sup.2]/g family and F for the 125 [m.sup.2]/g silica. Each silica is studied with different forms (powder, micropearl or granule). [TABULAR DATA 1 OMITTED]

We developed two "agglomerate strength" determination techniques which are based on the measurement of the effect of a controlled energy input onto precipitated silica agglomerates. We consider those methods describe quantitatively the ease of macrobreaking up of the agglomerates.

Ultrasonic method

A technique first developed by Cifticioglu et al. (refs. 7-9) was adapted to the study of the silica agglomerates breaking up. This technique consists of exposing water silica dispersion to an ultrasonic field and following the resultant changes via particle sizes analysis. We can notice that an easier test was previously used by Kraus (ref. 10) for carbon black.

It was shown that the technique gives a novel way to quantitatively discriminate precipitated silicas in terms of ease of dispersion.

The ultrasonic disruption was accomplished with a 600 W Vibracel, provided with a horn with 19 mm tip. The concentration of the sample was 5% wt. and the transmitted ultrasonic power was during 420 seconds. Powder and micropearl are tested without any specific sample preparation; granular forms need a slight manual breaking up before testing.

The precipitated silica was suspended in distillated water and the particle size distribution obtained by a Sympatec Laser particle size analyzer.

We retain the [D.sub.50] (in [mu]m) as the main parameter. In order to maintain the optical concentration constant we had to adjust for each sample, the concentration of suspension within the chamber of the particle size analyzer. We defined a second parameter we called "breaking up factor" ([F.sub.d]) given by the relation (1), where O.C is the optical concentration:

(1) [F.sub.d] = ([10.sup.*] introduced suspension volume)/O.C the easier the breaking up is, the lower is the [D.sub.50] and the higher is the [F.sub.d].

DOP viscosity

We developed a two-step method consisting of a disruption of the silica agglomerates under a mechanical grinding, and a viscosity measurement of the resulting powder suspended in phthalate dioctyl (DOP).

The mechanical crushing was accomplished with a grinding mill with a rotor speed of 20,000 rpm. The silica is introduced in the grinding mill with a delivery of 25 g/minute.

The ground precipitated silica was suspended in DOP at an 8% wt. concentration. 208.7 g of such a suspension are prepared by introducing 16.7 g of silica under mechanical stirring (about 800 rpm) into 192 g of DOP. The viscosity measurement is made after 25 minutes stirring of the suspension; the temperature stabilization of the suspension at 20[degrees]C is obtained after 30 minutes, then the viscosity measurement is made on a viscosimeter with the needle n[degrees]4. The final lecture is read after three minutes at 5 rpm following a first step of 50 rpm during three minutes.

We consider that an increase in the viscosity is mainly due to the smaller size particles resulting from the crushing. The higher the ease of breaking up is the lower the particle size is and the higher the viscosity is (ref. 11). The particle size is estimated by measuring the [D.sub.50] on the Sympatec just after grinding.

In rubber evaluation

Optical microscopy

We developed a specific formula (table 2) in order to assess clearly the macrodispersion level of the silica. The mixer used had a net chamber volume (dm3) of 1.6, ram pressure (MPa) of 0.8 and rotor speed (rpm) of 80/130. The mixing procedure is described in table 3. A raw rubber sheet is molded between a "terphane" sheet and black cardboard. 1.5 mm2 is observed by optical microscopy under incident light. The objects appear white and the rubber black. We used an image analyzer for counting silica agglomerates bigger than 125 [mu]m ("a" number) and between 50 and 125 [mu] ("b" number). From these results we define with the relation (2) a dispersion index:

(2) D.I = 4a + b the higher the D.I is the lower the dispersion state is.

Table 2 - macrodispersion
formula for optical
evaluation (phr)
IR 1205             60
SBR 1509            40
Silica              35
Oil                 15
PEG 4000           2.2
Activ zinc oxide     2
Stearic acid       0.5
Santocure NS         1
Accelerator TMTM   0.5
SUlfure T 300      1.5
Table 3 - mixing, compound preparation and
          slides exploitation procedures
Mixing procedure
First step: internal mixer
T = 0 min.      Introduction at 50[degrees]C and at a 80 rpm
                speed of IR and SBR
T = 1 min.      Silica, oil, NS, PEG 4000 and stearic
                acid
T = 3.5 min.    ZnO and Santocure NS
T = 5 min.      End of the compounding (temperature
                about 125[degrees]C
Second step on open mill      Introduction of S and TMTM
Sample preparation
Weight 2.7 g of compound, and press the sample, at cold
temperature inbetween two terphane sheets under 200 bars
Microscopic views
Low angled light; 100 times as final magnification; three
slides for each sample
Photographs exploitation
Number of the cumulated objects which have more than
100 [micro]m as a diameter (a), and those which have a diama-Index
of dispersion: D - 4a + b

Mechanical properties

There exists a certain number of properties depending more or less clearly on the dispersion level of the reinforcing particles within the rubber.

Funt (ref. 12) mentioned an increase in tensile strength, fatigue life and tear resistance with an improvement of the macrodispersion of a N 375 carbon black within a SBR 1502 formulation. Dizon and Papazian (ref. 13) observed an improvement in tensile strength, fatigue life and elongation at break with an increase of the macrodispersion of a carbon black (N 347) in a SBR 1712 recipe. We have studied those properties in quite simple compounds (tables 4-6).

Table 4 - EPDM formula used for vulcanizates
              evaluations
EPDM recipe (phr)
  EPDM 6630 (30 phr oil)               130
  Silica                       50(SiO2 with 170 [m.sup2]/g) or
                                    55(125[m.sup.2]/g)
Naphthenic oil                          50
Zinc oxide                               4
PEG 4000                                 3
Microcrystalline wax                     2
Polyethylene wax                         2
Stearic acid                             3
Coupling agent TESPT                     1
Accelerator MBT                          1
Accelerator DPTT                         2
Accelerator TMTD                         1
EPDM recipe mixing procedure
First step in the internal mixer at 60[degrees]C
T = 0 minute  EPDM
T = 1 minute  Silica + coupling agnet + St.ac + PEG + oil
T = 3 minute  ZnO + waxes
T = 5.5 min.  End of the compoundig (final temp. 130[degrees]C)
Second step on the open mill at 60[degrees]C
Sulfur + accelerators - three through the rolls (0.2
mm) and calendering 2.5 mm thick
Table 5 - SBR formula used for vulcanizates
         evaluations
SBR recipe (phr)
  SBR 1509                   100
  Silica                      40
  Aromatic rosin SP80         10
  Active zinc oxide          2.5
  PEG 4000                     2
  Stearic acid                 2
  Accelerator MBTS           1.80
  Accelerator DOTG           0.75
  Sulfur T300                  2
SBR recipe mixing procedure
First step in the internal mixer at 60[degrees]C
T = 0 min.    SBR 1509
T = 1 min.    2/3 Silica + St. ac + PEG + ZnO + SP80 + S
T = 2.5 min.  1/3 Silica
T = 5.5 min.  End of the compounding (final temp. 130[degrees]C)
Second step on the open mill at 60[degrees]C
Accelerators - three times through the rolls (0.2 mm) and
calendering 2.5 mm thick
Table 6 - NR formula used for vulcanizates
          evaluations
NR recipe
  NR SMR L                    100
  Silica                       25
  Carbon black (N 347)         35
  Aromatic oil 729 FC          10
  Zinc oxide                  3.5
  Antiozonant IPPD            1.5
  Antioxidant TQ              1.5
  Stearic acid             3.1(1) or 3.3(2)
  Accelerator CBS          1.75(1) or 1.6(2)
  Retarder PVI                0.15
  Coupling agent TESPT     2.81(1) or 2.2(2)
  Sulfur T300                  2
(1) - for silicas with 170 [m.sup.2]/g
(2) - for silicas with 125 [m.sup.2]/g according to the relations:
CBS = 1.25 (0.12(*)SS(*)10E-3); S.Ac = 3.0 (0.1(*)SS(*)10E-3);
C.Ag = 0.7(*)SS(*)10E-3
NR recipe mixing procedure
Three steps mixing sequences in int. mixed at 60[degrees]C
1st: NR + AOx + silica + silabe + ZnO + St.Ac + oil = MB1
2nd: MB1 + N347 = MB2
3rd: MB2 + S + CBS + PVI
Finalization on open mill at 60[degrees]C 10 cuts + calendering

Processing and curing characteristics

Processing and curing characteristics were determined on a rheometer at desired temperature according to ASTM D-2084. Physical testing was carried out on samples which had been cured as follows: For the EPDM recipe at 160'C with the T90 time, for the SBR recipe at 150[degrees]C with a T90 time and finally with the NR recipe at 140[degrees]C for 40 minutes.

Vulcanizate properties

Stress strain properties were measured according to ASTM D-412, hardness (Shore A) was obtained after 15 s according to ASTM D-2240. Cut growth resistance was determined following ASTM D-1052 55. Abrasion loss was measured by the DIN abrasion tester (DIN 53516). It is considered that the DIN technique measures only simple abrasive wear (ref. 2). Fatigue block tearing resistance was measured as mentioned in reference 14.

Results and discussion

Silica dispersibility

Ultrasonic test results

This test clearly distinguishes two families of precipitated silicas (table 7 and figure 1): The classical precipitated silicas with a low dispersibility (low [F.sub.d] and a high [D.sub.50]), the new generation precipitated silica with a high dispersibility, that is to say a high [F.sub.d] and a low [D.sub.50]. [TABULAR DATA 7 OMITTED]

For each specific surface area, [D.sub.50] and [F.sub.d] are well correlated as it is shown in figure 1. Unfortunately we were not able to measure with the same conditions the granular forms of the studied silicas, nevertheless the granules belong to their specific surface area family (figure 1). A new generation granular form shows the lowest [D.sub.50] and the highest [F.sub.d] (170 [m.sup.2]/g precipitated silicas) as previously observed for MP and P forms.

DOP viscosity results

In order to achieve the complete characterization of the studied silicas, the DOP viscosity test results were necessary insofar we had the same preparation procedure, that is to say the same way in order to evaluate all together the silicas (table 8). [TABULAR DATA 8 OMITTED]

Moreover, this test gives information about the breaking up kinetic. The new generation of silicas (refs. C and F) breaks up more after one grinding than the classical one: this observation confirms the results obtained with the ultrasonic test. In addition, the new generation precipitated silicas breaking up process continues during the next four grindings. It is not true for the other classical precipitated silicas. We have to mention for the new generation granular form (ref. C(Gr)) that its behavior, although it is not so good as the powder or micropearl forms, is quite better than the one of the classical powders.

Concerning more widely the influence of the final presentations of the silica on the ease of dispersion or the ease of breaking up, we observe the following classification (from the best to the worst ease of dispersion): MP # Gr; this classification is of course true for an identified generation (classical or new).

Conclusion

We distinguish the two different precipitated silica generations - the classical and the new one.

We observe a difference between a powder or a micropearl and a granular form, more precisely we are able to quantify the influence of the precipitation process and the influence of the final presentation of the silica on the ease of dispersion.

The dispersibility of the granular form is lower than those of the powder or the micropearl one.

For micropearls and powders, the two methods are equivalent in terms of characterization of the ease of dispersion.

The optimum from the point of view of the ease of dispersion is produced by a new precipitation process and presented under the MP form.

In rubber evaluation

State of macrodispersion (table 9)

[TABULAR DATA 9 OMITTED]

Owing to the sizes of powder form ([D.sub.50] under 50 [mu]m which is theoretically the detection limit), this technique applies only to the granular and the micropearl forms.

We obtained a good correlation between the DOP viscosity breaking up test results and the macrodispersion levels measured by this technique.

Again, we find the same classification as previously, that is to say the best behavior for the new generation silicas.

Conclusion for the silicas characterization

The breaking up tests performed (Ultrasonic and DOP viscosity) appear to measure effectively the ease of macrodispersion of the precipitated silicas.

Mechanical properties

The only significant affected properties in the EPDM recipe are the elongation at break and the tensile strength. All the others remain in the same range of values.

There is a linear relationship between tensile strength and elongation at break as it is shown in figure 2, and this relationship is dependent from the specific surface area of the silica.

We observe (figure 3) a well marked improvement in elongation at break and in tensile strength with the ease of dispersion (as measured by our DOP viscosity test). The same tendency was found by Dizon and Papazian for carbon black (ref. 13).

Both measurements take into account the effect of the "in rubber" contact area of the filler in the compound. The higher the "in rubber" contact area is the higher the tensile strength is and the higher the DOP viscosity is (ref. 1). Macrodispersion and microdispersion improvements result in tensile strength and elongation increase (ref. 12).

In SBR recipe

Again we observe an increase in tensile strength and in elongation at break when the ease of dispersion of the silica is improved. The same kind of variations in the SBR recipe are obtained as those we obtained in EPDM recipe, that is to say a well correlated relationship between elongation at break and tensile strength.

Without coupling agent, the main parameter influencing the tensile strength is the ease of dispersion of the filler.

Concerning the elongation at break, the specific surface area of the silica seems to be a second significant parameter. We consider it is the consequence of the ingredients adsorption (DOTG, ...) resulting in lower crosslinking level for the highest specific surface area. In fact, their delta torque as measured by the rheometer is lower.

For the De Mattia cut growth resistance (figure 4) the higher the ease of dispersion is the higher the cut growth resistance is. Again, the highest specific surface area gives the highest values, and again we consider that this effect is due to the ingredients adsorption.

In terms of abrasion resistance, we observe the same variations. The higher the "in rubber" contact area is the lower the abrasion loss is.

In NR recipe

In a NR formulation with a low silica / carbon black ratio, we do not observe any significant effect on the mechanical properties, nevertheless, we find again an improvement in abrasion resistance when the ease of dispersion increases.

With a coupling agent and an adjustment of the ingredients amounts in the NR recipe (refs. 1-4), the abrasion loss only depends on the total contact area of the filler in the rubber (ref. 1).

All the silicas (170 [m.sup.2]/g and 125 [m.sup.2]/g) follow the same relationship between abrasion loss and the DOP viscosity. We have always the same amount of silica, therefore we can conclude that the DOP viscosity takes into account both parameters: On one hand the specific surface area, and on the other hand the ease of dispersion; it does mean the real contact area of the filler in rubber. It is mentioned in the literature (refs. 12 and 13) that a higher carbon black dispersion level improves fatigue resistance, we observe the same effect with precipitated silica.

Conclusion

We have developed specific laboratory tests in order to access the ease of dispersion of precipitated silica in rubber. We have shown that those tests are correlated with the state of dispersion observed within the rubber: The DOP viscosity test appears to be particularly well correlated with tensile strength, elongation at break, cut growth and block tearing resistance, properties which are improved when the DOP viscosity increases, that is to say when the dispersion of the filler is better. In other words we confirm for silicas what was said by Dizon and Papazian for carbon black (ref. 13).

References

[1.] Y Bomal. Ph. Cochet, B. Dejean, ACS Rubber Division, Denver, CO, May 18-21, 1993. [2.] K.M. Davies, R. Lionnet, Rubbercon '81, Harrogate, England, June 8-12, 1981. [3.] S. Wolff, Tire Sci. Technol 15, 276 (1982). [4.] S. Wolff, Rubber Chem. Technol. 55, 967 (1987). [5.] S. Wolff, E. H. Tan, ACS Rubber Division, New York, AT April 8-11, 1986. [6.] M.J. Wang, S. Wolff, E.H. Tan, ACS Rubber Division Meeting, Louisville, KY, May 19-22, 1982. [7.] M. Ciftioglu, A. Akinc, L Burkhart, American Society Bull. 65, 1591, (1986). [8.] S.G. Thoma, M. Ciftioglu, D.M. Smith, Powder Technol., 68, 63, (1991). [9.] S.G. Thoma, M. Ciftioglu, D.M. Smith, Powder Technol., 68, 71, (1991). [10.] J. Jansen, G. Kraus, Rubber Chem. Technol., 53, 48, (1980). [11.] C. Parkinson, S. Matsumoto, P. Sherman, J. Coll. Interf. Sci., 33, 150, (1970). [12.] J.M. Funt, Rubber World, Feb, 21, 1986. [13.] E.S. Dizon, LA. Papazian, Rubber Chem. Technol., 50, 765, (1977). [14.] E. Bomal, I. Gelling, Rubber World, June, 33,1993.

Acknowledgements

"Characterization of physical properties of SBR/carbon black masterbatch" is based on a paper given at the May, 1993 meeting of the Rubber Division. "Dispersibility measurements of prec. silicas influence of dispersion on mech. properties" is based on a paper given at the October, 1993 meeting of the Rubber Division. "Short para aramid fiber reinforcement" is based on a paper given at the October, 1993 meeting of the Rubber Division. "Chloramine effects on elastomer degradation" is based on a paper given at the October, 1993 meeting of the Rubber Division.