Impact of silane on dispersion and performance of submicronic filled polymers.

The properties of filled or reinforced polymers generally depend on the size, shape and surface characteristics of the filler particles. For a given system, one of the most important parameters impacting the mechanical, electrical or optical performance is the filler dispersion, It is well known that large agglomerates tend to form a flaw, which leads to mechanical failure. Silanes are widely used to help the filler dispersion. Their role in the dispersion process is assumed, but there is no evidence for their mode of action.

Dispersion mechanism

The first challenge of processing the compound is the filler dispersion. The agglomerates have to be broken down into smaller parts, called aggregates, but not to the level of primary particles (figure 1). An optimal particle size distribution has to be achieved to get the best performance in a specific application.

[FIGURE 1 OMITTED]

During mixing of the compound, mechanical shear induces dispersion and distribution of the filler in the matrix. In dispersive mixing, the objective is to break down agglomerates, and possibly aggregates, to fragments of uniform size. These fragments must be evenly distributed within the matrix to insure uniform properties of the final compound. However, breaking down the filler particles in this manner often proves to be quite difficult. Even in the case of silica filled rubber for tire tread, where extra efforts are made to disperse the filler, as much as 15- 20% of the total amount of a highly dispersible silica remains in the macro dispersion state (agglomerate larger than 5 [micro]m) (ref. 1).

Dispersive mixing is governed by a competition between the hydrodynamic forces acting on the particulate agglomerate and the cohesive forces holding that agglomerate together.

Hydrodynamic forces are determined by the strength and geometry of the flow field produced by the shear stress, while particle-particle interaction forces and the packing arrangement of individual particles within the agglomerate determine agglomerate cohesivity.

The mechanism for dispersion has been extensively studied (refs. 2-4). Two modes of dispersion are identified: the bulk rupture (an abrupt breakage of the agglomerate into multiple large fragments) and erosion (a sequential removal of small fragments from the agglomerate periphery) (figure 2). The bulk rupture mechanism is favored with high hydrodynamic stress (high shear). This mechanism is sudden and leads quickly to an agglomerate size reduction. The erosion mechanism is slower but leads to a smaller final aggregate size. Increasing the hydrodynamic force leads to a shift from dispersion via erosion to dispersion via rupture.

[FIGURE 2 OMITTED]

Factors affecting filler dispersion

Several physical and chemical factors can affect the dispersion process, including: the structure and cohesivity of filler agglomerates, the surface activity of the filler, the chemical interactions between polymer and filler, and any infiltration of the polymer into the agglomerate.

* The structure, i.e., the packing arrangement of the primary particles of the filler, is an intrinsic parameter mainly dependent on the filler synthesis route. Mercury porosimetry characterizes this parameter and gives the pore size distribution with good resolution.

* The cohesivity of the agglomerates depends on the strength of the individual bonds between neighboring particles and the number of such bonds that must be broken to release a fragment. The strength of individual bonds is determined by particle composition and surface characteristics (electrostatic, Van der Waals and hydrogen-bonding forces), as well as interactions from secondary species, such as silanes, grafted or adsorbed onto the particles. For instance, silanes can decrease the agglomerate cohesivity through a shielding of inter-aggregate interaction, thus decreasing the hydrodynamic stress needed to break the agglomerate.

* The infiltration of the matrix into the agglomerate occurs as soon as filler and polymer are mixed together. The role of infiltration of the agglomerate by the polymer is one of the first order parameters of filler dispersion. This can have a positive or negative impact on the ability of the filler to disperse.

For instance, if it creates bridges between aggregates, the cohesivity of the agglomerates is reinforced, resulting in a delay of the dispersion (example: CaC[O.sub.3] and silica filled silicone resins [re. 5]).

* The surface activity of the filler is defined through its surface energy, [[gamma].sub.s], and is comprised of dispersive ([[gamma].sup.d.sub.s]) and specific ([[gamma].sup.sp.sub.s]components.

[[gamma] = [[gamma].sup.d.sub.s] + [[gamma].sup.sp.sub.s]

The dispersive component ([[gamma].sup.d.sub.s]) is based on London forces, for instance, and is representative of polymer-filler interaction.

The specific component ([[gamma].sup.sp.sub.s]) characterizes the interaction between filler particles. Table 1 gives some examples of [[gamma].sup.d.sub.s] for untreated and treated fillers.

Effect of silane on dispersion

Silane treatment of fillers is known to improve the dispersion, but the mechanisms of action are not fully understood. Does it modify the infiltration of the polymer into the agglomerates or does it only transform the surface chemistry of the filler?

Effect on infiltration

The silane impact on infiltration of a polymer into an agglomerate is not widely depicted in the literature. Scurati, et al (ref. 7), have investigated dispersion through erosion kinetics studies in which the reduction in size of a single agglomerate of known density, prepared by filler compaction, was monitored in a controlled flow field. Their work focused on the effect of silane pretreatment of the agglomerates. They observed that after treatment, the aggregate size decreases and the infiltration rate of SBR into TESPT (bis-triethoxysilyl-propyl-tetrasulfane) treated filler is slower than for untreated filler. These results can be interpreted as being due to the changes of polymer-filler interaction and agglomerate packing properties.

[FIGURE 3 OMITTED]

Effect on surface energy

Several studies on the influence of surface properties and surface treatment on filler-filler and filler-polymer interactions of silica filler treated with different silanes are reported in the literature (refs. 8 and 9). The silane treatment clearly decreases both the dispersive and specific components of the surface energy (ref. 8).

[[gamma].sup.d.sub.s ] represents the polymer/filler interaction, which is correlated to the polymer bound to the filler surface and that cannot be separated from the filler by solvent extraction. Thus, the impact of silane treatment of the silica used in a rubber compound can be determined by measuring the insoluble polymer after solvent extraction. Figure 4 shows an example of alkyl silane treatment of silica in SBR. The decrease of insoluble polymer can be attributed to a change in the dispersive component of the surface energy [[gamma].sup.d.sub.s].

[FIGURE 4 OMITTED]

Effect of silane on interface/interphase contribution to bulk performance

Submicronic fillers are generally classified as reinforcing fillers because they can develop, if well dispersed, a large interface with the polymer (table 2). For instance, a volume fraction of 20% of 200 [m.sup.2]/g silica could generate more than 5,000 [m.sup.2] of interface in 100 g of polymer. The silanization of the surface with an organosilane may influence a layer of 1 to 5 nm thick corresponding to 5 to 25 g of influenced polymer in every 100 g. Figure 5 illustrates the silane impact on the dynamic mechanical performance of silica filled rubber. The silane organic functionality is an octyl group. The non-linear behavior (Payne effect) is drastically reduced after silane treatment, without any change of silica dispersion (ref. 10).

[FIGURE 5 OMITTED]

The role of a silane is more complex than a single surface energy modification. Inter-diffusion phenomenon and interpenetrating network (IPN) formation in the interphase region occur and are critical for the final performance. Optimization of the IPN formation is therefore of primary importance, and hence selecting the proper organosilane with regards to its compatibility with the matrix and the filler is critical.

Conclusions

Organosilanes are widely used in filled polymer composites to help the dispersion of fillers, including submicronic fillers.

The exact role of the silane is not fully understood, but, depending on the functionality, amount and method of introduction, silane action can be observed at different levels, including:

* It can accelerate or slow down the infiltration kinetic of the polymer matrix into agglomerates;

* it can decrease the agglomerate cohesivity, decreasing the hydrodynamic stress needed to break the agglomerate and also limiting the re-agglomeration phenomenon; and

* it generates an interface that can have a significant impact on bulk performance.

The selection of the organic functionality of the silane allows the optimization of the final processing-performance compromise.

References

1. L. Revekamp, M. Debowski, PJ. van Swaaij and J. Vansco, Rubber Division, ACS, May 2005, San Antonio, TX.

2. J. Boyle, I. Manas-Zloczower and D.L. Feke, Part. Syst. Charact., 21, (2004), 205-21.

3. J.E Boyle, I.C. Manas-Zloczower and D. Feke, Filler Technology, 153 (2005), 127-133.

4. V. Collin, Ph.D., Ecole des Mines de Paris, CEMEF March 2004.

5. P. Levresse, I. Manas-Zloczower and D.L. Feke, Rubber Chemistry and Technology, 75 (1), (2002), 119.

6. Handbook of Fillers, G. Wypych, Plastics Design Library, 1999.

7. A. Scurati, IC. Manas-Zloczower and D.L. Feke, Rubber Chemistry and Technology, 75 (4), (2002), 725-737.

8. M.J. Wang and S. Wolff, Rubber Chemistry and Technology, 65 (4), (1992), 715-735.

9. J. Rainier, C. Gauthier, L. Chazeau, L. Stelandre and M.N. Bouchereau, to be published.

10. J. Rainier, C., Gauthier, L. Chazeau, L. Stelandre and L. Guy, to be published.

By S. Mealey, L. Stelandre, P. Chevalier and V. Smits, Dow Corning 

Table 1--dispersive component of the filler free surface energy
and effect of surface treatment (ref. 6)

Filler                                     [[gamma].sub.sd]
                                           (mJ/[m.sup.2])

Carbon black                                       51
Mg[(OH).sub.2]                                     95
Mica                                               70
Talc                                              130
Ti[O.sub.2]                                        76
CaC[O.sub.3]                                      200
CaC[O.sub.3] treated with stearic acid          48-53
Si[O.sub.2]                                       105
Si[O.sub.2] esterified with alcohols            46-87
  [C.sub.16]-[C.sub.1]
Si[O.sub.2] vinyl silane treatment                 84

Table 2--polymer--filler contact area of 100 g of polymer at
5, 10 and 20% filler volume fraction

                                             Volumetric
                              Specific          area
              Density           area         [m.sup.2]/
Filler      g/[cm.sup.3]     [m.sup.2]/g     [cm.sup.3]

Alumina         3.7              103           381.1
Silica          2.1              200             420
CB              1.8              120             216
Clays           2.6               25              65

           Filler/polymer contact area
           [m.sup.2]/100 phr of polymer

Filler       5.0%     10.0%     20.0%

Alumina       542     1,144     2,575
Silica      1,053     2,222     5,000
CB            632     1,333     3,000
Clays         132       278       625

This work also shows a dramatic effect of the silane on the
erosion kinetics (figure 3).