Silica wet masterbatch: a new process for pre-dispersion of silica in emulsion polymers.

There is a growing interest in silica-reinforced rubber compounds. For example, silica use in tire compounds has grown five to ten percent each of the past ten years. The reasons for this growth include better fuel economy through reduced rolling resistance, increased tread life, comparable or better traction for safety and a competitive cost/price structure - all of which lead to strong consumer appeal.

With the numerous advantages now realized by silica-based tire compounds, one might wonder about the delay of silica's entry into the history of commercial tire production. Besides cost, a major cause of this delay is the fundamental chemical difference between silica and other components of rubber compounds. Silica has a very polar surface while other components are relatively non-polar. To illustrate, precipitated silica dust readily disperses in water, a polar solvent, because the water easily wets the surface of each particle. Conversely, carbon black dust cannot be dispersed in water without assistance such as a wetting agent or steam grinding. The ease of dispersion of these fillers reverses when non-polar rubber is the dispersion medium. To make silica a competitive or superior filler to carbon black, the pioneers in the field developed specific chemistry which compatibilizes silica with rubber and, more importantly, binds silica chemically to the rubber (ref. 1). Key tire properties such as reduced rolling resistance and tread fife are highly dependent on this special silica chemistry.

It is specific chemistry with silica plus a modified, wet-masterbatch process that have enabled us to bring silica into a nearly standard, emulsion-polymer finishing process. Though we found many diverse ways of combining silica and polymer latex documented in the patent literature, none of these processes fit well with our current wet masterbatch process. We now report that we can chemically treat and uniformly incorporate silica slurry in a coagulation process that is so efficient and similar enough to our existing process that commercial development appears feasible. We anticipate that rubber masterbatch compositions can be produced with combined loadings of rubber. oil, silica dust and/or carbon black that are typical for carbon black alone.

Background

Dry-mix silica processing, challenges and history

Silica dust has highly polar hydroxy groups on its surface. This makes silica incompatible with non-polar polymers such as natural and synthetic rubber. Like droplets of water dispersed in oil, dry silica can be forced to disperse in rubber by mechanical means. However, the silica tends to agglomerate rather than freely disperse and maximize reinforcement. Physical properties of the cured rubber compound are not their best.

The pioneers in the field such as Siegfried Wolff found that dispersion and cured properties improved when the surface hydroxy groups were modified with silanes. These compounds reduce surface polarity, and some can chemically bind filler to polymer (ref. 1). An example of such a silane which has also become the standard of comparison in tire compounds, is bis-(3-triethoxysilylpropyl)-tetrasulfane (TESPT. Degussa Si69 or Silquest A-1289). As is typical, one group on the silicon is a hydrocarbon chain capable of binding with the polymer during vulcanization. This dual-reacting type of silane is commonly known as a coupling compound, a function not limited to just silanes, (refs. 2-5). Over 20 years of silica-based tire compound development by Wolff and others has produced a number of silane compounds that work in this capacity, e.g., 3-mercaptopropyltrimethoxysilane (MPTMS. Silquest A-189. OSi Specialties, Inc.) and 3-thiocyanatopropyltriethoxy silane (TCPTS, Degussa Si264).

The two reactions of silanes during compounding, hydrolysis and condensation, have been well studied and reviewed (ref. 6). Hydrolysis releases the alcohol attached to the silicon transforming less polar ether groups into more polar hydroxy groups. Condensation eliminates water to form an ether bond between the silane and the silica, or another silane. There is sufficient residual moisture available on the silica surface and in most rubber compounds to complete hydrolysis and condensation during the first stage of mixing. Up to two bonds to silica are believed possible, data of Gorl and Hunsche suggest that silane oligomers, are forming at the third site rather than a third bond to silica (ref. 7).

A critical part of the dry-mixing process is the migration of enough silane from the bulk rubber phase to the silica surface to be available for bonding and coupling. A question one might ask. therefore, is what percentage of the silane charged to an internal mixer migrates to the silica surface where it will presumably do the most good for its value? From the perspective of purely statistical mixing, one would expect uniform distribution of silane in the mix. This would dictate that the percentage of silane in contact with silica would be proportional to the volume percent of the mix represented by the silica. Logic suggests that statistical mixing does not control silane distribution. We suggest that the silane is at least partially directed from the bulk phase toward the silica surface where it accumulates because of the chemical and physical changes in the first mixing stage. One could envision this occurring by the following process.

The silane, initially non-polar, disburses statistically in the bulk phase. During the hydrolysis step, the silane reacts with moisture and possibly trace amounts of acidity, both of which are available in bulk rubber -- by-products of coagulation. As alcohol is released from the silane, the silane molecule becomes increasingly polar and less compatible with the bulk rubber phase. Migration of the silane is then directed strongly to the silica surface where it accumulates by association through hydrogen bonding. One would expect that the directed orientation of the silane molecule, even by association, would make it act as a wetting agent, pacifying the polar sites on the surface of the silica. Thus, the silica particles become compatible with and disperse in the rubber phase to the extent that they are coated with silane. As the wetting action of the silane removes the phase boundary between the once incompatible materials, the viscosity of the mixture also decreases, an observation reported for silica/rubber and other filler/media combinations. Time, temperature and silane level complete the surface reaction and determine the extent to which the silica is dispersed in and made capable of reacting with the rubber phase during the vulcanization process.

Certainly not all the silane can be expected to migrate to the silica surface, so some percentage is not capable of functioning as a coupling agent. The popular TESPT is a sulfur donor in its own right, so any TESPT not bonded to silica will still cure polymer and increase modulus. This costly silane can be fully utilized in one way or another during cure. However, other potentially useful silanes are not polysulfides and are not capable of independent function. These may be better utilized if pre-coated on the silica. Pre-coated, dry silicas are more costly, but the overall compound cost may be the same or lower for a given silane if performance is improved. One might expect either better properties at the same silane/silica loading or equivalent properties at lower silane/silica loading when the silane is precoated, on the silica. Some examples of these interesting products which show the loading and types of silanes available are Ciptane I (PPG, contains 3% MPTMS), Coupsil 8113 (Degussa, contains 11.3% TESPT) and Coupsil 6411 (Degussa, contains 9.7% TCPTS). These products do not appear to have become popular with the tire industry, presumably due to cost. In addition, pretreated, hydrophobic fumed silicas are not readily useful for coupling reactions in tire compounds because they have been highly and exhaustively treated with non-coupling silazanes to eliminate most or all reactive, hydrophilic sites. Sufficient coupling-type silane may not bond to these silicas, eliminating certain property advantages for the cured compounds. The much higher cost of fumed silica also diminishes its competing with the precipitated variety.

Wet masterbatch processes

Background

Perhaps the major objection heard about dry-mix processes involving either carbon black or silica and rubber is the dust exposure and contamination in the workplace. Mechanical and pneumatic methods of handling and containing both fillers have improved and increased in cost with time. For those willing to make the investment, dry mixing silica is still an option. Dry mixing silica has the further difficulty of wetting and dispersing an initially incompatible material in the rubber. The additional energy needed to mix silica and its coupling agent(s) requires significantly longer and more numerous mix cycles causing significantly more wear on mixing equipment per pound of compound processed.

The other, notably cleaner and more efficient approach to dispersing filler is the wet masterbatch process. The wet masterbatch process for carbon black has worked well and received wide acceptance since its invention and evolution over the past 35 years (refs. 8-13). Wet masterbatch processes for silica have taken diverse paths, no doubt to meet specific needs, but none of these approaches has ever been used for commercial production of SBR masterbatch. The following examples illustrate the variety of approaches.

Addition of dry silica

Some approaches bear little resemblance to traditional wet masterbatch coagulation. For example, oil-extended SBR latex can be blended with a mixture of dry precipitated and hydrophobic fumed silica to give semi-dried particulate that contains the precipitated silica (ref. 14). The fumed silica acts as a partitioning agent during drying and is about 98% recovered and recycled. Similarly, dry silica can be added to concentrated rubber latex, so dehydration causes coagulation into a paste or particulate (ref. 15). These approaches trap all the original components, e.g., water and salts, and prevent washing of the final product.

Co-precipitation of silica and rubber

Other approaches start with silicate solutions and co-precipitate the silica with the rubber blend (refs. 16 and 17). This added step introduces new control variables to the coagulation process. e.g., reproducing the same final silica structure from run to run. This process is further complicated if carbon black slurry is present and silica precipitation on the black particles alters their structure. These extra parameters create a more complex process that may be beyond reasonable expectation of control.

Special polymer agents

Two patents claim that butadiene-based, emulsion polymers with vinyl pyridine (ref. 18) or methacrylic acid (ref. 19) as co-monomer will efficiently incorporate silica. Both these polymers contain polar sites on the second monomers that would be compatible with the silica surface if the two could be brought into proximity. These approaches are obviously outside the realm of commercial SBR production.

Surface active agents on silica

The silica wet masterbatch processes most similar to a standard black masterbatch process include a surface-active additive in the silica slurry preparation. These additives have the necessary polar sites capable of associating with and rendering the silica less hydrophilic, thus, making it more compatible with the rubber and other non-polar organic phases. The earliest process used polyethylene polyamine, e.g., tetraethylene pentamine, at 0.1-2.0% w/w silica; higher amounts were said to give no advantage (ref. 20). This process made a white masterbatch with rubber; extender oil and carbon black were not present.

Two later processes used oleophilic quaternary ammonium salts, typical cationic surfactants, to pacify the silica (refs. 21 and 22). The levels used, 4-8% w/w of silica, were greater than the above levels of polyamine. These processes specify a never-dry silica precipitated prior to use. Extender oil and carbon black were included, giving these processes the desired broader utility.

The most recent reference drops the stipulation of neverdry silica and uses commercially available silica powder to prepare slurry (ref. 23). A typical quaternary ammonium salt is again specified, but the level is lower, 2-4% on silica. Other differences include a pH adjustment of the treated silica slurry to the alkaline range, and no carbon black or oil are used in the coagulation recipe. The inventors cite the earlier work above as not producing a commercial process, but historical evidence suggests that this process was no more successful.

Verification and improvement

We followed the procedure for the above process with typical levels of solid filler, with and without extender oil emulsion. We found that the process works marginally well with silica alone, but results become unacceptable when carbon black is present. Too much carbon black tends to remain dispersed, and building crumb size during coagulation, a critical step for black masterbatch. becomes increasingly difficult as more silica and quaternary ammonium salt are added. This behavior is not surprising if one considers the probable mechanism by which the quaternary ammonium salts work.

A quaternary ammonium salt is a cationic surfactant normally used at acid pH. A quaternary salt is rendered less water soluble at alkaline pH, the pH used in the patent, directing it to the silica surface where the polar head group should preferably associate. This would direct the non-polar tail of the molecule to be oriented away from the surface, similar to the directed association of hydrolyzed silane with silica, above. When enough surfactant coats the silica it appears non-polar, it mixes well with the rubber phase and it co-coagulates more efficiently. However, when the initially alkaline pH is reduced in coagulation to deactivate the anionic surfactant stabilizing the rubber latex, the quaternary salt becomes an active surfactant again. Since it is more soluble at low pH, some surfactant leaves the silica and associates in the normal manner, via the hydrophobic tail groups, with non-polar particles, e.g., polymer or carbon black. This action tends to keep polymer and carbon black dispersed at low pH when just the opposite effect is desired. We do not expect this process to be commercially viable, particularly with carbon black.

We found with further experiments that better compounds for treating silica slurry for efficient coagulation have no surfactant function that is pH sensitive. We also found that covalent bonding of the treatment chemical to silica is preferred to just associative, i.e., hydrogen bonding. The best candidates are coupling agents such as silanes which can give efficient incorporation during coagulation then render additional service during vulcanization. Both the cost and the benefits of the coupling agent can be more readily passed on to the compounder if the treatment chemical serves a dual function.

New wet masterbatch process

Handling and treatment of dry silica with silane appeared to be a good option for the first step of our wet masterbatch process. The (by process is simple and efficient, but estimated capital costs appeared too high for commercialization. A wet silica treatment process initially seemed very inefficient because we expected the excess water to keep this equilibrium reaction from working. However, the possibility of a successful, aqueous treatment process was supported by work of Pohl and Blackwell (ref. 24). They showed that at acidic pH and 25 [degrees] C, silane would react with silica in excess water. When we tried this reaction over a range of temperatures, we found by design of experiments that this reaction could be shortened from days to hours at an optimum pH and temperature.

Incorporation efficiency

After we established a reliable silica treatment process with silane, we determined the efficiency of silica incorporation into masterbatch as a function of silane charged to the silica treatment slurry. Similar to what previous workers found with polyamines (ref. 20) and quaternary ammonium salts (refs. 21-23) the minimum organic loading on the silica appears to be 2-4% for complete silica incorporation. A typical correlation is shown in figure 1.

[Figure 1 ILLUSTRATION OMITTED]

The molecular weight of the silane used in these experiments. MPTMS, is low compared to TESPT. If one accounts for the loss of the three methoxy groups during hydrolysis, close to half the weight of the MPTMS charge is lost even if all the silane binds to the silica. Thus, to reach the expected range of 2-4% organic loading on silica, roughly 4-8% silane needs to be charged, which is what we found. Efficient incorporation begins at about 4% charged silane under these particular conditions and composition. We have found dial the lower end of the range of good incorporation efficiency is highly influenced by coagulation conditions, masterbatch composition and the type of coupling agent on the silica. Optimum conditions need to be established for every combination of wet masterbatch components and coagulation conditions.

New compounding possibilities

More needs to be said to the compounder about the change in silica incorporation depicted in figure 1. There is a more significant change occurring than doubling silica incorporation after the first three parts of silane are added to the slurry. Without silane, the silica is a little better incorporated than if it were sprinkled by hand on the rubber crumb. The silica is just sporadically, non-uniformly trapped as pockets and streaks as the crumb is collected on screens for drying. With silane, both the incorporation and distribution of silica dramatically improve. The wet-treated silica is dispersed so uniformly that no white streaks or pockets of silica are visible in the black background. The only visible white color comes from the larger particles of silica normally present in the original bag of powder. These can probably be removed by filtration in our process if they are objectionable, but they are relatively few and disburse immediately on mixing.

The broad utility of our wet process suggests that we can add coupling agent more uniformly and exclusively to the silica than can be done in a dry-mix process. This can be done with silica in its smallest particulate form, so every available surface is treated. There is no lower size or upper surface limit to the silica particle we can treat and incorporate. We can now consider compounding silica types not seriously considered before due to their handling difficulties with rubber compounds.

Compounding studies

Mixing procedure

We prepared a series of wet masterbatch samples with target compositions. We determined the filler and oil content for each sample, then generated a free-mix compound recipe to match each one. No attempt was made to optimize these compounds.

We studied compound mixing using a lab internal mixer with a one-liter capacity mixing chamber. For free-mix samples, we added base polymer, coagulated from the same latex used to make the wet masterbatch, black, zinc oxide, silica and mercaptosilane at zero minutes, lowered the ram, added all remaining ingredients but curatives at 132 [degrees] C, and dumped the mix at 149 [degrees] C. We mixed curatives on the mill. Similarly, we added wet masterbatch samples and all ingredients except curatives to the mixer at zero minutes and dumped at 121 [degrees] C. We mixed curatives on the mill. We completed testing on standard laboratory equipment using ASTM procedures. We had several duplicate sample sets in the testing scheme. We indicate averaged results for duplicates by a (2) on the compound identifications.

Compound comparison

The data show a significant reduction in mix energy, figure 2, and mixing time, figure 3, for the wet masterbatch compounds compared to the corresponding free-mix compounds. The energy reduction averaged 225 watt-hours, and mixing time averaged 1.76 minutes. Note that the first compound listed, not the first compound mixed, has an unusually high mixing energy and longer mix time than would be expected compared to the other free-mixed compounds.

[Figures 2 and 3 ILLUSTRATION OMITTED]

When we discuss the physical properties of this compound, they also show an inconsistency when compared to the other pairs of free vs. wet masterbatch mixes.

The Mooney viscosity and scorch data show virtually the same viscosity and very similar scorch times. The Monsanto MDR data demonstrate equivalent cure initiation times, [t.sub.1] and [t.sub.2], and shorter [tc.sub.90] times, figure 4, indicating a slightly faster cure rate for the wet masterbatch compounds compared to the free mixes. The [M.sub.H] values were only 5% lower for the wet masterbatch compounds.

[Figure 4 ILLUSTRATION OMITTED]

Tensile strength, figure 5, and elongation, figure 6, of the wet masterbatch compounds were consistently higher than their free-mixed counterparts. Modulus values, figure 7, were higher also with the exception of the first compound listed. The first compound may have been compromised by the unusual mixing discussed above. Tensile sheet durometer values, figure 8, were equivalent.

[Figures 5-7 ILLUSTRATION OMITTED]

We tested Goodrich flexometer at two different laboratories using two sets of conditions. The two sets of data yield differing results. The 38 [degrees] C base data indicate approximately equivalent delta T and compression set measurements, whereas the 100 [degrees] C oven temperature tests indicate higher delta T and compression set for the wet masterbatch mixes. These discrepancies have not been resolved as yet.

Summary and conclusions

We have developed a pilot process for the efficient treatment and uniform incorporation of precipitated silica slurry into a wet rubber masterbatch. Our successful work has been done mainly with low molecular weight silanes and SBR latex, but some work shows success with other silanes and NBR latex. We are also working with natural rubber latex, latex rubber/plastic blends and other coupling agents. We have filed for patent protection of this process.

We draw several conclusions from a compounding study:

* Significant reductions in mixing time and energy are possible using silica in a wet masterbatch form. This indicates that one can use a single-pass mix in place of multipass mixes that are common with silica-containing free-mixed compounds.

* Curing characteristics are equivalent for the two mixes with the possible exception that the wet Masterbatch may have slightly faster cure rates.

* The stress/strain properties of the wet masterbatch compounds are superior to the freemixed compounds.

We believe our wet masterbatch technique offers flexibility to utilize many types of silica including those that cannot be satisfactorily incorporated using dry mixing techniques but which potentially have superior compound properties. This is our initial attempt to produce these masterbatches, and we immediately see a large potential which further development effort will attempt to exploit. However, the real proof will be in compounding larger test pieces and working with others in their specific applications.

References

(1.) S. Wolff, Rubber Chem. Technol. 55, 967 (1982).

(2.) P. H. Sandstrom and L. G. Wideman (to Goodyear Tire & Rubber Company), U.S. Patent 5,610,216 (March 11, 1997).

(3.) P.H. Sandstrom and L.G. Wideman (to Goodyear Tire and Rubber Company), U.S. Patent 5,605,951 (February 25, 1997).

(4.) H. Furokawa, A. Nakamura and A. Shirahata (to Dow Corning Toray Silicone Company), U.S. Patent 5,604,288 (February 18, 1997).

(5.) T.J. Segatta and P.H. Sandstrom (to Goodyear Tire and Rubber Company), U.S. Patent 5,405,897 (April 11, 1995). 6. F.E. Osterholtz and E.R. Pohl, J. Adhesion Sci. Technol. Vol. 6, No. 1, 127 (1992).

(7.) U. Gorl and A. Hunsche, paper No. 76 presented at a meeting of the Rubber Division, American Chemical Society, Louisville, Kentucky, October 8-11, 1996.

(8.) J.D. Sutherland, Jr. (to Copolymer Rubber & Chemical Corporation), U.S. Patent 3,055,856 (September 25, 1962).

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(10.) J.F. Brewer, R.E. Shrader and L.C. Schilling (to Copolymer Rubber & Chemical Corporation), U.S. Patent 3,525,162 (August 25, 1970).

(11.) R.A. Gerlicher (to Copolymer Rubber & Chemical Corporation), U.S. Patent 3,767,605 (October 23, 1973).

(12.) R.F. Karg and H.J. Gros (to Copolymer Rubber & Chemical Corporation), U.S. Patent 4,213,888 (July 22, 1980).

(13.) R.F. Karg (to Copolymer Rubber & Chemical Corporation [DSM Copolymer, Inc.]), U.S. Patent 4,233,365 (November 11, 1980).

(14.) T.R. Ten Broeck and R.E. Downey (to Goodyear Tire & Rubber Company), U.S. Patent 3,907,734 (September 23, 1975).

(15.) J. Machurat (to Rhone-Poulenc), FR 2,588,008 (April 3, 1987).

(16.) G. Maass, et al (to Chemische Werke Huls Aktiengesellschaft), U.S. Patent 3,849,363 (November 19, 1974).

(17.) J. Machurat and G. Vrisakis (to Rhone-Poulenc), GR 27 53 237 (June 1, 1978).

(18.) H.E. Haxo, Jr. (to United States Rubber Company), U.S. Patent 3,098,83 7 (July 23, 1963).

(19.) M.R. Ambler and G.L. Burroway (to Goodyear Tire & Rubber Company), U.S. Patent 4,481,329 (November 6, 1984).

(20.) W.H. Leukhardt, Jr., and J. W. Adams (to United States Rubber Company), U.S. Patent 2,616,860 (November 4, 1952).

(21.) Oliver W. Burke, Jr., U.S. Patent 3,686,113 (August 22, 1972).

(22.) Oliver W. Burke, Jr., U.S. Patent 3,689,452 (September 5, 1972).

(23.) E. Fischer and D.C. Edwards (to Polysar Limited), U.S. Patent 4,482,657 (November 13, 1984).

(24.) E.R. Pohl and C.S. Blackwell, in "Controlled interphases in composite materials, " H. Ishida, Ed., Elsevier Science Publishing Co., Inc, 1990, p. 37.