Introduction to mineral fillers for rubber.

In the most general terms, a filler is a finely divided solid that is used to modify the mechanical, electrical and/or optical properties of a material in which it is dispersed. From the inception of the rubber industry, the importance of fillers in providing durability and performance has been recognized. For instance, U.S. patents issued in 1907 list over 200 powdered materials for use in reinforcing natural rubber compounds.

In the early 1900s, materials that were used to reduce the tack, increase the hardness or reduce the cost of natural rubber compounds were predominantly finely-ground naturally occurring products such as clay and mica or flours of wood, wheat and bone; or manufacturing by-products such as factory dusts and zinc oxide.

As the rubber industry grew, fine particle materials such as precipitated calcium carbonate and titanium dioxide were added to the non-black fillers, while carbon black became the dominant filler in the tire industry.

After World War II, the advent of synthetic rubber, burgeoning industrial demands and increased mobility provided by modern automobiles required specialty fillers to provide increased levels of tensile strength and resistance to tear, abrasion and exposure to the environment. This led to the development of high structure and fine particle size particle blacks and synthetic fine particle non-black fillers, such as precipitated silica and silicate and fumed silica.

The next stage of development came with the universal adoption of radial tires which required the use of even finer particle size fillers, and saw the advent of treatment systems such as silane coupling agents to provide a chemical bond between non-black fillers and the rubber molecule.

Over the past century, two trends have dominated:

* Filler particle size has become smaller; and

* fillers have changed from naturally occurring materials to materials that have specific shapes to enhance the interaction with the rubber matrix. For instance, a particle with a high aspect ratio such as kaolin or talc has higher reinforcement than a more spherical particle.

Overview of non-black fillers (refs. 2 and 3)

Non-black fillers can be used to impart a number of desirable properties to rubber compounds, including:

* Increased tear strength;

* increased adhesion to fibers or adjoining compounds;

* lower hysteresis;

* increased resistance to abrasion;

* lower or higher coefficient of friction;

* chemical compatibility or chemical resistance;

* lower cost;

* white or colored compounds.

Fillers are broadly defined as falling into the general categories of diluent or degrading fillers, that is, fillers which lower the strength properties of the rubber compound to which they are added; extending or semi-reinforcing fillers which have little impact on the strength of the rubber compound and reinforcing fillers which increase the strength of the rubber compounds.

The two basic aspects of fillers that directly correlate to their reinforcement of rubber are the size and complexity of the particle that exists in the rubber compound. As particle size becomes smaller, the surface area increases, providing more available contact between the rubber molecule and the filler particle. At the same time, the smaller particles reduce the stress concentration that can lead to catastrophic failure initiation. The other parameter is the complexity of the particle shape, which can be a result of the shape factor of mineral particles (figure 1), or of the aggregation of small particles to make a reinforcing particle (figure 2). Figures 3-7 show general trends for rubber properties for a variety of non-black fillers.

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Mineral fillers (refs. 4 and 5)

Mineral fillers are naturally occurring materials that are mined and ground to a specified particle size. The grinding may be done dry using mechanical mills, or for finer product, the ore is ground wet. Wet grinding may be autogenous where the ore grinds by attrition with itself, or a grinding media may be employed.

Additional processing may include a combination of:

* Separation of fine and coarse particles by use of screens, or by air or water flotation, or centrifugal filtration;

* removal of impurities by washing, heat treatment, magnetic separation or chemical treatment;

* surface treatment with a variety of chemicals to improve the compatibility with the rubber matrix.

Calcium carbonate

Calcium carbonate (figure 8) that has been ground (GCC), also known as whiting, limestone, marble, chalk, dolomite or calcite, is added to rubber compounds to reduce cost and to impart hardness and opacity to rubber articles. The particle sizes range from 2 to 80 [micro]m for dry ground product, where the mined limestone or marble is crushed, pulverized and air classified. Wet grinding, where the product is refined from a slurry by centrifugal and/or screen fractionation and then dried, produces a product with particle sizes from 0.5 to 11 [micro]m. The chemical composition and crystalline nature depend on the limestone deposit that is mined, i.e., chalk or marble and color of the deposit. Typical filler loading ranges from 20 to 300 phr. The most important applications are in electrical wire and cable insulation, where the low moisture content and natural insulating properties make it a preferred filler, and in the production of articles where low cost and smooth surface appearance are desired, such as footwear and extruded hoses and automotive sealing parts.

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Baryte

Baryte is predominately barium sulfate that is available in particle diameters from 1 to 20 [micro]m. It is used as a filler when a high specific gravity is required of the rubber article. Typical loading level is 25 to 100 phr for articles such as rubber stoppers and seals.

Ground crystalline silica

Crystalline silica from sand or quartz can be ground to produce a degrading or extending filler for low cost rubber articles. In order to minimize the health hazards of exposure to airborne crystalline silica, particle sizes normally range from 2 to 20 [micro]m.

Biogenic silica

Naturally occurring silica is often referred to as diatomaceous earth, since the primary deposits are the exoskeletons formed by diatoms that have extracted silicic acid from sea water and formed amorphous silica shells. Diatomaceous earth is usually very high in surface area because the shells retain the radial and/or rod-like structures of the living creature. The largest deposits are several million years old and have been partially converted to crystalline silica over time so that the diatomaceous earth may need to be separated or treated to remove crystalline content. Biogenic silica is used as a semi-reinforcing filler or as a carrier for liquid compounding ingredients.

Kaolin clay

Kaolin clay (refs. 6 and 7), also called Kaolinite or China Clay, is hydrous aluminum silicate consisting of platelets with alternating layers of silica and alumina in the structure (figure 9). The fine particles of clay are formed by the weathering of granite. Clay deposits are classified as primary, secondary and tertiary. Primary deposits are mixtures of clay and granite that are found where the clay was originally weathered. They contain only 40-50% particles less than 2 [micro]m. Secondary deposits are formed when fine particles from primary deposits are carried by water flow and deposited in a new location. Tertiary deposits are the most important commercial deposits due to the fine particle size, with [is greater than] 80% of the particles less than 2 [micro]m in diameter, and high purity that results when water carries the fine particles of a secondary deposit to a new location. Clay is broadly divided into soft and hard clay (that is, they produce softer and harder rubber compounds, respectively, at a given loading level). Because clay is mined as a fine particle size material, it does not require significant grinding for use in rubber. There are five basic processes for producing clay for rubber reinforcement from the mined form:

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* Air-floated clay in which the ore is milled to break up lumps and air classified. It is the least expensive form of clay and has moderate reinforcement.

* Water washed clay involves gravity separation of impurities, bleaching and magnetic separation to improve color properties and centrifuging to produce the desired particle size range. Water washed clay has higher reinforcement with the ability to control pH, color and particle size. Table 1 shows a typical chemical analysis of the differences between the processes.

Table 1 - typical chemical analysis

Composition %           Airfloat   Water-washed   Calcined

[Al.sub.2][O.sub.3]      38.09         39.33        44.80
Si[O.sub.2]              45.08         44.98        52.80
Ti[O.sub.2]               1.52          1.61         1.40
[Fe.sub.2][O.sub.3]       1.69          0.36         0.30
CaO                       0.02          0.05        Trace
[K.sub.2]O                0.27          0.02        Trace
LOI @ 950 [degrees] C    13.80         13.59         0.10

* Delaminated kaolin (figure 10) uses chemical and/or mechanical means to break apart the platelet structure of the clay, which further increases the available surface area and reinforcement properties.

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Water vapor transmission rate

Fine particle   1.515
Talc            1.289
Delaminated     0.912

Note: Table made from bar graph.

* Metakaolin is partially calcined by heat treating to 600 [degrees] C.

* Calcined (figure 11) is heated to 1,000 [degrees] C, which produces a very white, high surface area mineral with an inert surface.

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Clay is a widely used filler for rubber compounds of all types, including components of tires such as fiber adhesive compounds, and the entire range of non-tire rubber applications where good reinforcement, moderate cost and good processability are desired. Clays are usually added to compounds at levels of 20 to 150 phr.

Talc

Talc is a platelet form of magnesium silicate with a high aspect ratio. Because the platelets can orient in the extrusion process, it provides rubber extrudates with smooth surfaces and which can be extruded at high rates. It is commonly used in compounds, which have critical surface appearance such as exterior automotive components or consumer goods. Talc is used in tires in white sidewall compounds to provide a smooth appearance to the buffed sidewall. The large platelets of talc provide a barrier to gas and moisture permeability in compounds which allows talc to be used in applications such as hydraulic and automotive hoses, barrier films and tire innerliners. Talc is usually used in addition to other fillers with total filler content of 30 to 150 phr.

Titanium dioxide

The rutile form of titanium dioxide is an important filler for white and colored rubber articles. The ability of the titanium to scatter light provides high whiteness and opacity, which gives the filler particle the ability to cover background colors. Titanium dioxide is usually used at 20 to 40 phr loading in conjunction with other fillers.

Aluminum trihydrate

Aluminum trihydrate (figure 12) or ATH is not a naturally occurring mineral, but rather an intermediate mineral that is formed in the conversion of bauxite to aluminum. ATH is refined using two major processes:

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* The Bayer process uses caustic soda to digest the ore and solubulize the alumina from the bauxite as sodium aluminate. The sodium aluminate is separated from the insoluble iron oxide and titanium impurities. Sodium aluminate hydrolyzes on the surface of aluminum trihydrate seed crystals to produce crystalline aluminum trihydrate.

* The bauxite may be sintered in a high temperature kiln with limestone and sodium carbonate to produce dicalcium phosphate and sodium aluminate. The soluble sodium aluminate is separated and precipitated as in the Bayer process. This process produces a crystal with lower organic content.

The aluminum trihydrate is then ground and classified using the same procedures as other minerals, and is available in particle sizes from less than 1 [micro]m to several micrometers. Aluminum trihydrate is used to provide flame-retardant properties to rubber properties and to suppress smoke formation. The water of hydration of the aluminum trihydrate crystal is released beginning at 230 [degrees] C, which absorbs heat and provides water vapor to cool the rubber article and disperse smoke. Typical applications are in the building industry for carpet backings, electrical wires and roofing membranes, in automotive interior foam parts and wiring, and in consumer goods such as furniture, flooring, electronics cables and components. Aluminum trihydrate is used at levels of 20 to 70 phr.

Precipitated calcium carbonate

Precipitated calcium carbonate is formed by dissolving limestone and precipitating Ca[CO.sub.3] as very fine particles, using [CO.sub.2] to precipitate particles for calcium hydroxide (Aroganite process) or by using [Na.sub.2][CO.sub.3] or ammonium carbonate to precipitate from a calcium carbonate solution (Solvay process). Typical particle sizes range from 0.02 to 2 [micro]m. Most commercially available precipitated calcium carbonate is spherical colloidal particles or aggregates of a few spherical particles. Additional shapes and aggregates of varying morphology are possible and provide increased reinforcement. Precipitated calcium carbonate is used as a semi-reinforcing filler in shoe products and industrial rubber goods, particularly when the compound requires low moisture levels or when resistance to alkali solutions is needed. The low moisture content and good reinforcement allow precipitated calcium carbonate to be used as a preferred filler in wire and cable insulation applications.

Acknowledgements

"Introduction to mineral fillers for rubber" is based on a paper given at the Latest Developments in Rubber Reinforcement seminar, November, 2000, Akron, OH.

"Compounding with para-aramid fiber engineered elastomers" is based on a paper given at the October, 2000 meeting of the Rubber Division.

"Powder rubber -- a new raw material generation for simplifying production - pt. 2" is based on a paper given at the April, 2000 meeting of the Rubber Division.

"RPA-studies into the silica/silane system" is based on a paper given at the April, 2000 meeting of the Rubber Division.

References

(1.) ACS Rubber Division, Intermediate Course "Compounding and Processing of Elastomers," 1983.

(2.) R.O. Babbit, Vanderbilt Rubber Handbook, Norwalk, CT: R.T. Vanderbilt Co., 1988.

(3.) M. Morton, Introduction to Rubber Technology, New York: Van Nostrand Reinhold Co. 1974.

(4.) R.A. Baker, "An overview of hidden minerals of polymer applications," J. M. Huber, Engineered Materials Division.

(5.) H.S. Katz and J.V. Milewski, Handbook of Fillers and Plastics, New York: Van Nostrand Reinhold Co. 1987.

(6.) T.J. Florea, "Elastomer reinforcement with chemically modified and specialty clays," Elastomerics, (July, 1986).

(7.) J.M. Huber, Kaolin Clays and Their Industrial Uses, New York, 1955.

Larry Evans is technology manager, rubber and AIC applications, J.M. Huber, responsible for research to understand the role of mineral and silica fillers in improved compound formulations for tires, manufactured rubber products, adhesives, sealants and coatings.