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FILLING IN THE GAPS
Sam Burgess
Careful use of fillers can enhance rubber compounds.
By SAM BURGESS
[Editor’s Note: Our normal Tech Session columnist, RL Hudson CEO Rick Hudson, has loaned his quill to our resident rubber chemist, Sam Burgess. Rick will return in our next issue.]
As the Director of Material Technology here at RL Hudson, I am constantly looking for ways to improve the materials we use, with the overall goal of improving product quality and reliability. One way I accomplish this is through the selective use of rubber fillers. I’d like to offer you a quick look at the history, purpose, and anatomy of rubber fillers, as well as an overview of how fillers work.
LOOKING BACK Rubber fillers date back to the Amazon Indians, who may have scraped the black deposit from rocks around a wood burning fire and used this deposit in rubber latex. This was to prevent light age cracking and reduce the surface tackiness of their basketballs when playing against Cortez in 1518-1521. However, this scraping process was not patented until 1830. By 1892, this filler became known as “channel black.”
By 1922, “carbon black” was manufactured by burning natural gas and later oil in oxygen poor furnaces. It was 1925 before carbon black was found to be better in tires than zinc oxide, thus ending white tires. The first silicate was introduced in 1939. Around 1947, the first filler was incorporated into the polymer during the latex stage, providing better dispersion and higher tensile strength. Though non-black fillers (e.g. silica, clay, metallic oxides, calcium carbonate) are used today, carbon black is the most important rubber filler.
CALLING IN REINFORCEMENTS Fillers are primarily used to reinforce the physical properties of polymers. Without fillers, some polymers have the strength for certain applications, but cannot be processed. Due to the inherent high viscosity of most polymers, thermoset rubber cannot be processed without the use of filler to reduce some or all of this viscosity, thereby smoothing the surface of the part. Other polymers are simply too weak for use without reinforcing fillers.
But though reinforcement is their primary role, fillers can also fulfill other functions. For example, a filled compound contains less base polymer and is therefore less expensive. Presence of filler can also reduce tack, block damaging ultraviolet light, and alter the color of the material.
MAKING THE GRADE The particle size or surface area of filler is critical to the physical property desired. Small filler particles provide greater reinforcement than large ones. Non-black fillers can vary from coarse grades of 1 m2/g to fine grades of 400 m2/g. Carbon blacks can be made up to 1000 m2/g, with most rubber grades varying from 6 m2/g to 250 m2/g.
The structure of carbon black is not unlike a bundle of grapes. The more grapes in the bunch, the more voids for oil or other plasticizer to be absorbed, allowing for greater extension or lower hardness. The amount of oil that filler can absorb is known as the DBP (DiButylPhthalate) absorption and is normally determined using an absorptometer.
BOOSTING PROPERTY VALUES Reinforcement occurs when two or more materials of different mechanical characteristics are combined, and the strength of one material is imparted to the other. Think of fabric reinforcement of rubber belting. In most cases, reinforcing filler enhances the tensile, hardness, modulus, tear, and abrasion properties of the polymer.
Tensile strength is the amount of force required to break a specimen. Some polymers (such as natural rubber) are strong enough as is, and adding filler actually decreases their inherent tensile strength, but filler is still required to make them processible. Some filled polymers show a tensile increase to a particular loading point, then a decrease due to the lack of enough polymer to hold the filler together.
Both hardness (resistance to indentation) and modulus (force required to produce a specific elongation) increase as filler loading increases. This can be nullified with the addition of oils or plasticizers. Tear resistance can also be augmented by adding filler, which acts as a patch to help prevent the polymer from being torn apart. Filler can also act to shield a rubber compound from abrasive contact with another surface.
In addition to the physical properties just mentioned, filler can also enhance resistance to creep, cold flow, hysteresis, and heat-build-up. When two or more fillers (such as silica and carbon black) are blended – as in off-the-road tires – a better product is obtained than if each were incorporated separately. Each filler has its own contribution.
FORMULATING SUCCESS Filler technology has come a long way since the time of the Amazon Indians, and this has been but a brief survey of a complex topic. Rest assured, we at RL Hudson understand the intricacies of formulating materials to meet specific applications. Please call us at 800-722-6766 if we can help you formulate success.
