FILLERS FOR RUBBER

When we think of fillers, we think of something added to cheapen the finished product cost or extend something else like talc to soap bar or talc to virgin polymers for master batches manufacturing.

But there’s another type of filler we seldom think about – the recycled version of the same item. In the glass industry they call this cullet and in ceramics it is called grog. In the rubber industry it is called rubber reclaim. It is made by devulcanising used rubber items (mostly tyres) under temperature and pressure.

There are many types and grades of reclaim rubber. It usually comes in rubber sheets packed inside bags, but can also be ground into fine particles called rubber crumb. The grades are strictly controlled as for any raw material. Each grade has its own data sheet listing its parameters like ash content, rubber hydrocarbon content and tensile strength.

Interestingly, this type of filler does not just cheapen; it also acts as a process aid by shortening mixing times.

The rubber industry is a great one for fillers – they use many types, divided broadly into black and non-black fillers. The dominant black filler is the widely-used carbon black. Its fine particle size and affinity for rubber gives it excellent reinforcing properties.

The non-black fillers are usually white mineral powders such as:

• Silica
• Kaolin
• Calcined kaolin
• Calcium carbonate
• Zinc oxide
• Barium sulphate (barytes)
• Talc
• Titanium dioxide
• Aluminium trihydrate (ATH)

Mineral fillers are designed to impart various desirable properties like increased tear strength and abrasion resistance, reduced coefficient of friction or lower cost. How well they do this depends very much on the minerals’ particle size and particle shape. The finer the particle, the higher its surface area so the more reactive it is. The shape of the particle determines how well it interacts with the rubber matrix.

The trend with rubber fillers is definitely towards finer particle sizes for better filler performance. Inorganic fillers and organic rubber don’t always bond comfortably together, so silane coupling agents are often used to provide a chemical bond between the rubber and the filler. Each filler has its own strengths and weaknesses. That’s why you may find a cocktail of them in rubber formulations. Fine silica is always reinforcing filler while kaolin is more of cheapening filler. Very fine kaolin (< 500nm) becomes semi-reinforcing and is called ‘hard” kaolin because it imparts hardness to rubber. Zinc oxide can reinforce but it also activates the vulcanization process. Fine talc of 4µ at d50 works as a reinforcing agent while with 350 mesh used as an anti sticking agent.

KAOLIN VS TALC

Many of the customers have confusion over difference between kaolin and talc.

Similarities:

They are both fine white powders that come from digging a mineral out of the ground and refining it. If you take a microscope and look at the particles close-up, they both look like tiny flat plates, which are called platelets. This platelet shape makes them good at cutting out the light, which is why we put them into paints and ceramic glazes to help with opacity.

Kaolin and talc have a lot more in common: They are both inert, which means they take no part in chemical reactions. They are also both insoluble in water and hence they do not dissolve.

Although both are minerals, only kaolin is called clay. In fact kaolin’s common name is china clay. The common name for talc is talcum powder and it is the softest mineral on earth. That must be why it is chosen for use as baby powder.

Kaolin is also a soft mineral and comes just above talc on the Mohs hardness scale of minerals:

Mohs hardness of minerals
Diamond	10
Silica	7
Kaolin	2
Talc	1

Differences:

The talc is much whiter compare to Kaolin.

The biggest difference between Talc and Kaolin is that talc stick to your blazes while kaolin makes our hands only white and dusty.

Talc is hydrophobic which means water-hating and hence it loves anything organic things. Kaolin, on the other hand, is hydrophilic. It makes it much easier to disperse into water than talc, which tends to sit on top of it. Talc may be water-hating but surprisingly, you can still disperse it into your water-based paint without too much of a problem. This water-hating property does come in useful in waterproofing products, where you need ingredients that help to reject water.

Kaolin is a clay mineral and falls into a family called alumino-silicates. Talc, however, is a magnesium silicate. This would explain their different behavior. However sometimes when you are just using them as inert fillers, their differences are not important and you would just buy whichever is cheapest – usually kaolin. Like when you are using them in laundry soap bars, for example. For white toilet soap, though, you will need whiteness, so then you would choose talc rather than kaolin.

It gets even more interesting when you melt these two minerals, as happens when they are used in ceramics. Kaolin is a basic ingredient of most ceramic bodies and glazes and is needed for its alumina content, while talc is used mainly as an additive which helps to control thermal expansion, reduce crazing and improve the whiteness of the ceramic article or glaze.

PARTICLE SHAPE

Have you ever thought about the shape of different mineral particles, and what impact the particle shape has on a particular mineral’s performance?

Many industries where minerals are used as fillers or opacifiers, the shape of the particle is very important, especially in terms of how much opacity one mineral delivers compared to another. The industries where this is particularly significant are the paint and paper industries, but particle shape can also have a bearing in rubber, adhesive and other applications.

Commonly-used mineral particles can generally be described as either platy or round. Round can be described as particles that have similar dimensions of height, width and length, so a cube or a rhombic shape, for example, would both be included in this simplistic definition of round.

Some common minerals and their general particle shape are given in this table:

Platy particles	Round particles
Talc	Calcium carbonate
Kaolin	Dolomite
Pyrophyllite	Silica
Mica	Feldspar

Platy particles in general give better hiding power (or opacity) than round particles. Imagine the platelets as a number of tiles lying on top of each other. Then it is quite easy to understand that this pile of tiles would block out the light better than a pile of the same number of round balls. Or if you want to block the light to the same extent, you probably need a larger number of round balls to get the same effect as the tiles.

So the general conclusion is that if you want to increase your opacity, go for the platy rather than the round mineral shape.

Interestingly, the production of synthetic precipitated calcium carbonate has now advanced to a point where the manufacturers now claim they can “engineer” a calcium carbonate particle with a platy shape.

In some industries, however, the mineral shape has no bearing. Like in Ceramic, what is important is the chemical composition of the mineral because everything gets melted together at high temperature. Particle size probably has more relevance than particle shape.

HOW WHITE IS WHITE?

If you deal with white minerals and fillers you have probably pondered this question at some time or other. Comparing the whiteness of one white powder with another is not so easy. Especially if you only have the data sheets to go on!

Measuring the whiteness of minerals is a complex subject. What we need are a few simple guidelines that are easy to use in our day-to-day working lives.

Simplest method:

If you have samples of the two minerals, you can do a comparison by eye. I suggest you pour out some powder on to a plain dark background. Press some flat with your finger. Wet a corner of it, and then compare the two. Which is whiter? Is it a greyish-white or a yellowish-white? The lighting in the room can affect this method a lot, so it is only a very rough test.

Another way is to keep a standard and compare your new sample by eye to your standard – a quick and easy way to check how different batches compare with each other.

In Paint Industry, chemists make up a paint sample with the standard and one with the new filler, and then they do a drawdown. This compares two films of paint against a black and white striped background and is a quick way to check whiteness as well as hiding power (opacity).

A more accurate approach is to measure the whiteness with a spectrophotometer. There are several types and makes of these; the one we use is the Elrepho spectrophotometer.

The measurements you get from a spectrophotometer are usually taken at different wavelengths. These are also manipulated to give further parameters. A list of these reads something like this: L*, a*, b*, x, y, Rx, Ry, Rz, R457, CIE Whiteness, ISO-Brightness, Yellowness. So which of these parameters should be used for comparing whiteness? More than one from this list can be used – the confusion arises because we don’t all use the same one! It doesn’t matter so much which one we use, but for comparing apples with apples, we should use the same parameter for the comparison.

The paper industry has standardized on ISO-Brightness, which is the same as the R457 Reflectance measurement, measured at 457nm. So if you find one mineral’s data sheet reports ISO-Brightness and another says Reflectance R457, you can use these as one and the same and easily compare one mineral to another.

Whiteness Ry is the other common parameter used as a measure of whiteness.  You will often find this parameter on talc and calcium carbonate data sheets. However it gives a different (usually higher) result to the R457 measurement.

Some mineral suppliers, however, just report the Y-value as a whiteness measurement.

So read the data sheet carefully. If you need a different parameter to make a particular comparison, ask the suppliers for it – he has probably measured it but has just not reported it on his data sheet.

If you are given the L*, a* and b* values (sometimes called Lab color) these can tell you a bit more about the color of that mineral than just whiteness. Lab color was designed to approximate human vision.

The L component closely matches human perception of lightnessand is given on a scale of 0 to 100.

The a-value is similarly a measure of greenness or redness as shown on this schematic:

The b-value gives a reading between -1 and +1 on the b-axis or the yellow/blue scale.

Look out for a high b-value, for example – it means the mineral is yellowish. Similarly, a positive a-value indicates reddishness.