When we think fillers, we think of something added to cheapen or extend something else. Like adding breadcrumbs to your meatballs or kaolin to your soap bar.
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 (I know what you're thinking but forget it – this grog is not quaffable!). In the rubber industry it is called rubber reclaim. So if you've gone onto our website and wondered what rubber reclaim is, now you know. 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 (click on the name to read more about it). This filler is so fine it reminds me of soot. Go near it and soon you look like a chimney sweep! Its fine particle size and affinity for rubber gives it excellent reinforcing properties.
The non-black fillers are usually white mineral powders such as:
The "Big Three" in terms of volumes used are calcium carbonate, kaolin clay and precipitated silica.
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. We used to sell 2-micron kaolin for rubber filler, now our 1-micron grade dominates.
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. Pity there isn't something like that to make humans get along!
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 a reinforcing filler while kaolin is more of a 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 vulcanisation process.
Look, I've simplified a very complex subject. If you're a technical guy and you want more detail, the best article I ever read on rubber fillers is here, courtesy of the R.T Vanderbilt company