The process of making rubber goods produces a certain amount of waste. Some of this will be legitimate, such as flash, sprue, runners, etc, and some (almost inevitably) will be due to rejects and other problems. As long as the waste is cured (vulcanised) it can usually be recycled by size reduction into usable materials again. The ideal situation is when the producer segregates and saves the waste and it is then recycled and used back into their product.

The incoming waste is identified and checked and is then progressively broken down in size by passing it through very high energy cutters and granulators. The granulators have a removable, perforated “basket” as the base and this can be changed as required. The full range of sizes runs from 2.5 to 30mm. The common, usual sizes would be 4 or 6mm, and larger chunks would “whirl” around inside, passing through the cutters again and again, until they were small enough to fall through the basket. Vibrating sieve units separate or grade the output. High strength magnets are used at every possible flow point to ensure that any residual metal particles are removed. The product may need multiple passes and will then be bagged (25kg or “big bags”).

At this stage the material is known as “granules” and has a ready outlet in safety surfacing and the like.  It is used in the same way as the stone “chips” in tarmac. A typical specification would be 6 – 2 mm rubber granules – that means that no particle will be larger than 6mm and anything smaller than 2mm will have been removed (i.e. no dust) – that would be used as Base Layer for a surface. Another grade would be 4 – 1mm (on a similar basis) and would be used for the Top Coat  i.e. the visible surface. The sizes can be varied as required and the material used could be Tyre Rubber, EPDM, Nitrile etc. This type of material has a relatively low added value.

Rubber Crumb

This is where the real value of this type of recycling can be found.

The waste rubber is first granulated to about 4mm and then passed to a series of rotary grinding machines. Again, magnets are used at every possible place to remove any particles of residual metal (mainly from the original tyre construction). Once started, the process continues automatically – the powder is fed to sealed sieve units where the only exits are:

• Correct sized material to the bagging units
• Oversize material, which has not passed through the sieve, is automatically returned to the system and is reground until it is the correct size.

The actual sieves can readily be changed to make a wide range of particle sizes, and unlike some processes, our stated sieve size means the largest possible size the crumb can be – not the average.

Basic properties

In essence, the true basic properties of the material are the same as the properties of the waste material used to make the crumb or granules. However, the real properties that we are interested in are as follows:-

1. A relatively inert material of known and controlled particle size (no oversize or dust)
2. A material that is compatible with the type of rubber compound it is going back into.
3. A material with a jagged, partially activated, very high surface area that will chemically bond into the rubber article when it is cured (vulcanised)  -   (crumb particles cannot be seen on the surface of the cured article, nor can they be “prised out” – crumb is used as an anti-tack dusted onto the surface of tyre sidewall veneer compound, that disappears completely when the tyre is cured).
4. A material that is very cost effective, environmentally sound and technically viable

(Note: For very high tech applications (FKM etc) the compounder / producer can have peace of mind knowing that the only chemical materials in the crumb are exactly the same as in the product they are making – no compatibility or contamination issues.  That is of course, as long as they make use of our bespoke service, where they supply THEIR waste material, and we use it (and nothing else) to make THEIR own crumb.)

Why should you use it?

For the compounder:-

a)     The low initial cost and the possible value recovery aspects will make you popular

b)     The technical aspects can be extremely useful –

In small additions of 5 to 10 phr (parts per hundred of rubber)

• It can reduce air trapping
• It can reduce “laking” on the surface of “flat” mouldings
• It can improve hot tear strength
• It can reduce flow, or excessive “creep” in the uncured rubber

In larger additions – above 10 phr, to as much as you like – see i.

• Physical properties will fall in relation to the amount of crumb added. If there are no physical requirements – cheap ladder feet, wheel barrow tyres, sleeping policemen  etc, over 50% of the total weight can easily be added.
• As a cheap filler it is a low S.G., readily compatible blend of hydrocarbons and mineral material.  In very high additions it may be necessary to increase the amount of sulphur / accelerators slightly.

For the accountant:-

a)     It is low price material in the first place.  The word “cheap” conjurers up the wrong idea.

b)     Taken in conjunction with a robust system of internal waste collection and segregation – which is then passed onwards to the recycler – hopefully to be returned as “house” crumb – the economic advantage is completely maximised :-

•  A useful, low cost raw material put back in stock
•  Value recovery of internal waste and scrap
•  Elimination of waste rubber disposal costs

How do you use it ?

Just like any other rubber filler.  If large amounts are used it will be better to add it after the addition of any oils, plasticisers or process aids.  It can have a tendency to absorb the oils resulting in slightly higher hardness than expected.