Rubber Compounding

Rubber Compounding

This is one of the subjects to master in the field of rubber technology. Compounding is not really a science . It is part art, part science. In compounding one must cope with literally hundreds of variables in material and equipments. There is no infallible mathematical formulation to help the compounder, that is why compounding is so difficult a task

What is rubber compounding and Why are we doing it ?
How can we do it successfully?

Rubber compounding is the art & science of selecting and combining elastomers and additives to obtain an intimate mixture that will develop the necessary physical & chemical properties for a final finished products as per specification desired to achieve the required useable properties in all types of rubber products used in various applications.

Objectives of Rubber Compounding:

1. To secure certain properties in the finished product to satisfy service requirements

2. To attain processing characteristics necessary for efficiant utilisation of available equipments

3. To achieve the desirable properties & process ability at the lowest possible cost. In other words, the most important criteria in compounding is, to secure an acceptable balance among demands arising from the above criteria.

Equipment For Rubber Compounding

What is mixing?

Mixing is defined as a process of incorporating the compounding ingredients into the rubber, done on an open rubber mixing mill at the beginning of a mixing cycle the rubber is forced or drawn in to the working area between pair of rolls on open mixing mill . The process of incorporation, mastication, & dispersion are the most important functions of mixing which contributes to a very good product as regards a compound to meet all the desired properties as per specification of required compound.

What is a mixing mill?

Is defined as the machine with two differential speed rolls, running in opposite direction, forming a nip for mastication mixing , or warming before extrusion . The rotation of the rolls pulls the ingredients through the nip, which is the clearance between the rolls. The remaining surface of the roll is used as means of transportation for returning stock to the nip for further mixing

Internal Mixer is a machine with profiled rotors rotating in an enclosed chamber in opposite direction , it is used for rapid mixing and large throughput due to its versatility, it is tending to replace the open mixing mill, Specially as a batch mixer. The basic purpose of all internal mixers are the ability to exert high localized shear stress to the material being mixed. A lower shear rate stirring, a homogenous action, that is the effective ness of dispersive mixing results from the combination of high shear stress and large shear deformation. . An internal mixer design is a compromise based on experience to best accommodate the wide range of compounds typical of a manufacturing operation involving mixed products.

Banbury Machine:

Banbury mixer consists of a completely enclosed mixing chamber in which two sprial shaped rotors operate a hopper at the top to receive compounding ingredients for mixing and a door at the bottom for discharging the mixed batch of compound .the bulk of the shearing action takes place between the rotor and the chamber wall

A Compounder must have the following knowledge:

• An understanding of the plant, processing methods available to him in the organization,

• Moulding unit knowledge to develop compounds for compression moulding, transfer moulding and injection moulding
• In a tyre industry a compounder must have knowledge of tyre machinery, tyre moulds

• In extrusion industry he must have knowledge of the type of extrusion process in operation.

• Up to date and elaborate knowledge of both the chemical and the physical properties of elastomers is most essential

• Knowledge of vulcanizing system available, knowledge of in- process physical testing and also testing of finished products

• A good comppunder with experience must be able to convert science in to art. So that he has the capability of producing material that will be easy to process and he must be able to produce and formulate economical, practical compounds as competition in today’s world is quite severe.

Scorch control in Peroxide Vulcanization

Using a stable free radical Peroxide vulcanization of rubbers offers various advantages over sulphur vulcanisation. Like rapid vulcanization without reversation at higher temperatures, good compression set, excellent heat ageing properties, possibility to co-vulcanise saturated and unsaturated rubber blends etc. These are attributed to the C- C cross – links formed during peroxide vulcanization, which have the same bond strength as the C – C bonds in the polymer back bone. As per the generally accepted mechanism, peroxide vulcanization is initiated by the thermal decomposition of organic paroxides to highly reactive free radicals which then abstract hydrogen atoms from the elestomer, yielding rubber to macro radicals. These micro radicals combine to form the cross – links. One outstanding problem that restricts the wider industrial use of peroxide vulcanization is the low scorch safety. If scorch control can be significantly improved, the range of industrial application of peroxide vulcanization can be broadened. Earlier works to use radical scavengers / co – agent combination to introduce scorch control has not succeeded in bringing about scorch to a level equilant to sulphur/accelerator combination. The present paper addresses it and shows that stable nitroxyl free radical, 2, 2, 6, 6,- tetramethylpiperidine-1- oxyl [TEMPO ], used in bringing about living free radical polymerization, can control scorch in peroxide vulcanization in a way similar to the sulphur/ accelerator system.

Use of TEMPO has led to the reduction in cross link density due to the reaction of it with the macro free radicals generated. A recent US patent described a method to over come this by modifying the TEMPO. THE PRESENT PAPER SHOWS THAT THE REDUCTION IN CROSS-LINK DENSITY CAN BE VERY WELL COMPENSATED USING APPROPRIATED CO-AGENTS.

What is rubber and why does it stretch ?

The terms Elastomer and Rubber are scientifically identical and inter changeable , although the letters used in some are as to refer only to natural rubber which comes from the latex contained by some trees and other plants as opposed to synthetic rubber which is generally an oil by product.

Some standards attempt to reserve the term elastomer for a cross linked material. Elastomer’s are a class of materials which differ quite obviously from all other solid materials in that they can be stretched easily and almost completely reversibly, to high extensions. An ordinary postal rubber band illustrate this behaviour. It will generally be made from natural rubber, and can be stretched to seven times its original length, after which – before reaching its ultimate breaking elongation –it can be released and will rapidly recover to almost exactly the original length that it had before stretching.

The material is said to be elastic. Most synthetic elastomers are not as elastic as natural rubber, but all can be stretched or other vice deformed in a reversible manner to an extent which easily distinguishes them from all other solid matrials. A metal spring exhibits a high reversible elasticity, but this is a feature of its wound shape. The actual metal itself of which the spring is made only deforms slightly, by twisting locally, of any particular point, nothing like the high deformation of which elastomers are capable.

Thank God for rubber!

Rubber is an extremely versatile and useful product which we use in our everyday lives. We probably don’t realise it, but so many things around us are rubber products, right from the wheels that get us around to parts of the furniture we use to sit when we are tired and want to relax. And these are just a couple of things. What about the pressure cooker that has the ring made from rubber or the soles of shoes that are often made from rubber? The list can go on and on. Read the rest of this entry »

Even Mayans used Rubber

The rubber industry in India is ever-growing with novel products and innovations hitting the shelves every month. But have you ever sat down and pondered about this wondrous product? Well today let’s take a look at a complete different theory on the evolution of rubber. Read the rest of this entry »