Rubber compounding

Rubber compounding is the complex,

multidisiplinary science of selecting and blending

the appropriate combination of elastomers and

other ingredients to meet the performance,

manufacturing, environmental, and cost

requirements for rubber goods made and used in

commerce. There is a wide variety of elastomers

and ingredients that are available in making rubber

goods, which include all of the following types of

products: tires, innertubes, retreaded tires,

footwear, rubber rolls, hoses, belts,

weatherstripping, O-rings, seals, diaphragms,

tubing, rubber and latex gloves, ball bladders,

medical devices, bumpers, and numerous other

products.

Compounding is a term that has evolved within

the plastics and rubber industry and in many

respects is a misnomer for the material science of

modifying a polymer or polymer blend through

addition of other materials to achieve a set of

mechanical properties for a specific service

application.

Raw materials for a compound are generally

selected in the following order:

(1) polymer (natural or synthetic rubber)

(2) fillers or reinforcing agent

(3) antioxidants and antiozonants

(4) plasticizers or oils

(5) bonding agent or adhesive (if needed)

(6) tackifer (if needed)

(7) vulcanization system [curing agent,

accelerator(s), or coagent]

Performance requirements of the final product

often dictate which specific type of elastomer can

be used. The expected usable life for the product is

controlled by many factors such as end-customer

needs, competitive situation in the marketplace,

safety, and reliability. Rubber products are almost

always used as a functional part of another system.

For example, tires, hoses, belts, O-rings, and

numerous rubber components are used in

manufacturing automobiles and trucks. The overall

life of the vehicle as well as its performance level

often relates to the service life or quality level of

the rubber parts.

Equipment used for Compounding

The rubber technologists mixing department has

bags of powders, drums of liquids and bales or

granules or chips of raw gum elastomer. These are

weighed out precisely, to match both the batch

weight needed and the ratio of ingredients in the

formulation. Machines are necessary to mix these

chemicals, resulting in a finely blended, solid

homogeneous mixture. In many cases, the

compounder and process operator expend their

energy reducing the elastic component of the

uncured rubber compound, to help it process, and

then increase that component again during

vulcanization. Mixing is accomplished using mills

and/or internal mixing machines. The resulting

compound is then preshaped by mills, extruders or

calenders, to prepare it for vulcanization. The latter

is achieved using molds (which further shape the

product),

autoclaves, and sometimes ovens. That just leaves

finishing operations, such as removing flash (see

section 4.6.1 for an explanation of flash), or maybe

the grinding of rubber rollers (cured in an

autoclave) to a finished dimension, and then

packaging the product.

Mills

These were used at the beginning of the rubber

industry and are still an essential piece of

rubber processing equipment.

A mill consists of two horizontally placed hollow

metal cylinders rotating towards each other (see

Figure ). The distance between the cylinders (mill

rolls) can be varied, typically between 0.25 to 2.0

cm. This gap between the rolls is called a nip.

Figure Conceptual view of rubber mill rolls

Operation

Raw gum elastomer is placed into the gap between

the two mill rolls, the mill nip. It then bands, as a

continuous sheet, onto one of the rolls. The speeds

of the two rolls are often different, the back roll

rotating faster than the front. The difference in

speed between the two rolls is called the friction

ratio and allows a shearing action (friction) at the

nip to disperse the ingredients and to force the

compound to stay on one roll, preferably the front

one. A friction ratio of 1.25:l is common. Powders,

liquids, etc., are then added to the nip in a specific

way. The process produces friction which creates

heat. This excess heat needs to be removed, either

by spraying or flooding the inside of the roll with

cooling water or by passing water through drilled

channels in the wall of the roll.

A device is necessary to prevent the rubber from

moving past the end of the rollers. This is

accomplished by a piece of metal called a guide,

positioned at each end of the roll, so as to

almost touch the surface. At the beginning of the

mixing process, pieces of material tend to

Mill processing

The following description relates primarily to

compounds which use sulfur as the crosslinking

agent. The key to mixing (in a Banbury mixer or a

mill) is to maintain sufficient viscosity to ensure an

adequate shearing action, to distribute the non-

rubber ingredients into the raw gum elastomer, or

to force the raw gum elastomer into the

microscopic spaces of each filler particle. Both

mechanisms have been hypothesized and one

typical mixing sequence might be as follows: The

raw gum elastomer is placed into the nip and

allowed to band onto the front roll. In the case of

NR, it needs to move though the nip quite a few

times to reduce its nerve (elasticity) and to lower

its high viscosity (low viscosity grades are

available). It then forms a smooth, more plastic,

band on the roll. Normally most powders (other

than accelerators and sometimes sulfur) are then

added. If significant heat is produced, then cross-

linking agents and accelerator addition will be

delayed to the last part of the mixing process. In

some cases, when excessive heat is produced, it

may be necessary to remove the compound from

the mill before the accelerator is added, to avoid

scorching (prevulcanization). The compound at this

point is known as a masterbatch.

Internal mixing machines

If the rolls of a mill are twisted to produce a

corkscrew effect (they would now be called rotors),

and then a block of steel is placed over the mill nip

with the block connected to a steel rod above it,

this would be called a ram. The ram would move

up, to allow addition of ingredients to the nip, and

it would move down to force the compound

ingredients into the nip. If the whole thing is

surrounded in a heavy metal jacket with a chute at

the top to put ingredients in and a door at the

bottom (underneath the rotors),

to let the mixed material out, the result will be an

internal mixing machine.

Operation

In 1916 Mr. Fernley H. Banbury, improved on an

internal mixing machine built by Werner &

Pfleiderer [ 11 by designing the Banbury mixer.

The Banbury mixer had modified rotors and the

addition of a floating weight. The internal mixer

rapidly became an essential part of the rubber

industry. At the present time, mixers are available

in sizes ranging from those capable of mixing a kg

or so, to those that can mix more than 500 kg per

load, equivalent to many large mills. The internal

mixer is faster, cleaner, (produces less dust from

powdery materials such as carbon black, silica and

clay), uses less floor space, and is probably less

operator sensitive. It has thus displaced the mill for

most compounding operations. However, the

variable nip opening on a mill, plus immediate

visual feedback of the state of the mix, allows a

good mill operator a high degree of control and

consequently dispersion. The internal mixer has a

fast mixing capability, from around two to ten

minutes, and thus requires an efficient cooling

system. This is provided by drilled channels in the

walls of the mixing chamber, through which water

passes to control the mix temperature. The rotors

and discharge door can also be water cooled. The

temperature of the compound being mixed is

measured by a thermocouple in the side of the

mixing chamber. Other

parameters which can be measured and controlled

during the mixing process are electrical

power (amperage or watts) and time.

Figure

)

Raw gum elastomer is dropped through the hopper

into the mixing chamber where it is mixed by the

rotors. The ram, pressing on to the rubber mixture,

is forced down by a pneumatically or hydraulically

controlled cylinder, whose pressure is adjusted to

give the best control of the mixing process. Oil may

be poured in from the hopper, or injected through a

valve in the hopper wall just above the mixing

chamber, Mixing can occur between the rotors

(intermeshing rotors) or between the mixing

chamber walls and the

rotors (tangential rotors), depending on the

machine. The rotor to rotor, or rotor to wall,

clearance is very important to correct mixing.

Recent modifications are the Banbury ST rotors

(synchronous technology) and Pominis VIC

(variable intermeshing clearance) design, where

the distance between the rotors can be varied.

Non-black fillers and other compounding ingredients

Silica

There are two types of silica, natural and precipitated. Natural silicas like diatomaceous earth Impart stiffness and give a very dead compound which will extrude without swelling. These are not considered among reinforcing fillers.

Precipitated silicas are the best's non-black reinforcing fillers so far developed and come closets to carbonblack properties. They have a particle size as fine that of carbon black and they also have an extremely reactive surcease. These are generally prepared from sodium silicate solution by precipitation.

Precipitated silicas are easily mixed and excellent dispersion is obtained. It is better to mix silica batches in a Danbury than on a roll mill. No special precautions need be taken for Danbury mixing,

procedures ordinarily used for other reinforcing fillers should be followed. If mill mixing is following for silica loaded NR compounds, some productions should be taken to ensure good properties and hence the complete batch should mixed and cut from the mill as quickly are possible. It is also preferable to mix rubber and silica alone and then to complete the mixing after maturing for one or two says. The probable reason for the improvement in properties is that in the absence of any other material, the rubber penetrates more into the filler and thus causes better filler-rubber interaction. Subsequent addition of other materials does not interfere with any existing rubber-filler bond.

Silicas stiffen rubber compounds to a considerable extent. The stiffening can be reduced by the addition of suitable plasticisers. Properties like tensile strength, hardness, tear resistance and abrasion resistance are improved very much. Compression set, however, is increased.

Excessive milling may cause reduction in properties Reinforcing silicas are highly adsorptive. Hence in formulating NR Compounds containing them, it is necessary to use more than the normal

quantity of accelerators or else to use certain activators like DEG of triethanolamine. In silica-filled NR compounds thiaz 01- thiuram combinations are not satisfactory as these are scorchy and are not flat coming. Thiazol-guanidine combinations are found to be more satisfactory. However in silicas filled SBR compounds both these combinations are satisfactory. Silicas are generally used in white or light coloured stocks Hence it is essential that non-staining and non-discoloring anti-oxidants are used in such compounds.

In SBR compounds containing silica, use of CD resin helps in realising the best tensile and year properties. About 10 parts of resin in a 30 volume loaded stock and proportionately smaller quantities for lower loading should be used. It is not necessary to observe any precautions white mixing silica filled SBR compounds, as in the case of NR.

Precipitated silicas impart very good properties to nitrile rubbers, giving good tensile strength and tear resistant. They are also excellent reinforcing fillers for neoprene's. Good tensile strength and tear resistance can be attained sacrificing elongation. It is also possible to get easily

prosecutable buty1 stocks with good physical properties by using precipitated silica as filler.

Precipitated silicas are suitable as filler for translucent compounds based on NR, SBR and nitrile rubbers.

Fillers of minor importance:

The following fillers find use only to a limited extent.Slate flour : May the used in cheap acid resistant stocks where whiting is not suitableBarytes : This is naturally occurring Barium sulphate. It is easily milled into rubber but is inert. Used in acid resistant compound and where weight is important.

Balance fixe : This is precipitated Barium Sulphate. Finer than barytes. These also come under inert fillers.

Magnesium : This gives very stiff compounds with high permanent set. Used in Carbonate : translucent compounds.

Organic fillers

1. Cork: Natural cork, ground to various degrees of fineness gives compounds with a high degree of resilience and compressibility and is used in flooring, gaskets etc.

2. Glue: Animal or fish glue added as powder or in hydrated form imparts a degree of oil and fuel resistance to NR for suck items as hose linings and gaskets, when cheapness and very moderate fluid resistance are required.

3. Cyclised natural rubber: This usually blended with NR, gives compounds of high modulus and hardness with low specific gravity. A moderate amount assists in giving a good surface finish to moulded goods.

4. High styrene resins: Copolymers of butadiene and styrene, with 50-80% bound styrene are alternatives to Cyclised rubber for high modulus, tear resistance and abrasion resistance. In combination with silica or silicates their main use is in hardwearing shoe soling.

Plasticisers and Extenders

Plasticisers are added in rubber compounds with the following objectives.1. Increase plasticity and workability of the compound.2. Aid in wetting and incorporation of fillers3. Provide lubrication to improve extrusion, moulding or other shaping operations.4. Reduce batch temperature and power consumption during mixing5. Modify the properties of the vulcanised products.

Plasticisers are divided broadly into two classes: chemical Plasticisers and physical Plasticisers. The former types act by reducing the molecular weight of the rubber, by chain scission. Physical platiciser not as intermolecular lubricants.

Chemical Plasticisers are appropriate when:a) The primary concern is for modifying the properties of the uncured stock rather than those of the vulcanisate.b) The type of rubber being used is one which responds to such agents. c) The other compounding ingredients and mixing

conditions are right for maximum activity of the plesticiserd) The cost is favourable in comparison with other methods of getting the same effect.

Physical plasticisers are used when:a) Modification of the vulcanisate properties is also desired b) Processing required the lubricating, tackifying and other special properties that can be obtained with different physical softeners.c) Cost is favourable in comparison with other approaches.

Different chemical Plasticisers and are available which are quite different from one another and are effective under different conditions. Some are listed below:1. Certain accelerators (eg. MBT & DPG) have mild Plasticising action on NR.2. Aromatic mercaptans (eg. Thionaphthol & Xylyl mereaptan) have strong softening effect on NR and reclaim.3. Phenyl hydrazine salts-strong in NR & SBR.4. Thiuram disulphides - Strong in neoprenes.5. Benzami dodipheny1 disulphide - effective only at high temperature.

6. Certain petroleum sulophenol, especially its zinc salt, is powerful in NR and at high temperature in SBR.

It is important to note that in the absence of a chemical plasticiser NR breaks down most rapidly at low mill temperatures and that the chemical plasticisers work most effectively at high temperatures. For this reason, the compounded should use a fully effective quantity of the chemical plasticiser and masticate at high temperatures or else leave it out altogether and masticate at as low a temperature as possible.

Most of the important physical plasticisers come from any one of the following sources.

1. petroleum (mineral oils, resins, waxes, asphalt) 2. pine tree (Pine tar, pitch, resins)3. Coal tar (coal tar oil, pitch, resins)4. Natural fats & Oils (Vegetable oil, blown Oils, fatty acids, fictive)5. Synthetic organic compounds (Ester Plasticiser, liquid polymers etc.)

Petroleum oils are the most common among the above. They range from highly aromatic to

aromatic, naphthenic and paraffin Oils. All these are available in various grades of viscosity and staining power. When softeners are used in small dosages, merely to improve processing, the aromatic oils are suitable. But for very high dosage of softeners along with high quantities of carbon black or mineral fillers, naphthenic oils are preferred from the viewpoint of compatibility and aging resistance. Paraffinic oils and petroleum jelly should be used only when internal lubrication and high gloss are required. Aromatic oils cause staining and discolouration while naphthenic and paraffinic types do not.

In general, the main factors considered for selection of physical plasticisers are as follows:1. Compatibility2. Cost3. Efficiency4. Staining characteristics5. Low temperature properties6. Effect on vulcanization and ageing characteristics of the rubber

Extenders

These are substances which are added to rubber

compounds in lage quantities so that the cost of the compound can be reduced, without seriously affecting the final properties. Important among the commercially used extenders are the following.

1. Rubber reclaim

This is a product resulting from the treatment of vulcanised scrap rubber tires, tubes etc. by the application of heat and chemicals, whereby a substantial degree of devulcanisation and regeneration of the rubber compound to its original plastic and state is affected, thus permitting the product to be processed, compounded and vulcvanised. Reclaiming is essentially depolymerisation, the combined sulphur is not removed. Different types are available, depending upon the original type of waste rubber employed. Whole tyre reclaim (WTR) contains about 50% rubber hydrocarbon and 20-25% carbon black, the rest being plasticisers and miscellaneous ingredients. While using whole tyre reclaim for every 1 part of rubber which is being replaced, 2 parts of reclaim is added. In addition to the economic advantage, use of reclaim has other beneficial affects such as short mixing time, low power consumption, low heat development during

processing, reduced swelling and shrinkage, higher cure rate, minimised reversion, good ageing etc.

Factice:

This is a class of elastic gums made by reacting certain vegetable oils with sulphur. Most familiars is the one used in erasers. Fictive can be blended with NR and with some synthetic rubbers is a rather high proportion and the blend will remain highly elastic. It will also be soft and suffer much loss of strength and abrasion resistance. Another way in which it is used is to increase the tolerance for liquid softeners in synthetic rubber compounds.

Mineral rubber

Airs blown petroleum asphalt are called mineral rubber. Products of this sort wold hand only moderate weakening effect on tensile and tear strength. But would relatively be poor in respect of resilience and heat build up and flexing.

White pigments and colours

White pigments are used in rubber compounds to which them. Its use in coloured compounds mains

the colours brighter important among the common white pigments are titanium dioxide and lithophone.

Pure titanium is extracted from minerals, precipitated as the hydroxide, calcined to the oxide and ground. It is a semireinforcing filler comparable on a volume basis with zinc oxide, but is mainly used for its whitening power in tyre sidewalks, hospital accessories, floor tiles etc. And as an excellent heat resistant filler for silicone rubber. The agnate form is preferred where extreme wittiness is required. The retile form gives a rather creamier colour, but is more state at high temperatures.

Lithophone is a mixture of zinc suphide and barium sulphate, usually coprecipitated in equimolecular proportions it is used mainly in cheaper white or coloured compounds as a whitening agent.

Colour

There are two types of colours: inorganic and organic. The inorganic colours are noted for their stability to curing conditions, and for their complete freedom from staining or 'bleeding'.

Important inorganic colours are antimony trisulphide (Crimson and Golden) (cadmium sulphide) (deep red to orange to yellow), Cadmium sulphoselenide (Colours similar to those of sulphides), Chromium oxide (dull green), Iron oxide (deep red yellow), mercuric sulphide (bright red), Nickel titillate (Yellow and ultramerineblue). The inorganic colouring agents in general give rather dull colours. Hence for brightly coloured material is desirable to use more expensive synthetic organic pigments which are available in a very large range of colours and shades. Pastel shades are generally obtained by combining such materials with light colourd inorganic pigments like titanium dioxide. Many of these pigments are available as pastes or as masterbatches in rubber, which greatly assists in dispersion. If powders are used as such, these shall be added early in the mixing cycle. Certain dyestuffs soluble in rubber are also used to produce dedicate shades in translucent materials.

Chemical blowing agents

In the manufacture of cellular rubbers from solid rubbers, many types of chemicals are used. The commonly used one in sponge is sodium bicarbonate, though ammonium carbonate and

bicarbonate are also used. These materials liberate carbon dioxide gas on heating. The blowing operation is pressure dependent and hence these are not favored in microcellular compounds. Organic chemicals such as those based on dinitrosopentamethylene tetramine, benzene sulphony1 hydroxide and azodicarbonamide liberate nitrogen gas on heating and find use either alone or in combination with inorganic blowing agents for applications where controlled cell structure is important. Decomposition of dinitrosopentamethylene treatment is accelerated by fatty acids like stearic acid. Hence it is customary to use a larger proportion of stearic acid than what is usually used, in compounds containing DNPT based blowing agent. The unpleasant odour that is associated with the stearic acid activated decomposition of DNPT can be minimised by the use of urea.