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 suitable
Barytes : 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 fillers
3. Provide lubrication to improve extrusion, moulding or other shaping operations.
4. Reduce batch temperature and power consumption during mixing
5. 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 plesticiser
d) 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. Compatibility
2. Cost
3. Efficiency
4. Staining characteristics
5. Low temperature properties
6. 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.