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Non-black fillers and other compounding     ingredients

Addition of fillers in rubber compounds is mainly for the following purposes.

1. Reinforcement
2. Economy
3. Improved processability

Fillers are of two classes the blacks and the non-black fillers. Carbon blacks in 
general are superior to the non-black fillers. But they are not suitable for the production of white or light coloured products. A number of materials are included among the non-black fillers. But only a few among tem such as clays, silicoses, silioas and carbonates are widely used.

General considerations

A rubber compounder must consider and control a variety of factors in developing a recipe for a rubber product. Therefore he must now how the various properties are affected by the addition of non-black fillers. Some of the important properties to be considered are: strength, stiffness, elongation, hardness, permanent set, resiscauce to tearing, flexing and abrasion, resistance to deterioration specific gravity, colour, processability, time and temperature of cure, compatibility with adjacent compounds, ahension to fabrics or metals and cost. Most of these of these properties are affected to varying degrees by the non-black fillers.

Tensile strength depends on many factors besides the filler. Some of these are type and quality or rubber, amount f softeners, degradation resulting from mixing and the degree of vulcanisation. The effect of the filler itself depends on many factors. The quantity used is very important. There is an optimum quantityfor any given reinforcing filler, above or below which maximum strength will not be obtained.

The physical and chemical properties of the filler play their part. From the physical standpoint, the factors are the particle size, surface area per unit eight, surface activity and the electrical charge on the particle. Chemically, it is important to know whether the material is acidic or alkaline and whether it will react chemically with the accelerators.

In general. The best reinforcing fillers are those having the smallest particle size. But this means only the effective particle size as it actually exists in the rubber compound. Not only the presence of individual course particles in the filler, but also the presence of agglomerated masses by particles will probably reduce tensile strength. Therefore, in addition to the particle size of a filler, one must also consider its dispensability, ie, how well the agglomerates of the filler are broken up by the shearing forces during mixing.

Two fillers may have the same particle size and yet not produce the same effect of tensile. Because they vary considerably in their surface area. Naterials with high surface area generally retard cure by dsorption of accelerators.

In addition to particle size and surface area there are other factors which are equally important. These are possibly the adhesion between the filler particles and the rubber matrix or the ability of the rubber to wet out the filler or something else which depends upon the surface activity of the filler regardless of whether that activity is chemical or electrical.

The effect of the filler on the accelerator and the vulcanizing agent must be considered from the chemical as well as the physical standpoint. Chemical activity of the filler may cause either undercure or overcure either of which may cause lower strength. An alkaline filler increases the rate of cure and an acidic one retards it. In some cases a filler may react with an accelerator as in the case of calcium silicate and ABT. The calcium MBT thus formed is an inactive accelerator.

Among the non-black reinforcing fillers, the best tensile strength is given by precipitated silica. Calcium silicate and chemically altered clacy would be next in line, followed by zinc oxide, ultrafine carbonates and hard clay. The same factors that influence tensile strength also affect elongation. Is reduced elongation is also reduced by fibrous materials such as asbestos. In general, the effect of the addition of fillers is to reduce the elongation. For high elongation it is best to use the medium particle size precipitated calcium carbonate, ground limestone or blane fixe. Stiffening pigments such as precipitate silica, silicates etc. should be avoided. If high modulus is sought, the above order should be reversed. Materials that favour high elongation have the least effect on modulus.

Resilience of compounds containing different fillers is generally inversally proportional t the reinforcement imparted by them. Whiting stocks have good resilience, while calcium silicates or clay stocks are relatively poor in this respect. Precipitated silicas and zinc oxide are exceptions as they impart good resilience and also good reinforcement. Resilience decreases with loading in the case of any filler. Fure gum stocks have the highest resilience.

Hardness is generally increased by increasing the proportion of filler to rubber. It also depends upon the shape and size of particles. Particles which are needle shaped or lamellar tend to arrange themselves in parallel lies during mixing, tubing or calendering and will produce harder stocks than spheres of the same material. This is shown clearly by calcium carbonates made from ground shell (lamellar) and from ground limestone (Sherica10).

Size and shape of the filler particle also determine how much tear resistance it imparts. The best tear resistance is obtained from fine particle size, spherically shaped fillers. Lamellar shaped particles give poor results. Among non-black fillers the highest tear resistance is given by precipitated silica followed by calcium silicate and chemically treated clays. Precipitated calcium carbonate a give fair results and are used where softness and reasonable tear resistance are required, as in hot water bottles. Ground whiting, clay, magnesium carbonates and similar filler give poor tour resis stance.

Fine particle size fillers have the best resistournce to flex apparently because coarser pargcles act as nucleia from which cracks will spread. Filler that produce a grain effect and overloading with any type of filler are to avoided. Zinc oxide, precipitated silica and calcium silicates are good fillers for resistance to flexing.

The highest resistance to abrasion is given by the finest particle size fillers. Precipitated silica gives the best results. Calcium silicate, zinc oxide and clay follow in that order, but considerably down the soale from silica. The calcium carbonates impart little resistance to abrasion.]
When acid resistance is required, acid soluble materials like the carbonates and zinc oxide shall be avoided. Instead materials which are inert in acid such as silica, clay or barytes are used.

To obtain the best electrical properties one should use fillers which are free of water-soluble material and which are not hydrated. Some carbonates are good in this respect, but not all. The acid washed clays are better in this respect that the dry ground clays. Hydrated silica and calcium silicates are not good for insulation purpose, although they are suitable for cable jacket stocks.

Some individual types of non-black fillers which are more important in the rubber industry are described in more detail below:

Zinc oxide

Zinc oxide used in the rubber industry are made by the American process, where it is made from zinc ore and the French process, where it is made from actallic zinc. The American process oxides often contain small quantities of lead.

Cadmium irons or sulphur. Besides being poisonous, the presence of lead ad cadmium can cause discoloration of vulanizates and affect the cure rate. Sulphur may be present as sulphite on the surface of zinc oxide. This may retard aldehyde-amine-accelerated systems.

Although, at present, the major use of zinc oxide in the rubber industry is as activator for organic accelerators, it as used as a filler in rubber where high resilience, heat resistance and thermal conductivity are required. Motor mounts vibration absorbers and similar items still use zinc oxide loading because of the high heat conductivity and low heat build-up. Wire and cable compounds also make use of zinc oxide because it imparts good electrical charateristios such as low power factor, high resistively and low water absorption. In addition to these zinc oxide is used as curing agent in polychloroprene, cholorobuty1 etc. Specific gravity of zinc oxide is 5.6 and hence compounds containing substantial loading of zinc oxide tend to be heavy. Dispersion of ordinary zinc oxide is a little difficult and hence it is preferable to aid it early in the mixing cycle. Coated zinc oxides dipper a more easily.

Calcium carbonates

There are two major classes of calcium carbonates: the ground whiting and the precipitated calcium carbonates. Ground whiting is available as dry ground or wet ground. Dry ground whiting, which is one of the cheapest rubber compounding ingredients is made by grinding soft limestone. It can be incorporated into rubber in very large quantities and are used in such items as mats and cheap moulded goods. Water ground whiting can be made wit more uniformity and in smaller particle size. This is slightly costlier than the dry ground type, but contains much less foreign matter and are often quite white in colour. It is still a poor reinforcing agent, but slightly better than the dryground type. Ground whiting made from oyster shells is laminar and is recommended for extruded goods and it gives the highest modulus of any calcium carbonates.

All the precipitated calcium of bonated are made from lime stone. The carbonate is first converted into another calcium compound and this is later converted back into the carbonate under conditions that permit control of particle size. The limestone is first burnt to quicklime which is then slake toget milk of lime. The milk of lime on then treated without carbon dioxide from the kiln or better with sodium carbonate solution to get calcium carbonate. The precipitate is then filtered, dried and powdered. In some processes some type a coating is given to the particles prevent coalescence the also to aid dispersion.

Calcium carbonates produce easily mixed, soft, tacky natural rubber stocks. It does not affect cure. Ground whiting impart very little tear resistance. But the precipitate types impart reasonably good tear resistance particularly to hot stocks. For this reason, it is used in products like hot water bottles.


Clay also is a low-cost filler out unlike whiting, it has a pronounced stiffening effect, it also imparts hardness and a fairly good abrasion resistance.

Clays are the natural decomposition products of feldspar and other alumina-silica minerals. The clay deposits occur in different regions of the world, usually to 60 ft. below the surface of earth. Missing is done using power shovels. The crude clay is then ground and pecked. Some clay are water ground to get more uniform particles. The brownish colour of some types of clay is due to iron. This can be removed by acid digestion and washing.

Rubber clays consist principally of flat platelets, which accounts for itseffect on modulus and hardness. The effect o modulus varies considerably with different clays, those causing the most stiffening being classed as 'hard clays' and those causing less stiffening being called 'soft clays'. The two types differ slightly in particle size. Hard clay is a good materiel for use where stiffness at low cast is desired. For this reason, it is used it footwear compounds, mechanical goods and wire compounds. It is useful in extrusion compounds to prevent sagging or collapsing during cure. The reinforcing value and abrasion resistance of soft clay is less when that of hard clay and it is used when lass stiffening is a needed. Both types are acid resistant and hence used in tank lining compounds. Clay loading usually ranges from 50 to 300 phy. 

Fixing of clay into rubber does not cause difficulties. It has a slight retarding effect on cure especially at higher loading, because they adsorb accelerators and generally have an acid pH. This tendency is overcome by the use of a small quantity of triethanolamine or diethylene glycol. 

Regenerated clay is produced by decomposing ordinary clay with sulphuric acid and the resulting aluminum sulphate solution containing silica is treated with sodium silicate solution. The precipitate formed is filtered, washed, dried and ground. Regenerated clay is having smaller particle size and is more reinforcing then ordinary clay.

Calcium Silicate

Hydrated calcium silicate is used in the rubber industry whenever hardness, stiffness, abrasion resistance and tear resistance are required. It is made by the reaction between calcium chloride and sodium silicate in solution under conditions which produce pigment grade calcium silicate. The precipitate is filtered, washed, dried and ground.

Generally there is retarding effect by calcium silicate especially in NR and SBR, because of the adsorption of accelerators. This is overcome by the addition of glycoks. The common accelerator except MBT is not satisfactory Probably because of the formation of the inactive calcium salt.

Reinforcement imparted by calcium silicate is much better than that by clay. In Neoprene compounds it does not affect cure rate, tensile strength and tear resistance. It increases hardness considerably and this does not change much during continuous exposure to boiling water.

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