The blending of different additives with raw rubber is known as rubber compounding.

1. Compounding Of Rubber
Presented by
Devansh Gupta
M.Sc Polymer Science
Semester 2

2. Introduction
• Rubber is a polymeric material that are characterized by it’s ability of
reversible deformation under influence of external deformation forces.
• It is classified as natural rubber and synthetic rubber.
• Natural rubber only contains the monomers of isoprene whereas; the
synthetic rubber contains different types of monomers.
• Natural rubber is a coagulated latex obtain from certain types of trees that
are grown in the tropical regions whereas; synthetic rubber are obtained
by mixing butadiene and styrene which are two by products of petroleum
refining.
• Manufacturers of synthetic rubbers use different chemical formulations in
order to meet specific requirements and properties.
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3. Concentration of Rubber Latex
• The natural rubber or latex obtained from the Hevea Brasiliensis plant is
known as field or fresh latex.
• The field latex which is known as unconcentrated latex is not suitable for
commercial use as it contains considerable amount of non-rubber
constituents and too much water in it.
• The latex is therefore concentrated to about 60% rubber solids before it is
used to make rubber products.
• This concentration process is carried by following processes.
1. Centrifuging
2. Creaming
3. Electro Decantation
4. Evaporation
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4. • Centrifuging and Creaming processes works on the principle of increasing
the gravitational force of the rubber particles, by applying centrifugal
force on these particles or by adding a creaming agent like sodium
alginate, gum tragacanth etc. Both these processes of concentration
result in a decrease of non-rubber content and water.
• In electrode cantation process a the negative charge is produced on the
tiny rubber particles, this charge formation is used to extract the rubber
particles from the field latex which makes it concentrated.
• By the evaporation process only the water molecules can be removed from
the field latex. After the evoporation latex contains all the non-rubber
constituents which were present in the original latex.
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5. Compound
• It is a homogenous mixture of ingredients used in a particular
product manufacture such that the properties of most of the
ingredients are unchanged in the final product whereby
providing a defined set of mechanical properties.
Compounding
• It is the science of selecting and combining polymers and
additives to obtain physical and chemical properties for a
finished products.
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6. Rubber Compounding
• The concentrated latex is first blended with the various additives as
required for different applications. The blending of different additives is
known as latex compounding.
• Latex compounding involves not only the addition of the proper chemicals
to obtain optimum physical properties in the finished product but also the
proper control of colloidal properties which enable the latex to be
transformed from the liquid state into finished product.
• Some other objectives of rubber compounding are as follows.
1. To secure certain properties in the finished product to satisfy the service
requirements.
2. To attain processing characteristics necessary for efficient utilization of
available equipment.
3. To achieve desirable properties and processability at the lowest cost.
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7. Major Objectives Of Compound
Development
• Process Requirements –
1. Proper mixing (Incorporation, dispersion, distribution and plasticization)
2. Process safety
3. Stickiness and tackiness
4. Shaping (Calendering, extrusion, assembling and moulding)
• Property Requirements –
1. Hardness
2. Stress-Strain properties
3. Abrasion resistance
4. Hysteresis & set properties
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8. • Resistance To Degradation –
1. Heat, oxygen, ozone
2. Flame
3. Liquids (Chemicals)
4. Light (UV & IR)
• Miscellaneous Property Requirements –
1. Low temperature flexibility
2. Electrical properties
3. Permeability
4. Contact with food stuff and drugs
5. Bonding to metal & non-metals
6. Swelling
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9. Raw Rubber Selection Criteria
• Cost
• Ease of mixing
• Strength requirements
• Modulus or stiffness requirements
• Abrasion resistance requirements
• Elongation requirements
• Oil resistance requirement
• Fatigue requirements
• Service temperature
• Flammability
• Chemical resistance requirement
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10. Compounding Ingredients
• Latex
• Thickening agents
• Wetting agents
• Vulcanizing Agents
• Antidegradants (Antioxidants, antiozonants, protective waxes)
• Processing aids (Lubricants & release agents)
• Fillers (Carbon black, non-black materials like calcium
carbonate)
• Softeners
• Dispersing Agents
• Colour pigments
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11. Thickening agents
• Sometimes, it is frequently necessary to increase the viscosity of latex
compounds. Thus, dipping mixes may require to be thickened so that
thicker deposits of rubber are obtained.
• Latex compounds may be thickened by adding thickening agents.
• A thickening agent or thickener is a substance which can increase
the viscosity of a liquid without substantially changing its other
properties.
• For thickening purposes a wide range of natural products are available,
e.g., gums, casein, glue and gelatine. These are all somewhat
unpredictable in effect, are subject to bacterial attack and although they
may cause high initial increase in viscosity.
• Now a days commonly used thickening agents are sodium carboxymethyl
cellulose, polyvinyl alcohol etc.
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12. Wetting Agents
• Sometimes the addition of a wetting agent to the latex mix is necessary for
successful impregnation of fabrics or fibres with latex.
• Though a medium speed wetting agent, Calsolene oil HS has been found to
assist in obtaining a complete penetration between textile fibres without
any danger of destabilising the latex.
Calsolene Oil HS
• Calsolene oil HS, a highly sulfonated oil, is available as a clear, amber
coloured liquid, readily soluble in water. Unlike some high speed wetting
agents, Calsolene oil HS does not give rise to viscosity changes of the latex
compounds.
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13. Vulcanizing Agents
• Vulcanizing agents react chemically with the raw gum elastomer forming
cross-links between the polymer chains, resulting in a more dimensionally
stable and less heat-sensitive product.
• This process generally involves the chemical reaction of the rubber with
sulphur in presence of an activator (such as zinc oxide) and accelerator.
• For vulcanization process Elastomers need to have unsaturated bonds.
• Its cost is relatively low but its function is essential.
• Amount of sulphur must be low because at high sulfur level, it can show
bloom on the surface.
• Sulfur donor like Tetramethyl thiuram disulphide (TMTD) or Zinc diethyl
dithiocarbamate (ZDC) can be also used.
• Some other Vulcanizing Agents are shown below.
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14. Type Of Vulcanizing Agent Common Use
Sulphur or Sulphur bearing
materials
Natural Rubber, Isoprene, SBR,
Butyl, Poly Butadiene, EPDM,
Nitrile
Organic Peroxides Urethane, Silicone, Chlorinated
Polyethylene, PVC/Nitrile
Metallic Oxide Polychloroprene,
Chlorosulphonated polyethylene,
polysulphide
Organic Amines Acrylic, Fluorocarbon,
Epichlorohydrin
Phenolic Resins Butyl
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15. Antidegadants
• Because of the great surface area exposure of most latex products,
protection against environment is very important.
• To extend the service life of vulcanized elastomer by protecting them from
oxygen, ozone, light, heat, and flex fatigue antidegadants are added.
• Many applications involve light colored products, which must not darken
with age or on exposure to light.
• Non-staining antioxidants such as hindered phenols (styrenated phenols)
must be used.
• Where staining can be tolerated, amine derivatives such as phenylene
diamines, phenyl beta-napthylamine, ketone-amine condensates may be
used. These have good heat stability and are also effective against copper
contamination, which cause rapid degradation of rubber.
• Selection Criteria for choosing accurate antidegredant involves Volatility,
Solubility, Chemical Stability, Concentration, Health & Safety.
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16. Processing Aids (Lubricants)
• Two types of lubricants are generally used in compounding.
1. External lubricants
2. Internal lubricants
• External lubricants prevent sticking of the compound to the
processing equipments by forming a film between the bulk of
the compound and the metal surface of the processing
equipment.
• Internal lubricants improves flow behavior of the materials.
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17. Fillers
• Fillers are materials used to extent the range of physical properties, to
reduce compound cost, modify the processing properties, to increase the
viscosity and to influence the chemical resistance of the compound.
• Most of the non black fillers such as china clay, mica powder, whiting
(calcium carbonate), Lithopone, Blanc Fixe (barium sulphate) may be
used in latex compounds.
• Carbon black does not reinforce latex in the manner that it does dry
rubber, and is used only in small amounts in latex for color, as are
various other dyes and pigments.
• Selection Criteria for choosing accurate filler involves cost, processing
requirements, colour requirement, reinforcement requirement and any
other special requirement.
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18. Softeners
• A Softener is an ingredient that –
1. Speeds up the rate of polymer breakdown
2. Helps to disperse the other compounding ingredients
3. Helps to reduce nerve within the compound
4. Can impart building tack
5. Improves the processing properties and modify the final compound
properties
6. Adjusts the compound cost and reduce power consumption during
processing
• In applications like toy balloons, softeners are added to soften them so
that they may be easily inflated.
• Generally paraffinic oil, naphthenic oil and aromatic petroleum mineral
oils and some plasticizers are used as softeners.
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19. Dispersing Agents
• The particle size of solid materials added to latex must usually be made as
small as possible to ensure intimate contact with the rubber particles.
• Solid materials are usually added to latex as dispersion.
• The material to be added is mixed with dispersing agents in deionized
water and ground to a small particle size in a ball mill or attritor.
• In these devices stones or other hard pebble-sized materials are made to
tumble and mix with chemicals reducing them to very small size.
• The concentration of dispersing agents rarely exceeds 2% except in special
circumstances.
• None of the common materials such as gelatin, casein, glue or soap such
as ammonium oleate possesses all the requisite properties and hence it is
necessary to use mixtures of two or more of them.
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20. Compounding Criteria
• During compounding, it is essential to avoid the addition of any material
liable to cause coagulation. As already discussed, the latex compound
should be properly stabilized. In general, the addition of water-soluble
organic liquids, salts of polyvalent metals and acidic materials are to be
avoided.
• Water-insoluble liquids and solids must be added as emulsions and
dispersions respectively, in which the size of the individual particle is of
the same order as that of the rubber particles in the latex.
• Care should be taken to avoid the use of hard water at any stage of latex
compounding as it has a destabilizing action on latex.
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21. • The containers for the latex storage must be made from stone, enamelled
iron, stainless steel, and wood lined with rubber or gutta-percha.
• During the addition of the compounding ingredients, the mix should be
stirred slowly but thoroughly. Slow stirring of the latex mix assists in the
removal of bubbles and minimizes the formation of a skin, which arises
from evaporation of water in the latex.
• It is important to avoid contact between the stirrer and the container, since
latex is readily coagulated by friction.
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