Rubber Chemicals & additives 

Significant advances in the field of rubber compounding over the years have helped expansion of the rubber chemical and additives manufacturing industry. 

Discovery of the process of vulcanization of rubber in 1839 by Charles Goodyear has enhanced the need for chemical and additives (C&As) for transforming rubber into more useful products.  A large number of C&As have helped to expand the application of rubber in variety of rubber compounding have enabled larger control over specifically desired end-characteristics in products with substantial cost savings in some cases.  These advances have helped expansion of the rubber C&A manufacturing industry. 

A large number of C&As are at present used for making rubber products, generally classified as chemicals/ additives used as aids and means of curing, chemicals used as aids means of curing.  Chemicals used as aids to processing, those used as aids to improving quality, those used as diluents, and special additives. 

Curing aids and means

  The materials coming under this group comprise vulcanising agents, accelerators and activators.  Vulcanising agents function as cross-linking agents.  Sulphure is the most common among them.   The normal rubber-grade sulphur is in the rhomic form, ground so fine to pass sieve of about 200 mesh/inch.  This sulphur is soluble in carbon disulphide and chloroform and insoluble in water. 

Another grade, known as insoluble sulphur, is also used in certain applications.  This type, known as MC sulphur, is especially suitable in butyl and nitrile mixes where good dispersion of sulphur is necessary.  In this material the Sulphure particles are coated with about 2.5% magnesium carbonate to reduce agglomeration and is as fine as the rhotmic form. 

Several other materials are also used as vulcanizing agents.  They are: sulphur monochloride, sulphur donors, metallic oxides, phenolic resins, pquinone dioxies, and peroxides. 

Accelerators

 Accelerators are organic chemicals that reduce the time and temperature for vulcanization of rubber.  They also modify the vulcanization (curing) reaction to produce cured (vulcanized) rubber of better quality.  The different materials used as accelerators can be classified in a variety of ways: 

·        Property classification – peaky, flat, curing, quick set-up, and delayed action. 

·        Speed classification – slow, medium, semi0ultra (fast-medium) and ultra (fast). 

·        Chemical classification – aldehyde reaction products such as amines, guanidines, thioureas, thiozoles, thiurams, sulphanamides, dithiocarbamates and xanthates. 

Activators 

These chemicals activate vulcanization reaction.  In other words, they trigger the accelerator and increase the vulcanization rate.  Activators in common use are classified into three groups as below: 

·        Inorganic compounds: Metal oxides such as zinc oxides, hydrated time, litharge, manganese, red lead, alkali carbonates and hydroxides. 

·        Fatty acids: Usually high-molecular weight monobasic acids or mixtures of the following types-stearic, oleic, lauric, palmitic acids-and hydrogenated oils- palm, gastor, fish and linseed oils.

 ·        Amines (alkaline substances): Diethanol amine, triethanol amine. 

Processing materials 

The materials used as aids to processing are known as Plasticisers/softeners and extenders.  Plasticisers are used to increase to plasticity and workability of the rubber compound, to aid the wetting and incorporation of fillers, provide lubrication, to improve extrusion, moulding or other shaping operations, reduce batch temperature and power consumption during mixing, and to modify the properties of rubber products.

 They are divided broadly into two chemical and physical Plasticisers.  The former acts by reducing the rubber’s molecular weight by “chain session” and the latter acts as an intermolecular lubricant. 

Chemical plasticiser are quite different from one another and are effective under different conditions.  Some of them are: Aromatic mercaptans (eg. Thionaphthol and xylyl mercaptan) which have a strong softening effect on NR and reclaim rubber: phenyl hydrazine salts, thiuram disulphides, benezamide-diphenyl disulphide, petroleum sulphonates and paentachloro-thiophenol.

Most of the important physical Plasticisers come from any one of the following sources: Petroleum (mineral oils, resins and waxes), pine tree (pinetar, pitch, resins), coal tar (tar oil, pitch and resins), natural fats and oils (vegetable oil, blown oils, fatty acids), and synthetic organic compounds. 

Petroleum oils are the most common among the above.  They range from highly aromatic naphtenic to paraffinic oils.  All these are produced in various grades of viscosity and staining power. 

Extenders

  These are substances added to rubber compounds in large quantities to reduce cost, without seriously affecting the final properties.  The most important commercially used extenders are relaimed rubber (RR) and Factice.  Use of RR as an extender has economic advantages and other advantages such as short mixing time, jow power consumption.  Low heat development, reduced swelling and shrinkage, reduced swelling and shrinkage, good ageing etc. 

Factice is a class of elastic gum made by reacting certain unsaturated, vegetable oils with sulphur. Factice can be blended with NR and with some synthetic rubbers in rather high proportions and the blend will remain highly elastic. 

Quality-enhancers 

Reinforcing fillers (RFs) and antioxidants come under this group.  The RFs can be broadly divided into two categories: a) Carbon blacks, silica and silicates of aluminum and calcium; b) Semi-reinforcing hared clays, activated calcium carbonates, magnesium carbonates etc. 

Among the RFs used in the rubber industry, a lion’s share is constituted by different types of carbon blacks. 

Carbon blacks 

Carbon blacks (CBs) are essentially carbon prepared by converting liquid or gaseous hydrocarbons into elemental carbon by partial combustion or thermal decomposition.  Depending on the process adopted for the preparation, CBs are grouped as furnace blacks, thermal blacks, channel blacks and lampblacks. 

Furnace blacks are produced by incomplete combustion of natural gas or heavy aromatic residue oils from the petroleum industry,  in refractory lined steel furnaces and separating the carbon by means of filter bags.  They are pelletised and packed. 

In the thermal process, natural gas or oil is thermally decomposed at about 1300 deg. C

in the absence of free air, in a cylindrical furnace filled with an open checker work of silica bricks. 

The channel blacks are produced by feeding natural gas or oil into thousands of small burner tips where the small flames impinge either on a large rotating drum or onto reciprocating channel irons.  The blacks deposited is scraped and collected. 

Lamp black is made by burning oil and allowing the black formed to settle down by gravity in a series of chambers. 

The commercially available carbon blacks used in rubber products manufacture are the following. 

Name                                      Abbreviations             ASTM D1765 Number

 

Intermediate super-

Abrasion furnace                      ISAF                                        N-220

Easy processing                        EPC                                         S-300

Channel

High abrasion furnace               HAF                                        N-330

Fine furnace                              FF                                            N-440

Fast extrusion furnace               FEF                                         N-550

General Purpose                       GPF                                         N-660

Furnace

Semi-reinforcing furnace           SRF                                         N-770

Fine thermal                              FT                                            N-880

Medium thermal                        MT                                          N-990

 Different grades of CB available are now considered the most versatile RFs for the rubber industry. 

Non-black RFs

 For reinforcement purpose, a number of non-black fillers can also be used.  Among the non-black RFs, the best is precipitated silica.  Calcium silicate and chemically altered clay would be next in line, followed by ultrafine carbonates and hard clay. 

Chemicals and additives used as diluents 

 A number of materials are used in the rubber industry as diluents.  They are generally termed as non-reinforcing fillers.  The most important among them are the following: 

Whiting, soft clays, barites and ithopone. Among these, whiting (mineral calcium carbonate) is the most important filler used in high loading in low cost compounds.  Soft clays which are of white powder are also used at high loading for low cost of coloured mechanical goods.  Barytes is a very high sp.gravity (4.5) material used as an inert filler particularly in chemical and acid (sulphuric) resistant tanks lining.  Lithophone is a mixture of zinc sulphide and barium sulphate (precipitated).

Special materials

 A number of materials generally termed as miscellaneous compounding ingredients are used in the rubber industry for manufacturing products requiring specific property.  A few typical examples are given below:

 Function Example 

 A brasive Pumice (ground) Ground glass

Blowing agent DNPT (DI nitroso pentamethy lene tetramine)

Colourants titanium dioxide (white) phthalocyanine blue (blue) iron oxide (red)

Flame retarder antimony oxide, chlorinated paraffin

Odorants methyl, salicylate, vanilla, lemon grass oil.

Promoter p-dinitrosobenzene

Retarder N-nitroso dipheny lamine salicy acid

Antimildew agent ortho phenyl phenol

Antioxidants condensation product of acetone and dipheny lamine phenyl-beta-napthy lamine

The details given above show that a number of chemicals/ additives are used of make rubber products.  The industries manufacturing these chemicals/additives are known as rubber chemicals/additives manufacturing industries.

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