Rubber: An Effective And Suitable Elastomer
For Manufacturing Engineering
And Metal Working Industries

By Dr Thoguluva Raghavan Vijayaram PhD

Senior Lecturer Department of Manufacturing Process and System Faculty of Manufacturing Engineering, UTeM Universiti Teknikal Malaysia Melaka Ayer Keroh, 75450 Melaka Malaysia Email: vijayaram1@gmail.com

Rubber is a collective term for macromolecular substances of natural origin known as natural rubber (NR) or synthetic origin or manmade known as synthetic rubber (SR). Rubbers are elastomers, whose dimensions can be greatly changed when stressed and which return to their original dimensions when the deforming stress is removed. A rubber is defined as being capable of recovering from large deformations quickly. The term rubber was originally used only for the natural product that is obtained from a thick, milky fluid called as latex that oozes from certain plants when they are cut. Most latex comes from the Para rubber tree that grows in South America, and Sri Lanka. Prior to Second World War, this material was important in the manufacture of many products, and sources of supply were considered to be valuable.

An indispensable product to modern society as steel, wood and mortar is rubber. Rubber is of two types—natural and manmade. All of us use products made of rubber at home, at work, at play and even when we travel. Automobiles, trains, aircraft, industries rely on it for variety of purpose. That is why it is rightly said that rubber in the modern world is omnipotent. Rubber is a yellowish, elastic, amorphous material obtained from the latex or milky sap of various tropical plants like the rubber tree. This latex is vulcanized, pigmented, finished and modified into various products like electric insulation, elastic bands, belts, tires, hoses, gaskets and containers.

The applications of rubber are wide spread. The material is used for coatings tubing, hose, and tires. Rubber processing methods include many of the same processes as used in plastics. Among these are various molding processes for products such as tires. Extrusion is used for tube moldings, and hose. Calendering is used to make sheet products and to coat fabrics and metal with powder. It is classified as natural rubber and synthetic rubber. Natural rubber is the oldest commercial elastomer and it is made from the processed sap of a tropical tree. In its crude form, it is an excellent adhesive and many cements can be made by dissolving it in suitable solvents. Its use as an engineering material dates from 1839, when Charles Goodyear discovered that it could be vulcanized by the addition of about 30% sulphur followed by heating to a suitable temperature. The cross linking restricts the movement of the molecular chains and imparts strength.

Rubber can now be compounded to provide a wide range of characteristics, ranging from soft and gummy to extremely hard. When additional strength is required, textile cords or fabrics can be coated with rubber. The fibers carry the load and the rubber serves as a matrix to join the cords while isolating them from one another to prevent chafing. For severe service, steel wires can be used as the load bearing medium. Vehicle tires and heavy duty conveyor belts are examples of this technology. Rubber exhibits unique physical and chemical properties. Rubber's stress-strain behavior exhibits the Mullins effect, the Payne effect and is often modeled as hyperelastic. Owing to the presence of a double bond in each and every repeat unit, natural rubber is sensitive to ozone cracking. The material properties of natural rubber make it an elastomer and a thermoplastic. However it should be noted that as the rubber is vulcanized it will turn into a thermoset.

When rubber is stretched the "loose pieces of rope" are taut and thus no longer able to oscillate. Their kinetic energy is given off as excess heat. Therefore, the entropy decreases when going from the relaxed to the stretched state, and it increases during relaxation. This change in entropy can also be explained by the fact that a tight section of chain can fold in fewer ways than a loose section of chain, at a given temperature. Relaxation of a stretched rubber band is thus driven by an increase in entropy, and the force experienced is not electrostatic, rather it is a result of the thermal energy of the material being converted to kinetic energy.

Rubber relaxation is endothermic, and for this reason the force exerted by a stretched piece of rubber increases with temperature. The material undergoes adiabatic cooling during contraction. This property of rubber can easily be verified by holding a stretched rubber band to your lips and relaxing it. Stretching of a rubber band is in some ways equivalent to the compression of an ideal gas, and relaxation in equivalent to its expansion. Note that a compressed gas also exhibits "elastic" properties, for instance inside an inflated car tire. The fact that stretching is equivalent to compression may seem somewhat counter-intuitive, but it makes sense if rubber is viewed as a one-dimensional gas. Stretching reduces the "space" available to each section of chain. The use of rubber is widespread, ranging from household to industrial products, entering the production stream at the intermediate stage or as final products.

Natural rubber is produced commercially from the latex of the Hevea brasiliensis tree which is cultivated in plantations mainly in the tropical regions in Southeast Asia, especially in Malaysia and Indonesia. The source of natural rubber is a milky liquid known as latex which is a suspension containing very small particles of rubber. The liquid latex is collected from the trees and taken to a processing center where the field latex is diluted to about 15% rubber content and coagulated with formic acid. The coagulated material is then compressed through rollers to remove water and to produce a sheet material. The sheets are either dried with currents of hot air or by the heat of a smoke fire. The rolled sheets and other types of raw rubber are usually milled between heavy rolls in which the mechanical shearing action breaks up some of the long polymer chains and reduces their average molecular weight. Natural rubber production in 1980 accounted for about 30 percent of the total world’s rubber market.

Natural rubber is mainly cis-1,4 polyisoprene mixed with small amounts of proteins, lipids, inorganic salts, and numerous other components. Cis-1,4 Polyisoprene is a long chain polymer, having an average molecular weight of about 500000 gm/mol. The polymer chains of natural rubber are long, entangled, and coiled, and at room temperature are in a state of continued agitation.

The tensile strength of natural rubber is increased by vulcanization. Vulcanization is a chemical process by which polymer molecules are joined together by cross linking into larger molecules to restrict molecular movement. In 1839, Charles Goodyear discovered a vulcanization process for rubber by using sulphur and basic lead carbonate. The word Vulcan indicates the roman god of fire. He found that when a mixture of natural rubber, sulphur, and lead carbonate was heated, the rubber changed from a thermoplastic to an elastomeric material. Although, even today the reaction of sulphur with rubber is complex and not completely understood. Rubber and sulphur react very slowly even at elevated temperatures, so that to shorten the cure time at elevated temperatures accelerator chemicals are usually compounded with rubber along with other additives such as fillers, plasticizers, and antioxidants. Usually, vulcanized soft rubbers contain about 3 weight % sulphur and are heated in the 100 to 200 degree C range for vulcanizing or curing. If the sulphur content is increased, the cross linking that occurs will also increase, producing harder and less flexible material. A fully rigid structure of hard rubber can be produced with about 45% sulphur. The use of fillers can lower the cost of the rubber product also strengthen the material. Carbon black is commonly used as a filler for rubber, the finer the particle size of the carbon black, the higher the tensile strength is. It also increases the tear and abrasion resistance of the rubber. Silica such as calcium silicate and chemically altered clay are also used as fillers for reinforcing rubber.

Some of the properties of vulcanized natural rubber: cis-polyisoprene is shown below in Table-1.

Table-1: Properties of vulcanized natural rubber: cis-polyisoprene

Tensile strength	2.50 to 3.50 ksi
Elongation	750 to 850 %
Density	0.93 gm/cc
Recommended operating temperature	-50 to 82 degree C

Natural rubber compounds are outstanding for their flexibility, good electrical insulation, low internal friction, and resistance to most inorganic acids, salts, and alkalies. However, they have poor resistance to petroleum products, such as oil, gasoline, and Naptha. In addition, they lose their strength at elevated temperatures, so it is advisable that they not to be used at temperatures above 80 degree C. They also deteriorate fairly rapidly in direct sunlight unless specially compounded. Natural rubber has good resistance to abrasion and fatigue, and it has high frictional properties, but it has low resistance to oil, heat, ozone, and sunlight. Typical applications are tires, seals, shoe heels, couplings, and engine mounts.

Natural rubber is a vital agricultural product or commodity which is used in the manufacture of a wide range of products. Rubber plays a major role in the socio-economic fabric of many developing countries. Products made from natural rubber, like tyres, engineering components and latex products are very essential to modern life.

Natural rubber is available in many grades. However, the most important distinction is that between latex and solid grades. Latex is the liquid which comes out of the tree. Solid grades are produced from latex which has coagulated either in a factory or in the field.

Natural rubber has certain unique properties such as follows:

Natural rubber combines high strength with outstanding resistance to fatigue.
It has excellent green strength and tack which means that it has the ability to stick to itself and to other materials which makes it easier to fabricate.
It has moderate resistance to environmental damage by heat, light and ozone which is one of its drawback.
The natural rubber has excellent adhesion to brass-plated steel cord, which is ideal in rubber tyres.
It has low hysteresis which leads to low heat generation, and this in turn maintains new tyre service integrity and extends retreadability.
The natural rubber has low rolling resistance with enhanced fuel economy.
It has high resistance to cutting, chipping and tearing.

Natural Rubber is used due to the following reasons:

Natural rubber forms an excellent barrier to water.
This is possibly the best barrier against pathogens, so latex is used in surgery as surgical and medical examination gloves.
Natural rubber is an excellent spring material.
Natural rubber latex is also used in catheters, balloons, medical tubes, elastic thread, and also in some adhesives.
Other than rayon, it is the sole raw material, which is used by the automotive industry.
Rubberwood is another byproduct of natural rubber which is growing in importance. It is a source of charcoal for local cooking.

The technical strengths of natural rubber matches well and to be used as a suitable tyre material. These have been succinctly summarized by Baker as follows:

high green strength, tack and cohesive properties: these are essential for maintaining green tyre uniformity and stability during building and shaping operations;
excellent adhesion to brass-plated steel cord;
low hysteresis which imparts low heat generation, which in turn maintains new tyre service integrity and extends retreadability;
low rolling resistance with enhanced fuel economy;
excellent snow and ice traction for winter tyres and all-season treads; and
high resistance to cutting, chipping and tearing.

Synthetic rubber is a white, crumbly, plastic mass which is processed and vulcanized in the same manner as natural rubber. In other words, synthetic rubber is an artificially produced material having properties similar to natural rubber. Most synthetic rubbers are obtained by polymerization or polycondensation of unsaturated monomers.

There are wide varieties of different synthetic rubbers, reflecting the various different applications and the chemical and mechanical properties they have. Co-polymerization of different monomers leads to the material properties to be varied across a wide range.

Though, World war II became the force for the emergence of synthetic rubber on a large-scale basis when governments began building plants to balance natural rubber shortages, there were other various reasons as well after the war which led to the development of an alternative or substitute for natural rubber.

Some important factors resulting to the production of synthetic rubber are

Rising prices for natural rubber on the world market in response to the general state of the economy
Political events which cut customers off from the suppliers of raw materials
Long transport distances
Regional constraints with respect to establishing rubber plantations
The increase in global demand for rubber.

Like natural rubber, synthetic rubber has a varied range of applications, such as follows:

In the tyre industry to manufacture car, aircraft and bicycle tires
Drive belts
Hoses
Medical equipment
Seals
Floor coverings
Conveyor belts
Molded parts etc.

There are different varieties of synthetic rubber, each having their unique properties. Some of the common properties of synthetic rubber are as follows:

Better abrasion resistance
Good Elasticity
Better heat and aging resistance
Electrical insulation material
Flexible at low temperatures
Flame retardant
Resistant to grease& oil etc.

Synthetic rubbers were first created in the early 1930s and were subsequently developed because of the uncertainty of the supply of natural rubber. This first rubbery synthetic was derived from acetylene gas and was a long chain molecule called polychloroprene, better known as neoprene. Synthetic rubbers in 1980 accounted for about 70 % of the total world’s supply of rubber materials.

Some of the important synthetic rubbers are styrene-butadiene, nitrile rubbers, and the polychlorprenes. Synthetic rubbers are further developed than natural rubbers. Examples are synthetic natural rubber, butyl, styrene butadiene, polybutadiene, and ethylene propylene.

Compared to natural rubbers, they have better resistance to heat, gasoline, and chemicals, and they have a higher range of useful temperatures. Examples of synthetic rubbers that are resistant to oil are neoprene, nitrile, urethane, and silicone. Typical applications of synthetic rubbers are tires, shock absorbers, seals, and belts.

Styrene-butadiene rubber (SBR) is an important synthetic rubber and also the most widely used one. Actually, it is a butadiene-styrene copolymer. After polymerization, it contains 20% to 23% styrene. SBR rubber is lower in cost than natural rubber and so is used in many rubber applications. For example, for tire treads, SBR has better wear resistance but higher heat generation.

Nitrile rubbers are copolymers of butadiene and acyrolonitrile with the proportions ranging from 55% to 82% butadiene and 18% to 45% acyrlonitrile. These rubbers are more costly than ordinary rubbers, so these copolymers are limited to special applications such as fuel hoses and gaskets where high resistance to oils and solvents is required.

The polychloroprene or neoprene rubbers are similar to isoprene. They also have fair fuel and oil resistance and increased strength over that of ordinary rubbers. However, they do have poorer low temperature flexibility and are higher in cost. As a result, neoprenes are used in specialty applications such as wire and cable covering, industrial hoses and belts, and automotive seals and diaphragms.

Silicone rubbers or Silicones have the highest useful temperature range up to 315 degree C, but such other properties as strength and resistance to wear and oils are generally inferior to those in other elastomers.

Typical applications of silicone rubbers are sealants, gaskets, thermal insulation high temperature electrical switches, electrical insulation, auto-ignition cable, spark-plug boots, and electronic apparatus.

A few fact sheets of rubber are summarized as follows:

Fact Sheet Number:1 List of Industrial Products

Anti vibration mountings	Automobile rubber products	Calendered rubber products
Extruded rubber products	Medical rubber products	Metal bonded components
Rubber adhesives	Rubber ball	Rubber bands
Rubber beading	Rubber bearing	Rubber belt
Rubber lining	Rubber buckets	Rubber magnets
Rubber bullets	Rubber molded products	Rubber cable
Rubber pads	Rubber coating	Rubber rollers
Rubber duct	Rubber stopper	Rubber expansion joints
Rubber suit	Rubber flooring	Rubber track
Rubber footwear	Rubber valve	Rubber gloves
Rubber injection parts	Rubber sealants	Rubber matting

Fact Sheet Number: 2 List of different synthetic rubbers

Acrylic rubber	Isoprene rubber	Silicone rubber
Butadiene rubber	Nitrile rubber	Styrene butadiene rubber
Butyl rubber	Perfluoroelastomer rubber	Neoprene rubber
Chlorosulfonated polyethylene / Hypalon	Polychloroprene rubber	Polysulfide rubber
Ethylene propylene diene monomer	Fluoroelastomers/Viton	Polychloroprene or polyneoprene rubber

Fact Sheet Number:3 List of Industries where Rubber is used

Agricultural industry	Defense industry	Printing and paper industry
Aerospace / aviation industry	Medical industry	Textile industry
Automobile industry	Mining industry	Petroleum industry
Chemical industry	Construction industry	Power generation industry

Raw rubber is composed of long molecules of high molecular weight and is therefore quite springy and resistant to being worked. First, these molecules must be broken up so the rubber can be molded and formed. For this purpose, crude rubber may be extruded in a plasticator, masticated by revolving beaters in a Banbury mixer, or plasticized between rolls in a rubber mill. The last may be performed alone or after one or both of the other treatments.

In the rubber mill, the rubber is squeezed between two rolls turning toward each other on the entering side. One revolves up to one third faster than the other and induces a severe shearing as well as compressive action in the rubber. The sheet coming out of the rolls is commonly fed back for a time into the entering mixture for thorough blending. In the mixer and mill, the rubber is impregnated with the substances added to it for the final product. The second step is to mold or form the rubber as desired and vulcanize it.

Compared to vulcanized rubber, uncured rubber has relatively few uses. It is used for cements; for adhesive, insulating, and friction tapes; and for crepe rubber used in insulating blankets and footwear. Vulcanized rubber, on the other hand, has numerous applications.

Resistance to abrasion makes softer kinds of rubber valuable for the treads of vehicle tires and conveyor belts, and makes hard rubber valuable for pump housings and piping used in the handling of abrasive sludge. The flexibility of rubber is often used in hose, tires, and rollers for a wide variety of devices ranging from domestic clothes wringers to printing presses; its elasticity makes it suitable for various kinds of shock absorbers and for specialized machinery mountings designed to reduce vibration. Being relatively impermeable to gases, rubber is useful in the manufacture of articles such as air hoses, balloons, balls, and cushions.

The resistance of rubber to water and to the action of most fluid chemicals has led to its use in rainwear, diving gear, and chemical and medicinal tubing, and as a lining for storage tanks, processing equipment, and railroad tank cars. Because of their electrical resistance, soft rubber goods are used as insulation and for protective gloves, shoes, and blankets; hard rubber is used for articles such as telephone housings, parts for radio sets, meters, and other electrical instruments. The coefficient of friction of rubber, which is high on dry surfaces and low on wet surfaces, leads to the use of rubber both for power-transmission belting and for water-lubricated bearings in deep-well pumps.

Other significant uses of rubber are door and window profiles, hoses, belts, matting, flooring and dampeners for the automotive industry in what is known as the "under the bonnet" products. Significant tonnage of rubber is used as adhesives in many manufacturing industries and products, although the two most noticeable are the paper and the carpet industry. Rubber is also commonly used to make rubber bands and pencil erasers.

About The Author:
Dr.Thoguluva Raghavan Vijayaram, currently working as Senior Lecturer in the Faculty of Manufacturing Engineering at UTeM, Universiti Teknikal Malaysia Melaka, Malaysia. He hails from India and he has completed his PhD Research Degree in Mechanical Engineering (Metal Matrix Composites: Materials Engineering) from the Faculty of engineering, Universiti Putra Malaysia. He has published quality research papers in reputed International journals, National journals, International conference proceedings and in the Malaysian broadsheet. He has a wide range of work experience, both in academics and as well as in industry, consultancy, and teaching and especially in research and development work. His areas of expertise include: Metallurgical Engineering, Mechanical Engineering and Manufacturing Engineering and his special areas of research interests are in the field of advanced casting technology and techniques, composite materials and processing, powder metallurgy, Ferrous and Non-Ferrous foundry metallurgy, solidification science and technology, solidification processing of metals, alloys and composites, microgravity solidification, squeeze casting, die casting die design, heat treatment, Metallography, microstructure-property correlation ship, new materials and process development, aerospace engineering materials, computer simulation of casting solidification, FEM analysis and advanced engineering mathematics. Besides, he is a prominent writer and possesses wider experience in editing technical papers, theses and dissertations.

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Rubber: An Effective And Suitable Elastomer For Manufacturing Engineering And Metal Working Industries

By Dr Thoguluva Raghavan Vijayaram PhD

Rubber: An Effective And Suitable Elastomer
For Manufacturing Engineering
And Metal Working Industries