Except for natural rubber, the rubber market is very important for today’s technology. Rubber materials constitute a very important branch of the polymer market also. Synthetic rubbers have the biggest share in the rubber market also. Here we explain the top ten available commercial synthetic rubber that is available in the polymer market today.
We use rubber in the production of lots of products. We produce these products with a bunch of production methods that we will describe here. They produce rubber products that you are seeing around you by one of these processes.
Types of Rubbers
Styrene-butadiene rubber materials are the most important elastomer material in polymer chemistry. It has specific properties and because of these properties, they use these materials in various kinds of engineering applications. Here, we will state these properties and applications of styrene-butadiene.
Properties of Styrene-Butadiene Rubbers
As you understand from its name, styrene-butadiene is the copolymer of butadiene and styrene. It has the biggest market share among other elastomers, which is reaching up to 40% of the total market share. They produce styrene-butadiene rubber materials alternative to natural rubber materials in WW2 by Germans. The most attractive property of styrene-butadiene is the low cost. We can reinforce it with a vulcanization process with carbon black. With that process, styrene-butadiene has similar properties to natural rubber material.
The mechanical properties of styrene-butadiene are lower than natural rubber material. But wear resistance and resistance to heat, ozone and water are superior.
General applications that styrene-butadiene that they use are; automobile tires, wire insulation, and footwear.
- Properties of styrene-butadiene: The symbol of styrene-butadiene is ‘SBR’ in the market. We stated that it is a copolymer of styrene and butadiene which have chemical representations C8H8 and C4H6 respectively. The elongation of styrene-butadiene rubber material at failure is around 700% according to its original shape. The tensile strength of styrene-butadiene rubber material is 20 MPa. The modulus of elasticity is 17 MPa. The service temperature range of styrene-butadiene rubber material is between -50C and 110C.
Butyl Rubber Materials
We produce butyl as copolymers of polyisobutylene and polyisoprene. The percentage of these two compounds in butyl is around 98-99% and 1-2% respectively. It has very low air permeability which leads to its use in such applications as inflatable tubes and tubeless tire liners.
The compound of isobutylene has chemical representation (C4H8)n and isoprene has (C5H8)n. The ‘n’ means the number of monomers that these two compounds formed. In the market, the symbol of butyl is BTL and you can find out commercial butyl products with this symbol. We can use the available temperature limit that butyl from -50C to 110C. At 300% elongation, butyl rubber material has a modulus of elasticity of 7 MPa. Also, the tensile strength of butyl rubber material is 20 MPa. The maximum elongation according to its original shape is 700%. The density of butyl is around 0.92 gr/cm^3.
Ethylene-Propylene Rubber Material
Ethylene-propylene is terpolymer. The third constituent monomer in this terpolymer is diene, which has a very small proportion compared with ethylene and propylene. This terpolymer is called EPDM also in this market. General applications of ethylene-propylene rubbers are; cable insulation and automotive parts.
Constituents ethylene and propylene have chemical representations of C2H4, and C3H6 respectively. We generally add diene monomer to obtain a cross-linked structure in this rubber material. The service temperature limits of ethylene-propylene are from -50C to 150C. Ethylene-propylene has a density of 0.86 g/cm^3. And tensile strength of ethylene-propylene 15MPa. The maximum elongation of ethylene-propylene rubber material according to its original shape is around 300%.
Properties and Applications of Isoprene Rubber Material
We synthesize this synthetic rubber as the equivalent of natural rubber. It has some characteristics of natural rubber material. Synthetic polyisoprene is also softer than natural rubber and much more easily molded. Some of the applications we use that isoprene in; caulking, footwear, conveyors etc.
The rubber material obtained from isoprene is polyisoprene. Polyisoprene has a chemical representation of (C5H8)n. ‘n’ means the number of copolymers in the polymer. The symbol of isoprene is ‘IR’ in the market that you can find it with this symbol. The temperature availability of isoprene is from -50C to 80C. The modulus of elasticity of polyisoprene is around 17 MPa at an elongation of 300% of its original shape. Tensile strength is around 25 MPa. Failure elongation of polyisoprene is 500%.
Properties of Ethylene-Propylene Rubber Material
Ethylene-propylene is terpolymer. The third constituent monomer in this terpolymer is diene, which has a very small proportion compared with ethylene and propylene. This terpolymer is called EPDM also in the rubber market. General applications of ethylene-propylene rubbers are; cable insulation and automotive parts.
Constituents ethylene and propylene have chemical representations of C2H4, and C3H6 respectively. We add diene monomer generally to obtain a cross-linked structure in this rubber material. The service temperature limits of ethylene-propylene are from -50C to 150C. Ethylene-propylene has a density of 0.86 g/cm^3. And tensile strength of ethylene-propylene is 15MPa. The maximum elongation of ethylene-propylene rubber material according to its original shape is around 300%.
They are thermoplastic materials but have elastomeric behaviors. In the elastomer market, they have a very important place. Thermoplastic elastomers do not take their elastomeric behavior from their cross-linked molecular structure, they take their elastomer characteristics from their chemical connections between different phases that exist. These phases are soft and hard phases that constitute the thermoplastic elastomer.
They have a block copolymer of styrene-butadiene-styrene block copolymer which is very characteristic for this group. Thermoplastic elastomers include different two materials that are not chemically incompatible with each other. So they for different phases. So chemical bondings between these phases constitute thermoplastic elastomer materials.
General applications of thermoplastic elastomers are; automotive molded parts, extruded tubes, wire coating, and footwear.
- Properties Of Styrene-Butadiene-Styrene: The symbol of this polymer in the market is ‘SBS’. It has a very important place in the elastomer market which has a market share of 12%. The service temperature limit is between -50C and 60C. At higher temperatures, it shows thermoplastic properties. Elongation at the failure of SBS is around 400% according to its original shape. Tensile strength is 14 MPa.
Vulcanization of Rubbers
The vulcanization process is a special application for rubbers to obtain more cross-linking structures between mers/molecules. These cross-linkings provide a much more stiff structure than thermoset plastics. As you know that also, elastomers are very flexible if we compare them with other polymers. With the vulcanization method, elastomers lose nothing from their flexibility but gain a stiffer structure.
How the Vulcanization is Applied?
In the most basic aspect, vulcanization is the application of sulfur to elastomers. Goodyear found this method first, in which nearly 8% of sulfur that they add inside the elastomer constitution.
But in today’s technology, that type of vulcanization process evolved. Because this old type of process takes very long hours. With the addition of very small amounts of zinc oxide(ZnO) and stearic acid(C18H36O2), the total vulcanization process reduces to minutes.
Vulcanization Applications in Different Rubber Shaping Processes
You know that there are various kinds of rubber shaping processes available in the manufacturing world to shape different kinds of rubber parts. According to this fact, various kinds of application styles are available in rubber shaping. But in general, they apply vulcanization in two different principles;
- Application of vulcanization to batch production processes of parts,
- Application of vulcanization to continuous production processes of parts.
Batch Production Vulcanization
In this kind of rubber shaping process, we obtain discrete parts from discrete molds or production machinery. So, we apply vulcanization to these discrete parts.
Generally, we apply autoclaves or gas curing methods to vulcanize the rubber parts. Autoclaves are the heated and pressurized vessels in which the vulcanization of parts takes place. In the gas curing method, we direct inert and heated gas on parts to make vulcanization.
Continuous Application Of Vulcanization
There is a lot of continuous production of rubber parts available in manufacturing. In these methods, produced parts or shapes are not discrete parts. They are in continuous shape such as wire coatings, extrudates, etc.
For these kinds of production processes, we apply continuous drums or hot-air tunnel types of vulcanization methods.
Additives and Compounding of Rubber Materials
In most rubber applications, we do not use both natural and synthetic rubbers single-handedly. Because there can be other mechanical and physical feature requirements changes from one application to another application that we use materials.
To obtain these additional features from rubbers, we use some additives or reinforcements. Here, you will find brief information on some additives and compounds that we use in part production.
Additives Used in Rubber Compounding
There are several additives that we use in rubber compounding applications. The most important compounding operation of rubber materials is vulcanization. Vulcanization is the chemical process that we apply to increase the density of cross-linking between molecules. So we can achieve rigidity and other mechanical properties with vulcanization. We make vulcanization generally with sulfur addition.
Another compounding process of rubber materials is the addition of carbon black which is a colloidal carbon mixed with water. We obtain carbon black by the thermal decomposition of soot which is a basic hydrocarbon. If we compound it with carbon black, tensile strength, and abrasion, we increase and tearing resistance of the produced product.
You know that most automobile tires are black. This is because of the addition of carbon black which is a compounding material of tires.
We can also use calcium carbonate used as a non-reinforcing filler to obtain much more volume of material. Another nonreinforcing filler is silica. But depending on the particle size, silica can act as a reinforcing filler inside the rubber material.
We also use hydrous aluminum silicates as a reinforcing agent in rubber material. When compounded with rubber material, it gives tensile strength and abrasion resistance. If we compare it with carbon black, carbon black is superior in terms of the addition of these features to rubber. But, we can prefer hydrous aluminum silicates if we do not desire the black color in the application.
We compound the rubber material also with other additives to prevent aging which we also call aging retarders. Ozone-protective additives are also very important compound elements of rubber materials.
When a rubber material part that we produce by using molds, we also add mold release compounds to rubber material.
Reinforcements in Rubber Products
Also, we use steel fibers in tire production. Steel fibers give lower extensibility to tires but better mechanical properties to resist conditions that dynamically use by automobiles.
Rubber Compound Mixing Operations
Cross-linking formation is a very important phenomenon for rubber materials. We must control the degree of cross-linking specifically to obtain the required properties. So, monitoring cross-linking formation is very important in the mixing operations of compounds.
The vulcanization process is the formation of cross-linking between molecules. Viscosity decreases immensely with cross-linking formation. So, we must make mixing with vulcanization agents after mixing with other agents.
We call the first mixing process masterbatch mixing which includes carbon black, colorants, and other agents. We accomplish very homogeneous mixing in this stage.
After the masterbatch mixing, we mixed with other agents that we mix with sulfur to obtain cross-linking formation.
We use two-roll mils or internal mixer systems to mix the rubber material with other components.
Natural Rubber Materials
The natural rubber material is the long chain elastomer that constitutes isoprene(C5H8). High molecular weight is the most important characteristic of isoprene. Latex is the source of isoprene that we obtain from trees that grow in tropical regions. Latex is not pure polyisoprene. It includes a high amount of water and other ingredients. We apply some processes to latex to extract the polyisoprene from it. These processes are generally coagulation, dying, and spraying.
We must apply the vulcanization process to this natural rubber material to obtain a useful material. Because natural rubber material is very elastic in hot weather and very stiff in cold weather.
In the vulcanization process, they enhance the cross-linkings in natural rubber material. With this application, we obtain an elastomer that has very good resilience properties.
In the vulcanization process, we mix sulfur and other chemicals with it. In sulfur addition with heat application, we obtain cross-linkings in them. After the process, we obtain vulcanized rubber. Sulfur itself also vulcanizes the natural rubber, but it will take hours to obtain desired cross-linked structure. We use additives to accelerate the cross-linking formations in them.
The vulcanized rubber material that we produce from them has very good tensile strength and resilience characteristics. But with heat, oil, and sunlight applications, vulcanized rubber can degrade. With the use of some additives, we prevent drawbacks for vulcanized ones.
Applications of natural ruıbbers are; show soles, seals, bushings, and shock absorption.
The most important application of vulcanized rubber is automobile tires. They have an additive called carbon-black to obtain wear and environmental-resistant material.
Properties of Polyisoprene Vulcanized Rubber Material
The modulus of elasticity of polyisoprene is around 18 MPa in %300 elongation. Also, its tensile strength of it is around 25 MPa. The elongation that we need to tear apart vulcanized polyisoprene is 700% according to its original shape. We can use it between temperatures of -50C to 80C. It has a very big market share in elastomer materials which is around 22% of the total elastomer market.
Natural Rubbers vs. Synthetic Rubbers
We obtain rubber material from both natural and synthetic sources. The main natural source is the ‘latex’ that we produce from special plants that grow in tropical regions of the World.
The synthetic source of these materials is petroleum. There are some kinds of processes that we apply to petroleum to obtain synthetic ones.
Production of Natural Rubber Material
We obtain natural rubber from special plants grown in tropical places in ‘latex’ form. This latex includes nearly 30% of polyisoprene which is the constituent of a natural one, a colloidal mixture with water. To obtain a 100% natural one from latex, there must be some processes and operations applied.
To decompose the natural ones from the colloidal mixture of latex, we apply the coagulation technique. In this process, we dilute latex with water, then we add acetic acid or formic acid. These acids lead to the formation of coagulation of natural ones in the form of thin slabs. The coagulation process generally takes around 12 hours.
We squeeze these thin slabs of natural rubber materials with rolls to decompose the remaining water. The obtained sheet must dry in special smokehouses. In these smokehouses, we cover natural rubber slabs with wooden frames to prevent mildew.
This smoke includes creosote which also prevents the natural rubber material from oxidation. After several days of the smoking and drying process, the produced natural rubber is sent to the manufacturer.
Better grades of natural rubber materials can be produced with the application of hot air instead of smoke. And also a much better grade of natural rubbers can be produced with the application of multiple coagulation processes and other additional processes after the air drying processes.
Production of Synthetic Rubber Materials
Synthetic rubbers have the biggest market in part manufacturing because of the abundance of the source material to produce synthetic rubber materials.
The production of synthetic rubbers depends on the same production characteristics as thermoplastic polymers. But the produced synthetic one is supplied to part manufacturers in the form is large bales.
There are several types of synthetic rubbers are produced and used by manufacturers.
Polychloroprene type of synthetic rubber material is generally used in sheet production.
You can find out the mechanical and material properties of other synthetic rubber from this link; Ethylene-Propylene and Isoprene.
Butadiene and butyl rubbers are also very important synthetic materials in the manufacturing market.
Rubber Shaping Processes
There are a bunch of shaping processes available in the industry. Let’s explain these types of these shaping processes.
Rubber Calendering Method
Calendering is a process that gives required thicknesses to these materials by passing them through a set of rolls. The distance between rolls defines the thickness of rubber material to be calendered. This method is also applied to thermoplastics, but the application temperatures of much lower than thermoplastics, for rubber materials.
Lower application temperatures are used because of the cross-linking occurrence after a specific temperature. This premature cross-linking is not desired in the shaping process.
Like thermoplastics, swelling occurs after the rolls. So, swelling effects must be considered in the calendering process design for rubbers.
Rubber Extrusion Method
The extrusion of rubber materials is nearly the same as the extrusion of thermoplastics. The main difference between these extrusion and thermoplastic extrusion processes is the barrel length.
Barrel lengths for extrusion machines are much lower than thermoplastic extrusion. This is because of preventing the premature cross-linking occurrence in rubber molecules which is not desired thing.
Swelling is also a problem after exiting from the extrusion machine die. This must be considered in the design stage.
Rubber molding application is slightly the same as thermoplastic injection molding processes. There is a barrel and screw mechanism that pushes the material inside molds. Like the other processes, there is a risk of premature cross-linking occurrence in molds.
There is another kind of molding process is called compression molding. The importance and difference of this process is the cross-linking formation inside the mold. Transfer molding is also used.
Mold expenses are one of the biggest downsides of molding processes. But if the serial production manner of high-quality parts is adopted, it can be profitable.
Gaskets, seals, shoe insoles, bottle stops, and parts like that from rubber material are produced with molding processes generally.
Rubber Coating Processes
Rubber coating processes comprise several methods to impregnate or coat the other materials or parts with rubbers to produce composite parts or products.
We use calendering operations also in coating and impregnating operations.
Skimming is a type of coating process that fabrics made from other materials are fed from spool to rubber material solution. And this rubber material and fabric coating passed through heat chambers to take the solvent from the rubber, then obtain cross-linkings. We skim the excessive rubber coating with special doctor blades.
Spraying is another process that we coat the materials with spray. This is simple like that.
Dip Casting of Rubber Material
In the dip casting or dipping method, we dip a type of negative mold inside a rubber bath. And it’s dried with cross-linking on that mold. This sequence can be repeated to obtain the desired thickness. With this method, hand gloves can be produced.
As you see above, these materials are very important in engineering. It is a very important class of polymers. And there are various types of them are available in nature.
These are the general points about rubber materials. So, do not forget to leave your comments and questions below about the rubber materials. Your precious feedbacks are very important to us.