Steps in Rubber Processing Compounding Vulcanization and Shaping Methods
Rubber Processing Comprises Four Basic Steps:
Mastication, the first step, occurs when the elastomer is removed, and the molecules are broken down to provide a more effortless flow.
Mixing, the second step, is usually carried out immediately after the mastication process when additives are incorporated.
Shaping, the third step, occurs for the dense mass obtained, for example, either by extrusion or moulding.
Curing, the final step, occurs when the polymer molecules become interlinked and the shape of the dense mass is fixed.
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Mastication
Mastication and softening are both steps that are usually carried out in batches. The operation of rubber is done either on rubber mills or in a large enclosed mixing machine.
A prominent example of an enclosed machine is the Banbury, which is a registered trademark mixer. It comprises heavy steel counter-rotating paddles in an hourglass-shaped chamber and can hold up to a one-half ton of rubber.
Rubber mills usually hold two large horizontally opposed yet closely spaced steel cylinders, which are almost up to three meters or ten feet long. They are rotated slowly in opposite directions and at somewhat varied speeds.
In the mastication process, the rubber is sheared and softened. This occurs between the wall of the Banbury mixer and the paddles. Another approach is between the two cylinders in the rolling mill.
Mixing
Mixing is a process carried out on machines similar to those used in the mastication process, sometimes immediately after softening. A few various kinds incorporated into the base elastomer by a combined mixing and shearing action are- reactive materials, protective chemicals, oils, and fillers.
An enclosed Banbury-type mixer is known to produce up to one-half ton of the mixed compound within a few minutes. The compound is then sheeted out and coated with a soap release. This prevents the compound from sticking and then stored until used on the steel pallets that can hold up to one ton of rubber.
Shaping
Shaping of the mixture into the desired form occurs in multiple ways. Extruders produce long and continuous products such as tire treads, wire coverings, and tubing in this step. Extruders are used to build several profiles that are cut to the actual length later. Multi-Roll calendars are commonly used to make wide sheeting.
In injection and transfer moulds, the rubber mix is enforced through the channels into a mould chamber to acquire the required shape, cured under immense pressure.
Rubber tires are composed of multiple components- sidewall compound, cord plies, bead wire, tread, inner liner, and belt package. These components are brought and assembled as a complete whole tire. This occurs before the transfer to the curing press.
Curing
Curing is the final process that is carried out in pressurized steel moulds. It is heated either by electricity or steam to the temperature at which the interlinking reaction takes place.
The standard curing conditions are a few minutes at 160 °C (320 °F) temperature. Because heat penetrates slowly, thick articles must be allowed to cure for longer time intervals, several hours, and lower temperatures.
The pressure of one megapascal or 145 pounds per square inch or more is customarily imposed to maintain the wanted shape. This forces the trapped air to dissolve in the compound. A few other methods of curing the rubber mix upon the compound that took shape include steam heating in autoclaves or microwave radiation or passage through a heated bath of molten metal salts or a fluidized bed.
In these methods, the curing process is carried out at near atmospheric pressure.
What is Rubber Chemical Compound Processing?
The blending or compounding of polymers, specifically the elastomers, is done for two reasons. The first reason is to improve the base elastomer's technical properties and the second for an enhanced rubber chemical compound processing behaviour.
Rubber compounding is a standard method that refers to adding specific chemicals to raw rubber material to obtain the desired properties. The well-known chemicals that are often used are cross-linking agents, colourants, and anti-degradants.
The cross-linking agents must establish cross-linking agents to interconnect at the molecular level to improve elasticity and strength.
During the vulcanization or curing process, the unformulated elastomers are linked together. They form networks, increasing the strength and modulus and decreasing the hysteresis. Sulphur is a joint and widely used rubber compounding agent.
Reclaimed rubber is a rubber compounding ingredient. The scrap rubber undergoes a unique process before it can be reused. The rubber obtained as the by-product in the process is known as reclaimed rubber.
Rubber compounding processes using carbon particles often involve intensive mechanical processing.
FAQs on Rubber Processing in Polymer Chemistry
1. What is rubber processing in chemistry?
Rubber processing is the series of chemical and physical operations used to convert raw natural or synthetic rubber into usable products with desired mechanical properties. It involves modifying the polymer structure to improve strength, elasticity, and durability.
- Raw rubber is mainly a polymer of isoprene (C5H8)n.
- Processing includes mastication, compounding, shaping, and vulcanization.
- Chemical additives such as sulfur, accelerators, and antioxidants are incorporated to enhance performance.
- The goal is to control elasticity, tensile strength, hardness, and resistance to heat and chemicals.
2. What are the main steps involved in rubber processing?
The main steps in rubber processing are mastication, compounding, shaping, and vulcanization. Each step alters the physical or chemical properties of the rubber polymer.
- Mastication: Mechanical shearing reduces molecular weight and softens rubber.
- Compounding: Mixing rubber with additives like sulfur (S8), fillers, and accelerators.
- Shaping: Extrusion, molding, or calendaring into required forms.
- Vulcanization: Heating with sulfur to form cross-links between polymer chains.
3. What is mastication in rubber processing?
Mastication is the mechanical process of breaking down long rubber polymer chains to reduce viscosity and improve workability. It prepares raw rubber for uniform mixing with additives.
- Performed using heavy rollers or internal mixers.
- Causes controlled reduction in molecular weight.
- Improves plasticity and processing efficiency.
4. What is compounding in rubber processing?
Compounding is the process of mixing rubber with chemicals and fillers to achieve desired mechanical and chemical properties. It determines the final performance of the rubber product.
- Vulcanizing agents: Sulfur (S8).
- Accelerators: Speed up cross-linking reaction.
- Fillers: Carbon black (C) increases strength.
- Antioxidants: Prevent oxidative degradation.
5. What is vulcanization of rubber?
Vulcanization is the chemical process of heating rubber with sulfur to form cross-links between polymer chains, improving elasticity and strength. It transforms soft, sticky rubber into tough, elastic material.
- Occurs typically at 140–160°C.
- Sulfur atoms form –S–S– cross-links between polyisoprene chains.
- Increases tensile strength and heat resistance.
- Reduces plasticity and stickiness.
6. What is the chemical structure of natural rubber?
Natural rubber is composed of cis-1,4-polyisoprene, a polymer of isoprene (C5H8). Its repeating unit contains a double bond that allows cross-linking during vulcanization.
- Monomer: Isoprene (2-methyl-1,3-butadiene).
- Polymer formula: (C5H8)n.
- Configuration: cis-1,4 addition.
- Contains C=C bonds that react with sulfur during vulcanization.
7. What is the difference between raw rubber and vulcanized rubber?
The main difference is that raw rubber has no cross-links, while vulcanized rubber contains sulfur cross-links between polymer chains. This chemical modification changes its mechanical properties.
- Raw rubber: Soft, sticky, low tensile strength, temperature-sensitive.
- Vulcanized rubber: Strong, elastic, heat-resistant, less soluble.
- Cross-linking restricts chain mobility.
8. What role does sulfur play in rubber processing?
Sulfur acts as a cross-linking agent in vulcanization, forming bridges between rubber polymer chains. These sulfur bridges enhance strength and elasticity.
- Forms monosulfide (–S–), disulfide (–S–S–), or polysulfide links.
- Improves tensile strength and abrasion resistance.
- Reduces tackiness and deformation.
9. What are the types of rubber used in rubber processing?
Rubber used in processing is classified into natural rubber and synthetic rubber based on its source and polymer chemistry.
- Natural rubber: cis-1,4-polyisoprene from latex.
- Styrene-butadiene rubber (SBR): Copolymer of styrene and butadiene.
- Nitrile rubber (NBR): Copolymer of butadiene and acrylonitrile.
- Neoprene: Polymer of chloroprene.
10. Why is carbon black added during rubber processing?
Carbon black is added as a reinforcing filler to improve the mechanical strength and wear resistance of rubber. It significantly enhances durability.
- Increases tensile strength and hardness.
- Improves abrasion resistance.
- Enhances UV stability and conductivity.
- Commonly used in tire manufacturing.
