Sulfur vulcanization of rubber

Sulfur vulcanization of rubber

Natural rubber is a naturally occurring material. It is made by trees, just like maple syrup. Trees produce latex, which is then processed into a useful natural rubber material. Natural rubber has been known for thousands of years. It was known to ancient civilizations such as the Incas, who used rubber to make balls for ancient sports. At that time, the biggest disadvantage of natural rubber was that it started to melt or stick when the temperature was too high. Natural rubber is a component of the milk sap of many species of dicotyledonous plants. Natural rubber is obtained from latex, an emulsion that exudes from the inner bark of rubber trees. The scientific name of the rubber tree is caoutchouc, which comes from the word caaochu meaning a weeping tree. Natural latex contains 30-35% rubber, 2-3% proteins and lipids, 0.3% resin and 1.5-4% glycosides. Coagulated rubber after separation and drying yields raw rubber. It is an isoprene (2-methyl-L,3-butadiene) polymer, with a molecular weight in the range of 300,000. Synthetic rubber is made from petroleum by-products by polymerizing 1,3-butadiene, chloroprene, isobutene, etc. Synthetic rubber refers to to virtually any man-made rubber material. Synthetic rubber mimics the desirable properties of natural rubber and is used in many of the same applications. There is a wide range of synthetic rubbers available on the market, many of which have unique performance characteristics that natural rubber does not. As mentioned earlier, the rubber at this stage is soft, sticky and thermoplastic. It has low tensile strength and low flexibility. It is easy to understand that, in essence, such rubber is a mixture of polymer chains of various lengths. Most importantly, it has no cross-linking at all and is therefore sticky and thermoplastic.

As a result, the material, known for several centuries, did not find any significant utility. The situation changed radically when Charles Goodyear discovered the vulcanization process in 1839, accidentally. While conducting an experiment, he spilled a mixture of rubber and sulfur with other ingredients on a hot kiln. And behold, the rubber formed into a hard material. Properties rubbers have been radically changed by cross-linking polymer chains. This process is called vulcanization.

The vulcanization process has undergone many modifications since its discovery by Charles Goodyear in 1839. For the vulcanization process to be useful and successful, it should be controlled, it should start at a certain point, speed up if necessary, and it must stop at the right time.

Sulfur vulcanization of rubber.

Vulcanization is a chemical process that converts natural rubber and other elastomers into cross-linked polymers Sulfur is the most commonly used vulcanizing agent When heated with rubber, sulfur forms bridges between individual polymer molecules A catalyst and initiator are often added to speed up the vulcanization process Cross-linked elastomers have much better mechanical properties In fact, unvulcanized rubber has poor mechanical properties and low durability The crosslinking process is quite complicated and involves a sequence of reactions.

Vulcanization is a technological process that consists in the transformation of a rubber mixture into rubber The effect of this process is the transformation of a material with poor mechanical properties into a structural material with the desired properties and shape Rubber obtained in the vulcanization process becomes durable, flexible and chemically resistant Vulcanized rubber does not show plasticity Up to the most important technological parameters necessary to carry out the process in question include: pressure, temperature, time and method of vulcanization The result of the vulcanization process is the formation of transverse chemical bonds between rubber molecules and the formation of a three-dimensional spatial network. This process is called cross-linking. be carried out by physical methods through the action of gamma radiation or fast electrons from accelerators actor.

Rubber vulcanization gives the rubber valuable physical properties such as flexibility, tensile strength, abrasion resistance and liquid resistance. Vulcanized rubber becomes less soluble and absorbs less moisture. the elastomer has favorable properties.

Stages of the rubber vulcanization process:

1Beginning of vulcanization.

The beginning of vulcanization is considered the state of the mixture in which
the thermoplastic flow rate begins to decrease.

2 Pre-vulcanized.

It is a slightly cross-linked rubber, which causes the formation of a gel. Pre-vulcanization does not give the product properties characteristic of rubber.

3 Optimum vulcanization.

A key part of the process to get the vulcanizate with the best selected properties. To obtain the best rubber properties, the optimum vulcanization temperature and the correct residence time of the vulcanized rubber at this temperature must be determined.


After exceeding the time or temperature of vulcanization adopted for
optimal, the properties of the vulcanizate usually deteriorate.

5 Reverse vulcanization process.

This term is used to describe the disintegration process that takes place simultaneously with cross-linking
network bonds.Simultaneous course of these two reactions makes it difficult to determine the optimal parameters of the vulcanization process Optimum vulcanization time is shifted, despite the fact that the cross-linking isomerism shows maximum Reversion process is easy in rubbers
containing sulfur and accelerators forming polysulfide cross-links.

Determination of the optimal vulcanization of a rubber mixture.

Currently, rheometers are commonly used for this purpose, measuring and recording changes in torque as a function of time for the appropriate temperature. On the basis of the analysis of the vulcanization curve, conclusions can be drawn about the properties of the tested rubber mixture.
Plasticity, mold filling ability, vulcanization rates, or overvulcanization behavior of the mix can be tested
Using rheometers, it is possible to determine the kinetics of vulcanization of rubber mixtures on the basis of the curve of changes in torque as a function of vulcanization time.

Rubber vulcanization process description.

Polysulfide crosslinks formed at low temperatures can contain four to six sulfur atoms, while shorter sulfur bridges are formed at higher reaction temperatures.
The vulcanization of rubber with sulfur alone is a chemical process that is extremely slow, it can take several hours at elevated temperatures of 140°C or more This is problematic because prolonged exposure to heat and oxygen leads to oxidative degradation, which in turn results in poor mechanical properties of the resulting rubberIs it is also an economical process. Accelerators are usually used to minimize rubber degradation and speed up the vulcanization process. Accelerator is defined as a compound that increases vulcanization rate, allowing vulcanization to occur at a lower temperature and with greater efficiency. The accelerator also reduces the amount of sulfur needed to cross-link the polydiene, thus improving aging resistance vulcanized rubber.

Accelerators for sulfur vulcanization of rubbers.

The vulcanization of rubber with sulfur alone is an extremely slow process and can take several hours at elevated temperatures (140°C or more) This is problematic because prolonged exposure to heat and oxygen leads to oxidative degradation, which in turn results in poor mechanical properties It is also not economical to minimize to degrade the rubber and speed up the vulcanization process, accelerators are typically used. The accelerator is defined as a compound that increases the vulcanization rate and allows the vulcanization to proceed at a lower temperature and with greater efficiency. The accelerator also reduces the amount of sulfur needed to crosslink the polydiene, which improves the aging properties of the vulcanized rubber.

Accelerators are essential components of all sulfur vulcanization systems. They not only increase the reaction speed and efficiency of sulfur curing, but also improve the aging properties and mechanical properties of the cross-linked rubber. and the tendency to bloom sulfur on the surface of the rubber product.

Typical rubber vulcanization systems consist of rubber, accelerator, sulphur, metal oxide and fatty acid, the latter two being the activator. organics require the addition of an organic activator to achieve the desired curing and end-use properties.

The most common activator is zinc fatty acid ester, which is often formed in the reaction of a fatty acid with zinc oxide. The most commonly used fatty acids are: stearic, lauric, palmitic, oleic and naphthenic acids. The fatty acid dissolves zinc and forms the actual catalyst.Zinc oxide can also act as a filler or white colorant in rubber products while the fatty acid improves filler incorporation and dispersion by wetting the zinc oxide particles and reducing interfacial tension The addition of activators in combination with secondary alkaline accelerators allows better control of the onset of curing very complex and not fully understood.

There are a large number of chemical compounds belonging to different classes that accelerate the rubber vulcanization process. However, only about 50 accelerators are used in industry on a commercial scale.

Types of rubber.

Rubber is an extremely versatile, versatile material that is used in many domestic and industrial applicationsFrom natural rubber derived from rubber trees to a wide variety of synthetic rubbers, there really is a rubber material for every occasion There are many varieties of rubber, each offering its own set of performance attributes and desirable properties As we know rubber is flexible Not only in terms of elasticity and ductility mechanical properties Because the chemical properties of rubber make it extremely attractive to construct a wide range of different types of synthetic rubber that combine the best properties of natural rubber with many beneficial additional features.

Natural rubber (NR).

Natural rubber (isoprene) is derived from the latex sap of the Pará rubber tree (hevea brasiliensis) Natural rubber has high tensile strength and is resistant to fatigue caused by wear such as chipping, cutting or tearing The downside is that natural rubber is only moderately resistant to damage caused by heat, light and ozone Natural rubber is used in gaskets, seals, shock mounts, hoses and hoses.


Butyl rubber is a copolymer of isobutylene and isoprene There are many uses for butyl rubber, but this synthetic rubber is popular in several key products Tire inner tubes, speakers, roofing and gas masks are often made of butyl Butyl is also used by the pharmaceutical and sports industries Butyl rubber is a great option for absorbent shock Offers extremely low gas and moisture permeability and outstanding resistance to heat, aging, weathering, ozone, chemical attack, flexing, abrasion and tearing Butyl is resistant to phosphate ester based hydraulic fluids and has excellent electrical insulation properties Typical applications include O-rings, inserts for tanks and sealants Its impermeability to gases makes butyl ideal for sealing in vacuum applications.


Easily identifiable by its trade name, neoprene, chloroprene was first developed in the 1930s.Many consumer products such as wetsuits, laptop covers, cycling clothes and costumes are made of chloroprene. Civil engineers also rely on chloroprene in bridge construction.

Chlorinated polyethylene (CPE).

Chlorinated polyethylene CPE is often added to other materials to improve their weather and impact resistance.

Ethylene propylene diene rubber (EPDM).

EPDM is an extremely popular synthetic rubber material that is used in many industries due to its excellent sealing properties, durability and resistance. The construction and automotive industries use EPDM for weatherproofing and sealing. It is also used for a variety of consumer products and electronics.EPDM rubber offers excellent resistance to heat, ozone, weathering and aging as well as low electrical conductivity, low compression set and low temperature properties. EPDM rubber is used in a range of HVAC and automotive applications as well as in electrical insulation products.

Fluoroelastomers (FKM).

Fluoroelastomers remain stable even after prolonged exposure to extreme temperatures and are often used in high temperature environmentsFKM is also highly resistant to flame, aging, oils, ozone and many other chemicals.
Viton® is a fluoroelastomer material that is used in a variety of applicationsViton® is a DuPont brand This durable synthetic rubber and fluoropolymer elastomer provides exceptional temperature stability from -20 degrees Celsius to +205 degrees Celsius A disadvantage of Viton® is that it can swell in fluorinated solvents, is relatively expensive and can deteriorate quickly if the wrong grade is used. Along with nitrile, it is one of the most widely used elastomers for seals, including o-rings, gaskets and seals.


Nitrile's resistance to oil, fuel and chemicals sets it apart from other synthetic rubbers that exhibits excellent resistance to petroleum oils, fuels, water, alcohols, silicone greases and hydraulic fluids Nitrile has a temperature range of -54 to +149 degrees Celsius and has a good balance of desirable properties such as low compression set, high abrasion resistance and high tensile strength Nitrile is not recommended for use with automotive brake fluid, ketones, hydraulic fluids containing phosphate esters, and nitro or halogenated hydrocarbons.

Hydrogenated nitrile (HNBR).

Hydrogenated nitrile rubber compounds exhibit better resistance to oils and chemicals than nitrile rubbers and can withstand much higher temperaturesHNBR provides excellent resistance to oils, fuels, many chemicals, steam and ozone It also provides exceptional tensile and tear strength, elongation and abrasion resistance However, HNBR is relatively expensive and offers limited flame retardancy, poor electrical insulation and is incompatible with aromatic oils and polar organic solvents.

Polybutadiene rubber.

Most polybutadiene rubbers are used to make tires, but this synthetic rubber can also be found in golf ball cores and plastic compounds.

Styrene butadiene rubber (SBR).

SBR offers excellent abrasion resistance and remains stable as it ages. SBR is found in roughly half of all tires, along with shoe soles, seals, sealing coatings and more.

Thermoplastic vulcanizates (TPV).

Known commercially as Santoprene, TPV is a close copy of EPDM rubber. TPV offers excellent sealing properties, is fully recyclable and has a pleasant feel. TPV is found in many automotive components, household appliances, building materials and more.

Silicone (Q).

Silicone copes well with water, steam, or petroleum-based fluidsAlthough it can operate in a temperature range of -84 to +232 degrees Celsius, silicone has been shown to withstand short-term exposure to low temperatures of -115 degrees Celsius making it better suited for static rather than dynamic applications.The chemical stability of silicone means that it is widely used in the food and medical industries, as well as in sealants, lubricants and circuit boards, to name a few.

Applications of natural and synthetic rubber.

The uses for rubber are almost limitless Perhaps the largest consumer of synthetic rubber is the automotive industry Tires, seals, o-rings, seals, hoses, belts and other components throughout the vehicle are often made of synthetic rubber Aerospace industry uses synthetic rubber for many of the same components Construction industry also uses synthetic rubber relies heavily on synthetic rubber for caulking, flooring, roofing and other applications.

Another major consumer of natural and synthetic rubbers is the medical industrySynthetic rubbers are safe for patients who are sensitive or allergic to latex, so they are often used in tubing and other products that come into direct contact with the skin Healthcare professionals also rely on synthetic rubber for protective equipment, medical components, and many other.

Everyday consumers will find synthetic rubber throughout their daily lives. Shoes, sporting goods, kitchen tools, and even chewing gum contain all kinds of synthetic rubber.

The production of vulcanized rubber products requires many competences in the field of rubber chemistry and the vulcanization process. The key factor is the ability to precisely control the process.

Zamak Mercator offers complete vulcanization systems consisting of a shock furnace and a segment vulcanization furnace. The vulcanization system can be configured according to the customer's expectations [length, width, speed, number and distribution of radiators].

Shock ovens designed for surface vulcanization and shape fixation of extruded rubber profiles.

The IR shock furnace is equipped with high-power short-wave infrared radiators. Infrared radiation is designed to quickly heat the surface of the extruded profile and, thanks to rapid vulcanization, pre-fix the shape and give the surface of the vulcanized profile a nice appearance. % of energy to "irradiated" products in the form of infrared radiation. Therefore, thermal energy reaches the profile immediately and the vulcanization process is very effective.

The profile formed in the extruder head is fed into the blast furnace chamber and under the influence of infrared radiation, its surface is initially cross-linked. As a result, we obtain a thin, pre-vulcanized coating on the profile before it goes to the proper curing furnace. This furnace improves the quality of the product by giving it smoothness and a delicate gloss of the profile surface and enables better control of the external dimensions of the extruded profile.

Tunnel curing ovens designed for continuous vulcanization of rubber.

Tunnel ovens built on the basis of infrared radiators ensure a short heating time and vulcanization in the entire volume of the extruded profile. The use of infrared radiators enables faster vulcanization and better temperature distribution throughout the section, thus ensuring high stability of product dimensions and higher quality. This is possible due to the fact that the radiators used in tunnel kiln emit about 60% of heat energy in the form of radiation, which reaches the vulcanized profile immediately and penetrates into it. Thanks to the optimized design of the reflector, we can reduce energy consumption. The furnace has separate heating zones, enabling selection of individual parameters of the vulcanization process, depending on the shape of the vulcanized gasket ) and the speed of extrusion and vulcanization.

The regulation of the vulcanization process is carried out by means of an advanced control system for the power of the radiators.The construction of the tunnel kiln allows the opening of individual segments, enabling easy changeover of the conveyor belt in the tunnel chamber and control of the process during its operation.

The advantage of infrared furnaces is their very short time necessary to obtain full readiness for operation. Thanks to this, the energy losses needed to heat the chamber are significantly reduced.


See more:

A library of scientific works for which extruders, rolling mills or other Zamak Mercator devices were used

Innovative solutions in furnaces for continuous vulcanization of rubber [IR]

PMMA depolymerization [recycling] reactor


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