Vulcanization

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Worker placing a tire in a mold before vulcanization. VulcanizationMold1941.jpg
Worker placing a tire in a mold before vulcanization.

Vulcanization (British English: vulcanisation) is a range of processes for hardening rubbers. [1] The term originally referred exclusively to the treatment of natural rubber with sulfur, which remains the most common practice. It has also grown to include the hardening of other (synthetic) rubbers via various means. Examples include silicone rubber via room temperature vulcanizing and chloroprene rubber (neoprene) using metal oxides.

Contents

Vulcanization can be defined as the curing of elastomers, with the terms 'vulcanization' and 'curing' sometimes used interchangeably in this context. It works by forming cross-links between sections of the polymer chain which results in increased rigidity and durability, as well as other changes in the mechanical and electrical properties of the material. [2] Vulcanization, in common with the curing of other thermosetting polymers, is generally irreversible.

The word was suggested by William Brockedon (a friend of Thomas Hancock who attained the British patent for the process) coming from the god Vulcan who was associated with heat and sulfur in volcanoes. [3]

History

Roller hockey ball obtained via vulcanisation. Roller-hockey-(Quad)-Ball.jpg
Roller hockey ball obtained via vulcanisation.

In ancient Mesoamerican cultures, rubber was used to make balls, sandal soles, elastic bands, and waterproof containers. [4] It was cured using sulfur-rich plant juices, an early form of vulcanization. [5]

In the 1830s, Charles Goodyear worked to devise a process for strengthening rubber tires. Tires of the time would become soft and sticky with heat, accumulating road debris that punctured them. Goodyear tried heating rubber in order to mix other chemicals with it. This seemed to harden and improve the rubber, though this was due to the heating itself and not the chemicals used. Not realizing this, he repeatedly ran into setbacks when his announced hardening formulas did not work consistently. One day in 1839, when trying to mix rubber with sulfur, Goodyear accidentally dropped the mixture in a hot frying pan. To his astonishment, instead of melting further or vaporizing, the rubber remained firm and, as he increased the heat, the rubber became harder. Goodyear worked out a consistent system for this hardening, and by 1844 patented the process and was producing the rubber on an industrial scale.[ citation needed ]

Applications

There are many uses for vulcanized materials, some examples of which are rubber hoses, shoe soles, toys, erasers, hockey pucks, shock absorbers, conveyor belts, [6] vibration mounts/dampers, insulation materials, tires, and bowling balls. [7] Most rubber products are vulcanized as this greatly improves their lifespan, function, and strength.

Overview

In contrast with thermoplastic processes (the melt-freeze process that characterize the behaviour of most modern polymers), vulcanization, in common with the curing of other thermosetting polymers, is generally irreversible. Five types of curing systems are in common use:

  1. Sulfur systems
  2. Peroxides
  3. Metallic oxides
  4. Acetoxysilane
  5. Urethane crosslinkers

Vulcanization with sulfur

The most common vulcanizing methods depend on sulfur. Sulfur, by itself, is a slow vulcanizing agent and does not vulcanize synthetic polyolefins. Accelerated vulcanization is carried out using various compounds that modify the kinetics of crosslinking; [8] this mixture is often referred to as a cure package. The main polymers subjected to sulfur vulcanization are polyisoprene (natural rubber) and styrene-butadiene rubber (SBR), which are used for most street-vehicle tires. The cure package is adjusted specifically for the substrate and the application. The reactive sites—cure sites—are allylic hydrogen atoms. These C-H bonds are adjacent to carbon-carbon double bonds (>C=C<). During vulcanization, some of these C-H bonds are replaced by chains of sulfur atoms that link with a cure site of another polymer chain. These bridges contain between one and several atoms. The number of sulfur atoms in the crosslink strongly influences the physical properties of the final rubber article. Short crosslinks give the rubber better heat resistance. Crosslinks with higher number of sulfur atoms give the rubber good dynamic properties but less heat resistance. Dynamic properties are important for flexing movements of the rubber article, e.g., the movement of a side-wall of a running tire. Without good flexing properties these movements rapidly form cracks, and ultimately will make the rubber article fail.

Vulcanization of polychloroprene

The vulcanization of neoprene or polychloroprene rubber (CR rubber) is carried out using metal oxides (specifically MgO and ZnO, sometimes Pb3O4) rather than sulfur compounds which are presently used with many natural and synthetic rubbers. In addition, because of various processing factors (principally scorch, this being the premature cross-linking of rubbers due to the influence of heat), the choice of accelerator is governed by different rules to other diene rubbers. Most conventionally used accelerators are problematic when CR rubbers are cured and the most important accelerant has been found to be ethylene thiourea (ETU), which, although being an excellent and proven accelerator for polychloroprene, has been classified as reprotoxic. From 2010 to 2013, the European rubber industry had a research project titled SafeRubber to develop a safer alternative to the use of ETU. [9]

Vulcanization of silicones

An example of a silicone rubber keypad typical of LSR (Liquid Silicone Rubber) moulding Silicone rubber keypad example 1.jpg
An example of a silicone rubber keypad typical of LSR (Liquid Silicone Rubber) moulding

Room-temperature vulcanizing (RTV) silicone is constructed of reactive oil-based polymers combined with strengthening mineral fillers. There are two types of room-temperature vulcanizing silicone:

  1. RTV-1 (One-component systems); hardens due to the action of atmospheric humidity, a catalyst, and acetoxysilane. Acetoxysilane, when exposed to humid conditions, will form acetic acid. [10] The curing process begins on the outer surface and progresses through to its core. The product is packed in airtight cartridges and is either in a fluid or paste form. RTV-1 silicone has good adhesion, elasticity, and durability characteristics. The Shore hardness can be varied between 18 and 60. Elongation at break can range from 150% up to 700%. They have excellent aging resistance due to superior resistance to UV radiation and weathering.
  2. RTV-2 (Two-component systems); two-component products that, when mixed, cure at room-temperature to a solid elastomer, a gel, or a flexible foam. RTV-2 remains flexible from −80 to 250 °C (−112 to 482 °F). Break-down occurs at temperatures above 350 °C (662 °F), leaving an inert silica deposit that is non-flammable and non-combustible. They can be used for electrical insulation due to their dielectric properties. Mechanical properties are satisfactory. RTV-2 is used to make flexible moulds, as well as many technical parts for industry and paramedical applications.

See also

Related Research Articles

<span class="mw-page-title-main">Neoprene</span> Chemical compound

Neoprene is a family of synthetic rubbers that are produced by polymerization of chloroprene. Neoprene exhibits good chemical stability and maintains flexibility over a wide temperature range. Neoprene is sold either as solid rubber or in latex form and is used in a wide variety of commercial applications, such as laptop sleeves, orthopaedic braces, electrical insulation, medical gloves, liquid and sheet-applied elastomeric membranes or flashings, and automotive fan belts.

<span class="mw-page-title-main">Thermosetting polymer</span> Polymer obtained by irreversibly hardening (curing) a resin

In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening ("curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and may be promoted by high pressure or mixing with a catalyst. Heat is not necessarily applied externally, and is often generated by the reaction of the resin with a curing agent. Curing results in chemical reactions that create extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network.

<span class="mw-page-title-main">Elastomer</span> Polymer with rubber-like elastic properties

An elastomer is a polymer with viscoelasticity and with weak intermolecular forces, generally low Young's modulus (E) and high failure strain compared with other materials. The term, a portmanteau of elastic polymer, is often used interchangeably with rubber, although the latter is preferred when referring to vulcanisates. Each of the monomers which link to form the polymer is usually a compound of several elements among carbon, hydrogen, oxygen and silicon. Elastomers are amorphous polymers maintained above their glass transition temperature, so that considerable molecular reconformation is feasible without breaking of covalent bonds. At ambient temperatures, such rubbers are thus relatively compliant and deformable.

<span class="mw-page-title-main">Cross-link</span> Bonds linking one polymer chain to another

In chemistry and biology, a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural polymers.

<span class="mw-page-title-main">EPDM rubber</span> Type of synthetic rubber

EPDM rubber is a type of synthetic rubber that is used in many applications.

<span class="mw-page-title-main">Polysulfide</span> Molecules derived from sulfur chains

Polysulfides are a class of chemical compounds derived from anionic chains of sulfur atoms. There are two main classes of polysulfides: inorganic and organic. The inorganic polysulfides have the general formula S2−
n
. These anions are the conjugate bases of polysulfanes H2Sn. Organic polysulfides generally have the formulae R1SnR2, where R is an alkyl or aryl group.

Accelerants, or accelerators, are substances that increase the rate of a natural or artificial chemical process. They play a major role in chemistry, as most chemical reactions can be hastened with an accelerant. Understanding accelerants is crucial in forensic science, engineering, and other fields where controlled chemical reactions are essential. Accelerants function by either altering a chemical bond, speeding up a chemical process, or changing the reaction conditions. Unlike catalysts, accelerants may be consumed during the process.

<span class="mw-page-title-main">Silicone rubber</span> Elastomer composed of silicone

Silicone rubber is an elastomer composed of silicone—itself a polymer—containing silicon together with carbon, hydrogen, and oxygen. Silicone rubbers are widely used in industry, and there are multiple formulations. Silicone rubbers are often one- or two-part polymers, and may contain fillers to improve properties or reduce cost. Silicone rubber is generally non-reactive, stable, and resistant to extreme environments and temperatures from −55 to 300 °C while still maintaining its useful properties. Due to these properties and its ease of manufacturing and shaping, silicone rubber can be found in a wide variety of products, including voltage line insulators; automotive applications; cooking, baking, and food storage products; apparel such as undergarments, sportswear, and footwear; electronics; medical devices and implants; and in home repair and hardware, in products such as silicone sealants.

<span class="mw-page-title-main">Tire manufacturing</span> Process of tire fabrication

Pneumatic tires are manufactured according to relatively standardized processes and machinery, in around 455 tire factories in the world. With over 1 billion tires manufactured worldwide annually, the tire industry is a major consumer of natural rubber. Tire factories start with bulk raw materials such as synthetic rubber, carbon black, and chemicals and produce numerous specialized components that are assembled and cured.

<span class="mw-page-title-main">Acrylate polymer</span> Group of polymers prepared from acrylate monomers

An acrylate polymer is any of a group of polymers prepared from acrylate monomers. These plastics are noted for their transparency, resistance to breakage, and elasticity.

Curing is a chemical process employed in polymer chemistry and process engineering that produces the toughening or hardening of a polymer material by cross-linking of polymer chains. Even if it is strongly associated with the production of thermosetting polymers, the term "curing" can be used for all the processes where a solid product is obtained from a liquid solution, such as with PVC plastisols.

Injection molding of liquid silicone rubber (LSR) is a process to produce pliable, durable parts in high volume.

Rubber Technology is the subject dealing with the transformation of rubbers or elastomers into useful products, such as automobile tires, rubber mats and, exercise rubber stretching bands. The materials includes latex, natural rubber, synthetic rubber and other polymeric materials, such as thermoplastic elastomers. Rubber processed through such methods are components of a wide range of items.

Polymer engineering is generally an engineering field that designs, analyses, and modifies polymer materials. Polymer engineering covers aspects of the petrochemical industry, polymerization, structure and characterization of polymers, properties of polymers, compounding and processing of polymers and description of major polymers, structure property relations and applications.

A thermoset polymer matrix is a synthetic polymer reinforcement where polymers act as binder or matrix to secure in place incorporated particulates, fibres or other reinforcements. They were first developed for structural applications, such as glass-reinforced plastic radar domes on aircraft and graphite-epoxy payload bay doors on the Space Shuttle.

Resin casting is a method of plastic casting where a mold is filled with a liquid synthetic resin, which then hardens. It is primarily used for small-scale production like industrial prototypes and dentistry. It can be done by amateur hobbyists with little initial investment, and is used in the production of collectible toys, models and figures, as well as small-scale jewellery production.

RTV silicone is a type of silicone rubber that cures at room temperature. It is available as a one-component product, or mixed from two components. Manufacturers provide it in a range of hardnesses from very soft to medium—usually from 15 to 40 Shore A. RTV silicones can be cured with a catalyst consisting of either platinum or a tin compound such as dibutyltin dilaurate. Applications include low-temperature over-molding, making molds for reproducing, and lens applications for some optically clear grades. It is also used widely in the automotive industry as an adhesive and sealant, for example to create gaskets in place.

<span class="mw-page-title-main">Charles Goodyear Medal</span> Award

The Charles Goodyear Medal is the highest honor conferred by the American Chemical Society, Rubber Division. Established in 1941, the award is named after Charles Goodyear, the discoverer of vulcanization, and consists of a gold medal, a framed certificate and prize money. The medal honors individuals for "outstanding invention, innovation, or development which has resulted in a significant change or contribution to the nature of the rubber industry". Awardees give a lecture at an ACS Rubber Division meeting, and publish a review of their work in the society's scientific journal Rubber Chemistry and Technology.

In polymer chemistry, materials science, and food science, bloom refers to the migration of one component of a solid mixture to the surface of an article. The process is an example of phase separation or phase aggregation.

<span class="mw-page-title-main">Sulfur vulcanization</span> Process to transform the material properties of natural rubber

Sulfur vulcanization is a chemical process for converting natural rubber or related polymers into materials of varying hardness, elasticity, and mechanical durability by heating them with sulfur or sulfur-containing compounds. Sulfur forms cross-linking bridges between sections of polymer chains which affects the mechanical and electronic properties. Many products are made with vulcanized rubber, including tires, shoe soles, hoses, and conveyor belts. The term vulcanization is derived from Vulcan, the Roman god of fire.

References

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  2. James E. Mark; Burak Erman; F. R. Eirich, eds. (2005). Science and Technology of Rubber. p. 768. ISBN   0-12-464786-3.
  3. Hancock, Thomas (1857). Personal Narrative of the Origin and Progress of the Caoutchouc Or India-Rubber Manufacture in England. London: Longman, Brown, Green, Longmans, & Roberts. p. 107.
  4. Tarkanian, M., & Hosler, D. (2011). America’s First Polymer Scientists: Rubber Processing, Use and Transport in Mesoamerica. Latin American Antiquity, 22(4), 469-486. doi:10.7183/1045-6635.22.4.469
  5. "Rubber processed in ancient Mesoamerica, MIT researchers find". News.mit.edu. July 14, 1999. Retrieved October 25, 2021.
  6. "A Guide to the Uses and Benefits of Vulcanised Rubber". Martins Rubber. January 27, 2020. Retrieved June 16, 2021.
  7. "Vulcanized Rubber" . Retrieved June 16, 2021.
  8. Hans-Wilhelm Engels, Herrmann-Josef Weidenhaupt, Manfred Pieroth, Werner Hofmann, Karl-Hans Menting, Thomas Mergenhagen, Ralf Schmoll, Stefan Uhrlandt “Rubber, 4. Chemicals and Additives” in Ullmann's Encyclopedia of Industrial Chemistry, 2004, Wiley-VCH, Weinheim. doi : 10.1002/14356007.a23_365.pub2
  9. "A Safer Alternative Replacement for Thiourea Based Accelerators in the Production Process of Chloroprene Rubber". cordis.europa.eu. Retrieved April 25, 2024.
  10. "MSDS for red RTV-Silicone" (PDF). Archived (PDF) from the original on October 9, 2022. Retrieved June 24, 2011.