Vulcanisation (American English: vulcanization) is a range of processes for hardening .
Vulcanisation can be defined as the curing of , with the terms 'vulcanisation' and 'curing' sometimes used interchangeably in this context. It works by forming 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.
The word was suggested by William Brockedon (a friend of Thomas Hancock who attained the British patent for the process) based on the god Vulcan who was associated with heat and sulfur in .
History
In ancient
Mesoamerican cultures, rubber was used to make balls, sandal soles, elastic bands, and waterproof containers.
[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] It was cured using sulfur-rich plant juices, an early form of vulcanisation.
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 vaporization, 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.
On 21 November 1843, British inventor, Thomas Hancock took out a patent for the vulcanisation of rubber using sulfur, eight weeks before Charles Goodyear did the same in the US (30 January 1844). Accounts differ as to whether Hancock's patent was informed by inspecting samples of American rubber from Goodyear and whether inspecting such samples could have provided information sufficient to recreate Goodyear's process.
Applications
There are many uses for vulcanised materials, some examples of which are rubber hoses, shoe soles, toys, erasers, hockey pucks, shock absorbers, conveyor belts,
vibration mounts/dampers, insulation materials, tires, and bowling balls.
Most rubber products are vulcanised 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), vulcanisation, in common with the curing of other thermosetting polymers, is generally irreversible.
Five types of curing systems are in common use:
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Sulfur systems
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Metallic oxides
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Acetoxysilane
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Urethane crosslinkers
Vulcanisation with sulphur
The most common vulcanising methods depend on sulfur. Sulphur, by itself, is a slow vulcanising agent and does not vulcanise synthetic
. Accelerated vulcanisation is carried out using various compounds that modify the kinetics of crosslinking;
[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. ] this mixture is often referred to as a cure package. The main polymers subjected to sulphur vulcanisation 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
hydrogen atoms. These C-H bonds are adjacent to
Alkene (>C=C<). During vulcanisation, some of these C-H bonds are replaced by
polysulfide atoms that link with a cure site of another polymer chain. These bridges contain between one and several atoms. The number of sulphur 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 tyre. Without good flexing properties these movements rapidly form cracks, and ultimately will make the rubber article fail.
Vulcanisation of polychloroprene
The vulcanisation of
neoprene or
polychloroprene rubber (CR rubber) is carried out using metal oxides (specifically
magnesium oxide and
zinc oxide, sometimes Pb
3O
4) rather than sulfur compounds which are presently used with many natural and
. 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.
Vulcanisation of silicones
Room-temperature vulcanizing (RTV)
silicone is constructed of reactive oil-based polymers combined with strengthening mineral fillers. There are two types of room-temperature vulcanising silicone:
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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.
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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 . Break-down occurs at temperatures above , leaving an inert Silicon dioxide 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
