![VulcanizationMold1941](https://upload.wikimedia.org/wikipedia/commons/2/2f/VulcanizationMold1941.jpg)
Vulcanization (British: Vulcanisation) is a range of processes for hardening
rubber
Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, an ...
s.
The term originally referred exclusively to the treatment of
natural rubber
Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, and ...
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.
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-link
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 ...
s between sections of
polymer chain
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
which results in increased rigidity and durability, as well as other changes in the mechanical and electrical properties of the material.
Vulcanization, in common with the curing of other
thermosetting polymer
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 ...
s, is generally irreversible.
The word vulcanization is derived from
Vulcan
Vulcan may refer to:
Mythology
* Vulcan (mythology), the god of fire, volcanoes, metalworking, and the forge in Roman mythology
Arts, entertainment and media Film and television
* Vulcan (''Star Trek''), name of a fictional race and their home p ...
, the Roman god of fire and forge.
History
Rubber—latex—had been known for thousands of years in
Mesoamerican
Mesoamerica is a historical region and cultural area in southern North America and most of Central America. It extends from approximately central Mexico through Belize, Guatemala, El Salvador, Honduras, Nicaragua, and northern Costa Rica. Withi ...
cultures, used to make balls, sandal soles, rubber bands, and waterproof containers. Rubber was processed for specific applications within the
Aztec Empire — rubber and latex goods were processed and constructed, and then shipped to the capital for use or further distribution.
Early rubber tube tires in the 19th century would grow sticky on a hot road, until debris would get stuck in them and eventually the tires would burst.
Charles Goodyear
Charles Goodyear (December 29, 1800 – July 1, 1860) was an American self-taught chemist and manufacturing engineer who developed vulcanized rubber, for which he received patent number 3633 from the United States Patent Office on June 15, 1844. ...
, in the 1830s, was working to improve those tube tires. He tried heating up 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
Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which inc ...
further or
vaporizing
Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenom ...
, the rubber remained firm and, as he increased the heat, the rubber became harder. Goodyear quickly worked out a consistent system for this hardening, which he called vulcanization because of the heat involved. He obtained a patent in the same year, and by 1844 was producing the rubber on an industrial scale.
Applications
There are many uses for vulcanized materials, some examples of which are rubber hoses, shoe soles, toys, erasers, shock absorbers, conveyor belts, vibration mounts/dampers, insulation materials, tires, and bowling balls. Most rubber products are vulcanized as this massively improves their lifespan, function, and strength.
Overview
In contrast with
thermoplastic
A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling.
Most thermoplastics have a high molecular weight. The polymer chains associate ...
processes (the melt-freeze process that characterize the behaviour of most modern polymers), vulcanization, in common with the curing of other
thermosetting polymer
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 ...
s, is generally irreversible.
Five types of curing systems are in common use:
# Sulfur systems
#
Peroxide
In chemistry, peroxides are a group of compounds with the structure , where R = any element. The group in a peroxide is called the peroxide group or peroxo group. The nomenclature is somewhat variable.
The most common peroxide is hydrogen ...
s
# Metallic oxides
# Acetoxysilane
# 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; this mixture is often referred to as a cure package. The main polymers subjected to
sulfur vulcanization are
polyisoprene
Polyisoprene is strictly speaking a collective name for polymers that are produced by polymerization of isoprene. In practice polyisoprene is commonly used to refer to synthetic ''cis''-1,4-polyisoprene, made by the industrial polymerisation of i ...
(
natural rubber
Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, and ...
) and
styrene-butadiene
Styrene-butadiene or styrene-butadiene rubber (SBR) describe families of synthetic rubbers derived from styrene and butadiene (the version developed by Goodyear is called Neolite). These materials have good abrasion resistance and good aging st ...
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
Magnesium oxide ( Mg O), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO and consists of a lattice of Mg2+ ions and O2− ions ...
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
In organic chemistry a diene ( ) (diolefin ( ) or alkadiene) is a covalent compound that contains two double bonds, usually among carbon atoms. They thus contain two alk''ene'' units, with the standard prefix ''di'' of systematic nomenclature. ...
rubbers. Most conventionally used accelerators are problematic when CR rubbers are cured and the most important
accelerant
Accelerants are substances that can bond, mix or disturb another substance and cause an increase in the speed of a natural, or artificial chemical process. Accelerants play a major role in chemistry—most chemical reactions can be hastened with an ...
has been found to be
ethylene thiourea
Ethylene thiourea (ETU) is an organosulfur compound with the formula . It is an example of an ''N'',''N''-disubstituted thiourea. It is a white solid. It is synthesized by treating ethylenediamine with carbon disulfide.
Ethylene thioureas are ...
(ETU), which, although being an excellent and proven accelerator for polychloroprene, has been classified as
reprotoxic
Reproductive toxicity refers to the potential risk from a given chemical, physical or biologic agent to adversely affect both male and female fertility as well as offspring development. Reproductive toxicants may adversely affect sexual function ...
. The European rubber industry has started a research project SafeRubber to develop a safer alternative to the use of ETU.
Vulcanization of silicones
![Silicone rubber keypad example 1](https://upload.wikimedia.org/wikipedia/commons/6/60/Silicone_rubber_keypad_example_1.jpg)
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:
# 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.
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.
# 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
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
deposit that is non-flammable and non-combustible. They can be used for
electrical insulation
An electrical insulator is a material in which electric current does not flow freely. The atoms of the insulator have tightly bound electrons which cannot readily move. Other materials—semiconductors and conductors—conduct electric current ...
due to their
dielectric
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
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
*
ISO 2921
*
Polymer stabilizers Polymer stabilizers (British: polymer stabilisers) are chemical additives which may be added to polymeric materials, such as plastics and rubbers, to inhibit or retard their degradation.
Common polymer degradation processes include oxidation, UV- ...
*
Sulfur concrete
*
Vulcanized fibre
Vulcanized fibre or red fibre is a laminated plastic composed of only cellulose. The material is a tough, resilient, hornlike material that is lighter than aluminium, tougher than leather, and stiffer than most thermoplastics. The newer wood-lamin ...
References
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Chemical processes