Vulcanization

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Worker placing a tire in a mold before vulcanization.

Vulcanization (British English: Vulcanisation) is a range of processes for hardening

chloroprene rubber
(neoprene) using metal oxides.

Vulcanization can be defined as 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

Rubber – latex – had been known for thousands of years in

Mesoamerican 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.[4]

Early rubber tube tires in the 19th century would grow sticky on a hot road. Debris would get stuck in them and eventually the tires would burst.

Charles Goodyear, 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 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 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,[5] vibration mounts/dampers, insulation materials, tires, and bowling balls.[6] 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

Main article: Sulfur vulcanization

The most common vulcanizing methods depend on sulfur. Sulfur, by itself, is a slow vulcanizing agent and does not vulcanize synthetic

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

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.[8]

Vulcanization of silicones

An example of a silicone rubber keypad typical of LSR (Liquid Silicone Rubber) moulding
Main article: RTV silicone

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.[9] 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

References

  1. doi:10.1016/S0079-6700(96)00015-9
    .
  2. 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. ^ "A Guide to the Uses and Benefits of Vulcanised Rubber". Martins Rubber. January 27, 2020. Retrieved June 16, 2021.
  6. ^ "Vulcanized Rubber". Retrieved June 16, 2021.
  7. doi:10.1002/14356007.a23_365.pub2
  8. ^ "A Safer Alternative Replacement for Thiourea Based Accelerators in the Production Process of Chloroprene Rubber". cordis.europa.eu. Retrieved April 25, 2024.
  9. ^ "MSDS for red RTV-Silicone" (PDF). Archived (PDF) from the original on October 9, 2022. Retrieved June 24, 2011.
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