Vulcanization

Worker placing a tire in a mold before vulcanization.

Vulcanization(British English:Vulcanisation) is a range of processes for hardeningrubbers.The term originally referred exclusively to the treatment ofnatural rubberwithsulfur,which remains the most common practice. It has also grown to include the hardening of other (synthetic) rubbers via various means. Examples includesilicone rubberviaroom temperature vulcanizingandchloroprene rubber(neoprene) using metal oxides.

Vulcanization can be defined as thecuringofelastomers,with the terms 'vulcanization' and 'curing' sometimes used interchangeably in this context. It works by formingcross-linksbetween sections of thepolymer chainwhich 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 otherthermosetting polymers,is generally irreversible.

The word was suggested byWilliam Brockedon(a friend ofThomas Hancockwho attained the British patent for the process) coming from the godVulcanwho was associated with heat and sulfur involcanoes.

History

In ancientMesoamericancultures, rubber was used to make balls, sandal soles, elastic bands, and waterproof containers.It was cured using sulfur-rich plant juices, an early form of vulcanization.

In the 1830s,Charles Goodyearworked 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 withsulfur,Goodyear accidentally dropped the mixture in a hot frying pan. To his astonishment, instead ofmeltingfurther orvaporizing,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.

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,vibration mounts/dampers, insulation materials, tires, and bowling balls.Most rubber products are vulcanized as this greatly improves their lifespan, function, and strength.

Overview

In contrast withthermoplasticprocesses (the melt-freeze process that characterize the behaviour of most modern polymers), vulcanization, in common with the curing of otherthermosetting 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 syntheticpolyolefins.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 tosulfur vulcanizationarepolyisoprene(natural rubber) andstyrene-butadienerubber (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—areallylichydrogen atoms. These C-H bonds are adjacent tocarbon-carbon double bonds(>C=C<). During vulcanization, some of these C-H bonds are replaced bychains of sulfuratoms 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 fle xing movements of the rubber article, e.g., the movement of a side-wall of a running tire. Without good fle xing properties these movements rapidly form cracks, and ultimately will make the rubber article fail.

Vulcanization of polychloroprene

The vulcanization ofneopreneorpolychloroprenerubber (CR rubber) is carried out using metal oxides (specificallyMgOandZnO,sometimesPb₃O₄) rather than sulfur compounds which are presently used with many natural andsynthetic 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 ofacceleratoris governed by different rules to otherdienerubbers. Most conventionally used accelerators are problematic when CR rubbers are cured and the most importantacceleranthas been found to beethylene thiourea(ETU), which, although being an excellent and proven accelerator for polychloroprene, has been classified asreprotoxic.From 2010 to 2013, the European rubber industry had a research project titled SafeRubber to develop a safer alternative to the use of ETU.

Vulcanization of silicones

An example of asilicone rubber keypadtypical of LSR (Liquid Silicone Rubber) moulding

Main article:RTV silicone

Room-temperature vulcanizing (RTV)siliconeis 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 formacetic 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. TheShore hardnesscan 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 inertsilicadeposit that is non-flammable and non-combustible. They can be used forelectrical insulationdue to theirdielectricproperties. 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
• Sulfur concrete
• Vulcanized fibre
• Vulcanizing shop