Sulfur vulcanization

This article is about processes specifically based around sulfur. For a wider range of processes, see Vulcanization.

Worker placing tire in a mold before vulcanization.

Sulfur vulcanization or sulfur vulcanisation is a chemical process for converting natural rubber or related polymers into more durable materials by heating them with sulfur or other equivalent curatives or accelerators. Sulfur forms cross-links (bridges) between sections of polymer chain which results in increased rigidity and durability, as well as other changes in the mechanical and electronic properties of the material. A vast array of 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.

The main polymers subjected to sulfur vulcanization are polyisoprene (natural rubber, NR), polybutadiene rubber (BR) and styrene-butadiene rubber (SBR), all of which are rich in unsaturated groups. Together these have a wide range of applications of which vehicle tires are perhaps the main example. Several other specialty rubbers may also be vulcanised, such as nitrile rubber (NBR), butyl rubber (IIR) and EPDM rubber.

Vulcanization, in common with the curing of other thermosetting polymers, is generally irreversible; however there has been significant effort to try and develop 'de-vulcanisation' processes in order to improve the recycling of rubber wastes.

Chemistry

General representation of the chemical structure of vulcanized natural rubber showing the crosslinking of two polymer chains (blue and green) with sulfur (n = 0, 1, 2, 3 …).

The chemistry of vulcanisation is complex, and there has long been uncertainly as to whether it proceeds in a radical or ionic manner. The reactive sites, often referred to as 'cure sites', are unsaturated groups such as alkenes and allyls. During vulcanization sulfur bridges are formed between these sites, crosslinking the polymer. These bridges may consist one or several sulfur atoms. Both the extent of crosslinking and the number of sulfur atoms in the crosslinks strongly influences the physical properties of the rubber produced:

• Excessive crosslinking can convert the rubber into a hard and brittle substance (i.e. ebonite).
• Short crosslinks, possessing lower numbers of sulfur atoms, give the rubber better heat resistance.
• Longer crosslinks, with higher numbers of sulfur atoms, give the rubber improved physical durability.

Sulfur, by itself, is a slow vulcanizing agent and does not vulcanize synthetic polyolefins. Even with natural rubber large amounts of sulfur as well as high temperatures and long heating periods are necessary, with the end products often being of an unsatisfactory quality.

Over the last 200 years various chemicals have been developed to improve the speed and efficiency of vulcanisation, as well as to control the nature of the cross-linking, in order to produce rubber articles with the desired properties. When used together to give a rubber with particular properties vulcanization reagents are generally referred to as a cure package.

Cure Package

The cure package consists of various reagents that modify the kinetics and chemistry of crosslinking. These include accelerators, activators, retarders and inhibitors. Note that these are merely the additives used for vulcanisation and that other compounds may also be added to the rubber such as fillers or polymer stabilizers.

Sulfur source

Ordinary sulfur (octasulfur) is by far the most commonly used reagent due to its low cost and low toxicity, however it is possible replace sulfur with other sulfur-donating compounds, for example accelerators baring disulphide groups, in what is often termed 'efficient vulcanization' (EV). Disulfur dichloride may also be used for 'cold vulcanization'. Under normal circumstances sulfur is not miscible in the polymer prior to vulcanization and attention is paid to prevent sulfur bloom, where it migrates to the surface of the article.

Accelerators

See also: Accelerant § Rubber_vulcanization

Accelerators act much like catalysts allowing vulcanisation to be performed cooler yet faster and with a more efficient use of sulfur. They achieve this by reacting with and breaking the sulfur ring to form a reactive intermediate, referred to as a sulfurating agent. This in turn reacts with cure site in the rubber to bring about vulcanization.

There are two major classes of vulcanization accelerators: primary accelerators and secondary accelerators (also known as ultra accelerators). Primary activators date back to the use of ammonia in 1881, with secondary accelerators have been developed from around 1920.

Primary (fast-accelerators)

Primary accelerators perform the bulk of the accelerating and mostly consist of thiazoles, often derivatised with sulfenamide groups. The principle compound is 2-mercaptobenzothiazole (MBT) which has been in use since the 1920s, it remains a moderately fast curing agent giving sulfur chains of a medium length but it's relatively short induction period can be a disadvantage. Other primary accelerators are essentially 'masked' forms of MBT which take time to decompose into MBT during vulcanisation and thus have longer inductions periods.

• 
  Mercaptobenzothiazole (MBT)
• 
  Mercaptobenzothiazole disulfide (MBTS)
• 
  Dicyclohexyl-2-benzothiazolesulfenamide (DCBS)

Oxidative coupling of MBT gives mercaptobenzthiazole disulfide (MBTS), sulfenamide derivatives are produced by reacting this with primary amines like cyclohexylamine or tert-butylamine. Secondary amines like dicyclohexylamine can be used and result in even slower accelerators. Such a slow accelerator is required in applications in which the rubber is being cured onto a metal component to which is it required to adhere, such as the steel cords in vehicle tires.

Secondary (ultra-accelerators)

Secondary or ultra-accelerators are used in small amounts to augment the behaviour of primary accelerators, they act to boost the cure speed and increase cross-link density but also shorten the induction time which can lead to premature vulcanization. Chemically, they consist mainly of thio-carbonyl species such as thiurams, dithiocarbamates, xanthates and organic thioureas. Many of these compounds need to be combined with activators, typically zinc ions, in order to be fully active. Aromatic guanidines are also used.

• 
  Thiuram
• 
  Zinc bis(dimethyldithiocarbamate) (Ziram)
• 
  diphenylguanidine (DPG)

Secondary accelerators have very fast vulcanization speeds with minimal induction time, making them unsuitable as primary accelerators in highly unsaturated rubbers such as NR or SBR, however they can be use as primary accelerators in compounds with fewer curing site such as EPDM. Xanthates (principally, zinc isopropyl xanthate) are important in the vulcanization of latex, which is cured at relatively low temperature (100-120°C) and therefore need an inherently rapid accelerator. The major thiurams used are TMTD (tetramethylthiuram disulfide) and TETD (tetraethylthiuram disulfide). The major dithiocarbamates are the zinc salts ZDMC (zinc dimethyldithiocarbamate), ZDEC (zinc diethyldithiocarbamate) and ZDBC (zinc dibutyldithiocarbamate).

Activators

Activators consist of various metal salts, fatty acids as well as nitrogen-containing bases, the most important these being zinc oxide. Zinc actives many accelerators by coordination, for example causing thiuram to convert into ziram. Zinc also coordinates to the sulfur-chains of sulfurating agents, changing the most likely bond to break during cross-link formation, untilmately they promote efficient use of sulfur to give a high density of cross-links. Due to the low solubility of ZnO it is often combined with fatty acids such as stearic acid to form more soluble metallic soap (i.e. zinc stearate).

Retarders and Inhibitors

Cyclohexylthiophthalimide is used to impede the onset of vulcanization.

In order for good quality vulcanisation to take place the rubber, sulfur, accelerators, activators and other compounds must be fully mixed to give a homogeneous liquid. In practise this can involve melting the sulfur (mpt. 115°C) and at these temperatures vulcanisation can begin prematurely. This is often undesirable as the mixture may still need to be pumped and moulded into its final form before it sets solid. Premature vulcanisation is often referred to as 'scorch' and can be prevented by the addition of retarders or inhibitors which increase the induction period before vulcanisation commences and thus provide scorch resistance. A retarder slows down both the onset and rate of vulcanisation, whereas inhibitors only delay the start of vulcanisation and do not effect the rate to any great extent. In general inhibitors are preferred, with cyclohexylthiophthalimide (often termed PVI - pre vulcanisation inhibitor) being the most common example.

Devulcanization

Main article: Tire recycling

The market for new raw rubber or equivalent is large. The auto industry consumes a substantial fraction of natural and synthetic rubber. Reclaimed rubber has altered properties and is unsuitable for use in many products, including tires. Tires and other vulcanized products are potentially amenable to devulcanization, but this technology has not produced material that can supplant unvulcanized materials. The main problem is that the carbon-sulfur linkages are not readily broken, without the input of costly reagents and heat. Thus, more than half of scrap rubber is simply burned for fuel.

Inverse vulcanization

Although polymeric sulfur is unstable and decomposes back to its monomer, it is possible to create stable polymers consisting mostly of sulfur via a reaction with low levels of unsaturated organic linkers (e.g. 1,3‐diisopropenylbenzene). This process is called inverse vulcanisation and produces polymers where sulfur is the main component. It is not currently of commercial importance but had been investigated as a means of producing polymers for lithium–sulfur batterys and for creating water filters with a high affinity for mercury.

History

The curing of rubber has been carried out since prehistoric times. The name of the first major civilization in Guatemala and Mexico, the Olmec, means 'rubber people' in the Aztec language. Ancient Mesoamericans, spanning from ancient Olmecs to Aztecs, extracted latex from Castilla elastica, a type of rubber tree in the area. The juice of a local vine, Ipomoea alba, was then mixed with this latex to create processed rubber as early as 1600 BC. In the Western world, rubber remained a curiosity, although it was eventually used to produce waterproofed products, such as Mackintosh rainwear, beginning in the early 1800s.

Modern developments

In 1832–1834 Nathaniel Hayward and Friedrich Ludersdorf discovered that rubber treated with sulfur lost its stickiness. It is likely Hayward shared his discovery with Charles Goodyear, possibly inspiring him to make the discovery of vulcanization.

Thomas Hancock (1786–1865), a scientist and engineer, was the first to patent vulcanization of rubber. He was awarded a British patent on May 21, 1845. Three weeks later, on June 15, 1845, Charles Goodyear was awarded a patent in the United States. It was Hancock's friend William Brockedon who coined term 'vulcanization'.

Goodyear claimed that he had discovered vulcanization earlier, in 1839. He wrote the story of the discovery in 1853 in his autobiographical book Gum-Elastica. Here is Goodyear's account of the invention, taken from Gum-Elastica. Although the book is an autobiography, Goodyear chose to write it in the third person so that the inventor and he referred to in the text are the author. He describes the scene in a rubber factory where his brother worked:

The inventor made experiments to ascertain the effect of heat on the same compound that had decomposed in the mail-bags and other articles. He was surprised to find that the specimen, being carelessly brought into contact with a hot stove, charred like leather.

Goodyear goes on to describe how his discovery was not readily accepted.

He directly inferred that if the process of charring could be stopped at the right point, it might divest the gum of its native adhesiveness throughout, which would make it better than the native gum. Upon further trial with heat, he was further convinced of the correctness of this inference, by finding that the India rubber could not be melted in boiling sulfur at any heat, but always charred. He made another trial of heating a similar fabric before an open fire. The same effect, that of charring the gum, followed. There were further indications of success in producing the desired result, as upon the edge of the charred portion appeared a line or border, that was not charred, but perfectly cured.

Goodyear then goes on to describe how he moved to Woburn, Massachusetts and carried out a series of systematic experiments to optimize the curing of rubber, collaborating with Nathaniel Hayward.

On ascertaining to a certainty that he had found the object of his search and much more, and that the new substance was proof against cold and the solvent of the native gum, he felt himself amply repaid for the past, and quite indifferent to the trials of the future.

Later developments

The discovery of the rubber-sulfur reaction revolutionized the use and applications of rubber, changing the face of the industrial world. Formerly, the only way to seal a small gap between moving machine parts was to use leather soaked in oil. This practice was acceptable only at moderate pressures, but above a certain point, machine designers were forced to compromise between the extra friction generated by tighter packing and greater leakage of steam. Vulcanized rubber solved this problem. It could be formed to precise shapes and dimensions, it accepted moderate to large deformations under load and recovered quickly to its original dimensions once the load is removed. These exceptional qualities, combined with good durability and lack of stickiness, were critical for an effective sealing material. Further experiments in the processing and compounding of rubber by Hancock and his colleagues led to a more reliable process.

Around 1900, disulfiram was introduced as a vulcanizing agent, and became widely used.

In 1905 George Oenslager discovered that a derivative of aniline called thiocarbanilide accelerated the reaction of sulfur with rubber, leading to shorter cure times and reducing energy consumption. This breakthrough was almost as fundamental to the rubber industry as Goodyear's sulfur cure. Accelerators made the cure process faster, improved the reliability of the process and enabled vulcanization to be applied to synthetic polymers. One year after his discovery, Oenslager had found hundreds of applications for his additive. Thus, the science of accelerators and retarders was born. An accelerator speeds up the cure reaction, while a retarder delays it. A typical retarder is cyclohexylthiophthalimide. In the subsequent century chemists developed other accelerators and ultra-accelerators, that are used in the manufacture of most modern rubber goods.