Sulfur vulcanization

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.^[6]

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.^[7]

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.^[8]

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.

Methods

Sulfur 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, and one obtains an unsatisfactory crosslinking efficiency with unsatisfactory strength and aging properties. Vulcanization accelerators improve the quality of the product. Diverse additives, comprising the cure package, are necessary. The combined cure package in a typical rubber compound consists of insoluble sulfur together with compounds that modify the kinetics of crosslinking. These additives include accelerators, activators like zinc oxide, and stearic acid. The accelerators and activators are catalysts. An additional level of control is achieved by agents that inhibit vulcanization until some optimal time or temperature, which allow the uncrosslinked polymer to adapt to the mold before crosslinking. Typical retarding agents are sulfenamides. Antidegradants such as aromatic amines are added to inhibit oxidative degradation of the vulcanized product oxygen, and ozone.^[9] Before vulcanization, the sulfur is not miscible in the polymer, and attention is paid to prevent sulfur bloom, where it migrates to the surface of the article.

Cyclohexylthiophthalimide is used to impedes the onset of vulcanization.^[9]