vulcanization
As vulcanization refers to methods in which thermoplastic natural rubbers or synthetic rubbers in elastomeric be converted plastics (rubber). During vulcanization, covalent cross-links are formed between the macromolecules of the rubbers , so that the molecules can no longer move freely against each other, which leads to elastic behavior .
Classic vulcanization is the conversion of natural rubber with sulfur , which was developed by Charles Goodyear in 1839 .
Procedure
Sulfur vulcanization
For vulcanization, a rubber mixture, consisting of raw rubber , sulfur or sulfur-donating substances such as. B. disulfur dichloride (S 2 Cl 2 ), catalysts (for example, 2-mercaptobenzothiazole or tetramethylthiuram disulfide and zinc oxide and fatty acids are used to increase the reaction rate ) and fillers. Nowadays vulcanization is usually done with 1.8 to 2.5% sulfur and a temperature of 120 to 160 ° C. The long-chain rubber molecules are bound by sulfur bridges networked . As a result, the plastic properties of the rubber or the rubber mixture are lost, and the substance is converted from a plastic to an elastic state by means of the vulcanization process .
The rubber produced in this process has permanently elastic properties compared to the original product, returns to its original position when subjected to mechanical stress, has higher tear resistance, elongation and resistance to aging and weathering.
The elasticity of the rubber material depends on the number of sulfur bridges. The more sulfur bridges there are, the harder the rubber is. The number of sulfur bridges in turn depends on the amount of sulfur added and the duration of the vulcanization. As the rubber ages, the sulfur bridges are replaced by oxygen bridges; the rubber becomes brittle and porous.
Other procedures
In addition to classic sulfur vulcanization, rubbers are often crosslinked using peroxides , metal oxides or high-energy radiation . Since vulcanization with sulfur requires the presence of double bonds , these processes are used especially for rubbers that do not contain double bonds (e.g. EPM , EVA ). Metal oxides are u. a. used in the crosslinking of chloroprene rubber . Compounds like dibutylamineaccelerate vulcanization. The course of vulcanization can be measured and graphically documented with a test or measuring device, the vulcanometer or a simple rheometer .
The vulcanization of chloroprene rubber or neoprene (CR rubber) is carried out using metal oxides (in particular MgO and ZnO, sometimes also PbO). In addition, due to other processing factors (mainly 'scorch' or 'singeing', the premature crosslinking of rubbers due to the influence of heat), the selection of accelerators is regulated by other parameters. Most commonly used accelerators are problematic when curing CR rubbers. The most important accelerator for this is ethylene thiourea(ETU) used. While this substance is an excellent and proven catalyst for polychloroprene, it is classified as toxic to reproduction. The European rubber industry has started the SafeRubber research project to develop a safer alternative to using ETU.
Styrene-butadiene rubber (SBR) and nitrile rubber (NBR) can also be vulcanized purely thermally (thermovulcanization).
Apprenticeship
Mechanic for tire and vulcanization technology is a recognized apprenticeship in Germany and Austria. In Germany, the apprenticeship "tire mechanic" is a trade that is subdivided into the fields of tire and chassis technology and vulcanization technology. As a process mechanic (specializing in plastics and rubber technology, with a focus on molded parts), the training lasts three years. Trained vulcanizers are employed in tire and chassis service or in retreading and conveyor technology. Here the main task in the vulcanization technology job is the repair of vehicle tires of all types, the repair of conveyor belts and the coating and adhesive connections in the industrial sector.
literature
- H.-W. Engels: Rubber , 4th Chemicals and Additives . In: Ullmann's Encyclopedia of Industrial Chemistry . Wiley-VCH Verlag, 2004 (detailed overview of rubber and vulcanization).
Web links
Individual evidence
- ↑ to post vulcanization . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.VT07153 Version: 2.3.3.
- ^ MD Lechner, K. Gehrke, EH Nordmeier: Makromolekulare Chemie . 4th edition. Birkhäuser Verlag, 2010, ISBN 978-3-7643-8890-4 , p. 485.
- ^ Karlheinz Biederbick: Plastics . 4th edition. Vogel-Verlag, 1977, ISBN 3-8023-0010-6 , p. 82.
- ↑ Joachim Buddrus: Fundamentals of organic chemistry . 4th edition. Walter de Gruyter Verlag, Berlin 2011, ISBN 978-3-11-024894-4 , p. 897.
- ↑ Safe Rubber, to alternative for accelerators in the production of rubber ( Memento of 14 April 2015, Internet Archive ).