Vulcanization - what is that?

•     The way of the natural caoutchouc
  
•     The chemistry of caoutchouc
  
•     Other synthetic caoutchoucs and their properties
  
•     Vulcanizing - a change in physical state
  
•     Vulcanization of a tube repair as an example  

 

From caoutchouc to rubber     The country of origin of caoutchouc is the brasilian jungle on the south-american continent. There caoutchouc can be found in high quantites (ca. 35%) in the white sap of a tree named hevea brasiliensis. The method of scratching the truncks of the up to 60m high trees in order to get the rap, sometimes also called latex or rubber milk, goes back to the aztecs. Nowadays hevea brasiliensis is cultivated, grown and regularly harvested within the so-called chaouchouc belt (30th parallel north to 30th parallel south) all around world. Furthermore the produce of the plant has been increased from originally 2-4 kg per plant to up to 23 kg (with help of chemical stimulants).   The caoutchouc belt - cultivation of hevea brasiliensis (sorry, only German)     To harvest the rubber milk screw-shaped or herringbone pattern-shaped stripes are scratched into the bark and the sap is collected in buckets placed under it. It is then collected within bigger tanks and been forced into koagulation (lumpping of the proteins of the latex).   The produced coutchouc lumps aer washed with water and then mixed in order to obtain a more or less uniform coutchouc. Then they are rolled to thin sheets with friction rollers or with and waver pattern and then dried or even smoked (depending on if they are needed as pale crêpe or smoked sheets). As bale of caoutchuc they are ready to be shipped.   The way of the caoutchuc (sorry, German again)     Depending on quality and cleanness the caoutchuc is used for producing rubber rings for jelly glases, milk tubes, baby comforter or caps for ferment bottles (high quality, light colors), for producing side wall rubber for tires, tubes or conveyor belts (medium quality, light-brownish colors) or for producing tread rubber for cars or trucks (low quality, darker colors).   For the last two products a series of substances are added to the caoutchuc so that at the end sometimes only 42% is still caoutchuc. A typical tire consists - besides caoutchuc - of synthetic caoutchuc (ca. 18%), filling substances as soot or silica (28%) - they are responsible for the adrasion resistence, the tensile strength and the temper - , peptisation agents (0,1%) and dispergence agents (1,2%) - for helping the mixing and the blending of the ingredients - , aging protectors (1,2%), anti-light protectors and waxes (0,9%), softeners (3%), interconnecting agents (normally sulfur; 1,5%) and vulcanization accelerators (0,6%) together with the vulcanization activators (3%).   The entire mixture is then finally put into form and vulcanzied onto the tread of a tire with 150 to 200 degrees Celsius. The rubber tire is finished.     The chemistry of the caoutchuc     Isoprene, its chemical name is methyl butadiene, is the basic ingredient of the caoutchuc. It consists of a ramified hydrocarbon chain, which is unsaturated - meaning that is has double binds - so that it can be polymerized (left picture below). Because of the facts that single binded carbon atoms be rotated arbitrarily and on the contrary double binded carbon atoms are fixed, there are four different isomeres for the polyisoprene: cis-1,4-polyisoprene, trans-1,4-polyisoprene, 1,2-polyisoprene and 3,4-polyisoprene (using one or both double binds).   Monomere form of isoprene or methyl butadiene      polymere isomeres of isoprene     The first two isomeres form by breaking both double binds, which partly flap to the middle and partly form the new polymerization single binds. The last two isomeres on the other hand only use one double bind which goes into the polymerization chain (so theoretically the other double bind can be integrated into another chain).   Because all these different isomeres are formed from methyl butadiene we can assume the polyisoprene is everything but a homogeneous substance (for example only cis-1,4-polyisoprene; see picture below), that it is more like a mixture of all isomeres (picture below that).       polyisoprene as a homogeneous substance? Probably not   polyisoprene in his natural, inhomogeneous form     The synthetic caoutchuc is build out of similar molecules, only that these molecules are produced in petroleum refineryies. Nowadays the most frequently used synthetic caoutchuc (SBR) consists of styrene (benzole and ethene under separation of a hydrogene molecule - in German it's called Styrol) and butadiene. These molecules again are able to form a polimerization chain by breaking up their double chains and forming new single binds. But the mechanical (and other) properties of this caoutchuc are hardly comparable wit the exellent properties of natural caoutchuc.   Basic ingredients of synthetic caoutchuc SBR (styrene butadiene rubber)   styrene-butadiene polymerization chain (SBR)   So the search for other synthetic caoutchucs became a great task; the resulting properties of the different types of rubber had to be optimized. Today we know more than one hundred different synthetic caoutchucs for diverse requirements (normally just one! See table below). There are caoutchucs which are resistant to acids or leaches, to cold or heat, to pressure or abrasion, but generally fail in all the other categories sometimes completely.   Silicone caoutchuc (Silastic)resistant to heat and cold VMQ fluor caoutchuc (Fluorel)resistant to acid and leach FKM polyurethane caoutchuc (Adipren)high abrasion restistence EU epichlorhydrine caoutchucozone resistant ECO styrole-butadiene-caoutchuchigh tensile strength SBR Other synthetic caoutchucs and their properties     Therefore a number of different sustances are added to the caoutchuc before the vulcanization to ensure that the rubber will be satisfying in lots of areas independently of the basic molecule of the corresponding caoutchuc (respectively the caoutchuc mixture, because for even a tire for example lots of different natural caoutchucs are normally mixed).   In any case everything depends on the right mixture, and the perfect caoutchuc does not exist and will never be found.     Vulcanizing - a change in physical state     As already explained above, the latex milk (resp. the liquid ingredients of synthetic caoutchucs) is transformed into a relatively solid but still deformable mass, the caoutchuc. Chemically speaking, the short, freely moving molecules (butadiene etc.) are now polymerized into long chains. These hydrocarbon chains have a length between some 50 and some 350 carbon atoms but are still relatively free to move because there are no chemical bonds between them. This mass (=caoutchuc) is kneadable or as the chemical engineer says plastic. The chains are able to glid along each other under outer force (i.e. pressure) but are still a solid because of the electrostatic attraction. But we are looking for a different physical state: the finished rubber has to be elastic, that is it has to be so solid that it will bounce back to its original form after releasing an outer force that deforms it.   caoutchuc: plastic state (green rings represent sulfur)     To get this property we have to interconnect the chains with chemical bridges. For that sulfur seems to be the best substance. The sulfur (normally in its solid state as the molcule ring S8) is added in form of a fine powder to the caoutchuc. But to break the sulfur rings and to place them between the caoutchuc chains energy is needed. Therefore vulcanization, the transition from the plastic into the elastic state, begins to be effective only at temperatures above 150ºC and under pressure. Analysis has shown that the sulfur rings open and even break apart and interact with the still existing double binds of the carbon chains to form the bridges. By doing so they inhibit the gliding of the chains.   rubber: elastic state (green rings of sulfur are broken and connect the double binds of the carbon chains)     Vulcanization of a tube repair as an example     But now there is a probleme with repairing an (already vulcanized) tube: the vulcanized rubber is elastic and the inner sulfur is used or inactive/fixed so that by vulcanizing new caoutchuc onto the old rubber, sulfur bridges cannot form between the old and the new carbon chains because the sulfur is not movable enough. How is a repair still possible?   Closer investication has revealed that really no sulfur bridges are formed between old and new rubber. The rubber is only hold together by the adhesion forces, the forces between different and reparated materials, and not by the cohesion forces, which can be found within homogene substances.   (between wet paper and window glass)          (within water drop on wax paper) adhesion and cohesion forces     The adhesion forces hold together two surfaces because they perfectly fit together. Tension between them will only lead to a vacuum which will then keep the two surfaces together. Enlarging the surface (by roughening it) will even enlarge the adhesion force.   (new rubber above, old rubber below; tension leads to vacuum) adhesion forces used for the vulcanization     On this idea a tube repairs is based on. After finding the hole the surface around it is roughened. To prevent air inclusion within the deeper grooves when putting on the new raw rubber (=caoutchuc) the spot is coated with rubber solution (in benzole soluted tire caoutchuc). This coating closes the grooves and also helps by sticking together the new caoutchuc and the old rubber until it is vulcanized.   (new raw rubber above, old rubber below, red is rubber solution) old rubber is roughened, coated and overlayed with raw rubber     Then the spot is put into a heating form and is heated under pressure onto 130ºC til 170ºC. The heating time depends on the thickness of the vulcanizing rubber (approximately 2 til 5 minutes for each millimeter). Now the sulfur rings within the caoutchuc and the rubber solution break up and interconnect their hydrocarbon chains. The newly formed surface fits exactely into the old roughened rubber.   after the vulcanization (large surface; only air inclusions)     This union can not be saparated by abrasion or tensile strength, the tube is repaired.