Lecture No. (12) Rubber Vulcanization

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1 Lecture No. (12) Rubber Vulcanization

2 History of Rubber Vulcanization Generally, the elastomers materials give the flexibility and elasticity properties, but in many cases the properties of elastomers must be modified in order to turn them into useful rubbery materials. Rubber has two disadvantages it becomes soft and sticky when heat, and it becomes hard and solid when cold. In the 18th century, the first modification was done on the natural rubber (polyisoprene), and these disadvantages were overcome in 1839, by the discovery of vulcanization process accidentally by American Charles Goodyear when heated natural rubber that contained sulfur with 8 percent. In 1941, Thomas Hancock used the same process in England, when crosslinking of free rubber chains to form three dimensional elastic networks by polysulfide bonds. Rubber Vulcanization Rubbers are generally very viscous elastic liquids which can flow, especially at high temperatures, this is because their long molecular chains can slip past one other when heating or applied force. and be very hard at low temperature because it is crystallize below about (5 C). There is a need to make bridges among these chains in order to prevent these slipping and make the rubber behave like elastic material, and returning to its original state after force remove. The vulcanization (curing) process of rubber is defined the process which involves the chemical reaction of the unsaturated rubber with small amount of sulfur about ( pphr) at high temperature in presence of an activator and accelerator, to form chemically cross-linking between the free rubber chains together to form a large three dimensional elastic network, the crosslinking process is often called curing or commonly vulcanization. The vulcanization process and the cross-linking reaction are illustrated in Figure (1). 1

3 Figure (1): Vulcanization Process and Cross-Links Reaction. This process results are transforming the original elastomer material from a soft, sticky and weak materials (viscous liquid) to a tough, elastic and resilient (hard solid) materials at high temperatures and stayed soft at low temperatures. Also give materials that returned to its original shape when force removes. As well as give certain properties such as higher tensile strength, elasticity, oxidation resistance, solvents resistance, swelling resistance, abrasion resistance, and ageing resistance over a wide range of temperatures, these above properties and other properties are shown in Figure (2). Figure (2): Properties of Valcaniced Rubber. 2

4 The original elastomeric material is converted from liquid state into solid state, that will not flow when heat, because the molecules chains are now permanently tied together. And this process gives elastomers their elasticity when cold, because when the elastomer chain try to slip over each other the crosslinking resist this movement and give force lead to chain back to their original position after the stress is relieved. Note, when over cured in this way, certain elastomers, particularly natural rubber, will revert to the soft, more plastic, less elastic condition same of the unvulcanization elastomer characteristics. Mechanisms of Vulcanization Process The vulcanization process occurs during sulfur reacts with unsaturated hydrocarbon elastomers with present of additives, this process mechanism as shown in Figure (3). Figure (3): Sulfur Interlinks are formed in Vulcanization Process. Notice that in the middle of the isoprene unit there are two carbon atoms linked together with double bonds while all the other bonds are singular. The natural rubber chain (polyisoprene chain) under the influence of heat, sulfur reacts with carbon atoms at the double bonds, and an indeterminate number of sulfur atoms (Sn) form crosslinkages between adjacent chains of polyisoprene. These double bonds allow elastomers to be elastic. That due to, all the double bonds are considered as potential sites, because it is not very stable. When sufficient heat energy applied on rubber, one of the double bonds can be disconnected from one of the carbon atoms. And refer to an open site that is available to attach with a 3

5 different sulfure atoms, at the combine elemental sulfur with rubber chains. This meaning the rubber has double bonds before vulcanization, but after vulcanization become have single bonds. This attachment leads to changing the uncured and useless material into the very useful and unique material. This reaction of "crosslinking" of two polyisoprene molecules with sulfur atoms, then a rubber network is formed with bridges of monosulfides. This is referred to vulcanization process. The vulcanization process occurs at approximately one out of every (200) potential sites in average sulfur cure system. Many mechanisms have been studied to explain the sulfur vulcanization process alone without additives such as (accelerators and activator), one of the most important mechanisms is thermally induced free radical reaction mechanism. When using sulfur is has a ring structure (S8) during the vulcanization, the sulfur rings firstly generate polysulfides radical at a high temperature. One radical polysulfide attach the free rubber chain removes hydrogen from carbon atoms near to double bonds and gives a chain radical, another polysulfide reacts with the chain radical and forms a new polysulfide radical. This polysulfide radical can react with another rubber chain to form a crosslink, and then a rubber network is formed with bridges of polysulfides, this type of vulcanization process as shown in figure (4). Figure (4): Sulfur Interlinks are formed in Vulcanization Process without Additives. 4

6 In Figure (A), free adjacent chains of polyisoprene rubber, made up of units containing carbon-carbon double bonds, are mixed with sulfur molecules. In Figure (B), the chain under the influence of heat, sulfur reacts with carbon atoms near to the double bonds, and an indeterminate number of sulfur atoms (Sn) form crosslinkages between adjacent chains of polyisoprene rubber. Types of Curing Systems Nowadays, many curing systems are used to vulcanize different types of elastomers materials. The type of curatives are used for vulcanization depends on the type of rubbers either saturated or unsaturated. The sulfur system is commonly used as curatives (vulcanized) for unsaturated rubbers. While saturated rubbers cannot be vulcanized by sulfur. Therefore the most common curatives (vulcanized) for saturated rubbers are peroxides, metal oxides, amines, isocyanates and urethane. Sulfur-Based Curing Systems The sulfur is the most widely used as curing agent for vulcanized unsaturated elastomers to effectively cross-linking the rubber chains, particularly the natural, styrene butadiene, and polybutadiene rubbers. Although the other curing agents can be used to vulcanization unsaturated elastomers, but sulfur dominates use because it is distinguish by have low cost, low toxicity, good compatible with other compounding additives, and able to provide the desired vulcanization properties such as high strength rubber. The most common vulcanizing methods are those dependent on sulfur and it consists principally of heating rubber with sulfur and other substances are added during vulcanization process such as (accelerators, activators, etc.). The sulfur does not simply dissolve or disperse in the rubber, but it will perform chemical bonds with rubber at double bonds to form of cross-links (bridges) between the long molecules chain of elastomer, function with sufficient heat and time provided. The crosslinking have about (45 to 55) sulfur atoms that combine with rubber chain. 5

7 Theory of Sulfur Vulcanization The vulcanization reaction can be modified in various desirable ways by using accelerators and activators. Because of when sulfur is used in the rubber vulcanization, the cure rate is slow to form polysulfide links and cross-linking with sulfur alone is quite inefficient and requires curing times of about (3 to 5) hours. But when addition of accelerators with sulfur lead to reduce the cure time between (2-5) min. Also, increase the sulfur crosslinking efficiency and cure rate is fast. There are different types of accelerators use during the vulcanization process, such as thiazoles (C 3 H 3 NS) and sulfonamides. Generally, the type of accelerator is much too important than the level of accelerator in controlling of vulcanization reaction. However, when increased the levels of accelerator lead to increase the degree of cross-linking done. Almost all accelerators require use of inorganic and organic activators to develop their full effectiveness. The main purpose of activator in rubber compounds is to support the vulcanization reaction, especially in the sulfur curing systems. The inorganic activators include zinc oxide and magnesium oxide, and the organic include stearic, palmitic, carboxylic acid and zinc salts. The efficiency of the polysulfide cross-links during rubber vulcanization can be controlled by the complex compounding formed by rubber with sulfur, activators (zinc oxide and carboxylic acid), and accelerators. It is that these additives combine to create soluble zinc ions that activate reactions involved in polysulfide crosslink formation. 6