Rubber compounding vulcanization system

Despite of 150-year s history of vulcanization process, it is impossible to consider that fundamental and applied researches in direction of vulcanization systems perfection are completed. For today one of the ways of rubbers properties improvement is the synthesis and application of the new chemicals-additives, including, vulcanization active, that is connected, first of all, with reduction of global stocks of zinc ores as basic raw material for reception of traditional activator - zinc oxide. Besides, modem increase of industrial potential and the accumulation of big quantity wastes derivate the problems of ecological character, which require the emergency decision. Therefore creation of resourcesaving technologies of the new compounds reception from products of secondary raw material processing has paramount importance. 

The basic compounding formulation specifies the minimum requirement of fillers, vulcanizing agents, and other substances that must be added to the rubber compound to achieve the desired properties. After the rubber, cure system and reinforcing filers have been selected it will be necessary to make several adjustments before all requirements are satisfied. It is generally sensible to start with the simplest mix formula for meeting the requirements. The recipe or the formula is usually written on the basis of hundred parts of rubber. For example if 5 parts of zinc oxide is to be added it is denoted as 5 phr (five parts per hundred rubber). Elementary compounding... 

It can be seen that every type of vulcanization system differs from every other type in the kind and extent of the various changes that together produce the vulcanized state. In the vulcanization processes, consideration must be made for the difference in the thickness of the products involved, the vulcanization temperature and thermal stability of the rubber compound. The word cure to denote vulcanization is believed to have been coined by Charles Goodyear and the same has been a recognized term in rubber industry circles [2]. The conditions of cure will vary over a wide range according to the type of vulcanizate required and the facilities available in a rubber factory. Many factors must be predetermined, including the desired hardness of the product, its overall dimensions, the production turnover required and the pretreatment of the rubber stock prior to vulcanization. Hardness will normally be determined by the composition of the stock but it can also be influenced by the state of cure. 

These observations make it highly probable that similar reactions occur when carbon black is exposed to thermally or shear-generated polymer free radicals during processing of rubber compounds or to polymeric radicals formed during vulcanization, particularly in such typically free radical cross-linking systems as peroxide or radiation. 

An irreversible process used to convert raw rubber compounds into a crosslinked elastomeric system, through sulfur or sulfur compounds. Arises from vulcan being the God of the fire and the smithy. 

Vulcanization, named after Vulcan, the Roman God of Fire, describes the process by which physically soft, compounded rubber materials are converted into high-quality engineering products. The vulcanization system constitutes the fourth component in an elastomeric formulation and functions by inserting crosslinks between adjacent polymer chains in the compoimd. A typical vulcanization system in a compound consists of three components (1) activators (2) vulcanizing agents, typically sulfur and (3) accelerators. 

Accelerators are products that increase both the rate of sulfur crosslinking in a rubber compound and crosslink density. Secondary accelerators, when added to primary accelerators, increase the rate of vulcanization and degree of crosslinking, with the terms primary and secondary being essentially arbitrary. A feature of such binary acceleration systems is the phenomenon of synergism. Where a combination of accelerators is synergistic, its effect is always more powerful than the added effects of the individual components. 

Sodium hexamethylene-l,6-bisthiosulflde dihydrate, when added to the vulcanization system, breaks down and inserts a hexamethylene-1,6-dithiyl group within a disulfide or polysulfide crosslink. This is termed a hybrid crosslink. During extended vulcanization periods or accumulated heat history due to product service, polysulfidic-hexamethylene crosslinks shorten to produce thermally stable elastic monosulfidic crosslinks. At levels up to 2.0 phr, there is little effect on compound induction or scorch times, nor on other compound mechanical properties (Rubber Chemicals, 1998). 

Depending on the complexity of the formulation, size of the internal mixer, and application for which the compound is intended, the mix cycle can be divided into a sequence of stages. For an all-natural-rubber compound containing 50 phr carbon black, 3 phr of aromatic oil, an antioxidant system, and a semi-EV vulcanization system, a typical Banbury mix cycle will be as follows ... 

This chapter has reviewed both the types and the properties of elastomers, compounding with a range of filler or reinforcement systems such as carbon black, and enhancement of tiller performance by novel use of compounding ingredients such as silane coupling agents. Other issues such as antioxidant systems and vulcanization systems were also discussed. The role of the modern materials scientist in the tire and rubber industry is to use materials to improve current products and develop new products. Four key parameters govern this development process ... 

Figure 10.1. Tensile stress-strain curves for four natural rubber compounds of different hardnesses 73 IRHD contains 50 parts of a reinforcing black, and different vulcanizing systems account for the different curves of the two gum compounds (47 and 33 IRHD). (Lindley, 1964.)...

The use of peroxide vulcanization systems has always been somewhat controversial among rubber compounders, largely based on its higher cost compared to sulfur. However, this needs to be examined on a compound by compound formulation basis. From the standpoint of thermal stability, the peroxide crosslink has a bond energy of about 82 kilocalories and is as stable as any of the carbon-carbon bonds in the polymer backbone. In contrast, the sulfur cross-link,... 

The vulcanization activator system consisting of zinc oxide and stearic acid has received much less research effort than other components in the rubber compound. Stearic acid and zinc oxide levels of 2.0 and S.Ophr, respectively. 

The heat resistance of NR is not good enough for many uses, and it is exceeded by many synthetic rubbers. It is affected by the choice of vulcanization system, vulcanization conditions, choice of protective agents, and even choice of filler. To obtain good aging resistance of NR vulcanizates, one must use protective agents in the compound and use relatively short curing cycles at relatively low temperatures. 

Most Rubber-based adhesives may be cured by a sulphur-based vulcanizing system (see Rubber-based adhesives compounding), however, as mentioned in Polychloroprene rubber adhesives applications and properties, CR adhesives are cross-linked by various reactions involving the labile chlorine atoms in the repeat unit. This is reflected in the additives used, as discussed below. ... 

The elastomers and elastomer blends used in tire compounds, and particularly tread compounds, are thus equally important to the structural design parameters of the tire (Table 4.7). A properly designed tread compound will ensure the tire can meet its performance targets. Tread compounding materials fall into one of five general categories polymers, fillers, protectants, vulcanization system, and various special purpose additives. Elastomers for tread compounds are typically natural rubber (NR), styrene-butadiene rubber (SBR), polybutadiene (BR), and in some instances isobutylene based polymers for winter and special performance tires. 

Reinforcing resins or novolacs form an interpenetrating network and can interact with other polymers. Hexamethylenetetramine-resorcinol systems do not start to react until temperatures exceed 120 °C. At vulcanization temperatures of 160 °C they can significantly increase the stiffness of rubber compounds though in dynamic service conditions such as in a tire tread compound the stiffness imparted by such novolac resins may be degraded. 

Thermoplastic vulcanizates (TPVs) are composed of a vulcanized rabber component, such as EPDM, nitrile rubber, and butyl rubber in a thermoplastic olelinic matrix. TPVs have a continuous thermoplastic phase and a discontinuous vulcanized rubber phase. The most common TPV polymer systan is PP/EPDM rubber however, a number of other polymer systems have been commercially developed. These include PP/NBR, PP/butyl and PP/halobutyl, PP/NR, and PP/EVA/EPDM. Producers include Advanced Elastomers Systems (Santoprene, Geolast, and Trefsin). The highly rubberlike properties of TPV have enabled than to perform as engineered thermoplastic rubbers. In numerous application areas they have directly replaced premium-performance thermoset rubber compounds. Prominent among these are dananding automotive applications, electrical insulation and connectors, compression seals, appliance parts, medical devices, and food and beverage contact applications. 

---