VULCANISATION RATE

Both these methods give zinc oxides of low activity. Zinc oxide from the American process can have a varying sulphur content, dependant upon the ore s source, and unless known and allowed for, this can affect the compound vulcanisation rate. 

Recently, it was shown using FT-IR that the decrease of vulcanisation rate and final crosslink density of sulfur vulcanised NR upon increasing silica content may be related to increased absorption of zinc stearate onto the silica surface [69]. 

The 13C NMR examination of CBS cured SBR was performed on unfilled, silica filled, and carbon black filled samples [41], Several different SBR samples with respect to styrene content and cis, trans, and vinyl BR content were used. The unfilled SBR samples gave 3 major peaks that appeared at 51.0, 50.2, and 49.3 in a spectrum similar to BR vulcanisation. Unfortunately, peaks below 45 ppm are obscured by the different main chain structural peaks of SBR. A difference was seen in the rate of formation of these peaks in filled samples with silica inhibit the vulcanisation rate compared to carbon black filled samples. 

The composition of the vulcanisation systems and the dynamics of vulcanisation can affect the bond strength significantly [6]. The flow behaviour of the stocks is controlled by the vulcanisation rate. During vulcanisation gradual transition from plastic flow... 

A low hardness EPDM/PP blend thermoplastic dynamic vulcanisate was produced by reactive compounding on a twin screw extruder. The effect of the dynamic vulcanisation rate on the average rabber particle diameter, and the average rubber phase crosslinking density of extruded vulcaiusates was investigated. 6 refs. 

The vulcanisation rate of a sulphur rubber composition is increased by heating the rubber composition to a temperature ranging from 100 to 200C. The rubber composition contains a sulphur vulcanisable rubber, a sulphenamide compound and a hydrated thiosulphate, the latter two compounds significantly increasing the vulcanisation rate of the rubber. 

Peroxides decompose when heated to produce active free radicals which ia turn react with the mbber to produce cross-links. The rate of peroxide cure is coatroUed by temperature and selection of the specific peroxide, based on half-hfe considerations (see Initiators, free-RADICAL Peroxy compounds, organic). Although some chemicals, such as bismaleimides, triaHyl isocyanurate, and diaHyl phthalate, act as coagents ia peroxide cures, they are aot vulcanisation accelerators. lastead they act to improve cross-link efftcieacy (cross-linking vs scissioa), but aot rate of cross-link formatioa. 

Accelerators are chemical compounds that iacrease the rate of cure and improve the physical properties of the compound. As a class, they are as important as the vulcanising agent itself. Without the accelerator, curing requires hours or even days to achieve acceptable levels. Aldehyde amines, thiocarbamates, thiuram sulfides, guanidines, and thiasoles are aU. classified as accelerators. By far, the most widely used are the thiasoles, represented by mercaptobensothiasole (MBT) and bensothiasyl disulfide (MBTS). 

Antioxidants may be assessed in a variety of ways. For screening and for fundamental studies the induction period and rate of oxidation of petroleum fractions with and without antioxidants present provide useful model systems. Since the effect of oxidation differs from polymer to polymer it is important to evaluate the efficacy of the antioxidant with respect to some property seriously affected by oxidation. Thus for polyethylene it is common to study changes in flow properties and in power factor in polypropylene, flow properties and tendency to embrittlement in natural rubber vulcanisates, changes in tensile strength and tear strength. 

Accelerated sulphur systems also require the use of an activator comprising a metal oxide, usually zinc oxide, and a fatty acid, commonly stearic acid. For some purposes, for example where a high degree of transparency is required, the activator may be a fatty acid salt such as zinc stearate. Thus a basic curing system has four components sulphur vulcanising agent, accelerator (sometimes combinations of accelerators), metal oxide and fatty acid. In addition, in order to improve the resistance to scorching, a prevulcanisation inhibitor such as A -cyclohexylthiophthalimide may be incorporated without adverse effects on either cure rate or physical properties. 

Vulcanisation being a chemical reaction, is time/temperature dependant. In factory operations, vulcanisation is usually carried out in an autoclave using steam under pressure at temperatures up to 160°C. If the lined unit is a vessel too large to fit in an autoclave and has an adequate pressure rating, all outlets can be sealed and it can itself be pressurised. If this technique is employed then care must be taken, as a failure of the steam supply with consequent condensation, can cause a vacuum and subsequent collapse of the vessel. 

Increase the rate of the cross-linking action with sulfur considerably allow for lower sulfur content to achieve optimum vulcanisate properties. Organic accelerators (e.g. thiuram, dithiocarbamate, etc.) are of major importance. In some cases it is necessary to retard the onset of vulcanisation to assure sufficient processing safety. The antioxidant 2-mercaptobenzimidazole (MBI) acts as a retarder for most accelerators. 

An accelerator which permits processing of rubber compounds to be carried out with less risk of scorching but which does not slow down the rate of cure at normal vulcanisation temperatures. Demoulding... 

This is the ratio of the rates of vulcanisation at two temperatures 10 °C (or 18 °F) apart. For rubber vulcanisation the coefficient is approximately 2 but it varies slightly with the temperature, the type of compound and the accelerator. 

The rate of vulcanisation of a rubber compound is controllable by the choice of accelerator. The range of products offered to the rubber industry has been categorised historically into recognised classes. New developments have resulted in products that improve compound performance and which overcome dermatological problems, and do not generate nitrosamines and other extractable or volatile decomposition products. 

Oxidative ageing of rubbers is limited by the rate of diffusion of oxygen into the rubber product and is usually confined to the outer 3 mm. Antioxidants are used to protect rubbers from the effects of thermal oxidation and the vast majority of compounds will contain one or more. Peroxide vulcanisates are usually protected with dihydroquinolines. Other antioxidants react adversely with the peroxide inhibiting the crosslinking reaction. 

The solubility of wax in vulcanised rubbers is low (of the order of 0.5% for NR) but enough wax has to be added to a rubber compound to ensure that once the compound has been vulcanised and the rubber cools, the rate of migrational movement of the wax from the rubber mass to the surface of the rubber is rapid. Dependant upon the application, the addition level of wax can be up to about 10 phr. Migration of the wax to the rubber surface will also carry other ingredients such as antioxidants, antiozonants and other materials (e.g., vulcanisation residuals), to enhance the surface protection. 

Systems are available which use molten salt as the curing medium. These systems can be either unpressurised or pressurised, and can have a salt recirculation system fitted. The advantage of this method of vulcanisation is the high temperatures which can be achieved with consequent greater product throughput rates. 

The pronounced efficiency of EPDM (POLY) grafted with TPA on the photoageing of the parent polymer is reported on Figure 7. Similar experiment after vulcanisation (not shown) reduced the photo-oxidation rate by a factor 1.5 both for virgin and stabilized films. 

If a fully compounded thermosetting rubber is subjected to a plasticity measurement at a high enough temperature and for long enough, it will cure and, consequently, there is not always a clear distinction between a plasticity test and a test for scorch or rate of cure. For example, the Mooney viscometer is used to measure scorch, i.e. the onset of vulcanisation, and an oscillating disc rheometer will measure the plasticity of the compound before the onset of cure as well as the increase in stiffness as curing takes place. 

Tests for scorch and rate of cure should be distinguished from tests for degree of cure or optimum cure measured on the vulcanised material. The latter type of test estimates degree of cure by measuring the physical properties of test pieces vulcanised for various times, tensile properties, swelling and set measurements being the parameters most commonly used. 

Rubbers are also being extruded, in a not essentially different way from plastics. Cooling of the extrusion cylinder is necessary to prevent premature vulcanisation as a result of the heat developed by internal friction. The extruder is fed by ribbons, obtained from milled sheets. End products are hoses, profiles, and cable mantles. On-line vulcanisation can be achieved by passing the extrudate through a steam channel, while the rate of extrusion is adjusted to the rate of curing. For this purpose high-rate vulcanisation recipes have been developed. Steam temperatures of about 200 °C are being applied (15 bars pressure). Treads for motorcar tyres are also extruded they are wrapped round the pre-formed carcass and then formed and vulcanised in a press. 

EPDM was developed and commercialised in the late 1950s. With an annual production capacity of more than 1,000 kt in 1998 [10]. EPDM is currently the fourth elastomer by volume and has become more or less a commodity rubber. Actually, EPDM is the largest non-tyre rubber. The annual growth rate is about 4%. DSM and Exxon are market leaders with a combined market share of approximately 40%. PP/EPDM-based thermoplastic vulcanisates which have currently the fastest growing rubber market (8% per year). 

The mechanism of peroxide crosslinking of elastomers is much less intricate than that of sulfur vulcanisation. Crosslinking is initiated by the thermal decomposition of a peroxide, which is the overall cure rate determining step. Next, the active radicals thus formed abstract hydrogen from elastomer chains to form macroradicals. Finally, crosslinking results either from the combination of two macroradicals or from the addition of a macroradical to an unsaturated moiety of another primary elastomer chain. 

Similarly, considering the swelling effect of vulcanised poly(butadiene), induced by a good solvent, the relaxation rate has been shown to obey the simple equation ... 

There is wide variety of vulcanisation agents and methods available for crosslinking rubber materials including peroxide, radiation, urethane, amine-boranes, and sulfur compounds [20]. Because of its superior mechanical and elastic properties, ease in use, and low cost, sulfur vulcanisation is the most widely used. Although vulcanisation with sulfur alone is not practical compared to the accelerated sulfur vulcanisation in terms of the slower cure rate and inferior physical properties of the end products, many fundamental aspects can be learned from such a simply formulated vulcanisation system. The use of sulfur alone to cure NR is typically inefficient, i.e., requiring 45-55 sulfur atoms per crosslink [21], and tends to produce a large portion of intramolecular (cyclic) crosslinks. However, such ineffective crosslink structures are of interest in the understanding of complex nature of vulcanisation reactions. 

Accelerators, e.g. zinc oxide and fatty acids, increase the rate of vulcanisation of rubber by sulfur and they reduce the amount of sulfur required from 10% to <3%. Certain sulfur-donating accelerators, like thiuram disulfides (1) and mercaptobenzothiazole (2), will effect vulcanisation without added sulfur to yield products with greatly enhanced ageing properties.1... 

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