HOT AIR VULCANISATION

The use of hydrosilylation-curing technology also allowed silicone rubber to attain a better surface cure, particularly when hot-air vulcanised. The combination of a polished extruder die and such materials gave an outstandingly smooth, glossy finish to extrusions. Figure 2 shows a selection of parts made from this type of silicone rubber. 

Vulcanisation of conductive silicone rubber compositions is, like conventional silicone rubber, by peroxide. However inhibition of dichlorobenzoyl peroxide by carbon blacks makes it necessary to use an alternative curing technique, i.e. hydrosilylation or addition curing, for hot-air vulcanisation. Applications for conductive or antistatic silicone rubber include ... 

Hot-air vulcanisation of silicone rubber is usually only possible if 2,4-dichlorobenzoyl peroxide is used as the curing agent. The acidic decomposition products of this peroxide will bloom to the surface of most silicone rubber compositions within one to two days after vulcanisation. For this reason it has been necessary to give a second (post) cure in an air-circulating oven at 200 °C or higher in order to remove these crystalline deposits. For many grades of silicone rubber physical properties, particularly compression set, were optimised by this post-cure. 

The general necessity for silicones to be vulcanised continuously at the point of extrusion presents no problem. With the correct choice of peroxide, cure can be obtained in less than 1 min for thin section profiles by passing through an infra-red heated unit or hot-air vulcaniser (HAV). Pressure is not necessary to suppress porosity and many simple heating techniques work satisfactorily. Both hot liquid baths and fluidised bed... 

Hot air, steam, and hot water vulcanisation is widely used in the latex industry, and fluid-bed heat transfer and electronic microwave curing has also been used. Cross-linking by electron radiation has been experimentally used, but has not yet been developed commercially. 

Any method of vulcanising rubber products which proceeds without interruption from start to finish as compared to the method of vulcanising separate batches of products or sections of a product. Continuous vulcanisation processes include the cold curing of proofed cloth, the vulcanisation of belting and flooring, of cables and certain extruded products by either the Liquid Curing Medium, Fluid Bed, Microwave, or Hot Air techniques. 

Also called hot air cure. A method of curing, mainly used for footwear. The articles are placed in a double-walled vulcaniser, the heat for curing being obtained from steam circulating between the double walls. Steam does not therefore come in contact with the articles in the vulcaniser itself. Dry Rubber Compound... 

A continuous process for the vulcanisation of extruded sections. The heating medium is a bed of tiny glass spheres fluidised by steam or hot air. Fluidisation... 

A hot air chamber used for heating or drying raw rubber, for vulcanising rubber products by the dry heat method, or for carrying out accelerated ageing by the air oven method. On the continent of Europe, the term oven is sometimes used in the sense of autoclave. 

Hot air tunnels are often used for vulcanisation of cable covered with rubbers such as silicone. These systems may also incorporate infrared radiation as a means of boosting heat transfer to the product. 

Vulcanisation using hot air systems remains the most important production system for profile production. Hot air can be used alone but can also be supplemented by the addition of infrared heaters. The systems usually consist of modular units which can be built up into the required length. Air speed can usually be controlled and the compound throughput can be adjusted from speeds of 2.5 m/sec up to 20 m/sec. These systems are sufficient for thin section profiles, but thicker articles will require slower transport speeds. Addition of microwave systems to these units... 

The integrated hot air/UHF units, at present in the development stage, will help to overcome some of the problems associated with vulcanisation of complex profiles with irregular wall thickness or asymmetrical geometries. 

DNPT)) were studied using a gas evolution apparatus. The decomposition temperature of ADC decreased with both DNPT and 4,4-oxybis(benzenesulphonyl hydrazide) (OBSH) blending and this affected the structure and properties of the resulting foams. Using a tube mould for an extrudate to vulcanise the NR/EPDM extradate in a hot air oven was found to control the expansion and foam dimensions. The NR compositions affected the foam structure and properties. 16 refs. 

A TG-DTA study of the thermochemical processes occurring at vulcanisation temperatures with N-oxydiethylene-2-benzthiazyl sulphenamide and N-cyclohexyl-2-benzthiazyl sulphenamide and their mixtures with sulphur showed the formation of high molecular weight polysulphides [73]. The influence of metallic oxides (Fe203, Sn02) on hot air ageing of one-pack room temperature vulcanised fluorosilicone rubber has been studied by means of TG-DTA [74, 75]. TG-DTA and TG were both applied to study the thermal characteristics of room temperature vulcanised silicone rubber [76]. 

The resistance of NBR to ageing in hot air is superior to that of natural rubber and polyisoprene. As will be seen (Section 6), much progress has been made in this respect in recent years by improving the choice of compounding ingredients and their ratios. Accordingly it is now possible to make NBR vulcanisates permanently resistant to hot air at about 90 °C. This means that they can be exposed continuously to air at this temperature for approximately 12 months. At 120°C a service life of about 40 days can be expected at 150°C it is likely to be about three days. 

The double bonds of NBR have been selectively hydrogenated for the same purpose, i.e. in order to improve the resistance of the vulcanisates to ageing in oils and hot air. Pyridine-cobalt complexes and complexes of rhodium, ruthenium, and iridium " have been described as hydrogenation catalysts. In some cases the hydrogenation is incomplete. The main obstacle to homogeneous catalytic hydrogenation of olefinic structures is the difficulty in obtaining adequate selectivity, but there are special catalysts based on transition metals which are almost entirely satisfactory in this respect. 

The main properties of some rubber compounds and vulcanised rubbers, as well as their applications, are given in Table 20. As seen from the table, silicone elastomer-based rubbers are designed for prolonged use in a wide range of temperatures from -50 to +250 °C, some from -70 to + 350 °C (for a short period of time). These rubbers are efficient in air, ozone and in an electric field rubbers based on IRP-1339 and IRP-1401 compounds are also efficient in case of limited air supply. They function well in high humidity and under the influence of oxidants, hot water, vapour and low pressure. They are stable in weak-acid and weak-alkali media and are nontoxic. 

The vulcanised carborane-siloxane rubber [530] has an unusual degree of thermal stability it does not show any persistent changes after 100 h at 250 °C in the open air. Hot-hardened rubbers are used mainly in electrical engineering and the aircraft industry. Those intended for medical applications may be vulcanised under radiation without a catalyst. 

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