Press curing, vulcanization

Vulcanization of conveyor belts may be carried out in sections using press cure or continuously by means of a Rotocure equipment. In press cure, vulcanization is done by heating in long presses, the belt... 

Industrially the curing (vulcanization) of diene homopolymers and copolymers with elementary sulfur is carried out in a heated press at 100-140 °C (hot curing) this cannot be done in a normal laboratory on account of the expensive apparatus required. However, the principle of curing can be illustrated by crosslinking a butadiene-styrene copolymer (SBR 1500) with disulfur dichloride (SjCy at room temperature (cold curing) ... 

The finished dry rubber and latex based rubber products can be vulcanized by several techniques depending on the type of rubber compound (dry rubber compound/latex compound), size of the finished product, and its shape and structure. Moulded rubber products are vulcanized by press curing using compression, transfer, or injection moulding presses. The vulcanization techniques other than moulding may be grouped into batch and continuous methods. The batch methods include the use of autoclaves, hot air/gas oven, and hot liquid/ water bath. Rubber products may be vulcanized at room temperature by cold curing either by immersion of rubber products in a carbon disulphide solution of sulphur chloride (SjCy, or by exposure to its vapour. 

In each step of vulcanization, the section of the belt to be vulcanized is gripped and stretched hydraulically to minimize or eliminate elongation during use. The difliculties of press cure may, however, be avoided by adopting continuous vulcanization with a Rotocure equipment in which the actual curing operation is carried out between an internally steam heated cylinder and a heated steel band. Rotocure is also useful for the vulcanization of transmission belts and rubber sheeting. 

The dynamic vulcanization of blends of EPDM rubber with polyolefins (PP or PE) has been described (Coran and Patel, 1980). The rubber-plastic proportions and the extents of vulcanization were varied. In a few instances the rubber was first press-cured and then ground to various particle sizes. The ground rubber particles were then mixed with molten polypropylene. It was found that the ultimate properties (UE and UTS) varied inversely with rubber particle size. Since the smallest particle sizes of vulcanized rubber were obtained by dynamic vulcanization (not by grinding of cured rubber), the more durable compositions were obtained by dynamic vulcanization. 

Examples of the use of blocked diisocyanates for rubber-fabric adhesion are as follows vulcanized polychloroprene and SBR can be adhered strongly to nylon and polyester fibre fabric by means of aqueous adhesive systems (Table 8.4). This combination is spread or roller coated on to the fabric which is then allowed to dry. Bonds to sheet rubber stock can be made immediately after the treated fabric is dried or at any time thereafter. When the sheet rubber is applied it should be held under moderate pressure to provide intimate contact with the treated fabric and to prevent lifting if any gases are emitted during cure. Press cures of 20-40 min at 140°C are sufficient to cure the adhesive and most elastomer compositions being adhered. If a latex film is applied to the treated fabric, the assembly can be cured in a hot-air oven at 120"C. A chemical bond results between fabric and the diphenylmethane-/7,/ -diisocyanate generated on the thermal cleavage of the blocked diisocyanate. 

Properties of vulcanized compounds (l[iitiaj/aged") press cured 20 min at 166 C... 

This reaction occurs during the press cure operation. The third step in the vulcanization process is considered to involve the elimination of hydrogen fluoride to form a diimine ... 

The specimen was prepared by the following method. After mixing HAF carbon black (50 phr) with natural rubber (NR) in a laboratory mixer, carbon gel was extracted from unvulcanized mixture as an insoluble material for toluene for 48 h at room temperamre and dried in a vacuum oven for 24 h at 70°C. We made the specimen as a thin sheet of the carbon gel (including carbon black) by pressing the extracted carbon gel at 90°C. The cured specimen was given by adding sulfur (1.5 phr) to the unvulcanized mixture and vulcanized for 30 min at 145°C. The dynamic viscoelastic measurement was performed with Rheometer under the condition of 0.1% strain and 15 Hz over temperatures. 

Cure characteristics and reinforcement parameter (aF) - The cure characteristics were determined using the RPA 2000 dynamic curemeter (Alpha Technologies). The increase in torque at a frequency of 0.833 Hz and 2.79% strain was measured. The optimal vulcanization time (f90) of the samples was determined and used for curing of the samples in the press. 

In the one-stage vulcanization process, NR and EPDM are first masticated separately and then mixed with each other. Additives such as ZnO, stearic acid, carbon black (or silica), and process oil are added. The mix thus obtained is allowed to cool to room temperature. Finally, coupling agent known as DIPDIS and sulfur are added to the mix on the cooled mill. The stocks are finally cured under pressure at 160°C (32-33). In the two-stage process, NR and EPDM are first masticated separately. Then, additives such as ZnO, stearic acid, DIPDIS, and sulfur are incorporated in the EPDM. The compounded EPDM mix is then heated at 160°C in the hydraulic press for the predetermined time to yield the grossly undercured mix. The undercured mix is then blended with NR to the required blend ratio. The blend compound is finally vulcanized to the optimum cure time values (32-33). 

Perhaps the more important drawback of this method is the fact that for a 0.5-mm-thick sample, the temperature through its thickness should instantaneously reach that of the heated slabs of the press. This assumption is not exactly true, as shown in Figure 1.6 with the curve expressing the increase in the state of cure at the mid-plane of the rubber sheet as a function of time. Thus, the S shape for the curve representing the cure (or rather the mean cure) versus time (Figure 1.1) is logical. Moreover, the scorch time [2,3], which is the time necessary for the onset of vulcanization to take place, will perhaps need another explanation. 

Curing method Solvent evaporation, vulcanizing type by heat press RT (two-part) Evaporation of solvent Solvent evaporation, chemical cross-linking with curing agents and heat Evaporation of solvent... 

This is followed by vulcanization to obtain vulcanized composite sheets using a compression moulding in hydraulic press. The cure time for the vulcanization is generally determined using a Monsanto rheometer and optimum curing times (tgo) is calculated. The sheeted rubber compounds are conditioned at a tem- perature of 25 °C for 24 h in a closed container before cure assessment. 

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