Vulcanization porosity

Zinc oxide is a common activator in mbber formulations. It reacts during vulcanization with most accelerators to form the highly active zinc salt. A preceding reaction with stearic acid forms the hydrocarbon-soluble zinc stearate and Hberates water before the onset of cross-linking (6). In cures at atmospheric pressure, such as continuous extmsions, the prereacted zinc stearate can be used to avoid the evolution of water that would otherwise lead to undesirable porosity. In these appHcations, calcium oxide is also added as a desiccant to remove water from all sources. 

Anofher imporfanf parameter fhaf has an effect on the overall performance of fhe fuel cell is fhe fype of carbon particle used in the MPL. Two of the most common carbon particle types used in this layer are Vulcan XC-72R and acetylene black (AB). Jordan et al. [154,159] were able to show that micropo-rous layers with AB (1.25 mg cm with 10 wt% PTFE) performed better than MPLs with Vulcan XC-72R carbon black. They suggested that the reason for this result was the lower porosity of the acetylene black, which made it better at removing water from the MEA, thus leading to improved gas flow and diffusion toward fhe cafalyst layer. 

The manufacture of sponge rubber products such as gaskets for heat exchangers is based on the inclusion in the compound of chemicals which cause gas formation during vulcanization and thereby produce the desired porosity. Sodium bicarbonate and ammonium bicarbonate are examples of popular blowing agents. 

This system involves the use of some form of heating by air or steam in a chamber in a manner such that the vulcanization occurs immediately after the rubber is formed in an extruder or calender. This is a suitable process for extruded profiles and calendered sheets and conveyor belts. Liquid curing method (LCM) is also a continuous process which involves the use of suitable hot liquid baths in which extruded profiles can be passed through and vulcanized continuously. Items can be cured rapidly at temperatures from 200°C to 300°C however the compounds must be suitably designed to prevent porosity as this is a common problem with any extrudate. Suitable materials for curing medium includes bismuth-tin alloys, an eutectic mixture of potassium nitrate and... 

The value of A is not related to sample porosity. For example, adsorption on either a nearly nonporous or a porous carbon adsorbent, e.g. carbon black or Vulcan 3D [ 11 ], gave similar A values, i.e. A = 2.07 and 2.04 respectively. 

The medium is prepared on a master form, consisting of a heavy fabric belt, surfaced on one side with a layer of rubber filled with small round pits imiformly spaced. These pits are 0.020 in. deep, and the number per unit area and their surface diameter determine the porosity of the sheet. A thin layer of latex is fed to the moving belt by a spreader bar so that the latex completely covers the pits, yet does not run into them. This process traps air in each pit. The application of heat to the under-surface of the blanket by a steam plate causes the air to expand, blowing little bubbles in the film of latex. When the bubbles burst, small holes are left, corresponding to the pits. The blown rubber film, after drying, is cooled and the process repeated until the desired thickness of sheet is obtained. The sheet is then stripped off of the master blanket and vulcanized. 

Approximately 95% of the pits are reproduced as holes in the rubber sheet. The holes are not exactly cylindrical in shape but are reinforced by slight constrictions which contribute to strength and tear resistance. This type is referred to as "plain," and can be made with fabric backing on one or both sides to control stretching characteristics. If the unvulcanized material is first stretched, and then vulcanized while stretched, it is called "expanded." Resulting holes are oval and have a higher porosity (sometimes up to 30%). Special compounds have been formulated for resistance to specific chemicals under high concentrations at elevated temperatures, such as 25% sulfuric acid at 180°F. 

The structural model of PPCs was fine-tuned to resemble the properties of Vulcan carbon XC-72 (hydrophobic carbon particles with low internal porosity) (Kinoshita, 1988). These PPC are included in the initial configuration of the simulation box. Although PPCs were allowed to relax during simulations, they retained their initial structure. 

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