Diffusion of moisture

The cure rate of a sihcone sealant is dependent on the reactivity of the cross-linker, catalyst type, catalyst level, the diffusion of moisture into the sealant, and the diffusion of the leaving group out of the sealant. For one-part sealants, moisture diffusion is the controlling step and causes a cured skin to form on the exposed sealant surface and progress inward. The diffusion of moisture is highly dependent on the temperature and relative humidity conditions. 

Surface evaporation can be a limiting factor in the manufacture of many types of products. In the drying of paper, chrome leather, certain types of synthetic rubbers and similar materials, the sheets possess a finely fibrous structure which distributes the moisture through them by capillary action, thus securing very rapid diffusion of moisture from one point of the sheet to another. This means that it is almost impossible to remove moisture from the surface of the sheet without having it immediately replaced by capillary diffusion from the interior. The drying of sheetlike materials is essentially a process of surface evaporation. Note that with porous materials, evaporation may occur within the solid. In a porous material that is characterized by pores of diverse sizes, the movement of water may be controlled by capillarity, and not by concentration gradients. 

The major function of cutin is to serve as the structural component of the outer barrier of plants. As the major component of the cuticle it plays a major role in the interaction of the plant with its environment. Development of the cuticle is thought to be responsible for the ability of plants to move onto land where the cuticle limits diffusion of moisture and thus prevents desiccation [141]. The plant cuticle controls the exchange of matter between leaf and atmosphere. The transport properties of the cuticle strongly influences the loss of water and solutes from the leaf interior as well as uptake of nonvolatile chemicals from the atmosphere to the leaf surface. In the absence of stomata the cuticle controls gas exchange. The cuticle as a transport-limiting barrier is important in its physiological and ecological functions. The diffusion across plant cuticle follows basic laws of passive diffusion across lipophylic membranes [142]. Isolated cuticular membranes have been used to study this permeability and the results obtained appear to be valid... 

The dynamic viscoelastic properties of acetylated wood have been determined and compared with other wood treatments in a number of studies. Both the specific dynamic Young s modulus (E /j) and tan S are lower in acetylated wood compared with unmodified wood (Akitsu etal., 1991, 1992, 1993a,b Korai and Suzuki, 1995 Chang etal., 2000). Acetylation also reduces mechanosorptive creep deformation of the modified wood (Norimoto etal., 1992 Yano etal, 1993). In a study of the dynamic mechanical properties of acetylated wood under conditions of varying humidity, it was concluded that the rate of diffusion of moisture into the wood samples was not affected by acetylation (Ebrahimzadeh, 1998). 

Bruises are clearly visible in NMR images of apples, peach, Asian pear, and onion [14]. A relatively new bruise is brighter than the surrounding tissue due to a decrease in the magnetic susceptibility variations in the tissue. This decrease is a result of cell damage and subsequent diffusion of moisture into air pockets in the tissue of fruit [15]. However, the very old bruise does not show up as a brighter region, but instead appears darker because of dehydration[14]. 

Through one hole is placed an inverted funnel for introduction of chlorine the stem of a dropping funnel is passed through the second hole and rests directly above the flared part of the funnel. The third hole contains an outlet tube for excess chlorine. To prevent diffusion of moisture back into the reaction chamber, small drying tubes containing calcium chloride should be placed on the end of this latter tube and also connected to the inlet of the dropping funnel. The whole apparatus should be set up in a well-ventilated hood. 

It reduces the rate of diffusion of moisture to the critical interphase between the substrate and the adhesive. 

Ghosh, V., Duda, J.L., Ziegler, G.R., and Anantheswaran, R.C. (2004). Diffusion of moisture through chocolate-flavoured confectionery coalings. FoodBioprod Process. 82(C 1), 35-43. 

Modeling of the falling-rate period is usually done by treating the drying problem as a diffusion problem, where the rate-limiting step is the diffusion of moisture from deep within the solid to the surface. 

Figure 8.7. Diffusion of moisture across a timber slab. The parabolic gradient reflects the fact that the diffusion coefficient increases with moisture content.

It is important to note that diffusion is not a universally defined term. In foods, the processes of self-diffusion of the polymer matrix molecules, selfdiffusion of solutes, translational diffusion of solutes, and diffusion of moisture and other liquids have not been well discerned among theorists. All of these diffusion or mobility-based processes may occur in foods and pharmaceuticals. Yet recent theories do not clearly and consistently address which of these processes are of significance to chemical reactions, and how changes in water content or a as well as T or Tg affect each of these types of diffusion. 

This water concentration is certainly far below any analytical detection limit and may be present in the equipment, despite most careful drying and baking, via diffusion of moisture from glass or metal walls. 

Nordon P. and David H.G. (1967) Coupled diffusion of moisture and heat in hygroscopic textile materials, Int. J. Heat Mass Transfer, vol. 10, pp. 853-866. 

P. Henry, Disc. Faraday Soc., 3(1948) 243., The diffusion of moisture and heat through textiles. 

Xji = initial moisture content at start of drying when t = 0 Xi = initial free-moisture content D = diffusivity of moisture through solid s = one-half slab thickness... 

One-component room temperature-vulcanizing, condensation-curing materials (RTV-1) based on di- or trialkoxysilyl terminated polymers cannot be used, as the vulcanizate formed on the surface of the applied material is water vapor inqrermeable. Further diffusion of moisture is prevented, and thus deep section cure does not take place (Scheme 2). The solution is to use mono-silanol-terminated polymers. 

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