Paint films, diffusion rate

It is worthwhile drawing attention to health hazards associated with film infected water systems which also cause corrosion. Two of the most common are Legionnaires disease and so called humidifier fever . Because of strong adhesion of biofilms and diffusion rates through the film treatment based on cleaners and chemical sterilisers such as chlorine often fail similar considerations apply to other systems in industry, e.g. food, paint, oil and gas are examples where biofilm activities have given massive problems. 

The diffusion of water through paint films has been measured by various workers. The weight of water which could diffuse through three clear vehicles and eight paint films, each 0-1 mm thick, at 85-100% r.h. has been calculated on the assumption that the water would be consumed as soon as it reached the metal surface, i.e. that the rate-controlling step was the rate of diffusion of water through the film, and is shown in Table 14.3 ... 

Kittelberger and Elm measured the rate of diffusion of sodium chloride through a number of paint films. Calculations based on their results showed clearly that the rate of diffusion of ions was very much smaller than the rate of diffusion of either water or oxygen. Furthermore, they found that there was a linear relationship between the rate of diffusion and the reciprocal of the resistance of the film. This relationship suggests that the sodium chloride diffused through the membrane as ions and not as ion pairs, since the diffusion through the film of un-ionised material would not affect the resistance, because if a current is to flow, either ions of similar charge... 

Figures 9 and 10 give the calculated carbon dioxide concentration in the paint film, using different values for the diffusion coefficient and reaction rate constants.

Solvents are selected such that some will escape relatively quickly from paint films to prevent excessive flow, while others will escape slowly to provide film leveling and adhesion. With typical alkyd coatings, the first 30% of solvent has been observed to evaporate as quickly as the neat solvents, essentially at a constant rate which is dependent on volatility. Later stage evaporation occurred several times more slowly and was rate-controlled by solvent diffusion to the surface of the paint film. The transition point between this behavior was defined as the resin solids content at which the evaporation rate due to volatility equaled that due to diffusion. Transition points have been observed to typically occur at a resin solids content of 40-50% v/v. Thus alkyd paints, normally formulated at 27-40% v/v resin solids, generally exhibit rapid initial solvent-release driven by volatility while high solids coatings (usually 65-75% v/v resin solids) dry solely by a diffusion-controlled process with negligible influence by solvent volatility (Ellis, 1983). 

At ambient temperature, the solubility of water in a typical paint film exposed to a relative humidity of 100%, reaches 0.5 to 3% by weight and the diffusion coefficient varies between 10 " and 10 m /s [12]. This corresponds to a water permeability of 10 to 10 mol/m s. The solubility of oxygen in paints is on the order of some 70 mol/m at atmospheric pressures, and the diffusion coefficient varies between 10 and 10 ° m /s [13]. This results in an oxygen permeability of 10 to 10 mol m s . These values indicate that thin paint films do not prevent water and oxygen from reaching the metal surface. However, they slow down the reaction rate. 

Water and oxygen diffuse through paint film at a relatively high rate. The protective ability of a paint-phosphate film is influenced by the properties of the phosphate film because the film is in contact with the diffused water and oxygen. Film properties of concern are solubility, uniformity, and porosity. 

Certain inorganic compounds, usually as the zinc salt, are used in paints as passivators to control rust formation. These inhibitors are slightly soluble in water, and are slowly dissolved from the paint film through moisture permeabihty. The ions formed are carried to the film-metal interface by moisture diffusion. There they cause the corrosion electrical potential to be elevated to the passive potential at which the corrosion rate is dramtically reduced by polarization at the anode. 

In suspension processes the fate of the continuous liquid phase and the associated control of the stabilisation and destabilisation of the system are the most important considerations. Many polymers occur in latex form, i.e. as polymer particles of diameter of the order of 1 p.m suspended in a liquid, usually aqueous, medium. Such latices are widely used to produce latex foams, elastic thread, dipped latex rubber goods, emulsion paints and paper additives. In the manufacture and use of such products it is important that premature destabilisation of the latex does not occur but that such destabilisation occurs in a controlled and appropriate manner at the relevant stage in processing. Such control of stability is based on the general precepts of colloid science. As with products from solvent processes diffusion distances for the liquid phase must be kept short furthermore, care has to be taken that the drying rates are not such that a skin of very low permeability is formed whilst there remains undesirable liquid in the mass of the polymer. For most applications it is desirable that destabilisation leads to a coherent film (or spongy mass in the case of foams) of polymers. To achieve this the of the latex compound should not be above ambient temperature so that at such temperatures intermolecular diffusion of the polymer molecules can occur. 

Most coatings are applied as solutions, emulsions, or suspensions of the pigment, and are converted to solid films after application, usually by allowing the solvent to evaporate. Latex, one of the simplest of paint formulations, is simply a dispersion of high molar mass polymer particles in water. In the first stage of solvent evaporation, the rate of evaporation is essentially independent of the presence of the dissolved or dispersed pigment. As the solvent evaporates, the viscosity increases and the free volume decreases, so that the rate of evaporation becomes dependent on how rapidly solvent molecules can diffuse to the surface of the film [782],... 

All adhesives absorb water, and water uptake data for a number of structural adhesives are collected in Table 31.3. Such data are obtained by measuring the weight of water absorbed by an immersed film, and include the diffusion coefficient D and the weight absorbed at equilibrium M. This means that adhesive layers in joints will absorb water and its vapor, and transmit it to the interface. This cannot be prevented by sealing the edges with a paint or lacquer, as these also absorb water. The data can be used to calculate the rate at which water will enter joints, and water concentration profiles within them (Comyn 1983). This is discussed in more detail in Sect. 31.5.2. 

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