Modelling biocide release based on coating properties

\u3cp\u3eGrowth of micro-organisms on coated substrates is a common problem, since it reduces the performance of materials, in terms of durability as well as aesthetics. In order to prevent microbial growth biocides are frequently added to coatings. Unfortunately, early release of these biocides reduces the biocidal protection of these coatings. Furthermore, since biocides leach to the environment, their release rate is a key parameter determining the environmental impact of biocides. As a result of legislation, the use of biocides and the corresponding concentrations is restricted. Understanding how coating and biocide properties determine biocide release rate is crucial to understand durability, and enable new product development and evaluation of environmental impact. The present study aims to establish this connection via modelling of the release process. The proposed model is presented with an initial validation. The release is viewed as a combination of water diffusion, dissolution of the embedded biocide and release from the material. The key parameters necessary for the model are bulk parameters determined from experiments. The resulting model shows that release can be proportional to water exposure time or to its square root, depending on the dominating processes in a certain regime. In application conditions high variabilities exist in terms of materials and conditions, therefore the effect of environmental conditions is investigated by a statistical approach. The results show that the variability in the time of protection (the time during which biocides are still present in the coating) is high, due to the variabilities in the weather conditions. A variation of 1 year (95% interval) was seen on an average time of protection of 1.5 years. The model can be used to assess new approaches towards material development and helps underpinning product performance claims.\u3c/p\u3

Numerical analysis of flow and mass transfer in humid fractal surfaces

\u3cp\u3eIn this paper we investigate the convective mass transfer process between a laminar air flow passing over a humid fractal surface. To determine the mass transfer convection coefficient, h\u3csub\u3em\u3c/sub\u3e, in complex 'wetted' fractal geometries, a simple numerical method is considered to obtain the rate of evaporation for any type of surface. This approach is validated considering two benchmark cases commonly discussed in laminar boundary layer theory, e.g., the flow over a flat plate and the flow over a cylinder. By considering different types of fractal geometries and different air flow speeds, we characterize the effects that such multi-scale fractals have on the convective mass transport driving the surface averaged rate of evaporation, n¯\u3csub\u3eA\u3c/sub\u3e. The results show the potential of fractals surfaces to enhance evaporation, where depending on the fractal shape considered, for instance, an increase by more than a factor of 3 in the rate of evaporation was obtained in comparison with a reference case where no fractals structures are imposed.\u3c/p\u3

Understanding the water absorption from MHEC modified glue mortar into porous tile:influence of pre-drying

\u3cp\u3eThis article presents an experimental investigation on the influence of Methylhydroxyethylcellulose (MHEC) modified mortar on water absorption into tile and base substrate using Nuclear Magnetic Resonance (NMR) imaging. Addition of MHEC shifts the absorption behavior from homogeneous to inhomogeneous as a result of increased viscosity of pore solution. The observed reverse flow from tile to mortar is a result of decrease in capillary pressure in mortar as a consequence of decrease in pore size due to hydration. Pre-drying of 1.3 wt% MHEC mortar with dry air shows little wetting by tile due to MHEC transport and formation of dry region at the mortar surface. On the contrary, pre-drying of 1.3 wt% MHEC mortar at 40% RH slows down MHEC transport and delays the formation of dry region at the mortar surface enabling higher wetting by tile.\u3c/p\u3

Transport of a water-soluble polymer during drying of a model porous media

\u3cp\u3eThis article presents an experimental investigation on transport of methylhydroxyethylcellulose (MHEC) during drying of a model porous material. Nuclear magnetic resonance imaging and thermogravimetric analysis are used to measure water and MHEC transport, respectively. MHEC is added to glue mortars to increase open time, i.e., the time period during which tiles can be applied with sufficiently good adhesion. Previous work showed that MHEC promotes a receding front during drying and therefore leads to differences in the degree of hydration throughout the mortar sample, i.e., the top surface shows poor hydration and the bottom surface shows good hydration. In this study, we investigate the transport of MHEC during drying of a model porous material, consisting of packed glass beads saturated with an aqueous MHEC solution. At MHEC concentration less than 1.3 wt%, homogeneous drying is observed, enabling advective transport of MHEC toward the drying surface. In this case, accumulation of MHEC may form a skin at the top surface and below this skin layer, a gel zone may form, which allows migration of water toward the evaporation surface. When the MHEC concentration is above 1.3 wt%, front receding drying is observed, which prevents transport of MHEC, resulting in a more homogeneous distribution of MHEC.\u3c/p\u3

Dissolution properties of cerium dibutylphosphate corrosion inhibitors

The corrosion inhibitor cerium dibutylphosphate, Ce(dbp)3, prevents corrosion by cerium and dbp deposition at the alkaline cathode and acidic anode respectively. The pH dependent Ce(dbp)3 solubility seems to play an essential role in the inhibition degree. We found that Ce(dbp)3 scarcely dissolves in water with a ~1 mM solubility limit between pH 2 and 9, whereas at pH 1, we found ~3 mM due to protonation of dbp, and for pH >9, the cerium precipitated as Ce(OH)3. We believe that the Ce(dbp)3 dissolution process is an important aspect for understanding its release from coatings and thus the corrosion inhibition. In case of a pressed Ce(dbp)3 tablet, its dissolution is dominated by transport of dissolved Ce(dbp)3 across a stagnant diffusion layer. The chemical bond strength between cerium and dbp is a major factor in their transport. The infrared spectrum of Ce(dbp)3 powder revealed a covalent-like bond, while the molar conductivity showed complete dissociation, so Ce(dbp)3 transports in water as separate ions

Inhibition of pH fronts in corrosion cells due to the formation of cerium hydroxide

The effect of cerium-based corrosion inhibitors on the pH front between the alkaline cathode and acidic anode in corrosion cells has been studied. The cerium component of these inhibitors can affect the pH front since it precipitates in an alkaline environment as cerium hydroxide, which is important since the corrosion inhibition mechanism of the cerium component is a result of its deposition as a highly electrical resistive (passivation) layer on the cathode. It is studied whether the cerium can reach the cathode when fed into the corrosion cell from an external source after the onset of corrosion. To this end a simulation model was set up that includes the Poisson–Nernst–Planck theory to describe ion transport and the Frumkin–Butler–Volmer equation to describe charge transfer at the electrodes. In this model both the self-dissociation of water and the formation of cerium hydroxide are taken into account. To support our findings experimentally a corrosion cell consisting of an aluminum and copper electrode was used, in which the pH fronts were visualized using a pH-indicator. Two types of inhibitors were used; namely, highly soluble CeCl3 and sparsely soluble cerium dibutylphosphate, Ce(dbp)3. The results show that CeCl3 can reduce the size of the alkaline region and reach the cathode to form a passivation layer, whereas the solubility in case of Ce(dbp)3 is too low to supply sufficient amounts of trivalent cerium cations to penetrate the alkaline region. This behavior can be explained by the simulation results, which reveal a threshold for the corrosion inhibitor solubility below which no passivation of the cathode occurs

Decreased bio-inhibition of building materials due to transport of biocides

\u3cp\u3eBio-inhibition of buildings and structures is an important issue. In many cases building materials have biocides added to prevent growth of micro-organisms. Growth of microorganisms on building materials has several negative effects; (1) Aesthetic damage, e.g. fungi, algae grow on the material, resulting in early replacement and high cleaning costs, (2) Material damage, and (3) Health problems. However, current legislation forces manufacturers to reduce the biocide load, which requires manufacturers to look for alternatives or other improvements. One way is to increase the efficacy of biocides. There are several factors which rule the efficacy of a biocide in a building material. In this paper we will give a short overview of the mechanisms that lead to a decrease in efficacy of biocides. One of the mechanisms, leaching into and from the materials is researched by using leaching experiments. This because leaching of biocides into and from building materials has not been researched to a great extent. In our experiments the leaching of Propiconazole (Wocosen 50TK) has been tested in gypsum layers applied on aerated concrete. The sample was then placed into an artificial rain setup which releases the biocides. The analyses of the samples show that the biocide leaches out of the gypsum layer and simultaneously into the aerated concrete. From the results it may be concluded that a biocide will leach from a plaster into an aerated concrete wall, which opens opportunities to improve the biocide efficacy by preventing this process from occurring.\u3c/p\u3

Enzymatically triggered release of dye model compounds from zein particles

\u3cp\u3eA responsive release system consisting of biocide encapsulated by the hydrophobic protein zein was investigated, in which a biocide will be released if growth of micro-organisms occurs. Because biocide release is difficult to detect, a model system using dyes with different size, polarity and charge was developed first to study the applicability of the zein matrix as an enzyme triggered responsive release system. The responsive release efficiency was highest for dyes with a positive charge, that have limited flexibility and the possibility of aromatic interactions with the matrix. The positively charged dyes showed a larger effect than negatively charged dyes. Additives were shown to enhance the release efficiency of the system. The most efficient release system combines the dye methyl violet with the additive oleic acid. These compounds interact to form a larger, more hydrophobic moiety. A plasticizing effect is thought also to be important, since even without clear specific interactions between dye and additives, release properties are enhanced. The charge of the additives seemed to be more important than their size, and both hydrophobic and ionic interactions were shown to play a role.\u3c/p\u3

Sorption of water-glycerol mixtures in porous Al\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e studied with NMR imaging

\u3cp\u3eThe penetration of water-glycerol mixtures in porous Al\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e is measured using NMR imaging to quantitatively describe the liquid penetration and possible front separation of liquids in non-transparent porous media. We have studied the dependency on viscosity over two orders of magnitude by changing the glycerol concentration and have investigated the effect of pore size in the micrometer and sub-micrometer regime. We show that the liquid dynamics can be accurately described by Darcy's law for all investigated liquid – media combinations and show that the permeability scales with the pore radius squared as predicted by permeability models by Lucas-Washburn and Katz-Thompson. Furthermore we have used T\u3csub\u3e2\u3c/sub\u3e relaxation analysis to investigate the homogeneity of the imbibing mixture during uptake and have found that water-glycerol mixtures penetrate the porous Al\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e samples as a single homogeneous liquid.\u3c/p\u3

Water permeability of pigmented waterborne coatings

Coatings are used in a variety of applications. Last decades more and more coating systems are transforming from solvent to waterborne coating systems. In this study the influence of pigments on the water permeability of a waterborne coating system is studied, with special interest in the possible interfacial layer of additives around the pigments. In our study an acrylate based binder is mixed with different glass sphere concentrations and sizes. Subsequently, the coating permeability is studied through wet-cup experiments and water uptake in the coating, either on a glass substrate or on wood, is monitored with NMR. These experiments show that water is absorbed more quickly by coatings containing glass spheres. From the experimental results it can be concluded that an interface layer around the pigment particles facilitates water migration and increases the effective diffusion. A modification of the effective medium theory is used to describe the observation and good agreement between experiments and theory is obtained. Both theory and experiment show that in a coating a layer around the pigments facilitates water transport and increases the effective diffusion constant