No description available.

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No description available.

The object of this research is to produce and characterise some coated powers for use in the powder metallurgy and related industries. The study introduces very briefly general powder metallurgy techniques and raw materials. A review of coating techniques is given, followed by a discussion of techniques for handling particulate substrates in chemical reaction systems. A discussion of the fluidised bed chemical reactor and electrode, and mechanically stirred vessels is presented.

This thesis describes investigations into a number of coating processes using experimental, analytical and computational techniques. The first problem, considered experimentally, is that of reverse roll coating with a liquid reservoir positioned directly above horizontally aligned rollers. Measurements of the film thickness as a function of the height of fluid in the reservoir and speed ratio are presented. When the wetting line is located downstream of the nip, either a decrease in the height of the associated hydrostatic head or an increase in the speed ratio causes a reduction in the thickness of the outgoing film. However, when the wetting line is located upstream of the nip the opposite is found to be true. The bead-break instability in forward meniscus coating is considered both experimentally and analytically. Agreement between predictions from a simple mathematical model of the stable bead and experimentally determined meniscus positions is seen to be excellent. A perturbation hypothesis is used to predict the onset of the bead-break instability, at which the upstream meniscus accelerates rapidly towards the downstream one, so the two collide and the bead collapses. The results from the model compare well with experiments. An outline of a method for using the bead-break instability as a design criterion is also presented. Typically in a slot, blade or knife coater the downstream meniscus is assumed to pin at a corner of the coating device. In chapter 5, a series of experiments and a corresponding computational study, are presented which illustrate that the meniscus can advance up the face of such coating devices (in this case a roll-flat plate system). Reducing the corner angle is seen to reduce the size of the climb region and the associated recirculation at this point at the downstream meniscus. It is also shown that the meniscus can detach from the corner and retreat into the gap, which can in turn give rise to the ribbing instability. An offset gravure coating arrangement is considered in chapter 6. The coating arrangement is split into two areas of study - the offset gravure nip and the kiss coating bead. An experimental investigation of the offset nip with the two rolls vertically aligned and running at the same speed in forward mode reveals two ways in which the metered film thickness can be influenced. Either increasing the nip force by pressing the two rolls together or decreasing the roll speeds causes a reduction in the metered film thickness. At higher speeds the metered film thickness is observed to asymptote to a limiting value, the value of which depends on the gravure pattern. The reverse mode kiss coating bead operating at speed ratios greater than one is also investigated. Experiments reveal that under these conditions, all the fluid is transferred from the roll surface to the web and the two make contact due to the generation of a sub-ambient pressure field within the bead. Two models based on lubrication theory are derived, one assuming an infinitely tensioned web and a second that incorporates the effect of web flexibility. The latter is found to give much better agreement with the experimental data. Finally a perturbation hypothesis is applied to these to models in order to predict the onset of the ribbing instability, both of which are found to give reasonable agreement with the experimental data. Finally, the results of a systematic experimental investigation of reverse mode direct gravure coating is reported, where the web runs directly over a gravure roll surface. This wide ranging parametric study illustrates the effect of the operating parameters on the final film thickness. Key findings are that speed ratio, fluid properties and cell shape and size can significantly influence the final film thickness. For a fixed roll speed it is observed that as the web speed is increased the gravure bead becomes unstable. This results in streaking on the web, and gives an upper limit to the speed ratio.

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No description available.

Alkyd emulsions are currently being developed for coating applications. These paints combine the technical advantages of alkyd polymers, which give coatings of a high gloss finish, with the environmental and health benefits of solvent-free systems. Nonetheless, the drying and film formation of these systems involve several steps, none of which is well understood yet. These include creaming or sedimentation, evaporation of the continuous water phase, coalescence of the alkyd droplets, a possible phase-inversion, and the oxidative cross-linking of the alkyd polymer to form a hard coating. In this thesis. Magnetic Resonance Microscopy is used to investigate the film formation of alkyd emulsion coatings. As the process is complex, studies on a different range of emulsions and related systems are performed to gain further insight into the different steps involved. A combination of Liquid State Microscopy, diffusometry and broad line imaging (Stray Field Magnetic Resonance Imaging) is employed, and it is shown that this combination provides results extremely rich in information. First, a study of the cream layer and the creaming dynamics of model bulk oil-in-water emulsions containing different concentrations of thickener is presented. The experimental data are compared to the predictions of a numerical model, and evidence is gained that the presence of the thickener induces flocculation according to a depletion mechanism. Then, the water evaporation from alkyd emulsion droplets is investigated and the water concentration across the droplet is modelled according to an original model based on lateral diffusion of water. There is no evidence from spectroscopy and diffusometry for phase-inversion during the drying of alkyd emulsion coatings. Finally the cross-linking of alkyds containing driers is found to be non-uniform across the film thickness.

A study of glass-ceramic coatings on metal substrates was carried out in order to understand how the coatings and substrates interact and how these interactions affect coating microstructures and properties. There are two systems involved. One is a lithium-silicate glass-ceramic coating on Ni/Cr/Co alloy and the other is a lithium-aluminosilicate glass-ceramic coatings (with some other optional compositions) on titanium metal. Simple techniques such as screen printing or the "droplet” method were used to coat the substrates with a layer of fine glass powder, and then a firing procedure followed to create a vitreous enamel layer on the metals. Heat treatment was usually applied to convert the vitreous coating into a highly crystallized, glass- ceramic coating since this generally results in superior properties. Because both the Ni/Cr/Co alloy and titanium metal are active metals, complicated interactions were observed at the coating/metal interface. In the Ni/Cr/Co case. Cr from the substrate tends to diffuse into the coating very rapidly at the firing temperature. The rapid diffusion of mainly Cr++ ions and subsequent change into Cr+++ ions results in saturation of Cr+++ ions in the glass coating. The second oxidation step proceeds more rapidly at the coating surface and LiCr(Si03)2 crystals start to precipitate on the surface of the coating. Mismatch of TEC, thermol expansion coefficient, between LiCr(Si03)2 and the coating causes severe disruption. In addition, some components in the coating, for instance, P2O5, react with Cr to form Cr^Pî. As a result, in the reaction zone, consumption of the intended nucleating agent P2O5 leads to a poorly crystallized structure affecting coating properties. In the titanium system, though long range diffusion across the interface was not observed, the major problem is the reactivity of titanium with coating components including Si( > 2. Direct reaction between titanium and silica gives TisSia as an interfacial layer and the gaseous O2 produced may disrupt the molten glass structure during firing. Other interactions such as Ti/P2Os can also proceed to give other damaging effects. In general, the coating on titanium after firing is usually very porous if these interactions are not prevented. Preoxidation of Ni/Cr/Co alloy created an adherent Cr203 layer on the alloy surface, and this layer is very stable in the coating glass due to its low diffusivity and solubility. The existence of this layer prevented direct contact of alloy and coating at the firing temperature, minimizing interfacial reactions and leading to the desired coating structure. From a chemical point of view, preoxidation of titanium metal can create a barrier of Ti02 between the coating and metal to hinder the formation of Ti5Si3. However, the poor bonding strength of this layer meant that it was ineffective in forming a transition layer between the metal and coating. Furthermore, the Ti02 can be readily dissolved by the coating glass during firing. Addition of an adherence oxide, CoO, was successful in 1) producing Ti02 in situ at the interfacial area and Co/Ti dendrites, both of which are necessary in maintaining chemical as well as mechanical bonding across the interface 2) diverting and hence minimizing the damaging direct reaction between Ti and SiC > 2 because the reaction between Ti and CoO always takes place prior to the reaction between Ti and Si02 thermodynamically. In the systems of glass-ceramic coatings for reactive metal substrates, various complications may occur. In this study, interactions of lithium-silicate/Ni/Cr/Co alloy and a wide range of glass-ceramic coatings/titanium have been studied. Detailed observation has been given together with explanation. Further work has also been suggested so that better understanding and application may generate from what has been observed in this project.

Poly(vinyl alcohol) (PVA) is a polymer used in numerous applications, principally those in which its high water solubility is a desirable asset. However there are also areas where PVA is limited by its inherent solubility (for example some specific environments in the biomedical field). This work has sought to overcome such limits by manipulating the surface of PVA in order to propose various means by which the surface solvent resistance might be increased while maintaining the bulk properties of the polymer. Both chemical and physical modifications have been tried and in each case progress has been made towards insolubilizing a single surface of the polymer when in film form. Grafting various species onto the surface of PVA was successfully performed. It is believed that such species bonded to the PVA via attachment to the hydroxyl groups (though this has not been proven conclusively). The data contained herein has led to the conclusion that the primary factor in reducing solubility this way is the removal of the hydroxyl groups, and not the attachment of specifically highly hydrophobic molecules. Introducing permanent cross-links into the surface region has been attempted via various routes. The data recorded shows promise however the system is far from optimised. The biggest challenge remaining is to optimise the depth of material cross-linked. Some steps have been made towards understanding and controlling this parameter though there is much scope for further investigation. The methods used have built on those used for bulk cross-linking and as such are new for the case of surface specific treatment. An interesting phenomenon in some semi-crystalline polymers reported in recent years is that of surface specific crystallization. This effect has been successfully induced and observed in PVA to produce what is believed to be a highly crystalline surface layer, and crystalline regions of PVA are generally accepted to be more water resistant than amorphous ones. In summary, in this work several surface-specific treatments for PVA have been trialled, providing options for post-film forming modification to reduce the surface water sensitivity whilst retaining the bulk properties of the polymer.