Films formation

The process of film formation is a crucial factor in achieving optimum performance from a water based emulsion. 

Ideally, the polymeric, spherical particles should coalesce to form a fully coherent film, free from imperfections. 

This coalescing process has been thoroughly studied by many workers over the years. Techniques such as scanning electron microscopes (SEM), small angle neutron scattering and luminescence spectroscopy have facilitated a greater understanding of the process. 

The film forming process is very different from a solvent soluble resin and is much more difficult to control. From polymer solutions, the polymer chains are already present in a fully interpenetrated network with the molecules of solvent fully solvating the polymer species. On drying, the solvent merely evaporates, leaving the fully interpenetrated network of polymer chain as a concentrated uniform layer. 

The particle coalescing process can be affected by a number of factors  

Coating a solid surface with a liquid film presents difficulties when 5 0 (partial and nonwetting conditions) and a thin film is desired, it will be necessary to force the liquid to spread and eventually, to avoid the subsequent retraction of the film. The ptecise meaning of thin film will be explained in Sec. 3.2. 

In any case, for partially wetting systems, the difference between the liquid and solid velocities should exceed a critical value in order to obtain a fiquid film 

In this situation, called brides tail , theTCL moves with almost the same veloc- 

If the relative velocity between the hquid and the solid is increased and Ca Cac, the TCL cannot hold the meniscus velocity any more and a macroscopic film of constant thickness h, develops behind the meniscus. Several works have been devoted to study the dependence of h with the velocity in multiple different geometries and in diverse systems. When inertial and gravitational effects are neghgible Ca 10 ), the results can be summarized in a simple equation 

Another feature of cylindrical geometry is the observation of a Hmiting maximum coating thickness equal to h/a = theoretically explained by Reinelt and 

As the paint dries on the substrate, a firmly bonded film is formed. The properties of this film are determined both by the substrate and its pretreatment (cleaning, degreasing) and by the composition of the coating and the application method used. Drying of the paint on the substrate takes place physically (1-3) or chemically (4)  

1) Evaporation of the organic solvents from solvent-containing paints 

4) Reaction of low molecular mass products with other low or medium molecular mass binder components (polymerization or cross-linking) to form macromolecules 

Plastisols and organosols are a special case of physically drying coatings systems in which the binders consist of finely dispersed poly( vinyl chloride) or thermoplastic poly(meth)acrylates suspended in plasticizers. Organosols also contain some solvent. On drying at elevated temperatures, the polymer particles are swollen by the plasticizer, a process known as gelation. 

Chemical Drying. Chemically drying paints contain binder components that react together on drying to form cross-linked macromolecules. These binder components have a relatively low molecular mass, so that their solutions can have a high solids content and a low viscosity. In some cases, solvent-free liquid paints are possible. Chemical drying can occur by polymerization, polyaddition, or polycondensation. 

Gels are frequently used for ceramic film formation. Many aspects of the processing of films are common to all the deposition techniques. Schroeder [48] has outlined the conditions necessary for thin film formation. The solution must wet the substrate, it must remain stable with aging, it should have some tendency toward crystallization into a stable high-temperature phase, and for miiltiple layers the previous layers must be either insoluble or heat treated to make them insoluble before subsequent depositions. 

For a solution to wet a substrate the contact angle 0 between the surface of a drop of solution and the substrate must be less than 90°. The conditions for this are described by Young s equation  

Chapter 8 Other Ceramic Powder Fabrication Processes 

FIGURE 8,20 Schematic diagram showing the relationship between gel structure and film structure. Redrawn from Gallagher and Ring [31]. A similar figure is given in Brinker [49]. 

Polymerizable systems, like the metal alkoxides, are interesting because it is possible to form all of these different film structures by simply manipulating the solution chemistry. Properties such as structure, viscosity, and concentration are easily controlled with polymers. 

Marschall [Ilk] describes methods which allow the undisturbed measurement of the local film thickness, the wave amplitude, wave frequency and wave length as well as the inclination of the film surface as a function of time. It is interesting to note that these methods make use of the scattering of laser beams. Vorontsov [llq] made a systematic study of the effect of the types and dimensions of regular surface features providing roughness on walls with vertical film flow. 

Starting from the hydrodynamic model of the liquid film generated mechanically in columns with rotating elements and using simplifying assumptions Dietz et al. [lid] deduced an equation for the calcidation of film thicknesses in the range 0 film thickness gap width  

The special flow processes in thin-film stills were studied by Godau [He]. Representing the relevant parameters and mathematical relations Billet [Hf] also deals with the continuous distillation from a thin film as exemplified by a Lipotherm thin-filni still. Arithmetical work with the relations obtained indicates that operating with reflux can yield maximum separating results under certain loading conditions. 

The possibilities of intensifying the transport of material in falling films were studied systematically by Wilnsch et al. [He]. Special consideration was given to effects due to the ripple of the film, the curvature of the phase interface, the roughness of the solid surface and the intentional disturbances of the film flow. 

In the following, several selected examples are given for the study of submonolayer adsorbates and very thin films on metal electrodes. These include 

For effective solid lubrication, it is not enough to have a material with low internal or external friction. It is also necessary for it to form films with sufficient adhesion to a substrate, and internal cohesion, to withstand rubbing under high loads. Molybdenum disulphide has this ability to a very high degree. It can be made to adhere readily and firmly to a substrate, forming a strong, cohesive film. 

Because of this ready adherence to a substrate, molybdenum disulphide films can be produced in a wide variety of different ways, including flotation from the surface of a liquid, spraying, brushing or dipping in a volatile dispersant, bonding with adhesive or polymeric compounds, rubbing with powder, transfer, and vacuum sputtering. The nature of the initial film produced depends on the way in which it is applied, and all the important types will be discussed in subsequent chapters. 

This chapter will be mainly concerned with the type of consolidated film produced by burnishing or running-in, and consisting mainly or entirely of molybdenum disulphide. The processes occurring in the production of such films 

In the drying stage at the end of water evaporation the particles adopt a hexagonal dose-packed geometry. Good subsequent film formation requires a high level of polymer particle deformability and the rapid interdiffusion of polymer chains between the particles. Emulsion polymers therefore possess a so-called minimum film formation temperature (MET), below which no compact film can be formed. The determination of the MET is discussed below. 

To create a film with a defined (dry film) thickness of up to about 200 jim, the dispersion is usually cast on to the substrate using either a drawdown film applicator or a roller applicator. Suitable substrates are glass, polyethylene, polyethylene tereph-thalate or teflon. Films with thicknesses in the millimeter range, such as are used for mechanical strength testing, can be formed by pouring the dispersion into flexible polyethylene or silicone rubber trays, which facilitate the removal of the film after drying. 

As an aqueous dispersion can only dry above 0 °C, the MFFT and white-point temperature are only defined above this value. The control of the polymer layer thickness is crucial for the measurements. Mechanical stress may develop during film formation (particularly when crosslinking is involved) which leads to crack formation above a certain layer thickness. A further point which should be considered is that very short drying times are often used in dispersion processing, for example on coating machines. In this case, the MFFT may well he above the value determined according to ISO 2115. The discrepancy is caused by kinetic limitations in water evaporation and polymer interdiffusion [24]. 

The process starts with contacting a porous support with the colloidal precursor solution in a dip-coating or a spin-coating process. Film formation starts either with a film-coating or a slip-casting mechanism as will be discussed below. 

In many applications, latexes are spread on substrates as coatings for protective purposes. In many latex applications (such as latex paints and industrial coatings), the formation of a cohesive film is the ultimate goal. In such cases, film formation takes place within several days. 

The early experiments and considerations of the condensation phenomena of vapours on solid surfaces are treated extensively by Holland [258] and Neugebauer in Ref. [252a] and will therefore not be repeated here in full detail. In condensation, 

Remarkable insights have been gained especially from electron microscopical in-situ experiments of nucleation and subsequent film growth under clean and controlled vacuum environment, see [290]. Many experiments have been performed with noble metals on mainly single crystalline substrates, but under other conditions also non-metals and compounds have been investigated, see [291]. 

As follows from electron diffraction, films of crystalline materials formed on amorphous substrates under normal conditions are generally polycrystalline in their structure. During further thickness growth, a columnar microstructure can often be observed and thicker film may develop a texture. 

Most films deposited even at room temperature are in a non-equilibrium state and highly imperfect containing vacancies, dislocations, stacking faults and grain boundaries as can be seen in Fig. 58a, unless there is some mechanism for achieving equilibrium. The method of approaching equilibrium is by movement of atoms in and on the surface layers. The most important parameter controlling the mobility of atoms in a solid film is diffusion. Therefore if the condensation process occurs 

Schematic representation of the stages of metal film growth and approximate relationship between dislocation density and film thickness according to Pashley [318,319]. 

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Still another manifestation of mixed-film formation is the absorption of organic vapors by films. Stearic acid monolayers strongly absorb hexane up to a limiting ratio of 1 1 [272], and data reminiscent of adsorption isotherms for gases on solids are obtained, with the surface density of the monolayer constituting an added variable. 

It is known that even condensed films must have surface diffusional mobility Rideal and Tadayon [64] found that stearic acid films transferred from one surface to another by a process that seemed to involve surface diffusion to the occasional points of contact between the solids. Such transfer, of course, is observed in actual friction experiments in that an uncoated rider quickly acquires a layer of boundary lubricant from the surface over which it is passed [46]. However, there is little quantitative information available about actual surface diffusion coefficients. One value that may be relevant is that of Ross and Good [65] for butane on Spheron 6, which, for a monolayer, was about 5 x 10 cm /sec. If the average junction is about 10 cm in size, this would also be about the average distance that a film molecule would have to migrate, and the time required would be about 10 sec. This rate of Junctions passing each other corresponds to a sliding speed of 100 cm/sec so that the usual speeds of 0.01 cm/sec should not be too fast for pressurized film formation. See Ref. 62 for a study of another mechanism for surface mobility, that of evaporative hopping. 

The energetics and kinetics of film formation appear to be especially important when two or more solutes are present, since now the matter of monolayer penetration or complex formation enters the picture (see Section IV-7). Schul-man and co-workers [77, 78], in particular, noted that especially stable emulsions result when the adsorbed film of surfactant material forms strong penetration complexes with a species present in the oil phase. The stabilizing effect of such mixed films may lie in their slow desorption or elevated viscosity. The dynamic effects of surfactant transport have been investigated by Shah and coworkers [22] who show the correlation between micellar lifetime and droplet size. More stable micelles are unable to rapidly transport surfactant from the bulk to the surface, and hence they support emulsions containing larger droplets. 

Exerowa and co-workers [201] suggest that surfactant association initiates black film formation the growth of a black film is discussed theoretically by de Gennes [202]. A characteristic of thin films important for foam stability, their permeability to gas, has been studied in some depth by Platikanov and co-workers [203, 204]. A review of the stability and permeability of amphiphile films is available [205]. 

Daniel M F, Lettington O C and Small M 1983 Investigation into the Langmuir-Blodgett film formation ability of amphiphiles with oyano head groups Thin Solid Films 99 61-9... 

IHIN FILMS - FILM FORMATION TECHNIQUES] (Vol 23) pLOWTffiASURETffiNT] (Vol 11)... 

Film applications Film bases Film fabrication Film factors Film formation... 

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