Molten films

Hot Corrosion. Hot corrosion is an accelerated form of oxidation that arises from the presence not only of an oxidizing gas, but also of a molten salt on the component surface. The molten salt interacts with the protective oxide so as to render the oxide nonprotective. Most commonly, hot corrosion is associated with the condensation of a thin molten film of sodium sulfate [7757-82-6], Na2S04, on superaHoys commonly used in components for gas turbines, particularly first-stage turbine blades and vanes. Other examples of hot corrosion have been identified in energy conversion systems, particularly coal gasifiers and direct coal combustors. In these cases the salt originates from alkali impurities in the coal which condense on the internal... 

The coated part is then exposed to infrared (IR) heat. The powder coat melts and forms a smooth and uniform film, which usually can take as little as only a few seconds to minutes, depending on the heat surface and the nature of the substrate. The molten film is then held at temperature until maximum flow and leveling is achieved, and then immediately exposed to a UV lamp while still molten. Cure usually takes less than 2 s. ... 

Extensional, elongational or shear-free flows play a dominant role in the post die-forming step, such as stretching of melt strands in spinning, uniaxial stretching of molten films... 

Flow and Heat Transfer in the Molten Film during Melt Removal Formulate equations of the coupled heat transfer and flow problems involved during the melt removal (by a simple shearing flow) in the conductive heating of a polymer sheet. If x is the direction of the melt removal and y the direction of the main temperature gradient, allow both vA and vy to be nonzero (because <5 = <5(x)) also, allow for a convective heat flux in the x direction. Assume that the polymer is crystalline, with constant average values for p, k, and Cp. 

Essentially, hot-melt adhesives are blends of polymers that are melted and kept at an elevated temperature until applied to a cooler substrate the second substrate is brought into contact immediately with the molten film and afterwards the adhesive cools rapidly, a bond being formed almost instantaneously. The temperature of the molten adhesive generally is in the region 160 to 200 °C, so that its viscosity is low enough for application—and this means that the substrates to be bonded must be capable of withstanding the effect of adhesive at this temperature (this can cause difficulties with some thermoplastics, especially expanded materials). 

At its operating temperature, 700-900 K, S02 oxidation catalyst consists of a molten film of V, K, Na, (Cs) pyrosulfate salt on a solid porous Si02 substrate. The molten film rapidly absorbs S02(g) and 02(g) - and rapidly produces and desorbs SC>3(g), Chapters 7 and 8. 

The IR spectra of polyester oligomers are almost identical to those of polyester fibres. The oligomers can be prepared as KBr pellets, as a molten film between NaCl or KBr platelets or else they can be investigated directly by the diffuse reflection or the diamond ATR method. 

Figure 2. Cross sectional microphotograph of molten films. 1 metal sheet, 2 insulator, 3 thermoelectric film.

Figure 3. Thermoelectric properties of the FcosjSi naos molten film.

Beam Penetration and Material Removal. Welding with electron, plasma, or laser beams can be modeled by considering the movement of a vertical cavity (along with the surrounding molten film) through the material to be joined (Fig. 18.7). The cavity depth-to-width ratio is usually about 10, and, to first approximation, the outer boundary of the molten liquid can be represented by a cylinder whose surface is at the melting temperature Tm. 

The equipment used for extrusion coating is similar to that used for the extrusion of flat film. Figure 2.30 shows a typical extrusion coating setup. The thin molten film from the extruder is pulled down into the nip between a chill roll and a pressure roll situated directly below the die. The pressure between these two rolls forces the film on to the substrate while the substrate, moving at a speed faster than the extruded film, draws the film to the required thickness. The molten film is cooled by the water-cooled, chromium-plated chill roll. The pressure roll is also metallic but is covered with a rubber sleeve, usually neoprene or silicone rubber. After trimming, the coated material is wound up on conventional windup equipment. 

The film is normally quenched in a water bath but may also be quenched on chilled rolls. The chilled rolls are expensive and their use is restricted. The quench tank for films is very similar to that used for monofilament. The molten film exits the die an inch or so above the water level and film guides route the sheet through the tank so that the tank water temperature is uniformly and constantly controlled and the water is free from turbulence. The film leaves the tank at a speed usually no greater than 45 m/min. The speed is limited because of water carry-over by the film. If the film is wet, difficulties will be encountered in drawing. Resin manufacturers control the wettability of films with additives and, consequently, the running speed. Drying methods, such as blowing with air, are applied as an added precaution just after the film leaves the water-quench tank. 

The molten film or web, when flowing freely, will have a thickness even larger than the slit width, owing to the so-called die-swell , the effect of the elastic recovery of the orientation in the die. Therefore a draw-down is always applied the material is removed from the die at a much higher... 

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