Dip-coating

Dip coating is a process frequently used for the production of gloves. Fillers play a prominent role in dip coating formulations. The formulation must be designed to impart two opposite properties to the coating. It should have a relatively low viscosity to assist dipping process and the viscosity should rapidly increase after the form is withdrawn from the dipping tank. 

Rheologically, these formulations are pseudoplastic with a significant yield value. The pseudoplastic characteristic is responsible for the performance of coating and wetting the form in the dipping tank (relatively low viscosity) and for providing a uniform coating of required thickness as form is withdrawn. The yield 

These new developments allow materials to be produced in an economical manner into products of high quality. However, the extrusion industry continues to use the traditional technology and, therefore, many of the old problems of filler incorporation remain. 

The production of foamed products is very sensitive to changes in composition and the parameters of processing. The inclusion of fillers complicates the process and careful consideration must be given to the effects that filler incorporation has on material properties. Experimental work must be done to verify process conditions and material performance. 

a homogeneous suspension of filler must be produced. This suspension should be sedimentation-lfee because it would influence foam properties. Rheological studies are used to select the appropriate dispersion agents which keep the viscosity close to the viscosity of the unfilled formulations. Complete filler wetting and network formation helps to make the stable suspensions. 

Dip-coating is a very simple and useful technique for preparing composite membranes with a very thin but dense toplayer. Membranes obtained by this method are used in reverse osmosis, gas separation and pervaporation. The principle of this technique is shown schematically in figure EH -11. 

If the solvent of the coating solution does not wet the porous substrate, no pore penetration will occur. This method can be applied to coat porous hydrophobic polymers such as polyethylene, polypropylene, polytetrafluoroethylene or poiyvinylidene fluoride with a water soluble polymer. Since water does not wet the membrane (at least if the Laplace pressure is not e.xceeded) the polymer wB definitely not penetrate. The. solution properties are very important and this is determined by four parameters 

The polymer, which is assumed to be linearly and amorphous, occurs in solution as a random coil. The dimensions of the coil depend on the type of solvent, in good solvents the coil dimensions are large whereas in poor solvents the coil dimensions are much smaller. If the solvent is very poor then the coils aggregate and precipitation may occur. The qualit) of the solvent can be expressed by the Flory-Huggins interaction parameter%. An increase of the molecular weight will also result in an increase in coil dimension. 

If the polymer concentration increases then the coils will overlap. This process always occurs in dipcoating where the solvent is being evaporating. At a certain moment the coils will overlap and will form an entangled network. The fonnation and morphology of this network strongly depend then on the solvent and polymer. 

Different processes use plastisols for coating molds. These processes are defined as follows  

Dip molding A process similar to hot dip coating, in which the finished product is the fused plastisol stripped from the dipped mold. Dip molding is used to produce such articles as gloves, grips, and protective caps. 

Slush molding Used to create hollow, flexible items. A quantity of plastisol is poured into a heated female mold, causing gelling of the plastisol directly in contact with the heated surfaces. The excess plastisol is poured off and the gelled layer is allowed to fuse. After fusing, the finished item is removed from the mold. 

In processes illustrated on these figures, plasticizers are required for final product performance to increase elastic properties of manufactured products. Gloves are coated with rubber latex. Its viscosity is adjusted by the plasticizer. Caplet coating for pharmaceutical applications is done by immersion in a gelatin solution where water is solvent and processing plasticizer. 

Selection of plasticizer or plasticizing system goes beyond lowering the viscosity. The amount of form coating, its nniformity, and appearance depend on rheological properties of plastisols. Forms are subjected to a controlled movement which has several steps  

Aluminum is the highest melting point metal (660°C) applied by hot dipping. Aluminized steel can be used at temperatures up to 550 C without appreciable oxidation. This steel has very good resistance to gases and vapors containing small quantities of sulfur dioxide and hydrogen sulfide [41,42,43]. 

One of the most important points to address when designing or purchasing a dip coater is the quality of the motion. The translation must be as homogeneous 

The object to be coated is dipped into the tank full of coating and pulled out, and the excess coating drains back into the dip tank. To minimize the thickness differential the rate of withdrawal is controlled. 

This equation is valid for any lateral section of the film. The uptake of fluid is given by 

For Crossian rheology, the momentum balance can be integrated to determine the velocity profile [33], which is given by 

FIGURE 13.18 Factor [= versus volume ftaction for dip coatiiig a 

The Crossian rheology, when compared to the Newtonian rheology using the low shear viscosity, has a laiger thickness and the same dependence on substrate velocity (Le., /U. To date, this theoiy 

The orientation of anisotropic particles during dip coating can be analyzed by considering the rotational diffusion of these particles in shear. Rotational diffusion in shear flow has been reviewed by Van de Ven [58]. The ratio of the shear rate, y, to the rotational diffusion coefficient, , defines the rotational Peclet number (Pe = y/ ). When the rotational Peclet number is small (i.e., near zero), the anisotropic particles are randomly oriented by diffusion. When the rotational Peclet number is large, the particles rotate but have a preferential orientation aligned with the shear. The period of rotation is given by 

The thickness, t, of the liquid film depends on the viscosity, q, of the solution and the speed, v, with which the object is withdrawn from the solution and is given by 

The green tape can easily be punched to produce holes or vias, which are an important part of the processing of 1C packages. In 

The high-tech version of the dipping process used to glaze pottery. 

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Laminates. Laminate manufacture involves the impregnation of a web with a Hquid phenoHc resin in a dip-coating operation. Solvent type, resin concentration, and viscosity determine the degree of fiber penetration. The treated web is dried in an oven and the resin cures, sometimes to the B-stage (semicured). Final resin content is between 30 and 70%. The dry sheet is cut and stacked, ready for lamination. In the curing step, multilayers of laminate are stacked or laid up in a press and cured at 150—175°C for several hours. The resins are generally low molecular weight resoles, which have been neutralized with the salt removed. Common carrier solvents for the varnish include acetone, alcohol, and toluene. Alkylated phenols such as cresols improve flexibiUty and moisture resistance in the fused products. 

Powder Coating. Nylon-11 and nylon-12 are used in powder form for anticorrosion coating of metals. Dip coating and electrostatic and flame spraying are used. Dip coating, which involves immersing a preheated article into fluidi2ed nylon powder, is most suitable for automation. 

Tire Cord. Melamine resins are also used to improve the adhesion of mbber to reinforcing cord in tires. Textile cord is normally coated with a latex dip solution composed of a vinylpyridine—styrene—butadiene latex mbber containing resorcinol—formaldehyde resin.. The dip coat is cured prior to use. The dip coat improves the adhesion of the textile cord to mbber. Further improvement in adhesion is provided by adding resorcinol and hexa(methoxymethyl) melamine [3089-11 -0] (HMMM) to the mbber compound which is in contact with the textile cord. The HMMM resin and resorcinol cross-link during mbber vulcanization and cure to form an interpenetrating polymer within the mbber matrix which strengthens or reinforces the mbber and increases adhesion to the textile cord. Brass-coated steel cord is also widely used in tires for reinforcement. Steel belts and bead wire are common apphcations. Again, HMMM resins and resorcinol [108-46-3] are used in the mbber compound which is in contact with the steel cord to reinforce the mbber and increase the adhesion of the mbber to the steel cord. This use of melamine resins is described in the patent Hterature (49). 

The productive stock, ie, the curable compound, is made up by mixing the nonproductive stock in the Banbury once more with the curative package (sulfur, accelerators, etc). This time the drop temperature is lower, in the range of 95—112°C. The productive stock is then sheeted or pelletized and coated with the dip coat, cooled, and finally stored, ready for further processing for final fabrication. 

Acetate propionate esters are nontoxic, exhibit excellent clarity and high tensile strength, and can be formulated into hot-melt dip coatings for food (168). Alternatively, they may be dissolved in volatile solvents and appHed to foods in the form of a lacquer coating (169). 

Solution Deposition of Thin Films. Chemical methods of preparation may also be used for the fabrication of ceramic thin films (qv). MetaHo-organic precursors, notably metal alkoxides (see Alkoxides, metal) and metal carboxylates, are most frequently used for film preparation by sol-gel or metallo-organic decomposition (MOD) solution deposition processes (see Sol-GEL technology). These methods involve dissolution of the precursors in a mutual solvent control of solution characteristics such as viscosity and concentration, film deposition by spin-casting or dip-coating, and heat treatment to remove volatile organic species and induce crystaHhation of the as-deposited amorphous film into the desired stmcture. 

Dip coating is very commonly used for coating continuous objects that are not flat, such as fibers, and for irregularly shaped discrete objects. Tears or drops of coating at the bottom of dip coated articles may be removed by electrostatic attraction as the article is moved along a conveyor. 

Dip and Spin Coating. The dip coating technique described for webs can also be used to coat discrete surfaces such as toys and automotive parts. The surface to be coated is suspended on a conveyor and the part dipped into the coating solution. The surface is then removed, the coating drains, then levels to give the desired coverage. The object is then dried or cured in an oven. 

Saturation and Coating Processes. Saturation coatings consist of impregnation, dip coating, and conformal and surface coatings. 

Although the above applications consume over 90% of the polyethylene produced there are a number of other important end-uses. Filament for ropes, fishing nets and fabrics are an important outlet for high-density polyethylene powdered polymers are used for dip coating, flame spraying, rotational moulding and other outlets, whilst fabricated sheet is important in chemical plant. 

Steel is by far the most important basis metal. Cast irons are also coated by hot dipping, and some copper is coated with tin or tin alloys. Other hot-dip coatings are only applied for special purposes. 

Up to 750°C, the performance of all aluminium diffusion coatings is considered to be very good, but above this temperature the results appear to be dependent on the coating thickness, diffusion treatment and the specific service environment. Sprayed aluminium coatings can be used up to 900°C after diffusion treatment. Hot-dipped coatings also benefit from additional diffusion treatment, and omission of silicon from the coating alloy improves performance at the elevated temperatures. 

Hot Dipped Coatings Major attempts have been made to improve the quality of aluminised steel strip. Requirements on coating thickness and uniformity have been imposed. It is the speed of sheet movement, length of path in the molten bath, temperature and composition of the bath that control the thickness of the intermetallic layer which lies below the aluminium outer surface. The process of intermetallic alloy formation is diffusion controlled, and it is usual that some dissolution of iron into the molten aluminium does occur at a rate, Ac/At, which is given by... 

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