Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 283 of 331

Film Casting Techniques Free Films Free films, that is, films not applied permanently to a substrate, are used for a wide variety of tests and, accordingly, vary in thickness and size. The most common castings of free films are for permeation testing or strength (usually tensile) and elongation tests. They can also be used for cold flex tests or moisture/solvent ab- sorption studies related to permeation. Probably the most common free film castings are laid down on a nonstick surface, such as the silicone-coated papers (available through Leneta and oth- er suppliers) or Teflon or poly (ethylene) sheets. The most common problem is that wetting to form a continuous film does not happen. Coatings (especially waterbornes) are difficult to formulate to get good wetting on low surface ten- sion substrates. One trick to use here is to make up the sample at higher-than- normal solids content (i.e., viscosity) so that the resistance to beading up will keep the film flat until it dries. An alternative is to apply several coats until the dried, beaded-up portions merge to form a continuous film — time-consum- ing, but patience is a virtue in the lab. For the thicker films (200 mils or more), a Pyrex or Teflon-coated baking pan can be used with some success. Occasionally, there is a "mud racking" problem, but you can reduce it by unsticking the edges of the film from the vertical sides of the dish with a spatula every morning to allow uninhibited shrinkage. This way, the mud cracks tend to leave a larger sized piece, and a square inch is all you need for the moisture absorption or solvent swell test. Hudson Machinery makes a dandy little 1-in. die for cutting out small tensile pieces from such a film, and it has been used quite successfully in many strength test studies with minimum sample availability. Mercury pool casting or tin-foil casting with mercury amalgamation to remove the tin without stressing the coating film is a technique no longer in use, because the mercury vapor is deemed a health hazard by regulatory agencies. Rarely, but occasionally, a free film of a polymer or binder material is needed (as a means of testing water permeation or absorption, for instance). Some of these tend to be tacky (oligomers? incomplete cure?), so powder the surface to make it easier to handle. Simple cornstarch is preferred to the talcs, baby powders, and other substances often used. Rotational casting is a means of obtaining nonporous nonmud cracked free films. The standard jar mill roller can be used with a variety of open con- tainers (quart or gallon jars, earless paint cans with a cut out of the bottom, etc.) to centrifugally cast the film with good thickness control. Sim- ply put 100 ml of coating in a 1-gal can and let it roll over night, and you have a reasonable film of 6 in. x 18 in. when you peel it off. That is enough for 6-in. tensile bars and 2-in. permeation circles, with some left over for a color card. You can get good, close control of film thickness by knowing the area of the cylinder and the weight of coating needed. To get a powder coating film cen- trifugally cast, simply hold a bunsen burner to the outside of the rolling can, and the powder coating melts to coat all over the inside. Watch out for scorch- ing. One can even cast pressure-sensitive adhesive formulations by this tech- nique. Spray-outs are occasionally used to make a free film (e.g., onto a silicone coated paper). There is no problem with that in the hands of a skilled practi- tioner, but the permeation test results for the spray-out may not be as good as 282

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