Urethane is just one more way to go in the world of mold making for special makeup effects. Actually, when we use the term urethane as it pertains to the work that we do and the uses we have for it, the name has been shortened from polyurethane. There is a specific substance called urethane, also known as ethyl carbamate, and the two should not be confused. For our purposes, when you see the word urethane in the context of special makeup effects, it means polyurethane.
Urethane formulas cover an extremely wide range of stiffness, hardness, and densities. These materials include low – and high-density flexible foam used in upholstery and bedding as well as in makeup effects prosthetics (known as cold foam), which will be discussed fully in Chapter 6; low – and high-density rigid foam used for thermal insulation; soft-solid elastomers used for gel pads, print rollers, and mold making as a substitute for some tin-cure RTV silicones; and hard-solid plastics used as electronic instrument bezels, structural parts, and mold making as well, as a substitute for epoxy and polyester resin molds. Urethanes are widely used in high-resiliency flexible foam seating, rigid foam
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insulation panels, microcellular foam seals and gaskets, durable elastomeric wheels and tires, electrical potting compounds, high-performance adhesives and sealants, Spandex fibers, seals, gaskets, carpet underlay, and hard plastic parts.1 There are a number of commercial urethane foam, urethane rubber, and urethane plastic products designed for use by makeup effects artists and mold makers. These are listed in the appendix at the back of this book.
For the most part, the use of these materials is the same as their counterparts, with a few notable exceptions. The reason we use silicones for mold making is that silicones are essentially self-releasing; that is, almost nothing will stick to silicone except other silicone. Urethane rubbers, on the other hand, are not self-releasing. In fact, they are the opposite: Urethane rubbers are adhesive. To prevent adhesion between urethane rubber and the porous surface of a cast (made of Hydrocal, plaster, Ultracal, or the like), it must be sealed and then released. Casts made of thermoplastic, which includes urethane plastic, epoxy, and polyester resin (fiberglass), must be sealed with shellac or PVA and then released. When in doubt about what to use, manufacturers recommend a small test application to determine the proper release agent. That is a good idea, considering how much work goes into a project prior to reaching this point. It’d sure be a shame to have to start over.
All liquid urethanes are moisture sensitive and will absorb atmospheric moisture. Using water-based clays is not recommended. Mixing tools and containers should be clean and made of metal or plastic; mixing urethane with a wooden stir stick can cause problems merely via atmospheric moisture retained in the wood, even if you’re working in a humidity-controlled environment. If you choose to make your mold using urethane rubber instead of silicone and you plan on building the dividing wall with WED or other water clay, it is absolutely imperative that the clay be well sealed with Crystal Clear, and dry, before applying the urethane! I strongly suggest a test with sealed clay and a bit of urethane before trying this on a project.
This is quite true also of urethane plastic used for a hard outer shell of a matrix mold, for example. If you are using WED clay or other water-based clay to form barriers and walls, it must definitely be well sealed to prevent moisture contact with the urethane. It will bubble and foam if there is any contact with any moisture from water. Use another material to form your walls.
Working with silicone does not require that you wear a respirator or work in a well-ventilated environment (though it’s not a bad idea to have good ventilation anyway), but it is strongly recommended that you wear safety glasses and gloves to minimize the risk of contamination and irritation. However, working with urethane shares essentially the same hazardous risks of working with fiberglass resins: epoxy and polyester. Mixing should only be done in a well-ventilated
‘Wikipedia contributors, “Polyurethane,” Wikipedia, The Free Encyclopedia, http://en. wikipedia. org/w/index. php? title=Polyurethane&oldid=177688842
environment while wearing a respirator. Rubber gloves and safety glasses are also strongly advised. Before working with any of these materials, you should read and be familiar with each product’s material safety data sheet (MSDS).
With these exceptions, working with urethane follows the same procedural steps as working with gypsum, silicone, and fiberglass. We won’t repeat the step-by-step process in this book. You can easily go back and figure it out on your own.