Rapid Prototyping involved creating a working model of various parts of the system at a very early stage, after a relatively short investigation. The method used in building it is usually quite informal, the most important factor being the speed with which the model is provided.
There are water-and solvent-based -- also called oil-based -- polyurethane varnishes. Water-based polyurethanes are more environmentally friendly and safer to use than solvent-based, but they are not as resistant to wear and tear as solvent-based products.
It is supplied in liquid form at > 98% non-volatile. It is composed predominantly of monomeric HMMM and lower levels of dimeric and trimeric analogs, which are linked either through methylene, -N-CH2-N-, or methyleneoxy, -N-CH2-O-CH2-, bridges. HMMM is also used as a crosslinking agent with polyurethanes. The polymers known as urethane oils3,4 include materials that incorporate the carbamate functional group as well as other functional groups. In the present article, castor oil5 is used as a hydroxyl group containing polyol and isophorone diisocyanate as polyfunctional diisocyanate.