PTM&W Urethane FAQs
- What is a urethane?
- What can cause urethanes to bubble?
- Are urethanes sensitive to mix ratio?
- How should I store my urethane between uses?
- How can I color my urethanes?
- What should I do for urethane cleanup?
- How much heat or cold will cured urethanes take?
- What kind of physical properties should I expect from a cured urethane?
What is a urethane?
Urethanes are named for the urethane group that is formed when an isocyanate component (urethane resin) and the hydroxyl or amine component (urethane hardener) react. This kind of plastic is known as a thermoset, because the urethane reaction is not reversible with heat or pressure, these thermoset urethanes do not melt or flow. Epoxies are also thermosets, but are named for the reactive epoxy group that you start with before cure. There are no epoxy groups left in cured epoxies.
What can cause urethanes to bubble?
Like many other 2 part systems, urethanes can trap the air stirred in during the mixing of the resin and hardener. This trapped air is easily removed by pulling a short vacuum on the mix before pouring the part. Additionally, bubbling in urethanes is caused when the isocyanate component (urethane resin) reacts with water. This isocyanate/water reaction generates carbon dioxide gas or bubbles. Water pick up can come from moist air, or even paper, or wood mixing equipment. The urethane resins and hardeners as supplied have extremely low moisture content, and will not bubble when mixed, so that very nice bubble free castings can be made with just a vacuum step after mix, if plastic or metal mixing equipment is used.
Are urethanes sensitive to mix ratio?
Two part urethanes, like two part epoxies, need the correct quantity of resin and hardener groups for best results. Unlike epoxies, there are no “variable mixing ratio” urethanes. Mixes using excess urethane hardener may cure to a solid, but the excess hardener groups will attract moisture into the cured part, and always give inferior physical properties. Larger excesses of hardener may give cures that are sticky, or semi-solid “goo’s”. Mixes with large excesses of isocyanate (urethane resin) may remain liquid after cure, or mixes with small excesses of isocyanate may form low melting thermoplastic waxes, or brittle solids. Carefully weighing the correct amounts of both urethane resin and urethane hardener will ensure best results.
How should I store my urethane between uses?
As the isocyanate (urethane resin) reacts with water and can skin over like a can of paint, or build up carbon dioxide pressure if stored with wet air in the head space of the container it is best to carefully purge, or displace the wet air with dry nitrogen, and then carefully seal the container between uses. The urethane hardener can also pick up moisture from wet air in the head space, and while no skin forms, or pressure build up occurs, in extreme cases enough water can build up, so that bubbling can occur when mixed with the urethane resin. For best storage put the dry nitrogen cap on both the resin and hardener between uses. Extremes of heat or cold for storage should be avoided, with the storage sweet spot being at 60°F to 90°F, and the use temperature sweet spot being 70°F to 80°F. Today’s urethanes, as formulated, will withstand a fair amount of abuse during storage or processing, but the very best results are achieved with a little extra care with the dry nitrogen cap, and with the use, and storage temperatures.
How can I color my urethanes?
Urethanes can be colored by adding a few percent of a paste color dispersion either to the mix just before the vacuum step, or by adding to the polyol (hardener side) before the mix if more time is needed to tint to the correct shade. Both the color dispersions made for urethanes, and those made for epoxies work well. Even the tubes of “Artists Oil Paints”, or universal color dispersions for oil based paint work well. Do not use universal color dispersions designed for water based paint, as these have water or alcohol’s that cause the urethanes to bubble. Do not use solvent containing color dispersions, as again this will cause bubbling and loss of properties. Do not use more color than you need to get the color depth you desire, because larger amounts generally cause some loss in properties. Be sure to maintain the correct Resin to Hardener ratio. For example, if the correct mix ratio is 100 parts by weight of urethane resin to 50 parts by weight of urethane hardener, and you are going to add 2 parts by weight of color dispersion to the urethane hardener, then the final mix ratio becomes 100 parts by weight of urethane resin to 52 parts by weight of the urethane hardener/color dispersion blend.
What should I do for urethane cleanup?
Urethanes should be cleaned promptly off equipment before cured, as cured residues are tough to remove. Promptly wipe, drain, or spin off the excess liquid, uncured urethane, and then use a small amount of solvent to remove any residues. Flammable solvents, like Acetone, require care to avoid static or any spark (such as an electric drill) in the area, as flammable fumes can accumulate giving rise to fire or explosion hazards. Non flammable solvents, like Methylene chloride, also require care as accumulated fumes can give rise to health hazards. Wherever possible, use disposable plastic coated paper mixing cups or tubs, to minimize cleanup problems.
How much heat or cold will cured urethanes take?
Soft urethanes will withstand extreme cold, and can remain flexible to far below sub zero. Hard urethanes can be formulated that will maintain their hardness, and other mechanical properties at upwards of 250°F.
What kind of physical properties should I expect from a cured urethane?
Urethanes can be formulated to cover a wide range of hardness, everything from very soft rubbers to hard plastics. In general, urethanes are known to excel at having better abrasion resistance at every hardness than a comparable rubber or plastic made from another kind of polymer. Urethanes can also exhibit very good low temperature properties, staying flexible when many other polymers become brittle. In addition, urethanes have an overall balance of high elongation, tensile strength, flexural strength, and tear strength that yields tough, durable products that can be processed by liquid casting without expensive tooling.