Polyurethane glues like the popular Excel and Gorilla Glue form bonds which in some applications are stronger than PVA glues or Cyanoacrylate glues. These are highly water resistant glues good for exterior applications and will bond wood, as well as non porous materials like stone, ceramics and some plastics and foams. For models and miniatures they are often used to glue dissimilar materials together, in settings where epoxies are not useful, or are too heavy.(/p>
Polyurethane glues foam and swell up as they cure. These glues do not clean up with water, so have denatured alchohol handy to clean up your joints before the glue sets, or you can scape and sand the glue after it has cured. Polyurethane glues become brittle as they cure, and are more easily sanded than more rubbery PVA glues.
Best Methods for Applying Polyurethane Glues
Polyurethane glues work best when the glue is applied in very thin layers to both sides of the join and the join is clamped firmly together. Use a plastic glue spreader to apply these glues in a thin layer. These glues must have moisture to work, so joins may need to be lightly dampened or misted with water five minutes before you use these glues in dry climates. The glue exands and foams as it cures, so use very sparing amounts, and make sure joins are clamped tightly, to prevent the glue's expansion from forcing the join apart as it cures. The glues take 20 minutes to set up, so you have some time to set your clamps and align your joints. Also, as your hands have some moisture, always wear gloves when using these glues, or the glue will react with your hands. Most solvents are inactive on polyurethane glue, so don't ever plan on removing them without sanding once they have bonded. Denatured alcohol can be used o wipe up joins before the glue sets
Storing Polyurethane Glues
For best shelf life wipe the nozzle clean after use with a dry rag and squeeze all the air from the bottle, then replace the lid while squeezing the bottle to keep the bottle as air free as possible. As the glue reacts with tiny amounts of water, open bottles of these glues should always be stored upside down, to keep the possibility of air meeting the glue to a minimum. Store in a cool, dry place away from direct sunlight. Most polyurethane glues have a shelf life of approximately 3 years unopened, or 6 months to 1 year after opening if properly stored.
Advantages of Polyurethane Glues for Models and Miniatures
- Good bonding of dissimilar materials, useful where bonds between dissimilar materials may need to be sanded. Great for gluing card to tile or porcelain, rocks to wood or metal hinges to wood.
- No mixing is required, the glue is used straight out of the bottle.
- Gap Filling - Polyurethane glues will fill small gaps of 1 to 2mm. For larger gaps, the air bubbles in the expanded glue create a weaker bond which will fail, so they are not advised for areas where strong bonds across gaps are required.
- Weather Resistant - although not completly waterproof these glues generally stand up to outdoor applications and can be used over a temperature range from 40 to 130 degrees Fahrenheit. They are best applied at room temperature
- Cure Time - one to two hours, allowing time to position parts and set clamps.
- Work on Polystyrene Foams - Polyurethane glues can be used with expanded styrene foams, commonly used for rc airplane wings and other models.
- Sand Easily - These glues can be easily sanded to remove any glue line. Sanding does not gum up sandpaper as with PVA glues.
- Glue takes most dyes and stains, does not accept aniline stains.
Safe Handling of Polyurethane Glues
- Keep Polyurethane glues well out of reach of children and animals. Issues concerning animals and polyurethane glues can be found in the article on Gorilla Glues and Pets
- Work in a well ventilated area. Polyurethane glues contain isocyanates. Avoid if you have respiratory issues.
- Always wear gloves when using these glues, or the glue will react with your hands. It can best be removed from skin using a pumice stone or pumice based soap.
- Wear old clothes when working with polyurethane glue, it cannot be removed from clothing easily