First, polyurethane, PU for short or sometimes PUR, is not a single material with a fixed composition. Rather it is a range of chemicals sharing similar chemistry. It is a polymer composed of units of organic chains joined by urethane or carbamate links. Most polyurethanes are thermosetting polymers and do not melt when heated, thermoplastic polyurethanes which do melt have also been made available.
Polyurethane polymers are formed by the reaction of isocyanate and a polyol. Both the isocyanates and polyols used contain two or more functional groups per molecule, usually.
THAT'S IT FOR THE CHEMISTRY. If you want to read more have a look at the Wikipedia Polyurethane Article.
What is Polyurethane Adhesives Used For?
Whatever the exact composition, polyurethanes have some things in common. They are STICKY. Reading the claims of PU adhesive manufacturers is like reading a who's who of materials. Very few things are omitted. Polyurethane glues will work on just about all normal materials, porous or not. Wood, metals, rubbers, cured epoxy, leather, tile and glass, many plastics, concrete and brick, the list goes on. It does not work well on polypropylene, polyethylene or on such substances as teflon or silicone. Nor does it like waxy or oily surfaces.
Polyurethane based adhesives can set solid and relatively inflexible, or can remain rubbery and flexible. Boat owners will have heard of 3M Marine Adhesive 4200 sold as sealant and somewhat removable adhesive or Sikaflex Polyurethane Marine Adhesives. These are commonly available as sealants and high quality bedding materials that can be painted, unlike silicone sealant.
Most polyurethane adhesives set more solidly without being brittle. They can be sanded, and a trueform will cut through the glue without dulling the blade as epoxy might.
What are the Advantages of Polyurethane Glue?
- It stick to most things and can glue different materials together. It is successful in bonding non porous materials such as metal to wood, or mirrors to walls.
- Some compositions have a nice work time of 20 or so minutes. This allows for leisurely working and clamping time.
- Depending on the glue it can set quite quickly after clamping and allow the user to continue work as the glue completes its curing.
- A major advantage over many adhesives is its ability to set in high moisture conditions. IN fact it needs moisture to set and will cure faster in conditions where other glues, such as epoxy, cannot be used.
- It is waterproof. Some brands are more than others. Passes ANSI Type I & II water-resistance testing. Vice brand makes SUPERGRIP, a marine polyurethane. Polyurethanes are generally not recommended for long term immersion in water or use below the waterline unless protected by paints or other protection.
- It requires no mixing or measuring and can be used directly from the bottle.
- It is available in several different viscosities and packaging. It is available in caulking tubes with filler which makes it stay and not sag, squeeze bottles, tubes, and single use package.
- Polyurethane is available as a hot melt glue that sets and holds parts together without the need for clamping. Once the glue has cooled and set it continues to cure for several days to attain its full strength. (see side bar for Titebond hot melt adhesive)
- If properly applied and cured polyurethane makes a strong bond.
- Will set in a wide range of temperatures.
- Suitable for outdoor projects Polyurethane has good UV resistance.
- Does not contain solvents and is a low Volatile Organic Compound producer. 100% solids so it does not tend to crack and shrink as it sets.
- It can be sanded (non clogging), stained and painted. Cleanup of squeezed out extra glue can be chiseled off easily. Set glue is slightly flexible and not brittle.
- Food safe after curing.
What are the Disadvantages of Polyurethane Glue
- It is not as strong as epoxy. On wood it is stronger than the wood so its strength is not an issue but on metal, epoxy provides a stronger bond.
It compares to other wood glues. Great claims have been made but tests seem to show about equal holding power. Polyurethane had better end grain gluing capability though because of better glue penetration.
- Parts being glued with polyurethane needs to have a tight fit to be strongly joined. polyurethane is not a good gap filler. Although it is sometimes advertised as gap filling, tests show that strength is reduced when the glue line is thick. When setting it froths up and the froth, although gap filling, is not strong. Some formulations have filler that allows for some gap filling.
- In very dry areas it requires moisture to set or it can set very slowly and not reach it's full potential.
- Because it is sensitive to humidity in the air and wood, setting times can vary considerably.
- It tends to foam and squeeze out. If not clamped the parts can be forced apart by the foam, weakening the joint. The foam gives the illusion of gap filling, but it is not strong. Foaming is greater when there is a lot of moisture. Some compositions foam more than others which are practically foam free such as the construction adhesives as PL Premium.
- After curing it is very chemically inert and safe but the intermediate phases are toxic, irritating or carcinogenic. This means polyurethanes have to be handled carefully, kept off your hands and not breathed in too much. See the side box for the Wooden boat forum link which has a good discussion on the safety of PU glues. You can google for the MSDS of various brands to get more information.
Warnings include: Contains isocyanate containing polymers. Contact causes eye irritation. Prolonged or repeated skin exposure may cause allergic reaction, irritation and sensitization. Contact may stain skin. Do not allow eye contact. Avoid prolonged or repeated contact with skin.
- Some brands and compositions are expensive.
- Polyurethane has a limited shelf life, less than a year, and once opened can go off quickly if moisture gets in.
- Polyurethane is difficult to clean off hands, (gloves are highly recommended). Acetone or lacquer thinner can be used to clean tools while still uncured.
So Is Polyurethane good for Boatbuilding??!
For many applications it is stellar. Anytime you need to attach a part that fits well with no gaps and that you can clamp securely, you can consider polyurethane. I would not hesitate to use it for gunnel spacers for example. It's hard to beat the convenience of being able to just use a small amount of strong waterproof glue right out of the tube. Some arguments from various boat forums on the left.
It has been used with great success in strip construction, (cedar strip building) AS LONG AS THE PARTS FIT WELL AND ARE CLAMPED AS THEY SET so as to not be weakened by the frothy foam that is generated. The main purpose of the wood strip is to act as a core for the fiberglass cladding that will be added later so there is not a great deal of stress on the glue and it gets encapsulated in epoxy to keep the moisture out anyway. It sands well and squeezed out glue and foam can be removed with a scraper. Because it sets quickly it allows the boatbuilder to add more strips in one sitting.
Several designers of simple boats have used it and report success. Countless Puddle Duck Racers and Mouse boats have been built using PU adhesives. Some builders have used PU building adhesive such as PL Premiumm, to make fillets and saturate fiberglass tape over fillets.
I would not have any qualms using polyurethane in places where screws supplement the joint.
In emergency repair of wet wood, polyurethane is the adhesive of choice, as long as the parts are kept together with clamping or with temporary screwing.
I would put most of my structural work together using Epoxy or Resorcinol glue. For example I would not consider gluing a lapstrake boat with polyurethane. I would use epoxy. It is a proven boatbuilding material, is a good gap filler when thickened and has superior strength and waterproof qualities.
Since Snowboards (see sidelinks) can be made using PUR adhesives then they have come of age and command some respect!
Howard Percival Johnson restores old wooden boats. Check him out at Old Time World Some eye candy for old motorboat lovers. He emailed me and said he had successfully redecked a large boat using PU glue.
email me if you find mistakes, I'll fix them and we'll all benefit: Christine