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Polyurethane Adhesives, Are they Good for Boat Building?

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. Thermosetting polymers, or thermoset plastic are polymers that becomes rigid and set when heated. Often room temperature is enough to provide the heat necessary to set thermosetting adhesives. The adhesive is applied as a liquid, or in some cases where it has been filled or modified as a paste. Heat, As the glue sets, it absorbs heat energy that powers the chemical reaction, cross linking, to make a strong network of chemical bonds.

Once set Polyurethane adhesives 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 solidly becoming relatively inflexible and brittle, or can remain rubbery and flexible depending on composition and fillers. Boat owners will have heard of 3M Marine Adhesive/Sealant Fast Cure 4200 Or Sikaflex 1A Polyurethane Premium Grade High Performance Elastomeric Sealant

These are commonly available as sealants, adhesives or high quality bedding materials that can be painted, unlike silicone sealant.

Most construction grade polyurethane adhesives set solidly without being brittle. They can be sanded, and a Trueform or Shinto rasp (9" Shinto Saw Rasp) will cut through the glue without dulling the blade as thickened epoxy might.

What are the Advantages of Polyurethane Glue?

What are the Disadvantages of Polyurethane Glue

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 as they set. Since the main purpose of the wood strip is to act as a core for the fiberglass cladding that will be added later 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 many 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.

Polyurethane glue can be used for laminations, it is messy and annoying to clean up but makes a good joint. The catch is always having good fitting surfaces and a good tight clamp. Here is Fine Woodworking page about bent lamination glueing.

email me if you find mistakes, I'll fix them and we'll all benefit: Christine

Small Print

I don't claim to be an expert. I'm not an engineer or a chemist. I make mistakes. This page is not specifically a recommendation of products, it is for information and entertainment. IF you want to build a boat with polyurethane make sure you use the material as instructed by the manufacturer. There are health concerns about uncured polyurethane, so wear gloves and work in a well ventilated area, or wear a vapor mask. Do your homework and be safe.