The big bonding guide, part 1: How adhesives work and their use in yacht building

The contents of this special

• Part 1: Mode of action of adhesives and use in yacht building
• Part 2: Basics of the miracle resin epoxy
• Part 3: Which resin for which purpose?
• Part 4: Coating, coving, holes
• Part 5: Tubes, fillers, beams, fittings

Bonding does not have a good image - the disappointments experienced are too great, and many people have too little confidence in the bonds. But this is completely unjustified. 20 years ago, there were around 3,000 screws in a car, today there are only around 1,000. The rest are glued. Even spot welds, which always cause corrosion problems, are increasingly held together chemically. Rust is therefore a thing of the past, with at least the same rigidity of the vehicles.

Even large aircraft such as the A 380 cannot be realised without bonding, as the materials that have to be joined together are too diverse: Aluminium, titanium, carbon fibre. Welding? Impossible! Lightweight construction is the keyword and bonding is the order of the day. After all, every German citizen consumes an average of around 6.5 kilograms of adhesive per year, and the trend is rising. So bonding is on the rise.

Products for use on board

Of course, this development does not stop at yacht building, supported by the fact that the handling of resin and hardener is part of the daily business in most shipyards anyway. Strongbacks are glued in and not necessarily always bonded to the hull using angle laminate. This is often not even necessary. Bulkheads are attached with MS polymers. Laminating? Unnecessary! And hull windows are also glued in. No screws, no mechanical fixings, just the adhesive. And soft is not necessarily the first word that comes to mind with modern ships, on the contrary. Gluing does not seem to harm the structure and therefore the rigidity of today's designs.

How does adhesive work?

Ceiling panels are also held in place with the miracle paste, as are the individual layers of laminate sails or the deck on the hull, which provides an extra portion of longitudinal rigidity. "This connection is so good when glued, screws would just get in the way," comments Dennis Hennevanger from the Saffier shipyard. The Dutch company glues the decks on all larger models. "No problems, always tight. And anyone who sails one of our boats in strong winds knows how tight they are."

The process is therefore part of everyday life for shipyards. But what exactly is bonding and when is it useful on board? What different types of adhesive are there and what are they used for? And is it possible for owners to work with adhesives themselves?

Adhesion and cohesion

Adhesives are usually liquid or pasty and become more solid or even hard during use. The bonding of the components and the associated hardening is the cohesion, i.e. the bond between the adhesive and itself.

This can be controlled quite well. Single-component products react with their environment, such as humidity. This is why the manufacturer's processing instructions are so important. Multi-component adhesives, such as resins, cure by reaction of the components with each other. This can be easily controlled, but requires a precise ratio of the two components to each other and the correct ambient conditions, such as the right temperature, for the reaction to take place completely. Epoxy resin, for example, usually needs at least 10 degrees Celsius to react. In any case, it is therefore crucial to adhere exactly to the manufacturer's processing specifications. This is the only way the adhesive will achieve the desired strength in the end.

The other part of a good bond is adhesion, i.e. the adhesion of the adhesive to the surface to be bonded. The preparation of both sides is extremely important. It is clear that nothing will adhere to a dusty or greasy surface. A particularly smooth contact layer is also unfavourable. Put simply, the adhesive needs to be able to cling to its counterpart. Wetting is the key word here.

The key question is: would the surface remain damp if you poured water over it? If the laminate is coated with topcoat, it will bead up immediately. However, if the colour has been sanded off, the laminate is rough and degreased and free of dust, then the surface will initially remain damp. Special primers can also prepare difficult surfaces for adhesives. Like dust, grease on a surface prevents water - and therefore adhesives - from adhering to it.

Here, solvents such as acetone help to prepare the surfaces as well as possible. Different substrates also need to be sanded differently: Wood, for example, with 80-grit paper, polyester with 120-grit, steel needs 100-grit and aluminium even 300-grit. These are rough guidelines for preparation.

Primary or secondary?

When bonding, a distinction is made between primary and secondary bonds. The former is when the substrate is still chemically active, i.e. curing is not yet complete when the element to be bonded is applied. In the case of epoxy resin, this is usually 24 hours. The next layer would then not only bond mechanically with the surface, but also form a chemical bond, as there are still reaction partners available in the substrate that are suitable for bonding - cohesion is created. This is also much stronger than purely mechanical adhesion.

This is not unimportant, because if the processing intervals are adhered to, intermediate sanding to enable mechanical adhesion is not necessary. This can be taken into account when planning the various layers when rebuilding the many layers on the hull after osmosis treatment. After all, who wants to sand the underwater hull again if this can be avoided through planning?

However, both in yacht building and for the owner who has to glue something to his boat, it will mostly be a matter of secondary bonding. This is because the substrate will usually already have hardened by the time something is attached. The deck of a yacht, for example, is only glued on many days after the hull has hardened. "There's no other way," says Hennevanger, explaining the shipyards' view. "To get the hull out of the mould, it has to be cured. And the mould is needed for the next build. Of course, the mould is removed before the deck is put on, so the marriage has to take place later. And then the gluing starts."

The experts agree that this is not a disadvantage or a weak point in the construction: "A well-calculated and executed bond is a good thing," says Uli Manigel. He is a structural expert at the IMCI certification office. Matthias Bröker from Judel/Vrolijk & Co agrees: "If the forces are known, and these can be easily determined today, the adhesive and surface can be designed accordingly. If the shipyard then works with sufficient precision, for example complies with the gap sizes specified by the adhesive manufacturer and the environmental conditions are right, then there is a lot to be said in favour of bonding." This is because the properties and capabilities of the materials are also known. So it's just a maths problem and a challenge to work precisely and carefully.

Knowing what to glue with

If you want to do your own bonding on board, you will always make a secondary connection, because all the bondings on a yacht are cured on delivery. It doesn't matter whether a new through-hull passage is sealed with MS polymer - this is also a bonded joint - or whether a platform for an autopilot drive needs to be installed in a more complex manner. Bonding is also the method of choice for replacing aged Plexiglas panes or simply attaching a bracket for a heating hose to the inside of the fuselage.

This raises the question of the right product, because it is not only the adhesive strength that makes the difference, but also the strength in the hardened state. Cured does not always mean actually hard. Permanently elastic sealants, as the name suggests, remain flexible and can therefore firstly seal very well and secondly compensate for any movements. For example, metal diffusers and the hull surrounding them have significantly different coefficients of thermal expansion.

"Thermal expansion is also a major problem with Plexiglas panes. For this reason alone, screws are very unfavourable, they get in the way of the expansion and the plexiglass cracks. But adhesives also reach their limits. It is better to use safety glass or Plexiglas panes prefabricated in metal frames," explains Bröker. A thicker layer and a wide adhesive seam help to compensate for movements caused by thermal expansion. In the case of surface-mounted windows, it is therefore advisable to go to the maximum limits of the manufacturer's specifications. If necessary, divide the window surfaces sensibly, which also reduces the expansion distance.

Use of MS polymers

If the sealing material cannot compensate for the movements occurring between minus 20 degrees in winter and plus 30 degrees in summer, the bond will leak. A hard-brittle mixed epoxy resin is completely unsuitable here. Conversely, a permanently elastic adhesive in structurally central areas would be nonsense. The keel, for example, should be bonded as firmly as possible to the hull and strongback; there is no room for flexible seams. The play that would result from the constant load changes, for example in rough seas, would constantly put excessive strain on the joints and bolts, resulting in premature failure. A very strong adhesive is the first choice here: for example, an epoxy resin with a filler that also guarantees the optimum seam thickness of often less than one millimetre due to the size of its components. This is particularly important for joints under pressure.

Equally important is the question of whether it should be possible to replace them at a later date. Seacocks, for example, need to be replaced regularly. This is another reason why the rather easy-to-remove connection using MS polymers is a good option. The platform of the autopilot, on the other hand, can remain in place; the solution with resin and possibly even angle laminates is then an option. If done properly, these can only be removed with brute force, i.e. using an angle grinder.

Angled laminate for the right connection

It is easy to calculate whether an angle laminate is required. A good epoxy resin adhesive offers a tensile shear strength of 35 Newtons per square millimetre. According to the manufacturer, the type 1 autopilot drive from Raymarine develops a maximum thrust of 295 kilograms. This has to be absorbed. Approximately 295 kilograms is 3000 Newtons, with a 15 per cent margin it is around 3500 Newtons. So if the bonding surface can be larger than 100 square millimetres, and it will be, bonding with epoxy and without angle laminate is already sufficient. However, more surface area is better, because the lever with which the force acts is not taken into account in this example. However, the example shows how strong bonds can be.

Epoxy resin is recommended to achieve optimum results. It has advantages over polyester resin in the secondary bonding area, as it wets better and therefore adheres better. It is also less harmful to health during processing. It is also important that the fillets of the emerging adhesive are clean: "If notches form there, this is the point at which cracking begins. A homogeneous fillet is therefore extremely important," says Matthias Bröker.

This also applies if an angled laminate is to be applied later. Without a cove, the corner would be too sharp for a glass fibre scrim; air bubbles would always remain in the radius, which would weaken the bond enormously.

Modern adhesives are sensible and sophisticated

Modern adhesives have become an integral part of yachts. And they don't have to, because they give designers more freedom for their designs. And they save the shipyards time in production. Take hull windows, for example, which would hardly be possible in their current size without these miracle products. Although not envisaged by the CE, it would even be conceivable to bond windows as structurally effective components.

In modern boatbuilding, it is easily possible to design joints that are easy to produce and optimised for adhesive bonding right from the design stage. Ultimately, it is the shipyards that have to adhere to the designer's specifications regarding the adhesive to be used. In addition, the manufacturers' processing instructions must be followed almost slavishly. Only then will the joint achieve the desired strength.

"We therefore regularly have samples analysed destructively to see whether the adhesive is OK and whether we are working correctly," says Hennevanger, speaking from practical experience. According to the shipyard manager, they have not had any abnormalities so far.