Sealant Selection

When we built our boat, Terrie (my wife) and I added a teak rubrail. It was bedded down in a generous bead of 3M’s Marine Adhesive Sealant 5200 and through-bolted on 12-inch centers. That was 16 years ago.

Two years ago we decided to paint the hull with one of the high-gloss two-part polyurethane paints. Rather than paint over the rub-rail, which was by now somewhat scarred, we decided to remove it and replace it. I pulled all the fasteners, expecting it to pop loose, but nothing moved. I beat on it as hard as I could, but still nothing moved. Finally, I hammered a pry bar under one end and levered. There was a splintering noise as a length of teak gave way, bringing the top layer of the fiberglass lay-up with it. It took a full day of hard grunt work to splinter the rest of the rail off, and another two days to repair the damage to the hull.

This was an unpleasant way to learn the difference between a sealant (which is what I have always considered 3M’s 5200 to be) and an adhesive (which I now know it to be). It was a noteworthy introduction to the realm of modern bedding compounds, which have come a long way from the days of hemp, tar, flax caulking, and putty/linseed oil formulations. These traditional substances have been replaced by three modern types of synthetic chemicals — polysulfides, polyurethanes, and silicones — with outstanding properties as far as the boat owner is concerned, but properties that clearly must be understood in order to select the correct material for a particular job.

Cohesion versus adhesion

My experience has led me to believe that one of the key properties to be considered is the bonding capability of the sealant. This can be broken down into cohesive strength and adhesive strength.

Loosely speaking, cohesive strength is the internal resistance within the sealant to being torn apart. If, for example, two pieces of wood are bonded together and then pulled apart, and the break point occurs within the bonding substance itself (i.e., the sealant ruptures), then there is a cohesive failure. This is in contrast to a break between the bonding substance and either piece of wood, in which case there is said to be an adhesive failure (i.e., the adhesion of the bonding substance to the substrate has failed).

One measure of cohesive strength is tensile strength. This is measured using a test known as ASTM D412 (developed by the American Society for Testing and Materials, a national standards-setting body). A sample of sealant is formed into a sheet, allowed to cure, and cut into strips with a special dog bone die. The dog bone sample is then gripped at both ends and stretched at a rate of 2.0 inches per minute until it fails. The tensile strength is the force required to tear the sample apart, measured in pounds per square inch (psi).

Turning now to adhesion, we find that it can take different forms. The four principal ones are: 1) mechanical anchorage, in which the adhesive grips a rough surface; 2) interpenetration, in which the adhesive diffuses into the surface of the material being bonded; 3) electronic attraction, in which the mutual attraction of positively and negatively charged particles at the atomic level is used to create a bond; and 4) chemical interaction, in which the adhesive chemically bonds at the molecular level with the surface being bonded. All sealants use mechanical anchorage for adhesion; depending on the sealant and the surface being bonded, there may also be a chemical bond.

The industry has developed two common tests designed to measure the adhesion properties of a sealant. These are the lap shear test and the peel test. The lap shear test consists of bonding together two overlapping pieces of material, allowing the bonding substance to cure, and then pulling the pieces apart, measuring the force it takes to do this. The peel test is similar, except that the two pieces of material are bonded together at right angles, and then one is peeled back, once again measuring the force required.

Clearly, the results of these tests will vary according to such factors as how thick a bead of sealant is used, at what temperature and humidity it is cured and for how long, and, most importantly, what materials are bonded together (for example, wood to wood or glass to plastic) and how the materials were prepared for the test (whether the surfaces to be bonded were sanded or primed, etc.). In order to make fair comparisons between different products, the industry uses a couple of standardized lap shear and peel tests developed by ASTM (D1002 and D1846). Most comprehensive product data sheets will give the results of these tests, providing a pretty fair measure of comparative adhesive strength.

When these figures are not available, as a general rule it seems that, if a product has high tensile strength, it also has high numbers when it comes to both lap shear and peel strength, so tensile strength can be used as a broad indicator of adhesive strength (although there are times when this does not hold true; 3M’s 5200, for example, has higher tensile strength than Sikaflex 240, but does not adhere as well to unprimed aluminum). If using tensile strength as a rough measure of adhesive properties, it should be borne in mind that the numbers for tensile strength are always considerably higher than those for lap shear and peel strength.

Finally, it is important to remember the fact that just because a product has a high adhesive strength does not necessarily mean that it is the best substance for a given application. Sometimes a lower strength, lower modulus (less adhesive) product is preferable. This is particularly the case when the sealant is not needed to provide mechanical strength, but is simply there to keep water out (our rub rail, for example, and much boat hardware), and also in sealing gaps where significant movement is expected. In such applications, flexibility becomes more important than adhesion.

Flexibility and elongation

Flexibility is a measure of how much movement can take place in a joint before the sealant/adhesive exceeds the limits of its elasticity (flexibility is sometimes referred to as elasticity, or joint-movement capability). Once this limit has been exceeded, the sealant/adhesive will not return to its original shape. It may still be well bonded and watertight, but there will have been some deformation within the product itself.

Flexibility must be distinguished from elongation, which is another property of sealants commonly given in technical literature. Elongation is a measure of how far a cured sample of the sealant can be stretched before it tears apart. In other words, it measures the maximum amount of stretch before the point of rupture during the tensile strength test. Long before the break occurs, the sealant will have been permanently deformed (i.e., it will have been stretched beyond the limits of its flexibility rating).

Both flexibility and elongation are measured as a percentage of the length of the sample being tested. For example, 50% flexibility means the sample can be stretched to one-and-a-half times its original length before deformation occurs; 500% elongation means it can be stretched to six times its original length before it tears apart (for example, if the original length is one inch, and the length at break is six inches, the elongation is 500%).

As a general rule, the greater the flexibility and elongation, the softer the product and the lower its cohesive strength. In many instances, such as bedding down hardware, this is a beneficial trade-off. The softer product will also be easier to remove than a hard one if the hardware has to be taken off at any time.

Tack, cure, and shelf life

Polysulfides and polyurethanes can be bought in one-part or two-part formulations; the silicones are invariably one-part. With the two-part formulations, mixing starts a chemical reaction that causes the substance to set up. For example, in the case of the polyurethanes, poleol and isocyanate are mixed and combine (polymerize) to form a single long chain substance (a tough molecular structure). If the two parts are measured accurately, properly mixed, and applied according to the directions, the results are predictable and a full and even cure is achieved.

One-part formulations require no mixing (which eliminates the possibility of operator error). They are formulated in such a way as to start to cure upon exposure to some external agent (for example, moisture, oxygen, or ultraviolet radiation). Moisture in the atmosphere is commonly used (what is known as a poly condensation reaction). In this case, water is incorporated into the substance, and carbon dioxide released as a part of the reaction. The carbon dioxide diffuses through the sealant to escape into the atmosphere.

If the product is left to cure in unfavorable conditions (such as application under water, or in direct sunlight, or if there is a chemical reaction with the substrate, or if the sealant is too thick), bubbles or even a carbon dioxide "tunnel" may form in the sealant, reducing adhesion. One-part formulations are not appropriate when large volumes or areas of sealant are used (such as when bonding togetherlaminatingsheets of plywood) because of the limited ability of moisture in the atmosphere to penetrate to the inner areas of the bond.

Some polyurethanes and the one-part (cartridge-type, as opposed to two-part) polysulfides are notoriously slow to skin over, tack up and cure, whereas silicones set up much faster. 3M’s 5200 takes a day or so to skin over, and seven days or more to fully cure (3M has recently produced a "fast-cure" version that skins over in two hours and cures in 24). The various Sikaflex polyurethanes take anywhere from 45 minutes to more than an hour to skin over, and from one day to seven days to cure. Boatlife’s LifeCalk polysulfide takes 24 hours to skin over, and 20 days to cure.

The thicker a bead, the longer these products take to set up. Since curing normally takes place through the absorption of moisture from the atmosphere, the higher the relative humidity, the faster the cure. Curing can always be sped up by spraying with water or by immersion, but even so it can be frustratingly slow, especially if the next job is waiting on the cure; attempts to speed it up too much risk the formation of CO2 bubbles.

Cure time can also be sped up at the manufacturing stage by changing the formulation of the product, but in this case the shelf life, and the working time once the sealant is extruded, are reduced. Some of the "fast-cure" products have a desiccant (a water-absorbing substance) built into the container, which slows the rate of moisture absorption when in storage and so extends the shelf life, but even so there is a limit to how much cure time can be reduced without producing an unacceptably short shelf life.

In general, the net result of these conflicting factors is that the one-part formulations have a rather short shelf life (generally less than a year, although 3M’s 5200 is guaranteed for at least one year, and will most likely last two to three). Shelf life can be extended by long-distance cruisers by keeping spare (or opened) tubes in a zip-lock bag containing some desiccant, and then storing the bag in as cold a place as possible (a fridge or freezerthe tube will have to be allowed to warm up before use). The two-part polysulfides and polyurethanes have an extended shelf life (often measured in years).

Pros and cons of different sealants

The polysulfides and polyurethanes have similar properties and tend to be used for the same kinds of jobs. The polysulfides are the older technology and have been steadily supplanted by polyurethanes, at least in part because of the slow cure rate of the one-part polysulfides and the fact that they have a rather obnoxious odor. Nevertheless, the polysulfides still have a role to play on boats, notably for filling deck seams, because they have a higher resistance to ultraviolet radiation, fuel and oil spills, and various teak cleaners (note that Sika manufactures a deck-seam polyurethane that is specially formulated to provide resistance to UV and teak cleaners, while BoatLife has a specially formulated silicone with the same properties). Since the two-part formulations are generally less viscous (i.e., more fluid) than the one part, and as a result can be poured, they are particularly suitable for filling horizontal seams; their major drawback is that once mixed they have a limited pot life.

Almost all the polysulfides and polyurethanes can be used below the waterline, whereas most silicones cannot. Following a cure, the polyurethanes and the polysulfides can be sanded (the polysulfides rather more easily than the polyurethanes) and painted, although, given the inherent movement capability of all the sealants, where movement does occur any paint job will sooner or later crack.

The silicones have lower cohesive and adhesive strength than the polyurethanes and polysulfides, and poor tear resistance. They are not paintable or sandable (unless specifically formulated with this in mind — e.g., Sandable Silicone from BoatLife, which is intended for deck seams only). For some reason, the mere presence of silicones in a boat shop can also cause problems with other painting or adhesive operations (causing fish eyes, for example, with some paints). Many silicones have an unpleasant odor when curing (giving off acetic acid), although this is not true of Boatlife’s Sandable Silicone (the company describes it as having been "de-skunked"!). On the plus side, the silicones adhere well to some hard-to-bond substances such as plastics and glass, with excellent flexibility and good UV stability. Silicones generally have better heat stability than polyurethanes and polysulfides, which is why they are found in high-temperature gasket compounds for engines and in top-of-the-line marine exhaust hoses. Silicone also has a relatively long shelf life and is easy to apply.

BoatLife has a hybrid sealant (LifeSeal) composed of a mixture of polyurethane and silicone, which, they claim, combines the best properties of both products. On the plus side, it has a moderately high tensile strength, yet is very flexible; it has good adhesive properties, but can still be removed without damaging the substrate; it skins over in 30 minutes, and cures in 24 to 36 hours; and it can be used above and below the waterline. On the negative side, it cannot be sanded or painted.

Irrespective of the properties of individual sealants, only marine-grade products should be used on boats (bathroom caulk from the local hardware store is not suitable). If an application is below the waterline, special chemistry is required to ensure long-term stability: the product must be specifically labeled for this purpose.

Installation techniques, tips, and clean-up

With all sealants, surface preparation is critical for effective adhesion. The primary adhesion mechanism is mechanical, which means the surface needs to be clean (and in particular free of grease, oil, and dust), devoid of loose material (such as poorly bonded paint), and preferably abraded (even if this is only at the microscopic level). All sealant manufacturers have proprietary chemicals for surface cleaning and preparation, but in practice these are often not needed if the surface is thoroughly cleaned by more traditional methods. Some situations that need particular thought are:

· Painted surfaces. If the sealant develops a higher adhesive strength than the underlying adhesion of the paint to its substrate, stresses in the joint are likely to rip the paint loose. In general, any sealant can be used on well-bonded, two-part epoxy and polyurethane paints, but products with a relatively low adhesive strength are recommended on all other paints (including one-part epoxies and polyurethanes) if the joint is expected to be stressed.

· New fiberglass. The fiberglass lay-up will still be undergoing internal chemical reactions, giving off certain chemicals, and in all probability shrinking. In addition, any gel coat is likely to contain traces of mold release agents, while the other side of the lay-up may have a surface coating designed to exclude air during the cure (if the air is not excluded, the surface remains tacky). If possible, fiberglass components should be allowed to age. In any event, they should be thoroughly cleaned, if necessary using a proprietary cleaner and priming agent, and, if possible, lightly abraded. Since there are so many different kinds of fiberglass, for critical bonds the effectiveness of a sealant should be pre-tested for adhesion to the fiberglass prior to the main bonding operation.

· Translucent materials. The polyurethanes, unless specifically inhibited, have less resistance to ultraviolet degradation than the polysulfides and silicones. If the sealant in the joint (as opposed to around the edges) will be exposed to sunlight, a polysulfide or silicone sealant should be used. If polyurethanes are used, particularly as an adhesive rather than simply as a sealant, the joint should be made opaque (for example, by fitting a masking strip on windows, or by painting).

· Plastics. There are so many different plastics in use, each with its own unique chemical formulation, that it is difficult to make generalizations concerning appropriate sealants. Some plastics require specialized treatment for effective bonding to take place with some sealants (e.g., polyethylenes with polyurethanes). Other plastics just need to be clean (and preferably lightly abraded). The silicones seem to have the best all-around adhesion to plastic while the polysulfides run into the most problems (including chemical incompatibility with some plastics, resulting in damage to the plastic). Chemicals in both polysulphides and polyurethanes, particularly some of the primers, may cause stress cracking with acrylics (such as Plexiglas). Sika recommends a proprietary primer on PVC, ABS, and acrylics, and advises that, in general, while the polyurethanes can be used as a sealant they should not be relied upon as an adhesive (in other words, the sealant should be used in conjunction with mechanical fastening). If in doubt about the appropriateness of a particular sealant on a particular plastic fitting, the fitting manufacturer should be consulted or a test made on a small area.

· Metals. Given a clean surface, most sealants will bond well to most metals. For example, I once stuck a stainless steel hatch runner on a teak bed log using 3M’s 5200. Later, I had to remove the runner. The teak splita substrate failurebefore I could engineer a cohesive failure! Steel needs to be cleaned of all mill scale and rust, while stainless steel simply needs to be well cleaned; both should, if possible, be abraded. Of the boatbuilding metals, aluminum is likely to give the most problems. If the aluminum has already been coated (anodizing, chromate, or paint), it just needs cleaning. Otherwise it will need to be cleaned, abraded, and preferably primed with a proprietary primer.

· Hardware installation. It is often a good idea to install hardware loosely and to tighten it down fully only after the bedding sealant has at least partially cured.

· Deck seams. Leaking deck seams on old boats are not easy to seal. The problem is the contaminants that will probably have saturated parts of the planking. Once old caulking has been dug out, it is advisable to run a router down the seams to expose fresh wood on both sides. Fairly aggressive solvents (such as acetone) should then be used for cleaning. Even on new wood, the natural oils should be cleaned out of the wood in the seams (acetone, once again). Sealant manufacturers then recommend a proprietary primer and seam sealer. Although primers can be dispensed with in most other sealing applications, they are highly recommended in deck seams. Next, a bond breaker, which is generally a narrow tape of cotton, polyethylene, or polypropylene, or else a band of caulking cotton, is laid in the bottom of the seam (Sika sells a special dispensing tool for this job). The purpose of the bond breaker is to prevent the sealant adhering at this point – the only place adhesion is wanted is on the sides of the planking (this reduces the likelihood of seam failure). Finally, the sealant is either poured in or pushed in with a caulking gun, moving the gun away from the operator, pushing the bead of sealant ahead of the gun, rather than drawing it towards the operator (which would result in trapped air). Once cured, the seam should be sanded with the grainand not with an oscillating sander, which might tear the material loose.

As noted, where critical bonds are concerned, it is normally advisable to use the manufacturer’s cleaning and priming agent. Unfortunately, these are generally expensive, with a limited shelf life and a short working time. Once opened they have to be used up or thrown away. Almost all include some powerful chemicals that require care in handling and for which a face mask or breathing apparatus is recommended.

Critical bonds should also be tooled, which is to say the sealant is squeezed down into contact with the mating surfaces with a knife or some other implement. This ensures a good contact between the sealant and the surface. The job will be made easier if the tool is periodically dipped in warm water.

All modern sealants are difficult to clean up. Careful masking will reduce the mess and the clean-up time, but in spite of this some additional cleaning is almost always required. I have found that it is best to scrape up the majority of the excess sealant, before curing, using a knife or chisel and then to clean up the rest with multiple small pieces of (cotton) rags or paper towels. I make one pass with the rag or towel and then throw it away — if I try to get a second wipe, I invariably end up smearing the goop all over the place.

Before curing, all sealants can be softened and wiped up with various proprietary solvents, or paint thinners in a pinch, but this is generally a messy business. Note, also, that alcohol and alcohol-based products should not be used on uncured or partially cured polyurethanes, since this will permanently inhibit a cure. Similarly, polyurethanes should not be painted until they are fully cured, since chemicals in the paint may permanently arrest any further cure.

Once a modern sealant has cured, any excess can only be cut or chiseled away. And, when it comes to taking off old fixtures, if the fitting will not pop loose after the fasteners have been removed, it is generally necessary to resort to brute force in order to get it off. However, it is sometimes possible to cut through cured sealant using a cheese wire, for example. Or you can try more specialized tools like one used by autoglass repair shops for cutting through windshield seams.

Contributing editor Nigel Calder latest book is Cuba, A Cruising Guide, published by Imray, Laurie, Norie and Wilson.

By Ocean Navigator