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Welding vs. Gluing

Why is welding better for inflatable manufacture than gluing?

Welding is a better process because it last indefinitely.  Glued parts are subject to the glue getting “tired” and letting go.  The process of welding, fuses two like coated materials together, and there is no adhesive interface.  A welded lap seam is stronger in a shear direction than the base fabric.  A glued lap seam is stronger in that direction too until the glue gets “tired” and will let go. 

Plasticizer migration is what is responsible for adhesive bonds losing their strength over time.  The plasticizer is the same thing in the coating that keeps the UV radiation from destroying your boat.  This is one reason why PVC boats last so long under UV, but also the reason why the glue can let go.  This migration process is accelerated with heat and Humidity.  So a glued boat left in a closet near the beach in the Mexican tropics may lose it’s glued parts in 1 year, and likewise if you store your boat in the attic of your house in Houston TX.  However if your glued raft lives in Alaska, you may never have a problem if it never gets hot.  Over 30 years in business we have experienced all these things.   Why do welded boats last longer?  They do not come apart from glue failure. We replace a lot of glued parts, and we see all kinds of conditions boats are exposed to.

Inflatable Boat Fabric Properties

Contrary to some literature that is our there, plastic coated fabrics are not weaker nor do they leak more than other coated fabrics. In many cases they are superior in air retention. I have considerable discussion about coated fabrics in our web site about materials. All coated fabrics are plastic. Some are simply Thermo Plastic, and some are Thermo Set Plastics. It is the fabric inside that adds the strength to the coated fabric, and even this is plastic. Polyester (the base fabric that we use) is a Thermo set plastic that does not stretch very much, and Nylon, which is a choice of most neoprene coated fabrics is a thermo plastic that stretches more. Thermo set plastic coated fabrics are Hypalon and Neoprene. Hypalon* can be welded if it is welded within a short time after the coating process. It goes through an age bonding process that makes it unweldable after a couple of hours or sometimes days. Some Hypalons for roofing applications can be welded indefinitely with the right technique. It is therefore a thermo plastic that turns into a thermo set plastic over time. Neoprene in the uncured state is soft, like latex, and will cure to a harder state over time. Uncured Neoprene is my choice of repair material for both Neoprene and hypalon, because it is so easy to prepare for gluing. When Neoprene is heated it turns into a thermo set plastic. I have never tried to weld it, but my guess is that the vulcanizing process that causes it to set would keep this form happening. If uncured neoprene is allowed to sit touching another piece of uncured neoprene, it will eventually weld to that other piece.

Thermo set plastics have a dry abrasion quality that makes them wear better when dry. A good example of this is Neoprene belting for the mining industry. In the water they are at a disadvantage because these materials grab on rocks and are prone to wear faster because they are not as slick in the water. Thermo plastic coated fabrics are weldable because the coatings can be melted and fused together. This means that heat of friction can also melt the coatings, and the consumer needs to be aware of this. However, in a water environment, heat of friction is not an issue, because water is a lubricant for Thermo Plastic coated fabrics, and they actually wear better in the water because of this.

The word plastic refers to a property that materials have. It means permanent deformation under stress. That is what plastics do. Thermo plastics deform more than Thermo set plastics under stress. That does not have much to do with how they perform as a coating. It has a lot to do with how they perform as a fabric under the coating. Nylon ( thermo plastic) stretches more, and has a little higher tensile strength. Polyester (thermo set plastic) stretches less, and therefore needs less air pressure behind it to keep the boat stiff. Tensile strength is also determined by how strong the fabric base is. Therefore, a polyester fabric may actually have a stronger tensile strength than a nylon base fabric.

Coating technology has come a long way in the last 5 years. Most of the thermo plastics have better adhesion to the base fabric than do thermo set plastic coatings. New processes have been implemented that allow for a solution coating that completely saturates and coats the base fabric before a plastic coating is melted into place. these types of fabrics provide great air retention and coating adhesion. As a result there are fewer problems associated with air loss and delamination of the coating form the base fabric.

The Welding Process
There are two processes that are used today for welding thermoplastic coated materials, Radio Frequency Welding (see the machine), and Hot Air Welding (welding a tape on a part). Both of these processes generate heat to fuse the materials together. both of these processes use pressure to force the materials to fuse. Therefore it is heat and pressure that make thermoplastic welding possible. Plastics do not melt at the same temperatures. Therefore it is difficult to fuse dissimilar materials together.

Radio frequency welding uses a press that applies pressure to a large surface area. The press has a table that the material is placed on. Dies are used to direct the welding process. When the press comes together, Micro waves are passed through the small area between the die and the table where the weld takes place. These micro waves heat the material and the combination of heat and pressure cause the weld to take the shape of the die. RF welding is fast.

Hot air welding uses hot air to heat the coating on the fabric where it is to be bonded together. A Nozzle is situated between two rollers that pull the material through the machine. As the material is pulled through the machine, hot air is applied to the surfaces to be fused together. Pressure from the rollers and heat form the hot air cause the plastic to fuse as the plastic cools. With the material moving through a machine like this, different shapes can be made. Patterns can then be put together to form the shape of an inflatable boat. This is also a fast process. This process is not subject to the intricacies of radio frequency. However the operator must know a whole different set of rules to make it work.

The Gluing Process
Gluing processes consist of preparation, application and bonding of the fabrics together. There are two more steps involved in the gluing process, and this makes the process more expensive, because there is more labor involved. The quality of a glued joint is determined by how hard the material is to prepare, how well it is prepared, and environmental factors that may affect the adhesive. Parts can be stockpiled at many different stages of production. I believe that this is how efficiency is obtained in this process. The preparation process is a lengthy one. There is plenty of room for human error, both in perpetration of the fabric, and application of the adhesive. This is the primary reason why glued products are more expensive. There is a lot of information about the actual process of gluing in our website that I will not go over here.

Which is better?
It is obvious that if there are two less steps in the process there is less chance for human error. At Jack’s Plastic Welding, we believe in both processes. However, we do not glue any air holding parts of any Inflatable boat, and as of 1999, all D ring patches are RF welded. If there is a glue failure, it will not be associated with the loss of an air chamber, and resultant loss of functionality. Not all parts can be welded and still look good and be functional. Still we have a very small warranty problem associated with glue failures, and virtually no warranty problems associated with weld failures.

Because welding actually matches and bonds coated surfaces together in a way that adhesives can not (no adhesive interface), it is a much more reliable method for constructing inflatable boats. Properties and strength of the original materials can be maintained, and not compromised by the presence of an adhesive and the labor to apply it. Add to this the savings in labor, and it becomes a very viable method of manufacture. We are constantly searching for methods to eliminate adhesives in our products, because of the reliability of welding, and of cost savings.

* Hypalon is a registered trademark of the dupont corporation.

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