WEST SYSTEM offers six different fillers—why so many? When is it critical to use a specific one? What are the differences? Let’s explore the details of WEST SYSTEM Fillers and discuss how they function. Understanding these products will enable you to take advantage of the versatility inherent in the WEST SYSTEM 105 System.
The primary purpose of all our fillers is to modify the viscosity of the liquid epoxy mixture. Still, other properties, such as strength, density, and surface texture, are affected when fillers are added. Having six fillers allows us to create a mixture with ideal properties for your application.
First, we will discuss the four fillers used for structural bonding and then the two fillers for fairing. Structural fillers provide a strong mixture after they are cured, and it is important to note they are hard and difficult to sand. We do not recommend them for surface fairing and finishing. Fairing fillers, conversely, are much easier to sand and shape to a smooth finish but should not be used for highly loaded structural bonds.
Adhesive Fillers
All WEST SYSTEM Fillers, except for 406 Colloidal Silica, are blends. These blends have been developed to create products that are easy to use and provide predictable properties. Custom manufacturing equipment is used to produce them, and the raw materials are not easily blended using hand tools, so we encourage our customers to take advantage of these time-tested filler formulations.
406 Colloidal Silica Adhesive Filler
We will start with one of our most versatile fillers, 406 Colloidal Silica. It is our only filler that is not a blend—it is 100% fumed silica. It is very difficult to buy in small quantities and obtain a grade suitable for use in epoxy. We add value by providing the ideal product for use in WEST SYSTEM Epoxy and providing it in convenient packaging.
The product has a very low density when in the container due to the particle’s unique nature. The particles are somewhat “cloudlike,” creating a significant surface area that enables the particle to efficiently saturate a mixture of liquid epoxy. The particle’s shape allows it to stay suspended in the epoxy and become entangled with other particles. This makes it the most effective at modifying the viscosity to the desired consistency and providing very smooth fillets. The resulting cured mixture is of high strength with the intent to be used as a structural adhesive.
403 Microfibers
Another filler that is well-suited for structural bonding is 403 Microfibers. 403 is a blend of different fillers, the majority of which is cotton flock fiber. These fibers blend in quickly and increase the viscosity with a relatively small amount of filler. Because the filler consists of actual fibers that reinforce the epoxy, the cured mixture will have strength that is suitable for use as a structural adhesive. However, it does cure with a smooth surface. We often describe it as having an oatmeal texture. 403 Microfibers is ideal for applications that require the epoxy to be spread thickly over a large bonding area, such as laminating two sheets of plywood. Because of its rougher texture, we do not recommend 403 for filleting.
405 Filleting Blend
When fillets are used to stitch and glue plywood boats and other applications where the epoxy will be visible on wood, we recommend 405 Filleting Blend. This filler is a recipe of nutshell flour and other fillers that give a wood tone color and create very smooth fillets. It is processed and filtered to ensure the particles stay consistent and suspended in the epoxy. We also add specific amounts of other fillers to create a color similar to mahogany and other common boatbuilding woods.
404 High-Density
The last of our structural fillers is 404 High-Density. This product is a mineral filler that, as the name describes, contains a very high-density particle. The particles are shaped like long needles made of a hard mineral. This provides strength to the thickened epoxy. We developed it for potting the mounting studs of wind turbine blades. It provides the highest strength out of all our fillers but it is not dramatically stronger. It requires more filler to increase viscosity than the other adhesive fillers, so it is ideal for highly stressed joints that are common in fastener bonding. It has the unique property of absorbing heat when the epoxy cures, reducing the chance of excessive temperatures in thick applications.
Low-Density Fillers
407 Low-Density Filler
407 is a blend of phenolic microspheres with other fillers. The blending is essential because if you attempt to use 100% microspheres as epoxy filler, the light filler particles will float to the surface and will not fully mix into the epoxy. Our blending solves this issue and makes using a low-density filler very easy. Blending of multiple fillers allows this filler to be used as an adhesive for lightly loaded bond lines. For example, 407 Low-Density Filler may be well-suited for a small stitch glue boat if the fillet radius is increased, possibly providing a weight reduction. Using 407 Low-Density Filler in this application is discussed in the Gougeon Brothers on Boat Construction, pages 102-103.
410 Microlight®
Finally, we have 410 Microlight. This highly formulated filler that takes advantage of multiple particle types and unique liquids to make an extremely low-density filler that can be easily dispensed and stirred into neat epoxy. It is lightly packed in the container which, makes it easy to scoop out with little mess, and it will not float out of the liquid epoxy. The ease of sanding comes from the use of thermoplastic micro balloons. Using these micro balloons requires caution when using the filler under a dark-colored surface that will get direct sunlight exposure. 407 Low-Density Filler has better temperature resistance for these applications. For other fairing applications, it is very easy to apply and sand, which is an important consideration. 105 Epoxy Resin and corresponding hardeners cure to very strong and durable materials that are difficult to sand without adding a low-density filler.
Safety
None of our fillers are carcinogenic. It is important to note that we do not sell crystalline silica, which requires special handling for safe use. It has a very different particle shape than 406 Colloidal Silica. A dust mask, safety glasses, and gloves will provide exposure protection when utilizing any WEST SYSTEM Filler.
WEST SYSTEM Fillers have an indefinite shelf life when stored properly. Store fillers in a dry environment because they absorb moisture and may become clumpy. If this occurs, run the filler through a flour sifter to break up the clumps and mix thoroughly to break up any remaining clumps.
Final Notes
We offer multiple fillers so our customers can have the ideal product for their application, but these products do overlap each other in where they can be used. If you keep 406 Colloidal Silica and 407 Low-Density Filler in your workshop, you will be well-equipped for most common applications. If the specific properties of another filler are required, the versatility of the 105 System enables you to purchase that filler and use it in the same resin and hardener.
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