Resin Additives – Why Use, What’s Out There and Pros & Cons – Part 2

In part two of this series on Resin Additives, we’ll dive into some specific resin additives, focusing on fillers and resin reinforcements.


Fumed Silica (aka Cabosil) is an extremely versatile thickening agent / additive for epoxy resin systems. Because of its highly hydrophilic nature and easily dissolving, it is used in products worldwide ranging from composites, paints, water and even the food industry. Fumed Silica is finer than dust, a nanometer scale size material. Silica’s low density material packs in high strength while providing quick thickening properties. Fumed Silica is a small particulate substance and may be harmful if inhaled. It is strongly advised to wear a mask when working with this material or to have adequate exhaust ventilation. 

Advantages: Extremely versatile in use, can be used as single additive or with other material mixtures as a thickening agent at all above consistencies. Very quick filling agent with great thixotropic properties at the right consistencies.

Disadvantage: As an additive, it may make sanding more difficult. Best used as a thickening agent along with other additive mixtures.

Glass / Plastic Microspheres are widely mixed with epoxy to provide light faring compounds. These microspheres are hollow with small air voids, making it an ideal light weight filler and fairing additive. This additive is larger in size compared to that of Fumed Silica but does not thicken to the same extent as Fumed Silica.

Advantages: Easy sanding ability & lightweight, ideal for fairing

Disadvantage: When used alone with epoxy, material does not cling well to vertical surfaces.

For vertical surface fairing, mix Microspheres with Resin at ~1:1, add a small amount fumed silica to thicken mixture until it will hold onto vertical surface. (Think plaster consistency) This method will still provide an easy surface to sand.

Resin Reinforcement

Carbon Fiber Nanotubes used as a resin additive are relatively new to composites considering other products. Carbon nanotubes are very small, known best for being one of the strongest materials known to man being the cylindrical version of graphene. When mixed in light volumes with epoxy (4-6% by volume), it can create reinforced resins great for castings and surface coats. The long thin carbon tubes make for a good surface coat additive because the nanotubes transfer energy better to the reinforcement than resin alone. There is still much to be learned from this unique material’s characteristics going into final use products.

Advantage: Surface abrasion, added durability to castings and surface coats, better energy transfer leading to tougher composite laminates

Disadvantage: Challenging to work with in traditional composite laminating

Glass Microfibers are made from the “left-overs” of producing fabrics or other various fiberglass materials. Instead of the scrap fiberglass being thrown away, it is finely chopped to various sizes for use as needed. Being left over does not make it use any less valuable, fiberglass microfibers offer stronger mechanical bonding than that of the resin alone, fumed silica, or microsphere additives. “Glass microfibers” are finely chopped to a micrometer sizing of ~2µm in diameter, basically the smallest glass sizing that will offer any substantial strength as a resin additive. Glass microfibers great resin additives for general hole filling, surface repairs and for secondary bonding with well-prepared like mating surface bonds.

Advantages: Offers higher mechanical properties over products such as wood filler, Fumed Silica, or microspheres.

Disadvantages: More difficult to work with as consistency is comparable to a runny jelly, some experience helps with processing this material.

Milled Carbon Fiber (MCF) is produced similarly to that of glass microfibers and is between 80-100 µm in length. When used even in small mixtures (~5%) in resin it adds a substantial increase to a resin’s mechanical properties. Because of CF’s low thermal expansion there is an evident increase the thermal properties of a given resin. MCFs can also be engineered to manipulate a surface’s electrical conductivities. It is used commonly in increasing various plastic properties, and in enhancing thermal properties of tools and molds. MCFs are also used in the same manner for general hole repairs, surface defects, and secondary bonding. Note: When mixed, the color of the resin will become a darker shade which may affect a desired surface finish.

Advantage: Additive results in better material properties than that of Glass Microfibers for stability, toughness, modulus, and strength. Can be used for additional enhancement of thermal & electrical properties in the resin.

Disadvantage: May be difficult to work with in higher mixt ratios, research proper mix ratios before use to ensure desired resin properties are reached for the task at hand.

The series will wrap up next week as we examine using Chopped Materials & Additive Powders as Resin Additives.