An Introduction to Fiberglass

The Fundamentals

In the United Kingdom, fiberglass is more aptly named « glass reinforced plastic » (GRP), which is litteraly what it is: fiberglass strands petrified in plastic.

Individually, both fiberglass strands and resins have weaknesses. Fiberglass strands are highly resistant along the strand, but break easily accross the fiber. Alone, the plastic is brittle and breaks under load. It is the combined force of the fiberglass and the plastic that gives fiberglass its solid mechanical properties (Elchakany, Yang and Pham, 2023).

The plastic used in fiberglass comes from resin, a chemically stable and viscous liquid. When the resin is combined with a catalyst or an hardener, the liquid becomes unstable and a chemical reaction occurs (often generating heat). Those interested in resins from an organic chemistry standpoint can read Wikipedia or CompositeWorld for a good summary (Wikipedia 1-3, n.d., CompositeWorld, n.d.). For the rest of us, the key point to remember is that resin is converted in solid plastic by mixing it with either a catalyst or an harderner. At the beginning of the curing process, which can last from minutes to hours, the resin is still liquid. During that period, one can mold several shapes of fiberglass before it solidifies.

It is also worth mentionning the obvious: neither plastic nor (fiber)glass rots, decays or corrodes. It is a very stable product. This is what led John Casey to write that « Wooden boats regularly die early deaths of natural causes; fiberglass boats must be assassinated. » (Casey, 2022).

When considering its strength, its ability to be shaped and its durability, it is understandable that fiberglass is the foundation of hulls and so many boat parts.

Fiberglass Strands

Fiberglass strands are usually organized like a fabric. They come in three basic forms: woven, chopped strand and combined. Woven fiberglass is such that the strands are knitted with a known pattern – most often perpendicular, similar to a chessboard – to increase its strenght in the direction of the pattern.

In a chopped strand fabric, the fiberglass strands are chopped and peppered randomly in a layer of polystyrene. If truly random, it increases the strength in all directions. The combined fabric is the combination of both: one layer of chopped strand combined with one layer of woven. Woven fabrics can be made of larger strands, allowing for tougher layers of fiberglass, while the chopped strand fabric is made of smaller strands. The « size » of the strands are referenced by their weight per unit of surface area: 18 oz fabric, 10 oz fabric, 6 oz fabric, and so on. The higher, the stronger is the fabric… and the harder it is to shape in place.

That is the advantage of the chopped strand mat. Some resins dissolve polystyrene (the ester type described below), freeing the chopped mat strands during the application. It thus makes the strands much more pliable during the application and thus fits acute angles better. If one needs to fiberglass two perpendicular pieces of wood together, chopped strand mat would be a very well suited material. For structural work such as on the hull or on engine bearers, woven fiberglass is better suited.

I am not aware of commercial differences regarding fiberglass mats and strands. I think of it like spaghettis: there may be small differences through brands, but in general terms, the sauce is what differentiates one spaghetti recipe from another. For fiberglass, resin differentiates the final product much more than the fiberglass brands.

Resin Types

Resins can be classified in two categories: epoxy and the « ester » types such as polyester or vinylester. There are different chemicals within the « ester » category, but bunching them together is a good first approximation. The general qualitative properties of the two categories are summarized in the table below.

Ester types work with a catalyst, which is added in terms of drops (e.g. five drops to a pint), meaning that the catalyst has a negligible impact on volume. A minimum quantity of catalyst is required for the reaction to occur evenly in the resin. Adding more, up to a maximum, can alter the time it takes for the chemical reaction to occur. In plain terms, adding more catalyst speeds up the process, up to a certain point.

One can thus change the amount of catalyst to match some circumstances. For instance, temperature has an impact on the curing time (the hotter, the faster), so adding less catalyst during hot days can somewhat counteract the impact of the temperature. If a fiberglass job requires more time, adding the minimum amount of catalyst may give this needed time.

Epoxy resins work with an hardener, which must be added in proportions that substantially changes the volume (e.g. 3 parts of resin for 1 part of hardener). Thus, calculating the final required volumes must account for the hardener volume. The exact proportions will vary with brands.

The strength of each type of resins is pretty well documented. A reccuring statistic found on most product description is that epoxy resin is (roughly) 20% stronger than polyester resin, which is the main type of ester (BoatUS, 2012; SPSystems, n.d.; Mantavoni et al., 2017; Wikipedia – 3, n.d.; El Wazery et al, 2017). This is accurate for both tensile and compression strength.

Along pretty much any dimension except the price, epoxy resins are superior. Perhaps the sole exception is when it comes to gelcoat, which is nothing but polyester resin with pigments. Because gelcoat is polyester in disguise, it does not adhere well to epoxy repairs. Interprotec, or other types of finishes, would be better suited, but if the use of gelcoat is an absolute must, then one binds itself to the ester family for better … or worse. It should however be noted that epoxy is water resistant and can be used as a « last coat » prior to painting.

Peanut Butter, Mayonnaise and Ketchup

Small jobs and repairs do not lend themselves to the use of fiberglass mats. A good example would be « fiberglassing » within drilled holes to avoid exposing the wood core. In no way a fiberglass fabric can enter a drilled hole, despite the need to protect the core from the elements. This is where « thickened epoxy » comes in play. For small repairs, it is the standard fiberglassing technique.

Thickened epoxy is epoxy resin with silica in powder form blended in the mix. The strands of silica are so small that they float in the air (and a mask should be worn when it is manipulated). The density of silica varies with the need: the more silica, the more the resin will behave like a solid. So when gravity is your ally, or when you must inject epoxy in small crevisses, less silica helps as the liquid form will fill it all. In any other case, more silica will turn the resin in a putty, which will help shaping it appropriately.

There is no doubt that the established nomenclature to describe densities is american. Densities, or consistenties, are refered as « peanut butter » (large amount of silica), « mayonnaise » (medium amount of silica) and « ketchup » (small amount of silica). The nomenclature conveys known (american) textures. For beginners, BoatWorks Today quantifies these textures in volume (table below), but they recognize that this is mostly to help get a sense of the textures rather than an hard rule.

Ester Family?

The three main types of resin in the ester family are polyester resin, vinylester resin and phenolester resin. The first resin is the main type, being used in boat construction, « puttys » and gelcoat. It is cheap and widely availlable. It is however less water resistant and, if used underwater, can cause blistering. Vinylester is chemically designed to be more water resistant and is slightly more robust than polyester. Phenolester is mainly used because of its fire retardant properties. Most often, phenol- and vinyl- esters must be ordered. In all likelihood, off the shelf ester resin is polyester.

Practical Knowledge and Techniques

The videos by BoatWorks Today are a good starting point for acquiring a practical sense of how fiberglass works. If you have time on hand, exploring the channel is very helpful. The channel has an experimental mindset and the lead person is a very good teacher. The video « How to Fiberglass Over Plywood » influenced me on how to think about fixing the engine bearers to the hull. In particular, the technique to smooth the angles, so as to ease the fiberglass into the shape, is a crucial technique to understand. Pre-heating the resin, as explained in the first video, is also of practical importance.

Want to Know More?

Have a look at the « boatwork » section of this website for more practical tutorials and techniques.

References

BoatUS (2012). Polyester or Epoxy resin?, retrieved online in October 2023 at this url.

BoatWorks Today (2022). Epoxy for Beginners, YouTube, retrieved online in October 2023 at this url.

Casey, D. (2005). Complete Illustrated Sailboat Maintenance Manual, McGraw Hill, 892 pages, ISBN 0-07-146284-8.

CompositeWorld (n.d.). Polyester resins: The Basics, retrieved online in October 2023 at this url.

Elchakany, M., Yang, B. and T. Pham (2023). Mechanical Properties of fiber reinforced polymer (FRP) and steel bars, Chapter 2 of Geopolymer Concrete Structures with Steel and FRP Reinforcements: Analysis and Design, Woodhead Publishing, Pp 75-135, ISBN 9780443188763.

El-Wazery, M.S., El-Elamy, M.I., Zoalfakar, S.H. (2017). Mechanical Properties of Glass Fiber Reinforced Polyester Composites, International Journal of Applied Science and Engineering, 14(3), pp. 121-131.

Fiberglass wharehouse (2022). Polyester resin vs Epoxy resin, retrieved online in October 2023 at this url.

Mantovani, D.P., Rohen, L.A., Neves, A.C.C., Vieira, J.S., Pontes, L.A.P., Viera, C.M.F, Margem, F.M. and S.M. Monteiro (2017). Comparative Analysis of the Tensile Properties of Polyester to Epoxy Matrixes Composites Reinforced with Hemp Fibers, International Workshop Advances in Cleaner Production Proceedings, retrived online in October 2023 at this url.

SP Systems (n.d.). The Advantages of Epoxy Resin versus Polyester in Marine Composite Structures, retrieved online in October 2023 at this url.

Wikipedia (n.d.). Polyester resins, retrieved online in October 2023 at this url.

________ (n.d.). Epoxy, retrieved online in October 2023 at this url.

________ (n.d.). Fiberglass, retrieved online in October 2023 at this url.