CEO, Hydroflex Technology
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The Inertia

Here is my challenge to you: learn more about your surfboard and boards in general. Being in the board building industry, I see how little surfers know about their most important piece of equipment. This blows my mind but at the same time isn’t surprising. I myself knew nothing about surfboards for the first five to six years that I surfed, so I know how easy it is to overlook when your attention is mainly focused on the amount of time spent in the water.

You may ask, why listen to me? I run a company that is pushing surfboard construction and whose goal is to build the best performing surfboards on the market. With an engineering background, I can understand how materials act and how they will affect the board you ride. I am also fortunate enough to work with some of the world’s best surfboard shapers who are pushing the design of boards to new heights.

Epoxy vs. Polyester

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Being in the surfboard industry, we are always surprised to find out how little most consumers know about the boards they are buying. Of course, when we were little groms and only cared about how much time we spent in the water, we didn’t know one board from the next either. Our goal is to educate you on surfboard construction and materials. Let me explain how the choice of materials and construction will change the way your board will ride as well as the durability of the board.

If you didn’t know, there are two main types of resin used in surfboard manufacturing, but what really sets epoxy and polyester apart? What does it mean for your board to be an “epoxy” board? Do you ever wonder how these two compare in terms of tensile strength, modulus, elongation or any other number of factors? More likely, you’re thinking, “which one is better?” We at Hydroflex want you to make an informed decision when it comes time to order your next stick.

When most surfers hear the word “epoxy,” what I believe comes to mind is an overseas pop-out board that you see at popular tourist spots. And that is partly true. Many of these boards are made using epoxy, but their riding characteristics don’t necessarily come from the epoxy resin used. This comes from their construction. But recently, many of the top pros have been using epoxy surfboards–Kelly Slater for one.

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Before we get all scientific on you, let me explain it in basic terms. First, epoxy resin is a superior resin compared to the standard polyester resin that the majority of standard short boards are built with. It’s stronger, lighter, can flex more before breaking and won’t break down over time like polyester does.

All sounds great, right? Well, why don’t all surfboard makers use it then? The answer is pretty simple: money. It’s more expensive and harder to work with than the cheaper polyester resin that more shortboards are made of. Epoxy is two to three times more expensive than polyester resin. The surfboard market is already a very tough market to make any money in so most surfboard builders use the cheapest materials, including fiberglass and resin. Obviously, that makes it so they can sell their products at a competitive price. Spending two to three times more money to make a better board is not something the market is willing to support at this time, so board builders are not doing it.

Now that we have a base understanding of the two materials used for board construction, let’s get technical. Here is a breakdown of important board building terms that, if you can master, will make your board building/buying experience a much simpler process.

Adhesive Properties. This is probably the most important factor when it comes to delamination and fiber bonding, which affects overall strength. How strong does the resin bond with the materials it comes into contact with? Epoxy resin has substantially better adhesion than polyester resin because of the existence of polar hydroxyl and specific ether groups. Laymen’s terms: epoxy resin allows for a stronger, lighter board with less delamination.

Tensile Strength. Likely the most quoted statistic for resin systems. Tensile strength is the amount of lateral force a material can withstand before failing (breaking). Epoxy resin has significantly higher tensile strength–up to 8.5MPa. Laymen’s terms: Epoxy makes boards stronger overall and are specifically less likely to buckle or snap.

Tensile Elongation. Your boards stretch and compress all the time. The amount that a material is able to stretch without failing is called tensile elongation. At 3.5-4.5% of overall length, epoxy resin is able to stretch more than polyester resin which can only stretch about 2% before it fails. Laymen’s terms: With epoxy boards, you get less delamination and buckling.

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Tensile Modulus. How much a material deforms when pushed on by, say, your heel or fingers. Epoxy resin stands up to impact force much better than polyester. Laymen’s terms: Epoxy boards sustain less pressure dings and other issues like tail rotation.

Micro-Cracking. Those little spider dings and cracks in your old board are actually tiny failures in the resin matrix. Epoxy resin has a greater bond at a molecular level, which reduces transverse micro-cracking. Laymen’s terms: epoxy resin is less susceptible to rail cracks, spider dings and other small failures.

Curing Shrinkage. Resin molecules reconfigure during curing and some mass is lost as a result of this process. Curing shrinkage causes structural weakness at a molecular level and can contribute to lower tensile strength and increased micro-cracking. Epoxy resin experiences 75% less curing shrinkage. Laymen’s terms: Epoxy boards are stronger and less susceptible to blemishes due to shrinkage.

Osmosis. All resins allow a small amount of water to pass through. This can cause a dramatic reduction in board strength and can produce small bubbles in the deck. Epoxy resin retains around 90% of its strength after heavy use, which is much more than polyester’s 65% post ingress strength. Laymen’s terms: Epoxy allows for stronger boards that don’t mind being put in the water a few hundred times a year.

Adhesive Properties. The adhesive properties of a resin system are important in achieving the full mechanical properties of a composite. The adhesion of the resin matrix to the fiber reinforcement or to a core material in a sandwich construction is important. Epoxy systems offer the best performance, and are frequently found in many high-strength adhesives. This is due to their chemical composition and the presence of polar hydroxyl and ether groups. The adhesive properties of epoxy are especially useful in the construction of honeycomb-cored laminates where the small bonding surface area means that maximum adhesion is required.

Mechanical Properties

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After a cure period of seven days at room temperature, a typical epoxy will have higher tensile and modulus properties than a typical polyester. The beneficial effect of a post-cure at 80°C for five hours can also be seen.

Micro-Cracking. The strength of a laminate is usually thought of in terms of how much load it can withstand before it suffers complete failure. This ultimate or breaking strength is the point at which the resin exhibits catastrophic breakdown and the fiber reinforcements break. However, before this ultimate strength is achieved, the laminate will reach a stress level where the resin will begin to crack away from those fiber reinforcements not aligned with the applied load, and these cracks will spread through the resin. This is known as transverse micro-cracking and, although the laminate has not completely failed at this point, the breakdown process has commenced.

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For brittle resin systems, such as most polyesters, this point occurs a long way before laminate failure. Recent tests have shown that for a polyester/glass-woven roving laminate, micro-cracking typically occurs at about 0.2% strain. This equates to a usable strength of only 10% of the ultimate strength. In an environment such as water or moist air, the micro-cracked laminate will absorb considerably more water than an un-cracked laminate. This will then lead to an increase in weight, moisture attack on the resin and fiber-sizing agents, loss of stiffness and, with time, an eventual drop in ultimate properties. Increased resin/fiber adhesion is generally derived from both the resin’s chemistry and its compatibility with the chemical surface treatments applied to fibers. Here, the well-known adhesive properties of epoxy help laminates achieve higher micro-cracking strains.

hydroflex3Fatigue Resistance. Generally, composites show excellent fatigue resistance when compared to most metals. However, since fatigue failure tends to result from the gradual accumulation of small amounts of damage, the fatigue behavior of any composite will be influenced by the toughness of the resin, its resistance to micro-cracking, and the quantity of voids and other defects, which occur during manufacture. As a result, epoxy-based laminates tend to show very good fatigue resistance when compared with polyester–this being one of the main reasons for their use in aircraft structures.

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Shrinkage. Also of importance to the composite designer and builder is the amount of shrinkage that occurs in a resin during and following its cure period. Shrinkage is due to the resin molecules re-arranging and re-orienting themselves in the liquid and semi-gelled phase. Polyester esters require considerable molecular rearrangement to reach their cured state and can show shrinkage of up to 8%. The different nature of the epoxy reaction, however, leads to very little rearrangement and with no volatile by-products being evolved, typical shrinkage of an epoxy is reduced to around 2%. The absence of shrinkage is, in part, responsible for the improved mechanical properties of epoxies over polyester as shrinkage is associated with built-in stresses that can weaken the material. Furthermore, shrinkage through the thickness of a laminate leads to “print-through” of the pattern of the reinforcing fibers, a cosmetic defect that is difficult and expensive to eliminate. 

Degradation from Water Ingress. All resins will absorb some moisture, thus adding to a laminate’s weight. What is more significant is how the absorbed water affects the resin and resin/fiber bond in a laminate, leading to a gradual and long-term loss in mechanical properties. Both polyester and vinyl ester resins are prone to water degradation due to the presence of hydrolysable ester groups in their molecular structures. As a result, a thin polyester laminate can be expected to retain only 65% of its inter-laminar sheer strength after immersion in water for a period of one year, whereas an epoxy laminate immersed for the same period will retain around 90%.

hydroflex4Osmosis. All laminates in a marine environment will permit very low quantities of water to pass through them in vapor form. As this water passes through, it reacts with any hydrolysable components inside the laminate to form tiny cells of concentrated solution. Under the osmotic cycle, more water is then drawn through the semi-permeable membrane of the laminate to attempt to dilute this solution. This water increases the fluid pressure in the cell to as much as 700psi. Eventually, the pressure distorts or bursts the laminate or gel coat, and can lead to a characteristic chicken-pox-like surface. Hydrolysable components in a laminate can include dirt and debris that have become trapped during fabrication but can also include the ester linkages in cured polyester. A polymer chain having an epoxy backbone is substantially better than many other resin systems at resisting the effects of water. Such systems have been shown to confer excellent chemical and water resistance, low water transmission rate and very good mechanical properties to the polymer.

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