Epoxy vs PU - Understand Surfboard Construction

Well now, this has also been a long time coming, the reason being that there are so many different people/shapers/pros who have a different point of view, a different bias and perspective on this subject. My quest is to take this unruly bull by the horns and discuss the facts of the matter and present you with a concise and honest way to judge which construction type is for you.

(spoiler alert) I have my prejudices and I found myself unmoved by the facts. I would still purchase the same sort of board even though I now know the facts, and my opinion is now an educated one.

I realise that this might not be the most interesting subject, but it is something that has always fascinated me and not because I am really into the 'nuts and bolts' of the materials and how they perform, but more because I never found two people who agreed about the whys of this strange subject.

To understand where the choice came from we have to go back, wayyyyy back, to a man called Bob Simmons who died in 1954 in a surfing accident in San Deigo at age 35. He is considered by most to be the inventor of the 'planing hull' surfboards among many other things surf. That, however is not why we are interested in him, Bob was also one of the first shapers to experiment with Styrofoam and the 'sandwich' construction that we take for standard in modern times. Some chaps called Joe Quigg and Pat Paterson who took it further, discovered that the lighter and far more manoeuvrable sandwich construction was an immediate success. Today the core (foam) of a modern sandwich construction surfboard would be roughly half the weight of the overall board. This is a kind of 'rule of thumb' measurement. For the same size board, a balsa core would be roughly six times the weight, to give you an idea. It's really no wonder then with the practicalities involved in surfing that the sandwich was an overnight success and has remained the majority shareholder in surfboard construction to this day.

Note: before we get started this is just a small note to admit that while I talk about different properties of these materials, I am being generic. It is possible to get sucked into a vortex of chemical formulae with different foam densities and variation in modulus but am not sure anyone really want to get molecular at this point. This is not a simple subject and I am trying to simplify it. If you would like me to go 'mega-geek' and in depth in another article, then leave me a comment below.

Cool, right, on with the story.

Understanding Cores in General

As I have alluded to above, PU (polyurethane), PS  and EPS (Polystyrene and Expanded Polystyrene) come in different densities from the 'blank' manufacturers. It is your choice what you order as a shaper. Greater density would generally mean more strength and rigidity and less flex (in the core). So to put it another way, the stiffer the core, the stronger the overall sandwich would be. We call the breaking point of the board (the whole sandwich) the 'face wrinkling load capacity'.

Stiffness & Cores - Are you still with me? Okay, so let's simplify this, doubling core stiffness would lead to double the overall 'wrinkling load capacity', but would give you the same overall flex of the sandwich. This is because the stiffness of the overall sandwich has little to do with the actual core. The skin (Poly or Epoxy) of the of the sandwich is much stiffer than the core and will negate any extra of less flex in the core.

To sum up :

A Stiffer Core = Bigger Wrinkling Load Capacity = Equal Overall Sandwich Flex

Less Stiff Core = Smaller Wrinkling Load Capacity = Equal Overall Sandwich Flex

Density & Cores - As you increase the density of the core it's stiffness is increased, and if you look at the above that means greater strength (Wrinkling Load Capacity) but no actual effect to the overall flex or stiffness of the sandwich as a whole. What it does effect, however is weight. The denser a board, the heavier it is, that's the trade off.

Denser Core = Stiffer = Bigger Wrinkling Load Capacity = Same Overall Flex of the Sandwich = Heavier

I am aware that some of you find more relevance in numbers and would rather understand the figures, so for every 7kg/m3 that you increase the core density, you can expect roughly a 4% increase in core weight (only the core weight- remember that core weight is typically accountable for half the overall construction weight).

Understanding Skin Materials

Fibre Thickness - Glass fibre comes in two thicknesses and you can choose a combination of these on top or bottom to attain the desired result. 6oz and 4oz cloth are the standards and here are some general rules for choosing your thickness. 6oz is stronger and stiffer but heavier than 4oz.

6oz Fibre = 1.5 times stronger = 1.5 times stiffer = 1.2 times heavier than 4oz Fibre

So, again we see the trade off, to get a stronger overall sandwich we have to sacrifice overall flex and weight. 6 oz Fibre gives you 50% greater strength, makes the overall flex 50% less more but will increase the weight by 20%.

Right, now that you understand the generics of what makes up a surfboard and how the different parts contribute to the end result as a whole, we can move on and compare some different materials used in construction as promised by the title of this article.

Different types of Glass

E Glass and S Glass are the industry standards, both have marginally different properties, S Glass is stiffer and stronger and E Glass has more flex:

E Glass = 10% more flex than S Glass

S Glass = Strength (Wrinkling Load Capacity) 10% increase but 10% stiffer (again the trade off)

Carbon is becoming more popular in the construction of surfboards, and improves the strength (wrinkling load capacity) by around 100%, with minimal weight compromise of around 10% plus to the sandwich. The inevitable trade off comes with flexibility, a carbon board will be around 3 times as stiff as the same board made with S Glass.

Polyester vs Epoxy Skin Plastics (Resin)

Let's take a moment to step back and suck in all that we have learnt thus far. If it is your first time reading and learning all this then great. It's awesome to know this stuff if you are looking to compare boards and understand not just the different shapes of a board but also these construction variables. If you know this already then I hope to have got most of it correct, please pull me up on any mistakes in the comments, my aim being to spread understanding and not confusion.

So Poly vs Epoxy, what an opinion most people have about this! Everyone has a view, or thinks they do, but when asked to explain it most make little or no sense. Here are some quick (and maybe surprising facts).

- Comparing Epoxy to Polyester as to the effect on the end result of the sandwich.

Strength (wrinkling load capacity) - Equal

Flex or Stiffness - Equal

Weight - Equal

There are a few differences however. Epoxy resin is stiffer than Polyester, but as the glass fibre (E Glass or S Glass) is much more important to flex this actually has a negligible effect on the overall flex of the sandwich. Epoxy has far greater adhesive properties than Polyester which makes delamination less likely and also is rumoured to have a slightly greater impact resistance, making it less likely to crack if 'bumped'.

As a side note (not really tackling eco in this article) the effect on a human handling epoxy is about 10 times less toxic than that of handling polyester resin.The scale of this should be kept in mind though as the process for making the core or 'blank' is around 10, 000 times more toxic than the resins and the cost of transporting aboard from one continent to another eclipses all of these. If you want to know more on this, check out a previous article on Eco Surfboards - Dispelling Myths. 

Getting to the Point

I realise that I have not really touched on the different types of core, the Polyester vs EPS vs what ever new stuff there is out there. Lib Tech's super secret recycled foam or the interesting looking Varial stringerless blanks to mention a couple. I have decided that this has enough juice in it to be another article entirely and I thought, hey, what if no one is interested, this is all fairly geeky stuff after all . So let me know if you found this interesting or informative and I will again don the 'nerd's hat' and get stuck into the inherent differences between the cores out there today.

As I have said, before I embarked on this journey to discover the facts about the different layers to construction and how they effect the overall surfboard, I had a bias as to which sort of construction I prefer. I have not been moved from this, even after possibly realizing that there are other options I could and should try. At the end of the day as a surfer it just comes down to what you feel like you prefer. We are all different and surf different waves.

As and Aussie friend once said to me, " James, it's just the same as if you prefer Sheela's that are redheads, blondes  or brunettes, what you like is what you like, simple as." and while I find his analogy a little sexist, I kind of see his point.

Hit us up on social media or email if you want to hear more geek stuff, otherwise it's back to the Kelly Slater Wavepool and what colour Alana's pants are.