Closing the Gap, How Your Kayak is Made- Resins

The feedback from the last installment was incredible. Who knew that Gelcoat was such an exciting subject!? Lots of great questions and comments came out on social media and I hope it keeps going. The whole big picture idea is to share what goes into a composite kayak and how it affects the final weight. There is more than just putting together the components that must be considered. Quality, cost and performance are the main factors to consider when designing the layup. Since Gelcoat was so exciting, lets dial it back a bit and delve into the scintillating topic of Resin.

Resin, you probably remember it as the stuff that will clear a room. Smelly, sticky and it always finds its way to the toe of my favorite shoes. Most of my boatbuilding life has been spent slinging epoxy. Polyester and Vinylester became second nature once I took on building fiberglass and carbon kayaks. There has been massive technology advancements on the resin front, mostly in the Vinylester and Epoxy realm. The main difference between Polyester, Vinylester and Epoxy is the Cure Mechanism. PE and VE use a catalyst, Methyl Ethyl Ketone Peroxide, or MEKP. Another nasty component to go along with the Styrene. Epoxy uses a hardener and cures at a much slower rate that is "tunable" by choice of hardener cure rate and based off of ambient temperature.

Not all resin is created equal. Take Polyester resin for an example. You can buy it at your local big box hardware, or auto parts store. Would you build a boat with it? It would make a perfectly functional boat for sure. Would I take it out into dicy conditions, maybe not. The two "esters" have a shelf life that greatly affects the quality of the layup. About 90 days is the mark at where they should not be used. After that, the performance starts to degrade and the components separate. The resin will cure, but it will be a "soft set" and never harden to specifications. I've seen this on boats coming through the shop. Set one on a sawhorse and it will permanently distort. The average retail outlet that sells Polyester Resin doesn't have the inventory control in place to keep track of shelf life and it is buyer beware. Composite suppliers have a date coding system in place that, if you ask, they will give you the system to track your own inventory. Sure, I could get a better deal buying in larger quantities, but quality would suffer.

I am a data geek. Give me a Technical Data Sheet for a few products and I am headed down the proverbial worm hole. There is no one perfect material, but the data sheets can reveal the best for your particular intended use. So for the remainder of this article, I am going to use three infusion resins to compare the difference on a variety of standard tests. All three are thermoset resins with similar flow characteristics. Remember, unsupported resin is weak and brittle. When it is reinforced with a cloth such as fiberglass, that is when it gains true strength. For the sake of the comparison, all of the data will be in an unsupported form and fully cured.

Barcol Hardness- The rating of how "hard" a material is when stricken with the pointy end of a tool called an Impressor. The depth of the impression is rated on a scale of 1-100. The higher the number, the harder the material is. So, when you've sat in your kayak and it's perched on a rock, you'll want the higher rating.

Tensile Strength- It is the amount of pressure the material can withstand before failure. The value (in PSI) when that the rock cracks the hull of your prized boat.

Tensile Elongation- This rating tests the materials ability to stretch and is performed to ultimate failure. Think of the last article about Gelcoat. It doesn't have any elongation rating due to its brittle nature.

Flexural Strength- This is the test that determines the ability to resist loads, or how much it can support under tension. So, when you are climbing on the back deck after an unplanned wet exit, this is a test of flexural strength.

Bond Strength- The test that determines how well resin bonds to a substrate. This one is really important in a layup. If the bond is less than the reinforcement can support, (i.e. Carbon Fiber) you've waisted you money on a very expensive material because it is only as strong as the bond strength.

Now that I have bored you to tears, here is how they stack up. Lets start with Polyester Resin.
Polyester resin has been around for a long time. It was first developed in 1847 and was granted a US Patent in 1933. The current form of Unsaturated Thermoset Resin surfaced in 1946 out of the Aircraft Industry. Since then, it has been used extensively in the Marine Industry. It just works, and has been used in kayak construction since the 50's. The plus side of Polyester Resin is cost. It is the least expensive of the three and therefore the most popular among the manufacturers. The down side, it comes up short in every category. It's brittleness makes it prone to cracking and it is also permeable, susceptible to water intrusion over time. The bond strength is poor in comparison, so using it with carbon fiber is a complete waste of an expensive material. So here is how it stacks up.

Barcol Hardness Rating- 40
Tensile Strength- 6800psi
Tensile Elongation- 0 ( I had to look at that twice!)
Flexural Strength- 10,000 psi
Bond Strength- >500psi

So, what does this tell us? It is reasonably hard and has good strength, well within the requirements of a kayak. It also chemically bonds to gelcoat by intertwining on a molecular level. Where it fails is in Elongation and Bond Strength. Poly needs more surface area to achieve a good bond. Fabrics like chopped strand mat  (CSM) or 1708 biax (with CSM face) are a good match for Polyester. It becomes the weakest link when coupled with more specialized fabrics such as tight weaves, Aramids (Kevlar) and Carbon Fiber.

Now lets move to Vinylester Resin. This resin is a hybrid of Polyester and Epoxy. It is NOT Epoxy! I've seen way too much misleading info on the interwebs with manufacturers calling it Epoxy/Vinylester resin. While it has many attributes of Epoxy, it will come up on the losing end in a  head to head comparison. Vinylester appeared in composites in the mid to late 60's. It addressed many shortcomings of PE Resin by being more resistant to chemicals and water intrusion. In the mid range as far as price, a good VE Resin will be far superior, for not much more money. Let's look at the data.

Barcol Hardness Rating- 40
Tensile Strength- 12,000 psi
Tensile Elongation- 5.5%
Flexural Strength- 22,000 psi
Bond Strength- 600 psi

As you can see, VE Resin is a strong performer that nearly doubles all values. For a modest increase in price you get a lot more for your money. There is one caveat that will become more apparent with the next set of data. Bond Strength is better, but still falls short when it comes to its ability to stick to a fabric. So, Carbon and VE is ok, the weakest part of the equation is the bond. Now, in a fiberglass layup, it is a rock star! The higher quality fabrics pair well with VE resin and it is superior in every way when compared to PE/Fiberglass. Remember the video we did last summer? We ran over a deck, that was infused with VE Resin, with a 4,000lb SUV. The only damage was a small gelcoat crack!

Epoxy is the Holy Grail of the resin world. It is also the most expensive. At three times the cost of PE Resin, it has not gained a lot of love in the kayak world. It cuts into the bottom line in a business where the retail price has not moved to match the rise in material cost. That's an altogether different discussion.... Epoxy appeared on the scene in the 30's, commercially in 1946. It has been slow to infiltrate the marine industry, but is now a staple at high end yacht manufacturers. The aerospace and automotive industries continue to advance modern composites at a rapid rate. What affect does this have on the paddle sports industry? Carbon Fiber and hybrid cloths coupled with Epoxy greatly reduce weight while increasing strength (when used properly). Epoxy does have a significant weakness though. UV exposure will degrade it. It needs to be protected with a UV resistant coating. Standard Gelcoat does not bond to it, so it needs a specialized Gelcoat system designed for it. The same goes for clear coats.

Barcol Hardness Rating- 78
Tensile Strength- 10,000 psi
Tensile Elongation- 6-8%
Flexural Strength- 16,300 psi
Bond Strength- 2,000 psi

The standout value of the Epoxy Resin is the bond strength. It exploits the qualities of Carbon Fiber while being more tolerant of flex. It also excels as being the hardest of the three resins. With the added cost of Epoxy, it is overkill in a fiberglass layup, but shines when coupled with a modern Aramid or Carbon Fiber. Since it has limitations with respect to compatibility to gelcoats, it requires a careful manufacturing process. 

As you can see, this is a complex variable when it comes to kayak construction. There is a delicate balance of positives and negatives that must be weighed. This is just a small part of the puzzle that need to be pieced together to get the most value for your money. One small change can have a major impact on durability. The old adage of "more is better" does not apply in the kayak world. Adding more of one component to make up for a compromise in another will have an impact with every stroke of your paddle.


Post a Comment