07.31.08
Sticky Stuff
I’m nearing the end of the clean-up phase of the fiberglass pieces and I’m approaching the point at which I’m going to need to start the fiberglass repair work. As such, I thought I’d post some information on the techniques that I’m going to use to do the fiberglass repairs.
As I’ve mentioned before, the method that I’m going to describe is known as “Moldless Composite Construction” — a technique developed by the German sailplane industry but popularized in the U.S. by Burt Rutan. The word “Composite” refers to any structure in which the end result is better than the sum of its parts. By this definition, a peanutbutter and jelly sandwich is a composite structure.
Expanding the idea a bit, composite structures are usually composed of a fabric, an adhesive and perhaps one or more types of core materials. The fabrics are usually fiberglass, carbon fiber or an aramid fiber such as Kevlar. Combinations of these fibers can also be used. The Bradley uses chopped strands of fiberglass for their “fabric.”
The adhesive is usually a polyester resin system, a vinylester resin system or an epoxy system. The Bradley kit car used a polyester resin system. Resin and epoxy systems must not be mixed before they cure. However, once cured, you can use any of the systems to adhere new parts or repair old ones. I’ll be using an epoxy system to repair the damage to my car over the Bradley’s polyester system.
The core materials can be any number of types of foams (urethane, PVC, polystyrene, or proprietary foams such as Clark Foam or Last-A-Foam), or they can be cork, balsa wood, plywood, or corregated cardboards or metals. Most of the Bradley was constructed without core materials. However, plywood is used as a core material in certain portions of the body such as the forward and aft longitudinal ribs.
The main thing to remember about a composite is that we build our structures so as to take advantage of the materials’ beneficial properties while down-playing their detrimental ones. For example, adhesives have both good properties and bad ones. On the positive side, they’re strong and hard when cured, and they flow easily so that we can wet out our fabrics efficiently. On the negative side, they can be brittle and heavy. Foams are lightweight, which is good, but they are weak, which isn’t. Fiberglass is strong in tension, but less so in torsion and shear and it is weak in compression. So, we need to design our structures so that the fiberglass is in tension as much as possible. We need to use the foams as lightweight spacers between the layers (plies) of fiberglass so that the fiberglass takes the loads and the foams do not. We need to use the adhesives to bond the fabrics to the foams so that there is minimal tendency for the fabrics to delaminate from the foam structures and weaken them. That’s a lot to ask from these materials but, by being clever in how how use them, we can maximize the properties of our composite structures in the areas where we need them to perform.
Today, we’ll start our discussion by looking at the adhesives. Adhesives come in three families — vinylester systems, polyester systems and true epoxies. That order is important — its an order of increasng cost and an order of decreasing weight. For an electric car, low weight is good. There is a natural tendency to think of any adhesive that is composed of two parts (which you mix together to activate) as an epoxy, but this is not true. Vinylester and polyester resin systems are catylized whereas true epoxies are mixed. What’s the difference?
Have you ever worked with Bondo? Bondo is a non-structural cosmetic filler that is a part of the polyester resin family. It is non-structural — meaning that it is not intended to take loads — it is intended to fill dips and depressions to produce a pleasing cosmetic appearance. Because it is part of the polyester resin family, it needs to be catylized to be activated. When you mix Bondo, you add a very small amount of a red cream “hardener” to activate the material. The use of the word “hardener” here is unfortunate because, from a chemist’s standpoint, it is not a hardener — it is a catylist called MEKP (methyl ethyl ketone peroxide). In the strict sense of the term, hardeners are components of true epoxy systems — not of catylized resin systems.
In resin systems, all of the chemicals needed to harden the mixture already exist in the mixture. In Bondo, the grey stuff that comes in the can has all of the chemicals that are needed to produce a hard sbstance — but, they need a kick to get the reaction started. That’s what the catylist does. A catylist is a material that promotes a chemical reaction but does not enter into the reaction itself. Technically, a catylist lowers the energy barrier that keeps the chemicals from reacting. When you mix in the catylist, the energy barrier that keeps the chemicals from reacting is lowered by the catylist and the chemicals react, forming a hard substance. The amount of catylist required to kick off the reaction is not critical — you need to use enough to do the job but if you add too much, it’s just wasted. That’s why the mix ratio for Bondo (around 1/2% to 3%) isn’t that critical.
Now, contrast that to a true epoxy system. True epoxies contain a resin and a hardener. Here, the use of the word “hardener” is appropriate. Both components are required to create the chemical reaction, so there is no need for a catylist to promote the reaction. The mix ratios for true epoxies are fairly critical. If you have too much resin or too much hardener, they go unreacted and weaken the physical properties of the hardened material.
The epoxy that I like to use is called MGS 335 and you can learn more about it here…
http://www.aircraftspruce.com/catalog/cmpages/mgsresin.php
I use the L335 resin with the H340S slow hardener and mix the material with a 100:45 resin:hardener ratio by volume. This gives me a pot life of over 2 hours before the epoxy starts to gel. The long pot life means that I can only do one lay-up per day on any one given piece. I have to let the material cure overnight in warm weather and for two days if the weather is cold, but that’s a condition that I’m willing to endure. The long pot life means that I can take my time doing each lay-up and do a good job. I don’t feel rushed as I would if my epoxy had a 30-minute pot life.
Now, how do you mix the stuff to get the exact ratio that you need? I use a Sticky Stuff Dispenser by Michael Engineering:
http://www.michaelengineering.com/sticky_stuff/SSD.html
You need to purchase the pump for the particular epoxy system that you plan on using, so that it will mix the materials in the proper ratio. If you change materials, you may need to have the pump modified to accomodate a different mixing ratio, but Michael Engineering will sell you the proper parts so that you can do this yourself. With the pump, you simply put the hardener in the smaller container, the resin in the larger one and then pump the handle. The materials are delivered into your mixing cup via the two aluminum spouts on the pump.
Use unwaxed paper cups to mix the epoxy — the wax from waxed cups will contaminate and weaken the epoxy. Popsicle sticks or tongue depressors work great as mixing sticks. Mix the material for 2 minutes, scraping the sides of the cup every 15 seconds — that’s the standard Rutan protocol. After your lay-up, wipe any excess epoxy off the mixing stick with a paper towel and let any excess epoxy harden in the cup. You can re-use the cup for the next lay-up after the epoxy hardens. Used cups and sticks actually work better than new ones because they tend to soak up less of the hardener (which has a lower viscosity than the resin.
In the photo, you’ll notice that I keep my pump — and a spare gallon of epoxy — and some 5-minute epoxy — and some 30-minute epoxy — we’ll talk about those another time — in a wooden box. We call these things epoxy incubators. There’s a 60-watt lightbulb in the box which is connected to a dimmer. There’s also a thermometer in the box. By futzing with the dimmer, I can heat the box to keep the epoxy warm. I keep mine at about 95F year round, which means that I need to crank up the dimmer in the winter. By keeping the epoxy warm, you lower the viscosity of the two components. This makes it easier to pump and mix the material and it seems to wet out the fiberglass a bit better.
One problem that many of us have had with the Sticky Stuff Dispenser is that some chemical in the hardener embrittles the plastic Tupperware container that holds it. You go down to the shop one morning to work on a lay-up and discover that your pump is leaking hardener into the bottom of your incubator. This results in a lot of colorful language, an abnormally large consumption of paper towels and wasted hardener. As a result, some of us have started to replace the hardener container on these pumps with 1-quart uncoated paint cans from the local big-box hardware store. The down side is that you can’t tell when your hardener container is nearing “empty.” I draw marks on the resin container to give me hints as to when I might need to replenish the hardener.
Another problem with the Sticky-Stuff Dispenser is that the Tupperware containers are apparently porous to carbon dioxide gas. CO2 filters in thru the container and reacts with the hardener, forming a crystalline sludge at the bottom of the container. This only happens if you let the hardener sit unused in the pump for months on end. If you’re a fairly regular user of the epoxy, you don’t need to worry about this. However, if it happens to you, you must throw out the hardener and clean your pump because the hardener is defective at that point. Using a meal paint can to replace the Tupperware container helps with this problem a lot. Some builders, including yours truly, keep an argon blanket over the hardener and this eliminates the problem.
If you don’t feel like ponying up $300 bucks for an epoxy pump, you can mix the epoxy by weight using a postal gram scale. Figure out how much epoxy you want (say, for example, 3 ounces) and then weight out 100 parts of the resin in your mixing cup. Re-zero the postage scale and then add in 38 parts of hardener and mix the material. Note that you have to use different mixing ratios if you’re mixing by weight instead of by volume. Builders who use this technique claim that it’s just as easy to use as a pump once they get into the rhythm of things. Most builders make up a chart that shows how many grams of resin and hardener are required to mix up a batch of a certain size.
By the way, it seems appropriate to mention that, when working with epoxy, get used to the idea of mixing many small batches instead of a few large ones. This extends your working time. Large batches can generate their own internal heat from the curing reaction. The heat accelerates the curing rate and, in extreme cases, can result in something called an exotherm where the epoxy flash-cures. In some cases, exotherms have been known to cause a fire.
The MGS epoxy requires a shop temperature of at least 70F. Anything lower than that and it won’t cure properly. It may look cured, but it won’t have developed the advertised strength. There are ways that you can overcome this by post-curing, but that’s an advanced technique for another day’s discussion. For now, just keep your shop over 70F.
This brings us to the last topic of the day: Safety. The hardeners in true epoxy systems contain a chemical which is an allergic sensitizer. Some people can work up to their elbows in this stuff and never have an allergic reaction. Others only need to walk into a room where the epoxy is curing and they start itching and caughing up green guck. I’m of the latter persuasion. Problem is — you don’t know what your own reaction will be until it happens. Like hay fever, some people never react, some react after a few years and some people get sensitized fairly quickly. Once you become sensitized, you’re sensitized for life. Lucky you. Reactions also vary from one individual to another. Some people itch a bit, some get holes in their fingernails and some wind up in the hospital for a week or more with bad reactions in their lungs. So, if you’re going to work with this material, you MUST protect yourself properly. Here’s my drill…
1.) A Tyvek suit with hood — always. No exceptions.
2.) An MSA-approved, carbon canister organic vapor respirator. No lee-way here either. Just use it.
3.) Butyl rubber gloves. They’re the only ones that have been proven to work. Not latex, not nitrile, not anything else. Butyl! Latex gloves are porous — they aren’t to bacteria and viruses (which are large) but they are to molicules (which are a LOT smaller).
4.) Latex rubber gloves over the butyl rubber gloves. This keeps the butyl gloves clean. The butyl gloves are expensive — about $20 bucks a pair, so we use inexpensive latex gloves over them and just throw out the latex gloves when they get messy.
5.) Some form of eye protection. I use labratory splash goggles.
If you have access to a system that feeds fresh outside air into a respirator, you can use that instead of the organic vapor mask. Just make sure that your fresh air supply doesn’t draw in vapors from the epoxy.
The MGS epoxy is a great material, but is has almost no odor, so you can’t use your nose to determine if you need the organic vapor mask. You also can’t use it to determine when the carbon canisters in the mask are shot, so change the canisters frequently. Once you’ve done your lay-ups for the day, keep the shop well ventilated and don’t re-enter the shop withour some form of lung protection until the epoxy has cured.
I have found that in winter, when the epoxy cures slowly, I need to let the material cure for 48 hours before I can sand it. It’s just too gummy otherwise. Also, you need to use your organic vapor mask when enterng the shop if it contains partially cured epoxy. Make sure to use a dust mask when sanding cured epoxy.
If you’d like to work with this material, I recommend that you purchase it from The Composite Store (CST):
http://www.cstsales.com/
For some reason that we haven’t been able to figure out, both Aircraft Spruce & Specialty and Wicks Aircraft Supply (who also sell MGS epoxy) have outrageous shipping charges for this material. They claim that they can’t ship it by UPS and that it must be shipped by truck. The Composite Store has worked out a system with UPS to ship it and buying it from them will save you mucho dinero. You do have to pay the hazmat charges (as you would with other vendors), but the shipping charges from CST are far more reasonable.
Enough for now. Next time out, we’ll look at the fabric side of the equation.
