Epoxy and Gel Coat

General Composites Discussions Composites Talk
#1

I will be making Carbon Fibre fuel tanks shortly and have a couple of questions.

  1. Which Epoxy’s are resistant to petrol? Are there addatives available to non- resistant epoxys?

  2. What can I use as a Gel Coat as polyester Gel Coat is not compatable with Epoxy. Can I use Epoxy mixed with Silica to make a compatable Gel Coat that I can spray on to mould and let cure, then lay my Epoxy? I could also do with it being UV resistant.

I need some sort of a Gel Coat type skin sprayed on first so that when I demould I am able to have a layer that I can sand and buff the flange lines out as the mould is a 4 piece and there will undoubtedly be join lines.

Thanks:rolleyes:

#2

Can I ask why you are trying to make a take out of CF? It would be far easier in addition to a lot safer to overlay an existing tank. While it can be done, it is definitely an advanced project.

#3

I am making a CF fuel tank to reduce the weight for racing, to overlay a tank would be a complete waste of time and money. I have just about finished the mould for the tank and am quite capable of making a carbon one. I have been a pattern and mould maker for over 11 years and would not concider this project to be too advanced.

My question was about Epoxy and Gel, so if you have any relivant information then I would be greatful.

#4

Many of the epoxy manufactures have suggested not to use their products to make a gasoline fuel tank. While their epoxy is resistant to gasoline, they can’t say how resistant it is due to all the different formulations which might contain chemicals that can attack the epoxy. No one knows what the long term effects are either.

If it is used for racing, have you check the rule book? Typically composite fuel tanks are not allowed. Ones that do allow a composite tank also requires a fuel cell bladder to be used. From a safety standpoint, a composite fuel tank is very dangerous because composites do not handle impacts well and tend to shatter, and they also don’t handle abrasions.

#5

mreitt, ouch! That seemed to be a rather curt response. “a complete waste of time and money” - I was seeking to clarify your intentions, not to insult your 11 years of experience.

As Scooter already mentioned, this type of construction should also plan for a bladder or inner cell. As for the actual construction material, I would suggest that you contact some of the major manufacturers for a specific recommendation. Also, the construction of composite fuel tanks that I have seem are almost always cylindrical. Who is your regular resin supplier? VER might be a possible solution due to its high chemical resistance.

#6

:slight_smile: I did not meen for my reply to sound bad, somtimes the problem is when you can’t actually talk to someone. Please accept my apologies if I sounded rude.

What do you meen by a bladder? I thought about using Vinyl Ester but it won’t give me as much strength as epoxy. My tank will have a layer of CF and a layer of Kevlar ( Probably 2 x Kevlar). The carbon is 155gsm 2K and the Kevlar is 320gsm 0/90 R=BN, not sure what the “0/90 R=BN” meens so I you have any ideas then let me know! I’m thinking that this with an Epoxy would be extreamly strong and would match the steel tank any day. I actually wanted some 3K or 6K but this is now rare and I am also now located in New Zealand so I don’t stand much chance! If you can help me with this problem then I would be forever in your favour!

I have seen some tanks made in a simular way, check out this link:-

http://www.oppracing.com/products.php?action=show_images&id=361&bodywork_id=79#image_7

I have been looking at some Epoxys, what do you think of this?:-

http://68.72.74.53/pae_index.htm

I look forward to your responses and I apoligise again if I came across rude. It’s actually nice to be able to chat to like minded people, oh and may I say I’m impressed with the Sabb parts, do you use epoxy for those?

Cheers

#7

The rubber bladdler is like what is used in race car fuel cells, a silicone or PUR sheeting sealed together. This is in case if the outer skin / shell cracks or gets spilt, the fuel does not leak out.

But with Composites, if you drop the MC tank (lay it down and I have…) and it hits the pavement, it will shatter! Without the bladder, then where is the fuel going to go? Up in flames!:eek:

Tig welded alum MIGHT be a better choice in this application…lighter than steel but more forgiving.

#8

No worries. What I was think was something more like an inner fuel bladder with a hard epoxy shell. In this way, you would not be as constrained by the resin type and would benefit for a high safety factor. I believe this design would also conform with racing safety regulations.

A custom inner fuel cell like this - .

I have never worked with these folks (http://www.racerpartswholesale.com/fsafe9b.htm), but you can Google it and see what comes up.

Also, these guys are located in Australia and make fiberglass motorcycle fuel tanks. Call them and inquire as to the type of resin they use.
http://www.classic-motorcycle-fiberglass.com/tanks/Fiberglass%20Motorcycle%20Fuel%20Tanks%20-%20Universal%20Tank%20Large%20LHS.htm

Here is another fiberglass fuel tank. But, interestingly enough, they say “This gas tank is not recommended for street use, this is a fiberglass gas tank that will not dent!!! It is possible that the gas tank will rupture and spill fuel possibly catching your motorcycle on fire if you happen to crash on the street. This gas tank is recommended for racing purposes only!!!”

#9

To tell you the truth, when I worked on the factory superbike teams. All the composite tanks we had would end up leaking. The heat of summer would cause the fuel to expand and blow the tank apart somewhere. Generaly you have to seam the two halves together (top and bottom) and it leaks from there. The tanks would also get soft with time as the fuel started attacking them.

Honestly Id love to just figure out how to do it right myself just for a show bike project Im doing. Id like to ride it once in awhile but not regularly so it would have to be functional.

Ive just made them with a wood buck and some aluminum skin these days. Its not that hard and its pretty light. Not to mention, aluminum doesnt spark when its dragging across the pavement.

#10

oh and don’t even think about spraying epoxy, it will make you dead. The hardeners will kill you if you inhale an atomized spray.

#11

I would attempt to find a supplier of a semi liquid form of Nitrile that could be thinned and activated and then poured into the finished tank and tumble the closed tank until such time your inside walls are fully lined and the liquid has gelled for an airtight seal.Much like sealing or leakproofing a car gas tank.
I’m assuming also that before the two halves are put together …you have an aluminum boss mounting ring dangling inside awaiting the fuel door to be screwed in place ?
…Vincent

#12

Find a fuel tank sealer to coat the inside of the tank…after the fuel surface has cured. You will have to scuff (with about 80 grit sand paper) the epoxy/composite surface before applying the sealer. I dont know if this is possible in your molding/laminating process. There are several sealers on the market made for old steel gas tanks that have holes from rust etc. Many laminating epoxy manufacturers that I have contacted about a fuel resistant coating for a composite tank…suggest using a polysulfide tank sealer if your using aviation fuel or fuel without alcohols. If your using a fuel that contains Ethanol or Methanol you will have to go with something else. Most of the gasoline you buy at the gas station in the US has Ethanol in it now which is very corrosive even to many epoxies…and those laminating epoxies will not hold up to extended exposure to Ethanol or Methanol (3-7 day exposure).

There are some epoxy manufactures that have epoxy formulations that are fully resistant to Methanol/Ethanol…these are basically the epoxy floor coverings that are brushed on concrete floors in chemical plants etc. Some last over a year while being fully immersed in Methanol. You could potentially use these to coat the inside of your tanks…they would most likely have very good adhesion too.

Ive done a bit of looking into this looking for something that was Methanol resistant for a CF part I am making that will come in contact with Methanol. Ive done a bit of questioning of the manufacturer on the product listed in the link below…they it is Methanol/Ethanol resistant. I havent tested it yet…but they recommended scuffing my composite/epoxy surface with the 80 grit and then painting the tank sealer on. Maybe worth a look for you.
http://www.prp-porstore.com/page/por15/PROD/TankSealers/TS

#13

Thanks for all of your replys!

The Fuel tank sealer looks like a good alternative to me. As far as the tank expanding is this caused by gasses? If so could you not put some sort of a breather somewhere at nr the top of the tank to allow the gasses to escape, sort of like the ones on moto x bikes? If the fuel tank sealer is as it states resistant to all fuels then this would surely provide a fully protective barrier and therefore stop the tank from being attacked and then becoming soft?

I thought about using a Vinyl Ester to seal the inside of the tank but someone said that this would not bond to the Epoxy, is this true?

Well alot of planning to do so keep the ideas flowing guys!

Cheers again everyone…

#14

We thought about doing composite tanks, but the liability, even if we were to sell on ebay, with a fake business name, with some kind of remote account with harvey the rabbit’s name on it, would be too much in the event of a death. even if you put “show purposes only” someone will get a good enogh lawyer to put you under

#15

We make fibreglass tanks for vintage off road bikes, but while they work fine for that application, I would think that the rules would make use of composite fuel tanks on a race bike, quite impossible.

As has already been stated you may be able to get round this by having a custom inner fuel cell made, but this would increase cost, to around double what an aluminuim tank would be.

Chris

#16

Wouldnt you have to have some kind of a venting system on a fuel tank anyway…most auto gas tanks have a vented fuel tank cap.

If you dont want to go with an inner bladder or inner fuel cell…I would find out what the contents of the fuel you are going to run are…if it is like aviation fuel which doesnt contain alcohols or high octane race gas which most also do not contain alcohols then there are many options for very good fuel tank sealers since the home built aircraft industry offers many high quality fuel tank sealers that meet industry regulations for composite tanks. There have been lots of fiberglass tanks for aircraft built using sealers like the ones listed in the link below. Check out the link below for some ideas and see if you can contact the manufacturesr or test the sealers before using them to make sure they will work for you:

http://www.aircraftspruce.com/menus/cs/fueltanksealants.html

Now if you find out the fuel you are going to use contains alcohols (methanol/ethanol) etc…then like was said things get tough. Like was pointed out the ethanol that is in most USA gas station pumps (some of the USA gasoline only contains 5% ehtanol but it still weakens most epoxies rapidly)…will soften the epoxy and weaken it rather rapidly…a few days even.

I guess it depends on how bad you want to get a composite tank that withstands the fuel you are going to use. There are solutions out there but it might take some $$ and effort to find what works for you. Plus if there is going to be legal obligations that you are concerned about…that is another consideration. Ive talked to a couple epoxy manufacturers that will formulate a specific epoxy for you that is alcohol (methanol/ethanol) resistant but that will cost some good $. Some had the special formulation on the shelf but there was a minimum order of like $500 for a large drum of it. THat was too much product and too much $$ for my small part.

#17

I think the more I ran all this through my head…My feeling would be to build the two part Composite tank,bond it together in a strong fashion once rough internally sanded…then pour the tank sealer into it and begin to tumble/roll the cell until you KNOW and can view that its been fully coated.
Then …simply have the dicipline to drain the tank after road/track fun with the nice little drain petcock that you thought to install before the two halves were bonded !! I’d leave the fuel cap off with just a towel laying over the filler hole to keep dust/dirt out of the tank ,all the while allowing it to vent off the attacking solvents.
…Cheers, Vincent

#18

The proper coatings linings like in a Fuel Safe fuel cell, is not just a rubber (silicone or Polyurathane) as it also has a woven reinforcements just like a composite product…

Yes, the old motorcycles like Penton, Montessa, Bultaco and others did have fiberglass tanks…but when I was much younger and working on those bikes, they did leak!

#19

I think I’m just going to use a good vinyl ester! I have found one that will resist all the chemicals in all the fuels. This will be slightly weaker than the Epoxy but it will still be stronger than a polyester.

My mould is nearly ready and my first tank should be out in 3/4 weeks. I shall post a picture of the finished tank. I have only been able to source 2k 155 gsm and 12k so I will be going with the 2k, bloody aeroplanes!:mad:

#20

Wonder if race rules in NZ allow composite fuel tanks? Not sure if they are allowed here in the UK.

Chris