Choosing an Epoxy

[Jim Cullen]

Hi Group!

 

I started bulding a Cozy Mk IV many years ago (Plans #0076), got about 70% of the airframe done, and then gave up because of a bad series of allergic reactions to the epoxies. I had used Saf-T-Poxy II, a Hexcel epoxy that went by a number (no name) which I forget, and a third epoxy that I also can't remember. I'll also confess that my personal protection was pretty poor (latex gloves, no respirator, etc.). The fuselage has a kind of bluish, greenish, brownish, yellowish color.

 

Anyway, I've been building a kit car (Classic Motor Carriages MG-TD) lately and have been using EZ-Poxy and far better protective equipment (butyl gloves and a carbon canister respirator -- religiously) and am happy to report that I have had no adverse reactions.

 

As a result, I'm thinking of going back and re-starting my Cozy Mk IV. Because my technique has improved over the intervening years, I'll probably start the project from scratch and not reuse much of the previously done fiberglass work (except the elevators and centersection spar, which came out beautifully).

 

My question is: What epoxy are you guys and gals using nowadays? Is EZ Poxy OK? I understand that there is a new epoxy that has a long pot life -- which is useful if you're skinning a wing by yourself. I wouldn't want to attempt this with EZ-Poxy -- it has too short a pot life. What do you like and how can I learn more about the properties of the currently advocated materials -- especially the temperatures that I have to hold while working the layups? Are the new materials approved by Nat Puffer?

 

Many thanks for your help.

 

Tailwinds,

 

Jim

[Jon Matcho]

Hi Jim, and welcome!

 

What epoxy are you guys and gals using nowadays?

Most are using either the MGS 285 or MGS 335 systems. The 285 has slightly better properties than 335, but costs a bit more.

 

Is EZ Poxy OK?

Yes, but with all the name, ownership, and formulation changes behind this one, you need to be sure you have a compatible system. Aircraft Spruce, now responsible for the Cozy plans, sells it and you can ask them directly.

 

I understand that there is a new epoxy that has a long pot life -- which is useful if you're skinning a wing by yourself.

That may be either of the MGS systems using the "slow" hardener. I don't know what you consider "long", but the slow hardener seems to allow upwards of 60 minutes of working time. When I need to loosen it up, I just hit the cup with a warm heat gun for a few seconds. Or, if too viscous when wetting out, I'll hit it with warm air again for a few seconds to thin it out.

 

What do you like and how can I learn more about the properties of the currently advocated materials -- especially the temperatures that I have to hold while working the layups?

I have found 65F degrees is fine, which I understand to be an absolute minimum, with time-to-sand in the 24 hour range. 70+F degrees is ideal.

 

Are the new materials approved by Nat Puffer?

Here's what Nat had to say way back in 2000 about the MGS epoxy systems: "It is my opinion, based upon my investigation and experience that the MGS resins are superior to what I have used in the past and would be my first choice if I were building another airplane today."

 

I look forward to hearing about your next steps!

[Jim Cullen]

Hi Jon:

 

Many thanks for the reply!

 

Here in las Vegas, things tend to get pretty hot in the hanger over the summer months. Can you think of any disadvantages of building the entire plane with MGS 285 and the slow hardner? Aircraft Spruce shows that the H-287-S slow hardener has a 4-hour pot life. That sounds ideal for doing fuselage sides or wing skins, if I'm working alone. I have one of the Sticky stuff dispensers and, rather than futzing with mixing the slow and fast hardeners, could I just stick (pun intended) with the slow hardener for the whole plane?

 

Tailwinds,

 

Jim

[Jon Matcho]

Here in las Vegas, things tend to get pretty hot in the hanger over the summer months.

You may know this already, but If you can get it up to 122-140 degrees, you can post cure your parts for "free". You don't need to post cure -- room temperature cure is just fine, but it does bring the Tg temp. of your parts up to higher limits.

 

Can you think of any disadvantages of building the entire plane with MGS 285 and the slow hardner?

There are none, other than your cures will take a bit longer than with the "fast".

 

Aircraft Spruce shows that the H-287-S slow hardener has a 4-hour pot life.

Yes, that's on the MGS label as well. My times are with using a combination of the fast and slow hardeners, which you can freely mix to get whatever times you need. I find the slow hardener cures much slower, compared with 100% fast hardener, which cures ready-to-sand in 12 hours. I used to want to get rid of all my fast hardener, but lately I've been mixing it 50/50 with the slow.

 

I haven't figured it out yet, but I think the fast hardener tends to exotherm much more quickly than the slow hardener. Last night my cup of micro was getting stiff, so I shot it with some warm air, which made it even more stiff. This didn't happen the other day when I did the same with the slow hardener. It may not be a good idea to warm the cup at all -- I'm unclear on this at the moment.

 

Another benefit of the MGS system is that it's rather odorless when mixed. The hardener smells something awful, but I tend not to stick my nose close to it.

 

Do you still have your 70% complete airframe? Why not continue with it? I'm sure you could do it over better the 2nd time around, but that would cause me some anguish.

[Jim Cullen]

Do you still have your 70% complete airframe? Why not continue with it? I'm sure you could do it over better the 2nd time around, but that would cause me some anguish.

 

Hi Jon:

 

Thanks again for the benefit of your wisdom.

 

I'll probably use some of the parts that I already have. The centersection spar, elevators, wheel pants and winglets turned out great and I'll probably use them.

 

I have concerns about the attach tabs on the main gear strut. When I clamped the tabs after the layups, I may have applied too much pressure and squeezed out too much of the epoxy, resulting in a dry layup for the tabs. I'll probably rebuild the main gear leg assembly.

 

The canard will also get rebuilt. It came out heavy and I got artsy by trying to put the position lights behind colored plastic lenses (similar to the ones in the Glasair wings) in the tips and I'm not happy with the way that they came out. Also, I was working too cold and the skins are heavy (and maybe weak).

 

The wings will get rebuilt. I got caught up in the middle of the hardshell debate and used the technique to build my wings. Honestly, I think they're fine and many Long-EZs were built with this technique. I even did a series of peel-strength tests that were reported in the Central States publication that showed that a hardshelled layup had better peel resistance than one normally made. The debate seems not to have been settled. Still, I think I'll redo the wings.

 

As for the fuselage, I tried a lot of modifications. some were great, others were crummy. Rebuilding the tub won't take that long and I can do a much better job by learning from my experinece. Also, I had a funny experience one day while standing in the tub while it was supported on sawhorses. I stepped on the floor in the area where the forward gear bulkhead meets the right fuselage side (I weigh about 210) and I heard a ripping sound. Don't know if it was delamination, tearing fiberglass or what. I examined the area visually and couldn't see any problems, but I think I'd feel more comportable redoing the fuselage tub.

 

I also got about half way thru the canopy. I was making the forward portion tilt forward and I was making the aft portion a permanent structure epoxied to the fuselage -- with a rollover structure and two gull-wing doors with Glasair latch hardware. I was pretty happy with the way things were going until I botched cutting out the gull-wing doors. I need to rebuild the canopy but will probably stay with my modified design.

 

Enough for now. I'm doing a parts inventory to see what I still have. I understand that Brock is closing its doors? Do they have any parts left if I need to reorder some?

 

Tailwinds,

 

Jim