Epoxy strengths
 

Hi,

we have been using z-poxy brand 30 min epoxy for some time on our field. However, after the LHS ran out of the same, I ordered Tower Hobbies 30 min epoxy. While using that for repair I noticed that, atleast when using microballoon filler, it forms a rubbery mass when dry that is no where as strong as what I used to get with the earlier stuff. What gives? Isint the Tower Epoxy as strong?

Ameyam

RE: Epoxy strengths
 

A lot depends on the proportions you mix it in.

Tom

RE: Epoxy strengths
 

You may have gotten the 50/50 mix off a bit. Not sure what the weather is like in India at this time but here in the states the weather is cold and damp, it does effect the cure time of epoxy. Cold damp weather means a longer cure time. I have never noticed any strength difference between any of the brands but during the summer it starts setting up a lot faster. One trick I use is my covering heat gun. I heat up both parts then blend it. If it starts to thicken up as I'm stirring I heat it up again in the cup. I also use acid brushes to mix it. I make sure it is a nice frothy off white color. I'm color blind so it may not be white but it is frothy and mixed 100%.

RE: Epoxy strengths
 

Make sure you are not mixing it in a wax coated paper cup or similar. Use a glass or plastic container that is wax free, even mixing on a piece of paper is okay but nothing with a wax coating or the cure will be jeopardized.

RE: Epoxy strengths
 

Not all epoxies are created equal. I personally stay away from the 1:1 ratio epoxies for the reasons you have just stated. They just don't cure up as well as the other resins do. The reason is that while making the mix ratios easy for us mix challenged guys thery have diluted the catilist agent by doing so. Z-poxy is a quality product but in this day and age most people don't know the difference so the hobby shop sells whatever is cheaper and when it's time to reorder, they order what sells. Myself I havent purchased and epoxy "glue " in about 10 years. I do alot of epoxy fiberglass layups so I always have epoxy laminating rersin available. To thicken it up for use as glue I add a little milled fiber and cabosil. If one were to buy a quart of epoxy resin and the additives you would end up with 3 pints of usable material for about 50.00. IMO that is much cheaper then what one would pay for just epoxy glue like the Tower stuff. Best thing is that once you have the resin kit you will start to find uses for it. Sheeting foam wings, sealing engine conpartments, glassing it landing gear plates. You could even make up some balsa/glass servo trays.............imagination is the only limit.

RE: Epoxy strengths
 

I've had epoxy that stayed rubbery. It can be caused by improper mixing.

I was told by a guy who worked at a defense contractor that they had to follow some pretty stringent rules when mixing epoxy. They were supposed to put measured amounts of resin and hardener in a cup, then mixed it for 45 seconds. Pour into a clean cup, get a new mixing stick, mix for 45 seconds, then it is ready to use.

RE: Epoxy strengths
 

I work for a defence contractor as a composits specialist and have never heard of such a thing. The ambient temp is going to play a part of this and one just has to use good judgment when mixing. We do howerver have to log in the paperwork the experation date, lot #, exact mix ratio by grams, how many grams of additives ( Cabosil, microballons, milleg fiber or glass beads ) then lastly what temp we cured the epoxy at. Heat curing add tensil strength. NEVER attempt to cure epoxy below 65 degrees. Humitity dosent have much effect as long as you are under 85%

RE: Epoxy strengths
 

It must be a 'company' thing rather than a DOD thing.

RE: Epoxy strengths
 

I agree, Uncle Sam is only concerned that the end product will perform

RE: Epoxy strengths
 

yeah the heat gun works wonders

RE: Epoxy strengths
 

Its quite hot here- it never drops much below 29 Deg C through the year. Its winter now, the nights are a bit cooler but I had this problem fixing a Reactor 46 more than a month back when the weather was a lot hotter- around 32 Deg C

Now, seriously, you cant expect someone to measure exactly how much of each part they add to the mix when doing hobby repairs, DOD is a diferent issue. I usually just squeeze the bottle till I feel I have got fairly the same amount of each. Did that with the earlier brand as well. I need to add in some filler- for easier repairs I used to use talcum powder earlier but I got a bottle of micro-balloon filler from Tower and use than when the repair is critical. Filler is required because I dont want the epoxy to flow as it cures. The Reactor has a frame construction and the repairing wood- tristock etc, doesnt always have a full bearing surface, so I need the epoxy to fill the gaps.

I cant use local epoxy- the only brand I know is Araldite from Pidilite and it comes in 8 hour and 5 minute varieties, not 30 minute type. It is not always possible to wait that long when repairing stuff and epoxy will simply flow out if it takes that long to cure. Also, I fear the 5 min variety will cure before I am done working with it.

I mix in a plastic cup- the drinking water type or or paper when mixing larger amounts. I usually dont need more than1/5th of the cup. If the amount is small or the wood allows, I just mix on the wood. I dont mix till I get a frothy appearence but I do mix till I am sure the stuff is fairly homogenous. Since I add filler, I can get the judgement of this- usually 10-15 seconds is enough to get a even colour. For applying, I simply use my fingers where I can reach. Its icky but gets the job done and its lot more accurate then using a stick or something. If I cant reach- inside firewalls etc, I use a brush. I simply rub off on a rag or tissue afterwards. I usually tack my stuff with CA immediately after I apply the epoxy so its held in place while the epoxy cures.

Still, the TH one leaves a lot to be desired in terms of strength. Wonder if the same problem makes TH's ARFs so poor in terms of glueing

Ameyam

RE: Epoxy strengths
 

With epoxies it is very important to get the mix correct. If you are say 5% off it in either direction, it will affect the strength. Mixing to a froth is not a good idea either. All that does is add air into the epoxy and weakens it. Micro balloons will also weaken epoxy. Milled fiber or cotton flox is best as an additive to increase strength and cabosil is best to keep it from penetrating too much or sagging. If possible, heat can be used to speed up the cure. The laminating resin I use allows me to lay up a part in this case a cowl and then move into a large box with a 40 watt light bulb. The interior of the box will get to around 110 degrees F. I come back to it in an hour and a half and am able to remove the part from the mold. The parts I heat cure are also stronger then if I allowed them to cure at room temp overnight.

RE: Epoxy strengths
 

CA glue both thick or thin could form rubbery lumps when it touches fresh uncured epoxy.

RE: Epoxy strengths
 

I mix in a plastic or paper cup most the time. I run a bead of part A the length of the cup then run a bead of part B right next to it. I have been doing it for so many years when I'm out of epoxy I'm out in both bottles. Being off a little bit isn't going to effect anything, being off a lot can cause problems. It sounds like your not getting the two parts blended enough. If part A&B are close and you still have tacky epoxy then you need to work on your mixing a bit more. Mix first then add the Micros and mix them in. I use several different brands bought from several different hobby shops. Never have noticed a nickles worth of difference in strength or hardness. When I ran out of epoxy on my last build a friend gave me some old epoxy he had on hand, it is Tower brand. It works just fine for me.

RE: Epoxy strengths
 

Humm.. that could be it. I usually just dump in all three and mix. In my earlier brand, I ran out of the yellow stuff earlier. In those days I didnt work much with epoxy so that bottle lasted a good year or so. In the new type I just use up equal amounts.

Someone mentioned milled fiber or cotton flox. Whats that? Can I use some fluffed cotton to ensure it doesnt flow much?

Ameyam

RE: Epoxy strengths
 

I've never had any problems with the Tower 30 min epoxy, and I tend to buy it a half dozen or more pairs of bottles at a time.

I've used it successfully with Microballoons many times.

It dries HARD and white with microballoons, so I suspect that you introduce too high of a ratio of Microballoons, and/or you may have not mixed things properly.

Did you use both components from Tower, and NOT mixed other epoxies you may have had around?

You may have to reduce the amount of Microballoons you mix in. Too high of a ratio may result in what you've mentioned.

RE: Epoxy strengths
 

Right, thats what I suspect- I am using too much microbaloons. What other ways are there to thicken epoxy? I am not sure what carbosil is called locally

Ameyam

RE: Epoxy strengths
 

I use the little mixing cups with the increments on the side, Usually 2 grams each for 4 grams of mix is what I use and it comes out perfect every time.  I use the TH stuff too and it is so hard it makes sanding difficult if I use too much.

RE: Epoxy strengths
 

OK, that got the issue out of my head. I am sure I am using too much filler. Luckily, the last batch I decided to pour into the gaps in the wood, I mixed without any filler. It flowed out of the gap but I am sure it is strong enough

Ameyam

RE: Epoxy strengths
 

The main thing, when using any type of filler, always thoroughly mix the epoxy parts together first then add the filler.

RE: Epoxy strengths
 

In regard to epoxy fillers, my favorite is Stits 'SuperFil'. I much prefer this to epoxy and microballoons. Although it seems a little pricey, it is extremely lightweight (lightest that I have found), sets up slowly, stays in place (does not run),and sands easily. Itcomes in a two-part and makes a lot of filler. One order has lasted me for several airplanes and I haven't even used one third of it yet. I've only been using it for about a year now, so I don't have any long term comments; but, I suspect that like most epoxy materials, it will hold up well.

RE: Epoxy strengths
 

Charles, is that stuff heavier than the Microballoons?

The Microballoons have a tendancy to waft into the air.

They are so darned small ( smaller than toner it seems ) that I'm sure people are breathing them in unintentionally!

RE: Epoxy strengths
 

I think you just discovered your problem all by yourself. You can be a little off with the parts and it usually just slows the cure time but if it isn't mixed completely it may never cure. If you add the MBs first it doesn't mix very well. I use 30 min epoxy and MBs for making fillets. I have a cup of alcohol on my bench, mix up the epoxy and MBs to about the thickness of soft tooth paste then start adding the mix and shaping with my fingers dipped in the alcohol. It's very easy to shape the stuff.

RE: Epoxy strengths
 

opjose

I have used both epoxy mixed with microballoons and the Stits SuperFil. Although I have never been able to measure the difference in weight (if any), the SuperFil seems lighter. My main reason for liking the SuperFil is that it stays in place when used, it sands very easily, and it does not seem to set up before I am finished smoothing a fillet. I still use epoxy and microballoons occasionally when I only need a very small amount, but for fillets and ding fillers, the SuperFil is hard to beat.

RE: Epoxy strengths
 

Ok, I repaired the Reactor with 5 minute epoxy yesterday. Actually, adding microballoons converted it to 30 second epoxy- after mixing on a piece of paper or wood (thankfully) and then applying it in place with a stick (again, thankfully) and then adding in the BMs and then pushing it into the cavities with the same stick (and not using fingers for a change), it dried even befor I had a chance to take the model off my lap!

The side of the Reactor is frame and rather bumpy due to the frame construction and there are a lot of debris from earlier repairs. I filled the epoxy into the gaps between the ply sheet I was using for repair and the fuse side and it has hardened. Will this have sufficient strength? The ply I used was 2mm thick fibrous ply from a US40 build by GP

Ameyam