'Epoxy' woes

[jorgoz]

I've recently bought some 'epoxy' through ebay.
http://cgi.ebay.de/GFK-Epoxidharz-Epoxydharz-Epoxi-Polyesterharz-3-25Kg_W0QQitemZ380126317721QQcmdZViewItemQQptZDE_Sp ort_Wassersport_Windsurfen?hash=item58814b4c99&_trksid=p3286.c0.m14&_trkparms=72%3A1229%7C66%3A2%7C65%3A12%7C39%3A1%7C 240%3A1318%7C301%3A0%7C293%3A1%7C294%3A50

I've used it as a first coating before putting down some fibreglass, then filling the weave. Seems to work ok.

But, this morning i wanted to put in some fillets for joining the the deck and hull of my clc wood duck (from plans).

I mixed up 90 grams of epoxy, i always work with small batches. Then added some thixotropypowder mixed with woodflour for a nice tan colour. After mixing it didn't sag on the mixing stick or in the paper cup. started filleting and after a couple of minutes i noticed the mix sagging, even getting quite runny. I removed it, mixed in some more fof my filleting mix, reapplied and same result.


I then mixed up a completely new batch, 60 grams. Heated it a little with a hairdryer, then mixed in my filleting mix, same result.

Then mixed a third batch, again 60 grams, again heated with a hairdryer, a bit more than the previous attempt, mixed in my filleting mix, heated a little more. Applied and it started to heat up on my quite quickly, but still sagging.

Thixotropypowder, i was sold this to thicken epoxy for filleting.
http://cgi.ebay.de/500g-Thixotropierpulver-fuer-EP-und-UP-Harze_W0QQitemZ350110018817QQcmdZViewItemQQptZKuns tstoffe_Chemie?hash=item51842eed01&_trksid=p4634.c0.m14.l1262&_trkparms=%7C301%3A0%7C293%3A1%7C294%3A30

Is this a problem with the supposed epoxy or is it my technique lakking ?

[JBreeze]

Several comments:

1) Have you used epoxy, wood flour and the white powder (which I'm guessing is "silica fume" (cab-o-sil)) for fillets before? You didn't state if this is the 1st time using epoxy for fillets, or if something seems unusual using this brand of epoxy. Here is a photo and a link to some pictures of thickened epoxy...

http://www.boatbuilder.org/epoxy3.jpg

www.boatbuilder.org/ epoxy.htm

2) From memory, I think I use about 1 part cab-o-sil, 2-3 parts woodflour and 1 part epoxy (by volume) to make a thick fillet paste.....mix up your epoxy, add the cab-o-sil, then keep adding wood flour while stirring until you get a thick paste that won't fall off the mixing stick.

3) Often, a fillet is covered by fiberglass tape and saturated with epoxy....if you apply the tape right after the fillet is made, the tape will help fillet stay in place. This is called "wet-on-wet"

4) The heat should accelrate the cure of the epoxy, but it may also be making it less viscous and run. My german isn't good enough to translate the ebay listings to determine if the thixotropic powder is silica fume, if it is compatible with epoxy, nor if the epoxy "blushes"....amine blush could be a problem with some brands of epoxy.

5) Sorry about my lack of knowledge about the epoxy you are using....tell us more about your experience with epoxy and fillets and I might be able to find some helpful links.:)


PS...Here are links in english regarding the epoxy and powder.....anyone here familar with this company?

http://www.hp-textiles.com/shop/index.php?cat=c21_Epoxyd-Resin.html

http://www.hp-textiles.com/shop/index.php?cat=c23_Fillers---Pigment.html

[jorgoz]

No probs on the german, my german is quiet lousy as well.

This isn't the first time i'm filleting. I wait about an hour, maybe 2 before putting on fibreglass tape, never tried the wet-on-wet though. My mix doesn't slide or fall of the mixing stick, it forms nice spikes which retain their shape, but after a while, when the resin heats up it looses it's consistency, that's why i tried the different approaches.

I bought the thixotropic powder with an epoxy producer (vosschemie) over here, Belgium. I asked for silica for applying fillets and that's what they sold me. It looks like cab-o-sil.

Room temp was around 19.5 degrees, 67 fahrenheit, which is around the ideal temp i believe.

It was also my understanding that a fillet should stay in place. And reading this article confirms that for me, http://www.westsystem.com/ss/bonding-gluing-clamping/

First time i'm using this resin though, previously used vosschemie epoxy bk
http://members.lycos.nl/vosschemie/epoxy.htm ,only in dutch though, and didn't have this kind of issues.

I've order cloth and tape from hp-textiles, but haven't tried their epoxy.

[Todd Bradshaw]

Generally, if the mix sags before it hardens up, the cure is to use more powder. Mix some small test batches, measuring the ratio of filler to resin carefully, do a test fillet or two on scrap wood and let them cure until stiff. Increase the powder ratio in your test batches until they no longer sag, then use that same ratio for the real fillets. Sometimes it takes enough powder that the stuff seems almost like dry dough on the mixing stick, but if that's what it takes to get one that doesn't eventually sag, then that's what it takes. The proof of a proper ratio is the end result once it's hardened, not what it looks like on the mixing stick. And put the hair dryer away. You shouldn't need it.

[Tom Lathrop]

The photo I saw on the website did not look like fumed silica at all , more like cotton fibers. I'd ask them just what it is.

[JBreeze]

Tom might be right!

Back of the envelope calculations indicate that 500grams of silica fume should be about about 11 liters in volume (very light, fluffy stuff).

Is your 500 gm thixotropy powder about 10-11 liters? If not, then perhaps you received something other than silica fume....as far as I know, silica fume is the best of the thixothropic fillers commonly available.

This is the reference I used for converting weight to volume:

http://www.uscomposites.com/fillers.html

[Dave Carnell]

The best, and cheapest, thixotropic powder is talc. Sands easily, too. Get generic baby powder. Check the label that the ingredient is talc, not cornstarch.

[Tom Lathrop]

The best, and cheapest, thixotropic powder is talc. Sands easily, too. Get generic baby powder. Check the label that the ingredient is talc, not cornstarch.

That brings up a question that I was intending to try. Have you tried cornstarch? Since cornstarch is a cook's favorite thickener, why would it not work in epoxy also?

[Cuyahoga Chuck]

The best, and cheapest, thixotropic powder is talc. Sands easily, too. Get generic baby powder. Check the label that the ingredient is talc, not cornstarch.

All the standard thixotropic fillers can be rated for strength or fairing ability. The strongest are fibers like chopped glass, cotton microfibers or coloidal silica. One step down is woodflour.
Microballoons is the easiest to sand but is lower on strength.
If fillets are struck off properly there should be no need to do any sanding. If the "wet on wet" technique is used the fillets have to accept the tape in short order.
Using anything that comes out of a kitchen or bathroom cabinet may work in a pinch but nobody knows what the engineering characteristics of this stuff is or what shortcomings any of these sundry powders may possess.

[JBreeze]

Chuck,

I don't think some of the fillers you mentioned are thixotropic, in the technical sense, i.e. viscosity decreases with shear.....

Here is a link:

http://en.wikipedia.org/wiki/Thixotropy

[RodB]

Dave's right... I think there is quite a history of builders using talc as a thickener... and it works fine. From personal experience for filleting... I do not think it gets as "stiff" as a 50:50 mix of cabosil and wood flour... so I usually use talc for fairing.... 1/4 cabosil-3/4 talc... it sands easily.

RodB

[George Roberts]

Epoxy warms as it cures and as it warms the viscosity drops. It is really hard to get the proper engineering properties and the correct viscosity.

Silica makes for a very tough epoxy.

[jorgoz]

Well guys thx for the input so far.

It's definitely silica, i didn't buy the silica through ebay, but a shop that specializes in polyester and epoxy over here, where i, up until now, always bought my epoxy.

This weekend i gave the resin another try, mixed it up real thick, (15% in weight filleting mix added). I didn't add the filleting mix in one go. Initially i couln't get it mixed properly, but eventually i got there. I applied a fillet. Looked good, but after a while i went back to check up on it and to my horror it had all run down to lower parts of the joint. The 15% (weight) seems a more than what i've read on the mas epoxy site as ratio for a filleting mix, http://mas.promorevolution.com/public/index.cfm?fuseaction=articles.view&id=2311.
qoute :
"Filleting: 1 Part Resin/Hardener mix using "Medium" hardener, 1 Part Cab-O-Sil M5 or Aerosil and 2-3 Parts Wood Flour "

I'll try some more test mixes as Todd proposes, only way to get a definite answer.

I sent the seller the question if this resin is suited for this application, but didn't get any response.

The hp-textiles people seem much more serious, they responded quite quickly to my enquire. They sent me a pdf-file with the technical data sheet for the resin they propose, but it's to large to upload here. Anyone would like to take a look at it ?

[jorgoz]

Well, yesterday evening i mixed up a test batch. I added 20% (by weight)of thixotropic powder and woodflour (western red cedar). The mix was very very hard to get mixed properly and i definitely can't use the pastry-style applying technique, to thick. The resulting fillet was quite coarse. The fillet stayed put, but i put a blob (about 2 cm x 2 cm x 2 cm) of the mix on a piece of ply, the piece of ply put at angle of about 45 degrees. Initially it stayed put but after about half and hour it started to slide down and when it touched it to move it a bit it completely collapsed and became a small puddle. I'm seriously starting to doubt this resin for filleting. Still haven't got an answer from the seller.

I asked hp-textiles about filleting with their resin and they told me to use cotton fibres, http://www.hp-textiles.com/shop/product_info.php?info=p253_hp-bf1-0100---baumwollflocken.html&XTCsid=36cf8281c6128ff6cd0822f32ce36e4b ???

[Stiletto]

I've used woodflour quite a bit for fillets etc. I mixed it in until it had only just lost its sheen when being mixed, it looked almost too dry. I think George is right about epoxy generating some temperature of its own. What is the ambient temperature of your workplace? That may be a contributing factor.
By mixing larger batches, you may find that the epoxy starts to 'kick' before it starts to sag.

[jorgoz]

I think it's definetely something with the resin.

Yesterday i received a set of HP-E45KL resin from hp-textiles.com and i mixed up a filleting batch and no problems whatsover.

The resin is thicker though.

Would the viscosity of the resin have to do something with it ?

[RodB]

Its not the viscosity of the resin most likely... I'm betting its just not the right substance. If you mix epoxy, even the thinner viscosity versions and than add wood flour and cabosil to proper thickness (50:50), it will thicken and stay the same consistency more or less until it becomes hard.

If I had to guess, sometimes, when you are a beginner, you may mix it at the start and think it is thick enough to stay put...but you soon realize it must be mixed a bit thicker to "stay put" on a fillet. This is learned by experience, but also, the behavior you have described does not following along with what I have experienced in any epoxy.

The descriptions you mention above just do not go along with epoxy and its usual behavior. Just be sure you mix to the consistency of peanut butter (with nuts) and you shouldn't have any problems.... IF you are mixing "epoxy".

R

[Boatmik]

Generally the difficulty of sanding the fillet gives you some indication of strength. After all you have to break bits out of the surface.

If a mix is easy to sand it won't be very strong and vice versa.

If there is a glass tape going over the fillet it probably doesn't matter too much as it acts pretty much like a sandwich construction - wood on one side of the fillet to hold it together, glass on the other. Just so long as the glass tape/s can stick to the fillet.

Like a surfboard. The foam is weak, but with glass on all sides it is remarkably strong.

However if using fillets without glass tape it is good to use a filler in the epoxy that is actually strong enough for the size of the fillet being used.

If talc/epoxy is easy to sand, then it is not very strong and the fillet will have to be bigger to match the strength of the hull material

So why do thixotropes work? it is because they are fine fibres that get all tangled up together when the resin tries to flow. As far as I know talc is not like a fibre at all, so I suspect it is not a very good thixotrope compared to the items on Chuck's list and would reiterate his comments that powders out of the kitchen might or might not have good properties for boat building.

One of the interesting comparisons is that it is not healthy to breath cotton fibres, colloidal silica or glassfibre even in modest quantities. This is because they ARE fibres and stick and poke into your lungs if you let them in there.

Talc, flour, Q-cels etc are harmless or relatively so because they are rounded in form, so they won't harm you much and also will not make high strength or high thixotropic mixes.

They might work enough .. but if you do a careful comparison they are likely to be not very good.

Most definite way is to make up a sample section, cut it into pieces and start to break them.

As far as cornflour or any other type of flour. It is a big jump to move from thickening a soup or custard and thickening epoxy!

You have to ask why the flour works to thicken your mum's custard!

The reason it does is because when the long gluten protein molecules join up into long chains and form a sticky bundle not unlike a big tangle of spagetti.

Epoxy usually has no water in it so you cannot rely on this mechanism.

I agree with RodB too. The runnyness of the epoxy does change the amount of powders a bit, but it is still easy enough to reach the right consistency.

Best wishes
Michael Storer

[the_gr8t_waldo]

i use cabosil as a thickener alot. but have never mixed it by ratios. i do it by hand/eye. nothing is added to the epoxie mixture, untill i'm sure that that is throuly mixed. and then i add the thickening agent, and mix that untill the mixture is about the consisity of peanut butter. i've found that once an inside fillet is just about 3/4 way hard, that is the time to start to lay cloth over it. not earlyer, cause the working of the resin into the cloth will push the fillet into strange and mostly unwanted forms.

[Todd Bradshaw]

I'm pretty sure they're still using talc as a common thickener for the gelcoats used on production fiberglass boats. It thickens resin, but the end result tends to be rather brittle, especially in any thickness that's more than a few mils. Other than being relatively cheap and capable of forming a somewhat thicker, but smooth, lump-free goo, I doubt it has much else going for it from a structural standpoint.