Epoxy / Polyester Resins Question

[Wotname]

Originally Posted by boatpoker

This is where RamblinRod and I disagree. I believe polyester can work even in amateur hands in non-structural or cosmetic applications. For structural or external below water applications I believe for a secondary bond, epoxy is far superior. I do not use polyester in those applications

Thanks and ahh, yes... I got you and RR mixed up in in my brain for a moment .

Perhaps RR will be kind enough to answer my question.

FWIW, I would default to using epoxy simply because I am very comfortable using it and know that it is going to work as I intended. Poly is more of a mystery to me...

[ramblinrod]

Originally Posted by Wotname

In general terms, what is the correct (or proper) way to carry out this work so that the secondary bond does not fail?

Well, there are a lot of variables.

But frankly, if one good practice, they will succeed.

What happens, is the avg DIYer, goes to a hardware store, picks some poly resin off the shelf (most likely waxed instead of unwaxed), preps the surface poorly, mixes the resin badly, applies incorrectly, and it fails (no surprise).

Assuming it is the material's fault (as nobody wished to accept blame), they investigate alternate materials, find West System (it's way more expensive so it has to be better) read the instructions, follow them to the letter (as they now have some real coin invested) and it works.

Surprise!

(If they had of followed the West System instructions to the letter for poly (other than mix ration of course), it would have worked.

Differences:

1. Poly catalyst ratio (never more than 1.5%, adjusted down only, based on temp).

That's about it.

I have performed several destructive tests, where when completed properly, the new FRP cannot be peeled from the old FRP, without transfer of at least some of the old FRP (or wood) onto the new. (ie. bond stronger than original material). Except in a tabbing application, the normal stress on a repair is not a peeling action, it is compression, tension, or sheer. In a tabbing application, the stress is often a peeling action, whether epoxy or poly, the wood (especially teak or other oily wood) should sanded, dusted, and then wiped (2 rag) repeatedly with acetone, to remove oil. First coat of poly should be thinned with acetone or styrene, and then subsequent poly full strength. Do this properly and your will tear the fibres of DRY balsa and fir plywood apart trying to peel it. (If the wood is wet, including teak oil rising to surface, both poly and epoxy will fail, most assuredly.)

The most common DIY poly mistake is to tab to wood with no prep at all. When that fails, the user blames the material, switches to epoxy, follows West System instructions and low and behold it works. (Again, had they employed the principles of the West System instructions the first time with the poly, it would have worked with the poly.)

[ramblinrod]

Originally Posted by NevisDog

I'm sure some folks (though perhaps very, very few folks) have figured how to "properly" complete a repair, that will not fail, using cheap polyester resins, and I take my hat off to those folks.

The more important point to get across here is that polyester resin is not, never was, never will be, and is never claimed to be, an adhesive; epoxy resin is. I have no idea how to repair with polyester but I imagine it's quite an achievement, to be admired by all. Lesser mortals should not even try it.

This "poly is not an adhesive" position is bogus.

Poly and epoxy have very similar adhesive qualities. According to the independent studies I have seen (sorry no link, I have to go fix boats) the adhesion properties of basic epoxy is about 20% stronger than poly.

Again, if done properly, one is only counting on 20% of the poly adhesion strength, there is an 80% overkill. If one is not preparing properly and needs 200% of poly strength, neither poly nor epoxy will work.

Beware of epoxy manufactures who market (very well I may add) to DIYers.

They (naturally) will do everything possible to cast their product in a favourable light.

Whereas poly manufactures don't really market to the DIYer.

Follow the epoxy guys instructions when using poly (other than catalyst ratio) and there is very little difference.

As I have said before in other threads, epoxy tends to mix with fillers better, but it is just a matter of another 30 seconds or so of stirring in the filler into poly. Meanwhile, the poly cures in a fraction of the time of the epoxy (unless too hot a hardener is used risking the epoxy pot spontaneously combusting).

[Sailmonkey]

Originally Posted by boatpoker

I see many secondary bonds fail right out of the factory. They lay these hulls up so fast and get them out of the mold well before they are ready.

A common bond failure occurs when a liner is fitted within the hull (often glued with plexus or the like) then the joints are glassed over (the secondary). If I spent a little time in my files I could pull out hundreds of factory secondary polyester bond failures like the one below. If they can't do it in the factory what are the odds of Bob the repair guy with bucket of resin in the yard.

That sure looks like tabbing applied over gelcoat. Almost born to fail regardless of the resin.

[ramblinrod]

Originally Posted by boatpoker

I see many secondary bonds fail right out of the factory. They lay these hulls up so fast and get them out of the mold well before they are ready.

A common bond failure occurs when a liner is fitted within the hull (often glued with plexus or the like) then the joints are glassed over (the secondary). If I spent a little time in my files I could pull out hundreds of factory secondary polyester bond failures like the one below. If they can't do it in the factory what are the odds of Bob the repair guy with bucket of resin in the yard.

OK, I think you are stretching this a bit too far.

There are millions and millions of factory secondary poly FRP bonds that are intact after 20 years, and longer.

But the best practice is to apply the new lay-up before the unwaxed previous lay-up has cured tack-free.

I can't speak to what cause the failure in the photo, other than to say, I am 100% sure it was an application error, and poly could have worked just fine if applied properly.

Remember that the guys in the factory are on the clock (both for lay-up schedule cure times and labour cost reduction), and sometimes, things are slapped together all wrong. Often, turn over is high, the wages are $#!+, and there is little dedication to the craft or quality. Consider low cost, high volume, prefab home construction to on-site custom premium home construction (which the latter can have problems too, but typically of a one-off nature that someone just goofed).

Fortunately, most high volume production boat builders that are still in business, have pretty significant quality control programs, to minimize individual worker error and design error. The review / revision cycle of the latter is much longer, so a fair number of inferior production models can hit the water before the fault is every discovered and corrected.

[DeepFrz]

So, it seems with poly, do everything exactly right and you may have a good bond. However, mess up a little and look out.

However, when you are doing a repair, you need for the resin to also function as an adhesive--gluing the patch to the surrounding surface. Polyester is an adequate adhesive but not as good as epoxy. As a general rule, the tensile strength of a polyester bond will be around 20 percent weaker than the same bond made with epoxy. -- Don Casey

After a cure period of seven days it can be seen that the tensile
strength of the epoxy resin is 20 to 30% higher than those of
polyester and vinyl ester. -- MJM Yachts

Although there are some polyester resins that demonstrate improved resistance to high temperature, standard polyester resins offer resistance to temperature up to (HDT) +/- 60oC. At more extreme temperatures the polyester composite often becomes pliable.
Epoxy resin offers resistance to temperatures of up to 300oC. Total hardening of epoxy resin products is possible after many hours baking inside a furnace. High temperatures inside the furnace provide tempering and improve a product’s resistance to high temperature. Therefore turbine air supply assemblies are made from epoxy resins. -- Dex Craft

And on and on and on... I have never seen a composite aircraft built with polyester resin, have you?

What are the laminate temps. on a sunny day in the tropics?

[ramblinrod]

Oh, and one more thing...

Sometimes a repair guy will recommend using epoxy because, either:

a) That is what they were trained on, and they don't know that either will work fine.

b) Maximize profit (both parts and labour).

c) It makes sense to the customer if the original failed, to use something "better" the next time. (Even though the something better, would have failed too, if used under the original conditions, but of course, the customer wouldn't understand that.)

After a period of time (days, weeks, months, or years) some of the epoxy users will develop a sensitivity to it, and then can't go near it while it is curing (up to 2 weeks). If they stay on career, they are then forced to use poly, and low and behold, find out that it works just as well (when applied using the same techniques used to apply epoxy except for catalyst ratio).

Fortunately, I have not developed a sensitivity to epoxy...yet, (knock on teak) so I use it only where absolutely necessary (application / conditions really dictate) so that I will hopefully never be restricted from using it where it is actually required (most commonly, in the classroom of the FRP seminars and demos I present, where the smell of poly would be prohibitive).

[ramblinrod]

Originally Posted by DeepFrz

So, it seems with poly, do everything exactly right and you may have a good bond. However, mess up a little and look out.

RR: With either poly or epoxy, if the material is applied improperly it may fail. Correct application is virtually identical to both materials.

And on and on and on... I have never seen a composite aircraft built with polyester resin, have you?

RR: Exactly as I said, epoxy has about 20% more strength. Anyone relying on more than 10% of breaking strength, for normal conditions is a dang fool.

What are the laminate temps. on a sunny day in the tropics?

RR: Straw men arguments. We are talking boats. Millions and millions of boats made of poly FRP have plied the waters the world in all climates.

[smj]

Originally Posted by ramblinrod

RR: Straw men arguments. We are talking boats. Millions and millions of boats made of poly FRP have plied the waters the world in all climates.

I don't think anyone would argue this statement, but that's not the subject of this thread.

[ramblinrod]

Originally Posted by smj

I don't think anyone would argue this statement, but that's not the subject of this thread.

Well if making and repairing boats out of FRP isnt relevant to this thread, I surely don't know what is.

FWIW, my reponses embeedded within the post I was replying to, referring to aircraft, is unclear, because the CF quotation function is too difficult to use with this iPhone virtual keyboard.

[smj]

Originally Posted by ramblinrod

Well if making and repairing boats out of FRP isnt relevant to this thread, I surely don't know what is.



FWIW, my reponses embeedded within the post I was replying to, referring to aircraft, is unclear, because the CF quotation function is too difficult to use with this iPhone virtual keyboard.

I think most people would agree that there's nothing wrong with a well built polyester hull. The disagreement comes into the appropriate resins to use on making a repair to a cured polyester hull.

[ramblinrod]

Originally Posted by smj

I think most people would agree that there's nothing wrong with a well built polyester hull. The disagreement comes into the appropriate resins to use on making a repair to a cured polyester hull.

OK, everyone is entitled to their opinion. But please consider this, if the repair will only be subject to forces equalling about 10% of the adhesive strength polyester resin, why would a material of slightly greater adhesive strength be deemed "mandatory".

Based on all of my research and experience, the two key reasons to use epoxy for marine FRP are

1. When the original construction was epoxy.
2. When the smell of poly is prohibitively offensive.

The reasons not to use epoxy when any property superiority is not required for the application:

1. Contact dermatitis and allergic response.
2. Slower curing.
3. Temp specific hardeners.
4. Propensity to spontaneously combust.
5. Material cost.
6. Poor compatibility with poly resin and gelcoat overcoat.
7. Contributing to obscene margins of
The Gudgeon Bros. ;-).

[transmitterdan]

Originally Posted by smj

I think most people would agree that there's nothing wrong with a well built polyester hull. The disagreement comes into the appropriate resins to use on making a repair to a cured polyester hull.

If you substitute vinyl for poly in the above statement I would agree. I don't think pure polyester resin has a good rep below the waterline. It requires an epoxy seal coat.

I have seen many polyester repairs above the waterline work quite well. But it does require someone of many years experience and innate skill to get it right the first time.

[smj]

Originally Posted by ramblinrod

OK, everyone is entitled to their opinion. But please consider this, if the repair will only be subject to forces equalling about 10% of the adhesive strength polyester resin, why would a material of slightly greater adhesive strength be deemed "mandatory".



Based on all of my research and experience, the two key reasons to use epoxy for marine FRP are



1. When the original construction was epoxy.

2. When the smell of poly is prohibitively offensive.



The reasons not to use epoxy when any property superiority is not required for the application:



1. Contact dermatitis and allergic response.

2. Slower curing.

3. Temp specific hardeners.

4. Propensity to spontaneously combust.

5. Material cost.

6. Poor compatibility with poly resin and gelcoat overcoat.

7. Contributing to obscene margins of

The Gudgeon Bros. ;-).

10% of the adhesive strength? I fully agree, polyester should be fine for cosmetic repairs.

[ramblinrod]

Originally Posted by smj

10% of the adhesive strength? I fully agree, polyester should be fine for cosmetic repairs.

Your statement is illogical. If a structural repair only requires 10% of the adhesive strength of polyester resin, then polyester resin is more than adequate for the structural repair.