Carbon Fiber Sizing

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#1

I was told that different carbon fiber has specific sizing and may not be compatible with epoxy resin for hand layup. I tried to search the internet but could not find anything.

We are in the process of creating a 22ft powerboat using epoxy resin and quadriaxial carbon fiber at 400g/m2. Is seems that the quadriaxial fabric does not wet out completely after the resin has been applied and roled. Whilst the fabric is being rolled, it seems that is soaked in resin, so it is not a question of lack of resin but a couple of minutes after the rolling stops, the fabric seems to go into a matt finish and although it is sticky with resin, it does not give the ‘wet’ finish as a 2x2 twill weave gives.

Could this be a problem with the resin or perhaps the fabric sizing as I was told?

Also, we are noting that the resin is taking a very long time to start gelling (when touching the epoxy does not leave a trace on your finger). It is taking about 7 hours to do this.

Any input will be greatly appropriated.

Thanks.

#2

Virtually all commecial carbon fibers have sizing that is compatible with epoxy (the actual sizings are generally epoxy-based in fact). The tow(s) used in stitched quadraxial fabrics are the same as those used in woven fabrics, so it sounds like you are having some issues with the epoxy curing correctly. The construction of the quad fabric will generally be tighter than a woven, so proper impregnation will take more time, but the fabric construction should have no bearing on resin gelling/curing.

#3

This is what I am suspecting. After 48 hours, the epoxy seemed to have cured but we tried a destructive test and the layers of quadriaxial carbon came apart with out bare hands! It seems to be quite strong in terms of rigidity but it terms of bonding, there is no strength whatsoever. We also tried at attach more carbon to the ‘supposidly’ cured resin to try another bonding test, however after 24 hours the bonding is very weak and can be destroyed easily. You can pull off a whole thread (string) of the quadriaxial carbon without any effort. We abandoned the project and contacted our epoxy supplier so see what exactly is happening. Now, 4 weeks later, we tested the epoxy again just for curiosity and found that it had cured quite well. We had to actually use great force to destroy the carbon. And the good thing is that it did not delaminate but destroyed in pieces when great force was used.

What are your opinions on how this epoxy is behaving? We ordered the epoxy with slow hardener but this is ridiculously slow (assuming it cured well)

I also noticed something strange with the hardener. As a few drops naturally drip out of the drum when pouring and settle on the top of the tank, the hardener turned in to a white powder substance after 24 - 48 hours.

Is this normal?

#4

What is the humidity in your place?

Does the same resin work well with 200 grams twill? Or when just brushing on a coating onto something? Some epoxy resins go hazy in contact with air.

On sizings, see attached.

#5

Humidity is around 80 - 85%

We easily managed to delaminate 2 layers of carbon when supposidly cured and this is what they look like:

Any comments?

#6

Can you tell what epoxy was used, or post a datasheet?

Also I see quite some voids, and a dart shaped white area that I do not understand. Did you get rid of airbubbles by rolling with a serrated roller (bubble buster)

#7

The Epoxy is European as I am from Malta in Europe and bought the resin from Italy. This is the data sheet for the Epoxy. However we opted for a slower hardener and he sent us a different hardener to that on the data sheet. Pot life of the slower hardener is more then 4 hours. I’m suspecting that the hardener he sent is is not adequate for the Epoxy.

We observed that a slight spillage of pure hardener on the tank, after a couple of days, dried into a white flaky substance. Is this normal?

#8

we spent long hours with a roller to get rid of air bubble but this still happened!

#9

What kind of roller? In my experience some rollers make more bubbles than they eliminate. If you are pressing really hard the fibers will get compressed, the excess resin is pushed out and the laminate then sucks air back in as the fibers rebound.

#10

3" wide splined roller: http://www.google.com.mt/imgres?um=1&hl=en&safe=off&sa=N&tbo=d&biw=1152&bih=773&tbm=isch&tbnid=mWDpzU1taZVVVM:&imgrefurl=http://www.henleycraftfibreglasssupplies.co.uk/&docid=BFNHJNyaj0wRoM&imgurl=http://www.henleycraftfibreglasssupplies.co.uk/Paddle%2520Roller%252070mm%2520x%252021mm.gif&w=406&h=336&ei=LLzvUJGIOY_Jswab6IG4Cg&zoom=1&iact=rc&dur=287&sig=113359253200795142811&page=1&tbnh=132&tbnw=142&start=0&ndsp=28&ved=1t:429,r:12,s:0,i:115&tx=62&ty=50

#11

Use them lightly. Better a couple of strokes lighthanded, than 1 heavy stroke.

As for the bubbles: It is possible that humidity plays a role here. Epoxy + water = bubbles.

Also, looking at the datasheet, I guess the epoxy that you tested still is in a brittle B-stage. I would toss a test sample in the oven at 80 degrees C for at least 12 hours, preferably 18 or 24 hours. This will dramatically change the properties.

The white substance is normal. The thin and slow hardener corrodes with air. It is non-reversible. Keep the pots closed at all times.

#12

Could it be possible that the hardener is not compatible to the Epoxy? If this is the case, will a normal good epoxy attach to the one I have (assuming the hardener i have is not compatible)

#13

The epoxy does look over worked a bristle roller should give better results but you will need to be careful not to pick up any loose ends of stitching when you roll.

Are you wetting out both sides of the cloth or just wetting out from one side? With Quad cloths sometimes it helps to lay in place then fold back half the cloth and wet out the underside then fold back in position fold back into correct position then do the same on the other half then go on to wet out the topside.

However I don’t think that is the main problem best guess I would say this looks hardener related wrong hardener or wrong mix ratio you need to be very accurate here close to the gram. I note the mixing ratio is given for both weight and volume could these amounts have been mixed up?

Also look at how your carbon cloth is stored moisture in the cloth can be indicated by milky looking patches when cured. Be careful what you roll the fabric out onto, fresh plastic is best. The cloth will pick up contaminates from benches oils grease etc from the wood. Perhaps he first meter off the roll was a little damp or contaminated.

You really should think about using wet lay up and vacuum bagging for your project.

#14

Not likely. I just think it is a brittle B-stage. If you toss a piece in the oven, and try to destroy it afterwards, we know more.

In response to WiMa:

The test looks like woven cloth to me.

#15

Would it be beneficial to use peel ply? Is so, how is the best way to use/apply it?

Thanks.

#16

herman

“The test looks like woven cloth to me.”

yes I agree

#17

Yes that is right, the pictures showed are of a woven cloth. We just presently tried a test sample with the quadriaxial carbon, one piece with peel ply, and another with out. The peel ply was just placed and rolled on the wetted quadriaxial. We did not add further epoxy on top of the peel ply as it already seemed wetted out. Do you recommend using peel ply for better bonding? This is a hand laminating job (no vacuum bagging)

#18

These are some samples we made using Epoxy and quadriaxial carbon 400g/m2 and biaxial glass at 600g/m2. All samples were layed on a flat surface. The smaller square samples consist of:

1st one from left (square): 1 layer ±45 degre 600g biaxial glass and 1 layer 0 - 90 degree 600g biaxial glass (total 1.2g/m2)

2nd one from left (square): 2 layers of 400g/m2 quadriaxial carbon (800g/m2 total) and peel ply on surface.

3rd piece from the left: 1 layer of 400g/m2 quadriaxial with peel ply on top for smoother finish.

4th piece from the left: 1 layer of 400g/m2 quadriaxial without peel ply.

The others are single layer samples of biaxial glass.

One must note that the samples are rotated from lay up position to show flat bottom part.

One notices that the quadriaxial carbon tend to leave marks and patterns (air bubbles perhaps?) where the epoxy did not seem to penetrate. I wetted the “bed” in epoxy before applying these layers then added more epoxy by brush then rolled for a few times each. The biaxial fiberglass does not to have this problem for some reason as seen from the first sample in the left.

The quadriaxial carbon is much stronger, stiffer and lighter then the biaxial glass used (to be expected) but there seems to be air bubbles when using the quadriaxial carbon. One also can note that the one with peel ply (3rd from left) caused less “bubbles” than the sample on the right, which was without peelply. However the second sample from left which has 2 layers of carbon still yielded these bubbles and peel ply was used too.

These samples have not been post cured yet.

Any comments will be greatly appreciated.

#19

That is just a matter of technique.

As for peel strength: postcure first.

#20

Some epoxy hardeners and resins need to be postcured before demoulding. We had similiar problem (brittle parts) with less known epoxy resins and hardeners.