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AlgoFlash (Aeronautics) (OP)
9 Aug 11 10:44
Hello,

I am using M-Foam corecell with prepreg to create a small sandwich but the corecell doesn't bond to the prepreg at all. I am using a press at a temperature slightly above 100C, which shouldn't be a problem according to what I was told my the corecell seller.

Has anybody got any clue on what could be wrong here?
Thank you for your help.
SWComposites (Aerospace)
9 Aug 11 11:36
You probably need an adhesive layer between the core and prepreg.  Even "self-adhesive" prepregs only have so-so bonding capability.

What specific prepreg are you using?

 
AlgoFlash (Aeronautics) (OP)
9 Aug 11 12:04
Actually what I forgot to say is that it used to bond perfectly just one month ago.

It's a carbon/epoxy Prepreg from Newport that I have been using since june. It was given to me by a teacher but he can't give me the reference so that's all I have... I don't think it is too old because it was left at 0F since January and wasn't used before that.

http://www.newportad.com/newport_woven.html

Thank you for your help, I might have been lucky with the first sandwiches I made and should add an adhesive now...

  
berkshire (Aeronautics)
10 Aug 11 0:34
Have you checked the basics:
1 Ensured that the foam is free of any dust or other surface contamination.?
 2 Checked that the pre-preg has reached room temperature before peeling the plastic back and exposing the resin.

 You say it has been stored at 0 degrees and it was not used before that. Was it purchased new, or was it older material that was donated to a school? Is there an out time log with it?
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them.  Old professor

AlgoFlash (Aeronautics) (OP)
10 Aug 11 9:12
I cleaned the surface of the foam with vacuum because it had small particles on it after machining, but I admit I didn't check if prepreg had reached RT before exposing the resin. It stayed at RT for maybe 10-15 minutes before I peeled the plastic, I wonder if this is enough.

This material was donated to school but there is no out time log with it. My teacher told me that it was little used (is that correct english?) but we have no log at all.

What makes me wonder is the fact that I used it one month ago and left it at 0F after that for the whole month. And between the samples made one month ago and those samples, there is a HUGE difference. No bonding at all.

I am considering adding an adhesive thanks to SWComposites, and I will make sure that it stays at RT for longer thanks to berkshire. But I feel that what I am experiencing is too extreme for those changes to have an effect (but hey I'm no expert).

Thanks a lot guys!
Compositepro (Chemical)
10 Aug 11 16:54
Is the prepreg tacky and drape-able? If not it has probably gone bad. Warm it with a blow dryer and if it becomes tacky and drape-able it might still be useable. I've also seen inexperienced people peel the poly-film liner from structural film adhesives and throw the adhesive away,thinking the poly-film was the adhesive.

Are you sure the press is actually applying pressure? You cannot use much pressure on foam and the press may not be controllable at such low pressures.  
ESPcomposites (Aerospace)
10 Aug 11 16:57
As SW stated, you should use an adhesive regardless.

How does the prepreg feel? Does it feel stiffer and with less tack than it did a month ago? Universities often get material that has technically been expired, but still has use...but only for a little while.

We use to give away material to the university because it "expired". It was actually cheaper that way. Otherwise you would have to cure it before disposal.

Brian
www.espcomposites.com

AlgoFlash (Aeronautics) (OP)
11 Aug 11 9:48
First, let me thank you very much for your comments.

I don't feel that the prepreg has changed since last month, but I may not be paying enough attention. I haven't bought any adhesive yet since the prepreg is probably expired as mentioned in several answers.
The press is applying pressure but you are right, maybe not enough. I don't think the pressure has changed since last month though.

As for the prepreg being expired it seems the most likely but I am shocked how extreme the shift was between unexpired and expired material. The bonding went from really good with a customized peel test to no bonding at all.

I have another question that was raised by berkshire's question about peeling the plastic back: this particular prepreg wasn't placed in a closed bag in the freezer whereas all the time I have used prepreg it was placed in a sealed bag. Do all prepregs have to be placed in sealed bags once in the freezer?

Thanks again.
  
allcomposites (Materials)
11 Aug 11 11:05
You can use FM 300 adhesive layer from Cytec between foam and prepreg.
 
rerig (Aerospace)
11 Aug 11 11:25
"Do all prepregs have to be placed in sealed bags once in the freezer?"
Yes. Prepreg MUST be sealed in a bag prior to re-freezing.
It Must be thawed to Room Temp prior to opening the sealed bag for use.

There are two clocks that must be tracked.
Shelf Life - Total life of a perishable item. (ie. 24 Months below x Deg)
Shop Life - Accumulated of Out of Freezer times.  (ie 10 days = 240 Hrs) Usually the first 4 hours of out time don't count, but I feel you are not the first to thaw the material.

My company also donates expired materials to the Uni. It can't be used any further for aircraft repairs, but can still give students an idea how to work with the material. (Adhesives & prepregs) We include the Certification & testing data for their interest, but we do note the material is expired for actual aircraft use.

Rerig
 
SWComposites (Aerospace)
11 Aug 11 11:27
Do all prepregs have to be placed in sealed bags once in the freezer?

its a darn good idea to keep it in bags.  if the prepreg was stored in the freezer without being in a bag, it may have absorbed moisture during that time.  also, when it was removed to room temp, moisture likely condensed on the surface of the prepreg and was absorbed intot he surface.  moisture in a bondline is a very good way of making poor bond.

 
AlgoFlash (Aeronautics) (OP)
11 Aug 11 11:47
Well ok, I guess you have found what was wrong with that prepreg.

I keep track of its out time but I have one more question about it: how do shelf and shop life cumulate with each other (if they do cumulate at all)?
 
SWComposites (Aerospace)
11 Aug 11 12:20
how do shelf and shop life cumulate with each other (if they do cumulate at all)?

they don't for most practical purposes.  freezer life is much, much longer than room temp (shop) life
 
Compositepro (Chemical)
11 Aug 11 13:07
Moisture is not necessarily the "kiss of death" for prepregs. Freezers, themselves are very dry environments. It is the sweating on cold surfaces after coming out of the freezer that causes moisture problems. Phenolic prepregs even release water as part of the cure process. I've found that moisture is often erroneously blamed for composite processing problems, and it becomes a 'red herring" which prevents identifying the true problem.
rerig (Aerospace)
12 Aug 11 15:44
Shelf Life is the Total "Drop Dead" life of the material if stored correctly.
Shop Life is the Accumulation of "Out Times" that the material has been out of the storage environment.

They don't interact with each other except for storage requirements.

Example: I am going to die in xx years if not exposed to radiation.
If accumulated exposure to radiation exceeds xxx, I die sooner.

Attachment is from a Major OEM specification.

Rerig
etchac (Aeronautics)
16 Aug 11 14:55
I also face problem of delamination between foam (corecell) and prepreg. I believe that delamination problem came from contamination. I would like to fing a way to clean foam before wrapping with prepreg. During machining, chip of foam are create. I would like to eliminate those foam chip by a chemical way. Can you suggest me products to use for cleaning (surface preparation) and avoid chemical reaction with the foam.  Can you gieve me some ideas?
etchac
berkshire (Aeronautics)
16 Aug 11 15:16
etchac
It would be better if you would start a new subject in this forum and red flag this post, instead of piggybacking onto this question.
 Also corecell is a brand name are you using that particular brand of foam?
Please start again.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them.  Old professor

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