Curing phenolic resin in conventional oven

[NurS]

Hi. I'm making some phenolic resol resin but the problem is, it generates a lot of voids. The water has been eliminated under reduced pressure but still when I cure it using oven, I got highly porous materials. Is there anyone can share tips, cure cycle or processing conditions that I can try on? I even tried the slow cure which took 4 days in the oven but failed! The samples came out very fragile and cracked! I did in autoclave too at 7 bar pressure but still got bubbles in it. Please anyone can help me, I'm dying to get good samples. Thank you.

 

[Compositepro]

Phenolic resins contain water as a solvent, but they are also condensation curing polymers.This means that the curing reaction creates water. To cure phenolic without pressure requires first removing air bubbles (or air trapped between plies) and then gelling the resin below 100C, before curing at higher temperatures. Tackless prepreg will help avoid trapping air, but that is generally not practical for contoured parts. Air bubbles will be greatly expanded by the presence of water.

 

[NurS]

Hi Compositepro, thank you for your reply. Can you explain how to remove the air bubbles? First let me explain how I did it. I made this resol resin by mixing everything in round bottom flask, heat it at 70ºC for an hour, and stir it mechanically. Then neutralised it before water elimination in rotavapor. After that, mix it with resorcinol at 50ºC/0.5h then cure it in the oven with this cure cycle (40ºC C/0.5h,50ºC/1 h,85ºC/1.5 h,125ºC/2 h,150ºC/2 h). And I put this resin in petri dish as it just contains around 10-15g. Where do you think I go wrong? Thank you in advance.

 

[Compositepro]

I assumed you were making a fiber reinforced composite panel. This makes it much simpler. You might need to vacuum degass after mixing in resorcinol and pouring into your dish. Again, the key is to sufficiently gel the resin before going above 100C, so that the vapor pressure of water cannot create steam bubbles. The water of cure will be trapped in the resin and will slowly diffuse out of the solidified resin. This does result in shrinkage of the resin, which can cause voids if the shrinkage is restrained by the mold. Your cure cycle steps from 85C to 125C. this is not good. You should have a dwell at 100C or slightly lower. How long you need to dwell at 100C depends on the reactivity of the resin. But it is very simple to visually verify when bubbles form.

Are you using a forced convection oven, which has a fan to circulate air inside? Many lab ovens are gravity convection with no fan. These are okay for keeping small samples at a constant temperature, but perform poorly in quickly heating samples to the target temperature.

 

[berkshire]

If you can find a copy , "Phenolic liquid resin, Hand layup techniques." by John A Wills Copyright TXU 753-568 . covers most of the problems and questions you are now asking. here is a link to the product on Amazon

http://www.turnshops.com/Phenolic-Liquid-Resin-Hand-Lay-UP-Techniques/

.
B.E.

You are judged not by what you know, but by what you can do.

 

[NurS]

Thank you Compositepro for the suggestion. Right now, I'm trying to make resins and figure out the optimum formulation before incorporating fibre. I'll modify the cure cycles. If we vacuum degass the mixture, is that fine? as I assume this is condensation process which yield water as byproduct...... And do you think it is necessary to use rotavapor?

Hi Berkshire, I couldn't find the book. Do you have the copy of it? Thanks in advance.

 

[berkshire]

Nurs,
I do have a copy That I bought from John a few months before he died.
B.E.

You are judged not by what you know, but by what you can do.

 

[Compositepro]

If there are any solvents in the resin, you do want to rotovap it to remove them.

 

[NurS]

Berkshire: Really? Well I guess I couldn't get it from you right, shame on me then. By the way thanks mate!

Compositepro: Just to make sure, I used liquified phenol, formaldehyde, sodium hydroxide, and hydrochloric acid (for neutralisation). There is no solvent, right? Thanks!

Thank you so much everyone. Need to work out what's best as been spending 10 months tryin' to produce good resin!

 

[Compositepro]

Water is your solvent in this case. Phenolic prepregs are often dried/B-staged to a target volatiles level. This is tested by weight loss of a sample after some time in an oven. About 4% weight loss is to be expected due to the condensation reaction and residual solvent removal.

How many books have you read about phenolic resins in those 10 months? Phenolics are one of the first plastics used commercially. There has been a lot published about them but most of the literature is well over 50 years old. The old books tend to be very descriptive compared to those of today. You may need to go to a research library to find them.

Phenolic parts are most often made by compression molding using over 1000 psi. Good parts are pretty easy to make this way. Non-pressurized cure is much more challenging.

 

[berkshire]

NurS
Since you seem to be in the UK and I am in the USA that would be a little difficult.
Since this resin originated in Europe , you might see what you can find on Bakelite
Check out Blagden Cellobond ( formerly B-P performance Plastics) Sully South Glamorgan CF645YU They used to sell Phenolic resins.
They may also offer support.
B.E.

You are judged not by what you know, but by what you can do.