Phenolic oven cure 1

[Anster25r9er]

I am working on an oven cure with phenolic resin and silica. I am running into trouble since phenolic resin is a condensation cure and am experiencing bubbles in the final cure. For the green state cure a vacuum bag is being used and then it has been taken off for the post cure. Does anyone have any suggestions on the green state cure to release the water and volatiles better? Also should the vacuum be maintained during the post cure? Mainly what is the best way to remove the volatiles to avoid bubbles and de-lamination?

 

[blakmax]

Oh dear. Cure of any condensation polymerising material under vacuum only will risk extensive voiding, and the same is true for addition polymerising materials which have been exposed to conditions where moisture is absorbed prior to cure. Firstly, do not fall for the age-old suggestion that applying a high vacuum will draw out the volatiles. All a high vacuum does is to reduce the local pressure which will INCREASE the size of the voids. You really need to look for positive pressure methods (mechanical clamping, weights etc.) maybe in conjunction with the intelligent use of vacuum. One measure we have employed for addition polymerising materials where there is a risk of moisture absorption is to hold high vacuum until the resin starts to gel, so that whatever volatiles that can be captured by escaping have the opportunity to do so. Then, just prior to gel, reduce the vacuum to a nominal level to maintain contact with the mould/surface. This allows the void size to reduce as the local pressure increases.

Hope this helps.

Blakmax

 

[materialsliz]

We cure phenolic glass prepreg in the oven under full vac (28 in Hg) during the entire cure, and also use bleeder and perforated release on the top and bottom of panels to draw away volatiles and resin. We don't use mechanical means to apply pressure, we use the vac. We haven't experienced any problems with voids or porosity in the laminates. If we need to post cure we demold and do a free air cure.

 

[Anster25r9er]

Thanks for the replies. We have some panels in the oven right now and we will see how they turn out.

 

[Compositepro]

Materialsliz, what is the part you are making? Under those cure conditions I suspect that your part is fairly porous but the pore size is microscopic. This type of porosity can be very useful in improving the fire resistance and ablative properties of phenolic composites because it allows gases to escape without blowing the plies apart. What is the finished resin content?

 

[materialsliz]

Compositepro, we do a variety. Some are honeycomb panels, others are solid laminates. They are interior panels so I'm sure there were flammability requirements that were proven out by our customer. All of them are build to print so we follow the customer's cure requirements. We haven't done resin content analysis on these panels.

 

[blakmax]

OK, my comments above were relevant for structural applications where micro-voids may significantly reduce bending and shear strength of solid laminates. Another example of significant strength loss due to micro-voids is for adhesive bonds, where peel strength losses exceeding 50% have been reported.

If it is not structural then who cares?

Regards

Blakmax

 

[Compositepro]

Well, rocket nozzles are somewhat structural. Large inter-laminar voids are unacceptable in any kind of part. The OP is about phenolic/silica which indicates it might be an ablative application. In that case the cure cycle and bagging procedure mentioned by materialsliz is probably very appropriate. The perforated release film will allow resin bleed which will remove all large bubbles around the perforation. It just will not prevent micro-porosity. But this can be essential to the proper performance of the composite.

 

[Anster25r9er]

Materialsliz,

I was wondering if you could share your cure temperature profile?

 

[materialsliz]

Pull full vac (25 in Hg min). Load part in oven. Heat part to 340-370F at about 4F/min based on part TC's. Hold for 120 minutes min at cure temp. Cool at 8F/min max to under 150F. Full vac on the entire time.

A key feature of the fab is on the top and bottom of the panels we place a layer each of dry glass, perforated FEP, and peel ply in order to draw out the volatiles. If we don't have these we will get porosity in the parts.