Phenolic Composites Manufacturing - Autoclave Cure

[7twofour]

We are currently evaluating the possibility of curing a phenolic-carbon and phenolic-aramid hybrid laminate in an autoclave versus the ovens that we typically run phenolic parts in.

I have heard of horror stories of the phenolic gasing causing vent lines and equipment to "clog up" over time needing replacement. I have also heard that there are certain techniques/equipment that can alleviate the issue. We don't seem have this issue with our oven cured phenolics.

Does anybody have any experience curing phenolic materials in an autoclave and can shed some light on the subject?

Thanks in advance!

 

[Compositepro]

Phenolic prepregs contain residual solvents for tack improvement and release water during cure. Some contain free phenol or formaldehyde which are some what volatile. This can create problems in vacuum systems. However, if you don't have problems with oven cure I don't see why an autocalve would cause problems unless you are not using vacuum-bags in you oven.

What do you expect to achieve by switching from oven to autoclave?

 

[7twofour]

Thanks for the quick response Compositepro.

The specific phenolic-carbon prepreg material we are purchasing hasn't been cured in the oven at our facility but we do a high volume of phenolic-aramid and phenolic-polyesters all under vacuum bags.

This is a new design part that the customer would prefer us to switch our epoxy-aramid/carbon autoclave cured design to a phenolic-aramid/carbon and retain the autoclave cure (due to toxicity/flamability/etc). Although we don't feel that the autoclave cure is necessary as this is an interior non-structural part, we still want to comply with the customer request but want to adjust as necessary given any additional issues the phenolic autoclave cure may bring.

Also, for the autoclave cure we are going to be investing in permanent bagging since this is a high turn part. Do you see this causing any issues?

Thanks!

 

[Compositepro]

I have no experience with oven cured phenolics but I would gues that the porostity level is relatively high. This can be good for flame resistance because to porosity allows the gasses generated by pyrolysis to escape rather than blistering and delaminating the composite.

In autoclave cured phenolics the condensation products (e.g., water) are usually trapped in composite and will slowly dry out after cure. You have to be very careful with post cures done soon after initial cure because the steam pressure can blow parts apart.

Reuseable vacuum bags will be a challenge. Phenolic resins are very corrosive to most bagging materials like nylon and silicone. You will need to use teflon barrier films to protect your bagging films. The fumes of phenolic resin can penetrate barrier films and attack bagging films after several cycles.

 

[avior]

We are curing phenolic/aramid prepreg parts in oven under vacuum and autoclave under 15, 30 and 45 psi pressure since 2003 and glass phenolic since 2006. No important problems occurred. Few bags puncture from time to time due to bridging or vacuum ports sliding during manipulation. Reusable silicone bags never puncture but need regular cleaning; especially the seal should be free of breather residue. We used phenolic hybrid glass-carbon reinforced also without any mentionable issues. About autoclave maintenance . . . the same as ovens used to cure phenolic prepregs, maybe you should pay more attention to vacuum ports fittings and filters, - periodic cleaning rather than vacuum pipe system.

 

[blakmax]

My understanding is that phenolics are condensation polymerisers not addition polymerisers. In addition polymerisers such as epoxies, A + B =C and there is no byproduct, whereas for condensation polymerisers A+B = C +D where D is a byproduct which is often water. That water turns to steam at the cure temperature and that leads to the high void content. It may also explain why excessive flow occurs. Contrary to popular belief, vacuum does not necessarily draw out such volatiles. Often, the low pressure causes the bubbles of volatiles to expand and the resin, not the volatiles is pushed out. I would expect that laminates cured in an oven under vacuum would have a very high void content. A micro-section of a sample should be used to assess the void content. Don't rely on visual inspection fo the surface because if the volatiles can reach the surface, the voids break out to vacuum and dissapear. It is those voids which do not make it to the surface that cause the problem.

Void content may be reduced by increased autoclave pressure to counteract the vapour pressure of the volatiles.

 

[7twofour]

Thank you all for your input and explanation on the subject.

We do replace/maintain our vacuum system filters/fittings/etc regularly so it seems that we shouldn't have any issues more than what we deal with now.

Again, thank you!