fire in our autoclave. Why?

[CompSource]

Recently we had a fire in our autoclave during a 120°C/180°C stepcure.to cure hexply. The cure cycle is more or less according to the product data sheet of Hexcel and has been used for quite some years now. As far as I could determine all these years the bagging materials have not changed. The autoclave is, unfortunately, filled with compressed air. (Few weeks ago we decided to change to N2 in the near future).
As far as I could examine no foreign objects has been put in the autoclave during the cure cycle.

I believe a fire requires three conditions:
1. Oxygen
2. Fuel
3. Ignition

We have oxygen but what about the fuel? Gasses of the some bagging materials? Gasses of the epoxy? Gasses of Airweave? (The mould is positioned on a steel rack. To protect the vacuum bag some polyester Airweave is between the vacuum bag and the rack. This means that the Airweave is positioned in the free air)

How about the ignition? Static electricity??

I hardly believe that all this is really the case because the process is running for some years know and nothing has changed. This means that I really do not have any idea. This is the first time that I have a fire during an autoclave process.

I hope that someone can help me.

 

[MikeHalloran]

This may not be a strictly technical problem.

Who stands to gain from the change to N2? I.e., was it an unsolicited suggestion, the virtue of which _just_happened_to_be_ demonstrated most vividly after the suggestion was rebuffed? I hate to say it, but I have seen this happen a couple of times.

You need to review the charts from your autoclave's process logger, and the surveillance videos from the vicinity.



Mike Halloran
Pembroke Pines, FL, USA

 

[Joshmo]

When curing large quantities of partiuclar resins they can exotherm but I don't think they would actually catch on fire. I think such exotherming is associated with cyanate esters and perhaps bmi's, not epoxies.

 

[Compositepro]

Exotherm of curing resins is a very real hazard. Most aerospace resins are highly cross-linked for high temperature resistance. This means that there is a lot of energy released during cure. For this reason cures are generally done slowly over a period of hours. Many aerospace epoxy resins will catch fire if they exotherm. I know people who have been killed by exotherms. The real danger is in working with pure resins. Prepregs are usually less than 40% resin and this limits the peak exotherm temperature significantly because of the heat capacity of the fiber.

The factors that will cause an exotherm are:
Too fast a temperature ramp to cure temperature.
Too thick of a part.
Resin bleed that pools.

Nitrogen will not stop an exotherm but it will prevent bagging materials from contributing to a fire.

Exotherms are most common in formulation labs where new formulations are not yet fully understood. 10 grams of resin exotherming will generate so such noxious smoke that the lab must be evacuated for several hours.

 

[PanelGuy]

I have done this before. We had a 40 g sample of a prepreg film in the oven below cure temp and it started to smoke. We evacuated the plant after cutting power to the lab. Everyone was fine, smoke everywhere. Best guesses were:

1 - Prepreg beyond life cycle developed hot spot and exothermed
2 - Thermal controller overran and created an exotherm.

No log so cannot prove or disprove. I know that when we were taken in that the 4"x4" x 5 ply resin film sample had intumesced to a black loaf of 12x12x4 and tar was running out of the oven and on to the lab floor.

Fire takes, fuel, energy and oxygen. Without the later you could very easily get a nice smoldering fire that would back draft when you opened the door. Pressurizing with air...hey guess what...

All it takes is heat and an exotherm can run up. Once the autoignition temperature of something nearby is reached...away you go. Happens all the time in POlyester lay-up facilities...well not all the time, but more than you would like to know.

 

[RPstress]

A possible factor is freshness of the resin. I have been told of development work being done with resin near the end of its outlife, and then when the production runs started in earnest, prepreg with a fresh batch of resin was used which exothermed more and caught fire. They apparently had two autoclave fires before they were able to tweak the cure cycle with appropriate dwells and heat up rates to control the exotherm.

 

[CompSource]

All of you thanks for the suggestions.
I do not believe it is exothermic reaction. The laminate thickness is thin. I have experience with exothermic reaction of epoxy resins because I have manufactured parts with a thickness of 6 (2.3 inches) till 9 cm (3.5 inches ). To exclude the exothermal reaction we could perform some DSC tests to see if there is an exothermic reaction around the moment of fire.

Has any of you experience in fire due to airweave in an autoclave? I do not mean the airweave in the vacuum bag but airweave in the free air of the autoclave? Could this be the start of the fire?

Regards,

Bert

 

[CompoMan]

Do you carry out QA test on the supplied prepregs. Sometime the resins/hardeners/catalysts may not be correct i.e not well mixed or dispersed. For a catalysed system (which I guess your hexply system probably is), localised high concentration of catalyst may create excessive heat especailly if the initiation temperature of the catalyst is within the range of your ramp up temperature.