Why are carbon fiber composites baked

[Quilbilly]

I am wondering why epoxy/carbon fiber composites are so often baked to cure as opposed to regular, like WEST System, room temperature curing. Does this mean one cannot use carbon fiberand epoxy to strengthen structures as effectively unless they are bake cured- Todd

[Ross M]

"I am wondering why epoxy/carbon fiber composites are so often baked to cure as opposed to regular, like WEST System, room temperature curing."

Primarily so that they can use pre-impregnated cloth, helpful for achieving consistent saturation levels with minimal weight.

Secondly, some use baked resin systems to improve high temperature performance.

"Does this mean one cannot use carbon fiberand epoxy to strengthen structures as effectively unless they are bake cured"

Not at all - but you might want to be careful. It is pretty easy to end up with something that acts like two independent structures, neither of which performs as expected.

The search thingy upper right is wonderful. Also peruse this thread: Woodenboat Forum FAQ (http://media5.hypernet.com/cgi-bin/UBB/ultimatebb.cgi?ubb=get_topic&f=1&t=008298&p=)

Ross

[bainbridgeisland]

1. There are literally thousands of epoxy compounds each with different properties. Only a minority will cure at room temperature. Thus your choice of cured properties is limited.

2. Larger epoxy molecules tend to produce better mechanical properties but need higher temperature to cure and are also more viscous at room temperature. They need elevated temperature to obtain a viscosity that can be processed with carbon fiber. Thus, if you want the best mechanical properties, elevated temperature cure is usually needed.

3. Higher temperature cured epoxies usually have higher service temperature. Many room temperature cured epoxies become much less effective about 140 degrees F. A dark painted deck or mast can exceed 140 F on a sunny day.

You can use room temperature cured epoxy with hand lay-up processing with carbon fiber but the mechanical properties will be significantly less than optimum. Even so, there are good places to use such material on modern wooden boats.

For example, large wooden boats use diagonal strapping to prevent hogging. This has been traditionally accomplished with bronze straps fastened to the planking. However, modern laminated wood boats have the option to use carbon fiber, laminated in place. Though carbon fiber is much more expensive than bronze per pound, the labor savings more than makes up for the materials cost. Here is an real-world example of labor expended: Carbon fiber strapping just before the last layer of planking = 2 Men, 5 hrs; Letting in bronze strapping and fastening it in place = 2 Men, 10 hours.

[ 07-20-2004, 01:28 PM: Message edited by: bainbridgeisland ]

[John E Hardiman]

As Ross pointed out, the strength of carbon fiber parts is very sensitive to resin content. Additionally, it is also extremely sensitive to fiber direction (i.e. 5 degrees off axis causes a 50% reduction in strength for some parts using unidirectional fibers). The use of pre-pregs that can be accurately positioned and then are then heat/pressure cured minimizes these problems.

Carbon fiber is one of the most mis-used modern materials IMHO.

[paladin]

when baked there is less oil than fried....

[John Meachen]

Perhaps a controversial subject to be discussing on the Woodenboat forum!As has been mentioned above,there are many different resin formulations and quite a number of types of carbon as well as a number of different weaves.The aerospace variety are prepregs and come from the factory with the resin already applied.The rolls of material are kept in freezers to prevent a cure taking place and are returned to cold storage as soon as the necessary pieces for the component have been cut.The material is applied to the mould and consolidated with a vacuum bag and subject to no leaks being found,is then cooked in an autoclave at a pressure that might be ninety psi and following the temperature gradients and dwells that the resin supplier specifies.
For conventional wet layups,a cure will take place but if you can determine the appropriate post cure treatment the physical properties of the laminate will be improved.A similar thing happens with ordinary poyester/glass layups-most of us have heard laminates described as "green" and this is the state they remain in until a full cure has taken place.A post cure treatment accelerates the cure and enhances the mechanical properties but I have never heard of a boat being post cured.

[George Roberts]

John Meachen ---

I have always been an advocate of post curing - often a black plastic wrap and some sunshine is enough.

I think there was a small piece on post curing in Wooden Boat a couple years ago. A large boat, West Epoxy, tent and heaters, and the necessary test core.

As for carbon fiber: It is unlikely that handlaid carbon fiber/room temp epoxy will be stronger than plain glass/room temp epoxy. Reasons follow.

Carbon has a different density than glass, so 6.0oz of carbon fiber with epoxy will weigh the same as 7.5oz of glass with epoxy. So comparisions are often a bit biased.

Carbon is more difficult to fit to curves because it is stiffer.

[John Meachen]

George,I have no doubt that your efforts to achieve a post cure will have improved the properties of the moulding.It would be good if everybody were able to do the same.I have to disagree with you about the relative densities of carbon and glass and their effect on strength.The weight of the material is just that-weight per unit area and the same weight of carbon will be much stiffer and stronger than its glass counterpart.On this side of the atlantic we use grams per square metre and given the difference you remind us of,300 gram/sq.metre carbon will produce a stiffer laminate that 300 gram/sq.metre glass.For wet layup work it is probaly safe to assume that both materials will require their own weight again in epoxy resin.This gives a fifty percent resin content and is subject to human error.Modern prepregs use less than 45% resin and it is evenly distributed which helps the performance of the moulded structure to behave as the calculations say it should.As I mentioned in my earlier post this is perhaps not the ideal arena in which to be discussing the materials but it is only in the last couple of decades that composite structures have managed to overtake the physical properties of nature's own composite wood.

[George Roberts]

John Meachen ---

(I sure hope I get these numbers right.)

On this side of the ocean we use 32% by VOLUME for plain weave. That is 50% epoxy by WEIGHT for glass and 60% epoxy by WEIGHT for graphite.

And a lot of experience helps one to lay carbon fiber better.

[Dan McCosh]

There are better answers above, but the main thing is that the issue isn't the carbon fiber, but the epoxies used that need the higher temperature to cure properly.