Naked Science Forum
General Science => General Science => Topic started by: Sciencem9 on 31/05/2017 14:50:20
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Hi there,
I currently work for a freezer rental company and I'm creating a report for my manager to help us understand why our customers need to store carbon fibre at low temperatures.
Is the temperature requirement something that's only essential during the storage stage or is it required during the manufacturing process as well?
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What they are most likely doing is storing uncured composite. This is carbon fibre inpregnated with resin but uncured - they might refer to it as prepreg. In this state the composite can be handled, cut and shaped before final curing. If it is stored at room temp it can have an 'outlife' of a few weeks, but if kept in a freezer at -20C it can be as long as a year - they might call this the freezer or shelf life.
Once the composite has been cured it no longer needs to be kept cold.
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Is the temperature requirement something that's only essential during the storage stage or is it required during the manufacturing process as well?
Carbon-fibre threads are weaved into cloth to make it more manageable, then manufacturers of products cut the cloth to the shapes they need and put it into moulds. There are three main approaches to how you do things though, two of them using dry cloth and one using cloth that already has resin all through it (the "prepreg" kind which needs to be stored at low temperature to stop the resin hardening).
One method is to stick dry cloth into a mould and then apply epoxy resin to it with a brush, being careful to wet the whole thing out. You then enclose the thing in plastic (sheeting) and suck all the air out with a vacuum pump to compress the cloth and squeeze as much resin out of it as possible in order to minimise the weight of resin left in it. This method is the least popular as it puts carcinogenic fumes into the air, is messy, slow, labour intensive, and it wasteful of resin. It can be used for test pieces though, and it's sometimes necessary to make additions to a piece after the bulk of the thing has already been made using a better method.
Another method is infusion, and again this starts with dry cloth. The cloth is put in the mould, then it's sealed in under plastic as before and the vacuum pump is attached. The resin is then sucked through the cloth by the vacuum. This takes careful planning to make sure resin is sucked into every part of the cloth to wet it out fully, and a lot of pieces may be ruined if it fails to get into every crucial section, while cosmetic surface imperfections can also occur if the infusion is incomplete, which isn't acceptable for the outside of things that are meant to look beautiful, like sports cars. Thick layers of carbon cloth often don't infuse well, particularly if a lot of unidirectional cloth is used (with most of the fibres running in the same direction), so it isn't always possible to build what you want to with this method.
The preferred method in most serious production uses prepreg cloth with the resin already all through it. With the other two methods, the resin is mixed with a hardener just before it's used and that puts a time limit on how long you have to get it into the cloth and to apply the vacuum before it reacts and turns solid, but you can store the cloth forever at room temperature. With prepreg though, the resin in the cloth has already been mixed with a hardener, but it won't react at low temperature (or will only do so extremely slowly), so you keep it refrigerated. It's taken out into the warm just before use, removed from the packaging, cut to shape and put in the mould, then it gets bagged up (enclosed in plastic sheet) and the vacuum is applied as normal to remove excess resin. An autoclave is used too so that the pressure on the outside of the "bag" can be increased to squeeze the cloth much more tightly and get more resin out of it - this leads to lighter products which are no less strong. The vacuum pump allows one atmosphere of pressure to be applied without an autoclave, but that pressure will vary depending on the weather. An autoclave allows several atmospheres of pressure to be applied and with no differences in the end product caused by weather, although the variation in the weight of the produced parts can still vary by as much as ten percent. (An autoclave can also be used with the other methods.)
In all three cases, once the resin starts to react and harden, it generates heat, and there's no need to try to remove that heat by cooling - heat can be added in autoclaves to speed up the reaction, but there are also ideal temperatures for curing the product in order to maximise the strength of the material, and that varies depending on the kind of resin used, so it may need to be kept at very high temperature for 24 hours.
For you, I suspect that the only part of this process that you need to worry about is the storage of prepreg cloth while it's still in its packaging, unless something's being built that's so complex and takes so long to lay the cloth in the mould that it needs to be put together in cold conditions to prevent parts of it from reacting before it's all bagged up and in the autoclave with the vacuum attached, in which case people might need to work in a freezer room, but I haven't ever heard of anyone actually doing that.