Yafmot
April 21st, 2008, 07:26 AM
Recently, in a post in the Pyro section, I broached the Idea of a thread devoted to advanced materials and processes, primarily geared toward composites. Bacon suggested I put it here (I think he's chomping at the bit to make some composite rocket motor casings), so here goes...
We're going to start with Polymer Matrix, Fiber Reinforced Composites, since that's what most people seem interested in. Don't worry, we'll get to the metals and ceramics soon enough. I'm going to break it down into four areas...
FIBERS: Their manufacture, properties and applications.
MATRIX MATERIALS: similar discussions.
PROCESSING: Vacuum bagging, autoclave curing etc.
DESIGN CONSIDERATIONS: Because it's possible to turn $50K worth of perfectly good graphite into a perfectly useless chunk of shit.
FIBERS: The oldest modern structural fiber is glass. You may have noticed that most of the glass fiber manufacturing plants are in the southeast, or anywhere sand pits can be found, specifically near-pure fused silica. Such is commonly found in South Carolina, which is why a lot of the plants are located there. The raw material is literally just laying around.
To turn the sand to glass, it is melted in a furnace to a viscous, flowing form. It is then forced through a device called a spinaret, which is much like a shower nozzle. As it comes out the other side, it is elongated as it cools, so that when it solidifies, it is very thin. These then are usually treated with a sizing of some sort, frequently a Silane. This allows for better handling during subsequent processing, as well as improving the "grip" of whatever resin system is ultimately used. The fibers are then gathered into bundles called "tows," with the number of individual fibers bunched in multiples of a thousand, expressed in "K" (2K, 3K, 4K, 6K, 8, 12 & so on). The smaller ones are used more for woven cloth, while the bigger ones are generally used for tooling, pultrusion, and filament winding (which will be covered in the processing section).
(By the way, these furnaces are run continuously for up to five years between mainenance shutdowns.)
There are three basic grades of structural glass. "E" glass is the cheap stuff used for secondary or noncritical applications such as wingtips, fairings, hot tubs, shower pans and the like. This is the type of stuff one would use in a chopper gun, along with cheap polyester resin.
"S" glass is used for highly stressed areas such as landing gear attatch points, spar caps, and helicopter rotor blades that can survive 20mm cannon hits. It's quite expensive, even more so than many grades of graphite (carbon) fiber. But for some applications, it's hard to beat.
"S-2" glass fits between the other two, toward the high side. It can be used for most of the applications that S glass can, at substantially less cost.
Graphite (carbon) Fiber. There are many more grades of this than there are glass fibers, but there are two basic families, delineated by their precursor materials. They are Polyacrylonitrile, or PAN based, and pitch based.
The PAN precursor is basically just a really fine, pure grade of Rayon (or Orlon; same stuff).Tows of it are unwound in a heated, inert atmosphere and pyrolyzed under tension. the carbon remaining to form Graphene rings, and the tension beginning to align the rings to within 15 or so degrees of the longitudinal axis. This is your basic structural carbon fiber, with a tensile strength of about 120 KSI, and a tensile modulus of about 45 MSI. (I'll explain these properties some more in a moment.) the fiber has now been elongated by a factor of 10.
If you want a higher performance fiber, both stronger and stiffer, you repeat the process with the temperature bumped up to around 2,200F. You also up the tension, in order to elongate the fiber by another 10x. This aligns the graphene rings to within 6 degrees of the longitudinal axis. Now you're looking at about 300 KSI tensile strength, and about 200 MSI tensile modulus. Of course, these are just approximations, to illustrate the effects of various process parameters. By juggling them around, fibers with much higher strength and stiffness can be had. Just bring money.
The other precursor is Pitch, which is the black, gooey stuff from the bottom of a fractional petroleum distillation tower. We're talkin' sub-asphalt here. It's purified, to get rid of the various phenols and other species that can interfere with the continuity of a fiber. Then it's squirted through a spinaret into a hot, inert atmosphere, where it solidifies. From there, it's handled pretty much like PAN carbon, but at a much higher temperature and elongation. The finished fiber is generally used for very high end applications such as satellite chassis and highly stressed, dimensionally critical Carbon/Carbon composites (I'll cover these later).
There's another type of "black stuff" out there. It's amorphous Carbon fiber made by Nippon Graphite. It's not graphitic at all, though. All the Carbon atoms are randomly oriented, with no crystalline structure whatsoever. Structurally, its properties are roughly that of E glass, or slightly less. But it does have one very strange & possibly useful feature. On a demonstration I saw, there was a wedge drop aparratus that would release a 1 Kilo, wedge shaped weight 20 inches onto a bridged, 2x8 cm. specimen of unidirectional laminate of regular stuff with 1 or 2 plies 90 degrees to the length, just to keep it in one piece. the wedge just fell right through the thing, fracturing all but a couple of plies. When the demo was performed again, with an almost identical specimen, the wedge just bounced, and kept bouncing for a good 15 or 20 seconds. The only difference was ONE unidirectional ply of XN-05 amorphous carbon on the compression(!) side of the stack. I'm gonna' get some and try making a crossbow out of it. With that kind of rebound, it should be awesome!
SYNTHETIC FIBERS: The most well known of these is Kevlar, DuPont's brand of Aramid fiber. The term "Aramid" is a contraction of Aromatic Polyimid. It was first synthesized in the early '60s, for use in automobile tires. There are different grades of it depending primarily on the application. It's very tough, but not that stiff. Which is to say it'll make a hell of a kayak, but not much of a wing spar.
Of course, its antiballistic properties are legendary. the last time I worked with any was about a year ago, using some off the shelf Aramid/PVB-Phenolic spall liner as a backing for some Aluminum/Carbon vehicle armor I can't talk about.
It works with a wide range of polymeric matrix materials, but is somewhat limited in its high temperature capabilities; it starts to let go around 350F.
Spectra fiber: Originally developed by Allied Signal back in the '80s, it's an elongated, crosslinked polyethylene fiber that has an extremely high strength to weight ratio. Oddly enough, the stuff is crosslinked after it's elongated, unlike an elastomer, which is crosslinked beforehand. So instead of deforming and then rebounding to its original shape, the links help it resist deformation in the first place.
Unfortunately, there are enough problems to damn the stuff in most applications. For one thing, it is just polyethylene, the same basic stuff as a trash bag. That means its thermal properties are for shit. It starts to weaken around 240F, and by 260 or so it's just mush. That's why it was abandoned for firefighting vehicles in urban areas. In riot situations, the local scum will actually shoot at the firefighters. Their vehicles are frequently parked in close proximity to burning buildings, and the armor can easily reach 300F through radiant heating, instantly ruining the ballistic resistance.
Another problem is that PE is a polyolefin, and like most of these, it is formed in a melt-phase polymerization process. This means that, as it cools, the high molecular weight, crystalline material (the good stuff) tends to migrate towards the center of the fiber. Meanwhile, the low molecular weight material is rejected toward the surface in an amorphous state. This stuff has no shear strength, for all practical purposes. That means that the resin system, whatever it might be, has nothing to grab onto. You guys with some exposure to composites may be aware of interlaminar shear problems. well, this is an inTRAlaminar shear problem. It's exactly analogous to trying to pick up a block of ice on a hot day. Ice is strong stuff. Just ask any Titanic survivor. If you haul off & punch that block, you'll likely bust a knuckle or two. But if you try to pick it up by placing your hands on either side and lifting, forget it. Your hand will just slide up, and you might thumb yourself in the eye. This is because, as strong as the crystalline interior of the ice might be, the stuff on the surface is amorphous, with randomly oriented molecules just bouncing around off of each other.
There are a couple of ways around this. Both involve etching the weak stuff off the surface. One is to send the material out to a shop such as 4th State, who will plasma etch it off. It'll cost money though. They've got to amortize the acquisition and operational costs, and they've got kids to feed and bills to pay just like anybody else.
The other way is to chemically etch the bad stuff. For this process, you put a whole bunch of Potassium Dichromate in a tank of H2SO4 and soak the fiber in it. This will attack the low molecular weight stuff, but leave the crystalline material intact, as long as you don;t leave it too long.. Then, of course, you've got to do repeated washings, and then all of that liquid has to be disposed of, which can get ugly expensive. You're probably much better off with the plasma etch.
This is about as much as I can handle in one sitting. After we hash this part out, I'll cover the matrix materials (the gooey stuff).
We're going to start with Polymer Matrix, Fiber Reinforced Composites, since that's what most people seem interested in. Don't worry, we'll get to the metals and ceramics soon enough. I'm going to break it down into four areas...
FIBERS: Their manufacture, properties and applications.
MATRIX MATERIALS: similar discussions.
PROCESSING: Vacuum bagging, autoclave curing etc.
DESIGN CONSIDERATIONS: Because it's possible to turn $50K worth of perfectly good graphite into a perfectly useless chunk of shit.
FIBERS: The oldest modern structural fiber is glass. You may have noticed that most of the glass fiber manufacturing plants are in the southeast, or anywhere sand pits can be found, specifically near-pure fused silica. Such is commonly found in South Carolina, which is why a lot of the plants are located there. The raw material is literally just laying around.
To turn the sand to glass, it is melted in a furnace to a viscous, flowing form. It is then forced through a device called a spinaret, which is much like a shower nozzle. As it comes out the other side, it is elongated as it cools, so that when it solidifies, it is very thin. These then are usually treated with a sizing of some sort, frequently a Silane. This allows for better handling during subsequent processing, as well as improving the "grip" of whatever resin system is ultimately used. The fibers are then gathered into bundles called "tows," with the number of individual fibers bunched in multiples of a thousand, expressed in "K" (2K, 3K, 4K, 6K, 8, 12 & so on). The smaller ones are used more for woven cloth, while the bigger ones are generally used for tooling, pultrusion, and filament winding (which will be covered in the processing section).
(By the way, these furnaces are run continuously for up to five years between mainenance shutdowns.)
There are three basic grades of structural glass. "E" glass is the cheap stuff used for secondary or noncritical applications such as wingtips, fairings, hot tubs, shower pans and the like. This is the type of stuff one would use in a chopper gun, along with cheap polyester resin.
"S" glass is used for highly stressed areas such as landing gear attatch points, spar caps, and helicopter rotor blades that can survive 20mm cannon hits. It's quite expensive, even more so than many grades of graphite (carbon) fiber. But for some applications, it's hard to beat.
"S-2" glass fits between the other two, toward the high side. It can be used for most of the applications that S glass can, at substantially less cost.
Graphite (carbon) Fiber. There are many more grades of this than there are glass fibers, but there are two basic families, delineated by their precursor materials. They are Polyacrylonitrile, or PAN based, and pitch based.
The PAN precursor is basically just a really fine, pure grade of Rayon (or Orlon; same stuff).Tows of it are unwound in a heated, inert atmosphere and pyrolyzed under tension. the carbon remaining to form Graphene rings, and the tension beginning to align the rings to within 15 or so degrees of the longitudinal axis. This is your basic structural carbon fiber, with a tensile strength of about 120 KSI, and a tensile modulus of about 45 MSI. (I'll explain these properties some more in a moment.) the fiber has now been elongated by a factor of 10.
If you want a higher performance fiber, both stronger and stiffer, you repeat the process with the temperature bumped up to around 2,200F. You also up the tension, in order to elongate the fiber by another 10x. This aligns the graphene rings to within 6 degrees of the longitudinal axis. Now you're looking at about 300 KSI tensile strength, and about 200 MSI tensile modulus. Of course, these are just approximations, to illustrate the effects of various process parameters. By juggling them around, fibers with much higher strength and stiffness can be had. Just bring money.
The other precursor is Pitch, which is the black, gooey stuff from the bottom of a fractional petroleum distillation tower. We're talkin' sub-asphalt here. It's purified, to get rid of the various phenols and other species that can interfere with the continuity of a fiber. Then it's squirted through a spinaret into a hot, inert atmosphere, where it solidifies. From there, it's handled pretty much like PAN carbon, but at a much higher temperature and elongation. The finished fiber is generally used for very high end applications such as satellite chassis and highly stressed, dimensionally critical Carbon/Carbon composites (I'll cover these later).
There's another type of "black stuff" out there. It's amorphous Carbon fiber made by Nippon Graphite. It's not graphitic at all, though. All the Carbon atoms are randomly oriented, with no crystalline structure whatsoever. Structurally, its properties are roughly that of E glass, or slightly less. But it does have one very strange & possibly useful feature. On a demonstration I saw, there was a wedge drop aparratus that would release a 1 Kilo, wedge shaped weight 20 inches onto a bridged, 2x8 cm. specimen of unidirectional laminate of regular stuff with 1 or 2 plies 90 degrees to the length, just to keep it in one piece. the wedge just fell right through the thing, fracturing all but a couple of plies. When the demo was performed again, with an almost identical specimen, the wedge just bounced, and kept bouncing for a good 15 or 20 seconds. The only difference was ONE unidirectional ply of XN-05 amorphous carbon on the compression(!) side of the stack. I'm gonna' get some and try making a crossbow out of it. With that kind of rebound, it should be awesome!
SYNTHETIC FIBERS: The most well known of these is Kevlar, DuPont's brand of Aramid fiber. The term "Aramid" is a contraction of Aromatic Polyimid. It was first synthesized in the early '60s, for use in automobile tires. There are different grades of it depending primarily on the application. It's very tough, but not that stiff. Which is to say it'll make a hell of a kayak, but not much of a wing spar.
Of course, its antiballistic properties are legendary. the last time I worked with any was about a year ago, using some off the shelf Aramid/PVB-Phenolic spall liner as a backing for some Aluminum/Carbon vehicle armor I can't talk about.
It works with a wide range of polymeric matrix materials, but is somewhat limited in its high temperature capabilities; it starts to let go around 350F.
Spectra fiber: Originally developed by Allied Signal back in the '80s, it's an elongated, crosslinked polyethylene fiber that has an extremely high strength to weight ratio. Oddly enough, the stuff is crosslinked after it's elongated, unlike an elastomer, which is crosslinked beforehand. So instead of deforming and then rebounding to its original shape, the links help it resist deformation in the first place.
Unfortunately, there are enough problems to damn the stuff in most applications. For one thing, it is just polyethylene, the same basic stuff as a trash bag. That means its thermal properties are for shit. It starts to weaken around 240F, and by 260 or so it's just mush. That's why it was abandoned for firefighting vehicles in urban areas. In riot situations, the local scum will actually shoot at the firefighters. Their vehicles are frequently parked in close proximity to burning buildings, and the armor can easily reach 300F through radiant heating, instantly ruining the ballistic resistance.
Another problem is that PE is a polyolefin, and like most of these, it is formed in a melt-phase polymerization process. This means that, as it cools, the high molecular weight, crystalline material (the good stuff) tends to migrate towards the center of the fiber. Meanwhile, the low molecular weight material is rejected toward the surface in an amorphous state. This stuff has no shear strength, for all practical purposes. That means that the resin system, whatever it might be, has nothing to grab onto. You guys with some exposure to composites may be aware of interlaminar shear problems. well, this is an inTRAlaminar shear problem. It's exactly analogous to trying to pick up a block of ice on a hot day. Ice is strong stuff. Just ask any Titanic survivor. If you haul off & punch that block, you'll likely bust a knuckle or two. But if you try to pick it up by placing your hands on either side and lifting, forget it. Your hand will just slide up, and you might thumb yourself in the eye. This is because, as strong as the crystalline interior of the ice might be, the stuff on the surface is amorphous, with randomly oriented molecules just bouncing around off of each other.
There are a couple of ways around this. Both involve etching the weak stuff off the surface. One is to send the material out to a shop such as 4th State, who will plasma etch it off. It'll cost money though. They've got to amortize the acquisition and operational costs, and they've got kids to feed and bills to pay just like anybody else.
The other way is to chemically etch the bad stuff. For this process, you put a whole bunch of Potassium Dichromate in a tank of H2SO4 and soak the fiber in it. This will attack the low molecular weight stuff, but leave the crystalline material intact, as long as you don;t leave it too long.. Then, of course, you've got to do repeated washings, and then all of that liquid has to be disposed of, which can get ugly expensive. You're probably much better off with the plasma etch.
This is about as much as I can handle in one sitting. After we hash this part out, I'll cover the matrix materials (the gooey stuff).