Kevlar fibers in Rubber matrix? 5

[Onemorechance]

Hi there,

I need to make a very light reservoir, the differential pressure should be 30 atmosphere, it should be very flexible and foldable when empty.
I was thinking of using a combination of Kevlar tissue in a Rubber matrix . Is this a good idea?
Would it be possible (instead of using Kevlar tissue) to wind the reservoir with a matrix-wetted kevlar fiber ?

At least when using rubber as a matrix I would not have matrix failure because its stiffness is very much lower than the fiber stiffness. But maybe winding Kevlar fiber in this case is not a good idea? Using Kevlar tissue is maybe a better idea? How much lower is the tensile allowable stress for a tissue versus UD fibers?
Is it even possible to use rubber as a matrix with kevlar?

Are ther maybe other solutions?

What do you think?

Onemorechance.

 

[JimMetalsCeramics]

You can put Kevlar in rubber. You should squeeze the pre-set rubber into the fabric (one sheet at a time) degas in vacuum, and then cure.

 

[Onemorechance]

Thank you Jim,

1. But do you know if it is a good choice to use this combination (kevlar fabric with rubber matrix) for a pressure reservoir? It would be a cylindrical reservoir of 2 m long and 0.14 m in diameter. It would have to hold pressurized air at roughly 3 MPa (or 30 Atmosphere).

2. Is there another way than using Kevlar fabric, like using wounded UD fibers?

Onemorechance

 

[JimMetalsCeramics]

It's not an easy question. You should find an adequately strong rubber. You may have to play with bonding between the rubber and the Kevlar. Do an online search for reinforced-rubber.

 

[HDS]

Of course you can bond Kevlar to rubber. One of the main placees it has been used is for the most common and complex of composite structures. Tires.

 

[Onemorechance]

Yes, I know, but in this case the kevlar is mainly used as a protective layer keeping sharp objects from penetrating, not as a reinforcement of the rubber.
I have done some looking around on the internet, but the results are not satisfactory. Seems like nobody realy is interested in using reinforced rubber for pressure vessels?
Of course rubber has rather non linear elasticity characteristics, therefore predictions are probably not so easy to perform. Maybe the rubber ages more quickly than other matrix materials? I don't know.

 

[CoryPad]

Onemorechance,

Fuel tanks for racing vehicles sometimes use Kevlar-reinforced elastomers. ATL is a specialist in this area - they may be able to supply a suitable product, material, or info. You can learn more at:

http://www.atlinc.com

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[orangepeel]

Onemorechance,

I have done quite a bit of work with non-tire flexible composites or fiber-reinforced elastomers.
a) It is possible to reinforce a rubber with Kevlar if you can get a compatible sizing on the fiber, and use the proper fabrication/processing method.... but I might suggest other fibers may do better for many requirements.
b) what type of rubber do you want to use, and what kind of enviroment will it be in? Corrosive? High Temp? chemicals?
c) Do you want to use liquid 2-part RTV - type elastomers? or Do you want to use a calendering method like that used with tires?
d) Does the fiber angle matter, or do you want a general quasi-isotropic reinforcement? ie, what kind of response do you want?

Silicone typically require special sizings on most commercial aero-space fibers such as graphite and fiberglass. Polyurethane elastomers bond well with most standard sizings on fiberglass and graphite.... need to degas before laminating or infusion in most cases.

I made a "rubber muscle" a few years ago with kevlar and polyurethane..... and had problems with adhesion between fiber and matrix, and appeared to have tow separation.

good luck,

ldp

 

[Onemorechance]

Thanks everybody for the advice.
Sorry for replying so late, but I was away on vacation for a while.

Let me specify a bit more what I would like. The pressure reservoir would have to be a long tube of ~ 5 m in length and ~ 200 mm in diameter. Originally I figured I would need a differential pressure of 30 atmosphere, but now I see I can have less (about half that value, say 15 Atmosphere). It is part of an inflatable structure and has to function as a beam picking up shear and bending. It should be flexible enough so if the bending is too high it would not break but fold. There is no corrosive environment, no chemicals, temperatures may range between –40°C to +70°C but will mostly be around 16°C. In order to have this structure as light as possible I would like to wind this reservoir with UD fiber running through a wet matrix (liquid 2-part RTV - type elastomer ?). I would need fibers in longitudinal (0°) direction in transverse direction (90°) to pick up all the tensile stresses due to the pressure and bending loads and some (+- 45°) fibers to pick up shear stresses due to shear and torsion. All the strength calculations and so I can figure out myself, it is the practical advice I need.

What kind of fiber do I best use?
What kind of matrix?
How do I determine for a given matrix a compatible sizing on a given fiber?
What is a proper fabrication/processing method?

I am asking a lot am I not?

By the way, thanks a lot for your answer Orangepeel I feel I can learn a lot from you.

Regards,

Onemorechance

 

[orangepeel]

Onemorechance,

Your design will not be as simple as it seems. If you just put axial or longitudinal fibers in your inflatable tube, the tube will contract axially under pressure, creating a "muscle". If you apply only circumferential windings, the tube will expand axially. The rubber hose manufacturers know that you must wind at least one ply at about +/- 55 degrees, relative to the long axis, as well as the 0's and 90's. It appears that you have done FEM... but you may have problems getting some good initial orthotropic material properties for a fiber-reinforced elastomer. It also appears that Kevlar may be overkill for your application, that fiberglass would be cheaper and easier to work with. Although not as elegant, chopped strand mat/tape may be an alternative to filament winding at a specific angle... in that case, kevlar tissue may be fine if you can ensure good bonding. If you want to wet wind a part, viscosity of your rubber resin will be a factor, as well as pot life. I assume your "bladder" needs to be without leaks... which means you may need a layer of virgin rubber on the inside. Some urethane rubbers will degrade under UV, the silicone rubbers will require an alternate fiber sizing....., I haven't played with any florine-based elastomers. Fabrication? Cost? If you have to apply a fiber at a specific angle, filament winding or fiber placement may be your best bets, as I am not aware of any unidirectional fiber-reinforced elastomer prepreg. Perhaps you could wind dry and then infuse.

Good luck, I think that I have raised as many questions as I have solved. If you are serious about this, perhaps we could talk offline.

-ldp

 

[Onemorechance]

Yes, I am serious about it, but I think we are not supposed to exchange phone nrs or email addresses. I also think we better stick to the rules or we will not be allowed to use this site any more.

Do you think fiberglass has good enough bonding with an elastomer like rubber? How about carbon fiber?

I would like to use a filament winding technique and I was intending to use 0° as well as 90° as well as +-45° fibers.
The 0° fibers would have to pick up longitudinal stresses originating from the differential pressure and the bending.
The 90° fibers would have to pick up hoop stresses from the differential pressure. Finally the +-45° fibers are there to pick up shear and torsion.

The structure would be deployed most of the time which means that the reservoir would be under pressure. It would not be inflated and deflated constantly like a 'muscle'.

You say that the viscosity of the rubber resin may be too high in order to use wet winding. How about winding over a layer of virgin rubber on the inside and then somehow putting rubber on the outside and vulcanizing under pressure and heat?


Regards, Onemorechance

 

[orangepeel]

Onemorechance,
No problem on the rules, ... just thought it would be a little faster to email directly back and forth, rather than come here and submit.... but this is more educational for everyone else. It appears to me that sharing email addresses is okay.... but ??... okay.

Note: I tend to use the terms rubber and elastomer interchangably.... for any purists out there.

The sizings on fiberglass and carbon that are typically used in connection with polyester resins or epoxy systems seem to work well with the urethane rubber/elastomer systems that I used (Renco RP series, Adiprene L100, etc). When I used Silastic S silicone rubber, I had to strip the existing sizing off with toluene?? or acetone... and apply another suitable sizing.

Filament winding: unless you use "pegs" or "peg strips" it will be virtually impossible to wind 0 degree fiber on your mandrel.... you can get close with a long mandrel, proper design and perhaps a lot of friction. An alternative is to apply unidirectional cloth, and wind hoops over it. The reason I said that you should wind a layer at +- 54.7 degrees, is then you are assured that your inflatable tube will not either contract, or expand, under pressure.... but then maybe you want that.... or it doesn't matter.

The two-part rubber resins that I have used, tend to be more viscous, and when I tried to do the traditional wet wind, with a bath, I didn't get good wet-out of the fiber, and I got a lot of smearing of fibers and resin on the associated rings and surfaces before it got to the mandrel. Typically I do smaller parts on the winder. Now I wind dry, and after the pattern has started, I use a squeegee or brush and apply the resin directly to the mandrel with fiber..... I can work the resin in with the brush, and move in the direction of the fibers. That wouldn't work in a production setting, but works for my jobs. If properties transverse to the fiber are important, one must be careful that "hair" or "fuzzies" don't form, and mess up transverse stiffness or deformation.

For hand lay-ups, I have used a similar methods, although the Naval Surface Warfare Center - Carderock (West Bethesda, MD) uses Adieprene L100 quite a bit, and when it is used with a Kaytur curing agent, the cross-linking or curing doesn't set in until about 210F. This gives one an essentially indefinite pot life, and if the resin is pre-heated, you can get some very good laminates with resin infusion.

Hope this helps.... so much for a short response.

have a good day!

-ldp

 

[Compositepro]

I just came across this posting and have several comments. Structures like you describe have been made and can be found on the web if you look for "inflatable strucures".
For the diameter and pressure of tube you will have a hoop load of 900 lbs./inch of length in the wall membrane. Safety factors for this type of application are often 4 or 5 times. As you see, the loads can be very high and dangerous amounts of energy can be stored in the compressed air.

The load carrying element is the fiber that is used and not the rubber. The rubber is mainly a gas barrier. Also the most important aspect of the fiber is its tensile modulus rather than strength. In order to load a material up to its tensile strength it must be near its strain at failure. If you use a material with 10% strain your tube diameter will increase by 10% before you exceed the strength of the material. This is usually unacceptable so strain and stiffness are the design drivers.

You could use aramid (Kevlar), glass, carbon fibers, or even steel wire can work. The best way to apply a thick layer of rubber is by using calendered rubber sheet stock or stips. Home Depot carries self fusing rubber tape in the electrical section that will give a idea of what it looks like. For your application you must use a curing type of rubber that cross-links.

The high-modulus fibers must be twisted appropriately if want to be able to fold the membrane. Normally these fiber are in tow form where the fibers are all parallel. This is fine for epoxy matrix parts. But in rubber if you bend the tow the fibers on the inside of the bend will kink and be damaged. You can find twisted fibers under carbon or glass string or rope on the web. Carbon fibers generally have less than 2% strain to failure and 700,000 psi tensile strength.

The fiber can be preimpregnated by the fiber supplier or you can impregnate the with latex dispersions of the same type of rubber used for the sheet. Latex is a dispersion of submicron solid rubber particles in water. When the water evaporates the partcles fuse together into a nonporous film. The latex is very low viscosity even though it is 50-60% rubber, and it soaks into fibers easily. Latex could also be used in place of sheet rubber but it can only be applied and dried in thin layers. This takes time but is very simple to do.

Winding with continuous fibers will give you the best strength and design flexibilty but realy requires automated equipment. But fabrics can be used and would be
the easiest approach for manual fabrication.

 

[Onemorechance]

Thanks, this is very helpful information.

Regards,

Onemorechance