Advanced fibers in boats???

[coldandhardy]

This is an old thread and I am new to this forum (in fact it is my first post) but I believe that I have something of credibile interest that could be added here. To the non-technical this post will be tedious and boring!!!!

I recently retired from an "advanced composites" business and I am seriously considering the pruchase of a boat. In my business I used Kevlar (and Twaron) for several balistic applications and I used fiberglass a lot (I always spelled it that way)! I even used carbon fiber in many applications (in fact my spelling is generally bad and I rarely use spell check for blogs).

When it comes to boats I believe it is important that one considers carefully blending advanced materials like Kevlar and Twaron (aramid) and Carbon fibers with fiberglass and resins. From a physics and applications perspective one has to be particularly concerned about the aramid group of fibers.

These fibers typically have a wetting characteristic that repels the resin that is trying to bond to the fibers (it is of little consequence whether it is vacuum bagged or laid up by hand/this is a matter of physics/surface energy) . Sort of like oil and water! There will be those of U that may choose to challenge me on this one but I suggest that U tell that one of the larges aircraft manufacturers in the world that had to replace the leading wing edges of a popular model that they made with aramid fibers and resins to cleverly survive abrasive and impact failures in this area encountered on runways (they replaced them).

The leading edges of the wings failed from freeze-thaw cycling of absorbed moisture (not abrasion or impacts) and much sooner than their life expectancy. Simply stated water expands and contracts as it freezes and thaws. It turns out that the fibers repelled the resins during vacuum bagging (physics relating to surface tension mechanisms) and as the resins shrank on curing they pulled away minutely from the fibers. The resulting emply space provided a capillary space for moisture ingression at an atomic level. The freeze-thaw cycle thereafter can be a very destructive process (and was) to composites as the aircraft manufacturer discovered. Water expands when it freezes. This is a huge force. It can split steel barrels!! Resins and plastics have relativly little strength.

Aircraft parts experience multiple freeze-thaw cycles during take offs and landings. The effect was that the aramid fibers delaminated from the matrix. The laminate puffed up (a bit like pop corn) and took on more moisture (compounding the problem) and resulted in laminate failures.

I believe that some waterlines on boats experience this same cycling but at a slower pace albeit that there is more water present for obvious reasons (certainly not when the boats are parked where freezing never occurs). I would be very mindful of this potential problem in boats if re-inforced with aramid fibers!!! IMHO they will not be as strong as intended in a few years as they were engineered and tested to be when constructed!

Aramid and carbon also have another characteristic about them that is purely physics (that is to say that clever design cannot outsmart Einstein completly)! This is called CTE (coefficient of thermal expansion). Fiberglass is a very elastic material (sort of a perfect spring) - (it also intimately wets out and can bond elemently with the resins with no separation space (elemental or otherwise) between resin and fiber). Carbon and aramid are very strong and rigid when compared to the reisn that they are embeded in. In plain english these fibers have a relatively neutral CTE - close to 0 - (effectivley they do not expand and contract in a meaniningful way when compared to resins and fiberglass). What does this mean??

They can add a lot of stress to the internal matrix (the resin to fiber bond/if there is one at all) when compare to fiberglass when variable temperatures are encountered. This is probably not a huge effect in boats but internal stresses in the laminate do take away from the ultimate strength of the laminate during temperature fluctuations.

Polyester and vinylester resins shrink while they cure to varing degrees but the fibers do not. Epoxy, on the other hand, expands while curing and shrinks during the cool down after cure (the shrinkage is small and has very little effect because the bonds with the fibers are formed during the expansion pahase of the cure cycle. The effect is that epoxy bonds very well to the glass fibers (and other fibers as well) when compared to polyester and vinyl ester. The expansion of the epoxy during cure forces the resin at a molecular level into the fibers (in a mechanical way) such that it produces physical properties substantially greater (upon cure) than most other resins by far!

Finally carbon fibers are a galvanic material. That is to say that they contribute to galvanic corrosion. Most boaters already know a lot about galvanic corrosion so I won't embarrass myself by attempting a technical description of the pitfalls of such.

Coming back to aramid fibers in my summary I would personally be cautious. I do like their impact characteristis and their outstanding resistance to penetration failure. I simply don't know if they bring a bigger problem than they solve with them. The galvanic corrosion properties of carbon fibers can be engineered around but when it comes to aramid pure physics is a more dominant player that cannot easily be engineered around!

[sailingdog]

Coldandhardy-

Thank you for your post. Although it may be a bit more technical than most of the users would appreciate, I am glad you posted it. If someone were looking to reinforce a fiberglass hull for additional abrasion and impact resistance, what fiber would you recommend be used? I've been looking at using some of the UHMW PE fibers for doing such a thing, and wanted your feedback on it.

[Giulietta]

Vacuum is not used to impregnate fibers, its to remove air pockets...boats don't fly.

[sailingdog]

Giu-

Actually, it is generally used to remove air pockets, but also to draw the resin in to the laminate and help fully impregnate the laminate with resin to avoid dry voids.

From the Reichold's website—Reichold is a vacuum infusion supplier.

The vacuum infusion process uses vacuum pressure to drive resin into a laminate. Materials are laid dry into the mold and the vacuum is applied before resin is introduced. Once a complete vacuum is achieved, resin is "pulled" into the laminate via carefully placed tubing. Reichhold is a leading supplier of resins for vacuum infusion.

[Giulietta]

SD...that is a side effect (used as a sales thing by them)...on my boat vaccum was used only to help the Divinycell adhere to the fiber, not to impregnate fibers, thus avoiding later core delamination.

Better impregnation is still done with a brush and a good resin layer man.

[sailingdog]

Giu-

I've done a bit of research on the different vacuum infusion processes used in laminate construction, and believe that it is used in many of them for resin transport and impregnation, rather than as just a side effect.

Take a look at this website, which is one of the companies I've dealt with previously.

[Giulietta]

OK, I'll look at it...but site does not work...

[sailingdog]

I think you're confusing the vacuum bagging process with vacuum infusion, which are slightly different processes.

I think what the people who made your boat used was Vacuum Bagging, which is described as:

This is basically an extension of the wet lay-up process described above where pressure is applied to the laminate once laid-up in order to improve its consolidation. This is achieved by sealing a plastic film over the wet laid-up laminate and onto the tool. The air under the bag is extracted by a vacuum pump and thus up to one atmosphere of pressure can be applied to the laminate to consolidate it

Vacuum Infusion is described as:

In other infusion processes (SCRIMP, RIFT, VARTM), fabrics are laid up as a dry stack of materials as in RTM. The fibre stack is then covered with peel ply and a knitted type of non-structural fabric. The whole dry stack is then vacuum bagged, and once bag leaks have been eliminated, resin is allowed to flow into the laminate. The resin distribution over the whole laminate is aided by resin flowing easily through the non-structural fabric, and wetting the fabric out from above.

[Giulietta]

Ahhh thanks SD yes I was...sorry about me being stubborn..the English....
Vaccum Bagging it was for me

[sailingdog]

Giu-

Always glad to help my friends... de nada.

Dan