Kevlar Summary - long

Kevlar Summary - long

Post by Glesh » Sun, 21 Sep 1997 04:00:00

Thanks much for the kevlar contributions.  

Please read the entire summary for complete information since some of the
replies don't agree.

Off the top of my head I have learned:
- Kevlar is hard to cut.  One company suggests  a set of scissors for
kevlar and a different set for carbon and regular glass because of the cut
angle.  After trying it for myself,  I have concluded that cutting the
kevlar is 95% of the job.  Also, I have found that it was impossible to trim.
- Kevlar needs UV protection
- Kevlar has no shape memory and should have carbon under it in case a
dent occurs.  
- Kevlar does not sand so cut it precisely and cover it with regular
fiberglass for sanding.

For my lamination repair, I used polyester resin, 1/4 foam, carbon,
kevlar, and regular fiberglass.  Tomorrow I will test it in good size
waves, I hope.

Below are the responses:

Kevlar has really great 'impact' strenght. Kevlar laminates have greater
tensile strenght than fiberglass but not necessarlilly as strong in
compression or bending. Kevlar can be used with both epoxy and polyester
resins whereas carbon is primarilly used with epoxy only. You will need
REAL sharp scissor to cut Kevlar and its still a b_tch.

If your board must be repaired with epoxy, Kevlar is not recemended (it
will certainly still be strong but you arent getting what you paid a lot
for). If you need to use '***' material, use carbon. Since epoxy is
probably the best material mechanically for reparing ANY type of board
(even if the board was built in polyester - unless cosmetics are an
issue - epoxy must be protected from UV) your probably best off using
carbon and glass.

Don't bother with the kevlar. Like the other guy said, it absorbs water.
In fact, it should be dried in an oven for a couple of hours before it
is wetted with epoxy. Stay away from the carbon too. Sure it might be
better in a lot of ways, but you are making a repair. Why do you need a
high performance fabric on a repair? (unless the board is carbon) Stick
with the glass. It is cheap, easy to use and more flexible than carbon.
Which is good, because your repair will flex and be less likely to pop
off. Also, when finished, paint the repair for UV protection.
P.S. Kevlar acts like leather (tough), Carbon acts like glass (stiff).

Kevlar is an organic fiber with tubular microstructur (i.e. the
single kevlar fibers are actually hollow tubes). It has low
specific weight, high tensile strength, very high impact strength /
resistance and low pressure strength (*). It is hydrophob (is this
the word, i mean it attracts water) and not very UV stable.
Kevlar can only be cut with special heavy duty scissors or very
sharp knive blades (still difficult) and can not be sanded at all.
Thick kevl;ar is fairly difficult to wet with resin, take
a very thin, viscous type of resin, for larger laminates i'd
also use vacuum (or an autoclave :-)) for curing to get optimal results.
For small patches one can go without.

I did use Kevlar on the decks of 2 of my boards, mainly for increased
resistance. Works so far, not a single ding or scratch yet.
(layup is glass-airex-carbon-kevlar-glass).

If your dings were caused by "hard impact" (boom, mast, rock) rather
"soft impact" (heel dents) then it should work fine.
Just be warned, it is a b***h to work with and it should be covered
by some paint to protect it from UV light (although i don't have
any hard data how much kevlar really degrades when exposed to sunlight)

Since yopu've already routered out 1/8 inch of foam you could also
consider placing some pvc foam (airex, divinycell) in the ding area.

(*) For comparison: glass is a mineralic fiber, has high specific
medium tensile , pressure strength and impact resistance, is easy to
cut, sand and laminate and is failry UV resistant.
Carbon has high tensile and pressure strength , very high Elasticity
(i.e. is very stiff), low impact resistance (i.e. breaks easily when
impcted by hard things), is easy to work with.

The advice you got in other follow-ups is, in general, good advice, but
there are some factual errors. For example, Kevlar fibers are not
tubular, but rather consist of fibrils (smaller fibers) in a softer
matrix. For more info on Kevlar chemistry and general properties, see my
Kevlar article at

As for working with Kevlar, it is a pain. You will need sharp scissors,
but they don't have to be expensive. I use the $14.95 Featherweight
Scissors from Fibre Glast (
Although I haven't been using them a long time, they so far have worked
just as well as my $100 ceramic shears. For your small project, they
should be just fine. (Fibre Glast also sells Kevlar and resins, though
they are a bit on the expensive side; see my list of suppliers at

As someone else pointed out, UV degredation is a significant issue. You
can lose 50% or more of the properties after exposure to the sun. Fresh
Kevlar should be a bright yellow. Exposure to UV turns it brown, so it's
easy to tell when the material has been damaged. And it doesn't take a
lot of light to damage it: I had some spools of Kevlar stored in a room
which didn't get direct sunlight, and the fibers were noticeably
discolored within a year.

yes, kevlar is "very" strong, but you will need to use special shears to
cut it. THe trouble in your case is that the board would end up very stiff
in one place AND FLEX ALL OVER THE REST. You should never be in a situation
where you need to sand into any glassfibre  or kevlar strands, this should
only be on resin or hardened gellcoat. So make sure you leave a recess to
flow on resin afterwards, for later sanding/compounding. You should borrow
shears from your supplier of glassfibre (care: some glassfibre
manufacturing companies get pissed off by the amount of small enquiries
they recieve from individuals for private/hobby use). My dad was in this
business for 20 years and a consultant to Lola racing cars and several
performance yacht companies, I don't see him much but recall a lot from his

Thats the first thing, you cannot sand kevlar, it goes *** (for the
benefit of those who don't know..)Kevlar is almost completely stretch
free, but has little memory. If you wet out a strip of kevlar with epoxy
resin, and after it had gone hard tried to bend it, it would stay in the
bent shape. Carbon however will flex back into shape and do it really
fast. Herein lies the difference, and the reason using them together is
so good. Kevlar on top for the impact resistance, and carbon
underneath(carbon can shatter like porcelain).
kevlar under traps (to revent the sort of impacts you have) is pretty
effective, but carbon is good too. Together they are awesome...