| What is Kevlar?
Kevlar is a long-chain synthetic polyamid,
whose particular architecture qualifies it as an "aramid". Aramid's
are close cousins to Nomex, it is used in structural honeycomb
cores, and in fireproof garments worn by race car drivers and rescue
personnel. Bottom line - Kevlar is
tough, abrasion and puncture resistant.
What is the Mythos
?
Let's get rid of some crab! "Ballistic-grade"
Kevlar ( Kevlar-29 ) is very different from "structural-grade"
Kevlar ( Kevlar-49 ). Kevlar-29 is not a laminate - but multi-layer
soft "blankets" of Kevlar plies sewn together - known as "soft
armor" - and then mechanically fastened to the "what so ever " work
piece - not very useful in our Hovercraft ! So if some Hovercraft's
/ Boats have Kevlar components and get advertised as
bulletproof - just walk away from that crab - it is just a mythos -
but not the fact . Sorry if I killed
somebody's advertising slogan .
Design and Application
!
A big advantage is Kevlar's specific gravity
of 1.5 in relation to E-glass with about 2.4 that allows you to
produce a lighter work piece - on the other hand is there is the
problem that E-glass will sink easier in the resin than Kevlar and
it is most important to apply extra care while wetting out and
getting rid of excess resin - either by hand with squeegee's or with
the application of Peel Ply! But the first step is still to
cut dry Kevlar !What is easy on E-glass - since you break the fibers
more than cutting them is hard on Kevlar - Sharp tools are critical
to successfully cutting Kevlar.Don't use the same tool for cutting
Kevlar and E-glass since the E-glass will quickly kill the tool edge
reducing its effectiveness in cutting the Kevlar fibers.
Resins which are very suitable for E-glass applications might
not work well with Kevlar - you need lower viscosity epoxy resins to
get the most out of your work piece. It is recommended to use
Epoxies with viscosity's under 300 centipoints - in relation
"general - purpose" styrenated laminating resins are 800 centipoints
or above ! Keep in mind that the biggest downside of Kevlar is its
compression strength ( about 30% less than plain old E-glass)
- but in the lift - thrust duct it would give great safety.
Safety - Although Kevlar dust and fibers won't make you
itch like E-glass does, inhaling yellow fuzz simply isn't a smart
idea. Wear suitable respiratory protection and a good fitting set of
safety glasses or goggles. Even your nose and lungs won't know if
you're laminating Kevlar or last weeks newspaper, a good respirator
and adequate shop air-exchange rates obviously are recommended
during cutting, wet-layup and finishing work!
Grinding - Sanding
of the work piece is some
headache since grits much coarser than 180 really "fuzz up " Kevlar
- which is no big deal in case of a repair since the fibers mingle
nicely with the patch and increase adhesion - just for the finishing
up, you have to sand with 180 - 200 grit to reduce fuzing and last
not least make a finish with ScotchBrite. Cutting, boring and
machining is the best way
to call 1-800-4-KEVLAR and ask for DuPont's product literature - and
make use of the recommendations for tool vendors since it would take
me 6 month to get all their links here together! Just for the moment
to satisfy you : Basic rule of thumb for almost all cutting,
machining, and boring operations on Kevlar laminates is
high speed, low feed!
Backing up the area which is machined
is important to avoid
delamination. |