Details of KevlarTM usage and abusage



1.    Use

Kevlar or, for the technical purests, aramid fibre, was used for side-panels on Demolition Doris.  With, it has to be said, rather disappointing results.  Certainly strong, and certainly light-weight.  But see below for photos of an impact by Tornado at Worthing earlier in 2002.  Suspect the results were partly to do with the choice of resin - this was governed largely by the lack of specialist manufacturing facilities and equipment and hence the need to select a room-temperature resin.  May use it again for non- or low-impact components (battery holders or retainers spring to mind).  But as someone at Worthing pointed out, Kevlar is fine until it gets damaged...then it's not exactly repairable on the day.  Be warned, as well, it burns nicely.

If anyone's interested in experimenting, I've set out below what's involved.  No doubt there are many other ways, but this is how I fabricated the side panels and this method was based on a long conversation with a Kevlar boffin at SP Engineering, on Isle of Wight.

2.    Materials

Principle

Similar to fibreglass - building up layers of aramid cloth impregnated with resin.  Recommended minimum thickness is about 5mm (experience says go thicker for areas at risk from severe impact).  Top and bottom layers should be glass-fibre as this apparently gives better adhesion if bonding (gluing) other components.

Cloth

RA320H5  320gms/sq.m  0.4mm thick satin weave (SPEngineering)

Resin

Ampreg 20 Epoxy Laminating System (standard cure) (SPEngineering)

Fibreglass

RE292  300gms/sq.m 0.3mm thick quadran weave (SPEngineering)

Aramid-metal bonding

Spabond 125 Epoxy Adhesive System (SPEngineering)

Cutting

Aramid cloth is horrible to cut...and hard work.  Best way is to purchase aramid shears, which are serated and  teflon-coated.  Ordinary (smooth) shears don't stand a prayer since the fibres are so strong.  Watch for the fabric de-weaving, particularly on long narrow sections.  Another piece of received wisdom is to keep the fibres as long as possible, to maximize strength.

Final Weight

Epoxy roughly doubles weight of cloth.  So to get an estimate of final weight of Kevlar component in Kg:  (Total area in sq.m) x ((aramid cloth weight/sq.m)(no. of layers) + (fibreglass cloth weight/sq.m)(no. of layers)))/1000

3.    Suppliers 

Manufacturer

SP Systems
St. Cross Business Park
Newport
Isle of Wight
PO30 5WU

Tel. 01983 828000
Fax. 01983 828100
Email:  marketing.services@spsystems.com
Web:   www.spsystems.com


Supplier


(of SP systems products)

MarineWare
(Southampton)
Tel. 02380 330208

4.    Process 

This is what I used, but may vary depending on type of resin...RTFM!

Mould

Normal fibreglass mould techniques apply, with the very obvious criterion of ensuring you can separate mould from Kevlar by using plenty of release agent.  And use the proper stuff.  Melamine finish on hardboard may feel slippery but the resin easily sticks to it, (& then hardboard had to be chiselled away).  What did work was 50mm insulating tape - I covered the 2nd mould with this & it separated easily.

I made the side-panels slightly convex for strength (!) - bent a strip of hardboard over a thin wooden strip, stapled it down, and used this as the mould.  Worked a treat.




Preparation

Get everything ready before mixing the resin, i.e,

  1. Cut all of the cloth to more or less the right size - much easier at this stage than when the resin is hard.  If strips, roll loosely - easier to then use.

  2. Make sure all tools etc. are within reach (sounds obvious but...)

  3. Safety:  well-ventilated room, wear barrier cream and gloves (e.g. latex ), mask capable of filtering fumes and not just dust (Halfords sell them).

  4. Mixing resin:  ratio of resin to hardener is crucial to final strength so don't guess it.  I bought a set of digital kitchen scales with 1gram resolution, and mixed 500grams at a time.  So accuracy is 0.2%.  And another obvious point...the resin is expensive so only mix what you will comfortably use before it starts to go off.

  5. Resin+ hardener=exothermic reaction, i.e, it gets hot.  So mix in wide flat container rather than tall thin, to maximize surface area.

Layering

5mm final thickness = 10 layers = 1 layer glass fibre, 8 layers aramid, 1 layer glass fibre.  Process was:

  1. Paint resin onto mould.

  2. Unroll cloth onto mould

  3. Stipple more resin

  4. Repeat 1-3 for about 3 layers.  Then lay strip of polythene from dust-sheet (very thin) on top and use small roller to compress layers and to ensure thorough wetting.

  5. Remove polythene & discard;  repeat 1-4 until all layers used.

  6. Leave to harden.

Hardening

Takes about 24 hours for medium-cure resin- again dependent on type of resin.

Curing

Strength can be improved by c. 30% by post-curing with heat.  50-60 degrees Centigrade is ok, but piece needs to be cooked for about 24 hours.  Again on the advice of the SP Engineering fella, I constructed a bespoke "oven" heated by two 100W light-bulbs wired to a dimmer switch.  Via a digital thermometer, the temperature could be controlled.  My side- panels cooked for 24 hours at almost spot-on 50oC.  Construction of oven as follows:

NB:  If you attempt the same, MAKE SURE IT'S ELECTRICALLY SAFE AND THERE'S NO FIRE RISK.


5.    Machining
 

Drilling

Not too difficult but use sharp drill bits.  Bear in mind it's the resin you'll be drilling - the aramid fibres don't tend to be cut.

Sawing

I used a metal bandsaw to trim panels to size.  Worked, but kept clogging blade.  Many stops to unclog!

Sanding

Avoid.  You sand the resin away but leave the aramid fibres.  Hence one furry (literally) finish.

Fibres

Odd fibres can be singed away with a soldering iron but WATCH THE FUMES - they're toxic

6.    Results 

Judge for yourselves whether it's worth the trouble! Incidentally it was the resin which gave way - the Kevlar fibres were unbroken.

7.    Further info. 

Tons of info on the web, but search for "aramid" rather than "Kevlar" since latter is trade-name.  One of best sources is SP Engineering, who will also send you a CD-ROM with tech-spec's of all of their resins and cloths.