If you wish to find out whether the use of peel ply adds weight in the form
of extra resin to your wet lay up, tried this simple experiment:
Prepare the lay up, ensuring that the glass cloth is fully wetted out, but
using no more resin than usual. Instead of leaving it to dry in air,
immediately cover it with a sheet of peel ply, using a brush or squeegee to
eliminate air and wet out the peel ply as far as possible, but without
adding any extra resin. Allow to cure, then peel off the ply and examine the
surface. You will find that a large proportion of the area has the smooth
surface to be expected from use of peel ply, but it is likely that there
will be a few rough patches where there was insufficient resin. The peel ply
itself will now contain resin which would otherwise have remained as part of
the lay up.
This demonstrates that the peel ply can both remove excess resin and provide
a smoother surface. Adding more resin to ensure that the peel ply is fully
wetted out and no rough patches are left is likely to add less weight than
that of the resin removed, so that a fully smooth surface is possible with
no weight gain using peel ply.
The disadvantages of using a peel ply to cover large surface areas are: the
difficulty (especially for novices) of ensuring that the glass cloth is
fully wetted out and there are no trapped air bubbles; the extra material
cost and effort during lay up; and the possibility of contamination with
some types of peel ply that contain release agents.
Some people have expressed concern that peel ply may draw too much resin out
of the lay up and weaken it. Remember, however, that any resin in excess of
that needed to wet out the glass fibres is excess weight and does not add
strength. High technology processes, such as those using autoclaves under
pressure, seek to maximise the fibre/resin ratio. Problems with hand lay ups
using peel ply are more likely to result from inadequate wetting out before
the peel ply was applied, rather than too much resin being drawn out of the
finished lay up by the peel ply.
Although peel plies are often used to cover areas which will subsequently be
bonded, it is important that the cured surface is properly abraded to ensure
that it is chemically active. The following advice was received from those
professionally involved in composites:
"The peel ply problem is probably not contamination if it is a corona
treated polyester. These rely on the slick glazed surface after corona
discharge treatment to enable them to be removed. However the same slick
surface is cast into the laminate bonding surface. That is why it won't
stick.
"The problem is that many people believe that you need a clean surface for
adhesion. That is true, but it is also not sufficient. The secret to
bonding success is to have a chemically active surface, so that chemical
bonds can be formed between the adhesive and the substrate. (For metals, it
is essential that the chemically active surface is resistant to the
formation of hydrated oxides because this is the most common cause of
disbonding.) Simply removing a peel ply is not sufficient to provide the
chemical activity. The only reason these things appear to work in the short
term is that there is a small amount of fracture of the resin which occurs
at weave cross-overs, and that small fracture surface is chemically active.
"Unfortunately, in service water migrates along the bond interface through
the chemically inactive sites, and this leads to disbonding.
"I suggest you get a copy of Hart-Smith, L.J., Redmond, G, and DAVIS, M.J.,
The Curse of the Nylon Peel Ply, 41st. Int SAMPE Symp. and Exhib., Anaheim
25-28 Mar 1996"
I hope this lump of information doesn't give anyone indigestion after their
New Year revels. Remember that there is a large safety factor in the design
of composite bonds for amateur construction, so that a less than perfect lay
up is still likely to be more than strong enough to keep your aeroplane
together.
Fly safely and have a happy New Year
Mike
Europa Club Safety Officer
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