Most high performance gliders use laminated sailcloth (commonly
referred to as Mylar), a sandwich of polyester film, tafetta, scrim, and/or
unidirectional fibers. Hang glider manufacturers purchase laminated sailcloth
from Dimension-Polyant GmbH, Sailcloth Technology,
http://www.dimension-polyant.com/en/Company_7.php, Porcher (http://www.porcher-sport.com/en/produits/paragliding.html),
Contender, Bainbridge, and perhaps other companies. You'll find the materials
here:
http://www.dimension-polyant.com/en/Laminate_Racing_2_4_1.php.

Wills Wing explains why they use laminated sailcloth on their T2C's here:
http://willswing.com/features/sailcloth.asp

Laminated sail materials using polyester film have been used for
many years in hang glider sails. The original inspiration for the use of these
fabrics was an attempt to achieve superior performance by using a lower stretch
material that was still relatively light in weight. In high performance hang
gliders, limiting spanwise twist is a key component of performance, and twist is
generally controlled by sail tension. The disadvantages of the polyester film
fabrics have been that the reduced elasticity under load has generally resulted
in "stiffer," less responsive handling qualities, and that the polyester
laminated fabrics have been, historically, not as durable or long lasting as
woven fabrics. The current generation of laminated fabrics are vastly improved
over those of even ten years ago, however it is still generally the case that a
premium woven fabric will outlast even the best laminated materials.

Laminated sailcloth degrades in sunlight. Steve Pearson at Wills
Wing writes:

When new, the structural properties of the same laminates, but
with or without an additional UV film, are the same.  But, with UV
exposure, laminates without UV films (on top) degrade.

There’s no easy correlation between the lab tests and results in the field. In
my experience, I would say that 300 hours UV lab was comparable to a glider that
was flown and exposed continuously though a season in Florida, Big Spring,
Owens, Forbes type conditions. After 300 hours of UV exposure the outer layer of
non UV film was brittle but not fractured.

The UV exposed sample starts letting go at .05 in/in strain and completely
fractures by .1 in/in strain, or at about half the strain (and strength) of the
unexposed film. Only the outer layer of film is fractured and the remaining 3
layers of scrim, film and taffeta continue to support load.

300 hrs of lab UV exposure correspond to about 150 hrs of airtime and associated
setup in a high UV environment (as above). At that point the outer layer of film
has lost 50% of its original strength. At 300 hours of airtime and exposure in a
high UV environment (two seasons), the outer layer of most conventional
laminates without UV film are fractured and beginning to delaminate.

In lab tests, after 300 hours of UV exposure, laminates with UV film (again on
top) are as good as new.

Laminates with UV film is available from Dimension-Polyant, but
according to Steve Pearson, you have to order a custom laminate, with a premium
price for custom construction and another premium for the UV film. He states
that as far as he can tell only Wills Wing uses UV film on their laminates. I
have asked Moyes (Gerolf), Icaro (Christian), and Aeros (Oleg) if they use
laminates with UV film. See below.

As a hang glider pilot it would seem to me that I would want a sail with UV film
on the top (unlike recent sails that were sewed for Highland Aerosports where
the UV film got placed on the bottom).

Wouldn't you want to know if the sailcloth used on your glider was topped with
UV film? When I asked Steve about this, he wrote:

These are the problems I started trying to solve twenty five years
ago.

Until we made the first UV film laminates we recommended woven materials for
recreational gliders that were used in hi-UV exposure climates. It’s rare for a
laminate film glider to last more than 300 hours (maybe in low UV Europe or
northern latitudes). Woven gliders last twice that long. UV-film laminates seem
to do even better than woven sailcloth.

There is a long history of the progression of laminate construction that
gradually improved durability and structural performance from the first
practical laminate “Surfcote” which was simply a layer of taffeta to one layer
1.5 mils – 2 mils of film, to the current state of the art with 4-5 layers,
high-strength fibers and UV film.

Oleg Bondarchuk <<aerosint>>
writes:

We've started using Porcher NCV laminates with UV protection film
more than ten years ago for all our trike wings and Combat hang gliders (http://www.aeros.com.ua/structure/hg/cbt07_en.php).
A bit later we have started using PX laminates with UV protection film for our
Combats and Discuses. We are still using NCV with UV protection for our trike
wings and PX with UV protection for Discuses and for some Combats.

We have been asking Dimension-Polyant about ODL laminate with UV protection, but
they wouldn't sell it to anyone except Wills Wing, because of exclusive contract
with them for selling this laminate. At the moment we are in process of getting
UV protected laminate from Dimension-Polyant.

Steve Pearson writes:

I wasn’t aware that Dimension-Polyant was restricting sales of
custom laminates with UV film. I was asked several years ago by Dimension-Polyant
for permission by another company to use a custom laminate configuration that we
were purchasing and I told them that they were free to share our configurations
with anyone. I would certainly extend that approval to Aeros, Moyes and Icaro.

You have to distinguish between “UV protection” which is incorporated into
almost all laminates in the adhesive and UV film. Dimension-Polyant laminates
all have “UV protection” but none except their custom D4 laminates have UV film.

There is no visible distinction between the films. UVODL06 and ODL06 are
identical looking. ODL06 has “UV protection”. UVODL06 is far superior with
respect to UV deterioration but otherwise the two are structurally identical.