Introductory
notes
As generally known, modern mountaineering ropes are
composed by a core and a surrounding sheath; both components
are made of very fine nylon threads – most frequently
polyamide 6 - with a diameter of 30 micron (equivalent
to the 30 thousandth part of a millimetre or half the
size of a normal hair). A “simple” UIAA
rope contains as many as 60 – 70 thousand [1]
threads.
Nylon is the best material for mountaineering ropes,
on account of its excellent tensile properties: high
breaking strength, large elongation at rupture and good
elastic recovery (in general, capacity to retain its
mechanical and dimensional properties even after a large
strain), and a very good manageability which implies
good functionality of the rope. However, it cannot be
said that the ultimate solution for the composition
of mountaineering ropes has been found yet. Indeed the
nylon threads are very sensitive to side-pressure (tight
knots, sharp edges, etc.), their filaments break easily
by rubbing against rock, their physical and mechanical
properties are strongly affected by the UV radiation
of sunlight; in addition, water intake causes reduction
of the dynamic performance of ropes (number of falls
held in the Dodero test[a] ).
These properties – common to all synthetic fibres
- are qualitatively well known, but their quantitative
aspects require further investigation, particularly
so for mountaineering equipment. Consequently, the Safety
Commission of the Italian Alpine Club has programmed
a wide research on some of the mentioned problems. Experiments
are being carried out in laboratory and in the field,
i.e. mountains or crags and rock faces.
Part 1 - Ropes and
sunlight: a matter of ... colour[2]
As already mentioned, exposure of nylon ropes to the
sun causes an irreversible deterioration of their physical
and mechanical properties due to reactions induced by
UV radiation. Photo-oxidation modifies the chemical
structure of the nylon macromolecules, since it initiates
their de-polymerisation, which leads to loss of resistance
and elasticity of the material. This process can be
reproduced in laboratory using a suitable artificial
light.
It is possible to reduce these reactions by means of
photo-chemical stabilization of the nylon fibres; this
can be accomplished using either UV protecting agents
similar to those used in sun creams (products that act
as filters for a radiation of specific wavelengths),
or anti-oxidation products [3] [4].
These processes - photo degradation of nylon and its
stabilization - have been well studied, but it is still
very difficult to clearly anticipate their development.
Particularly on materials used for mountaineering knowledge
is very scarce, so that mountaineers cannot answer questions
such as: How big is the reduction of the mechanical
characteristics of the core and sheath of a rope due
to sunlight, as a function of usage? What is the influence
of this degradation in terms of resistance to wear and
dynamic behaviour of the rope ?
Laboratory experiments
Ropes of five different makes, chosen among the most
commonly used, where exposed to artificial light as
well as to natural sunlight.
The exposure to artificial light was performed by means
of an apparatus usually employed in specialised laboratories
to produce accelerated photo chemical degradation, the
so called xenotest. This apparatus contains a xenon
lamp, which produces a radiation spectrum nearly identical
to the solar one; its radiation flux is about 10 times
that obtained by natural sunlight at sea level.
Experiments in-the-field
Exposure to natural sunlight occurred:
- at an altitude of 2550 m, on a wall of the Kostner
hut in Vallon (Sella group)
- at an altitude of 1843 m, on a wall of the Carestiato
hut (Moiazza group)
two representative places in the Dolomites area, very
popular with mountaineers.
Rope samples with a length of 15-16 m were wound in
a spiral within a metallic structure, a sort of cage
with a diameter of approximately 1 m. The resulting
“piece of folk art” (see PHOTOS 1 and 2)
was exposed on the southern wall of the huts during
the whole summer season, from June to the end of September.
To ensure a homogeneous exposure of the ropes to sunlight,
the hut keepers kindly agreed to give the cage half
a turn once a week.
Photo 1: Kostner hut
The tests
At the end of the season, the performance of the ropes
on the Dodero (number of sustained falls) as well as the
mechanical properties of sheath and core filaments were
tested.
It was obvious that the degradation of core filaments
was going to be small compared to sheath filaments, due
to the protection provided by the sheath.
Photo 2: The piece of folk art
Synthesis of the results
Colour degradation.
The following general remarks apply:
- ropes decolourise much more rapidly under irradiation
with the xenon lamp than under natural sunlight (PHOTO
3)
- on some ropes all colours fade uniformly whereas on
other types of rope some colours remain stable and others
disappear nearly completely (PHOTO 4)
- the ropes exposed at 2550 m on the Kostner hut lost
more colour compared to those exposed at 1834 m on the
Carestiato hut (PHOTO 5).
Decay in mechanical properties of the filaments.
A correlation was found – as expected - between
the fading of colour and the mechanical properties of
the filaments: the higher the loss of colour, the higher
the degradation of the mechanical characteristics. This
seems to concern mainly the brilliant colours and the
colours a-la-mode, say acid green and fuchsia. This may
be caused by either a rather vaguely described catalytic
action due to the chemical structure of the colour itself,
a process mentioned in literature [5], or by the reduction
of the filtering action of the colour against UV.
Core and sheath.
The degradation of filaments due to UV radiation is
more contained and more uniform throughout the core
than in the sheath, irrespective of the type of exposure
(sun/rain or xenotest). The larger sensitivity of the
sheath fibres to radiation is due not only to the
Photo 3: Rope 1
Photo 3: Rope 2
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