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EuroWire – July 2008
60
technical article
High cavitation strength
primary coatings for
optical fibres
By Huimin Cao
1
, DSM Desotech Inc, Elgin, Illinois, USA;
and Markus Bulters
2
and Paul Steeman
2
, of DSM Research, Geleen, Netherlands
Abstract
It is well known that the design of soft
primary coatings in combination with
hard secondary coatings provide good
micro-bending protection for dual-coated
optical fibres. However, this dual layer
design also introduces thermal stress in
the coating system due to the mismatch
of the thermal expansion/contraction of
the two coating layers. Under the tri-axial
tensile stress, the soft primary coating may
form internal ruptures. The cavitation of the
primary coating is a possible defect mode
that can be detrimental to fibre attenuation
performance. In this paper, the mechanism
for coating cavitation in terms of different
types of driving forces is discussed. Cavitation
strength of the primary coating is introduced
as a key property for achieving a robust,
high-performance coating system with the
desired low micro-bending sensitivity in
combination with high cavitation resistance.
1. Introduction
One of the major advantages of the
dual-layer coating design for optical fibres is
to provide better micro-bending protection
than that afforded by a single layer coating.
Soft primary coating, acting as a buffer layer,
combined with hard secondary coating,
acting as a shielding layer, provides ideal
bending resistance for the fibres to withstand
external stresses in a cable environment.
[1]
Thermal stress in the dual-layer coating
system is inevitable due to the different
thermal expansions and contractions of
the glass, primary coating, and secondary
coating. Standard single mode or multi-mode
fibres with high quality dual-layer coatings
do not exhibit out-of-spec attenuation
increase during temperature cycling, because
the thermal stress is distributed uniformly
around the fibre. However, for fibres having
a certain amount of defects in the coating
system, especially in the primary coating, a
high level of attenuation from micro-bending
loss can be present at room temperature, and
the attenuation can increase dramatically as
temperature drops due to the non-uniform
thermal stress imparted by the defects.
Potential defects in the primary coating include
particles and gels, crystal formation, geometry
irregularities, de-lamination, and cavities.
De-lamination and cavities are both associ-
ated with tensile stresses in the primary
coating introduced thermally or mechani-
cally. While the de-lamination of primary
coating from glass has been well studied,
[3,4]
the
possibility of cavity formation from internal
rupture of the primary coating has not been
adequately addressed. Although primary
coatings usually have very high elongation
under uni-axial tensile stress, the coating
material may develop internal ruptures under
a tri-axial tensile stress. In-depth research
work has been conducted at DSM Desotech
in recent years to study this possible failure
mode. The mechanism of cavity formation in
the primary coating has been investigated
and the development of primary coatings
with high cavitation resistance has been
achieved through proper molecular design
of the cross-linking network structure of the
coatings.
2.
Mechanism of cavity
formation in the
primary coating layer
The driving force for cavity formation in
the primary coating is the tri-axial tensile
stress, which at a high level may exceed the
cavitation strength of the coating and cause
cohesive failure of the coating structure.
Two types of tri-axial stresses can be present
in the coating depending on different
origins. The stress can be thermally induced
from temperature variation or induced from
external mechanical forces.
2.1 CavitiesInducedbythethermalstress
2.1.1 Thermal stresses in a dual-layer coating
system.
It has been well understood that
thermal stresses are present in the coated
fibre system.
[2-5]
The tri-axial stress in the
primary coating, as illustrated in
Figure 1
,
is caused by the mis-match between the
thermal expansion coefficients of the glass,
primary coating and secondary coating.
Based on the theory of material mechanics,
the tri-axial stress, consisting of radial stress
σr, tangential stress σ
θ
and axial stress σ
z
components can be calculated.
Figure 2
shows
the calculated stress distribution in a typical
dual-layer coating system where coating layer
thickness is 30 μm each, Young’s modulus
E1=1MPa, E2=1GPa, linear thermal expansion
coefficients α1=3x10-4/K, α2=1x10-4/K and
Poisson ratios ν1=0.5, ν2 =0.4.
The system is exposed to a temperature
change of -30ºC, to simulate the stress in
the coating system when the coated fibre is
cooled down from the drawing process to
room temperature. Although the temperature
in the coating during UV-curing could be as
high as 100ºC, the thermal stress only starts
to build up when the temperature drops
below the secondary coating T
g
(~50ºC).
The three stress components in the primary
coating are tensile and all at the same level as
shown in
Figure 2
.
Figure 1
▲
▲
:
Tri-axial thermal stresses in a dual-layer
coating system
Figure 2
▲
▲
:
Calculated thermal stresses in a dual-layer
coating system