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EuroWire – July 2008 63
technical article
For example, the coatings shown in Figure 10
were reported as having cavitation strength
values of 0.96 MPa and 1.49 MPa, respectively.
3.2 High cavitation strength primary
coatings
As discussed in 2.1.2, coating cavitation occurs
when the tri-axial tensile stress exceeds the
cavitation strength of the coating material. In
order to reduce the risk of coating cavitation,
the two effective approaches are to 1) reduce
the level of thermal stress, and/or 2) increase
the coating cavitation strength. The thermal
stress level is affected by both coating layers
where secondary coating plays a more
important role than the primary coating.
On the other hand, cavitation strength is an
intrinsic property of the primary coating. A
high cavitation strength primary coating is
always desired to ensure the robustness of
the coated fibre, under conditions of thermal
stress and any possible mechanical stresses
encountered during processing, handling
and deployment in the field.
Table 1 gives several examples of primary
coatings with different cavitation strength
behaviour. The cavitation strength (σcav) was
measured using the test method described in
3.1. The values of storage modulus E’ at room
temperature from DMA and the ratios of
σcav /E’ are also listed.
As discussed in 2.1.2, the cavitation strength
of an ideal rubber should be (5/6)E. From
Table 1, each of the coatings has a cavitation
strength higher than its modulus, which
indicates the coatings do not comply with
perfect elasticity.Themodulus, corresponding
to the crosslink density of the coating, still
plays an important role in determining the
cavitation strength of a coating material.
However, through proper molecular level
design of the polymer network structure,
high cavitation strength can be achieved
independently of the coating modulus.
In other words, ideal soft, but tough coatings
having a high ratio of cavitation strength/
modulus can be realised. The low modulus
is for the benefit of better micro-bending
performance.
From Table 1, Coating A has the lowest
modulus, however, its cavitation strength is
also the lowest (<1 MPa). In fact, the fibre with
this coating showed severe cavities from the
cooling process after fibre drawing. Coating
B, with cavitation strength equal to 1.21 MPa,
is considered strong enough to withstand
the thermal stress encountered during fibre
cooling. No cavities were observed on the
fibre with Coating B. Also from theoretical
analysis, this cavitation strength level is
sufficiently higher than the calculated ~0.8
MPa thermal stress in the primary coating.
However, the ratio of σcav/E’ of Coating B is
only 1.2, the lowest among all the coatings.
This type of coating is considered adequate
to withstand the regular stress situations, but
did not realise its full potential to become a
highly robust coating material.
On the other hand, Coatings C, D, E and F
exhibit the desired high cavitation strength
properties. The modulus of Coating C or
Coating D is at the typical level among
commercial primary coatings. However, their
cavitation strength is designed to be at an
exceptionally high level through optimum
molecular structure of the crosslinking net-
work. Coating E has a medium-low modulus
level (combined with low T g), which was
developed to be applied on both single
mode and multi-mode fibres. The cavitation
strength of this coating is still at very high
level (2.1 MPa) and allows for a high ratio of
σcav/E’ (2.3). Coating F provides excellent
micro-bending resistance attributed to the
ultra-low modulus (and low Tg). In the mean
time, a sufficiently high level of cavitation
strength (1.51 MPa) has also been achieved
with the ratio of σcav/E’ being as high as
2.4. For ultra-soft coatings like this, special
precautions must be taken to incorporate
the property of good cavitation strength
into the coating structure. Otherwise, the
pitfall of developing coating cavitation and
deteriorating fibre attenuation performance
is a possible risk.
Situations such as Coating A where cavities
were already present in the fibre after drawing
can be easily identified. The hidden risk lies
in situations where cavities in the coating
can gradually form and cause attenuation
increase in the field, when the fibre goes
through environmental temperature cycles
or stays at low temperatures for a long
time period, ie in submarine cables. A
carefully designed high quality coating
system not only contributes to premium
fibre performance but also provides better
long-term reliability of the optical fibres.
4. Conclusions
Primary coating cavitation has been studied
comprehensively as a possible failure mode
in dual-layer coated optical fibres.
The driving force for coating cavitation is a
tri-axial tensile stress, which can be induced
by internal thermal stress or external
mechanical impact. The coating ruptures
cohesively when the tri-axial tensile stress
exceeds the coating cavitation strength. A
test method was developed to quantitatively
evaluate the cavitation strength of a coating
material.
Through understanding of the coating
cavitationmechanism and insights on coating
cavitation resistance, it has been possible to
design coating materials with high cavitation
strength to provide robustness to coated
fibre under potential thermal and mechanical
stresses. High cavitation strength/modulus
ratios have been obtained, to afford the
desired low modulus/low Tg primary coat-
ings, for improved micro-bending protec-
tion, in combination with the high cavitation
strength. n
5. References
[1] D Gloge, ‘Optical-fiber Packaging and Its Influence
on Fiber Straightness and Loss’, The Bell System
Technical J, 54(2), 245-262 (1975)
[2] W W King, ‘Thermally Induced Stresses in an
Optical - Fiber Coating’, J of Lightwave Technology,
9(8), 952-953 (1991)
[3] W W King and C J Aloisio, ‘Thermomechanical
Mechanism for Delaminations of Polymer Coatings
from Optical Fibers’, J of Electronic Packaging, 119,
133-137 (1997)
[4] P L Tabaddor, C J Aloisio, C H Plagianis, C R
Taylor, V Kuck and P G Simpkins, ‘Mechanics of
Delamination Resistance Testing’, International
Wire and Cable Symposium Proceedings, p 725
(1998)
[5] C J Aloisio, WW King and R C Moore, ‘A Viscoelastic
Analysis of Thermally Induced Residual Stresses in
Dual Coated Optical Fibers’, International Wire and
Cable Symposium Proceedings, p 139 (1995)
[6] A N Gent and P B Lindley, ‘Internal Rupture of
Bonded Rubber Cylinders in Tension’, Proc Roy Soc
A, 249, 1958
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Coating E' σcav Ratio
(MPa) (MPa) σcav /E'
A 0.37 0.95 2.6
B 0.97 1.21 1.2
C 1.33 2.5 1.9
D 1.2 2.8 2.3
E 0.9 2.1 2.3
F 0.64 1.51 2.4
Table 1 ▲ ▲ : The measured cavitation strength
properties of the selected primary coatings
Figure 9 ▲ ▲ : Example of cavities in a sample recorded
by the camera (20x) at certain stress level
Figure 10 ▼
▼ : Tensile stress in relation to the number of
observed cavities in two coating materials