EoW July 2008

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 T g ). 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.

Coating E'

σ

Ratio

cav

/E'

(MPa)

(MPa)

σ

cav

A 0.37 0.95

2.6 1.2 1.9 2.3 2.3

B C

0.97 1.21

1.33

2.5 2.8 2.1

D 1.2

E F

0.9

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 T g primary coat- ings, for improved micro-bending protec- tion, in combination with the high cavitation strength. n 2.4 Table 1 ▲ ▲ : The measured cavitation strength properties of the selected primary coatings [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 0.64 1.51 5. References

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

1 DSM Desotech Inc 1122 St Charles Street Elgin, IL 60120 USA Tel : +1 847 214 3836 Email : huimin.cao@dsm.com Website : www.dsm.com 2 DSM Research Geleen, The Netherlands Tel : +31 46 476 1853 Email : markus.bulters@dsm.com

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EuroWire – July 2008