EoW July 2008

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.

Figure 1 ▲ ▲ : Tri-axial thermal stresses in a dual-layer coating system

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 2 ▲ ▲ : Calculated thermal stresses in a dual-layer coating system

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EuroWire – July 2008