EoW July 2008

technical article

This means the stress in the primary coating at room temperature is a hydrostatic tensile stress. It increases as temperature decreases further until reaching the primary coating T g (typically ~-20ºC), when the primary coating also turns into the glassy state. The calculated tensile stress in the primary coating is ~0.8 MPa at room temperature as shown in Figure 2 . Due to the visco-elastic property of the secondary coating, the actual stress level should be lower than the calculated stress and decrease with time as the secondary coating undergoes stress relaxation at sub-T g temperatures. [5] While the risk of coating cavitation by thermal stress is low for typical dual-coated fibres, precautions must be taken to evaluate certain types of coating systems discussed below. The new developmental trend for primary coatings is to further reduce their modulus and T g to provide improved micro-bending buffering protection over a wide temperature range. In this type of coating system, the tensile stress keeps building up as temperature begins to drop, yet the primary coating remains in its rubbery state. As shown in Figure 3 , the calculated tensile stress increases linearly with tem- perature decrease. The stress relaxation of the secondary coating is also much slower at low temperatures. In addition to the risk of high thermal stress, a lower modulus primary coating may also be more prone to cavitation, due to its lower crosslink density. It is therefore very important that primary coatings with low modulus and low T g be carefully designed to have high cavitation strength through optimum structure of the crosslinking network. In-depth knowledge of the cavitation resistance of UV curable coating materials at the molecular level allows the development of coating systems having improved micro- bending performance combined with high cavitation strength, to assure robust fibre performance over a wide temperature range. Another example of a high-risk situation with regard to cavity formation is fibre with thicker than standard coating layers.

The tensile stress in the primary layer of a fibre with the glass/coating OD structure of 125/350/500 μm is calculated and also plotted in Figure 3 . The tensile stress in the primary coating of this fibre is 2.8 times the stress level of that in the primary coating of a standard 245 μm OD coated fibre. Therefore, fibres having thicker coating layers should be composed of a primary coating having high cavitation strength in combination with a secondary coating having faster stress relaxation. 2.1.2 Cavity formation in the primary coating. Figure 4 shows microscope images of some cavities formed in a 500 μm OD coated fibre, after temperature cycling between 85ºC and -60ºC. Irregularly shaped coating ruptures of different sizes can be observed in the primary coating layer. The fact that the coating ruptures are wide open, shown as voids, indicates the presence of a tri-axial tensile stress in the primary layer at room temperature. From fracture mechanics theory, the parameter representing the cavitation resistance of a material is called cavitation strength. When the tri-axial stress reaches this critical point, the material starts to rupture and form internal cavities. It has been calculated and proved experimentally that for an ideal rubber, the tri-axial stress for a very small spherical hole to be inflated unboundedly is (5/6)E, where E represents the Young’s modulus. [6] Any microscopic network defect in the material may serve as the initial rupture site. This means for a 1 MPa primary coating, a tri-axial tensile stress of 0.83 MPa can already cause cavity formation according to the un-bounded growth mechanism, if the coating material behaves like an ideal rubber. By proper molecular design of the coating’s cross-linked network structure, the desired high cavitation resistance can be achieved, with the cavitation strength significantly exceeding the coating modulus. In this type of high cavitation strength primary coatings, small cavities will not grow

Figure 4 ▲ ▲ : Cavities in the primary coating layer induced by temperature cycling in a 500µm fibre (left) 40x (right) 200x

Figure 5 ▲ ▲ : A schematic diagram of the localised tensile stresses in the primary coating by a mechanical lateral force

un-boundedly and the material will not rupture even under a relatively high tensile stress level that could be present in the primary coating. 2.2 Cavities induced by the mechanical stress In addition to the hydrostatic thermal tensile stress, cavity formation in primary coatings can also be driven by anisotropic tri-axial stress resulting from a mechanical impact on the coated fibre. It has been previously reported that coating tears were observed under high tension, when pulling fibre through a re-winder assembly to test the coating’s resistance to de-lamination. [4] When an external mechanical force is exerted on a coated fibre, the coating layers will de-form and result in a non-uniform stress field in the coating material. Figure 5 schematically illustrates the defor- mation of the coating layers under a lateral force F. Since the secondary coating is a much harder material than the primary coating, the secondary layer behaves like a hollow tube being pressed under the lateral pressure with the shape of the tube changing to oval, but with no deformation on coating thickness. The primary coating is bonded on both sides with glass and secondary and is forced to deform internally. The areas of primary coating along the force direction are compressed and the areas perpendicular Figure 6 ▲ ▲ : Mean normal stress in the primary coating layer induced by a mechanical lateral force calculated by Finite Element Analysis

Figure 3 ▼ ▼ : Calculated thermal stress vs temperature for a regular 250µm fibre (assuming the stress starts to develop below secondary Tg ~50°C)

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EuroWire – July 2008