EoW November 2008

technical article

The acrylategroups respondby crosslinking via the freeradical mechanism of polymeri- sation, following photo-initiation induced by the UV curing lamps at draw. The kinetics dictates a reduced cure speed during processing, unless steps are taken to modify the process for optimum cure. This is achievable by an understanding of the character of the primary coating curing process. There are at least two components of the curing process that act to retard the rate of polymerisation of the soft primary coating. First, the high temperature of the curing coatings induced by exposure to the high intensity UV lamp environment and the exotherm of polymerisation reactions slows the overall observed rate [8] . Second, it has been demonstrated that close proximity of stacked UV lamps create in effect rapidly superposed, repeated photo-initiation periods. The rate of disappearance of acrylate groups under this condition is again retarded. The disposition of UV lamps is such that time is maximised between repeat UV exposures resulting in a significant increase in the coating degree of cure, comparing processes with the same speed and overall UV dose [9], [10] . Thus it is possible to deal effectively with a reduced modulus primary coating and achieve a near-complete cure at required fibre draw speeds. A second aspect of the primary coating for enhanced micro-bending protection in FTTx applications is the temperature dependence of the modulus. While a low modulus may be a characteristic at room temperature, deployment in the field will find fibre exposed to temperature extremes where microbend-inducing stresses may be present. Therefore, a lowest possible glass transition temperature T g is required so that the primary coating remains soft and protective in all situations. A tough secondary coating is necessary to protect the primary coating and glass from damage during handling and installation. This coating may be designed to be inked as a colour code or it may be colour-inclusive to provide identification without the necessity of a separate inking process. 3 Results A new primary coating, a development based on the commercial graded index multi-mode product’s coating, has been adapted for application to single mode fibre designs, particularly targeting extreme deployment environments such as FTTx.

Figure 2 ▲ ▲ : Dynamic mechanical properties of a commercial singlemode primary coating, stress rate at 1 Hz

Figure 3 ▲ ▲ : Dynamic mechanical properties of the new singlemode primary coating, stress rate at 1 Hz

On fibre, this coating typically cures well to a modulus of about 0.8 MPa, a level characteristic of most singlemode fibre primary coatings in the industry. The reasons for the discrepancy between the film modulus and in-situ modulus are detailed in references [8] through [10] . ’, estimated by the peak in tanδ, is at approximately -30°C. Thus the coating, and other similar formulations, will respond as a glass at extreme low temperatures like -40 to -50°C. (This is an incomplete picture, as there is a time dependency to the stress induced by strain at low temperature, but the ‘T g ’ remains a useful metric for comparison). Figure 3 shows the dynamic mechanical properties of the new primary, using a film sample made similarly to the above example. In Figure 3 , the new primary coating exhibits an equilibrium modulus at just under 1 MPa in the cured film, and on fibre the insitu modulus is typically measured at 0.3 to 0.4 MPa, the target. The ‘T g

The preferred secondary coating pro- tecting the fibre structure features an optimised colouring system inclusive in the bulk, not requiring an extra layer of ink to be added for colour-coding. The new colours are enhanced for brightness and visibility under dim lighting situations, eg in deep shade or in manholes. 3.1 Mechanical properties The dynamic mechanical properties of a typical commercial primary coating are shown in Figure 2 . The data was obtained on a TA DMA at 1 Hz oscillatory stress rate, taking care that the strain is kept within the linear region of stress-strain behaviour. The sample of coating was cured on polyester in a 75-micron film with a UV dose of 1 J/cm 2 . The lamp used is a mercury-halide bulb operating at 300 W/ inch output. This UV exposure is sufficient to ensure the material is on the plateau of the dose-modulus curve. The data shows the equilibrium modulus to be approximately 1.5 MPa.

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EuroWire – November 2008