EuroWire September 2020

Technical Article

Analytical study of soft coating material for optical fibre By Kumpei Kobayashi, Takumi Nakajima, Hirofumi Uchida and Noriyasu Shinohara, Tsukuba Laboratories, JSR Corporation, Japan

Abstract Mechanical properties of UV curable urethane acrylates having low Young’s modulus for optical fibre were studied. Two kinds of urethane acrylate oligomer were applied to UV curable urethane acrylate coatings. The analysis of dynamic mechanical responses by tanδ curves suggested the presence of phase separation and different phase separation status. It was found that the nanoscale morphology of phase separate film was visualised by atomic force microscopy (AFM). 1 Introduction Recently UV curable urethane acrylates have been widely used as coating materials for optical fibre and they are basically formulated with urethane acrylate oligomers (UA), acrylic monomers and photo initiators. One of the strong demands for optical fibre is a low micro bending attenuation. The fibre can be obtained by double-layered coatings of a soft material and a hard material on a glass fibre. In recent years, lower Young’s modulus for a soft coating material and higher Young’s modulus for a hard material have been required to suppress micro bending under harsh conditions. In general, a soft coating material having lower Young’s modulus tends to show lower mechanical strength, and this low mechanical strength leads to a bad yield ratio in producing optical fibre because of the formation of cavities in the soft

coating material. Therefore, it is known that to achieve high mechanical strength with low Young’s modulus is difficult. In a previous study, we reported that the induction of the phase separation to a coating was one good solution to obtain high mechanical strength with low Young’s modulus [1] . However, in this report, the presence of a separated phase for the films was confirmed only by analysis of dynamic mechanical response, and little was known about the detail of the morphologies. It is necessary to examine the nanoscale morphologies in order to elucidate the phenomenon. As a first step, we visualised the morphologies of phase separate films with low Young’s modulus by atomic force microscopy (AFM). In this paper, we describe our recent work and information obtained about mechanical properties and morphologies of UV curable soft urethane acrylates. All samples of soft coatings consisting of urethane acrylate oligomers, acrylic monomers and photoinitiators were prepared in our laboratories. Urethane acrylate oligomers (UA-1, UA-2) Di-functional urethane acrylates (UA-1, UA-2) were prepared from a polypropylene diol and 2,4-toluene diisocyanate end-capped with 2-hydroxyethyl acrylate and an alcohol. Monomers A mixture of two mono-functional acrylates was used. The ratio of the two monomers was kept constant at 40:60. In addition to the acrylates-2, 6 wt % of N-vinyl monomer was included in the formulations. Experiment 1.1 Materials

Photoinitiator and additives As a photoinitiator, 2 wt % of 2,4,6- trimethylbenzoyl diphenyl phosphine oxide was used. Di-functional monomer 0~3.5 wt % of di-functional monomer derived from hydrocarbon based di-alcohol was added. 1.2 Preparation of physical properties The formulated resin was drawn on a glass plate by using a 381-μm (15 MIL) applicator. The resin on the glass plate was passed under metal-halide lamps (275 mW/cm 2 ) with a UV dose of 500mJ/cm 2 . The cured film was peeled off from the glass plate and turned over, and passed under the same lamps once again. The film was conditioned for 24 hours at 23°C, 50% RH. The thickness of the film was about 200 μm. 1.3 Evaluation of physical properties Tensile properties were measured at 23°C, 50% RH. Young’s modulus was measured by using a tensile testing machine at tensile speed of 1mm/min with crosshead distance of 25mm. The modulus was defined by the secant modulus at 2.5% elongation. The tensile strength at break and tensile elongation were measured under the same condition except the tensile speed of 500mm/min was applied. 1.4 Analysis of dynamic mechanical response Dynamic mechanical response of the cured film was measured by using a visco-elastometer in the temperature range of -100°C to +100°C, with heating rate of 2°C/min and at a frequency of 3.5 Hz. Temperature dependence of storage modulus (E’) and loss tangent (tanδ) for the cured film was measured.

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September 2020