Hydrogen evolution from silicone resinsfor primary coating applications

S.R. Barnes, Ph.D., S.P. Riley, Ph.D., and S.V. Wolfe, Ph.D.

Indexing terms: Optical fibres, Optics

Abstract: The evolution of hydrogen from a thermally cured silicone primary coating has been evaluated.Using the experimental data, the maximum amount of hydrogen generated from this source in the lifetime of acable has been estimated. This has been shown to have a negligible effect upon cable attenuation.

1 Introduction

Molecular hydrogen can cause increases in attenuation inoptical fibres at wavelengths corresponding to character-istic absorption spectra [1, 2]. One such wavelength isadjacent to the 1300 nm window used extensively in single-mode systems. It is therefore desirable to quantify thesources of hydrogen in any particular cable design.

It has recently been suggested that the thermally curedsilicones could be a major source of hydrogen, and, if thiswere to be true, such materials would have to be avoidedin the design of optical-fibre cables [3]. One particularthermally cured silicone (Sylgard 182, Dow Corning) hasbeen extensively evaluated at STL. It has been shown thathydrogen release from Sylgard 182 is limited, and thematerial can therefore be used in optical-fibre cables.

2 Theoretical

The cure of Sylgard 182 proceeds by the establishment ofcrosslinks between vinyl groups attached to poly(dimethylsiloxane) of the base resin and Si-H groups (silylidyne)present in the curing agent. The crosslinking occurs by anaddition reaction catalysed by a soluble platinum com-pound.

I II I- S i - C H = CH2 + H - S i - ^ - S i - C H 2 - C H 2 - S i -

The base resin and catalyst are both comparativelyviscous liquids requiring efficient mixing, and, as the reac-tion proceeds, the viscosity increases.

Studies on crosslinked silicone networks by Flory andErman [4] at Stanford and by Kirk et al. [5] at MIT haverevealed steric limitations to the Si-vinyl Si-H additionreaction. When reactive sites occur on adjacent siliconeatoms, size limitations prevent poly(dimethyl siloxane)chains from assembling on every site. Because of this effect,the reaction is unlikely to proceed to completion [4, 5]. Asa consequence, there are residual Si-H groups in cured sili-cone coatings. These groups are liable to hydrolytic attack,especially alkali catalysed, with the generation of hydro-gen.

I I- S i - H + H2O— - S i - O H + H2

Therefore, in optical fibres comprising a silicone primarybuffer coating and a secondary coating of nylon, there ispotential for hydrogen generation due to the presence ofwater in the nylon extrudate.

By this mechanism, the generation of hydrogen is

Paper 3852J (E13), first received 31st December 1984 and in revised form 5th March1985

The authors are with Standard Telecommunication Laboratories Ltd., LondonRoad, Harlow, Essex, United Kingdom

limited by the availability of Si-H groups, and hence levelsoff with time.

3 Experimental procedure and results

In order to demonstrate the link between hydrogen gener-ation and concentration of Si-H, the Sylgard 182 catalyst/base ratio was varied. Separate fibres were coated in anidentical fashion with Sylgard 182 while decreasing theratio of catalyst/base resin from the recommended 1 partdown to 0.5 parts per 10 parts of base resin.

Fig. 1 shows the amount of hydrogen generated from

200

150

100

50

10:1

50 100 150 200 250 300test duration , hours

350

Fig. 1 Hydrogen evolution from fibres coated with Sylgard 182 withvarying base-resin/curing-agent ratio

equal lengths of such fibres after heating in a copper tubein a 100°C oven in a wet environment for periods of up totwo weeks.

The results show a steady reduction in hydrogen gener-ation as the ratio falls. They do not reduce to zero becauseof the steric effect referred to in the preceding text. Theresults also indicate the predicted levelling off with time, inless than two weeks, confirming the self-limiting nature ofthe hydrogen source. A decrease in measured hydrogenwas noticed in fibres which had been stored in ambientconditions for some time before testing, indicating thatsome hydrogen evolved at an early stage had already dissi-pated. Another significant means of reducing hydrogengeneration is to cure at higher temperatures. With a mod-erate increase in the temperature of the curing oven, thelevel of hydrogen generation can be reduced by at least afactor of two.

Extensive tests on hydrogen generation were run on oneparticular batch of production fibre pulled and coated withSylgard 182 at the STC Optical Fibre Unit, Harlow, half ofwhich was then secondary coated with nylon. The hydro-

IEE PROCEEDINGS, Vol. 132, Pt. J, No. 3, JUNE 1985 169

gen generation was measured at different oven tem-peratures at normal humidities.

•7 0.0035

E

'2 0.0030

- 0.0025

2 0.0020p

£ 0.0015

0.0010

0.0005

050 100 150 200 250 300 350

test duration, hours

Fig. 2 Hydrogen evolution from Sylgard 182 primary coated fibres atdifferent oven temperatures

-- 0.0035

100*C

50 150 200 250 300 350test duration , hours

Fig. 3 Hydrogen evolution from secondary coated fibres {SCF) at differ-ent oven temperatures

Figs. 2 and 3 show the hydrogen generation with timefor primary coated fibre and for secondary coated fibre,respectively. It appears that the hydrogen generated tendsto reach the same level from the two lengths. This suggeststhat the nylon has no inherent hydrogen production.

0.01

0.001

at

cit

8 0.0001

0.000011 10 100

test duration , hours1000

Fig. 4 Hydrogen outgassing of SCF at 100° C

+ UK-Belgium 5 fibre* fibre 1:343014

The results have been used to construct a master curvewhich can be used as a model for the behaviour of Sylgard182/nylon coated fibres.

As a practical illustration of the relationship betweenfibre and cable design, tests have also been carried out onsamples of fibre allocated for use in the proposed UK-Belgium 5 submarine optical cable.

The measurements are grouped together in Fig. 4. Therange of hydrogen generation is shown, and all the valuesfall below the master curve.

4 Discussion

From the master curve, values for the asymptotic level andtime constant for a simple experimental relationship canbe estimated. These are then used to obtain an asymptoticlimiting value for hydrogen generation from the popu-lation of UK-Belgium 5 fibres tested after 16 h and 60 h.The value is 1.3 ml/km per fibre.

It is now possible to estimate the maximum contribu-tion of hydrogen from Sylgard/nylon coated fibres in anyparticular design of cable over its lifetime.

Fig. 5 shows the cross-section of the UK-Belgium 5

Fig. 5 Cross-section of UK-Belgium 5 cableSDS 2551: two-component welded copper tube2: nylon-coated steel kingwire3: six nylon-coated optical fibres and two fillers4: water-blocking material

cable. The central core of the cable consists of six second-ary coated fibres plus two fillers around a nylon-coatedsteel king wire. This is contained within a welded thick-walled composite copper tube 70% filled with a high-viscosity waterblocking material. This tube provides abarrier against ingress of externally generated hydrogen.There are no dissimilar metal surfaces inside the hydrogenbarrier so that there will be no hydrogen generation due toelectrolytic effects. In addition to the secondary coatedfibres, all other materials within the hydrogen barrier havebeen experimentally assessed and have been shown toexhibit negligible outgassing or hydrogen generation fromchemical changes.

This cable has a free volume of 1.4 lkm"1 around thefibres. For the worst case of no solubility of hydrogen inthe other cable materials, all the gas will occupy the freevolume. Using the pessimistic estimate of hydrogen gener-ation of 1.3 mlkm"1 per fibre for the secondary coatedfibre and a negligible contribution from other sources, the

170 IEE PROCEEDINGS, Vol. 132, Pt. J, No. 3, JUNE 1985

maximum partial pressure in the free space of the cablewill be 577.55 Nm' 2 (5.7 x 10"3 Atm). Using test datadescribed by Pitt and Marshall [1], the likely maximumattenuation change resulting from all hydrogen effects atthis partial pressure over a period of 25 years will thereforebe no more than 1.4 x 10"3 dB km"1 at 1310 nm. Siliconeprimary coated fibres can therefore be used with con-fidence in current undersea cable designs.

5 Acknowledgments

The authors would like to thank the directors of StandardTelecommunications Laboratories Ltd. (and STC Tech-nology, Ltd.) for permission to publish this paper. Theywould also like to acknowledge the assistance of Mr. J.H.Rhys-Jones in the pulling of fibres and Mr. D.K. Patel inhydrogen evolution measurements.

6 References

1 PITT, N.J., and MARSHALL, A.: 'Prediction of long term lossincrease in single mode optical fibres exposed to hydrogen'. IEE Collo-quium, London,June 1984

2 MOCHIZUCHI, K., NAMIHARA, Y., and YAMAMOTO, H.:'Transmission loss increase in optical fibres due to hydrogen per-meation', Electron. Lett., 1983, 19, p. 743

3 MIES, E.W., PHILEN, D.L., REENTS, W.D., and MEADE, D.A.:'Hydrogen susceptibility studies pertaining to optical fibre cables'.OFC '84, New Orleans, W13-1-W13-4

4 FLORY, P.J., and ERMAN, B.: 'Silicone networks with junctions ofhigh functionality and the theory of rubber elasticity', J. Poly. Sri.Polym. Phys. Ed., 1984, 22, pp. 49-55

5 KIRK, K.A., BIDSTRUP, S.A, MERRILL, E.W., and MEYERS,K.O.: 'Tensile and swelling behaviour of model silicone networks withlow extense of crosslinks', Macromol., 1982,15, pp. 1123-1128

6 BARTLETT, K., DOBSON, J.V., and EASTHAM, E.: 'A new methodfor the detection of hydrogen in breath and its application to acquiredand inborn sugar malabsorption', Clinica Chimica Act, 1980, 108, pp.189-194

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