New Methods for Evaluating Optical Fiber Coating Materials
by Takayori Ito *, Hidefumi Miyagi *, Dai Fujiwara * and Yoshihisa Kano *UV-curable resins are used as the coating material for optical fibers, and their
physical properties change with the degree of curing, thereby influencing thetransmission characteristics of the fiber. In order to maintain stable quality of the finished productit is necessary to evaluate the degree of curing of the coating resin while it is on the fiber. Theauthors have measured the logarithmic damping ratio for optical fibers double-coated with UVresin using a rigid-body pendulum-type viscoelasticity testing apparatus, and, from the results,
have clarified the relationship between the dose of UV irradiation and the degree of curing of thecoating resin on the optical fiber.
It is also necessary, as a means of improving the efficiency of manufacturing optical fibers, todevelop a rapidly curable UV coating material, but since the reaction proceeds in such a shorttime, it has been difficult to establish an effective method of evaluating the degree of UV curing.
In this work it has been possible, using a rheometer incorporating a fast oscillation mode program, to evaluate changes over time in the modulus of elasticity of UV-curable resins under irradiation.

ABSTRACT1.

INTRODUCTION

Optical fibers are crucial in the field of telecommunications, offering the advantages of low loss. broad bandwidth, fine diameter, light weight, freedom from electromagnetic induction, freedom from crosstalk, and efficiency
in resource consumption. In comparison to the copper
conductors used in conventional telecommunications
cables, however, they are at a disadvantage in terms of
their low mechanical strength, which makes them brittle.
From the processes of fiber drawing, coating and cabling
to the laying of the cables, they are subjected to tensile
force, and subsequently, under a variety of environments,
they must have superior static and dynamic fatigue characteristics. For this reason optical fibers, immediately afterdrawing, are covered by a coating of resin to improve theirmechanical strength. This paper describes methods forevaluating the degree of curing of optical fiber coating
materials, and then introduces a method for evaluating thedegree of resin curing using a rigid-body pendulum-type
viscoelasticity testing apparatus.

2.

EVALUATING DEGREE OF CURING OF
COATING MATERIALS

Optical fibers are so designed that their transmission characteristics will not change despite fluctuations in the temperature and humidity of their environment or the passageof time, and in this the material used as the coating plays* Polymer Engineering Center, Ecology and Energy Lab.

an important role. The most generally used fibers are silica-based, and they have superior transmission characteristics, but silica glass is easily fractured by external forces.
The fibers are therefore covered with a coating of resin
which is usually of a 2-layered structure. The primarylayer--the layer directly in contact with the glass--has thepurpose of dispersing external force, and is made of aresin having a low modulus of elasticity. It must alsoadhere closely and form a stable interface. Outside thisprimary layer is the secondary layer, which, to protect it
and compensate for its mechanical strength, is made of aresin having a comparatively large modulus of elasticity(see Figure 1). Also, in multifiber cables, there may be apigmented layer on the outside for color coding.
In recent years, to improve manufacturing efficiency, thetrend has been to use UV resins that cure in an extremely
short time. It has therefore become extremely important,from the standpoints of assuring the quality of the finishedproduct and achieving stable production, to obtain anunderstanding of the rheological properties of the resin-coating characteristics, curing behavior, curing conditions,and so on (see Figure 2).
Pigmented layer
Secondary layer
Primary layer
CladdingCore

Figure 1

Structure of coated optical fiber.

1Fiber preform

Heating furnace

Curing behavior:degree, rateUV
irradiator

Fiber diameter gauge

Coating characteristics:
shear viscosity, wetness

Die
Control system

Bare fiber
Dancer rollers

Fiber coating material properties:
interface stability, mechanical strength

Figure 2

CapstanTake-up drum

Process model of optical fiber drawing/coating apparatus and required rheological properties.

Table 1

Methods of evaluating degree of curing for optical fiber coatings.

Mechanical methods

Spectroscopic methods

Thermoanalytical methods

Qualitative / quantitative analysisDynamic viscoelasticity

ATR-FTIR

Photo DSC

GC-MSTMA

Microscopic FTIR

Gel fraction measurement

Push-in modulus

Time-resolved FTIR

Photochemical reaction
calorimetry

Dynamic cure meter
Rigid-body pendulum-type
viscoelasticity testing apparatus

Methods of evaluating the degree of curing may bedivided broadly into spectroscopic methods, thermoanalytical methods, mechanical methods and qualitative/quantitative analysis. Table 1 lists the most important methods.
The degree of curing of a UV-curable resin varies depending on the intensity of irradiation, the dose of irradiatingenergy, the temperature during curing, and the curing
atmosphere. It is therefore commonly seen that evaluations differ when based on a bench test of the resin alone
or on the cured coating of a finished product. The authors
have given particular emphasis to evaluating the degreeof curing of resin as a finished product--when it forms thecoating on a silica-glass fiber.

Oscillation
SampleMetal

Figure 3

Rigid-body pendulum type viscoelasticity testing
apparatus.

A1

3.

A2

A4

EVALUATING DEGREE OF CURING OF
COATING MATERIAL
P1Using an A&D Model RPT-3000 rigid-body pendulum typeviscoelasticity testing apparatus (Figure 3), the authors
measured the logarithmic damping ratio ∆ of the UV-curable coating resin on an optical fiber and the period T of
the pendulum, end evaluated the degree of curing. Thefulcrum of the pendulum was placed on the surface of thesample and the pendulum was made to oscillate, and byanalyzing the mechanism of the damping of the amplitudeof oscillation and the period of oscillation it is possible toevaluate the degree of curing and other mechanical properties of the sample. At the temperature at which the sample undergoes a phase change from a vitreous state tobecome a rubbery elastic body, the loss in pendulum
motion increases and its amplitude is rapidly damped. The
waveform of the pendulum damping curve is modeled in

2

A3

P2

P3

Adsorption time

Adsorption time
Measuring interval

Figure 4

Model of pendulum damping curve.

Figure 4. The logarithmic damping ratio ∆ can be calculated by Equation (1)∆ = {ln(A1/A2)+ln(A2/A3)+......+ln(An/An+1)}/n(1)and oscillation period T (in seconds) by
T = (P1+P2+......+Pn)/nwhere: n is the number of oscillations

Furukawa Review, No. 22(2)2002Logarithmic damping ratio ( ∆ )

0.35

4.

Irradiation dose:
Large (T1 = -8.3; T2 = 138.5)
Medium (T1 = -9.6; T2 = 134.0)
Small (T1 = -10.73; T2 = 1308.5)

0.3
0.25
0.2
0.15
0.1
0.05
-100

-50

050

100150Temperature (°C)

Figure 5

Temperature dependence of logarithmic dampingratio ∆ of a prototype UV-resin coated optical fiber.

1.E+06
1.E+05G' (Pa)1.E+04
Intensity of irradiation

1.E+0332 mW/cm2
1.E+0245 mW/cm2
63 mW/cm2

1.E+01

88 mW/cm2

1.E+00
1.E-01012345Time (s)Figure 6

Change in modulus of elasticity G' under UV irradiation for UV-curable resin.

Figure 5 shows the temperature dependence of the logarithmic damping ratio ∆ of a prototype UV-resin coatedoptical fiber. The peaks corresponding to the glass transformation temperature Tg of the materials of the primarycoating and secondary coating were observed in the vicinity of -10°C and 130°C respectively. Correlations between
these peaks and the glass transformation temperatures
measured using other viscoelasticity testing equipment
have been confirmed. When the dose of UV irradiation
was increased the peak temperatures using the present
evaluation method shifted higher. The shape of the peaks
also became broader, indicating that it is the peaks in logarithmic damping ratio ∆ that shows the condition of curing. Since the present method makes it possible for thecondition of curing of the materials of both the primary andsecondary coatings on the fiber to be evaluated directly, it
promises to be applicable to on-site quality control during
manufacture.

EVALUATING CURING BEHAVIOR OF
RESINS UNDER UV IRRADIATION

Understanding the behavior that takes place in theprocess of UV curing is of major concern with respect tooptimizing the conditions under which optical fibers aremanufactured and improving product reliability. Changes
in the modulus of elasticity were measured during thecourse of the resin curing process using Rheologica's
DAR-100 rheometer, which incorporates a fast oscillation
mode program enabling evaluation to be accomplishedwith extremely short sampling times. It can therefore beapplied to the behavior of optical fiber coating materials
that exhibit very quick UV-curing.
This program samples each sinusoidal wave cycle during dynamic measurement of viscoelasticity at 512 points,
enabling the calculation/detection of the torque at each ofthese points. Thus if dynamic viscoelasticity is measured
at a frequency of 1 Hz, data can be obtained for 512
points per second. Figure 6 shows changes in the stored
modulus of elasticity G' during the curing of coating resin
under constant UV irradiation. It was observed that in aperiod of approximately 1 second, UV irradiation produced
an increase in G' of some five orders of magnitude--from 1
Pa to 105 Pa. And by measuring the change over time in
the modulus of elasticity while changing the intensity ofirradiation, it was also possible to confirm that an increasein irradiation intensity produced faster curing of the resin.
These results mean that this system clarified the changes
in G' during the curing process, and has proved to behighly effective for evaluating coating resins that are cured
very quickly under UV irradiation.

5.

CONCLUSION

This paper has touched briefly on methods used to evaluate the degree of curing of optical fiber coating materials.
It then describes an evaluation method advanced by theauthors that uses a rigid-body pendulum-type apparatus
for testing viscoelasticity and presents the results of theuse of a stress-control rheometer in evaluating the curing
behavior of a fiber-optic coating material cured under UV
irradiation.
It is believed that evaluating viscoelasticity with respect
to optical fiber coating materials will contribute significantly
to developing new products and maintaining product reliability.

3