Special Issue

Polymer Materials Technology

A Method Related to Adhesion of Glass / Primary Interface
for Long-term Reliability
Minoru Kasahara*, Yoshihiro Arashitani*, Hiroki Tanaka*
Kouji Mochizuki*, Yasuo Nakajima*, Masaki Nishiguchi*

A typical optical fiber is coated with UV curable resins in order to protect theglass. The coating consists of two layers, a soft inner primary layer and a hardouter secondary layer. The coatings are cured using ultraviolet irradiation. The polymerization
reaction generates heat and because the coefficient of thermal expansion of the primary layer isgreater than that of the secondary layer, a negative hydrostatic pressure is generated in the primary layer during the cooling process that has the potential to delaminate the primary layer fromthe glass. In order to ensure the long-term reliability of the optical fiber, it is important to accurately determine the adhesive force of the glass/primary interface. In this paper, we will report ona new glass/primary adhesion measurement method that has been developed for that purpose.

ABSTRACTINTRODUCTION
1.1. INTRODUCTION
Glass optical fibers are drawn with protective polymer
coatings. In general, the protective polymer coating is a
UV (ultraviolet light) curable resin. Most optical fibers
have at least two coating layers. Each layer has a specificrole to protect the glass. The first layer (in contact with theglass fiber), called the primary coating, works as a buffering layer against outside stresses that can cause bending
loss. Therefore, the primary coating should be soft. The
Young's modulus should be less than 1 MPa and theglass transition temperature (Tg ) should be below theoperating temperature range. The second layer, called
the secondary coating, should be sufficiently hard to protect the glass from external mechanical stresses. The secondary coating should have a Young's modulus of morethan 500 MPa and the Tg above 60°C.
In fiber drawing process, two coating layers of opticalfiber are together heated by the curing exothermic reaction of the UV curable resin and the radiant heat from the
UV lamp and the temperature of the coating layer isincreased above Tg of the secondary layer. After that, thefiber is wound onto the spool while being naturally cooling to the room temperature. Therefore, whereas thephase of the secondary layer changes from a rubbery
state to a glass state in the cooling process, the primarylayer which Tg is lower than room temperature is present
in a rubbery state. Since the coefficient of thermal expansion (CTE) of polymer materials changes greatly at theboundary of Tg, it means that the heat shrinkage of the
primary layer is larger than that of the secondary layer inthe temperature range below Tg of the secondary layer.
Such mismatch of the coefficient of thermal expansion
generates a stress at the primary/secondary interface anda negative hydrostatic pressure in the primary layer. And
also it leads to generate the force which delaminates off
the primary layer from the glass at the primary/glass interface1)-4).
Therefore, it is estimated that the thermal stress of thesecondary layer generated during the natural cooling process of the coating layer is accumulated due to theshrinkage of the outside diameter of the primary layer.
While the secondary layer is cooled to the room temperature from around Tg, the outside diameter of the primarylayer tends to become smaller than the inner diameter ofthe secondary layer. This is because the coefficient ofthermal expansion of the primary layer is larger than theone of the secondary. Figure 1 illustrates a schematic diagram of the volume change occurring due to the mismatch in the coefficient of thermal expansion of the primary layer and the secondary layer. In Figure 1, β represents the coefficient of volume expansion, its subscripts
represent the kind of coating material and ΔT represents
the temperature difference between the room temperature and Tg of the secondary layer.

* Research & Development Div.
Polymer Materials R&D Laboratories

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Volume change

A Method Related to Adhesion of Glass / Primary Interface for Long-term Reliability

y
er
bb
u
R

Secondary coating
Glassyyerbb Primary coatinguR（βp – βs ）ΔT

coolingRT

Tg of S-coating

Figure 1 Schematic diagram of the volume change due to theCTE mismatch between the primary and the secondary
layer.

In order to ensure the reliability of the optical fiber, it isvery important to know the thermal stress of the coating
layer. In other words, it is to estimate the thermal stressand the residual stress generated in the coating layers bymeasuring the CTE of each coating layer from the actual
optical fiber. Those studies were performed by Nakajima
et al and the contents are summarized in Furukawa
Review of No. 34 5).
Nowadays optical fibers, which are widely used in avariety of fields and long term optical and mechanical reliability, are requiring special features, such as various
aging conditions under extreme environments typicallyapplied to ensure optical fiber properties such as strip
force, pullout force, water or solvent resistance, are maintained. When an external stress is applied on an opticalfiber, coating delamination can sometimes occur at theglass/primary interface that can cause microbending loss,a failure mode in optical fibers. If the adhesive force of theglass/primary interface becomes lower than the force todelaminate the primary layer from the glass, for example,
by immersing the fiber in water, it may result in generatinga micro-bending loss. Therefore, it is also important toaccurately determine the adhesive force of the glass/primary interface, especially, if the fiber is immersed inwater, in order to ensure the long term reliability.
A pullout force test is a useful metheology to measure
glass/primary coating adhesion strength. The pullout testis the only available method for measuring the adhesion
at the glass/primary interface of an optical fiber in situ, but
the pullout force is obtained as the one combining themechanical properties such as the tightening force of thesecondary coating, the shear strength of the primarycoating and coating friction with other interactions at theglass/primary interface. In other words, the pullout force
covers many non-adhesive interfaces in addition to thepure adhesion at the glass/primary interface. Moreover,the pullout force depends on the tensile speed of pulling
the glass fiber out of the coating as described at Section
4.1.
In view of this, the development of a new measurement
method was required in order to know the correct adhe-sive force of the glass/primary interface, in particular,when the fiber is immersed in water.

THE
PULLOUTTESTINGMETHOD2.2. The
Pullout
Testing
Methodfor Optical
FOR OPTICAL FIBERS
Fibers
2.1 Fiber Sample Preparation
The UV curable resins used for primary and secondary
coatings were commercially available materials. We prepared the fiber samples with values for the glass/secondary coating O.D. of 125/245 μm, respectively.
2.2 Sample Preparation Method for the Pullout Test
and the Measurements
1 cm length of coating is adhered to a rigid substrate.
Figure 2 shows the schematic diagram. The coating issevered at the end of the substrate and by pulling theother end of the fiber sample at a constant rate using
Tensilon Type tensile tester (A&D Company, Limited, RTC1310A), the force needed to break the adhesive bond
between the glass and the coating was measured. The
measurement was performed under the ambient condition of 23℃/50% relative humidity based on the FOTP105, 1993 draft.

1 cm

Secondary coating
Primary coating
Glass fiber

Figure 2 The schematic diagram of the sample for the pullout
test.

2.3

Sample Preparation Method for the “Modified”
Pullout and the Measurement Method
The “modified” pullout test sample is prepared in a similar
way as the “standard” pullout test sample. For the modified pullout test we developed, a constant load is appliedon the fiber, compared to the constant rate of force of thestandard pullout test, and the time to pull out the glass
fiber is measured. Therefore, hanging a various load tothe samples, the time when the strip at the glass/primaryinterface begins was measured by the tags and the optical sensors. These samples are setup in the temperature/humidity chamber and the measurement was performed
under a desired temperature/humidity ambient condition.
The relationship between the time when the glass waspulled out from the coating and the load was determined
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A Method Related to Adhesion of Glass / Primary Interface for Long-term Reliability

Loading, N

by changing the load gradually. In Figure 3, the typicalmeasurement result is shown. As the load applied to theglass/primary interface decreases, the time when theglass is pulled out from the fiber increases, but when theload becomes below a certain value, the time when thefiber pullout increases rapidly and the slope becomessmooth. The load at the knee point where the slope
becomes smooth is defined as the “limit adhesive force”
and thereafter it is called the “static pullout force”.

THERESULTSRESULTSAND
AND CONSIDERATIONSCONSIDERATIONS
4.4. THE
4.1

The Relationship Between the Pullout Force andthe Tensile Speed
In Figure 4, the measurement results of the pullout force
when the fibers coated with two kinds of UV curable resins are pulled by changing the tensile speed are shown.
As it is clear from Figure 4, the pullout force depends on
the tensile speed when the glass is pulled out from thecoating. And it can be seen that as the tensile speed
decreases, it approaches to a certain saturated value.
The measurement in Figure 4 was performed under theambient condition of 23℃, but even when the temperature increased to 60℃, almost same result was obtained.
In other words, although the tensile test is performed at
the slowest speed which the tensile tester can control inorder to minimize the influence of the tensile speed asless as possible, the pullout force still decreases so it isunderstood that there is a limit in the existing pullout test
method.

Limit adhesion strength
20ab

Figure 3 Typical measurement data of the static pullout force.

OPTICALFIBER
FIBERPROPERTIESPROPERTIES AFTER
3.3. OPTICAL
AFTERUNDER
AGING EXTREMEUNDER EXTREMEAGING
ENVIRONENVIRONMENTMENTThe Transmission Properties of the Ribbon After
Hot Water Immersion for the Long Term
The trial fibers in section 2.1 were colored with a 5 μm
thickness for identification and four fibers were arrangedin parallel. The commercially available UV curable resin isapplied to form 4-fiber ribbon. This 4-fiber ribbon was
immersed in 60℃ hot water and the attenuation increasein the elapsed day was monitored in comparison to theinitial value before water immersion.
Measurement was performed using optical pulse tester(Anritsu Corp. MW9060A). Attenuation for each fiber wasmeasured at 1.55 μm by using optical time domain reflectometry (OTDR) method.
3.2 The Fiber Strength After Temperature and
Humidity Aging
As the test sample, the trial fibers in section 2.1 wereaged under zero stress in the condition of 85℃/85% relative humidity. The fibers which were cut to a length of 2 m
were prepared. Both ends were fixed by wrapping onto
the mandrel (φ100 mm). The distance between the mandrels is 500 mm. The test was based on TIA/EIA 455-28C
and the fibers were pulled at a tensile speed of 2.5%/min.
and the breaking strength was measured. The measurement was performed under the ambient condition of
23℃/50% relative humidity.

Pullout force, NTime, hrs
15

10

5

0
0.01

3.1

0.1

1

10

100

Tensile speed, mm/min.
Figure 4 The pullout force vs. tensile speed.

4.2

The Measurement Condition for the Static
Pullout Force
The purpose of this study is to measure the “pure” adhesive force by removing the non-adhesive factors as muchas possible from the pullout force which includes many ofthem. By improving the dynamic pullout test where theforce is measured at a constant tensile speed, and bymeasuring the time when the glass fiber is pulled out fromthe coating under the condition that a certain load isapplied to the fibers, the static pullout force test method
has been developed. It can be thought that the influenceof the shear strength of the primary layer, the friction of
the coating and etc. can be minimized as much as possible.
In order to remove the non-adhesive factors further, atfirst, the influence which the temperature affects the static
pullout force was investigated. The testing equipment of
the static pullout force is setup in the temperature/humidity chamber and the relative humidity was fixed at 50%.
The temperature was set at 23℃, 45℃, 60℃ and 85℃.
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A Method Related to Adhesion of Glass / Primary Interface for Long-term Reliability

2

Loading, N2

1.5

85℃/95% RH
60℃/98% RH
60℃/50% RH
60℃/30% RH

1

0.5

0
0.001

0.01

0.1

1

10

100

Time, hrs
Figure 6 The relationship between the static pullout force andthe humidity.

4.3

1.5

1

85℃/50% RH
60℃/50% RH
45℃/50% RH
23℃/50% RH

0.5

0
0.001

consideration of workability, the ambient condition of
60℃/98% relative humidity was set as the optimum measurement condition for static pullout force.

Loading, N

The measurements were performed at each temperature.
As the test results are shown in Figure 5, at any temperatures, the static pullout force was about 1.1 N so the temperature did not affect the values in this temperature
range. However, it can be understood that concentratingon the time when the glass is pulled out from the coating,the time decreases as the temperature increases. In otherwords, it was found that the temperature does not affectthe static pullout force, but it leads to shorten the measurement time. It is estimated that this time reduction isdue to the reduction of the tightening force by the secondary layer. As for the test result of 60℃ and 85℃,
around the static pullout force of one minute values forboth temperatures were obtained. And also the subsequence values look same. Therefore, in consideration ofoperations, the 60℃/50% relative humidity is set as theprovisional optimum ambient parameters for these conditions.

0.01

0.1

1

10

100

The Comparison of the Pullout Force and theStatic Pullout ForceIn Figure 7, the measurement results of the static pullout
force for the fiber coated with two kinds of UV curable resins are being superimposed to the measurement results ofthe pullout force shown in Figure 4 when changing the tensile speed. In Figure 7, it can be seen that the up anddown relationship with pullout force is reversed in the static
pullout force. This result suggests that the high pullout
force does not always mean the high adhesive force.

Time, hrs
Figure 5 The relationship between the static pullout force andthe temperature.

Pullout force, N

2

Loading, N

Next, in order to investigate the influence in which thehumidity affects the static pullout force, on the basis of theprovisional optimum condition (60℃/50% relative humidity)
obtained in Figure 5, the relative humidity was changed to
30% and to 98% and further, the condition of the temperature of 85℃ that shows the highest saturated steam quantity within the controllable range of temperature/humidity
chamber, 95% relative humidity was added and the measurement was performed. Figure 6 show the test results.
As the humidity increases, the pullout force decreases andthe pullout force at 60℃/98% relative humidity and
85℃/95% relative humidity was about 0.75 N. On the other
hand, regarding the time when the glass is pulled out
from the coating layer, no significant difference appeared.
The time that static pullout force is obtained was only several minutes’ difference in these conditions. It is estimatedthat the factor of the pullout force reduction at highhumidity is due to the weakening of the hydrogen bond at
the glass/primary interface. From the results of Figure 6,
the test results for both the condition of 60℃/98% relative
humidity and 85℃/95% relative humidity were around 6
minutes and also other values look same. Therefore, in

20

a
b

1.5

15
10
5

1
0
0.01

0.5
0
0.001

(1)
1 (2)

0.1

10

100

Tensile speed, mm/min.
0.01

0.1

1

10

100

Time, hrs
Figure 7 The comparison between the pullout force and thestatic pullout force.

Table 1 shows the difference between the pullout test
and the static pullout test. The pullout test is shown by
the “arrow” which means the dynamic tensile test and thestatic pullout test is shown by the “picture of the weight”
which means the static load in a simplified manner. The
pure adhesive force at the glass/primary interface is contained in both test results, but in contrast with the“dynamic” pullout test to measure the force at a constanttensile speed, the “static” pullout test is to measure thetime when the glass is pulled out from the coating at a
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A Method Related to Adhesion of Glass / Primary Interface for Long-term Reliability

certain load. By this modification of the testing method,
the dynamic influence such as the shear strength of the
primary layer, the coating friction and etc. can be minimized. Further, by performing the static pullout force test
under the ambient condition of high temperature andhumidity, in reality, 60℃/98% relative humidity, the tightening force of the secondary can be reduced and thehydrogen bonding strength can be weakened. In otherwords, by performing the static pullout test and removing
the non-adhesive factors which, consequently included,
as much as possible, the pure adhesive force can beobtained. Because the measurement is performed under
the ambient condition of high temperature and humidity, it
can be said that the adhesive force at the glass/primaryinterface, particularly when the optical fiber is immersed
in water, can be obtained.

Pure adhesion at G/PHydrogen bonding
strength
Tightening force of
S coating
Shear strength of
P coatingCoating friction etc.
Table 15 mm/min.

60ºC/98% RH













The comparison between the pullout test and the staticpull out test.

The Relationship Between the Pullout Test andthe Long-term Hot Water Immersion Test for
Ribbon Fiber
The fibers which are coated with two kinds of UV curable
resins (A, B) and have almost the same pullout force wereprepared. Each fiber was colored and 4-fiber ribbon was
fabricated.
The attenuation of those fibers after 60℃ hot water
immersion was monitored periodically. Also, the static
pullout forces for these two kinds of optical fibers were
measured. These results are summarized in Figure 8. In thesame way as Table 1, the pullout force and the static pullout
force are shown by the “arrow” and the “picture of weight”
respectively in a simplified manner.

Added loss at 1550 nm, dB/km

4.4

0.16

A
A
A
B
B
B

0.14
0.12
0.1
0.08
0.06
0.04
0.02
0

0

200

400

600

800

12

1.2

10

1

Static pullout force, N

Pullout force, N

days@60℃ water

8
6
4
2

0.8
0.6
0.4
0.2
0

0
A

B

A

B

Figure 8 The relationship between the pullout test and the long term hot water immersion test for ribbon fiber.

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A Method Related to Adhesion of Glass / Primary Interface for Long-term Reliability

Although the optical fibers which have almost the same
pullout force are selected, the attenuation increase did
not occur even after about 800 days hot water immersion
in one of the ribbon fibers (A in the figure), but otherfibers (B in the figure) attenuated after about 400 days of
hot water immersion. In this experiment, the intent wasthat the difference between two kinds of samples is indicated clearly as the extreme environments. From theseresults, it is clear that the pullout force does not indicate
the adhesive force at the glass/primary interface accurately when the optical fiber is immersed in water, but it canbe seen that the static pullout force agrees well with theresult of the optical fiber attenuation under the hot water
immersion. In other words, these results are one of theresults which support that there is a correlation between
the static pullout force and the adhesive force at theglass/primary interface when the optical fiber is immersed
in water.
4.5 The Relationship Between the Pullout Test andthe Fiber Strength After Temperature and
Humidity Aging
Another result which supports that there is a correlation
between the static pullout force and the adhesive force at
the glass/primary interface when the optical fiber isimmersed in water is the fiber tensile strength after thetemperature and humidity aging. In Figure 9, the result of
the optical fiber coated with another different kind of UV
curable resin (C in the figure) is added. The lower left

graph shows the Weibull plots of the fiber strength measured after 30 days zero stress aging under the ambientcondition of 85℃/85% relative humidity. If the distribution
is wider, it means that there is a higher possibility of the
weak fibers. However, it can be understood that the resultof the fiber strength after temperature and humidity aging
agrees well with the result of the static pullout force (right
below graph).
Here, the worth noting further is the time required forthe accelerated test. In IEC 60793-1-50, in order to measure the optical fiber strength after temperature andhumidity aging, 30 days aging period under the ambientcondition of 85℃/85% relative humidity was required. On
the other hand, the required time to measure the static
pullout force was only several minutes and even the time
to confirm the knee point of the curve for the time when
the fiber is pulled out from the coating vs. added load
was considered, it can be thought that 30 minutes isenough. When compared to the typical accelerated aging
period of 30 days for evaluation of the optical fiber properties, it is an overwhelming time reduction and the static
pullout force is the one which can predict the optical fiber
properties after the aging under these extreme environments in a very short evaluation period. In other words,
the static pullout test can provide us a better screening
method to select a reliable optical fiber than the conventional method.

30 days
11 12 1
10
9
8
10
9
8
10
9
8
10
9
8

12
11
7 6 511211
7 6 511211
7 6 51

210
93
48
210
93
48
210
93
48
210
93
48

11 12 11211
7 6 511211
7 6 511211
7 6 51

210
93
48
210
93
48
210
93
48
210
93
48

11 12 11211
7 6 511211
7 6 511211
7 6 51

210
93
48
210
93
48
210
93
48
210
93
48

11 12 11211
7 6 511211
7 6 511211
7 6 5130 minutes
210
93
48
210
93
48
210
93
48
210
93
48

11 12 11211
7 6 511211
7 6 511211
7 6 51

210
93
48
210
93
48
210
93
48
210
93
48

11 12 111 12 123

12
11
7 6 51412
11
7 6 51412
11
7 6 51

4

10
9
8

2
3
2
3

7 6

5

2
3
4

2
3

4
12
12
12
12
12
12
11
11
11
11
11
11
7 6 517 6 517 6 517 6 517 6 517 6 51
210
210
210
210
210
2
10
9
93
93
93
93
93
3
8
48
48
48
48
48
4
7 6 5
7 6 5
7 6 5
7 6 5
7 6 5
7 6 5

A
B
C

1

l n (- l n( 1 - F ))

0
-1
-2
-3
-4
-5
94%

96%

98%

ln(strength)

100%

102%

1.2

Static pullout force, N

2

1
0.8
0.6
0.4
0.2
0

A

B

C

Figure 9 The relationship between the pullout test and the fiber strength after temperature and humidity aging test.

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A Method Related to Adhesion of Glass / Primary Interface for Long-term Reliability

CONCLUSION
5.5. CONCLUSION
In order to accurately determine the adhesive force theglass/primary interface, in particular, when the opticalfiber is immersed in water, the existing pullout test has
been modified and a new measurement method has
been developed. It is the second method which can measure the adhesive force at the glass/primary interface insitu. The adhesive force at the glass/primary interface
called the static pullout force can be considered as the“pure” adhesive force from which the non-adhesive factors inherently included during the “dynamic” pullout test
to measure at a constant tensile speed are removed asmuch as possible. This testing method not only shows
the correlation between the static pullout force and theoptical fiber properties, but also is very useful as themethod which is able to select the reliable optical fiber
because the evaluation period can be shortened.

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Delamination of Polymer Coatings From Optical Fibers”, Journal of
Electronic Packaging, 119(1997), 133.
3) Win-Jin Chang: “Viscoelastic analysis of hydrostatic pressure
induced optical effects in double-coated optical fibers”, J. Appl.
Phys., 93(2003), 1.
4) M. Kasahara, Y. Arashitani, H. Tanaka, K. Mochizuki,Y. Nakajima
and M. Nishiguchi:“Investigation of Optical Fiber Performanceafter Accelerated Aging and Adhesion Strength at Glass/PrimaryInterface”, Proc. 62nd IWCS(2013), 336.
5) Y. Nakajima, H. Tanaka, K. Mochizuki, K. Fuse, E. Arashiya, T.
Nishimoto, A. Seno, M. Okada: “A Study of Thermal Strain andThermal Stress in Optical Fiber Coatings” Furukawa Review No.
34 (2008)

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