Three Bond Technical News
Issued July 1, 2004

63

Resins for Optics
1. Light-Curing Resin
2. Heat-Curing Resin
Introduction ________________________________________________As the word "optoelectronics is popular," the fields of electronics and optics now have a close
relationship each other. Optical field includes precision components such as optical pickup devices,liquid crystal devices, micro lenses, prisms, and optical wave guides. The resin materials that are used
for these components require peculiar optical properties that have not been needed in the electric andelectronic fields. In this issue, we summarize and introduce our newly developed products that are
applicable to these optical components.

Contents
Introduction ................................................1
1. Light-Curing Resin .................................2
1-1. Brief Description of the Refractive
Index ...............................................2
1-2. Dispersion of the Refractive
Index ...............................................3
1-3. Brief Description of the 2P Molding
Method............................................4
1-4. Light Transmittance ........................4

2. Heat-Curing Resin................................. 5
2-1. Brief Description of the Optical
Components ................................... 5
2-2. Requirement Characteristics ofSealants for the Optical
Components ................................... 5
2-3. Characteristics of TB2960 and
TB2960B ........................................ 5
3. Conclusion............................................. 811.

Light-Curing Resin

Table 1 lists the general refractive indexes.

1-1. Brief Description of the Refractive Index
The refractive index shows the optical density ofa material and it is generally represented by “n”. In
other words, the refractive index is the resistanceunder which light passes through a material. A
greater resistance results in an increased refractiveindex, while a smaller resistance results in adecreased index.
Light has a property, which is traveling straight ina single material, but deflecting the path at thesurface if entering different material. As shown in
the example in Fig. 1, when the light emitted fromthe light source (A), travels straight through the air
and reaches the surface (B) of the water, it enters
the water in a deflected direction and then travels
straight through the water as well.
As discussed above, the property in which thetravel direction is deflected when the light enters adifferent material is called "refraction." Degree ofrefraction differs by material, and numericrepresentation of this is the refractive index
As the refractive index changes depending on the
wavelength of the light, the standard value ismeasured using a sodium lamp, which is called a "Dray" (wavelength: 589 nm), as the light source. A
refractive index measured using this D ray isexpressed in units of "nD." The calculation formulafor nD is as follows.

The refractive index nD in the air (or in avacuum) is 1.0, while it is 1.3 in water. A higher nD
results in a larger refractive index.
Incident light

Air
Water

Refracted light

Fig. 1 Refractive Index

2

Table 1 General Refractive Index
MaterialnD

Air

1.0

Water

1.3

Glass

1.5

The ThreeBond (hereinafter referred to as "TB")
3078 series includes UV-curing resins that therefractive index is controlled. The TB3078 series
covers nD 1.41 to 1.59, and it is also possible tominutely control the refractive index to the desiredvalue.
To control the refractive index, the chemicalcomposition of the UV-curing resin is changed. TheUV-curing resin consists of various molecules, eachhaving a unique refractive index.
The refractive index can theoretically becalculated from the following Lorentz-Lorentz
formula, without conducting actual measurement.
Lorentz-Lorentz formula

Molecular refraction
Molecular volume

Atomic refraction determines the molecularrefraction, and the refraction index can be obtained
from the sum of these refractions.
It can be seen from the above formula thatenlarged molecular refraction results in an increased
refraction index, while an increase in molecularvolume results in a decrease in the index. Therefore,
the refraction index of the entire resin compound
can be controlled.
Application designs of UV-curing resins for
optics are broadly classified into two categories:
one is to match the refractive index with that of thepart material (to be bonded), and the other is toutilize the difference in the refractive indexes byusing a resin with the bonded material that has
significantly different refractive index.
Table 2 summarizes the refractive indexes ofvarious polymers.

Table 2 Refractive Indexes of Polymers
Polymer

Refractive index, nDPolybenzyl methacrylate

Refractive index, nD

Polytetrafluoroethylene

1.35 to 1.38

Stylene-acrylonitrile copolymer

1.57

Poly-4-methylpentene-11.47

Polyphenylene methacrylate

1.57

Polymethyl methacrylate

1.50

Polydiallyl phthalate

1.57

Polyvinyl alcohol

1.49 to 1.53

Polyethylene terephthalate

1.57

Diethylene glycol bis allyl carbonate

1.50

Polystyrene

1.58

Polycyclohexyl methacrylate

1.51

Polyvinyl chloride

1.59

Polyethylene

1.51

Polyvinyl naphthalene

1.63

Polyacrylonitrile

1.52

Polyvinyl carbazole

1.68

Nylon 6

1.53

1.68

Table 3 Various characteristics of UV-Curing Resins for Optics
Item

Unit / examination
methodTB3078

TB3078B

TB3078C

TB3078D

VisualTransparent

Transparent

Transparent

Transparent

Viscosity

mPa•s

150

180

300

100Hardness

JIS

D 80

D 80

A 74

A 80

1.56

1.47

1.44

1.39

1.59

1.51

1.46

1.41

53

75

201

142

220

135

278

280

Refractive index (before curing)
Refractive index (after curing)
Coefficient of linear expansion (α1)
Coefficient of linear expansion (α2)

nD 25°C

ppm/°C

1-2. Dispersion of the Refractive Index
1) Dispersion by wavelengthFig. 2 plots the refractive indexes of TB3078 atvarious wavelengths. It can be seen that a longer
wavelength decreases the refractive index. The
refractive index depends on the wavelength. In
optical systems, this dispersion is desired to besmall, and by reducing this dispersion, opticalaberration can be decreased.

Refractive index, nD

Appearance

Wavelength, nm

Characteristic of TB3078 Dispersion by Wavelength

Refractive index, nD

Fig. 22) Dispersion by temperature
Fig. 3 plots the refractive indexes of TB3078 at
various temperatures. It can be seen that a higher
temperature decreases the refractive index. This
demonstrates that the refractive index depends onthe density of the medium, as previously discussed.
When the temperature is increased, the density
drops then, as a result, the refractive index isdecreased.

Measured temperature, CFig. 3

Characteristic of TB3078 Dispersion by Temperature

3

The TB3078 series can be used as a 2P molding
material.
1-3.

Brief Description of the 2P Molding
Method"2P" is an acronym for Photo Polymerization.
Photo polymerization is a molding process utilizing
the light-curing property. Fig. 4 simply illustrates
this process.
(1) The optically designed stamper (casting mold)is filled with UV-curing resin (TB3078 series).
(2) Press bond a base plate such as a transparent
acrylic plate against the die, then,(3) Irradiate ultraviolet rays through thetransparent plate to polymerize-cure the resin.
(4) Any pattern on the stamper can be copied tothe transparent base plate.
The size of the patterns on the casting moldgenerally ranges from submicrons to several
hundred microns.

Transparent base plate(1)

Acrylic plate

Press-fitting and spreading

Releasing (finished product)
(4)

(2)

Fig. 4 2P Molding Process
1-4. Light Transmittance
The TB3078 series features high lighttransmittance and superior heat resistance. Fig. 5
plots the light transmittances measured on TB3078
samples cured and aged at various test temperatures.
Within the wavelength range at or above 400 nm,
the light transmittance is significantly decreased
with aging at 250°C, while it remains constant withaging at or below 200°C.

Room temperature

Wavelength, nmXmin

Xmin

XminXmin

Xmin

Film thickness: 50 mFig. 5 Characteristic of TB3078 - Light Transmittance

4Irradiation of ultraviolet rays
(3)

Stamper (nickel-plated)

Light transmittance, %Advantages of 2P molding• UV-curing resins can be polymerized at lower
temperatures than those used in the heatpolymerization method, minimizing the optical
distortions.
• The light curing process provides excellentproductivity.
• The stamper (mold) is fabricated in an easymanner, resulting in lower costs compared tometallic molding dies.

2.

Heat-Curing Resin

The Internet is now entering into broadband era,and progressing day by day. Optical fiber cables areindispensable components for the popularization ofbroadband Internet services. Communications using
fiber optics cables require devices that control
amplification, wave division, and switching of
optical signals. In the optic communications, theoptical characteristics are impaired with humidity,
therefore, effects of external moisture must be shutoff. Three Bond developed TB2960 and TB2960B,
which are sealants for optical components.
2-1.

Brief Description of the OpticalComponents
Optical fiber has been developed as acommunication medium to replace metallic cables,and it is expected to be used in various fields.
Signal attenuation of optical fiber is small and it
allows long distance and large capacity, compared
to conventional metallic cables. Therefore, if opticalfiber cables are connected to general houses, its
advantages are inestimable. To complete fiber-optic
networks that covers general houses, it is necessary
to install a splitter that divides optical signals, andan optical device that sends and receives opticalsignals (i.e., a terminal). For example, the opticaldevice consists primarily of the following
components.
• Optical waveguideSame as electrons flow through electric circuits,
optical waveguides lead optic signals to optic
circuits formed on base boards by utilizing
differences in refractive indexes. The principals aresame as fiber optics, but the optical waveguide isplain surface structure while fiber optics is fabric.
• Optical switch
The optical switch is a device for switching optic
signals, as it is, without converting into electricalsignals.
• Photocoupler
Photocoupler is a device to branch optical powerthat transmits on a optical fiber, with set ratio.
• Optical transceiver
Optical transceiver is a device that integrates
optical transmitter and receiver, converting electricand optic signals in both directions.
Lenses, optical glasses, and prisms which areused in optical equipment such as cameras, are also
optical components.
2-2.

Requirement Characteristics of Sealants
for Optical Components
Optical components such as an optical fiber cantransmit light without attenuation. However, light isaffected and attenuated in moisture, which causes
transmission loss. Sealants are used to protect thedevices, discussed earlier, from the degradation
caused by external moisture and to enhance thedurability. (See Fig. 6.)
Optical fiber
Optical component
Potting agent

Sealant

Fig. 6 Application Example of SealantConventionally, UV-curing resins and epoxy
resins have been used as sealants. However,Shrinkage of UV-curing resins are large during
curing, and internal stress remains inside thecomponent. Epoxy resins, due to their higher
hardness, occasionally suffer from cracking during
the heat cycle. Taking into consideration thesedrawbacks of current resins, we have beendeveloping resins for optical components, aiming
following items as requirement characteristics.
• Low permeability:
Preventing penetration of moisture into opticalcomponents and maintaining their originalperformance• Flexibility:
Reducing internal stress during the heat cycle• Low curing shrinkage:
Mitigating damages to optical components during
curing2-3. Characteristics of TB2960 and TB2960B
Three Bond investigated and researched newmaterials that satisfy the requirements items
described in the previous section, and ultimately
developed TB2960 and TB2960B. These sealants
consist primarily of polyolefine polymers that havea reaction group at both ends and, this component ispolymerized and bridged to form elastic rubber,when heated,

5Generally, resins with high gas-sealing
performance are often hard, and it is a cause ofinternal stress and the cracks that occur when
exposed to heat cycle. TB2960 and TB2960B are
flexible and low-permeable sealants. (SeePhotograph 1.)
Table 4 shows the physical properties of both
TB2960 and TB2960B. As the permeability istemperature dependent, a higher temperature results
larger permeability. Table 5 shows the permeabilityof TB2960 and TB2960B, and their temperaturedependency.

Photograph-1 Cured Objects of TB2960 andTB2960B

Table 4 General Characteristics of TB2960 and TB2960B
ItemUnit

TB2960

TB2960B

Application-Sealant

Potting agent

Test method-Appearance

-

White

Black

3TS-201-02

Viscosity

Pa•s

280

14

3TS-210-02

Specific gravity

-

0.96

0.98

3TS-213-02

Standard curing conditions-

100°C × 30 min

120°C × 30 min-Hardness

-

A 20

C 25

3TS-215-01, 02

Peeling property

MPa

1.6

0.5

3TS-320-01

Elongation

%

290

120

3TS-320-01

Cure contraction%

1.0, max.

1.0, max.

3TS-228-02

Glass transition point°C-58.7-55.0

3TS-501-04

Coefficient of linear
expansionppm/°C

α157.4
α2 260.7

α151.9
α2 271.9

3TS-501-05

Volume resistivityΩ•m

1.8 × 1013

6.6 × 1013

3TS-401-01Ω

8.5 × 1014

1.2 × 10113TS-402-01

Dielectric loss tangent-0.009 1kHz
0.004 1MHz0.008 1kHz
0.002 1MHz

3TS-405-01

Dielectric breakdown voltage

kv/mm

32.3

17.5

3TS-406-01

TB2960

TB2960B

Test method

0.9

1.1

1.9

1.8

2.9

2.4

Surface resistivity

Table 5 Permeability Measurements
Test conditions

Unit

40 °C × 95%RH
60 °C × 95%RH
85 °C × 85%RH

2g/m /24h

JIS Z 0208

Resin thickness: 3 mm

6

Peeling property, MPa

Studying the resistances of the cured object undervarious environment conditions shows any
degradation of the tension strength with dumbbell
property on any of them, including humidity
resistance (85°C × 85 %RH) (see Fig. 9), heat-cycle
resistance (40°C × 2 h ⇔ 120°C × 2 h) (see Fig. 10),acid resistance (90°C, pH-1 sulfuric-acid solution)(see Fig. 11), alkali-resistance (90°C, 10 % sodiumhydroxide solution) (see Fig. 12), and heat
resistance (120°C) (see Fig. 13). It can keep stablerubber material properties for a long time
TB2960 is designed for sealing and TB2960B isfor potting applications in order to meet the usageof each component. TB2960B is blackened so that it
will be suitable for light-shielding applications.

Time, hTime, hPeeling property, MPa

Peeling property, MPa

Fig. 11 Acid ResistanceTime, hFig. 9 Moisture Resistance

Cycle

Fig. 10 Heat-Cycle ResistancePeeling property, MPa

Peeling property, MPa

Fig. 12 Alkali ResistanceTime, hFig. 13 Heat Resistance73.

Conclusion

Recently, the optics related business has been developing remarkably. We have developed new materials to meetthe requirements of the market. We hope that the resins for optics introduced in this issue contribute to thedevelopment of the optoelectronics field.

Photograph-2 TB3078-Series UV-Curing Resins for Optics

Kazuhiro Kojima
Electrical Material Development Section, Development Department
Koichi Kaneko
Transport Material Development Section, Development Department
Research Laboratory
Three Bond Co., Ltd.

1456 Hazama-cho, Hachioji-shi, Tokyo 193-8533, Japan
Tel: 81-426-61-1333

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