Three Bond Technical News
Issued January 1, 2001

56

Functions and Evaluation of Damper Agentfor Optical Pickups
Introduction ________________________________________________For years, optical devices such as CDs, MDs, and DVDs have been incorporated in home electricalappliances, PCs, games, and car navigation systems. The production of these general-storage devices
in 1999 was over 0.3 billion.
The core of such a device, an optical pickup lens, reads laser ray reflected from a disk surface. To
immediately suppress the vibration caused inside or outside of the device and the counteraction of theservo function, a damper agent that absorbs vibration is used.
This issue focuses on a damper agent developed in 1998, introducing its purpose, its required
properties, and the method by which it has been evaluated.

Contents
Introduction.......................................................................................................... 1
1. The function of damper agents ....................................................................... 2
2. Features of Three Bond's damper agents....................................................... 4
3. Evaluation methods......................................................................................... 5(1) Penetration............................................................................................... 5(2) Laser Doppler vibrometer ........................................................................ 5(3) Rheometer ............................................................................................... 6
Conclusions......................................................................................................... 81

1.

The function of damper agents

An OPD is a device that plays back (or records)
information by detecting bumps or pits formed on a
disk. The distance between pits for CDs is 1.6 µm,
equivalent to 30 tracks within the width of a single
strand of human hair. Since a CD contains
approximately a billion pits, the pits are extremely
densely distributed. The focus of the laser (with awavelength of 0.78 µm) on the disk is precise.
Reading a signal requires positioning resolution
within a margin of error of ±1µm. The CD control
consists of a rotary servo, a focus servo, and a
tracking servo1). The rotary servo adjusts thenumber of revolutions for the inner and outer radiusof the disk to read the pits arranged along a spiral
track with constant linear density. The focus servo
moves the pickup up and down to focus the laser
beam. The tracking servo adjusts the lateral motionof the laser to track the pit train. To ensure theaccuracy of the pickup unit, the damper agent
constantly absorbs the vibrations and stresses
generated by the three servos (the vibration frommotor rotation and the reaction in position
correction by focus and tracking servos) as well as
external vibrations.
There are four main techniques for reducing
vibration: (1) isolation, (2) damping, (3) dynamic
damping, and (4) shock absorption2). (1) Vibration
isolation minimizes the transmission of incoming
and outgoing vibration. (2) Vibration damping

reduces vibration by converting vibrational energy
into thermal energy using materials of high internal
damping directly attached to the vibration transfer
path. (3) Dynamic damping transfers vibrationalenergy in the main vibration system to a dynamic
damper attached to the vibration source with mass(m), spring constant (k), and viscous damping
coefficient (c). The damper absorbs and damps thevibration of a specific frequency. (4) Shockabsorption reduces the impact force when shock isapplied to a unit. The shock absorber converts
shock energy into heat via internal damping andreduces rebound energy. Among these, the damper
agents for pickup systems must provide functions(1) and (4).
Recent trends in pickup system structures havebeen to support the pickup lens with 4 thin wires.
The damper agent is deployed near the base of thewires. Damper agents are categorized roughly into
solid and liquid types, of which the liquid types areapplied and then cured. Ultraviolet-curing resins are
popular for bonding around the pickup mechanism.
Three Bond provides the market with bothheat-curing and ultraviolet-curing resins as damper
agents.
Resonance frequencies are important vibrational
characteristics for the pickup actuator, particularly
the first resonance frequency (F0) and its peakheight, referred to as the Q value (Figure 2).

Damper agent

Backsurface

Pits

Figure 1: Pits and central part of pickup

2dB

Q value

F0

Hz

Figure 2: F0 and Q value of the actuator

Here, we will briefly discuss the first resonance.
Figure 3 shows a fixed metal plate with a single
degree of freedom (indicated by the arrow). Whenthe metal plate is hammered (i.e., when vibration is
induced), the metal plate oscillates. The oscillation
is divided into the sum of components (1), (2), (3),etc3). Figure 4 shows the frequency response curvefor the metal plate oscillations. The first peakmarked as (1) is referred to as the first resonance.
Generally in the low frequency region, the firstresonance has large amplitude and tends to generatelarge-scale mechanical effects.

(1) First bending(2) Second bendinglarge-scale mechanical effects if applied to the
pickup continuously, the system is designed toblock this resonance frequency. Thus, F0 (generally
30 Hz to 50 Hz) must remain constant at the initialvalue after the endurance test, and the Q value mustbe at or below 10 dB (ideally 5 dB to 10 dB), sincevibrations are absorbed faster if the Q value issmaller, and to suppress first resonance amplitude.
Having two degrees of freedom, the actuator
shown in Figure 1 is known as a biaxial actuator. It
can be modeled as a wire spring with a single
degree of freedom, fixed at one end and supporting
the lens at the other. F0 can be theoreticallycalculated from the mass of support (m), Young's
modulus (E), diameter (d), and length (l). The Q
value is known to be inversely proportional to thedamping ratio (ζ) and proportional to the loss factor(tan δ) of the material.

(3) First torsion

Gain

Figure 3: Metal plate oscillation(1)

ONO SOKKI CF-5200 MULTI-PURPOSE FFT ANALYZER
100.0 Hz A: AC/10.0mVr
B: AC/11.00Vr

(2)
(3)

28.3E-3
TIME A
REAL
V

AVERAGEOFF
32
32
LENGTH2048
MEM No.
118.0000 s REC ADS
0REC GAP2

0.5-28.3E-3

Frequency

Figure 4: Frequency response curveX:

941.40625ms

X:

941.40625ms

[s]

Y:

-10.918mV

Y:

159.708mV

2.83
TIME B
REAL
V
0.5
-2.83

We will now provide a similarly brief overview of
pickup systems. Figure 5 shows the frequency
response curve for a pickup system. From top tobottom, the figures indicate the input signal (asweep sine wave), the output signal, the gain
characteristic of the frequency response function,and the phase characteristic of the frequency
response function. The gain characteristic shows
how the amplitude changes as the signal passes
through the system. The x-axis corresponds tofrequency while the y-axis corresponds to the
output/input amplitude ratio (dB). The phasecharacteristic indicates the advance or delay of thephase of the output signal relative to the input signal.
The x-axis indicates frequency, while the y-axis
indicates the phase shift in degrees or radians.
Since the first resonance oscillation generates[s]

Wed Nov 15 20: 45: 39 2000

Input signal (upper) and output signal (lower)
ONO SOKKI CF-5200 MULTI-PURPOSE FFT ANALYZER
100.0 Hz A: AC/31.6mVr
B: AC/3.1-70.0
CRS PWR
MAG
dbVr
1/jw
5.0db/-100.00.0000F0[Hz]

X: 100.000 Hz

rc-rc
Y:

-92.67 dBVr

100.00rc-rc
Y:

1.34 deg

AVERAGEOFF
32
32
LENGTH2048
MEM No.
11
REC ADS
0
REC GAP2180
CRS SP
PHASE
deg
1/jw
-180
0.0000[Hz]
X: 100.000 Hz

100.00
Wed Nov 15 20: 32: 50 2000

Gain (upper) and phase (lower)
Figure 5: Actuator characteristics

3

The vibrational characteristics of a gel material, aviscoelastic body, depend largely on its molecular
structure, average molecular weight, and polarstructure, as well as the shape, diameter, and surface
structure of the filler particles added. When stressand strain are applied to the gel, friction between
the molecules or between the molecules and thefiller arising from macro-Brownian motion of themain chain and the micro-Brownian motion of theside chains converts vibrational energy into thermalenergy, absorbing vibration.
The design of damper agents is based on thistheoretical underpinning. The major factors are not
only to soften the damper agents with increasing
dilution oil but to control the oligomer structure, toadjust crosslinking density, and to optimize filler
content, particle shape, dilution oil molecularweight, and its polarity.

2.

Features of Three Bond's damper agents

Damper agents are resins consisting ofsilicone-based pre-polymers. Various materials areused to absorb vibrations. Among them, silicone isnotable for its low temperature dependence withrespect to elasticity and viscosity compared to otherpolymer materials. While silicone is generally
regarded as a rubber material, the silicone used in
damper agents has the special properties of forminggels after they are cured. This morphology increases
vibration absorption at the frequency at which
damping is required. Improved vibration absorption
depends on both the elasticity of the rubber and theviscosity of the liquid. The specifics of elasticity
and viscosity are discussed later.
Three Bond produces two different types ofresins: heat-curing and ultraviolet-curing types.
Both types exhibit extremely small changes incharacteristics (particularly damping characteristics),
and cure rapidly in curing media. The heat-curingtypes cure through reactions known as addition
reactions (also known as hydrosilylation reactions).
Heat-curing resins are available as one-part or
two-part resins. One-part resins are easier to handle
and can be applied as is. On the other hand, theymust be stored at lower temperatures than two-partresins. Two-part resins cure at a certain temperaturewhen mixed at a 1:1 ratio. The curing rate is similar
to that of one-part resins, but the shelf life is longer,
since they do not cure until mixed.
Ultraviolet-curing resins cure through the radical
polymerization induced by ultraviolet irradiation.

4(Please refer to Technical News No. 10 for specificsof the reaction mechanism.) Depending on theworking environment of optical pickup production,
we recommend ultraviolet-curing resins forautomated lines, and heat-curing resins for batchprocesses when ultraviolet irradiators cannot be
installed due to line equipment restrictions.
Two heat-curing resins, TB1230H (two-partresin) and TB1238 (one-part resin), and an
ultraviolet-curing resin, TB3168, were released in
1999. Their properties are given below.
TB1230H and TB1238 characteristics:(1) A major component is a heat-curing silicone
resin
The major component is a heat-curing silicone
resin cured by addition reactions.
(2) Low curing temperature
TB1238 cures at lower temperatures and more
quickly than conventional one-part addition-curing
silicone.
• Caution:
Hydrosilylation reactions involve highly active
metal catalysts. Do not mix or allow the resin tocontact sulfur, phosphor, nitrogen compounds, ororganic metal salts, or it may affect theeffectiveness of the catalysts, impairing curing.
TB3168 characteristics(1) A major component is an ultraviolet-curing
silicone resin
TB3168 is a prepolymer consisting of UV
reactive groups added to a silicone resin that offers
the same flexibility across a wide temperature
range.
(2) Fast curing and stable characteristics underexcessive UV exposure
Because gels have a small number of reactive
groups that form crosslinking points, they tend tocure slowly under UV irradiation. Three Bondachieves fast curing via catalysts that offer superior
photo-curing
characteristics
andstructuralformation.
(3) High durability
Reducing unreacted component content by fastcuring and effective reinforcement with fillers
reduce variations in durability testing.
(4) High reliability
The rheological properties (discussed later) of
each lot of the gel are assessed to minimize
variations.

• Caution:
The filler components in the resin tend to form sediments. Stir well before use.
Table 1: Properties
Product nameThreeBond 1230H

Color and appearance
Curing method
Viscosity

Colorless

ColorlessWhite liquid

One-part heat-curing

Ultraviolet-curing0.44 (4.4) for both
A and B agents

0.52 (5.2)

15 (150)

0.97

1.01

90

80

100

(80°C x 60 minutes)(100°C x 60 minutes)(3 kJ/m )

6100

5500

6000

1.1

1

0.97 for bothSpecific gravity

A and B agents

Penetration *G * (30Hz)Pa

tanδ (30Hz)Ω•m

Volume resistivityΩ

Surface resistivity

ThreeBond 3168

Two-part heat-curing
P a•s ( P )(Curing conditions)ThreeBond 1238

Dielectric constantDielectric loss tangent

1.13×10
2.7×10

13

14

2

0.9

3.30×1013

2.74×1012

9.45×1014

5.40×10

13

2.39

2.01

2.31

-

-

0.0032*1/4 cone with 9.38 g loading

3.

Evaluation methods

Three Bond uses the three methods outlined
below to assess the characteristics discussed in theprevious section. As essential elements of the resin
design process, these three methods measure thedegree of cure and the reliability of the damper
agents.
8
2

4
9

1

Cone

2

Dial gage3

Sample container

64

Support5

5

Holder

1

6

Latch

3

7

Level adjustment screw

10

8

Rack for measurement

7

9

Level detection device

10 Fine adjustment knob

Figure 6: Penetrameter(1) Penetration
Penetration is a typical method used to measure
the hardness of the resin itself. We use a methodthat conforms to JIS K 2220: A cone is inserted into
the cured object, and hardness is expressed in termsof penetration depth in millimeters multiplied by 10(mm x 10). For example, if the cone penetrates 5
mm into the material, the penetration is given as 50.
We use two types of cones, weighing 9.38 g and 50
g.
(2) Laser Doppler vibrometer
The Laser Doppler vibrometer measures thefrequency characteristics (the F0 and the Q value
discussed earlier) of the pickup system bygenerating oscillations electronically. The actuator
is offered by the manufacturer. This test requires
manufacturers to provide actuators. The equipment
measures the frequency and velocity of theoscillator via FFT and calculates displacement andacceleration by use of differentials and integrals4).
Measurement using actual products offers
benefits, since it is the same evaluation method usedby manufacturers. On the other hand, the data will
also include various factors inherent in the actuator
tested, making these special tests that differ from
evaluations of the resin itself. Naturally, the test
cannot be performed if the design of the actuator isin the early stage of development and in flux, or if
the actuator cannot be provided due to security
concerns. Security agreements are concluded incertain cases when actuators are provided. (SeeFigure 5.)5(3) Rheometer
Rheometers were originally designed to measure
the viscosity and viscoelasticity of fluids in theuncured state. The viscosity of fluids differs
depending on various conditions: whether the fluidis thixotropic, during coating (under stress) or atrest, and the temperature of the resin. Coating
methods include screen printing, dispense coating,and transcription. Viscosity must be designed andadjusted to match the coating method used.
Rheometers provide more information thanconventional rotational viscometers and are used toevaluate the coating performance of liquids. They
are also used to evaluate damper agents, which havea typical viscoelastic behavior, for resin design,
product improvement, and inspections.
The measurement detects the stress (strain)
generated when mechanical strain (stress) is applied
to the sample as a function of time. These quantities
can be used to express viscosity and viscoelasticity.
The elastic modulus of the polymer material itself
that makes up the damper agent (known as thestorage elastic modulus and expressed as G') andthe viscosity that relaxes the elasticity (known as
the loss elastic modulus and expressed as G'')

significantly
affectvibration
absorption
characteristics. The ratio of G'' to G' is the loss
tangent (expressed as tan δ)5), which provides a
guide value when designing damper agents. These
parameters are used in measuring the degree ofcuring of the resin, changes before and after the
reliability test, and optimum ultraviolet exposure.
Compared to the evaluation of damper agentswith a Laser Doppler vibrometer, rheometermeasurements are simpler and provide moreevaluation items to evaluate more details of theresin's intrinsic properties.
Excitation
Probe
Resin

Sample stage

Figure 7: Rheometer probe

80

5

60

2

40

1

100

20

10

80

5

Phase degree

10

G* kPa100

20Phase degree

G* kPaExample of resistance measurement with rheometer
Before and after 80°C × 1.2 W

60

2

40

1
20

0.5

20
0.5

0
0.5

1

2

5

10

20

50

0
0.5

1

2

5

10

20

50

Frequency Hz

Frequency Hz

TB3168

Product provided by Company A

Change in viscoelasticity before and after test at 80°C × 1.2 W
Figure 8: Resistance data at 80°C
Measurement conditions
Equipment:
REOLOGICA DAR-100
Measurement mode:
Oscillation strain control
Geometry:
P25 Gap: 1.00 mm
Frequency:
0.5-50 Hz
Measurement temperature: 25°C
Strain:0.007

6Figure 8 shows the change in G* (complex elastic
modulus: discussed later) and the Phase degree(phase angle in degree, δ: discussed later) before
and after an environmental test measured with arheometer. TB3168's viscoelastic characteristics
change less during testing than the product provided
by Company A, indicating that TB3168 is highly
heat-resistant. This characteristic is naturally
reflected in performance when used as a damper
agent for pickup systems.

Figure 9 shows the degree of ultraviolet-curing measured with the rheometer. The charts superpose themeasured graphs for 4 samples with varied ultraviolet exposures ranging up to 15 J/cm2, where the unit exposureis 3 J/cm2. For TB3168, the curves for 3 J/cm2 through 15 J/cm2 virtually overlap, indicating that rheologicalcharacteristics remain constant through accumulated exposure. On the other hand, the curves for the productprovided by Company B (especially G* curve) spread widely across a band, indicating that this product cures
slowly under ultraviolet exposure and that rheological characteristics do not stabilize even after exposure at 15
J/cm2.

5
70
2

60

1

50
40

0.5

10
80
5
70
2

60

1

50

Phase degree

80

G* kPa10

Phase degree

G* kPaExample 2 of resistance measurement with rheometer

40

0.5

30

30
0.2

0.2
20

20
0.1

0.1

10
0.5

1

2

5

10

20

50

10
0.5

1

2

5

10

20

50

Frequency Hz

Frequency Hz

TB3168

Product provided by Company B

Change in viscoelasticity after ultraviolet exposures of 3, 6, 9, and 15 J/cm2
G*(30Hz)

8000
7000
6000
5000

Company A

4000

Company B

TB31683000
1

Exposure (J/cm2)

10

100

Figure 9: Degree of ultraviolet curing and overexposure resistanceThe basic formula for dynamic viscoelasticity is σ
= G*γ (stress = (complex elastic modulus) × strain).
More simply, this expresses the same relationship as
Hooke's law, F = kχ (force = (spring constant) ×(displacement)). Rheometers measure G* underconstant stress or strain, after which parameters
including G' and G'' are calculated. The relationshipbetween G*, G' and G'' is given by:

The delay in response when stress is applied to a
viscoelastic body such as a polymer material isexpressed in terms of the phase (the phase degree =δ), where δ ranges between 0 and π/2. The
characteristics of a material change at G' = G'', δ =π/4, and tan δ = 1, with the material being solid for
G' > G'' and liquid for G' < G''.

F0=

G* = G' + iG''
Imaginary axis

G*

G'
G''

t
Figure 10: Relationship between G*, G', and G''

1

k

2

m

We are currently undertaking a comprehensive
investigation of the correlation between datameasured with a rheometer and data measured byother methods, including the F0 and the Q value
measured with the actuator and the low frequency
sensitivity. F0 is currently believed to correlate with
G*, which corresponds to the spring constant for agel, since F0 is proportional to spring constant k, as
indicated in Equation (4). The Q value is considered
proportional to tan δ (as discussed earlier). However,these quantities may eventually prove not to becorrelated, and further data is urgently required.

7

Summary
A number of optical pickup manufactures have selected resins released by Three Bond in 1999, resins that now
represent our standard-grade products. Products of all grades offer excellent curing and resistance properties, as
demonstrated by the fact that the products have passed the reliability tests of our customers. Building on theseachievements, we continue to pursue further developments in damper agents offering even better damping
properties, particularly vibration absorption (reduced Q value), as well as optimized optical pickup characteristics
for each manufacturer's products.
We also plan to pursue applications of resins for vibration prevention, sound insulation, and shock absorption in
devices other than OPDs.
Tsunetoshi Fujisawa
Katsuhiko Kishi
Electric Development Division
Hiromasa Kitazawa
Research Planning Division
Development Department
Research Laboratory
Three Bond Co., Ltd.
Reference documents
1) Heihachiro Nakajima and Hiroshi Ogawa, Handbook of Compact disk, Ohmsha Co., Ltd.
2) Yoshihito Osada and Kanji Kajiwara (Representative editing), Handbook of gel, NTS Co., Ltd.
3) Kihachiro Tanaka and Nobuyuki Okubo, Observing oscillation, Ohmsha Co., Ltd.
4) Hands-on seminar document: FFT analyzer, Ono Sokki Manufacturing Co., Ltd.
5) Lecture: Rheology, High Polymer Molecules Publishing Group, the Society of Rheology, Japan

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

8