ThreeBond Technical News
Issued July 1, 2015

86

Curing Degree Evaluation of Reactive
Adhesives Using Measurement Devices
Introduction
Reactive adhesives demonstrate their function upon transition from liquid to solid during the curing reaction.
Generally, adhesive strength measurement is used to judge curing of reactive adhesives. However,as applications expand beyond adhesion, the cured state and precise degree of curing required make it
impossible to accurately judge curing on adhesive strength alone. Instead, we look at phenomena occurring
during the adhesive curing process, investigating an evaluation method that uses measurement devices.
This article describes how curing degree evaluation of reactive adhesives using measurement devices isperformed at ThreeBond.
We believe that understanding the state of the curing process and precise degree of curing for customers
using adhesives in manufacturing processes will provide effective evaluation for effective information
when setting up curing conditions.
Hereafter, reactive adhesive is shortened to adhesive.

Table of Contents

Introduction.................................................1
1. Background.............................................2
2. Rheometers ............................................3
3. FT-IR (Fourier-Transform Infrared)
Spectrophotometers...............................3

4. DSC (Differential Scanning Calorimetry)....6
5. Curing Degree Evaluation Summary......7
Closing........................................................81

1.

Background

1-1 Necessity of Checking the Degree ofCuringVarious phenomena occur in the adhesive during thecuring process, and these can be roughly divided into four
categories.
• Changes in adhesive strength• Changes in viscoelasticity• Changes in functional group• Changes in heating value
Generally, these changes in curing strength are used as themethod by checking the degree of adhesive curing. Thisis then used to measure adhesive strength for the curingconditions. However, the following issues also arise.
• Fluctuations regularly occur within the breakage test
itself• Testing is readily influenced by adherend type andsurface conditions
Because of these factors, this method is not suitable for
judging the degree of curing for adhesive independently.
This is why checking the degree of curing by using theanalysis equipments is needed.

However, solids with a modulus of rigidity (shear modulus)of 107 Pa or more cannot be measured, making it difficult
to measure the end point of the reaction in some adhesives.
Therefore, this method is mainly used to evaluate the initialprocess during which a solid begins to form.
In curing degree evaluation using an FT-IR spectrophotometerto observe functional group changes, shifts in the amountof functional groups are checked under determined curingconditions. Curing behavior can also be checked throughreal-time FT-IR measurement which measures continuouschanges in functional group amount. In either case, therequired curing conditions are obtained by assuming thatcuring is completed when there is no further Change in thecharacteristic functional Groups.
In curing degree evaluation using DSC to observe heating
value change, changes in the heating value are comparedfrom before and after reaction under determined curingconditions. In either case, the required curing conditions
are obtained by assuming that curing is completed when
the heating value reaches zero. However, curing behavior
cannot be checked through changes in heating value.
The three evaluation methods shown in Table 1 are explained
in detail in the sections to follow.

1-2 Using Measurement Devices to Judge theDegree of Curing
At ThreeBond, there are three ways with measurement
devices are used to determine the degree of curing in theadhesive (Table 1). The analysis method (and measurement
device) is chosen based on the phenomena occurring during
the adhesive curing process.
In curing degree evaluation with a rheometer to observechanges in viscosity, changes in viscosity due to reactivity
are continuously measured in order to check curing behavior.
Table 1 Curing Degree Evaluation Using Measurement Devices
Phenomena Occurring During
the Curing Process

Changes in viscoelasticity

Rheometer

Changes in functional group

FT-IR (real-time FT-IR)
spectrophotometer

Changes in heating value

2

Measuring Device

DSC

Obtainable Information

1. Changes in viscoelasticity due to the reaction (viscosity
change, curing behavior)
2. Gel point and crosslink initiation point due to thereaction
1. Reduction in the functional group amount due to thereaction
2. Continuous change in the functional group amount(curing behavior)
1. Comparative heating value before and after reaction(reaction ratio).

2.

Rheometers

2-1 Rheometer Curing Degree Evaluation
Rheometers are used to evaluate the degree of hardening
through changes in viscosity during the curing process.
The adhesive changes from a liquid to a solid during thecuring process. The phenomenon during curing where
the adhesive solidifies indicates a change in viscosity, andtherefore, a change in viscoelasticity. Rheometers quantify
viscoelasticity with storage modulus (G’), loss modulus(G”), and tanδ (= G”/G’), and record its change in time. Here,storage rigidity (G’) and loss rigidity (G”) are indicated bysolidity and liquidity, respectively.

We must be aware of measurement limitation when using
a rheometer to perform Evaluation of the curing behaviour.
Rheometers designed to evaluate liquids are limited tostiffness between 106 and 107 Pa. When measuring curing
behavior, a stiffness balance of 106 to 107 Pa is maintainedin hard materials (Fig. 2). In such cases, the graph may notshow complete rigidity (reaction ratio 100%).
Rheometer measurement of the changes in viscoelasticity
during the curing process such as those detailed above
clarify partway cured adhesive viscosity changes and enableconditions where flow stops to be checked. This information
can be used to set curing conditions in manufacturing
processes using adhesives.

Below we explain how to read the chart using the results ofmeasuring changing in curing behavior of epoxy resin as an
example (Fig. 1).

Section B: Increase as temperature increased, but at a
certain point the curing reaction begins and the cured object
forms part by part as it thickens and tanδ decreases.
Section C: As the reaction progressing and the crosslink
reaction begins, a three-dimensional crosslinked structure
forms and storage rigidity (G’) increases sharply. When theratio of solid to fluid is equal (G’ = G”), tanδ = 1, called
the gel point for the sake of convenience, the cured object
begins to form as a single mass. From this point, a threedimensional crosslinked structure forms rapidly, storage
rigidity (G’) increases and tanδ (G”/G’) decreases further.

G'
G''
tan
Viscosity*
Temperaturetan

G',G'' [Pa] / Viscosity* [Pa·s]

150°C

Rubber Flexibility Range(Measurement Limits)

10 7
10 6

80°C

10 5

G' [Pa]Section A: In the initial curing stage liquidity is high, andloss rigidity (G”) expressing liquidity grows larger thanstorage rigidity (G’) expressing solidity.

10 8

120°C

10 4
10 3
10 2
10 1

Observable Range

10 0
0

10

20

30

40

50

60

Time[min]Fig. 2 Rheometer Measurement Limits

3.

FT-IR (Fourier-Transform Infrared)
Spectrophotometers

3-1 Overview
Adhesives contain functional groups tied to the reaction,such as epoxy or acrylic. This functional group change
due to the polymerization reaction when heat or UV rays
are applied, so when an FT-IR spectrophotometer is used
to measure infrared absorption spectrum, reduction in thepeak of the functional group involved in the reaction isobserved.

10°C

Time[min]Fig. 1 Rheometer Epoxy Resin Measurement
Results3

Functional Groups and Reaction Mechanisms

A0 =zEpoxy
z
resin

H
R N

O

+

Epoxy base(glycidyl ring)

At =
OH
OH

O
H N

OH
* Peak change derived from epoxy base near 910 cm-1

Acrylic base(vinyl base)* Peak change derived from acrylic base near 1635 cm-1 or
810 cm-1

3-2 FT-IR Spectrophotometer Curing Degree
Evaluation
Evaluating the degree of curing via changes in functionalgroup is based on comparison of changes in the functionalgroup before and after reaction. The spectrum of infraredabsorption is measured for a “blank sample (before
reaction)” and “survey sample (after reaction)”, then thepeak area and change in height of the functional groupare used to calculate the degree of curing (reaction ratio).
In these measurements, absorbance completely changes
depending on the thickness of the measurement samples.
As a result, when calculating the reaction ratio, benzene
rings and others with no reaction-related change in peakarea or height are used as a reference to compare the output
intensity ratio*1 to. If there are no components usable as areference, calculation can be performed through changes in
the functional group peak only.
*1:

Absorption peak area and height involved in reaction(functional group peak)
Absorption peak area and height not involved in reaction(reference peak)Calculating the Reaction Ratio
A 0 AtA0× 100

A 0 : Blank sample peak intensity ratio
A t : Survey sample peak intensity ratio

4r

For A 0 : 0.3550

H2
RO C CC O+ H2C CC O

Reaction Rate (%) =rtr0 : Blank sample peak value
rt : Survey sample peak valuer : Reference peak value

A t : 0.0686

Reaction Rate (%) =zAcrylic
z
resin
RO

r

Using epoxy resin infrared absorption spectrum (Fig. 3) asan example, material is cured at 80°C × 60 min to calculate
the reaction ratio below.

0.35500.0686

0.3550×100 = 81%: Functional group peak contributing
to reaction (epoxy base): Peak not contributing to reaction(benzene ring)

Absorbance

R NH2

r0

Blank Sample

Material cured at 80°C×60 min(survey sample)

4000

3500

3000

2500

2000

1500

1000

Wavenumbers[cm-1]

Fig. 3 Epoxy Resin Infrared Absorption
Spectrum Measurement

However, measurement is difficult for resins with total
amount of the functional group involved in the reaction thatis very small, such as silicone resins.

3-3 Real-Time FT-IR Measurement Curing
Behavior Evaluation
The FT-IR software enables continuous measurementof the infrared absorption spectrum using real-time FTIR measurement. Using this method allows changes infunctional groups to be monitored continuously (Fig. 4) socuring behavior is evaluated from reaction ratio changes
calculated over time.
Additionally, an FT-IR spectrophotometer equipped with an
ATR stage on the measuring part can measure the infraredabsorption spectrum using attenuated total reflection, where
only the area where the sample and the measuring partinterface is observed. Using this, curability of deep areas
can be evaluated by adjusting the film thickness of the UV
curing resin on the ATR stage (Fig. 7).

Using an ATR stage that heat can be applied to enables
measurement of the infrared absorption spectrum andevaluation of curing behavior at any temperature applied toheat curing resin (Fig. 7).

Epoxy Base
(910cm -1)

Absorbance

3-4 Real-Time FT-IR Measurement
Time

Wavenumbers[cm-1]

Fig. 4 Real-Time FT-IR Epoxy-Base Absorption
Measurement Peak Change vs Time
Elapsed

3-4-1 Deep Area UV Curing Resin Evaluation
Curing degree is different on the surface and in deep areasof UV curing resins. The curing reaction occurs later indeeper parts, this is because the surface absorbs the UV
needed to cure, making it difficult for deeper areas to obtaina sufficient amount of light. The degree of curing must be
checked (deep area curing) where it interfaces with theadherend when design change of the workpiece changes thefilm thickness of the UV curing resin, etc.
Because of this, real-time FT-IR spectrometry can be used
for evaluation other than the physical process of change infilm thickness.
The example of measurement in Fig. 8 shows real-time FTIR measurement results for UV curing resin by thickness

ATR Stage
Measurement Part

Fig. 5 ATR FT-IR Spectrophotometer

This measurement method can be used to check the degreeof curing for each thickness of film.
100

ATR Measuring Part

Conversion[%]80

Infrared Ray

60

0.2mm
0.4mm
0.6mm
0.8mm
1.0mm

40

To the Detector

20

0
0

Fig. 6 ATR Technique Measurement Equipment
Schematic (observing information in thearea where the sample and measuring
part interface)
UV Light
Sample

10

20
30
40
Exposure dose[kJ/m2]

50

60

Fig. 8 Real-Time FT-IR by Resin Thickness
Measurement Results
Lamp Type: High-Pressure Mercury Vapor Lamp
Illuminance: 100 mW/cm2(UV Curing Resin)

Temperature of Measuring Part is Controllable(10 to 300°C)

3-4-2 Setting Curing Condition of Heat Curing
Resin
When setting curing conditions for heat curing resins, thecuring temperature and time must be set in order to ascertain
the curability of the resin itself independent of the cured
material. In these cases, curing behavior can be evaluatedby curing temperature with real-time FT-IR measurement.

Fig. 7 ATR Stage Measuring Part Measurement
Schematic (above: UV light measurement,
below: heating measurement)

The example of measurement in Fig. 9 shows real-time
FT-IR measurement results for epoxy resin by curing
temperature Graphing the reaction ratio change for each

SampleSpacer foradjusting thickness

5

curing temperature makes it possible to easily check curing
behavior over time.

DSC [mW]

Because FT-IR measurement is used to calculate the degreeof curing based on change in the amount of the functionalgroup, the reaction ratio may not be 100% depending on thetype of resin as in Fig. 8, 9, etc. This is likely because as
curing progresses, steric hindrances cause remnants of thefunctional group within the three-dimensional crosslinked
structure, and these are observed.

24
22
20
18
16
14
12
10
8
6
4
2
0
-2

100

231mJ/mg
Pre-Curing

80°C×30min

26.6mJ/mg

80°C×60min

19.5mJ/mg

Curing Complete

50

Glass Transition Temperature

100

150

40

Take the following example for a reaction ratio for curing at
80°C × 60min.

Reaction Ratio (%) =
0
20

40

Fig. 9 Real-Time FT-IR by Curing Temperature
Measurement Results (Epoxy Resin)

DSC (Differential Scanning
Calorimetry)

4-1 DSC Curing Degree Evaluation
DSC analysis measures the balance of heat flow when heat is
applied to a sample. There are two types of equipment, heat
flux and input compensation, through which reaction starttemperature, heating value, glass transition temperature,
specific heat and other parameters can be measured. Amongthese, heating value can be used to evaluate the degreeof curing (reaction ratio). Next, we will explain how thereaction rate is calculated using epoxy resin as an example.
Heat is generated during the epoxy resin curing process.
Therefore, total heating value (H0) and residual heating
value (Ht) are measured from the pre-cured sample andcured sample, respectively, using DSC (Fig. 10). The heating
value varies depending on curing conditions. The reaction
ratio is calculated by inputting obtained heating values into
the following equation.
Calculating the Reaction Ratio
H0 Ht
H0

19.5
231

× 100 = 9260

Time[min]Reaction Ratio (%) =231

As an example of evaluation, the degree of curing can bechecked for its corresponding part, and that result can beused to set the optimum curing conditions.

4-2 DSC Curing Conditions Estimation:
Kinetic Analysis
Although curing behavior is not directly observable using
DSC, it can be inferred through kinetic analysis.
Measurement is performed at at least three different
temperature elevation speeds. Measurement is performed ateach temperature elevation rate, the heating peaks obtainedin those measurements are divided by each reaction fraction,and the temperature reached is recorded (Fig. 11).
60% Reaction Fraction
DSC [mW]

0

Temperature[°C]Fig. 11 Heating Peak Reaction Fraction Division

× 100

H 0 : Total Heating Value
H t : Residual Heating Value

6

250

Fig. 10 Heating Value by Curing Condition
Measurement Results (Epoxy Resin)

120°C
100°C
80°C
60°C

60

20

4.

200

Temperature[°C]80Conversion[%]

116mJ/mg

80°C×10min

The temperature and heat elevation speed are converted to
an Arrhenius plot, and data is obtained via value lines foreach reaction fraction as below (Fig. 12).

Using this formula, the relationship between temperatureand time for each fraction is calculated to obtain the analysis
results (Fig. 13).
A reaction ratio of 90% can be predicted at 140°C × 10 min
for these materials.

logB [°C/min]

10 3

90%
50%

1000/T [1/K]

Fig. 12 Data for Each Reaction Fraction

Life Time [min]

10 2

10 0

The slope of the line is the activation energy. Activationenergy is the minimum amount of energy required forthe chemical reaction. The relationship between eachtemperature and the time required is calculated by inputting
numerical values into the following state equation.

dαdt= A exp

10 1

10 -1
100

120

140

160

180

Temperature[°C]f (α)

Fig. 13 Analysis Results (Reaction Fraction,
Curing Temperature, Relation to Curing
Temperature)α : Reaction Valuet: TimeA : Frequency Factor
Ea : Activation Energy
R : Gas Constant
T : Absolute Temperaturef(α) : function of α

5.

Curing Degree Evaluation Summary

Below is a summary of how we utilize phenomena occurring
during the adhesive curing process and measurement
devices that determines the degree of curing to perform
evaluation (Table 2).

Table 2 Curing Degree Evaluation Summary
Measurement
DeviceScope of Application

1. Changes in viscoelasticity due tothe reaction (viscosity change,
curing behavior)
Rheometer
2. Gel point and cross-linking
reaction initiation point
1. Reduction in the functional group
amount due to the reaction
FT-IR
2.
Continuous change in the
Real-time FT-IR
functional group amount (curing
behavior)

Specific Points for
Determining Curing Degree
Difficult to judge reaction
end point due to device
mechanics
Enables initial processevaluation of solid formation

Initial process evaluation of
curing speed evaluated due todevice measurement limitations(G’ = up to 107 Pa)* Curing behavior evaluation

100% cured when thefunctional group related to
the reaction is completely
depleted(when change stops)

Curing reaction is observablefrom start to finish* Curing behavior evaluation,
curing degree measurement

1. Comparative heating value before
and after reaction.

100% cured when heat is
no longer generated

Curing range when heating value
can be observed* Curing degree evaluation

DSC

Obtainable Information

7

Closing

We introduced our evaluation methodology for checking the degree of curing in reactive adhesives by utilizing analyzers
different from those usually employed in physical tests of adhesive strength measurement. In order to make accurate curing
judgements, evaluation methods must be selected and combined according to usage objectives and applications.
For example, production processes optimization for parts using adhesives can be furthered by evaluating the degree of curing
using a rheometer or real-time FT-IR spectrophotometer. Furthermore, these techniques can be used to clarify the cause of any
issues that arise due to curing conditions.
At ThreeBond, we understand customer needs and select the optimal evaluation method to provide technical information usefulwhen checking the degree of curing or setting curing conditions.

ThreeBond Fine Chemical Co., Ltd. R&D Headquarters
Technical Service Department, Inspection and Analysis Division
Shigeo Tanaka
Tatsuhiro Kiryu
Hiroshi Takebe

ThreeBond Co., Ltd.
8

4-3-3 Minamiosawa, Hachioji,
Tokyo 192-0398
Tel: +81-42-670-5333