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Rheology characterization of UV curable silicone elastomers
Article in Rubber World · August 2014
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AUGUST 2014
125
years
The Technical Service Magazine For The Rubber Industry
Rheology characterization
of UV curable silicone elastomers
by Huiping Zhang, Mary Krenceski and Beate Ganter,
Momentive Performance Materials
www.rubberworld.com
Volume 250, No. 5
Rheology characterization of UV curable
silicone elastomers
by Huiping Zhang, Mary Krenceski and Beate Ganter,
Momentive Performance Materials
Silicone rubber is vulcanized through either addition
cure, condensation cure or free radical cure. Additioncured silicone generally offers better mechanical, electrical and optical properties than other silicones. Platinum is
a commonly used catalyst for addition cure. It is normally activated by heat to facilitate the hydrosilylation
reaction between vinyl containing polysiloxane and hydride containing polysiloxane that crosslinks the rubber
(refs. 1 and 2). Typical curing temperatures for fabricating
silicone articles are in the range of 120-200°C. Such high
temperatures tend to exclude silicone from consideration
in many applications involving low heat materials (such
as polyolefins) or temperature-sensitive substances (such
as pharmaceutical ingredients). It is also challenging to
find a desired balance of fast cure and long work life for
silicone, which are two seemingly contradictory but important properties.
A new class of photo-sensitive catalysts is now found
to cure the silicone at ambient temperature, where the
hydrosilylation reaction is triggered by UV rather than
by heat (ref. 3). This UV catalyst enables command cure
for silicone, where the crosslinking occurs only when
the reactive mixture of vinyl and hydride polysiloxanes
is exposed to UV radiation. It helps solve the longstanding dilemma of fast cure and long work life. It
makes it possible to include silicone in designs involving temperature-sensitive materials. It also helps elimi-
nate the scorching issue commonly encountered in heatcured silicone rubber, where premature vulcanization
occurs when a cold material is injected into a hot mold.
All the features that UV technology can offer open
doors to new applications and product innovations,
while the resultant UV-cured rubber still maintains the
same superior properties of a conventional additioncured silicone elastomer (ref. 3).
It then becomes important to understand the curing
characteristics of UV-curable silicone rubber. Rheology
is employed to accomplish this challenging task, as will
be discussed in this article.
Experimental
All chemicals used in this article were from Momentive
Performance Materials. All rheology measurements
were performed on an ARES LS2 rheometer (TA Instruments) outfitted with a UV-curing option (TA Instruments) that brings UV light to the test samples (figure
1). OmniCure S2000 from Lumen Dynamics Group
served as the UV light source. (OmniCure is a trademark of Lumen Dynamics Group.) It contains a high
pressure 200 watt mercury vapor short arc lamp and a
320-500 nm filter. The UV intensity on the sample was
determined on a Silver Line UV radiometer.
A dynamic time sweep was utilized to characterize the
vulcanization behavior of UV-curable silicone rubber.
The oscillation angular frequency was chosen at 200 rad/s
and the strain level was set at 4% to ensure that the rheological behavior of the test samples stayed within the
linear viscoelastic region.
Figure 1 - schematic of ARES UV option (TA
Instruments) for rheology measurement
Transducer
(torque/normal)
Mirror
Quartz plate
UV light
source
Collimator
Sample
Steel plate
Motor
(angular displacement)
Crosslinking percentage (%)
Figure 2 - representative curing curves of
different silicone rubbers
UV-curable silicone, tested at 25°C
90%
Standard
platinum-cured
silicone, tested
at 150°C
Peroxide-cured
silicone, tested at
150°C
1
UV or heat applied
5
Time (min.)
Note: Test results. Actual results may vary.
2
RUBBER WORLD
Figure 3 - effect of UV intensity on vulcanizaton - (a) shear storage modulus G’ vs.
time at various UV intensities; (b) an example of gel point when G’ and G” cross over;
(c) gel time vs. 1/intensity; and (d) curing time t90 vs. 1/intensity
Moduli G’, G” (Pa)
1.0E+05
350 mW/cm2
G’ (Pa)
1.0E+06
(a)
1.0E+04
170 mW/cm2
80 mW/cm2
45 mW/cm2
1.0E+03
UV
1.0E+02
0
(b)
1.0E+05
120
180
G”
1.0E+04
Tan (delta)
Gel point
1.0E+03
240
0
3.0
1.5
UV gel time
0.0
60
Time (s)
120
180
240
Time (s)
30
120
(c)
25
100
20
80
t90 (s)
Gel time (s)
4.5
G’
1.0E+02
60
6.0
Tan (delta)
1.0E+06
15
(d)
60
40
10
R2
5
= 0.9688
R2 = 0.9012
20
0
0
0
0.005
0.01
0.015
0.02
0.025
(cm2/mW)
1/intensity
Note: Test results. Actual results may vary.
Results and discussion
Representative curing curves of various silicones shown
in figure 2 demonstrate that the UV-curable silicone
vulcanizes similarly as conventional platinum-catalyzed
silicone, except that the catalyst is activated by UV at
ambient temperature rather than by heat. The effect of
UV intensity, catalyst loading, sample thickness and irradiation time on the curing behavior of UV-curable
silicone rubber was further examined. Fillers were purposely excluded in the test samples so that the outcome
observed was from the addition cure reaction between
vinyl and hydride polysiloxanes.
Effect of UV intensity
Since the catalyst is activated by UV, it is expected that
the higher the UV intensity the sample receives, the
faster the reaction proceeds. Experimental results suggest the same. As illustrated in figure 3a, the slope of
storage modulus G’ vs. time was indeed steeper at
higher UV intensities.
To further quantify the effect of UV intensity, both
gel time and t90 were determined at various radiation
intensities. The gel point is characterized by the cross-
August 2014
0
0.005
0.01
1/intensity
0.015
0.02
0.025
(cm2/mW)
over between shear storage modulus G’ and shear loss
modulus G” (figure 3b) (ref. 4), and t90 is defined as the
time when the complex shear modulus G* reaches 90%
of its ultimate value. It was found that both gel time and
t90 were reciprocally proportional to the UV intensity,
as demonstrated in figures 3c and 3d, respectively.
These results provide guidance in selecting appropriate
UV radiation conditions for various applications.
Effect of catalyst loading
In addition to UV intensity, the curing speed could also
be adjusted by loading different amounts of catalyst in
the system. As illustrated in figure 4a, the higher the
catalyst loading, the faster the sample cured, as anticipated. However, the correlation between curing time t90
and catalyst loading was found to be non-linear (figure
4b). When the t90 starts to level off, adding more catalyst would become ineffective in accelerating the reaction. The suggested catalyst loading would be where the
t90 starts to reach a plateau, and it is likely to increase
as UV intensity decreases.
Effect of sample thickness
3
Figure 4 - effect of catalyst loading on vulcanization - (a) shear storage modulus G’ vs. time at
various catalyst loadings and UV intensities, (b) curing time t90 vs. catalyst loading
(Note: catalyst loading at 1 or 1X represents the recommended catalyst amount under
350 mW/cm2 UV radiation)
1.0E+06
300
(a)
(b)
250
45 mW/cm2
1.0E+05
350 mW/cm2
0.25X cat. 45 mW/cm2
1.0E+04
1X cat. 45 mW/cm2
t90 (s)
G’ (Pa)
200
5X cat. 45 mW/cm2
100
0.25X cat. 350 mW/cm2
1.0E+03
150
R2 = 0.9489
1X cat. 350 mW/cm2
0
60
R2 = 0.9348
5X cat. 350 mW/cm2
UV
1.0E+02
50
120
180
240
0
300
0
1
Time (s)
2
3
4
5
6
Catalyst loading
Note: Test results. Actual results may vary.
Silicone rubber is known as a poor thermal conducting
material. It takes time for heat transfer to occur across
the thickness. Therefore, the inner section of a molded
part is usually cured slower than the outer section for a
typical heat-cured silicone, and the curing time tends to
increase as its thickness increases.
For UV curable silicone rubber, however, the limiting
factor is UV transmission across the thickness, rather
than the heat transfer. As silicone rubber is highly transparent to UV due to its lack of UV-absorbing groups on
its chemical structure, UV light can penetrate deep into
silicone without losing much of its intensity, although it
will eventually attenuate over distance due to scattering.
Experimental data shown in figure 5 reveal that the curing rate of UV-curable silicone stayed constant as sample thickness increased from 0.2 mm to 2 mm, unlike
Figure 5 - effect of sample thickness on vulcanization - (a) shear storage modulus G’ vs. time
at various sample thicknesses under 170 mW/cm2 UV radiation, (b) gel time vs. thickness
at various UV intensities
1.0E+06
30
(a)
(b)
25
20
Gel time (s)
G’ (Pa)
1.0E+05
0.2 mm
1.0E+04
0.4 mm
0.6 mm
1.0E+03
1.0 mm
UV
0
60
0
2 mm
120
180
Time (s)
240
10
5
1.5 mm
1.0E+02
15
0
300
0.5
1
1.5
2
2.5
Thickness (mm)
350 mW/cm2
170 mW/cm2
80 mW/cm2
Note: Test results. Actual results may vary.
4
45 mW/cm2
RUBBER WORLD
Figure 6 - dynamic time sweep of UV curable
silicone rubber at different exposure times
under 350 mW/cm2 UV radiation
1.0E+06
G’ (Pa)
1.0E+05
1.0E+04
on
1.0E+03
6s
2s
UV
1s
1.0E+02
0
300
600
900
1,200
Time (s)
Note: Test results. Actual results may vary.
standard platinum-catalyzed silicone. The UV intensity
appeared to remain across the thickness to enable identical curing speeds.
It should be noted that the curing reaction is expected
to slow down as the thickness further increases and UV
radiation starts to weaken. Nonetheless, the new UV
curing approach can allow fast cure of thick articles,
which is often difficult to achieve for heat-cured silicone. In fact, a molded part with a cross-section of 100
mm could be completely vulcanized in five minutes
under UV, as compared to more than 30 minutes in a
regular heat curing process (ref. 5).
Effect of irradiation time
In addition to keeping UV radiation on until the sample
fully cured, different UV exposure times were also applied to the sample to examine its curing behavior. The
result shown in figure 6 reveals that the reaction proceeded toward completion long after the UV radiation is
terminated, unlike most other UV-curing systems where
the reaction would stall once the UV radiation was shut
off. The UV-curable silicone appeared predestined to
reach completion, even when only a small fraction of
the catalyst had a chance to get activated; although it did
take a significantly longer time to cure at shorter irradiation times.
This interesting phenomenon may be understandable,
given that a potent catalyst could facilitate a reaction at
extremely low loading due to its almost unlimited regeneration capability and high efficacy. Even though
shorter irradiation time produces fewer amounts of catalytic centers, these active centers would stay live
through countless regenerations and would eventually
August 2014
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drive the reaction to completion. The overall reaction
rate would suffer though, as hydrosilylation is mainly
diffusion controlled, and thus its rate is a function of
active catalyst concentration.
The photo-catalyst system employed in the UV curable silicone rubber is radically different from a typical
photo-initiator system, in that the latter usually lacks a
regeneration mechanism. As a result, the reaction would
soon stop for the latter once the radiation is terminated
and the activated initiator is depleted, whereas the reaction will continue on for the former. This unique feature
of photo-sensitive catalyst may enable silicone rubber to
adapt to a wide variety of applications and curing processes.
The result shown in figure 6 also indicates that only
a certain amount of UV dose was needed to fully activate the catalyst, as suggested from the data of 6s timed
radiation versus continuous radiation. Once almost all
the catalysts are activated, further UV radiation would
not help accelerate the reaction. In fact, a UV dosage of
5J/cm2 is generally recommended for UV curable silicone products to ensure sufficient catalyst activation
(ref. 4).
Conclusions
UV curable silicone rubber offers many unique curing
properties, such as command cure, low temperature
vulcanization, relative thickness independence and postradiation curing capability. These features should allow
silicone to penetrate into previously limited applications
involving two-component molding or complex geometries, as well as potential new applications.
Rheology is instrumental in elucidating and quantifying vulcanization characteristics. It also helps provide
processing guidelines for this new class of silicone materials. Further investigation in filled systems will be
performed, as most silicone products contain fillers or
other additives.
This article is based on a paper presented at the 184th
Technical Meeting of the Rubber Division, ACS, October 2013.
References
1. Comprehensive Handbook on Hydrosilylation, B.
Marciniec, Ed., Pergamon Press, Oxford, England,
1992.
2. L.N. Lewis, R.E. Colborn, H. Grade, G.L. Bryant,
C.A. Sumpter and R.A. Scott, Organometallics 14, 2,202
(1995).
3. B. Ganter, Medical Device & Diagnostic Industry,
July 2013, 40.
4. C.Y.M. Tung and P.J. Dynes, J. Appl. Polym. Sci. 27,
569 (1982).
5. B. Ganter, Rubber World, 248, 26 (2013).
5