JULY 2013
The technical service magazine for the rubber industry
Volume 248, No. 4
www.rubberworld.com
Putting silicone rubbers
into a new light
Putting silicone rubbers into a new light
by Beate Ganter, Momentive Performance Materials
Silicone rubbers are still a niche in the overall rubber market
at 1.5% of the global rubber market, but their demand is growing fast due to megatrends and product innovations supported
by their superior properties. Properties such as excellent UV
stability, clarity, purity, biocompatibility, strong heat aging,
high chemical resistance, low compression set and the ability
to keep their mechanical properties stable at a temperature
range from -40°C to 200°C have made silicone elastomers an
attractive material for many industries. In fact, silicone elastomers are the preferred material in many consumer goods, automotive, energy and health care applications.
Momentive Performance Materials (MPM) has recently
developed a new family of silicone rubbers that can be cured
by UV light where the crosslinking is initiated by a photochemical reaction rather than heat. Until now, heat curing by
using peroxides or a platinum-catalyzed hydrosilation reaction
has been the curing process for silicone elastomers. The UV
technology offers two huge breakthroughs, including the benefit of on-demand high curing speed at low or slightly elevated
temperatures and the ability to cure silicone rubbers combined
with temperature-sensitive materials.
Until MPM introduced UV-curing silicone rubbers, heat
was always a limiting factor for designers and manufacturers
when selecting a second material to combine with silicone.
Customers had always been limited to using more costly, heatstable thermoplastics, a certain impediment to realizing the full
Table 1 - overview of the typical properties
of silicone elastomers
Benefits
• High thermal stability
• Excellent dielectric properties
• Sterilizable with steam, ETO and gamma radiation
• Highly transparent vulcanizates
Main features
SiO backbone
• Mechanical properties stay stable
from -40°C to 200°C
• USP Class VI possible and ISO 10993 compliance
• Neutral odor and taste
• FDA, BfR, KTW and WRAS approvals possible
Stable material
• Resiliency, high recovery after flexing
• Long service life at dynamic stress
• Reduced material fatigue
Elastic memory
• Minimal insertion force,
smooth operation
• Ergonomics and comfort
• No plasticizers or other organic additives
Low modulus, durometer
Ease of fabrication
• Design versatility with
tight tolerances
• Excellent processability
• Transparency, easily pigmented
innovative potential of silicone elastomer’s unique properties,
as shown in table 1.
With UV curing, the silicone rubbers can be processed via
extrusion or in an injection molding process with special
molds. The shortened curing times during heating can reduce
energy usage, resulting in potential energy savings. The UV
cured silicone elastomer typically retains the same typical
properties and benefits of silicone elastomers that are thermally cured.
With UV cure LSR, the application set is expanded for a
great number of industries, taking silicone rubber to an entirely different platform.
Crosslinking of UV curing silicone rubbers by UV light
The UV curing of liquid silicone rubber (LSR) and heat cured
silicone rubber (HCR) still involves an addition curing mechanism like the heat curing silicone rubbers, but with a photosensitive catalyst instead. This catalyst is activated at a wavelength in the UVA area between 320 and 400 nm. UV irradiation can be generated by gas discharge lamps emitted from a
microwave or an electrode system. Both have shown comparable spectra dependent on the dotation of the lamp (D-bulb,
V-bulb or H-bulb).
For the UV curing silicone rubbers, the D-bulb (iron dotation) has shown the most effective curing. The lamp systems
are differentiated by the position of the generator and the lifetime of the bulb. For the extrusion of UV HCR, either system
typically can be used. The electrode system is preferred for the
UV LSR because the generator does not need to be directly
connected to the lamp. The microwave system is, in most
cases, too big to be integrated into the injection molding machine.
For the curing of solid silicone rubbers (HCR) through an
extrusion process, UV lamps with a power of 200 W/cm and a
length of 20 cm are typically used. In the injection molding
process, the type of lamp and the power is dependent upon the
geometry and size of the article to be cured. For large articles,
2-3 lamps with 200 W/cm and a length between 20 and 30 cm
are typically needed to ensure that the whole article is irradiated. Smaller articles can typically be cured with an LED lamp
emitting a wavelength around 365 nm and an output of more
than 4,000 mW/cm2. The LED lamps are very small and can
be easily integrated into most molds and injection molding
machines. Overall, a minimum dose energy of 5 J/cm2 is
needed to ensure efficient activation to get a fast curing. Less
UVA dosage would tend to generate less amount of activation,
therefore requiring longer curing times or higher curing temperatures to ensure full vulcanization.
Molding of UV curing liquid silicone rubbers (LSR)
Injection molding process
For the injection molding process of a standard LSR, the steel
mold is typically heated up to 180-200°C to get a fast curing
and productive cycle time. Some production processes require
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Figure 1 - principal process of the UV
curing injection molding
Material feeding
Pump
Figure 2 - demolding of a cured test sheet
from a PMMA mold
Shape and cure
UV additive
UV transparent mold
Base
material
UV-source
Screening
• Material pumped into transparent mold
• Curing initiated with start of UV light
a mold temperature around 120°C, which leads to significantly longer cycle times due to the low thermal conductivity
of silicone rubber.
UV curing LSRs can be molded in the same injection
molding machines, but the mixing ratio is different (100:2 instead of 1:1 for thermal curing LSRs), and a special mold is
required. Figure 1 shows the principal process of the UV curing injection molding process.
Mold material and low temperature UV curing
One of the major challenges of this process is that the mold
material needs to be UV transparent and UV stable. Quartz
glass and special PMMA grades are the most common materials that offer these properties. Because quartz glass is very
brittle, PMMA is often preferred as the mold material. PMMA
can offer easier handling, increased design freedom and heat
stability up to 70°C, which is generally sufficient for the injection molding process.
When an electrode UV lamp is used to initiate the curing,
the mold material needs to be cooled. Otherwise, the PMMA
mold will be deformed due to the heat and high amount of
infrared (IR) irradiation which is generated by the lamp. This
cooling can be achieved by a water channel in front of the
mold.
If an LED lamp is used for the curing, cooling generally is
not required because the LED lamp emits less heating and a
low amount of IR. The small wavelength area of the LED
typically increases the lifetime of the mold, and cycle times of
more than 15,000 shots have been achieved without damaging
the PMMA.
The UV curing is very fast at a relatively low temperature
(up to 40°C). Due to the speed of the process, the cycle times
can be dramatically reduced, especially when curing thick
JULY 2013
parts, because the low thermal conductivity of silicone rubber
is no longer a limiting factor for the curing. Even cross-sections up to 100 mm typically cure in less than five minutes; the
same section would take more than 30 minutes in a heat curing
process.
Due to the low temperature, there is typically less shrinkage
of the article, no air entrapment and more control of the scorch
performance. Also, the curing reaction only starts when the
light is activated. Overall, this process can yield high quality
articles, especially those large in size, in productive cycle
times.
Coloring of UV LSR is also possible, as shown in figure 3
with the blue insulator. The colors need to be translucent so
that the UV light still can pass through the article, but most of
the standard colors may be used. The activation time is sometimes extended by using color pigments.
Two-component injection molding with UV light
Two-component injection molding has been utilized for a
Figure 3 - insulators made from UV LSR
Figure 4 - molded article from PP and
UV LSR 2060
Figure 6 - thermal curing extrusion process
compared to UV curing
HCR
Extruder
Shock heat >450°C
Heating tunnel
2-10 m >250°C
Material extruded through die over thermal
shock heater in heating tunnel
HCR
UV source
Extruder
>30 cm
Material extruded through die in UV source.
number of applications in the healthcare, consumer goods and
automotive industries; examples are shower heads, air diffusors, sensors, spatulas, shutter breaks and valves.
One of the limiting factors in selecting the polymer for the
second component is the temperature stability. As is the case
with standard LSR curing, two-component injection molding
uses heat to cure the silicone and requires a high temperature
above 120°C for short and efficient cycle times. Nylon, polyester and other comparable plastic polymers are established for
the two-component injection molding process, but inserts
made from polyolefins or other standard thermoplastics are
not suitable for the two-component (2C) process with LSR
because they would melt.
The UV curing technology with mold temperatures of 3040°C can enable new potential two-component combinations
of silicone elastomers with polyolefins or standard thermoplastics. The process can allow for the use of different, and
often less expensive, plastics, and is no longer limited by heatFigure 5 - mold concept of PP/UV LSR
molding
Polypropylene insert
PMMA cavity
UV
LED
UV LSR
injection
point
Steel base mold
UV LSR
stable polymers, so it opens up new design opportunities for
kitchenware, labware, teething rings/infant care, and softtouch handles and grips. It can also become possible to implement electronic parts or co-mold with acrylic resins for LED
applications (ref. 1).
The example in figure 4 shows a wine plug molded from
PP and UV LSR 2060 in an Engel injection molding machine.
The mold was designed by Elmet, an Austrian mold maker.
Figure 5 shows that the PMMA insert was built on one side of
the cavity. The injected UV LSR was then cured via an integrated UV light LED system in 20 seconds. For stable and
optimum curing with good de-molding properties, it is recommended to use tool temperatures in the range of 30-40°C. The
mold temperature still impacted the process. Due to the small
temperature increase in the curing process, the cavity pressure
did not rise due to the lack of thermal expansion normally seen
in the standard heat cure LSR process. The expected pressure
in the cavity is <50 bar.
UV curing solid silicone rubbers
Solid silicone rubbers or heat cured rubbers (HCR) are mainly
cured in an extrusion process using long ovens (up to 12 m)
with temperatures above 250°C, which consumes considerable energy during manufacturing. The UV curing solid silicone rubbers (Addisil UV EX) can be extruded and shaped
with standard silicone rubber extruders. For the curing, a UV
lamp as explained previously with an irradiation power of 200
W/cm2 and an arc length of 20 cm is used to cure the rubber.
This can reduce the production footprint significantly because
the UV lamp with an overall length of about 30 cm can replace
long curing ovens. The UV extrusion process can also be more
energy-efficient because the UV lamp typically consumes less
energy than the long heat ovens. Figure 6 shows the comparison of a thermal curing extrusion process to a UV curing silicone rubber process.
The UV curing of the solid silicone rubbers can be very fast
at the ambient temperature (about 70°C), due to the IR irradiation and heat produced by the UV lamp. This temperature
varies, depending on the geometry and size of the article, type
of UV lamp and its set-up.
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Figure 7 - UV extrusion of a rope
This fast or on-demand curing at low temperatures typically results in high quality extruded products with tight tolerances without formation of bubbles, even with articles with
cross-sections up to 50 mm. Those thick articles or profiles can
be cured with double extrusion speed as compared to thermal
curing HCRs. As mentioned previously, coloring is also possible for the solid rubbers; a variety of pigments can be used,
as shown in figure 8.
By curing at temperatures around 70°C, the following can
be possible: The co-extrusion of silicone rubber with thermally-sensitive plastics; the incorporation of thermally-sensitive
ingredients into the silicone rubber; or the encapsulation of
electronic parts in a continuous process.
An interesting application is encapsulation, as shown in
figure 9. The company, Luxall s.r.l., offers a variety of different encapsulated LED strips with various colors, effects in
customized profiles or shapes (ref. 2). The Addisil (a trademark of Momentive Performance Materials) UV EX rubbers
have enabled the continuous manufacturing of those LED
strips. Typically, the strips are made in a multi-step process
with casting techniques. The UV technology helps to more
productively manufacture high-quality encapsulated LED
strips while maintaining the superior silicone elastomer properties, including enhanced UV stability, wide temperature
performance and stability, critical to their use outdoors.
Figure 8 - color examples
JULY 2013
Figure 9 - different types of UV silicone
encapsulated LED strips - pictures courtesy
of Luxall s.r.l.
Potential applications in the healthcare industry
Since the UV curing rubbers cure by a hydrosilation reaction, certain harmful by-products such as peroxide are not
formed. The UV LSR 20X0 and Addisil UV EX elastomers,
cured only by UV light, have passed several regulatory compliance tests (ISO 10993, USP Class VI, extraction limits
according to 21CF177.2600), which enable these elastomeric parts to be considered for use in certain healthcare
applications.
Potential applications could be plastic/silicone co-extruded
articles used as catheters, tubes or profiles, or encapsulated
cables or other electronic parts for medical devices.
Summary and outlook
The current demands of the marketplace extend past what
conventional thermal curing elastomers can provide. The new
UV curing silicone elastomers developed by Momentive Performance Materials open up new potential fields of application
in the healthcare, consumer goods and energy industries,
among many others. UV curing silicone elastomers can provide the unique set of properties to which these industries are
already accustomed with conventional thermal curing silicone
elastomers, while adding advantageous manufacturing characteristics.
The combination of on-demand curing at low temperatures
and other technical advantages can provide for shorter cycle
times, reduced work steps, better handling under clean room
conditions, lower energy consumption, low deviation from
tolerances, the ability to produce complex geometries, fewer
Figure 10 - combination of technical
advantages opens up new fields of
application
Low tolerances
Complex geometries
Curing at low
temperatures
Combinability with
other materials
Less bubbles
No scorch
bubbles or scorch, as well as the ability to combine with temperature-sensitive materials.
With the new UV silicone elastomers, certain hazardous
chemicals are typically not formed as side-products during the
curing process. Several regulatory compliance tests (ISO
10993, USP Class VI, extraction limits according to
21CF177.2600) have been passed successfully, which allow
these products to be considered for use in certain healthcare
applications.
In many industries, the possibility of curing at low temperatures could be the enabler for product innovations that
require combining silicones with temperature-sensitive materials.
UV cure LSR might, therefore, replace thermal curing silicones for some applications, but more importantly open the
door to innovative new ones.
References
1. Medical Materials, March 12, 2013.
2. Plastruction, Vol. 1, No. 11+12/11, S. 22.
3. Medical Plastic News, July/August 2012, page 12.
4. Medical Device Developments, pp. 39-40, 2010.
Beate Ganter, Ph.D.
Senior Project Manager UV
Momentive Performance Materials Inc.
Dr. Beate Ganter is the senior project manager for UV-curing Silicone Rubbers at Momentive Performance
Materials Inc. (MPM). Her global career in product development and marketing spans more than a decade.
Before joining MPM in 2007, Beate worked with Bayer AG, later GE Bayer Silicones, in the development of
silicone rubber. She earned her bachelor’s degree in chemistry and her Ph.D. in metal organic chemistry
from the Technical University of Aachen in Germany.
For more information, visit Momentive.com or contact a Momentive Performance Materials Inc.
customer service center.
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