UV-curing silicone rubbers
Magazine for the Polymer Industry
Special Reprint
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2013
1
UV-curing silicone rubbers
UV-Curing Silicone Rubbers find
uses in new of application fields
The vulcanization of the UV-curing silicone rubber compounds starts when the
light is turned on.
B. Ganter, S. Boßhammer, U. Irmer*
3. UV-curing
3.1 UV light
Currently, the demand for product combinations of elastomers with plastics,
electronic parts or other temperature-sensitive ingredients is increasing significantly, driven by a large number of new industrial trends in consumer goods,
medical and lighting technology. Silicone elastomers are well known for their
outstanding UV stability, very good heat ageing properties, chemical resistance, low compression set and that they keep their mechanical properties stable
within a temperature range from -40°C to 200°C. These properties favor silicone
elastomers for applications in the consumer goods, automotive, energy, and
healthcare industries. A detailed overview is given in the publication by
O. Franssen and H. Bayerl [1].
Various curing methods are used for the
vulcanization of silicone elastomers. Usually peroxides or platinum catalysts are
used, which enable a curing reaction at
temperatures of over 120 °C. Momentive
developed a new family of silicone elastomers that are vulcanized by means of
UV light. The advantage is that they cure
very fast, even at low temperatures, and
thus enable the combination of silicone
elastomers with temperature-sensitive
materials. It is possible to continue to
process these materials by extrusion or injection molding – using special translucent
molds. This technology enables fast and
energy-efficient processing as long heating times are not more required.
2. Curing characteristics of
different silicone rubbers
The chart compares the vulcanization graphs
of UV-curing silicone rubber, a platinum-curing liquid silicone rubber (LSR) and a peroxidecuring high consistency rubber (HCR) to show
the vulcanization advantages by means of UV
light as mentioned in the introduction (fig. 1).
Both create analog spectra depending on
the gases in the lamps (Hg, Hg with Fe doping, Hg with Ga doping). For the UV-curing
silicone elastomers, mercury vapor lamps
with Fe doping are preferred. The various gas
discharge lamp systems differ by the position of the generator, the lifecycle and the
power-up mode (stand-by or on/off). For the
injection molding units only lamps with electrode beam technology are suitable as the
generator can remain outside of the mold
and does not have to be connected directly
with the lamp (fig. 2). For extruding, one
UV-curing silicone rubber measured at 25 °C
Silopren LSR measured at 150 °C
Fig. 1:
The vulcanization of the
UV-curing silicone rubber
compounds starts immediately when the light is
turned on
Crosslinking level / %
1. Introduction
To create UV rays, traditional gas discharge lamps can be used. It is possible to
use lamps with electrode beam technology
as well as lamps with excitation via microwaves.
90 %
crosslinking level
Silopren HCR,
peroxide-curing,
measured at 150 °C
1
Start-up UV light
Time / min
5
* Dr. Beate Ganter,
beate.ganter@momentive.com,
Dr. Stephan Boßhammer,
Uwe Irmer
Momentive Performance Materials GmbH,
Leverkusen
Paper, DKT 2012, 2 – 5 July 2012, Nürnberg,
Germany
Published with kind permission of Deutsche
Kautschuk-Gesellschaft e. V., Frankfurt,
Germany
2
Fig. 2:
UV LSR injection molding unit during exposure
to light
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can use lamps with microwave technology
as well as with electrode beam technology
(fig. 3).
Due to the progressive development of
LED lights, these can also be used now.
UV-LEDs emit light with a specific wavelength so no unnecessary radiation ranges
are developed (e. g., IR radiation) and this
in turn considerably extends the lifecycle
of the mold material. In addition, energy
consumption and space requirements of
LED lights are significantly less and they
can therefore easily be integrated into an
injection molding unit.
3.2 UV-curing liquid
silicone rubbers (LSR)
Liquid silicone elastomers are usually processed by injection molding at mold temperatures of 180 – 200 °C. UV-curing LSRs
can be processed with the same machines,
however, a transparent mold is required,
allowing the UV light to pass through. SpeFig. 3:
UV-curing of an extruded round profile
cial types of acrylic glass can be used as
material, which allow a large part of the UV
light to penetrate and also have a higher
UV stability versus traditional acrylic glass.
Due to the low vulcanization temperature, the manufactured parts show almost no
shrinkage and also have fewer air bubbles. As
the vulcanization only starts once the light is
turned on, scorching can be avoided. This is often a problem with thermally curing systems.
This technology enables the manufacture of
high quality products that usually require no
finishing. In addition, with UV-curing, the cycle times can be reduced significantly. Silicone
parts with wall thicknesses of up to 100 mm
can be vulcanized in less than 5 min. On the
other hand, with the traditional thermal procedure, curing can take up to 30 min. Therefore, a significant amount of time and energy
can be saved. The process is shown in figure 4.
Examples are items for the high-voltage
industry where such wall thicknesses are
manufactured from silicone elastomers for
insulators or cable fittings. The UV-curing
silicone elastomer UV Electro 2X5 helps to
manufacture these items quickly and energy
efficiently and with extremely high quality
(fewer air bubbles, no shrinkage) (fig. 5).
With the UV technology, it is also possible
to process LSRs together with thermally sensitive plastics in traditional injection molding
machines. An example is the combination of
LSRs with polypropylene; their joint processing in a mold was already demonstrated [2].
This opens the possibility to combine nearly
Fig. 4:
Diagram for UV-curing of LSR
Fig. 5:
all and moreover all inexpensive plastics with
silicone elastomers (fig. 6). In addition, liquid silicone rubber can be mixed with temperature-sensitive ingredients, such as active
pharmaceutical ingredients (API), and vulcanized under UV light.
3.3 UV Curing High Consistency Rubbers
In extrusion processes, high consistency
rubbers (HCR) are often vulcanized in hot air
channels at temperatures above 250 °C. They
usually require a line of hot air ovens exceeding a length of 10 m. UV-curing Addisil UV EX
silicone rubbers are processed and shaped by
means of standard extruders. For vulcanization, only a UV light is required, which usually
has a maximum length of 30 cm. This saves
energy, as long heating segments at high temperatures are no longer required, as well as
space in the manufacturing area as the vulcanization line is significantly shorter (fig. 7).
During UV curing, the vulcanization process is extremely fast (Command Cure) so that
there is no formation of bubbles, even in
thick-walled parts. Extrudates, such as complicated profiles, can be manufactured with
very precise dimensional stability.
In addition, extruding at lower temperatures also allows for a co-extrusion of silicone rubbers and thermally sensitive plastics or incorporating temperature-sensitive
ingredients/materials. A very interesting application in this regard is the continuous sheathing of LEDs [3]. Up to now, manufacturing
such light tubes has only been possible using
Insulator made from UV-cured LSR
Pump
UV-additive
UV-transparent mold
Base material
UV-source
UV-shield
Material
Shaping and curing
• Material is transferred in a transparent mold
• Curing starts by switching on UV-source
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3
UV-curing silicone rubbers
complex procedures with casting technologies
that are limited to a specific length (fig. 8).
As already implied in the introduction,
UV-curing of silicone rubbers is based on a
platinum-catalyzed hydrosilylation, which
does not cause any breakdown products
during extruding or injection molding. Biocompatibility tests (ISO10993, USP Class VI,
volatile components) were performed with
various UV materials and passed so that an
application in the healthcare, food, and consumer goods industries is possible.
The process enables combinations of silicone elastomers with thermally sensitive
plastics, components, chemicals, or active
pharmaceutical ingredients, which previously lead to the destruction of the material or
active agent due to the high curing temperatures. Thus, generally new 2K applications
present themselves for silicone elastomers,
which were previously inaccessible.
[2] Doug Smock, PlasticsToday, July 9th, 2012
[3] Plastruction, Vol. 1, No. 11+12/11, pg. 22
Fig. 6:
Plugs for wine bottles made from PP/LSR
5. References
[1] O. Franssen, H. Bayerl, GAK 7/2012, pg.
462 – 466
4. Summary and outlook
HCR
Vulcanizing silicone rubbers with UV light
opens up new areas of application for silicone
elastomers as the UV vulcanization enables
extremely fast curing at low temperatures.
Silicone elastomers vulcanized by means of
UV light show the same properties as thermally cured elastomers. Manufacturing processes remain similar, existing injection molding
machines or extrusion systems can further be
used. UV lights have to be integrated into the
process and in the case of injection molding,
special transparent molds have to be made.
On the other hand, long heating times (LSR)
and long vulcanization ovens with high temperatures (HCR) can be eliminated.
Extruder
Shock
channel
> 450 °C
Hot air channel
> 250 °C
Length: 2–12 m
Extrusion and thermal curing
Fig. 7:
Comparison of thermal
vulcanization and UV
vulcanization during
extrusion
HCR
Extruder
UVsource
≤ 30 cm
Extrusion and UV-curing
Fig. 8:
LED light tube encased in
UV-cured silicone rubber
(courtesy of Luxall s.r.l)
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4 issues 2013
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