The technical service magazine for the rubber industry Volume 248, No. 4

JULY 2013

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Putting silicone rubbers into a new light

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 ther-mally cured. With UV cure LSR, the application set is expanded for a great number of industries, taking silicone rubber to an en-tirely different platform.

Crosslinking of UV curing silicone rubbers by UV lightThe UV curing of liquid silicone rubber (LSR) and heat cured silicone rubber (HCR) still involves an addition curing mecha-nism like the heat curing silicone rubbers, but with a photosen-sitive catalyst instead. This catalyst is activated at a wave-length in the UVA area between 320 and 400 nm. UV irradia-tion can be generated by gas discharge lamps emitted from a microwave or an electrode system. Both have shown compa-rable 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 dota-tion) has shown the most effective curing. The lamp systems are differentiated by the position of the generator and the life-time 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 ma-chine. 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 irradi-ated. 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 tem-peratures to ensure full vulcanization.

Molding of UV curing liquid silicone rubbers (LSR) Injection molding processFor 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

Putting silicone rubbers into a new lightby 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 grow-ing 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 elasto-mers are the preferred material in many consumer goods, au-tomotive, 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 photo-chemical 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 ben-efit 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, heat-stable thermoplastics, a certain impediment to realizing the full

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Table 1 - overview of the typical properties of silicone elastomers

Main featuresSiO backbone

Stable material

Elastic memory

Low modulus, durometer

Ease of fabrication

Benefits• High thermal stability

• Excellent dielectric properties• Sterilizable with steam, ETO and gamma radiation

• Highly transparent vulcanizates

• 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

• Resiliency, high recovery after flexing• Long service life at dynamic stress

• Reduced material fatigue

• Minimal insertion force, smooth operation

• Ergonomics and comfort• No plasticizers or other organic additives

• Design versatility with tight tolerances

• Excellent processability• Transparency, easily pigmented

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a mold temperature around 120°C, which leads to signifi-cantly 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 in-stead of 1:1 for thermal curing LSRs), and a special mold is required. Figure 1 shows the principal process of the UV cur-ing 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 materi-als 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 injec-tion 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

parts, because the low thermal conductivity of silicone rubber is no longer a limiting factor for the curing. Even cross-sec-tions 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 some-times extended by using color pigments.

Two-component injection molding with UV lightTwo-component injection molding has been utilized for a

Figure 1 - principal process of the UV curing injection molding

Material feeding Shape and cure

PumpUV additive

UV transparent mold

Base material

UV-sourceScreening

• Material pumped into transparent mold• Curing initiated with start of UV light

Figure 2 - demolding of a cured test sheet from a PMMA mold

Figure 3 - insulators made from UV LSR

number of applications in the healthcare, consumer goods and automotive industries; examples are shower heads, air diffu-sors, 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, poly-ester 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 30-40°C can enable new potential two-component combinations of silicone elastomers with polyolefins or standard thermo-plastics. The process can allow for the use of different, and often less expensive, plastics, and is no longer limited by heat-

stable polymers, so it opens up new design opportunities for kitchenware, labware, teething rings/infant care, and soft-touch handles and grips. It can also become possible to imple-ment 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 inte-grated UV light LED system in 20 seconds. For stable and optimum curing with good de-molding properties, it is recom-mended 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 consider-able energy during manufacturing. The UV curing solid sili-cone 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 compari-son of a thermal curing extrusion process to a UV curing sili-cone 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 irradia-tion 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 4 - molded article from PP andUV LSR 2060

Figure 5 - mold concept of PP/UV LSR molding

Polypropylene insert PMMA cavity

UV LSRinjection point

UV LED

Steel base mold UV LSR

Figure 6 - thermal curing extrusion process compared to UV curing

HCR Extruder

Material extruded through die over thermal shock heater in heating tunnel

UV source

>30 cm

Material extruded through die in UV source.

HCR Extruder

Heating tunnel2-10 m >250°C

Shock heat >450°C

This fast or on-demand curing at low temperatures typi-cally results in high quality extruded products with tight toler-ances 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 pos-sible 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 thermal-ly-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 differ-ent encapsulated LED strips with various colors, effects in customized profiles or shapes (ref. 2). The Addisil (a trade-mark 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 prop-erties, including enhanced UV stability, wide temperature performance and stability, critical to their use outdoors.

Potential applications in the healthcare industrySince the UV curing rubbers cure by a hydrosilation reac-tion, 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 com-pliance tests (ISO 10993, USP Class VI, extraction limits according to 21CF177.2600), which enable these elasto-meric 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 outlookThe current demands of the marketplace extend past what conventional thermal curing elastomers can provide. The new UV curing silicone elastomers developed by Momentive Per-formance Materials open up new potential fields of application in the healthcare, consumer goods and energy industries, among many others. UV curing silicone elastomers can pro-vide the unique set of properties to which these industries are already accustomed with conventional thermal curing silicone elastomers, while adding advantageous manufacturing charac-teristics. 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

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Figure 7 - UV extrusion of a rope

Figure 8 - color examples

Figure 9 - different types of UV siliconeencapsulated LED strips - pictures courtesy

of Luxall s.r.l.

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bubbles or scorch, as well as the ability to combine with tem-perature-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 tem-peratures could be the enabler for product innovations that require combining silicones with temperature-sensitive mate-rials. UV cure LSR might, therefore, replace thermal curing sili-cones for some applications, but more importantly open the door to innovative new ones.

References1. 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.

Figure 10 - combination of technical advantages opens up new fields of

application

Low tolerances Complex geometries

Less bubbles

No scorch

Curing at lowtemperatures

Combinability withother materials

Beate Ganter, Ph.D.Senior Project Manager UVMomentive 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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