LSR7000 SPE. Ultra Clear Liquid Silicone Rubber for Innovative Optical Solutions in Lighting. Durable Versatile Optically Clear

Transcription

1 LSR7000 SPE Ultra Clear Liquid Silicone Rubber for Innovative Optical Solutions in Lighting Durable Versatile Optically Clear WRITTEN BY: Hee-Seok Hwang (speaker), Momentive Performance Materials Inc. Oliver Franssen, Momentive Performance Materials GmbH Heiko Bayerl, Momentive Performance Materials GmbH REV 1015

2 LSR7000 SPE Ultra Clear Liquid Silicone Rubber for Innovative Optical Solutions in Lighting Durable Versatile Optically Clear Hee-Seok Hwang (speaker), Momentive Performance Materials Inc. Oliver Franssen, Momentive Performance Materials GmbH Heiko Bayerl, Momentive Performance Materials GmbH Abstract For many years, polycarbonate (PC) and polymethylmethacrylate (PMMA) have been the materials of choice, other than glass, for optical applications. These polymeric materials have advantages such as weight reduction, increased freedom in design complexity, and better manufacturing economics due to lower energy consumption, as well as less post processing when compared to glass. However, there are trade-offs when choosing a thermoplastic due to its less superior thermal, UV and chemical resistance, compared to glass. A new optical polymer is now available: the Ultra Clear Silopren* LSR 7000 Series from Momentive Performance Materials Inc. (MPM). This clear as glass liquid silicone rubber (LSR) series combines the physical property benefits of silicones, the ease and high productivity process advantage of LSR, and an optical transparency of 94 percent. LED lighting is one of the most innovative and dynamic industry segments, driven by efficiency and design. Often, new technologies require new materials, especially high-power LEDs, which demand that optics possess a combination of extreme material properties. Materials close to the LED have to withstand harsh UV and blue light radiation in combination with a temperature of up to 150 C for 100,000 hours, the lifetime of a typical LED system. Due to its inorganic backbone, the Silopren LSR 7000 series offers the ability to survive this extreme environment. The Silopren LSR 7000 series provides outstanding thermal, UV, and blue light stabilities, which make this material an ideal candidate to consider for the production of lenses for high power LEDs in professional lighting, automotive lighting, and other lighting applications. In addition to the long-term optical stability of LSR 7000, the LSR injection molding process can enable true freedom of design in manufacturing. For example, thick-walled parts, extreme changes from thin to thick sections and ultra- precision surface structures are possible. Furthermore, the molded parts are typically free of frozen stresses and any birefringence effects, and resistant to micro-cracks. Material waste is reduced to a minimum through cold runner technology. This paper gives an overview of the special material properties of the Ultra Clear Silopren LSR 7000 series and compares its physical data to commercial thermoplastics used for optical applications. Examples highlight existing and potential applications that demonstrate the specific properties of this special material and the benefits created that are not possible with traditional materials. Introduction LSR is a member of the hot-vulcanizing silicone elastomers group. This group of materials is made up of an inorganic basic network of silicone and oxygen. Its elastomeric properties are achieved by the addition of organic methyl groups which only interact to a minor extent. The inherent chemical properties of silicone make LSR a strong, durable material as compared to thermoplastics such as PC and PMMA. The bonding energy of the inorganic Si-O chain of a silicone is 451kJ/mol., which is at least 30 percent higher than the carbon C-C chain of an organic plastic material of 352 kj/mol. Figure 1 shows the typical structure of a silicone polymer chain as well as possible variations concerning organic side groups attached to it. The polymer chains form a three-dimensional network and are thus characterized by very little creep under temperature and load. Silicone elastomers, with a glass transition temperature below -80 C, offer reliable low-temperature

3 flexibility in applications down to -40 C and below. The typical basic properties of silicone rubbers include high heat resistance up to over 200 C, high resistance to ozone and ultraviolet radiation as well as special characteristics that are attributable to the high bonding forces of the siloxane chain. R R - Si - O - Si - O - Si - R R R Bonding energy: c -- c = 352 kj/mol Si -- O = 451 kj/mol R n R R R = - CH 3 = - CH=CH 2 = - Ph = - CH 2 -CH 2 -CF 3 Silicone Elastomers: MQ Methyl-Silicone VMQ Vinyl-Methyl-Silicone PVMQ Phenyl-Vinyl-Methyl-Silicone FVMQ Fluoro-Vinyl-Methyl-Silicone Figure 1. Basic chemical structure of silicone rubber. While thermoplastics may be used to mass produce optical parts, the materials do not typically possess long-term durability to thermal or UV light exposure, whereas the Silopren* LSR 7000 series does. Additional typical properties of the Silopren LSR 7000 series as compared to other transparent materials are detailed below. Additionally, the Silopren LSR 7000 series may be considered for the manufacture of physiologically inert, biocompatible articles for medical applications such as catheters and tubes or for consumer articles like baby pacifiers since LSR parts are generally neutral in both taste and odor. Liquid silicone rubbers cure without creating decomposition products via an addition cure mechanism using a platinum catalyst. Furthermore, very low hardness values are possible without the need of plasticizers. Silicone materials for lighting applications In lighting applications, highly transparent, silicone-based casting systems have been used for a number of years in the chip-onboard (COB) or surface-mounted-device (SMD) technology for sealing the LED semiconductor while acting at the same time as an integrated light feed-out. Silicones have proven themselves in this application with their outstanding aging resistance and color consistency when in direct contact with the high temperature of the semiconductor and the radiation of the LED in the UV and blue-light range. The high-power LEDs of the COB type, in particular, cause conventional plastics to age and yellow rapidly. Table 1. Silopren LSR 7000 series in comparison with other transparent materials. Figures may vary by specific grades. Typical product data values are averages and should not be used as specifications. Category Property Silicone (LSR 7000) PC PMMA Class Properties of Optical Clarity, 2mm Transmission [%] 94 86~89 Standard low duro Index of Refraction (RI) Standard low duro Haze % <1 1~ Abbe number Yellowness Index <1 1~ Durability Heat resistance Excellent Poor Poor Excellant UV resistance Excellent Poor Poor Excellant Design Freedom Complex/micro-design Excellent Good Good Poor Material flexibility Excellent Poor Poor Poor Light weight design Excellent Good Good Poor

4 Figure 2 shows a typical silicone application for LED chip packaging. In this case, RTV type silicones usually are dispensed over the LED chip. Table 2. Typical properties of Silopren* LSR 7000 Series [1]. LSR 7005 LAR 7030 LSR 7060 LSR 7070FC LSR 7080J LSR 7090 Shore A hardness Shore D hardness Viscosity (mixed) at 23 C [Pa s] Tensile strength Elongation at break [%] Figure 2. Silicone-based casting system of the LED package. Injection Molded LSR for Lighting Applications The transparent silicones used for casting the LED chips have now been combined with the processing characteristics of classical LSRs. This development has yielded the highly transparent, injection moldable Silopren LSR 7000 series. The highly transparent liquid silicones are available in a hardness range of 5 to 90 Shore A. Silopren LSR 7005 grade, which at 5 Shore A is very soft, has been developed for adaptive optical interfaces. With very small installation forces, Silopren LSR 7005 grade is able to adapt to different structures and tolerances. The harder Silopren LSR 7060, LSR 7070FC and LSR 7080J grades, in the range of 60, 70 and 80 Shore A, balance requirements regarding flexibility and geometric stability. The Silopren LSR 7090 grade at a hardness of 90 Shore A is the latest development and combines the best dimensional stability with optical durability. The complete Silopren LSR 7000 series enables optic designers and engineers to consider primary and secondary optics for future professional lighting and automotive lighting applications. All products offer a combination of the following benefits: transmission rates over 94 percent (measured at 2 mm wall thickness and at 598 nm); the excellent typical properties that silicone elastomers possess; and compatibility with the cost-effective injection molding technology. Tear strength [N/mm] Transmission at 2 mm [%] Refractive Index at 20º C / 589 nm Typical product data values are averages and should not be used as specifications. The Silopren LSR 7000 series exhibits a constant light transmission property for wavelengths in the UV region from 300 nm up to the near infrared region above 1000 nm. Measured at 2 mm wall thickness, this material has a light transmission of at least 94 percent over a wide range of the light spectrum without absorption cut-offs. Figure 4. Light Transmission properties [2]. The refractive index of Silopren LSR 7000 materials is around 1.41 at room temperature and at a wavelength of 589 nm. This indicates low Fresnel losses giving excellent transmission and efficient optics. Normally the refractive index falls with increasing light wavelengths.

5 Consequently, different light wavelengths in an optical component will refract differently (optical dispersion). This behavior is known as chromatic aberration and is quantified by the Abbe number. The Abbe number of Silopren LSR 7000 materials is typically about 50 [4], making the optical dispersion relatively low for a polymer material. injection-compression molding. A lens made of Silopren LSR 7070 grade with a diameter of 70 mm and a thickness of 13 mm exhibited no voids or sink marks unlike a corresponding molding made of a PMMA. The silicone lens was produced without gatingsystem waste via cold-runner injection molding. The gate cross-section was considerably less than 1 mm² while shrinkage compensation and packing in the cavity is ensured by the thermal expansion of the liquid silicone from a barrel temperature of approximately 25 C to a mold temperature above 150 C. Since the flexibility of the material does not cause internal stresses, even in thick-walled components, the lens can be injection-molded in comparatively short cycle times with very good, optically homogeneous properties. LSR 7010 PC COP PMMA PMMI *Abbe number (v) *v =(nd-1)/ (nf-nc), where (nd-1) = Refractivity, (nf-nc) = Principal Dispersion Figure 5. Dispersion properties [2]. Figure 6 shows two injection molded parts that can be seen in front of bipolar light: Silopren* LSR 7070 grade and PMMA. It can be noted that the PMMA sample shows residual stresses frozen as result of the cooling step of the injection molding process unlike the lens made of LSR The dependence of the refractive index on temperature is a more serious matter that has to be taken into account, especially when designing refractive lens systems. The change in refractive index with temperature - dn/dt - is relatively high: with a temperature increase of 100 C, the refractive index falls from at 25 C to at 125 C [5]. With temperature fluctuations, the focal point of a refractive lens will shift accordingly. This must be taken into consideration in material selection and/or in optical design. Due to their cure mechanism characteristic and comparatively low moduli, liquid silicone components are distinguished by having optically homogeneous properties. Stress-free moldings are possible with pure injection molding without the need for long cooling- or holding-pressure times, an overdimensioned gating system, or a special variant of injection molding techniques, such as, Figure 6. Under polarized light, injection -molded lens made of Silopren LSR 7070 grade in comparison with PMMA [6]. One of the distinctive characteristics of the Silopren LSR 7000 series is that its outstanding

6 optical properties are virtually unaffected after heat exposure to 150 C for 6480 hours. Unlike organic plastics, the optical transparency of the liquid silicone elastomers hardly changes. Figure 9. Test results from MPM showed yellowing after 6,480 hours at 150 C on a variety of transparent materials, including the Silopren LSR 7000 series. Figure 7. Transmission change at 589 nm over 6480 h at 150 C [7]. The manifestation of aging against different materials was easily observed with yellowing after heat exposure for 6480 hours at 150 C. While the yellowness index of the thermoplastic materials under consideration is above 40 and in some cases 54, the yellowness index of Silopren LSR 7060, LSR 7070 and LSR 7080 grades after this heat exposure was comparatively low at 3-4. The yellowing comparison can be seen at Figure 8 below: SAE J2527 is an acceleration weathering exposure test protocol widely used to simulate actual sun exposure for many external parts such as automotive headlamp application. This test simulates aging in a changing hot humid climate at 47 C with 50 percent relative humidity (RH) and additional irradiation with ultraviolet light at an intensity of 0.55 W/m*m at 340 nm and 38 C / 95% RH in the dark phase. After exposure to Xenon accelerated weathering for 9,089 kj, which corresponds to a Florida exposure equivalent of 3.27 years, the haze value of the LSR 7070 increased by 9.6 percent. The corresponding change in polycarbonate tested for comparative purposes was 87 percent. The transparency and color of the liquid silicone rubber was minimally affected. Figure 8. Yellowness index change over 6480 h at 150 C [8]. Figure 10. Light transmission after weathering as specified in SAE J2527 [9]. The Kunststoff-Institut für die mittelständische Wirtschaft NRW GmbH (K.I.M.W.), Germany, investigated the influence of production conditions and processing parameters on the molding properties of lenses made of PC, PMMA, PMMI and LSR7000. Lens design was characterized by thin and thick sections with 9mm thick that need to be filled through

7 1.7mm thin sections. In a standard injection molding process, it is challenging to maintain the holding pressure on the thermoplastic melt at thick areas during cooling time. The thin section tends to freeze before the thick section is cooled, resulting in sink marks and warpage. In order to achieve a steady holding pressure throughout, the thermoplastic polymer injection mold was equipped with an embossing core and an innovative Variotherm temperature control system in order to produce moldings with as little stress as possible in feasible cycle times. In the LSR injection molding process, the low viscous LSR easily fills all sections of the cavity at a low pressure. The heat-curing silicone is injected at a low temperature into a hot cavity, eliminating the need for holding pressure to compensate shrinkage. The LSR lenses, which were comparable both functionally and geometrically to the thermoplastic lenses, were produced using a simple LSR injection mold with conventional electrical heating. These tests also confirmed that geometrically perfect, stress-free parts can be produced from LSR even without using expensive temperature control technology and without complex injection-compression processes. In addition, it was possible to process the LSR within a broad processing window to produce high quality parts. These lenses were then exposed for 700 hours at 85 C and 85 percent humidity. The lenses made of LSR 7070 were the only ones to deliver a perfect light pattern and that exhibited no deformations or cracks unlike lenses made of PC, PMMI or PMMA [11]. It is worth noting that all of the LSR lenses were the only ones produced with no sprue waste by direct gating. Figure 11. Test lens made of Silopren* LSR 7070 grade [12] Source: KIMW, Germany, 2011 Figure 12. Light distribution before and after storage for 700 h at 85 C and 85% RH [13]. Photo source: KIMW, Germany, 2011 Processing in Injection Molding The Silopren LSR 7000 series is processed exclusively by injection molding in accordance with LSR technology and heat-cured during the molding process by an addition cure mechanism with no decomposition by-products. Using a suitable metering and pumping system, the two-component system is fed into the chilled barrel of the injection-molding machine with a metering screw at a mix ratio of 1:1 via a static mixer. Mixing component A (which contains a platinum complex as a chemical catalyst) with component B (which contains the cross-linking agent) produces a reactive mixture that then enters the injection unit with a maintaining temperature of C Here, a comparatively short metering screw equipped with a spring-

8 operated, disk-type, non-return valve dispenses the necessary shot weight. This precise shot volume is then injected into a mold which, in most cases, is electrically heated to between 120 C and 200 C, depending on the application and necessary process. The optimized and tested temperature range for this product family is between 130 C and 150 C for a normal size application of one to 20mm thickness. The process should run in a lower temperature range for anything larger. Generally, molds are vented due to the very short mold filling time and the extremely small mold gap (permissible with this technology). Curing takes place very quickly, generally at about five seconds per millimeter of wall thickness, at a temperature of 180 C. The molds are, in most cases, equipped with cold runner technology, thereby helping achieve a fullyautomated process without sprue waste. Due to LSR s low viscosity characteristics, (between 50 to 500 Pa*s measured at room temperature and with shear rates of 10*1/s), it can fill a very long flow path with a length to cross-section ratio up to 500 to one, even at low injection pressure. In addition, very complicated mold designs and surfaces can be produced with precision. Furthermore, the material s flexibility allows demolding of geometrically difficult structures and even undercuts without the use of splits or core pullers. Figure 13. Simplified view of a LSR injectionmolding machine with mold and metering system. Table 3. Comparison between Silicone and Thermoplastics for sample lens. Category Silicone (LSR 7000) PC PMMA Advantage of LSR 7000 No. of cavity Excellent Poor Poor High yield/shot Cycle time [min] Cooling time Curing time - 12~15 12~15 5~6 - - High productivity Gate size Excellent Poor Poor Easy Optic design Sprue Excellent Poor Poor Less material waste Filling pressure [bar] 25~ ~ ~550 Holding pressure [bar] Machine pressure [bar] Design Diameter Thickness Volume 150~300 >>800 >>600 60~100 60~150 60~150 Size 90 mm Max. 45 mm ~160 ccm Small machine possible Ultra Clear Liquid Silicone Rubber in Lighting Applications The development of the ultra clear Silopren* LSR 7000 series coincides with an era of rapid technological development in the general and automotive lighting industries. The general lighting industry, driven by LED technology, strongly needs more durable materials to enhance the capability of fixture systems, which require more effective luminosity, a long life span and a flexible design. In the automotive lighting industry, safety, energy efficiency and downsizing are key factors advancing the technologies. Furthermore, light is increasingly being used as a design element for the interior passenger compartment. External automobile lights are becoming a design element to showcase and draw attention to brand. Simultaneously, the most diverse sensor and camera technologies in the vehicle are becoming increasingly advanced, requiring high optical substrates [14]. LEDs are increasingly used as light sources

9 since they offer advantages over conventional luminaires with regards to service life, efficiency, rapid response time, and a robust and compact configuration. The latest generation of high power LEDs provides increased light intensity but also reaches higher temperatures, emitting strong blue light radiation, which pushes the limit of many thermoplastics. Optical components made of glass would then be the expensive stand-in solution [15]. Potential Application Examples Since the introduction of the Silopren LSR 7000 series, engineers have been utilizing this innovative material for a wide array of unique applications. The series has evolved into a technology enabler where ideal design was unachievable with traditional materials due to limitations caused by their physical properties. Below are examples of existing and potential applications that demonstrate the specific properties of the Silopren LSR 7000 series and the benefits created that are not possible with traditional materials. Many more designs are currently in development across the globe as engineers begin to incorporate the series into even more innovative applications. Figure 15. Optical lenses for automotive lighting [17]. Isotropic: minimal to no stress even in complex and micro-structure designs Figure 16. Fresnel lens with micro-structure. High flowability: easy implementation of micro-structure optical design like Fresnel lenses Figure 14. Secondary lenses of outdoor lighting. High hardness up to 90 Shore A: dimensional stability and function to protect inner devices High flowability: Design freedom for complex and micro-structure optical parts High temperature and UV light stability: long-term optical property retention

10 benefits, such as homogenous optical properties, increased precision, and enhanced part integration options to better meet the needs of current and future professional and automotive lighting designs and technologies. References Figure 18. Adaptive optical interface [19]; Source: KOSTAL Ultra-low modulus: easy to adapt to different shapes, tolerances and surfaces 1. [1, 7-9] Laboratory results from Momentive Performance Materials, [2, 4, 5] Laboratory results, Fraunhofer Institute IOF, Germany, [3] Professor Neyer, A., University of Dortmund, Germany, [6] IKV, Germany, [10-13] Kunststoff-Institut für die mittelständische Wirtschaft NRW GmbH (K.I.M.W.), Germany, Optical Technologies Joint Project No. 2, documents relating to project meeting on 28th/29th November [14, 15] Berlitz, S.: Design und Techniktrends in der Lichttechnik. Kunststoffe im Automobilbau, VDI-Verlag, 2008, pp [16] REDA S.p.a., Brugherio, Italy 8. [17] Eschenbach Optik GmbH, Nuremberg, Germany 9. [18] Lee&Lee High-tech, Seoul, Korea 10. [19] Leopold Kostal GmbH & Co. KG, Germany Contact Information Figure 19. Light guide Optical clarity: uniform and efficient light distribution over a broad range of wavelengths Conclusions With the advancement and increased use of LED technologies in general lighting, automotive lighting and electronic applications, Momentive Performance Materials Inc. s Silopren* LSR 7000 series offers a new alternative to glass and thermoplastics. It represents the best combination of properties, such as heat resistance, and UV resistance, compared to glass, with the productivity and design freedom of thermoplastics. Due to its elastomeric properties, Silopren* LSR 7000 offers additional Hee-Seok Hwang Senior Global Project Manager Elastomer, Marketing Momentive Performance Materials Inc. Tel.: Hee-seok.hwang@momentive.com Oliver Franssen Senior Global Marketing Leader Elastomer Transportation, Marketing Momentive Performance Materials GmbH Tel: Oliver.Franssen@momentive.com Heiko Bayerl Regional Marketing Manager Elastomers, Marketing Momentive Performance Materials GmbH Tel: Heiko.Bayerl@momentive.com

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