Now that the means by which actives can move through silicone have been established, a closer look will be taken at the factors that control their diffusivity and rate of release. While we’ve talked briefly about how matrix and reservoir designs impact release rates, there are other factors to be considered in how an active will move throughout a cured silicone system and release into or onto the body. When considering diffusivity, one must realize that variables associated with both the active and the silicone medium have a part to play. On a very basic level the molecular weight and/or the molecular volume (or spatial dimensions) of an active will have an impact on how readily it migrates through a cured silicone. The bulkier the molecule the slower the progress. Similarly, the amount of crosslinking that a given silicone formula provides will impact diffusion and release. The more functional groups on the polymer (and often the lower the molecular weight of the polymer) will yield a greater crosslink density and, in effect a denser web through which to pass. Then there are also filler levels to consider. Silicone elastomers are most commonly filled with silica, which provides the mechanical, rubber-like strength of an elastomer, as opposed to the soft and easily torn consistency of a silicone gel. The greater the loading of a reinforcing filler, such as silica, the greater the steric hindrance to slow down the active. However, it’s not simply the steric hindrance of the reinforcing filler; the pendant groups coming off the backbone of the polymers themselves will also impact the progress of an active. Moreover, very large groups such as diphenyl, will provide a much greater degree of hindrance than smaller groups such as dimethyl. Lastly, there is the polarity of the pendant groups versus the polarity of the active to be considered. Slight inconsistencies in polarity may result in further slowing of the active through silicone.
All of the above variables may be controlled through custom formulation to optimize the diffusivity and permeability of an active through a silicone.
Silicones are well established as the elastomeric biomaterials of choice for long-term implants, and are also ideal for use as platforms for drug delivery. As indicated above, a host of APIs are soluble in silicones. Additionally, the material’s inherent microporous structure provides a means of transporting soluble APIs through the cured material and delivering them to their targeted location. Lastly, silicone chemistry offers a variety of methods by which to control permeation and elution rates. Taken as a whole, these characteristics distinguish silicone as a versatile raw material that is tailor-made to facilitate the needs of the emerging targeted-release and combination product markets.
Recent Developments in Ultra Low OutgassingTM Silicones for Space Applications
Bill Riegler(1) Brian Burkitt(2), Vincent Malave(3), Roy Johnson(4), and Rob Thomaier(5)
NuSil Technology, Product Director – Engineering Materials, 1050 Cindy Lane, Carpinteria, CA 93013, USA,
(2)NuSil Technology, Technical Sales Supervisor
(3)NuSil Technology, Technical Sales
(4)NuSil Technology, Chemist
(5)NuSil Technology, Research Director
Presented at the 11th Japan International SAMPE Conference (JISSE-11)
November 25-27, 2009, Tokyo, JAPAN
On spacecraft, silicones are imperative materials for use as protective coatings, encapsulants, adhesives, and sealants. Silicones also provide broad operating temperature ranges and are excellent materials for CTE mismatch in bonding applications. Nonetheless, there is concern that silicones may contribute to contamination of sensitive equipment when exposed to harsh environments and vacuum conditions. Contamination from polymeric materials
usually results from outgassing of low molecular polymers not crosslinked into the polymer matrix. Subsequently, these contaminants may condensate on nearby surfaces during thermal cycling. Controlled volatility silicones formulated to reduce the levels of outgassed materials have been in use for decades. These materials are required to meet standards set by NASA and the European Space Agency (ESA), including
1.1 Situation Background and Overview of Contents
With the large amounts of adhesive/sealants currently utilized on spacecraft, manufacturers must pay close attention to the materials’ outgassing levels and carefully monitor their cumulative contaminant levels. Selecting a material with outgassing levels that exceed the industry standard of ≤1 % TML and ≤0.1% CVCM could prove beneficial for many reasons. For instance, by employing materials with lower outgassing levels manufacturers could use more of a material when necessary for mechanical reasons or as required by production techniques. An Ultra Low OutgassingTM material keeps processing time down as no additional conditioning is required to achieve the desired outgassing values, nor are the physical properties altered in the conditioning. In addition, if device contamination is based on cumulative contaminant levels from all material within the device’s vicinity, using a material with exceptionally low levels of outgassing could allow use of other materials with higher outgassing levels in the same vicinity. An exemplary potential application for using Ultra Low OutgassingTM materials can be found in solar cell arrays. The lower outgassing level produces lower contamination, subsequently producing a potential more efficient solar cell by extending the life of a cell, especially in Low Earth Orbit (LEO) where equipment is exposed to the detrimental effects of molecular oxygen.3 The spacecraft could potentially last longer in space, therefore displacing the huge cost to build and transport spacecraft over longer periods of time.
In recent years, the industry need for lower levels of outgassing led to the development of several silicone material systems that exceed the standard criteria for outgassing and meet the Ultra Low OutgassingTM requirements of ≤0.1 % TML and ≤0.01 % CVCM. Originally, these materials were all resin-reinforced silicone elastomers. While silica- reinforced systems generally have improved physical properties over resin-reinforced systems, silica contains a volatile component and chemists were initially unable to develop a silica-reinforced material that met the Ultra Low OutgassingTM requirements. Extensive research and development provided SCV-2585, the first silica-reinforced Ultra Low OutgassingTM silicone system. This report compares the cured physical properties of a standard silica filled, low outgassing material, CV-2289, with SCV-2585. CV-2289, like all CV- materials, meets the NASA standard of TML and To date, CV10-2568 is a widely used low outgas silicone material for space applications offered by NuSil Technology. It incorporates the iron oxide/microballoon filler and is an ideal material for bonding, sealing, or potting due to its silica-reinforcement and subsequent mechanical properties. This includes low density and low modulus for CTE mismatch and excellent thermal stability over a broad operating temperature range required for materials used in extreme environments such as space, where thermal temperatures can range from -115°C to 300°C. Nevertheless, a low modulus silica reinforced, iron oxide and microballoon elastomeric silicone that meets Ultra Low OutgassingTM requirements has not previously been achieved. This report compares the cured physical properties of the CV10-2568 with a comparable Ultra Low OutgassingTM developmental iron oxide and microballoon filled silicone elastomer, LSR2-9860-30. The outgassing profiles of each material based on ASTM E 1559 test method are examined in detail.
Table 1. List of material samples used and their basic chemical makeup.
Table 1. List of the materials that are compared in this study.
2.1.1 Processing for Controlled Volatility
The critical contaminating species typically outgassed from silicone materials are primarily caused by the low to middle molecular weight silicone cyclics and polymers that are not covalently bonded into the silicone matrix. These species are eliminated in NuSil CV materials to prevent subsequent outgassing and contamination. A refinement process can remove low to middle molecular weight linears and cyclics from the polymer formed during the polymerization process. To produce the Ultra Low OutgassingTM materials, SCV-, extensive processing time is needed to achieve lower levels.
2.2.1 ASTM E 595
This test method is used to determine the volatile content of materials when exposed to a vacuum environment (i.e. space). The two parameters measured are TML and CVCM. Water vapor recovery (WVR) is an additional parameter that can also be obtained after the completion of the exposures and measurements required for TML and CVCM.
2.2.2 ASTM E 595 Test Parameters
Each material sample is preconditioned at 50 % relative humidity and ambient atmosphere for 24 hours. The sample is weighed and loaded into the test chamber within the ASTM E 595 test stand. The sample is then heated to 125°C and vacuum is pulled to less than 5×10-5 torr. These conditions are held for 24 hours. The volatiles that outgas under these conditions escape through an exit port, and condense on a collector plate maintained at 25 C. Once the test is complete, the samples are removed from the chamber and the collector plate and samples are then weighed.
2.2.3 Data Analysis
The CVCM is the quantity of material outgassed from the sample that condenses on the condenser plate and is presented as a percentage calculated from the difference in mass on the collector plate before and after the test. The percent TML, the percent total mass of the material outgassed from the initial sample is calculated from the mass of the sample measured before and after the test. After the specimen is weighed to determine the TML, the WVR can be determined. The specimen is stored at 50% humidity for 24 hours at 25°C to permit sorption of water vapor. The specimen is then weighed again which is then subtracted by the mass determined after vacuum exposure to obtain the WVR.
2.2.4 ASTM E 1559
OSI laboratories4 conducted ASTM E1559 experiments and provided test reports. The isothermal outgassing test apparatus is explained in detail by Garret et al. and will only be discussed here briefly.5,6 The material sample can range from 0.5 g to 10 g and is placed in a temperature-controlled effusion cell in a vacuum chamber. All samples are preconditioned in accordance with ASTM E 595 unless otherwise specified. Outgassing flux leaving the effusion cell orifice condenses on four Quartz Crystal Microbalances (QCMs) that are controlled at selected temperatures. The QCMs and effusion cell are surrounded by liquid nitrogen shrouds to ensure the molecular flux impinging on the QCMs is due only to the sample in the effusion cell. The TML and outgassing rate from the sample are determined as functions of time from the mass deposited on an 80 K QCM and normalized with respect to the initial mass of the sample. The amount of condensable outgassing species (VCM), is measured as a function of time from the mass collected on the 298 K QCM. After the outgassing test is complete, the QCMs are then heated to 398 K at a rate of 1K/min. As the QCM heats the deposited material evaporates. The species that evaporate can be analyzed by a quadruple mass spectrometer to quantitatively determine the species observed.
126.96.36.199 ASTM E 1559 Sample Preparation
The CV-2289 and SCV-2585 samples were cured at 150°C for 15 minutes into discs 1.49 inches in diameter by 0.125 inches thick. CV10-2568 and LSR2-98960-30 were cured at 150°C for 30 minutes into discs. The surface area is calculated for both faces and the edge of the disc. One of the supplied discs of material was placed in the effusion cell as the test sample. The samples were tested with no additional preconditioning.
188.8.131.52 Test Parameters
The following parameters were set for each sample:
Chamber Pressures: 10-8-10-10 torr
View Factor from QCM to sample: 415 cm2
Test Duration: 72 hours
Sample Temperature: 125 °C
QCM Temperatures: 80 K, 160 K, 220 K, and 298 K
QCM Sensitivity: 4.43 x 10-9 g/cm2/Hz
2.2.5 Data Analysis
184.108.40.206 Outgassing Rate
The outgassing rates for species condensable on the warmer QCMs can be calculated from curve fits to the data. The total outgassing rate from the 80 K QCM then can be compared to the outgassing rates for species condensable on the warmer QCMs to determine the rates of very high volatility species (water and solvents) and the rates of the remaining species (high, medium, and low volatility).
3. RESULTS AND DISCUSSION
3.1 Comparison of Physical Properties
In Table 2 the results for the typical material properties for each sample were measured according to ASTM protocols, are listed and compared. These properties were measured at NuSil Technology.
3.2 % Total Mass Loss
3.2.1 Comparison of % TML from ASTM E 595 and %TML at the end of the ASTM E 1559 Test
The %TML of each sample for the results obtained from ASTM E595 is compared to the results obtained from ASTM E1559.
3.2.2 % TML ASTM E 1559
220.127.116.11 Amount of TML Due to Different Outgassed Species
The volatility of a material is directly related to its molecular weight (MW): the lower the molecular weight, the higher the volatility, and vice versa. Molecular weight ranges for the species in the different volatility categories can be estimated based upon engineering experience related to species condensability and mass spectrometer data. The “extremely low molecular weight” group of species in Table 3 is primarily due to water and solvents, highly volatile materials. The “low molecular weight” species most likely have molecular weights of 50 to 200 amu, the “medium molecular weight” species fall in the 200 to 400 amu range, and the “high molecular weight” species have estimated molecular weights above 400 amu7 and are the least volatile of all the species.
Table 3. Cumulative Amounts of Estimated Volatility of Different Outgassed Species
3.2.3 Total Mass Loss as a Function of Test Time
Fig. 2 & 3 is a plot of the TML of each sample over the 72 hour period. After the initial loss and compared to CV-2289 & CV10-2568 respectively, the Ultra Low OutgassingTM materials, SCV-2585 & LSR2-9860-30, show relatively no mass loss over the entire course of the experiment.
3.3 % Volatile Condensable Material (ASTM E 1559)
Figures 4 & 5 are plots of the volatile condensable material from the 298 QCM as a function of time.
3.4Comparison of the Total Outgassing Rate as a Function of Time
Fig. 6 & 7 shows the total outgassing rate data as a function of test time for CV-2289, SCV-2585, CV10-2568, & LSR2-9860-30. These outgassing rates are for species condensable at 80 K and thus would not include certain gases such as nitrogen and oxygen.
3.3 Desorption Rate at 80 K QCM
The QTGA test data can be used to determine the relative amounts of the species outgassed. Recall that at the conclusion of the isothermal outgassing test, the 80 K QCM is heated at increments of 1K/min while the outgassed species that deposit on the surface evaporate from the crystal. As the temperature of the QCM is increased during QTGA, the collected species will evaporate from the QCM in order of their relative volatilities. As the temperature of the QCM increases, the evaporation rate of the species also increases until it reaches a peak. The slope of the leading edge is characteristic of the species being volatilized. In Fig. 8 & 9, the QTGA data for all materials are plotted together as evaporation rate from the QCM as a function of QCM temperature.
As devices and processes become more advanced and sensitive to molecular contamination, more details of characterization of the construction materials must be obtained. Ultra Low OutgassingTM specification requirements of ≤ 0.1% TML and ≤ 0.01% CVCM can be useful in the overall management of outgassing species. The results from kinetic outgassing data allow engineers to better predict the levels of contamination, migration, and deposition once the materials are in space. Achieving these lower levels does not compromise physical properties and thus a broad range of silicone elastomers with unique and specific properties can be developed. We have shown that the addition of unique filler packages such as iron oxide and microballoons do not change the outgassing kinetics.
1. ASTM E-595, “Standard Test Method for Total Mass Loss and Collected Condensable Materials from Outgassing in a Vacuum Environment.”
2. ASTM E 1559, “Standard Test Method for Contamination Outgassing Characteristics of Spacecraft Materials.”
3. Banks, B.A., de Groh, K.K., Rutledge S.K., Haytas, C.A. “Consequences of Atomic Oxygen Interaction with silicone contamination on Surfaces in Low Earth Orbit,” Proc of SPIE, 8784, 62-71, 1999.
4. OSI Labs
5. J.W. Garrett, A.P.M. Glassford, and J. M. Steakley, “ASTM E 1559 Method for Measuring Material Outgassing/Deposition Kinetics”, Journal of the IEST, pp. 19-28, Jan/Feb 1995
6. A.P.M.Glassford and J.W.Garrett, “Characterization of Contamination Generation Characteristics of Satellite Materials”, Final Report WRDC-TR-89-4114, Jun 82 – Aug 89
7. S.L Sivas, B. Riegler, B. Burkitt, and R. Thomaier. “Testing Ultra Low OutgassingTM Silicone Materials,” SAMPE Journal, Jan/Feb 2008.
Bill Riegler is the General Manager Asia for NuSil Technology, the eighth largest silicone manufacturer in the world. Bill has been in the silicone industry for almost twenty five years with various positions at NuSil and the silicone division of Union Carbide, which became the OSi Specialties Group of GE Silicones, now Momentive. Bill has a B.S. in Chemistry from the University of California at Santa Barbara and a Masters in Business from Pepperdine University. He began his career in Research and Development and held several technical sales positions before managing NuSil’s domestic technical sales force & Engineering Materials Product Line. Bill is now directing NuSil’s worldwide efforts into Asia for Healthcare & Engineering Materials, which includes the Aerospace, Photonics and Electronics Industries.
A Silicone-Based Ice-Phobic Coating for Aircraft
Summer L. Sivas, Ph.D., Technical Specialist,
Bill Riegler, Product Director-Engineering Materials,
Rob Thomaier, R&D Director,
NuSil Technology LLC, Carpinteria, CA., U.S.
Kelly Hoover, Senior Engineer, Pratt & Whitney, East Hartford, CT., U.S.
The problem of ice build-up in an engine or on the leading edges of a plane is a significant problem in the aircraft industry. Ice build up can affect many aspects of flying such as lift, drag, and thrust. Under the direction of Pratt & Whitney in 2004, the Department of the Army, Engineer Research and Development Center, Cold Regions Research and Engineering Laboratory (CRREL) in Hanover, NH, U.S, performed adhesion strength tests designed to measure the adhesion strength of ice to several different commercially available ice-phobic coatings. The results show that the silicone material, R-2180, dropped the adhesion strength by a factor of 40 over bare steel and aluminum surfaces. These results were the lowest mean failure stress that CRREL has ever seen at 37 kPa while Teflon was previously the lowest tested value measured at 238 kPa.
Key words: Silicone coating, ice-phobic coating, de-icing
1.1 Background. Ice control is a significant and practical concern spanning over many industries. To address these issues the U.S. Army Corps of Engineers of the Department of the Army has written a twenty-chapter manual to provide guidance in this area.1 Titled Ice Engineering, this manual covers ice buildup on lock walls, hydropower intakes, navigation channels, hydraulic systems, etc. Ice engineering is not only a structural engineering concern in cold regions but also a major area of significance in the aircraft industry affecting many aspects of flying. For instance, when ice builds up on the wings or leading edges of airplanes it may decrease lift and increase drag. In fact, wind tunnel tests have shown that very thin ice sheets can reduce lift by as much as 30% and drag by 40%.2 These consequences of ice build-up are extremely hazardous and may occur during any season, anywhere in the country. In order to avoid hazardous situations the Federal Aviation Administration (FAA) has administered Airworthiness Directives (Ads), like Docket # FAA-2004-19078, that identifies the limitations of certain commercial aircraft to icing problems.
Controlling ice from building up on static surfaces is not a trivial task. There are multiple ways to control ice build up on an aircraft and often more than one is used for maximum benefit. The most effective approach is to keep the temperature of the surface above 0ºC. Often times the aircrafts are stored in heated hangars or heaters are built into areas where ice tends to form. Chemicals such as derivatives of glycol ethers have also been used to de-ice aircrafts, as they effectively lower the freezing point. Recently, however, Canada has banned 2-methoxyethanol as a de-icing chemical because of environmental concerns.3 Unfortunately, both these solutions are costly and impractical in many situations.
An ideal and more economical de-icing solution would be to apply a material that prevents ice from building in the first place rather than taking it off after the fact. However, this is a difficult undertaking considering that the adhesion strength of the ice must be less than the shear stress that the ice exerts on the substrate. In general terms adhesion is defined as the physical and chemical bonding of two substrates. Substrates that have reactive groups available for bonding like OH or C=O groups on glass, plastics, and aluminum make this chemical attraction greater through van der Waals forces or weak hydrogen attraction. Substrates with limited available bonding sites make adhesion difficult, such as Acetal, Nylon 24, or PTFE. Numerous other substrates fit somewhere in-between. Currently there are many materials commercially available and marketed as ice-phobic. Many of these materials have been tested and ranked by the 1998 study by Haehnel and Mulherin.4 More recently, another round of materials were tested and reported in Laboratory Ice Adhesion Test Results for Commercial Icephobic Coatings for Pratt & Whitney at CRREL, including a silicone coating from NuSil Technology, LLC.4 1.2 Silicone Chemistry. Silicone, or more appropriately named ‘Polyorganosiloxanes’, are over sixty years old.6 The diagram below shows a typical structure, Figure 1, where the R groups represent functional organic constituent groups such as methyl, phenyl or trifluoropropyl. Silicones have very unique properties compared to organic based rubbers. Their ability to remain elastic at low temperatures and resistance to breakdown at high temperatures make its use valuable in harsh environments. The typical glass transition point (Tg) of many silicones is less than -115 ºC. Other properties silicone offers are low modulus, resistance to moisture (< 0.4 %), high dielectric strength of 500 V/mil, low shrinkage (< 1%), low ionic content (< 20 ppm) and formulation flexibility. Unlike one-part moisture cure silicone adhesives, this mechanism involves no leaving group allowing these systems to cure in closed environments. Most platinum systems can fully cure at room temperature in twenty-four hours, or the cure can be accelerated with heat.
Twelve replicates each of six different ice-phobic coatings were spray applied to aluminum test piles. The tested coatings are given in Table 1. Each sample was rinsed at least two times with isopropyl alcohol except for the Microphase Coatings, Cary, NC based on the companies recommendation prior to ice adhesion tests.
The adhesion strengths of listed ice-phobic coatings were tested and compared using a CRREL developed test method to measure the bond strength of ice to a substrate adapted from the Zero-Degree Cone Test for adhesive joints.4 A schematic of the test stand is shown in Figure 3. Ice is grown in a gap between two concentric, cylindrical surfaces. The force required to push the inner cylinder out of the ice collar is measured determining the adhesive strength. An O-ring placed at the bottom of the inner cylinder, keeps any water from leaking out while it is freezing. The samples are frozen for eight hours at -10ºC and allowed to rest for another 40 hours. The samples are then tested on the equipment with a constant rate of 0.06 mm/min until the ice adhesion fails.
4. RESULTS AND DISSCUSSION
Laboratory Ice Adhesion test results for commercial ice-phobic coatings, Table 1, were tested for Pratt & Whitney. These materials include: MegaGuard LiquiCote; Phasebreak B-2; Microphase ESL; RIP-4004, 51PC951; and R-2180. Load-displacement plots for each test performed were collected and are available with the CRREL report.5 Figure 5 summarizes these results. The Y-axis describes the shear stress required for ice release, the value that is calculated from the measured maximum load required to remove the ice from the surface. The mean and standard deviation were derived from 12 replicates. The error bars indicate the range in stress values for each group of samples.
Prior to this study, Teflon consistently expressed the lowest failure values of 238 kPa. However, it is apparent that the silicone polymer, R-2180, demonstrates the least amount of nominal stress of 37 kPa compared to the other commercially available icephobic coatings tested. Phasebreak B2 and ESL also had low adhesion strength at 117 and 295 kPa respectively. However, both coatings show high variability and standard deviation. These discrepancies are associated with the observation that the ice was in various states of solidification. It was observed in the CRREL report that several of the replicates from both Phasebreak B-2 and ESL had observable traces of unfrozen water on the top and bottom of the samples.4 These materials are suspect to inconsistencies due to solutes leaching from the coatings into the water that lower the freezing point of the surrounding water. It would be valuable to determine if these coatings would stabilize if the tests were conducted under conditions where the ice completely freezes at lowertemperatures.
Figure 4: Laboratory Ice Adhesion test Results for Commercial Ice-phobic Coatings for Pratt & Whitney, May 2004, CRREL.4
A comparable study conducted by the US Army Corps of Engineers of the Department of the Army evaluated the adhesion strengths of several commercially available materials, coatings, and paints known to have low friction or non-stick properties, see Figure 5.1 In this test the coatings were applied over samples having highly durable Corp paints previously applied because it was considered highly unlikely that the low-adhesion coatings would actually replace the existing paints and more likely would be applied directly over the Corps paints. Similar to the results presented in this study, R-2180 again performed superior to the other coatings and materials tested. In fact, it decreased the adhesion strength over bare steel by a factor of 40.
Another critical issue addressed is how well a coating will perform after exposure to extreme environmental conditions and wear. Figure 6 shows how R-2180 performs after exposure to various environmental conditions including physical wear, thermal cycling, humidity cycling, and salt spray. The durability of R-2180 coated aluminum pins were tested by roughening the surface with sand paper prior to CRREL ice adhesion to simulate the influence of wear. Figure 6 shows that although the worn R-2180 surface does not perform as remarkably as a freshly applied coating of R-2180, it still outperforms the Teflon. Furthermore when coated pins were exposed to extreme thermal conditions, humidity cycles, and sprayed with a salt-water solution prior to ice adhesion testing, R-2180 continues to perform better than Teflon (Figure 5).
Ice build up is a serious problem and major economic impact in the aircraft industry resulting in multiple efforts to understand and improve the problem. Tests performed by the Department of the Army, Engineer Research and Development Center, Cold Regions Research and Engineering Laboratory (CRREL) have shown that the R-2180 silicone coating significantly reduces ice adhesion when applied on aluminum surfaces compared to other commercially marketed ice-phobic coatings and other materials known to have non-stick properties. In particular, R-2180 performed almost 10 times better than many other commercially available materials typically used for aircraft purposes. In addition, R-2180 silicone coating continues to show favorable performance after wear and exposure to extreme environmental conditions such as heat, humidity, and salt water. It is believed that surface energy of the material or coating can have a dramatic effect on ice adhesion and perhaps a complex interaction between the ice and the silicone coating that reduces adhesion. Thus it would be valuable in future studies to measure the surface energies of the several materials tested to determine any correlation between the ice adhesion and surface energy of the coating.
(1) EM 1110-2-1612, Engineering and Design – Ice Engineering, U.S. Army Corps of Engineers, Department of the Army, October 20, 2002, UPDATE VERSION September2006.
(2) Mulherin, ND, RB Haehnel, JF Jones (1998) Toward developing a standard shear test for ice adhesion. Proceedings, 8th International Workshop on Atmospheric Icing Structures, Reykjavik, Iceland, June 8-11, 1998. IWAIS ’98.
(3) CNN online, December 11, 2006, Canada Bans Ethylene Glycol Monomethyl Ether.
(4) Laboratory Ice Adhesion test Results for Commercial Icephobic Coatings for Pratt & Whitney, May 2004, CRREL.
Using Optical Index Matching Silicone Gels to Improve Outdoor
Viewing and Ruggedness of Displays
Bill Riegler and Michelle Velderrain
NuSil Technology LLC
1050 Cindy Lane
Carpinteria, CA 93013
Portable computing, military environments, and demanding outdoor activities like fire rescue require a reliable and accurate display. Most commercial, off-the-shelf displays are not produced for direct viewing outdoors. The bright ambient light outdoors can cause high reflection losses and subsequently “display washout.” Companies can increase display brightness, however this typically results in added power consumption and poor aspect ratios.
An alternative and growing trend is use of optical index matching silicone gels to reduce reflection losses and also aid in durability. Commercial displays typically have the display protected by a coverglass with a small air gap between the display and the coverglass (see Figure 1). This gap leads to reflection loss as much as 8.5%, dependent on the difference of refractive indices between the surfaces. This reflection loss can be decreased by filling the gap with an optical index matching silicone gel, thus greatly increasing viewing outdoors in bright ambient light. These optical index matching silicone gels are especially designed for protection of sensitive photonics assemblies. The encapsulation materials help improve impact resistance and protect the assembly from dust and mechanical and thermal shock. Optical silicone gels are also transparent and their softness easily allows the LCD panel to be disassembled for rework. There are several materials to choose from depending on the refractive index matching: 1.52 to match BK7 glass or 1.46 to match silica glass. In addition, these materials bond well to glass and other substrates used in the display.
Most optical silicones cure at room temperature within 24 hours, or heat can be used to accelerate the cure. Curing at lower temperatures minimizes stress on the display by reducing the thermal expansion between materials with different Coefficients of Thermal Expansion (CTE). No UV exposure or temperature bake is required to initiate the cure process. These materials also remain fluid for at least eight hours. The long pot life and low viscosity enable the materials to wet out and fill in voids in complex assemblies and permit time for any trapped air bubbles to float to the surface and escape (see Table 1 for properties).
Epoxy and acrylate encapsulants have also found use for gap filling in displays. However, these materials’ rigid nature causes cracking when thermally cycled and they yellow when exposed to UV light from the sun.
These optical gels have specialized optical properties that allow them to provide the needed optical benefits for displays (see Figures 1-2).
Optical Silicones for use in Harsh Operating Environments
By Bill Riegler, Product Director-Engineering Materials, Stephen Bruner, Marketing Director, Rob Thomaier, Research Director,
NuSil Technology, Carpinteria, CA.
Optical Polymers, Gels, and Thermosets for Index Matching Applications
A poster presentation at Optics East, Philadelphia, PA, October 25-28, 2004
The optics industry widely uses silicones for various fiber optic cable potting applications and light emitting diode protection. Optics manufacturers know traditional silicone elastomers, gels, thixotropic gels, and fluids not only perform extremely well in high temperature applications, but also offer refractive index matching so that silicones can transmit light with admirable efficiency. However, because environmental conditions may affect a material’s performance over time, one must also consider the conditions the device operates in to ensure long-term reliability. External environments may include exposure to a combination of UV light and temperature, while other environments may expose devices to hydrocarbon based fuels. This paper will delve into the chemistry of silicones and functional groups that lend themselves to properties such as temperature, fuel, and radiation resistance to show why silicone is the material of choice for optic applications under normally harmful forms of exposure. Data will be presented to examine silicone’s performance in these environments.
The production of electronics and sensors used in harsh environments is expected to be an $887 million dollar market by 2008. Today’s electronics and sensors encounter a variety of harsh environments, including: extremes in temperature or pressure, chemical aggressiveness, acceleration/deceleration, radiation, cyclic operation and shock. Devices employed in such environments must endure these harsh conditions, enabling them to perform their functions over time. A brief list of current optical sensing technology and developing applications is listed below:
• Vision sensors for industrial applications that include the identification and measurement of production outputs, verify accurate assembly, and guide production equipment.
• Weather satellites employ the use of optical sensors technologies that continuously expose their position-sensors to sunlight or radiation. These sensors operate under extremely high temperatures in order to sense the satellite is in its proper position.
• The Department of Energy’s National Energy Technology Laboratory is developing novel sensors able to withstand high temperatures and harsh environments. These microsensors will be used in fuel cell, turbines, gasification, and combustion systems.
• NASA’s Marshall Space Flight Center utilizes the Advanced Video Guidance Sensor in conjunction with a computer program for the autonomous rendezvous of a spacecraft with a target satellite.
• NASA is investigating hybrid Peizoelectric/fiber optic sensors for aerospace, aeronautical and automotive applications.
The term “Silicone” is actually a misnomer. Normally the suffix ‘-one’ delineates a substance has a double bonded atom of oxygen in its backbone. Scientists initially believed that silicone materials contained double bonded oxygen, hence the use of ‘silicone.’ However, silicones are really inorganic polymers, having no carbon atoms in the backbone, and therefore should be named ‘Polysiloxanes.’ The diagram below shows their typical structure:
This polymeric structure allows polysiloxanes to be used in a wide array of applications because it allows different types of polysiloxanes groups to be incorporated. Different polysiloxanes can provide a variety of excellent properties that can be chosen according to the specific application, temperature stability (-115 to 260°C), fuel resistance, optical clarity (with refractive indexes as high as 1.60), low shrinkage (2-%), and low shear stress. Different types of silicones, or polysiloxanes, and their property advantages include: Dimethyl silicones, or dimethylpolysiloxanes, are the most common silicone polymers used industrially. These types of polymers are typically the most cost effective to produce and generally yield good physical properties in silicone elastomers and gels. The polymer pictured below contains vinyl endgroups that participate in a platinum catalyzed addition reaction (see section on Cure Chemistry for more information).
For optics purposes, all dimethylpolysiloxanes have a refractive index of 1.40, 25ºC at 598nm. Methyl phenyl silicone systems contain diphenyldimethylpolysiloxane co-polymers. The phenyl functionality boosts the refractive index of the polymers from 1.40 upwards to 1.60. There are limitations, the steric hinderance of the large phenyl groups prohibit significantly high concentrations of diphenyl units on the polymer chain. Silicone polymers with diphenyl functionality with refractive index of 1.43 to 1.46 are useful in bio-optic applications (e.g., intraocular lenses) in creating a thin lens. The diagram below shows a typical structure for a methyl phenyl silicone:
Fluorosilicones are based on trifluoropropyl methyl polysiloxane polymers and historically are used for applications that require fuel or hydrocarbon resistance. The trifluoropropyl group contributes a slight polarity to the polymer, resulting in swell resistance to gasoline and jet fuels. For optic applications, the refractive index is 1.38 at 25ºC at 598nm. While some fluorosilicones contain 100% trifluoropropylmethylpolysiloxane repeating units, other systems contain a combination of the fluorosiloxane units and dimethyl units to form a co-polymer. Adjusting the amount of trifluoropropyl methyl siloxane units in the polymerization phase provides optimal performance in specific applications. The diagram below shows a typical structure for a fluorosilicone copolymer:
Silicone materials, with the chemistries described above appear in a wide variety of material compositions. This broad range of material compositions makes silicone a Silicone Thixotropic Gels are comprised of an optical fluid immobilized in a nanoparticle powder. These gels have no curing characteristics and by nature they are thixotropic and do not have well-defined viscosities. At rest they are mechanically stable and will not migrate. Due to the index matching limitations of the nanoparticle powder, these materials are available at 1.46 to 1.59 refractive indices only. Their primary use is for improving the return loss in a single mode mechanical fiber splice. Silicone Curing Gels contain reactive silicone polymers and reactive silicone crosslinkers in a two-part system. When mixed together these materials are designed to have a very soft and compliant feel when cured and will stick to substrates without migrating. Viscosities can be adjusted with the molecular weight of the polymers from 200 – 10,000cP. Depending on the functionality of the polymer, optical index matching can be formulated from 1.38-1.57. For HBLED applications this allows for the optimal light to come out of the die while protecting it from dust, moisture, vibration and changes in temperature. The yield strength of the gel is low enough to permit wire bonds to slice through during thermally induced micromotion without risking wire bond failure. Steps need to be taken to manufacture these materials with minimal outgassing and low ionic species. Other applications besides encapsulating HBLEDs include potting of packaged modules such as transponders, transceivers and detector arrays. Silicone Thermosets fall into two categories, moldable elastomers and adhesives. Like the Gels, these two-part systems contain reactive polymers and crosslinkers that cure up to a rubbery type hardness. Most will cure at room temperature, however some need heat to cure. To impart increased physical properties, sometimes these materials have higher viscosities. The moldable materials can be casted or injection molded into optical lenses.
They have inherently stronger physical properties than the gels and can work as excellent adhesives in optical applications. Special versions of these can be produced to have extremely low outgassing for electronic and aerospace applications. These also can have the broad refractive index range of 1.38 – 1.57. Silicone resins, also called Polysilsesquioxanes, are highly crosslinked siloxane systems with the empirical formula: R – Si – O1.5 Both the Polysilsesquioxane and T-resin names can be derived from the empirical formula. The root “sesqui” indicates the one and a half stoichiometry of the oxygen bond to silicon. T-resin indicates the trisubstitution of silicon by oxygen. Silicone resins are also named by the organic, or “R,” group.
Sample Resin Structures:
These materials when cured can give very hard durometers, Shore D. The phenyl content can be adjusted providing refractive index from 1.40-1.57.
When a manufacturer in the optics industry chooses a material for a specific application, material properties aren’t the only deciding factor. That manufacturer also has to examine how the material is used. Inconvenience in production or material by-products can make a chosen material ineffective for a specific application. Silicones, however, can be designed around various cure chemistries to accommodate different production needs. Silicone systems can cure by platinum catalyzed addition cured systems, tin condensation cure systems, peroxide cure systems, or oxime cure systems. Some of the oldest cure chemistry used with silicones utilizes an acetoxy tin condensation cure system, such as used in household bathroom caulk. These systems yield a vinegar-like smell (acetic acid), a byproduct of the reaction. For various reasons as described below, platinum systems are the most appropriate for optics applications.
Tin condensation systems involve hydroxyl functional polymers and alkoxy- functional crosslinking compounds. The alkoxy functional crosslinker first undergoes a hydrolysis step and is left with a hydroxyl group. This hydroxyl group then participates in a condensation reaction with another hydroxyl group attached to the polymer. The reaction can proceed without the assistance of the tin catalyst, but the presence of the catalyst boosts the rate of reaction. The reaction mechanism is pictured below:
The main disadvantages of condensation systems are the leaving group, shrinkage and long cure time, as several days are often required for to completely cure an elastomer. Peroxide catalyzed systems, have a reaction mechanism that involves a peroxide catalyst and either methyl groups or vinyl functional groups. The peroxide catalysts create free radical species of the methyl and vinyl that can then form covalent bonds. Pictured below is the reaction mechanism involving a peroxide catalysis of two methyl groups:
Disadvantages include a lengthy post-curing step at high temperatures in order to remove the reaction’s byproducts. Other disadvantages include the possibility of the catalyst interacting with active agents.
Platinum catalyzed silicones utilize a platinum complex to participate in a reaction between a hydride functional siloxane polymer and a vinyl functional siloxane polymer. The result is an ethyl bridge between the two polymers. The reaction mechanism is pictured below:
Platinum systems are often cured quickly with heat, but can be formulated to cure at low temperatures or room temperature if necessary. The advantages of these systems include a faster cure and no volatile byproducts. The possibility of inhibiting the cure is the main disadvantage of platinum systems. Inhibition is defined as either temporarily or permanently preventing the system from curing. Some types of inhibitors are purposefully added to these systems to control the rate of cure. However, contact with tin, sulfur, and some amine containing compounds may permanently inhibit the cure. Compounds that inhibit the cure can be identified easily by attempting to cure a platinum catalyzed system in contact with the compound, as inhibition results in uncatalyzed regions of elastomer systems or inconsistency in cure over time.
Typical Testing For Optical Materials
Optical sensing devices operate most efficiently when photons can pass freely through the optics of the device. For this reason, components used in the assembly of the optics should have similar refractive indexes. The refractive index of silicone that can be effective in harsh operating environments can range from 1.38 to 1.60. We describe some of the testing methods to characterize silicones for optical applications.
Refractive Index, the measurement of the speed of light traveling through a transparent material. It is measured at 589 nanometer (nm) wavelength (a.k.a. “the Sodium D line”, or “nD”) with a refractometer using the method of the American Society for Testing and Materials’ ASTM D-1218 at a fixed temperature of 25.0ºC. As previously mentioned, silicones have a refractive index range of 1.38-1.60.
Refractive index versus change in temperature, is conducted at 25ºC to 50ºC in 5-degree steps. Data is reported as shown in the graph below, and the least squares linear regression fit to the data for the thermo-optic coefficient (in units of dn/dT) is also calculated and provided on the chart.
Refractive Index versus change in Wavelength, data are measured at 411nm, 589nm, 833nm, 1306nm and 1550nm at 25.0ºC and presented as shown in the graph below. Coefficients are also printed on the plot for a Sellmeier dispersion curve fit.
Optical Absorption versus change in Wavelength, is measured across the wavelength range of 300nm to 1700nm, with 2nm resolution, using a spectrophotometer with the sample temperature at 25ºC. The graph is presented below:
The general properties of silicones, by product type, are described briefly above. For our purposes, we will present and discuss data that relates specific properties of silicones that are useful in harsh operating environments. While the stability of the siloxane bond bodes well for silicone material performance in a diverse range of operating conditions, we have chosen to focus on three types of environments that can adversely affect materials in a sensor assembly; extreme temperatures, exposure to fuels, and radiation exposure.
Extreme temperatures can cause outgassing of volatile components with temperatures ranging from -115ºC to 260ºC. In closed electronic and opto-electronic systems, volatile components can contaminate sensitive components. Smaller electronic packages with higher voltage requirements can produce excessive heat, and any components containing volatile chemical species can outgas under these conditions. Electronic systems can contain a variety of components at various operating temperatures. Cooler components behave like a cold soda can on a warm humid day, as they can become a conduit onto which these volatile components re-condense.
Traditionally, applications on satellites and shuttles employ low outgassing materials, as these applications encounter extraterrestrial environments that cause outgassing. These environments typically undergo extreme temperature cycling in a vacuum. The ASTM test method E595 provides a standard for testing all silicone materials for extraterrestrial use. Total Mass Loss (TML) and Collected Volatile Condensable Material (CVCM) testing can be useful in screening low outgassing materials from other materials. NASA has set current limits of Total Mass Loss not to exceed 1.0% and Collected Volatile Condensable Materials not to exceed 0.1%. The materials listed below demonstrate the physical similarities between low outgassing silicone materials and other silicone materials:
Another factor to consider is the coefficient of thermal expansion or CTE in electronic packages. Silicone materials typically have large CTE values compared to filled and unfilled epoxies. This large CTE, combined with and related to low modulus values, creates an environment where low stress is imparted on mismatched CTE components. This low stress results in fewer component fractures over large temperature ranges. Radiation Applications that are exposed to radiation, such as weather sensors, can adversely affect the materials used. Silicone materials are affected as well by radiation but these effects can be minimized with adjustments to the type of polymer used. Radiation can create free radical groups on polymers within the elastomeric matrix. These free radicals readily form linkages to other polymers creating a higher crosslink density. A higher crosslink density in silicone systems generally translates to higher durometer or hardness, higher modulus and lower elongation. As briefly described above, higher modulus values can create stress in temperature cycling applications where CTE mismatches are observed. While the increase in modulus from radiation exposure may still be lower than epoxies, certain silicone chemistries can be effective in resistance to radiation. Several sources point to phenyl based silicones as having greater stability than standard polysiloxane systems. Walter Noll states, “resistance increases with increasing content of phenyl groups, copolymers with diphenylsiloxane units and methylphenylsiloxane units have proved particularly suitable”. In potting applications where elongation is not a significant factor, like electric wire and cable, electrical properties are “little affected by radiation” Handbook pg 16. The table belowdemonstrates the radiation resistance of phenyl containing silicone rubbers (PMQ) compared to dimethyl silicone rubbers (VMQ).
Radiation Resistance Silicone Rubber
When we refer to fuel resistance, we are specifically citing the resistance of silicones to swell when put in contact with hydrocarbon based fuels and fluids. While silicone materials do not undergo structural breakdown when exposed to hydrocarbon-based compounds, some systems expand several times their original size and could create problems in lens and other potting applications. Fluorosilicones, with the chemistry described above, perform effectively in a fuel and fluid environment by retaining geometry and resisting absorption of hydrocarbons. The table below lists one fuel (JP 5) and two hydraulic fluids (Hydrol and Skydrol) common to the aerospace industry. The table also lists the percentage of trifluoropropylmethylsiloxane units used in the polymer of the elastomer system tested. The results are listed as the percent swell after exposure:
The table demonstrates that different fluorosilicone compositions perform optimally in different systems and we point out that more trifluoropropylmethylsiloxane units are not always better.
Silicone elastomers, gels, thixotropic gels, and fluids not only perform extremely well in high temperature applications, but also offer refractive index matching so that silicones can transmit light with admirable efficiency. The examples of emerging and existing sensor technology provided the basis on which optics engineers must also consider the conditions the device operates in to ensure long-term reliability. External environments may include exposure to a combination of UV light and temperature, while other environments may expose devices to radiation and hydrocarbon based fuels. This paper demonstrated that the chemistry of silicones and functional groups that lend themselves to properties such as fuel and radiation resistance show why silicone is the material of choice for optic applications under normally harmful forms of exposure.
M. Lin, X. Qing, Hybrid Peizoelectric/Fiber Optic Sensor Sheets, Nasa Tech Briefs, July 2004: pg.28.
A. Whitehead, Novel Sensor Diagnoses Flight Optical Transceiver Problems, Photonics Tech Briefs, July 2004: pg. 11a.
J. Lombaers, J. Bullema, Micro Harsh Environment Actuators and Sensor Technology,http://www.microned.nl
W. Lynch, Handbook of Silicone Rubber Fabrication, Van Nostrand Reinhold Company, New York, 1978.
W. Noll, Chemistry and Technology of Silicones, Academic Press, New York, 1968
Technews, NETL Selects Sensor Research Projects, National Energy Technology Laboratory, http://www.netl.doe.gov
R. Jones, W. Ando, J. Chojnowski, Silicon-Containing Polymers, Kluwer Academic Publishers, Dordrecht, 2000.
S. Clarkson, J. Semlyen, Siloxane Polymers, PTR Prentice Hall, New Jersey, 1993
NEW LOW STRESS FILM ADHESIVE FOR AEROSPACE APPLICATIONS REQUIRING LOW OUTGASSING
Bill Riegler, Product Director-Engineering Materials,
Dee Anne Long, R&D Supervisor,
NuSil Technology LLC, Carpinteria, CA.
1050 Cindy Ln, Carpinteria, CA, US 93013
Presented at the 2006 SAMPE Conference, May 1-4, Long Beach, CA.