Low Outgas Silicone Pressure Sensitive Adhesive for Aerospace Applications II By Bill Riegler, Product Director-Engineering Materials, Joan Meyer, R&D Supervisor, NuSil Technology LLC, Carpinteria, CA Presented at the 2005 Fall SAMPE Conference, October 31-November 3, Seattle, WA..
ABSTRACT The aerospace industry, primarily satellite manufacturers, have expressed the need for a low outgas, thermally stable, adhesive tape. The current products in the marketplace have limitations at both high, 175ºC, and low, -100ºC, temperatures. A new silicone Pressure Sensitive Adhesive (PSA) was developed to pass ASTM E-595(1), low outgassing requirements of 1% or less Total Mass Loss (TML) and 0.1% or less Collectable Volatile Condensable Materials (CVCM). This PSA was then fabricated into a tape and measured for performance and compared to other non-low outgassing silicone PSAs. The basics were outlined in a presentation presented at the SAMPE technical conference in Long Beach, CA, May 2004(2). This paper will expand on this technology by providing testing data at both high and low temperatures, comparative testing under different conditions with industry standard acrylate tapes, and more details on the fabrication of the tape. KEY WORDS: Silicone, PSA, ASTM E-595, low outgas, Aerospace 1. INTRODUCTION Let us first review many of the factors that brought us to developing this low outgas silicone pressure sensitive adhesive that were brought up in the paper presented at SAMPE 2004 in Long Beach, CA. The National Aeronautics & Space Administration (NASA) recommends all adhesives used in extraterrestrial environments be tested by ASTM E-595. This test method is primarily a screening technique and very useful in identifying materials with relatively low potential for contamination, verifying material quality, and aiding in material selection and qualification for the space, electronics, cleanroom, or other high vacuum applications such as leak detectors and particle accelerators. The criteria used for acceptance or rejection of material is determined by the user and based upon specific component and system requirements. Historically a maximum Total Mass Loss (TML) of 1.00% and a maximum Collected Volatile Condensable Materials (CVCM) of 0.10% have been used as screening levels for rejection of materials. Pressure Sensitive Adhesives (PSA) are a whole category of adhesives defined as materials that adhere to a substrate temporarily when given some pressure and can be removed when needed without damaging the original substrate. They do not require activation by heat or water. A band-aid is a basic example of a pressure sensitive adhesive. All tapes are made from a PSA applied and cured on backing film. The chemistry used for the adhesive varies depending on the environment and application. Silicone PSA s incorporate a high molecular weight polydimethylsiloxane polymer and a tackifying silicone resin dispersed in a solvent system. As with any silicone adhesive, silicone PSA s are most appropriate when used in extreme temperatures ranging from -115 to 260 C. A very unique and useful satellite repair product can be produced by combining the low outgas properties needed for a material used in space with the PSA technology incorporated into tapes. Hence the development of an experimental low outgas silicone PSA that can be fabricated into a tape, and the various testing performed.
2. SILICONE PSA CHEMISTRY The diagram below shows a typical polydimethyl siloxane, silicone structure: R=CH 3, phenyl, Fl 3 CCH 2 CH 2 Polysiloxanes offer excellent elastomeric properties, a wide range of temperature stability (-115 to 260 C when phenyl substituted), fuel resistance (when Trifluoropropyl substituted), optical clarity (with refractive indexes as high as 1.60), low shrinkage (2%), and low shear stress. Silicones are used in a wide array of applications due to these property advantages(3). Silicone PSA s incorporate a polydimethylsiloxane polymer with a high molecular weight and a tackifying silicone resin dispersed in a solvent system(4). The main goals for resin manufacture are finding the ideal molecular weight and resin functionality. Resins with high molecular weights and increased functionality give a PSA very good cohesive properties, but may compromise the adhesive properties. On the other hand, resins with low molecular weight and decreased functionality give better adhesive properties but compromise cohesive properties. Adjusting the amounts and types of starting materials for the resin can alter these properties. Adding resin provides several benefits to a silicone polymer: (1.) Covalent bonding through the hydrosilation, condensation or peroxidic crosslinking of functional groups on the resin with functional groups on the polymer. (2.) Hydrogen bonding of the silicone polymer with silanol groups on the surface of the resin. (3.) Chain entanglement of the resin and the polymer. The low intrinsic viscosity of the resin can also provide viscosity control by decreasing the polymer s viscosity. Finally, the resin provides optical clarity. Polymer and resin refractive indexes can be matched to result in an optically clear material. Silicone PSA s will typically crosslink further by curing after solvent removal. Two systems are currently available: platinum catalyzed and peroxide catalyzed. The platinum catalyzed system follows the same reaction as shown in Figure 1 and the curing is typically done in a single oven at 100 to150 C.
CROSSLINKER CH 3 SiO H + CH 2 CH SiO CH 3 POLYMER Pt CH 3 SiO CH 2 CH 2 SiO CH 3 CROSSLINKED SILICONE N ETWORK FIGURE 1. Platinum cure reaction. The peroxide cure system is more common and employs benzoyl peroxide, or 2,4- dichlorobenzoyl peroxide, as a catalyst to drive a free-radical reaction and achieve cure. The curing is normally done in a multi-zoned oven. Solvent removal is achieved through a gradual increase in temperature, starting at 60 to 90 C to ensure that the peroxide catalyst does not cure solvent into the PSA. The temperature is then increased to 130 to 200 C, eliminating the peroxide through decomposition. A high crosslink density PSA can be better achieved through peroxide curing due to the ability to increase peroxide levels up to 4%. However, as discussed in the introduction, an increase in cohesive strength will lower tack performance. These trade-offs are adjusted based on the application. 3. TAPE FABRICATION Fabricators of tapes and adhesive backed components, take the liquid PSA and either wet coat in sheet form, for small applications or in roll form (pilot coaters and full width production coaters) when large quantities are required. The PSA adhesive may be applied on one or both sides of a substrate such as Kapton, Mylar, Nomex, foils, foams, and rubbers or it can be coated directly onto a release film (See Figure 2). Coat weights on supported film range from 0.0003 inch to over 0.010 inch thick. When the adhesive is coated directly onto a release film, this is called an unsupported PSA transfer film. Common post production processes include: die cutting, laser cutting, component assembly and automated pick and place solutions for difficult to apply parts and materials. Figure 2. The layers in a psa tape.
4. PSA APPLICATIONS There are many benefits to using a PSA instead of liquid adhesives. Possibly the most important being the cleanliness of using PSA s. Liquid adhesives are messy and difficult to apply exactly where needed. Therefore they usually require cleanup afterwards. Also, using a PSA tape requires no cure time and can be dye cut to match any unique configuration. Another big benefit is the consistent bond line offered by the PSA. Obtaining a consistent bond line with a liquid adhesive can pose a challenge, especially when adhering a large surface area. 5. LOW OUTGAS TESTING As mentioned in the Abstract, ASTM E-595 is used to verify all silicone adhesives for extraterrestrial use. The test involves each material sample undergoing preconditioning, conducted at 50% relative humidity and ambient atmosphere for twentyfour hours. The sample is weighed and loaded into a compartment (see Figure 3) within a test stand (Figure 4). The sample is then heated to 125 C at less than 5 x 10-5 torr for 24 hours. Any volatile components of the sample outgas in these conditions. The volatiles escape through an exit port, and if condensable at 25 C, condense on a collector plate maintained at that temperature. The samples are post-conditioned in 50% relative humidity and ambient atmosphere for a twenty-four hour minimum. The collector plate and samples are then weighed again to determine the percentage of weight change, determining TML% and CVCM%. Standard criteria for low outgas materials limit materials Total Mass Loss (TML) to 1.0% and Collected Volatile Condensable Material (CVCM) to 0.10%. To adhere to these requirements, NuSil Technology performs this as a standard, lot-to-lot test.
Figure 3. Low outgas test chamber. Figure 4. Low outgas test stand.
6. PSA PROPERTY TESTING Based on test methods used by the Pressure Sensitive Tape Council (PSTC), made up of 29 North American PSA tape manufactures, to control and monitor the PSA market and its products, three test were used to characterize this PSA(5). The first is T-peel adhesion, ASTM D 1876, this measures the bond strength of the PSA to a substrate. We chose Aluminum as the substrate to adhere the PSA to, a common substrate used in the spacecraft industry. The panel was lightly sanded and cleaned prior to adhereing the Kapton backed PSA cut to 1 inch x 6 inch. An Instron tester was used to pull the PSA tape away from the aluminum panel at a 90 degree angle. Measurement is reported in units of pounds per inch (see Figure 5). PSA tape Aluminum Figure 5. Schematic of T-peel test Force applied The second is tack. Every PSA has a different level of tackiness and this can be measured and compared by performing a Blunt Probe tack-test, reference ASTM D 2979(6). A 0.020 inch thick sample of the PSA tape is set horizontal onto a fixture. A metal probe the diameter and length of a pencil is pressed into the PSA using a load cell set at 1lb of force and held for 1 second, the probe is then pulled upwards out of the PSA at a speed of 10 inches/minute (See Figure 6). The drag force on the probe is measured which characterizes the tackiness of the PSA. Probe Force applied PSA Figure 6. Schematic of Blunt Probe Tack test. The third test is Static Shear, PSTC 107A(5). Shear is defined as the strength of the PSA itself, or the material s strength. This test demonstrates the ability of the adhesive to resist slippage at extreme temperature while under load. At also shows that the adhesive is resistant to flow or migration from the bondline to other surfaces. Two standard aluminum panels, 4 inch x 1inch x 0.6 inch are overlapped to attach the PSA cut
in ½ inch x ½ inch wide strips. The panels are clamped vertically and a 50gram weight is attached to the hanging end (See Figure 7). We exposed the set-up to 175ºC for 24 hours and recorded whether the PSA was able to hold the weight. We also tested the set-up at reduced temperature, -105ºC. Base Panel PSA Panel Weight Figure 7, Schematic for Shear test. 7.TESTING RESULTS Upon following the test methods provided in Section 6, Table 1 summarizes the testing results. From these results we find that this tape holds weight at low temperatures, -105ºC, as well as high temperature, 175ºC. This is a critical parameter for this tape being used in space applications. Tabel 1, Testing results of CV4-1161-5* 25ºC -105ºC 175ºC T-Peel (ppi) 2.2 3.5 Tack Test (lbs) 0.7 Shear Test pass pass pass Pass/fail Other Testing Outgassing TML CVCM 0.10 % 0.017 % Tg -122ºC 8. COMPARATIVE ANALYSIS By comparison to an industry standard acrylate tape, the results in Table 2, show that the acrylate material does not hold weight at low temperature, and although at high temperature it maintains weight, if any slight flex were put on the material it would
shatter. These temperature tests demonstrate the ineffectiveness of this tape under extreme temperature requirements. Because this test was the most critical, any additional comparison tests were unnecessary. Table 2, Testing results of a Low Outgas Acrylate PSA 25ºC -105ºC 175ºC T-Peel (ppi) 1.75(8) 0.03(8) Tack test (lbs) Shear Test pass fail pass Pass/fail Other Testing Tg -41ºC(8) Material held on to panels however was extremely brittle and would shatter with the slightest flex. 9. CONCLUSION The development of a low outgas silicone PSA, combining the low outgas properties needed from a material used in space with the PSA technology incorporated into tapes, was concluded and characterized. The low outgassing requirements of 1% or less Total Mass Loss (TML) and 0.1% or less Collectable Volatile Condensable Materials (CVCM) were acheived. The performance goals were also successful, to maintain thermal stability at extreme temperatures, 175ºC to 105ºC. All of these goals were met and when compared with industry standard acrylate tape, proved superior under extreme environments often found in space. The aerospace industry now has a thermally reliable, low outgassed PSA tape that can be used in a variety of applications common to satellite manufacturing. The material comes in a number of forms and thicknesses. Future work could be performed to incorporate electrically and /or thermally conductive fillers into the PSA to develop further unique tapes. *CV4-1161-5 is 0.002inch Kapton with both sides coated with CV2-1161 at a thickness of 0.0015. Total thickness of the tape is 0.005. 3M 966 is 0.002 inch Acrylate adhesive between 2 liners which were removed (8).
10. REFERENCES (1) ASTM E-595 (2) B.Riegler, J.Meyer, Low Outgas Silicone Pressure Sensitive Adhesive for Aerospace Applications, 36 th International SAMPE Technical Conference, May 2004. (3) B. Riegler, R. Thomaier, H. Sarria, Accelerating Cure of Silicone Adhesives, 34 th International SAMPE Technical Conference, November 2002. (4) Chemistry and Technology of Silicones by Walter Noll (Academic Press Inc. Copyright 1968) (5) D. Varanese, The Fundamentals of Selecting Pressure-Sensitive Adhesives, Medical Plastics and Biomaterials, January 1998. (6) ASTM 1876 (7) ASTM 2979 (8) 3M Data Sheet 70-0709-3907-2
Bill Riegler is the Product Director-Engineering Materials for NuSil Technology LLC, the eighth largest silicone manufacturer in the world. Bill has been in the silicone industry for twenty years with various positions at NuSil and the silicone division of Union Carbide, which has become the OSi Specialties Group of GE Silicones. 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. Bill is now directing NuSil s worldwide efforts into the Aerospace, Photonics, Electronics and Automotive Industries. Joan L. Meyer has been a chemist in the research and development department at NuSil Technology LLC for six years. Her current work focuses on synthesis of novel polymers and resins for use in the medical and aerospace fields. She attended the University of California at Santa Barbara receiving her B.S. in chemistry and was involved in materials research with both the Chemistry and Chemical Engineering departments.