Patent Application
20080286473
The present invention relates generally to ice adhesion mitigation and, in particular, to the reduction of ice adhesion through the use of hydrophobic coatings containing siloxanes and fluoropolymer resins.
Ice adhesion is undesirable in a variety of situations. Ice formation on aircraft wings can detrimentally alter their aerodynamic properties. Ice formation on ships sailing in sub-freezing conditions can affect their seaworthiness. In fact, ice formation on a variety of industrial and exterior equipment, such as locks, valves, gauges, etc. can hinder their use or performance.
Ice adhesion is also a key concern in the NASA Space Shuttle program. The Space Shuttles utilize solid rocket boosters and an external fuel tank during launch. The external fuel tank delivers 535,000 gallons of liquid hydrogen and oxygen propellants to the three Space Shuttle Main Engines (SSME). The tank, the only component that cannot be reused, is covered by polyisocyanurate and polyurethane foam materials that insulate the propellants, in part, to help keep ice from forming on the tank's exterior. However, there are areas of the tank that form ice and the formed ice can be a source of debris and hazard of flight. The umbilical structures that currently feed LO2 and LH2 to the SSMEs often ice up wherein a nominal 50% portion or less of such ice build-up is actually liberated at Solid Rocket Booster (SRB) start up. Use of a coating formed in accordance with the present invention should significantly increase the amount of ice liberated prior to flight, directly improving flight safety.
For the reasons stated above, and for other reasons that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a current need in the art for successful approaches to mitigating ice adhesion.
The various embodiments employ a hydrophobic coating containing siloxanes and fluoropolymers to hinder the accumulation of moisture and to reduce the adhesion of any ice formed onto a coated surface or substrate. The invention includes methods and compositions of varying scope.
In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific preferred embodiments in which the present invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that process and chemical changes may be made without departing from the spirit and scope of the present invention. It is noted that the drawings are not to scale unless a scale is provided thereon. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof.
Umbilical structures which feed the Space Shuttle Main Engines LO2 and LH2 from the external fuel tank of NASA's Space Shuttle are known locations of frost and ice growth during the pre-launch period following fuel loading. At Solid Rocket Booster (SRB) start, a nominal 50% portion or less of the ice build up is released due to the vibro-acoustic energy created during the launch sequence. If most or all of the ice was released at SRB start then little or no ice would be available to adversely impact Shuttle tiles. New physics-based models indicate that ice debris from the umbilical structure is a serious flight hazard which may well be mitigated if ice release occurs before flight or very early in flight. Such early ice release can be achieved by appropriate application of the present invention's unique coating which can reduce ice adhesion by as much as 90%.
The various embodiments of the present invention employ a hydrophobic coating containing siloxanes and fluoropolymers to hinder the accumulation of moisture and to reduce the adhesion of any ice formed on the coating. By using a hydrophobic coating formed in accordance with the present invention, moisture can bead off the surface before it has an opportunity to freeze. And by reducing adhesion, any formation of ice ultimately may be dislodged with less force. One preferred embodiment of the present invention hereafter referred to as the Shuttle Ice Liberation Coating I (SILC-I) comprises a dilute solution of siloxanes in acidified alcohol and fluoropolymer powder. In another preferred embodiment of the present invention, hereinafter referred to as the Shuttle Ice Liberation Coating II (SILC-II) comprises the same major ingredients as SILC-I with the addition of an ultra-violet light absorbing chemical.
Embodiments of the invention were tested against uncoated surfaces using a double lap shear testing protocol (ASTM [2002] D3528-96). In this protocol, a single coupon was encased on two sides by ice 0.125 in. (0.317 cm) thick.
As the coatings were originally designed for use with the External Tank LO2 feed line brackets, double lap shear tests were performed at the hardware operating temperatures. Bracket temperatures obtained by NASA were used to design the temperature regime for testing of various embodiments of the present invention. In the various embodiments, compositions containing siloxanes and fluoropolymer resin particles in solvent were prepared. The source of the siloxanes for testing was RAIN-X Original Glass Treatment available from SOPUS Products, Houston, Tex. The composition of the RAIN-X solution, according to the MSDS, is as follows: ethanol/SD alcohol 40 86%; isopropanol 4%; ethyl sulfate 1%; polydimethylsiloxanes (silicone oil) <9%; silicic acid (H4SiO4), tetraethyl ester, hydrolysis products with chlorotrimethylsilane <9%; and siloxanes and silicones, di-Me, hydroxy-terminated <9%. More generally, such a solution can be thought of as essentially a dilute solution, i.e., approximately 5%, of poly alkyl hydrogen siloxanes in an acidified alcohol carrier for the purposes described herein. Composition percentages are in weight percent unless otherwise noted. The compositions of the various embodiments may contain additional chemical components that do not materially affect the basic and novel properties of the compositions disclosed herein. Some examples include dyes, stabilizers, preservatives, and antimicrobial agents.
Sources for the fluoropolymer resin particles may include polytetrafluoroethylene (PTFE) powders or other fluorinated polymer powders. UF-8TA is a functional ultra-fine PTFE powder with an average agglomerated particle size of 4.0 μm, average particle size of 0.3 μm, and component particle sizes as small as 200 nm (0.2 μm). The density of UF-8TA is 450 g/L. It is specially treated for compatibility and made for release applications where superior dispersion is needed. Improved properties over standard PTFE include extremely good release, chemical inertness to all industrial chemicals and solvents, wide range of service temperatures from −240 to 250° C. (−400 to 482° F.), low friction, and excellent non-stick properties. In comparison, MP-55 is a white, fine particle PTFE micro-powder with an average particle size of 4.0 μm, minimum particle size of 0.2 μm, and density of 300 g/L. It is used as an additive in several applications, including dry film lubricants and coatings. As used with various embodiments of the invention, the MP-55 PTFE powder produces performance levels significantly improved over similar compositions having UF-8TA PTFE powder. Both materials are available through Laurel Products, LLC, Elverson, Penn.
Testing of coated coupons of the type depicted in
Testing of compositions in accordance with embodiments of the present invention included several variations on formulations and coated substrates. The following examples show the dramatically reduced ice adhesion results for samples of various weight percent compositions tested once, then again a second and third time without reapplying the coating. The repeated ice tests demonstrate the durability of the coating systems. In creating and testing the various samples, the following procedural steps were followed for both SILC-I and SILC-II test samples:
KOROPON primed aluminum coupons with dimensions of 2.00 inches (5.08 cm)×1.0 inch (2.54 cm) were wiped clean with an isopropyl alcohol (IPA) solution prior to application of the coating. 5 grams of original RAIN-X was weighed into a mixing container. To the RAIN-X was added 5 grams of Laurel Products MP-55 PTFE powder. The contents of the container were rapidly mixed for approximately 30 seconds or until the powder was visibly wet out. Extra care was taken to ensure that the contents of the container were not over mixed. The mixture was then applied to the surface of the KOROPON primed aluminum coupon with a foam brush. A thin even coating was accomplished by first applying ample amount of the mixture to both sides of the foam brush and then with one side of the foam brush apply one stoke up, flip the brush to utilize the other side and apply another stroke up. This process is repeated for all surfaces to be coated. The resultant wet coating should be about 0.02 g/cm2. The preferable coating should appear even, thin, and smooth. The coating was allowed to cure for a minimum of 3 hours prior to handling and 24 hours prior to use. Cure time was determined by use of Fourier Transform Infrared Spectroscopy (FTIR), which shows no peak change after 24 hours at the 1220 cm−1 wavenumber. The KOROPON primed aluminum coated with the SILC coating was placed into a holder where ice was allowed to grow, see
KOROPON primed aluminum coupons with dimensions of 2.00 inches (5.08 cm)×1.0 inch (2.54 cm) were wiped clean with an isopropyl alcohol (IPA) solution prior to application of the coating. 4.3 grams of original RAIN-X was weighed into a mixing container. To the RAIN-X was added 5.7 grams of UF-8TA functional ultra-fine PTFE powder. The contents of the container were rapidly mixed for approximately 30 seconds or until the powder was visibly wet out. Extra care was taken to ensure that the contents of the container were not over mixed. The mixture was then applied to the surface of the KOROPON primed aluminum coupon with a foam brush. A thin even coating was accomplished by first applying ample amount of the mixture to both sides of the foam brush and then with one side of the foam brush apply one stoke up, flip the brush to utilize the other side and apply another stroke up. This process is repeated for all surfaces to be coated. The resultant wet coating should be about 0.02 g/cm2. The preferable coating should appear even, thin, and smooth. The coating was allowed to cure for a minimum of 3 hours prior to handling and 24 hours prior to use. Cure time was determined by use of Fourier Transform Infrared Spectroscopy (FTIR), which shows no peak change after 24 hours at the 1220 cm−1 wavenumber. The KOROPON primed aluminum coated with the SILC coating was placed into a holder where ice was allowed to grow, see
KOROPON primed aluminum coupons with dimensions of 2.00 inches (5.08 cm)×1.0 inch (2.54 cm) were wiped clean with an isopropyl alcohol (IPA) solution prior to application of the coating. 5.0 grams of original RAIN-X was weighed into a mixing container. To the RAIN-X was added 3.0 grams of Laurel Products MP-55 PTFE powder and 2.0 grams of UF-8TA functional ultra-fine PTFE powder. The contents of the container were rapidly mixed for approximately 30 seconds or until the powder was visibly wet out. Extra care was taken to ensure that the contents of the container were not over mixed. The mixture was then applied to the surface of the KOROPON primed aluminum coupon with a foam brush. A thin even coating was accomplished by first applying ample amount of the mixture to both sides of the foam brush and then with one side of the foam brush apply one stoke up, flip the brush to utilize the other side and apply another stroke up. This process is repeated for all surfaces to be coated. The resultant wet coating should be about 0.02 g/cm2. The preferable coating should appear even, thin, and smooth. The coating was allowed to cure for a minimum of 3 hours prior to handling and 24 hours prior to use. Cure time was determined by use of Fourier Transform Infrared Spectroscopy (FTIR), which shows no peak change after 24 hours at the 1220 cm−1 wavenumber. The KOROPON primed aluminum coated with the SILC coating was placed into a holder where ice was allowed to grow, see
KOROPON primed aluminum coupons with dimensions of 2.00 inches (5.08 cm)×1.0 inch (2.54 cm) were wiped clean with an isopropyl alcohol (IPA) solution and dip coated with organosilicate-organotitanate primer, DC-1200 and allowed to dry prior to application of the coating. 20.80 grams of original RAIN-X was weighed into a mixing container. To the RAIN-X was added 5.1 grams of a polymethyl hydrosiloxane fluid, DF 1040, which was mixed with a spatula for 100 strokes. To the mixture was added 24.27 grams of Laurel Products MP-55 PTFE powder. The contents of the container were mixed for 300 strokes then transferred to a mortar and mixed with a pestle for 300 additional strokes. The mixture was then transferred to a seal container prior to application to the KOROPON/DC-1200 primed surface. The mixture was then applied to the surface of the KOROPON/DC-1200 primed aluminum coupon with a foam brush. A thin even coating was accomplished by first applying ample amount of the mixture to both sides of the foam brush and then with one side of the foam brush apply one stoke up, flip the brush to utilize the other side and apply another stroke up. This process is repeated for all surfaces to be coated. The resultant wet coating should be about 0.02 g/cm2. The preferable coating should appear even, thin, and smooth. The coating was allowed to cure for a minimum of 3 hours prior to handling and 24 hours prior to use. Cure time was determined by use of Fourier Transform Infrared Spectroscopy (FTIR), which shows no peak change after 24 hours at the 1220 cm−1 wave number. The KOROPON / DC-1200 primed aluminum coated with the SILC+DF1040 coating was placed into a holder where ice was allowed to grow, see
KOROPON primed aluminum coupons with dimensions of 2.00 inches (5.08 cm)×1.0 inch (2.54 cm) were wiped clean with an isopropyl alcohol (IPA) solution prior to application of the coating. 5.5 grams of original RAIN-X was weighed into a mixing container. To the RAIN-X was added 3.7 grams of Laurel Products MP-55 PTFE powder. The contents of the container were rapidly mixed for approximately 30 seconds or until the powder was visibly wet out. To the mixture was added 0.8 grams of Tinuvin 400, UV absorber, and mixture thoroughly. Extra care was taken to ensure that the contents of the container were not over mixed. The mixture was then applied to the surface of the KOROPON primed aluminum coupon with a foam brush. A thin even coating was accomplished by first applying ample amount of the mixture to both sides of the foam brush and then with one side of the foam brush apply one stoke up, flip the brush to utilize the other side and apply another stroke up. This process is repeated for all surfaces to be coated. The resultant wet coating should be about 0.02 g/cm2. The preferable coating should appear even, thin, and smooth. The coating was allowed to cure for a minimum of 3 hours prior to handling and 24 hours prior to use. Cure time was determined by use of Fourier Transform Infrared Spectroscopy (FTIR), which shows no peak change after 24 hours at the 1220 cm−1 wavenumber. The KOROPON primed aluminum coated with the SILC coating was placed into a holder where ice was allowed to grow, see
KOROPON primed aluminum coupons with dimensions of 2.00 inches (5.08 cm)×1.0 inch (2.54 cm) were wiped clean with an isopropyl alcohol (IPA) solution and then coated with a thin layer of FIRE-X paint and allowed to dry prior to application of the coating. 6.0 grams of original RAIN-X was weighed into a mixing container. To the RAIN-X was added 4.0 grams of Laurel Products MP-55 PTFE powder. The contents of the container were rapidly mixed for approximately 30 seconds or until the powder was visibly wet out. Extra care was taken to ensure that the contents of the container were not over mixed. The mixture was then applied to the surface of the KOROPON/FIRE-X primed aluminum coupon with a foam brush. A thin even coating was accomplished by first applying ample amount of the mixture to both sides of the foam brush and then with one side of the foam brush apply one stoke up, flip the brush to utilize the other side and apply another stroke up. This process is repeated for all surfaces to be coated. The resultant wet coating should be about 0.02 g/cm2. The preferable coating should appear even, thin, and smooth. The coating was allowed to cure for a minimum of 3 hours prior to handling and 24 hours prior to use. Cure time was determined by use of Fourier Transform Infrared Spectroscopy (FTIR), which shows no peak change after 24 hours at the 1220 cm−1 wavenumber. The KOROPON/FIRE-X primed aluminum coated with the SILC coating was placed into a holder where ice was allowed to grow, see
Various foams were machined for testing with the ASTM D1623 ‘Type A’ double-conical specimen 20 as shown in
X-ray photoelectron spectroscopy (XPS) also called electron spectroscopy for chemical analysis (ESCA) is a chemical surface analysis method. XPS measures the chemical composition of the outermost 100 Å of a sample. Measurements can be made at greater depths by ion sputter etching to remove surface layers. All elements except for H and He can be detected at concentrations above 0.05 to 1.0 atom %, depending on the element. In addition, chemical bonding information can be determined from detailed analysis. XPS measurement of fluorocarbon, a principal component of compositions of the various embodiments, remaining on the surface of SILC-I coupons following three cycles of double lap shear testing were compared to similar measurements of a coated, but untested, SILC-I coupon. The cycled coupon surfaces contained less MP-55 than that of the untested coupon, indicating a loss of some of the coating. However, a significant amount of coating remained on the surfaces, and the ice adhesion performance had not been compromised. Consistent with the observed loss of coating at each test cycle, these results suggest that the failure plane between the tested coupons and the ice occurred within the coating. Areas where coating was not visible by eye, XPS measured significant amounts of SILC-I and SILC-II to be present.
Ten droplets of deionized water were placed along the length of each of these same coupons for contact angle analysis. Mean angle±standard deviation obtained for a KOROPON-surface control coupon was 81°±3.3°, increasing to 104°±1.8° for a KOROPON-surface control coupon with RAIN-X coating. In contrast, the tested RM samples (Table 9) were 137°±6.6°, 142°±8.2°, and 137°±11.9°, respectively, and the untested RM sample was 143°±10.9°. It is noted that the contact angle of PTFE is approximately 112°, thus leading to the unexpected result that the contact angle of compositions in accordance with embodiments of the invention combining siloxanes and fluoropolymer resin exceeds the contact angles of each constituent of the composition.
Table 9 herebelow provides contact angle data of various icephobic formulations showing surface energy interactions with water. Coating formulations are made by weight percent addition of constituent materials as described in listed examples (i.e. RAIN-X/MP-55/Tinuvin 400 55-37-8 is 55% by weight RAIN-X, 37% by weight MP-55 PTFE powder, and 8% by weight Tinuvin 400 UV absorber). The data shows the resulting coatings to be super-hydrophobic and durable after weathering in the Kennedy Space Center launch pad environment.
Table 10 herebelow provides contact angle data of various icephobic formulations showing surface energy interactions with water. Spray-On Foam Insulation (SOFI) used for the coupons and on the External Tank are closed cell polyurethane and polyisocyanurate foam materials. Coating formulations are made by weight percent addition of constituent materials as described in listed examples (i.e. RAIN-X/MP-55/Tinuvin 400 55-37-8 is 55% by weight RAIN-X, 37% by weight MP-55 PTFE powder, and 8% by weight Tinuvin 400 UV absorber).
Table 11 herebelow provides contact angle data of various icephobic formulations showing surface energy interactions with water. Coating formulations are made by weight percent addition of constituent materials as described in listed examples (i.e. RAIN-X/MP-55/Tinuvin 400 55-37-8 is 55% by weight RAIN-X, 37% by weight MP-55 PTFE powder, and 8% by weight Tinuvin 400 UV absorber). Samples 116 and 117 were made in the same manner as that described for Example 5 with the exception that RAIN-X and MP-55 were allowed to sit in a closed container for 2 hours before the addition of Tinuvin 400 and the substrate was SOFI not Koropon primer. The data show the resulting coatings to be super-hydrophobic and durable after weathering in the Kennedy Space Center launch pad environment.
Contact angle data concur with the XPS results indicating that the applied hydrophobic coating remains present on the tested coupon surfaces, and indicate that the hydrophobic performance of the untested surface is largely preserved after three test cycles.
Scanning electron microscopy (SEM) coupled with an energy dispersive spectroscopy (EDS) elemental map of fluorine was used to visualize conditions on coupon surfaces coated with a composition in accordance with an embodiment of the invention. A test coupon was thinly coated and left to cure for more than 2 days prior to analysis, abrasive wiping, and re-analysis. SEM analysis indicated that abrasive wipe did not remove all PTFE from the surface of the sample. The diameters of the MP-55 beads, measured by the SEM, fell below the wavelength of visible light. This result infers that, even when it may appear that the coating has been removed, it actually remains present. These SEM results verifying the presence of MP-55 on the coupon surface following abrasion support those of the XPS and contact angle analyses of tested coupons.
Although the aforementioned testing utilized a gloved finger to apply the compositions to the test surfaces, other application techniques are suitable. For example, the compositions may be sprayed, brushed, or rolled onto a surface. Commonly available foam paint brushes, i.e., a tapered foam pad on a wooden handle, were found to produce a thick and uniform coating with some streaking. Note that streaking is not a concern to the functionality of the compositions.
The compositions in accordance with embodiments of the invention can range from pastes to fluids suitable for use in common industrial spray equipment, depending upon the level of solids. For example, a composition containing approximately 40% PTFE would generally be in a paste form while a composition containing approximately 20% PTFE would be sufficiently liquid as to be suitable for spraying. Various embodiments can range from approximately 5% fluoropolymer resin to approximately 60% fluoropolymer resin. Such embodiments would contain from approximately 9% siloxanes to approximately 4% siloxanes, respectively, with the remainder being essentially a carrier solvent. As an example, one embodiment may be formulated using approximately 95% RAIN-X solution and 5% MP-55 PTFE powder, another embodiment may be formulated using approximately 80% RAIN-X solution and 20% MP-55 PTFE powder, another embodiment may be formulated using approximately 60% RAIN-X solution and 40% MP-55 PTFE powder, and another embodiment may be formulated using approximately 40% RAIN-X solution and 60% MP-55 PTFE powder. Each of the preceding examples may further include a UV light absorber.
The fluoropolymer resin should include fine particles. The various embodiments may include particles of less than 1 mm, preferably having an average particle size less than about 200 μm, more preferably having an average particle size of approximately 4 μm. For at least some embodiments, a minimum particle size of the fluoropolymer resin may be 200 nm or less.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.
The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
