Patent Application
20200115593
The present disclosure relates generally to pressure sensitive adhesives (PSAs), and more particularly to PSAs used for bonding an active element panel to an aircraft structure.
PSAs are used to adhere a wide variety of aircraft components, including but not limited to deicers, anti-icers, and heated floor panels. PSAs used for deicing and anti-icing applications must have a high shear strength to maintain durability during actuation (i.e., inflation of a pneumatic deicer) and across a service temperature range of −40° F. to 160° F. (−40° C. to 71° C.). Current PSAs used to attach deicers to aircraft surfaces cannot be removed without the use of solvents, scrapers, and abrasives. A need exists for a PSA that can behave as a high shear PSA when in service and as removable PSA at the end of the lifetime of the component or if repositioning is needed during installation.
In one aspect, a device comprises an active element panel and a pressure sensitive adhesive layer bonded to the active element panel and defining a bonding surface bondable to an aircraft structure. The pressure sensitive adhesive layer comprises a mixture of a pressure sensitive adhesive and a solid solvent.
In another aspect, a method for removing an active element panel on an aircraft structure includes applying heat to a pressure sensitive adhesive layer of the active element panel and separating the pressure sensitive adhesive layer from the aircraft structure. The pressure sensitive adhesive layer includes a mixture of a pressure sensitive adhesive and a solid solvent.
The present summary is provided only by way of example, and not limitation. Other aspects of the present disclosure will be appreciated in view of the entirety of the present disclosure, including the entire text, claims and accompanying figures.
While the above-identified figures set forth embodiments of the present invention, other embodiments are also contemplated, as noted in the discussion. In all cases, this disclosure presents the invention by way of representation and not limitation. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the invention. The figures may not be drawn to scale, and applications and embodiments of the present invention may include features, steps and/or components not specifically shown in the drawings.
A solid solvent can be incorporated into a high shear pressure sensitive adhesive (PSA) to produce a thermally switchable PSA, which exhibits properties of a high shear PSA below a melting temperature of the solid solvent and exhibits properties of a removable PSA above the melting temperature of the solid solvent. Heat can be applied to the thermally switchable PSA to allow for removal and repositioning during installation without destroying the PSA and without leaving a residue on an aircraft surface to which the PSA was bonded. As used herein, “pressure sensitive adhesive” refers to an adhesive that bonds to a surface upon application of pressure without the use of solvents to apply or activate the adhesive, and includes one or two-sided adhesive transfer tape. The term “thermally switchable” refers to the change in rheological behavior of the PSA upon application or removal of heat. Various examples of thermally switchable PSAs are provided herein. The use of thermally switchable PSAs can facilitate the installation and removal of various aircraft components, which are adhesively bonded to a substrate. The thermally switchable PSAs of the present disclosure are particularly suited to the installation of aircraft components requiring high shear strength across a wide range of service temperatures (e.g., −40° F. to 160° F. (−40° C. to 71° C.)), such as anti-icers and deicers used to inhibit atmospheric ice accumulation on aircraft structures. Examples of deicers suitable for use with the thermally switchable PSAs disclosed herein include, but are not limited to, elastomeric pneumatic deicers, elastomeric electrothermal deicers, and electromechanical deicers, as known in the art and described in U.S. Pat. Nos. 6,250,587 and 6,352,601. Those skilled in the art will also appreciate that the thermally switchable PSAs of the present disclosure can also replace conventional PSAs used in other applications that do not necessarily require high shear strength, but may benefit from switchable high shear strength and removable rheological behavior. Such applications include, but are not limited to, heated floor panels for aircraft cabin temperature control.
In some embodiments, active element panel 14 can be an anti-icer or a deicer, such as an elastomeric pneumatic deicer, elastomeric electrothermal deicer, and electromechanical deicer, as known in the art. In alternative embodiments active element panel 14 can be a heated floor panel for use in an aircraft cabin. Active element panel 14 can comprise a variety of materials depending on the application. Those skilled in the art will appreciate that active element panel 14 is not limited to the components disclosed herein and that PSA layer 16, while particularly suited to applications requiring high shear strength, is not limited to such applications, nor limited to use on an aircraft. In some embodiments, active element panel 14 can be flexible and capable of conforming to the shape of aircraft structure 12 with reasonable force and without damage to active element panel 14. Aircraft structure 12 can be constructed from a metal, such as aluminum, or a composite material as known in the art.
Active element panel 14 can be stored for use with PSA layer 16 installed. Bonding surface 22 of PSA layer 16 can be provided with a release layer (not shown), which can be removed immediately before adhering bonding surface 22 to aircraft structure 12. In a non-limiting example of device installation, aircraft structure 12 can be prepared for bonding by first scrubbing the surface clean with a cloth soaked in a solvent, such as toluene or MEK, and then wiping the surface with another solvent, such as ethyl alcohol. Primer layer 20 can be optionally utilized to provide an enhanced adhesive bond between PSA layer 16 and aircraft structure 12. A non-limiting example of a suitable primer is Tape Primer 94 available from 3M™. Primer 20 provides an interface between PSA layer 16 and aircraft structure 12 and can increase an adhesion surface area by filling in areas where the two surfaces would not otherwise come into contact due to surface abnormalities or discontinuities. Primer 20 may or may not be necessary depending on surface energy of aircraft structure 12 and viscoelastic properties and thickness of PSA layer 16. If primer 20 is used, it can be applied to aircraft structure 12 and allowed to dry before device installation. Device 10 can be bonded to aircraft structure 12 by first pressing a selected area of bonding surface 22 (PSA) against aircraft structure 12 and subsequently pressing bonding surface 22 against aircraft structure 12 with increasing distance from the selected area. In such manner, boding surface 22 is bonded to aircraft structure 12 merely by applying pressure. A rubber roller can be used to press device 10 against aircraft structure 12.
Thermally switchable PSA layer 16 can comprise an acrylic, rubber, or silicone PSA material combined with a solid solvent. The type of PSA material can be selected based on the application and/or material to which the PSA is bonding (e.g., acrylic PSAs generally exhibit better adhesion to metals than silicone PSAs). The addition of the solid solvent to the PSA material allows device 10 to be removed or repositioned with the application of heat. Heat can be provided by heat gun or other energy source capable of providing sufficient thermal energy to PSA layer 16. As referred to herein, “solid solvent” refers to a low molecular weight additive that is a viscosity-reducing solvent above the melting temperature of the solid solvent and a solid reinforcement below the melting temperature. The presence of the solid solvent thereby changes the rheology of PSA layer 16 with the addition of heat, dropping high and low strain rate storage and loss moduli to the extent that the high shear rheology of PSA layer 16 below the solid solvent melting temperature transitions to become a removable PSA above the melting temperature. Solubility of the solid solvent can be chosen relative to the PSA material (e.g., acrylic, rubber, or silicone) such that as PSA layer 16 cools from the solid solvent melting temperature, the solid solvent phase separates and becomes a reinforcing solid particle. In this state, the solid solvent increases a high strain rate storage and loss moduli and peel strength above that of the PSA material alone and without affecting low strain rate storage and loss moduli. The two states of PSA layer 16 can be illustrated using viscoelastic windows on a Chang plot, as shown in
Quadrant 1 is defined to be above a horizontal line at a G′ value of 30,000 Pa and to the left of a vertical line at a G″ value of 32,000 Pa. PSAs in quadrant 1 have a high storage modulus G′ and a low loss modulus G″. If the viscoelastic window is within quadrant 1, the material is rubbery, has a high storage modulus and does not dissipate energy. The overall behavior is that of a non-PSA or release film (i.e. the material will be unlikely to stick to a surface and will debond easily).
Quadrant 2 is defined to be above a horizontal line at a G′ value of 30,000 Pa and to the right of a vertical line at a G″ value of 32,000 Pa. PSAs in quadrant 2 have high storage and loss moduli G′ and G″. If the viscoelastic window is within quadrant 2, the material will have high cohesive strength due to the high storage modulus G′ and high dissipation due to high loss modulus G″. The overall behavior is that of a strong high shear PSA. As long as the low frequency G′ is below the Dahlquist criterion of 3×105 Pa at the temperature of application (usually about 25° C.), the PSA will be tacky and stick easily to the substrate surface.
Quadrant 3 is defined to be below a horizontal line at a G′ value of 30,000 Pa and to the left of a vertical line at a G″ value of 32,000 Pa. PSAs in Quadrant 2 have low storage and loss moduli G′ and G″. If the viscoelastic window is within quadrant 3, the material is a soft, highly viscous fluid. The material will stick well to a surface because the storage modulus G′ is well below the Dahlquist criterion, and will not have much strength. Because the loss modulus G″ is also low, materials with a viscoelastic window in quadrant 3 will not dissipate much energy when peeled. The resulting behavior is an easily removable PSA.
Quadrant 4 is defined to be below a horizontal line at a G′ value of 30,000 Pa and to the right of a vertical line at a G″ value of 32,000 Pa. PSAs in Quadrant 4 have a low storage modulus G′ and a high loss modulus G″. If the viscoelastic window is within quadrant 4, the material is a pressure sensitive adhesive capable of operating at very low temperatures. Because the loss modulus G″ is high, materials with a viscoelastic window in quadrant 4 will have a high dissipation. Coupled with a low storage modulus G′, the resulting behavior is a material that can stick with short contact time and at low temperatures.
A central area is defined as a box bounded by an upper horizontal line at a G′ value of 130,000 Pa, a lower horizontal line at a G′ value of 8,000 Pa and a pair of vertical lines at G″ values of 6,500 Pa and 150,000 Pa. The central area is characterized by medium storage and loss moduli G′ and G″. If the viscoelastic window is within this central area, the material behaves as a middle of the road or general purpose PSA.
The Dahlquist criteria line is defined as a horizontal line at a G′ value of 300,000 Pa as illustrated in
Thermally switchable PSA layer 16 can be designed to exhibit properties of a high shear PSA at a temperature below the melting temperature of the solid solvent and exhibit properties of a removable PSA at and above the melting temperature of the solid solvent. PSA layer 16 can be further designed to have high shear strength over a service temperature range, such that PSA layer 16 will not fail due a change in temperature within the service temperature range during operation. For aircraft deicer applications, the service temperature can generally range from −40° F. to 160° F. (−40° C. to 71° C.). For deicing applications, PSA layer 16 has high frequency storage and loss moduli G′ and G″ located in quadrant 2 of the Chang plot within a range of G′ values greater than 30,000 Pa and G″ values greater than 32,000 Pa across the service temperature range. Preferably, the high frequency storage modulus G′ will be above 130,000 Pa and the high frequency loss modulus G″ will be above 32,000 Pa across the service temperature range. To prevent failure within this temperature range, the solid solvent selected for PSA layer 16 has a melting temperature above the service temperature range. Preferably, the melting temperature is significantly above the service temperature, but below a temperature at which PSA layer 16 could be degraded by excessive heat (approximately 175° C. for acrylic based PSAs) or at which a material forming the active element layer degrades or melts. For example, the melting temperature of the solid solvent can be above the service temperature of a polyurethane deicer, but below a melting temperature of the polyurethane. In the present example, the solid solvent melting temperature can be between approximately 100° C. and 160° C.
For good tack upon installation of device 10, the lower left corner storage modulus G′ (low frequency storage modulus G′) is less than the Dahlquist criteria at 25° C. (temperature at which PSA layer 16 is applied to aircraft structure 12). For removability, PSA layer 16 is designed such that the viscoelastic window moves to a position low within quadrant 3 of the Chang plot when PSA layer 16 is exposed to heat at or above the melting temperature of the solid solvent. Preferably, the low frequency storage and loss moduli G′ and G″ (lower left hand corner of the viscosoelastic window), and the high frequency storage and loss moduli G′ and G″ (upper right corner of the viscoelastic window) are as low as possible within quadrant 3, but above the tan δ=1 line (i.e., the ratio of G′ to G″ is greater than one), such that PSA layer 16 can be peeled off with low force with a reduced likelihood of leaving a residue on the substrate (e.g., aircraft structure 12). As the ratio of G′ to G″ drops below 1 (below the tan δ=1 line), PSA layer 16 can experience cohesive failure. As temperature increases, adhesive softening, a reduction in shear strength, and a reduction in cohesive strength can be observed, as illustrated in
The solid solvent of PSA layer 16 has a solubility parameter close to the solubility parameter of the PSA material in PSA layer 16 to enable melting, but greater than the solubility parameter of the PSA material, such that the solid solvent is able to precipitate out of the PSA material and crystallize below the solid solvent melting temperature. The separation of the solid solvent into reinforcing solid particles within PSA layer 16 can increase the shear strength of PSA layer 16 as compared to PSA materials without the incorporation of a solid solvent. Preferably, the difference between the two solubility parameters is less than 6 MPa1/2 and, preferably, with in a range of approximately 2 to 5 MPa1/2. The solubility parameter of a typical acrylic PSA is between 18.1 MPa1/2 and 25.1 MPa1/2.
The following are non-limiting examples of thermally switchable PSA layer 16. Isothermal frequency sweeps of the PSA materials were performed using an ARES G2 RDA manufactured by TA Instruments LLC, 159 Lukens Drive, New Castle, Del. 19720. Isotherm temperatures were 25° C., 50° C., 100° C., and 150° C. Storage modulus G′ and loss modulus G″ were measured from angular frequency of 0.01 rad/s to 100 rad/s taking five data points per decade of frequency. The geometry was a 25 mm diameter parallel plate.
The solid solvent powder was crushed to form a fine powder and combined with a 25 mm diameter×2.5 mm thick disc of the 3M VHB™ PSA, according to the table below. The mixture was heated to 175° C. for 10 minutes in a convection oven. Upon heating above the solid solvent melt temperature, the solid solvent melted to become a liquid and was absorbed into the 3M VHB™ PSA. The sample was cooled to 25° C. Weight percent of the solid solvent in the PSA was calculated according to Equation 1.
Experimental results presented in Tables 2-7 indicate that a solid solvent content above 6 wt % and below 21 wt % is preferable for the solid solvents tested. At 5.95 wt % adipic acid and 5.91 wt % suberic acid, a low reduction in viscosity was observed, but not sufficient enough to effectively switch the high shear PSA to a removable PSA above the solid solvent melting temperature. Above 21 wt %, the solid solvent increased stiffness to an extent that the material no longer performed as a PSA (with the low frequency storage and loss moduli G′ and G″ increased above the Dahlquist criteria line) at 25° C. (21.3 wt % suberic acid) or softened only enough to become a general purpose PSA above the melting point of the solid solvent (21.1 wt % adipic acid). The PSAs investigated performed effectively as a high sheer PSA at room temperature and removable PSA above the solid solvent melting point with solid solvent contents of 12.75 wt % adipic acid and 10.6 wt % suberic acid.
As disclosed, a solid solvent can be incorporated into a high shear PSA to produce a thermally switchable PSA, which exhibits properties of a high shear PSA below a melting temperature of the solid solvent and exhibits properties of a removable PSA above the melting temperature of the solid solvent. Heat can be applied to the thermally switchable PSA to allow for removal and repositioning during installation without destroying the PSA and without leaving a residue on an aircraft surface to which the PSA was bonded. The use of thermally switchable PSAs can facilitate the installation and removal of various aircraft components, which are adhesively bonded to a substrate, including aircraft components requiring high shear strength across a wide range of service temperatures (e.g., −40° F. to 160° F. (−40° C. to 71° C.)), such as anti-icers and deicers used to inhibit atmospheric ice accumulation on aircraft structures.
Summation
Any relative terms or terms of degree used herein, such as “substantially”, “essentially”, “generally”, “approximately” and the like, should be interpreted in accordance with and subject to any applicable definitions or limits expressly stated herein. In all instances, any relative terms or terms of degree used herein should be interpreted to broadly encompass any relevant disclosed embodiments as well as such ranges or variations as would be understood by a person of ordinary skill in the art in view of the entirety of the present disclosure, such as to encompass ordinary manufacturing tolerance variations, incidental alignment variations, transient alignment or shape variations induced by thermal, rotational or vibrational operational conditions, and the like. Moreover, any relative terms or terms of degree used herein should be interpreted to encompass a range that expressly includes the designated quality, characteristic, parameter or value, without variation, as if no qualifying relative term or term of degree were utilized in the given disclosure or recitation.
Discussion of Possible Embodiments
The following are non-exclusive descriptions of possible embodiments of the present invention.
A device comprises an active element panel and a pressure sensitive adhesive layer bonded to the active element panel and defining a bonding surface bondable to an aircraft structure. The pressure sensitive adhesive layer comprises a mixture of a pressure sensitive adhesive and a solid solvent.
The device of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations, and/or additional components:
A further embodiment of the device of any of the preceding paragraphs, wherein the pressure sensitive adhesive exhibits properties of a high shear pressure sensitive adhesive at a temperature below a melting temperature of the solid solvent and exhibits properties of a removable pressure sensitive adhesive at a temperature above the melting temperature of the solid solvent.
A further embodiment of the device of any of the preceding paragraphs, wherein the pressure sensitive adhesive material is selected from a group consisting of an acrylic pressure sensitive adhesive, a rubber pressure sensitive adhesive, and a silicone pressure sensitive adhesive.
A further embodiment of the device of any of the preceding paragraphs, wherein the pressure sensitive adhesive has a first solubility parameter and the solid solute has a second solubility parameter, and wherein a difference between the first and second solubility parameters is less than 6 MPa1/2.
A further embodiment of the device of any of the preceding paragraphs, wherein the second solubility parameter is greater than the first solubility parameter.
A further embodiment of the device of any of the preceding paragraphs, wherein the difference between the first and second solubility parameters is within a range of approximately 2 to 5 MPa1/2.
A further embodiment of the device of any of the preceding paragraphs, wherein the solid solvent has a melting temperature above a service temperature of the device in operation and below a melting temperature of a material forming the active element panel.
A further embodiment of the device of any of the preceding paragraphs, wherein the pressure sensitive adhesive material is an acrylic pressure sensitive adhesive and the solid solvent is selected from a group consisting of carboxylic acids and L-lactide.
A further embodiment of the device of any of the preceding paragraphs, wherein the solid solvent precipitates out of the pressure sensitive adhesive and crystallizes at a temperature below the solid solvent melting temperature.
A further embodiment of the device of any of the preceding paragraphs, wherein the solid solvent increases a shear strength of the pressure sensitive adhesive at a temperature below the solid solvent melting temperature.
A further embodiment of the device of any of the preceding paragraphs, wherein the pressure sensitive adhesive layer has a high frequency storage modulus above 130,000 Pa and a high frequency loss modulus above 32,000 Pa across a temperature range of −40 to 71 degrees Celsius (−40 to 160 degrees Fahrenheit).
A further embodiment of the device of any of the preceding paragraphs, wherein the pressure sensitive adhesive layer has a viscoelastic window low frequency and high frequency storage and loss moduli below 30,000 Pa at a temperature above the melting temperature of the solid solvent.
A further embodiment of the device of any of the preceding paragraphs, wherein the pressure sensitive adhesive layer has low frequency storage and loss moduli below 30,000 Pa with the low frequency storage modulus being greater than the low frequency loss modulus at a temperature above the melting temperature of the solid solvent.
A method for removing an active element panel on an aircraft structure includes applying heat to a pressure sensitive adhesive layer of the active element panel and separating the pressure sensitive adhesive layer from the aircraft structure. The pressure sensitive adhesive layer includes a mixture of a pressure sensitive adhesive and a solid solvent.
The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations, additional components, and/or steps:
A further embodiment of the method of any of the preceding paragraphs, wherein applying heat comprises raising a temperature of the pressure sensitive adhesive layer above a melting temperature of the solid solvent.
A further embodiment of the method of any of the preceding paragraphs, wherein the pressure sensitive adhesive layer comprises a pressure sensitive adhesive having a solubility parameter within approximately 2 to 5 MPa1/2 of a solubility parameter of the solid solute.
A further embodiment of the method of any of the preceding paragraphs, wherein the pressure sensitive adhesive layer has a low frequency storage modulus below 30,000 Pa with the low frequency storage modulus being greater than the low frequency loss modulus at a temperature at a temperature above the solid solvent melting temperature.
A further embodiment of the method of any of the preceding paragraphs further including repositioning the active element panel on the aircraft structure by cooling the pressure sensitive adhesive layer to a temperature below the melting temperature of the solid solvent, and attaching the pressure sensitive adhesive layer to the aircraft structure.
A further embodiment of the method of any of the preceding paragraphs, wherein the cooled pressure sensitive adhesive layer has a high frequency storage modulus above 130,000 Pa and a high frequency loss modulus above 32,000 Pa.
A further embodiment of the method of any of the preceding paragraphs, wherein the pressure sensitive adhesive layer has a high frequency storage modulus above 130,000 Pa and a high frequency loss modulus above 32,000 Pa across a temperature range of −40 to 71 degrees Celsius (−40 to 160 degrees Fahrenheit).
While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
