REFLECTIVE FILM FOR THERMAL CONTROL

Abstract

A reflective film includes a polymeric layer and a reflective component. The polymeric layer includes a polyimide polymer, where the polyimide polymer has a backbone with at least one non-terminal phenyl group. A linkage is connected to the non-terminal phenyl group, where the linkage can be an amide linkage or an ester linkage. An oligomeric silsesquioxane compound is connected to the linkage through an organic substituent, where the oligomeric silsesquioxane is not incorporated into the polymer backbone.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a reflective film for thermal control, and more particularly to thermal control in high altitude environments.

2. Description of the Related Art

Polyimides are an important class of polymeric materials and are known for their superior performance characteristics. These characteristics include high glass transition temperatures, good mechanical strength, high Young's modulus, good UV durability, and excellent thermal stability. Most polyimides are comprised of relatively rigid molecular structures such as aromatic/cyclic moieties.

As a result of their favorable characteristics, polyimide compositions have become widely used in many industries, including the aerospace industry, the electronics industry and the telecommunications industry. In the electronics industry, polyimide compositions are used in applications such as forming protective and stress buffer coatings for semiconductors, dielectric layers for multilayer integrated circuits and multi-chip modules, high temperature solder masks, bonding layers for multilayer circuits, final passivating coatings on electronic devices, and the like. In addition, polyimide compositions may form dielectric films in electrical and electronic devices such as motors, capacitors, semiconductors, printed circuit boards and other structures. Polyimide compositions may also serve as an interlayer dielectric in both semiconductors and thin film multichip modules. The low dielectric constant, low stress, high modulus, and inherent ductility of polyimide compositions make them well suited for these multiple layer applications. Other uses for polyimide compositions include alignment and/or dielectric layers for displays, and as a structural layer in micromachining applications.

In the aerospace industry, polyimide compositions have been used for optical applications as reflective membranes. In application, a polyimide composition is secured by a metal (often aluminum, copper, or stainless steel) or composite (often graphite/epoxy or fiberglass) mounting fixture that secures the polyimide compositions. Such optical applications may be used in space, where the polyimide compositions and the mounting fixture are subject to repeated and drastic heating and cooling cycles in orbit as the structure is exposed to alternating periods of sunlight and shade. Satellites can remain in orbit for extended periods, so the materials used should survive as long as the satellite remains functional.

One important factor of a material in space is its service temperature. Generally, there are three types of heat transfer: conductive heat transfer where heat passes to or from a first object to a second object touching the first object; convective heat transfer where heat passes from a first object to a gas, such as the atmosphere; and radiant heat transfer, where heat passes to or from an object through electromagnetic radiation, such as infrared (IR) light. Radiant heat transfer controls the temperature of an object in space. Radiant heat transfer can also be significant for objects in high altitude environments, because the atmosphere for convective heat transfer is thin and no objects are present for conductive heat transfer. “High altitude environment” is defined to include altitudes above 20,000 feet, the stratospheric near-space environment, and space.

Polyimide polymers are subject to rapid degradation in a highly oxidizing environment, such as an oxygen plasma or atomic oxygen [AO], as are most hydrocarbon- and halocarbon-based polymers. AO is present in low earth orbit [LEO], so many spacecraft experience this highly oxidizing environment. The interactions between the oxidizing environment and the polymer material can erode and reduce the thickness of the polymer material. To protect from erosion, protective coatings including metals, metal oxides, ceramics, glasses, and other inorganic materials can be applied as surface treatments to polyimides subjected to the oxidizing environment.

While these coatings are effective at minimizing the oxidative degradation of the underlying material, they often experience cracking from thermal and mechanical stresses, mechanical abrasion, and debris impact. After cracking, the protective surface is compromised and the underlying polymeric material can be degraded from additional exposure to the oxidizing environment. Therefore, the availability of polymers which are able to resist AO degradation is very desirable.

Oligomeric silsesquioxanes [OS] can be incorporated into a polyimide matrix to improve the durability of polyimides in these environments. Polyimides with incorporated OS demonstrate excellent resistance to AO degradation prevalent in LEO environments. Polyimides with incorporated OS provide additional benefits as well. Polyhedral OS are referred to by the trademark POSS™, and are a common form of OS.

Rockets, aircraft, other lift engines, and lighter than air gases are often used to carry a cargo into space or other high altitude environments. Much of the total mass of a space rocket is the fuel used to propel the rocket into space, and the mass of any cargo carried into high altitude environments is limited. Cargo carried into high altitude environments is typically limited to a certain size and weight to fit within the confines and capabilities of the vehicle carrying the cargo. The cost per unit weight of delivering a vehicle, cargo, and payload into high altitude service is high, so many efforts are made to reduce the total weight. Reductions in the total volume occupied by the cargo are also beneficial. The various components of a vehicle, cargo, and/or payload should be able to withstand the harsh high altitude environment for the expected life. Reducing the cargo weight can save costs, or allow for additional components to be included with the vehicle. Efforts to find new materials or combinations of materials which can withstand the high altitude environment and which can reduce the weight and volume of the vehicle, cargo, and/or payload continue. Materials which can withstand high altitude environments may also be beneficial in other environments, such as certain industrial applications.

SUMMARY OF THE INVENTION

A reflective film includes a polymeric layer and a reflective component. The polymeric layer includes a polyimide polymer that has a backbone with at least one non-terminal phenyl group. A linkage is connected to the non-terminal phenyl group, where the linkage can be an amide linkage or an ester linkage. An oligomeric silsesquioxane compound is connected to the linkage through an organic substituent, where the oligomeric silsesquioxane is not incorporated into the polymer backbone. The reflective component can be used for thermal control in structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the formation of an amic acid.

FIG. 2 depicts the formation of an imide bond from an amic acid.

FIG. 3 depicts an oligomeric silsesquioxane (OS) compound.

FIG. 4 depicts a polyhedral shaped OS compound.

FIG. 5 depicts 4-4′4-[hexafluoroisopropylidene]diphthalic anhydride [6-FDA]

FIG. 6 depicts 4,4′-oxydiphthalic anhydride [ODPA]

FIG. 7 depicts 1,3-diaminobenzoic acid [DBA]

FIG. 8 depicts 4,4′-diaminobiphenyl-3,3′-dicarboxylic acid [DBDA].

FIG. 9 depicts para-phenylene diamine [p-PDA]

FIG. 10 depicts the formation of a tethering amide bond by the reaction of an amine with a carboxylic acid.

FIG. 11 depicts a section of a representative polyimide polymer with the OS connected with an amide.

FIG. 12 depicts a section of a representative polyimide polymer with the OS connected with an ester.

FIG. 13 depicts a non-terminal phenyl group of the polymer backbone, with an OS compound labeled X connected with an ester or amide linkage, labeled XX.

FIG. 14 depicts different structures that represent the symbol YY in FIG. 13, with ZZ representing a direct bond, —O—, —S—, —SO—, —SO₂—, —CH₂—, —CF₂—, —C(CH₃)₂—, —(CF₃)₂—, —(CH₂)_n—, —(CH₂CHCH₃O)_n—, —((CH₂)₄O)_n—, —(Si(CH₃)₂O)_n—, —(SiH(CH₃)O)_n—, —(SiH(C₆H₅)₂O)_n—, or —(Si(C₆H₅)₂O)_n—.

FIG. 15 depicts possible structures which are represented by the “WW” in FIG. 14.

FIG. 16 depicts two types of vehicles with a reflective film on each.

FIG. 17 depicts a reflective film with reflective components within the polymeric layer.

FIG. 18 depicts a perspective view of a flexible mirror, which can be a second surface mirror or a first surface mirror.

FIG. 19 depicts a side view of electromagnetic radiation passing through the polymeric layer and reflecting off the reflective layer of a second surface mirror.

NOTE: The use of waved lines “” indicates the molecule continues, but does not necessarily repeat. The use of square brackets “[” and/or “]” indicates that the structure repeats beyond the bracket. The use of round brackets “(” and/or “)” indicates substructures within a repeat unit and does not indicate the substructure repeats beyond the round brackets. In this description, an atom AA shown connected to a phenyl group through a bond, instead of at the angles representing carbon atoms, is meant to depict the atom AA connects to any available carbon atom in the phenyl group, and not to a specific carbon atom. Therefore, such a drawing does not specifically denote an ortho, meta, or para positioning of the bond to the AA atom.

DETAILED DESCRIPTION

Polyimide

Polyimides are a type of polymer with many desirable characteristics. In general, polyimide polymers include a nitrogen atom in the polymer backbone, wherein the nitrogen atom is connected to two carbonyl carbons, such that the nitrogen atom is somewhat stabilized by the adjacent carbonyl groups. A carbonyl group includes a carbon, referred to as a carbonyl carbon, which is double bonded to an oxygen atom. Most polyimides are considered an AA-BB type polymer because two different classes of monomers are used to produce the polyimide polymer. One class of monomer is called an acid monomer, and is usually in the form of a dianhydride. The other type of monomer is usually a diamine, or a diamino monomer. Polyimides may be synthesized by several methods. In the traditional two-step method of synthesizing aromatic polyimides, a polar aprotic solvent such as N-methylpyrrolidone (NMP) is used. First, the diamino monomer is dissolved in the solvent, and then a dianhydride monomer is added to this solution. The diamine and the acid monomer are generally added in approximately a 1:1 molar stoichiometry.

Because one dianhydride monomer has two anhydride groups, different diamino monomers can react with each anhydride group so the dianhydride monomer may become located between two different diamino monomers. The diamine monomer contains two amine functional groups; therefore, after one amine attaches to the first dianhydride monomer, the second amine is still available to attach to another dianhydride monomer, which then attaches to another diamine monomer, and so on. In this matter, the polymer backbone is formed. The resulting polycondensation reaction forms a polyamic acid. The reaction of an anhydride with an amine to form an amic acid is depicted in FIG. 1. The high molecular weight polyamic acid produced is soluble in the reaction solvent and, thus, the solution may be cast into a film on a suitable substrate such as by flow casting. The cast film can be heated to elevated temperatures in stages to remove solvent and convert the amic acid groups to imides with a cyclodehydration reaction, also called imidization. Alternatively, some polyamic acids may be converted in solution to soluble polyimides by using a chemical dehydrating agent, catalyst, and/or heat. The conversion of an amic acid to an imide is shown in FIG. 2.

The polyimide polymer is usually formed from two different types of monomers, and it is possible to mix different varieties of each type of monomer. Therefore, one, two, or more dianhydride-type monomers can be included in the reaction vessel, as well as one, two or more diamino monomers. The total molar quantity of dianhydride-type monomers is kept about the same as the total molar quantity of diamino monomers. Because more than one type of diamine or dianhydride can be used, the exact form of each polymer chain can be varied to produce polyimides with desirable properties.

For example, a single diamine monomer AA can be reacted with two dianhydride comonomers, B₁B₁ and B₂B₂, to form a polymer chain of the general form of (AA-B₁B₁)_x-(AA-B₂B₂)_y in which x and y are determined by the relative incorporations of B₁B₁ and B₂B₂ into the polymer backbone. Alternatively, diamine comonomers A₁A₁ and A₂A₂ can be reacted with a single dianhydride monomer BB to form a polymer chain of the general form of (A₁A₁-BB)_x-(A₂A₂-BB)_y. Additionally, two diamine comonomers A₁A₁ and A₂A₂ can be reacted with two dianhydride comonomers B₁B₁ and B₂B₂ to form a polymer chain of the general form (A₁A₁-B₁B₁)_w-(A₁A₁-B₂B₂)_x-(A₂A₂-B₁B₁)_y-(A₂A₂-B₂B₂)_z, where w, x, y, and z are determined by the relative incorporation of A₁A₁-B₁B₁, A₁A₁-B₂B₂, A₂A₂-B₁B₁, and A₂A₂-B₂B₂ into the polymer backbone. Therefore, one or more diamine monomers can be polymerized with one or more dianhydrides, and the general form of the polymer is determined by varying the amount and types of monomers used.

The dianhydride is only one type of acid monomer used in the production of AA-BB type polyimides. It is possible to used different acid monomers in place of the dianhydride. For example, a tetracarboxylic acid with four acid functionalities, a tetraester, a diester acid, or a trimethylsilyl ester could be used in place of the dianhydride. In this description, an acid' monomer refers to either a dianhydride, a tetraester, a diester acid, a tetracarboxylic acid, or a trimethylsilyl ester. The other monomer is usually a diamine, but can also be a diisocyanate. Polyimides can also be prepared from AB type monomers. For example, an aminodicarboxylic acid monomer can be polymerized to form an AB type polyimide.

The characteristics of the polyimide polymer are determined, at least in part, by the monomers used in the preparation of the polymer. The proper selection and ratio of monomers are used to provide the desired polymer characteristics. For example, polyimides can be rendered soluble in organic solvents by selecting the monomers that impart solubility into the polyimide structure. It is possible to produce a soluble polyimide polymer using some monomers that tend to form insoluble polymers if the use of the insoluble monomers is balanced with the use of sufficient quantities of soluble monomers, or through the use of lower quantities of especially soluble monomers. The term especially soluble monomers refers to monomers which impart more of the solubility characteristic to a polyimide polymer than most other monomers. Some soluble polyimide polymers are soluble in relatively polar solvents, such as dimethylacetamide, dimethylformamide, dimethylsulfoxide, tetrahydrofuran, acetone, methyl ethyl ketone, methyl isobutyl ketone, and phenols, as well as less polar solvents, including chloroform, and dichloromethane. The solubility characteristics and concentrations of the selected monomers determine the solubility characteristics of the resultant polymer. For this description, a polymer is soluble if it can be dissolved in a solvent to form at least a 1 percent solution of polymer in solvent, or more preferably a 5 percent solution, and most preferably a 10 percent or higher solution.

Most, but not all, of the monomers used to produce polyimide polymers include aromatic groups. These aromatic groups can be used to provide an attachment point on the polymer backbone for a tether. A tether refers to a chain including at least one carbon, oxygen, sulfur, phosphorous, or silicon atom that is used to connect the polymer backbone to another compound or sub-compound. Therefore, if the polymer backbone were connected through the para position on a phenyl group, wherein the para position refers to the number 1 and the number 4 carbons on the benzene ring, the ortho and meta positions would be available to attach a tether to this polymer backbone. The ortho position to the number 1 carbon refers to the number 2 and number 6 carbons, whereas the meta position to the number 1 carbon refers to the number 3 and number 5 carbons.

Many polyimide polymers are produced by preparing a polyamic acid polymer in the reaction vessel. The polyamic acid is then formed into a sheet or a film and subsequently processed with heat (often temperatures higher than 250 degrees Celsius) or both heat and catalysts to convert the polyamic acid to a polyimide. However, polyamic acids are moisture sensitive, and care must be taken to avoid the uptake of water into the polymer solution. Additionally, polyamic acids exhibit self-imidization in solution as they gradually convert to the polyimide structure. The imidization reaction generally reduces the polymer solubility and produces water as a by-product. The water produced can then react with the remaining polyamic acid, thereby cleaving the polymer chain. Moreover, the polyamic acids can generally not be isolated as a stable pure polymer powder. As a result, polyamic acids have a limited shelf life.

Sometimes it is desirable to produce the materials for a polyimide polymer film, but wait for a period of time before actually casting the film. For this purpose, it is possible to store either a soluble polyimide or a polyamic acid. Soluble polyimides have many desirable advantages over polyamic acids for storage purposes. Soluble polyimides are in general significantly more stable to hydrolysis than polyamic acids, so the polyimide can be stored in solution or it can be isolated by a precipitation step and stored as a solid material for extended periods of time. If a polyamic acid is stored, it will gradually convert to the polyimide state and/or hydrolytically depolymerize. If the stored material becomes hydrolytically depolyermized, it will exhibit a reduction in solution viscosity, and if the stored material converts to the polyimide state, it will become gel-like or a precipitated solid if the polyimide is not soluble in the reaction medium. This reduced viscosity solution may not exhibit sufficient viscosity to form a desired shape, and the gel-like or solid material cannot be formed to a desired shape. The gradual conversion of the polyamic acid to the polyimide state generates water as a byproduct, and the water tends to cleave the remaining polyamic acid units. The cleaving of the remaining polyamic acid units by the water is the hydrolytic depolymerization referred to above. Therefore, the production of soluble polyimides is desirable if there will be a delay before the material is formed for final use.

Soluble polyimides have advantages over polyamic acids besides shelf life. Soluble polyimides can be processed into usable work pieces without subjecting them to the same degree of heating as is generally required for polyamic acids. This allows soluble polyimides to be processed into more complex shapes than polyamic acids, and to be processed with materials that are not durable to the 250 degree Celsius minimum temperature typically required for imidizing polyamic acids. To form a soluble polyimide into a desired film, the polyimide is dissolved in a suitable solvent, formed into the film as desired, and then the solvent is evaporated. The film solvent can be heated to expedite the evaporation of the solvent.

Oligomeric Silsesquioxane

OS compounds or groups are characterized by having the general formula of [RSi]_n[O_1.5]_n wherein the R represents an organic substituent and the Si and the O represent the chemical symbols for the elements silicon and oxygen. R can be aliphatic or aromatic, and includes a wide variety of organic compounds. The silicon atoms are connected together through the oxygen atoms, with the R groups connected to the silicon atoms, as seen in FIG. 3. These OS compounds have hybrid organic and inorganic properties. The Si—O groupings provide the inorganic properties, and the attached R groups provide the organic properties. Frequently, these OS compounds exist in a cage form such that a polyhedron is created by the silicon and oxygen atoms, as shown in FIG. 4. When the OS compound is in the cage form or the polyhedral form, the R groups are exterior to the cage, with the Si atoms generally forming corners of the cage.

Frequently, the OS compound will have an organic substituent which has a functional group. These OS compounds can therefore have organic substituents with varying structures connected to the different Si atoms within a single OS compound. A typical example would be a polyhedral OS represented by the formula [RSi]_(N-1) [R′A]₁[O_1.5]_n, wherein R′ symbolizes an organic substituent with a functional group which can be used to connect the OS compound to a polymer backbone or some other molecule. In this case, the A is used to represent an element. This element is usually Si, but can also be other elements, including aluminum (Al), boron (B), germanium (Ge), tin (Sn), titanium (Ti), and antimony (Sb). These different atoms incorporated into the OS compound provide different characteristics which will be imparted to the polymer.

Attaching an OS group to a polyimide polymer can affect many characteristics of the polymer, including oxidative stability, temperature stability, glass transition temperature, solubility, dielectric constant, tensile properties, thermomechanical properties, optical properties, and other properties. One significant characteristic improved by incorporation of OS in a polyimide polymer is increased resistance to degradation in oxidizing environments, such as oxygen plasma and AO, as discussed above. Oligomeric silsesquioxanes [OS] can be incorporated into a polyimide matrix to improve the durability of polyimides in these environments. Therefore, polyimide polymers with incorporated OS are desirable.

When a hydrocarbon or halocarbon polyimide that includes OS is exposed to an oxygen plasma or AO, the organic substituent portions of the OS oxidize into volatile products while the inorganic portion forms a passivating silica layer on the exposed surface of the polyimide polymer. This process is referred to as the glassification of the polymer. The silica layer tends to protect the underlying polyimide material from further degradation by the oxidizing environment. Additionally, the silica layer absorbs at least a portion of the ultraviolet [UV] light, especially the UV light with a wavelength shorter than about 256 nm. Therefore, the silica layer also protects the polymer film from radiation, because UV light is a form of electromagnetic radiation. Additionally, if the silica layer exhibits sufficient thickness, it reduces gas and water permeability through the polyimide film. It is possible to deposit additional silica on a polyimide film to produce the thickness necessary to significantly reduce gas and water permeability through the film.

OS has been blended with polymers to provide a degree of protection herein described, but the amount of OS which can be blended with a polymer is limited. Typically, the amount of OS incorporated into the polymer by blending methods is limited to a concentration where the OS compounds do not aggregate into domains large enough to scatter visible light. Incorporation of additional OS above this level typically results in a reduction in optical and/or mechanical properties. However, it has been found that chemically reacting the OS with the polymer reduces the OS aggregation and provides more uniform distribution of the OS within the polymer. As such, more OS can typically be incorporated into a polymer matrix via covalent bonding than by simpler blending methods. This results in a polymer which is better able to withstand exposure to oxygen plasma, AO, and UV radiation.

It is possible to attach the OS groups to a polyimide polymer by reacting the OS with one of the monomers before polymerization. In practice, however, this method is difficult owing to the high cost of OS and number of reaction and purification steps needed to obtain a usable monomer of sufficient purity. High monomer purity is required for the formation of sufficient molecular weight polyamic acids and polyimides. For example, others have described a method to incorporate a polyhedral OS into the polyimide backbone requiring four reaction steps. The fewer the reaction and purification steps, the lower the cost and the greater the efficiency of the entire process, as a general rule. Therefore, a method of producing a desired polymer with fewer reaction and purification steps is desirable, because such a method would probably reduce the cost and improve the overall efficiency of the process. The current disclosure describes a polyimide composition incorporating OS and a method of synthesizing that polymer that uses two reaction steps and zero to two purification steps.

Selection of Monomers

The characteristics of the final polymer are largely determined by the choice of monomers which are used to produce the polymer. Factors to be considered when selecting monomers include the characteristics of the final polymer, such as the solubility, thermal stability and the glass transition temperature. Other factors to be considered include the expense and availability of the monomers chosen. Commercially available monomers that are produced in large quantities generally decrease the cost of producing the polyimide polymer film since such monomers are in general less expensive than monomers produced on a lab scale and pilot scale. Additionally, the use of commercially available monomers improves the overall reaction efficiency because additional reaction steps are not required to produce a monomer which is incorporated into the polymer. One advantage of the current invention is the preferred monomers are generally produced in commercially available quantities, which can be greater than 10,000 kg per year.

One type of monomer used is referred to as the acid monomer, which can be either the tetracarboxylic acid, tetraester, diester acid, a trimethylsilyl ester, or dianhydride. The use of the dianhydride is preferred because it generally exhibits higher rates of reactivity with diamines than tetrafunctional acids, diester acids, tetraesters, or trimethylsilyl esters. Some characteristics to be considered when selecting the dianhydride monomer include the solubility of the final polymer as well as commercial availability of the monomers.

Certain characteristics tend to improve the solubility of the polyimide polymer. These characteristics include flexible spacers, so-called kinked linkages, and bulky substituents. The flexible spacer is an atom covalently bonded between two separate phenyl groups. The phenyl groups are relatively ridged, so the flexible spacer allows for increased motion between separate phenyl groups. Alkyl linkages are not as stable as the phenyl groups, so the use of simple hydrocarbon alkyl groups between phenyl groups can reduce the stability of the polymer. The stability of the overall polymer can be improved if the linkage is saturated with fluorine instead of hydrogen. Also, the use of other atoms, such as Oxygen or Silicon, can result in a more stable polymer.

The term kinked linkages refers to a meta connection on a phenyl group. This means the polymer backbone would be connected through a number 1 and number 3 carbon on a phenyl group. The use of kinked linkages, or meta linkages, in the polymer backbone tends to result in a higher coefficient of thermal expansion, as well as greater solubility.

Bulky substituents in the polymer also tend to increase the overall polymer solubility. Bulky substituents are compounds which are large and tend to interfere with intramolecular and intermolecular chain association because of their size. The bulky substituents can be included between phenyl groups in the backbone, connected directly to a phenyl group, or they can be tethered to the polymer backbone. The bulky substituents tend to reduce the ability of adjacent polymer chains to tightly associate. This tends to allow solvent molecules to enter between adjacent polymer chains, which increases the polymer solubility.

Urging phenyl groups in the backbone to align in different planes also tends to increase the polymer solubility, and bulky substituents can be used for this purpose. If two phenyl groups in the backbone are relatively close together, and each has a bulky substituent connected or tethered to it, those bulky substituents will sterically interfere with each other and tend to urge one phenyl group into a plane that is perpendicular to the plane of the other phenyl group.

The flexible spacers can be saturated with larger components such as fluorine to improve the solubility of the resulting polyimide polymer. Other preferred atoms for separating the phenyl groups include oxygen and sulfur. The preferred dianhydride monomers of the current invention are 6-FDA and ODPA, as seen in FIGS. 5 and 6, but other dianhydride monomers may also be used.

The OS group is usually attached to the diamine monomer owing to the greater availability of diamine architectures as compared to dianhydrides. Therefore, one of the diamine monomers used should have a functional group independent of the two amines, so when the two amines are incorporated into the polymer backbone the functional group is still available for a subsequent chemical reaction. This functional group is referred to as an attachment point because this is the point where the OS group is attached to the polymer backbone. A wide variety of attachment points can be used, but a carboxylic acid is preferred. The attachment point is incorporated into the length of the polymer backbone, not just at the ends or terminus of each chain, so that more OS compounds can be attached to the polymer. Therefore, the preferred attachment point is a carboxylic acid connected by a single bond to a phenyl group, wherein the phenyl group is non-terminal and the phenyl group is part of the polymer backbone.

The monomers DBA and DBDA, as shown in FIGS. 7 and 8, are utilized heavily because of the presence of a carboxylic acid group, which serve as attachment points for the OS groups. Other diamino monomers without a free attachment point can also be used. One example of such a monomer is p-PDA, as seen in FIG. 9. The higher the concentration of diamino monomers without an attachment point, the lower the concentration of diamino monomers with a free attachment point. Since the OS is primarily incorporated into the polymer through the attachment point, the overall concentration of OS compounds in the final polymer can be controlled by varying the ratio of diamino monomers with and without a free attachment point. Many other diamino monomers can be used, including but not limited to 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (BDAF), 1,3 bis(3-aminophenoxy)benzene (APB), 3,3-diaminodiphenyl sulfone (3,3-DDSO2), 4,4′-diaminodiphenylsulfone (4,4′-DDSO2), meta phenylene diamine (m-PDA), oxydianiline (ODA), the isomers of 4,4′-Methylenebis(2-methylcyclohexylamine) (MBMCHA), the isomers of 4,4′-Methylenebis(cyclohexylamine) (MBCHA), the isomers of 1,4-cyclohexyldiamine (CHDA), 1,2-diaminoethane, 1,3-diaminopropane, 1,4-diamonbutane, 1,5-diaminopentane, 1,6-diaminohexane, and diamonodurene (DMDE).

Any remaining free carboxylic acid groups at the end or terminus of a polymer chain also serve as attachment points. These terminal carboxylic acid groups are generally connected to a terminal phenyl group. Attachment points connected to non-terminal phenyl groups are needed to increase the ratio of OS incorporation into the final polymer. A non-terminal phenyl group is a phenyl group in the polymer backbone which is between two imide bonds in the polymer backbone.

Oligomeric Silsesquioxane Considerations

Incorporation of the OS group into the polymer matrix is often beneficial for the reasons previously described. Usually, the OS group is incorporated in a polyhedral cage structure, and the polyhedral OS is attached to the polymer. In such instances, the OS group will have at least one organic substituent with a functional group for attaching to the polymer. It is also possible for the polyhedral OS to have two or more functional groups, in which case it can be used as a monomer or as a crosslinking component. For example, a polyhedral OS group including two amine functional groups could be incorporated into the backbone of the polymer as a diamine monomer. Alternatively, if one OS functional group were to attach to a first polymer chain, the second OS functional group could attach to a different polymer chain, so the two polymer chains were cross-linked. The polymer chains can also be crosslinked by the inclusion of other organic substituents which will crosslink the polymer chains.

The current invention considers the attachment of the OS groups to the backbone by a tether such that these OS groups do not form part of the backbone of the polymer. This provides advantages as compared to incorporating the OS groups in line with the polymer backbone. One advantage of pendant attachment is that OS and polyhedral OS are rigid. When rigid OS is incorporated in line with the polymer backbone, it generally increases the material stiffness and ultimately limits the amount of polyhedral OS incorporated. By attaching the OS groups to the backbone by a tether, the bulk polymer stiffness is less affected with increasing levels of OS incorporation.

It is advantageous to use a tether when attaching an OS group to a polyimide polymer backbone. Steric factors favor the use of a tether because the OS group is usually bulky and the tether provides an attachment point apart from the bulk of the OS. The tether needs to be long enough to allow the OS group to react with the polyimide backbone, but it should be short enough to restrict the OS from aggregating into large domains that reduce the mechanical and optical properties. Keeping the OS group close to the attachment point provides for the OS being relatively evenly distributed throughout the polymer. Also, keeping the OS group close to the attachment point provides for a bulky constituent near the polymer backbone, which tends to improve the solubility of the polymer. Some desired properties of the tether itself include the provision of a stable and secure attachment, as well as many degrees of freedom of movement for the OS group. The tether can include functional groups, such as amides or esters. A tether or chain about 5 atoms long provides enough length to allow attachment while keeping the OS group close enough to the attachment point.

Preferably, the OS compound will include one organic substituent with a functional group for attachment to the polymer backbone. This organic substituent becomes the tether connecting the polymer backbone to the OS compound. The preferred functional groups for this attachment are either an amine or an alcohol. The amine or alcohol on the OS compound reacts with a carboxylic acid on the polymer backbone to form an amide or ester connection, with the amide reaction shown in FIG. 10. The amide or ester thus formed includes either a carbonyl carbon and a linking nitrogen, as seen in FIGS. 10 and 11, or a carbonyl carbon and a linking oxygen, as seen in FIG. 12. The carbonyl carbon is connected by a single bond to a phenyl group, and the phenyl group is part of the polymer backbone. The linking nitrogen or oxygen is connected by a single bond to the carbonyl carbon, and the linking nitrogen or oxygen is also connected to the organic substituent tether for the OS compound.

By using the process described above, a polyimide polymer is formed which contains either an amide or ester linking a non-terminal phenyl group to the OS compound. This section of the polymer is shown in FIG. 13. The nitrogen in FIG. 13 is from the imide bond in the polymer backbone, XX represents either an oxygen atom or a nitrogen atom with an attached hydrogen, depending on if the OS group is attached by forming an ester or an amide, respectively. The symbol X represents an OS compound, including a tether between the XX atom and the OS compound. FIG. 14 depicts different structures that represent the symbol YY in FIG. 13. These different structures depend on the diamino monomers selected for the polyimide polymer. The symbol WW in FIG. 14 represents either a hydrogen atom, a free carboxylic acid, or another amide or ester linkage with an attached OS compound, as shown in FIG. 15, wherein XX again represents either nitrogen with an attached hydrogen or oxygen. The symbol ZZ in FIG. 14 represents a direct bond, —O—, —S—, —SO—, —SO₂—, —CH₂—, —CF₂—, —C(CH₃)₂—, —(CF₃)₂—, —(CH₂)_n—, —(CH₂CHCH₃O)_n—, —((CH₂)₄O)_n—, —(Si(CH₃)₂O)_n—, —(SiH(CH₃)O)_n—, —(SiH(C₆H₅)₂O)_n—, or —(Si(C₆H₅)₂O)_n—. In these Figs., an atom shown connected to a phenyl group through a bond, instead of at the hexagon angles representing carbon atoms, is meant to depict that atom connected to any available carbon atom in the phenyl group, and not to a specific carbon atom.

Process

The process for creating the final polymer should involve as few reactions and as few isolations as possible to maximize the overall efficiency. Minimization of the number of vessels or pots which are used during the production process also tends to improve efficiency, because this tends to minimize the number of reactions and/or isolations of the polymer.

The first step is forming the polyimide polymer backbone. Some of the basic requirements for this polymer backbone are that it be soluble, that it includes an attachment point, and that it has many of the desirable characteristics typical of polyimide polymers. Typically, the diamino monomers will be dissolved in a solvent, such as dimethylacetamide [DMAc]. After the diamino monomers are completely dissolved, the dianhydride monomer is added to the vessel and allowed to react for approximately 4 to 24 hours. The use of an end capping agent, such as a monoanhydride or a monoamine, is not preferred until after the polymerization reaction is allowed to proceed to completion. At that point, the addition of phthalic anhydride or other monoanhydride end-capping agents can be used to react with remaining end group amines. Adding end capping agents during the polymerization reaction tends to shorten the polymer chains formed, which can reduce desirable mechanical properties of the resultant polymer. For example, adding end capping agents during the polymerization reaction can result in a more brittle polymer.

At this point the monomers have reacted together to form a polyamic acid. It is desired to convert the polyamic acid to a polyimide. The conversion of the polyamic acid to the polyimide form is known as imidization, and is a condensation reaction which produces water, as seen in FIG. 2. Because water is a by-product of a condensation reaction, and reactions proceed to an equilibrium point, the removal of water from the reaction system pushes or drives the equilibrium further towards a complete reaction because the effective concentration of the by-product water is reduced. This is true for chemical reactions generally, including condensation reactions.

The water can be removed from the reaction vessel chemically by the use of anhydrides, such as acetic anhydride, or other materials which will react with the water and prevent it from affecting the imidization of the polyamic acid. Water can also be removed by evaporation. One imidization method involves the use of a catalyst to chemically convert the polyamic acid to the polyimide form. A tertiary amine such as pyridine, triethyl amine, and/or beta-picolline is frequently used as the catalyst. Another method previously discussed involves forming the polyamic acid into a film which is subsequently heated. This will vaporize water as it is formed, and imidize the polymer.

A third imidization method involves removing the formed water via azeotropic distillation. The polymer is heated in the presence of a small amount of catalyst, such as isoquinoline, and in the presence of an aqueous azeotroping agent, such as xylene, to affect the imidization. The method of azeotropic distillation involves heating the reaction vessel so that the azeotroping agent and the water distill from the reaction vessel as an azeotrope. After the azeotrope is vaporized and exits the reaction vessel, it is condensed and the liquid azeotroping agent and water are collected. If xylene, toluene, or some other compound which is immiscible with water is used as the azeotroping agent, it is possible to separate this condensed azeotrope, split off the water for disposal, and return the azeotroping agent back to the reacting vessel.

If water is removed from the reaction vessel as a vapor it is possible to proceed with the addition of the OS group without isolating the polyimide polymer formed. However, if the water is chemically removed, it is preferred to precipitate, filter, and possibly wash the polyimide before adding the OS group. If the polymer is precipitated, the filtered solid polyimide is used as a feed stock in the next step. It should be noted that when reference is made to isolating a polyimide, it refers to the isolation of a polyimide at one point in the entire production process. For example, if a polyimide was formed from the basic monomers, then precipitated and filtered, and then washed several times, this would count as a single isolation and purification, even though several washes were performed. Two isolations would be separated by a reaction step.

The OS group is connected to the polyimide back bone previously formed. One of the first steps is finding a suitable solvent to dissolve the OS group as well as the polyimide polymer. Some examples of suitable solvents include: methylene chloride, chloroform, and possibly tetrahydrofuran (THF). One way to control the amount of OS that is incorporated into the polymer is by controlling the amount of monomer with an attachment point which was included in the formation of the polyimide polymer. Another way would be to limit the amount of OS added. If OS is added at a stoichiometric quantity lower than the available attachment points, then there would be free attachment points remaining. These free attachment points could be used for cross linking with other polymer chains, or they could be used for other purposes. Preferably, the OS group is a monofunctional group with one functional group for attachment to the polymer back bone. Preferably this functional group is an amine or alcohol, which will combine with the preferred carboxylic acid that is available as the attachment point on the polymer back bone.

If the OS group includes two or more functional groups, different polymer chains can be cross linked through this OS group. For this to happen, the OS group would be linked to two separate polymer chains. An amine on the OS group can be reacted with the carboxylic acid on the polymer chain to form an amide bond. This is a condensation reaction in which water is produced as a byproduct. In order to drive the reaction to completion it is desirable for the water to be removed from the reaction system. This can be facilitated with a dehydrating agent, such as dicyclohexylcarbodiimide, followed by subsequent isolation of the polyimide polymer. Additionally, this amide-forming reaction can be facilitated with an acid catalyst. If the OS compound contains an alcohol for reacting with the carboxylic acid, water is still produced as a byproduct and removing the water will tend to drive the reaction to completion.

An alternate possibility is to remove the water via azeotropic distillation from the reaction vessel. This can be done by adding or by continuing to use an azeotroping agent such as xylene or toluene, then vaporizing the water, separating the water from the reaction vessel, and discarding the water after it has exited the reaction vessel. This is similar to the process described above for the imidization reaction.

The current process includes up to two isolations of the polyimide polymer. The first possible isolation is after the polymer imidization reaction, and the second possible isolation is after the OS group has been attached to the polyimide polymer. The azeotropic removal of water in the vapor form during a condensation reaction eliminates the need for a subsequent isolation. Therefore, if vaporous water is azeotropically removed during one of either the polymer imidization reaction or the OS attachment reaction, the number of isolations needed for the production of the final OS containing polymer is reduced to one. If vaporous water is removed after both of the above reactions, it is possible to produce the final product with no isolations.

By using a carboxylic acid as the attachment point on the polyimide polymer and an amine or an alcohol as the functional group on the OS, it is possible to connect the OS group to the polymer back bone using gentle reaction conditions. For example, the reaction conditions for a carboxylic acid/amine attachment do not require rigorous material purity or dryness, and/or could be completed at the relatively low temperature of approximately 25 degrees Celsius. Similar conditions are possible if an alcohol is reacted with the carboxylic acid attachment point.

It should be noted that the cost of the OS compounds can be high and so the use of a very efficient means for connecting the OS group to the polyimide polymer is desired. The carboxylic acid and amine group connection is very efficient in terms of OS attachment to the polyimide, and can be utilized to produce connection efficiencies in the range of about 73% to about 99%. After the OS group has been connected to the polyimide polymer, the product can be stored for use at a later time or it can be used immediately to produce polymer films or other desired polymer articles.

Polymer Uses

The polyimide polymer produced as described above can be used for several specific purposes. One important characteristic to consider is the color of the polymer. Polyimide polymers usually absorb the shorter wavelengths of light up to a specific wavelength, which can be referred to as the 50% transmittance wavelength (50% T). Light with wavelengths longer than the 50% transmittance wavelength are generally not absorbed and pass through the polymer or are reflected by the polymer. The 50% T is the wavelength at which 50% of the electromagnetic radiation is transmitted by the polymer. The polymer will tend to transmit almost all the electromagnetic radiation above the 50% T, and the polymer will absorb almost all the electromagnetic radiation below the 50% T, with a rapid transition between transmittance and adsorption at about the 50% T wavelength. If the 50% T can be shifted to a point below the visible spectrum, the polymer will tend to be very clear, but if the 50% T is in or above the visible spectrum, the polymer will be colored.

Generally, the factors that increase the solubility of a polymer also tend to push the 50% T lower, and thus tend to reduce the color of a polymer. Therefore, the factors that tend to reduce color in a polymer include flexible spacers, kinked linkages, bulky substituents, and phenyl groups which are aligned in different planes. The current invention provides a polyimide polymer with very little color.

A polyimide polymer with low color is useful for several applications. For example, if a polyimide is used as a cover in a multi layer insulation blanket on a satellite, the absence of color minimizes the amount of electromagnetic radiation that is absorbed. This minimizes the heat absorbed when the polymer is exposed to direct sunlight. Temperature variations for a satellite can be large, and a clear polyimide polymer, especially one that is resistant to AO degradation, provides an advantage.

Display panels need to be clear, so as not to affect the quality of the displayed image. The current invention is useful for display panels. In addition to optical clarity, a display panel should have low permeability to water and oxygen, a low coefficient of thermal expansion, and should be stable at higher temperatures. Thermal stability at 200 degrees centigrade is desired, but stability at 250 degrees centigrade is preferred, and stability at 300 degrees centigrade is more preferred. The surface of the current polymer can be glassified by exposing the surface to an oxygen plasma. This causes the OS groups to degrade to a glass type substance, which tends to coat and protect the surface of the polymer. This glass layer is thin enough that the layer bends and flexes without breaking. The glassified layer protects the polymer, and if the glassification layer is thick enough it could lower the permeability of the resultant polymer to water and oxygen. After the surface of the OS containing polymer is glassified, the permeability to oxygen and water vapor tends to be reduced, so the need for additional measures to lower this permeability are reduced or eliminated. One example of an additional measure which could be used to lower permeability is to thicken the glassification layer by depositing silicon oxide on the surface of the film. The effects of any traces of color in the polymer are minimized by supplying the polymer in a thin film, such as 1 mil thick. Thicker films are more durable, but they are also heavier and tend to have greater color effects, with the opposite being true for thinner films.

Polyimide films tend to be very strong, so they can be used as protective covers. For example, sheets of polyimide film can be placed over solar panels to protect the panels from weather and other sources of damage. For a solar panel to operate properly, it has to absorb sunlight. Polyimide polymers with low color are useful to protect solar panels and other items where a view of the protected object is desired.

EXAMPLES

Example 1

To a clean, dry, 5 liter (l) reactor equipped with an overhead stirrer, thermometer, and rubber septa were added 428.26 grams (g) APB, 81.88 g DBA, and 5591.08 g DMAc. The reactor was sealed and purged with dry argon while the solution was stirred vigorously with the overhead stirrer until the regents dissolved. To this solution was added 898.76 g 6FDA, and the resultant slurry was stirred for 24 hours, at which point 13.78 g phthalic anhydride (PA) was added to the reaction vessel and allowed to react for 4 hours. 501.18 g pyridine and 646.84 g acetic anhydride were added to this solution. The solution was stirred for 24 hours at room temperature, 24 hours at 70° C., then cooled to room temperature and precipitated in deionized water. The recovered polymer, which is referred to as SPC, was washed three times with water, and dried overnight in a vacuum oven at 100° centigrade (C) to yield 1374.81 g dry SPC (98% yield).

To a clean, thy, 5 L reactor equipped with an overhead stirrer, thermometer, and rubber septa were added 900.00 g of SPC prepared as described above and 5406.75 g dichloromethane (DCM). The reactor was sealed and purged with dry argon while the solution was stirred until homogeneous. 398.75 g aminopropylisobutyl polyhedral OS, 86.55 g Dicyclohexyl carbodiimide (DCC), and 1802.25 g DCM, and 891.00 g dimethylacetamide (DMAc) were added to this solution. The reaction was allowed to proceed for 24 hours at room temperature. The reactor was then cooled to 0° C. for three hours during which time the contents became heterogeneous. The contents of the reactor were drained, and filtered to remove the precipitate. The recovered polymer solution was precipitated into ethanol, recovered, and rinsed three times with deionized water. The OS-containing polymer was then dried in a vacuum oven at 110° C. for 48 hours to yield 1148.30 g dry polymer (94% yield).

Example 2

To a clean, dry, 5 L reactor equipped with an overhead stirrer, thermometer, and rubber septa were added 1050.00 g of SPC prepared in Example 1 and 4000.00 g tetrahydrofuran (THF). The reactor was sealed and purged with dry argon while the solution was stirred until homogeneous. 403.18 g aminopropylisobutyl polyhedral OS, 87.51 g DCC, and 2090.00 g THF were added to this solution. The reaction was allowed to proceed for 24 hours at room temperature. The reactor was then cooled to 0° C. for three hours during which time the contents become heterogeneous. The contents of the reactor were drained, and filtered to remove the precipitate. The recovered polymer solution was precipitated into deionized water, recovered, and rinsed three times with deionized water. The OS-containing polymer was then dried in a vacuum oven at 110° C. for 48 hours to yield 1207.90 g dry polymer (98% yield).

High Altitude Environments

High altitude environments can degrade many materials over a relatively short period of time. “High altitude environments” is defined to include altitudes above 20,000 feet, the stratospheric near space environment, and space. In these high altitude environments, there is less atmosphere than at lower altitudes to filter the UV light, or to filter the vacuum ultraviolet light (VUV). Direct exposure to the UV and VUV light emitted by the sun can be very damaging for many materials, including most polymers. On Earth's surface, much of the UV and VUV are filtered by the atmosphere, which makes this form of radiation far less intense and damaging than in high altitude environments. Atomic oxygen presents an additional hazard in space. Atomic oxygen is present in low earth orbit, and atomic oxygen can corrode and degrade materials quickly. Spacecraft orbiting in low earth orbits should have materials capable of withstanding exposure to atomic oxygen. The harsh nature of high altitude environments requires special materials to survive and function for an extended time period.

Atomic oxygen and UV radiation degrade most known hydrocarbon-based or organic materials, including high strength fibers used in composites such as polyaramide fibers. Other' materials degraded in high altitude environments include polyamides, polyesters, polybenzoxazole, epoxies, natural rubbers, synthetic rubbers, silicones, polyurethanes, polyureas, polyacrylates, polymethacrylates, polyolefins, polycarbonates, cellulosics, polyethylenes, ultra high molecular weight polyethylene, carbon fibers, and many more. Many polyimides are damaged by atomic oxygen, but are resistant to damage from UV radiation. Some metals, such as silver, are rapidly corroded by atomic oxygen. The thin or non-existent atmosphere in high altitude environments does not provide for convective cooling. Consequently, the temperature of the structure is largely affected by radiative heat transfer via the absorption and emission of light and other thermal radiation. Exposure to sunlight can result in rapid heating, and periods of shade can result in rapid cooling. The temperature differentials for an object present in the high altitude environment can be large, and the associated thermal expansion and thermal contraction can present design challenges for high altitude vehicles 24. Brittle materials can present extra challenges to avoid cracks and/or breaks.

Aircraft and/or spacecraft are often assembled on the ground, and then launched into service, typically from a runway, another aircraft, or a rocket, where the term “aircraft” includes airplanes, airships, and other objects that travel through the atmosphere, as seen in FIG. 16. The term “spacecraft” includes vehicles 24 intended for use above the earth's atmosphere. In this description, the term “vehicles 24” includes both aircraft and spacecraft. This subjects the aircraft or spacecraft to stress and vibration from ground handling and assembly, and the thrust of the launch. The cost of launching a vehicle 24, cargo, and payload into service is relatively high, so products which reduce weight are desired. Once in service, the airplane, airship, satellite, or other spacecraft can be placed in orbit, or it can be moved to various destinations as desired.

Many events can damage various components in a high altitude environment. The various components may be re-arranged or re-positioned once in place, such as when a satellite unfurls an array of solar panels. Different components can be damaged during any re-positioning operations, which may occur for various reasons. Many vehicles 24, once in service, have a relatively large surface area. The large surface area increases the chances of impact from a micro meteor, micrometeorite, or other debris. Stresses from the launch can also damage a component or part. A component on an aircraft or spacecraft has many opportunities to be damaged, including stress from the launch, pre-launch damage, re-positioning issues, and impacts from objects once in service. Besides damages from impacts, launch stresses, and ground handling, vehicles 24 in high altitude environments are exposed to relatively high levels of UV radiation, and may also be exposed to atomic oxygen.

Radiant Heat Transfer Fundamentals

Visible light is one form of electromagnetic radiation. Other types of electromagnetic radiation include infrared, ultraviolet, radio waves, microwaves, X-rays and gamma rays. The different types of electromagnetic radiation are differentiated by wavelength, where ultraviolet light has wavelengths between approximately 100 nanometers and 400 nanometers, visible light has wavelengths between approximately 400 nanometers and approximately 750 nanometers, and infrared radiation has wavelengths between approximately 750 nanometers and 300 micrometers.

Radiant heat is transferred from one object to another by the emission or absorption of electromagnetic radiation. A significant portion of radiant heat transfer of objects in high altitude environments involves infrared radiation (also referred to as infrared light), visible light, and ultraviolet light. A “radiant heat transfer environment” is defined as an environment where radiant heat transfer is the dominant form of heat transfer, so conduction and convection are less important to overall heat transfer than radiant heat transfer. High altitude environments separated from land can be radiant heat transfer environments, because there is no object present for conduction and the air can be thin enough that convective heat transfer is not very effective. High altitude environments include space, where there is no air and radiant heat transfer tends to be dominant. Although the other types of electromagnetic radiation also transfer heat, most radiant heat is transferred with ultraviolet, visible, and infrared light. Often, a primary heating source is the sun, wherein the solar radiation comprises ultraviolet, visible, and infrared radiation. Other heat sources are the earth, the moon, and other structural objects within a local vicinity. The difference in the rate of absorption of electromagnetic radiation by an object as compared with the rate of emission of electromagnetic radiation by the same object in a radiant heat transfer environment, such as many high altitude environments, affects the temperature of the object. So, if an object absorbs electromagnetic radiation at a rate faster than it emits in a radiant heat transfer environment, the object tends to increase in temperature, and vice versa.

Incident electromagnetic radiation from various sources can interact with the object in one or more of three modes: it can be absorbed by the object; it can be reflected by the object, or it can be transmitted through the object. Different objects have different tendencies to absorb, reflect, or transmit electromagnetic radiation, and an object can have different tendencies to absorb, reflect, or transmit electromagnetic radiation of different frequencies. For example, certain polyimides as described above tend to transmit ultraviolet and visible light with a wavelength higher than the 50% transmittance, and the same polyimide tends to absorb electromagnetic radiation with a wavelength shorter than the 50% transmittance. If a polyimide has a 50% transmittance of about 399 nanometers or less, the polyimide will appear to exhibit low or no color to the human eye. However, if a polyimide has a 50% transmittance greater than about 399 nanometers, the polyimide may appear colored. Polyimides which appear colored tend to absorb more ultraviolet and visible radiant heat than clear polyimides.

The tendency of an object to absorb, reflect, or transmit electromagnetic radiation of a specific frequency can be measured. For a given object, the absorptivity is the ratio of incident electromagnetic radiation which is absorbed divided by the total incident electromagnetic radiation, where incident electromagnetic radiation is the total amount of electromagnetic radiation striking the object. The incident electromagnetic radiation can be measured for a set wavelength, or for a range of wavelengths. In the same manner, for a given object the reflectivity is the ratio of incident electromagnetic radiation reflected divided by the total incident electromagnetic radiation, and the transmissivity is the ratio of the incident electromagnetic radiation transmitted, or passing through the object, divided by the total incident electromagnetic radiation. The absorptivity, reflectivity, and transmissivity for a given object at a set wavelength or range of wavelengths add to one, because all incident electromagnetic radiation is either absorbed, reflected, or transmitted. Since a primary heating source is often the sun, the solar absorptivity, solar reflectivity, and solar transmissivity are important parameters that affect the temperature of objects, especially objects in high altitude environments.

Objects also emit electromagnetic radiation. The tendency of an object to emit electromagnetic radiation is referred to as the emissivity of the object, where “emissivity” is the ratio of electromagnetic radiation emitted by an object per unit area divided by the electromagnetic radiation emitted by a black body per the same unit area at the same temperature. A black body is a reference which emits a maximum amount of electromagnetic radiation at a given temperature. Reflected and transmitted electromagnetic radiation do not significantly change the temperature of an object, but the rate at which electromagnetic radiation is absorbed as compared with the rate at which electromagnetic radiation is emitted affects the temperature of an object in a radiant heat transfer environment, such as many high altitude environments. The emissivity of an object in the wavelengths from approximately 1 micron to approximately 35 microns is often referred to as the thermal emissivity of an object. The thermal emissivity is therefore an important parameter that affects the temperature of objects.

An object will tend to approach an equilibrium temperature, so objects hotter than the equilibrium temperature tend to cool down and vice versa. The equilibrium temperature in a radiant heat transfer environment is the temperature at which the rate of energy absorption is equal to the amount of energy emission. In a radiant heat transfer environment, the rate of heat emission depends primarily on the temperature of the object and the emissivity of the object, and the rate of heat absorbed depends on the absorptivity of the object and the amount of incident electromagnetic radiation. Both a higher emissivity and a lower absorptivity tend to result in an object having a lower equilibrium temperature.

Several factors can influence the transmissivity, reflectivity, absorptivity, and emissivity of objects. Many of these factors depend on the surface of the object. For example, clear glass has a high solar transmissivity, but a black coating may decrease the solar transmissivity of the object. In general, black objects tend to have higher solar absorptivities than white objects. An object with one surface which is black and the other surface which is white may have different solar absorptivities when measured at the different surfaces. The same is true for solar reflectivity, solar absorptivity, and thermal emissivity, so these properties can depend on the surface, and these properties can vary on different surfaces and different parts of an object.

Some properties depend on the thickness of a material. A thin layer of polymer or glass may exhibit a high solar transmissivity, but a thicker layer of the same material may begin to appear colored and exhibit a lower solar transmissivity. Similarly, thermal emissivity for uncoated polymeric layers tends to increase with increased thickness of the polymeric layer, so thicker layers of polymer films tend to emit more electromagnetic radiation than a thinner film layer of the same polymer material at the same temperature. In a radiant heat transfer environment in which the dominant heating source is the sun, the thermal emissivity, solar absorptivity, solar transmissivity, and solar reflectivity affect the temperature of the object. Many metallic materials, including aluminum, silver, and gold, and many surfaces coated with metallic materials tend to exhibit lower solar absorptivity than nonmetal materials. However, many metals also exhibit lower thermal emissivities than nonmetal materials, which means that the rate of electromagnetic radiation energy emitted between approximately 1 micron and approximately 35 microns is lower than the rate of electromagnetic radiation energy emitted in the same bandwidth for nonmetal materials. For many metals, the solar absorptivity is higher than the thermal emissivity, so without additional temperature control, metals will tend to increase in temperature when exposed to sunlight in a radiant heat transfer environment, including many high altitude environments. Many polymers tend to have a high thermal emissivity, and clear polymers can have a low solar absorptivity and high solar transmissivity. Transmissivity of a polymer or other materials is frequency dependent, so visible light may readily pass through the polymer but UV light may not. For many clear polymer materials in a radiant heat transfer environment, its thermal emissivity is higher than its solar absorptivity. Therefore, the temperature increase that occurs upon exposure to sunlight is lower than that of many uncoated metals in the same environment.

Reflective Film

A reflective film 10 can be used for thermal control in radiant heat transfer environments, as seen in FIG. 17. The reflective film 10 can have a front side 12 and a back side 14, and it can be flexible or rigid. The reflective film 10 includes a polymeric layer 16 and a reflective component 17. The polymeric layer 16 includes a polyimide as described above. In one embodiment, the reflective component 17 is within the polymeric layer, as shown in FIG. 17, with continuing reference to FIG. 16. In an alternative embodiment referred to as a second surface mirror 11, there is a reflective layer 18 adjacent to the polymeric layer 16, as shown in FIGS. 18 and 19, and the electromagnetic radiation passes through the polymer until reaching the reflective layer 18. In the second surface mirror 11 embodiment, the reflective component 17 is within the reflective layer 18, but there can also be reflective component 17 within the polymeric layer 16 as well.

The second surface mirror 11 is one embodiment of a reflective film 10, and the reflective layer 18 includes a reflective component 17. Reflective components 17 incorporated into the polymeric layer 16 are referred to in this description as “additives” to reduce confusion with reflective components 17 within a reflective layer 18. Additives 19 are referred to with the reference number 19, but it is to be understood that all additives 19 discussed in this description are also described by the term reflective components 17. A reflective film 10 with additives 19 in the polymeric layer 16 may not include a reflective layer 18. The additives 19 can prevent most or all incident electromagnetic radiation from passing through the polymeric layer 16, so the reflective layer 18 may not be needed. In fact, if a reflective layer 18 is adjacent to a polymeric layer 16 including additives 19, the absorptivity of the reflective film 10 can be increased due to the internal reflectance between the additive 19 and the reflective layer 18.

There is another embodiment of a reflective film 10 referred to as a first surface mirror. The structure of the first surface mirror can be the same as the second surface mirror 11, except the reflective layer 18 faces the source of electromagnetic radiation. This can mean the back side 14 faces the source of electromagnetic radiation for a first surface mirror, and the front side 12 faces the source of electromagnetic radiation for a second surface mirror 11. The reflective surface 18 of a first surface mirror tends to reflect much of the incident electromagnetic radiation without the radiation passing through the polymeric layer 16. The reflective component 17 in the reflective layer 18 can be the same or different than any reflective component 17 which may be within the polymeric layer 16. In yet additional embodiments, there can be a reflective layer 18 sandwiched between two or more polymeric layers 16 or there can be more than one polymeric layer 16 positioned on the same side of the reflective layer 18.

A wide variety of reflective components 17 can be incorporated into the polymeric layer 16. Some examples of additives 19 include titanium dioxide (TiO₂), silicon dioxide (Si_O2), tin oxide (SnO), antimony oxide (Sb₂O₃), silver, aluminum, gold, tin, platinum, and palladium, and the additives 19 can be present as particles, flakes, strands, powders, or other forms. Additives 19 can be included in the polymeric layer 16 in various ways. Non-limiting examples include dissolving the additives 19 in the polymer, suspending the additives 19 in the polymer, or entrapping the additives 19 in a polymeric matrix. There are other additives 19 which can be included in a polymeric layer 16 which do not significantly change the reflectivity of the polymeric layer 16. These other additives 19 can modify many different polymer properties, as desired.

Additives 19 can impact the physical properties of the polymeric layer 16, and different additives 19 can have different effects. For example, aluminum flakes tend to increase the solar reflectivity of the polymeric layer 16 and decrease the thermal emissivity of the polymeric layer 16. In contrast, titanium dioxide tends to increase the solar reflectivity of the polymeric layer 16 and increase the thermal emissivity of the polymeric layer 16. Additives 19 can be incorporated into the polymeric layer 16 in various concentrations. It has been found that additive 19 to polymer ratios of 0.1 to 1 can reduce solar absorptivity, but in some embodiments the additive 19 to polymer ratio can be as high 10 to 1. In some instances, an additive 19 to polymer ratio greater than 1 to 1 can result in degraded reflective film 10 properties such as strength or elasticity. It is also possible to use additive 19 to polymer ratios higher or lower than those mentioned here.

The reflective layer 18 of the first or second surface mirror 11 includes a reflective component 17 and tends to have a high solar reflectivity. The reflective component 17 in the reflective layer 18 can include certain metals, such as aluminum, silver, gold, and other metals. The reflective layer 18 can also include non-metallic materials with a relatively high reflectivity. The reflective layer 18 can be a few angstroms thick, or it can be thicker or thinner as desired. The reflective layer 18 can be relatively thin and still reflect most incident electromagnetic radiation. The reflective layer 18 can be the back side 14 of the first or second surface mirror 11, but it is also possible to apply one or more coating layers 20 to the reflective layer 18 so a coating layer 20 becomes the back side 14 of the first or second surface mirror 11. A coating layer 20 can also be applied to the back side 14 of a reflective film 10 which does not have a reflective layer 18. The side of the reflective layer 18 facing the source of electromagnetic radiation should be reflective, but in some embodiments it is not necessary or even desirable for the side of the reflective layer 18 facing away from the electromagnetic source to be reflective.

The polymeric layer 16 can be the outermost layer on the front side 12 of the reflective film 10, but it is also possible to provide a cover layer 22 over the polymeric layer 16 as desired. The cover layer 22 can provide protection from environmental hazards, additional thermal emissivity, or other properties to the front side 12. For example, the cover layer 22 can provide spectrally selective reflectivity or absorptivity, such as by reflecting wavelengths below 400 nanometers, or by reflecting wavelengths between 350 nanometers and 550 nanometers, or by reflecting wavelengths greater than 5 microns, or by absorbing wavelengths below 300 nm. The cover layer 22 can provide protection to the underlying polymer structure, including the polymeric layer 16, from environmental hazards by reducing the exposure of the polymer structure to AO, ozone, dirt, debris, or other hazards.

In another embodiment, the polymeric layer 16 can form the front side 12, so the polymeric layer 16 can provide the surface properties of the front side 12. Preferably, the polymeric layer 16 and any cover layers 22 present do not absorb significant amounts of solar ultraviolet, visible, or infrared light to minimize solar heating gain. In one embodiment, the polymeric layer 16 and any cover layers 22 present are reflective, but in another embodiment the polymeric layer 16 and/or any cover layers present have a high transmissivity, appear clear, and allow most heat-producing electromagnetic radiation to pass through with low or no absorption. In the second surface mirror 11 embodiment where the polymeric layer 16 and any cover layers 22 present have a low solar absorptivity, incident electromagnetic radiation striking the front side 12 tends to pass through the polymeric layer 16 with low or no absorption to the reflective layer 18, reflect from the reflective layer 18, pass back through the polymeric layer 16 with low or no absorption, and pass out of the second surface mirror 11, as shown in FIG. 19, with continuing reference to FIGS. 15-18. In many applications, minimizing the weight of a reflective film 10 is desirable, so reducing or eliminating any cover layer 22 and/or any coating layer 20 may reduce total weight and therefore be desirable.

In the first or second surface mirror 11 embodiment, the low solar absorptivity and high solar transmissivity of the polymeric layer 16 combined with the high solar reflectivity of the reflective layer 18 makes the solar absorptivity of the first or second surface mirror 11 low, at least from the side facing the electromagnetic radiation source. Providing the polymeric layer 16 with the reflective layer 18 also tends to produce a relatively high thermal emissivity. In any embodiment, the reflective film 10 can have a thermal emissivity to solar absorptivity ratio of greater than 1, so the thermal emissivity is actually greater than the solar absorptivity. This allows for the reflective film 10 to operate in a radiant heat transfer environment at a lower temperature than a similarly configured reflective film 10 in the same environment with a thermal emissivity to solar absorptivity ratio of less than 1. Reflective films 10 without the polymeric layer 16 or with a polymeric layer 16 that exhibits lower thermal emissivity are examples of reflective films 10 which may have a thermal emissivity to solar absorptivity ratio of less than 1. A reflective film 10 can be used with aircraft or spacecraft for cooling purposes in radiant heat transfer environments based on the emissivity to absorptivity ratio being greater than 1. This type of cooling is referred to as passive cooling, because there are no moving parts, and there is no fluid flow needed for the cooling. Passive cooling can be the primary type of cooling for aircraft and spacecraft, so effective passive cooling surfaces are desirable.

The polyimide described above can be included in the polymeric layer 16. The polymeric layer 16 can be entirely made of the polyimide described above, but it is also possible for other materials to be present in the polymeric layer 16, such as fillers, additives 19, or even other polymers. Several properties of the polyimide described above can be advantageous for the polymeric layer 16. The polyimide can make a polymeric layer 16 that is strong enough to support the reflective layer 18, so no other materials is needed for overall strength. The polyimide can be flexible, so the reflective film 10 can be flexible. A flexible reflective film 10 can be easier to store and deploy than a rigid one, and may be able to withstand stress without breaking better than a rigid material. The polyimide can provide protection from UV and VUV light, and atomic oxygen. (AO), which can reduce the need for a cover layer 22 and may reduce the weight of the reflective film 10. The polymeric layer 16 can have a 50% transmittance of 399 nanometers or less, so the polymeric layer 16 appears essentially clear to the human eye and does not absorb significant heat from solar electromagnetic radiation.

The polymeric layer 16 provides additional benefits as well. The polyimide is soluble, so it can be dissolved in a solvent and applied in solution. The polymeric layer 16 can then be cured by simply evaporating the solvent, which can be at lower temperatures than those necessary for imidizing a polyamic acid. For example, some solvents such as tetrahydrofuran, methylene chloride, chloroform, acetone, methyl ethyl ketone, methyl isobutyl ketone, monoglyme, diglyme, and many other solvents can be evaporated from a polymeric solution at a temperature not exceeding 150 degrees centigrade, so the polymeric layer 16 can be cured at a temperature not exceeding 150 degrees centigrade. A solvent can be evaporated at a temperature below the solvent boiling point, so low temperatures can be used to evaporate the solvent and cure the polymeric layer 16. Generally, lower temperatures require a longer period to evaporate solvent, so a lower cure temperature may increase the required cure time. For example, a polymeric layer 16 can be cured at a temperature not exceeding 100 degrees centigrade, or at a temperature not exceeding 50 degrees centigrade, or at a temperature not exceeding 23 degrees centigrade, as desired. Other factors can also impact the cure time, such as air flow over the curing polymeric layer 16 and the polymeric layer thickness.

The current invention has additional favorable properties. In some embodiments, the polyimide can be easily transported and stored as a solid for extended periods, which provides simpler logistics than for a relatively unstable polyamic acid. Oligomeric silsesquioxane is incorporated into the polyimide, so the polymeric layer 16 will exhibit self-healing upon exposure to atomic oxygen. The OS forms a passivating silica layer when exposed to atomic oxygen. If this OS layer is scratched or broken, the exposed polyimide below forms another passivating silica layer when exposed to atomic oxygen. This helps to minimize harm caused by scratches or other damage to the polymer layer 16. Additionally, the polyimide will not yellow as rapidly as many other polymers when exposed to UV, so the effective life of the reflective film 10 may not be shortened as much as with other polymers. Yellowing is a gradual increase in the color of a polymer over time.

Reflective Film Use on Vehicles

The reflective film 10 can be used for temperature control on vehicles 24 in high altitude service. The back side 14 of a reflective film 10 can be placed in contact with certain components in the vehicle 24, so the reflective film 10 can serve as a radiator for cooling. Some components in a vehicle 24 can generate heat, such as certain electronic equipment or devices with moving parts, so a cooling surface can be desirable. Also, surfaces of vehicles 24 can absorb heat from the sun, so cooling can be helpful. In other embodiments, the reflective film 10 can be a structural part of the vehicle, such as a face of a satellite or the skin of an airship. For airships, which are also called dirigibles, blimps, and zeppelins, heating of the gas inside the airship can increase pressure and if left unmitigated can cause a rupture. Alternatively, heating of gases inside an airship can require stronger material to prevent a rupture, and stronger materials are often heavier. In use, the first side 12 of the reflective film 10 generally faces away from the vehicle 24. Second surface mirrors 11 can also be used to reflect light without overheating, such as a mirror used to concentrate sunlight on a solar panel.

A reflective film 10 can degrade over time from exposure to contaminants and the environment. Yellowing from exposure to UV is one example of a reflective film 10 degrading over time, but corrosion from atomic oxygen can reduce the film thickness and thereby it's thermal emissivity, and degrade the thermal control performance of the reflective film 10. Collection of dirt, exhaust, and other particles on the reflective film front side 12 can also darken and thereby degrade the reflective film 10. Providing a polymeric layer 16 resistant to attack from atomic oxygen and degradation from UV can extend the life of the reflective film 10. Often, a gradual degradation of a reflective film 10 is planned for by design engineers, and extra cooling capacity is included in the vehicle 24 for decreased cooling capacity from degraded reflective film 10. This requires different levels of cooling for different components in a vehicle 24 at different points in the life of the vehicle 24, because the initial cooling capacity can be greater than needed and can actually cool a component below the desired operating temperature. Providing a reflective film 10 which degrades at a slower rate can simplify design considerations, and can save on weight be requiring less surface area devoted to cooling from reflective film 10.

The reflective film 10 can be used for thermal control on a vehicle 24, where the reflective film 10 includes a polymeric layer 16 and may include a reflective layer 18. The polyimide polymer described above is included in the polymeric layer 16. Therefore, the method for supplying thermal control components includes providing a reflective film 10. The polymer included in the polymeric layer 16 is produced by reacting at least one acid monomer with at least one diamine monomer to form a polyamic acid polymer backbone, where the monomers are selected such that the polyamic acid polymer backbone includes a non-terminal attachment point. Next, the polyamic acid polymer backbone can be imidized to produce the polyimide polymer. An oligomeric silsesquioxane (OS) compound can be attached to the attachment point on the polyimide polymer backbone. The OS compound can be attached by reacting a functional group connected to the OS compound with the attachment point. The attachment point can be a carboxylic acid, and the functional group can be an amine, an alcohol, an epoxy, or other functional groups. In some embodiments, the polymer backbone, in either the polyamic acid form or the polyimide form, is isolated a maximum of one time. Other embodiments of polyimides with OS are also possible.

This provides a reflective film 10 as described. The polymeric layer 16 includes a polyimide polymer with at least one non-terminal phenyl group. The non-terminal phenyl group is connected to a linkage, which can be an amide linkage formed by the reaction of the carboxylic acid attachment point with an amine functional group connected to the OS. The linkage can also be an ester linkage formed by the reaction of the carboxylic acid attachment point with an alcohol functional group connected to the OS. The polyimide polymer may also contain a mixture of amide and ester linkages by adding OS with both alcohol and amine functional groups, either simultaneously or sequentially.

CONCLUSION

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed here. Accordingly, the scope of the invention should be limited only by the attached claims.

Claims

1. a method for supplying thermal control components comprising: (a) preparing a reflective film having a front side, an emissivity and an absorptivity, where the emissivity is greater than the absorptivity from the front side, where the reflective film includes a polymeric layer, and where the polymeric layer includes a polyimide polymer produced by: (i) reacting at least one acid monomer with at least one diamine monomer to form a polyamic acid backbone, where the monomers are selected such that the polyamic acid backbone includes a non-terminal attachment point;(ii) after step (i), imidizing the polyamic acid backbone to form a polyimide polymer backbone; and(iii) after step (ii), attaching an oligomeric silsesquioxane (OS) compound to the attachment point on the polyimide polymer backbone, such that the OS compound is connected to the polyimide polymer backbone.

2. The method of claim 1 where the polymeric layer includes a reflective component within the polymeric layer.

3. The method of claim 1 where the reflective film further comprises a reflective layer positioned adjacent to the polymeric layer.

4. The method of claim 1 where the polymer backbone is isolated a maximum of one time.

5. The method of claim 1 further comprising supplying the reflective film for use with a vehicle.

6. The method of claim 1 where the monomers are selected such that the polymeric layer is soluble.

7. The method of claim 1 where the OS is in the form of a polyhedron.

8. The method of claim 1 where the attachment point is comprised of a carboxylic acid, where a functional group is connected to the OS, and the functional group is reacted with the attachment point to attach the OS to the polyimide polymer backbone, and where the functional group is selected from the group consisting of an amine and an alcohol.

9. The method of claim 1 where the polymeric layer produces a layer of silicon dioxide when exposed to atomic oxygen.

10. The method of claim 1 where the polymeric layer has a maximum 50% transmissivity of 399 nanometers.

11. A reflective film comprising: (a) a reflective component;(b) a polymeric layer contacting the reflective component, the polymeric layer including a polyimide polymer comprising: (b1) a polyimide polymer backbone, where the polymer backbone includes at least one non-terminal phenyl group;(b2) a linkage selected from the group consisting of: (i) an amide linkage comprising a carbonyl carbon and a linking nitrogen atom, wherein the carbonyl carbon is connected by a single bond to the non-terminal phenyl group in the polymer backbone and the linking nitrogen atom is connected by a single bond to the carbonyl carbon;(ii) an ester linkage comprising a carbonyl carbon and a linking oxygen atom, wherein the carbonyl carbon is connected by a single bond to the non-terminal phenyl group in the polymer backbone and the linking oxygen atom is connected by a single bond to the carbonyl carbon; and(iii) a combination of the amide linkage and the ester linkage;(b3) at least one oligomeric silsesquioxane (OS) compound comprising silicon atoms and oxygen atoms connected together; and(b4) an organic substituent connected to each silicon atom of the OS compound, wherein at least one organic substituent is connected to the linkage.

12. The reflective film of claim 11 where the OS is in the form of a polyhedron.

13. The reflective film of claim 11 where the reflective component is within the polymeric layer.

14. The reflective film of claim 11 further comprising a reflective layer positioned adjacent the polymeric layer, where the reflective layer includes the reflective component.

15. The reflective film of claim 14 where the polymeric layer protects the reflective layer from atomic oxygen.

16. The reflective film of claim 11 further comprising a front side, a solar absorptivity, and a thermal emissivity, and where the thermal emissivity is greater than the solar absorptivity from the front side.

17. A reflective film comprising: (a) a reflective component;(b) a polymeric layer contacting the reflective component, the polymeric layer including a polyimide polymer having a backbone including at least one unit represented by the following formula:

18. The reflective film of claim 17, where the polyimide polymer includes at least one unit selected from the group consisting of the compounds represented by the following formulas:

19. The reflective film of claim 17 where the reflective component is within the polymeric layer.

20. The reflective film of claim 17 further comprising a reflective layer adjacent to the polymeric layer, where the reflective layer includes the reflective component.