Patent Description:
Transparencies used for modern aircraft often require a protective coating or film located on the outer surface to prevent damage to fragile underlying metal or ceramic or polymeric conductive coatings, such as gold, silver or indium tin oxide (ITO) or carbon nanotube (CNT). Such surface protective coating and the underlying conductive layers form a multilayer electrostatic dissipative (ESD) coating system to provide the transparencies with erosion resistance, abrasion/scratch resistance, electrostatic dissipation capability and environmental durability. Organic polyurethane coating and sol-gel based polysiloxane coating are the two major families of such surface protective coatings used for these applications with respect to their super light transmission and high optical clarity.

Polyester-based polyurethane coating possesses elastomeric behaviors demonstrating excellent flexibility, toughness, tear, erosion resistance, and impact resistance while it is low hardness and thus lacks abrasion/scratch resistance. Sol-gel based polysiloxane coating, on the other hand, bears glassy behaviors exhibiting high hardness, abrasion resistance, and scratch resistance while it is brittle and thus lacks erosion resistance and impact resistance. Such property differences result in application differentiations of polyurethane coating and polysiloxane coating. Polyurethane coating is selected as surface protective coating of ESD coating system for such transparencies as military canopies, pilot windshields, and cockpit windows where high erosion resistance is a necessity. Polysiloxane coating is preferably used as surface protective coating for passenger cabin windows or side windows where erosion resistance is less demanding. Therefore, either the organic polyurethane coating or the sol-gel based polysiloxane coating is considered not fully satisfactory as a universal surface protective coating of ESD coating system for transparencies used for modern aircraft. This is the current state-of-the-art with surface protective coating for aircraft transparencies. It can readily be appreciated that there is a need for a transparent coating composition having the combined advantageous properties of good erosion resistance and impact resistance as well as good abrasion resistance and scratch resistance, in addition to electrostatic dissipation capability and environmental durability.

<CIT> relates to two-component siloxane-based coatings containing polymers with urea linkages and terminal alkoxysilanes. <CIT> relates to siloxane-based coatings containing polymers with urea linkages and terminal alkoxysilanes. <CIT> relates to durable, electrically conductive transparent polyurethane compositions and methods of applying same.

The present invention provides a two-part curable coating composition comprising: (A) a composition comprising an alkoxysilane terminated polyester urethane prepolymer, and (B) a composition comprising a silanol terminated polysiloxane prepolymer. In one aspect, the two-part curable coating composition comprises:.

The composition preferably further comprises one or more additives selected from: one or more of the following: solvent; urethane forming catalyst; antistatic agent; stabilizer selected from one or more of a UV absorber, light stabilizer and thermal stabilizer; and surface active agent selected from one or more of a flow or levelling agent and surfactant. Preferably, the composition comprises at least two solvents, a urethane forming catalyst, an antistatic agent, an UV absorber, a light stabilizer, a thermal stabilizer, and a surfactant.

The alkoxysilane-terminated polyester urethane prepolymer (A) may be obtained by a process comprising: (i) reacting an aliphatic diisocyanate with a polyester diol to form an isocyanate-terminated polyester urethane prepolymer; and (ii) silanization of the isocyanate-terminated polyester urethane prepolymer with an amino-functional alkoxysilane.

The silanol terminated polysiloxane prepolymer (B) may be obtained by sol-gel process of an alkoxysilane selected from the group consisting of: a trialkoxysilane, an alkylorthosilicate, or a bis(trialkyloxysilyl)alkane; and preferably an alkyl orthosilicate.

The present invention further provides a polyurethane-polysiloxane hybrid coating composition (PUPSHCC) precursor, which is obtainable by a process comprising mixing or compounding the composition (A) and composition (B). A polyurethane-polysiloxane hybrid coating composition (PUPSHCC) according to the present invention is obtainable by a process comprising thermally curing the PUPSHCC precursor.

A further aspect of the present invention provides a process for preparing a two-part curable coating composition comprising: (<NUM>) preparing an alkoxysilane-terminated polyester urethane prepolymer (A) by a process comprising (i) reacting an aliphatic diisocyanate with a polyester diol to form an isocyanate-terminated polyester urethane prepolymer; and (ii) silanization of the isocyanate-terminated polyester urethane prepolymer with an amino-alkoxysilane as described herein; and (<NUM>) preparing a silanol terminated polysiloxane prepolymer (B) by sol-gel process of an alkoxysiloxane with at least one acid, in the presence of a solvent comprising an alcohol and water as described herein.

The present invention additionally provides the use of the two-part curable coating composition as a coating for a substrate such as in aircrafts, particularly aircraft transparencies. The invention further provides a substrate which is coated with the polyurethane-polysiloxane hybrid polymer coating composition of the invention.

In a further aspect, a substrate which is coated with the polyurethane-polysiloxane hybrid polymer coating composition of the invention may be prepared by a process comprising applying the polyurethane-polysiloxane hybrid coating composition precursor disclosed herein to the surface of a substrate, and thermally curing the coating composition precursor.

The present invention utilizes the erosion- and impact-resistance properties of organic polyurethane coatings with the abrasion and scratch-resistance properties of sol-gel based polysiloxane coatings by providing unique, modified hybrid compositions. The compositions of the present invention can advantageously be applied as a single coating onto the outer surface of a substrate such as an aircraft transparency, to provide a transparency having a surface protective layer with desirable erosion resistance and abrasion resistance properties, in addition to electrostatic dissipation capability and environmental durability.

The present invention provides a polyurethane-polysiloxane hybrid coating composition (PUPSHCC) having advantageous properties of erosion resistance, impact resistance as well as abrasion resistance and scratch resistance, in addition to the electrostatic dissipation capability and environmental durability, and thus is highly suitable as a surface coating, particularly as a universal surface protective coating of ESD coating system, for transparencies such as those used in modern aircraft.

Unless otherwise indicated, the term "hydrocarbyl" used herein throughout refers to monovalent group formed by removal of a hydrogen from a hydrocarbon group. The term "hydrocarbyl" includes a saturated or unsaturated hydrocarbyl group, which may be aliphatic, cyclic, or aromatic. The term "hydrocarbyl" preferably refers to monovalent hydrocarbyl radicals containing <NUM> to <NUM> carbon atoms, particularly <NUM> to <NUM> carbon atoms, more particularly <NUM> to <NUM> carbon atoms, and most particularly <NUM> to <NUM> carbon atoms, which may be linear, branched, cyclic, saturated and unsaturated species, such as alkyl, alkenyl, cyclic alkyl or cyclic alkenyl, aryl, alkaryl or aralkyl groups. Particularly preferred hydrocarbyl groups are methyl and ethyl.

The term "hydrocarbylene" used herein, unless otherwise indicated, refers to a divalent group formed by removal of two hydrogens from a hydrocarbon group. The term "hydrocarbylene" includes a saturated or unsaturated hydrocarbylene group, which may be aliphatic, cyclic or aromatics. The term "hydrocarbylene" preferably refers to divalent hydrocarbylene radicals preferably an alkylene or arylene group containing <NUM> to <NUM> carbon atoms, particularly <NUM> to <NUM> carbon atoms, more particularly <NUM> to <NUM> carbon atoms, and most particularly <NUM> to <NUM> carbon atoms, which may be linear, branched, cyclic, saturated and unsaturated species, such as alkylene, alkenylene, cyclic alkylene or cyclic alkenylene, arylene, alkarylene or aralkylene groups. Particularly preferred hydrocarbyl groups are methylene, ethylene and propylene, and especially propylene.

The term "urethane grade solvent" refers to a solvent having <NUM> wt. % or less of water. The invention generally provides a two-part curable coating composition comprising: (A) a composition comprising an alkoxysilane terminated polyester urethane prepolymer and (B) a composition comprising a silanol terminated polysiloxane prepolymer obtainable by sol-gel process of an alkoxysilane. The compositions (A) and (B), when compounded to form a polyurethane-polysiloxane hybrid coating composition (PUPSHCC) precursor, which can be applied to a surface of a substrate, and cured, optionally in the presence of a catalyst, in order to form a polyurethane-polysiloxane hybrid coating composition having advantageous properties such as one or more of high erosion resistance, high abrasion or scratch resistance and impact resistance.

In one aspect, the invention provides a two-part curable coating composition comprising:.

In any aspect or embodiment of the present invention, preferably R<NUM> at each occurrence is the same or different, and each is independently selected from: a C<NUM> to C<NUM> alkyl, or a C<NUM> to C<NUM> alkyl group. Preferably R<NUM> at each occurrence is the same, and each represents a C<NUM> to C<NUM> alkyl, or a C<NUM> to C<NUM> alkyl group, and preferably a methyl or ethyl group.

In any aspect or embodiment, the group Y is preferably H. In order to provide additional advantageous properties to the coating compositions, various additives may be added to the compositions. The additives may be added to the compositions (A) and/or (B), either during the process for their preparation, or after their preparation. Preferably, the compositions (A) and (B) include at one or more of the following: one or more solvents; a urethane forming catalyst; an antistatic agent; one or more stabilizers selected from one or more of a UV absorber, light stabilizer and thermal stabilizer; and surface active agent selected from one or more of a flow or levelling agent and surfactant.

In any embodiment of the present invention, composition (A) preferably further comprises an alkoxysilane terminated polyester urethane prepolymer, at least one solvent, a flow or levelling agent, at least one stabilizer, a urethane forming catalyst and a surface additive. In a preferred embodiment, the solvent, flow or levelling agent, stabilizer, urethane forming catalyst and surface additive are added during the reaction to prepare the alkoxysilane terminated polyester urethane prepolymer in component (A).

In any embodiment of the present invention, composition (B) further comprises a silanol terminated polysiloxane prepolymer obtainable by sol-gel process of an alkoxysilane selected from a compound of general formula (II), (III) or (IV), at least one solvent, and an antistatic additive. In a preferred embodiment, the solvent is present in the reaction mixture, and the antistatic additive is added after the sol-gel process.

Preferably, in any embodiment of the present invention, composition (A) comprises an alkoxysilane terminated polyester urethane prepolymer, at least one solvent, a flow or levelling agent, at least one stabilizer, a urethane forming catalyst and a surface additive; and composition (B) further comprise at least one solvent, and an antistatic additive.

The various additives are discussed in more detail below.

The compositions of the present invention preferably comprise solvents as the composition media and carrier. The solvents act as a media to improve flow properties and to facilitate the application of the compositions as a coating to the surface of a substrate. Urethane grade solvents are preferably used.

In any aspect or embodiment of the present invention, the two-part curable composition preferably comprises at least one solvent, and more preferably comprises two or more solvents. Preferably, composition (A) comprises an aprotic solvent and a protic solvent. Particularly suitable aprotic solvents may be selected from the group consisting of ethyl <NUM>-ethoxypropionate (EEP), n-pentyl propionate (nPP), <NUM>-butoxyethyl acetate (BEA), di-isobutyl ketone (DIBK), and methyl isobutyl ketone (MIBK), or a combination thereof. EEP is a particularly preferred aprotic solvent. Preferred protic solvents include a hydroxyketone, preferably a β-hydroxyketone, and more preferably diacetone alcohol (DAA). The solvents may be present as a reaction solvent during the preparation of the alkoxysilane terminated polyester urethane prepolymer and/or may be added after their preparation. For example, during the preparation of the isocyanate-terminated polyester urethane prepolymer, the aprotic solvent as described above is used to dilute the prepolymer and serve as the prepolymer media.

In a preferred embodiment, the composition (A) comprises an alkoxysilane terminated polyester urethane prepolymer and a mixture of ethyl <NUM>-ethoxypropionate and diacetone alcohol.

Preferably, composition (B) comprises one or more protic solvents, such as water, or an alcohol, and more preferably water and ethanol. Typically the solvents are present in the sol-gel process for preparing the silanol terminated polysiloxane prepolymer.

Protic solvents such as diacetone alcohol (DAA) are good solvents for both the organic polyurethane portion and the sol-gel based polysiloxane portion. DAA may be advantageously used as a co-solvent with EEP for the PUPSHCC. The protic solvent, such as DAA, is mixed with the isocyanate-terminated polyester urethane prepolymer mixture as media for alkoxysilane-terminated polyester urethane prepolymer formation by silanization of the isocyanate-terminated polyester urethane prepolymer with the amino-functional alkoxysilane coupling agent <NUM>-aminopropyltrimethoxysilane (APTMS).

A urethane forming catalyst is preferably used for preparation of the isocyanate-terminated polyester urethane prepolymer [the precursor to the alkoxysilane terminated polyester urethane prepolymer of composition (A)] and for the thermal curing of the polyurethane-polysiloxane hybrid coating composition.

The urethane forming catalyst accelerates the urethane linkage formation. Representative catalysts which are suitable for this invention include organo-metallic compounds dibutyltin dilaurate and dibutyltin diacetate available from Sigma-Aldrich and bismuth and zinc compounds available from Shepherd Chemical Company. The catalyst is typically used in the range of <NUM> ppm to <NUM> ppm, and more typically in the range of <NUM> to <NUM> ppm, based on the total weight of solids in the composition.

Preferably, the catalyst is selected from the group consisting of: dibutyltin dilaurate (DBTDL) or dibutyltin diacetate (DBTDA).

In some embodiments, the urethane forming catalyst is present in composition (A) of the two-part curable composition, for example, as the catalyst used to prepare the isocyanate-terminated polyester urethane prepolymer precursor to the alkoxysilane terminated polyester urethane prepolymer. The catalyst present in the preparation of the isocyanate-terminated polyester urethane prepolymer may be carried over to alkoxysilane terminated polyester urethane prepolymer in sufficient quantities to catalyse the thermal curing of the polyurethane-polysiloxane hybrid coating composition. In some embodiments, additional catalyst may be added prior to the final curing step (e.g. during the compounding step as describe below) in order to accelerate the curing process.

When the coating compositions of the present invention are to be utilized as protective coatings for aircraft, it is desirable to provide static dissipative properties, since the outer surface of an aircraft transparency is typically subject to electrostatic charging, especially in high performance modern aircraft. This charging is caused by contact with ice crystals and other particles during flight, which results in transfer of a charge to the surface via triboelectric or frictional effects. This phenomenon is called precipitation charging, or p-static charging, in the industry.

P-static charging of a non-conductive (dielectric) outer surface of an aircraft transparency can create several serious problems affecting aircraft performance, transparency service life, and personnel safety. Discharge during flight can result in damage to outer coating layers from dielectric breakdown or can result in electronic interference with instruments. Such charge accumulation can also create shock hazards for flight and ground personnel.

To prevent these problems caused by p-static charging, the outer layer of an aircraft transparency should be sufficiently conductive to allow the charge to drain across the surface to the airframe or through the thickness of the layer to an underlying grounded conductive layer. In such applications, antistatic agents are used in the two-part curable compositions of the present invention.

Thus, in any aspect or embodiment of the present invention, the two-part curable composition may further comprise an antistatic agent, preferably wherein the antistatic agent is a hydrophilic or a hydrophobic antistatic agent, more preferably wherein the antistatic agent is a salt of (bis)trifluoromethane-sulfonimide, particularly wherein the antistatic agent is: lithium (bis)trifluoromethanesulfonimide , tri-n-butylmethylammonium bis-(trifluoromethanesulfonyl)-imide, or a quaternary alkyl ammonium salt of (bis)trifluoromethane-sulfonimide. These antistatic agents are available from <NUM> as Fluorad HQ <NUM>, FC-<NUM> or FC-<NUM>, respectively. Antistatic additive Fluorad HQ <NUM> is a hydrophilic ionic solid salt of lithium and fluorinated imide with formula Li+ -N(SO<NUM>CF<NUM>)<NUM>. Antistatic additive FC-<NUM> is a hydrophobic ionic liquid salt of quaternary ammonium and fluorinated imide with formula (n-C<NUM>H<NUM>)<NUM>(CH<NUM>)N+ -N(SO<NUM>CF<NUM>)<NUM>. Antistatic additive FC-<NUM> is a hydrophobic ionic liquid salt of quaternary ammonium and fluorinated imide with single primary alcohol group on quaternary ammonium with formula R<NUM>N+ -N(SO<NUM>CF<NUM>)<NUM>. The antistatic additive is typically used in the range of <NUM> to <NUM>, and more typically in the range of <NUM> to <NUM> weight percent, based on the total weight of solids in the composition. Most preferably lithium (bis)trifluoromethanesulfonimide (Fluorad HQ <NUM>) is used as the antistatic agent.

The two-part curable composition according to any aspect or embodiment of the present invention preferably comprises an antistatic agent in an amount of <NUM> wt% to <NUM> wt%, and preferably <NUM> wt% to <NUM> wt%, based on the total weight of solids in the composition.

The use of stabilizers in the compositions of the present invention can significantly enhance the environmental durability of the resulting PUPSHCC when used as a surface protective coating of ESD coating system for transparencies used for modern aircraft. Hence, in any aspect or embodiment of the present invention, the compositions may further comprise one or more stabilizers such as: a UV absorber, a light stabilizer, and a thermal stabilizer. These stabilizers, particularly when used in combination (i.e. as a package of stabilisers), provide effective stabilization against the detrimental effects of light and weathering.

Preferably, the compositions of the present invention comprise at least one UV absorber. Suitable UV absorbers are preferably selected from the hydroxyphenyl-triazine, hydroxyphenyl-benzotriazole or hydroxyphenyl-benzophenone classes of UV absorbers. The UV absorber competitively absorbs the UV light that may be detrimental to the hybrid coating composition. Representative UV absorbers which are suitable for this invention include Tinuvin <NUM>, Tinuvin <NUM>, Tinuvin <NUM>, Tinuvin <NUM>, and Tinuvin <NUM> of the hydroxyphenyl-triazine class and Tinuvin <NUM>, Tinuvin <NUM>, Tinuvin <NUM>, Tinuvin <NUM>-<NUM>, and Tinuvin <NUM>-<NUM> of the hydroxyphenyl-benzotriazole class, available from BASF. The preferred UV absorber for this invention is Tinuvin <NUM>, which advantageously has an extremely high extinction coefficient in the UV-B and UV-A range. The high extinction coefficient allows formulation of the composition with reduced UV absorber content.

Thus, in any aspect or embodiment of the present invention, the compositions comprise at least one UV absorber, preferably selected from:.

Tinuvin <NUM> is a particularly preferred UV absorber.

Preferably, the compositions of the present invention comprise at least one light stabilizer (light absorber). The light stabilizer is preferably a hindered amine light stabilizer (HALS). Such stabilizers are capable of trapping free radicals and acts as radical scavengers in the autoxidation cycle and inhibits the photo-oxidative degradation of polymeric materials. Representative light stabilizers which are suitable for this invention include Tinuvin <NUM>, Tinuvin <NUM>, Tinuvin <NUM>, Tinuvin <NUM>, or Tinuvin <NUM>, available from BASF. The preferred light stabilizer for this invention is Tinuvin <NUM> having reactive primary hydroxyl which enables Tinuvin <NUM> to be cured into the polyurethane-polysiloxane networks so as to improve compatibility and resistance to migration.

In any aspect or embodiment of the present invention, the compositions of the present invention preferably comprise a light absorber selected from the group consisting of:.

Particularly, the light absorber is selected from the group consisting of: Tinuvin <NUM>, Tinuvin <NUM>, Tinuvin <NUM> and Tinuvin <NUM>; or Tinuvin <NUM>, Tinuvin <NUM> and Tinuvin <NUM>.

According to any aspect or embodiment of the present invention, the compositions disclosed herein preferably comprise at least one thermal stabilizer. The thermal stabilizer is typically a sterically hindered phenolic antioxidant that protects the composition against thermo-oxidative degradation. Representative thermal stabilizers which are suitable for this invention include IRGANOX <NUM>, IRGANOX <NUM>, IRGANOX <NUM>, or IRGANOX <NUM>, available from BASF. Low color, good compatibility, high resistance to extraction and low volatility are typical selecting standard for the thermal stabilizer for application in this composition and IRGANOX <NUM> is the preferred one.

Preferred thermal stabilizers are selected from the group consisting of: pentaerythritol tetrakis(<NUM>-(<NUM>,<NUM>-di-tert-butyl-<NUM>-hydroxyphenyl)propionate) (IRGANOX <NUM>);; octadecyl <NUM>-(<NUM>,<NUM>-di-tert-butyl-<NUM>-hydroxyphenyl)propionate (IRGANOX <NUM>); benzenepropanoic acid, <NUM>,<NUM>-bis(<NUM>,<NUM>-dimethylethyl)- <NUM>-hydroxy-, C7-<NUM>-branched alkyl esters (IRGANOX <NUM>);; and triethylene glycol bis(<NUM>-tert-butyl-<NUM>-hydroxy-<NUM>-methylphenyl)propionate(IRGANOX <NUM>); and more preferably wherein the thermal stabilizer is pentaerythritol tetrakis(<NUM>-(<NUM>,<NUM>-di-tert-butyl-<NUM>-hydroxyphenyl)propionate) (IRGANOX <NUM>).

In any aspect or embodiment, the compositions of the present invention preferably comprises a stabilizer combination (i.e. a package of stabilizers) comprising: a UV absorber, a light absorber and a thermal stabilizer, and preferably wherein the composition comprises a stabilizer combination of: <NUM>-methylheptyl <NUM>-[<NUM>-[<NUM>,<NUM>-bis(<NUM>-phenylphenyl)-<NUM>-<NUM>,<NUM>,<NUM>-triazin-<NUM>-ylidene]-<NUM>-oxocyclohexa-<NUM>,<NUM>-dien-<NUM>-yl]oxypropanoate (Tinuvin <NUM>), <NUM>,<NUM>-bis[N-butyl-N-(<NUM>-cyclohexyloxy-<NUM>,<NUM>,<NUM>,<NUM>-tetramethylpiperidin-<NUM>-yl)amino]-<NUM>-(<NUM>-hydroxyethylamine)-<NUM>,<NUM>,<NUM>-triazine (Tinuvin <NUM>) and pentaerythritol tetrakis(<NUM>-(<NUM>,<NUM>-di-tert-butyl-<NUM>-hydroxyphenyl)propionate) (IRGANOX <NUM>).

The package of stabilizers with combination of Tinuvin <NUM> / Tinuvin <NUM> / IRGANOX <NUM> provides a particularly effective stabilization against the detrimental effects of light and weathering. The package of stabilizers such as the combination of Tinuvin <NUM> / Tinuvin <NUM> / IRGANOX <NUM>, can be added during the isocyanate-terminated polyester urethane prepolymer preparation.

In any aspect or embodiment of the present invention, the compositions of the present invention preferably comprise one or more stabilizers in a total amount of <NUM> wt% to <NUM> wt%, and preferably <NUM> wt% to <NUM> wt%, based on the total weight of solids in the two part curable composition.

Surface active agents or surfactants may be favorably and optionally employed in the compositions of the invention in order to control the wetting or spreading action of the coating, provide the coating surfaces with a satisfactory flow-and-leveling properties, eliminate coating defects such as ruptures and craters, and/or increase surface slip. The surface active agents may include flow or levelling agents and/or surfactants.

According to any aspect or embodiment of the present invention, the compositions may optionally comprise a surface active agent which is a flow or levelling agent, preferably wherein the flow or levelling agent is a silicone-based flow or levelling agent, particularly a modified siloxane silicone based flow or levelling agent and more preferably wherein the flow or levelling agent is selected from the group consisting of:.

available from Momentive Performance Materials. The Silwet silicone surfactants are modified trisiloxanes with outstanding wetting, spreading and leveling properties. The powerful Silwet L-<NUM> that combines a very low molecular weight trisiloxane with a polyether group is preferred for this invention. Hence, the compositions of the present invention most preferably comprises as a flow or levelling agent, polyethylene glycol mono(<NUM>-(tetramethyl-<NUM>-(trimethylsiloxy)disiloxanyl)propyl)ether (Silwet L-<NUM>).

According to any aspect or embodiment of the present invention, the compositions may optionally comprise a surface active agent which is a surfactant. Representative surfactants which are suitable for this invention include BYK silicone surface additives which are modified polydimethylsiloxanes for increasing surface slip. More preferably, the surfactant is a polyether modified polydimethylsiloxane, or a polyester-modified polydimethylsiloxane. Particularly, the surfactants include those selected from the group consisting of:.

The reactive BYK-<NUM> that is a polyester modified hydroxyl functional polydimethylsiloxane is particularly preferred. BYK-<NUM> can be crosslinked into the polyurethane-polysiloxane networks via its primary OH-groups and can increase the surface slip permanently.

In any aspect or embodiment of the present invention, the compositions of the present invention comprises a flow or levelling agent which is polyethylene glycol mono(<NUM>-(tetramethyl-<NUM>-(trimethylsiloxy)disiloxanyl)propyl)ether Silwet L-<NUM>, and a surfactant which is BYK-<NUM>. The use of Silwet L-<NUM> with BYK-<NUM> provides an excellent combination of spreading/flow/leveling and permanent surface slip.

Preferably, each surface active agent is used in the range of <NUM> wt% to <NUM> wt%, more preferably in the range of <NUM> wt% to <NUM> wt%, based on the total weight of solids in the two part curable composition.

In a preferred embodiment, composition (A) comprises at least one flow or levelling agent (preferably polyethylene glycol mono(<NUM>-(tetramethyl-<NUM>-(trimethylsiloxy)disiloxanyl)propyl)ether Silwet L-<NUM>). The flow or leveling agent may be added to the reaction mixture for preparing the isocyanate-terminated polyester urethane prepolymer precursor to the alkoxysilane-terminated polyester urethane prepolymer. In another preferred embodiment, composition (A) further comprises at least one surfactant (preferably BYK-<NUM>), which may be added to the alkoxysilane-terminated polyester urethane prepolymer after the silanization reaction.

Preferably, the alkoxysilane-terminated polyester urethane prepolymer in composition (A) is obtainable by a process comprising:.

Steps (i) and (ii) are described in more detail below.

The aliphatic diisocyanate in step (i) is preferably a monomeric aliphatic diisocyanate, or a monomeric cycloaliphatic diisocyanate. Particularly suitable aliphatic diisocyanates include those selected from the group consisting of: hexamethylene diisocyanate, methylene bis (<NUM>-cyclohexylisocyanate), and isophorone diisocyanate. An especially preferred aliphatic diisocyanate is methylene bis (<NUM>-cyclohexylisocyanate), available from Covestro as Desmodur W.

The polyester diol is preferably a polycaprolactone-based polyester diol, preferably a linear polyester diol derived from caprolactone monomer which is terminated by primary hydroxyl groups, more preferably wherein the polyester diol is obtainable by catalytic ring-opening polymerization of caprolactone monomer in the presence of a diol initiator. Preferably, the diol initiator is alkylenediol or an dialkyleneglycol initiator. More particularly, the diol initiator is selected from the group consisting of <NUM>,<NUM>-butanediol (BDO), diethylene glycol (DEG) and neopentyl glycol (NEO), and most preferably, diethylene glycol.

Preferably, the polyester diol has a molecular weight of <NUM>/mol to <NUM>/mol, preferably <NUM>/mol to <NUM>/mol, or <NUM>/mol to <NUM>/mol, or particularly <NUM>/mol to <NUM>/mol, and especially <NUM>/mol to <NUM>/mol.

Suitable polyester diols are preferably those obtainable by catalytic ring-opening polymerization of caprolactone in the presence of <NUM>,<NUM>-butanediol, preferably having a molecular weight of <NUM>/mol to <NUM>/mol. Representative polyester diols which are suitable for preparation of the isocyanate-terminated polyester urethane prepolymer of this invention include polycaprolactone-based Capa <NUM>, Capa <NUM>, Capa <NUM>, Capa <NUM>, Capa <NUM>, Capa <NUM>, Capa <NUM>, and Capa <NUM>, with average molecular weight (g/mol) in the range of <NUM> to <NUM>, available from Perstorp Specialty Chemicals. The preferred polyester diol for this invention is Capa <NUM>. Preferably, the molar amount of the diisocyanate is in excess of the polyester diol. More preferably, the molar ratio of the polyester diol to aliphatic diisocyanate in step (i) is from: <NUM>:<NUM> to <NUM>:<NUM>, abut <NUM>:<NUM> to <NUM>:<NUM>, <NUM>:<NUM> to <NUM>:<NUM>, <NUM>:<NUM> to <NUM>:<NUM>, or <NUM>:<NUM>.

Typically, in the reaction step (i), <NUM> to <NUM> percent of isocyanate functionality of the aliphatic diisocyanate is converted to urethane linkage by the polyester diol and the unreacted isocyanate functionality terminates the urethane prepolymer.

A urethane forming catalyst is advantageously used for preparation of the isocyanate-terminated polyester urethane prepolymer in step (i), and optionally in the subsequent thermal curing of the PUPSHCC. The catalyst accelerates the urethane linkage formation. Representative catalysts which are suitable for this invention include organo-metallic compounds dibutyltin dilaurate and dibutyltin diacetate available from Sigma-Aldrich and bismuth and zinc compounds available from Shepherd Chemical Company. Preferably the catalyst is dibutyltin dilaurate (DBTDL) or dibutyltin diacetate (DBTDA).

In a preferred embodiment, step (i) is carried out in the presence of a urethane forming catalyst (preferably dibutyltin dilaurate or dibutyltin diacetate). The catalyst may be used in used in an amount of <NUM> to <NUM>, and more typically in the range of <NUM> to <NUM> ppm, based on the total weight of solids in the composition.

The urethane-forming catalyst is preferably carried through to the isocyanate-terminated polyester urethane prepolymer product in step (i) and to the alkoxysilane-terminated polyester urethane prepolymer component of composition (A) after the silanization step (ii). Advantageously, the urethane-forming catalyst present in the resulting composition (A) may enhance the curing step to prepare the final polyurethane-polysiloxane hybrid coating composition (PUPSHCC) without the need to add further catalyst prior to curing. However, if required a further quantity of the urethane forming catalyst may be added to prior to the curing step.

Where additives as described above are employed in the compositions of the invention to provide enhanced properties for certain applications of the coating compositions (such as for aircraft transparencies), some of the additives may be added to the reaction mixture for preparing the isocyanate-terminated polyester urethane prepolymer in step (i). Preferably, the reaction mixture in step (i) includes at least one additive selected from the group consisting of: one or more stabilizers, preferably a UV absorber, a light stabilizer and a thermal stabilizer preferably as described as above, and a flow/levelling agent as described above. More preferably the reaction mixture in step (i) includes a UV absorber, a light stabilizer, thermal stabilizer, and a flow/levelling agent.

In a particularly preferred embodiment, the reaction mixture in step (i) comprises the following additives: a stabilizer package comprising a combination of Tinuvin <NUM> / Tinuvin <NUM> / IRGANOX <NUM>; and a flow/levelling agent which is polyethylene glycol mono(<NUM>-(tetramethyl-<NUM>-(trimethylsiloxy)disiloxanyl)propyl)ether (Silwet L-<NUM>).

Step (i) preferably comprises heating the reaction mixture. The reaction mixture in step (i) is preferably heated to a temperature of: <NUM> to <NUM>, or <NUM> to <NUM>, or <NUM> to <NUM>. Preferably, the reaction time in step (i) is <NUM> to <NUM> hours, or preferably <NUM> to <NUM> hours. During the reaction, the reaction mixture is preferably continuously agitated. Preferably the reaction is carried out in an inert atmosphere, and preferably under nitrogen.

Following the reaction, the reaction mixture is preferably cooled. The resulting isocyanate terminated polyester urethane prepolymer mixture may be diluted with a carrier solvent, step (i) may further comprise adding an aprotic solvent to the isocyanate-terminated polyester urethane prepolymer to form a homogeneous mixture having a solid content of <NUM> to <NUM> wt%, preferably <NUM> to <NUM> wt%, more preferably <NUM> to <NUM> wt% (particularly <NUM> to <NUM> wt%). The aprotic solvent may be the same solvent as employed in the reaction mixture, and is preferably selected from the group consisting of: ethyl <NUM>-ethoxypropionate (EEP), n-pentyl propionate (nPP), <NUM>-butoxyethyl acetate (BEA), di-isobutyl ketone (DIBK), and methyl isobutyl ketone (MIBK), and more preferably <NUM>-ethoxypropionate (EEP).

The product mixture comprising the isocyanate terminated polyester urethane prepolymer from step (i) may advantageously be stored in a seal container until required for the preparation of the alkoxysilane-terminated polyester urethane prepolymer. Typically, the product mixture from step (i) may have a pot life of up to about <NUM> months, or up to about <NUM> months.

In an illustrative example of the preparation of the isocyanate-terminated polyester urethane prepolymer of step (i), the aliphatic diisocyanate [such as methylene bis (<NUM>-cyclohexylisocyanate) - Desmodur W] and the polyester diol (such as Capa <NUM> to <NUM> (<NUM>°F) may be heated in a stainless steel mixing pot equipped with hot plate, agitator, nitrogen inlet and thermometer by heating held at this temperature for <NUM> hours with continuous agitation and nitrogen blanketing. The ratio of Desmodur W to polyester diol Capa <NUM> is selected typically in the range of <NUM> to <NUM>, and more typically in the range of <NUM> to <NUM> percent of isocyanate functionality of Desmodur W is converted to urethane linkage by the polyester diols and the unreacted isocyanate functionality terminates the urethane prepolymer. A urethane forming catalyst such as dibutyltin dilaurate (DBTDL) or dibutyltin diacetate (DBTDA) is added for accelerating the reaction of Desmodur W with Capa <NUM>. A package of stabilizers including a UV absorber, a light stabilizer, and a thermal stabilizer in the range of <NUM> to <NUM> weight percent for environmental durability enhancement and optional surface active agents or surfactants for flow/leveling control and enhancing coating surface cosmetics can also be conveniently added during the isocyanate-terminated polyester urethane prepolymer preparation. The resulting isocyanate-terminated polyester urethane prepolymer can be diluted with urethane grade aprotic solvent ethyl <NUM>-ethoxypropionate (EEP) as a clear homogeneous mixture with solid content of approximately <NUM>-<NUM> weight percent.

Step (ii) of the above process involves silanization of the isocyanate terminated polyester urethane prepolymer with an amino-functional alkoxysilane coupling agent to form the alkoxysilane-terminated polyester urethane prepolymer component (A), which can be crosslinked with the separately prepared polysiloxane prepolymer in component (B) to form the PUPSHCC.

In any embodiment of the present invention, the amino-functional alkoxysilane in step (ii) is selected from the group consisting of: an aminoalkyltrialkoxysilane, an aminoaryltrialkoxysilane, an aminoalkyl(alkyl)(dialkoxyl)silane, a [bis(trialkoxylsilyl)-alkyl]amine, an N-(aminoalkyl)-aminoalkyltrialkoxysilane, or an N-alkyltrialkoxylsilyl-N,N-dialkylamine; and preferably wherein the amino-functional alkoxysilane is an aminoalkyltrialkoxysilane.

More preferably, the amino-functional alkoxysilane is selected from the group consisting of <NUM>-aminopropyltrimethoxysilane, <NUM>-aminopropyltriethoxysilane, <NUM>-aminobutyltrimethoxysilane, <NUM>-aminobutyltriethoxy-silane, aminophenyltrimethoxysilane, aminophenyltriethoxysilane, <NUM>-aminopropyl-methyldiethoxysilane,<NUM>-aminopropylmethyldimethoxysilane, bis(trimethoxysilylpropyl)amine, bis(triethoxysilylpropyl)amine, and N-ethyl-<NUM>-trimethoxysilyl-<NUM>-methylpropanamine.

<NUM>-aminopropyltrimethoxysilane (APTMS) or <NUM>-aminopropyltriethoxysilane (APTES) (available from Gelest or Sigma-Aldrich); and especially <NUM>-aminopropyltrimethoxysilane (APTMS) are particularly preferred amino-functional akoxysilanes for step (ii).

In a preferred embodiment, step (ii) comprises addition of the amino-functional alkoxysilane to the isocyanate-terminated polyester urethane prepolymer. The silanization reaction is typically a fast, spontaneous reaction between the amino functionality of the amino-functional alkoxysilane and the isocyanate terminals of the isocyanate-terminated polyester urethane prepolymer.

Preferably, step (ii) is conducted in the presence of at least one aprotic solvent and a protic solvent preferably as described above. The aprotic solvent may be the same solvent as employed in the reaction mixture in step (i) as described above, with EEP being a particularly preferred aprotic solvent. A protic solvent may be added to the reaction mixture in step (ii). Preferred protic solvents include a hydroxyketone, preferably a β-hydroxyketone, and more preferably diacetone alcohol (DAA). Step (ii) is most preferably carried out in a combination of an aprotic solvent and a protic solvent, preferably in a combination of EEP and DAA.

Step (ii) is preferably conducted at room temperature. The reaction is preferably conducted in an inert atmosphere, preferably under nitrogen.

The silanization reaction between the amino functionalities of the amino-functional alkoxysilane coupling agent and the isocyanate terminals of the isocyanate-terminated polyester urethane prepolymer is typically a fast spontaneous reaction with heat generation and urea linkage formation. The silanization reaction typically is complete within <NUM> hour and typically generates the alkoxysilane-terminated polyester urethane prepolymer as a clear homogeneous mixture in the aprotic/protic solvent (e.g. EEP/DAA).

Where additives as described above are employed in the compositions of the invention to provide enhanced properties for certain applications of the coating compositions (such as for aircraft transparencies), some of the additives may be added to the reaction mixture for preparing the alkoxysilane-terminated polyester urethane prepolymer in step (ii). In a preferred embodiment, at least one surface active agent which is preferably a surfactant, more preferably a BYK silicone surface additive (modified polydimethylsiloxanes), and most preferably BYK-<NUM> is added to the reaction product of step (ii).

The composition (A) comprising an alkoxysilane-terminated polyester urethane prepolymer typically has a pot life of <NUM> to <NUM> hours at room temperature in a sealed container.

In any aspect or embodiment of the present invention, the silanol terminated polysiloxane prepolymer of composition (B) may be obtainable by sol-gel process of an alkoxysilane of formula (II), (III) or (IV):
<CHM>
<CHM>
<CHM>
wherein:.

In preferred embodiments, m represents an integer from <NUM> to <NUM>.

Preferably in any embodiment of the present invention, each X<NUM>, X<NUM>, X<NUM>, X<NUM>, X<NUM>, and X<NUM> represents an alkoxy group having <NUM>-<NUM> carbon atoms, more preferably <NUM>-<NUM> carbon atoms and more preferably methoxy or ethoxy.

Representative preferred alkoxysilanes which are suitable for the polysiloxane prepolymer formation of this invention include tetraethyl orthosilicate, tetramethyl orthosilicate, methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, <NUM>,<NUM>-bis(triethoxysilyl)ethane, and <NUM>,<NUM>-bis(trimethoxysilyl)ethane.

Fully hydrolyzed tetraethyl orthosilicate and tetramethyl orthosilicate have potential to generate <NUM> silanol functionalities which can participate in the subsequent condensation among themselves or with alkoxysilane-terminated polyester urethane prepolymer for network formation. Fully hydrolyzed methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane and ethyltriethoxysilane have potential to generate <NUM> silanol functionalities which can participate in the subsequent condensation among themselves or with alkoxysilane-terminated polyester urethane prepolymer for network formation. Fully hydrolyzed <NUM>,<NUM>-bis(triethoxysilyl)ethane and <NUM>,<NUM>-bis(trimethoxysilyl)ethane have potential to generate <NUM> silanol functionalities which can participate in the subsequent condensation among themselves or with alkoxysilane-terminated polyester urethane prepolymer for network formation. Single alkoxysilane or alkoxysilane combination can be used for the polysiloxane prepolymer preparation. In any embodiment of the present invention, the alkoxysilane is preferably tetramethyl orthosilicate or tetraethyl orthosilicate, and more preferably tetraethyl orthosilicate, which are available from Gelest or Sigma-Aldrich.

The sol-gel process is preferably conducted in an acidified aqueous-organic solvent mixture. The water in the acidified aqueous-organic solvent mixture is to hydrolyze the alkoxysilane and to form the polysiloxane prepolymer. The actual amount of water in the acidified aqueous-organic solvent mixture can vary widely and can be readily determined empirically.

In a preferred embodiment, the silanol terminated polysiloxane prepolymer in composition (B) is obtainable by a process comprising: reacting an alkoxysilane, preferably an alkyltrioxysilane, an alkylorthosilicate or a bis(trialkyloxysilyl)alkane, and more preferably an alkylorthosilicate (particularly TEOS), with at least one acid, in the presence of a solvent comprising an alcohol and water.

The solvent constituent of the acidified aqueous-organic solvent mixture for formation of the silanol terminated polysiloxane prepolymer in composition (B) of the present invention can be any solvent or combination of solvents which is compatible with the alkoxysilanes, their hydrolytes, the formed polysiloxane prepolymer and the alkoxysilane-terminated polyester urethane prepolymer to be compounded with the formed polysiloxane prepolymer.

Representative solvents which are suitable for the polysiloxane prepolymer formation of this invention include alcohols (preferably methanol, ethanol, n-propanol, isopropanol, n-butanol); glycol ethers (preferably ethylene glycol dimethyl ether, propylene glycol dimethyl ether, propylene glycol methyl ether); diacetone alcohol, ketones (preferably methyl ethyl ketone, methyl isobutyl ketone, di-isobutyl ketone), and esters (preferably ethyl acetate, n-propyl acetate, n-butyl acetate, <NUM>-butoxyethyl acetate, and ethyl <NUM>-ethoxypropionate) and combinations thereof. Particularly preferred solvents for the aqueous-organic solvent mixture are alcohols, more preferably wherein the alcohol is selected from a C1-C6 alcohol, particularly a C1-C3 alcohol, and more preferably ethanol.

An organic acid or inorganic acid, or a combination thereof is used to acidify the aqueous-organic solvent mixture and as a catalyst to accelerate hydrolysis of alkoxysilanes for the silanol terminated polysiloxane prepolymer formation. Representative organic or inorganic acids which are suitable for the silanol terminated polysiloxane prepolymer formation of this invention include acetic acid, formic acid, malic acid, succinic acid, malonic acid, hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, etc..

Preferably, the acid comprises a carboxylic acid and/or a mineral acid, preferably wherein the acid comprises a carboxylic acid and a mineral acid. The carboxylic acid is a C2-C6 alkylcarboxylic acid, preferably acetic acid.

Preferably, the mineral acid is selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, hydrobromic acid, hydrofluoric acid, and phosphoric acid, and preferably wherein the mineral acid is hydrochloric acid.

In a particularly preferred embodiment, the polysiloxane prepolymer can be conveniently prepared at room temperature by sol-gel process of the alkoxysilane, preferably tetraethyl orthosilicate (TEOS) in an acetic acid or hydrochloric acid or their combination acidified aqueous-organic solvent mixture.

The process preferably comprises adding the alkoxysilane to a mixture comprising the at least one acid, water and an alcohol.

Preferably the reaction mixture is held at a temperature of: <NUM> to <NUM>, <NUM> to <NUM> or <NUM> to <NUM>, preferably for: <NUM> to <NUM> hours, <NUM> to <NUM> hours, <NUM> to <NUM> hours, or <NUM> to <NUM> hours. The reaction mixture is preferably stirred continuously during the reaction time.

The acid is preferably present in an amount to maintain a pH in the range of <NUM> to <NUM>, preferably <NUM> to <NUM>, and more preferably <NUM> to <NUM>. The acid preferably comprises a combination of acetic acid and hydrochloric acid.

Where additives as described above are employed in the compositions of the invention to provide enhanced properties for certain applications of the coating compositions (such as for aircraft transparencies), some of the additives may be added to the reaction mixture for preparing the silanol terminated polysiloxane prepolymer. In a preferred embodiment, at least one antistatic agent (preferably wherein the antistatic agent is a hydrophilic or a hydrophobic antistatic agent, more preferably wherein the antistatic agent is a salt of (bis)trifluoromethanesulfonimide, particularly wherein the antistatic agent is: lithium (bis)trifluoromethanesulfonimide, tri-n-butylmethylammonium bis-(trifluoromethanesulfonyl)imide, or a quaternary alkyl ammonium salt of (bis)trifluoromethanesulfonimide, and most preferably lithium (bis)trifluoromethanesulfonimide) is added to the reaction product.

The composition (B) prepared as described above may be stored in a sealed container for up to about <NUM> week at room temperature.

The PUPSHCC according to the invention is prepared by combining the compositions (A) and (B) of the two-part curable composition as described in any aspect or embodiment herein and curing the composition. The two-part curable composition when combined, forms a curable composition (i.e. a polyurethane-polysiloxane hybrid coating composition precursor), which is thermally curable. The PUPSHCC precursor, may be applied to a substrate, and cured in situ on the substrate to form a coated substrate. The curing process results in the crosslinking of the alkoxysilane-terminated polyester urethane component of composition (A) with the silanol terminated polysiloxane prepolymer component of composition (B).

In a preferred embodiment, the PUPSHCC precursor is prepared by compounding the compositions (A) and (B) of the two-part curable composition as described in any of the aspects and embodiments described herein, optionally with a urethane forming catalyst. The compounding may be carried out by continuous agitation (e.g. in a stainless steel pot). During the compounding step, the mixture is preferably degassed to obtain a homogeneous mixture.

Optionally, a urethane forming catalyst may be added to the mixture to be compounded. The urethane forming catalyst may be the same as the catalyst used for preparing the isocyanate terminated polyester urethane prepolymer as described above. Preferably, the catalyst is selected from the group consisting of dibutyltin dilaurate (DBTDL) or dibutyltin diacetate (DBTDA). However, in some cases, component (A) may include sufficient urethane forming catalyst from the preparation of the isocyanate terminated polyester urethane prepolymer, such that additional catalyst in the mixture to be compounded may not be necessary.

Preferably, to facilitate application of the polyurethane-polysiloxane hybrid coating composition precursor to a substrate, the PUPSHCC precursor has a solids content of <NUM> wt% to <NUM> wt, and preferably <NUM> wt% to about wt% or <NUM> wt%.

In order to form the PUPSHCC, the PUPSHCC precursor prepared by the above process is thermally cured, optionally after removal of at least a portion (preferably at least <NUM> wt%, at least <NUM> wt%, at least <NUM> wt%, or at least <NUM> wt%), of solvent from the PUPSHCC precursor. The solvent may be removed by exposure to air for a sufficient amount of time. Prior to thermal curing, the PUPSHCC precursor is preferably dried to a tack-free consistency.

In preferred embodiments, the thermal curing comprises heating the polyurethane-polysiloxane hybrid prepolymer to a temperature of: <NUM> to <NUM>, <NUM> to <NUM> or <NUM> to <NUM>.

The present invention further provides a process for preparing a two-part curable coating composition as defined in any aspect or embodiment herein, comprising:.

The components (A) and (B) of the two-part curable coating composition as defined in any aspect or embodiment may be compounded and applied to a substrate, and the resulting mixture may be thermally cured to provide a coated substrate.

The present invention further relates to the use of a composition as described herein (i.e. the two-part curable composition, the PUPSHCC and PUPSHCC precursor) as a coating for a substrate, preferably wherein the substrate is selected from the group consisting of an aircraft, a spacecraft, a marine craft or vehicle part, preferably an aircraft part, and more preferably an aircraft transparency, and most preferably an aircraft window or an aircraft windscreens.

Following application of the PUPSHCC precursor to a surface to be coated, the coating is subjected to thermal curing (optionally following solvent removal) to provide a polyurethane-polysiloxane hybrid coating composition which is a crosslinked alkyloxysilane-terminated polyester urethane with the polysiloxane.

The invention further provides a substrate, preferably an aircraft, spacecraft, marine craft or vehicle part, which is coated with a polyurethane-polysiloxane hybrid polymer coating. The substrate is preferably an aircraft part, preferably an aircraft transparency, more preferably an aircraft window or windscreen (windshield).

The invention additionally provides a process for preparing a PUPSHCC coated substrate comprising applying a polyurethane-polysiloxane hybrid coating composition precursor as defined in any embodiment herein, to a surface of the substrate, removing at least a portion of the solvent, and preferably at least <NUM> wt%, at least <NUM> wt%, at least <NUM> wt%, or at least <NUM> wt%, of the solvent, from the polyurethane-polysiloxane hybrid coating composition precursor, preferably by air drying, and thermally curing the coating composition precursor to provide a substrate coated with a polyurethane-polysiloxane hybrid coating composition.

The PUPSHCC precursor may be cured in situ on the substrate by heating to a temperature of: <NUM> to <NUM>, <NUM> to <NUM> or <NUM> to <NUM>, for example in an oven.

In preferred embodiments, the substrate is an aircraft transparency, preferably an aircraft window or windshield, and wherein the surface of the substrate comprises an electrically conductive layer, preferably wherein the conductive layer is an indium tin oxide layer, and optionally a silicon binder layer over the conductive layer, onto which the polyurethane-polysiloxane hybrid coating composition precursor is applied.

In a preferred embodiment, the formulation of the PUPSHCC coated substrate comprises:.

In this process, the alkoxysilane-terminated polyester urethane prepolymer is crosslinked with the silanol terminated polysiloxane prepolymer in the subsequent compounding/flow coating/air drying/thermal curing steps. In the case of an aircraft transparency, the surface typically contains an outer surface electrically conductive indium tin oxide (ITO) or carbon nanotube (CNT) layer. The PUPSHCC precursor is applied to the surface to form a wet topcoat on the ITO or CNT layer. Subsequent drying and thermal curing steps convert the wet topcoat to a durable surface protective coating over the electrically conductive ITO or CNT layer so that a desirable ESD coating system is formed on the outer surface of aircraft transparencies to provide the transparencies with enhanced erosion resistance, abrasion/scratch resistance, electrostatic dissipation capability and environmental durability.

Also provided by the invention is a substrate, preferably an aircraft transparency, and more preferably an aircraft window or an aircraft windshield, obtainable by the process described above.

The present invention describes a PUPSHCC which may be modified with antistatic additives for conductivity promotion, and a package of stabilizers including an UV absorber, a light stabilizer, and a thermal stabilizer for environmental durability enhancement and application of such coating composition as a universal surface protective coating of ESD coating system for transparencies used for modern aircraft to provide the transparencies with enhanced erosion resistance, abrasion/scratch resistance, electrostatic dissipation capability and environmental durability without increase in the coating system complexity and without compromise of any performance properties compared to current state-of-the-art materials.

Surface protective coating formed from the PUPSHCC of this invention inherited the excellent rain erosion resistance from the comparative organic polyurethane and the superior abrasion resistance from the comparative polysiloxane. The preferred embodiments as described in the following illustrative examples <NUM>-<NUM> showed that surface protective coating formed from the optimized PUPSHCC had Taber Abrasion as percent haze increase ΔHz100 in the range of <NUM>-<NUM> which was even superior to the typical result of the comparative polysiloxane while keeping excellent rain erosion resistance as the comparative polyurethane. Surface protective coating formed from the PUPSHCC of this invention also demonstrated the necessary electrostatic dissipation capability. Such surface protective coating is therefore considered suitable as surface protective coating of ESD coating system for transparencies used for modern aircraft to provide the transparencies with enhanced erosion resistance, abrasion/scratch resistance, electrostatic dissipation capability and environmental durability.

Other features and advantages of the invention should become apparent from the following description of the preferred embodiments.

Aircraft-grade (per MIL-PRF-<NUM>) stretched acrylic sheets of dimension <NUM>"x32" were cleaned with a standard cleaning procedure with cerium oxide, detergent water, tap water, DI water, heptane and IPA in sequence. In Examples <NUM>-<NUM> and <NUM>-<NUM> (i.e. ITO as conductive layer), the cleaned acrylic sheets were first primed with a polyurethane primer and air dried for <NUM> hours in clean room. The primed acrylic sheets were then flow-coated with a conductive indium tin oxide (ITO) layer, air dried for <NUM> hour and heated in an air oven at <NUM> (<NUM>° F) for <NUM> hour. The ITO coated acrylic sheets were then used as substrates for evaluating the transparent PUPSHCC. In Example <NUM> (CNT as conductive layer), the cleaned acrylic sheets were first flow-coated with a carbon nanotube (CNT) conductive coating and air dried for <NUM> hour in clean room. The CNT coated acrylic sheets were then capped with a polyurethane layer, air dried for <NUM> hour and heated in an air oven at <NUM> (<NUM>° F) for <NUM> hour. The ITO or CNT coated acrylic sheets were then used as substrates for evaluating the transparent PUPSHCC.

Compounding the sol-gel polysiloxane prepolymer with the alkoxysilane-terminated polyester urethane prepolymer was performed in a stainless steel mixing pot with continuous agitation and degassing. The resulting PUPSHCC precursor is a clear homogeneous coating mixture which can be conveniently flow-coated over the outer surface of aircraft transparencies which contain an outer surface electrically conductive ITO or CNT layer to form a wet topcoat on the ITO or CNT layer. Subsequent air drying and thermal curing steps converted the wet topcoat to a durable transparent surface protective coating over the electrically conductive ITO or CNT layer so that a desirable ESD coating system is formed on the outer surface of aircraft transparencies to provide the transparencies with enhanced erosion resistance, abrasion/scratch resistance, electrostatic dissipation capability and environmental durability.

Table <NUM> highlights typical results of the critical performances of rain erosion resistance, abrasion/scratch resistance, and electrostatic dissipation capability (refer to the section of measurements and performance tests for test methods and procedures) for surface protective coating formed from the PUPSHCC of this invention with surface protective coating formed from the comparative state-of-art systems of the organic polyurethane and sol-gel based polysiloxane.

The critical performances showed that surface protective coating formed from the comparative organic polyurethane had excellent rain erosion resistance while having inferior abrasion/scratch resistance. On the other hands, surface protective coating formed from the comparative polysiloxane had superior abrasion/scratch resistance while failing rain erosion resistance with surface pitting on tested samples. Neither of these prior art systems is considered fully satisfactory as surface protective coating of ESD coating system for transparencies used for modern aircraft.

The PUPSHCC of this invention can best be understood and illustrated by reference to the following examples.

Into a stainless steel mixing pot equipped with hot plate, agitator, nitrogen inlet and thermometer were charged with <NUM> of polyester diol Capa <NUM>, <NUM> of methylene bis (<NUM>-cyclohexylisocyanate) Desmodur W, <NUM> of flow/leveling agent Silwet L-<NUM>, <NUM> of antioxidant thermal stabilizer Irganox <NUM>, <NUM> of UV absorber Tinuvin <NUM>, <NUM> of light stabilizer Tinuvin <NUM> and <NUM> of <NUM>% dibutyltin dilaurate (DBTDL) diluted in aprotic solvent ethyl <NUM>-ethoxypropionate (EEP). The above components were heated to <NUM> (<NUM>° F) and held at this temperature for <NUM> hours with continuous agitation and nitrogen blanketing. The resulting isocyanate-terminated polyester urethane prepolymer was cooled down to room temperature. <NUM> of aprotic solvent ethyl <NUM>-ethoxypropionate (EEP) was then added under agitation to dilute the prepolymer as a clear homogeneous mixture with solid content of approximately <NUM> weight percent. The isocyanate-terminated polyester urethane prepolymer mixture can be stored in a sealed container with pot life up to <NUM> months.

The isocyanate-terminated polyester urethane prepolymer mixture prepared in Step (A) was further diluted with <NUM> of diacetone alcohol (DAA) with continuous agitation and nitrogen blanketing. <NUM> of amino-functional alkoxysilane coupling agent <NUM>-aminopropyltrimethoxysilane (APTMS) was added to the urethane prepolymer mixture over a ten minute period to silanize the isocyanate-terminated polyester urethane prepolymer. The silanization between the amino functionalities of APTMS and the isocyanate terminals of the polyester urethane prepolymer is a fast spontaneous reaction with heat generation and urea linkage formation. The silanization reaction completed within <NUM> hour and generated trimethoxysilane-terminated polyester urethane prepolymer mixture in EEP/DAA. <NUM> of surface additive BYK-<NUM> was then added in the prepolymer mixture with agitation. The trimethoxysilane-terminated polyester urethane prepolymer had pot life <NUM>-<NUM> hours at room temperature in a sealed container.

Into a plastic container equipped with lid, stirring plate and magnetic stirring bar were charged with <NUM> of ethanol, <NUM> of DI water, <NUM> of acetic acid and <NUM> of hydrochloric acid. The above components were stirred at room temperature for <NUM> minutes for an acidified aqueous-alcoholic mixture. <NUM> of tetraethyl orthosilicate (TEOS) was added to the acidified aqueous-alcoholic mixture over a ten minute period with continuous stirring for sol-gel polysiloxane prepolymer formation. The sol-gel process was held for <NUM> days at room temperature with gentle stirring. <NUM> of hydrophilic antistatic additive Fluorad HQ <NUM> was then added. The resulting polysiloxane prepolymer can be stored in a sealed container with pot life up to <NUM> week at room temperature.

In clean room the trimethoxysilane-terminated polyester urethane prepolymer prepared in Step (B), the polysiloxane prepolymer prepared in Step (C) and <NUM> of <NUM>% dibutyltin dilaurate (DBTDL) diluted in aprotic solvent ethyl <NUM>-ethoxypropionate (EEP) were transferred into a stainless steel mixing pot equipped with agitator and vacuum line for compounding. The compounding was performed for <NUM> minutes with continuous agitation and degassing. The resulting PUPSHCC precursor was a clear homogeneous mixture with solid content of approximately <NUM> weight percent. The PUPSHCC precursor had pot life <NUM>-<NUM> hours.

The PUPSHCC precursor was then flow-coated over the previously prepared acrylic sheets of dimension <NUM>"x32" which had an outer surface electrically conductive ITO layer to form a wet topcoat on the ITO layer. Subsequent <NUM> hours of air drying and <NUM> hours of <NUM> (<NUM>° F) oven curing steps converted the wet topcoat to a transparent conductive surface protective coating over the electrically conductive ITO layer with coating thickness approximately <NUM>, light transmittance <NUM> %, percent haze <NUM>, crosshatch adhesion <NUM>%, and Taber Abrasion as percent haze increase ΔHz100 = <NUM>.

In this example, all coating materials and processing procedures including isocyanate-terminated polyester urethane prepolymer, trimethoxysilane-terminated polyester urethane prepolymer, polysiloxane prepolymer, compounding and flow-coating of the PUPSHCC procedures were same as those in example <NUM> except that there was a silicone binder over ITO layer on the acrylic substrate. The resulting transparent conductive surface protective coating over the electrically conductive ITO layer had coating thickness approximately <NUM>, light transmittance <NUM>%, percent haze <NUM>, crosshatch adhesion <NUM>%, and Taber Abrasion as percent haze increase ΔHz100 = <NUM>.

The isocyanate-terminated polyester urethane prepolymer was prepared as Example <NUM>, Step (A).

The PUPSHCC precursor was then flow-coated over the previously prepared acrylic sheets of dimension <NUM>"x32" which had an outer surface electrically conductive ITO layer to form a wet topcoat on the ITO layer. Subsequent <NUM> hours of air drying and <NUM> hours of <NUM> (<NUM>° F) oven curing steps converted the wet topcoat to a transparent conductive surface protective coating over the electrically conductive ITO layer with coating thickness approximately <NUM>, light transmittance <NUM>%, percent haze <NUM>, crosshatch adhesion <NUM>%, and Taber Abrasion as percent haze increase ΔHz100 = <NUM>.

Into a stainless steel mixing pot equipped with hot plate, agitator, nitrogen inlet and thermometer were charged with <NUM> of polyester diol Capa <NUM>, <NUM> of methylene bis (<NUM>-cyclohexylisocyanate) Desmodur W, <NUM> of flow/leveling agent Silwet L-<NUM>, <NUM> of antioxidant thermal stabilizer Irganox <NUM>, <NUM> of UV absorber Tinuvin <NUM>, <NUM> of light stabilizer Tinuvin <NUM> and <NUM> of <NUM>% dibutyltin diacetate (DBTDA) diluted in aprotic solvent ethyl <NUM>-ethoxypropionate (EEP). The above components were heated to <NUM> (<NUM>° F) and held at this temperature for <NUM> hours with continuous agitation and nitrogen blanketing. The resulting isocyanate-terminated polyester urethane prepolymer was cooled down to room temperature. <NUM> of aprotic solvent ethyl <NUM>-ethoxypropionate (EEP) was then added under agitation to dilute the prepolymer as a clear homogeneous mixture with solid content of approximately <NUM> weight percent. The isocyanate-terminated polyester urethane prepolymer mixture can be stored in a sealed container with pot life up to <NUM> months.

In clean room the trimethoxysilane-terminated polyester urethane prepolymer prepared in Step (B), the polysiloxane prepolymer prepared in Step (C) and <NUM> of <NUM>% dibutyltin diacetate (DBTDA) diluted in aprotic solvent ethyl <NUM>-ethoxypropionate (EEP) were transferred into a stainless steel mixing pot equipped with agitator and vacuum line for compounding. The compounding was performed for <NUM> minutes with continuous agitation and degassing. The resulting PUPSHCC precursor was a clear homogeneous mixture with solid content of approximately <NUM> weight percent. The PUPSHCC precursor had pot life <NUM>-<NUM> hours.

The PUPSHCC precursor was then flow-coated over the previously prepared acrylic sheets of dimension <NUM>"x32" which had an outer surface electrically conductive CNT layer to form a wet topcoat on the CNT layer. Subsequent <NUM> hours of air drying and <NUM> hours of <NUM> (<NUM>° F) oven curing steps converted the wet topcoat to a transparent conductive surface protective coating over the electrically conductive CNT layer with coating thickness approximately <NUM>, light transmittance <NUM>%, percent haze <NUM>, crosshatch adhesion <NUM>%, and Taber Abrasion as percent haze increase ΔHz100 = <NUM>.

An aliphatic polyester urethane composition with GKN code SS7008 was flow-coated over previously prepared acrylic sheet of dimension <NUM>"x32" which had an outer surface electrically conductive ITO layer to form a wet topcoat on the ITO layer. The coated acrylic sheet was then air dried and oven cured according to the process sheet. The resulting transparent surface protective coating over the electrically conductive ITO layer had coating thickness <NUM>, light transmittance <NUM>%, percent haze <NUM>, crosshatch adhesion <NUM>%, and Taber Abrasion as percent haze increase ΔHz100 = <NUM>.

A polysiloxane composition obtained from SDC Technologies with GKN code SS6782 was flow-coated over previously prepared acrylic sheet of dimension <NUM>"x32" which had an outer surface electrically conductive ITO layer to form a wet topcoat on the ITO layer. The coated acrylic sheet was then air dried and oven cured according to the process sheet. The resulting transparent surface protective coating over the electrically conductive ITO layer had coating thickness <NUM>, light transmittance <NUM>%, percent haze <NUM>, crosshatch adhesion <NUM>%, and Taber Abrasion as percent haze increase ΔHz100 = <NUM>.

Coating thickness of the surface protective coating over the electrically conductive ITO layer was measured with QuintSonic Thickness Gauge. Light transmittance and percent haze were measured with Gardner haze-gard according to ASTM D-<NUM>. Crosshatch adhesion was measured according to ASTM D-<NUM>.

Abrasion resistance of the coated acrylics was tested as Taber Abrasion according to ASTM D-<NUM> using a Taber Abrader with CS-10F grinding wheels and <NUM>/arm by abrading a sample of the coated acrylics of approximately <NUM> inches by <NUM> inches. Percent haze increases in the wear pattern were measured for <NUM> cycles and showed as ΔHz100, %.

Electrostatic dissipation capability testing of the coated acrylics was carried out at <NUM> (<NUM>° F), -<NUM> (<NUM>° F), -<NUM> (-<NUM>° F), and -<NUM> (-<NUM>° F) using the method described in ASTM D-<NUM>. The coated acrylic sheet of dimension <NUM>"x32" was first painted with a silver electrode <NUM> square foot (<NUM>"x12") on surface in the bottom area. The silver electrode painted acrylic sheet was then put in a temperature controlled chamber and the surface electrode and the underneath ITO conductive layer were connected to an electric power supplier for recording the relationship of current density and applied voltage at various temperatures. The applied voltages for setting current densities <NUM>µA / sq. at <NUM> (<NUM>° F), <NUM>µA / sq. at -<NUM> (<NUM>° F), <NUM>µA / sq. at -<NUM> (-<NUM>° F) and <NUM>µA / sq. at -<NUM> (-<NUM>° F) were measured in the range of <NUM> V to <NUM>,<NUM> V without dielectric breakdown for surface protective coating formed from the PUPSHCC of this invention. The surface protective coating with such electrostatic dissipation capability is sufficiently conductive to allow the charge on the surface to drain through the thickness of the surface protective coating to the underlying grounded ITO conductive layer thus it can be used satisfactorily as surface protective coating of ESD coating system for transparencies used for modern aircraft where static dissipative properties are required.

Specimen of the coated acrylics were tested rain erosion resistance. The coated acrylics were first cut into <NUM>"x1. <NUM>" dimension then step machined across upper edge, filled with gray elastomeric material flush to remaining sample surface of <NUM>/<NUM>" thick x <NUM>/<NUM>" wide to fit with the tester holder. Rain erosion test parameters are <NUM> degree impact angle, one inch per hour rainfall and test velocity <NUM> and <NUM> miles per hour. Average rain drop size is <NUM>. Test specimen were inspected initial and after <NUM> exposure to the rainfall for topcoat delamination or damage. Less than <NUM>% coating loss or delamination or damage after test is viewed as pass.

Samples of the coated acrylics were tested light transmittance, percent haze, crosshatch adhesion, and overall appearance with accelerated QUV exposure for up to <NUM> weeks. The accelerated QUV equipment was operated in a cycle of <NUM>° C / <NUM> hours UV exposure and <NUM>° C / <NUM> hours condensation (UV off, high humidity) with UVB-<NUM> EL Fluorescent Lamps. Testing samples were removed from the QUV equipment weekly to check percent light transmission, percent haze, percent adhesion, and overall appearance. Percent light transmittance and percent haze are measured according to ASTM D-<NUM>. Percent adhesion is measured according to ASTM D-<NUM>. The surface protective coating formed from PUPSHCC exhibited substantially no degradation in light transmission, percent haze, percent adhesion, and overall appearance during the QUV exposure and shows excellent environmental durability against the detrimental effects of light and weathering.

Claim 1:
A two-part curable coating composition comprising:
(A) a composition comprising an alkoxysilane terminated polyester urethane prepolymer, comprising a prepolymer chain having a terminal group of general formula (I):
<CHM>
wherein:
Y represents: H; R<NUM> wherein R<NUM> represents hydrocarbyl, preferably alkyl or aryl, optionally wherein R<NUM> at each occurrence is the same or different, preferably the same, and each is independently selected from: a C<NUM> to C<NUM> alkyl, or a C<NUM> to C<NUM> alkyl group, more preferably each is a methyl or ethyl group; or wherein Y represents the group:
<CHM>
and preferably wherein Y is H;
L represents a hydrocarbylene group, preferably an alkylene or an arylene group, preferably a C<NUM> to C<NUM> hydrocarbylene group, and preferably methylene, ethylene, or propylene and most preferably propylene;
W<NUM>, W<NUM>, W<NUM>, W<NUM>, W<NUM> and W<NUM> may be the same or different and each independently represents -OR<NUM>, wherein R<NUM> at each occurrence may be the same or different, and each R<NUM> is independently selected from an alkyl group, preferably wherein the alkyl group has <NUM> to <NUM> carbon atoms, and preferably wherein the alkyl group has <NUM> to <NUM> atoms, preferably methyl or ethyl, and most preferably methyl;
and
(B) a composition comprising a silanol terminated polysiloxane prepolymer obtainable by sol-gel process of an alkoxysilane selected from a compound of general formula (II), (III) or (IV):
<CHM>
<CHM>
<CHM>
wherein:
X<NUM>, X<NUM>, X<NUM>, X<NUM>, X<NUM>, and X<NUM> each represents -OR<NUM>;
R<NUM> at each occurrence is the same or different, and each is independently selected from an alkyl group, wherein the alkyl group has <NUM> to <NUM> carbon atoms, preferably <NUM> to <NUM> atoms, more preferably <NUM> to <NUM> carbon atoms, and most preferably <NUM> to <NUM> carbon atoms; and
m represents an integer from <NUM> to <NUM>, preferably <NUM> to <NUM>.