Abstract:
A protective device for a glazed structure, in particular an aircraft windscreen  20,  comprises at least one removable sacrificial sheet of transparent composite  10.  The composite  10  comprises a transparent polymeric film  11  having on one side an electrically conductive layer  12  formed from a dispersion of electrically conductive particles and which is coated with a transparent hard coat  13,  with the other side having adhesive layer  14  thereon. Sheets of the composite  10  may be arranged in a stack so that each sheet adheres to the adjacent underneath sheet with the uppermost sheet of each stack being removable as the sheet becomes damaged and/or dirty.

Description:
FIELD 
       [0001]    This invention relates to a transparent polymeric film composite which is suitable for the protection of glazed surfaces which are particularly susceptible to a build up of electrostatic charge. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is well known to protect a vehicle windshield by the use of a protective film cover that overlies the windscreen and is adhered to the windscreen, see for example WO 99/2840. The protective cover can be easily removed by peeling from the windscreen after use. DE-A-3221-766 discloses a self adhering transparent film that is used to protect glass surface on motor vehicles and aeroplanes and which allows the glass surface to be cleaned by removal of the film. 
         [0003]    A multilayer protective film composite for an automobile windshield is disclosed in U.S. Pat. No. 5,002,326. The different layers of film are removed successively as each film surface becomes dirty to expose a new clean film surface to improve visibility for a driver. 
         [0004]    The transparent windshield or canopies of aircraft, in particular, helicopters are expensive and may become abraded or scratched when the aircraft is used in a harsh environment in which the air may filled with dust or sand particles such as may be found when operating is deserts. 
         [0005]    The exposed surfaces of helicopter windscreens, in particular, accumulate large amounts of static electricity during their operation of the helicopter and this static electricity is dissipated to earth when the helicopter touches down. 
         [0006]    It has been found that if the windshield or window of a helicopter is protected from scratches and abrasions by means of a polymeric film over layer there is an electrostatic charge build up on the outer surface of the film which discharges into the helicopter windscreen on landing and consequently burns holes in the protective over layer. Similar problems may arise when the helicopter rotors are running whilst the aircraft is sitting on the ground. 
         [0007]    EP1154000 describes a polymeric film having a conductive thin film for static electricity prevention. The thin film comprises a layer of a resinous binder containing metal oxide particles and has a superior transparency with a total light permeability of at least 80% and a haze value of no greater than 5%. The conductive thin film is applied to glass cases, CRT screens, and as an antistatic material to clean room floor and walls. 
         [0008]    The present invention provides a film composite which can be used as an overlay to protect aircraft windscreen from abrasions and scratches and which also prevents a build up of static electricity especially in dry and desert conditions. 
       STATEMENTS OF THE INVENTION 
       [0009]    According to one aspect of the present invention there is provided a transparent protective polymeric film composite for laying over a surface, for example glazing, the protective composite comprising a transparent polymeric film having on one side an electrically conductive layer formed from a dispersion of electrically conductive particles, the layer being coated with a transparent hard coat, said one side in use facing away from said surface. 
         [0010]    The conductive layer may be formed from a dispersion of electrically conductive nanoparticles comprising at least one of carbon, a metal or metal oxide. The metal nanoparticles may comprise nanoparticles of aluminium, silver, gold, platinum or metal coated nanoparticles. The metal oxide nanoparticles may comprise nanoparticles of ITO (indium tin oxide), fluorine doped tin oxide, tin oxide, titanium oxy nitride, antimony doped zinc oxide and preferably the metallic oxide is ATO (antimony tin oxide). The nanoparticle size should be of less than  0 . 1  microns diameter. 
         [0011]    Alternative conductive layer may be provided by a dispersion of a electrically conductive polymer such as polythiophene e.g PEDOT-PSS (Baytron) available from Bayer. 
         [0012]    In general, the transparent composite can be used as an overlay for aircraft windows, canopies etc. to combine the advantages of a removable protective film with static electricity prevention, without loss of window transparency. 
         [0013]    A protective device for an aircraft window may also comprise a plurality of sheets of transparent composite according to the Invention arranged in a stack so that each sheet adheres to the adjacent underneath sheet. Such an arrangement is described in EP 1489 147. The hard coat may contain a siliconized acrylate resin to assist in removal of the adjacent upper sheet. 
         [0014]    The polymeric film may comprise one of polycarbonate, acrylic, polypropylene and PET, the preferred film being PET. The film is preferably a PET (polyethylene terephthalate) film, preferably having a thickness of between 4 mil to 7 mil (0.1 to 0.175 mm) and which m-lay contain a UV absorbing material as is disclosed in U.S. Pat. No. 6,221,112. 
         [0015]    The composite preferably has a surface resistivity of less than 1×10 9  ohms/square at 100 volts and when applied to glass the film composite/glass combination has optical properties such that it has a % VLT of at least 75%, preferably greater than 80%, a Haze value of less than 5%. 
         [0016]    The conductive nanoparticles may be dispersed in a layer on one side of the film with the hard coat being coated onto said layer. Preferably, the conductive layer comprises ATO dispersed on the surface of the film with a maximum areal density of 1.0 gms per m 2  and preferably between 0.16-1.0 gms per m 2 . Such a layer has a surface resistivity of 3.3×10 7  ohms/square at 10 volts. 
         [0017]    The conductivity of the composite is influenced by the thickness of the conductive layer, however the thicker the ATO layer then the lower the adhesion of the hard coat to the polymeric film. An increase in thickness of the conductive layer also affects the optical properties of the film composite. 
         [0018]    In the preferred embodiment the hard coat is a UV curable acrylate based resin as is described in U.S. Pat. No. 4,557,980 the contents of which are hereby incorporated into the present specification. The hard coat after curing and drying has a thickness of about 1.8 microns and a pencil harness of about 2 H. The composite will have a surface resistivity of about 1.9×10 8  ohms/square at 100 volts. 
         [0019]    The other side of the PET film is coated in a suitable adhesive for adhering the composite to the glazing, and is preferably a pressure sensitive adhesive such as the solvent based adhesives including National Starch 80-1057. Suitable releasable clean peel adhesives may also be used, for example Gelva GMS 3149 (available from Cytec Inc. Surface Specialities), which in use adhere to the film layer. 
         [0020]    A release liner may be laminated over the adhesive coating. 
         [0021]    Glazing includes any suitable transparent material which may be used for vehicle windscreens, aircraft canopies and windscreen and windows etc. and which include glass, acrylic sheet, polyester sheet, polycarbonate sheet. 
         [0022]    Such a composite provides a sacrificial layer which protects a windscreen from damage due to abrasion by dirt etc. and which dissipates static electricity, and has good optical properties in the visible and near infra-red. Good IR transmission allows for the use of night vision goggles or other night vision instruments through a protected screen. 
         [0023]    Another aspect of the invention provides an aircraft window protector comprising at least one sheet of transparent composite itself comprising a transparent polymeric film having on one side an electrically conductive layer coated with a transparent hard coat, said one side in use facing away from said surface, the film having on its other side an adhesive for adherence to the window. 
         [0024]    The protector may comprise a plurality of sheets of transparent composite comprising a transparent polymeric film having on one side an electrically conductive layer coated with a transparent hard coat containing a surface energy reducer, said one side in use facing away from said surface, the film having on its other side an adhesive, the sheets being arranged in a stack so that each sheet adheres to the adjacent underneath sheet with the uppermost sheet of each stack being removable. 
         [0025]    Yet another aspect of the Invention provides a method of protecting an aircraft windscreen or canopy from damage due to abrasion by dirt etc. wherein in said method the windscreen is provided with a removable sacrificial layer comprising a transparent polymeric film composite which protects a windscreen, has good optical properties has optical properties in the visible such that it has a % VLT of at least 80%, a Haze value of less than 5%, and is sufficiently transmissive in the visible/near infra-red wavelengths (600-1000 nm) to allow unimpaired use of night vision goggles, and dissipates static electricity. 
         [0026]    Also according to yet another aspect of the present invention, there is provided a method of manufacture of an anti-static protective transparent film composite in which method an aqueous dispersion of an electrically conductive material is mixed with a suitable liquid and applied to a surface a transparent film, the dispersion is dried, and then coated with a scratch resistant coating. 
         [0027]    The dispersion preferably comprises a nanoparticle dispersion, preferably of metal or metal oxide, mixed with an organic solvent. 
         [0028]    The metal oxide is preferably ATO and the aqueous dispersion is mixed with water miscible solvents such as methanol, isopropanol, and pyrol, to form a liquid composition having a lower surface tension and increased viscosity, thereby improving the quality of the coating and eliminating “dewets”. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0029]    The invention will be described by way of example and with reference to the accompanying drawings in which: 
           [0030]      FIG.1  is a cross-section through a first protective film composite according to the present invention, 
           [0031]      FIG. 2  is a cross-section through the composite of  FIG. 1  shown in situ on glazing, and 
           [0032]      FIG. 3  is a cross-section through a second composite according to the present invention. 
           [0033]      FIG.4  is a graph of static charge retention versus time for a windscreen and windscreen covered with prior art protective film, and 
           [0034]      FIG. 5  is a graph of static charge retention for a windscreen and for windscreen covered with film according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    With reference to  FIG. 1  there is shown a protective film composite  10  comprising a suitable transparent polymeric film  11  coated on one side with an electrically conductive layer  12 , preferably of a conductive metal oxide, which in turn is over coated with a scratch resistant hardcoat  13 . The other side of the polymeric film is coated with a transparent adhesive layer  14  covered with a protective release liner  15 . 
         [0036]    Suitable transparent films  11  are polycarbonate film, acrylic film and polyester film, preferably a polyethyleneterephthalate (PET) film treated with a UV absorber as described in U.S. Pat. No. 6,221,112B so as to absorb up to 99% of UV radiation. A suitable PET film is DuPont Teijin Films&#39; Melinex 454. The film has a thickness of about 7 mil (175 microns). 
         [0037]    The electrically conductive layer  12  is formed from nanoparticles of ATO (antimony doped tin oxide). A 22% aqueous dispersion of ATO (available from LWB Einhoven BV, Netherlands) is modified by the addition of water miscible solvents for example, methanol, isopropyl alcohol, and pyrol. The resulting liquid composition has a lower surface tension coupled with a higher viscosity allowing the mixture to be coated into the PET film  11  using known coating techniques, for example, roller coating, reverse and forward gravure techniques, and slot die coating. In the present example the coating was applied by reverse gravure techniques. 
         [0038]    The composition typically comprises (% by weight) 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 100 
                 parts of 
                 22% ATO aqueous dispersion 
               
               
                 8.9 
                 parts 
                 N-methyl pyrrolidone 
               
               
                 22.3 
                 parts 
                 methanol 
               
               
                 13.2 
                 parts 
                 isopropyl alcohol 
               
               
                   
               
             
          
         
       
     
         [0039]    The coating was dried at 140° F. (60° C.) and has an areal density of ATO of between 0.16-1.00 gsm. The surface resistivity was measured at 3.3×10 7  Ohms/square at 10 volts using a Keithley Model 6517A High Resistance Meter connected to a Model 8009 Resistivity Fixture. 
         [0040]    Although the conductivity of the ATO layer may be increased by increasing the thickness of the layer  12  if the areal density of ATO is greater than 1.00 gsm the adhesion of the hard coat  13  becomes unacceptably low and the optical properties of the composite  10  are adversely affected. 
         [0041]    In order to provide for a good dissipation of static electricity coupled with good optical properties, that is a % VLT (Visible light transmission) of better than 80% with a % haze &lt;2, and high transmission in range 600-1000 nm, the areal density of ATO should be between 0.16-1.00 g/m 2 . 
         [0042]    The hard coat  13  is a UV cured acrylate based resin which is formed from a liquid composition which is applied over the dried ATO dispersion by any suitable process. The coating composition may comprise a resin and solvent base as is described in U.S. Pat. No. 4,557,980. The coating composition used for the hard coat layer  13  is formed from a liquid composition which is applied to the surface of the PET film by a reverse gravure process. The coating composition may comprise a resin and solvent base as is described in U.S. Pat. No. 4,557,980 and typically comprise the constituents of Table  1  below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Acrylate resin 
                 30–75% 
               
               
                   
                 Acrylic Acid 
                  0–45% 
               
               
                   
                 Solvent 
                  0–40% 
               
               
                   
                 Photoinitiator 
                 2.4–5.0% 
               
               
                   
                   
               
             
          
         
       
     
         [0043]    The percentages are weight percentages of the coating mixture. 
         [0044]    The acrylate resin is preferably a mixture of pentaerythritol tetraacrylate and triacrylate. A suitable material is Sartomer SR-295 available from Sartomer (Total). Suitable solvents, in addition to the acrylic acid which acts as a solvent, are isopropyl alcohol and MEK (methylethyl ketone). 
         [0045]    The ingredients for the coating are mixed together and the stable mixture is stored for later use. 
         [0046]    If a siliconized acrylate resin is to be added to the hardcoat, then 0.04-0.7% siliconized acrylate (Ebercryl 1360 available from UCB Chemical Corp) should be added to the hard coat composition. 
         [0047]    The hard coat composition is applied using a reverse gravure process in a thickness of about 1-6 microns and coats evenly and levels smoothly. After application to the PET film the coating remains stable until drying, and UV curing after drying. The final cured dried hard coat has a thickness of about 2 microns, more typically between 1.5-2.5 microns. 
         [0048]    The hard coat has the following typical physical properties: 
         [0049]    Haze &lt;1%, 
         [0050]    Gloss 60 degree gloss 100 gloss units 
         [0051]    Scratch resistant to 0000 Steel Wool 
         [0052]    Abrasion &lt;12% change in Tabor haze. 
         [0053]    Pencil Hardness 2H-3H 
         [0054]    Pencil Hardness is measures according to ASTM D3363-92a 
         [0055]    The Gloss was measured using a Byk Gardner Glossmeter. 
         [0056]    The haze was measured using a Hunter Laboratories Ultrascan XE and calculated according to (Diffuse Transmittance/Total Transmittance)×100 over a light range of 380-780 nm. 
         [0057]    The scratch test is a subjective test in which the coating is rubbed with steel wool and viewed for scratching. 
         [0058]    The abrasion test uses a Taber Abrader in accordance with ASTM D1044-93 using a CS10 wheels each loaded with 1 kg. The results are quoted in an increase in haze after 100 cycles. 
         [0059]    The adhesive layer  14  is a solvent based pressure sensitive adhesive applied to the underside (in use) of the film  11  using slot die coating, or other suitable techniques and dried at 60° C. A suitable adhesive is National Starch 80-1057 modified with Tinuvin 328 to improve durability. As an alternative, the adhesive  14  could comprise an easy peel type adhesive for example Gelva GMS 3149 which adheres preferentially to the film. 
         [0060]    The adhesion of the film composite to any underlying glazing must lie between particular limits. The adhesion must be sufficient to prevent easy release of the film composite during use but must not be so adhesive as to damage the glazing when the composite is removed. 
         [0061]    The release liner  15  may comprise a polyethylene coated paper, or PET film with a silicone release coating, which can be peeled from the adhesive leaving the adhesive layer on the film  11 . 
         [0062]    The application of the film composite  10  to a windscreen  20  comprises are series of steps. The windscreen is cleaned using a non-hazardous film application solution comprising at least a mixture of detergent and water. The film composite sheet is cut to size and moulded to the shape of the surface to be protected. The release liner  15  is removed from the composite  10  and both the windscreen and adhesive layer  14  are sprayed with said solution. The film is placed over the surface and smoothed into place, expelling all air pockets. The adhesive layer  14  is then allowed to cure for  24  hours. 
         [0063]      FIG. 2  shows a composite  10  in place on a windscreen shown with the release liner  15  removed and the composite  10  adhered to glazing  20  for example a helicopter windscreen. 
         [0064]    In use, the protective film composite  10  may be cleaned using the standard windscreen cleaning techniques. The composite  10  is not harmed by standard window cleaning chemicals, for example Windex. 
         [0065]    Film Clarity 
         [0066]    The optical clarity of the windscreen protected by composite  10  was tested for comparison with a unprotected screen, by means of a subjective test in which an observer viewed optical charts through the screens at various distances. There was no noticeable difference between the two windscreens. 
         [0067]    Compatibility with Night Vision Goggles 
         [0068]    The film composite  10  was tested by means of subjective test in which pilots equipped with night vision goggles flew helicopters having half the windscreen covered in the composite film. The pilots flew for periods of 1.5 hours in various light levels from rural dark to well lit urban environments. The testing showed that the composite film  10  did not affect night vision goggle performance. 
         [0069]    Electrostatic Testing 
         [0070]    Electrostatic Testing was performed by applying static charge using a high voltage, low current device. The induced charge and charge decay characteristics were measured on the bare windscreen and windscreen covered with prior art none conductive protective film for a 35 kV induced charge. 
         [0071]    The results of the test showed that the protective film acted as a capacitor, storing up charge until a level was reached and the built-up charge would arc to the nearest conductive-material, the windscreen. The windscreen dissipated the 35 kV-induced charge in less than five minutes, whereas the very insulative protective film effectively held a charge greater than 8 kV for more than five minutes, as seen in  FIG. 4 . The film was also observed to hold a charge of 8 kV for up to 30 minutes. The protective film held the charge in pockets until enough was built-up, where it would then arc through the film to the windscreen. There was no visible arcing, however small burns ranging in size from a pencil tip to an eraser were evident in the protective film. There were 5 to 10 noticeable burn holes generated on each protective film after one full charging test. Analysis determined that locations of the holes were driven by underlying contaminants, acquired during installation. 
         [0072]    Electrostatic Testing of Film with Conducting Protective Coating. 
         [0073]    Two 8 inch×4.5 inch (200 mm×112.5 mm) samples of composite  10  according to the present invention, as well as a sample of the prior art non-conductive film were installed on an aircraft windscreen. The non-conductive control film test data obtained on the second test exactly matched the data collected during the first test. The sample of composite  10  were comparable to the plain windscreen in the manner in which the static electricity discharged, as seen in  FIG. 5 . 
         [0074]    The composite  10  has a resistivity less than 1×10 9  ohms/square, preferably about 2.0×10 8  Ohms/square, and typically 1.9×10 8  Ohms/square in order to dissipate static electrical charge from the windscreen and prevent damage to the composite. 
         [0075]    Resistance to Use of Wipers 
         [0076]    Wipers of an aircraft fitted with the protective film were operated for 1 minute on each a dry and moist windscreen while the aircraft was parked on the ground. A follow-on test evaluated the same criteria during in-flight operation of the dry wipers. The tests were also repeated under a moist windscreen/wiper condition. The results of the test showed that the composite  10  was not affected by the usage of dry or moist wipers on the ground or during flight. 
         [0077]    Resistance to Windspeed 
         [0078]    A speed sweep was performed on aircraft that had the protective film installed to evaluate the films ability to stay adhered to the windscreen. The tested speeds ranged from hover to 310 knots. The testing showed that the protective film was not affected by the speed of the aircraft. The film remained clear and attached under each flight speed. 
         [0079]    Resistance to Temperature 
         [0080]    The protective film&#39;s ability to remain clear and adhered to the windscreen was evaluated for low temperatures. The aircraft windshield&#39;s operating temperature range is from −65-160° F. (−55-70° C.). 
         [0081]    A sample of the 7 mil protective film was attached to a piece of glass. A preliminary test was performed where the sample was cold temperature cycled numerous times from −15-70° F. (−25-21° C.) to determine if there was any shrinkage or peeling in the protective film and if any discoloration, bubbling, or hazing occurred. The test results showed no anomalies and the film adhesive strength was not affected by the cold temperature. 
         [0082]    Through flight-testing, the protective film was evaluated in the temperature range of 35° C. to −35° C. The results of the temperature evaluation showed that the protective film was not affected by temperature. The film remained clear and attached under each evaluated temperature. 
         [0083]    Durability 
         [0084]    The protective film&#39;s durability and ability to protect the underlying windscreen was evaluated throughout the test program. Sand blasting testing showed that a film covered windscreen could last nearly twice as long as the glass windscreen alone before needing maintenance. The film was flight tested for more than 100 hours between the three test aircraft. The protective film has flown in harsh operating extremes, such as brown-out and hot/old temperature conditions. The film&#39;s durability was evaluated during the brown-out condition testing. Two aircraft, were submitted to brown-out conditions during landings and take-offs, where the windscreens were blasted with dust, sand, and rocks. Throughout the 7 days of testing, each aircraft logged over 50 take-offs and landings. After each test day the aircraft&#39;s windscreens were evaluated and a comparison was made between the windscreen with the protective film and the one without. Throughout the test, the windscreens began to pit and show damage. The windscreen with the protective film was protected and remained unaffected by the elements, where as the windscreen without began to become more difficult to see through due to the pitting and other damages. 
         [0085]    Another embodiment of the Invention is shown in  FIG. 3 , which shows a plurality of sheet of composite  10  stacked one on the other on a windscreen  20 . The hard coat layers  13  incorporate a siliconized acrylate resin to reduce the surface energy to enable upper sheets of composite to be removed from underlying sheets as the upper most sheet becomes damaged and difficult to see through.