Abstract:
A transparent porous SiO2-coating for a transparent substrate material has improved optical properties. These properties can be obtained, in particular, by plasma treatment.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and hereby claims priority to PCT Application No. PCT/EP2008/060592 filed on Aug. 12, 2008 and DE Application No. 10 2007 043 653.1 filed on Sep. 13, 2007 the contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to the field of transparent materials, in particular transparent materials whose reflection is reduced. 
         [0003]    With many transparent materials, in particular transparent materials based on plastic, such as for instance polycarbonate etc., the difficulty arises that these have, in part, unwanted reflection properties, which hamper or even render impossible their use in many applications. 
         [0004]    Numerous attempts were thus undertaken to design transparent materials to be less reflective, in particular by applying further layers. 
         [0005]    To this end, it was proposed for instance, by a so-called “flower-like alumina” layer, to reduce the reflection (see Yamaguchi et al, Journal of Sol-Gel Science &amp; Technology, 2005,33, 117-120). This procedure nevertheless requires a tempering step with increased temperatures (approx. 400° C.). 
         [0006]    Other proposed coatings include multi-layer systems with changing refractive indices, e.g. multi-layer systems made of SiO 2  and TiO 2 . Tempering steps, generally at temperatures above 400° C., are also needed here (see M. Walther, OTTI-Seminar Regensburg, September 2005). Other systems use layers made of TiO 2  and MgF 2  (see EP 564 134 B1), in which additional fluorocarbon resins are provided. This system is in turn disadvantageous in respect of poor applicability. 
       SUMMARY 
       [0007]    The object is therefore to create a transparent coating for a transparent substrate material, which overcomes the afore-cited disadvantages at least partially and can in particular be applied in a simple manner. 
         [0008]    Accordingly, the inventor proposes a transparent coating for a transparent substrate material, which is characterized in that the coating is based on SiO 2 , has a porosity of ≧35% and ≦65% and the transmission according to the wipe test amounts to ≧95%. 
         [0009]    The term “based on SiO 2 ” within the meaning of this description means or includes in particular that the coating contains SiO 2  as a main component. Preferably ≧70%, even more preferably ≧80% and most preferably ≧90% to ≦100 of the coating is made of SiO 2 . 
         [0010]    The term “transparent” within the meaning of this description means or includes here in particular a permeability of ≧90% in the wavelength range used respectively, in particular in the visible wavelength range. 
         [0011]    The term “transmission according to the wipe test” relates here in particular to the measurement of the transmission according to the wipe test, as described in the method part. 
         [0012]    In many applications, such an SiO 2  coating can achieve one or several of the following advantages:
       the coating is essentially homogenous to the human eye and a single coating is sufficient for many applications (contrary to the multi-layer systems cited above).   the coating can, as described again below, with the majority of applications, be applied by simple dip-coating and subsequent plasma treatment in terms of its desired porosity can be introduced so that complicated application steps to be implemented in particular at a high temperature can be avoided.   the thickness of the manufactured coating amounts, as described again below, in many applications in the range of 50-200 nanometers. It is therefore largely insensitive to thermal and mechanical stress (in particular bending stress) and only marginally influences component dimensions and tolerances.       
 
         [0016]    According to a more preferred embodiment, the porosity of the coating amounts to between ≧40% and ≦60%, still preferred between ≧45% and ≦55%. 
         [0017]    According to a preferred embodiment, the transmission according to the wipe test amounts to ≧97%. 
         [0018]    A preferred embodiment is characterized in that the thickness of the coating lies between ≧90 nm and ≦130 nm. This has proven particularly favorable for many applications. The thickness of the coating preferably lies between ≧95 nm and ≦115 nm. 
         [0019]    A preferred embodiment is characterized in that the refractive index n 1  of the coating lies between ≧0.8*√{square root over (n 2 )} and ≦1.2*√{square root over (n 2 )}, with n 2  being the refractive index of the substrate. In this case, the reflection can be reduced again for many applications. The refractive index n 1  of the coating preferably lies between ≧0.9*√{square root over (n 2 )} and ≦1.1*√{square root over (n 2 )}. 
         [0020]    A preferred embodiment is characterized in that the coating is essentially a porous full mold body, in particular a homogenous porous full mold body, and/or forms such a body. 
         [0021]    The term “essentially” refers here in particular to ≧90 vol %, preferably ≧95 vol % of the coating. 
         [0022]    As a result, an easily producible and further reflection-reducing coating can be achieved in many applications. 
         [0023]    A preferred embodiment is characterized in that the coating has transmission-increasing properties, in particular for light in the visible wavelength range. 
         [0024]    The coating is preferably able to increase the transmission of the substrate by ≧2%, preferably by ≧4% in the wavelength range used respectively, in particular in the visible wavelength range. 
         [0025]    A preferred embodiment is characterized in that the average diameter of the pores amounts to between ≧1 nm and ≦50 nm. With many applications, this has shown to be particularly favorable for the anti-reflective properties of the coating. The average diameter of the pores preferably amounts to ≧2 nm to ≦40 nm, still preferably ≧3 nm to ≦20 nm. 
         [0026]    A preferred embodiment is characterized in that the diameter of ≧90% of the pores amounts to between ≧1 nm to ≦50 nm. 
         [0027]    A preferred embodiment is characterized in that the diameter distribution of the pores of the coating essentially follow a log-normal distribution with a half-width of ≦20 nm, preferably ≦10 nm, even more preferably ≦8 nm. 
         [0028]    “Essentially” means here that ≧90% of the pores, preferably ≧95% of the pores and most preferably ≧98% of the pores follow this distribution. 
         [0029]    A distribution of this type has proven particularly favorable for many applications, since an optically special homogenous coating can thus be achieved. 
         [0030]    A preferred embodiment is characterized in that the coating is manufactured by a Sol-Gel method and subsequent plasma treatment. 
         [0031]    The inventor also proposes a method for manufacturing a transparent coating for a transparent substrate material, characterized in that the method relates to a Sol-Gel process. 
         [0032]    The term “Sol-Gel process or Sol-Gel method” within the meaning of this description means or includes in particular all processes and/or all methods, in which metal precursor materials, in particular metal halogenide and/or metal alkoxide in solution are subjected to hydrolysis and subsequent condensation. 
         [0033]    The term “plasma treatment” within the meaning of this description means or includes in particular all processes and/or methods, in which ionized molecules, in particular radicals of a gas, which was generated by an energy source such as high frequency or microwaves, act on the substrate. This is mostly associated with an increase in temperature. 
         [0034]    In the case of coatings and/or substrates, it has surprisingly transpired that a significant improvement in the surface quality and further properties can again be achieved in many applications by plasma treatment. 
         [0035]    It is noted that within the meaning of this description “plasma treatment” also relates in particular to a corona treatment, even if this is often handled differently in terms of linguistic use. A plasma treatment within the meaning of this description thus explicitly also includes a corona treatment; this represents in particular a preferred embodiment. 
         [0036]    According to a preferred embodiment, the plasma treatment is implemented as an atmospheric pressure plasma. This has proven very advantageous in many applications. 
         [0037]    The term “atmospheric pressure plasma” within the meaning of this description means or includes in particular all processes and/or methods, in which a plasma is applied to a substrate under atmospheric environmental conditions. 
         [0038]    The plasma treatment is preferably implemented at a process gas pressure of ≧2 bar to ≦8 bar. This has proven to be advantageous with many applications. The process gas pressure preferably lies at ≧3 bar to ≦6 bar, preferably ≧3.5 bar to ≦5 bar. 
         [0039]    The plasma treatment is preferably implemented with high voltage and/or rotation of the nozzle. 
         [0040]    A preferred embodiment is characterized in that during at least one part of the Sol-Gel process, at least one porosity-producing component is present, which is removed and/or destroyed upon termination of the Sol-Gel process. 
         [0041]    The porosity-producing component is preferably removed and/or destroyed at least partly by the plasma treatment. 
         [0042]    A preferred embodiment is characterized in that the at least one porosity-producing component is a polymer, with the average molar mass of the polymer preferably amounting to between ≧5000 Da and ≦50000 Da, still preferably between ≧10000 Da and ≦35000 Da. 
         [0043]    A preferred embodiment is characterized in that the polymer is an organic polymer, preferably selected from the group containing polyethylene glycol, polypropylene glycol, copolymers made of polyethylene glycol and polypropylene glycol, polyvinylpyrrolidone, polyether, alkyl, cycloalkyl and/or aryl-substituted polyether, polyester, alkyl-, cycloalkyl- and/or aryl-substituted polyester, in particular polyhydroxy butanoic acid or mixtures thereof. 
         [0044]    General groups/molecule definition: General groups or molecules, such as alkyl, alkoxy, aryl etc. are described. If not described otherwise, the following groups are preferably used within the generally described groups/molecules.
   Alkyl: linear and branched C1-C8 alkyls,   Long-chain alkyls: linear and branched C5-C20 alkyls   Alkenyl: C2-C6 alkenyl,   Cycloalkyl: C3-C8 cycloalkyl,   Alkoxide/alkoxy: C1-C6 alkoxy, linear and branched   Long-chain alkoxide/alkoxy: linear and branched C5-C20 alkoxy   Aryl: selected from aromatic compounds with a molecular weight below 300 Da.   Polyether: selected from the group containing H—(O—CH 2 —CH)R)) n —OH and H—(O—CH 2 —CH(R)) n —H, with R being selected independently from: hydrogen, alkyl, aryl, halogen and n from 1 to 250.   Substituted polyether: selected from the group containing R 2 —(O—CH 2 —CH(R 1 )) n —OR 3  and R 2 —(O—CH 2 —CH(R 2 )) n —R 3 , with R 1 , R 2 , R 3 , being selected independently from: hydrogen, alkyl, long-chain alkyls, aryl, halogen and n from 1 to 250.   Ether: the compound R 1 —O—R 2 , with each R 1  and R 2  being selected independently from the group containing hydrogen, halogen, alkyl, cycloalkyl, aryl, long-chain alkyl.   
 
         [0055]    Except as mentioned otherwise, the following groups/molecules are more preferred groups/molecules within the general groups/molecule definition:
   Alkyl: linear and branched C1-C6 alkyl,   Alkenyl: C3-C6 alkenyl,   Cycloalkyl: C6-C8 cycloakyl,   Alkoxy, Alkoxide: C1-C4 alkoxy, in particular isopropyloxide   Long-chain alkoxy: linear and branched C5-C10 alkoxy, preferably linear C6-C8 alkoxy   Polyether: selected from the group containing H—(O—CH 2 —CH(R)) n —OH and H—(O—CH 2 —CH—(R)) n —H, with R being selected independently from: hydrogen, alkyl, aryl, halogen and n from 10 to 250.   Substituted polyether: selected from the group containing R 2 —(O—CH 2 —CH(R)) n —OR 3  and R 2 —(O—CH 2 —CH(R 2 )) n —R 3 , with R 1 , R 2 , R 3 , being selected independently from: hydrogen, alkyl, long-chain alkyls, aryl, halogen and n from 1 to 250.   
 
         [0063]    A preferred embodiment is characterized in that the silicon is added in the form of a silicon alkoxide precursor solution. 
         [0064]    A preferred embodiment is characterized in that the pH value of precursor solution containing silicon amounts to between ≧1 to ≦6. 
         [0065]    The inventor also proposes a transparent coating for a transparent substrate, manufactured in accordance with the method. 
         [0066]    The inventor further proposes an optical component comprising a transparent substrate and a coating applied and/or arranged on the substrate. 
         [0067]    A preferred embodiment is characterized in that the substrate is selected from the group containing glass, transparent plastics, preferably selected from the group containing polycarbonate, polyacryl, PMMA and mixtures thereof, as well as mixtures thereof. 
         [0068]    The inventor further proposes a method for manufacturing an optical component, characterized in that the coating is applied to the substrate by dip-coating and/or spin-coating and is subsequently subjected to a plasma treatment. 
         [0069]    The inventor further proposes the use of a coating and/or an inventive optical component for
       optical instruments   spectacles   headlamp housings in automotive engineering   window glass, particularly in automotive engineering   cockpit glazing       
 
         [0075]    The afore-cited components to be used and described in the application examples are not subject to any particular exceptional conditions in terms of size, design, material choice and technical conception, so that the selection criteria known in the application field can be used unrestrictedly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0076]    These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
           [0077]      FIG. 1  a diagram with three transmission measurements of a polycarbonate substrate coated in accordance with a first potential embodiment of the invention, a polycarbonate substrate coated in accordance with a comparative method and an uncoated polycarbonate substrate. 
           [0078]      FIG. 2  shows the head of a plasma cannon with emerging plasma and an overall view of the plasma cannon. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0079]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0080]      FIGS. 1 and 2  relate to the example described below: 
       EXAMPLE I 
       [0081]    An optical component based on polycarbonate was manufactured as follows: 
         [0082]    Two solutions were initially prepared: 
         [0000]    Solution 1: 4 g polyethylene glycol (molar mass 35000 Da) was prepared in 50 ml of ethanol and slowly added to water while being stirred until a complete solution emerges. 4 drops of 1 N HCl are then added.
 
Solution 2: 4 ml tetraethoxysilane in 20 ml EtOH
 
         [0083]    Solution 2 was then filled up with solution 1 to 50 ml and stirred for 2 hours. 
         [0084]    The polycarbonate substrate was then dip-coated with the solution (speed 55 mm/min). It was then air-dried and tempered at 100° C. for 1 hour in the oven. After cooling, it was placed in water for one minute. 
         [0085]    The coating was subsequently subjected to a plasma treatment, a device by the company Plasmatreat was used with a high frequency generator FG 3001 and a rotational nozzle RD 1004, which was equipped with a nozzle head of type AGR123. 
         [0086]    The porosity of the coating amounted here to 55%, the transmission in accordance with the wipe test amounted to 97%. 
       COMPARATIVE EXAMPLE 1 
       [0087]    For comparison purposes, a second coating was produced, with the plasma treatment having been left out. 
         [0088]      FIG. 1  shows a diagram with three transmission measurements of a polycarbonate substrate according to example I (“TPC plasma”), a polycarbonate substrate according to comparative example I (“TPC wash”) and an uncoated polycarbonate substrate (“TPC”). There is a clear improvement in the transmission, particularly in the wavelength range around 400-450 nm both in comparison with the comparison method and the uncoated substrate. 
         [0089]      FIG. 2  shows the functional principle of an atmospheric pressure plasma, which relates primarily to a plasma being generated at normal pressure, without an additional reaction vessel, which assists with maintaining a pressure level which differs from the atmospheric pressure. 
         [0090]    The core is the plasma generator, in which the central electrode  3 , the outer, potential-free electrode  5  and the insulator  4  form a discharge zone. The high voltage generator  8  now converts the mains voltage into the high voltage (5-15 kV, 10-100 kHz), which generates a pulsed arc  7 . The supply voltage  13  and the process gas  10 , which is usually oil-free pressurized air, are fed to the discharge zone through the flexible gas duct  6 . The pressurized air firstly reaches the gas control block  12  by way of the pressurized air connector  11 . The gas is passed through a flexible tube  8 , in which gas and current supply lines to the discharge zone are accommodated, via the gas duct  6  to the discharge path and is excited in the discharge zone into the plasma state. The air flow transports the developing active species (i+, e−, r*) which develop in the arc  7 , out of the discharge zone. A potential-free plasma jet (active gas jet) is produced. 
       Wipe Test 
       [0091]    The transmission according to the wipe test of coatings was measured in the following way: 
         [0092]    A 180 g hammer, on the end of which was fastened, by adhesive tape, a standard cleaning fleece from the company Clear Clean “Absormat” saturated with a standard cleaning agent “Sidolin Streifenfrei Zitrus” (Henkel), was pulled across the respective coating 50 times depending on the sample side, without a further vertical force effect. 
         [0093]    The transmission of the coating was then measured in the “Haze-Guard PLUS” (Byk-Gardner). 
         [0094]    The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in  Superguide  v.  DIRECTV , 69 USPQ2d 1865 (Fed. Cir. 2004).