The invention relates to a method of producing oxidic, antireflective coatings on transparent plastic substrates of the acrylic glass group, especially on sheets and lenses of polymethylmethacrylate, in which the substrate is vacuum coated with a first layer of high refractive index followed by a second layer of low refractive index.
The demand for stable antireflective coatings on transparent substrates has long existed. The fact that they increase transmission by about 8 to 10% is less important than the fact that they eliminate undesirable light reflections, which is especially important when the substrate in question is situated between an observer or viewer and an object. This is the case, for example, with the windshields of aircraft and motor vehicles and with eyeglass lenses. Especially in darkness, light rays coming from behing the observer and striking the substrate produce reflections which are several times brighter than the object being observed and they virtually blind the observer. The rays of the sun coming from behind and striking the glazing of a helicopter cockpit, or headlight beams striking the windshield of a car from behind, are examples of this situation. But even light rays coming from the direction of the object, that is, coming from ahead of the observer, produce undesirable reflections when, after passing through the substrate, they are reflected from behind, as for example from bright clothing or from the cornea of the human eye. The latter circumstance is especially important in the case of eyeglasses.
While the problem of the antireflective coating of inorganic substrates has been satisfied in a substantially satisfactory manner, the situation is still poor in the case of plastic substrates. The reason for this is to be found in the fact that, on the basis of physical laws, the index of refraction of the antireflective coating must be lower than the index of refraction of the substrate. This excludes a large number of oxidic materials from the production of antireflective coatings. Of the remaining materials, some are hard to vaporize, and in the case of others the substrate has to be heated to between 300.degree. and 350.degree. C. either during the vapor coating or subsequently thereto, in order to achieve the desired coating properties. Such substrate temperatures, however, cannot be applied to plastic substrates for obvious reasons. For example, magnesium fluoride, which can be used on inorganic substrates, cannot be used on plastic substrates, because microfissures occur in the coating, and the coating is removed by the sweat test. Furthermore, the single-layer coating does not produce a sufficient antireflective effect. The reasons for this are familiar to the average technical person. There is a decided tendency, therefore, to look for multi-layer or laminated systems.
In the book, "Die Fachvortrage des WVAO-Jahreskongresses 1973 in Berlin," published by the Wissenschaftliche Vereinigung fur Augenoptik und Optometrie e.V., of Bad Godesberg, in connection with the antireflective treatment of plastic glasses and the above-mentioned physical laws with regard to the indices of refraction, it is recommended that first a coating having a higher refractive index of about n=2 be applied, followed by a silicate coating. The following are given as required characteristics of the coating:
(a) Low reflection at the maximum light sensitivity of the eye; PA1 (b) Freedom from absorption; PA1 (c) Strength of adhesion; PA1 (d) Hardness; PA1 (e) Resistance to chemicals, sweat and fungi; PA1 (f) Resistance to wiping (scratch resistance); PA1 (g) Temperature stability; PA1 (h) Low aging effect.
It has been found that, in the coatings on plastic glasses which are described therein, do have a temperature stability up to about 90.degree. to 100.degree. C., but only under dry conditions, not, for example, in the sweat test.
For example, the attempt has been made to improve the teaching of the above-cited literature by vacuum coating the substrate first with a high-refraction layer of titanium dioxide and then with a second, low-refraction layer of magnesium fluoride. However, it has been found that the second layer is easily removed during the sweat test and upon heating, while the first layer usually remains intact. This layer alone, however, does not fulfill the purpose of producing a great reduction of reflection. Cryolite could also serve for the second layer with regard to the refractive index, but its hygroscopicity precludes its use.
It has furthermore been found that plastics display widely varying behavior under vapor coating conditions. This applies, for example, with regard to their surface qualities before and after any necessary glow discharge treatment, and with regard to the condensation of monomers etc., and thus also to the strength of adhesion. Also the aging behavior during the later use of the vapor coated objects differs entirely from one plastic to another. Lastly, even the refractive index, which determines the optical properties in conjunction with thin coatings, and the thermal expansion coefficient and modulus of elasticity, which are important to mechanical strength, differ greatly. For each plastic or for each group of plastics, therefore, a tailored-to-measure vapor coating process must be developed, especially when the plastic involved must comply with special requirements having no connection with the coating, such as for example resistance to shattering, etc.