Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
1998-09-28
2001-09-04
Pianalto, Bernard (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S296000, C427S302000, C427S322000, C427S385500, C427S399000, C427S536000, C427S569000
Reexamination Certificate
active
06284326
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for producing a firmly adhering coating on polymethyl metacrylate [sic] (PMMA) substrate surfaces.
In addition to glass, transparent plastics are increasingly being used for the production of optically imaging constructional elements, such as lenses and prisms. PMMA in particular has proven itself in this context. A key disadvantage of the optical components produced from PMMA is their relatively soft surface, which can easily be damaged by scratches. It is therefore common to protect the surface by vapor deposition of quartz (SiO
2
) or else by plasma-polymerized silicon compounds. It is also possible to coat the surfaces with optical functional layers in order, for example, to reduce reflection.
It has unfortunately turned out to be the case that the coatings chosen to date do not adhere very firmly. Under thermal stress, the layers become detached or cracks form which, although without disruptive effect in the case of spectacle lenses, for example, are not tolerable in the case of precision optics. In the case of injection molded optical components in particular the so-called spray skin is subject to higher mechanical stress and exhibits poor adhesiveness for applied coatings.
From EP 0 482 615 A1 it is known to provide the surfaces of various polymer materials with a poly(hydroxyethyl acrylate) coating which is intended to provide improved wettability, printability, adhesiveness, antistatic properties and freedom from misting. The special coating is to be produced by one of the known processes of addition polymerization. According to the stated process parameters the surface to be coated is first of all exposed under vacuum to the poly(hydroxyethyl acrylate) in vapor form, after which a plasma discharge is initiated by means of which the coating is polymerized. The coating operation constitutes a final treatment of the polymer surface.
SUMMARY OF THE INVENTION
The invention was therefore based on the object of specifying a process for producing, on PMMA surfaces, a coating which is resistant to thermal and mechanical stresses, is largely neutral from an optical standpoint and which adheres firmly even to injection molded components. The coating itself should, moreover, be able to be coated again with optical functional layers.
With a process of the type specified at the outset this object is achieved in accordance with the invention by the features of claim
1
. Advantageous developments are evident from the features of the subclaims.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
A key concept of the invention is to provide a coating which adheres firmly to PMMA and which itself serves as an adhesion-promoting interlayer for further coatings. Organic starting compounds which can be addition polymerized by a free-radical mechanism are chosen as the coating substance, and are normally referred to as monomers. These monomers are, in accordance with the invention, caused to undergo addition polymerization by means of initiator radicals produced from a plasma. The process is therefore one of plasma-induced addition polymerization.
The process of causing monomers to add onto other macromolecules by addition polymerization is known per se and is also referred to as graft polymerization. The textbook MAKROMOLEKÜLE, Hans-Georg Elias, 3rd edition (1975), pages 680-683, gives examples of reactive groups. This reference also reveals that free radicals are able to initiate the addition polymerization and that polymers can be activated by means, for example, of UV radiation or gamma radiation.
It has surprisingly now been found that the plasma-induced addition polymerization of monomers on PMMA leads to a very firmly adhering and adhesion-promoting coating which is capable of compensating mechanical stresses in the interfaces. The PMMA substrate surface that is to be coated, and the reactive starting compounds for the coating (monomers), are exposed only briefly to the plasma. In this context, a low-pressure plasma has been found particularly suitable. The duration and timing of the plasma exposure depend on the nature of the coating and on the layer thickness of the monomer used, and can be optimized by experimentation.
The coating of the optical components, produced from PMMA, by a plasma-induced addition polymerization of a monomer does not alter the optical quality of the surface of precision optical components and advantageously permits subsequent coating with optical and physico-mechanical functions, such as antireflection, optical splitter, mirror, dichroic and dielectric layers, and also abrasion and wear layers, or layers for increasing the environmental stability.
REFERENCES:
patent: 5007928 (1991-04-01), Okamura et al.
patent: 5015523 (1991-05-01), Kawashima et al.
patent: 5451453 (1995-09-01), Gagnon et al.
patent: 0 482 615 (1992-04-01), None
patent: 58-147431 (1982-02-01), None
patent: 62-092250A (1987-04-01), None
patent: 05179034A (1993-06-01), None
Makromolekuele, Hans-Georg Elias, 3rd Edition (1975), pp. 680-683 (No month avail).
Yasuda, “Glow Discharge Polymerization”, Thin Film Processes, pp. 360-369, (1978) (No. month avail).
Goetzelmann Rainer
Gruenwald Heinrich
Scheerer Friedrich
Terhuerne Joerg
Zoeller Alfons
Foley & Lardner
Pianalto Bernard
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