WO2001008455A1

WO2001008455A1 - Method of promoting uniform process conditions in pecvd reactor for batch production of glazing panels

Info

Publication number: WO2001008455A1
Application number: PCT/US2000/019954
Authority: WO
Inventors: Jeffrey R. Dykhouse; Daniel V. Raney; Stephen M. Jaffe; Steven M. Gasworth
Original assignee: Exatec, Llc.; Metroline Industries, Inc.
Priority date: 1999-07-21
Filing date: 2000-07-21
Publication date: 2001-02-01
Also published as: AU6363400A

Abstract

A batch process for plasma coating polycarbonate panels (16) for automotive glazing panels in which plasma enhanced reactant is propagated through a vacuum reaction chamber (10) containing a number of panels predominantly by diffusion to insure uniformity of conditions and thereby avoid optical flaws. This is done by setting the spacing of the electrodes (12, 12A) a distance apart sufficient to reduce convection and to establish activated reactant propagation predominantly by diffusion; and, by consistently aligning the hole pattern (24) in each cleanable electrode cover with the matching electrode hole pattern (26) to insure uniform reactant inflow emanating from holes in each cover.

Description

METHOD OF PROMOTING UNIFORM PROCESS CONDITIONS IN PECVD

REACTORFORBATCH PRODUCTION OF GLAZING PANELS

Cross Reference to Related Application

This application claims the benefit of the filing date of provisional application 60/144,756, filed on July 21, 1999.

Background of the Invention

This invention concerns a process for coating plastic panels to be used as glazing for automotive vehicles. It has long been proposed to use injection molded polycarbonate panels as a replacement for glass glazing conventionally used in automotive vehicles. Such plastic glazing offers significant advantages over glass for this application, including weight reduction, improved protection from collision injury for the occupants, and greater design freedom due to the more pronounced curvatures made possible by molded panels.

A potential drawback arising from the use of plastic such as polycarbonate for glazing is that it is less scratch resistant than glass. However, there have heretofore been developed silicon dip coatings for polycarbonate which substantially increase its scratch resistance. Such dip coatings successfully improved the abrasion resistance of polycarbonate, but the scratch resistance was still insufficient for automotive use. Furthermore, polycarbonate panels are not sufficiently resistant to weathering to be suitable for automotive glazing, and in particular, polycarbonate is subject to yellowing when exposed to sunlight for long periods, requiring that this dip coating also provide UV blocking to improve weatherability of the polycarbonate. See U.S. patent 4,842,941 for a description of such dip coatings. To obtain adequate abrasion resistance for automotive applications, an additional process has heretofore been developed involving a plasma enhanced chemical vapor deposition (PECVD) often simply referred to as a plasma coating. In the PECVD process, reactants such as organosilicon monomers are activated by a plasma to form a coating on panel surfaces.

Such plasma coatings are designed to be applied over the previously described dip coating to substantially further improve the abrasion resistance of the glazing over that obtained the previously known dip coatings.

Reference is made to U.S. patents 5,298,587; 5,320,875; 5,433,786; 5,494,712; 5,718,967 and 5,900,284 which set forth further details of plasma coating processes and apparatus which are hereby incorporated by reference.

In the PECVD process, hollow electrodes mounted in a vacuum chamber are used to create a plasma, and are formed with a hole pattern so that reactants introduced into the interior of the electrodes pass out through the holes and into the chamber. In passing out from the electrodes, the reactants become activated by the plasma so as to be able to form a durable coating on the surfaces of a panel in the chamber.

While such PECVD processes have now been successfully reduced to practice for coating individual pieces, commercial production of polycarbonate automotive glazing panels would require that PECVD processing of the panels must be carried out on large numbers of such panels.

Problems have arisen in attempting to set up volume production of PECVD due to the need for almost perfect optical clarity of coated glazing panels to achieve consumer acceptance of such coated panels used for automotive glazing. Optical clarity is also important for safety considerations.

Practice of this PECVD process in volume production necessitates a resort to batch production, due to the slow nature of the PECVD process in plasma coating polycarbonate glazing. Batch production involves simultaneously coating a number of panels in a vacuum chamber in order to achieve reasonable production rates. In a batch processing set up, a number of flat electrodes are mounted spaced apart in the reaction chamber to form intervening bays in which are suspended the panels to be coated. The electrode spacing is normally set to achieve maximum utilization of the available space.

In practicing such batch methods, it has been found that an unacceptably high incidence of visible "white spots" or hazing will often occur on the panels when batches of panels are processed at the same time.

It is the object of the present invention to improve the PECVD batch processing of panels so as to minimize the incidence of such optical flaws as white spots and hazing.

Summary of the Invention

This object and others which will become apparent upon a reading of the following specification and claims have been discovered to be achieved by firstly establishing propagation of the plasma enhanced reactants issuing from the electrode hole patterns into the spaces between the panels predominantly by diffusion, rather than by convection. This is believed to occur because diffusion maximizes the uniformity of the concentrations of the activated reactants coming into contact with the panel surfaces to be coated. Uniformity of conditions in turn insures that the coated panel surfaces will be substantially free of such optical defects as "white spots" or hazing thought to occur as a result of locally nonuniform concentrations of the activated reactants, as will be discussed further herein.

Predominance of reactant propagation by diffusion is accomplished by increasing the spacing of the electrodes over that set by merely maximizing the utilization of the chamber space, to thereby reduce the flow velocity of the activated reactants in the space between electrodes to a sufficiently low level to prevent the formation of these optical flaws.

A further improvement in this PECVD process contributing to minimizing such optical flaws comprises the step of consistent positioning of the hole patterns in cleanable covers with respect to the hole patterns in the electrodes in a multiple electrode, multiple panel installation. A consistent alignment results in uniform flow resistance presented by the aligned hole patterns in each electrode-cover assembly, to in turn achieve uniformity of the rates of inflow of the reactants through the hole pattern from each of the covered electrodes. This in turn insures uniformity of the rates of inflow of the reactants into each of the spaces in the chamber, promoting the achievement of uniform conditions in each bay, so that the critical conditions necessary to avoid white spots and haze are consistently maintained.

Description of the Drawings

Figure 1 is a cross sectional view of a PECVD vacuum chamber having a number of glazing panels disposed therein for batch production of coated panels, with diagrammatic representations of associated equipment.

Figure 2 is an enlarged diagrammatic representation of a deposition bay defined between two electrodes, with a glazing panel disposed therein. Figure 3 is a simplified exploded perspective view of an electrode and a cover showing the hole pattern of each.

Detailed Description

In the following detailed description, certain specific terminology will be employed for the sake of clarity and a particular embodiment described in accordance with the requirements of 35 USC 112, but it is to be understood that the same is not intended to be limiting and should not be so construed inasmuch as the invention is capable of taking many forms and variations within the scope of the appended claims.

As discussed above, multistep dip and plasma coating processes have been developed for increasing the abrasion resistance of polycarbonate in order to allow the use of such material as automotive glazing panels. These dip coatings have also functioned as UV blockers to improve panel weatherability.

It should be understood that the present invention does not lie in the details of such dip or plasma coatings or processes per se, which are described in the patents referenced above.

It is critical for acceptability for automotive use that the coated plastic glazing panels be completely free from noticeable optical flaws.

As discussed above, sufficiently abrasion resistant coatings for use of polycarbonate to construct automotive glazing panels can be achieved by coatings applied by plasma enhanced chemical vapor deposition (PECVD). In this process, coating reactants are subjected to a plasma so as to be activated to promote the deposition of the material onto the panels. In this process, a plasma is generated by an array of hollow electrodes mounted spaced apart in a vacuum reaction chamber, reactants are continuously introduced into a space within the electrodes, and passing out through holes in the electrode and electrode covers, in a plasma-induced activated state. The activated reactants disperse through the spaces between the electrodes and form an abrasion resistant coating on the panels disposed in bays defined by the spaces between the electrodes. Propagation of a fresh supply of reactants through the chamber is induced by continuous exhausting of the previously introduced reactants from the chamber.

Such plasma coatings typically involve a multistep process, using argon, oxygen and organosilicon reactants as described in the above referenced U.S. patents.

As noted above, this particular PECVD process is slow, requiring about two hours to be completed, and thus volume production of the panels has led to the use of a batch method in which a number of panels are arranged within the vacuum chamber, suspended in the bays between the electrodes for simultaneous coating of all the panels in one batch.

Figure 1 illustrates this arrangement, in which a vacuum reaction chamber 10 is shown, enclosing an array of hollow, generally planar electrodes mounted therein, spaced apart from each other and vertically parallel with each other. Two or more of the electrodes 12A may be of greater height to more completely occupy the interior space within the vacuum chamber 10.

The spaces between the adjacent electrodes 12, 12A define bays 14 within which are suspended polycarbonate glazing panels 16, as by means of hangers (not shown).

A partial vacuum is created in the chamber 10 by connection to a vacuum source 18, and reactant gases are introduced into the electrodes from a source 20. As seen in Figure 3, the electrodes 12, 12A have opposite faces which are perforated to create a hole pattern 26 and reactants are drawn into the chamber 10 through the holes 26 by the partial vacuum. To allow cleaning of deposited material and prevent clogging of the electrode holes, cover panels 22, 22A overlie each electrode face, each cover panel also having a hole pattern 24, the cover panel hole pattern 24 matching the electrode hole pattern 26 to allow the reactants to pass through both sets of aligned holes and out into the interior of the vacuum reaction chamber 10.

Referring to Figure 2, it has been found that the spacing S between adjacent the electrodes 12 (or 12 A) creating the bay for receiving a panel must be set at a certain minimum for a given inflow rate in order to avoid having any white spots, which rate will vary with the parameters of each application. That is, the electrode and chamber configuration, glazing panel configuration, gas pressure, etc. will affect the particular spacing required to carry out the process. In a typical system, a minimum spacing of 7.5 to 9.5 inches was successfully used with polycarbonate panels using tetramethyldisiloxane (TMDSO) combined with argon and oxygen respectively as reactants for successive coatings.

The reason for the need to maintain a minimum electrode spacing is believed to arise from an increase in reactant gas flow velocities through the spaces 14 with decreased spacings.

Increased flow velocities tend to create local nonuniform conditions in the reactor, in turn leading to localized variations in reactant concentrations and other process conditions. Such localized conditions in the chamber may not be within the narrow range of critical conditions necessary for proper coating, and are believed to sometimes create the white spots or hazing of the coated panels described above. By propagating the reactants throughout the chamber spaces predominately by diffusion an improved degree of uniformity of activated reactant concentrations in the panel bays will be achieved compared to that resulting when dispersal of the activated reactants into those spaces is primarily by convection.

Increased uniformity of conditions will also improve the uniformity of the coating thickness on the panel to make it easier to insure that a proper thickness is achieved over the entire panel surface.

Another critical factor is the need to consistently align the hole patterns 24 on each of the covers 22, 22a with the hole patterns 26 in the electrodes 12, 12A (Figure 3). It has been discovered that any variation in the alignment between the cover hole patterns 24 and the electrode hole patterns 26 on different electrode-cover assemblies may create different flow restrictions presented by the holes on different panels, causing variations in the rates at which that the reactants flow into the panel bays, potentially creating local conditions under which optical flaws will appear.

Accordingly, consistent alignment of the hole patterns is necessary to achieve proper coating conditions throughout the regions on either face of each of the panels 16 to minimize the formation of optical flaws in the coatings.

Claims

1. In a process for the production of coated plastic glazing panels including the steps of placing a plurality of plastic panels side-by-side in a vacuum reaction chamber, and establishing plasma enhanced chemical vapor deposition with vaporized and gaseous reactants introduced through a series of spaced apart hollow electrodes formed with a hole pattern allowing said reactants to be drawn into said vacuum chamber, said plastic panels each disposed in a respective bay defined by a space between adjacent electrodes, the improvement comprising the step of establishing propagation of plasma excited reactants predominantly by diffusion through said spaces between adjacent sets of electrodes to effect substantially uniform distribution of plasma-excited reactants in said spaces, whereby optical flaws in said coating are avoided.

2. The process according to claim 1 wherein said improvement includes the step of separating said electrodes apart a sufficient distance to insure propagation of reactants predominantly by diffusion.

3. The process according to claim 2 wherein said electrodes are provided with covers overlying each of said hole patterns, and wherein said step of establishing propagation of said plasma-excited reactants further includes the step of providing hole patterns in said covers consistently aligned with said hole patterns in said electrodes in each electrode-cover assembly to cause substantially uniform inflow of plasma enhanced reactant into said spaces through each of said covers.

4. The process according to claim 2 wherein said plastic glazing panels are constructed of polycarbonate.

5. The process according to claim 4 wherein said reactants include an organosilicon monomer.

6. The process according to claim 5 wherein said organosilicon monomer is tetra-methyldisiloxane.

7. In a process for the production of coated plastic glazing panels including the steps of placing a plurality of plastic panels side-by-side in a vacuum reaction chamber, and establishing plasma enhanced chemical vapor deposition with vaporized and gaseous reactants introduced through a series of spaced apart hollow electrodes formed with a hole pattern, allowing said reactants to pass into said vacuum chamber, said plastic panels each disposed in a respective bay defined by a space between adjacent electrodes, each of said electrodes provided with a cover overlying each of said hole patterns, the improvement wherein said step of establishing propagation of said excited reactant includes the step of providing hole patterns in said covers consistently aligned with said hole patterns in said electrodes to cause substantially uniform inflow of plasma enhanced reactant into said spaces through each of said covers.