Chemistry: electrical and wave energy – Processes and products – Vacuum arc discharge coating
Reexamination Certificate
1998-08-03
2001-06-26
McDonald, Rodney G. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Vacuum arc discharge coating
C204S298410, C118S7230VE, C427S580000
Reexamination Certificate
active
06251233
ABSTRACT:
TECHNICAL FIELD
This invention generally relates to vacuum vapor deposition coating of substrates and methods and systems involved in vacuum vapor deposition. More particularly, this invention relates to the production of a highly-active and energized plasma-enhanced vapor from a solid source, such as silicon, and to the application of the plasma within a continuously-operating high-speed coating system.
BACKGROUND OF THE INVENTION
The plasma-enhanced vapor may be used for deposition onto plastic articles, particularly for depositing a glass-like coating onto plastic bottles. The coating provides an enhanced gas-barrier and better adhesion compared with prior art coatings, and is suitable for pressurized containers, whose surface flexes and stretches, and whose internal pressure acts against an external coating. The primary component of the vapor is produced by evaporating, in an evaporative source, one or more solids and the deposition of the coating may be applied in conjunction with a reactive gas, or gases, to provide desired coating clarity or colorization. Further, it may be produced by using more than one evaporation source and solids of different boiling points.
Commercial applications of plastic articles have experienced a growth, because of the properties of these articles such as low-cost, light weight, flexibility, resistance to breakage, and ease of manufacture and shaping. However, plastics also have the disadvantage of relatively low abrasion-resistance and poor barrier properties against the permeation of vapors such as water, oxygen, and carbon dioxide. In food packaging applications, limitations in barrier properties have limited the use of plastics. For example, in the case of beverage bottles, inadequate barrier properties have restricted the use of smaller bottles required in some markets. Solutions to this problem, including the use of high-barrier plastics and coatings of various types, have been either uneconomical or have provided inadequate barrier-improvement or add expense to the known recycling processes.
A number of processes have been developed for the application of coatings on plastic, but these have been mainly for plastic films. Relatively few processes have been developed which allow the economic application of a glass-like coating onto preformed plastic containers such as PET bottles, where the demands on the coating's barrier performance are increased by the flexing of the walls of the bottle, the stretching of said walls under pressure, and the delaminating force due to the in-bottle pressure. Also, most processes are on the batch-production principle, and very few processes exist which can be applied to a continuously-running process.
U.S. patent application Ser. No. 08/818,342 filed by Plester et al on Mar. 14, 1997 allowed, and PCT International Application PCT/US98/05293 filed on Mar. 13, 1998 describe the use of an anodic arc for externally-coating beverage bottles and their disclosures are incorporated by reference herein in their entirety. Anodic arc systems are also described by Ehrich et al in U.S. Pat. Nos. 4,917,786; 5,096,558; and 5,662,741, the disclosures of which are also incorporated herein by reference.
The basic anodic system, as described by the prior art, has the following disadvantages:
a) The crucibles evaporative material content, such as silicon, cannot be replenished continuously when this evaporative material is in powder or pellet/chip form.
b) The quantity of vapor evolved from the crucible depends partly on the degree of filling of the crucible with evaporative material. Since the degree of crucible-filling is a variable which constantly changes, this could present a control problem.
c) The distribution, at various angular displacements, of the quantity of vapor evolved from the crucible, also depends partly on the degree of filling of the crucible with evaporative material. This makes it difficult to use the vapor from the crucible for the purpose of coating several articles simultaneously, without the risk that these will all receive different amounts of coating.
d) The lips of the crucible are eroded by the anodic arc. This not only presents a maintenance problem, but it also means that the material of the crucible may thus be included in the coating composition and thereby reduce the performance of the coating. For example, crucibles for holding silicon are normally constructed of carbon, which is eroded and vaporized by the anodic arc and the carbon vapor is free to form a contaminant in the desired silicon or silicon dioxide coating.
e) The said crucible lip erosion further affects the quantity of vapor evolved and the distribution of this vapor at various angular displacements around the crucible.
f) Even where the crucible is independently heated (rather than intentionally heated by the anodic arc), the anodic arc represents a second and uncontrolled source of heating. This second source of heating partly affects the quantity of vapor evolved, irrespective of any control device for the crucible's independent heating system. This makes process control of evaporation rate difficult, whilst evaporation rate is an important parameter.
g) The anodic arc energizes the plasma, but since an uncontrolled and unknown portion of this arc's energy is dissipated by evaporation of the material in the crucible, this makes the process control of the critical parameter of plasma-enhancement difficult.
h) Since part of the energy of the anodic arc inadvertently causes evaporation, even in anodic arc systems with independent crucible heating, this limits the amount of energy available for plasma enhancement.
i) Anodic arc systems employing independent crucible heating have complicated designs around the crucible in view of the conflicting needs, on the one hand to heat the crucible and on the other hand to provide a cooled anodic connection. This can result in additional cost and complication, oversized heating systems, and energy waste, as well as lead to crucible-damage on shut-down due to the cooling-effect of the anodic connection.
j) Many applications, particularly those involving colored coatings, require the simultaneous evaporation of more than one solid substance. For barrier enhancement, it can also be desirable to add other substances to the base coating. Since such substances differ in boiling point, they cannot be combined in a single evaporating crucible, because evaporative fractionation within the crucible would lead to poor coating composition control. Therefore, multi-component coatings using the anodic arc system must be produced by a multi-series of anode-cathode couples, since one separate anodic arc source for each crucible is needed for process-control purposes. This not only makes a multi-component coating systems complicated and expensive, but also risks interference between the closely positioned array of anodic arcs.
k) The cathode's evaporative material cannot be replenished continuously and it is therefore desirable in practice to use materials which erode slowly. This acts contrary to the desire to use the cathode for optimum plasma enhancement and ionization, since materials which achieve this often have a high erosion rate. The use of Zn, Cu, Al, noble metals, alkaline earths, and particularly Mg, has been found to be highly desirable, and in most cases continuous cathode replenishment is needed for economic operation.
Prior art exists (German Patent DE 4440521C1, Hinz et al) where the crucible is independently heated by electrical resistance or by thermal radiation, and where the anodic arc plasma-enhancement is provided separately by means of a cathode and a separate anode. However, the anode of such systems quickly becomes coated with the evaporated material from the crucible, or with plasma particles, or with the reaction product when a reactive gas is used. Such systems are therefore only usable where the coating is electrically conducting, since the anode would otherwise quickly become inoperative and the system would shut down. Since the barrier coating of plast
Ehrich Horst
Plester George
McDonald Rodney G.
Sutherland & Asbill & Brennan LLP
The Coca-Cola Company
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