Coating processes – Interior of hollow article coating – Spraying
Patent
1986-06-11
1989-03-07
Bell, Janice
Coating processes
Interior of hollow article coating
Spraying
427250, 427251, C23C 1600
Patent
active
048105317
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of vapour deposition of tin on to a surface, e.g. the surface of a film of plastics material to make a metallised film suitable for packaging.
In known methods of vapour deposition of metals, the metal is melted in a receptacle and evaporated therefrom and subsequently deposited from the vapour on to a surface to be coated. Conventional apparatus for metallising plastics film comprises a vacuum chamber containing a reel-to-reel transport mechanism which passes the film over one or more receptacles, known as evaporation "boats" or heaters. The boats are often made of boron nitride materials. The boats are heated by electric current while metal wire is fed into the boats from driven spools within the vacuum chamber.
Aluminium is commonly used for metallising packaging films. Aluminium is particularly suitable for vapour deposition because it has a relatively low melting point, favourable vapour pressure characteristics and acceptable cost. When molten, aluminium wets boron nitride completely and evaporates rapidly so that large areas of film can be coated quickly and processing costs are minimised.
However, there is a need for films bearing a vapour deposited coating of tin, because tin has the property of helping to preserve the colour and flavour of food products by preventing oxidative degeneration. Japanese patent publication No. 52 37170 describes various uses of tin coatings on organic film in containers for storing food and drink.
A polymeric film can be metallised with tin by using resistance heating to melt and evaporate the tin to create tin vapour. Although tin has favourable vapour pressure characteristics when molten, it unfortunately does not wet evaporation boats, particularly boron nitride boats, and consequently its evaporation rate is very low. The speed of transport of the film over the evaporation boats has therefore to be reduced so as to increase the residence time over the evaporation boats, to permit deposition of coating of useful thickness. This increased residence time over the evaporation boats gives rise to a risk of film damage as a result of excessive exposure to heat and to droplets of molten tin which may be thrown out of the boats. At such low speeds, the overhead costs of the apparatus become prohibitive to successful commercial working.
This invention provides a method of vapour deposition of tin on to a surface wherein the tin is melted in a receptacle and evaporated therefrom and subsequently deposited from the vapour on to the surface, characterised in that a coating of titanium hydride is applied to the receptacle before the tin is melted in it. This pre-treatment of the receptacle results in its being wetted by the molten tin, so as to enhance the rate of evaporation of the tin and thereby permit economical deposition.
Preferably the coating is applied by applying to the receptacle a dispersion of titanium hydride in a volatile dispersant which does not react with the titanium hydride or the material of the receptacle, and then evaporating the dispersant.
The dispersant may be a chlorinated hydrocarbon, e.g. from the group consisting of carbon tetrachloride, 1,1,1-trichloroethane and dichloromethane.
The receptacle may be of an electrically conducting material, heated by passing an electrical current through it to melt the tin. For example, the receptacle may be of a boron nitride material.
The tin may be fed in the form of a wire into the heated receptacle.
Alternative boat materials include graphite and silicon carbide. The invention may be used to deposit tin on to a film of polymeric material, for example a film of polyethylene or polypropylene or a polyester.
Various embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which:
FIG. 1 is a diagrammatic representation of a side view of apparatus for vapour depositing a coating of metal on to a continuous web, and
FIG. 2 is an enlarged view of an evaporation boat also seen in FIG. 1.
In FIG. 1 a vacuum chamber 1, ev
REFERENCES:
patent: 2860075 (1958-11-01), Alexander et al.
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patent: 3205086 (1965-09-01), Brick et al.
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patent: 3457784 (1969-07-01), Bolter
patent: 3649734 (1972-03-01), Wilson
patent: 3730507 (1973-05-01), Montgomery
Archibald et al., "Source Evaporant Systems for Thermal Evaporation", Solid State Technology, vol. 19, No. 7, Jul., 1976, pp. 32-40.
The Condensed Chemical Dictionary, 6th ed., N.Y. Rheinhold, 1961, p. 1146.
Bell Janice
Metal Box plc
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