Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Forming nonmetal coating using specified waveform other than...
Reexamination Certificate
2000-01-18
2002-04-09
Jones, Deborah (Department: 1775)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Forming nonmetal coating using specified waveform other than...
C205S121000, C205S129000, C205S139000
Reexamination Certificate
active
06368483
ABSTRACT:
This invention concerns aluminium workpieces, particularly aluminium sheet, mainly thin sheet used for containerstock or as foil. Containers are often subjected to abrasion during transportation prior to filling.
Foil containers are frequently formed, stacked together and then transported to a separate site for filling. The process of transportation results in fretting between adjacent containers. Cleaning the foil stock and then coating the surface with an abrasion resistant coating can prevent fretting, but the economics of foil production require that any coating process be of low cost.
Cleaning of foil stock at high speed is difficult. Annealing alone does not give adequate cleaning, and chemical rinses are often too mild, i.e. they only degrease. Electrolytic processes are probably the only ones that can clean a foil surface in the short process times required.
Currently, foil for food use is sold as a commodity product mainly in the bare state or backed with paper or polymer as a laminate, sometimes with simple lacquer printing. Generally, printed or laminated products are suitable only for cold applications, e.g. sandwich wrap, while bare metal is also frequently employed in food ovens. Few coatings are available that on heating, do not at least partially discolour or act as a source of chemicals for migration into food. Pretreatments such as chromating, which could also colour the surface, tend not to be approved for direct food contact. However, premium foil producers wish to differentiate their product with a pleasing visual design without risking migration of foreign species into food.
The present invention addresses these problems. In one aspect the invention provides an aluminium workpiece having on a surface thereof an anodic oxide film, the thickness of the anodic oxide film being different at different predetermined locations on the surface, wherein optical interference colours are visible when the surface is viewed in white light with different colours visible at the said different predetermined locations on the surface, and wherein a metal deposit is not present within the aluminium oxide film.
A workpiece is a body of indeterminate size and shape. For example extrusions, both continuous extrusions and cut lengths, are workpieces. Formed units e.g. for architectural use or as vehicle panels are workpieces. Another example of a workpiece is a container. Sheet and plate, both in continuous form and in the form of cut pieces, are further examples of workpieces. As previously noted, the invention is mainly concerned with thin sheet of the type used for containerstock or as foil.
As discussed in more detail below, interference colour effects arise due to interference between light reflected from a metal/oxide interface at the bottom of the anodic oxide layer, and light reflected from the air/oxide interface at the top of the anodic oxide layer and/or from some semi-transparent reflective layer supported on or in the anodic oxide layer. Interference colour contrasts are generated when the thickness of the anodic oxide film is different at different predetermined locations on the surface.
The invention provides aluminium foil in the form of a continuous strip or of cut sheet at least 1 m
2
in area, and having on a surface thereof an unsealed anodic oxide film 5-1000 nm thick. The anodic oxide film gives an attractive finish that also imparts wear resistance and results in a clean finish.
Aluminium sheet has in the past been continuously anodised mainly for two purposes. In one application, continuous anodising is performed at low line speeds to make thick protective anodic oxide films for architectural use. Such films comprise a barrier layer and an overlying porous layer, and the pores are sealed prior to use. The second application is performed at higher line speeds to make thin pretreatment layers to promote the adhesion of organic coatings to the sheet. Such films are typically 100 nm or less thick and are made in phosphoric acid or a similar electrolyte having substantial dissolving power for aluminium oxide, so that the resulting film is extremely rough with pillars or whiskers which provide a keying effect and promote adhesion. The anodic oxide films carried by aluminium sheet of the present invention are quite different from either of these prior art structures.
In another aspect the invention provides a method of anodising aluminium foil, which method comprises passing the foil round at least one roll immersed in anodising electrolyte and facing a series of at least three pairs of electrodes with anodising current applied between the two electrodes of each pair.
In another aspect the invention provides a process for cleaning aluminium foil, which process comprises anodising the foil in the form of continuous strip or of cut sheet at least 1 m
2
in area so as to form on a surface thereof an unsealed anodic oxide film 5-1000 nm thick.
The word aluminium is herein used to cover the pure metal and alloys in which Al is a major component. The alloy may be for example in the 1000 or 3000 or 5000 or 6000 or 8000 series of the Aluminum Association Register. For example, AA
3003
and
8008
are often used for containerfoil, and AA
1100
and
1200
and
8006
are often used for foil.
Containerstock is thicker gauge foil often 80-150 &mgr;m or thicker that is formed into usually open containers for food or into food trays. The term aluminium foil is generally used to denote sheet below 150 &mgr;m thick. The invention is more particularly concerned with foil below 85 &mgr;m thick. Domestic cooking foil is typically 8-40, and usually 10-20, &mgr;m thick. Thin aluminium foil may be laminated onto a carrier such as plastic sheet for some applications such as packaging and food wrapping. The aluminium foil in this laminate is about 3-20 &mgr;m usually 5-20 &mgr;m thick.
Present on one or both surfaces of the sheet (but usually on one surface only) is an anodic oxide film. The film needs to be thick enough to provide fretting resistance, and should not be so thick that there is any risk of it spalling off when the foil is folded or flexed. A preferred range is 50-500 nm particularly 100-500 nm thick. Within this, films at least 125 nm thick have a major additional advantage that constructive interference colours are generated, between the metal/oxide interface on one side of the film and the oxide/air interface on the other and markedly enhance the appearance of the sheet. The pores are unsealed, i.e. they have not been formally sealed e.g. by being exposed to boiling water or steam nor to a proprietary cold sealing solution, although the pores may sometimes be found to be partly or wholly closed at their outer ends. Sealing would be impractical at the high line speeds required for economic operation.
Fretting resistance is usually an important requirement, especially in food containers. It is therefore generally preferred that such workpieces according to this invention have either one or both surfaces completely covered with an anodic oxide film. For household foil below about 40 &mgr;m thickness, fretting is less likely to be a problem. Foil or sheet according to the invention may have one surface (or even both surfaces) partly or preferably wholly covered with an anodic oxide film, which film may vary in thickness over different regions of the surface as discussed below.
The anodic oxide film could in principle be of the barrier layer type, grown in an electrolyte having no dissolving power for aluminium oxide. In such systems, film thickness is proportional to anodising voltage, and inconveniently high voltages might be needed to generate films of the required thickness. So preferably the anodic oxide film is of the type formed in an electrolyte having some dissolving power for aluminium oxide and comprises a barrier layer and an overlying porous layer. It is desired that the anodic oxide film reduces, rather than increases, any tendency for food to adhere to the aluminium sheet. With this in mind, the anodic oxide film should preferably have a rather smooth outer
Ball Jonathan
Barlow Eric
Davies Nigel Cleaton
Jones Rodney Charles
Alcan International Limited
Cooper & Dunham LLP
Jones Deborah
Savage Jason
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