The invention relates to a process for testing multi-layer composites, for example metal-plastic composite films, and containers produced therefrom. Furthermore, the invention relates to an apparatus which is suitable for implementing the process.
The aim of the invention is to specify a process via which damaged points can be identified with great sensitivity in a layer which is not electrically conductive, this process being suitable for large-scale manufacture.
The expression xe2x80x9cdamaged pointxe2x80x9d designates an area of the layer which is not electrically conductive, whose size is in the microscopic or sub-microscopic range and which is more pervious for molecules of a material than the diffusion constant of the material in the present layer which does not have such a damaged point.
Metal-plastic composites or composites of an electrically-conductive plastic and an electrically non-conductive plastic, or form-stable containers coated on the inside with an electrically non-conductive plastic are used for packaging which is to protect the packaged products, for example foodstuffs or pharmaceutical preparations in fluid or solid form to some special degree, for example against the inward diffusion of air, against moisture in the air, against light or against the loss by diffusion of volatile contents from the packaging.
Composites of this type comprise a thin metal foil, generally aluminium foil, which is generally coated on both sides with one or more plastic films. Generally, one side of the aluminium foil is provided with a sealable layer which permits hermetically-sealed containers to be produced from such composites, for example packaging bags.
Pure plastic films are generally permeable to gases and steam. However, composite films of metal and plastic are practically diffusion-proof. In the case of containers produced from such composite films, the diffusion is generally determined by the quality of the seam.
In order that the composite films are sufficiently flexible, the individual films, especially the plastic inner film which faces the packaged product, are kept very thin. When processing into corresponding containers, these thin interior films can easily be damaged and at gas-permeable damaged points in the inner film, contact may occur between the metal foil which lies under the inner film and the packaged product. In the case of sensitive products, this metal contact can affect the packaged product. Furthermore, the packaged product can destroy the metal foil by corrosion, whereupon the composite film looses its proof against diffusion.
A plurality of processes are known for testing whether a composite is undamaged and that the integrity of seals on containers produced from, or comprising, such a composite is not jeopardised. In the case of one widely-used process, the filled and sealed container is placed in a chamber which is at reduced pressure, this chamber being provided with a sensitive manometer, and also being connected with a vacuum pump via a valve. When the reduced-pressure chamber is evacuated to a specified pressure and the valve closed, if a seal of the container fails a part of the contents leaves the container and evaporates corresponding to its vapour pressure in a vacuum, whereupon the pressure shown on the manometer increases. The pressure shown on the manometer can be used as an indicator for a failed seal.
DE-196 51 208 describes a test process wherein the brightness of a gas discharge in a vacuum is used as an indicator for the quantity of leakage leaving a filled container.
In the case of another very sensitive process, helium is charged into the container. The helium leaving the closed container is detected with a mass spectrometer.
With another process, plastic coatings on metal surfaces are tested for pore freedom. Here, the metal surface is connected with a terminal of a voltage source. A flexible electrode, which can comprise, for example, an electrically-conductive elastomer, is connected via a measuring device to the other terminal of the voltage source. The coated surface which is to be tested is scanned with the flexible electrode. If there is a damaged point in the coating between the flexible electrode and the conductive substrate, electrical current flows into the circuit which serves as an indicator for the gas-permeable damaged point.
According to a further process, the plastic coating on a metal surface which is to be tested is wetted with an electrolyte in which an electrode is submerged. The other electrode is formed by the metal surface. Both electrodes are placed in an electrical circuit which contains a measuring device and a voltage source. If there is a damaged point in the area of the coating which is wetted with the electrolyte, an electrical current flows in the circuit which is provided with a voltage source, this current serving as an indicator for the damaged point. The metal foil is used as the electrode in the case of composite films of plastic and metal.
Form-stable containers with an electrically-insulating inner layer can be tested in a similar manner. Here, the container is filled with an electrolyte in which an electrode is submerged.
Scanning the surface with an electrode is only possible with flat or slightly-bent and accessible surfaces. Small containers can not be tested in this manner.
When using an electrolyte, small containers can also be tested. Containers which are tested and found to be trouble-free should, however, be carefully cleaned and dried, wherein the examination of large numbers of containers is made very difficult. In the case of miniaturised and sterilised disposable containers for pharmaceutical products, scanning processes and electrolyte processes are not practicable.
Hence the object is to specify a process and an apparatus for detecting gas-permeable damaged points in test subjects which comprise an electrically non-conductive layer, wherein the electrically non-conductive layer is opposite an electrically-conductive layer on the one side and on the other side is opposite a gas environment. The test subjects can be present as an individual electrically non-conductive layer or as a multi-layer composite, or as containers produced therefrom.
With the process, the undamaged nature of the electrically non-conductive layer and the soundness of containers produced from the electrically non-conductive layer or from the multi-layer composite can be tested, and this is also the case if only one of the non-conductive layers in a multi-layer composite is damaged. The process should also be applicable for miniaturised containers and larger production runs, and should take place automatically if possible.
This object is solved by a process having the following characterising features. At least one test subject is disposed in a test chamber which has been evacuated to a given pressure and is maintained at this pressure. The gas in the gas environment is ionised by means of an electrical gas discharge on the other side of the electrically non-conductive layer, preferably by impact-ionisation during dark discharge. An electrical voltage is applied between the electrically-conductive layer which lies on the other side of the electrically non-conductive layer, and a counter-electrode disposed in the test chamber, this electrical voltage generating a field strength in the area of the electrically non-conductive layer which lies beneath the dielectric strength of the electrically non-conductive layer without gas-permeable damaged points. An electrical current flows between the electrically-conductive layer and the counter-electrode disposed in the test chamberxe2x80x94if a gas-permeable damaged point is present in the electrically non-conductive layerxe2x80x94which serves as an indicator for the gas-permeable damaged point in the electrically non-conductive layer. The electrical gas discharge is maintained during measurement of this current.