Measuring and testing – With fluid pressure – Porosity or permeability
Reexamination Certificate
2002-09-11
2003-12-09
Williams, Hezron (Department: 2856)
Measuring and testing
With fluid pressure
Porosity or permeability
Reexamination Certificate
active
06658919
ABSTRACT:
The invention concerns a method of measuring the permeability of a coating on a plastic wall to low-molecular gases, for example oxygen, wherein the diffusion coefficient and/or solubility coefficient of the coating with respect to the low-molecular gas is approximately an order of magnitude lower than that of the plastic wall, in which the space on the coating side is subjected to the action of a first fluid and is flushed through until substantially only said first fluid is in said space.
It is known for fluid packs to be made from plastic material, for example in the form of bottles with an opening at the top. Such bottles are produced from polyethylene terephthalate (PET) and are used in many cases for packaging for example water. It is known that the walls of the plastic packs are permeable in relation to low-molecular gases, and for that reason the periods of time for which it is possible to keep liquid foodstuffs, in particular juices and CO
2
-containing mixed drinks, are limited. In order to prevent the oxygen which is responsible for this from entering the pack or the carbon dioxide from escaping and in order in that way to make it possible to make better use of such plastic packs for liquid foodstuffs, manufacturers have occasionally gone over to coating the inside or the outside of the walls of those plastic packs. An example of such a known coating process for improving that so-called barrier property of plastic packs is plasma-enhanced coating from the gaseous phase (the English abbreviation used is PECVD). In that respect, in the case of the inner coating, a gas mixture is introduced into the pack or the hollow body. Electromagnetic energy is radiated thereinto, and fires the plasma, and in that treatment procedure, for example a vitreous silicon oxide or an amorphous carbon layer is applied in the interior of the hollow body. Low-molecular gas, for example oxygen or carbon dioxide, by virtue of that additional barrier, can penetrate through the plastic walls substantially less to not at all, and low-molecular organic compounds can also no longer penetrate into the plastic material, with the consequence that, in the case of packaged liquid foodstuffs, the drinks can thereby keep their taste and their quality for substantially longer. There are however various factors which involve a coating of different thicknesses on the wall surface, for example the differing configuration of the coated bodies.
In accordance with another known method, X-ray fluorescence is used to measure on a wall portion which is cut out of the bottle, whether an application has taken place. This method also suffers from the disadvantage that it is not possible to establish whether the application is homogenously distributed on the surface and the coating therefore enjoys a good barrier property. In addition this elementary analysis can only be established on a stamped-out portion of the wall of the pack.
Therefore a further method has been developed whereby the barrier property of a coating on a plastic wall is measured by means of the oxygen which diffuses through the wall and the coating. For example a pack which is in air is flushed in the interior with nitrogen for a period of one day until the interior of the pack is certain to contain almost no longer any oxygen, while outside the pack there is a gas with 21% oxygen. If, after that flushing operation with nitrogen, the internal atmosphere of the hermetically sealed pack is left at rest, then the oxygen which possibly accumulates there in the course of time originates from the atmosphere surrounding the pack. That oxygen has then migrated from the outside through the plastic wall inwardly. After a waiting time of at best about twenty-four hours (typically up to three weeks), this measuring method provides that a steady final value in respect of the oxygen content in the atmosphere in the interior of the pack has become established. If that value is compared to a predetermined barrier value, it is then possible to establish whether the measured pack has been coated with a sufficiently good barrier. There is the disadvantage here that this known checking method requires a very long period of time so that the method can scarcely be used in the course of pack production as real time measurement is practically impossible to implement.
Therefore the object of the invention is to provide a method of the kind set forth in the opening part of this specification, by means of which the barrier property of the coating can be measured in a relatively short time, reliably and possibly for the entire surface of the coating, without destroying the body or the pack having the plastic wall.
In accordance with the invention that object is attained in that:
firstly the plastic wall is subjected to the action of a fluid which contains a test medium with at least one component which can diffuse into the plastic wall and be absorbed there,
after the step of subjecting the coated space to the action of the fluid test medium deposited on the coated surface is removed therefrom,
thereafter the coated space is subjected to the action of an extraction medium in which the test medium is well soluble and the test medium can be measured, and
the concentration of the test medium in the extraction medium is measured.
This novel measuring method can provide for measuring the coating of the entire surface of a plastic wall, for example on the inside or the outside of a pack, which for example can be in bottle form, in regard to the barrier properties. It is therefore possible to measure whether low-molecular gases, preferably oxygen or carbon dioxide, can or cannot diffuse through the coating. If the coating is non-homogenous or if there are indeed places at which the wall of the pack is not coated at all, a markedly greater degree of diffusion of oxygen will be measured than if the coating is homogenous on the entire surface area in such a way that the oxygen cannot pass through the wall, without overcoming the barrier.
Both in the known method and also in the method according to the invention the space on the coating side, in the case of an internal coating therefore the internal space of the pack, is admittedly subjected to the action of a first fluid, for example flushed therewith, and is then flushed through until there is no longer any low-molecular gas in that space, which is intended to be prevented by the coating from passing through or diffusing through the plastic wall. In the known case the coated space is subjected to the action of nitrogen which is not a test medium. In accordance with the invention in contrast the first fluid contains a test medium for example acetaldehyde or hydrogen peroxide (H
2
O
2
). In the known method the flow of the low-molecular gas, for example oxygen, is always measured in one direction. For example the method involves measuring whether oxygen outside the plastic pack (in the air) diffuses through the plastic wall into the interior thereof, in spite of the coating. A reversed flow is neither produced nor measured. In accordance with the invention in contrast the coated space is admittedly also flushed with the first fluid containing the test medium, for example H
2
O
2
, but no account whatsoever is taken of the oxygen outside the pack. Rather, such a test medium is selected, which can diffuse into the plastic wall and also be absorbed there. If the coating were theoretically very good with a 100% barrier property, then no test medium would be absorbed in the coating and definitely not in the plastic wall, it could thereafter also not be extracted and consequently it could not be measured. The gas flow is therefore provided in two directions, more specifically for example from the interior of the pack in the plastic walls outwardly and thereafter back again out of those walls or the coating back into the internal space in the pack. The advantageous independence of low-molecular gases which are outside the pack is recognised, insofar as the internal space is coated (and vice-versa). It would in that respect moreover be possible
Politzer Jay L
Tetra Laval Holdings & Finance S.A.
Williams Hezron
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