WO1998003850A1

WO1998003850A1 - A process for measuring the gas permeability and an apparatus that carries out this process

Info

Publication number: WO1998003850A1
Application number: PCT/IT1997/000176
Authority: WO
Inventors: Paolo Manini
Original assignee: Saes Getters S.P.A.
Priority date: 1996-07-18
Filing date: 1997-07-18
Publication date: 1998-01-29
Also published as: ITMI961489A1; IT1283445B1; ITMI961489A0

Abstract

A process for measuring the gas permeability of a film (10) or other thin materials, comprising the following operative steps: dividing by means of said film (10) the interior of a tank (4) into a load chamber (5) and a measure chamber (6), whereby said chambers (5, 6) are not in fluid contact with each other; lowering the pressure in the load chamber (5) down to a value of less than 0,1 mbar and the pressure in the measure chamber (6) down to a value of less than 10-6 mbar; continuously evacuating the measure chamber (6) by means of a high-vacuum pump (18) connected to said chamber (6) through a duct (17) provided with a choking neck (22) having a known conductance C; introducing a gas into the load chamber (5); continuously measuring in the duct (17), upstream of the neck (22), the partial pressure of said gas as long as a constant pressure value P¿2? is determined; measuring the pressure P1 inside the load chamber (5); and calculating the permeability X of the film (10) as a function of P1, P2 and C. An apparatus that carries out this process is also described.

Description

"A PROCESS FOR MEASURING THE GAS PERMEABILITY AND AN APPARATUS THAT CARRIES OUT THIS PROCESS"

The present invention relates to a process for measuring the gas permeability, in particular of thin materials such as films, membranes, sheets and the like, and an apparatus that carries out such a process in an easy and precise way.

It is acknowledged that polymeric films comprising metal layers obtained by means of vacuum deposition or rolling become more and more important with respect to the industrial applications thanks to their remarkable barrier effect against the flow therethrough of gases. For example, an important application field of these films concerns thermal insulation panels, which are principally used in the refrigerators. These panels are made of insulating materials, usually polymeric foams, inorganic powders or glass wool, contained in an envelope obtained by sealing to each other two sheets of multilayer film along the edge of the panel. Vacuum is made inside these panels for thermal insulation, and furthermore there can be inserted a gas sorbing material for fixing the gases, if any, released by the materials composing the panel or penetrated from the outside. Vacuum inside the panel is principally maintained by the metallized film forming the envelope. This film can be made by means of deposition with physical techniques or by means of evaporation of metal layers onto polymeric sheets; the thus obtained metal layers generally are fractions of micron thick. As an alternative, such film- can be obtained by joining metal sheets together with plastic sheets; in this case the metal layers generally are 5 to 20 micron thick. Anyway, the multilayer film containing one or more metal layers has a permeability remarkably lower than multilayers obtained by means of deposition of thin films. However, because of the intrinsic characteristics of the manufacturing process of the thin metal layer, it is necessary to verify by means of sampling the absence of microdefects, such as for instance very small passing through holes generally called pin-holes. In order to determine the permeation due to the presence of such microdefects, it is necessary to have available a precise measure, possibly easy to carry out, of the gas permeability of these film-; even if it is very small.

A known process for measuring the gas permeability of the films is for instance that one described in the D-1434 test according to the ASTM standard. However, such a process has a sensitivity suitable for measuring the gas permeability of non-metallized films, but unsuitable for metallized multilayers, whose permeability is so low that cannot be measured either with the above process or with other traditional processes. It is therefore an object of the present invention to provide a process for measuring the gas permeability, in particular of thin materials such as films, membranes, sheets and the like, which has a remarkably higher sensitivity with respect to the known processes. Another object of the present invention is to provide an apparatus suitable for carrying out such a process in an easy and precise way. Said objects are achieved by means of a process whose features are described in claim 1 and an apparatus whose features are described in claim 6.

The process according to the present invention can be advantageously employed in the industry of metallized multilayer films and in the research and development field, as it allows to estimate at previously unreachable levels changes and improvements, if any, of the permeability of new films. Furthermore, thanks to its easiness, the process according to the present invention can be advantageously used for the quality control of a multilayer film already in a production line.

Further advantages and features of the process and apparatus according to the present invention will become clear to those skilled in the art from the following detailed description of an embodiment thereof by referring to the attached drawing wherein the single figure shows a schematic view of the apparatus for carrying out the process according to the present invention.

Referring to such a figure, it can be seen that the apparatus according to the present invention comprises a cylinder 1 filled with helium gas. Such a cylinder is connected through a feed duct 2, provided with an intercepting valve 3, to a gasproof test tank 4 (shown in a longitudinal section), preferably made of steel. This tank is suitably divided into a load chamber 5 and a measure chamber 6 joined together along electropolished gas-proof flanges 7 and 8. The inner walls of chambers 5 and 6 are also electropolished and have a very low surface roughness in order to make easier the degassing once vacuum is made therein.

Load chamber 5 is separated from measure chamber 6 by a perforated plate 9, also preferably made of steel, that supports the film sample 10 whose permeability has to be measured. Plate 9 and film 10 are arranged between flanges 7 and 8 and are firmly kept in position thereby with film 10 suitably placed toward load chamber 5. The total area value of the holes of plate 9 is known as it is indispensable for measuring the permeability of film 10. As a matter of fact, such a value coincides with the free surface of film 10 separating load chamber 5 from measure chamber 6. Therefore, it is clear that, besides system leaks that are in any case negligible, a gas flow between load chamber 5 and measure chamber 6 can only occur through permeation of film 10 at the areas corresponding to the holes of plate 9. Plate 9 provides a mechanical support to film 10 in order to avoid that the latter warps or, at the worst, tears when the pressure inside load chamber 5 is higher than the pressure inside measure chamber 6. Load chamber 5 is connected to a low-vacuum manometer 1 1 suitable for measuring pressures between 1 mbar and atmosphere pressure. Both chambers 5 and 6 are connected through two ducts 12 and 13, provided with intercepting valves 14 and 15, to a low-vacuum air-pump 16, for example a rotary pump, that is suitable for reducing the pressure inside chambers 5 and 6 to values lower than 0,1 mbar, preferably to about 0,01 mbar.

Measure chamber 6 is suitably connected through duct 17 to a high-vacuum pump 18, for example a diffusion pump, preferably provided with an auxiliary low- vacuum rotary pump (not shown in the drawing). Duct 17 is divided into two sections by a branching comprising two parallel ducts 19 and 20. Duct 19 is suitably provided with a valve 21, while duct 20 is provided with a choking neck 22 whose conductance C is known. The dimension of the gas flow F through neck 22 is a gas quantity per time unit, for example (mbar x l)/s. Such a flow depends on conductance C, measured in 1/s, according to the formula:

Fc=(P₂-P₃) x C, wherein P₂ is the pressure in mbar measured upstream of neck 22 and P₃ is the pressure in mbar measured downstream of neck 22.

In order to determine these pressures, duct 17 is connected to a mass spectrometer 23 arranged between measure chamber 6 and the branching comprising neck 22, and to a high-vacuum manometer 24, for example a Bavard- Alpert ionization manometer, arranged between the branching comprising neck 22 and the high-vacuum pump 18.

First of all, for measuring the permeability of film 10 by means of the apparatus according to the present invention, such a film must be placed between chamber 5 and chamber 6, as shown in Fig. 1. The pressure inside the whole apparatus is then reduced to values lower than 0,1 mbar by operating the low- vacuum pump 16. During this operation valve 3 is closed, while valves 14, 15 and 21 are open to allow the gas outflow from the whole apparatus. Thereafter, the valves 14 and 15 are closed and the high-vacuum pump 18 is operated. Through this operation the pressure inside chamber 6 and duct 17 is further reduced to values lower than 10^'' mbar, preferably to magnitudes of about 10^"7 mbar, corresponding to the condition of dynamic balance between the degassing speed of the inner walls of the system and the pumping speed of pump 18. By closing valve 21, with the high- vacuum pump 18 still nuining, all the gases flow along the duct 17 only through neck 22. At this time valve 3 is opened and helium contained in cylinder 1 enters the load chamber 5. Such a valve is closed when manometer 1 1 detects a pressure Pi between 1 and 1000 mbar, preferably comprised between 300 and 700 mbar. The pressure inside the measure chamber 6, measured with mass spectrometer 23, rises up to reach a constant value P₂. In this balance state the helium flow Fx through film 10 and the helium flow F_c through neck 22 are the same. If Pi and P₂ are the pressures respectively upstream and downstream of film 10 and X is its permeability, the flow F_x is obtained according to the formula:

As it is

the value of X, i.e. the permeability of film 10, is:

X = C(P₂-P₃)/(P,-P₂). Such a value can therefore be easily and exactly calculated by simply measuring the pressure values Pi, P₂ and P₃ and the conductance C of choking neck 22. Furthermore, as it is P₂»P₃, X can be determined in an approximate way according to the following formula:

X = (CP₂)/(P,-P₂). Helium is used for the measurement according to the present invention because this gas is present in the atmosphere with a partial pressure of about 5*10^"6 atm. When measure chamber 6 is evacuated at the above pressures, the residual helium is reduced to a pressure lower than 10^" mbar, in particular 10^"" mbar. Although an apparatus under high vacuum anyway has an atmospheric leakage with flows usually comprised between 10^"9 and 10^"10 (mbar x l)/s of air, such flows correspond to about 10^"14 - 10^"' (mbar x l)/s of helium, whereby the disturbances of this leakage to the helium measurement is completely negligible. With the process according to the present invention it is therefore possible to measure in a precise way permeation flows through film 10 even of magnitudes of 10^"π (mbar x l)/s. Helium is also used because its permeability in polymeric films coupled with metal layers, if any, is proportional to the permeability of the other gases. Consequently, the process according to the present invention allows to have an approximate measure of the permeability of gases other than helium. On the contrary, a direct measure of the permeability of the specific gas could not have been possible as the known processes have a remarkably lower sensitivity.

The apparatus according to the present invention has, as an accessory equipment, a plurality of perforated plates 9, which are interchangeable and different from each other with regard to the shape and the size of the holes. It is therefore possible to change the surface of film 10 exposed during the measurement simply by choosing a plate whose holes are more or less in number and/or larger or smaller. The gas flow through film 10 can be thereby maintained in an optimum values range even when the apparatus is used for measuring the permeability of films much different from each other, such as films made only of polymeric material and films coupled with metal layers. It is clear that chambers 5 and 6 of the apparatus according to the present invention may have a different shape and/or size, but it is anyway preferable that they have a reduced volume. Indeed, a load chamber 5 having a small volume allows to reach pressure Pj at the beginning of the measurement with small amounts of helium and thus saving this gas, which is rather expensive Furthermore, thanks to the small size of measure chamber 6, pressure P₂ becomes stable faster, thereby reducing the measuring times.

In another embodiment of the apparatus according to the present invention, neck 22 is also interchangeable with other choking necks having a different conductance in order to adapt the measurement to different kinds of films. Such an interchangeable neck is made in its simplest form by a piece of pipe having a cross- section smaller than duct 17.

Another embodiment of the apparatus according to the present invention is provided with temperature detecting and controlling means (not shown in the 6 -

drawing). Firstly, said means are suitable for taking chambers 5 and 6 to a temperature different from the room temperature in order to determine the permeability of film 10 in function of the temperature. This is useful both for testing the film sample at conditions similar to the actual use conditions, for instance at the low temperatures in the refrigerators, and for carrying out accelerated tests at higher temperatures, generally comprised between 50° and 80° C. Secondly, said temperature detecting and controlling means can enhance the degassing of the inner walls of the system during its evacuation before the measurement. Such an operation is carried out by raising the temperature of the system up to some hundreds of degrees, excluding the area near film 10 to avoid that the latter is altered by too high temperatures.

Further variations and/or additions can be made by those skilled in the art to the embodiment hereinabove described and shown without departing from the scope of the invention itself

Claims

1. A process for measuring the gas permeability of a film ( 10) or other thin materials, characterized in that it comprises the following operative steps:

- dividing by means of said film (10) the interior of a tank (4) into a load chamber (5) and a measure chamber (6), whereby said chambers (5, 6) are not in fluid contact with each other;

- lowering the pressure in the load chamber (5) down to a value of less than 0, 1 mbar and the pressure in the measure chamber (6) down to a value of less than 10^'6 mbar; - continuously evacuating the measure chamber (6) by means of a high- vacuum pump ( 18) connected to said chamber (6) through a duct ( 17) provided with a choking neck (22) having a known conductance C;

- introducing a gas into the load chamber (5);

- continuously measuring in the duct (17), upstream of the neck (22), the partial pressure of said gas as long as a constant pressure value P₂ is determined;

- measuring the pressure Pi inside the load chamber (5); and

- calculating the permeability X of the film ( 10) as a function of Pi, P₂ and C.

2. A process according to the previous claim, characterized in that the permeability X of the film ( 10) is calculated according to the formula:

X = (CP₂)/(P,-P₂).

3. A process according to claim 1, characterized in that it also comprises a measurement of the pressure P₃ in the duct ( 17) downstream of neck (22), whereby the permeability of the film ( 10) is calculated according to the formula:

X = C (P₂-P₃)/(P,-P₂).

4. A process according to one of the previous claims, characterized in that the gas introduced into the load chamber (5) is helium.

5. A process according to one of the previous claims, characterized in that the partial pressure P₂ is measured by means of a mass spectrometer (23).

6. An apparatus for carrying out the process according to one of the previous claims, characterized in that it comprises:

- a test tank (4) divided into a load chamber (5) and a measure chamber (6) by a perforated plate (9) suitable for supporting the film ( 10);

- a gas cylinder (1) connected to said load chamber (5) through a duct (2) provided with an intercepting valve (3);

- a low- vacuum pump (16) connected to said load chamber (5) through a duct (12) provided with an intercepting valve (14);

- a manometer ( 1 1) connected to said load chamber (5);

- a high-vacuum pump ( 18) connected to said measure chamber (6) through a duct (17) provided with a choking neck (22); and

- a mass spectrometer (23) connected to the duct (17) upstream of the neck (22).

7. An apparatus according to the previous claim, characterized in that it comprises a manometer (24) connected to the duct (17) downstream of the neck (22).

8. An apparatus according to claim 6 or 7, characterized in that the pump (16) is connected to the measure chamber (6) through a duct ( 13) provided with an intercepting valve (15).

9. An apparatus according to one of claims 6 to 8, characterized in that the duct ( 17) has a branching formed by two ducts (19,20) in parallel, whereby one is provided with an intercepting valve (21) and the other is provided with the neck (22).

10. An apparatus according to one of claims 6 to 9, characterized in that it comprises temperature detecting and controlling means.