Membrane system, membrane and metering membrane pump

Abstract

The invention relates to a membrane system, a membrane and a metering membrane pump. The invention also relates to a metering diaphragm pump having a diaphragm system of the type described or having a corresponding diaphragm, the diaphragm being clamped at the outer section thereof, and having a chamber which is divided by the diaphragm into a metering chamber and a working chamber. An actuator is provided that moves the core back and forth between a first position and a second position. By means of the measure of the invention, the metering diaphragm pump can be designed such that the ratio of the outer diameter of the deflection section to the distance of the core between the first position and the second position is less than 15, preferably less than 12.5, particularly preferably less than 10, and greater than 5.

Description

[0001] The present invention relates to a membrane system, a membrane and a metering diaphragm pump.

[0002] A membrane is used in a metering diaphragm pump, the membrane having an outer section for being clamped, a central section and a flexure section surrounding the central section, the flexure section being connected to both the outer section and the central section. Inside the metering pump, the membrane is clamped at its outer section in such a way that the membrane divides the chamber into a metering chamber and a working chamber, in which a dosing fluid is sucked in through a suction inlet and discharged through a pressure outlet.

[0003] The flexure section is designed to allow the central section to move back and forth relative to the outer section along a longitudinal axis between a first position and a second position.

[0004] An actuator engages with the central section, the actuator causing the central section to move back and forth along the longitudinal axis. Since the outer section is clamped, the flexure section must deform to enable the central section to move back and forth. The membrane has a first side and a second side, the first side forming the wall of the metering chamber and thus being for contact with the dosing fluid, and the second side forming the wall of the working chamber and not being in contact with the dosing fluid. The actuator engages with a core that abuts against the second side at the central section.

[0005] In order to attach the membrane to the core, the membrane has a fastening section located on the second side. A clamping piece is provided, the clamping piece holding the fastening section between the core and the clamping piece, allowing the central section abutting against the core to move back and forth between the two positions as the core moves.

[0006] The demand for such metering pumps is growing, especially with regard to the service life of the membrane and the increased delivery capacity while maintaining a compact design. It has been attempted to increase the stroke length relative to the outer diameter of the flexure section in order to increase the dosing volume during the pressure stroke, i.e., during the movement of the core or the central section from the first position to the second position. However, this results in premature failure of the membrane, limiting the extent to which the stroke length can be increased. Similarly, increasing the membrane surface area is only feasible to a limited extent, as this would reduce the compactness of the metering pump.

[0007] Starting from this prior art, the object of the present invention is to provide a membrane system of the type mentioned at the beginning, the membrane system including a membrane, a core and a clamping piece, the membrane system allowing a greater stroke length without reducing the service life of the membrane system.

[0008] According to the present invention, this object is achieved by providing a support element having a support surface, wherein, compared to the first position, in the second position, the support surface contacts the second side of the membrane to a greater extent in the region of the flexure section. It has been found that during the movement from the first position to the second position, i.e., during the so-called pressure stroke, the maximum stress on the membrane occurs in the flexure region, so the membrane is supported by the support element during this movement. During the return movement, i.e., from the second position back to the first position, the so-called suction stroke, the stress is significantly lower. Therefore, the membrane does not need to be supported here and can move away from the support element to allow for a greater movement of the core relative to the outer section.

[0009] The support surface should not extend parallel to the longitudinal axis. Furthermore, in a cross-sectional view containing the longitudinal axis, the support surface preferably extends in a direction perpendicular to the longitudinal axis by a length exceeding 10% of the extension of the flexure section in this direction.

[0010] In a preferred embodiment, the support surface is arranged on the clamping member. For example, the core can have a section with an external thread, and the clamping member can be screwed onto the external thread.

[0011] Especially when dispensing substances that are harmful to the environment or health, a membrane with a protective layer covering the second side of the membrane is usually used, at least in the unclamped area and the area not in contact with the core. The protective layer is arranged such that in the case of membrane fatigue, the dispensed fluid penetrates through the now leaking membrane into the space between the protective layer and the second side of the membrane, where the dispensed fluid can be detected, usually resulting in an immediate stop of the dispensing process.

[0012] Especially when using such a protective layer, and in the case of not using a protective layer, the constant contact and separation between the second side of the membrane (or the protective layer) and the support surface in this area result in particularly high stresses.

[0013] Therefore, in a preferred embodiment, the support surface is at least partially made of an elastic material, preferably made of plastic, and most preferably made of an elastomer (such as EPDM (ethylene-propylene-diene rubber) or NBR (nitrile rubber)).

[0014] By providing an elastic support surface, the stress on the flexure section is significantly reduced, thereby increasing its service life. In a preferred embodiment, the support surface is convexly curved, which means the support surface bulges outwards. This allows an increase in the dispensed volume per stroke because the flexure section is concavely curved in the first position and convexly curved in the second position. Since the flexure section is at least partially located on the support surface in the second position, the support surface is also convexly curved in this position.

[0015] In a preferred embodiment, the core is also convexly curved on the side abutting the central section. Particularly preferably, the entire outer surface of the core is convexly curved. However, in some embodiments, it may be sufficient if at least the edge region of the central section (i.e., the region closest to or facing the support surface) is convexly curved.

[0016] In another preferred embodiment, it is provided that the following two are substantially equal, namely: the sum of the radius of curvature of the support surface and the thickness of the film at the section in contact with the support surface, and the sum of the radius of curvature of the convexly curved section of the core and the thickness of the film at the section in contact with the convexly curved section of the core. If the film thickness is the same in both the central section and the flexure section, then the radius of curvature of the support surface and the radius of curvature of the convexly curved section of the core are also equal. However, if the thicknesses are different, the radius of curvature is adjusted accordingly. It is advantageous for these two sums to be exactly equal. The smaller the difference between these sums, the greater the effect of the present invention.

[0017] When the difference between the following two sums is less than 25%, and preferably less than 15%, of the film thickness at the section in contact with the support surface, it is considered to be substantially equal, namely: the sum of the radius of curvature of the support surface and the thickness of the film at the section in contact with the support surface, and the sum of the radius of curvature of the convexly curved section of the core and the thickness of the film at the section in contact with the convexly curved section of the core.

[0018] The advantage of this measure is that the film can be rolled onto the support surface particularly easily, thereby further extending the service life of the film.

[0019] In another preferred embodiment, it is provided that the film has a thickness d at the edge of the central section R , and a thickness d at the section in contact with the support surface in the second position S , where in a cross-sectional view, at least one edge of the central section and the difference in spacing from the support surface |d S - d RThe imaginary line of | extends substantially along a continuous and inflection - free section of a mathematical function. Similarly, when the film thickness in the central section and the flexure section is the same, the situation is simplified because at least one edge of the central section and the support surface both follow the mathematical function. The mathematical function can be of any type, but good results have been obtained with polynomial functions, trigonometric functions, elliptic functions, and circular functions. It is advantageous when the edge of the central section and the imaginary line are exactly on a continuous and inflection - free section of the mathematical function. However, a small deviation only slightly reduces the effect of the present invention. Thus, if the deviation of the edge of the central section and / or the imaginary line from the mathematical function is not greater than 25%, preferably not greater than 15%, and particularly preferably not greater than 5% of the film thickness at the section in contact with the support surface, the edge of the central section and the imaginary line extend substantially along a continuous and inflection - free section of the mathematical function.

[0020] In a preferred embodiment, the support surface is rotationally symmetric or substantially rotationally symmetric. This is particularly advantageous for films that are substantially circular because the flexure section is then supported uniformly.

[0021] In a preferred embodiment, the fastening section is located at the boundary between the central section and the flexure section. Since the fastening section is intended to fix the film to the core, this position is particularly advantageous because it ensures that all parts of the film on the side of the fastening section are firmly connected to the core or permanently supported by the core, while the parts on the other side of the fastening section are part of the flexure section and are thus movable relative to the core.

[0022] The present invention also relates to a film for a film system of the type described. The film can be multilayered. For example, the film can have two layers spaced apart by a fabric.

[0023] Furthermore, alternatively or in combination, the film can have a protective layer on its second side that extends over the outer section, the flexure section, and the fastening section but not over the central section. Preferably, the protective layer abuts against the second side of the film in the first and second positions, at least in the area where the protective layer can contact the support surface, but is not bonded to the second side of the film. The protective layer is intended to trap the dispensing fluid that penetrates the damaged film in the event of a film rupture (i.e., a point - type failure). A corresponding sensor can be provided between the protective layer and the film to detect the presence of the dispensing fluid, thereby triggering the pump to shut off or at least emit a fault signal. However, the sensor does not necessarily have to be located between the protective layer and the film. For example, the sensor can be located near the outer section. The sensor can be a liquid sensor or a pressure sensor that detects the bulging of the film caused by the fluid entering between the protective layer and the other film layers.

[0024] In another preferred embodiment, as an alternative to or in combination with the multi-layer structure and / or the protective layer, it is provided that the second side has a groove-shaped recess on the side of the flexure section facing the central section. The groove-shaped recess allows the flexure section to better follow the support surface during the movement of the core between the first position and the second position, thereby increasing the service life of the membrane in this area.

[0025] In another preferred embodiment, the first side of the membrane has a convex curved section at the transition from the outer section to the flexure section, a convex curved section at the transition from the central section to the flexure section, and a concave curved section connecting the two convex curved sections. In other words, the membrane has a bead located in the flexure section on its first side, allowing the core to move a greater distance relative to the clamped outer section without excessive elastic or plastic deformation of the membrane or its individual layers.

[0026] In a preferred embodiment, when the membrane is observed in a stress-free state, i.e., in a state where no force is applied to the membrane via the core, the tangent of the inflection point between the convex curved section and the concave curved section located at the transition from the outer section to the flexure section forms an angle of less than 30° with the longitudinal axis. Alternatively or in combination, the tangent of the inflection point between the convex curved section and the concave curved section located at the transition from the central section to the flexure section forms an angle of less than 65° with the longitudinal axis.

[0027] The position of this inflection point further increases the movement range of the flexure section, thereby allowing a greater stroke and thus a greater dosing volume per stroke.

[0028] The aim is to increase the dosing volume per stroke without increasing the lateral expansion of the membrane, and thus without increasing the expansion of the dosing chamber in this direction.

[0029] The present invention also relates to a metering diaphragm pump having a membrane system of the described type or having a corresponding membrane, wherein the membrane is clamped at its outer section, and the metering diaphragm pump has a chamber divided by the membrane into a metering chamber and a working chamber. An actuator is provided that moves the core back and forth between a first position and a second position. By the measures of the present invention, the metering diaphragm pump can be designed such that the ratio of the outer diameter of the flexure section to the distance between the first position and the second position of the core is less than 15, preferably less than 12.5, particularly preferably less than 10, and greater than 5. Description of the Drawings

[0030] Other features, advantages and applications of the present invention will become apparent from the following description of the preferred embodiments and the drawings.

[0031] In the drawings:

[0032] Figure 1A cross-sectional view of the metering head of a prior art metering diaphragm pump is shown.

[0033] Figure 2 A cross-sectional view of a first inventive embodiment of the membrane system is shown.

[0034] Figure 3 Shows Figure 2 an enlarged detail.

[0035] Figure 4 A cross-sectional view of a second inventive embodiment of the membrane system is shown, and

[0036] Figure 5 shows Figure 4 an enlarged detail.

[0037] Figure 1 A cross-sectional view of the dosing head of a prior art metering diaphragm pump is shown. The dosing head includes a working chamber element (5) and a dosing chamber cover (4), and the membrane system is clamped between the working chamber element (5) and the dosing chamber cover (4). The membrane system includes membranes (7, 8, 9), which have an outer section (7), a central section (9), and a flexure section (8) connecting the outer section (7) to the central section (9), and the outer section is clamped between the working chamber element (5) and the dosing chamber cover (4). The core (2) is positioned with its front surface abutting against the central section (9).

[0038] The membranes (7, 8, 9) divide the cavity formed by the dosing chamber cover (4) and the working chamber element (5) into a dosing chamber (6) and a working chamber (12). The core (2) has a threaded hole (15) for connecting the push rod of the actuator. With the help of this actuator, the core (2) can move from Figure 1 its first position shown in to the second position along the longitudinal axis (16) to the left, in which the volume of the dosing chamber (6) is smaller than in the first position. When the core (2) moves from the second position to the left to the first position, the pressure in the dosing chamber (6) decreases, allowing the dosing fluid to be sucked into the dosing chamber (6) through the suction valve (1). When the core (2) moves from the first position to the right to the second position, the suction valve (1) closes, and the dosing fluid in the dosing chamber is pressed out through the pressure valve (3) into a pressure line (not shown).

[0039] The membranes (7, 8, 9) have a first side facing the dosing chamber (6) and a second side facing the working chamber (12), the first side being intended to be in contact with the dosing fluid and the second side not being intended to be in contact with the dosing fluid. The central section (9) abuts against the core (2). To attach and seal the membranes (7, 8, 9) to the core (2), a fastening section (10) is arranged on the second side of the membrane, and the fastening section (10) is clamped between the core (2) and the clamping member (13).

[0040] A protective layer (11) is provided on the second side of the membranes (7, 8, 9), and the protective layer (11) together with the outer section (7) is clamped between the dosing chamber cover (4) and the working chamber element (5), and together with the fastening section (10) is clamped between the clamping element (13) and the core (2). A cavity (14) is formed between the second side of the membranes (7, 8, 9) and the protective layer (11). If the membranes (7, 8, 9) rupture or perforate during operation, the dosing fluid may enter the cavity (14), where it can be detected as a fault condition.

[0041] Figure 2 and Figure 3 FIG. and partial cross-sectional views show a first embodiment of the membrane system of the present invention. The membrane system can be used in the dosing head of a metering diaphragm pump as shown in Figure 1 The depicted embodiment is rotationally symmetric, which means that the membrane is substantially circular. The membrane has an outer section (27), which can be clamped in the usual way between the dosing chamber cover (see reference numeral 4 in Figure 1 the drawings) and the working chamber element (see reference numeral 5 in Figure 1 the drawings).

[0042] In addition, a spherical core (22) is provided, against which the central section (29) of the membrane abuts. A flexure section (28) connects the outer section (27) to the central section (29). The side of the membrane facing to the right in Figure 2 is the first side, which is intended to be in contact with the dosing fluid. The side of the membrane facing to the left in Figure 2 is the second side, which points to the working chamber and is thus not intended to be in contact with the dosing fluid.

[0043] Additionally, a fastening section (30) is provided on the second side of the membrane, and the fastening section (30) is clamped and sealed between the core (22) and the clamping element (23). To enhance the sealing effect, the clamping element (23) is characterized by sealing ribs on its surface facing the fastening section (30), such that the protective layer (31) is pressed against the fastening section (30), so that the fastening section (30) and the protective layer (31) are clamped between the clamping element (23) and the core (22).

[0044] Compared with the clamping element (13) of the prior art (see Figure 1 ), the clamping element (23) of the present invention is significantly larger. The clamping element (23) provides a rotationally symmetric support surface (39). The support surface (39) is convexly curved. In Figure 2In the position of the core (22) shown, the support surface (39) does not contact or hardly contacts the flexure section (28) of the membrane. When the core (22) moves to the right along the longitudinal axis (16) during operation, a part of the flexure section (28) will roll onto the support surface (39). Thus, during the pressure stroke, the flexure section (28) is supported by the support surface (39) at least on the side adjacent to the central section (29), thereby extending the service life of the membrane, especially during large stroke movements.

[0045] To enhance this support effect, the second side of the membrane features a groove-shaped recess (37) located at the boundary between the central section (29) and the flexure section (28). The clamping member (23) has a corresponding rib (38) on which the membrane rests.

[0046] The membrane in this embodiment is multilayered and comprises two layers, with a fabric layer embedded between the two. A protective layer (31) is provided, and Figure 1 different from the known embodiment shown, this protective layer (31) directly abuts against the second side of the membrane. This ensures that the protective layer (31) can roll onto the support surface (39) without being damaged. However, the protective layer (31) is not bonded to the second side of the membrane, so in the case of membrane rupture, the dosing fluid can still enter the space between the protective layer (31) and the second side of the membrane.

[0047] The support surface does not extend parallel to the longitudinal axis. Additionally, in a cross-sectional view Figure 2 containing the longitudinal axis, the support surface extends perpendicular to the longitudinal axis for a length l S , and this length l S is greater than 10% of the extent l W of the flexure section in this direction.

[0048] Figure 3 Shows Figure 2 an enlarged partial cross-sectional view. This view clearly shows that due to the protective layer (31), the membrane is thicker in the flexure section (28) than in the central section (29). In this example, the membrane thickness in the central section (29) is 2.2 mm, while the protective layer (31) has a thickness of 0.75 mm, resulting in a membrane thickness of 2.95 mm in the flexure section. The core (22) is convexly curved, and the support surface (39) of the clamping member (23) is also convexly curved. The curvatures of the central section (22) and the support surface (39) match to ensure a smooth transition between the support surface (39) and the front surface of the core (22).

[0049] In this embodiment, the front surface of the core (22) and an imaginary line spaced from the support surface (39) by the membrane thickness difference between the flexure section and the central section are locatedFigure 3 On the same circle in the cross-sectional view shown.

[0050] For the embodiment without the protective layer (31) provided, the surface of the support surface (39) and the front surface of the core (22) in contact with the central section (29) are located on the same circle. In Figure 3 In the embodiment shown, this also applies to the second position, that is, when the protective layer (31) is not considered, the position where the core is shifted to the second side of the membrane to the right along the longitudinal axis (16). The protective layer (31) rests on the support surface (39) in the second position, such that the surface of the protective layer (31) is located on the circle formed by the radius of curvature of the core. Therefore, the radius of curvature of the support surface (39) marked 45.55 mm in the drawing is different from the radius of curvature of the central section (29) marked 48.5 mm on its first side in the drawing. The difference between the two radii exactly corresponds to the membrane thickness in the flexure section (2.2 mm + 0.75 mm = 2.95 mm).

[0051] It should be understood that the main effect of the present invention is achieved through the transition between the central section (29) and the flexure section (28) that rolls onto the support surface (39). Therefore, the entire front surface of the core (22) does not have to be strictly located on the circle. It is sufficient if the edge region adjacent to the flexure section (28) is located on the circle.

[0052] The front surface of the core and the imaginary line spaced apart from the support surface also do not have to be located on the circle. In principle, any mathematical function can be used as long as it is continuous and has no inflection points in the section where the support surface (39) or the imaginary line spaced apart from the support surface (39) by the membrane thickness and the edge region of the front surface of the core (22) are located.

[0053] Figure 4 A cross-sectional view of a second inventive embodiment of the membrane system is shown. This figure is similar to Figure 2 the figure shown in Figure 2 so only the differences between the embodiment shown in Figure 4 and the embodiment shown in

[0054] In Figure 4 the embodiment shown, the clamping member (33) has a coating (40) made of an elastic material (such as, EPDM). The coating (40) is integrally bonded to the clamping member (33). The shape of the surface of the coating (40) is exactly similar to Figure 2 the surface of the clamping member (23) shown in

[0055] Figure 5 Shows Figure 4Magnified details of an embodiment. Clearly visible is the outer section (27), which is clamped and thus does not deform during the movement of the core (22). The central section (29) is also visible, which abuts against the front surface of the core (22) and similarly does not deform during the movement of the core (22) from the first position to the second position and back again. Between the two sections is the flexure section (28), which undergoes significant deformation during the movement of the core (22).

[0056] Along Figure 5 the path of the middle film from top to bottom, there is a flat section corresponding to the outer section. This is followed by a convex curved section, which transitions to a concave curved section, and near the central section, transitions back to a convex curved section. The inflection points where the convex curvature becomes concave curvature or the concave curvature becomes convex curvature are marked with reference numerals 41 and 42.

[0057] A tangent line (43) is also drawn at the inflection point 41 between the convex curved section and the concave curved section located at the transition from the outer section to the flexure section. This tangent line (43) forms an angle β less than 30° with the longitudinal axis (16). In addition, a tangent line (44) is drawn at the inflection point 42 between the convex curved section and the concave curved section located at the transition from the central section to the flexure section. This tangent line (44) forms an angle α less than 65° with the longitudinal axis (16). The position of the inflection point is independent of the choice of the elastic support surface and is thus also advantageous in embodiments without an elastic support surface.

[0058] List of reference numerals

[0059] 1 Suction valve

[0060] 2, 22 Core

[0061] 3 Pressure valve

[0062] 4 Dosage chamber cover

[0063] 5 Working chamber element

[0064] 6 Dosage chamber

[0065] 7, 27 Outer section of the diaphragm

[0066] 8, 28 Flexure section of the diaphragm

[0067] 9, 29 Central section of the diaphragm

[0068] 10, 30 Fastening section

[0069] 11, 31 Protective layer

[0070] 12 Working chamber

[0071] 13, 23, 33 clamping parts

[0072] 14 cavities

[0073] 15 threaded holes

[0074] 16 longitudinal axis

[0075] 36 fabric

[0076] 37 groove-shaped recess

[0077] 38 rib

[0078] 39 support surface

[0079] 40 elastic coating

[0080] 41, 42 inflection points

[0081] 43 sealing rib.

Claims

1. A membrane system for a metering pump, the membrane system comprising: A membrane having an outer section intended to be clamped, a central section, and a flexure section surrounding the central section, the flexure section connecting both the outer section and the central section, wherein the flexure section is designed to allow the central section to move back and forth between a first position and a second position relative to the outer section along a longitudinal axis, wherein the membrane has a first side intended to be in contact with a dosing fluid and a second side not intended to be in contact with the dosing fluid; and a core, the core abutting against the second side at the central section, wherein the membrane has a fastening section located on the second side and is provided with a clamping piece, wherein the core, the fastening section and the clamping piece are arranged and designed so that the fastening section is held between the core and the clamping piece, characterized in that A support element having a support surface is provided, wherein the support surface in the second position is in contact with the second side of the membrane in the region of the flexure section and in the first position is not in contact with the second side of the membrane in the region of the flexure section.

2. The membrane system according to claim 1, characterized in that The support surface is arranged on the clamping piece, wherein preferably the core has a section with an external thread and the clamping piece is screwed onto the external thread.

3. The membrane system according to claim 1 or 2, characterized in that: The support surface is at least partially made of an elastic material, preferably of plastic, and particularly preferably of an elastomer such as EPDM or NBR.

4. The membrane system according to any one of claims 1 to 3, characterized in that The support surface is convexly curved.

5. The membrane system according to claim 3, characterized in that The core is at least partially convexly curved on the side resting on the central section, wherein preferably at least the section facing the supporting surface is convexly curved.

6. The membrane system according to claim 5, characterized in that The sum of the radius of curvature of the support surface and the film thickness at a section in contact with the support surface, and the sum of the radius of curvature of the convexly curved section of the core and the film thickness at a section in contact with the convexly curved section of the core are substantially equal.

7. The membrane system according to claim 5, characterized in that The film has a thickness d at the edge of the central section. R , and has a thickness d at a section in contact with the support surface in the second position. S , wherein in cross-sectional view, at least one edge of the central section is spaced apart from the support surface by a difference |d S -d R The imaginary line | extends along the continuous and inflection-free segment of the mathematical function.

8. The membrane system according to any one of claims 1 to 7, characterized in that The support surface is rotationally symmetric.

9. The membrane system according to any one of claims 1 to 8, characterized in that The fastening section is located at a boundary between the central section and the flexure section.

10. A membrane for use in a membrane system according to any one of the preceding claims, characterized in that The membrane has two layers separated by a fabric.

11. The film according to claim 10, characterized in that The membrane has a protective layer on its second side, which extends over the outer section, the flexure section and the fastening section but not over the central section, wherein preferably, the protective layer abuts against the second side of the membrane but is not bonded to the second side of the membrane in both the first position and the second position, at least in the area where the protective layer can contact the supporting surface.

12. The membrane according to claim 10 or 11, characterized in that The second side has a groove-shaped recess on a side of the flexure section facing the central section.

13. The membrane according to any one of claims 10 to 12, characterized in that The first side of the membrane has a convex curved section at the transition from the outer section to the flexural section, a convex curved section at the transition from the central section to the flexural section, and a concave curved section connecting the two convex curved sections, wherein preferably, the tangent of the inflection point between the convex curved section and the concave curved section at the transition from the outer section to the flexural section forms an angle of less than 30° with the longitudinal axis, and / or the tangent of the inflection point between the convex curved section and the concave curved section at the transition from the central section to the flexural section forms an angle of less than 65° with the longitudinal axis.

14. A metering diaphragm pump having a membrane system according to any one of claims 1 to 9 and preferably having a membrane according to any one of claims 10 to 13, wherein: The membrane is clamped at its outer section, the metering diaphragm pump has a chamber divided by the membrane into a metering chamber and a working chamber, an actuator is arranged therein, the actuator moves the core back and forth between the first position and the second position, characterized in that The ratio of the outer diameter of the flexure section to the distance of the core between the first position and the second position is less than 15, preferably less than 12.5, particularly preferably less than 10, and greater than 5.

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