CH687969A5 - Faserverstaerkte membrane. - Google Patents

Description

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description

Technical field

The present invention relates to the field of technical membranes. It relates to a fiber-reinforced membrane for the separation of two spaces in which different pressures prevail, which membrane comprises a flat membrane body made of an elastomer, which can be deflected elastically in the direction perpendicular to the membrane surface at least in some areas, the membrane body being provided by a large number of elastomers embedded short fibers is reinforced.

Such a membrane is e.g. from a lecture by Monsanto, “Overview of industrial applications of Santoweb® fiber in the rubber industry”, held at the 8th International Rubber Symposium, Gottwaldow (CSSR), October 1981.

State of the art

Technical membranes are basically movable, sealing walls that separate two rooms with generally different media (gases, liquids) and / or different pressure conditions. The special feature of a membrane is its mobility, in particular its ability to perform a more or less large stroke in either direction perpendicular to its clamping surface (membrane surface) - mechanically driven or as a result of a pressure difference.

The ability of a membrane to move essentially depends on the properties of its material (in the case of a composite structure: its materials), its dimensions and, for larger strokes - on the shape imparted during manufacture. Essential criteria for membrane materials are sufficient

- flexibility (agility),

- elasticity (sealing function),

- tensile strength (pressure absorption), and

- Chemical resistance within the application-related temperature and pressure difference range as well as in the presence of appropriate media to be separated by the membrane. The basic materials used are mainly elastomers, which can be reinforced with fabrics or fiber admixtures for certain applications to achieve higher tensile strength.

There are three general areas of application for membranes: the conversion of a pressure difference into a mechanical force (hydraulically or pneumatically actuated actuators, pressure or flow regulators, valves), the reverse conversion of a mechanical force into a pressure (pumps, compressors) and the pure one Separation of different media without significant pressure difference (expansion vessels, pressure accumulator).

The large number of membrane designs required for this can be attributed to two basic shapes, the flat membranes and shaped membranes, the shaped membranes in turn being subdivided as follows:

- beading membranes

- plate membranes

- rolling membranes and

- dome membranes.

The flat membranes can be punched out of prefabricated sheet material in a particularly simple manner, but must be pretensioned during installation in order to enable perfect operation. The shaped membranes have the advantage of a larger stroke, a smaller change in effective area over the entire stroke range and easier installation (without a pre-tensioning operation). They are either made from web material by contour embossing or deep drawing, or directly by mold vulcanization.

The following materials, for example, have proven themselves as elastomers in membranes:

- acrylonitrile butadiene rubber (NBR)

- Ethylene propylene terpolymer rubber (EPDM)

- fluorine rubber (FKM)

- Fluorosilicone rubber (FVMQ)

- silicone rubber (VMQ)

- Chlorobutadiene rubber (CR)

- Epichlorohydrin rubber (ECO)

- Butyl rubber (HR)

- Isoprene rubber (IR)

- natural rubber (NR)

- Polyurethane (PUR).

In the form of reinforcing fabrics, which are either placed on one or both sides of the membrane or placed directly in the membrane, e.g. the following fiber materials have proven themselves:

- polyamides

- aromatic polyamides (aramids)

- Glass

- polyester

- rayon.

The above-mentioned production of fabric-reinforced membranes (in particular shaped membranes) by means of shape vulcanization leads to various problems: since in shape vulcanization the reinforcing fabric is introduced into the corresponding vulcanization mold together with the elastomer, in the case of more complicated shapes, the fabric is easily displaced or localized, which lead to the fact that the mechanical properties of the finished membrane at these points either deviate from the desired values, or the fabric undesirably comes to the surface, where it is exposed to the attack of the adjacent medium, and thus forms a weak point for the membrane . There is also the reinforced fabric

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Basically, membranes have the disadvantage that they form multilayer composite bodies, the individual layers (elastomer layers and fabric layers) of which have different mechanical properties, so that the vulcanized tissue can detach itself over time from the overlying elastomer layers and the membrane due to the flexing movement of the membrane during operation in this way becomes unusable.

Finally, the insertion of a continuous, reinforcing fabric means that essential mechanical properties of the membrane, such as e.g. the stretchability is predetermined and constant by the type of fabric. It is therefore not possible to design the membrane so that these mechanical properties in predetermined functional areas of the membrane, e.g. in a bead or in a rolling area, be set differently from the start. For example, Due to the reinforcement of the fabric, the elongation at break across the entire membrane surface is usually limited to a value of less than 10%, while the elongation at break in the case of conventional rubber without reinforcement is of the order of 500%.

In order to specifically solve the above-mentioned problem of the shifted textile insert, it was proposed in the publication cited at the beginning to use short fibers made of cellulose, which are preferably used in injection molding, for rubber pressed parts such as seals or membranes instead of being reinforced by textile fabric in the rubber matrix aligned in a certain way to achieve optimal mechanical properties. However, information on parts by weight and the length and diameter of the cellulose fibers or the mechanical properties of the resulting membranes cannot be found in the publication.

Finally, it should be pointed out that it has been known for a long time to incorporate synthetic short fibers, in particular in the form of an aramid fiber pulp, as reinforcing filler in rubber mixtures (see the article by LL Outzs “Use of aramid fiber pulp / neoprene masterbat-ches”, GAK 5 / 1991, pp. 226-234). The aramid pulp has a fiber diameter of approximately 12 μm and an average fiber length of 1 mm. The available surface of the aramid staple fiber is only 0.1 m2 / g, while the manufacturing process of the pulp brings this value to 7-9 m2 / g by fibrillating the surface. Areas of application for such masterbatches with aramid fiber pulp are hoses, drive belts, solid tires, caterpillar pads, seals including oil drilling packs, roller covers, shoe soles and generally products made from sheet material. This type of fiber reinforcement is not intended for dynamically loaded membranes.

Problems with reinforcement using short fibers are generally caused by the fact that the short fibers are mechanically connected to one another only via the elastomer matrix. As a result, the modulus and elongation at break in the

As a rule, deviate significantly from the values for fabric-reinforced material and only approach these values with relatively high fiber shares, which in turn worsens other properties of the material. This is probably the reason why reinforcement with short fibers has so far not been successful in practice in the case of dynamically loaded membranes.

Presentation of the invention

It is an object of the invention to provide a fiber-reinforced membrane that can be loaded dynamically and is characterized by improved mechanical properties with a simplified production.

The object is achieved in the case of a membrane of the type mentioned at the outset in that the membrane has an elongation at break of greater than or equal to 50% and less than or equal to 150% and the stress value for an elongation of 50% or greater is equal to 5 N / mm 2.

The invention is based on the surprising finding that in the case of dynamically loaded membranes it is sufficient for the full functionality to leave the elongation at break at a comparatively high value if the module is simultaneously raised to a sufficiently large value. In this way it is possible to make do with relatively small proportions of fiber material.

A first preferred embodiment of the invention is characterized in that highly tear-resistant synthetic fibers and / or carbon fibers are used as short fibers.

Such fibers, which have already been used, for example in the form of aramid fibers, as an asbestos substitute to increase the abrasion resistance in elastomers, are notable for very good mechanical properties, so that a high reinforcing effect in the membrane is achieved even with comparatively small fiber fractions. This makes it possible, in particular also in the injection molding process, to produce highly flexible, highly structured membranes, which can also replace the conventional fabric-reinforced membranes without restriction, but are much easier to manufacture.

A second preferred embodiment of the membrane according to the invention is characterized in that

(a) the short fibers are made from an aromatic polyamide (aramid);

(b) the short fibers are fibrillated;

(c) the average length of the short fibers is between 0.8 and 1.5 mm; and

(d) the ratio of fiber length to fiber diameter of the short fibers is in the range of about 100.

The manufacture and mechanical properties of the membrane are particularly favorable if, according to another preferred embodiment, aramid fibers are added to the elastomer in a weight fraction of between 3 and 8%.

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Another preferred embodiment of the invention is finally characterized in that the short fibers within the membrane body are oriented with their fiber axes predominantly in the direction of a tensile load on the membrane. In this way, direction-dependent mechanical properties can be advantageously realized, which are optimized with regard to the loads occurring in the application.

Further embodiments result from the dependent claims.

Brief description of the drawings

The invention will be explained in more detail below using exemplary embodiments in conjunction with the drawings. Show it

1 is a detail and in cross section of the known application for a dynamically loaded membrane in a safety valve;

2 shows in cross section the inner structure of the membrane from FIG. 1 with fabric reinforcement according to the prior art;

3 shows a section of the membrane according to FIG. 2 with incorrectly embedded tissue reinforcement;

4a shows the cross section through a membrane reinforced with short fibers, in which the fibers are embedded with (schematically indicated) isotropic directional distribution;

4b shows the cross section through a membrane reinforced with short fibers, in which the fibers are embedded with (schematically indicated) uniaxial directional distribution; and

4c shows the cross section through a membrane according to a preferred embodiment of the invention, in which different mechanical properties are achieved at different points by changing the thickness.

Ways of Carrying Out the Invention

1 shows an exemplary application of a dynamically loaded, elastic membrane in a safety valve in cross section. A membrane 10 with a plate-shaped, rotationally symmetrical membrane body 1 is clamped on the edge between two flanges 7a and 7b. The membrane 10 separates two spaces R1 and R2, in which media with different pressures and pressure differences of a few bar are generally present.

For this application, a sealing bead 2 is preferably formed on the outer circumference of the membrane body 1, which comes to rest in a specially shaped groove in one of the flanges 7a, b and takes over a corresponding sealing function. In the central part of the membrane body 1, a metallic support plate 4 is vulcanized in, which picks up the pressure of a vertically displaceable pressure stamp 5 and introduces it over a wide area into the membrane. The part of the membrane body 1 located under the support plate forms an elastic sealing surface 3 which, when the

Pressure stamp 4 is pressed sealingly against an opposite valve seat 6. During this closing process of the valve, the part of the membrane body 1 located between the central part (3, 4) and the sealing bead 2 usually bulges out and forms a bead (not shown in FIG. 1 for the sake of simplicity).

So that the membrane 10 can withstand the changing forces occurring even at higher pressure differences between the spaces R1 and R2, the membrane body 1 in the prior art - as shown in FIG. 2 - is equipped with a fabric insert 8 running largely in the center of the body, which mechanically reinforces the elastomer body with relatively high elasticity.

Because of the vulcanized-in support plate 4 and the molded-on sealing bead 2, it is generally necessary to manufacture the membrane shown in an injection molding process, in which the fabric insert 8 is first inserted into the injection mold and then surrounded with the elastomer. The elastomer injected into the mold may shift the fabric insert 8, so that - as indicated in FIG. 3 - there is either an upsetting (area A) or a deviation from the central layer (area B). In both cases, the shift of the tissue insert from the ideal position results in a weakening of the mechanical properties of the membrane, which cannot be tolerated.

In the membrane according to the invention, the fabric insert is therefore completely replaced on the one hand by short fibers distributed in volume. On the other hand, the elongation at break with values between 50% and 150% and the module with a tension value greater than or equal to 5 N / mm2 at an elongation of 50% are selected for the finished membrane material. Under these conditions, there are surprisingly sufficient mechanical properties for dynamic loading with good manufacturability.

The short fibers preferably consist of a highly tear-resistant synthetic material and / or carbon, in particular of an aromatic polyamide, and are mixed with the elastomer in certain proportions by weight (in particular 3 to 8%). The special type of fiber material, especially fibrillation, allows a very high binding-active surface, so that with fiber lengths between 0.8 and 1.5 mm and fiber length to diameter ratios of around 100, the fibers are very well integrated into the elastomer matrix is possible. In this way, a high reinforcement effect can be achieved without significantly impairing the processing properties of the elastomer / fiber mixture. It has also proven to be particularly favorable for use to set the membrane (10) to a hardness between 60 and 90 Shore “A”.

The directional distribution of the short fibers 11 in the membrane body 1 depends largely on the type of manufacture (FIGS. 4a and 4b). In Fig. 4a, the short fibers 11 are oriented isotropically, which is due to the alternately perpendicular to each other

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Fiber triple should be hinted at. This type of alignment is not optimal for a membrane because the tensile loads occur predominantly in the radial direction and in the plane of the membrane body. It is therefore advantageous to orient the short fibers 11 - as shown in FIG. 4b - with their fiber axes predominantly radially to the axis of symmetry of the membrane body 1. This can be done e.g. can be achieved by injecting the elastomer / fiber mixture from the center into the membrane shape in such a way that corresponding radial shear forces occur which orient the fibers.

Since the short fibers 11 - unlike a fabric insert - change the mechanical properties of the elastomer in the entire volume, on the one hand sealing beads 2 can be easily molded onto the membrane, which are also fiber-reinforced and form a unit with the rest of the membrane body. On the other hand, it is also possible to adjust the mechanical properties of the membrane differently in different areas by the choice of the material thickness in order to take into account different loads and the required flexibility. In this way e.g. the membrane - as is clear from FIG. 4c - in the functional area 9 between the center and the outer edge, in which the membrane bulges during operation of the valve, can be deliberately weakened by reducing the thickness from d2 to d1 in order to produce a more favorable bulging behavior . In this context, it has proven particularly useful to select the thickness d1 of the membrane in its functional area 9 to be less than or equal to 2.5 mm.

It goes without saying that the invention is not limited to the membrane shape shown by way of example in the figures, but can also be used successfully with other dynamically loaded membranes. In addition, a large number of combinations of elastomers and short fibers is conceivable within the scope of the invention.

Overall, the invention results in a membrane for dynamic loads, which can be produced easily and with little effort, can be flexibly adapted to the requirements of the application and has excellent mechanical properties.

List of designations:

1 membrane body

2 sealing bead

3 sealing surface

4 support plates

5 stamps

6 valve seat

7 a, b flange

8 fabric insert

9 functional area

10 membrane

11 short fiber d1, d2 thickness

R1, R2 space

Claims (10)

Claims 1.Fiber-reinforced membrane (10) for the separation of two spaces (R1, R2) in which different pressures prevail, which membrane (10) has a flat membrane body (1) which can be deflected elastically in the direction perpendicular to the membrane surface, at least in some areas Elastomer comprises, the membrane body (1) being reinforced by a plurality of short fibers (11) embedded in the elastomer, characterized in that the membrane (10) has an elongation at break of greater than or equal to 50% and less than or equal to 150% and the stress value at an elongation of 50% is greater than or equal to 5 N / mm2. 2. Fiber-reinforced membrane according to claim 1, characterized in that the membrane (10) has a hardness between 60 and 90 Shore "A". 3. Fiber-reinforced membrane according to one of claims 1 and 2, characterized in that highly tear-resistant synthetic fibers and / or carbon fibers are used as short fibers (11). 4. Fiber reinforced membrane according to claim 3, characterized in that (a) the short fibers (11) are made from an aromatic polyamide; (b) the short fibers (11) are fibrillated; (c) the length of the short fibers (11) is between 0.8 and 1.5 mm; and (d) the ratio of fiber length to fiber diameter of the short fibers (11) is in the range of about 100. 5. Fiber-reinforced membrane according to claim 4, characterized in that the proportion of short fibers (11), based on the total weight, is between 3% and 8%. 6. Fiber-reinforced membrane according to one of claims 1 to 5, characterized in that the short fibers (11) within the membrane body (1) are oriented with their fiber axes predominantly in the direction of a tensile load on the membrane (10). 7. Fiber-reinforced membrane according to one of claims 1 to 6, characterized in that the membrane body (1) is rotationally symmetrical, and that the short fibers (11) with their fiber axes are predominantly oriented radially to the axis of symmetry of the membrane body (1). 8. Fiber-reinforced membrane according to one of claims 1 to 7, characterized in that the membrane body (1) is formed with a locally different thickness (d1, d2). 9. Fiber-reinforced membrane according to one of claims 1 to 8, characterized in that the membrane (10) has a functional area (9), and that the thickness (d1) of the membrane (10) in the functional area (9) is less than or equal to 2.5 mm is. 10. Fiber-reinforced membrane according to one of claims 1 to 9, characterized in that the membrane body (1) is formed on the outer circumference of a sealing bead (2). 5 10th 15 20th 25th 30th 35 40 45 50 55 60 65 5