DE4231927C2 - Plastic membrane for pressure accumulators - Google Patents

Description

The invention relates to an elastic, high gas-tight plastic membrane for pressure accumulator.

It is generally known in pressure storage systems, e.g. B. Hydro bubble or hydromembrane accumulators or accumulators, Rubber or elastomeric membranes that are used as hollow spheres or flat membranes formed and with gas, preferably air or stick filled with fabric. Your way of working is based on the great compressibility of gases, through energy is saved. Your functionality therefore depends on it from that the predetermined amount of gas remains as constant as possible and no gas losses occur during operation. Such Accumulators can be used as shock absorbers to To dampen pressure surges in a working group. You can also used as an energy source for a pumpless emergency circuit are and are ultimately suitable for. B. also as hydraulic Compression springs (cf. company lettering from Hydac Technology GmbH, 6603 Sulzbach / Saar; Hydak International, "Storage, Accumulators, Accumulateurs, brochure No. 3.000.0 / 3.87).

It is also known e.g. B. from EP-A-0 360 648, in pressure storage systems rubber or elastomeric membranes made from a copolymer of butadiene with styrene and acrylonitrile with the short name NBR and a nitrogen permeation coefficient Q = 15 m ² / s / Pa × 10 ^{- 17} (20 ° C) to 35 m ² / s / Pa × 10 ^-17 (80 ° C) according to DIN 53536.

There is a serious disadvantage of such elastomeric membranes in that when used depending on the chemical Composition of the elastomer and the operating time for the enclosed gas become permeable. The nitrogen or the Air initially dissolves in the elastomer and diffuses under Evaporation into the environment ("gas permeability of rubber elastic materials for nitrogen "in colloid magazine and Zeitschrift für Polymer 220, Issue 2, pp. 97ff, 1967, Dr. Lockpick Steinkopf publishing house Darmstadt). In this way, the the amount of gas in the storage system and the functionality speed decreases rapidly, making the storage system unusable is and is not reusable, but disposed of must become.

There has been no shortage of attempts, the permeation of the gases to prevent or at least by modifying the membrane belittling. So z. B. from DE-OS 36 28 608 after a complicated process known membrane known from five bonded layers of different materials consists. These layers have a different Gas permeability, which should ensure that the Gas is discharged and does not diffuse into the surrounding oil can.

From DE-OS 25 22 380 is also known to increase the Permeation resistance for nitrogen not only elastomers use from multiple layers, but in a complex manner a space filled with liquid between the To create layers of elastomer in which the nitrogen dissolve can.

EP-A-0 360 648 is also used to increase the permeance ation resistance known a membrane, which is also made of consists of several layers, initially in one complex chemical process an elastomer is made into which a second polymer is embedded, which the diffusion path of the Gases bothers.  

Finally, from DE-PS 28 21 671 is a membrane pressure Storage for liquids with a pressure-resistant housing base part and a pressure-resistant upper housing part, which housing parts are inextricably linked, continue with two between the housing parts parallel to each other, elastic deformable membranes, which is a pre-stressed Separate the gas cushion from the liquid to be stored, and with a gas-permeable arranged between the membranes Liner known, which is characterized in that the Liner consists of a solid and between the Housing parts is gas permeable to the outside. over the structure or composition of the membranes and The patent specification does not contain any intermediate layer.

Another serious disadvantage of the previously known systems is that they are not thermally deformable. This means, they can either be used only as flat surfaces or need z. B. by strong mechanical stress on the geometric Shape, e.g. B. the bladder memory, adapted and by additional constructive measures are recorded.

From DE 41 17 411 A1 are elastic known high gas-tight plastic membranes for pressure accumulators, made of a layer of plastic resin and on one side or have a rubber layer on both sides.

The thickness of the synthetic resin layer is from approximately 50 µm to 2000 µm, that of the rubber layer of about 200 µm to 2 mm. Despite the large layer thickness, it is Gas permeability increases, especially if it is at deep Temperatures to embrittlement of the membrane material and it fails.

The object of the invention is an elastic, high gas tight Specify plastic membrane, which is characterized by high permeations resistance corresponding to a low permeation coefficient and good durability with comparatively easy to manufacture is marked and due to these properties in the most diverse storage systems, such as z. B. also in the above-mentioned literature references can be used are.

According to the invention, this object is achieved with a plastic membrane as defined in the claims is drawing.

Plastic membranes according to the invention have a very high Permeation resistance to nitrogen, i. H. one very low permeation coefficients Q according to DIN 53536 and  53380. The permeation coefficient Q indicates the gas volume at a given pressure difference in a certain time passes through a specimen of known area and thickness.

Plastic membranes according to the invention have a permeation coefficient Q in m ² / s. Pa ^-1 of at least less than 0.1 m ² / s. Pa. 10 ^-17 on.

Membranes that can be produced according to the invention are also opposite Mineral oil resistant and can therefore be found in oily liquids save without additional measures, can be used directly.

For the manufacture of the plastic membranes of the invention Suitable heat-crosslinkable (addition-crosslinking) polysiloxanes:

Silanes and siloxanes which are suitable for producing the plastic membranes and which crosslink to polysiloxanes are commercially available. They are usually offered together with a crosslinking agent, a catalyst and, if necessary, diluents. A typical addition-crosslinkable siloxane system which can be used according to the invention consists, for. B. from:

• a) Dehesiv 920 (siloxane monomer)
• b) Crosslinker V24
• c) catalyst OL and optionally
• d) diluents, e.g. B. from aliphatic coals hydrogen.

Manufacturer company Wacker AG Munich.

Manufacture of the plastic membranes of the invention:

According to the invention, the solution of monomers, crosslinking agents, Catalyst and optionally diluent e.g. B. with a Doctor blade or by spraying or pouring or brushing, preferably with roller systems, as in 7. Nentwig, Lexikon for film technology, Verlag Weinheim 1991, p. 33, fig. 1ff, described, applied.

Then the solution applied to the polymer film at temperatures between 80 and 150 ° C, preferably at 120 ° C, vulcanized for 30 seconds to 60 minutes, preferably 20 minutes. The desired layer thickness is expedient by diluting the Vulcanizing mixture in a manner known per se by addition of diluents, e.g. B. aliphatic hydrocarbons, such as B. petroleum ether (boiling point 40-60 ° C) in a ratio of 1: 1 set to 1:10.

After vulcanization, a thermoplastic process is created bare composite film, the two layers firmly attached to each other are welded.

It follows from the following test results that the siloxane modes Composite foils significantly smaller permeation coefficient cations against nitrogen have as pure polyethylene terephthalate films.

Measurement of the permeation coefficient:

To determine the permeation coefficient Q, a Device as shown in the drawing is used.  

The membrane to be tested is clamped between the measuring chambers K1 and K2 so that no gas can escape to the outside at the clamping points. The test gas N ₂ enters the measuring chamber K2 through a valve. The gas pressure can be read on the manometer. A gas-permeable support plate is attached so that the membrane does not bend under the influence of the gas pressure. A measuring tube calibrated in volume units is connected to the measuring chamber K1 and is filled with colored water.

The volume of liquid V _F , which is displaced by the gas from the U-tube, is determined as a function of the test duration t at a constant temperature T, (room temperature 23 ° C.) and is shown graphically in order to check the proportionality.

Since the test gas only has to diffuse into the sample, it is created a non-linear start-up process. Because of this, the sample conditioned for about 30 minutes, d. H. it will be without actual Measurement exposed to the test conditions.

The measured volume of liquid that is displaced in the U-tube is not equal to the volume of gas diffused through the sample, because the gas in the chamber K1 is due to the height difference L the liquid in the two U-tube legs under one higher pressure than the outside air pressure. That is why it is needed here a correction.

The permeation coefficient is to be determined from the measured pair of values using the equation below.

Q = permeation coefficient in [m ² + s ^-1 + Pa ^-1 ]
T = temperature of the sample in [° C]
ΔV _F = volume of the liquid displaced by the gas from the U tube after Δt seconds in [m ³ ]
b = thickness in [m]
A = effective area of the membrane in [m ² ]
Δp = pressure difference between chamber K2 and external air pressure in [Pa]
Vu = volume of chamber K1 and the U-pipe supply at the start of the test in [m ³ ]
A _U = cross section of the U-tube in [m ² ]
D = density of the liquid in the U-tube in [kg / m ³ }
ΔL = height difference of the liquid in the U-tube between the start and end of the measurement in [m]
p ₀ = standard pressure: 101 325 Pa
T ₀ = standard temperature: 273.15 K.
g = gravitational acceleration: 9.80665 m / s ²

When using these units, the permeation coefficient indicates how much m ^{3 of} gas, based on 0 ° C at 101 325 Pa, can pass through a test specimen of 1 m thickness and 1 m ² surface at 1 Pa pressure drop at the temperature of the test specimen in one second .

The following examples are intended to illustrate the invention vivid.

example 1

A polyethylene terephthalate film (hostaphan film) with a A thickness of 19 µm was stretched on a glass plate. A solution of 100 parts by weight of Dehesiv 920 was with 2.5 parts by weight Crosslinker V24 and 1.0 part by weight of OL Catalyst (Wacker) mixed in the order given. The separately The mixture of crosslinker and catalyst was slow added to the Dehesiv 920 with stirring, with stirring Blistering was avoided.  

The solution thus prepared was clamped onto the hostaphan film bubble-free (doctor blade height 100 µm) and in one Vulcanized at 120 ° C for 25 minutes.

Then the permeation coefficient Q for nitrogen certainly.

Results:
Q at 23 ° C: No gas penetration was found within the measuring accuracy of the method used.
Q at 80 ° C: 0.05 m ² /s.Pa × 10 ^-17

Comparative Example 1

Under the same conditions, an uncoated hostaphan foil measured.

Results:
Q at 23 ° C: 0.11 m ² /s.Pa × 10 ^-17
Q at 80 ° C: 0.02 m ² /s.Pa × 10 ^-17

Example 2

The procedure was as in Example 1, but with a Doctor blade height of 50 microns worked.

Results:
Q at 23 ° C: No gas penetration was found within the measuring accuracy of the method used.
Q at 80 ° C: 0.03 m ² /s.Pa.10 ^-17

Example 3:

The procedure was as in Example 1, but with a Doctor blade height of 20 microns worked.

Results:
Q at 23 ° C: No gas penetration was found within the measuring accuracy of the method used.
Q at 80 ° C: 0.05 m ² /s.Pa.10 ^-17

Example 4:

The procedure was as in Example 1, but one was Hostaphan film with a thickness of 12 µm is used.

Results:
Q at 23 ° C: No gas penetration was found within the measuring accuracy of the method used.
Q at 80 ° C: 0.01 m ² /s.Pa.10 ^-17

Comparative Example 2:

An uncoated Hostaphan film of 12 µm was tested.

Results:
Q at 23 ° C: 0.15 m ² /s.Pa.10 ^-17
Q at 80 ° C: 0.24 m ² /s.Pa.10 ^-17

Example 5

The procedure was as in Example 1, but the solution Dehesiv 920 diluted 1: 1 with petroleum ether, boiling point 40-60 ° C. the doctor blade height was 50 µm.

Results:
Q at 23 ° C: No gas penetration was found within the measuring accuracy of the method used.
Q at 80 ° C: 0.003 m ² /s.Pa.10 ^-17

Example 6:

The procedure was as in Example 1, but the Dehesiv 920 solution diluted 1:10 with petroleum ether. The Doctor blade height was 50 µm.

Results:
Q at 23 ° C: No gas penetration was found within the measuring accuracy of the method used.
Q at 80 ° C: 0.004 m ² / s - Pa.10 ^-17

Example 7:

The procedure was as in Example 1, but one was Hostaphan film of 4 µm. Thickness used and the solution Dehesiv 920 diluted 1:10. The doctor blade height was 50 µm.

Results:
Q at 23 ° C: 0.0004 m ² /s.Pa.10 ^-17

Comparative Example 3:

An uncoated Hostaphan film of 4 µm was tested.

Results:
Q at 23 ° C: 1.2 m ² / s • Pa • 10 ^-17

Claims (2)

1. Elastic high-gastight plastic membrane for pressure accumulator, consisting of a carrier film made of polyethylene terephthalate a layer thickness of 4-25 µm and with a glass wall temperature of at least -40 ° C on one side or a layer of a cross-linked polysiloxane on both sides with a doctor blade height less than 100 microns. 2. plastic membrane according to claim 1, characterized in that the polysiloxane is warm cross-linked dimethylpolysiloxane.