DE4231927A1 - Elastic, gas-tight membrane for use in pressure accumulators - comprises polyester base film, esp. PET, coated on one or both sides with layer of crosslinked polysiloxane - Google Patents

Abstract

Elastic, highly gas-tight plastic membranes (I) are claimed, consisting of a base film (A) made of plastic with Tg at least -40 deg. C (II) coated on one or both sides with a layer (B) of plastic material (III). Also claimed is prepn. of (I), by coating film (A) on one or both sides with a polysiloxane-forming soln. contg. silane and siloxane monomer, crosslinker, catalyst and opt. other components such as diluents, and then heating, pref. at up to 120 deg. C., to evaporate the volatiles and crosslink the monomer(s). Pref., (II) is polyester, pref. PET, and (III) is a crosslinked polysiloxane. Pref., film (A) is 4-25 microns thick and is coated on one or both sides with a layer of thermally-crosslinked dimethylpolysiloxane (IIIA). USE/ADVANTAGE - Membranes (I), esp. coated Teflon (RTM) and/or polyethylene terephthalate (PET) films, are useful in pressure accumulators, esp. bubble and membrane accumulators (claimed). Applications include shock absorbers, energy sources for pumpless emergency circulation systems, pressure springs etc. The invention provides elastic, gas-tight membranes with high permeation resistance (low permeation coefft.), good stability, and comparative ease of prodn.

Description

The invention relates to an elastic, high gas-tight plastic membrane, a process for its manufacture and its Use, especially in pressure storage systems or hydro to save.

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 ", prospectus 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 elastomer membranes from a copolyiner 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 53 536.

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.

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 or composite film 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 53 536 and  53 380. 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 .

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

Carrier films suitable for the manufacture of the plastic membranes of the invention:
Although polyester films are particularly suitable for producing the plastic membranes according to the invention, other elastic films, eg. B. from polyether ether ketones, polyvinylidene fluoride and polypropylene can be used. It has proven to be expedient to use foils with a layer thickness of 4 to 25 μm, since thicker foils are generally not elastic enough.

The use of poly has proven particularly advantageous proven ethylene terephthalate films, the z. B. under the goods Hostaphan are commercially available.

Heat-crosslinkable (addition-crosslinking) polysiloxanes suitable for the production of the plastic membranes of the invention:
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 appropriate, 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 carbons.
  Manufacturer company Wacker AG Munich.

Manufacture of the plastic membranes of the invention:
According to the solution of monomers, crosslinking agent, catalyst and optionally diluent, for. B. with a doctor blade or by spraying or pouring or brushing, preferably with roller systems, as described in J. Nentwig, Lexikon für Folientechnik, Verlag Weinheim 1991, p. 33, Fig. 1ff.

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.

Membranes with polysiloxane have proven to be particularly advantageous layers (measured dry) of 10 to 200 µm proven.

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

Measurement of the permeation coefficient:
A device as shown in the drawing was used to determine the permeation coefficient Q.

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 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.

With the help of the measured pair of values is the following Equation to determine the permeation coefficient.

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 is 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 .

The test results show that the siloxane modes Composite foils significantly smaller permeation coefficient cations against nitrogen have as pure polyethylene terephthalate films.

Claims (9)

1. Elastic high-gastight plastic membrane, consisting of a carrier film made of plastic with a glass conversion temperature Tg of at least -40 ° C on one side or a layer of plastic material on both sides material. 2. Plastic membrane according to claim 1, characterized in that the carrier film is a polyester film and that Plastic material made from a cross-linked polysiloxane stands. 3. Plastic membrane according to claim 2, characterized in that the carrier film is a polyethylene terephthalate film. 4. Plastic membrane according to one of claims 1 to 3, characterized characterized in that the carrier film has a layer thickness of Has 4 to 25 microns. 5. Plastic membrane according to one of claims 1 to 4, characterized characterized in that the carrier film on one or both Sides a layer of thermally cross-linked dimethyl poly has siloxane. 6. Process for the production of a plastic membrane one of claims 1 to 5, characterized in that one on a carrier film made of plastic with a glass conversion temperature Tg of at least -40 ° C on one side or one on both sides to form a polysiloxane layer suitable solution of silane and siloxane monomer, crosslinked agent, catalyst and optionally further compo nents, such as diluents, and the applied brought solution with evaporation of volatile components and Crosslinking of the monomer or monomers heated.   7. The method according to claim 6, characterized in that one the applied vulcanization solutions on temperatu heated up to 120 ° C. 8. Use of plastic membranes according to claim 1 to 5 in Pressure accumulators, especially bladder and membrane accumulators. 9. Use of coated Teflon films and / or poly ethylene terephthalate films in pressure accumulators, in particular Bubble and membrane storage.