RU2801653C2 - Single-layer membrane for expansion tank - Google Patents

Abstract

FELID: membranes.

SUBSTANCE: invention relates to a single-layer membrane for an expansion tank. The single layer membrane comprises a thermoplastic olefin block copolymer. The properties of the membrane are at least comparable to those of conventional single-layer membranes for expansion tanks made from vulcanized rubber.

EFFECT: single layer membrane is recyclable and easy to manufacture.

15 cl, 5 dwg, 1 tbl

Description

The field of technology to which the invention belongs

The present invention relates to a single-layer membrane for an expansion tank, to the use of a thermoplastic olefin block copolymer to obtain a single-layer membrane for an expansion tank, to a method for producing a single-layer membrane, and to an expansion tank containing a single-layer membrane.

State of the art

An expansion tank or expansion vessel protects closed (not open to atmospheric pressure) fluid systems from pressurizing the system. The tank absorbs excess fluid pressure caused by thermal expansion. Expansion tanks are often used in home central heating systems, but they have been known to serve other purposes as well, such as vehicle suspension systems.

The operating principle of the expansion tank is based on the principle of gas compressibility, as opposed to liquid. The expansion tank consists of two compartments separated by a flexible membrane. One side of the expansion tank is connected to the heating system piping and therefore contains a liquid, which is often water. On the other side is pressurized air and usually a valve, such as an automotive-type nipple for checking pressure and adding air. When the heating system is not filled or is at the lower limit of normal operating pressure, the membrane is pressed against the fluid inlet. As the volume of the liquid, and therefore the pressure, increases, for example due to an increase in temperature, the membrane moves and thereby compresses the air on its other side. When using expansion tanks in domestic drinking water systems, the membrane and the tank must comply with the requirements for drinking water quality.

Obviously expansion tank diaphragms must withstand a large number of pressure cycles in which the diaphragm expands and retracts as a result of pressure changes in the system. After a certain number of cycles, fatigue of the membrane material can cause damage to the membrane, which increases the water and gas permeability of the membrane.

The number of cycles that a membrane must withstand without significant loss of barrier properties is specified in NEN 13831:2007. In this test, the diaphragm expansion vessel is subjected to continuous cyclic loading, possibly at elevated temperatures. Pressurized water is pumped into the tank until it is filled to 50% of the contents of the chamber. The membrane expands and then the pressure is released again. After the cycle test, when the tank has cooled down (if the test was carried out at an elevated temperature, eg 75°C), the gas side of the vessel is filled with air up to 1.5 bar. The pressure drop over the next hour must not exceed 0.15 bar.

Expansion tank membranes often consist of a vulcanized rubber such as styrene butadiene (SBR) rubber or bromobutyl rubber (BiiR). In addition, membranes for expansion tanks containing EPDM rubber are also known (for example, presented in documents US 9366373 B2, EP 0193077 A1, US 2013/180993 A1 and WO 2016/203275 A2), as well as other membranes containing EPDM (for example, presented in documents US 6016841 A, US 2007/167569 A1 and US 2011/223439 A1). However, due to the need for vulcanization, the manufacture of such membranes can be cumbersome, and due to the presence of vulcanizing agents and vulcanization accelerators, the resulting membranes are not always suitable for use with potable water. Other materials are preferred for use with potable water. Moreover, vulcanized rubber tends to leak small amounts of oxygen and water after a while. This leads to the formation of air bubbles in the system and to water in the air compartment of the expansion tank. This causes the expansion tank to gradually degrade to the point where too little air is contained in the air compartment and the pressure falls below the critical value of 0.2 bar. In this case, the expansion tank no longer performs its function. Expansion tank diaphragms based on vulcanized rubber have a maximum service life of about 15 years under standard conditions. Due to the cross-linking of rubber molecules, conventional expansion tank membranes cannot be easily recycled. For example, they cannot be melted down and recycled.

Membranes with improved gas barrier properties can be made from thermoplastic materials such as thermoplastic polyurethane (TPU) as disclosed in WO 2013/170323. TPU is relatively cheap, easy to process, flexible, and less susceptible to bacteria compared to alternative thermoplastic materials. However, TPU membranes are still relatively water permeable.

Also known membranes for expansion tanks containing several layers. By using multiple layers, the membrane can be given properties that one layer lacks, such as water tightness, by adding an additional barrier layer on top of another layer so that the entire membrane has the desired water and gas tightness and durability. However, the manufacture of membranes for expansion tanks containing several layers is difficult. For example, it may be necessary to manufacture two or more separate membranes, which are then superimposed on each other. The present invention aims to overcome the aforementioned disadvantages of expansion tank membranes, or at least to provide a useful alternative. Thus, it is an object of the present invention to provide a membrane for an expansion tank with proper waterproof properties. Another object of the present invention is to provide a membrane for an expansion tank with proper gas barrier properties. Another object of the present invention is to provide a membrane for an expansion tank that can be easily manufactured. Another object of the present invention is to provide a membrane for an expansion tank with appropriate mechanical properties. Another object of the present invention is to provide a membrane for an expansion tank that can be recycled. In addition, an expansion tank is also provided.

The essence of the invention

To solve at least one of these objects in the first aspect, the present invention provides a single-layer membrane for an expansion tank containing at least a thermoplastic olefinic block copolymer TOBC (which is the same as an olefinic thermoplastic elastomer copolymer (OTPE)).

The expansion tank membrane can separate two expansion tank compartments. This means that such a membrane requires a peripheral edge which can be clamped between the first housing part and the second housing part of the expansion tank. The membrane may have, for example, a beaded peripheral edge.

Expansion tanks may be cylindrical (FIG. 1) or rectangular (FIG. 2), depending on the application.

Since the membrane consists of only one layer, it can be easily produced in one step, for example using injection molding or extrusion blow molding. Since it is not necessary to stack different membranes on top of each other, for example for multilayer membranes, the manufacturing process is relatively quick and simple. In addition, since the membrane contains a thermoplastic elastomer, more specifically an olefin thermoplastic elastomer copolymer, the expansion tank membrane can be obtained with desirable properties (such as flexibility, low gas permeability, low water permeability) without any chemical crosslinking.

Thus, instead of chemical crosslinking, thermoplastic elastomer copolymers, as well as olefinic thermoplastic elastomer copolymers, are crosslinked through physical interactions. The polymer chains of thermoplastic elastomeric copolymers contain blocks of at least two different incompatible repeating units, so-called soft segments and hard segments. At least two rigid segments are present to form a crosslinked mesh. The hard segments will phase separately from the soft segments, forming physical crosslinks (as opposed to chemical crosslinks in the case of, for example, vulcanized rubber) in the soft segment polymer matrix, provided that the soft segment polymer is the dominant segment.

General classes of thermoplastic elastomeric copolymers are: styrenic block copolymers, thermoplastic polyurethanes, thermoplastic copolyesters, thermoplastic polyamides, and olefinic thermoplastic elastomeric copolymers (OTPEs), otherwise known as thermoplastic olefinic block copolymers (TOBCs).

OTPE (TOBC), which are the subject of the present invention, are a class of olefin polymers having crystallizable blocks, such as polyethylene blocks with low comonomer content, if any, and high melting point, alternating with amorphous blocks, for example, based on propylene, mixtures of ethylene and olefin with a high content of comonomer, etc. and low glass transition temperature.

Thus, olefinic thermoplastic elastomeric copolymers may differ from other classes of thermoplastic elastomeric copolymers in that both the soft segments and the hard segments are formed by olefinic repeat units. The olefin thermoplastic elastomer consists essentially of olefin repeat units, such as propylene and ethylene repeat units, and forms a crosslinked network of olefin hard segments in a matrix of olefin soft segments. In this context, the term "essentially" means that the content of olefin repeating units in the copolymer is more than 90 wt.%, preferably more than 95 wt.%, even more preferably more than 99 wt.%, and the rest is obtained from copolymerizable non-olefin monomers, initiators , chain transfer agents and/or other functional groups. Preferably, the amount of non-olefin repeating units in the olefin thermoplastic elastomer is no more than 10 wt.%, more preferably no more than 5 wt.%, most preferably no more than 1 wt.%. It is particularly preferred that the olefin thermoplastic elastomer be free of any non-olefin repeating units. Olefinic thermoplastic elastomer copolymers belong to the general class of polyolefins and may differ from the general class of polyolefins in their ability to phase separate into a structure containing physical crosslinks of hard segments in a matrix of soft segments.

Thermoplastic elastomer copolymers should be considered separately from other classes of thermoplastic elastomers such as thermoplastic vulcanizates (eg elastron, forpren, santoprene, trefsin, etc.) and thermoplastic polyolefin elastomers. Such thermoplastic elastomers are actually mixtures of several components themselves. For example, thermoplastic polyolefin elastomers are blends of a thermoplastic polymer such as polypropylene or polyethylene and pre-crosslinked rubber particles.

Unlike conventional vulcanized and/or otherwise crosslinked membranes, which require chemical crosslinking to ensure their mechanical properties, the membranes of the invention can be melted down and recycled. In addition, since crosslinking chemicals are not required, fewer toxic chemicals are required in the manufacturing process.

Single-layer expansion tank membranes made from a thermoplastic elastomer copolymer are known, the elastomer copolymer being thermoplastic polyurethane (TPU) as proposed in WO 2013/151441. However, while such membranes may have the necessary flexibility and strength, they are relatively permeable to water.

Polyolefins are known for their waterproof properties. Therefore, in multilayer membranes for expansion tanks, as a rule, a polyolefin layer is used to provide waterproof properties.

Surprisingly, there is no need to use multiple layers for an expansion tank membrane when taking advantage of the waterproof properties of polyolefins. By using a polyolefin that is an olefin thermoplastic elastomer copolymer, a single layer expansion tank membrane with reduced water vapor permeability can be obtained compared to a single layer expansion tank membrane containing only non-olefin or partially olefin thermoplastic elastomer copolymers.

In a second aspect, the invention provides a method for producing a single-layer membrane for an expansion tank, comprising:

heating an olefinic thermoplastic elastomeric copolymer and optionally mixing with a neo-olefinic or partially olefinic thermoplastic elastomeric copolymer and/or an additional polyolefin,

injection molding of a heated copolymer or mixture of a) in a membrane mold to form a single layer membrane,

optional cooling of the membrane in the membrane mold,

removal of the membrane from the mold.

Heating and mixing can be done in an extruder. Preferably, in step a) the polyolefin and the thermoplastic elastomeric copolymer are heated to a temperature in the range of 200-220°C. The injection molding in step b) preferably takes place at a pressure of 130 to 150 bar and an injection time of 1 second to 4 seconds.

In a third aspect, the invention provides for the use of an olefinic thermoplastic elastomer copolymer to form a single layer membrane for an expansion tank.

Finally, an expansion tank is provided containing a single layer membrane according to the invention.

Description of Embodiments

Preferably the membrane is a diaphragm membrane. Such a membrane has a substantially spherical periphery which can be sandwiched between the two halves of the expansion vessel. The diaphragm membrane may be at rest essentially hat-shaped, containing an outer region that is essentially flat, and an inner region that is at least partially curved and defines the volume. The thickness of such a membrane is preferably at least 0.8 mm. It should be noted that the diaphragm membrane is different from the membrane in the form of a bubble or balloon.

Preferably, the olefinic thermoplastic elastomeric copolymer contains propylene repeating units with a propylene content of at least 80% by weight. Preferably, the polyolefin is an ethylene-propylene copolymer due to the superior waterproof properties of polypropylene compared to other polyolefins. More preferably, the copolymer has a melt flow index of 1 to 20 g/10 min according to ASTM D1238 (200°C/2.16 kg).

Preferably, the olefinic thermoplastic elastomeric copolymer contains repeating isotactic propylene units with a random distribution of ethylene.

Preferably, the olefinic thermoplastic elastomeric copolymer has an ethylene content of 4 to 20% by weight, preferably 13 to 18% by weight, more preferably 15 to 17% by weight. Copolymers with these compositions have an optimal balance between low gas and water permeability and mechanical properties.

Preferably, the membrane contains an additional polyolefin. The additional polyolefin does not itself form a phase-separated structure of hard segments and soft segments, but may be included in the soft segment matrix of the olefin thermoplastic elastomeric copolymer. The additional polyolefin may be any polymer or copolymer consisting essentially of olefin repeating units, ie C2-Cx olefin repeating units, preferably C2-C4 olefin units, more preferably propylene and/or ethylene repeating units. In this context, the term "essentially" means that the content of olefinic repeating units in the (co)polymer is above 90%, preferably above 95%, even more preferably above 99%, and the remainder is derived from copolymerizable non-olefin monomers. For example, the polymer may contain a small amount of non-olefin moieties. For example, the polyolefin may be grafted with other monomer units such as maleic anhydride. Preferably, the amount of non-olefin repeating units in the additional polyolefin is not more than 10 wt.%, more preferably not more than 5 wt.%, more preferably not more than 1 wt.%. Preferably the polyolefin is free of any non-olefin repeating units. Most preferably, the additional polyolefin is polypropylene, since polypropylene has excellent waterproof properties.

Preferably, the membrane further comprises a non-olefinic or partially olefinic thermoplastic elastomeric copolymer. The addition of such a non-olefin or partially olefin thermoplastic elastomer enhances the processability of the polymer blend. Examples of non-olefinic or partially olefinic copolymers of thermoplastic elastomers are styrenic block copolymers, thermoplastic polyurethanes, thermoplastic copolyesters and thermoplastic polyamides.

Styrene block copolymers are block copolymers of styrene and a diene such as polyisoprene and/or polybutadiene. The diene may be hydrogenated. Block copolymers are multiblock copolymers such as triblock copolymers. Examples of styrene block copolymers include poly(styrene-co-ethylene/propylene-styrene) (SEPS), poly(styrene-isoprene-styrene) (SIS), poly(styrene-co-ethylene/butylene-styrene) (SEBS ) and poly(styrene-butylene/styrene) (SBS). The polymers may be linear or branched. Moreover, they can be mixtures of linear and branched block copolymers and/or mixtures containing diblock copolymers having one soft segment and one hard segment. Such styrene block copolymers are available from, for example, Kraton, Kuraray, TSRC and LCY.

Thermoplastic polyurethanes (TPUs) are block copolymers composed of alternating sequences of hard and soft segments or domains formed by the reaction of (1) diisocyanates with short chain diols (chain extenders) and (2) diisocyanates with long chain diols. By changing the ratio, structure and/or molecular weight of the reaction compounds, a variety of TPUs with different structures can be obtained. This allows the structure to be fine-tuned according to the desired final material properties. For example, a higher ratio of hard to soft segments will result in a more rigid thermoplastic polyurethane (TPU). Desmopan or elastollan are examples of commercially available TPUs. Preferably the TPU contains a polytetramethylene ether glycol (PTMEG) polyol.

Thermoplastic copolyesters are multiblock copolymers of chemically different segments of polyesters and polyethers linked by ester bonds. In a typical example, the polyester-ether copolymer consists of poly(butylene terephthalate) (PBT) as a rigid and short segment connected by ester groups to flexible and long poly(tetramethylene oxide) segments.

Finally, thermoplastic polyamides are block copolymers with hard and soft segments, while the block copolymer contains amide linkages. For example, they are based on nylon and polyethers or polyesters.

All of these styrenic block copolymers, thermoplastic polyurethanes, thermoplastic copolyesters and thermoplastic polyamides are non-olefinic or partially olefinic, with the term "non-olefinic" meaning that these polymers are free of any olefinic repeating units. The term "partially olefinic" means that the polymers, although they may contain olefinic repeating units, may also contain non-olefinic repeating units. Thus, the non-olefin or partially olefin thermoplastic elastomeric copolymer contains a significant amount of non-olefin repeating units. In this context, the term "significant" means more than 10 wt.% non-olefin repeating units. All of the above classes of thermoplastic elastomeric copolymers contain non-olefin repeating units.

Preferably, the non-olefinic or partially olefinic thermoplastic elastomeric copolymer is a styrenic block copolymer. More preferably, the styrene block copolymer is a styrene-isoprene block copolymer and most preferably a poly(styrene-isoprene-styrene) triblock copolymer or a mixture of said triblock copolymer with a diblock copolymer. Ideally, the block copolymer has a melt flow index (ASTM D1238, 200° C./5 kg) of 8 to 19 g/10 min.

Preferably, the poly(styrene-isoprene-styrene) triblock copolymer has a polystyrene content of 10 to 21 wt.%, preferably 12 to 19 wt.%, more preferably 14 to 17 wt.%. Without wishing to be limited by theory, it is believed that due to the small number of polystyrene blocks compared to the average isoprene block, the triblock copolymer is very flexible, resulting in good mechanical properties of membranes for expansion tanks.

The composition used for the membrane may contain additional components, ie additives, up to 50% by weight. Such additives may include fillers, colorants and other polymers, and the like.

Preferably, the expansion tank single layer membrane is free of ethylene vinyl alcohol (EVOH) or EVOH copolymers. EVOH has excellent gas barrier properties. However, it has poor mechanical properties and is not resistant to water and/or steam. Therefore, EVOH is often used as the middle layer in multilayer membranes for expansion tanks, the outer layers of which provide a water barrier. Moreover, it is a relatively expensive polymer. EVOH or EVOH copolymers can be blended into the polymer blend that is used to make a single layer membrane for an expansion tank. Thus, the addition of EVOH or EVOH copolymers may be advantageous for expansion tank membrane properties. However, it is a relatively expensive polymer. Therefore, the use of EVOH or EVOH copolymers is not preferred.

Preferably, the single-layer diaphragm for the expansion vessel is oil-free. Oils are often added to polymer blends to improve processability, for example as process oils. However, such oils may leak from the membrane. This is undesirable, especially in the case of drinking water applications.

Preferably, the membrane contains at least 10%, more preferably at least 20% olefinic thermoplastic elastomeric copolymer.

Preferably, the membrane consists of 10-60 wt.% non-olefinic or partially olefinic thermoplastic elastomeric copolymer, 10-90 wt.% olefinic thermoplastic elastomeric copolymer, 0-80 wt.% additional polyolefin and 0-20 wt.%, when added in total amount up to 100 wt.%.

Preferably the membrane is composed of one or more olefinic thermoplastic elastomeric copolymers. Since such a membrane consists of only one type of olefinic material, recycling is highly advantageous.

Alternatively, the membrane consists of a mixture of one or more olefinic thermoplastic elastomeric copolymers and an additional polyolefin.

As used herein, singular nouns mean "one or more than one". As used herein, the terms "including" and/or "having" mean "comprising" (ie, an open language that does not exclude other elements or steps). The mere fact that certain measures are set forth in mutually distinct dependent claims does not mean that a combination of these measures cannot be used to advantage.

Brief description of the drawings

1 is a schematic side view of a cylindrical expansion tank.

Fig. 2a is a three-dimensional view of a rectangular expansion tank.

Fig.2b is a three-dimensional view of two parts of the housing of a rectangular expansion tank.

Fig. 2c is a three-dimensional cross-sectional view of a rectangular expansion vessel with a membrane.

Fig. 2d is a schematic cross-section of a rectangular expansion vessel with a membrane.

Detailed description of the invention

Figure 1 shows a schematic side view of a cylindrical expansion tank with a first housing part (1') and a second housing part (2').

On figa shows a three-dimensional view of a rectangular expansion tank. On fig.2b shows the first part (1) of the body and the second part (2) of the body.

Figure 2c shows a three-dimensional cross-sectional view of a rectangular expansion vessel with a membrane. The first part of the body (1), the second part of the body (2) and the membrane (3) are shown in the drawing.

Figure 2d shows a schematic cross-section of a rectangular expansion vessel with a membrane. The first part of the body (1), the second part of the body (2), the membrane (3) and the peripheral edge (4) of the membrane are indicated. The edge (4) is made with the possibility of clamping between the first part (1) of the body and the second part (2) of the expansion tank body.

Examples

Expansion vessel membranes were made by heating the membrane materials in an extruder to a temperature of 210°C, injection molding the material or mixture of materials into a membrane mold at 140 bar for about 1-4 seconds, cooling the mixture into a membrane mold, and extracting membranes from the mold.

Membranes 1 to 2.5 mm thick and about 30 cm in diameter were pressure cycle tested in accordance with NEN 13831:2007. The strength was determined by repeating the pressure cycle test for 1000 cycles, after which the gas side of the vessel was filled with air to 1.5 bar. If the pressure drop over the next hour did not exceed 0.15 bar, the test was continued for another 500 cycles.

The N ₂ and H ₂ O penetration rates were determined separately from the cycling tests. The method used was a combined permeability test by measuring the diffusion of N ₂ and H ₂ O over time in one test setup. In the test setup, the membrane was placed between a vacuum chamber and a chamber filled with pressurized water and nitrogen. Diffusion rate was measured by measuring weight and pressure as a function of time. The relationship between pressure drop and weight versus time was the calculated membrane permeability to water and nitrogen.

Table 1.

Tensile strength (N/mm ² ) Elongation at break (%) Tensile strength (N/mm) Durability (cycles) Permeability H ₂ O (g/m ² /h*mm) Permeability N ₂ (g/m2/h*mm) Compound 30°C 70°C 30°C 70°C 1 100wt% BiiR* 8.5 1050 18 >60000 0.05 0.47 0.006 0.31 2 100wt% SBR* >50000 0.23 1.31 0.022 0.083 3 100wt% TPU* (a) 44 1000 115 >50000 0.31 5.83 0.006 0.31 4 100wt% TPU* (b) 56.3 1000 98 >30000 0.31 2.63 0.004 0.021 5 80wt% OTPE1 20wt% OTPE2 0.12 0.83 0.018 0.086 6 64wt% OTPE1
18wt% OTPE2
18wt% PP 17.2 1445 63 >10000 0.17 0.75 0.014 0.074 7 38wt% SIS
38wt% PP
23wt% OTPE1
(a') 11.10 1075.00 40.00 >8000
<10000 0.09 0.63 0.02 0.083 8 38wt% SIS
38wt% PP
23wt% OTPE1
(b') 14.9 1150 43 >15000 <43000 0.12 0.49 0.005 0.03 9 25wt% SIS
25wt% PP
50wt% OTPE1 11.9 1444 40 >10000 0.11 0.88 0.02 0.078 7 100wt% Elastron* 4 850 23 >1000 <0.1 <1.0 >0.025 >0.085 8 100wt% Forpren* 4.8 550 23 >15000 <0.1 <1.0 >0.025 >0.085

* comparative examples

a and b, a' and b' - repeated measurements of similar compositions

materials

All materials are standard commercially available products.

BiiR: vulcanized bromobutyl rubber

SBR: SBR vulcanized rubber

TPU: Polyether urethane based on MDI (methylene diphenyl diisocyanate) + PTMEG (polytetramethylene ether glycol)

SIS: Poly(styrene-isoprene-styrene) triblock copolymer with MFI from 8.5 to 18.5 g/10 min as measured by ASTM D1238 (200°C/5 kg) and polystyrene content from 14.0 to 17, 0 wt%.

PP: polypropylene with MFI from 10 to 100 g/10 min, as measured by ASTM D1238 (200°C/2.16 kg).

OTPE1: Olefinic thermoplastic elastomeric copolymer with MFI 1.4 g/10 min as measured by ASTM D1238 (190°C/2.16 kg) containing repeating units of isotactic propylene with a random distribution of ethylene and an ethylene content of 16 wt.%.

OTPE2: Olefinic thermoplastic elastomeric copolymer with MFI from 1 to 20 g/10 min as measured by ASTM D1238 (190°C/2.16 kg) containing repeating units of isotactic propylene with a random distribution of ethylene and an ethylene content of 6 wt.%.

Conclusion

In experiment 5, a membrane composed of a blend of OTPE has an improved (ie, lower) water penetration rate compared to a membrane made only of thermoplastic polyurethane. Moreover, the water permeation rate is comparable to a standard SBR membrane. Experiment 6 shows that a similar blend additionally containing polypropylene has improved water penetration rate at high temperatures and improved nitrogen permeability (at both temperatures) compared to Experiment 5. Compositions that additionally contain SIS (Experiments 7, 8, 9) , were easier to process and also had excellent barrier properties.

Claims (25)

1. Expansion tank containing a single-layer membrane, and the membrane contains polypropylene and at least one thermoplastic olefin block copolymer, which is a copolymer of ethylene and propylene, with a propylene content of at least 80 wt.% and with a melt flow index of 1 to 20 g /10 min according to the ASTM D1238 method (200°C/2.16 kg), while the thermoplastic olefin block copolymer contains repeating isotactic units of propylene with a random distribution of ethylene, and the thermoplastic olefin block copolymer has an ethylene content of from 4 to 20 wt.%. 2. A tank as claimed in claim 1, wherein the membrane has a peripheral edge that is configured to be clamped between the first body part and the second body part of the expansion tank. 3. Tank according to claim 1, wherein the thermoplastic olefin block copolymer has an ethylene content of 13 to 18% by weight, preferably 15 to 17% by weight. 4. Tank according to any one of the preceding claims, wherein the membrane further comprises a non-olefinic or partially olefinic thermoplastic elastomeric copolymer. 5. Tank according to claim 4, wherein the non-olefinic or partially olefinic thermoplastic elastomeric copolymer is a styrenic block copolymer. 6. Tank according to claim 5, in which the styrene block copolymer is a block copolymer of styrene and isoprene, preferably a poly(styrene-isoprene-styrene) triblock copolymer or a mixture thereof with a diblock copolymer, more preferably with an MFI of 8 to 19 g/10 min as measured by ASTM D1238 (200°C/5 kg). 7. Tank according to claim 6, wherein the poly(styrene-isoprene-styrene) triblock copolymer has a polystyrene content of 10 to 21 wt.%, preferably 12 to 19 wt.%, more preferably 14 to 17 wt.% . 8. Tank according to any one of the preceding claims, wherein the membrane is free of EVOH or EVOH copolymers. 9. Tank according to any one of the preceding claims, in which the membrane is free of oil. 10. Tank according to any one of the preceding claims, wherein the membrane contains at least 10%, more preferably at least 20%, of a thermoplastic olefin block copolymer. 11. A tank according to any one of the preceding claims, wherein the membrane consists of: a) 10-60% by weight of a non-olefinic or partially olefinic thermoplastic elastomeric copolymer, b1) 10-90% by weight of a thermoplastic olefin block copolymer, b2) 0-80 wt% polypropylene, c) 0-20 wt.% additives when added in a total amount of up to 100 wt.%. 12. Tank according to any one of claims 1 to 3, wherein the membrane consists of one or more thermoplastic olefin block copolymers. 13. Tank according to any one of claims 1 to 3, wherein the membrane consists of a blend of one or more thermoplastic olefin block copolymers and an additional polyolefin. 14. A method for producing a single-layer membrane for an expansion tank, the method comprising: a) heating the thermoplastic olefin block copolymer and optionally blending with the neo-olefin or partially olefin thermoplastic elastomer copolymer and/or additional polyolefin, b) injection molding the heated copolymer or mixture from step a) in a membrane mold to form a single layer membrane, c) optionally cooling the membrane in a membrane mold, d) removing the membrane from the mold, while the thermoplastic olefin block copolymer is an ethylene-propylene copolymer with a propylene content of at least 80 wt.% and a melt flow index of 1 to 20 g/10 min according to ASTM D1238 method (200°C/2.16 kg), while the thermoplastic olefin block copolymer contains repeating isotactic units of propylene with a random distribution of ethylene, and the thermoplastic olefin block copolymer has an ethylene content of from 4 to 20 wt.%. 15. The use of a thermoplastic olefin block copolymer to obtain a single-layer membrane for an expansion tank, while the thermoplastic olefin block copolymer is an ethylene-propylene copolymer with a propylene content of at least 80 wt.% and a melt flow index of 1 to 20 g/10 min in accordance with the ASTM D1238 method (200°C/2.16 kg), while the thermoplastic olefin block copolymer contains repeating isotactic units of propylene with a random distribution of ethylene, and the thermoplastic olefin block copolymer has an ethylene content of from 4 to 20 wt. %.

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