EP3781873A1 - Single layer expansion tank membrane - Google Patents

Single layer expansion tank membrane

Info

Publication number
EP3781873A1
EP3781873A1 EP19726191.0A EP19726191A EP3781873A1 EP 3781873 A1 EP3781873 A1 EP 3781873A1 EP 19726191 A EP19726191 A EP 19726191A EP 3781873 A1 EP3781873 A1 EP 3781873A1
Authority
EP
European Patent Office
Prior art keywords
membrane
expansion tank
olefinic
copolymer
styrene
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP19726191.0A
Other languages
German (de)
French (fr)
Inventor
Jan Hendrik Timmerman
Herman Reezigt
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Flamco BV
Original Assignee
Flamco BV
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Flamco BV filed Critical Flamco BV
Publication of EP3781873A1 publication Critical patent/EP3781873A1/en
Pending legal-status Critical Current

Links

Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24—HEATING; RANGES; VENTILATING
    • F24D—DOMESTIC- OR SPACE-HEATING SYSTEMS, e.g. CENTRAL HEATING SYSTEMS; DOMESTIC HOT-WATER SUPPLY SYSTEMS; ELEMENTS OR COMPONENTS THEREFOR
    • F24D3/00—Hot-water central heating systems
    • F24D3/10—Feed-line arrangements, e.g. providing for heat-accumulator tanks, expansion tanks ; Hydraulic components of a central heating system
    • F24D3/1008—Feed-line arrangements, e.g. providing for heat-accumulator tanks, expansion tanks ; Hydraulic components of a central heating system expansion tanks
    • F24D3/1016—Tanks having a bladder
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16J—PISTONS; CYLINDERS; SEALINGS
    • F16J3/00—Diaphragms; Bellows; Bellows pistons
    • F16J3/02—Diaphragms
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
    • F15B—SYSTEMS ACTING BY MEANS OF FLUIDS IN GENERAL; FLUID-PRESSURE ACTUATORS, e.g. SERVOMOTORS; DETAILS OF FLUID-PRESSURE SYSTEMS, NOT OTHERWISE PROVIDED FOR
    • F15B2201/00—Accumulators
    • F15B2201/30—Accumulator separating means
    • F15B2201/315—Accumulator separating means having flexible separating means
    • F15B2201/3151—Accumulator separating means having flexible separating means the flexible separating means being diaphragms or membranes
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
    • F15B—SYSTEMS ACTING BY MEANS OF FLUIDS IN GENERAL; FLUID-PRESSURE ACTUATORS, e.g. SERVOMOTORS; DETAILS OF FLUID-PRESSURE SYSTEMS, NOT OTHERWISE PROVIDED FOR
    • F15B2201/00—Accumulators
    • F15B2201/60—Assembling or methods for making accumulators
    • F15B2201/61—Assembling or methods for making separating means therefor
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16L—PIPES; JOINTS OR FITTINGS FOR PIPES; SUPPORTS FOR PIPES, CABLES OR PROTECTIVE TUBING; MEANS FOR THERMAL INSULATION IN GENERAL
    • F16L55/00—Devices or appurtenances for use in, or in connection with, pipes or pipe systems
    • F16L55/04—Devices damping pulsations or vibrations in fluids
    • F16L55/045—Devices damping pulsations or vibrations in fluids specially adapted to prevent or minimise the effects of water hammer
    • F16L55/05—Buffers therefor
    • F16L55/052—Pneumatic reservoirs
    • F16L55/053—Pneumatic reservoirs the gas in the reservoir being separated from the fluid in the pipe

Definitions

  • the current invention relates to a single layer membrane for an expansion tank, use of a styrenic block copolymer for manufacturing a single layer membrane for an expansion tank, to a method for manufacturing the single layer membrane, and to an expansion tank comprising the single layer membrane.
  • An expansion tank or expansion vessel protects closed (not open to atmospheric pressure) liquid systems from excessive pressure building up in the system.
  • the tank absorbs excess liquid pressure caused by thermal expansion.
  • Expansion tanks can often be found in domestic central heating systems, but they are also known to serve other purposes, for example in suspension systems for vehicles.
  • An expansion tank consists of two compartments separated by a flexible membrane.
  • One side of the expansion tank is connected to the piping of the heating system and therefore contains a liquid, which is often water.
  • the other side contains air under pressure, and normally a valve, such as e.g. a Schrader valve for checking pressures and adding air.
  • a valve such as e.g. a Schrader valve for checking pressures and adding air.
  • expansion tank membranes should be able to withstand a large number of pressure cycles, in which the membrane is expanded and retracted as a result of pressure variations within the system. After a certain amount of cycles, fatigue can cause damage to a membrane, which increases the water and gas permeability of a membrane.
  • the number of cycles a membrane should be able to withstand without a significant loss of barrier properties is determined by NEN 13831 :2007.
  • an expansion tank with a membrane is subjected to continuous cyclic stressing, optionally at an elevated temperature. Pressurised water is pumped into the tank until it is filled to 50% of the content of the chamber. The membrane expands and then the pressure is released again.
  • the cyclic test once the tank has cooled down (if the test was performed at elevated temperature, e.g. 75X), the gas side of the vessel is filled with air to 1 ,5 bar. The pressure drop within the following hour shall not exceed 0,15 bar.
  • Expansion tank membranes often consist of vulcanized rubber, such as styrene- butadiene (SBR) rubber or bromobutyl rubber (BiiR).
  • SBR styrene- butadiene
  • BiiR bromobutyl rubber
  • the manufacturing of such membranes may be cumbersome, and due to the presence of the vulcanizing agents and accelerators, the resulting membranes are not always suitable for use with potable water.
  • other materials are preferred.
  • vulcanized rubber tends to let through small amounts of oxygen and water over time. This leads to air bubbles in the system, and to water in the air compartment of the expansion tank.
  • Vulcanized rubber based expansion tank membranes have a maximum lifespan of about 15 years under standard conditions. Due to the cross-linking of the rubber, conventional expansion tank membranes cannot easily be recycled. For example, they cannot be molten and reshaped.
  • TPU thermoplastic polyurethane
  • US 2010/006532 which is in a different field, discloses a retort liner for a bottle comprising styrenic block copolymers and one or more polyolefin polymers.
  • US 2008/017653 discloses a thermoplastic diaphragm assembly for use in a pressure vessel.
  • the diaphragm is formed from a thermoplastic elastomeric material selected from the group consisting of 1. ethyl vinyl acetate (EVA), 2. rubber, 3. rubber blends, and 4. polypropylene-rubber blends.
  • EVA ethyl vinyl acetate
  • Expansion tank membranes comprising multiple layers are also known, e.g. from US 2010/209672. By using multiple layers, the properties that one layer is lacking, e.g. water impermeability, may be given to the membrane by adding an extra barrier layer on top of another layer, such that the total membrane has the desired water and gas impermeability, as well as durability.
  • the production of expansion tank membranes comprising multiple layers is complicated.
  • the present invention aims to overcome the abovementioned drawbacks in expansion tank membranes, or at least to provide a useful alternative. Therefore, it is an objective of the present invention to provide an expansion tank membrane with adequate water barrier properties. It is another objective of the current invention to provide an expansion tank membrane with adequate gas barrier properties. It is a further objective of the current invention to provide an expansion tank membrane which can easily be produced. It is a further objective of the current invention to provide an expansion tank membrane with adequate mechanical properties. It is another objective of the present invention to provide a membrane for an expansion tank which can be recycled. Also an expansion tank is provided.
  • the present invention provides a single layer membrane for an expansion tank, comprising a mixture of a polyolefin and a non-olefinic or partially olefinic thermoplastic elastomer copolymer.
  • a membrane for an expansion tank can separate two compartments of an expansion tank. This means that such a membrane requires a peripheral edge which is configured to be clenched between a first housing part and a second housing part of an expansion tank.
  • the membrane may for example have a beaded circumferential edge.
  • Expansion tanks may be cylindrical (Fig. 1) or have a rectangular shape (Fig. 2), depending on the application. Because the membrane only consists of a single layer, it can easily be produced in a single step by, for example, injection molding or blow molding.
  • the membrane comprises a thermoplastic elastomer
  • an expansion tank membrane with the desired properties can be obtained without any chemical cross-linking.
  • thermoplastic elastomer copolymers are cross- linked by physical interactions.
  • the polymer chains of the thermoplastic elastomer copolymers comprise blocks of at least two different incompatible repeating units, so called soft segments and hard segments, of which at least two are present in order to form a cross- linked network.
  • the hard segments will phase separate from the soft segments, forming physical cross-links (as opposed to chemical cross-links in the case of e.g. vulcanized rubber) in a matrix of the soft segment polymer, provided the soft segment polymer is the dominant segment.
  • Generic classes of thermoplastic elastomer copolymers are: styrenic block copolymers, thermoplastic polyurethanes, thermoplastic copolyesters, and
  • thermoplastic polyamides These classes are all non-olefinic or partially olefinic, wherein non-olefinic means that these polymers do not comprise any olefinic repeating units.
  • Partially olefinic means that the polymers, although they may comprise olefinic repeating units, also comprise non-olefinic repeating units.
  • a non-olefinic or partially olefinic thermoplastic elastomer copolymer comprises non-olefinic repeating units. All the above classes of thermoplastic elastomer copolymers comprise non-olefinic repeating units.
  • Styrenic block copolymers are block copolymers of styrene and a diene, such as polyisoprene, and/or polybutadiene. The diene may be hydrogenated.
  • the block copolymers are multiblock copolymers, such as triblock copolymers.
  • styrenic 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 may be mixtures of linear and branched block copolymers, and/or mixtures comprising diblock copolymers having a single soft segment and a single hard segment.
  • Such styrenic block copolymers are manufactured by, e.g. Kraton, Kuraray, TSRC, and LCY.
  • TPUs Thermoplastic polyurethanes
  • the TPU comprises a polytetramethylene ether glycol (PTMEG) polyol.
  • PTMEG polytetramethylene ether glycol
  • Thermoplastic copolyesters are multiblock copolymers of chemically different polyester and polyether segments connected by ester linkages.
  • a typical example of a poly (ether ester) consists of poly(butylene terephthalate) (PBT) as the hard and short segment connected by ester groups with flexible and long poly(tetramethylene oxide) segments.
  • Thermoplastic polyamides finally, are block copolymers with hard and soft segments, the block copolymer comprising amide bonds. They are for example based on nylon and polyethers or polyesters.
  • thermoplastic elastomer copolymers are to be seen separately from other classes of thermoplastic elastomers, such as thermoplastic vulcanizates (e.g. Elastron, Forprene, Santoprene, Trefsin, etc.) and thermoplastic polyolefinelastomers.
  • thermoplastic vulcanizates e.g. Elastron, Forprene, Santoprene, Trefsin, etc.
  • thermoplastic polyolefinelastomers are in fact themselves mixtures of multiple components.
  • thermoplastic polyolefinelastomers are mixtures of a thermoplastic polymer such as polypropylene or polyethylene and pre-cross-linked rubber particles.
  • the membranes according to the invention can be molten and reshaped. Furthermore, as no cross-linking chemicals are required, less toxic chemicals are required in the production process.
  • Single layer expansion tank membranes from a thermoplastic elastomer copolymer, the elastomer copolymer being a thermoplastic polyurethane (TPU) are suggested in WO 2013/151441. However, although such membranes may have the required flexibility and durability, they are relatively permeable to water.
  • Polyolefins are known for their water barrier properties. That is why in multi-layer expansion tank membranes, there usually is a polyolefin layer providing the water barrier properties.
  • the polyolefin can be any polymer or copolymer consisting essentially of olefinic repeating units, i.e. C 2 - C x olefinic repeating units, preferably C 2 - C 4 olefinic repeating units, more preferably propylene and/or ethylene repeating units.
  • olefinic repeating units i.e. C 2 - C x olefinic repeating units, preferably C 2 - C 4 olefinic repeating units, more preferably propylene and/or ethylene repeating units.
  • the combined olefin repeating unit content of the (co)polymer is higher than 90 wt.%, preferably higher than 95 wt.%, even more preferably higher than 99 wt.%, with the remainder being derived from copolymerizable non-olefinic monomers.
  • the polymer may comprise a small amount of non-olefinic moieties.
  • the polyolefin may be grafted with other monomeric units, such as maleic anhydride.
  • the amount of non-olefinic repeating units in the polyolefin is not higher than 10 weight%, more preferably not higher than 5 weight%, most preferably not higher than 1 weight%.
  • the polyolefin does not comprise any non-olefinic repeating units.
  • the invention provides a method for manufacturing a single layer membrane for an expansion tank, comprising
  • Heating and mixing may be performed in an extruder.
  • the polyolefin and the thermoplastic elastomer copolymer are heated to a temperature of between 200 - 220 °C.
  • the injection moulding in step b) preferably takes place at a pressure of between 130 - 150 bar, and an injection time of between 1 seconds and 4 seconds.
  • the invention provides for the use of a styrenic block copolymer for manufacturing a single layer membrane for an expansion tank.
  • an expansion tank comprising the single layer membrane according to the invention.
  • the membrane is a diaphragm membrane.
  • a diaphragm membrane has a substantially spherical circumference which can be clenched between two halves of an expansion tank.
  • the diaphragm membrane may have a substantially hat-shaped form in rest, comprising an outer region which is substantially flat and an inner region which is at least partially curved and defines a volume.
  • the thickness of such a membrane is preferably at least 0.8 mm. It is noted that a diaphragm membrane differs from a bladder or balloon shaped membrane.
  • the polyolefin polymer or copolymer comprises propylene repeating units with a propylene content of at least 80 wt.%. More preferably, the polyolefin is a copolymer of ethylene and propylene. Alternatively, the polyolefin is a propylene homopolymer, because of polypropylene’s excellent water barrier properties as compared to other polyolefins. Even more preferably, the polyolefin is polypropylene with a Melt Flow Index of between 5 and 100 g/10 min, most preferably between 10 and 100 g/10 min according to ASTM D1238 (200 °C/2.16 kg).
  • the non-olefinic or partially olefinic thermoplastic elastomer copolymer is a styrenic block copolymer. More preferably, the styrenic block copolymer is a block copolymer of styrene and isoprene, 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 between 2 and 24 g/min.
  • ASTM D1238, 200 °C/5 kg Melt Flow Index
  • the block copolymer has a Melt Flow Index (ASTM D1238, 200 °C/5 kg) of between 8 and 19 g/min.
  • expansion tank membranes which comprise a mixture of a styrenic block copolymer, notably a poly(styrene- isoprene-styrene) triblock copolymer, and a polyolefin, notably polypropylene, have mechanical properties which are comparable to conventional vulcanized expansion tank membranes, while showing favorable gas and water permeation rates over extended periods of time.
  • the poly(styrene-isoprene-styrene) triblock copolymer has a polystyrene content of between 10 and 25 wt.%, preferably between 12 and 21 wt.%, most preferably of between 14 and 17 wt.%.
  • the triblock copolymer is very flexible, which results in adequate mechanical properties for expansion tank membranes.
  • composition used for the membrane may comprise further components, i.e. additives, up to 50 % by weight.
  • additives may include fillers, colorants, and further polymers and the like.
  • the single layer expansion tank membrane does not comprise ethylene vinyl alcohol (EVOH) or EVOH copolymers.
  • EVOH has excellent gas-barrier properties. However, its mechanical properties are poor and it is not resistant to water and/or water vapor. Therefore, EVOH is often used as a middle layer in multilayer expansion tank membranes, wherein the outer layers provide the water barrier. Moreover, it is a relatively expensive polymer. It may be possible to blend EVOH or EVOH copolymers into the polymer mixture that is used to produce a single layer expansion tank membrane. Addition of EVOH or EVOH copolymers in that way may be advantageous for the properties of an expansion tank membrane. However, it is a relatively expensive polymer. Therefore, use of EVOH or EVOH copolymers is not preferred.
  • the single layer expansion tank membrane does not comprise an oil.
  • Oils are often added to polymer blends in order to increase processability, e.g. as processing oils. However, such oils may leak out of the membrane. Especially in the case of potable water applications, this is undesirable.
  • the membrane comprises between 10 and 60 wt.%, preferably between 20 and 60 wt.%, more preferably between 25 and 50 wt.% non-olefinic or partially olefinic thermoplastic elastomer copolymer.
  • the membrane consists of 10 - 60 wt.% non-olefinic or partially olefinic thermoplastic elastomer copolymer, 40 - 90 wt.% polyolefin, and 0 - 20 wt. % additives, the total adding up to 100 wt.%.
  • Figure 1 is a schematical side view of a cylindrical expansion tank.
  • Figure 2a is a 3D view of a rectangular expansion tank.
  • Figure 2b is 3D view of the two housing parts of a rectangular expansion tank.
  • Figure 2c is a 3D view of a cross-section of a rectangular expansion tank with a membrane.
  • Figure 2d is a schematical cross-section of a rectangular expansion tank with a membrane.
  • Figure 1 is a schematical side view of a cylindrical expansion tank, with a first housing part (1’) and a second housing part (2’).
  • Figure 2a is a 3D view of a rectangular expansion tank.
  • the first housing part (1), and second housing part (2) are indicated.
  • Figure 2c is a 3D view of a cross-section of a rectangular expansion tank with a membrane.
  • the first housing part (1), second housing part (2), and the membrane (3) are indicated in the figure.
  • Figure 2d is a schematical cross-section of a rectangular expansion tank with a membrane.
  • the first housing part (1), second housing part (2), the membrane (3), and the peripheral edge (4) of the membrane are indicated.
  • the edge (4) is configured to be clenched between the first housing part (1) and the second housing part (2) of the expansion tank.
  • Expansion tank membranes were produced by heating the materials for the membrane in an extruder to a temperature of 210°C, injection moulding the material or mixture of materials into a membrane mould at a pressure of 140 bar for about 1 to 4 seconds, cooling the mixture in the membrane mould, and releasing the membrane from the mould.
  • Membranes with a thickness of between 1 and 2.5 mm and a diameter of about 30 cm were subjected to cyclic pressure testing according to NEN 13831 :2007. Durability was determined by repeating the cyclic pressure test for 1000 cycles, after which the gas side of the vessel was filled with air to 1 ,5 bar. If the pressure drop within the following hour did not exceed 0,15 bar, the test was continued for another 500 cycles.
  • N 2 and H O permeation rates were determined separately from the cyclic testing.
  • the used method was a combined permeation test by measuring over time the diffusion of N 2 and H O in one test unit.
  • a membrane is placed between a vacuum chamber and a chamber filled with water and nitrogen under pressure.
  • the rate of diffusion is measured by measuring the weight and pressure over time.
  • the relation between the pressure reduction and weight in time is the calculated permeation coefficient of the membrane for water and nitrogen. Table 1.
  • TPU Polyether urethane, MDI (Methylene diphenyl diisocyanate) + PTMEG
  • SIS Poly(styrene-isoprene-styrene) triblock copolymer with a MFI of between 8.5 and 18.5 g/10 min as measured by ASTM D1238 (200 °C/5kg), and a polystyrene content of between 14.0 and 17.0 mass%
  • PP polypropylene with a MFI of between 10 and 100 g/10 min as measured by ASTM D1238 (200 °C/2.16 kg)

Abstract

The invention relates to a single layer membrane for an expansion tank. The single layer membrane comprises a mixture of a polyolefin and a non-olefinic or partially olefinic thermoplastic elastomer copolymer. The properties of the membrane are at least comparable to those of conventional single layer expansion tank membranes made from vulcanized rubber. The present single layer membrane is recyclable and easy to produce.

Description

Title: Single layer expansion tank membrane
Technical Field
The current invention relates to a single layer membrane for an expansion tank, use of a styrenic block copolymer for manufacturing a single layer membrane for an expansion tank, to a method for manufacturing the single layer membrane, and to an expansion tank comprising the single layer membrane.
Background art
An expansion tank or expansion vessel protects closed (not open to atmospheric pressure) liquid systems from excessive pressure building up in the system. The tank absorbs excess liquid pressure caused by thermal expansion. Expansion tanks can often be found in domestic central heating systems, but they are also known to serve other purposes, for example in suspension systems for vehicles.
The working principle of an expansion tank is based on the principle that a gas, different from a liquid, is compressible. An expansion tank consists of two compartments separated by a flexible membrane. One side of the expansion tank is connected to the piping of the heating system and therefore contains a liquid, which is often water. The other side contains air under pressure, and normally a valve, such as e.g. a Schrader valve for checking pressures and adding air. When the heating system is empty or at the low end of the normal range of working pressure, the membrane is pushed against the liquid inlet. As the liquid volume, and thereby the pressure increases, for example due to a temperature increase, the membrane moves and thereby compresses the air on its other side. When expansion tanks are used in domestic drinking water systems, the membrane and the tank must conform to drinking water regulations.
It will be clear that expansion tank membranes should be able to withstand a large number of pressure cycles, in which the membrane is expanded and retracted as a result of pressure variations within the system. After a certain amount of cycles, fatigue can cause damage to a membrane, which increases the water and gas permeability of a membrane.
The number of cycles a membrane should be able to withstand without a significant loss of barrier properties is determined by NEN 13831 :2007. In this test, an expansion tank with a membrane is subjected to continuous cyclic stressing, optionally at an elevated temperature. Pressurised water is pumped into the tank until it is filled to 50% of the content of the chamber. The membrane expands and then the pressure is released again. After the cyclic test, once the tank has cooled down (if the test was performed at elevated temperature, e.g. 75X), the gas side of the vessel is filled with air to 1 ,5 bar. The pressure drop within the following hour shall not exceed 0,15 bar.
Expansion tank membranes often consist of vulcanized rubber, such as styrene- butadiene (SBR) rubber or bromobutyl rubber (BiiR). However, due to the need for vulcanization, the manufacturing of such membranes may be cumbersome, and due to the presence of the vulcanizing agents and accelerators, the resulting membranes are not always suitable for use with potable water. For use with potable water, other materials are preferred. Moreover, vulcanized rubber tends to let through small amounts of oxygen and water over time. This leads to air bubbles in the system, and to water in the air compartment of the expansion tank. This causes a gradual decrease of the function of the expansion tank, up to the point where the air compartment contains too little air and the pressure drops under a critical value of 0,2 bar. At that point the expansion tank does not perform its function anymore. Vulcanized rubber based expansion tank membranes have a maximum lifespan of about 15 years under standard conditions. Due to the cross-linking of the rubber, conventional expansion tank membranes cannot easily be recycled. For example, they cannot be molten and reshaped.
Membranes with improved gas barrier properties may be made of thermoplastic materials, e.g. thermoplastic polyurethane (TPU), such as disclosed in WO 2013/170323. TPU is relatively cheap, easy to process, elastic and less sensitive to bacteria in comparison to alternative thermoplastic materials. Nevertheless, TPU membranes are still relatively permeable to water.
US 2010/006532, which is in a different field, discloses a retort liner for a bottle comprising styrenic block copolymers and one or more polyolefin polymers.
US 2008/017653 discloses a thermoplastic diaphragm assembly for use in a pressure vessel. The diaphragm is formed from a thermoplastic elastomeric material selected from the group consisting of 1. ethyl vinyl acetate (EVA), 2. rubber, 3. rubber blends, and 4. polypropylene-rubber blends. Expansion tank membranes comprising multiple layers are also known, e.g. from US 2010/209672. By using multiple layers, the properties that one layer is lacking, e.g. water impermeability, may be given to the membrane by adding an extra barrier layer on top of another layer, such that the total membrane has the desired water and gas impermeability, as well as durability. However, the production of expansion tank membranes comprising multiple layers is complicated. It may for example require the production of two or more separate membranes which are then stacked on top of each other. The present invention aims to overcome the abovementioned drawbacks in expansion tank membranes, or at least to provide a useful alternative. Therefore, it is an objective of the present invention to provide an expansion tank membrane with adequate water barrier properties. It is another objective of the current invention to provide an expansion tank membrane with adequate gas barrier properties. It is a further objective of the current invention to provide an expansion tank membrane which can easily be produced. It is a further objective of the current invention to provide an expansion tank membrane with adequate mechanical properties. It is another objective of the present invention to provide a membrane for an expansion tank which can be recycled. Also an expansion tank is provided.
Summary of invention
To reach at least one of said objectives, in a first aspect the present invention provides a single layer membrane for an expansion tank, comprising a mixture of a polyolefin and a non-olefinic or partially olefinic thermoplastic elastomer copolymer.
A membrane for an expansion tank can separate two compartments of an expansion tank. This means that such a membrane requires a peripheral edge which is configured to be clenched between a first housing part and a second housing part of an expansion tank. The membrane may for example have a beaded circumferential edge.
Expansion tanks may be cylindrical (Fig. 1) or have a rectangular shape (Fig. 2), depending on the application. Because the membrane only consists of a single layer, it can easily be produced in a single step by, for example, injection molding or blow molding.
Because there is no need to stack different membranes on top of each other, such as for multi-layer membranes, the production process is relatively quick and easy. Furthermore, because the membrane comprises a thermoplastic elastomer, an expansion tank membrane with the desired properties (flexibility, low gas permeation, low water permeation) can be obtained without any chemical cross-linking.
Instead of chemically cross-linked, thermoplastic elastomer copolymers are cross- linked by physical interactions. The polymer chains of the thermoplastic elastomer copolymers comprise blocks of at least two different incompatible repeating units, so called soft segments and hard segments, of which at least two are present in order to form a cross- linked network. The hard segments will phase separate from the soft segments, forming physical cross-links (as opposed to chemical cross-links in the case of e.g. vulcanized rubber) in a matrix of the soft segment polymer, provided the soft segment polymer is the dominant segment. Generic classes of thermoplastic elastomer copolymers are: styrenic block copolymers, thermoplastic polyurethanes, thermoplastic copolyesters, and
thermoplastic polyamides. These classes are all non-olefinic or partially olefinic, wherein non-olefinic means that these polymers do not comprise any olefinic repeating units.
Partially olefinic means that the polymers, although they may comprise olefinic repeating units, also comprise non-olefinic repeating units. Thus, a non-olefinic or partially olefinic thermoplastic elastomer copolymer comprises non-olefinic repeating units. All the above classes of thermoplastic elastomer copolymers comprise non-olefinic repeating units.
Styrenic block copolymers are block copolymers of styrene and a diene, such as polyisoprene, and/or polybutadiene. The diene may be hydrogenated. The block copolymers are multiblock copolymers, such as triblock copolymers. Examples of styrenic 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 may be mixtures of linear and branched block copolymers, and/or mixtures comprising diblock copolymers having a single soft segment and a single hard segment. Such styrenic block copolymers are manufactured by, e.g. Kraton, Kuraray, TSRC, and LCY.
Thermoplastic polyurethanes (TPUs) are block copolymers consisting 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 varying the ratio, structure, and/or molecular weight of the reaction compounds, a variety of differently structured TPUs can be produced. This allows for fine tuning of the structure to the desired final properties of the material. For example, a greater 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 comprises a polytetramethylene ether glycol (PTMEG) polyol.
Thermoplastic copolyesters are multiblock copolymers of chemically different polyester and polyether segments connected by ester linkages. A typical example of a poly (ether ester) consists of poly(butylene terephthalate) (PBT) as the hard and short segment connected by ester groups with flexible and long poly(tetramethylene oxide) segments.
Thermoplastic polyamides finally, are block copolymers with hard and soft segments, the block copolymer comprising amide bonds. They are for example based on nylon and polyethers or polyesters.
The thermoplastic elastomer copolymers are to be seen separately from other classes of thermoplastic elastomers, such as thermoplastic vulcanizates (e.g. Elastron, Forprene, Santoprene, Trefsin, etc.) and thermoplastic polyolefinelastomers. Such thermoplastic elastomers are in fact themselves mixtures of multiple components. For example, thermoplastic polyolefinelastomers are mixtures of a thermoplastic polymer such as polypropylene or polyethylene and pre-cross-linked rubber particles.
As opposed to conventional vulcanized and/or otherwise cross-linked membranes, which require chemical cross-linking for their mechanical properties, the membranes according to the invention can be molten and reshaped. Furthermore, as no cross-linking chemicals are required, less toxic chemicals are required in the production process. Single layer expansion tank membranes from a thermoplastic elastomer copolymer, the elastomer copolymer being a thermoplastic polyurethane (TPU) are suggested in WO 2013/151441. However, although such membranes may have the required flexibility and durability, they are relatively permeable to water.
Polyolefins are known for their water barrier properties. That is why in multi-layer expansion tank membranes, there usually is a polyolefin layer providing the water barrier properties.
Surprisingly, it is not needed to utilize multiple layers for an expansion tank membrane when it is required to take advantage of the water barrier properties of polyolefins. By mixing a thermoplastic elastomer copolymer with a polyolefin, a single layer membrane for an expansion tank can be obtained with a decreased water vapor
permeability as compared to a single layer expansion tank membrane which only comprises a thermoplastic elastomer copolymer. Although the durability is somewhat decreased as compared to that of prior art single layer expansion tank membranes, they still meet the demands for expansion tank membranes.
The polyolefin can be any polymer or copolymer consisting essentially of olefinic repeating units, i.e. C2 - Cx olefinic repeating units, preferably C2 - C4 olefinic repeating units, more preferably propylene and/or ethylene repeating units. Essentially in this respect means that the combined olefin repeating unit content of the (co)polymer is higher than 90 wt.%, preferably higher than 95 wt.%, even more preferably higher than 99 wt.%, with the remainder being derived from copolymerizable non-olefinic monomers. For instance, in order to increase the compatibility with the thermoplastic elastomer, the polymer may comprise a small amount of non-olefinic moieties. For example, the polyolefin may be grafted with other monomeric units, such as maleic anhydride. Preferably, the amount of non-olefinic repeating units in the polyolefin is not higher than 10 weight%, more preferably not higher than 5 weight%, most preferably not higher than 1 weight%. Preferably, the polyolefin does not comprise any non-olefinic repeating units.
In a second aspect, the invention provides a method for manufacturing a single layer membrane for an expansion tank, comprising
a) heating and mixing a polyolefin and a non-olefinic or partially olefinic thermoplastic elastomer copolymer,
b) injection moulding the mixture of a) into a membrane mould to form the single layer membrane,
c) optionally cooling the membrane in the membrane mould,
d) releasing the membrane from the mould.
Heating and mixing may be performed in an extruder. Preferably, in step a) the polyolefin and the thermoplastic elastomer copolymer are heated to a temperature of between 200 - 220 °C. The injection moulding in step b) preferably takes place at a pressure of between 130 - 150 bar, and an injection time of between 1 seconds and 4 seconds.
In a third aspect, the invention provides for the use of a styrenic block copolymer for manufacturing a single layer membrane for an expansion tank.
Finally, an expansion tank is provided comprising the single layer membrane according to the invention.
Description of embodiments
Preferably, the membrane is a diaphragm membrane. Such a membrane has a substantially spherical circumference which can be clenched between two halves of an expansion tank. The diaphragm membrane may have a substantially hat-shaped form in rest, comprising an outer region which is substantially flat and an inner region which is at least partially curved and defines a volume. The thickness of such a membrane is preferably at least 0.8 mm. It is noted that a diaphragm membrane differs from a bladder or balloon shaped membrane.
Preferably, the polyolefin polymer or copolymer comprises propylene repeating units with a propylene content of at least 80 wt.%. More preferably, the polyolefin is a copolymer of ethylene and propylene. Alternatively, the polyolefin is a propylene homopolymer, because of polypropylene’s excellent water barrier properties as compared to other polyolefins. Even more preferably, the polyolefin is polypropylene with a Melt Flow Index of between 5 and 100 g/10 min, most preferably between 10 and 100 g/10 min according to ASTM D1238 (200 °C/2.16 kg).
Preferably, the non-olefinic or partially olefinic thermoplastic elastomer copolymer is a styrenic block copolymer. More preferably, the styrenic block copolymer is a block copolymer of styrene and isoprene, 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 between 2 and 24 g/min. Most preferably, the block copolymer has a Melt Flow Index (ASTM D1238, 200 °C/5 kg) of between 8 and 19 g/min. In addition to favourable water permeation rates, expansion tank membranes which comprise a mixture of a styrenic block copolymer, notably a poly(styrene- isoprene-styrene) triblock copolymer, and a polyolefin, notably polypropylene, have mechanical properties which are comparable to conventional vulcanized expansion tank membranes, while showing favorable gas and water permeation rates over extended periods of time.
Preferably, the poly(styrene-isoprene-styrene) triblock copolymer has a polystyrene content of between 10 and 25 wt.%, preferably between 12 and 21 wt.%, most preferably of between 14 and 17 wt.%. Without wishing to be bound by theory, it is speculated that due to the low amount of the polystyrene blocks as compared to the middle isoprene block, the triblock copolymer is very flexible, which results in adequate mechanical properties for expansion tank membranes.
The composition used for the membrane may comprise further components, i.e. additives, up to 50 % by weight. Such additives may include fillers, colorants, and further polymers and the like.
Preferably, the single layer expansion tank membrane does not comprise ethylene vinyl alcohol (EVOH) or EVOH copolymers. EVOH has excellent gas-barrier properties. However, its mechanical properties are poor and it is not resistant to water and/or water vapor. Therefore, EVOH is often used as a middle layer in multilayer expansion tank membranes, wherein the outer layers provide the water barrier. Moreover, it is a relatively expensive polymer. It may be possible to blend EVOH or EVOH copolymers into the polymer mixture that is used to produce a single layer expansion tank membrane. Addition of EVOH or EVOH copolymers in that way may be advantageous for the properties of an expansion tank membrane. However, it is a relatively expensive polymer. Therefore, use of EVOH or EVOH copolymers is not preferred.
Preferably, the single layer expansion tank membrane does not comprise an oil. Oils are often added to polymer blends in order to increase processability, e.g. as processing oils. However, such oils may leak out of the membrane. Especially in the case of potable water applications, this is undesirable.
Preferably, the membrane comprises between 10 and 60 wt.%, preferably between 20 and 60 wt.%, more preferably between 25 and 50 wt.% non-olefinic or partially olefinic thermoplastic elastomer copolymer.
Preferably, the membrane consists of 10 - 60 wt.% non-olefinic or partially olefinic thermoplastic elastomer copolymer, 40 - 90 wt.% polyolefin, and 0 - 20 wt. % additives, the total adding up to 100 wt.%.
The terms "a'V'an", as used herein, are defined as one or more than one. The terms including and/or having, as used herein, are defined as comprising (i.e., open language, not excluding other elements or steps). The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.
Brief description of the figures
Figure 1 is a schematical side view of a cylindrical expansion tank.
Figure 2a is a 3D view of a rectangular expansion tank.
Figure 2b is 3D view of the two housing parts of a rectangular expansion tank. Figure 2c is a 3D view of a cross-section of a rectangular expansion tank with a membrane.
Figure 2d is a schematical cross-section of a rectangular expansion tank with a membrane.
Detailed description of the figures
Figure 1 is a schematical side view of a cylindrical expansion tank, with a first housing part (1’) and a second housing part (2’).
Figure 2a is a 3D view of a rectangular expansion tank. In figure 2b, the first housing part (1), and second housing part (2) are indicated.
Figure 2c is a 3D view of a cross-section of a rectangular expansion tank with a membrane. The first housing part (1), second housing part (2), and the membrane (3) are indicated in the figure.
Figure 2d is a schematical cross-section of a rectangular expansion tank with a membrane. The first housing part (1), second housing part (2), the membrane (3), and the peripheral edge (4) of the membrane are indicated. The edge (4) is configured to be clenched between the first housing part (1) and the second housing part (2) of the expansion tank.
Examples
Expansion tank membranes were produced by heating the materials for the membrane in an extruder to a temperature of 210°C, injection moulding the material or mixture of materials into a membrane mould at a pressure of 140 bar for about 1 to 4 seconds, cooling the mixture in the membrane mould, and releasing the membrane from the mould.
Membranes with a thickness of between 1 and 2.5 mm and a diameter of about 30 cm were subjected to cyclic pressure testing according to NEN 13831 :2007. Durability was determined by repeating the cyclic pressure test for 1000 cycles, after which the gas side of the vessel was filled with air to 1 ,5 bar. If the pressure drop within the following hour did not exceed 0,15 bar, the test was continued for another 500 cycles.
N2 and H O permeation rates were determined separately from the cyclic testing. The used method was a combined permeation test by measuring over time the diffusion of N2 and H O in one test unit. In the test unit a membrane is placed between a vacuum chamber and a chamber filled with water and nitrogen under pressure. The rate of diffusion is measured by measuring the weight and pressure over time. The relation between the pressure reduction and weight in time is the calculated permeation coefficient of the membrane for water and nitrogen. Table 1.
* comparative examples
a and b are repeat measurements of similar compositions
Materials
All materials are standard commercially available products.
BiiR: Vulcanized bromobutyl rubber
SBR: Vulcanized SBR rubber
TPU: Polyether urethane, MDI (Methylene diphenyl diisocyanate) + PTMEG
(polytetramethylene ether glycol) based
SIS: Poly(styrene-isoprene-styrene) triblock copolymer with a MFI of between 8.5 and 18.5 g/10 min as measured by ASTM D1238 (200 °C/5kg), and a polystyrene content of between 14.0 and 17.0 mass%
PP: polypropylene with a MFI of between 10 and 100 g/10 min as measured by ASTM D1238 (200 °C/2.16 kg)

Claims

C L A I M S
1. Single layer membrane for an expansion tank, comprising a mixture of a polyolefin and a non-olefinic or partially olefinic thermoplastic elastomer copolymer.
2. Membrane according to claim 1 , wherein the non-olefinic or partially olefinic thermoplastic elastomer copolymer is a block copolymer.
3. Membrane according to claim 1 or 2, wherein the membrane has a peripheral edge which is configured to be clenched between a first housing part and a second housing part of an expansion tank.
4. Membrane according to any one of the preceding claims, wherein the polyolefin consists essentially of C2 - C4 repeating units.
5. Membrane according to any one of the preceding claims, wherein the polyolefin comprises at least 80 wt.% propylene repeating units.
6. Membrane according to any one of the preceding claims, wherein the polyolefin is polypropylene homopolymer, preferably with an Melt Flow Index of between 5 and 100 g/10 min, more preferably between 10 and 100 g/10 min according to ASTM D1238 (200 °C/2.16 kg).
7. Membrane according to any one of the preceding claims, wherein the non-olefinic or partially olefinic thermoplastic elastomer copolymer is a styrenic block copolymer.
8. Membrane according to claim 7, wherein the styrenic block copolymer is a block copolymer of styrene and isoprene, preferably a poly(styrene-isoprene-styrene) triblock copolymer or mixture thereof with a diblock copolymer, more preferably with a MFI of between 8 and 19 g/10 min as measured by ASTM D1238 (200 °C/5 kg).
9. Membrane according to claim 8, wherein the poly(styrene-isoprene-styrene) triblock copolymer has a polystyrene content of between 10 and 21 wt.%, preferably between 12 and 19 wt.%, most preferably of between 14 and 17 wt.%.
10. Membrane according to any one of the preceding claims, not comprising EVOH.
1 1. Membrane according to any one of the preceding claims, not comprising an oil.
12. Membrane according to any one of the preceding claims, wherein the membrane comprises between 10 and 60 wt.%, preferably between 20 and 60 wt.%, more preferably between 25 and 55 wt.% non-olefinic or partially olefinic thermoplastic elastomer copolymer.
13. Membrane according to any one of the preceding claims, wherein the membrane consists of
a) 10 - 60 wt.% non-olefinic or partially olefinic thermoplastic elastomer copolymer, b) 40 - 90 wt.% polyolefin,
c) 0 - 20 wt. % additives,
the total adding up to 100 wt.%.
14. Method for manufacturing a single layer membrane for an expansion tank, comprising
a) heating and mixing a polyolefin and a non-olefinic or partially olefinic thermoplastic elastomer copolymer,
b) injection moulding the mixture of a) into a membrane mould to form the single layer membrane,
c) optionally cooling the membrane in the membrane mould,
d) releasing the membrane from the mould.
15. Use of styrenic block copolymer for manufacturing a single layer membrane for an expansion tank.
16. Use according to claim 15, wherein the styrenic block copolymer is a block copolymer of styrene and isoprene, more preferably a poly(styrene-isoprene-styrene) triblock copolymer.
17. Expansion tank comprising the membrane according to any one of claims 1 - 13.
EP19726191.0A 2018-04-19 2019-04-18 Single layer expansion tank membrane Pending EP3781873A1 (en)

Applications Claiming Priority (2)

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PCT/NL2019/050231 WO2019203647A1 (en) 2018-04-19 2019-04-18 Single layer expansion tank membrane

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20040173624A1 (en) * 2003-03-05 2004-09-09 Polymer & Steel Technologies Holding Company, L.L.C. Vessel diaphragm and method
US7960007B2 (en) * 2008-07-11 2011-06-14 Teknor Apex Company Retortable liners and containers
US20100209672A1 (en) 2009-02-17 2010-08-19 Yahya Hodjat Metallic Layer Membrane
CN101701144B (en) * 2009-10-30 2012-08-22 华南理工大学 Sealing material and application thereof
US8378025B2 (en) * 2010-03-12 2013-02-19 Equistar Chemicals, Lp Adhesive composition
CH702905A1 (en) * 2010-03-26 2011-09-30 Olaer Schweiz Ag Pressure compensating device for liquid flowed through systems.
NL2008613C2 (en) 2012-04-06 2013-10-09 Flamco Bv Expansion vessel.
BE1020694A5 (en) 2012-05-17 2014-03-04 Covess N V BALG SYSTEM FOR EXPANSION VESSEL.

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