Methods for the production of glass fibers used in the manufacture of a support structure for vacuum insulation panels, as well as glass fibers produced therewith.

Abstract

Method for producing glass fibers used in the manufacture of a support structure for vacuum insulation panels, wherein the surface of glass fibers or the surface of glass fibers with which a textile structure has been formed is coated with a porous oxide layer after or during their production and Subsequently, the surface of the porous oxide layer is immersed or sprayed with a solution of a salt, whereby the salt crystallizes in pores of the oxide layer during a thermal treatment and is chemically bonded to the surface of the porous oxide layer and A textile structure is produced using the coated glass fibers as a support structure.

Description

[0001] The invention relates to methods for producing glass fibers used in the manufacture of a support structure for vacuum insulation panels (VIPs), as well as glass fibers produced therein, and the development of a new and cost-effective method for the surface treatment of the glass fibers that can be used as a support core in the manufacture of highly insulating thin vacuum-insulated panels for various applications.

[0002] The operating principle of vacuum insulation panels (VIPs) is based on the principle of Dewar flasks: the suppression of heat transfer mechanisms by evacuating an intermediate space. Compared to alternative high-performance insulation materials, VIPs currently exhibit the lowest thermal conductivity values.

[0003] Unlike Dewar flasks, the space between the walls of VIPs is not completely evacuated, but filled with a porous core material. This allows thin films to be used as the outer layer, since the core can now withstand atmospheric pressure and also acts as a spacer between the film walls.

[0004] This primarily reduces gas heat conduction, as this accounts for the main part of the heat transport mechanisms when using commercially available porous materials.

[0005] This is primarily related to the pore size of the material used. The larger the pores, the higher the required vacuum level. Therefore, the most suitable material is one that still exhibits good insulating properties even at "high" residual gas pressures (10-50 mbar).

[0006] The pressure difference between the external pressure (barometric air pressure) and the internal pressure (vacuum) is crucial for the insulating effect and, above all, the lifespan of VIPs, as this directly affects the membrane. The greater the pressure difference, the shorter the lifespan of a conventional VIP.

[0007] Furthermore, the thermal conductivity of the solid also plays a crucial role. Based on these findings, pyrogenic silica has proven to be the most suitable material for the supporting core to date.

[0008] Currently available VIPs with a supporting core made of various glass fibers are offered with a short lifespan (maximum 5 years) due to the increase in thermal conductivity caused by outgassing of the glass fibers and, above all, by gas permeation through the cladding film. Such materials are not suitable for areas requiring a long lifespan, such as construction, container and tank manufacturing, and can only be used for short-life applications, such as disposable transport boxes.

[0009] After the evacuation of the glass fiber fleece in the VIP, the physically bound water is released from the glass surface in the form of water vapor as a result of the partial pressure difference over time (outgassing), which results in an increase in the overall thermal conductivity until such a VIP becomes unusable due to the increase in gas thermal conductivity and consequently a loss of vacuum within the panel.

[0010] Typically, during manufacturing, the glass fibers are coated with a so-called "sizing" (an aqueous emulsion of various organic polymers with a multitude of additives) to improve processability and maintain mechanical strength. A nonwoven fabric is then produced from these glass fibers and used as a support core for the production of VIPs. Outgassing also occurs from this sizing.

[0011] The majority of gases that cause a pressure increase within the VIP pass through the outer film. This is primarily due to the high permeability of water vapor. The highest permeation rates occur in the area of ​​the seal seams.

[0012] To reduce vacuum loss due to outgassing, a so-called getter is often used in VIPs. Calcium oxide (CaO) is primarily used as the getter material for water vapor. Other suitable getter materials include, for example, Al₂O₃, CaCl₂, BaO, MgO, sodium hexametaphosphate, silica gel, and others.

[0013] The solutions known so far have the following disadvantages: - complex manufacturing process for the support core material made of glass fibers - Increased thermal conductivity in VIPs due to incomplete suppression of outgassing from the support core material - separately used getter materials in VIPs that do not function permanently and reliably - resulting in an insufficient lifespan for VIPs (maximum ~ 5 years)

[0014] DE 11 2004 001 930 T5 discloses a vacuum heat insulator as well as a freezing device and a cooling device in which the insulator is used.

[0015] A porous coated fiber, a porous coated particle and a product that uses them are described in EP 1481 955 A2.

[0016] DE 10 2017 126 461 A1 relates to sound-absorbing textiles with improved thermal insulation and methods for manufacturing them.

[0017] The present invention is based on the objective of improving thermal insulation values ​​by preventing or significantly reducing outgassing processes from the glass fibers and consequently extending the service life of the glass fiber VIP, whereby this effect is to be achieved over the entire volume of a textile structure in a long-term and uniform manner.

[0018] According to the invention, this problem is solved by a method having the features of claim 1. Claim 8 defines glass fibers produced by the method. Advantageous embodiments and further developments of the invention can be realized with features specified in dependent claims.

[0019] According to the invention, the disadvantages, such as complex manufacturing processes and increased thermal conductivity in VIPs due to outgassing of the glass fiber support core material, can be avoided by providing the glass fibers with the appropriate surface treatment.

[0020] Conventionally manufactured glass fibers can be used as semi-finished products, provided their surface is uncoated and, in particular, has no sizing on it.

[0021] The surface of glass fibers, or the surface of glass fibers used to form a textile structure, is coated with a porous oxide layer after or during their production. The surface of the porous oxide layer is then immersed or sprayed with a salt solution. During thermal treatment, the salt crystallizes in the pores of the oxide layer and forms a metallurgical bond with the surface. This is preferably achieved by drying followed by crystallization. A textile structure is then produced using these coated glass fibers as a support structure. Examples of textile structures produced from these coated glass fibers include woven fabrics, knitted fabrics, or felts.

[0022] The support structure can then be inserted into a vacuum-sealed container for a vacuum insulation panel.

[0023] The porous oxide layer can be formed with silicon dioxide, aluminum oxide, boron oxide or zinc oxide.

[0024] During thermal treatment after immersion or spraying with the salt solution, the liquid components can be removed, preferably by evaporation.

[0025] The oxide layer should cover the surface of the glass fibers as completely as possible and have a porosity in the range of 10% to 90%.

[0026] An aqueous solution of a salt should preferably be used as the saline solution. The concentration should be between 1-15%. In a suspension used to form the porous oxide layer, small amounts of an organic or, preferably, at least one inorganic binder and a surfactant may be present in addition to oxide particles, but their combined proportion should be less than 1% by volume. Examples of suitable binders include polyethylene glycol, polyethylene oxide, and / or polyacrylates, and suitable surfactants include, for example, Dehypon® LS54 from BASF or sodium lauryl sulfate. The suspension can also be applied to uncoated glass fiber surfaces by immersion or spraying.

[0027] The thermal treatment that results in a metallurgical bond after immersion or spraying of the glass fibers coated with the porous oxide layer should consist of a thermal treatment process that is first dried and then sintered. During sintering, a maximum temperature of 300 °C should not be exceeded, as otherwise the strength of the glass fibers could be reduced.

[0028] A phosphate, borate, or sulfate can be used as the salt. The specific salt can be selected from an alkali phosphate, in particular sodium hexametaphosphate or sodium tripolyphosphate, an alkali borate, in particular sodium tetraborate, and an alkali sulfate, in particular sodium sulfate.

[0029] The pores on the surface of the oxide layer can be filled with crystallized salt to a minimum of 20% and a maximum of 95%.

[0030] Both the porous oxide layer and the salt crystals are firmly bonded to the glass fibers and do not change their position during the use of a vacuum insulation panel.

[0031] The double coating of the glass fibers simultaneously provides a protective function for the glass fiber surface, a significant reduction in outgassing from glass fibers, and a getter function for the glass fibers.

[0032] The invention enables an increase in the porosity and thus also the specific surface area of ​​the support core material, while simultaneously reducing outgassing from the coated glass fibers. Furthermore, the hygroscopic salt binds the water vapor contained in the VIP within the coating. This leads to an increase in the service life of a VIP, which can be very thin yet still exhibit high thermal insulation, high strength, and high vacuum resistance. The uniform distribution of the crystallized salt on the surface of the glass fibers also results in uniform binding of water within the volume of the support structure.

[0033] The following significant advantages can be achieved through the present invention: - Reduction of outgassing from the glass fibers - No additional getter system is required (thermal bridge) → Increased lifetime of the VIPs → Maintenance of a permanently very low thermal conductivity.

[0034] The invention will now be explained in more detail using an example.

[0035] The starting point for a support core material for vacuum insulation panels (VIP) is unlined glass fibers as a semi-finished product.

[0036] The first coating of the glass fibers is achieved by immersion in a coating suspension. Alternatively, the glass fibers can also be coated online during the manufacturing process. The coating suspension can have the following composition: SiO₂ 1–5 vol%; polyethylene glycol as a binder 0.01–0.6 vol%; Dehypon® LS54 as a surfactant 0.001–0.06 vol%; and the remainder being 100% deionized water. The pH value is preferably between 5 and 6. After immersion, the glass fibers are withdrawn from the suspension, whereby the coating forms on the entire surface of the glass fibers. The resulting coating is air-dried at a temperature of 70–120°C and then sintered at a temperature of 250–300°C.

[0037] The thickness of the porous silicon dioxide layer on the surface of the glass fibers is approximately 100 nm - 200 nm.

[0038] The second coating of the glass fibers can be carried out immediately after the cooling of the porous SiO2-coated glass fibers by immersing them in an aqueous salt solution. Alternatively, the second step can be carried out by spraying the glass fibers.

[0039] The salt solution can be formed with water and sodium hexametaphosphate or sodium tripolyphosphate as the salt, which is present in the salt solution with at least 80% of its maximum solubility.

[0040] After the second coating, the glass fibers are air-dried at a temperature of 90°C - 150°C. During this process, the salt crystallizes and the salt crystals become chemically bonded within the pores of the porous oxide layer.

[0041] The applied layers perform two functions simultaneously: protecting the surface of the glass fibers and acting as getter material.

[0042] The getter function of the glass fibers was demonstrated by measuring water absorption after 24-hour and 72-hour storage of samples in a water vapor-saturated atmosphere. The water absorption of one sample after subsequent drying reached up to 12%.

Claims

[1] Method for producing glass fibers used in the production of a support structure for vacuum insulation panels, wherein the surface of glass fibers or the surface of glass fibers with which a textile structure has been formed is coated with a porous oxide layer after or during their production and Subsequently, the surface of the porous oxide layer is immersed or sprayed with a solution of a salt, whereby the salt crystallizes in pores of the oxide layer during a thermal treatment and is chemically bonded to the surface of the porous oxide layer and A textile structure is produced using the coated glass fibers as a support structure. [2] Method according to claim 1, characterized by that the porous oxide layer is formed with silicon dioxide, aluminum oxide, boron oxide or zinc oxide. [3] Method according to any one of the preceding claims, characterized by , that the porous oxide layer is formed by means of a thermal treatment up to a maximum temperature of 300 °C. [4] Method according to any of the preceding claims, characterized by , that for the formation of the porous oxide layer, an aqueous suspension containing particles of the respective oxide is used when coating the surface of glass fibers by immersion or spraying. [5] Method according to any one of the preceding claims, characterized by that a phosphate, borate or sulfate is used as the salt. [6] Method according to the preceding claim, characterized by , that the salt used is a salt selected from an alkali phosphate, in particular sodium hexametaphosphate or sodium tripolyphosphate, an alkali borate, in particular sodium tetraborate and an alkali sulfate, in particular sodium sulfate. [7] Method according to any one of the preceding claims, characterized by, that in the thermal treatment which leads to the material bond after immersion or spraying of the glass fibers coated with the porous oxide layer, drying is carried out first and then sintering is performed. [8] Glass fibers produced by a method according to any of the preceding claims, characterized by , that a porous oxide layer is formed on the surface of the glass fibers and that crystallized salt adheres to the surface of the porous oxide layer in a metallurgically bonded manner.