Use of a silicone rubber mixture for the production of silicone rubber components by extrusion and device for the production of silicone rubber components

Abstract

Use of a silicone rubber compound for the production of silicone rubber components using the extrusion process, the silicone rubber mixture, dispersed within the mixture, forms an emulsion with water, wherein the silicone rubber mixture is fed to an extruder (5) for shaping, characterized by that the silicone rubber mixture, after leaving the extruder (5), has a material cross-sectional area of ​​≤ 1400 mm² as a raw component 2 , preferably of ≤ 1200 mm 2 or has a material thickness of 50 mm or preferably of 40 mm, wherein, for the purpose of shock evaporation of the water of the emulsion, the raw component, depending on the size of the cross-sectional area, is exposed in a tunnel oven (7) with two sections (7a, 7b) in a first section for a period of 10 to 110 seconds, preferably for a period of 20 to 90 seconds, to a temperature of 180°C to 250°C.

Description

[0001] The invention relates, on the one hand, to the use of a silicone rubber mixture for producing components by extrusion, wherein the silicone rubber mixture contains a water emulsion distributed throughout the mixture, and the silicone rubber mixture is fed into an extruder for shaping. On the other hand, the invention relates to an apparatus for producing silicone rubber components.

[0002] The use of a silicone rubber mixture for the production of components by extrusion is well known in the art. As an example, reference is made here to EP 3 238 904 B1, which deals with the production of foamed silicone rubber components using water. The advantage of this process for producing silicone rubber components using water is that water is not harmful to the environment in any way. This is in stark contrast to conventional blowing agents, whose environmental compatibility is often questionable.

[0003] US 7,683,125 B2 discloses a process for producing a silicone rubber comprising a first stage and a second stage. The first stage comprises the step of preparing an aqueous dispersion of a water-soluble polymer, a finely powdered silica filler, a conductive filler, and a non-conductive inorganic filler; and the step of preparing an aqueous emulsion by stirring and mixing the aqueous dispersion with the following components: an organopolysiloxane containing silicon-bonded alkenyl groups, an emulsifier, and a curing agent. In the second stage, the silicone rubber is obtained either by curing and drying a molded article formed from the aqueous emulsion or by dehydrating and curing the aqueous emulsion.

[0004] EP 1 602 693 A1 discloses a process for producing a silicone rubber, comprising: a first step comprising the step of preparing an aqueous emulsion from a water-soluble polymer, a finely powdered silica filler, a conductive filler, and a non-conductive inorganic filler, and the step of preparing an aqueous emulsion by stirring and mixing the resulting aqueous dispersion with the following components: an organopolysiloxane containing silicon-bonded alkenyl groups, an emulsifier, and a curing agent; and a second step of obtaining a silicone rubber either by curing the aqueous emulsion prepared in the first step to form a wet silicone rubber-like cured body and then removing water from the body, or by dehydrating and curing the aqueous emulsion prepared in the first step.

[0005] Silicone rubber is expensive. Therefore, efforts are made to ensure that components made from silicone rubber are lighter while maintaining essentially the same mechanical properties as solid components. Foamed components made from silicone rubber are also known.

[0006] US 8,227,520 B2 discloses a composition of an emulsion for producing a silicone rubber sponge, comprising - a liquid diorganopolysiloxane having at least two alkenyl groups per molecule; or a paste-like mixture of a liquid diorganopolysiloxane having at least two alkenyl groups per molecule and a reinforcing filler, an organopolysiloxane having at least two silicon-bonded hydrogen atoms per molecule and a platinum catalyst, - an aqueous solution of a water-soluble polymer, and - an emulsifier.

[0007] Furthermore, the production of a silicone rubber sponge based thereon is disclosed, which is based on curing and heating a molded part made of the said composition.

[0008] Now, it's a fact that components made of silicone rubber foamed using state-of-the-art technology often do not have a uniform pore structure. Rather, there are large and small pores distributed irregularly throughout the silicone rubber component. This means that the components are softer in some places and harder in others, where fewer and smaller pores are found. However, varying mechanical properties within a component can be disadvantageous depending on the intended use.

[0009] The object underlying the invention is therefore to remedy this situation. In particular, the object of the invention is to provide a foamed silicone component that exhibits essentially the same mechanical properties throughout its entire volume. To save material and thus for cost reasons, it should have a high porosity.

[0010] To achieve the object, when using a silicone rubber mixture for the production of components in the extrusion process, wherein the silicone rubber mixture has an emulsion with water distributed in the mixture, wherein the silicone rubber mixture is fed to an extruder for shaping, it is provided that the silicone rubber mixture, after leaving the extruder as a raw component, has a material cross-sectional area of ​​≤ 1400 mm 2 , preferably ≤ 1200 mm 2or has a material thickness of 50 mm or preferably 40 mm, wherein for the purpose of the shock evaporation of the water of the emulsion, the raw component is exposed to a temperature of 180°C to 250°C in a first section in a tunnel furnace with two sections for a period of 10 to 110 seconds, preferably 20 to 90 seconds, depending on the size of the cross-sectional area or the material thickness.

[0011] It has been found that such a shock heating of the component from a silicone rubber mixture results in a component with high porosity, with the maximum pore volume of almost 85% of the pores being between 10 and 200,000 µm 3 Due to the uniform distribution of approximately 85% of pores with similar or nearly identical pore volumes, such a component has virtually identical mechanical properties throughout.

[0012] The uniform distribution of the pores and their predominantly similar pore volumes can be explained by the fact that, due to the high temperatures, the water evaporates explosively and simultaneously with respect to a given extrudate cross-section or thickness. Due to the short residence time in a tunnel kiln at the specified high temperatures, there is also no risk of decomposition of the silicone structure.

[0013] The uniform pore distribution is also achieved by the homogeneous distribution of the blowing agent, water with silica. The surface structure of the silica promotes the existence of a very large number of microscopic bubble nucleation sites. The higher the temperature to which the silicone is exposed, the more these nucleation sites tend to be activated and form bubbles or pores. The stage of growth at which the pore structure is fixed depends largely on the interplay between evaporation and the crosslinking reaction.

[0014] It is also advantageous if crosslinking already occurs in the outer area of ​​the component, such as an extruded rod, during heating in the first section of the tunnel kiln. This means that deformation of the outer surface caused by gripping or transport devices, for example, is virtually eliminated.

[0015] Advantageous features and embodiments of the invention emerge from the subclaims.

[0016] For example, following shock heating in a first section of the tunnel kiln, the silicone rubber mixture of the component produced in the extruder is crosslinked in a second section of the tunnel kiln. Crosslinking can take place in a temperature range from 90°C to 250°C, with the higher the crosslinking temperature, the shorter the component's residence time in the second section of the tunnel kiln where crosslinking takes place. The time required for crosslinking the silicone rubber of the component also depends on the crosslinking agent and the amount added to the mixture, as well as the material's cross-sectional area or thickness.It is important to ensure that both during the pore formation process, as described above, and during crosslinking, the temperature and dwell time in the furnace are coordinated so that the entire volume of the component is affected during both the pore formation process and the crosslinking process. In principle, two separate furnaces can be used instead of a tunnel furnace with two sections.

[0017] It has already been explained elsewhere that the silicone rubber mixture has an emulsion, wherein the emulsion is advantageously formed as a mixture of water and silica before the mixture is mixed with the silicone rubber.

[0018] The rubber mixture may also contain crosslinking agents, such as peroxide, or the mixture may be crosslinked with platinum. The use of silanes as crosslinking agents is also known.

[0019] Additives can also be included in the silicone rubber mixture in order to impart the desired properties to the rubber mixture, for example with regard to color design, mechanical properties (friction reduction, hardness, tensile strength), flame retardancy, heat resistance, media resistance.

[0020] To produce a particularly closed-cell silicone rubber component, for example with a cross-sectional area of ​​120 - 180 mm 2When the silicone rubber mixture is suddenly heated to temperatures exceeding 100°C, particularly between 180°C and 250°C, the starting mixture contains the following components: 100 phr silicone rubber, 0.5–2 phr crosslinker, 0–3 phr water, and 0–10 phr hydrophilic, fumed silica. The residence time in the first section of the tunnel kiln is between 20 and 90 seconds. This clearly shows that residence time and temperature influence one another. This means that high temperatures result in shorter residence times. Temperature and residence time also depend on the composition of the silicone rubber mixture. For example, an HCR [High Consistency Rubber] with 2,4-dichloroperoxide has a TC 90 time (meaning 90% of crosslinking is complete) of approximately 15 seconds at a tunnel kiln temperature of 180°C. If the dichloroperoxide is replaced by dicumyl peroxide, the residence time in the tunnel kiln is 90 seconds, all other conditions being equal.The temperature in the workpiece is about 230° C.

[0021] The invention also relates to an apparatus for producing high-porosity silicone rubber components. A mixer is provided for producing a silicone mixture, followed by an extruder. The extrudate from the extruder is fed to a tunnel oven, with a vulcanization device downstream of the tunnel oven. The tunnel oven can also have two sections. The first section serves to form pores, the second section for crosslinking.

[0022] The silicone component is heated in the first and / or second section using infrared radiation. It has been shown that with infrared radiation in the wavelength range between 3 and 50 µm in the size range of the raw component output from the extruder specified elsewhere, the raw component is penetrated by the radiation and thus the raw component is heated shock-like across its volume, i.e. within a very short time, which leads to uniform pore formation across the entire material thickness or surface. Pore formation takes place in the first section with the high temperatures, while vulcanization takes place in the second section. According to the invention, one IR radiation source is provided for each section, or even several IR-based radiation sources. For crosslinking, vulcanization can also take place in a salt bath, with a microwave, or in a hot air tunnel.

[0023] Downstream of the tunnel kiln, a cutting device for producing silicone rubber components of the same length or size, as well as a tempering device, can be provided. Tempering can take place at a temperature range of approximately 200°C for four hours.

[0024] It has already been explained that the tunnel kiln, and in particular a tunnel kiln designed as a continuous furnace, has a temperature of 180°C to 250°C in the first section. In the second section, temperatures between 90°C and 250°C are used for vulcanization; the vulcanization time at approximately 200°C, for example, is 10 to 30 seconds. During the annealing, the vulcanization is completed if this has not already occurred during the vulcanization in the tunnel kiln.

[0025] Heating with IR radiation in the 3-50 µm range has the advantage of penetrating the silicone component so quickly, and pore formation begins and completes almost explosively at the same time. However, this also means that the convection component in the heating process is relatively low.

[0026] The invention is explained in more detail below using the drawing and the two tables as examples. Fig. 1 shows the entire device schematically; Fig. 2 shows a table of the porosity of the finished component; Fig. 3 shows a table of the volume of pores and their amount in the total volume of pores.

[0027] The device for producing silicone rubber components shown in the drawing is designated 1. In a mixer 3, a strand with a cross-sectional area of ​​approximately 165 mm 2A silicone rubber mixture was prepared at room temperature of approximately 20°C. The silicone rubber mixture in its initial state is as follows: 100 phr silicone rubber, 0.5-2 phr crosslinker, 0-3 phr water, 0-10 phr hydrophilic, fumed silica. Example mixture: Wacker Elastosil 401 / 60 base polymer + 1.5 phr 2,4-dichlorobenzoyl peroxide (DCLBP) + 1 phr hydrophilic silica + 1 phr water. [The commercially available Elastosil base polymer contains silanes and silica.]

[0028] The strand produced in the mixer is fed to an extruder 5, which is water-cooled to prevent crosslinking of the material in the extruder. The temperature in extruder 5 is therefore a maximum of 50°C to 60°C.

[0029] Downstream of the extruder 5 is a so-called tunnel furnace 7 with two sections 7a, 7b. In section 7a, the temperature is approximately 100°C to 180°C, for a strand with a cross-sectional area of ​​165 mm2 The residence time in the first section 7a of the tunnel kiln during the test at 160°C is 30 seconds. The surface temperature of the strand is 210°C. In the first section of the tunnel kiln, the pore formation of the extrudate from extruder 5 takes place, as already described elsewhere.

[0030] The tunnel oven 7 has a second section 7b where vulcanization takes place. Vulcanization takes place between 90°C for approximately 90 seconds and 250°C for approximately 10 seconds. In the present example, vulcanization took place at 250°C for approximately 10 seconds.

[0031] Downstream of the tunnel furnace 7 is a discharge device 11, in which the silicone rubber strand can still have a temperature of approximately 240°C.

[0032] Finally, the silicone rubber component, e.g., as a strand with a specific shape, is cut to length after the take-off device 11 using a cutting device 12 and annealed at approximately 200°C for four hours in an annealing device 13. Before annealing, the component can be assembled (arrow 15). The assembly and cutting device areas are at room temperature. This means that annealing takes place from room temperature. Here, the component is fully crosslinked if this has not yet occurred in the second section of the tunnel kiln.

[0033] Out of Fig. 2 shows that the porosity in this case is θ = 0.36 (we should already explain how the value of θ = 0.36 is arrived at). The volume fraction of the silicone molded part is 487 mm 3 and the volume fraction of the pores 117 mm 3 If you put both volume fractions in relation you get a porosity of 0.36.

[0034] The size distribution and its proportion of the total pore volume is determined from Fig. 3. This means, for example, that pores with a volume of 1 - 100,000 µm 3 64% of the total pore volume of approximately 177 cm 3 turn off. List of reference symbols: 1 device for the production of silicone rubber components 3 mixers 5 extruders 7 tunnel kiln 7a First section (pore formation) 7b Second section (vulcanization) 11 Extraction device 12 Cutting device 13 Tempering device 15 Arrow

Claims

[1] Use of a silicone rubber mixture for the production of silicone rubber components by extrusion, wherein the silicone rubber mixture has an emulsion with water distributed in the mixture, wherein the silicone rubber mixture is fed to an extruder (5) for shaping, characterized by , that the silicone rubber mixture, after leaving the extruder (5), as a raw component has a material cross-sectional area of ≤ 1400 mm 2 , preferably ≤ 1200 mm 2 or has a material thickness of 50 mm or preferably 40 mm, wherein for the purpose of the shock-like evaporation of the water of the emulsion, the raw component is exposed, depending on the size of the cross-sectional area, in a tunnel oven (7) with two sections (7a, 7b) in a first section over a period of 10 to 110 seconds, preferably over a period of 20 to 90 seconds, to a temperature of 180°C to 250°C. [2] Use of a silicone rubber mixture for the production of components in the extrusion process according to claim 1, characterized by that following the evaporation of the water, the cross-linking of the raw component produced in the extruder (5) from the silicone rubber mixture takes place by heating the component. [3] Use of a silicone rubber mixture for the production of components in the extrusion process according to claim 2, characterized by that crosslinking occurs with the addition of crosslinking agents, e.g. peroxide or silanes. [4] Use of a silicone rubber mixture for the production of components in the extrusion process according to claim 2 or 3, characterized by that crosslinking takes place in the temperature range from 90°C to 250°C for a maximum of 90 seconds. [5] Use of a silicone rubber mixture for the production of components in the extrusion process according to one of the preceding claims, characterized bythat the emulsion contains water and silica. [6] Use of a silicone rubber mixture for producing components by the extrusion process according to claim 5. characterized by that the silicone rubber mixture contains, in addition to silicone rubber, an emulsion with silica and water and agents for crosslinking the rubber, other additives such as color. [7] Use of a silicone rubber mixture for the production of components in the extrusion process according to claim 6, characterized by that the silicone rubber mixture in its initial state has the following components: 100 phr silicone rubber, 0.5 - 2 phr crosslinker, 0 - 3 phr water, 0 - 10 phr hydrophilic fumed silica. [8] Device for producing silicone rubber components (1), wherein for producing a silicone rubber component according to one or more of claims 1 to 7, a mixer (3) is provided, which is followed by an extruder (5), wherein the extrudate is fed to a tunnel oven (7) with two sections (7a, 7b), wherein the pore formation takes place in the first section (7a) and the vulcanization takes place in the second section (7b), and wherein the tunnel oven (7) has at least one infrared radiator per section to generate the required heat, such that the tunnel oven (7) can be heated to a temperature of 180°C to 250°C in the first section (7a). [9] Device for producing silicone rubber components (1) according to claim 8, characterized by that the second section (7b) can be heated to a temperature between 90°C and 250°C for vulcanization. [10] Device for producing silicone rubber components (1) according to claim 8, characterized by that a tempering device (13) is arranged downstream of the tunnel furnace (7). [11] Device for producing silicone rubber components (1) according to one of claims 8 to 10, characterized by that the arrangement of several infrared radiators is provided in the tunnel furnace (7), wherein the arrangement of the infrared radiators is selected such that the silicone raw component is irradiated from all sides. [12] Apparatus for producing silicone rubber components (1) according to claim 10 or 11, characterized by that the infrared radiator(s) operate in a wavelength range of 3 to 50 µm.

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