Silicone rubber composition for producing components by extrusion

Abstract

The invention relates to the 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 throughout the mixture, wherein the silicone rubber mixture is fed to an extruder (5) for shaping, wherein the silicone rubber mixture, after leaving the extruder (5), has a material cross-sectional area of ​​≤ 1400 mm², preferably ≤ 1200 mm², or a material thickness of 50 mm or preferably 40 mm, wherein, for the purpose of shock-induced evaporation of the water in the emulsion, the raw component, depending on the size of the cross-sectional area, is subjected in a tunnel oven (7) with two sections (7a, 7b) in a first section to a temperature of 10 to 300°C, preferably 180°C, for a period of 10 to 110 seconds, preferably 20 to 90 seconds. is exposed to 250°C.The invention further comprises a device for the production of silicone rubber components (1), wherein, for the production of a silicone rubber component according to one or more of claims 1 to 7, a mixer (3) is provided, to which an extruder (5) is connected, wherein the extrudate is fed to a tunnel furnace (7) with two sections (7a, 7b), wherein pore formation takes place in the first section (7a) and vulcanization takes place in the second section (7b).

Description

[0001] The invention relates, on the one hand, to the use of a silicone rubber mixture for the production of components by extrusion, wherein the silicone rubber mixture has an emulsion with water dispersed within the mixture, and wherein the silicone rubber mixture is fed to an extruder for shaping. On the other hand, the invention relates to a device for the production of silicone rubber components.

[0002] The use of a silicone rubber compound for the production of components using extrusion is well known in the art. By way of example, reference is made here only to EP 3 238 904 B1, which deals with the production of foamed silicone rubber components using water. The advantage of this method for producing silicone rubber components using water is that water is in no way harmful to the environment. This is in stark contrast to other commonly used blowing agents, whose environmental compatibility is often questionable.

[0003] Silicone rubber is expensive. Therefore, efforts are made to ensure that components made from silicone rubber are lighter while possessing essentially the same mechanical properties as solid components. Consequently, foamed components made of silicone rubber are also known.

[0004] Currently, silicone rubber components manufactured using state-of-the-art foaming techniques often lack a uniform pore structure. Instead, they contain large and small pores that are irregularly distributed throughout the component. This means that the components are softer in some areas and harder in others where fewer and smaller pores are present. However, these varying mechanical properties within the component can be a disadvantage depending on the intended application.

[0005] 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 reduce costs, it should have a high porosity.

[0006] To solve the problem, when using a silicone rubber mixture for the production of components by extrusion, wherein the silicone rubber mixture has an emulsion with water distributed throughout the mixture, and 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 mm², preferably ≤ 1200 mm², or a material thickness of 50 mm, or preferably 40 mm. For the purpose of the rapid evaporation of the water in the emulsion, the raw component is exposed, depending on the size of the cross-sectional area or the material thickness, in a tunnel oven with two sections, in a first section for a period of 10 to 110 seconds, preferably 20 to 90 seconds, to a temperature of at least 90°C, up to 300°C, preferably 180°C to 250°C.

[0007] It has been found that such a sudden heating of a component made from a silicone rubber compound results in a component with high porosity, where the maximum pore volume of almost 85% of the pores lies between 10 and 200,000 µm. Due to the uniform distribution of approximately 85% of pores with similar or nearly identical pore volumes, such a component exhibits almost identical mechanical properties throughout.

[0008] The uniform distribution of the pores and their predominantly similar pore volume 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. Because of the short residence time in a tunnel oven at the specified high temperatures, there is also no risk of decomposition of the silicone structure.

[0009] The uniform distribution of pores is also achieved through 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 of these nucleation sites tend to be activated and form bubbles or pores. The stage of growth at which the pore structure becomes fixed depends largely on the interplay between evaporation and the cross-linking reaction.

[0010] It is also advantageous if, during the heating of the component in the first section of a tunnel kiln, cross-linking already occurs in the outer area of ​​the component, for example, an extruded rod. This results in minimal deformation of the outer surface due to, for example, gripping or transport devices.

[0011] Advantageous features and embodiments of the invention are set out in the dependent claims.

[0012] For example, following the rapid heating in a first section of the tunnel oven, the crosslinking of the silicone rubber mixture of the component produced in the extruder takes place in a second section of the tunnel oven. Crosslinking can occur within a temperature range of 90°C to 250°C, with the higher the crosslinking temperature, the shorter the residence time of the component in the second section of the tunnel oven where crosslinking occurs. The time required for the crosslinking of the silicone rubber of the component also depends on which crosslinking agents and in what quantity they have been added to the mixture, as well as on the cross-sectional area or thickness of the extrudate.It is important to ensure that, both during the pore formation process as described above and during crosslinking, the temperature and residence time in the oven are precisely coordinated so that the entire volume of the component is affected during both processes. Alternatively, instead of a tunnel oven with two sections, two separate units can be used.

[0013] It has already been explained elsewhere that the silicone rubber mixture has an emulsion, the emulsion being advantageously formed as a mixture of water and silica before the mixture is blended with the silicone rubber.

[0014] It is also possible for the rubber mixture to contain crosslinking agents, such as peroxide, or for the mixture to be crosslinked with platinum. The use of silanes as crosslinking agents is also known.

[0015] Additives can also be included in the silicone rubber mixture to give the rubber mixture the desired properties, for example in terms of color, mechanical properties (friction reduction, hardness, tensile strength), flame protection, heat resistance, and media resistance.

[0016] To produce a closed-cell silicone rubber component, for example with a cross-sectional area of ​​120–180 mm², by rapidly heating the silicone rubber mixture to > 100°C, particularly to 180°C–250°C, the starting mixture contains, in particular, the following proportions: 100 phr silicone rubber, 0.5–2 phr crosslinker, 0–3 phr water, and 0–10 phr hydrophilic, pyrogenic silica. The residence time in the first section of the tunnel oven is between 20 and 90 seconds. This clearly shows that residence time and temperature are mutually dependent. Higher 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 that 90% of the crosslinking is complete, of approximately 15 seconds at a temperature of 180°C in the tunnel oven.If the dichloroperoxide is replaced by dicumyl peroxide, the residence time in the tunnel furnace, under otherwise identical conditions, is 90 seconds. The temperature in the workpiece is approximately 230°C.

[0017] The invention also relates to a device for producing high-porosity silicone rubber components, wherein a mixer is provided for producing a silicone mixture, followed by an extruder, the extrudate being fed from the extruder to a tunnel oven, and a vulcanization unit being arranged downstream of the tunnel oven. The tunnel oven can also have two sections. The first section serves for pore formation, the second section for crosslinking.

[0018] The silicone component is heated with infrared radiation in the first and / or second stage. It has been shown that infrared radiation in the wavelength range between 3 and 50 µm, within the size range specified elsewhere for the raw component emerging from the extruder, penetrates the raw component, thus heating it rapidly throughout its volume. This results in uniform pore formation across the entire material thickness or surface area. Pore formation occurs in the first stage at high temperatures, while vulcanization takes place in the second stage. Each stage can utilize one or more IR radiation sources. Curing can also be achieved through vulcanization in a salt bath, microwave oven, or hot air tunnel.

[0019] Downstream of the tunnel oven, a cutting device for producing silicone rubber components of the same length or size, as well as a tempering unit, can be provided. Tempering can take place at a temperature of approximately 200°C for four hours.

[0020] As already explained, the tunnel kiln, and in particular a continuous tunnel kiln, has a temperature of > 100°C, preferably between 180°C and 250°C, in the first section. In the second section, temperatures between 90°C and 250°C are provided for vulcanization; the vulcanization time is, for example, 10 to 30 seconds at approximately 200°C. During tempering, vulcanization is completed if it has not already been completed during vulcanization in the tunnel kiln.

[0021] Heating with IR radiation in the 3-50 µm range has the advantage that it penetrates the silicone component very quickly, and that pore formation begins and ends almost explosively at the same time. However, this also means that the convection component during heating is relatively low.

[0022] The invention is explained in more detail below by way of example, with reference to the drawing and the two tables. Fig. 1 shows the entire device schematically; Fig. 2 shows a table about the porosity of the finished component; Fig. 3 shows a table about the volume of the pores and their quantity in relation to the total volume of the pores.

[0023] The apparatus for manufacturing silicone rubber components, shown in the drawing, is designated 1. In a mixer 3, a strand with a cross-sectional area of ​​approximately 165 mm² of a silicone rubber mixture is produced at a room temperature of approximately 20°C. The initial silicone rubber mixture consists of: 100 phr silicone rubber, 0.5–2 phr crosslinker, 0–3 phr water, and 0–10 phr hydrophilic, pyrogenic silica. Example mixture: Base polymer Wacker Elastosil 401 / 60 + 1.5 phr 2,4-dichlorobenzoyl peroxide (DCLBP) + 1 phr hydrophilic silica + 1 phr water. [The commercially available base polymer Elastosil contains silanes and silica.]

[0024] The strand produced in the mixer is fed into an extruder 5, which is water-cooled to prevent cross-linking of the material within the extruder. The temperature in extruder 5 is therefore a maximum of 50°C to 60°C.

[0025] Downstream of extruder 5 is a so-called tunnel oven 7 with two sections 7a and 7b. In section 7a, the temperature is approximately 100°C to 180°C, with a strand having a cross-sectional area of ​​165 mm². The residence time in the first section 7a of the tunnel oven is 30 seconds at 160°C during the experiment. The surface temperature of the strand is 210°C. In the first section of the tunnel oven, the pore formation of the extrudate from extruder 5 takes place, as has already been described elsewhere.

[0026] Tunnel kiln 7 has a second section 7b where vulcanization takes place. Vulcanization occurs between 90°C for approximately 90 seconds and 250°C for approximately 10 seconds. In this example, vulcanization took place at 250°C for approximately 10 seconds.

[0027] Downstream of the tunnel furnace 7 is an exhaust device 11, in which the silicone rubber strand can still have a temperature of about 240°C.

[0028] Finally, the silicone rubber component, e.g., as a strand with a specific shape, is cut to length after the discharge unit 11 using a cutting unit 12 and tempered at approximately 200°C for 4 hours in a tempering unit 13. The component can be pre-assembled (arrow 15) before tempering. Room temperature prevails in the pre-assembly and cutting unit areas. This means that tempering takes place from room temperature. Here, the component is fully cross-linked if this has not already occurred in the second section of the tunnel oven.

[0029] Out of Fig. 2 This results in a porosity of θ = 0.36 in the present case (we should explain how the value of θ = 0.36 is derived). The volume fraction of the silicone molded part is 487 mm³ and the volume fraction of the pores is 117 mm³. The ratio of these two volume fractions yields a porosity of 0.36.

[0030] The size distribution and their proportion of the total pore volume result from Fig. 3 This means, for example, that pores with a volume of 1 - 100,000 µm 3< make up 64% of the total pore volume of approximately 177 cm 3<. Reference symbol list:

[0031] 1 Device for the production of silicone rubber components 3 Mixer 5 Extruder 7 Tunnel furnace 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 dispersed 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), 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 at least 90°C to 300°C, preferably 180°C to 250°C.

2. Use of a silicone rubber compound for the production of components by extrusion according to claim 1, characterized by that Following the evaporation of the water by heating the component, the cross-linking of the raw component produced from the silicone rubber mixture in the extruder (5) takes place.

3. Use of a silicone rubber compound for the manufacture of components by extrusion according to claim 2, characterized by that The crosslinking is carried out with the addition of crosslinking agents, e.g. peroxide or silanes.

4. Use of a silicone rubber compound for the manufacture of components by extrusion according to claim 2 or 3, characterized by that The crosslinking process takes place in a temperature range of 90°C to 250°C for a maximum of 90 seconds.

5. Use of a silicone rubber compound for the production of components by extrusion according to one of the preceding claims, characterized by thatThe emulsion contains water and silica.

6. Use of a silicone rubber compound for the production of components by extrusion 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 compound for the manufacture of components by extrusion 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 pyrogenic silica.

8. Device for the production of silicone rubber components (1), wherein, for the production of a silicone rubber component according to one or more of claims 1 to 7, a mixer (3) is provided, to which an extruder (5) is connected, wherein the extrudate is fed to a tunnel furnace (7) with two sections (7a, 7b), wherein in the first section (7a) pore formation takes place and in the second section (7b) vulcanization takes place.

9. Device for manufacturing silicone rubber components (1) according to claim 8, characterized by that the tunnel kiln (7) in the first section (7a) has a temperature of 100° to 300° C, in particular 180°C to 250°C.

10. Device for the production of silicone rubber components (1) according to one of claims 8 to 9, characterized by that The second section (7b) for vulcanization has a temperature between 90°C and 250°C.

11. Device for manufacturing silicone rubber components (1) according to claim 8, characterized by that A tempering unit (13) is located downstream of the tunnel furnace (7).

12. Device for the production of silicone rubber components (1) according to any one of claims 8 to 11, characterized by that the tunnel furnace (7) has at least one infrared emitter for generating the required heat in each section.

13. Device for the production of silicone rubber components (1) according to any one of claims 8 to 12, characterized by that the arrangement of several infrared emitters in the tunnel oven (7) is provided, wherein the arrangement of the infrared emitters is chosen such that the silicone raw component is irradiated from all sides.

14. Device for manufacturing silicone rubber components (1) according to claim 12 or 13, characterized by that The infrared emitter(s) operate in a wavelength range of 3 to 50 µm.