Casting process for the production of organic synthetic leather for vehicle interiors and a casting frame

Abstract

The invention relates to a manufacturing method for bio-synthetic leather that enables a more homogeneous and compact structure of the bio-synthetic leather through mechanical post-processing of bacterial nanocellulose, in particular by centrifugation and degassing by shaking. The invention also includes a casting frame suitable for carrying out the method.

Description

[0001] The invention relates to a process for producing organic synthetic leather for vehicle interiors, a corresponding process product and a cast frame suitable for the process.

[0002] Due to their animal and petrochemical origins, leather and synthetic leather have specific disadvantages, which negatively impact their ecological footprint, particularly due to their large carbon footprint. Leather, the traditional upholstery material, poses a high environmental impact due to the chemicals used in tanning, particularly during its processing from raw leather to the final product. For example, the waste products from leather production, which often contain heavy metals, pose a serious and real environmental risk if they potentially contaminate wastewater. The polyvinyl chloride (PVC) used in PVC synthetic leather is a fossil, thus finite petrochemical resource and, as a composite material, is difficult to recycle. Plasticizers for petrochemical plastics can also have negative effects on human health.

[0003] In this respect, there is a need for sustainable alternative materials, such as organic synthetic leather, which can be produced from renewable raw materials and using microorganisms.

[0004] With regard to use in the automotive industry, particularly for vehicle interiors as covers for seats, steering wheels and gear levers, high quality requirements exist with regard to the durability, resilience and odor emissions of the materials.

[0005] However, these requirements are only inadequately met by currently available alternative materials.

[0006] An exemplary method for producing a bio-synthetic leather fabric is disclosed in patent WO 2022 / 177528 A1. This describes a wet coating process for microorganism-produced cellulose onto a substrate such as paper or woven fabric. The starting material, cellulose, is broken down, mixed with additives, and then applied to the substrate without further treatment, dried, and pressed.

[0007] The invention is based on the object of developing a process that makes bio-artificial leather with improved stress properties and low odor emission accessible.

[0008] According to the present invention, the process for producing organic synthetic leather for vehicle interiors comprises the following steps: S01 - Providing bacterial nanocellulose synthesized by cellulose-producing bacteria; S02 - Washing the bacterial nanocellulose with distilled water; S03 - mechanical grinding of the washed bacterial nanocellulose in distilled water to obtain a cellulose dispersion; S04 - Adding a plasticizer and a crosslinker to the cellulose dispersion; S05 - centrifuging the resulting mixture and discarding a liquid supernatant resulting from the centrifugation to obtain a casting mass; S06 - Pouring the casting compound onto a base element (20) of a provided casting frame (100); S07 - Shaking the filled casting frame (100); and S08 - multi-step drying of the casting mass at a temperature below 100 °C.

[0009] First, in step S01, bacterial nanocellulose (BNC) is provided as the starting material, which was synthesized using cellulose-producing bacteria. Such bacteria can be, for example, acetic acid bacteria (K. xylinus), which can produce bacterial nanocellulose from glucose as a carbon source, peptone and yeast extract as nitrogen sources, as well as sodium hydrogen phosphate and citric acid.

[0010] In step S02, the provided bacterial nanocellulose is washed with distilled water and then mechanically crushed in distilled water in step S03 to obtain a cellulose dispersion (cellulose suspension).

[0011] In step S04, at least one plasticizer and at least one crosslinker are added to the cellulose dispersion.

[0012] The cellulose dispersion containing plasticizer and crosslinker is centrifuged in step S05, and the supernatant liquid resulting from the centrifugation is discarded. The residue is used as the casting compound for the subsequent process. The casting compound therefore comprises comminuted bacterial nanocellulose, plasticizer, crosslinker, and water. Centrifugation not only reduces the water content of the cellulose dispersion, but also compacts the casting compound, homogeneously distributes the plasticizer and crosslinker, and reduces bubbles in the casting compound. Discarding the excess water also simply removes excess or unbound plasticizer and crosslinker, thus reducing undesirable odors in the final product.

[0013] In step S06, the casting compound is poured onto a base element of a provided casting frame. In other words, the base element of the provided casting frame is coated with the casting compound.

[0014] Subsequent shaking (vibration) of the filled casting frame in step S07 further reduces (air) bubbles in the casting compound. This is followed in step S08 by multi-step drying of the casting compound at a temperature below 100°C. Multi-step drying is understood to mean drying in several steps, each with different temperature levels, each below 100°C. The temperature range according to the invention has the advantageous effect of preventing potential bubble formation due to excessively rapid evaporation of the water. The combined measures to reduce bubbles, as well as the evenly distributed plasticizers and crosslinkers in the casting compound, result in a homogeneous and compact fiber composition, which results in high mechanical strength of the dried process product.

[0015] The plasticizer can preferably be selected from at least one of the following substances: glycerin, triacetin, polyethylene glycol, linseed oil, and castor oil. These plasticizers are considered toxicologically safe, making them suitable for vehicle interiors. The crosslinker can preferably be selected from at least one of the following substances: citric acid and tannic acid. The crosslinker can preferably be a polycarboxylic acid, for example, a tricarboxylic acid. A polycarboxylic acid is understood here to be an organic compound with a plurality of carboxyl groups, for example, with 3, 4, 5, or 6 carboxyl groups.

[0016] Preferably, in step S04, 20 wt.% bacterial nanocellulose, 5 wt.% to 10 wt.% glycerol, and 3 wt.% to 5 wt.% crosslinker in distilled water (65 wt.% to 72 wt.%) are added. The percentage by weight of bacterial nanocellulose refers to its dry mass. Fillers and fibers can also be added to the mixture. Examples of fillers include chitosan, polyvinyl alcohol, chalk, sawdust, and coffee bean particles. Examples of fibers include plant fibers such as hemp fiber, flax fiber, jute fiber, and cotton fiber. Fibers of animal origin, such as wool and leather scraps, are also preferred. The fibers can also comprise synthetic fibers of plant origin, such as viscose fiber and modal fiber.

[0017] Preferably, the average fiber length of the comminuted bacterial nanocellulose is between 2 µm and 20 µm. If the average fiber length is within the preferred range, not only are the flow properties of the casting compound improved, but it also promotes effective (i.e., clear and rapid) settling (sedimentation) of the casting compound after centrifugation, thus saving time and material.

[0018] Washing the bacterial nanocellulose in step S02 may include, in addition to washing with distilled water, neutralization with a basic solution, preferably with 0.1 M NaOH solution, with the final washing step being carried out with distilled water. In this way, synthesis residues such as acetic acid can be neutralized.

[0019] The multi-step drying of the casting compound in the casting frame in step S08 preferably comprises a first drying step S08a at room temperature and a subsequent second drying step S08b at 80°C to 98°C. The first drying step particularly preferably lasts 20 h to 24 h at room temperature. The first drying step at room temperature can also be carried out in air, i.e., in no oven and at normal pressure (hydrostatic pressure). The second drying step particularly preferably lasts 1 h to 2 h at 80°C to 98°C. The second drying step can also be carried out in an oven. The second drying step can preferably be carried out in a vacuum oven at reduced pressure, preferably at 60 mbar.

[0020] If the crosslinker requires activation temperatures higher than 100°C, the crosslinker can be activated after the multi-step drying in step S09 by heating the dried casting compound at above 100°C, preferably at 110°C to 190°C. For example, the crosslinker can be citric acid, and the crosslinker can be activated at 120°C to 190°C. Optionally, the process comprises a second washing step S10 with distilled water, which takes place after the activation of the crosslinker. Furthermore, the product washed in this way can again be subjected to a multi-step drying step. This can comprise a first drying step S011 at room temperature and a second drying step S012 at 60°C to 90°C.

[0021] In a further preferred embodiment of the invention, it is provided that the dried casting compound is detached from the floor element.

[0022] In a further preferred embodiment of the invention, it is provided that the dried casting compound is pressed with the floor element.

[0023] Another aspect of the present invention relates to a bio-artificial leather obtainable by the process described above. The bio-artificial leather may preferably comprise 40 wt% to 65 wt% bacterial nanocellulose, 25 wt% to 40 wt% glycerol and crosslinking agent, and 0 to 10 wt% water.

[0024] The resulting organic synthetic leather may further comprise a floor element pressed with the dried casting compound.

[0025] A further aspect of the invention relates to a cast frame for carrying out the method described above. The cast frame comprises a first frame element with an opening, a second frame element with an opening, and a base element which is detachably arranged between the first frame element and the second frame element and is accessible through the openings of the first and second frame elements. In addition, the cast frame comprises a plurality of fixing elements which are designed to fix the first frame element against the second frame element and the base element arranged therebetween. In other words, the fixing elements are designed to press the first frame element against the second frame element and thus to fix the (clamped) base element arranged therebetween as well as the first and second frames. The opening of the first frame element is shaped in such a way that a coating of a surface of the base element is possible.

[0026] The base element is preferably a textile mesh or a perforated plate. This has the advantageous effect of allowing excess water to drain from the casting compound. The base element is preferably liquid-permeable. The base element can preferably be designed such that the casting compound, or the organic synthetic leather, can be easily removed after drying. To facilitate even removal of the dried product, the material of the base element is preferably selected from polyamide, polyester, polyethersulfone, ceramic, glass, and metal.

[0027] For example, the textile mesh can be a fabric made of polyamide, polyester, or polyethersulfone. For example, the perforated plate can be made of a material selected from polyamide, polyester, polyethersulfone, ceramic, glass, and metal.

[0028] In a further preferred embodiment, the floor element can have structural elements on its surface, which are shaped to impart a structural embossing to the casting compound on its contact surface with the floor element, for example, to imitate the surface structure of genuine leather. If the floor element is pressed with the dried casting compound, the structural elements can improve the adhesion between the floor element and the dried casting compound.

[0029] Furthermore, the first frame element can comprise a locking receptacle and the second frame element can comprise a locking device that can be inserted into the locking receptacle. The locking receptacle and the locking device are designed to releasably connect the first frame element to the second frame element. The locking device can comprise, for example, a pin, a bolt, or a screw. The locking receptacle can comprise a bore or a bore with an internal thread, each of which is suitable for receiving the locking device. The first frame element can preferably comprise a plurality of locking receptacles and the second frame element can comprise a corresponding plurality of locking devices in order to increase the stability of the cast frame.

[0030] One of the plurality of fixing elements can comprise, for example, a clamp, preferably a spring clamp or screw clamp. The plurality of fixing elements are preferably designed to be detachable.

[0031] The various embodiments of the invention mentioned in this application can be advantageously combined with one another, unless stated otherwise in the individual case.

[0032] The invention is explained below in exemplary embodiments with reference to the accompanying drawings. They show: Fig. 1 a process diagram of an embodiment of the method according to the invention, Fig. 2 a process diagram of another preferred embodiment of the method according to the invention, Fig. 3A Images of the fiber structure of a mechanically untreated bacterial nanocellulose; image taken with a scanning electron microscope and Fig. 3B Images of the fiber structure of a bio-synthetic leather produced according to the invention; image taken with a scanning electron microscope, Fig. 4 a photograph of a bio-artificial leather produced according to the invention (right in the picture) and a bio-artificial leather produced without centrifugation in step S05 and shaking of the frame in S07 (left in the picture). Fig. 5 a schematic representation of the separate components of the casting frame 100 according to the invention (fixing elements not shown), Fig. 6 a schematic side view of an embodiment of the assembled cast frame 100: The base element 30 is arranged (clamped) between the first frame 10 and the second frame 30, which are connected by locks 32 embedded in locking receptacles 12 (fixing elements are not shown), Fig. 7 a schematic plan view of an embodiment of the assembled casting frame 100: The fixing elements 40 exert a force on the first frame 10, the base element 20 and the second frame 30 (not shown) so that they are pressed against each other and thus fixed.

[0033] In the context of the present invention, bio-synthetic leather is understood to mean a leather substitute that does not contain or consist of tanned animal hide. The basic material of bio-synthetic leather is biosynthetically produced materials, i.e., materials produced by microorganisms (e.g., bacteria or fungi), such as cellulose, especially bacterial nanocellulose.

[0034] Bacterial nanocellulose (BNC) is cellulose with a fiber diameter of up to 100 nm, produced by bacteria.

[0035] Bacterial nanocellulose can be produced using acetic acid bacteria (K. xylinus) on the surface of a nutrient medium. The nutrient medium consists, for example, of glucose as a carbon source (20 g / L), peptone (5 g / L), yeast extract as a nitrogen source (5 g / L), as well as sodium hydrogen phosphate (2.7 g / L), and citric acid (1.15 g / L). The bacterial nanocellulose can be produced in static or dynamic processes at an ambient temperature of 20 °C to 30 °C. The cultivation time for static cultivation at 30 °C is approximately 14 days until a layer thickness of 1.5 cm is reached. The bacterial nanocellulose can preferably be cultivated to a layer thickness of 4 cm to 5 cm. Dynamic cultivation is carried out with a uniform oxygen supply and a constant temperature of 30 °C over a period of 4 to 5 days to achieve a layer thickness of 1.5 cm.

[0036] Fig. 1 shows a process diagram of a preferred embodiment of the method according to the invention, according to which the bacterial nanocellulose is first provided in step S01.

[0037] In step S02, the bacterial nanocellulose is washed with distilled water. The distilled water preferably has a temperature of 80°C to 90°C to effectively remove residues from the synthesis of the bacterial nanocellulose without damaging it.

[0038] Subsequently, in step S03, the washed bacterial nanocellulose is ground in distilled water to obtain suitable flow / casting properties in a later step S06. This can be done using a homogenizer.

[0039] After comminution, at least one plasticizer and at least one crosslinker are added in step S04 and evenly distributed throughout the mixture. The crosslinker is preferably a polycarboxylic acid, for example a tricarboxylic acid such as citric acid.

[0040] In step S05, the mixture is centrifuged to separate the casting compound and excess water (possibly with excess unbound plasticizer or crosslinker) and to compact and degas the casting compound.

[0041] The separated casting compound is poured onto a base element 20 of a provided casting frame 100 in step S06 and the casting compound is then degassed by shaking the casting frame 100 in step S07.

[0042] Only then does the multi-step drying S08 take place, which preferably comprises air drying at room temperature S08a and subsequent oven drying at 90°C to 98°C S08b. The air drying in step S08a preferably takes place over 20 to 24 hours. The oven drying in step S08b preferably takes place over 1 to 2 hours.

[0043] The product is an organic synthetic leather with a compact and homogeneous fiber structure.

[0044] Fig. 2 shows a process diagram of a further preferred embodiment of the method according to the invention, according to which, after the previously described steps S01 to S08, an additional activation of the crosslinker in step S09, a subsequent washing step S10 and subsequent drying steps S11 and S12 are provided.

[0045] In drying step S08, the cast casting compound, after degassing by shaking the casting frame in step S07, is first pre-dried by air drying in step S08a at room temperature for preferably 20 h to 24 h in order to cause evaporation and drainage / drip-off of the unbound water.

[0046] Subsequently, in step S08b, the casting compound is dried in an oven at 80 °C to 98 °C for preferably 1 h to 2 h.

[0047] In step S09, the crosslinker is then activated at temperatures above 100°C, preferably between 110°C and 190°C, particularly preferably at 150°C. This means that the crosslinking reaction of the bacterial nanocellulose by the crosslinker particularly preferably takes place at 150°C. For example, citric acid can be activated as a crosslinker at temperatures between 120°C and 190°C.

[0048] After cooling, the mass is washed with distilled water at preferably 60 °C in step S10 and then dried first by air drying at room temperature in step S11, preferably for 20 h to 24 h, and then oven drying at 60 °C to 90 °C in step S12, preferably for 2 to 3 days, to obtain a compact and homogeneous organic synthetic leather.

[0049] In another exemplary embodiment of the method according to the invention, the provided bacterial nanocellulose is purified alternately with 0.1 M NaOH solution and distilled water in several washing steps, for example, in 2 to 6 washing steps (preferably 3 to 4), while stirring. The temperature of the corresponding washing medium is preferably 80°C to 90°C.

[0050] Subsequently, the synthesized bacterial nanocellulose is mechanically ground using a homogenizer in distilled water (30 wt%) for 5 min to obtain an average fiber length of 2 µm to 20 µm.

[0051] Then, preferably 20 wt% BNC (as dry matter), 5 wt% to 10 wt% glycerol and 3 wt% to 5 wt% crosslinker are added in distilled water (65 wt% to 72 wt%).

[0052] In another embodiment, 20 wt.% BNC (as dry matter), 5 wt.% to 10 wt.% glycerol, 3 wt.% to 5 wt.% crosslinker, and 3 wt.% to 5 wt.% polyvinyl alcohol in distilled water (60 wt.% to 69 wt.%) can be added. Polyvinyl alcohol serves to further improve grip.

[0053] The mixture of crushed bacterial nanocellulose and the additives is preferably mixed for 2 to 7 days at room temperature.

[0054] In the next step, the mixture is centrifuged preferably for 10 to 15 min at 4000 rpm to 6000 rpm, uncooled at room temperature (for example with Centrifuge Mikro 220R, Hettich).

[0055] The aqueous supernatant is discarded after centrifugation and after the casting mass has settled.

[0056] The casting compound is then transferred to a provided casting frame (mold) and dried therein. Depending on the layer thickness, the casting compound can be dried in air at room temperature for up to 24 hours to remove free, unbound water from the product (by dripping and evaporation). This is followed by oven drying at 80°C to 98°C. Oven drying is preferably carried out at 80°C to 98°C for 1 to 2 hours to achieve a residual water content of less than 10 wt.% in the dried BNC. Temperatures should remain below 100°C during this step to avoid additional blistering due to evaporation.

[0057] If the crosslinker requires an activation temperature higher than 100°C to crosslink the bacterial nanocellulose, crosslinking can be initiated after drying by heating to > 100°C, preferably at 110°C to 190°C, particularly preferably at 130°C to 150°C. The heating time at the aforementioned temperatures is preferably 30 min to 45 min. The crosslinked material is washed in distilled water for 10 min to 15 min at 50°C to 70°C. As an optional additional step, the crosslinked material can then be washed again with distilled water, preferably at 50°C to 70°C. The washed product can be dried by air drying and subsequent oven drying, preferably at 60°C to 90°C, particularly preferably at 70°C. The humidity in the oven may preferably be 50% to 80%, particularly preferably 62.5% at 70°C.Adjusting the humidity in the oven has the advantage that no cracks form on the surface of the material during the drying process.

[0058] The Fig. 3A and Fig. 3B are images of an untreated bacterial nanocellulose ( Fig. 3A) and a bio-synthetic leather produced by the process according to the invention ( Fig. 3B). The images were taken using scanning electron microscopy and show that the fiber structure of the inventive bio-synthetic leather is significantly more homogeneous and compact due to the uniform distribution of the plasticizer and crosslinker compared to untreated bacterial nanocellulose, thus achieving greater mechanical strength. Furthermore, the material is more transparent.

[0059] Fig. Figure 4 shows a comparison of a bio-synthetic leather produced according to the invention (right in the image) and a bio-synthetic leather (left in the image) produced without steps S05 and S07. The bio-synthetic leather produced according to the invention exhibits significantly fewer air pockets and a more compact, lamellar, and straight layering.

[0060] Fig. 5 shows the individual, unassembled components (10, 20, 30) of a preferred embodiment of the cast frame 100. The components correspond to the first frame 10 with an opening and with a plurality of locking receptacles 12, the base element 20 and the second frame 30 with an opening and with a plurality of locking devices 32. The fixing elements 40 are not shown.

[0061] As in Fig. As shown in Figure 6, the base element 20 is arranged between the first frame 10 and the second frame 30. The second frame 30 can be releasably connected to the first frame via the locking devices 32. For this purpose, the locking devices 32 of the second frame 30 are inserted into the locking receptacles 12 provided for this purpose in the first frame 10. This prevents the first frame 10 from being displaced laterally (i.e., orthogonally to the layer direction) relative to the second frame 30.

[0062] As shown in the top view of the casting frame 100 in Fig.7, the cast frame comprises a plurality of fixing elements 40 which exert a force on the first frame 10, the base element 20 and the second frame 30 (not shown here), so that they are pressed against one another and thus fixed. For example, the first frame 10 and the second frame 30 can be pressed against one another by means of a spring clamp as a fixing element 40, so that the base element 20 arranged therebetween and the first and second frames 10, 30 are fixed. The stability of the cast frame can thus be ensured by the plurality of fixing elements 40. In a preferred embodiment, the base element 20 is a textile, for example a textile net, which is arranged, preferably clamped, between the first frame 10 and the second frame 30 as described above. In this embodiment, the plurality of fixing elements 40 also ensure the tightening of the base element. List of reference symbols S01 Provision of bacterial nanocellulose S02 Washing with distilled water S03 Crushing in distilled water S04 Addition of the plasticizer and the crosslinker S05 Centrifuging and separating the casting mass S06 Casting in sprues S07 Shaking the casting frame S08 multi-step drying of the casting mass at a temperature below 100 °C S08a Air drying at room temperature S08b Oven drying at 80 °C to 98 °C S09 Crosslinking reaction at over 100 °C S10 Wash with distilled water at 60 °C S11 Air drying at room temperature S12 Oven drying at 60 °C to 90 °C 100 sprues 10 first frame element 12 locking receptacle 20 floor element 30 second frame element 32 Locking 40 Fixing element QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] WO 2022 / 177528 A1

[0006]

Claims

[1] A process for producing organic synthetic leather for vehicle interiors, comprising the following steps: S01 - Providing bacterial nanocellulose synthesized by cellulose-producing bacteria; S02 - Washing the bacterial nanocellulose with distilled water; S03 - mechanical grinding of the washed bacterial nanocellulose in distilled water to obtain a cellulose dispersion; S04 - Adding a plasticizer and a crosslinker to the cellulose dispersion; S05 - centrifuging the resulting mixture and discarding a liquid supernatant resulting from the centrifugation to obtain a casting mass; S06 - Pouring the casting compound onto a base element (20) of a provided casting frame (100); S07 - Shaking the filled casting frame (100); and S08 - multi-step drying of the casting mass at a temperature below 100 °C. [2] The method according to claim 1, wherein the bacterial nanocellulose is mechanically comminuted to an average fiber length of 2 µm to 20 µm. [3] The method according to any one of claims 1 or 2, wherein the multi-step drying in step S08 comprises a first drying step at room temperature and a second drying step at 80°C to 98°C. [4] Method according to one of the preceding claims, further comprising following step S08: S09 - Activation of the crosslinker by heating the dried casting compound above 100 °C, preferably between 110 °C and 190 °C. [5] The method of claim 4, further comprising following step S09: S10 - Washing the crosslinked casting compound with distilled water; following step S10: S11 - Air dry at room temperature; and following step S11: S12 - Dry at 60 °C to 90 °C. [6] Method according to one of the preceding claims, wherein the dried casting compound is detached from the base element (20) of the casting frame (100) or is pressed with the base element (20) of the casting frame (100). [7] Organic synthetic leather obtainable by the process according to any one of claims 1 to 6. [8] Casting frame (100) for carrying out the method according to claim 1, comprising: a first frame element (10) having an opening; a second frame member (30) having an opening; a floor element (20) detachably arranged between the first frame element (10) and the second frame element (30) and accessible through the openings of the first and second frame elements; and a plurality of fixing elements (40) designed to fix the first frame element (10) against the second frame element (30) and the floor element (20) arranged therebetween. [9] Casting frame (100) according to claim 8, wherein the floor element (20) is a textile net or a perforated plate. [10] Cast frame (100) according to one of claims 8 or 9, wherein the first frame element (10) comprises a locking receptacle (12) and the second frame element (30) comprises a locking device (32) which can be inserted into the locking receptacle (12).

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