DE102010041635A1 - Impregnated cellulosic material, use of this cellulosic material and process for its preparation - Google Patents

Abstract

The invention relates to a cellulose material consisting of cellulose fibers (12) which are impregnated. According to the invention, an impregnation (13) of the cellulose fibers made of electrically conductive polymers, such as in particular PEDOT: PSS, is provided. The impregnation forms a type of network (14) which, due to the electrical conductivity of this network, can reduce the specific resistance of the cellulose material. As a result, the cellulose material can be advantageously adapted to corresponding applications in terms of its electrical properties. One use of the cellulose material is therefore also the electrical insulation of transformers, the cellulose material being soaked with transformer oil and an adaptation of the specific resistance of the cellulose material to the specific resistance of the oil leads to an improved dielectric strength of the transformer insulation. In addition, the invention relates to a method for producing the cellulose material described above, which has a suitable impregnation step for the cellulose material.

Description

The invention relates to a cellulosic material which is provided with an impregnation which increases the electrical conductivity of the cellulosic material. Moreover, the invention relates to a use of this cellulosic material. Finally, the invention also relates to a process for producing this cellulosic material.

From the US 4,521,450 It is known that a impregnable solid material made of cellulose fibers in an aqueous oxidizing agent, such as. As a weakly acidic solution of iron (III) chloride solution, cerium (IV) sulfate, potassium hexacyanoferrate (III) or molybdatophosphoric acid can be immersed. Subsequently, the wet cellulosic material is treated with either liquid or vapor pyrrole compounds at room temperature until the pyrrole is polymerized depending on the concentration of the oxidizing agent. The thus impregnated cellulosic material is dried at room temperature for 24 hours. The oxidizing agent ensures on the one hand for the polymerization of the pyrrole compounds, in addition to an increase in electrical conductivity. The specific resistance ρ of such impregnated cellulosic materials can thus be influenced by the concentration of pyrroles and the type of oxidizing agent. In the preparation of the impregnated cellulose material, the toxic effect of pyrrole must be taken into account by appropriate working conditions and appropriate waste disposal.

In general, the use of electrically conductive polymers, for example, from DE 10 2007 018 540 A1 known to use transparent, electrically conductive layers, for example for the heating of automotive glass. Examples of such electrically conductive polymers are polypyrroles, polyaniline, polythiophenes, polyparaphenylenes, polyparaphenylene-vinylenes and derivatives of said polymers. A specific example of such polymers is PEDOT, which is also sold under the trade name Baytron by Bayer AG. PEDOT is also referred to by its systematic name as poly (3,4-ethylene dioxythiophene).

The object of the invention is to provide a cellulose material with an electrical conductivity of this cellulose material enhancing impregnation or a method for its production, which allows a simplified production. The object of the invention is also to specify a use for this cellulosic material.

This object is achieved by the above-mentioned cellulose material in that the impregnation consists of a polymer which is crosslinked from a negative ionomer, in particular PSS, and a positively charged ionomer. As the cellulosic material, materials of all known forms can be used. Cellulosic materials are preferably produced as paper, paperboard or pressboard. These cellulosic materials may be semi-finished products for technical components, which are advantageously used in the impregnated variant. As positively charged ionomers preferably PEDOT or PANI can be used. PEDOT refers to the already mentioned poly (3,4-ethylene-dioxydthiophene). PANI is polyaniline and PSS is polystyrene sulfonate.

The use of negatively charged and positively charged ionomers advantageously makes it particularly easy to produce the cellulosic material. The ionomers can be easily dissolved in water and thus fed to the process of making the cellulosic material, which is also water-based. By crosslinking the ionomers following preparation of the cellulosic material, the resistivity of the cellulosic material can be lowered. The ionomers polymerize and form in the cellulosic material an electrically conductive network, which is responsible for the reduction of the specific resistance. Advantageously, the process of production with relatively non-toxic substances, such as PEDOT, PANI and PSS, are performed, so that in comparison to the use of pyrroles much lower demands on the process reliability must be made. In addition, no toxic waste, whose disposal would mean an additional expense.

According to an advantageous embodiment of the invention, it is provided that the specific resistance ρ _{p of} the impregnated cellulosic material is 10 ¹² Ωcm. This has the advantage, for example, when using the cellulosic material in oil, that the specific resistance of the cellulosic material is comparable to that of the oil on the order of magnitude, so that during operation dielectric stress spreads evenly over the transformer oil and the insulating material. Therefore, according to the invention, a solution to the problem is provided in that the cellulosic material having an electrical conductivity of the cellulosic material enhancing impregnation, which consists of polymer, which is crosslinked from a negatively charged ionomer and a positively charged ionomer, is used as insulation material for a transformer.

The cellulosic material is preferably flat and provided over the entire thickness with an at least substantially constant concentration of the polymer intended for impregnation. This can be achieved that the specific resistance in particular the use of the cellulosic material as an electrical insulation in oil over the entire cross-section of the insulation produces a uniform voltage drop (in addition to the ratios present in the electrical insulation in transformers).

Furthermore, the object is achieved by a method for producing an impregnated cellulosic material, in which an impregnation with organic substances takes place in the following steps. First, an aqueous electrolyte is prepared from a positively charged ionomer and a negatively charged ionomer, particularly PSS. Cellulose fibers are added to this electrolyte. This results in a pulp, the starting material for a production of paper, which is drawn from the pulp. Alternatively, the cellulose fibers can also be soaked with the electrolyte. This presupposes that a raw material containing cellulose fibers is already present, which is preferably dry or in dehydrated state, so that the electrolyte can be supplied to this intermediate product. In the next step, the water of the electrolyte is at least so far removed that the cellulosic material is formed. By this is meant that the cellulosic material already forms a handleable dressing, which can be used as a basis for further processing. This is done by dewatering, for example, as known from papermaking, by draining the pulp on a sieve. Finally, the ionomers are crosslinked. For this purpose, preferably a heat treatment above the crosslinking temperature of the respective ionomers is required. This forms the already mentioned above network of polymers, which is electrically conductive, and therefore reduces the resistivity of the cellulosic material.

According to an advantageous embodiment of the method is provided that are used as positively charged ionomers PEDOT and / or PANI. The advantages associated with the selection of these ionomers have already been explained above.

Furthermore, it can be advantageously provided that the removal of the water before the crosslinking of the ionomers is supported by a rolling of the cellulosic material. This is particularly advantageous in a continuous production of the cellulosic material, since a long web can be produced by rolling the cellulosic material. It is furthermore advantageous if, after crosslinking of the ionomers, complete drying of the cellulosic material takes place. By the complete drying, the required value for the resistivity can be achieved, whereby advantageously a comparatively precise adjustment of the specific resistance over the concentration of the impregnating substance is possible.

It is also advantageous if the removal of the water and / or the crosslinking of the ionomers is effected by pressing heated rollers against the cellulosic material. Due to the contact with the heated rollers, the heat can advantageously be entered into the cellulosic material particularly effectively. In this case, the necessary crosslinking temperature can be set. By heating the cellulosic material it comes at the same time to evaporate the residual water and thus to a drying process. This can be at least initiated by the heated rollers, with a final drying can also be done for example in a heat chamber.

To be able to produce cellulosic materials of greater thickness, it can be advantageously provided that the cellulosic material is produced by layers of several previously impregnated layers. This makes it possible to ensure that the individual layers of cellulose material are so thin that impregnation is made possible at least substantially over the entire thickness of the layer. This is also possible if the layers produced are soaked in the manner described above by the electrolyte and not the individual cellulose fibers are impregnated. In order subsequently to come to a cellulose material of a greater thickness, this is layered after the treatment with the electrolyte to a multilayer cellulose material. In this case, crosslinking and / or drying can already be started before the layers are layered. It is advantageous, however, to complete the cross-linking only after the layers have been layered, since then a cross-linking of the polymers of different layers can take place with one another, so that the network of polymers already described is formed across all layers. As a result, the resistivity of a multilayer cellulosic material can advantageously be increased to a relatively high degree with comparatively little polymer material.

The cellulosic material according to the invention was prepared according to an embodiment under laboratory conditions, the process being explained in more detail below. A commercially available pressboard was used (hereinafter referred to as "cellulose state of delivery"). This was first cut into 90 x 50 mm pieces with a thickness of 3.1 mm. These were heated in distilled water at temperatures between 95 and 99 ° C with stirring by means of a magnetic stirrer until individual layers began to dissolve in the edge region of the press chip. At this stage, the pressboard was completely soaked with water. The wet pressboard was out taken from the water and separated into its individual layers. The separated layers were reheated in distilled water at 95-99 ° C with stirring until more single leaves dissolved. The individual layers and leaves were removed again from the water and separated to the thinnest separable layer. The very thin, mechanically no longer separable layers were heated with stirring in distilled water (temperature see above) until individual cellulose fiber was present.

In a next step, the filtration of the resulting pulp was carried out to thin fabric layers. The individual cellulose threads were filtered off, with the aid of a Buchner funnel under application of a negative pressure. The filter paper used was a black belt filter from Schleicher & Scholl No. 589 or 595. The fabric layers thus obtained still contained from 270 to 300% of water, based on the original weight of the pressboard used. The fabric layers were easily separated from the black band filter.

In a next step, the individual fabric layers were dissolved in individual cellulose threads in an aqueous solution containing one percent by weight PEDOT: PSS solution while stirring with a magnetic stirrer at room temperature. These were impregnated with PEDOT: PSS during the stirring process. After one hour of stirring, the impregnated cellulose threads were filtered off under the principle already described under reduced pressure on a black belt filter. The resulting fabric layer was easily detached from the black belt filter. The filtered PEDOT: PSS solution was collected in a feeding bottle, and after a recovery of the required concentration of PEDOT: PSS this solution could be recycled.

In a next step, the impregnated fabric layers should be smoothed by rollers. For this purpose, the individual fabric layers were superimposed and pressed together slightly with a flat object. Subsequently, the stack of impregnated fabric layers was compressed several times with increasing pressure by a roller. The individual fabric layers were compacted to an impregnated fiber felt, with excess liquid was pushed out. The fabric stack was compacted until the thickness of the fiber felt obtained was about 4-4.5 mm.

In a next step, the polymers should be crosslinked and carried out a drying of the cellulosic material. For this purpose, the remaining water was removed by evaporation in a drying oven between steel plates under pressure. The temperature for drying was chosen so that initially a cross-linking of the polymer took place with each other. For this purpose, the impregnated fiber felt was placed between steel plates. The steel plates were compressed at a pressure of 2.4 KPa. The contact surfaces with which the impregnated fiber felt came into contact were coated with Teflon in order to prevent caking of the unpolymerized starting materials to the metal plates. The crosslinking of the polymer was carried out at 82 ° C and took between 30 and 90 minutes. Once crosslinking was complete, metal bearing surfaces were used for final drying. The final drying was carried out at 104 ° C and a pressure of 4.22 KPa and was carried out until the weight and the thickness of the cellulose material did not change.

The cellulosic materials produced under laboratory conditions gave the following advantages. By impregnating the cellulose material with PEDOT: PSS or with PANI: PSS, the electrical properties could be controlled so that the specific resistance of the cellulosic material could be changed. The apparatus construction could be kept relatively low compared to the use of pyrrole compounds because of the toxic safety of the polymers used.

Further details of the invention are described below with reference to the drawing. Identical or corresponding drawing elements are each provided with the same reference numerals in the individual figures and will only be explained several times to the extent that differences arise between the individual figures. Show it:

1 Cellulose fibers surrounded by a network of polymers according to an embodiment of the cellulosic material according to the invention in three-dimensional view,

2 3 shows a schematic section through a multilayer cellulose material according to another embodiment of the cellulose material according to the invention,

3 an embodiment of the cellulosic material according to the invention in section, as this is used as insulation in a transformer, wherein the applied voltages are shown schematically on the insulating material,

4 an embodiment of the method according to the invention, shown schematically by a manufacturing plant as a side view and

5 graphically the resistances achieved different embodiments of the cellulosic material according to the invention.

According to 1 is a cellulosic material through two cellulose fibers 12 represents on which polymers 13 a fragmented network 14 form. This network 14 is obtained by polymerizing the polymer only after impregnation of the cellulose fibers 12 and preparation of the cellulosic material. The network 14 Penetrates the cellulosic material, so that an electrically conductive connection of the electrically conductive polymer is ensured. Therefore, the network decreases 14 of the polymer 13 the resistivity of the cellulosic material in the manner described above.

In 2 It can be seen that a cellulosic material also consists of several layers 13 can be constructed. In this case, these are layers whose impregnation has taken place only after their production. Therefore, it can be seen that the network 14 from the polymers in each case only near the surface of the layers 13 is because the electrolyte with which the layers 13 were soaked, only had penetrated into the surface of the individual layers. However, polymerization of the polymer did not occur until the layers 13 already joined to the cellulosic material, so that the forming networks extend over the layers and so on the one hand to a better cohesion of the cellulosic material and on the other hand contribute to a reduction of the resistivity of the cellulosic material.

An electrical insulation material 18 according to 3 consists of several layers of paper 19 as cellulosic material, between which are oil layers 20 lie. Also the papers 19 are soaked in oil, which is in 3 not shown in detail. This is in 3 within the papers the impregnation with BNNT 11 to recognize. The according to 3 insulation shown surrounds, for example, in a transformer there coming to use windings, which must be electrically insulated to the outside and each other.

The electrical insulation of a transformer must prevent electrical breakdowns in the event of an AC voltage being applied. In this case, the isolation behavior of the insulation depends on the permittivity of the components of the insulation. For oil, the permittivity ε _{o is} approximately 2, for the paper ε _p at 4. When the insulation is subjected to an alternating voltage, the load on the individual insulation components results in the voltage U _o applied to the oil being approximately twice as high , like the voltage applied to the paper U _p . If the nanocomposite according to the invention is used, in which the paper 19 in the in 3 is impregnated with BNNT, the BNNTs do not influence the stress distribution in the insulation according to the invention since the permittivity ε _{BNNT is} also approximately 4 and therefore the permittivity ε _{comp of} the impregnated paper is also approximately 4. Thus, even with the insulation according to the invention, the voltage U _o applied to the oil is approximately twice as great as the voltage U _comp applied to the nanocomposite (paper).

If faults occur in the transformer, the dielectric strength of the insulation when DC voltages are present can also be significant. The distribution of the applied voltage to the individual insulation components is then no longer dependent on the permittivity, but on the resistivity of the individual components. The specific resistance ρ _o of oil is 10 ¹² Ωcm. In contrast, ρ _p of paper is three orders of magnitude higher and is 10 ¹⁵ Ωcm. This causes the voltage at the oil U _o is a thousand times the voltage on the paper U _p when DC voltage is applied. This imbalance involves the risk of breakdowns in the insulation when the insulation is subjected to DC voltage and electrical insulation fails.

The invention in the paper 19 introduced BNNT 11 be z. B. by a suitable coating of PEDOT: PSS and possibly by an additional doping with dopants with their specific resistance (between 0.1 and 1000 Ωcm) adjusted so that the specific resistance of the paper ρ _{p is} reduced. This makes it possible to set a specific conductivity ρ _comp for the composite according to the invention, which is approximated to the specific resistance ρ _o and ideally corresponds approximately to this. With a specific resistance ρ _comp of approximately 10 ¹² Ωcm, the voltage U _o applied to the oil is in the region of the voltage U _comp applied to the composite, so that a balanced voltage profile is established in the insulation. This advantageously improves the dielectric strength of the insulation, since the load on the oil is appreciably reduced.

In 4 is a production plant for a cellulose material in the form of a paper web 22 which is suitable for carrying out an embodiment of the method according to the invention. This system has a first container 23 for an electrolyte 24 in which in the electrolyte ionomers of PEDOT and PSS are included. In addition, from a reservoir 25 cellulose fibers 12 in the electrolyte 24 is trickled. In this way, a pulp in the electrolyte is in a conventional manner and therefore not shown in detail 24 made on a sieve-shaped treadmill 26 is deposited. This treadmill leads into a second container 27 where the electrolyte 24 can drain, resulting in the cellulose fibers an already partially drained mat is created. The electrolyte is pumped 28 a reprocessing plant 29 fed, where the required concentration of PEDOT and PSS is reset. The treated electrolyte can flow through an inflow 30 the first container 23 be supplied.

From the cellulose material obtained in the further course of the process, the paper web 22 produced. First, a further drainage via a pair of rollers 31 wherein also in this dehydration step released electrolyte in the container 27 is caught. Then the paper web happens 22 a next pair of rollers 32 , wherein a comparatively large wrap angle is achieved by the S-shaped guidance of the paper web around the pair of rollers. The pair of rollers is namely on the indicated heaters 33a heated, so that a heat transfer to the paper web is possible. This can also include additional heating 33b supportive use. About the heaters 33a . 33b The paper web is brought to polymerization temperature, so that the ionomers polymerize to PEDOT: PSS and forms the network already described above. This treatment also involves further drainage.

After polymerizing the ionomers can via another feed device 34 again electrolyte are applied to the paper web, the now largely dewatered paper web is absorbent enough so that the cellulose fibers can be soaked with the electrolyte. Subsequently, the paper web passes through 22 another pair of rollers 35 and is thereby drained again. Further dehydration and polymerization of the additionally introduced ionomers is via a pair of rollers 36 achieved, this in the pair of rollers 32 described manner via heaters 33a . 33b is heated.

Once the paper web 22 the roller pair 36 leaves, the paper web is largely drained. However, this still contains a residual content of water and is therefore a drying device 37 supplied and can be dried in this drying device as needed.

It should be noted that the specific resistance ρ of the produced paper web 22 not only depends on the content of PEDOT: PSS but also on the residual water content. If the paper web is to be used, for example, as electrical insulation in a transformer, it must be impregnated with oil and therefore may no longer contain any water. This is due to the subsequent drying in the drying device 37 sure. The drying device 37 may for example be designed as an oven.

From the cellulosic material according to the invention several samples have been prepared, which were examined for their specific resistance ρ. These investigations are in 5 shown. The specific resistance of the samples ρ is plotted in Ω · cm. First, the as-received cellulosic material was examined to obtain a reference value. Under the above laboratory conditions, a non-impregnated sample was further prepared and tested as well. Furthermore, samples were prepared using an electrolyte with PEDOT and PSS. To this electrolyte, in each case at a concentration of 1% by weight, a stock solution of PEDOT and PSS was fed, which in one case was characterized by an electrical conductivity of 1 × 10 ^-5 S / cm and in the other case by 1 S / cm.

The resistivity of the tested samples was respectively measured in the ambient air, after drying at 104 ° C for 6:30 hours, and after drying at 104 ° C for 96 hours. The columns in 5 are shown in the above order for each sample, which is also reflected in the legend at the right end of the illustration 5 can be removed.

If one first compares the non-impregnated sample with the cellulosic material in the delivery state, it is noticeable that the specific resistance of the non-impregnated sample is higher. This can be explained by the fact that pure water was used in the production of the non-impregnated sample under laboratory conditions, which can not be assumed for the cellulosic material as delivered. If one compares the impregnated samples with the unimpregnated sample, it becomes clear that the polymerized polymer PEDOT: PSS leads to a reduction of the specific resistance in the dried samples as well as in the moist sample. It should be noted that the concentration of PEDOT: PSS in the samples is still comparatively low. It is expected that an increase in the concentration of PEDOT: PSS in the samples will also lead to a further reduction in resistivity. In particular, by increasing the content of PEDOT: PSS, the cellulosic material in the dry state can be adjusted to an electrical resistivity of 10 ¹² Ω · cm. This is particularly advantageous for the above-mentioned reasons for the use of the cellulosic material as insulation of transformers when the cellulosic material is impregnated with oil. Namely, the specific resistance of transformer oil is also in the range of 10 ¹² Ω · cm.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• US 4521450 [0002]
• DE 102007018540 A1 [0003]

Claims (11)

Cellulosic material ( 19 . 22 ), which with a the electrical conductivity of the cellulosic material ( 19 . 22 ) impregnation is provided, characterized in that the impregnation of a polymer ( 13 ), which is crosslinked from a negatively charged ionomer, in particular PSS, and a positively charged ionomer. Cellulosic material ( 19 . 22 ) according to claim 1, characterized in that the positively charged ionomer is PEDOT or PANI. Cellulosic material ( 19 . 22 ) according to one of the preceding claims, characterized in that the specific resistance ρ _{p of} the impregnated cellulosic material ( 19 . 22 ) is 10 ¹² Ωcm. Cellulosic material ( 19 . 22 ) according to one of the preceding claims, characterized in that it is of planar design and over the entire thickness of an at least substantially constant concentration of the intended polymer for impregnation ( 13 ) having. Use of a cellulosic material ( 19 . 22 ) with a the electrical conductivity of the cellulosic material ( 19 . 22 ) increasing impregnation consisting of polymer ( 13 ), which is crosslinked from a negatively charged ionomer, in particular PSS, and a positively charged ionomer, as insulating material ( 18 ) for a transformer. Process for producing an impregnated cellulosic material ( 19 . 22 ), in which an impregnation with organic substances takes place, characterized in that • an aqueous electrolyte ( 24 ) is made from a positively charged ionomer and a negatively charged ionomer, in particular PSS, cellulose fibers ( 12 ) the electrolyte ( 24 ) or with the electrolyte ( 24 ), • the water of the electrolyte ( 24 ) is removed at least so far that the cellulosic material ( 19 . 22 ) and • the ionomers are crosslinked. A method according to claim 6, characterized in that as a positively charged ionomer PEDOT or PANI is used. Method according to one of claims 6 or 7, characterized in that the removal of the water prior to crosslinking of the ionomers by rolling the cellulosic material ( 19 . 22 ) is supported. Method according to one of claims 6 to 8, characterized in that after the crosslinking of the ionomers, a complete drying of the cellulosic material ( 19 . 22 ) he follows. Method according to one of claims 6 to 9, characterized in that the removal of the water and / or the crosslinking of the ionomers by pressing heated rollers ( 29 . 31 . 32 . 36 ) is supported. Method according to one of claims 6 to 0, characterized in that the cellulosic material by layers of several previously impregnated layers ( 15 ) will be produced.

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