EP0041205B1 - Process for the manufacturing of filter materials - Google Patents

Description

The invention relates to a method for producing filter materials for oil and air filters for internal combustion engines by impregnating a porous paper substrate based on cellulose with a dispersion of at least one resin and / or monomer curable by electron beams and subsequent curing with electron beams.

Filters are required for various technical purposes, which on the one hand should have a high retention capacity and on the other hand are resistant to the medium to be filtered. In the modern air filters known today for cleaning the combustion air from engines, for example, star-folded filter papers with special impregnation are used. The structure of this filter paper is such that on the one hand it opposes the passage of air due to innumerable pores, but on the other hand only allows the smallest and therefore harmless dust particles to pass through because of the fineness of the pores. Filters with the same structure are also used as oil filters in internal combustion engines.

The filters were manufactured from porous paper webs that were impregnated with uncrosslinked phenolic resins. The crosslinking or curing of the phenolic resins takes place after soaking at an elevated temperature. This manufacturing process was very complex overall, since the resins were cured at temperatures in the order of 160 ° to 180 ° C. and with considerable residence times, so that curing systems of 30 to 50 meters in length are required. In addition, the phenolic resin vapors released during the curing process have a considerable environmental impact. The environmental protection problems essentially lie in the fact that the alcohol-soluble phenolic resins or phenolic resin vapors can reach the exhaust air or waste water.

The process was further complicated by the fact that the curing process could not be carried out in one process step, but had to be interrupted during the filter manufacture. The hardening or condensation process was interrupted to allow the paper to be folded or pleated. The final condensation was only carried out after the folding, in some cases with tears in the paper at the folded edges. This final condensation itself is necessary so that the paper no longer contains any volatile constituents that can migrate into the medium to be filtered. The precondensation is carried out to such an extent that the paper or the resin contained therein no longer sticks to one another and can therefore be processed.

Another problem in the manufacture of the filters is the fact that the phenol-impregnated and hardened filters are very difficult to bond to their abutting edges.

In order to remedy the disadvantages described above, EP-A-24703, which represents an earlier right under Article 54 (3) and (4) EPC, has already proposed a method in which the impregnating agent is a solution or dispersion of a resin and / or monomers based on acrylic acid and / or methacrylic acid esters with polyols. In addition, it is proposed to effect curing by electron radiation. According to this process, the phenolic resins are thus replaced by other resins, and the impregnated paper web is cured by electron radiation instead of the conventional heat curing in a drying channel.

The invention is therefore based on the object of specifying a method for producing filter materials for oil and air filters for internal combustion engines. These filter materials should be very economical to manufacture and enable high quality filters. The method is intended to avoid environmental pollution and to be able to be carried out with the lowest possible system expenditure.

The object is achieved in a method of the type mentioned in the invention in that the impregnation is carried out on a paper web in the paper machine with an aqueous emulsion with a resin and / or monomer content of 15 to 70 wt .-%, wherein the paper web consists of a predominantly long-fiber cellulose with a low freeness of 13 to 15 Schopper-Riegler.

In the method according to the invention, all that is required to carry out the method is the paper machine, which is anyway necessary for producing the paper web, and an installation for electronic radiation curing, the paper machine being provided at a suitable point with a device which permits impregnation of the paper web. The impregnation can be done through a commission, e.g. B. a size press can be performed. However, it is also possible to supply the impregnating agent by spraying on the paper web. An aqueous emulsion of an electronically curable resin is used as the impregnating agent.

From DE-A-2 551 479 it was already known to impregnate in the paper machine during paper production for the production of impregnated and surface-treated foils for the coating of chipboard and the like. The purpose of this known method is to create a paper in which the finished impregnated web has the lowest possible porosity, so that a dense paper is produced which is well suited for the subsequent coating of a particle board, for which a good appearance is required. The urea and / or melamine resins used are heat-curable and give the paper an extremely high hardness and tightness after drying. Papers of this type can therefore not be used as filter papers. As a rule, they are airtight and watertight and take on the character of a plastic with increasing resin content.

FR-A-2 241 384 has also made known a process for the production of impregnated papers, in which the impregnation can take place in the paper machine.

The curing can then be carried out using electron beams. This process is also not concerned with the production of porous paper for further processing into filters for internal combustion engines. Rather, the aim is to improve the wet strength of the paper, while still obtaining papers which have a relatively high water absorption capacity. The papers obtained in this way have good air permeability, but also a relatively high absorption capacity for water, which makes them unsuitable as filter papers for motor vehicle purposes.

Finally, from FR-A-1 359 241 a process for the production of impregnated papers is known, the paper machine being subjected to an impregnation with an aqueous emulsion of at least one resin and / or monomer that can be hardened by high-energy radiation in the paper machine and the impregnated paper web is then subjected to curing by means of β-rays. The manufactured product replaces certain fabrics and contains long fibers. The production of filter paper is not addressed.

The resin to be used in the process according to the invention is added to the paper web in the paper machine in the form of an aqueous emulsion. Resins and / or monomers based on acrylic acid and / or methacrylic acid esters with polyols are used. The emulsion itself has a resin content of 15 to 70% by weight. The use of an emulsion has the advantage that the resin particles can penetrate the paper very well.

The resin is attached to the paper in an amount of 10 to 30% by weight, preferably 20% by weight. The distribution of the resin should be even, so that the filter effect is even across the entire filter.

The preferred resin content in the paper for the filters is 20% by weight. For reasons of economy, the resin content should be as low as possible. The resin content can vary depending on the intended use of the filter material and the specified values such as burst pressure, rigidity of the filter, etc. With high strengths, a higher resin content of up to 30% is introduced, while with low strengths a resin content of up to 10% is sufficient. These statements also apply to papers for other areas of use. With a view to safe operation, no solvents should be used that could cause an explosion.

The resin components are fully cross-linked in the electron beam curing system. This is very important, as any non-cross-linked portions, as noted above, can be absorbed by the filtered medium. The content of soluble components in the finished filter should be as low as possible.

In the case of special types of filters, the production according to the invention has the advantage that the resins used are not soluble in alcohols. For example, in filters for internal combustion engines that are operated with new fuels, the components of which include alcohols, the alcohols can lead to the dissolution of conventional resins. For example, alcohol fuels can also remove uncondensed phenolic resin from the filters.

In order to achieve good porosity of the paper web and, above all, high strength, the paper web is made from a predominantly long-fiber cellulose. The proportion of long-fiber cellulose is preferably more than 90%. It is also favorable to use a cotton cellulose as cellulose.

The cellulose used has a low freeness of 13 to 15 Schopper-Regier. Due to the low degree of grinding of the cellulose used, which, as already mentioned above, is preferably cotton cellulose, a so-called cotton linter, the paper web and also the subsequent filter are given high strength. In order to increase the strength of the paper web, customary so-called wet-strength agents can also be added to the pulp.

The resin uptake by the paper web is preferably controlled via the initial moisture of the paper web when it enters the size press. The higher this initial moisture content, the lower the resin absorption. Furthermore, the speed of the paper web is used to control the resin uptake. At higher paper web speeds, smaller amounts of resin are taken up.

After radiation curing, the paper web is cut into individual sheets corresponding to the filter sizes. These sheets are folded, provided with an adhesive line on one edge and glued together. It should be noted that the porosity of the filter papers produced in this way is very remarkable. The less resin is used, the better the filter effect. However, the burst pressure is all the less. Depending on the purpose for which the filter is used, more or less resin can consequently be added to the paper web, i. that is, the strength of the paper web can be increased more or less.

It was found that the paper webs produced according to the invention can be subjected to folding without damage occurring in the paper web at the folded edges. Consequently, the folded air filters required for internal combustion engines can be produced particularly well from the paper web.

The filters produced in this way also have a number of other advantages; hold like that the filters put out a high pressure. For filters for combustion plants, for example, a burst pressure of 2.2 to 2.5 bar is required. The lower limit is generally considered to be 1.8 bar. Filters manufactured using the new process have burst pressures that are significantly higher.

While the filters previously produced from phenol generally had a brown color, the filters produced by the method according to the invention have no coloration due to the method, i. that is, the filters are white. This is very cheap, for example, if the color alone is used to determine whether the filter was already in use or not.

In the context of the invention, aqueous emulsions of monomers or resins which can be hardened by electron radiation or other high-energy radiation are used as impregnation agents. Particularly suitable monomers are polyacrylated and / or polymethacrylated polyols, polyacrylated polyols being preferred because of the higher reaction rate. Suitable resins for preparing the emulsions are, for example, the resins and / or monomers proposed in the aforementioned EP-A-24703.

• Trimethylolpropan, Pentaerythrit bzw. Pentaerythritol, Hexandiol, insbesonders Hexan-1, 6-diol und Polyäthylenglykole oder Propylenglykole, wie Triäthylenglykol oder Tripropylenglykol.

Polyols on which the monomers which can be used according to the invention are based can be straight or branched chain and one or more heteroatoms, e.g. B. in the form of oxygen bridges. They preferably have 2 to about 10, in particular 5 to 9, and particularly preferably ₅ or 6 carbon atoms. They preferably contain 2 to about 5 and in particular 2, 3 or 4 hydroxy functions. Preferred examples of such polyols are:

• Trimethylolpropane, pentaerythritol or pentaerythritol, hexanediol, in particular hexane-1, 6-diol and polyethylene glycols or propylene glycols, such as triethylene glycol or tripropylene glycol.

The hydroxyl groups of the polyols mentioned can be wholly or partially esterified by acrylic acid and / or methacrylic acid.

Specific examples of the present invention usable monomers are trimethylolpropane triacrylate, hexanediol diacrylate, pentaerythritol tetraacrylate, pentaerythritol triacrylate, hexane-1, 6-diol diacrylate, Diäthylenglykoldiacrylat, Triäthylenglykoldiacrylat, Tetraäthylenglykoldiacrylat, tripropylene glycol diacrylate, 2-Hydroxäthylacrylat, 2-Hydroxypropoylacrylat, Hexandiolmonoacrylate and butandiolmonoacrylate and the corresponding methacrylates or mixed acrylate Methacrylates.

The monomers can be used alone or in a mixture, if appropriate with high or low molecular weight prepolymers.

Particularly suitable monomers are pentaerythritol triacrylate or pentaerythritol acrylate mixtures with an average degree of esterification of 3 to 3.3 and with a viscosity of 600-1 200 mPa.s (measured at 20 ° C. with the Brookfield viscometer, type RVT).

High or low molecular weight resins or oligomers are suitable as resins. Examples of these are soluble polyurethane acrylates formed from the aforementioned hydroxy-functional acrylate monomers.

The molecular weight of such resins or oligomers is preferably in the range from 800-8,000 and their viscosities go from 1,000-50,000 mPa.s. The content of acrylic and / or methacrylic unsaturated monomer units is, for example, 2-6 per molecule.

Resins of this type are produced by customary procedures, for example as described in DE-OS-2 530 896 and DE-OS-2 542 314. For example, a polyol is reacted with a di-isocyanate. The content of free NCO groups reached is then saturated with hydroxy-functional acrylate or methacrylate monomers. The person skilled in the art can choose amounts and reaction conditions such that the desired molecular weights and the desired content of acrylic and / or methacrylic unsaturated monomer units are achieved. The viscosities can be varied, for example, by appropriate dilution of the resins with monomers.

• Isocyanate wie :
  • a) 4,4-Diphenylmethandiisocyanat
  • b) Toluylendiisocyanat
  • c) Hexamethylendiisocyanat
  • d) 4,4-Dicyclohexylmethandiisocyanat
• Polyole wie:
  • a) Polypropylenglykole, Polyäthylenglykole mit den Molekulargewichten 400, 1 000, 2000, 3 000, 4000
  • b) Polyester auf der Basis aliphatischer und aromatischer Dicarbonsäuren, vorzugsweise Adipinsäure und Sebacinsäure und difunktionellen aliphatischen Alkoholen, wie Äthylenglykol und Neopentylglykol, Diäthylenglykol, Hexandiol-1,6 sowie polyfunktionellen Alkoholen wie Trimethylolpropan und Pentaerythritol.
    • Hydroxyfunktionelle, mit NCO-Gruppen reaktive Acrylatomonomere und Methacylatmonomere wie :
    • 2-Hydroxyäthylacrylat
    • 2-Hydroxyathylmethacrylat
    • 2-Hydroxypropylacrylat
    • 2-Hyd roxypropylmethacrylat
    • Butandiolmonoacrylat
    • Hexandiolmonoacrylat
    • Pentaerythritoltriacrylat

The following raw materials can be used as starting materials for the polyurethane acrylates which can be used, for example, as resins or oligomers:

• Isocyanates such as:
  • a) 4,4-diphenylmethane diisocyanate
  • b) tolylene diisocyanate
  • c) hexamethylene diisocyanate
  • d) 4,4-dicyclohexylmethane diisocyanate
• Polyols such as:
  • a) Polypropylene glycols, polyethylene glycols with the molecular weights 400, 1,000, 2,000, 3,000, 4,000
  • b) polyester based on aliphatic and aromatic dicarboxylic acids, preferably adipic acid and sebacic acid and difunctional aliphatic alcohols, such as ethylene glycol and neopentyl glycol, diethylene glycol, 1,6-hexanediol and polyfunctional alcohols such as trimethylolpropane and pentaerythritol.
    • Hydroxy-functional acrylic atoms and methacylate monomers reactive with NCO groups such as:
    • 2-hydroxyethyl acrylate
    • 2-hydroxyethyl methacrylate
    • 2-hydroxypropyl acrylate
    • 2-hydoxypropyl methacrylate
    • Butanediol monoacrylate
    • Hexanediol monoacrylate
    • Pentaerythritol triacrylate

• Hexandiol-1,6-diacrylat
• Butandiol-1,4-diacrylat
• Triäthylenglykoldiacrylat ,
• Tetraäthylenglykoldiacrylat
• Dipropylenglykoldiacrylat
• Tripropylenglykoldiacrylat
• Tetrapropylenglykoldiacrylat

Depending on the required viscosity, the resins produced are diluted with monomers to concentrations of 40-80% with:

• Hexanediol 1,6-diacrylate
• Butanediol 1,4-diacrylate
• Triethylene glycol diacrylate,
• Tetraethylene glycol diacrylate
• Dipropylene glycol diacrylate
• Tripropylene glycol diacrylate
• Tetrapropylene glycol diacrylate

The acrylic or methacrylic, unsaturated monomers and / or oligomers explained above on the basis of examples are processed into oil-in-water (O / W) emulsions using emulsifiers. The total solids content of these emulsions, which mainly consists of the monomer and / or oligomer together with the emulsifiers, is 15 to 70% by weight, preferably 20 to 60% by weight and in particular approximately 40% by weight, based on the finished emulsion. For reasons of stability, the particle size of the resulting emulsion is <1 μm and preferably 0.4 to 0.6 μm. This particle size is achieved by conventional measures, such as the selection of suitable emulsifiers and emulsification conditions.

Suitable emulsifiers for the emulsions used according to the invention are in principle nonionic emulsifiers, anionic and cationic emulsifiers, alone or in a mixture with one another.

Surprisingly, however, unexpectedly good results with regard to particle size and stability were found when using nonionic emulsifiers.

For example, emulsions with a particle size of <1 µ, m and a heat storage stability of 2 to 6 months at temperatures of 30 to 60 ° C were produced.

The amount of emulsifier used depends on the monomers and / or oligomers used and on the emulsifying conditions. In general, 1 to 7% by weight, preferably 1 to 3% by weight, of the emulsifiers or mixtures thereof, calculated on the inner phase or the total solids content of the OIW emulsion, is required.

• Alkylarylpolyglykoläther, wie z. B. Octylphenolpolyätheralkohole mit 10 bis 40 Mol Äthylenoxid pro Molekül (z. B. 16 oder 40 Mol/Molekül), Nonylphenolpolyätheralkohole mit 10 bis 40 Äthylenoxfd/Molekül (z. B. mit 35 Mol/Molekül) sowie äthoxylierte Polypropylenglykol-Blockpolymerisate mit mehr als 40 Mol Äthylenoxid/Molekül. Hiervon besonders geeignet sind die äthoxylierten Nonylphenole.

Nonionic emulsifiers, alone or in a mixture with anionic emulsifiers, are preferably used. Examples of such nonionic emulsifiers are:

• Alkylaryl polyglycol ethers, such as. B. octylphenol polyether alcohols with 10 to 40 moles of ethylene oxide per molecule (e.g. 16 or 40 moles / molecule), nonylphenol polyether alcohols with 10 to 40 moles of ethylene oxide / molecule (e.g. with 35 moles / molecule) and ethoxylated polypropylene glycol block polymers with more than 40 moles of ethylene oxide / molecule. Of these, the ethoxylated nonylphenols are particularly suitable.

• a) Alkylphenoläthersulfate
• b) die Alkalisalze der Sulfobernsteinsäure.

Mixtures of the above-mentioned, nonionic emulsifiers with anionic emulsifiers such as, for. B.

• a) Alkylphenol ether sulfates
• b) the alkali salts of sulfosuccinic acid.

• a) o-Phenylphenol
• b) p-Chlor-m-kresol
• c) Cloracetamid
• d) p-Hydroxybenzoesäureester
• e) 1,2-Benzisothiazol-3-on
• f) Kupfer-8-hydroxychinolin
• g) Thiocarbamate

In addition, an effective preservative, e.g. B. a fungicide, but this must not interfere with the stability of such an O / W emulsion. The usual compounds used for this purpose are suitable as preservatives, e.g. B.:

• a) o-phenylphenol
• b) p-chloro-m-cresol
• c) Cloracetamide
• d) p-hydroxybenzoic acid ester
• e) 1,2-benzisothiazol-3-one
• f) copper-8-hydroxyquinoline
• g) thiocarbamates

Amounts of 0.02-0.4% by weight have proven successful, but particularly amounts of 0.1-0.3% by weight, based on the total batch, have proven successful.

• a) Tributylphosphat und/oder
• b) Organopolysiloxanen
• c) Gemische von Organopolysiloxanen mit aliphatischen Kohlenwasserstoffen.

It can also be beneficial to incorporate a highly effective defoamer into the emulsions that can prevent foaming in high-speed machines. Conventional compounds or mixtures, for example of, are particularly suitable as defoamers

• a) tributyl phosphate and / or
• b) organopolysiloxanes
• c) Mixtures of organopolysiloxanes with aliphatic hydrocarbons.

The amount used is preferably 0.05-0.5% by weight, based on the total batch, in particular 0.1-0.4% by weight.

• 1. Das Monomere und/oder Oligomere wird in einen Behälter mit Mischwerk und Hochleistungshomogenisator (Prinzip Stator/Rotor o. ä., Fa. IKA o. ä.) gegeben.
• 2. Der für das System optimale Emulgator, z. B. nichtionogene dem Monomer, Oligomergemisch zugesetzt.
• 3. Das Monomer und/oder Oligomergemisch wird mit dem Emulgator intensiv vermischt.
• 4. Bei weiterlaufendem Mischwerk wird nun deionisiertes Wasser zugesetzt. Die Emulsionsbildung erfolgt durch Phasenumkehr, die nach der Zugabe von ca. 40-70 % des Wassers eintritt.
• 5. Die Zugabe weiterer notwendiger Additive, wie Entschäumer und Fungizide findet nach der Phasenumkehr statt.
• 6. Nach der Zugabe aller benötigten Rezepturbestandteile erfolgt die Feinstverteilung der inneren Phase (Monomer/Oligomer) mit dem Homogenisator.

The impregnating resin emulsion used according to the invention can be prepared by any process suitable for the preparation of oil-in-water emulsions, which is the desired one Particle size leads. For example, the above-mentioned monomers and / or oligomers can be converted into an O / W emulsion as follows.

• 1. The monomer and / or oligomer is placed in a container with a mixer and high-performance homogenizer (principle stator / rotor or the like, from IKA or the like).
• 2. The optimal emulsifier for the system, e.g. B. non-ionic added to the monomer, oligomer mixture.
• 3. The monomer and / or oligomer mixture is mixed intensively with the emulsifier.
• 4. With the mixer still running, deionized water is now added. The emulsion is formed by reversing the phase, which occurs after the addition of approx. 40-70% of the water.
• 5. The addition of other necessary additives, such as defoamers and fungicides, takes place after the phase reversal.
• 6. After the addition of all required formulation components, the inner phase (monomer / oligomer) is finely distributed with the homogenizer.

As mentioned, the aqueous emulsions used according to the invention have a solids content (monomer and / or oligomer plus emulsifier and further additives or auxiliaries) of 15 to 70% by weight. The concentration of the monomer and / or oligomer or resin ultimately depends on the desired degree of coverage of the substrate with the monomer, oligomer or resin.

The fleece or paper materials impregnated according to the invention are hardened by electron radiation, as mentioned at the beginning. The unsaturated acrylate resin is cured by radial chain polymerization, which leads to ketene growth with a high three-dimensional degree of branching. For this purpose, there is a short-term irradiation with electron beams of a low dose. Electron or beta radiation is used. The radiation can be generated by conventional electron sources. In general, one or more stage electron accelerators are used, for example the electron accelerators described in EP-A-24703. The paper webs impregnated according to the invention are optionally passed under an inert gas, such as nitrogen, past a window from which the electron radiation emerges. The acceleration voltages of the single-stage or multi-stage electron accelerators used are, for example, about 150 to 500 kV, the radiation dose is, for example, about 0.1 to about 16 Mrad.

example 1

• a) The following is a special recipe for a resin / monomer mixture emulsifiable according to the invention:
  • 1 equivalent of polypropylene glycol MG 400
  • 2 equivalents of tolylene diisocyanate 80/20
  • 1 equivalent of 2-hydroxyethyl acrylate
  • The product obtained from this is dissolved 50% in hexanediol 1,6-diacrylate.
  • MG resin = 980 Viscosity: 600-1 200 mPa.s Functionality: 2
• b) An example of an emulsifiable monomer is: pentacrythritol triacrylate (average degree of esterification 3.3, viscosity 600-900 mPa.s).

Example 2

• a) Rezeptur :
  
  
• b) Herstellungsvorschrift

Preparation of an oligomer suitable for emulsification and impregnation.

• a) Recipe:
  
  
• b) Manufacturing instructions

Items 1 and 2 are placed in a 2 1 three-necked flask.

The flask is equipped with a condenser (water-cooled), an agitator with a patent plug, which allows the flask to be covered with nitrogen during the manufacturing process (because of the NCO / water reaction), as well as a thermometer for temperature control.

Items 1 and 2 are heated to 75 ° C with stirring for 2 hours.

Now the NCO number is determined, NCO target = 6.24 ± 0.1%.

When the NCO value is reached, 5-20 ppm nitrobenzene are added (thermal stabilizer for 4-hydroxybutyl acrylate) and item 3 is added. The temperature is now kept at 75 ° C. for 3 hours.

After 3 hours, the NCO number is determined, NCO target = 0.00%.

If there is no free NCO left, item 4 is added and mixed thoroughly. After mixing, the reactor contents are cooled to approx. 40 ° C and the resin is filled.

Example 3

• 1. 25,00-63,00 Teile Monomer und/oder Oligomer
• 2. 2,00- 5,00 Teile äthoxyliertes Nonylphenol
• 3.40,00-48,00 Teile deionisiertes Wasser
• 4. 0,01- 0,30 Teile 1,2-Benzisothiazol-3-on
• 5. 0,05- 0,500 Teile Polysiloxan

Recipe example for an emulsion:

• 1. 25.00-63.00 parts of monomer and / or oligomer
• 2. 2.00-5.00 parts of ethoxylated nonylphenol
• 3.40.00-48.00 parts deionized water
• 4. 0.01-0.30 parts of 1,2-benzisothiazol-3-one
• 5. 0.05-0.500 parts of polysiloxane

• a) erreichte Teilchengröße ≦1µm (durch Feinstverteilung der inneren Phase im Homogenisator)
• b) pH-Wert : 6-8
• c) Aussehen : milchig-weiss
• d) Viskosität : 10-250 mPa.s
• e) Festkörpergehalt : 30-65 %

The parts refer to the weight.

• a) particle size achieved ≦ 1µm (due to fine distribution of the inner phase in the homogenizer)
• b) pH: 6-8
• c) Appearance: milky white
• d) Viscosity: 10-250 mPa.s
• e) Solids content: 30-65%

Example 4

A paper web is made from a cellulose with a long fiber content of 80% in a paper machine. To impregnate the web, an aqueous emulsion according to Example 3 with a monomer content of 30% is filled into the size press of the paper machine. The paper web is passed through the size press and impregnated. At the end of the paper machine, an electron radiation curing system is set up, through which the paper web is passed. The result is an impregnated paper that has a resin content of 21% and is porous with high strength. The burst pressure is over 3 atmospheres. The resin is fully cured.

Claims (6)

1. A process for making filter materials for oil and air filters of internal combustion engines by impregnating a porous cellulose-based paper substrate with a dispersion of at least one resin and/or monomer which can be hardened by electron beam irradiation and subsequently hardening with electron beam irradiation, characterised in that the impregnation is conducted on a paper web in the papermaking machine with an aqueous emulsion having a resin and/or monomer content of 15 to 70 weight %, wherein the paper web consists predominantly of long-fibred cellulose which has a low beating degree of 13 to 15 SR°.

2. A process according to claim 1, characterised in that aqueous emulsions or resins and/or monomers based on esters of acrylic acid and/or of methacrylic acid with polyols are used.

3. A process according to one of claims 1 or 2, characterised in that the resin and/or monomer is added to the paper in a quantity of 10 to 30 weight %, preferably 20 weight %.

4. A process according to one of claims 1 to 3, characterised in that the proportion of long-fibred cellulose amounts to over 90 %.

5. A process according to one of claims 1 to 4, characterised in that the cellulose is cotton cellulose.

6. A process according to one of claims 1 to 5, characterised in that after the hardening by irradiation the paper web is cut up into individual sheets corresponding to the filter size and that the individual sheets are folded, provided with a strip of adhesive at one edge and adhered together.