EP0862671B1 - Process for preserving paper - Google Patents

Abstract

PCT No. PCT/EP96/05176 Sec. 371 Date May 21, 1998 Sec. 102(e) Date May 21, 1998 PCT Filed Nov. 22, 1996 PCT Pub. No. WO97/19224 PCT Pub. Date May 29, 1997A process is disclosed for preserving paper involving the following steps: (a) a plurality of paper layers in contact with one another at least in regions is simultaneously brought into contact with a preservative fluid containing polycondensates based on one or more hydrolytic polycondensation-capable compounds of elements M of main groups III-V and sub-groups II-IV of the Periodic Table; in said polycondensates, at least one organic group (G) with at least two carbon atoms and for each carbon atom at least one atom selected from hydrogen or fluorine and without polar substituents is bound, either directly via one of the carbon atoms or via a connecting group A, to at least 0.1% of the central atoms M; and (b) the paper thus treated with the preservative fluid is dried and/or hardened.

Description

The invention relates to a method for preserving paper and to a preserved paper obtainable by the method.

Although the destruction or decay of paper, not least due to environmental and environmental influences, is little in the public consciousness, it is a fact that this decay of paper causes damage in the billions annually worldwide, quite apart from that In some cases, material that is valuable in terms of culture and art history is irretrievably lost. The main reason for this is the sulfuric acid content caused by paper production, which leads to a slow but steadily progressing, catalytically induced hydrolysis of the cellulose structure.

Although there are deacidification processes that can slow down or hide this destruction of paper, it does stop the decay but does not achieve any solidification. Each leaf (e.g. a book) must be treated individually for consolidation. Therefore, the methods previously used for this purpose are very complex and also only of limited effectiveness.

There is therefore an urgent need for a process using paper or paper products, in particular (valuable) books, old (historical) documents (such as certificates, maps, etc.) but also works of art (drawings, etchings, ink engravings, art prints, etc.) and printed matter that is subjected to intensive use (eg books in libraries, banknotes) can be deacidified easily and effectively and at the same time solidified (hereinafter generally referred to as "preserved"). Although solidification is possible in principle by impregnation with organic polymerizable monomers and subsequent polymerization of the same, touching pages, for example of a book, stick irreversibly in the process.

EP-A-0 582 265 describes a process for the re-solidification of damaged paper, in which paper is treated with an isocyanate-containing solution or an isocyanate vapor. There is no sticking of the sides.

The object of the present invention is thus in particular to provide a process which, by simple impregnation, causes the paper to solidify without the paper layers touching one another, with less toxic and less volatile compounds than the isocyanates being able to be used. It should also be possible to effect a simultaneous deacidification by adding a deacidifying agent.

• (a) das gleichzeitige Kontaktieren einer Mehrzahl von sich zumindest in Teilbereichen berührenden Lagen (z.B. Blätter, Bögen oder Bahnen) aus Papier mit einer Konservierungsflüssigkeit, die Polykondensate auf der Basis von einer oder mehreren zur hydrolytischen Polykondensation befähigten Verbindungen der Elemente M der Hauptgruppen III bis V und der Nebengruppen II bis IV des Periodensystems der Elemente umfaßt, wobei in diesen Polykondensaten an mindestens 0,1% der Zentralatome M mindestens eine organische Gruppe G, die mindestens 2 Kohlenstoffatome, an die jeweils mindestens ein aus Wasserstoff- und Fluoratomen ausgewähltes Atom gebunden ist, umfaßt und keine polaren Substituenten aufweist, entweder direkt über eines der Kohlenstoffatome oder über eine Verbindungsgruppe A gebunden ist; und
• (b) die Trocknung und/oder Härtung des mit der Konservierungsflüssigkeit behandelten Papiers.

Surprisingly, it has been found that the above object can be achieved by a method for preserving paper, which comprises

• (a) the simultaneous contacting of a plurality of layers (e.g. sheets, sheets or webs) made of paper, at least in some areas, with a preserving liquid, the polycondensates based on one or more compounds of the elements M of the main groups III to which are capable of hydrolytic polycondensation V and the subgroups II to IV of the Periodic Table of the Elements, wherein in these polycondensates at least 0.1% of the central atoms M at least one organic group G, the at least 2 carbon atoms, is bonded to the at least one atom selected from hydrogen and fluorine atoms is comprised, and has no polar substituents, is bonded either directly through one of the carbon atoms or through a linking group A; and
• (b) drying and / or curing the paper treated with the preserving liquid.

In the present invention, the term “paper” is understood to mean products based on matted plant fibers, in particular based on cellulose, wood pulp, straw cellulose, rag pulp and waste paper pulp. For example, this term includes graphic papers (such as writing and printing papers). Of course, other products such as For example, kraft paper, cardboard and cardboard are treated, although in the latter cases effective preservation will not be necessary in most cases.

The above elements M from the main groups III to V and the subgroups II to IV of the periodic table of the elements are e.g. by at least one element selected from silicon, aluminum, boron, tin, zirconium, vanadium and tin, with Si, Al and Zr being preferred. In particular, preferably 75 to 100 and particularly preferably 90 to 100% of all central atoms of the polycondensates present in the coating composition are silicon, aluminum and / or zirconium atoms.

According to the invention, at least 0.1% (preferably at least 0.5% and in particular at least 1%) of all central atoms M present in the above polycondensates M are bound to organic groups G which have at least 2 (preferably at least 3 and in particular at least 5) carbon atoms each having at least one and preferably at least two atoms selected from hydrogen and fluorine atoms, and which have no polar substituents. These groups G are preferably partially fluorinated aliphatic groups, in particular alkyl and / or alkenyl groups. These groups can be, for example, those of long-chain, saturated or unsaturated fatty acids such as, for example, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, etc. These chains can have been introduced into the polycondensates, for example, by chelation of the corresponding acid with a starting compound of, for example, Al or Zr capable of hydrolysis. In this case, the above linking group A is thus a chelating group that belongs to the structure leads.

Examples of partially fluorinated groups G which can be used according to the invention (and are also preferred according to the invention) are groups which have aliphatic carbon atoms to which a total of 2 to 30 (preferably 3 to 25, more preferably 5 to 20 and particularly preferably 8 to 18) fluorine atoms are bound, and which (at least in the case of direct binding of G to M) are each separated from the central atom M by at least 2 atoms (preferably carbon atoms).

In the above-mentioned number of fluorine atoms which are bound to aliphatic carbon atoms, those fluorine atoms which are possibly bound in some other way, for example to aromatic carbon atoms, are not taken into account (for example in the case of C ₆ F ₄ ). The fluorine-containing group can also be a chelating ligand. It is also possible for one or more fluorine atoms to be located on a carbon atom from which a double or triple bond originates. In particular with regard to the accessibility of the corresponding starting substances (ie the (partially) hydrolyzable compounds of the elements M), fluorine-containing, non-hydrolyzable groups are preferred, which are preferably bonded to silicon atoms. Specific examples of such fluorine-containing, non-hydrolyzable groups are, for example:
CF ₃ CH ₂ CH ₂ -, C ₂ F ₅ CH ₂ CH ₂ -, C ₄ F ₉ CH ₂ CH ₂ -, nC ₆ F ₁₃ CH ₂ CH ₂ -, nC ₈ F ₁₇ CH ₂ CH ₂ -, nC ₁₀ F ₂₁ CH ₂ CH ₂ - and iC ₃ F ₇ O- (CH ₂ ) ₃ -.

Among these groups, nC ₆ F ₁₃ CH ₂ CH ₂ -, nC ₈ F ₁₇ CH ₂ CH ₂ - and nC ₁₀ F ₂₁ CH ₂ CH ₂ - are particularly preferred. Of course, other fluorine-containing groups G can also be used according to the invention, as can mixtures of different fluorine-containing groups G.

Fluorine-containing polycondensates, as can be used according to the invention, and their preparation are described in detail in WO-A-92/21729 (EP-A-587 667). This application also specifies specific compounds suitable for producing the polycondensates used according to the invention, in particular Si, Al and Zr.

At least 25% of the central atoms M of the polycondensates are preferably at least one (for example one or two) non-hydrolyzable or non-hydrolyzing organic group, including the above groups G. In the present context, “non-hydrolyzing” means a group which, in principle, is accessible to hydrolysis, but actually does not hydrolyze under the given conditions during the production of the polycondensates, the treatment of the paper and the subsequent drying and / or curing. Examples of the latter groups can also include, for example, the above chelating groups A. One or two (preferably one) non-hydrolyzable or non-hydrolyzing groups are preferably bonded to 50 to 100 and in particular 75 to 100% of the central atoms M in the polycondensates. These groups can be selected, for example, from alkyl (especially C _1-4 alkyl, such as methyl, ethyl, propyl and butyl), alkenyl (especially C _2-4 alkenyl such as vinyl, 1-propenyl, 2-propenyl and Butenyl), alkynyl (especially C _2-4 alkynyl, such as ethynyl and propynyl) and aryl (especially C _6-10 aryl, such as phenyl and Naphthyl), where the groups just mentioned or at least those which differ from group G may optionally have one or more substituents, such as chlorine, bromine, hydroxyl, alkoxy, epoxy, (optionally substituted) amino, etc. The above alkyl radicals also include the corresponding cyclic and aryl-substituted radicals, such as cyclohexyl and benzyl, while the alkenyl and alkynyl groups can also be cyclic and the aryl groups mentioned should also include alkaryl groups (such as tolyl and xylyl). Particularly preferred groups are groups H 'which have an (optionally multiply) unsaturated carbon-carbon bond and / or have a highly reactive functional group such as epoxy. Among the unsaturated groups, particular mention should be made of groups which have a (meth) acryloxy radical (in particular a (meth) acryloxy-C _1-4 -alkyl radical such as (meth) acryloxypropyl). The presence of such groups in the corresponding polycondensates has the advantage that after the paper has been treated with the preserving liquid, it can be cured twice, namely thermally or photochemically induced linking of the unsaturated organic residues by (radical) polymerization (or ring-opening polymerization in the case) of epoxy rings) and a thermal completion of the polycondensation (for example by elimination of water from still existing M-OH groups).

According to the invention, it is therefore preferred if 1 to 100, in particular 5 to 85 and particularly preferably 20 to 70, mol percent of the non-hydrolyzable or non-hydrolyzing organic groups (including groups G) and at least one carbon-carbon double or triple bond and / or have an epoxy ring.

The (partially) hydrolyzable starting compounds used to prepare the preservation liquid or polycondensates used according to the invention are those of elements of main groups III to V and subgroups II to IV of the periodic table. Of course, other hydrolyzable compounds can also be used, in particular those of elements of main groups I and II of the periodic table (e.g. Na, K, Ca and Mg) and subgroups V to VIII of the periodic table. However, the compounds just mentioned preferably make up no more than 10 and in particular no more than 5 mol percent of the hydrolyzable monomeric compounds used overall.

Examples of hydrolyzable groups in the starting compounds (which can not necessarily be used as monomeric compounds, but can already be used as corresponding precondensates of compounds of one or more of the elements M) are - depending in part on the central atom M - halogen (F, Cl, Br and I, especially Cl and Br), alkoxy (especially C _1-4 alkoxy, such as methoxy, ethoxy, n-propoxy, i-propoxy and butoxy), aryloxy (especially C _6-10 aryloxy, e.g. phenoxy), acyloxy (especially C _1-4 acyloxy, e.g. acetoxy and propionyloxy) and acyl (e.g. acetyl).

In addition to the above-mentioned hydrolyzable groups, other suitable groups which may also be mentioned are hydrogen and alkoxy radicals having 5 to 20, in particular 5 to 10, carbon atoms and halogen- and alkoxy-substituted alkoxy groups (e.g. β-methoxyethoxy).

Since the hydrolyzable groups are practically no longer present in the preserved paper, but are lost through hydrolysis (and condensation), the Hydrolysis products may sooner or later also have to be removed in any suitable manner (especially if such a hydrolysis product in the preservation liquid used according to the invention would have an unfavorable influence on the paper or its components to be preserved), those hydrolyzable groups which do not carry any substituents are particularly preferred and lead to low molecular weight hydrolysis products such as lower alcohols such as methanol, ethanol, propanol and butanols. The latter hydrolysable groups are also preferred because they have practically no influence on the pH during hydrolysis (in contrast to, for example, halogen).

In addition to the above-described polycondensates and, if appropriate, water and hydrolysis products from the hydrolysis of the starting compounds, other components may also be present in the preservation liquid used according to the invention, which of course do not have any harmful influence on the paper to be preserved or its constituents (such as glue, filler and Dyes, resins, etc.) and the substances applied to the paper (e.g. printing ink, ink, graphite, etc.).

In most cases, the preservation liquid used according to the invention will contain a solvent for adjusting the viscosity of the preservation liquid. This solvent can be water and / or organic solvents. The organic solvents include, in particular, the lower alcohols, such as methanol, ethanol, propanol and the butanols, since these are usually already formed as by-products of the polycondensation reaction using the preferred starting materials (see above). Of course, mixtures of Solvents are used, for example mixtures of the by-products formed by the hydrolysis reaction (for example alcohols) and other (preferably volatile) solvents such as for example ethers, ketones, esters and (aliphatic or aromatic) hydrocarbons. The total solids content of the preservation liquid used according to the invention is generally 10 to 75, preferably 15 to 50 and particularly preferably 20 to 40 percent by weight.

In particular in the case where at least some of the non-hydrolyzable or non-hydrolyzing organic groups which are bonded to central atoms M have reactive multiple bonds and / or other reactive groups (for example epoxy rings), the preservation liquid can also contain compounds that can react with such groups or groupings in the context of a thermally or photochemically induced reaction. It can therefore prove useful, for example, in the presence of non-hydrolyzable or non-hydrolyzing organic groups with a carbon-carbon double or triple bond (such as (meth) acryloxyalkyl groups) to add organic unsaturated compounds to the preserving liquid, which then can copolymerize with these unsaturated groups of the polycondensates. Examples of such unsaturated organic compounds are styrene, acrylic acid, methacrylic acid or corresponding derivatives (eg esters, amides, nitriles) of the acids just mentioned. Such compounds can also be partially or perfluorinated. It is also possible to use compounds which have (per) fluorinated groups which react with non-hydrolyzable or non-hydrolysing non-fluorinated groups during the preparation of the preservation liquid or the polycondensates and thereby provide fluorinated groups (for example by reaction of SH - or NH groups with hexafluoropropene oxide).

Further e.g. in the case of epoxy-containing organic groups on central atoms M, the preservation liquid contains compounds which can undergo (catalytic) ring-opening polymerization with the epoxy rings, e.g. Compounds containing hydroxyl and amine groups (e.g. phenols).

By adding the above-mentioned organic compounds, which can react with organic groups present on the central atoms M of the polycondensates used according to the invention, it is possible to adjust the properties of the resulting impregnation to the specific circumstances (type and nature of the paper to be preserved, etc.).

In particular if the organic groups in the polycondensates of the preservation liquid used according to the invention include those with unsaturated carbon-carbon bonds, the preservation liquid preferably also contains a catalyst for the thermally and / or photochemically induced curing of the polycondensates applied to the paper. For example, a photopolymerization initiator can be added. The commercially available photoinitiators can be used, for example. Examples include Irgacure ^(R) 184 (1-hydroxycyclohexylphenyl ketone), Irgacure ^(R) 500 (1-hydroxycyclohexylphenyl ketone, benzophenone) and other photo initiators of the Irgacure ^(R) type available from Ciba-Geigy; Darocur ^(R) 1173, 1116, 1398, 1174 and 1020 (available from Merck), benzophenone, 2-chlorothioxanthone, 2-methylthioxanthone, 2-isopropylthioxanthone, benzoin, 4,4'-dimethoxybenzoin, benzoin ethyl ether, benzoin isopropyl ether, Benzyldimethylketal, 1,1,1-trichloroacetophenone, diethoxyacetophenone and dibenzosuberone.

Suitable thermal initiators include organic peroxides in the form of diacyl peroxides, peroxydicarbonates, alkyl peresters, dialkyl peroxides, perketals, ketone peroxides and alkyl hydroperoxides. Specific examples of such thermal initiators are dibenzoyl peroxide, tert-butyl perbenzoate and azobisisobutyronitrile.

In the presence of epoxy rings, the usual ring opening catalysts may be present in the preservation liquid, e.g. tertiary amines (e.g. imidazole and its derivatives).

The above initiators are added to the preservation liquid in the usual amounts. For example, a preservation liquid containing 30 to 50 percent by weight solid may contain initiator in an amount of e.g. 0.5 to 2 percent by weight (based on the total amount) can be added.

Finally, the preservation liquid preferably also contains at least one substance which (at least partially) neutralizes the acid contained in the paper, ie a deacidifying agent and / or a buffer. All compounds known to the person skilled in the art for the neutralization of acid are suitable for this purpose, provided that they do not interfere with the reactions to be carried out subsequently (for example curing by further condensation, polymerization, etc.). Examples include inorganic and organic bases (for example amines), ammonium salts and other salts which have a basic reaction in aqueous solution.

The preservation liquid produced can come into contact with the paper either as such or after partial or almost complete removal of the solvent used or of the solvent formed during the reaction (for example the alcohols formed by hydrolysis of the alkoxides) and / or after addition of a suitable solvent in order to lower the viscosity to be brought. At the latest before the use of the preservation liquid, additives that are still customary (not harmful to the paper and its components) can be added, e.g. Colorants, oxidation inhibitors, leveling agents, UV absorbers and the like.

The preserving liquid, optionally (and preferably) provided with an initiator and optionally other additives, is then brought into contact with the paper, preferably by immersing the paper in the preserving liquid. Before drying and / or curing, excess preservative liquid is preferably removed (e.g. by draining or squeezing). Drying takes place at room temperature or slightly elevated temperature.

After drying, if appropriate, the preserving liquid impregnating the paper, depending on the type or the presence of an initiator, can be cured thermally and / or by irradiation in a manner known per se.

In the case of thermal curing, the curing temperatures are preferably at least 50 ° C., in particular at least 90 ° C. The maximum curing temperature is determined, among other things, by the thermal load capacity of the paper to be treated. Curing temperatures of 180 ° C. and in particular 150 ° C. are preferably not exceeded. The Hardening time is usually 1 to 2 hours.

The following examples illustrate the present invention.

example 1

To a solution of 55.8 g (0.2 mol) of γ-glycidoxypropyltriethoxysilane and 5.1 g (0.01 mol) of 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane (FTS) were added with stirring 27 g (1.5 mol ) Water was added, whereupon the reaction mixture was heated under reflux for 6 hours. 20 g (0.08 mol) of bisphenol S were added to the then single-phase solution, followed by dilution with 100, 150 or 200 g of ethanol. After bisphenol S had dissolved, 0.8 g (0.001 mol) of 1-methylimidazole was added as the crosslinking catalyst, and the reaction mixture was stirred for 10 minutes. The solution was placed in a crystallizing dish for coating.

Three sheets of paper (10 cm x 10 cm) were cut and stored in the sol for 10 minutes with complete coverage. Then the paper sheets were placed between two glass plates (10 cm x 10 cm) and freed of excess sol by pressing them together. The soaked paper sheets were then cured between the glass plates for 1 hour in a preheated drying cabinet at 130 ° C.

The sheets did not stick to each other or to the glass plate in the above dilutions.

Example 2

Example 1 was repeated, except that no bisphenol S was used.

As a result, it was also found that the sheets did not stick to each other or to the glass plate.

Example 3

17 g (0.94 mol) of water were added to a solution of 49.7 g (0.2 mol) of 3-methacryloxypropyltrimethoxysilane and 5.1 g (0.01 mol) of FTS with stirring, and the resulting mixture was then stirred for 24 hours Was heated to reflux. The then single-phase solution was diluted with 100, 150 or 200 g of ethanol and then 0.16 g (0.001 mol) of azobisisobutyronitrile (AIBN) was added as a polymerization catalyst. The resulting sol was stirred for another 20 minutes until the AIBN had completely dissolved and then placed in a crystallizing dish for coating. The further procedure was as described in Example 1.

As a result, it was found that the sheets did not stick to each other or to the glass plate.

Example 4

The procedure of Example 3 was repeated, except that an additional 1.8 g (0.01 mol) of 3-aminopropyltrimethoxysilane was used and the amount of water added was 17.8 g (0.99 mol).

No sticking of the paper sheets, neither with one another nor with the glass plate, was observed.

Example 5

To a solution of 24 g (0.2 mol) of dimethyldimethoxysilane, 4.36 g (0.02 mol) of methyloctyldimethoxysilane and 5.1 g (0.01 mol) of FTS were added 12.9 g (0.72 mol) with stirring Water and 50 g of ethanol were added, followed by refluxing for 6 hours. The then single-phase solution was diluted with 50, 100 or 150 g of ethanol and placed in a crystallizing dish for coating. The further procedure was as described in Example 1.

The sheets did not stick to each other or to the glass plate at the specified dilutions.

Example 6

The procedure of Example 5 was repeated, except that an additional 7.2 g (0.04 mol) of 3-aminopropyltrimethoxysilane was used and the amount of water used was 15.1 g (0.84 mol).

The sheets did not stick to each other or to the glass plate.

Example 7

In a solution prepared as in Examples 1 to 6, a book with sheets, the fragility of which just allowed careful turning of the pages, was immersed and the solution was then left to act for about 30 minutes. The book was then removed from the solution, excess solution was removed by hand pressure and dried in an oven at 60 ° C. for several hours. The individual book pages were not glued. The wet tensile strength had quadrupled. The instillation of H ₂ SO ₄ did not cause holes to form.

Claims (20)

1. Process for preserving paper, comprising

   (a) simultaneous contacting of a plurality of paper layers which are in contact with each other at least in partial regions with a preservative fluid comprising polycondensates based on one or more hydrolytically polycondensable compounds of the elements M of the main groups III to V and the sub-groups II to IV of the Periodic Table, where in said polycondensates at least one organic group G which comprises at least two carbon atoms to each of which at least one atom selected from hydrogen and fluorine atoms is bonded and which organic group G has no polar substituents, is bonded, either directly via one of the carbon atoms or via a linking group A, to at least 0.1% of the central atoms M; and

   (b) drying and/or curing of the paper treated with said preservative fluid.
2. Process according to Claim 1, characterized in that the elements M are selected from Si, Al, B, Sn, Zr, V and Zn, particularly Si, Al and Zr, or mixtures of said elements.
3. Process according to any one of Claims 1 and 2, characterized in that at least 0.5% and particularly at least 1% of the central atoms M have organic groups G.
4. Process according to any one of Claims 1 to 3, characterized in that said organic groups G have at least 3 and preferably at least 5 carbon atoms to each of which at least one and preferably two atoms selected from hydrogen and fluorine atoms are bonded.
5. Process according to any one of Claims 1 to 4, characterized in that the groups G are aliphatic groups, preferably alkyl and/or alkenyl groups, which groups may optionally be partially fluorinated.
6. Process according to any one of Claims 1 to 5, characterized in that the groups G comprise groups wherein 2 to 30 fluorine atoms are bonded to aliphatic carbon atoms, which carbon atoms, in the case of the direct bonding of the group G, are separated from M by at least 2 atoms.
7. Process according to any one of Claims 1 to 5, characterized in that the linking group A forms a chelate complex with M.
8. Process according to Claim 7, characterized in that in said chelate complex M is coordinated via at least 2 atoms of said group A which are selected from oxygen and nitrogen atoms.
9. Process according to any one of Claims 7 and 8, characterized in that the chelate complex has the structure or
10. Process according to any one of Claims 1 to 9, characterized in that in addition to the groups G, organic groups H are present on the central atoms M, which groups H are different from the groups G and particularly have multiple bonds, functional groups and or hetero atoms in, respectively at the carbon skeleton.
11. Process according to Claim 10, characterized in that the groups H comprise groups H' which are capable of undergoing, optionally in the presence of catalysts, a thermal and/or photochemical curing reaction.
12. Process according to Claim 11, characterized in that the groups H' comprise groups having polymerizable carbon-carbon double or triple bonds and/or epoxide rings.
13. Process according to any one of Claims 11 and 12, characterized in that the preservative fluid additionally contains one or more catalysts, selected from thermal and photochemical polymerization catalysts and catalysts for the epoxide ring opening polymerization.
14. Process according to any one of Claims 11 to 13, characterized in that the preservative fluid additionally contains at least one organic compound which can take part in a polymerization reaction.
15. Process according to any one of Claims 1 to 14, characterized in that the preservative fluid additionally contains water and/or an organic solvent, preferably selected from aliphatic alcohols.
16. Process according to any one of Claims 1 to 15, characterized in that the preservative fluid additionally contains a deacidifier and/or a buffering compound.
17. Process according to any one of Claims 1 to 16, characterized in that the contacting is carried out by immersing the paper layers in the preservative fluid.
18. Process according to any one of Claims 1 to 17, characterized in that the drying and/or curing operation is carried out in an oven.
19. Process according to any one of Claims 1 to 18, characterized in that said plurality of paper layers is a whole book.
20. Preserved paper, obtainable by the process according to any one of Claims 1 to 19.