EP0779942B1 - Process for manufacturing cellulose fibres - Google Patents

Abstract

Prodn. of soln. of spun cellulose fibre with reduced fibrillation tendency is by treatment of the fibres with (A) a N-methylol ether of a carboxamide, urethane, urea or aminotriazine; (B) a cyclic hydroxy or alkoxy ethylene urea N-substd. by 1 or more alkyl gps.; (C) a hydrophilic modified polyisocyanate; and/or (D) a polyurethane/isocyanate mixt..

Description

The present invention relates to a new method of manufacture of cellulose fibers spun from solvents reduced tendency to fibrillate by treating the fibers with certain reactive compounds.

GB-A-2 043 525 describes the production of cellulose fibers by spinning a cellulose solution in a suitable solvent, e.g. an N-oxide of a tertiary amine, such as N-methylmorpholine-N-oxide, known. In such a spinning process the Cellulose solution extruded through a suitable nozzle and the resulting fiber precursor washed in water and then dried. Such fibers are called "solvent spun Fibers ".

Such cellulose fibers spun from solvents offer many application advantages, but tend to fibrillate. This is the splicing of the finest fiber fibrils, the processing of cellulose fibers in textile production can lead to problems.

WO-A-92/07124 recommends the to solve this problem Treatment of the cellulose fibers with an aqueous solution or Dispersion of a polymer that has a variety of cationically ionizable Groups, e.g. a polyvinylimidazoline.

Furthermore, EP-A-538 977 teaches the use of compounds which have 2 to 6 functional groups with cellulose can react, e.g. Products based on dichlorotriazine, for this purpose.

The object of the present invention was to develop a new method for the production of cellulose fibers spun from solvents with reduced tendency to fibrillate, that comes from other chemical defibrillation reagents.

It has now been found that the preparation of solvents spun cellulose fibers with reduced tendency to fibrillate succeeds advantageously if one with or the fibers several compounds from the group of treated hydrophilically modified polyisocyanates.

The hydrophilically modified polyisocyanates are usually in the form of aqueous dispersions, which are essentially free of organic solvents and other emulsifiers are used in the method according to the invention.

As a basis for the hydrophilically modified used according to the invention Polyisocyanates serve the usual diisocyanates and / or usual higher functional polyisocyanates with a medium NCO functionality from 2.0 to 4.5. These components can alone or in a mixture.

Examples of common diisocyanates are aliphatic diisocyanates such as tetramethylene diisocyanate, hexamethylene diisocyanate (1,6-diisocyanatohexane), Octamethylene diisocyanate, decamethylene diisocyanate, Dodecamethylene diisocyanate, tetradecamethylene diisocyanate, trimethylhexane diisocyanate or tetramethylhexane diisocyanate, cycloaliphatic Diisocyanates such as 1,4-, 1,3- or 1,2-diisocyanatocyclohexane, 4,4'-di (isocyanatocyclohexyl) methane, 1-isocyanato-3,3,5-trimethyl-5- (isocyanatomethyl) cyclohexane (Isophorone diisocyanate) or 2,4- or 2,6-diisocyanato-1-methylcyclohexane and aromatic diisocyanates such as 2,4- or 2,6-tolylene diisocyanate, Tetramethylxylylene diisocyanate, p-xylylene diisocyanate, 2,4'- or 4,4'-diisocyanatodiphenylmethane, 1,3- or 1,4-phenylene diisocyanate, 1-chloro-2,4-phenylene diisocyanate, 1,5-naphthylene diisocyanate, Diphenylene-4,4'-diisocyanate, 4,4'-diisocyanato-3,3'-dimethyldiphenyl, 3-methyldiphenylmethane-4,4'-diisocyanate or Diphenyl ether 4,4'-diisocyanate. Mixtures of mentioned diisocyanates are present. Of these, aliphatic are preferred Diisocyanates, especially hexamethylene diisocyanate and Isophorone diisocyanate.

Examples of suitable, higher-functionality polyisocyanates are Triisocyanates such as 2,4,6-triisocyanatotoluene or 2,4,4'-triisocyanatodiphenyl ether or the mixtures of di-, Tri and higher polyisocyanates by phosgenation of corresponding aniline / formaldehyde condensates are obtained and Represent polyphenyl polyisocyanates having methylene bridges.

(a) Isocyanuratgruppen aufweisende Polyisocyanate von aliphatischen und/oder cycloaliphatischen Diisocyanaten. Besonders bevorzugt sind hierbei die entsprechenden Isocyanato-Isocyanurate auf Basis von Hexamethylendiisocyanat und Isophorondiisocyanat. Bei den vorliegenden Isocyanuraten handelt es sich insbesondere um einfache Tris-isocyanatoalkyl- bzw. Triisocyanatocycloalkyl-Isocyanurate, welche cyclische Trimere der Diisocyanate darstellen, oder um Gemische mit ihren höheren, mehr als einen Isocyanuratring aufweisenden Homologen. Die Isocyanato-Isocyanurate haben im allgemeinen einen NCO-Gehalt von 10 bis 30 Gew.-%, insbesondere 15 bis 25 Gew.-%, und eine mittlere NCO-Funktionalität von 2,6 bis 4,5.

(b) Uretdiondiisocyanate mit aliphatisch und/oder cycloaliphatisch gebundenen Isocyanatgruppen, vorzugsweise von Hexamethylendiisocyanat oder Isophorondiisocyanat abgeleitet. Bei Uretdiondiisocyanaten handelt es sich um cyclische Dimersierungsprodukte von Diisocyanaten.

(c) Biuretgruppen aufweisende Polyisocyanate mit aliphatisch gebundenen Isocyanatgruppen, insbesondere Tris(6-isocyanatohexyl)biuret oder dessen Gemische mit seinen höheren Homologen. Diese Biuretgruppen aufweisenden Polyisocyanate haben im allgemeinen einen NCO-Gehalt von 18 bis 25 Gew.-% und eine mittlere NCO-Funktionalität von 3 bis 4,5.

(d) Urethan- und/oder Allophanatgruppen aufweisende Polyisocyanate mit aliphatisch oder cycloaliphatisch gebundenen Isocyanatgruppen, wie sie beispielsweise durch Umsetzung von überschüssigen Mengen an Hexamethylendiisocyanat oder an Isophorondiisocyanat mit einfachen mehrwertigen Alkoholen wie Trimethylolpropan, Glycerin, 1,2-Dihydroxypropan oder deren Gemischen erhalten werden können. Diese Urethan- und/oder Allophanatgruppen aufweisenden Polyisocyanate haben im allgemeinen einen NCO-Gehalt von 12 bis 20 Gew.-% und eine mittlere NCO-Funktionalität von 2,5 bis 3.

(e) Oxadiazintriongruppen enthaltende Polyisocyanate, vorzugsweise von Hexamethylendiisocyanat oder Isophorondiisocyanat abgeleitet. Solche Oxadiazintriongruppen enthaltenden Polyisocyanate sind aus Diisocyanat und Kohlendioxid herstellbar.

(f) Uretonimin-modifizierte Polyisocyanate.

Common aliphatic higher functional polyisocyanates of the following groups are of particular interest:

(a) Isocyanurate group-containing polyisocyanates of aliphatic and / or cycloaliphatic diisocyanates. The corresponding isocyanato-isocyanurates based on hexamethylene diisocyanate and isophorone diisocyanate are particularly preferred. The present isocyanurates are, in particular, simple tris-isocyanatoalkyl or triisocyanatocycloalkyl isocyanurates, which are cyclic trimers of the diisocyanates, or mixtures with their higher homologues containing more than one isocyanurate ring. The isocyanato-isocyanurates generally have an NCO content of 10 to 30% by weight, in particular 15 to 25% by weight, and an average NCO functionality of 2.6 to 4.5.

(b) uretdione diisocyanates with aliphatic and / or cycloaliphatic isocyanate groups, preferably derived from hexamethylene diisocyanate or isophorone diisocyanate. Uretdione diisocyanates are cyclic dimers of diisocyanates.

(c) Polyisocyanates containing biuret groups with aliphatically bound isocyanate groups, in particular tris (6-isocyanatohexyl) biuret, or mixtures thereof with its higher homologues. These polyisocyanates containing biuret groups generally have an NCO content of 18 to 25% by weight and an average NCO functionality of 3 to 4.5.

(d) Polyisocyanates containing urethane and / or allophanate groups with aliphatic or cycloaliphatic isocyanate groups, such as are obtained, for example, by reacting excess amounts of hexamethylene diisocyanate or isophorone diisocyanate with simple polyhydric alcohols such as trimethylolpropane, glycerol, 1,2-dihydroxypropane or mixtures thereof can. These polyisocyanates containing urethane and / or allophanate groups generally have an NCO content of 12 to 20% by weight and an average NCO functionality of 2.5 to 3.

(e) Polyisocyanates containing oxadiazinetrione groups, preferably derived from hexamethylene diisocyanate or isophorone diisocyanate. Such polyisocyanates containing oxadiazinetrione groups can be prepared from diisocyanate and carbon dioxide.

(f) Uretonimine-modified polyisocyanates.

For the use according to the invention, aliphatic diisocyanates are used and aliphatic, higher functional polyisocyanates in particular prefers.

The described diisocyanates and / or more functionalized Polyisocyanates become non-ionic hydrophilic for conversion modified polyisocyanates for use in the invention are particularly preferred with NCO-reactive compounds implemented the hydrophilic structural elements with nonionic Groups or with polar groups that are not in ion groups can be transferred. That’s it Diisocyanate or polyisocyanate in a stoichiometric excess before so the resulting hydrophilically modified polyisocyanate still has free NCO groups.

R⁸

für C₁- bis C₂₀-Alkyl, insbesondere C₁- bis C₄-Alkyl, oder C₂- bis C₂₀-Alkenyl, Cyclopentyl, Cyclohexyl, Glycidyl, Oxethyl, Phenyl, Tolyl, Benzyl, Furfuryl oder Tetrahydrofurfuryl steht,

E

Schwefel oder insbesondere Sauerstoff bezeichnet,

D

Propylen oder vor allem Ethylen bedeutet, wobei auch insbesondere blockweise gemischt ethoxylierte und propoxylierte Verbindungen auftreten können, und

n

für eine Zahl von 5 bis 120, insbesondere 10 bis 25 steht,

in Betracht.Hydroxyl group-terminated polyethers of the general formula VII in particular come as such NCO-reactive compounds with hydrophilicizing structural elements R ⁸ -E- (DO) _n -H in the

R ⁸

represents C ₁ to C ₂₀ alkyl, in particular C ₁ to C ₄ alkyl, or C ₂ to C ₂₀ alkenyl, cyclopentyl, cyclohexyl, glycidyl, oxethyl, phenyl, tolyl, benzyl, furfuryl or tetrahydrofurfuryl,

E

Denotes sulfur or especially oxygen,

D

Propylene or especially ethylene means, and in particular mixed ethoxylated and propoxylated compounds can also occur in blocks, and

n

represents a number from 5 to 120, in particular 10 to 25,

into consideration.

The use of non-ionically hydrophilically modified polyisocyanates, which contain the polyether VII built in therefore also a preferred embodiment.

These are particularly preferably ethylene oxide or propylene oxide polyethers started on C ₁ to C ₄ alkanol with average molecular weights of 250 to 7000, in particular 450 to 1500.

One can from the described diisocyanates and / or more functionalized Polyisocyanates also first by reaction with a deficit of hydroxyl-terminated polyesters other hydroxyl-terminated polyethers or on polyols, e.g. Ethylene glycol, trimethylolpropane or butanediol, prepolymers generate and then these prepolymers or simultaneously with the polyethers VII in deficit to the hydrophilic implement modified polyisocyanates with free NCO groups.

It is also possible to prepare non-ionically hydrophilically modified polyisocyanates from diisocyanate or polyisocyanate and polyalkylene glycols of the formula HO- (DO) _n -H, in which D and n have the meanings given above. Both terminal OH groups of the polyalkylene glycol react with isocyanate.

The enumerated types of non-ionic hydrophilic modified Polyisocyanates are described in DE-A 24 47 135, DE-A 26 10 552, DE-A 29 08 844, EP-A 0 13 112, EP-A 019 844, DE-A 40 36 927, DE-A 41 36 618, EP-B 206 059, EP-A 464 781 and EP-A 516 361 described in more detail.

The described diisocyanates and / or more functionalized Polyisocyanates become anionically hydrophilic for conversion modified polyisocyanates reacted with NCO-reactive compounds, the hydrophilizing anionic groups, in particular Acid groups such as carboxyl groups, sulfonic acid groups or Phosphonic acid groups. The diisocyanate or Polyisocyanate in a stoichiometric excess, so that resulting hydrophilically modified polyisocyanate is still free Has NCO groups.

As such, NCO-reactive compounds with anionic groups come especially hydroxycarboxylic acids like 2-hydroxyacetic acid, 3-hydroxypropionic acid, 4-hydroxybutyric acid or hydroxylpivalic acid and 2,2-bis- and 2,2,2-tris (hydroxymethyl) alkanoic acids, e.g. 2,2-bis (hydroxymethyl) acetic acid, 2,2-bis (hydroxymethyl) propionic acid, 2,2-bis (hydroxymethyl) butyric acid or 2,2,2-tris (hydroxymethyl) acetic acid. The carboxyl groups can be partially or completely by a base be neutralized to in a water soluble or water dispersible Form. The base is preferred here a tertiary amine, which is known to Isocyanate is inert.

The described diisocyanates and / or more functionalized Polyisocyanates can also be mixed with a non-ionic hydrophilically modifying and anionically hydrophobically modifying Compounds that are added sequentially or simultaneously are implemented, for example with a deficit the polyethers VII and the hydroxycarboxylic acids described.

The enumerated types of anionically hydrophilically modified polyisocyanates are in the documents DE-A 40 01 783, DE-A 41 13 160 and DE-A 41 42 275 described in more detail.

The described diisocyanates and / or more functionalized Polyisocyanates become cationically hydrophilic for conversion modified polyisocyanates reacted with NCO-reactive compounds, the chemically built-in alkylatable or protonable Functions with the formation of a cationic center included. In particular, such functions are tertiary nitrogen atoms, which are known to be inert to isocyanate and can be easily quaternized or protonated. When implementing of diisocyanate or polyisocyanate with these NCO-reactive Connections exist in excess so that the resulting hydrophilically modified polyisocyanate is still free Has NCO groups.

in der

R⁹ und R¹⁰

lineares oder verzweigtes C₁- bis C₂₀-Alkyl, insbesondere C₁- bis C₅-Alkyl, bedeuten oder zusammen mit dem N-Atom einen fünf- oder sechsgliedrigen Ring bilden, der noch ein O-Atom oder ein tertiäres N-Atom enthalten kann, insbesondere einen Piperidin-, Morpholin-, Piperazin-, Pyrrolidin-, Oxazolin- oder Dihydrooxazin-Ring, wobei die Reste R² und R³ noch zusätzlich Hydroxylgruppen, insbesondere jeweils eine Hydroxylgruppe, tragen können, und

R¹¹

eine C₂- bis C₁₀-Alkylengruppe, insbesondere eine C₂- bis C₆-Alkylengruppe, die linear oder verzweigt sein kann, bezeichnet,

in Betracht.Such NCO-reactive compounds with tertiary nitrogen atoms are preferably amino alcohols of the general formula VIII

in the

R ⁹ and R ¹⁰

are linear or branched C ₁ - to C ₂₀ -alkyl, in particular C ₁ - to C ₅ -alkyl, or together with the N atom form a five- or six-membered ring which is also an O atom or a tertiary N atom may contain, in particular a piperidine, morpholine, piperazine, pyrrolidine, oxazoline or dihydrooxazine ring, where the radicals R ² and R ^{3 may} additionally carry hydroxyl groups, in particular in each case one hydroxyl group, and

R ¹¹

denotes a C ₂ to C ₁₀ alkylene group, in particular a C ₂ to C ₆ alkylene group, which may be linear or branched,

into consideration.

Particularly suitable amino alcohols VIII are N-methyldiethanolamine, N-methyldi (iso) propanolamine, N-butyldiethanolamine, N-butyldi (iso) propanolamine, N-stearyldiethanolamine, N-stearyldi (iso) propanolamine, N, N-dimethylethanolamine, N, N-dimethyl (iso) propanolamine, N, N-diethylethanolamine, N, N-diethyl (iso) propanolamine, N, N-dibutylethanolamine, N, N-dibutyl (iso) propanolamine, triethanolamine, tri (iso) propanolamine, N- (2-hydroxyethyl) morpholine, N- (2-hydroxypropyl) morpholine, N- (2-hydroxyethyl) piperidine, N- (2-hydroxypropyl) piperidine, N-methyl-N '- (2-hydroxyethyl) piperazine, N-methyl-N' - (2-hydroxypropyl) piperazine, N-methyl-N '- (4-hydroxybutyl) piperazine, 2-hydroxyethyl oxazoline, 2-hydroxypropyl oxazoline, 3-hydroxypropyl oxazoline, 2-hydroxyethyl dihydrooxazine, 2-hydroxypropyl dihydrooxazine or 3-hydroxypropyl dihydrooxazine.

in der R⁹ bis R¹¹ die oben genannten Bedeutungen haben und R¹² C₁- bis C₅-Alkyl bezeichnet oder mit R⁹ einen fünf- oder sechsgliedrigen Ring, insbesondere einen Piperazin-Ring, bildet, in Betracht.Furthermore, such NCO-reactive compounds with tertiary nitrogen atoms are preferably diamines of the general formula IXa or IXb

in which R ⁹ to R ^{11 have} the abovementioned meanings and R ¹² denotes C ₁ to C ₅ alkyl or forms a five- or six-membered ring, in particular a piperazine ring, with R ⁹ .

Particularly suitable diamines IXa are N, N-dimethylethylenediamine, N, N-diethyl-ethylenediamine, N, N-dimethyl-1,3-diamino-2,2-dimethylpropane, N, N-diethyl-1,3-propylenediamine, N- (3-aminopropyl) morpholine, N- (2-aminopropyl) morpholine, N- (3-aminopropyl) piperidine, N- (2-aminopropyl) piperidine, 4-amino-1- (N, N-diethylamino) pentane, 2-amino-1- (N, N-dimethylamino) propane, 2-amino-1- (N, N-diethylamino) propane or 2-amino-1- (N, N-diethylamino) -2-methylpropane.

Particularly suitable diamines IXb are N, N, N'-trimethyl-ethylenediamine, N, N, N'-triethylethylenediamine, N-methylpiperazine or N-ethylpiperazine.

Furthermore, polyether (poly) oles can also be used as NCO-reactive compounds with built-in tertiary nitrogen atoms, the by propoxylation and / or ethoxylation of amine nitrogen having starter molecules can be used become. Such polyether (poly) ole are, for example Propoxylation and ethoxylation products of ammonia, Ethanolamine, diethanolamine, ethylenediamine or N-methylaniline.

Other usable NCO-reactive compounds are tertiary nitrogen atoms containing polyester and polyamide resins, tertiary Polyols containing urethane groups and nitrogen atoms Polyhydroxypolyacrylates containing tertiary nitrogen atoms.

The described diisocyanates and / or more functionalized Polyisocyanates can also be mixed with a non-ionic hydrophilically modifying and cationically hydrophilically modifying Compounds that are added sequentially or simultaneously are implemented, for example with a deficit the polyethers VII and the amino alcohols VIII or the diamines IXa or IXb. Mixtures of non-ionic hydrophilic modifying and anionically hydrophilically modifying Connections are possible.

The listed types of cationically hydrophilic modified polyisocyanates are in the documents DE-A 42 03 510 and EP-A 531 820 described in more detail.

Since the hydrophilically modified polyisocyanates mentioned in usually used in aqueous media is sufficient for To ensure dispersibility of the polyisocyanates. Preferably act hydrophilic within the group of described modified polyisocyanates from certain reaction products Di- or polyisocyanates and hydroxyl-terminated polyethers (Polyether alcohols) such as compounds VII as emulsifiers for this purpose.

The good results achieved with the hydrophilically modified Polyisocyanates in aqueous media are all the more surprising since it was to be expected that isocyanates would rapidly dissolve in an aqueous environment decompose. Nevertheless, the polyisocyanates used according to the invention have a pot life of several in the aqueous liquor Hours on, i.e. the present polyisocyanate dispersions are stable within the usual processing time. From one Dispersion is said to be stable when its components remain dispersed in each other without being discrete Separate layers. The term "pot life" means the time during which the dispersions remain processable before they jell and set. Gel aqueous isocyanate dispersions and set because of a reaction between the water and the Isocyanate takes place, forming a polyurea.

The compounds can be used in the process according to the invention for the production of cellulose fibers generally in an aqueous System, preferably in aqueous solution or emulsion, for Application, the aqueous system in general, based on the weight of the aqueous system, 0.1 to 20% by weight, preferably 0.5 to 10 wt .-%, which has compounds.

Stufe 1:

Lösen der Cellulose in einem mit Wasser mischbaren Lösungsmittel

Stufe 2:

Extrudieren der Lösung durch eine Düse unter Bildung der Faservorstufe.

Stufe 3:

Behandlung der Faservorstufe mit Wasser um Lösungsmittel zu entfernen und die Cellulosefaser auszubilden

Stufe 4:

Trocknung der Faser

The manufacturing processes for cellulose fibers spun from solvents generally run in 4 stages.

Step 1:

Dissolve the cellulose in a water-miscible solvent

Level 2:

Extrude the solution through a die to form the fiber precursor.

Level 3:

Treatment of the fiber precursor with water to remove solvents and form the cellulose fiber

Level 4:

Drying the fiber

The preferred solvent in stage 1 is N-methylmorpholine-N-oxide used.

The wet fiber obtained in Step 3 is called undried Fiber is usually referred to and referred to the dry weight of the fiber, 120 to 150 wt .-% water.

The water content of the dried fiber is generally based on the dry weight of the fiber, 60 to 80 wt .-%.

The treatment according to the invention with the compounds can either on the wet fiber (during or after level 3) or on the dried fiber (after step 4). It is but also a treatment in the fiber production stage (Level 2), e.g. in a precipitation bath, possible.

If the treatment is done on the moist fiber, it can for example by adding the aqueous system of the compounds happen to a circulating bath that is the fiber precursor contains. The fiber precursor can e.g. as staple fiber available.

If the treatment is done on the dried fiber, it can these e.g. as staple fiber, fleece, yarn, knitwear or fabric available. The treatment of the fibers in this case can e.g. in aqueous liquor.

In contrast to the method described in EP-A-538 977 can in the process of the invention to the presence of Alkali should be avoided.

Treatment is usually at a temperature of 20 to 200 ° C, preferably 40 to 180 ° C, made. There is a chemical reaction of the compounds with the hydroxy groups of cellulose, with a chemical link between Hydroxy groups of different cellulose fibrils possible is. This increases the stability of the fiber.

The duration of treatment is usually 1 second to 20 minutes, preferably 5 to 60 seconds and in particular 5 to 30 seconds.

In the impregnation process, the treatment can be done both at room temperature (20 ° C) with subsequent drying up to 100 ° C as well Condensation at temperatures up to 200 ° C, in particular at 150 to 180 ° C.

The treatment of the moist or dried fiber can be 0.1 up to 10% by weight, preferably 0.2 to 5% by weight, in particular 0.2 up to 2% by weight, based in each case on the dry weight of the fiber, of connections. In some cases it can however, it may also be advantageous to increase the amounts mentioned, e.g. up to approx. 20% by weight.

In the treatment, other aids that are customary here can be used in the usual quantities used for this purpose. In particular are anti-migration agents, for example based on oxethylation products, to mention.

As already stated, the connections can be made the compounds described in EP-A-538 977 pure can be fixed thermally (without alkali), making them optimal have it integrated into the fiber production process. The dyeability the fibers treated in this way with all the usual cellulose fiber dyes, reactive dyes are generally also possible.

Using the test methods described in EP-A-538 977, to which express reference is made here can be advantageous Results are achieved.

Claims (4)

1. A process for producing solvent-spun cellulosic fibers having a reduced tendency to fibrillate, which comprises treating the fibers with one or more compounds from the group of the hydrophilic modified polyisocyanates.
2. A process as claimed in claim 1, wherein the compounds used are nonionically hydrophilic modified polyisocyanates containing hydroxyl-terminated polyethers of the general formula VII R⁸-E-(DO)_n-H where

   R⁸

   is C₁-C₂₀-alkyl, C₂-C₂₀-alkenyl, cyclopentyl, cyclohexyl, glycidyl, hydroxyethyl, phenyl, tolyl, benzyl, furfuryl or tetrahydrofurfuryl,

   E

   is sulfur or oxygen,

   D

   is propylene or ethylene, and

   n

   is from 5 to 120.

3. A process as claimed in claim 1 or 2, wherein the fibers are treated with from 0.1 to 10% by weight, based on the dry weight of the fibers, of the compounds.
4. A process as claimed in any of claims 1 to 3, wherein the treatment is carried out at from 20 to 200°C.