WO1996000323A1

WO1996000323A1 - Process for producing para-aromatic polyamide paper

Info

Publication number: WO1996000323A1
Application number: PCT/EP1995/002406
Authority: WO
Inventors: Tsutomu Takahashi; Masanobu Iwama
Original assignee: Akzo Nobel N.V.; Sumitomo Chemical Company, Limited
Priority date: 1994-06-23
Filing date: 1995-06-22
Publication date: 1996-01-04
Also published as: TW318193B; AU2922495A

Abstract

A process for producing a para-aromatic polyamide paper which comprises impregnating a web prepared by papermaking from a papermaking material comprising as an essential component at least one component selected from the group consisting of pulp, short fibers and stable fibers of a para-aromatic polyamide with or without a particulate polymer of a para-aromatic polyamide with a binder which is a solution consisting of a polar amide solvent having dissolved therein 0.5 % to 10 % by weight of a chloride of an alkali metal or an alkaline earth metal and 0.5 % to 10 % by weight of a para-aromatic polyamide having an inherent viscosity of 1.0 to 2.5 dl/g. According to the above process, a high performance para-aromatic polyamide paper consisting essentially of a para-aromatic polyamide is obtained. Said paper has agglutinate portions of fibers while keeping the heat resistance and stiffness which are the characteristic features of para-aromatic polyamide and also has such excellent properties that the paper quality is uniform, the breaking length is long and the paper strength is high.

Description

PROCESS FOR PRODUCING PARA-AROMATIC POLYAMIDE PAPER

BACKGROUND OP THE INVENTION

Field of the invention

This invention relates to a process for producing a paper consisting essentially of a para-aromatic polyamide (referred to hereinafter as para-aramid in some cases). The para-aramid paper obtained by the process of this invention is useful particularly for an insulating paper, a composite material having a honeycomb structure and the like in application fields requiring heat resistance and high strength.

Related Art

Para-aramid fibers have excellent properties such as high strength, high stiffness and high heat resistance, and para-aramid pulp prepared from para-aramid fibers has been widely used as a substitute for asbestos. However, said para-aramid fibers are not melted and hence the para-aramid paper prepared from said para-aramid pulp has no agglutinate portions (in some cases, called entangled portions). As a result thereof, the para-aramid paper has low strength, so that it has been considered that the use of the para-aramid paper as an insulating paper and the like be difficult.

Extensive research was made to solve the above problems. For example, JP-A-59-163,418 discloses that the agglutinate portions between fibrils can be improved by using pulp obtained by fibri Hating fibers consisting of the para-aramid and aliphatic polyamide. In JP-B-3-39,539, a sheet-like paper comprising para-aramid short fibers is proposed for improving the heat resistance and strength of a paper composed of meta-aromatic polyamide (in some cases, referred to hereinafter as meta-aramid) . Extremely fine short fibers (in some cases called fibrils) composed of the meta-aramid are obtained by violently agitating a meta aramid solution under high shearing in a non-solvent (usually aqueous medium). Said fibrils agglutinate upon drying, and as a- result, a meta-aramid paper comprising as an essential component fibrils composed of a meta-aramid has a high strength. On the other hand, the para-aramid fibers have higher fiber strength than the meta-aramid fibers, so that a paper having higher strength and higher heat resistance can be obtained by combining para-aramid short fibers with fibrils composed of the meta-aramid. The above-mentioned system is a system in which a component other than the para-aramid is added to the para-aramid paper in order to form agglutinate portions in the para-aramid paper; however, the addition of said other components results in such a disadvantage that the high stiffness and high heat resistance which are characteristic features of the para-aramid fibers are impaired.

JP-A-3-14,832 discloses a method for preparing a para-aramid paper using as a binder a para-aramid fi-brous gel composition consisting of the para-aramid, an amide solvent, an alkaline earth metal and N methylpyrrolidine. However, the working examples thereof show that the breaking length is 0.34 lb/in/oz/yd² (corresponding to 0.18 km), and this value is only about two times the breaking length of a paper prepared from commercially available para-aramid pulp. Moreover, JP-A-3-90,693 discloses coating cone, sulfuric acid on the surface of a para-aramid paper, partially dissolving the para-aramid in the surface layer and then coagulating the same to form connected portions. As a similar technique, JP-A-52-34,399 discloses an insulating material consisting essentially of the meta-aramid. That is to say, it discloses an insulating material prepared by laminating to an aromatic polyamide paper a synthetic resin layer obtained by coating a solution consisting of an aromatic polyamide and an amide solvent. Each of the above cases is characterized by using a solvent in which the aromatic polyamide is dissolved. That is, in JP-A-3-90,693, it is characteristic to use an amide solvent in which the meta-aramid can be dissolved.

Also, JP-A-6-41,298 discloses a para-aramid dope having a lower polymerization degree, characterized in that 4 to 10 by weight of a para-aramid having an inherent viscosity of 1.0 to 2.5 dl/g and 2 to 10 by weight of a chloride of an alkali metal or an alkaline earth metal are dissolved in a polar amide solvent and the para-aramid dope has an optical anisotropy. Moreover, it is disclosed that industrially useful fibers and pulp are produced from the above dope. This invention aims at improving the mechanical strength of a para- aramid paper consisting essentially of the para-aramid. As mentioned above, in the prior art, the main current is that various binders other than the para-aramid are used to introduce agglutinate portions for the purpose of enhancing the paper strength. However, in this case, the heat resistance and high stiffness which are the characteristic features of the para-aramid are impaired. In the case of a paper prepared by subjecting commercially para-aramid pulp to papermaking, its breaking length is 0.1 km or less. In the case of fibrils consisting of the meta-aramid, the wet paper obtained by subjecting them to papermaking forms, when dried, agglutinate portions between the fibrils. The agglutinate portions are made stronger by subjecting the dried paper to calender roll at a high temperature. As a result, the breaking length of the paper consisting of the meta-aramid becomes 3 to 12 km. On the other hand, in the case of commercially available para-aramid pulp, no agglutinate portions are formed. If a paper consisting essentially of the para-aramid and having agglutinate portions should be prepared, the said paper would have high strength and keep the characteristic features of the para-aramid.

In the method by which a solvent in which the constituents of the paper can be dissolved is used to form the agglutinate portions, the coated surface becomes film-like. As a result thereof, it becomes difficult for the paper to have homogeneous agglutinate portions even in the direction of the paper thickness.

The problem of this invention is how to obtain a paper having high heat resistance, high strength and uniform quality,

SUMMARY OF THE INVENTION

It is an objective of this invention to provide a process for producing a para-aramid paper consisting essentially of a para-aramid and having excellent mechanical strength.

It is another objective of this invention to provide a process for preparing a para-aramid paper having a breaking length of 3 km or more.

Other objectives and advantages of this invention will become apparent from the following description.

According to this invention, there is provided a process for producing a para-aromatic polyamide paper which comprises using as a binder a solution in a polar amide solvent of 0.5 to 10 by weight of a chloride of an alkali metal or an alkaline earth metal and 0.5 to 10 by weight of a para-aromatic polyamide having an inherent viscosity of 1.0 to 2.5 dl/g.

This invention further provides a process for producing a para- aromatic polyamide paper which comprises subjecting to papermaking a paper-making material comprising as essential components at least one component selected from the group consisting of pulp, short fibers and staple fibers of a para-aromatic polyamide with or without a particulate polymer of a para-aromatic polyamide and impregnating the thus obtained web with the solution mentioned above as a binder. DETAILED DESCRIPTION OF THE INVENTION

The term "para-aromatic polyamide" or "para-aramid" used herein means a polyamide obtained by polycondensing a para-oriented aromatic diamine and a para oriented aromatic dicarboxylic acid halide and consisting essentially of recurring units in which an amido linkage is bonded to the aromatic ring in its para-position or corresponding orientation position (namely, orientation positions existing opposite to each other on the same axis or on parallel axes, for example, 4,4'-biphenylene, 1,5-naphthalene, 2,6-naphthalene or the like), and includes specifically aromatic polyamides having the para-orientation type structure or a structure close thereto, for example, poly(paraphenylene terephthalamide) , poly-(4,4'-benzanilide terephthal amide) , poly(paraphenylene-4,4'-biphenylene dicarboxa ide) , poly(paraphenylene-2,6-naphthalene dicarboxamide) and the like. These aromatic polyamides are produced by polymerizing a para-oriented aromatic diamine with a para-oriented aromatic dicarboxylic acid halide.

The para-oriented aromatic diamine used in this invention includes paraphenylenediamine (referred to hereinafter as PPD in some cases), 4,4'-diamino biphenyl, 2-methyl-paraphenylenediamine, 2-chloro paraphenylenediamine, 2,-naphthalenediamine, 1,5 naphthalenediamine, 4,4'-diaminobenzanilide and the like. The para-oriented aromatic dicarboxylic acid halide used in this invention includes terephthaloyl chloride (referred to hereinafter as TPC in some cases), 4,4'-benzoyl chloride, 2-chloroterephthaloyl chloride, 2,5-dichloroterephthaloyl chloride, 2-methylterephthaloyl chloride, 2,6-naphthalenedicarboxylic acid chloride, 1,5-naphthalene dicarboxylic acid chloride and the like.

The binder used in this invention is prepared by adding 0.94 to 0.99 mole, preferably 0.95 to 0.98 mole, of a para-oriented aromatic dicarboxylic acid halide to 1.00 mole of a para-oriented aromatic diamine so that the para-aramid concentration becomes 0.5 to 10 by weight in a polar amide solvent in which 0.5 to 10 by weight of a chloride of an alkali metal or an alkaline earth metal has been dissolved, and subjecting the resulting mixture to polymerization at a temperature of -20 to 50°C.

The para-aramid in the binder used in this invention is a para-aramid whose inherent viscosity (the term "inherent viscosity" used herein means that defined hereinafter) is 1.0 to 2.5 dl/g, preferably 1.5 to 2.2 dl/g.

When the inherent viscosity is less than 1.0 dl/g, a sufficient paper strength-increasing effect as a binder is not obtained. When the inherent viscosity is more than 2.5 dl/g, a stable liquid binder is not formed and a gel is formed whereby the impregnation of a web with the binder becomes difficult.

In the binder used in this invention, the concentration of the para- aramid is 0.5% to 10% by weight. When the concentration is less than 0.5% by weight, a sufficient paper strength-increasing effect as a binder is not obtained. When the concentration is more than 10% by weight, a stable liquid binder is not formed and a gel is formed whereby the impregnation of a web with the binder becomes difficult.

The chloride of an alkali metal or an alkaline earth metal used in this invention includes lithium chloride, calcium chloride and the like. The amount of the chloride added to the polymerization system is 0.5 to 2.5 moles, preferably 0.5 to 1.5 moles, and more preferably 0.7 to 1.3 moles, per mole of the amido group formed by polycondensation. When the amount is less than 0.5 mole, the solubility of the para- aramid produced becomes insufficient, and when it exceeds 2.5 moles, the viscosity of the polymerization mixture (binder solution) becomes high and the degree of impregnation of a web with the binder becomes low. This is not desirable. The concentration of the chloride of an alkali metal or an alkaline earth metal is 0.5% to 10% by weight. When this concentration is less than 0.5% by weight, the solubility of the para-aramid produced by polycondensation becomes insufficient and when it exceeds 10% by weight, the dissolution of the chloride in the polar amide solvent becomes difficult. This is not desirable.

The polar amide solvent used in this invention includes, specifically

N,N-dimethyl formamide, N,N-dimethylacetamide, N-methyl -2-pyrrol idone

(referred to hereinafter as NNP in some cases), N,N,N' ,N'-tetramethylurea and the like.

In this invention, the web is produced by dispersing in water a papermaking material composed of at least one component selected from the group consisting of pulp, short fibers and stable fibers obtained from the para-aromatic polyamide with or without a particulate polymer obtained from the para-aromatic polyamide, and subjecting the dispersion to papermaking in a known manner followed by drying the resulting paper. Then, said paper is impregnated with the above mentioned binder which is the characteristic feature of this invention. The impregnated paper is coagulated with aqueous medium and subjected to washing and drying under pressure. The kind, combination and amount of the para-aromatic polyamides in various forms as mentioned above can be appropriately selected depending upon the purpose. For example, for obtaining a para-aramid paper having a good formation, the pulp form is preferred, and when the paper strength is important, it is preferable to use short fibers and/or stable fibers in combination with pulp.

Here, the pulp, short fiber, stable fiber and particulate polymer obtained from the para-aromatic polyamide may be those produced in a known manner. The web is prepared by subjecting to paper making a dispersion of the papermaking material in water. The papermaking method is not critical, and the papermaking can be effected by a conventional paper making machine. The papermaking can be effected by hand making; however, a Fourdrinier paper machine or cylinder paper machine can be used in industry. Furthermore, the papermaking can be effected by a paper machine provided with Rotoformer or the like. The method of impregnating the web with the binder is not critical and, for example, a conventional impregnating machine can be used. Specifically, Yoshihito Miura, "Fushokufu Yoron" published by Kobunshi Kankokai states machines and methods. The important point in this invention is to sufficiently remove free water. 'When a large amount of free water is present, the binder contacts with the free water, upon which coagulation starts from the surface to make it impossible to impregnate the interior of the web with the binder. As a result, the binder attaches only the surface of the web and the paper strength-increasing effect becomes small.

The web impregnated with the binder is immersed in a binder- coagulating solution and then sufficiently washing the web and then the solvent is removed. The coagulating solution to be used is preferably an aqueous N-methylolpyrrolidone solution, a mixed solution of N-methylolpyrrolidone and methanol or the like. The thus obtained web is dried under pressure. Consequently, agglutination portions are formed between the para-aramids in various forms constituting the web. For example, when the web is dried by calendering under pressure, preferably, the free water attaching to the wet paper is removed, and thereafter, the paper is heat-pressed to be dried, upon which a para- aramid paper having excellent formation and paper strength is obtained. According to a small scale method, the above-mentioned purpose can be achieved by heating under pressure the web covered with a metal foil or a heat-resistant resin film. However, it is also possible to heat the web under pressure while continuously removing the free water in the above calendering. The heat-drying conditions are preferably such that the drying is effected at a temperature of 80°C or more under pressure, to form a para-aramid paper having agglutinate portions. When the temperature is less than 80°C, it takes a longer time for the drying and hence such a temperature is disadvantageous in industry.

The drying conditions in an industrial scale production, namely temperature and pressure, are determined depending upon the density, strength and the like of the desired para-aramid paper. For example, the temperature and pressure conditions in the so-called calendering are preferably 130°C or more and 10 kg/cm or more. By the production process of this invention, a para-aramid paper consisting essentially of para-aramid and having a breaking length of 3 km or more can be produced though the breaking length may be varied depending upon the kind and blending composition of para-aramid short fibers and the like.

The para-aramid paper obtained by the process of this invention can be used as an insulating paper. When the para-aramid paper is used as an insulating paper, mica, ground quartz, glass fibers, alumina, talc and the like can be incorporated into the paper for improving the insulating properties. On the other hand, alumina flakes, carbon black, stainless steel short fibers or the like may be incorporated into the paper to prepare a para-aramid paper having electrical conductivity.

Other uses include composite materials having a honeycomb structure in which the para-aramid paper prepared by the process of this invention is used as a reinforcing material. Also, the para-aramid paper can be applied to uses such as particle boards and the like as a substrate for adiabatic and fireproofing walls. DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is further explained in detail below by way of Examples and Comparative Examples. The test method, the evaluation method and the judgement criteria in the Examples and Comparative Examples were as stated below.

(1) Method of measuring inherent viscosity

Flow time of each of a solution of 0.5 g of the para-aramid in 100 ml of 96-98% sulfuric acid and 96-98% sulfuric acid per se was measured by means of a capillary viscometer at 30°C, and the inherent viscosity was determined from the ratio of the two flow times according to the following equation:

Inherent viscosity = ln(T/To)/C [unit dl/g] wherein T and To are the flow time of the para-aramid solution in sulfuric acid and the flow time of the sulfuric acid, respectively, and C is the concentration of the para-aramid solution (g/dl).

(2) Specific surface area

The specific surface area (m²/g) of the para-aramid pulp was determined by the BET specific surface area method from the amount of nitrogen adsorbed measured using Flowsoap 112300 manufactured by Micromeritics.

(3) Breaking length

The breaking length was determined by use of an Instron tensile tester according to JIS P8113.

(4) Observation of agglutinate portions (entangled portions) Using a scanning type electron microscope (SEM) manufactured by Hitachi Limited, the agglutinate portions of the para-aramid paper were observed. The case that in the portions in which the para-aramid short fibers or pulp are contacted with one another, their interface was not observed at a magnification of 5000, was judged that there was an agglutinate portion.

Synthesis Example 1 (Synthesis of Binder A)

In a 500-ml separable flask equipped with an agitating blade, a thermometer, a nitrogen-introducing tube and a powder-feeding inlet, poly(paraphenylene terephthalamide) was produced by polymerization. The flask was first dried, 25.3 9 of calcium chloride dried at 200°C for two hours was placed therein and thereafter 390 g of NMP was placed therein, after which the inner temperature of the flask was elevated to 85°C. After the calcium chloride was completely dissolved, the contents of the flask were allowed to cool to room temperature, and then, 12.5 g (0.117 mole) of paraphenylene-diamine (PPD) was added thereto and dissolved completely. The resulting solution was further cooled with ice-water at about 2-3°C, and when the inner temperature reached 5°C, about 1/3 of the total amount [22.74 g (0.112 mole)] of terephthaloyl chloride (TPC) to be added was gradually added to the solution. The inner temperature was elevated by the heat of polymeri¬ zation; however, after ten minutes, it lowered again to 3°C. At this time, about 1/3 of the total amount of TPC to be added was gradually added, upon which the inner temperature was elevated to 10°C; however, after ten minutes, it lowered again to 5°C, upon which the remaining TPC was gradually added. At this time, the viscosity of the polymerization mixture increased and no rapid polymerization occurred and the inner temperature was substantially 5°C and constant. After the TPC had been completely dissolved, the polymerization mixture was aged at 5°C for two hours. The polymerization mixture (polymer dope) after the ageing exhibited optical anisotropy and was a so-called lyotropic solution. This solution is referred to hereinafter as Binder A. The inherent viscosity of the poly(para phenylene terephthalamide) contained in Binder A was 1.54 dl/g.

Synthesis Example 2 (Synthesis of Binder B)

In the same manner as in Synthesis Example 1, except that 5.69 g of calcium chloride, 3.244 g (0.030 mole) of PPD and 5.888 g (0.029 mole) of TPC were used, polymerization was conducted. The polymerization mixture obtained was an isotropic solution. This solution is referred to hereinafter as Binder B. The inherent viscosity of the poly(paraphenylene phthalamide) contained in Binder B was 1.66 dl/g.

Synthesis Example 3 (Preparation of para-aramid pulp)

Short fibers of water-swollen poly(paraphenylene terephthalamide) were prepared by the following method:

1. Polymerization

In a 500-ml separable flask equipped with an agitating blade, a thermometer, a nitrogen-introducing tube and a powder-feeding inlet, paraphenylene terephthalamide was polymerized.

After the flask was sufficiently dried, 0.135 mole of dried calcium chloride was added to 300 g of NMP, and the calcium chloride was completely dissolved in NMP at an inner temperature of 85°C. Subsequently, 0.120 mole of PPD was added to the solution and the contents of the flask were cooled until the inner temperature reached -6°C, after which 0.115 mole of TPC was gradually added thereto while the inner temperature was kept at 5°C or less. After completion of the addition of TPC, ageing was effected at a temperature of -6° to 0°C for two hours to obtain a stable, liquid polymer dope. The inherent viscosity of the poly(paraphenyleneterephthalamide) contained in this dope was measured to find that it was 1.8 dl/g.

2. Spinning

The liquid polymer dope of the para-aramid thus obtained was spun using as a coagulating bath an aqueous aolution containing 20% by weight of NMP. The spinning nozzle used had a cone-shaped hole with a cylindrical end, the L/D of the cylindrical hole portion being 1 and the hole diameter in this portion being 0.07 mm. After the spinning, the filaments obtained were sufficiently washed with water and then cut to a fiber length of about 6 mm to obtain short fibers of the water-swollen para-aramid. A part of the short fibers were sampled and dried at 120°C for two hours to find that its solid content was about 20% by weight.

3. Preparation of pulp

Subsequently, the water-swollen para-aramid short fibers thus obtained were pulped in PFI mill manufactured by Kumagai Riki Kogyo K.K. The pulp thus obtained was in the wet state, and dispersed as it was in water and stored. A part of the pulp was sampled, dried and measured for specific surface area to find that it was about 2 m²/g. The solid content was about 20 by weight.

Example 1

Into 1,300 cc of deionized water were poured 1.6 g of short fibers (fiber length: mm) of para-aramid fibers (Twaron 1000, a trade name of Akzo Nobel), and they were treated by a home mixer for three minutes. Thereto were added 25 g (solid content: 3.5 g) of the water-swollen para-aramid pulp obtained in Synthesis Example 3 from which the free water had been sufficiently removed and 700 cc of deionized water. The resulting dispersion was subjected to beating by means of a standard pulper manufactured by Kumagai Riki Kogyo K.K. and then to papermaking by a standard square shaped sheet machine manufactured by Kumagai Riki Kogyo K.K. The web obtained by the papermaking was dried at 50°C for 60 minutes in an air-circulating type drier to completely remove the free water. The remaining water content of the dried web was 4.1% by weight. A coating film was formed on a flat glass plate using the Binder A of Synthetic Example 1 and the dried web was placed thereon, after which another flat glass plate was placed on the dried web. The resulting assembly was lightly pressed by hand to impregnate the web ith the binder and then the assembly was immersed in a 10% aqueous NMP solution, in which the glass plate was gradually peeled. Poly(paraphenylene terephthalamide) was coagulated from Binder A to obtain a sheet-shaped web. This sheet-shaped web was sufficiently washed with water and then sandwiched in between two sheets of copper foil having a thickness of 35 μm, and the resulting assembly was subjected to hot-calendering under pressure by means of a midget rolling mill manufactured by Kabushiki Kaisha Daito Seisakusho. The roll temperature at this time was 165°C and the roll nip was 340 μ .

The basis weight of the para-aramid paper thus obtained was 178 g/m² and the breaking length of the para-aramid paper was 8.2 km. The para- aramid paper was observed by SEM to find that agglutinate portions in which the fiber interface between the para-aramid pulp and the Twaron short fiber was indefinite were present all over the paper.

Examples 2 to 4

In accordance with Example 1, the thickness (basis weight) of the coating film of Binder A was varied to prepare a para-aramid paper. The measurement 15 results obtained are shown in Table 1. Table 1

Example No. Amount of Density Breaking Basis resin ^'from length weight of binder A paper

(g/m²) (g/cc) (km) (g/cm²)

2 28 0.73 4.2 111

3 72 0.85 7.5 156

4 109 0.92 8.1 193

Example 5

The same procedure as in Example 1 was repeated, except that Binder B was substituted for the Binder A, to prepare a para-aramid paper.

The basis weight of the para-aramid paper was 146 g/m² and the breaking length of the paper was 7.9 km.

Comparative Example 1

In accordance with the method of Example 1, 30 g of the para-aramid paper obtained in Synthesis Example 3 was subjected to papermaking and dried in a hot oven without being hot pressed. The dried paper obtained had a breaking length of 0.11 km. By the SEM observation, no definite agglutinate portions were found.

Comparative Example 2

In the same manner as in Comparative Example 1, a dried paper was obtained from 6 g of a commercially available para-aramid pulp (Twaron 1097, a trade name of Akzo) having a specific surface area of about 6 m²/g. The breaking length of the said paper was 0.08 km.

Claims

A process for producing a para-aromatic polyamide paper, characterized in that a solution consisting of a polar amide solvent having dissolved therein 0.5% to 10% by weight of a chloride of an alkali metal or an alkaline earth metal and 0.5% to 10% by weight of a para-aromatic polyamide having an inherent viscosity of 1.0 to 2.5 dl/g is used as a binder.

A process for producing a para-aromatic poly amide paper, characterized in that a web obtained by papermaking from a papermaking material comprising at least one component selected from the group consisting of pulp, short fibers and stable fibers of a para-aromatic polyamide is impregnated with the binder re-cited in Claim 1.

3. The process according to Claim 1 or 2, wherein the para-aromatic polyamide is poly(paraphenylene terephthalamide), poly(4,4'-benzanilideterephthalamide) , poly-(paraphenylene-4,4-biphenylene dicarboxamide) or poly-(paraphenylene-2,6-naphthalene dicarboxamide) .

4. The process according to Claim 1 or 2, wherein the para-aromatic polyamide paper has a breaking length of 3 km or more.

5. The process according to Claim 1, wherein the inherent viscosity of the para-aromatic polyamide is 1.5 to 2.2 dl/g.

6. The process according to Claim 1, wherein the chloride of an alkali metal or an alkaline earth metal is lithium chloride or calcium chloride.

CONFIRMATION θnpγ

7. The process according to Claim 1, wherein the polar amide solvent is N,N-dimethylformamide, N,N dimethylacetamide, N-methyl -2-pyrrol idone or N,N,N',N' tetramethylurea.