CA2075894A1 - Process for the in-line drying of polyamide/imide wire enamels containing n-methylpyrrolidone in enamel-application units - Google Patents
Process for the in-line drying of polyamide/imide wire enamels containing n-methylpyrrolidone in enamel-application unitsInfo
- Publication number
- CA2075894A1 CA2075894A1 CA002075894A CA2075894A CA2075894A1 CA 2075894 A1 CA2075894 A1 CA 2075894A1 CA 002075894 A CA002075894 A CA 002075894A CA 2075894 A CA2075894 A CA 2075894A CA 2075894 A1 CA2075894 A1 CA 2075894A1
- Authority
- CA
- Canada
- Prior art keywords
- enamel
- drying agent
- polyamide
- drying
- methylpyrrolidone
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B13/00—Apparatus or processes specially adapted for manufacturing conductors or cables
- H01B13/06—Insulating conductors or cables
- H01B13/065—Insulating conductors with lacquers or enamels
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
- B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
- B05D7/00—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials
- B05D7/20—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials to wires
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
- B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
- B05D3/00—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials
- B05D3/007—After-treatment
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B3/00—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties
- H01B3/18—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances
- H01B3/30—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes
- H01B3/303—Macromolecular compounds obtained by reactions forming a linkage containing nitrogen with or without oxygen or carbon in the main chain of the macromolecule, not provided for in groups H01B3/38 or H01B3/302
- H01B3/306—Polyimides or polyesterimides
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- Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- Spectroscopy & Molecular Physics (AREA)
- Wood Science & Technology (AREA)
- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Manufacturing & Machinery (AREA)
- Life Sciences & Earth Sciences (AREA)
- Organic Insulating Materials (AREA)
- Paints Or Removers (AREA)
- Macromolecular Compounds Obtained By Forming Nitrogen-Containing Linkages In General (AREA)
- Insulated Conductors (AREA)
- Solid-Sorbent Or Filter-Aiding Compositions (AREA)
- Polyamides (AREA)
- Processes Of Treating Macromolecular Substances (AREA)
- Treatments For Attaching Organic Compounds To Fibrous Goods (AREA)
- Drying Of Solid Materials (AREA)
- Manufacture Of Porous Articles, And Recovery And Treatment Of Waste Products (AREA)
- Application Of Or Painting With Fluid Materials (AREA)
Abstract
22.01.90/fe Process for in-line drying of polyamide-imide wire enamels containing N-methylpyrrolidone in enamelling plants Abstract The present invention relates to a process for drying polyamide-imide wire enamels which contain N-methylpyrrolidone, have viscosities of at least 100 mPas at 23°C and are passed in circulation between the enamel applicator and reservoir tank in enamelling plants, in which the enamel is pumped through one or more tanks containing a drying agent.
Description
PA~ 90 163 2g~ 22.01.gO/fe Dr. Beck & Co., Aktiengesellschaft, Hamburg Process for in-line dryinq of polyamide-imide wire enamels containinq N-methylpyrrolidone in enamellinq plants The present invention relate~ to a process for drying polyamide-imide wire enamels containing N-methyl-pyrxolidone which are passed in circulation between the enamel applicator and reservoir tank in enamelling plants.
When hygroscopic enamels based on moisture-sensitive binders and/or hardeners are proce~sed in enamelling plants with a large open enamel surface and at :~ 15 the same time a high absolute atmospheric humidity, troublesome gel formation and thus considerable economic loss due to loss of production, loss of material and additional personnel costs often OGCur.
-~ Such problems arise in particular when processing polyamide-imide wire enamels containing N-me~hylpyr-rolidone as the solvent. Various problem already occur at water contents of these wire enamels above about 5 %
by weight. The increase in the water content can thus lead to an increase in the viscosity of the enamel, which has an adverse effect on its processibility. The water content of the enamels moreover leads to surface defects in the stoved enamel films. Under adverse conditions, - such as, for example, too high an absolute atmospheric h~midity, con~iderable economic ~s~ ~ conse~uen~ly occurred to date.
Although the uptake of water by the enamel can be re-duced, for example, by increasing the content of blender in the enamel and the content of N-methylpyrrolidone is therefore reduced, the content of blenders cannot be increased to an unlimited extent. Another possibility comprises encap~ulating the application system and blanketing with dry inert gas. However, even this does not provide complete protection.
It is known from Japanese Published Specification 49,048,727 that surface defects of clear enamel coatings based on acrylate copolymer solutions can be avoided by adding a molecular sieve to the acrylate copolymer solution be~ore mixing with the crosslinking agent and application of the clear enamel, stirring the mixture for some time and then removing the molecular sieve by filtration. This drying of enamels by means of a molecular sieve is also described in Japanese Published Specification 59,102,932 for drying electrically conduct-ive ~ransparent coating age~t~. ~Iowever, th se coating agents de6cribed in that specification are preferably applied by mean~ of spray application and therefore have a low viscosity which i8 absolutely essential or this application. Moreover, they do not contain N-methylpyr-rolidone as a solvent, precisely this solvent being responsible for the problems with polyamide-Lmide wire enamels.
As is known, molecular sieves are also added to ~7-~
enamels directly as a drying agent. Thus, for example -as described in German Auslegeschrift 1,143,634 - alkali metal aluminum silicates having a zeolite structure are incorporated directly into polyurethane compositions as a drying agent, in order thus to avoid premature gelling of the compo~itions by reaction of the free NC0 groups with moisture originating from the pigments employed.
However, since these silicates are not removed before application of the coating compositions, this drying method is limited to a few intended uses where these substances in the coatings do not cause trouble.
The use of this process for wire enamels, for example, is not possible.
It is furthermore known that moisture also leads to changes in the dielectric properties of electrical insulation liquids and moisture must therefore also be excluded when filling, for example, distribution trans-formers with electrical insulating liquid. These electrical in~ulating liquids are therefore dried direct-ly after their preparation and before being introducedinto the transformer. Thi is effected, for example, by channelling the electrical insulating liquid several times, by means of a pump device, through a column filled with molecular sieve. Since electrical insulating liquids are usually of low viscosity (dynamic viscosity ~ 100 mPas at 23~C), thi~ drying causes no problems.
Finally, various possibilities are known for drying hygroscopic ~olvent~. Thus, drying of N-methylpyrrolidone - 4 ~
by means of a process which is practicable in industry is described, for example, in German Offenlegungs-schrift 2,709, 679. In this process, the water is removed by stripping with an inert gas at a temperature above 100C. However, it is not possible to use this process on enamels containing N-methylpyrrolidone, since the solid and therefore the viscosity of the enamels would be changed due to the losses of N-methylpyrrolidone which ari~e during this process.
The present invention was based on the object of providing a process which enables hygroscopic polyamide-Lmide wire enamels containing N-methylpyrrolidone to be processed in plants with a larga free enamel surface even under a high absolute atmospheric humidity without deteriorations occurring in the industrial properties of the enamel. In particular, the gelling of the enamels which often oCcurc in this case should be a~oided. -It should furthermore be possible to carry out this process inexpen ively and without a high expenditure on mainten-ance. Above all, it should be possible to carry out thisprocess in ~he customary existing enamelling plants, without ma~or conversion work on ~he existing enamelling plants being nece~sary. Moreover, in spite of integration of the drying process, continuous automated operation of the enamelling plant should continue to be ensured and the additional maintenance expenditure on the plant required because of the integration of the drying process should be as low as possible.
Surprisingly, thi~ object is achieved by a - 5 - ~ 3 .~
process for drying polyamide-imide wire enamel~ which contain N-methylpyrrolidone, have viscosities of at least 100 mPa~ at 23C and are pa~sed in circulation beween the enamel applicator and reservoir tank in enamelling plants by pumping the enamel through one or more tank~ contain-ing a drying agent.
On the basis of the very high viscosity of the polyamide-imide wire enamels containing N~methylpyr-rolidone, it is surprising and was not foreseeable that the water content of enamels passed in circulation between the enamel applicator and reservoir tank in anamelling plants can be reduced by the process according to the invention, so that these hygroscopic polyamide-imide wire enamels with SQnSitive contents can also be 1~ processed in plants with a large free enamel surface, even under a high ab olute atmospheric humidity, without deteriorations occurring in the industrial properties of the enamel.
The process according to the invention moreover : 20 has the advantage that it can be carried out inexpens-ively, no ma~or conversion work on existing enamelling plants is needed for carrying out the process, continuous automated operation of the enamelling plant continues to be ensured and the additional e~penditure on maintenance of the plant caused by integration of the enamel drying is low.
It is furthermore also surprising inasmuch as the water content of the enamel~ can be reduced by the process according to the invention, since the - 6 ~
N-methylpyrrolidone itself is dried not by treatment with molecular sieves but by means of expensive processes, such as, for example, the process described in German Offenlegungsschrift 2,709,679.
All the known drying agents which are inert toward~ the enamel to be dried, such as, in particular, various molecular sieves and silica gel, are suitable for use in the process according to the invention.
The silica gels employed as the drying agent, which are often also called silicic-acid gels, are colloidal silicic acid, the pore structure of which can be controlled by appropriate choice of the preparation conditions. The so-called narrow-pored silica gels having pore openings of 3 to 5 10-1 m, preferably 3 to 4 10-1 m, are preferably employed in the process according to the invention.
The molecular sieves employed accordi~g to the invention a~ the drying agent are synthetic or naturally occurring alkali metal and alkaline earth metal alumin~m silicates having a zeolite structure, of the general formula Me2/zO Al2O3 xSiO2. Me in this formula repre-sents an alkali metal or alkaline earth metal r X repre-sen~s the valency of the metal and x preferably assumes values between 1.8 and 2. The so-called narrow-pored zeoli~es of the A ~ype with pore openings of 3-5 10-1 m are thus employed. The pore openings can be changed in a controlled manner for a given x value through the choice of alkali metal or alkaline earth metal Me employed. The zeolite of the A type where x = 2 thu~ has pores of .
3 101 m in the K form (Me = potassium), pores of 4 10-1 m in the Na form and pores of 5 10-1 m in the Ca form. The silicates which are preferably employed in the process according to the invention are the alkali metal alumosilicates, particularly preferably the sodium alumosilicates, in each case - as already stated - having an x value of preferably 1.8 to 2Ø
When choosing the particular molecular sieve employed, it should also be remembered that many chemical ; 10 and physical properties, that is to say not only the pore width, are influenced by the aluminum content of the zeolites. Under certain circumstances, various molecular sieves in each case produce optLmum results, depending on the chemical build-up of the enamel to be dried. However, the particular optimum molecular sieve can easily be determined by means of a few experiments on the basis of the abovementioned values.
The drying agent is in general employed in the pxoce~s according to the invention in the form of bead granules. The average size of the granules is preferably a particle diameter of be~ween 1 and 5 mm. In order to ensure a sufficiently high flow rate of the enamel through the tanks filled with the drying agent, it is nece~sary to choo~e drying agents have a larger particle diameter, the higher the viscosity of the enamel to be dried.
Synthetic zeolite~ of the A type in the sodium form having a pore width of 3-4 10-1 m in the form of granules having an average particle diameter of 2 - 3 mm - 8 - ~ ~f~ s~
are preferably employed for drying wire enamel~ which contain N-methylpyrrolidone, are based on polyamide-imide~ and usually have viscosities at 23C of at least 100 mPas, preferably 100 to 10,000 mPas.
The molecular sieves or silica gels are advant-ageously activated by being heated at temperatures of 350 to 400C for several hours before being used in the ; process according to the invention. The water uptake ; capacity of the molecular sieves is then usually about 15 to 20 ~ of the intrinsic weight at 23C.
The regeneration of the drying agent, that is to say release of the water adsorbed, is also effected analogously, simply by heating at about 350 - 400C for several hours. This requires very easy and inexpensive maintenance of the device for carrying out the process according to the invention which i~ integrated in~o the enamelling plant.
The amount of drying agents to be used and the time intervals within which the drying agents must be regenerated depend on the amount of moisture to be removed from the enamels and thus depend on a large number of factors, ~uch as, for example, the ~ize of the open enamel ~urface, the level of the absolute atmosp-heric humidity of the environment, the maximum water content which can be tolerated by the enamel and the tendency of the enamel to take up water. The amount of drying agent employed i~ in general between 1 and 10 parts by weight per 100 parts by weight of enamel. The amount which i~ the most favorable in an individual case can ea~ily be determined with the aid of a few experi-ment~. The amount of drying agent employed i8 usually cho~en so that regeneration is necessary within a time interval of 3 to 4 weaks.
The choice of tanks which contain the drying agent is not critical. Commercially available flow-through cells, columns, cartridges or the like can thus be used. It is possible to use either several smaller tanks or one or a few larger tanks here.
Because of the high viscosity of the enamels to be dried, it is necessary to pump the enamel through the tank or tanks. Examples of suitable pumps are the gear pumps and piston pumps usually employed as enamel pumps.
The flow rate of the en~mel to be dried through the tank or tanks containing the drying agent is optimized with the aid of these pump5 as a function of the viscosity and temperature of the enamel, the diameter and height of the tanks containing the drying agent and the particle diameter of the drying agent.
The ~ank or tanks containing the drying agent i5 or are preferably incorporated directly into the enamel cixculation of the enamelling plant. This tank or these ta~ks is or are particularly preferably installed in the enamel reflux from the application sy3tem to the enamel -~ 25 reservoir tank. However, it is of course also possible for the tanX or tanks to be installed in a secondary ; circulation and for the enamel to be pumped through this secondary circulation as required.
To avoid contamination of the enamel by any r 0 2~
abraded drying agent it is appropriate for one or more ~ilters to be included downs~ream of the drying tanks.
Suitable filters are the filters usually employed in the enamel industry, such as, for example, bag filters. The pore width of these ~ilters is usually between 1 and 15 ym.
The enamelling plants into which this in-line drying device in question is integrated are the enamel]ing plants usually employed. They are described, for example, in the journal beck isolier technik, Volume 23, May 1975, issue 50, page 57 et seq.
The process according to the invention can be used analogous for all hygroscopic enamels which are based on moisture-sensitive binders and/or hardeners and are processed in enamelling plants. However, in order to ensure that these enamels can be adequately pumped or flow adequately, the viscosity of these enamels should as far as possible be below 1,000 mPas at 23C.
A typical field of use of the process according to the invention is the drying of polyamide-imide wire enamels containing N-methylpyrrolidone [sic]. The wire enamels usually have viscosities at 23C of between 100 and 10,000 mPas at a solids content of 20 to 40% by weight.
The polyamide-imides used in wire enamels are - known and described, for example, in United States Patent Specification 3,554,984, German Offenlegungsschrift 2,556,523, German Auslegeschrift 1,266,427 and German Offenlegungsschrift 1,95~,512. The polyamide-imides are prepared in a known manner from polycarboxylic acids or anhydrides thereof in which 2 carboxyl groups are in the vicinal position and which must also have at least one further functional group, and polyamines containing at least one primary amino group which is capable of forming an imide ring, or compounds having at least 2 isocyanate groups. The polyamide-imides can also be obtained by reaction of polyamides, polyisocyanates which contain at least 2 NCO groups and cyclic dicarboxylic anhydrides which contain at least one other g.roup which i~ capable of condensation or addition~
It is furthermore also possible for the polyamide-imides to be prepared from diisocyanates or diamines and dicarboxylic acids if one of the components ; already contains the imide group. Thus, ln particular, a tricarhoxylic anhydrlde can first be reacted wlth a dlprlmary diamlne to give the correspondiny dllmido-.~
~ 12 -carboxylic acid, which is then reacted with a diisocyan-ate to give the polyamide-imide.
Tricarboxylic acids or their anhydrides in which 2 carboxyl groups are in the vicinal position are prefer-ably employed f or the preparation of the polyamide-imides. The corresponding aromatic tricarboxylic an-hydrides, such as, for example, trimellitic anhydride, naphthalenetricarboxylic anhydrides, bisphenyltri-carboxylic anhydrides and other tricarboxylic acids having 2 benzene nuclei in the molecule and 2 vicinal carboxyl groups, such as the examples listed in German Offenlegungsschrift 1,956,512, are preferred. The tri-mellitic anhydride is especially pref erably pref erred .
The diprimary diamines already described f or the poly-amidocarboxylic acids can be employed as the amine component. It is furthermore also possible to employ aromatic diamines which contain a thiadiazole ring~ such as, f or example, 2, 5-bis- ( 4 -aminoph nyl ) -1, 3, 4 -thiadia-zole, 2, 5-bis- ( 3 aminophenyl ) -1, 3, 4-thiadiazole, 2- ( 4-aminophenyl)-5-(3-aminophenyl)-1,3,4-thiadiazole and mixture~ of the various isomers.
Suitable diisocyanates for the preparation of the polyamide-imides are aliphatic diisocyanates, such as, f or example, tetramethylene, hexamethylene, heptamethyl-ene and trimethylhexamethylene diisocyanates; cycloali-phatic diisocyanates, such a~, for example, isophorone diisocyanate, ~, '-diisocyanato-1,4-dimethylcyclohexane cyclohexane 1, 3-diisocyanate, cyclohexane 1, 4-diisocyanate ,- l-methylcyclohexane 2, 4-diisocyanate - 13 - ~ ~t~
and dicyclohexylmethane 4,4'-diisocyanate; aromatic diisocyanates, such as, for example, phenylene diisocyan-ates, toluylene diisocyanates, naphthalene diisocyanates and xylene diisocyanates and substituted aromatic systems, such as, for example, diphenyl ether diisocyan-ates, diphenyl sulfide diisocyanates, diphenyl sulfone diisocyanates and diphenylmethane diisocyanates; and mixed aromatic-aliphatic andaromatic-hydroaromatic diisocyanates, such as, for example, 4-phenyl isocyanate methyl [sic] isocyanate and tetrahydronaphthylene 1,5-diisocyanate and hexahydrobenzidine 4,4'-diisocyan-ate. Diphenylmethane 4,4'-diisocyanate, toluylene 2,4-diisocyanate and toluylene 2,6-diisocyanate and hexamethylene diisocyanate are preferably employed.
Suitable polyamides are those polyamides which have been obtained by polycondensation of dicarboxylic acids or derivatives thereof with diamines, or of amino-carboxylic acids and their derivatives, such as, for example, lactams.
The following polyamides may be mentioned as examples: dimethylenesuccinic acid amide, pentamethylene-pimelic acid amide, undecanemethylenetridecanedicar-: boxylic acid amide, hexamethyleneadipic acid amide ard polycaproic acid amide. Hexamethyleneadipic acid amide and polycaproic acid amide are particularly preferred.
Heavy metal salts which are soluble in the wire enamels, such as, for example~ zinc octoate, cadmium octoate, tetraisopropyl titanate or tetrabutyl titanate, can be employed as crosslinking catalysts in the hardening - 14 - ~r~ a~
of the polyamide-imides in an amount of up to 3 % by weight, based on the binder.
The invention will now be illustrated in more detail with reference to the following drawing and an S example.
Figure 1 shows the diagram of an enamelling plant in which the enamel is passed in circulation between the enamel applicator and reservoir tank. In this plant, the enamel is pumped out of the reservoir vessel (1) into the distributor vessel (3) by means of the gear pump (2). The enamel runs through the lines (4) which lead out of the interior of the distributor vessel into the application devices (5). If appropriate, some of the enamel can also be recycled from the distributor vessel directly into the reservoir vessel via a line (6). In the present plant, in each case 4 application devices lying one under the other are supplied from the distributor vessel. The enamel issuing from the lowest application devices is collected and pumped by means of a gear pump (7) through a tank (8) illed with drying agent, and from there is passed to the reservoir vessel (1).
The invention will now be illustrated in more detail in the following example. All the data on parts and percentages are weight data, unless expressly stated otherwise.
Preparation of a polyamide-imide wire enamPl A polyamide-imide is prepared, by the method described in German Auslegeschrift 1,266,427 from:
38.5 parts of trimellitic anhydride, - ' -"
, - 15 - 2 ~
60.0 parts of diphenylmethane diisocyanate.
The wire enamel is a 30 % strength solution of ` this polyamide-imide in a mixture of 65 parts of N-methylpyrrolidone and 35 parts of xylene. This wire enamel has a viscosity of 230 mPas at 23C.
Checking the water uptake of this wire enamel 1,500 g of enamel are heated at 35C in a open vessel (diameter of 150 mm) on a magnetic stirrer, while stirring. 330 ml of enamel per hour are pumped through a glass column (height of 350 mm, diameter of 30 mm, filled with 300 ml of glass beads having a diameter of 2-3 mm) by means of a piston pump. The enamel runs back over the column into the vessel. The room temperature is 23 - 24C
and the relative atmospheric humidity is 40 %.
Determination of the water content and viscosity of this enamel in the reservoir vessel as a function of time gives the values shown in Table 1. The water content was determined here by measurement of the refractive index, with the aid of a calibration curve.
Table 1 Time Water content Viscosity at 23C
in hours in ~ in mPas , - 0 1.74 300 1 2.12 420 2 2.62 445 3 3.12 510 4 3.62 560 4.02 620 - 16 - ~37 6 4.52 710 Example 1 The polyamide-imide wire lacquer is brought to a water content of about 6 % by addition of water. (Compare S Table 2).
Analogously to the procedure for checking the water uptake of the wire enamel, 1,500 g of the water-containing enamel are now heated to 35C, while stirring.
- The enamel is pumped through a glass column (height of 350 mm, diameter of 30 mm) using a piston pump (flow rate 330 ml of enamel/hour). The glass column is filled with 300 ml of a commercially available zeolite (bulk density about 480 g/l, nominal pore diameter about 4 lD-l m, oil number about 48, water uptake capacity about 24 %, average particle size 2 - 3 mm; commercial product "Bayer Zeolith T ~ 343" from Bayer). The enamel runs back over the column into the vessel. The room temperature is 23 -24C and the relative atmospheric humidity is 40 %.
Determination of the water content and the viscosity of the enamel in the reservoir ves~el as a function of time gives the values ~hown in Table 2. The water content~was ;~ determined here by measurement of the refractive index, with the aid of a calibration curve.
Table 2 Time Water content Viscosity at 23C
; in hours in ~ in mPas 0 6.22 1,018 1 4.92 730 - 17 -2J ~ ;~h~ ~
2 3.22 520 3 2.15 420 4 1.30 295 1.05 280 6 1.06 280 Example 1 shows that pumping the polyamide-imide enamel through the glass column filled with the molecular sieve is an effectiv2 method for xeducing the water content. By installing one or more tanks filled with drying agent into customary enamelling plants, the water content of the enamel passed in circulation between the enamel applicator and the reservoir tank is thus always effectively kept low, and the problems caused by water uptake which are usually observed, such as, for example, the increase in the viscosity of the enamel and the occurrence of surface defects on the stoved enamel films, are eliminated.
:
,
When hygroscopic enamels based on moisture-sensitive binders and/or hardeners are proce~sed in enamelling plants with a large open enamel surface and at :~ 15 the same time a high absolute atmospheric humidity, troublesome gel formation and thus considerable economic loss due to loss of production, loss of material and additional personnel costs often OGCur.
-~ Such problems arise in particular when processing polyamide-imide wire enamels containing N-me~hylpyr-rolidone as the solvent. Various problem already occur at water contents of these wire enamels above about 5 %
by weight. The increase in the water content can thus lead to an increase in the viscosity of the enamel, which has an adverse effect on its processibility. The water content of the enamels moreover leads to surface defects in the stoved enamel films. Under adverse conditions, - such as, for example, too high an absolute atmospheric h~midity, con~iderable economic ~s~ ~ conse~uen~ly occurred to date.
Although the uptake of water by the enamel can be re-duced, for example, by increasing the content of blender in the enamel and the content of N-methylpyrrolidone is therefore reduced, the content of blenders cannot be increased to an unlimited extent. Another possibility comprises encap~ulating the application system and blanketing with dry inert gas. However, even this does not provide complete protection.
It is known from Japanese Published Specification 49,048,727 that surface defects of clear enamel coatings based on acrylate copolymer solutions can be avoided by adding a molecular sieve to the acrylate copolymer solution be~ore mixing with the crosslinking agent and application of the clear enamel, stirring the mixture for some time and then removing the molecular sieve by filtration. This drying of enamels by means of a molecular sieve is also described in Japanese Published Specification 59,102,932 for drying electrically conduct-ive ~ransparent coating age~t~. ~Iowever, th se coating agents de6cribed in that specification are preferably applied by mean~ of spray application and therefore have a low viscosity which i8 absolutely essential or this application. Moreover, they do not contain N-methylpyr-rolidone as a solvent, precisely this solvent being responsible for the problems with polyamide-Lmide wire enamels.
As is known, molecular sieves are also added to ~7-~
enamels directly as a drying agent. Thus, for example -as described in German Auslegeschrift 1,143,634 - alkali metal aluminum silicates having a zeolite structure are incorporated directly into polyurethane compositions as a drying agent, in order thus to avoid premature gelling of the compo~itions by reaction of the free NC0 groups with moisture originating from the pigments employed.
However, since these silicates are not removed before application of the coating compositions, this drying method is limited to a few intended uses where these substances in the coatings do not cause trouble.
The use of this process for wire enamels, for example, is not possible.
It is furthermore known that moisture also leads to changes in the dielectric properties of electrical insulation liquids and moisture must therefore also be excluded when filling, for example, distribution trans-formers with electrical insulating liquid. These electrical in~ulating liquids are therefore dried direct-ly after their preparation and before being introducedinto the transformer. Thi is effected, for example, by channelling the electrical insulating liquid several times, by means of a pump device, through a column filled with molecular sieve. Since electrical insulating liquids are usually of low viscosity (dynamic viscosity ~ 100 mPas at 23~C), thi~ drying causes no problems.
Finally, various possibilities are known for drying hygroscopic ~olvent~. Thus, drying of N-methylpyrrolidone - 4 ~
by means of a process which is practicable in industry is described, for example, in German Offenlegungs-schrift 2,709, 679. In this process, the water is removed by stripping with an inert gas at a temperature above 100C. However, it is not possible to use this process on enamels containing N-methylpyrrolidone, since the solid and therefore the viscosity of the enamels would be changed due to the losses of N-methylpyrrolidone which ari~e during this process.
The present invention was based on the object of providing a process which enables hygroscopic polyamide-Lmide wire enamels containing N-methylpyrrolidone to be processed in plants with a larga free enamel surface even under a high absolute atmospheric humidity without deteriorations occurring in the industrial properties of the enamel. In particular, the gelling of the enamels which often oCcurc in this case should be a~oided. -It should furthermore be possible to carry out this process inexpen ively and without a high expenditure on mainten-ance. Above all, it should be possible to carry out thisprocess in ~he customary existing enamelling plants, without ma~or conversion work on ~he existing enamelling plants being nece~sary. Moreover, in spite of integration of the drying process, continuous automated operation of the enamelling plant should continue to be ensured and the additional maintenance expenditure on the plant required because of the integration of the drying process should be as low as possible.
Surprisingly, thi~ object is achieved by a - 5 - ~ 3 .~
process for drying polyamide-imide wire enamel~ which contain N-methylpyrrolidone, have viscosities of at least 100 mPa~ at 23C and are pa~sed in circulation beween the enamel applicator and reservoir tank in enamelling plants by pumping the enamel through one or more tank~ contain-ing a drying agent.
On the basis of the very high viscosity of the polyamide-imide wire enamels containing N~methylpyr-rolidone, it is surprising and was not foreseeable that the water content of enamels passed in circulation between the enamel applicator and reservoir tank in anamelling plants can be reduced by the process according to the invention, so that these hygroscopic polyamide-imide wire enamels with SQnSitive contents can also be 1~ processed in plants with a large free enamel surface, even under a high ab olute atmospheric humidity, without deteriorations occurring in the industrial properties of the enamel.
The process according to the invention moreover : 20 has the advantage that it can be carried out inexpens-ively, no ma~or conversion work on existing enamelling plants is needed for carrying out the process, continuous automated operation of the enamelling plant continues to be ensured and the additional e~penditure on maintenance of the plant caused by integration of the enamel drying is low.
It is furthermore also surprising inasmuch as the water content of the enamel~ can be reduced by the process according to the invention, since the - 6 ~
N-methylpyrrolidone itself is dried not by treatment with molecular sieves but by means of expensive processes, such as, for example, the process described in German Offenlegungsschrift 2,709,679.
All the known drying agents which are inert toward~ the enamel to be dried, such as, in particular, various molecular sieves and silica gel, are suitable for use in the process according to the invention.
The silica gels employed as the drying agent, which are often also called silicic-acid gels, are colloidal silicic acid, the pore structure of which can be controlled by appropriate choice of the preparation conditions. The so-called narrow-pored silica gels having pore openings of 3 to 5 10-1 m, preferably 3 to 4 10-1 m, are preferably employed in the process according to the invention.
The molecular sieves employed accordi~g to the invention a~ the drying agent are synthetic or naturally occurring alkali metal and alkaline earth metal alumin~m silicates having a zeolite structure, of the general formula Me2/zO Al2O3 xSiO2. Me in this formula repre-sents an alkali metal or alkaline earth metal r X repre-sen~s the valency of the metal and x preferably assumes values between 1.8 and 2. The so-called narrow-pored zeoli~es of the A ~ype with pore openings of 3-5 10-1 m are thus employed. The pore openings can be changed in a controlled manner for a given x value through the choice of alkali metal or alkaline earth metal Me employed. The zeolite of the A type where x = 2 thu~ has pores of .
3 101 m in the K form (Me = potassium), pores of 4 10-1 m in the Na form and pores of 5 10-1 m in the Ca form. The silicates which are preferably employed in the process according to the invention are the alkali metal alumosilicates, particularly preferably the sodium alumosilicates, in each case - as already stated - having an x value of preferably 1.8 to 2Ø
When choosing the particular molecular sieve employed, it should also be remembered that many chemical ; 10 and physical properties, that is to say not only the pore width, are influenced by the aluminum content of the zeolites. Under certain circumstances, various molecular sieves in each case produce optLmum results, depending on the chemical build-up of the enamel to be dried. However, the particular optimum molecular sieve can easily be determined by means of a few experiments on the basis of the abovementioned values.
The drying agent is in general employed in the pxoce~s according to the invention in the form of bead granules. The average size of the granules is preferably a particle diameter of be~ween 1 and 5 mm. In order to ensure a sufficiently high flow rate of the enamel through the tanks filled with the drying agent, it is nece~sary to choo~e drying agents have a larger particle diameter, the higher the viscosity of the enamel to be dried.
Synthetic zeolite~ of the A type in the sodium form having a pore width of 3-4 10-1 m in the form of granules having an average particle diameter of 2 - 3 mm - 8 - ~ ~f~ s~
are preferably employed for drying wire enamel~ which contain N-methylpyrrolidone, are based on polyamide-imide~ and usually have viscosities at 23C of at least 100 mPas, preferably 100 to 10,000 mPas.
The molecular sieves or silica gels are advant-ageously activated by being heated at temperatures of 350 to 400C for several hours before being used in the ; process according to the invention. The water uptake ; capacity of the molecular sieves is then usually about 15 to 20 ~ of the intrinsic weight at 23C.
The regeneration of the drying agent, that is to say release of the water adsorbed, is also effected analogously, simply by heating at about 350 - 400C for several hours. This requires very easy and inexpensive maintenance of the device for carrying out the process according to the invention which i~ integrated in~o the enamelling plant.
The amount of drying agents to be used and the time intervals within which the drying agents must be regenerated depend on the amount of moisture to be removed from the enamels and thus depend on a large number of factors, ~uch as, for example, the ~ize of the open enamel ~urface, the level of the absolute atmosp-heric humidity of the environment, the maximum water content which can be tolerated by the enamel and the tendency of the enamel to take up water. The amount of drying agent employed i~ in general between 1 and 10 parts by weight per 100 parts by weight of enamel. The amount which i~ the most favorable in an individual case can ea~ily be determined with the aid of a few experi-ment~. The amount of drying agent employed i8 usually cho~en so that regeneration is necessary within a time interval of 3 to 4 weaks.
The choice of tanks which contain the drying agent is not critical. Commercially available flow-through cells, columns, cartridges or the like can thus be used. It is possible to use either several smaller tanks or one or a few larger tanks here.
Because of the high viscosity of the enamels to be dried, it is necessary to pump the enamel through the tank or tanks. Examples of suitable pumps are the gear pumps and piston pumps usually employed as enamel pumps.
The flow rate of the en~mel to be dried through the tank or tanks containing the drying agent is optimized with the aid of these pump5 as a function of the viscosity and temperature of the enamel, the diameter and height of the tanks containing the drying agent and the particle diameter of the drying agent.
The ~ank or tanks containing the drying agent i5 or are preferably incorporated directly into the enamel cixculation of the enamelling plant. This tank or these ta~ks is or are particularly preferably installed in the enamel reflux from the application sy3tem to the enamel -~ 25 reservoir tank. However, it is of course also possible for the tanX or tanks to be installed in a secondary ; circulation and for the enamel to be pumped through this secondary circulation as required.
To avoid contamination of the enamel by any r 0 2~
abraded drying agent it is appropriate for one or more ~ilters to be included downs~ream of the drying tanks.
Suitable filters are the filters usually employed in the enamel industry, such as, for example, bag filters. The pore width of these ~ilters is usually between 1 and 15 ym.
The enamelling plants into which this in-line drying device in question is integrated are the enamel]ing plants usually employed. They are described, for example, in the journal beck isolier technik, Volume 23, May 1975, issue 50, page 57 et seq.
The process according to the invention can be used analogous for all hygroscopic enamels which are based on moisture-sensitive binders and/or hardeners and are processed in enamelling plants. However, in order to ensure that these enamels can be adequately pumped or flow adequately, the viscosity of these enamels should as far as possible be below 1,000 mPas at 23C.
A typical field of use of the process according to the invention is the drying of polyamide-imide wire enamels containing N-methylpyrrolidone [sic]. The wire enamels usually have viscosities at 23C of between 100 and 10,000 mPas at a solids content of 20 to 40% by weight.
The polyamide-imides used in wire enamels are - known and described, for example, in United States Patent Specification 3,554,984, German Offenlegungsschrift 2,556,523, German Auslegeschrift 1,266,427 and German Offenlegungsschrift 1,95~,512. The polyamide-imides are prepared in a known manner from polycarboxylic acids or anhydrides thereof in which 2 carboxyl groups are in the vicinal position and which must also have at least one further functional group, and polyamines containing at least one primary amino group which is capable of forming an imide ring, or compounds having at least 2 isocyanate groups. The polyamide-imides can also be obtained by reaction of polyamides, polyisocyanates which contain at least 2 NCO groups and cyclic dicarboxylic anhydrides which contain at least one other g.roup which i~ capable of condensation or addition~
It is furthermore also possible for the polyamide-imides to be prepared from diisocyanates or diamines and dicarboxylic acids if one of the components ; already contains the imide group. Thus, ln particular, a tricarhoxylic anhydrlde can first be reacted wlth a dlprlmary diamlne to give the correspondiny dllmido-.~
~ 12 -carboxylic acid, which is then reacted with a diisocyan-ate to give the polyamide-imide.
Tricarboxylic acids or their anhydrides in which 2 carboxyl groups are in the vicinal position are prefer-ably employed f or the preparation of the polyamide-imides. The corresponding aromatic tricarboxylic an-hydrides, such as, for example, trimellitic anhydride, naphthalenetricarboxylic anhydrides, bisphenyltri-carboxylic anhydrides and other tricarboxylic acids having 2 benzene nuclei in the molecule and 2 vicinal carboxyl groups, such as the examples listed in German Offenlegungsschrift 1,956,512, are preferred. The tri-mellitic anhydride is especially pref erably pref erred .
The diprimary diamines already described f or the poly-amidocarboxylic acids can be employed as the amine component. It is furthermore also possible to employ aromatic diamines which contain a thiadiazole ring~ such as, f or example, 2, 5-bis- ( 4 -aminoph nyl ) -1, 3, 4 -thiadia-zole, 2, 5-bis- ( 3 aminophenyl ) -1, 3, 4-thiadiazole, 2- ( 4-aminophenyl)-5-(3-aminophenyl)-1,3,4-thiadiazole and mixture~ of the various isomers.
Suitable diisocyanates for the preparation of the polyamide-imides are aliphatic diisocyanates, such as, f or example, tetramethylene, hexamethylene, heptamethyl-ene and trimethylhexamethylene diisocyanates; cycloali-phatic diisocyanates, such a~, for example, isophorone diisocyanate, ~, '-diisocyanato-1,4-dimethylcyclohexane cyclohexane 1, 3-diisocyanate, cyclohexane 1, 4-diisocyanate ,- l-methylcyclohexane 2, 4-diisocyanate - 13 - ~ ~t~
and dicyclohexylmethane 4,4'-diisocyanate; aromatic diisocyanates, such as, for example, phenylene diisocyan-ates, toluylene diisocyanates, naphthalene diisocyanates and xylene diisocyanates and substituted aromatic systems, such as, for example, diphenyl ether diisocyan-ates, diphenyl sulfide diisocyanates, diphenyl sulfone diisocyanates and diphenylmethane diisocyanates; and mixed aromatic-aliphatic andaromatic-hydroaromatic diisocyanates, such as, for example, 4-phenyl isocyanate methyl [sic] isocyanate and tetrahydronaphthylene 1,5-diisocyanate and hexahydrobenzidine 4,4'-diisocyan-ate. Diphenylmethane 4,4'-diisocyanate, toluylene 2,4-diisocyanate and toluylene 2,6-diisocyanate and hexamethylene diisocyanate are preferably employed.
Suitable polyamides are those polyamides which have been obtained by polycondensation of dicarboxylic acids or derivatives thereof with diamines, or of amino-carboxylic acids and their derivatives, such as, for example, lactams.
The following polyamides may be mentioned as examples: dimethylenesuccinic acid amide, pentamethylene-pimelic acid amide, undecanemethylenetridecanedicar-: boxylic acid amide, hexamethyleneadipic acid amide ard polycaproic acid amide. Hexamethyleneadipic acid amide and polycaproic acid amide are particularly preferred.
Heavy metal salts which are soluble in the wire enamels, such as, for example~ zinc octoate, cadmium octoate, tetraisopropyl titanate or tetrabutyl titanate, can be employed as crosslinking catalysts in the hardening - 14 - ~r~ a~
of the polyamide-imides in an amount of up to 3 % by weight, based on the binder.
The invention will now be illustrated in more detail with reference to the following drawing and an S example.
Figure 1 shows the diagram of an enamelling plant in which the enamel is passed in circulation between the enamel applicator and reservoir tank. In this plant, the enamel is pumped out of the reservoir vessel (1) into the distributor vessel (3) by means of the gear pump (2). The enamel runs through the lines (4) which lead out of the interior of the distributor vessel into the application devices (5). If appropriate, some of the enamel can also be recycled from the distributor vessel directly into the reservoir vessel via a line (6). In the present plant, in each case 4 application devices lying one under the other are supplied from the distributor vessel. The enamel issuing from the lowest application devices is collected and pumped by means of a gear pump (7) through a tank (8) illed with drying agent, and from there is passed to the reservoir vessel (1).
The invention will now be illustrated in more detail in the following example. All the data on parts and percentages are weight data, unless expressly stated otherwise.
Preparation of a polyamide-imide wire enamPl A polyamide-imide is prepared, by the method described in German Auslegeschrift 1,266,427 from:
38.5 parts of trimellitic anhydride, - ' -"
, - 15 - 2 ~
60.0 parts of diphenylmethane diisocyanate.
The wire enamel is a 30 % strength solution of ` this polyamide-imide in a mixture of 65 parts of N-methylpyrrolidone and 35 parts of xylene. This wire enamel has a viscosity of 230 mPas at 23C.
Checking the water uptake of this wire enamel 1,500 g of enamel are heated at 35C in a open vessel (diameter of 150 mm) on a magnetic stirrer, while stirring. 330 ml of enamel per hour are pumped through a glass column (height of 350 mm, diameter of 30 mm, filled with 300 ml of glass beads having a diameter of 2-3 mm) by means of a piston pump. The enamel runs back over the column into the vessel. The room temperature is 23 - 24C
and the relative atmospheric humidity is 40 %.
Determination of the water content and viscosity of this enamel in the reservoir vessel as a function of time gives the values shown in Table 1. The water content was determined here by measurement of the refractive index, with the aid of a calibration curve.
Table 1 Time Water content Viscosity at 23C
in hours in ~ in mPas , - 0 1.74 300 1 2.12 420 2 2.62 445 3 3.12 510 4 3.62 560 4.02 620 - 16 - ~37 6 4.52 710 Example 1 The polyamide-imide wire lacquer is brought to a water content of about 6 % by addition of water. (Compare S Table 2).
Analogously to the procedure for checking the water uptake of the wire enamel, 1,500 g of the water-containing enamel are now heated to 35C, while stirring.
- The enamel is pumped through a glass column (height of 350 mm, diameter of 30 mm) using a piston pump (flow rate 330 ml of enamel/hour). The glass column is filled with 300 ml of a commercially available zeolite (bulk density about 480 g/l, nominal pore diameter about 4 lD-l m, oil number about 48, water uptake capacity about 24 %, average particle size 2 - 3 mm; commercial product "Bayer Zeolith T ~ 343" from Bayer). The enamel runs back over the column into the vessel. The room temperature is 23 -24C and the relative atmospheric humidity is 40 %.
Determination of the water content and the viscosity of the enamel in the reservoir ves~el as a function of time gives the values ~hown in Table 2. The water content~was ;~ determined here by measurement of the refractive index, with the aid of a calibration curve.
Table 2 Time Water content Viscosity at 23C
; in hours in ~ in mPas 0 6.22 1,018 1 4.92 730 - 17 -2J ~ ;~h~ ~
2 3.22 520 3 2.15 420 4 1.30 295 1.05 280 6 1.06 280 Example 1 shows that pumping the polyamide-imide enamel through the glass column filled with the molecular sieve is an effectiv2 method for xeducing the water content. By installing one or more tanks filled with drying agent into customary enamelling plants, the water content of the enamel passed in circulation between the enamel applicator and the reservoir tank is thus always effectively kept low, and the problems caused by water uptake which are usually observed, such as, for example, the increase in the viscosity of the enamel and the occurrence of surface defects on the stoved enamel films, are eliminated.
:
,
Claims (7)
1. A process for drying a polyamide-imide wire enamel which contains N-methylpyrrolidone, has a vis-cosity of at least 100 mPas at 23°C and is passed in circulation between the enamel applicator and the reser-voir tank in enamelling plants, in which the enamel is pumped through one or more tanks containing a drying agent.
2. The process as claimed in claim 1, wherein naturally occurring or synthetic zeolites having an average pore diameter of 3 to 5 10-1° m and an average particle diameter of 1 to 5 mm are employed as the drying agent.
3. The process as claimed in either of claims 1 and 2, wherein an alkali metal alumosilicate is employed as the drying agent.
4. The process as claimed in any one of claims 1 to 3, wherein the drying agent is employed in an amount of 1 to 10 parts by weight per 100 parts by weight of enamel to be dried.
5. The process as claimed in any one of claims 1 to 4, wherein one or more filters are included downstream of the tank or tanks containing the drying agent.
6. The process as claimed in any one of claims 1 to 5, wherein the tank or tanks containing the drying agent is or are incorporated directly into the enamel circulation of the enamelling plant.
7. The process as claimed in claim 6, wherein the tank or tanks containing the drying agent is or are installed in the enamel reflux from the application system to the enamel reservoir tank.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DEP4004462.9 | 1990-02-14 | ||
| DE4004462A DE4004462A1 (en) | 1990-02-14 | 1990-02-14 | METHOD FOR INLINE DRYING POLYAMIDIMIDE WIRE VARNISHES CONTAINING N-METHYLPYRROLIDONE IN LACQUERING PLANTS |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2075894A1 true CA2075894A1 (en) | 1991-08-15 |
Family
ID=6400076
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002075894A Abandoned CA2075894A1 (en) | 1990-02-14 | 1991-01-22 | Process for the in-line drying of polyamide/imide wire enamels containing n-methylpyrrolidone in enamel-application units |
Country Status (10)
| Country | Link |
|---|---|
| EP (1) | EP0515405B1 (en) |
| JP (1) | JPH0630729B2 (en) |
| KR (1) | KR950004147B1 (en) |
| AT (1) | ATE115010T1 (en) |
| BR (1) | BR9106042A (en) |
| CA (1) | CA2075894A1 (en) |
| DE (2) | DE4004462A1 (en) |
| DK (1) | DK0515405T3 (en) |
| ES (1) | ES2067922T3 (en) |
| WO (1) | WO1991012087A1 (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2006346521A (en) * | 2005-06-13 | 2006-12-28 | Mitsui Chemicals Inc | Method for coating resin, film, and its use |
| EP3185259A1 (en) * | 2015-12-21 | 2017-06-28 | General Electric Technology GmbH | Method for regulating the humidity content level in an electrical transformer having oil-impregnated cellulosic insulating elements |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| NL277805A (en) * | 1961-07-21 | |||
| DE2432911A1 (en) * | 1974-07-09 | 1976-01-29 | Trafofit Gmbh | METHOD AND DEVICE FOR DRAWING WATER FROM DIELECTRIC LIQUIDS |
| GB2151501A (en) * | 1983-12-22 | 1985-07-24 | Coal Ind | Continuous sorption process |
| US4726818A (en) * | 1984-12-20 | 1988-02-23 | Union Carbide Corporation | Bulk removal of water from organic liquids |
-
1990
- 1990-02-14 DE DE4004462A patent/DE4004462A1/en not_active Withdrawn
-
1991
- 1991-01-22 EP EP91903197A patent/EP0515405B1/en not_active Expired - Lifetime
- 1991-01-22 DE DE59103814T patent/DE59103814D1/en not_active Expired - Fee Related
- 1991-01-22 AT AT91903197T patent/ATE115010T1/en not_active IP Right Cessation
- 1991-01-22 CA CA002075894A patent/CA2075894A1/en not_active Abandoned
- 1991-01-22 JP JP3503077A patent/JPH0630729B2/en not_active Expired - Lifetime
- 1991-01-22 KR KR1019920701935A patent/KR950004147B1/en not_active IP Right Cessation
- 1991-01-22 WO PCT/EP1991/000113 patent/WO1991012087A1/en active IP Right Grant
- 1991-01-22 DK DK91903197.1T patent/DK0515405T3/en active
- 1991-01-22 ES ES91903197T patent/ES2067922T3/en not_active Expired - Lifetime
- 1991-01-22 BR BR919106042A patent/BR9106042A/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| DE59103814D1 (en) | 1995-01-19 |
| EP0515405B1 (en) | 1994-12-07 |
| DE4004462A1 (en) | 1991-08-22 |
| DK0515405T3 (en) | 1995-04-18 |
| ATE115010T1 (en) | 1994-12-15 |
| JPH0630729B2 (en) | 1994-04-27 |
| JPH05506396A (en) | 1993-09-22 |
| KR920703214A (en) | 1992-12-17 |
| ES2067922T3 (en) | 1995-04-01 |
| WO1991012087A1 (en) | 1991-08-22 |
| KR950004147B1 (en) | 1995-04-27 |
| BR9106042A (en) | 1993-02-02 |
| EP0515405A1 (en) | 1992-12-02 |
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