Note: Descriptions are shown in the official language in which they were submitted.
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-~
-- 3 --
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
.
-- 7 --
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
- 9
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.
:
,
