Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 163739
Process for producing a polymerized, heat-resistant lacquer
The invention concerns a novel process for producing a polymeriæed, heat-
resistant, cellulose acetate based lacquer with a high carbon black
compatibility and a high pigment absorption for record carriers coated with
aluminium, as are used, for example, for electroerosion printers.
For coating record carriers, both nitrocellulose lacquers and cellulose
acetate lacquers have been used so far. Nitrocellulose lacquers have
proved to be little temperature-resistant leading to the print electrodes
being covered with a tough to firm cake of combustion products, such as
lacquer pyrolizate, moisture, and dust, so that they failed after some
time. In the presence of such hard pigments in the lacquer it is also
possible for the lacquer to be subject to automatic erosion in the area of
the pigment grains, so that the latter are torn from the lacquer film,
rolling under the print electrodes on the lacquer surface and interrupting
the print process. Of the cellulose acetate lacquers only the following
lacquers are suitable for such record carriers:
a) Cellulose propionates.
These lacquers have a moderate thermal stability with a relatively high
softening point of about 200 C to 220 C but a poor carbon black
compatibility and a low filler absorption, thus being little suited for
pigmentation.
b) Cellulose acetate butyrates.
Cellulose acetate butyrates (CAB) with a butyryl content of about 38 per
cent (dibutyrate) have a melting point of 160 C to 180 C, a good carbon
black and pigment compatibility and a good film elasticity.
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Cellulose acetate butyrates with a butyryl content of 15
percent to 17 percent (monobutyrates) have a high thermal
stability. An advantage is that the melting point and the
decomposition point practically coincide [220C to 240C~.
However, as the molecular weight decreases, there is also a
drop in the melting point of the monobutyrates. The
disadvantage of this polymer is its poor adhesion to paper
in conjunction with carbon black and its low pigment
absorption.
c) Pure cellulose lacquers.
These lacquers have the highest temperature resistance of
the cellulose acetate lacquers and a melting and
decomposition point of 240C. However, as lacquers for
record carriers, they are even less suited than the
above-mentioned monobutyrates.
The brittleness of cellulose acetate lacquers can be reduced
by the addition of plasticisers and by a low molecular
weight, but this simultaneously leads to a reduction in
their temperature resistance. Even the use of phosphoric
acid esters which are not easily inflammable does not
improve the temperature resistance of such lacquers. After
differently long printing periods, all of these lacquers
lead to the feared caking, i.e., the occurrence of hard
layers on the print electrodes, as described above.
As for cellulose based lacquers, such lacquers are for
technical and economic reasons superior to the other
lacquers used for the production of record carriers.
It is the object of the invention to provide a high
temperature resistant lacquer, which has a high carbon black
compatibility and a high pigment absorption, and which also
has a high elasticity and a good adhesion to paper and
aluminum. Such a temperature-resistant polymer can be
produced in accordance with the invention in that,
initially, the higher melting polymer is homogenized by
kneading at softening temperature, whereby the quantity of
plasticer added is such that the mechanical energy of the
kneading process is still transferred in full to the
polymer, that,
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1 1~373~
subsequently, the lower melting polymer is added and this mixture is
kneaded up to homogenization at about the mean softening temperature, and
that the copolymerizate thus obtained is processed to form a lacquer,
adding pigments, if required.
In this context the term "cellulose acetate" is to cover all cellulose
acetates on the one hand and cellulose acetate monobutyrates and cellulose
acetate propionates on the other.
The procedure preferably adopted is such that solvent-free cellulose
acetate and high-polymer solvent-free ethylene vinyl acetate copolymerizate
are processed in a kneader by kneading at about the mean softening
temperature to form a copolymerizate. It is particularly advantageous for
the cellulose acetate/polyvinyl acetate copolymerizate thus obtained to be
pigmented in a 5 per cent to 20 per cent solution in higher ketones or
esters with 3 per cent to 4 per cent carbon black and, depending upon the
degree of pol~ymeriz~tion, to be mixed with 0.5 per cent to 5 per cent of a
phosphoric acid ester as a plasticer.
As in this case radical polymerization is concerned, the actual polymer-
ization process is initiated and kept going by kneading a mixture of 50 per
cent to 90 per cent of the cellulose acetate with 50 per cent to 10 per
cent of the polyvinyl acetate or of the ethylene vinyl acetate copolymer-
izate, whereby kneading tears the lony molecule chains, thus producing the
radicals necessary for polymerization. At temperatures above 95 C bloc'.i
polymerization predominates but must not exceed a moderate degree to retain
the positive lacquer characteristics, such as elasticity, good solubilitv,
etc., and to raise the decomposition and melting temperature. The degree
of subsequent cross-linkage is determined by the addition of peroxides.
Therefore, it is advantageous for the mean molecular weight of the
copolymerizate to be reduced by further cold kneading or mastication to
such a degree that the viscosity and solubility required are obtained.
From this polymer the actual lacquer is subsequently produced as follows.
The cellulose acetate polyvinyl acetate copolymerizate and the copolymer-
izate obtained from the ethylene vinyl acetate copolymerizate, respect-
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ively, is pigmented as a 5 percellt to 20 percent solution inhigher ketones or esters with 3 percent to 4 percent carbon
black and, depending upon the degree of polymerization of
the copolymeri~ate, is mixed with 0.5 percent to 5 percent
of a phosphoric ester as a plasticer. For this purpose, a
phosphoric acide cresol ester is preferably used.
The electrode wear is at its lowest with purely carbon black
pigmented lacquers.
With lacquers pigmented in another manner it is advantageous
to retain at least 1 percent carbon black in the lacquer.
For this purpose, about 5 percent to 20 percent of an
anorganic pigment, e.g., calcium carbonate, can be added.
To make up for the missing blackening and covering power of
carbon black, several percent bitumen or coal tar pitch are
preferably added. Black organic dyestuffs are also suitable
for this purpose. Finally, it is pointed out that all
details refer to the liquid finished lacquer.
The process in accordance with the invention will be
described in detail below by means of several tests carried
out and the results obtained.
As starting materials the following polymers were used:
1. A cellulose acetate butyrate with a viscosity of a 20
percent solution in acetone ethanol 9 : 1 measured at 23C
from 2500 to 4600 mPa.s and a butyryl content of 15 percent
and a softening temperature of 220C as well as a molecular
weight of about 40 000,
2. a cellulose acetate butyrate with a viscosity of 10 000
to 15 000 mPa.s, a butyryl content of 16.5 percent, a
softening temperature of 240C, and a molecular weight of
about 55 000,
3. an ethylene vinyl acetate copolymerizate with a vinyl
acetate content of 60 percent, a Mooney viscosity ML4 at
100C of 40 + 5,
4. an ethylene vinyl acetate copolymerizate with a vinyl
acetate content of 70 percent and a Mooney viscosity ML4 at
100C of 60 + 5,
5. polyvinyl acetate with a viscosity of a 10 percent ethyl
acetate solution of 700 mPa.s and a softening temperature of
200C as well as a molecular weight of 1 100 000.
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1 ~637~9
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With these materials the following tests were carried out.
Tc5t lio. 1:
lo an oil-heated kneader with a capacity of 1 liter 480 g of a cellulose
monobutyrate were added in portions and heated up to the softening
temperature of 220 C. At the same time, 60 g of a plasticer were added in
portions, taking care that the softness of the mixture is such that the
mechanical energy of the kneader is still transferred to the polyrer.
Energy transfer is prevented if the quantity of plasticer added or the
temperature used for kneading is too high. This adversely affects the
0 number of radicals which are required and available for subsequent block
polymerization and which are obtained by tearing the molecule chains. The
polymer was kneaded for about 15 minutes until the mass had a silky
appearance and all grains or inhomogeneities had disappeared.
Subsequently, about 110 g of an ethylene vinyl acetate copolymer of no. 4
were added and kneading continued for another 20 minutes until the
corJol-~nerizate obtained during that process was completely homogeneous.
Tests have shown that the copolymerizate thus obtained is a block polymer-
izate. After 24 hours, the polymer product was dissolved in ethyl acetate
(10 per cent solution) and lcft for two days. During that time the
unreacted polymer parts collected at the top as a turbid layer. The clear,
pure copolymer solution was drained off mixed with 3 per cent carbon black
and 5 per cent CaC03, and processed by meanâ of a stirrer to form a
lacquer. A comparison with a pure cellulose acetate butyrate lacquer
showed a noticeably reduced shrinkage of the new lacquer, which ~mongst
other characteristics is indicative of an improved pigment compatibility.
~dhesion was improved, too. The new polymer has also a much ilighcr
hardness.
Test No. 2:
This test was carried out in the same manner as test no. 1. The cellulose
acetate butyrate no. 2 was homogenized in a kneader at 2~0 C and mixed
with 110 g polyvinyl acetate of the starting material no. 5. The total
kneading period was 45 minutes, then the mixture was cooled to about
150 C, and kneading continued until a coarse-grained granular material had
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been obtained. The pure copolymer has a high hardness, a good solubility
in ethyl acetate, and good film formin~ properties. The lacuer ~roduced
therefron has a very good adhesion to paper and very little shrinkage even
without the addition of a plasticer.
Test No. 3:
560 g of the starting material no. 2 were homogenized by kneading at 240 C
and mixed with 110 g of the starting material no. 3, and copolymerized in
accordance with the same process as in test no. 1. After about ~5 minutes
kneading was completed.
During this test only very little copolyrner was obtained after dissolut on
in ethyl acetate after 48 hours. A longer reaction period would lead to a
higher yield, particularly if the subsequent cross-linkage is effected
by means of a sufficiently temperature stable (200 - 220 C) orsanic
peroxide. It is also possible to obtain a higher yield by adding the
starting polyrners at a ratio of 1 : 1 and by subsequently separating the
unreacted parts.
All copolyrners thus produced are at least 20 C to 30 C more temperature-
resistant than previously used cellulose acctate butyrates and have a much
higher hardness, a substantially improved carbon black compatibility, and
a far higher pigment absorption than previously known cellulose acetate
butyrates.
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