Patent Application
20220403170
The invention relates to new polyimide solutions having ecological and toxicological benefits compared to existing polyimide solutions as well as to their use, in particular for coating applications.
Several soluble aromatic polyimides have been developed and are commercially available for different applications. WO2011/009919A1 for example discloses several soluble polyimides of the P84® type as well as processes for their production and their use to manufacture membranes. According to WO2011/009919A1 P84® type polyimides can be dissolved in DMF (dimethyl formamide), DMAc (Dimethylacetamid), NMP (N-Methyl-2-pyrrolidon), NEP (N-Ethyl-2-pyrrolidon), Sulfolan (Tetrahydrothiophen-1,1-dioxid) and DMSO (dimethyl sulfoxide).
Solvents of highest commercial relevance for polyimides are DMF, DMAc, NMP and NEP. These solvents, however, are classified CMR (Carcinogenic, Mutagenic and Reprotoxic) substances. DMF and NMP are in addition listed on the SVHC (Substances of Very High Concern) candidate list. As consequence, processing, i.e. manufacture, transport and use, of polyimide solutions comprising such solvents is subject to strict safety requirements. In fad, these limitations are a serious problem for many potential customers, in particular small and mid-size companies.
Due to the ecological, toxicological and administrative problems caused by NMP as solvent for polyimide or polyamidimide resins, paint manufacturers are looking for alternatives. DE 10 2014 104 223 A1 for example suggests use of 3-methoxy-N, N-dimethylpropane amide as a solvent in paint applications instead of NMP.
Among the solvents listed in WO2011/009919A1, DMSO is a less critical alternative. There are, however, a number of disadvantages for processing solutions of polyimides in DMSO. On the one hand, the high melting temperature of 18° C. may lead to freezing-out of the polyimide solutions in DMSO, for example during transport. On the other hand, P84® solutions in DMSO show a poor storage stability in air. In contact with humidity, phase inversion due to water absorption occurs within a short time (<1 h). Phenomenologically, this can be perceived by the formation of a white, firm skin on the surface of the solution. Especially for coating applications, this prohibits use of polymide solutions in DMSO.
US 2004/0266979 discloses a process for producing solvent soluble polyimides. Polyimides are produced by polycondensing at least one tetracarboxylic acid component with at least one diamine component in a solvent in the presence of a tertiary amine as catalyst. Analogue to WO2011/009919A1 theist of solvents that can be used comprises the CMR classified compounds DMF (dimethyl formamide), DMAc (dimethylacetamid). NMP (N-methyl-2-pyrrolidon), hexamethylphosphoramid, sulfolan, but also DMSO (dimethyl sulfoxide). According to the examples of US 2004/0266979 an inhomogeneous reaction mixture is obtained at the end of the polycondensation reaction. Large amounts of DMAc were added to the reaction mixture after completion of the polycondensation to obtain a uniform solution. Without use of DMAc as main solvent no uniform solution was obtained in US 2004/0266979. As consequence, US 2004/0266979 suffers from the same disadvantages as described for WO2011/009919A1 before.
U.S. Pat. No. 6,916,772 B2 discloses compositions for stripping photoresist compounds, such as polyimides, from substrates, comprising: a) about 5% to about 50% by weight of an alkyl substituted pyrrolidone, an alkyl substituted piperidone, or a mixture thereof, b) about 0.2% to about 20% of one or more alkanolamines, and c) about 50% to about 94% of a sulfoxide, a sulfoxide, or mixture thereof. Most of the compositions disclosed in U.S. Pat. No. 6,916,772 B2 comprise the CMR classified solvent NMP. The solvent used in US '772 therefore causes the same ecological, toxicological and administrative problems as discussed before. U.S. Pat. No. 6,916,772 B2 does not provide no hint that the polyimide solutions disclosed therein can be used for coating applications or that the polyimide solutions solve any of the problems solved by the present invention. U.S. Pat. No. 6,916,772 B2 relates to a totally different technical field.
As consequence, a strong demand exists for new solvents or solvent mixtures for polyimides, in particular for aromatic polyimides.
The problem to be solved by the present invention was to provide a new polyimide solution respectively new solvents or solvent mixtures for polyimides, not having the disadvantages of the polyimide solutions of the prior art or having such disadvantages only to a reduced degree. Further problems were to provide a process for manufacture such new polyimide solutions as well as to enable the use of the new polyimide solutions in different application fields, in particular in coating applications.
A first particular problem to be solved by the present invention was to provide new solvents or solvent mixtures for polyimides. Preferably these new solvents or solvent mixtures shah be less toxic and/or having beneficial ecological properties compared to solvents used in commercially available polyimide solutions at the time the invention was made respectively compared to polyimide coating solutions disclosed in the prior art cited above. Preferably use of CMR substances should be reduced or avoided.
A second particular problem to be solved by the present invention was to provide new solvents or solvent mixtures for polyimides allowing to produce polyimide solution with good storage stability in air, preferably with improved storage stability in air compared to solutions in pure DMSO, more preferred having a skin formation time if stored in air of at least 2 hours, even more preferred at least 4 hours, particular preferred at least 6 hours and most preferred of at least 8 hours.
A third particular problem to be solved by the present invention was to provide new solvents or solvent mixtures allowing to produce, transport and use polyimide solutions at a broad temperature range, in particular at low temperatures, especially at temperatures below 0° C.
A fourth particular problem to be solved by the present invention was to provide new solvents or solvent mixtures for polyimides having high solubility, i.e. allow to produce polyimide solutions with a high solid content and more preferred also with a low viscosity.
A firth particular problem to be solved by the present invention was to provide new solvents or solvent mixtures for polyimides allowing to produce polyimide solution which can be used in a wide variety of applications, especially in coating applications. Preferably it should be possible to obtain high quality coatings with good performance properties.
A sixth particular problem to be solved by the present invention was to provide new solvents or solvent mixtures for polyimides allowing to produce, transport and use polyimide solutions with less safety and administrative requirements.
Further problems not mentioned before will become obvious in view of the subsequent description, examples and claims.
The inventors surprisingly found out that a solvent mixture comprising DMSO and co-solvents as defined in claim 1 or one of the dependent claims can be used to produce polyimide solutions that solve the problems of the present invention.
The solvent mixture of the present invention can be used to dissolve polyimides, in particular aromatic polyimides, without use of any CMR and SVHC classified substances, lice NMP, NEP, DMAc and DMF as solvent or co-solvent, and are itself not classified as toxic.
Even though the solvent mixtures of the invention are based on DMSO, the formulations, i.e. polyimide solutions, of the present invention show significantly better properties in processing than polyimide solutions hi pure DMSO. Melting points below 0° C. down to −24° C. were achieved, which allows to produce, transport and use polyimide solutions of the invention in regions with very low temperature.
Even after a duration of 2 up to more than 8 hours in air, inventive polyimide formulations do not show any adverse skin formation if in contact with humidity.
Solutions with a polyimide content of at least 25% by weight with good or even very good, i.e. low, viscosity were obtained.
Further benefits not mentioned before will become obvious in view of the subsequent description, examples and claims.
Subject of the present invention is a polyimide solution of claim 1, a process for manufacture the polyimide solution according to claim 13 or 14 and the use of the inventive polyimide solution according to claim 16 or 17. Preferred embodiments are claimed in the dependent claims.
The terms “polyimide solution” and “polyimide formulation” are used synonymously in the present invention and relate to a formulation comprising a solvent mixture and at least one polyimide, wherein the term “solution” means that the polyimide(s) is/are dissolved in the solvent mixture and no undissolved polyimide particles are visible for the naked eye.
The present invention provides polyimide solution comprising
Preferably the amounts of the components a. to h. are selected such that they total in sum to 100 wt. % of the solvent mixture, i.e. no further co-solvents i. are comprised.
As polyimide a), aliphatic or aromatic polyimides can be used. Preferably aromatic polyimides are used. Aromatic polyimides, preferably made from dianhydrides and diisocyanates respectively diamines, as well as methods for their manufacture are known to a man skilled in the art. Homo-, random-, or copolymers or mixtures or blend of different polyimide polymers may be used. All kinds of solvent soluble aromatic polyimides can be used in the present invention.
As described before, especially in the prior art section, a strong demand exists for solvents or solvent mixtures for aromatic polyimides, being free of respectively having a very low content of solvents being classified as CMR and of SVHC substances. As shown in the examples below, the solvent mixture of the present invention fulfills these requirements and show excellent performance in combination with aromatic polyimides comprising at least 80 mol % aromatic monomers. It is thus preferred that the inventive polyimide solutions comprise as polyimide component a) a polyimide comprising 80 to 100% aromatic monomers, more preferred comprising 90 to 100% aromatic monomers, even more preferred comprising 95 to 100% aromatic monomers, particular preferred comprising 97 to 100% aromatic monomers and most preferred comprising 100% aromatic monomers.
Preferably the aromatic polyimide(s) comprise(s) identical or different recurring units of Formula (1)
wherein
the functional group RA represents one or more, identical or different moieties selected from the group consisting of the moieties RA1, RA2 and RA3
where
the functional group R8 represents one or more, identical or different moieties selected from the group consisting of the moieties RB1, RB2 and RB3
wherein Y1, Y2, Y3 and Y4 are either H or CH3 or alkyl radicals with 2 to 4 carbon atoms and Z═—CH2—, —(CH3)2C—, SO2. —(CF3)C—. —CO—, —COO—, —CONH—, —O—.
More preferred the polyimide(s) a) is/are prepared by reacting
with
or
Even more preferred
or
In a first particular preferred embodiment of the invention the polyimides having the following structure according to Formula (2):
where 0≤x≤0.5 and 1≥y≥0.5, the sum of x and y=1 and R represents one or more, identical or different moieties selected from the group consisting of the moieties L1, L2, L3 and L4.
The polyimide is very particularly preferably a polymer according to Formula (2) where x=0, y=1 and R consists of 64 mol % L2, 16 mol % L3 and 20 mol % L4. Such solvent soluble polyimide is known as P84® or P84® type 70 and has the following CAS number: 9046-51-9.
Also, very particular preferred the polyimide of Formula (2) is a polymer having the composition x=0.4, y=0.6 and R consists of 80 mol % L2 and 20 mol % L3. This solvent soluble polyimide is known as P84® HT or P84® HT 325 and has the following CAS number: 134119-41-8.
Details regarding the production of these and further similar polyimides according to Formula (2) can be extracted from WO 2011/009919 A1, the whole content of the documents is hereby explicitly incorporated in the description of the present invention by reference. All polymers described in the examples of WO 2011/009919 A1 are particularly preferably used in the process of the present invention.
DE 21 43 080 describes the manufacture of solvent soluble polyimides made from BTDA and mixtures of toluene-2,4-diisocyanate, toluene-2,6-diisocyanate and 4,4′-methylenediphenyl-diisocyanate. It also describes the manufacture of solvent soluble polyamic acid from BTDA and mixtures of toluene-2,4-diamine, toluene-2,6-diamine, 4,4′-methylenediphenyl-diamine as well as the subsequent imidation to the corresponding polyimide. Details regarding the production of these and further similar polyimides and polyamic acids can be extracted from DE 21 43 080, the whole content of both documents is hereby explicitly incorporated in the description of the present invention by reference. All polymers described in the examples of DE 21 43 080 are particularly preferred used in the process of the present invention.
Further preferred polyimides that can be used in the present invention are block-copolyamides, i.e. copolymers comprising, preferably consisting of, the blocks (A) as per the ensuing formulae (3), and (B) as per the ensuing formulae (4):
Said blocks A and B have a differing composition, i.e. the pairs R1 and R3 on the one hand and R2 and R4 on the other cannot each be identical at one and the same time.
The block copolyimide comprises a continuous phase of block A. The functional group R1 therein comprises either or both of the following functional groups:
R2 comprises at least one or 2 or 3 of the following functional groups
Block A has the following compositions in embodiments that are most preferable:
The recited mole percentages relate to the functional groups R1 and R2 such that the amounts of the various units are each selected such that the sum is 100 mol % for each of these groups.
where
R4 comprises at least one or more of the following functional groups
where Y1, Y2, Y3 and Y4 are either H or CH3 or alkyl radicals with 2 to 4 carbon atoms and Z═—CH2—, —(CH3)2C—, SO2. —(CF3)C—. —CO—, —COO—, —CONH—, —O—, with the proviso that at least one of the radicals Y1 to Y4, preferably at least two of the radicals Y1 to Y4, more preferably at least three of the radicals Y1 to Y4 and most preferably all the radicals Y1 to Y4 are equal to CH3 or a C2 to C4 alkyl radicals.
Z in R4c is preferably —CH2—,—(CH3)2C—, —(CF3)2C— or —O—, more preferably Z═—CH2— or —(CH3)2C—. It is very particularly preferable for R4c to have the following composition: Y1, Y2 and Y3═H, Y4═CH3 or a C2 to C4 alkyl radical and Z═—CH2— or —(CH3)2C— or, respectively, Y1 and Y3═CH3 or a C2 to C4 alkyl radical, Y2 and Y4═H or CH3 and Z═—CH2— or —(CH3)2C—. It is most preferable for R4c to have the following composition: Y1, Y2, Y3 and Y4═CH3 or a C2 to C4 alkyl radical and Z═—CH2— or —(CH3)2C—, preferably —CH2—. It is most preferable for the radicals Y1 to Y4 in the abovementioned preferred embodiments to be CH3 if they are not H.
In one particularly preferred embodiment, block (B) has the following composition:
The mole percentages stated for AF3 and AF4 relate to the functional groups R3 and R4, respectively, in total, so the amounts of the various units are each selected such that they sum to 100 mol % for each of these groups.
Very particular preference is given to the combinations of the abovementioned AF1 and/or AF2 with AF3 and/or AF4. Combinations of AF1 or AF2 with AF4 are most preferable.
The block lengths n and m of blocks A and B are preferably in the range from 1 to 1000, more preferably in the range from 1 to 500, yet more preferably in the range from 1 to 200, yet still more preferably in the range from 5 to 150, yet still more preferably in the range from 10 to 100, yet still even more preferably in the range from 10 to 50 and most preferably in the range from 10 to 40.
The block lengths of blocks A and B may be the same or different. The block-copolyimide may further exhibit some distribution with respect to the particular block lengths of blocks A and B; that is, not all blocks A or all blocks B need to have the same length. The ratio between blocks A and B may thus be varied across a wide range. Proportions in the block copolyimide of this second preferred embodiment of the present invention may be from 5 to 90% for block B and from 10 to 95% for block A. Particular preference is given to the ratio of A:B=80:20 or 70:30 or 60:40 or most preferably 50:50.
Details regarding the production of the block-copolyimides can be extracted from WO 2015/091122, the whole content of which is hereby explicitly incorporated in the description of the present invention by reference. All polymers described in the examples of WO 2015/091122 are particularly preferably used as polymer in the process of the present invention.
Further preferred polyimide(s) that can be used in the invention is/are selected from the group consisting of
or one of its derivatives like Br-Matrimid,
The solvent mixture b) of the invention comprises as main component a. dimethylsulfoxide (DMSO) in an amount of from 55 to 90 wt. %, preferably 55 to 85 wt. %, more preferred 80 to 80 wt. % of the total weight of the solvent mixture b). If the content of DMSO is too low, it was found that the solvent mixture has insufficient solubility for the polyimides. Depending on the solvent mixture only partial dissolution or only swelling of the polyimides was found. If the content is too high, the solubility can be very good, but the resulting polyimide solutions showed poor application properties, in particular a high melting temperature, resulting in potential freezing of the solution during transport.
As component b. the solvent mixture b) comprises one or more primary or secondary and/or tertiary alkanolamine(s) having 3 to 15, preferably 4 to 10, more preferred 4 to 9 carbon atoms, or a mixture thereof.
Inventors have found that if component b. is not comprised in the solvent mixture, either poor solubility or, if good solubility could be achieved, rapid skin formation in air within 1 hour was observed. It is possible but not preferred to provide a solvent mixture b) consisting of components a. and b. wherein a. and b. are comprised in amounts specified below. Inventors, however, found out that in terms of skin formation and melting point it is preferred that the solvent mixture b) comprises at least one, preferably at least two, more preferred at least three of components c. to e. as co-solvents. In this case, component b. can be used also in low amounts. If the content of component b. In solvent mixture b) is too high, the solubility of the solvent mixture decreases. It is, thus, preferred that component b. is comprised in the solvent mixture b) in sum in an amount of 0.1 to 20 wt. %, preferably 0.5 to 19 wt. %, more preferred 1 to 15 wt. %, even more preferred 2 to 13 wt. % of the total weight of the solvent mixture b).
Most preferred alkanolamines, in terms of solubility, application properties and toxicity classification, are selected from the group consisting of triethanolamine and 2-(dimethylamino)ethanol and mixtures thereof, wherein
Component c., i.e. benzyl alcohol, can be used as co-solvent, too. Best results with regard to solubility and skin formation of the polyimide solution have been found if component c. is used in combination with components a. and b. respectively in combination with components a., b, and d. Also combinations of components a., b., c., d. and e. can be used. It is therefore preferred if component c. is comprised in the solvent mature b) in an amount of 0 to 40 wt. %, preferably 0.1 to 25 wt. %, more preferred 1 to 20 wt. % and even more preferred 2 to 15 wt. % of the total weight of the solvent mixture b).
Component d., i.e. cyclohexanone, can be used as co-solvent, too. Best results with regard to solubility and skin formation of the polyimide solution have been found if component d. Is used in combination with components a. and b. or in combination with components a., b. and c. or if combinations of components a., b., c., d. and e. are used. It is, however, also possible to use solvent mixtures b) comprising components a., b., d., e. and f. It is therefore preferred if component d. is comprised in the solvent mixture b) in an amount of 0 to 25 wt. %, preferably 1 to 25 wt. %, more preferred 2 to 20 wt. % and even more preferred 2 to 15 wt. % of the total weight of the solvent mixture b).
Component e., i.e xylene, can be used as co-solvent, too. Best results with regard to solubility and the application properties of the polyimide solution in coating applications have been found if component e. is used in combination with component a., b., c. and d. or combinations of components a., b. e. and f. are used. It is, however, also possible to use solvent mixtures b) comprising components a., b., d., e. and f. It is therefore preferred if component e. is comprised in the solvent mixture b) in an amount of 0 to 15 wt. %. preferably 0.1 to 10 wt. %, more preferred 0.5 to 8 wt. %, most preferred 1 to 5 wt. % of the total weight of the solvent mixture b).
Component f., i.e. acetophenone and/or an alkylene carbonate, can be used as co-solvent together with component a. and b., preferably it is used in combination with components a., b., d. and/or e., to obtain polyimide solutions with good solubility and application properties. Component f. in sum is preferably comprised in the solvent mixture b) in an amount of 0 to 20 wt. %, more preferred 1 to 15 wt. % and even more preferred 5 to 15 wt. % of the total weight of the solvent mixture b).
With regard to component g., i.e. propylene glycol, it has been found that this component is disadvantageous for the solubility of the solvent mixture for polyimides. Thus, component g. is comprised in the solvent mixture b) in a maximum amount of 5 wt. %, preferably 0.1 to 5 wt. %, more preferred 0.01 to 3 wt. %, even more preferred 0.001 to 1 wt. %, especially preferred 0 to 0.5 wt. %. Most preferred propylene glycol is not comprised, i.e. its content is 0 wt. % of the total weight of the solvent mixture b).
DMF, NEP, NMP and/or DMAc are critical solvents because they are classified as CMR and partially SVHC substances. Even though these co-solvents can be comprised in the solvent mixture in minor amounts, it is preferred that their amount is minimized and most preferred that these compounds are not comprised. In sum DMF, NEP, NMP and/or DMAc may be comprised in the solvent mixture b. as component h. in an amount of 0 to 10 wt. %, preferably 0 to 5 wt. %, more preferred 0 to 3 wt. %, even more preferred 0 to 1 wt. %, especially preferred 0 to 0.1 wt. % and most preferred 0 wt. % of the total weight of the solvent mixture b).
Optionally co-solvents i. can be comprised, preferably glycol ethers, especially propylene glycol ethers commercially available under the trade name Dowanol®, for example, 1-methoxy-2-propanol (Dowanol® PM) as component i. Preferably component i. is comprised in solutions specifically prepared for spray coating applications.
The solvent mixtures b) preferably comprises or consists of the following combination of components:
In solvent mixtures consisting of components a. and b. or solvent mixtures comprising components a. and b. and optionally one or more of components c. to f., preferably in solvent mixtures I) to VI), each component a. to f. is comprised in an amount selected from the ranges defined above and the amount of each component is selected such that the amounts of the components in sum total to 100% of the solvent mixture b).
More preferred are combinations I), III) and IV) and most preferred combinations III) and IV).
To reduce transport costs and for ecological reasons it is preferred to reduce the amount of solvent mixture b) in the polyimide solution. Inventors have found that with the inventive solvent mixture b) high concentrated polyimide solutions with good, i.e. low, to acceptable viscosity, can be prepared. Preferably the polyimide solutions of the invention comprise it sum 5 to 35 wt. %, preferably 10 to 30 wt. % and more preferred 15 to 25 wt. %, referred to the overall weight of the polyimide solution, of polyimides(s) a). For higher polyimide contents a strong increase in viscosity was found.
Also preferred polyimide solution of the invention comprise component b) in an amount of 95 to 65 wt. %. preferably 90 to 70 wt. % and more preferred 85 to 75 wt. % of the overall weight of the polyimide solution.
In an alternative embodiment the polyimide solutions are used to prepare dispersions containing inorganic particles. The dispersions comprise as additional component c) one or more fillers, preferably selected from the group consisting of graphite, metal sulfides such as molybdenum disulfide and tungsten disulfide, metal oxides such as aluminum oxide, inorganic nitrides such as aluminum nitride, silicon nitride and hexagonal boron nitride, silica, abrasive particles such as cubic boron nitride, diamond. Fillers can be comprised in an amount of from 0 to 20% by weight of the overall solid components of the dispersion. Solid of the dispersion means weight of the residue obtained after evaporation of all solvents at 230° C. for 2.5 h in a vacuum oven. If fillers are comprised the above given amounts of components a) and b) are adapted accordingly.
The polyimide solutions of the invention can be prepared by a process comprising the steps
In an alternative embodiment the polyimide solutions of the invention can be prepared by a process comprising the steps
If dispersions comprising fliers are prepared, fillers are preferably added after step iii., respectively III., Alternatively fillers can also be added between steps i. and ii., respectively I. and II.
In order to minimize dissolution time and increase process efficiency it is preferred to carry out step iii. or step II. at a temperature of from 25 to 90° C., more preferred 50 to 70° C. Dissolution of the polymide can be done easily with technology and equipment known to a man skilled in the art. Preferably preparation and/or storage of the polyimide solution is done under inert conditions.
The polyimide solution of the invention can be used to coat substrates, preferably steel, aluminum, copper and PEEK (polyetheretherketone). Inventors have found that the polyimide solution of the invention can be applied via spray coating, dip-coating, roller coating or with a squeegee.
The examples below are intended to illustrate and describe the present invention in more detail but shall not be construed in any way to limit the scope of the claims.
A number of polyimide solutions according to the invention and of comparative polyimide solutions were prepared and tested. For production of the polyimide solutions a dry polyimide powder is dissolved with a weight content of up to 25% in different solvent mixtures at 60° C. with stirring. The components used and their amounts are summarized in Table 1.
P84® type 70 obtained from Evonik Fibres GmbH, Austria
P84® HT obtained from Evonik Fibres GmbH, Austria
Block-co-PI prepared according to example 13 in WO2015/091122
All of the following chemicals were used as lab grade chemicals:
DMSO obtained from abcr GmbH
2-(dimethylamino)ethanol obtained from Merck KGaA
Triethanolamine obtained from Sigma-Aldrich Chemie GmbH
Acetophenone obtained from Merck KGaA
Benzyl alcohol obtained from Sigma-Aldrich Chemie GmbH
Cyclohexanone obtained from Sigma-Aldrich Chemie GmbH
Propylene glycol obtained from Sigma-Aldrich Chemie GmbH
Xylene obtained from Sigma-Aldrich Chemie GmbH
Propylene carbonate obtained from Carl Roth GmbH+Co. KG
Storage Stability
The prepared polyimide solutions were tested as coating solution as follows:
The polyimide solution was applied as coating to a steel or aluminum surface via a doctor blade or after dilution, via a spray coating process. The adhesion of coatings was investigated via cross-hatch tests.
After drying, the tested inventive coatings have shown a quality comparable to coatings prepared by use of P84® Type 70 solutions in NMP. The assessment and comparison of the different coatings was carried out by cross-cutting tests according to ISO 2409
Melting temperatures for different solutions 20% PI solutions were determined via DSC measurements.
Table 1 shows that none of the polyimide solutions of the comparative examples solves all problems of the present invention. Most comparative solvent mixtures had poor solubility with undissolved polyimides and/or swelling. Inventive example E15 shows that inventive solutions comprising a binary solvent mixture consisting of DMSO and a tertiary alkanolamine in amounts according to the invention, show excellent storage stability, excellent solubility and significantly improved melting point compared to comparative example CE5. CE5 is a direct comparison to E15, i.e. also a binary solvent mixture consisting of DMSO and 2-(dimethylamino) ethanol but with non-inventive contents of both solvents. While both E15 and CE5 show good solubility; CE15 shows skin formation after 4 h, indicating that the storage stability is worse if a binary solvent mixture consisting of DMSO and a tertiary alkanolamine is used and if the content of the amine in the solvent mixture is too low. CE9 shows very good solubility and low melting point but skin formation started already after 1 hour. In contrast thereto, all inventive examples have at least moderate but most have good or very good solubility. The inventive examples with good and very good solubility were further tested for their viscosity, skin formation time, melting point and in a cross-cutting test after coating of a substrate. It turned out that all tested inventive examples have a melting point below 0° C. All tested inventive examples have skin formation times of at least two hours, most of more than 8 hours, which is a significant benefit for their application. The viscosities of the tested inventive examples are good or even very good and the quality of the coatings ins the cross-cutting test is also good or very good. Examples E1 to E15 show that the present invention offers high flexibility for a commercial application. While solutions can be provided having very low melting point but limited storage stability (4 h), it is also possible to provide solutions with very good storage stability but melting point slightly above 0° C. and finally it is also possible to provide solutions with both, very low melting point and excellent storage stability. Thus, a man skilled in the art can choose the right solvent mixture depending on the commercial requirements. In any case, however, the inventive solvents showed good or even very good solubility performance and are classified as non-toxic, which is a significant benefit compared commercially available polyimide solutions in DMF, DMAc, NMP and NEP.
Table 1 confirms that the solvent mixtures of the invention solve all invention and provide significant advantages over prior art solutions as well as commercially available polyimide solutions as well as to alternative polyimide solutions that could be classified non-toxic (CE2 to CE9).
| Number | Date | Country | Kind |
|---|---|---|---|
| 21179346 | Jun 2021 | EP | regional |
