Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-10-26
2002-02-26
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S410000, C523S415000, C523S423000, C523S428000
Reexamination Certificate
active
06350796
ABSTRACT:
RELATED APPLICATIONS
This application claims priority to Austrian application No. A 1796/98, filed Oct. 28, 1998, herein incorporated by reference.
FIELD OF THE INVENTION
This invention relates to two-component isocyanate crosslinkable systems based on aqueous dispersions of hydrophilic epoxy adducts. This invention also relates to binders containing the systems according to the present invention and a method for coating material using said binders.
Description of the Related Art
In unmodified form, epoxy resins, especially those based on bisphenol A which are commonly used commercially, are insoluble or of very low solubility in water. This means that an aqueous phase which is in equilibrium with the epoxy resin has a mass fraction of less than 1% of the epoxy resin. In principle, it is possible to obtain water-dilutable, cationically stabilized base resins by reacting epoxy resins with amines and then protonating the basic groups. By modifying the epoxy resin with nonionic hydrophilic groups or with anionic groups, it is likewise possible to achieve a limited solubility which is sufficient to give adequate stability to a dispersion of the particular modified epoxy resin in water. Such dispersions can be diluted with (additional) water. The modified epoxy resin can then be processed from the aqueous dispersion; after the water fraction has been removed by evaporation or after penetration into the substrate, the resin remains on the surface and, given a adequate composition of the disperse phase, forms a coalesced film which can be crosslinked chemically by adding appropriate curing agents.
In the case of the cationically modified epoxy resins, dilutability in water is achieved by neutralizing some or all of the basic groups of the epoxy-amine adduct with acids, such as formic acid, acetic acid or lactic acid, for example. In this case the level of basic amine groups in the epoxy-amine adduct (measured, for example, by way of the amine number; see below) and the degree of neutralization of said groups (i.e., the fraction of ionic groups) are critical for the extent of dilutability in water.
When formulating the batches care must be taken to ensure that the base resins have the level of amine groups required to ensure sufficient stability of the aqueous solution of the binder. Experience indicates a target amine number in the range from 30 to 100 mg/g.
A degree of neutralization of from 20 to 60% of the basic amine groups is usually sufficient for practical dilutability. The base resins are then diluted to the desired concentration using deionized water. The resin dispersions obtained in this way range from virtually clear to highly opaque and should have a readily manipulable viscosity in the range from about 100 to 4000 mPa.s at ambient temperature.
Cationically stabilized epoxy adducts of this kind are part of the prior art and have already been described on numerous occasions in the patent literature. In the field of cataphoretic electrodeposition coating, in particular, they are successfully employed in combination with blocked difunctional or oligofunctional isocyanates (see, for example, Austrian Patent Applications AT 1665/86, AT 1766/78, and German Patent Applications DE-A 30 41 700, DE-A 33 00 583, DE-A 33 11 513). If desired, they are subsequently processed further with crosslinking catalysts, pigments, fillers and other additives to form pigmented paints.
In contrast to electrodeposition coating applications, where the formulation of the base resins and thus the number and nature of the ionogenic groups strongly influence the electrophoretic applicability, the criteria governing, for example, the interior spray coating of drums are different.
One particular problem affecting the combination of water-diluted epoxy-amine adducts with unblocked isocyanates is the often very short processing time (pot life). Because of the basic medium, the unblocked isocyanate groups react with water even before baking, during application. This results in film defects due to gas bubbles (the reaction of isocyanate with water produces the corresponding amines and carbon dioxide) and reduced film crosslinking, since part of the crosslinking agent is consumed by the reaction with the water.
The epoxy-amine adducts which arc commonly employed in cataphoretic electrodeposition coating generally include a fraction of strongly basic amino groups with little steric hindrance, since such groups result in particularly favorable electrochemical deposition properties. For the reason given above, such adducts are poorly suited to combination with unblocked isocyanates.
It has surprisingly now been found that, under certain conditions, water-diluted epoxy adducts can be combined with unblocked isocyanates to form 2-component systems without entailing problems with film defects due to gas bubbles or inadequate film crosslinking.
SUMMARY OF THE INVENTION
The present invention therefore provides two-component systems based on aqueous dispersions of hydrophilic epoxy adducts Ah having hydroxyl groups as reactive groups and unblocked difunctional or polyfunctional isocyanates B. The hydrophilic epoxy adducts Ah can be selected from cationically stabilized hydrophilic epoxy adducts Ak, anionically stabilized hydrophilic epoxy adducts Aa and nonionically stabilized epoxy adducts An.
DETAILED DESCRIPTION OF THE INVENTION
For this purpose it is necessary first of all, in the case of the cationically stabilized epoxy-amine adducts Ak, that only tertiary amino groups and not primary or secondary amino groups are present in the epoxy-amine adduct. The tertiary amino groups present must, furthermore, be sterically hindered. This is achieved by careful selection of the amines used and, in the course of synthesis, by direct attachment of the amino groups to epoxy groups without the formation of terminal amine groups in aliphatic side chains.
Owing to these conditions, amines such diethylaminopropylamine or dimethylaminopropylamine, for example, are unsuitable as raw materials since epoxy-amine adducts prepared from them contain terminal amino groups in aliphatic side chains. Amines which have proven particularly suitable, on the other hand, are those which carry either short-chain branched alkyl groups or hydroxyalkyl groups. When an adduct is formed with oxirane groups of the epoxy component, the result is then tertiary amine structures with strong steric hindrance which can nevertheless still be neutralized effectively with acids and so contribute to the water-solubility of the resins. In the case of the amines which carry hydroxyalkyl groups, additional OH groups are incorporated into the resin assembly and may act as crosslinking sites with, for example, isocyanate groups.
An important criterion for the epoxy-amine adducts Ak suitable for the invention is a sufficiently high number of OH groups, which are groups that are capable of crosslinking with isocyanates. OH numbers in the range from about 150 to about 400 mg/g have been found suitable.
The invention therefore provides two-component binders for the interior coating of drums, consisting of a cationically stabilized epoxy component Ak and a crosslinking agent B, wherein component Ak comprises exclusively those tertiary amino groups which are formed in the addition reaction from the amine component A2 and the epoxy component A1, and wherein the amine component A2 has a primary or secondary amino group and has no tertiary amino groups.
Compounds which can be employed as crosslinking agent B are in principle all those which undergo addition or condensation reactions with hydroxyl-containing compounds. Crosslinking agents B which have been found suitable in the context of the invention are primarily those which carry at least two unblocked isocyanate groups per molecule. Only using isocyanate crosslinking agents of this kind is it possible to produce chemically resistant coatings which cure rapidly.
Particularly suitable isocyanates are low molar mass isocyanates which are liquid at room temperature, having a viscosity of from about 50 to about 10,000 mPa.s.
Dworak Gert
Feola Roland
Gerlitz Martin
Weinberger Manfred
Aylward D.
Dawson Robert
Frommer & Lawrence & Haug LLP
Vianova Resins AG
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