Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2000-08-31
2002-08-06
Cameron, Erma (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S393000, C427S393500
Reexamination Certificate
active
06428854
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a new one-component stoving composition which can be baked at low temperatures into a coating without the release of volatile compounds and which also has a good storage stability at room temperature.
BACKGROUND OF THE INVENTION
The coating systems used for the protection of substrates against external influences can on the whole be subdivided into two-component systems and one-component systems. For the two-component technique, the two essential starting components—binder and resin—have to be stored separately because of their high reactivity and then mixed together by the technician on site. This requires an exact measurement and often permits only short processing times.
One-component systems do not have this disadvantage. Besides one-component systems which dry by physical means and one-component reactive systems which are cured by atmospheric moisture (curing sensitive to ambient conditions and film thickness) also known are mixtures of at least two reactive components containing at least one of the components in blocked or unreactive form. In these, the blocking agents are eliminated by heating and the reaction between the components then takes place. The terms “baked” and “cured” are synonymous in this application.
Examples of these which may be mentioned are the one-component PU systems based on the very early blocked polyisocyanates described, for example, in DRP 72898. These systems have been widely adopted, above all in the field of industrial baking enamels, for example, for the enamelling of automobiles on production lines and coil coating, and are distinguished by having generally very good film properties, such as resistance to chemicals, scratch resistance and resistance to prolonged weathering. During the baking process, the cross-linking is accompanied by the release of the blocking agent (see, for example, Progr. Coatings 3, (1975), 73 and 9, (1991), 3, which, for reasons explained in more detail below, has to be regarded as a fundamental disadvantage compared with systems where no such release takes place. The same disadvantage is shown by the blocked polyisocyanates recently referred to as carbamates, for example in EP-A 624,577, which can be prepared, for example, from melamine and organic carbonates.
Examples of other baking systems which release decomposition products during the baking process are melamine-formaldehyde resins and urea-formaldehyde resins. The products released here are the alcohols used for etherification and, to a greater or lesser extent, formaldehyde. An additional disadvantage is the lack of resistance to hydrolysis, particularly under acid conditions, which increasingly limits the possible applications of these products.
Recently, there have been intensified endeavors to further improve the acid-resistance of coatings cured by melamine resin. Proposed solutions include additions of binders containing alkoxysilane groups, for example, in U.S. Pat. No. 4,772,672 and EP-A 627,474. Apart from the fact that decomposition products are also formed here, coating systems of this kind have to be specially protected from water, e.g., atmospheric moisture, owing to the resistance to hydrolysis of the alkoxysilyl unit. Furthermore, in view of other important film properties such as flexibility, crack-formation and resistance to the impact of stones, such additives can be used only to a limited extent. Such limited use limits the acid resistance benefits the coatings can provide.
All these systems have the disadvantage that volatile constituents are released in the course of curing, which increases the volatile organic compound (VOC) value. In addition, at greater film thicknesses there is the risk of bubble formation as a result of the substances released. One-component systems which do not release substances do not have these disadvantages. Hitherto only a few cross-linking mechanisms have proved suitable for the preparation of baking enamels which do not release substances.
An important class of products which may be particularly mentioned here are the epoxy resins, which participate in addition reactions, for example, with polyesters or polyacrylates containing carboxylic groups. In particular the aromatic epoxy resins prepared, for example, from bisphenol A have been widely used for powder coatings and tris-glycidyl isocyanurate (TGIC) has found wide application where greater resistance to weathering is required.
One disadvantage of the aromatic epoxy resins is their lack of resistance to light, so that the use of their resulting coating is possible only to a limited extent. There are no such problems with aliphatic TGIC, but doubts regarding its toxicological nature have prompted searches for other alternatives.
TGIC-free aliphatic epoxy-carboxylic acid systems are described inter alia in DE-A 2,240,312, 2,240,314, 2,457,826 or U.S. Pat. No. 4,091,048, both for powder coating applications and for solvent-containing and aqueous liquid coatings. A disadvantage of these systems is their frequently limited storage stability, so that storage for a prolonged period is possible only under cooling conditions. Neither is the standard of their properties adequate for a number of applications, or it is clearly in need of improvement. For application as varnish for automobiles, the relevant properties are lack of resistance to the impact of stones, susceptibility to scratches and yellowing due to heat.
In EP-A 639,598 and 727,452, resins containing uretdione groups are described as another possible route to the preparation of baking systems which do not release substances. These resins can be reacted with compounds containing OH groups with the aid of appropriate catalysis under the conventional conditions for industrial baking enamels. A fundamental disadvantage of such systems is that the resins containing uretdione groups, owing to the chain-extending effect of this internal isocyanate blocking, necessarily have high molecular weights, which limits their possible uses.
Systems which do not release substances and do not have the disadvantages described above can be formulated on the basis of cyclic carbonates. The preparation of compounds containing several carbonate groups in the form of five-membered rings are described, for example, in DE-A 272,665 or 274,721. The opening of the ring (cross-linking) is effected here by means of amino groups. Hydroxyl groups are not reactive enough at the required baking temperatures.
Thus coating materials for electrodip coating which consist of a component containing several carbonate groups in the form of five-membered rings and a component having several primary or secondary amino groups are described, for example, in DE-A 1,953,0,225, DE-A 1,953,0,226, EP-A 661,354 or EP-A 661,355. However, compounds containing several cyclic carbonate groups in the form of six-membered rings are only described in the patents cited above in connection with amines.
DE-A 4,432,647 describes oligourethanes containing 1,3-dioxan-2-one groups, a method for their preparation and their use in combination with compounds having active hydrogen atoms for the production of plastics, or as binders, or binder components in coating materials (one-component baking systems or two-component systems). It emerges from the examples that where polyhydroxy compounds are used as the compounds having active hydrogen atoms, it is possible to formulate one-component systems which are stable in storage at room temperature and can be cured at 155° C. to form hard coating films having good solvent resistance (Example 5).
Baking temperatures of around 155° C. are not a decisive advance in the field of one-component PU baking enamels. Where the compounds having active hydrogen atoms are compounds bearing primary or secondary amino groups, baking temperatures of 80° C. can be achieved (Examples 6, 7). But these systems, owing to the high reactivity of the polyamines, are not stable in storage at room temperature and should therefore be classified as two-component systems. However, DE-A 4,432,647 does
Brück Jochen
Buysch Hans-Josef
Engbert Theodor
Hovestadt Wieland
Melchiors Martin
Bayer Aktiengesellschaft
Cameron Erma
Gil Joseph C.
Roy Thomas W.
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