Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Compositions to be polymerized by wave energy wherein said...
Reexamination Certificate
2000-04-17
2002-04-30
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Compositions to be polymerized by wave energy wherein said...
C522S104000, C522S109000, C522S179000, C522S182000, C525S165000, C525S168000, C525S170000, C427S512000, C427S508000, C428S480000, C428S482000
Reexamination Certificate
active
06380279
ABSTRACT:
DESCRIPTION
The present invention relates to novel radiation-curable powder compositions comprising a mixture of at least one semi-crystalline polyester containing end methacryloyl groups and of at least one amorphous polyester containing end methacryloyl groups, as well as to the preparation and to the uses of the said compositions.
More particularly, the present invention relates to powder compositions curable by ultraviolet irradiation or by accelerated electron beams, the binder of which is composed of a mixture of at least one semi-crystalline polyester containing end methacryloyl groups and of at least one amorphous polyester containing end methacryloyl groups, and which lend themselves to the production of paint and varnish coatings exhibiting a unique array of properties, inter alia excellent hardness and flexibility, excellent mechanical properties, notable resistance to solvents and to weathering, and good stability on storage.
Heat-curable powder compositions are well known in the state of the art and are widely used as paints and varnishes for coating the most diverse objects. The advantages of these powders are manifold; on the one hand, problems due to solvents are completely eliminated and, on the other hand, the powders are 100% used, insofar as only the powder in direct contact with the substrate is retained by the latter, the excess powder being, in principle, fully recoverable and reusable. This is why these powder compositions are preferred with respect to coating compositions which are provided in the form of solutions in an organic solvent.
Heat-curable powder compositions have already found a wide outlet in the coating of domestic electrical appliances, automobile industry accessories, metal furniture, and the like. They generally contain heat-curable organic compounds which constitute the binder for the paint, fillers, pigments, catalysts and various additives for adapting their behaviour to their use.
Different types of heat-curable powder compositions exist. The most well known compositions contain, as binder, either a mixture of polyesters containing carboxyl groups with epoxide-containing compounds, such as triglycidyl isocyanurate, or &bgr;-hydroxyalkylamides, or alternatively a mixture of polyesters containing hydroxyl groups with isocyanates, which are or are not blocked, glycoluril or melamine resins, polycarboxylic acid anhydrides, and the like.
Currently, the great majority of polyesters used in heat-curable powder compositions are amorphous polyesters. Now, when the polyester is amorphous, it is difficult to prepare perfect heat-curable pulverulent compositions because they have to meet often contradictory criteria. Thus, these powders may not reagglomerate during their handling, their transportation and their storage, which implies that the amorphous polyester must possess a sufficiently high glass transition temperature (Tg). On the other hand, in order for the powder particles to be able to coalesce and to form a perfectly homogeneous and uniform coating, it is necessary for the glass transition temperature (Tg) of the polyester to be sufficiently low to ensure a low viscosity in the molten state which, itself, ensures good wetting of the pigments and other solid materials accompanying the polyester in the formulation of the said heat-curable powder compositions.
Moreover, the powder must be capable of melting at the stoving temperature in order to form an even film before the crosslinking reaction begins which results in the final curing. In order to obtain good spreading of the molten film over the surface of the substrate, it is therefore necessary for the viscosity of the polyester in the molten state to be sufficiently low. This is because a very high viscosity in the molten state prevents good spreading of the molten film and is reflected by a loss in the evenness and in the gloss of the coating. Finally, the rate of the crosslinking reaction of the composition can only be controlled by varying the temperature, as well as the amount and/or the nature of the crosslinking agent and of the crosslinking catalyst which is optionally used. Taking into account all the preceding requirements, it is generally accepted that a suitable amorphous polyester must exhibit a glass transition temperature (Tg) which is greater than 50° C. but less than 85° C. and a viscosity in the molten state of between 100 and 10,000 mPa·s, measured at a temperature of 200° C. on a cone/plate viscometer according to ASTM D 4287-88.
For all these reasons, it is not generally recommended to produce coatings from compositions based on such amorphous polyesters by stoving at temperatures below 160° C. for a time of approximately 10 to 20 minutes. In fact, at lower stoving temperatures, coatings of poorer quality having unsatisfactory physical properties are generally obtained. Moreover, the difficulty which is encountered in establishing a glass transition temperature (Tg) which is sufficiently high, in order to eliminate the tendency towards reagglomeration during storage, constitutes a lack of flexibility for the user of the powder.
In order to solve these problems, heat-curable powder compositions have more recently been provided in which the binder is composed of a semi-crystalline polyester containing carboxyl groups or hydroxyl groups. This is because the properties of semi-crystalline polyesters make it possible to overcome, to a large extent, the disadvantages described above which are brought about by the establishment of the glass transition temperature (Tg), of the viscosity in the molten state and of the reactivity of the amorphous polyesters.
Semi-crystalline polyesters generally have a melting point greater than 40° C. and a low glass transition temperature (Tg) generally of less than 30° C. This has the consequence that the viscosity in the molten state of semi-crystalline polyesters is much less than that of amorphous polyesters of comparable molecular weight commonly used in heat-curable powder compositions.
Heat-curable compositions based on semi-crystalline polyesters have already formed the subject of a certain number of publications in the form of papers and patents. In the patent literature, mention will in particular be made of International Patent Application PCT WO 91/14745, European Patent Application 70.118 and U.S. Pat. Nos. 4,352,924, 4,387,214, 4,937,288 and 4,973,646.
Semi-crystalline polyesters and the heat-curable powder compositions prepared from the latter therefore possess, on the whole, properties which are more advantageous than those of amorphous polyesters. However, in spite of their advantageous properties, the semi-crystalline polyesters of the state of the art also exhibit significant disadvantages on an industrial scale, even if these semi-crystalline polyesters are used as an additional component for modifying commercially available conventional amorphous polyesters.
Firstly, the coatings obtained from such compositions containing semi-crystalline polyesters exhibit a low surface hardness (HB pencil hardness). This defect of semi-crystalline polyesters greatly restricts the technical applications of the compositions containing these polyesters.
Secondly, in order to be curable at a moderate temperature, generally at least 150° C., polyesters require the presence of a crosslinking agent (epoxide-containing compounds, &bgr;-hydroxyalkylamides, and the like) and of a catalyst. Now, for the preparation of the powder, the polyester must be melted with the crosslinking agent, the catalyst and the other additives in an extruder at a temperature in the region of the crosslinking temperature of the system. It follows that, without specific precautions, an undesirable premature crosslinking of the binder, by reaction between the polyester and the crosslinking agent, already takes place during the preparation of the powder. A powder thus prepared produces defective coatings because of the presence of gelled particles and because, at the time when the molten film has to be spread over the surface of the substrate to be coated, instead
Loosen Patrick
Loutz Jean-Marie
Maetens Daniel
Moens Luc
Van Kerckhove Marc
McClendon Sanza L.
Seidleck James J.
UCB S.A.
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