Stock material or miscellaneous articles – Composite – Of polyester
Reexamination Certificate
2000-10-13
2002-04-09
Acquah, Samuel A. (Department: 1711)
Stock material or miscellaneous articles
Composite
Of polyester
C528S272000, C528S491000, C528S503000, C524S081000, C524S700000, C524S783000, C524S801000, C524S847000, C525S437000, C427S207100, C427S212000, C427S340000, C427S384000, C428S304400, C428S402000
Reexamination Certificate
active
06368725
ABSTRACT:
The invention relates to the preparation of coloured powder lacquers in particular for use in powder coating compositions. The invention also relates to the use of the powder lacquers obtained for the coating of substrates, as well as to the coated substrates thus obtained.
Powder lacquers are generally prepared by conventional extruder technology. For this, the constituents of the powder lacquer, that is to say the binders, curing agents and optionally pigments and further additives, as solids, are pre-mixed intensively in the dry state in the requisite quantitative ratios, and are then melted at the lowest possible temperature in an extruder and intermixed intensively. The binders and curing agents are thereby plasticised, and the optionally contained pigments and extenders wetted. The optionally coloured extrudate which is obtained is then rolled out to a thin layer, cooled and crushed into coarse granules which are ground to the finished powder lacquer in a mill.
Difficulties can arise in this process, in particular with regard to the extrusion and grinding processes. The extrusion process may proceed too slowly for highly reactive binder mixtures, such that partial gelation occurs during which a reduced residence time in the extruder negatively influences pigment dispersion in the binder. Furthermore, only binders within a certain viscosity range can be processed by extruder. The broad particle size spectrum within the range 0.1 to 500 micrometers, for example, which forms during grinding necessitates additional sieving and milling processes for specific applications. Furthermore, the fines which arise are disadvantageous in terms of health and processing technology.
It is furthermore known to utilise as an auxiliary agent for the preparation of powder lacquers low molecular weight inert compounds in the form of compressible fluids. Here, the solution or suspension formed by the compressible fluid and the components of the powder lacquer composition is sprayed under expansion, such that the cooling brought about by the process of expansion causes the powder lacquer particles to form. Particle sizes of from 5 to 150 micrometers, but also very fine particles within the range 1 to 5 micrometers, can be obtained by utilising corresponding nozzles.
High pressures and large quantities of gas are generally necessary in such processes in order to dissolve the relevant solids in the supercritical fluid.
Further methods for preparing powders are based on the so-called “non-aqueous dispersion (NAD) processes”. Here, a polymer or polymer precursor, for example a polyester, is incorporated in a dispersing medium at a temperature below 200° C., for example. In order to prepare coloured powders pigments are added to the dispersion, preferably after cooling of the dispersion. The corresponding powder particles are obtained by further cooling, separation and drying of the resulting particles.
The addition of the pigments at a reduced temperature, for example room temperature or slightly higher, gives rise to problems with regard to cohesion between the polymer particles and the pigment particles, in that separation occurs between the particles. If, on the other hand, the pigments are added to the dispersion at a relatively high temperature, particle sizes are altered. It is in general difficult with such processes to obtain a narrow particle size distribution as well as homogeneously organised particles. Coatings with powder lacquer compositions based on such NAD lacquer powders are unsatisfactory owing to lacquer film defects, for example cratering.
The object of the present invention is therefore to develop a process based on an NAD process for pigmented lacquer powders, by which powder particles having a narrow grain size range and homogeneous particle organisation result, and powder coatings with powder lacquer compositions based on pigmented lacquer powders prepared in this manner are obtained, which have the desired good lacquer characteristics.
The object is achieved by a process provided by the invention for the preparation of a powder lacquer composition based on polyester by
a) common or separate heating to a temperature of 120-200° C. of
A) 99-40 wt. % of one or more polyester precondensates, and/or of monomers usable for the preparation thereof,
B) 1-60 wt. % of one or more pigments and
C) 0-10 wt. % of one or more wetting additives and/or further additives which are conventionally used for lacquers, wherein the percentages by weight total 100%,
b) homogeneous mixing of the components A), B) and C) with the application of shear forces and with the heating temperature maintained,
c) dispersion in a dispersing medium of the mixture obtained, at a temperature equal to or above the melting temperature of the starting components A), B) and C) with further polycondensation of the polyester precondensate A), and/or of the monomers thereof until a polyester of a desired molecular weight is obtained,
d) cooling of the dispersion and addition of a conventional cross-linking agent for the polyester and optionally of conventional additives, within the temperature range 60 to 140° C.
The homogeneous mixing of the components with the application of shear forces may take place, for example, within a period of from 10 to 100 minutes, depending on the type of starting components, in particular depending on the type and quantity of pigments utilised.
In order to achieve the desired molecular weight of the polyester up to a region of, for example, 50 000 g/mol, the temperature in the dispersing medium may be within the range 120 to 280° C. after the addition of the composition. After the desired molecular weight has been achieved the reaction may be terminated by lowering the temperature. The reaction may be terminated at any time by corresponding cooling.
In order to prepare powder lacquer compositions the process may be carried on in continuous manner to the desired final molecular weight before the addition of the cross-linking agent(s).
The process may be interrupted after the composition to be used according to the invention has been heated and mixed homogeneously with the application of shear forces, that is after a coloured polyester precondensate mixed homogeneously after the addition of pigments and optionally wetting additives has been obtained, and the coloured polyester precondensate may undergo interim storage until it is reacted further.
The process is preferably carried on in continuous manner until a polyester of the desired final molecular weight is obtained. This is achieved by maintaining the temperature within a range, for example between 120 and 280° C., in the dispersing medium and distilling off the condensation products such as, for example, water, methanol. In order to prepare powder lacquer compositions the dispersion is then cooled with the composition to be used according to the invention to temperatures of between 60 and 140° C., and the cross-linking agent(s) as well as optionally further additives is/are added to the dispersion bath under these conditions. The finished coloured powder lacquer results after cooling of the dispersion, separation and drying by conventional processes.
In order to process further the coloured polyester precondensate which has optionally undergone interim storage it may, after being heated to a temperature of from 120 to 200° C., be dispersed in a dispersing medium at a temperature equal to or above the melting temperature of the starting components, for example from 180 to 280° C., optionally with dispersion stabilisers, until the final molecular weight is achieved. After cooling of the dispersion to a temperature of from 60 to 140° C., the cross-linking agent and optionally the additives may be added to the dispersing medium and the finished coloured powder lacquer can result after cooling, separation and drying.
After cooling to, for example, room temperature or below 60° C., separation and drying of the particles there results in accordance with the process according to the invention a coloured powder lacquer the particle structure
Gräf Knut
Jacobs Alexandra
Mörs Volker
Simmrock Hans-Ulrich
Ward Bennett
Acquah Samuel A.
Deshmukh Sudhir G.
E. I. du Pont de Nemours and Company
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