Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2002-11-13
2004-11-02
Toomer, Cephia D. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C516S099000
Reexamination Certificate
active
06812269
ABSTRACT:
The invention pertains to a process for preparing an aqueous powder coating dispersion comprising components A and B, wherein component A is a binder resin or a mixture of binder resins and component B is a crosslinker or a mixture of crosslinkers or component A is a crosslinker or a mixture of crosslinkers and component B is a binder resin or a mixture of binder resins. Optionally the aqueous powder coating dispersion contains a pigment, a thickener, a filler, a catalyst, an initiator, and/or one or more additives.
Solid powder coating compositions comprising a suitable resin/crosslinker combination are commonly used for coating a substrate with a crosslinked film, by evenly distributing the composition over the surface of the substrate by spraying and curing the resulting layer on the substrate. Curing on a metallic substrate, for example in industry in general, most commonly takes the form of applying an elevated temperature, for example from 160 to 200° C. However, curing can also take the form of applying radiation/light, for example ultraviolet light or electron beam radiation, a process which is especially suitable for coating substrates that cannot withstand high temperatures (plastics, wood, etc.).
When a solid powder coating composition is used to coat a substrate, special application equipment is required. Furthermore, it is difficult to obtain a thin crosslinked film with a uniform thickness and appearance on the substrate, due to the relatively large size and irregular shape of the particles in the powder coating composition. Reducing the particle size in such a composition, e.g. by jet milling, is not only costly but often leads to caking and other handling/application problems of the resulting fine powder. For example, it is difficult to charge the very small particles during electrostatic application, and elaborate steps need to be taken to cope with dust when such a composition is applied on a substrate. Further, once a solid powder coating composition is prepared, due to its solid character there is no flexibility to adapt properties like for example colour, gloss etc.
Hence there are considerable advantages to applying a particulate powder coating as a dispersion for coating a substrate. No dust is formed when the dispersion is spread on the surface of the substrate, and it was found that the particles in the dispersion can be much smaller than in a solid powder coating composition without creating handling/application problems.
As indicated above, powder coating dispersions have a number of advantages over particulate powder coating compositions. Further, these dispersions have an advantage over solvent-borne coating compositions since there is no need for the evaporation of any organic solvent. The emission of most organic solvents contributes to the VOC level of the coating composition. The VOC level is restricted by present day legislation. Such restrictions are expected to be tightened in the near future.
Also, in the case of a liquid dispersion it is possible to use equipment commonly used for handling/applying conventional wet paints, while special equipment is needed for handling/applying solid powder coating compositions. Application on non-conductive substrates like wood and plastics is also easier.
In general, the T
g
of the components which can be used for preparing the aqueous powder coating dispersion can be much lower than in solid powder coatings, because there is no need for high T
g
binders that are used in a standard (solid) powder coating to ensure sufficient physical stability.
The possibility to incorporate binders with much lower T
g
's increases the freedom of binder formulation. This additional degree of freedom in binder formulation has a positive effect on final film properties like flow, appearance etc.
Furthermore, paint additive(s) can be added to a final dispersion without any problems, making the preparation of a paint formulation from a dispersion much more flexible. It is also relatively easy to filter a dispersion in order to remove impurities/large particles.
For example, WO 96/37561 discloses a wet milling process for the production of an aqueous powder coating dispersion, in which process a solid powder coating composition is wet milled in an aqueous medium. However, the powder coating dispersion obtained has a comparatively wide particle size distribution (3-20 &mgr;m), and the particles are still irregularly shaped and relatively large.
Preparation of smaller particles than described by traditional wet-milling processes is in general very difficult and economically not attractive. Larger particles in powder coating dispersions have a strong tendency to sediment which is undesirable because this limits the stability of the final dispersion. The sediments formed are normally hard to re-disperse and in practice additional measures have to be taken to prevent sedimentation for example by adjusting the rheology of the coating formulation by adding thickeners or other rheology modifiers. A disadvantage of adding thickeners in general is that the solids content of the final coating formulation decreases which is not desirable. Also the film formation is negatively influenced by the presence of bigger particles which is observed by phenomena like mud-cracking, poor flow, no uniform layer thickness etc. Furthermore, the process cannot be carried out continuously, and obtaining particles with the above-mentioned size requires a comparatively long milling time (about 2 hours).
Similar sized/shaped particles are obtained in U.S. Pat. No. 5,379,947, where a jet milling process for the production of an aqueous powder coating dispersion is disclosed. Powder coating flakes are jet milled, and the resulting powder is dispersed in an aqueous medium to obtain a dispersion. Such a process cannot be carried out continuously either, and jet milling until particles of the desired size are formed also requires a comparatively long period of time. Other disadvantages of such a jet milling process are the requirement to use expensive equipment and a relatively high energy consumption.
WO 97/45476 discloses a process for the preparation of a powder coating dispersion wherein a molten-like substance comprising a resin and a crosslinker is dispersed in a liquid medium under extrusion conditions. In this process the starting materials such as binder resin(s) and crosslinker(s) are mixed and molten. Then water is added to the molten mixture and a powder coating dispersion is obtained. Since the obtained powder coating dispersions are cured by heat, this process is not suited for the preparation of powder coating dispersions comprising a binder/crosslinker mixture that start to react at a temperature close to or below the melting or softening temperature of the mixture.
It is the objective of the present invention to provide a process for preparing an aqueous powder coating dispersion without having the above-mentioned disadvantages.
A process was found for preparing an aqueous powder coating dispersion comprising components A and B, wherein component A is a binder resin or a mixture of binder resins and component B is a crosslinker or a mixture of crosslinkers or component A is a crosslinker or a mixture of crosslinkers and component B is a binder resin or a mixture of binder resins.
This process is characterised in that the process comprises the steps of:
a) dispersing a composition comprising component A by addition of an aqueous medium in a phase inversion emulsification process,
b) adding component B simultaneously with or after the addition of the aqueous medium, and
c) optionally mixing the obtained composition to obtain a dispersion comprising components A and B
Since components A and B are dispersed separately or mixed at relative low process temperatures, this process is particularly suited for preparation of aqueous powder coatings dispersions based on a wide range of binder/crosslinker combinations, including combinations that start to react at temperatures below or close to the processing temperature.
For the purpose of the prese
Beijers Gerard Henk
Borgström Johan Eric
Koenraadt Martinus Adrianus Antonius Maria
Sörensen Kent Raabjerg
Akzo Nobel N.V.
Fennelly Richard P.
McGillycuddy Joan M.
Parker Lainie E.
Toomer Cephia D.
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