Dispersion process

Details Dispersion process Dispersion process

2001-02-07

2003-01-28

Cain, Edward J. (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C524S611000, C524S612000

Reexamination Certificate

active

06512025

ABSTRACT:

This invention relates to a process for preparing reactive particulate dispersions and reactive powders, in particular a process for manufacturing aqueous dispersions of suspended particulates in a continuous process.
A reactive particle consists of a resin (that is to say a natural or synthetic polymer having reactive functional groups) and a crosslinker (that is to say a monomer, an oligomer or other polymer having reactive functional groups capable of reacting with the reactive functional groups on the resin) which can be reacted together to form a crosslinked network. In a reactive particle a proportion of the functional groups in the resin and the crosslinker will be un-reacted, and be capable of undergoing crosslinking reactions with functional groups in resin and crosslinker molecules in the same or other reactive particles. It is also a feature of such particles that they will be an intimate mixture of ingredients, with each particle having essentially the same composition.
Reactive particulate dispersions are typically used in the preparation of coating compositions where, following application to a surface to be coated, any carrier liquid is either allowed to evaporate, or will be driven off, and the particles caused to form a crosslinked film, for example by stoving. Reactive powders are used in powder-coatings where the dry powder is applied to a surface to be coated, and then caused to crosslink in the same way.
Conventionally, powder coatings are prepared by melting together a resin and a crosslinker for a time and at a temperature such that no substantial reaction takes place between them. The melt is then extruded, allowed to solidify as a mass, and the solid extrudate is then pulverised, with the powder so obtained then being classified. Such a powder can be applied directly or as a dispersion in a liquid carrier. However, a problem with such processes is the need for the grinding and classification steps, which are relatively expensive. In particular, the grinding step requires the use of expensive cooling equipment, particularly with low Tg polymers, to ensure both that the temperature of the resin does not exceed that at which further reaction may occur, and also to ensure that the resin remains brittle, in order that satisfactory particle size reduction can occur.
It is also a disadvantage of such a process that the resin particles so produced tend to have irregular particle shapes and sizes, and that there is poor control over these parameters. Regular particle shape and size is advantageous, as is the provision of substantially spherical particles, since such particles can have advantages in terms of their ability to retain electrostatic charge when they are used in particle coatings. They can also be more fluidizable.
It is also known to produce certain types of dispersions for use as water based paints. For example, U.S. Pat. No. 5,087,645 (Toyo Seikan Kaisha Ltd.) discloses a batch process for producing a water based paint composition which comprises an epoxy resin, a curing agent and an acrylic resin, wherein the acrylic resin has an acid number between 2 and 30, and wherein the acrylic resin has been modified by converting the carboxyl groups on it to amine or ammonium salts, by the addition of an aqueous solution of ammonia or an amine. The resulting solution undergoes phase inversion to provide the water based paint.
We have now discovered that reactive dispersions and powders can be made much more simply, from a wider range of materials, and to a higher degree of uniformity by a melt/dispersion process which is carried out on a continuous basis (i.e. preferably in an extruder), and also has certain other advantageous aspects.
Accordingly the present invention provides a continuous process for preparing a reactive particulate dispersion in a liquid carrier which process comprises mixing together under extrusion conditions to form a molten mixture a resin and a crosslinker under shear at a temperature and for a time such that substantial crosslinking potential is retained between the resin and the crosslinker, and thereafter dispersing the mixture whilst still molten into a liquid carrier, and allowing the molten dispersed mixture to form particles.
By carrying out the process under extrusion conditions, this allows the operator to heat and mix rapidly the components of the composition, but also to rapidly cool the mixture once an intimate blend is attained, thereby minimising the extent of reaction that occurs. It is highly preferred that the reaction is carried out in an extruder, and also highly preferred that the extruder is a twin screw extruder, to ensure that the desired temperature control and intimate mixing is achieved.
According to a further aspect of the invention, there is provided a continuous process for preparing a reactive particulate dispersion in a liquid carrier in an extruder equipped with a main intake, an exit port, and an intermediate liquid injection port between the main intake and the exit port, and heating means for heating material as it passes through the extruder between the main intake and the liquid injection port, in which a resin and a cross linker are introduced into the extruder through the main intake, and are heated and mixed together as they pass through the extruder so as to form a molten mixture before they reach the liquid injection port, and a liquid carrier is introduced into the extruder through the injection port, and the molten mixture becomes dispersed in the liquid medium, the temperature and the throughput of the extruder being such that substantial crosslinking potential between the resin and the crosslinker remains in the final dispersion, and the dispersion is allowed to cool on leaving the extruder such that the molten mixture solidifies to form a particulate dispersion.
Preferably, where the reactive particulate dispersion is intended for use as a coating composition, other coating composition components such as pigments, flow agents and catalysts can be introduced into the main intake at the same time as the resin and the crosslinker. Preferably the components are subjected to a short physical pre-mixing step at ambient temperature prior to being introduced to the extrusion conditions. Also, where a dispersion agent is used, it is preferred that this is introduced into the injection port at the same time (e.g. predispersed) with the liquid carrier.
A highly preferred aspect of the process according to the present invention is that it can be carried out without the addition of, and in the absence of any solvents which are capable of dissolving the resin in the composition, such as volatile organic solvents. This provides a clear advantage of the process over other known processes, such as for example that described in the above mentioned U.S. patent (which in practice requires the use of volatile organic solvents). In particular, the use of such solvents can cause the Tg of the resin to decrease, and also decreases the blocking temperature of the final powder composition. This can lead to unacceptable agglomeration of the resin in the powder state, making it less suitable for use. Preferred coating compositions do not contain any solvent which is capable of dissolving the resin in the composition, and also have not had any such solvent used in their manufacture, as in practice if such a solvent has been used in their manufacture, some is inevitably retained in the resin. This leads to the undesirable attributes described above. As such, preferably the blocking temperature of the resin powder which can be isolated from the dispersion produced according to the invention is at least is 40° C.
Consequently, it is preferred that the liquid carrier used for the dispersion step is not capable of dissolving the resin in the composition, and is therefore immiscible with the polymer components of the reaction mixture. The use of such an immiscible liquid carrier would not significantly reduce the viscosity of the polymer in the dispersion being processed, and also would not significantly affect the Tg

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