Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-07-28
2004-01-06
Short, Patricia A. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S439000, C525S440030, C525S444000, C525S452000, C525S454000, C525S466000, C528S272000, C528S296000
Reexamination Certificate
active
06673874
ABSTRACT:
The present invention relates to a process of preparing a coating composition and to a coating composition prepared by the process.. The invention also relates to a process of coating using the coating composition and to a coated substrate obtainable by the coating process.
Powder coatings are traditionally made by mixing powdered resin, pigment, cross-linking agent and additives uniformly in a mixer. The mixture is then usually extruded as a melt, cooled to harden the extrudate, broken up and finally milled and sieved to produce a powder. This method possesses a number of problems however. The resin types are limited to those that are brittle and amenable to dry milling when cold. The breaking up and milling steps are difficult and expensive, especially for polymers with a low glass transition temperature (Tg) or of a high molecular weight, and result in particles having a broad distribution range and an irregular shape.
Dispersion processes have enabled polymer particles of uniform size and shape to be produced without the need for a grinding and milling step. Typically, these processes involve heating the polymer to a temperature above its softening point, and dispersing in an inert liquid in the presence of a dispersing agent using high shear agitation to form a dispersion with the polymer in finely divided form, after which the dispersion is cooled to below the softening point of the polymer to harden the particles. The process is carried out in a reactor equipped with a variable speed rotor and agitator, and the reactor is heated and cooled to the desired temperatures. This heating and cooling aspect of the process can be long, often taking several hours. Another problem with this process is that the reactor cannot be used for dispersing resins of high viscosity because of poor energy transfer around the stirring device so that the dispersion efficiency decreases considerably. Also, the process can only be carried out in batches, and not on a continuous basis.
More recently, polymer dispersions have been made in an extruder as shown, for example, in EP 246,729 and U.S. Pat. No. 5,124,073. Extruders are conveying single or multi-screw mixers. In some types there may be two or more screws which rotate in the same direction, and in other types the screws may rotate in opposite directions. The use of an extruder enables polymers of high viscosity to be readily dispersed and processes to be carried out continuously. In a typical process, a polymer is melt kneaded in a first zone of the extruder and liquid and dispersant are added to the melt as it enters a second zone with the result that a dispersion of the polymer in the liquid is formed. The dispersion is then cooled and uniform spherical polymer particles of controlled dimensions are formed.
It is known from other prior art documents to process polymers in extruders. For example, EP 477,634 (BASF Corporation) describes a process for introducing additives into a polyamide or polyester melt, comprising dispersing a pigment or non pigment colorant in an at least partially volatile liquid vehicle, with a water dispersible or soluble polyamide or polyester, or copolymers thereof, feeding the resulting dispersion to a vented extruder which is extruding the polymer, and removing the volatiles through an extruder vent to obtain a substantially homogenous system, which contains the polymer and an additive. Suggested stabilisers for the system include polyalcohols.
U.S. Pat. No. 4,996,259 (BASF Aktiengesellschaft) describes aqueous synthetic wax dispersions which can be used as coating films, and which comprise a partially or completely neutralized copolymer of a C
2
-C
4
olefin, and an unsaturated mono or dicarboxylic acid or dicarboxylic anhydride. The preparation occurs by neutralizing and dispersing the solid substance in an aqueous medium at above the melting point of the copolymer, by a method in which the process of neutralization and dispersion are carried out in an extruder.
U.S. Pat. No. 5,001,194, (Dow Chemical Company), describes a functionalized polymeric composition having separate phases, the first phase comprising a copolymer of monovinylidene monomer and a co-monomer, selected to have a reactive epoxide functionality. The second dispersed phase is a copolymer of an olefin monomer, and one or more ethylenically unsaturated monomers copolymerizable with the olefin monomer. An extruder can be used for the shear mixing required in the manufacturing process.
EP 173,456 (Mitsui) and EP 246,729 (Mitsui) both describe an aqueous dispersion, and process for the preparation thereof, comprising a hydrophobic thermoplastic resin, which is preferably an olefin resin, and a water insoluble or non water swelling polymer comprising a carboxylic acid salt group. The process is a dispersion one, in which the resin and polymer are melt kneaded in the presence of water, wherein phase inversion occurs, to form a stable dispersion of the resin particles in water.
U.S. Pat. No. 4,359,557 (Watkins et al), and reported in “Reactive Extrusion Principles and Practice”, Polymer Processing Institute, edited by M. Xanthos, Hanser Books, 1992, describes a process for making polyethylene terephthalate (PET) fibres having low pilling properties, in which ethylene glycol and PET are admixed in an extruder, with the resultant processed fibre being held at a constant length whilst being heat set to a controlled length. A larger viscosity reduction was observed when the processed polymer contained ethylene glycol than when it was processed without ethylene glycol.
Reactions to increase the molecular weight of certain polymers by reacting them with low molecular weight materials using an extruder to carry out mixing of the components are known. U.S. Pat. No. 5,376,734 (Al Ghatta) and U.S. Pat. No. 5,334,669 (Ghisolfi) relate to processes of producing high molecular weight polyesters for castings or mouldings from polyesters with a low molecular weight by mixing with an acid or anhydride in a heated extruder and then heating the extruded mixture for a long period, such as 12 hours at 150-170° C., in an oven. The examples show that the intrinsic viscosity of the polymers is greater after passing through the extruder which indicates an overall increase in molecular weight. A similar process is disclosed by U.S. Pat. No. 4,132,707 (Borman) relating to preparing a branched PET with high melt viscosity for plastic mouldings by reacting a polyester having a low melt viscosity with a branching component such as pentaerythritol by mixing the components in an extruder, pellitizing the extrudate and then heating it at around 200° C. for about 7 hours in an oven. Another similar process is disclosed in EP 442 282 (Phobos) which is concerned with making high molecular weight polyesters for mouldings or yarn, from low molecular weight polyesters, particularly from recycled PET, by reaction with a dianhydride involving mixing the polymer and dianhydride in an extruder, pellitizing the extrudate and then heating at about 200° C. for about 5 hours. The examples show that intrinsic viscosity of the polymers is greater after passing through the extruder which indicates an overall increase in molecular weight.
Further problems encountered in the art also relate to the preparation of functionalized polymers. It is not generally possible to make these (i.e. polymeric units with functional end groups, which are for example capable of cross-linking) by building a polymer from a mixture of suitable functional monomers and structural monomers because they are randomly dispersed along the chain. They also have a tendency to form branched polymers, which have unacceptably high viscosity for use in coating compositions.
It is also possible that functionalized polymers containing cleavable groups such as ester groups can be hydrolysed to form smaller molecular weight fragments, but in such instances the residual functional group on the cleaved polymer is a hydroxy. Previous techniques which have carried out this reaction and not quenched it have produced an equilibrium polymer co
Imperial Chemical Industries PLC
Short Patricia A.
Stachel Kenneth J.
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