Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-02-14
2002-01-22
Yoon, Tae H. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S422000, C524S429000, C524S904000, C524S567000, C525S934000, C427S458000, C427S459000
Reexamination Certificate
active
06340720
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns powder coating comprising polyvinylidene fluoride (“PVDF”), and in particular, a co-coagulation process for making powder coating compositions comprising PVDF. PVDF is a very important raw material for coating compositions based largely on ultraviolet (“UV”) light transparency. PVDF, for example, provides long term protection for metal surfaces against exterior exposure, much better than other known organic coating materials. In order to balance the performance of PVDF coatings, a secondary resin normally is needed to provide good adhesion to substrate, to reduce the shrinkage of the polyvinylidene fluoride due to excess crystallization, to increase the dispersability of a pigment in a coating and to obtain good optical properties. Thermoplastic polymers, such as acrylic polymers, can be used in the invention as a secondary resin for the PVDF-based powder coating compositions made from the co-coagulation process.
The process involves the co-coagulation of a blend comprising PVDF latex and a thermoplastic polymer latex, such as an acrylic polymer latex, to produce a PVDF-based powder coating that can be applied to a substrate. The co-coagulation process is less expensive and less time consuming than other processes and provides a coating composition that has a higher amount of homogeneity. Homogeneous coating compositions, including those in the form of a powder, are preferred because the higher degree of homogeneity in a coating blend provides for better performance, such as better coating properties, hardness, adhesion to substrate, and abrasion and mar resistance.
Industrial and commercial users of coating compositions, as well as the general public, have increasingly demanded coating preparations and paints with reduced or no volatile organic content (“VOC”). VOC is a concern to industrial and commercial applicators of paint and coating compositions, as well as the general public, because of the health risks that have been associated with VOC in coatings and paints. Also, industrial and commercial users prefer coating compositions with little or no VOC because of disposal and waste concerns that arise with material having VOC. Thus, there is consumer demand for high quality and weather or corrosive resistant coating materials that can be applied without the use of solvents, such as powder coatings including PVDF-based powder coatings.
2. The Related Art
Polyester powder coatings have been traditionally used for outdoor applications and such coatings have evolved to triglycidylisocyanuarate and, recently, superpolyesters. However, these polyester coatings tend to undergo photo-oxidation and hydrolysis when used as exterior protective coatings, and do not meet the highest performance standards. PVDF coatings have better weather resistance than polyesters. PVDF has been used in architectural coating formulations since early 1960, and is demonstrated to be a long lasting protective and decorative coating for outdoor applications. In binder matrix, PVDF based coating compositions usually contain 70 weight percent of PVDF and 30 weight percent of a secondary polymer. Pigments may be added for decorative purpose.
Commercial PVDF paints are generally solvent dispersions and release solvent during coating preparation. The solvent emission is currently managed by the end user through the installation and operation of an incineration system in a coating line. The incinerator will burn the VOC of the solvent emission to reduce or preclude harmful emission of VOC to the atmosphere. The need for an incinerator is, generally, a regulatory requirement. Thus, PVDF based paints in a solvent dispersion will require the end user to invest capital to install air pollution control apparatus, such as an incinerator, and incur costs to maintain the equipment and comply with regulatory requirements. These costs can be avoided by the use of solvent free coatings, such as PVDF-based powder coating preparations.
PVDF-based powder coatings wherein PVDF is mixed with at least one inorganic coating, such as silica sand and glass spheres, are described in U.S. Pat. No. 3,824,115 and U.S. Pat. No. 4,185,000. The methods for making these coating compositions do not involve co-coagulation. The powder coating compositions described in these patents result in a product having non-uniform color when applied to a substrate, due to the separation of the particles in the composition, because the particles within the powder coating composition have different densities. Also, the pigment is not well wetted by the resin and, thus, does not produce a coating having desired long-term weatherability. U.S. Pat. No. 4,391,763 discusses PVDF powder having spherical particles suitable for powder coating application. The coatings described in this patent lack homogeneity, which is a concern with this approach to obtaining PVDF-based powder coating compositions.
PVDF-based powder coating compositions can be made by cryogenic grinding methods. U.S. Pat. No. 4,770,939 describes a process comprising mixing pigment with PVDF and a compatible thermoplastic using an extruder at melt condition, pelletizing the resulting extrudate, and cryogenically grinding the pellets. European Patent 284,996 uses the same process to produce PVDF powder coatings containing solid coalescent to improve flow characteristics during a baking process. U.S. Pat. No. 5,346,727 discusses the use of low molecular weight PVDF and its copolymer to produce powder coating using cryogenic grinding processes. Cryogenic powder fabrication processes require large energy consumption for extrusion and pelletization. The cryogenic grinding step requires a large quantity of liquid nitrogen as coolant. Thus, cryogenic processes tend to be expensive and difficult to manage, and not as well suited for large scale manufacturing as other processes, including the co-coagulation process described herein. Cryogenic grinding processes can generally be characterized as time consuming and costly.
U.S. Pat. No. 5,229,460 describes a grinding process that does not involve cryogenic processing at about −50° C. The process involves mixing and heating fluoropolymer, such as, PVDF, and other ingredients in an extruder. The molten mixture is then cooled slowly to obtain a solid mass having a high degree of crystallinity to reduce the toughness of the solid mixture. Because of the high degree of crystallinity, the solid mass can be ground to powder at a temperature above −50° C. The solid mass may also comprise a terpolymer of three monomers. This process tends to be time consuming and costly, and requires energy expenditures to obtain a molten PVDF mixture.
A method for making a pigmented PVDF powder coating using a solvent removal process, but without coagulation of a polymer latex mixture, is described in U.S. Pat. No. 5,739,202. The method involves solvent dissolution of a fluorine-free polymer and then dispersing a PVDF polymer and pigments in the solution. Solvent removal and grinding is required to obtain the powder coating product.
A process for making a powder composed of a poly (meth)acrylate and fluoropolymer involving steps of (1) mixing together a water based latex of a first polymer and a water-based latex of a second polymer; (2) spray drying the mixture of step (1) to form substantially spherical particles; and (3) optionally passing the particles through a sieve was recently described in U.S. Pat. No. 5,827,608. This process does not involve co-coagulation of polymer latex to achieve a desired product, and is quite expansive from the viewpoint of water removal.
It has now been found that powder coatings comprising PVDF which are homogeneous and exhibit superior mechanical and physical properties, and exemplary performance, can be made by a co-coagulation method. This method involves co-coagulation of a latex mixture comprising polyvinylidene fluoride (or copolyvinylidene fluoride) latex and a compatible thermoplastic latex with, optionally, additives including pigments. The co-coagul
Kelly Michelle
Kent Bradley
Lin Shiow-Ching
Ausimont USA, Inc.
Norris & McLaughlin & Marcus
Yoon Tae H.
LandOfFree
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