Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
1998-09-25
2001-08-21
Yoon, Tae H. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C528S492000, C528S499000, C523S340000, C523S501000, C524S556000, C524S832000, C524S845000
Reexamination Certificate
active
06277953
ABSTRACT:
The present invention relates to stable aqueous polymer dispersions and a process for their preparation. More particularly, aqueous polymer dispersions are prepared by forming a polymer salt in a hydrophilic organic solvent and removing the hydrophilic organic solvent from a solvent blend of water and the hydrophilic organic solvent at temperatures which minimize the mean particle size of the polymer in the resulting aqueous dispersion of the polymer and/or removing the organic solvent from the polymer/organic solvent/water blend such that inversions do not occur.
BACKGROUND OF THE INVENTION
Protective and decorative coatings based on polyester, alkyd and acrylic polymers have been increasingly used in many applications because they provide a wide range of strength, flexibility, toughness, adhesion, degradation resistance and other film properties. Many are solution polymers because they are prepared and applied as solutions of organic polymers in organic solvents.
Environmental concern over the use of organic solvents has become increasingly important to the coating industry. This concern not only extends to preservation of the environment for its own sake, but extends to public safety as to both living and working conditions. Volatile organic emissions resulting from coating compositions which are applied and used by industry and by the consuming public are not only often unpleasant, but also contribute to photochemical smog. Governments have established regulations setting forth guidelines relating to volatile organic compounds (VOCs) which may be released to the atmosphere. The U.S. Environmental Protection Agency (EPA) established guidelines limiting the amount of VOCs released to the atmosphere, such guidelines being scheduled for adoption or having been adopted by various states of the United States. Guidelines relating to VOCs, such as those of the EPA, and environmental concerns are particularly pertinent to the paint and coating industry which uses organic solvents that are emitted into the atmosphere.
The use of aqueous dispersions of polymeric vehicles, or alternatively, high solids polymeric vehicles are two general approaches that have been used to reduce VOCs in coating compositions. Previous approaches for conventional aqueous systems have limited the molecular weights of the polymers used in the polymeric vehicle, which limits the impact resistance and other properties of the coating binders and films resulting from the polymeric vehicles.
The conventional process for the preparation of aqueous polymer dispersions is emulsion polymerization, in which hydrophilic assistants (emulsifiers and/or protective colloids) are used in order to ensure the stability of the dispersion. These assistants make the films produced from the dispersions somewhat sensitive to water.
Another way to reduce VOCs is to use water as a medium for the film-forming components in the polymeric vehicle. One approach is to make a polymer such as a polyester, alkyd, acrylic or epoxy polymer having carboxyl groups, or other ionizable groups and acid number in amounts effective such that when the carboxyl groups are converted into salts with a neutralizer such as an amine, they will permit the polymer or oligomer to be dispersed in a mixed solvent system which includes an organic solvent and water. The salt, such as an amine salt, of the oligomer or polymer disperses into the mixed water/organic solvent system with the formulation of a dispersion as opposed to a solution. This is commonly known as a water reducible system.
Organic solvent has typically been removed from water reducible systems through two routes. The first route utilizes low temperature boiling solvents that are water miscible and often form low temperature boiling azeotropes with water. These types of processes result in the loss of neutralizer and require addition of make-up neutralizer in the process. Further, use of insufficient solvent in these systems results in the polymer salt being insoluble with addition of water. Hence, an inversion takes place as solvent is stripped from the system.
The second route typically utilizes organic solvents that boil at temperatures greater than 100° C. and also often form azeotropes with water. Distillation proceeds at higher temperatures which, if the salt is an amine, will result in loss of amine, cause undesirable particle size variation in the dispersion and undesirably increase the average particle size of the resin in the dispersion. If distillation continues too long and too much amine is stripped, the dispersion will become unstable and fall apart.
To keep viscosities low, in some water reducible systems molecular weights of polymers have been kept low. As a result, these systems have required large amounts of cross-linker and cross-linking to achieve coatings performance. The use of higher molecular weight polymers in water reducible systems often results in unacceptably high processing viscosities due to the inversion from water in oil to oil in water. In addition, removal of water and solvent results in loss of neutralizer as described above, destabilization of the dispersion and undesirable large resin particle size.
Examples of processes for forming acrylic polymer dispersion are described in U.S. Pat. Nos. 5,319,019, 5,356,988 and 5,380,771. In each of these processes water immiscible solvents are used and solvent is stripped at high temperatures. High temperature stripping results in inversions which increase viscosity and result in increased particle size.
An object of the invention is to provide a polymeric vehicle which will reduce or minimize VOCs.
Another object of this invention is to provide polymeric vehicles which are not only low in VOCs, but which provide coating binders with good film properties such as hardness and impact resistance.
Another object of this invention is to provide a process for preparing stable aqueous polymer dispersions having higher solids content with resins having low acid values and higher molecular weights, yet also retain low processing viscosities while preparing the dispersions.
Yet another object of this invention is to control the viscosity to low levels of a liquid polymeric vehicle or liquid formulated coating composition with the use of water and with the minimization of organic solvents for such control.
Other objects, advantages, features and characteristics of the present invention will become more apparent upon consideration of the following description and the appended claims.
SUMMARY OF THE INVENTION
The present invention is directed to a dispersion process effective for providing a stable water dispersion of high molecular weight polymers. The aqueous dispersions of the invention have less than about 2 weight percent organic solvent, at least about 30 weight percent solids, and a viscosity of less than about 20.0 poise at about ambient temperature. In an important aspect of the invention, processing temperatures are minimized, mean particle size of the resins in the dispersions are kept to size of not more than about 300 nm and the dispersion process is inversionless.
In accordance with the dispersion process of the invention, an ionic functional polymer is synthesized in a hydrophilic organic solvent, or synthesized neat and subsequently mixed with organic hydrophilic solvent. In an important aspect of the invention, the polymer which is formed has an acid value of at least about 4, and has a solubility of at least about 50 weight percent in the hydrophilic solvent, based on the weight of the polymer and solvent, and the hydrophilic solvent has a solubility in water of at least about 5 weight percent at processing temperatures, based on the weight of the solvent and water mixture.
After the polymer is fully dissolved in the organic solvent, an amount of neutralizer is added effective for providing a dispersion of the polymer upon addition of water and removal of solvent. The neutralizer may be any salt-forming base compatible with the ionizable functional polymer such as sodium hydroxide or an amine. In a very important
Coad Michael Davis
Nothnagel Joseph Leo
O'Callaghan Kevin Jude
Sterzer Carlos Alfredo Kienzie
Fitch Even Tabin & Flannery
McWhorter Technologies Inc.
Yoon Tae H.
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