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
1999-09-24
2004-05-04
Egwim, Kelechi C. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C524S501000, C524S512000, C524S514000, C524S521000, C524S522000, C524S538000, C524S539000, C524S542000, C525S153000, C525S155000, C525S218000, C525S220000, C525S221000, C525S163000
Reexamination Certificate
active
06730740
ABSTRACT:
The present invention pertains to aqueous cross-linkable polymer compositions for use in coatings and to a process for preparing the same.
BACKGROUND OF THE INVENTION
Due to increasingly stringent environmental regulations concerning the allowable content of volatile organic compounds in coatings, major efforts have been made to minimize the use of organic co-solvents in water borne coatings based on acrylic binders. However, in water borne coatings that use acrylic polymers as the main binder, the final hardness after curing of an applied coating film is often limited by the amount of organic co-solvent in the coating formulation. The organic solvent is required to lower the film-formation temperature in order to ensure proper coalescence of the polymer particles during the film-forming process. For some coating applications a minimal hardness is required, for example, if a high blocking resistance or scratch resistance is important. The hardness can be increased by raising the glass transition temperature of the acrylic polymer. However, when this coating is applied and dried at ambient temperature this often leads to poor film formation because of the limited amount of coalescing aid allowed. This poor film formation results in reduced gloss, low chemical resistance, and weathering.
Attempts have been made to lower the necessary content of volatile organic compounds in coatings by using two-component coating systems. In order to increase hardness, the two components should cross-link, for which a cross-linker is needed. Such cross-linkers, e.g., polyaziridine or polyisocyanate, often have a toxic nature. Two-component coating systems also show the general drawback that after mixing of the two components the pot life is limited.
A composition and a process of the type according to the opening paragraph are known from PCT patent application WO 95/29944. This publication discloses a process for the production of an aqueous polymer composition for use as a coating with improved hardness and low film forming temperature. Use is made of a cross-linking agent which links the first, water-soluble polymer with the second, hydrophobic polymer. The cross-linking agent reacts by condensation. The lower the water concentration, the more this reaction takes place. It was found that the cross-linking reaction proceeds relatively slowly and begins only after evaporation of all the water in the film. As a result, the early hardness and water resistance are relatively low.
European patent application EP-A 0 587 333 discloses a water-resistant polymer dispersion containing a carboxylated water-soluble polymer which is neutralized and solubilized with, e.g., ammonia. An alkali-insoluble emulsion polymer is prepared in the presence of the water-soluble first polymer. The alkali-insoluble polymer may contain an amine functionality. The composition may comprise a water-soluble polymer with an amine functionality, which serves to neutralize the soluble polymer, as an alternative for the ammonia. During preparation, the two stages are grafted together. Metal ions may be incorporated into the monomer mixture so as to create ionic metal/carboxylate cross-links. The water-soluble stage polymer remains susceptible to bases, which may cause resolubilization when a second layer of paint is applied. The hydrophobic polymer remains thermoplastic and does not cross-link, so that after application the resulting coating film shows a limited hardness.
A common problem with aqueous polymer compositions comprising polymer dispersions with polymers having carboxylic acid-functional groups, is their poor water resistance and poor recoatability, since the water-soluble polymer will easily redissolve in the presence of water or, when a second layer of water-borne paint is applied, under the influence of the present neutralizing base.
The object of the present invention is a polymer composition which can be used as a binder in water borne coatings with a low content of volatile organic compounds, which shows rapid hardness development and good film formation. Another object of the present invention is a process for making such a composition.
This object of the invention is achieved with a composition of the above-described type, wherein the composition further comprises a water-soluble third polymer with an amine functionality.
SUMMARY OF THE INVENTION
One embodiment of the invention relates to an aqueous cross-linkable polymer composition comprising:
a) a water-soluble first polymer comprising acid groups, wherein said first polymer is at least partially neutralized with a volatile base;
b) a dispersion of a substantially water-insoluble second polymer, said second polymer comprising carbonyl-functional groups; said second polymer produced by emulsion polymerization in the presence of an aqueous solution of the first polymer;
c) a cross-linking agent capable of undergoing a condensation reaction with the carbonyl-functional groups of the second polymer; and
d) a water-soluble, amine-functional third polymer.
In another embodiment, the invention relates to a process for preparing such an aqueous polymer composition.
DETAILED DESCRIPTION OF THE INVENTION
More specifically, the present invention pertains to an aqueous cross-linkable polymer composition comprising:
a) a first polymer, which comprises acid groups and is made water-soluble by the addition of a volatile base;
b) a dispersion of a second polymer, which comprises carbonyl-functional groups and which is water-insoluble;
c) a cross-linking agent which can react by condensation with the carbonyl-functional groups of the second polymer; and
d) a third polymer, which is water-soluble and has an amine functionality.
As a result, two different cross-linking reactions will occur after application of the composition according to the invention as a coating. Upon evaporation of the volatile base used to at least partially neutralize the acid groups of the water-soluble first polymer, the amine functionality of the water-soluble third polymer will form ionic bonds with the acid groups of the first polymer. The second cross-linking reaction takes place after evaporation of the water via reaction of the carbonyl-functional groups of the hydrophobic second polymer with the cross-linking agent. The first curing reaction is fast and is responsible for rapid initial hardness development. The second curing reaction is a slow covalent cross-linking and is responsible for the ultimate mechanical properties of the coating. Using a polymer composition according to the present invention, clear as well as pigmented coatings with volatile organic contents below 100 g/liter can be formulated, which have a high hardness, high gloss, high chemical resistance, and good weathering properties after curing.
Coatings comprising a polymer composition according to the present invention can be applied to various substrates, such as metal, wood, paper, cardboard, gypsum, concrete, plastic, etc. Various known application methods may be used, such as brushing, spraying, rolling, dipping, printing, etc. In particular, a polymeric composition according to the present invention can be used in pigmented coating compositions for application directly onto metal (self-priming systems) or as a primer or a top coat for metal, or as a primer, clear coat, or top coat for wood.
The object of the invention is also achieved with a process comprising the steps of:
a) preparing an aqueous solution of an acid-functional first polymer, which is made water-soluble by the addition of a volatile base;
b) preparing by emulsion polymerization in the presence of the aqueous solution of the first polymer, a dispersion of a substantially water-insoluble second polymer comprising carbonyl-functional groups;
c) adding a cross-linking agent which can react with the functional groups of the second polymer,
d) adding a third polymer with an amine functionality to the polymer dispersion after the preparation of the dispersion of the second polymer, optionally after addition of volatile base to the dispersion of the first polyme
Delaunoit Genevieve
Mestach Dirk Emiel Paula
Pilaszek Wincenty Lambertus Stanislaw
Weber Maarten Johannes Jacobus
Akzo Nobel N.V.
Burke Michelle J.
Egwim Kelechi C.
McGillycuddy Joan M.
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