Coating compositions prepared with a sterically hindered...

Details Coating compositions prepared with a sterically hindered... Coating compositions prepared with a sterically hindered...

1999-01-08

2001-07-10

Dawson, Robert (Department: 1712)

Stock material or miscellaneous articles

Composite

Of silicon containing

C528S032000, C427S387000, C524S501000, C524S506000, C524S804000, C524S806000, C524S812000, C524S178000, C524S401000, C524S800000

Reexamination Certificate

active

06258460

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a coating composition having increased block resistance comprising a polymer prepared by emulsion polymerizing at least one ethylenically unsaturated non-carboxy functional monomer, from 0.01 to 2 pphm of a sterically hindered alkoxylated silane monomer, and at least one anionic surfactant, wherein the polymer is crosslinked during film formation.
BACKGROUND OF THE INVENTION
Dry paint often comes in contact with itself especially in window and door areas and, depending on its hardness, the pressure, temperature, humidity, and duration of time which the surfaces are in contact, the painted surfaces sometimes stick together. This undesirable sticking together of two painted surfaces when pressed together or placed in contact with each other is referred to as “blocking”. Thus, an important characteristic of coatings is block resistance.
A crosslinked coating composition with a high degree of entanglement (higher viscosity) would be expected to have greater block resistance than a coating composition with few entanglements (low viscosity). For these reasons, silane monomers have been used to provide crosslinking in polymer compositions. U.S. Pat. No. 3,729,438 describes an aqueous dispersion of a crosslinkable emulsion polymer prepared from vinyl acetate and a vinyl hydrolyzable silane. Upon removal of water from the emulsion polymer, the polymer crosslinks.
U.S. Pat. No. 3,706,697 describes an emulsion polymerization product of an acryloxyalkylsilane monomer with an alkyl acrylic ester wherein a silane is introduced to the polymerization after a portion of the other monomers are polymerized. International Patent Application WO 97/15624 describes curable aqueous compositions prepared with an emulsifiable polymer having pendant and/or terminal silyl ester groups and pendant terminal acidic groups neutralized with fugitive bases.
Japanese Patent Application No. 6025502 describes a composition containing a polymer emulsion obtained by adding a tin catalyst (a diorganotin carboxylate) which is insoluble in water, to silylated vinyl polymers after emulsion polymerization.
The disadvantages of using such alkoxy silane monomers, however, are that (1) the alkoxy silane monomers may crosslink during the emulsion polymerization and form grit; (2) the alkoxy silane monomers may hydrolyze during the emulsion polymerization so that even if the alkoxy silane monomer was polymerized the reactivity necessary for subsequent cross-linking would be destroyed; or (3) the alkoxy silane modified emulsion polymer may hydrolyze during storage.
Copending commonly assigned U.S. patent application Ser. No. 09/166,050, filed Oct. 5, 1998, describes a pressure sensitive adhesive which is prepared by emulsion polymerizing an ethylenically unsaturated monomer and 0.01 to 2 pphm of a sterically hindered alkoxylated silane monomer.
There continues to be a need for coating compositions which provide improved block resistance, and provide increased shelf life without the disadvantages present in prior art coating compositions.
SUMMARY OF THE INVENTION
The present invention provides a coating composition having increased block resistance comprising a polymer prepared by emulsion polymerizing at least one ethylenically unsaturated non-carboxy functional monomer, from 0.01 to 2 pphm of a sterically hindered alkoxylated silane monomer, and at least one anionic surfactant, wherein the polymer is crosslinked during film formation, and the sterically hindered alkoxylated silane monomer has the structure
R—Si—(OR
2
)
n
(R
3
)
3-n
wherein R
1
is selected from the group consisting of an alkylene, arylene, and aralkylene group; R
2
is independently a sterically hindered alkyl group having 3 to 10 carbon atoms in a straight or branched chain configuration; R
3
is a monovalent alkyl group having 1 to 10 carbon atoms; and n is an integer of from 1 to 3.
According to another aspect the invention provides a solvent-free paint formulation comprising at least one pigment, rheology modifier, and surfactants, wherein the improvement comprises adding a polymer prepared by emulsion polymerizing at least one ethylenically unsaturated non-carboxy functional monomer, from 0.01 to 2 pphm of a sterically hindered alkoxylated silane monomer, and at least one anionic surfactant, wherein the polymer is crosslinked during film formation.
According to another aspect the invention provides a method of making a coating composition having increased block resistance comprising:
(I) preparing an aqueous emulsion polymer comprising at least one ethylenically unsaturated non-carboxy functional monomer, from 0.01 to 2 pphm of a sterically hindered alkoxylated silane monomer, and at least one anionic surfactant; (II) mixing a Lewis acid or a tin catalyst with the emulsion polymer to form a coating composition; (III) applying the coating composition to at least one surface of a substrate; and (IV) drying the coating composition to form a film. Preferably the Lewis acid is selected from carboxylic acids, dicarboxylic acids, mineral acids, sulfuric acid and phosphoric acid.
According to another aspect the invention provides a method of making a coating composition having increased block resistance comprising: (I) preparing an aqueous emulsion polymer comprising at least one ethylenically unsaturated non-carboxy functional monomer, from 0.1 to 5 pphm of at least one ethylenically unsaturated carboxy-functional monomer, from 0.01 to 2 pphm of a sterically hindered alkoxylated silane monomer, and at least one anionic surfactant; (II) applying the emulsion polymer to at least one surface of a substrate; and (III) drying the emulsion polymer to form a film.
The sterically hindered alkoxysilane functionality of the silanes crosslink during film formation, most probably after particle coalescence, resulting in a crosslinked coating with improved block resistance. Moreover, the presence of the sterically hindered alkoxy groups in the polymer minimizes crosslinking during emulsion polymerization and storage of the coating composition.
DESCRIPTION OF THE INVENTION
The coating compositions of the present invention are prepared from an aqueous emulsion polymer which is curable to form a film. The polymer is the reaction product of an ethylenically unsaturated non-carboxy functional monomer, from 0.01 to 2 pphm of a sterically hindered alkoxylated silane monomer, optionally from 0.1 to 5 pphm of an ethylenically unsaturated carboxy-functional monomer, optionally from 0.1 to 5 pphm of a ureido-functional monomer, and an anionic surfactant. The sterically hindered alkoxylated silane is incorporated in the backbone of the polymer. The steric hindrance of the alkoxylated silane minimizes hydrolysis of the alkoxylated group during polymerization and storage.
While not wishing to be bound by any particular theory, the present inventors believe that crosslinking occurs between the sterically hindered alkoxysilane functionality on the polymer by means of a hydrolysis reaction to give silanols with subsequent condensation reaction between silanols and/or carboxyl groups on the polymer. Such crosslinking occurs during film formation, most probably after particle coalescence or drying of the coating.The advantage of preparing the coating composition with sterically hindered alkoxylated silane monomers is that crosslinking during the emulsion polymerization of the polymer and storage of the coating, especially in the presence of carboxyl groups, is minimized.
The sterically hindered alkoxylated silane monomer is present in a critical amount of from 0.01 to 2 pphm, preferably 0.05 to 1 pphm. The sterically hindered alkoxylated silane monomer has the structure:
R′—Si—(OR
2
)
n
(R
3
)
3-n
wherein R
1
is selected from the group consisting of an alkylene, arylene, and aralkylene group; R
2
is independently a sterically hindered alkyl group having 3 to 10 carbon atoms in a straight or branched chain configuration; R
3
is a monovalent alkyl group having 1 to 10 carbon atoms; and n is an integer of from 1 to 3.
Suitable stericall

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