Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1998-10-30
2001-03-20
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S505000
Reexamination Certificate
active
06204319
ABSTRACT:
TECHNICAL FIELD
This invention relates to an aqueous coating composition and in particular to an aqueous coating composition containing a carbon black pigment dispersion formed with an AB-block copolymer that has improved black color jetness.
BACKGROUND OF THE INVENTION
AB copolymer dispersants are known in the art and are used to form pigment dispersions that are used in a variety of coating compositions and for inks, in particular for inks for ink jet printers. U.S. Pat. No. 4,656,226 issued Apr. 7, 1987, to Hutchins et al shows an AB block copolymer dispersant having polar groups such as acid or amine groups that attach to a pigment's surface and the other block of the polymer keeps the pigments stabile in the dispersion or coating composition. U.S. Pat. No. 5,231,131 to Chu et al issued Jul. 27, 1993 shows the use of graft copolymers to form aqueous pigment dispersions. U.S. Pat. No. 5,272,201 issued Dec. 21, 1993 to Ma et al and U.S. Pat. No. 5,221,334 issued Jun. 22, 1994, to Ma et al show AB or BAB block polymers that are used in aqueous printing inks to keep pigments dispersed in the inks.
A number of the above polymer dispersants are made via group transfer polymerization (GTP). GTP is a “Living” free radical polymerization technique and is a well-known technique that is used to form the aforementioned block polymers. In GTP, an initiator splits with one end going to the tail of an acrylic monomer unit and the other to the head. The group at the head transfers to the head of a second monomer unit as that unit is polymerized with the first. Such techniques are described in U.S. Pat. Nos. 4,417,034 issued Nov. 22, 1983, and 4,508,880 issued Apr. 2, 1985, to O. W. Webster and U.S. Pat. Nos. 4,414,372 issued Nov. 8, 1983, and 4,524,196 issued Jun. 18, 1985 to Farnham et al. The above patents are hereby incorporated by reference.
Conventional truck and automobile finishes comprise a clear coat applied over a pigmented base coat that is applied over a primer layer. The polymer dispersant used in the base coat must not only keep the pigment dispersed but needs to contain a reactive group that will react with the crosslinking agent used in the coating composition which on curing makes the dispersant an integral part of the resulting finish. Polymer dispersants can cause intercoat adhesion failures and usually are used only in limited amounts. It is desirable that the polymer dispersant have reactive groups that will bond with the primer layer to improve intercoat adhesion of the base coat to the primer rather than cause intercoat adhesion problems that cause flaking and chipping of the resulting finish. Carbon black pigments are particularly difficult to disperse and a dispersant is needed that disperses the carbon black pigment but also has the above properties.
SUMMARY OF THE INVENTION
An aqueous coating composition containing:
(a) an aqueous carrier medium:
(b) a pigment having a pH below 7.0;
(c) an AB block copolymer pigment dispersant resin of
(1) an A block of a hydrophobic polymer having amine groups; and
(2) a B block of a hydrophilic polymer having hydroxyl groups and carboxyl groups;
the AB block copolymer has a number average molecular weight of 5,000-20,000 and contains 20-80% by weight of a polymeric A segment and correspondingly 80-20% by weight of a polymeric B segment;
wherein the pigment and the dispersant are present in a pigment to dispersant weight ratio of 0.1/1-20/1;
(d) a film forming polymeric binder containing carboxyl groups and
(e) a crosslinking agent which on curing of the coating composition will react with the carboxyl and hydroxyl groups of the AB block copolymer and the carboxyl groups of the film forming polymeric binder.
Pigment dispersions of the AB block copolymer and process for forming the dispersions also are part of this invention.
DETAILED DESCRIPTION OF THE INVENTION
The novel coating composition of this invention is an aqueous based composition containing 10-70% by weight of film forming polymeric components which include the film forming polymeric binder, the crosslinking agent and the AB-block copolymer dispersant and a corresponding amount of 30-90% by weight of an aqueous carrier which is primarily water. Acidic difficult to disperse pigments can be used in this coating composition, such as carbon black. The AB block copolymer used in the composition has an A block which is hydrophobic and has amine functionality and is attracted to the acidic pigment. The B block of the copolymer is hydrophilic, contains hydroxyl and carboxyl groups which can be formed into a salt by the addition of ammonia or amine to make the copolymer easily dispersible in the aqueous medium and keeps the polymer and associated pigment dispersed in the aqueous medium.
The novel coating composition is stable and in general is non-flocculated or agglomerated. The AB block copolymer dispersant used in the composition is compatible with a variety of polymeric film forming binders that are conventionally used in coating compositions and in particular, compatible with acrylic polymers that are used in the novel coating compositions. Upon curing of the coating composition, the AB block copolymer reacts with other film forming components of the coating composition and becomes part of the film and does not cause deterioration of the film upon weathering as may occur if it remained an unreacted component of the film. Also, since the AB block copolymer is an excellent dispersant, the ratio of polymer to pigment is less than is used with conventional dispersants and a variety of difficult to disperse acid pigments can be dispersed thereby enlarging the number and types of pigment dispersions that can be formulated. Polymers also can be dispersed in an aqueous medium using the AB block copolymer. The AB block copolymer has reactive groups that improve adhesion to a primer layer thereby reducing flaking and chipping of the coating composition.
The coating composition contains a film forming binder, typically an acrylic polymer and a curing agent, such as a blocked polyisocyanate or an alkylated melamine. These provide excellent coating properties desirable for automotive finishes. Base coats of conventional base coat/clear coat finishes currently used on automobiles and trucks containing a carbon black pigment dispersion that uses the AB-block copolymer have significantly improved black jetness in comparison to base coats pigmented with carbon black pigment dispersions that utilize conventional random acrylic polymers.
The AB block copolymer used herein can be prepared by anionic polymerization or by Group Transfer Polymerization (GTP) techniques or by any other process, such as atom transfer, that can be used to prepare block copolymers. Preferably, the polymer is prepared using GTP techniques as described in the aforementioned GTP patents (U.S. Pat. Nos. 4,417,034, 4,508,880, 4,414,372 and 4,524,196). These polymers have a number average molecular weight of about 5,000-20,000, preferably about 7,500-15,000 and a polydispersity of 1.8-2.0. The A block or segment of the polymer comprises about 20-80% by weight of the polymer and correspondingly, the B block or segment of the polymer comprises about 80-20% by weight of the polymer.
All molecular weights referred herein are determined by GPC (gel permeation chromatography) using a polystyrene standard.
The term (meth)acrylate refers to both the acrylate and methacrylate esters.
The AB block copolymer has a relatively non-polar anchoring A segment and a relatively high polar stabilizing B-segment. The A-segment is designed to absorb on the surface of the pigment and the B-segment provides a steric stabilization of the pigment particle against flocculation. The A-segment must be available for interaction with the pigment surface; (2) of sufficient size to provide irreversible adsorption; and (3) not so large as to completely cover the pigment surface or cause collapse of the B-segment. The B-segment must be: (1) of sufficient size to provide steric stabilization (approximately number average molecular weight of 1500 or
Houze Eric Charles
McIntyre Patrick F.
Spinelli Harry J.
Yokoyama Ayumu
Benjamin Steven C.
Cain Edward J.
E.I. Du Pont de Nemours and Company
Fricke Hilmar L.
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