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
1995-12-14
2001-10-30
Reddick, Judy M. (Department: 1713)
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...
C524S361000, C524S365000, C524S504000, C524S538000, C525S183000
Reexamination Certificate
active
06310127
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to improved coating compositions. More particularly, the present invention relates to rheological additives for coating compositions. Even more particularly, the present invention relates to thixotropic additives which control sag, levelling and pigment suspension in a coating composition. The rheological additives of the present invention are copolymers having a backbone of recurring &agr;-olefin and carboxylic acid monomeric units, having grafted thereto one or more polymeric grafts with the following recurring monomeric units:
—NHC(O)RC(O)NHR
1
and—NH—R—C(O)—
or a combination thereof, wherein R is an alkylene group of at least about 2 carbon atoms, and R
1
is R or aryl.
2. Description of Related Art
In the formulation of coatings, it is well known that rheological modifiers may be added to control the flow properties of the final product for a particular application. A large number of rheological additives are available to control properties of a coating formulation during storage, transportation, processing, application, and post-application to a particular surface. These additives include modified and unmodified organic clays, and a large variety of organic compounds, as set forth, for example, in
Rheology Handbook,
Rheox, Inc., Hightstown, N.J. 1991.
The control of sag, levelling and pigment suspension in a coating formulation requires additives which maintain viscosity of the coating at a variety of shear rates. This is difficult, since control of these properties involves opposing phenomena. For example, total freedom from sag requires a coating with extremely high viscosities during application; however, the low flow rate of such a material means that the coating may not flow out completely to form a smooth film. The development of an enveloping rheological structure to control pigment suspension during storage may result in coating which exhibits levelling problems during application.
To maintain viscosity under these diverse conditions, the prior art has suggested incorporation of additives to control pigment suspension, such as surfactants and thickeners, along with additional additives intended to control levelling and sag, such as organic clays. However, production of a coating with such multiple additives is a difficult and complex procedure, sometimes requiring separate processing conditions for each rheological additive. A correct balance of properties must also be achieved among the various additives. In the prior art, even processing parameters of single rheological additives have proven quite difficult to control. If these precise formulation requirements are not employed, coatings with inconsistent properties may be produced. These precise formulations are also difficult to manufacture on a large scale, and require maintenance of a large variety of additive products.
The rheological modifier utilized in the improved coating composition of the present invention performs the functions of both an anti-sag additive and an anti-settling additive. In addition, the process of producing the coating composition of the present invention does not require the multiple formulation steps or precise parameter control necessary to compound the coatings of the prior art.
SUMMARY OF THE INVENTION
The present invention is an improved coating composition. In addition to the polymeric binder and liquid medium characteristic of typical coatings, the present composition includes an anti-sag and anti-settling effective amount of a thixotropic rheological modifier. The rheological modifier of the present invention comprises two essential components, a backbone polymeric block and a graft polymeric block. The polymeric backbone comprises recurring monomeric units derived from monomers selected from the group consisting of one or more &agr;-olefins and one or more unsaturated carboxylic acids. The polymeric backbone may include unreacted pendent carboxylic acid functions, which may be further reacted with an alkaline substance to be partially or fully neutralized; or, may be un-neutralized. Connected to the backbone are reaction residues formed by the reaction of the acid functions of the backbone and amine functions of one or more polymeric grafts comprising recurring monomeric units derived from monomers selected from the group consisting of:
—NHC(O)RC(O)NHR
1
—
and
—NH—R—C(O)—
or a combination thereof, wherein R is an alkylene group of at least about 2 carbon atoms, and R
1
is R or aryl.
The rheological modifier utilized in the present composition improves both the anti-sag and anti-settling characteristics of the coating composition.
The present invention further includes a process for producing the improved coating composition above, which comprises the following steps:
(1) dispersing the rheological modifier of the present invention in a liquid medium to form a gel;
(2) incorporating the gel into a coating composition.
The rheological additive of the present invention may be incorporated into a paint composition in the pigment grinding step, and preparation of a coating composition containing the present additive requires fewer precise process steps than preparation of coatings with other known rheological modifiers, so production on a commercial scale is simplified. The present process provides an anti-sag and anti-settling effective composition whose performance is less affected by processing parameters such as heat, shear and air entrapment, than other known additives. In addition, the chemical structure of the rheological additive utilized in the present invention evolves fewer harmful by-products during processing than known additives, and provides a coating composition which releases fewer deleterious solvents during drying.
DETAILED DESCRIPTION OF THE INVENTION
The rheological modifiers utilized in the improved coatings of the present invention may generally be described as thixotropic in nature. When the modifiers of the present invention are incorporated into a coating, a wide range of coating viscosities may be produced, depending on shear rate.
In the improved coatings of the present invention, when shear forces are applied to the coating in the unsheared state, as shear rate is increased, viscosity decreases. The longer the material is sheared, the lower the viscosity, until some limiting value is reached. If the shear rate is then decreased, viscosity rebuilds slowly. This increase in viscosity is time dependent; and, given sufficient time (seconds to many hours) the viscosity returns to the value originally observed in the original unsheared state.
While not wishing to be bound by any theory, it is believed that intermolecular attractions between highly polar carboxyl and amide functionalities in the rheological additive of the present invention create a complex colloidal network in the unsheared state during storage. As the shear rate increases, the weak intermolecular attractions in the colloidal structure break down, and the coating exhibits excellent high-shear viscosity properties necessary for effective processing and application. As the rate of shear is decreased, the intermolecular structure again begins to rebuild and predominate. At low post-application shear rates, the intermolecular attractions again predominate to provide a coating viscosity which is high enough to prevent sag during the drying of the coating, but low enough to provide excellent levelling and film-forming properties. At the extremely low shear rates experienced during storage, the intermolecular attractions form a colloidal network which acts to reduce flocculation of any dispersed pigments present in the composition. As understood, this colloidal network envelops the pigment particles and reduces the interaction of electrostatic charges on the surface of the particles.
The rheological additive of the present invention improves the sag resistance of a coating composition. Following application on a surface, the coating must maintain sufficient viscosity during the drying process to prevent unsightly ru
Allied-Signal Inc.
Reddick Judy M.
Szuch Colleen
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