Coating processes – With post-treatment of coating or coating material – Solid treating member or material contacts coating
Reexamination Certificate
2002-02-25
2004-06-29
Cameron, Erma (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Solid treating member or material contacts coating
Reexamination Certificate
active
06756075
ABSTRACT:
The present invention relates to a method of manufacturing an in-press coated composite substrate having an aqueous composition applied as a combined sealer and topcoat finish. More particularly the present invention relates to the incorporation of ion exchange resin into the aqueous coating composition to achieve (i) improved initial whiteness in a white pigmented composition, (ii) excellent color retention/stability and (iii) excellent press release properties; in a single coating composition for pressboard applications.
PCT Patent Application Ser. No. PCT/US99/25959, published as WO 00/27635, discloses the use of a quick or rapid setting primer coating composition for in-mold or pressboard applications to improve the surface quality and release characteristics of the coated substrate. Such characteristics include low porosity, smoothness, hardness, flexibility, blocking resistance, moisture resistance and adherence to subsequently applied coating compositions. However, such compositions can exhibit insufficient uniformity of the coating across the surface of the substrate, in that the coating composition needs to obtain uniform fiber coverage across the substrate. Additionally, such compositions can exhibit yellowing and color stabilization problems. Moreover, the use of ion exchange resin in aqueous coating compositions for in-mold or pressboard applications is not disclosed.
The problem addressed by the invention is to provide a single in-mold or pressboard coating composition that exhibits excellent initial whiteness, color stability/retention and press release properties along with traditional surface characteristics required of in-mold pressboard coating applications. The incorporation of ground ion exchange resin into an aqueous pigmented pre-press sealer coating composition achieves this objective.
The present invention provides a method for the manufacture of an in-press coated composite substrate, comprising:
(a) applying an aqueous coating composition to a surface of a compressible mat comprising fibers, chips or particles and a resin;
(b) compressing the mat and applied coating composition between heated metal surfaces in a press; and
(c) releasing the compressed, coated composite substrate from the press;
characterized in that the aqueous coating composition comprises an aqueous emulsion copolymer and a ground ion exchange resin. The present invention further provides an aqueous coating composition for in-press molded composite substrates comprising an emulsion polymer and ground ion exchange resin wherein said coated composite substrate exhibits improved mold release properties, color stability and initial whiteness as compared with identical coatings without the ion exchange resin.
In the manufacture of consolidated wood products, including hardboard, fiberboard, particleboard and the like (also referred to as “pressboard” or as a “composite substrate”), wood fibers, chips or particles are mixed with a thermoset resin, formed into a mat and then compressed at high temperature and pressure to yield the desired product. Other types and combinations of fibers, chips, particles and resin may be used in the composite substrate, including cellulose, glass, synthetic polymers, carbon and organic or inorganic cementitious compositions. The product may take many forms but is usually in the form of a sheet or board. In many applications, prior to compression, a waterborne pre-press sealer is applied to the wet or dry fiber/chip/particle mat to provide hot press release and to aid in the consolidation of the board surface. After removal from the press, a pigmented, aqueous topcoat, or aqueous sealer followed by an aqueous topcoat, may be further applied to improve aesthetic and surface characteristics of the unfinished board. Such characteristics include low porosity, smoothness, hardness, flexibility, blocking resistance, moisture resistance and adherence to subsequently applied coating compositions, in order to protect the board during storage and handling. A further goal of the sealer/topcoat is to provide color stabilization, that is, to prevent the migration of tannins and other undesirable colored wood extracts from the wood fiber/chip/particle board into the coating, thereby preserving the aesthetic appearance of the coated wood under adverse conditions, including, humidity, heat and water contact.
Manufacturers of fiber/chip/particle board have long sought after a single waterborne pre-press coating that would serve the dual purposes of both the pre-press and post-press finishes. In so doing, both the sealer and topcoat application steps could be eliminated from the post-press board manufacturing process, promoting faster line speeds and higher productivity while reducing energy consumption. One problem faced by the industry is that the thicker coating required to perform both pre-press and post-press functions often leads to unacceptable adhesion to the press plates. Another problem is color stability/retention in that the usually white coating emerges from the hot press yellowed or darkened, most likely as a result of contamination/leaching of tannins and other colored species from the wood fiber/chip/particle substrate. The darkened coatings vary in intensity and can “bleach out” when exposed to ultraviolet (UV) light, creating undesirable variations in color across the substrate surface during storage. Another problem is the uniformity of the coating across the surface of the substrate, in that the coating composition needs to obtain uniform fiber coverage across the substrate.
The present invention addresses the above problems by providing a single in-mold or pressboard coating composition that exhibits excellent initial whiteness (for a white pigmented composition), color stability/retention and press release properties along with traditional surface characteristics required of in-mold pressboard coating applications. The coating composition of the present invention incorporates ground ion exchange resin in the form of pre-slurry into an aqueous pigmented pre-press sealer coating composition. The particle size of the ground ion exchange resin ranges from 0.1 to 50 microns and can be an anion or cation exchange resin. The ion exchange resin may also be classified as many types of resins, including mixed bed resins, macroreticular resins or other known resins, including a combination of resins. The ion exchange resin is effective at low levels, including at least 1.5 percent solid ion exchange resin on coating polymer solids.
Coating compositions frequently contain emulsion polymers. Emulsion polymers as used herein are defined as compositions containing an emulsion-polymerized, water-insoluble addition polymer with a glass transition temperature (“Tg”) of from −50 degrees centigrade (“° C.”) to 150° C., more particularly from −10° C. to 120° C. “Glass transition temperature” or “T
g
” as used herein, means the temperature at or above which a glassy polymer will undergo segmental motion of the polymer chain. Glass transition temperatures of a polymer can be estimated by the Fox equation [Bulletin of the American Physical Society 1, 3, page 123 (1956)] as follows:
1
T
g
=
w
1
T
g
(
1
)
+
w
2
T
g
(
2
)
For a copolymer of monomers M
1
and M
2
, w
1
and w
2
refer to the weight fraction of the two co-monomers, and T
g(1)
and T
g(2)
refer to the glass transition temperatures of the two corresponding homopolymers in degrees Kelvin. For polymers containing three or more monomers, additional terms are added (w
n
/T
g(n)
). The T
g
of a polymer can also be measured by various techniques including, for examples, differential scanning calorimetry (“DSC”). The particular values of T
g
reported herein are calculated based on the Fox equation.
The glass transition temperatures of homopolymers may be found, for example, in “Polymer Handbook”, edited by J. Brandrup and E. H. Immergut, Interscience Publishers.
The emulsion polymer may be prepared by the addition polymerization of at least one ethylenically unsaturated monomer such as, f
DeMasi Anne Seton
Lauer Rosemarie Palmer
Cameron Erma
Rohm and Haas Company
Stauss Karl
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