Compositions and thermal mass transfer donor elements for...

Details Compositions and thermal mass transfer donor elements for... Compositions and thermal mass transfer donor elements for...

1995-02-09

2001-02-20

Hess, Bruce H. (Department: 1774)

Stock material or miscellaneous articles

Structurally defined web or sheet

Discontinuous or differential coating, impregnation or bond

C040S612000, C116S06300T, C359S584000, C428S484100, C428S500000, C428S913000, C428S914000

Reexamination Certificate

active

06190757

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to thermal mass transfer compositions and articles for use in producing signage articles. In particular the present invention relates to the use of thermal mass transfer compositions which employ a binder which does not significantly absorb ultraviolet radiation, thereby avoiding substantial heat absorption and degradation of the binder, and including durable, stable pigments which do not exhibit significant color change or loss of brightness.
2. Related Art
Thermal mass transfer processes use a donor sheet (commonly denoted a “ribbon” or “foil”) and a receptor sheet or substrate. The thermal mass transfer donor sheet normally comprises a carrier layer and a colorant layer with at least a thermally transferable colorant (a dye or preferably a pigment) in a heat softenable binder. The colorant layer typically consists of a pigment dispersed in a binder (the binder transferring with the pigment during thermal transfer). Thermal mass transfer sheets are used with the donor surface (colorant layer) in intimate contact with a receptor material, and the donor sheet is heated in an imagewise manner (e.g., by thermal printheads, irradiation as by a laser or high intensity radiation transmitted through a mask or stencil) to transfer the image forming material. In the thermal mass transfer system, the colorant layer is softened by the imagewise heating (and sometimes a receptor layer on the receptor sheet is contemporaneously softened), and the softened area is transferred to the receptor sheet. The ultimate use of the substrate having the transferred image thereon frequently dictates the durability requirements of the image.
Thermal mass transfer is useful for preparing outdoor durable signage articles, such as automobile registration tags which are adhered to license plates. See, for example, Patent Cooperation Treaty applications WO 94/19710 (claiming priority from U.S. Ser. Nos. 08/017,573 and 08/033,627) and WO 94/19769 (claiming priority from U.S. Ser. Nos. 08/017,573 and 08/033,625), both published Sep. 1, 1994. For the articles described therein, thermal mass transferred indicia was printed onto a specially formulated polyurethane “multifunction” layer, an attribute of which was to eliminate the need for a cover layer for resin-based binder printed indicia. Many commonly used binders employ large amounts of UV stabilizers and UV absorbers, which tend to produce heat in the binders and cause their early degradation, thereby decreasing brightness and/or increasing color change.
There is a need in the signage art, particularly those meant for outdoor usage, to be able to apply color images to many different substrates without loss of brightness or color change, and without having to use complex processes.
SUMMARY OF THE INVENTION
The present invention overcomes many deficiencies of the prior art in providing good quality, durable (i.e. minimal color change and minimal loss of brightness) thermal mass transfer images. The durability of thermal mass transfer images produced by the methods of the invention is improved by use of thermal mass transfer precursor compositions including dispersions of organic and/or inorganic pigments. As used herein, the term thermal mass transfer precursor composition means a coatable, film-forming composition (preferably aqueous) comprising binder precursors and pigments. Binder precursors are present in the coatable compositions, whereas the term binder means the solid remnants of the binder precursors. The term thermal mass transfer composition means a solid composition present either on an inventive donor element or as an image on a substrate.
In accordance with one aspect of the present invention, coatable, film-forming thermal mass transfer precursor compositions are described comprising:
a) a polyalkylene latex binder precursor;
b) an acrylic latex binder precursor;
c) an effective amount of a pigment to provide the desired color to a thermal mass transfer composition using the composition; and
d) a diluent (preferably water), in which the polyalkylene latex binder precursor, acrylic latex binder precursor, and pigment are all dispersed,
wherein the pigment and the acrylic binder precursor are present at a weight ratio of pigment to acrylic binder precursor ranging from about 0.5:1.0 to about 1.5:1.0, and the polyalkylene latex binder precursor is present in an amount to provide the desired low UV and visible light transparency to the thermal mass transfer composition. It will be understood that optional ingredients such as emulsifiers, dispersing aids, surfactants and the like, will normally be included in commercial embodiments of the inventive coatable, film-forming compositions, as further described herein, as long as the resulting thermal mass transfer composition (other than pigments, dyes, or colorants) is substantially UV transparent.
The term polyalkylene not only includes polyethylene, polypropylene, polybutadiene, and the like, but also polyalkylene-like oligomers and polymers having a high percentage of methylene units, such as oligomeric compositions comprising the reaction product of a low molecular weight organic acid, such as acrylic acid, and a high molecular weight alcohol or short chain diol.
As used herein the term acrylic includes copolymers and terpolymers of an alkylene monomer and an acidic copolymerizable monomer. Examples of alkylene monomers include ethylene, propylene, and the like, and examples of acidic copolymerizable monomers include acrylic acid and alkylacrylic acids such as methacrylic acid, ethacrylic acid, and the like.
The term coatable means the compositions of the invention have viscosity no greater than 50 centipoise, measured using a Brookfield viscometer, #2 spindle, at room temperature (about 20° C.).
Another aspect of the invention is a thermal mass transfer donor element comprising a dried version of the inventive composition (i.e. a thermal mass transfer composition) adhered to a carrier, wherein the carrier is preferably a film, more preferably a polymeric film.
The compositions and donor elements of the invention may be used in conventional processes for providing a thermal mass transfer image on a substrate. Thus, another aspect of the invention is a signage article comprising a thermal mass transfer composition of the invention adhered to a substrate, thus forming a colorant layer. The thermal mass transfer processes used to produce these articles typically comprises the steps of placing the colorant layer of a thermal mass transfer donor element in contact with a second surface, and transferring at least a portion of the thermal mass transfer composition from the donor element to the second surface by heating at least a portion of the thermal mass transfer donor layer. Preferred substrates are paper, metal, and polymeric, with polymeric substrates particularly preferred, for example the polymeric surface of a retroreflective sheeting. Particularly preferred signage articles include outdoor durable signage such as highway signs, automobile registration validation stickers and window stickers, and license plates. The colorant layer of signage articles of the invention may be exposed or buried beneath one or more polymeric (particularly polymethylmethacrylate (PMMA)) or glass cover layers, but for uses where prolonged outdoor exposure is expected, a cover layer is particularly desirous.
The colorant layer thickness in articles of the invention is preferably from about 1 to about 10 micrometers, more preferably from about 2 to about 8 micrometers, and most preferably from about 3 to about 6 micrometers. The colorant layer (on both the thermal mass transfer donor elements and the signage articles of the invention) has a softening or melting temperature between 50° C. and 140° C., preferably from 60° C. and 120° C., more preferably from 65° C. and 110° C. and most preferably from 70° C. and 100° C.
In donor elements of the invention which employ a polymeric film carrier (preferably polyethylene terephthalate (PET))

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