Fluorescent pigments or daylight fluorescent pigments have been produced commercially since the late 1940's for use in paints, printing inks, and plastics. U.S. patent literature from 1938 forward describes the preparation and use of various fluorescent pigments. Patton, in his Pigment Handbook, (Volume 1, pp 891-903) describes the chemistry, the production, the properties, the major uses, and some of the limitations of fluorescent pigments. An early advancement in the use of fluorescent colorants for plastics is described in U.S. Pat. No. 2,119,189 in which Widmer taught the use of a resinous molecule as a carrier or suitable medium for fluorescent dyes. Later, Switzer, Kazenas, and others utilized extremely friable organic glass-like compounds as carriers for fluorescent dyes. These glass-like compounds included modified sulphonamide resins, urea-melamines, glyceryl phthalates, polyesters, polyamides, vinyl resins, and silica gels.
U.S. Pat. No. 3,922,232 describes a fluorescent colorant comprising particles of a resinous precondensate colored with a fluorescent dye. The precondensate consists of 0.5 to 2 molecules of a carboxylic acid, ester, or anhydride and one mole of a polyhydroxy compound. It is mentioned that additives, such as polyethylene wax and ethylene-acrylic acid copolymers, can be added to the precondensate or to the final colorant. The final colorant is useful for coloring polyethylene.
U.S. Pat. No. 4,911,830 teaches the preparation of fluorescent pigment concentrates by mixing about 5 to 40 wt % fluorescent pigment, 5 to 20 wt % inorganic fillers, 2 to 10% silica gel or precipitated silica, 1 to 10% dispersant comprising at least 2 of (a) oxidized polyethylene wax, (b) unoxidized polyethylene wax, (c) ethylene-acrylic acid copolymers, and (d) bivalent metal salts of (a) or (c) and the remainder an ethylene polymer of substantially higher molecular weight up to 50% by weight. Certain types of fluorescent pigments, especially those useful at elevated temperatures, were not effective using this composition, resulting in excessive plate-out.
Although several advancements have been made in the state of the art for the use of fluorescent pigments to color plastics, most commercial fluorescent pigment concentrates continue to have only limited compatibility with a wide range of plastics. This limited compatibility results in many plate-out problems during the compounding of the concentrates and during the extrusion and molding of fluorescent pigmented products.
The detailed explanation of fluorescents and the chemistry involved is outside the scope of this discussion. However, an excellent description is given by Patton in his Pigment Handbook, Volume 1, pp 891-903. Many daylight fluorescent dyes are based on aromatic structures such as the xanthenes, rhodamines, aminonaphthalimides, perinones, and thioindigos.
Fluorescent dyes usually must be in dilute solution in order to fluoresce. Excessive concentration levels sometimes results in a quenching of the fluorescence due to molecular collisions, reabsorption of emitted light, and other interactions. If the dyes are stabilized within a rigid glass-like resin, the undesirable deactivation is greatly reduced. Certain resin matrices are preferred for this immobilization of the dye molecules since these resins not only contribute more intense fluorescence, but also provide greater fade and thermal resistance. An example of this glass-like resin medium is one formed by cocondensing a toluene sulfone amide-formaldehyde within a triazine such as melamine or benzoguanamine.
The use of fluorescent pigments in plastics has been slow to develop because of plate-out problems during pigment compounding and during subsequent extruding and molding operations. Plate-out is the undesirable separation of the pigment from the base plastic and its deposition on screws and other metal processing equipment. This phase separation results from the extreme incompatibility of the fluorescent pigment binder and the plastic to be colored. Another limiting factor in the growth of fluorescent colorants in plastics is the relatively poor heat resistance of the glass-like binders for these pigments. Most commercially available fluorescent pigments can withstand temperatures up to 425.degree. F. for only a very short time. High shear processing conditions which exist in many plastic compounding operations also adversely affect the color stability of many fluorescent pigments.