Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
2002-12-19
2004-05-04
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From silicon reactant having at least one...
C528S026000, C528S032000, C528S038000, C528S024000, C525S100000
Reexamination Certificate
active
06730765
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to organosiloxane oligomers and curable compositions containing them. The organosiloxane oligomers contain maleamic acid functionality. Blends of these organosiloxane oligomers, electron poor vinyl compounds, and free radical photoinitiators are curable or crosslinkable upon exposure to ultraviolet light or radiation. The resulting cross-linked films are particularly useful in silicone based release coating applications.
BACKGROUND
Silicone compositions have long been used to make release liners for pressure sensitive tapes and labels, wall coverings, and other tacky substances. Silicone release coatings are commonly available in four families: solvent based silicone release coatings, 100% solids silicone release coatings, water-based silicone release coatings and UV/EB curable silicone release coatings.
Solvent-based silicones consist of dispersions of reactive high molecular weight polysiloxane gums in organic solvents. A crosslinking catalyst is added to the dispersed low-solids mixture just prior to coating. The mixture is then applied to the substrate as a very thin layer. Common coating methods include direct or reverse gravure, Mayer rod, size press, roll coating and the like. The web is then passed through an oven. The heat from the oven removes the solvent and accelerates the cross-linking or curing reaction. This process requires a large amount of thermal energy to evaporate the solvent and drive the curing reaction at commercially viable rates. Environmental regulations require that solvent vapors cannot escape into the atmosphere. The solvent vapors must either be trapped or incinerated. This requires considerable capital and operating expenditure. Rising energy costs and stringent environmental regulations are causing this once dominant technology to be replaced with more environmentally friendly alternatives.
Water-based silicones or emulsion silicones consist of dispersions of reactive silicone polymers in water. A cross-linking catalyst is added to the emulsion just prior to coating. The mixture is then applied to the substrate as a very thin layer. Common coating methods include direct or reverse gravure, Mayer rod, size press, roll coating and the like. The web is then passed through an oven. The heat from the oven removes the water and accelerates the cross-linking or curing reaction. Water-based silicones do avoid the environmental regulations associated with solvent systems. However, evaporating the water and driving the curing reaction at commercially viable rates is capital and energy intensive.
100% solids thermally cured silicones consist of a mixture of low molecular weight silicone polymers, a cross-linker and a catalyst. Optionally, a cross-linking reaction inhibitor is added to the mixture to extend the bath life. Once these components are combined, the cross-linking reaction begins. The mixture is applied to the substrate before the cross-linking reaction increases the viscosity of the bath to unusable levels. After coating, the substrate is heated to accelerate the cross-linking reaction. Common coating methods include direct or reverse gravure, Mayer rod, size press, roll coating and the like. 100% solids, thermally cured silicones typically have no solvent associated environmental limitations. However, driving the curing reaction at commercially viable rates is capital and energy intensive.
Another class of 100% solids release layer compositions is UV or FB (electron beam) curable. As with all 100% solids systems, solvent associated limitations typically are not present. UV curable compositions are applied to the substrate using the common coating methods mentioned previously. Two curing mechanisms are prominent in the UV cured silicone release liner market. One is a free radical polymerization; the other is a cationic mechanism.
Free radical photoinitiators generate free radicals on exposure to UV light. The free radicals initiate a chain reaction or polymerization of acrylate functional groups. This polymerization nearly instantly transforms the liquid coating to a solid release layer. Commonly available free radical photoinitiators are insoluble or very slightly soluble in polydimethylsiloxane (PDMS) fluids. Functionalized PDMS fluids are a major component in all silicone release compositions. Low solubility causes problems when selecting photoinitiators for this application. A further difficulty is that the free radical cure mechanism is inhibited by oxygen. These systems typically require nitrogen purge at the point of UV exposure. Providing an inert atmosphere at the point of cure adds expense and complexity to the coating and curing process. Another feature of the free radical system is the lack of a cold or dark cure. The free radical polymerization proceeds while the substrate is exposed to UV light. Once the light is removed, the reaction stops.
Cationic photoinitiators generate acids on exposure to UV light or an electron beam (EB). The acid initiates a chain reaction or polymerization of epoxy (oxirane) functional groups. This polymerization transforms the liquid coating to a solid release layer. This reaction is much slower than the free radical polymerization. Commonly available cationic photoinitiators are insoluble or very slightly soluble in the epoxy-functionalized PDMS fluids that are the base-polymers for this type of coating. Photoinitiators were developed specifically for cationic silicone release compositions. A full discussion of cationic photoinitiators and epoxy-silicones is set forth in U.S. Pat. Nos. 4,279,717 and 5,814,679 both of which are incorporated by reference. Cationic silicones are very sensitive to the pH of the substrate. Even slightly basic substrates will greatly impede or even quench the cure. Substrates used for other thermally cured silicones or free radically cured silicones typically will not work with cationic silicones. Cationic silicones require special acidic substrates.
The following references relate to the field of the invention: U.S. Pat. Nos. 5,527,578; 6,008,267; J. Ericsson, M, Nilsson, S. Lundmark, L. Svensson, S. Jonsson, K. Lindgren, “Radtech 2000 Conference Proceedings”, 2000, pages 173-195; H. K. Hall, A. B. Padias, Aldrichimica Acta, Vol. 28, No 2, 1995, Pages 37-44.
SUMMARY OF THE INVENTION
The present invention describes 100% solids UV cured chemistry useful in creating release layers. As with the release layers described above, the release properties are provided by the PDMS portion of the release layer. Charge-transfer complexes can provide the curing or reactive portion of the release compositions.
A charge-transfer (Ci) complex, which is also called an electron-donor-acceptor (EDA) complex, is formed when two olefins of differing polarities are mixed. The individual olefins are referred to as the electron donor and the electron acceptor. A CT complex is in equilibrium with the parent olefins. The paper by Hall and Padias in Aldrichimica Acta, Vol. 28 No. 2, 1995 describes such CT complexes. The reactivity of a CT complex depends on the electron density donated from the electron donor to the electron acceptor. The greater the electron donation, the greater the reactivity. In the case of large electron density donation, spontaneous polymerization will take place. This is undesirable for the present invention. This invention describes olefin functional groups that do not spontaneously polymerize at room temperature.
CT complexes are more reactive via a free radical mechanism than either of the uncomplexed olefins. A great advantage of free radical reactions with CT complexes is that inhibition by oxygen is greatly reduced or even eliminated. Such a system overcomes the two major disadvantages or deficiencies of the cationic and free radical systems commercially available. Neither oxygen, nor basic materials inhibit the polymerization of CT. Thus the present invention provides UV cured silicone release formulations that do not suffer the major limitations of UV cured free radical systems or the major limitation of UV cured cationic sy
Dawson Robert
Millen White Zelano & Branigan PC
NCR Corporation
Zimmer Marc S.
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