Compositions: coating or plastic – Coating or plastic compositions – Marking
Reexamination Certificate
2000-07-07
2002-10-29
Klemanski, Helene (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Marking
C106S031610, C106S031580, C106S031860
Reexamination Certificate
active
06471758
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an ink composition for a meltable ink usable in a printing device in which ink drops are ejected from ink ducts. The present invention comprises an agent which reversibly cross-links the fluid ink. The present invention also relates to a method of printing a substrate with such an ink composition.
Inks of this kind, which are solid at room temperature and liquid at an elevated temperature are known from U.S. Pat. No. 5,380,769. These inks are reactive compositions which contain at least two components, i.e. a basic ink component and a reagent, the components being transferred to a receiving medium separately from one another. Exposure of the basic ink component to the reagent results in the formation of polymers which form a network in the fluid ink by means of reversible bonds.
An important disadvantage of an ink composition of this kind is that it consists of two separate components which have to be transferred successively to a receiving medium. This makes the printing device complex and the production and processing of the ink expensive.
SUMMARY OF THE INVENTION
The object of the present invention is to obviate these disadvantages. To this end, an ink composition has been invented in which the agent which reversibly cross-links the fluid ink comprises a gelling agent. A gelling agent is capable, as such, to thicken a liquid by forming a three-dimensional structure therein. The liquid thus passes over into the form of a gel. A gelling agent can consist, inter alia, of high and low molecular compounds, a mixture of compounds, or of discrete particles. The molecules or particles of the gelling agent so interact with one another in the gel that a network is formed in the liquid. During this network formation, it is not necessary for the molecules or particles of which the gelling agent consists to be actually chemically bonded or have physical contact. All that is required is that they should have a physical interaction such as to result in a reinforcing effect in the liquid. As a result the viscosity of the liquid increases without it passing over into a solid phase.
Typical gelling agents are high-molecular, elongated molecules which form an elastic network in a medium, the interstices of the network being filled with the medium which can be in a liquid or a solid state. If the medium in the interstices is in a liquid state, a gel arises which has some fluid-like properties, such as the property that molecules can diffuse relatively easily into the continuous liquid matrix and some solid-like properties such as the property that the gel can withstand a certain shear stress without deformation occurring, before the gel starts to flow like a liquid. When the liquid in the interstices of the network solidifies, the gel passes over to the solid state.
The ink composition is preferably a gel at a temperature equal to or higher than room temperature. In this way a network can be formed by a printing ink on a receiving medium at ambient or elevated temperatures. In another preferred embodiment, the ink composition is a gel at a first temperature and a sol at a second temperature higher than the first temperature. A sol is a low viscosity, homogeneous liquid, which may contain colloidal particles. The ink is cross-linked in such a fashion that the links are broken when the temperature is elevated above the second temperature, also called the gel-transition temperature. This is a very important advantage for the inks of the present invention because in this way it can be assured that the inks have a low viscosity at the jetting temperature. A low viscosity ameliorates the jetting characteristics of an ink jet ink.
The use of gelling agents is of course known from liquid inks (aqueous and solvent inks), but these gelling agents cannot be directly used in a meltable ink. It is in fact impossible to select a molecule or particle that can cause a liquid to pass over into a gel. Whether a specific compound has the property of being able to cause a liquid to pass over into the form of a gel also depends on the properties of the liquid. Moreover, in the case of meltable inks it is difficult, accurately, to co-ordinate the different solubilities of the different components in such a manner as to form a homogenous melt at the temperature at which ink drops are ejected from the ink ducts, and a specific solidification and crystallisation behaviour is exhibited on cooling of the melted ink. This makes the development of meltable inks very complex.
Known gelling agents such as carragenan, laminarane, pectin and gums such as gum arabic, xanthane and guar gums, are high-molecular polymers. Since meltable inks of themselves are already more viscous than liquid inks at the temperature at which the ink drops are ejected from the ink ducts, the addition of a small quantity of such high-molecular gelling agents can lead to an unacceptably high viscosity in the melted state. This means that there is an adverse effect on the jet properties of the ink. Oligomer gelling agents, i.e. gelling agents with a molecular weight of less than 10,000, which are less common are therefore preferably used, so that a higher percentage of gelling agent can be added to the ink composition without having an adverse effect on the melt viscosity of the ink composition. In a further preferred embodiment, low-molecular gelling agents are used, i.e. gelling agents with a molecular weight of less than 1,000.
The fact that oligomers and low-molecular weight compounds can have gelling properties despite their relatively low molecular weight can be explained as follows. In the case of oligomers and low-molecular gelling agents, the molecules separate from the melted ink on an adequate reduction of the temperature, and form long compound chains via mutual non-covalent interactions, said chains behaving in accordance with the high-molecular polymers in the previously mentioned gelling agents. The compound chains can form a network which causes the melted ink composition to pass over into a gel. Since the molecules of the gelling agent form a network structure, concentrations of the gelling agent as low as 25% can be sufficient to gel the liquid ink composition. When the gel is heated up, the interactions between the molecules of the gelling agent are interrupted and a sol re-forms. A supplementary advantage of the use of oligomers and low molecular weight gelling agents is that the gel-sol transition takes place relatively quickly. Because of this transition, it is only necessary to break the relatively weak non-covalent bonds between the compound molecules of the polymer chains. In addition, small molecules will be mixed homogeneously in the melted ink matrix more rapidly. This is an important advantage because a meltable ink frequently has to be brought into the melted state quickly, e.g. when the printing machine is started up and the user wants to print an image immediately. For use in a hot melt ink, a gelling agent is preferably used which has amphiphilic properties, i.e. partly polar and partly apolar properties. An example is a gelling agent with a straight alkane backbone and some additional polar groups. Because of the amphiphilic properties the combining of the gelling agent with various hot melt ink compositions is simplified. Gelling agents having an optimal function according to the present invention only need to be present in a quantity of less than 10% based on the total weight of the ink composition. Finding such gelling agents is difficult but has the advantage that no adverse effects on the viscosity of the melted ink and other physical properties, such as the melt-temperature and the adhesive and mechanical properties of the solidified ink, nor the solubility properties of the liquid ink are expected as a result. In a further preferred embodiment, the gelling agent constitutes less than 5% of the total ink composition.
To ensure that the ink does not gel when it is situated in an in duct prior to the ejection of an ink duct from said duct, the ink is pr
Haas Albin Bernhard Werner
Huinck Marcus Petrus Leonardus
Kelderman Erik
Kortenhoeven Nicolina Margriet
Suilen Frederik Leonardus Everardus Marie
Faison Veronica F.
Klemanski Helene
Océ Technologies B.V.
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