Cleaning and liquid contact with solids – Processes – Using sequentially applied treating agents
Reexamination Certificate
2001-07-02
2003-06-17
Hardee, John (Department: 1751)
Cleaning and liquid contact with solids
Processes
Using sequentially applied treating agents
C510S172000, C510S176000
Reexamination Certificate
active
06579381
ABSTRACT:
The present invention relates to the field of the maintenance of the screens used in printing, and more particularly in screen printing.
In screen printing, screens are used which consist of a frame in which a fabric is stretched, originally made of silk but nowadays more usually made of nylon/polyester, which has very fine mesh cells (of the order of 100 &mgr;m). A masking product, intended to fill some of the mesh cells of the fabric, is firstly applied to the screens and then ink is applied, which passes through the mesh cells which are left free and which correspond to the impression of the pattern to be reproduced on the printing medium.
For each new pattern to be printed using this technique, it is necessary to clean the screen, removing the ink and the masking product.
Conventionally, the screen is cleaned in three phases, which are as follows:
in a first phase, the screen is treated with an organic solvent which allows the ink to be removed and then it is rinsed with high-pressure water in order to remove the solvent and the ink residues;
in a second phase, an aqueous solution of an oxidizing agent, such as sodium periodate, called a degritting solution, is applied in order to make the masking product disappear from the obstructed mesh cells, and then a further rinsing with high-pressure water is carried out; and
in a third phase, the ghost image remaining on the screen is removed by means of a chlorinated product and a final rinse with high-pressure water is carried out.
This cleaning is carried out either manually, or automatically in washing tunnels in which the steps take place in succession in different compartments.
The machines currently used for this cleaning have four compartments. Associated with each compartment is a tank designed to contain a product corresponding to the aforementioned phases.
In the first compartment there is a tank containing a solvent able to dissolve the inks used in screen printing. In this compartment, the screen is sprayed with the product found in the tank by nozzles which are mounted on a vertical spray bar. The vertical spray bar moves in a horizontal direction in order to spray the entire area of the screen. The sprayed product falls back into the tank, by gravity, with the ink.
The second compartment also has a tank containing solvent. The latter is also sprayed using a vertical spray bar which moves in a horizontal direction over the screen. This solvent is cleaner than that of the first tank. Solvent-recycling machines in the first and second tanks are provided in order to increase the lifetime of the product contained in these tanks.
Before the screen passes from the second to the third compartment, it is washed with high-pressure water.
The corresponding tank in the third compartment contains a degritting product allowing the masking product to be removed. This product is applied to the screen, also using nozzles placed on a vertical spray bar which moves horizontally.
In the final, fourth compartment, the screen is washed with high-pressure water.
The screen is then ready to undergo a new step, that of removing the ghost image.
While the screen is being cleaned, it is therefore out of commission for a period which may range up to one day, or even longer.
This is because, and by way of example to illustrate the loss of time devoted to cleaning a screen, the final phase consisting in removing the ghost image is carried out by applying, to both sides of the screen, a product based on chlorine and sodium hydroxide, which is left to act for at least half a day. After this reaction time, the screen is washed with a solvent, in order to remove the ink, masking product and chlorinated product residues, the solvent is left to react and then the screen is rinsed with high-pressure water. After the screen is dried, it is again available for the next use.
Another drawback with the cleaning of the screens is the very high level of pollution that they produce, since the effective solvents such as, for example, halogenated hydrocarbons are harmful and nonbiodegradable and, moreover, the residual mixture of water, solvents and ink that are collected cannot easily be recycled. In addition, these solvents are inflammable and handling them, especially in large quantities, is dangerous.
The problem of the complexity and the duration of the cleaning process has already been studied. Thus, according to document FR-A-2 354 377, a composition for cleaning a screen is known which allows both the ink and the masking product to be removed, this composition consisting of a mixture of an organic solvent, especially gamma-butyrolactone, N-methyl-2-pyrrolidone or dimethylformamide (DMF), an oxidizing agent such as sodium periodate, and optionally a surfactant.
However, the treatment of the screen by this composition is not completely satisfactory as a prior step of treating the screen with white spirit proves to be necessary in order to remove the surplus ink. Furthermore, this solution does not solve the problem of pollution by the residual organic solvents, and it may involve particularly toxic solvents such as DMF.
To limit the pollution caused by the cleaning compositions and their consequent effects on the environment, document WO-A-90/08603 provides a composition in which the organic solvents conventionally employed are replaced with a mixture of dibasic esters, such as dimethyl esters chosen from dimethyl succinate, adipate and glutarate, which is used in the presence of a surfactant. This ester mixture, which acts on the printing ink, is not toxic and is biodegradable. The preferred surfactant is nonionic, such as an ethoxylated aliphatic alcohol or an ethoxylated alkylphenol. One advantageous composition comprises 90-100% by weight of the aforementioned ester mixture and 10-0% by weight of an ethoxylated aliphatic alcohol.
However, this composition does not dissolve the masking product and its action must be supplemented in a conventional manner with a degritting step and then a step of treatment with a chlorinated product in order to remove the ghost image.
For want of a really effective solution, the screen cleaning processes have remained the same for many years, that is to say for at least thirty years, and have been widely employed in screen printing, despite their numerous drawbacks.
According to the invention, a cleaning composition has been developed which makes it possible to solve all the disadvantages encountered with the conventional cleaning methods, including the partial solutions of the prior art.
The invention provides a composition and a process for cleaning a screen-printing screen which are effective and result in a single step, in the removal of both the ink and the masking product. This composition is highly advantageous as it has a specific activity with respect to the products to be removed from the screen, but has no corrosive effect liable to damage the fabric or to result in its premature aging.
Furthermore, the cleaning according to the invention leaves no or virtually no ghost image behind so that the screen, after drying, can be reused immediately.
The benefit of the cleaning power of the composition is twofold. Firstly, and as indicated above, this power is exerted against all the products and residues to be removed from the mesh cells of the fabric, but, in addition, it is almost immediate, and therefore the period of decommissioning of the screen for the sake of cleaning is considerably reduced.
Despite its effectiveness, this composition is not polluting. It is highly advantageous as it is based on nontoxic products presenting no hazard either to the user or to the environment.
Thus, a first subject of the invention is a cleaning composition comprising:
an aqueous solution comprising from 0.05 to 5% by weight of sodium metaperiodate and from 0.05 to 0.75% by weight of sulfuric acid, and
a solution containing from 0.5 to 90% by weight of a mixture of dialkyl esters and from 0.5 to 50% by weight of a nonionic surfactant.
The percentages mentioned in the present description and the claims are expressed by weight of
Chim 92
Hardee John
LandOfFree
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