The invention relates to a desensitizing ink for wet. offset printing on an acceptor surface o . a chemical duplicating set comprising at least two superposed sheets, one of the facing surfaces of which has an electrophilic acceptor coating and the other a nucleophilic coating capable of producing a chromogenic reaction with said electrophilic acceptor coating.

According to the British Pat. No. 1,433,469, desensiti inks are already known that contain polypropyleneglycol with a molecular weight between 400 and 5,000, aking it possible to pr in letterpress, flexography or gravure on the acceptor surface o a duplicating set, and thus to prevent any color formation in th areas thus printed when a subsequent writing or typing is perfor on the duplicating set.

As mentioned in the British Pat. No. 1,525,269, the ho of British Pat. No. 1,433,469 did not succeed in formulating a desensitizing ink based on polyalkyleneglycol making it possible to print in wet offset the acceptor surface of a chemical dupli¬ cating set.

As described in Swiss Pat. 628,288, the holder of the present invention has worked out a desensitizing ink based on polyalkylene glycol or other nucleophilic alkoxylated compounds of high molecular weight, making it possible to print the accept surface of a chemical duplicating set by wet offset printing, following an alkoxylation to such a degree that their hydrophili lipophilic balance (HLB) is between 2 and 9 in order to allow on the one hand, a continuous ink transfer from the inking roller to the hydrophobic areas of the wetted printing plate of an offse

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press, and on the other hand, to desensitize the corresponding areas of the acceptor surf ca of the chemical duplicating set.

In order to increase the molecular weight of the de¬ sensitizing compound, the ink, according to the invention, is characterized in that the nucleophilic alkoxylated compound is bridged by an aliphatic or aromatic polyisocyanate derivative or again by an aliphatic or aromatic polyacid derivative which further makes ^" it possible to improve the transfer of the ink on the inking rollers of the wet offset press. As a result, an. ink flow of constant thickness is produced by the inking rollers. This ink film withstands particularly well the resulting impact the emulsion of." the daπvpening water in the ink, before being transferred successively to the offset printing plate then to th blanket and finally as a desensitizing ink layer, at a rate of 1,5 - 5g/m2, on a paper support.

According to the invention, a desensitizing ink for wet offset printing of the acceptor surface of a chemical dupli cating set, cän be formulated the following way:

EXAMPLE 1

On a 3 roll πiill, 70 parts of a varnis obtained by the reaction of 63 parts of polypropylene glycol (M.W. 2,500) with 5 parts of hexamethylene triisocyanate and the addition of 32 parts of an acid modified phenolic resin are ixe _d with

13 parts of titanium dioxide, 1 part of Silicon oxi _d e and 6 parts of calciu carbonate.

12 parts of the alkoxylated derivative are a ded to the ^' ground ink. The 11LB value of the ink a ount to 3 ( - 1) . This ink.: shows a suitable reheology for wet offset printing

EXAMPLE 2

On a 3 roll ill, 68 parts of a varnish obtained by the reaction of 66 parts of propoxylated diethylene triamine (M.W. 2,500 with 9 parts of trimethylol propane treated with 3 moles of toluyl diisoeyanate and the addition of 25 parts of an acid modified. phenolic resin are mixed with 12 parts of titanium dioxide, 2 parts of Silicon oxide and 6 parts of calcium carbonate.

12 parts of the alkoxylated derivative are added to . the ground ink. • The HLB value of the ink amounts to 4 (- 1) .

This inks shows a suitable rheology for wet offset printing.

EXAMPLE 3

On a 3 roll mill, 60 parts of a- varnish obtained by the reaction of.58 parts of polypropylene glycol (M.W. 2,500) with 8 parts of ricinoleic acid, 10 parts of trimethylol propane treated with 3 moles of toluyl diisoeyanate and the addition of 24 parts of a neutral esterified collophony resin are mixed with 20 parts of titanium dioxide, 2 parts of Silicon oxide and 6 parts of calcium carbonate. ^"•

12 parts of the alkoxylated derivative- are added to the ground ink. The HLß value of the ink amounts to 3 (+ 1) . This ink shows a suitable rheology for wet offset printing.

EXAMPLE 4

On a 3 roll mill, 60 parts of .a varnish obtained by the reaction of 72 parts of propoxylated diethylene triamine (M.W. 2,500) with 5 parts of castor oil, 5 parts of hexamethylene triisocyanate and the addition of 10 parts of neutral esterfied collophony resin are mixed with 20 parts of titanium dioxide, 2 parts of Silicon oxide and 6 parts of calcium carbonate.

12 parts of the alkoxylated derivative are added to the ground ink. The HLB value of the ink amounts to 4 (- 1) . This ink shows a suitable rheology for wet offset printing. EXAMPLE 5

On a 3 roll mill, 68 parts of a varnish obtained by th reaction of 75 parts of polypropyleneglycol (M.W. 2,500) with 5 parts of a long oil alkyd, 5 parts of hexamethylene triisocyanat and the addition of 15 parts of an acid modified phenolic resin are mixed with 12 parts of titanium dioxide, 2 parts of Silicon oxide and 6 parts of calcium carbonate.

12 parts of the alkoxylated derivative are added to th ground ink. The HLB value of the ink amounts to 3 (- 1) . This ink shows a suitable rheology for wet offset printing .

In the desensitizing ink formulated as in Examples 3, 4 and 5, a Substitution has been made with either a fatty acid containing hydroxyl groups (.Example 3) , or a fatty oil (Exa ple 4) or a long oil alkyd (.Example 5) , for a part of the nucleophilic alkoxylated compound, which makes it possible to increase the viscosity of the ink.

A stoichimometric amount of diacid can be reacted with the alkoxylated derivative which can be written HO (R) OH; a Polyester is obtained in the presence of p-toluene-sulfonic acid as the catalyst. As the reaction temperature is over 100°C, the ester forimation is acco panied by losing water which can be collected in a Dean-Starke colu n. The product increases molecu weight which is reflected by an increase in viscosity.

EXAMPLE 6

96 parts of polypropyleneglycol are reacted with 4 parts of succinic acid at a temperature above 100°C in the presence of 0.2 parts of p-toluene sulfonic acid and a succinic polyester is obtained. 35 parts of an acid modified phenolic re are added under stirring to 65 parts of the succinic polyester and the mixture is heated to the co plete solubilization of the resin. This varnish shows 120 poises of viscosity Measured on a Haake cone-plate viscosimeter at 25°C.

On a 3' roll mill, 70 parts of this polyester varnish i mixed with 10 parts of titanium dioxide, 2 parts of Silicon oxid 6 parts of calcium carbonate and 10 parts of the liquid succinic polyester.

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The ink exhibits very good desensitizing properties, a good hydrophiliclipophilic balance (HLB) of 4 (1 1) , a viscosi of 120 poises and initial t ck of 150 units (measured on a Tack- Scope of Testprint at a speed of 100 m/min at 25°C) and a good transfer on the inking rollers.

EXAMPLE 7

94 parts of polypropylene glycol are reacted with 6 parts of citric acid at a temperature above 100°C in the presenc of 0.2 parts of p-toluene sulfonic acid and a citric polyester i obtained.

38 parts of an acid modified phenolic resin are added under stirring to 68 parts of the citric polyester and the mixtu is heated to the complete solubilization of the resin. The varn thus obtained has a viscosity of 140 poises at 25°C.

On a 3 rdll mill, 67 parts of the polyester varnish are mixed with 12 parts of titanium dioxide, 2 parts of Silicon acid 6 parts of calcium carbonate and 13 parts of the liquid citric polyester.

The ink thus obtained exhibits very good wet offset desensitizing properties, (.HLB: 4 (.- 1) , a viscosity of 110 pois at 25°Cand initial tack of ^' 155 units.

As a Variation, synthesis of a polyester can be done with an aromatic polyacid, such as isophtalic or terephtalic aci

As an other Variation, synthesis of polyester can be done starting with the propoxylated diethylene triamine of similar molecular weight.

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Still another Variation can be the Substitution of part of the aliphatic or aromatic polyacid by:

- either fatty acids,.such as ricinoleic acid, oleic acid etc.

- or fatty oils which react around 250°C by transesterificatio

- or with fatty älkyds containing acid groups.

In order to make the w.etting of the hydrophilic areas of the wet offset printing plate easier, a portion or all of the water required to- insure the Saturation of these areas can be emulsified in the ink mass.

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