The present invention concerns a method for preparing a coating on a web-formed material. In the method a coating hardenable by irradiation is first spread onto the web , and the web is thereupon made to pass by a source of radiation, whose radiation causes hardening of the coating.

In the hardening of coatings taking place by means of irradiation a chain reaction is concerned whose initiators are primary radicals produced by radiation. This technique has been used, e.g. , when coating record envelopes or book covers of paper with a glossy protec¬ tive lacquer hardened by UV-light. The commonest substances used in coatings hardenable by irradiation are

- aerylate-modified oligomers, such as acrylated epoxy and polyester compounds , by means of which the chemical and physical properties of the coating are controlled,

- difunctional and trifunctional vinylmonomers, such as hexanedioldiacrylate (HDDA) and trimethylolpropanetri- acrylate (TMPTA) , whose quantity and quality have the strongest effect on the hardening dosage, as well as - monofunctional monomers, such as ethylhexylacrylate

(EHA) , by means of which, if necessary, the viscosity of the coating is adjusted to the level required by the spreading apparatus .

The coating agents are chosen in accordance with the properties that are required from the face.

The present coatings hardenable by irradiation are relatively expensive.

Another problem has been the shrinkage taking place on vinyl polymeration; monomers may shrink by up to 20 per cent. This causes curling up of the web as well as poor adhesion of the coating.

Also, the use of acrylmonomers as softeners is

problematic because of their toxicity.

The object of the present invention is above all to provide a method for the coating of a web-formed material, in particular paper or plastic, wherein the coating is hardened by irradiation and wherein the polymerizable compound used is of lower cost than the prior-art compounds but by whose means a coating of good quality and above all a non-shrinking coating is provided. In the method now invented a coating is used which contains natural resins modified in a certain way to make them reactive. The resin is modified to make it reactive by linking acrylic groups polymerizable by irra¬ diation to it. The acrylic groups may be linked directly to the resin, or to a resin that has been first modified in a suitable way.

Natural resins are amorphous mixtures which contain carboxylic acids, oils and terpenes as essential constituents. As examples should be mentioned tall rosin, gum rosin, and wood rosin obtained from softwood. The natural resins contain unsaturated cyclic resin acids as their main component, of which acids as the most im¬ portant ones should be mentioned abietic, dehydroabietic, palustric, dihydroabietic, neoabietic, pimaric, isopimaric, 13- ,7 ,9 (11 ) -abietic, sandracopimaric , 7 ,9 (11 ) -abietic , 8, 15-isopimaric, 8 ,15-pimaric, 8 , 12-abietic, and dehydro- dehydroabietic acid. Natural resins also contain fatty acids.

From the published Patent Application JP-A-59-170165 (ref. Japan Patent Abstracts 59-170165 (A) or Che . Abstr. 102:97107a) it is known to use UV-harden- ing mixture for etching of circuit cards, which mixture contains a product obtained from fumarized natural resin by acrylation with glycidylacrylate or methylmethacrylate. In the present invention, above all natural resins can be concerned which have been first esterified and then acrylated. For esterification of a natural resin, it is possible to use suitable mono-, di-, tri-,

tetra- or polyfunctional hydroxy compounds or their reac¬ tive derivatives. Of these compounds should be mentioned: triols, such as g ycerol; polyols, such as pentaerythritol; as well as diols, such as ethyleneglycol , diethylene- glycol, and triethyleneglycol . In particular acrylic- acid-chloride is suitable for acrylation of esterified resin. In coatings, it is also possible to use resin derivatives that have been carboxylated before acrylation. The carboxylation is carried out best with unsaturated carboxylic acid or with a reactive derivative of same.

Suitable acids are in particular unsaturated dicarboxylic acids, especially fu aric, aleic and oxalic acid. For the acrylation of a carboxylated derivative, preferably an acrylic compound containing a hydroxy group is used, e.g. hydroxyalkylacrylate , in particular (2-hydroxyethyl) - acrylate.

In some applications it is also possible to use, e.g., acrylic derivatives of resins that have been modi¬ fied to make them hydroxyfunctional. A resin can be modi- fied to make it hydroxyfunctional, e.g. , by ethoxylation . A hydroxyl group can be acrylated in particular by means of acrylic acid.

In principle, an unmodified resin can also be acrylated directly. This is also carried out preferably by using an acrylic compound containing a hydroxyl group.

A further possibility is to subject the resin first to carboxylation, then to esterification, and only thereupon to acrylation.

In view of the invention, the most essential feature is modification of the resin acids contained in the resin. At the same time, however, e.g. on esterifi- cation, the fatty acids in the mixture may also react.

The accompanying reaction formula illustrates some possible reactions of modification with abietic acid used as a model.

In a coating mixture in accordance with the invention, it is possible to use derivatives of, e.g., tall, gum or wood rosin.

The suitable concentration is chosen on the basis of the desired properties of the mixture and of the coating. Of course, the concentration also depends on the resin used and on its modification as well as on the other components of the mixture. A mixture may contain 0.5 to 100 per cent of modified natural resin. A recom- mended concentration is 1 to 50 % , in particular 5 to 30 %, and especially 10 to 20 %.

Various mixtures hardenable by irradiation can be concerned, and they can be used for the coating of many sorts of web materials, but in particular for the coating of papers and plastics. The mixtures are partic¬ ularly well suitable for the coating of paper, as will be described in more detail in the examples.

Hydroxyfunctionalized and acrylated resin derivatives are suitable in particular for resilient lacquers from- which good resistance to solvents is not required. They are also suitable for pigmented mixtures hardenable by irradiation, such as printing colours, as well as for coating mixtures in which good adhesion is required. Directly acrylated derivatives are suitable for lacquers from which good resistance to moisture and solvents as well as hardness are required.

Esterified and acrylated as well as also carboxylated and acrylated derivatives are again partic- ularly suitable for coatings from which long-term good properties, resistance to heat, or complete hardening are required, especially when a glossy coating is desired,

For the hardening of the coatings, suitable irradiation can be used, such as EB- or UV-irradiation. The coating mixtures in accordance with the invention are significantly less expensive than the mixtures that are in use at present.

The mixtures are also technically more advan¬ tageous, because their shrinkage is reduced. Moreover, the quantity of detrimental acrylmonomers can be reduced.

The following -examples illustrate some ^~ embodiments of the invention.

Example 1. Fumaration of tall rosin. A-rosin (manufactured by Yhtyneet Paperitehtaat Oy Valke, acid number 162, saponification number 174, proportion of resin acids 83 %, softening point 65 C) was reacted with fumaric acid at 220 °C for 2.5 h. Fumaric acid was used as a quantity less than 15 % of the quantity of rosin, depending on the desired degree of fumaration, typically 5 to 10 %.

Example 2. Esterification of fumarized rosin. A-rosin fumarized in accordance with example 1 (quantity of fumaric acid 6.0 %, acid number 200) was esterified with ethyleneglycol, whereby the acid number was lowered to 16.

Example 3. Acrylation of fumarized rosin. 150 g of A-rosin fumarized in accordance with example 1 (quantity of fumaric acid 8.0 %, acid number 215), 0.45 g p-toluene-sulphonic acid, 0.4 g hydroquinone were heated in a reactor to 230 C, and 69 ml (2-hydroxy- ethyl) aer late (2-HEA) was added to the mixture over 2.5 h. The acid number was lowered from 215 to 49. Herein¬ after another 10 ml 2-HEA was added during 1 h whereby the acid number was lowered to 38. The product was washed 5 times with water.

In order to establish the reactivity, a film was prepared of the mixture, and the film was extracted in boiling toluene in a Soxhlet apparatus. It was noticed that acrylated rosin does not dissolve to an equally high extent as ordinary esterified rosin does.

Example 4. Ethoxylation of tall rosin.

A mixture of 1.35 kg A-rosin, 1.6 dm^ (5 mol) ethyleneoxide, and 0.1 .% of sodium acetate was heated for 6 h at 180...200 °C under a pressure of 4 bars.

Example 5. Acrylation of ethoxylated rosin.

500 g A-rosin ethoxylated in accordance w.ith the example, 1.2 g hydroquinone, and 1.2 g p-toluene- sulphonic acid were weighed into a pressure-proof reactor, The temperature was raised to about 230 C. A total of

80 ml acrylic acid was added to the mixture as 4 batches. In connection with the addition, the temperature in the reaction chamber was lowered to 120 C, and the pressure was lowered so that the water produced could be removed. The reaction time was about 6 h. The acid number was lowered from 105 to 50. The acrylated rosin was washed with water three times.

Example 6. Acrylation of tall rosin. Into a reactor 80 g A-rosin, 0.2 g p-toluene- sulphonic acid, and 0.6 g hydroquinone were weighed. The temperature was raised to 250 C, and 31 ml 2-hydroxy- ethylacrylate (2-HEA) was added during 3 h. After 2 h, when the acid number had been lowered from "165 to 24, a further 10 ml 5 2-HEA was added during 0.5 h, whereby the acid number was lowered to 12. The product was washed with water 5 times.

Example 7. Esterification of resin acid. Monoabietic acid was esterified with tri ethylodipropane .

Example 8. Acrylation of esterified resin acid.

Esterified resin acid prepared in accordance with example 7 was acrylated with acrylic-acid chloride, so that a 2-functional acrylated rosin derivative was obtained.

Example 9. Coating with acrylated ethoxy derivative.

SAP paper (Tervasaari semi alkaline pulp, grammage 60 g/m ² ) was coated with a mixture of 40 g EA-81 , 20 g A-rosin ethoxylated and acrylated in accord¬ ance with examples 4 and 5, and 0.9 g photoinitiator M 1173 (Merck, 2-hydroxy-2-methyl-1-phenylpropan-1-one) . The coating was hardened by means of a UV-apparatus (80 /cm, 18 m/min, 4 hardening times) . A glossy, hard lacquer layer was obtained.

Example 10. Coating with acrylated rosin.

Coating like in example 7 was performed with a mixture of 20 g EA-81, 10 gA-rosin acrylated with 2-HEA in accordance with example 6, and 0.45 g M 1173, and hardening was carried out 5 times. A glossy, hard lacquer layer was obtained.

Example 11. Coating with esterified and acrylated rosin.

10 % of esterified and acrylated rosin in accordance with example 8 was added to epoxyacrylate (EA-81 BASF) . The mixture was hardened by means of an Electrocurtain ^® electron accelerator. The mixture was hardened to a hard glossy face with a dosage of 6 Mrad.

The test was repeated by using a rosin deriv¬ ative alone as a coating. Thereat a radiation dosage of 20 Mrad was required for hardening.