DE2412571A1 - PROCESS FOR CURING A LIGHT-CURABLE POLYMER LAYER OF A PRINTING PLATE - Google Patents

Description

Case 297

Dr. Be / ba

V /. R. Grace & Co., 1114 Avenue of the Americas, New York, N. Y. Ioo36, (U.S.A.)

Method of curing a photocurable polymer layer of a printing plate

The invention relates to the production of polymer printing plates. In particular, the invention relates to a particular method of curing a layer of a liquid, photocurable polymer preparation of a polymer printing plate before developing the plate by removing the incompletely hardened layer areas.

Time plays an essential role in printing technology. Heretofore, relief printing plates have usually been made by this Made that a layer of metallic zinc or magnesium is light engraved and etched became. These old procedures required one

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relatively long time to produce a satisfactory Embossing plate. For more recently, the use of polymerizate materials has been considered as Proposed substitutes for the metals used to form the printing plates. For example, are photopolymerized or light cured printing elements in U.S. Patents 3,129,098, 3,537,853 and 3 615 '45o. Practical experience in performing the procedures outlined in these patents has shown that the time it takes for the pol7 to polymerize or harden Printing layer is necessary before the development of the finished printing plates is still longer, than it is desired for many purposes int. A Λ / 0-itoroi: problem that arises in the making. polymeric printing plots occasionally encountered is a phenomenon known as "cupping" or "dishing". This Phenomenon, if it occurs, gives a concave depression of certain appearance in relatively extensive certain areas of coinage. Another problem that sometimes occurs in manufacturing of polymeric embossing plates or polymeric pattern plates or master plates, such as, for example in FTP-A Patent 3,666,461, lui'tri l '^^ results in an instability of the brightest picture dots, whereby these brightest light image points are not fully connected to one another at their Bnas and are foreroded or not properly on the dies or during pressing Stereo printing plates made from the polymeric master plate.

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The object of the invention is to provide a method for producing polymeric prints or Sample plates, with the total curing time or polymerization time for comparison with previously known method is reduced. Another aim of the invention is to provide a method for reducing or reducing the instability of the brightest light image points in polymeric embossing printing plates and / or polymeric pattern plates. Yet another object of the invention is to provide of improved polymeric printing or pattern plates.

It has been found that the tasks mentioned and other tasks can be achieved if the Hardening in the step of the image application in the production of the plates in two separate from each other Stages is carried out. In the first stage, the layer is made of the photo-curable polymer directly exposed to high-energy radiation for a short period of time. After that, a The slide bearing the image is inserted and the photo-curable polymer continues to be the High-energy radiation is exposed through the image-bearing slide until the photo-curable Polymer cured practically completely in the areas exposed to the image is.

The minimum duration for direct exposure (hereinafter sometimes referred to as the "burst exposure time" is the one that is needed to achieve the desired reduction in "cupping" or

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To achieve "dishing" in large relief areas of the plate. The maximum "shock exposure time" is just less (eg not more than 9 o $) than the exposure time after which the photopolymer begins to gel. The onset of gelation can easily be determined from a visual observation of the wrinkling on the surface of the polymer layer as the "skin" cures. These general parameters will vary depending on the particular photohardenable composition used, the type and intensity of the high energy radiation, and other factors. When working with the commercially available "LETTERFLEX" systems and using commercially available "LETTERFLEX" photo-curable preparations, which are widely used, for example, in the USA, it has been found that excellent results are achieved if the immediate exposure time is 0.5 to 4, ο seconds. Even very short "burst exposure times" result in an improvement by reducing the "cupping" or "dishing". An optimal improvement in every respect (ie the greatest reduction in "deepening" and the greatest possible increase in the stability of the brightest light image points) is achieved with the maximum "burst exposure time". In practice, this point in time is determined by observing the time at which a sample of the photopolymer begins to gel, as determined under the prevailing lighting conditions and adjusting the shock exposure time to such a time duration which is approximately Io io below the obtained exposure for initiating the G-elierung.

The subsequent exposure to generate the image (hereinafter referred to as "main exposure time"

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referred to) is continued until 'the exposed by the image parts of the photo-curable preparation are practically completely hardened. Also ■ this process changes with the particular preparation, the type and intensity of the source of the high energy radiation; and other similar factors. Experiences in the laboratory and in the technical production with the mentioned commercial ones "LETTERPLEX" systems and the commercially available "LETTERPLEX" polymers have been shown to last for at least 15 seconds, preferably 20 to 50 seconds to be exposed to a main exposure for every second "burst exposure time". With "burst exposure times" of 0.5 to 4.0 seconds, this means that the main exposure time is at least 7.5 to 60 seconds and preferably 10 to 200 seconds, whereby total exposure times can be obtained within a wide range of 8 to 24 seconds.

When working according to the invention, it is found that the combination of a short direct "burst exposure time" and the subsequent "main ^{exposure time 11"} or the exposure time described in more detail above reduces the total exposure time or curing time to less than half and often only a third of the exposure time that would be required, if the overall exposure were to be carried out through a slide bearing the image. At the same time, the brief "shock exposure" reduces the "cupping" or "dishing" to a point at which this cupping becomes irrelevant for all practical purposes. At the same time, the stability is the brightest light image points in the hardened and developed

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Embossing plate or original plate as good or "better than the one that has been carried out with the" so far Process can be achieved in which only one image exposure stage is carried out and the entire Exposure is done through a slide that bears the image ..

Virtually any liquid photopolymer can be used in accordance with the invention. E.g. the preparations are those in U.S. Patents 3,661,744, 3,676,195, 3,676,283, 3,697,395 to 3,697,397, 3,697,4o2 and 3,7oo,574 are useful. The type of photopolymer is not critical as long as it is This creates an embossing plate for letterpress printing machines with the formation of hardened and unhardened ones Surfaces.

The photopolymer can actually be a solid substance in advance if it is heated by heating can be liquefied, e.g. to a temperature in the BeMch of 21 to 94 ° C, until a viscosity in Relationship from loo to lo.ooo cP is reached.

The photopolymer preferred for the purposes of the invention is of the polyene-polythiol type according to US Pat. No. 3,661,744. Such photopolymers which are a photosensitizer for Photo-curing rate acceleration are commercially available from W. R. Grace & Co available. When reference is made in the following to polyene-polythiol photopolymers, those in the photopolymers described above are understood to be.

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The following examples are intended to illustrate, but not “limit, the invention. In all cases All parts and percentages are based on weight, unless otherwise stated.

example 1

Polymeric embossing plates of the type in which a liquid photopolymer is used, have been described in detail in the United States patent 3 615 45o. Polymeric door panels are described in U.S. Patent 3,666,461.

Examples of certain suitable photopolymers and their use in making such plates are given in Examples 2 and 3 below.

In the process of processing the plate, it is necessary to go through the following stages:

(l.) The liquid photopolymer is formed on a support or a support plate a thin film.

(2.) This photopolymer film on the substrate is then exposed to ultraviolet light, namely first directly for a short period of time ("burst exposure time") and then through a Image, e.g. a photographic slide. At this point, according to the procedure of Prior art the incompletely cured polymer in those not exposed through the image

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Bodies removed.

An expedient.s method for carrying out the invention is to begin with briefly and directly to expose high-energy radiation and then a stencil carrying the image or a positive or

/ The negative slide with lines / as a halftone image parallel to the surface of a layer of a photohardenable (i.e., a photohardenable, through Light crosslinkable or photopolymerizable) preparation ("Potopolymer") to be arranged which directly on a carrier plate had been cast. The slide bearing the picture and the surface of the preparation can be in contact with each other or have an air gap between them, as just now is appropriate. The photo-curable layer is then further through the slide by means of a Source of high-energy radiation, preferably a point light source or collimator light source exposed until the layer is practically completely hardened or polymerized to form a insoluble layer in the areas exposed by the image. The thickness of the after all Embossing obtained by this method can be regulated by the fact that the thickness of the layer the preparation is varied. It is preferred to use UV light to harden the entire embossed image Post-exposed for about 1 minute. The resulting hardened plate is then developed, for example one Airflow exposed to an air knife to the extent necessary to remove the imperfectly cured Remove polymers in the parts not exposed by the image. After that, the light cured Printing plate for printing e.g. as an embossing plate or according to the USA patent

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3 666 461 ready to use as a sample for the production of other panels.

The following examples are intended to illustrate the preparation of a "certain photopolymer. All parts and percentages." are based on weight ".

Example 2

It becomes tetraene by reacting tolylene-2,4-diisocyanate prepared with trimethylolpropane diallyl ether, a compound of the following formula being obtained will:

CH ₀ = CHCH ₀ -OCH ₀ 2 2.2

CH ₂ OCH ₂ CH = CH ₂

H ₅ C ₂ -P-CH ₂ OC-NH

CH ₂ -CHCH ₂ -OCH ₂

HC-OCH ₂ -CC ₂ H ₅

CH ₂ OCH ₂ CH = CH ₂

A round bottom flask comes with a stirrer, thermometer, dropping funnel, and inlet and outlet tubes provided for nitrogen. The flask can be placed in a heating mantle or in a water bath be dived as desired.

There were two moles (428 g) of trimethylolpropane diallyl ether with 0.2 cm of dibutyltin dilaurate under nitrogen mixed. To the mixture was added 1 mole of tolylene-2,4-diisocyanate at the rate of addition and the water cooling were adjusted so that the temperature was below 70 ° C for another Hour remained. An analysis for isocyanate groups showed

that at that time the implementation was practically complete. As an antioxidant for prevention 0.6 g of 2,6-di-tert-butyl-p-methylphenol were then added to a vinyl polymerization.

Example 3

• There was a haul (602 g) of the tetraene prepared according to Example 2 with one mole (488 g) of pentaerythritol tetrakis-3-mercaptopropionate and 15 g of benzophenone mixed together. The mixture hardens when it is at the prescribed level Thickness of e.g. 0.5 mm is spread as a layer in ultraviolet light very quickly and results a tough strong product. It makes excellent embossing plates.

Example 4

A plate suitable for air etching was prepared in the following manner.

A device was used that was available as a commercially available "LETTERPLEX I machine" from the company ¥. R. Grace & Co known and widespread in the United States. This device comprises various basic device parts. The primary part is the exposure tower. The exposure tower consists of a plate, a Holder for glass frame negatives, an ultraviolet light source and a system with which the polymer is distributed and spread by means of a squeegee. The plate is made of cast aluminum and contains a recess that is etched on the surface and connected to a vacuum source in order to this plate to hold a pressure plate in place, as will be explained later. Located below the carrier plate a heating device such as a bench

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from infrared lamps or a series of electrical heating wires to make sure the plate is on is kept constant temperature. The holding device for the negative consists of a plate made of "Pyrex" glass with a vacuum grille. The holding device can also be a transparent overlay sheet made of clear plastic that is a polyester can be include.

The negative is embedded between the glass plate and the plastic cover film. The glass plate is held in place by a metal frame. The storage device for the negative is in pivot on the back is hung in such a way that it is lifted for the insertion of the llegative and over the Base plate can be lowered.

In the first stage the negative is between the Inserted the glass plate and the plastic cover sheet of the carrier for the negative. After that, will the support for the printing plate (which is commercially available as a gold-colored aluminum plate, 0.25 mm thick is formed) applied to the base plate. This plate was finished with a conversion coating treated of a known type and then coated with a resin to prevent the last To improve coating from the photopolymer described below. To make sure, that the negative lies completely flat within the plastic cover sheet and with the glass plate is in contact, can over the surface of the holding device for the negative a Squeeze roller to be rolled. Similarly, you can use the squeegee roller over the platter

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if this is applied to the base plate. Independent vacuum systems work in such a way that the negative is attached tightly to the glass plate and the aluminum carrier plate rests close to the base plate.

The actual holding device for the negative against the glass plate is the plastic cover film which is held even by the applied vacuum.

The holding device for the negative, which is still in the upright position, is now on the Operator moved the device too. A series of outlet nozzles in a linear tube in front of the device now lets the Fotopolymerp emerge, z. B. the mixture described in Example 3. It becomes quite a linear mass of the photopolymer measured exactly. The holding device for the negative now moves back to the starting position, while at the same time the deposited mass of the photopolymer is spread into a film by means of a doctor blade is, which has a uniform thickness of 0.5 to 0.58 mm, the thickness of the choice of Operation depends. The backsheet is then removed, protective doors on the front of the device are now closed to protect the operator from the UV light that is now turned on will. Initially, the plate is exposed to a burst of UV light according to the invention in order to Phenomenon known as "cupping" or "dishing" will eliminate. If this does not happen, this phenomenon occurs and results in one

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concave characteristic depression in certain larger embossed letters. The use of the "burst exposure time" prevents the later development of this undesirable phenomenon. The doors are now lifted and the operator lets the holding device for the negative down onto the base plate which carries the pressure plate. At this point it must be noted that the holding device for the negative does not come into contact with the photopolymer, i.e. an air gap from 0.3 to 0.45 now between the surface of the photopolymer and the bottom surface of the plastic cover film consists. This air gap is created by washers around the outer edges of the base plate maintain.

The doors are closed again and the UV light source is automatically set for a predetermined period Period of time (the "main exposure time"), which depends on the end use intended for the printing plate depends. In a typical case for newspaper printing plates this "main exposure time" is about 3o to 60 seconds.

The UV light is now switched off, the doors are raised and the operator lifts the holding device for the negative. The obtained pressure plate is now ready for development.

This description gives a summary including the inventive modifications of the The apparatus and method of U.S. Patent 3,597,080. It is also possible under

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Use of this device or the related automated device according to US Pat. No. 3,635,711 to connect an appropriate source of high energy radiation to the spreading device and the doctor blade for the lOtopolymer, as can be seen from FIGS. 2, 6 to 8 and Io to 13 of the USA Patent Specification 3,597,080 and Figures 3, 4 and 7 to 9 of US Pat. No. 3,635,711, so that the "shock exposure time" can be carried out simultaneously with the spreading of the photohardenable preparation through the doctor blade.

Example 5

A series of four embossing printing plates were made from the same batch of a photohardenable preparation manufactured in practically identical ways. In one approach, the hardening stage was complete effected by exposure through a slide bearing the image using a bright source of xenon light. For the other three plates, a "shock exposure time" was initially used applied without a slide, after which the main exposure or picture exposure with the slide and the same light source was used in each case. The deepening was measured by making cross-sections through large parts of the image and then microscopic Determination of surface tolerances or by means of microphotographs. The results are in the following Table I compiled.

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Table I.

example
Ur. photo exposure
duration
(Pre-exposure)
(express) 6th MAIN REPORTING
duration
(Image exposure)
(Seconds) surfaces
tolerance
(mm) 5A no 15o + o, o38 5B 2 4o + o, oo635 5C " o, 8 58 + 0.0089 5D 1.6 56 + o, oo635 example

A number of embossing plates were produced according to example b in order to test the phenomenon of "deepening". No attempt was made to determine an optimal ratio of "shock exposure time" to "main" exposure time. The results are given in Table II.

Table II

Example pre-exposure no. duration

(express)

Image exposure time (seconds)

Surface tolerance (mm)

no

0.8 1.6

60

56 56

+ o, o38 to o, o5

+ o, o2 + o, ol5

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Examples 7 to 9

Experience has shown that the best ratio of "shock exposure time" to "main exposure time" about 2o to 5o seconds "main exposure time" for every second "burst exposure time" is to one to achieve good stability of the brightest light image points (0.1 to 0.13 mm). When evaluating an approach a commercially available "LETTERFLEX" light-curable Polymers using laboratory equipment was ensured that the optimal exposure conditions were 3.1 seconds "burst exposure time" or Direct exposure and "main exposure" or image exposure time were 68 seconds. Both Same experiments found that 228 seconds of exposure time were needed to make a printing plate of the same quality if all exposure was through the image slide.

In experiments with two further batches of the light-curable polymer "LETTERFLEX", optimal ratios were found as follows: 2 seconds "shock exposure time" and 40 seconds "main exposure time" for one batch and 2 1/4 seconds "shock exposure time ¹¹ with 48 seconds" main exposure time for the other approach, which took about 115 to 125 seconds of exposure to produce the same quality plate when all exposure was through the image slide.

Subsequent work in the production of polymeric original plates or "original plates" according to U.S. Patent 3,666,461 gave similar results

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en

surprising "reduction / of the total exposure time, when the two step process of the invention was used.

- patent claims -

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Claims (7)

Claims fl ·)) Method of curing a photo-curable Polymer layer of a printing plate or pattern plate made of a polymer by irradiating the polymer layer with high-energy Radiation, characterized that you first get the light-curable polymer layer of high-energy radiation exposing the photocurable polymer layer for a short period of time high-energy radiation projected through a slide carrying the image or pattern is suspended until the 'polymer layer has cured practically completely in the exposed areas of the image or pattern. 2.) The method according to claim 1, characterized in that the immediate Exposure time is not more than 90 $ of the time within which the photohardenable Polymers begin to gel when it is immediately exposed to the prevailing exposure conditions is exposed. 3.) The method according to claim 1 or 2, characterized in that the immediate Exposure time is 0.5 to 4 seconds. - 19 409846 / UB9Ü 4.) Method according to any one of claims 1 to 3, characterized in that the image exposure time at least 15 seconds for every second of the immediate exposure time amounts to. 5.) The method according to claim 4, characterized in that the image exposure time 2o to 5o seconds for every second of the immediate exposure time. 6.) The method according to any one of claims 1 to 5, characterized in - that a practically uniform layer of a liquid, photohardenable polymer preparation is spread on a support plate before the immediate exposure and the image exposure done, and that an embossing plate by removing the parts of the incompletely cured photohardenable preparation which during the image exposure stage were not exposed, is developed from the carrier plate. 7.) Polymeric embossing plate or pattern plate with increased stability of the brightest light image points and reduced "cupping" or "dishing" as determined by surface tolerance measurement, which are less than + o, o25 mm in the large Embossing areas is produced by the method of claim 6. - 2o A09846 / Ü690 - 2ο - δ.) Polymeric embossing printing plate according to claim 7, characterized in that the surface tolerance is less than + 0.01 mm in the large embossed areas. 4098A6 / 069Ü