Method for pretreating materials for flexography by means of mobile low-temperature plasma application

Abstract

Where printing plate is to be secured to a plate cylinder by means of double-sided adhesive tape, the surface of the printing plate to be secured to the plate cylinder is, prior to bonding, partially pre-treated by means of a low-temperature plasma pencil after cleaning with isopropanol.

Claims

1. A method for securing a printing plate to a plate cylinder using a double-sided adhesive tape, comprising: cleaning the printing plate to be secured to the plate cylinder with isopropanol; partially treating a surface of the printing plate to be secured to the plate cylinder with a low-temperature plasma pencil after the cleaning of the printing plate with isopropanol.

2. The method of claim 1, wherein the treatment with the low-temperature plasma pencil is applied in two continuous lines across a width of the printing plate at a width of about one centimeter at a beginning of the printing plate and at an end of the printing plate at a location where the printing plate is to be secured around the plate cylinder.

3. The method of claim 1, comprising: bonding the printing plate to the plate cylinder after the surface of the plate cylinder is partially treated with the low temperature plasma pencil.

4. The method of claim 3, wherein the partial treating of the surface of the plate cylinder comprises, prior to the bonding, partially treating the surface of the plate cylinder with the low-temperature plasma pencil in two continuous lines across a width of the plate cylinder at a width of about one centimeter at a beginning of the printing plate and an end of the printing plate at a location where the printing plate is to be secured around the plate cylinder.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Embodiments of the present disclosure are illustrated in further detail by the subsequent description of the Figures. In this:

(2) FIG. 1 is a diagram illustrating the adhesive force in relation to the pre-treatment method;

(3) FIG. 2 is a diagram illustrating the T peel values in relation to the pre-treatment method;

(4) FIG. 3 is a diagram illustrating the contact angle in relation to the pre-treatment method;

(5) FIG. 4a shows the printing process simulation test set-up relevant for flexographic printing, seen from the angle of the plate cylinder;

(6) FIG. 4b shows the printing process simulation test set-up relevant for flexographic printing, seen from the angle of the printing plate;

(7) FIG. 5 shows the test results of the technical test during use in respect of the peel adhesion properties of the adhesive tape when being removed from the printing plate after completion of the printing process.

(8) FIG. 6a shows the treatment of an unjoined printing plate with a low-temperature plasma pencil.

(9) FIG. 6b shows a joined printing plate after treatment with a low-temperature plasma pencil.

DETAILED DESCRIPTION OF EMBODIMENTS

(10) Various embodiments now will be described more fully hereinafter with reference to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific embodiments by which the invention may be practiced. The embodiments may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the embodiments to those skilled in the art.

(11) Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The term “herein” refers to the specification, claims, and drawings associated with the current application. The phrase “in an embodiment” as used herein does not necessarily refer to the same embodiment, though it may. Furthermore, the phrase “in further embodiment” or “a further development” as used herein does not necessarily refer to a different embodiment, although it may. Thus, as described below, various embodiments of the invention may be readily combined, without departing from the scope or spirit of the invention.

(12) In addition, as used herein, the term “or” is an inclusive “or” operator, and is equivalent to the term “and/or,” unless the context clearly dictates otherwise. The term “based on” is not exclusive and allows for being based on additional factors not described, unless the context clearly dictates otherwise. In addition, throughout the specification, the meaning of “a,” “an,” and “the” include plural references.

(13) In the case in hand, the adhesive force values were measured between the covered side of the adhesive tape and a surrogate material for photo-polymer plates, i.e. a 125 μm thick PET mylar film (PET-0083) as surrogate substance for the actual plate backing base material, 10 minutes after application (cf. FIG. 1, minimum, maximum and mean value, respectively). It was the objective of this measurement to determine general differences in terms of adhesive force between the two adhesive tapes, irrespective of the actual intended use case, so as to use the results as a reference to analyse differences manifesting themselves down-stream during and also after the actual flexographic printing step in a more objective fashion. The measurements were repeated for a total of 50 times so as to increase reliability of the measurement results. For this purpose, the mylar film was specially pre-treated: in one case it was just cleaned with isopropanol (IPOH), in one case it was pre-treated with isopropanol and a primer pencil (“Lohmann Primer Pen”), in one case it was pre-treated with isopropanol and the low temperature plasma device PZ2 by the company “Relyon Plasma GmbH”, and in one case it was pre-treated isopropanol and a conventional roll corona. Ultimately, it turned out that the primer pencil treatment resulted in significantly higher adhesive force values compared to the other three pre-treatment methods.

(14) By way of comparison and as illustrated in FIG. 2, the peel-strength values were measured using the generally recognised T peel test, again 10 minutes after application, and the illustration shows the respective minimum, maximum and mean value. The T peel test is used for characterising the peel properties of peel systems and is carried out with reference to ASTM D 1876 and DIN 55529. The “T” indicator refers to the presence of two peel arms that are bent °90 during the peel test, resulting in a T shape. The T peel test is carried out on a universal test machine. In this, the clamping length is I.sub.0=50 mm and the peel speed is vT=100 mm/min. Generally speaking, the conditions were similar to the results as illustrated in FIG. 1 relating to the “cylinder peel method”: The measurements regarding the samples pre-treated with a primer pencil showed significantly higher adhesive forces compared to the other three other pre-treatments.

(15) FIG. 3 shows the contact angle in relation to the surface pre-treatment. As a matter of principle, surface wettability of constitutes a characteristic that provides information as to whether it is a low-energy surface and thus less suitable for bonding or a high-energy surface and thus better suitable for bonding. Wettability is assessed using a water droplet applied to the surface. If an actual water droplet forms after application, the surface is low-energy, if the water droplet dissolves, however, it is a high-energy surface. Conventionally, test inks are used for a more in-depth analysis regarding wettability, and the contact angle of the droplet is measured (test methods according to DIN 53364 and ASTM D 2578-84). For improved surface adhesion, the contact angle measured should be as small as possible. In this context, the contact angle of the liquid droplet depends on the surface energy of the liquid and the surface to be glued. The four different pre-treatment methods referenced in this application (additionally, the contact angle was measured for a film surface without any surface treatment whatsoever, for comparison) have different effects on the surface energy of the surface to be glued, the best potential adhesion values were reflected in the surface pre-treated with the primer pencil.

(16) The linked FIGS. 4a and 4b show the test set-up relevant for flexographic printing for the simulation of a printing process, once seen from the side of the plate cylinder (FIG. 4aa) and once seen from the printing plate from (FIG. 4b). FIG. 5 shows the test results of the technical test during use in respect of the peel adhesion properties of the adhesive tape being removed from the printing plate after completion of the printing process. Four different alternatives were tested at identical temperature and humidity parameters. The plate sleeve/sleeve “Rubin” of the company “Polywest Sleeve Systems” with a diameter of 380 mm and a printing plate “ACE” by the company “Flint Group Flexographic Products” with a thickness of 2.54 mm and a shore hardness A of 66 were used, respectively. A conventional hard rubber roller with a shore hardness A of 20 was used both for applying the adhesive tapes to the plate sleeve and for applying the printing plate to the adhesive tape on the sleeve, and a pressure of 4 bar was used, for two revolutions each.

(17) The materials were measured for hardness in accordance with DIN ISO 7619-1. In this set-up, the indentation depth/penetration depth of a steel pin into the material under examination is measured—for shore A the steel pin is shaped like a truncated cone and for shore D it is shaped like a needle tip.

(18) The shore hardness A of 66 of the printing plate is of significance in the case in hand because, given that it is a relatively hard material, interacting with the relatively small circumference of the sleeve of 380 mm and the relative substantial thickness of the printing plate of 2.54 mm, it simulates challenging printing conditions: The printing plate's elasticity properties in connection with the circumference of the sleeve in general cause an increased tendency of the edges of the printing plate to lift off of the sleeve.

(19) The following variations were tested: One printing plate was pre-treated only with IPOH, three printing plates were each cleaned with IPOH and additionally pre-treated essentially continually at the respective beginning and end points to be secured to the sleeve at a width of about one centimetre, i.e. once with a primer pencil, once with a roll corona and, as shown in FIGS. 6a and 6b once with a low-temperature plasma pencil. FIG. 6a shows the treatment of the unjoined printing plate with a low-temperature plasma pencil. FIG. 6b shows the joined printing plate after treatment with a low-temperature plasma pencil.

(20) After the simulated printing processes under realistic conditions (i.e. over 5 hours and at a speed of 5 m per minutes), the following parameters in particular were examined and identified (the respective column indicating the values shown in brackets):

(21) The subjective force to be exerted after printing (“force”) to remove the adhesive tape from the printing plate on a scale from 1 (very little) to 5 (no longer demountable).

(22) The subjective haptics after printing (“haptics”) during removal of the adhesive tape from the printing plate on a scale of 1 (very soft peel adhesion properties) to 5 (very hard peel adhesion properties).

(23) In both tests, only the covered side was relevant (“c.s.”=closed side) because the open side (“o.s.”=open side) was glued to the sleeve and presently was not the subject matter of the test.

(24) The differences in lift-off of the plate edges (“edge lift-off printing plate”) once with reference to the type of pre-treatment and once also with respect to the beginning of the tape (“upper”) and its end (“lower”), respectively. The plate edge lift-off is referenced in mm.

(25) Potential residual adhesive on the printing plate that actually was a mix of adhesive, foam backing material of the adhesive tape as well as residual primer (cf. “comment”).

(26) The printing process as described was repeated five times (once cleaned with IPOH only, once IPOH+partially primered, once IPOH+partially treated with roll corona or IPOH+partially treated with a low temperature plasma pencil), the results were essentially identical in all tests and corresponded to the sample results listed here.

(27) The following conclusions could then be drawn in respect of the requirements specified at the beginning concerning secure and reliable bonding in flexographic printing, repositioning and re-usability, respectively as well as releasability.

(28) 1) The adhesive force of the adhesive surface was significantly higher in the material to be glued that was pre-treated with a primer in comparison to the three other surface types. This manifested itself firstly in the fact that the edge lift-off was only 1 mm (upper edge) and 2 mm (lower edge), whereas it was 3 and 2 mm, respectively in the surface treated with the plasma pencil and at least 2.5 times higher in the surface pre-treated with corona and the entirely non-pre-treated surface, compared to the primered surface. On the other hand, in the latter surface, the biggest force was required to remove the tape from the printing plate after the printing process—the force required was estimated to be about twice as large compared to the three other surface types, and also repositioning was significantly impaired as a consequence of the increased adhesive force: On a scale of 1=mild via 2=moderately impaired to 3=severely impaired, it was classed as “severely impaired”.

(29) 2) This increased adhesive force exhibited significant disadvantages during delamination after the printing process in comparison to the three alternative applications in that residual adhesive, foam and primer remained on the edge of the printing plate, namely exactly in the area where the primer was applied prior to bonding. Consequently, the materials could no longer be used (i.e. adhesive tape) or, in any case, only following extensive cleaning (i.e. the printing plate).

(30) 3) The preparation time until securing the printing plate to the plate cylinder was significantly longer when the material surfaces were primered compared to the alternatives, caused in particular by the requisite drying time of the primer and the effort required for cleaning the surfaces that are unintentionally contaminated by the primer—it was about eight times longer compared to the three alternatives.

(31) 4) Whilst most of the values of the majority of the relevant criteria (preparation time, repositioning, residue, peeling force required after the printing process as well as haptics) were on a similar and comparable level in the surface cleaned only with IPOH and the surface additionally treated with corona or plasma pencil, respectively, the values regarding potential edge lift-off of the printing plate from the plate cylinder during or after completion of the printing process showed significant differences: In that respect, the pre-treatment with the plasma pencil was associated with substantial advantages over the two remaining methods: Edge lift-off of the upper edge was at least four times less and edge lift-off of the lower edge was at least 2.5 times less.

(32) Several repetitions of the tests ultimately showed that only after the fourth repetition of the printing process (i.e. the fifth simulated printing process) using the low-temperature plasma pencil pre-treatment, the results deviated negatively from the results as illustrated, especially in that after that time residual adhesive tape remained on the specimen.

(33) Hence, it is the general conclusion that in respect of said relevant assessment criteria the use of the low-temperature plasma pencil for pre-treating the printing plate after cleaning with isopropanol constitutes the most efficient method for executing flawless flexographic printing. Even more so, in view of the fact that the straightforward multiple re-usability of the components involved in the printing process and the associated reduction of cleaning agents/solvents required for cleaning results in a substantial reduction of environmental impact due to decreased generation of waste products, not to mention significant cost-saving effects.

(34) Generally and applicable in analogy with the tests as illustrated, in respect of the requirements of flexographic printing the plasma pencil can be also used for pre-treating a plate cylinder, as derives also from analogous tests and results not shown in the present application.

(35) As far as applicable, all individual features shown in the sample embodiments can be combined and/or exchanged without leaving the scope of the present disclosure.