WO2009148303A1

WO2009148303A1 - Method for manufacturing a stencil for screen printing

Info

Publication number: WO2009148303A1
Application number: PCT/NL2009/000128
Authority: WO
Inventors: Marcel Dialma Everaars; Arnold Antony Koetsier
Original assignee: Stork Prints B.V.
Priority date: 2008-06-04
Filing date: 2009-06-03
Publication date: 2009-12-10
Also published as: NL2001744C2

Abstract

The invention relates to a method for manufacturing a stencil for screen printing, comprising the steps of a) applying a resist layer of photosensitive material on a screen, comprising a network of dykes delimiting openings; b) applying a mask layer of masking material; c) structuring of the mask layer into a mask; d) exposing the resist layer through the mask using radiation for which the photosensitive material is accessible; e) removing the mask and parts of the exposed resist layer to obtain a stencil for screen printing with at least partially open screen openings, said opening defining the image to be printed, wherein the masking material is applied in a first medium having a poorer solubility for photosensitive material than for the masking material, and wherein the mask and parts of the exposed resist layer are removed by a second medium.

Description

Short title: Method for manufacturing a stencil for screen printing

The invention relates to a process for manufacturing a stencil for screen printing. From EP1860499 such a method is known and comprises the steps of:

- applying a photosensitive layer to a screen;

- applying a mask layer on top of the photosensitive layer, also called black mask;

- structuring the mask layer into a mask;

- exposing and developing the photosensitive layer.

Now it has appeared that the mask is not properly removed when developing the photosensitive layer. The water-based developer generally used to rinse (resuspend) the non-exposed parts of the photosensitive layer, is unable to remove the mask. When developing the photosensitive layer, remnants of the mask remain, the subsequent removal of which is time-consuming. This phenomenon is caused by the fact that the formulation of the mask layer is chosen such as to prevent a harmful effect on or resuspension of the photosensitive layer when the mask layer is applied to the photosensitive layer. This is realized by selecting the solubility/resuspendability range of the mask layer differently than the solubility/resuspendability range of the photosensitive layer.

It is an object of the invention to provide a process in which during the development step the mask is solubilised in the developer, in particular a water-based developer.

This object is achieved by means of a method as defined in claim 1. Preferred embodiments are defined in the dependent claims.

In the method according to the invention a screen is provided with a layer of photosensitive material. The screen comprises a network of dykes delimiting openings, like a (metal) sieve, gauze or fabric. The resist layer fills the screen openings and forms a continuous layer at least at one side of the screen (the later printing side when printing). On top of the resist layer a mask layer is coated which is subsequently converted into a mask, usually by means of a laser, by removing masking material. Next, the resist layer is exposed and developed. The masking material is opaque for the radiation used for exposure of the underlying layer of photosensitive material.

According to the invention the masking material is coated by means of a first medium, which shows a poorer solubility (resuspendability) for the photosensitive material of the resist layer than for the masking material. In order to remove the mask and exposed parts of the resist layer, a second medium is used in which both the masking material and said parts of the resist layer are soluble/resuspendable. When a negative photoresist is used as the photosensitive material the unexposed parts of that layer are removed, while in the case that a positive photoresist is used the exposed parts become soluble/resuspendable in the developer.

In a preferred embodiment a binder is used for the mask layer which is soluble in water (second medium) and, at the same time, has a higher solubility in polar solvents (first medium) than the photosensitive material of the resist layer.

In this application the expression 'solubility of the masking material in the first medium' refers to the solubility/resuspendability of the binder in the masking material of the first medium. The binder in the masking material will be referred to as binder M. The expression 'solubility/resuspendability of the photosensitive material of the resist layer' refers to the solubility/resuspendability of the binder as used in the photosensitive material of the resist layer in the first medium (see below). The binder as used in the photosensitive material will be referred to as binder L.

Preferably, the proportion between the solubility of the binder of the masking material in the first medium and the binder of the masking material of the photosensitive material in that medium is as large as possible, like at least 10:1. In addition, the solubility of binder L in the first medium is poor, preferably less than 1 g/L, more preferably less than 0.1 g/L.

During the formulation of the mask layer, the binder is dissolved in a polar organic solvent, such that during the application of the mask layer on top of the resist layer, the resist layer is not damaged or resuspended. The actual resist layer is advantageously prepared by applying a water-based emulsion, preferably a negative type resist emulsion, on a screen and allowing the water to evaporate. The water-based emulsion comprises a continuous water phase containing solid particles, advantageously with a diameter in the range of 1 to 30 micrometer. These solid particles consist for example of polyvinyl acetate. In the water phase a water-soluble polymer is dissolved, e.g. polyvinyl alcohol with a saponification degree of 88%. When the emulsion is drying, a hard, friable resist layer is formed, which consists of solid particles, which are mutually connected by a small layer of water-soluble polymer. The solid particles are thus not blended, as a consequence of which the resist layer is well- resuspendable in water. In the water phase a so-called sensitizer like potassium dichromate can be dissolved. The sensitizer is able to form an insoluble complex with the water-soluble polymer, when exposed to light, usually having a wavelength of 360 nm. In this case, the exposed parts of the resist layer are then non-resuspendable, the unexposed parts are.

It is mentioned that from EP 0785474 A1 a photosensitive layer is known which is used in the fabrication of a plate for flexoprinting. This photosensitive layer consists of a photo-elastomer, which is not applicable for use in the present invention. Namely, a photo- elastomer for a plate for flexoprinting has a glass transition temperature (T₉) which is below room temperature (T_k)(20 ⁰C), as a consequence of which the photo-elastomer, under the circumstances of use of the stencil for screen printing according to the invention, has elastic properties. These elastic properties would cause the openings in the screen printing stencil, through which the ink paste is forced, to be deformed or even pressed closed, which is undesirable when regarding the desired image resolution and quality. Besides, the above photo-elastomer is, after exposure, not rinsable with water. A mixture of butanol and perchloric ethylene should be used as developer.

When the mask layer is applied, the resist layer has not yet been exposed and is thus easily resuspendable in water. This way the mask layer cannot be applied from a water- based solution, yet the mask layer should be water-soluble during development. For this purpose binders with a solubility range which partly overlaps with the solubility range (in fact the resuspendability range) of the resist layer, are used for the mask layer. This means that these are binders which are soluble in water, but also in non-aqueous media, in particular organic solvents, while the resist layer is only resuspendable in water. Examples of such binders are hydroxypropyl cellulose; polyvinyl pyrrolidone; copolymers of vinyl pyrrolidone and vinyl acetate, methacrylic acid, vinyl imidazole or vinyl caprolactam; copolymers of maleϊc acid and alkenes or arylenes. When formulating the mask layer the above binders are dissolved in polar solvents like lower alcohols like for example ethanol, 1-propanol, 2- propanol, or acetates like ethyl acetate, butyl acetate and propylene glycol diacetate.

Example 1

This example consists of the description of the resist layer and the description of the mask layer.

First of all a formulation for a resist layer of photosensitive material is prepared. The preparation of which comprising the steps of:

1) preparing the water phase;

2) preparing the resin phase;

3) emulsifying the resin phase into the water phase.

Ad 1) 600 kg of a 10% solution of polyvinyl alcohol (Mowiol 26-88 from Clariant) in tap water is prepared at room temperature while stirring.

Ad 2) An amount of 250 kg of an epoxy resin (Epicote 828 from Brenntag Volker) is mixed with 2 kg of a dispersing agent (Triton CF10 from Epenhuyzen) at 120 ⁰C, after which the resin is cooled to 60 ⁰C.

Ad 3) The water phase is pumped into a reactor with a content of 1 cubic meter. A propeller stirring device rotates with such a rotational velocity that a vortex is formed. Next, 1 kg of dispersing agent is added (Triton CF10 from Epenhuyzen), after which the warm, liquid resin phase is added while stirring to the water phase in 30 minutes. The mixture is stirred another 60 minutes at 70 ⁰C. A formulation for the mask layer is prepared as follows. A dispersion of 6 g INXEL™ BLACK A903 (carbon black concentrate) in 25 g of 2-propanol is prepared. At the same time a solution of 2,5 g copolymer of vinyl pyrrolidone and vinyl acetate (S-630 from ISP) in 100 g of 2-propanol is prepared. Next, both mixtures are combined, ultrasonically shaken and filtrated. The proportion of the solubility of binder M (S-630) and the solubility of binder L (Mowiol 26-88) in 2-propanol is larger than 67:1. The solubility of binder M and binder L is determined by adding an aliquot of the involved binder to a quantity of first medium (in this case about 17 g accurately weighed 2-propanol) and stirring the obtained mixture for an hour at room temperature, after which it is visually determined whether the resulting solution is clear.

Just before the application of the prepared emulsion of photosensitive material as a resist layer on an electroformed nickel screen, 1% of potassium dichromate is added. After the resist layer has been dried the formulation of the masking material is applied as a mask layer on top of the resist layer en subsequently dried. By means of a computer controlled laser parts of the mask layer are removed (ablated) to obtain a mask. Next, the underlying resist layer is exposed through the open areas of the mask layer, after which the mask en the underlying unexposed parts of the resist layer are removed with tap water. Thus, a stencil for screen printing was obtained with at least partially open (meaning not covered or filled with material of the resist layer ) screen openings, defining an image to be printed.

Example 2

In this example the formulation of the resist layer is prepared as described above under example 1.

A formulation for a mask layer is prepared as follows. A dispersion of 6 g INXEL™ BLACK A903 (carbon black concentrate) in 25 g of 2-propanol is prepared. At the same time a solution of 2,5 g of polyvinyl pyrrolidone K30 (ISP) in 100 g of 2-propanol is prepared. Next, both mixtures are combined, ultrasonically shaken and filtrated. The proportion of the solubility of binder M (K30) and the solubility of binder L (Mowiol 26-88) in 2-propanol is larger than 70:1. The solubility of binder M and binder L is determined as described in example 1.

Following the same route as described in Example 1 the photoresist layer and the mask layer from the prepared formulations were applied on an electroformed nickel screen and subsequently, by means of a laser, a mask was formed from the mask layer, the photoresist layer was exposed through the mask and subsequently rinsed to obtain a stencil for screen printing.

Example 3 In this example the preparation of the resist layer is also carried out as described in example 1.

A formulation for a mask layer is prepared as follows. A dispersion of 6 g INXEL™ BLACK A903 (carbon black concentrate) in 25 g of 2-propanol is prepared. At the same time a solution of 2,5 g of hydroxypropyl cellulose (Klucel E CS from Hercules) in 100 g of 2- propanol is prepared. Next, both mixtures are combined, ultrasonically shaken and filtrated. The proportion of the solubility of binder M (Klucel E CS) and the solubility of binder L (Mowiol 26-88) in 2-propanol is larger than 14:1. The solubility of binder M and binder L is determined as described in example 1.

The procedure of Example 1 is followed to prepare a stencil for screen printing.

Claims

1. Method for manufacturing a stencil for screen printing, comprising the steps of a) applying a resist layer of photosensitive material on a screen, comprising a network of dykes delimiting openings; b) applying a mask layer of masking material; c) structuring the mask layer into a mask; d) exposing the resist layer through the mask using radiation for which the photosensitive material is accessible; e) removing the mask and parts of the exposed resist layer to obtain a stencil for screen printing with at least partially open screen openings, said opening defining the image to be printed, wherein the masking material is applied in a first medium having a poorer solubility for the photosensitive material than for the masking material, and wherein the mask and parts of the exposed resist layer are removed by a second medium.

2. Method according to claim 1 , wherein the second medium comprises a water- based developer.

3. Method according to one of the preceding claims, wherein the first medium comprises an organic solvent, preferably a polar organic solvent.

4. Method according to one of the preceding claims, wherein the masking material comprises a water-soluble binder which is also soluble in a polar organic solvent.

5. Method according to one of the preceding claims, wherein the proportion of the solubility of the binder of the masking material in the first medium and the solubility of the binder of the photosensitive material in that medium is larger than 10:1.

6. Method according to one of the preceding claims, wherein step a) is carried out with a water-based emulsion of the photosensitive material.