Abstract:
The invention relates to a process and a device for manufacturing segmented pressure-sensitive adhesive layers and applying the same to a substrate, involving the use of a reaction medium containing radiation-inducible polymers and/or prepolymers of olefinically unsaturated compounds and the addition of a photoinitiator.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a process and a device for manufacturing segmented pressure-sensitive adhesive layers and applying the same to a substrate, involving the use of a reaction medium containing radiation-induced monomers and/or prepolymers of olefinically unsaturated compounds, and optionally the addition of a photoinitiator. 
     DESCRIPTION OF THE PRIOR ART 
     Segmented pressure-sensitive adhesive layers are widely used in practice, for example, in such pressure-sensitive adhesives which are required to be water- or gas-permeable. Also, in some cases, reducing the consumption of pressure-sensitive adhesive is a reason for the segmentation of a pressure-sensitive adhesive layer. 
     Furthermore, segmentation is required if small areas are stuck to substrates. Here, it is only the area to be adhered which is to be provided with a segment of the pressure-sensitive adhesive layer. 
     The processes for segmentation, punching, knife application, and screen printing are known above all. Among these, punching has the disadvantage of entailing large amounts of waste. Knife application only permits production of limited layer thicknesses, which also applies to screen printing. The latter moreover requires a considerable expenditure in terms of the provision of contours. 
     U.S. Pat. No. 5,344,681 describes a process for segmentation wherein pressure-sensitive adhesive segments are produced in recesses of a carrier material. This involves particular difficulties in connection with the removal of the segments from the recesses. 
     SUMMARY OF THE INVENTION 
     Starting from the aforementioned prior art, it is the object of the invention to provide a process and a device which enable the manufacture of segmented pressure-sensitive adhesive layers of any desired thickness and contour in an economical manner as well as in a manner which can be readily realized in technical terms, and which are suitable for mechanization with high output using comparatively uncomplicated devices. 
     To achieve this object in a process for manufacturing segmented pressure-sensitive adhesive layers and for applying the same to a substrate, using a reaction medium containing radiation-inducible polymers and/or prepolymers of olefinically unsaturated compounds under addition of photoinitiators, the following sequence of operational steps is proposed with the present invention: 
     that the reaction medium is kept ready in a metering and filling station under absence of oxygen, 
     that the empty templates are conveyed, preferably intermittently one after another, with a lower covering of their segment-defining opening put against the said templates, to a filling station wherein they are filled with a dosable amount of the reaction medium, 
     that the filled templates are passed in continuous conveyance below a UV section, whereby the reaction medium is solidified by means of radiation-induced reaction involving at least partial polymerisation, 
     that after removal of the lower covering, the reaction medium, which is then exposed on the upper side and the bottom side, is, during the further transport between an upper and a lower UV section, fully cured from above and from below by continued radiation-induced polymerisation reaction to form the finished pressure-sensitive adhesive layer, 
     that the template, containing the finished pressure-sensitive adhesive layer, is conveyed to a transfer station wherein the pressure-sensitive adhesive layer is pushed out of the template and is combined with a substrate, 
     that, optionally, the substrate which has been equipped with the pressure-sensitive adhesive layer is conveyed by a conveyor belt to a final-manufacturing station (not shown). 
     The process is suitable for economically and continuously manufacturing segmented pressure-sensitive adhesive layers of any desired thickness and contour in a manner which is comparatively uncomplicated and can be readily realized in technical terms. More particularly, the process is suitable for use in a fully-mechanized device. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One embodiment of the process provides that the thickness of the segmented pressure-sensitive adhesive layer is defined by the thickness of the template. 
     Furthermore it is provided by the invention that the template and the lower covering are rendered anti-adhesive, with the template and lower covering being made of dehesive material or the inner walls of the opening and the covering preferably being provided with a dehesive coating. 
     A further embodiment of the process provides that the polymerization reaction is performed by irradiating the reaction medium with ultraviolet light or electron radiation, and that the ultraviolet light is preferably generated by a laser. 
     The process is further characterized in that to produce the pressure-sensitive adhesive layer, acrylate-based pressure-sensitive adhesives are selected with preference, and that the mixture of monomers is principally employed under addition of a photoinitiator. 
     Also, a prepolymer may be used for polymerization. Since during polymerization, the reaction medium is subjected to a volume shrinkage, a corresponding embodiment of the process provides for the addition to the reaction medium of an additive compensating the volume shrinkage occurring in the course of polymerization. 
     Since the reaction medium must be protected from access of oxygen during its handling up to the time of performing the polymerization reaction, it has proved advantageous to render the environment of the template inert by means of nitrogen, in order to increase the degree of reaction. Since in the case that the reaction medium is metered into an opening of the template it is not absolutely necessary that said opening be filled-up completely, the thickness of the pressure-sensitive adhesive can be adjusted in accordance with the filling amount to be metered, i.e. the thickness can be smaller than the thickness of the template. As a consequence, in the manufacture of the adhesive layer it is possible to vary the thickness within relatively broad limits without the need of making a new template for each thickness. 
     A device for manufacturing segmented pressure-sensitive adhesive layers and applying the same to a substrate, more particularly for realizing the process according to the invention, is characterized by the following structural features: 
     a continuous conveyor belt or chain for templates to be conveyed in circulation, 
     a base or support belt running synchronously within the said conveyor belt, under section-wise formation of a lower covering, 
     a first, upper UV section and a subsequent lower UV section, 
     a transfer station comprising a template-emptying device. 
     As a transfer station, a turnstile is suitable. 
     In the vertical axis, a turnstile is mounted above the template to be emptied. From below, the contoured adhesive tape is pushed upwards out of the template by means of a plunger or punch. The turnstile consists of 4 arms fitted at a distance of 90°, each arm having a suction foot and a pneumatic cylinder. 
     A cycle starts at 0° at the magazine ( 4 ), from which the suction device takes out the piece which is to be rendered self-adhesive. The arm is rotated through 90° counter-clockwise and reaches a station ( 5 ) where the surface is processed (e.g. corona discharge pre-treatment). After a rotation of altogether 180°, at the push-out and transfer station ( 3 ) the piece is placed onto the template in exact position. With the aid of the push-out plunger provided at the bottom side, the piece, which has been rendered self-adhesive, is then jointly removed from the template. The said piece, after a further rotation of altogether 270°, is placed on a dehesive conveyor-belt ( 6 ) and is subsequently subjected to final manufacturing processes. After the final 90° rotation, the cycle is repeated, respectively, started anew (360°=0°). 
     The device is comparatively uncomplicated and is suitable for mechanized, continuous production of segmented pressure-sensitive adhesive layers using uncomplicated technological means. 
     One embodiment of the device provides that the templates have openings to receive the reaction medium, said openings having unrestricted segment-defining geometric shapes. 
     Furthermore, an embodiment of the device may be characterized in that the base or support belt in parts has segment-shaped protuberances forming pressure-sensitive adhesive-free volume portions by projecting into openings of templates and preventing the filling thereof with reaction medium. These protuberances have been rendered dehesive. They offer the possibility of forming areas which are surrounded on all sides by pressure-sensitive adhesive. Finally, the device provides that the templates have a thickness of between 0.5 to 6 mm. 
     Further details, features and advantages of the invention will become apparent from the following explanation of an embodiment example which is schematically represented in the drawings. The device shown in the Figure comprises a continuous conveyor belt or chain  14  for templates  10  to be conveyed in circulation. These are attached, at pre-determined distances from each other, to the transport chain  14  and circulate clockwise according to arrow  19  about a respective pair of deflector rolls. Each of the said templates  10  has segment-defining openings  18  which are closed in the region of the filling station  1  at their bottom side by a support belt  15  under formation of a lower covering  11 , so that at the filling station  1  the still liquid reaction medium can be filled in portions into each opening  18 . The reference numeral  20  designates the filling amount in each template  10 . The filling amount may take up only part of the thickness of the template—as illustrated —but it may also from case to case fill up the template  10  in its full height. After a template  10  has been filled with reaction medium at the filling station  1 , it runs in continuous transport initially under a UV section A, in a section  2  of the line, and subsequently thereto in a section  2 ′ of the line between an upper UV section A and a lower UV section B, with the reaction medium, which is then exposed at the upper side and the bottom side, being fully cured from above and from below by continued radiation-induced polymerisation reaction to form the finished pressure-sensitive adhesive layer  13 . 
     At the intersection of the transport distances  2  and  2 ′ the support belt  15  had previously run off downwards, thus removing the lower covering  11 , and thereby exposed the underside of the reaction medium for polymerization reaction from below. 
     Then the template  10  containing the finished pressure-sensitive adhesive layer  13  is conveyed to the transfer station  3 , where the pressure-sensitive adhesive layer  13  is pushed out from the template  10  with the aid of a plunger- or punch-shaped emptying device  16 , and is delivered to the transfer station  3 . 
     In the transfer station  3 , with the aid of the template-emptying device  16  the ready-polymerized pressure-sensitive adhesive layer  13  is delivered, after it has been pushed out of the template  10 , to an arm of the conveyor means  17  configured as a turnstile, and is held therein, for example, by action of suction. 
     The turnstile  17  takes individual substrates  12  from a stock (not shown in detail) and delivers them to an arm, each arm being provided with a suction foot and compressed-air cylinder. 
     A cycle starts at 0° rotation at the magazine or depot  4 , from which the suction arm  21  removes the piece  12  which is to be rendered self-adhesive. The arm is then rotated through 90° anti-clockwise and reaches station  5 , where the surface of a piece  12  is processed, for example, by coronary pre-treatment. After a further rotation through altogether 180°, at transfer station  3  the piece  12  is placed in exact position on the template  10 , whereafter, with the aid of the push-out plunger  16  provided at the bottom side, joint removal of the piece which has been rendered self-adhesive takes place. 
     Following a further rotation of the turnstile arm of altogether 270°, the said piece  12 , together with the pressure-sensitive adhesive layer  13 , is placed on a conveyor belt  6  which has been adjusted to be dehesive, and is subsequently subjected to final-manufacturing processes. After the final 90° rotation, the cycle is completed and a new cycle of 360° will be started. 
     The invention will be illustrated by means of the following examples. 
    
    
     EXAMPLE 1 
     From 
     60 parts by weight of 2-ethylhexyl acrylate 
     30 parts by weight of methyl acrylate 
     10 parts by weight of acrylic acid 
     0.01 parts by weight of Irgacure 184 (1-hydroxy-cyclohexyl-phenyl ketone) 
     an approx. 8% polymer-monomer solution is prepared. At a wavelength of approx. 280 nm, the photoinitiator disintegrates, the reaction is started, which reaction is stopped by switching off the UV lamps when a viscosity of approx. 3 Pa·s has been reached. 
     Subsequently, 0.6%-wt. of DPGDA (dipropylene glycol diacrylate) and 0.6%-wt. of the photoinitiator Irgacure 1800 (75%-wt. of 1-hydroxy-cyclohexylphenyl ketone and 25%-wt. of Bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl phosphine oxide) are added to the prepolymer. By adding 2-ethylhexyl acrylate, the viscosity is controlled and the dosable, reactive polymer-monomer mixture is obtained, which is stored under absence of oxygen. 
     This adhesive was dosed into the cavity of a segment-defining template wherein the letter “L” had been formed. The basic body of the “L” is 6 mm in width, the horizontal and vertical limbs have a length of 20 and 30 mm, respectively. This cavity was milled into a 1.5 mm-thick Teflon plate. 
     The polymer solution is dosed by means of a piston pump and an aluminium pipe (inner diameter 2 mm) into the cavity between siliconized polyester film and the opening (“L-mold”) seated thereon. The mold is filled completely, the volume amounts to 0.35 cm 3 . The filled template is irradiated for 15 seconds with a UV laser in a chamber 15 which has been rendered inert with nitrogen. 
     Type: Lextra 200 (by the firm of Lambda Physik, Göttingen), wavelength: 351 nm, impulse rate 5-30 Hz, mean power 10 W, splaying 30 mm×40 mm, 
     whereby the segmented adhesive is polymerized and largely cured. Subsequently, the template is rotated through 180° and again irradiated for 15 seconds from the opposite side, 
     whereby the polyacrylate basic body is fully cured (degree of reaction &gt;99%). With the aid of a negative punch (L-shape having a slip of 0.05 mm) the segmented transparent pressure-sensitive adhesive tape is liberated from the template and placed on a siliconized PETP film. An L piece made of rigid PVC and having the same dimensions is subjected to a corona discharge pre-treatment and stuck to the segmented pressure-sensitive adhesive layer, so that a self-adhesive 1.5 mm-thick element results. 
     EXAMPLE 2 
     Free-radical substance polymerization as in Example 1. 
     From 
     90 parts by weight of 2-ethylhexyl acrylate 
     10 parts by weight of acrylic acid 
     0.02 parts by weight of Irgacure 500 
     an approx. 8% polymer-monomer solution is prepared. A target viscosity of 1.5 Pa·s is adjusted. At a wavelength of approx. 200-280 nm the photoinitiator disintegrates, the reaction is started, which reaction is stopped upon reaching the viscosity of approx. 1.5 Pa·s by switching off the UV lamps. 
     Subsequently, 0.6% HDDA (hexanediol diacrylate) and 0.8% of the photoinitiator Irgacure 1700 (75%-wt. of 1-hydroxy-2methyl-1-phenyl-propane-1-one and 25% of Bis(2.6-dimethoxy-benzoyl)-2,4,4-trimethylpentyl phosphine oxide) are added to the prepolymer. By adding 1.5%-wt. of micro-hollow glass balls Q-CEL® 300 (U.S. Pat. No. 4,223,067) a filled, dosable and reactive polymer-monomer mixture is obtained, which is stored under absence of oxygen. 
     The L-template of Example 1 is used. The mold is filled only partly, the volume amounts to 0.24 cm 3 . The polymer solution is dosed by means of a piston pump and an aluminium pressure pipe (inner diameter 2 mm) into the cavity between siliconized polyester film and the opening seated thereon (“L mold”). The mold is filled up completely, the volume amounts to 0.35 cm 3 . The filled template is irradiated with a UV lamp for 30 seconds in a chamber which has been rendered inert with nitrogen. 
     Type: UVA fluorescent lamp TL-K40W/10R with reflector, power 40 W, wavelength 315-400 nm, Philips, 
     whereby the segmented adhesive is polymerized and largely cured. Subsequently, the template is rotated through 180° and again irradiated for 15 seconds from the opposite side, 
     whereby the polyacrylate basic body is fully cured (degree of reaction &gt;99%). With the aid of a negative punch (L shape having a slip of 0.05 mm) the segmented, white adhesive tape is liberated from the template and placed on a siliconized PETP film. An L piece made of rigid PVC and having the same dimensions is subjected to a corona discharge pre-treatment and stuck to a segmented pressure-sensitive adhesive layer, so that a 1.0 mm-thick self-adhesive element is obtained. 
     Owing to the increase in density during the transformation from monomer to polymer, volume shrinkage occurs and thereby a slight curvature in the upper surface of the segmented pressure-sensitive adhesive layer. This phenomenon can be compensated with small doses of common expansion agents such as baking powder or Expancel 551DU by the firm of Nobel Industries, Sweden. 
     The process according to the invention and the device provided for performing the same are uncomplicated and useful, and can be realized by economical means. In this respect the invention constitutes an optimum solution to the task presented at the outset. 
     The invention has been described with particular emphasis on the preferred embodiments, but variations and modifications within the spirit and scope of the invention may occur to those skilled in the art to which the invention pertains.