AN AQUEOUS ADHESIVE WHICH CAN BE APPLIED TO A SUBSTRATE BY SCREEN PRINTING, A METHOD FOR PREPARING SAME AND THE USE OF SUCH AN ADHESIVE

Abstract: The invention relates to a aqueous adhesive which can be applied to a substrate by screen printing, whereby said adhesive essentially comprises the following components: rubber, a cellulose derivative, and a water-miscible co-solvent, and also to a method for preparing such an aqueous adhesive and to the use of such an aqueous adhesive on a substrate, in particular a blister pack.

Description: An aqueous adhesive which can be applied to a substrate by screen printing, a method for preparing same and the use of such an adhesive. The present invention relates to an aqueous adhesive which can be applied to a substrate by screen printing, a method for preparing said aqueous adhesive, and furthermore to the use of such an aqueous adhesive on a substrate, in particular a blister pack. An aqueous adhesive of this type is known from Japanese Patent Application 62048780 (Toa Gosei). This Japanese patent application relates to water-dispersable, pressure-sensitive adhesive compositions which can be used for being applied to substrates, such as films, textiles and paper by screen printing. Adhesive compositions of this type are obtained by polymerizing in an aqueous medium a monomer mixture consisting of (meth)acrylic acid esters. Polymerization in an aqueous medium requires the use of emulsifiers or dispersing agents. The polymers formed have a relatively high molecular weight and are dispersable, but they are not water-soluble any more, as a result of which it is difficult to obtain a uniform distribution over the substrate. In addition to that the use of an adhesive composition comprising polymers having a high molecular weight in practice often leads to clogging of the screen to be used in screen printing. Due to the fact that partial polymerization of the adhesive already takes place on the screen, a skin is formed, as a result of which the mesh of the screen wire becomes clogged, which leads to incomplete application of the aqueous adhesive to the substrate. In addition to that the aqueous adhesive according to said Japanese patent application produces a strongly and permanently tacky surface after being applied to a substrate, which is undesirable in practice. Accordingly there is a need for an aqueous adhesive which can be applied to a substrate by screen printing and which does not present any problems when the adhesive is squeezed through the screen, such as clogging of the screen and the formation of surface faults of the adhesive on the substrate. In addition to that the aqueous adhesive must exhibit an adequate adhesive power and be capable of being applied to a substrate by screen printing at great speed, which makes demands of the viscosity and solid matter content of the adhesive. The aqueous adhesive as referred to in the introduction is according to the present invention characterized in that said adhesive essentially comprises the following components: (A) rubber, (B) a cel l ul ose deri vati ve , and (C) a water-mi sci bl e co-sol vent . After the aqueous adhesive according to the present invention has been applied to a substrate and subsequently been dried by using drying in the air, a drying tunnel or another drying treatment as will be apparent to those skilled in the art, the surface of the dried aqueous adhesive will be substantially non-tacky. A bond is obtained by pressing two substrates provided with films of a dried aqueous adhesive together under the application of a force, for example by means of mechanical pressure rollers or a vacuum. The amount of rubber (A) in the aqueous adhesive according to the present invention is 40-60% by weight, based on the total weight of the aqueous adhesive. When the amount of rubber (A) is more than 60% by weight, the viscosity of the resulting adhesive will decrease undesirably, as a result of which the aqueous adhesive will present problems when being processed by screen printing. When the amount of rubber is less than 40% by weight, the resulting aqueous adhesive will contain insufficient active material , in particular rubber substances, as a result of which the desired adhesive power will be insufficient. Such a low rubber content will furthermore lead to absorption of the adhesive into the substrate, as a result of which the contact percentage between the substrate layers to be bonded will be too low. In addition to that it has become apparent that surface faults, such as blistering, will occur in the adhesive film applied to the substrate, which surface faults have an adverse effect on the adhesive power and on the outward appearance. The amount of cellulose derivative (B) in the aqueous adhesive according to the present invention is 0.2-8% by weight, based on the total weight of the aqueous adhesive. When the amount of cellulose derivative (B) is more than 8% by weight, the amount of solid matter in the resulting aqueous adhesive will be too high, so that screen printing of the aqueous adhesive is no longer possible, in particular because of the strongly increased viscosity of the adhesive. Such a high percentage of cellulose derivative (B) will moreover lead to a reduced adhesive power of the resulting aqueous adhesive. When the amount of cellulose derivative (B) is less than 0.2% by weight, the viscosity of the eventual aqueous adhesive will be too low, as a result of which the adhesive cannot be satisfactorily distributed over the screen with the scraping means, for example a doctorblade or a squeegee. In addition to that the presence of a cellulose derivative prevents the aqueous adhesive from drying on the screen already, whilst it is furthermore prevented that film formation takes place too quickly. The amount of water-miscible co-solvent (C) in the aqueous adhesive according to the present invention is 4-20% by weight, based on the total weight of the aqueous adhesive. When the amount of water-miscible co-solvent (C) is more than 20% by weight, the viscosity of the resulting aqueous adhesive will be too high, as a result of which screen printing of the aqueous adhesive is no longer possible. In addition to that fiocculation may occur. When the amount of water-miscible co¬ solvent (C) is less than 4% by weight, drying of the adhesive will become problematic. It is preferred to use natural rubber as the rubber component (A). It is also possible, however, to use a mixture of natural rubber and synthetic rubber as the rubber component (A) in the aqueous adhesive according to the present invention, but the natural rubber content of such a mixture must be at least 70%. When the amount of natural rubber is less than 70%, the adhesive power of the resulting aqueous adhesive will be insufficient. It is preferred to use an aqueous natural rubber dispersion in the aqueous adhesive according to the present invention, in particular a aqueous natural rubber dispersion with a low ammonia content. A low ammonia content is preferred because the emission of ammonia into the atmosphere will be low when such a rubber dispersion is used, as a result of which there will be no problems in connection with the legal provisions with regard to the maximum allowable ammonia content in closed rooms when the adhesive according to the present invention is processed by screen printing. According to the present invention it is preferred to use methyl hydroxy ethyl cellulose as the cellulose derivative (B) . Methyl hydroxy ethyl cellulose is a molecule surrounded by a water jacket, which prevents the dispersion of the aqueous adhesive from drying on the screen already, because such a cellulose derivative "retains " the water. In addition to that methyl hydroxy ethyl cellulose exhibits strong swelling by nature and prevents the rubber present in the adhesive from forming a film too quickly. Methyl hydroxy ethyl cellulose is marketed by Hoechst, Frankfurt am Main, Germany, under the brand name Tylose MH (brand). In another embodiment of the present invention carboxymethyl cellulose is used as the cellulose derivative (B). According to the present invention it is preferred to use an organic compound having at least one OH-functionality as the water- miscible co-solvent (C) . An organic compound of this type is miscible with water, keeps the rubber in dispersion and in addition must not have an adverse effect on the rubber, such as swelling. It is preferred to use an organic compound which forms an azeotrope with the aqueous adhesive. The advantage of this is that the co-solvent evaporates from the adhesive simultaneously with the water present in said adhesive, thus ensuring the stability of the adhesive on the screen. In addition to that the formation of an azeotrope will lead to an even evaporation speed. It is preferred to use a polyvalent alcohol, in particular hexylene glycol, in the aqueous adhesive according to the present invention. In addition to that it is also possible to use other organic compounds which form an azeotrope with the aqueous adhesive, such as compounds having at least one OH- functionality and one or more functional groups selected from ketone and aldehyde groups. The aqueous adhesive according to the present invention preferably comprises an agent for regulating the pH-value, in particular AMP 90 (marketed by BASF of Germany). The pH-value of the resulting aqueous adhesive must vary between 8.5 - 12. With a pH-value outside this range fiocculation will occur. The addition of such an agent moreover has a buffering effect on the aqueous adhesive. The aqueous adhesive according to the present invention may furthermore comprise agents selected from the group consisting of antistatic agents, anti-oxidants, flow control agents, pigments. The method for preparing the aqueous adhesive according to the present invention is characterized in that an aqueous solution containing a cellulose derivative is mixed with a solution containing a co-solvent, after which the rubber component is added to the solution thus obtained. It is to be preferred to add the rubber component to the solution of the cellulose derivative and the co-solvent, because an aqueous adhesive forming a homogeneous solution is thus obtained. When first the rubber component is added to the aqueous solution containing a cellulose derivative, after which a solution containing a co-solvent is mixed therewith, the rubber component will adhere to the cellulose derivative, as a result of which the resulting aqueous adhesive will have an unsatisfactory homogeneity. Moreover, it is to be preferred to add the agent for regulating the pH-value to the solution containing the co¬ solvent. The present invention furthermore relates to a substrate coated with an aqueous adhesive by screen printing, in particular a blister pack, whereby the aqueous adhesive according to the present invention has been applied thereon by screen printing. A blister pack provided with an adhesive applied by screen printing is known from European Patent Application 0 642 988. According to said European patent application an adhesive is applied to the blister pack by screen printing, but there is no data available on the composition of the adhesive. Moreover, the adhesive according to European Patent Application 0 642 988 is an adhesive which provides a bond by using a high temperature and a high pressure, whilst the aqueous adhesive according to the present invention provides a bond by pressing together two substrates provided with dried films of aqueous adhesive. The present invention will be explained in more detail hereafter by means of an example. Preparation of the aqueous adhesive according to the present invention. An aqueous solution containing a cellulose derivative is prepared by adding 2 kg of Tylose MH (brand) to 30 kg of water under continuous stirring at a speed of 1500 revolutions per minute. After the full amount of Tylose MH (brand) has been added, the resulting solution is stirred for another 15 minutes, using a speed of 1750 revolutions per minute, after which a homogeneous solution is obtained. The temperature during mixing is lower than 35 °C. Then 6 kg of AMP 90 is dissolved in 14 kg of hexylene glycol in a separate vessel under continuous stirring, using 1500 revolutions per minute. The processing temperature is lower than 30 °C and a homogeneous solution is obtained. Then the solution containing hexylene glycol is mixed with the solution containing Tylose MH (brand) under continuous stirring at a speed of 1000 revolutions per minute for 10 minutes. The processing temperature is lower than 30 °C. Finally 50 kg of natural rubber are added. After stirring at a speed of 1500 revolutions per minute for about 10 minutes the aqueous adhesive according to the present invention is obtained as a homogeneous solution. The homogeneous solution thus prepared is then applied to a blister pack by screen printing.

Claims: CLAIMS 1. An aqueous adhesive which can be applied to a substrate by screen printing, characterized in that said adhesive essentially comprises the following components: (A) rubber, (B) a cellulose derivative, and (C) a water-miscible co-solvent. 2. An aqueous adhesive according to claim 1, characterized in that the amount of rubber (A) is 40-60% by weight, that the amount of cellulose derivative (B) is 0.2-8% by weight, and that the amount of water-miscible co-solvent (C) is 4-20% by weight, based on the total weight of the aqueous adhesive. 3. An aqueous adhesive according to claims 1 - 2, characterized in that the rubber component (A) used is natural rubber. 4. An aqueous adhesive according to claims 1 - 2, characterized in that the rubber component (A) used is a mixture of natural rubber and synthetic rubber, whereby the natural rubber content of the mixture is at least 70%. 5. An aqueous adhesive according to claims 1 - 4, characterized in that an aqueous natural rubber dispersion is used. 6. An aqueous adhesive according to claim 5, characterized in that an aqueous natural rubber dispersion having a low ammonia content is used. 7. An aqueous adhesive according to claims 1 - 2, characterized in that the cellulose derivative (B) used is methyl hydroxy ethyl cellulose. 8. An aqueous adhesive according to claims 1 - 2, characterized in that the cellulose derivative (B) used is carboxymethyl cellulose. 9. An aqueous adhesive according to claims 1 - 2, characterized in that the water-miscible co-solvent (C) used is an organic compound having at least one OH-functionality. 10. An aqueous adhesive according to claim 9, characterized in that the water-miscible co-solvent (C) used is a compound which forms an azeotrope with the aqueous adhesive. 11. An aqueous adhesive according to claims 9 - 10, characterized in that a polyvalent alcohol is used. 12. An aqueous adhesive according to claims 9 - 10, characterized in that the water-miscible co-solvent (C) used is an organic compound having at least one OH-functionality and one or more functional groups selected from ketone and aldehyde groups. 13. An aqueous adhesive according to claim 11, characterized in that hexylene glycol is used. 14. An aqueous adhesive according to claims 1 - 13, characterized in that said aqueous adhesive furthermore comprises an agent for regulating the pH-value. 15. An aqueous adhesive according to claim 14, characterized in that AMP 90 is used. 16. An aqueous adhesive according to claims 1 - 15, characterized in that said aqueous adhesive furthermore comprises agents selected from the group consisting of antistatic agents, anti-oxidants, flow control agents, pigments. 17. A method for preparing an aqueous adhesive according to any one of the preceding claims, characterized in that an aqueous solution containing a cellulose derivative is mixed with a solution containing a co-solvent, after which said rubber component is added to the solution thus obtained. 18. A method according to claim 17, characterized in that said agent for regulating the pH-value is added to the solution containing a co-solvent. 19. A method according to claims 17 - 18, characterized in that the pH-value of the solution thus obtained varies between 8.5 - 12. 20. A substrate coated with an aqueous adhesive by screen printing, characterized in that an aqueous adhesive according to any one of the preceding claims 1 - 16 has been applied to said substrate. 21. A blister pack provided with an aqueous adhesive by screen printing, characterized in that an aqueous adhesive according to any one of the preceding claims 1 - 16 has been applied to said blister pack.

Inventors: LINSSEN RONALDUS JOSEPHUS MARI, WOUTERS GERARDUS WILHELMUS, LINSSEN, RONALDUS, JOSEPHUS, MARIE, WOUTERS, GERARDUS, WILHELMUS
Applicants: LIGRA B V, LINSSEN RONALDUS JOSEPHUS MARI, WOUTERS GERARDUS WILHELMUS, LIGRA B.V., LINSSEN, RONALDUS, JOSEPHUS, MARIE, WOUTERS, GERARDUS, WILHELMUS
Application year: 1996
Main classification: C09J121
IPC codes: B65D75, C09J121