Adhesive composition and coated paper sheets

Abstract: An adhesive composition exhibiting an adequately controlled adhesive bonding strength when paper sheets coated therewith are bonded together and adapted for subsequent separation of the bonded sheets of paper by peeling comprises as essential ingredients: (a) a rubber latex; (b) a silica gel powder having a specified average particle diameter; and (c) a starch, preferably having a specified particle diameter, e.g. wheat starch, each in a specified weight proportion. In addition to good balance between reliability of the adhesive bonding and subsequent peelability, a paper sheet coated with the adhesive composition is protected from decrease in adhesiveness even when the surface is contaminated with a silicone oil and from problems due to blocking when adhesive-coated sheets are stacked one on the other with the adhesive-coated surfaces in direct contact with each other.

Description: The present invention relates to a novel adhesive composition for temporary adhesive bonding of paper sheets or, more particularly, the invention relates to an adhesive composition capable of adhesively bonding sheets of paper and the like with such an adequate adhesive bonding strength by pressing without heating of the adhesive layer that the sheets once adhesively bonded can be separated by peeling without exhibiting excessively high peeling resistance. Along with the revolutionary progress in information-transmission systems, paper-based information-transmission systems, such as mailing of printed matter, is also experiencing a departure from its traditional style. For example, foldable postcards as a system is already an acceptable mailing system under the postal laws of several countries. In this system, a sheet of paper having a size substantially larger than that of regular postcards specified by the postal regulations is printed on its surface and then folded in two or three by the sender of the mail with an object to reduce the sheet size to the regular postcard size and to keep the printed information in confidence while areas on the sheet which come into contact with each other by folding are coated with a special adhesive so that, when the sheet is folded and pressed, the flaps of the folded sheet coming into contact with each other can be adhesively bonded to each other temporarily enabling the foldable postcard to be handled in just the same way of mailing as ordinary postcards and keeping the printed information in confidence. The receiver of the thus folded postcard separates the adhesively bonded flaps by peeling into the unfolded sheet so that the printed information on the areas heretofore concealed by bonding of the flaps is now readable. Similar foldable sheets of paper are now acquiring prevalent use for recording and transmitting computerized information printed out by using various types of printer machines. As is understood from the description given above of the foldable postcard system, the performance of the system depends heavily on the properties of the adhesive with which the foldable postcard is coated on some areas in order that reliable adhesive bonding can be obtained between the flaps of the folded sheet at least while the postcard is on the mailing route but the bonded flaps can be readily opened by the receiver by peeling without an excessively high peeling resistance eventually causing rupture of the paper sheet per se. Specifically, the adhesive used in this application, referred to as a peelable adhesive hereinbelow, must give reliable but temporary adhesion between sheets of paper. It is known empirically that the peeling resistance of paper sheets coated with a peelable adhesive composition and bonded together by pressing should be in the range from 30 to 120 g or, preferably, from 60 to 80 g per 25 mm width although the exact optimum peeling resistance may depend on the quality of the paper. Moreover, the surface of a paper sheet coated with a peelable adhesive composition is sometimes contaminated with a silicone oil in the printing process, in particular, using a non-impact printer. As is well known, silicone oils usually give strong surface releasability to the surface coated or contaminated therewith so that a sheet coated with a peelable adhesive no longer can exhibit reliable adhesion when the surface is contaminated with a silicone oil. It is also important that the surface of a sheet coated with the peelable adhesive is free from problems caused by the phenomenon of so-called blocking which means that, when adhesive-coated surfaces are kept in contact with each other, the surfaces spontaneously become bonded together even without intentional application of a compressive force to effect bonding by means of the pressure-sensitive adhesive. Thus, a peelable adhesive must satisfy the two contradictory requirements of reliability of adhesive bonding and peelability to ensure safe opening of the folded sheet by the receiver or reader of the information. Various types of peelable adhesives have been proposed in the prior art with an object to control or limit the adhesive bonding strength of the adhesive composition using a pressure-sensitive, heat-sensitive or remoistening adhesive as the base adhesive, of which pressure-sensitive adhesives belong to the only practically promising class as compared with the other types. A conventional way to control the adhesive bonding strength of a pressure-sensitive adhesive is to compound the adhesive with a substantial amount of a fine solid particulate material although no practically satisfactory peelable adhesive compositions have yet been obtained by this means. For example, Japanese Patent Kokai 62-158770, -158771, -158772 and -158773 disclose pressure-sensitive adhesive compositions comprising 100 parts by weight as solid of an emulsion of a synthetic rubber such as SBR, NBR and polychloroprene rubber, optionally, in combination with a tackifier and 5 to 40 parts by weight of a fine silica gel powder. These adhesive compositions, however, are not suitable for use as peelable adhesives since a good balance cannot be obtained between reliability in adhesive bonding and subsequent peelability. JP-57192474 describes a pressure-sensitive adhesive having resistance to blocking, heat and abrasion and suitable for an electronic printer which comprises a synthetic resin and non-thermoplastic fine particles with a particle size of 1-20 µm selected inter alia from silica gel and starch. The present invention accordingly has an object to provide a novel and improved peelable adhesive composition capable of giving adhesive-coated paper sheets which can be temporarily but reliably bonded together by pressing leaving subsequent peelability with an adequate peeling resistance and are free from problems due to the adhesion-inhibiting effect of contamination with a silicone oil and due to the phenomenon of blocking. The invention also has an object to provide a sheet of paper coated with such a peelable adhesive composition. The peelable adhesive composition of the present invention comprises, as a mixture: (a) 100 parts by weight as solid of an aqueous emulsion or latex of a rubber; (b) from 30 to 150 parts by weight of particles of silica gel having an average particle diameter in the range from 1 µm to 20 µm; and (c) from 70 to 250 parts by weight of starch the total amount of the components (b) and (c) being in the range from 100 to 400 parts by weight, characterised in that the starch has an average particle diameter larger than that of the silica gel particles and the silica gel has an oil absorption of at least 100 ml/100 g. The starch particles should preferably have an average particle diameter in the range from 5 to 25 µm. Most satisfactory results can be obtained by using wheat starch as the component (c). As is described above, the essential ingredients in the inventive peelable adhesive composition include (a) a rubber emulsion or latex, (b) a silica gel powder and (c) a starch each in a specified amount although it is optional that the adhesive composition is further admixed with various kinds of known additives such as tackifiers and the like. The rubber in the form of an aqueous emulsion or latex as the component (a) is not particularly limited and includes natural rubber, SBR, NBR, polychloroprene rubber and the like conventionally used as a base constituent of pressure-sensitive adhesives. It is preferable, however, that the rubber is a natural rubber modified by the graft polymerization of styrene and methyl methacrylate in respect of resistance against blocking, heat and abrasion of the adhesive-coated surface. The contents of the styrene moiety and methyl methacrylate moiety in the graft-modified natural rubber should preferably be in the range from 2 to 10% by weight and from 10 to 40% by weight, respectively, based on the natural rubber calculated as solid in the latex. The aqueous emulsion or latex containing from 40 to 60% by weight of these rubbers are commercially available and can be used as such or after modification by graft polymerization as the component (a) in the inventive peelable adhesive composition. The component (b) in the inventive adhesive composition is a silica gel powder which should have an average particle diameter in the range from 1 µm to 20 µm or, more preferably, from 1 µm to 5 µm. So-called fumed and precipitated silica fillers such as those commercially available under the tradename of Aerosil and the like can also be used as the component (b) with an object to improve the abrasion resistance of the adhesive layer provided on a paper sheet although the particles thereof are sometimes too fine to be used alone. Another important characteristic parameter of the silica gel powder as the component (b) is the amount of oil absorption which can be determined, for example, according to the testing procedure specified in JIS K 5101. This parameter is particularly important in order to mitigate the adverse influences caused by contamination of the adhesive-coated surface with a silicone oil presumably because the silica gel particles serve to absorb the silicone oil which otherwise causes a heavy decrease in the adhesiveness of the surface. In this regard, the silica gel powder as component (b) should have an oil absorption of at least 100 ml/100 g or, preferably, from 200 to 300 ml/100 g. The amount of the silica gel powder as the component (b) in the inventive adhesive composition should be in the range from 30 to 150 parts by weight or, preferably, from 50 to 100 parts by weight per 100 parts by weight of the rubber latex or emulsion calculated as solid. When the amount of the silica gel powder is too small, the adhesive bonding strength between the bonded surfaces would be too high to allow the bonded sheets to be readily peeled off subsequently in addition to the problem of deficiency of oil absorption capacity to prevent the adverse effect caused by contamination with a silicone oil. When the amount of silica is too large, on the other hand, the adhesive bonding strength is too low, so that the reliability of adhesive bonding is lost so as to cause eventual spontaneous separation of the sheets once bonded. The third of the essential ingredients, i.e. component (c), in the inventive adhesive composition is a starch. Needless to say, starch is a natural product obtained from various kinds of plants and the properties of starches including not only the particle diameter but also morphology of particles depend on the kind of the plant from which the starch is obtained. Specifically, some of starch particles may be coarse or fine and may be angular or spherical. It has been reported that the average particle diameters of starches of various origins are about 30 µm in potato starch, 15 µm in corn starch, 10 µm in wheat starch, 20 µm in tapioca starch and 15 µm in waxy corn starch. Although it is presumable that the performance of a starch powder as component (c) in the inventive adhesive composition depends on various parameters other than the average particle diameter and the morphology of particles, the average particle diameter is the most important parameter according to the results of the detailed experiments conducted by the inventor. Specifically, the starch powder as component (c) should have an average particle diameter in the range from 5 µm to 25 µm or, preferably, from 8 µm to 20 µm. In this regard, starches of corn, wheat, tapioca and the like can be used as component (c) with less preference for potato starch, of which most satisfactory results can be obtained with wheat starch although the exact reason is not well understood. In any event, use of a starch of an excessively large average particle diameter is undesirable because the minimum thickness of the adhesive coating layer must be equal to or larger than the largest particle diameter of the starch particles necessitating a correspondingly large coating amount. Importantly, it was discovered that quite satisfactory results could be obtained when the starch particles as component (c) had an average particle diameter substantially larger than that of the silica gel particles as the component (b). Although the reason therefor is also not well understood, it is presumable that, when the particle diameter of the starch is subtantially larger than that of the silica gel particles, the mimimum thickness of the adhesive layer formed on the surface of a sheet is limited by the diameter of the coarser starch particles while the finer silica gel particles may sink in the adhesive layer between the starch particles and do not appear on the surface of the adhesive layer so that the possible adverse influences caused by silicone oil absorbed by and retained on the surface of the silica gel particles may be further reduced. The amount of the starch as the component (c) in the inventive adhesive composition should be in the range from 70 to 250 parts by weight or, preferably, from 70 to 200 parts by weight or, more preferably, from 120 to 200 parts by weight per 100 parts by weight of the rubber emulsion or latex as the component (a) calculated as solid provided that the total amount of the silica gel powder as the component (b) and the starch as the component (c) is in the range from 100 to 400 parts by weight or, preferably, from 170 to 300 parts by weight per 100 parts by weight of the rubber emulsion or latex as the component (a) calculated as solid in view of the balance between reliability in the adhesive bonding and subsequent peelability. When the total amount of the particulate constituents, i.e. components (b) and (c), in the adhesive composition is too large, the consistency of the composition is too high to be evenly applied on to the surface of a paper sheet. When the total amount is too small, on the other hand, the adhesive bonding strength obtained by using the adhesive composition is too high so that difficulties are encountered in separating by subsequent peeling the sheets of paper once they are adhesively bonded. The peelable adhesive composition of the present invention comprising the above described essential components (a), (b) and (c) can be prepared by merely uniformly blending the components each in a specified amount according to a conventional procedure. It is of course optional that the inventive adhesive composition contains various known additives including tackifiers such as terpene resins and the like each in a limited amount. The peelable adhesive composition of the invention prepared as described above can be applied to at least one surface of paper sheet of various grades by using a conventional coating machine followed by drying with heating so that an adhesive-coated paper sheet suitable for temporary adhesive bonding can be obtained. The coating amount is usually in the range from 2 to 12 g/m² or, preferably, from 4 to 8 g/m² calculated dry. The resulting adhesive-coated paper sheets can be bonded together reliably but in a subsequently peelable fashion when they are laid one on the other with the adhesive-coated surfaces in direct contact with each other and pressed under an appropriate pressure of, for example, at least 20 kg/cm² at room temperature. In the following description, peelable adhesive compositions of the present invention and performance of paper sheets coated therewith are illustrated in more detail by way of examples and comparative examples. The adhesive-coated paper sheets as the test sample were prepared by coating a sheet of woodfree paper having a basis weight of 70 g/m² on one surface with the adhesive composition using a wire-bar coater in a coating amount of 5 g/m² as solid followed by drying for 1 minute at 100 °C and evaluated for the following items in the respectively described testing procedures. Test I. Adhesive bonding strength Two adhesive-coated test sheets were heated for 2 seconds at 180 °C and, after cooling, they were laid one on the other with the adhesive-coated surfaces in direct contact with each other and press-bonded by pressing in a dry sealer under a compressive force of 50 kg/cm² at room temperature. The thus adhesively bonded paper sheets were conditioned by standing for 30 minutes in an atmosphere of 65% relative humidity at room temperature and were then subjected to determination of the peeling resistance using an automatic tensile tester (Model Autograph AGS 50, manufactured by Shimadzu Seisakusho Co.) by pulling in a 90° direction at a pulling velocity of 30 cm/minute at 20 °C . The results of this peeling-resistance test were given in g per 25 mm width of the test sample. The results were evaluated to be excellent when the peeling resistance was from 60 to 80 g/25 mm, good when the peeling resistance was from 30 to 60 g/25 mm or from 80 to 120 g/25 mm, fair when the peeling resistance was from 15 to 30 g/25 mm or from 120 to 180 g/25 mm and poor when the peeling resistance was smaller than 15 g/25 mm or larger than 180 g/25 mm. Test II. Adhesive bonding strength after contamination with silicone oil Two adhesive-coated test sheets after heating for 2 seconds at 180 °C and cooling in the same manner as in Test I were each coated with a dimethyl silicone oil in a coating amount of 0.5 g/m² using a printability testing machine and then adhesively bonded together also in the same manner as in Test I except for an increase of the pressure in the dry sealer to 100 kg/cm² Thereafter, the thus adhesively bonded sheets were subjected to determination of peeling resistance in the same manner as in Test I and evaluated according to the same criteria as above. Test III. Resistance against blocking Two adhesive-coated test sheets as prepared were laid one on the other with the adhesive-coated surfaces in direct contact with each other and kept standing for 30 minutes at 50 °C under a load of 500 g/cm² followed by determination of peeling resistance in the same manner as above. The test sample was evaluated as satisfactory with respect to resistance against blocking when the peeling resistance determined in the above described manner did not exceed 10 g/25 mm or, desirably, 5 g/25 mm. Test IV. Abrasion resistance at elevated temperature The test was performed according to the procedure specified in JIS P 8136 at a temperature of 150 °C by rubbing with a metal probe under a load of 200 to 500 g/cm². The test sample was evaluated as satisfactory when no changes were found on the surface of the adhesive layer after 20 rubbing cycles in the above described manner. When defects were found on the surface of the adhesive layer after rubbing a number of times smaller than 20, the number of rubbing cycles after which defects were first found, was recorded. Example 1. A graft-modified natural rubber latex was prepared by conducting emulsion polymerization of 4 parts by weight of styrene and 16 parts by weight of methyl methacrylate in a natural rubber latex in an amount of 100 parts by weight calculated as solid. This rubber latex was admixed with an aqueous emulsion of a terpene resin as a tackifier in an amount of 5 parts by weight as solid to give a base adhesive in the form of an aqueous emulsion, of which the solid content was adjusted to 50% by weight. Into 100 parts by weight as solid of this base adhesive were added 50 parts by weight of a silica gel powder having an average particle diameter of about 4 µm and an oil absorption of 240 ml/100 g and 150 parts by weight of wheat starch having an average particle diameter of about 15 µm to prepare a peelable adhesive composition. The results of the evaluation tests of the thus prepared peelable adhesive composition are shown in Table 1. Example 2. The experimental procedure was exactly the same as in Example 1 described above except that the amount of the wheat starch was increased to 250 parts by weight. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Example 3. The experimental procedure was exactly the same as in Example 1 described above except that the amounts of the silica gel powder and wheat starch were increased to 100 parts by weight and 200 parts by weight, respectively. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Example 4. The experimental procedure was exactly the same as in Example 1 described above except that the amounts of the styrene and methyl methacrylate used in the graft-polymerization for the modification of the natural rubber latex were increased to 8 parts by weight and 25 parts by weight, respectively. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Example 5. The experimental procedure was exactly the same as in Example 4 described above except that the amount of the wheat starch was increased to 250 parts by weight. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Example 6. The experimental procedure was exactly the same as in Example 4 described above except that the amounts of the silica gel powder and wheat starch were increased to 100 parts by weight and 200 parts by weight, respectively. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Comparative Example 1. The experimental procedure was exactly the same as in Example 1 described above except that the amounts of the silica gel powder and wheat starch were decreased to 30 parts by weight and 60 parts by weight, respectively. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Comparative Example 2. The experimental procedure was exactly the same as in Example 1 described above except that the amount of the silica gel powder was increased to 60 parts by weight and the amount of the wheat starch was decreased to 30 parts by weight. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Comparative Example 3. The experimental procedure was exactly the same as in Example 1 described above except that the amounts of the silica gel powder and wheat starch were increased to 200 parts by weight and 250 parts by weight, respectively. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Example 7. The experimental procedure was substantially the same as in Example 1 described above except that the base adhesive was prepared by admixing 100 parts by weight as solid of an aliphatic petroleum-based hydrocarbon resin in the form of an aqueous emulsion containing 46% by weight of solid (Nopco B-1, a product by Sun Nopco Co.) as a tackifier with 100 parts by weight as solid of an NBR latex of 40% nitrile ratio containing 41% by weight of solid (Nipol 516, a product by Nippon Zeon Co.). The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Example 8. The experimental procedure was substantially the same as in Example 7 described above except that the NBR latex was replaced with the same amount as solid of an SBR latex of 55% styrene ratio containing 41% by weight of solid (Nipol LX 204, a product by Nippon Zeon Co.). The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Example 9. The experimental procedure was substantially the same as in Example 7 described above except that the NBR latex was replaced with the same amount as solid of a polychloroprene rubber latex containing 50% by weight of solid (Denka Chloroprene LA-50, a product by Denki Kagaku Kogyo Co.). The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Example 10. The experimental procedure was substantially the same as in Example 1 described above except that the wheat starch was replaced with the same amount of potato starch having an average particle diameter of about 30 µm. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Example 11. The experimental procedure was substantially the same as in Example 10 described above except that the amount of the potato starch was increased to 250 parts by weight. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Example 12. The experimental procedure was substantially the same as in Example 10 described above except that the amounts of the silica gel powder and the potato starch were increased to 100 parts by weight and 200 parts by weight, respectively. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Example 13. The experimental procedure was substantially the same as in Example 1 described above except that the silica gel powder having an average particle diameter of 4 µm was replaced with the same amount of another silica gel powder having an average particle diameter of about 10 µm and an oil absorption of 220 ml/100 g and the wheat starch was replaced with the same amount of tapioca starch having an average particle diameter of about 20 µm. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Example 14. The experimental procedure was substantially the same as in Example 13 described above except that the amount of the tapioca starch was increased to 250 parts by weight. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Example 15. The experimental procedure was substantially the same as in Example 13 described above except that the amounts of the silica gel powder and the tapioca starch were increased to 100 parts by weight and 200 parts by weight, respectively. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Comparative Example 4. The experimental procedure was substantially the same as in Example 1 described above except that the silica gel powder having an average particle diameter of 4 µm was replaced with the same amount of another silica gel powder having an average particle diameter of about 10 µm and an oil absorption of 220 ml/100 g and 150 parts by weight of the wheat starch having an average particle diameter of 15 µm was replaced with the same amount of another wheat starch having an average particle diameter of 4 µm. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Comparative Example 5. The experimental procedure was substantially the same as in Comparative Example 4 described above except that the amount of the wheat starch was decreased to 50 parts by weight. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1. Comparative Example 6. The experimental procedure was substantially the same as in Comparative Example 4 described above except that the amount of the silica gel powder was increased to 100 parts by weight and the amount of the wheat starch was decreased to 20 parts by weight. The results of the evaluation tests of the thus prepared peelable adhesive composition are also shown in Table 1.

Claims: An adhesive composition which comprises, as a mixture: (a) 100 parts by weight as solid of an aqueous emulsion or latex of a rubber; (b) from 30 to 150 parts by weight of particles of silica gel having an average particle diameter in the range from 1 µm to 20 µm; and (c) from 70 to 250 parts by weight of starch the total amount of the components (b) and (c) being in the range from 100 to 400 parts by weight, characterised in that the starch has an average particle diameter larger than that of the silica gel particles and the silica gel has an oil absorption of at least 100 ml/100 g. An adhesive composition as claimed in claim 1 in which the particles of the starch have an average particle diameter in the range from 5 to 25 µm. An adhesive composition as claimed in claim 2 in which the starch is wheat starch. An adhesive composition as claimed in any one of the preceding claims in which the rubber is a natural rubber modified by graft polymerization of styrene and methyl methacrylate. An adhesive composition as claimed in claim 4 in which the natural rubber modified by graft polymerization of styrene and methyl methacrylate contains from 2 to 10% by weight from 10 to 40% by weight of the styrene moiety and methyl methacrylate moiety, respectively, based on the moiety of natural rubber. An adhesive composition as claimed in any one of the preceding claims in which the silica gel has an oil absorption of from 200 to 300 ml/100 g. An adhesive-coated paper sheet for temporary adhesive bonding which is a sheet of paper coated on at least one surface with an adhesive composition which comprises, as a mixture: (a) 100 parts by weight as solid of an aqueous emulsion or latex of a rubber; (b) from 30 to 150 parts by weight of particles of silica gel having an average particle diameter in the range from 1 µm to 20 µm; and (c) from 70 to 250 parts by weight of starch the total amount of the components (b) and (c) being in the range from 100 to 400 parts by weight, characterised in that the starch has an average particle diameter larger than that of the silica gel particles and the silica gel has an oil absorption of at least 100 ml/100 g. An adhesive-coated paper sheet for temporary adhesive bonding as claimed in claim 7 in which the coating amount of the adhesive composition is in the range from 2 to 12 g/m² dry. An adhesive-coated paper sheet as claimed in claim 7 or claim 8 wherein the adhesive composition is as claimed in any one of claims 2 to 6.

Inventors: SAKAI TSUTOMU, SAKAI, TSUTOMU
Applicants: TOPPAN MOORE KK, TOPPAN MOORE COMPANY, LTD.
Application year: 1991
Main classification: C09J121
IPC codes: C09J103, C09J7, C09J121, C09J151