A METHOD OF JOINING TOGETHER PAPER

Abstract: A method of joining together at least two paper sheets (1, 2). The method comprises the steps of moistening each paper sheet (1, 2) with a determined amount of water within the region of the intended join (9); placing the paper sheets together with the moistened regions in abutment with one another; and processing the moistened regions of the combined paper sheets in an ultrasound welding device (7, 8) such that at least 80 percent by weight of the water applied will be removed. The invention also relates to apparatus for carrying out the method and to a join produced by means of said method.

Description: A METHOD OF JOINING TOGETHER PAPER The present invention relates to a method of joining together at least two paper sheets, a device for carrying out the method, and a join produced by means of the method. Paper sheets are joined together at present with the aid of different adhesives or fasteners, such as staples. It is also known to join paper together mechanically, by deforming the paper; c.f. coffee filter paper, for instance. Joins of this latter nature are beneficial from an environmental aspect, since paper products that are joined together in this way can be recycled in their existing state, without needing to first remove glue residues or metal elements. Joins of this nature that can be effected easily also constitute a cost saving in comparison with conventional joins, since distinct from conventional joins neither glue nor metal elements need be used. The object of the present invention is precisely one of producing paper joins in the absence of glue or metal elements . This object is achieved in accordance with the invention by means of a method of joining together at least two paper sheets which is characterised by the steps of moistening or wetting each paper sheet with a determined amount of water in the region of the intended join, placing the sheets together with the moistened regions in abutment with one another, and processing the moistened regions of the thus combined paper sheets in an ultrasound welding device such as to remove at least 80 percent by weight of the added water. Since the only substance added to the paper sheets that are joined together in this way is water, the paper can be recycled directly after use. The method can also be carried out in conventional apparatus in a simple and cost-effective manner. In one preferred embodiment, the paper sheets are calendered prior to being processed in the ultrasound welding device. Appropriate moistening of the paper sheets is determined empirically, by moistening the paper sheets to different degrees of wetness, although under the same conditions in other respects, wherewith the strength of the join is determined for each degree of moistening. An appropriate degree of processing in the ultrasound welding device is then determined empirically, by processing paper sheets that have been moistened to the same degree with different energy inputs to said ultrasound welding device, wherewith the strength of the join is determined for each energy input value . The invention also relates to apparatus for joining together at least two paper sheets in the form of two running paper webs taken from storage reels, said apparatus being characterised by a moistening unit, paper-web combining means, and an ultrasound welding device, these apparatus components being disposed downstream of one another in the direction of web movement, and also by means for feeding the paper webs past the moistening unit and through the web combining means and the ultrasound welding device . In one preferred embodiment, the web combining means is a calendering device. The invention also relates to a join which joins together at least two paper webs in the absence of a binding agent, said join being characterised in that it includes regions that have a greaseproof paper-like structure and a thickness smaller than 50%, preferably smaller than 30%, of the combined thickness of said webs . The invention will now be described with reference to the accompanying drawings, in which Fig. 1 is a schematic illustration of inventive apparatus for joining paper sheets together; Fig. 2 illustrates schematically and in perspective part of a paper web comprised of two paper sheets that have been joined together in the apparatus shown in Figure 1; Fig. 3 is a graph showing the tensile strength and moistening of the paper sheets as a function of the speed at which the paper sheets are advanced; Fig. 4 is a graph showing the tensile strength as a function of ultrasound pressure in respect of different feed speeds and degrees of wetness; and Fig. 5 is a graph showing the tensile strength as a function of wetness. The apparatus shown in Figure 1 for joining together two paper webs 1, 2 taken from storage reels 3, 4 includes a moistening or wetting unit 5, a calendering device 6, an ultrasound welding unit 7, and an anvil roll or counterpressure roll 8, these components being disposed in the above order sequentially in the feed direction of the webs 1 , 2. The modus operandi of the apparatus is as follows: The paper webs are wetted with a determined amount of water as they pass the moistening unit 5. The moistened paper webs 1 , 2 then pass through a calendering device 6 with the moistened regions of the webs in mutual abutment. The thus combined webs 1, 2 then pass through the nip defined by the horn of the ultrasound unit 7 and the anvil roll 8. It has surprisingly been found that the two paper webs are mutually joined in those regions moistened by the unit 5, after having passed the ultrasound welding unit and the anvil roll. Figure 2 shows in perspective a piece that has been cut from the webs 1, 2 subsequent to passing the ultrasound welding unit 7. In the illustrated example, the anvil roll includes rows of projections that form rows of mutually connected regions 10 in the resultant join 9, as evident from Figure 2. The mutually joined paper webs are very thin in the regions 10, and have a so-called greaseproof paper-like structure. The strength of the join obtained is contingent on the extent to which the paper has been moistened and also on the mechanical energy applied in the ultrasound processing operation. It would appear that the major part of the join 9 is obtained by chemical bonds (of which hydrogen bonds probably form a predominant part) , although the join will also probably include mechanical bonds. It can be assumed from the greaseproof paper-like structure in the regions 10 that the ultrasound horn disintegrates the paper fibres within the regions of the projections on the anvil roll, such as to obtain short fibres in said regions. The presence of water and the heat generated by the mechanical working process also constitute necessary conditions for obtaining a join. Because the manner in which the join between the paper webs 1, 2 is actually established is not properly understood at present, it is impossible at this moment in time to optimise the degree to which the paper should be moistened and the amount of energy that shall be supplied. It is known, however, that excessive moistening or wetting of the paper and no wetting at all will not result in a join when the paper is subjected to said ultrasound working process. It has also been found that an effective join can be obtained when the moisture content of the paper webs is as low as 5% and that no join could be obtained at a web moisture content of 50%. The moisture content of the webs is measured by weighing the join region both before and after applying water and is constituted by the weight of the water applied divided by the weight of the paper prior to application of the water, multiplied by one-hundred. The conditions in which a join of optimum strength is obtained must therefore be determined empirically. Figures 3 and 4 illustrate how the degree of moisturisation and energy input are determined in order to obtain a joint of maximum strength between two paper webs comprised of a paper having a weight per unit area of 60 g/m 2 designated CREPE PAPER WHITE and manufactured by Arjo Wiggins, Paris, France. These paper webs were allowed to run through apparatus constructed in accordance with Figure 1. The moisturising unit was designed to deliver to the passing paper webs a determined amount of water per unit of time. The calendering pressure was 0.5 bar, which is an optimum pressure for this type of paper. The calendering process is intended to facilitate absorption of surface water into the paper. The ultrasound welding unit comprised a unit from Dukane Corp., Illinois, USA, with the designation 20A2000. The ultrasound horn had a constant amplitude of 15 μm during all tests. The amount of energy supplied to the ultrasound welding device was varied, by varying its counterpressure (UL pressure) . The strength of the joins obtained was measured by means of SCAN P44:81. The strength of the join and the degree of moisturisation (g/m 2 ) was first measured as a function at which the paper webs were advanced, at constant UL-pressure. The moisturising unit delivered a constant amount of water per unit of time, causing the extent to which the papers were moisturised to fall with increasing paper- feed speeds. The result of these tests is shown in Figure 3. The curve F denotes join strength (N) while the curve B denotes the degree of moisturisation (g/m 2 ) . In a first test series, the paper webs were each wetted with 0.82 g water/min over a width of 0.025 m and the web-feed speed was increased in increments from 5-20 m/min. The strength of the resultant join was measured at each web-feed speed. As will be evident from Figure 3, the greatest join strength was obtained at a web-feed speed of 11 m/min and a degree of moisturisation of 2.98 g/m 2 . In a second test series using web-feed speeds in the range of 18-30 m/min, the paper webs were wetted with 1.66 g water/min. At this speed range, the strongest join was obtained at the web speed of 22 m/min and the moisturising degree of 3.02 g/m 2 . In a third test series using web-feed speeds in the range of 28-40 m/min, the webs were wetted with 2.56 g water/min. In this speed range, the strongest join was obtained at a web speed of 31 m/min and a moisturising degree of 3.30 g/m 2 . In a fourth test series using web-feed speeds in the range of 38-50 m/min, the webs were wetted with 3.51 g of water/min. The strongest join was obtained in this speed range at a web speed of 42 m/min and a moisturising degree of 3.34 g/m 2 . The test series illustrated in Figure 3 illustrate that the strength of the join obtained is dependent on the extent to which the paper is wetted, i.e. the degree of moisturisation. Figure 4 illustrates a number of tests in which the UL pressure, by which is meant the counterpressure of the ultrasound horn, was varied while holding the degree of moisturisation of the paper webs and the speed at which said webs were fed through said welding device constant . As in the earlier case, the curves F and B denote join strength and degree of moisturisation respectively, while the curve UL shows the UL pressure . As will be evident from Figure 4, the strongest join, in excess of 7 N, was obtained at web-feed speeds of 11 m/min, 22 m/min and 31 m/min at respective UL pressures of 1.6 bar, 2.4 bar and 1.8 bar, and that a join strength of 6 N was obtained at a web-feed speed of 42 m/min and at a UL pressure of 3.6 bar. The aforesaid UL pressures corresponded to a welding unit power of 0.2 kW, 0.217 kW, 0.221 kW and 0.408 kW at respective web-feed speeds. It is thus evident from the tests illustrated in Figure 4 that the strength of the join is also dependent on the amount of energy delivered. It will also be evident that the energy delivered by the welding unit to the paper webs is highly significant to the strength of the join - the strength of the join decreases when the energy delivered is too low and too high. There is no explanation why this is so, although it may be possible that when too little energy is delivered the chemical reactions that create the chemical bonds (these bonds probably corresponding to the major part of the strength of the join) are too slow, whereas excessive energy will cause the reactions to take place too quickly, or not at all. It is also possible that some of the resultant bonds in the join are broken when mechanical working is too intensive. The described tests, or trials, show that the conditions which will provide a maximum join strength can be determined empirically relatively easily. Such empirical trials can also be applied to determine conditions under which desired join strengths that are weaker than the maximum join strength can be obtained. An example in this respect is when it is desired to obtain joins that can be easily opened. The aforedescribed method thus enables joins of mutually different strengths to be readily made. Figure 5 shows two curves I and II which illustrate the join strength as a function of the moisture content in the join region of the paper webs . The webs were advanced at a speed of 30 m/min and the UL pressure was 1.2 bar in respect of curve I and 1.8 bar in respect of curve II. It will be evident from the Figure that good joins can be achieved within a relatively broad moisture-content range. When carrying out the described method, it has been found important to remove the major part of the water applied to the join region. For instance, not more than 20 percent by weight of the water applied to the paper may remain in the join region after the webs have passed the ultra-sound welding device. The thickness of the greaseproof paper-like regions 10 in the join have been measured and found to be less than 30% of the original paper webs . It will be understood that the described embodiment of the invention can be modified within the scope of the invention. For instance, other types of ultrasound welding devices than the described device can be used. The paper sheets may, of course, be stationary and the moistening unit and welding device may be movable. Furthermore, other types of paper moistening or wetting devices may be used. The method can also be applied to join together more than two paper sheets. Other paper than the paper used in the trials may, of course, be joined together in accordance with the invention. The invention is therefore restricted solely by the contents of the following Claims.

Claims: CLAIMS 1. A method of joining together at least two paper sheets (1, 2) , comprising the steps of moistening each paper sheet (1, 2) with a determined amount of water within the region of the intended join (9) ; placing the paper sheets together with the moistened regions in abutment with one another; and processing the moistened regions of the combined paper sheets in an ultrasound welding device (7, 8) such that at least 80 percent by weight of the water applied will be removed. 2. A method according to Claim 1, characterised by wetting said join regions so as to obtain a moisture content of between 5-50 percent by weight. 3. A method according to Claim 1 or 2 , characterised by calendering the paper sheets (1, 2) prior to processing in the ultrasound welding device. 4. A method according to Claim 3, characterised by carrying out the method while moisturising (B) the paper sheets to different extents but under the same conditions in other respects, such as to determine appropriate moisturisation of the paper sheets (1, 2) empirically, and measuring the strength of the join (F) for each degree of moisturisation. 5. A method according to Claim 4, characterised by processing the paper sheets at different energy inputs (UL) to the ultrasound welding device, such as to determine an appropriate degree of processing in said welding device (7, 8) empirically, and measuring the join strength (F) for each energy input value . 6. Apparatus for joining together at least two paper sheets (1, 2) in the form of two running paper webs taken from storage reels (3, 4), characterised by a moisturising unit (5) , means (6) for combining the paper webs (1, 2) , and an ultrasound welding device (7, 8) , said apparatus components being disposed mutually sequentially in the direction of web movement, and by means of feeding the paper webs past the moisturising unit and through the web-combining means and the ultrasound welding device. 7. Apparatus according to Claim 6, characterised in that the web combining means (6) is a calendering arrangement. 8. A join (9) which joins together at least two paper webs (1, 2) and which lacks binding means, characterised in that the join (9) includes regions (10) of a grease-proof paperlike structure and a thickness which is smaller than 50%, preferably smaller than 30%, of the combined thickness of the paper webs .

Inventors: BERDAL OVE, ZAWADZKI WALDEMAR, BERDAL, OVE, ZAWADZKI, WALDEMAR
Applicants: BERDAL OVE, MOELNLYCKE HEALTH CARE AB, ZAWADZKI WALDEMAR, BERDAL, OVE, ZAWADZKI, WALDEMAR
Application year: 1998
Main classification: B31F5
IPC codes: B31F5, D21G1, D21H27