IMPROVEMENTS TO THE PATTERNING OF TISSUE PAPER
This patent relates to the patterning of tissue paper which term is used herein to represent wet or dry laid paper having a basis weight between 6 and 50 g/m2 preferably between 10 and 26g/m2 and used for example as filtration bags used for the infusion of products such as tea or coffee.
There exists within the tea and coffee processing industries considerable interest in providing brand identification upon product bags for security and aesthetic appeal. This can take the form for example of patterning or discrete logo images introduced into the tissue paper at the production stage.
Commercial techniques in the past have centred upon water jet technology, a process whereby fibre distribution is disrupted at the papermaking formation stage to create regions of high and low fibre density which when viewed against the darker tea or coffee bag content provide a pattern by optical contrast.
However, it is recognised by those skilled in the art that the subsequent regions of low fibre density result in significant reductions in paper strength and moreover give rise to the loss of ingredient particles due to the occurrence of voids within these regions.
It is well known within the paper industry that the combined action of heat and pressure on a uniform tissue paper web results in a compaction of the fibres and a reduction in thickness.
This process commonly referred to as hot calendering also affords improved surface smoothness and moreover a reduction in opacity, and is most conveniently achieved by passing the tissue paper through the nip of hot rolls.
These properties as illustrated in Table 1 are achievable for tissue papers constructed from predominantly cellulosic fibres as well as mixtures of cellulosic, synthetic and thermoplastic fibre compositions.
TABLE 1: HOT CALENDERED TISSUE PAPERS
The given optical transmission values were obtained by a parallel beam of light from a first light source transmitted perpendicularly through the sample and recorded by a photocell whilst optical surface scatter was determined by the proportion of light from a second light source reflected back to a photocell placed close to a second light source and inclined at an angle of 45° to the paper surface.
The hot calendering was conducted on a commercial two plain steel roll stack over the full width of the paper at the given temperature with the nip pressure exerted by hydraulic rams.
A further advancement of the technique has been realised by the paper industry whereby the pressure rolls' smooth continuous surfaces are replaced by surfaces incorporating castellations or impressions which to all intents and purposes provide a localised calendering on discrete areas of the paper as it progresses through the nip.
Such a process is commonly referred to as embossing which term is used herein to refer to any localised calendering technique resulting in a profile in the thickness of paper in the longitudinal or transverse direction of the paper between the affected areas. It is also recognised that the application of heat during the embossing process improves fibre compaction giving rise to the terminology "hot embossing".
Experiments conducted with a commercial hot embossing unit further demonstrated that the hot calendering physical changes exemplified in Table 1 could be duplicated within the embossed regions as illustrated in Table 2. In these examples, the emboss consisted of a diamond shaped lined structure occupying 32% of total surface area.
TABLE 2: HOT EMBOSSED TISSUE PAPERS
Enhanced optical transmission coupled with reduced optical backscatter resulted on visual inspection in a high degree of contrast between the embossed region and remaining bulk areas when viewed against a dark background such as tea or coffee.
Whilst the optical properties afford adequate contrast for pattern definition, the described processes unfortunately result in significant reductions in strength properties of the tissue as demonstrated also in Tables 1 and 2. However, depending upon the basis weight or substance of the tissue paper and the product into which the paper is to be converted, the reduction in strength may be acceptable.
According to a first aspect of the present invention there is provided a process for patterning tissue paper as herein defined by embossing a continuous strip of paper characterised in that embossing is carried out in the course of conversion of the strip into tissue paper products.
According to a second aspect of the present invention there is provided apparatus for converting tissue paper into patterned product, comprising means for converting a continuous strip into said product, characterised in that embossing means is provided for embossing the strip in the course of such conversion.
According to a third aspect of the present invention there is provided a process for patterning tissue paper as herein defined comprising subjecting the paper to the action of a heated embossing roller coated with a liquid adapted to enhance the patterned effect obtained by the embossing operation and/or the strength of the paper in the embossed area.
According to a fourth aspect of the present invention there is provided apparatus for patterning tissue paper as herein defined, comprising an embossing roller defining a nip with a coacting pressure roller, means for heating the embossing roller, means for guiding the paper into the nip, and means for coating the embossing roller with a liquid adapted to enhance the patterned effect obtained by the embossing operation.
The invention will now be further described by way of example only with reference to the accompanying drawings, in which :-
Fig. 1 is a schematic view of one embodiment of apparatus in accordance with the invention;
Fig. 2 is a schematic view of one embodiment of tea bag conversion machine illustrating the invention;
Fig. 3 is a schematic view of a second embodiment of tea bag conversion machine illustrating the invention, and
Fig. 4 is a further embodiment of the invention incorporated in a schematically illustrated tea bag conversion machine.
Referring now to Fig. 1 of the drawings, a papermaking machine is indicated diagrammatically at PM and produces paper P which is guided by suitable guide means (not shown) through the nip of a roller pair in which the lower roller 1 provides a counter-surface for an upper, heated embossing roller 2 having a profiled surface including raised portions or projections indicated diagrammatically at 2a.
The papermaking machine produces tissue paper having a basis weight of 6 to 50 g/m2 preferably 10-26 g/m2. The papermaking machine may use a wet or dry laying process to produce tissue paper of the kind generally known as long fibred paper or non-woven material and the term "tissue paper" is used herein to include all of these materials.
A wet laid tissue paper may, for example, be made on a conventional inclined wire machine used for making long-fibred tissue paper suitable for conversion into infusion bags.
The tissue paper may have any desired fibre composition. For example, the paper may be made entirely from cellulosic fibres. Alternatively, the paper may comprise a mixture of cellulosic fibres and synthetic plastics material which may be in the form of plastics, e.g. polypropylene, fibres. One example of tissue paper that may be treated by the process according to the invention has a layer (upper or lower) of predominantly synthetic plastics fibres, e.g. polypropylene, and a layer (lower or upper) of predominantly cellulosic fibres.
As the paper P passes through the nip of the roller pair 1, 2, the projections 2a of the embossing roller 2 produce compressed areas in the paper which in the finished product reflect and/or transmit light differently from the remainder of the paper so as to achieve a patterned effect. By changing the pattern of projections on the embossing roller 2 any desired patterning effect of the paper P may be achieved, e.g. geometric patterns, trade marks, company logos or other markings or combinations thereof so as to achieve an aesthetically pleasing effect. The patterning produced by the embossing process is enhanced by the controlled application of an additive, e.g. a synthetic plastics material such as polypropylene, in a molten state to the compressed areas so as to fuse together the bulk web within these regions thereby enhancing the patterned effect. This is achieved by a reduction in light scattering within the fibres of the web by the continuum of additives such as polypropylene surrounding the fibres and thereby enhancing the transmission of light. A major advantage is that the strength of the paper within the compressed regions is enhanced by the additive.
The additive may be a clear or colourless material or, alternatively, it may be coloured to alter the light transmission and/or
reflection characteristics of the compressed regions thereby further enhancing the patterned effect. The colouring may be achieved by the use of dyes which may be encapsulated in known manner for physiological acceptability. Alternatively, colouring may be achieved by the use of fine particulate material or dust obtained from the infusion material, e.g. tea dust.
The molten additive, with or without colouring, is applied to the embossing roller 2 by a heated transfer roller 3 in rolling contact with the embossing roller 2 and a coating roller 4 which is partially immersed in a heated bath 5 of molten additive. A doctor blade 6 in contact with the coating roller 4 ensures that a uniform layer of additive is picked up by the roller 4 and transferred via the transfer roller 3 to the projections of the embossing roller 2. These projections deliver the additive in the course of creating the locally compressed areas in the paper.
By way of an example, granules of a thermoplastic polyolefin (hereafter referred to as hot melt) are melted in the metal reservoir using a hot plate located beneath the reservoir heated to 200°C. At the same time, the transfer rollers (3 and 4) are heated to approximately 120°C by small cartridge heaters mounted in their core. The transfer rollers are set in motion while the doctor blade 6 is adjusted to provide an even coating of hot melt on the roller surfaces. The coating roller 3 then transfers to the protrusions of the embossing roller which is heated to 120°C. Sample paper from a coil is fed between the embossing roller and the lower roller at a speed of approximately 10 metres/min with a force of approximately 30N/mm being applied between them. It is observed that the hot melt from the protrusions of the embossing roller is impregnated into the paper where it rapidly solidifies.
Fine tea dust added to the reservoir of hot melt as a suspension and to a concentration of 20% by weight, results in a coloured embossing pattern being impregnated into paper by encapsulating the dust into the thermoplastics material as it solidifies.
A further advantage has been realised by the significantly improved paper strength characteristics within the coated and compressed regions as demonstrated in Table 3.
TABLE 3: HOT COATED TISSUE PAPERS
Although the hot melt dose (coat application gsm) appears high it applies to the patterned areas which occupy a small proportion of the bulk area.
In the illustrated examples the patterning technique has been applied to paper in the final stage of manufacture. It will be appreciated that finished paper may be treated in the same way in which case the paper P will be supplied to the roller set 1, 2 from a reel or coil which may be prepared in dimensions suitable for use on the subsequent conversion machine for converting the tissue paper into the bag product.
It has been found that very accurate registration of discrete pattern images can be achieved by scaling down the embossing machinery to a size such that it can conveniently be integrated with the bag conversion machinery and which enables the embossing rolls to be synchronised with the sealing and slitting mechanism of such machinery.
In the apparatus of Fig. 2 a heated embossing die in the form of a disc 11 with equispaced image recesses 12 (each consisting of the word TEA) engraved onto its periphery is pressed against a plain roll 13 whilst tissue paper 14 from a reel is fed into the nip 15. A threaded collar nut 16 locates the die which also facilitates convenient die pattern exchange. The embossed paper 17 is then fed to folding 18, tea dosing 19, sealing 20 and cutting 21 stages of a
SUBSTITUTE SHEET
standard tea bag conversion machine (for example a single lane TEEPACK CONSTANTA machine for cellulosic paper or a multi-lane SIG heat seal conversion machine for heat sealable paper). In use, the embossing die 11 is pressed with a force of 10-50N/mm against the plain roll 13 whilst the tissue paper 14 is fed to the nip 15 to provide a transparent strip 22 in the tissue paper containing opaque imprints 23 of the engraved images 12 registered on the final product bag 26. The embossing rolls 13, 24 are electrically heated to a temperature of 180°C by cartridge elements 25 mounted in their core, whilst the embossing die 11 is fixed to the roll 24 in heat exchanging relationship therewith by the threaded collar 16.
It was found that in use of the apparatus of Fig. 2 the embossed image remained in a constant position on the tea bags manufactured. In a further experiment, a similar sample of paper was embossed using the same embossing unit but subsequently reeled to normal tension prior to converting separately into tea bags on the same tea bag machine. In this case the embossed image ran out of registration by 5mm after conversion of only 200 metres of paper. Bearing in mind that conversion paper lengths of 3000 metres are common, the requirement for precision registration of images is obviously of importance.
Surprisingly it was found that tissue paper comprising solely of long cellulosic wood fibres together with wet strength agents e.g. kymene (Hercules Chemicals Ltd) and sodium carboxy-methyl-cellulose, could be embossed to provide an adequate embossed transparent stripe interrupted by the discrete images. Similar results were achieved for tissue papers containing thermoplastics in their construction.
It was also found that paper strength characteristics in the machine direction was not reduced unduly by the embossing process since the transverse extent of the embossed pattern occupies only a small proportion of the total paper width.
Naturally a technique such as that described above by reference to Fig. 1 could be adapted to this paper processing on the conversion machine to enhance transparency and paper strength as shown in Table 3. Fig. 3 illustrates the apparatus of Fig. 2 incorporating a device as shown in Fig. 1 with reference numerals increased by 100 for similar parts.
In a further embodiment illustrated schematically in Fig. 4, parts corresponding to those of Fig. 2 are given reference numerals increased by 200 and are not further described. A thin strip 230 40μm thick x 3mm wide of thermoplastic material e.g. polyamide is fed from a reel 231 into the nip 215 beneath the tissue paper 214 where it is laminated into the tissue paper by the action of the heated embossing die 211.
It was found that preferred results were obtained when the lower roll temperature was maintained at a temperature below the melting point of the thermoplastic strip (e.g. 60°C for polyamides) to facilitate release of the paper from the lower rolls. An impressively shiny transparent stripe together with an image emboss was achieved on the outside of the subsequently converted tea bags. It was also found that the strength of the laminated region was significantly greater than the remaining tissue paper as demonstrated in Table 4.
TABLE 4: HOT LAMINATED TISSUE PAPERS
By embossing the tissue paper in the course of conversion of a continuous strip into tea ba'gs or other paper products it is possible to avoid the registration problems that would inevitably occur as a result of stretching if embossing were carried out at an earlier stage, e.g. during paper manufacture or during subsequent reeling from machine rolls to coils of suitable width for use on standard conversion machines. As described above, such stretching causes a failure of registration of the embossed pattern with the final product.
It will be appreciated that the invention is concerned broadly with changing the appearance of tissue paper products, particularly but not exclusively tea bags, by a pressing technique which has been referred to as localised calendering or embossing, these terms being used synonymously to mean a pressing operation resulting in deformation of the paper profile and a consequent change in the optical properties of the compressed area or areas. A simple example of such embossing is a continuous strip extending lengthwise of a tea bag and having greater transparency than the remainder of the bag. Such a strip enables closer visual inspection of the tea bag contents than has hitherto been possible. The strip may of course be replaced by discontinuous areas or a single embossed area serving the same purpose or intended merely as pattern or ornament. As described, the embossing or localised calendering may produce its useful and/or decorative effect by using an embossing die in which significant parts, such as letters, of the final pattern are provided by projections or recesses in the die with the background against which such significant parts are viewed being provided by recesses or projections respectively. For example, the embossing die of Fig. 3 produces a continuous stripe by localised calendering and the TEA characters, which are seen in the product against the background of this relatively transparent stripe, are produced by the recesses 22 and remain of the same opacity as the remainder of the paper.
Although the description has been confined to hot embossing it will be appreciated that similar effects may be achieved, possibly with application of greater pressure, alternative additions or embossing at ambient temperatures. Also, embossing may be carried out simultaneously on a plurality of paper strips derived from a common paper web, for example in the multi-lane SIG machine mentioned above.