Methods for making paper or board, a board tray and pulp particles coated with foamable polymer for use in the same

Abstract

The invention concerns a method of manufacturing paper or board, the paper or board obtained by the method, and a method of producing a tray from the board. In a fibrous stock for papermaking is incorporated an amount of polymer coated fibrous particles, which even have a foaming agent such as moisture contained therein. A web is formed from the fibrous stock on a forming fabric of a paper or board machine, followed by pressing and drying to a finished paper or board. As the paper or board is sufficiently heated the foaming agent within the particles evaporates, causing expansion and foaming of the polymer coating, which adds to malleability of the board as it is moulded into a shaped article such as a board tray. The invention even covers production of doubly coated fibrous particles for use in said methods, the inner coating layer being of a foamable polymer with a higher melt flow rate and the outer coating layer being of a non-foaming polymer of a lower melt flow rate.

Claims

1. A method of manufacturing paper or board, comprising the steps of (i) coating pulp fibres and a foaming agent with polymer, to produce polymer-coated foamable particles, (ii) incorporating an amount of said particles into a fibrous stock for papermaking, (iii) forming a web from said fibrous stock on a forming fabric of a paper or board machine, (iv) pressing and drying said web to a paper or board layer, and (v) heating said paper or board layer to have the foaming agent contained in the polymer-coated foamable particles to produce vapour or gas, which foams said polymer.

2. The method of claim 1, wherein the foaming agent is moisture, which evaporates and foams the polymer.

3. The method of claim 1, wherein a melt flow rate of the polymer is at least about 15 g/10 min at 190° C.

4. The method of claim 1, wherein the polymer comprises low density polyethylene (LDPE).

5. The method of claim 1, wherein at step (i) said polymer-coated foamable particles are provided with a further polymer to form a double-layer coating having an inner layer and an outer layer, a melt viscosity of the polymer of the inner layer being higher than a melt viscosity of the further polymer of the outer layer, and at step (v) the polymer of the inner layer is foamed by means of the foaming agent while the further polymer of the outer layer remains substantially unfoamed.

6. The method of claim 1, wherein the pulp fibres in said polymer-coated foamable particles have a length of 1 to 50 mm.

7. The method of claim 1, wherein a share of polymer in said polymer-coated foamable particles is 5 to 65 wt-%.

8. The method of claim 1, wherein said polymer-coated foamable particles are added to the fibrous stock at a headbox of the paper or board machine, by mixing the polymer-coated foamable particles with liquid circulated in the process, to form 5 to 25% of the dry weight of the fibrous stock supplied onto the forming fabric.

9. The method of claim 1, wherein said polymer-coated foamable particles are incorporated in a middle layer of a multilayer board.

10. The method of claim 1, wherein a temperature at step (v) to foam the polymer is about 110 to 150° C.

11. The method of claim 1, wherein the paper or board is provided with a further polymer coating layer on both sides thereof before the heating, which induces foaming in the polymer-coated foamable particles incorporated in the fibrous stock.

12. The method of claim 11, wherein the further polymer used for coating the paper or board has a lower melt viscosity than the polymer of the particles incorporated in the fibrous stock.

13. The method of claim 11, wherein foaming of the polymer is carried out before the newly produced paper or board is reeled.

14. The method of claim 1, wherein a temperature at step (v) to foam the polymer is 115 to 125° C.

15. A method of manufacturing a board article, such as a tray or a plate, from board produced in accordance to step (i) to (iv) in claim 1, comprising the steps of (i) placing the board produced in step (iv) in claim 1 over a mould, which has a recess shaped to accord with the article being produced, (ii) drawing the board against the mould by means of vacuum, (iii) during step (ii), heating the board to induce foaming in the polymer-coated foamable particles incorporated in the board, and (iv) cooling the newly moulded article to solidify the polymer within the board.

16. A method of producing polymer coated particles for use in the method of claim 5, comprising the steps of (i) suspending fibrous cellulosic particles of a length of 1 to 50 mm in a solution containing a foamable first thermoplastic polymer and a first solvent, (ii) precipitating said first thermoplastic polymer onto said fibrous particles by means of added water and phase inversion, to obtain fibrous particles with enclosed humidity and a first polymer coating layer, (iii) suspending the polymer-coated particles obtained at step (ii) in a solution containing a second thermoplastic polymer and a second solvent, said second thermoplastic polymer having a melt viscosity lower than the melt viscosity of said first thermoplastic polymer, and (iv) precipitating said second thermoplastic polymer onto said fibrous particles by means of added water and phase inversion, to obtain fibrous particles with a double-layer polymer coating having an inner first polymer coating layer and an outer second polymer coating layer, in which the inner first polymer coating layer is foamable while the outer second polymer coating layer remains unfoamed.

Description

DETAILED DESCRIPTION

(1) Polymer Coated Particles

(2) To produce polymer coated fibrous particles cellulosic fibres or bunches of such fibres of a length of 1 to 50 mm, preferably 1 to 5 mm, are suspended in a solution containing a foamable thermoplastic polymer and an organic solvent. The fibres can be of chemical pulp, chemithermomechanical pulp (CTMP), thermomechanical pulp (TMP) or the like, produced from birch, pine, spruce, aspen, eucalyptus etc. A preferred polymer for the purpose is LDPE with a melt flow rate of about 15 g/10 min at 190° C. or more, and a preferred solvent is decalin. Water is then added to the solution to bring about a phase inversion and precipitation of the polymer onto the cellulosic fibres. Drop-like particles are thereby obtained, which comprise humid cellulosic fibres coated with a layer of polymer. These particles can be used as an additive to the fibrous stock for papermaking or boardmaking and subsequent making of moulded plates or trays according to the teachings of the invention.

(3) Optionally the fibrous particles may be provided with a double-layer coating of two different polymers. The coating obtained as described above will serve as a foamable inner coating layer of a first polymer in contact with the enclosed humid fibres. An outer, preferably non-foamable outer coating of a second polymer is produced by suspending said singly coated particles in a solution containing said second polymer and an organic solvent. A preferred polymer for the purpose is LDPE with a melt flow rate less than 15 g/10 min at 190° C., and a preferred solvent is decalin. Water is then added to the solution to bring about a phase inversion and precipitation of the second polymer onto the singly coated particles. The result is doubly coated drop-like particles, the double-layer polymer coating enclosing the humid cellulosic fibres or bunches of such fibres. The polymers are selected so that when heated the inner coating layer melts and foams as the moisture in the fibres is vaporized, while the outer coating layer remains unmelted and unfoamed. The doubly coated particles with such behaviour are the preferred additive for use in the making of paper or board, as well as in subsequent making of moulded plates or trays according to the teachings of the invention.

(4) Alternatively, singly coated fibrous particles for use in the invention could be produced by extrusion of a melted blend of fibres and polymer. The particles thus produced are elongate, yarn-like and may comprise a plurality of fibres or bundles of fibres embedded in a continuous polymeric phase. The elongate yarn as produced may then be cut into shorter pieces for incorporation in the fibrous stock.

(5) Making of the Paper or Board

(6) A single layer or multilayer paper or board may be produced by use of an ordinary paper or board machine. A fibrous stock for papermaking is prepared, and an amount of the singly or doubly coated drop-like particles are added thereto. The stock with an even distribution of said particles is supplied from a headbox to a forming fabric of the paper or board machine, turned to a fibrous web, and pressed and dried to finished paper or board product. The products include single or double layer papers of a weight of 40 to 130 g/m.sup.2 and multilayer boards of a weight of 130 to 500 g/m.sup.2. Boards of a weight of 190 g/m.sup.2 or more, which are suitable for being turned to trays, are preferred. The density of the products is preferably about 280 g/dm.sup.3 or more. In case of a multilayer board the drop-like foamable particles are preferably incorporated in an inner layer only, even if it is possible to have such particles in each layer of a board product.

(7) Foaming of the drop-like polymer coated particles by heating may be performed before the finished paper or board web is reeled into rolls for transport. However, it is preferable to leave the particles unfoamed at this stage, and have them foamed first when the paper or board is moulded into articles such as plates and trays.

(8) The fibrous stock for use in the invention may have an ordinary constitution, that is about 0.2 to 1 wt-% of fibres in an aqueous medium. However, for achieving an even distribution of the drop-like particles it is preferable to apply the well-known foam forming technique, in which fibers are supplied to the forming fabric of the paper or board machine as a foamed dispersion. A surface active agent is incorporated in the stock, and air is blown to produce foam of an air content of about 60 to 70 vol-% or more. The basic technique of foam forming is described e.g. in the publication GB 1 395 757, and WO 2013/160553 discloses a further development with microfibrillated cellulose (MFC) included in the stock. In the practice of the present invention such a foamed stock would receive the drop-like coated particles as an additive before it is fed onto the forming fabric. Use of MFC as a component of the stock is also an option within the metes and bounds of the invention.

(9) Addition of the drop-like coated particles to the fibrous stock is preferably carried out immediately before the paper or board machine headbox by means of flash mixing technique. Equipment for flash mixing under trade mark TrumpJet is available from Wetend Technologies Ltd.

(10) Moulding of Trays

(11) Production of a moulded tray from the board obtained as described above uses a mould, which has a cavity shaped to accord with the tray that is being produced. The board web may be cut into blanks, which are positioned to face the mould cavity one by one, followed by the moulding step which turns them to trays, or a web may be fed to the moulding tool on a continuous basis, to have the trays separated from each other first after having been moulded.

(12) The moulding operation is preferably carried out by drawing the board against the bottom of the mould cavity by means of vacuum. Heat and moistening are applied to aid in adapting the board to the contours of the mould cavity. Furthermore, heating of the board induces foaming of the polymer coated drop-like particles embedded in the board. The expanding particles are more or less brought into contact with each other and add to the malleability of the board during the moulding operation, thus avoiding the tendency of wrinkling. In case the particles are doubly-coated, melting and foaming occurs in the inner polymer coating layer whereas the outer polymer coating layer is stretched as the foaming inner layer expands but remains solid and unfoamed. Finally the newly moulded tray is cooled and the foamed polymer within the board is solidified in its expanded state.

(13) The finished tray is particularly useful as a package for various foodstuffs. Avoidance of wrinkles on the tray rim flanges allows providing the tray with a heat-sealed lid without risk of leaks and thereby adding to the shelf-life of the package.

(14) Plates as disposable tableware and other moulded paper and board articles covered by the invention may be manufactured by similar techniques merely by varying the shape of the mould cavity as required.