Field of the invention

The present invention relates to translucent fibrous products, and to a method for the manufacture of translucent fibrous products. Accordingly, a translucent fibrous product, such as translucent paper web or a cardboard web or a paper sheet or a cardboard sheet or the like may be obtained.

Backg round of the invention

Translucent fibrous products are used widely in various packaging materials. Some methods have been proposed in the art for improving the translucency of fibrous webs.

WO2010/046534 relates to a fibrous product, which has at least one transparent or translucent area that comprises a carbohydrate derivative, which is plasticized with a plasticizer. Said carbohydrate derivative may be long-chained carbohydrate, such as starch, dextrin, cellulose, hemicellulose, cellulose acetate, starch acetate and corresponding polymers. It is also possible to use other carbohydrate derivative-like thermoplastic biopolymers, such as polylactic acid.

GB 1 012 120 relates to a method where watermark-like transparent areas are formed by impregnating the paper with different chemicals, such as sucrose acetate isobutyrate.

In JP 2665566, the transparency of paper is improved by applying onto its surface paraffin, a higher fatty acid or a fatty acid alcohol ester or a similar material, which is diluted in a solvent, such as benzene, toluene or xylene, or in chlorinated hydrocarbon or in alcohol. There are substantial disadvantages associated with the translucent products and methods for their manufacture, according to the state of the art. They typically concern thin and rigid paper products and the properties of said products are not satisfactory, for example with respect to strength. The purpose of the present invention is thus to eliminate at least part of the disadvantages associated with the known technology and to provide an improved solution for translucent fibrous products and their manufacture.

Object of the invention An object of the invention is to provide translucent fibrous products.

Another object of the invention is to provide moldable and translucent fibrous products.

Another object of the invention is a method for the manufacture of translucent fibrous products. A still further object of the invention is a method for the manufacture of moldable and translucent fibrous products.

Definitions

Unless otherwise specified, the terms, which are used in the specification and claims, have the meanings commonly used in the field of paper, board, cardboard industry, particularly in the field of paper and pulp chemistry and industry. Specifically, the following terms have the meanings indicated below.

The term "fibrous material" refers here to fibrous web, fibrous sheet, fibrous mat or blanket comprising fibers.

The term "translucent product" refers here to a product, which permits light to pass through, but the object on the opposite side are not clearly visible. Light is transmitted through, but diffusion prevents perception of distinct images.

With the term "translucent" is meant that a symbol which is situated under the product or the surface (on the other side in relation to the viewer), such as text or figure or color or similar marking, is visible or readable or otherwise optically detectable through the modified area of the fibrous product. In general, the present invention aims at essentially changing or controlling, or both, the permeability of visible light and of UV and IR radiation. Thus, the term "translucent" refers to products which have been prepared to be either "transparent" or "translucent".

Summary of the invention

The present invention is based on the idea that fibrous material is impregnated with at least one polymer to obtain translucent fibrous products having improved translucency and strength properties. Said translucent fibrous products may comprise at least one plasticizer. Said translucent fibrous products may comprise at least two layers of fibrous material.

By this impregnation treatment the fibrous material is transformed to a translucent fibrous product having improved elongation properties at elevated temperatures and/or in the presence of humidity. The translucent fibrous products are also suitable for molding, particularly using thermoformation, for providing molded products of predetermined shape and size.

Figures Transparency of impregnated one sheet, two sheets laminated together and three sheets laminated together is illustrated in Figure 1.

The molded product is illustrated in Figure 2.

Detailed description of the invention

It was surprisingly found that translucent, strong and durable fibrous products can be obtained, which are particularly suitable in various applications in the fields of packaging, advertising, composite materials, interior design, furniture, etc. Further, said translucent fibrous product may easily be molded to molded translucent fibrous products having predesigned shape, form and size, suitably to three-dimensional products. Translucent fibrous product

The translucent fibrous product comprises fibrous material selected from fibrous webs, paper webs, board webs, tissue webs or sheets cut from any of said webs. Said fibrous material may be formed from plant derived (natural fibers) or synthetic fibers, or any combinations thereof. Natural (plant derived) fibers may be selected from chemical pulp, such as sulphate and sulphite pulp, organosolv pulp, recycled fibers, and/or mechanical pulp including e.g. refiner mechanical pulp (RMP), pressurized refiner mechanical pulp (PRMP), pretreatment refiner chemical alkaline peroxide mechanical pulp (P-RC APMP), thermomechanical pulp (TMP), thermomechanical chemical pulp (TMCP), high-temperature TMP (HT-TMP) RTS-TMP, alkaline peroxide pulp (APP), alkaline peroxide mechanical pulp (APMP), alkaline peroxide thermomechanical pulp (APTMP), Thermopulp, groundwood pulp (GW), stone groundwood pulp (SGW), pressure groundwood pulp (PGW), super pressure groundwood pulp (PGW-S), thermo groundwood pulp (TGW), thermo stone groundwood pulp (TSGW), chemimechanical pulp (CMP), chemirefinermechanical pulp (CRMP), chemithermomechanical pulp (CTMP), high-temperature CTMP (HT-CTMP), sulphite-modified thermomechanical pulp (SMTMP), reject CTMP (CTMPR), groundwood CTMP (G- CTMP), semichemical pulp (SC), neutral sulphite semi chemical pulp (NSSC), high-yield sulphite pulp (HYS), biomechanical pulp (BRMP), pulps produced according to the OPCO process, explosion pulping process, Bi-Vis process, dilution water sulfonation process (DWS), sulfonated long fibres process (SLF), chemically treated long fibres process (CTLF), long fibre CMP process (LFCMP), Kraft wood pulp, mdf-fibers, nanocellulose, and modifications and combinations thereof. The pulp may be a bleached or non-bleached pulp. The pulp may originate from hardwood or softwood, including birch, beech, aspen such as European aspen, alder, eucalyptus, maple, acacia, mixed tropical hardwood, pine such as loblolly pine, fir, hemlock, larch, spruce such as Black spruce or Norway spruce, and mixtures thereof.

Also non-wood plant raw material, such as seed hair fibers, leaf fibers, bast fibers, plant fibers can be provided from e.g. straws of grain crops, wheat straw, reed canary grass, reeds, flax, hemp, kenaf, jute, ramie, seed, sisal, abaca, coir, bamboo, bagasse, cotton kapok, milkweed, pineapple, cotton, rice, reed, esparto grass, Phalaris arundinacea, or combinations thereof may be used.

The synthetic fibers may comprise fibers of polyester, polyethylene, polypropylene, polylactide, rayon, lyocell, nylon, glass, polyacetate, aramide, carbon and any combinations thereof. Additionally, optional additives may be used . Said additives may for example comprise wetting agents, wet-strengtheners, coloring agents, fire protection agents (e.g. borates, phosphates, magnesium trihydrate), softening agents, inorganic fillers and any combinations thereof. The grammage of the fibrous material is suitably 40-500 g/m ² , preferably 80- 400 g/m ² . The fibrous material may comprise a chemical cellulose mass, mechanical mass or chemi-mechanical mass or a combination of two or more masses.

The translucent fibrous product comprises at least one polymer. Said polymer may be selected from carbohydrate derivatives, polylactic acid, polyurethane and polyolefins. Examples of suitable carbohydrate derivatives are cellulose derivatives, starch and dextrin derivatives and mixtures of two or more derivatives. Examples of such derivatives are: cellulose Cl-4 alkyl ester, oxidized cellulose Cl-4 alkyl ester, starch Cl-4 alkyl ester, oxidized starch Cl-4 lower alkyl ester, and corresponding ethers and mixtures of esters and/or ethers. Suitable derivatives are cellulose and starch esters and ethers, especially lower alkyl esters, such as methyl, ethyl, propyl and butyl esters (cellulose or starch formate, -acetate, -propionate and -butyrate). Suitable polymers are also polyurethane and polyolefins, preferably said polymer is polyurethane or polyethene. The translucent fibrous product comprise may comprise 15 - 65 % by weight, calculated from the dry product, preferably from 20 to 40 % by weight of the polymer.

The translucent fibrous product suitably may comprise also at least one plasticizer, which is hydrophilic or hydrophobic or both. The plasticizer improves the compatibility of the polymer with the fibers, particularly with cellulose fibers, and provides in combination with the selected polymer uniform and desired translucency. The combination of the plasticizer with said polymer may also affects the viscoelastic behavior of the fibers at the softening and melting temperatures of the polymer, whereby the molding and particularly thermoformation of the moldable fibrous product is improved, and the separation of the polymer from the fibers can be avoided.

Suitably said plasticizer is selected from mono-, di- and triglyceric ester of acetic acid, C2-4 alcohols comprising 1-5 hydroxyl groups, and esters of these, mono-, di- or trialkyl esters of citric acid, particularly mono-, di- or tri-Cl-4-alkyl esters of citric acid, propylene glycol, dipropylene glycol, glycerol and mixtures thereof, and mixtures thereof. Preferably, a biodegradable plasticizer is used. According to a preferable embodiment of the present invention, the plasticizing material is triethyl citrate, glycerol or glycerol monoacetate, which is a non-toxic, nonvolatile water-soluble liquid.

The amount of the plasticizer is from 10-30 % by weight of the amount of the polymer, calculated by dry weight.

Said translucent fibrous product may comprise at least one layer or more than one layers. The layered structure may also comprise a polymeric layer between layers of fibrous material, and said polymer layer or film may act as a barrier. Suitably polyethene and the like may be used as barrier materials or barrier layers. Preferably said translucent fibrous product is a multilayer product comprising at least 2 layers of the fibrous material, preferably from 2 to 5 layers of the fibrous material, suitably laminated together.

The translucent fibrous product may comprise from 0.1 to 74 % by weight of fibers.

Suitably the grammage of each layer in the multilayer product is 40-500 g/m ² , preferably from 50 to 200 g/m ² , particularly preferably from 80 to 150 g/m ^2, Method for the manufacture of the translucent fibrous product

The method for the manufacture of the translucent fibrous product comprises the steps of:

Impregnating at least part of at least one fibrous web or sheet with a liquid comprising at least one polymer. Optionally at least one plasticizer is mixed in an amount of 10 - 30 % by weight with the polymer melt or polymer dispersion or polymer suspension, or alternatively said plasticizer is applied on the web or sheet after impregnation with the polymer. Optionally the plasticizer is applied only at selected locations of the fibrous web or sheet impregnated with the polymer, whereby locations with increased translucency and decreased or no translucency can also be created. Optionally drying is carried out after the impregnation step.

Optionally at least two or more impregnated fibrous webs or sheets are combined, suitably by lamination at elevated temperature and pressure, or by using adhesives, or other suitable means known as such, to obtain multi-layer structures comprising at least two layers, preferably from 2 to 5 layers of fibrous material. The layered structure may also comprise a polymeric layer between layers of fibrous material, and said polymer layer or film may act as a barrier. Suitably polyethene and the like may be used as barrier materials or barrier layers. The applying of the polymer may be carried out by coating using spray coating, roller, extrusion coating, curtain coating or foam coating.

Suitably the grammage of each layer in the multilayer product is 40-500 g/m ² , preferably from 50 to 200 g/m ² , particularly preferably from 80 to 150 g/m ^2.

Optionally at least one plasticizer is mixed with the polymer melt or polymer dispersion or polymer suspension, or alternatively said plasticizer is applied on the web or sheet after impregnation with the polymer.

Impregnating at least part of a fibrous web or sheet means here that the fibrous web may be impregnated completely, or only pre-selected parts of the fibrous web may be impregnated whereby the final product contains locations with translucency and without translucency.

Said fibrous web or sheet may be obtained using any traditional methods use for the manufacture of paper webs and cardboard webs. In the method, any equipment and apparatus used in processes in the paper or cardboard manufacture can be utilized here. Foam formation methods are here not regarded as traditional methods.

The optional drying of the impregnated web or sheet is suitably carried out for example by heating with means conventionally used in the manufacture of paper and cardboard products.

The liquid comprising at least one polymer and at least one plasticizer refers to a melted mixture of the polymer and plasticizer, or an aqueous dispersion comprising polymer particles and the plasticizer dispersed therein, or to a suspension comprising polymer particles and the plasticizer suspended therein. Said dispersion or suspension may comprise 20 - 50, preferably 30-40 % wt of the polymer. Said melted mixture of the polymer and plasticizer may comprise the plasticizer in an amount of from 10-30 % by weight of the amount of the polymer, calculated by dry weight.

The polymer is selected from carbohydrate derivatives, polylactic acid, polyurethane and polyolefins. Examples of suitable carbohydrate derivatives are cellulose derivatives, starch and dextrin derivatives and mixtures of two or more derivatives. Examples of such derivatives are: cellulose Cl-4 alkyl ester, oxidized cellulose Cl-4 alkyl ester, starch Cl-4 alkyl ester, oxidized starch Cl-4 lower alkyl ester, and corresponding ethers and mixtures of esters and/or ethers. Suitable derivatives are cellulose and starch esters and ethers, especially lower alkyl esters, such as methyl, ethyl, propyl and butyl esters (cellulose or starch formate, -acetate, -propionate and -butyrate). Suitable polymers are also polyurethane and polyolefins, preferably said polymer is polyurethane or polyethene or cellulose acetate.

The impregnation of the fibrous web or sheet may be carried out by pressing, using spray coating, roller, extrusion coating, curtain coating, foam coating, through tanks containing the impregnation solution, flexo printing, screen printing, transfer film techniques or other such techniques, on one side or on both sides. Optionally elevated temperature (20 - 250°C) and pressure (0.1-20 Mpa) or vacuum may be used. In pressing a polymer film is pressed to the surface of the fibrous web or sheet at an elevated temperature, whereby said polymer melts into a liquid. The impregnation may suitably be followed by passing through any of pressing, calendering, glazing, drying and winding stations.

The plasticizer may be applied on the web or sheet after impregnation with the polymer, using methods descried above in connection with the impregnation step.

Additionally, in addition to translucency, also good thermoforming properties may be achieved, whereby products with higher elongation properties at elevated temperatures are obtained, particularly improving the moldability properties, for example during thermoformation.

The average particle size of the polymer in dispersion is preferably from 0.5 to 800 nm, preferably from 1 to 600 nm. The average particle size may be measured using methods known in the art, suitably with Coulter-Counter. The use of the specific particle size improves the impregnation rate and provides very homogeneous translucency at the location of the fibrous web where it is applied. It also provides good thermoforming properties to the product. Products with higher elongation properties at elevated temperatures are obtained, having improved moldability properties during for example thermoformation. Said fiber material contains pores and cavities. Thus particularly polymers having smaller average particle size have better access to the cavities than the ones with larger particle size and provide improved and more homogeneous impregnation. On the contrary, larger particles may clog the pores and cavities and obstruct smaller particles entering, whereby the poor impregnation, typically remaining on the surface only is obtained.

Suitably said plasticizer is selected from mono-, di- and triglyceric ester of acetic acid, C2-4 alcohols comprising 1-5 hydroxyl groups, and esters of these, mono-, di- or trialkyl esters of citric acid, particularly mono-, di- or tri-Cl-4-alkyl esters of citric acid, propylene glycol, dipropylene glycol, glycerol and mixtures thereof, and mixtures thereof, suitably in the form of aqueous solutions or blended with polymer liquid. Preferably, a biodegradable plasticizer is used.

According to a preferable embodiment of the present invention, the plasticizing material is triethyl citrate, glycerol or glycerol monoacetate, which is non-toxic, non-volatile water- soluble liquid.

The method may optionally further comprise coating step, whereby coating methods known in the art may be carried out using coating dispersions.

The translucent fibrous product may optionally be subjected steps selected from cutting, undulating, thermoforming or molding in a molding device, with the aid of heat and optionally moisture, to obtain products with predetermined form and size.

Considerable advantages can be achieved with the present invention. Thus, by impregnating a fibrous material, such as paper or cardboard web, it is possible to manufacture translucent and also according preferable embodiments moldable fibrous products which may be processed further to various products. Said translucent fibrous products may be used as packages, cardboard applications, advertising material, consumer packages, in blister packages of pharmaceuticals and other products requiring such packages.

In addition, the embodiment using biodegradable plastics in said moldable fibrous products makes it easier to recycle those packages.

The adhesion of cellulose fibers with the polymer is improved with selected plasticizer, and surprisingly homogeneous and translucent products are achieved.

Packages with local or complete translucency can be obtained if desired. Impregnation of the surface or dosing of the polymer and plastiziser onto the surface of the paper, onto an area of a desired size, may be carried out by means of an orifice or a roller, followed by a compressing stage. Alternatively the polymer and plasticizer may be applied also at least partly on different locations.

The invention provides a translucent and optionally moldable single-layer or multilayer fibrous product, with improved elongation properties at elevated temperatures. It was also surprising that the fibrous products can be very easily molded or thermoformed into desired shape and size, to yield product with high strength properties, appealing finishing, and additionally good barrier properties if needed.

According to a preferable embodiment the translucent fibrous product comprising may be molded even with very mild external pressure without any excess heat to the desired product.

The product can easily be molded for example using moisture and/or heat to a desired form or structure. It may be used for replacing packaging materials based on non-renewable sources, such as plastics widely used in food and pharmaceutical industry. The product may be molded to trays suitable for packaging of food, such as meat products, it may be used for form-and-seal packages, in modified atmosphere packages, in blister packages for replacing plastic and aluminum foil blisters.

The present invention can be used to produce foodstuff packages, consumer packages, transport packages, and products which include figures formed of translucent areas. Also sufficiently strong moldable fibrous products are provided for producing molded packaging materials for larger and heavier pieces.

It was surprising that a fibrous web, particularly a multilayer impregnated fibrous web may be molded without losing its translucency, even at elevated temperature at essentially dry conditions. Typically the temperature is above the temperature used in the impregnation step (180°C for cellulose acetate). Even extensive and deep molding can be carried out without tearing the web.

The following illustrating examples do not restrict the scope of protection of the present invention.

Examples Example 1. Multilayer product

In this example it is shown that a multilayer product containing three 100 g/m ² fibrous layers may be molded to a translucent and strong molded product whereas a similar product cannot be made of a traditional 300 g/m ² fibrous web.

A product with molded layer structure was obtained from fibrous sheets. The fibrous sheets were impregnated, the impregnated sheets were laminated together with the aid of pressure and heat, and the laminated structure was molded with the aid of pressure and heat to desired form. Fibrous sheet (traditionally manufactured sheet, grammage 100 g/m ² ) was impregnated with cellulose acetate (20 x 20 cm film) at 180°C temperature and 2200 psi pressure using a press, 15 s. Sheets of baking paper were used on both side of the test sample for preventing the evaporation of humidity.

Two impregnated sheet were laminated together and then a third sheet was limited thereon at 180°C temperature and 2200 psi pressure using a press, time 15 s for two sheets, 30 s for three sheets. Sheets of baking paper were used on both side of the test sample for preventing the evaporation of humidity. Transparency of impregnated one sheet, two sheets laminated together and three sheets laminated together is illustrated in Figure 1.

The obtained three layer product was molded in a hemisphere mold having a radius of 50mm and depth of 10 mm. The 3-layer laminated structure was placed in the mold between two sheets of baking paper and the mold was placed in an oven at 180°C temperature fo 10 minutes, then the mold was placed in a press, pressed under 2200 psi pressure for 50 s. The molded product is illustrated in Figure 2.