PAPER BASED COMPOSITE PLANAR MATERIAL

Abstract

 The present invention relates to a paper-based composite planar material, which contains cellulose fibers and filler particles, selected from kaolin, kaolinite, TiO₂, Al₂O₃ and mixtures thereof, of sizes in the range of from 50 nm to 200 µm, whereas the filler particles content is in the range of from 5 to 65 % (w/w). It relates also to a packaging material for microwave applications, which contains the composite material according to the present invention.
Furthermore, a method of preparation of the composite material is disclosed, as well as its uses.

Description

Field of Art

[0001] The present invention relates to a modification of paper-based materials for applications in packaging, in particular during microwave heating.

Background Art

[0002] The quantity of packaging material production grows exponentially worldwide. The materials based on cellulose, essentially paper, have the highest quantity in production. Progressively, important changes occur in structure and properties of the paper thin layer. Also the generaly known paper applications change towards new technical fields, e.g. structured foils with phase inclusions, microheterogenic foil materials with visual signalization (wetness, microbe presence, enzyme, etc.), foil materials with specific biological activity (intracellular chemical information systems, control systems for transport flow regulation, immobilization layers of specific cultures, etc.), foil materials for economic ayay of agricultural production, foil and dispersive materials for ecological applications.

[0003] First paper, produced by felting of fibres, was invented in China at the beginning of this era. Around the year 600 A.D., this production technology got into Japan and Middle Asia, and around the year 900 A.D. to North Africa. In 11^th century, paper production began in Europe - in Spain and Italy. Mechanical paper production began to evolve following the invention of paper machine around the year 1800 A.D. Paper is a product made by felting of fine plant fibres in aqueous dispersion on a web. Surface paper mass is approximately up to 150 g/m². Paper exceeding this surface mass is called carton, and above 250 g/m² board. Papers can be divided to graphical papers, used for different kinds of printmaking and writting, wrapping papers, used to protect different kinds of goods, technical papers, used in technical practices, and special papers for special technical applications. From the paper production point of view, it can be divided into two basic phases: paper pulp preparation, including defibering, grinding, filling, gluing and coloring, and paper pulp processing, using a paper machine. During the paper pulp preparation, cellulose fibres are machined, which leads to their structural changes, e.g. fiber dimension changes, increase in active surface of fibers and swelling of fibres. Individual fibres undergo phases of defibering, spinning, grinding, egalization and homogenization. The paper machine is a complex technological device, comprising a head box, web, pressing and drying parts. Furthermore, it may contain gluing presser, calendering and winding of paper. In order to obtain a constant quality, paper pulp must be spread over the web of the paper machine evenly (quantity and speed) along its whole width. Once the paper pulp spreads over the web, water is removed from it. The head box contains paper pulp with 0.6 % of dry mass, while by the end of the web the dry mass content is about 50 %. After water removal of a paper sheet on the web of the paper machine, more water is being removed by pressing, generally in several consecutive pressers with gradually increasing pressures. Drying part removes water residues and produces paper with dry-mass of about 95 %. Furthermore, during certain types of paper production, coating of paper is often required. Coating is performed directly in the paper machine by gluing pressers. This process is followed by calendering, which gives the paper its required smoothness and thickness. Winding of paper is the last operation during paper production using paper machine. It might be followed by cutting into sheets, grading, wrapping and weighting.

[0004] Issues concerning paper based packaging materials (cellulose pulp) and their modifications for microwave applications are discussed worldwide for several decades. The principal solution for this kind of packaging is in preparation of paper composed from a plurality of layers and an inserted metalized PET-based (e.g. EP 642989), LDPE (EP 437946, CN 202295622) polymeric layer, enabling a selective absorbance of microwave energy and a volume extensibility change (shrinkage) once the final temperature is reached (JP 2010001041).

[0005] From a practical application point of view, concerning a heat preparation of food in a microwave field, also the ability of the pagkaging to resist the infiltration of moisture, oil or grease should be considered important, besides the above mentioned aspects of efficiency of microwave energy absorption and its transformation to heat.

Disclosure of the Invention

[0006] The subject of the present invention is a paper-based composite planar material, which contains cellulose fibers and filler particles, selected from kaolin, kaolinite, TiO₂, Al₂O₃ and the mixtures thereof, of size in the range of from 50 nm to 200 µm, preferably of size from 50 nm to 5 µm, more preferably from 0.5 µm to 5 µm. The filler particles content is in the range of from 5 to 65 % (w/w).

[0007] Filler particles are preferably deposited on the surface of the cellulose fibers. This is achieved by an addition of colloid dispersion of nano/micro-particles of the filler, based on kaolinite, TiO₂, Al₂O₃, SiO₂ and/or kaolin, into a dispersion of paper pulp, which is based on cellulose pulp, during the preparation procedure.

[0008] The described material has significantly better values of vapor, water and oil permeability.

[0009] In order to maintain a perfect dispersion of filler particles, their stabilization is necessary, using for example a polymeric surfactant of the type of polyoxyglycol, preferably of an weight average molecular mass from 5 kDa to 1.8 MDa.

[0010] In a preferred embodiment, the composite material may further contain refining additives based on acrylate dispersions or vinyl acetate, preferably in the range of from 0.001 to 20 % (w/w), to enable modulation of strength and wettable characteristics.

[0011] The composite material can further contain a surface layer containing a mixture of at least one hydrophilic polymer, preferably a cellulose derivative, more preferably hydroxyethyl cellulose or carboxymethyl cellulose, of the weight mean molecular mass from 10 kDa to 1.5 MDa, with iron oxide, aluminium oxide, titanium oxide and/or silicon oxide particles. Said metal oxide may contain the metal in various oxidation states (for example Fe₂O₃, Fe₃O₄, FeO, FeO₂, Al₂O₃, TiO₂, SiO₂). Starch might be used instead of the hydrophilic polymer. Metal oxide particles are individually dispersed in the surface layer, and ensure the antimicrobial properties related to following photochemical, microwave or plazmochemical generation of active forms of oxygen. Preferably, metal oxide particles might be present in the surface layer in the amount of from 0.1 to 15 % (w/w) relative to the dry mass of the composite material.

[0012] Preferably, the surface layer might be solidified by physical or chemical procedures by the addition of cross-linking agent, thermally or plasmachemically (for example by corona treatment or microwave plasma). For example divinyl sulfone, glutaraldehyde and others might be used as cross-linking agents, in concentrations in the range of from 0.01 to 7 % (w/w), and optionally other known procedures might be used.

[0013] In another preferred embodiment, the surface layer might further contain a photochemically active substance enabling to monitor sterility changes visually by color changes, which is particularly preferable for example in food-processing and food-storing applications.

[0014] The invention further contains a packaging material for microwave applications, which contains the composite material according to the present invention.

[0015] The present invention also relates to use of the composite material for microwave applications and for food-processing and food-storing applications.

[0016] The present invention further encompasses a method for preparing the composite material, wherein a colloid dispersion of nano/micro-particles of the filler, based on kaolinite, TiO₂, Al₂O₃, SiO₂ and/or kaolin, is added into a dispersion of paper pulp based on cellulose pulp, and the resulting mixture is placed on a wire (paper web). Preferably, the colloid dispersion is stabilized by a surfactant, for example a polymeric surfactant of the type of polyoxyglycol, preferably of a weight average molecular mass from 5 kDa to 1.8 MDa. In one preferred embodiment, refining additives based on acrylate dispersions or vinyl acetate can be added to the mixture.

[0017] In one preferred embodiment, the composite material may be further coated with a surface layer containing a mixture of at least one hydrophilic polymer, preferably a cellulose derivative, more preferably hydroxyethyl cellulose or carboxymethyl cellulose, of the weight mean molecular mass from 10 kDa to 1.5 MDa, with iron oxide, aluminium oxide, titanium oxide and/or silicon oxide particles. Said metal oxide may contain the metal in various oxidation states (for example Fe₂O₃, Fe₃O₄, FeO, FeO₂, Al₂O₃, TiO₂, SiO₂). Starch might be used instead of the hydrophilic polymer. Preferably, the surface layer might be solidified by physical or chemical procedures by the addition of cross-linking agent, thermally or plasmachemically (for example by corona treatment or microwave plasma).

Brief Description of Figures

[0018] 

Fig. 1. Depiction of original cellulose fibers (by SEM).

Fig. 2. Depiction of covering of cellulose fibres by microparticles of kaolinite (by SEM).

Fig. 3. Detailed surface structure of cellulose fibers covered by a kaolinite based filler (by SEM).

Fig. 4. Dependence of equilibrium contact angle of wetting upon the age of a glycerol drop for unmodified paper.

Fig. 5. Dependence of equilibrium contact angle of wettability upon the age of a glycerol drop for a modified paper sample.

Fig. 6. The influence of kaolin addition upon kinetics of glycerol wettability on studied paper samples. The experimental point empty circle with a cross has been excluded from the linear regression calculation. Full circle: critical time t₁, empty circle: critical time t₂.

Fig. 7. The influence of size of the kaolin filler particles (diameter of the particles) on the kinetics of glycerol wettability on studied paper samples. The experimental point empty circle with a cross has been excluded from the linear regression calculation. Full circle: critical time t₁, empty circle: critical time t₂.

Fig. 8. Mössbauer spektra of a solidified surface layer of carboxymethylcellulose filled with dispersed nanoparticles of iron(III) oxide with the size of 60 nm (left) and 40 nm (right): (a) transmission and (b) conversion electrone ⁵⁷Fe Mössbauer spektra. Measurements were carried out at room temperature of 23 °C. The concentration of iron(III) oxide nanoparticles was 5 % (w/w).

Examples

Example 1: Preparation of the composite material

[0019] The preparation procedure of the composite material is based on classical paper technologies, where an additional element (a colloidal dispersion of filler nano/micro particles based on kaolinite, TiO₂, Al₂O₃, SiO₂ and kaolin in a concentration range of from 2 to 65 % (w/w) and of an exactly defined mean particle diameter in a range of from 50 nm to 5000 nm) is added to the system of paper dispersion on the basis of cellulose pulp. The filler nano/micro particles are stabilized electrostatically, eventually sterically, for example by a polymeric auxiliary preparation addition, based on amphoteric polymeric polyoxyglycol type surfactant (e.g. polyethylene oxide (PEO)) of the mean molecular mass in a range of from 5 kDa to 1.8 MDa and in a concentration range of from 0.0001 to 12.0 % (w/w) of the mass of the dispersion.

[0020] The dispersion treated by the above-described method is further mixed with paper pulp and deposited on a paper web. By this method, a selective coverage of individual cellulose fibers by filler particles of exactly defined distribution of particle sizes is achieved, enabling formation of a complex nano-structured layer in a cross-section of the final paper sheet. After water removal, common further paper processing is applied, including pressing and other common paper processing procedures.

Example 2: Paper coating procedure

[0021] Subsequently to the preparation of nano-structured paper according to Example 1, a coating procedure takes place, using a water soluble dispersion of cellulose derivatives (preferably hydroxyethyl cellulose, carboxymethyl cellulose) of the mean molecular mass in a range of from 10 kDa to 1.5 MDa mixed with nanoparticles of iron, aluminium or silicium oxides of variable oxidation stages or combinations thereof (variable mass ratio lies in the range of from 0.01 to 30 % (w/w)). The water soluble dispersion of hydrophile polymers might be replaced by a starch dispersion (e.g. Perlsize or Perlcoat (Lyckeby Amylex)).

Example 3: Solidification of surface layer

[0022] After the paper coating procedure according to Example 2, its solidification is then performed using physical or chemical procedures with the addition of a suitable cross-linking agent. The solidification is performed thermally or plasmachemically (e.g. by corona treatment or microwave plasma). Divinylsulphon, glutaraldehyde etc. in concentrations in a range of from 0.01 to 7 % (w/w) might be used as cross-linking agents, or, alternatively, other known procedures.

Example 4: Material properties evaluation

[0023] In order to evaluate the wettability of the tested papers, a method of following the kinetics of sinking-in was chosen, based on monitoring of the contact angle of wetting changes with the age of a glycerole drop. It is possible to expect that when the contact wetting angle is 0°, the surface is perfectly wet. The time needed to reach this situation - critical time - corresponds to the time on the x axes (age of a drop) necessary to obtain a perfect wettability. By comparison of individual critical times it is possible to compare the resistance of individual papers tested towards liquid infiltration. As can be seen from kinetic curves, in most cases the dependence of contact wetting angle of glycerole on studied papers decreased with increasing time, showing a characteristic linear progression in the starting stages of infiltration and limit linear progression at the end of the experiment. Therefore a method of tangents was chosen, and a critical time was calculated as a cross-section of tangents with the time axes at the beginning and the end of the experiment (see Fig. 4 for ordinary paper and Fig. 5 for a paper according to Example 3). The first tangent (from the beginning of the experiment) can be associated with the property characterizing surface roughness of the sample, influencing the speed of equilibrium state establishement. The second tangent (from the end of the experiment) can be associated with the property characterizing the inner structure of the paper, related to the infiltration into a microporous structure of the paper layer.

[0024] Fig. 6 shows the influence of a degree of paper filling (expressed as a kaolin concentration within a working dispersion) on the critical times t₁ and t₂. As it is apparent from this dependence, the critical time t₁ is not influenced by the filler concentration. On the contrary, the critical time t₂ depends on the microporous structure of the paper layer, and linearly increases with increasing concentration of fillers. Such linear increase confirms the increasing resistance against the infiltration of testing liquid of 75 % (t₂ = 32 s, t₂ of an unfilled paper is 8 s).

[0025] The influence of additives in coating of cellulose fibres is demonstated by 15-times higher increase (from 8 s to 120 s) of both critical times (t₁ and t₂) upon acrylate dispersion addition, comparing with unmodified paper.

[0026] The last monitored parameter was the size of filler particles (kaolin). The summary of the results is in Fig. 7. The granulometry of particles does not influence the surface roughness of the sample (t₁), related to the technological procedure of paper preparation. However, a significant increase of paper resistance towards infiltration of liquids for particles with diameter less than 200 µm (increase of critical time t₂ from 4 s to 39 s) was observed. In order to characterize the infiltration of vapours, experiments with hexane vapours were performed using selected paper samples using iGC method (inversion gas chromatography).

[0027] The influence of mineral composition, used as microwave radiation absorption, is important for the efficiency of the change of microwave energy into heat. In our case, we focused on minerals of the type of kaolin, as a frequent filler in paper applications, but with an eye on the dependence of dielectric constant (imaginary part) on the size of the particles, moisture and shape of the particles. Kaolinite exhibits characteristics suitable for its applications in packaging for microwave heating. Slight warming (temperatures around 110 °C) takes place.

[0028] From the efficient transformation of microwave radiation into heat point of view, the most suitable layer seems to be a layer of dispersed iron oxide (Fe₃O₄, magnetite), or eventually Fe₂O₃ or other metal oxides in different oxidation states, which are deposited on the surface of the above mentioned paper, i.e. a composite of cellulose/filler, forming a homogeneous distribution of magnetite nano particles. Such layer contains a solidified layer of hydrophilic polymer of a defined molar mass, which is further solidified by physical or chemical procedures in order to exhibit exactly defined swelling characteristics when exposed to moisture. This ensures the ability of selective surface wrinklage of the above described sandwich structure paper/coating, influencing the ability of microwave radiation absorption by this system. This combination of nano structured cellulose layer with fibres selectively coated by defined filler particles (with relatively narrow distribution of particle sizes) based on kaolinite, and a hydrophilic layer of dispersed magnetite particles of exact granulometry (in the range of from units to hundreds of nanometers), enabling an efficient absorption of microwave radiation and its transformation into heat. It can be assumed a selective absorption of the magnetic, as well as the electrical component of the microwave field by the specifically organized metal oxide nanoparticles in the prepared planar sandwich composite, e.g. in octahedral or tetrahedral configuration of iron atoms within the crystal lattice of its oxide, as it is for a layer of Fe₂O₃ nanoparticles in a solidified carboxymethylcellulose (Fig. 8).

Industrial Applicability

[0029] Coated and filled papers, according to the present invention, can find their applications mainly in packaging of food used for microwave heating, e.g. roasted corn, pizza, ready-to-cook food, prepared meals, etc.

Claims

1. A paper-based composite planar material, characterized in that it contains cellulose fibers and filler particles selected from kaolin, kaolinite, TiO₂, Al₂O₃ and mixtures thereof, said filler particles having a size in the range of from 50 nm to 200 µm, whereas the filler particle content is in the range of from 5 to 65 % (w/w).

2. The composite material according to the claim 1, characterized in that the filler particles are deposited on the surface of the cellulose fibers.

3. The composite material according to any one of the preceding claims, characterized in that it further contains at least one polymeric surfactant of the polyoxyglycol type, preferably of an weight average molecular mass from 5 kDa to 1.8 MDa.

4. The composite material according to any one of the preceding claims, characterized in that it further contains at least one acrylate- or vinyl acetate-based refining additive.

5. The composite material according to any one of the preceding claims, characterized in that it further contains a surface layer containing a mixture of at least one hydrophilic polymer or starch, preferably a cellulose derivative, with iron oxide, aluminium oxide, titanium oxide and/or silicon oxide particles.

6. The composite material according to any one of the claims 1 to 4, characterized in that it further contains a surface layer containing a mixture of starch with iron oxide, aluminium oxide, titanium oxide and/or silicium oxide particles.

7. The composite material according to any one of the preceding claims, characterized in that the surface layer is solidified by cross-linking, or thermally or plasmachemically.

8. The composite material according to any one of the preceding claims, characterized in that the surface layer further contains a photochemically active substance adapted for monitoring sterility changes visually using color changes.

9. Packaging material for microwave applications, characterized in that it contains the composite material according to any one of the preceding claims.

10. A method for preparing the composite material according to any one of claims 1 to 8, characterized in that a colloid dispersion of nano/micro-particles of a filler, based on kaolinite, TiO₂, Al₂O₃, SiO₂ and/or kaolin, is added into a dispersion of paper pulp based on cellulose pulp, and the resulting mixture is placed on a paper web, wherein the colloid dispersion is preferably stabilized by a surfactant.

11. The method according to claim 10, wherein the resulting composite material is further coated with a surface layer containing a mixture of at least one hydrophilic polymer or starch with iron oxide, aluminium oxide, titanium oxide and/or silicon oxide particles.

12. The method according to claim 11, wherein the surface layer is solidified by addition of a cross-linking agent, thermally or plasmachemically.

13. Use of the composite material according to any one of claims 1 to 8 for microwave applications and/or for food-processing and/or food-storing applications.

Drawing