FIELD OF THE INVENTION

[0001 ] The present invention is directed to a material composition and method of producing a material composition with enhanced barrier properties. While the present invention will be described in relation to its use for packaging

applications, it is to be appreciated that the invention is not limited to this application, and that other applications are also envisaged.

BACKGROUND TO THE INVENTION

[0002] There is a need to develop sustainable packaging with controlled gas and water vapour permeability. Nanocellulose/nanofibre composites/paper offer good potential, but the porosity of the non-woven composite is bigger than the length scale of most gas molecules, making it unsuitable for gas and water vapour permeability applications such as food packaging. Conventional paper is made of micron size cellulose fibres (macro fibres). The hydrophilic nature of cellulose and poor water-vapour and oxygen barrier properties due to high network porosity limits papers use in certain packaging applications, which are increasingly becoming more demanding. Paper packaging also easily absorbs water from the environment, or from the food it contains, thereby losing its physical and mechanical strength. Water vapour and oxygen can easily diffuse through the void spaces in paper, and as well as water in its condensed form through the fibre cell walls. To mitigate these limitations, common plastic materials, such as low density polyethylene (LDPE), wax and aluminium are added to the paper substrate, leading to multilayer or composite materials.

However, these materials can have serious environmental issues and the addition of these materials to paper makes the paper very difficult and inefficient to recycle as it is very difficult to separate the components. [0003] Cellulose macro fibres are a bundle of nanofibres. Cellulose nanofibres (CNF) can be produced from macro fibres through mechanical fibrillation combined with chemical or enzymatic pre-treatment. Films/sheets prepared with CNF show superior barrier properties compared to macro fibres because of reduction in network porosity and pore size. Figure 1 shows the barrier properties oxygen permeability (OP) vs. water vapour permeability (WVP) for many materials including cellulose nanofibres prepared from carboxymethylated softwood fibres. Cellulose nanofibre sheets have low OP due to low porosity compared to films made from both commercially available petroleum based polymeric materials such as low density polyethylene (LDPE) and high density polyethylene (HDPE) and biopolymers. However, WVP of nanofibre sheet is high because of the inherent sensitivity to water vapour (hydrophilic nature) of cellulose. Barrier properties of nanofibre sheets depend on the size of

nanofibres, sample crystallinity, and hydrophilicity. WVP increases with

temperature and pressure as diffusion of moisture increases with temperature and pressure. WVP typically decreases with increase in film density and film thickness. Therefore, modifying the structure of the pore network by decreasing pore size or increasing sample crystallinity should reduce the WVP. Reducing the WVP while keeping low OP is a key challenge to the utilisation of cellulose nanofibres for barrier applications.

[0004] To enhance the barrier properties of nanocellulose sheet, gas and water vapour permeability, porosity and pore size of the nanocellulose sheet has to be minimized. Traditionally, nanoparticles or disk type materials such as clay platelets are added to the suspension prior to the sheet making, either to reduce the pore size of nanofibre sheet or to increase tortuosity. Although these techniques improve the barrier properties, random arrangement of these materials in the nanofibre matrix limits the composite performance.

[0005] The above discussion of background art is included to explain the context of the present invention. It is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge at the priority date of any of the claims of this specification.

[0006] It is therefore an object of the present invention to provide an improved material composition and method of producing the material composition with enhanced barrier properties.

SUMMARY OF THE INVENTION

[0007] According to one aspect of the present invention, there is provided a material composition with enhanced barrier properties; including:

a porous substrate having a void fraction, the void fraction of the porous substrate being at least partially filled with an inorganic precipitate to thereby reduce the porosity and permeability of the porous substrate.

[0008] The porous substrate is preferably selected from paper, nanofibre paper, microfibre paper and their composites, foam, gels, fabrics and wood. The porous substrate is preferably formed with cellulosic fibres or cellulosic nanocrystals having a diameter range from 1 nm to 30 μιη.

[0009] The inorganic precipitate is preferably formed from at least one salt solution selected from metal halides, metal carbonates, metal hydrogen carbonates, metal hydroxides and metal nitrates. According to the preferred embodiment, the inorganic precipitate is formed from a combination of the above noted salt solutions. Alternatively, the inorganic precipitate may be formed from a combination of a gas and an above noted salt solution. Preferably, the salt solution is selected from CaCI ₂ , NaCO ₃ , Ca(HCO ₃ )2, Ca(OH) ₂ , and Ag (NO) ₃ . The gas may preferably be CO ₂ .

According to a preferred embodiment, the inorganic precipitate may be [001 1 ] The void fraction of the porous substrate preferably includes pores having a size ranging from 1 nm to 10 μιη. The porous substrate may be formed using roll to roll technology. Alternatively, the porous substrate may be formed by filtration of nanofibre suspensions. According to another alternative, the porous substrate may be formed by spraying or coating a nanofibre suspension onto another substrate.

[0012] The porous substrate may preferably have a thickness ranging from 1 μιη to 1 cm. The porous substrate may be flexible having an elastic modulus (E) less than 100 MPa. According to a preferred embodiment, the material

composition may have an oxygen permeability lower than 3 cc/m ² day or 1 .2 cc^m/(m ² .day.kPa) and a water vapour permeability lower than 5 gm/m ² day or 1 x10 ^"11 g/(m.s.Pa). The material composition may preferably have a basic weight ranging from 10 to 200 g/m ² .

[0013] The material composition may be recyclable by soaking and

mechanical disintegration thereof.

[0014] According to another aspect of the present invention, there is provided a method of producing a material composition with enhanced barrier properties; including:

a) exposing a porous substrate having a void fraction to at least one salt solution; and

b) allowing the salt solution to precipitate and at least partially fill the void fraction of the porous substrate with an inorganic precipitate to thereby reduce the porosity and permeability of the porous substrate.

[0015] A preferred embodiment of the method may include exposing the porous substrate with a first salt solution, and then a second salt solution or gas, the first and second salt solution or gas reacting together to form the inorganic precipitate. The method may preferably include exposing the porous substrate to the at least one salt solution by dipping the porous substrate within the salt solution, coating the porous substrate with the salt solution, surface sizing the porous substrate with the salt solution, or spraying the salt solution onto the porous substrate. Alternatively, the salt solution may be simultaneously or sequentially applied to at least one side of the porous substrate.

[0016] The salt solution may be selected from metal halides, metal

carbonates, metal hydrogen carbonates, metal hydroxides and metal nitrates. The salt solution may preferably be selected from CaC^, NaCO3, Ca(HCO3)2, Ca(OH) ₂ , and Ag (NO) ₃ . The gas may preferably be CO ₂ . The molarity of the salt solution may preferably be in the range of 0.05 and 0.5 M.

[0017] According to a further aspect of the present invention, there is provided a material composition produced according to the above described method.

[0018] The at least partial filling of the void fraction of the porous substrate by an inorganic precipitate according to the present invention therefore enhances the barrier properties of the porous substrate by reducing its porosity and

permeability.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] It will be convenient to further describe the invention with respect to the accompanying drawings which illustrate preferred embodiments of the present invention. Other embodiments of the invention are possible, and consequently, the particularity of the accompanying drawings is understood as not superseding the generality of the preceding description of the invention.

[0020] In the drawings:

[0021 ] Figure 1 is a diagram showing the oxygen permeability and water permeability of various materials; [0022] Figure 2 is a SEM image of nanoparticles precipitated on the surface of a nanocellulose sheet;

[0023] Figure 3 is a table showing the barrier properties of a nanocellulose sheet and composite prepared according to a method 1 ;

[0024] Figure 4 is a table showing the barrier properties of a nanocellulose sheet and composite prepared according to a method 2;

[0025] Figure 5 is a table showing the mechanical properties of a

nanocellulose sheet and composite; and

[0026] Figure 6 is a SEM image showing the appearance of a composite suspension after disintegration.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention relates to a material composition having enhanced barrier properties, and to a method of producing such a material composition. While this invention is specifically described herein for non-woven composite preparations using cellulose nanofibres and its use in packaging applications as an example, it is to be appreciated that the invention is indeed a general method of composite preparation, which encompasses different classes of materials and their specific applications.

[0028] The invention consists of exposing a porous web or material, such as paper, nanofibre paper or foam and their composites to a single or multiple salt solutions. When two salt solutions are used, a porous web or material is first exposed to a first salt solution A and then to a second salt solution B. The salts A and B are selected such as A and B react or ion-exchange together and the resulting product A-B is insoluble and precipitates into particles of variable and controllable length scale within the porous structure of the material/web. The salt solutions A and B are absorbed and retained into the pores of the web or material. Upon precipitation, the resulting particles or solid precipitates remain preferably within the material pores. Therefore, the particles fill the pores of the composite, drastically decreasing porosity and gas/liquid permeability. The present invention provides for the particle/solid precipitate to be preferably found in the bigger pores of the porous material, exactly where it is needed the most to significantly reduce porosity/permeability of the porous material. This significantly decreases the porosity of paper and cellulose nanofibre composites for packaging applications.

[0029] The invention enables the filling with inorganic particles of a porous material after it has been made. It therefore enables the filling of internal big pores inaccessible from the surface of the materials (i.e. if surface pores are smaller than internal pores).

[0030] The combination of salt A-B includes, but is not limited to CaCI ₂ , Na ₂ CO ₃ , Ca(HCO ₃ ) ₂ and Ca(OH) ₂ . In some instances a salt solution can be replaced by a gas, for example, if Ca(OH) ₂ is used as first salt solution, CO ₂ gas will be used instead of another salt solution to precipitate inorganic particles.

[0031 ] When a porous web or material is exposed to a single salt solution, salt solution precipitates insoluble particles in the pores upon drying. Salts of such type include, but are not limited to Ca(HCO ₃ ) ₂ and Ca(OH) ₂ .

[0032] Methods of exposing the web to a single or multiple salt solutions include, but are not limited to: immersion/dipping, and surface treatment

(coating/spraying).

[0033] Porous materials to be treated include, but are not limited to: paper, nanofibre paper, microfibre paper and their composites, foams, gels, fabrics and wood. EXAMPLES

[0034] Example 1 a. MATERIALS: Sodium Carbonate (Na ₂ CO ₃ ), Calcium Chloride (CaCI ₂ ), Calcium Hydroxide (Ca(OH) ₂ ).

EXAMPLE 1 NANO CELLULOSE SHEET PREPARATION

[0035] Micro fibrillated cellulose (MFC) supplied from Daicel Chemical Ltd (Celish KY-100G grade) was homogenized at 1000 bar pressure using a GEA high pressure homogenizer for 5 passes after diluting the sample from 25 wt% solids concentration to 0.5wt% solids concentrations. 60 g/m ² basis weight sheets/films were prepared with homogenized fibres using standard automatic British hand sheet maker.

EXAMPLE 2 NANO CELLULOSE COMPOSITE PREPARATION THROUGH IN- SITU PRECIPITATION

[0036] Nanocellulose composite were prepared in two different ways. In both methods, in-situ precipitation of CaCO ₃ nanoparticles in the nanofibre sheet was carried with Na ₂ CO ₃ and CaCI ₂ using the following reaction:

Na ₂ CO ₃ + CaCI ₂ CaCO ₃ + 2NaCI

METHOD 1

[0037] Initially Na ₂ CO ₃ and CaCI ₂ solutions with same molarity such as 0.2M and 0.5M were prepared. First, prepared nanofibre sheet was dipped in Na ₂ CO ₃ solution until it was saturated and then the nanofibre sheet dipped in CaCI ₂ solution and then dried using hotplate at 140°C temperature.

METHOD 2 [0038] Na2CO3 and CaC^ solutions with different molarity, for example Na2CO3 solution with 0.5M and CaC^ solution with 0.2M, were prepared and then precipitation was carried as described in the method 1 .

EXAMPLE 3 ESTIMATION OF THE AMOUNT OF CACO ₃ PRECIPITATED

[0039] Ash test was conducted according to the standard method TAPPI T21 1 to estimate the amount of CaCO ₃ precipitated in the composites.

EXAMPLE 4 BARRIER PROPERTIES

[0040] Barrier properties of nanofibre sheets and composites, WVP and OP, were measured using MOCON PERMATRAN 3/31 model and OX-TRAN machines, respectively, at 23°C and 50%RH. Sheets were dried at 105°C for 4 hours prior to testing.

EXAMPLE 5 MECHANICAL PROPERTIES

[0041 ] The thickness, apparent density and tensile strength of sheets were measured according to Australian/New Zealand Standard Methods 426s, 208s and 437s, respectively. Sheets were conditioned for 24 hours at 23°C

temperature and 50% RH prior to testing the mechanical properties.

[0042] As described in example 2, nanocellulose sheet was dipped in Na ₂ CO ₃ solution first to completely fill the nonwoven pores with Na ₂ CO ₃ solution. When the sheet was dipped in the CaC^ solution, the Na ₂ CO ₃ reacted with CaC^ and formed CaCO ₃ nano particles in the nanofibre matrix and on the nanofibre surface. These nanoparticles reduced the pore size and porosity of nanofibre composite. The nanoparticles precipitated on the surface can be seen clearly in the SEM image of nanocellulose composite showed in Figure 2. [0043] Reduction in pore size and porosity with in-situ precipitation decreases both OP and WVP of nanocellulose composite. This trend was observed in Tables 1 and 2 shown in Figures 3 and 4 respectively. WVP and OP of nanocellulose sheet are 44.7 gm/m ² day and 20 ccl m ² day, respectively. These values are reduced by one order of magnitude with the precipitation of CaCO ₃ with 0.2M reactants. WVP and OP increased to 10.6 gm/m ² .day and 3.02 cc/m ² .day with a higher reactant solution concentration of 0.5M. Increasing reactants concentration increases the precipitation of NaCI on the sheet, which increases the absorption and diffusion of water vapour.

[0044] Mechanical properties of nanocellulose sheet and composites are given in Table 3 shown in Figure 5. Results showed a slight reduction in tensile strength and modulus. This may be due to rewetting of sheet during in-situ precipitation. Interestingly, precipitation of CaCO ₃ did not affect the bonding between the nanofibres.

[0045] The materials produced are repulpable and recyclable. The

recyclability of the composites was checked by soaking the composite in water overnight and disintegrated it with a hand blender. The appearance of

disintegrated suspension is similar to the original nanofibre suspension used for preparing composites. SEM image of this suspension cast on metal plate given in Figure 6.

[0046] Modifications and variations as would be deemed obvious to the person skilled in the art are included within the ambit of the present invention as claimed in appended claims.