Gas permeability Greaseproof paper was coated with chitosan to obtain a packaging material with good barrier properties towards oxygen, nitrogen, and carbon dioxide (Kjellgren and others 2006). The oxygen permeability in the same range as the polyethylene terephthalate was obtained at coat weights exceeding 5 g/m2. The oxygen permeability was not substantially affected by temperature changes, provided that the air permeance of the base paper was low. A barrier against nitrogen and carbon dioxide required a coat weight exceeding 5 g/m2. Trezza and others (1998) reported a reduction in the oxygen permeability of paper coated with corn zein. Furthermore,Gällstedt and others (2005) studied the effects of coating procedures on oxygen barrier properties of paper and paperboard coated with chitosan, WPI, WPC, and WG. Paper sheets were solution-coated using a hand applicator, WG was compression-molded onto paper and paperboard, and chitosan solution was also applied on paperboard using curtain-coating. The coatings on the applicator-coated sheets were too thin and discontinuous to improve the oxygen barrier properties. Because of the higher amount of WG material in the compression-molding process, coatings were thick and continuous, resulting in low oxygen permeability. Chitosan-curtain-coated paperboard showed the highest oxygen-barrier properties, which are comparable to those of commonly used packaging oxygen-barrier polymers. On the other hand, Khwaldia (2004) studied the combined effects of mica, carnauba wax, glycerol, and NaCAS concentrations on oxygen-barrier properties. Coating significantly increased oxygen-barrier property. The oxygen permeability of the coated paper was 13 to 90 times lower than that of the uncoated paper.
Water vapor permeability A water barrier can be formed by changing the wettability of the paper surface with sizing agents or through coating with hydrophobic materials. Paper is often coated with paraffin wax, applied in a molten form, to produce a water vapor barrier. Han and Krochta (1999) studied the wetting properties and WVP of whey-protein-coated paper. They reported that the whey protein coating increased the water-vapor-barrier property of pulp paper. The WVP decreased by 44.8% compared to the uncoated paper after WPI coating with 10 g/m2. The properties of the NaCAS-paper bilayers were investigated by Khwaldia (2009). The WVP of NaCAS-coated paper was decreased consistently by increasing coating weight from 3 to 18 g/m2. NaCAS coating on paper reduced WVP by 75% for 18 g/m2 coating weight compared to that of the uncoated paper. In a previous study, Khwaldia and others (2005) showed that the WVP of NaCAS-coated papers decreased as the amount of wax in the coating increased. The addition of hydrophobic substances to this hydrophilic matrix provides the moisture barrier properties. The substantial reduction in WVP of paper by incorporation of waxes was expected because waxes are most efficient substances to reduce moisture permeability due to their high hydrophobicity.
Parris and others (1998) evaluated coating formulations composed of the corn protein zein and paraffin wax for their water-vapor-barrier properties. The water vapor transmission rates for paper coated with paraffin wax were found to be significantly lower than those measured using the zein-coated paper. Coating the paper with a 2% solution of zein in paraffin wax reduced the water vapor transmission rates by approximately half the values obtained for wax-coated paper. Water vapor transmission values were strongly dependent on the amount of wax in the coating. On the other hand, Rhim and others (2006) showed that water barrier properties of paperboards increased by SPI or alginate coating with CaCl2 posttreatment. Biopolymer-coated paperboards can be used in the preparation of water-resistant corrugated fiberboard boxes for the storage of high-moisture foods. Larotonda and others (2005) reported that Kraft paper impregnation with cassava starch acetate is an interesting alternative for improving the hygroscopic properties and obtaining a waterproof paper. Furthermore, hydroxypropyl methylcellulose (HPMC)-based coatings reduced WVP and further reduction was obtained when beeswax was incorporated in the HPMC-lipid composite-coated paper (Sothornvit 2009). Using HPMC as a coating material for paper has a benefit in terms of lower concentration of coating solution, while providing desirable mechanical properties. Indeed, a low concentration of HPMC is adequate to provide the appropriate viscosity for coating on paper. Further investigation is still needed to verify the properties of HPMC-based coated paper with specific products.
Bordenave and others (2007) evaluated the barrier properties against moisture and the liquid water sensitivity of chitosan-coated papers. They showed that the chitosan coating led to a significant decrease of the paper moisture transfer but the surface hydrophilicity remained high.
Gas permeability Greaseproof paper was coated with chitosan to obtain a packaging material with good barrier properties towards oxygen, nitrogen, and carbon dioxide (Kjellgren and others 2006). The oxygen permeability in the same range as the polyethylene terephthalate was obtained at coat weights exceeding 5 g/m2. The oxygen permeability was not substantially affected by temperature changes, provided that the air permeance of the base paper was low. A barrier against nitrogen and carbon dioxide required a coat weight exceeding 5 g/m2. Trezza and others (1998) reported a reduction in the oxygen permeability of paper coated with corn zein. Furthermore,Gällstedt and others (2005) studied the effects of coating procedures on oxygen barrier properties of paper and paperboard coated with chitosan, WPI, WPC, and WG. Paper sheets were solution-coated using a hand applicator, WG was compression-molded onto paper and paperboard, and chitosan solution was also applied on paperboard using curtain-coating. The coatings on the applicator-coated sheets were too thin and discontinuous to improve the oxygen barrier properties. Because of the higher amount of WG material in the compression-molding process, coatings were thick and continuous, resulting in low oxygen permeability. Chitosan-curtain-coated paperboard showed the highest oxygen-barrier properties, which are comparable to those of commonly used packaging oxygen-barrier polymers. On the other hand, Khwaldia (2004) studied the combined effects of mica, carnauba wax, glycerol, and NaCAS concentrations on oxygen-barrier properties. Coating significantly increased oxygen-barrier property. The oxygen permeability of the coated paper was 13 to 90 times lower than that of the uncoated paper.
Water vapor permeability A water barrier can be formed by changing the wettability of the paper surface with sizing agents or through coating with hydrophobic materials. Paper is often coated with paraffin wax, applied in a molten form, to produce a water vapor barrier. Han and Krochta (1999) studied the wetting properties and WVP of whey-protein-coated paper. They reported that the whey protein coating increased the water-vapor-barrier property of pulp paper. The WVP decreased by 44.8% compared to the uncoated paper after WPI coating with 10 g/m2. The properties of the NaCAS-paper bilayers were investigated by Khwaldia (2009). The WVP of NaCAS-coated paper was decreased consistently by increasing coating weight from 3 to 18 g/m2. NaCAS coating on paper reduced WVP by 75% for 18 g/m2 coating weight compared to that of the uncoated paper. In a previous study, Khwaldia and others (2005) showed that the WVP of NaCAS-coated papers decreased as the amount of wax in the coating increased. The addition of hydrophobic substances to this hydrophilic matrix provides the moisture barrier properties. The substantial reduction in WVP of paper by incorporation of waxes was expected because waxes are most efficient substances to reduce moisture permeability due to their high hydrophobicity.
Parris and others (1998) evaluated coating formulations composed of the corn protein zein and paraffin wax for their water-vapor-barrier properties. The water vapor transmission rates for paper coated with paraffin wax were found to be significantly lower than those measured using the zein-coated paper. Coating the paper with a 2% solution of zein in paraffin wax reduced the water vapor transmission rates by approximately half the values obtained for wax-coated paper. Water vapor transmission values were strongly dependent on the amount of wax in the coating. On the other hand, Rhim and others (2006) showed that water barrier properties of paperboards increased by SPI or alginate coating with CaCl2 posttreatment. Biopolymer-coated paperboards can be used in the preparation of water-resistant corrugated fiberboard boxes for the storage of high-moisture foods. Larotonda and others (2005) reported that Kraft paper impregnation with cassava starch acetate is an interesting alternative for improving the hygroscopic properties and obtaining a waterproof paper. Furthermore, hydroxypropyl methylcellulose (HPMC)-based coatings reduced WVP and further reduction was obtained when beeswax was incorporated in the HPMC-lipid composite-coated paper (Sothornvit 2009). Using HPMC as a coating material for paper has a benefit in terms of lower concentration of coating solution, while providing desirable mechanical properties. Indeed, a low concentration of HPMC is adequate to provide the appropriate viscosity for coating on paper. Further investigation is still needed to verify the properties of HPMC-based coated paper with specific products.
Bordenave and others (2007) evaluated the barrier properties against moisture and the liquid water sensitivity of chitosan-coated papers. They showed that the chitosan coating led to a significant decrease of the paper moisture transfer but the surface hydrophilicity remained high.
การแปล กรุณารอสักครู่..
