Coating composition and coated paper

This application claims the benefit of priority to People's Republic of China Patent Application No. 201210288758.0, Ser. No. 2012081500481420, filed Aug. 14, 2012, which is hereby incorporated by reference herein in its entirety.

1. Technical Field

The present disclosure relates to a coating composition in the field of papermaking and a coated paper applied with the coating composition.

2. Description of Related Art

In the field of papermaking, coatings are commonly applied to one or both surfaces of paper sheet. Many printed paper materials, for example, rely on coatings to provide a suitable surface and background for the printed ink. However, coatings on paper may lack sufficient strength, flexibility, or other useful properties. Limitations of prior technology may include, for example, either or both of the following: (1) the inability to apply a heavy coat to paper without high risk of the coat cracking or otherwise failing in use; and (2) mechanical failure of the coat such as spallation, blistering, tearing, peeling away from the substrate in printing, etc., due to inadequate tensile strength or other strength properties of the coating.

Therefore, there is room for improvement within the art.

Cellulose is an organic compound with the formula (C 6 H 10 O 5 ) n , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β-1,4 linked D-glucose units. Nano-fibrillated cellulose (NFC) is nanoscale cellulose microfibrils defibrated from cellulosic materials. The NFC microfibrils have dimensions of about 100 nm to about 2000 nm in length, and about 3 nm to about 200 nm in diameter. The NFC typically has a property that if it is mixed with water to obtain an aqueous dispersion with the NFC having a weight percentage of 1%, the aqueous dispersion at room temperature has a viscosity of 50 cps to 300 cps.

NFC may also be known as nanocellulose, nanofibrillar cellulose, microfibrillar cellulose, cellulose nanofiber (CNF), or microfibrillated cellulose (MFC).

The NFC may be prepared by a TEMPO catalytic oxidation method, a biological enzyme method, a mechanical homogenization method, a ball milling method, or a high-pressure homogenization method. In the preparing process, some hydroxyl groups (—OH) on the chain of glucose of the NFC compound are selectively oxidized to carboxyl groups (—COOH) and aldehyde groups (—CHO). The NFC has a high aspect ratio and small dimensions, and has hydroxyl groups, carboxyl groups, and aldehyde groups. Thus, NFC has good compatibility with various aqueous coating compositions, and can be dispersed in an aqueous coating composition stably and prevent or inhibit flocculation of the coating composition particles. In addition, NFC can help the coating composition to display good film-forming properties.

One example for preparing NFC by the tempo catalytic oxidation method may include the following steps: (1) providing bleached kraft pulp (BKP) and beating the pulp until the freeness (Canadian Standard Freeness)of the pulp is less than 200 ml; (2) adding sodium hydroxide solution (4 wt %) to 6 g of the beaten pulp until the pH value of the slurry reaches 9, and then adding 10 mg TEMPO ((2,2,6,6-Tetramethylpiperidin-1-yl)oxyl) free radical solution (0.1 wt %) and 100 mg catalyst of sodium bromide solution (1 wt %); (3) adding 90 ml sodium hypochlorite solution (10 wt %) to the above pulp and stirring; (4) adding sodium hydroxide solution (4 wt %) again to the pulp until the pH value of the stuff reaches 10 to 10.5, and maintaining stirring for about 5 hours; (5) adding ethanol, and then adding hydrochloric acid solution (4 wt %) to the pulp until the pH value reaches 7 and a fiber suspension is made; (6) removing the inorganic salt in the fiber suspension by centrifugal washing; and (7) ultrasonically treating the above fiber suspension using a frequency of 20 kHz and power of 25 W for 35 minutes, at which time a translucent or transparent NFC aqueous dispersion is made.

A coating composition contains NFC, pigment, latex, auxiliary additive, and water; wherein on a dry weight basis, the NFC is in an amount by weight of 0.02 parts to 10 parts in the total composition, the pigment is in an amount by weight of 75 parts to 95 parts in the total composition, the latex is in an amount by weight of 5 parts to 15 parts in the total composition, and the auxiliary additive is in an amount by weight of 0.35 parts to 10 parts in the total composition. Optionally, the coating composition may further contain alkali, giving the coating composition a pH value of 8 to 10.5. The alkali in the coating composition can be sodium hydroxide or potassium hydroxide, but is not limited the two above-mentioned alkalis.

The pigment is selected from the group consisting of ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), kaolin, talc, and any combination thereof. In one embodiment, at least 65% of the pigment by weight has a particle size of less than 2 μm. More specifically, at least 95% of the pigment by weight has a particle size of less than 2 μm.

The latex can be usual latex for paper coating, such as any one or more of styrene butadiene latex, propyl benzene latex, methyl methacrylate styrene butadiene latex, styrene acrylic resin, acrylic emulsion, and polyurethane emulsion.

The auxiliary additive can be any known auxiliary additive for paper coating. Generally the auxiliary additive is selected from the group consisting of starch, water proofing agents, lubricants, defoamers, dispersants, rheology modifiers, fluorescent whitening agents, dyes, and any combination thereof. For example, the auxiliary additive may contain starch in an amount by weight of 0 parts to 7 parts, water proofing agent in an amount by weight of 0.15 parts to 1 part, lubricant in an amount by weight of 0.25 parts to 1 part, defoamer in an amount by weight of 0.05 parts to 0.5 parts, dispersants in an amount by weight of 0.05 parts to 0.5 parts, rheology modifier in an amount by weight of 0 parts to 1 part, fluorescent whitening agent in an amount by weight of 0 parts to 3 parts, and dye in an amount by weight of 0 parts to 1.5 parts.

The coating composition is prepared by mixing NFC, pigment, latex, alkali, auxiliary additive and water together. The coating composition generally contains solids in an amount of 25% to 75% by weight. The coating composition may have a pH value of 8 to 10.5.

The coating composition may also be prepared by adding the above-mentioned raw materials of the coating composition in a certain order. One such example for making the coating composition includes the following steps: (1) mixing pigment in an amount by weight of 75 parts to 95 parts and some water together; (2) adding alkali in an amount by weight of 0.1 parts to 1 part, to make the pH value of the mixing system from 8.5 to 10; (3) adding auxiliary additive in an amount by weight of 0.35 parts to 10 parts; (4) adding latex in an amount by weight of 5 parts to 15 parts; (5) adding NFC in an amount by weight of 0.02 parts to 10 parts; and (6) adding water if necessary to adjust the mixing system's solids content; and thus the coating composition is made.

Benefits of adding the NFC in the coating composition may include one or more of the following: (1) the NFC has a high aspect ratio and good flexibility, which can effectively improve the fracture resistance of the coating composition and provide capability to apply heavy coats to paper without high risk of the coat cracking; (2) the small amount of the NFC can change the viscosity and high-shear viscosity of the coating composition, and the NFC can act as a rheology modifier and film-forming auxiliary of the coating composition; (3) the carboxyl groups and the aldehyde groups of the NFC can crosslink with functional groups of the components in the coating composition, and improve the quality of the coated paper; (4) the NFC can substitute a portion of the latex required in the coating composition; and (5) the negatively charged NFC can attract some of the positively charged particles in the coating composition, which can improve the dispersion of the coating composition particles.

A coated paper applied with the above-mentioned coating composition includes a paper substrate and at least one coat formed on one or both surfaces of the paper substrate. The coat formed on one surface of the paper substrate can be a monolayer or a stacked multilayer. The coating composition containing NFC applied on the surface of the paper substrate can effectively improve the fracture resistance of the paper, and can reduce the risk of heavy coat cracking and mechanical failure of the coat such as spallation, blistering, tearing, and peeling away from the paper substrate. Furthermore, the coating composition containing NFC can also reduce the roughness of the coated paper and make the surface of the coated paper much smoother. Therefore, the coated paper applied with the coating composition containing NFC can be used as high-grade printing paper, wrapping paper for foods or cosmetics, etc.