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In this experiment, we explored how to control the penetration of palmitoyl chloride in the gas grafting of PVOH-coated paper. When calcium carbonate was pre-coated, the gas grafting reaction of the PVOH-coated paper tended to be inhibited. It is believed that palmitoyl chloride is additionally consumed by moisture generated when calcium carbonate neutralizes hydrochloric acid gas, a by-product of the graft reaction, thereby inhibiting the graft reaction and reducing the hydrophobization efficiency by generating unreacted residual free fatty acids. Unlike calcium carbonate, the clay coating layer blocks the penetration of vaporized palmitoyl chloride and improves the gas grafting density of PVOH without moisture generation. The PVOH coating layer, where vaporized palmitoyl chloride penetrates, is thinly processed to avoid unnecessary graft reactions in the thickness direction, and the propagation of palmitoyl chloride in the gas phase is blocked by the clay-coating layer. In previous studies, an excess of 1400 mg/m2of palmitoyl chloride was required to secure strong hydrophobicity of less than 10 g/m2of Cobb hot water absorption. However, by pre-coating the clay, only 650 mg/m2of palmitoyl chloride, which was about a 53% reduction compared to previous studies, was sufficient to perform a strong hydrophobic treatment.

Gas grafting treatment with fatty acid chloride is a hydrophobization method based on the principle of forming ester bonds by reacting hydroxyl groups on the surface of hydrophilic substances with a gaseous fatty acid chloride. In order to increase the hydrophobization efficiency through gas grafting treatment, it is necessary to form an ester bond concentrated on the surface of the polyvinyl alcohol (PVOH) coating layer. When palmitoyl chloride, a fatty acid chloride, reacts with the hydroxyl groups of PVOH to form polyvinyl palmitate, the by-product hydrochloric acid promotes the dehydration of PVOH, causing discoloration while producing polyacetylene. Using this discoloration principle of polyene, it was estimated whether evaporated fatty acid chloride penetrated into the PVOH coating layer. The top layer was coated with PVOH having a saponification degree of 89 %, and the bottom layer was coated with PVOH having a saponification degree of 99 %. There was a difference in the discoloration according to the degree of saponification of PVOH, even the amount of palmitoyl chloride was small as much as 300 mg/m2. It was confirmed that the vaporized fatty acid chloride during gas grafting treatment permeates the PVOH layer of 7 g/m2even in a very small amount.

During the manufacture of low basis weight tissue paper, it is difficult to efficiently use the dry strength agent (DSA) because a large amount of DSA adsorbed fines releases in forming roll by centrifugal forces. In this study, cationic polyvinylamine (C-PVAm) was used as a DSA in an environment where the retention of fines was weak. Addition of C-PVAm to the thermomechanical pulp (TMP) or TMP mixed with softwood bleached kraft pulp (SwBKP) improved the turbidity of filtrate from sheet former, however, the strength of handsheet paper was similar to that without C-PVAm. When C-PVAm was selectively added to SwBKP as much as 4 %, the tensile index could be improved by approximately 10.6 % without changing the retention of fines. In addition, C-PVAm added before the beating of SwBKP showed better results than C-PVAm added after the beating in terms of fines retention, tensile index, and formation. In particular, the tensile index was improved about 7.7 % by adding of C-PVAm 4 % as is before beating of SwBKP. Consequently, it was found that C-PVAm with a high reaction rate can be added before beating of SwBKP to improve the physical strength.