| The sizing agents which can be used in neutral/alkaline condition have been used more and more widely as the sizing process changed from acidic to neutral/alkaline. Alkyl ketene dimer(AKD) was widely used in the paper industry, owing to its high sizing efficiency, appropriate hydrolytic stability, without emulsification in situ, less stickies and scale forming on roller. However, AKD size has the following disadvantages:(1) the solids content of commercial AKD emulsion is about 15%, and its period of validity is only for three months under 5-32℃, which increase the costs of transportation and preservation;(2) AKD molecule will hydrolysis in aqueous system influenced by temperature and pH, and generates β- keto acid, which causes the sizing reversion in sized paper.(3) It needs a period of time for AKD to obtain a final sizing degree. Therefore, it is significantly important to reduce the hydrolysis of AKD during the transportation, storage and sizing process, and thus improving its sizing efficiency.In this paper, cationic starch and polyvinyl alcohol(PVA) were used as protective colloids to prepare AKD powder. The effect of reaction parameters on the water dispersibility of AKD emulsion, water re-dispersibility and sizing performance of prepared AKD powder, and mechanism thereof were investigated.First, cationic starch was used as a protective colloid to study the influence of surface modifying conditions, such as dosage of protective colloid, reaction temperature, reaction time, stirring speed and amount of aluminum sulfate on AKD emulsion characteristics and the sizing efficiency. And then the emulsion was dried using oven to obtain a starch modified AKD powder. The water re-dispersibility and sizing performance of AKD powder were discussed. The results showed that, the mixture of AKD and 5% starch and 0.2% aluminum sulfate(both based on the total emulsion) were stirred at 10000r/min at 70℃ for 10 min, the resulting AKD emulsion can be stably dispersed in water. While the water re-dispersibility and sizing performance of AKD powder still need to be further improved.Based on the study above, both cationic starch and PVA were used as protective colloid, aiming to synthesis a stable and re-dispersible AKD powder. Zeta potential analysis, particle size analysis, infrared spectrum analysis, scanning electron microscopy and Cobb60 analysis were performed to study the influence of dosage of PVA, drying methods and drying temperature on the dispersion property and sizing efficiency. Results showed that, due to its good film-forming and hydrophilic properties, PVA can improve the redispersibility of AKD/PVA powder. At the dosage of PVA was 3%(to the total emulsion), the best re-dispersible sizing agent powder was obtained. Compared with oven drying and freeze drying, spray drying was more suitable to dry the emulsion, and better re-disperbility and sizing efficiency was achieved. As the dosage of re-dispersible sizing agent powder was 0.5%, the Cobb60 value was 32.66 g/m2, which was close to that of commercial AKD emulsion at dosage of 0.25%.In order to improve the sizing effect of AKD powder and improve the sizing lag phenomenon, in this paper, we also used cationic starch as single protective colloid to prepare a re-dispersible AKD/starch powder by strengthening the protective colloid effect of starch through prompting the esterification between AKD and cationic starch. Results indicated that the suitable parameters for the reaction were as follows: the mass ratio of AKD to starch was 15%, reaction temperature was 65℃, the pH was 8.5, reaction period was 2.0h. When the dosage of the prepared AKD/starch powder was 1.25%, the Cobb60 value reached up to 22.5g/m2, and the curing rate of paper off-machine was 90-100% without any lag. It showed that, AKD/starch powder significantly improved both the sizing effect and sizing lag. Moreover, AKD/starch powder has a good storage stability, it still has good redispersibility and sizing performance after being stored for 4 months. |
PDF Full Text Request