Colloids and Surfaces A: Physicochemical and Engineering Aspects

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1 Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (211) Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects journa l h omepa g e: Roles of short amine in preparation and sizing performance of partly hydrolyzed ASA emulsion stabilized by Laponite particles Pengxiang Ding, Wenxia Liu, Zhenhuan Zhao Shandong Provincial Key Laboratory of Fine Chemicals, Shandong Polytechnic University, Daxue Road, Western University Science Park, Jinan, Shandong 25353, PR China a r t i c l e i n f o Article history: Received 28 January 211 Received in revised form 23 March 211 Accepted 24 March 211 Available online 2 April 211 Keywords: Sizing ASA Hydrolyzed ASA Laponite Short amine Emulsification Solid stabilized emulsions a b s t r a c t Alkenyl succinic anhydride (ASA) is an internal sizing agent used to hydrophobize paper and paperboard in paper making process. Partly hydrolyzed ASA (hereinafter referred to as PH-ASA) emulsions can be stabilized by Laponite and short amines such as, and. It is found that the short amines may improve the affinity of Laponite particles to PH-ASA, favoring PH- ASA emulsification, forming emulsion of high stability and small droplets. It is further observed that the sizing development of the PH-ASA on bleached chemi-mechanical pulp (BCTMP) is promoted when aluminum sulfate is added. Amine with longer carbon chain is more efficient both in promoting PH-ASA emulsification and in its sizing process. The amine molecules adsorbed on Laponite are more effective in stabilizing PH-ASA emulsions while the free amine in the dispersion work more efficiently in promoting sizing performance through a mechanism of accelerating the reaction between hydrolyzed ASA and aluminum sulfate. However, the PH-ASA emulsions lose most of their sizing effect by hydrolysis within 6 min. 211 Elsevier B.V. All rights reserved. 1. Introduction Internal sizing is a process to hydrophobize paper and paperboard by introducing sizing agent in the fiber substrate. Alkenyl succinic anhydride (ASA) is commonly used in the neutral/alkaline papermaking process. It is an oily liquid at room temperature and is added to fiber slurry in a form of aqueous emulsion. Because of its relatively high chemical reactivity as an anhydride, it is possible to develop adequate sizing degrees online of paper machine by reacting with the hydroxyl groups on polysaccharide components in the paper substrate [1 3]. It is generally accepted that the sizing mechanism of ASA involves esterification of the hydroxyl groups of cellulose. However, most early researches use cationic starch which also contains hydroxyl groups as ASA emulsifier. It is questionable whether most of the ester bonds occur between ASA and cellulose or between ASA and cationic starch [1,4]. In addition, the high reactivity of ASA also promotes its rapid hydrolysis in aqueous solution. The hydrolyzed product alkenyl succinic acid (ASAcid), after being saponified, may render water resistance on paper substrate in the same way using rosin as sizing agent [5]. It was also found that, although the Corresponding author at: School of Light Chemistry and Environment Engineering, Shandong Polytechnic University, Daxue Road, Western University Science Park, Jinan, Shandong 25353, PR China. Tel.: ; fax: addresses: liuwenxia@spu.edu.cn, liuwenxia@sdili.edu.cn (W. Liu). reactive ASA is necessary for efficient sizing, most of the ASA retained in sized paper is hydrolyzed [6 8], which can be removed by nonionic surfactant [8]. Furthermore, Scalfarotto found that the aluminum salts generated from reaction between hydrolyzed ASA and aluminum ion can improve the sizing efficiency [9]. Our early research also showed that the addition of aluminum sulfate can boost the sizing performance of ASA [1]. Therefore it is logical to speculate that hydrolyzed ASA may have a significant contribution to ASA sizing [11], and ASAcid might hydrophobize paper the same way like dispersed rosin size. Controversially, numerous researches show that, when added to fiber slurry, hydrolyzed ASA emulsion does not produce any sizing effect although the hydrolyzed ASA is more hydrophobic than ASA itself [7]. As a free acid, the hydrolyzed ASA even acts as a de-sizing agent [12]. Isogai and coworkers ascribed such interfering effect to the coalescence between free ASAcid and ASA, which results in the formation of large flocs of size particles and consequently inefficient distribution on paper web [6 8]. Furthermore, our research found that even partly hydrolyzed ASA can be well emulsified and stabilized by either carboxymethyl cellulose or methyl cellulose, such emulsion can not render any sizing on bleached chemi-mechanical pulp (BCTMP). However, when the partly hydrolyzed ASA is emulsified and stabilized by Laponite combined with short amine, it performs rather well on sizing BCTMP. Therefore, it is interesting to know what roles the short amine play during the emulsification and sizing of partly hydrolyzed ASA /$ see front matter 211 Elsevier B.V. All rights reserved. doi:1.116/j.colsurfa

2 P. Ding et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (211) Experimental 2.1. Materials Laponite was provided by Rockwood Additives Ltd. with a trade name of Laponite RD. It is disc-shaped crystal with a thickness of 1 nm and a diameter of 25 3 nm. Ethylamine, and are all extra pure (+99%) grades provided by Sinopharm Chemical Reagent Co. Ltd. Partly hydrolyzed ASA (referred to as PH-ASA) is a commercial product from Hengtai Chemical Reagent Co. Ltd., Liaoning, China. It was a mixture of hexadecenyl succinic anhydride, octadecenyl succinic anhydride and their hydrolyzed products. The melting point is around 35 C. Aspen bleached chemi-mechanical pulp (BCTMP) was supplied by Shandong Huatai Paper Co. Ltd. After being torn to pieces and immersed in tap water overnight, the aspen BCTMP was first disintegrated at a concentration of 12 g/l for 3, revolutions with a pulp disintegrator (Lorentzen and Wettre) and refined to 35 SR with a laboratory valley beater. Cationic acrylamide copolymer (CPAM) with trade name Percol 292 was provided by Ciba Specialty Chemicals Ltd. Modified bentonite was obtained by purifying and modifying calcium bentonite with sodium fluoride according to reference [13]. All the other chemicals are analytical reagents Preparation and characterization of PH-ASA emulsion 2 g/l Laponite dispersion and 1 g/l amine solution were made by directly disperse Laponite and short amine in deionized water. The Laponite dispersion was then mixed with amine solution to obtain amine-modified Laponite. The free amine in the mixture was removed by a centrifuger and the product was washed with distilled water until the mixture can no longer be laminated with centrifuging at 4 rpm for 1 min. The negative surface charge density of the obtained modified Laponite was measured by polyelectrolyte titration with a Mütek PCD-3 particle charge detector (Mütek, Herrsching, Germany). Without free amine removed, the interface tensions between the modified Laponite and PH-ASA were determined by an automatic interfacial tensiometer (JYW-2, Chengde Dingsheng Tester Co. Ltd.) at the emulsifying temperature of 35 C. The amine modified Laponite dispersion, with or without containing free amine, was employed as water phase. After being adjusted to ph 6 with 2 wt% HCl solution and warmed up to 35 C, the water phase was added to the melted PH-ASA with a temperature of 4 C. The mass fraction of the PH-ASA was set at 25%. Then the resulting mixture was treated at 1 rpm for 3 min with a FM2 high shear dispersing emulsifier (FLUKO Equipment Shanghai Co. Ltd.). The emulsifying effect of the modified Laponite on PH-ASA was evaluated by final emulsion volume fraction, defined as the ratio of emulsion volume to the total volume of emulsified system. The droplet size and morphology of PH-ASA emulsions were analyzed by BK2 optical biomicroscope (Chongqing Optical and Electrical instrument Co. Ltd.). Average droplet size and droplet size distribution were obtained by application of microscopic image analysis software Handsheet making and analysis Aspen BCTMP was disintegrated and diluted to form a 1 wt% pulp suspension and adjusted to around ph 6 with 5 wt% HCl solution, then aluminum sulfate (Al 2 (SO 4 ) 3 18H 2 O) was added at a stirring speed of 5 rpm. After a period of 6 s stirring, the PH-ASA emulsion was introduced followed by addition of CPAM. After each addition, the pulp suspension was stirred for 6 s. Then the stirring speed was enhanced to 12 rpm, having the mixture undergo Fig. 1. Infrared spectrum of partly hydrolyzed ASA. 3 s of high shearing. After that, the stirring speed was returned to 5 rpm, and sodium modified bentonite was added and mixed for 6 s. Handsheets with a basis weight of approximately 6 g/m 2 were prepared using PTI Rapid-Kőthen Blattbildner-sheet Former (ISO ). Finally the handsheets were dried in an oven at 15 C for 3 min. The charge level of aluminum sulfate, CPAM and bentonite were 1 wt%,.5 wt% and.2 wt%, respectively, based on oven-dry BCTMP. The sizing degree of handsheets was measured by the liquid permeation method in accordance with China National Standard GB/T [14]. To measure the sizing degree, the handsheets were cut into square paper samples of 3 mm 3 mm, and conditioned at 23 ± 1 C and 5 ± 2% relative humidity for over 24 h. A paper sample was folded into a square vessel with a bottom of 2 mm 2 mm. The paper vessel was floated on the surface of 2 wt% ammonium thiocyanate solution, and a drop of 1 wt% ferric chloride solution was dripped on the bottom of the vessel at the same time. The penetration time was determined until red ferricthiocyanate spots appeared. The sizing degree of the handsheets was the average permeation time of ten paper vessels, for which half of their inner bottom are wire-side of the handsheets. The reaction of PH-ASA with aluminum sulfate and as well the influence of amine on sizing of paper substrate were characterized by a Shimadzu IRPrestige-21 FTIR spectrophotometer. The infrared spectra of PH-ASA emulsions/aluminum sulfate mixtures were done in the presence of carboxylmethyl cellulose (CMC). The samples were first coated on glass slide and dried at 15 C for 3 min before infrared analysis. CMC was used as a model compound of cellulose fiber. 3. Results and discussion In infrared spectrum, the carbonyl stretching frequencies of ASA are usually located at approximately 1782 and 1863 cm 1 while the adsorption frequency of the hydrolyzed product ASAcid appears at approximately 171 cm 1 [3]. Fig. 1 confirms the existence of the hydrolyzed product in the PH-ASA Role of short amine played in preparation of ASA emulsion Short amines such as, and n- butylamine are amphiphilic after being ionized at ph 6. They carry positive charges, and may be attached to negatively charged Laponite surface through electrostatic attraction, reducing the negative surface charge density of Laponite, and increasing the affinity of Laponite towards PH-ASA, both of which consequently favors the emulsification of PH-ASA. Fig. 2 shows the effect of amine dosage on the final emulsion volume fraction of emulsified PH-ASA. The negative surface charge density of Laponite and the interface tension between the modified Laponite dispersion and melted PH- ASA as the function of amine dosage were shown in Fig. 3. The charge of the modified Laponite based on ASA is 2 wt%. Apparently, the introduction of short amine can significantly improve

3 152 P. Ding et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (211) Fig. 2. Effect of amine dosage on emulsion volume fraction of PH-ASA. Laponite dosage (wt% based PH-ASA) the final emulsion volume fraction of emulsified PH-ASA possibly via a mechanism of reducing the interface tension between PH- ASA and water phase. The reduction of Laponite negative surface charge density also promotes the adsorption of nanoparticles at PH-ASA and water interface [15]. Short amine bearing more carbon atoms is more efficient in reducing the surface negative charge density and the interface tension thanks to its longer hydrophobic chain [16]. For, and at the dosage levels of 12 wt%, 8 wt% and 6 wt% based on Laponite, respec- Negative surface charge density (µeq/g) Interface tensions between Laponite dispersions and ASA (mn/m) (a) Amine dosage (wt% based on Laponite) (a) Amine dosage (wt% based on Laponite) Fig. 3. Effect of amine dosage on (a) negative surface charge density of Laponite and (b) interface tensions between Laponite dispersions and PH-ASA. Fig. 4. Effect of the dosage of amine modified Laponite on (a) PH-ASA emulsion volume fraction and (b) average particle size. tively, the modified Laponite correspondingly demonstrate the best performance both on stabilizing the emulsions and on lowering the interface tension while their negative surface charge densities also reach their inflection points. Therefore, lowering the interface tension between oil and water, and promoting the adsorption of Lapontie particles at the oil-water interface are the important roles that short amine, together with Laponite particles, play in stabilizing PH-ASA emulsions. Since the performance of a sizing agent depends mainly on its emulsion stability and particle size, a series of emulsification experiments were carried out to investigate whether a stable PH-ASA emulsion with appropriate particle size can be prepared through optimizing the addition level of amine modified Laponite. The results are shown in Fig. 4. The charge amount of, n- propylamine and based on Laponite were 12 wt%, 8 wt% and 6 wt%, respectively. As shown in Fig. 4, the amine with longer carbon chain shows better improvement on the emulsification effect of the modified Laponite on PH-ASA at lower Laponite dosage. When the dosages exceed 3 wt%, all the three modified Laponite are able to emulsify PH-ASA into stable emulsions, meanwhile the average size of emulsion droplet is reduced to several micrometers. This indicates that all these three amine modified Laponite are competent stabilizers of PH-ASA emulsions at appropriate dosages. For solid particle stabilized oil-in-water emulsions, the solid particles with bigger diameter stabilize emulsions with larger average droplet [17]. Laponite particle size is far less than 1 nm while the Laponite is easily dispersed. Theoretically, Laponite can stabi-

4 P. Ding et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (211) Fig. 5. Optical microscope image and droplet size distribution of PH-ASA emulsions stabilized by amine modified Laponite before (top) and after (bottom) the free amine were removed: (a) modified (b) modified (c) modified. lize PH-ASA emulsions of particle size less than 1 m unless the latter aggregates into large particles, or, if the emulsification process is interfered by free amine. Since the primary amine group of the short amines used in current research is only partly ionized at ph 6, unionized amine will stay as free molecules in the modified Laponite dispersions. Since amphiphilic nature of free amine may promote their adsorption preferentially on PH-ASA-water interfaces [18,19], it is interesting to know whether it favors the formation of stable PH-ASA emulsions. Therefore, the dispersions of Laponite, modified by 12 wt%, 8 wt% and 6 wt%, respectively, were centrifuged and washed to have free amine removed. The resulting products were employed to emulsify the PH-ASA. Fig. 5 displays the optical micrograph and droplet size distribution of PH-ASA emulsions stabilized by amine modified Laponite before and after the free amine were removed. Table 1 lists the Table 1 Average droplet size of PH-ASA emulsions. With free amine Stabilized by Laponite modified with Ethylamine n-propylamine n-butylamine Not removed Removed average droplet size of the corresponding PH-ASA emulsions stabilized by modified Laponite with and without free amines. The Laponite dosage was 3 wt% based on PH-ASA. All thus prepared PH- ASA emulsions have a 1% emulsion volume fraction. As shown in Fig. 5 and Table 1, after the free amine were removed, the PH-ASA emulsions stabilized with modified Laponite have much smaller droplets and more uniform size distribution. It is hypothesized that the amine modified Laponite increases its affinity towards PH-ASA by adsorbing amine as intermediate so to facilitate the emulsification of melted PH-ASA. The free amine in aqueous phase otherwise affect adversely on the emulsification process. The reason may be that the preferential adsorption of free amine molecules at PH-ASA and water interface interferes with the adsorption of the modified Laponite particles Sizing performance of PH-ASA emulsions and the role amine played in sizing process Fig. 6 shows the sizing performance of PH-ASA emulsions stabilized by 3% of modified Laponite, before and after free amine were removed. The concentration of, and based on Laponite was 12 wt%, 8 wt% and 6 wt%, respectively. As shown in Fig. 6, the PH-ASA emulsions stabilized by amine modified Laponite demonstrated better sizing perfor-

5 154 P. Ding et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (211) Sizing degree (s) (a) unmodified PH-ASA dosage (wt% based on BCTMP) Sizing degree (s) without retention agent with retention agent Aluminum sulfate dosage (wt% based on BCTMP) 5 (b) Fig. 7. Effect of aluminum sulfate dosage on sizing degree of handsheet sized with PH-ASA emulsion. Sizing degree (s) emulsions without free amine provide much smaller particle size than those with free amine, as shown in Fig. 5. Therefore, the higher hydrophobicity of amine with long carbon chain and/or promoting effect of free amine may account for the superior performance of the PH-ASA emulsion stabilized with modified Laponite Roles of aluminum sulfate and amine played in promoting PH-ASA sizing PH-ASA dosage (wt% based on BCTMP) Fig. 6. Sizing performance of PH-ASA emulsions stabilized by amine modified Laponite (a) before and (b) after free amine removed. mance, in contrast, the ASA emulsion prepared by unmodified Laponite did not offer any sizing on paper substrate. The amine with longer carbon chain performed better in promoting sizing development. After free amine removed, the PH-ASA emulsions stabilized with modified Laponite gave lower sizing. The sizing improvement of modified PH-ASA emulsions may be attributed to the better emulsion stability and smaller particle size. However, the emulsions stabilized by different amines have similar particle size and stability, and furthermore modified Laponite stabilized PH-ASA Fig. 7 shows the results of handsheets sized by PH-ASA emulsion stabilized with modified Laponite. The sizing degree was plotted as a function of aluminum sulfate dosage with or without retention aid (CPAM/bentonite microparticle retention system, if applicable). The amount of PH-ASA is set at.4% based on BCTMP. Apparently, the performance of PH-ASA emulsion stabilized with amine modified Laponite depends in great extent on the employment of both aluminum sulfate and retention aid. Aluminum sulfate may partially play the role of retention aid. However, in the case using CPAM/bentonite retention system only, the PH-ASA emulsion can not develop measurable sizing without aluminum sulfate, implying that aluminum sulfate must have participated in the sizing reaction once adequate sizing is developed. Fig. 8 shows the infrared spectra of PH-ASA emulsion/cmc mixture dried at 15 C with or without presence of aluminum sulfate. As shown in Fig. 8, after being dried with CMC, the carbonyl stretching frequency of ASA at 1782 and 1863 cm 1 disappeared, while the stretching frequency of hydrolyzed ASA at 1712 cm 1 strengthened. This is evident that PH-ASA had completely hydrolyzed after Fig. 8. Infrared spectra of PH-ASA emulsion/cmc mixture. (1) Laponite stabilized emulsion, (2) emulsion stabilized with modified Laponite after removing free amine, and (3) emulsion stabilized with modified Laponite before removing free amine.

6 P. Ding et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (211) drying. The adsorption at 1554 cm 1 may be due to the existence of calcium ions in commercial CMC [2]. In presence of aluminum sulfate, the stretching at 1597 cm 1 which is attributed to carboxyl groups coordinately bond with aluminum ions, appeared [2]. This indicates that a reaction of hydrolyzed ASA and aluminum sulfate takes place. The weakest adsorption at 1597 cm 1 was given by PH- ASA stabilized with unmodified Laponite, in contrary, the strongest adsorption was given by emulsion stabilized with modified Laponite without removing free amine. The sizing performance follows the same pattern. Therefore, it was concluded from the infrared spectra that the sizing effect of PH-ASA is facilitated by the reaction of aluminum sulfate and hydrolyzed ASA. Moreover, the presence of amine, especially the free amine, promotes the said reaction, improving the sizing efficiency of PH-ASA emulsions Effect of storage time on stability of PH-ASA emulsions Solid particles may stabilize emulsions by forming a dense film around dispersed droplets impeding coalescence [21]. Amine modified Laponite as a solid stabilizer may form mechanical barrier for PH-ASA droplets to both coalescence and hydrolysis. Therefore, the effect of storage time on the morphology, average droplet size and sizing performance of amine modified Laponite stabilized PH-ASA emulsions were investigated. Fig. 9 shows the variation of the average droplet size as function of storage time. Fig. 1 shows the optical microscope image of PH-ASA emulsions stabilized by modified Laponite after being prepared from 1 min to 6 min. The charge amount of, and based on Laponite were 12 wt%, 8 wt% and 6 wt%, respectively. The Laponite dosage was 3 wt% based on PH-ASA. The free amine in Laponite dispersion did not removed. Apparently, the droplets of the PH-ASA emulsions grew larger with time when the storage time was less than 3 min but still sustained their spherical shape to some extent. Non-spherical emulsion droplets emerged at the storage time of 4 min resulting in rapid increase of the emulsion droplet size. With further extending of the storage time, the non-spherical emulsion droplets spread/coalesced to form extended domains with irregular shapes Average droplet size (µm) Storage time (min) Fig. 9. Average droplet size of PH-ASA emulsions stabilized by amine modified Laponite as function of storage time. while the appearance of these emulsions did not change significantly, i.e., neither water nor oil was released. In addition, when the emulsions were stored for 9 min, they could be well dispersed in both water and PH-ASA, and gradually jelled, indicating the formation of a fluid-bicontinuous gel [22]. This suggests that the variations of the emulsion droplet size and morphology originate from ASA hydrolysis rather than emulsion instability since nonspherical emulsions or bicontinuous structure are formed as oil and water are partially miscible [23], or the interface tension between oil and water is very low [24]. The ASAcid in PH-ASA may promote hydrolysis of the remained ASA leading to the partial wetting of PH-ASA on water and formation of non-spherical emulsions. Therefore, amine-modified Laponite may provide stability for PH-ASA emulsions but cannot prevent the hydrolysis of ASA. Fig. 11 shows the effect of storage time on the sizing performance of PH-ASA emulsions stabilized by amine modified Laponite. The amount of PH-ASA and aluminum sulfate based on BCTMP were Fig. 1. Optical microscope image of PH-ASA emulsions stabilized by modified Laponite after being prepared from 1 min to 6 min.

7 156 P. Ding et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (211) Province. The authors are also grateful to Dr. Guohong Yin for language revision, Mr. Mingzhao Ding and Miss Jinxia Wang for the preparation of handsheets. References Fig. 11. Effect of storage time on sizing performance of PH-ASA emulsions stabilized by amine modified Laponite..5 wt% and 1 wt%, respectively. As shown in Fig. 11, the sizing performances of the PH-ASA emulsions deteriorated remarkably with time, especially as the time exceeded 3 min, which correlated well with the variation of the emulsion droplet size and morphology as shown in Figs. 9 and 1. n-butylamine modified Laponite stabilized PH-ASA emulsion performed better but deteriorated quicker. At the storage time of 6 min, the PH-ASA emulsions lost most of their sizing performances, indicating well dispersed emulsions are prerequisite for the sizing development of the PH-ASA. 4. Conclusions Short amines noticeably improve the emulsifying performance of Laponite on PH-ASA, and also the sizing performance of Laponite stabilized PH-ASA emulsions. Among the three short amines investigated, is the most efficient one, in promoting the emulsification of PH-ASA and in the sizing performance as well. The amine molecules adsorbed on Laponite are more efficient in stabilizing PH-ASA emulsions owing to the increased affinity, while the free amine in modified Laponite dispersions are more efficient in promoting the sizing performance of PH-ASA emulsions due to the accelerated reaction between hydrolyzed ASA and aluminum sulfate. However, the PH-ASA emulsions lose most of their sizing performances within 6 min by hydrolysis. Acknowledgments The project was funded by National Natural Science Foundation of China (297699), Shandong Natural Science Foundation (ZR21CM8) and Taishan Scholar Program of Shandong [1] J.M. Gess, D.S. 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