Decorative paper, widely used in the surface finishing of wood-based panel, can not only protect the wood-based panel from wear, heat and contamination, but also endow the products with excellent visual effects(Paula, 2012; Tan, 2006). The properties of decorative paper are determined by the properties of the decorative base paper directly. Among the performance indices of decorative base paper, the two key indices are printability and absorbability. The former affects the print quality and the latter affects the absorption of melamine in the production of decorative paper(Santos et al., 2005; Liu et al., 2007). Hence, a huge dem and for decorative base paper with good absorbability and printability exists.

Surface treatment is considered as an effective method to meet the dem and , because it is crucial in obtaining good printability, surface property, shape stability and excellent physical strength of paper. However, good water-resistant and oil-resistant ability of paper is got by traditional manufacturing method, which adversely affects the absorbability of decorative paper(Hu et al., 2008; Xu et al., 2012). In the present study, a series of self-emulsifying SAE were synthesized by introducing acrylic acid as the functional monomer. The carboxyl of acrylic acid not only provides the hydrophilicity for latex but also reacts with other monomers. The crosslinking reaction between acrylic acid and other monomers endows the product with good film forming property, which leads to excellent printability of paper sheets(Zhang et al., 2011; Wang et al., 2006). Subsequently, the effect of dosages of acrylic and initiator and the mass ratio of hard monomer(styrene)withsoft monomer(butyl acrylate)on the performance of SAE and its surface-treatment behavior were determined. Finally, the emulsion properties of self-emulsifying styrene acrylate and the paper properties were determined.

The styrene(St), acrylic acid(AA), butyl acrylate(BA), methyl methacrylate(MMA), ammonium persulfate(APS) and sodium bicarbonate used were all analytically pure. The starch used in surface treatment was cassava starch and the α-amylase was BR. Decorative base paper with base weight of 110 g ·m^-2 was supplied by Sh and ong Zhengda Paper Co., Ltd. SAE was synthesized in laboratory.

Firstly, an amount of distilled water was poured into a four-port flask with a stirring device, a condensing device and a thermometer. After adding a certain quantity of sodium bicarbonate to the reactor, 1/3 mixed monomers(St, MMA, AA and BA, MMA was used as a type of hard monomer with the dosage of 2wt%) and 1/3 initiator(APS, 10wt%)were added to the flask while stirring, keeping the temperature 80 ℃. When the blue light appeared, the residual monomers were added dropwise within 60 min, whereas the residual initiator was added within 90 min. The temperature was then raised to 85 ℃ and maintained for 1.5 h. Finally, the self-emulsifying SAE latex was obtained and purified, by st and ing, separating by separating funnel and filtering, for further evaluation.

The reaction of AA was as follows:

A microscopic infrared spectroscopy(Nicolet iN10)was utilized to characterize the chemical groups on the paper. The emulsion particle morphology and the surface morphology of paper were both observed under a scanning electron microscopy(SEM)system(JEOL JSM-5310) and a laser particle sizer(Brookhaven 90Plus)was used to measure the particle size at a concentration of 0.01%. The charge density of latex was measured by a particle charge detection(PCD)system(BTG, PCD03)at a concentration of 10^-4g ·L^-1. The viscosity was measured by a rotational viscometer(Chuang Yu, NDJ-79)at the rate range of 1-100 r ·min^-1. The emulsion stabilities, including mechanical, storage, pH, calcium ion and dilution stability, were also investigated(Warson et al., 2001).

A certain amount of starch was cooked with the addition of α-amylase and the concentration was regulated to 10% by adding water. The gelatinized starch and self-emulsifying SAE were mixed with the weight ratio 20 :1. Surface treatment was performed at a concentration of 10% from 60 ℃ to 70 ℃ and the coating weight was 7-8 g ·m^-2. Based on a previous study(Xu et al., 2013), the films were prepared by a drawdown using wire-wound bars and dried on a glazer at 105 ℃, kept for 5 min.

The surface strength and absorbability(capillary rise)of the treated paper sheets were measured according to ISO 3783—2006 and ISO 8787: 1989, respectively.

The single factor experiment was designed and the influencing factors, such as the dosage of acrylic acid, the dosage of initiator, the mass ratio of hard monomer with soft monomer, were studied.

In the preparation of SAE, acrylic acid was introduced as the functional monomer. The carboxyl of acrylic acid not only provides the hydrophilicity for latex but also reacts with other monomers. The crosslinking reaction between acrylic acid and other monomers endows the product with good film forming property, which leads to excellent printability of paper sheets.

The particle size affects the emulsion performance and its application. In general, small and narrowly distributed particle size contributes to excellent emulsion performance. In addition, moderate charge density of latex leads to stronger interaction between emulsion and electronegative paper fiber. Thus, these two indices are of particular interest.

When the APS dosage was 0.3%, the ratio of St with BA was 1.5 :1 and the dosage of MMA was 2wt%, the effect of the AA dosage on the performance of SAE was investigated and the result was shown in Fig. 1.

	Fig.1 Effect of the AA dosage on the particle size and charge density of SAE

Fig. 1 showed that with the increased AA dosage, the emulsion particle size decreased at first then increased and the charge density increased. The finding was due to the increased carboxyl with increased dosage of AA, which led to an increased anionic degree of emulsion. The hydrophilic groups also increased and the polymer chains easily dispersed(Xu et al., 2013; Chen et al., 2002; Dziomkina et al., 2006), so the SAE particle size decreased. When the dosage of AA was larger than 8%, with increased charge density of polymer molecular chains, the electrostatic repulsion of emulsion particles increased, which led to decreased interaction between the particles(Wu et al., 2009). As a result, the emulsion particle size became larger.

The surface strength of paper sheet was selected to represent the printability in terms of the pick velocity and pick resistance of paper and in terms of the delamination resistance of the liner on paper, by simulating the behavior in the printing process. The capillary rise was selected to indicate the absorbability of paper sheet. Thus, the surface strength and capillary rise of decorative base paper are of particular interest.

Under the same reaction conditions as Fig. 1, the effect of the AA dosage on the surface-treatment behavior of SAE was investigated and the result was shown in Fig. 2.

	Fig.2 Effect of the AA dosage on the surface- treatment behavior of SAE

As shown in Fig. 2, increased AA dosage led to increased capillary rise of paper sheet. This finding can be attributed to the increased AA dosage causing the amount of hydrophilic carboxyl to increase on the emulsion molecular chains. As a result, the absorbability of paper sheet, treated by SAE, increased.

Fig. 2 also showed that with the increase of AA dosage, the surface strength of paper sheet initially increased and then decreased. It was because that the crosslinking reaction between AA with carboxyl and other monomers could endow the product with good film forming property. Within the certain range(5wt%-9wt%), the dosage of AA was larger, the film forming property was better and the surface strength of paper sheet increased. However, when the AA dosage was larger than 8%, the particle size increased(shown in Fig. 1). Owing to the big particle size, the polymer film on paper surface was not compact and poor film was got, the surface strength of paper sheet decreased.

Finally, the optimum AA dosage of 8wt% was obtained with smaller particle sizes and moderate charge density, which led to higher absorbability and higher surface strength of paper sheet.

The dosage of initiator has critical effect on the performance of latex. In this study, APS was chosen as the initiator and its dosage was investigated.

When the AA dosage was 8wt%, the ratio of St with BA was 1.5 :1 and the dosage of MMA was 2wt%, the effect of the APS dosage on the performance of SAE was investigated and the result was shown in Fig. 3.

	Fig.3 Effect of the APS dosage on the particle size and charge density of SAE

From Fig. 3, it was shown that with the increase of the APS dosage, the particle size decreased at first then increased and the opposite trend of charge density of emulsion was observed.

With the increase of the APS dosage, the formation rate of free radicals and the chain termination rate increased, leading to lower average molecular weight of polymer, and small particle size was got. When the APS dosage was larger than 0.4%, with the increased APS dosage, the termination rate of polymerization increased and the reaction rate relatively decreased. Thus the amount of latex particles, which participated in the reaction, became small and the particle size of latex became bigger.

On the other h and , when the dosage of APS was small, the reaction rate was low, resulting in the residual of some of the monomers. Because of the presence of residual monomers, the conversion of AA with carboxyl, was low, leading to low charge density of emulsion. And with the increase of APS dosage, the amount of the free radicals became larger and the rate of polymerization became faster, leading to high conversion of AA. Because of more anionic groups provided by AA on the polymer molecular chain, higher charge density of emulsion was obtained. When the APS dosage of was larger than 0.4%, the conversion of AA increased little and the charge density of latex almost unchanged.

Under the same reaction conditions as Fig. 3, the effect of the APS dosage on the surface-treatment behavior of SAE, including the surface strength and capillary rise, of decorative base paper were investigated.as shown in Fig. 4.

	Fig.4 Effect of the APS dosage on the surface-treatment behavior of SAE

It was shown in Fig. 4 that the surface strength of paper sheet, treated by SAE, increased at first then decreased and the opposite trend of capillary rise was observed. It was because with the increase of the APS dosage(from 0.1% to 0.4%), the particle size of latex decreased(shown in Fig. 3). So the permeation and film formation of polymer on the paper surface was easy, which was helpful for the formation of good polymer film. As a result, the surface strength of paper sheet increased. When the APS dosage was larger than 0.4%, the particle size of SAE became bigger and it was not close for the stacking of the particles when the polymer film formed, leading to the formation of poor film. So the surface strength of paper sheet decreased.

In addition, with the increase of the APS dosage, the particle size of SAE decreased at first then increased(shown in Fig. 3). When the particle size was small, it was close for the stacking of the particles when the polymer film formed, so it was difficult for the water’s permeation into the film, which was presented as low capillary rise. Oppositely, when the particle size became larger, poor film on the paper surface was got and it was easy for the permeation of water into film. So the capillary rise increased.

Finally, the optimum APS dosage of 0.4% was obtained with smaller particle sizes and moderate charge density, which led to higher absorbability and higher surface strength of paper sheet.

Different monomers have different film-forming property and hydrophobic property. So the dosage of each monomer is crucial for the performance of SAE. When the AA dosage was 8wt%, APS dosage was 0.4% and the dosage of MMA was 2wt%, the effect of St with BA on the performance of SAE was investigated and the result was shown in Fig. 5.

	Fig.5 Effect of the mass ratio of St with BA on the particle size and charge density of SAE

From Fig. 5, we could see that the particle size of emulsion changed little with the change of the mass ratio of St with BA, whereas the charge density decreased. The finding was due to the higher reactivity of St than other monomers(including BA), which led to the self-polymerization of St, rather than the copolymerization of AA and St. When the mass ratio of St with BA increased, the anionic groups provided by AA on the polymer molecular chain decreased and the smaller charge density of emulsion was obtained. And the ratio of St with BA had little effect on the emulsion particle size.

Under the same reaction conditions as Fig. 5, the effect of the St with BA on the surface-treatment behavior of SAE, including the surface strength and capillary rise, of decorative base paper were investigated, as shown in Fig. 6.

	Fig.6 Effect of the mass ratio of St with BA on the surface-treatment behavior of SAE

It was shown in Fig. 6 that with the increase of the mass ratio of St with BA, the surface strength of paper sheet, treated by SAE, increased at first then decreased and the capillary rise increased.

The more the hard monomer(St)was, which endowed the polymer film high strength and hardness, the better film was got. So the surface strength of paper sheet increased. When the mass ratio of St with BA was larger than 1.5 :1, the hardness of polymer film became too high and the film became brittle, shown as decreased surface strength.

On the other h and , the film-formation property of BA was better than St, so when the mass ratio of St with BA increased, looser film was obtained and the capillary rise of paper sheet decreased.

Finally, the optimum mass ratio of St with BA of 1.5 :1 was obtained with smaller particle sizes and moderate charge density, which led to higher absorbability and higher surface strength of paper sheet.

The performance of self-emulsifying SAE prepared under the optimum conditions, such as 8wt% AA, 0.3wt% ammonium persulfate, mass ratio of styrene(St)to butyl acrylate(BA)1.5 :1, was studied and the results were shown in Tab.1. Excellent emulsion performance was obtained in the laboratory, i.e., small particle sizes, low viscosity, moderate charge density and good stability. Fig. 7 showed that the SAE particles were spherical and with core- shell structure, which endowed the emulsion low film forming temperature, good stability and excellent mechanical properties(Zhang et al., 2008). From Fig. 8, we observed that the particles were well distributed, with the average diameter around 130 nm.

Tab.1 Performance of SAE prepared in lab

Tab.1 Performance of SAE prepared in lab Property Solids content(%) 35 Particle size/nm 130 Charge density/(mmol·L-1) 0.70 Viscosity/(mPa·s) 20 Mechanical stability Stable Storage stability stability Stable during 90 days pH stability 9-1 Calcium ion stability Stable, has no layered Dilution stability Stable

	Fig.7 Particle morphology of SAE

	Fig.8 Particle size distribution of copolymer latex

A microscopic infrared spectroscopy was utilized to characterize the chemical groups on the paper and the surface morphology of paper sheets were observed under a scanning electron microscopy. Fig. 9 showed that compared to untreated paper and the paper treated by starch, the peak at 1 727 cm^-1 was observed on the paper sheet treated by SAE, which indicated there was carbonyl of carboxylic. From Fig. 10, we could see that compared to the other paper sheets, more surface of paper, treated by SAE, was covered by film. For the paper sheet, treated by SAE, carboxyl on the paper endowed good absorbability and the polymer film endowed excellent printability.

	Fig.9 Microscopic infrared spectroscopy of paper sheets

	Fig.10 SEM of paper sheets

And the surface-treatment effect of SAE was shown in Tab.2. From the table, we could see that for the paper treated by SAE, the capillary rise had little change and the surface strength and smoothness had been improved greatly, compared with the values of untreated paper and national st and ard. The paper treated by starch had poor absorbability(shown as low capillary rise) and surface strength.

Tab.2 Physical properties of decorative base paper after surface treating ^①

Tab.2 Physical properties of decorative base paper after surface treating ① Capillary rise/mm·(10 min)-1 Surface strength/(m·s-1) Smoothness/s Untreated paper 20 0.8 105 Paper treated by self-made SAE 19 1.1 130 Treated by starch 16 0.9 110 National standard 20 — 100 ①The national standard is GB/T 24989—2010.

Self-emulsifying latex with excellent performance was obtained under the following conditions: 8wt% AA, 0.3wt% ammonium persulfate, mass ratio of styrene(St)with butyl acrylate(BA)1.5 :1, 130 nm emulsion particle size, moderate charge density of the emulsion and low viscosity. An excellent surface-treatment effect was obtained under these conditions.

The surface-treated paper was covered by film partially and carboxyl was observed on the paper. The printability of decorative base paper, treated by self-emulsifying SAE, significantly improved and the absorbability of paper sheet was maintained.