EPS was obtained from the local market. Decalin, cyclohexane and methylcyclohexane with reagent grade were bought from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). The concentration of polystyrene was determined with a UV-7504C UV-vis Spectro-photometer at 261.5 nm. The ultrasonic wave was generated by BL10-250A ultrasonic generator.

The EPS was dissolved in different cycloalkane solutions and diluted to form solutions with different polystyrene concentrations. The data of the concentration (c) of polystyrene and absorbance (a) were fitted to a standard curve, namely a = 2.39702c − 0.02258 (R = 0.99967).

A certain amount of cycloalkane solution was put into a beaker and then the beaker was placed in a water bath with stirring and temperature control device. EPS was made into a rectangular strip with a cross-sectional area of 2 cm × 2 cm and inserted vertically into the solvent (cycloalkane solution). After isolation, only one side of the EPS was in contact with the solvent in the dissolution process. Sampling and analysis were carried out to calculate the thickness (d) of the EPS.

The thickness of a single EPS face would decrease in the process of dissolution. In this condition, mass transfer is the rate control step. The dissolution rate per unit cross-sectional area of EPS is expressed as:

− 1 A d W d t = − ρ s d L d t = k Δ C (1)

where A is the cross-sectional area of the solvent surface in the unit of cm², W is the weight of EPS in the unit of g, L is the thickness of EPS in the unit of cm, ρ_s is the density of EPS in the unit of g/cm³, ΔC is the difference between the saturation concentration of solute in the sphere and the overall concentration in the solvent in the unit of mg/ml, k is a constant.

In the initial condition of t = 0 and d = 0, the expression of (1) could be rewritten as:

d = k Δ C ρ s t (2)

where d has a linear relationship with t, and the slope is mass transfer coefficient (m).

EPS strips were dissolved in three cycloalkane solvents (decalin, cyclohexane and methylcyclohexane) at the temperature of 25˚C and stirring rate of 200 rpm, respectively. The effect of solvent on EPS dissolution strips is shown in Figure 1.

As shown in Figure 1, the dissolution rate of EPS in the three solvents increases linearly in the first 4minutes, and then gradually become constant, respectively. It can be seen that the dissolution rates of EPS in the three solvents are in the order of decalin (m = 0.19525) > methyl cyclohexane (m = 0.13650) > cyclohexane (m = 0.11372). A large number of bubbles rise during the dissolution process, which will adhere to the dissolving face of EPS, preventing the dissolution process. Meanwhile, the increment of the distance between the blades of the mixer and the dissolving face weaks the influence of the stirring on the dissolution. So the dissolution curve gradually becomes stable. Decalin was used as solvent for subsequent experiments as the dissolution rate of EPS in it is higher than that of the other two solvents.

The dissolution of EPS at temperatures of 15˚C, 25˚C and 35˚C was carried outin 300 ml of decal insolvent with stirring rate of 200 rpm. The experimental result is shown in Figure 2.

The temperature has a significant impact on dissolution rate. It is observed that there is an obvious increase of dissolved thickness of EPS at the temperature

of 25˚C and 35˚C. The smooth curve of 15˚C indicats that low temperature is not good for EPS dissolving. The effect of temperature was confirmed by the value of tree mass transfer coefficients, m = 0.03582, 0.19525 and 0.32996, representing for that of 15˚C, 25˚C and 35˚C, respectively.

To compare the effect of mechanical agitation andultrasonic wave on EPS dissolution. The dissolution process was conducted in 300 ml of decal insolvent at temperature of 25˚C. The result is shown in Figure 3.

At the same condition, the dissolution curve of mechanical stirring indicated a faster dissolution rate (m = 0.19525) than that of ultrasonic wave (m = 0.10404). This could be attributed to the fact that the bubble appeared in the ultrasonic wave-assisted dissolution process attached finely to the dissolution interface, which impeded the mass transfer.

The EPS dissolution at stirring rate of 200 rpm, 300 rpm and 400 rpm was studied in 300 ml decalin solvent at 25˚C. The effect of stirring rate on EPS dissolution is shown in Figure 4.

The dissolution rate of EPS increased steadily with dissolution time, and higher dissolution rate appeared at the faster stirring rate. The fitted mass transfer coefficients for three dissolution process is 0.19525 (200 rpm), 0.22792 (300 rpm) and 0.38521 (400 rpm), respectively. The transition of the curve of 200 rpm after 3 minutes indicates that low stirring rate could not cause enough disturbance to the EPS dissolution. It is more difficult to dispel the attached bubbles attached at the lower stirring rate as the dissolving face gradually become farther from the stirring blade.

The effect of dissolution face on EPS dissolution was conducted under temperature of 25˚C and stirring rate of 200 rpm in 300 ml of decal insolvent. The result is shown in Figure 5. It is found that the dissolution rate increases similarly for

the three faces in the first 3 minutes. However, the dissolution rate increases higher for the upper face in the stage of 3 to 5 minute. This trend can be explained by the observation that the number of attached bubbles to the upper face is less than that of the down face. While disturbance generated by bubbles in the process of upward floating could enhance mass transfer of the side face and top face of EPS. The mass transfer coefficient (m) of three dissolving face are 0.19525 (down face), 0.24372 (side face) and 0.27775 (top face), respectively.