Preparation and application of polystyrene nanoparticles and paraffin emulsions

XUE Zhongheng, REN Qinyi, LIU Haijuan, KE Wentao, XIN Yue, ZHU Xinsheng

(Faculty of Textile and Clothing Engineering, Soochow University, Suzhou 215021)

Abstract: Sulfonated polystyrene nanoparticles (nano-SPS) were prepared by soap-free emulsion polymerization using styrene (St) and sodium styrene sulfonate (NaSS) as monomers. A paraffin Pickering emulsion was formed from nano SPS and paraffin under strong shear and was used to impregnate and finish viscose nonwoven materials to prepare phase change material. The morphology of nano-SPS, the stability of paraffin Pickering emulsion and the thermal properties of the phase change material were investigated. The results showed that NaSS participated in the copolymerization of St and nano-SPS was in monodispersion with a particle size about 40 nm and exhibited a relatively good hydrophilicity with a water contact angle of 60.4° when the volume fraction of NASS based on St was 25%; paraffin emulsion exhibited good storage stability when the conditions for emulsifying paraffin with nano-SPS were optimized as follows: nano-SPS mass fraction 1.5%, paraffin/water volume ratio 55/45; and the phase change material had the enthalpy of phase change up to 105.0 J/g, indicating a good phase change heat storage capacity, when the paraffin loading rate of the phase change material prepared by impregnating and finishing nonwovens with the above paraffin emulsion was 351.6%.

Key words: nonwoven; polystyrene; soap-free emulsion polymerization; paraffin emulsion；phase change materials

Paraffin emulsion is a new type of fluid with waterproofing, water retention, and heat transfer and storage functions[1]. It can be potentially used for wood protection and waterproofing[2]as well as solar energy storage[3]. Traditional paraffin emulsions often present supercooling while being cooled[4], however, a paraffin Pickering emulsion was reported to eliminate supercooling and leakage shortcomings since solid particles were adsorbed firmly on the oil-water interfaces by forming a rigid barrier with long shelf life[5-7]. Therefore, the research and development of environmentally friendly and practical paraffin Pickering emulsions has become a research hotspot in recent years[8-13]. Although a variety of particles have been used to stabilize paraffin Pickering emulsions, organic polymer costabilizers are always needed to complement because of extremely high hydrophilicity and rigidity of nanoparticles. In addition, the existing solid particles are not electrically charged, and thus high solid content Pickering emulsions are difficult to be produced.

Soap-free emulsion polymerization is a technique to prepare organic nanoparticles. In this study, a water-soluble monomer was adopted to copolymerize with an oil-soluble one to adjust the hydrophilicity, water contact angle, particle size and surface potential of sulfonated polystyrene nanoparticles (nano-SPS). Paraffin Pickering emulsion was obtained from nano-SPS particles and paraffin under strong shear, and a phase change material was made by dipping nonwoven in paraffin Pickering emulsion.

1 Experimental

1.1 Materials

Styrene (St) and potassium persulfate (KPS) with a purity of 99.5% were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and St was purified by distilling prior to use. Sodium styrenic sulfonate (NaSS) comonomer and paraffin were obtained from Aladdin Industrial Corporation (Shanghai, China) and used without further purification. Commerically distilled and deionized water (DDI) was used, and the viscose nonwoven fabric with an area density ca. 70 g/m2was supplied by Changshu Yongdeli Spun-laced Nonwoven Fabric Co., Ltd. (Jiangsu, China).

1.2 Instrument

Zetasizer Nano ZS90 particle size instrument was made by Malvern Co., Ltd., Nicolet iS5 Fourier infrared spectrometer by Thermo Electron Scientific Corporation, vltra 55 thermal field emission by Carl Zeiss(Shanghai) Co., Ltd., OCA 20 contact angle tester by DataPhysics Instruments GmbH, Germany, Scientz-N vacuum freeze-drying machine by Ningbo Scientz Biotechnology Co., Ltd, FA-18 high-shear instrument by Fluko Equipment Shanghai Co., Ltd, and DSC 250 differential scanning calorimeter by TA Instruments-Waters (Shanghai) LLC.

1.3 Methods

1.3.1 Preparation of nano-SPS

First, the monomer dispersion was purged with nitrogen gas for 30 min before initiation. The reaction temperature and stirring rate were set at 70 ℃ and 250 r/min, respectively. The reaction lasted for 3 h after adding the intiator KPS and proceeded for another 2 h when the reaction temperature was raised to 85 ℃, so as to increase the monomer conversion rate[14]. After polymerization completion, the emulsion was pre-frozen below -50 ℃ for 8 h. The samples were then placed in a vacuum freeze-drying machine at a vacuum below 5 Pa and temperature around -50 ℃. The freeze-dried nano-SPS samples were ready after 48-h treatment. The ingredients are shown in Tab.1.

Tab.1 Chemical components of sulfonated nano-SPS samples

1.3.2 Preparation of paraffin Pickering emulsions

The nano-SPS was dispersed in water in the presence of ultrasonication, hydrochloric acid was used to adjust the pH of the dispersion to 3, and finally the agglomerates were filtered with paper. Paraffin Pickering emulsion was obtained by shearing the dispersion with FA-18 high-shear instrument at a rate of 10 000 r/min while adding paraffin. The effects of NaSS content, nano-SPS dosage and paraffin-water volume ratio on the shelf life of paraffin Pickering emulsion were investigated in details.

1.3.3 Preparation of nonwoven phase change materials

The viscose nonwoven fabric was immersed in the paraffin Pickering emulsion for 30 min and then dried at 40 ℃ under vacuum after removing the excess emulsion with tissue paper. Thus the nonwoven phase change material was obtained.

1.3.4 Thermal cycling treatment of paraffin-loaded nonwoven phase change materials

The phase change material of paraffin loaded nonwovens was put into the oven at 70 ℃, taken out after holding for 30 min, and then kept in air at 10 ℃ for 30 min. The treatment process was repeated for 20 times.

1.4 Analysis and measurement

Infrared spectroscopy (FTIR): The microstructure of nano-SPS was analyzed with a Nicolet iS5 instrument over a frequency range of 4 000 cm-1to 400 cm-1with the resolution 4 cm-1and 32 scanning times.

Particle size test:The nanoparticles were dispersed in water at a mass fraction of 1%. The particle size distribution was obtained with dynamic light scattering (DLS) using a ZS90 instrument at a scattering angle of 90° at room temperature.

Scanning electron microscopy (SEM):The micromorphology of nano-SPS was observed by SEM under an accelerated voltage of 3 kV. The particle surfaces were sputter coated with gold before observation.

Contact angle measurements:The three-phase contact angle of nano-SPS was measured across the aqueous phase using Data Physics OCA 20 instrument at room temperature after the sample diskette was pressed at a pressure of 20 MPa.

Differential scanning calorimetry (DSC):The phase change heat and melting temperature of nano-SPS nonwoven phase change materials were measured with a DISCOVERY DSC 250 differential scanning calorimeter at a heating rate of 10 ℃/min in the range of 10-150 ℃.

Paraffin load rate(k):Thekvalue of paraffin-loaded nonwoven phase change material was calculated based on weight gain according to the following equation:

(1)

Where,m1is the mass of unimpregnated nonwovens andm2is the mass of nonwovens after impregnation and drying.

Leakage rate of paraffin(L):The paraffin-loaded nonwoven samples were weighed after themal cycling treatment, andLwas calculated according to the following equation:

(2)

Where,M1is the mass of nonwovens without heating and cooling andM2is the mass of nonwovens after cycling.

2 Results and discussion

2.1 Microstructure of nano-SPS

As shown in Fig.1, there are several absorption peaks near 695, 754, 1 451, 1 492 cm-1, which indicates the presence of polystyrene due to its benzene ring and substitution mode. The S—O vibration peaks from the sulfonic acid group appear at 536 cm-1, 1 068 cm-1and 1 190 cm-1. The peak at 1037 cm-1is significantly enhanced with increasing NaSS content, indicating involution of NaSS in copolymerization. Therefore, hydrophilic modification of polystyrene nanoparticles is achieved[14].

Fig.1 FT-IR spectra of nano-SPS samples

As shown in Fig.2, the average particle size first decreases and then increases with increasing NaSS content. The polydispersity index of all samples is less than 0.45, indicating good dispersion of the nanoparticles in water. According to the mechanism of emulsion polymerization[15], KPS usually produces free radicals and easily initiates the reaction of the water soluble monomer NaSS in the aqueous phase, and the resultant hydrophilic copolymer chains tend to lie on the surface of micelle particles, reducing the interfacial tension of the dispersed phases during polymerization. NaSS is believed to improve the stability of primary particles and avoid their agglomeration in polymerization due to the repulsive force of the particles, thus the particle size gradually decreases. And the polymerization is also considered to be initiated in styrene-dissolved micelles with NaSS monomers acting as surfactant molecules. More comonomers are adsorbed on the surfaces of St droplets and significantly generate more monomer droplets at a higher content of NaSS. Finally, polymerization may occur in monomer droplets when the number and volume of St droplets are close to the solubilized micelles, therefore,

Fig.2 Size distribution of nano-SPS samples

larger latex particles will be produced markedly. The relationship between particle diameter and comonomer content is consistent with the results of literature [16].

As shown in Fig.3,the nano-SPS samples with 5% and 10% NaSS by mass volume fraction show slight agglomeration,and this may ascribe to the less efficient repulsion between particles with a particle size of ca.200 nm.The nano-SPS samples with 20% and 25% NaSS by mass volumn fraction have uniform particle sizes of ca.40 nm and good monodispersity.

Fig.3 SEM images of nano-SPS samples

2.2 Hydrophilicity of nano-SPS

As shown in Tab.2, the water contact angle of nano-SPS decreases with the increase of NaSS content, which is because more comonomers will lie at the surfaces of latex particles with increasing NaSS content, thus enhancing the hydrophilicity of the particles. The water contact angle of sample 1#is 105.4°, showing hydrophobicity, whereas that of sample 2#is 86.8°, indicating a certain hydrophilicity. Finally, the strong hydrophilicity is obtained for sample 5#with a contact angle of 55.4°.

Tab.2 Water contact angles of nano-SPS samples

2.3 Shelf life of paraffin Pickering emulsions

The mass fraction of nano-SPS was fixed at 1% (on the basis of the total emulsion volume), and the oil-to-water volume ratio was assigned as 50/50. Six nano-SPS samples were used to prepare paraffin Pickering emulsions. As shown in Fig.4, stable paraffin Pickering emulsions are obtained except for the emulsion with the nanoparticles with 5% NaSS by mass volume fraction, which presents rapid phase separation with the upper layer of paraffine and the lower layer of aqueous solution containing solid particles and a small amount of emulsion between the two layers;however, the nano-SPS samples with 10% and 15% NaSS by mass volume fraction give two-layer liquids, namely,the upper emulsion layer and the lower aqueous solution layer in a small amount;and stable and uniform paraffin Pickering emulsions are finally generated by adding 25% and 30% NaSS by mass volume fraction into nano-SPS,which are verified to be the oil-in-water emulsions by dosing water into the emulsion without de-emulsifying[17].

Fig.4 Shelf life of paraffin Pickering emulsion at different NaSS contents

The nano-SPS sample with 25% NaSS by mass volume fraction and oil-to-water volume ratio of 50/50 was selected to investigate the effect of the dosage of nano-SPS on the shelf life of the Pickering emulsion. As shown in Fig.5, the emulsions with 0.5% and 1.0% nano-SPS by mass fraction present a small amount of paraffin layer over the emulsion phase, whereas the emulsions with 1.5% and 2.0% nano-SPS by mass fraction have good stability. Furthermore, the viscosity of the emulsions increases with increasing nano-SPS.

Fig.5 Shelf life of paraffin Pickering emulsion at different nano-SPS dosages

The nano-SPS sample with 25% NaSS by mass volume fraction and 1.5% nano-SPS by mass fraction was selected to investigate the effect of the oil-to-water volume ratio on the shelf life. The oil-to-water ratio was set at 50/50, 55/45, 60/40 and 70/30. As shown in Fig.6, the emulsions are quite stable initially, after 24-h standing and inversion state after 24-h standing at the oil-to-water volume ratios of 50/50 and 55/45; the viscosity of the emulsions become higher with increasing oil phase ratio, but a small amount of paraffin escapes from the emulsions at a oil-to-water volume ratio of 70/30.

Fig.6 Shelf life of paraffin Pickering emulsion at different oil-to-water ratios

2.4 Application of paraffin emulsions for nonwoven phase change materials

The emulsion with 25% NaSS by mass volume fraction, 1.5% nano-SPS by mass fraction and oil-to-water volume ratio of 55/45 was selected. The paraffin Pickering emulsion was used for impregnation. As shown in Fig.7, paraffin and paraffin-loaded nonwoven fabrics have the same melting temperatures about 49.0 ℃. The wide but shallow endothermal peak of raw nonwoven at 116.5 ℃ is related to the volatilization of moisture in the fabric. However, the supposed moisture evaporation peak disappears after paraffin loading, which implies that the paraffin emulsion may penetrate into the inner part of the nonwoven and generate a hydrophobic protection layer after drying, which restricts moisture intrusion[2].

Fig.7 DSC curves of blank nonwoven and raw paraffin and paraffin-loaded nonwovens1-Raw nonwoven; 2-Nonwoven with a k value of 137.1%; 3-Nonwoven with a k value of 351.6%; 4-Paraffin

It can be seen from Tab.3 that compared with that of raw paraffin (151.1 J/g), the phase change enthalpy of paraffin decreases generally after being loaded, and the phase change enthalpies are 82.1 J/g and 105.0 J/g whenkof nonwoven fabric is 137.1% and 351.5%, respectively; the phase change enthalpy of nonwovens increases when the relative content of paraffin increases with the increase ofk, which means that there may be some physical interaction among paraffin, nano-SPS and nonwoven. It can be sure that the paraffin-loaded nonwoven possesses good heat storage capacity.

Tab.3 Melting enthalpy of raw paraffin and paraffin-loaded nonwovens

To verify the coating and enwrapping stabilities of paraffin droplets on the nonwoven fabric, the paraffin-loaded fabrics were heated and cooled for 20 cycles.Lof three different paraffin-loaded fabrics is listed in Tab.4. The resultantkis 137.1% andLis only 1.67% when a paraffin emulsion with 1.5% nano-SPS by mass fraction and oil-to-water volume ratio of 10/90 was used for nonwoven fabric, indicating a stable paraffin coating on the fabric;kis 351.6% andLincreases up to 20.1% when the emulsion with 1.5% nano-SPS by mass fraction and oil-to-water volume ratio of 55/45 was used; however,Lis only 1.94% andkis 386.8% when the emulsion with 4.5% nano-SPS by mass fraction and oil-to-water volume ratio of 55/45 was used. Conclusively,Lis higher at higherkat the same particle dosage, whereas an increase in nano-SPS dosage diminishes the leakage at the samek, which is consistent with literature [18].

Tab.4 L of paraffin-loaded nonwovens after cycling

DSC was used to investigate the changes of the melting and crystallization temperatures of the fabrics impregnated with different emulsions after 20-repetition of the heating and cooling treatment. Tab.5 shows that the paraffin supercooling increases from 3.9 ℃ to 4.7 ℃ whenkis increased at the same nano-SPS dosage, whereas the supercooling decreases from 4.7 ℃ to 2.8 ℃ when the nano-SPS dosage is increased at the same oil-to-water volume ratio. Although the optimum content of nano-SPS for the long shelf life of paraffin Pickering emulsion is 1.5%, the continuous addition rather than decrease of nano-SPS certainly does not spoil storage but leads to the increases of cost and viscosity of the emulsion. Namely, the dosage of nano-SPS will be enhanced for preparing nonwoven phase change materials having less supercooling and leakage, but will not make any conflict on the shelf life of emulsion.

Tab.5 Supercooling of paraffin in nonwoven fabrics with different paraffin loading after cycling

CHEN L et al.[19]found that the supercooling of a traditional paraffin emulsion was ca. 15 ℃, far higher than that of paraffin Pickering emulsion, at a heating/cooling rate of 10 ℃/min. In fact, in the traditional paraffin emulsion, the paraffin crystallizes via homogeneous nucleation and the carbon long-chain oil-philic group of emulsifier is soluble in paraffin, which lead to the remarkable supercooling[20]. However, in the paraffin Pickering emulsion, nano-SPS firmly adsorbs on the surfaces of the paraffin droplets. During the cooling process of the paraffin droplets, nano-SPS acts as heterogeneous crystalline nuclei to induce paraffin crystallization and to eliminate supercooling to some extent. The heterogeneous nuclei on the surfaces of the paraffin droplets increase with the dosage of nano-SPS, and thus the paraffin crystallizes easily[21-22].

3 Conclusions

a.A nano-SPS Pickering emulsifier was prepared by soap-free emulsion polymerization and used for preparing paraffin emulsion and viscose nonwoven phase change materials. With increasing comonomer NaSS dosage, the diameter of nanoparticles first decreases and then increases, and the water contact angles of nanoparticles decreases constantly. The size and water contact angle of the monodisperse nanoparticles are ca. 40 nm and 60.4° at the dosage of 25%, respectively.

b.The shelf life of paraffin Pickering emulsions is greatly improved both at the increased nano-SPS dosage and the enhanced comonomer NaSS content. The optimized emulsion ingredients are a oil-to-water volume ratio of 55/45 with a nano-SPS dosage of 1.5% by mass fraction and a comonomer NaSS content of 25% by mass volume fraction. The oil-in-water emulsion is verified by the dilution of disperse-phase liquid.

c.The nonwoven phase change enthalpy reaches 105.0 J/g at akvalue of 351.6%.Lis 1.94%,and the supercooling is 2.80 ℃ when the nonwoven exhibits akvalue of 386.8% and is exposed to 20-repetition treatments at 10 ℃ and 70 ℃.