The 5th ISFR (October 11-14, 2009, Chengdu, China)

PREPARATION OF POLYSTYRENE HOLLOWMICROSPHERES FROM WASTE FOAMED POLYSTYRENE
PLASTICS BY MICROENCAPSULATION METHOD
Wei Bin*1, Wang Shujun1, Liu Hongyan2, Liu Ning1
1

State Key Laboratory of Heavy Oil Processing, China University of Petroleum,
Beijing 102249, China
2
School of Chemical Science and Engineering, China University of Petroleum,
Beijing 102249, China
E-mail: weibin421@163.com
Tel:: +86-10-89733638, 13488758176

Abstract: Waste foamed polystyrene plastics were used as the main raw materials for the
preparation of polystyrene hollow microspheres with high additional value by
microencapsulation method. Firstly the preparation conditions were studied, such as theemulsifying time, the temperature and time of the solidification of oil phase, and then themethod of removing the encapsulated water was discussed. The factors which influenced theaverage diameter and tap density of polystyrene hollow microspheres were also investigated,
and finally the recycling technology of reagents used in experiment was discussed. Under
optimum conditions, polystyrene hollow microspheres with the lowest tap density of
0.12g·cm-3 and average diameter from 40µm to 650µm were successfully produced.
Keywords: waste foamed polystyrene plastics; polystyrene hollow microspheres;
microencapsulation method; preparation conditions; tap density;average diameter
1. Introduction
As waste foamed polystyrene plastics (FPS) have the characteristics of stable chemical
properties, large volume, aging resistance, corrosion resistance, they can not degrade bythemselves in natural environment, thus destroying ecological balance and causing “whitepollution” at every corner of the society.[1,2]This paper introduces a modified reuse method of waste FPS— Microencapsulation
method, preparing polystyrene (PS) hollow microspheres with high additional value fromwaste FPS. PS hollow microsphere, characterized by its unique properties such as low density,
optical scattering, high specific surface area and good surface permeability, has attracted
considerable attention.[3-5]It can be utilized in inertial confinement fusion (ICF) targets, lightweight fillers, high performance opacifying coating and anti-ultraviolet filler.[6-8] The- 160 -PARTⅢ MECHANICAL RECYCLING
technological process of microencapsulation method to prepare PS hollow microspheres isshown in Fig.1.[6,9].
O-phaseO-phaseW1-phase
W1-phase
W1/O
emulsion

W1-phase
Solid PS shellW2-phase

Solidification of O-phase

W2-phase
W1/O/W2
emulsion
VoidPS shell
Final PS hollow microsphere

Fig.1 Schematic diagram for preparing PS hollow microsphereswith the microencapsulation method
2. Definition of the preparation conditions and investigation of factors influencing PShollow microspheres
2.1 Selection of emulsifierFig. 2 shows the relationship between the volume ratio of water/oil phases (VW1/VO)
when Span 80 is used as emulsifier in the experiment.
0.55

0.40
0.35

Tap density/g·cm-3

tap density/g·cm-3

0.50
0.45
0.40
0.35

0.30
0.25
0.20
0.15

0.30

0.10
0.25
0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

VW1/VO

Fig. 2 Relationship between VW1/VO andtap density of PS hollow microspheres(Span 80 as emulsifier)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

WW1/WOFig. 3 Relationship between WW1/WO
and tap density (sodium dodecylbenzene
ulfonate as emulsifier)From Fig. 2 we can find that the density of PS hollow microspheres decreases when thevolume ratio of water/oil phases increases. When the tap density is about 0.28 g·cm-3, it isvery hard to decrease. This shows that the cavity volume of PS hollow microspheres is small.
Fig. 3 shows the relationship between the mass ratio of water/oil phases and the tap density ofPS hollow microspheres when emulsifier is sodium dodecylbenzene sulfonate. Aftercomparison with Fig. 2, it is found that the tap density of PS hollow microspheres decreases
obviously when sodium dodecylbenzene sulfonate is used as emulsifier. Therefore sodium
dodecylbenzene sulfonate is selected as emulsifier in this experiment.

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2.2 Selection of the main dispersant agents of W2-phase
Dispersant agent directly influences the dispersity of prepared microspheres. Improper
selection of dispersant agent will cause agglomeration phenomenon. Gelatin and polyvinyl
alcohol are selected as the main dispersant agents of W2-phase in this experiment.
2.3 Factors influencing the average diameter of PS hollow microspheres
Varied factors of the average diameter of PS hollow microspheres are studied in thisresearch. It defines that the second emulsifying rotating speed, the concentration of sodium
dodecylbenzene sulfonate in W1-phase, and the concentration of PS in O-phase play majorroles in influencing the average diameter (choosing more than 100 microspheres from sample
at random and working out the average diameter of these hollow microspheres as the average
diameter of the sample). The concrete relationship is: 1) The average diameter of hollow
microspheres decreases as the emulsifying rotating speed increases; 2) When theconcentration of sodium dodecylbenzene sulfonate in W1-phase is within a certain range(0.5wt%~2wt%), the average diameter of PS hollow microspheres goes up with concentrationand it is a basically rectilinear relationship. While the concentration of sodium
dodecylbenzene sulfonate is high enough (>2wt%), the average diameter of PS hollow
microspheres stays the same; 3) The average diameter of PS hollow microspheres goes up
with the increase of mass fraction of PS in O-phase.
2.4 Factors influencing the tap density of PS hollow microspheres
This experiment investigates the factors influencing the tap density of PS hollow
microspheres and defines that the mass ratio of water/oil phases and the concentration of PS
in O-phase are the factors which influence the tap density. The concrete relationship is: 1) thetap density of hollow microspheres decreases when the mass ratio of water/oil phases
increases. When it arrives at certain value, PS hollow microspheres can not be preparedbecause water in oil type of emulsion cannot be formed; 2) in certain concentration range, thetap density of PS hollow microspheres increases when the concentration of PS in O-phaseincreases. The lowest tap density of PS hollow microspheres prepared at present is
0.12g·cm-3.
2.5 The second emulsifying time
Since the W/O emulsions prepared by using sodium dodecylbenzene sulfonate as
emulsifier are unstable, the second emulsifying time should be lessened in order to avoid thedemulsification of emulsion. It is experimentally found that W/O emulsion in the system will
demulsify if stirred for over 30 minutes. Therefore, it should be heated immediately afterW1/O emulsion being added and the W1/O emulsion should be stirred in the heating process.

- 162 -PARTⅢ MECHANICAL RECYCLING
2.6 Temperature and time of the solidification of O-phase
The solidification temperature should be higher than the boiling point of organic solventof O-phase by 4°C~6°C, and heating lasts for two hours; then the system is heated to70°C~75°C and is solidified for one hour to improve the compressive strength of PS hollow
microspheres. During the second solidification, the temperature shall not exceed 80°C;
otherwise parts of the microspheres will be softened and deformed.
2.7 Method of removing the encapsulated water
After the solidification of O-phase, we can get the particle of W1-phase encapsulatedwith solid PS spherical shell. If W1-phase is directly dried and removed, it is hard to spread on
the surface of PS, because PS has a strong hydrophobicity and the contact angle of W1-phaseand PS is comparatively big. After being dried, surfactant in W1- phase and microimpurity
stay on the shell of microspheres and form spot, thus damaging the surface finish ofmicrospheres. Therefore, first we should immerse the particle of W1-phase encapsulated withsolid PS spherical shell into absolute ethanol for one day and exchange the W1-phase in thePS spherical shell and then we can dry it in WG 2003 type desktop drying oven for one day,PS hollow microspheres are obtained.
3. Recycling of experimental raw materials
The method adopted in the experiment to recycle the waste FPS is a kind of modified
reuse method. Except for FPS, all other raw materials can be recycled. The organic solvent ofO-phase can be recycled by connecting a reflux condensation equipment beside the four-port
flask. Experiments found that the recovery ratio of solvent can reach up to 87%. Moreover,W2-phase can also be used for several times. Because after the solvent evaporates, W1-phasewhich is encapsulated by solid PS shell won’t have a great influence on the mass ratio ofwater, gelatin and sodium dodecylbenzene sulfonate in W2-phase.
3. Characteristics of PS hollow microspheres
Under optimum conditions, PS hollow microspheres with the lowest tap density of
0.12g·cm-3 and average diameter from 40µm to 650µm are prepared.

Fig. 4 SEM diagram of thehollow microspheres

Fig. 5 SEM diagram of incised PShollow microspheres- 163 -The 5th ISFR (October 11-14, 2009, Chengdu, China)

Fig. 6 SEM diagram of of smaller
particle size microspheres

Fig. 7 SEM diagram of the bigger
particle size microspheres

Fig.4 is the SEM diagram of the hollow microspheres randomly extracted in sample. It is
used to calculate the average diameter of samples; Fig.5 is the section SEM diagram ofincised PS hollow microspheres. From the diagram we can see that there is a comparatively
big cavity in the middle and many other small cavities; Fig.6 is microsphere of smaller
particle size (PS hollow microsphere is prepared by using polyvinyl alcohol as dispersantagent). Fig.7 is microsphere of bigger particle size (PS hollow microsphere is prepared byusing gelatin and sodium dodecylbenzene sulfonate as dispersant agents)
4. Conclusions
Microencapsulation method is an effective way to recycle waste FPS to prepare PS
hollow microspheres. With the characteristics of easy operation, simple equipment and low
cost, the microencapsulation method is suitable for batch production of PS hollow
microspheres with various particle sizes.
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