PARTⅢ MECHANICAL RECYCLING

SELECTIVE SURFACE OZONATION OF POLYVINYLCHLORIDE FOR ITS SEPARATION FROM WASTE PLASTICMIXTURE BY FROTH FLOATATION
Tetsuji Okuda,*1 Keisuke Kurose,2 Mallampati Srinivasa Reddy,2 Tsung-Yueh Tsai,1Satoshi Nakai,2 Wataru Nishijima,1 Mitsumasa Okada2
1Environmental Research and Management Center, Hiroshima University, 1-5-3
Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8513, Japan.
2
Department of Chemical Engineering, Graduate School of Engineering, Hiroshima
University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan.
E-mail: aqua@hiroshima-u.ac.jp
Abstract: The separation of waste plastics is important for its feedstock and mechanicalrecycling. The selective surface hydrophilization of polyvinyl chloride (PVC) by ozonation
was studied as the pre-treatment of its separation by the following froth flotation. Theozonation could selectively increase the hydrophilicity of the surface of PVC among plastic
mixture with three other plastics. The hydrophilization of PVC by surface analysis would be
due to the hydrophilic groups produced by ozonation. XPS and AFM analysis revealed thatthe increase was due to the structural change in chlorine group in PVC to hydroxylic acid,ketone and carboxylic groups by ozonation. The separation of PVC from other plastics could
be achieved by the combination of the pre-ozonation and the following froth floatation. The
effect of some condition onto separation efficiency in the froth floatation was alsoinvestigated.
Keywords: Froth floatation; hydrophilization; ozonation; PVC
1. Introduction
Synthetic organic polymeric materials (plastics) are widely used in our daily life andvarious industrial fields. Therefore, large amounts of waste plastics were produced andreached about 9.9 million tons/y in Japan [1,2]. Before, the most of waste plastics weredisposed to landfill site directly or after incineration, because many kind of plastics was mix
in the waste plastics. However, the recycle is recently desired, and several kinds of low
promoting plastic recycle had been established in Japan. One of them was the Containers andPackaging Recycling Law, and it was established in Japan and came into effect in 2000 toimpel recycling of the containers and packaging waste[3]. The problems associated with each- 141 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
recycling method, including the thermal, chemical or material recycling, of waste plastics are
the productions of hydrogen chloride and chlorine gas in addition to the formation of dioxins
and furans[4,5]. These problems caused by chlorine in the waste plastics can be avoided by
the removal of PVC from waste plastics.
About 75% of polymer produced in Japan consists of six polymers, i.e. polyethylene(PE), polypropylene (PP), acrylonitrile-butadiene-styrene resin (ABS), polystyrene (PS),poly(ethylene terephthalate) (PET) and PVC. These typical densities are approximately 0.9,0.9, 1.1, 1.1, 1.3 and 1.4 g/cm3, respectively [1,6,7]. Therefore, the separation of heavypolymers (PVC and PET) from light ones (PE and PP) can be achieved on the basis of theirdensity. However it is difficult to separate the heavy polymers from middle density polymers(PS and ABS). Especially, the separation of PVC from PET would be practically impossibleon the basis of density. There are no simple and inexpensive techniques to separate PVC fromothers. If hydrophobic the PVC surface is selectively changed into hydrophilic, where otherpolymers still have hydrophobic surface, flotation process can separate PVC from others[8,9].
One of the commonly used methods for the hydrophilization is a chemical conditioning withhydrophilization agents, e.g. the selective adsorption of a surfactant onto polymer. Anothermethod is physical modification, such as flame, heating and plasma treatments [7,11,12]. It is
well known that chlorinated organic compounds are dechlorinated by ozone in water andwastewater treatment processes. The separation of PVC from mix waste plastics would bepossible by froth flotation, if ozone can dechlorinate PVC and produce hydroxyl and carboxylgroups in the surface and the hydrophilicity of the PVC surface would be selectivelyincreased. Therefore, the selective surface modification of PVC surface by ozone wasconfirmed and its mechanism was studied in this research. The separation of PVC from otherplastics was also studied by the combination of the pre-ozonation and the following froth
floatation, and the effect of floatation condition onto separation efficiency was investigated.
2. Materials and methods
2.1 Materials
Three types of virgin PVC plates (Takiron Ltd, 10×10mm with 2 mm thickness) wereused for the separation study. The rigid and flexible PVC plates contained 8 and 36wt% ofadditives, respectively (Tab.1). Although rigid PVCs are known to have the smaller amount ofplasticizers, rigid PVCs without plasticizers were used to confirm the correlation betweenhydrophilization and plasticizers as additives. The plasticizer was Dioctyl phthalate (DOP)
and its double bond could react with ozone. Colorant and stabilizer are titanium dioxide andlead compounds (3PbO･PbSO4･H2O), respectively, and these inorganic oxides are expected
to be less-reactive with ozone.

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Tab. 1 Properties of three PVCs[12,13]
Rigid PVC
Content(wt%)

Flexible PVC

Vinyl chloride

92

90

64

Additives

81136

Plasticizer

0427

Colorant and stabilizer

87

9

1.43

1.40

1.24

3Density (g/cm )2.2 Contact angles and ozonation
To evaluate the effectiveness of ozonation on the increase of hydrophilization for PVCs,
the contact angles of plastic plates were measured before and after ozonation. Contact angle
meter (Kyowa Interface Science, CA-XP) was used for the analysis. Sessile bubble method
was used to analyze the contact angles.
Surface modification of plastics by ozonation was conducted in a glass reactor, 18 cm
high with 5.0 cm inner diameter, giving a volumetric capacity of 0.35 dm3, with a glass
bubble diffuser plate at the bottom (pore diameter of 10-16 µm). Ozone gas (90-150 mg-O3/L)
was produced by a ozonaizer (POX-10, Fuji-electric) and introduced into the cell through thediffuser at a flow rate of 50 mL/min in ultra pure water at around pH 7.
2.3 Surface analysis
The changes of chemical and physical structure on surface were observed by XPS (X-ray
Photoelectron Spectrometer) and AFM (Atomic Force Microscopy) analysis, respectively.
Nano Scope Ⅲ multimode (Veeco Instruments Inc.) was used for AFM analysis with scansize of 20×20 µm2, x-range of 2 µm/div and y- and z- range of 700 nm/div. A S-Probe
ESCA (Surface Science Instrument) was used for XPS analysis, each binding energies at288.5, 531.8, 533.3 and the sum of 200.0 and 201.6eV were used for the analysis of O=C-O,
C=O, C-O and -CHCl-CH2-, respectively.
2.4 Froth flotation
Flotation experiments were conducted in the same equipment as the ozonation and aglass cylinder with 15.0 cm inner diameter. Nitrogen gas was used for the flotation
experiments. A paddle was installed into the cell for mixing. MIBC (Methyl Isobutyl Carbinol,4-Methyl-2-pentanol) was added to be 36 µg/L as a frother.[3] The nitrogen gas was supplied
to the reactor for 15 seconds.
3. Results and Discussion
3.1 Hydrophilicity change of surface by ozonation
The contact angles in sessile bubble method of PVCs with 8-36% of additives at 0-60
minutes of ozonation are shown in Fig. 1. The increase in the contact angle indicates the- 143 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
increase of surface hydrophilicity, rapid increase was observed until 15 minutes in every PVC.
The contact angle change during 15 minutes ozonation was summarized in Fig 2, and highest
increase was observed in the rigid PVC with 8% of additives, its contact angles was increased
from 108 deg to 157 deg. The results confirm the hydrophilization on the surface of PVCs asshown in previous repot [14], and it was found that rigid PVC containing low additives waseasily modified in comparison with flexible PVC containing high additives. This would bedue to the replacement of the chloride groups on the surface of PVC into hydrophilic
functional groups.
60180140120

Contact angle changeby ozonation（degrees）

Contact angle (degrees)

160

100
80
8%
11%
36%

60
4020050
4030
201000102030
4050
Ozonation time （min）

60

Fig. 1 Contact angles of PVCs with differentadditive ratio during ozonation.

0102030
Percentage additives（%）

40Fig. 2 Contact angles change of PVCs
with different additive ratio
during ozonation.

3.2 Chemical and physical structure change
The changes of electron counts of each functional groups carbonyl, as like carboxyl andchlorine groups, in XPS analysis before and after ozonation are summarized in Fig.3 . It isclear that the hydrophilic groups on PVC surfaces, especially ketone group including a part ofcarboxylic groups, increased by ozonation, whereas the vinyl chloride group decreased. Thechanges were more significant in the rigid PVC with low additives coincident with the higherhydrophilization in the rigid PVC surface (Fig. 1 and Fig. 2). These results suggest that thehydrophilization of PVC was not caused by the oxidation of additives but the oxidation ofvinyl chloride itself because the vinyl chloride group decreased with the increase in thehydrophilic groups.
AFM was conducted to study the physical surface change by ozonation. The surfacehydrophilicity may increased by the decrease in surface roughness.[15] The surface roughness
expressed by the root mean square (RMS) of AFM height images of PVC before and afterozonation (Fig. 4) indicated that the hydrophilization of PVC surface was not caused by thechange in the surface roughness in both rigid and flexible PVC.

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1220000

O=C-O
C=O
C-O-CHCl-CH2-15000
10000

Average roughness (nm)Electronic count difference (count/sec)

PARTⅢ MECHANICAL RECYCLING

5000
0108%
36%

8642-5000

0-10000
010203040

without ozonation

with ozonation

Percentage additives (%)

Fig. 3 Changes of electron counts associatedwith each functional groups in XPSbefore and after ozonation

Fig. 4 Average roughness of PVC
before and after ozonation

3.3 Selectivity of ozonation
Fig. 5a shows the comparison of froth floatation among 4 plastics include rigid PVC
with 8% additives without ozonation. Each plastic can be settled at certain mixing speed in
the froth floatation, and the mixing speed depends on surface hydrophilicity. We could not
find any specific mixing speed where PVC could be separated from other three plastics, that is,
all PVC tubes settled down and all other plastic tubes floated. On the other hand, the perfect
removal of PVC from other three plastics could be achieved at 190-210 rpm of mixing speed
in case of pre-ozonation as shown in Fig.5b. The separation was summarized in Fig. 6 whichshows the floatation percentage of three other plastics when all PVC pieces was settled down.
It indicates that selective surface hydrophilization could be happen on PVC, and theseparation could be achieved in froth floatation.
3.4 Effect of conditions of froth floatation
The condition of froth floatation could affect on the separation efficiency, therefore theeffects of reactor size, pellet size, water temperature and mixing time onto the separation
efficiency was investigated. The results were summarized in Fig. 7, and the result of PET wasshown. The increase of cylinder reactor size from 5 cm (Fig. 6) to 15 cm (Fig. 7) of diameter
was not affect on the separation efficiency for 5×5×5mm pellets. The increase of pellet size
from 5×5×5mm to 5×15×15mm was dramatically decrease the separation efficiency. It would
be caused by the difficulty of keeping floatation on the balance of bubbling and mixing when
the relative surface area of plastic was increased. However, it was found that the increase oftemperature and mixing time could improve the balance, and could improve the separation
efficiency.

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100
Percentage of float (%)

Percentage of float (%)

100
806040PVC
PC
PETPMMA

205060PVCPCPETPMMA

40
2000080

100 150 200 250
Mixing speed (rpm)030050100

150200250300

Mixing speed (rpm)(a) without pre-ozonation(b) with 10 minutes pre-ozonation

10010080PETPMMAPC

Floatation percentage of PET
at the mixing speed of all PVC settled (%) .

Floatation percentage of three other plastics atthe mixing speed of all PVC settled（％）

Fig. 5 The effect of ozonation on the selective separation of PVC in the froth floatation.[14]60402080

60

40

20

0
5*5*5mmpellets0
without ozonation

10*10*5mm
pellets

15*15*5mm
pellets

with ozonation

Fig. 6 Separation efficiency of threeplastics when all PVC pieces was settled(5×5×5mm pellets in 5cm cylinder)

Temperature(20ºC→40ºC) &Mixing period(20→30s) increase

Fig. 7 Effects of pellets size andfloatation condition on theseparation efficiencies. (15cm cylinder)

4. Conclusions
The increase was due to the structural change in chlorine group in PVC to hydroxylicacid, ketone and carboxylic groups by ozonation. The separation of PVC from other plastics
could be achieved by the combination of the pre-ozonation and the following froth floatation.
The increase of temperature and mixing time could improve the separation efficiency.
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