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Materials Chemistry and Physics 124 (2010) 163–167

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

hemical modification of flexible and rigid poly(vinyl chloride) by nucleophilicubstitution with thiocyanate using a phase-transfer catalyst

omohito Kameda, Yuuzou Fukuda, Guido Grause, Toshiaki Yoshioka ∗

raduate School of Environmental Studies, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan

r t i c l e i n f o

rticle history:eceived 28 August 2009eceived in revised form 1 June 2010ccepted 4 June 2010

eywords:oly(vinyl chloride)

a b s t r a c t

The purpose of this study was to examine the effect of a phase-transfer catalyst on the chemicalmodification of flexible and rigid poly(vinyl chloride) (PVC) by substituting chloride with thiocyanate(SCN) in order to develop a new process for recycling PVC. The effects of temperature and time onthe reaction of a SCN/ethylene glycol (EG) solution on PVC were investigated in the presence andabsence of tetrabutylammonium bromide (TBAB) as a phase-transfer catalyst. TBAB was found to accel-erate the dehydrochlorination of both flexible and rigid PVC, thus allowing the reaction to take place

lexibleigiducleophilic substitutionhiocyanatehase-transfer catalyst

over shorter reaction times. The substitution yield and substitution/dehydrochlorination ratio werehigher in the presence of TBAB than in its absence. By reducing the reaction temperature, the substi-tution/dehydrochlorination ratio increased, and substitution occurred more rapidly when TBAB waspresent. The differences between flexible and rigid PVC were negligible. Together, these results indi-cate that the phase-transfer catalyst TBAB is effective in accelerating the substitution of chloride by SCN.This two-phase reaction allows for the easy separation of the polymer from the solvent without using

al pro

other chemicals or therm

. Introduction

Poly(vinyl chloride) (PVC), a common commodity plastic, cane classified according to its plasticizer content into flexible andigid PVC. Flexible PVC has excellent processability and is com-only used as agricultural foil, wire coating material, and floor

overing. On the other hand, rigid PVC is characterized by its excel-ent mechanical strength, making it useful for pipes, joints, and as

construction material. A reasonable percentage of agriculturaloils is recycled into floor coverings, and wire coatings and pipesan be reused after mechanical recycling. The VINYLOOP process,eveloped by SOLVAY in Belgium, uses a solvent for the separa-ion of PVC and additives. However, most of the PVC is landfilled,ince thermal processes such as incineration and pyrolysis resultn the release of HCl, which tends to cause corrosion of the facility.urthermore, the products of liquid pyrolysis such as oil containhlorinated organic compounds, making it impossible to use thems fuel or feedstock. Accordingly, various methods for separating

he chlorine from organic products have been studied to promotehe recycling of waste PVC [1–9].

In our laboratory, we have demonstrated wet treatmentrocesses for waste PVC, using either aqueous NaOH at high

∗ Corresponding author. Tel.: +81 22 795 7211; fax: +81 22 795 7211.E-mail address: yoshioka@env.che.tohoku.ac.jp (T. Yoshioka).

254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2010.06.011

cesses.© 2010 Elsevier B.V. All rights reserved.

temperature and pressure [10–13] or NaOH/ethylene glycol (EG)solution under atmospheric pressure [14,15]. In the latter case,we found that the hydroxide-catalyzed dehydrochlorination reac-tion occurred due to a combination of E2 and SN2 mechanisms(Scheme 1) [14,15]. This discovery allowed us to develop newpolymers from waste PVC by introducing new functional groups(Scheme 2). In a previous study, we found moderate substitution ofCl by thiocyanate (SCN) in addition to the elimination of HCl [16].The obtained polymer is expected to have antibacterial properties.

The reaction time and temperature of this hydroxide-catalyzeddehydrochlorination can be reduced by employing a phase-transfercatalyst. Even under mild conditions (70 ◦C and under atmosphericpressure), we observed considerable dehydrochlorination in aque-ous NaOH at a yield of up to 80% [17–19]. We also found that theefficiency of the catalyst depended strongly on the size of the cata-lyst molecule. Since tetrabutylammonium bromide (TBAB) was themost effective phase-transfer catalyst, we have also used this cata-lyst in the present work. Thiobenzoate was used as a nucleophile forinvestigating the PVC chain configuration [20,21]. Various nucle-ophiles were used for the surface modification of PVC [22–24].

Scheme 3 shows the reaction mechanism for the phase-transfer−

catalyzed substitution (e.g. of Cl in PVC by SCN). Since SCN is

unable to penetrate the organic phase due to the size of the sol-vation shell and its own polarity, a phase-transfer catalyst is usedas a vehicle for the SCN− ion. The catalyst is generally the halide ofa quaternary amine or phosphine, bearing lipophilic alkyl groups.

164 T. Kameda et al. / Materials Chemistry and Physics 124 (2010) 163–167

Scheme 1. Mechanism for the dehydrochlorination of PVC in a NaOH/EG solution.

Scheme 2. Reaction formula for the substitution of Cl in PVC by Nu.

Sp

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cwdt

2

2

TfiT(ps

Js

TC

D

Table 2Composition of the rigid PVC used.

(wt.%)

PVC 82.4MBS 13.2Sn stabilizer 2.47Monoglycelide 0.99Processing aid 0.82LDPE 0.082

addition of TBAB, a dehydrochlorination yield of almost 50% wasachieved after 12 h. Increasing the temperature to 190 ◦C resultedin further improvement of the dehydrochlorination yield, whichreached 93.4% after 6 h without TBAB and 98.7% after 3 h withTBAB. Therefore, in the presence of TBAB, nearly complete dehy-

cheme 3. Reaction mechanism for the substitution of Cl by SCN in PVC in theresence of a phase-transfer catalyst.

t can easily penetrate the organic phase with SCN− as the counteron and replace the Cl−, which in turn is transported back into theiquid phase. This can lead to a reduced reaction time and hence ton improved substitution yield, since the zipper mechanism [25],esponsible for the fast dehydrochlorination of PVC, does not occur.

This study reports the effect of a phase-transfer catalyst on thehemical modification of flexible and rigid PVC by substituting Clith SCN. The effects of the temperature and reaction time on theehydrochlorination of PVC in a SCN/EG solution in the presence ofhe phase-transfer catalyst TBAB were investigated.

. Experimental

.1. Materials

The compositions of the flexible and rigid PVC samples used are presented inables 1 and 2, respectively. The flexible PVC consisted primarily of PVC, CaCO3 as aller, and diisononyl-phthalate (DINP) as a plasticizer. The Cl content was 20.6 wt.%.he rigid PVC consisted primarily of PVC, methyl methacrylate/butadiene/styreneMBS) as an impact modifier, and a Sn stabilizer. The Cl content was 46.5 wt.%. PVCellets 4 mm in size were ground while being cooled with liquid nitrogen, and then

ieved to obtain particles between 150 and 250 �m in size.

The PVCs and other reagents were purchased from Kanto Chemical (Tokyo,apan) and Wako Pure Chemical Industries (Osaka, Japan). KSCN was used as theource for SCN and as a nucleophile.

able 1omposition of the flexible PVC used.

(wt.%)

PVC 36.8CaCO3 28.3DINP 23.9Chlorinated paraffin 6.99Alkylbenzene 1.84Pb stabilizer 1.10Calcium stearate 0.74Wax 0.37

INP: C6H4(COOC9H20)2.

Pigment 0.012

MBS: methyl methacrylate/butadiene/stylene = 15/70/15.LDPE: low density polyetylene.

2.2. Methods

The SCN/EG solution was prepared by dissolving KSCN (molar SCN/ClPVC ratio:4) in EG. A SCN/EG solution (50 mL) was placed in a 100 mL flask, to which wasadded the phase-transfer catalyst tetrabutylammonium bromide (TBAB) (molarTBAB/ClPVC ratio: 0.25). The flask was heated to the required temperature usinga silicone oil bath under a N2 flow of 100 mL min−1. After reaching the requiredtemperature, 1.0 g of PVC powder was added to the solution, and the mixture wasstirred for a specified time. After cooling the flask with water, the reaction solutionwas filtered. The solid residue was washed with deionized water and methanol, anddried under reduced pressure.

2.3. Characterization

The Cl concentration of the filtrate was determined by a Dionex DX-100 ionchromatograph and a Dionex model AS-16A column (eluent: 35 mM NaOH). Theresidue was burnt at 850 ◦C under an air flow of 100 mL min−1, and the evolved gaswas quenched by water and hydrogen peroxide water traps. The Cl in the residuewas determined by analyzing the solutions in the traps using ion chromatography.The contents of C, H, N, and S in the residue were determined by combustion analysis.The residue was also analyzed by Fourier transform infrared spectroscopy (FT-IR).

3. Results and discussion

3.1. Flexible PVC

Fig. 1 shows the effect of TBAB on the dehydrochlorination yieldfor flexible PVC in a SCN/EG solution. The dehydrochlorination yieldwas calculated as the ratio of the Cl content in the filtrate to that inthe original PVC. The addition of TBAB significantly accelerated thereaction. The reaction rate at 150 ◦C was low, resulting in a dehy-drochlorination yield of only 7.9% after 24 h. In contrast, after the

Fig. 1. Effect of TBAB on the dehydrochlorination yield of flexible PVC in a SCN/EGsolution.

T. Kameda et al. / Materials Chemistry and Physics 124 (2010) 163–167 165

Fig. 2. Effect of TBAB on the substitution of Cl by SCN and the elimination of HCl,resulting in the dehydrochlorination of flexible PVC. Conditions: (a) 150 ◦C, 24 h; (b)150 ◦C, 12 h + TBAB; (c) 190 ◦C, 6 h and (d) 190 ◦C, 3 h + TBAB.

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da

ineoatibTs2etsftaq

◦ ◦

F(

cheme 4. Reaction formula for the substitution of Cl by Nu and the elimination ofCl in the dehydrochlorination of PVC.

rochlorination was achieved within half the time required in itsbsence.

As shown in Scheme 1, the dehydrochlorination of flexible PVCs caused by the substitution of Cl by Nu, resulting in the elimi-ation of HCl. Based on the elemental analysis of the residue, theffects of TBAB on the substitution of Cl by SCN and the eliminationf HCl were examined at 150 and 190 ◦C (Fig. 2). The substitutionnd elimination yields were calculated from the ratios of y and zo n, according to Scheme 4. The sum of the substitution and elim-nation yields is the dehydrochlorination yield of PVC, expressedy the ratio of (y + z) to n. The substitution yield in the absence ofBAB was 3.2% and 22.4% at 150 and 190 ◦C, respectively. The sub-titution yield rose after the addition of TBAB and reached 24.6% and6.1% at 150 and 190 ◦C, respectively. It is noteworthy that the pres-nce of TBAB resulted in higher substitution yields independent ofemperature. The ratio of substitution to dehydrochlorination (theum of substitution and elimination) increased after adding TBAB

◦

rom 0.41 to 0.50 and from 0.24 to 0.26 at 150 and 190 C, respec-ively. This increase might be caused by the catalytic effect of TBAB,llowing the substitution to approach equilibrium conditions moreuickly while not affecting the elimination of HCl.

ig. 4. Photographs of (a) flexible PVC, as well as its products obtained from reacting thec) at 130 ◦C for 24 h.

Fig. 3. Yields of the substitution of Cl by SCN and the elimination of HCl in thepresence of TBAB, resulting in the dehydrochlorination of flexible PVC at low tem-peratures: (a) at 100 ◦C for 48 h and (b) at 130 ◦C for 24 h.

The results above clearly show that substitution works better atlower temperatures. Further reductions in the temperature causeda further rise in the substitution/dehydrochlorination ratio, whichreached 0.75 after 24 h at 130 ◦C and 0.86 after 48 h at 100 ◦C (Fig. 3).It should be noted, however, that these high yields came at the costof significantly longer reaction times. In both cases, the degree ofdehydrochlorination was low while the substitution ratio was high,indicating that it is possible to upgrade the PVC and maintain itsoriginal properties.

It is known that increasing the length of the conjugated C Cdouble bonds results in a change in the color of the product fromwhite to yellow, orange, red, brown, and black. Fig. 4 shows samplephotographs of a flexible PVC and its products obtained after thereacting the former in a SCN/EG solution in the presence of TBABat 100 ◦C (48 h) or 130 ◦C (24 h). The color of the product changedto dark gray at 130 ◦C (Fig. 4(c)), indicating the presence of conju-gated C C double bonds and progress of the E2 reaction. In contrast,the color of the product obtained at 100 ◦C (Fig. 4(b)) was similarto that of the original flexible PVC (Fig. 4(a)). This indicates theabsence of the zipper mechanism, which is known to be responsi-ble for the formation of conjugated C C double bonds during thedehydrochlorination of PVC; it corresponds to the decline in thenumber of double bonds observed at lower temperatures (Fig. 3)[21].

Fig. 5 shows the FT-IR spectra of flexible PVC and its prod-ucts obtained after reacting the former in a SCN/EG solution in the

presence of TBAB at 100 C after 48 h and at 130 C after 24 h, respec-tively. The absorption bands corresponding to the C−H stretchingvibration (around 2900 cm−1), derived mainly from PVC and DINP,and the C O stretching vibration (around 1700 cm−1), derived from

former with a SCN/EG solution in the presence of TBAB: (b) at 100 ◦C for 48 h and

166 T. Kameda et al. / Materials Chemistry and Physics 124 (2010) 163–167

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Dpo((mS2ai

3

otdwa1

ig. 5. FT-IR spectra of (a) flexible PVC, as well as its products obtained from reactinghe former with a SCN/EG solution in the presence of TBAB: (b) at 100 ◦C for 48 hnd (c) at 130 ◦C for 24 h.

INP, are visible in all the FT-IR spectra. The FT-IR spectrum of theroduct obtained at 130 ◦C (Fig. 5(c)) differs markedly from thosef the flexible PVC and the product obtained at 100 ◦C. Fig. 5(a) andb) shows the absorption bands of the C C stretching vibrations1600–1750 cm−1), consistent with the elimination of HCl by an E2

echanism. The FT-IR spectra also confirm the substitution of Cl byCN following an SN2 mechanism. Both the –S–C N group (around150 cm−1) and the –N C S group (around 2050 cm−1), obtaineds a result of the isomerization of the –S–C N group, were observedn the FT-IR spectra of both products.

.2. Rigid PVC

Fig. 6 shows the effect of TBAB on the dehydrochlorination yieldf rigid PVC in a SCN/EG solution. Rigid PVC reacted in a way similar

o flexible PVC. The addition of TBAB resulted in a more rapid dehy-rochlorination. At 190 ◦C, nearly complete dehydrochlorinationas achieved at after 3 h (99.7%) and 6 h (96.7%) in the presence and

bsence of TBAB, respectively. The reaction accelerated markedly at50 ◦C, increasing the dehydrochlorination yield from 2.2% in the

Fig. 6. Effect of TBAB on the dehydrochlorination yield of rigid PVC.

Fig. 7. Effect of TBAB on the substitution of Cl by SCN and the elimination of HClfrom rigid PVC. Conditions: (a) 150 ◦C, 24 h; (b) 150 ◦C, 24 h + TBAB; (c) 190 ◦C, 6 hand (d) 190 ◦C, 3 h + TBAB.

absence of TBAB to 44.5% in its presence after a reaction time of24 h. Elemental analysis of the residue clarified the effect of TBABon the substitution of Cl by SCN and the elimination of HCl at 150and 190 ◦C (Fig. 7). The substitution yield increased robustly from0.9% in the absence of TBAB to 23.2% in its presence at 150 ◦C, whileonly a moderate rise from 23.3% to 28.4% was observed at 190 ◦C.The substitution/dehydrochlorination ratio also increased slightlyin the presence of TBAB from 0.43 to 0.52 and from 0.24 to 0.28at 150 and 190 ◦C, respectively. As was the case with flexible PVC,the reaction shifted strongly towards substitution when the tem-perature was reduced. The substitution/dehydrochlorination ratioincreased to 0.91 at 100 ◦C after 48 h and to 0.74 at 130 ◦C after 24 h(Fig. 8). The increase in the ratio of substitution was also visiblein the product colors (Fig. 9). The product at 100 ◦C (Fig. 9(b)) wasalmost exactly the same color as that of the rigid PVC (Fig. 9(a)).Even at 130 ◦C (Fig. 9(c)), there was only a slight change in prod-uct color; the orange tinge indicated that the zipper mechanism,responsible for fast dehydrochlorination of PVC [21], did not occurat this low temperature. That is, the formation of conjugated C Cdouble bonds was not observed. (For interpretation of the refer-ences to color in this text, the reader is referred to the web version

of the article.)

The FT-IR spectra show the substitution of Cl by SCN (Fig. 10).The absorption bands derived from the –S–C N group (around2150 cm−1) and the –N C S group (around 2050 cm−1) werepresent in products obtained at 100 ◦C after 48 h and at 130 ◦C after

Fig. 8. Yields of the substitution of Cl by SCN and the elimination of HCl from rigidPVC in the presence of TBAB: (a) at 100 ◦C for 48 h and (b) at 130 ◦C for 24 h.

T. Kameda et al. / Materials Chemistry and Physics 124 (2010) 163–167 167

Fig. 9. Photographs of (a) rigid PVC, and its products after dehydrochlorination

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[24] N.R. James, A. Jayakrishnan, Surface thiocyanation of plasticized poly(vinyl

ig. 10. FT-IR spectra of (a) rigid PVC, and its products after dehydrochlorination inhe presence of TBAB: (b) at 100 ◦C for 48 h and (c) at 130 ◦C for 24 h.

4 h. The rising intensities of their absorption bands with increas-ng temperature can be explained by an acceleration of the SN2eaction under these conditions.

. Conclusions

TBAB was found to accelerate the dehydrochlorination of flexi-le and rigid PVCs in a SCN/EG solution. Both the substitution yieldnd the ratio of the substitution to dehydrochlorination increasedn the presence of TBAB. A further reduction of the temperatureaused a shift in the reaction towards one of substitution.

This behavior can be explained by the lipophilic character of thehase-transfer catalyst; that is, by increasing the catalyst concen-ration in the PVC phase, the concentration of SCN also increasesScheme 3). Although the equilibrium remains unaffected by theatalyst, this fosters an accelerated exchange between Cl and SCN.he increase in the substitution/dehydrochlorination ratio in theresence of the catalyst occurs because the acceleration of substi-ution is more than that of elimination.

Overall, the phase-transfer catalyst, TBAB, was found to be veryffective in accelerating the substitution of Cl in PVC by SCN.

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[

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