European Polymer Journal 48 (2012) 885–895

Contents lists available at SciVerse ScienceDirect

European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Macromolecular Nanotechnology

Gelation/fusion behavior of PVC plastisol with a cyclodextrin derivative andan anti-migration plasticizer in flexible PVC

Byong Yong Yu, Ah Reum Lee, Seung-Yeop Kwak ⇑Department of Materials Science and Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151–744, Republic of Korea

NA

NO

TECH

NO

LOG

Y

a r t i c l e i n f o

Article history:Received 27 October 2011Received in revised form 4 January 2012Accepted 5 February 2012Available online 24 February 2012

Keywords:PVC plastisolPlasticizerCyclodextrin derivativeMigration inhibitorGelation and fusion process

0014-3057/$ - see front matter Crown Copyright �doi:10.1016/j.eurpolymj.2012.02.008

⇑ Corresponding author. Tel.: +82 2 880 8365; faxE-mail address: sykwak@snu.ac.kr (S.-Y. Kwak).

MO

LECU

LAR

a b s t r a c t

We successfully evaluated the effects of 2,3,6-per-O-benzoyl-b-cyclodextrin (Bz-b-CD) onthe rheological properties of PVC plastisols and the migration behavior of plasticizer fromflexible PVC. Two types of plasticizer, di-isononyl phthalate (DINP) and diisononyl cyclo-hex-4-ene-1,2-dicarboxylate (Neocizer), along with Bz-b-CD as a migration inhibitor weremechanically mixed into an emulsion grade PVC resin to prepare plastisols. The presence ofBz-b-CD was expected to facilitate formation of stable complexes with DINP or Neocizer inthe flexible PVC. It was necessary to determine whether changes in the processing condi-tions of the PVC plastisol were needed for use in this application. To this end, the viscoelas-tic properties of the plastisols, including the elastic modulus, G0, and the viscous modulus,G00, were continuously monitored as a function of temperature during the gelation andfusion processes using rheological analysis techniques. The results showed that completegelation was slightly delayed and both moduli (G0 and G00) decreased upon addition ofBz-b-CD to the PVC matrix. FE-SEM images yielded insight into the gelation and fusionprocesses. The curing conditions and physical properties of the flexible PVCs containingBz-b-CD were optimized, and the influence of Bz-b-CD on the migration of the plasticizersand the stability of the flexible PVC was studied. The results showed that Bz-b-CD reducedmigration of DINP and Neocizer from the flexible PVC by almost 40% and 25%, respectively,thereby favorably restricting migration within the flexible PVC.

Crown Copyright � 2012 Published by Elsevier Ltd. All rights reserved.

CRO

MA

1. Introduction

Plasticizers are additives that can improve the flexibilityand workability of poly(vinyl chloride) (PVC) by formingsecondary bonds (dipole–dipole interactions) with thepolymer chains. The high density of the ends of the plasti-cizer chains introduces additional free volume into the PVCmatrix [1]. PVC can be plasticized with varying amounts ofplasticizers to form flexible PVC. Phthalate plasticizers,such as di-isononyl phthalate (DINP) and di-(2-ethylhexyl)phthalate (DEHP), are common plasticizers that are widelyused in industrial applications [2]. Phthalate plasticizersweakly interact with PVC chains and may migrate out of

2012 Published by Elsevier L

: +82 2 885 1748.

the plasticized PVC products into the environment, forexample, into the materials that contact the PVC. Healthrisk assessments over the past few decades have examinedthe health effects associated with phthalate leaching fromPVC [3,4]. Plasticizer leaching can render the materials incontact with the PVC useless for some applications dueto plasticizer effects on these materials’ mechanical prop-erties or appearance, and the mechanical properties ofthe plasticized PVC products themselves can degrade dueto plasticizer loss. Industries have begun to exploit newalternative non-phthalate plasticizers (e.g., di-isononylcyclohexane-1,2-dicarboxylate (DINCH) and diisononylcyclohex-4-ene-1,2-dicarboxylate (Neocizer)) to alleviatelegal concerns. Unfortunately, several popular alternativeplasticizers are structurally similar to phthalate plasticiz-ers and, consequently, also have the potential to migrateand introduce toxic compounds into the environment.

td. All rights reserved.

Fig. 1. Chemical structure of (a) DINP and (b) Neocizer.

886 B.Y. Yu et al. / European Polymer Journal 48 (2012) 885–895

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

The use of nanoscale particles as functional additiveshas attracted significant attention in several polymermaterial research fields in recent years [5,6]. b-Cyclodex-trin (b-CD) has an internal cavity shaped like a truncatedcone of about 0.8 nm deep and 0.60–0.64 nm in diameter.This cavity possesses a relatively low polarity that canaccommodate guest organic molecules inside, resulting ininclusion complexes. In particular, the high density of hy-droxyl groups on the exterior of b-CD can easily be modi-fied with various functional groups to endow the b-CDwith specific interactions with other molecules [7]. Grantet al. recently described the effect of b-CD and hydroxypro-pyl b-CD incorporation into plasticized PVC on leaching ofDEHP and biocompatibility [8]. In previous reports, we suc-cessfully prepared a plasticizer with reduced DEHP migra-tion by directly incorporating nanoscale b-CD derivativesinto DEHP. The addition of a b-CD derivative decreasedthe levels of DEHP migration from the flexible PVC samplesby almost 40%. The reduction in DEHP migration from theflexible PVC was due to the formation of p–p stabilizedcomplexes and inclusion of DEHP molecules by suitablyoriented b-CD derivatives [9]. In this context, and moti-vated by the technological and scientific value of thesestudies, we intensively pursued a rheological study of theinfluence of b-CD derivatives during the gelation andfusion of PVC plastisols. PVC plastisols are suspensionsconsisting of an emulsion-type PVC resin in a liquid contin-uous phase formed mainly by a plasticizer and a thermalstabilizer. PVC plastisols are used today to produce manycommercially important products [10]. All industrial pro-cesses for preparing plastisols involve heating the plastisolto 150–200 �C. During heating the plastisol undergoes twoprocesses, gelation and upon cooling the plastisol is trans-formed into a relatively soft, flexible substance [11]. Addi-tional components, such as b-CD derivatives, can be addedto modify the rheological properties of the PVC plastisol.From a commercial perspective, it is useful to optimizeplastisol processing conditions for various PVC formula-tions. A useful method for monitoring gelation and fusionis to characterize the viscoelastic properties. When com-bined with field-emission scanning electron microscopy(FE-SEM), viscoelastic characterization provides a betterunderstanding of the viscoelastic behavior, such as gela-tion and fusion of PVC plastisol. The parameters most fre-quently used to quantify rheological properties are theelastic (or storage) modulus, G0, and the viscous (or loss)modulus, G00. These properties show distinct changes atthe initiation and termination of plastisol gelation, and atthe temperature at which fusion becomes complete [12].

In this study, viscoelastic measurements permitted con-tinuous monitoring of the changes in the G0 and G00 as afunction of temperature during gelation and fusion. Therheological analysis quantified the influence of Bz-b-CDon the gelation and fusion behavior of the PVC plastisols.A phthalate or a non-phthalate plasticizer, DINP or Neociz-er, respectively, along with Bz-b-CD as a migration inhibi-tor, were mechanically mixed with emulsion-grade PVCresin to prepare a PVC plastisol. It is important to note thatthe non-phthalate plasticizer, Neocizer, was structurallysimilar to the most widely used DINP (see Fig. 1). Theinfluence of Bz-b-CD on the physical properties of the flex-

ible PVC was experimentally investigated. Migration testsof the flexible PVC were conducted according to theInternational Organization for Standardization (ISO)3826:1993(E) method to measure the plasticizer migrationbehavior. Gas chromatography (GC) equipped with a flameionization detector (FID) was employed to identify andquantify which migrated to the surface of the PVC partand dissolved in the contacting solvent.

2. Experimental

2.1. Materials

b-Cyclodextrin (b-CD) was obtained from Tokyo Chem-ical Industry Co., Ltd. and dried in a vacuum oven at 60 �Cfor 7 days prior to use. Benzoyl chloride and anhydrouspyridine were purchased from Sigma–Aldrich (stated pur-ity P99%). Di-isononyl phthalate (DINP) and diisononylcyclohex-4-ene-1,2-dicarboxylate (Neocizer) were kindlyprovided by Aekyung Petrochemical Co., Ltd., Korea. Emul-sion grade poly(vinyl chloride) (PVC) resin (LG PB1752, de-gree of polymerization: 1700 ± 50 and kwert (k) value: 76)was provided by LG Chem. Ltd., Korea. Epoxidized soybeanoil (ESO) and a thermal stabilizer (methyl tin, trade nameMT-800) were purchased from Yakuri Pure Chemicals Co.,Ltd., Japan and Songwon Co., Ltd., Korea, respectively, andused to prepare the PVC plastisol. All chemicals except b-CD were used as received without further purification.

2.2. Preparation of cyclodextrin derivative as a migrationinhibitor

b-CD was modified with benzoyl chloride, resulting in2,3,6-per-O-benzoyl- b-cyclodextrin (Bz-b-CD), in accor-dance with the procedures described in the literature[13]. Purified and dried b-CD (11.35 g, 10 mmol) was stir-red into 240 mL anhydrous pyridine, and 160 mL benzoylchloride (1.44 mol) was added. The solution was stirredat 50 �C for 72 h. Termination of the reaction was accompa-nied by a solution color change from bright pink to orange–brown, with some precipitate. The mixture was evaporatedat 50 �C under reduced pressure until it reached about halfthe volume. The thick solution was cooled in an ice bath,and 500 mL anhydrous methanol was added very slowlywith stirring. The copious white precipitate was filteredoff, and the crude product was resuspended in methanol.

B.Y. Yu et al. / European Polymer Journal 48 (2012) 885–895 887

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

The white powders in methanol were filtered out andwashed several times with, alternately, distilled water ormethanol. Finally, the product was dried in a vacuum ovenand ground to a fine white powder. The modification of b-CD was verified using Fourier-transform infrared (FT-IR)spectroscopy using a Perkin-Elmer GX IR spectrophotome-ter with a spectral resolution of 4 cm�1 over the range4000–400 cm�1. All samples were prepared by compres-sion molding, and potassium bromide (KBr) powder wasused as the sample matrix and reference material. The de-gree of substitution of Bz-b-CD was confirmed by 1H nucle-ar magnetic resonance (NMR) spectroscopy using a BrukerAvance spectrometer 500 with dimethyl sulfoxide-d6 asthe solvent. The resulting Bz-b-CD particles were incorpo-rated with DINP or Neocizer as follows. To allow for aquantitative comparison, the mass of the primary plasti-cizer, either DINP or Neocizer, was 70 g in each sampleand the mass of Bz-b-CD, where used, was 10 g. To theplasticizer was carefully added dried Bz-b-CD with stirringat room temperature. The mixtures were sonicated in a335 W sonicator bath (Bransonic Co., Ltd., Model 3510R)for 30 min at room temperature until clear, resulting in atransparent colloidal solution. The particle sizes and sizedistributions of Bz-b-CD in the prepared samples weremeasured using dynamic light scattering (DLS) methodswith a Photal DLS-7000 spectrophotometer equipped witha Photal GC-1000 digital auto-correlator (Otsuka Electron-ics Co., Ltd., Osaka, Japan). In this procedure, the wave-length (k) of the argon (Ar) laser was 488 nm, and thescattering angle was 90� with respect to the incident beam.The correlation functions were analyzed using the con-strained regularized CONTIN method to determine the dis-tribution decay rates. The experiments were conducted atroom temperature, and each experiment was repeatedtwo or more times.

2.3. Rheological measurements of the PVC plastisolscontaining Bz-b-CD

PVC pastes, also known as plastisols, were preparedusing a dried emulsion PVC resin, DINP or Neocizer, Bz-b-CD, epoxidized soybean oil (ESO), and a thermal stabilizer(TS), as listed in Table 1. The PVC plastisols were preparedby slowly adding Bz-b-CD and the other additives to thedry emulsion PVC resin. A mechanical stirrer with a two-blade propeller was then used to further homogenize thepaste. After mixing had been completed, air bubbles inthe plastisols were removed by applying a vacuum. The

Table 1Compositions of the PVC plastisols.

Sample codes Components (phr)a

PVC resin DINP or Neocizer

PVC/DINP 100 70PVC/DINP-CDPVC/NeoPVC/Neo-CD

a Parts per hundred resin of PVC.b Epoxidized soybean oil used as a secondary plasticizer.c Thermal stabilizer.

plastisols were then aged for 2 weeks at room temperatureprior to use. The aging is necessary because viscosity in-creases initially primarily because of de-agglomerationbut it stabilizes after 2 weeks [14]. The viscoelastic behav-ior of the PVC plastisols was examined to investigate theinfluence of Bz-b-CD on the properties and to determinethe optimal processing conditions. A parallel-plate rota-tional rheometer (Rheometer AR2000, TA instrumentsInc.) was used in the dynamic oscillatory mode with a con-trolled heating rate. The 40 mm diameter parallel-platedisks were used with a gap setting of 1 mm. The frequencyof oscillation and shear strain amplitude were kept at 1 Hzand 2.5%, respectively. The measurement temperature wasvaried from 25 to 200 �C, with a programmed rate of in-crease of 5 �C min�1. Morphological analysis of the PVCplastisols at various stages of gelation and fusion was con-ducted using an FE-SEM, JEOL JEM-6700F, which used sec-ondary electrons with an acceleration voltage of 15 kV. TheFE-SEM samples were prepared by loading a plastisol be-tween the plates of the rheometer and heating the appara-tus to the desired temperature without application of shearstrain. The sampling temperatures were chosen based onthe characteristic features of the viscoelastic curves.

2.4. Physical properties of the flexible PVC sheets containingBz-b-CD

Flexible PVC samples were fabricated through gelationand fusion of the PVC plastisols upon heating. The sampleswere then cooled to room temperature. The flexible PVCsheets containing Bz-b-CD were denoted PVC/DINP-CDand PVC/Neo-CD, respectively. For comparison, PVC/DINPand PVC/Neo without Bz-b-CD (blank sample) was alsoprepared according to the same method (see Fig. S1). Theinfluence of Bz-b-CD on the physical properties of the flex-ible PVCs was assessed experimentally. The glass transitiontemperatures, Tg, of the flexible PVCs were determined rel-ative to the indium standards using a TA Instrument 2920differential scanning calorimetry (DSC) system at a heatingrate of 5 �C min�1 over the temperature range �80 to150 �C. The temperature programmed procedure was per-formed under a stream of nitrogen. As a measure of theflexibility of the PVC samples, we measured the% elonga-tion at break using tensile tests performed on a LLOYDLR10K universal testing machine (UTM). The tests wereconducted at a strain rate of 50 mm min�1 with a 1 kNstatic load cell. The test specimens assumed dumbbellshapes with a width of 9.5 mm and a thickness of 2 mm,

ESOb TSc Bz-b-CD

3 2 –10–10

Fig. 2. FT-IR spectra of (a) b-CD, (b) benzoyl chloride, and (c) Bz-b-CD.

888 B.Y. Yu et al. / European Polymer Journal 48 (2012) 885–895

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

in accordance with the American Standard Testing Method(ASTM) D-638 [15]. The thermal decomposition studieswere performed over the temperature range 25–600 �Cusing the TA Instrument Q500 thermogravimetric analysis(TGA) system under flowing nitrogen at a scan rate of10 �C min�1. The masses of the flexible PVC samples wereapproximately 5–10 mg. In addition, the optical propertiesof the flexible PVC sheets with a thickness of 0.40 mm weremeasured using haze tests (BYK Gardner Co., Ltd., Haze-Gard Plus) with a Commission Internationale de l’Eclairage(CIE) standard illuminant C (320 < k < 780 nm) as the lightsource.

2.5. Migration tests

Migration tests were carried out on the prepared flexi-ble PVC sheets according to the International Organizationfor Standardization (ISO) test method 3826:1993(E) [16].Distilled water and ethanol were mixed to prepare theextraction solution. The ratio of distilled water to ethanolwas set to 124:100 by the volume ratio, with a density of0.9374 g/mL at 25 �C by pyknometer. The migrated plasti-cizers in the extraction solutions were quantitatively ana-lyzed using a Hewlett Packard model 6890 Series II PlusGC system with a flame ionization detector and a DB-5capillary column (30 m � 0.32 mm I.D. with a film thick-ness of 0.25 lm). The column was maintained at 80 �Cfor 3 min, ramped up to 320 �C with a heating rate of10 �C min�1, and finally maintained for 13 min. The gaschromatograph was operated in the splitless injectionmode at a temperature of 320 �C. Helium was used as thecarrier gas at a flow rate of 1.8 mL min�1. A calibrationcurve was constructed by plotting the ratio of the peakarea of several DINP or Neocizer standard solutions as afunction of concentration between 1 and 20 mg/100 mL.A summary of the detailed condition of GC analysis is givenin Table S1. The flexible PVC sheets were fabricated in sizesof 30 � 30 � 2 (L � H � D mm3). The PVC sheets (i.e., PVC/DINP, PVC/DINP-CD, PVC/Neo, and PVC/Neo-CD) wererinsed with distilled water to remove dust and impuritiesfrom the surface, and then immersed in 100 mL of theextraction solution. The testing temperature was main-tained at 37 ± 1 �C and the samples remained in solutionfor 1–7 days without shaking during the tests. The flaskswere then removed from the oven, inverted gently 10times, and the contents were transferred to a sample cell.Quantitative analysis of each sample was repeated threetimes to construct a calibration curve and twice for theother samples. The concentrations of the plasticizer thathad migrated into the extraction solution from the flexiblePVC samples were expressed in mg/100 mL.

3. Results and discussion

3.1. Bz-b-CD as a migration inhibitor

Fig. 2 shows three FT-IR spectra of neat b-CD, benzoylchloride, and Bz-b-CD. The broad absorption band in therange of 3000–3600 cm�1 displayed by neat b-CD, shownin Fig. 2(a), corresponded to the stretching vibrations ofthe hydroxyl (–OH) group. Several intense bands in the

range 1029–1157 cm�1 were assigned to primary and sec-ondary C–OH stretches, and to the C–O–C antisymmetricstretches, respectively [17]. Modification of the neat b-CDwith benzoyl chloride, however, introduced significantchanges to the FT-IR spectrum of Bz-b-CD, as can be seenin Fig. 2(c). The broad peak corresponding to the hydroxylgroup stretch disappeared and was accompanied by theappearance of peaks corresponding to the sp2 C–H stretchat 3075 cm�1 and the aromatic C@C at 1603 and1448 cm�1. These bands corresponded to stretches of thearomatic ring of the benzoyl group. In addition, the conju-gated C@O stretches at 1729 cm�1 and the C–O stretches inthe range 1000–1300 cm�1 resulted from formation of anester. Therefore, the Bz-b-CD spectrum clearly indicatedthat b-CD had been modified. Fig. 3 shows the 1H NMRspectra of neat b-CD, benzoyl chloride, and Bz-b-CD. 1HNMR spectroscopy provided additional evidence for themodification and indicated the degree of substitution.The Bz-b -CD spectrum showed that the hydroxyl grouppeaks of b-CD disappeared as the peaks attributed to theprotons of the benzoyl group appeared (2,3,6-COC6H5 at6.9–8.1 ppm), indicating the substitution of the three hy-droxyl groups of b-CD. Each proton (3-H triplet at6.2 ppm, 1-H doublet at 5.6 ppm, 2-H doublet of doubletsat 5.1 ppm, 5-, 6a-, 6b-H, at 4.8–5.0 ppm, and 4-H tripletat 4.5 ppm, respectively) of the Bz-b-CD was detected,and the peaks were shifted downfield upon modification(see Fig. 3(c)). The number of benzoyl groups, x, introducedto neat b-CD (which included 21 hydroxyl groups) was eas-ily calculated from the relative peak integrals in the 1HNMR spectrum. The value of x was determined to be 20.2(around 96%), which confirmed the successful modificationof neat b-CD to yield Bz-b-CD. Additional discussion of themodification and general characterization of Bz-b-CD isprovided in our previous publication [9].

Fig. 3. 1H NMR spectra of (a) b-CD, (b) benzoyl chloride, and (c) Bz-b-CD.

B.Y. Yu et al. / European Polymer Journal 48 (2012) 885–895 889

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

3.2. Dispersion of Bz-b-CD in plasticizers

Several thermodynamic methods can be used to predictand explain miscibility in multiple phase systems, includ-ing the solubility parameter, the equation-of-state, andthe lattice theory [18]. The solubility parameter method(d (cal/cm3)1/2) was chosen for this study. The solubilityparameters of Bz-b-CD, DINP, and Neocizer can be calcu-lated based on the following equation, d = qRG/M, whereq represents the density, G is the set of group molar attrac-tion constants, and M is the molecular mass of the repeat-ing unit. For simplicity, room temperature was used as thestandard condition. The group molar attraction constants(commonly Small’s constants) of the specific functionalgroups, which are useful in determining the solubilityparameter, have been calculated by Small and Hoy [19].Compounds with similar solubility parameters (±1.8 (cal/cm3)1/2) are likely to be moderately miscible [20]. This isbecause the energy of mixing two components is balancedby the energy released by interactions among the purecomponents. PVC has a solubility parameter of 9.66 (cal/cm3)1/2. The calculated solubility parameters for DINP,Neocizer, and Bz-b-CD were 8.83, 8.72, and 8.01 (cal/cm3)1/2, respectively. Thus, it was expected that Bz-b-CDcould be well-dispersed in both plasticizers. As shown inFig. 4, the quality of the Bz-b-CD dispersion in DINP orthe Neocizer solution was examined by DLS analysis. Thecorrelation functions were analyzed by means of the con-strained regularization method to determine the distribu-tion decay rate. This method analyzes the intensity ofscattered laser light over time, which depends on the par-ticle size. It was very difficult to disperse b-CD in the plast-icizers, as predicted by the solubility parameter and

hydrophilicity of b-CD and microscale (around 0.4 lm)agglomeration of b-CD was observed in both plasticizers.The DLS results indicated formation of a nanoscale disper-sion of Bz-b-CD in both DINP and Neocizer(d = 2.1 ± 0.6 nm in DINP and 2.3 ± 0.8 nm in Neocizer).These results showed that, as expected, Bz-b-CD could bewell-dispersed in both plasticizers.

3.3. Rheological behavior of PVC plastisols containing Bz-b-CD

PVC resins obtained by the emulsion polymerizationprocess are usually used in the preparation of plastisols.Generally, flexible PVC products are fabricated through aplastisol by heating briefly to the fusion temperature andthen cooling [21]. As shown in Figs. 5 and 6, G0 and G00 ini-tially decreased as the plastisol was heated from roomtemperature. The system behaved as a suspension ofnon-interacting PVC particles in the plasticizer, whichformed a continuous phase with a viscosity that decreasedwith increasing temperature [22]. During the later stagesof heating, the PVC particles dissolved into the plasticizerfrom their outer surfaces, which glued the particles to-gether. In this stage, G0 reached a maximum correspondingto the complete absorption of plasticizer by the PVC parti-cles. When a PVC plastisol is heated, PVC particles swellwith the plasticizer as the plasticizer is absorbed, and asteep increase in G0 and G00 is observed. This process iscalled gelation. Further temperature increases produceadditional swelling and dissolution of more polymers,and the microcrystallites of the PVC melt and both G0 andG00 begin to drop off. This process is called fusion [23].Fig. S2 illustrates the concept of gelation as part of thecomplete fusion scheme. The temperature at which fusion

Fig. 4. Particle size distributions of Bz-b-CD (solid black bar) and neatb-CD (shaded gray bar) in (a) DINP and (b) Neocizer.

Fig. 5. Viscoelastic profile of (a) PVC/DIN

890 B.Y. Yu et al. / European Polymer Journal 48 (2012) 885–895

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

became essentially complete is indicated by the intersec-tion of the moduli (G0 and G00). The processability require-ments for PVC plastisol processing emphasize theviscoelastic behavior, such gelation and fusion, of theplastisol [24]. During gelation and fusion, the elastic andviscous modulus in the rheological measurements under-went important changes. Fig. 5 shows the changes in G0

and G00 for the PVC/DINP and PVC/DINP-CD plastisols,respectively, which were recorded as a function of temper-ature. The viscoelastic behavior obtained from PVC/DINP-CD was similar to that of PVC/DINP. The maximum of themodulus corresponded to the complete gelation and onsetof fusion. In this stage, the PVC particles became swollen asthe plasticizer was taken up, and some portion of the PVCmolecules may have dissolved into the plasticizer from thesurface layers of the particles. Further temperature in-creases resulted in a decrease in the modulus, indicatingthe dominance of fusion and melting of the PVC microcrys-tallites. The decreased modulus was due to two processes:(i) the normal temperature-dependent changes were pri-marily attributable to thermal expansion, and (ii) meltingof the PVC microcrystallites [25]. As can be seen Fig. 5,complete gelation of the PVC/DINP-CD plastisols occurredat 158 �C, a gelation temperature that was slightly higherthan that of the PVC/DINP plastisol (150 �C). Gelation ofthe PVC plastisols was delayed upon addition of Bz-b-CDparticles, possibly because of the decreased interaction ofthe PVC with the plasticizer. In the Neocizer series, thegelation point of the PVC/Neo-CD plastisol was 169 �C,slightly higher than the gelation point of the PVC/Neo plas-tisol (162 �C). Gelation of the PVC plastisol was alsodelayed by the addition of Bz-b-CD particles, as shown inFig. 6. The formation of entanglements among the PVCchains may have been blocked by the Bz-b-CD nanoparti-cles (see Fig. 7(b)). During processing, partial melting ofthe crystallites in the primary PVC particles occurred,allowing the macromolecules to diffuse through the

P and (b) PVC/DINP-CD plastisol.

Fig. 6. Viscoelastic profile of (a) PVC/Neo and (b) PVC/Neo-CD plastisol.

B.Y. Yu et al. / European Polymer Journal 48 (2012) 885–895 891

NA

NO

TECH

NO

LOG

Y

boundaries and entangle among the other macromole-cules, as shown in Fig. 7(a). The melted crystalline compo-nents then recrystallized during cooling to form newlycreated ordered domains of secondary crystallites [26].The presence of well-dispersed Bz-b-CD nanoparticles inthe PVC matrix may have disrupted the physical crosslinksbetween neighboring PVC molecules, thereby partiallyinterrupting formation of the 3-D macromolecular

Fig. 7. Formation of molecular entanglement;

network. The fusion process completed, according to theviscoelastic data, at almost the same temperature (about187 �C) as was observed for Figs. 5 and 6, suggesting thatthe macromolecular network reached an equilibrium stateat around 187 �C. These results showed that the presenceof Bz-b-CD did not alter the fusion point of the PVC plasti-sol. As shown in Fig. S3, the viscoelastic behavior of theNeocizer series was similar to that of the DINP series.

(a) neat PVC and (b) PVC with Bz-b-CD.

MA

CRO

MO

LECU

LAR

Fig. 8. DSC curves for non-plasticized PVC and flexible PVC samples.

892 B.Y. Yu et al. / European Polymer Journal 48 (2012) 885–895

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

However, the temperatures of complete gelation differedbetween the DINP and Neocizer series. This differencecould be explained in terms of the compatibility of theplasticizers and PVC. The solubility parameter and thepolarity parameter, /, can predict the compatibility. Thepolarity parameter can be obtained from the equation /= (Ap/Po)M/1000, where Ap is the number of carbon atomsin the molecule, excluding aromatic and carboxylic carbonatoms, Po is the number of polar groups, and M is themolecular weight. The factor 1000 is used to convenientlyrescale / [27]. Small polarity parameters for the plasticizerand similar solubility parameters for the plasticizer andPVC predict good PVC/plasticizer compatibility [28]. Thecalculated solubility parameters and polarity parameterwere 8.83 (cal/cm3)1/2, 3.78 for DINP and 8.72 (cal/cm3)1/2,5.07 for Neocizer. The relative values predicted that DINPwas more compatible with PVC due to the presence of thearomatic ring. The aliphatic ring in Neocizer decreasedthe compatibility with PVC. Therefore, the absorption ofDINP reached completion faster than that of Neocizer, andthe gelation and fusion processes of the DINP series ap-peared at lower temperatures (12 �C lower) as a conse-quence of the higher solvent power of the plasticizer. Themorphologies of the PVC plastisols at various stages of gela-tion and fusion were examined by FE-SEM to investigatethe structural changes associated with the PVC/DINP andPVC/Neocizer interactions, and the morphologies provideda qualitative analysis of the gelation and fusion processes.These results were compared with and used to interpretthe changes in the viscoelastic behavior. Starting from atwo-phase system comprising solid particles dispersed inthe liquid, the plastisol transitioned into a one-phase rub-bery solid though gelation and fusion. Figs. S4 and S5 showSEM images of the PVC/DINP-CD and PVC/Neo-CD system.The images show a magnification of �5000. At 70 �C, thePVC particles were clearly identifiable, and the presenceof agglomerates was apparent. At the complete gelationtemperature, 158 or 169 �C for the PVC/DINP-CD or PVC/Neo-CD systems, respectively, few particles were identifi-able, and interparticle boundaries were obscured by entan-glement. At 170 �C, fusion occurred, and the particulatemorphology was almost absent. Finally, at 187 �C, fusionhad completed, and recrystallization followed, upon cool-ing, to form a 3-D structure held together by crystallitesand elastomeric molecules. The fracture surface was con-tinuous, and no domain boundaries could be identified.During gelation and fusion, the PVC plastisols changed fromPVC particles in DINP or Neocizer to a uniform mass. Over-all, the disappearance of the PVC particulate boundaries in-creased the homogeneity of the PVC plastisols.

3.4. Physical properties of the flexible PVC

Fig. 8 shows the DSC thermograms of the flexible PVC.The glass transition temperatures, Tg, determined duringthe second runs, corresponded to the mid-points of thesmall endothermic rises in the pre- and post-transitionbaselines. A single Tg for each of the flexible PVCs studiedsupported the miscibility among PVC, the plasticizers,and Bz-b-CD. The flexible PVC samples containingBz-b-CD exhibited slightly higher Tg values in comparison

with those of pure flexible PVCs. The addition of plasticiz-ers to a PVC resin increased the free volume of the PVC,thereby lowering the PVC Tg. The flexibility of PVC is amain reason why these materials are used so widely. Thepercent plasticization efficiency, EDTg, can be calculatedaccording to the following equation:

EDTg ¼DTg;PVC=plasticizer�CD

DTg;PVC=plasticizer� 100

where DTg is the reduction in Tg. The calculated EDTg valuesare listed in Table 2 with the glass transition temperaturesof the flexible PVC samples. The plasticizing efficiency ofPVC/DINP-CD was found to be comparable to that of PVC/DINP, with the EDTg value of PVC/DINP-CD reaching up to97.8%. EDTg of the PVC/Neo-CD was also found to be com-parable to that of PVC/Neo, differing by only 1.7%. Thesematerials, therefore, were completely flexible at room tem-perature. Fig. 9 shows the stress–strain curves of the PVC/DINP and PVC/DINP-CD samples incorporating Bz-b-CD.These results are typical of soft tough materials. As shownin Fig. 9, although all samples behaved similarly, the ulti-mate strength of the stress and elongation at the breakslightly decreased as the Bz-b-CD content in the flexiblePVC sheets increased. Therefore, Bz-b-CD was well-dis-persed in the PVC matrix without forming agglomerates,which act as significant defects. Bz-b-CD apparently inter-acted with the polar groups of the PVC chains and impededthe motions of the PVC chains. The plasticization efficien-cies, EDeb, were estimated by comparing EDeb due toDINP-CD and Neo-CD with those of DINP and Neocizer:

EDeb¼ Deb;PVC=plasticizer�CD

Deb;PVC=plasticizer� 100

The eb values of the flexible PVC samples and the calcu-lated EDeb values are also presented in Table 2, and thetrends differ only slightly from those observed for the EDTg

data. The transmittance and haze of the flexible PVC sheetscontaining Bz-b-CD were investigated and compared withtheir counterparts, PVC/DINP and PVC/Neo. In general,

Table 2Glass transition temperatures, ultimate elongation, and percent plasticization efficiency.

Data PVC PVC/DINP PVC/DINP-CD PVC/Neo PVC/Neo-CD

Tg (oC)a 85.6 �47.3 �44.4 �40.9 �38.8EDTg (%)b 0 100.0 97.8 100.0 98.3eb (%)c 163.2 767.6 710.1 657.3 622.7EDeb (%)d 0 100.0 90.5 100.0 92.9

a Glass transition temperatures.b Percent plasticization efficiencies estimated from the lowering of the glass transition temperatures.c Ultimate elongation.d Percent plasticization efficiencies estimated from the improving of ultimate elongation.

Fig. 9. Tensile stress–strain curves for the flexible PVC sheets.

Fig. 10. Transmittance (striped bar) and haze (shaded bar) of the flexiblePVC sheets.

B.Y. Yu et al. / European Polymer Journal 48 (2012) 885–895 893

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

agglomerates can seriously deteriorate the clarity of PVCsheets. As shown in Fig. 10, no deterioration in the trans-mittance of the PVC sheets was observed upon additionof Bz-b-CD nanoparticles. Bz-b-CD was endowed withhydrophobic benzoyl groups that limited formation ofagglomerates among Bz-b-CD particles in PVC/DINP-CDand PVC/Neo-CD. In other words, Bz-b-CD was well-dispersed in the plasticized PVC matrix on the nanoscale.However, the dispersed Bz-b-CD particles slightly in-creased the haze of the sheets because the tiny Bz-b-CDparticles scattered light in the plasticized PVC matrix.The thermal stability of each PVC sheet was analyzed byTGA, and the results are shown in Fig. S6. All sample curvesshowed similar trends, and each curve presented two dis-tinct stability stages corresponding to a first weight loss ofabout 75%, between 210 and 320 �C, and a second weightloss of about 15% between 410 and 470 �C. The analysis re-sults suggest that the first weight loss corresponds to theelimination of HCl with some benzene traces. The secondstage corresponds to the polyacetylene sequences formedby elimination of HCl from adjacent carbon atoms duringthe first stage [29]. At the temperature above 470 �C, theweight is almost unchanged. These results confirm thatthe thermal stability of the PVC/DINP-CD and PVC/Neo-CD is similar to that of the PVC/DINP and PVC/Neo samples.

3.5. Anti-migration of the plasticizer in flexible PVC

Two specific types of plasticizer, DINP and Neocizer, in-cluded alkyl chains comprising a mixture of C8–C10 chain

isomers, and the largest component of the mixtureincluded C9 chains [30]. As seen in Fig. S7, the mixture ofisomeric alkyl chains introduced several peaks into theGC–FID chromatogram. A calibration curve was con-structed by plotting the sum of the GC–FID peak areas cor-responding to the DINP and Neocizer plasticizers relativeto the peak area of an internal standard for plasticizer con-centrations of 1, 2, 5 and 10 mg/100 mL (shown in Fig. S8).The plasticizer response was linear, with a correlation coef-ficient, r, of r > 0.9997 for DINP and r > 0.9995 for Neocizer.The migration behavior of the plasticizers from the flexiblePVC containing Bz-b-CD was examined by extracting into amixed water/ethanol solution at 37 �C. The plasticizers thatmigrated during extraction of each PVC sample over 1, 3, 5,and 7 days were detected by GC–FID, and the sum of thepeak areas was compared with the calibration curve todetermine the plasticizer concentration. The total amountof the migrated plasticizers as a function of time is illus-trated in Fig. 11. The figure shows that plasticizer migra-tion dramatically increased during the initial stages, andthe rate of migrated plasticizer extraction slowed remark-ably after 24 h. After a seven day extraction, the concentra-tion of the migrated plasticizer extracted from allplasticized PVC sheets incorporating Bz-b-CD was lowerthan that extracted from the neat plasticized PVC sheet,as listed in Table 3. The plasticizer on the surface of thePVC sheet diffused through the interface between the sheetand the extraction solution, and the vacancies left by themigrated plasticizers were exchanged with the extractionsolution [31]. For thermodynamic reasons, the penetrated

Fig. 11. The cumulative amount of migrated plasticizer as a function oftime at 37 �C.

Table 3Concentration of the plasticizer and anti-migration efficiencies as afunction of time.

Samples Concentration of plasticizer in themigration medium (mg/100 mL)

1 day 3 day 5 day 7 day

PVC/DINP 4.89 6.26 6.66 6.86PVC/DINP-CD 2.89 3.66 4.10 4.25Anti-migration efficiencies (%) 40.86 41.64 38.46 38.11PVC/Neo 7.03 8.84 9.75 10.11PVC/Neo-CD 5.27 6.56 7.13 7.70Anti-migration efficiencies (%) 24.98 25.83 26.88 23.86

Fig. 12. Schematic illustration of the anti-migration m

894 B.Y. Yu et al. / European Polymer Journal 48 (2012) 885–895

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

extraction solution facilitated the diffusion of plasticizerfrom within the PVC sheet to the surface of the PVC sam-ple. During this process, Bz-b-CD prevented interactionsbetween the plasticizer and the extraction solution. There-fore, Bz-b-CD in the PVC matrix played a key role in inhib-iting migration and reducing plasticizer extraction. Theanti-migration efficiency (%) was calculated according to:

Anti-migration efficiencyð%Þ¼ 1�CPVC=plasticizer�CD

CPVC=plasticizer

� ��100

where CPVC/plasticizer-CD and CPVC/plasticizer are the concentra-tion of the migrated plasticizer during migration in thePVC/plasticizer-CD and PVC/plasticizer samples, respec-tively. The anti-migration efficiencies of PVC/DINP-CDand PVC/Neo-CD were 38.11% and 23.86%, respectively.This indicated that the presence of Bz-b-CD in the flexiblePVC matrix dramatically reduced plasticizer migration,which was attributed to the formation of an inclusion com-plex between the Bz-b-CD cavity and the plasticizer, andstabilizing p–p associations between the benzoyl groupsof Bz-b-CD and the aromatic ring of the plasticizer mole-cules. Bz-b-CD, which hindered migration of the plasti-cizer, also will block the plasticizer molecules by windingpathway (also called tortuous pathway) effect. In otherwords, it would promote the path length for transportingplasticizers and results in a decrease of plasticizer migra-tion (see Fig. 12). Neocizer did not introduce p–p contacts,and only van der Waals interaction stabilized the interac-tions between Bz-b-CD and Neocizer. For this reason, thereduction in the rate of Neocizer migration (with an ali-phatic ring) was smaller than the reduction in the rate ofDINP migration (with an aromatic ring). Differences inthe structure strongly affected the plasticizer/Bz-b-CDinteraction process. In addition, gel permeation chroma-tography (GPC) results confirmed that Bz-b-CD was not re-leased from the flexible PVC sheets during the migration

echanism of the plasticizer in the PVC matrix.

B.Y. Yu et al. / European Polymer Journal 48 (2012) 885–895 895

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

process (see Fig. S9). The results showed that Bz-b-CDnanoparticles reduced the rate of migration of both phthal-ate and non-phthalate plasticizers, thereby preserving thecomposition of flexible PVC over longer periods of time.

4. Conclusion

We present, here, a study of the influence of Bz-b-CD onthe rheological properties of PVC plastisols and the preven-tion of plasticizer migration from flexible PVC. Rheologicalanalysis showed that the gelation of the PVC plastisols wasslightly delayed upon addition of Bz-b-CD particles, whichobstructed the absorption of both plasticizers. Entangle-ment among the PVC chains was blocked by the Bz-b-CDnanoparticles. However, completion of the fusion process,as observed in the viscoelastic data, occurred at almostthe same temperature (around 187 �C). No significantchanges in the physical properties of the flexible PVC wereobserved upon addition of Bz-b-CD, possibly due to thegood dispersion of Bz-b-CD nanoparticles in the PVC ma-trix on the nanoscale. The presence of Bz-b-CD in the PVCmatrix played a key role in inhibiting plasticizer migrationwhich may explain the improved stability of the flexiblePVC.

Acknowledgment

This research was supported by the Eco-InnovationProject through the Korea Ministry of Environment.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.eurpolymj.2012.02.008.

References

[1] González N, Fernández-Berridi MJ. Fourier transform infraredspectroscopy in the study of the interaction between PVC andplasticizers: PVC/plasticizer compatibility. J Appl Polym Sci 2008;107(2):1294–300.

[2] Rahman M, Brazel CS. The plasticizer market: An assessment oftraditional plasticizers and research trends to meet new challenges.Prog Polym Sci 2004;29(12):1223–48.

[3] Hakkarainen M. Migration of monomeric and polymeric PVCplasticizers. Adv Polym Sci 2008;211(1):159–85.

[4] Tickner JA, Schettler T, Guidotti T, McCally M, Rossi M. Health risksposed by use of di-2-ethylhexyl phthalate (DEHP) in PVC medicaldevices: a critical review. Am J Ind Med 2001;39(1):100–11.

[5] Yang B, Bai Y, Cao Y. Effects of inorganic nano-particles onplasticizers migration of flexible PVC. J Appl Polym Sci 2010;115(4):2178–82.

[6] Fenyvesi E, Balogh K, Siro I, Orgovanyi J, Senyi JM, Otta K, et al.Permeability and release properties of cyclodextrin-containingpoly(vinyl chloride) and polyethylene films. J Incl PhenomMacrocycl Chem 2007;57(1–4):371–4.

[7] Dodziuk H. Cyclodextrins and their complexes, Chemistry, AnalyticalMethods, Applications. Weinheim: Wiley-VCH Verlag GmbH & Co;2006. p. 147–190.

[8] George SM, Gaylor JDS, Leadbitter J, Grant MH. The effect ofbetacyclodextrin and hydroxypropyl betacyclodextrin incorporation

into plasticized poly(vinyl chloride) on its compatibility with humanU937 cells. J Biomed Mater Res B Appl Biomater 2011;96B(2):310–5.

[9] Yu BY, Chung JW, Kwak S-Y. Reduced migration from flexiblepoly(vinyl chloride) of a plasticizer containing b-cyclodextrinderivative. Environ Sci Technol 2008;42(19):7522–827.

[10] Fenollar O, García D, Sánchez L, López J, Balart R. Optimization of thecuring conditions of PVC plastisols based on the use of an epoxidizedfatty acid ester plasticizer. Euro Polym J 2009;45(9):2674–84.

[11] Persico P, Ambrogi V, Acierno D, Carfagna C. Processability andmechanical properties of commercial PVC plastisols containing low-environmental-impact plasticizers. J Vinyl Addit Technol 2009;15(3):139–46.

[12] Daniels PH, Brofman CM, Harvey GD. Meaningful evaluation ofplastisol gelation and fusion temperatures by dynamic mechanicalanalysis. J Vinyl Addit Technol 1986;8(4):160–3.

[13] Boger J, Corcoran RJ, Lehn J-M. Selective modification of all primaryhydroxyl groups of a- and b-cyclodextrin. Helv Chim Acta1978;61:2190–218.

[14] Nakajima N, Harrell ER. Rheological observation of gelation andfusion process of poly(vinyl chloride) plastisol. Adv Polym Tech1986;6(4):409–40.

[15] ASTM D638–02a, Standard Test Method for Tensile Properties ofPlastics.

[16] ISO 3826, Plastics Collapsible Containers for Human Blood and BloodComponents. 1993.

[17] Villaverde J, Morillo E, Perez-Martinez JI, Gines JM, Maqueda C.Preparation and characterization of inclusion complex ofNorflurazon and b-cyclodextrin to improve herbicide formulations.J Agric Food Chem 2004;52(4):864–9.

[18] González N, Fernández-Berridi MJ. Applications of fourier transforminfrared spectroscopy in the study of interactions between PVC andplasticizers: PVC/plasticizer compatibility versus chemical structureof plasticizer. J Appl Polym Sci 2006;101(3):1731–7.

[19] Small PA. Some factors affecting the solubility of polymers. J ApplChem 1953;3:71–80.

[20] Jang BN, Wang D, Wilkie CA. Relationship between the solubilityparameter of polymers and the clay dispersion in polymer/claynanocomposites and the role of the surfactant. Macromolecules2005;38(15):6533–43.

[21] García JC, Marcilla A. Rheological study of the influence of theplasticizer concentration in the gelation and fusion processes of PVCplastisols. Polymer 1998;39(15):3507–14.

[22] García-Quesada JC, Marcilla A, Beltran M. Study of the processabilityof commercial PVC plastisols by rheology. J Vinyl Addit Technol1999;5(1):31–6.

[23] Kwak S-Y. Structural changes of PVC plastisols in progress ofgelation and fusion as investigated with temperature-dependentviscoelasticity, morphology, and light scattering. J Appl Polym Sci1995;55(12):1683–90.

[24] Nakajima N, Kwak S-Y. Effect of plasticizer type on gelation andfusion of PVC plastisol, dialkyl phthalate series. J Vinyl Technol1991;13(4):212–22.

[25] Daniels PH. Optimization of plastisol processes by dynamicmechanical analysis. J Vinyl Addit Technol 2007;13(3):151–4.

[26] Fillot L-A, Hajji P. U-PVC gelation level assessment, Part 2:Optimization of the differential scanning calorimetry technique. JVinyl Addit Technol 2006;12(3):98–107.

[27] Ramos-Devalle L, Gilvert M. PVC/plasticizer compatibility:evaluation and its relation to processing. J Vinyl Technol 1990;12(4):222–5.

[28] Ramos-Devalle L, Gilvert M. Compatibility between PVC andplasticizer blends and its relation with processing. J Vinyl Technol1992;14(2):74–7.

[29] Shah BL, Shertukde VV. Effect of plasticizers on mechanical,electrical, permanence, and thermal properties of poly(vinylchloride). J Appl Polym Sci 2003;90(12):3278–84.

[30] Koch HM, Angerer J. Determination of secondary, oxidised di-iso-nonylphthalate (DINP) metabolites in human urine representativefor the exposure to commercial DINP plasticizers. J ChromatographyB 2007;847(2):114–25.

[31] Rahman M, Brazel CS. Ionic liquids: New generation stableplasticizers for poly(vinyl chloride). Polym Degrad Stabil 2006;91(12):3371–82.