fusion behavior of PVC plastisol with a cyclodextrin derivative and an anti-migration plasticizer in flexible PVC

European Polymer Journal 48 (2012) 885–895

Contents lists available at SciVerse ScienceDirect

European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Macromolecular Nanotechnology

Gelation/fusion behavior of PVC plastisol with a cyclodextrin derivative and an 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

i n f o

Article history: Received 27 October 2011 Received in revised form 4 January 2012 Accepted 5 February 2012 Available online 24 February 2012 Keywords: PVC plastisol Plasticizer Cyclodextrin derivative Migration inhibitor Gelation and fusion process

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) on the rheological properties of PVC plastisols and the migration behavior of plasticizer from flexible PVC. Two types of plasticizer, di-isononyl phthalate (DINP) and diisononyl cyclohex-4-ene-1,2-dicarboxylate (Neocizer), along with Bz-b-CD as a migration inhibitor were mechanically mixed into an emulsion grade PVC resin to prepare plastisols. The presence of Bz-b-CD was expected to facilitate formation of stable complexes with DINP or Neocizer in the flexible PVC. It was necessary to determine whether changes in the processing conditions of the PVC plastisol were needed for use in this application. To this end, the viscoelastic 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 and fusion processes using rheological analysis techniques. The results showed that complete gelation was slightly delayed and both moduli (G0 and G00 ) decreased upon addition of Bz-b-CD to the PVC matrix. FE-SEM images yielded insight into the gelation and fusion processes. The curing conditions and physical properties of the flexible PVCs containing Bz-b-CD were optimized, and the influence of Bz-b-CD on the migration of the plasticizers and the stability of the flexible PVC was studied. The results showed that Bz-b-CD reduced migration of DINP and Neocizer from the flexible PVC by almost 40% and 25%, respectively, thereby favorably restricting migration within the flexible PVC. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction Plasticizers are additives that can improve the flexibility and workability of poly(vinyl chloride) (PVC) by forming secondary bonds (dipole–dipole interactions) with the polymer chains. The high density of the ends of the plasticizer chains introduces additional free volume into the PVC matrix [1]. PVC can be plasticized with varying amounts of plasticizers to form flexible PVC. Phthalate plasticizers, such as di-isononyl phthalate (DINP) and di-(2-ethylhexyl) phthalate (DEHP), are common plasticizers that are widely used in industrial applications [2]. Phthalate plasticizers weakly interact with PVC chains and may migrate out of

⇑ Corresponding author. Tel.: +82 2 880 8365; fax: +82 2 885 1748. E-mail address: [email protected] (S.-Y. Kwak).

the plasticized PVC products into the environment, for example, into the materials that contact the PVC. Health risk assessments over the past few decades have examined the health effects associated with phthalate leaching from PVC [3,4]. Plasticizer leaching can render the materials in contact with the PVC useless for some applications due to plasticizer effects on these materials’ mechanical properties or appearance, and the mechanical properties of the plasticized PVC products themselves can degrade due to plasticizer loss. Industries have begun to exploit new alternative non-phthalate plasticizers (e.g., di-isononyl cyclohexane-1,2-dicarboxylate (DINCH) and diisononyl cyclohex-4-ene-1,2-dicarboxylate (Neocizer)) to alleviate legal concerns. Unfortunately, several popular alternative plasticizers are structurally similar to phthalate plasticizers and, consequently, also have the potential to migrate and introduce toxic compounds into the environment.

0014-3057/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2012.02.008

MACROMOLECULAR NANOTECHNOLOGY

a r t i c l e

MACROMOLECULAR NANOTECHNOLOGY

886

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

The use of nanoscale particles as functional additives has attracted significant attention in several polymer material research fields in recent years [5,6]. b-Cyclodextrin (b-CD) has an internal cavity shaped like a truncated cone of about 0.8 nm deep and 0.60–0.64 nm in diameter. This cavity possesses a relatively low polarity that can accommodate guest organic molecules inside, resulting in inclusion complexes. In particular, the high density of hydroxyl groups on the exterior of b-CD can easily be modified with various functional groups to endow the b-CD with specific interactions with other molecules [7]. Grant et al. recently described the effect of b-CD and hydroxypropyl b-CD incorporation into plasticized PVC on leaching of DEHP and biocompatibility [8]. In previous reports, we successfully prepared a plasticizer with reduced DEHP migration by directly incorporating nanoscale b-CD derivatives into DEHP. The addition of a b-CD derivative decreased the levels of DEHP migration from the flexible PVC samples by almost 40%. The reduction in DEHP migration from the flexible PVC was due to the formation of p–p stabilized complexes and inclusion of DEHP molecules by suitably oriented b-CD derivatives [9]. In this context, and motivated by the technological and scientific value of these studies, we intensively pursued a rheological study of the influence of b-CD derivatives during the gelation and fusion of PVC plastisols. PVC plastisols are suspensions consisting of an emulsion-type PVC resin in a liquid continuous phase formed mainly by a plasticizer and a thermal stabilizer. PVC plastisols are used today to produce many commercially important products [10]. All industrial processes for preparing plastisols involve heating the plastisol to 150–200 °C. During heating the plastisol undergoes two processes, gelation and upon cooling the plastisol is transformed into a relatively soft, flexible substance [11]. Additional components, such as b-CD derivatives, can be added to modify the rheological properties of the PVC plastisol. From a commercial perspective, it is useful to optimize plastisol processing conditions for various PVC formulations. A useful method for monitoring gelation and fusion is to characterize the viscoelastic properties. When combined with field-emission scanning electron microscopy (FE-SEM), viscoelastic characterization provides a better understanding of the viscoelastic behavior, such as gelation and fusion of PVC plastisol. The parameters most frequently used to quantify rheological properties are the elastic (or storage) modulus, G0 , and the viscous (or loss) modulus, G00 . These properties show distinct changes at the initiation and termination of plastisol gelation, and at the temperature at which fusion becomes complete [12]. In this study, viscoelastic measurements permitted continuous monitoring of the changes in the G0 and G00 as a function of temperature during gelation and fusion. The rheological analysis quantified the influence of Bz-b-CD on the gelation and fusion behavior of the PVC plastisols. A phthalate or a non-phthalate plasticizer, DINP or Neocizer, respectively, along with Bz-b-CD as a migration inhibitor, were mechanically mixed with emulsion-grade PVC resin to prepare a PVC plastisol. It is important to note that the non-phthalate plasticizer, Neocizer, was structurally similar to the most widely used DINP (see Fig. 1). The influence of Bz-b-CD on the physical properties of the flex-

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

ible PVC was experimentally investigated. Migration tests of the flexible PVC were conducted according to the International Organization for Standardization (ISO) 3826:1993(E) method to measure the plasticizer migration behavior. Gas chromatography (GC) equipped with a flame ionization detector (FID) was employed to identify and quantify which migrated to the surface of the PVC part and dissolved in the contacting solvent. 2. Experimental 2.1. Materials b-Cyclodextrin (b-CD) was obtained from Tokyo Chemical Industry Co., Ltd. and dried in a vacuum oven at 60 °C for 7 days prior to use. Benzoyl chloride and anhydrous pyridine were purchased from Sigma–Aldrich (stated purity P99%). Di-isononyl phthalate (DINP) and diisononyl cyclohex-4-ene-1,2-dicarboxylate (Neocizer) were kindly provided by Aekyung Petrochemical Co., Ltd., Korea. Emulsion grade poly(vinyl chloride) (PVC) resin (LG PB1752, degree of polymerization: 1700 ± 50 and kwert (k) value: 76) was provided by LG Chem. Ltd., Korea. Epoxidized soybean oil (ESO) and a thermal stabilizer (methyl tin, trade name MT-800) were purchased from Yakuri Pure Chemicals Co., Ltd., Japan and Songwon Co., Ltd., Korea, respectively, and used to prepare the PVC plastisol. All chemicals except bCD were used as received without further purification. 2.2. Preparation of cyclodextrin derivative as a migration inhibitor b-CD was modified with benzoyl chloride, resulting in 2,3,6-per-O-benzoyl- b-cyclodextrin (Bz-b-CD), in accordance with the procedures described in the literature [13]. Purified and dried b-CD (11.35 g, 10 mmol) was stirred into 240 mL anhydrous pyridine, and 160 mL benzoyl chloride (1.44 mol) was added. The solution was stirred at 50 °C for 72 h. Termination of the reaction was accompanied by a solution color change from bright pink to orange– brown, with some precipitate. The mixture was evaporated at 50 °C under reduced pressure until it reached about half the volume. The thick solution was cooled in an ice bath, and 500 mL anhydrous methanol was added very slowly with stirring. The copious white precipitate was filtered off, and the crude product was resuspended in methanol.

The white powders in methanol were filtered out and washed several times with, alternately, distilled water or methanol. Finally, the product was dried in a vacuum oven and ground to a fine white powder. The modification of bCD was verified using Fourier-transform infrared (FT-IR) spectroscopy using a Perkin-Elmer GX IR spectrophotometer with a spectral resolution of 4 cm1 over the range 4000–400 cm1. All samples were prepared by compression molding, and potassium bromide (KBr) powder was used as the sample matrix and reference material. The degree of substitution of Bz-b-CD was confirmed by 1H nuclear magnetic resonance (NMR) spectroscopy using a Bruker Avance spectrometer 500 with dimethyl sulfoxide-d6 as the solvent. The resulting Bz-b-CD particles were incorporated with DINP or Neocizer as follows. To allow for a quantitative comparison, the mass of the primary plasticizer, either DINP or Neocizer, was 70 g in each sample and the mass of Bz-b-CD, where used, was 10 g. To the plasticizer was carefully added dried Bz-b-CD with stirring at room temperature. The mixtures were sonicated in a 335 W sonicator bath (Bransonic Co., Ltd., Model 3510R) for 30 min at room temperature until clear, resulting in a transparent colloidal solution. The particle sizes and size distributions of Bz-b-CD in the prepared samples were measured using dynamic light scattering (DLS) methods with a Photal DLS-7000 spectrophotometer equipped with a Photal GC-1000 digital auto-correlator (Otsuka Electronics Co., Ltd., Osaka, Japan). In this procedure, the wavelength (k) of the argon (Ar) laser was 488 nm, and the scattering angle was 90° with respect to the incident beam. The correlation functions were analyzed using the constrained regularized CONTIN method to determine the distribution decay rates. The experiments were conducted at room temperature, and each experiment was repeated two or more times. 2.3. Rheological measurements of the PVC plastisols containing Bz-b-CD PVC pastes, also known as plastisols, were prepared using a dried emulsion PVC resin, DINP or Neocizer, Bz-bCD, epoxidized soybean oil (ESO), and a thermal stabilizer (TS), as listed in Table 1. The PVC plastisols were prepared by slowly adding Bz-b-CD and the other additives to the dry emulsion PVC resin. A mechanical stirrer with a twoblade propeller was then used to further homogenize the paste. After mixing had been completed, air bubbles in the plastisols were removed by applying a vacuum. The

plastisols were then aged for 2 weeks at room temperature prior to use. The aging is necessary because viscosity increases initially primarily because of de-agglomeration but it stabilizes after 2 weeks [14]. The viscoelastic behavior of the PVC plastisols was examined to investigate the influence of Bz-b-CD on the properties and to determine the optimal processing conditions. A parallel-plate rotational rheometer (Rheometer AR2000, TA instruments Inc.) was used in the dynamic oscillatory mode with a controlled heating rate. The 40 mm diameter parallel-plate disks were used with a gap setting of 1 mm. The frequency of oscillation and shear strain amplitude were kept at 1 Hz and 2.5%, respectively. The measurement temperature was varied from 25 to 200 °C, with a programmed rate of increase of 5 °C min1. Morphological analysis of the PVC plastisols at various stages of gelation and fusion was conducted using an FE-SEM, JEOL JEM-6700F, which used secondary electrons with an acceleration voltage of 15 kV. The FE-SEM samples were prepared by loading a plastisol between the plates of the rheometer and heating the apparatus to the desired temperature without application of shear strain. The sampling temperatures were chosen based on the characteristic features of the viscoelastic curves. 2.4. Physical properties of the flexible PVC sheets containing Bz-b-CD Flexible PVC samples were fabricated through gelation and fusion of the PVC plastisols upon heating. The samples were then cooled to room temperature. The flexible PVC sheets containing Bz-b-CD were denoted PVC/DINP-CD and PVC/Neo-CD, respectively. For comparison, PVC/DINP and PVC/Neo without Bz-b-CD (blank sample) was also prepared according to the same method (see Fig. S1). The influence of Bz-b-CD on the physical properties of the flexible PVCs was assessed experimentally. The glass transition temperatures, Tg, of the flexible PVCs were determined relative to the indium standards using a TA Instrument 2920 differential scanning calorimetry (DSC) system at a heating rate of 5 °C min1 over the temperature range 80 to 150 °C. The temperature programmed procedure was performed under a stream of nitrogen. As a measure of the flexibility of the PVC samples, we measured the% elongation at break using tensile tests performed on a LLOYD LR10K universal testing machine (UTM). The tests were conducted at a strain rate of 50 mm min1 with a 1 kN static load cell. The test specimens assumed dumbbell shapes with a width of 9.5 mm and a thickness of 2 mm,

Table 1 Compositions of the PVC plastisols. Sample codes

PVC/DINP PVC/DINP-CD PVC/Neo PVC/Neo-CD a b c

Components (phr)a PVC resin

DINP or Neocizer

ESOb

TSc

Bz-b-CD

100

70

3

2

– 10 – 10

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

MACROMOLECULAR NANOTECHNOLOGY

887

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

888

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

in accordance with the American Standard Testing Method (ASTM) D-638 [15]. The thermal decomposition studies were performed over the temperature range 25–600 °C using the TA Instrument Q500 thermogravimetric analysis (TGA) system under flowing nitrogen at a scan rate of 10 °C min1. The masses of the flexible PVC samples were approximately 5–10 mg. In addition, the optical properties of the flexible PVC sheets with a thickness of 0.40 mm were measured using haze tests (BYK Gardner Co., Ltd., HazeGard Plus) with a Commission Internationale de l’Eclairage (CIE) standard illuminant C (320 < k < 780 nm) as the light source.

MACROMOLECULAR NANOTECHNOLOGY

2.5. Migration tests Migration tests were carried out on the prepared flexible PVC sheets according to the International Organization for Standardization (ISO) test method 3826:1993(E) [16]. Distilled water and ethanol were mixed to prepare the extraction solution. The ratio of distilled water to ethanol was set to 124:100 by the volume ratio, with a density of 0.9374 g/mL at 25 °C by pyknometer. The migrated plasticizers in the extraction solutions were quantitatively analyzed using a Hewlett Packard model 6890 Series II Plus GC system with a flame ionization detector and a DB-5 capillary column (30 m  0.32 mm I.D. with a film thickness of 0.25 lm). The column was maintained at 80 °C for 3 min, ramped up to 320 °C with a heating rate of 10 °C min1, and finally maintained for 13 min. The gas chromatograph was operated in the splitless injection mode at a temperature of 320 °C. Helium was used as the carrier gas at a flow rate of 1.8 mL min1. A calibration curve was constructed by plotting the ratio of the peak area of several DINP or Neocizer standard solutions as a function of concentration between 1 and 20 mg/100 mL. A summary of the detailed condition of GC analysis is given in Table S1. The flexible PVC sheets were fabricated in sizes of 30  30  2 (L  H  D mm3). The PVC sheets (i.e., PVC/ DINP, PVC/DINP-CD, PVC/Neo, and PVC/Neo-CD) were rinsed with distilled water to remove dust and impurities from the surface, and then immersed in 100 mL of the extraction solution. The testing temperature was maintained at 37 ± 1 °C and the samples remained in solution for 1–7 days without shaking during the tests. The flasks were then removed from the oven, inverted gently 10 times, and the contents were transferred to a sample cell. Quantitative analysis of each sample was repeated three times to construct a calibration curve and twice for the other samples. The concentrations of the plasticizer that had migrated into the extraction solution from the flexible PVC 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, benzoyl chloride, and Bz-b-CD. The broad absorption band in the range of 3000–3600 cm1 displayed by neat b-CD, shown in Fig. 2(a), corresponded to the stretching vibrations of the hydroxyl (–OH) group. Several intense bands in the

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

range 1029–1157 cm1 were assigned to primary and secondary C–OH stretches, and to the C–O–C antisymmetric stretches, respectively [17]. Modification of the neat b-CD with benzoyl chloride, however, introduced significant changes to the FT-IR spectrum of Bz-b-CD, as can be seen in Fig. 2(c). The broad peak corresponding to the hydroxyl group stretch disappeared and was accompanied by the appearance of peaks corresponding to the sp2 C–H stretch at 3075 cm1 and the aromatic C@C at 1603 and 1448 cm1. These bands corresponded to stretches of the aromatic ring of the benzoyl group. In addition, the conjugated C@O stretches at 1729 cm1 and the C–O stretches in the range 1000–1300 cm1 resulted from formation of an ester. Therefore, the Bz-b-CD spectrum clearly indicated that b-CD had been modified. Fig. 3 shows the 1H NMR spectra of neat b-CD, benzoyl chloride, and Bz-b-CD. 1H NMR spectroscopy provided additional evidence for the modification and indicated the degree of substitution. The Bz-b -CD spectrum showed that the hydroxyl group peaks of b-CD disappeared as the peaks attributed to the protons of the benzoyl group appeared (2,3,6-COC6H5 at 6.9–8.1 ppm), indicating the substitution of the three hydroxyl groups of b-CD. Each proton (3-H triplet at 6.2 ppm, 1-H doublet at 5.6 ppm, 2-H doublet of doublets at 5.1 ppm, 5-, 6a-, 6b-H, at 4.8–5.0 ppm, and 4-H triplet at 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, introduced to neat b-CD (which included 21 hydroxyl groups) was easily calculated from the relative peak integrals in the 1H NMR spectrum. The value of x was determined to be 20.2 (around 96%), which confirmed the successful modification of neat b-CD to yield Bz-b-CD. Additional discussion of the modification and general characterization of Bz-b-CD is provided in our previous publication [9].

889

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

3.2. Dispersion of Bz-b-CD in plasticizers Several thermodynamic methods can be used to predict and explain miscibility in multiple phase systems, including the solubility parameter, the equation-of-state, and the lattice theory [18]. The solubility parameter method (d (cal/cm3)1/2) was chosen for this study. The solubility parameters of Bz-b-CD, DINP, and Neocizer can be calculated based on the following equation, d = qRG/M, where q represents the density, G is the set of group molar attraction constants, and M is the molecular mass of the repeating unit. For simplicity, room temperature was used as the standard condition. The group molar attraction constants (commonly Small’s constants) of the specific functional groups, which are useful in determining the solubility parameter, 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 is because the energy of mixing two components is balanced by the energy released by interactions among the pure components. 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-CD could be well-dispersed in both plasticizers. As shown in Fig. 4, the quality of the Bz-b-CD dispersion in DINP or the Neocizer solution was examined by DLS analysis. The correlation functions were analyzed by means of the constrained regularization method to determine the distribution decay rate. This method analyzes the intensity of scattered laser light over time, which depends on the particle size. It was very difficult to disperse b-CD in the plasticizers, 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 dispersion 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 be well-dispersed in both plasticizers. 3.3. Rheological behavior of PVC plastisols containing Bz-b-CD PVC resins obtained by the emulsion polymerization process are usually used in the preparation of plastisols. Generally, flexible PVC products are fabricated through a plastisol by heating briefly to the fusion temperature and then cooling [21]. As shown in Figs. 5 and 6, G0 and G00 initially decreased as the plastisol was heated from room temperature. The system behaved as a suspension of non-interacting PVC particles in the plasticizer, which formed a continuous phase with a viscosity that decreased with increasing temperature [22]. During the later stages of heating, the PVC particles dissolved into the plasticizer from their outer surfaces, which glued the particles together. In this stage, G0 reached a maximum corresponding to the complete absorption of plasticizer by the PVC particles. When a PVC plastisol is heated, PVC particles swell with the plasticizer as the plasticizer is absorbed, and a steep increase in G0 and G00 is observed. This process is called gelation. Further temperature increases produce additional swelling and dissolution of more polymers, and the microcrystallites of the PVC melt and both G0 and G00 begin to drop off. This process is called fusion [23]. Fig. S2 illustrates the concept of gelation as part of the complete fusion scheme. The temperature at which fusion

MACROMOLECULAR NANOTECHNOLOGY

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

MACROMOLECULAR NANOTECHNOLOGY

890

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

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

became essentially complete is indicated by the intersection of the moduli (G0 and G00 ). The processability requirements for PVC plastisol processing emphasize the viscoelastic behavior, such gelation and fusion, of the plastisol [24]. During gelation and fusion, the elastic and viscous modulus in the rheological measurements underwent 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 temperature. The viscoelastic behavior obtained from PVC/DINPCD was similar to that of PVC/DINP. The maximum of the modulus corresponded to the complete gelation and onset of fusion. In this stage, the PVC particles became swollen as the plasticizer was taken up, and some portion of the PVC molecules may have dissolved into the plasticizer from the surface layers of the particles. Further temperature increases resulted in a decrease in the modulus, indicating the dominance of fusion and melting of the PVC microcrystallites. The decreased modulus was due to two processes: (i) the normal temperature-dependent changes were primarily attributable to thermal expansion, and (ii) melting of the PVC microcrystallites [25]. As can be seen Fig. 5, complete gelation of the PVC/DINP-CD plastisols occurred at 158 °C, a gelation temperature that was slightly higher than that of the PVC/DINP plastisol (150 °C). Gelation of the PVC plastisols was delayed upon addition of Bz-b-CD particles, possibly because of the decreased interaction of the PVC with the plasticizer. In the Neocizer series, the gelation point of the PVC/Neo-CD plastisol was 169 °C, slightly higher than the gelation point of the PVC/Neo plastisol (162 °C). Gelation of the PVC plastisol was also delayed by the addition of Bz-b-CD particles, as shown in Fig. 6. The formation of entanglements among the PVC chains may have been blocked by the Bz-b-CD nanoparticles (see Fig. 7(b)). During processing, partial melting of the crystallites in the primary PVC particles occurred, allowing the macromolecules to diffuse through the

Fig. 5. Viscoelastic profile of (a) PVC/DINP and (b) PVC/DINP-CD plastisol.

891

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

boundaries and entangle among the other macromolecules, as shown in Fig. 7(a). The melted crystalline components then recrystallized during cooling to form newly created ordered domains of secondary crystallites [26]. The presence of well-dispersed Bz-b-CD nanoparticles in the PVC matrix may have disrupted the physical crosslinks between neighboring PVC molecules, thereby partially interrupting formation of the 3-D macromolecular

network. The fusion process completed, according to the viscoelastic data, at almost the same temperature (about 187 °C) as was observed for Figs. 5 and 6, suggesting that the macromolecular network reached an equilibrium state at around 187 °C. These results showed that the presence of Bz-b-CD did not alter the fusion point of the PVC plastisol. As shown in Fig. S3, the viscoelastic behavior of the Neocizer series was similar to that of the DINP series.

Fig. 7. Formation of molecular entanglement; (a) neat PVC and (b) PVC with Bz-b-CD.

MACROMOLECULAR NANOTECHNOLOGY

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

MACROMOLECULAR NANOTECHNOLOGY

892

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

However, the temperatures of complete gelation differed between the DINP and Neocizer series. This difference could be explained in terms of the compatibility of the plasticizers and PVC. The solubility parameter and the polarity parameter, /, can predict the compatibility. The polarity parameter can be obtained from the equation / = (Ap/Po)M/1000, where Ap is the number of carbon atoms in the molecule, excluding aromatic and carboxylic carbon atoms, Po is the number of polar groups, and M is the molecular weight. The factor 1000 is used to conveniently rescale / [27]. Small polarity parameters for the plasticizer and similar solubility parameters for the plasticizer and PVC predict good PVC/plasticizer compatibility [28]. The calculated solubility parameters and polarity parameter were 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 DINP was more compatible with PVC due to the presence of the aromatic ring. The aliphatic ring in Neocizer decreased the compatibility with PVC. Therefore, the absorption of DINP reached completion faster than that of Neocizer, and the gelation and fusion processes of the DINP series appeared at lower temperatures (12 °C lower) as a consequence of the higher solvent power of the plasticizer. The morphologies of the PVC plastisols at various stages of gelation and fusion were examined by FE-SEM to investigate the structural changes associated with the PVC/DINP and PVC/Neocizer interactions, and the morphologies provided a qualitative analysis of the gelation and fusion processes. These results were compared with and used to interpret the changes in the viscoelastic behavior. Starting from a two-phase system comprising solid particles dispersed in the liquid, the plastisol transitioned into a one-phase rubbery solid though gelation and fusion. Figs. S4 and S5 show SEM images of the PVC/DINP-CD and PVC/Neo-CD system. The images show a magnification of 5000. At 70 °C, the PVC particles were clearly identifiable, and the presence of agglomerates was apparent. At the complete gelation temperature, 158 or 169 °C for the PVC/DINP-CD or PVC/ Neo-CD systems, respectively, few particles were identifiable, and interparticle boundaries were obscured by entanglement. At 170 °C, fusion occurred, and the particulate morphology was almost absent. Finally, at 187 °C, fusion had completed, and recrystallization followed, upon cooling, to form a 3-D structure held together by crystallites and elastomeric molecules. The fracture surface was continuous, and no domain boundaries could be identified. During gelation and fusion, the PVC plastisols changed from PVC particles in DINP or Neocizer to a uniform mass. Overall, the disappearance of the PVC particulate boundaries increased 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 during the second runs, corresponded to the mid-points of the small endothermic rises in the pre- and post-transition baselines. A single Tg for each of the flexible PVCs studied supported the miscibility among PVC, the plasticizers, and Bz-b-CD. The flexible PVC samples containing Bz-b-CD exhibited slightly higher Tg values in comparison

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

with those of pure flexible PVCs. The addition of plasticizers to a PVC resin increased the free volume of the PVC, thereby lowering the PVC Tg. The flexibility of PVC is a main reason why these materials are used so widely. The percent plasticization efficiency, EDTg, can be calculated according to the following equation:

EDT g ¼

DT g;PVC=plasticizerCD  100 DT g;PVC=plasticizer

where DTg is the reduction in Tg. The calculated EDTg values are listed in Table 2 with the glass transition temperatures of the flexible PVC samples. The plasticizing efficiency of PVC/DINP-CD was found to be comparable to that of PVC/ DINP, with the EDTg value of PVC/DINP-CD reaching up to 97.8%. EDTg of the PVC/Neo-CD was also found to be comparable to that of PVC/Neo, differing by only 1.7%. These materials, therefore, were completely flexible at room temperature. 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 shown in Fig. 9, although all samples behaved similarly, the ultimate strength of the stress and elongation at the break slightly decreased as the Bz-b-CD content in the flexible PVC sheets increased. Therefore, Bz-b-CD was well-dispersed in the PVC matrix without forming agglomerates, which act as significant defects. Bz-b-CD apparently interacted with the polar groups of the PVC chains and impeded the motions of the PVC chains. The plasticization efficiencies, EDeb, were estimated by comparing EDeb due to DINP-CD and Neo-CD with those of DINP and Neocizer:

EDeb ¼

Deb;PVC=plasticizerCD  100 Deb;PVC=plasticizer

The eb values of the flexible PVC samples and the calculated EDeb values are also presented in Table 2, and the trends differ only slightly from those observed for the EDTg data. The transmittance and haze of the flexible PVC sheets containing Bz-b-CD were investigated and compared with their counterparts, PVC/DINP and PVC/Neo. In general,

893

B.Y. Yu et al. / European Polymer Journal 48 (2012) 885–895 Table 2 Glass transition temperatures, ultimate elongation, and percent plasticization efficiency.

c d

PVC

PVC/DINP

PVC/DINP-CD

PVC/Neo

PVC/Neo-CD

Tg (oC)a EDTg (%)b eb (%)c EDeb (%)d

85.6 0 163.2 0

47.3 100.0 767.6 100.0

44.4 97.8 710.1 90.5

40.9 100.0 657.3 100.0

38.8 98.3 622.7 92.9

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

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

agglomerates can seriously deteriorate the clarity of PVC sheets. As shown in Fig. 10, no deterioration in the transmittance of the PVC sheets was observed upon addition of Bz-b-CD nanoparticles. Bz-b-CD was endowed with hydrophobic benzoyl groups that limited formation of agglomerates among Bz-b-CD particles in PVC/DINP-CD and PVC/Neo-CD. In other words, Bz-b-CD was welldispersed in the plasticized PVC matrix on the nanoscale. However, the dispersed Bz-b-CD particles slightly increased the haze of the sheets because the tiny Bz-b-CD particles scattered light in the plasticized PVC matrix. The thermal stability of each PVC sheet was analyzed by TGA, and the results are shown in Fig. S6. All sample curves showed similar trends, and each curve presented two distinct stability stages corresponding to a first weight loss of about 75%, between 210 and 320 °C, and a second weight loss of about 15% between 410 and 470 °C. The analysis results suggest that the first weight loss corresponds to the elimination of HCl with some benzene traces. The second stage corresponds to the polyacetylene sequences formed by elimination of HCl from adjacent carbon atoms during the first stage [29]. At the temperature above 470 °C, the weight is almost unchanged. These results confirm that the thermal stability of the PVC/DINP-CD and PVC/NeoCD 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, included alkyl chains comprising a mixture of C8–C10 chain

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

isomers, and the largest component of the mixture included C9 chains [30]. As seen in Fig. S7, the mixture of isomeric alkyl chains introduced several peaks into the GC–FID chromatogram. A calibration curve was constructed by plotting the sum of the GC–FID peak areas corresponding to the DINP and Neocizer plasticizers relative to the peak area of an internal standard for plasticizer concentrations of 1, 2, 5 and 10 mg/100 mL (shown in Fig. S8). The plasticizer response was linear, with a correlation coefficient, r, of r > 0.9997 for DINP and r > 0.9995 for Neocizer. The migration behavior of the plasticizers from the flexible PVC containing Bz-b-CD was examined by extracting into a mixed water/ethanol solution at 37 °C. The plasticizers that migrated during extraction of each PVC sample over 1, 3, 5, and 7 days were detected by GC–FID, and the sum of the peak areas was compared with the calibration curve to determine the plasticizer concentration. The total amount of the migrated plasticizers as a function of time is illustrated in Fig. 11. The figure shows that plasticizer migration dramatically increased during the initial stages, and the rate of migrated plasticizer extraction slowed remarkably after 24 h. After a seven day extraction, the concentration of the migrated plasticizer extracted from all plasticized PVC sheets incorporating Bz-b-CD was lower than that extracted from the neat plasticized PVC sheet, as listed in Table 3. The plasticizer on the surface of the PVC sheet diffused through the interface between the sheet and the extraction solution, and the vacancies left by the migrated plasticizers were exchanged with the extraction solution [31]. For thermodynamic reasons, the penetrated

MACROMOLECULAR NANOTECHNOLOGY

a b

Data

894

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

extraction solution facilitated the diffusion of plasticizer from within the PVC sheet to the surface of the PVC sample. During this process, Bz-b-CD prevented interactions between the plasticizer and the extraction solution. Therefore, Bz-b-CD in the PVC matrix played a key role in inhibiting migration and reducing plasticizer extraction. The anti-migration efficiency (%) was calculated according to:

  C PVC=plasticizerCD  100 Anti-migration efficiencyð%Þ ¼ 1  C PVC=plasticizer

MACROMOLECULAR NANOTECHNOLOGY

Fig. 11. The cumulative amount of migrated plasticizer as a function of time at 37 °C.

Table 3 Concentration of the plasticizer and anti-migration efficiencies as a function of time. Samples

Concentration of plasticizer in the migration medium (mg/100 mL)

PVC/DINP PVC/DINP-CD Anti-migration efficiencies (%) PVC/Neo PVC/Neo-CD Anti-migration efficiencies (%)

1 day

3 day

5 day

7 day

4.89 2.89 40.86 7.03 5.27 24.98

6.26 3.66 41.64 8.84 6.56 25.83

6.66 4.10 38.46 9.75 7.13 26.88

6.86 4.25 38.11 10.11 7.70 23.86

where CPVC/plasticizer-CD and CPVC/plasticizer are the concentration of the migrated plasticizer during migration in the PVC/plasticizer-CD and PVC/plasticizer samples, respectively. The anti-migration efficiencies of PVC/DINP-CD and PVC/Neo-CD were 38.11% and 23.86%, respectively. This indicated that the presence of Bz-b-CD in the flexible PVC matrix dramatically reduced plasticizer migration, which was attributed to the formation of an inclusion complex between the Bz-b-CD cavity and the plasticizer, and stabilizing p–p associations between the benzoyl groups of Bz-b-CD and the aromatic ring of the plasticizer molecules. Bz-b-CD, which hindered migration of the plasticizer, also will block the plasticizer molecules by winding pathway (also called tortuous pathway) effect. In other words, it would promote the path length for transporting plasticizers and results in a decrease of plasticizer migration (see Fig. 12). Neocizer did not introduce p–p contacts, and only van der Waals interaction stabilized the interactions between Bz-b-CD and Neocizer. For this reason, the reduction in the rate of Neocizer migration (with an aliphatic ring) was smaller than the reduction in the rate of DINP migration (with an aromatic ring). Differences in the structure strongly affected the plasticizer/Bz-b-CD interaction process. In addition, gel permeation chromatography (GPC) results confirmed that Bz-b-CD was not released from the flexible PVC sheets during the migration

Fig. 12. Schematic illustration of the anti-migration mechanism of the plasticizer in the PVC matrix.

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

[9]

[10]

4. Conclusion [11]

We present, here, a study of the influence of Bz-b-CD on the rheological properties of PVC plastisols and the prevention of plasticizer migration from flexible PVC. Rheological analysis showed that the gelation of the PVC plastisols was slightly delayed upon addition of Bz-b-CD particles, which obstructed the absorption of both plasticizers. Entanglement among the PVC chains was blocked by the Bz-b-CD nanoparticles. However, completion of the fusion process, as observed in the viscoelastic data, occurred at almost the same temperature (around 187 °C). No significant changes in the physical properties of the flexible PVC were observed upon addition of Bz-b-CD, possibly due to the good dispersion of Bz-b-CD nanoparticles in the PVC matrix on the nanoscale. The presence of Bz-b-CD in the PVC matrix played a key role in inhibiting plasticizer migration which may explain the improved stability of the flexible PVC. Acknowledgment This research was supported by the Eco-Innovation Project through the Korea Ministry of Environment. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.eurpolymj. 2012.02.008.

[12]

[13]

[14]

[15] [16] [17]

[18]

[19] [20]

[21]

[22]

[23]

References [24] [1] González N, Fernández-Berridi MJ. Fourier transform infrared spectroscopy in the study of the interaction between PVC and plasticizers: PVC/plasticizer compatibility. J Appl Polym Sci 2008; 107(2):1294–300. [2] Rahman M, Brazel CS. The plasticizer market: An assessment of traditional plasticizers and research trends to meet new challenges. Prog Polym Sci 2004;29(12):1223–48. [3] Hakkarainen M. Migration of monomeric and polymeric PVC plasticizers. Adv Polym Sci 2008;211(1):159–85. [4] Tickner JA, Schettler T, Guidotti T, McCally M, Rossi M. Health risks posed by use of di-2-ethylhexyl phthalate (DEHP) in PVC medical devices: a critical review. Am J Ind Med 2001;39(1):100–11. [5] Yang B, Bai Y, Cao Y. Effects of inorganic nano-particles on plasticizers 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-containing poly(vinyl chloride) and polyethylene films. J Incl Phenom Macrocycl Chem 2007;57(1–4):371–4. [7] Dodziuk H. Cyclodextrins and their complexes, Chemistry, Analytical Methods, Applications. Weinheim: Wiley-VCH Verlag GmbH & Co; 2006. p. 147–190. [8] George SM, Gaylor JDS, Leadbitter J, Grant MH. The effect of betacyclodextrin and hydroxypropyl betacyclodextrin incorporation

[25] [26]

[27]

[28]

[29]

[30]

[31]

895

into plasticized poly(vinyl chloride) on its compatibility with human U937 cells. J Biomed Mater Res B Appl Biomater 2011;96B(2):310–5. Yu BY, Chung JW, Kwak S-Y. Reduced migration from flexible poly(vinyl chloride) of a plasticizer containing b-cyclodextrin derivative. Environ Sci Technol 2008;42(19):7522–827. Fenollar O, García D, Sánchez L, López J, Balart R. Optimization of the curing conditions of PVC plastisols based on the use of an epoxidized fatty acid ester plasticizer. Euro Polym J 2009;45(9):2674–84. Persico P, Ambrogi V, Acierno D, Carfagna C. Processability and mechanical properties of commercial PVC plastisols containing lowenvironmental-impact plasticizers. J Vinyl Addit Technol 2009; 15(3):139–46. Daniels PH, Brofman CM, Harvey GD. Meaningful evaluation of plastisol gelation and fusion temperatures by dynamic mechanical analysis. J Vinyl Addit Technol 1986;8(4):160–3. Boger J, Corcoran RJ, Lehn J-M. Selective modification of all primary hydroxyl groups of a- and b-cyclodextrin. Helv Chim Acta 1978;61:2190–218. Nakajima N, Harrell ER. Rheological observation of gelation and fusion process of poly(vinyl chloride) plastisol. Adv Polym Tech 1986;6(4):409–40. ASTM D638–02a, Standard Test Method for Tensile Properties of Plastics. ISO 3826, Plastics Collapsible Containers for Human Blood and Blood Components. 1993. Villaverde J, Morillo E, Perez-Martinez JI, Gines JM, Maqueda C. Preparation and characterization of inclusion complex of Norflurazon and b-cyclodextrin to improve herbicide formulations. J Agric Food Chem 2004;52(4):864–9. González N, Fernández-Berridi MJ. Applications of fourier transform infrared spectroscopy in the study of interactions between PVC and plasticizers: PVC/plasticizer compatibility versus chemical structure of plasticizer. J Appl Polym Sci 2006;101(3):1731–7. Small PA. Some factors affecting the solubility of polymers. J Appl Chem 1953;3:71–80. Jang BN, Wang D, Wilkie CA. Relationship between the solubility parameter of polymers and the clay dispersion in polymer/clay nanocomposites and the role of the surfactant. Macromolecules 2005;38(15):6533–43. García JC, Marcilla A. Rheological study of the influence of the plasticizer concentration in the gelation and fusion processes of PVC plastisols. Polymer 1998;39(15):3507–14. García-Quesada JC, Marcilla A, Beltran M. Study of the processability of commercial PVC plastisols by rheology. J Vinyl Addit Technol 1999;5(1):31–6. Kwak S-Y. Structural changes of PVC plastisols in progress of gelation and fusion as investigated with temperature-dependent viscoelasticity, morphology, and light scattering. J Appl Polym Sci 1995;55(12):1683–90. Nakajima N, Kwak S-Y. Effect of plasticizer type on gelation and fusion of PVC plastisol, dialkyl phthalate series. J Vinyl Technol 1991;13(4):212–22. Daniels PH. Optimization of plastisol processes by dynamic mechanical analysis. J Vinyl Addit Technol 2007;13(3):151–4. Fillot L-A, Hajji P. U-PVC gelation level assessment, Part 2: Optimization of the differential scanning calorimetry technique. J Vinyl Addit Technol 2006;12(3):98–107. Ramos-Devalle L, Gilvert M. PVC/plasticizer compatibility: evaluation and its relation to processing. J Vinyl Technol 1990; 12(4):222–5. Ramos-Devalle L, Gilvert M. Compatibility between PVC and plasticizer blends and its relation with processing. J Vinyl Technol 1992;14(2):74–7. Shah BL, Shertukde VV. Effect of plasticizers on mechanical, electrical, permanence, and thermal properties of poly(vinyl chloride). J Appl Polym Sci 2003;90(12):3278–84. Koch HM, Angerer J. Determination of secondary, oxidised di-isononylphthalate (DINP) metabolites in human urine representative for the exposure to commercial DINP plasticizers. J Chromatography B 2007;847(2):114–25. Rahman M, Brazel CS. Ionic liquids: New generation stable plasticizers for poly(vinyl chloride). Polym Degrad Stabil 2006; 91(12):3371–82.

MACROMOLECULAR NANOTECHNOLOGY

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