Synthesis of Chlorinated Poly(vinyl chloride) with Uniform Distribution

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Synthesis of Chlorinated Polyvinyl Chloride with Uniform Distribution of Cl Assisted by SCCO2 and Co-solvent Ying-Hui Qian, Gui-Ping Cao, Xue-Kun Li, and Nan Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 28, 2017

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Industrial & Engineering Chemistry Research

Synthesis of Chlorinated Polyvinyl Chloride with Uniform Distribution of Cl Assisted by SCCO2 and Co-solvent

Ying-Hui Qian, Gui-Ping Cao∗, Xue-Kun Li, Nan Wang

UNILAB, State Key Lab of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

KEYWORDS: PVC chlorination, SCCO2, co-solvent, kinetics

∗

To whom correspondence should be addressed. Tel.:86-21-64253934. E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT

For the first time, a preliminary foaming treatment for polyvinyl chloride (PVC) was adopted to facilitate the chlorination of PVC with the assistance of supercritical carbon dioxide and acetone. The chlorine content of chlorinated polyvinyl chloride (CPVC) obtained from pretreated PVC had exceeded 0.66 g/g, while that of CPVC using non-pretreated PVC only reached about 0.60 g/g. The analysis of particle diameter and morphology of pretreated PVC showed that the preliminary treatment had enlarged particle diameter and inner pore structure. GPC, DSC, and 13

C-NMR analysis showed that the obtained CPVC had narrow molecular weight distribution,

excellent thermal properties, and uniform chlorine distribution. Furthermore, an improved kinetics model was proposed to indicate the crucial step of the chlorination. The frequency factor was expressed as the ratio of the particle volume of the foamed PVC to that of original PVC. Consequently, the model values of chlorine content agreed well with experimental values.


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1. INTRODUCTION Polyvinyl chloride (PVC), with the annual production capacity exceeding 43 million tons in 2015, is one of the most widely used plastics all over the world. However, its Vicat softening temperature (VICAT) is around 65 - 75 ºC1, which severely limits its application at the temperature higher than 65 ºC. If the chlorine content of PVC can be elevated to more than 0.65 g/g by chlorinating it to CPVC, the VICAT will exceed 100 ºC2, 3 and its application will be widely broadened. Benefiting from its strong mechanical properties, flame retardant ability, and acid- and base-proof, CPVC has been considered as one of the high performance thermoplastics and been used as engineering plastics4. Three methods, i.e. solvent method, solid-phase method, and aqueous suspension method, have been proposed to chlorinate PVC. For solvent method, PVC is dissolved in CCl4 or other chlorohydrocarbons, and chlorinated by chlorine through free radical reaction at 80 ºC - 100 ºC. This method is mature and the distribution of chlorine on CPVC chains is uniform. However, the improvements in thermal stability and mechanical properties of the obtained CPVC are hindered by the residual solvent in CPVC. Furthermore the utilization of chlorohydrocarbon is not environmentally benign5. In the case of the solid-phase method, PVC is chlorinated in a fluidized bed under the UV-light6, in which the separation of remained Cl2 and vice product HCl from CPVC is facile. However, this method encounters great difficulties in removing the reaction heat inside PVC particles and industrialization7. Concerning the aqueous suspension method, PVC is dispersed in water and consequently chlorinated, which is a green process and produces CPVC with considerable heat resistance and high mechanical strength8. Therefore, aqueous suspension method has become the predominant technology for PVC chlorination.


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According to typical free radical mechanism, −CH2CHCl− is preferentially chlorinated to −CHClCHCl− in the process of PVC chlorination via aqueous suspension method. However, the diffusion of chlorine from bulk solution into PVC particle is impeded by both the outside thick film and the inside dense structure of PVC particle, resulting in the attack of chlorine free radicals on −CHClCHCl− to generate −CHClCCl2−9 and lots of un-reacted −CH2CHCl− which are deeply embedded. The non-uniform chlorine distribution and the low chlorine content of the obtained CPVC are responsible for the its thermodynamic un-stability9. It can be seen that producing CPVC with high chlorine content and uniform chlorine distribution is the great difficulty. Therefore, in industrial chlorination of PVC, PVC raw materials with highly porous film and loose structure are specially manufactured to ensure both high degree of chlorination and the uniform chlorine distribution. However, except special PVC, almost all the other types of commercial PVC with thick outer film and the dense inner structure cannot be applied to chlorination up to now. Therefore, we deem that activating the film and structure of commercial PVC is the key to suit commercial PVC to chlorination. Hence, the supercritical carbon dioxide (SCCO2) is proposed to "break" the film and "loose" the matrix of commercial PVC in this work for the first time. SCCO2 is widely used as an environmentally benign solvent in polymer applications, such as formation of polymer composites, polymer modification, microcellular foaming, polymer blending, polymerization, and particle production10-14. SCCO2 can be used to replace conventional solvents (often noxious) because of its liquid-like density and gas-like diffusivity in the supercritical phase. As a green foaming agent, SCCO2 has been utilized as a revolutionary invention in the polymer industry. In recent years, SCCO2 foaming has been the hotspot, which


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has already been reported in polystyrene15, polycarbonate16, poly(ethylene terephthalate)17, and poly(ethylene-co-vinyl acetate)18. Diaz and his coworkers19 studied the cell morphology and density of rigid PVC foamed by SCCO2 in a continuous extrusion process. In their work, fast fusion and rheological characteristics were noted as key parameters in the continuous foaming process. The structure of the film and matrix of the PVC resin were changed by SCCO2. Wu and his coworker20 investigated the effects of processing parameters, such as temperature, processing pressure, mixing time, and rotor speed on PVC foams, they had produced PVC foams with the cell density of 1.0 × 107 - 3.5 × 108 cells/cm3, average cell size of 15 - 60 µm and bulk density of 0.6 - 0.87 g/cm3. Kumar and his coworkers21 studied the influence of temperature on foaming PVC, they noted that the density of microcellular decreased with temperature. Meanwhile, according to the nucleation theory22, they declared that CO2 swelled the matrix of PVC in the foaming process and the nucleation of gas bubbles would be promoted during the quick depressurization. Small addition of co-solvent into SCCO2 can greatly improve the performance of foaming. Morisaki and his coworker23 prepared skinless poly(methyl methacrylate) (PMMA) with SCCO2 and ethanol which acted as co-solvent. They believed that ethanol could enhance the affinity between PMMA and SCCO2. Ding et al.24 pointed out that ethanol could obviously improve the solubility of CO2 in cellulose acetate, and the cell densities increased with the increasing content of ethanol. Lora et al.25 reported the cloud-point data for poly(vinyl fluoride) (PVF) in SCCO2 and in the mixture of SCCO2 and acetone, they discovered that the solubility of PVF in SCCO2 would increase when acetone was added in SCCO2. Sun et al.26 found that small addition of cosolvent could increase the solubility and sorption diffusivity of CO2 into nitrocellulose.


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Therefore, it is obvious that small addition of suitable co-solvent can increase the solubility of CO2 in polymer. Whereas, foaming PVC using SCCO2 assisted with co-solvent and its special application in the chlorination have not been reported previously. In this work, the commercial PVC resin, which cannot be chlorinated to CPVC with chlorine content of more than 0.65 g/g, is foamed by SCCO2 assisted with acetone to break the outer film and enlarge the inner pores to prepare CPVC with chlorine content of more than 0.65 g/g via aqueous suspension method. Moreover, the particle size, surface and cross-sectional morphologies of the foamed PVC resin will be analyzed. Furthermore, the glass transition temperature, molecular weight, and chlorine distribution of the obtained CPVC will be analyzed by DSC, GPC, and 13C-NMR respectively. Additionally, an improved kinetics model is proposed. The frequency factor in the Arrhenius equation is expressed by the ratio of the particle volume of PVC after foaming to that of original PVC. 2. EXPERIMENTAL SECTION Materials. PVC (GB/T5761-2006) was presented by Gansu Yinguang Juyin Chemicals Co., Ltd., whose average diameter was 198.86 µm determined using Mastersize 2000 Laser Analyser (Malvern Instruments, Worcestershire, UK) and number and weight average molecular weights ( M n and M w ) were 40.89 kg/mol and 101.65 kg/mol, respectively, determined by GPC (Polymer Laboratories, GPC 50 plus. system, UK). Hydrochloric acid, sodium hydroxide (≥98.0%), CHCl3 (≥99.9%), methanol (≥99.5%) and acetone (≥99.7%) as the co-solvent of CO2 were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Carbon dioxide (CO2) and nitrogen gas (N2) with the purity of 99.99% were purchased from Wugang Gas Co., Ltd. Chlorine generated from MnO2-HCl reaction, describing as Devi et al27.


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Experimental procedure. The experimental procedure consisted of two stages: (1) the foaming of PVC with SCCO2 and acetone (Ace), (2) the chlorinating of foamed PVC. For the procedure of foaming PVC, 300 g PVC was foamed by SCCO2 with 60 g acetone as co-solvent (PVCSCCO2-Ace) at 50 ºC and 16 MPa for 15 h in a reaction kettle equipped with pressure gauge and stirrer. For comparison, another 300 g PVC foamed by SCCO2 without any co-solvent (PVCSCCO2) was carried out under the same conditions. In the processes of both series, CO2 was used to adjust the pressure. In the procedure of chlorinating PVC, 60 grams of foamed PVC grain was immersed in 120 g aqueous hydrochloric acid (15 wt %) solution for 10 h in the reactor equipped with a stirrer, thermometer, gas delivery tube, and drain sleeve. The reactor was heated to the predetermined temperature after the air in the reactor was completely replaced with pure N2. Cl2 was continuously inlet and irradiated simultaneously with ultraviolet light (4 W, Philips Company). PVC was chlorinated to CPVC via free radical reaction. The by-produced HCl and un-reacted Cl2 flowing out of the reactor were absorbed by sodium hydroxide solution (30 wt %). When the chlorination was finished, the reactor was cooled to room temperature and N2 was charged to exhaust the residual HCl and Cl2. Sample Analysis. The the particle size and its distribution of all PVC samples were determined using Mastersize 2000 Laser Analyser (Malvern Instruments, Worcestershire, UK). The samples were ultrasonically dispersed in an aqueous solution before analysis. The surface and cross-sectional morphologies of each PVC series were characterized by the scanning electron microscope (SEM, SM-6360LV, JEOL, Japan). To obtain the cross-sectional samples of PVC particles, all samples were first fixed in polyvinyl alcohol (PVA) films, and then


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the films were rapidly freezed in liquid nitrogen for 3 min. The films were sequentially broken into two pieces to expose the cross-section with desire integrity. The chlorine contents of all CPVC samples were analyzed by the oxygen flask combustion method and titration. After being washed by re-distilled water and dried, the sample was weighted accurately, folded in the filter-paper and placed in a platinum basket. And it was burned in an oxygen combustion flask containing KOH solution as the absorbent of HCl. After completely burning, the solution containing KOH and KCl was neutralized by HNO3, and then titrated by standard AgNO3 solution. Each CPVC sample was analyzed for three times to get an averaged result. And the standard deviation of the measured chlorine contents was less than 0.5 %. The chlorine content of original PVC was also analyzed with the same method. The number average molecular weight ( M n ), weight average molecular weight ( M w ), and molecular weight distribution ( PDI = M w M n ) of all samples were measured by GPC (Polymer Laboratories, GPC50 plus. system, UK). The column was calibrated by commercially available narrow distributed polystyrene. Tetrahydrofuran (THF) was used as the mobile phase and the flow rate was 1.00 mL/min. The standard PS samples with different molecular weights and distribution index of 1.0 were purchased from Shanghai Seebio Biotech, Inc. The molecular weights of CPVC were calculated as follows αCPVC α PS kCPVC M CPVC = kPS M PS

(1)

where, the Mark-Houwink constants of CPVC, kCPVC and αCPVC , were 1.50×10-4 (cm3×mol1/2×g-3/2) and 0.717 at 25 ºC in THF, respectively28, kPS = 1.17×10-4 (cm3×mol1/2×g-3/2) and α PS = 0.583 were the Mark-Houwink constants of PS at 25 ºC in THF.


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The thermal behaviors of the CPVC samples were determined with a differential scanning calorimeter (DSC 200 PC/1/H, USA). Each sample (about 6 mg) encapsulated in a platinum pan was heated from 20 to 150 ºC at a rate of 10 °C/min under nitrogen atmosphere. The position distribution of Cl atom on CPVC chains was characterized by 13C-NMR using a Varian Unity-500 spectrometer (US) at 400 MHz. The scanning time and number were 3600 s and 1024, respectively. Tetrahydrofuran-d8 99.5 atom %D was used as solvent. 3. RESULTS AND DISCUSSION Particle size analysis of PVC. The average particle size of PVC is an important parameter in the chlorination process. Tadashi Tadokoro et al.29 noted that PVC with an average particle size larger than 200 µm was more preferable for the floating resin in the aqueous suspension method. Table 1 and Figure 1 summarized the particle size and particle size distribution of all PVC samples. The average particle size of PVC foamed by SCCO2 (203.5 µm) was 2.36 % larger than that of original PVC (198.8 µm). If acetone was added to cooperate with SCCO2 to foam PVC, the increment in the particle size was more obvious (about 9.96 %). Thus, it is obvious that the acetone can improve the foaming effect to obtain the larger particles due to the synergistic effect of acetone and CO2 on PVC. Furthermore, the larger particle size implied larger pores in the matrix of PVC resin, which can be observed in the next section. Consequently, chlorine could easily diffuse deep into formed PVC resin, which will be proved in the reaction kinetics analysis of PVC chlorination. Morphology characterization of PVC. In general, PVC particles with loose structure are chosen as preferred raw materials to manufacture CPVC, because the migration of chlorine in the matrix of these PVC particles will be much easier, and the chlorination inside particles is more uniform6. Figure 2 and Figure 3 illustrate the cross-section of original PVC and PVCSCCO2.


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Comparatively, the inner structure of original PVC is tight. If the PVC is foamed by SCCO2, the inner structure becomes loose. The diameter of pores in PVCSCCO2 ranged between 0.1 µm and 5 µm, which was larger than that of original PVC (between 0.1 µm and 2 µm). According to the research of Siripurapu et al.30, the above results could be caused by the diffusion of SCCO2 into the matrix of PVC. A homogeneous PVC/CO2 mixture was prepared at high pressure, and the nucleation of gas bubbles would be promoted during the quick depressurization until the temperature of the mixture drops below the glass transition temperature (Tg) of the plasticized PVC, after which the pores would be immobilized in the PVC matrix. Meanwhile, according to the study of Tomasko et al.31, the Tg of PVC will be reduced due to the interaction between CO2 and PVC. Sefcik et al.32 proposed that the intermolecular interaction of CO2 and PVC, dispersion force and induction force, can decrease the energy which is necessary to activate the main - chain molecule of PVC, resulting in reduced Tg of PVC. Kasturirangan33 provided evidence at molecular level for the interactions between CO2 and PVC by transmission spectra, and he discovered that chlorine atoms in PVC constituted weak basic sites, and the enthalpy of interaction between C-Cl and CO2 was – 3.0 kJ/mol. Figure 4 shows the cross-section of PVCSCCO2-Ace. Compared with PVCSCCO2, most of the pores with an average diameter ranging from 0.1 µm to 15 µm in PVCSCCO2-Ace were larger than those in PVCSCCO2 (from 0.1 µm to 5 µm). The formation mechanism of the pores in PVCSCCO2Ace

and PVCSCCO2 was similar. However, the intermolecular interaction among CO2, acetone and

PVC will enhance the depression of Tg of PVCSCCO2- Ace. The interaction between CO2 and acetone was studied by Besnard and his coworkers34. The strongly attractive interactions between acetone and CO2, predominantly due to electron donoracceptor interaction, were measured by Raman spectra. Meanwhile, San-Fabián et al.35 simulated


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the electron donor-acceptor interaction between CO2 and acetone via ab initio and density functional theory methods. They confirmed the existence of Lewis acid - Lewis base interaction between the oxygen of acetone and the carbon of CO2. Wu et al.36 noted that the addition of small amount of polar co-solvent to non-polar SCCO2 would give a higher polarity of supercritical mixed solvents, and acetone is a typical polar compounds (dipole moment µ = 2.72 D)37. Therefore, it is reasonable to consider SCCO2 and acetone (CO2/acetone) as a polar supercritical mixed solvent. The intermolecular forces, dispersion force, induction force, and orientation force, between PVC and CO2/acetone will enhance the main - chain molecular motions of PVC, resulting in the depression of Tg of PVCSCCO2- Ace. The film covered PVC particles always limits the diffusion of chlorine into the matrix of PVC. If the film is highly porous, the chlorine would diffuse into the matrix of PVC easily, which will lead to highly chlorine content of CPVC and more uniform distribution. Figure 5, Figure 6, and Figure 7 show the external surface morphology of original PVC, PVCSCCO2, and PVCSCCO2- Ace, respectively. Compared with the smooth external surface of original PVC, the external surface of PVCSCCO2-Ace was rough and porous. The reason is described above, the surface layer of PVC was plasticized by SCCO2 and acetone, and then a porous structure was formed upon depressurization. The results show that it is effective to break the film and loose the matrix of commercial PVC using SCCO2 and acetone as foaming media. Chlorine content analyzing of CPVC. Figure 8 shows a schematic diagram of the process of PVC chlorination to CPVC. The chlorine content (f) of CPVC obtained by the chlorination of original PVC (CPVC) rose from 0.567 g/g to 0.605 g/g. It indicates that the chlorination procedure could be initiated by UV light. However, the chlorine content of the CPVC was limited. Barriere and coworkers38 investigated the kinetics of PVC chlorination. They found that


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the chlorine content of CPVC particles increased fast at the beginning, because the surfaces of particles were chlorinated fast. However, the diffusion of chlorine was limited by the film which covered the bulk material of PVC particles, leaving decreased chlorination rate. Meanwhile, they pointed out that the chlorine content of CPVC would achieve the higher, if the chlorination carried out for fairly long time Then, we employed SCCO2 to foam original PVC before chlorination. The chlorine content of CPVC obtained by chlorination of PVCSCCO2 at 80 ºC for 6 h (CPVCSCCO2) reached 0.617 g/g. However, the chlorine content of CPVCSCCO2 increased only 0.012 g/g compared with CPVC. In the third series of experiments, acetone was employed to assist SCCO2 in foaming original PVC. The chlorine content of CPVC obtained by chlorination of PVCSCCO2-Ace at 80 ºC for 6 h (CPVCSCCO2-Ace) rose to 0.664 g/g. The above results demonstrated that the chlorine migration and diffusion in the matrix of PVC particle were important, which benefited from the highly porous film and loose structure of PVC which was foamed by SCCO2 and acetone. Molecular weight and molecular weight distribution analysis. The molecular weight and molecular weight distribution are very important parameters to the chemical and mechanical properties of polymer materials. Figure 9 and Table 2 show M n , M w , and polydispersity index ( PDI = M w /M n ) of original PVC and CPVC samples (CPVC, CPVCSCCO2, and CPVCSCCO2-Ace). Compared with original PVC, the M n and M w of all CPVC samples increased, while PDI decreased. And, the PDI of CPVCSCCO2-Ace ( PDI = 1.92) was relatively narrow, while the PDI of CPVC and CPVCSCCO2 were 2.20 and 2.15, respectively. Lukas and coworkers28 noted that chlorination was a radical reaction, and the increase of molecular weight was the combined result


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of Cl addition, chain breakage, and chain extension. Lattimer et al.39 deemed that the increase of molecular weight was responsible for the improvement of the thermal properties of CPVC, which broadened the application of CPVC. These researches suggest that the chemical and mechanical properties of CPVCSCCO2-Ace are best. Glass transition temperature analysis. The glass transition temperature (Tg) is an important parameter to characterize the thermal properties of polymer and can be obtained by the DSC curves40. Figure 10 (A) - (C) show the DSC curves and results of all the PVC samples chlorinated at 80 °C for 2 h, 4 h, and 6 h, respectively. As shown in Figure 10 (C), the Tg of CPVC-6h-0.605 (chlorination of original PVC for 6h with the 0.605 g/g chlorine content), CPVCSCCO2-6h-0.617 (chlorination of PVCSCCO2 for 6h with the 0.617 g/g chlorine content), and CPVCSCCO2-Ace-6h-0.664 (chlorination of PVCSCCO2-Ace for 6 h with the 0.664 g/g chlorine content) were illustrated in the following order: CPVCSCCO2-Ace-6h-0.664 (112.4 ºC) > CPVCSCCO2-6h-0.617 (86.9 ºC) > CPVC-6h-0.605 (85.0 ºC). It is concluded that the thermal properties of CPVCSCCO2-Ace were improved compared to the CPVC and CPVCSCCO2. Lattimer39 noted that this was attributed to (i) a cross-linking reaction (ii) elimination of reactive defects in the PVC structure. Meanwhile, the relationship between Tg and chlorine content is shown in Figure 10 (D). We can see that with the process of chlorination, the Tg increases with the increasing of chlorine content. The PVC with the highly porous film and larger pores was obtained after the PVC was foamed by SCCO2 and acetone, reducing the internal diffusion limitation to chlorination, and therefore, leading to the preferential formation of -CHClCHCl- units of CPVCSCCO2-Ace and increasing the Tg of CPVC. As Wansoo Huh and coworkers41 noted: chlorination increased the number of -CHCl- units and Tg.


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Chlorine distribution analysis. Uniformed chlorine distribution in the CPVC chain is usually difficult to achieve in the procedure of chlorination PVC. However, it would be conducive to obtain the CPVC with uniformed chlorine distribution if the PVC is foamed by SCCO2 and acetone. The results of 13C-NMR of original PVC and CPVC samples chlorinated at 80 ºC for 6 h are shown in Figure 11. According to the integral area of corresponding structures with different chemical shift and total integral area, the structure of molecular chain of all CPVC samples can be determined. In 13C-NMR spectrum, broad at 45-50 ppm and 55-60 ppm are observed, which are derived from -CH2- and -CHCl- segments42, respectively. Chemical shifts at 67 ppm and 22 ppm are attributed to the solvent (tetrahydrofuran-d8). As shown in Figure 11, no obvious peak at 89-96 ppm assigned to -CCl2- segments42 were observed, indicating that few -CCl2- segments were formed in the chlorination. Therefore, it can be concluded that the distribution of chlorine in CPVC chains is uniform and the CPVC obtained is thermal stability when the chlorine content is more than 0.65 g/g. Furthermore, the integral area and mole ratio of different segments calculated from Figure 11 and the chlorine content obtained by 13C-NMR (fNMR, expressed as Equation (2)) and titration (f) are shown in Table 3. It can be seen that the results of fNMR and f were in well agreement.

f NMR =

35.5 × A-CHCl- + 71.0 × A-CCl283.0 × A-CCl2- + 48.5 × A-CHCl- + 14.0 × A-CH2-

(2)

Reaction kinetics of PVC chlorination. It has been proved by Barrière et al38 that the chlorination of PVC was a free radical chain reaction. And according to the results of 13C-NMR, it can be concluded that one hydrogen of

has been substituted by a chlorine atom,

thus, the chain reaction proceeds can be shown as follows. Firstly, under UV light, the chlorine decomposes into chlorine radicals.


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(3) Then, chlorine radicals attack and substitute the hydrogen atoms on the

of

to form

(4)

( Cl1M2 ⋅)

( Cl1M )

(5)

( Cl1M2 ⋅)

( Cl12 M )

Finally, the termination step most likely to occur is (6) where k1 , k2 , k3 ,and k4 are the reaction rate constants in the reactions (3), (4), (5), and (6), respectively. Based on the pseudo-steady-state hypothesis to the reactive radicals

and

(hereafter denoted as Cl1M2 ⋅ ), it follows from Equations (3)-(6) that dc Cl⋅ 2 = 2 k1c Cl 2 + k3cCl1M 2 ⋅c Cl 2 − k2 cCl⋅cCl1M − 2 k 4 cCl ⋅ =0 dt

(7)

and dc Cl1M 2 ⋅ dt Therefore, the concentration of

= k2 cCl⋅cCl1M − k3c Cl1M 2 ⋅cCl 2 = 0

(8)

and Cl1M2 ⋅ were described as

c Cl⋅ =

k1 0.5 c Cl k4 2


(9)

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k2 k3

k1 −0.5 c Cl M c Cl 2 k4 1

(10)

= k2

k1 0.5 c Cl M c Cl 2 k4 1

(11)

k1 k4

(12)

c Cl1M2 ⋅ = then, the reaction rate is expressed as

dcCl12M dt Defining a new parameter k as

k = k2 meanwhile

cCl1M +cCl12 M = cCl1M ,0

(13)

therefore

(

0.5 c Cl 12 M = c Cl1M ,0 − c Cl1M ,0 e xp − kc Cl t 2

(

0.5 cCl1M = cCl1M ,0 e xp − kc Cl t 2

)

)

(14) (15)

where cCl1M and c Cl12 M is the concentration of un-reacted and reacted chlorinated PVC repeat unit, namely

, respectively. c Cl1M ,0 is the concentration of un-

and

reacted chlorinated PVC repeat unit at t = 0 h . The chlorine content f (t ) in the model is calculated as

f (t )=

35.5cCl1M + 70.9cCl12M 62.5cCl1M + 96.9cCl12M

(16)

Combining Equations (14), (15), and (16), chlorine content can be expressed as f (t ) =

( ) 96.9 − 34.5exp ( −kc t ) 70.9 − 35.5exp −kcCl0.52 t 0.5 Cl2


(17)

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where the solubility of chlorine in aqueous hydrochloric acid solutions measured by Alkan43 is shown in Table 4. Meanwhile, according to the Arrhenius equation, k can be presented as k = k0 exp( −

E ) RT

(18)

where k0 is the frequency factor, which describes the effective collision frequency of and

, and is affected by the internal structure of PVC. And the change of internal

structure of PVC can be described by the ratio of the particle volume of PVC after foaming process to that of original PVC (V/V0). Therefore, the frequency factor k0 is expressed as 2

V  V k0 = exp( a + b + c   ) V0  V0 

(19)

where a, b, and c are the model parameters. V is the volume of PVCSCCO2 or PVCSCCO2-Ace, V0 is the volume of original PVC. Meanwhile, the particles are assumed to be sphere with the volume of original PVC, PVCSCCO2 or PVCSCCO2-Ace described as Equation (20) 1 3 V = π (d) 6

(20)

where, d is the average particle size of PVC, PVCSCCO2 or PVCSCCO2-Ace. To obtain the kinetics model of PVC chlorination, four parameters, that is, a, b, c, and E, should be estimated by fitting the experimental data. The data used for fitting in this work were over the ranges of 65 - 80 ºC, 0 - 6 h, and 198.8 - 216.8 µm. Based on the Levenberg-Marquardt algorithm, these parameters can be calculated by a nonlinear least-squares method. And, the results are shown in Table 5.


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Page 18 of 40

The frequency factor for PVC chlorination can be obtained by Equations (19), and the results are shown in Table 6. From Table 6, it can be seen that the frequency factor for PVC chlorination increased with increasing V/V0, which indicated that the effective collision of and

will increase. The model and experimental values of chlorine content of PVC chlorination are shown in

Figure 12. The correlation quality is evaluated by average absolute relative deviation (AARD) which is expressed by Equation (21). The AARD of CPVC, CPVCSCCO2, and CPVCSCCO2-Ace were 0.2774 %, 0.3679 %, and 0.1596 %, respectively. It can be concluded that model values of chlorine content of PVC chlorination agreed well with experimental values.

AARD=

f − f mod 1 × ∑ exp × 100% N f exp

(21)

4. CONCLUSIONS In this work, the commercial PVC, which was foamed by SCCO2 assisted with acetone to break the film and enlarge the inner pores, was chlorinated to prepare CPVC with the chlorine content of 0.664 g/g via aqueous suspension method. The average particle size of PVCSCCO2-Ace was 218.6 µm, which was 9.96 % larger than that of original PVC. The morphology characterization results illustrated roughness porous film and loose interior structure of PVCSCCO2-Ace. The chlorine content of CPVCSCCO2-Ace obtained by chlorination of PVCSCCO2-Ace at 80 ºC for 6 h has reached 0.664 g/g. This result was mainly due to the highly porous film and loose structure of PVCSCCO2-Ace. The M n , M w , PDI = M w M n of CPVCSCCO2-Ace were 75.65 kg/mol, 145.38 kg/mol, and 1.92, respectively. Therefore, the molecular weight distribution of CPVCSCCO2-Ace was relatively narrow. The Tg of CPVCSCCO2-Ace was 112.4 ºC. Moreover, the


18

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relationship between Tg and chlorine content has been obtained. The results of

13

C-NMR

indicated that the chlorine distribution of CPVCSCCO2-Ace chains was uniform. Therefore, the CPVCSCCO2-Ace obtained by this methodology had good thermal properties and was thermal stability. The improved kinetics model for PVC chlorination was derived from the experimental data over the ranges of 65 - 80 ºC, 0 - 6 h, and 198.8 - 216.8 µm, and the model agreed well with all the experimental data, with average absolute relative deviation less than 0.4 %. In this model, the frequency factor in the Arrhenius equation was expressed by the ratio of the particle volume of PVC after foaming process to that of original PVC (V/V0). In the whole process of chlorination, the frequency factor for PVC chlorination increased with increasing V/V0, indicating the increased of effective collision of

and

.


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Page 20 of 40

FIGURES

Figure 1.Particle size distribution described of PVC, PVCSCCO2, and PVCSCCO2-Ace

(a)

×1000

(b)

10 µm

×5000

5 µm

Figure 2.Internal morphologies of original PVC, (a) × 1000 and (b) × 5000.


20

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(a)

(b)

×1000

×5000

5 µm

10 µm

Figure 3.Internal morphologies of PVC foamed by SCCO2, (a) × 1000 and (b) × 5000.

(a)

(b)

×1000

10 µm

×5000

5 µm

Figure 4.Internal morphologies of PVC foamed by SCCO2 and acetone, (a) × 1000 and (b) × 5000.

(a)

×1000

(b)

50 µm

×5000

10 µm

Figure 5.External morphologies of of original PVC, (a) × 1000 and (b) × 5000.


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(a)

×1000

(b)

50 µm

Page 22 of 40

×5000

10 µm

Figure 6.External morphologies of of PVC foamed by SCCO2, (a)× 1000 and (b) × 5000.

(a)

×1000

(b)

×5000

10 µm

50 µm

Figure 7.External morphologies of of PVC foamed by SCCO2 and acetone, (a)× 1000 and (b) × 5000.


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Figure 8.Schematic diagram of the process of PVC chlorination to CPVC, f means chlorine content (80 °C, 6 h)

Figure 9.The GPC curves of PVC, CPVC, CPVCSCCO2, and CPVCSCCO2-Ace


23


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Figure 10.The DSC curves of (A)：CPVC-2h-0.586 (chlorination of original PVC for 2 h with the 0.586 g/g chlorine content), CPVCSCCO2-2h-0.597 (chlorination of PVCSCCO2 for 2 h with the 0.597 g/g chlorine content), CPVCSCCO2-Ace-2h-0.615 (chlorination of PVCSCCO2-Ace for 2 h with the 0.615 g/g chlorine content), (B)：CPVC-4h-0.598 (chlorination of original PVC for 4 h with the 0.598 g/g chlorine content), CPVCSCCO2-4h-0.610 (chlorination of PVCSCCO2 for 4 h with the 0.610 g/g chlorine content), CPVCSCCO2-Ace-4h-0.646 (chlorination of PVCSCCO2-Ace for 4 h with


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Page 26 of 40

the 0.646 g/g chlorine content), (C)：CPVC-6h-0.605 (chlorination of original PVC for 6 h with the 0.605 g/g chlorine content), CPVCSCCO2-6h-0.617 (chlorination of PVCSCCO2 for 6 h with the 0.617 g/g chlorine content), CPVCSCCO2-Ace-6h-0.664 (chlorination of PVCSCCO2-Ace for 6 h with the 0.664 g/g chlorine content), (D) the relationship between Tg and chlorine content

Figure 11.The 13C-NMR analysis of CPVC, CPVCSCCO2, and CPVCSCCO2-Ace


26

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Page 28 of 40

Figure 12.Chlorinated of PVC vs. time at different temperatures, (a) original PVC, (b)PVC foamed by SCCO2, (c) PVC foamed by SCCO2 and acetone


28

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TABLES Table 1. Particle size analysis of the PVC, PVCSCCO2, and PVCSCCO2-Ace d(10), µm d(50), µm d(90), µm d, µm ϕ, %

No.

Name

1

PVC

140.5

192.8

265.1

198.8

0

2

PVCSCCO2

143.8

197.5

271.1

203.5

2.36

3

PVCSCCO2-Ace

158.2

213.0

288.1

218.6

9.96

The d(10), d(50), and d(90) mean the diameter at 10% undersize, 50% undersize, and 90% undersize, respectively. d means the volume-average particle diameter. For PVCSCCO2, ϕ can be expressed as

ϕ=

dPVCSCCO − dPVC 2

dPVC

× 100%

For PVCSCCO2-Ace, ϕ can be expressed as

ϕ=

dPVC SCCO

2 -Ace

dPVC

− dPVC

× 100%

Table 2. The molecular weight and molecular weight distribution of CPVC, CPVCSCCO2, and CPVCSCCO2-Ace Mn ,

Name

kg/mol

Mw

, kg/mol

Mw / Mn

PVC

40.89

101.65

2.49

CPVC

78.35

172.25

2.20

CPVCSCCO2

73.75

158.60

2.15

CPVCSCCO2-Ace

75.65

145.38

1.92


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Page 30 of 40

Table 3. The 13C-NMR analysis and results of CPVC, CPVCSCCO2, and CPVCSCCO2-Ace, chlorine content obtained by 13C-NMR (fNMR), chlorine content obtained by titration (f) name

ACCl2: ACHCl: ACH2

nCCl2: nCHCl: nCH2

fNMR, g/g f, g/g

PVC

0.00 : 1.00 : 1.01

0.000 : 0.498 : 0.502

0.566

0.567

CPVC

0.00 : 1.00 : 0.73

0.000 : 0.578 : 0.422

0.604

0.605

CPVCSCCO2

0.00 : 1.00 : 0.68

0.000 : 0.595 : 0.405

0.612

0.617

CPVCSCCO2-Ace

0.00 : 1.00 : 0.37

0.000 : 0.730 : 0.270

0.661

0.664

ACCl2, ACHCl and ACH2 represent the integral area of peak of -CCl2-, -CHCl- and -CH2- segments, respectively. nCCl2: nCHCl: nCHCl represents molar ratio of -CCl2-, -CHCl- and -CH2- segments. The chlorine content fNMR can be obtained from 13C-NMR. Table 4. Solubility of chlorine in aqueous hydrochloric acid solutions at different temperature No. T, ºC

cCl2 , mol/m3

1

65

42.87

2

70

39.34

3

75

35.35

4

80

31.37

Table 5. Estimated parameters for the reaction kinetics model No. Parameter

Value

1

a

– 4.06

2

b

13.67

3

c

– 4.27

4

E

30.82 kJ/mol


30

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Table 6. The V/V0 and model values of frequency factor (k0) for PVC chlorination V/V0, µm3/µm3 k0, 1/(s×(mol/m3)0.5)

No.

Samples

1

CPVC

1.00

2.09×102

2

CPVCSCCO2

1.07

2.96×102

3

CPVCSCCO2-Ace

1.33

7.11×102


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Page 32 of 40

AUTHOR INFORMATION Corresponding Author *Tel: 0086-21-6425 3934. Fax:+86-21-6425 3934. E-mail: [email protected].

Notes The authors declare no competing financial interest.

ABBREVIATIONS PVC , Polyvinyl chloride; VICAT , Vicat softening temperature; CPVC, chlorinated polyvinyl chloride; Ace, acetone; PVCSCCO2-Ace, PVC foamed by SCCO2 and acetone; PVCSCCO2, PVC foamed by SCCO2; M n , number average molecular weights; M w , weight average molecular weights; PDI = M w M n , molecular weight distribution; Tg, glass transition temperature; f , chlorine content obtained by titration; fNMR, chlorine content obtained by

13

C-NMR; CPVC,

chlorination of original PVC; CPVCSCCO2, chlorination of PVCSCCO2; CPVCSCCO2-Ace, chlorination of PVCSCCO2-Ace; AARD, average absolute relative deviation.


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Page 38 of 40

Contents of Figures Figure 1.Particle size distribution described of PVC, PVCSCCO2, and PVCSCCO2-Ace Figure 2.Internal morphologies of original PVC, (a) × 1000 and (b) × 5000. Figure 3.Internal morphologies of PVC foamed by SCCO2, (a) × 1000 and (b) × 5000. Figure 4.Internal morphologies of PVC foamed by SCCO2 and acetone, (a) × 1000 and (b) × 5000. Figure 5.External morphologies of of original PVC, (a) × 200, (b) × 1000, and (c) × 5000. Figure 6.External morphologies of of PVC foamed by SCCO2, (a) × 200, (b) × 1000, and (c) × 5000. Figure 7.External morphologies of of PVC foamed by SCCO2 and acetone, (a) × 200, (b) × 1000, and (c) × 5000. Figure 8.Schematic diagram of the process of PVC chlorination to CPVC, f means chlorine content (80 °C, 6 h) Figure 9.The GPC curves of PVC, CPVC, CPVCSCCO2, and CPVCSCCO2-Ace Figure 10.The DSC curves of (A)：CPVC-2h-0.586 (chlorination of original PVC for 2h with the 0.586 g/g chlorine content), CPVCSCCO2-2h-0.597 (chlorination of PVCSCCO2 for 2h with the 0.597 g/g chlorine content), CPVCSCCO2-Ace-2h-0.615 (chlorination of PVCSCCO2-Ace for 2h with the 0.615 g/g chlorine content), (B)：CPVC-4h-0.598 (chlorination of original PVC for 4h with the 0.598 g/g chlorine content), CPVCSCCO2-4h-0.610 (chlorination of PVCSCCO2 for 4h with the 0.610 g/g chlorine content), CPVCSCCO2-Ace-4h-0.646 (chlorination of PVCSCCO2-Ace for 4h with the 0.646 g/g chlorine content),

(C)：CPVC-6h-0.605 (chlorination of original PVC for 6h

with the 0.605 g/g chlorine content), CPVCSCCO2-6h-0.617 (chlorination of PVCSCCO2 for 6h with


38

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the 0.617 g/g chlorine content), CPVCSCCO2-Ace-6h-0.664 (chlorination of PVCSCCO2-Ace for 6h with the 0.664 g/g chlorine content), (D) the relationship between Tg and chlorine content Figure 11.The 13C-NMR analysis of CPVC, CPVCSCCO2, and CPVCSCCO2-Ace Figure 12.Chlorinated of PVC vs. time at different temperatures, (a) original PVC, (b)PVC foamed by SCCO2, (c) PVC foamed by SCCO2 and acetone

Contents of Tables Table 1. Particle size analysis of the PVC, PVCSCCO2, and PVCSCCO2-Ace Table 2. The molecular weight and molecular weight distribution of CPVC, CPVCSCCO2, and Table 3. The 13C-NMR analysis and results of CPVC, CPVCSCCO2, and CPVCSCCO2-Ace, chlorine content obtained by 13C-NMR (fNMR), chlorine content obtained by titration (f) Table 4. Solubility of chlorine in aqueous hydrochloric acid solutions at different temperature Table 5. Estimated parameters for the reaction kinetics model Table 6. The V/V0 and model values of frequency factor (k0) for PVC chlorination


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A Table of Contents (TOC)

External Surface

Chlorine Content

Cross Section

Chlorination

PVC

0.605 g/g

PVC Chlorination

Foamed by SCCO2

Foamed by SCCO2

0.617 g/g

×1000 Chlorination Foamed by Foamed by SCCO2 and Ace SCCO2 and Ace


0.664 g/g

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Synthesis of Chlorinated Poly(vinyl chloride) with Uniform Distribution

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