Plasma-assisted synthesis of chlorinated polyvinyl chloride (CPVC

Chemical Engineering Journal xxx (2012) xxx–xxx

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Chemical Engineering Journal

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Plasma-assisted synthesis of chlorinated polyvinyl chloride (CPVC) characterizedby online UV–Vis analysis

Wei Lu, Qianli Yang, Binhang Yan, Yi Cheng ⇑Department of Chemical Engineering, Beijing Key Laboratory of Green Reaction Engineering and Technology, Tsinghua University, Beijing 100084, PR China

h i g h l i g h t s

" Process decoupling of plasma assisted PVC chlorination: plasma initiation and chlorine migration." An online UV–Vis spectral analysis method: revealed dynamic characteristics of PVC chlorination process." Plasma initiation: demonstrated highly effective shown by cyclic curves of chlorine consumption." Temperature and particle properties: key factors influencing the PVC chlorination." Pyrolysis GC–MS: helpful to identify the molecular structure of CPVC.

a r t i c l e i n f o

Article history:Available online xxxx

Keywords:Plasma assisted polymer chlorinationChlorinated polyvinyl chloride (CPVC)Process decouplingDielectric barrier discharge (DBD)Gas–solid method

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⇑ Corresponding author. Tel.: +86 10 62794468; faxE-mail address: [email protected] (Y. Chen

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a b s t r a c t

An online UV–Vis analysis system is established to reveal the dynamic characteristics of plasma-assistedsynthesis of chlorinated poly vinyl chloride (CPVC). Dielectric barrier discharge (DBD) plasma is gener-ated in a vibrated-bed reactor intermittently so that the decoupled processes, i.e., the plasma initiatedchlorination step and the chlorine migration step, are mimicked and operated repeatedly at atmosphericpressure. The instant chlorine consumption rate shows corresponding cyclic curves, illustrating evidentplasma initiation followed by chlorine migration inside particles. The mechanism of process decouplingconcept is therefore demonstrated vividly. Under the same power density of plasma, temperature isascertained as a key factor to influence the PVC chlorination process. PVC particles with smaller sizes tendto be chlorinated easily, while other particle properties such as specific surface area and microstructurealso exert complicated effect on the chlorination process. SEM results show that plasma can destroy thefilm layer on the particle surface, which makes the secondary sub-particles inside a PVC particle exposeto the environmental chlorination gas and plasma. A pyrolysis GC–MS analysis helps to identify themolecular structure of CPVC product in terms of the compositions of chlorobenzene and dichlorobenzenein the pyrolized products, which are formed by chlorine atoms bonded on polymer chains.

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1. Introduction

Chlorinated polyvinyl chloride (CPVC) is a high-performancethermoplastic produced by further chlorination of PVC resin witha rise of chlorine content from 56.8 wt% to 63–69 wt%. There aretwo important reasons for pursuing large-scale, clean productionof CPVC. First, CPVC has many superior characteristics to PVC suchas the excellent mechanical properties, flame retardant and corro-sion resistance. Most of the CPVC products are suitable for a widevariety of applications, e.g., cold and hot water pipes, industry li-quid handling, membrane, building materials, etc. [1]. Thus, a high-er net added value is superimposed on the CPVC products. Second,the production of CPVC immobilizes excess chlorine from the

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asma-assisted synthesis of chloj.cej.2012.06.158

chlor-alkali industry into the solid-state CPVC. At the same time,the mass ratio of carbon per kilogram CPVC becomes much lowerfor the addition of the chlorine element into the resin. In this sense,CPVC is a typical low-carbon product with high performance invarious applications [2].

The state of the art process to commercially synthesize CPVCadopts the aqueous-suspension method using batch operation[3,4]. Whereas, this process inevitably brings severe environmentalconcern for the discharged waste liquids, gases as well as the cor-rosion on the equipment due to the moist chlorine. In comparison,a gas–solid process is acknowledged as a much cleaner one: drygases environment during the chlorination allows for the easy han-dling of solid particles and effluent gases (i.e., HCl/Cl2); especially,there is no stringent demand on the equipment materials in dryatmosphere at around 100 �C. However, the main challenges ofthe gas–solid process are (1) to find an effective initiator to

rinated polyvinyl chloride (CPVC) characterized by online UV–Vis analysis,

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Nomenclature

A instant absorption rate (–)A1 absorption rate without reaction (–)b length of measurement cell (mm)c0 chlorine concentration in the reactant gas (mol/L)c1 chlorine concentration in effluent gas (mol/L)c2 chlorine concentration in effluent gas (mol/L)I1 flow rate of dilution Ar stream (L/min)I0 initial Ar flow rate (L/min)k coefficient of Lambert–Beer law (L/(mol mm))m the volume of 1 mol gas at a specific temperature and

pressure (L/mol)

n0 initial chlorine gas flow rate, (L/min)n1 chlorine flow rate in the effluent gas (L/min)n2 HCl flow rate in the effluent gas (L/min)R chlorine consumption rate (–)Tg glass transition temperature (�C)

Greek Lettersa constant (–)b constant (–)

2 W. Lu et al. / Chemical Engineering Journal xxx (2012) xxx–xxx

improve the process efficiency, and meanwhile (2) to create a pro-cess design to help the homogenization of chlorine element insideparticle products [5]. Some patents declared to use UV light or F2 asthe initiator to synthesis CPVC in fluidized bed reactors [6,7]. But,as a matter of fact, UV light can be easily shielded by the movingparticles in a gas–solid fluidized bed so that the UV initiation can-not uniformly exert onto all the particles in a large-scale fluidizedbed. This causes the low efficiency of a conventional gas–solid con-tacting process, and furthermore has a negative effect on the qual-ity of CPVC products.

It is reported that the typical mechanism of PVC chlorination isa series of free radical reactions [8,9]. We therefore proposed usingcold plasma as the effective initiator instead of conventional UVlight or thermal energy [10]. Cold plasma is comprised of highlyenergetic ions, radicals, UV light, etc., which can accordingly pro-mote the radical reactions efficiently at low temperature. On theother hand, cold plasma can be generated in the void space ofthe packed or fluidized particles, therefore avoid the uneven initi-ation effect and the blocking effect of particles on the plasmaintensity in large-scale reactors. Several successful applicationshave been reported to use cold plasma to treat the surface of poly-mer materials at atmospheric and vacuum conditions [11–14]. Itcan be therefore deduced that cold plasma would promote PVCchlorination because it can not only decompose chlorine to freeradicals but also activate the surface of PVC particles simulta-neously. Thus, chlorination reaction would be accelerated signifi-cantly. While, different from the simplex surface treatment ofpolymer materials, the activated chlorine need further migratesinto the core of the particles to homogenize its spatial distribution,which is assumed to be a slow diffusion-like process. Therefore, thewhole chlorination process can be decoupled into two steps: thefirst step is the plasma initiated chlorination on the surface ofPVC particle, and the second step is the chlorine homogenizationinside particles. In our previous work, we have demonstrated thatbased on the principle of process decoupling method for plasma-assisted chlorination process. CPVC with chlorine content up to69 wt% had been synthesized successfully in a plasma fixed bed[10] and a plasma circulating fluidized bed [15], both operated atatmospheric pressure. Characterization results by SEM/EDS, TGAand Raman spectra showed that CPVC made by this brand-newprocess had the desired microstructure and the thermal stabilitysimilar to that of the commercial CPVC products.

This work aims to further disclose the dynamic characteristicsof the whole chlorination process using a newly established onlineUV–Vis spectral analysis system. The effects of several key factors(e.g., temperature and particle size) on the dynamic chlorinationprocess will be quantitatively illustrated. SEM and pyrolysisGC–MS are employed to characterize the morphology and micro-structures of CPVC products.

Please cite this article in press as: W. Lu et al., Plasma-assisted synthesis of chloChem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.158

2. Experimental section

2.1. Experimental apparatus

The whole experimental system includes the gas supply (Cl2:99.999%; Ar: 99.99%) using cylinders, mass flow controllers, gaspreheating facility, DBD plasma reactor and its control system, on-line UV–Vis spectral analysis, effluent gas absorption, etc. The reac-tor unit and the online UV–Vis spectral analysis system are shownin Fig. 1. The reactor system consists of a thin quartz DBD plasmareactor, a vibration platform and a box with an infrared light to con-trol the system temperature. The DBD plasma reactor is 7 mm inthickness, in which about 5 g PVC particles are filled in each exper-iment and shaken by the vibration platform in order to improve thecontact efficiency between chlorine gas and particles and avoidpolymer sintering. Both sides of the reactor are agglutinated bybronze sheets as electrodes which are connected to a sinusoidalhigh-voltage AC power supply (Nanjing Coronalab, CTP-2000P) togenerate plasma inside the reactor with vibrated particles.

The UV–Vis analysis system includes a Dark-UV Deuterium–Halogen light source (Avalight-DH-S, AVANTES Company) to gen-erate a straight beam of light with a wavelength between 190and 1100 nm, a high sensitivity spectrometer (Avaspec-2048,AVANTES Company) and a computer to sample the data. The lightpasses through the measurement cell and then is received by thespectrometer. The computer records the light intensities beforeand after passing through the measurement cell, which is madeof glass with a dimension of 150 mm in length and 7 mm in diam-eter. The size of the measurement cell is determined based on theexperimental observation by achieving reasonable response timeand measurement precision.

2.2. Basic principles of online UV–Vis analysis system

During the chlorination process, PVC particles react with Cl2 andrelease HCl at the same amount. Lukas et al. [16] proposed themain reactions involved in the PVC chlorination process asfollowing:

½�CH2 � CHCl� �n þ Cl2 ! ½�CHCl� CHCl� �n þHCl ð1Þ

½�CH2 � CHCl� �n þ Cl2 ! ½�CH2 � CCl2 � �n þHCl ð2Þ

Since there are no other side reactions, it is easy to obtain thereaction progress by analyzing the concentrations of Cl2 or HCl inthe effluent gas. According to the spectral database, Cl2 has anobvious absorption peak at the wavelength of 330 nm, while HClnot. It is feasible to obtain chlorine concentration by analyzingthe absorbance intensity at this wavelength by UV–Vis analysismethod in terms of Lambert–Beer’s law.



Fig. 1. Schematic drawing of experimental platform: plasma vibrated-bed reactor and online UV–Vis analysis system.

W. Lu et al. / Chemical Engineering Journal xxx (2012) xxx–xxx 3

As mentioned above, 1 mol HCl is generated as 1 mol Cl2 is con-sumed. Hence, the system pressure is constant in the reactor andanalysis systems during the reaction. Assume that the chlorineconcentration in the reactant gas is c0 mol/L, the initial Cl2 andAr flow rates are n0 and I0 L/min; in the effluent gas, chlorine con-centration and flow rate are c1 mol/L and n1 L/min while HCl flowrate is n2 L/min. Before entering the measurement cell, anotherflow stream of Ar is pumped into dilute the effluent gas in orderto ensure that chlorine concentration is dilute enough to fit theLambert–Beer’s law. The flow rate of this Ar stream is I1 L/minand the chlorine concentration obtained by the UV–Vis analysissystem is c2 mol/L. Since the reactor and pipes are all inert to chlo-rine and HCl gases, there is no other chlorine-absorption substancein the system except for PVC particles. Hence it is easy to calculatethe instant chlorine consumption rate according to the absorptionrate which is recorded as A. The calculation steps are listed asfollow.

Before entering the reactor, the relationship between c0 and n0

is shown in Eq. (3). In this equation, m stands for RT/P with a unit L/mol. During each experiment, m can be regarded as a constant forR, T, P are all unchanged.

c0 ¼n0

ðn0 þ I0Þmð3Þ

During the chlorination, the volume of gas is unchanged,n1 + n2 = n0

c1 ¼n1

ðn1 þ n2 þ I0Þm¼ n1

ðn0 þ I0Þmð4Þ

When dilution gas Ar is introduced in,

c2 ¼n1

ðn0 þ I0 þ I1Þmð5Þ

c1

c2¼ n0 þ I0 þ I1

n0 þ I0¼ 1þ I1

n0 þ I0ð6Þ


Because I1, n0, I0 are all constants,

c1

c2¼ const ð7Þ

c1 ¼ ac2 ð8Þ

So the chlorine consumption rate R can be calculated as:

R ¼ n0 � n1

n0¼ 1� n1

n0ð9Þ

Based on Eqs. (3) and (4), R is displayed as Eq. (11).

c1

c0¼ n1

n0ð10Þ

R ¼ 1� c1

c0¼ 1� ac2

c0ð11Þ

So the chlorine consumption rate in chlorination is linear to c2,and the relationship between c2 and the absorption rate A is lineardue to the principle of Lambert–Beer’s law. A is recorded by onlineUV–Vis analysis system. Meanwhile, k is the coefficient and b is thelength of measurement cell. Both k and b are constants.

A ¼ kbc2 ð12Þ

R ¼ 1� aAkbc0

ð13Þ

So R is linear to A. When R1 = 0, A = A1, the coefficient b can becalculated as b ¼ a

kbc0. However, it is unnecessary to determine

the value of b for the chlorine consumption rate R can be calculatedthrough comparing A with A1.

A1 ¼kbc0

að14Þ

R ¼ 1� aAaA1¼ 1� A

A1ð15Þ



Table 1Properties of PVC raw materials used in the experiments.

PVC 1# 2# 3# 4# 5# 6#

Particle size (lm) 228 297 150 169 150 106Mesopore (cc/g) 0.002 0.004 0.002 0.003 0.002 0.002Surface area (m2/g) 1.025 1.29 0.835 0.762 0.504 0.874Polymerization degree 750–850 750–850 750–850 650–750 1000–1100 750–850Surface morphology


Before chlorination experiments, reactant gases pass throughthe whole system without reaction and the absorption rate A1 is re-corded as a constant. Then, it is feasible to obtain the instant valueof chlorine consumption rate R by recording the absorption rate A.

2.3. Experimental procedure

The physical properties of PVC raw materials used for chlorina-tion are listed in Table 1, which are provided by several major PVCmanufacturers in China. The PVC particle samples have differentcharacteristics such as particle size, specific surface area, secondaryparticles distribution, and surface morphology. PVCs named as 1#,2#, 3#, 4# and 5# are produced by suspension method, while PVC 6#

is produced by bulk polymerization method. Different from otherparticles, PVC 6# has a smaller average particle size of 100 lmand no surface film. On the other hand, PVC 3# is produced spe-cially for CPVC synthesizing process.

In the experiments, PVC particles of about 5 g are put in the DBDvibrated bed reactor. The particles are vibrated in the reactor to en-sure excellent gas–solid contact, especially to avoid the potentialagglomeration at high temperatures. The vibration amplitude isabout 40 mm in horizontal direction and frequency is about 4 Hz.The system temperature is raised gradually from 25 to 100 �C ina box with temperature control using an infrared heating light. Inall the experiments, power density of DBD plasma is fixed at�2.4 W/cm3, and the plasma frequency is 12.5–15.8 kHz, whichcan be finely regulated to optimize the discharge. Every 4 minthe plasma is turned on and lasts for 1 min. This is to mimic theprocess decoupling concept in the plasma vibrated bed reactor.The online UV–Vis instrument then records the chlorine concentra-tion in the effluent gas to reveal the decoupled steps, i.e., plasmainitiated chlorination and the chlorine migration inside particle.Normally, we operate the chlorination process of each PVC samplefor about 1.5 h. In some cases, the chlorination process is recordedfor about 7 h in order to fully reveal the characteristics of PVC chlo-rination. The chlorine content of CPVC product is analyzed by astandard method, i.e., oxygen flask combustion and potentiometrictitration method [10].

It should be noted that many factors will influence the chlorina-tion process. In this work, we pay the major attention to the effects ofsystem temperature and particle properties on the PVC chlorination.The power density of DBD plasma and the operation mode of plasmaare both fixed based on our previous experience [10]. And the chlo-rine concentration in the feed gases is kept 30% in volume ratio.

3. Results and discussion

3.1. Dynamic characteristics of chlorination process

A typical result of plasma assisted PVC chlorination processcharacterized by the online UV–Vis analysis system is illustrated


in Fig. 2. In the very beginning, the system temperature graduallyrises to 60 �C. However, no obvious chlorine consumption can bemeasured until plasma is turned on. This indicates that the effi-ciency of PVC chlorination using gas–solid method would be lowby individual thermal effect. When the plasma is turned on, thechlorine consumption increases markedly, demonstrating theeffective initiation of the chlorination process by plasma. As shownin Fig. 2a, the chlorine consumption keeps rising in the 1 min ofplasma affiliation time, then decreases back to a low chlorine con-sumption rate of 10–15% in the next 4 min when plasma is turn off.Subsequently, the cyclic curves are repeated corresponding to theintermittent operation of plasma, as shown in Fig. 2b. With the in-crease of the temperature from 60 to 100 �C, the chlorine con-sumption rate increases accordingly. When more chlorine isadded into the PVC particles, the overall chlorination reaction be-comes slower. This can be identified from the trend of the cycliccurves shown in Fig. 2c. As the result of the experiment, the chlo-rine content in the product is 67.8 wt%, measured by oxygen flaskcombustion and potentiometric titration method.

As a whole, the online UV–Vis analysis helps to demonstrate theproposed concept of process decoupling in the plasma-assistedCPVC synthesis. That is, the chlorination process is decoupled toplasma initiation step and chlorine migration step; these two stepsare proceeded intermittently so that the chlorine is graduallybonded to the PVC raw materials after the cyclic operation. It canbe seen that the plasma initiation is very effective, and can be evenpromoted by increasing the power density of plasma. However, thepromotion effect must be carefully manipulated. If the plasmas ini-tiation is too weak, one can imagine that the efficiency would betoo low or even zero to help the chlorination process. If the plasmainitiation is too strong, PVC particles may be decomposed, or thechlorine element may be firmly bonded on the surface layer of aPVC particle so that the chlorine is not homogeneously distributedinside the particle. Both cases have been encountered in our exper-iments. For example, chlorine content of PVC particles can rise to65 wt% in 30 min with the aid of continuously pulsed DBD plasma,but the CPVC particles are partly sintered and turn yellow in color.To sum-up, the newly established online UV–Vis analysis methodis very helpful to optimize the operation of the plasma-assistedPVC chlorination process.

Following each plasma initiation step, a lower reaction rate stilloccurs to further help the chlorination during the chlorine migra-tion step. Chlorine can react with the long-life macromolecularradicals formed in plasma initiated step. This does not occur unlessPVC particles are treated by plasma, which is a direct proof of plas-ma initiation effect. Active sites on PVC particles generated by plas-ma etching, grafting and destroying C–H bonds promote thechlorine absorption and reaction on particle surface. Some articleshave reported similar research using atmosphere DBD to treatpolymers [17,18], such as the grafting of monomers on PMMAthrough a two steps DBD plasma enhanced method [19]. As



Fig. 2. Chlorination of PVC 3# at 2.4 W/cm3 and 30% of chlorine gas: (a) the firstcycle of plasma enhanced chlorination step and chlorine migration step; (b) onehour’s chlorination while temperature rise from 60 to 100 �C; and (c) a 7 hours’reaction in order to show the characters of the whole chlorination process.

Fig. 3. Effect of system temperature on the chlorination process: (a) chlorinationcycles at 25, 60 and 100 �C; (b) comparison of chlorination rate at temperaturesfrom 25 to 100 �C.


another matter of fact, a higher temperature would promote thechlorination reaction together with the homogenization of chlorineelement inside a PVC particle [20,21]. This behaves like a slow dif-fusion process. Although the chlorination rate is not fast, the chlo-rine migration step cannot be ignored and actually plays asignificant role to manipulate the process efficiency and the qualityof CPVC products. This chlorination rate is affected by temperatureand particle properties which would be discussed in the followingsection.

Furthermore, we try to understand the mechanism of plasma-assisted PVC chlorination from the viewpoint of radical chemistry.Huang [22] revealed that bond dissociation energy of Cl–Cl is208.8 kJ/mol which could be destroyed when wavelength of lightis shorter than 524.4 nm; while C–H bond dissociation energy inPVC is estimated to be 377 kJ/mol which needs a UV wavelengthshorter than 317.6 nm. A conventional gas–solid method to chlori-nate PVC uses UV light as initiator for it is able to decompose chlo-rine gas to chlorine radicals. As a comparison, DBD plasma withdifferent atmosphere emits UV light with different wavelengths[23,24]. We measured the spectrum of Cl/Ar plasma by a spectrom-eter with 0.12 nm resolution and found that the main emission


spectrum is in the range of 240–260 nm wavelengths with498.9–460.5 kJ/mol of photon energy. This is able to activate bothchlorine gas and PVC macromolecules. Meanwhile, there are manyother active species existing besides UV light in plasma atmo-sphere, which would also promote chlorination through etching,grafting and other interaction. In addition, plasma with a higherpower density is able to penetrate through the particle bed homog-enously, which avoids the uneven distribution of light source (i.e.,the initiation effect). All the advantages of plasma over UV light re-sult in the higher chlorine consumption rate. Therefore, plasma canact as a more effective initiator for PVC chlorination.

3.2. Effect of system temperature on chlorination process

In our previous work, chlorination temperature was proved tobe an important factor for efficient chlorination [10,15]. Here, weuse the online UV–Vis analysis method to characterize the temper-ature effect straightforwardly. As shown in Fig. 3a, chlorinationdoes not take place at low temperature of 25 �C, for the chlorineconsumption rate is merely zero. This temperature is far belowthe glass transition temperature (Tg) of PVC. Then chlorinationtemperature is raised to 60 �C. Under this situation, cycles of obvi-ous chlorine absorption peak with the value of about 40% are ob-served, which is a direct proof that chlorination is promoted byraising temperature. At last, the chlorination temperature is raisedto 100 �C. Accordingly the highest instant chlorine consumptionrate is about 20% higher than that at 60 �C.

For a better discussion of the temperature effect, we carried outthe plasma-assisted chlorination reaction by raising the tempera-ture gradually by 10 �C from 60 �C to 100 �C as shown in Fig. 3b.It is clearly shown that with the increase of the system temperaturethe chlorination process is enhanced. For example, the peak value ofthe chlorine consumption rate rises from 22% (at 60 �C) to 65% (at100 �C). That is, the chlorination process is significantly accelerated.

These results illustrate that temperature is a very important fac-tor in plasma-assisted chlorination process. It is noticed that a



Fig. 4. Effect of particle properties on the chlorination process: (a) chlorination ofPVCs 1# and 2# with large particle sizes; (b) chlorination of PVCs 3#, 4# and 5# withsimilar particle sizes (�150 lm); (c) chlorination of PVCs 2#, 5# and 6# withdifferent particle size; and (d) chlorination of PVCs 3# (special PVC) and 6#.


higher temperature results in a higher chlorination rate in plasmainitiation step and a faster decrease of reaction rate in chlorinemigration step shown in Fig. 3. Under a higher temperature, chlo-rine radicals have higher activity and are easier to extinguish bycolliding with each other or PVC particles. On the other hand, theproperties of PVC particles would change at higher temperatures.Different from inorganic particles, PVC particles are at glassy statewhen the temperature is lower than Tg and at rubber state whenthe temperature is higher than Tg. In glassy PVC particles, polymerchains are fixed so that particles are hard and easy to be fluidized.However, the free volume in PVC particles is small, which more orless prohibits the chlorine migration inside PVC particles. Raisingtemperature would promote vibration of polymer chains, accord-ingly enlarge the free volume in PVC particles and reduce the dif-fusion resistance of chlorine migration process. All these aspectsat allowed higher temperatures would benefit to achieve the fasterchlorination rate and deeper chlorination in the secondary parti-cles because the chlorine diffusion and migration is strongly en-hanced as well as the chlorination rate itself. It is able to obtainCPVC products with a higher chlorine content by raising tempera-ture, even higher than 120 �C at the end of chlorination. However,since PVC particles are sensitive to the temperature and easy todecompose when temperature is too high, temperature must bekept stable and around the Tg of CPVC product. In the experiments,we noticed the fast decomposing and sintering of particles whentemperature is higher than 150 �C.

3.3. Effect of particle properties on chlorination process

PVC particles can be characterized by their properties such assurface area, pore volume, surface morphology, particle size, poly-merization degree, and stacked structure of secondary particles[5]. In general, producers like to choose PVC particles with largesurface area, loose structure and adaptive polymerization degreeas the raw materials to make CPVC because chlorine migration pro-cess in bulk of these PVC particles would be much easier, and thechlorination inside particles is more uniform under these situa-tions. BET results show that there are merely no meso-pores inPVC particles, but the porosity of PVC particles is about 60% ana-lyzed by mercury porosimetric method. Thus we suggest that mostof the pores in PVC particles are macro-pores larger than 50 nm andhave little resistance to chlorine diffusion in gas–solid chlorination.

As shown in Fig. 4a, PVCs 1# and 2# with larger particle sizesboth have a chlorination rate of about 30%, which is lower thanPVCs 3#, 4# and 5# with smaller particle sizes of about 150 lmshown in Fig. 4b. It seems that particle size is a possible influencingfactor in PVC chlorination. Thus, commercial PVCs 2#, 5# and 6#

with different particle sizes are chlorinated and the results are gi-ven in Fig. 4c. In these experiments, PVC 6# with the smallest par-ticle size among the others has the best performance in thisplasma-assisted chlorination process. The chlorination consump-tion rate even reaches 70% at 100 �C in plasma initiated chlorina-tion step. In contrast, though the other two PVC particles arechlorinated fast in the beginning, the chlorination rate quickly de-creases later on. Though PVCs 1# and 2# have larger specific surfaceareas listed in Table 1, chlorine consumption rates are lower thanPVC particles with smaller particle sizes. These results indicate thatsmaller particles can be more easily chlorinated in general.

However, since the structure of PVC particles is complex, manyother properties of PVC particles would influence chlorination pro-cess in addition to the influence of particle size. In Fig. 4b, PVC 4#

has a highest chlorine consumption rate in chlorine migration stepas its surface is exposed outside without little film covered; PVC 5#

has the weakest reaction ability for its small specific surface area.At the same time, PVC 3# which is produced specially for CPVC syn-thesis has the best chlorination ability, even better than PVC 6# as


shown by Fig. 4c. It is noticed that PVC 3# has a much larger spe-cific surface area than other PVCs with the same polymerizationdegree and particle size. Therefore, the influence of PVC propertieson the chlorination process could be very complicated.

This work and the previous ones [2,10,15] demonstrated thatplasma helps enhancing the chlorination process in terms of thechlorine content inside the particles for any PVC products we canobtain from industry. However, the process efficiency differs fromeach other greatly. And also, the corresponding CPVC products



Fig. 5. SEM images (a) surface morphology of PVC 5# before plasma treatment; (b)film on the particles surface is destroyed by plasma; (c) magnificent of surface film;and (d) secondary particles expose outside after plasma treatment.

Table 2Pyrolysis components of PVC 6#, CPVC produced by plasma assisted method andcommercial CPVC by a pyrolysis GC–MS method.

Product Retention time PVC 6# CPVC(This work)

CPVC (EC950)

HCl 1.645 63.48 73.04 80.94Benzene 2.886 27.62 14.11 9.35Toluene 4.257 3.19 2.65 0.94Chlorobenzene 5.633 0 6.74 6.73m-Dichlorobenzene 9.064 0 0.42 0.61p-Dichlorobenzene 9.206 0 0.49 0.73o-Dichlorobenzene 9.666 0 0.56 0.70Naphthalene 12.887 1.03 1.03 0Chloronaphthalene 16.839 0 0.96 0

Data is not normalized so that it could only be used for relative comparison.Pyrolysis products of PVC 6# are complex and this table only presents several mainproducts for comparison with CPVC.


would have different properties which directly link to the practicalapplications. As a novel technique to make chlorinated polymers, alot of fundamental work is still expected to understand the plasma-assisted chlorination process with different raw materials of PVCs.

3.4. Characterization of CPVC products

In our previous work [10,15], SEM/EDS, TGA, Raman spectralanalysis had been used to characterize the final products from


the plasma-assisted chlorination process. The results indicatedgood micro-structure of the CPVC particles when comparing withthe commercial ones. In this work we have revealed the dynamiccharacteristics of chlorination process using the online UV–Visanalysis on the effluent gases. As discussed above, particle proper-ties of PVCs have great influence on the instant chlorination perfor-mance. Based on the updated knowledge of the plasma effect, weuse SEM to visualize the surface change of the chlorinated PVC par-ticles. Here we take PVC 5# as an example. Originally, the PVC par-ticles have smooth and thick film at the surface due to the addeddispersant in the production process using suspension polymeriza-tion method. The film covers the inner secondary particles tightly,as shown in Fig. 5a. It is interesting to see from Fig. 5b that the sur-face of PVC particles is seriously damaged by plasma after 1 hintermittent treatment. It is considered that PVC particles withthick and smooth film are not easy to be chlorinated using conven-tional processes because the film would prevent chlorine gas fromdiffusing inward. Thus producers tend to choose loose PVC parti-cles with thin, broken film in aqueous suspension method. In thisplasma-enhanced gas–solid chlorination process, the film can bedestroyed with the aid of cold plasma without influencing the sec-ondary particles in the PVC particles. Then the secondary particlesare exposed. Accordingly, chlorine gas is easier to diffuse insideand reacts with PVC.

A pyrolysis GC–MS was further employed to analyze the micro-structure of PVCs and CPVCs [25]. PVC and CPVC samples werepyrolyzed at 600 �C in the atmosphere of He and a series of pyro-lysis products such as HCl, benzene and toluene were released.Based on the analysis of these pyrolysis products, it is feasible todetermine the amount and position of extra chlorine atoms fixedon the polymer chains. The results are listed in Table 2, includingPVC 6#, CPVC produced in this work and CPVC EC-950 producedby PolyOne Company using aqueous suspension method. It is clearto see that CPVC products emit more HCl, chlorobenzene anddichlorobenzene but less benzene and toluene than PVC after thepyrolysis operation. Though the formation mechanism of toluenein PVC pyrolysis is not clear, introduction of chlorine atoms inthe macromolecular chains would result in the formation of chlo-robenzene by adding one chlorine atom in a 6-carbon sequenceand form dichlorobenzene in case of adding two. The results in Ta-ble 2 demonstrate that the plasma-assisted PVC chlorination usinggas–solid method can successfully fix chlorine atoms on the poly-mer chains, which is another proof besides the measurement ofchlorine content and SEM/EDS analysis [25,26].

4. Conclusions

In this work, an online UV–Vis spectral analysis method wasnewly established to reveal the dynamic characteristics of PVC




chlorination process with the aid of plasma by measuring the chlo-rine concentration in the effluent gas. PVC particles were chlori-nated in a DBD vibrated-bed reactor, which provided excellentgas–solid contact, especially avoided the potential polymeragglomeration at high temperatures. The plasma was turned on/off intermittently, e.g., 1 min on and 4 min off, to mimic the pro-cess decoupling design, i.e., the fast plasma initiation and the slowchlorine migration of the plasma-assisted chlorination process.Being the first contribution, this work successfully proved the pro-cess decoupling concept and its dependence on the system tem-perature and particle properties by straightforward illustrationfrom the cyclic curves representing chlorine consumption rate intime. Plasma initiation was evidently seen, followed by the chlo-rine migration step. A higher temperature promoted the chlorina-tion efficiency and PVC particles with smaller sizes tended to bechlorinated easily. In general, PVC particles with larger surfacearea, looser structure and adaptive polymerization degree werechosen as the raw materials to make CPVC. The existence of filmon the surface of PVC particles would prohibit the chlorination pro-cess. However, it was found that plasma exerted strong effect onthe film layer, and as a result secondary particles inside a PVC par-ticle were exposed to the environmental chlorination gas and plas-ma. For this reason, the plasma-assisted PVC chlorination did notshow dominant effect of particle morphology on the chlorinationability. A pyrolysis GC–MS was applied to analyze the PVC andCPVC particles. The results showed that many chlorine atoms werefixed on the polymer chains which represented as the composi-tions of chlorobenzene and dichlorobenzene in the pyrolized prod-ucts. It should be mentioned that the influences of PVC properties,the mechanism of chlorination in chlorine migration step andfurther optimization of chlorination conditions for this plasma-assisted CPVC synthesis method are still needed to explore. Thesame methodology can be applied to make chlorinated polyethyl-ene (CPE), chlorinated polypropylene (CPP), and so on. The onlineUV–Vis analysis method provides a common technique to charac-terize the chlorination process, while the pyrolysis GC–MS is help-ful to identify the molecular structure of chlorinated polymers.

Acknowledgements

Financial supports from National Science and Technology KeySupporting Project (No. 2009BAC64B09), National Natural ScienceFoundation (No. 21176137) and the Program for New CenturyExcellent Talents in University are acknowledged.

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Plasma-assisted synthesis of chlorinated polyvinyl chloride (CPVC