Qianli Yang,Wei Lu,Lin Bai,Binhang Yan,Yi Cheng*

Department of Chemical Engineering,Tsinghua University,Beijing 100084,China

Keywords:Polymer Photochemistry UV–Vis online analysis Dynamic characteristics Chlorinated poly vinyl chloride

ABSTRACT Dynamic characteristics of UV enhanced gas–solid PVC chlorination process were revealed by a UV–Vis spectral online analysis method.Experimental results showed an instantaneous increase of the chlorination rate as soon as UV light was affiliated,which demonstrated the intensified effect of UV radiation on PVC chlorination directly.Different affiliation methods of UV light were then studied,proving that continuous UV radiation could enhance the chlorination process significantly while intermittent UV radiation was able to initiate the chlorination reaction once it was conducted.Besides,experiments were carried out to study the in fluences of parameters on the chlorination process such as UV wavelength,chlorination temperature,partial pressure of chlorine gas and PVC raw materials.Among all the parameters,chlorination temperature and partial pressure of chlorine gas were testified as two key factors to determine the chlorination performance.Thermal analysis of CPVC products showed that their corresponding properties such as the glass transition temperature(T g)and the homogeneity of chlorine distribution in polymer phase were improved with the increase of chlorine content.

1.Introduction

Chlorinated poly vinylchloride(CPVC)is a kind ofhigh-performance thermoplastic.It has better physicochemical properties than poly vinyl chloride(PVC),such as higher vicat softening temperature(105–125 °C),Rockwell hardness(R117–122)and oxygen index(＞50).Thus itcan be utilized to produce high-quality cold and hotplastic liquid pipes, fittings,liner of chemical columns,building materials,etc.

CPVC was firstly synthesized by I.G.Farben AG company in 1959[1].Since then it has been acknowledged as an important modified product of PVC[2].The most widely used synthesizing process of CPVC is the aqueous suspension method,which has been adopted by many international companies like Lubrizol,Sekisui,and Polyone.Though highquality CPVC products with various trademarks are produced by this method,the process is encountered with inevitable environmental problems such as the disposal of a large amount of waste acid liquid(i.e.,about 20 tons per ton CPVC)[3].As an alternative process,gas–solid method is much cleaner with the advantages of no waste water discharge,easy after-treatment system of CPVC product,low corrosion of apparatus and so on.This method was proposed by Weben et al.[4]in 1966,where PVC grains were chlorinated in a gas–solid reactor with lines of UV lamps settled in.Some other patents[5,6]also reported the gas–solid CPVC synthesis method using UV lamps.However,this method is still not widely used,which could be attributed to the lack of fundamental studies,especially the direct and quantitative experimental data to reveal the characteristics of the UV enhanced gas–solid chlorination process.

The available studies on the mechanism of the PVC chlorination process in solvents and aqueous phase with swelling agents would help to understand the basic principles of chlorination.Using the pyrolysis–gas chromatography,Tsuge et al.[7]studied the structure of CPVC through analyzing the pyrolysis products.They pointed out that CPVC is composed of four kinds of units:[– CH2–CHCl–],[– CHCl–CHCl–],[– CH2–CCl2–],and[– CCl2–CHCl–].Lukas et al.proposed a possible PVC chlorination mechanism in a series of articles[8–11].They pointed out that the PVC chlorination process is a radical reaction,in which hydrogen elements on polymer chains are substituted by chlorine atoms and each[– CH2–CHCl–]group receives at most two chlorine atoms.These researches disclosed the basic mechanism of the PVC chlorination processes.

Gas–solid chlorination is a combined process of diffusion and reaction,so that the distribution of chlorine elements on the polymer chains as well as the PVC inner structures is complex.Wachi et al.[12]proved that the substituted chlorine elements distribute unevenly in CPVC grain due to the mass transfer limitation of chlorine gas.They utilized an online chlorination process analysis by continuously recording the change of a sample's mass in chlorine gas with UV radiation at room temperature.Barriere et al.[13]derived a series of equations to describe the in fluence of chlorine diffusion on the chlorination process.These researches testified that the gas–solid chlorination process has unique characteristics because of the coupling of diffusion and reaction,but neither of them revealed the dynamic details through reliable experiments operated attemperatures higherthan 80°C(i.e.,PVC's glass transition temperature,Tg)with the aid of UV radiation.

In our work,a lab-scale vibrated-bed reactor equipped with UV radiation was established together with a UV–Vis online analysis system to reveal the dynamic characteristics of the chlorination process.The vibration of the reactor helps to intensify the gas–solid contact under the conditions ofslow reactantgas flow rate and relatively high temperatures.The UV–Vis online analysis system records the concentration of chlorine in the exhaust gas continuously,which allows access to the information of chlorination reaction.By utilizing this platform properly,the dynamic characteristics of the UV intensified PVC chlorination process can be revealed quantitatively.

2.Experimental

2.1.Chlorination apparatus and the online analysis system

The schematic diagram of the experimental platform is shown in Fig.1.The experimentalapparatus consists of two parts:a UVfacilitated vibrated bed and a UV–Vis online analysis system.A thin quartz cylindrical vibrated bed reactor with 6 mm thickness and 50 mm inside diameter is fixed on a vibration platform with the frequency of 5 Hz and horizontal amplitude of 20 mm.Both sides of the reactor are transparent,allowing UV radiation to pass through them with little reduction.A UV lamp whose main radiation wavelength is either 253.7 nm(4 W,Philips Company)or 350 nm(4 W,Toshiba Company)is set in the gap beneath the reactor.Two thermocouples are settled on both sides of the reactor to measure and controlthe reaction temperature by regulating the infrared light on the top of the container box,which is used for heating the reactor and the PVC grains.The data acquisition and treatmentmethod by the UV–Visonline analysissystemhasbeen introduced in our former work[14].

Fig.1.UV enhanced vibrated-bed chlorination apparatus and UV–Vis online analysis system.

2.2.Experimental design

QS-C62 PVC resin(Qilu Petrochemical Company),which has been adopted by many manufactures to synthesize CPVC products in China,is chosen as the raw materialin most of the experiments.In each experiment,about 4 g of PVC grains is packed in the reactor,occupying about half of its volume.A mixture of chlorine gas(99.999%)and argon gas(99.9%)is supplied by cylinders.Before the initiation of chlorination reaction,PVC grains are pretreated by chlorine and argon mixture gas(PCl2=25 kPa,and chlorine flow rate=25 ml·min-1)for 30 min to remove oxygen and water adsorbed in their pores.The chlorination rate,r,is expressed by r=,where[MCl]stands for the concentration of[– CHCl–CHCl–]or[– CH2–CCl2–]groups generated during chlorination.Similarly,the unreacted[– CH2–CHCl –]unit in the PVC polymer phase is written as[MH].In the experiments,a parameter J is defined to represent the ratio of PVC mass W(g)and chlorine flow rate F(ml·min-1)as J=(g·min·ml-1).

The UVradiation intensity(μW·cm-2)inside the reactor is measured by the irradiation module of a spectrometer(Avaspec-ULS2048*64,Avantes company).The total radiation power I(W)can be calculated by the integration of radiation intensity with transmission area,and it can be considered as the totalenergy of UV radiation exerted on chlorine gas and PVC grains.In this process,chlorine gas is more inclined to absorb UV radiation,so that we defined a parameter,E=I/N(J·mol-1)to representthe effectofUVradiation,where E is the UVradiation energy received by chlorine gas and N is the mole flow rate of chlorine gas.

Several series of experiments were conducted to study the dynamic characteristics of the UV enhanced chlorination process.In fluences of UV wavelength,radiation mode,chlorination temperature(T),partial pressure of chlorine gas(PCl2),and particle properties on the chlorination process were also studied.In the end,thermal properties of CPVC products synthesized in the lab and commercial products from market were characterized by Differential Scanning Calorimetry(DSC)and Thermal Gravimetric Analysis(TGA).

3.Results and Discussion

3.1.Dynamic characteristics of UV enhanced PVC chlorination process

In order to have a general understanding on the dynamic characteristics of the UV enhanced PVC chlorination process,T was raised gradually by 10 °C every 10 min from ambienttemperature to 80 °C under the conditions of PCl2=25 kPa and J=0.16 g·min·ml-1.

When T reached 80°C,UV light was turned on and maintained for 1 h with E=156 J·mol-1.The dynamic curve of such a process is depicted in Fig.2.Based on the experimental results,PVC particles adopted in this work cannot be chlorinated at 80°C without UV radiation.Hence Fig.2 can clearly distinguish the intensification effect of UV light on the chlorination reaction from the thermal chlorination.

Fig.2.Dynamic characteristics of UV enhanced chlorination process under conditions of 350 nm UV.(P Cl2=25 kPa,J=0.16 g·min·ml-1,E=156 J·mol-1 and T=80 °C).

The solid line in Fig.2 shows the change of r with time.At the beginning of chlorination,r remains zero for about 1 h in the temperature range from 25 °C to 80 °C.In this period,though PVC grains are soaked in chlorine atmosphere,merely no reaction takes place.When T rises to 80°C,r starts to increase at a slow speed.This phenomenon shows that PVC grains cannot be chlorinated at 80°C when there is no extra intensified method.Thermal chlorination occurs only when the temperature exceeds 80°C.This process is slow in the beginning and has low efficiency as reported by formerresearches[15].As soon as UVradiation is affiliated,r rises to a large value of about 0.07 mol·h-1immediately,and then decreases gradually in the following 1 h.This interesting phenomenon directly shows the significant intensification of UV radiation on the PVC chlorination process,which is similar to the effect of cold plasma[14,15].On the other hand,the dashed line displays the change of chlorine content of CPVC with time,which is an exponential-like curve similar to what was reported by former articles[13,16].After 1 h of chlorination,the chlorine mass content of CPVC product reaches 66%with the average chlorine conversion ratio of 41.0%.

The significant acceleration on chlorination brought by UV radiation proves that it can generate a large amount of active species as soon as it is affiliated.It has been acknowledged that UV radiation is able to break chemicalbonds and generate free radicals fast[17].We listthe bond energies ofmain groups in the PVC chlorination process in Table 1.Among all the groups,the defect structures in PVC molecules like allyl chloride and tertiary chloride are the easiestparts to be activated because oftheir lowest bond dissociation energy,but the amount of these structures is so limited[18,19]that they have little contribution to the fast chlorination process.Meanwhile chlorine gas can be decomposed into radicals by UV radiation shorter than 491.5 nm,indicating that a considerable amount of chlorine radicals can be generated by the 350 nm UV radiation used in this experiment.These radicals are supposed to be the main active speciesin this processbecause oftheirinstantaneousgeneration and high reactivity.

Table 1 Bond energies of groups in PVC polymer chains and gaseous species

3.2.In fluence of UV wavelength and intensity

Ithas been proved above that UVproperties(i.e.,wavelength and intensity)have certain effects on the PVC chlorination process.In this section,chlorination experiments were firstly conducted using UV wavelengths of 253.7 nm and 350 nm under the conditions of E=156 J·mol-1,70 °C,and J=0.09 g·min·ml-1.PCl2was varied from 15 kPa to 75 kPa.The in fluence of radiation energy was also studied individually with other parameters fixed.

As shown in Fig.3,under different partial pressure conditions both 253.7 nm and 350 nm UV radiations have similar intensification ability with the same radiation energy input.Based on bond energy data of groups listed in Table 1,the two UV radiations are both able to activate chlorine gas.The mechanism of the UV excited chlorine decomposition process can be written in the form of the photo-reaction Arrhenius formula based on the theory of photochemistry as below[20]:

In Eq.(3),n represents the number density of photon.The reaction rate constant k0is only related to the radiation energy nhv at the same temperature,so that UV radiations with different wavelengths but similar radiation energies should have similar ability in decomposing chlorine gas.However,since the UV radiation can also break up the molecularstructure to weaken the performance ofCPVC resin[17],the better UV wavelength and UV intensity for the gas–solid chlorination process should be further optimized.

Fig.3.Chlorination ability of UV radiations with 253.7 nm and 350 nm.(T=70°C,J=0.09 g·min·ml-1).

3.3.Initiation of PVC chlorination by intermittent UV radiation

Exceptforcontinuous UVradiation,another UVaffiliation mode called“intermittent radiation”was also adopted in this series of experiments to study the initiation ability of UV radiation.PVC grains were chlorinated under the conditions of τ =0.2,T=70 °C,J=0.09 g·min·ml-1,E=156 J·mol-1and PCl=50 kPa.The parameterτ =was defined to rep-

2resent the ratio of time with and without UV radiation during the reaction.

Dynamic characteristics of intermittent UV radiation are revealed in Fig.4(a).When τ=0.2,reaction rate r rises as soon as UV light is affiliated,and then decreases immediately as the UV lamp is shut off.Many chlorination cycles are recorded in this process,among which the peak value of r in each cycle increases from 0.05 mol·h-1to 0.075 mol·h-1in the beginning,and decreases gradually to 0.025 mol·h-1in the end.At the same time,r maintains a low value in the period of toff,which further proves that the UV light,similar to cold plasma,is also capable of initiating the PVC chlorination process[14].The mechanism of the initiation effectby externalintensification methods such as cold plasma and UVlightis believed to be the slow chain transfer ofradicals.Main routes of this process can be represented in the following Eqs.(4)and(5).Besides the generation of chlorine radicals by UV radiation,the first step is the generation of large molecular radical M·.

These large molecular radicals have longer life than chlorine radicals because they are fixed in the polymerphase,butthey can reactwith chlorine gas to generate chlorine radicals again.For this reason chlorination can sustain even after the UV radiation is terminated.However,since the concentration of M·is much smaller than the concentration of chlorine radical generated by UV radiation,the chlorination rate is much slower.

Based on the analysis above,when UV radiation is affiliated,a large number of chlorine radicals are generated immediately.The produced radicals attack polymer chains by substituting hydrogen atoms.HCl and M·are formed consequently to start the initiation of radical chain.When the UV lamp is off,chlorine radicals generated in the gas phase or on the surface of the polymer phase quickly annihilate by colliding with each other or react with MH in the PVC polymer phase,but the M·still exists and keeps reacting with chlorine gas.However,in comparison with the chlorine radicals produced by UV radiation,the concentration of M·is much lower and the activity is much weaker,so that the chlorination rate decreases greatly.

Fig.4.(a)Intermittent UV radiation(τ=0.2)and(b)comparison of intermittent UV radiation(τ=0.017),continuous UVradiation and thermalchlorination.(T=70–90°C,J=0.09 g·min·ml-1,E=156 J·mol-1 and P Cl2=50 kPa).

The solid line in Fig.4(a)depicts the change ofchlorine contentwith time under intermittent UV radiation.Differentfromthe linearrelationship reported by Wachi et al.[12],it is a wave-like curve resulting from the fast change of the chlorination rate.

On the other hand,a short-time UV radiation was studied to make a comparison with former results.In this experiment,the reactor was exposed to UVradiation foronly 5 s each time withτ=0.017.The in fluence of UV light was much weaker because of the shorter UV affiliation time.The comparison of the chlorination process under the conditions of(a)continuous UVintensification,(b)short-time intermittent UVintensification and(c)thermal chlorination is shown in Fig.4(b).It is clear that continuous UV affiliation has the highest chlorination rate and the other two are similar.The short affiliation of UV radiation can barely promote the chlorination process,showing that though UV radiation can initiate PVC chlorination,its effectgets weaker when affiliation time is shortened.Generally,contribution of the intermittent radiation to the PVC chlorination process is not as great as that of the continuous radiation,but it can initiate chlorination through providing initial radicals,making the fast chlorination period occur earlier than ordinary thermal chlorination.

3.4.In fluences of temperature and partial pressure of chlorine gas

As a non-catalytic gas–solid reaction,the PVC chlorination process is controlled by both reaction and diffusion of chlorine gas in the polymer phase[13].Thus we suggested thattemperature(T)and partialpressure(PCl2)are key factors during chlorination.In the following section,the in fluences of T and PCl2are exhibited for fullunderstanding ofthe UVenhanced gas–solid PVC chlorination process.PVC grains were chlorinated respectively at fixed chlorination temperatures(30–100°C)or fixed PCl2(33–101 kPa),and the values of J and E were 0.09 g·min·ml-1and 156 J·mol-1,respectively.

Fig.5.Effects of temperature and partial pressure of chlorine gas on PVC chlorination(a)dynamic curves of different temperatures when P Cl2=50 kPa,(b)dynamic curves of different P Cl2 when T=90°C and(c)different temperatures and P Cl2.(350 nm UV radiation,J=0.09 g·min·ml-1,E=156 J·mol-1).

In Fig.5(a)and(b),the changes of chlorine contents with time show thatthe in fluence of T on chlorination issignificant,because free volume of the polymer phase[21]is expanded at higher temperature so that chlorine gas is much easier to diffuse in,and the movement of polymer chains increases the colliding possibility of chlorine radicals and MH.Meanwhile,the in fluence of partial pressure of chlorine gas is evident only before 33 kPa.This result proves that chlorination develops much deeper under the conditions of high chlorine concentration because the diffusion and dissolution ofchlorine gas in polymer phase are intensified at high chlorine partial pressure.

We notice that most of the curves of chlorine content changing with time are exponential-like.This phenomenon indicates thatthe chlorination rate in the early period of chlorination is fast,and then slows down when most of the hydrogen elements in PVC molecules are substituted by chlorine.The kinetic equation has been proposed by Barriere et al.[13]as below:

It is easy to derive the relationship between[MCl]and reaction time t from this equation.At the beginning of chlorination,the concentration of active sites[MH]in PVC molecularchains is 0.0224 mol·L-1,thus the concentration of[MCl]can be written as:

And then,chlorine content w changes with time:

The form of Eq.(8)validates that the intrinsic kinetics of PVC chlorination should be exponential,which is similar to the experimentaldata.However,the mass transfer during the gas–solid chlorination process cannot be ignored,thus a kinetic model including both intrinsic kinetics and diffusion equations should be developed in detail.This will be further discussed in our future work.

In summary,the above experiments show that raising T and PCl2can promote PVC chlorination significantly.The reason may be thatincreasing T and PCl2would intensify the chlorine diffusion and solution by expanding the free volume and improving the chlorine equilibrium concentration in the polymer phase,which enhances the[Cl]in Eq.(8).On the other hand,higher T also enhances the reaction constant k in Eq.(8).Moreover,the tendency can also be applicable in thermal chlorination because the mechanism of enhancement for T and PCl2has no relationship with the UV light.Based on the experimentalresults,chlorination is sensitive to temperature,so that the chlorination temperature should be controlled precisely in the range of 80–120 °C during the scale-up process.Researches have proved that when CPVC resin stays long in the atmosphere higher than 130°C,its polymer chains will be broken to form many shorter sequences[22],which is harmful for the mechanical properties of CPVC products.

3.5.Properties of PVC grains

It is acknowledged that choosing suitable PVC raw materials is very importantfor the quality ofsynthesized CPVC products,because propertiesofPVC grains willaffectthe chlorination process[14].Researches on the morphology of PVC grains(100–200 μm)produced by suspension polymerization have proved that they are formed by coalescence of primary particles(0.2–1.5 μm)and their agglomerates(1–10 μm).Threedimensional networks of upper structures are then wrapped by a thin polymer“skin”named outer pericellular film(0.2–0.5 μm)[23].Thus a PVC grain has a complex morphology including parameters such as the grain size,porosity and pore size distribution,and thickness of the outer pericellular membrane.These properties determine the accessibility of the grain's internal pores and in fluence the diffusion of chlorine gas and hydrogen chloride.

Except for QS-C62,PVC grains labeled as HNLC(Henan Lianchuang Company),SCJL(Sichuan Jinlu Company)and TYSG-5(Xinjiang Tianye Company)were chlorinated under the conditions of T=70°C,J=0.09 g·min·ml-1,E=156 J·mol-1and PCl2=50 kPa.Fig.6 shows that the chlorination abilities of different PVC grains are different,and QS-C62 is the best one.This result validates that,as the raw material of commercial CPVC products,QS-C62 has better performance in chlorination than general PVC resin.However,there seems to be no obvious difference on the basic properties of PVC resins listed in Table 2 to distinguish QS-C62 from the others.

Fig.6.Chlorination dynamic curves of PVC grains produced by different companies.(T=70 °C,J=0.09 g·min·ml-1,E=156 J·mol-1 and P Cl2=50 kPa).

Table 2 Porosity of PVC grains and chlorine content after 1 hour chlorination

Itis difficultto gauge the in fluence ofa single parameteron the chlorinating ability of PVC resins produced by different companies,because they utilize individual polymerization processes which result in various grain morphologies.In order to exclude the in fluences of differentpolymerization processes,we chose four kinds of PVC grains produced by one company(Xinjiang Tianye Company)named as TYSG-3,TYSG-5,TYSG-7 and TYSG-8 to study the in fluences of porosity and pore distribution.These grains are produced by the same series of polymerization processes,so that they have similar grain morphology,but their polymeric degree and oil absorption values are different.It is acknowledged that the polymeric degree does not in fluence the chlorination process,but oil absorption value reflects both the porosity and the specific surface area of PVC grains.We employed a mercury intrusion porosimeter(Autopore IV 9500,Micromeritics Company)to characterize PVCs'porosity and pore distribution,which is shown in Fig.7(a).The first large peak represents the space between PVC grains which has little influence on chlorination and can be ignored.The second peak depicts the information of inner pore of PVC grains,whose diameters are between 1000 and 100 nm.Porosity values and chlorine contents after chlorination of four kinds of PVCs are listed in Table 2,and it is clear that PVCs with higher porosity are much easier to be chlorinated.

Fig.7.Pore distribution(a)and chlorination dynamic curves(b)of PVC grains with trademarks from TYSG-3 to TYSG-8.(T=70 °C,J=0.09 g·min·ml-1,E=156 J·mol-1 and P Cl2=50 kPa).

Dynamic curves illustrated by Fig.7(b)show thatthe effectofporosity of PVC grains on chlorination is embodied in the middle period of chlorination.The main trend is that PVC with larger porosity results in higher chlorination rate,which is probably because the PVC grains with large porosity can maintain high chlorination rate during chlorination because of the faster transportation of chlorine and chlorine hydrogen.It can be inferred that reaction of thermal chlorination obeys the same rule because the UV light cannot change pore structures of PVC materials.Thus we suggest that the PVC grain with loose structure and high porosity is better for chlorination.Patents claimed that the value of porosity of chlorinated-suited PVC should be controlled at 0.15–0.20 ml·g-1[24–26].However,the in fluence of PVC grains on chlorination is complex and should be further discussed,especially the accessibility of inner pores which is determined by the polymerization processes.

3.6.Thermal properties and characterization of CPVC products

CPVC samples synthesized by the UV enhanced gas–solid method with different chlorine contents were characterized by Differential Scanning Calorimetry(DSC)and Thermal Gravimetric Analysis(TGA)to study their thermal properties.These methods have been utilized by researchers to analyze the glass transition temperature(Tg)and homogeneity of CPVC products[12,27].In this section,seven CPVC products named CPVC1#–7#with different chlorine contents as well as three kinds of commercial CPVC products named GX-J(Weifang Gaoxin Company),HA-58K(Sekisui Chemical Company)and EC-950(Polyone Company)were characterized.

Fig.8.DSC analysis of CPVC products with different chlorine contents.(a)and(b)CPVC synthesized by gas–solid method,(c)CPVC synthesized by aqueous suspension method.(N2 atmosphere and 10 °C·min-1 heating rate without temperature cycle).

Fig.8(a)and(b)represents the DSC curves of CPVCs with different chlorine contents without the elimination of thermal history.Generally,apparent Tgmeans the homogeneity of chlorine distribution in CPVC grains.Fig.8 shows that Tggets much more precise as the chlorine content of CPVC samples increases.Samples with chlorine mass content lower than 66%do not have apparent phase transition temperature,and their DSC curves are irregular.However,when chlorine mass content is higher than 66%,a significant Tgwhich increases as long as the rise of chlorine content can be observed.This phenomenon proves two facts:1)CPVC with higher chlorine content has a higher Tg,which has been accepted by many researches；and 2)the homogeneity of CPVC resin is improved with the increase of chlorine content.It has been proved thatin a CPVC grain synthesized by the heterogeneousprocess some agglomerates still have low chlorine contentbecause of chlorine diffusion.In the heating process,agglomerates with different chlorine contents gradually turn soft,resulting in a long endothermic period.However,when the average chlorine content of a CPVC grain is higher,the difference of chlorine content among the agglomerates becomes less apparent,since the chlorine mass content of CPVC cannot be higher than 73.2%[10].CPVC7#with 71.2%chlorine mass content has an apparent phase transition temperature of 186.7°C which is similar to the value reported by Wachi et al.[12].In this sense we suggest that CPVC7#can be regarded as homogeneous polymer.Fig.8c represents the DSC curves of commercial CPVC products,and they also have long phase transition periods which indicate their heterogeneous structures.

TGA results(see Fig.9)show that regulation of decomposition of CPVC samples with differentchlorine contents is as follows.When chlorine mass content is lower than 65%,two stages exist in the decomposition process,which is similar to PVC resin.The first stage can be regarded as the removalof HClduring the PVC and CPVC decomposition processes[22],in which about 65%of mass is lost.When chlorine mass content rises higher than 65%,only one stage can be observed from TGA curves.The initial decomposition temperature of all the samples is about 280°C with little difference.

Fig.9.TGAanalysis ofCPVCproducts with differentchlorine contents.(N2 atmosphere and 10 °C·min-1 heating rate).

Both the DSC and TGA results indicate that thermal properties of CPVC resin change greatly after chlorine mass content reaches 65%–66%.The thermalproperties ofCPVC resin is similarto thatofPVC before chlorine mass content rises to 65%–66%in which one chlorine element is introduced to a triad in PVC polymer chains by theoretical calculation[28].We suggest thatthe properties ofCPVC resin would change greatly when more triads are substituted by two chlorine elements or more.

4.Conclusions

The dynamic characteristics of the UV enhanced gas–solid PVC chlorination process,especially the instantaneous acceleration effect of UV light on chlorination,were experimentally investigated with the aid of the UV–Vis online analysis method.Experiments showed that UV lights with 253.7 and 350 nm wavelengths had similar intensification ability.The initiation ability of UV by intermittent radiation could also be observed,which displayed a synchronous change of chlorination rate.Exceptfor UVrelated parameters,chlorination temperature(T)and partial pressure of chlorine gas(PCl2)were validated to be key factors in chlorination because of their in fluences on reaction and chlorine diffusion.At the same time,PVC grains with higher porosity were proved to be much easier to be chlorinated.

Thermal properties of CPVC products characterized by DSC and TGA showed that(1)the glass transition temperature(Tg)rises with the increase of chlorine content；(2)the homogeneity of CPVC product increases as chlorine content rises；and(3)the thermal decomposition behavior changes from two stages to one after chlorine mass content reaches 65%–66%,at which point each triad is supposed to receive one chlorine atom.Generally,the results disclosed the in fluence of newly added chlorine elements on the macroscopic properties of CPVC resin.

Nomenclature

h Planck's constant

K Boltzmann constant

k rate constant of the chlorination reaction,s-1·mol-0.5·L-0.5

k0rate constant of the reaction for chlorine radical production,s-1

n number density of photon,m-3

T temperature for the chlorination reaction,K

TgThe glass transition temperature,K

t time for the chlorination reaction,s

v frequency of photon,Hz

w chlorine mass content of CPVC production,%

[Cl2] local concentration of chlorine,mol·L-1

[MCl] local concentration of chlorinated PVC repeat unit,mol·L-1

[MH] local concentration of un-chlorinated PVC repeat unit,mol·L-1