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Unified production of chlorinated isotactic polypropylene and chlorinated paraffin via a solvent free chlorination process.

INTRODUCTION

Chlorinated isotactic polypropylene (CIPP) with chlorine percentage of 28-35 by weight and random chlorine substitution in polymer has been widely used as adhesives of polypropylene materials (1-5). For such application, CIPP should be completely soluble in toluene or xylene solvent, which requires a homogeneous chlorination process of the polymer. At present, most of CIPP in China is produced via carbon tetrachloride (CTC) solvent method, where CTC is used as solvent for its full inertness to chlorine, good solubility for chlorine, isotactic polypropylene (IPP) and CIPP, as well as its ease of separation from CIPP product. For the same reason, many patents and papers have been reported on the preparation of CIPP and other chlorinated polyolefin by CTC solvent method (6-11). However, such application of CTC as a solvent in chlorination reaction has to be eradicated under the Montreal Protocol for phasing out ozone depleting substances (ODS) by 2010 (12). Therefore, it is imperative to find an alternative solvent to substitute the application of CTC. For this purpose, much research has been carried out and some solvents have been investigated (13-15). These solvents can be divided into four categories, viz. (1) chlorinated methanes, e.g., chloroform and methylene dichloride, (2) chloro-substituted ethane or ethylene, e.g., trichloroethylene, tetrachloroethane and ethylene tetrachloride. (3) chloro-substituted benzene, e.g., chlorobenzene, monochloromonofluorobenzene and mixtures thereof, and (4) water. It can be seen that all these attempts are directed to find a solvent which is fully or at least partially inert to chlorine so as to mimic the attributes of CTC. However, further chlorination of chloro-substituted hydrocarbons is inevitable especially when chlorine percentage of CIPP exceeds 30 by weight, and accordingly CTC, hexaehloroethane and polychlorinated benzene can be formed for the first three kinds of solvents, respectively. This is not acceptable in view of ODS reduction or unwanted toxic residues in CIPP product. Water is totally inert to chlorine, but it fails to provide a homogeneous chlorination condition for suspended polypropylene particles. Thus, it is less appropriate as a solvent for the production of low chlorine percentage of CIPP with uniform chlorination although many researchers have attempted so (16-19).

To address the difficulty in finding a solvent that is comparable with CTC in terms of inertness to chlorine, cost-effectiveness, and ease of separation, we have tried a solvent that can be chlorinated simultaneously with IPP, forming two useful chlorinated products, i.e., CIPP and chlorinated solvent, in one run. In this way, the reactor can be used much more efficiently than the conventional method, in which the mass fraction of CIPP is only about 10% of the solution and the other 90% is CTC solvent. Toward this aim, no report has been available till now to our best knowledge. In this report, paraffin is chosen as a reactive solvent, because it can dissolve IPP at higher temperature, and its chlorinated form, chlorinated paraffin (CP), can dissolve CIPP. Therefore, the reaction solution remains homogeneous throughout, which is helpful for CIPP product with uniform chlorination. Considering that CP with chlorine percentage of 10-65 wt is a volume chemical widely used in plastic industry (20), (21), and that paraffin is sparsely volatile and less toxic than traditional volatile organic solvents like CTC, the present unified production method is more efficient and environmentally friendly.

EXPERIMENTAL

Materials and Apparatus

All the solvents and reagents were obtained from commercial sources in China and were used without further purification. IPP (melting point 170[degrees]C. density 0.91 g/ [cm.sup.3]) was kindly supplied by Jincheng Chem. Co., a CIPP producer. Chemical grade paraffin (a mixture of aeyelic compounds with boiling point above 300[degrees]C and carbon chain length ranging from 12 to 15), and AR grade AIBN and acetone were obtained from Tianjin Damao Chemical Reagent Factory. Chlorine gas is industrial grade gained from Beijing Second Chemical Factory. Caution: gaseous chlorine can powerfully irritate the throat and should be used with adequate ventilation and precautions against inhalation.

The reaction was carried out at atmospheric pressure in a four-hatch flask immersed in thermostatic oil bath. The reaction mixture was vigorously stirred using a mechanical blender, the reaction temperature was monitored with glass thermometer, and the flow rate of chlorine gas into the reactor was monitored with a flowmeter. The effluent gas containing unreacted chlorine and the product gas. hydrogen chloride (HCl), was scrubbed by dilute solution of sodium hydroxide (NaOH).

Preparation of CIPP and CP

To a 250-ml flask. 5 g of granular IPP and 100 ml of paraffin were added. After 1 h stirring of the mixture at 150[degrees]C. IPP particle dissolved completely into paraffin forming a homogeneous and transparent solution. Then 0.5 g of AIBN was added to the flask and chlorine gas was introduced at a constant flow rate of 6.0-6.5 g/h for 6 h. Later, the reaction temperature was decreased gradually to 70[degrees]C and chlorine gas was continuously fed into the reactor at the said flow rate for 10 h until percentage chlorine of CIPP reached 28-35 by weight. When the chlorination finished, nitrogen gas was introduced to strip the residual chlorine and HCl gases in the reactor. Then the reaction mixture was cooled to room temperature, to which two times volume of acetone was added, forming CIPP solid precipitate and CP solution of acetone. The CIPP precipitate was filtered and washed twice with small amounts of acetone and once with deionized water, and then dried overnight in an oven at 80[degrees]C. CP and acetone were separated via simple distillation followed by rotary vacuum evaporation to remove ail volatile components.

Properties Measurement of CIPP and CP

The chlorine percentage of CIPP and CP are quantitatively determined using the combustion of the product sample, i.e., CIPP or CP. followed by titration of the chloride anion (22). For this measurement, 20-30 mg of CIPP or CP was weighed with an analytical balance and wrapped with a filter paper. The wrapped sample was ignited and quickly put into a flask containing 25 ml of 1.0 mol/l NaOH solution and pure oxygen gas, and sealed for at least half an hour to ensure the complete absorption of HCl gas by NaOH solution. In pure oxygen atmosphere, CIPP or CP was easily combusted involving flame. The chloride ion concentration was analyzed according to the standard procedure using silver nitrate as titration solution and potassium chromate as indicator.

Viscosity of CP liquid at different temperatures was determined using rotary viscometer (NDJ-1, Shanghai Scale & Instrument Co.).

The absorbance of 20% (by weight) CIPP solution of toluene was measured using Ultraviolet-Visible spectrophotometer (752 model, Shanghai Precision & Scientific Instrument Co.) at a wavelength of 450 nm.

Differential Scanning Calorimetry

Calorimetric measurements were performed in a differential scanning calorimeter (NETZSCH DSC 204 Fl, Germany) at a heating rate of 10[degrees]C/min under nitrogen atmosphere in a temperature range of 40-200[degrees]C. The crystallinity of the samples (IPP. C1PPI and CIPP2) was calculated using Eq. l.

Crystallinity = [[[delta][H*.sub.f]]/[[delta][H.sub.f.sup.0]]] x 100% (1)

where [delta][H.sub.f.sup.0](138 kJ/kg) is the heat of fusion of 100% crystalline of IPP and [delta][H*.sub.f]is the heat of fusion of samples.

IR Spectroscopy

Infrared spectra of IPP and CIPP were recorded using a single bounce attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectrometer (Thermo Electron Nexus 8700, USA). The polymers were directly placed on the diamond crystal and then pressed tightly. Data were collected over 32 scans with a resolution of 8 [cm.sup.-1] at room temperature.

NMR Spectroscopy

High-resolution proton magnetic resonance ([.sup.1]H-NMR]) and [.sup.13]C-NMR spectra were recorded using a 600 MHz Bruker av600 spectrometer (Switzerland), in [CDCl.sub.3], at room temperature and tetramethylsilane as an internal reference.

RESULTS AND DISCUSSION

Asynchronous Chlorination of IPP and Paraffin

The paraffin used in this study is a mixture of acyclic compounds with boiling point above 300[degrees]C and carbon chain length ranging from 12 to 15. IPP is obtained via thermal degradation of its commercial grade. The formula of CIPP and CP with chlorine percentage of 30 by weight can be roughly represented by [[([C.sub.12][H.sub.22])[Cl.sub.2]].sub.n] and [C.sub.12][H.sub.24][Cl.sub.2], respectively. However, due to the structural difference between paraffin and IPP, their chlorinating rates are different. Figure 1 shows the instantaneous chlorine percentage of CIPP and CP for the samples taken at different reaction times. It is seen that the chlorinating rate of paraffin is faster than that of IPP, which may be attributed to the activity order of available hydrogens to be substituted in a particular hydrocarbon species, i.e., [-CH.sub.2] > -CH > [-CH.sub.3] (23), meanwhile the ratio of secondary hydrogen in paraffin is much higher than that in IPP. The experimental results indicate that CIPP with chlorine percentage of about 30 wt can be produced successfully along with a coproduction of CP with chlorine percentage ranging from 35 to 40 by weight.

[FIGURE 1 OMITTED]

It is well known that higher temperature results in the reduction of liquid viscosity and increase in chlorination rate, but can also result in a higher energy cost and thermal decomposition of the chlorinated products. Therefore it is better to carry out the reaction at lower temperature. For this purpose, some other chlorination conditions are studied, and the corresponding instant chlorine percentage of CP and CIPP is measured and listed in Table1. As seen from Table 1, the chlorine percentage differences between CP and CIPP are similar for all reaction conditions except e, which suggests that at least 6 h high temperature chlorination is necessary for destroying the crystal structure of polymer and then the chlorination reaction can proceed in a homogeneous solution at lower temperature, e.g., 70[degrees]C. Among the chlorination conditions a through d studied, d is recommended considering its longer chlorination time at lower temperature which is likely to decrease energy cost and prevent thermal decomposition of the chlorinated products.
TABLE 1. Instantaneous chlorine percentage of CP and CIPP by weight at
different reaction condition.

Chlorination condition      a             b              c

Sampling time/h         CP     CIPP   CP     CIPP   CP     CIPP

 3                      10.28   5.06  10.78   5.18   8.92   3.94

 6                      18.31  12.11  18.76  11.78  15.78  10.64

9                       26.82  17.45  25.45  18.43  21.39  16.31

12                      32.02  23.45  32.48  24.05  28.65  22.19

15                      37.19  28.74  36.28  28.58  34.81  28.20

Chlorination condition       d             e

Sampling/h               CP    CIPP    CP    CIPP

 3                      11.18   5.15   7.97   3.94

 6                      19.32  11.34  14.44   8.01

9                       25.15  17.73  21.98  12.22

12                      31.49  23.23  29.92  15.43

15                      37.29  28.38  37.60  19.04

In the reaction process, the chlorination temperature and duration are
as follows: (a) 150[degrees]C (15 h), (b) 150[degrees]C (9 h) +
110[degrees]C (6 h), (c) 150[degrees]C (3 h) + 110[degrees]C (12 h),
(d) 150[degrees]C (6 h) + 70[degrees]C (9 h), (e) 150[degrees]C (3 h) +
70[degrees]C (12 h).


Crystallizability of CIPP

Isotactic polypropylene is a highly crystalline polymer. To make it soluble in organic solvent at room temperature, the crystalline structure has to be destroyed. For this purpose, chlorination modification is a commonly used method (24). The crystallizability of CIPP products, viz. CIPPI prepared using paraffin solvent and C1PP2 prepared using CTC solvent, is determined through DSC analysis and compared with IPP raw material. As shown in Fig. 2. the melting point of IPP is about 170[degrees]C, and the apparent crystalline degree is 68.6% as estimated by the fusion heat of DSC measurement (25). However, no endothermic peak is found for CIPP1 and CIPP2 samples in the vicinity of the melting point, suggesting that the CIPPs with chlorine percentage of about 30 wt become noncrystalline. This may be attributed to the steric hindrance of the randomly distributed chlorine atoms in the polymer chains, which prohibited the crystallization of the polymer.

[FIGURE 2 OMITTED]

Homogeneity of Chlorine Distribution in CIPP

The microstructure of IPP, CIPP1, and CIPP2 is studied by infrared spectra, as shown in Fig. 3. From Fig. 3, it is evident that the IR spectra of CIPP1 and CIPP2 are virtually same, whereas the difference of IR spectrum between IPP and CIPPs is noticeable. In the IR spectrum of IPP, four absorption frequencies, namely, 2953.6, 2874.1, 2918.5, and 2838.4 [cm.sup.-1] were present in the range of 2960-2830 [cm.sup.-1] The first two absorptions can be assigned to C-H stretching vibration of methyl group [-CH.sub.3], and the last two absorptions can be attributed to the C-H stretching vibration of methylene group [-CH.sub.2]-. In the spectrum of CIPPs, although the absorption frequencies for the first three bands also appeared, they are higher than those in IPP due to the presence of C-Cl bond. It is also observed that the absorption at 2838.4 [cm.sup.-1] in IPP became too weak to be detected in CIPP, indicating that the chlorination took place at [-CH.sub.2] group. The absorption at 1457.5 and 1375.8 [cm.sup.-1] for IPP can be assigned to bending vibration of the methyl and methylene group respectively in the polymer, and this absorption is corresponding to 1457.5 and 1380.1 [cm.sub.-1] for CIPP1 and 1456.6 and 1380.4 [cm.sup.-1] for CIPP2, respectively. This demonstrates the presence of [-CH.sub.2] and [-CH.sub.3] groups in the chlorinated polymer because of the low chlorine percentage in CIPP and random chlorine distribution.

[FIGURE 3 OMITTED]

To examine the fine structure of the chain segment with respect to IPP, the fingerprint region of IPP, CIPP1, and CIPP2 in the frequency range of 500-1300 [cm.sup.-1] is portrayed in Fig. 4. As a rule, the absorptions of IPP at 1165, 998, 895, and 840 [cm.sup.-1] can be ascribed to helical structure of IPP (26) due to the crystallite in the polymer. However, in the chlorinated polymers, i.e., CIPP1 and CIPP2, the characteristic absorptions disappeared. Moreover, the band at 1162.2 [cm.sup.-1] in IPP shifted to 1147.6 [cm.sup.-1] for CIPP1 and to 1146.1 [cm.sup.-1] for CIPP2. It has been reported that band at 1168 [cm.sup.-1] in solid IPP shifts to 1151 [cm.sup.-1] in liquid IPP and asymmetry of the 970 [cm.sup.-1] band is sensitive to helical structure (27). This indicates that the crystal structure may be destroyed in CIPPs, which is consistent with the DSC observation. The results suggest that the chlorine is evenly distributed in the polymer chain as a result of thorough dissolution and nonfolding structure of polymer. Therefore, the present method can be deemed as a homogeneous solution chlorination process similar to the conventional CTC solvent method.

[FIGURE 4 OMITTED]

It is noted that in comparison with the spectrum of IPP, new absorptions, especially absorbance at about 756 [cm.sup.-1] a distinct peak was recorded for CIPP1 and CIPP2 in the range of 800-550 [cm.sup.-1] (far-infrared region), which is probably related to the stretching vibration mode of C-Cl bond in the chlorinated polymer (27). The marginal difference of CIPP1 and CIPP2 in IR spectra. caused by the different chlorine content in CIPP1 and CIPP2, may be ascribed to the similar chlorination conditions provided by paraffin and CTC solvent, i.e., they are both of chlorination reaction in a homogeneous solution following radical substitution mechanism.

Figure 5 shows the 600 MHz [.sup.1]H-NMR spectra of CIPP1 and CIPP2. From Fig. 5, it can be found that the spectra for CIPP1 and CIPP2 are virtually same, which also indicates the similar microstructure of CIPP1 and CIPP2. In the assignment of the chemical shifts of CIPP, the [.sup.1]H-NMR spectra can be divided into three regions of [delta] values which are named low magnetic fields (LMF) 3.4-4.2 ppm, middle magnetic fields (MMF) 1.4-2.5 ppm and high magnetic fields (HMF) 0.75-1.4 ppm. In LMF, the appearance of new peaks can probably be assigned to chlorine-substituted methyl and methylene proton resonances from the spectra of ethylene-vinyl chloride copolymers (28), (29) and CPE (30). For example, the line at about 3.7 ppm is ascribed to the -CHCl- group formed by the chlorination of the syn proton (31). In addition, in the MMF region, the observed broad absorptions are arisen from methylene and melhine protons adjacent to chlorine-substituted groups, The methyl proton resonances which are adjacent to the chlorinated methane and methylene groups fall in the region of HMF. The doublet centered at about 1.0 ppm is due to methyl protons which are two carbons away from the chlorinated group, and the shoulder at about 1.2 ppm is attributed to methyl protons adjacent to tertiary chloride (31)

[FIGURE 5 OMITTED]

Determination of Chlorine Distribution in CIPP

Figure 6 shows the 600 MHz [.13.sup]C-NMR spectra of CIPP1 and CIPP2. From Fig. 6, it can be found that the spectra for CIPP1 and CIPP2 are virtually .same. The chemical shifts of peaks a, b, and c reflect the nonchlorinated group of [-CH.sub.3], -CH- and [-CH.sub.2]-, respectively in the structure of CIPP1 and C1PP2. The other peaks are ascribed to the chlorinated group, such as -CHCl-, [-CH.sub.2]Cl-, and so on. For example, the chemical shift of peak i is ascribed to [-CH.sub.2]Cl-. More details are summarized in Table 2.
TABLE 2. Peak assignment for CIPP with the corresponding structure.

Peak  Chemical shift               Structure assignment
          (ppm)

d         16.65       [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN
e         18.50       ASCII]

f         34.00       [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN
                      ASCII]

g         40.02       [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN
h         41.94       ASCII]

i         49.86       [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN
                      ASCII]

j         70.40       [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN
                      ASCII]


[FIGURE 6 OMITTED]

Technical Comparison for CTC Solvent and Paraffin Solvent Methods

To show the feasibility of the present method for the production of CIPP, the operating conditions and the characteristics are outlined in Table 3, It is seen that although the initial reaction temperature in the present method is somewhat higher for the sake of complete dissolution of IPP, the pressure is reduced dramatically due to the negligible volatility of paraffin. This is helpful for reducing the risk of toxic gas leakage and increasing the safety of industrial operation. More importantly, the paraffin can be used to substitute the solvent CTC. an ODS, which is being banned by the Montreal Protocol for the protection of ozone layer. In addition, some other advantages of the present method are also deserved mention. First, the emission of VOC is reduced greatly from 350 to 500 kg CTC to an estimated 30 kg acetone per ton of CIPP product due to the difference in emission ways. That is, the majority of CTC solvent is emitted via off gas of the chlorinating and purging process at high temperature, whereas acetone is emitted only in the separation process of CIPP at low temperature. Second, efficiency of the reactor is enhanced from 10 to 100%, because in the conventional method. 90% of the reactor is taken up by CTC solvent, while in the present method 90% is taken up by paraffin solvent that will be converted concurrently to another useful product of CP. Therefore, the present method is more efficient and energy-saving. Finally, the usability of chlorine and the quality of CIPP is virtually same between these two methods.
TABLE 3. Technical comparison for conventional method and present
method.

Comparison items        Conventional method  Present method

Reaction temperature    From 110 to          From 150 to 70[degrees]C
                        78[degrees]C

Reaction                0.3 MPa              0.1 MPa
pressure(abs.)

Emission rate of ODS,   385-550              0
kg/t-CIPP

Emission rate of        350-500 kg CTC       <30 kg acetone
solvent, kg/t-CIPP

Efficiency of the       10%                  100%
reactor

Output                  CIPP                 CIPP and CP

Usability of chlorine   Near to 100%         Near to 100%

Percentage chlorine of  20-40                20-40
CIPP

Product separation      Distillation/vacuum  Solvent
                        drying               precipitation/distillation

Reaction time           Short                Long


Properties of CIPP Produced by CTC and Paraffin Solvent Methods

To compare the quality of CIPP and CP produced by CTC solvent and paraffin solvent methods, some properties are measured, e.g., chlorine percentage by weight, solubility of CIPP in toluene, absorhance of 20% (by weight) CIPP solution, and viscosity of CP at different temperatures. It is well-known that the chlorinated polymer can be decomposed at high temperature, forming conjugated double bonds as a result of elimination of hydrochloride and thus presenting color under visible wavelength. The absorbance of a 20% CIPP solution of toluene under a wavelength of 450 nm is used to represent the chroma of a CIPP product. And accordingly, the chroma can be deemed as an index of the degree of thermal degradation of the polymer, and the lower the chroma the better the product quality.

Table 4 shows the quality indices of CIPP prepared by the present method and that obtained from Guangzhou Jincheng Chem. Co. using CTC solvent method. As shown in Table 4, the CIPP produced by the present method has a lower chroma in comparison with the CIPP produced by the CTC solvent method, which may be attributed to its lower percentage of chlorine. Besides, no significant difference is observed for the properties of CIPP produced by these two methods. This suggests that paraffin can be used as a promising reactive solvent to replace CTC, an inert solvent to chlorine, for the production of CIPP. In this way, CIPP can be produced in a more environmentally benign and cost-effective way.
TABLE 4. Properties comparison of CIPP produced by different methods.

Quality indices          Conventional method   Present method

Percentage chlorine of        33.86                30.44
CIPP

Solvency in toluene      Complete dissolution  Complete dissolution

Homogeneity of CIPP (a)      Transparent           Transparent

Absorbance (b)                 1.452                1.189

pH value in water (c)          6.47                 6.39

(a) Homogeneity of a 20% CIPP solution of toluene is measured via
eye-measurement.

(b) The absorbance of a 20% CIPP solution of toluene is measured with
752-photometer at wavelength of 450 nm.

(c)To 35 g of 20% toluene solution. 28 g of deionized water was added,
stirred 30 min and settled 60 min, and then the PH value of the water
layer is determined using PH-meter.


Properties of CP Coproduced With CIPP

As we know, CP with chlorine percentage of 10-65 wt is commercially produced by chlorinating paraffin with chlorine at elevated temperature in the presence of radical initiator of AIBN. This process is nearly identical to the present method for the unified production of CP except the presence of some amount of IPP or its chlorinated form. Therefore, the CP product produced by these two methods is of marginal difference, as shown in Table 5 for the viscosity of CP al different temperatures.
TABLE 5. Viscosities of CP made by different methods at different
temperatures.

Temperatures (C)                        20    30    40   50   60

Viscosity (Pa.s)  Conventional method  73.3  24.7  7.6  3.1  1.4

                  Present method       75.4  27.2  8.2  3.4  1.5


However, the viscosity of CP coproduced is slightly higher than that produced by conventional method, which is likely due to the residue of the chlorinated low molecular IPP in CP.

As an extension of the present philosophy, CIPP may either be produced as a byproduct by the present manufacturers of CP, or paraffin may be replaced with other reactive solvents provided that their chlorinated form is of value and great demand in industry.

CONCLUSIONS

An environmentally friendly method is proposed, which utilizes paraffin as a reactive solvent to replace CTC, an ODS solvent, for the unified production of CIPP and CP. Microstructure and properties of the products are analyzed by DSC, ATR-FTIR, [.sup.1]H-NMR, and [.sup.13]C-NMR. It is shown that the product quality of CIPP and CP with the present method is comparable with that of the conventional method. Further, the present method is superior to the conventional one for the manufacture of chlorinated polymers in terms of VOC reduction, efficiency of the reactor, operating conditions, and more importantly the ODS abatement.

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Kui-Long Tan, (1), (2) Chun-Xi Li, (1), (2) Ying-Zhou Lu (1), (2) Zi-Hao Wang (2)

(1) State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, 100029 Beijing, China

(2) College of Chemical Engineering, Beijing University of Chemical Technology, 100029 Beijing, China

Correspondence to: Chun-Xi Li; e-mail: liex@mail.buet.edu.cn

DOI 10.1002/pen.21379

Published online in Wiley InterScience (www.interscience.wiley.com).

[C] 2009 Society of Plastics Engineers
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