Anal. Chem. 1997, 69, 618-622
Composition and Microstructure Analysis of Chlorinated Polyethylene by Pyrolysis Gas Chromatography and Pyrolysis Gas Chromatography/Mass Spectrometry Frank Cheng-Yu Wang* and Patrick B. Smith
Analytical Sciences Laboratory, Michigan Division, The Dow Chemical Company, Midland, Michigan 48667
A pyrolysis gas chromatography method was developed to determine the composition and microstructure of chlorinated polyethylene (CPE). This method utilized specific aromatic compounds which were formed through dehydrochlorination of trimers after pyrolysis of CPE polymers at elevated temperatures. The composition and microstructure calculation was based on the difference between the levels of ethylene and vinyl chloride trimers formed. This method is valid for CPE polymers containing between 25 and 48 wt % chlorine. The composition of CPE polymers used in this study was corroborated with 13C-NMR results and the manufacturer’s product specification. Chlorinated polyethylene (CPE) is a polyethylene that has random chlorine substitution. The use and commercialization of halogen substitution (especially chlorine substitution) to improve the toughness and barrier properties, as well as ignition resistance, of long-chain hydrocarbon polymers originated in the United States in the early 1950s. Depending on the degree of chlorination, CPE polymers can have elastomeric and thermoplastic forms which have extraordinary compatibility with a range of other materials. This makes CPE readily adaptable to common compounding and curing techniques. CPE polymers can produce end products that are hard and tough or soft and flexible. Most commercial CPE products contain 25-50 wt % chlorine. CPE is an ignition-resistant polyethylene. It is also used in blends to change the ignition characteristics of other polymers. CPE has no unsaturation in the polymer backbone, giving it excellent ozone and weathering properties. The saturated backbone also results in a temperature stability that allows CPE to perform well continuously at temperatures of 150 °C. CPE can provide satisfactory resistance to most acids, bases, oils, and alcohols. CPE polymers are well known for their impact resistance, abrasion resistance, and resistance to mechanical damage. Because CPE polymers have good heat resistance, chemical resistance, and mechanical properties, their major applications are in automotive hose, tubing, and wire jacketing. Other applications include roofing, gaskets, tank linings, and molded goods. The only disadvantage of CPE is its sensitivity to temperatures above 200 °C. The chlorination reaction of polyethylene can be expressed as
618 Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
where one molecule of chlorine reacts with polyethylene to yield one molecule of hydrogen chloride and one atom of chlorine substituted onto the polymer. This chlorination can be achieved by solution (solvent), aqueous suspension, or fluidized bed processes. The distribution of chlorine atoms in the CPE polymer chain and the properties of CPE are greatly affected by the morphology of the polyethylene used and the accessibility of the polyethylene chain to the chlorination. To better control the chlorination processes, understand the chlorine atom distribution, and develop improved structure/property relationships, it is necessary to have an analytical method that is capable of a quantitative determination of the composition and microstructure in the CPE. The distribution of chlorine atoms in CPE was studied by infrared spectroscopy (IR).1-3 The CH2 bending bands in the 1400 to 1475 cm-1 range were used to determine the proportion of methylene groups centered in -CH2CH2CH2-, -CH2CH2CHCl-, and -CHClCH2CHCl- triads.3 The chlorine atom distribution in CPE has also been intensively investigated by nuclear magnetic resonance spectroscopy (NMR).4-12 There have been numerous structural studies of polymeric materials using dimers and trimers produced by pyrolysis gas chromatography (Py-GC) and pyrolysis gas chromatography/mass spectrometry (Py-GC/MS).13-20 In CPE, the dehydrochlorination reaction is the dominant reaction in pyrolysis. Several diagnostic (1) Quenum, B. M.; Berticat, P.; Vallet, G. Polym. J. 1975, 7 (3), 277. (2) Lindberg, J. J.; Stenman, F.; Laipio, I. J. Polym. Sci., Polym. Symp. 1973, 42 (2), 925. (3) Oswald, H. J.; Kubu, E. T. Soc. Plastics Eng. Trans. 1963, 3, 168. (4) Hoesselbarth, B.; Keller, F.; Findeisen, M. Acta Polym. 1989, 40 (6), 371. (5) Chang, B. H.; Zeigler, R.; Hiltner, A. Polym. Eng. Sci. 1988, 28, 1167. (6) Busch, W.; Kloos, F.; Brandrup, J. Angew. Makromol. 1982, 105, 187. (7) Pinther, P.; Keller, F.; Hartmann, M. Acta Polym. 1980, 31 (5), 299. (8) Keller, F.; Michajlov, M.; Stoeva, S. Acta Polym. 1979, 30 (11), 649. (9) Keller, F. Plaste Kautsch. 1979, 26 (3), 136. (10) Keller, F. Faserforsch. Textiltech. 1978, 29 (2), 133. (11) Abu-Isa, I. A.; Myers, M. E., Jr. J. Polym. Sci., Polym. Chem. Ed. 1973, 11 (1), 225. (12) Saito, T.; Matsumura, Y.; Hayashi, S. Polym. J. 1970, 1, 639. (13) Kalal, J.; Zachoval. J.; Kubat. J.; Svec. F. J. Anal. Appl. Pyrol. 1979, 1, 143157. (14) Shimono, T.; Tanaka. M.; Shono. T. J. Anal. Appl. Pyrol. 1979, 1, 77-84. (15) Alajberg, A.; Arpino. P.; Deur-Siftar. D.; Guiochon. G. J. Anal. Appl. Pyrol. 1980, 1, 203-212. (16) Wang, F. C.-Y.; Gerhart. B. B.; Smith. P. B. Anal. Chem. 1995, 67, 353640. (17) Wang, F. C.-Y.; Smith. P. B. Anal. Chem. 1996, 68, 425-30. (18) Wang. F. C.-Y.; Smith. P. B. Anal. Chem. 1996, 68, 3033-37. (19) Tsuge, S.; Ohtani. H.; Takeuchi, T. Macromolecules 1969, 2, 200. (20) Tsuge, S.; Ohtani. H. Pyrolysis Gas Chromatography of High Polymers Fundamentals and Data Compilation; Techno-System: Tokyo, 1989; p 178. S0003-2700(96)00947-X CCC: $14.00
© 1997 American Chemical Society
was heated to 700 °C at 20 °C/ms and held at 700 °C for a 20 s interval. The pyrolysis products were split in the 250 °C injection port, with 10 psi head pressure, 30:1 split ratio, separated on a fused-silica capillary column (J & W Scientific DB-5, 30 m × 0.25 mm i.d., 1.0 µm film) using a linear temperature program (35 °C/ 10 min, 10 °C/min to 200 °C/4 min, then 20 °C/min ramp to 320°C/23.5 min), and detected using a FID. In the PY-GC/MS system, the FID was replaced with a VG Trio-1 mass spectrometer. The output from the GC was transferred through a transfer line (280 °C) to the ion source of the mass spectrometer. An electron ionization mass spectrum was obtained every second over the mass range of 29-500 Da.
Figure 1. Calibration curve for the set vs actual temperature of the pyroprobe.
pyrolysis products such as benzene, toluene, styrene, and naphthalene have been observed. The amount of these aromatic compounds formed directly reflects the concentration of chlorine atoms and their distribution in the CPE. In this study, both Py-GC and Py-GC/MS techniques in conjunction with four commercially available CPE samples (chlorine content 25%, 36%, 42%, and 48% by weight) were used to develop a method to investigate the composition and microstructure of CPE. The major pyrolysis products of CPE have been identified by Py-GC/MS. The composition and structure calculations were based on those degraded trimer peak intensities obtained by Py-GC. This Py-GC method can be used to quantitatively determine the chlorine content in CPE. The same method can also explore the microstructure through number-average sequence length of ethylene and vinyl chloride monomers. Other structure-related terms, such as the percentage of grouped vinyl chloride monomers, i.e., the percentage of chlorine atoms structured as poly(vinyl chloride)-like (PVC-like) structures, can also be calculated. EXPERIMENTAL SECTION Sample Preparation. All four CPE samples (chlorine content 48%, 42%, 36%, and 25% by weight), labeled as samples A, B, C, and D, and a PVC polymer were purchased (Catalog Nos. 184, 185, 186, 327, and 355) from Scientific Polymer Products Inc. (Ontario, NY). All the samples were in the powder form and were used without further purification. Py-GC and Py-GC/MS Conditions. Samples of CPE polymer powder (∼2.5 mg) were put into a quartz tube. The quartz tube was equilibrated for 5 min in a 180 °C interface connected to the injection port of a Hewlett Packard Model 5890 gas chromatograph equipped with a flame ionization detector (FID). Samples were pyrolyzed (CDS 120 Pyroprobe Pt coil) at a set temperature of 700 °C. The calibration curve for the set vs actual temperature of the pyroprobe is plotted in Figure 1. The 700 °C (actual 690 °C) pyrolysis temperature was chosen because the yield of trimers of CPE was higher at that temperature. The coil
RESULTS AND DISCUSSION The reproducibility of pyrolysis data is always a concern when applying the technique to any kind of quantitative study. The reproducibility of pyrolysis results is tested with five consecutive runs of 48 wt % of CPE polymers. The average of all peaks which resulted from the pyrolysis of trimers in the polymer showed a relative standard deviation below 3%, which demonstrates the reliability of the pyrolysis method. The sample stability in the pyroprobe before pyrolysis is very important in a pyrolysis investigation. Pyrolysis of a CPE polymer was performed on the dried powder. The powder was heated in the pyrolysis chamber at 180 °C with helium carrier flow for 5 min. A volatility experiment showed there were no detectable materials released during this period. Our studies also showed that CPE will start evolving hydrogen chloride above 250 °C as a result of the dehydrochlorination reaction. The rate of hydrogen chloride evolution increases with temperature. This study showed that there is no significant effect on the composition for the first 6 h. Figure 2 shows the pyrograms of all four CPE samples. The characteristic components of pyrolyzed CPE are benzene, toluene, styrene, naphthalene,19,20 and a series of aliphatic hydrocarbons (sets of dialkenes, alkenes, and alkanes). The aromatic hydrocarbons are the result of chain scission followed by dehydrochlorination of the chlorinated part of the polymer chain. They are also the major pyrolysis products of poly(vinyl chloride) (PVC).20 The aliphatic hydrocarbons result from the pyrolysis of the polyethylene (PE) portion of the polymer chain. If a CPE pyrogram is displayed together with those of PE and PVC, as in Figure 3, it is clear that the CPE pyrogram is almost a combination of the PE and PVC pyrograms. The relative intensities of aromatic hydrocarbon peaks and aliphatic hydrocarbon peaks in the CPE pyrogram depend on the degree of chlorination and the distribution of chlorine atoms. One way to conceive of the distribution of chlorine atoms in the polymer is to view it as the combination of monomers of ethylene, vinyl chloride, 1,2-dichloroethylene, 1,1-dichloethylene, 1,1,2-trichloroethylene, and tetrachlorethylene. When the CPE contains no more than 50 wt % chlorine, the polymer can be considered as a copolymer of ethylene and vinyl chloride.19 A few 1,2-dichloroethylene, 1,1-dichloethylene, and 1,1,2-trichloroethylene monomer-like units may exist in the CPE with 48 wt % of chlorine, but the level is so small that they do not have a significant effect on this assumption. The composition and microstructure calculation is based on the triad distribution of CPE pyrolysis fragments. If the CPE is considered as a copolymer of ethylene (E) and vinyl chloride (V), the possible triad combinations of these two monomers are the Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
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Figure 3. Pyrograms of PVC, CPE, and PE. The pyrogram of CPE (center) can be viewed as the combination of the pyrogram of PVC (top) and the pyrogram of PE (bottom). Figure 2. Pyrogram of CPE polymers. The pyrogram of the 25%Cl polymer (top) possesed considerable PE-like characteristics. As the Cl level increased, the pyrogram gradually acquired more PVC-like characteristics, such as that of the 48% Cl polymer (bottom).
ethylene triad (labeled EEE), the triad of two ethylene and one vinyl chloride (labeled EEV, VEE, and EVE), the triad of one ethylene and two vinyl chloride (labeled VVE, EVV, and VEV), and the vinyl chloride triad (labeled VVV). The pyrolysis products of these four kinds of triads are 1-hexene, cyclohexene, 1,3cyclohexadiene, and benzene, respectively. The number-average sequence length (NASL) of both monomers can be calculated on the basis of the following formulas:21
N(E) )
N(V) )
NEEE + NVEE+EEV + NVEV (1/2)NVEE+EEV + NVEV NVVV + NEVV+VVE + NEVE (1/2)NEVV+VVE + NEVE
(1)
(2)
where N(E) and N(V) are the number-average sequence lengths of monomers E and V. NEEE, NEEV+VEE, NVEV, NEVE, NEVV+VVE, and NVVV are the experimentally derived six distinguishable triad molar fractions or number of molecules. From the formulas above, two major factors dominate the relationship between triad distribution and trimer production. The first is the pyrolysis efficiency, which (21) Randall, J. C. Polymer Sequence Determination; Academic Press: New York, 1977; pp 41-69.
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represents the probability/efficiency of breakdown of a specific triad configuration to produce the corresponding trimer. The second is the detection efficiency, resulting from variable FID responses for the trimers. These two factors cannot be separated in most cases of composition and microstructure determination. The relationship between trimer production and triad distribution can be expressed as
experimental trimer peak intensity × Kn f triad distribution in the polymer where Kn is equal to the combination of pyrolysis efficiency and detection efficiency. Because the trimers EEV, VEE, and EVE all form cyclohexene, an assumption (the first assumption) needs to be made that Kn is the same for EEV, VEE, and EVE. The same assumption also needs to be made for 1,3-cyclohexadiene with the trimers of VVE, EVV, and VEV. The triad distribution in the polymer and the trimer peak intensities from pyrolysis can be related by four proportionality factors, K1, K2, K3, and K4, which correspond to 1-hexene, cyclohexene, 1,3-cyclohexadiene, and benzene. The K1, K2, K3, and K4 values can be determined by analyzing four different known compositions of CPE polymer standards. The Kn values used in this calculation are K1 ) 1.0, K2 ) 1.0, K3 ) 1.0, and K4 ) 0.4. The determination of number-average sequence lengths for ethylene and vinyl chloride in CPE polymers is challenging because six of eight trimers are not resolved by pyrolysis gas
Table 1. Composition and Microstructure Values Calculated from Pyrolysis Peak Intensities of Four Different Compositions of CPE Polymers and One Blended CPE/PVC Polymera sample
normalized data
NASL composition GNASL
PVC-like Py-GC result NMR result manufacturer
hexene (EEE) cyclohexene (EEV) benzene (VVV) 1,3-cyclohexadiene (VVE) N(E) N(V) mol % E mol % V GN(E) GN(V) %G(E) %G(V) %Cl/total Cl wt % Cl wt % Cl wt % Cl
A
B
C
D
E
0.238 0.076 0.523 0.162 4.31 8.25 34 66 11.36 11.65 84 96 80 46 47 48
0.333 0.070 0.428 0.169 5.48 7.10 44 56 16.30 9.62 87 96 76 42 41 42
0.494 0.080 0.220 0.206 6.47 4.03 62 38 20.45 5.21 89 93 57 33 34 36
0.657 0.060 0.119 0.164 10.06 3.32 75 25 34.80 4.17 93 92 48 24 26 25
0.136 0.053 0.716 0.094 4.14 16.24 20 80 9.65 24.93 85 98 90 51 49
a NASL ) number average sequence length; GNASL ) grouped number average sequence length; GN(E) ) grouped number-average sequence length, ethylene monomer; GN(V) ) grouped number-average sequence length, vinyl chloride monomer; %G(E) ) percent grouped monomers, ethylene monomer; %G(V) ) percent grouped monomers, vinyl chloride monomer; and PVC-like ) the percentage of chlorine atoms in PVC-like structures.
chromatography. From a theoretical point of view, there is no way to know how much cyclohexene peak intensity is contributed from the EEV and VEE or from EVE triads. The same situation exists for the 1,3-cyclohexadiene peak from the VVE, EVV, and VEV triads. To utilize eqs 1 and 2, another (the second) assumption needs to be made to obtain all six terms of triad intensities. The second assumption results from the mechanism of the halogenation of aliphatic hydrocarbons.22 In the chlorination of polyethylene, “the presence of a chlorine on a carbon reduces the substitution rate at that carbon and at the neighboring carbon”.23 In CPE polymers, the benzene, toluene, styrene, and naphthalene peak intensities increased when the chlorine level in the CPE increased. This indicated that most of the chlorinated polyethylene chain possessed a PVC-like structure. A hypothesis can be made to explain this phenomenon: there is a strong tendency for chlorine atom substitution to occur in such a way as to form PVC-like structures. If this hypothesis is true, the six unresolved trimer terms can be separated. The triad distribution can be expressed as follows:
NEEE ) normalized/corrected 1-hexene peak intensity NEEV + NVEE )
These terms are subsequently used to determine the numberaverage sequence length. The number-average sequence can be further used to calculate the composition. The formula related to the calculation can be expressed as
mol % E )
N(E) × 100% N(E) + N(V)
(3)
mol % V )
N(V) × 100% N(E) + N(V)
(4)
There are other important microstructure-related terms that can be calculated from the number-average sequence length. These terms are the percentage of grouped monomers and the numberaverage sequence length of grouped monomers. For vinyl chloride (V) monomer in the CPE, these two terms can be expressed as follows:
% of grouped V ) %G(V) ) (NVVV + NEVV+VVE) × 100% GN(V) )
NVVV + NEVV+VVE (1/2)NEVV+VVE
(5) (6)
(2/3)normalized/corrected cyclohexene peak intensity NEVE ) (1/3)normalized/corrected cyclohexene peak intensity NEVV + NVVE ) (2/3)normalized/ corrected 1,3-cyclohexadiene peak intensity NVEV ) (1/3)normalized/ corrected 1,3-cyclohexadiene peak intensity NVVV ) normalized/corrected benzene peak intensity
In the case of CPE, it is helpful to define another term, called the percentage of chlorine atoms in PVC-like structures, as the percentage of chlorine atoms which exist in the polymer chain as three or more vinyl chloride monomer units polymerized together. This term can be used to indicate the degree/ percentage of crystalline (PE-like) or amorphous (PVC-like) structures. The percentage of chlorine atoms in PVC-like structures can be expressed as (22) Potter, A.; Tedder, J. M. J. Chem. Soc., Perkin Trans. 2 1982, 12, 1689. (23) Marchand, G. R. Chlorinated Polyethylene. In Polymeric Materials Encylopedia; Salamone, J. C., Ed.; CRC Press Inc.: New York, 1996; p 1234.
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% Cl in PVC-like )
NVVV 2
NVVV + ( /3)NEVV + (1/3)NEEV
× 100% (7)
Table 1 lists all composition and microstructure terms calculated from this Py-GC method plus the composition from manufacturer product specifications and 13C-NMR results as reference. The only terms for which a direct comparison can be made between the Py-GC results, 13C-NMR results, and manufacturer product specifications is the weight percent chlorine in the sample. These values are in excellent agreement. The chlorine level determined by this Py-GC method closely matched that given by the manufacturer product specifications and 13C-NMR results for all CPE samples evaluated. The numberaverage sequence length of ethylene and vinyl chloride calculated from this method also reasonably corresponds to the composition of CPE. However, the best way to prove this Py-GC method capable of defining the microstructure of the polymers is to analyze two samples which are similar in composition but different in microstructures. To do this, a blend of CPE/PVC (50 wt % of sample B and 50 wt % of PVC) sample was prepared (sample E). The total chlorine content in this blended sample is 49 wt %, which is very close to that of sample A (48 wt %), but obviously there is a significant difference in the microstructure of these samples. Figure 4 shows the pyrograms of sample A and sample E. Examination of the Py-GC results for samples A and E in Table 1 shows that they are different in almost every term of this calculation. The number-average sequence length of vinyl chloride is almost double in sample E, indicating longer vinyl chloride repeat unit blocks. Even through the percentage of grouped (two or more units together) vinyl chloride units is similar in both samples A and E, the number-average sequence length of grouped vinyl chloride units shows that the continuity of the vinyl chloride unit in sample E is much longer than that in sample A. The percentage of PVC-like structure values clearly showed that the arrangement of the vinyl chloride repeat units of sample E is closer to a PVC structure than sample A. These results demonstrate that this Py-GC method is capable of obtaining not only the composition but also the microstructural information for CPE. CONCLUSIONS A Py-GC method has been developed that can be used to determine both the composition and microstructure of chlorinated polyethylene (CPE). The determination of the composition and structure of CPE is achieved through the detection of the pyrolysis trimers and the application of two critical assumptions for the polymer system. The chlorine contents of all polymers tested are in excellent agreement with NMR measurements and the product composition specification. To distinguish the structural differ-
622 Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
Figure 4. Pyrograms of sample A (bottom; 48 wt % Cl) and sample E (top; 50 wt %/50 wt % blend of sample B and PVC).
ences of polymers with the same composition, there are several structure-related terms that have been derived to reveal the structural difference, such as the percent of grouped monomers, the number-average sequence length of grouped monomers, and the percentage of chlorine atoms in PVC-like structures. The composition and microstructure of CPE is a direct reflection of the CPE preparation method and the degree of chlorination. In other words, the composition and microstructure of CPE can be used to rationalize the physical-mechanical properties obtained from different chlorination processes. This method extends the capabilities of pyrolysis from the quantitative and structural study of copolymer systems into the realm of chemically modified homopolymers. ACKNOWLEDGMENT The authors thank Dr. G. R. Marchand for his helpful suggestions about the mechanism and reactivity of the chlorine substitution reaction in CPE. Received for review September 17, 1996. December 2, 1996.X
Accepted
AC960947C X
Abstract published in Advance ACS Abstracts, January 15, 1997.
