The Dehydrochlorination of PVC An Introductory Experiment in Gravimetric Analysis Hugh D. ~ u r r o w s ' Departamento d e Quimica, Universidade d e Coimbra, 3049 Coimbra, Portugal Henry A. Ellis Chemistry Department, University of the West Indies, Mona, St. Andrews, Jamaica, W.1 Christopher A. Odilora Department of Chemistry, Bendei, State University, P.M.B. 14. Ekpoma, Bendel State, Nigeria Experiments i n gravimetric analysis a t t h e general chemistry level acquaint students with the application of reaction stoichiometry in quantitative analysis and provide oractice in the techniaues of weizhinz. .. .. Common techniques involve either precipit:ition ofspec~esfrom solution, followed b\. weichinc of tht. dried preripitnte, or menaurement of thk mass loss of a substance that hasundergone a thermal decomposition (thermogravimetric analysis). Although thermogravimetric analysis (TGA) is often associated with relatively sophisticated thermobalances, the basic application requires just a n analytical balance, a system that undergoes a fairly reproducible thermal degradation, and a controlled heating of samples. This simplicity has considerable advantages, since, apart from the analytical balance, the necessary apparatus i s cheap, and i n addition, t h e experiment can be carried out easily within a normal laboratory period. A good experiment of this type involves t h e dehydration of BaClz.2Hz0 a n d other salt hydrates ( 1 , Z ) . However, although this produces good results, and fulfills the above criteria, students often do not r e a d favorably, because of supposed lack of relevance. We have been involved in teaching experimental chemistry, to large numbers of first-year students in science, engineering, agriculture, etc., who possess widely valying educational backgrounds, many with no practical experience i n chemistry. For these, we have derived a n experiment i n gravimetric analysis which is cheap, relevant, easy to carry out, and yet capable of producinggood experimental results. This involves the thermal degradation of poly (vinyl chloride) (PVC). PVC: Origins, Applications, and Thermal Behavior (3) PVC is one of the most widely used plastics and finds applications in areas a s diverse as water pipes, floor and roof tiles, electrical plugs and cables, records, bottles, packing films and sheet, etc. The monomer, vinyl chloride, was first prepared by Liebig i n 1835 via the dehydrochlorination of ethylene dichloride CH,CICH,CI
KOHIethanol solution
,CH,=CHCI .
(1)
while its polymerization was first noted in 1872 by Baumann, following observations on the effect of sunlight on the monomer. Klatte (1912) found that a n economically viable route t o t h e monomer involved reaction between acetylene and hydrogen chloride. (2) HCSH + HCI + CH,CHCI '~ddressto which correspondence shold be addressed
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Journal of Chemical Education
However, subsequent routes to polymerization favored chemical initiators, such a s organic peroxides, CH2CHCI + initiator -t -(CH2CHCI),-
(3)
Pure PVC is a hard, brittle solid, and its commercial success stems largely from the discovery by Semon in 1926 that compounds such as dialkyl phthalates made the polymer soft and plastic, so that i t can be moulded into various shapes. I t is now one of the most important thermoplastics ( 3 , 4 ) .However, i t suffers two serious drawbacks for commercial usage. Firstly, i t is unstable with respect to both heat and light, and i t degrades a t relatively low temperatures by dehydrochlorination ICH2CHCI),-
heat
ICH=CH),-
+ HC1
(4)
Apart from the release of the potentially toxic HC1, this leads to coloration and loss of mechanical properties. The actual temperature for onset of decomposition depends upon many factors but is usually lower than the temperatures of 150-200 OC normally used for thermal processing. The second problem is that both the monomer (vinyl chloride) and some of the olasticizers used with PVC are carcinogens, while the polymer itselfmay be a health hazard This has led to calls lo rcstrict it.; auolicution and uswc. .. However, its good material properties a t room temperature and its relativelv low cost have meant that PVC i s still r l major polymer. In fact, t h dtm:~nd ~ for it is continuing to in incn:aic, and in 1 9 W sale.; for PVC and its c~~oolsmrrs the united States alone were about 4.5 x lo9 kg (51,making i t the second most important synthetic polymer, after polyethylene. To combat its thermal instability, a number of good thermal stabilizers have been introduced. Commercial PVC products all now contain such stabilizers, often in combinations of two or more compounds which have a combined synergistic effect. We will not attempt to discuss this stabilization but will concentrate on dehydrochlorination. The steps occurring upon thermal decomposition of PVC can be seen most clearly by TGA with a n automatic thermohalance, where the sample is heated a t a constant rate, and the weight loss is measured a s a function of temperature. A good description of the technique is given elsewhere (8).I n this method, weight loss is generally observed over a fairly wide temperature range, which is governed both by the decomposition kinetics and the rate of heating, of the sample. Some of our results (9)for the TGAof purified PVC i n a nitrogen atmosphere are shown in the figure. (The PVC was purified by dissolving in tetrahydrofuran and precipitating with methanol). At the heating rates used, the first step is
observed between about 600 and 800 K, and the mass loss corresponds to the evolution of one hydrogen chloride molecule per CH2CHC1- unit. Qualitative confirmation of the dehydrochorination comes from passing the evolved gas into water, noting the decrease i n pH resulting from hydrochloric acid formation, or analyzing for Cl-by AgCl formation on addition of silver nitrate solution. We have carried out more quantitative studies by passing the evolved gas from the TGA experiment into water, and measuring the chloride ion formation a s a function of PVC sample temperature, using an iou-selective electrode. These results are also included i n the figure, and closely follow the weight loss data. An alternative quantitative method we have used involves titration of t h e hydrochloric acid produced with standard sodium hydroxide solution. The percentage of acid produced i s i n good agreement with the weight loss and chloride ion data. Above 800 K, further weight loss is observed. This corresponds to t h e overall breakdown of the polymer chain. In this experiment we will just concentrate on the dehydrochlorination. Before describing the gravimetric procedure, some brief qualitative tests will be given to show students some of the common plastics where PVC, or related chlorinated copolymers, are used. A recent article in this Journal has described TEMPERATURE K related qualitative tests using combustion Solid line represents thermogram observed upon heating purified poly (vinyl chloride) in a and density measurements for the identifi- nitrogen atmosphere on a themobalance: heating rate: 20 K min-'. Circles represent chlocation of a number of polymers (10).Practi- ride ion concentration detected by an ion-selectiveelectrode upon passing the evolved gas cal classes combining these experiments from the TGA experiment into water. with those described i n the present study would provide a valuable way of introducing mation, while the change in color of the litmus paper supGeneral Chemistry students to the important area of polyports this. mer chemistry. Repeat with various samples, including plastic bottles Exoerimental Section (for example those used for cooking oil), plastic drain pipe, records, plastic food wrappings, bags, etc. Note from the Safety Precautions production of HCI those samples t h a t contain PVC, or Gloves should be worn for all manipulations of PVC, far some related chlorinated polymer. Althoush the experiit has been implicated as a carcinogen and also can ment is designed for a r e ~ a t i v &unsophist&ated ~ general cause allergic dermatitis. chemistr/ laborator/, if a simple IR spectrometer is availHydrogen chloride is a powerful irritant. able, i t is-useful to run the spe&ra of some of these samples All PVC degradation studies should be (preferably as films), and compare them with the literacarried out in eficient fume hoods. ture spectrum for PVC (4). As this experiment is designed for relatively inexperiGravirnetric Analysis of the Chlorine Content of PVC enced students, i t does provide a good opportunity for discussing laboratory safety. I t i s imperative before starting A clean porcelain crucible, with lid, is dried to constant these experiments that students are aware of the potential weight in a n oven a t 150 "C, allowed to cool in a desiccator, hazards with both poly (vinyl chloride) and hydrogen chloand-then weighed on an analytical balance. About 0.5 g of ride (11). PVC is a very widely used polymer. We have used PVC powder is introduced and the sample weighed. The Sigma low molecular weight PVC, which is a powder, and sample is heated in the crucible a t about 250-300 "C until is easily handled. evolution of HC1 starts. Great care should be taken not to heat samples to excessively high temperatures. Heating is Tests for the Presence of PVC in Plastic Materials continued for about another five minutes, and then the These depend upon the detection of HC1 evolved in the sample i s allowed to cool to room temperature in a desiccadehydrochlorination. Introduce approximately 0.1 g of tor. The sample is reweighed, and the heating procedure PVC powder into a test tube, heat strongly with a Bunsen repeated until constant mass i s observed between one burner under a fume hood, and test the gas liberated value and the next. From the mass of the crucible (m,), the with: crucible with PVC (me)and the mass of the crucible and decomposition (mJ, the percentage of HC1, and 1. moist blue litmus . o a.m r hence the ~ e r c e n t a a e of chlorine in the PVC can be calcu2. a few drops of concentrated NH3 solution on a glass stopper. lated: The observation of the characteristic white fumes of Percentage of CI = 35.5136.5 x (mP- md)/(mp- m,) x 100 NH4C1 in the latter case gives good evidence for HC1 for-
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Number 5 May 1995
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Because of cost limitations with large numbers of students, we normally have used Bunsen burners for heating. Students must take care to obtain reasonable temperature control, and it may be preferable to use sand baths. However, reasonable electrical heating systems, such a s muffle furnaces, do provide greater rigor and temperature control. Although the method is very simple, the experimental results are surprisingly good, and analysis of data from -200 students gave mean (59.3 i5.6%) and median (60%) values which are close to the theoretical value (58.3 %) for the weight loss of HCI from PVC. I t is worth noting that this was the first experiment that any of these students had done in gravimetric analysis, and that many had only done five or six chemistry experiments before this. We feel, therefore, that this does fulfill the necessary attributes for an introductory experiment in gravimetric analysis. I t is worthwhile pointing out two oversimplifications. Firstly, we have implied that the PVC has a pure head-totail structure of the vinyl chloride units. If this were so, the polymer would be expected to be stable to much higher temperatures than observed experimentally. I t is generally believed that the thermal instability stems from stmctural abnormalities such a s branching, head-to-head sections, choroallyl groups, etc. Secondly, there is a tendency to get weight losses slightly higher than the theoretical one for loss of HCI. In part, this is due to inexperience and a tendency to overheat samples. However, loss of hydrogen chloride is not the only process, and there is a small sys-
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Journal of Chemical Education
tematic error resulting from competing thermal breakdown pathways, such a s loss of benzene, or other organic molecules (12).Nevertheless, these reactions are much less important than the dehydrochloriuation, and the reproducibility of this experiment supports its value for introducing general ideas about gravimetric analysis. Acknowledgment
We are grateful to the Quimica Laboratorial students and staff in Coimbra for trying out these experiments. H. D. B. thanks INIC for financial support. We also express our sincere gratitude to the Chemistry Department, Obafemi Awolowo University, Ile-Ife, Nigeria, where the preliminary studies on this work were carried out. Literature Cited
vogei's 7kzfbook 0iQ"onltatim how,,"icAmly*k revised by Bass&, J.; Denney: R. C..;Jeffen, G.H.; Mendhsm J.;4th ed.;Langnan: London; 1978: p 432. 2. Vineent,A.Educ.Cham. 1981,28,39. 1.
3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Stinaon, S. 0 . C k m . ondEng. Ne~s June 18, 1984.21. Chan. W. H.J Chem Educ 1987.64.897. C h ~ m and . Ens News June 28,1993.44. Wypyeh. J. Polyvinyl Chlorida Sfobilizoflon; Elseuier: Amsterdam: 1986. Miehel, A,; n a n van Hoang: Guyot, A. Stabilization ondD8grdoflon ofPdymrs: All-, D. L., Hawkins, W L., Eds; A.C.S. Advances in Chemisly Seties: 1978. 169, 386. Pope, M. I.; Judd, M. D. Educ Chem 1911.8.89. Odilors, C. A. M S c . Thesis; Univemityof Ife, 1983. Blumberg,A.A.J Ckm. Educ 1993.70.399, S a , A 1.DangemvsPm~~~soflndvsftiaiMclferimb 5th ed.; Van Noatrand Fainhold: New York, 1979: pp 492,728. See, for example, Matausska, K: Sugirnoto,Y;Murakemi, I. Polymer Comm 1985, 26, 371.
