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Plasticizers in PVC
A Combined IR and GC Approach W. H. Chan Hong Kong Baptist College. 224 Waterloo Road. Kowloon, Hong Kong Although poly(viny1chloride) (PVC) was first prepared in the last century, efforts to replace celluloid by this new material w r e impeded by its thcrmal instability during tho fabrication process. It was Semon's discovery of the plasticization 01' I'VC by ohthalates in 1926 that marked oft the long successful story of the use of PVC plastk. To improve the flow and softness of the plastic, di(2-ethylhexyl) phthalate (DEHP) has gradually emerged as the choice of plasticizer for PVC.' Recently, DEHP has been found to be a human carcinogen by the US Consumer Product Safety Commision.^ Therefore, the content of plasticizer in a consumer plastic product has become a serious concern of our local i n d u ~ t r i e sWe . ~ have therefore devised the following experiment in our instumental analvsis lahoratorv to investigate the plasticizer content in PVC-products. By the end of the experiment, students should he able to distinguish different kinds of plastic consumer products commonly encountered in dailv life. Thev should also aonreciate how the content of aEfects the apia;ent plasticity of the PVC products. Most importantly, they should learn IR and GC analytical techniques through a real-world application, instead of from two separate, unrelated IR and GC experiments that are commonly designed for an instrumental analysis laboratory. In our experiment, IR spectroscopy was used to screen plastic samples for phthalate plasticized PVC products, whereas GC was used to identify the isolated phthalates. The open-ended nature of this study is worthy of mention, too. Students dcked their own plastic samoles and subsequently shared their results cobperatively: Two different isolation methods were provided for different erouns . of students. Comparisons andcontrssts uf these two methods created an atmosphere of lively disrussions. BY using GC uualitative analysis, they might eventually find that their^^^ contain different kinds of phthalates.
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sampling Screening Five suspected PVC samples (e.g., plastic toys, packaging materials, bottles, etc.) were chosen by each group of students for analysis. About 0.05 g of each sample was dissolved in 10 mL of THF. A few drops of the solution were transferred onto a microsco~icslide bv a dis~osahlepipet. T o facilitate the evaporati& of solveni, the film of the solution was irradiated under a lamp. After 10 min, the uolvmer film formed was stripped off a i d mounted in an IR cell holder. Then the IR spectrum of the sample was recorded. Those samples with characteristic PVC absorptions (2950 cm-1,
1450 cm-I, 1285 cm-I, and 1095 em-') and an ester peak a t 1740 cm-', which indicated the presence of phthalate, were selected for further study. Isolation and IR Characterization of Phthalates4 Two methods were assigned to different groups of students for the isolation of phthalates from the PVC samples. (1)Soxhlet extraction. The plasticized PVC sample was cut into small pieces. A weighed sample (-1 g) was extracted with 100 mL of hot ether in a Soxhlet extractor overnight. After removal of solvent by rotary evaporator, the residue, which should predominantly be phthalate, was weighed. (2) Precipitation method. Approximately 1 g of weighed ilasticized PVC sample was cut into small pieces and dissolved in 15 mL of hot THF. After the sample had completely dissolved, 100 mL of methanol was added slowly to precipitate out the PVC. The granular PVC precipitate was separated by filtration. Concentration of the filtrate under reduced pressure gave the phthalate as the residue, which was then weighed.
The weight percentage of plasticizer in PVC was calculated based on the weight of the isolated phthalates from both methods. The completeness of the extraction methods was ensured by the absence of IR ester absorption in the residual PVC. The IR spectra of the isolated phthalates were compared with that of an authentic DEHP sample. Identification of the Phthalates by GC on Their Hydrolyzed Products Approximately 100 mg of the extracted phthalates was refluxed for 3 h in 5 mL of 2N aqueous NaOH solution with 5 mL of ethanol added to facilitate the homogeneity of the reaction. The alcohols released were extracted by three portions of ether (3 X 20 mL). The combined ether solutions were dried (Na?SOd) . . and subiected directlv to GC analvsis. ~,. usin:! pure isimctanol and 2-elhvlh~aanolas the standards. Thc (>(' experiments w r e oerfcrrmell usine a 0.31 mm X 25 m fused silica capillary column (with 0.52 &of cross-linked methyl silicon) installed in Hewlett-Packard 5890A gas chromatograph equipped with flame ionization detector.
' Chem. Eng. News 1984, (June 18). 27. 'Griesemer. R. A. Repon to the US Consumer Product Safety Commssion oy theChromc Hazard Advisory Panel on DEHP. Seotember 1985. Industrial Support Division Health and Safety Standards Circular No. 18/85. issued by the Industry Department of the Hong Kong Government. Crompton, T. R. Chemical Analysis of Additives in Plastics, 2nd ed: Pergamon: Oxford. 1977. Volume 64
Number 10 October 1987
897
Results and Discussion Four of the 10 consumer plastic samples collected by two groups of students were found to be phthalate-plasticized PVC by comparing their IR spectra with that of the authentic PVC sample. A typical IR spectrum of a PVC beach ball sample, together with that of the PVC standard, is shown in Figure 1. The rest of the plastic samples are made from polystyrene, polyethylene, and polypropylene as indicated by their IR spectra. Each group of students used a different isolation technique to analyze the plasticizer content of the plastic. T o reduce the duplication, students selected only three of the four presumed PVC samples for investigation, with two of those in common for the sake of comparison. Both precipitation and extraction methods were satisfactory in isolating phthalate from the PVC products. The completeness of the isolation methods can he ensured by checking the IR of the residual PVC as mentioned ahove. The following results were obtained: Phthalate in PVC SarnDles
Extraction Method Precioitation Method
Beach ball
Ruler bag
30.9% 31.8%
16.0% -a
Soda bonle -a
7.2%
Ruler 4.0% 3.8%
Plasticizer coment of mls sample was not determined by mis goup of studsm
The results of the two methods are quite consistent. The soft beach hall contains the highest percentage of plasticizer whereas the stiff plastic ruler contains the lowest. From the ahove results, students should appreciate the relationship between the content of plasticizer and the softness of the olastic. The hiaher the content of the plasticizer, the softer the material wil he. Identification of the extracted phthalates by IR, though seemingly simple, was found to present difficulties. Phthalates with different alcohol moiety give very similar IR spectra. T o identify the alcohol portion of the phthalate required
898
Journal of Chemlcal Education
Figure 2. Gas chromatqram of hydrolyzed phthalates (detector and in]ector = 250DC, initial temp. = 7 5 % temp. ramp = iO'CImin, finaltemp. = 14OoC, a = isooctanol, b = 2-ethylhexanoi).
the introduction of other analytical methods. Direct application of GC was used to the identification of various phthalates. It was, however, quite difficult to distinguish phthalates of similar structure. A more definite identification of the kind of phthalate~used was ohtilined by hydroly7iny the phthnlate and the runstiruenc nlcohols were then identified t ~ GC. y For instance, thi. retention tirnesoiIlEHPand D101' ~aiisoortylphthalate)were too close ro allow baseline resolution of h e i r GC signals; however, as shown in Figure 2, isooctanol and 2-ethylhexanol resulting from the hydrolysis of DIOP and DEHP, respectively, were easily identified by GC. In some cases, minor unidentified plastic additives with longer retention time were also detected. Results of the exoeriment indicate that DEHP and DIOP are the must common plsstirizers 11scdin PVC formulation. I
