Antioxidants in Plastic An Instrumental Analysis Project Wing Hong Chan, King Sum Lam, and Wai Keung Yu Hong Kong Baptist College, 224 Waterloo Road, Kowloon. Hong Kong Many polymers are subject to oxidation during manufacture and processing and in their end use. In the case of polypropylene (PP. I), C",
I
GCH-CH~+ 1
the tertiary carbons are known to provide a site for oxidative deeradation. In the absence of ~ o l v m e radditives. no PP c o k d withstand a critical produckon step such as high temperature extrusion or outdoor exposure to ultraviolet radiation in end-use service without molecular breakdown. The antioxidants, mainly the hindered phenols such as butylated hydroxytoluene (BHT, 2) and butylated hydroxyanisole
Figure 1. IR
spectrum ol polypropylene.
Three to five suspected PP plastic samples were chosen by each group of students for analysis. About 0.05 g of solid PP samples, each in a whole piece, were pressed into s transparent film 100 pm thick by an IR KBr press. The PP samples were identified by comparing their IR spectra with the authentic PP IR spectrum as shown in Figure 1.
@HA, 3),
I CH,
I
OCH,
impart to PP processing stability and long-range thermal1 photo oxidative endurance. Several experiments have been reported in this Journal'-3 in dealing with the applications of polymer additives. Due t o the importance of the antioxidants in polymer formulation, we have devised the following project in our instrumental analysis laboratory to investigate the antioxidant content in PP products. This oroiect offers valuable learnine,. ex~eriences to stu. dents: (i)'fhrough the sampling process, students have the o~oortunitvto familiarize different kinds of ~ l a s t i consumc e;broducts commonly encountered in the& daily life. (2) Stndenta learn to use the IR s ~ e c t r o s c o ~ iaas c , chromatographic, and UV spectroscopic methods c&plementarily through a real world application. (3) Students are greatly benefited by the open-ended nature of the project. Different groups of students can select their own plastic samples and subsequently share their findings. (4) By comparing the antioxidant content of various PP samples, students may realize the importance of the plastic formulating to its application.
Isolation of the Antloxldant4 Handling Precoutious: Chloroform is a flammable, hazardous chemical. All operations involving the transfer of solutions of chloroform should be carried out in a ventilation hood. About 3-5 g each of PP sample was cut into small pieces and placed in the thimble of a Soxhlet extractor. The antioxidants were isolated by Soxhlet extraction with 150 mL of chloroform for 2 h. The coal extracts were concentrated by a rotary evaporator, and the residue was diluted with chloroform to 25 mL in a 25-mL volumetric flask.
UV Spectroscopic Method The identity of the extracted antioxidants was established by the TLC technique (i.e., comparing with the authentic BHA and BHT standards). The absorbance of the sample extracts was measured in a 10-mm cell over the range 255 nm to 320 nm against a pure chloroform blank in the comparison cell. A calibration graph was prepared using known concentrations (2 to 20 ppm) of the antioxidant in chloroform. The concentration of antioxidant in the chloroform extract of the unknown samnles was determined from an appropriate calibration graph. From this concentration, the wt % of the antioxidant in the samples is calculated.
Gas Chromatographic Method5 Three microliters of the extracted solution was chromatographed using a Hitachi gas chromatograph equipped with a SK-30 column
' Allen, N. S.; Mckeller, J. F. J. Chern. Educ. 1979, 56,273-274.
Sample Screening Hint: In order to facilitate the sample screening process, students were informed that PP is a fairly expensive consumer plastic and its use is confined to some specific applications. In addition to its use as the container for delicate products, due to its excellent temperatureresistance property, PP is selected to make products that require to sterilization at boiling water temperature.
172
Journal of Chemical Education
Caspar, A,; Gillois. J.; Guillerm. G.; Savignac, M.; Vo-Quang,L. J. Chern. Educ. 1986,63,811-812. Chan, W. H. J. Chem. Educ. 1987,64,897-898. Haslam. J.; Willis, H. A.; Squirrel. D. C. M. Identification and Analysis of Plastics. 2nd ed.; Iliffe: London, 1972, p 345. Horwity,W.; Ed. Official Methods of Analysis of the Association of Official Analytical Chemist. 13th ed.; Association of OfficialAnalytical Chemists: Washington, DC, 1980: p 324.
The Antloxldant Content (wt %)
in PP Samples
photographic feeding precooked disposable sweet battle food tray syringe container
samples
film container
UV memod GC method
0.03Ia -
0.085E
-
0.04Ss
-
-
O.03Ss
0.054#
O.OOBB 0.0056
'As BHT. As BHA.
210
280
320
380
nm
WAVELENGTH
Figure 2. me UV spechum of BHT standards.
Figure 3. The calibration curve of BHT.
and e flame ionization detector. During operation the column was
tivity of the antioxidant renders the UV spectroscopic determination highly sensitive. Figure 2 shows the UV spectrum of different BHT standard solutions; i t is obvious that the determination of BHT can he made to part-per-million levels. A typical UV calibration graph for BHT can thus he constructed in Fieure 3. In contrast to the IJV method, the gas chromnuyrnrn of BHT and BHA can be easils resolved bv adiustine the flow rate of the carrier gas and the column iemperature. Under the conditions described in the experimental section, the chromatogram of BHA and BHT exhibits a peak at 4.8 and 5.8 min, respectively. The detection limit hy this method is about 10 ppm, which is comparable with that of the UV method. The antioxidant content of the PP samples determined by the GC and UV method are shown in the table. The antioxidant content in the collected samples was about 0.05 wt %. Except one PP sample that contains mixed antioxidants, BHT was the only antioxidant found in the samvles. This is vrobablv attributable to the cheaver price of BHT. Among all samiles, the highest amount of antioxidant content found in the feedine bottle is easilv rationalized by the fact that i t requires repeated steriliza&on. In short, this proiect students with the opportu- . provides nity of utilizing various instruments to solve real:world problems. In addition to its analytical value, students acquire a great deal of knowledge of polymer chemistry.
maintained at 16U°C while both the injector and the detector were maintained at 240 'C. Calibration graphs for H H A and RHT were prepared using known concentrations of the antioxidant in chloroform. The antioxidant content of the unknown samples was calculated with reference to the calibration graph. Results and Dlscusslon
The vroiect is desiened for two erouws of students to work in parahefand shouG be completed in three 3-h laboratory periods. Initiallv, students should work toeether, accordine to the hint in the experimental, coll&t enough samples for the IR spectroscopic screening process. After seleEting the PP samples, students should be-split into two groups for the antioxidant analysis. One group employs the UV spectroscopic method, whereas the other group utilizes the gas chromatographic method. The UV spectra of BHA and BHT are fairly similar, their absorption maximum appear a t 290 nm and 280 nm, respectivelv. Although a difference of 10 nm in the UV svectrum maybe sufficrently large to distinguish these two common antioxidants. the TLC techniaue can be utilized to establish further the identity of the antioxidant extracts. Eluted with 1:4 ethvl acetate:wetruleum ether, theR,valuen for BHA and BHT are 0.52 and 0.91, respectively. BY using this simple technique, i t is interesting to note that almost all antioxidant extracts are found to be BHT. The large molar ahsorp-
Volume 66
Number 2 February 1989
173
