The minimum time required for complete diffusion of silicon was not evaluated. It was convenient in this laboratory to have the diffusion period cover between 16 and 20 hours: which was overnight, and the results of Tables I, 11, and I11 represent these time periods. I t is not believed that the addition of tartaric acid in the color development of silicon in this procedure is necessary because little or no phosphate is believed to ‘diffuse into the sodium hydroxide portion. However, tartaric acid was added throughout these determinations as a safety feature. S o study was made, a t this time, to ascertain the maxiinurn amounts of SiF4 t,hat, can be diffused and collected by the amounts of S a O H t’hat are used throughout this procedure. Likewise, no study was undertaken a t this time to
determine silicon in organic materials; however, since fluoride is determined using diffusion procedures ( I I ) in organics, it would seein likely that silicon in organosilicons known as silicones, silanes, or siloxanes could be determined by this procedure. LITERATURE CITED
(1) AATlll Methods, “Chemical Analysis of Metals,” p. 368 (1956). (2) Carlson, A. B., Banks, C. V.,ANAL. CHEM.24, 472 (1952). (3) Ilozinel, Charles >I., “Modern IIethods of Analysis of Copper and Its Alloys,” Elsevier, New York, 1963. (4) Fowler, 11. >I., Industrial Eng. Chem., Anal. Ed. 4, 382 (1932). (5) Hill, U. T., ANAL. CHEM.21, 589 (1949). (6) Hoffman, J. E., Lundell, G. E. F., J . Res. 2Yatl. Bur. of Std. 3 , 581 (1929). ( 7 ) Honda, 31. J., Chem. SOC.(Japan) 70, 103 (1949). [8) King, E. J., Stacy, B. I)., Holt, P. F.,
Yates, 1). X, Pickles, D., Analyst 80, 441 (1955). ( 9 ) Knudson, H. T.,Juday, C., Meloche, V. LV., Ind. Eng. Chem., Anal. Ed. 12, 270 (1‘340). (10) Lundell, G. E. F., Hoffman, J. 1.1 “Outlines of 1Iethods of Chemica iinalysis,” Wiley, New York, 1938. (11) Rowley, R. J., Farrah, G. H., Am. Ind. Hyg. dssoc. J . 23, 314 (1962). (12) Shell, H. R., ANAL.CHEM.27, 2006 (1955). (13) Shell, H. R., Craig, R. L., “Synthetic Mica Investigation, VII, Chern. Anal. and Calculation to Unit Formula of Fluorsilicates,” C. S. Bur. Mines Rept. Invest. 5158 (1956). (14) Shell, H. R., Martin, G. W., V. S . Bur. Afines R e p t . Invest. 5557 (1959). (15) Vail, J. G., “Soluble Silicates,” Vol. 1, p. 40, Reinhold, New York, 1952. (16) Woods, J. T., XIellon, l i . G., Ind. Eng. Chem., .4nal. Ed. 13, 760 (1940). RECEIVEDfor review July Accepted August 18, 1964.
1,
1964.
Determination of Residual Monomers and Other Volatile Components in Styrene Based Polymers by Gas Chromatography PETER SHAPRAS and GEORGE C. CLAVER Plastics Division, Research Department, Monsanto Co., Springfield, Mass.
b A gas chromatographic method was developed for the direct determination of residual monomers, nonpolymerizable volatiles, and volatile additives in styrene based polymers. The method i s simple, highly sensitive, and i s applicable to many polymeric systems. The method i s based on the direct injection of solutions or dispersions of polymers in N,N’-dimethylformamide (DMF)using toluene as an internal standard. Additives, pigments, and stabilizers do not interfere in the monomer determination.
K
of residual monomer levels in polymeric systems is of great interest in the plastics industry. Fabricating, strength, and permanence properties, as well as taste and odor of certain foods in contact with the polymer, may be affected. In recent years residual monomer levels in polymers deci eased because of improved polymerization technology. Previouqly used methods, such as titration of double bonds (4,5 ) , or ultraviolet spectrophotometric methods ( I ) , are not e a d y applicable to the determination of monomeis at low concentrations The determination of such nionoiners in pigmented, stabilized, or partially soluble polyiners is very lenqthy and tediouq. The determination of monoNOWLEIIGL
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ANALYTICAL CHEMISTRY
mers in copolymers and terpolymers presents an even more difficult, problem. Polarography has been used for the determination of acrylonitrile (S). The gas chromatographic determination of residual monomers in latex systems has been reported ( 2 , 6-8). In this work, a gas chromatographic method was developed for the determination of residual monomers and other volatile components in styrenebased polymers. N,,V’-dimethylformamide (DMF) was chosen as solvent because it interferes least with the direct determination of monomers of interest. Monomers-such as acrylonitrile, butadiene, styrene, alpha-methylstyrene, and cresylic additives-were determined to parts per million. EXPERIMENTAL
h IVilkens Model 600 HY-Fi gas chromatograph equipped with straight bore injection block and hydrogen flame detector is used. T h e gas chromatograms are recorded on 0-1 mv. f3rown strip chart recorder. Two columns in series are used: a-foot, ‘/*-inch 0.d. 20% Tween 81, followed by 10-foot, ‘V8-inch o.d. 10% Resoflex 446 on Chronisorb W! 30 to 60 mesh. Sitrogen is used as the carrier gas with a flow rate of 30 cc./ minute measured with floivniet,er at room temperature. The columns are held at 120’ C. and the injection block Apparatus.
held at 210” C. 1 hydrogen flow rate of 23 cc./niinute is used for the detector. The detector is operated a t a n impedance of IOg ohms, a n output sensitivity of 1x and attenuated between 1 x and 128 x , depending on monomer concentration. ’The sample as a solution or dispersion (rubber-containing polymer^ form dispersions rather than solutions) is injected with a 50-pl. syringe (Hamilton S-705). Reagents. A\crylonitrile, 99.8%, commercial grade; >tyrene, 99.8%; alpha-methylstyrene, commercial grade; ethylbenzene, reagent grade; toluene, reagent grade; and S,S’dimethylformamide (DMF), technical grade. Commercial DMF contains low boiling point impurities that must be removed. This can easily be done by distillation. An 85 to 90% center cut through la 15-inch S‘igreux column is usually found to be chromatographically clean. IS
Table I. Relative Retention Data
Compound Butadiene Acrylonitrile Toluene Ethylbenzene Styrene D Yi I’
Relative retention times (toluene = 1) 0 40 0 76 1 00 1 40 2 25 3 75
Pol\ mer
Pol) s t j iene ~t\iene-acr~lonitrile cwpol>iner ht\ lene - x r ) lonitrile butadiene tei poll mer
Table II. Weight Per Cent Volatiles -4crj lonitrile EthSl Benzene Added Found Added F o u n r 0 087 0 084 f 0 003 0 043 0 043 f 0 002 0 043 0 046 f 0 003 0 008 0 0074 f 0 0003 0 069 0 069 f 0 002
Added 0 183 0 091 0 045 0 091
Styrene____ Found 0 178 f 0 007 0 090 f 0 003 0 047 f 0 003 0 093 f 0 008
0 024 0 008
0 137 0 064
0 140 f 0 004 0 068 f 0 007
Preparation of hllonomer ContainPCRIFICAing Polymer Standards. TIOX O F P O L Y M E HPolymers .~. are dissolved in chloroform a n d precipit a t e d with methanol while mixing in a high-speed blender. T h e precipitated polymers are dried a t 60' C. for 2 1 hours in a vacuum oven. T h e purity of polymers used for standardization is checked by t h e gas chromatographic method d'escribed below. PREPARATION OF STAXDARD SOLUTIONS. Purified polymer. 0.500 gram, is dissolved in 5 ml. of distilled D M F t o which 1 ml. of a 0.5%)solution of toluene in distilled D l I F is added. Known levels of monomer are then established by adding appropriate aliquots of standard monomer solutions. The amount of monomer added in this manner is varied between 0.00005 to 0.005 gram. This corresponds to 0.01 to l.OYGof monomer in the polymer. Calibration graphs are prepared by plotting the ratio of peak height' of 100
90
80
70
z I !60 I! i 4 50
n &
40
a
sa YI
30
20
10
a 3:
RETENTlOlN TIME MIN. Figure 1. Gas chromatogram of styrene-acrylonitrile-butadiene terpolymer Identified components are: 1 . Butadlene; Acrylonitrile; 3. Toluene; 4. Ethylbenzene; Styrene; 6. Solvent
2.
5.
0 022 f 0 003 0 008 f 0 0008
0 043 0 07
0 04.5 f 0 002 0 073 f 0 003
monomer to toluene peak height us. per cent monomer. Straightline relationships are obtained for the range of concentration of interest. Procedure. T h e sample, 0.500 gram, is weighed into a 10-ml. glassstoppered Erlenmeyer flask a n d dissolved in 5 ml. of purified D l I F and 1 ml. of 0.5yGtoluene in D M F . Five microliters of solution are injected into t h e gas chromatograph, set u p as outlined above. T h e polymer remains in t h e injection block, a n d is removed periodically by reaming. The frequency of cleaning depends on the work load; cleaning every fift'y injections is preferred. RESULTS A N D DISCUSSION
Good resolution is obtained for the monomers of interest in several polymeric systems. The relative retention tim,es for the components of interest are shown in Table 1. The polymers analyzed were : Polystyrene, styreneacrylonitrile copolymer, high-impact polystyrene (containing styrene-butadiene rubber), and styrene-acrylonitrile-butadiene terpolymer. Gas chromatograms, showing the components of interest, are seen in Figure 1 for the styrene-acrylonitrilebutadiene terpolymer. The sensitivity of t,he instrument is capable of detecting less than 10 p.p.m. of monomer. The resolution of minor components is improved, if necessary, by using temperature 1irop;ramming. D N F was chosen as a solvent because it offers several advantages, as compared to other solvents. It is not only a good solvent for many styrene-based polymers, but its boiling point' (154' C.) is higher than the monomers of interest, thus the monomers move through the instrument ahead of the solvent. hnother advantage is that the DMF is easy to purify. The purification is a simple distillation, as outlined above, and the solvent is stable after purification. A series of polymers was analyzed using this method. The precision of the method is indicated in Table 11. This method is also equally applicable to other polymers soluble in DMF. *in example of such polymer is polyvinyl chloride-vinylacetate copolymer. Figure 2 is the gas chromatogram of volatiles in a sample of this copolymer.
80 I L
70
-
z
-
g60
-
u4 , Y
g50
a
-
-
E40
-
a30
-
20
-
10
-
0"-
L 3
6
9
12
RETENTION TIME MIN. Figure 2. Gas chromatogram of polyvinylchloride-vinylacetate copolymer in DMF solution Toluene as an interval standard Components: 1 -vinylchloride, 2-vinylacetate, 3-toluene Column: 10-foot, '/c-inch o.d., 10% Resoflex 446 on Chromosorb W, 3 0 to 60 mesh Carrier gos: NP Flow rate: 25 cc./minute Detector: t i 2 flame Sample size: 5 MI.
LITERATURE CITED
It. H., Boyer, R. F., "Styrene and Its Polymers, Copolymers and Derivatives," p. 317, Reinhold, New York, 1952. ( 2 ) Brodsky, J., Kunststofe 51, 20 (1061). (3) Claver, G. C., Murphy, M. E., ANAL. CHEW31, 1682 (1969). ( 4 ) Critchfield, F. A., Zbzd., 31, 1406 f 1959). ( 5 ) Critchfield, 1;. A . , Funk, G. L., Johnson, J. B., Ibzd., 2 8 , 76 (1956). ( 6 ) Selsen, F. Jl.,Eggertsen, F. T., Holst, J. J., Ibzd., 33, 1150 (1961) ( 7 ) Shauras, P., Claver, G.C., I b i d . . 34. 433 (i962). 18'1 Tweet. 0.. lliller, W K.. Ibzd.. 35. 852 (1963)
(1) Boundy,
RECEIVEDfor review July 1, 1964. Accepted September 2, 1964. VOL. 36, NO. 12, NOVEMBER 1964
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