Determination of Residual Monomer in Polymer Emulsions by Rapid Distillation and Gas Chromatography ORLANDO TWEET and WILLIAM K. MILLER
S . C. Johnson & Son, Inc., Racine, Wis. ,A method has been developed for the determination of residual monomer in polymer emulsions by isolation of the monomer from the polymer by distiliation in the presence of a measured amount of toluene and chromatographing the organic layer. The monomer content i s calculated b y measuring the peak area ratio of monomer to ihe internal standard (toluene) and referring to a calibration curve. The method, exemplified in a procedure for determining ethyl acrylate in ethyl acrylatestyrene copolymer emulsions, i s rapid, sensitive, and precise.
T
HE DETERMINATION of small amounts of residual monomer in polymer emulsions is essential for the control of objectionable odors in the commercial products in which the emulsions are often used. Several recently published methods invoh e the use of gas chromatography. Instruments with flame ionization detectors can be used for analyzing the emulsion directly (1, 6). The method of Yelsen, Eggertsen, and Holst ( 5 ) separates the volatile components on a column, oxidizes them over hot copper oxide, removes water ~ i t hcalcium sulfate, and measures carbon dioxide in a thermal conductivity detector. Another method (9) eliminates water with a drying tube before chromatographing the other volatile components. Distillation is a commonly used method of making a preliminary separation of aqueous and organic phases but often results in a lengthy procedure. Brobst (1) reports the separation of monomers b y steam distillation followed by extraction with cyclohexane and gas chromatographic analysis of the extract. Various chemical methods are available and suitable for certain monomerq. Critchfield ( 3 ) refers to several of these methods and proposes a modified bromination procedure. A method based on a reaction with morpholine (4) has been tried in our laboratories. If the monomeric compound is one in which the carbon to carbon double bond is conjugated with a carboxyl, carhalkouy, or nitrile group, a tertiary amine is formed which can be titrated, in many cases, in the presence of the
852
ANALYTICAL CHEMISTRY
sample. This method as well as most of the other chemical method. is lengthy and lacks seiisiti1ity arid rcproducibility. I n the present method, the monomer is isolated from the aqueous emulsion by an extractive distillation in the presence of a measured amount of toluene. The organic layer is collected in a crank case dilution type distilling receiver and chromatographed. With the toluene acting a \ an internal standard, the monomer content can be readily determined. The work reported herein describes the use of the method for determining ethyl acrylate in ethyl acrylate-styrene copolymer emulsions. The monomer is separated in a one-step procedure ready for chromatographing in about 20 minutes. It avoids the problems connected with the direct injection of an aqueous polymer emulsion sample into the instrument and is specific for the monomer under consideration since the sample does not undergo chemical change. The use of an internal standard makes it unnecessary to chromatograph an accurately measured sample of uniform size. Either thermal conductivity or ionization detector instruments can be used in the analysis. EXPERIMENTAL
Apparatus. A Barber-Colman Model 20 gas chromatograph was modified b y adding a Gow-Mac Model 9999 thermal conductivity detector and power supply. T h e detector was operated a t 150' C. with a bridge current of 200 ma. T h e carrier gas was Table 1. Determination of Ethyl Acrylate in Copolymer Emulsion
Monomer, grams RecovInitially Recov- erp, present Added ered % 0,0903 0.1085 105.5 0.0125 o.. n125 0.0842 0,0990 102.4 . _ ~ . 0.0847 0,1008 103.7 0.0125 0.0128 0.0887 0.0973 95.8 0.0128 0.0863 0.0943 95.2 0.0876 0.0972 9 6 . 8 0.0128 0.0777 0.1336 106.5 0.0478 h-one detecteda 0.0485 0.0490 101.0 Xone detecteda 0.0174 0.0168 96.6 These two samples were vacuumstripped initially to free them of monomer.
helium, and the 6-ft. packed column was 30'% silicone stopcock grease on acid-mashed Celite 545 (30- t o 60mesh). T h e column temperature was 87' C. The distillation apparatus consisted of a 500-ml. boiling flask, a 12.5-ml. crank case dilution type distilling receiver such as Corning No. 3582 and a condenser, all connected with 24/40 ground glass joints and assembled in the same manner as the well known Dean-Stark apparatus for water. Procedure. A 50-ml. sample of emulsion is diluted with 125 ml. of water in a 500-ml. boiling flask. EYactly 3 ml. of reagent grade toluene, 20 mg. of hydroquinone, a few boiling chips, a n d a small amount of antifoam are added. T h e mixture is refluxed until 3 ml. of organic layer are collected in t h e receiver. T h e upper layer is cooled, and a portion is pipetted into a vessel containing a few crystals of anhydrous sodium sulfate. A 0.5-pl. sample of the organic layer is injected into the instrument using a 1-p1. Hamilton syringe. The monomer is eluted at a high sensitivity, and the toluene a t a lower sensitivity which will allow nearly full-scale deflection on the chart. The peak areas for the toluene and the monomer are measured and their ratio is calculated. The amount of monomer present can then be obtained from a calibration curve obtained by recovering known amounts of monomer from water under the same conditions as those used for the sample. The calibration curve of peak area ratio v s . grams of monomer is a straight line within the limits of interest. RESULTS A N D DISCUSSION
This method has been used for the determination of residual ethyl acrylate in more than 100 samples of ethyl acrylate-styrene copolymer emulsions from two different sources. A typical chromatogram is shown in Figure 1. Weighed amounts of ethyl acrylate were added to 50 ml. of emulsion previously analyzed by the procedure described herein and then reanalyzed for total free monomer. The recoveries (shown in Table I) were near 100% A calibration curve prepared by adding known amounts of ethyl acrylate to 3 ml. of toluene and chromatographing directly is identical to that prepared from the distilled standard.. Recovery
of ethyl acrylate in the extractive distillation step is 10~070. Preparation of the calibration curves from distilled standards is recommended as a general procedure since recovery of many inoiioiners in the distillation step may not be complete. The sensitivity of the method for ethyl acrylate according to the procedure used for quality control analysis (0.5-pl. sample and a 10 to 1 attenuation ratio) is approximatldy 40 p.p.m. of nionomer. By injecting a 5-pi. sample and using a n attenusition ratio of 128 to I , 0.0002 gram of ethyl acrylate equivalent to 4 p.p.m. in a 50-gram sample, has been detected. Under the latter conditions, ben?#ene,present a,s an inipurity in reagent grade toluene, Iroduces a larger peak than ethyl acrylate, but the two peaks are completely separated. The precision of tlie method was demonstrated by mab:ing a series of I O determinations on a typical emulsion. Xri average value of 0.194yo ethyl acrylat,e was obtained with a standard deviation of f 0 . 0 1 6 . The column which we iised does not tli.;tinguish between ethyl acrylate and cthyl propionate, an impurity soinetinies found in ethj.1 acrylate. The two rnaterials have lieen separated in our laborstory, lioivrtver, on a liquid substrate of tetraethylene glycol diniethyl ether. Since ethyl acrylate is aln-ays the dominating odor in tlic ethyl acrylates t p w criiulaions iiivcstigatctl, thcrc
toluene at this temperature, but by temperature programming from 50" to 150" C., separate and symmetrical peaks for ethyl acrylate, toluene, and styrene were obtained. Acrylonitrile and ethyl acrylate \Iere determined simultaneously in a n exlwimental polymer by analyzing the emulsion under exactly the same conditions of distillation and chromatography used for the ethyl acrylate-stj rene copolymers. .icrylonitrile is allpreciably soluble in nater, but the satisfactory calibration curve obtained indicates that the partition coefficient favors the extraction of acrylonitrile into toluene. The method can be extended to other monomers by proper choice of naterimmiscible organic solvent, column, and instrument parameters. The method is l i m i t d , lionever, to monomers which can be readily extracted from nater.
TOLUENE
1
ETHYL ACRYLATE
LITERATURE L
x5
XI0
u_ 0
1
2
l
l
1
4
1
1
l
l
I
5
6
1
8
9
ml""1.t
Figure 1. Chromatogram of acrylate and internal standard
ethyl
\vas little interest in determining sty-
rene. Styrene monomer is rwovered in the toluene layer and can be chromatographed and separated from the toluene a t approximately 1%" C. Ethyl acrylate is iiot q a r a t c t l fruin
CITED
(1) Brobst, I
