Determination of Additives in P by Absorption Spectroscopy H. L. SPELL and R. D. EDDY The Dow Chemical Co., Freeport, Tex.
b Methods were developed to determine 2,6-di-tert-butyl-p-cresol, 9-octadecenamide, and 4,4-thiobis(6-fertbutyl-m-cresol), at concentrations of 1 to 500 p.p.m. in polyethylene. The additives are extracted by shaking powdered polyethylene with a solvent in a stoppered bottle a t room temperature. Carbon disulfide and carbon tetrachloride extracts are examined b y infrared spectroscopy in cells 1 to 10 cm. thick to determine all three additives. When interferences are present, the alkylat'ed phenolics may be determined from the ultraviotet scans of iso-octane extracts. The minimum detectability is about 1 p.p.m. and the precision is within * 2 p.p.m.
T
WO THERhL4L ANTIOXIDANTS COM-
monly used in commercial polyethylenes are 2,6-di-tert-butyl-p-cresol (Ionol, Shell Chemical) and 4,4thiobis(6-tert-butyl-m-cresol) (Santonox, Monsanto Chemical). The most widely used slip agent is 9-octadecenamideJ commonly referred to as oleamide. A rapid and accurate method of determining these additives in polyethylene was needed to control their additions in the manufacturing process and in research operations. Spectroscopic methods sometimes permit the determination of additives directly in the polyethylene by examining thick
films, either solid or melt. The infrared spectrum is obtained by a differential method using a compensating film which is devoid of the component to be measured. Miller and Willis (4) described a unique technique for this type of analysis. This approach was not entirely successful for the accurate determination of most of the additives a t the concentration levels of interest (I to 500 p.p.m.). Another method used in our laboratory involved dissolving the polyethylene in a solvent by boiling under reflux. After cooling, the solvent was separated from the precipitated polyethylene and examined by spectroscopic methods. This procedure is excessively time-consuming and unsuited for production control analysis. Ionol has been determined in polyethylene by mass spectrometry as reported by LeBlanc (3) in 1956, but oleamide and Santonox cannot be accurately measured a t these low concentrations. The method described in this report is based upon the extraction of the additives from the polyethylene a t room temperature, followed by the infrared or ultraviolet spectroscopic examination of the extract, from which the concentration of the additive may be calculated. A similar extraction procedure has been applied to the determination of stabilizers in rubber, most recently by Nawakowski ( 6 ) and Hilton ( I ) , but no
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100
methods for the determination of additives in polyethylene by absorption spectroscopy have been reported. EXPERIMENTAL
Solvent Extractions. To extract the additives from pelletized polyethylene 5 grams of sample were shaken by a wrist-action shaker with 10 ml. of solvent in a 2-ounce stoppered bottle for the prescribed time at room temperature. The extraction rates of Ionol, Santonox, and oleamide from pelletized polyethylene are shown in Figure 1. Carbon disulfide and isooctane were used to extract both Ionol and Santonox, while carbon tetrachloride was used to extract oleamide. These rates were determined with low density (0.920 gram per cc.) conventional polyethylene in 7-mesh pellets. The additive concentration levels were approximately 300 p.p.m. As indicated by Figure 1, the carbon disulfide extractions required 1s/4hours to recover Ionol and 21/2 hours to recover Santonox. A shaking period of 21 hours was necessary to extract oleamide with carbon tetrachloride. The iso-octane extractions were considerably slower, taking 50 hours to recover Ionol, and 76 hours to recover Santonox. Approximately 98% of the additive was recovered in each case. For rapid extractions, the polyethylene was ground to 50 mesh with a Braun pulverizer Type UA (Braun Corp. , Los ilngeles, Calif .). Five grams of the sample were shaken with 25 ml. of solvent. The extraction rates are shown in Figure 2. Forty minutes of shaking Jyere sufficient to extract Santonox and
SOLYEN1
/ 40
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i
+i2
ADDIlIVE 1. Ionol
SOLVENT
2. Ole.m,de
cc14
3. s.nt.nox
cs2
4. I0"d 5. S a n t o n ~ r
Ilo
