Mikio Sazuki Research Department, Nichiban Company, Nerima-ku, Tokyo, Japan Shin Tsuge and Tsagio Takeuchi Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Nagoya, Japan
A new method for estimating occiuded solvents in coated materials such as adhesive tapes was proposed. For removal of the solvent, a vaporizing attachment was constructed and directly set to the inlet port of a gas chromatograph. It was operated at vaporizing temtures ranging from 50 to 110 "6. Removed vapor introduced periodically into the gas chromatoh by stream of Nzcarrier gas. From a series of gas chromatograms obtained by the periodic introduction method, it was possible to estimate the content of the solvents in the base polymer even before the solvents were entirely removed. The principle and the versatility of this method were also discussed. DETERMINATION of the residual solvents in high polymers and resins has become increasingly important, especially in the field of adhesives, bonding and packing materials, and plastic paints since the occluded solvents often give various effects on the physical and chemical characteristics of the polymeric materials. In addition, the qualitative and quantitative analysis of the composition of the solvents is also necessary for the process control of their manufacturing. However, difficulties are generally encountered in the quantitative recovery of the occluded solvents because of the different degree of sorption of the solvents to the polymers and frequently the formation of protective film over the surface. Usually, in a low temperature vaporizer, the solvents dry too slowly to yield adequate concentration of vapor for analysis. On the other hand, high temperature generally facilitates the vaporization of the solvents, but the resulting vapor may often be complicated with the additional degradation products from the base polymers and resins. Consequently, great care has to be taken in the selection of vaporizing conditions for complete removal of the solvents. Gas chromatography, which has high potentialities of separating and sensing trace organic compounds, has often been applied to the analysis of the residual solvents (1-4). The solvents are usually separated prior to the gas chromatographic analysis as follows : by overnight-vacuum distillation with water or oil bath for plastic adhesives, spray lacquers, etc. ( I ) , by solvent extraction for 1 hr at 70 and 130 "e, for cellophane coated with nitrocellulose and vinylidene chloride copolymers, respectively (2), and by dry-distillation in a jar with a special cover at 170 "F (76.6 "C) for 5 min in a forced air circulation oven for packing materials (3). However, these indirect methods generally have some disadvantages in carrying out rapid and quantitative analysis of a slight amount of the occluded solvents because of their timeconsuming process and difficulties of quantitative recovery. (1) J. Haslam and A. R. Seffs, Analyst, 83,455 (1958). (2) S. 6. Gilbert, L. 1. Oetzel, W. Asp, and L. Brazier, Mud. Puckag., May, 167 (1965). (3) L. H. Phifer, ibid., Nov., 154 (1964). (4) J. A. Hudy, J. Gus Chrurnarogr., 4, 350 (1966).
Recently, a direct method was reported for the gas chromatographic examination of common lacquer solvent using a sealed capillary capsule ( 4 ) . The application of this method, however, is restricted to liquid samples such as lacquers which contain solvents as major components. In this paper, a new gas chromatographic method is described for the estimation of residual solvents in adhesive tape without prior separation of the solvents from the base polymer. A vaporizing attachment was designed and set to the inlet port of a common gas chromatograph. By periodic introduction of the vaporized solvents into the gas chromatograph, it was possible to obtain reproducible and quantitative data even before the solvents are entirely removed from the base polymer. EXPERIMENTAL
Apparatus, The vaporizing attachment shown in Figure 1 was constructed at Jeolco, Inc., according to the original design by one of the authors (M.S.). It consists of a sample charging drum which is set in a heating block of aluminum alloy and a six-way stopcock. It is possible to hold a tape sample with dimensions up to 10 mm x 275 mm along the ditch on the drum. The temperature of the block is variable from room temperature up to 200 "C. Fluctuation of the temperature was within A0.5 "C at 200 "C. Dead volume of the vaporizer is about 5 ml. This apparatus is directliy attached to the inlet port of a gas chromatograph, Shimadzu GG1B with flame ionization detectors. Gas chromatographic analysis was carried out under the following operational conditions: Separation column: Stainless tube (4 mm i d . X 1.5 m> packed with 15 w t z of squalane on Shimalite (60-80 mesh). Column temperature: 56.5 or 85.5 "C, isothermal. Carrier gas: Nz 1. kg/cm2 (about 30 ml./min). Hydrogen flame: HZ25 ml/min, air 0.6 l./min.
Preparation of Analytical Sample. Natural rubber masticated at 70 "C for 10 min was used as typical adhesive material. Five kinds of special grade solvents were utilized: n-pentane, n-hexane, benzene, cyclohexane, and toluene. Thirty grams of the rubber was dissolved in 400 ml of each solvent or mixed solvent. For simplicity of the system, adhesive tape for this examination was prepared merely by coating the polymer solution on a thin polyethylene terephthalate film one side of which was so treated as to facilitate adhesion of the polymer. After being air-dried, the adhesive tape was cut into appropriate size (10 mm X 257 mm) for the examination, on which about 40 mg of the adhesive polymer was coated. Procedure. To illustrate the procedure, three positions of the stopcock are shown in Figure 2 . After charging the tape sample on the drum at room temperature, it is rapidly set in the heating block maintained at a fixed temperature between 50 and 110 "C and is allowed to stand for 10 min. At this stage the stopcock is in position A (Stand by). The
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
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4 E
I IOQC
2 3
9Q°C
4
70 O C
Y
-6
a e
W I 50°C I
0 Figure 1. Schematic diagram of vaporizing attachment
-
A . Sample charging drum, B. Six-way stopcock, C. Heater, D. Thermocouple, E. Pre-
I
4 8 HEATING
I
I
12 16 TIME ( m i n )
Figure 3. Relationships between heating time and peak area with n-hexane at n = 1
heater-coil for carrier gas
EXHAUST c OUT LET VAPORlZlNO CHAMBER
RESULTS AND DISCUSSIOK
A
STAND-BY
Ons INLET
B INJECTION
4
!-'--'---'
-Q
7,
6
EXHAUST
Figure 2. Schematic diagram of flow system at three positions of a stopcock stopcock is turned to position B (Injection) for a fixed period
of 3 sec and again returned to position A. By this operation, a part of the vaporized solvents is introduced into the separation column to give the first gas chromatogram within 4 to 18 min. The second and the subsequent gas chromatograms with the same sample are obtained by repeating the same procedure mentioned above periodically [A + B(3 sec) + A]. The periodic cycle of the measurement was altered between 4 and 18 min depending on the component with the highest retention time in the system. 1706
ANALYTICAL CHEMISTRY, VOL. 42,
Sample Heating and Injection. A preliminary study was made on the effect of temperature and time on removal of the solvents from the adhesive tape. A blank analysis was carried out using the apparatus with a piece of thoroughly dried adhesive tape which was maintained at 90 "C for 10 hr in a flow of N:! carrier gas prior to the examination. The resulting chromatogram did not show any appreciable peaks of the solvents 01depolymerization products up to 110 "C of vaporizing temperature. Above the limiting temperature, a slight drift of the base line was observed. Consequently, in the following measurements, vaporizing temperatures below 110 "C were used. The effect of heating time on the amount of vaporized solvents was examined at various temperatures, where the injection time was fixed at 3 sec. Figure 3 shows a typical result with the solvent of n-hexane. The higher the temperature, the more amount of the solvent is removed. Generally, the time required to reach near-equilibrium at a given temperature increases with the rise of boiling points in a homologous series of solvent. However, in this periodic introduction method, it is not always necessary to inject the equilibrated vapor. In the following, 10 min of the first heating time was used, within which the vapor attains over 80% of the equilibrated composition at 70 "C or above with every solvent utilized. The experimental result also shows that vaporizing temperature of 50 "C or below is too low to remove solvents with relatively high boiling point. The injection time of the vaporized solvents into the gas chromatograph significantly affects the analytical result. From the standpoint of increasing the vapor concentration, a longer time is recommended. On the other hand, by increasing the sampling time, lower theoretical plate number (TPN) and poorer resolution are obtained. Preliminary experiments were carried out on the change of TPN at various injection time between 1 and 10 sec. The TPN for a 3-sec injection time is nearly as high as that obtained when the pure solvents are injected by means of a microsyringe. At the same time, 3-sec injection give good resolution and enough peak on the resulting gas chromatogram with every solvent.
NO. 14, DECEMBER 1970
12
b Figure 4. Gas chromatogram of mixed system of benzene, cyclohexane, and toluene from adhesive tape at 85.5 "C of column temperature 1. benzene, 2. cyclohexane, 3.
t
0
toluene
.
"
"
2 4 6 8 1 0 RETENTION. TIME (mid I
5
1
Figure 5. Gas chromatogram of mixed system of n-pentane and hexane isomers from adhesive tape at 57.5 "C of column temperature
t
1. rz-pentane, 2. 2-RIP, 3.3-MP, 4. n-hexane, 5. MCP O
2 4 6 8 IO RETENTION TIME (mid
Table I. a-values at Various Vaporizing Temperatures Vaporizing temperature, "C Boiling 50 70 90 110 point, "C Solvents +Pentane 36.1 0.1149 0,1242 0.1323 0.1449 60.27 0.0906 0,1267 0.1316 0.1381 2-MP 63,27 0.0930 0,1160 0.1225 0.1356 3-MP 0.0880 0.1266 0.1300 0.1352 68.74 n-Hexane 0.0860 0.1175 0.1275 0.1304 71.8 MCP 80.10 0.0854 0.0922 0.0931 0.0984 Benzene 84.16 0,0828 0,1026 0.1042 0.1069 Cyclohexane Toluene 110.62 ... 0.0383 0.0385 0.0388
Accordingly, 3 sec of the injection time was used for the following measurements. Typical Gas Chromatogram. Figure 4 shows a typical gas chromatogram obtained with a mixed system of benzene, cyclohexane, and toluene at the vaporizing temperature of 70 "C and 3-sec injection. Similarly, Figure 5 illustrates one with a mixed system of n-pentane and n-hexane, where the peaks of 2-methylpentane(2-MP) and 3-methylpentane(3MP) are the isomers of n-hexane and methylcyclopentane(MCP) is an impurity from n-hexane. Quantitative Determination. Peak area, which was measured from a series of gas chromatograms obtained with one sample by means of the periodic introduction method, was plotted as a function of the associated number of injections on semilogarithmic graph. Typical results shown in Figure 6 with n-hexane indicate that a linear relationship is valid at every temperature and that its slope becomes steeper with the rise of vaporizing temperature. The relationship can generally be written as follows: log A , = log Ao
-
an
n
=
1,2,. . . .
(1)
or A,
=
Ao lo-""
where n is the associated number of injection, AOand A , are the peak area in cm2 at n = 0 and n, and a is the gradient of
0
I
2
4
b
5
7
6
INJECTION
8
NUMBER
9
1011 12
(nl
Figure 6. Logarithm of observed peak area as a function of the associated injection number with n-hexane at vaporizing temperature of 50,70,90, and 110 "C the straight line. The same trend mentioned above was observed with the other solvents utilized. Table I shows a-values of the various solvents at 50, 70, 90, and 110 "C of vaporizing temperature. Once the experimental conditions are fixed, the a-value is almost specific for a given solvent regardless of its initial content in the polymer. Although the gradient generally decreases as the rise of boiling point of the solvent, some reversed data are observed with cyclohexane and benzene. This phenomenon indicates that the removal of the residual solvent in the polymer is affected not only by its boiling point but also by the molecular interaction between the occluded solvent and the polymer. As the number of injections are increased, A , becomes progressively small until no peak is observed on the gas chromatogram. Consequently, the total amount of the occluded solvents is considered to be proportional to the summation ( A ) of A,, as shown in Equation 3. c
A
=
n=l
C
A,
=
Ao
n=l
lo-"%
(3)
where the critical number of injection (c) and A. are determined experimentally by extrapolation of the straight line to zero area (Ad = 0) and zero number (n = 0), respectively. Peak area can easily be converted into the quantity of each solvent by multiplying the factor (f)which is preliminary determined by the calibration curve for the pure solvent and has dimensions of mg,/cmz. Accordingly the content (S) of the occluded solvent can be calculated by the next equation.
fA
S (ppm) = W*
10-6
where W is the weight of the adhesive polymer in mg. Provided that a linear relationship holds between logarithm of peak area (log A,) and the associated number of injection (n), a few measurements are sufficient to estimate the value of
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
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Table 11. Precision Data Sample no.
Solvents
XI
x2
2-Meth ylpentane
11
3-Methylpentane n-Hexane Methylcyclopentane Cyclohexane
40 23 47 42
12 41 26
i
1
2
CY and Ao, even if the experimental conditions such as heating temperature, time, and injection time are altered. Evaluation of the Method. To evaluate the precision of this periodic introduction method, duplicate measurements were carried out with two kinds of adhesive tape prepared from solutions of hexane isomers and cyclohexane, respectively. The measurements were performed at 70 "C of vaporizing temperature. The data presented in Table I1 indicate that the residual solvents ranging in content from 10 to 50 ppm can be determined within 8.3 % of relative standard deviation. On considering the difficulties of preparing adhesive tape with constant level of the residual solvent, the relatively large deviations are rather satisfactory for the determination in this field. Since no alternative absolute method to estimate the con-
49
47
Content found, ppm xa 13 44 30 46 43
x4
12 42 30 42 42
Av(n) 12 42 27 46 44
Re1 std dev, % 7.5 4.7 8.3
6.0 5.2
tent of the occluded solvents in polymers is available, accuracy of this method cannot be discussed. However, the obtained quantitative data can reasonably be considered to be close to the true value, judging from the principle implicit in this method. Though this study was carried out using the solvents with relatively low polarity, further work on the other polar solvents system is currently in progress. This periodic introduction method is also applicable to the investigation of the residual solvent in plastics and organic coatings by modification of the sample holder.
RECENED for review June 24, 1970. Accepted August 24, 1970.
Automatic Data Recording of Unsaturation uantitative yd rogenation J. J. Szakasits Houston Research Laborafory, Shell Oil Company, P. 0. Box 100, Deer Park, Texas 77536 The measurement of titrant uptake during hydrogenation of olefinic materials using the Brown catalytic hydrogenation technique has been automated. A highly sensitive pressure transducer with linear output (0-10 volts) in the range of interest is coupled to the buret of the analytical hydrogenator. Hydrostatic pressure in the buret i s converted to a corresponding electrical signal through the pressure transducer, and displayed on a chart recorder. Usefulness of this apparatus i s eminent when olefinic materials possessing differing hydrogenation rates are analyzed routinely. Other advantages include the capability of detecting small amounts of easy-to-hydrogenate olefinic components, a reduction in operator time, and the attainment of a permanent record. The apparatus is sturdy and easily constructed.
A QUANTITATIVE HYDROGENATION METHOD has been successfully adapted ( I ) to determine the unsaturation in gas oils and gasolines with relatively high sulfur content (up to 4 wt This technique is based on the method developed by Brown et al. (Z), and involves the generation of a highly active platinum catalyst on activated carbon in a specially designed hydrogenation apparatus. The Brown analytical hydrogenator is automatic in the sense that the generation of hydrogen takes place at a rate regulated by a mercury valve
z).
(1) J. L. S. Curtis and M. 0.Baker, ANAL.CHEM., 42,278 (1970). (2) C. A. Brown, S. C . Sethi, and H. D. Brown, ibid., 39,823 (1967). 1708
*
which controls the flow of sodium borohydride (NaBH4) to the hydrogenator flask as a function of the internal pressure of the system. The hydrogenator flask pressure is limited by a mercury bubbler permitting the titration to proceed at near atmospheric pressure. The quantity (milliliters) of titrant of known molarity consumed during hydrogenation is read at predetermined time intervals during hydrogenation, and a curve based on these point readings is plotted. An instrument has been developed which automatically records on a chart recorder and simultaneously displays on a digital panel meter the milliliters of titrant consumed during hydrogenation. The hydrogenation process of materials is unchanged from the techniques described in References I and 2. The value of this apparatus is most apparent, when materials possessing differing hydrogenation rates are titrated routinely (10-20 minutes titration). Also, with this apparatus, small amounts of easy-to-hydrogenate components in a sample can be measured. This method requires less operator time and gives a uniform and permanent record. EXPERTMENTAL
In order to be able to measure and record automatically the milliliters of titrant consumed by the analytical hydroanalyzer upon the injection of a quantity of sample, the liquid pressure in the buret is converted to a corresponding electrical signal by means of a highly accurate pressure transducer with linear output in the range of interest. The pressure ( P )
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
