AIDS FOR THE ANALYST Apparatus for Automatic Determination of Gel Time
tion sample contained in a vial suspended in a constant temperature bath. The dispensed beads sink to the bottom of the vial as long as the sample is fluid. After a gel has formed, subsequent beads are retained on the top surface of the sample. The gel time is thus indicated by the number of beads in the bottom of the gelled mass, each bead representing one time interval
John S. Billheimer and Richard Parrette, Aerojet-General Corp., Azusa, Calif.
couise of development and control work on bulk polyI merixation . . . in this laboratory, the need arose for automatic K THE
determination of the gel time of many samples. Although a practiced observer can duplicate gel times precisely by manually probing the polymerizing mixture from t'ime to t'ime, the end point, is subjective and the method is inconvenient when the gel time is long. A n automatic probing machine which drives a wire loop through the sample in a reciprocating motion ( 4 ) is suit,able for polymerizations characterized by rapid formation of the gel structure, but Jvith slowly reacting systems, where the gel att,ains strength on]!- gradually, the reciprocating probe tears the gel structure as it forms. The probe then continues to move in this channel without indicating an end point. A torque-measuring probe or rotor operat.ed in the polymerizing mixture likewise results in channeling, and is therefore of limited applicability.
CLOCK M O T O R
SYNCHRONOUS
5-6 5 4-5 4 3 - 4 L G E L 'IME,HOURS -
S
A
M
P
L
E
The apparatus, shown in the diagram, performs five gel-time determinations simultaneously. The upper or rotating disk is fabricated of '/*-inch aluminum sheet and contains five concentric rings of 16 or 24 equally spaced l3/@-inchholes. The bed plate contains a single aperture for each of the five rings of holes in the rotary disk. The shaft for the rotary disk rotates within t u o Marlin-Rockwell Corp. K-5 ball bearings, mounted so that the rotary disk turns with a 1/32-inch clearance from the stationary bed plate. A synchronous clock motor drives the rotary disk by means of suitable sprockets so as to bring a row of beads into dropping position every 1/3, 1, or 3 hours as desired. The upper disk is loaded with 4 m m . glass beads in the holes. When the clock drive brings a radial r o v of five beads into coincidence with the apertures in the base plate, the beads fall through these apertures and are conducted by means of flexible tubing to the surface of the polymerization specimen. The flexible tubing permits positioning of the gel sample in a constant temperature bath or under an inert atmosphere as necessary for gelation of the resin under controlled conditions. Typical gelation specimens into which beads were dropped at 1hour intervals for 6 hours are shown in the diagram. If a bead is captured in fall, the gel time is reported as the time corresponding to the dispensing of the captured bead. In general, however, a bead is not captured in fall, and the gelation is known only to have occurred between the arrival of two sequential beads, so that the gel time is reported as the mean of the times for the last bead to sink through the resin and the first bead to be retained on the surface. With a suitable selection of sprockets and starting time of the apparatus, the gel time of slow polymerization processes can be measured to the nearest 10 minutes in 8 hours, the nearest 0.5 hour in 24 hours, or the nearest 1.5 hours in 7 2 hours, as the case may warrant. A limitation on the frequency of bead dispensing is the impracticability of counting a large number of beads within the gelled specimen. A preliminary run or general familiarity with the resin system will usually indicate the appropriate choice of bead interval and starting time of bead dropping.
-1
HOLDER
,4-
'
One of the chief advantages of this instrument is the ease of disposing of the completed sample. More elaborate viscometers involve a difficult and impractical resin removal after each test. With the bead-dispensing apparatus, both the vial and the beads are inexpensive and expendable. An evample of the applicability of this apparatus to development studies in resin formulation is shown in Table I.
0
OROPPiNGi
Table I.
Effect of Inhibitor Concentration on the Gel Time of a Resin Copolymer Gel Time. Hours 3 12 28
Concentration of Inhibitor, %
0.0075 0.020 0,040 0.080 0.150
The practice of determining gel point by measuring viscosities up to the point of maximum rate of increase in viscosity is general, and a wide variety of viscometers has been employed ( 1 4 , 6 ) 6). Although the method serves t o characterize the gelation thoroughly from a quantitative standpoint, the labor involved becomes prohibitive when applied t o routine control determinations. Automatic recording adaptations of some of the viscosimeters for gel-time determinations are conceivable, but in general require expensive instrumentation. A simple, inexpensive automatic gel-time apparatus has been developed which avoids many of the difficulties encountered in the methods discussed above. I n principle, the apparatus periodically dispenses a glass bead to the surface of a polymeriza-
Table 11.
272
84 100
Control Gel-Time Determinations on a Resin Copolymer
Bbl. No.
1st run
A B C D E F G
6.0 5.5 4.1 4.8
3.5 6.2 3.5
Gel Time, Hours 2nd run 6.0 5.3 3.9 5.0 3.5 6.2
3.5
hleali 6.0
5.4 4.0 4 9 3 . .5 8.2 3 5
273
V O L U M E 28, N O . 2, F E B R U A R Y 1 9 5 6 It is evident that the apparatus is capable of measuring a broad range of gel times. Table I1 gives an example of a study of quality control, in which the gel times of resin from seven barrels were determined in duplicate. The estimated standard deviation for a single determination from this series of duplicate determinations, calculated from the formula
where k is the difference betneen duplicate determinations, and m is the number of duplicate determinations, is 0.11 hour, or 2.3% of the average value of 4.8 hours. ACKNOWLEDGMENT
This paper is based upon work performed for the Xavy Department, Bureau of Aeronautics, under Contract NOa(s) 10146 with the A4erojet-GeneralCorp. LITERATURE CITED
(1) Foord, Y. G., J . Chem. SOC. (London) 40, 48 (1940). ( 2 ) €Eggens, H. G., and Plomley, K. F., Xature 163, S o . 4131, 22 (1949). ( 3 ) Nichols, P. L., and Yanovsky, E., J . Am. Chem. SOC.66, 1625 (1944.
Rohm & Haas Co., Resinous Products Division, Philadelphia, Pa., private communication. (5) Toy, -4.D. F., and Brown, L. V.,I n d . Eng. Chem. 40, 2276 (4)
soil and rock samples per year, preparat,ory to the determination of many trace constituents. Previously, a small sample n-as mixed with the appropriate flux in a borosilicate glass test tube, the fusion was made by heating in a rack over a Bunsen burner, and finally the tube was rotated manually as it cooled, to obtain a thin coating of the melt, on the sides of the test tube to facilitate dissolving the product. By means of the multiple-unit fusion rack, in use for the past gear, 11 fusions can be made simultaneously. The equipment serves three purposes: It provides a uniform sample treatment to improve the precision of the analytical methods ; it automatically rotates the cooling tubes so that the molten flux solidifies in a thin layer; and it saves much time and effort. The apparatus (shown in Figures 1 and 2) heats each test tube uniformly by constaritly passing the tube, rotating on the axis of the unit,, over a series of burners for a definite time. The rotation of each test tube keeps the sample in intimate contact with the molten flux t,o provide adequate sample treatment, and during the cooling cycle causes the melt to coat the wall of the tube with a thin layer of the fusion product a b it solidifies. Neither of these procedures requires the attention of an operator; the operator is relieved of the manual procedure that was necessary prior to thr drvelopnient of the multiple-unit fusion rack. CONSTRUCTION
.1 perspective dimc~rsionaldrawing is shown in Figure 1. The entire unit is 23 inches long, 12 inches wide, and 16 inches high, and reighs approximately 45 pouiirls.
(1948). ((j)
Vincent. €1. L.. Ihiil., 29, 1267 (1937).
Multiple-Unit Fusion Rack A. P. Marranzino and William H. Wood, U. S. Geological Survey, Denver Federal Center, Denver, Colo.
fusion rack was developed in the U. S. Geo14.mnTIPLE-unit logical Surve>-to facilitate the fusion of approximately 20,000 '
Figure 2.
Figure 1.
Drive mechanism
Burner assembly
The carriage assembly for holding tho test tubes is rotatecl a t 15 r.p.m. by a 1/75-h.p., 115-volt, alternating current, continuous-action, gear-reducing motor connerted to the drive shaft of the carriage by a chain drive (a S o . 1A steel ladder chain having a yield point of 37 pounds). A sprocket (ao-teeth, 1.18inch pitch diameter n-ith a '/&-inch hub t h a t has a S/le-inch hole) is mounted on the drive shaft of the motor and is connected by the chain to a sprocket of the same size betueen the aluminum bearing posts of the carriage. This arrangement gives the drive unit a 1t o 1 ratio; other ratios may be obtained by a suitable selection of sprockets and chains. A fan is mounted on the motor and serves both to cool the
