Anal. Chem. 1981, 53, 2380-2381
2380
Gel Time Measurement of Resins Philipp wh Schuessler” and William J. Dobbin IBM, Bodle Hill Road, Owego, New York 13827
Industry has long sought an inexpensive and reproducible method for determining the gel time of various resins. Presently available techniques have various shortcomings. Methods based on cone-plate viscometry, while accurate and precise, require relatively expensive instrumentation. Other commercial instruments based on rotor/shear techniques require large volumes of resin which may be costly or difficult to obtain. Rotor techniques require 50-100 cm3 of material while shear techniques require less than 1 cm3 but use very expensive instrumentation to achieve the high test temperatures. Both methods suffer the disadvantage of physical damage and cost when a thermosetting resin is allowed to cure before it is removed from test apparatus. A third, inexpensive method in popular use is the stroke or “lollipop stick” technique. While widely used as an acceptance test, interpretation of gel time is extremely operator dependent. The technique disclosed below requires small volumes of resin (1-2 cm3) and uses inexpensive “gel” cells fabricated from Pyrex test tubes.
Stopcock
c Atmosphere VAC Source
Constant Temperature Bath
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RESULTS AND DISCUSSION This technique has been used in several areas of the polymer chemistry. The most common use has been in quality control of incoming adhesive systems as lot variances are readily detected. Gel curves have been established for an epoxy resin based on Epon 1001 and diaminodiphenyl sulfone (DADPS), using several accelerator concentrations. Results are shown in Figure 3. A second application has been in the evaluation of accelerator effectiveness studies where various molecular structures of the same parent molecule have been quantitatively ranked according to their ability to accelerate polym-
Plate and Magnetic Stirrer
Flgure 1. Essential parts of the gel cell test apparatus.
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EXPERIMENTAL SECTION Apparatus. The essential parts of the test apparatus are depicted in Figure 1. The “gel” cell is fabricated from a length of capillary tubing blown into the bottom of a Pyrex test tube (Figure 2). This is readily made in the laboratory or may be purchased through a glass fabricating house. The stopcock and vacuum gauge are necessary as variations in the pressure drop can cause significant shifts in the observed time. The drying tube on the air inlet prevents atmospheric moisture from contaminating the sample or interfering with anhydride cross-linking agents. Procedure. Liquid polymers are placed directly into the test tube. Systems impregnated with glass cloth or fillers are separated by solvent extraction and centrifuging. The solvent selected for the extraction should: (1)have a boiling point lower than the test temperature of the analyses, (2) have a significant volatility at room temperature, (3) be capable of dissolving all the active components in the system. The resin-solvent mixture is placed in the gel cell and the bulk of the solvent removed at temperatures below the onset of cure by vacuum stripping the solvent at room temperature. The gel cell is lowered into the constant temperature bath, held to A 1 “C, and timing is started. (Any residual solvent will flash off at this point.) With the stopcock adjusted to give a 20 torr (266Pa) pressure drop, air bubbles can be seen flowing upward through the liquid resin. Timing is completed when these bubbles cease flowing. Close observation may be required in determing flow cessation. The elapsed time is defined as the gel time at the average temperature of the test. By repetition of the above steps at various temperatures, a gel curve can be generated for the polymer system. Reproducibility of data is presented in Table I where different lots of a commercial resin were tested by use of the discussed method.
Drying Tube
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1.
L
-1 5 M E Diameter Teat Tube
Figure 2. Typical gel cell.
80
li
90
T B ~ . P ~ % YOC IB
iia
ria
130
------+
Figure 3. Gel as a function of heat and accelerator concentration. ~
Table I. Precision of the Gel Method as Determined on Different Lots of an Epoxy Adhesive temp, “C gelation time, min 89* 1 104t 1 113 t 1 115+ 1 118+ 1
84, 84 32.5,31.5, 32.0; 29.5, 30.5 19.5. 18.7 18.8;18.5, 21.5, 18.6 14.0. 14.5. 15.2
erization. For example, the electron-withdrawing characteristics of chlorophenylated urea accelerators have been studied. A third area of investigation in which this technique has been
0003-2700/81/0353-2380$01.25/00 1981 American Chemical Society
2381
Anal. Chem. 1981, 5 3 , 2381-2383 0 Sunrhlna Gelornster Data X Gel all Data
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Tempenturn
Figure 4. Gel
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times via two techniques.
used is in optimizing accelerator concentrations for various polymers. As in Figure 3, it was readily shown that accelerator concentrations in excess of 1.5% did not yield a faster cure
schedule and that, in fact, the polymerization rate after 1.5% accelerator concentration becomes kinetic dependent. Data generated by this technique compares well with data generated with other types of apparatus. Specifically, comparison curves for the gelation of a liquid epoxy and dicyandiamide as the curing agent are shown in Figure 4. The instrumental data were generated by use of a Sunshine Gelometer. Excellent agreement is shown between the two curves for a major portion of the gel curve. A difference does exist between the curves at temperatures in excess of 170 “C.These may be attributed to several factors, such as residual solvent quenching the system during the first minutes of test as it is volatilized off. RECEIVED for review June 16,1981. Accepted September 17, 1981.
Comparison of Two Materials for Storage of Nitrogen Standard Solutions Kathleen A. Mitchell Philip Morris U.S.A., Research Center, P. 0.Box 26583, Richmond, Virginia 2326 1
Most methods for the determination of nitrogen are labor-intensive and numerous references to procedures using features of automation are evidence of the importance of being able to handle large numbers of samples (1-4). One procedure offering high sample throughput is a modified Kjeldahl digestion in a block digestor with an automated colorimetric readout using the Technicon AutoAnalyzer system (5-9). A method modified for this laboratory is based on Wall and Gehrke’s semiautomated procedure (6). The suggestion l(6, 7,9) that standards may be stored and reused until exhausted has been adopted as a labor-saving measure to eliminate daily preparation of the standards. The standards, prepared by the addition of ammonium sulfate to the Kjeldahl digestion matrix where selenium serves1 as the catalyst, are compared to digested samples using a colorimetric readout based on a modified Berthelot reaction. It was observed that the response of 6 week old standards was greater than the response for the freshly prepared standards. This “agiing” phenomenon was evaluated as function of storage conditions. Pyrex glass storage bottles, routinely used for storage of standard solutions, and Nalgene (linear polyethylene) storage bottles are compared for their ability to preserve ammonium sulfate standards prepared in a sulfuric acid digestion matrix. The re3ults presented here describe those conditions which are suitable for storage of nitrogen standard solutions.
prepared as follows: A 1.7688-gportion of dried ammonium sulfate (21.20% N) was weighed and transferred to a 500-mL volumetric flask. The ammonium sulfate was dissolved and brought to volume with deionized water. The working nitrogen standards were prepared in a digestion matrix as follows: 25-mL aliquots of 36 N H2S04were transferred to five 250-mL digestion tubes and 6 g of potassium sulfate and 2 selenized Hengar granules were added to each digestion tube. The digestion tubes were then placed in the block digestor preheated to 200 “C and heated for 15 min. The tubes were allowed to heat to 400 “C for a total “Auto” cycle period of 2.25 h. With caution the digestion tubes were removed from the block digestor and allowed to cool for 30 min. Approximately 20 mL of deionized water was added to each tube. Aliquots of 2, 5, 10, 15, and 20 mL of the stock standard nitrogen solution were pipetted into the separate BD-20 digestion tubes containingthe digestion matrix. The contents of each BD-20 tube were diluted to the 250 mL volume mark with deionized water. The standards were equivalent to 6, 15,30,45, and 60 pg of nitrogen/mL. The total volume of 250 mL was fiitered through Whatman no. 42 fiiter paper and collected in four containers-two Pyrex glass and two Nalgene. Procedure. Each working standard was inverted before a portion was transferred to an AutoAnalyzer sample cup. The working standards were submitted to the AutoAnalyzer system in a random order at various times in a 7-week period. The standard calibration setting of the single-channel colorimeter was held at 775 for the analysis which was equivalent to 0.175 total absorbance units full scale. “Response”, as used in the context of this report, is defined as percent of full scale absorbance units.
EXPERNMENTAL SECTION
RESULTS AND DISCUSSION
Apparatus. The Technicon BD-20 block digestor is used to prepare the digestion matrix for standard8 in the same manner as the digestion of a sample. BD-20 digestion tubes, calibrated to 250 mL are used. The Technicon AutoAnalyzer I1 system includes sampler, proportioning pump, ammonia determination analytical cartridge No. 116-D531-01,AAII single-channel colorimeter, and recorder (available from Technicon Instruments Corp., Tarrytown, NY). Glass storage bottles (Corning Catalog no. 1500-125) and Nalgene bottles (Catalog no. 2104-0004) are available from most laboratory suppliers. Reagents. Reagents for the determination of nitrogen in the BD-20 digests are prepared as described by Technicon (6). Standards. Stock standard nitrogen solution, 750 pg/mL, was
Responses for each concentration level of nitrogen standards stored in four containers were obtained on the AutoAnalyzer system. Paired responses representing the same concentration and same storage material, and their average, are given in Table I for day 1and day 51 of the testing period. Responses for all four bottles a t each concentration are similar on day 1 as shown in Table IA and IB. In Table IC the response of the pairs stored in Nalgene on day 51 continue to show reproducibility. Further, those responses show little differences from the initial responses as seen in Tables IA and IB. Table ID demonstrates the “aging” effect of the standard solutions stored in glass. Large differences in response are
0003-2700/81/0353-2381$01.:!5/00 1981 American Chemical Society
