Determination of Degree of Cure of Polymeric Materials through Evaporative Rate Analysis Eugene Cerceo Vertol Division, The Boeing Co., Philadelphia, Pa. 19142 ANALYSIS OF THERMOSET POLYMERS has always been difficult, especially the study of the degree of cross-linking. The difficulty arises mainly because polymers are insoluble in all solvents owing to their three-dimensional chemically bonded structure. Therefore, solution can only arise through the cleavage of chemical bonds. This also explains the inability of polymers to melt; instead they undergo preferential decomposition at higher temperatures. The most thorough and sophisticated analysis to date on such a material has involved controlled pyrolysis followed by chromatographic separation in which the effluent is allowed to pass through a rapid scanning infrared spectrometer and, thence, into a mass spectrometer. The data from these instruments can be correlated with nuclear magnetic resonance information (1) and the total output from all the instruments computer-analyzed ( 2 ) . When an analysis of the degree of cross-linking is considered, the approach that can be taken is a controlled pyrolysis at a temperature in the order of 400 “C from a radio frequency source. Specific bonds absorb energy up to cleavage. This bond breakage may involve time in the order of sec, which is instantaneous in relation to the time involved for cure, Thus, the curing reaction is unaffected. A relatively simple and straightforward method known as evaporative rate analysis has recently been introduced into the field of polymer chemistry (3). The technique is based upon the placement of a special test solution onto the surface in question. The test solution is composed of a solvent plus a carbon-14-labeled compound (half life = 5770 years) ( 4 ) . In polymeric materials, the function of the solvent is to swell uncured polymer areas, thus producing a greater susceptibility to ( 1 ) Ronald A. Hites and K. Biemann, ANAL.CHEM.,39, 965
(1967). (2) Jack W. Frazer, ibid., 40, 31A-37A (July 1968). (3) “Application of Test Solutions to Measurement of Cross-
Linking of Thermosetting Resins,” Ametek Technical Products Summit, N. J. 07901, Section D, pp 4.4.1.1, 1968. (4) Specification Sheet Bulletin 967, Meseran Brand Surface Analyzer, Ametek Technical Products, Summit, N. J. 07901, 1967.
Table I.
Po1y mer E-705 BP-907
AF-30
Description Thin, flat epoxyfiber glass laminate Thin, flat sheet of epoxy adhesive Thin, flat sheet of phenolic-nitrile
interaction with the radiochemical. The mechanism of the attraction of the radiochemical to the polymer surface probably involves a type of short order electrostatic attraction of the labeled compound to unreacted functional groups or bonding sites at uncured or partially cured polymer areas. In actuality, the solvent is permitted to evaporate and the residual radioactivity on the surface is measured by a thin end-window Geiger-Muller detector tube positioned just above the radioactive material. EXPERIMENTAL
Reagents. The resin-glass fiber composite studied was E-705 supplied by U. S. Polymeric, Santa Ana, Calif., and the adhesives investigated were BP-907 epoxy from American Cyanamid, Havre de Grace, Md., and AF-30 phenolicnitrile from Minnesota Mining and Manufacturing, Minneapolis, Minn. The solvent-radiochemical selection for the three polymeric materials were as follows: for E-705 and BP-907, the solvent was 98 trifluorotrichloroethane and 2 chloroform, and the radiochemical was tetrabromoethane; for AF-30 the solvent was 90% .trifluorotrichloroethane and 10 tetrahydrofuran, and the radiochemical was tetrabromoethane. Apparatus. The MESERAN brand (Ametek Technical Products) of surface analyzer Model 700-A provided for a controlled passage of nitrogen gas over the test surface to produce a uniform evaporation of the radiochemical. Although temperature was also a variable affecting the evaporative process, normal laboratory temperature controls prevented distortions due to temperature variations. The analysis of the evaporative data was conducted by suitable electronic equipment which recorded a continuous plot of count rate US. time (5). Sequential areas under the evaporative curve were analyzed based on the pulse information observed by the detector. Such a setup provided a numerical expression of the evaporative rate phenomena. Instead of using the total number of counts, a function of these counts was expressed as A d , AB, AC, and AD. These rep-
z
z
Meseran Technical Manual, Bulletin 9167, Ametek Technical Products, Summit, N. J. 07901, 1967.
(5)
Cure Conditions for Polymeric Samples Numerical graphical Cure designation temp, “C 1 ... 2 121 3 177 4
Cure pressure, psi ...
Cure time, min
200
15
100
60
...
5
149
6
149
7
177
50 3 50 200
8
...
...
15 30
60
9
116
100
160
60
10 11
100
204
100
60 60
VOL. 41, NO. 1, JANUARY 1969
191
150 150
100
100
COUNTS PER 14-SEC INTERVAL
COUNTS PER 14-SEC INTERVAL
50
50
*
0
AA1 AB1 AC1 AD1 AA2 AB2 AC2 AD2
Figure 1. Evaporative rate curves of E-705 epoxy epoxy resented counts per 14-sec intervals, and by resetting the counters, additional sequential areas could be examined. To permit evaporation of the highly volatile solvent before the start of the sequential area determination, a deliberate and external time delay was provided. This time delay was necessary because carbon-14 emissions are of relatively low energy (0.156 MeV) and, in the initial stages of analysis, were absorbed by the solvent and not detected. The peak of the rate curve signified the end of solvent evaporation and initiation of the evaporation of radiochemical from the surface in question. Procedure. The epoxy laminate and the two adhesives were cured in a small hot press according to the conditions specified in Table I. The thin, flat samples, approximately 2 in.2 in area, were investigated on the instrument described.
I50
100 COUNTS PER 1CSEC INTERVAL
RESULTS AND DISCUSSION
The evaporative rates were strongly influenced by the degree of solvent attack, which in turn was a function of the degree of cure. Also, the greater the degree of solvent attack, the greater the retention of radiochemical. Specifically, to illustrate the point with E-705 epoxy having the following structural backbone,
50
r 0
C" 3
and with the hydroxyl and epoxy functional groups crosslinked with diamines and anhydrides, solvent attack by chloroform and trifluorotrichloroethane gave a clear separation of the evaporative rate curves (Figure 1). The lowest curve 192
ANALYTICAL CHEMISTRY
Figure 3. Evaporative rate curves of AF-30 phenolic-nitrile represents the fully cured sample, signifying that minimum sites were available from unlinked chains for the retention of radiocompound. Thus, the counters, which record the
amount of radioactivity on the sample surface, register the minimum quantity of beta emissions. The same reasoning applied to BP-907 epoxy adhesive, which is thought to be structurally similar to E-705,and to AF-30 phenolic-nitrile adhesive (Figures 2 and 3). A further investigation is planned to obtain evaporative rate curves on overcured samples. It is surmised here that, because chain cleavage is being initiated, the curves will begin to rise because of an increase in attractive sites and, thus, a greater retention of radiochemical. Thus, through use of very little time (approximately 2 min per analysis) and comparatively simple electronics, the rate of cure of polymeric materials may be followed as a function of pressure, temperature, and time. The most attractive features are the economical aspect and the simplicity of the method in-
volved. Although the sophisticated computer analysis through complex chemical instrumentation will remain the number one analytical tool in the evaluation of polymeric structure in relation to cure, a straightforward method such as evaporative rate analysis may initiate a trend toward simplicity in the investigation of long chain molecules. ACKNOWLEDGMENT
The author is grateful to Gary Greene of Ametek Technical Products for his assistance in setting up the experimental procedures. RECEIVED for review July 24, 1968. Accepted September 13, 1968.
Determination of Tantalum and Its Separation from Niobium and Other Closely Associated Elements Using Tetra-N-Pentylammonium Bromide R. G . Dosch Sandia Laboratory, Albuquerque, N.M . NUMEROUS TECHNIQUES and methods have been proposed for separating tantalum and niobium from closely associated elements, as well as from one another; the latter has proved to be the more difficult task. A long list of gravimetric procedures for the separation of tantalum from niobium can be found in the literature. The more popular of these appear to be the tannin procedure as described by Schoeller ( I ) , the N-benzoyl-N-phenylhydroxylamineprocedure (2, 3), and the selenous acid procedure (4). These gravimetric methods have proven unattractive for reasons such as being extremely time consuming and tedious; giving incomplete separation or incomplete recovery of the precipitated element; and having precipitation conditions involving very careful pH control in a relatively narrow range, high complexing ion concentration, and rigid limitations as to amounts and ratios of elements present in the sample to be analyzed. For these reasons, most of the current analytical work involving tantalum-niobium systems is being done using ion exchange techniques to effect the necessary separations. Ion exchange techniques for separating niobium from tantalum, which are valid for any ratio of the two metals, are outlined in a general review of the analytical chemistry of niobium and tantalum by Kallmann ( 5 ) . These techniques have the added advantage of separating niobium and tantalum from closely associated elements in Groups IV and V as well as from each other. The main disadvantage results when these analyses are done on a nonrepetitive basis. In this situation, preparation of an ion exchange system becomes time consuming, while the separation time may only take 4-6 hours. (1) A. R. Powell and W. R. Schoeller, Analyst, 50, 485 (1925). (2) A. K. Majumdar and A. K. Mulherjee, Nufurwissenschuffen, 44, 491 (1957). (3) R. W. Moshier and J. E. Schwarberg, ANAL.CHEM., 29, 947 (1957). (4) F. S. Grimaldi and M. M. Schnepfe, ibid., 30, 2046 (1958). (5) S. Kallmann in “Treatise of Analytical Chemistry,” Part 11,
Vol. 6, Section A, I. M. Kolthoff and P. J. Elving, Eds., Interscience, New York, N. Y.1962, pp 177-406.
Techniques such as cellulose chromatography and solvent extraction, which have been successfully used in separating Nb from Ta, present somewhat the same problem when applied to a nonroutine or nonrepetitive analytical situation. In the work reported here, tetra-n-pentylammonium bromide is proposed as an analytical reagent for the isolation and determination of tantalum by gravimetric techniques. Tantalum is quantitatively precipitated as white, easily filterable, tetra-n-pentylammonium tantalum hexafluoride (TPATF), which is subsequently fired, and the tantalum weighed as Tanor. Interfering substances are limited to anions such as chromate, permanganate, perrhenate, tungstate, vanadate, and molybdate which form nonvolatile tetra-n-pentylammonium compounds (6). Elements which form insoluble fluorides also interfere; however, these can be easily eliminated by filtration prior to precipitation of the tantalum. Elements closely associated with Ta, those in Groups IV, V, and VI, were studied relative to their quantitative effect upon the precipitation of the TPATF. Quantitative separations were obtained in mixtures containing Ta to impurity ratios ranging from 10 :1 to 1 :10 with amounts of tantalum ranging from approximately 10-150 mg. Larger ratios of impurity to tantalum or larger amounts of tantalum have not been attempted at this time. Total time required for the separation and determination of Ta is approximately 2 1 / 2hours. This allows 1 / 2 hour for dissolution of the sample, precipitation of TPATF, and the filtering process, and 2 hours for firing the precipitate to Ta205. EXPERIMENTAL
Reagents. Tetra-n-pentylammonium bromide (TPABr) was used as received from Eastman Organic Chemical Co. without further purification. The metals used in this work (6) R. G . Dosch, ANAL.CHEM., 40,829 (1968). VOL. 41, NO. 1, JANUARY 1969
193
