John Chih-An Hu
The Analytical Approach
Quality Assurance Laboratories Boeing Aerospace Company M/S 23-22 P.O. Box 3999 Seattle, Wash. 98124
Edited by Jeanette G. Grasselli
Chromatopyrography for Polymer Characterization This one-step two-shot analytical technique is a new adaptation of pyrolysis gas chromatography An urgent problem arose in the Air Launched Cruise Missile (ALCM) project of Boeing Aerospace Company during the crucial competition for a major defense contract. During the final inspection before the scheduled delivery of the missiles to the U.S. Air Force, the quality assurance engineers discovered that one rubber part of the missile was not stamped with identification marks and its composition was unknown. The delivery was held up. The competition between Boeing and another aircraft company had reached a decisive stage, and it was critical to meet the delivery schedule. The part
in question was an expensive item, and the nondestructive sampling analysis necessary allowed only a tiny amount of sample to be obtained. Conventional mechanical and instrumental analytical methods were undesirable because (a) they were not specific enough for characterization; (b) they were too slow to meet the delivery schedule; or (c) they required a large sample size, which would result in destruction of the part. The urgency of the problem prompted the ALCM management team responsible for delivery to make an unprecedented visit to the laboratory to personally re-
quest a miraculous analysis within the hour to meet the delivery schedule. A new analytical approach—a technique called chromatopyrography (CPG), which had been developed in our Boeing Quality Assurance Laboratories—made this speedy analysis a reality. This article will describe the new approach and some of its applications to actual problem-solving. Chromatopyrography, a one-step two-shot analytical technique, is a new adaptation of pyrolysis gas chromatography. Boeing's analytical chemists had originally developed it to modernize the quality control test methods
Figure 1 . C h r o m a t o p y r o g r a p h y
Rubberlike Material
A Chromatopyrogram
Formulation
Volatile Ingredients
Vaporization First Shot 270 °C
Pyrolysis
Second Shot 1000 °C
Polymer
Ash
Nonvolatile Ingredients
Elemental Analysis
used in the inspection of incoming nonmetallic industrial goods and poly meric materials. This modernization was necessary because the ever-increasing number of test samples from incoming materials mandated faster and more economical test methods. In the aerospace indus try today, polymeric materials are widely used, especially those with rel atively low density and consequent light weight. Traditional Methods of Analysis
Traditional mechanical tests (i.e., tensile, elongation, etc.) are not satis factory for the complete characteriza tion of polymeric materials since they do not define a unique chemical iden tity of the material being tested. Also, they can be time-consuming, costly, and they often require large sample size and test specimens in specific forms. Instrumental methods such as chromatography, mass spectrometry, infrared spectrometry, and thermal analysis are faster than mechanical tests, and the data are more suitable and reliable for quality control pur poses. But academic instrumental meth ods often have to be modified for the solution of industrial problems. New approaches are sometimes required because industrial materials tend to be diverse and complex in composi tion, often making sample preparation troublesome. It was a new approach to a conventional instrumental tech nique—pyrolysis gas chromatogra phy—that resulted in the develop ment of chromatopyrography. It was used first for quality control purposes, but later it was applied to the solution of problems such as the unidentified missile part. Polymer analysis by conventional pyrolysis gas chromatography suffers from problems with interlaboratory reproducibility and standardization. Results of cooperative studies among many laboratories in the U.S. (1 ) and in Europe (2-5) indicated that repro ducible pyrograms could be obtained from pure polymers, but that commer cial polymeric materials give poor re sults because of their chemical com plexity. Unfortunately, the majority of samples received in industrial labora tories fall into this latter category. Most efforts to improve reproducibili ty in recent years have been concen trated on the design of an effective pyrolyzer with a fast pyrolysis tempera ture rise time (6). But little attention has been directed toward other weak nesses in the method. The first of these weaknesses in volves the solid sample introduction technique. The traditional procedure required a prepyrolysis waiting period
Time (mln)
Time (min)
Figure 2. Chromatopyrogram o i silicone rubber A
after the sample was inserted into the injection port, but this waiting period resulted in sample losses. The amount of sample lost depended on the injec tion port temperature, boiling points of the volatile constituents of the sam ple, length of the waiting period, and the carrier gas flow rate. Also, compounded polymeric mate rials contain definite amounts of vola tile ingredients. Some volatile ingredi ents were lost during the waiting peri od, and the remaining volatiles caused irreproducibility problems. In some procedures, attempts were made to re move the volatiles by solvent extrac tion, but in addition to being timeconsuming, this created problems of its own since residual solvent and im purities from the solvent complicate the analysis. The analysis of these volatile ingre dients is just as important as analysis of the polymer because, frequently, it is the formulation containing volatile ingredients that is unknown rather than the polymer itself. A large num ber of polymeric materials can be manufactured from a single type of polymer by varying the ingredients in these formulations. We felt that an on-line process, which could be carried out in the car rier gas flow system, would be more desirable than elimination of the vola tiles by extraction or use of a waiting period. Chromatopyrography (7-11) is such a process. The CPG Approach
CPG, as shown in Figure 1, involves
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only one step but is actually a twoshot process. The first shot is designed to determine the volatile ingredients and the specific formulation. The sec ond shot identifies the polymeric structure. The procedure is actually quite sim ple. The injection port is preheated to 270 °C and maintained at that tem perature. This heat along with a car rier gas flow of 30 mL/min causes an effective flash vaporization. The anal ysis begins as soon as the sample is in serted. This is the first shot, which im mediately drives out all of the volatile ingredients and results in chromatogram A, which serves as a fingerprint of the formulation. After chromatogram A is complete, the second shot is fired simply by pushing the pyrolysis button. The thermally purified poly mer is then pyrolyzed at 1000 °C for 15 s to develop pyrogram B, which serves as a fingerprint of the polymer. After pyrogram Β is complete, the in organic residue can be isolated for ele mental analysis if desired. The combi nation of chromatogram A and pyro gram Β forms what we call a chroma topyrogram. These two shots are more effective in completely characterizing a polymeric material than either shot alone. The CPG approach effectively meets a number of requirements. First, the labyrinth-type design of the injection port is such that sample losses do not occur due to back flush of the carrier gas (9). During sample inser tion, the carrier gas is momentarily re leased into the atmosphere along the outermost peripheral edge of the in0003-2700/81/0351-311 A$01.00/0 © 1981 American Chemical Society
mercury vapor spectra 1 st Order Spectrum Grating: 118Qg/mm 4000 A Blaze Slits: lOO^im χ 12 7mm
1st Order Spectrum Grating 1180 g/mm 4000 A Blaze Slits 10um χ 5mm
Time (min)
3021 5 A 3023 5 A 3025 6 A 3027 5 A (Left to Right)
Hg 3021 3027 Quadruplet
3650 1 A 3654 8 A 3663 3 A (Lett to Right)
Hg 3650 3663 Triplet
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Time (min)
Figure 3. Chromatopyrogram of silicone rubber Β
jector barrel and the space inside the injector liner where the sample is lo cated is in a static condition without back-flush. As soon as the pyrolysis probe is sealed, the normal carrier gas flow is resumed and gas chromato graphic analysis begins. All volatile in gredients of the sample are thus subjected to CPG analysis. Another factor is that the dynamic percolating conditions in the heated injection port substantially lower the boiling tem peratures of the monomeric ingredi ents to such an extent that practically all volatile ingredients are vaporized instantaneously at 270 °C while the high polymers are not affected (11). The requirement of only a minute sample size in the range of sub-milli grams to micrograms also favors a rapid and complete vaporization of the volatile ingredients. And the fast vaporization results in sharp peaks and a stable recorder baseline during sample insertion. Meeting all of these conditions means CPG can also be used for the direct introduction of heterogeneous viscous liquid samples for GC analysis (9), simplified headspace-type analy sis, and splitless sample injection for capillary GC (9). CPG Applications
CPG has modernized industrial quality control test procedures, and it also has problem-solving applications. Many real-world analyses involving rubberlike polymeric materials, which in the past required long research ef forts, can now be completed by CPG
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316 A • ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
within an hour. It is CPG that solved the urgent identification problem we described earlier concerning the unstamped mis sile part. We knew the part should have been fabricated from an ap proved Boeing proprietary formula tion that was based on a specific siloxane polymer. Figure 2 is the chromato pyrogram of the sample, which was re moved from the missiles by a pseudonondestructive technique. It is identi cal to that of the specified Boeing pro prietary material (silicone rubber A). The unmarked missile part had been unambiguously identified within the time limit and the missiles could be delivered on schedule! Figure 3 shows the chromatopyrogram of a different silicone rubber (silicone rubber B) for comparison purposes. A second case that required a CPG solution occurred in the Boeing AWACS project. The AWACS (Air borne Warning and Control System) is the aircraft with a flying-saucerlike radome mounted on top of a Boeing 707 air frame. During a routine inspection of the prototype AWACS aircraft it was discovered that some rubber sheaths covering high voltage electric cables showed signs of crazing while other similar sheaths didn't show such failure signs. Failure of electric insula tion might result in serious difficulties in operating the AWACS system, so the problem was immediately investi gated. The approach was to first find out what materials were involved by chemical analysis and then to deduce the cause of failure. The sample of the failed sheath was
Time (min)
Time (min)
Figure 4. Chromatopyrogram of failed nitrile sheath
subjected to our former conventional rubber analysis: extraction with sol vents, then pyrolysis and infrared (IR) spectrometry, a time-cpnsuming pro cedure. The IR results of the failed material indicated a nitrile type of rubber, but the formulation of the sample was not positively identified. Therefore crazed and noncrazed rub ber sheaths were reexamined with CPG, which immediately character ized both the formulation and poly
mer structure of the materials. From the "fingerprints" of the polymer structure the failed sheath was posi tively identified as a nitrile rubber and the good sheath as neoprene. The chromatopyrogram shown in Figure 4 is from the failed nitrile sheath, and Figure 5 is that of the intact neoprene sheath. The analyses were fast and the results were unambiguous. It was deduced that the high voltage environment had apparently resulted
in a high ozone concentration that at tacked the nitrile rubber and caused the crazing of the sheath. The neo prene rubber was more ozone resistant and was not adversely affected. The problem was solved by replacing the failed nitrile materials with neoprene rubber sheaths. The CPG technique, when compared to the conventional method, was much faster, less costly and more specific. A third case was a problem encoun tered in the hammer shop, where air plane metal parts are shaped in vari ous forms. The rubber forming pad is one of the essential tools needed in the shaping process. One batch of rubber forming pads was found defective. The defective pads had torn and cracked after only three days in use while the previous pads had been used for months without cracking. The shop supervisor wondered why this had suddenly happened, so both the cracked new material and a used good pad were analyzed by CPG. The re sults revealed that the used good pad was made of a natural rubber while the cracked one was made of a syn thetic nitrile rubber. The problem was solved by replacing the nitrile rubber forming pads with natural rubber ma terials. These three cases exemplify how a new analytical approach can modern ize quality control test methods and then be used for other industrial prob lem-solving applications. CPG has al ready proven itself useful in charac terizing rubberlike materials and un doubtedly will be used to solve addi tional real-world problems in the fu ture. After all, two shots at any analytical problem are better than one!
References
Time (min)
Time (min)
Figure 5. Chromatopyrogram of intact neoprene sheath 318 A • ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
(1) J. Q. Walker, J. Chromatogr. Sci., 15, 267-74 (1977). (2) N. B. Coupe, C. E. R. Jones, and S. G. Perry, J. Chromatogr., 47, 291-6 (1970). (3) C. E. R. Jones, S. G. Perry, and Ν. Β. Coupe, in "Gas Chromatography 1970," R. Stock, Ed., Elsevier Publishing Com pany, Amsterdam, 1971, pp 399-406. (4) N. B. Coupe, C. E. R. Jones, and P. B. Stockwell, Chromatographic!, 6, (11), 483-8(1973). (5) T. A. Gough and C. E. R. Jones, Chro matography, 8,696-8(1975). (6) E. J. Levy, 26th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 3-7, 1975, Paper No. 19. (7) J. C. A. Hu, 30th Pittsburgh Confer ence on Analytical Chemistry and Ap plied Spectroscopy, Cleveland, Ohio, March 5-9, 1979, Paper No. 092. (8) J. C. A. Hu, U.S. Patent 4 159 894, 1979. (9) J. C. A. Hu, Anal. Chem., 51, (14), 1295-7 (1979). (10) W. Worthy, Chem. Eng. News, 58, (22), 26-27(1980): (11) J. C. A. Hu, Anal. Chem., 49, (4), 537-40(1977).
