John Chih-An Hu Quality Assurance Laboratories
Boeing Aerospace Company MIS 23-22 P.O. Box 3999
Seanle, 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 i n the Air Launched Cruise .Missile (ALCM, projert of Boeing Aerospace CGmpany during the crucial competition for a major defense cmtract. During the final inspection heiwe the scheduled deliveryof 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 s t e e , and it was critical to meet the de1i;ery schedule. The part
in question was an expensi\,e item, and the nondestructive dampling analysia necessary allowed only a tiny amvunt of sample LO he obtained. Conventional mechanical and instrumental analvtical 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 deliverv to make an unmecedented visit to thelaboratory to personally re-
quest a miraculous analysis within the hour to meet the deliver) schedule. A new analytical approach-a technique called chromatopyrngraphy (CI'CJ, which had heen 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 orieinallv develoDed it to modernize thequality control test methods
gure 1. Chromatopyrography A Chmrnalopyrograrn
Rubberlike Material
Vaporizatinn
A
Second 1000 "C
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Nonvolatile Ingredients
Pyrolysis
used in the inspection of incoming nonmetallic industrial g o d s and polymeric 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 industry today, polymeric materials are widely used, especially those with relatively low density and consequent light weight.
Traditional Methods of Analysis Traditional mechanical tests (i.e., tensile, elongation, etc.) are not satisfactory for the complete characterization of polymeric materials since they do not define a unique chemical identity of the material being tested. Also, they can he 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 aualitv . . control DUIposes. But academic instrumental methods often have to be modified for the solution of industrial problems. New approaches are sometimes required berause industrial materials tend to be diverse and complex in composition, often making sample preparation troublesome. It was a new approach to a conventional instrumental technique-pyrolysis gas chromatography-that resulted in the development of chromatopyrography. I t 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 prohlems with interlaboratory reproducibility and standardization. Results of cooperative studies among many laboratories in the U.S. ( 1 ) and in Europe (24)indicated that reproducible pyrograms could be obtained from pure polymers, but that rommercia1 polymeric materials give poor results because of their chemical complexity. Unfortunately, the majority of samples received in industrial lahoratories fall into this latter category. Most efforts to improve reproducibility in recent years have been concentrated on the design of an effective py. rolyzer with a fast pyrolysis temperature rise time ( 6 ) .But little attention has been directed toward other weaknesses in the method. The first of these weaknesses involves the solid sample introduction technique. The traditional procedure required a prepyrolysis waiting period 312A
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Figure 2. Chromatopyrogram ai 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 injection port temperature, boiling points of the volatile constituents of the sample, length of the waiting period, and the carrier gas flow rate. Also, compounded polymeric materials contain definite amounts of volatile ingredients. Some volatile ingredients were lost during the waiting period,and the remaining volatiles caused irreproducihility problems. In some procedures, attempts were made to remove the volatilesby solvent extraction, but in addition tQ being timeconsuming, this created problems of its own since residual solvent and impurities from the solvent complicate the analysis. The analysis of these volatile ingredients is just as important as analysis of the . Dolvmer it . because.. freauentlv. . is the formulation containing volatile ingredients that is unknown rather than the polymer itself. A large number 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 carrier gas flow system, would be more desirable than elimination of the volatiles by extraction or use of a waiting period. Chromatopyrography (7-11) is such a process. ~~
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The CPG Approach CPG, as shown in Figure 1,involves
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2. FEBRUARY 1981
only one step hut is actually a twoshot process. The first shot is designed to determine the volatile ingredients and the specific formulation. T h e second shot identifies the polymeric structure. The procedure is actually quite simple. The injection port is preheated to 270 OC and maintained at that temperature. This heat along with a carrier gas flow of 30 mL/min causes an effective flash vaporization. The analysis begins as soon as the sample is inserted. This is the first shot, which immediately 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 polymer is then pyrolyzed at 1000 "C for 15 s to develop pyrogram B, which serves as a fingerprint of the polymer. After pyrogram B is complete, the inorganic residue can be isolated for elemental analysis if desired. T h e combination of chromatogram A and pyrogram B forms what we call achromawpyrogram. These two shots are more effective in comdetelv characterizing a polymeric matkrial than either shot alone. T h e CPG approach effectively meets a number of requirements. First, the labyrinth-type design of the injection port is such that sample lossea do not occur due to back flush of the carrier gas (9).During sample insertion, the carrier gas ismomentarily released into the atmosphere along the outermost peripheral edge of the in0003-2700/81/035%31 lASO1.00/0 1981 American Chemical Society
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jector barrel and the space inside the injector liner where the sample is located is in a static condition without hack-flush. As soon as the pyrolysis probe is sealed, the normal carrier gas flow is resumed and gas chromatographic analysis begins. All volatile ingredients 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 temperatures of the monomeric ingredients to such an extent that practically all volatile ingredients are vaporized instantaneously a t 270 OC while the high polymers are not affected ( J J ) . The requirement of only a minute sample size in the range of sub-milligrams 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 he used for the direct introduction of heterogeneous viscous liquid samples for CC analysis (91,simplified headspace-type analysis, and splitless sample injection for capillary CC (9).
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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 efforts, can now be completed by CPG
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ANALYTICAL CHEMISTRY, VOL. 53. NO. 2. FEBRUARY 1981
within an hour. It is CPC that solved the urgent identification problem we described earlier concerning the unstamped missile part. We knew the part should have been fabricated from an approved Boeing proprietary formulation that was based on a specific siloxane polymer. Figure 2 is the chromatopyrogram of the sample, which was removed from the missiles by a pseudonondestructive technique. It is identical to that of the specified Boeing proprietary material (silicone rubber A). The unmarked missile part had been unambiguously identified within the time limit and the missiles could he 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 (Airborne 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 ruhher sheaths covering high voltage electric cables showed signs of crazing while other similar sheaths didn't show such failure signs. Failure of electric insulation might result in serious difficulties in operating the AWACS system, so the problem was immediately investigated. 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
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Figure 4. Chromatopyrogram of failed nitrile sheath subjected to our former conventional rubber analysis: extraction with solvents, then pyrolysis and infrared (IR) spectrometry, a time-cpnsuming procedure. The IR results of,the failed material indicated a nitrile type of rubber, hut the formulation of the sample was not positively identified. Therefore crazed and noncrazed ruhher sheaths were reexamined with CPG, which immediately character. ized hoth the fnrmulation and poly-
mer structure of the materials. From the "fingerprints" of the polymer structure the failed sheath was positively 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 attacked the nitrile ruhher and caused the crazing of the sheath. The neoprene ruhher 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 encountered in the hammer shop, where airplane metal parts are shaped in various forms. The rubber forming pad is one of the essential tools needed in the shaping process. One hatch 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 results revealed that the used good pad was made of a natural rubber while the cracked one was made of a synthetic nitrile rubber. The problem was solved by replacing the nitrile rubber forming pads with natural rubber materials. These three cases exemplify how a new analytical approach can modernize quality control test methods and then be used for other industrial problem-solving applications. CPG has already proven itself useful in characterizing rubberlike materials and undoubtedly will be used to solve additional real-world problems in the future. After all, two shots a t any analytical problem are better than one!
References ( I ) J.Q. Wa1ker.J. Chromotogr. Sei.. 15.
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267-74 (1977). (2) N. B. Couw. C. E.R. Jones. and S.G .
..
malograph& 8,696-8 (1975). 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(6)
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Flgure 5. Chromatopyrogram of intact neoprene sheath 318,.
ANALYTICAL CHEMISTRY, VOL. 53. NO. 2, FEBRUARY 1981
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J. C. A. Hu. U.S. P
1979
(9i>."C. A. Hu,Anal. Chem., 51, (14), 129E-7 (1979). (IO) W.Worthy, Chem. Enp. News, 58, (22). 2 6 2 7 (1980): (11) J. C.A. Hu.Anol. Chem.. 49,(4). 537-40 (1977).
