EFFECT OF TIME ON INFRARED SPECTRA
OF EPOXY PYROLYZATES EUGENE CERCEO
Vertol Division, The Boeing Co., Philadelphia, Pa. 19142
Changes in the infrared spectra of the pyrolyzates of epoxy-glass fiber composites were studied as a function of time. A relatively simple and straightfoward laboratory technique w a s adopted; in all cases the spectra were reproducible when the resins were analyzed immediately following pyrolysis. When the pyrolyzates were exposed to a laboratory environment for a prolonged period, spectral changes occurred in the fingerprint region (below 1500 cm.-’). These changes were attributed to the atmospheric oxidation and loss of lighter molecular weight species, such as cresols and xylenes.
INFRARED
SPECTROSCOPY using reference spectra provides an excellent and economic means for the rapid and positive identification of organic compounds and commercial materials. Success in infrared analysis depends on the degree to which the materials in question can be separated from other components. Frequently, polymeric materials cannot be analyzed by the usual infrared techniques, such as the mulling (using Nujol or perfluorokerosine), solution, melting, mechanical film, and film from solution methods (Hausdorff, 1951). When such cases are encountered, a thermal method known as pyrolysis has been successful. Pyrolysis has been defined as a method whereby a usually high molecular weight material is subjected to an intense heat for a short time in an oxygen-free atmosphere (Barnes Engineering Co., 1965). I n the case of polymers, fragmentation of the chains occurs, usually at preferred sites, with collection of the condensate in the gaseous or liquid phase. The residues remaining from the process may consist of plasticizers, fillers, and other inert additives. The dark color is primarily due to fragmented and carbonized polymer chains. This technique has become invaluable in determining the polymer backbone of a highly filled sample. Materials with a high percentage of carbon filler produce poor infrared spectra, while analysis of certain rubbers by infrared attenuated total reflectance spectroscopy (ATR) frequently yields spectra indicating essentially the presence of filler and only minutely the presence of polymer. When thermal excitation of polymers leads to bond cleavage, the pyrolysis products appear to be distributed in a certain definite ratio. The assumption is that operating conditions have been closely controlled; and, therefore, the pyrolysis products from a series of determinations on the same material will yield identical spectra. The expectation might be that thermal degradation will produce a wide variety of fragmentation products, yielding
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Ind. Eng. Chern. Prod. Res. Develop., Vol. 9,No. 1 , March 1970
very complex spectra. Yet this is not the case, for the spectra obtained are both characteristic and reproducible. I n various cases the pyrolysis spectra are almost identical to that of the parent long-chain polymer. I n most cases there is no change in the infrared spectra when pyrolysis conditions are slightly altered, although more sensitive techniques such as high resolution nuclear magnetic resonance and mass spectrometry should be capable of detecting such changes, should they occur. The spectral similarity of the pyrolyzate to the parent polymer is due to the fact that characteristic units cleaving from the high molecular weight polymer retain their individual character. Thus changes in molecular weight have no effect on these spectra (Harms, 1953). The pyrolysis method has been used for the identification of other crosslinked polymeric materials, such as polyurethane foams, phenolic-nitrile adhesives, and silicone rubber tapes. I n all cases there has been speculation that the pyrolyzates should be analyzed immediately for fear of atmospheric interaction, specifically oxidation. If such reactions were to occur, identification would naturally become ambiguous. So, in an effort to measure the effects of such interactions, a time study was conducted using thermoset epoxy-glass fiber composites. Structurally, the epoxy resins investigated can be characterized by the presence of ether linkages formed from the reaction of a comonomer with the epoxy ring,
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These widely used resins are formed from the addition of bisphenol-A to epichlorohydrin, the molecular weight of the resulting polymer being a function of the monomer ratio. The polymer can be represented by the structure:
c H, The liquid epoxy resins ( n = 0 to 1) are converted through the reactive epoxy sites into tough, rigid, and insoluble solids ( n > 1), in two ways: increase in molecular weight and therefore chain lengthening, and crosslinking using such agents as amines, anhydrides, and carboxylic acids (Dow Chemical Co., 1966). Experimental Proceduire
The epoxy-glass fiber composites studied were E-705 supplied by the U. S. Polymeric Co., Stamford, Conn., and BP-907 supplied by the American Cyanamid Co., Havre de Grace, Md. The instrument used for analysis was the Perkin-Elmer 521 infrared spectrophotometer scanning from 4000 c m - ' ( 2 . 5 microns) to 250 cm.-' (32 microns), although the spectra were found to be useful down to 400 cm.-' (25 microns). The actual method involved taking approximately 0.1 to 0.2 gram of sample and placing it in a small thickwalled test tube. The tube was then heated over the flame of a Bunsen burner. The upper part of the tube was kept relatively cool, but warm enough to avoid condensation of water vapor. The process was continued until a substantial amount of condensate had been collected. The pyrolyzate was then transferred by a closed-end capillary tube, and spread thinly and evenly on an optical potassium bromide window. The window was clamped in place in its holder ;and inserted into the sample beam of the spectrophotomeiter. An infrared spectrum was then obtained immediately and a t various times. Results and Discussion
I t is extremely difficult if not impossible to analyze thermoset resins by methods other than pyrolysis, especially if the polymeric materials appear in the form of glass fiber-reinforced epoxy resins. The problem arises in separating the crosslinked polymer from a glass fiber matrix followed by analysis for positive identification. Although seeming crude, infrared spectroscopy has given accurate and reproducible results. For such a simple procedure it is unsurpasijed for speed and accuracy. And though a more sophisticated capability such as pyrolysis chromatography followed by mass spectral analysis is the ultimate in polymer identification, still the amount of data obtained from this simple technique is rather amazing. Experimentally, the results indicate that even though different manufacturers and possibly different crosslinking agents are involved, the spectra of E-705 and BP-907 are essentially the same (Figure 1 and Table I ) , because of the presence of the same polymer backbone. The insensitivity to the presence of the crosslinking agent is probably caused by two factors: the inherent low crosslinking agent concentration and the overwhelming
bonding characteristics of the polymer backbone compared with those formed from the crosslinks. The spectra of the polymers after 7 hours indicate definite changes in band intensities. The most pronounced changes occur in the low frequency end of the spectra, specifically a group of absorptions a t 690, 750, 830, and 880 cm.-' These four bands were most seriously diminished after 24 hours. It is surmised that, because of their sharper disappearance, these bands are caused by cleavages giving rise to lighter molecular weight fragments, such as cresols, phenols, and xylenes (Flom et al., 1965). Lee (1965) along with Bishop and Smith (1967) reported that the products obtained from the pyrolysis of epoxies are: hydrogen, methane, water, acetylene, ethylene, carbon monoxide, ethane, formaldehyde, allene, propylene, acetylene, propane, carbon dioxide, methyl chloride, acrolein, acetone, propionaldehyde, vinyl chloride, cyclopentadiene, benzene, and methylcyclopentadiene, along with various types of aromatics such as phenols and cresols. At higher pyrolysis temperatures (1200" C.) Madorsky and Strauss (1961) found that hydrogen, ethylene, cyclopentadiene, and benzene started to predominate. Under these conditions such products escape as gases, and thus do not complicate the spectra. Spectroscopists generally consider frequencies below 1500 cm.-' as due to vibrations of all or parts of a molecule. This area is known as the fingerprint region and is very sensitive to structural changes. Above 1500 cm.-' is considered the functional group region, sensitive to organic functional groups. This latter area is more widely used for identification, although the fingerprint region can become useful. Specifically, it is possible to have polymers with identical functional groups but different molecular structures. At times, these can be separated on the basis of their fingerprint spectra. Experimentally, it was observed that the low boiling volatiles were only a small portion of the total pyrolyzate. The majority of the pyrolyzed products were tarry residues whose infrared spectra conformed closely to the spectra of cresols and phenols. Xylenes also fitted nicely into the spectral patterns. The presence of these cresols and phenols indicated cleavage of the methylene linkage and the removal of water. Thus, this infrared investigation correlated rather well with the findings of Lee (1965) in that the degradation products of the epoxy resins were mostly phenolic-type compounds. Theoretically, the decrease in band intensity as a function of time can be attributed to the slow loss of the light molecular weight fragments which have relinquished their sites of mutual electrostatic attraction because of atmospheric reactions (Figures 2 and 5). Most of these sites were formed as a result of thermal degradation. I n competition is the atmospheric water absorption on hydrophilic sites. Figures 2 and 4 show that despite the evaporation of the lighter pyrolyzate fragments, the hydroxyl band intensified. Again, this was attributed to water absorption from the atmosphere and also from water and other enol products formed as a result of interactions within the pyrolyzate. After approximately 50 hours the pyrolyzates essentially stabilized, as evidenced by the absence of further appreciable intensity changes. Using area determinations instead of absorptions for the specified bands yielded no added advantage. I n most cases it was more accurate to use the total absorption of the peaks, since area determinations were not possible Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 1, March 1970
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Figure 1 . Infrared spectra of epoxy pyrolyzates of E-705 and BP-907 epoxy-glass fiber composites taken immediately and a t selected times after pyrolysis
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Table 1. Assignment of Bands from Infrared Spectra of Thermoset Epoxy Pyrolyzates from E-705 and BP-907 Composites
Relative Descript ion"
Location, Cm.-'
Assignment'
3350
Broad and strong
3050 2980 1715
Hidden and fairly strong Fairly sharp triplet and strong Partially hidden with medium intensity Partially hidden and strong
1650 1600 1500 1450 1370
880 830
Sharp doublet and strong Sharp and strong Fairly broad and strong Fairly broad, strong and partially hidden Broad and strong Sharp and partially hidden with medium intensity Sharp, but weak Shitrp and strong
730 690
Sharp and strong Sharp and weak
1230 1170
Predominately atmospheric water absorption. Also, possible hydroxyl groups from fragmented polymer chains. Aromatic carbon-hydrogen stretching mode Aliphatic carbon-hydrogen stretching mode Carbonyl formation attributed to the degradation of amine cured resins. Possible nitrogen-type linkage due to the reaction of the crosslinking agent with the polymer Epoxy aromatic band Epoxy aromatic band Methylene bending mole Indicates presence of secondary alcohol functional groups bonded to fragmented polymer chains Aromatic ether probably due to the aromatic stretching mode Indications of aromaticity. Also, epoxy group region Indicates presence of cresols Para-substituted phenyl ring plus possible presence of cresol Indicates presence of cresols and xylols Indicates presence of cresols and xylols
a Relative descriptiow taken from infrared spectra obtained immediately after pyrolysis. 'Band assignments aided through consultation with standard references (Bellamy, 1962; Sadtler Research Laboratories, 1963).
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Figure 2. Absorbance as a function of time for selected peaks in infrared fingerlprint spectrum of pyrolyzate of E-705
Figure 3. Absorbance as a function of time for selected peaks in infrared functional group spectrum of pyrolyzate of E-705
because of the crowding and overlapping of the bands. Finally, degradation readily occurs a t the epoxide ring, and, therefore, if the polymer is to have thermal stability these sites should completely react. The resulting ether linkages require greater energetics for cleavage; thus when bonds weaker than carbon-oxygen are present-for example, carbon-nitrogen-it seems likely that these bonds will
be broken preferentially. Detecting the products of this cleavage in the pyrolyzate spectrogram may be difficult. Thus, for epoxies and a wide variety of other polymer types, delay in the spectral analysis following pyrolysis may adversely affect reproducibility. I t is recommended that for this type of analysis, spectra be obtained as soon as possible, since atmospheric oxidation and hydrolyInd. Eng. Chem. Prod. Res. Develop., Vol. 9 , No. 1, March 1970
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Figure 4. Absorbance as a function of time for selected peaks in infrared fingerprint spectrum of pyrolyzate of
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sis may cause loss or rearrangement of the pyrolysis products. A reasonable situation can be given. Consider the presence of fragments still containing hydroxyl ethers from the initial polymerization. These can dehydrate and yield unsaturated ethers. I n turn the unsaturated ethers may cleave or rearrange into unsaturated phenols which may cause some polymerization of the residue, and a subsequent loss in spectral integrity. Thus the spectrograms may become ambiguous or useless. As a further step, an in-depth investigation from the viewpoint of mass spectral analysis and possibly high resolution NMR would seem feasible. Such a study should lead t o the identification of the products of pyrolysis as a function of time, and toward the elucidation of the mechanisms involved when a pyrolyzate is left standing for a long period. Such a study would undoubtedly shed more light into the nature of pyrolyzates, their reactions and mechanisms. Literature Cited
Barnes Engineering Co., Instrument Division, Stamford, Conn., 06902, 1965, Applications Data, File AD-7, p. 1. Bellamy, L. J., “Infrared Spectra of Complex Molecules,” pp. 118-9, Wiley, New York, 1962.
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Figure 5. Absorbance as a function of time for selected peaks in the infrared functional group spectrum of pyrolyzate of BP-907
Bishop, D. P., Smith, D. A., Id.Eng. Chem. 59, No. 8, 32-9 (1967). Dow Chemical Co., Plastics Department, Midland, Mich. 48640, “DOWLiquid Epoxy Resins,” p. 4, 1966. Flom, D. G., Speece, A. L., Schmidt, G. A,, “Investigation of Catalytic Thermal Oxidative Degradation of Organic Polymers a t Elevated Temperatures,” Air Force Materials Laboratory, Research and Technology Division, U. S. Air Force, Wright-Patterson Air Force Base, Ohio, Tech. Rept. AFML-TR-65-235, 36-7, 41-3 (July 1965). Harms, D. L., Anal. Chem. 25, 1140 (1953). Hausdorff, H. H., Perkin-Elmer Corp., Nonvalk, Conn. 06852, “Analysis of Polymers,” Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 7, 1951. Lee, L. H., J . Polymer Sci. 62A, 859 (1965). Madorsky, S. L., Strauss, S., Modern Plastics 38, No. 6, 134 (1961). Sadtler Research Laboratories, Philadelphia, Pa., 19104, “Sadtler Standard Spectra,” “Pyrolyzates,” Vol. I, p. 2, 1963.
RECEIVED for review July 16, 1969 ACCEPTED November 16, 1969
