Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 68-73
08
Table I. ,OperatingEnergy Cost to Remove 1000 lb/h Water from PVC Latexa spray dryer energy rel o 6 Btu/h quired energy source 1000 SCF/h natural gas energy unit $2.50/103 SCF cost annual energy $20 000 cost 8000 h P.A.
evaporatorb
ultrafilterC
333.3 x 103 3 7 . 2 ~103 Btu/h Btu/h 333.3 lb/h 10.9 kW steam electricity $4.00/103lb $0.03/kWh $10 656
$2616
Dewatering PVC latex from 38% to 50% TS. Assuming a 3-effects evaporator, Assuming 41 gfd average flux and 70% pumping efficiency. a
as brand new membranes, it was clearly demonstrated that over 80% process flux recovery could be achieved consistently with periodical solvent cleaning. Conclusion It can be concluded that significant progress in product and process development over the past several years in ultrafiltration technology has resulted in the feasibility of concentrating industrial latex streams via large-diameter tubular ultrafiltration systems on a commercial scale. Techniques to enhance latex stability with respect to ultrafiltration processing conditions have been established. Effective cleaning and reuse of fouled tubular membranes by a solvent soaking and mechanical cleaning have been demonstrated. Due to the high flux performance obtainable with stable latex feeds, especially PVC latex emulsions, the tubular ultrafiltration process is also very energy
efficient. Table I presents a comparison of operating energy costs using ultrafiltration and two evaporative methods for PVC latex dewatering. Based on some reasonable energy unit costa, it is seen that the annual energy cost of using an ultrafilter is only 13% of that for a spray dryer and approximately one-quarter of that for a threeeffects evaporator. Thus, recent advances in ultrafiltration technology have made it a technically viable and economically attractive alternative for industrial latex applications. Literature Cited Barclay, L.; Harrlngton, A.; Ottewiii, R. H. Ko//o/d2. Z . Polym. 1972, 250, 655. Brian, P. L. T. In ”Desalination by Reverse Osmosis”; U. Merten, Ed.; M.I.T. Press: Cambridge, MA, 1966; Chapter 5. DeIPlco, J. US. Patent 4 160726, July 10. 1979. DelPlco, J.; Sternberg, S. U.S. Patent 3 956 114, May 11, 1976. Kozinski, A. A.; Lightfoot, E. N. A I C M J . 1972, 18, 1030. Michaels, A. S. Ghem. Eng. Prw. 1968, 64(12), 31. Porter, M. C. Ind. Eng. Chem. Prod. Res. Dev. 1972, 1 1 , 234. Probstein, R. F.; Shen. J. S. S.; Leung, W. F. Desalination, 1978, 24, 1. Shen, J. S. S., Hoffman, C. R. “A Comparison of Ultrafiltration of Latex Emulsions and Macromolecular Solutions"; presented at the 5th Membrane Seminar, Clemson University, Clemson, SC, May 12-14, 1980. Thomas, D. 0. Ind. Eng. Chem. Fundem. 1973, 12, 396. Utracki, L. A. J . Coibid Interface Scl. 1973, 42, 185. Viiker, V. L.; Colton, C. K.; Smith, K. A. A I C M J . 1961, 2 7 , 632. Wales, M. “Pressure Drop Across Polarized Layers in Ultraflltration”. presented in the Second Chemical Congress of the North American Contenent, Las Vegas, NV, Aug 24-29, 1980. Zahka, J.; Mir, L. Chem. Eng. Prog. 1977, 73(12), 53.
Received for reuiew December 10, 1980 Accepted September 8, 1981 A preliminary part of this work entitled, “Ultrafiltration of Industrial Latex Streams: Data, Analysis and Design”, was presented by one of us (J.S.) at the 88th AIChE National Meeting held in Philadelphia, June, 1980.
High Performance Liquid Chromatography. A Reliable Technique for Epoxy Resin Prepreg Analysis Gary L. Hagnauer’ and Davld A. Dunn Pohmer Research Dlvlslon, U.S. Army Materials and Mechanics Research Center, Watertown, Massachusetts 02172
The application of high performance liquid chromatography (HPLC) techniques for monitoring the chemical compositions of epoxy resin prepregs and for quantitativelyanalyzing specific resin components is discussed. Case studies are presented which demonstrate the versatility and reliability of HPLC for quality assurance. Using HPLC fingerprinting procedures, variations in the compositions of commercial prepreg materials are detected and related to problems with processability and to the failure of composites manufactured from particular prepreg batches. Examples are given to show how HPLC may be used for trouble-shooting prepreg problems and for developing acceptance criteria for use in prepreg specifications. HPLC is used to investigate the aging and curing behavior of an epoxy resin prepreg.
Introduction As part of the Army’s Materials Testing Technology program, high performance liquid chromatography (HPLC) techniques are being developed and evaluated for monitoring the compositions of epoxy resin prepregs (Hagnauer, 1980; Hagnauer and Dunn, 1980; Hagnauer and Setton, 1978). HPLC quality assurance test procedures are being designed and incorporated into specificationsfor epoxy resin prepregs used in the production of structural composites. The prepregs consist of glass or graphite fibers impregnated with 30-40 wt % formulated resin. Typically the resin formulations are complex mixtures of epoxy re-
sins, curing agents, diluents, accelerators, etc., and the surfaces of the fibers are “modified”or treated chemically to enhance bonding with the resins during the curing process. Furthermore, the resins are usually ”staged” or partially reacted during prepreg manufacture, and since the resins are reactive, prepregs may undergo compositional changes during transport and storage. The processability and properties of epoxy-based composites depend upon the composition of the prepreg materials from which they are manufactured. Therefore, quality assurance methods are required to guarantee that the type, purity, concentration, and distribution of chemical con-
This article not subject to U.S. Copyright. Published 1982 by the American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982 69
stituents in a supplier’s prepreg are consistent from batch to batch. HPLC is the most versatile and economically viable approach for analyzing epoxy resin prepregs. Sample solutions are injected into a liquid mobile phase which is pumped through column(s) packed with a stationary phase to facilitate separation and then into a detector which monitors the concentrations of the separated components. The theory and instrumentation are described in the literature (Snyder and Kirkland, 1979; Simpson, 1976). Recent advances have resulted in improved and automated instrumentation that is relatively low cost and simple to operate and maintain. In this paper, case studies are discussed in which HPLC procedures were applied to monitor the chemical compositions of batches of epoxy resin prepregs purchased over a period of years from three different manufacturers. The prepregs had continuous glass fibers and were processed as laminates at 250 OF. Liquid size exclusion, adsorption, and reverse bonded-phase HPLC methods are described for “fingerprinting” chemical compositions and for analyzing specific prepreg Components. HPLC analyses show that the epoxy resin formulations provided by the three prepreg manufacturers are quite different from one another. Correlations are established between the results obtained by HPLC analysis and changes in prepreg formulations, prepregging operations, and prepreg aging. HPLC is used to help develop acceptance criteria for prepreg procurement specifications and to investigate the curing behavior of an epoxy resin prepreg.
Experimental Section Prepreg batches or lots from batches were purchased at 3-4 month intervals from three different manufacturers over a period of 3-5 years. Upon receiving a batch of prepreg, HPLC analyses were run immediately and, in case further analysis might be required, specimens were stored in air-tight containers at -13 OC. Prepregs from the different manufacturers are designated PMA, PMB, and PMC. Reagent grade water was prepared from distilled water using a Millipore Milli-Q2 water purification system. Distilled-in-glass 2,2,4-trimethylpentane (C8) and tetrahydrofuran (THF) were used as received from Burdick & Jackson Labs. Prepreg solutions were prepared by extracting weighed sections of the prepregs with THF. Samples weighing 3 g (ca. 2 X 6 in.) were found to be representative of the prepregs within the precision of the techniques used for analysis. The solutions were prepared in volumetric flasks and were filtered through 0.2-pM Millipore membrane filters. Solution concentrations (ca. 10 pg/pL) were calculated from the difference between the initial weight of the prepreg and the final weight of the fibers after extraction, drying, and treatment for 4-6 h in a muffle furnace at 700-800 “C. The weight of the residue from the evaporation of a measured volume of the filtered prepreg solution may be determined to verify the resin concentration. Indeed the weight percent of insoluble reaction products (gel content) in the resin portion of the prepreg may be calculated using these data. Solutions were prepared from standards of identified prepreg components and were used for calibration in procedures involving quantitative analysis. The prepreg curing study was run using 0.2 g prepreg specimens. The specimens were placed into heated Pyrex tubes in a constant temperature bath (120 f 0.02 “C and were removed at l-min intervals. The immediate reduction in temperature followed by treatment with THF effectively
quenches the reaction. Solutions were prepared for HPLC analysis and gel contents were determined. A Waters Associates ALC/GPC-244 instrument with M 6 0 A solvent delivery system, 660 solvent programmer, 710A WISP auto-injection system, 440 dual wavelength UV absorbance detector, and R-400 differential refractive index (RI) detector was used with pStyragel(30 cm X 3.9 mm i.d.) and with pPorasil and pBondapak C18columns for the HPLC analyses. A Spectra Physics SP4000 data system was used for peak integration and data formatting. The analyses were run at ambient temperature, and mobile phase gradients were initiated automatically upon injection. Typically, injection volumes of 20 pL and mobile phase flow rates of 2 mL/min were used. Specific test conditions for PMA and PMB are described in the figure captions. The components in PMC were analyzed using the following conditions: mobile phase, (40% THF/GO% HzO) to (80% THF’/20% HzO) 50 min, gradient 8; column, pBondapak Cu; detector, UV 280 nm; analysis time, 1h; calibration, standard formulation solution. The test conditions for analyzing dicyandiamide (dicy) are as follows: mobile phase, HzO; column, pBondapak C18;detector, UV 230nm (Perkin-Elmer LC75 variable UV absorbance detector); analysis time, 2 min; calibration, series of standard dicy solutions or standard formulation solution.
Results and Discussion Quality Assurance. In developing quality assurance procedures for a prepreg where no information is available concerning resin composition, first the solubility characteristics of the resin are evaluated and then a set of HPLC fingerprinting techniques is applied. Ideally the resin will be fully soluble and the resin content of the prepreg can be determined gravimetrically by solvent extraction. If the prepreg has “advanced” such that insoluble reaction products (gel) are present, the extent of gelation may be analyzed (see Experimental Section). Generally, the best solvent for extraction and HPLC analysis is THF. The fingerprints shown in Figures 1,2, and 3 were obtained by analyzing the THF extract of prepreg PMA. The resin was fully soluble in THF. An HPLC chromatogram is a recorder trace of detector signal vs. time. The positions (retention times) and the heights or areas of the peaks provide a fingerprint of the resin composition. For a given set of experimental conditions, each component of the resin has a characteristic retention time and detector response. The retention time is a function of the separation mechanism and the size of the peak is proportional to the amount of the component in the sample. Differences in the compositions of prepregs may be revealed when the chromatograms are overlaid. The absence of peaks, appearance of new peaks, and changes in peak size indicate disparities in prepreg composition. A variety of techniques and detectors may be employed to analyze prepreg resin compositions. For a particular prepreg, certain techniques may be preferred for fingerprinting while special methods may need to be developed for analyzing specific components. Size exclusion chromatography (SEC) involves the separation of molecular species according to their size in solution. The separation takes place predominantly in the pores of the column packing where the molecules permeate into and out of pores to a greater or lesser extent depending upon their sizes and upon the distribution of pore sizes available to them. The larger the size of a molecule in solution, the shorter its retention time will be since the total pore volume available to it will be smaller. Hence for the conditions used in Figure 1,components having a molecular weight greater than about 500 g/mol will have
70
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21,
No. 1, 1982
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Figure 1. SEC analysis of PMA using RI and UV detectors. Columns, &pagel lo3, 500, 100, 100 A; mobile phase, THF.
retention times between 10 and 14 min and lower molecular weight components will elute between 14 and 18 min. Components having the same size in solution will have the same retention time but may have quite different detector responses as shown in Figure 1 (viz., the large peak at 16.5 min which is detected using a 254-nm monitor). The fingerprints in Figure 2 illustrate the application of adsorption HPLC. In this case the mobile phase is usually a solvent of low or intermediate polarity and separation depends upon specific interactions between the solute molecules and the surface of a polar packing material (e.g., silica). Retention times increase with the polarity of the solute molecules and are highly dependent upon selection of the mobile phase. If the mobile phase is not sufficiently polar, extremely long retention times, peak tailing, and irreversible adsorption on the column packing may occur. If the mobile phase is too polar, poor resolution is obtained. To improve resolution, solvent programming techniques may be used to adjust the polarity of the mobile phase during analysis. The chromatograms in Figure 2 were obtained by programming the mobile phase from 40% THF (60% C8) to 100% THF along a linear gradient (gradient 6) over a period of 30 min. Solvent programming with methylene chloride-THF mixtures also affords excellent resolution for many resins. Again it is noted that use of a dual wavelength detector provides more detailed fingerprints and clues to molecular differences in components. For example, significant differences in the molecular structures of components pro-
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ducing peaks at 7.1, 9.1 and 9.5 min. in Figure 2 are indicated because of the great disparity between their UV absorbance characteristics at 254 and 280 nm. Reverse bonded-phase HPLC is a type of liquid partition chromatography in which a relatively nonpolar stationary phase is chemically bonded to silica support material and the mobile phase is polar. Separation is based upon the relative solubility and distribution of the solute between the mobile and bonded phases such that solutes which are
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982 71
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more soluble in the bonded phase tend to have longer retention times. The main advantage of this technique is that the column packing is quite stable and may accomodate a variety of mobile phases and operating conditions. Excellent resolution is achieved, especially when solvent programming is employed (Figure 3). The fingerprints in Figures 1,2, and 3 are complementary in that, if one technique does not detect a difference in resin composition, another might. Indeed, even if certain components are not monitored because of poor detector response or insolubility, differences in their concentration may indirectly be observed by changes in the sizes of the peaks for components that are detectable. When run under identical conditions, fingerprints for prepregs PMA, PMB, and PMC are very different from one another. Therefore, special HPLC procedures were developed to optimize fingerprinting and specific component analysis for each type of prepreg. Case Study-Prepreg PMA. Batches of prepreg PMA received at 3 month intervals have been monitored over a period of three years using the procedures described in Figures 1, 2, and 3. The prepreg resin was fully soluble in all cases and no changes in the types of constituents were evident. However, when the sizes of the peaks were compared, differences were apparent between the fingerprints obtained from specimens of the two initial batches and later batches of PMA. In particular the peaks at 15.7, 4.9 and 8.9 min in Figures 1,2 and 3, respectively, were larger for the initial samples. Upon analyzing epoxy resin standards, it was found that the retention times and UV absorbance characteristics of the peaks which showed change were identical with those of the para, para isomer of the monomer (n = 0) of diglycidyl ether of bisphenol A (DGEBA). The peak for DGEBA was fully resolved in the adsorption and reverse bonded-phase analyses but not by SEC. The retention times of other components in PMA were s i m i i to that of DGEBA in the SEC analysis. Using purified DGEBA (n = 0) as a standard and the conditions in Figure 3, the resin in the initial prepreg batches had 36.8 and 31.3 wt % DGEBA; whereas, the resin in the later batches of prepreg was more consistent and contained
20-23 w t % DGEBA. Finally, it is noted that there were problems in processing the initial prepreg batches and that the HPLC analysis shows how the prepreg manufacturer “solved” the problem by reducing the DGEBA monomer content. Case Study-Prepreg PMB. Preparative HPLC techniques were used to isolate components of PMB for spectroscopic analysis (Hagnauer and Setton, 1978). The principal epoxy component, tetraglycidyl methylene dianiline (TGMDA), the curing agent dicy, and the accelerator, 3-(3,4-dichlorophenyl)-1,l-dimethylurea (Diuron), were identified. Fingerprinting procedures and processing conditions were developed based upon the composition and properties of initial batches of the prepreg. Prepreg batches were received and processed with no difficulties over a period of 3 years. However, after the first year, composites manufactured from PMB were no longer able to meet the mechanical test specifications. Fourier transform infrared (FTIR) spectroscopy showed that composites manufactured after the first year were incompletely cured (Thomas et al., 1979). HPLC analysis (Figure 4) shows that there are significant differences in the composition of the prepreg resin. The unacceptable prepreg resin contains about 2 % gel and 31% TGMDA compared with no gel and 51% TGMDA in the acceptable prepreg. The HPLC peaks at 5.3, 5.8, 6.4, and 6.8 min are much larger for the unacceptable prepreg and are attributed to reaction products between TGMDA oligomers and dicy. Also, according to HPLC analysis, the dicy and Diuron concentrations in the prepregs are nearly identical. Hence the failure of the composite to meet mechanical test requirements seemed to be directly related to a change in the chemical composition of PMB as determined by HPLC, and the problem was traced back to a change in the process used to manufacture the prepreg. Case Study-Prepreg PMC. The principal components in an unstaged sample of the PMC resin formulation were separated using preparative liquid chromatography and identified by spectroscopic analysis (Hagnauer and Setton, 1978). The resin formulation was found to consist of epoxy cresol novolac (ECN) and DGEBA resins, the
72
Ind. Eng. Chem. Prod. Res. Dev., Vol.
21, No. 1, 1982
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Table I. HPLC Analysis of PMC Prepregs (Weight Percent) %
sample resin 10176 9/77 4/78 1/79 6/80
unstaged
35.0 33.1 33.5 32.4 31.8
formulation std dev
ECN
Monuron 3.89 4.25 2.73 4.34 4.32
dicy
Epon 828
1273
total
6.46 7.21 4.48 5.96 7.05
33.8 34.5 27.3 37.6 32.0
39.8 36.8 31.7 41.9 40.6
84.0 82.8 66.2 89.8 84.0
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curing agent dicy, the accelerator 3-@-chlorophenyl)-l,ldimethylurea (Monuron), plus an epoxy flexibilizer additive. To determine which commercially available ECN and DGEBA resins were formulated in PMC, reverse bonded-phase HPLC procedures (see Experimental Section) were developed to separate the epoxy oligomers and to analyze the oligomer peak areas (Figure 5 ) . Then samples of commercial ECN and DGEBA epoxy resins were analyzed, and the oligomer retention times and peak areas were compared with those in PMC. In this way it was determined that PMC probably contains a mixture of the Shell Chemical Epon 828 DGEBA resin and the Ciba Geigy ECN 1273 resin. The average weight percentages of ECN 1273, Epon 828, and Monuron in the unstaged resin formulation were established from peak area measurements (Table I). A separate procedure was used to determine the weight percentage of dicy (see Experimental Section). Although not specifically identified, the epoxy flexibilizer additive was estimated to constitute ca. 5 wt % of the prepreg resin formulation. The standard deviation for the analysis of each component is given in Table I. To implement the HPLC procedures for quality assurance of PMC prepregs, a calibration standard based upon the composition of the unstaged resin formulation is used. The Epon 828 concentration is calculated from the area of the p,p-DGEBA (n = 0); whereas the ECN (n = 2) peak area is used to calculate the wt % ECN 1273. Monuron and dicy calibration constants are determined from their respective peak areas in the formulation standard. Prepreg samples are analyzed in triplicate and the prepreg standard is run before and after each set of prepreg analyses.
Lots from more than 20 different PMC prepreg batches were obtained from the prepreg manufacturer over a period of 5 years. During this time, HPLC was used for quality assurance to monitor prepreg composition and the prepregs were evaluated with respect to processability and properties of the cured laminates. The results shown in Table I are typical of those obtained from PMC prepregs which produce acceptable composite materials. The wt % values represent weight percentages of the unreacted components in the organic portion of the prepreg. Hence, the values reflect not only difference in the resin formulation but also changes in composition due to the prepregging process and prepreg aging. The wt % values of dicy, ECN 1273, and Epon 828 in the prepregs are generally less than in the unstaged formulation and indicate extents of reaction ranging from 5 to 30% for different prepreg batches. Such large variations in the extent of reaction suggest problems in controlling staging during prepregging. HPLC fingerprints of acceptable and unacceptable PMC prepregs are compared in Figure 5. The unacceptable prepreg was difficult to process and produced laminates with poor mechanical properties. A large peak at 5 min and the appearance of new peaks a t 2.5,5.5,12.5, and 27 min are indicative of an unacceptable prepreg. Using HPLC to quantitatively analyze the resin components, a prepreg is considered unacceptable if its ECN 1273 and Epon 828 components are found to be less than 30 and 28 wt %, respectively. The 4/78 sample (Table I) is marginally acceptable; whereas a sample with 28.2 wt % ECN 1273and 23.8 wt % Epon 828 is unacceptable even though the % resin, Monuron, and dicy may be within acceptable limits. Acceptance limits for Monuron and dicy are set at 2.5-4.5 wt % and 4-8 wt %, respectively. Finally, PMC prepregs are found to be unacceptable for most applications if their resin content is below 30 wt %. Aging and Cure Behavior of Prepreg PMC. Prepregs undergo reactions at room temperature which may be quite different from the types of reactions occurring at their cure temperatures. In addition, the presence of moisture accelerates the rate of reaction and affects the type of reaction products formed at room temperature. The appearance of a large peak with a retention time of 5 min (Figure 5) indicates that the prepreg has been exposed to moisture. This peak represents the dihydroxy product formed in the epoxy ring-opening hydrolysis of
Ind. Eng. Chem. Prod.
aging
Monumonths %gel ron dicy 3.86 7.42 1 0 6.75 3 0 3.94 6.51 6 0 3.98 4.41 56.2 3.48 18 2.2 3.68 5.86 lBa
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Table 11. Effect of Aging (21 "C) on PMC Prepreg Composition (Weight Percent) period,
Res. Dev., Vol. 21, No. 1, 1982 73
-80
Epon
ECN
828
1273
total
31.0 27.0 17.5 0.6 18.9
41.0 29.2 19.2 6.6 14.5
83.3 66.9 47.2 15.1 42.9
-60 9 z
a Desiccator.
DGEBA ( n = 0) and grows with prepreg aging at room temperature if no precautions are taken to eliminate moisture. The effect of moisture is evident when the gel content and w t 9% values of unreacted components in PMC prepregs aged in the open and in a desiccator at 21 "C for 18 months are compared (Table 11). More than half of the resin in the prepreg exposed to the atmosphere has formed gel, whereas only a slight amount of gel is evident in the desiccated sample. The soluble portion of the exposed prepreg contains about 13 wt % hydrolyzed DGEBA ( n = 0). Desiccating the prepreg effectively prevents hydrolysis but does not prevent other reactions which may occur due to the existence of previously hydrolyzed components. As shown in Table 11,the epoxy components are reactive at 21 "C and it is noted that hydrolysis reactions prevail prior to gel formation. Dicy reacts to a limited extent and little, if any, Monuron undergoes reaction. The prepreg would be considered unacceptable after aging 3 months at 21 OC. The prepregs lose their "tack" and become noticeably brittle during the 4-6 month intervd. Finally, no changes in composition were evident for PMC prepregs kept at -13 "C in air-tight containers for 5 years. Using reverse bonded-phase HPLC (see Experimental Section), the change in the composition of the PMC prepreg was monitored as a function of reaction time at 120 "C. Concomitant to HPLC analysis, the gel content (wt % gel) was determined for each sample. The weight percentages of the unreacted components and the wt % gel are plotted vs. reaction time in Figure 6. The results are reproducible and allow one to accurately analyze polymerization kinetics in the early stages of the cure reaction. Dicy, ECN 1273, and Epon 828 react at approximately the same rate and, assuming first-order kinetics, have a rate constant of about 0.31 min-'. Monuron has a somewhat larger rate constant of 0.37 mi&, and the rate constant for gelation determined in the 30-70 w t % gel region is about 1.4 min-'. Evaluation of the data suggests that the predominant species formed in the early stages of reaction are the 1:l reaction products between the primary amine on dicy and the epoxy groups on ECN and DGEBA oligomers. Additional information may be obtained by using HPLC to study the formation of secondary reaction products and by correlating gel content with the fraction of reacted components. Further insight regarding the reaction pathway and the formation or structure of the cross-linked resin matrix may be gained by comparing the HPLC results with those obtained using other techniques (e.g., spectroscopic, dielectric, calorimetric, and torsional braid analyses).
Conclusions HPLC techniques can be used to monitor the chemical compositions of epoxy resin prepregs for quality assurance.
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If the composition of the prepreg is unknown, it is best to apply several different HPLC techniques and utilize different detectors. The application of HPLC is most effective when at least some of the resin components are known. Indeed it has been shown that HPLC or HPLC supported by spectroscopic techniques may be used to isolate and identify resin components. The reliability of HPLC quality assurance procedures for monitoring prepregs has been demonstrated in three separate instances. In application, HPLC detected changes in resin compositions that were related to problems in processing prepregs and to mechanical failures in composites manufactured from prepregs. HPLC quantitative analysis procedures are particularly useful in determining precisely the compositional differences between acceptable and unacceptable prepregs and therefore in establishing chemical tolerance limits for prepreg specifications. Finally, the versatility of HPLC is noted in that procedures developed for quality assurance may also be applied to study the aging and cure behavior of prepregs. Literature Cited Hagnauer, G. L. Polym. Compos. 1080, 1 , S I . Hagnauer, G. L.; Dunn, D. A. "Materials 1980"; 12th National S A M E Tech nicai Conference, Seattle, 1980, p 648. Hagnauer, G. L.; Setton. I. J . LlquMChromtogr. 1078, 1 , 55. Simpson, C. F., Ed. "Practical High Performance Liquid Chromatography"; Heydon 8 Son, Ltd.: London. 1978. Snyder, L. R.; Kirkiand, J. J. "Introduction to Modern Liquid Chromatography", 2nd ed.;Wiiey-Interscience: New York, 1979. Thomas, 0. R.; Halpin. B. M.; Sprouse. J. F.; Hagnauer. G. L.; Sacher, R. E. Proceedings of the 24th National S A M E Symposium, San Francisco, May 1979, p 458.
Received for review February 25, 1981 Accepted August 19, 1981
