Letter Cite This: ACS Macro Lett. 2019, 8, 984−988
pubs.acs.org/macroletters
In Situ Vibrational Probes of Epoxy Gelation Lérys Granado,*,†,‡,§ Stefan Kempa,*,† Laurence John Gregoriades,† Frank Brüning,† Anne-Caroline Genix,‡ Jean-Louis Bantignies,‡ Nicole Fréty,§ and Eric Anglaret‡ †
Atotech Deutschland GmbH, Erasmusstraβe 20, 10553 Berlin, Germany L2C and §ICGM, University of Montpellier, CNRS, 34095 Montpellier, France
‡
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S Supporting Information *
ABSTRACT: This paper presents an efficient way to measure the curing kinetics and gel point, αgel, in epoxy resins from one single experiment. The epoxy curing reaction is herein monitored using in situ and time-resolved near-infrared absorption spectroscopy (NIR). The curing profiles over different isothermal conditions are in good agreement with DSC. Furthermore, the increase of the NIR absorption bands of aromatic rings (unreactive throughout curing) probe the cure shrinkage, as more and more aromatic rings are localized within the fixed sample volume. Therefore, the gel point is determined using the onset of the aromatic absorption increase. The results are in good agreement with the theoretical gel point, as well as DMA results. This innovative approach enables gelation measurements on epoxy neat resins and film composites with an easy-to-perform, accurate, robust, and versatile method.
E
poxy thermosetting polymers1 are extensively found in both large-volumes and high-performance manufactured goods. They are cost-efficient materials used in many fields, such as aerospace,2 microelectronics,3 automotive,4 energy,5 and so on. Epoxy thermosets present well-balanced performances, including good adhesion6 and thermomechanical properties, wear and chemical resistance, and thermal and electrical insulation. Epoxy polymers can be found as freestanding resins or used as binders in polymer−matrix composites as bulk materials as well as coatings or thin films.7 Epoxy curing consists of triggering the polymer crosslinking, usually by thermal or ultraviolet energy sources. The epoxy cross-linking involves the oxirane ring opening and addition on functional groups of the hardener (mainly amines,8 anhydrides,9 and phenols10). The cure stage is the key step to ensure good adhesion, spatial homogeneity, and a maximal reliability.11 It is therefore of utmost importance to control the curing kinetics (i.e., degree of curing, α, evolution with the time, and the temperature) and the associated polymer transitions throughout the manufacturing processes. The rheological and thermomechanical behaviors of the epoxy matrix change during the curing process, for example, the glass transition temperature, Tg, and viscosity significantly increase.12 One important transition is the gelation, occurring when the cross-linking polymer network percolates at the macroscopic level.13 Gelation is associated with the transition from the liquid-like to the rubber-like state when the matrix is devitrified. Gelation is conversion-dependent, so it is associated with the gel point, αgel, which is theoretically described by the Flory−Stockmayer equation:14 αgel =
where r is the stoichiometric ratio (equivalent hardener/ epoxy), and f and g are the functionality of the hardener and epoxy, respectively. The gel time (tgel) at a certain temperature, is typically determined using dynamic15 or thermomechanical analysis16 (DTMA) or rheology. Then, αgel is determined a posteriori with the help of the kinetic profiles measured by another technique, typically differential scanning calorimetry (DSC), associating the gel time to the conversion (tgel → αgel).17 For example, Yu et al. measured the gel time using the onsets in cure-shrinkage curves found in TMA and calculated αgel with the DSC curing kinetic profiles.18 These approaches present important drawbacks, such as being destructive and timeconsuming and requiring two independent experiments (having their own errors and biases). Furthermore, this methodology is not suited to thin films, due to the lack of sensitivity of rheological tools. A sensitive technique providing directly, from one single experiment, the gel point is crucially desirable. In response, we demonstrate in this Letter that in situ and time-resolved near-infrared (NIR) absorption spectroscopy can simultaneously monitor the reaction kinetics19−22 and gelation on three epoxy systems. NIR technique is demonstrated to be reliable, easy-to-perform, and nondestructive. Herein, the aromatic rings are considered as in situ molecular probes of the structural evolution of the sample throughout the curing process. A selection of NIR spectra recorded during isothermal epoxy curing of system (A) is presented in Figure 1a. We monitor the epoxy content decrease during cure from the absorbance of the Received: July 5, 2019 Accepted: July 19, 2019
1 r(f − 1)(g − 1) © XXXX American Chemical Society
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DOI: 10.1021/acsmacrolett.9b00508 ACS Macro Lett. 2019, 8, 984−988
Letter
ACS Macro Letters
Figure 1. In situ NIR data of the system (A) curing at 170 °C: (a) raw spectra at increasing curing time with epoxy band* and aromatic twin band** and (b) an example of the peak deconvolution.
combination band at 4528 cm−1. This intense band (characteristic of glycidyl oxirane (GO) rings) arises from the combination of fundamental stretching and deformation of CH (3050 cm−1) and CH2 (1460 cm−1).23 Besides, a twin band (Twin) is observed at 4611 and 4667 cm−1. This aromatic twin band is assigned to the combination of the aromatic conjugated CC stretching (1625 and 1626 cm−1) with the aromatic −CH fundamental stretching vibration (3050 cm−1).23 The GO band decreases throughout the curing process, as the epoxy moieties react with phenols. In opposition, the aromatic twin band displays only small, though significant, variations with the curing time, as the aromatic rings are not involved the curing process. We will further discuss the information available from these small variations in the next section. A routine was developed to subtract a straight baseline over the spectral range 4480−4780 cm−1 and to fit the experimental data. Due to molecular disorder in glass-former materials,24 Gaussian rather than Lorentzian functions describe well the IR bands (Figure 1b). The good correlation coefficients of the fits attest the good reliability of our deconvolution routine (reduced-R2 > 0.992). For the determination of the degree of curing, the aromatic twin band area is therefore used as an internal reference of primary choice, being intense, well resolved from GO band and desirably close to it. The degree of curing, α, as a function of the time, t, for a given isothermal condition is defined as α (t ) = 1 −
Figure 2. Comparison of isothermal kinetic profiles of system (A) determined using NIR and DSC, in the temperature range of 160− 180 °C. The lower plot traces, as representative examples at 170 °C, the standard errors (Δα) for NIR (propagated uncertainties arising from the fits) and DSC (standard deviations over three measurements).
confirming that NIR is well-suited to curing kinetics measurement under isothermal conditions. Gelation of system (A): The unreactive aromatic rings were used as an internal reference to assess curing kinetic profiles by NIR. However, the area of the twin band does increase during the isothermal cure process of system (A), see Figure 3a. First, the aromatic twin band area significantly increases for α < 0.2 (vide infra), which can be ascribed to temperature equilibration or volatile evaporation in the early curing stage, then it levels off prior to drastically increase again for about α > 0.58. Slight offsets are measured for different curing temperatures, nevertheless the general profiles are very close. The reason for such increases is readily found in the well-known shrinkage (densification) which occurs during epoxy cure. As the thermosets shrink, a higher content of aromatic rings absorbs the IR photons within a fixed interacting area. Thus, the absorbance of the aromatic rings can be used as internal and molecular indirect probes of the cure shrinkage. Indeed, the evolution of the twin band area is remarkably analogous to shrinkage curves,18 which can be used to measure the epoxy gel point. Furthermore, the onsets of increase in Figure 3a are nearly identical for all temperatures and in line with the theoretical gel point. This allows defining a NIR gel point at αgel = 0.58, which is independent of the curing temperature, as expected.
( AGO ATwin )t ( AGO ATwin )t = 0
(2)
where AGO and ATwin are the band areas of the glycidyl oxirane and the aromatic twin band, respectively. Kinetic profiles: Based on eq 2, the NIR cure kinetic profiles (α vs t) of system (A) are calculated for a series of isothermal conditions and compared to our previous isothermal DSC data,27 shown in Figure 2. The precision is satisfactory for the two methods (deviations, Δα ≤ 0.1, for most of the cases). An overall good accordance is found, especially during the first 15 min when the degree of curing increases rapidly. Then, the curves level-off at longer time exposure. At this point, NIR profiles generally fall below the DSC ones. The discrepancies are assigned to a lack of sensitivity of the isothermal DSC technique for lower temperatures (as the heat is released over longer periods of time). However, the difference in the values obtained between the two methods are relatively low (≤0.05), 985
DOI: 10.1021/acsmacrolett.9b00508 ACS Macro Lett. 2019, 8, 984−988
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ACS Macro Letters
Figure 3. (a) Aromatic twin band area as a function of the degree of curing for 160−180 °C, the gel point is determined by means of onsets, and (b) the gel point as determined using the maximum loss angle values with α, assessed by DMA. Examples of loss tangent traces are displayed in the inset, with the associated degrees of curing reported next to the curves. Vertical bold-dashed lines mark the theoretical gel point. The data were recorded on system (A).
We now compare in situ NIR to DMA results assessed on partially cured samples. Gelation is usually determined with in situ rheological tests when the loss tangent becomes independent of the frequency.25 Yet, the very high adhesion of the epoxy matrix made the in situ approach difficult. Rather, the samples were partially cured in an oven at 180 °C for various time, then α was measured by DSC (by mean of the residual enthalpy). DMA (ARES, TA Instruments, using rectangular torsion (0.56 × 12.00 × 40.00 mm3), with a strain of 0.1% (within linearity domain), at 1 Hz and 3 °C·min−1) was assessed on partially cured samples (ex situ tests) near the glass transition region, hence, avoiding dramatic adhesion on DMA clamps, which occurs while the polymer was fully relaxed (T ≫ Tg). The associated damping peaks in glass transition region (which maximum temperature define Tα) are displayed in the inset of Figure 3b, at various degrees of curing. Associating δmax the maximum loss angle at T = Tα, the partially cured samples presenting a damping peak maximum greater than one are considered ungelled (tan(δmax) ≥ 1, at Tα, i.e., G′′ crosses G′), whereas the others are considered gelled (tan(δmax) < 1, at Tα, i.e., G′′ ≠ G′). The Figure 3b shows that tan(δmax) decreases as a function of the degree of curing. The gel point is herein estimated as the associated degree of curing when tan(δmax) = 1. The estimation of the DMA gel point is found at αgel ∼ 0.58, in line with the NIR and theoretical value. Versatility of the method: The approach developed for system (A) was also applied to (B) another epoxy-phenol glass-filled composite (lower filler amount and epoxy/phenol stoichiometry) and (C) a model epoxy/amine neat resin. The NIR spectra and calculated kinetic profile of (B) are very similar to (A), due to the comparable formulations and spectroscopic configurations (Figure S5). This confirms the insignificant influence of the glass fillers to the method. Considering (C), the NIR signals are similar to (A) and (B) spectra (Figure S6), but the baseline is less intense and flatter (due to less light scattering). The steeper slope toward lower wavenumbers is mainly due to the DTGS detector response. Still, the epoxy and aromatic bands are observed within the same spectral range than (A) and (B). They are sharper and more intense because of the absence of the filler in (C). Figure 4 compares the twin band area as a function of the degree of curing for the three systems (A), (B), and (C). The area increases for all samples, as the thermosets shrink during the curing process. The evolutions are comparable for (A) and
Figure 4. Determination of the gel point using an in situ NIR-gelation experiment for the systems (A), (B), and (C) at 160, 190, and 60 °C, respectively. Aromatic twin band area is shown as a function of the degree of curing. The onsets are determined using the tangent method. The vertical dotted lines represent the theoretical gel points. For the sake of clarity, all the curves are offset and the curve (C) is scaled ×2.
(B), but the increase is stronger for (C), likely due to the absence of fillers. For the three systems, the onsets of increase are in good agreement with theoretical gel points, at αgel = 0.58, 0.46, and 0.58, respectively. The presented method is shown to be accurate, robust, and versatile. To conclude, the gel point measured by NIR with the onset of aromatic band increases provide an accurate description of the gelation. NIR absorption spectroscopy stands as a powerful alternative tool to measure simultaneously the curing kinetics and evaluate the cure-shrinkage and gelation from one single experiment, readily applicable on aromatic epoxy polymer and polymer−matrix composites. This method was proven to be accurate, reproductible, robust, and versatile. Herein the aromatic moieties are considered as in situ molecular probes of shrinkage. To the best of our knowledge, this is the first time that epoxy gelation is directly probed by vibrational spectroscopy. The same analysis is likely to hold for any aromatic polymer or composite whose gel point can be measured as an onset of cure-induced shrinkage/dilatation. We expect similar results using the reflection mode, which should enable gelation measurements in polymer thin films and coatings, which were so far difficult to assess. 986
DOI: 10.1021/acsmacrolett.9b00508 ACS Macro Lett. 2019, 8, 984−988
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ACS Macro Letters
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
(3) Lu, D. D.; Wong, C. P. Materials for Advanced Packaging. Mater. Adv. Packag. 2009, 1−719. (4) Karkanas, P. I.; Partridge, K.; Attwood, D. Modelling the Cure of a Commercial Epoxy Resin for Applications in Resin Transfer Moulding. Polym. Int. 1996, 41, 183−191. (5) Hardis, R.; Jessop, J. L. P.; Peters, F. E.; Kessler, M. R. Cure Kinetics Characterization and Monitoring of an Epoxy Resin Using DSC, Raman Spectroscopy, and DEA. Composites, Part A 2013, 49, 100−108. (6) Hu, M.; Guo, Q.; Zhang, T.; Zhou, S.; Yang, J. SU-8-Induced Strong Bonding of Polymer Ligands to Flexible Substrates via in Situ Cross-Linked Reaction for Improved Surface Metallization and Fast Fabrication of High-Quality Flexible Circuits. ACS Appl. Mater. Interfaces 2016, 8 (7), 4280−4286. (7) Morsch, S.; Lyon, S.; Greensmith, P.; Smith, S. D.; Gibbon, S. R. Water Transport in an Epoxy-Phenolic Coating. Prog. Org. Coat. 2015, 78, 293−299. (8) Cole, K. C.; Hechler, J. J.; Noël, D. A New Approach to Modeling the Cure Kinetics of Epoxy Amine Thermosetting Resins. 2. Application to a Typical System Based on Bis[4-(Diglycidylamino)Phenyl]Methane and Bis(4-Aminophenyl) Sulfone. Macromolecules 1991, 24 (11), 3098−3110. (9) Musto, P.; Abbate, M.; Ragosta, G.; Scarinzi, G. A Study by Raman, near-Infrared and Dynamic-Mechanical Spectroscopies on the Curing Behaviour, Molecular Structure and Viscoelastic Properties of Epoxy/Anhydride Networks. Polymer 2007, 48 (13), 3703−3716. (10) Tyberg, C. S.; Bergeron, K.; Sankarapandian, M.; Shih, P.; Loos, A. C.; Dillard, D. A.; McGrath, J. E.; Riffle, J. S.; Sorathia, U. Structure-Property Relationships of Void-Free Phenolic-Epoxy Matrix Materials. Polymer 2000, 41 (13), 5053−5062. (11) Aldridge, M.; Wineman, A.; Waas, A.; Kieffer, J. In Situ Analysis of the Relationship between Cure Kinetics and the Mechanical Modulus of an Epoxy Resin. Macromolecules 2014, 47 (23), 8368− 8376. (12) Pascault, J. P.; Williams, R. J. J. Glass Transition Temperature versus Conversion Relationships for Thermosetting Polymers. J. Polym. Sci., Part B: Polym. Phys. 1990, 28 (1), 85−95. (13) Alonso, M. V.; Oliet, M.; García, J.; Rodríguez, F.; Echeverría, J. Gelation and Isoconversional Kinetic Analysis of Lignin-PhenolFormaldehyde Resol Resins Cure. Chem. Eng. J. 2006, 122 (3), 159− 166. (14) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press, 1953. (15) Stark, W.; Jaunich, M.; McHugh, J. Carbon-Fibre Epoxy Prepreg (CFC) Curing in an Autoclave Analogue Process Controlled by Dynamic Mechanical Analysis (DMA). Polym. Test. 2013, 32 (8), 1487−1494. (16) Wingard, C. D. Characterization of Prepreg and Cured Epoxy/ Fiberglass Composite Material for Use in Advanced Composite Piping Systems. Thermochim. Acta 2000, 357−358, 293−301. (17) Stark, W. Investigation of the Curing Behaviour of Carbon Fibre Epoxy Prepreg by Dynamic Mechanical Analysis DMA. Polym. Test. 2013, 32 (2), 231−239. (18) Yu, H.; Mhaisalkar, S. G.; Wong, E. H. Observations of Gelation and Vitrification of a Thermosetting Resin during the Evolution of Polymerization Shrinkage. Macromol. Rapid Commun. 2005, 26 (18), 1483−1487. (19) Mijovic, J.; Andjelic, S.; Kenny, J. M. In Situ Real-Time Monitoring of Epoxy/Amine Kinetics by Remote near Infrared Spectroscopy. Polym. Adv. Technol. 1996, 7 (1), 1−16. (20) Mijovic, J.; Andjelic, S. A Study of Reaction Kinetics by NearInfrared Spectroscopy. 1. Comprehensive Analysis of a Model Epoxy/ Amine System. Macromolecules 1995, 28, 2787−2796. (21) Sangermano, M.; Pegel, S.; Pötschke, P.; Voit, B. Antistatic Epoxy Coatings with Carbon Nanotubes Obtained by Cationic Photopolymerization. Macromol. Rapid Commun. 2008, 29 (5), 396− 400. (22) Pandita, S. D.; Wang, L.; Mahendran, R. S.; MacHavaram, V. R.; Irfan, M. S.; Harris, D.; Fernando, G. F. Simultaneous DSC-FTIR
Epoxy systems: The main studied system (A) was epoxy-phenol/glass filler film composites, 560 μm thick (63 wt %, polydisperse spheres, ca. 300 nm average diameter, Figure S1). The Tg of the films was found at 15, 72, and 155 °C for α = 0, αgel, and 1, respectively (Figure S2). The cross-linking reaction involves the opening of bisphenolic glycidyl oxirane ring with phenol-formaldehyde oligomers with the presence of a Lewis acid catalyst.26 The stoichiometry (r ∼ 1) was estimated using 1H NMR (Figure S3) and the average hardener functionality, f = 4, by mass spectrometry (Figure S4). According to eq 1, the theoretical gel point was therefore evaluated at αgel ∼ 0.58. To check its versatility, we applied the presented method to two others epoxy systems: (B) a similar epoxy-phenol film composites (400 μm thick), but with a different formulation (42 wt % of glass fillers) and a different stoichiometric ratio: r ∼ 1.6 (g = 2 and f = 4, yielding a theoretical gel point at αgel ∼ 0.46) and (C) a model neat epoxy/amine resin: Epon 828/meta-xylene diamine (m-XDA), r = 1, g = 2, and f = 4, theoretical gel point of αgel = 0.58. NIR method: For the systems (A) and (B) the method was the following. In situ micronear-infrared experiments were carried out in transmittance using a Bruker IFS 66 V spectrometer coupled to a Hyperion 2000 microscope (Bruker, Inc.) using 15× Cassegrain objectives. The beam size was 100 × 100 μm2. A mercury cadmium telluride detector, a KBr beam splitter and a globar source were used. For each spectrum, 32 scans were coadded (acquisition time: 30 s/ spectrum), with a resolution of 4 cm−1 in the NIR range. The samples (6 mm diameter) were placed in an Instec hot stage regulated at 160.0, 170.0, or 180.0 ± 0.1 °C for (A) and 190.0 ± 0.1 °C for (B); the self-heating during curing was previously checked to be not significant (using the same sample geometry and DSC).27 T > Tg for both, avoiding vitrification during isothermal experiments. Three different areas of the samples were probed to check the reproducibility (deviations < 5%). Note that this NIR method was proven to remain quantitative, even considering the diffuse-reflection of the IR beam caused by the multiple matrix−filler interfaces.28 Considering system (C), after mixing the epoxy and m-XDA at RT during 5 min, 500 μL was placed into a glass cuvette. The cure monitoring was performed at 60.0 ± 0.1 °C for 1 h on an ABB MB3600 NIR spectrometer, using a deuterated triglycine sulfate (DTGS) detector with a resolution of 4 cm−1 and 32 scans/spectrum (acquired every 2 min). The data throughout this Letter are presented along with the standard uncertainty (±), with 95% confidence, involved in the measurements based on two standard deviations (2 σ). S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.9b00508. SEM micrographs; Glass transition temperatures; Proton NMR spectra; Mass spectra; and NIR spectra (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Lérys Granado: 0000-0001-5812-8777 Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acsmacrolett.9b00508 ACS Macro Lett. 2019, 8, 984−988
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ACS Macro Letters Spectroscopy: Comparison of Cross-Linking Kinetics of an Epoxy/ Amine Resin System. Thermochim. Acta 2012, 543, 9−17. (23) Poisson, N.; Lachenal, G.; Sautereau, H. Near- and MidInfrared Spectroscopy Studies of an Epoxy Reactive System. Vib. Spectrosc. 1996, 12 (2), 237−247. (24) Vandeginste, B. G. M.; De Galan, L. Critical Evaluation of Curve Fitting in Infrared Spectrometry. Anal. Chem. 1975, 47 (13), 2124−2132. (25) Eloundou, J. P.; Feve, M.; Gerard, J. F.; Harran, D.; Pascault, J. P. Temperature Dependence of the Behavior of an Epoxy−Amine System near the Gel Point through Viscoelastic Study. 1. Low- T g Epoxy−Amine System. Macromolecules 1996, 29 (21), 6907−6916. (26) Nair, C. P. R. Advances in Addition-Cure Phenolic Resins. Prog. Polym. Sci. 2004, 29, 401−498. (27) Granado, L.; Kempa, S.; Gregoriades, L. J.; Brüning, F.; Genix, A.-C.; Fréty, N.; Anglaret, E. Kinetic Regimes in the Curing Process of Epoxy-Phenol Composites. Thermochim. Acta 2018, 667, 185−192. (28) Granado, L.; Maurin, D.; Kempa, S.; Gregoriades, L. J.; Brüning, F.; Fréty, N.; Anglaret, É .; Bantignies, J.-L. L. NonDestructive DRIFT Spectroscopy Measurement of the Degree of Curing of Industrial Epoxy/Silica Composite Buildup Layers. Polym. Test. 2018, 70 (April), 188−191.
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DOI: 10.1021/acsmacrolett.9b00508 ACS Macro Lett. 2019, 8, 984−988
