UV Fluorescent Epoxy Adhesives from Noncovalent and Covalent

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UV fluorescent epoxy adhesives from non-covalent and covalent incorporation of coumarin dyes Peter Daniel McFadden, Kevin M Frederick, Liliana A Argüello, Yizheng Zhang, Pamela Vandiver, Nancy Odegaard, and Douglas A. Loy ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 24, 2017

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ACS Applied Materials & Interfaces

UV fluorescent epoxy adhesives from non-covalent and covalent incorporation of coumarin dyes Peter D. McFadden,1 Kevin Frederick,2 Liliana A. Argüello,1 Yizheng Zhang,1 Pamela Vandiver,2 Nancy Odegaard,3 Douglas A. Loy1,2* 1

Department of Chemistry and Biochemistry, The University of Arizona, Tucson, AZ 85721, USA

2

Materials Science and Engineering Department, The University of Arizona, Tucson, AZ 85721, USA

3

Arizona State Museum, The University of Arizona, Tucson, AZ, 85721, USA

Correspondence to: Douglas A. Loy (E-mail: [email protected]) ABSTRACT Epoxies are commonly used in art conservation as adhesives for artifact reconstruction and repair. However, with the development of colorless epoxies, it has become more difficult to detect repair work. Fluorescent epoxies would allow for easy detection of the epoxy joints by simple visual inspection under UV light while remaining unnoticeable under normal display lighting. Coumarins are natural dyes that can be added in very small amounts to make thermosets fluoresce. Depending on the functionality of the coumarin used, the dye may be physically encapsulated in the cross-linked polymer or it may be bound to the polymer through covalent bonds. In this paper, we examine the efficacy of coumarin (1) and coumarin 480 (2) as physically encapsulated dyes and 7-hydroxycoumarin (3) and 7-glycidyloxycoumarin (4) as covalently bound dyes in a commercial epoxy thermoset, Epo-Tek 301. All four dyes could be used to make the epoxy fluorescent, but coumarins 1 and 2 slightly reduced the lap shear strength of the thermoset and could be extracted with solvent. In contrast, coumarins 3 and 4 had little effect on the mechanical properties of the epoxy and only minute amounts could be extracted.

KEYWORDS Fluorescent, Epoxy, Adhesive, Art conservation, coumarin

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1. Introduction Epoxy thermoset adhesives are attractive for art conservation because of their strength and good adhesion1. Originally brown from oxidation2-4, newer epoxies remain colorless longer5-7, making it difficult to identify where or even if adhesives have been used to repair an artifact. Making the epoxy UV fluorescent is attractive because the thermoset can remain invisible under normal light, but fluoresce brightly with ultraviolet light. Bisphenol A based epoxies exhibit a slight innate fluorescence8, 9, but it is not enough to be readily visible to the eye. However, the addition of small amounts of a fluorescent dye can make the epoxy fluoresce10-16. Non-covalently attached dyes can leach or be extracted from the resin17 attenuating or eliminating the fluorescent signature. In addition, leaching of dyes from the resins can potentially cause environmental or health problems. Furthermore, adding even small amounts of organic additives to the epoxy can change the mechanical properties of the resulting thermoset, often weakening it through plasticization18. Covalent incorporation of dyes, relying on carboxylic acid11, hydroxyl, or amine12-15groups on the dye to react with epoxide groups in the resin formulation or epoxy or other electrophilic groups on the dye reacting with the amine groups in the hardener19, 20, should eliminate plasticization while maintaining fluorescence. Figure 1. In this study, we compared noncovalent and covalent incorporation of fluorescent coumarins 1-4 (Figure 1) into an epoxy thermoset, Epo-Tek 301. Coumarins have long been used as fluorescent dyes for lasers, sensors, and to monitor the cure kinetics of thermosets. Coumarins 1 and 2 are unreactive with the diglycidyl ether bisphenol A (DGEBA) and 2,4,4trimethylhexane-1,6-diamine and 2,2,4-trimethylhexane-1,6-diamine mixture used as a hardening agent. Dissolving around a milligram of dye per gram of epoxy will afford a fluorescent thermoset with 1 or 2 physically encapsulated in the network created by the reaction of the DGEBA and the bis-amine hardening agent (Figure 2A). In contrast, similar amounts of 3 and 4 will react with the epoxide and amine groups, respectively, during curing to create covalent attachments (Figure 2B). 7-Hydroxycoumarin 3 will react with the epoxide groups in the thermoset precursor, particularly in the presence of the amine hardening agent. Covalent incorporation in epoxies through reaction with the epoxy groups has been reported with hydroxyl21 and carboxylic acid22 modified coumarins. 7-Glycidyloxycoumarin, 4, prepared by reacting 7-hydroxycoumarin, 3, with epichlorohydrin (Scheme 1)23, will covalently incorporate when its epoxide group reacts with the amine groups of the hardening agent. Coumarin 4, used in synthesis of pharmaceuticals23, 24 and to modify polymers with the photocross-linkable coumarin group25, has not been reported as a fluorescent additive or monomer in epoxy thermosets. Furthermore, we found no previous reports comparing the influence of covalent and non-covalent dye incorporation on the mechanical properties of the thermosets or on the extent to which the dyes can be leached out of the resins with solvent.

Figure 2. 2 ACS Paragon Plus Environment

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2. Experimental Section 2.1 Materials. Coumarin 480 (2) was purchased from Exciton Corporation (Dayton, Ohio) and was used as received. The epoxy and hardener (Epo-tek 301) were obtained from Epoxy Technologies Inc. (Billerica, MA). The remaining chemicals and solvents used were purchased from Sigma-Aldrich (St. Louis, MO) and used without purification unless otherwise stated. The epoxy is a mixture of DGEBA and a small amount of oligomers of the same. NMR analysis of the epoxy give an equivalent epoxy weight of 173.4 g/equiv epoxide26. The hardener is a mixture of 2,4,4-trimethylhexane-1,6-diamine and 2,2,4-trimethylhexane-1,6-diamine. Glassware was oven-dried prior to use. All reactions were carried out under an atmosphere of argon. Glass substrates were microscope slides (25 x 75 x 1 mm) purchased from VWR International and have a typical composition of borosilicate glass. 2.2 Instrumentation. 1H NMR data were obtained on an AVIII-400 NMR spectrometer and chemical shifts were reported in part per million (δ). 1H NMR was internally referenced to tetramethylsilane (TMS) (δ 0.0) and 13C NMR chemical shifts were reported relative to the center peak of the multiplet for CDCl3 (δ 77.0 (t)). Attenuated total reflectance–Fourier transform infrared (ATR–FTIR) analysis of neat samples were performed on a Thermo NicoletFTIR Spectrometer iS10 with a ZnSe ATR crystal apparatus. Melting points were determined with a Mel Temp apparatus and are uncorrected. Fluorescence spectra of coumarins 1-4 (4 x 104 M)in methylene chloride were taken using PTI Quanta Master 40 Steady State Spectrofluorimeter with slit widths of 0.25 mm. Fluorescence of thermoset samples was measured by illuminating the samples with long wave ultraviolet and using an Ocean Optics Jaz spectrometer to measure the fluorescence with a QP 400-2-UV-BX optical fiber probe. Scheme 1. 2.3 Synthesis of 7-glycidyloxycoumarin (4)23 To 7-hydroxycoumarin (1.624 g, 10.02 mmol) and epichlorohydrin (20 mL, 260 mmol) in ethanol (25mL) in 100 mL round bottom flask equipped with magnetic stirrer, addition funnel, condenser and drying tube, KOH in ethanol (25 mL, 0.5 M) was added dropwise at room temperature. The mixture was refluxed for 2.5 h during which time the solution became transparent and dark red-brown in color. After cooling to room temperature, the solvent was removed by rotary evaporation and the brown solid residue remaining was extracted with chloroform (50 mL) and distilled water (40 mL). The red-brown organic phase was washed twice with water, dried over anhydrous MgSO4, and the solvent was rotary evaporated. The remaining light red-brown solids were recrystallized in ethanol to give short colorless needlelike crystals (1.71 g, 7.84 mmol, 78% yield). Melting Point: 115-116 °C (Literature 114-116°C)23; 1 H NMR (400 MHz, CDCl3): 2.79 (dd, 3J = 4.8, 2.6 Hz, 1H), 2.94 (dd, 3J = 4.9, 4.1 Hz, 1H), 3.38 (ddt, 3 ACS Paragon Plus Environment


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3

J = 5.9, 4.1, 2.7 Hz, 1H), 3.97 (dd, 3J = 11.1, 5.9 Hz ,1H), 4.33 (dd, 3J = 11.1, 2.8 Hz, 1H), 6.26 (d, 3 J = 9.5 Hz, 1H), 6.82 (d, 3J = 2.5 Hz, 1H), 6.88 (dd, 3J = 8.6, 2.5 Hz, 1H), 7.37 (dd, 3J = 8.6, 0.3 Hz, 1H), 7.63 (d, 3J = 9.5 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ 44.66, 49.89, 69.39, 101.79, 113.02, 113.08, 113.60, 128.99, 143.42, 155.90, 161.16, 161.68. IR(ATR) 3085, 3051, 3004, 2942, 1711, 1613, 1513, 1403, 1354, 1284, 1231, 1211, 1127, 1027, 991, 845, 617, 455 cm-1. 2.4 Epo-tek 301 thermoset without dye Epo-tek Part B (0.5046 g, 3.188 mmol) was mixed with Epo-tek Part A (2.0229 g, 5.832 mmol). The mixture was applied to coupons and oven cured for 2 hours at 65 °C. The cured epoxy was clear and colorless. 2.5 Epo-tek 301 thermoset with coumarin 1 (0.01 M). Coumarin (0.0034 g, 0.023 mmol) was dissolved in Epo-tek Part B (0.5013 g, 3.168 mmol). This was then mixed with Epo-tek Part A (1.9992 g, 5.764 mmol). The mixture was applied to coupons and oven cured for 2 hours at 65 °C. The cured epoxy was mostly clear and colorless with a moderate, yellow color. 2.6 Epo-tek 301 thermosets with coumarin 2-4 The remaining experimentals for preparing Epo-tek 301 resins with 2 (0.01 M), 3 (0.01 M), 4 (0.001 M), 4 (0.01 M), and 4 (0.1 M) are included in supporting materials. 2.7 Photodegradation study of coumarin 1 in Epotek 301. A cured sample of Epo-Tek 301 epoxy with 10-2 M coumarin (1) was subjected to long wave UV irradiation in the ADAC Systems Cure Zone 2 for one hr. The sample was compared to a control sample of identical composition and found to have significantly yellowed under the exposure. The fluorescence of both samples was compared and while the intensity was slightly lower for the exposed sample, both were strongly fluorescent. The exposed sample was irradiated in the cure lamp for an additional 22 hrs and the following pictures were taken to show the degree of yellowing and of the decrease in fluorescent intensity. The yellowing of the sample with UV exposure matches expectations and shows that the UV lamp was able to accelerate degradation and simulate a significant degree of aging. However, even under such extreme conditions, the sample remains strongly fluorescent and proves to be a robust fluorophore in the cured epoxy. ADAC system flux is 27 mW/cm2 measured between 300-400 nm. 2.8 Leaching experiments In a representative experiment, the amount of coumarin dye that can be leached from the epoxies was determined as follows: Coumarin 480 (2, 5.7 mg) was reacted with Epo-Tek 301 hardener (0.4971 g) and epoxy (2.0194 g). The cured coumarin epoxy (10-2 M) was shaved with 4 ACS Paragon Plus Environment

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a razor blade to collect 200 mg of shavings. A dichloromethane soxhlet extraction was run for 12 hours and the solvent was diluted to 25 mL. Fluorescence spectroscopy was done to determine the amount of coumarin (2) extracted based on a series of standards whose concentrations ranged from 0.022 – 44 ppm. Dilution information and fluorescence spectra for coumarin 2 are in supporting materials. Coumarin 1 did not fluoresce well in dichloromethane so it was not possible to. Coumarins 3 and 4 were determined to leach out in amounts less than 1% of the original amounts of the dyes used. 2.9 Lap Shear tests Lap shear tests involved bonding of two aluminum coupons via an overlap using a standard ASTM method for testing adhesive strengths27. The coupons used were 1” x 4” x 0.064”. Coupons were rinsed twice with acetone to remove any oils and all ink marks, then chromic acid etched at 70 °C to remove excess surface oxide and give a uniform surface. The coupons were rinsed with DI water and acetone and dried before applying adhesive formulations. Six different formulations were tested. In every case, test adhesive samples were prepared from 2.5 g Epo-Tek 301 resin was prepared by mixing the hardening agent (0.5 g) with the epoxy component (2 g). Dyes were pre-dissolved in the hardening agent before mixing with the epoxy. The control formulation was Epo-Tek 301 resin (2.5 g) and the amine terminated hardening agent (0.5 g) without any dye. The next three formulations had 1, 2, or 3 (10-2 M) in Epo-Tek 301. The last two formulations studied used 4 at two different concentrations (10-2 M and 10-3 M). At least six coupons were prepared for each formulation. The coupons were marked at 0.2 inch from edge for overlap and 0.25 inch for space beyond the overlap that also was covered with adhesive. A thin adhesive layer was applied on both coupons and overlap was hold with two binder clips, as shown in (Figure 3) Coupons were cured for 2 hours at 65°C. Tension tests were done with a MTS Criterion stress strain analyzer with aluminum coupons (width was 1”, lap length was 0.2) at 0.13 cm/min,” with a data acquisition rate of 100 Hz. Samples were clamped at 63mm from the overlap end, and tested with the program “MTS EM Tension (Simplified) Epoxy LapShear 1”.

Figure 3.

3. Results 3.1 Formation of fluorescent thermosets Epo-tek 301 is prepared by mixing 2.28 equivalents of DGEBA with the 1 equivalent of hardening agent in a scintillation vial until homogeneous and curing at 65 °C for 2h28. The Epo5 ACS Paragon Plus Environment


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tek 301 formulation has an excess (14.1%) of epoxide groups relative to the amine groups. Cross-linking occurs in this system because each primary amine group can react with two epoxy groups. The resulting network (Figure 2 & Scheme 2) has amine groups attached to the terminal carbons in as many as two ring opened epoxy groups in the DGEBA members of the thermoset. Fluorescent epoxy thermosets were prepared by dissolving coumarin dyes 1-4 in the hardening agent prior to mixing it with the epoxy component. With coumarins 1 & 2, where there are no expected covalent bonds between the dye and the epoxy resin, the order of addition is not important. Both 1 & 2 were soluble in the hardener and epoxide insuring that the dyes would be well dispersed and the resulting yellow tinted resins were transparent enough for fluorescence to be visible. However, 1 & 2 can leach from the thermosets after curing. Extraction of Epo-tek 301 shavings with dichloromethane afforded 31.7% of 2 used in the formulation. It was more challenging to measure the extraction of 1 from the epoxy, as the coumarin fluorescence intensities out of the resin were less intense. Comparing the fluorescence of the Epo-tek resin with 1 to the resin after extraction indicates that 23±3% was extracted. Leaching is reduced with covalent incorporation using 7-hydroxycoumarin 3, or 7glycidyloxycoumarin 4. Typically phenolic groups, like that in 3, are less reactive with epoxides than alcohols or amines29. Addition of coumarin 3 to the hardener results in deprotonation of the phenolic hydroxyl by the excess diamine, making the dye more reactive with the epoxy groups in the DGEBA. Furthermore, the elevated cure temperature used in this study would also ensure reaction of 3 with the epoxide groups. At 10-2 M concentration of 3, there is one 7hydroxycoumarin 3 for every 254 molecules of the DGEBA (Scheme 3). Scheme 2. Scheme 3. Thus, the addition of such a small amount of 3 reduces the concentration of epoxy groups less than 1mol%. As a result, no difference in cure time in the presence of 3 was observed. The glycidyloxy group on 4 reacts directly with the amine groups in the hardener (Scheme 4) so this dye should be easier to covalently link to the epoxy network than 3. In the case of the 10-2 M 4, about one out of every 140 hardener molecules have a coumarin attached. Under these conditions, the glycidyloxy group reacts quickly with amines only reducing the number of amine equivalents available to react with DGEBA by less than 1%. Since secondary amines, such as that in the product of 4 reacting with the diamine, can still react a second time, the addition of small amounts of 4 should have even less impact on network formation than 3. The addition of 4 to the formulations did not noticeably slow the curing of the Epo-tek 301 resins. No detectable amounts of 3 or 4 could be detected leaching from thermoset samples.


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Scheme 4.

3.2 Fluorescent properties Coumarins, well known as fluorescent dyes30, generally absorb in the ultraviolet between 300-400 nm and emit between 350-500 nm, appearing blue or blue-green in color. The spectra (Figure 4A) of 1, 3, and 4 were obtained with 10-4 M solutions in methylene chloride. Coumarin 2, due to much its stronger fluorescence, was measured at 10 -5 M and its excitation and emission spectra still had to be normalized by a factor of 10X to fit in the graph. In contrast, coumarin 1 in organic solvents emission intensity was two orders of magnitude less than 3 or 4 due to quenching31, 32,33. Coumarins bearing the tertiary amine, hydroxyl, and ether groups in 2, 3, and 4, respectively, generate emissive intramolecular charge transfer states resulting in more intense and red-shifted spectra, particularly for the amine-substituted 4 (Figure 4A)13, 33-35. Figure 4. For the fluorescent Epo-tek 301 adhesives to be useful for art conservationists, the fluorescence has to be visible and relatively stable. Because, it was the first coumarin to be studied, the optimum concentration for forming a visibly fluorescent, epoxy thermoset was determined by varying the concentration of coumarin 4 from 10-3 to 10-1 M. In epoxy samples with as little as 10-3 M 4, fluorescence was clearly visible with long wave UV, despite the coumarin being undetectable by infrared spectroscopy (Figure 5). The fluorescence was noticeably stronger at 10-2 M 4, whereas difference between the fluorescence with 10-2 and 101 M 4 in the epoxies was minute. Based on this, we chose to conduct comparisons of the fluorescence for coumarins 1-4 at 10-2 M (Figure 6). The cured epoxy without any coumarin appears dark in the photograph, though fluorescence from the DEGBA can be detected spectroscopically (Figure 4B). The samples doped with coumarins 2 and 3 are the brightest while the 4 modified thermoset is the least fluorescent of the coumarin modified epoxies. In contrast to the solution spectra of 1, Epo-tek 301 with 1 appeared to be strongly fluorescent and clearly green tinted (Figure 6). Its fluorescence spectrum (Figure 6) reveals its emission to be at 491 nm, 11 nm longer than the sample with 4. This effect is consistent with prior report of more intense, red-shifted emissions from 1 through intermolecular charge transfer from amines32. Stability of the coumarins in Epo-tek 301 was evidenced by their fluorescence after over 1 year exposed to lab lighting. Exposure of resin with 0.01 M 1 to long wave ultraviolet light (27 mW/cm2) for 22 h turned the epoxy brown, but the materials were still fluorescent though diminished in emission intensity (Supporting information, Figure 1S).



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Figure 5.

3.3 Adhesive studies To obtain adhesive strengths of the fluorescent epoxy thermosets, lap shear measurements on aluminum coupons were made (ASTM D1002), since measurements on ceramics are more difficult to make with precision. The aluminum coupons were glued together with a thin film of the epoxies cured at 65 °C for 2 h. After cooling, the lap shear adhesion measurements were taken six duplicate samples. In samples with and without coumarin dyes, only adhesive failure was observed. Epo-tek without additives exhibited a lap shear strength of 27.56 MPa and a Young’s modulus of 675.6 MPa (Table 1) which is twice the 13.5 MPa tensile strength specified by the manufacturer28 and is comparable to reports for similarly prepared epoxies. Addition of 10-2 M coumarin 1 to the epoxy resulted in an 11.6% decrease in lap shear strength and a 10.3% decrease in the shear modulus. Unpaired t-test analyses showed that the lap shear strength of Epo-tek with 1 was not significantly different from Epo-tek, but the shear modulus was significantly lower than that of Epo-tek alone. Epo-tek with 2 samples was significantly lower in lap shear strength (-8.5%) and modulus (-16%) compared to Epo-tek 301 alone. In contrast, addition of 3, which reacts with DGEBA epoxy groups, resulted in a slight decrease in lap shear strength (-5%) and a modest increase in shear modulus (+5.3%). With 10-2 M 4 the average lap shear strength (+10.4%) increased and the shear modulus (+1%) did not, but neither were not statistically different from the Epo-tek alone. At 10-3 M 4, the lap shear strength (+4.9%) and the shear modulus (+ 1.5%) were even closer to the unmodified thermoset.

3.4 Application of fluorescent Adhesive The same formulations with 4 used in the lap shear study were used to repair a broken ceramic pot, but the curing was allowed to progress at 25 °C for 48h. The surfaces of the break were completely covered with two thin film applications of the mixed epoxy and hardener, the pieces were fit together, and held in place in a bed of 1mm glass beads. Lower temperature curing allows the pieces to be re-positioned or even cleaned of the epoxy. Both the 10-3M and 10-2 M coumarin 4 epoxies were applied to ceramic pieces, cured and inspected under visible light and long wave ultraviolet light to determine which concentration would be adequate to visibly fluoresce. The adhesive bonds with 10-3 M coumarin 4 in epoxy were visible but did not show up well in photographic images. Figure 6 shows a clay ceramic pot joined with the epoxy 8 ACS Paragon Plus Environment

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with the 10-2 M 4. Under visible light, the epoxy can be seen running in a broad, slightly darker curve from the upper left side to the middle right side of the pot. Under UV, the bond is extremely visible and it is possible to see additional fluorescence on the upper left, surface of the pot from epoxy that was deposited slightly away from the actual joining. Thus, the fluorescent epoxy can not only be used in identifying adhesive joins, but detect sub-optimal application of the adhesive, and allow for its removal from the ceramic prior to curing. Figure 6.

4. Discussion A fluorescent epoxy should have similar coloration and transparency to commercial variants and clearly emit visible light upon excitation with ultraviolet light. In this regard, all of the coumarin dyes in this study were successful. Coumarin dyes 1 – 4 are soluble in the Epo-tek 301 formulation and afford transparent, slightly yellow tinted thermosets. Curing times are essentially the same as without the coumarins indicating that the small quantities of coumarins added had negligible effect on the curing chemistry. The slight yellow color appears with mixing of the coumarins into the hardener and remains in the cured resin. The transparency of the resins indicates that phase separation didn’t occurring during curing. All of the coumarinmodified thermosets were fluorescent with less than 1 milligram of the dyes per gram of thermoset. Even at 0.1M 4 the coumarin couldn’t be detected by infrared spectroscopy (Figure 6). The most intense fluorescence was from the epoxies prepared with 2 and 3. Deprotonation of hydroxycoumarin 3 is also known to intensify the fluorescence emission36, but results from the solvent extraction suggest that most of 3 is covalently incorporated. As expected, the physically entrapped dyes, 1 and 2, are more easily extracted from the thermosets with solvent and would likely pose greater risk of leaching in the presence of water or solvents. Covalent attachment of the dyes (3 and 4) does reduce their extraction from the thermosets making them the choice for applications where leaching might occur. Room temperature curing was only conducted with 4 for the purposes of applying the fluorescent epoxy to ceramics (Figure 7). The phenolic hydroxyl group in 3 may require high temperature and catalysts to react with epoxies, but the dye would be suitable for high temperature cured phenolic, carboxylic acid or alcohol cured epoxies. Unpaired t-tests analyses of the mechanical strength data (Table 1) revealed that the amounts of coumarins added were small enough that the changes to the lap shear strength and shear moduli of the samples were slight if at all. While the slight decreases observed for the non-covalently bound coumarin dyes (1 and 2) are consistent what is expected with plasticization, the amounts of dyes used are far less than what would normally be required for 9 ACS Paragon Plus Environment


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plasticization. Lack of plasticization is supported by the fact that the glass transition temperatures of the Epotek 301 with and without coumarin 2 remain at the cure temperature (65 °C). Similarly, improvement in mechanical properties, attributed to an anti-plasticization effect, has been observed with the addition of covalent modifiers of epoxies37, but the changes here were mostly statistically irrelevant. It is reassuring to note that adding the reactive dyes (3 and 4) which compete with the adhesive amine and epoxide groups, respectively, did not cause any dramatic changes to the mechanical properties of the Epo-tek 301 adhesive. Figure 7.

5. Conclusions A Fluorescent formulation of Epo-tek 301 was successfully prepared through the addition of small quantities of coumarins 1-4 to the hardener before mixing. Addition of the fluorescent dyes at concentrations adequate to be easily visible under UV, did not measurably change curing of the thermoset and only marginally change the lap shear strength. Covalently attaching the coumarins reduced leaching and slightly increased the strength The coumarin dyes should be compatible with most commercial epoxies, including higher temperature epoxies utilizing hardeners based on carboxylic acids, phenols, alcohols or isocyanates. The fluorescent epoxy adhesive should provide art conservators with a new tool to aid in the reconstruction or repair of artifacts.

Table 1. Lap shear strength and moduli of epoxies shown as the mean of six measurements with standard deviations. Sample

Coumarin Conc. (M)

Lap Shear Strength (MPa)

Shear Moduli (MPa)

EpoTek

0

27.56 ± 2.38

675.62 ± 24.10

EpoTek + 1

10-2

24.36 ± 3.93

606.04 ± 64.02

EpoTek + 2

10-2

25.20 ± 0.77

566.35 ± 95.46

EpoTek + 3

10-2

26.18 ± 2.78

711.80 ± 10.06

EpoTek + 4

10-2

30.43 ± 3.11

682.35 ± 16.67

EpoTek + 4

10-3

28.86 ± 2.52

686.08 ± 16.72


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Supporting Information. Experimental procedures for preparing the remaining fluorescent epoxy resins, photographs showing the photobleaching of the fluorescence with exposure to ultraviolet light, dye leaching experimentals and fluorescence measurements for the case with leaching of coumarin 2 from the Epotek 301 resin are included. Acknowledgements We acknowledge the National Science Foundation (DMR 1241783) for institutional and financial support and the University of Arizona with its Nuclear Magnetic Resonance (NMR) Facility and the Mass Spectrometry (MS) Facility for technical and institutional support. References and Notes 1. Werner, A., Synthetic Materials in Art Conservation. J. Chem. Educ. 1981, 58, (4), 321-324. 2. Down, J. L., The yellowing of epoxy resin adhesives: report on high-intensity light aging. Stud. Conserv. 1986, 31, (4), 159-170. 3. Mailhot, B.; Morlat-Therias, S.; Ouahioune, M.; Gardette, J.-L., Study of the Degradation of an Epoxy/Amine Resin, 1 Photo- And Thermo-Chemical Mechanisms. Macromol. Chem. Phys. 2005, 206, (5), 575-584. 4. Down, J. L., The Yellowing of Epoxy Resin Adhesives: Report on Natural Dark Aging. Stud. Conserv. 1984, 29, (2), 63-76. 5. Chuang, Y.-W.; Yen, H.-J.; Wu, J.-H.; Liou, G.-S., Colorless Triphenylamine-Based Aliphatic Thermoset Epoxy for Multicolored and Near-Infrared Electrochromic Applications. ACS Appl. Mater. Interfaces 2014, 6, (5), 3594-3599. 6. Maehara, T.; Takenaka, J.; Tanaka, K.; Yamaguchi, M.; Yamamoto, H.; Ohshita, J., Synthesis and Polymerization of Novel Epoxy Compounds having an Adamantane Ring and Evaluation of Their Heat Resistance and Transparency. J. Appl. Polym. Sci. 2009, 112, (1), 496-504. 7. Edwards, M.; Zhou, Y., Comparative Properties of Optically Clear Epoxy Encapsulants. Proc. SPIEInt. Soc. Opt. Eng. 2001, 4436, (Wave-Optical Systems Engineering), 190-197. 8. Allen, N. S.; Binkley, J. P.; Parsons, B. J.; Phillips, G. O.; Tennent, N. H., Spectroscopic Properties and Photosensitivity of Epoxy Resins. Polym. Photochem. 1982, 2, (2), 97-107. 9. Sigaev, V. Y.; Belous, D. A.; Shtan'ko, V. I., Effect of Photolysis on the Fluorescent Properties of Bisphenol A-based Epoxy Resins. Zh. Prikl. Khim. (S.-Peterburg) 1998, 71, (6), 1012-1015. 10. Theiss, F. J.; Weber, J., Fluorescence Lifetimes in Cooled Dye Matrixes. Opt. Commun. 1974, 12, (4), 368-369. 11. Sawicz-Kryniger, K.; Ortyl, J.; Popielarz, R. In The Performance of 7-(Diethylamino)coumarin-3carboxylic Acid as a Chemically Bondable Fluorescent Probe and a Fluorescent Marker for Epoxy Compositions, 2010; Wydawnictwo Naukowo-Techniczne TEZA: 2010; pp 129-140. 12. Kwak, S.-Y.; Yang, S. C.; Kim, N. R.; Kim, J. H.; Bae, B.-S., Sol-Gel Derived Dye-Bridged Hybrid Materials for White Luminescence. J. Sol-Gel Sci. Technol. 2013, 65, (1), 46-51. 13. Fernandez, R.; d'Arlas, B. F.; Oyanguren, P. A.; Mondragon, I., Kinetic Studies of the Polymerization of an Epoxy Resin Modified with Rhodamine B. Thermochim. Acta 2009, 493, (1-2), 6-13.



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14. Caselis, J. L. V.; Serrano, R. A.; Rosas, E. R., Preparation and Characterization of Epoxy-Silica Coatings using Rhodamine 6G as Dye. MRS Online Proc. Libr. 2014, 1613, (New Trends in Polymer Chemistry and Characterization-2013), 1-7. 15. Salisu, Z. M.; Yakubu, M. K.; Nkeonye, P. O.; Abba, H., The Synthesis of Diaminoanthraquinone Coloured Cross Linked Epoxy Resins and Their Application in Paint and Selected Polymers. Open J. Appl. Sci. 2014, 4, (1), 13-21. 16. Liu, G.; Wheat, H. G., Use of a Fluorescent Indicator in Monitoring Underlying Corrosion on Coated Aluminum 2024-T4. J. Electrochem. Soc. 2009, 156, (4), C160-C166. 17. D'Alpino, P. H. P.; Pereira, J. C.; Svizero, N. R.; Rueggeberg, F. A.; Pashley, D. H., Factors Affecting Use of Fluorescent Agents in Identification of Resin-Based Polymers. J. Adhes. Dent. 2006, 8, (5), 285-292. 18. Borodulin, A. S., Plasticizers for Epoxy Adhesives and Binders. Polym. Sci., Ser. D 2013, 6, (1), 5962. 19. Maslosh, V. Z.; Ivanov, V. N.; Izyneev, A. A.; Mognonov, D. M.; Korshak, V. V., Synthesis and Study of Structurally Colored Epoxy Resins. Dokl. Akad. Nauk SSSR 1977, 232, (5), 1138-41 [Chem. Tech.]. 20. Davis, K. B.; Braasch, D. A.; Pramanik, M.; Rawlins, J. W., Use of Fluorescent Probes to Determine Molecular Architecture in Phase Separating Epoxy Systems. Ind. Eng. Chem. Res. 2014, 53, (1), 228-234. 21. Sawicz, K.; Ortyl, J.; Popielarz, R., Applicability of 7-Hydroxy-4-Methylcoumarin for Cure Monitoring and Marking of Epoxy Resins. Polimery 2010, 55, (7-8), 539-544. 22. Li, D. W.; Bu, Y. Z.; Zhang, L. N.; Wang, X.; Yang, Y. Y.; Zhuang, Y. P.; Yang, F.; Shen, H.; Wu, D. C., Facile Construction of pH- and Redox-Responsive Micelles from a Biodegradable Poly(beta-hydroxyl amine) for Drug Delivery. Biomacromolecules 2016, 17, (1), 291-300. 23. Chen, Y. L.; Wang, T. C.; Lee, K. H.; Tzeng, C. C., Synthesis of Coumarin Derivatives as Inhibitors of Platelet Aggregation. Helv. Chim. Acta 1996, 79, (3), 651-657. 24. Singh, L. K.; Priyanka; Singh, V.; Katiyar, D., Design, Synthesis and Biological Evaluation of Some New Coumarin Derivatives as Potential Antimicrobial Agents. Med. Chem. (Sharjah, United Arab Emirates) 2015, 11, (2), 128-134. 25. Wang, B.; Guan, X.; Hu, Y.; Su, Z., Preparation and Fluorescent Properties of Poly(vinyl alcohol) Bearing Coumarin. Polym. Adv. Technol. 2007, 18, (7), 529-534. 26. Pramanik, M.; Mendon, S. K.; Rawlins, J. W., Determination of Epoxy Equivalent Weight of Glycidyl Ether Based Epoxides Via Near Infrared Spectroscopy. Polym. Test. 2012, 31, (5), 716-721. 27. ASTM D 1002-10 Standard Test Method for Apparent Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (metal to metal). In ASTM International: West Conshohocken, PA, 2010. 28. EpoxyTechnology Epo-Tek 301 Technical Data Sheet; 2015. 29. Mohan, P., A Critical Review: The Modification, Properties, and Applications of Epoxy Resins. Polym.-Plast. Technol. Eng. 2013, 52, (2), 107-125. 30. Cooke, D.; Fitzpatrick, B.; O'Kennedy, R.; McCormack, T.; Egan, D. In Coumarins - Multifaceted Molecules with Many Analytical and Other Applications, 1997; Wiley: 1997; pp 303-332. 31. Seixas de Melo, J. S.; Becker, R. S.; Macanita, A. L., Photophysical Behavior of Coumarins as a Function of Substitution and Solvent: Experimental Evidence for the Existence of a Lowest Lying 1(n,.pi.*) State. J. Phys. Chem. 1994, 98, (24), 6054-6058. 32. Jhun, B. H.; Ohkubo, K.; Fukuzumi, S.; You, Y., Synthetic Control over Intra- and Intermolecular Charge Transfer Can Turn on the Fluorescence Emission of Non-Emissive Coumarin. J. Mater. Chem. C 2016, 4, (20), 4556-4567. 33. Yip, R. W.; Wen, Y. X., Photophysics of 7-(Diethylamino)-4-methylcoumarin: Picosecond TimeResolved Absorption and Amplified Emission Study. J. Photochem. Photobiol., A 1990, 54, (2), 263-70.


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34. Setsukinai, K.-i.; Urano, Y.; Kikuchi, K.; Higuchi, T.; Nagano, T., Fluorescence Switching by ODearylation of 7-Aryloxycoumarins. Development of Novel Fluorescence Probes to Detect Reactive Oxygen Species with High Selectivity. Perkin 2 2000, (12), 2453-2457. 35. Piedras, A.; Gomez, B.; Carmona-Espindola, J.; Arroyo, R.; Gazquez, J. L., Intramolecular Charge Transfer Model in Fluorescence Processes. Theor. Chem. Acc. 2016, 135, (10), 1-9. 36. Fink, D. W.; Koehler, W. R., pH Effects on Fluorescence of Umbelliferone. Anal. Chem. 1970, 42, (9), 990-3. 37. Haldankar, G. S.; Garton, A., Antiplasticization of Epoxy Resins by Monoepoxy Additives. Polym. Mater. Sci. Eng. 1993, 69, 12-13.

TOC Graphic

Under 354 nm UV



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Coumarin dyes used in this study: coumarin (1), coumarin 480 (2), 7-hydroxycoumarin (3) and 7glycidyloxycoumarin (4). Figure 1 38x12mm (600 x 600 DPI)

ACS Paragon Plus Environment

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Synthesis of 7-glycidyloxycoumarin, 4, from 7-hydroxycoumarin. Scheme 1 352x264mm (72 x 72 DPI)



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Non-covalent (A) versus covalent (B) incorporation of coumarin dyes into epoxy thermosets prepared from DGEBA and a hardening agent composed of a mixture of 2,4,4-trimethylhexane-1,6-diamine and 2,2,4trimethylhexane-1,6-diamine. Figure 2 217x289mm (600 x 600 DPI)


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Figure 3. Lap shear test samples prepared with epoxy thermoset on aluminum coupons. Figure 3. 352x264mm (72 x 72 DPI)



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Curing of epoxy with hardener. Scheme 2 139x108mm (600 x 600 DPI)


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Reaction of 7-hydroxycoumarin 3 with epoxy groups in DGEBA Scheme 3. 163x112mm (600 x 600 DPI)



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Preparation of fluorescent hardener with one of the two diamine isomers present. The 4:diamine ratio was 1:140 for 0.01 M and 1:1060 for 0.001 M. Scheme 4 352x264mm (72 x 72 DPI)


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Fluorescence spectra for coumarins 1-4 in methylene chloride (A) and in Epo-tek 301 (B). Figure 4 352x264mm (72 x 72 DPI)



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Infrared spectra of cured Epotek 301 with 0.1 M and 0.01 M coumarin 4. Figure 5 352x264mm (72 x 72 DPI)


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Scintillation vials containing epoxy resins prepared from unmodified Epo Tek epoxy (A) and Epo Tek with 0.01 M coumarin (1) (B), coumarin 480 (2) (C), 7-hydroxycoumarin, 3, (D), 7glycidyloxycoumarin (4) (E), under normal light (top) and under long wave ultraviolet light (bottom). Figure 6 352x264mm (72 x 72 DPI)



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Ceramic with 0.01 M coumarin 4 in Epo-tek 301 adhesive bond under visible light (left) and long wave UV light (right). Figure 7 352x264mm (72 x 72 DPI)


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UV Fluorescent Epoxy Adhesives from Noncovalent and Covalent

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