Determination of bisphenol A (BPA) by gas chromatography–mass spectrometry and 1H NMR spectroscopy during curing of epoxy–amine resins

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POLYMER TESTING Polymer Testing 25 (2006) 912–922 www.elsevier.com/locate/polytest

Analysis Method

Determination of bisphenol A (BPA) by gas chromatography–mass spectrometry and 1H NMR spectroscopy during curing of epoxy–amine resins F.X. Perrina,, Thi Minh Hanh Nguyena,b, Thi My Linh Tranb, J.L. Verneta a

Laboratoire Mate´riaux a` Finalite´s Spe´cifiques (UPRES no. 1356), Universite´ de Toulon et du Var, BP 132, 83957 La Garde Cedex, France b Faculte´ de Chimie – Laboratoire de Chimie analytique, Universite´ Nationale de Hanoı¨, Vietnam Received 6 April 2006; accepted 20 May 2006

Abstract Two analytical methods were developed to characterize the fate of bisphenol A (BPA) added in low amounts (1–4 wt%) in two different epoxy–amine–BPA systems, one based on a mixture of aliphatic amine curing agents, diethylene triamine (DETA) and poly(oxypropylene)diamine (Jeffamine) and the other on the less reactive diaminodiphenylsulphone (DDS) hardener. The method involves a liquid–liquid extraction (LLE) procedure up to gelation. Identification and quantification was performed by gas chromatography–mass spectrometry (GC–MS) and proton nuclear magnetic resonance (1H-NMR) spectroscopy. The data obtained by the two methods are in agreement. The main purpose of this study was to determine the extent of aryl-etherification in actual complex epoxy/amine formulations. It serves notably to detect potential contamination of the atmosphere, water, and soil in the area surrounding where the unbound BPA is released. r 2006 Elsevier Ltd. All rights reserved. Keywords: Bisphenol A; Epoxy; Curing; GC–MS; NMR spectroscopy

1. Introduction Epoxy resins are one of the most important classes of thermosetting polymers. The addition of additives or modifiers (catalysts, diluents, fillers, resinous modifiers, elastomeric modification, etc.), to the two main ingredients is frequently adopted to change the characteristics of the uncured resin (rheology, curing kinetics, etc.), or the properties of the final material (fracture behaviour for Corresponding author. Tel.: +33 494 142580; fax: +33 494 142448. E-mail address: [email protected] (F.X. Perrin).

0142-9418/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.05.015

example). Some formulations proposed in industry were found to contain small amounts of phenolic compounds mixed with the amine hardener component. Phenolic compounds are hydrogen donor molecules and thus could theoretically catalyse the epoxy-amine reaction by forming a trimolecular complex facilitating the amine attack, as illustrated in Scheme 1. The same type of push-pull mechanism accounts for the autocatalytic nature of the curing process observed with epoxy/amine systems, which is related to the accumulation of hydroxyl groups during curing [1,2]. The formation of ether links during epoxy/amine cure by reaction between an epoxide group and a

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O

R1 HN

H2C

CH2

CH

CH O

R2

H O

H O

O

R1

R2

H Scheme 1. Trimolecular intermediate amine–epoxy–phenol showing the catalytic effect of a phenol compound on the amine–epoxy reaction.

hydroxyl group is significant only with certain epoxy/amine systems and in certain conditions (stoichiometric excess of epoxide groups, high cure temperatures, etc.) [3]. The etherification reaction is also accelerated by tertiary amine (Scheme 2) and hydroxyl groups (Scheme 3) [4]. Both the tertiary amine and hydroxy catalysis etherification proceed through the formation of trimolecular complex, as shown in Schemes 2 and 3, respectively. Schemes 2 and 3 suggest that phenol compounds in epoxy–amine–phenol ternary systems can be incorporated in the epoxy network through aryl ether bonds (Schemes 2 and 3 with R1 ¼ phenyl). Eventually, they may also accelerate etherification of aliphatic hydroxyl (present in the starting epoxy material or formed during the epoxy–amine reaction) with epoxide (Scheme 2 with R1 ¼ epoxy chain and R2 ¼ phenyl). Currently, our knowledge about the reactivity of phenol compounds with epoxy and epoxy/amine systems is mainly based on the pioneering work by Shechter and Wynstra [5] and Shechter et al. [6]. The catalytic effect of phenol for the alcohol– epoxide etherification [5] and for the amine–epoxy addition [6] were demonstrated. These studies were performed on soluble model compounds to permit the use of conventional analytical methods to elucidate the reaction mechanism. The danger with this approach is that the reaction rates and the reaction paths observed with these monofunctional compounds may be different from the results obtained on real polyfunctional systems. For instance, the addition of phenol through aryl ether bonds was found to strongly depend on the presence of a base catalyst [6]. Without a catalyst, phenol reacts with epoxy at a temperature of 200 1C or higher [5], while the reaction of phenol with epoxide proceeded at a reasonable rate at 50 1C in phenol–phenyl glycidyl ether–diethylamine (or

Scheme 2. Trimolecular intermediate showing the catalytic effect of a hydroxy compound on the etherification reaction (R1 ¼ or 6¼R2).

O CH

CH2

O H

R1 NR3

Scheme 3. Trimolecular intermediate showing the catalytic effect of a tertiary amine on the etherification reaction.

N-methyl aniline) mixtures [6]. In this context, one of the principle uncertainties arises from the fate of a phenol compound added to a given polyfunctional epoxy–amine system. In other words, does the hindered amine in polyfunctional epoxy–amine systems exert any catalytic effect toward the aryl etherification? It also appears fundamental to know if phenol compound is chemically bound into the cross-linked network or if it is only physically bound (hydrogen bonds) for two main reasons: (1) the way phenol is incorporated into the cured resin may affect the physical and mechanical properties of the final material in a different way; (2) the toxicological activity of phenols should be strictly considered if a significant amount of phenol remains not chemically bound to the network. At the present time, we found that some commercial formulations contain low amounts of bisphenol A (4,40 -isopropylidenediphenol) (BPA) added in the hardener component. The low amount of BPA in these formulations (1–4 wt%) suggest that BPA mainly serves here as a catalyst for the amine–epoxy reaction. As far back as 1936, Doods and Lawson noted oestrogenic activity of BPA [7]. More recently, BPA was found to be leached from a polycarbonate flask [8]. It was also found in saliva collected from patients who were treated with a DGEBA-based dental sealant [9]. In both cases, the

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leaching of BPA was ascribed to the hydrolysis of the polymer. In this paper, we developed analytical tools to investigate the fate of low amounts of BPA (1–4 wt%) in two formulations of different reactivity: an epoxy–aliphatic amine–BPA commercial formulation and a less reactive diglycidyl ether of bisphenol A (DGEBA)–diaminodiphenylsulphone (DDS)–BPA formulation. We will also propose a methodology for the determination of epoxide equivalent weight (EEW) and component composition in epoxy resins based on DGEBA/DGEBF mixtures using 1H NMR spectroscopy. 2. Experimental 2.1. Materials The epoxy resin was a DGEBA–diglycidyl ether of bisphenol F (DGEBF) mixture (see the next section for the characterization data) supplied by REA Industrie (France). DGEBF is used as a functional diluent to prepare solventless epoxy formulations. Three different amine hardeners were tested: a diethylene triamine (DETA)/polyoxypropylene amine/BPA commercial hardener (CH) having an amino hydrogen equivalent weight (AEW) of 59 g, jeffamine (Aldrich) with an AEW of 100 g and 4,40 -diaminodiphenylsulfone (Aldrich, 97%) with an AEW of 62 g. BPA and naphtalene were supplied by Aldrich. Water was purified by means of a water purification system. All other solvents were of analytical reagent grade (Aldrich) except dichloromethane used for the LLE of BPA (Fluka, 95%). 2.2. Methods The samples were prepared by adding appropriate amounts of an amine/BPA solution or BPA powder to the DGEBA/DGEBF resin either at room temperature (aliphatic amine-based epoxy) or at 120 1C (DDS-based epoxy). The specified quantities for the epoxy, amine hardener and BPA correspond to their molar equivalent of glycidyl ether, hydrogen amines, hydroxy functionalities, respectively. After intimate mixing, the viscous mixture was cast in a teflon mould and isothermally cured at a temperature related to the reactivity of the hardener (not greater than 60 and 200 1C, respectively, for aliphatic and aromatic amines). Samples (5 g) were taken at definite times during

curing, introduced into an Erlenmayer flask and cooled quickly to room temperature while placing the flask in a ice bath. 2.2.1. LLE procedure (before gelation) About 50 ml of an aqueous solution containing sodium hydroxide (0.3 M) and sodium chloride (100 g/l) was added to a 50 ml ether solution of 5 g of the epoxy/amine/BPA sample. After vigorous stirring for 10 min at room temperature, the aqueous phase was extracted and treated with 10% hydrochloric acid to obtain an acidic pH (
1–2) solution. NaCl was added to the acidic solution to obtain a 200 g/l NaCl concentration. Then, the aqueous extract was extracted three times with 50 ml of dichloromethane by vigorous shaking for 10 min. The extracts were treated with 1 g of magnesium sulphate and then concentrated to near dryness on a rotary evaporator under a gentle flow of N2. 2.2.2. Solid– liquid extraction (SLE) (after gelation) About 1–5 g of the epoxy/amine/BPA gelled material was ground to fine powder at the temperature of liquid nitrogen using a freezer mill SPEX Certiprep 6750. The powder was extracted twice with 20 ml of methanol by shaking overnight. The combined extracts (40 ml) were either concentrated (for NMR analysis) or used to prepare methanol solutions for GC/MS analysis. 2.2.3. 1H NMR analysis 1 H NMR spectra were measured at room temperature on a Brucker AV400 spectrometer operating at 400 MHz, using deuterated dimethylsulphoxide (DMSO-d6) as a solvent and naphtalene as an internal standard. 2.2.4. GC-MS analysis Chromatographic measurements were performed with a gas chromatograph HP 6890 equipped with a mass spectrometer HP 6890. The carrier gas was helium and the flow rate was 1 ml/min. The column was a fused silica capillary (30 m  0.25 mm i.d.  0.25 mm film thickness). The temperature of the injection port was 280 1C using a split ratio of 40/1. The oven temperature was held at 150 1C for 2 min, then elevated to 280 1C at 30 1C/min and finally held at 280 1C for 2 min. The interface temperature was 300 1C and the energy of ionizing electron was 70 eV. The MS was operated in scan

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mode with a scan range from m/z 45 to 300. Quantification was done by an internal standard method using naphthalene as an internal standard. Monitored ions were m/z (molecular ion of naphthalene) and 228.1 (molecular ion of BPA).

complicated by the presence in the resin of two different epoxies of very close chemical structure. The average molecular weight M n of the DGEBA/ DGEBF resin may be defined as Mn ¼

3. Results and discussion 3.1. Characterization of the DGEBA/DGEBF commercial mixture The DGEBA/DGEBF commercial mixture was characterized by 1H NMR spectroscopy. Fig. 1 illustrates the 1H-NMR spectrum of the DGEBA/ DGEBF mixture. All the resonance signals have been attributed to the corresponding protons of the DGEBA and DGEBF structures. Recently, the 1H NMR was found to constitute an alternative to the classical titration methods to determine the EEW of low molecular weight DGEBA resins [10]. Here, the problem is somewhat

915

x1 M n1 þ x2 M n2 , x1 þ x2

(1)

where x1 and x2 are mole number of DGEBA and DGEBF contained in 1 g of mixture, respectively, and M n1 and M n2 are the average molecular weight of DGEBA and DGEBF, respectively. M n1 and M n2 are related to the polymerization degree n1 and n2 by the relation: M n1 ¼ 284n1 þ 340,

(2)

M n2 ¼ 256n2 þ 312.

(3)

Considering the idealized structures in Fig. 1 where one epoxy group terminates each end of the DGEBA and DGEBF resins, the average EEW is one-half the average molecular weight of the

25000

20000

15000

10000

50000

0

7.0

6.0

5.0

4.0 ppm (t1)

3.0

2.0

1.0

Fig. 1. 1H NMR spectrum of the DGEBA/DGEBF commercial mixture.

0.0

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DGEBA/DGEBF mixture: EEW ¼

x1 ð284n1 þ 340Þ þ x2 ð256n2 þ 312Þ . 2ðx1 þ x2 Þ

(4)

EEW can be determined measuring the integrals I1, I2 and I3 corresponding, respectively, to the aromatic protons (peaks a and b), the epoxide group protons (peaks e, f and g) and the benzylic methyl protons (peak h). Indeed, the idealized structures shown in Fig. 1 give R1 ¼

R2 ¼

I 1 8½x1 ðn1 þ 1Þ þ x2 ðn2 þ 1Þ , ¼ 6ðx1 þ x2 Þ I2

(5)

I3 6x1ðn1 þ 1Þ ¼ . I2 6ðx1 þ x2Þ

(6)

Substituting Eq. (6) into Eq. (5) and re-arranging, we get 3I 1  4I 3 x2 ðn2 þ 1Þ R3 ¼ . ¼ ðx1 þ x2 Þ 4I 2

(7)

Eq. (4) can be re-arranged to be expressed as function of the integral ratios R1 and R2 (Eqs. (5) and (6)): EEW ¼ 96R1 þ 14R2 þ 28.

(8)

The EEW of the DGEBA/DGEBF commercial mixture calculated from Eq. (8) was 171.271.2 (value at 95% confidence interval determined from five different aliquots). An EEW of 17472.7 (value at 95% confidence interval determined from five chemical analysis) was found by acid titration using the pyridinium chloride–pyridine method [11]. The data found by 1 H NMR spectroscopy are in good agreement to those obtained from the classical chemical titration. The average polymerization degree, n¯ , can be defined by the following equation: x1 n1 þ x2 n2 n¯ ¼ . x 1 þ x2

½x1 ðn1 þ 1Þ þ x2 ðn2 þ 1Þ  ðx1 þ x2 Þ . ðx1 þ x2 Þ

(9)

(10)

(11)

The average polymerization degree was estimated by Eq. (11): n¯ ¼ 0:08.

y1 , 284n1 þ 340

(12)

x2 ¼

y2 . 256n2 þ 312

(13)

Re-arranging Eqs. (12) and (13), we get x1 ½284ðn1 þ 1Þ þ 56 ¼ y1 ,

(14)

x2 ½256ðn2 þ 1Þ þ 56 ¼ y2 .

(15)

Substituting Eq. (6) into Eq. (14) and Eq. (7) into Eq. (15), the following equations can be obtained: 284R2 ðx1 þ x2 Þ þ 56x1 ¼ y1 ,

(16)

256R3 ðx1 þ x2 Þ þ 56x2 ¼ y2 .

(17)

Addition of Eqs. (16) and (17) gives ðx1 þ x2 Þ ¼

1 . 284R2 þ 256R3 þ 56

(18)

Besides, from Eqs. (6) and (7), it is clear that x1 oR2 ðx1 þ x2 Þ,

(19)

x2 oR3 ðx1 þ x2 Þ.

(20)

Substituting the inequality Eqs. (19) and (20) in Eqs. (16) and (17), then, considering Eq. (18) we get: 312R3 340R2 oy1 o . 284R2 þ 256R3 þ 56 284R2 þ 256R3 þ 56

(21)

Therefore, n¯ ¼ 68R1  1.

x1 ¼

1

Eq. (8) can be re-written in order to reveal the expression of the integral ratio R1: n¯ ¼

The low value of n¯ agrees with the low intensity of the multiplet at d ¼ 4.1 ppm (peak i ascribed to the aliphatic carbinol methine) and confirms that the epoxy resin contains a small fraction of oligomers (n1 and n240). The mole number x1 and x2 are related with the weight fraction of DGEBA, y1, and DGEBF, y2, as follows:

Eq. (21) is a general expression that can be used for estimating the interval of the weight fractions of DGEBA and DGEBF in mixtures of these components. For the commercial epoxy used in this study, the width of the interval was found to be very low: 0:34oy1 o0:35. These data are in excellent agreement with the data given by the supplier: 65 wt% of DGEBF (and 35% of DGEBA).

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3.2. Preconcentration procedure

three successive extractions (see the next section for the determination of the extraction yield). We noted that the ionic strength of the aqueous phase was one of the key parameters that affects both the yield of BPA recovery and the efficiency of the separation. The ionic strength was adjusted using NaCl to obtain NaCl concentrations from 0 to 300 g/L. A 100 g/L NaCl concentration in the aqueous alkaline phase at the first step of the LLE ensures that a major fraction of the epoxy resin remains in the ether phase. Besides, a 200 g/L NaCl concentration in the acidic aqueous phase at the second step of the LLE was found to be necessary to totally extract BPA in dichloromethane by a salting-out effect. 3.3. Analytical determination of BPA

1.514

3.3.1. 1H NMR spectroscopy: extraction yield Fig. 2 shows a typical 1H NMR spectrum of a complex epoxy–amine–BPA mixture (Fig. 2(a)) together with the spectrum of the organic phase

6.640

6.977

7.506

7.905

9.137

A preconcentration procedure was essential to avoid strong interferences of amine and epoxy components during analysis. We adopted a liquid–liquid extraction method and a SLE method, respectively, before and after the gelation point of the mixture. The optimization of the extraction procedure was carried out on a stoechiometric DGEBA/DGEBF–jeffamine–BPA (1 wt%) mixture. For the LLE procedure, BPA was extracted in two steps: in a first step, BPA was totally removed as its dissociated form in the alkaline aqueous phase. The relatively hydrosoluble amine hardener was found to be present in great amounts in the aqueous phase. In the second step, the aqueous phase was acidified to form the amine salt and, thus, to selectively extract BPA in an organic solvent. Ethyl ether, n-hexane and dichloromethane were tested as extraction solvents. Dichloromethane was the most efficient, giving nearly 100% recovery of BPA after

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naphtalene

Hc

biphenol A (a) Hd

Hao

Hbo

Ha

Hb

H20 DMSO x

9.0

8.0

7.0

6.0

5.0 ppm (t1)

4.0

(b)

3.0

2.0

1.0

Fig. 2. 1H NMR spectrum of a stoechiometric DGEBA/DGEBF–jeffamine–BPA (1 wt%) mixture before (a) and after (b) the LLE procedure.

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Fig. 3. Typical chromatogram of a LLE residue (shown here is a DGEBA/DGEBF–jeffamine–BPA(1%) mixture after 2 h at room temperature): (1) naphthalene, and (2) BPA.

Fig. 4. Mass spectra obtained in scan mode for BPA.

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(Fig. 2(b)) after the LLE procedure. The 1H NMR spectrum of Fig. 2(b) is dominated by the proton signals of BPA (aromatic doublets at 6.64 and 6.98 ppm and methyl singlet at 1.51 ppm), while the proton signals of BPA are barely perceptible in Fig. 2(a). This reveals the efficiency of the separation, although minor amounts of the epoxy resin (small peaks at 7.05 and 6.77 ppm) are still present in the organic extract after extraction. The wt% recovery of BPA was determined by adding naphthalene as an internal standard to the organic residue obtained from the LLE procedure. The two doublets at 7.5 and 7.9 ppm are the absorption of the aromatic protons of the standard. The wt% of BPA in the epoxy/amine mixture is calculated by comparing the integral of the four aromatic protons of BPA at 6.98 ppm (Ia) with the integral of the four aromatic protons of

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naphthalene at 7.9 ppm (Iao): %BPA ¼

W N Ia M BPA  100, W R I ao M N

(22)

where WN is the weight of naphthalene, WR is the weight of resin and MBPA and MN are the molecular weight of BPA and naphthalene, respectively. The calculated wt% of BPA from Eq. (22) was compared to the theoretical amount of BPA in different epoxy–amine–BPA mixtures of known composition (0.5–2 wt% BPA) and just prepared at room temperature: the extraction yield was systematically higher than 95%. The same methodology was followed to determine the efficiency of SLE from a stoichiometric epoxy/CM blend cured at 20 1C for 14 days. The extraction yield of BPA was found to be near 100% as could be forecast from the low curie temperature.

1 GCMS

(1-αBPA)

0.8

RMN

0.6

0.4

0.2

0 0

500

1000

1500

2000

(a)

2500 3000 time (min)

3500

4000

4500

5000

1 GCMS

(1-αBPA)

0.8

RMN

0.6

0.4

0.2

0 0 (b)

1000

2000

3000

4000

5000

time (min)

Fig. 5. Fraction of non-reacted BPA (1aBPA) in (a) epoxy/BPA (1/0.076) and (b) epoxy/DDS/BPA (1/1/0.076) blends both cured at 100 1C.

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3.3.2. GC/MS analysis GC/MS has been used to analyse phenolic analytes either directly [12] or after silyl derivatization [13]. By incorporating a silyl group to a phenolic compound, better reproducibility, sensitivity and resolution can be achieved for the gas chromatographic analysis. This work is not dedicated to analysis of traces of polar analytes or to complex mixtures of phenol compounds with very similar chemical structure. Thus, a direct analysis of the LLE and SLE residues was performed. A typical chromatogram of the phenolic analyte obtained after the LLE procedure is shown in Fig. 3. Only 7 min were necessary to complete the analysis. The mass spectrum obtained in scan mode for the compound (2) of Fig. 3 is shown in Fig. 4 and reveals the molecular ion of BPA at m/z 228 and a base peak at m/z 213 corresponding to loss of a methyl group. The GC-MS and 1H NMR analysis were found to give very similar results, as inferred from Fig. 5 which compares the BPA content in epoxy/BPA and epoxy/DDS/BPA blends determined from both methods. The fate of BPA in both blends will be discussed in the next two sections. 3.4. The fate of BPA in epoxy/BPA and epoxy/ amine/BPA blends Fig. 6 compares the fate of BPA in stoichiometric epoxy/amine blends from either aliphatic or aromatic hardener.

BPA is integrally recovered from the totally cured epoxy/jeffamine blend. In such resins, BPA is only linked to the polymer network though physical bonds (hydrogen bonds) and is thus prone to be readily released in the environment. Conversely, the higher temperature required to cure epoxy resins with aromatic hardener like DDS favours the etherification between BPA and epoxy. DSC analysis (not shown) showed that BPA does not induce a change in Tg for epoxy/jeffamine blends (Tg onset measured during the second ramp near 32 1C) while a 10 1C decrease of the Tg of epoxy/DDS/BPA (1/1/ 0.076) compared to the Tg of epoxy/DDS (1/1) was noted (respective Tg onset of 164 and 174 1C). Owing to the low amount of free BPA (o10% of the initial content from Fig. 6) in the totally cured DDS based resins, the plasticization induced by BPA can undoubtedly be related to the chemical alteration of the polymer network (internal plasticization) rather than an external plasticization. Low amounts of BPA (few wt%) incorporated in epoxy/ aliphatic amine blends only acts to catalyse the epoxy–amine reaction, as suggested by the measured gel time of 161 min in epoxy/jeffamine (1/1) blend compared to 82 min in epoxy/jeffamine/BPA (1/1/0.076) blend, both cured at 60 1C. Fig. 6 also reveals a clear increase in the rate of etherification in the course of the reticulation at the three investigated temperatures. This acceleration must be related to the accumulation of hydroxy

1 DDS (200°C) DDS (150°C)

0.8

DDS (100°C) Jeffamine (60°C) (1-αBPA)

0.6

0.4

0.2

0 0

100

200

300

400 time (min)

500

600

700

800

Fig. 6. Fraction of non-reacted BPA (1aBPA) in epoxy/amine/BPA (1/1/0.076) blends isothermally cured.

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and/or tertiary amine during the epoxy/amine reaction. This result suggests that trimolecular complexes such as shown in Schemes 2 and 3 are involved during the epoxy/BPA reaction. It is noteworthy that this is not strictly speaking an autocatalytic effect. Further experimental evidence of such a catalytic effect is provided by comparing the change in BPA content in epoxy/BPA (1/0.076) and epoxy/DDS/BPA (1/1/0.076) mixtures cured under the same conditions (Fig. 7). Irrespective of 1

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the cure temperature, the rate of the etherification reaction between epoxy and BPA is found to be higher when the amine hardener is incorporated into the epoxy/BPA blend. Since BPA/epoxy/BPA trimolecular complexes may be potentially involved as soon as the initial times of the reaction, it is believed that tertiary amines, despite their steric crowding, exert a more efficient catalytic effect than hydroxy groups.

without DDS with DDS

(1-αBPA)

0.8

200°C

0.6 0.4 0.2 0 0

1

200

400 600 time (min)

800

1000

without DDS with DDS

0.8 (1-αBPA)

150°C 0.6 0.4 0.2 0 0

1

500

1000

1500 time (min)

2000

2500

3000

without DDS with DDS

(1-αBPA)

0.8

100°C

0.6 0.4 0.2 0 0

1000

2000 3000 time (min)

4000

5000

Fig. 7. Fraction of non-reacted BPA (1aBPA) in isothermally cured epoxy/BPA (1/0.076) and epoxy/DDS/BPA (1/1/0.076) blends.

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4. Conclusion In the first part of this paper, we showed that 1H NMR spectroscopy was an efficient tool to characterize unknown DGEBA/DGEBF epoxy mixtures: the general equations allowing the estimation of the EEW (Eq. (8)), the average polymerization degree (Eq. (11)) and the weight fraction of each component (Eq. (21)) have been determined. The values obtained by the proposed methodology agreed well with the EEW value obtained from the classical pyridinium chloride– pyridine titration method and with the weight fraction data given by the supplier. The main purpose of this work was to develop analytical tools to determine the fate of phenolic compounds incorporated to complex epoxy/amine mixtures. Both GC/MS and 1H NMR spectroscopy gave reproducible and convenient results. However, 1 H NMR seems more appropriate to determine the fate of BPA during curing owing to the relatively high amount of analytes on looking at the high sensitivity of GC/MS analysis. 1H NMR spectroscopy results are also less dependant than GC/MS on the interaction of other chemicals such as epoxy or amine residues not separated during the LLE procedure. Conversely, GC/MS, being a more sensitive method, should be preferred to 1H NMR spectroscopy when the aim is to determine the extent of leaching of BPA from the cured material into a specific environment such as water for example. In this study, the BPA was found to act only as a catalyst for the epoxy–amine reaction in the reactive aliphatic amine system raising the problem of its release in the environment. Apart from its catalytic effect, BPA was incorporated as aryl-ether bonds to the epoxy–DDS network when the cure temperatures are higher than 100 1C. Besides, a clear catalytic effect of tertiary amine and hydroxy groups for the etherification reaction has been revealed. It must finally be noted that the analytical methodology developed here may also be adapted to phenol compounds other than BPA, either to determine their fate during curing or their release in

a specific environment. An example is nonyl phenol which has found wide use in many aminecured epoxy formulations, functioning as a blush preventative, flexibilizer and polymerization accelerator.

Acknowledgement Grateful acknowledgement is due to AUF for its financial support.

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