Application of alizarin colorimetric measurements for quantification of amine extraction by model food simulants from epoxy polymer
© Jeliński et al.; licensee Springer. 2013
Received: 28 May 2013
Accepted: 23 October 2013
Published: 6 November 2013
A simple and straightforward method has been proposed for quantification of residual amine in cured epoxy resin. Non-bounded triethylenetetramine was extracted from epoxy polymer and determined via spectrophotometry using alizarin chromophore. Four solvents commonly used as food simulants, namely water, 95% ethanol, 10% ethanol and 3% acetic acid were examined. Released amine induces changes in the absorption spectrum of alizarin, by decreasing the intensity of the maximum at 430 nm band and mutually increasing the 527 nm band. These changes were proportional to the amounts of amine concentration in samples. The statistical significance of obtained calibration curves was validated. Among studied solvents, the highest amine release was observed for water solution and 3% acetic acid, that is approximately 7% w/w. The maximal amount of residual amine extracted with 95% ethanol was about 1.25%, while for 10% ethanol this amount was 2%. The effect of aging of the samples and exposure to artificial sunlight were also examined. The proposed method has been proven to be fast, low cost and directly applicable for analysis of typical epoxy resins.
Epoxy resins are widely applied as coatings, adhesives, construction composites etc. (Muskopf and McCollister ; Petrie ; Wicks et al. ). Common curing agents used with epoxy resins are aliphatic and aromatic polyamines (Petrie ; Prolongo et al. ; Wicks et al. ). However, the reaction of amine groups with oxirane rings is never complete, because the conversion degree is restricted by resin vitrification and therefore, residual amine hardeners can migrate from the cured polymer. This may cause serious problems due to the known toxicity of aliphatic and aromatic amines (Hughes et al. ; Patnaik ; Smith ). There are several examples of contact dermatitis resulting from the use of amine compounds in modified polymers (Conde-Salazar et al. ; Soto et al. ), and also the health problems caused by amine hardeners in epoxy systems have been studied shortly after the introduction of epoxy resins into modern industry (Bourne et al. ). After many years of usage of epoxy resins in different industrial and commercial applications, the problem of health hazards caused by such systems is still not completely examined, although many published works have focused on allergies, caused by both the epoxy resin itself (Akita et al. ; Kumar and Freeman ; Sasseville ) as well as resin modifiers, such as hardeners (Bachanek et al. ; Bray ). For that reason, determination of non-bounded amine hardener is an important analytical task. Chromatographic methods are the ones mostly used (Dopico-Garcia et al. ; Paseiro-Cerrato et al. ) however, gas chromatography has limited applicability because of low volatility of multifunctional amines. Near-infrared spectroscopy is a convenient and rapid technique offering reliable assessments of amine conversion degree (Escola et al. ; Prolongo et al. ). Direct spectrophotometry and spectrofluorimetry are rather less applicable to aliphatic amines, because they do not possess characteristic UV–VIS absorbance or fluorescence properties. On the other hand, both the techniques became fully applicable when appropriate colored indicator is used (Ajayakumar and Mukhopadhyay ; Bao et al. ; Basurto et al. ; Comes et al. ; García-Acosta et al. ; Liu et al. ; Oliveri and Di Bella ; Staneva et al. ). One of the promising indicators for low-polar solvents is alizarin (1,2-dihydroxy-9,10-anthraquinone) (del Río et al. ). Anthraquinone derivatives in general are known as analytical reagents (Hosseini and Asadi ; Mitic et al. ). Alizarin itself is applied in chemical analysis as well (Chamsaz et al. ; Feng et al. ). Alizarin exists in three forms of different color, namely the protonated form and two deprotonated forms corresponding to the monoanion and dianion (Das et al. ; Cysewski et al. ; Preat et al. ). The absorption maximum of the neutral non-dissociated form is located at 430 nm (Preat et al. ; Say-Liang-Fat and Cornard ) and the monoanionic form is characterized by an absorption maximum at around 530 nm (Quinti et al. ; Savko et al. ). The color change of alizarin can be modeled quite accurately based on simple protocol within TD-DFT framework, which has been shown by us in one of our earlier studies (Cysewski et al. ). This makes the use of alizarin chromophore very convenient since it is sensitive to the presence of proton acceptor agents even in non-water solutions.
Reagents and chemicals
Mid-viscosity epoxy resin with trade name Epidian 5 (Cedar, Poland) was used in the studies. Triethylenetetramine (TETA), known as one of the most widely used hardeners (Czub ; Manthey et al. ; Roskowicz and Smal ; Tripathy et al. ) has been purchased also from Cedar under trade name Z1. The above compounds were used without further purification. Analytical grade ethanol and acetic acid were supplied by POCH S.A. (Poland). Alizarin has been supplied by Sigma-Aldrich.
Stock solutions of TETA in widely accepted food simulants such as water, 95% ethanol, 10% ethanol and 3% acetic acid were prepared with the concentration of 0.160 mg/ml each. A series of diluted solutions was then prepared by adding different volumes of the solvent to a standard volume of the stock solution. Alizarin stock solutions in 95% ethanol, 10% ethanol, 3% acetic acid and methanol were prepared with the concentration of 0.4685 mmol/l. Aqueous solution was not used because the solubility of alizarin in water is very limited. Calibration samples were prepared by mixing 1.5 cm3 of each diluted amine solution with 1.5 cm3 of alizarin stock solution directly in a spectrophotometric cuvette. In such a way, three sets of calibration curves were obtained in water–methanol (50% v/v), 95% ethanol and 10% ethanol solutions. For 3% acetic acid, the alizarin stock solution was alkalized to pH equal to 12 and then the acidic solution of TETA was added as described above. Absorption spectra were recorded using a single-beam UV/VIS spectrophotometer (Merck, Spectroquant Pharo 300) with a wavelength resolution of 1 nm.
Series of epoxy polymer samples has been prepared at the hardener content equal to 8, 10, 12, 14 and 16 phr. Pre-weighted amounts of the epoxy resin and TETA were thoroughly mixed and left for 24 hours at ambient temperature for the completion of the curing process. Cured polymer pellets of about 50 mg were weighed and placed in sealed tubes filled with 3 ml of solvent for extraction. Preliminary tests showed that the extraction process is complete after 3 hours, which is documented in the Appendix. Next, 1.5 ml of the extract was taken and put into a cuvette containing 1.5 ml of the alizarin stock solution (alkalized in the case of 3% acetic acid solution). Then, the spectra were recorded in the same way as in the case of the calibration curves.
Aging effect and sunlight exposure
In order to investigate the effect of aging of the samples on the amine release process, one series of the samples was examined immediately after preparation, while the pellets from the second series were left for six months in room temperature and then examined.
An artificial sunlight lamp was used for simulating the effect of sunlight exposure. The power density of this lamp was measured as a function of the length from the lamp to the surface containing the examined epoxy pellets. Also, the temperature was measured in order to avoid overheating of the samples. The length of 12.5 cm was chosen, giving a power density of 9.2 kW/m2. Since the annual solar irradiance in middle Europe (e.g. Poland) is about 1000 kWh/m2, a series of time intervals from 1 to 100 hours mimicked the amount of energy distributed to the samples throughout the one year span.
Statistical analysis has been conducted as follows. Linear equations and R2 coefficients were used to determine the variance of the y-intercept (denoted as C).
where Yi are the experimentally obtained values and Yi,calc are the ones estimated on the basis of the obtained linear equation.
Results and discssion
Addition of increased amounts of TETA to alizarin solution results in substantial changes of UV–VIS spectra. The absorption maximum of alizarin at 430 nm is decreased and the height of the peak at 527 nm is increased. It is obvious, that the observed spectral changes are caused by the increase of solution alkalinity and the formation of alizarin amine complex with pseudo-dissociation to anionic form. This is discussed in the Appendix section. These regularities related to color change from colorless to magenta-red are observed in all of the solvents studied.
Linear equations, R 2 coefficients, values of the standard deviation of y-intercept (SD(A)) and the values of limits of detection (LOD) and quantification (LOQ) for the calibration curve utilizing the absorbance at 527 nm
3% acetic acid
y = 5.544x + 0.242
y = 1.389x + 0.128
y = 11.293x + 0.1644
y = 9.6858x + 0.165
From the data provided in Table 1 and Appendix one can see that the absorbance values at 527 nm proved to have the best linearity among the three curves. Although the curve utilizing the ratio of absorbencies has a slightly better linearity in the case of 10% ethanol and 3% acetic acid, it performs much worse in the case of 95% ethanol and water. On the other hand, the curve utilizing the absorbance values at 430 nm has a good linearity for 95% ethanol and water, although worse than the '527 nm’ curve, but it performs much worse in the case of 10% ethanol and 3% acetic acid. The above observations are also confirmed by the LOD and LOQ values for these curves. Also, the solvents used influenced both the detection and quantification limits. The best sensitivity is obtained in the case of 3% acetic acid, while the worst sensitivity characterizes 10% ethanol, although the limits of detection and quantification are satisfactory for all the solvents. One can infer, that the most appropriate calibration curve for TETA determination is provided with the absorbance at 527 nm. Besides, all four food simulant solvents may be used for analytical purpose, with conjunction of alizarin chromophore.
Quantification of amine extraction
The polarity of TETA results in very high affinity towards water, which was documented by extraction experiments. Having the above in mind, the lack of differences in the amounts of released amine for fresh samples and samples aging for six months suggests that there was no further progress in cross-linking reaction during this period of time. However, it is interesting to see, that such statistically significant differences occur in the case of much less polar 95% ethanol solution. This may suggest that during the aging of the sample partial penetration of the amine inside the polymer takes place. However, this physical process leads to such a small change of affinity towards water, that the extraction with the use of this polar solvent causes the release of all, or at least the majority, of the unbounded amine. Also, it can be suggested that the amine sorption process is caused by intermolecular interactions weaker than those stabilizing amine solutions in water, but stronger than those of TETA-ethanol system. Thus, residual amounts of amine that are not chemically bounded can be released to water. Also, wet surfaces coated with epoxy polymers are probably rich in extracted portions of amine, which poses some ecological challenge if such polymers are used. On the other hand, this releasing process occurs only for limited time period that is restricted to total amount of free amine in samples. Since water is a very effective extraction media it seems to be rational to propose a post-polymerization process of immersion in water as a natural way of freeing the polymer from the residual amine.
A new, simple and accurate method of determination of residual amine in epoxy polymer has been developed. The method utilizes the susceptibility of alizarin to undergo color changes in proton accepting media. THETA, acting as a base, induces changes in the alizarin chromophore which enables the use of spectrometric methods in quantitative determinations. The calibration plot corresponding to the absorbance at 527 nm was identified as being particularly suitable for determination of residual amine amount. The sensitivity of the developed method is fully adequate for typical epoxy resin formulations. It has been shown that the quantity of released amine is the largest in the case of water and 3% acetic acid(reaching 7%) and the smallest in the case of 95% ethanol (slightly above 1%). Such results indicate that the amounts of released amine are relatively large and therefore the release of amine from the epoxy polymer is ecologically important, since amines create significant health hazards for humans, such as dermatitis. The post-polymerization immersion of epoxy polymer in water may be used as a simple method of freeing the polymer from the residual amine. The proposed analytical method proved its usefulness in examining two different effects, namely the aging of the samples and their exposure to artificial sunlight. It was also demonstrated that the aging of the samples causes a statistically important difference in the residual amine extraction for 95% ethanol but no such difference can be observed in the case of water. This might suggest, that there is no further progress in cross-linking reaction during this time and the nature of this effect is only diffusional. Also, the exposure of the samples to artificial sunlight caused a progress in the cross-linking reaction and a resulting decrease in the amounts of extracted residual amine.
Extraction time measurements
Calibration curves less suitable for amine determination
Linear equations, R 2 coefficients, values of the standard deviation of y-intercept (SD(A)) and the values of limits of detection (LOD) and quantification (LOQ) for the calibration curve utilizing the absorbance at 430 nm
3% acetic acid
y = -2.823x + 0.860
y = -0.272x + 0.127
y = -0.612x + 0.837
y = -3.857x + 1.068
Linear equations, R 2 coefficients, values of the standard deviation of y-intercept (SD(A)) and the values of limits of detection (LOD) and quantification (LOQ) for the calibration curve utilizing the ratio of absorbencies
3% acetic acid
y = 15.131x + 0.131
y = 18.583x + 0.9191
y = 2.0863x + 0.1079
y = 21.016x-0.0621
The pH values of Alizarin with TETA stock solutions
Quantification of amine release from 3% acetic acid
where: AH2O (λ530) is the absorbance of water–methanol solution for given amine concentration; AAc(λ527,Cx) is the absorbance of the 3% acetic acid solution for the same amine concentration and AAc(λ527,C0) is the absorbance of the 3% acetic acid modified with NaOH. This means that the relative change of absorption in the case of acetic acid solution is used instead of a direct one.
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