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© IWA Publishing 2015 Water Science & Technology
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Analysis of acid transport through multi-phase epoxy mortars for wastewater structures M. Valix
ABSTRACT The characteristics of acid migration through epoxy mortars were examined. Diffusion coefficients of typical sewer bio-metabolised acids: sulphuric, nitric, citric and oxalic acids were determined by gravimetric sorption method and fitted to the multi-phase Jacob–Jones model. Acid permeation was characterised by hindered pore diffusion with the extent being determined by the polarity of the acid
M. Valix School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia E-mail: [email protected]
and epoxy, and by the microstructure of the epoxy. Epoxy with higher polarity was able to reduce the diffusion coefficients by 49, while dense phases of the coating reduced the diffusion coefficient by 5,100. These results reflect the relative influence of epoxy polarity and microstructure on their performance as protective liners in sewers. Key words
| biometabolised acids, density, epoxy, hindered diffusion, polarity
INTRODUCTION Polymeric resins mortars are widely used in mitigating the corrosive effects in wastewater structures because of their good mechanical properties and acid permeation resistance. As coatings, they prolong the life of the wastewater structures, by providing a barrier between the corrosive environment and the concrete substrate. The effectiveness of a lining-based protective system relies on appropriate selection of the coating material that best fits the needs of the environment. Currently, this is challenged by insufficient knowledge in predicting their performance and durability in real sewer environments. Biogenic attack of concrete sewer pipes occurs as a result of the actions of various organisms. These organisms grow successively to generate acidic metabolites that corrode the concrete substrate (Okabe et al. ; Lamberet et al. ). Fungi, such as Aspergillus niger, produces various organic acids including citric and oxalic acids (Gu et al. ; Lamberet et al. ); chemoautotrophic nitrifying bacteria, such as Nitrobacter, converts amine species to nitric acid; and acidophiles, such as Acidithiobacillus thiooxidans, oxidise H2S and sulphur compounds to form sulphuric acid (Sand & Bock ). To understand the performance of lining materials in sewer environments, this study examined the migration of biogenic acids through various epoxy mortars. Although numerous studies of water transportation through polymeric liners have been conducted (Gu et al. ; Maggana & Pissis ; Feng et al. ), little has been conducted in the study of acid permeates. In our investigation, acid permeation through epoxy doi: 10.2166/wst.2015.214
coatings was performed by gravimetric method. The measured mass transport parameters were correlated to the polarity and microstructure of the epoxy and polarity of the acids.
EXPERIMENTAL Coatings Two commercially available epoxy mortars were used in this study. They consisted of bisphenol A and a mixture of bisphenol A and F resins cured with amine curing reagents using instructions provided by the manufacturers. The chemical properties of the coatings are summarised in Table 1. The loss of ignition (LOI), which reflects the organic content of the coatings, was determined using a Philips PW2400 XRF with an Rh end-window tube at 1,050 C (Philips, Almelo, The Netherlands). The filler size and concentration were as provided by the manufacturers. The dimensions of the epoxy specimen are 5 cm × 5 cm × 5 mm. W
Fourier transform infrared analysis Fourier transform infrared (FTIR) spectra of raw and acid immersed coatings were obtained in KBr pellets using a Thermos Nicolet 6700 FTIR spectrometer with attenuated
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Table 1
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Acid transport through multi-phase epoxy
Water Science & Technology
Chemical properties of the epoxy mortars
Coating
Epoxy type
LOI (wt.%)
Filler (wt. %)
Filler size (Dmean, μm)
A
A
58.7
41.3
198.9
B
Mixture of A & F
12.1
87.9
350
total reflection (Thermo Fisher Scientific, Waltham, MA, USA). Gravimetric sorption method Testing of acid permeation was carried out by the gravimetric method (Fiore et al. ). Dry square epoxy coupons were immersed in 300 ml reagent grade acids. The list of acids used and their wetting properties are shown in Table 2. The pKa values reflected acid strength while the acid solubility in water reflected relative polarity. All acids are polar, with oxalic acid having the least polarity. Acidic solutions with concentrations of 1, 5, 10 and 20 (g/ml)% were prepared with deionised water and reagent grade sulphuric, nitric, citric and oxalic acids. The immersed coupons were maintained at 25 C in a temperature controlled chamber. At appropriate time intervals, samples were removed from the acid bath, blotted dry, and weighed on an analytical balance. Epoxy acid uptake with time, Mt, was calculated as follows: W
Mt ¼
wt wd × 100 wd
(1)
where wt and wd are the weight of the specimen at time t and of the dry specimen, respectively. The samples were reimmersed after weighing. The pH of the acid baths was monitored and adjusted to maintain the solution pH.
RESULTS AND DISCUSSION FTIR analysis of the epoxy mortars The FTIR surface functional groups analysis of coatings A and B (Figure 1) are summarised in Table 3. The relative Table 2
|
Acidity and solubility of the acids in water
Permeate
Formula
pKa (Haynes 2014)
Solubility in water
Citric acid
C6H8O7
3.14, 4.77, 6.39
147.76 g/L (20 C)
Oxalic acid
C2H2O4
1.23, 4.19
14.3 g/L (20 C)
Nitric acid
HNO3
1.4
miscible
Sulphuric acid
H2SO4
3, 1.99
miscible
W
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quantities of these functional groups were determined by normalising their peak areas with the area under the Si-O-Si peak. As shown, various polar groups that could form hydrogen bonding, including hydroxyls, carboxylate, amines and sulphide groups, are present in both coatings. It is apparent that the proportion of polar groups in coating A is greater than in B, suggesting its greater polarity. This could also be attributed the higher epoxy content of coating A (see Table 1). The polar interaction between the citric acid and the epoxy coatings was examined by FTIR in Figure 2. Coating A was immersed in 5% citric acid for up to 18 months. To examine the interaction more fully, the multicomponent bands were resolved into their individual peaks by using a curve resolving algorithm based on the Levenberg–Marquardt method (Marquardt ). Figure 2(a) shows the full FTIR spectra and Figure 2(b) the resolved peaks. The region from 3,700 to 3,100 cm1 was de-convoluted into three peaks. The partially resolved peak at 3,627–3,563 cm1 was attributed to free permeate. The band at 3,450 cm1 was assigned to polar permeate hydrogen-bonded to hydroxyl groups, and 3,220 cm1 to polar permeate hydrogen-bonded to amine groups (Bellenger et al. ; Musto et al. ; Feng et al. ). Figure 2(b) shows the absence of free acid in the raw coating, which increased with immersion to 12 months then declined from 12 to 18 months. The permeation of water does not disrupt the hydrogen bonding of water already sorbed in the epoxy (Musto et al. ); our results however, show acids can displace sorbed water. Permeates bonded to the OH showed a slight shift in wavelength from 3,415 to 3,390 cm1 with acid immersion. A similar shift from 3,250 to 3,210 cm1 was noted for permeate bonded to the amine group after acid immersion. Both shifts were attributed to the replacement of the bound water with the acid (Omoike & Chorover ). The peak area at 3,390 cm1 increased in 6 months, but little increase occurred from 6 to 12 months. This was attributed to size exclusion of the acids as the amount of acid adsorbed to OH may have promoted pore blockage. However, between 12 and 18 months, a further rise in the peak area was observed, suggesting that additional OH sites, perhaps in the more dense phase, have been accessed. The area at 3,220 cm1 increased in 6 months and declined thereafter. This reflects the reversibility of citrate adsorption on amine groups consistent with the reversible sorption of water on this group (Bellenger et al. ; Musto et al. ). These results suggest that acid transportation through epoxy was dictated by the rate of acid adsorption and desorption, which in turn was controlled by the strength of
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Figure 1
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FTIR spectra of (a) coating A and (b) coating B.
the polarity interactions between acid and epoxy. The relatively higher areas associated with acids’ hydrogen bonded to both the OH and NH group show the fraction of acid bound by polarity interaction is greater than acids present in free state in the epoxy. The effects of polarity interactions on acid transport will be further examined below. Effect of acid and epoxy polarity on the mass transport of acids The migration of acids through the coatings was assessed by fitting gravimetric immersion data to the multiphase Jacobs and Jones model ( Jacobs & Jones ). The formation of
different phase structures in epoxies is well established and provides a suitable explanation for the observed permeation behaviour of acids (Vanlandingham et al. ; Feng et al. ). To take into account the presence of the two phases, the normalised permeate content M(t) ¼ Mt/M∞ as a function of time was described by the following morphology dependent equation ( Jacobs & Jones ): ( M(t) ¼ Vd þ
"
#) Dd t 0:75 1 exp 7:3 b2 ( " 0:75 #)! Dl t 1 Vd 1 exp 7:3 2 b
(2)
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Table 3
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Characteristic FTIR bands of coatings A and B
Coating A 1
Vibrations
% Normalised relative areas
3,390
O2 2H stretch
87.2
2,919–2,887
C–H stretch methylene
24.3
2,854
Stretching C2 2H of CH2 and CH aromatic and aliphatic
9.6
Wavenumber (cm
)
1,608–1,508
C5 5O2 2O carboxylate ion, aromatic band stretching
1.6
1,596
NH deformation primary amine
0.4
Coating B
7.4
4.3
1,509
Stretching C2 2C of aromatic
5.5
1,463
CH3 symmetric deformation of Si-CH3
6.1
1,168
C5 5S
10.5
3.4
1,060–1,039
Si2 2O2 2Si stretching vibration
100.0
100.0
830
C2 2O2 2C oxirane
0.9
3.5
636
C2 2H out of plane bending for aromatics
0.9
0.9
where Mt and M∞ are the percentage permeate content with time and at equilibrium, Dd and Dl are the diffusion coefficients in the dense and less dense phases, respectively, and Vd is the volume fraction of the dense phase. The diffusion coefficients were estimated from the equilibrium permeate concentrations of the dense and less dense phase, Md and Ml. The value of Ml was estimated by extrapolating the slope of the plot of M(t) as a function of t 1/2 back to the M(t) axis. The diffusion coefficients of the various acids at different concentration in both dense and less dense phases of coatings A and B are reported in Table 4. As shown, the diffusion coefficients in the less dense phase were generally faster than the dense phase in both coatings, by a ratio of up to 5,100. The presence of two phases has been proposed to occur via initial formation of microgels, which deplete their immediate neighbourhood of reactants. At the later stages of cure, the regions between the microgel particles cross-link to form the soft phase (Zhou & Lucas ). The high cross-linking density is responsible for the greater packing effect resulting in reduced free volume, thus providing a higher resistance to permeation (Feng et al. ). The role of acid polarity in acid transport is exemplified by the faster diffusion coefficients exhibited by oxalic acid in the less dense phase of the epoxies. Its diffusion coefficients are 7–49 times faster compared to the other acids. Because oxalic acid is the least polar among the acids tested (see Table 2), its polarity interaction with other polar species in the epoxy resin was also the least. This permitted oxalic acid to diffuse faster compared to the other acids tested.
The role of epoxy polarity was demonstrated by the lower diffusion coefficients of the acids in the less dense phases of coating A. Coating A, which was relatively richer in polar sites (see Table 3), appears to promote greater hydrogen bonding with the acids that in turn restricted acid diffusion. The diffusion coefficients of the acids in the dense phase of the epoxies, as shown in Table 4, were not significantly influenced by the type of acids and the epoxy polarity. These results suggest that the acid transport through the epoxy occurs by hindered diffusion (Yang & Guin ). In the less dense phase, hindered diffusion occurred as a result of polarity interaction between acids and by the polar groups in the epoxy (Soles et al. ). Highly polar acids exhibited lower diffusion coefficients compared to the less polar acids, such as oxalic acid; while in the more dense phase, hindered diffusion appeared to occur primarily as a result of steric exclusion because of the greater packing effect in these crystalline phases (Deen ). The eventual outcome of acid permeation will be the failure of the protective coating system manifested by its delamination. As the acids reach the coating-concrete interface, it will begin corroding the concrete. The resulting loss of concrete integrity will promote coating delamination by either the cohesive failure of the concrete and/or adhesion failure. The rate of delamination, in the absence of interfacial flaws and osmotic pressures from the concrete substrate, will thus be primarily dictated by the permeability of the coating material (Raupach & Wolff ). These results show that coatings containing higher concentrations of polar groups and higher density phases, as demonstrated by coating A, will
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provide greater resistance to acid permeation and in turn will be able to prolong coating adhesion.
CONCLUSIONS The transport of sewer metabolised acids through twophased epoxy mortar linings was examined by the JacobsJones model (Jacob & Jones ). Acid migration was characterised by hindered diffusion. Hindered diffusion in the less dense phase occurred as a result of polarity interactions between the epoxy and acids by hydrogen bonding. This reduced the diffusion coefficient by a ratio of up to 49. In the more dense phase, hindered diffusion occurred as a result of size exclusions from the greater packing or reduced pore volume, resulting in reduction of the diffusion coefficient by a ratio of up to 5,100. These results demonstrate the roles of the lining polarity and polymer density relative to their resistance to acid permeation.
ACKNOWLEDGMENTS The author acknowledges the financial support provided by the Australian Research Council and many members of the Australian water industry through LP0882016 the Sewer Corrosion and Odour Research (SCORe) Project (www.score.org.au). Figure 2
Table 4
|
|
(a) FTIR spectra of raw coating A and samples immersed in 5% citric acid for up to 18 months and (b) resolved FTIR peaks.
Acid diffusion coefficients in the less dense and more dense phase of coating A and coating B Dl × 1010 cm2/s
Dd × 1012 cm2/s
Acids
Coating A
Coating B
Coating A
Coating B
1% citric acid
8.61
5.64
2.12
2.56
5% citric acid
8.66
14.88
2.03
2.90
10% citric acid
8.26
17.48
1.69
2.74
1% oxalic acid
96.72
77.41
2.32
1.68
5% oxalic acid
90.62
148.40
2.05
2.88
10% oxalic acid
98.55
128.43
2.01
2.39
1% nitric acid
2.56
6.43
1.61
2.19
5% nitric acid
6.71
8.42
1.44
2.66
10% nitric acid
9.80
10.41
1.44
1.47
5% sulphuric acid
4.94
8.92
1.57
2.12
10% sulphuric acid
2.50
3.04
1.54
2.09
20% sulphuric acid
4.95
5.89
1.80
1.87
REFERENCES Bellenger, V., Verdu, J. & Morel, E. Structure-properties relationships for densely cross-linked epoxide amine systems based on epoxide or amine mixtures. 2. Waterabsorption and diffusion. Journal of Materials Science 24 (1), 63–68. Deen, W. M. Hindered transport of large molecules in liquidfilled pores. Aiche Journal 33 (9), 1409–1425. Feng, J. X., Berger, K. R. & Douglas, E. P. Water vapor transport in liquid crystalline and non-liquid crystalline epoxies. Journal of Materials Science 39 (10), 3413–3423. Fiore, A., Marano, G. C., Monaco, P. & Morbi, A. Preliminary experimental study on the effects of surface-applied photocatalytic products on the durability of reinforced concrete. Construction and Building Materials 48, 137–143. Gu, J. D., Lu, C., Mitchell, R., Thorp, K. & Crasto, A. Fungal degradation of fiber-reinforced composite materials. Materials Performance 36 (3), 37–42. Haynes, W. CRC–Handbook of Physics and Chemistry. 95th edn, CRC Press, Boca Raton, FL, USA. Jacobs, P. M. & Jones, F. R. Diffusion of moisture into 2-phase polymers. 1. The development of an analytical model and its
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application to styrene ethylene butylene styrene block copolymer. Journal of Materials Science 24 (7), 2331–2336. Lamberet, S., Guinot, S., Lempereur, E., Talley, J. & Alt, C. Field investigations of high performance calcium aluminate mortar for wastewater applications. In: Calcium Aluminate Cements, Proceedings of the Centenary Conference 2008, IHS BRE Press, Palais des Papes, Avignon, France. Maggana, C. & Pissis, P. Water sorption and diffusion studies in an epoxy resin system. Journal of Polymer Science Part B-Polymer Physics 37 (11), 1165–1182. Marquardt, D. W. An algorithm for least-squares estimation of nonlinear parameters. Journal of the Society for Industrial and Applied Mathematics 11 (2), 431–441. Musto, P., Ragosta, G. & Mascia, L. Vibrational spectroscopy evidence for the dual nature of water sorbed into epoxy resins. Chemistry of Materials 12 (5), 1331–1341. Okabe, S., Odagiri, M., Ito, T. & Satoh, H. Succession of sulfur-oxidizing bacteria in the microbial community on corroding concrete in sewer systems. Applied and Environmental Microbiology 73 (3), 971–980. Omoike, A. & Chorover, J. Spectroscopic study of extracellular polymeric substances from Bacillus subtilis:
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aqueous chemistry and adsorption effects. Biomacromolecules 5 (4), 1219–1230. Raupach, M. & Wolff, L. Durability of adhesion of epoxy coatings on concrete; causes of delamination and blistering. Sand, W. & Bock, E. Biodeterioration of mineral materials by microorganisms – biogenic sulfuric and nitric-acid corrosion of concrete and natural stone. Geomicrobiology Journal 9 (2–3), 129–138. Soles, C. L., Chang, F. T., Gidley, D. W. & Yee, A. F. Contributions of the nanovoid structure to the kinetics of moisture transport in epoxy resins. Journal of Polymer Science Part B-Polymer Physics 38 (5), 776–791. Vanlandingham, M. R., Eduljee, R. F. & Gillespie, J. W. Relationships between stoichiometry, microstructure and properties for amine-cured epoxies. Journal of Applied Polymer Science 71 (5), 699–712. Yang, X. F. & Guin, J. A. Effects of solute adsorption on hindered diffusion uptake rates in finite bath experiments. Chemical Engineering Communications 154, 101–118. Zhou, J. M. & Lucas, J. P. Hygrothermal effects of epoxy resin. Part I: the nature of water in epoxy. Polymer 40 (20), 5505–5512.
First received 2 January 2015; accepted in revised form 21 April 2015. Available online 8 May 2015
© IWA Publishing 2015 Water Science & Technology
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72.2
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2015
Analysis of acid transport through multi-phase epoxy mortars for wastewater structures M. Valix
ABSTRACT The characteristics of acid migration through epoxy mortars were examined. Diffusion coefficients of typical sewer bio-metabolised acids: sulphuric, nitric, citric and oxalic acids were determined by gravimetric sorption method and fitted to the multi-phase Jacob–Jones model. Acid permeation was characterised by hindered pore diffusion with the extent being determined by the polarity of the acid
M. Valix School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia E-mail: [email protected]
and epoxy, and by the microstructure of the epoxy. Epoxy with higher polarity was able to reduce the diffusion coefficients by 49, while dense phases of the coating reduced the diffusion coefficient by 5,100. These results reflect the relative influence of epoxy polarity and microstructure on their performance as protective liners in sewers. Key words
| biometabolised acids, density, epoxy, hindered diffusion, polarity
INTRODUCTION Polymeric resins mortars are widely used in mitigating the corrosive effects in wastewater structures because of their good mechanical properties and acid permeation resistance. As coatings, they prolong the life of the wastewater structures, by providing a barrier between the corrosive environment and the concrete substrate. The effectiveness of a lining-based protective system relies on appropriate selection of the coating material that best fits the needs of the environment. Currently, this is challenged by insufficient knowledge in predicting their performance and durability in real sewer environments. Biogenic attack of concrete sewer pipes occurs as a result of the actions of various organisms. These organisms grow successively to generate acidic metabolites that corrode the concrete substrate (Okabe et al. ; Lamberet et al. ). Fungi, such as Aspergillus niger, produces various organic acids including citric and oxalic acids (Gu et al. ; Lamberet et al. ); chemoautotrophic nitrifying bacteria, such as Nitrobacter, converts amine species to nitric acid; and acidophiles, such as Acidithiobacillus thiooxidans, oxidise H2S and sulphur compounds to form sulphuric acid (Sand & Bock ). To understand the performance of lining materials in sewer environments, this study examined the migration of biogenic acids through various epoxy mortars. Although numerous studies of water transportation through polymeric liners have been conducted (Gu et al. ; Maggana & Pissis ; Feng et al. ), little has been conducted in the study of acid permeates. In our investigation, acid permeation through epoxy doi: 10.2166/wst.2015.214
coatings was performed by gravimetric method. The measured mass transport parameters were correlated to the polarity and microstructure of the epoxy and polarity of the acids.
EXPERIMENTAL Coatings Two commercially available epoxy mortars were used in this study. They consisted of bisphenol A and a mixture of bisphenol A and F resins cured with amine curing reagents using instructions provided by the manufacturers. The chemical properties of the coatings are summarised in Table 1. The loss of ignition (LOI), which reflects the organic content of the coatings, was determined using a Philips PW2400 XRF with an Rh end-window tube at 1,050 C (Philips, Almelo, The Netherlands). The filler size and concentration were as provided by the manufacturers. The dimensions of the epoxy specimen are 5 cm × 5 cm × 5 mm. W
Fourier transform infrared analysis Fourier transform infrared (FTIR) spectra of raw and acid immersed coatings were obtained in KBr pellets using a Thermos Nicolet 6700 FTIR spectrometer with attenuated
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Table 1
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Acid transport through multi-phase epoxy
Water Science & Technology
Chemical properties of the epoxy mortars
Coating
Epoxy type
LOI (wt.%)
Filler (wt. %)
Filler size (Dmean, μm)
A
A
58.7
41.3
198.9
B
Mixture of A & F
12.1
87.9
350
total reflection (Thermo Fisher Scientific, Waltham, MA, USA). Gravimetric sorption method Testing of acid permeation was carried out by the gravimetric method (Fiore et al. ). Dry square epoxy coupons were immersed in 300 ml reagent grade acids. The list of acids used and their wetting properties are shown in Table 2. The pKa values reflected acid strength while the acid solubility in water reflected relative polarity. All acids are polar, with oxalic acid having the least polarity. Acidic solutions with concentrations of 1, 5, 10 and 20 (g/ml)% were prepared with deionised water and reagent grade sulphuric, nitric, citric and oxalic acids. The immersed coupons were maintained at 25 C in a temperature controlled chamber. At appropriate time intervals, samples were removed from the acid bath, blotted dry, and weighed on an analytical balance. Epoxy acid uptake with time, Mt, was calculated as follows: W
Mt ¼
wt wd × 100 wd
(1)
where wt and wd are the weight of the specimen at time t and of the dry specimen, respectively. The samples were reimmersed after weighing. The pH of the acid baths was monitored and adjusted to maintain the solution pH.
RESULTS AND DISCUSSION FTIR analysis of the epoxy mortars The FTIR surface functional groups analysis of coatings A and B (Figure 1) are summarised in Table 3. The relative Table 2
|
Acidity and solubility of the acids in water
Permeate
Formula
pKa (Haynes 2014)
Solubility in water
Citric acid
C6H8O7
3.14, 4.77, 6.39
147.76 g/L (20 C)
Oxalic acid
C2H2O4
1.23, 4.19
14.3 g/L (20 C)
Nitric acid
HNO3
1.4
miscible
Sulphuric acid
H2SO4
3, 1.99
miscible
W
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quantities of these functional groups were determined by normalising their peak areas with the area under the Si-O-Si peak. As shown, various polar groups that could form hydrogen bonding, including hydroxyls, carboxylate, amines and sulphide groups, are present in both coatings. It is apparent that the proportion of polar groups in coating A is greater than in B, suggesting its greater polarity. This could also be attributed the higher epoxy content of coating A (see Table 1). The polar interaction between the citric acid and the epoxy coatings was examined by FTIR in Figure 2. Coating A was immersed in 5% citric acid for up to 18 months. To examine the interaction more fully, the multicomponent bands were resolved into their individual peaks by using a curve resolving algorithm based on the Levenberg–Marquardt method (Marquardt ). Figure 2(a) shows the full FTIR spectra and Figure 2(b) the resolved peaks. The region from 3,700 to 3,100 cm1 was de-convoluted into three peaks. The partially resolved peak at 3,627–3,563 cm1 was attributed to free permeate. The band at 3,450 cm1 was assigned to polar permeate hydrogen-bonded to hydroxyl groups, and 3,220 cm1 to polar permeate hydrogen-bonded to amine groups (Bellenger et al. ; Musto et al. ; Feng et al. ). Figure 2(b) shows the absence of free acid in the raw coating, which increased with immersion to 12 months then declined from 12 to 18 months. The permeation of water does not disrupt the hydrogen bonding of water already sorbed in the epoxy (Musto et al. ); our results however, show acids can displace sorbed water. Permeates bonded to the OH showed a slight shift in wavelength from 3,415 to 3,390 cm1 with acid immersion. A similar shift from 3,250 to 3,210 cm1 was noted for permeate bonded to the amine group after acid immersion. Both shifts were attributed to the replacement of the bound water with the acid (Omoike & Chorover ). The peak area at 3,390 cm1 increased in 6 months, but little increase occurred from 6 to 12 months. This was attributed to size exclusion of the acids as the amount of acid adsorbed to OH may have promoted pore blockage. However, between 12 and 18 months, a further rise in the peak area was observed, suggesting that additional OH sites, perhaps in the more dense phase, have been accessed. The area at 3,220 cm1 increased in 6 months and declined thereafter. This reflects the reversibility of citrate adsorption on amine groups consistent with the reversible sorption of water on this group (Bellenger et al. ; Musto et al. ). These results suggest that acid transportation through epoxy was dictated by the rate of acid adsorption and desorption, which in turn was controlled by the strength of
334
Figure 1
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Acid transport through multi-phase epoxy
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FTIR spectra of (a) coating A and (b) coating B.
the polarity interactions between acid and epoxy. The relatively higher areas associated with acids’ hydrogen bonded to both the OH and NH group show the fraction of acid bound by polarity interaction is greater than acids present in free state in the epoxy. The effects of polarity interactions on acid transport will be further examined below. Effect of acid and epoxy polarity on the mass transport of acids The migration of acids through the coatings was assessed by fitting gravimetric immersion data to the multiphase Jacobs and Jones model ( Jacobs & Jones ). The formation of
different phase structures in epoxies is well established and provides a suitable explanation for the observed permeation behaviour of acids (Vanlandingham et al. ; Feng et al. ). To take into account the presence of the two phases, the normalised permeate content M(t) ¼ Mt/M∞ as a function of time was described by the following morphology dependent equation ( Jacobs & Jones ): ( M(t) ¼ Vd þ
"
#) Dd t 0:75 1 exp 7:3 b2 ( " 0:75 #)! Dl t 1 Vd 1 exp 7:3 2 b
(2)
335
Table 3
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Characteristic FTIR bands of coatings A and B
Coating A 1
Vibrations
% Normalised relative areas
3,390
O2 2H stretch
87.2
2,919–2,887
C–H stretch methylene
24.3
2,854
Stretching C2 2H of CH2 and CH aromatic and aliphatic
9.6
Wavenumber (cm
)
1,608–1,508
C5 5O2 2O carboxylate ion, aromatic band stretching
1.6
1,596
NH deformation primary amine
0.4
Coating B
7.4
4.3
1,509
Stretching C2 2C of aromatic
5.5
1,463
CH3 symmetric deformation of Si-CH3
6.1
1,168
C5 5S
10.5
3.4
1,060–1,039
Si2 2O2 2Si stretching vibration
100.0
100.0
830
C2 2O2 2C oxirane
0.9
3.5
636
C2 2H out of plane bending for aromatics
0.9
0.9
where Mt and M∞ are the percentage permeate content with time and at equilibrium, Dd and Dl are the diffusion coefficients in the dense and less dense phases, respectively, and Vd is the volume fraction of the dense phase. The diffusion coefficients were estimated from the equilibrium permeate concentrations of the dense and less dense phase, Md and Ml. The value of Ml was estimated by extrapolating the slope of the plot of M(t) as a function of t 1/2 back to the M(t) axis. The diffusion coefficients of the various acids at different concentration in both dense and less dense phases of coatings A and B are reported in Table 4. As shown, the diffusion coefficients in the less dense phase were generally faster than the dense phase in both coatings, by a ratio of up to 5,100. The presence of two phases has been proposed to occur via initial formation of microgels, which deplete their immediate neighbourhood of reactants. At the later stages of cure, the regions between the microgel particles cross-link to form the soft phase (Zhou & Lucas ). The high cross-linking density is responsible for the greater packing effect resulting in reduced free volume, thus providing a higher resistance to permeation (Feng et al. ). The role of acid polarity in acid transport is exemplified by the faster diffusion coefficients exhibited by oxalic acid in the less dense phase of the epoxies. Its diffusion coefficients are 7–49 times faster compared to the other acids. Because oxalic acid is the least polar among the acids tested (see Table 2), its polarity interaction with other polar species in the epoxy resin was also the least. This permitted oxalic acid to diffuse faster compared to the other acids tested.
The role of epoxy polarity was demonstrated by the lower diffusion coefficients of the acids in the less dense phases of coating A. Coating A, which was relatively richer in polar sites (see Table 3), appears to promote greater hydrogen bonding with the acids that in turn restricted acid diffusion. The diffusion coefficients of the acids in the dense phase of the epoxies, as shown in Table 4, were not significantly influenced by the type of acids and the epoxy polarity. These results suggest that the acid transport through the epoxy occurs by hindered diffusion (Yang & Guin ). In the less dense phase, hindered diffusion occurred as a result of polarity interaction between acids and by the polar groups in the epoxy (Soles et al. ). Highly polar acids exhibited lower diffusion coefficients compared to the less polar acids, such as oxalic acid; while in the more dense phase, hindered diffusion appeared to occur primarily as a result of steric exclusion because of the greater packing effect in these crystalline phases (Deen ). The eventual outcome of acid permeation will be the failure of the protective coating system manifested by its delamination. As the acids reach the coating-concrete interface, it will begin corroding the concrete. The resulting loss of concrete integrity will promote coating delamination by either the cohesive failure of the concrete and/or adhesion failure. The rate of delamination, in the absence of interfacial flaws and osmotic pressures from the concrete substrate, will thus be primarily dictated by the permeability of the coating material (Raupach & Wolff ). These results show that coatings containing higher concentrations of polar groups and higher density phases, as demonstrated by coating A, will
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provide greater resistance to acid permeation and in turn will be able to prolong coating adhesion.
CONCLUSIONS The transport of sewer metabolised acids through twophased epoxy mortar linings was examined by the JacobsJones model (Jacob & Jones ). Acid migration was characterised by hindered diffusion. Hindered diffusion in the less dense phase occurred as a result of polarity interactions between the epoxy and acids by hydrogen bonding. This reduced the diffusion coefficient by a ratio of up to 49. In the more dense phase, hindered diffusion occurred as a result of size exclusions from the greater packing or reduced pore volume, resulting in reduction of the diffusion coefficient by a ratio of up to 5,100. These results demonstrate the roles of the lining polarity and polymer density relative to their resistance to acid permeation.
ACKNOWLEDGMENTS The author acknowledges the financial support provided by the Australian Research Council and many members of the Australian water industry through LP0882016 the Sewer Corrosion and Odour Research (SCORe) Project (www.score.org.au). Figure 2
Table 4
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(a) FTIR spectra of raw coating A and samples immersed in 5% citric acid for up to 18 months and (b) resolved FTIR peaks.
Acid diffusion coefficients in the less dense and more dense phase of coating A and coating B Dl × 1010 cm2/s
Dd × 1012 cm2/s
Acids
Coating A
Coating B
Coating A
Coating B
1% citric acid
8.61
5.64
2.12
2.56
5% citric acid
8.66
14.88
2.03
2.90
10% citric acid
8.26
17.48
1.69
2.74
1% oxalic acid
96.72
77.41
2.32
1.68
5% oxalic acid
90.62
148.40
2.05
2.88
10% oxalic acid
98.55
128.43
2.01
2.39
1% nitric acid
2.56
6.43
1.61
2.19
5% nitric acid
6.71
8.42
1.44
2.66
10% nitric acid
9.80
10.41
1.44
1.47
5% sulphuric acid
4.94
8.92
1.57
2.12
10% sulphuric acid
2.50
3.04
1.54
2.09
20% sulphuric acid
4.95
5.89
1.80
1.87
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First received 2 January 2015; accepted in revised form 21 April 2015. Available online 8 May 2015
