Elimination of doubts about the suitability of epoxy resin in pipe and water storage systems
By Guido Deussing
Source: istock / Mumemories
What would a town be like if it had no electricity? Dark at night, but still populated. A town without a water supply, on the other hand, would be a desolate, deserted place. The first people who left a life of hunting and collecting behind them made sure that there was a river near their settlement. They were quick to learn how to transport water over long distances – above ground, initially, then mainly underground later on and concealed in buildings. The problem here: mending leaks means, first of all, exposing hidden pipes on a large scale, so that the damaged section can be found and repaired. The CIPP (cured-in-place pipe) principle is an attractive alternative. What is involved here is simply the coating of entire sections of pipe with plastic or, more precisely, epoxy resin from the inside. The process is effective but not without its critics. For no good reason? Let us take a look at the details.
The more people live in one place and the greater the level of urbanisation, the more important a sound water supply is. What is meant here is not merely general availability, i.e. the availability of sufficient quantities of high-quality water that does not represent a health risk. What is also meant is the installation of an intact storage and piping system, in order to be able to transport fresh water to the place where it is needed free of undesirable pollutants and odorants and to take waste water away again without any leakage loss.
In the Roman Empire, aqueducts were a common sight both in towns and out in the country; some of these extremely impressive structures still exist today and testify to the architectural skills of the era in which they were built. Nowadays, most water pipes are installed underground and, in houses, concealed underneath plaster. When corrosion and leakage occur due to wear and tear, which can always be expected after a certain amount of time in systems that transport liquids, repairs are essential, in order to limit liquid loss as well as the extent of collateral damage. While it is comparatively easy to carry out maintenance work on pipe systems that are above ground, laborious excavation work can be expected if water pipes under streets or in-house walls have to be reached.
The amount of work and expense needed can, however, be reduced if complete sections of piping are repaired rather than – quite literally – plugging individual holes, with the help of liner technology (relining) that does not require complex construction work. In this process, a liner impregnated with epoxy resin made, for example, of felt is pulled into the entire length of the corroded pipe. Experts are of the opinion that this CIPP principle is just as reliable as a completely new water pipe where imperviousness and load-bearing capacity are concerned, as soon as the epoxy resin has cured completely. Pipes can also be coated directly with epoxy resin, which is only advisable inside houses, however, due to the lack of additional stabilisation by an appropriate liner.
More than just an attractive repair material
Pont du Gard is an old Roman aqueduct near Nimes in Southern France. Source: istock / Bertl123
Let us take a look at epoxy resins, without getting too chemical about them: epoxy resins are synthetic resins that are used primarily as coating material due to their properties. Synthetic resins generally consist mainly of two free-flowing or viscous components that react with each other in different ways, become increasingly viscous in the course of time or under certain conditions and finally cure to become a thermosetting polymer. Chemically speaking, most epoxy resins in commercial use are manufactured by means of a reaction between a compound with a hydroxy group (R-OH) [this is the functional group of alcohols and phenols] and epichlorohydrin [a colourless liquid that smells like chloroform]. Epoxy resin was invented by the German chemist and inventor Paul Schlack, who obtained a German patent for it in 1939. The special feature of epoxy resins is: once they have cured, thermosetting polymers have good mechanical properties as well as high temperature and chemical resistance levels, so that they are considered to be high-quality plastics. They are used for such products as reaction varnishes, stove enamels, adhesives, laminates, mounting agents in metallography and moulding compounds for components in electrical and electronic engineering.
Focus on thermosetting polymers
Getting back to thermosetting polymers: they are plastics that are hardened by curing, a process that cannot be reversed by heat or any other means. Thermosetting polymers made from synthetic resins were, incidentally, among the first plastics to be produced industrially. “Bakelite, which was discovered in 1909, is the oldest example. Originally, they could only be manufactured from their source materials (e.g. phenoplastics made from phenolic resins) in moulds and were therefore known as moulding compounds too, in contrast to thermoplastics, which were known as injection moulding compounds. It was not until the mid-1960s that processes started to be developed that made it possible to manufacture thermosetting polymers by the injection moulding process. Thermosetting polymers are processed in numerous different ways today. The emphasis here is always on their high thermomechanical strength and their low specific component weight by comparison with metal.” .
Figure: Markomolecular structure of various types of plastic: A. Thermosets consist of closely-meshed polymers. Networks are shown in the figure as red dots. B. Elastomers consist of wide-meshed polymers. The wide mesh allows the material to be stretched under tensile load. C. Thermoplastics consist of uncrosslinked polymers, often with a partially crystalline structure (shown in red). They have a glass transition temperature and are meltable. (Source: wikipedia)
Epoxy resin as coating
Without mentioning specific figures, it can be said that the price of epoxy resin coating is relatively low, while the manpower for and expense of repair projects can be kept within reasonable limits. Estimates made in July 2015 indicate that about 80,000 tonnes of epoxy resin have been used to line the inside of water pipes in Europe since 1990: 60,000 tonnes in underground pipes and 20,000 tonnes in housing .
Most of the commercially used epoxy monomers are produced by the reaction of a compound with acidic hydroxy groups and epichlorohydrin: First the hydroxy group reacts in a coupling reaction with epichlorohydrin, followed by dehydrohalogenation. (Source: wikipedia)
The figures suggest that a success story has been written here. The use of epoxy resins in water pipes raises critical questions too, however, which relate to the material itself and/or its source materials. This is because practically all the epoxy resins used in water pipes are the product of a two-stage reaction between bisphenol A (BPA) and epichlorohydrin (ECH), as the Epoxy Resin Committee (ERC) – an alliance of national and international epoxy resin manufacturers – explains. The result of the addition and subsequent condensation reaction is a thermosetting polymer, i.e. an irreversibly hardened plastic not affected by heat that has good mechanical properties and similarly high temperature and chemical resistance levels. Epoxy resin is used not only to coat water pipes but also to line the inside of food and beverage cans.
Let us take another look now at the reactants separately and the warnings that have been issued about them: BPA is considered to be a “substance of very high concern” (SVHC) that has hormone-like and reprotoxic (CMR) properties [2-5]. ECH has a comparable profile and has also turned out to be carcinogenic in animal testing [6, 7].
Within permissible limits
In view of potential harm to health and the environment, environmental associations are demanding a prompt end to the coating of water pipes with epoxy resin . Epoxy resin manufacturers, on the other hand, claim that their products are highly beneficial in this application while representing no risk to health at the same time. They consider the probability that DBP will be released during the useful life of the epoxy resin coating due to improper installation or maintenance to be low. It is said that the material is not a significant source of potential human contact, so that it does not as a result represent a risk to health. The concentration of the reactants released during the coating process and in the course of useful life is, in turn, said to be uncritical too. There is apparently an estimated 200 kilograms of potentially free BDA in pipes above the ground and 600 kilograms in underground pipes – insignificant amounts in relation to the total volume of the overall amount of epoxy resin used. There is therefore no question of the limit at which a health risk arises being exceeded.
However, what if water is transported through a pipe system coated with epoxy resin that has been treated beforehand with chlorine dioxide (ClO2), a chemical that is approved for the disinfection of drinking water? Noyon et al.  have made an analysis of this process. They were prompted to do this by reports in which there was talk of water disinfected in this way that was transported through pipes repaired with epoxy resin and came out of the tap in a pinkish colour and smelling musty. The scientists explain that the discolouration indicated that the epoxy resins in the coating had been degraded and depolymerised to a large extent. In this context, Noyon et al. draw attention to reports according to which ClO2 is also made responsible for the accelerated degradation of polyethylene (PE) pipes. An initial GC/MS analysis had, finally, confirmed the presence of BPA in the discoloured water, while references had strengthened the assumption that degradation had taken place.
Research needs to be carried out
A similar GC/MS system was use to analyse bisphenols in water. Source: GERSTEL GmbH & Co. KG
In the course of an in-depth examination of the matter, Noyon et al. asked themselves how a test needed to be configured in order to be able to reliably detect or rule out possible migration of BPA from an epoxy resin coating to water storage systems and water pipes. In 2018, incidentally, the German Environment Agency (UBA) reduced the maximum migration of BPA into drinking water from materials that come into contact with drinking water to 2.5 µg/l . So far, there are no legal regulations about bisphenol F, bisphenol S and further bisphenols . Noyon et al. finally developed a migration test in which they brought neutral water as well as water with added chlorine and chlorine dioxide (0.25 mg/l and 0.5 mg/l) into contact with various epoxy resins that are approved for use in water, tested them on an ongoing basis for a period of several months and determined the enriched analytes via solvent-free, “green” analytical procedures. [Details of these analytical procedures can be obtained from the author [E-mail contact, keywort: BPA].
Insight into experimental details
BPA migration continues to increase after five months: Noyon et al. come to the conclusion that the period during which epoxy resins that are used in water are evaluated should be lengthened considerably. Source: Noyon et al.
Noyon et al. carried out different tests with a selection of epoxy resins, in water treated with chlorine and chlorine dioxide as well as in water that was not disinfected or treated. It was replaced several times a week. Specimens were tested that had been in contact with the epoxy resins for 24 hours in each case. The scientists report that the untreated water samples had relatively low BPA concentrations close to the detection limit of 10 ng/l in the course of the overall six-month monitoring process. One of the epoxy resins examined had, however, released up to 20 ng/l of BPA on an ad hoc basis at both the beginning and the end of the six-month monitoring period. Another epoxy resin had reached a maximum migration level of 180 ng/l shortly before the end of the period, before dropping to 40 ng/l.
The analysis of water specimens from a test configuration in which epoxy resins were in constant contact with chlorine and/or chlorine dioxide for a period of four months produced different results: Noyon et al. write that neither BPA nor BPF were detected; what was detected instead, however, was 2,4,6-trichlorophenol (TCP) at a concentration level that decreased over time after chlorination was discontinued. This substance is linked to a pharmaceutical taste in drinking water. At the microbiological level, 2,4,6-TCP can be converted into 2,4,6-trichloroanisole (TCA), which in turn causes organoleptic problems due to the musty taste and/or smell associated with it.
What remains to be said at the end
Noyon et al. report that their tests have demonstrated that the current evaluation period of a few days or weeks for epoxy resins that are used in water applications is simply too short to provide meaningful data about the migration of, for example, BPA, and about the development of organoleptically relevant compounds. The research scientists stress that the periods should be lengthened in order to obtain objective results. With respect to the application of their method to real samples, Noyon et al. come to the conclusion that 2,4,6-TCP was detected sometimes, but not BPA or BPF.
In addition to this, less BPA was detected in water storage systems than in repaired pipes, which is associated with various factors, including the surface volume ratio of water storage systems, the duration of the contact between the water and the epoxy resin, which may be longer in parts of the distribution network, and the quality of the epoxy resin used. To increase certainty, an obvious solution is to increase the length of time that these materials are evaluated experimentally. The scientists say that the analytical technology available is definitely highly suitable for the detection of BPA and chlorophenols and thus for evaluating the suitability of materials for water applications too.
 Naïke Noyon, Nora Elyasmino and Auguste Bruchet, Les Risques sanitaires et organoleptiques liés au Bisphénol A relargués par les résines époxy, GERSTEL Solutions worldwide – SBSE Special (2015) 25-28