FRP solutions for service with sodium hypochlorite

Commercial solutions of sodium hypochlorite (usually with 9-15% of active chlorine) are stabilized with sodium hydroxide. This and its strong oxidizing power make him very corrosive for many building materials. However, the stability of sodium hypochlorite depends on several factors, such as concentration, pH, temperature, and the impurities like metals. For example, if hard water is used for the preparation of the hypochlorite, this will not be as stable due to contamination by metals such as iron, calcium, and other metals, what will this be more aggressive against the building materials of tanks of storage (2.18). The pH can vary significantly when form hypochlorite, for example in the chemical processes and the chlorine cleaners. The following diagram shows the chemical composition on the pH, as the sodium hypochlorite (NaOCI) versus pH and chlorine, hypochlorous acid (HOCI) balance.

When the stability of the system is compromised, various mechanisms can be activated. (HOCI) hypochlorous acid and sodium hypochlorite (such as ionClO) break down through several possible reactions that can occur depending on the temperature, even in the absence of any catalyst (3-8). Table 1 summarizes these reactions and their reaction energies registered, without going into further details.

It is assumed that any intermediate product which is formed during previous reactions, can have significant effects on various materials. It is therefore very important to know the more possible conditions of operation, and this way the stability of the hypochlorite, before selecting a building material. Priority should be given to avoid the formation of unstable products or to modify the parameters of the process to improve the stability of the hypochlorite.

Studies of chemical resistance of the FRP (according to ASTM C581) with sodium hypochlorite, have traditionally been carried out at high temperatures, in an attempt to detect clearly the differences between the tested systems. This led to the conclusion that the resins with a high resistance to alkali made with a free system of cobalt, in laminates with a double synthetic veil Nexus, behave better (9), as it can be seen in Appendix I. Cobalt-free curing systems continue to be the preferred option as long as possible, and also the epoxy-resin vinylester bromadas, as we will see later.

In addition, ways to reduce the amount of cobalt (and thus the detrimental effect) have been studied, in standard curing systems, either due to the synergy with the potassium, either replacing him with vanadium. Both methods have shown positive aspects, but for the moment have not used in practice.

Many building materials have been used to transport and store the sodium hypochlorite at room temperature, for example special grades of polyethylene PEX (LHPDE), CPVC, FRP and titanium. Titanium is considered the best, but its high cost and availability are factors that limit its use. The EP can last between 7 and 11 years. As it has been confirmed by an industrial survey in 2004 (1), the FRP based on special Epoxy vinylester resins, are most common building materials used for the transport and storage, based on historical cases of more than 20 years of service. A tank made of FRP, well specified and built, can last from 20 to 30 years or more, with regular inspections of chemical barrier every two years and with the need of small repairs. Inappropriate design and manufacturing can lead to a failure of the chemical barrier and damage to the structure in less than 5 years, which will require the replacement of the tank (1, 2, 18). A special case are coatings FRP tanks of transport manufactured in steel or stainless steel. The service life of these solutions, rely on the mechanical integrity of the tank to prevent the separation of the laminate and steel, or cracking of the surface.

Experience tells us the following key elements to a good outcome of the FRP with sodium hypochlorite to ambient serving temperature:

-Use a resin epoxy vinylester appropriate, preferably bromada.

-Properly designing chemical barrier (for example with a double veil of surface, without the use of charges, additives and pigments), and a good structural design.

-A formulation without cobalt (or very low in cobalt).

-A good curing resin (would be desirable for a post-curado on the DIN 18820 recommendation).

-Carry out inspections regularly.

-Stable solutions of sodium hypochlorite (pH > 11, T)<40°c), sin="" metales="" contaminantes="" y="" agua="" blanda="" para="" la="" dilución,="" protección="" del="" sol="" directo="" sobre="" el="" tanque="" (especialmente="" la="" fase="" de="" vapor),="" recubrimiento="" externo="" de="" las="" tuberías,="">

The studies indicated here include test tubes used in the laboratory, as well as within the storage tanks of hypochlorite, in two treatment plants of water in Colorado, and for 12 months. The purpose of these studies was the identification of the best system of resins and the construction of the chemical barrier to get more life in the service of these tanks (2).

Include subsequent studies with alternative systems of resins and at lower temperatures.

Laboratory studies to 50 and 65 ° C followed by the standard ASTM C-581 on chemical resistance tests in FRP. Laminates of evidence consisted in 3 layers of Mat of 450 g/m² with a veil on each side. The panels were cured at room temperature environment during the night, followed by a postcurado at 94 ° C for 8 hours. After cut custom, the edges of the panels recubrieron with resin to prevent chemical attack to the fiber. The panels are submerged in a hypochlorite solution of 10% to 15%, and between 50 and 65 ° C. The hypochlorite solution was changed once per week to maintain the concentration of chlorine on 9 per cent at all times during the test. After 1, 3, 6 and 12 months, the panels were removed and assessed the Barcol hardness, resistance to bending, flexural module, and also visually.

Laminates were also placed in the interior of two storage tanks of hypochlorite, in the water treatment plant. These laminates were extracted from the tanks and sent for evaluation after an exhibition of 3, 6 and 12 months.

Laboratory tests and those of the two storage tanks were made with resin epoxy vinylester DERAKANE 1 411-350 (1 EVER), HETRON 1 922 (EVER 2) epoxy vinylester, HETRON FR992 epoxy bromada (BREVER1), DERAKANE 510A - 40 epoxy vinylester vinylester bromada (BREVER2), DERAKANE MOMENTUM 470-300, a novolaca epoxy and vinylester (NEVER). Resin BREVER1 was tested with 2 layers of veil of polyester, 1 layer of veil of polyester and 1 layer of glass veil C. Curing systems evaluated included a system of peroxide of methyl ethyl ketone (MEKP) /Cobalto (Co) and one (BPO) /Dimetil benzoyl peroxide aniline (DMA). Other resins were tested only with a layer of glass C veil, and cured with BPO/DMA. All test tubes were post-curadas for 8 hours at 94 ° C.

The results of laboratory tests in sodium hypochlorite stabilised at 50 ° C are shown in table 2.

None of the tests on the test showed a significant reduction of bending properties after 12 months. The attack surface of the test tubes varied, and was determined by visual inspection. The surface of the test tubes made of resin NEVER cured by BPO/DMA was that suffered more. Sixty per cent of the coating of edges disappeared during testing. These two observations suggest a chemical attack by what this resin is not seen as the best for a long time of life. The test piece based on the BREVER1 with two veils of polyester and cured by 0.15% of Co 6% / MEKP also showed surface attack.

While there was some loss of brightness, the analysis under a microscope showed the attack on the veil of polyester. Some of the fibers of polyester disappeared leaving hollow channels instead. This indicated that the polyester fibers had been attacked by the sodium hypochlorite 50 ° c. The test tubes made of BREVER1 and a veil of polyester cured with BPO/DMA retained a better surface appearance against the same cured with Co / MEKP. Cobalt has a catalytic effect on sodium hypochlorite which increases with temperature. The products of decomposition of hypochlorite is believed are harmful to the veil of polyester and resin. Laminates based on BREVER1 and BREVER2 with a veil of glass C and cured with BPO/DMA retain a surface finish of semibrightness, and there is no attack on resin after 12 months. The test tubes axes with EVER1 and EVER2 and a veil of glass C, cured with BPO/DMA have a smooth flat surface after 12 months. They showed a lesser attack than the one seen with the curing system based in cobalt, but slightly higher than the test made with BREVER1 and BREVER2.

The same test tubes were placed in tanks of storage in Thornton, Colorado, USA (table 3) and in the city of Westminster, Colorado, USA (table 4), to compare the results of the laboratory with the real life.

NEVER lost surface brightness and showed slight signs of attack to the resin. No differences were found in the other test tubes that were evaluated in these tanks. This is probably due to the low temperatures where the test tubes were exposed.

These data were compared with the results of a study conducted in laboratory 2 year (s) at 40 ° C and in a stabilized solution of sodium hypochlorite to 8% (5.25% for the last 18 months of exposure due to the difficulty of finding commercial NaOCl of 8%). The resins used were as follows:

EVER3 = DERAKANE MOMENTUM 411-350 epoxy vinylester.

BREVER3 = DERAKANE MOMENTUM 510C-350 epoxy vinylester.

These were cured with normal and low concentrations of cobalt, and with a system based BPO/DEA in one case, 2 layers of veil of polyester NEXUSTM were implemented by comparison. The test tubes were post-curadas at 100 ° C for 5 hours.

All the test tubes of evidence have been a brilliant surface after 12 months and have not deteriorated significantly. The study confirmed that, at 40 ° C, the impact of the cobalt in the sodium hypochlorite is less pronounced. However, you can determine that there is a difference between a 6% Cobalt 0.03%, compared to 0.2%. Both rolled with EVER3 and BREVER3 resins and cured with 0.03% cobalt 6%, best kept the set of properties. Therefore we can conclude by saying that they can afford small amounts of cobalt in the storage of sodium hypochlorite stabilised at room temperature. Content in active in a system curing cobalt can be minimized through synergies with potassium. There are commercial mixtures of promoters (cobalt) and potassium in the market.

The effect of the concentration of cobalt has been discussed in a previous publication (12). She was tested with sodium hypochlorite to 5.25 per cent to 65 ° C for 10 months. Three probes were built using the EVER1 resin and cured by 0.1 per cent of cobalt 6%/MEKP, 0.3% of cobalt 6%/MEKP and BPO/DMA. Figure 3 shows the graph of the weight loss versus exposure time for curing systems. Weight loss was directly related to the amount of cobalt. Thus, while the BPO/DMA system only lost 2% of its weight, System cured with a 0.3% lost 18% and system cured with 0.1% cobalt 6% lost nearly 7%.

In order to study the influence of the type of veil, made additional probes with BREVER1 resin and were in contact with sodium hypochlorite and 19 per cent to 65 ° C. The first test tube with a layer of glass C veil was cured by BPO/DMA, and the second with a layer of synthetic polyester standard veil and also cured by BPO/DMA. The results are shown in table 6.

The test tubes made of veil of standard polyester, after 12 months have been left without surface hardness and hold 28% of its properties of bending. The test tubes made of glass C veil retained 47% of its surface hardness and 70% of their bending properties. High temperature accelerates the decomposition of sodium hypochlorite and makes the tests harder in tests at 50 ° C, the veil of standard polyester was visibly attacked by sodium hypochlorite, which explains the differences seen at 65 ° C.

Other applications involving sodium hypochlorite or derivatives to variable pH (see Figure 1) include caustic cleaners to remove chlorine from the waste or exhaust while the typical cleaners for the reduction of chlorine, for example in the alkali plants are working in a satisfactory manner with well-defined chemical (sodium hydroxide)(, chlorine, sodium hypochlorite), gases from combustion, for example, of the industrial waste incinerators are often necessary to study and find individual solutions for a long service life. Installing classic for such flue gas purification systems consists of a cooler, a washer of acids to absorb the HCl with hydrochloric acid, followed by a caustic washer to neutralize the residual HCl and eliminate chlorine. However, the pH at this second stage fits 8-9 to limit the consumption of NaOH (higher pH also eliminate CO₂).As a result, the chlorine doesn't become completely stable sodium hypochlorite, a very high chemical, acting especially at the stage the Raschig rings inside the washer and gas aggressively. A high temperature (> 50 ° C) may also increase aggressiveness. An example of the washing system is shown in Figure 4.

While curing with BPO can help improve the lives of service with a laminated cured with cobalt, it may be required and accepted occasional renewal of chemical barrier. However in many cases the addition of a reducing agent such as sodium bisulfite is used to prevent chemical instability and improve the efficiency of the washer. 15, 16 And 17 references reviewed the chemistry involved in this reaction of "decloración". The more effective reducing agent on the basis of the cost is the sodium metabisulfite. It is suggested to dissolve the sodium metabisulfite in water to obtain a solution of sodium bisulfite and dosed at a rate of 3 grams of metabisulfite 1 chlorine, although in theory the required relationship would be of only 1.34: 1. Using a reducing agent, life in the service of the alkaline washer FRP will increase considerably and usually avoids the need to renew the chemical barrier during the life of the equipment.

Testing at room temperature in a sodium hypochlorite storage tank and laboratory, shows little difference between a resin vinylester bromada epoxy and vinylester standard Bisphenol an epoxy. One resin vinylester of epoxy novolaca showed some signs of attack...Laboratory tests conducted at 50 ° C, accelerated the attack and showed an advantage of the epoxy-resin vinylester bromadas on the vinylester based Bisphenol A. A veil of standard polyester did not have any advantage over the veil of glass (c) in the test, while a synthetic veil thermally coupled such as NEXUS, repeatedly to the veil of synthetic polyester and the veil of glass C. This can also be attributed to the relatively thick layer rich in resin that is achieved with this particular veil.

The main factors influencing the service life of sodium hypochlorite storage tanks are:

• The amount of cobalt available in resin, especially when the temperature increases.
• The degree of curing as demonstrated in historical studies and experiences in the field.
• The conditions of service which have an impact on the stability of the product (pH, pollutants, temperature, sunlight...).

A system of curing free Cobalt (BPO/amine) generally requires a post-curado to get the best results. For this reason it is not necessary to choose a system curing, e.g. for the rolling of steel tanks, then it is not possible to do a post-curado at 80 ° C. A system of curing MEKP/bass classic in cobalt, often gives a better result for this application, taking into account that the solution of sodium hypochlorite is stabilized and below 40 ° C.

The use of a reducing agent such as sodium bisulfite in alkaline cleaners of the combustion gases of incinerators of waste containing chlorine, can dramatically improve the life of the equipment and eliminate the need for the renewal of the chemical barrier.

References

1. Industry survey performed by Dow Chemical, 2004. Internal Information.
2. Michael g. Stevens, ' What is the Best Resin for FRP Sodium Hypochlorite Storage Tanks?', born 2008
3. Farr, j. P.; Smith, w. L.; Steichen, D. S. 'bleaching agents, survey'. Kirk-Othmer Encyclopedia of Chemical Technology. John Wiley & Sons, 1996. (Article online posting date December 4, 2000). See also references therein.
4. Lister, M. w. The Decomposition of Hypochlorous Acid. Can't. J. Chem. 1952, 30, 879-889.
5. Lister, d. w. "The Decomposition of Hypochlorite: The Uncatalyzed Reaction" Can. J. Chem. 1956, 34, 465-478.
6. Cotton, f. A..; Wilkinson, G.; Murillo, C. A..; Bochmann, M. Advanced Inorganic Chemistry. 6th ed. John Wiley & Sons, New York: 1999.
7. Anbar, m.; Ginsburg, D. 'organic hypohalites'. Chem. Rev. 1954, 54, 925-928. See also references therein.
8. Abramovici, S.; Neumann, R.; Sasson, y. ' Sodium Hypochlorite as Oxidant in Phase Transfer Catalytic Systems. Part i. protects of Aromatic Aldehydes'. J. Mol. Catal. 1985, 29, 291-297. ' Part II. Protects of Aromatic Alcohols'. J. Mol. Catal. 1985, 29, 299-303.
9. T. w. Cowley and M. a. Robertson, ' The effect of pH and temperature on Fiberglass Reinforced Composites in Sodium Hypochlorite solutions', was born, 1991
10. Sodium Hypochlorite General Information for the Consumer', Odyssey Manufacturing Co., March 21, 2004.
11. ASTM C-581 Standard Practice for Determining Chemical Resistance of Thermoset Resins used in Glass-Fiber Reinforced Structures Intended for Liquid Service, Annual Book of ASTM Standards, July, 2003.
12. Don Kelley, ' Fiberglass Reinforced Plastic Equipment for Treating Waste Incineration gas ', Corrosion 2004 paper # 04617, (Houston, TX, is born, 2004)
13. Jonathon Mason, Paul Kelly, ' Low Cobalt Add-ons Glass Reinforced Plastic Systems for Bleach Service', 10th International Symposium on Corrosion in the Pulp and Paper Industry (ISCPPI, 2001).
14. DERAKANE Resin Case History E-131-2005, Ashland Inc.
15. Richard Grubbs and Tom Ladshaw, ' Low Cost Approach for Dechlorination', Proceedings of the Georgia Water Resources Conference, 1991, University of Georgia
16. Kinetic Systems Inc., "Technology Update: Use of Sulfites to remove Chlorine or Chloramine in High Purity Water", June 28, 2002, www.kineticsgroup.com.
17. Filmtec Membranes, ' Water Chemistry and Pretreatment: Biological Fouling Prevention, Chlorination/Dechlorination', Form No. 609-02034-1004, The dow chemical company, www.dow.com.
18. Powell, 'sodium hypochlorite fiberglass reinforced plastic (frp) storage tank specification' and 'sodium hypochlorite General information handbook', www.powellfab.com.

This article is part of the presentation delivered by Michael Jaeger, Ashland Performance materials, in the 20th International Conference on composite materials, organized by the Spanish plastics Centre (CEP), the last 23 and 24 November in Barcelona.