after more than 25 Absorption/Desorption Characteristics of Organic Tank Lining Systems

Transcription

1 Absorption/Desorption Characteristics of Organic Tank Lining Systems By Trevor Parry Scientific & Technical Services Ltd., Blaydon, UK after more than 25 years of investigating coating failures in cargo tanks of chemical tankers, it is clear that in many instances the coatings failed even though the ship s operators strictly followed the coating manufacturer s guidelines for cargo compatibility. The simple tests involving static immersion of coated panels in cargoes and visual examination of the panels have limited value. Therefore, a test programme was developed to compare the results of simple static immersion tests in single cargoes with those of cyclic tests involving repeated periods of immersion and cleaning to simulate typical voyages involving sequential cargo carriage. The test programme evaluated the coatings absorption/desorption characteristics with a range of cargoes, since retention of cargo within a coating can have a detrimental effect on the coating performance and contaminate a subsequent cargo by desorption. This article deals with results of the absorption/desorption characteristics of a number of the tank linings examined. Absorption/Desorption Potential Although the fact that organic coatings absorb and desorb cargoes is not new, only recently has information on the subject been published. Most of the work has been done by a few researchers, mainly in the employ of paint manufacturers and often limited to their own coating systems. Consequently, many ship owners and operators are unaware of the potential for cargo retention in tank linings until they are faced with 24

2 the problem. This frequently occurs with high purity cargoes and cargoes designated for the food chain, particularly those subjected to sophisticated modern techniques that enable content analysis down to the level of parts per billion. Although organic coatings contain a degree of extractable material from solvents, additives, etc., it is present in a defined quantity and poses a limited risk of contamination, since once the material has been extracted generally in the early life of the coating it is no longer present. The contamination potential of cargoes absorbed into a coating is far greater because large quantities can be retained; the amount retained after different time periods is not well defined; very variable absorption/desorption characteristics are found between different coating types and within the same generic types of coatings from different manufacturers; and different rates of absorption/desorption are found between different cargoes. Factors such as coating thickness, temperature, and tank cleaning also affect the potential for absorption and desorption. Test Programme Seventeen coating systems of three generic types isocyanatecured epoxy, amine-cured epoxy, and phenolic epoxy were evaluated in the test programme. The cargoes involved in the absorption/desorption work were methanol, ethylene dichloride (EDC), acetone, acrylonitrile (ACN), caproic acid, methylene chloride, trichloroethylene, distilled water, ethyl acetate, ethylene glycol monomethyl ether, methyl tertiary butyl ether (MTBE), styrene monomer, vinyl acetate monomer Fig. 1: Isocyanate-cured epoxy Fig. 2: Amine-cured epoxy Fig. 3: Phenolic epoxy (VAM), and gasoline. The test method used was similar to one based on ASTM D570, Standard Test Method for Water Absorption of Plastics. It involved immersing coated test panels into the cargoes and then weighing the METHANOL EDC STYRENE VAM METHANOL EDC STYRENE VAM METHANOL EDC STYRENE VAM panels to monitor the increase in weight over a time period. After constant weight was achieved, the panels were allowed to naturally desorb and the weight change was monitored over another time period. The following criteria were used. 25

3 ISOCYANATE EPOXY PHENOLIC EPOXY AMINE EPOXY 26 Fig. 4: Variation of coating types to EDC Fig. 5: Variation of coating types to styrene Fig. 6: Variation of coating types to VAM All coatings were applied to steel panels blast cleaned to Sa 3 (white metal). Coatings were applied to the manufacturer s recommended thickness and cured to the manufacturer s requirements. ISOCYANATE EPOXY PHENOLIC EPOXY AMINE EPOXY ISOCYANATE EPOXY PHENOLIC EPOXY AMINE EPOXY Coatings were pre-extracted with methanol to remove residual material that could affect the test results and then dried to a constant weight to remove the methanol. The results described here are for desorption into an atmosphere of 25 C and 50% relative humidity. Contributory factors that could affect the absorption/desorption rates, including temperature, humidity, and simulated tank cleaning, were evaluated. Results Due to the very large amounts of data produced in the study, the focus of this article is limited to only four cargoes: methanol, EDC, styrene monomer, and vinyl acetate monomer. The results demonstrate the variation in absorption/desorption rates for the different cargoes; the variation that occurs between coating types; and the variation that occurs within the same generic coating types produced by different manufacturers. In Figures 1, 2, and 3, it can be seen that irrespective of coating type, EDC is absorbed to a far greater degree than the other cargoes and, except for styrene monomer in amine-cured epoxy and isocyanate-cured epoxy, the initial absorption rate is rapid. The initial desorption of all cargoes is rapid, but in the case of styrene monomer, it slows down over a longer period of time with generally more cargo being retained in the coating at equilibrium, particularly in the case of isocyanate-cured epoxy and phenolic epoxy types. With respect to EDC, Figure 4 shows that the isocyanate-cured epoxy absorbed approximately twice the amount of cargo as the other coating types. However, the release rates were rapid in all cases, with approximately the same amount of cargo being retained in the coating at equilibrium. Figure 5 demonstrates the large

4 variation among the coatings in the absorption of styrene monomer. The isocyanate epoxy attained an initial equilibrium state and then showed a further rapid increase that did not attain equilibrium. The test was terminated at 6,000 hours to determine the desorption from that point. The initial desorption initially occurred quickly, but after approximately 3,000 hours, 2.5 8% of styrene monomer remained in the coatings. The absorption characteristics of vinyl acetate monomer are shown in Figure 6. The isocyanate epoxy absorbed the greatest amount of cargo at equilibrium. The initial absorption and desorption rates were rapid for all coating types, but the percentage remaining at equilibrium varied considerably, with the isocyanate epoxy, having absorbed the highest amount, retaining the least. The three amine-cured epoxy coatings performed similarly with respect to initial absorption and desorption rates of EDC, but they showed a marked difference in the maximum amounts absorbed and retained at equilibrium (Fig. 7). In Figure 8, very large variations can be seen in the shape of the absorption/desorption curves for the three amine-cured epoxies with respect to styrene. The amount absorbed at equilibrium varied from approximately %, and the amount retained after desorption varied from approximately 1 5.5%. In coating 2, the desorption stage was initially rapid, with approximately half of the cargo being desorbed in a short time period, after which the desorption slowed dramatically. With coatings 1 and 3, however, the desorption Fig. 7: Variation within amine epoxies to EDC Fig. 8: Variation within amine epoxies to styrene Fig. 9: Variation within amine epoxies to VAM process was much slower and a longer time period was required to reach equilibrium. Figure 9 shows the different curves for three amine-cured epoxy coatings with respect to vinyl acetate monomer with the COATING 1 COATING 2 COATING 3 COATING 1 COATING 2 COATING 3 COATING 1 COATING 2 COATING 3 significant difference being the quantity of cargo absorbed at equilibrium (8.5 14%) and remaining after desorption (1.5 6%). Four phenolic epoxy coatings are shown in Figure 10 with respect to EDC. Initial rapid absorption is 27

5 COATING 10 COATING 11 Fig. 10: Variation within phenolic epoxies to EDC Fig. 11: Variation within phenolic epoxies to styrene Fig. 12: Variation within phenolic epoxies to VAM COATING 12 COATING 13 COATING 10 COATING 11 COATING 12 COATING 13 COATING 10 COATING 11 COATING 12 COATING 13 demonstrated with coatings 10, 11, and 12, and they also show a rapid desorption of more than half the absorbed cargo in a short time period before the process slows. Coating 13 has different absorption characteristics rapid for a comparably short time period and subsequently reaching equilibrium more slowly and gradually. The desorption is similarly rapid with approximately half of the absorbed cargo being desorbed in a short time period. It is interesting to note that coatings 11 and 13, which absorbed the greatest and least amount of cargo at equilibrium, retained a similar amount when desorption was complete. Figure 11 shows a large variation in the behaviour of the four phenolic epoxy coatings in styrene monomer. Coating 13, which absorbed the lowest quantity of cargo, retained the lowest quantity after desorption, whereas coating 12, which absorbed the second highest amount, retained almost 70% of the absorbed cargo on completion of the desorption phase. Variations in the absorption/desorption of the four phenolic epoxy coatings in vinyl acetate monomer are shown in Figure 12. Three of the coatings (10, 11, and 13) retained approximately the same amount after desorption at approximately 4% even though the amount absorbed varied considerably, between 6.5 and 13%. Although the rate of absorption took different forms, the shape of the desorption curves was similar. Discussion All the coatings that were examined absorbed and released the cargoes to varying degrees and at varying rates and confirmed in general terms the previous work undertaken by other researchers. Although it is difficult to draw conclusions from an extract of a comprehensive test programme, it can be seen that there was a large variation in the characteristics of the coatings of different types and also a large variation within coatings of the same generic type. The amounts of cargoes absorbed at equilibrium varied considerably, and major differences 28

6 were observed between the individual cargoes and the coatings themselves. The desorption phase was initially rapid with most combinations of cargoes and coatings, but it did not proceed to zero in all but a few instances, hence leaving residual cargo within the coating system. It must be noted that desorption into an atmosphere of 20 C and 50% relative humidity can provide a competing mechanism whereby water vapour from the atmosphere can absorb into the coating system. In none of the illustrated results was a weight gain found during the desorption phase, and analysis of paint flakes confirmed the presence of the cargo as the major absorbed constituent. The variable response to absorption and desorption of cargoes within coatings of the same generic type places the purchaser in a difficult position. Take, for example, phenolic epoxy materials where the purchaser must choose based on published data, and the principal information available is the paint manufacturer s data sheets and cargo resistance lists. Both the data sheets and cargo resistance lists could, in effect, provide very similar information, and to the purchaser there could appear to be little or no difference in the coating from a number of manufacturers. Figure 11, however, indicates that the choice of coating 12 would pose a much higher risk of retention of styrene monomer with the subsequent higher risk of contamination of a future cargo than, for example, coating 13. A similar situation exists with A cargo tank coating in good condition (top) and a test panel showing coating breakdown in the form of blistering (bottom), which increases the tendency of the coating to absorb cargo (Photos and figures courtesy of the author) amine-cured epoxies where Figure 8 illustrates that coating 1 poses a greater risk for potential cargo contamination than coating 3. In the case of EDC, which can react with water to form acidic components, paint manufacturers generally have a clause in their cargo resistance lists to say that the cargo should be completely removed from the coating prior to tank cleaning, loading an aqueous cargo, or other contact with water. Figures 7 and 10 illustrate that some coating systems retain more EDC than others at equilibrium and, hence, there is a greater possibility of reaction with water and breakdown of the coating system. The amount of absorbed material at equilibrium must be viewed in each individual cargo and coating combination because of the variable lengths of time to reach equilibrium. The amount of cargo absorbed into the coating will depend on carriage time, which is often considerably less than the time taken to reach the equilibrium absorption state. Hence, the amount of retained cargo after discharge and that available for potential cargo contamination will be influenced by the carriage time of the cargo. This article has only dealt with a small amount of the data available from the test programme. It is expected that future works will include information on other cargoes and the effects of temperature and tank cleaning. References 1. O.B. Sorensen, The Factors Influencing Ventilation and Drying of Epoxy Coated Cargo Tanks, proceedings of Mari-Chem 89, Gastech Ltd. (ed.), W. Woods, Retention of Chemicals by Tank Linings, F.O.S.F.A. International Symposium on Edible Oils How Pure are They? C.E.M. Van der Kolk, OCL Epoxy Tanklining A New Contribution to Operational Flexibility in Chemical Cargo Transport, proceedings of MariChem 89, W. Woods, Lining Up for the Nineties, Hazardous Cargo Bulletin, W. Woods, Tank Coatings An Absorbing Subject, Ship Repair and Marine Maintenance 94, New Orleans, LA, USA, O.B. Sorensen, Marine Paints: Coating Systems for Ballast and Product Tanks, Second Middle East Conference on Technological Developments in Coatings: Their Application and Uses, Bahrain, January