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Koninklijke Vereniging - Société Royale

DOSSIER

 

Water absorption in tank coatings


by Reian, G, Jotun Performance Coatings R&D Jotun AS, Sandefjord, Norway.


The absorption-desorption of fresh water and seawater was investigated for four commercially available epoxy tank coatings, each based on a slightly different binder technology.

It was shown that there are considerable differences between the various coatings when it came to water absorption and subsequent desorption. The pure epoxy and novolac epoxy coating absorbed less water overall and desorbed faster and more completely than the coatings based on novolac-RDGE or epoxy-silicon.

The latter two coatings retained a lot of water, even after as much as 10 days of perfect ventilation. As to the difference between fresh water and seawater, the coatings generally absorbed a higher amount of the fresh water than they did seawater. Based on this, it can be argued that fresh water is the better choice for cleaning purposes.

A cargo tank coating is generally exposed to water more frequently than to any single cargo, as water is used to clean the tank between different cargoes. Water retained in the coating after cleaning could potentially cause several problems; one being failure of the coating by reaction of water with subsequent cargoes, causing acidic or other harmful species to form within the coating; another problem could be water contamination of the cargo itself.

To shed more light on this, a test was initiated to study the amount and rate of, water absorption of four commercially available epoxy tank coatings and, more importantly, how long ventilation time the absorbed water needs to fully leave the coating.


Experimental procedures

Four different epoxy tank coatings were tested. The coatings are listed in Table 1 with technology base and curing regime. The coatings were applied onto patented Mylar polyester films for easy removal of free films. The coatings were applied using a fixed frame applicator, aiming to get a dry film thickness per the respective product’s technical data sheet (TDS).

After application, the coatings were cured per their respective TDS and/or application guidelines. The free coating films were then cut using a scalpel in squares of approximately 2 x 2 in and each square was weighed before immersion. During the absorption period the coating squares were immersed in fresh (tap) water or seawater2, in closed containers. The samples were removed at intervals for weighing.

All excess surface liquid was removed with a dry paper towel before each weighing. Weighing was performed using an analytical balance to ~0.1 mg. During the desorption period the coating was left to ventilate on the bench under ambient conditions (21 deg C - 23 deg C, 20 - 40% relative humidity) and weighed periodically. The absorption-desorption experiment was run over three cycles to study the accumulated effects of water uptake.


Definitions

Sorption describes the combined processes of adsorption and absorption . Adsorption is the physical adherence or bonding of ions and molecules onto the surface of another phase. Absorption is the incorporation of a substance in one state into another of a different state. For the sake of this article, we will assume that the migratory process is purely absorptional and all the molecules contributing to weight gain are incorporated in the free volume of the coating film.

Desorption is a phenomenon whereby a substance is released from or through a volume. The process is the opposite of sorption (ie, either adsorption and absorption). This occurs in a system being in the state of sorption equilibrium between the bulk phase and an absorbing volume (coating film). When the concentration (or pressure) of substance in the bulk phase is lowered, some or all of the sorbed substance returns to the bulk state.

The absorption and diffusion of water in polymeric materials such as epoxy systems is related to the free volume and the polymer-water affinity The amount of free volume depends on the molecular packing and is affected by both the crosslink density (and therefore the extent of curing) and physical ageing .

The polymer-water affinity is significantly influenced by the presence of hydrogen-bonding sites within the polymer (6). Water can sometimes be absorbed without causing swelling; when this happens, it is suggested that it remains unbound to the polymer and is effectively accommodated within the free volume .


Results

The test results for water absorption-desorption of the four tested tank coatings are summarised in Figure 1 for fresh water and in Figure 2 for seawater. Each absorption phase lasted for 12-14 days and each desorption phase for eight to 10 days (varying to fit in with weekends).

The results were plotted as percent weight gain as a function of time. It should be noted that there are no results presented for coating B with seawater, as the free film broke into several small pieces shortly after starting the test. It was considerably more brittle and therefore more difficult to handle than the other samples.

It can be seen from both Figure 1 and Figure 2 that for all coatings, and for all cycles, absorption stabilizes quite fast (reaches equilibrium) within a few days. The absorbed amount of water is considerably higher for coating B and D. This indicates that these coatings have a higher molecular affinity for water and/or a higher free volume. Previous absorption testing (not published) indicates a predominance of the first effect (higher affinity towards water) for these two products.
From both figures, it can also be seen that for all coatings, the desorption rate is fast and an equilibrium is reached within 24 hours of perfect ventilation3. For coating A and C all the absorbed water has left the coating within 24 hrs of ventilation, as can be seen by the graphs coming to equilibrium around the horizontal axis. For coating B and D, there seems to be a considerable retention of water. Even after 10 days of perfect ventilation there is still water left in the paint film and it even seems to accumulate from the second to the third test cycle.

It is difficult to predict the exact effect of water retention in a tank coating, but one has to wonder if it will affect coating lifetime and performance, especially if the coating is exposed to a water sensitive or an easily hydrolysable cargo.

                           

 

                          

Looking at the difference between fresh water (Figure 1) and seawater (Figure 2), all coatings absorb a higher amount of fresh water. In other words, fresh water penetrates more easily into the coatings film than does seawater. This could be a good reason to choose fresh water over seawater when cleaning coated cargo tanks.

 

 

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