Chapter 14 Paint failure – the science of blistering
Water blistering in polymeric coatings generally requires the presence of water locally within the film in an amount close to its saturation solubility. The films swell because of the absorbed water and there is an increase in the compressive stresses as emphasized by Brunt from the TNO in the Netherlands in the 1960’s. Hydrophilic components can collect water and initiate blisters. If the adhesion is poor, the blisters that form can remove the coating from the substrate, either isolated as blisters, or by total delamination of the coating. Isolated water filled blisters are usually formed in softer coatings, remembering that water has a significant plasticizing effect, whereas more rigid ones tend to delaminate, not being able to yield enough to accommodate local blisters. Both of these types of failure are called blisters here. Once formed, blisters can grow during continued water exposure, for example, by osmotic effects. There are more subtle mechanisms of blister formation, however, and hydrophilic components are not necessary. The solubility relations of water in the polymer in question are important, not just at room temperature but also as they are affected by changes in temperature. The δH parameter changes more rapidly than the other HSP, and the HSP for water approach those of the polymer more closely as temperature increases so the water solubility increases. As discussed in the following, this can lead to water blisters if the temperature falls rapidly. In order to minimize or prevent blistering at substrates the adhesion must be such that water at the interface cannot cause local loss of adhesion. Anchor-type adhesion or covalent bonding to a substrate is recommended if possible. Physical bonds across an interface are not nearly so resistant (see the chapter on adhesion). Under special conditions of rapid temperature changes it is possible for blisters to occur in the middle of polymer films, or even near the air surface, as described in detail below. This mechanism is responsible for a potential problem of excess water within a polymer be they elastomers or rigid plastics as described in the following. Water blistering can potentially occur in films applied to any substrate, but the majority of the practical problems are found at interfaces for coated metals and wood. The following general cases will be discussed:
· Cause 1: Presence of hydrophilic components
· Cause 2: Substrate below the local dew point
· Cause 3: Rapid temperature changes leading to “fog”
· Cause 4: Inverted primer (normally higher equilibrium water uptake) and topcoat (normally lower water uptake)
The problem of the whitening of restored paintings is discussed in the context of blister formation in coatings, even though the “blisters” remain very small, more like a fog. A final section discusses methods to alleviate the blistering problem.
Cause 1: Presence of hydrophilic components
It has generally been recognized in the coatings literature in the 1960’s and later that the presence of hydrophilic components could lead to blisters. Water-soluble components of pigments and fillers, hydrophilic pigments, and salts have been cited. A worst case scenario is the presence of salts in the film or at a substrate. Sodium chloride, for example, collects water at a relative humidity of 75% or above. Water molecules diffuse into all coatings (and plastics) at some relatively rapid rate compared to larger molecules. The rate is faster for films at temperatures above the glass transition temperature, and the amount of water at saturation also generally increases with increasing temperature as mentioned above. It is only a matter of time before a given film becomes saturated when it is in contact with liquid water or saturated steam. It should also be recognized that even on exposure to normal air, there will be a significant amount of water in the film at equilibrium with the water at some relative humidity in the air. Since this mechanism is fairly obvious, it will not be discussed in further detail, other than to point out that the hydrophilic sites may include substrate factors such as weathered wood, rust, or other effect.
Cause 2: Substrate below the local dew point
The satisfactory coating of cold water pipes has always been a problem of some significance, particularly in warm and humid climates. The condensation of water at or near the pipe can only be delayed by most coatings, and the anchor adhesion mechanism or covalent bonding, if possible, are suggested for best results. The blistering of cars can be a result of the same mechanism. On those days where water drips from under carports and in sheds, the cold metal under the coating on a car can also cause condensation of water from the increasingly warmer and more humid air present as the day grows older. There is a balance between how quickly the metal can rise to a temperature above the dew point, and how quickly the water can absorb into the film and diffuse in sufficient amount to the substrate. There are clearly times when the water gets there first to form blisters, even though temperature change generally occurs more rapidly than water transport.
Cause 3: Rapid temperature changes leading to “fog”
When a film that is saturated, or nearly saturated, at a higher temperature is cooled rapidly, water can remain in the film in excess of that soluble at the lower temperature. This water precipitates, much like fog. If the film is sufficiently resistant it may recover after this water finally escapes to the air. If there are hydrophilic components, the water will preferentially collect at such sites, and blisters are nucleated. The blisters can then grow on subsequent temperature cycling or because of osmotic effects. The testing of coatings in so-called blister boxes involves a combination of the causes 2 and 3. The sample is placed at an angle to the horizontal so that the condensed water can run off. The substrate will be colder than the interior of the cabinet invoking cause 2. At the same time the water periodically running off will induce local temperature changes that presumably enhance the severity of the test method. An exceptionally severe test of this kind involves putting the films on top of a container in which there is boiling water.
As shown in Charles M. Hansen, New developments in corrosion and blister formation in coatings, Progress in Organic Coatings, Vol 26, 113-120, 1995, blisters were formed near the air surface of epoxy-coated thick steel panels during an attempt to measure the diffusion coefficients for methanol in the coating at 50°C. The panels were removed from the methanol bath and weighed at room temperature. It only took a few cycles before sizeable methanol blisters near the air surface were formed. The methanol absorbed near the surface would be near the saturation value at 50°C, whereas there may still not be methanol at the metal substrate. This concentration of methanol exceeded what the surface region of the film could truly dissolve upon its removal from the methanol bath. Blisters formed and grew on subsequent cycling.
Two other situations exemplifying this cause are cited in the Handbook on pages 238-240. These are excess water in free films of EPDM rubber and in poly(phenylene sulfide) (PPS). The temperature cycling with water exposure for the EPDM was from 120°C to 15°C simulating a problem of a failure in a gasket subjected to decontaminating steam with subsequent hosing in a dairy. The PPS study was to demonstrate that even such rigid polymers could be made to fail by this mechanism. Here the temperature cycling with water exposure was between 90°C and 23°C using 2 mm films. Normal absorption curves were found initially in both cases, but late in the approach to equilibrium a more rapid water uptake was suddenly encountered as the polymers started to have the excess water problem. Control experiments at the higher temperature did not show uptake of excess water when the samples were left in the test for very long time periods. It took 5 days to rupture the EPDM gasket in the middle. The excess water started appearing in the PPS film after about 40 days.
Cause 4: Inverted primer and topcoat systems
Blisters are often encountered after repairing older paint, even shortly after the repair work has been completed. The work in Klaus Lampe and Charles M. Hansen, Blæredannelse i malingfilm (Blistering in Paint Films), Rapport T 16-83 M, Nordisk Forskningsinstut for Maling og Trykfarver (Scandinavian Paint and Printing Ink Research Institute), Hørsholm, 1983, 58 pages, helps explain how this can happen. Even though these studies were on metal substrates, the results are still applicable to coatings on wood.
Coatings were cycled between 40°C and 12°C. The cycled coatings blistered in half the time required for the non-cycled systems that were held at a constant 40°C. The blistered coatings had absorbed about 5%w water at the time of blistering. The non-cycled systems blistered when the primer became saturated with water, which required about 8%w (40°C). A topcoat with low water uptake and low permeability prolongs the time for the blisters to occur. Longer periods of room temperature drying after oven cure improved blister resistance, since the films were cured more thoroughly. Clear films had an initial milky appearance with blisters appearing later. This is a manifestation of the “fog” discussed above. One coating had the same water uptake at three different (40°C, 23°C, 12°C) temperatures. This coating could not be made to blister. The individual layers in these systems were between 25 and 50 microns as recommended by the suppliers.
A usual topcoat was applied as a primer and a usual primer was applied as a topcoat over this to see the effect of what might happen in a faulty repair situation. Blisters appeared rapidly at the substrate in such a “repair” coating with subsequent rusting. This occurred as a rule when the equilibrium water uptake in the topcoat was larger than that in the primer. The water in the lower layer could not escape rapidly enough in unfavorable situations such as rapid cooling, and blisters at the substrate were common. The only safe practice when in doubt is to remove the old paint.
To sum up this section, it can be concluded that blister formation is favored when more water can be taken up at equilibrium in a topcoat than in a primer. Such conditions can easily be found in repair coatings that are not tuned to the given paint to be repaired.
Whitening of restored paintings
Older repaired paintings occasionally develop whiteness at the places where they have been repaired. The reasons for this are thought to be based on the same mechanisms as those described above. There are examples of closed storerooms where the climate is not controlled and there are examples of paintings on cold walls. In every case the cause of the whiteness is condensed, fog-like water droplets. The water droplets have forced the repair paint apart and upon ultimate evaporation there are small holes where the water once resided. These holes have light scattering properties with whiteness being the visual result. This phenomenon would not even be recognized in a traditional white coating on steel or wood unless the water contact lasted long enough to produce true, water-filled blisters or delamination. For colored paints it might lead to pastel color version of the original color.
The things to think about are maintaining a more stable climate and making sure that the repair paint has lower water solubility at equilibrium than the original paint. This implies repair coatings that would be characterized as hydrophobic in nature, and that do not contain hydrophilic entities.
The cases and mechanisms described above are helpful in understanding some undocumented observations made by those in the coatings industry:
Why is pure water more severe in attacking coatings than salt water?
The answer is that the salt reduces the water activity, and the coating absorbs water at equilibrium with this activity. The water content in the coating only approaches the total water saturation possible when in contact with pure water, and there is excess capacity to truly dissolve the water freed during temperature cycling with water exposure.
Why do panels perform better when left alone for the full test period in water exposure tests?
The impatient formulator (or boss) who repeatedly removes panels from such testing causes a temperature change every time the panel is removed. This is particularly important for higher water temperature tests. This phenomenon should also be remembered in any cyclic testing procedure with changes between exposure to water, “sunshine”, and dry periods, with or without temperature change.
Why are there more problems with blistering with darker colors?
Darker colors have higher temperatures than lighter ones on sunny days. The larger temperature change on rapid cooling (clouds or night) after a moist and sunny period creates a larger amount of water that is in excess of that soluble at the lower temperatures. If this water cannot diffuse rapidly from a film, blisters will form. The higher temperature also leads to softer films that are not as resistant to mechanical effects (stresses) that can lead to loss of adhesion.
The temperature changes required for the formation of blisters in a water saturated film are not large. Common margins of control over the temperature changes in a hot water bath (perhaps +/- 1°C) are large enough to induce the effect.
Why is a hydrophilic topcoat the worst possible case? The usual topcoat/primer systems have the topcoat with less water solubility. Brunt emphasized that there has to be swelling stresses to cause the blisters (and then poor adhesion). In the usual topcoat/primer systems the swelling in the primer produces tensile stresses in the topcoat that is being pulled apart by the swelling beneath it. The tensile stresses produce a resistance against blistering. A 2% linear swelling by water is said by Brunt to be a minimum condition for blistering (though, of course, the degree of adhesion will affect the resistance to blistering). This would have to be a differential swelling between the topcoat and primer. The Hansen studies could not blister a coating system with equal water uptake in topcoat and primer. One can blister primers in the conventional systems, but here the temperature cycling comes in, and presumably not the swelling differential between the coatings. The excess phase separated water in the primer, probably collected at hydrophilic sites, produces swelling stresses that can lift the whole system from the substrate, again this being initiated at points of weak adhesion to the substrate.
How to minimize blistering
There are a number of factors that can help to minimize blistering. The mechanisms above explain how these function.
· Do not apply a coating with high equilibrium water solubility onto one with low equilibrium water solubility.
· Use anchor adhesion (pretreatments such as zinc phosphate) or covalent bonding to the substrate if possible. Epoxy coatings may simply delaminate without blisters if the adhesion is not suitable under conditions that otherwise would form local, water-filled blisters..
· Avoid hydrophilic components within the coating or contamination at a substrate.
· If a coating has the same equilibrium water uptake at the different temperatures of its use, it will presumably not blister. Just how to create such a coating is not known to the authors, but the ceramics industry was able to create products that did not change dimension with temperature, so why should the coatings industry not be able to something similar?
The use of thicker coatings will delay the onset of blistering, all else being equal. This is not cost effective, and the problem is not solved, although the external conditions may change for the better if the delay is long enough. It is also conceivable that a particularly thick, water-saturated coating will not be able to lose water fast enough on a rapid cooling cycle, and blisters would then form near the substrate, since this is where the water content remains high for the longest time. Delaying blistering is also possible by increasing the diffusion resistance in the coating. A topcoat with low permeability over a primer with high water solubility will extend the “safe” period. The film will be able to restore a normal condition when the unfavorable water exposure is no longer present. A creative suggestion is to include suitable holes in the coating by a controlled mechanism. Excess water will be able to be accommodated in such cases, at least up to a given amount.
Blisters have been all too common in many coatings. Among the major causes are cold substrates below the dew point of moist and warmer air. This condition is common after cold nights in the Spring and in the Fall. A similar situation exists for cold water pipes in warm and moist climates, and special measures must be taken to improve adhesion. A rapid decrease in the temperature of essentially water-saturated polymers can also lead to excess water in the bulk of the polymer since equilibrium water solubility is generally lower at lower temperatures. Excess water is precipitated like fog in the films. This can collect into blisters in weaker films and even cause delamination in more rigid coatings. This is particularly problematic for repair coatings when a repair topcoat with high water equilibrium solubility is used on top of a primer or previous coating with lower equilibrium water solubility. The water in the primer cannot escape readily in the event of a rapid decrease in temperature since there is too much water to remove from the layer above it with the time allowed to avoid blisters.