Osmotic Blisters in Coatings and Adhesives

Metal Finishing's adhesion and bonding expert discusses how blistering can be reduced or even eliminated by the proper selection of coating materials and substrate surface preparation.

Osmotic blistering is one of the most common failure modes for coatings in wet service environments, yet it is not a well understood failure mechanism. Osmotic blistering is all too familiar in certain applications, such as industrial marine coatings and linings and gel coats on boats and swimming pools. It can also affect adhesive bonds of decorative plastic shells to spas, hot tubs, and other applications where moisture is prevalent. Osmotic blistering is also often evident in coatings on concrete. Heavy organic coatings and liners are frequently thought to be barriers to the passage of moisture. But, in fact, all polymers are permeable to moisture and gases, to some extent. This seemingly small degree of moisture permeation and the soluble contaminants that can be carried with the moisture is the main cause for osmotic blistering.

PERMEATION AND OSMOSIS Osmotic blistering generally relates to thick polymeric coatings, thin films, or thin decorative laminates that are bonded to a more structural material. Osmotic blistering is often attributed to poor interfacial adhesion, but it is not so much an adhesion failure mechanism as a failure of the substrate or coating. The result can be devastating—chicken pox-like blisters on a surface that is generally prized for having a faultless appearance. The mechanism that causes osmotic blistering is the same no matter what the product. Osmotic blistering requires the following: (1) a permeable film; (2) a relatively impermeable substrate; (3) water dissolved solids; and (4) a concentration gradient. The concentration gradient is the driving force for the creation and expansion of a blister. Perhaps, the easiest way of understanding osmotic blistering is to consider one of its most notorious and destructive problems. Since the 1980s, blistering has been especially well recognized and understood by the manufacturers of marine boat hulls and suppliers of resins that go into aquatic products. This is due to the near-constant water contact in these applications. Using a boat hull as an example, the exterior gel coat is usually a polyester coating having properties different than that of the glass reinforced polyester laminated below it. The gel coat has pigment for color and gloss, UV resistance, abrasion resistance, and good moisture resistance, as it provides the outside surface that is next to the water. However, the gel coat does not provide a finite barrier to the passage of moisture. The mechanism contributing to the formation of boat hull blisters has been reported in the technical literature. In simple terms, what happens is water penetrates the outside coating both as vapor and as liquid water. All polymers are permeable to water to some degree due to the particularly small size of the H2O molecule. The rate of permeability will depend on the nature of the polymer, its barrier characteristics, its thickness, and temperature. Once the water has penetrated the outside coating, it then passes into the fiber-filled polyester laminate structure in similar fashion. However, once within the laminate structure, the glass fibers promote the permeation by acting as capillary tunnels to transport the water molecules further into the laminate. The permeated water will either: (1) degrade (hydrolyze) the polyester resin at a rate that depends on its molecular structure; and/or (2) go into chemical solution with water- soluble materials within the resin. These water-soluble materials include products in the formulation that have not fully reacted. Temperature will accelerate both the permeation and the degradation/solubilization processes. To varying degrees, these water-soluble materials are present in all cured polyester resins. Their presence does not necessarily result in a composite having low blister resistance. However, if they are concentrated beyond normal limits, or if they are concentrated in a given area, then blistering will result. On the other hand, potential hydrolysis products from an inferior resin in the backing composite could provide a constant supply source for osmotic pressure build-up. A driving force for water to enter the backing laminate is caused by the water solution in the resin having a greater concentration of water-soluble material than the water on the outside of the structure. One of the strongest drives in nature is to reach equilibrium, and this is what happens in this case. The water from the outside is “driven” into the areas that have a greater concentration of water soluble materials to dilute the solution. This is called an “osmotic force.”  In essence, the flow into the laminate exceeds the flow out, so that the fluid builds and the pressure increases to form a blister. With time, the blister grows larger and slowly starts separating the coating or outer layers of laminate from the bulk laminate causing a domed shape. As time goes on, the blister may break from the internal pressure and form a pinhole or crack in the exterior surface. The blistering of decorative gel coats on other glass-reinforced resin structures such swimming pools, bathroom appliances, and spas are a similar problem. All of these products utilize a two-part laminate: one part provides a functional and aesthetic appearance (a thin water-permeable layer) while the other part provides structural integrity. In hot tubs, blistering has been noticed even when the outer layer is a thin acrylic shell and not a gel coating. BLISTERING OF COATINGS ON METAL SUBSTRATES The same forces are at work to create blisters in coatings on metal substrates. Paint films are semi-permeable membranes that are permeable to water, but impermeable to dissolved solids. The driving force here is a water-soluble material at either the interface or within the interface. The water-soluble material that causes the osmotic gradient is generally either an inorganic salt, the products of corrosion, or retained solvents in the coating. Osmotic blistering has been related to chlorides, sulfates, and other inorganic solubles often found on substrates. These materials could result from environmental contamination or from depassivating salts such as chlorides and sulfates. Water extractable materials accidentally or deliberately entrained in the coating may also cause problems. Osmotic gradients may also arise from corrosion products such as Fe(OH)2. Where chlorides and sulfates are simultaneously present, the corrosion products are even more soluble in water. With certain coating binders the high pH generated at the cathode may hydrolyze the binder and this can be a source of solubles as well. Such blistering can also be due to hydrophilic solvents and other diluents being entrapped in the coating film. If these products are miscible with water, they could be drawn through the film osmotically. A greater concentration then occurs at the interface which needs to be compensated by an osmotic pressure, allowing more water through the film to reduce the concentration at the interface.

SUMMARY Blistering is one of the main causes of the failure of heavy industrial coatings and of decorative coatings on polyester laminates. When one sees blisters forming, the immediate thought is generally that this is either due to poor chemical resistance of the coating or to poor adhesion at the interface. However, the driving mechanism is the solubilization of materials, water diffusing into the coatings to equilibrate the concentration, and then water collecting at the interface to initiate and grow delamination. Blistering can generally be reduced or eliminated by the proper selection of coatings materials and substrate surface preparation. Efforts also need to be made to choose materials or to provide processing methods that do not leave water soluble components. One can also choose coatings that are good barriers to the transfer of moisture. This is usually done by the proper selection of polymeric resin binder (greater hydrophobicity and higher crosslink density). This is why epoxy resins are a major source of industrial coatings. Coatings and adhesive based on vinyl esters rather than more conventional polyester have also been associated with the near elimination of osmotic blistering in boat hulls. Barrier properties can also be enhanced by the addition of insoluble pigments in coatings or adhesives that provide a more tortuous path for the moisture to transverse. For an excellent and much more detailed discussion of osmotic blistering of paint films on metal, the reader is directed to a comprehensive article (Reference 4). For additional reading regarding osmotic blistering of coated polyester composites, the reader is referred to Reference 1. In the next article in this series, we will discuss how moisture ingress into an adhesive, coating, or other polymeric material can be measured.

BIO Edward M. Petrie is the sole proprietor of EMP Solutions, a Cary, N.C.–based consulting firm focused on solving problems in the adhesives and sealants industry. He also works as a technical expert for SpecialChem. For more information, visit www.specialchem4adhesives.com. REFERENCES

  1. Antonio, S.C., “Fiberglass Blisters and Barrier Coatings”, Ocean Navigation, No. 136, March/April 2004, pp. 38.
  2. Abeysinghe, H.P., et. al., “Substances Contributing to the Generation of Osmotic Pressure in Resins and Laminates”, Composites, January 1983, pp. 57-61.
  3. Rockett, T.J., et. al., “Method of Preventing Gel Coat Blistering in Fiber Glass Reinforced Polymers”, US Patent 4,724,173, February 9, 1988.
  4. Hare, C.H., “Blistering of Paint Films on Metal, Part 1: Osmotic Blistering”, Journal of Protective Coatings and Liners, February 1998, pp. 45-63.