A fiberglass boat is a laminated structure consisting of layers of various reinforcing fabrics and core materials, typically bonded together with a polyester resin. Gelcoat blistering, a frequent problems with fiberglass boats, occurs when moisture is allowed between the gelcoat and the laminate, or within the laminate itself moisture can enter the laminate from the inside or outside of the hull, passing through the gelcoat or via cracks or other flaws. The moisture reacts with water soluble materials remaining within the laminate from the curing process. The result is a pocket containing acids and under pressure a gelcoat blister.
Gelcoat blistering has affected many fiberglass hulls. Damage may range from a few large isolated blisters to an entire hull peppered with thousands of imperfections. On the basis of a 1990 survey, PRACTICAL SAILOR suggested that one in four boats can be expected to blister in its lifetime. Research is underway to investigate the causes of blistering but relatively little information is available to the boat owner about how to repair and prevent blistering.
Typical Fiberglass Boat Construction
A fiberglass boat is a composite structure, made of many layers of various reinforcing fabrics and core materials, typically bonded together with a polyester resin. There are as many lay-up schedules as there are boats. A typical hull section might consist of a layer of polyester gelcoat, several alternating layers of mat and woven roving, and in many cases a core material such as end-grain balsa or foam, followed by several more alternating layers of mat and woven . Gelcoats may be anywhere from 12 to 22 mils thick. They start as pigmented, unsaturated polyester resin and are designed to act as a moisture barrier for the underlying laminate, as well as to provide a smooth, glossy cosmetic finish. Generally, production fiberglass boats are built in a female mold. A release agent is first applied to the surface of the mold, over which the gelcoat material is applied. Subsequent layers of the laminate are laid up over the gelcoat. Hull thickness may vary from boat to boat. Older boats were often laid up with a solid glass laminate hull thickness of 1 1/2" (3.8cm) to as much as 5" (12.7cm) in the keel areas of the more heavily-built boats. Today, however, the trend is toward thinner, lighter laminates, a fact that makes the structural integrity of each of the laminate components all the more critical. Modern unsaturated polyesters used in boat construction are made up of three basic components: glycol, organic acid and reactive diluents (usually styrene). If you were to look at uncured polyester resin at a molecular level, you would see what appear to be thousands of chains made up of alternating glycol and acid units. Adding a peroxide catalyst, typically MEKP, to the polyester resin mixture initiates a cross linking reaction, in effect, creating bridges which link adjacent chains together. As the mixture cures, more and more bridges are established, and the free-moving glycol acid chains begin to gel, becoming a solid mass. Eventually, enough bridges are built to form a rigid, three-dimensional grid-the mixture has become a solid thermoset plastic.
Factors Affecting Blister Formation
As a building material, the unsaturated polyesters used in fiberglass construction seem to be a logical choice. They offer relative ease of handling, reasonable cost, and, what appears to be, an acceptable working lifetime. Unfortunately, there are other important characteristics that we now know are working against the polyester structures which lead to problems like blistering and delimitation. Many variables affect the formation of blisters including the formulation of the resin for specific applications, manufacturing quality assurance, and the environment the boat lives in. The chemical stability of the polymer and the permeability of the matrix are the key items affecting the durability of the fiberglass hull. The common thread is the ease with which moisture can enter the laminate and alter the chemistry of the resin matrix.
First, the unsaturated polyesters used in laminating resins and gelcoats are not waterproof. In fact, they are quite permeable and will allow water to migrate through the cured resin at a consistent, predictable rate. The permeability of polymer matrices involves a number of factors.
Thickness of the gelcoat membrane is an important element in diffusion of water through the membrane all the physical laws of diffusion use thickness as a variable). A thick coating of a stable polymer is a very capable moisture barrier, while a thick layer of an unstable polymer contains more of the elements that will facilitate its own breakdown.
The warmer the ambient temperature, the higher the rate of permeation. An increase in temperature can boost the rate of permeation through an unstable matrix by intensifying the molecular motion of both the polyester and the water. This means boats in the Caribbean are more likely to have problems than ones in Lake Superior.
A primary consideration is the distribution of free volume (voids) in the matrix. In any polymer, the free volume can be everything from the gaps (measured in angstroms) between and within the molecules to manufacturing artifacts such as entrapped air bubbles, cracks or dry fabric. The solidification (cure) rate, degree of cross-linking and the crystallinity variation of the cured matrix all affect this void distribution while also contributing to the overall chemical stability of the finished laminate. As a gathering point for migrating water molecules, voids provide ideal conditions for hydrolysis to take place and the initiation site for blister formation. It's common to find a high number of voids between the gelcoat and the laminate as a result of manufacturing practices and also find a high percentage of blisters occurring in this layer.
A polymer matrix of polyester resin may decompose when exposed to water and conditions are right; this chemical reaction is called hydrolysis. The presence of water in concert with resident unreacted resin components of the laminate can provide an acidic condition which can cause a breakdown of susceptible ester linkages which comprise the majority of bonds in polyester polymers. As the resin breaks down, many contaminants may be present in a heavy solution containing water, unreacted glycols, metallic promoters and acids. Very simply, this aqueous solution trapped between the laminates, or between the laminate and the gelcoat, contains chemicals which are primarily large molecules. Your boat, meanwhile, is sitting in relatively clean water; a molecule of water is small. The gelcoat thus becomes a semi-permeable membrane which allows the small water molecules to pass through without allowing the contaminated solutes to pass out. Water passing through the gelcoat into voids and resin-starved pockets helps break down more of the unsaturated polyester molecular chains, which in time allows more water to pass into the laminate. This one-sided movement of water into the laminate is known as osmosis; the water molecules move from an area of greater concentration into an area of lesser concentration. Research has shown that osmotic forces increase in direct proportion to the concentration of solutes (excess glycols, acids, metals, etc.) in the water within the laminate. The process, in effect, feeds on itself, creating hydraulic pressure between the gelcoat and laminate, or the laminates themselves. The result is a gelcoat blister. Temperature increases can cause increased sensitivity to degradation of the ester linkages in the polyester, additional swelling of the matrix and can accelerate hydrolysis where it is already occurring in the laminate. Another factor in the water transport mechanism across gelcoat membranes is surface oxidation. Sunlight with its UV degradation potential takes the gloss from gelcoats and destroys some of their water-exclusion capability.
Formulation Variables Influencing Blister Formation
A large number of formulation variables influence the susceptibility or resistance of cured polyester laminates to degradation and blistering. Many different types and combinations of glycols, acids and reactive diluents can be used by the resin manufacturer when developing a formulation. Each ingredient alters the basic physical characteristics of the cured resin, including hydrolytic stability, strength and elongation. The mixing process can also have an impact if it leaves improperly mixed and unreacted glycols trapped in the resin after cure. The use of a particular unsaturated polyester type and the choice of promoter and catalyst can act as blister initiators in poorly mixed or incompletely reacted matrices. Theoretically, a wide variety of additives (air-release agents, leveling additives, UV-resistant additives, surfactants, abrasion-resistant additives, fire retardants, anti-oxidants and co-monomers) have the potential to affect blister resistance in the cured laminate. Thixotropic agents, hydrophilic fillers, pigments, color paste vehicles, and the use of solvents as diluents can change the sensitivity to moisture and aid in the formation of blisters. The inclusion of any moisture sensitive materials could stimulate hydrolysis of the matrix materials and promote the osmotic pressure which causes blisters. With any particular formulation, the polarity of the polymer can affect how freely water will pass through the matrix by assisting or hindering the hydrogen bonding of the water molecule.
Post Construction Factors
Poor quality manufacturing practices, material limitations and the rigors of the boating environment can have an adverse effect on the interfacial adhesion between the polyester and fiber reinforcement in the laminate. In addition to poor wet-out during fabrication, high stress or strain in the laminate during use can cause a loss of adhesion or initiate micro-cracking at the interface. Micro-bubbles and multiphase interfaces within the matrix (due to different cure and shrinkage rates) are all points of stress concentration and, as such, are areas vulnerable to loss of adhesion or cohesion. The resulting voids promote water migration, leading to hydrolysis and the concentration of any soluble materials in the laminate. With all these variables, it is probably impossible to define a laminate schedule, polyester matrix formulation and manufacturing plan that is impervious to attack.
Source: West Systems "Gelcoat Blister Diagnosis, Repair & Prevention"