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.
Permeability
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.
Gecoat Thickness
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.
Temperature
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.
Voids
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.
Hydrolysis
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"