There are volumes of info out there on different resins, here is one I found particularly interesting comparing epoxy vs polyester resins.
The Advantages of Epoxy Resin versus
Polyester in Marine Composite
S t r u c t u re s
Introduction
In any high-tech structural application, where strength, stiffness,
durability and light weight are required, epoxy resins are seen as the
minimum standard of performance for the matrix of the composite.
This is why in aircraft and aerospace applications, as well as
offshore racing boats, epoxies have been the norm for years.
However 95% of pleasure boats under 60 feet today are still made
with polyester resin. The main consideration for materials selection
for most composite builders is cost, with performance and more
importantly value for money often being a secondary consideration.
As a general rule epoxy resins are twice as expensive as vinyl ester
resins and vinyl ester resins are twice as expensive as polyesters.
Since the resin can constitute 40 to 50% of the weight of a
composite component, this price difference is seen as having a
significant impact on the cost of the laminate.
However, when considered against the cost of the whole structure
(the boat) the cost is relatively insignificant, and the value of higher
quality and long term gain of better durability (therefore better resale
value) can be tremendous.
What contributes to this better value…..?
Epoxy resins have performance advantages over polyester and vinyl
esters in five major areas:
■ Better adhesive properties (the ability to bond to the
reinforcement or core)
■ Superior mechanical properties (particularly strength and
stiffness)
■ Improved resistance to fatigue and micro cracking
■ Reduced degradation from water ingress (diminution of
properties due to water penetration)
■ Increased resistance to osmosis (surface degradation due to
water permeability)
Adhesive Properties
Epoxy resins have far better adhesive properties than polyester and
vinyl ester resins. However many times have you known a polyester
car body filler fall off a ding repair? The superior adhesion of epoxy
is due to two main reasons. The first is at the molecular level, where
the presence of polar hydroxyl and ether groups improves adhesion.
The second is at the physical level - as epoxies cure with low
shrinkage, the various surface contacts set up between the liquid
resin and the reinforcement are not disturbed during cure. The result
is a more homogenous bond between fibers and resin and a better
transfer of load between the different components of the matrix.
High adhesion is especially important in resistance to micro-cracking
(see later) and when using sandwich construction. The bond
between the core and the laminate is usually the weakest link of the
laminate, and the superior adhesive properties of the epoxy resin
greatly increase the strength of the interface between skins and
core.
Mechanical Properties
Two important mechanical properties of any resin systems are its
tensile strength and stiffness. The figure below shows results of
tests carried out on commercially available polyester, vinyl ester and
epoxy resin systems, either cured at room temperature or post
cured at 175°F.
After a cure period of seven days it can be seen that the tensile
strength of the epoxy resin is 20 to 30% higher than those of
polyester and vinyl ester. More importantly, after post cure the
difference becomes ever greater. It is to be noted that boats built
with polyester resins are rarely post cured in the workshop while
boats built with epoxy quite often are. However, in practice all boats
can quite often see “natural” post cures – particularly dark coloured
surfaces under a Caribbean sun!
The consequences are two fold:
Structurally
A post-cured epoxy laminate will exhibit tensile strength and
modulus (stiffness) close to double that of a non-post cured
polyester or vinyl ester laminate.
Cosmetically
Polyester and vinyl ester resins shrink up to 7% volumetrically and
because the resin continues to cure over long periods of time this
effect may not be immediately obvious. This cure accounts for the
print through effect observed on a lot of older polyester boats. In
comparison, epoxies shrink less than 2% and an epoxy laminate will
be a lot more stable and have better cosmetics over a long period
of time than a polyester one.
Comparative Tensile Strength of
Resins
Comparative Stiffness of ResinsFatigue Resistance and Micro-Cracking
In most cases a properly designed hull laminate will never be
subjected to its ultimate strength so physical properties of the resin
matrix, although important, are not the only criteria on which a
selection has to be made. Long before ultimate load is reached and
failure occurs, the laminate will reach a stress level where the resin
will begin to crack away from those fiber reinforcements not aligned
with the applied load. This is known as ‘transverse micro-cracking’
and although the laminate has not completely failed at this point, the
breakdown process has commenced.
The strain that a laminate can take before micro cracking depends
strongly on the toughness and adhesive properties of the resin
system. For relatively more brittle resin systems, such as many
polyesters, this point occurs a long way before laminate failure, and
so severely limits the strains to which such laminates can be
subjected. In an environment such as water or moist air, the microcracked laminate will absorb considerably more water than an
uncracked laminate. This will then lead to an increase in weight,
moisture attack on the resin and fiber sizing agents, loss of stiffness
and with time, an eventual drop in ultimate properties.
The superior ability to withstand cyclic loading is an essential
advantage of epoxies vs. polyester resins. This is one of the main
reason epoxies are chosen almost exclusively for aircraft structures.
Typical FRP Stress/Strain Graph Typical Resin Stress/Strain
Curves (Post-Cured for
5 hrs & 176°F)
Degradation from Water Penetration
An important property of any resin, particularly in a marine
environment, is its ability to withstand degradation from water
penetration. All resins will absorb some moisture, adding to a
laminate’s weight, but what is more significant is how the absorbed
water affects the resin and resin/fiber bond in a laminate, leading to
a gradual and long-term loss in mechanical properties.
Both polyester and vinyl ester resins are prone to water degradation
due to the presence of hydrolysable ester groups in their molecular
structures. As a result, a thin polyester laminate can be expected to
retain only 65% of its inter-laminar shear strength after immersion
over period of one year, whereas an epoxy laminate immersed for
the same period will retain around 90%.
Osmosis
All laminates in a marine
environment will permit very low
quantities of water to pass through
them in vapor form. As this water
passes through, it reacts with any
hydrolysable components inside
the laminate to form tiny cells of
concentrated solution. Under the
osmotic pressure generated, more
water is then drawn through the semi-permeable membrane
provided by the gelcoat in an attempt to dilute this solution. This
water increases the fluid pressure in the cell. Eventually the pressure
will distort or burst the gel coat, leading to a characteristic “chickenpox” surface.
To delay the onset of osmosis, it is necessary to use a resin that has
both a low water transmission rate and a high resistance to attack
by water. A polymer chain having epoxy linkages in its backbone is
substantially better than polyester or vinyl ester systems at resisting
