A composite consists of a resin and a reinforcement. This week
we look at the characteristics of different resins and then next week at the
Any resin system for use in a composite material will require
the following properties:
1. Good mechanical properties
2. Good adhesive properties
3. Good toughness properties
4. Good resistance to environmental degradation
This diagram shows the stress / strain curve for an ‘ideal’
resin system. The curve for this resin shows high ultimate strength, high
stiffness (indicated by the initial gradient) and a high strain to failure. This
means that the resin is initially stiff but at the same time will not suffer
from brittle failure.
It should also be noted that when a composite is loaded in
tension, for the full mechanical properties of the fibre component to be
achieved, the resin must be able to deform to at least the same extent as the
fibre. This diagram gives the strain to failure for Eglass, S-glass, aramid
(Kevlar) and high-strength grade carbon fibres on their own (i.e. not in a
composite form). Here it can be seen that, for example, the S-glass fibre, with
an elongation to break of 5.3%, will require a resin with an elongation to break
of at least this value to achieve maximum tensile properties.
2 Adhesive Properties
High adhesion between resin and reinforcement fibres is
necessary for any resin system. This will ensure that the loads are transferred
efficiently and will prevent cracking or fibre / resin debonding when stressed.
3 Toughness Properties
Toughness is a measure of a material’s resistance to crack
propagation, but in a composite this can be hard to measure accurately. However,
the stress / strain curve of the resin system on its own provides some
indication of the material’s toughness. Generally the more deformation the resin
will accept before failure, the tougher and more crack-resistant the material
will be. Conversely, a resin system with a low strain to failure will tend to
create a brittle composite, which cracks easily. It is important to match this
property to the elongation of the fibre reinforcement.
4 Environmental Properties
Good resistance to the environment, water and other aggressive
substances, together with an ability to withstand constant stress cycling, are
properties essential to any resin system. These properties are particularly
important for use in a marine environment.
The resins that are used in fibre reinforced composites are
sometimes referred to as ‘polymers’. All polymers exhibit an important common
property in that they are composed of long chain-like molecules consisting of
many simple repeating units. Manmade polymers are generally called ‘synthetic
resins’ or simply ‘resins’.
Polymers can be classified under two types, ‘thermoplastic’ and
‘thermosetting’, according to the effect of heat on their properties.
Thermoplastics, like metals, soften with heating and eventually melt, hardening
again with cooling. This process of crossing the softening or melting point on
the temperature scale can be repeated as often as desired without any
appreciable effect on the material properties in either state. Typical
thermoplastics include nylon, polypropylene and ABS, and these can be
reinforced, although usually only with short, chopped fibres such as glass.
Thermosetting materials, or ‘thermosets’, are formed from a
chemical reaction in situ, where the resin and hardener or resin and catalyst
are mixed and then undergo a nonreversible chemical reaction to form a hard,
infusible product. Once cured, thermosets will not become liquid again if
heated, although above a certain temperature their mechanical properties will
change significantly. This temperature is known as the Glass Transition
Temperature (Tg), and varies widely according to the particular resin system
used, its degree of cure and whether it was mixed correctly. Above the Tg, the
molecular structure of the thermoset changes from that of a rigid crystalline
polymer to a more flexible, amorphous polymer. This change is reversible on
cooling back below the Tg. Above the Tg properties such as resin modulus
(stiffness) drop sharply, and as a result the compressive and shear strength of
the composite does too. Other properties such as water resistance and colour
stability also reduce markedly above the resin’s Tg.
Although there are many different types of resin in use in the
composite industry, the majority of structural parts are made with three main
types, namely polyester, vinylester and epoxy.
1 Polyester Resins
Polyester resins are the most widely used resin systems,
particularly in the marine industry. By far the majority of dinghies, yachts and
work-boats built in composites make use of this resin system.
Polyester resins such as these are of the ‘unsaturated’ type.
Unsaturated polyester resin is a thermoset, capable of being cured from a liquid
or solid state when subject to the right conditions.
Most polyester resins are viscous, pale coloured liquids
consisting of a solution of a polyester in a monomer which is usually styrene.
The addition of styrene in amounts of up to 50% helps to make the resin easier
to handle by reducing its viscosity. The styrene also performs the vital
function of enabling the resin to cure from a liquid to a solid by
‘cross-linking’ the molecular chains of the polyester, without the evolution of
any by-products. These resins can therefore be moulded without the use of
pressure and are called ‘contact’ or ‘low pressure’ resins. Polyester resins
have a limited storage life as they will set or ‘gel’ on their own over a long
period of time. Often small quantities of inhibitor are added during the resin
manufacture to slow this gelling action. For use in moulding, a polyester resin
requires the addition of several ancillary products.
These products are generally:
o Chemical/fire resistance
A manufacturer may supply the resin in its basic form or with
any of the above additives already included. Resins can be formulated to the
moulder’s requirements ready simply for the addition of the catalyst prior to
moulding. As has been mentioned, given enough time an unsaturated polyester
resin will set by itself. This rate of polymerisation is too slow for practical
purposes and therefore catalysts and accelerators are used to achieve the
polymerisation of the resin within a practical time period. Catalysts are added
to the resin system shortly before use to initiate the polymerisation.
The catalyst does not take part in the chemical reaction but
simply activates the process. An accelerator is added to the catalysed resin to
enable the reaction to proceed at a cold workshop temperature and/or at a
greater rate. Since in the absence of a catalyst, accelerators have little
influence on the resin, they are sometimes added to the resin by the polyester
manufacturer to create a ‘pre-accelerated’ resin.
Great care is needed in the preparation of the resin mix prior
to moulding. The resin and any additives must be carefully stirred to disperse
all the components evenly before the catalyst is added. This stirring must be
thorough and careful as any air introduced into the resin mix affects the
quality of the final moulding. This is especially so when laminating with layers
of reinforcing materials as air bubbles can be formed within the resultant
laminate which can weaken the structure. It is also important to add the
accelerator and catalyst in carefully measured amounts to control the
polymerisation reaction to give the best material properties. Too much catalyst
will cause too rapid a gelation time, whereas too little catalyst will result in
Colouring of the resin mix can be carried out with pigments.
The choice of a suitable pigment material, even though only added at about 3%
resin weight, must be carefully considered as it is easy to affect the curing
reaction and degrade the final laminate by use of unsuitable pigments.
Filler materials are used extensively with polyester resins for
a variety of reasons including:
1 To reduce the cost of the moulding
2 To facilitate the moulding process
3 To impart specific properties to the moulding
Fillers are often added in quantities up to 50% of the resin
weight although such addition levels will affect the flexural and tensile
strength of the laminate. The use of fillers can be beneficial in the laminating
or casting of thick components where otherwise considerable exothermic heating
can occur. Addition of certain fillers can also contribute to increasing the
fire-resistance of the laminate.
2 Vinylester Resins
Vinylester resins are similar in their molecular structure to
polyesters, but differ primarily in the location of their reactive sites, these
being positioned only at the ends of the molecular chains. As the whole length
of the molecular chain is available to absorb shock loadings this makes
vinylester resins tougher and more resilient than polyesters. The vinylester
molecule also features fewer ester groups. These ester groups are susceptible to
water degradation by hydrolysis which means that vinylesters exhibit better
resistance to water and many other chemicals than their polyester counterparts,
and are frequently found in applications such as pipelines and chemical storage
3 Epoxy Resins
The large family of epoxy resins represent some of the highest
performance resins of those available at this time. Epoxies generally
out-perform most other resin types in terms of mechanical properties and
resistance to environmental degradation, which leads to their almost exclusive
use in aircraft components. As a laminating resin their increased adhesive
properties and resistance to water degradation make these resins ideal for use
in applications such as boat building. Here epoxies are widely used as a primary
construction material for high-performance boats or as a secondary application
to sheath a hull or replace water-degraded polyester resins and gel coats.
Usually identifiable by their characteristic amber or brown
colouring, epoxy resins have a number of useful properties. Both the liquid
resin and the curing agents form low viscosity easily processed systems. Epoxy
resins are easily and quickly cured at any temperature from 5 degrees C to 150
degrees C, depending on the choice of curing agent.
One of the most advantageous properties of epoxies is their low
shrinkage during cure which minimises fabric ‘print-through’ and internal
stresses. High adhesive strength and high mechanical properties are also
enhanced by high electrical insulation and good chemical resistance.
Gelation, Curing and Post-Curing
On addition of the catalyst or hardener, a resin will begin to
become more viscous until it reaches a state when it is no longer a liquid and
has lost its ability to flow. This is the ‘gel point’. The resin will continue
to harden after it has gelled, until, at some time later, it has obtained its
full hardness and properties. This reaction itself is accompanied by the
generation of exothermic heat, which, in turn, speeds the reaction. The whole
process is known as the ‘curing’ of the resin.
The speed of cure is controlled by the amount of accelerator in
a polyester or vinylester resin and by varying the type, not the quantity, of
hardener in an epoxy resin. Generally polyester resins produce a more severe
exotherm and a faster development of initial mechanical properties than epoxies
of a similar working time. With both resin types, however, it is possible to
accelerate the cure by the application of heat, so that the higher the
temperature the faster the final hardening will occur. This can be most useful
when the cure would otherwise take several hours or even days at room
temperature. A quick rule of thumb for the accelerating effect of heat on a
resin is that a 10 degrees C increase in temperature will roughly double the
reaction rate. Therefore if a resin gels in a laminate in 25 minutes at 20
degrees C it will gel in about 12 minutes at 30 degrees C, providing that no
extra exotherm occurs.
Curing at elevated temperatures has the added advantage that it
actually increases the end mechanical properties of the material, and many resin
systems will not reach their ultimate mechanical properties unless the resin is
given this ‘postcure’. The postcure involves increasing the laminate temperature
after the initial room temperature cure, which increases the amount of
crosslinking of the molecules that can take place. To some degree this postcure
will occur naturally at warm room temperatures, but higher properties and
shorter postcure times will be obtained if elevated temperatures are used. This
is particularly true of the material’s softening point or Glass Transition
Temperature (Tg), which, up to a point, increases with increasing postcure
Comparison of Resin Properties
The choice of a resin system for use in any component depends
on a number of its characteristics, with the following probably being the most
important for most composite structures:
Degradation from water ingress
It has already been discussed how the adhesive properties of
the resin system are important in realising the full mechanical properties of a
composite. The adhesion of the resin matrix to the fibre reinforcement or to a
core material in a sandwich construction are important. Polyester resins
generally have the lowest adhesive properties of the three systems described
here. Vinylester resin shows improved adhesive properties over polyester but
epoxy systems offer the best performance of all, and are therefore frequently
found in many high-strength adhesives. As epoxies cure with low shrinkage, the
various surface contacts set up between the liquid resin and the adherends are
not disturbed during the cure. The adhesive properties of epoxy are especially
useful in the construction of honeycomb-cored laminates where the small bonding
surface area means that maximum adhesion is required. The strength of the bond
between resin and fibre is not solely dependent on the adhesive properties of
the resin system but is also affected by the surface coating on the
Two important mechanical properties of any resin system are its
tensile strength and stiffness. These diagrams show results for tests carried
out on commercially available polyester, vinylester and epoxy resin systems
cured at 20 degrees C and 80 degrees C.
After a cure period of seven days at room temperature, it can
be seen that a typical epoxy will have higher properties than a typical
polyester and vinylester for both strength and stiffness. The beneficial effect
of a post cure at 80 degrees C for five hours can also be seen.
Also of importance to the composite designer and builder is the
amount of shrinkage that occurs in a resin during and following its cure period.
Polyester and vinylesters can show shrinkage of up to 8%. The typical shrinkage
of an epoxy is only around 2%. The absence of shrinkage is, in part, responsible
for the improved mechanical properties of epoxies over polyester, as shrinkage
is associated with built-in stresses that can weaken the material. Furthermore,
shrinkage through the thickness of a laminate leads to ‘print-through’ of the
pattern of the reinforcing fibres, a cosmetic defect that is difficult and
expensive to eliminate.
The strength of a laminate is usually thought of in terms of
how much load it can withstand before it suffers complete failure. This ultimate
or breaking strength is the point it which the resin exhibits catastrophic
breakdown and the fibre reinforcements break.
However, before this ultimate strength is achieved, the
laminate will reach a stress level where the resin will begin to crack away from
those fibre reinforcements not aligned with the applied load, and these cracks
will spread through the resin matrix. This is known as ‘transverse
micro-cracking’ and, although the laminate has not completely failed at this
point, the breakdown process has commenced. Consequently, engineers who want a
long-lasting structure must ensure that their laminates do not exceed this point
under regular service loads.
The strain that a laminate can reach before microcracking
depends strongly on the toughness and adhesive properties of the resin system.
For brittle resin systems, such as most polyesters, this point occurs a long
way before laminate failure, and so severely limits the strains to which
such laminates can be subjected. As an example, recent tests have shown that for
a polyester/glass woven roving laminate, micro-cracking typically occurs at
about 0.2% strain with ultimate failure not occurring until 2.0% strain. This
equates to a usable strength of only 10% of the ultimate strength.
As the ultimate strength of a laminate in tension is governed
by the strength of the fibres, these resin micro-cracks do not immediately
reduce the ultimate properties of the laminate. However, in an environment such
as water or moist air, the micro-cracked 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 fibre sizing agents, loss of stiffness and,
with time, an eventual drop in ultimate properties.
Increased resin/fibre adhesion is generally derived from both
the resin’s chemistry and its compatibility with the chemical surface treatments
applied to fibres. Here the well-known adhesive properties of epoxy help
laminates achieve higher microcracking strains. As has been mentioned
previously, resin toughness can be hard to measure, but is broadly indicated by
its ultimate strain to failure. A comparison between various resin systems is
shown in this diagram.
Generally composites show excellent fatigue resistance when
compared with most metals. However, since fatigue failure tends to result from
the gradual accumulation of small amounts of damage, the fatigue behaviour of
any composite will be influenced by the toughness of the resin, its resistance
to microcracking, and the quantity of voids and other defects which occur during
manufacture. As a result, epoxy based laminates tend to show very good fatigue
resistance when compared with both polyester and vinylester, this being one of
the main reasons for their use in aircraft structures.
Degradation from Water Ingress
An important property of any resin, particularly in a marine
environment, is its ability to withstand degradation from water ingress. 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/fibre
bond in a laminate, leading to a gradual and long-term loss in mechanical
properties. Both polyester and vinylester resins are prone to water degradation
- a thin polyester laminate can be expected to retain only 65% of its
inter-laminar shear strength after immersion in water for a period of one year,
whereas an epoxy laminate immersed for the same period will retain around
This diagram demonstrates the effects of water on an epoxy and
polyester woven glass laminate, which have been subjected to a water soak at 100
degrees C. This elevated temperature soaking gives accelerated degradation
properties for the immersed laminate.
Resin Comparison Summary
- Easy to use
- Lowest cost
- Only moderate mechanical properties
- High styrene emissions in open moulds
- High cure shrinkage
Limited range of working time
- Very high chemical/environmental resistance
- Higher mechanical properties than polyesters
- Postcure generally required for highest properties
- High styrene content
- Higher cost than polyesters
- High cure shrinkage
- High mechanical and thermal properties
- High water resistance
- Long working times available
- Temperature resistance can be up to 140 degrees C wet / 220 degrees C
- Low cure shrinkage
- More expensive than vinylesters
- Critical mixing
- Corrosive handling
Other Resin Systems used in Composites
Besides polyesters, vinylesters and epoxies, there are a number
of other specialised resin systems that are used where their unique properties
Primarily used where high fire-resistance is required,
phenolics also retain their properties well at elevated temperatures. For
room-temperature curing materials, corrosive acids are used which leads to
unpleasant handling. The condensation nature of their curing process tends to
lead to the inclusion of many voids and surface defects, and the resins tend to
be brittle and do not have high mechanical properties.
2 Cyanate Esters
Primarily used in the aerospace industry. The material’s
excellent dielectric properties make it very suitable for use with low
dielectric fibres such as quartz for the manufacture of radomes. The material
also has temperature stability up to around 200°C wet. Typical costs: £40/kg. GTC-1a-1098 - 21
Synthetic resin using silicon as the backbone rather than the
carbon of organic polymers. Good fire-resistant properties, and able to
withstand elevated temperatures. High temperature cures needed. Used in missile
applications. Typical costs: >£15/ kg.
High toughness materials, sometimes hybridised with other
resins, due to relatively low laminate mechanical properties in compression.
Uses harmful isocyanates as curing agent.
5 Bismaleimides (BMI)
Primarily used in aircraft composites where operation at higher
temperatures (230 degrees C wet/250 degrees C dry) is required. e.g. engine
inlets, high speed aircraft flight surfaces.
Used where operation at higher temperatures than bismaleimides
can stand is required (use up to 250 degrees C wet/300 degrees C dry). Typical
applications include missile and aero-engine components. Extremely expensive
resin which uses toxic raw materials in its manufacture. Polyimides also tend to
be hard to process due to their condensation reaction emitting water during
cure, and are relatively brittle when cured. PMR15 and LaRC160 are two of the
most commonly used polyimides for composites.
Next week we’ll look at the different reinforcing fibres