Phenolic resins offer properties like high mechanical strength, long-term thermal and mechanical stability, excellent fire resistance and low smoke and toxicity. They exhibit a glass transition temperature above 150°C. Phenolic resins are mainly used in wood materials, insulating products, and as binders in the building and automotive industries. They are also used in applications that require high thermal stability like friction linings, carbon-carbon composites, and as ablative materials in re-entry vehicles.

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Thermoset Resins
2.1.6 Properties of Phenolic Resins
Phenolic resins offer many useful properties such as high mechanical strength, long-
term thermal and mechanical stability, excellent fire, smoke and toxicity characteristics,
and excellent thermal insulating capability. Because of its aromatic structure, phenolic
resin offers flame- and smoke-resistance. Smoke density (ASTM E662) of phenolic
resin is 16 compared with 480-515 for epoxy and 530 for vinyl ester. The cured resins
exhibit a glass transition temperature (Tg
) of >150°C. Phenolic resins are widely used
as moulding compounds. The thermomechanical properties of commercial phenolic
moulding compounds are presented in Table 2.3 [14].
Table 2.3 Mechanical properties of phenolic moulding compounds [14]
Moulding
compound
Tensile
strength
(MPa)
Tensile
modulus
(GPa)
Strain
at break
(%)
Flexural
strength
(MPa)
Compressive
strength
(MPa)
RX-630 82.7 13.8 0.6 193 275.8
RX-660 55.2 11.7 0.5 137.9 227.5
RX-865 68.9 17.2 0.4 186 241.3
XB-22 75.8 7.6 1.0 151.7 234.4
2.1.7 Applications of Phenolic Resins
Phenolic resins are mainly used in wood materials and insulating products. Such
materials are mostly used in the building and automotive industries. In wood
composites, about 10% binder resin is used which is mostly the aqueous solution of
resole type phenolic resins. The driving force for such application is that phenolic
resins can offer durable bonding that is resistant to heat and moisture. Phenolic-
bonded wood materials exhibit good resistance to moisture and other environmental
effects, and retain these properties even after lengthy exposure. Acoustic and thermal
insulating materials should retain a constant level of functional properties over long
periods of time. For example, materials used as insulating materials and in automobile
constructions should provide a load-bearing and/noise damping function throughout
the life of the product or vehicle. Phenolic resin-bonded textile mats are used for

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Chemistry, Properties and Applications of Thermoset Resins
such purposes. The other applications are moulding compounds, foundry/refractory
materials, abrasives, and friction linings. Phenolic resin moulding compounds continue
to be in electrical and household appliance areas. One of the oldest fields of application
for phenolic resin-glass fabric prepregs is the production of fishing rods [15].
The resin content in abrasive and friction lining materials is about 10%, whereas
binder content is much less (approximately 2%) in foundry materials. For friction
lining applications (e.g., automobile braking operation), the surface temperature of
the lining may increase up to 800 °C for a short time due to tremendous friction.
Phenolic resins are most suitable for such applications because of their high thermal
stability.
Phenolic resin is soluble in low boiling alcohol, impregnated papers, and fabric. It can
be made easily by soaking the fabric in resin solution followed by drying and curing.
Such impregnated papers are used as separators in lead–acid storage batteries and fuel
cells to separate positive and negative electrodes. Phenolic resin as an impregnation
material offers adequate insulation, high resistance to acids, oxidation and mechanical
strength. Because of inherent fire-retardant properties, they are used to make heat-
and fire-resistant panels.
Honeycomb structures and paper honeycombs also represent an application for
phenolic resins as impregnating and dipping agents. Honeycomb structures are very
effective for development of lightweight structures. Though epoxy and vinyl ester
resins are used for such applications, phenolic is also preferred due to its inherent
fire-resistant property and low cost. Honeycomb structures were initially intended for
only high-tech applications such as the flooring and interior paneling of aircrafts, but
such structures find application even in passenger and freight train cars to reduce the
weight penalty. Sliding doors for subways and ship buildings are other applications.
The panels must be coated with suitable flame-retardant materials to reach the required
fire protection standard.
Braking straps and friction pads are used in power shift transmissions, ship winch
transmission, and automobile clutches. Special types of papers impregnated with
special phenolic resins are used for this application. Adequate adhesion of the paper
to the metal parts is required to satisfy the parameters required for friction lining
compounds. In general, phenolic resins in combination with epoxy resins are used to
develop a wide range of friction lining materials. Another application of impregnated
fabric is in the foundry industry (e.g., to filter aluminium melts). For the preparation
of such impregnated fabric, woven glass fabrics is passed through an impregnating
bath containing an aqueous solution of resole, dried and finally cured in stages up
to 300 °C.

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Thermoset Resins
Because of their high aromatic content, phenolic resins absorb a lot of heat for their
degradation. This is why they are used as an ablative material in re-entry vehicles.
When a space vehicle re-enters the earth’s atmosphere, due to tremendous friction
with air, a lot of heat is generated. Phenolic resins can absorb the heat at the cost of
its degradation, and stops the vehicle from deteriorating as a result of the extremely
high temperature.
Phenolic resins are used as an alternative to conventional precursor (pitch, tar)
materials for the development and production of carbon-based engineering
materials (e.g., glassy carbon, carbon fibres, carbon–carbon composites [16–18]).
This application is related to the high yield of carbon (60–70%) produced as a
result of pyrolysis. Glassy carbon, which is prepared by pyrolyzing phenolic resin
at about 600 °C at a heating rate of 2–5 °C per hour, exhibits distinct properties
such as thermal resistance up to 3000 °C, very low density, alkali resistance, gas
permeability and biocompatibility. Because of high thermal stability, carbon-
based engineering materials are not only used for conventional applications like
electrode materials, slip ring seals and bearing materials, but also in nuclear
reactors, missiles and rocket engineering. Carbon–carbon composites are made by
curing the carbon fibre/phenolic prepreg followed by carbonisation at 500–1000
°C by heat/pressure and densification by chemical vapour deposition or chemical
vapour infiltration. During carbonisation, the precursor is converted to a porous
carbon fibre matrix. The unique feature of carbon–carbon composite is that the
matrix and reinforcement phases are carbon. The carbon–carbon composites are
suitable if high temperature strength and high strength to weight ratio are of
utmost importance. They can work at very high temperature (2200 °C) as well
as at subzero temperatures. Another important property that carbon–carbon
composites exhibit is very low high temperature creep. On reentry of the space
shuttle, the nose cone and leading edges of the shuttle experience hot gases rushing
through the nozzle/cone at high velocity, stressing and eroding the nozzle wall.
Carbon–carbon composites can resist high temperature erosion without burning
away. Most carbon–carbon composites are used in commercial and military aircraft
brakes, and in rockets and missiles.
Phenolic resins exhibit excellent dimensional stability with a constant use temperature
range of 180–200 °C, excellent resistance to chemicals, moisture, and heat; and
favorable behaviour against fire and smoke. The predominant consideration
in the use of phenolic resins as a matrix resin in fibre-reinforced composites is
fire behaviour. Phenolic-based composites perform better under fire conditions
compared with epoxy- or vinyl ester-based composites. This is due to their delayed
ignitability coupled with low heat release, low smoke evolution with little or no
toxic gas emission, and capability to provide significant strength retention (70%)