25
1
liquid/moisture under a specific temperature. Composite materials
are porous and have breathability. In respect to composite materials, the absorption of only small amounts of liquid may have a significant effect on their performance. The composites get saturated if
they are exposed to a liquid for an extended period. The absorbed
moisture plasticises and swells the matrix, resulting in a reduction
in mechanical properties, glass transition temperature, and service
temperature. Hygrothermal ageing deteriorates physico-chemical
and thermomechanical properties of the composite over time.
Moisture gets absorbed onto the composite via surface defects (such
as crazing, cracks, and voids) in the matrix and diffuses into the
bulk matrix causing swelling and plasticization. This results in
swelling stresses, causing the matrix to crack or the fibre/matrix to
debond. Fibre reinforcement is also adversely affected by moisture
due to leading effect that causes erosion of the coupling agent or
sizing on the fibre surface [20]. For example, water molecules penetrating glass fibre via surface microcracks result in the degradation
of the surface treatment on glass fibres, leading to the loss of
mechanical properties of polymeric composite.
1.7.5 Chemical and UV Resistance
The chemical resistance of polymeric composites is primarily dependent on the matrix. The role played by the reinforcement is minimal
and to the extent that it only allows the ingression of chemicals
slowly due to the physical resistance offered by them. In the case of
alkali, the glass gets corroded by alkalis; therefore, glass- reinforced
composites get corroded by alkalis. Polymeric composites can be
designed for excellent corrosion resistance (i.e. resistance to specific
hostile environments). The selection of a matrix that is resistant to a
specific environment plays an important role to ensure the composite’s optimum performance. The selection of reinforcing fibres is the
next important thing to make composites resistant to chemicals/
UV. The other influencing factors for chemicals/UV resistance of
composites are the moulding process, service environment, and
expected service life. Once operating and service parameters are
established, the appropriate materials can be selected to design the
composite [21]. Typical chemical and UV resistance of unidirectional (UD) polymeric composites are provided in . Table 1.13.
All composites are porous and breathable, as mentioned previously. The extent of porosity and breathability varies from materials
and processes. Therefore, composites are permeable to water and a
number of other liquids to a certain extent. The material selection
for composites depends on the severity of the environment in which
the composites are likely to be exposed. For a weak acidic environment, chlorinated or long-chain iso polyester would be suitable.
Vinyl esters, which have increased chemical resistance, are preferred for aggressive environments of caustics and solvents. Epoxies
have some of the best solvent and thermal resistances and are much
better than vinyl esters. However, epoxies are comparatively more
difficult to process and are more expensive.
1.7 · Properties of Polymeric Composites
1
liquid/moisture under a specific temperature. Composite materials
are porous and have breathability. In respect to composite materials, the absorption of only small amounts of liquid may have a significant effect on their performance. The composites get saturated if
they are exposed to a liquid for an extended period. The absorbed
moisture plasticises and swells the matrix, resulting in a reduction
in mechanical properties, glass transition temperature, and service
temperature. Hygrothermal ageing deteriorates physico-chemical
and thermomechanical properties of the composite over time.
Moisture gets absorbed onto the composite via surface defects (such
as crazing, cracks, and voids) in the matrix and diffuses into the
bulk matrix causing swelling and plasticization. This results in
swelling stresses, causing the matrix to crack or the fibre/matrix to
debond. Fibre reinforcement is also adversely affected by moisture
due to leading effect that causes erosion of the coupling agent or
sizing on the fibre surface [20]. For example, water molecules penetrating glass fibre via surface microcracks result in the degradation
of the surface treatment on glass fibres, leading to the loss of
mechanical properties of polymeric composite.
1.7.5 Chemical and UV Resistance
The chemical resistance of polymeric composites is primarily dependent on the matrix. The role played by the reinforcement is minimal
and to the extent that it only allows the ingression of chemicals
slowly due to the physical resistance offered by them. In the case of
alkali, the glass gets corroded by alkalis; therefore, glass- reinforced
composites get corroded by alkalis. Polymeric composites can be
designed for excellent corrosion resistance (i.e. resistance to specific
hostile environments). The selection of a matrix that is resistant to a
specific environment plays an important role to ensure the composite’s optimum performance. The selection of reinforcing fibres is the
next important thing to make composites resistant to chemicals/
UV. The other influencing factors for chemicals/UV resistance of
composites are the moulding process, service environment, and
expected service life. Once operating and service parameters are
established, the appropriate materials can be selected to design the
composite [21]. Typical chemical and UV resistance of unidirectional (UD) polymeric composites are provided in . Table 1.13.
All composites are porous and breathable, as mentioned previously. The extent of porosity and breathability varies from materials
and processes. Therefore, composites are permeable to water and a
number of other liquids to a certain extent. The material selection
for composites depends on the severity of the environment in which
the composites are likely to be exposed. For a weak acidic environment, chlorinated or long-chain iso polyester would be suitable.
Vinyl esters, which have increased chemical resistance, are preferred for aggressive environments of caustics and solvents. Epoxies
have some of the best solvent and thermal resistances and are much
better than vinyl esters. However, epoxies are comparatively more
difficult to process and are more expensive.
1.7 · Properties of Polymeric Composites