100
4
consumption of electricity adversely affects the cost of production
and environmental safety.
There are other important process conditions, such as pressure
and temperature. A low viscous medium creates uniform pressure
inside the mould during the processing of polymeric composites.
Control of the mould temperature determines the rate of cooling of
the matrix, and low temperatures facilitate a faster solidification of
the matrix. The balance between solidification and complete wetting of the reinforcement or filling of the mould must be ensured.
4.5.2 Determination of Processing Window
The determination of the processing window for composites
depends upon the following factors:
1. Type of Reinforcement—Choice of reinforcements influences
the processability of polymeric composites. The volume
fraction of the reinforcement and its surface treatment
influence the overall flow of the resin in the polymeric composites. Short fibres/staples are suitable for injection moulding,
whereas long fibres in any form and shape (such as fabrics or
rovings/tows) are used in all other processing technologies.
The material of fibres is another determining parameter to be
used for a particular processing technology. Processing
technologies that employ high temperatures use primarily
carbon or graphite, glass, and aramid (Kevlar®) fibres. Of late,
with the invention of new reinforcing materials (such as
alumina whiskers, tungsten carbide, polycarbosilanes, etc.), the
range of reinforcing material for high-temperature processing
has increased. These materials have not been exploited for their
full commercial potential due to their high costs, although they
have good properties. Some fibres (such as polyethylene/
UHMWPE or polypropylene fibres) are low melting temperature fibres and are not suitable for high-temperature processing
technologies. Their processability is limited to moderate
temperatures (i.e. < 120 °C). Natural fibres (such as jute, flex,
bamboo, etc.) are not processed at high temperature either.
Particulate reinforcements (such as carbon black, minerals,
and most of the chemical compounds) are stable at high
processing temperatures; therefore, these are used widely. In
short fibre- or particulate-reinforced composites, the size of a
suspended particle, size distribution, and volume fraction have
influence over rheology concerning the processability of
polymeric composites.
2. Type of Resin System—Resin systems affect the processability of
polymeric composites in two ways: affinity to form bonds with
fibre and by providing a desired shape of the product. All
organic resins are hydrophobic in nature; therefore, they don’t
have affinity towards inorganic fibres that are hydrophilic in
nature. The process of transforming the nature of inorganic
fibre from hydrophilic into hydrophobic will be discussed in
the next section.
Chapter 4 · Rheology in Processing of Polymeric Composites
4
consumption of electricity adversely affects the cost of production
and environmental safety.
There are other important process conditions, such as pressure
and temperature. A low viscous medium creates uniform pressure
inside the mould during the processing of polymeric composites.
Control of the mould temperature determines the rate of cooling of
the matrix, and low temperatures facilitate a faster solidification of
the matrix. The balance between solidification and complete wetting of the reinforcement or filling of the mould must be ensured.
4.5.2 Determination of Processing Window
The determination of the processing window for composites
depends upon the following factors:
1. Type of Reinforcement—Choice of reinforcements influences
the processability of polymeric composites. The volume
fraction of the reinforcement and its surface treatment
influence the overall flow of the resin in the polymeric composites. Short fibres/staples are suitable for injection moulding,
whereas long fibres in any form and shape (such as fabrics or
rovings/tows) are used in all other processing technologies.
The material of fibres is another determining parameter to be
used for a particular processing technology. Processing
technologies that employ high temperatures use primarily
carbon or graphite, glass, and aramid (Kevlar®) fibres. Of late,
with the invention of new reinforcing materials (such as
alumina whiskers, tungsten carbide, polycarbosilanes, etc.), the
range of reinforcing material for high-temperature processing
has increased. These materials have not been exploited for their
full commercial potential due to their high costs, although they
have good properties. Some fibres (such as polyethylene/
UHMWPE or polypropylene fibres) are low melting temperature fibres and are not suitable for high-temperature processing
technologies. Their processability is limited to moderate
temperatures (i.e. < 120 °C). Natural fibres (such as jute, flex,
bamboo, etc.) are not processed at high temperature either.
Particulate reinforcements (such as carbon black, minerals,
and most of the chemical compounds) are stable at high
processing temperatures; therefore, these are used widely. In
short fibre- or particulate-reinforced composites, the size of a
suspended particle, size distribution, and volume fraction have
influence over rheology concerning the processability of
polymeric composites.
2. Type of Resin System—Resin systems affect the processability of
polymeric composites in two ways: affinity to form bonds with
fibre and by providing a desired shape of the product. All
organic resins are hydrophobic in nature; therefore, they don’t
have affinity towards inorganic fibres that are hydrophilic in
nature. The process of transforming the nature of inorganic
fibre from hydrophilic into hydrophobic will be discussed in
the next section.
Chapter 4 · Rheology in Processing of Polymeric Composites