96
4
4.4.1 Fibre Aspect Ratio
The fibre aspect ratio in the rheology of polymeric composites is
important because it describes the behaviour of a material while
undergoing processing as well as the sensitivity of the material to
the microstructure and morphology. Although the length of the
fibres is crucial for the reinforcing effect, the more important
parameter is the fibre aspect ratio (length-to-diameter ratio).
Usually, the fibres get aligned in the direction of flow. In the injection moulding processes, the alignment of fibres takes place along
the wall of the mould due to shear force. Whereas in the core of the
moulding, it is observed that the alignment of fibres is comparatively less when due to a reduction in shear force. The higher the
fibre aspect ratio, the better the properties of composites. The most
effective fibre reinforcement is achieved if the length of the fibre
exceeds the critical fibre length, which is also dependent on the
diameter of the fibre. Excessive shear force breaks the brittle fibres;
therefore, the overall properties of the composite are reduced. In
order to take advantage of the fibre length, the diameter of the fibre
ideally should be decreased in order to increase the fibre aspect
ratio, which governs the reinforcement efficiency of the fibres in the
composites.
The use of rheological techniques enables a real-time monitoring of the processing influence on the microstructure of materials
that has been extensively used in the study of composites in conjunction with basic characterization techniques (e.g. SEM, mechanical properties). The rheological methods enable the measurements
of the properties of a polymer melt in response to both linear and
nonlinear deformations.
4.4.2 Fibre Content
Due to fibre loading in composite mix, the viscosity of composite
mix is expected to increase rapidly as the filler packs the composite
mix more closely. Therefore, collisions among fibres increase, as
discussed in 7 Sect. 4.2. The apparent viscosity, η ∗ , increases at a low
frequency because the fibre and polymer chains are not completely
oriented during the dynamic frequency test. The addition of fibres
in polymers perturbs the flow of the polymer and hinders the
mobility of the polymer chain during the flow. With an increase in
fibre loading, this phenomenon becomes more predominant, and
the viscosity of polymer systems further increases. This observed
behaviour is due to the formation of a strong network structure
among fibres and polymeric chains. This causes resistance in flow
because the rheological behaviour is sensitive to the molecular
structure.
Continuous fibres or fabric offers very high resistance for the
flow of matrices (i.e. melted thermoplastic as well as thermosetting
prepolymers). The low viscosities of thermosetting prepolymers
offer an advantage for the percolation of resin around the fibres in
Chapter 4 · Rheology in Processing of Polymeric Composites
4
4.4.1 Fibre Aspect Ratio
The fibre aspect ratio in the rheology of polymeric composites is
important because it describes the behaviour of a material while
undergoing processing as well as the sensitivity of the material to
the microstructure and morphology. Although the length of the
fibres is crucial for the reinforcing effect, the more important
parameter is the fibre aspect ratio (length-to-diameter ratio).
Usually, the fibres get aligned in the direction of flow. In the injection moulding processes, the alignment of fibres takes place along
the wall of the mould due to shear force. Whereas in the core of the
moulding, it is observed that the alignment of fibres is comparatively less when due to a reduction in shear force. The higher the
fibre aspect ratio, the better the properties of composites. The most
effective fibre reinforcement is achieved if the length of the fibre
exceeds the critical fibre length, which is also dependent on the
diameter of the fibre. Excessive shear force breaks the brittle fibres;
therefore, the overall properties of the composite are reduced. In
order to take advantage of the fibre length, the diameter of the fibre
ideally should be decreased in order to increase the fibre aspect
ratio, which governs the reinforcement efficiency of the fibres in the
composites.
The use of rheological techniques enables a real-time monitoring of the processing influence on the microstructure of materials
that has been extensively used in the study of composites in conjunction with basic characterization techniques (e.g. SEM, mechanical properties). The rheological methods enable the measurements
of the properties of a polymer melt in response to both linear and
nonlinear deformations.
4.4.2 Fibre Content
Due to fibre loading in composite mix, the viscosity of composite
mix is expected to increase rapidly as the filler packs the composite
mix more closely. Therefore, collisions among fibres increase, as
discussed in 7 Sect. 4.2. The apparent viscosity, η ∗ , increases at a low
frequency because the fibre and polymer chains are not completely
oriented during the dynamic frequency test. The addition of fibres
in polymers perturbs the flow of the polymer and hinders the
mobility of the polymer chain during the flow. With an increase in
fibre loading, this phenomenon becomes more predominant, and
the viscosity of polymer systems further increases. This observed
behaviour is due to the formation of a strong network structure
among fibres and polymeric chains. This causes resistance in flow
because the rheological behaviour is sensitive to the molecular
structure.
Continuous fibres or fabric offers very high resistance for the
flow of matrices (i.e. melted thermoplastic as well as thermosetting
prepolymers). The low viscosities of thermosetting prepolymers
offer an advantage for the percolation of resin around the fibres in
Chapter 4 · Rheology in Processing of Polymeric Composites