95
4
can give a totally different set of properties to the overall composites in comparison to fillers with sizes in the scale of nanometres.
Today, composites based on fillers with sizes in the nanometres
have been classified as a separate stream and are known as nanocomposites. The processing of composites with micron-sized fillers
is known to be much easier than that of fillers in the nanometre
scale.
Fibre-filled polymer melts have essential properties characteristic to them. Fibres are known to impart a mechanical integrity to
the compound. Often, certain fibres may contribute to properties
like conductivity owing to their nature and orientation within the
polymer matrix. However, the role of fibres in the eventual rheological properties of the composite can be immense due to their
higher L/D ratio. Higher L/D ratios sometimes cause problems with
poor dispersion in the polymer matrix. This calls for an increase in
the shear rate requirement for dispersing fibres within the polymer
matrix. It has been found that sometimes—at a superior loading of
fibres—the viscosity of fibre-based composites drops significantly at
high shear rates.
4.4 Flow Behaviour When Processing
Composites
The penetration of resin is more difficult in thermoplastics than in
thermosets because the thermoplastics have inherently high
molecular weights which lead to higher viscosities compared to
thermosetting liquid resins. High viscosities of matrices cause difficulties in mixing and coating fibres. Whereas, thermosetting
resins develop high molecular weight during the curing operation
which is after the fibres have been impregnated/ coated by thermosetting prepolymer. Typical viscosities of matrices are given in
. Table 4.1.
Apart from the viscosity, the flow behaviour in processing composites depends upon the following parameters:
5 Pressure difference applied
5 Drag force assisting the flow
. Table 4.1 Typical viscosities of matrices
Serial No.
Matrices
Viscosity
(Pa.s)
1.
Thermosetting prepolymer
(e.g. epoxy, phenol formaldehyde,
polyester, polyimides, etc.)
0.5–15
2.
Thermoplastic polymer melt
(e.g. polypropylene, polyethylene, nylon,
polycarbonate, polyether ether ketone,
polymethyl methacrylate, etc.)
1.5–30
4.4 · Flow Behaviour When Processing Composites
4
can give a totally different set of properties to the overall composites in comparison to fillers with sizes in the scale of nanometres.
Today, composites based on fillers with sizes in the nanometres
have been classified as a separate stream and are known as nanocomposites. The processing of composites with micron-sized fillers
is known to be much easier than that of fillers in the nanometre
scale.
Fibre-filled polymer melts have essential properties characteristic to them. Fibres are known to impart a mechanical integrity to
the compound. Often, certain fibres may contribute to properties
like conductivity owing to their nature and orientation within the
polymer matrix. However, the role of fibres in the eventual rheological properties of the composite can be immense due to their
higher L/D ratio. Higher L/D ratios sometimes cause problems with
poor dispersion in the polymer matrix. This calls for an increase in
the shear rate requirement for dispersing fibres within the polymer
matrix. It has been found that sometimes—at a superior loading of
fibres—the viscosity of fibre-based composites drops significantly at
high shear rates.
4.4 Flow Behaviour When Processing
Composites
The penetration of resin is more difficult in thermoplastics than in
thermosets because the thermoplastics have inherently high
molecular weights which lead to higher viscosities compared to
thermosetting liquid resins. High viscosities of matrices cause difficulties in mixing and coating fibres. Whereas, thermosetting
resins develop high molecular weight during the curing operation
which is after the fibres have been impregnated/ coated by thermosetting prepolymer. Typical viscosities of matrices are given in
. Table 4.1.
Apart from the viscosity, the flow behaviour in processing composites depends upon the following parameters:
5 Pressure difference applied
5 Drag force assisting the flow
. Table 4.1 Typical viscosities of matrices
Serial No.
Matrices
Viscosity
(Pa.s)
1.
Thermosetting prepolymer
(e.g. epoxy, phenol formaldehyde,
polyester, polyimides, etc.)
0.5–15
2.
Thermoplastic polymer melt
(e.g. polypropylene, polyethylene, nylon,
polycarbonate, polyether ether ketone,
polymethyl methacrylate, etc.)
1.5–30
4.4 · Flow Behaviour When Processing Composites