93
4
where
5 x = Penetration depth
5 k = Media permeability
5 P = Pressure
5 t = Time under pressure
5 μ = Viscosity
This law shows that high viscosities will reduce the penetration
depth unless the porosity, pressure, or time of the fibre bundle is
increased accordingly.
The width of the molecular weight distribution also plays an
important role in fibre wet out. If molecular weight distribution
width is narrow, polymer will melt over a narrow range of temperature, and fibre wet out will be easier. This is seen in nylon matrix
composites because nylon is highly crystalline thermoplastic; therefore, it shows sharp melting points which is a characteristic of crystalline material. Therefore, crystallinity also affects the ease of
wetting out the fibre in composites.
A process for improving the ability of selected high-viscosity
polymers to penetrate and coat fibres is based on the chemical technology known as ‘cyclics’ . In this technology, a high-viscosity thermoplastic polymer is initially formulated in a cyclical, short chain
structure that has an inherent lower viscosity than the normal linear structure, and the molecular weight is simultaneously increased
by heating the resin in the presence of an ionic catalyst. This ringopening polymerization usually can be accomplished during the
normal shaping in the moulding step.
4.3.1 Rheology in Deciding Processing
Conditions
Rheology comes into the picture even at the compounding stage in
an extruder where maintaining a temperature profile along the flow
path is a stringent requirement. Processing polyolefins does not
pose much difficulty due to their inherent ease of processability.
Industries all around generally tend to maintain a linear temperature profile for them.
Materials that are inherently hard, such as polycarbonate, need
to be processed in such a way that they do not affect the extruder.
Hence, it is advisable to maintain a temperature profile with a small
range that is much higher than their glass transition temperature
(melting temperature for crystalline polymers).
A further advantage of a rheological study prior to processing is
in the case of polymers like polyamides. Polyamides, commonly
known as nylon, are blessed with an inherent structure of strong
inter-chain hydrogen bonding, which is the main reason for their
ultimately high mechanical properties. However, the polymer has a
strange tendency to flow above its melting point. Therefore, it is
very important to judge the temperature required to process nylon,
as inadequate ideas of viscosity at various temperatures beyond its
melting point may lead to a ‘drooling’ phenomena while processing,
4.3 · Rheology in Material Selection
4
where
5 x = Penetration depth
5 k = Media permeability
5 P = Pressure
5 t = Time under pressure
5 μ = Viscosity
This law shows that high viscosities will reduce the penetration
depth unless the porosity, pressure, or time of the fibre bundle is
increased accordingly.
The width of the molecular weight distribution also plays an
important role in fibre wet out. If molecular weight distribution
width is narrow, polymer will melt over a narrow range of temperature, and fibre wet out will be easier. This is seen in nylon matrix
composites because nylon is highly crystalline thermoplastic; therefore, it shows sharp melting points which is a characteristic of crystalline material. Therefore, crystallinity also affects the ease of
wetting out the fibre in composites.
A process for improving the ability of selected high-viscosity
polymers to penetrate and coat fibres is based on the chemical technology known as ‘cyclics’ . In this technology, a high-viscosity thermoplastic polymer is initially formulated in a cyclical, short chain
structure that has an inherent lower viscosity than the normal linear structure, and the molecular weight is simultaneously increased
by heating the resin in the presence of an ionic catalyst. This ringopening polymerization usually can be accomplished during the
normal shaping in the moulding step.
4.3.1 Rheology in Deciding Processing
Conditions
Rheology comes into the picture even at the compounding stage in
an extruder where maintaining a temperature profile along the flow
path is a stringent requirement. Processing polyolefins does not
pose much difficulty due to their inherent ease of processability.
Industries all around generally tend to maintain a linear temperature profile for them.
Materials that are inherently hard, such as polycarbonate, need
to be processed in such a way that they do not affect the extruder.
Hence, it is advisable to maintain a temperature profile with a small
range that is much higher than their glass transition temperature
(melting temperature for crystalline polymers).
A further advantage of a rheological study prior to processing is
in the case of polymers like polyamides. Polyamides, commonly
known as nylon, are blessed with an inherent structure of strong
inter-chain hydrogen bonding, which is the main reason for their
ultimately high mechanical properties. However, the polymer has a
strange tendency to flow above its melting point. Therefore, it is
very important to judge the temperature required to process nylon,
as inadequate ideas of viscosity at various temperatures beyond its
melting point may lead to a ‘drooling’ phenomena while processing,
4.3 · Rheology in Material Selection