109
4
4.6.2 Surface Properties
The surface properties are affected by the flow behaviour of the
matrix. In short fibre composites, the alignment of the fibres occurs
along the walls of the mould due to a shearing mechanism. The
properties of the composites on the surface are superior in the longitudinal direction compared to the properties at the core of the
moulding. The shear flow experienced at the core is the minimum
flow; therefore, the orientation of short fibres is often random. The
presence of solvent in the matrix causes pinholes and bubbles. The
presence of moisture in the matrix leads to bubbles. The quality
and properties of the surface of the moulding are directly dependent on the processability of the materials under desired conditions. High- temperature curing or moulding cycles are common
with polymeric composites; therefore, in order to improve the processability of the polymeric composites, the raw materials must be
dry so that surface or core defects do not occur. The presence of
reactive fillers and additives must be avoided. Any impurity is a
main hindrance in the processability. Impurities act as the nucleation point for surface and core defects in the matrix and polymeric composites.
4.6.3 Rheology in Structure–Property
Correlation
High flow rates cause stretching and elongation of the polymeric
molecules. These thermodynamic unstable polymeric chains have a
high energy content and a high configuration of entropy. On curing, these molecules will relax and stabilize, attaining low entropy
and causing shrinkage. This phenomenon also causes warpages and
morphological irregularities in the polymeric composite.
4.7 Rheological Measurement
Rheological measurement can be divided into three major categories: viscoelastic, steady shear, and elasticity. The difference between
each of these tests is in the way the material is deformed.
4.7.1 Viscoelastic Measurements
Viscoelastic measurements reveal the flow and recovery behaviour
of polymers and composites under stress and strain.
1. Creep and Stress Relaxation—Polymers are generally viscoelastic; hence, they exhibit creep and stress relaxation behaviour.
Creep and stress relaxation tests measure the viscoelastic
behaviour by monitoring changes in elongation at a constant
stress and by measuring stress reduction at a constant elongation, respectively. Both tests permit the calculation of the same
Point to Ponder…
Viscoelasticity is a unique property
of polymers that shows viscous
behaviour in the form of flow and
elastic behaviour in the form of
rigidity simultaneously.
4.7 · Rheological Measurement
4
4.6.2 Surface Properties
The surface properties are affected by the flow behaviour of the
matrix. In short fibre composites, the alignment of the fibres occurs
along the walls of the mould due to a shearing mechanism. The
properties of the composites on the surface are superior in the longitudinal direction compared to the properties at the core of the
moulding. The shear flow experienced at the core is the minimum
flow; therefore, the orientation of short fibres is often random. The
presence of solvent in the matrix causes pinholes and bubbles. The
presence of moisture in the matrix leads to bubbles. The quality
and properties of the surface of the moulding are directly dependent on the processability of the materials under desired conditions. High- temperature curing or moulding cycles are common
with polymeric composites; therefore, in order to improve the processability of the polymeric composites, the raw materials must be
dry so that surface or core defects do not occur. The presence of
reactive fillers and additives must be avoided. Any impurity is a
main hindrance in the processability. Impurities act as the nucleation point for surface and core defects in the matrix and polymeric composites.
4.6.3 Rheology in Structure–Property
Correlation
High flow rates cause stretching and elongation of the polymeric
molecules. These thermodynamic unstable polymeric chains have a
high energy content and a high configuration of entropy. On curing, these molecules will relax and stabilize, attaining low entropy
and causing shrinkage. This phenomenon also causes warpages and
morphological irregularities in the polymeric composite.
4.7 Rheological Measurement
Rheological measurement can be divided into three major categories: viscoelastic, steady shear, and elasticity. The difference between
each of these tests is in the way the material is deformed.
4.7.1 Viscoelastic Measurements
Viscoelastic measurements reveal the flow and recovery behaviour
of polymers and composites under stress and strain.
1. Creep and Stress Relaxation—Polymers are generally viscoelastic; hence, they exhibit creep and stress relaxation behaviour.
Creep and stress relaxation tests measure the viscoelastic
behaviour by monitoring changes in elongation at a constant
stress and by measuring stress reduction at a constant elongation, respectively. Both tests permit the calculation of the same
Point to Ponder…
Viscoelasticity is a unique property
of polymers that shows viscous
behaviour in the form of flow and
elastic behaviour in the form of
rigidity simultaneously.
4.7 · Rheological Measurement