128
5
through amines or anhydrides. Only when all linkable sites in the
matrix are connected the reaction is said to be complete. When
linkable sites are still present or unreacted styrene is still present in
polyester, it is said that reaction is not complete. Therefore, further
heating is required to complete the curing. This subsequent heating
to complete the cure is called post curing.
? Example 5.2 Explain the significance of the peak in a DSC curve
v Answer
The peak in a DSC curve signifies the following:
5 It indicates at what temperature maximum heat flow
occurred in the polymer. Maximum heat flow is the value at
which maximum exotherm occurred due to the optimum
reactivity causing cross-linking.
5 The peak indicates the average temperature between the
curing regimes of the cure cycle.
5 The temperature associated with peak is the cure temperature. Below this temperature, if curing takes place, the
degree of cross-linking will not be optimum.
5.3.2 Selection Criteria for Thermosetting
Matrices
The selection criteria for thermosetting matrices are dependent on
the processability factors of resin as well as processing technology.
A summary of processability factors already has been outlined in
7 Chap. 1 (7 Sect. 1.3). The processability factors for thermosetting
resins are detailed here.
1. Properties Driven by Selection Criteria
5 Viscosity—The viscosity of resin during processing has an
important effect on maintaining the even volume fractions
of resin and reinforcement. Low viscosities help in processes involving low pressures like vacuum-assisted resin
transfer moulding. This in turn helps in the reduction of
shear forces experienced by resin and reduces the possibility of disturbing the reinforcement layup orientation.
Therefore, low viscosities or low viscosities at high temperature (below the gelation temperature) are preferred for
processing. Resins having low viscosities are preferred for
the resin selection criteria.
5 Wettability of Reinforcement—The processability of resin
only becomes improved with its capability to wet the
reinforcement. Resin must have the capability to percolate around every fibre and particle. The surface tension
of the resin must be low so that it smears the surface of
the fibre, as shown in . Fig. 5.6. Low wettability of the
resin results in resin-rich and resin- starved areas in
composites.
Chapter 5 · Processability of Thermosetting Composites
5
through amines or anhydrides. Only when all linkable sites in the
matrix are connected the reaction is said to be complete. When
linkable sites are still present or unreacted styrene is still present in
polyester, it is said that reaction is not complete. Therefore, further
heating is required to complete the curing. This subsequent heating
to complete the cure is called post curing.
? Example 5.2 Explain the significance of the peak in a DSC curve
v Answer
The peak in a DSC curve signifies the following:
5 It indicates at what temperature maximum heat flow
occurred in the polymer. Maximum heat flow is the value at
which maximum exotherm occurred due to the optimum
reactivity causing cross-linking.
5 The peak indicates the average temperature between the
curing regimes of the cure cycle.
5 The temperature associated with peak is the cure temperature. Below this temperature, if curing takes place, the
degree of cross-linking will not be optimum.
5.3.2 Selection Criteria for Thermosetting
Matrices
The selection criteria for thermosetting matrices are dependent on
the processability factors of resin as well as processing technology.
A summary of processability factors already has been outlined in
7 Chap. 1 (7 Sect. 1.3). The processability factors for thermosetting
resins are detailed here.
1. Properties Driven by Selection Criteria
5 Viscosity—The viscosity of resin during processing has an
important effect on maintaining the even volume fractions
of resin and reinforcement. Low viscosities help in processes involving low pressures like vacuum-assisted resin
transfer moulding. This in turn helps in the reduction of
shear forces experienced by resin and reduces the possibility of disturbing the reinforcement layup orientation.
Therefore, low viscosities or low viscosities at high temperature (below the gelation temperature) are preferred for
processing. Resins having low viscosities are preferred for
the resin selection criteria.
5 Wettability of Reinforcement—The processability of resin
only becomes improved with its capability to wet the
reinforcement. Resin must have the capability to percolate around every fibre and particle. The surface tension
of the resin must be low so that it smears the surface of
the fibre, as shown in . Fig. 5.6. Low wettability of the
resin results in resin-rich and resin- starved areas in
composites.
Chapter 5 · Processability of Thermosetting Composites