130
5
5 Cross-Link Density—The cross-link density of a thermosetting resin governs the property of the composite.
Uniform cross- linking throughout the matrix gives
uniform properties. Uneven cross-linking occurs primarily due to the uneven curing of the resin or the presence of
feebly cross-linked moieties in the resin that occurred due
to the expiration of the resin life or improper storage
conditions. Uneven cross-linked density has different sizes
of molecular networks in the resin, which grossly affects
the properties of the composite, as shown in . Fig. 5.7.
The resin matrix must be checked for its consistency
before processing the composite so that the cross-link
density is uniform.
5 Gelation Time—This is one of the most important
parameters from the point of view of processing. This
parameter provides the processing window for making
composites using any method and technology. The
gelation time is predominantly the resin’s property, and it
is affected by the catalyst used; temperature of processing;
reaction—if any—of the reinforcement; etc. Depending
on the processing technology used, resin with an optimum gel time to provide the maximum processing
window must be selected. For example, a hand layup
process is manual and slow; thus, it requires large
processing window. Therefore, resins with a high gelation
Fibres
Uniform cross-link
density
a
b
Non uniform
cross-link density
Resin
. Fig. 5.7 a Uniform and b nonuniform cross-link density of resin
Chapter 5 · Processability of Thermosetting Composites