198
7
7.2 Processing-Induced Defects in Polymeric
Composites
Defects in composites occur either during processing or in their
service life. Processing-induced defects in composites may occur
due to the following reasons.
7.2.1 Unprepared Reinforcement or Matrix
The surface of the reinforcement must be hydrophobic for the compatibility with a hydrophobic matrix. If the surface treatment for
compatibility with the matrix is poor, the composite will be of poor
quality, having low strength, low modulus, and low interlaminar
shear strength (ILSS). Also, the surface treatments are responsible for
efficient handling of the reinforcement. If the surface coating of the
carbon fibre is defective, a fuzzing of carbon fibre is observed during
the filament winding at the payout eye. Usually, a coating of polyimide or epoxy resin is added to the carbon-fibre surface to make it
compatible with organic resins and to provide ease of handling.
As far as matrices are concerned, the mixing of all the components (in two- or three-component systems) must be mixed in correct proportions to have the optimum properties. A high content of
hardner causes high exothermic reactions and a high cross-link
density, leading to brittleness and ultimately to poor-quality composites. Similarly, a low hardner content leads to under curing or no
curing of the composite, affecting its glass transition temperature
and mechanical properties. In one component matrix system, such
as phenol formaldehyde resin (popularly known as phenolic resin),
lumps of resin due to ageing are observed that also cause poor composite quality. The processing-induced defects for open and closed
mould processing are given next.
7.2.2 Improper Storage Conditions
Storage of resin is important because resins mature with heat and
time. The maturity of resin is the initiation of a low degree of crosslinking that increases the viscosity of the resin. Resin matrices are
susceptible to moisture absorption. The ingress of moisture causes
the material to bulk and creates surface defects from high- temperature
curing due to the formation of water vapours. The accumulation of
dust and foreign particles on the surface of the reinforcement or
inside the matrix reduces all of the properties of composites.
7.2.3 Wrong Placement of Fibres
Tailoring the in-plane stiffness of composite laminates can improve
the buckling and post-buckling characteristics by redistributing the
applied loads. The fibre placement technique is the core technology
Chapter 7 · Processability in Open Mould Processing of Polymeric Composites
7
7.2 Processing-Induced Defects in Polymeric
Composites
Defects in composites occur either during processing or in their
service life. Processing-induced defects in composites may occur
due to the following reasons.
7.2.1 Unprepared Reinforcement or Matrix
The surface of the reinforcement must be hydrophobic for the compatibility with a hydrophobic matrix. If the surface treatment for
compatibility with the matrix is poor, the composite will be of poor
quality, having low strength, low modulus, and low interlaminar
shear strength (ILSS). Also, the surface treatments are responsible for
efficient handling of the reinforcement. If the surface coating of the
carbon fibre is defective, a fuzzing of carbon fibre is observed during
the filament winding at the payout eye. Usually, a coating of polyimide or epoxy resin is added to the carbon-fibre surface to make it
compatible with organic resins and to provide ease of handling.
As far as matrices are concerned, the mixing of all the components (in two- or three-component systems) must be mixed in correct proportions to have the optimum properties. A high content of
hardner causes high exothermic reactions and a high cross-link
density, leading to brittleness and ultimately to poor-quality composites. Similarly, a low hardner content leads to under curing or no
curing of the composite, affecting its glass transition temperature
and mechanical properties. In one component matrix system, such
as phenol formaldehyde resin (popularly known as phenolic resin),
lumps of resin due to ageing are observed that also cause poor composite quality. The processing-induced defects for open and closed
mould processing are given next.
7.2.2 Improper Storage Conditions
Storage of resin is important because resins mature with heat and
time. The maturity of resin is the initiation of a low degree of crosslinking that increases the viscosity of the resin. Resin matrices are
susceptible to moisture absorption. The ingress of moisture causes
the material to bulk and creates surface defects from high- temperature
curing due to the formation of water vapours. The accumulation of
dust and foreign particles on the surface of the reinforcement or
inside the matrix reduces all of the properties of composites.
7.2.3 Wrong Placement of Fibres
Tailoring the in-plane stiffness of composite laminates can improve
the buckling and post-buckling characteristics by redistributing the
applied loads. The fibre placement technique is the core technology
Chapter 7 · Processability in Open Mould Processing of Polymeric Composites