143
5
materials. For high-performance composites and some of the engineering thermoplastics, the mechanisms of adhesion and matrixreinforcement bonding are poor. Adhesion between thermoplastic
resins and fibres is usually weak because of poor adsorption and
wetting, the absence of chemical bonding, a lack of mechanical
adhesion, limited or no interdiffusion, and poor electrostatic attraction. The surface free energies, γ, for thermoplastics are usually
much lower than those for conventional thermosetting resins,
resulting in a low W adhesion . Wetting of fibres with thermoplastics is
also poor because of their high viscosities. High viscosity, low solubility, and high crystallinity of thermoplastics also limit the interpenetration of the interphase regions. Thermoplastic resins have
few active sites—especially in highly aromatic resins—that can
form chemical bonds with the surface of the reinforcement.
Mechanical bonding does not take place for smooth surfaces and
reinforcements free of cracks. Also, highly viscous thermoplastic
resins flowing into small pockets or crevices on the surface of the
reinforcement make it difficult to create good mechanical bonding.
Generally, no electrostatic bonding is present in thermoplastic
composites [23–25].
In order to achieve good bonding between thermoplastic
matrices and fibre reinforcements, several techniques providing a
conventional layer to reduce interfacial stress concentration are
used. Example of such techniques are the electrodeposition of a
polymer on the fibre surface, improvement in the coupling agents,
improved surface oxidation treatments, matching mechanical
properties (especially the modulus) of the outer surface of the
fibre and matrix to lower the stress concentration that occurs at
the surface during loading, and using a ductile matrix in the interphase. Matching the modulus of the outer surface of the fibre and
the matrix leads to graded reinforcements throughout the cross
section.
It is a fact that in composites, the properties of reinforcement
are far, far superior to matrices in most cases. According to the
rule of mixtures, the overall properties of composites are compromised due to this fact. The reasons for these facts lay in the
following:
5 Selecting or developing a matrix that matches the high
properties of the reinforcement
5 Improvement of the interphase
5 Removal of surface and core defects in injection-moulded
composites
5 Achieving perfection in the desired orientation of the reinforcement
New developments have suggested better matrices for the composites—especially for carbon fibre-reinforced composites where
epoxy resins are primarily used. Higher functioning epoxy resins
have been developed to improve the properties of the resultant
composites. These epoxy resins are tri-functional and tetrafunctional resins. A new variety of polyester resins, such as vinyl
ester resins, has changed the scenario of polyester-based com5.4 · Methods to Improve Processability in Thermosetting Composites
5
materials. For high-performance composites and some of the engineering thermoplastics, the mechanisms of adhesion and matrixreinforcement bonding are poor. Adhesion between thermoplastic
resins and fibres is usually weak because of poor adsorption and
wetting, the absence of chemical bonding, a lack of mechanical
adhesion, limited or no interdiffusion, and poor electrostatic attraction. The surface free energies, γ, for thermoplastics are usually
much lower than those for conventional thermosetting resins,
resulting in a low W adhesion . Wetting of fibres with thermoplastics is
also poor because of their high viscosities. High viscosity, low solubility, and high crystallinity of thermoplastics also limit the interpenetration of the interphase regions. Thermoplastic resins have
few active sites—especially in highly aromatic resins—that can
form chemical bonds with the surface of the reinforcement.
Mechanical bonding does not take place for smooth surfaces and
reinforcements free of cracks. Also, highly viscous thermoplastic
resins flowing into small pockets or crevices on the surface of the
reinforcement make it difficult to create good mechanical bonding.
Generally, no electrostatic bonding is present in thermoplastic
composites [23–25].
In order to achieve good bonding between thermoplastic
matrices and fibre reinforcements, several techniques providing a
conventional layer to reduce interfacial stress concentration are
used. Example of such techniques are the electrodeposition of a
polymer on the fibre surface, improvement in the coupling agents,
improved surface oxidation treatments, matching mechanical
properties (especially the modulus) of the outer surface of the
fibre and matrix to lower the stress concentration that occurs at
the surface during loading, and using a ductile matrix in the interphase. Matching the modulus of the outer surface of the fibre and
the matrix leads to graded reinforcements throughout the cross
section.
It is a fact that in composites, the properties of reinforcement
are far, far superior to matrices in most cases. According to the
rule of mixtures, the overall properties of composites are compromised due to this fact. The reasons for these facts lay in the
following:
5 Selecting or developing a matrix that matches the high
properties of the reinforcement
5 Improvement of the interphase
5 Removal of surface and core defects in injection-moulded
composites
5 Achieving perfection in the desired orientation of the reinforcement
New developments have suggested better matrices for the composites—especially for carbon fibre-reinforced composites where
epoxy resins are primarily used. Higher functioning epoxy resins
have been developed to improve the properties of the resultant
composites. These epoxy resins are tri-functional and tetrafunctional resins. A new variety of polyester resins, such as vinyl
ester resins, has changed the scenario of polyester-based com5.4 · Methods to Improve Processability in Thermosetting Composites