101
4
High-temperature curing matrices adversely affect low melting temperature fibres. All of the organic matrices (barring
polyolefins, which are inherently inert) form a good bond with
commercially available reinforcing fibres. Polymeric matrices
depend upon the polymer type, chain architecture, molecular
weight, and distribution. The role of branching in polymeric
chains has an influence on the viscosity of the resin. Highly
branched resin systems have high viscosities due to the formation of large macromolecules of the polymer. The degree of orientations is determined by the rheological behaviour of the
polymer and nature of the flow in the fabrication process.
Molecular orientation hence influences the mechanical properties of moulded composite products.
3. Type of Coupling Agent—Inorganic fibres are treated with
coupling agents or provided with surface treatments to make
their surfaces hydrophobic. The hydrophobic surface of the
fibre allows it to bond with a hydrophobic resin system. In the
case of carbon and with the help of chemical processes,
reactive groups (such as -OH, -COOH, -HCO, -CO, etc.) are
attached. Once the surface of carbon fibre is reactive, a coating
with a thickness of a few micron (2–3 μm) of either epoxy resin
or polyimide is applied to make it hydrophobic. The surface of
an aramid is inherently chemically active and hydrophobic, but
it has very high smoothness; therefore, the surface energy is
the least, which does not allow the matrix to adhere to it.
Therefore, a surface treatment is used to roughen up the
surface of aramid fibre with the help of an emery stone. The
roughening of an aramid surface causes the undulation and
surface defects that may lead to a weak composite. In view of
this, another technique is used for making the surface of an
aramid fibre bond to the resin. In this technique, an aramid
fibre is oxidized by passing it through bromine water or a very
dilute nitric acid. The oxidized surface of the aramid fibre has
an even roughening in this process. In order to avoid deterioration of its surface further, a thin coating (2–3-μm-thick) of
polyimide is applied over its roughened surface.
In the case of glass fibres, the sizing is applied over the fibres.
The sizing materials used are starch or polyvinyl alcohol. The
role of the sizing material is to prevent the breakage of the glass
fibres while handling or weaving. No sizing material is needed
for surface treatment of carbon or aramid fibres owing to their
strengths and moduli. Once glass fibres are brought into the
desired form of fabric or rovings, the sizing is washed away, and
the washed fabric is treated with silane or titanate coupling
agents. These organometallic compounds are attached to the
surface of glass from its inorganic part and leave open the
organic part to make a bond with the organic resin system.
Fibres such as polyethylene and polypropylene are highly
inert, although they are organic in nature. The surfaces of these
fibres are discharged with a corona to make them rough so that
mechanical locking with the resin takes place. Hardly any chem4.5 · Rheology and Processability
4
High-temperature curing matrices adversely affect low melting temperature fibres. All of the organic matrices (barring
polyolefins, which are inherently inert) form a good bond with
commercially available reinforcing fibres. Polymeric matrices
depend upon the polymer type, chain architecture, molecular
weight, and distribution. The role of branching in polymeric
chains has an influence on the viscosity of the resin. Highly
branched resin systems have high viscosities due to the formation of large macromolecules of the polymer. The degree of orientations is determined by the rheological behaviour of the
polymer and nature of the flow in the fabrication process.
Molecular orientation hence influences the mechanical properties of moulded composite products.
3. Type of Coupling Agent—Inorganic fibres are treated with
coupling agents or provided with surface treatments to make
their surfaces hydrophobic. The hydrophobic surface of the
fibre allows it to bond with a hydrophobic resin system. In the
case of carbon and with the help of chemical processes,
reactive groups (such as -OH, -COOH, -HCO, -CO, etc.) are
attached. Once the surface of carbon fibre is reactive, a coating
with a thickness of a few micron (2–3 μm) of either epoxy resin
or polyimide is applied to make it hydrophobic. The surface of
an aramid is inherently chemically active and hydrophobic, but
it has very high smoothness; therefore, the surface energy is
the least, which does not allow the matrix to adhere to it.
Therefore, a surface treatment is used to roughen up the
surface of aramid fibre with the help of an emery stone. The
roughening of an aramid surface causes the undulation and
surface defects that may lead to a weak composite. In view of
this, another technique is used for making the surface of an
aramid fibre bond to the resin. In this technique, an aramid
fibre is oxidized by passing it through bromine water or a very
dilute nitric acid. The oxidized surface of the aramid fibre has
an even roughening in this process. In order to avoid deterioration of its surface further, a thin coating (2–3-μm-thick) of
polyimide is applied over its roughened surface.
In the case of glass fibres, the sizing is applied over the fibres.
The sizing materials used are starch or polyvinyl alcohol. The
role of the sizing material is to prevent the breakage of the glass
fibres while handling or weaving. No sizing material is needed
for surface treatment of carbon or aramid fibres owing to their
strengths and moduli. Once glass fibres are brought into the
desired form of fabric or rovings, the sizing is washed away, and
the washed fabric is treated with silane or titanate coupling
agents. These organometallic compounds are attached to the
surface of glass from its inorganic part and leave open the
organic part to make a bond with the organic resin system.
Fibres such as polyethylene and polypropylene are highly
inert, although they are organic in nature. The surfaces of these
fibres are discharged with a corona to make them rough so that
mechanical locking with the resin takes place. Hardly any chem4.5 · Rheology and Processability