Chemistry Reference
In-Depth Information
where
φ
F
5
A
F
/A
C
for
continuous fibers). This is the “law of mixtures” rule for composite properties.
With the cited values,
φ
F
is the volume fraction of fiber in the composite (since
φ
F
in the present example is 0.40 and the modulus of the
composite is about 43% of the fiber modulus. The fiber alignment is also a signif-
icant factor in composite properties. If the fibers in the foregoing example were
randomly oriented their
reinforcing effect would be less than 0.2 of
the
figure calculated above.
Discontinuous fibers are used when the manufacturing process prohibits the
application of continuous fibers, for example, in injection molding. In composites
of discontinuous fibers, stress cannot be transmitted from the matrix polymer to the
fibers across the fiber ends. Under load, the polymer is subjected to a shear stress
because the stress along each fiber will be zero at its ends and a maximum,
σ
m
,at
its center. The shear stress at the fiber
polymer interface transmits the applied
force between the components of the composite. The shear strength of this interface
is typically low and reliance must therefore be placed on having sufficient interfa-
cial area to transmit the load from the polymer to the fiber. This means that the dis-
continuous fibers must be longer than a certain critical minimum length, l
c
, which
depends on the interfacial shear stress,
τ
, fiber diameter, and applied load.
Experience shows that this minimum length is not difficult to exceed in dough or
sheet molding compounds, where unsaturated polyesters are mixed with chopped
fiber mats, with fiber lengths about 5
14 mm. These composites are usually com-
pression molded and cured hot in the mold. The process does not damage the fiber
to any significant extent. In injection molding, on the other hand, the initial fibers
are likely to be shortened by the mechanical action of the compounding process
and the shearing action of the reciprocating screw in the injection molder. They are
thus less likely to be effective than in sheet molding formulations.
The properties of fiber
polymer composites are influenced by the strength of
the bond between the phases, since stresses must be transmitted across their
boundaries. Some problems have been encountered in providing strong interfacial
bonds because it is difficult to wet hydrophilic glass surfaces with generally
hydrophobic viscous polymers. Coupling agents have therefore been developed to
bind the matrix and reinforcing fibers together.
These agents often contain silane or chromium groupings for attachment
to glass surfaces, along with organic groups that can react chemically with the
polymer. Thus vinyltriethoxysilane [H
2
C
Q
CHSi(OC
2
H
5
)
3
] is used for glass-
unsaturated polyester systems and
-aminopropyltriethoxysilane [H
2
NCH
2
CH
2
CH
2
Si (OC
2
H
5
)
3
] is a coupling agent for glass-reinforced epoxies and nylons. The
silanes which seem to couple effectively to glass are those in which some groups
can be hydrolyzed to silanols. Si
γ
a
O bonds are presumably formed across the
interface between the glass and coupling agent. Coupling agents for more inert
polymers like polyolefins are often acid-modified versions of the matrix polymer,
with maleic acid
grafted polypropylene as a prime example.
More recently, an emerging technique for preparing polymer composites is by
incorporating nano-sized inorganic fillers into polymeric materials. Several such
