Manufacturing defects in polymeric medical devices - Durability and Reliability of Medical Polymers - page 224

of the product may not cause failure. However, if it falls along a load path,
it can act as a nucleus for a brittle crack.
The polymer melt viscosity is an important variable in the process because
it effectively controls not just the way the process works, but the strength of
the final product. The root variable is molecular weight M which controls
both these properties; the greater the molecular weight of the polymer, the
greater both the melt viscosity and the product mechanical strength. Molecular
weight is a simple measure of chain length. The shear stress t of a Newtonian
fluid such as water is related to the shear rate g and the viscosity h by the
simple equation

t = hg [10.1]
However, polymer melts are governed by a so-called rate law, where
the shear stress is more sensitive to shear rate, or the rate at which the melt
moves when sheared:
t = h ( g ) n [10.2]

or

log t = log h + n log g [10.3]

and

log h = log t - n log g [10.4]

with the exponent n being a negative value. in other words, the melt viscosity
decreases with increasing shear rate, and such fluids are generally known as
'pseudoplastic' in nature, a specific example of a non-Newtonian liquid (Fig.
10.3). Thus, as the shear rate increases, the shear stress falls. Figure 10.3
shows how several different polymers react as shear rate rises, and there are
considerable variations between them. Thus, acrylics such as poly(methyl
methacrylate) (PMMA) and poly(vinyl chloride) (PVC) fall faster than
more rigid chains such as those of polybutylene terephthalate (PBT) and
polyethersulfone. The melt flow index (MFI) is an empirical measure of melt
viscosity used by moulders, which is inversely related to molecular weight.
The shear rates encountered in the pipes of moulding machines is typically
above 10 3 , so the melt viscosities used in moulding are those to the right of
Fig. 10.3. There is a subtle implication that some polymers are more difficult
to mould than others, those polymers with inflexible chains generally being
more difficult than simple chains such as low-density polyethylene (LDPE).
Greater care is needed for such polymers, which includes polysulfones and
polycarbonates as well as PBT since the melt is more Newtonian than simpler
polymers. Similar care is needed with composites such as short glass fibre
(GF) reinforced materials such as GF nylon owing to the thickening effect
of the short fibres present in the melt.

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