Page 202 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section Thermoplastics I 8 I

strain is linear, as shown in Fig. 2.2.) The particular behavior
depends on the polymer. For example, PMMA is glassy below
its Tg, whereas polycarbonate is not glassy below its T8: Rigid and
When the applied stress is increased further, polycarbonate brittle
eventually fractures, just as a piece of glass does at ambient (melamine. Tough and ductile
temperature. U, phenohc) (ABS, nylon)
Typical stress-strain curves for some thermoplastics and §
thermosets at room temperature are shown in Fig. 7.10. Note 5
than metals. Their ultimate tensile strength is about one order l/
that these plastics exhibit various behaviors, which may be de-
scribed as rigid, soft, brittle, flexible, and so on. The mechani-
cal properties of several polymers listed in Table 7.1 indicate
that thermoplastics are about two orders of magnitude less stiff
Soft and flexible
(polyethylene, PTFE
)
of magnitude lower than that of metals (see Table 2.2). 0
Strain
Effects of Temperature. If the temperature of a thermo-
FIGURE 1.I0 General terminology describing the
plastic polymer is raised above its Tg, it first becomes lent/very
behavior of three types of plastics. PTFE (poly-
and then, with increasing temperature, rubbery (Fig. 7.6).
tetrafluoroethylene) has Teflon as its trade name.
Finally, at higher temperatures (e.g., above Tm for crystalline
Source: After R.L.E. Brown.
thermoplastics), it becomes a viscous fluid, and its viscosity
decreases with increasing temperature. As a viscous fluid, it
-25°C
70
can be softened, molded into shapes, resolidified, remelted, f 00
and remolded a number of times. In practice, however, re-
60 I
peated heating and cooling causes degradation, or thermal Q; 50
aging, of thermoplastics. é 250
The typical effect of temperature on the strength and 5 40
elastic modulus of thermoplastics is similar to that of metals: Q 30
With increasing temperature, the strength and the modulus of U3 A 50°
20 -\ 65.
elasticity decrease and the ductility increases (Fig. 7.11). The 7800
effect of temperature on impact strength is shown in 10
Fig. 7.12; note the large difference in the impact behaviors of 0
various polymers. O 5 1 O 1 5 20 25 30
Strain (%)
Effect of Rate of Deformation. When deformed rapidly, the
FIGURE 1.l| Effect of temperature on the stress-
behavior of thermoplastics is similar to metals, as was indicated
strain curve for cellulose acetate, a thermoplastic.
by the strain-rate sensitivity exponent nfz in Eq. (2.9).
Note the large drop in strength and the large increase
Thermoplastics in general have high 771 values, indicating that
in ductility with a relatively small increase in
they can undergo large uniform deformation in tension before
temperature. Source: After T.S. Carswell and H.K.
fracture. Note in Fig. 7.13 how (unlike in ordinary metals) the Nason.
necked region elongates considerably. This phenomenon can be
easily demonstrated by stretching a piece of the plastic holder for a 6-pack of beverage
cans. Observe the sequence of necking and stretching behavior shown in Fig. 7.13a.
This characteristic (which is the same in the superplastic metals, Section 2.2.7) enables
the thermoforming of thermoplastics (Section 19.6) into such complex shapes as meat
trays, lighted signs, and bottles for soft drinks.

Orientation. When thermoplastics are deformed (say, by stretching), the long-chain
molecules tend to align in the general direction of the elongation; this process is called
orientation. As in metals, the polymer becomes anisotropic (see also Section 1.6), so
the specimen becomes stronger and stiffer in the elongated (stretched) direction than
in its transverse direction. Stretching is an important technique for enhancing the
strength and toughness of polymers and is especially exploited in producing high-
strength fibers for composite materials, as discussed in Chapter 9.