Materials for engineering, 3rd Edition - (Malestrom)

Organic polymeric materials 181
lost upon contact with microbial and/or invertebrate activity in a natural
environment within a limited period of time. At the limit, this represents
conversion of the material to carbon dioxide, water, inorganic salts, microbial
cellular components and miscellaneous by-products.
Most plastics at present used for packaging consist of high- and lowdensity
PE, which do not degrade by microbiological action. Due to their
large chain lengths and high molecular weight, most widely used alkanederived
plastics may have lifetimes of hundreds of years when buried in
typical solid-waste disposal sites. Low molecular weight hydrocarbons can
be degraded by microbes, but the rate of degradation becomes very slow
when the length of the polymer chain exceeds 24 to 30 carbon atoms. Decreasing
typical polymer molecules to biologically acceptable dimensions requires
extensive destruction of the PE matrix. This destruction can be partly
accomplished in blends of PE and biodegradable natural polymers by the
action of macroorganisms such as arthropods, millipedes, slugs and snails.
Starch–polyethylene complexes have been manufactured which exhibit
physical properties approaching those of low density PE. The starch is present
as a separate phase and is attacked by fungi and bacteria. This weakens the
polymer matrix and the eventual breakdown of the polymer chains reduces
the molecular weight which enhances microbial attack.
5.6.5 Chemical attack
Plastics are susceptible to environmental failure when exposed to certain
organic chemicals and this limits their use in many applications. Even aqueous
media can cause degradation, however, although the processes involved differ
from the corrosive attack of metals in such environments.
Diffusion of species into plastics is common and adverse effects can arise
which are not chemical in nature. In most interactions of water with structural
plastics no chemical bonds are altered, but damage known as ‘physical
corrosion’ may occur. Absorbed moisture has been shown to act as a plasticizer,
reducing the glass transition temperature and the strength of the polymer.
These effects are essentially reversible, although other, irreversible, effects
may be encountered, such as microcracking or crazing, as well as chemical
degradation of the polymer structure.
Organic liquids, such as cleaning fluids, detergents, petrol and lubricants
may seriously reduce the mechanical properties of plastics. As already
discussed, the most serious problems arise when a material is exposed to
aggressive fluids when it is under stress. Organic liquids may interact both
chemically and physically with a polymer. Chemical interactions may involve
a decrease in the molecular weight by chain breakage; this in turn may cause
a reduction in mechanical properties such as tensile strength, stiffness and
fracture toughness. Figure 5.11 illustrates the decrease in tensile strength of