204 Chapter 8
to the touch and embrittled. Low-density polyethylene is an effective absorber
of oily materials, including plasticisers as demonstrated by a polyethylene
fishing-box used to store plasticised PVC lures in Figure 3.
In addition, as esters, phthalate plasticisers are susceptible to hydrolysis
when exposed to strongly acidic or alkaline environments. Acid hydrolysis
causes the formation of crystalline phthalic acid, volatile 2-ethyl hex-1-ene
and 2-ethylhexanol. Acidic environments may develop when the PVC poly-
mer degrades to form hydrogen chloride. Oxygen attack on alkyl groups in
the DEHP molecule also results in the formation of phthalic acid.
4.4 Degradation of Polyurethane Foam
Polyurethanes are mainly degraded by oxidation, resulting in discolouration
and a loss of mechanical properties. Since oxygen comprises 21% of air, it is
difficult to prevent its contact with objects. Polyester-based urethanes are less
stable with time than polyether-based urethanes. Polyurethane foams degrade
more rapidly than films or fibres since their many cells offer a greater sur-
face area-to-volume ratio over which oxygen can have contact with the poly-
mer. In addition, the processing of foams may involve blowing air through
polyurethanes in liquid form, providing conditions favourable to oxidation
Figure 3A polyethylene fishing-box has absorbed plasticiser from plasticised PVC lures
and formed polyethylene ‘fossils’ over 20 years