Embrittlement of polymers

A significantly high rate of deformation leads to complete embrittlement of polymers which results in a lower threshold of elongation at break. On the other hand, the stiffness and the stress at break of the material under consideration increases with the rate of deformation. Figure 1 shows the stress-strain and fracture behavior of a polymer tested at various rates of deformation. The area under the stress-strain curves represent the volume-specific energy to fracture. 

The manner in which small amounts of colloidal filler destroy the cohesion of low-molecular-weigiht polyethylenes was thought by Kendall and Sherliker [61] to be similar to the phenomenon of environmental stress cracking, in which minor amounts of detergents or alcohol produce inordinate embrittlement of polymers, especially of low molecular weight [63]. In order to check their hypothesis, a drop of isopropanol was placed at die tip of the tear, and the fall in force at constant speed was measured. The results were plotted as a function of... 

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

Glass that has been under stress for a period of time may fracture suddenly. Such delayed fracture is not common in metals (except in cases of hydrogen embrittlement of steels) but sometimes does occur in polymers. It is often called static fatigue. The phenomenon is sensitive to temperature and prior abrasion of the surface. Most important, it is very sensitive to environment. Cracking is much more rapid with exposure to water than if the glass is kept dry (Figure 15.11) because water breaks the Si-O-Si bonds by the reaction — Si-O-Si—H H2O -> Si-OH + HO-Si. 

The conditions of processing and finishing can have influence but in essence the properties of a plasticized composition are linked with those of the original polymer and the types and amounts of plasticizer and other additives present. As a general comment, plasticized compositions are more flexible and less resilient than unplasticized PVC, less resistant to attack by other substances, to permeation, and to elevated temperatures under some circumstances of use the plasticizer can be extracted, leading to the gradual embrittlement of an item. 

Both physical aging and chemical side reactions result in a pronounced embrittlement of thermally treated polymers. Young moduli of these polymers usually become a little higher, ay passes through a maximum (in air much sooner than in vacuum or dry Ar atmosphere) and elongation at break becomes lower. Chemical embrittlement is more pronounced in polymers with an initial excess of the epoxy component. 

The findings of Hutson and Scott on embrittlement of inhibited polyethylene shown in Figure 7 also illustrate this point their presentation is unusual in that the point of embrittlement is indicated on the curve (26). As can be seen, this occurred before Stage III was well advanced. A similar situation can be found in the data of Wilson and Forshee the intrinsic fluidity of polymer that had degraded to a highly brittle state is indicated in Figure 6 (16). 

While it may be true that many materials will become severely embrittled or discolored before Stage III is far advanced, some special situations exist in which a polymeric system can be regarded as being in Stage IV. For example, in the life history of polymers that tend to crosslink when exposed to near ultraviolet radiation, the conservator may have to remove them or at least have a sound understanding of their swelling properties in solvents under conditions in which the films are in Stage IV with respect to the formation of insoluble matter (27). In a similar sense,... 

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