Embrittlement polyolefins

Oridation. This is caused by contact with oxidising acids, exposure to u-v, prolonged application of excessive heat, or exposure to weathering. It results in a deterioration of mechanical properties (embrittlement and possibly stress cracking), increase in power factor, and loss of clarity. It affects most thermoplastics to varying degrees, in particular polyolefins, PVC, nylons, and cellulose derivatives. 

Like other polyolefins, the polyethylenes are chemically inert. Strong oxidizing agents will eventually cause oxidation and embrittlement. They have no known solvent at room temperature. Aggressive solvents will cause softening or swelling, but these effects are normally reversible. 

The time to embrittlement of polyolefins during photo-oxidation is not directly related to initiation carbonyl concentration (9-11). This is exemplified for HOPE in Figure 3 and a similar relationship exists for LDPE and... 

Chemical breakdown usually involves oxidative chain reactions that cause embrittlement of semicrystalline polymers and discoloration of poly(vinyl chloride) and polymers with aromatic groups. The reactions are complicated by the presence of transient intermediates and by rates that depend on minute concentrations of molecular defects, impurities and additives. They also depend on several important piiysi-cal factors outlined in this brief overview of polyolefin degradation, two of these factors, the transfer of excitation energy and the transport of products and protectants, play a major role in stabilization processes. 

The polyolefin life trajectory is also called ageing, the term that is preferred if the long-term changes of polyolefin properties due to weathering come into the play. It may involve the participation of physical processes such as recrystallization, loss of stabilizers by bleaching and mechanically initiated embrittlement. In the older literature, the reader may encounter also the term corrosion that was implemented from metals. 

In semi-crystalline polymers such as polyolefins, initial oxidation occurs in the amorphous tie molecules between crystallites and sometimes even in the inter-lamellar amorphous chains within crystallites. This allows a relatively low levels scission to cause a disproportionately large changes in mechanical properties due to brittle failure in the amorphous regions (Celina, 2013). In PP for instance, beginning stages of bulk embrittlement corresponded to only 0.01% of oxidation (Fayolle et al., 2004). 

Light stabilizers are used to protect plastics, particularly polyolefins, from discoloration, embrittlement, and eventual degradation by UV light. The three major classes of light stabilizers are UV absorbers, excited state quenchers, and free-radical terminators. Each class is named for the mechanism by which it prevents degradation. The major types included in each light stabilizer class may be categorized by their chemistries, as shown in Table 4.17. 

Tables IV and V illustrate the pattern of dramatically reduced total elongations and increased embrittlement that accompanies most plastics upon cooling to cryogenic temperatures. These changes are most pronounced with the low-density, low-modulus polyolefins. A high-density, biaxially-oriented laminated polypropylene tape...

In use, plastics usually fail because of localized stresses in combination with some weakening of the material. Embrittlement is the most important practical consequence of any reaction of the polymer that reduces its molecular weight and/or cross-links the chains. A second common consequence of degradation is yellowing. Polyolefins are especially vulnerable to oxidation. If they are not stabilized in some way, they may have a lifetime of only a few days in bright simlight before they become weakened and useless. 

In semicrystalline polyolefins, failure is also often by embrittlement, but for different reasons. The main mechanism is chain scission, with lowering of molecular weight and of Tg. Failure occurs at very low levels of oxidation the molecular weight typically falls by less than a factor of 2 and there is less than one O2 molecule per 100 carbon atoms. The reason for this sensitivity lies in the complex morphology of many polymers. 

Heterogeneous oxidation of solid polymers, as apparent from mechanical failure and embrittlement very soon after the induction period when the overall damage of a material is still relatively small, is well established, especially for polyolefins, and is caused... 

The polyolefins, which contain relatively few double bonds are much more stable and their inherent oxidizability depends on the number of tertiary carbon atoms in the chain the order of oxidizability is therefore polypropylene (PP) > low density polyethylene (LDPE) > high density polyethylene (HOPE) [9]. However, HOPE and PP have a higher degree of crystallinity than LDPE and for the reasons discussed above, they undergo chemicrystallization and embrittlement more rapidly than LDPE. Electron-attracting groups in the polymer chain or pendant to the polymer chain increase oxidative stability, so... 

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