Polymeric materials aging

Obtained results allow to conclude that the dynamic indentation method can be applied to periodical express evaluation of polymeric material state being exposured to the radiation or temperature aging on purpose to early diagnostic of products to avoid emergency situations. 

Polylactides, 18 Poly lactones, 18, 43 Poly(L-lactic acid) (PLLA), 22, 41, 42 preparation of, 99-100 Polymer age, 1 Polymer architecture, 6-9 Polymer chains, nonmesogenic units in, 52 Polymer Chemistry (Stevens), 5 Polymeric chiral catalysts, 473-474 Polymeric materials, history of, 1-2 Polymeric MDI (PMDI), 201, 210, 238 Polymerizations. See also Copolymerization Depolymerization Polyesterification Polymers Prepolymerization Repolymerization Ring-opening polymerization Solid-state polymerization Solution polymerization Solvent-free polymerization Step-grown polymerization processes Vapor-phase deposition polymerization acid chloride, 155-157 ADMET, 4, 10, 431-461 anionic, 149, 174, 177-178 batch, 167 bulk, 166, 331 chain-growth, 4 continuous, 167, 548 coupling, 467 Friedel-Crafts, 332-334 Hoechst, 548 hydrolytic, 150-153 influence of water content on, 151-152, 154... 

The changes in properties observed on aging of different elastomers and their vulcanizates, and of many other polymeric materials, are well known. Antiozonants and antioxidants are employed to limit these changes. However, the most effective antioxidant for one material may be ineffective. 

ESRI is a nondestructive method for the study of degradation, which is an important advantage, especially for crystalline polymers. The major advantage of ESRI compared with FTIR methods is its sensitivity to early events in the aging process. Further developments of ESRI methods are expected to be of help in the ultimate goal accurate predictions of lifetimes for polymeric materials and a better understanding of the environmental factors. 

ISO 2440 1997 Flexible and rigid cellular polymeric materials - Accelerated ageing tests ISO 6914 2004 Rubber, vulcanized - Determination of ageing characteristics by measurement of stress at a given elongation... 

In comparison, no structural modification of model B was seen before 120 h of aging (80 °C). However, after 120 h two small doublets appeared in the NMR spectrum and several additional peaks became noticeable in the NMR spectrum. It was determined by NMR and IR spectroscopy that the hydrolysis products were an imide/carboxylic acid and an imide/anhydride. Model B was then aged for 1200 h at 80 °C to quantitatively determine the amount of hydrolysis products as a function of time. The relative intensity of the peaks due to carboxylic acid is constant after some time. The authors suggest that an equilibrium occurs between model B and the products formed during hydrolysis, and therefore, the conversion to hydrolysis products is limited to about 12%. This critical fraction is probably enough to cause some degradation of polymeric materials, but research on six-membered polyimides has remained active. 

Note 1 Thermodynamic processes that produce reversible changes in the physical structure of a polymeric material are termed physical aging. 

Unwanted degradation and oxidation processes can be avoided or at least suppressed for some time either by structural modiflcation of the polymer or by special additives. In practice, the addition of so-called antioxidants is particularly effective. Chemical substances that slow down oxidations and the following aging phenomena serve for this purpose. Antioxidants are sufficiently effective even in concentrations below 1 wt% and are added as early as possible to the polymer to be protected, e.g., already during the drying of powdery polymeric materials or during the preparation of granulates. Some of the most important so-called primary antioxidants are sterically hindered phenols and secondary aromatic amines secondary antioxidants are thioethers as well as phosphites and phosphonites. 

Heat aging Exposure of polymeric materials under specified conditions (temperature, time, presence or absence of air or oxygen, etc.), then testing them in stress-strain and hardness, determining the change of properties in comparison to the original (unaged materials). 

Figure 15.13 illustrates the hydrolytic stability of various polymeric materials, determined by a hardness measurement after exposure to high-RH aging. A period of 30 days in the 100°C, 95 percent RH test environment corresponds approximately to a period from 2 to 4 years in a hot, humid climate such as that of southeast Asia. The hydrolytic stability of urethane potting compounds was not believed to be a problem until it resulted in the failure of many potted electronic devices that were noticed first during the military action in Vietnam in the 1960s. 

Celluloses are similar to other linear polymeric materials in that they can possess one-dimensional order within an individual chain as well as three-dimensional order within an aggregate of chains. Increments in the levels of order occur during the isolation of native celluloses and also as a result of exposure to conditions that promote molecular mobility, such as elevated temperatures and immersion in plasticizing fluids. These increments generally result in embrittlement of the cellulosic materials. Similar effects are expected to occur upon aging of cellulosic textiles and papers over extended periods, and may be accelerated by hydrolytic cleavage of cellulosic chains. The implications of these effects for conservation practices, both with respect to recovery of function as well as in the assessment of deterioration, are reviewed. 

Chemical aging or chemical degradation is distinct from physical aging in that it involves often irreversible changes to the chemical structure of the polymer. Examples include oxidative crosslinking, de-polymerization, and UV-induced chain scission. These chemical changes can alter many of the physical and chemical properties of a polymeric material and again, often occur over extended timescales. 

The aging behavior of conventional filled polysiloxane materials has been relatively well studied. However, there is little currently known about the long term stability and aging behavior of polysiloxane nanocomposites. This is a key issue that must be addressed if polysiloxane nanocomposites are to become part of the next generation of polymeric materials. 

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