Shelf-aging

In general, one day of oven aging at 70°C corresponds to one year of natural or shelf aging (a minimum requirement for mbber products), whereas the oxygen and air bomb methods are more drastic. By varyiag the amounts and types or combiaations of antioxidants the relative effectiveness of these materials against normal oxygen deterioration can be determined. 

Shelf aging - The natural deterioration of rubber articles kept in storage or on the shelf under normal atmospheric conditions. This slow deterioration is due primarily to oxygen and ozone attack. 

H.H. Trieu, R. Avent, Effect of sterilization on shelf-aged UHMWPE tibial inserts, in Transactions of the 21st Annual Meeting of the Society for Biomaterials,... 

Clear-liquid separation is the appearance of a clear supernatant layer above the bulk of opaque paint after shelf-aging. This separation can occur within 1 day and be several inches thick in a 1-gal can. In tinted paints, colorant may float up into this layer and produce a very displeasing in-can appearance. Reincorporation of the supernatant layer by hand stirring or mechanical agitation usually solves the problem only temporarily because the clear-liquid layer returns after several days. HEUR/HEC- and HEUR/hydro-phobically modified HEC-thickened paints are particularly prone to this separation. 

Blend Ratio, HEUR Effective Molecular Weight, and HEC Molecular Weight. Table HI presents data that show the effect of these variables on the clear-liquid separation that occurs in a vinyl-acrylic flat after 1 month of shelf-aging. Generally, clear-liquid separation was greatest... 

In addition, oxidative degradation related to poor consolidation has been found in both in vivo and shelf-aged prostheses that were either gamma sterilized or ethylene oxide gas (EtO) sterilized (Costa et al. 2002). When oxidation related to consolidation occurs, the oxidation profile through the cups section is inhomogeneous and the maximum oxidation is observed near the center of the prosthesis. It is worth mentioning that this consolidation-related oxidation mechanism is often accompanied by whitening of the material, visible to the naked eye. 

FTIR map collected along the cross-section of an EtO-stehlized, shelf-aged UHMWPE acetabular cup. 

Figure 11.1 shows a collection of FTIR spectra through the thickness of a shelf-aged, EtO-sterilized cup where bulk oxidation was evident. Gamma irradiation of the UHMWPE was ruled out by examination of the transvinylene region of the ETIR spectra. The absorption at 1718 cm i is attributed to ketones. In this case, the oxidation was associated with poor consolidation of the UHMWPE, rather than the result of the sterilization process itself. These results suggest that prosthetic UHMWPE needs stabilization, as the totality of commercial polyolefines. Biocompatible stabilizers, such as vitamin E (an a-tocopherol), are easily available on the market and already employed in a number of different applications (Costa et al. 1998b, Costa et al. 2000). 

Shelf aging the change in a material s properties that occurs over time in storage. 

Although polymer degradation is more commonly assessed in inert atmosphere, the TGA in air may provide relevant and useful information to the evaluation of the oxidative stability of polymeric materials under natural conditions (weathering, shelf aging, in vivo, etc.). 

These materials are stable in storage for many months or years either as one-package or as two-package systems. In general, many two-package systems tend to have longer shelf age capabilities because the chemically reactive components can be separated until the time of use. However, some one-component products, such as the acetoxy sealants, also have excellent shelf stability and remain useful for up to three or four years after manufacture. 

Epoxy Sealant (Cabot, 1994) - Hydrophilic grades of fumed sihca are widely used as thickening agents for epoxy resin adhesives and sealants. They impart excellent initial viscosity and sag resistance. After shelf aging for several months, however, the epoxy systems lose part or all of their sag resistance. A simple two part epo sealant formulation to test the utihty of a series of mineral thixotropes is shown in Table 13. 

Greer K. A prospective study of the effects of five years shelf aging on the properties of UHMWPE following gamma sterilization in air or in a vacuum foil pouch. Transactions of the 28th Society for Biomaterials, 2003. p. 378. 

Lu S, Orr JF, Buchanan FJ. The influence of inert packaging on the shelf ageing of gamma-irradiation sterilised ultra-high molecular weight polyethylene. Biomaterials 2003 January 24(l) 139-45. 

Gsell RA, Taylor GK, Lin ST. Shelf aging of net shape compression molded 1900H ultra-high molecular weight polyethylene (UHMWPE) components. Transactions of the 44st Orthopedic Research Society, New Orleans, LA 1998 February 13-16, p. 364. 

TABLE 14.3 Oxidation index and tensile properties for X3 as manufactured and after 5 years shelf aging... 

Recent studies have shown that a large variability in the extent and in the distribution of oxidative degradation is observed in shelf-aged UHMWPE components dependent on the previously-mentioned parameters [38-42]. 

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