Elastomers, thermal aging

Structure—Property Relationships The modem approach to the development of new elastomers is to satisfy specific appHcation requirements. AcryUc elastomers are very powerhil in this respect, because they can be tailor-made to meet certain performance requirements. Even though the stmcture—property studies are proprietary knowledge of each acryUc elastomer manufacturer, some significant information can be found in the Hterature (18,41). Figure 3a shows the predicted according to GCT, and the volume swell in reference duid, ASTM No. 3 oil (42), related to each monomer composition. Figure 3b shows thermal aging resistance of acryHc elastomers as a function of backbone monomer composition. 

Finally, all the additives prepared by hydroperoxide intermediates increase dramatically the life time (both in photo and thermal ageing) of elastomers such as EPDM(ENB) and polyoctenamer whose the original stabilities are well known to be very low. 

In order to homogenize the elastomer surface properties, thermal aging of PDMS was sometimes carried out (at 115°C for 12 h) [170]. 

The characterization of the physical and chemical changes that occur in montmorillonite/PDMS nanocomposite elastomers as they are thermally aged is reported. Broadband Dielectric Spectroscopy (BDS) was used to track changes in the physical interaction between the polymer and clay associated with increases in non-oxidative thermal stability (as determined by TGA). The evolution of volatile siloxane species from the elastomers was characterized with Thermal Volatilization Analysis (TVA). Results suggest that the improved thermal stability and the increases in polymer/clay association are a result of significant re-structuring of the polymer network. 

It is proposed that thermal aging of the Cloisite/PDMS elastomers promotes the reformation of the siloxane network into a more thermodynamically stable form though a series of catalytically driven chain backbiting, hydrolysis and recombination reactions. This produces a siloxane network with increased thermal stability which is more intimately associated with the nano-filler. 

Curing and thermal ageing of EPDM synthetic elastomers has been studied using both high-resolution and H wideline solid-state NMR experiments based on structural and dynamic parameter measurements. ... 

Three methods that were used to measure the chemical changes associated with oxidative degradation of polymeric materials are presented. The first method is based on the nuclear activation of lsO in an elastomer that was thermally aged in an, 802 atmosphere. Second, the alcohol groups in a thermally aged elastomer were derivatized with trifluoroacetic anhydride and their concentration measured via 19F NMR spectroscopy. Finally, a respirometer was used to directly measure the oxidative rates of a polyurethane foam as a function of aging temperature. The measurement of the oxidation rates enabled acceleration factors for oxidative degradation of these materials to be calculated. 

PA-6 or PA-66 and >50% acrylic elastomers has been introduced by DuPont (Zytel FN) as a plasticizer-free, low modulus composition with good low-temperature toughness, resistance to thermal aging and solvents (particularly to fluorocarbon refrigerants) (Table 15.17). 

PC/LDPE, PC/elastomer, etc.) Improve low temperature toughness Improve thermal-aging resistance... 

The polyacrylate and ethylene-acrylic copolymers and one of the ethylene-propylene terpolymers (Nordel) were the best of the Intermediate temperature elastomers. Except for resistance to compression set, these materials were Inferior to the silicones in thermal stability as measured by their retention of tensile properties. The other EPDM compounds and butyl rubber were considerably inferior to the above-mentioned elastomers. It is not expected that the service life of the tested materials will be limited solely by their ability to resist hydrolytic degradation. The only caulking compositions which retained moderate physical integrity on thermal aging were the silicones. 

When elastomers are subjected for extended periods of time at elevated temperatures to a compressive load while being permitted to distort, they will not recover to their original dimensions when the load is removed. Measuring the compression set of compressed elastomers during thermal aging is a very useful and realistic means of evaluating materials for gasket applications. 

Figure 11. Arrhenius plot of thermal aging in air of a silicone elastomer.

Elastomers derived from isoprene and butadiene are readily crosslinked by peroxides but many of the vulcanizate properties are inferior to those of accelerated-sulfur vulcanizates. However, peroxide vulcanizates of these diene rubbers may be desirable in applications where improved thermal aging and compression set resistance are required. 

Bhargava S, Kubota M, Lewis RD, Advani SG, Prasad AK, Deitzel JM. Ultraviolet, water, and thermal aging studies of a waterborne polyurethane elastomer-based high reflectivity coating. Prog Org Coat 2015 79 75-82. 

Dielectric analysis has also been applied to study polymer thermal ageing [894], e.g. of adhesive bonded structures [895]. Dielectric loss is a means for detecting early steps in polymer degradation by oxidation, although nowadays CL is a strong competitor. The dielectric behaviour of post-irradiation oxidised PP/HALS was also reported [796]. Dielectric analysis has not been extended to many elastomer applications. Dissociation and mobility of salt complexes as heat stabilisers in PA4.6 have been investigated by means of DIES the technique was also used to study thiourea-doped PVAL [903]. 

Macaione et al [235] have used TG for the characterisation of SBR, BR and NR in mono-, di-, or triblend rubber systems and carbon-filled rubber composites and determined the percentage of highly volatile organics, elastomer(s), carbon-black, and inorganic residue for each sample. Lochmiiller et al [194] applied factor analytical methods to evaluate TG results of a series of rubber blends and mixtures composed of chloroprene rubber, NBR, and common rubber additives. TG and measurements of toluene extractable matter of cured siloxane rubbers thermally aged in inert gas atmosphere at 80° C showed a build-up of low-MW fragments in the rubber network with age [244]. 

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