Service environment aging

Special surface modifications are available to further improve reinforcement. The objective of the surface treatment is to increase filler loading and/or improve physical properties without loss of rheological characteristics. A variety of surface-modified kaolins have been introduced including clays treated with silane, titanate, polyester, and metal hydroxide. Silane-treated kaolin is used in applications requiring maximum aging characteristics in the service environment. 

Environmental aging is usually less severe in service (laboratory tests tend to accelerate aging so that the testing can be completed in a reasonable time). However, the effects of the actual service environment are generally more complex. For example, there may be simultaneous exposure to cyclic stress, cyclic temperature, and humid environments. 

A relatively new analytical technique, chemiluminescence (CL), is an ultrasensitive technique, and it has been reported that reaction rates as low as 10 mole/year can be measured (1-5). Thus, it could monitor the aging reactions on a real-time basis while the resins are exposed to a simulated service environment. If the method can be shown to be sufficiently sensitive and reliable, the errors inherent in extrapolating accelerated aging data to the actual conditions encountered can be eliminated (6-8). 

The data indicate that the mechanism responsible for the CL production in this resin system does not change appreciably within the temperature range investigated, i.e., up to 90 C. This result suggests that CL may be a good method to monitor real-time aging within the service environment. 

Stress Is an Important parameter In the service environment of loadbearing polymeric materials (1). The accelerating effect of stress on polymer-aging reactions Is recognized however, no experimental methods exist for direct determination of the effect of stress on the rates of real-time aging reactions. Cheoilluml-nescence offers potential for direct determination of the rates of polymer aging reactions. 

Rubbers are used in a wide variety of service environments and share many of the requirements and features of other materials discussed in this book, although resistance to light aging is generally much less serious a problem than in plastics because of the extensive use of carbon black as a filler or pigment. Attention is therefore drawn to other chapters on environmental resistance and weathering. This section focuses on properties more closely identified with rubbers. 

The nature of the polymeric material could also change with the service environment. Parts may swell in solvent, become brittle when exposed to UV, eliminate plasticizer on aging, gain a plasticizer (water) during exposure to humidity, and go through many other changes. All of these will have an effect on the joint, whether it is bonded with adhesives or mechanically fastened. 

Although of medium-to-high cost and of low initial tack, so as to need addition of tackifiers, NBR has a number of properties needed in specific service environments. Its temperature resistance is good if cured, NBR can easily be used at 150-175°C. Similarly, ageing properties, when compounded with adequate antioxidants, are excellent. 

Given the strength of most Pb-free solders and the stress levels that often prevail in an interconnection as a result of service environments or accelerated aging tests, the solder will deform primarily in the primary or transient stage. Therefore, it is important to understand the underlying deformation mechanisms that are active at this stage. A brief synopsis follows, describing several proposed theories for primary creep. A detailed account of those theories can be found in Ref 54. 

The addition of an aggressive chemical environment will greatly complicate the testing. All the problems of ageing tests apply, the uncertainty and variation in service conditions, the time scales involved, the validation of any accelerating process and the extrapolation to service conditions. Additional environmental effects will escalate the costs and the uncertainty of truly matching service rises steeply. 

Oilfields in the North Sea provide some of the harshest environments for polymers, coupled with a requirement for reliability. Many environmental tests have therefore been performed to demonstrate the fitness-for-purpose of the materials and the products before they are put into service. Of recent examples [33-35], a complete test rig has been set up to test 250-300 mm diameter pipes, made of steel with a polypropylene jacket for thermal insulation and corrosion protection, with a design temperature of 140 °C, internal pressures of up to 50 MPa (500 bar) and a water depth of 350 m (external pressure 3.5 MPa or 35 bar). In the test rig the oil filled pipes are maintained at 140 °C in constantly renewed sea water at a pressure of 30 bar. Tests last for 3 years and after 2 years there have been no significant changes in melt flow index or mechanical properties. A separate programme was established for the selection of materials for the internal sheath of pipelines, whose purpose is to contain the oil and protect the main steel armour windings. Environmental ageing was performed first (immersion in oil, sea water and acid) and followed by mechanical tests as well as specialised tests (rapid gas decompression, methane permeability) related to the application. Creep was measured separately. 

The second class includes tests for ageing, oxygen and ozone attack, adhesion, resistance to wear and tear, fatigue, etc. This class also includes tests under simulated service conditions under laboratory environments and immersion in fluids. These two classes of tests are conducted on vulcanized rubber. 

Stress relaxation measurements can also be used as a general guide to ageing, and it is particularly relaxation due to chemical effects which is then studied. Such measurements are normally made in tension and will be considered in Chapter 15 as an ageing test. Hence, in this section, only relaxation tests in compression will be discussed as this mode of deformation is the only one commonly used and standardised to directly estimate the relaxation of rubbers in service. For an application in tension, the methods described in Chapter 15 could, of course, be adapted. It must be appreciated that the methods in compression do not only measure relaxation due to physical effects, especially when elevated temperatures and liquid environments are used, so that the distinction is a little blurred. 

Finally, the rate of change in the critical property must be measured relative to expected environments of different severity and time intervals. If measurement cannot be made at the service temperature in the time that is available (as is generally the case), then accelerated tests may be used at elevated temperature or increased frequency. However, it is extremely important that care be taken to match the accelerated test conditions to the service conditions in as realistic a way as possible. For example, if accelerated aging by elevated temperatures is being used, the temperature must not be so high as to begin a degradation mechanism that would not normally be seen in service. 

Adhesives may experience many different and exotic environments. Environmental aging of adhesives is accomplished by exposing a stressed or unstressed joint to simulated operating conditions. Exposure is typically to elevated temperature, water, salt spray, or various chemical solutions that simulate the service conditions. A number of standard chemicals that are used to soak bonded specimens for 7 days at room temperature are... 

Smaller reactor size reduces the cost, improves control, and isolates process variables, however, effects of catalyst aging/deac-tivation as a function of time are not similarly reduced. These effects can be accelerated in the laboratory environment by increased temperature, water partial pressure, contaminant gas partial pressure, and various contaminant metals. As with scaled down equipment, these efforts are not without problems, however, when some catalyst lifetimes are measured in years, this is the only viable solution to meaningful catalyst research and development. This type of testing, coupled with characterization, has resulted in FCC catalysts with less resistance to coking and thus longer service life. 

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