Durability, environmental mechanisms

Durability in its broadest sense covers all aspects of irreversible property change with time and use. This includes all types of environmental agent that contribute to degradation and all aspects of mechanical action. This guide seeks to be comprehensive but concentrates on the most common environmental effects and the most important mechanical properties. More details of the test procedures used can be found in text books and the relevant international standards as referenced. 

Although numerous studies (1-3) have described work aimed at establishing criteria for the durability of adhesive joints, a thorough understanding of effects of the chemical and mechanical properties, on the durability of adhesive bonds is lacking. More specifically, the effects of surface preparation and dynamic loading, especially under environmental service conditions, has not been explored in detail for automotive structures. In this paper, a description of the effects of environment on the durability of adhesive bonds is presented. Particular attention is given to... 

Recently, many research efforts have been directed at developing pretreatments for metal surfaces which produce oxide layers with pores, fibrous projections, or microscopic roughness which can enhance metal/polymer adhesion by mechanical means. In order for the pretreatments to lead to an increase in durability, the oxide layers formed must be stable under environmental conditions. The bulk 3l-33 S2-128> of the research in this area has been completed in an attempt to increase the durability of... 

The increasing use of high-performance fibrous composites in critical structural applications has led to a need to predict the lifetimes of these materials in service environments. To predict the durability of a composite in service environment requires a basic understanding of (1) the microscopic deformation and failure processes of the composite (2) the significance of the fiber, epoxy matrix and fiber-matrix interfacial region in composite performance and (3) the relations between the structure, deformation and failure processes and mechanical response of the fiber, epoxy matrix and their interface and how such relations are modified by environmental factors. 

Results of these on-going experiments are helping to clarify the mechanisms acting to produce environmentally induced embrittlement of Fe and Ni and suggest different roles for various segregated impurities. It is expected that understanding of the important mechanisms will help the designing of new alloys which are more durable in particular environments. 

Most structural adhesives are, therefore, formulated to provide the best compromise between environmental resistance and the desired mechanical properties. Experience has generally revealed that although the moisture ingress of the adhesive or sealant does affect the durability, it is seldom the dominant factor. Generally, of greater importance is how the moisture influences the adhesive-adherend interface region. Table 15.16 summarizes the moisture resistance and performance properties of some of the more common structural adhesives. 

The objective of this research was to gain a better understanding of the oxidative-cleavage pathway. If this mechanism could be comprehended, it is possible that a relatively low-cost and environmentally safe process could be developed for producing a durable, commercially acceptable, lignin-based adhesive. 

It can be concluded that there is a potential for membrane separation of almost any gas from a mixture of gases if physical and chemical properties are carefully considered as well as material properties and durability, possible transport mechanisms, and optimum process conditions evaluated. Creative reflection and advanced research will be able to develop this environmental friendly separation technique for applications within many areas in the future, and hopefully be able to displace old, energyconsuming (and not so clean) technology or combine with them in hybrid process solutions. The costs of the final solution will always be a major issue for commercialization. 

Several other tests are used to correlate properties with the working conditions. The dynamic mechanical analyzer (DMA) is used to measure modulus and viscoelastic properties related to ductility. By using the DMA test method, the degradation in modulus could be imderstood and applied to the end use application. The effect of the environmental conditions is also tested the esthetic durability in heated environments is not restricted to warp and relaxation attributed to creep. Creep and stress relaxation tests are also done on SMC/BMC for their structural applications. The retention of color is critical for a heated appliance application. BMC/SMC can be formulated to offer excellent color stability when exposed to high temperatures. 

Fillers provide films with conductive properties, influence their surface properties, affect their permeability, mechanical and optical properties, and affect their durability against environmental exposure. Various technologies are used to produce conductive films. These include lamination to metal foils (in-plant, using pressure sensitive adhesives), surface coating, and addition of conductive materials. Conductive films are widely used in packaging to limit static electricity. 

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