Polymer components failure

Adhesion depends on a number of factors. Good adhesion is defined by most customers as substrate failure. The major adhesive manufacturers possess equipment that allows them to make bonds with customer substrates under conditions that closely simulate actual packaging lines. These bonds are peeled either automatically or by hand to gauge adhesion. The most important factors influencing adhesion are the wet-out of the substrate, partieularly by the polymer component of the adhesive system, and the specific adhesion with the substrate. Choice of resin is critical for both. Rosin, rosin esters and terpene phenolics are eommonly added for these purposes in EVA and EnBA-based systems. Adhesion at low temperatures is also influenced by the overall toughness of the system at the test temperature. 

Plastics. Part of the trend to substitute plastic and composite substrates for metals can be attributed to a desire to avoid the process of metallic corrosion and subsequent failure. Relatively little attention has been called to the possible failure modes of plastics under environments considered corrosive to metals. More extensive work should be conducted on the durability and life expectancy of plastic and composite materials under end-use environments. A further consideration is the potential for polymer degradation by the products of metal corrosion in hybrid structures comprising metal and polymer components. Since it is expected that coatings will continue to be used to protect plastic and composite substrates, ancillary programs need to be conducted on the mechanisms by which coatings can protect such substrates. 

With increasing use of polymeric materials in industry the corrosion engineer is faced with the need to have knowledge of the basic types of polymers, their characteristic features, modes of failure of polymeric components and the methods involved in characterization of polymers in failure analysis. Some characteristics of engineering polymers are as follows ... 

During creep, a loaded polymer component will gradually increase in length until fracture or failure occurs. This phenomenon is usually referred to as creep rupture or, sometimes, as static fatigue. 

In practice, nearly all polymer components are subjected to impact loads. Since many polymers are tough and ductile, they are often well suited for this type of loading. However, under specific conditions even the most ductile materials, such as polypropylene, can fail in a brittle manner at very low strains. These types of failure are prone to occur at low temperature and at very high deformation rates. 

Figure 8.14 The separation of polystyrene and polymethylmethacrylate samples of similar molecular mass (shown in the figure) by thermal FFF is based on the difference in the chemical composition of the two polymers. The failure to separate these two components by SEC shows that the two cannot be separated on the basis of size. Reprinted with permission from Macromolecules 19, 2618 1986 American Chemical Society.

Abstract Injection moulding, extrusion and other processes for manufacturing polymer components are discussed. Several case studies are presented where manufacture of polymer components is crucial to product performance, such as injection-moulded polycarbonate coimectors in a catheter, cylinder guard fractures, sight tubes, a crutch failure, and ozone cracking of tubing and a condom. 

Thermal analysis techniques will continue to play a vital role in assisting polymer scientists in all of their endeavours fundamental characterisation work and failure diagnosis studies the development of better polymers, polymer blends and compoimds the addressing of pressing current environmental concerns, such as the recycling of used tyres and the polymer components of electrical devices and the substitution of renewable raw materials (e.g. fibres such as hemp and natural rubber) for synthetic materials derived from petroleum products. 

The material in use as of the mid-1990s in these components is HDPE, a linear polymer which is tough, resiUent, ductile, wear resistant, and has low friction (see Olefin polymers, polyethylene). Polymers are prone to both creep and fatigue (stress) cracking. Moreover, HDPE has a modulus of elasticity that is only one-tenth that of the bone, thus it increases the level of stress transmitted to the cement, thereby increasing the potential for cement mantle failure. When the acetabular HDPE cup is backed by metal, it stiffens the HDPE cup. This results in function similar to that of natural subchondral bone. Metal backing has become standard on acetabular cups. 

Since most polymers, including elastomers, are immiscible with each other, their blends undergo phase separation with poor adhesion between the matrix and dispersed phase. The properties of such blends are often poorer than the individual components. At the same time, it is often desired to combine the process and performance characteristics of two or more polymers, to develop industrially useful products. This is accomplished by compatibilizing the blend, either by adding a third component, called compatibilizer, or by chemically or mechanically enhancing the interaction of the two-component polymers. The ultimate objective is to develop a morphology that will allow smooth stress transfer from one phase to the other and allow the product to resist failure under multiple stresses. In case of elastomer blends, compatibilization is especially useful to aid uniform distribution of fillers, curatives, and plasticizers to obtain a morphologically and mechanically sound product. Compatibilization of elastomeric blends is accomplished in two ways, mechanically and chemically. 

This topic has been mentioned in Section V, Failure, Defect and Contaminant Analysis, in Chapter 15, where a number of typical practical problem invetsigations were presented. Obviously the potential list of examples exhibiting different characteristics and requiring a different type of analysis is lengthy. When the sample is heterogeneous, e.g., a polymer blend or a composite, the study of the surface of a failed piece of material may reveal whether the problem is the interface of the components or that failure occurred within one of these. In particular in the case of crazing or necking orientation may have been induced, the way this can be analysed is discussed in Chapter 8. 

In practice, most lifetime prediction is based on service experience. Depending on the industry concerned, this can take the form of planned examination of components at the end of their service life or be limited to the explanation of warranty returns. Experience with polymers is now sufficiently long for service experience to be a prime source of information for components with lifetimes of up to 35 years. The construction industry provides a good example of systematic listing of component lifetimes, related to minimum quality levels and modified according to the service conditions. The electrical industry applies statistical methods to life components and predict failures. This, however, strays into the general field of engineering component lifetimes. In this book we are concerned with materials rather than components. 

FE data have been collected from the fracture of a wide variety of single and multi-component solids, ranging from single crystals of molecular solids to fiber-reinforced composites, and also from the peeling of adhesives 0-16 ). In this paper, we will restrict our attention to FE arising from the failure of polymer composites (fibrous and particulate), and the individual components thereof (fibers and matrix resins). 

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