Other Dimensional Changes

A partially crystalline polymer that is not fully crystallized during processing will shrink when heated to a certain temperature. When the chain mobility is increased enough for the chain to begin to form into crystallites, the polymer will shrink, since the crystallites occupy less volume than the amorphous regions. This behavior can be seen if one heats a nonoriented PET or nylon film. Most often it is associated with nonpackaging applications like the shrinkage of polyester fabrics when washed in water that is too hot. 

Oriented materials (see Section 7.2.6) can be caused to shrink by increasing the temperature, even if they are totally amorphous polymers like polystyrene. When orientation occurs, the amorphous regions are stretched into closer packing than is energetically at equilibrium. For partially crystalline polymers, the crystalline regions can be disrupted by orientation forces also. Therefore, when heat is applied and chain motion increases, the chains try to return to their low energy state. 

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Deterioration. Paintings are composite objects that have high vulnerabiUty. The various materials are adhered to each other, especially in a laminated stmcture, to form a source of potential trouble. Any dimensional change in one of the components or between the components as a consequence of changes in environmental conditions results in a strain on the adhesion of the various parts. Strains can lead to failure of the adhesion. This is one of the principal causes of losses in panel paintings, where the dimensional changes in the wooden support cause losses in adhesion between the paint layer and the support. 

Acrylic Resins. The first synthetic polymer denture material, used throughout much of the 20th century, was based on the discovery of vulcanised mbber in 1839. Other polymers explored for denture and other dental uses have included ceUuloid, phenolformaldehyde resins, and vinyl chloride copolymers. Polystyrene, polycarbonates, polyurethanes, and acryHc resins have also been used for dental polymers. Because of the unique combination of properties, eg, aesthetics and ease of fabrication, acryHc resins based on methyl methacrylate and its polymer and/or copolymers have received the most attention since their introduction in 1937. However, deficiencies include excessive polymerization shrinkage and poor abrasion resistance. Polymers used in dental appHcation should have minimal dimensional changes during and subsequent to polymerization exceUent chemical, physical, and color stabiHty processabiHty and biocompatibiHty and the abiHty to blend with contiguous tissues. 

These properties of integration with the underlying metal and negligible dimensional change are the most important features which distinguish diffusion coatings from other types of protection. 

Zinc diffusion sherardisingY " is mainly used for protection of ferrous metals against atmospheric corrosion. It has, in some respects, properties related to other types of zinc coating such as galvanising, but owing to the small dimensional change involved, it is of particular value for the treatment of machined parts, bolts, nuts, etc. 

In many cases, a product fails when the material begins to yield plastically. In a few cases, one may tolerate a small dimensional change and permit a static load that exceeds the yield strength. Actual fracture at the ultimate strength of the material would then constitute failure. The criterion for failure may be based on normal or shear stress in either case. Impact, creep and fatigue failures are the most common mode of failures. Other modes of failure include excessive elastic deflection or buckling. The actual failure mechanism may be quite complicated each failure theory is only an attempt to explain the failure mechanism for a given class of materials. In each case a safety factor is employed to eliminate failure. 

In addition to dimensional changes from changes in temperature, other types of di-... 

Thermoanalytical methods comprise a series of techniques in which a property is determined at different temperatures or as the temperature changes continuously. The property measured may include the mass of the sample (TGA), the heat flow to the sample (DSC), the magnetic character of the sample (TMA), or some other property such as dimensional changes. Each of these types of measurements gives information on some change undergone by the sample, and if the change is followed over time, it is possible to derive kinetic information about the transformation. 

Exception to this inert adsorbent assumption has been taken by Cook et at 101) and by Brunauer 24, 102). It has been stated 24), The theoretical arguments advanced in favor of this assumption were inadequate, and experimental data wholly lacking. The dimensional changes which take place when rare gases are adsorbed on to rigid adsorbents provide conclusive evidence that the assumption of inert adsorbents (for physical adsorption) is invalid. Many of the experiments on dimensional changes are not relevant here, in some cases because of the lack of heats of adsorption and in others because of the somewhat ill-defined nature of carbon and charcoal surfaces, a small amount of chemisorption may have taken place. It is generally accepted that adsorbents are not inert when chemisorption occurs. 

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