Differences from Metals

Before commencing any machining operations, it is best to consider how polyurethanes differ from metals and the general effect this difference has on the process. 

The thermal conductivity of metals is much greater than that of all polyurethanes. The impact of this is that in the case of metals the heat is readily removed by the coolant into the stock and machine. The lower thermal conductivity of the polyurethane means that the heat remains near the surface of the part. Without proper care, the part can melt quite easily. Most polyurethanes start softening at about 135°C and melt to a gummy state by 180°C. 

Dimensional measurements taken when the material is hot will be much larger than when the material is at room temperature and 50% relative humidity. This means that parts that may be in specification if measured during machining may be too small when reinspected later. Experience is needed to know how much extra must be allowed so that on cooling the dimensions will be correct. 

The use of water-based oil coolants and light oils will help to remove some of the heat generated at the cutting tool. Care must be taken to match the coolant 

When subjected to force, polyurethanes will distort. When compressed, the volume of the polyurethane remains, for practical purposes, the same. If the polyurethane is attached too firmly to the machine, it may be distorted. After machining when the part has been removed, the shape will change, giving a faulty part. 

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Polymeric surfaces are fundamentally different from metal oxide surfaces, and consequently the technical challenges to obtaining strong and durable adhesive... 

There are, of course, many more aspects of composite hardware design that differ from metallic bonded structure but do not necessarily involve adhesive bonding. For instance there are many types of reinforcement tape and fabric to choose from, the orientation of the plies must be chosen, the ply stackups must be balanced to avoid part warping after cure, a minimum number of plies must be used to prevent non-visible impact damage that significantly affects the load carrying capability of the part, etc. 

It is usual to think that plastics are a relatively recent development but in fact, as part of the larger family called polymers, they are a basic ingredient of animal and plant life. Polymers are different from metals in the sense that their structure consists of very long chain-like molecules. Natural materials such as silk, shellac, bitumen, rubber and cellulose have this type of structure. However, it was not until the 19th century that attempts were made to develop a synthetic... 

Adaptation of metal tolerances to plastics is not advisable. With plastics reaction to moisture and heat, for example, is drastically different from metals, so that pilot testing under extreme use conditions is almost mandatory for establishing adequate tolerance requirements. Also important to control cost is that close tolerances should be indicated only where needed, carefully analyzed for their magnitude, and proven out as to their usefulness. 

NOTE Electrolytes differ from metallic conductors in that the current is carried by ions, not by electrons (ion from the Greek word meaning to go ), so the passage of an electric current through an electrolyte is always accompanied by the transfer of matter. 

The unique catalytic properties of supported metal complexes, compared with different from metal and metal oxides, have been devoted to various chemical processes, including selective oxidation catalysis, shape selective catalysis and... 

Newtonian Viscosity in Glasses. As we saw in Chapter 1, the structure of glasses is fundamentally different from metals. Unlike metals and alloys, which can be modeled as hard spheres, the structural unit in most oxide glasses is a polyhedron, often a tetrahedron. As a result, the response of a structural unit to a shear force is necessarily different in molten glasses than in molten metals. The response is also generally more complicated, such that theoretical descriptions of viscosity must give way completely to empirical expressions. Let us briefly explore how this is so. 

Metal displacement in die flo-tum process differs from metal displacement in the cold rolling mill-process in that flo-tuming displaces the metal in el spiral manner as work revolves as compared to the displacement of metal in a longitudinal direction in the cold rolling process... 

Oxide Electrodes - Oxide electrodes are fundamentally different from metal electrodes in that the electrode itself exhibits oxygen-ion conductivity. This means that ionic diffusion through the electrode to the electrode/electrolyte interface may be of importance (see Figure 6). 

Ion-selective electrodes, discussed in the remainder of this chapter, respond selectively to one ion. These electrodes are fundamentally different from metal electrodes in that ion-selective electrodes do not involve redox processes. The key feature of an ideal ion-selective electrode is a thin membrane capable of binding only the intended ion. 

Simple ionic solids such as NaCl and KBr are like metals in that the individual ions are spheres that pack together in a regular way. They differ from metals, however, in that the spheres are not all the same size—anions are generally larger than cations (Section 6.2). As a result, ionic solids adopt a variety of different unit cells, depending on the size and charge of the ions. NaCl, KC1, and a number of other... 

I have attempted to create ceramics from the atom, the smallest building block of matter. Atoms are linked to bigger building blocks. How does that link lead to the formation of plastics in the one case, and ceramics in the other And in which ways do these material groups differ from metals and other materials ... 

In the previous section we saw that the experimental heats of chemisorption of many gases are highest on tantalum and lower on other metals, the decrease following a certain order. The order, mentioned there, is not exactly the same as the order for the heats of sublimation (78). If, however, Eq. (32) is valid and the term of the electronegativities may be neglected or is the same for all metals, the term containing the bond strength between metal atoms would be the only one which differs from metal to metal, and the term would follow the heat of sublimation. 

Section III is a discussion of HDN by catalysts different from metal sulfides, such as metals, metal carbides, and metal nitrides. The latter two classes of compounds have drawn much attention in the preceding decade in several fields of catalysis. Their possible role in hydrotreating is analyzed critically. 

Such electrodes differ from metalic ones only in the fact that the solution due to the negative charge of the ions formed gets negatively charged against the electrode, when the electrolytic solution pressure of the element is greater than the osmotic pressuro of its anions in the solution. 

Beachmarks appear as in the case of metals. Detailed mechanism of crack growth differs from metals and fatigue failure in polymers is not characterized by an endurance limit... 

Covalent bonding and metallic bonding both involve electron sharing. Explain how covalent bonding is different from metallic bonding. 

Polymeric materials show a wide range of stress-strain characteristics. One characteristic of polymers that is markedly different from metals and ceramics is that their mechanical properties are highly time- and temperature-dependent. An elastomer or a rubbery polymer shows a stress-strain curve that is nonlinear. 

Thermal expansion often very different from metals, so that clearances can be lost with temperature change. 

The nature of esters or ethereal salts has been fully discussed already in connection with the esters of inorganic acids and alcohols (p. 102). The name salts applies because they are formed by neutralizing an alcohol, acting as a base, with an acid. It must be emphasized, however, that in so terming these compounds salts we do not mean this to apply in a physical chemical sense as describing their properties in solution in accordance with the electrolytic theory of ionic dissociation. We are dealing here with questions of composition and constitution. Ethereal salts differ from metal salts, at least as to the degree of their dissociation into ions when in solution. 

It is important to note at the outset of this discussion that membrane electrodes are fundamentally different from metal electrodes both in design and in principle. We shall use the glass electrode for pH measurements to illustrate these differences. 

The rest of the elements are nonmetals or semimetals. Nonmetals are different from metals in many ways. Most nonmetals are gases, like oxygen. Solid nonmetals are hard and brittle, like carbon. They break apart easily. Carbon is used to make some pencil tips. Bromine is the only liquid nonmetal. Semimetals have traits of both metals and nonmetals. 

Effects of concentration. Anions also differ from metal ions in that they appear to react with uniform sites. That is, there is no evidence for the existence of a range of sites differing in affinity. This was noted by Dzombak and Morel [3] and has recently been considered in detail by Strauss [37]. 

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