Materials alternative

Heat/Solvent Recovery. The primary appHcation of heat pipes in the chemical industry is for combustion air preheat on various types of process furnaces which simultaneously increases furnace efficiency and throughput and conserves fuel. Advantages include modular design, isothermal tube temperature eliminating cold corner corrosion, high thermal effectiveness, high reHabiHty and options for removable tubes, alternative materials and arrangements, and replacement or add-on sections for increased performance (see Furnaces, fuel-FIREd). 

Competitive uses exist for almost every chemical a purchasing department buys. Often the demands of other uses and users are larger and influence supply and prices. Therefore, it is necessary to monitor these uses and especially to determine whether alternative materials might come into use. 

New Materials Technology. The unique chemical compatibihty of Si02 with sihcon and aluminum has been a significant factor in the dominance of siUcon-based semiconductor technology for MOSFETs in particular and integrated circuits in general. Two enhancements of conventional bipolar or MOS processes have been studied siUcon on insulator (SOI) and SiGe. An alternative material of importance is SiC. 

The use of calcium carbonate in thermosets continues to grow as these plastics replace alternative materials, especially in automotive appHcations. Increased loadings of calcium carbonate in thermosets reduce cost and provide better surface characteristics. 

Cadmium. In 1989, U.S. consumption of cadmium for coatings was 1474 t (7), compared to 1552 t in 1970, 2089 t in 1979, and 1230 t in 1985. Cadmium plating amounts to about 15% of total cadmium production (see Cadmiumand cadmium alloys). Of the cadmium being plated in 1989, 30% was for automotive parts, over 22% for electronics, and 18% for industrial fasteners. Because of cadmium s high and weU-pubHcized toxicity and very tight waste restrictions, there are considerable efforts to develop alternative materials, and the quantities of cadmium used in electroplating ate expected to decrease. The price of cadmium anodes in early 1993 was about l/kg. 

Choices of alternative materials. Corrosion probes are carefully chosen to be as close as possible to the alloy composition, heat treatment, and stress condition of the material that is being monitored. Care must be taken to ensure that the environment at the probe matches the service environment. Choices of other alloys or heat treatments and other conditions must be made by comparison. Laboratory testing or coupon testing in the process stream can be used to examine alternatives to the current material, but the probes and the monitors can only provide information about the conditions which are present during the test exposure and cannot extrapolate beyond those conditions. 

In most instances, there will be more than one alternative material which may be considered for a specific application. Calculation of true long-term costs requires estimation of the following ... 

The relatively long service life of the duplex tubes suggested that similar tubes be used for retubing. Because of supplier delays in securing duplex tubing, however, alternative materials, including admiralty brass, were considered. 

Engineering design, then, involves many considerations (Fig. 1.7). The choice of a material must meet certain criteria on bulk and surface properties (strength and corrosion resistance, for example). But it must also be easy to fabricate it must appeal to potential consumers and it must compete economically with other alternative materials. In the next chapter we consider the economic aspects of this choice, returning in later chapters to a discussion of the other properties. 

Cost rules out almost all alternative materials for long-distance pipe lines it is much cheaper to build and protect a mild steel pipe than to use stainless steel instead - even though no protection is then needed. The only competing material is a polymer, which is completely immune to wet corrosion of this kind. City gas mains are now being replaced by polymeric ones but for large diameter transmission lines, the mechanical strength of steel makes it the preferred choice. 

Although resistance to deflection and plastic yielding are obviously of first importance in choosing alternative materials, other properties enter into the selection. Let us look at these briefly. Table 27.4 lists the conditions imposed by the service environment. 

Notable among the alternative materials are the MBS polymers, in which methyl methacrylate and styrene are grafted on to the polybutadiene backbone. This has resulted in two clear-cut advantages over ABS. The polymers could be made with high clarity and they had better resistance to discolouration in the presence of ultraviolet light. Disadvantages of MBS systems are that they have lower tensile strength and heat deflection temperature under load. 

The electrical insulation properties of the acetal resins may be described as good but not particularly outstanding. There are available alternative materials which are better insulators and are also less expensive. There are, however, applications where impact toughness and rigidity are required in addition to good electrical insulation characteristics, and in these instances acetal resins would be considered. Table 19.3 lists some of the more important electrical characteristics of Delrin acetal resin. Data for the trioxane-based copolymer resin (e.g. Celcon) are virtually identical. 

Only one other eellulose ether has been marketed for moulding and extrusion applieations, benzyl cellulose. This material provides a rare example of a polymer whieh although available in the past is no longer commercially marketed. The material had a low softening point and was unstable to both heat and light and has thus been unable to compete with the many alternative materials now available. 

The principal application of melamine-formaldehyde moulding compositions is for the manufacture of tableware, largely because of their wide colour range, surface hardness and stain resistance. The stain resistance does, however, leave something to be desired and one aim of current research is to discover alternative materials superior in this respect. Cellulose-filled compositions also find a small outlet for trays, clock cases and radio cabinets and other purposes. The mineral-filled powders are used in electrical applications and knobs and handles for kitchen utensils. 

Ashby has taken his approach a stage further with the introduction of physically based estimates of material properties where these have not been measured (Ashby 1998. Bassett et al. 1998), where an independent check on values is thought desirable or where property ranges of categories of materials would be useful. Figure 5.5(c) is one example of the kind of estimates which his approach makes possible. A still more recent development of Ashby s approach to materials selection is an analysis in depth of the total financial cost of using alternative materials (for different number of identical items manufactured). Thus, an expanded metallic foam beam offers the... 

If you find that you cannot absorb increases in labor and raw material costs, then you may have to look for alternative approved sources, alternative materials, alternative methods or consider alternative designs. By including price in the improvement formula, it will act as a driving force. 

Serious considerations of alternative materials Adequate containment of identified carcinogens Medical screening of employees at risk ... 

Alternative materials may be used, including stainless steel and plastic. Sectional tanks may be installed for convenience in restricted places, but great care must be taken if they are subject to temperature variations, which can give rise to joint problems. 

Cost Cost of any task of project may exceed budget. When purchase agreements are made. First, seek alternative suppliers, then, consider alternative materials. 

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