Thermal processing foams

Foams, in the form of froths, are intimately involved and critical to the success of many mineral-separation processes (Chapter 10). Foams may also be applied or encountered at all stages in the petroleum recovery and processing industry (oil-well drilling, reservoir injection, oil-well production and process-plant foams). A class of enhanced oil recovery process involves injecting a gas in the form of a foam. Suitable foams can be formulated for injection with air/nitrogen, natural gas, carbon dioxide, or steam [3,5]. In a thermal process, when a steam foam contacts residual crude oil, there is a tendency to condense and create W/O emulsions. Or, in a non-thermal process, the foam may emulsify the oil itself (now as an O/W emulsion) which is then drawn up into the foam structure the oil droplets eventually penetrate the lamella surfaces, destroying the foam [3], See Chapter 11. 

Foamed carbon is also nongraphitizable. The cell structure of the polymer foam remains intact upon careful carbonization and densities lower than 0.1 g/cm- are obtainable. The thermal conductivity is just above that of plastic foams, but foamed carbon can be used at much higher temperatures. Its relatively low compressive strength can be increased by impregnation with pyrolytic carbon, although the thermal conductivity increases at the same time. By comparison with glassy carbon, foamed carbon is easy to work, so that the shape does not have to be established at the start of the process. Foamed carbon is corrosion resistant, as are all carbon modifications. 

The density measurements on thermally processed polymers clearly show the formation of a foamed polymer. The initial density values for selected foams together with the respective polyimide homopolymers are shown in Tables 4 and 5. The density values for the ODPA/FDA and PMDA/FDA homopolyimides were both 1.28 g cm 3 and 3FDA/PMDA is 1.34. Most of the propylene oxide-based copolymers derived from these copolymers ranged from 1.09 to 1.27gem-3, which is -85-99% of that of the polyimide homopolymers, irrespective of the architecture of the copolymer (i.e., triblock vs graft). This is consistent with 1-15% of the film being occupied by voids. From these data (i.e., comparison of Tables 2 and 3 with Tables 4 and 5, respectively), it appears that the volume fraction of propylene oxide in the copolymer (i.e., -80% or less). Thus, the efficiency of foam formation is poor, irrespective of the copolymer architecture. Conversely,... 

Comparing to the standard spray drying process, the addition of a foaming gas increases dryer throughput, intensifies thermal process efficiency, reduces thermal degradation of the product, and increases retention of highly volatile substances [111]. 

Biodegradable PUs can be processed into various products such as freeze-dried foams [67], electrospun fibers [115], and 3D-printed scaffolds [68], by the use of solvent or heat. In the case of thermal processing, the moisture must be removed to avoid heat-induced hydrolysis [72], Biodegradable PU has been used as scaffolds for the repair of bones, cartilages, and blood vessels [68,116,117], demonstrating the potential in a wide range of medical applications. 

Intumescent fire retardant additives undergo a thermal degradation process on heating, which produces a thermally stable, foamed, multicellular residue called intumescent char. When these substances are added to a polymeric material which is later involved in a fire, they produce an intumescent char which accumulates on the surface, while the polymer is consumed, providing insulation to the underling materials and partially protecting it from the action of the flame. 

Prasad et al. [106] reported a linear relationship between A7/d (0-425 J/g) and azodicarbonamide content (0-36%). DSC thus allows detection of the level of undecomposed CBAs present in processed foam products and establishes the onset temperature for the decomposition. Advantages of DSC over EGA techniques are ease of operation, shorter analysis time, and detection of azodicarbonamide concentrations as low as 1%. Dixon etal. [107] have correlated thermal analysis data (DSC, TGA) of a variety of CBAs with cell morphology of extruded, expanded PP rod samples. CBAs with a higher temperature and rate of gas evolution lead to foams displaying a finer cell size structure and higher cell density. 

Polyphenols. Another increa singly important example of the chemical stabilization process is the production of phenoHc foams (59—62) by cross-linking polyphenols (resoles and novolacs) (see Phenolic resins). The principal features of phenoHc foams are low flammabiUty, solvent resistance, and excellent dimensional stabiUty over a wide temperature range (59), so that they are good thermal iasulating materials. 

Other. Because a foam consists of many small, trapped gas bubbles, it can be very effective as a thermal insulator. Usually soHd foams are used for insulation purposes, but there are some instances where Hquid foams also find uses for insulation (see Eoamed plastics Insulation, thermal). Eor example, it is possible to apply and remove the insulation simply by forming or coUapsing the foam, providing additional control of the insulation process. Another novel use that is being explored is the potential of absorbing much of the pressure produced by an explosion. The energy in the shock wave is first partially absorbed by breaking the bubbles into very small droplets, and then further absorbed as the droplets are evaporated (53). 

Mechanical Properties and Structural Performance. As a result of the manufacturing process, some cellular plastics have an elongated cell shape and thus exhibit anisotropy in mechanical, thermal, and expansion properties (35,36). Efforts are underway to develop manufacturing techniques that reduce such anisotropy and its effects. In general, higher strengths occur for the paraHel-to-rise direction than in the perpendicular-to-rise orientation. Properties of these materials show variabiUty due to specimen form and position in the bulk material and to uncertainty in the axes with respect to direction of foam rise. Expanded and molded bead products exhibit Httie anisotropy. 

Thermal stabihty of the foaming agent in the presence of high temperature steam is essential. Alkylaromatic sulfonates possess superior chemical stabihty at elevated temperatures (205,206). However, alpha-olefin sulfonates have sufficient chemical stabihty to justify their use at steam temperatures characteristic of most U.S. steamflood operations. Decomposition is a desulfonation process which is first order in both surfactant and acid concentrations (206). Because acid is generated in the decomposition, the process is autocatalytic. However, reservoir rock has a substantial buffering effect. 

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