Foaming processes, plastics

When an engineering plastic is used with the structural foam process, the material produced exhibits behavior that is easily predictable over a large range of temperatures. Its stress-strain curve shows a significantly linearly elastic region like other Hookean materials, up to its proportional limit. However, since thermoplastics are viscoelastic in nature, their properties are dependent on time, temperature, and the strain rate. The ratio of stress and strain is linear at low strain levels of 1 to 2%, and standard elastic design... 

Surfactants used as lubricants are added to polymer resins to improve the flow characteristics of the plastic during processing they also stabilise the cells of polyurethane foams during the foaming process. Surfactants are either nonionic (e.g. fatty amides and alcohols), cationic, anionic (dominating class e.g. alkylbenzene sulfonates), zwitterionic, hetero-element or polymeric (e.g. EO-PO block copolymers). Fluorinated anionic surfactants or super surfactants enable a variety of surfaces normally regarded as difficult to wet. These include PE and PP any product required to wet the surface of these polymers will benefit from inclusion of fluorosurfactants. Surfactants are frequently multicomponent formulations, based on petro- or oleochemicals. 

We have already mentioned in Sect. 6.2, 6.3 that the physicochemical conditions of the foaming process and the foam stability criteria determine the upper and lower limits of cell sizes so that, depending on the polymer type, composition, and foaming process conditions, the upper limit of size may be as large as a few millimeters 36,83-85) recently, it was believed that the minimum size of a plastic foam cell cannot be less than several dozens of microns (Table 2). However, by the application of scanning electron microscopy and the mercury penetration method, plastic foam structures were found to incorporate gas voids whose minimum dimensions were fractions of a micron, i.e. 2 or 3 orders of magnitude smaller than could be observed earlier in cellular polymers... 

Ahnemiller, J., "Process and Performance Optimization of Chemically Foamed Polyesters," Plastics Engineering, 39 (2) 22-23 (February 1983). 

Schwartz, S.S. and Goodman, S.H., Chapter 15, "Foam Processes," in Plastic Materials and Processes, S.S. Schwartz and S.H. Goodman, editors. Van Nostrand Reinhold, New York (1982). 

Brabandt, H., Jr., "Multi-Component Foams Processing", Part I in Section III, "Manufacturing Equipment for Foamed Plastics", Handbook of Foamed Plastics, Ed. by R.J. Bender, Lake Publishing Corp., Libertyville, 111., (1965), p. 19-38. 

This is an extremely useful book devoting equal coverage to both plastics materials and processes. The book has 23 chapters including one. Chapter 15, on Foam Processes. Other chapters also have useful information on cellular plastics and elastomers. Ihe book is well illustrated and has a detailed table of contents and index. 

Yasuda, K. Yamaki, H. Injection molding technology with CO2 as a plasticize of resin. Proc. of international workshop of foam processing and supercritical fluid aided polymer processing. Tokyo, 2003 113-116. 

The relative strength of hollow-sphere foams lies between the theoretical performance of open- and closed-cell foams. The performance of optimized truss structures is similar to that of closed-cell foams and, for the Kagome truss, approaches the behavior of a Hashin-Shtrikman porous material. Honeycombs are the most efficient structures when loaded purely out-of-plane. However, plastic buckling can decrease its performance at low relative densities. Further, since honeycomb is highly anisotropic, any inplane loading results in severely reduced performance. Although the theoretical performance of closed-cell foams far exceeds that of open-cell foams, processing defects result in commercially available material that behaves similar to an open-cell material at low relative densities. Commercially available samples of other types of low-density metallic structures behave nearly as predicted. [17]... 

Polystyrene can be polymerised by foam processes, mass polymerisation, solution polymerisation, suspension polymerisation and emulsion polymerisation. Each contributes to the final characteristics of the plastics and the likely residues. A limit for styrene monomer may be imposed. Styrene can be readily analysed down to a 0.01% level. Polystyrene exhibits low shrinkage, hence is an excellent moulding material. 

Special Processes. Several more recent developments have opened the possibility of special foam processes that may grow to tremendous importance and perhaps even change the entire concept of foamed plastics. These are structural foam, reaction injection molding (RIM), and syntactic foam. 

These are typical of some of the novel foam processes that may be unknowingly changing the entire concept of foamed plastics. 

The fir.st, and obvious, area of interest in thermal properties is for applications involving thermal insulation. The polymer type most frequently involved is foams. For plastics and rubbers there is also a need for transport properties, particularly diffusivity. in the prediction of processing behavior. Most processes for forming these materials involve heat, often for quite short times, and the rate at which heat is transferred can be critical. 

Foamed plastics can be classified according to the nature of cells in them into closed-cell type and open-cell type. In a closed-cell foam each individnal cell, more or less spherical in shape, is completely closed in by a wall of plastic, whereas in an open-cell foam individual cells are inter-connecting, as in a sponge. Closed-cell foams are usually produced in processes where some pressure is maintained during the cell formation stage. Free expansion during cell formation typically produces open-cell foams. Most foaming processes, however, produce both kinds. 

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