Microcellular Foamed Polymers

The concept of microcellular thermoplastic foam was developed in the 1980s by researchers at the Massachusetts Institute of Technology and was based on the idea that the creation of a very large number of microbubbles, smaller than the preexisting natural flaws in a polymer, can reduce the material cost and consumption in mass produced plastic parts without compromising mechanical properties [15, 16]. 

Microcellular foams can be produced by noncontinuous processes such as a batch process [2, 12, 15, 16, 31, 32, 34, 35], continuous processes such as extrusion and injection molding [24,33,36,37], orby asemicontinuousprocess [38]. Since the semicontinuous process is not extensively used in the scientific community or in the industry, it will not be described in this chapter. Readers are encouraged to refer Ref. 38 for detailed information on this process. To date, microcellular foams have been produced in amorphous polymers [12, 31, 32, 34], semicrystalline polymers [35], and in elastomers [16]. Recently, MCF structures have also been produced in plastics filled with inorganic nanoparticles (montmorillonite) [39-43], as well as organic cellulosic fiber filled plastic composites [12, 31, 32, 34]. 

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A number of processing techniques are currently under development which enhance material properties by generating thin polymer layers within components, either via the formation of microcellular foams or microlayered structures. Microcellular foamed polymers contain high concentrations (> 10 bubbles cm ) of gas bubbles of the order of 1-10 pm in diameter.Such materials can provide a reduction in density of 20 to 40% in combination with increased fatigue and impact resistance without significantly compromising modulus and yield strength and will be processed by either extrusion or... 

Zhai, W. T., H. Y. Wang, J. Yu, J. Y. Dong, and J. S. He. 2008a. Cell coalescence suppressed by cross-linking structure in polypropylene microcellular foaming. Polym Eng Sci 48 1312-21. 

Kumar V., Wella- J. E., A Model for the Unfoamed Skin on Microcellular Foams, Polymer Engineering and Science, 34 (3) 169-173 (1994). 

Ramesh, N. S., Rasmussen, D. H., and Campbell, G. A., The Heterogeneous Nucleation of Microcellular Foams Assisted by the Survival of Microvoids in Polymers Containing Low Glass Transition Particles. Part 1 Mathematical Modeling and Numerical Simulation, Polym. Eng. ScL, 34, 1685 (1994)... 

Olefins or alkenes are defined as unsaturated aliphatic hydrocarbons. Ethylene and propylene are the main monomers for polyolefin foams, but dienes such as polyisoprene should also be included. The copolymers of ethylene and propylene (PP) will be included, but not polyvinyl chloride (PVC), which is usually treated as a separate polymer class. The majority of these foams have densities <100 kg m, and their microstructure consists of closed, polygonal cells with thin faces (Figure la). The review will not consider structural foam injection mouldings of PP, which have solid skins and cores of density in the range 400 to 700 kg m, and have distinct production methods and properties (456). The microstructure of these foams consists of isolated gas bubbles, often elongated by the flow of thermoplastic. However, elastomeric and microcellular foams of relative density in the range 0.3 to 0.5, which also have isolated spherical bubbles (Figure lb), will be included. The relative density of a foam is defined as the foam density divided by the polymer density. It is the inverse of the expansion ratio . 

Journal of Applied Polymer Science 88, No. 12, 20th June 2003, p.2842-50 MICROCELLULAR FOAM OF POLYMER BLENDS OF HDPE/PP AND THEIR COMPOSITES WITH WOOD FIBER Rachtanapun P SeUce S E M Matuana L M Michigan,State University... 

Boston, Ma., 7th-llth May 1995, p.2183-8. 012 EFFECT OF MORPHOLOGY ON MICROCELLULAR FOAMING OF SEMICRYSTALLINE POLYMERS Doroudiani S Park C B Kortschot M T Cheung L K Toronto,University (SPE)... 

Difficulties in producing microcellular foams from semicrystalline polymers are discussed and nucleation... 

MAKING MICROCELLULAR FOAMS FROM CRYSTALLINE POLYMERS... 

Microcellular foams can be obtained by flash spinning and are usually prepared at relatively high polymer concentrations in the spinning solution, i.e., at least 40% synthetic fiber-forming polyolefin. 

Microcellular foaming, bimodal cell size distributions, and high open-celled contents of molecular composites of HT-polymers were reported by Sun et al. [33], investigating blends of a rod-like polymer polybenzimidazole with an aminated PSU and poly(phenyl sulfone) by using carbon dioxide as a blowing agent. The complex foaming behavior was related to phase separation within the otherwise... 

Siripurapu S, Gay YJ, Royer JR, DeSimone JM, Spontak RJ, Khan SA (2002) Generation of microcellular foams of PVDF and its blends using supercritical carbon dioxide in a continuous process. Polymer 43 5511-5520... 

Huang HX, Wang JK (2007) Improving polypropylene microcellular foaming through blending and the addition of nano-clacium carbonate. J Appl Polym Sci 106 505-513... 

Rachtanapun P, Selke SEM, Matuana LM (2003) Microcellular foam of polymer blends of HDPE/PP and their composites with wood fiber. J Appl Polym Sci 88 2842-2850... 

Ramesh NS, Rasmussen DH, Campbell GA (1994) The heterogeneous nucleation of microcellular foams assisted by the survival of microvoids in polymers containing low glass-transition particles. 1. Mathematical-modeling and numerical-simulation. Polym Eng Sci 34 1685-1697... 

Sun HL, Mark JE, Tan SC, Venkatasubramanian N, Houtz MD, Arnold FE, Lee CYC (2005) Microcellular foams from some high-performance composites. Polymer 46 6623-6632... 

For example, twin-screw extruders with 80 tons/hr throughput and injection (100,000 kN) molding presses with shot size of 100 liters of polymer are available. Composites where the matrix is a polymer blend that comprises six different polymers have been introduced. Gas and multiple injection processes, melt-core technology, solid-state forming, microcellular foams all lead to new products with advanced performance. The polymer industry is becoming increasingly sophisticated. 

Roweton, S. L. and Shalaby, S. W., Microcellular Foams, in Polymers of Biological and Biomedical Significance, Shalaby, S. W., Ikada, Y., Langer, R. and Williams, J. Eds., Vol. 520, ACS Symposium Series, American Chemical Society, Washington, DC, 1993. 

Sintering is another method of polymeric microcellular foam production. Polymer beads can be partially melted to join the spheres at their surfaces, leaving behind a porous material. Fibers can be joined in the same fashion to produce a cellular solid (1). Emulsions can be used to produce foams by incorporating monomers in the oil phase of a water-oil emulsion with subsequent polymerization. A foam results upon water removal. Instead of a gas or a solvent, water serves as the pore-former around which polymeric cell walls develop (3),... 

H.-B. Zhang, Q. Yan, W.-G. Zheng, Z. He, Z.-Z. Yu, Tough Graphene-Polymer Microcellular Foams for Electromagnetic Interference Shielding. ACS Appl Mater Interfaces 2011,3,918-924. 

R. Mettinkhof, N. F.A. van der Vegt, and M. Wessling, Microcellular foaming of amorphous high-Tg polymers using carbon dioxide, pp. 874-884, Copyright (2001) American Chemical Society). 

Supercritical C02-assisted extrusion applications mainly involve polymer blending, microcellular foaming, particle production, and reactive extrusion. Of course, supercritical CO2 can also be used as an interfacial agent, foaming agent, or plasticizer in other appUcations. 

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