Microcellular extrusion

Heterogeneous nucleating agent improves cell morphology of the polymer foams made from poly(lactic acid) by microcellular extrusion. Mesoporous sihca was surface modified with heptadecafluoro-l,l,2,2-tetradecyl trimethoxy silane to decease nucleation energy barrier. The modified silica was found to be excellent nucleating agent for PLA foam production. ... 

The continuous microcellular process is also based on the concept of thermodynamic instability and a much shorter time is needed for saturation of polymer with gas [67]. As described by Park, when the polymer is melted in the extrusion barrel, a metered amount of gas is delivered to the polymer melt [67]. The injected gas diffuses into the polymer matrix at a much higher rate because of convective diffusion induced in the extmsion barrel at an elevated temperature [65]. As for a batch process, there are three specific steps in a continuous microcellular extrusion process formation of polymer/gas solution, cell nucleation, and shaping and cell growth [23,24,65-71]. A typical schematic of the overall continuous microcellular experimental equipment is illustrated in Figure 17.6. 

While the focus of this chapter is on injection molding, microcellular extrusion is also briefly discussed—primarily in the context of the historical development of tiie technology. Common traits are discussed as well as how the two processes differ, relative to the most important processing parameters that apply to each version of the microcellular process. 

With microcellular extrusion, management of the pressure drop is a major focus in process development and die design. There is a significant pressure... 

As with microcellular extrusion, the standard injection molding process parameters such as melt temperature and cooling temperatures also affect the cell sizes and the level of weight reduction achieved as well as the other typical characteristics, such as shrinkage. The fill rates and pressure levels that develop in the injection process have an additional major effect on microcellular molded parts as compared to standard molding in that they affect the cell structure. As described in the original research, the rate of pressure drop and subsequently the number of nucle-ation sites will determine how many cells are formed and how uniform they will be. 

PiUa, S., Kim, S. G., Auer, G. K., Gong, S., and Park, C. B. 2009. Microcellular extrusion-foaming of polylactide with chain-extender. Polymer Engineering and Science 49 1653-1660. 

Table 2 Die dimensions and residence time for microcellular foam extrusion ...

STUDY OF SHEAR NUCLEATION THEORY IN CONTINUOUS MICROCELLULAR FOAM EXTRUSION... 

A theoretical analysis based on shear energy was carried out with the aim of illustrating the mechanism of shear nucleation through a slit channel die in continuous microcellular foam extrusion. The shear energies for different positions and for different experimental conditions were calculated to demonstrate the various contributions to the surface free energy required for bubble nucleation. 14 refs. 

An alternative process for the production of PVC foam using microcellular foam technology, not requiring impact modifier, for thin wall profile (280) has been reported. Proof of concept experiments confirm the satisfactory solid state extrusion of PVC pellets, prefoamed in a batch solid state microcellular process (109). 

Two notable methods to produce microcellular foams include gas supersaturation in combination with an extrusion process developed by MIT/Trexel [84-86] and the continuous extrusion process by Dow [87,88]. Super-insulating materials are made by the encapsulation of a filler material inside a barrier film, aluminum foil, or metallized film. These materials exhibit 5-7 times the R-value of typical nonvacuum insulating materials depending on vacuum level and barrier and filler type. Uses for these VIPs (vacuum insulation panels) include refrigeration and controlled-temperature shipping containers. 

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... 

Microcellular foamed WPCs are fighter and feel more like real wood [271]. Both extrusion and injection molding have been used to produce foamed WPCs [272], producing materials with a density reduction of approximately 25%. 

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. 

Many industrial firms worldwide are now making microcellular products through extrusion and injection molding (imder license from Trexel, Inc.). Trexel, as the sole licensee of MITs has developed the MIT technology further for commercial applications. The trade name is MuCell. It is very likely that the number of new applications that use the microcellular technology will continue to increase at a rapid rate in the years to come. 

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]. 

Compared with the batch foaming process, relatively few publications feature continuous microcellular foaming of PLA in extrusion or injection molding equipment [3, 5, 73-79]. 

While these previous studies offered valuable insights, demonstrating the continuous extrusion process of PLA foamed with supercritical CO2, the morphology achieved in foamed PLA failed to satisfy the definition of microcellular foam due to lower cell population density (less than 10 cells/ cm ) with cells being larger than 60 m in average size. Distinctions between these features and microcellular foam... 

Typical processing methods - extrusion, extrusion coating, injection molding, microcellular foaming, spinning ... 

While virtually every thermoplastic material can be foamed, the specific characteristics that can be attained with microcellular processing (extrusion or injection molding) are very much dependent on the type of material, type of SCF, and the product design. 

Guo, M. C. and Y. C. Peng. 2003. Study of shear nucleation theory in continuous microcellular foam extrusion. Polym Test 22 705-9. 

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