Foamed polymers mechanical properties

Polyolefin foams are easier to model than polyurethane (PU) foams, since the polymer mechanical properties does not change with foam density. An increase in water content decreases the density of PU foams, but increases the hard block content of the PU, hence increasing its Young s modulus. However, the microstructure of semi-crystalline PE and PP in foams is not spherulitic, as in bulk mouldings. Rodriguez-Perez and co-workers (20) showed that the cell faces in PE foams contain oriented crystals. Consequently, their properties are anisotropic. Mechanical data for PE or PP injection mouldings should not be used for modelling foam properties. Ideally the mechanical properties of the PE/PP in the cell faces should be measured. However, as such data is not available, it is possible to use data for blown PE film, since this is also biaxially stretched, and the texture of the crystalline orientation is known to be similar to that in foam faces. 

Leduc S., Rodrigue D., Effect of Weld Lines on the Injection Moulding of Structural Foams. II Mechanical Properties, Cellular Polymers, 25 (2) 63-84 (2006). 

The mechanical piopeities of stmctuial foams and thek variation with polymer composition and density has been reviewed (103). The variation of stmctural foam mechanical properties with density as a function of polymer properties is extracted from stress—strain curves and, owkig to possible anisotropy of the foam, must be considered apparent data. These relations can provide valuable guidance toward arriving at an optimum stmctural foam, however. 

The effect of oxidative irradiation on mechanical properties on the foams of E-plastomers has been investigated. In this study, stress relaxation and dynamic rheological experiments are used to probe the effects of oxidative irradiation on the stmcture and final properties of these polymeric foams. Experiments conducted on irradiated E-plastomer (octene comonomer) foams of two different densities reveal significantly different behavior. Gamma irradiation of the lighter foam causes stmctural degradation due to chain scission reactions. This is manifested in faster stress-relaxation rates and lower values of elastic modulus and gel fraction in the irradiated samples. The incorporation of O2 into the polymer backbone, verified by IR analysis, conftrms the hypothesis of... 

Polymer International 51,No.7, July 2002, p.601-6 PHYSICAL STUDIES OF FOAMED REINFORCED RUBBER COMPOSITES. PART I. MECHANICAL PROPERTIES OF FOAMED ETHYLENE-PROPYLENE-DIENE TERPOLYMER AND NITRILE-BUTADIENE RUBBER COMPOSITES... 

The ability to modify their structure and the good cost/ performance ratio makes polyolefins technically and commercially attractive for mechanical energy absorption. This is especially true for High Melt Strength (HMS) PP which allows continuous extrusion foaming. Physical expansion of PP, properties of foamed PP, and application examples are considered in detail, mechanical properties in particular being compared with other polymer foams. 

The applicability of the Gibson and Ashby approach, whereby deformation mechanisms are identified, to a range of thermoplastic polymer foams is explored. LDPE, EVA and PP foams were produced by the BXL Plastizote nitrogen expansion process. A full range of mechanical properties is discussed from the simpler aspects of modulus and strength to the complexities of creep and recovery performance. 8 refs. 

Microcellnlar foams are reported to offer superior mechanical properties, while retaining the insulation properties of conventional foams. Low gas solubility resulting from the rigidity of the crystal lattice has precluded their production in semi-crystalline polymers, but a new process is said to have been experimentally proven in PP. Aspects covered include production difficulties, experimental details, results and conclusions. USA... 

The mechanical properties are also affected by the surfactant concentration in the emulsion precursor. Maxima in both crush strength and Young s modulus were shown at the surfactant concentration for optimum emulsion stability. Foams prepared from 100% styrene were found to have much lower compressive moduli than those containing DVB [130], This was attributed to plasticisa-tion of the polymer by the surfactant. 

EVA reins can be conveniently crosslinked by both peroxide or irradiation to enhance the mechanical properties and the heat resistance. It is the crosslinking, coupled with the inherent rubbery nature of the polymer, which makes EVA suitable for production of tough and abrasion resistant foams, especially suitable for footwear applications. Crosslinked EVA foams can be manufactured by two methods (8) ... 

Reichelt N, Stadlbauer M, Folland R, Park CB, Wang J (2003) PP-blends with tailored foam-ability and mechanical properties. Cell Polym 22 315-327... 

Dispersions of gas in solids are also called foams but the foam cells (bubbles) formed are isolated from one another. An example of such foams are the natural porous materials, cellular concrete, cellular glass and polymer foams. However, if in such disperse systems both phases are continuous (such as in many foamed polymers), they are called sponges. Many porous materials are partially sponge and partially solid foam. The properties of solid foams differ drastically from those of foams with liquid dispersion medium. At the same time the strength and other physical and mechanical characteristics of solid foams depend significantly... 

Flame-retardant polystyrene is used primarily in expanded foam for building insulation. Rubber-modified styrenic polymers are flame retarded for use in a number of applications, such as enclosures for electronics and business equipment. By far the largest volume flame-retardant HIPS application is television enclosures (Figure 29.1) these are made primarily from flame-retardant HIPS [3]. Flame-retardant HIPS has an attractive balance of mechanical properties, processability and cost. Flame-retardant styrenic blends such as HIPS-PPO and PC-ABS also find utility in a number of electrical applications such as printers, computers and monitors. These blends have received increasing attention recently because of their ability to be flame retarded with nonhalogen flame retardants (see Section 7). 

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