Properties foam density

The properties that are achieved in commercial stmctural foams (density >0.3 g/cm ) are shown in Table 3. Because these values depend on several stmctural and process variables, they can be used only as general guidelines of mechanical properties from these products. Specific properties must be deterrnined on the particular part to be produced. A good engineering guide has been pubHshed (103). 

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. 

Coefficient of Linear Thermal Expansion. The coefficients of linear thermal expansion of polymers are higher than those for most rigid materials at ambient temperatures because of the supercooled-liquid nature of the polymeric state, and this applies to the cellular state as well. Variation of this property with density and temperature has been reported for polystyrene foams (202) and for foams in general (22). When cellular polymers are used as components of large stmctures, the coefficient of thermal expansion must be considered carefully because of its magnitude compared with those of most nonpolymeric stmctural materials (203). 

The relationship between swell ratio and foam properties was initially established for LDPE crosslinked by DCP alone. Figure 15.7 shows that an increase in swell ratio is accompanied by a decrease in foam density and an increase in mean cell size. The higher swell ratios are associated with a more loosely crosslinked network (i.e. lower crosslink density) that has a greater ability to expand and hence lowers the foam density. 

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 . 

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. 

Blends of poly (ethylene terephthalate) (PETP) and polypropylene (PP) with different rheological properties were dry blended or compounded, and extrusion foamed using both physical blowing and chemical agents, and the foam properties compared with those of foam produced from the individual components in the absence of compatibilisers and rheology modifiers. The foams were characterised by measurement of density, cell size and thermal properties. Low density foam with a fine cell size was obtained by addition of a compatibiliser and a co-agent, and foamed using carbon dioxide. The presence of PP or a polyolefin-based compatibiliser did not effect... 

One of the great benefits of polyurethane is versatility. With only slight changes in chemistry, one can make products ranging from soft furniture cushions to automobile bumpers and infinite numbers of other products. Depending on the application, a polyurethane chemist can vary density and stiffness to achieve acceptable product performance. The chemistry is in fact much more versatile than is required. Figure 2.19 covers soft foams, rigid foams, and other polyurethanes. We will provide more details later in this chapter, particularly as to how the independent properties of density and stiffness relate to end uses. 

The properties that are achieved in commercial structural foams (density >0.3 g/cm3) are shown in Table 4. 

The properties and densities of the mixtures and their resultant syntactic foams not only depend on the binder/filler ratio but also on the microspheres themselves, their size, sphericity, polydispersity, apparent and bulk density, the thickness and uniformity of their shells. Thus, at a given binder/filler ratio, the fluidity of a mixture depends on the size of the microspheres (Fig. 2) and the apparent density depends on their bulk density (Fig. 3)l). As the bulk density of the microspheres increases (the filler particles become larger), the final strength of the material decreases3 76>. 

In the present case, the foam density relates perfectly with the previously observed rheological properties, as a transition in the flow behavior was detected at approximately 20 wt% of PPE (Fig. 13). In the viscoelastic case (below the percolation limit), the PPE content neither significantly influences the foamability nor the blend rheology. At elevated contents (beyond percolation), however, the PPE content strongly affects the rheological response of the blend and, subsequently, degrades the foaming behavior, which is verified by a reduced expandability. 

Bian, XC Tang, JH Li, ZM. et al. Dependence of flame-retardant properties on density of expandable graphite filled rigid polyurethane foam. J. Appl. Polym. Sci. 2007, 104, 3347-3355. 

Properties of Poly butene Sulfone Foam. Many properties of polybutene sulfone foam are similar to those of polystyrene foam. Mechanical properties are a little lower for the same foam density, but the bulk density of polybutene sulfone is 1.37 compared with 1.05 for polystyrene. Figure 6 shows that mechanical properties vary in the same ratio as density The insulating properties of polybutene sulfone foam are very good, somewhat better than polystyrene foam (Figure 7). Polybutene sulfone has a good solvent resistance as shown in Table I. In particular, styrene, benzene, and toluene do not attack polybutene sulfone but attack polystyrene. 

PEs provide many unusual properties to the cellular plastics industry. These foams are tough, flexible and chemical and abrasion resistant. They are known to have superior electrical and thermal insulation properties. Their mechanical properties are intermediate between rigid and highly flexible foams. Densities are 2 lb/ft3 and higher, approaching that of the solid plastics. The highly expanded polyolefin foams are potentially the least expensive of the cellular plastics. However, they require expensive processing techniques and for this... 

The manufacture of cellular PVC/wood composites has been studied. The properties achieved, as foam density was reduced, were examined showing suitability for many wood replacement applications (196). 

A major disadvantage of composites of wood with thermoplastics materials is a relatively high specific gravity compared with those of many natural wood products. A PVC-wood composite, for example, has a specific gravity of about 1.3 g/cc. The manufacture of cellular PVC-based wood composites was studied and the properties that were achieved as the foam density was reduced were examined. Overall, even with densities as low as 0.6 g/cc, the physical properties should be adequate for many wood replacement applications. The composites also exhibited the aesthetics of wood and economics that were favourable compared with those of both rigid and cellular PVC. 6 refs. 

Recycled PVC supermarket trays have been used successfully in production-scale trials to make the foamed layer in coextruded cladding. The extruded product had satisfactory foam density, foam structure and colour. Impact properties were better than those of the control made from virgin PVC this is attributed to the high levels of impact modifier used in tray formulations. These trials demonstrated that PVC straight-on trays can be recycled into foamed extrusions for wood replacement products. 8 refs. 

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