Polyolefin foams mechanical properties

Denning s three papers in the late 1960s (a.2-a.4) reviewed the development of closed-cell polyolefin foams, and their mechanical properties. Some of his predictions on materials development turned out to be true. In Part I he explains that non-crosslinked polyethylene (PE) foams have inferior creep properties to crosslinked foams this appeared to be the tensile creep of the melt, rather than compressive creep of the... 

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. 

Polyolefins have a spectmm of mechanical properties, ranging from almost-rigid EPP mouldings for helmets and factory containers to mbbery high density EPDM foams for mouse mats and grips on hand tools. Rather than list all these areas, five specific areas are described in detail. 

Becanse there are many factors involved in the dynamic mechanical compression of polyolefin foams, the Taguchi method was employed in a Perkin Elmer DM A7 dynamic mechanical analyser to establish a method to improve the measurement process. The signal-to-noise ratio was measured to determine how the variability could be improved. Control and noise factors were evaluated and levels chosen, with details being tabulated. Appendix A describes some of the factors. Tests were conducted on two closed cell foams. NA2006 foam is 48 kg/cu m LDPE and NEE3306 foam is 32 kg/cu m EVA. Different factors were shown to influence results for E and tan delta but an optimum combination is proposed for the simultaneous measurement of both properties. The results were less variable as frequency was increased. Small differences in the dynamic response of different materials should be measurable because of the low variability in the experimental results. 18 refs. 

The static and dynamic mechanical properties, creep recovery behaviour, thermal expansion and thermal conductivity of low-density foams made of blends of LDPE and EVA were studied as a function of the EVA content of the blends. These properties were compared with those of a foam made from a blend of EVA and ethylene-propylene rubber. A knowledge of the way in which the EVA content affects the behaviour of these blend foam materials is fundamental to obtaining a wide range of polyolefin foams, with similar density, suitable for different applications. 9 refs. 

A preliminary stndy on the viscoelastic behaviour of polyolefin foam sheets with different chemical (PE and PP) and cellular structure by DMA, in the low freqnency and low compression ranges, is presented. DSC and SEM are also used to determine the morphological parameters of the samples. A connection between the morphological properties (apparent degree of crystallinity), type of cellular structure, homogeneity, cell size and shape, cell wall thickness) and the viscoelastic behavionr, a basic key for the development of mechanical and insnlating applications, has been established. 9 refs. 

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. 

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... 

Polyolefin foams can be produced with closely controlled density and cell structure. Generally the mechanical properties of polyolefins lies between those of a rigid and a flexible foam. Polyolefin foams have a very good chemical and abrasion resistance as well as good thermal insulation properties. Cross-linking improves foam stability and polymer properties. 

The commercially available ferroelectret film is based on polyolefin foam manufactured in a continuous biaxial orientation process, followed by an expansion process to adjust the mechanical and the piezoelectric properties (Fig. 2 left). The equivalent circuit of a ferroelectret film sensor consists of a current source in parallel to the sensor capacitance (Fig. 2 right). The sensor capacitance Cp follows fromCp = eoeA/s, where sq is the permittivity of vacuum, e is the dielectric constant of the ferroelectret foam, A is the electrode area, and s is the thickness of the film. With a typical thickness of S = 70 pm and a dielectric constant of e = 1.5, the sensor capacitance is 19 pF/cm, a small capacitance in comparison to other piezoelectric sensor elements. The ferroelectret sensor element produces a current i(t) when a force F(t) is applied to the sensor element, according to i t) = where the dot... 

EFFECTS OF PROCESSING PARAMETERS ON THE CELLULAR MORPHOLOGY AND MECHANICAL PROPERTIES OF MINERAL-FILLED THERMOPLASTIC POLYOLEFIN MICROCELLULAR FOAMS... 

Ceramic foams are produced from organic precursor foams such as polyurethane or polyolefins. Their pores are then filled with an aqueous slurry of the ceramic typically containing 20 wt.% of ceramic particles in the size range of from 0.1 to 10 pm [461]. Wetting agents, dispersion stabilisers and viscosity modifiers are added to the slurry. Suitable ceramics are alumina, alumina silicates, zirconia, stabilised zirconia and titania, amongst others. The pores ofthe precursor foam may be filled completely or only coated on their surface by the ceramic particles. The foam is then dried and calcined at 1000 °C, which removes the polymer and sinters the ceramic. Metallic foams have similar properties compared with ceramic foams, but superior mechanical stability and improved heat conductivity. 

---