Polyolefin foams

Polyolefin foams are somewhat higher in cost than PS foams. They are also more flexible and better able to provide protection from multiple impacts. Typical densities are 16 to 32 kg/m (1 to 2 Ib/ft ). Polypropylene foams have somewhat greater rigidity than polyethylene foams. Polyolefin foams, like PS foams, are available in two varieties expanded (generally termed moldable) and extruded. PE/PS copolymer foams are also available, with characteristics generally intermediate between PS and PE foams, but with outstanding toughness. 

The manufacture of polyolefin foams is very similar to manufacture of extruded PS foam. Hydrocarbons or blends of hydrocarbons and carbon dioxide are used as 

Cellular polyolefins contribute many unusual properties to the cellular plastics industry. These foams are tough, flexible and chemical and abrasion resistant. They ate known to have superior electrical and 

Because cellular polyethylene is comprised of roughly equal volumes of resin and gas, its properties are different from those of ordinary unfoamed polyethylene. The cellular product has a much lower dielectric constant and therefore lower electrical losses. The composition of polyethylene (dielectric constant 2.3) and an inert gas (dielectric constant 1.0) has a dielectric constant of 1.5. In terms of electrical insulation the lower dielectric constant permits a reduction in space between inner and outer conductors without changing the characteristic impedance. For this reason it is possible to reduce the attenuation by 

Cellular polyethylene is particularly useful in a number of electrical applications, such as in coaxial cables (CATV, military and other) and in twin leads. The foamed material is particularly useful in modem wire-coating equipment because of the ease of handling and the economies it provides in size and weight of insulated conductors (6). 

---

There are three basic types of polyethylene foams of importance (/) extmded foams from low density polyethylene (LPDE) (2) foam products from high density polyethylene (HDPE) and (J) cross-linked polyethylene foams. Other polyolefin foams have an insignificant volume as compared to polyethylene foams and most of their uses are as resia extenders. 

Rodriguez-Perez, M. A. Crosslinked Polyolefin Foams Production, Structure, Properties, and Applications. Vol. 184, pp. 97-126. 

Polyolefin Foams, N.J. Mills, Metallurgy and Materials, University of Birmingham. 

In the manufacturing of crosslinked polyolefin foam, the foam expansion and cell stabilization highly dependent on the degree of crosslinking. It has been reported that at higher crosslinking level the foam produce have higher density [1, 2]. 

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 . 

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. 

Harden s (27) market survey of the growth of polyolefin foams production and sales shows that 114 x 10 kg of PE was used to make PE foam in 2001. The growth rate for the next 6 years was predicted as 5-6% per year, due to recovery in the US economy and to penetration of the automotive sector. In North America, 50% of the demand was for uncrosslinked foam, 24% for crosslinked PE foams, 15% for EPP, 6% for PP foams, 3% for EVA foams and 2% for polyethylene bead (EPE) foam. As protective packaging is the largest PE foam use sector, PE foam competes with a number of other packaging materials. Substitution of bead foam products (EPP, EPE, ARCEL copolymer) by extruded non-crosslinked PE foams, produced by the metallocene process was expected on the grounds of reduced costs. Compared with EPS foams the polyolefin foams have a lower yield stress for a given density. Compared with PU foams, the upper use temperature of polyolefin foams tends to be lower. Eor both these reasons, these foams are likely to coexist. 

MELT STRENGTH ANALYSIS FOR EXTRUDED POLYOLEFIN FOAM DEVELOPMENT... 

Houston, Tx., 25th-28th Eeb. 2001, p.431-40 MUCELLTHERMOFORMABLE POLYOLEFIN FOAM SHEET Blizard K Chapman B Trexel Inc. 

CROSSLINKING OF POLYOLEFIN FOAMS. I. EFFECT OF TRIALLYL CYANURATE ON DICUMYL PEROXIDE CROSSLINKING OF LOW-DENSITY POLYETHYLENE... 

---