Foamed density, reduced

In general the foam density reduces as the amount of blowing agent is increased, with a lower limit set by foam stability. It is possible to model the factors which affect the final density Mahapatro and co-workers (206) used a regular Kelvin foam model to analyse the expansion of PE foams. The foam has uniform sized cells, each with eight hexagonal faces and four square... 

The number of PPE particles dispersed in the SAN matrix, i.e., the potential nucleation density for foam cells, is a result of the competing mechanisms of dispersion and coalescence. Dispersion dominates only at rather small contents of the dispersed blend phase, up to the so-called percolation limit which again depends on the particular blend system. The size of the dispersed phase is controlled by the processing history and physical characteristics of the two blend phases, such as the viscosity ratio, the interfacial tension and the viscoelastic behavior. While a continuous increase in nucleation density with PPE content is found below the percolation limit, the phase size and in turn the nucleation density reduces again at elevated contents. Experimentally, it was found that the particle size of immiscible blends, d, follows the relation d --6 I Cdispersed phase and C is a material constant depending on the blend system. Subsequently, the theoretical nucleation density, N , is given by... 

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. 

Selective blending the more viscous PPE phase with PS allowed one to tailor the processing window of the miscible PPE/PS blend phase and the microstructure of the immiscible blend system. Following this approach, simultaneous foaming of both blend phases and more homogeneous cell structures could be achieved. Additionally, the overall foam density could be reduced. 

At a foaming time of 30 s, the differences in expansion behavior become more evident. The increase in SBM content leads to an increasing foam density for all foaming temperatures. Besides the previously mentioned reduction of the glass transition temperature of the more viscous PPE phase by the selective blending with PS, subsequently reducing the stabilization effect, the fine blend structure is responsible... 

Fire-fighting foams greatly reduce the density of water and allow it to spread easily over the fuel. These foams kill fires through a combination of the following [44,796-799] ... 

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. 

Discontinuous pre-foamers are more costly, but because of the higher steam pressure, lower foam densities can be achieved. In Europe, more and more discontinuous pre-foamers are being used, the reason being the availability of low pentane grades for reduced pentane emission. 

In addition to water, auxiliary blowing agents may be included in the foam formulation to further reduce the foam density (16) (17). These agents can be used in addition to, or as part replacement for the water in developing special foam properties. An example is the use of methylene chloride or CFC-11 in either polyether- or polyester-based systems for softening the resulting foam. A number of other volatile solvents are known to have been used also. 

The foam expansion ratio can be characterised by the liquid volume fraction in the foam, which is the sum of the volume fractions of the films, plateau borders and vertexes. Alternatively, the foam density can be used as a measure of the foam expansion ratio. The reduced pressure in the foam plateau border can be measured using a capillary manometer [4], while the bubble size and shape distribution in a foam can be determined by microphotography of the foam. Information about the liquid distribution between films and plateau borders is obtained from the data on the border radius of curvature, the film thickness, and the film-to-plateau border number ratio obtained in an elementary foam cell. 

Incorporation of elastomeric particles improves nucleation of gas bubbles, hence it stabilizes the foaming process, reduces bubble size and the final foam density. 

There is no optimum design, since the shape of the surface struck is unpredictable. A foam that is ideal for an impact with a flat surface has a too low yield stress to be ideal for an impact on a kerbstone. Designers must also consider ventilation, and compensate for the lower radius of curvature R at the front and rear of the helmet. Ventilation holes reduce the contact area of the foam on the head, so the foam density must be increased to compensate. Ventilation holes are particularly important at the front and rear, to achieve the required air flow. However, since accident surveys show the front and sides of helmets are the most frequently impacted sites, it is important that there is a good thickness of foam in these regions. 

Trichlorofluoromethane (Freon 11, Gene-tron ) n. CCI3F. A chemically inert blowing agent used until recently with water in flexible polyurethane formulations to control foam density and load-bearing properties. It is also a refrigerant and former aerosol propellant. Freon 11 has been phased out of most applications since 1995, in the general push to reduce fluorocarbons in the atmosphere. 

---