Foaming, blend systems

Reducing the SAN content promotes a finer dispersed and more homogeneous SAN phase, as can be seen by the TEM micrographs in Fig. 26. The altered microstructure shows a strong impact on the cell structure of the foamed blend systems at a PPE/PS ratio of 75/25 and a SAN content of 30 wt%, the cell size decreases,... 

Beside the development of new strategies for controlling cellular structure, the potential of foamed blend systems needs to be transferred into industrial applications. As demonstrated, blending allows one to reduce the cell size and to increase the cell density, beneficial for thermal insulation applications, while the mechanical or barrier properties can be improved via multiphase cell walls. 

The efficiency of azodicarbonamide and sodium bicarbonate blowing agents for PE foams was considered (253). These systems, which generate GO2 gas, are more suitable for compression moulding of foams. Blends of the blowing agents have a reduced exotherm, so are more suitable for polymer systems that are temperature sensitive, such as ethylene copolymers. 

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... 

The fundamental relationships between compatibilization and selective blending on the blend characteristics and the foaming behavior, as demonstrated in the following, will not only be valid for this particular blend system, but will help to understand and control the foaming behavior of multiphase polymer blends in general. 

The foam processing window of the blend systems is also controlled by the glass transition temperature of the blend phases. With regard to the neat blend system, the addition of PS continuously lowers the glass transition temperature of the PPE/PS phase, as predicted by the Couchman equation [77] (Fig. 25). For the carbon dioxide-laden case, the plastifying effect needs to be taken into account, which lowers the glass transition temperature of both PPE/PS and SAN. 

As previously shown for PPE/SAN blends, the foaming behavior of immiscible blend systems is affected by both the properties of the blend phases and the overall blend structure [1], In the present blend system, the viscosity of one specific blend phase is varied as a result, not only the foaming behavior of the blend phase is altered but also the microstructure of the blend is affected [94]. By investigating blend systems with constant PPE to PS ratios of 75/25 and 50/50, and varying the SAN content in the range of 20-40 wt%, the influence of both the microstructure and the viscosity ratio can be analyzed (Table 3). 

By foaming an immiscible blend system of a poly(ethylene glycol)PEG/ polystyrene (PEG/PS), Taki et al. detected a similar foaming behavior as well as a bimodal cell size distribution [78], While smaller cells formed in the more... 

Table 4 Cell size and cell density as function of the PPE/PS ratio and SAN content of the foamed (PPE/PS)/SAN blend systems. Foaming temperature 180°C, foaming time 10 s...

When increasing the foaming time to 30 s, the minimum in density is shifted from a foaming temperature above 180°C to temperatures between 160 and 180°C. The overall foaming behavior is similar to a foaming time of 10 s. Elevated SAN contents still tend to collapse at elevated foaming temperatures, while an elevated PPE/PS content stabilizes the foam structure. For a PPE/PS ratio of 75/25 the blend system containing 30 wt% SAN shows the best compromise between the density reduction and the cell structure. 

As a result of the finely dispersed SAN phase for a PPE/PS ratio of 50/50, the density reduction is almost independent of the SAN content for a foaming time of 10 s. The foam density is generally lower compared to the blend systems with a... 

For further understanding the influence of the viscosity ratio on the foaming behavior, an additional blend system with a PPE/PS ratio of 25/75 and a SAN content of 40 w% was investigated. Due to the high PS content, the PPE/PS matrix phase shows a lower viscosity and a similar glass transition temperature when compared to the dispersed SAN phase. As can be seen in Fig. 29a, the decrease in viscosity of the PPE/PS clearly promotes the formation of elevated SAN phase in comparison to the previously shown blends. 

For establishing a descriptive foam model of such blend systems, the (PPE/PS)/SAN blend with a PPE/PS ratio of 75/25 is exemplarily used (Fig. 30), as it further reveals pronounced differences in microstructure ... 

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. 

As previously demonstrated, the shear rheological properties are an important factor relevant for the processing and foaming. In addition, morphological features of the blend system can be detected at low shear rates [95], The shear viscosity and the storage modulus of the present blends are highlighted in Fig. 32. An in-... 

The cellular structure of the quaternary blend systems after foaming at 180°C for 10 s is highlighted in Fig. 33. An excellent homogeneity down to the microscale can be detected for all foamed blend compositions. As already discussed in the previous section, simultaneous foaming of the PPE/PS and the SAN phase in the noncompatibilized blend leads to a bimodal cell size distribution. Besides larger cells induced by the highly expanded SAN phase, smaller cells are formed in the PPE/PS phase (Fig. 33a). 

Descriptive Foam Model of the Microstructured and Nanostructured Blend Systems... 

Gutmann P, Ruckdaschel H, Bangarusampath DS, Altstadt V, Schmalz H, Muller AHE (2009) Influence of the microstructure on the foaming behavior of an immiscible blend system. Antec... 

Blended nonionic surfactants. Extremely low foaming detergent systems for removal of oils, lubricants and foreign materials from polyester double knit fabrics. Low foaming characteristics make these products suited for equipment where high turbulence is present. Extremely efficient between 90F and 140F. COG-500 is a lower concentration of COG. 

Using compressed gas instead of chemicals to foam polyester resins can reduce control problems and heat reactions. Mechanically blended foam processing systems use a non-reacting additive pre-mixed into the resin... 

---