**3.2 Foaming agents**

Physical blowing agents comprise of gases and low boiling hydrocarbons or their halogenated derivatives. Initially used blowing agents (pentane, butane, chlorofluoro hydrocarbons) are withdrawn because of ecological reasons (the Montreal Protocol Agreement) and fire hazard and replaced by noble gases (argone, nitrogen, carbon dioxide). Interesting properties have also hydrofluoro olefines, which have been used for manufacturing of polyurethane and EPS foams (Rosato, 2010).

Unfortunately several safe blowing agents exhibit either too low solubility in polymers or too high heat coefficient, which deteriorates thermo-insulating properties of foams. Thermal insulation expressed with the heat transfer coefficient λ depend on the cell size and density (Schellenberg & Wallis, 2010), but also on a nature of gas in the cells. Nitrogen and oxygen have comparable λ values (at 0°C respectively 22,7 and 23,2 mW/m K), however that of carbon dioxide equals to 13,7 mW/m K. Thus, the foams filled with CO2 exhibit much better thermo-insulating properties than others.

amount of 2 - 8% and such mixture is processed by extrusion or injection molding technology. Decrease in a density for 30% was reported after addition of 3% microspheres,

Fig. 9. Structure of foam manufactured with Expancel technology (www.akzonobel.com)

blowing agents produce foams density of 500 - 750 kg/m3.

manufacturing of polyurethane and EPS foams (Rosato, 2010).

thermo-insulating properties than others.

**3.2 Foaming agents** 

Undoubtedly the principal polymer foaming technology is that involving a gas delivered to a polymer by means of the chemical (CFA) or physical foaming agent (PFA). Low density foams (2 - 500 kg/m3) are manufactured with physical blowing agents, whereas chemical

Physical blowing agents comprise of gases and low boiling hydrocarbons or their halogenated derivatives. Initially used blowing agents (pentane, butane, chlorofluoro hydrocarbons) are withdrawn because of ecological reasons (the Montreal Protocol Agreement) and fire hazard and replaced by noble gases (argone, nitrogen, carbon dioxide). Interesting properties have also hydrofluoro olefines, which have been used for

Unfortunately several safe blowing agents exhibit either too low solubility in polymers or too high heat coefficient, which deteriorates thermo-insulating properties of foams. Thermal insulation expressed with the heat transfer coefficient λ depend on the cell size and density (Schellenberg & Wallis, 2010), but also on a nature of gas in the cells. Nitrogen and oxygen have comparable λ values (at 0°C respectively 22,7 and 23,2 mW/m K), however that of carbon dioxide equals to 13,7 mW/m K. Thus, the foams filled with CO2 exhibit much better

however the cells were of diverse size (Fig. 9).

Foaming with gases results mostly in foams of large cell size, however using supercritical liquids bring about manufacturing of microfoams (Cooper, 2000). At critical conditions (temperature and pressure) the density of a liquid and a gas equals. Above the critical temperature condensation of a gas is impossible, independing on a pressure applied. From that reason carbon dioxide is most appropriate for transportation, storage and dosing conditions, since its critical temperature is +31.1, whereas that for nitrogen is –146,9ºC and argone –122,3ºC.

Chemical blowing agents decompose within a specific temperature range, emiting a stechiometric amount of gases (usually nitrogen or carbon dioxide). Chemical blowing agents are classified as egzo- or endothermic, depending on the effect of a decomposition process. Due to a vigorous character of the decompositin reaction, egzothermic CBAs produce large size cells (>100 μm) of a non-uniform size distribution and cause a high overall expansion of the material (Fig. 10).

Fig. 10. Stucture of polypropylene foam produced with egzothermic CBA

The most popular egzothermic chemical blowing agent is azodikarbonamide (ADC) H2N-NH-(CO)-N=N-(CO)-NH2. ADC decomposes at 200-220ºC with emission of gases in the amount of 220 cm3/g. The mixture of gases comprises of nitrogen (65%), carbon monoxide (24%), carbon dioxide (5%) and ammonia (5%).

Endothermic chemical blowing agents need heat to continue decomposition, therefore it is easier to control the process just by changing its temperature. For that reason one can produce with endothermic CBA foams of lower cell size. Most popular endothermic blowing agent is a mixture of sodium hydrogen carbonate and citric acid. It decomposes to carbon dioxide and water in a two-stage reaction: first at 130-140ºC, second at 180-200ºC (Fig. 11).

Lightweight Plastic Materials 301

Cellular polymers may be manufactured by saturation of a solid polymer with gas in a high pressure vessel at elevated temperature. First trials concerned foaming of polystyrene with carbon dioxide. It was evidenced that the equilibrium amount of CO2 adsorbed by PS at 80ºC under 240 bar pressure equals to 11.8%. The foam density varied within 0.05-0.85 g/cm3 depending on the applied temperature, pressure and pressure drop rate. The cell size

The foam morphology is related to a structure of polymers – the most important are branchings in the polymer chain and crystallinity (Huang et al., 2008; Rachtanpun et al., 2004; Su & Huang, 2010; Li et al., 2007). It is well established that foaming is easier with amorphous polymers like PS than with crystalline ones like polyolefines. Amorphous polymers usually have higher melt strength and are more viscous, therefore cell growth is more difficult, but they hold gas pores better. Crystalline resins are less viscous but difficult to foam due to their chain entanglement and crystals formation at cooling, which disturbs the cell growth process. Foaming of semicrystalline polymers is more complicated than the amorphous polymer foaming, because the gas dissolves exclusively in amorphous regions.

Temperature range of the efficient foaming is limited from above by the polymer degradation temperature and from a bottom by the polymer melt viscosity, which allows for a cell growth. Because it takes place only in the amorphous regions, fast increase in a viscosity of the semi-crystalline polymers upon cooling makes the available temperature

Technology of a direct polymer saturation with a gas is useful rather for niche products due to a long time required for saturation because diffusivity of a gas in a solid polymer is low. Nevertheless, it may be used a. o. for scaffold manufacturing. Mooney et al. (1996) have shown that after treating copolymer of D,L lactic acid and glycolic acid with carbon dioxide under pressure of 5.5 MPa for 72 hours followed by fast decompression one observes cells in a polymeric material. Their size equals to ca. 100 μm, while the cell density depends on the

Cell nucleation in a polymer starts spontaneously after a sudden change of a thermodynamic state of the system (homogeneous nucleation) or may be induced with addition of small amount of a filler (heterogeneous nucleation) (Lee, 2000; Lee et al., 2006). Technology of foaming by means of extrusion or injection molding is more widely used, because a gas diffusion in molten polymers is faster and it is facilitated by mixing. These technologies were applied at late nineties at Massachussetts Institute of Technology (MIT) after the successful research on manufacturing PS microfoams by a solid state saturation with supercritical CO2. Since then the foaming injection molding received an industrial maturity, providing remarkable material savings. Chen et al. (2006) presented an example of polypropylene foaming, showing that the material savings for thin wall (0.5 mm) items equals to 4-9% and for the thick wall (15 mm) samples reached 50%. Unfortunately, in parallel a deterioration of mechanical properties was reported. Similar findings were presented by Bielinski (2004), who tested foaming of polypropylene and polyvinyl chloride using chemical blowing agents. He has found that depending on the CBA content (0,5–2

wt.%) and injection molding parameters the cell size varied in a range of 10-350 µm.

That causes a non uniform cell nucleation and irregular foam morphology.

range small in comparison to that of amorphous polymers.

process parameters and crystallinity of a polymer, reaching 93%.

amounted to 1-70 µm (Arora et al., 1998).

Fig. 11. TG analysis of endothermic chemical blowing agents

Decomposition takes place according to a reaction:

$$\rm C\_6H\_8O\_7 + 3\ NaOHO\_3 = \left(C\_6H\_5Na\_3O\_7\right) \cdot 2\ H\_2O + 3\ CO\_2 + H\_2O \tag{1}$$

Total amount of gases emitted in the above reaction equals to 120 cm3/g.

Solubility of gases in polymers is a crucial factor for foaming. If it is high, the saturation time of a polymer with a gas is shorter and lower pressure level is required to keep the gas in the melt. Solubility of carbon dioxide in several polymers is higher than that of nitrogen, therefore it generates more cell nuclei, which is essential for the foam structure. The size of a foaming gas particle is also important for the foaming technology. Since CO2 particle is small, it diffuses fastly through a polymer, which means that the cell growth rate should be high. However, from the other side, carbon dioxide may escape more easily to atmosphere, thus causing a collapse of the foamed material.

#### **3.3 Technology of polymer foaming**

Foaming process consists of four stages:


0 25 50 75 100 125 150 175 200 225 250 **temperature [0**

Fig. 11. TG analysis of endothermic chemical blowing agents

Total amount of gases emitted in the above reaction equals to 120 cm3/g.

high temperature and high pressure (e.g. in a molten polymer);

resulting from its decompression or temperature change;

Decomposition takes place according to a reaction:

thus causing a collapse of the foamed material.

parameters and properties of the polymer;

**3.3 Technology of polymer foaming**  Foaming process consists of four stages:

or crosslinking.

**C]**

C H O 3 NaHCO C H Na O · 2 H O 3 CO H O <sup>687</sup> 3 65 37 2 2 2 (1)

Solubility of gases in polymers is a crucial factor for foaming. If it is high, the saturation time of a polymer with a gas is shorter and lower pressure level is required to keep the gas in the melt. Solubility of carbon dioxide in several polymers is higher than that of nitrogen, therefore it generates more cell nuclei, which is essential for the foam structure. The size of a foaming gas particle is also important for the foaming technology. Since CO2 particle is small, it diffuses fastly through a polymer, which means that the cell growth rate should be high. However, from the other side, carbon dioxide may escape more easily to atmosphere,

1. dissolving of a gas in a polymer under high pressure (if polymer is in a solid state) or at

2. cell nucleation due to a sudden change in the thermodynamic state of a material

3. cell growth – their size and density depend on the blowing agent content, process

4. morphology fixation by polymer solidification, e.g. cooling below the glass temperature

**CF - 40 PLC - 742**

75

80

85

90

**mass loss [%]**

95

100

Cellular polymers may be manufactured by saturation of a solid polymer with gas in a high pressure vessel at elevated temperature. First trials concerned foaming of polystyrene with carbon dioxide. It was evidenced that the equilibrium amount of CO2 adsorbed by PS at 80ºC under 240 bar pressure equals to 11.8%. The foam density varied within 0.05-0.85 g/cm3 depending on the applied temperature, pressure and pressure drop rate. The cell size amounted to 1-70 µm (Arora et al., 1998).

The foam morphology is related to a structure of polymers – the most important are branchings in the polymer chain and crystallinity (Huang et al., 2008; Rachtanpun et al., 2004; Su & Huang, 2010; Li et al., 2007). It is well established that foaming is easier with amorphous polymers like PS than with crystalline ones like polyolefines. Amorphous polymers usually have higher melt strength and are more viscous, therefore cell growth is more difficult, but they hold gas pores better. Crystalline resins are less viscous but difficult to foam due to their chain entanglement and crystals formation at cooling, which disturbs the cell growth process. Foaming of semicrystalline polymers is more complicated than the amorphous polymer foaming, because the gas dissolves exclusively in amorphous regions. That causes a non uniform cell nucleation and irregular foam morphology.

Temperature range of the efficient foaming is limited from above by the polymer degradation temperature and from a bottom by the polymer melt viscosity, which allows for a cell growth. Because it takes place only in the amorphous regions, fast increase in a viscosity of the semi-crystalline polymers upon cooling makes the available temperature range small in comparison to that of amorphous polymers.

Technology of a direct polymer saturation with a gas is useful rather for niche products due to a long time required for saturation because diffusivity of a gas in a solid polymer is low. Nevertheless, it may be used a. o. for scaffold manufacturing. Mooney et al. (1996) have shown that after treating copolymer of D,L lactic acid and glycolic acid with carbon dioxide under pressure of 5.5 MPa for 72 hours followed by fast decompression one observes cells in a polymeric material. Their size equals to ca. 100 μm, while the cell density depends on the process parameters and crystallinity of a polymer, reaching 93%.

Cell nucleation in a polymer starts spontaneously after a sudden change of a thermodynamic state of the system (homogeneous nucleation) or may be induced with addition of small amount of a filler (heterogeneous nucleation) (Lee, 2000; Lee et al., 2006).

Technology of foaming by means of extrusion or injection molding is more widely used, because a gas diffusion in molten polymers is faster and it is facilitated by mixing. These technologies were applied at late nineties at Massachussetts Institute of Technology (MIT) after the successful research on manufacturing PS microfoams by a solid state saturation with supercritical CO2. Since then the foaming injection molding received an industrial maturity, providing remarkable material savings. Chen et al. (2006) presented an example of polypropylene foaming, showing that the material savings for thin wall (0.5 mm) items equals to 4-9% and for the thick wall (15 mm) samples reached 50%. Unfortunately, in parallel a deterioration of mechanical properties was reported. Similar findings were presented by Bielinski (2004), who tested foaming of polypropylene and polyvinyl chloride using chemical blowing agents. He has found that depending on the CBA content (0,5–2 wt.%) and injection molding parameters the cell size varied in a range of 10-350 µm.

Lightweight Plastic Materials 303

dominating stresses are related with shearing and elongational forces, therefore a knowledge of a dependence of the polymer melt viscosity on temperature and on the shear rate is essential. In Figs. 14 and 15 the basic characteristics for three different LDPE grades

Fig. 14. Viscoelastic characteristics of different LDPE grades (130ºC)

Fig. 15. Melt strength and elongation for different LDPE grades (130ºC)

0 40 80 120 160 200 240 280 320 360 400 440 480 Time [s]

The examples show that different grades of the same polymer differ in pseudoplasticity, therefore they should exhibit diverse melt reaction at low and high deformation rates. That means a different mixing efficiency of the melt with a gas (torque level and gas diffusion

MGNX LDPE A LDPE E

0,0 2,0 4,0 6,0 8,0 10,0

Force [cN]

have been presented.

The material savings and lower amount of waste cause that the chemical blowing agents are widely used. In Fig. 12 a yogurt cup made of PS foamed with CBA has been presented. The mass of a cup was decreased for 15-20% in comparison to the non-foamed item. Even if a foam structure is not uniform, the economical and ecological advantages are obvious.

Fig. 12. Foamed polystyrene cup and its wall structure (acc. to www.adeka-palmarole.com)

Technology of microcellular foam extrusion has been extensively studied a.o. by Park and co-workers (Park, 2000; Lee & Park, 2006). After injection of a gas into the polymer melt its diffusion is intensified by mixing, which results in a complete gas dissolution. The equilibrium reached in the extruder is lost after the melt exits the die (Fig. 13). Sudden pressure decrease causes also decrease in a gas solubility, which has to evolve from a polymer in a form of microcells. These sites form nuclei, of which the larger cells grow as more gas appears in the system as the polymer melt-gas solution decompression proceeds. That process is continued until the new equilibrium state is reached or the polymer solidifies.

Fig. 13. Polymer foaming extrusion principle

Since foaming involves several processes occuring in a fluid state, therefore knowledge of the viscoelastic characteristics of a molten polymer is very important. Because the

The material savings and lower amount of waste cause that the chemical blowing agents are widely used. In Fig. 12 a yogurt cup made of PS foamed with CBA has been presented. The mass of a cup was decreased for 15-20% in comparison to the non-foamed item. Even if a foam structure is not uniform, the economical and ecological advantages are obvious.

Fig. 12. Foamed polystyrene cup and its wall structure (acc. to www.adeka-palmarole.com)

Technology of microcellular foam extrusion has been extensively studied a.o. by Park and co-workers (Park, 2000; Lee & Park, 2006). After injection of a gas into the polymer melt its diffusion is intensified by mixing, which results in a complete gas dissolution. The equilibrium reached in the extruder is lost after the melt exits the die (Fig. 13). Sudden pressure decrease causes also decrease in a gas solubility, which has to evolve from a polymer in a form of microcells. These sites form nuclei, of which the larger cells grow as more gas appears in the system as the polymer melt-gas solution decompression proceeds. That process is continued until the new equilibrium state is reached or the polymer

Since foaming involves several processes occuring in a fluid state, therefore knowledge of the viscoelastic characteristics of a molten polymer is very important. Because the

solidifies.

Fig. 13. Polymer foaming extrusion principle

dominating stresses are related with shearing and elongational forces, therefore a knowledge of a dependence of the polymer melt viscosity on temperature and on the shear rate is essential. In Figs. 14 and 15 the basic characteristics for three different LDPE grades have been presented.

Fig. 14. Viscoelastic characteristics of different LDPE grades (130ºC)

Fig. 15. Melt strength and elongation for different LDPE grades (130ºC)

The examples show that different grades of the same polymer differ in pseudoplasticity, therefore they should exhibit diverse melt reaction at low and high deformation rates. That means a different mixing efficiency of the melt with a gas (torque level and gas diffusion

Lightweight Plastic Materials 305

The most important stages of the foaming process (e.g. cell nucleation and growth) have

Fig. 17. Cell size and amount in LDPE foamed at different melt pressure

30 rpm 40 rpm 50 rpm

Critical factor for foaming is the cell nucleation rate, which is related to a change in the thermodynamic state of a system and phase separation. The gas dissolved formerly in a

Fig. 18. Cell density in LDPE foam at different processing parameters

no screen 1 screen

0

50000

100000

150000

Cell density, 1/cm3

200000

250000

300000

350000

been presented schematically in Fig. 16.

rate) and easy or more difficult cell grow (melt viscosity) and resistance to rupture (melt strength). One can expect that significant differences in viscoelastic properties should have an impact on morphology of cellular plastics.

Fig. 16. Schematic representation of extrusion foaming stages

rate) and easy or more difficult cell grow (melt viscosity) and resistance to rupture (melt strength). One can expect that significant differences in viscoelastic properties should have

an impact on morphology of cellular plastics.

(b)

(a)

(c)

Fig. 16. Schematic representation of extrusion foaming stages

The most important stages of the foaming process (e.g. cell nucleation and growth) have been presented schematically in Fig. 16.

Fig. 17. Cell size and amount in LDPE foamed at different melt pressure

Fig. 18. Cell density in LDPE foam at different processing parameters

Critical factor for foaming is the cell nucleation rate, which is related to a change in the thermodynamic state of a system and phase separation. The gas dissolved formerly in a

Lightweight Plastic Materials 307

Fig. 20. Mixing torque and temperature change after addition of endothermic CBA to

Fig. 21. Foam morphology generated in LDPE of different melt strength (130ºC) – foam

The process of cell growth outlined in Fig. 16 stops as soon as the polymer glass temperature reaches the lower foaming limit. It may proceed fastly, because Tg increases with decreasing

density 0.576 g/cm3 (left) and 0.447 g/cm3 (right)

polymer

molten polymer is evolving simultaneously at several sites of the material (a). Since the nucleation rate is much higher than the diffusion rate, the cell nuclei arise first, and only after some time they start growing due to a diffusion of next gas particles which appear as the gas solubility in a polymer melt falls due to the pressure and temperature decrease in a material after it exits the extrusion die.

Number of cells nucleated in the polymer depend on the pressure difference in the melt and atmospheric pressure. High difference developed by a change in the processing parameters or equipment configuration facilitates generation of higher cell density and their smaller size (Figs. 17 and 18).

Cell growth process (b) and (c) depends on several parameters, a.o. on the melt viscosity, melt strength and dynamics of cooling. For lower viscous melts the cells grow faster and are larger than these in a more viscous system (Fig. 19).

Fig. 19. Cell size and population for different LDPE grades (130ºC)

One should consider that the gas dissolved in a polymer causes its plasticization, therefore the melt viscosity decreases markedly, thus modifying the foaming progress. In Fig. 20 a drop in the torque measured at kneading of the polymer melt after addition of an endothermic chemical blowing agent has been presented.

Total amount of cells and their size are related to the polymer melt strength. In case it is high the neighbouring cells may grow individually, however if it is low, the cell walls may disrupt due to an internal gas pressure and coalescence of cells occurs (Fig. 21). Another issue is a gas escape from the polymer melt to atmosphere and a resulting surface warpage.

molten polymer is evolving simultaneously at several sites of the material (a). Since the nucleation rate is much higher than the diffusion rate, the cell nuclei arise first, and only after some time they start growing due to a diffusion of next gas particles which appear as the gas solubility in a polymer melt falls due to the pressure and temperature decrease in a

Number of cells nucleated in the polymer depend on the pressure difference in the melt and atmospheric pressure. High difference developed by a change in the processing parameters or equipment configuration facilitates generation of higher cell density and their smaller size

Cell growth process (b) and (c) depends on several parameters, a.o. on the melt viscosity, melt strength and dynamics of cooling. For lower viscous melts the cells grow faster and are

material after it exits the extrusion die.

larger than these in a more viscous system (Fig. 19).

Fig. 19. Cell size and population for different LDPE grades (130ºC)

endothermic chemical blowing agent has been presented.

One should consider that the gas dissolved in a polymer causes its plasticization, therefore the melt viscosity decreases markedly, thus modifying the foaming progress. In Fig. 20 a drop in the torque measured at kneading of the polymer melt after addition of an

Total amount of cells and their size are related to the polymer melt strength. In case it is high the neighbouring cells may grow individually, however if it is low, the cell walls may disrupt due to an internal gas pressure and coalescence of cells occurs (Fig. 21). Another issue is a gas escape from the polymer melt to atmosphere and a resulting surface

(Figs. 17 and 18).

warpage.

Fig. 20. Mixing torque and temperature change after addition of endothermic CBA to polymer

Fig. 21. Foam morphology generated in LDPE of different melt strength (130ºC) – foam density 0.576 g/cm3 (left) and 0.447 g/cm3 (right)

The process of cell growth outlined in Fig. 16 stops as soon as the polymer glass temperature reaches the lower foaming limit. It may proceed fastly, because Tg increases with decreasing

Lightweight Plastic Materials 309

Cell size depend on the pressure within the cells, the melt strength and interfacial tension. The higher is the pressure, the smaller is the melt strength and interfacial tension, the larger cells are generated. As the cell grow and they wall thickness decreases, the coalescence probability of neighbouring cells increases. Behravesh at al. (1998) have found that a coalescence is facilitated with a high shear stress during processing, however its probability decreases with lower melt temperature. Therefore cooling of the gas-polymer solution in a

Polymer foaming may be performed with single screw extruders of high L/D ratio equiped with mixing elements at a last section of the screw, or with the twin screw or tandem extruders. In any case very important is a precise dosing of a gas, since its surplus causes

In a tandem system (Fig. 24) the first extruder serves for polymer melting and mixing it with the injected gas. In the second extruder further homogenisation of the temperature and gas

heat exchanger or within a die is advantageous.

distribution within a polymer melt should be performed.

Fig. 24. Tandem extruder system for foaming (acc. to Lee & Park, 2006)

by Xu et al. (2003) at the example of PS foaming with carbon dioxide.

temperature the melt pressure becomes high and the cell density is also high.

Proper design of every detail of the extrusion foaming set, the processing parameters and composition of the foamed material are crucial factors for the final morphology of a foam. The die geometry is of high importance, because the cell nucleation takes place there. Extensive discussion of the die role for generation of a high cell density has been presented

Pressure in a die depends on the temperature and shear forces in the molten polymer. Provided the die temperature is high, the melt viscosity is low, which causes a low pressure drop in the melt after it exits the die and a low number of nuclei. However, at the low die

large cells formation.

concentration of a gas in the polymer. At that stage a cooling dynamics is essential for the foam morphology (Fig. 22). Provided the cooling is fast in the cooled calibrator, cells remain at the size generated shortly after exiting a die. However, if cooling proceeds slowly, the cells continue growing until the material solidifies by heat exchange with the ambient atmosphere.

Fig. 22. LDPE foam morphology after cooling in calibrator and cooled freely (bottom)

Research on extrusion microfoaming of polystyrene with supercritical carbon dioxide has shown high influence of the extrusion die temperature and the dynamics of cooling on the resulting foam density and structure. The foaming level amounted to 15-25% (Sauceau et al., 2007).

Extruded microfoams have small cell size and exhibit low cell size distribution (Fig. 23).

Fig. 23. Structure of microfoam manufactured by extrusion foaming (Park, 2000)

concentration of a gas in the polymer. At that stage a cooling dynamics is essential for the foam morphology (Fig. 22). Provided the cooling is fast in the cooled calibrator, cells remain at the size generated shortly after exiting a die. However, if cooling proceeds slowly, the cells continue growing until the material solidifies by heat exchange with the ambient atmosphere.

Fig. 22. LDPE foam morphology after cooling in calibrator and cooled freely (bottom)

Extruded microfoams have small cell size and exhibit low cell size distribution (Fig. 23).

Fig. 23. Structure of microfoam manufactured by extrusion foaming (Park, 2000)

2007).

Research on extrusion microfoaming of polystyrene with supercritical carbon dioxide has shown high influence of the extrusion die temperature and the dynamics of cooling on the resulting foam density and structure. The foaming level amounted to 15-25% (Sauceau et al., Cell size depend on the pressure within the cells, the melt strength and interfacial tension. The higher is the pressure, the smaller is the melt strength and interfacial tension, the larger cells are generated. As the cell grow and they wall thickness decreases, the coalescence probability of neighbouring cells increases. Behravesh at al. (1998) have found that a coalescence is facilitated with a high shear stress during processing, however its probability decreases with lower melt temperature. Therefore cooling of the gas-polymer solution in a heat exchanger or within a die is advantageous.

Polymer foaming may be performed with single screw extruders of high L/D ratio equiped with mixing elements at a last section of the screw, or with the twin screw or tandem extruders. In any case very important is a precise dosing of a gas, since its surplus causes large cells formation.

In a tandem system (Fig. 24) the first extruder serves for polymer melting and mixing it with the injected gas. In the second extruder further homogenisation of the temperature and gas distribution within a polymer melt should be performed.

Fig. 24. Tandem extruder system for foaming (acc. to Lee & Park, 2006)

Proper design of every detail of the extrusion foaming set, the processing parameters and composition of the foamed material are crucial factors for the final morphology of a foam. The die geometry is of high importance, because the cell nucleation takes place there. Extensive discussion of the die role for generation of a high cell density has been presented by Xu et al. (2003) at the example of PS foaming with carbon dioxide.

Pressure in a die depends on the temperature and shear forces in the molten polymer. Provided the die temperature is high, the melt viscosity is low, which causes a low pressure drop in the melt after it exits the die and a low number of nuclei. However, at the low die temperature the melt pressure becomes high and the cell density is also high.

Lightweight Plastic Materials 311

Another technology of cellular profiles extrusion offers Sulzer Chemtech (Switzerland). OptifoamTM technology anticipates a gas injector between the screw and extruder head (www.sulzerchemtech.com). Gas (CO2 or N2) is injected into the gas melt through a fluid injection nozzle made of sintered metal. The mixture of a gas and a polymer is next

Fig. 26. Extrusion foaming set for OptifoamTM technology (www.sulzerchemtech.com)

Fig. 25. Gas injection system in MuCell® technology (acc. to www.trexel.com)

thoroughly homogenized in the static mixer section (Fig. 26).

1. die closure 2. control ring

4. gas injector 5. valves

3. screw mixing section

Shear stress level in the melt also influences pressure in the die. High pressure is beneficial to the cell density, however its too high level may cause the melt instabilities and cells rupture.

The nuclei number in a foamed polymer melt may be increased by means of fillers addition (Antunes et al., 2009; Khorasani et al., 2010). Foaming of polypropylene with carbon dioxide and addition of talc has shown that the cell density depends on both the filler and foaming agent content, however the nucleating effect of talc was observed only at low talc loading. Similarly, the positive effect of talc on the cell nucleation process and total cell density has been noted at foaming of PP with isopentane. It has been concluded that the nucleation mechanism depends on a size of the gas particles used for foaming. Recently also nanofillers have been reported as efficient cell nucleants for PP and HDPE foams (Khorasani et al., 2010).

Interesting properties exhibit cellular plastics manufactured from polymer blends. Basing on a knowledge on polymer melt rheology and blends morphology one can generate bi-modal distribution of cells located in different polymer domains. That idea has been presented for PPE/SAN blend foaming with carbon dioxide (Ruckdaeschel et al., 2007).

Another modification of foaming technology presents mixing of polymer with CBA, partial crosslinking of the polymer and than decomposition of the blowing agent with evolution of a gas. Cell structure can be limited by means of the crosslinking level, thus microfoams can be manufactured with such technology (Rodriguez-Perez et al., 2008). Microcellular polyethylene is used a.o. for pipe insulations, gaskets and in a healthcare as the wound dressing or sensitive skin protection.
