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The plasticization effect of carbon dioxide used as PBA is not only seen in the pressure decrease but also confirmed by a decrease in viscosity and increase in volume flow of the externally plasticized CA melt at constant processing conditions temperature, throughput, slit die gap. Since the effective dissolution of a PBA in the polymer melt is mostly pressure controlled, a certain viscosity of the polymer melt is required to achieve sufficient pressure in the extruder for dissolution of the PBA.

An explanation for this could be the continuous increase in volume flow and decrease in melt viscosity. As a result, the pressure inside of the extruder is too low to keep carbon dioxide dissolved in the CA melt. To minimize premature supersaturation of carbon dioxide and to avoid prenucleation in the extruder, the temperature of the extruder and the rheometer can be reduced.

This leads to an increase in the viscosity of the gas loaded CA melt. As a result, the pressure in the extruder also increases, keeping the carbon dioxide dissolved in the CA melt and avoiding premature supersaturation in the extruder.

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This is clearly shown in Figure The results for carbon dioxide used as a PBA show the general complexity between gas loaded polymer melts and the extrusion process. To get more information on the melt flow and viscosity behavior of gas loaded externally plasticized CA melts, further studies have to be conducted using additional blowing agents, for example butane or nitrogen N 2.

For the preliminary foam extrusion tests, the same CA compound composition 20 wt. Table 5 shows the PBAs used for these preliminary foam extrusion tests. The talc used has platelet geometry with a diameter d 0.

Lezersrecensie van 'Foam Extrusion'

The extruder is equipped with a mixing screw optimized for the foam extrusion process. The last 11 D of L before the die are temperature controlled in order to cool the polymer melt. The blowing agent was compressed and injected into the extruder barrel through a pressure hole at 16 D of L using a metering system equipped with a diaphragm pump. By means of a static mixer, the PBA is dispersed in the melt.

As seen from Figure 12 , the blowing agent not only influences the melt rheology of CA but also the foam extrusion and expansion behavior at the die. Influence of blowing agent and nucleating agent content on foam extrusion process of CA plasticized with 20 wt. Carbon dioxide shows rapid expansion at the die in comparison to HFO ze.

Extrusion line for tube / branch production in polyethylene foam

An explanation for this could be the higher diffusivity and permeability of carbon dioxide due to its lower molecular size when compared to longer chain PBAs such as HFO ze. Blowing agents having a high diffusivity will be phased out in a shorter time [ 54 ]. Additionally, the solubility of carbon dioxide is restricted in most polymer melts when compared to other conventional blowing agents such as hydrofluorocarbons [ 54 ]. High pressure in the extruder is required to keep carbon dioxide dissolved in the externally plasticized CA melt.

The high pressure gradient at the die in combination with the high vapor pressure and high diffusion rate of carbon dioxide causes a strong expansion process at the die. Not only the PBA but also the addition of nucleating agents, for example talc, affects the foaming behavior at the die.

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By adding talc to HFO ze, the foaming process of externally plasticized CA is significantly improved that can be seen in Figure 12 c and d. The expansion directly starts at the die due to better nucleation more and faster nucleation in conjunction with stable cell growth as both processes start simultaneously.

The investigation of the rod diameter D and the foam density as a function of PBA content and talc content verify the results got from high speed camera images. Generally, the higher the PBA concentration, the more PBA is dissolved in the polymer melt that can cause stronger foaming. The type of PBA and its solubility in the externally plasticized CA melt influences the intensity of the slope.

The addition of talc used as a nucleating agent shows a selective influence. For butane no significant influence is observed whereas for HFO ze the addition of talc 0. This is in good agreement with the visual results obtained with the high speed camera system. From Figure 13 one may also conclude that an increase in nucleating agent content from 0. An explanation for this could be the heterogeneous nucleation process in presence of talc. Therefore, the cell nucleation predominates the cell growth. Contrary, homogeneous nucleation occurs in absence of talc. Consequently, cell nucleation rate is limited and the following cell growth process predominates.

The nucleating agent is therefore more important for controlling the nucleation rate, the cell density, the foam morphology, and the stabilization of the foam network. It is one parameter that significantly affects the foam density and foam ratio, this is the ratio between the density of the foamed and the unfoamed polymer. As expected, increasing talc content leads to a steady decrease in foam density and continuous increase in the foam ratio, as shown in Table 6.

These observations agree well with results from literature [ 55 , 56 ]. However, too high talc contents can lead to agglomeration effects of the nucleated cells [ 57 , 58 ] or can cause cell collapse [ 59 ]. Therefore, an increase in foam density and a decrease in foam ratio may occur at too high talc contents [ 60 ].

The influences of PBA and talc on the foaming process, the foam density, and the foam ratio are supported by investigations of the foam morphology using optical microscopy OM. Figure 14 shows selected microscopy images of extrusion foamed CA rods. Butane as well as nitrogen causes a coarse inhomogeneous morphology with a broad cell size distribution and large partially opened cells to some extent.

This can be explained by premature phase separation supersaturation and cell coalescence due to the poor solubility of butane and nitrogen in the externally plasticized CA melt. Li [ 61 ] found that less soluble blowing agents tend to diffuse out more rapidly than the more soluble one. Consequently a smaller amount of these blowing agents is dissolved in the polymer melt for foaming. These investigations agree well with the detected foam density and foam ratio. Scanning electron microscopy SEM images are of further evidence of the previous results.

Blowing agents which show limited solubility in the externally plasticized CA melt such as butane or nitrogen lead to heterogeneous foam morphologies with large and partially opened cells, which is seen in Figure 15 b. These large cells can act as voids. As a result, final properties such as mechanical performance of these foams may be poor when compared to foams which have a fine and uniform morphology. Influence of blowing agent on the foam morphology of selected CA foam rods at constant talc content 0.

The limited solubility of butane and nitrogen can be explained by their nonpolar character.

Foam Extrusion: Principles and Practice, Second Edition - CRC Press Book

As seen from Table 7 , only the dispersion part of the Hansen solubility parameter is accessible for dissolving butane and nitrogen in the polar externally plasticized CA melt. In contrast, carbon dioxide is a more polar or hydrogen bonding gas. Therefore, the polar part and hydrogen bonding part are also available for dissolution of carbon dioxide in the CA melt. Hansen solubility parameter of selected physical blowing agents [ 43 ].

For butane, cell coalescence and partially opened cell structures are evident, especially in the center of the sample cross-section. Similar results were obtained for nitrogen. Due to the limited solubility in the externally plasticized CA melt, premature supersaturation of butane and nitrogen from the melt occurs. Thus, coalescence primarily in the flow center is observed due to this being the area of the lowest flow resistance. Conversely, carbon dioxide causes fine cell morphology with homogeneous cell size distribution and high cell density across the sample.

To obtain further information about the influence of the blowing agents and the nucleating agent, cell size and cell density N f were studied. Cell density Nf in cells cm -3 was calculated according to Eq. As expected from literature [ 55 , 60 ], the addition of a nucleating agent such as talc leads to a considerable decrease in cell size Figure This is in good agreement with the obtained results for the foam density and foam ratio. From Figure 17 , one may conclude that no tremendous influence of the PBA concentration on the cell size was found at a talc content of 0.

Similar results were found in [ 55 ] for PP foamed with carbon dioxide above 0. It can be assumed that heterogeneous cell nucleation predominates in presence of talc and therefore cell nucleation rate is controlled by the high talc content regardless of the increase in concentration of the PBA used. As the cell size continuously decreases with increasing talc content, cell density basically increases at the same time due to higher cell nucleation rate.

Lezersrecensie van 'Foam Extrusion'

This is shown in Figure For HFO ze, which shows good solubility in the externally plasticized CA melt, an exponential increase in cell density is observed with continuous increase in talc content due to an increase in heterogeneous cell nucleation rate. The higher cell density is directly seen in the finer foam morphology.

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In contrast, no excessive increase in cell density is observed for butane even at 0. This agrees well with the previous results obtained for the foam density, the foam ratio, and the foam morphology.