Blowing Agent Processing

4.5.1 Plasticization

Blending BAs affects foam processing in many ways. Viscosity, for instance, will be impacted differently from one type of BA to another. Ultrasonic monitoring performed on PS melts plasticized with either carbon dioxide or HFC-134a clearly shows that on an equivalent molar basis, HFC-134a has a greater plasticizing effect than CO2 (Figure 4.15). Simultaneous independent results obtained from conventional rheological measurements are in agree­ment with what was observed by the ultrasonic sensors. At a constant shear rate, shear stress gradually decreases as a function of gas concentration with lower values obtained for PS/HFC-134a. An increase of 50% in molar con­centration of the carbon dioxide would be required to match the viscosity decrease induced by a given amount of HFC-134a (Figure 4.16). Globally, larger BA molecules tend to plasticize polymer melts more efficiently than small ones, probably owing to a more important spacing of the polymer chains. This plasticization can be observed by the depreciation of the glass transition temperature (Tg) for doped systems. Figure 4.17 reports variations of Tg on an equivalent molar basis for different gases. It can be clearly seen

Blowing Agent Processing

FIGURE 4.15

Sound velocity (homogeneous phase) of polystyrene-blowing agent mixtures as a function of concentration. Measurements were conducted at a melt temperature of 190°C. All data normal­ized at P = 8.3 MPa. (From Vachon, C. and Gendron, R., Cell. Polym., 22, 75, 2003 [51]. With permission.)

Blowing Agent Processing

FIGURE 4.16

Semi-logarithmic plot of shear stress as a function of blowing agent concentration. Measure­ments were conducted at a melt temperature of 190°C. (From Vachon, C. and Gendron, R., Cell. Polym., 22, 75, 2003 [51]. With permission.)

that the lowering of Tg is directly correlated to molecular weight, i. e., molec­ular size.

Polymer type will also greatly influence the extent of plasticization. In comparison, plasticization by HFC-245fa was evaluated both in LDPE and in PS. Figure 4.18 reports the sound velocity and shear stress of

LDPE /HFC-245fa mixtures at 190°C prior to degassing. It can be observed that both sound velocity and stress decrease with concentration indicating that HFC-245fa plasticizes the polymer matrix. Results previously reported for the same blowing agent in PS [50] showed that the decrease of sound velocity and shear stress was not as important in LDPE as in PS. Sound

Blowing Agent Processing

FIGURE 4.19

Variation of the glass transition temperature (Tg) as a function of various mixture concentrations of blowing agents isopropanol and HFC-134a. The thin slash-dotted line corresponds to Chow’s estimate for isopropanol. The solid line corresponds to experimental results for mixtures of polystyrene (PS) with isopropanol and PS with HFC-134a. The bold dotted line corresponds to mixtures of PS with isopropanol and HFC-134a, where one of the two diluents is maintained at a constant fraction — for example, 5.12%H stands for 5.12 wt% of HFC-134a (H stands for HFC-134a, and i for isopropanol) combined with a variable concentration of isopropanol. (From Gendron, R. et al., Cell. Polym., 23, 1, 2004 [55]. With permission.)

velocity change associated with the plasticizing effect of HFC-245fa in PS is

7.6 m/s/wt%, while it is only of 6.03 m/s/wt% in LDPE.

Plasticization studies were also performed on the binary systems of blow­ing agents. Of course, blending can offer some technological or economical advantage, but the main interest is to understand how these agents interact with one another, and probe these systems for possible synergistic effects. An example of that is given in the rheological study of blends based on HFC – 134a and isopropanol in polystyrene. The analysis of the raw data was performed according to a procedure detailed in Gendron et al. [85] and includes corrections for pressure, temperature, and the Rabinowitsch factor for the apparent shear rate. Modeling of the results through the Williams – Landel-Ferry (WLF) equation (Equation 2.7) finally leads to the correspond­ing glass transition temperature that should be associated to each plasticized system, according to its PFA composition (Figure 4.19).

For the polymer systems containing only isopropanol, the Tg computed using Chow’s predictive equation based on the molecular weight of the solvent and of the repeat unit of the polymer [85,86] are indicated using a thin slash-dotted line. The solid and dotted lines correspond, respectively, to mixtures of PS with isopropanol only, and to different systems with one diluent at a fixed concentration. The agreement between Chow’s estimates and experimental results for HFC-134a is excellent over the investigated range of concentrations. Experimental results for mixtures of PS with isopropanol exhibit Tg values much higher than those predicted by Chow’s equation. According to results derived from Chow’s equation and based solely on its molecular weight, isopropanol (Mw = 60.096 g/mol) should be a better plasticizer than HFC-134a (Mw = 102.3 g/mol). Through exper­imental results, we see that in fact, both HFC-134a and isopropanol behave similarly.

Several combinations of isopropanol and HFC-134a have been attempted, and many observations can be made on the rheological response of these mixtures. The first two sets of data under investigation are those containing a fixed fraction of isopropanol (1.48 and 2.96 wt%), while the concentration of HFC-134a is varied. These are illustrated in Figure 4.19 by the two dotted lines labeled 1.48% i and 2.96% i. The two lines are quite parallel, and their slopes are smaller than that of sole HFC-134a. The Tg for mixtures involving the two diluents are always higher than the results obtained for HFC-134a, at identical total concentrations. Similar conclusions can be drawn for the set of data involving a fixed fraction of isopropanol. This example shows that combinations of BAs do not always behave in ways predictable from the properties of the pure components. In this particular system, the combi­nation of the two diluents seem to present some interactions that slightly inhibit the plasticizing effect.

Studies of the plasticization of gas-polymer systems probed by ultrasonic and rheological measurements are generally in agreement: lowering of shear stress is accompanied by lowering of ultrasonic velocity, as was seen for single BA systems (CO2 or HFC-134a in PS). However, an unusual discrep­ancy has been recently noted in blends of CO2 and (cyclohexane or isopro­panol) in PS. Figure 4.20(a) and (b) shows the variation of shear stress and sound velocity as a function of CO2 ratio in equivalent molar blends.

Figure 4.20(a) and (b) shows that when cyclohexane or isopropanol is gradually replaced by CO2, shear stress and ultrasonic velocity vary in oppo­site ways. It would be expected from previous studies that larger molecules are better plasticizers and should cause a more important drop of viscosity. In that sense, the macroscopic data provided by the on-line slit die follow that trend. The ultrasonic results, which are more sensitive to the microscopic entanglement of the polymer chain, suggest something entirely different. As described in Chapter 5, sound velocity Vus is directly proportional to the elastic modulus (L’) and specific volume (Vsp) (or density [p]) of the poly­mer-gas solution, following Equation 4.5:

Vus = 4LJp=4 L’-Vp (4.5)

In accordance with the rheological measurements reported in Figure 4.20(a), which measure the pressure drop (i. e., viscous loss) encountered in the slit die, it is expected that the elastic modulus will also be lower for mixtures containing increasing proportions of cyclohexane or isopropanol. However, when measuring the sound velocity, one has to account for the

(a)

 

62 –

60

58 –

56 –

54

52

50

48

46

44

42

 

ф CO2+ cyclohexane T CO2 + isopropanol

 

40 60

CO2 ratio (%)

 

(b)

 

• CO2 + cyclohexane v CO2 + isopropanol

 

— 1120

 

.

effect of specific volume. It may be postulated that even at constant pressure, the swelling of the polymer-gas solution is so important in the presence of cyclohexane or isopropanol that, overall, the sound velocity is higher than with pure CO2.

Leave a reply

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>