Cloud-point pressures

Trichlorodifluoroethane (HCFC-122) is a co-spin agent, which lowers the cloud-point pressure. The cloud-point pressure means the pressure at which a single phase liquid solution begins to phase separate. At temperatures above the critical point, there cannot be any liquid phase present and therefore a single phase, supercritical solution phase separates into a polymer-rich/spin fluid-rich, two-phase gaseous dispersion. 

Relatively low spinning temperatures and pressures that are above the cloud-point pressure can be used. Microcellular foam fibers may be obtained rather than plexifilaments, even at spinning pressures slightly below the cloud-point pressure of the solution. 

If the solvent size is again kept essentially constant, but now the solvent can hydrogen bond to the copolymer, very low cloud-point pressures are observed as long as the temperature remains high as shown with ethane compared to methanol. At temperatures below 125 °C, methanol prefers to self-associate... 

Figure 2 shows a series of cloud-point curves determined for the system ethylene-2-ethylhexyl acrylate-poly(ethylene-co-2-ethylhexyl acrylate). Each cloud-point curve corresponds to one stationary copolymerization condition in CSTR1. The compositions and concentrations referring to the five monomer-polymer mixtures, including one ethylene homopolymerization reaction (Experiment 1), are listed in Tab. 1. FA is the concentration of the acrylate units within the copolymer (in mole-%),/P and/A denote the concentrations of polymer and of acrylate monomer in the monomer-polymer mixture, respectively. As can be seen from Fig. 1 and from Tab. 1, increasing acrylate content in the copolymer lowers the cloud-point pressure. 

Cosolvents can enhance solubility of compounds in CO2, a topic beyond the scope of this chapter. It is useful, however, to point out some details on cosolvents in CO2. McHugh et al. demonstrate that a cosolvent can provide the specific interactions that are necessary to solubilize a compound in CO2 (70,71). For instance, butyl acrylate (BA) and ethyl hexyl acrylate (EHA) decrease the cloud point pressure of acrylate polymers owing to the specific polar interactions between the cosolvent with the acrylate backbone of the polymer (70). Addition of ethyl methacrylate (EMA) and butyl methacrylate (BMA) reduces the pressure needed to solubilize poly(ethyl methacrylate) (PEMA) and poly(butyl methacrylate) (PBMA) in CO2 (71). 

Kinzl, M. et al., SAFT modeling of inert-gas effects on the cloud-point pressures in ethylene copolymerization systems poly(ethylene-co-vinyl acetate)- -vinyl acetate-i-ethylene and poly(ethylene-co-hexene-l)-thexene-l-tethylene with carbon dioxide, nitrogen or -butane, Ind. Eng. Ghent. Res., 39, 541-546, 2000. 

Figure 3.28 shows the P-T diagram for four polyethylene-low molecular weight hydrocarbon mixtures. The cloud point pressures decrease substantially with increasing carbon number, or conversely polarizability, as a result of increased dispersion interactions between polyethylene and the solvent. Free volume differences between polyethylene and the hydrocarbons also decrease as the carbon number is increased. Even though ethane and ethylene have virtually identical polarizabilities, the cloud point curve with ethane is at a much lower pressure than that with ethylene, since the quadrupole moment of ethylene enhances ethylene-ethylene interactions relative to ethylene-polyethylene interactions because polyethylene is a nonpolar polymer. The two cloud point curves for polyethylene with propane and propylene are virtually identical. Evidently, the quadrupole moment for propylene is weak enough that propylene-propylene polar interactions do not substantially influence the strong dispersion interactions between polyethylene and each of these two solvents of virtually identical polarizabilities. 

Figure 3.36 The effect of ethanol and acetone cosolvents on the cloud point pressure of the poly(ethylene-co-methyl acrylate) (90mol% ethylene and 10 mol% methyl acrylate)-propane system. The copolymer concentration is fixed at 5 wt% and the weight-average molecular weight of the copolymer is 34,000 with a molecular weight polydispersity of 2.0. This copolymer is —15% crystalline. (Hasch et al., 1993.)...

Figure 7. Plot of the cloud-point pressure versus the weight percentage of the carbohydrate-derivative for AGLU (te) BGLU ( ) and BGAL (A) in supercritical CO2 at a temperature of 40.0X2 (40).

Comments. The cloud-point pressures are independent on Wb between wb = 0.001 and... 

HAN Han, S.J., Lohse, D.J., Radosz, M., and Sperling, L.H., Short chain branching effect on the cloud-point pressures of ethylene copolymers in subcritical and supercritical TproTpans, Macromolecules, 31, 2533, 1998. 

TUM Tumakaka, F., Sadowski, G., Latz, H., and Buback, M., Cloud-point pressure curves of ethylene-based terpolymers in fluid ethene and in ethene-comonomer-mixtures. Experimental study and modeling via PC-SAFT, J. Supercrit. Fluids, 41, 461, 2007. 

CH2 Chan, A.K.C., Russo, P.S., and Radosz, M., Fluid-liquid equilibria in poly(ethylene-co-hexene-1) + propane a light-scattering probe of cloud-point pressure and critical polymer... 

BUB Buback, M. and Latz, H., Cloud-point pressure curves of ethene ly[etltylene-co-((meth)acrylic acid)] mixtures, Macra/wo/. Chem. Phys., 204,638,2003. 

Cosolvent effect of alkyl acrylates on the phase behaviour of poly(alkyl acrylate)supercriti-cal CO2 mixtures has been reported. Cloud-point data to 220 and 2000 bar are presented for ternary mixtures of poly(butyl acrylate)-C02-butyl acrylate (BA) and poly(ethylhexyl acrylate)-C02-ethylhexyl acrylate) (EHA). The addition of either BA or EHA to the respective polymer-solvent mixtures decreases the cloud-point pressures by as much as 1000 bar and changes the pressure-temperature slope of the cloud-point curves from negative to positive, which significantly increases the single-phase region. 

However, the influence of polar comonomer units on polymer solubihty is in general neither Hnear nor necessarily monotonic. Fig. 2.6a shows the ethylene solubility of poly(ethylene-co-methyl acrylate) copolymers for different amounts of the methyl acrylate monomer in the copolymer from 0 mol% (corresponds to LDPE) to 44 mol%. For small amounts of the methyl acrylate monomer, favorable interactions of the methyl acrylate units of the copolymer with the quadru-pole moment of the ethylene enhance the solubility of the copolymer. Here, the copolymers first show a decreasing cloud point pressure. However, upon further increase of the methyl acrylate contents (above 13 mol%), the importance of the polar intermolecular interactions between the different methyl acrylate units of the copolymer molecules becomes dominant, leading to decreasing solubility. However, for the similar system poly(ethylene-co-propyl acrylate), very different behavior is observed. Here, the solubility of the copolymer increases with in-... 

The ceU, which is provided with sapphire windows and magnetic stirring, is a modification of the one described by Van Hest and Diepen [4]. A detailed description of this apparatus and the experimental techniques used is given by De Loos et al. [5]. The cloud-point pressures of mixtures of known composition have been measured as a function of temperature by visual observation of the onset of phase separation of the homogeneous phase by lowering the pressure (cloud-point isopleths). The cloud-points have been determined with an absolute error of 0.03 K in temperature and 0.1 MPa in pressure. 

In this context, poly (propylene) (PP) + pentane + carbon dioxide is an interesting system. F igure 10.11 shows the phase behavior of this system, where the cloud-point pressure at constant composition is plotted against the temperature. If no CO2 is present, the demixing pressure connected with LCST behavior increases slightly with increasing pressure. With increasing CO2 amount, the pressure increase with temperature is much more pronounced. 

Figure 10.13 Comparison between experimental and calculated cloud-point pressures [38] for the system ethylene -f poly(ethylene-co-methyl acrylate), where the polymer weight fraction is 0.05.

Figure 10.16 Cloud-point pressures in the system LLDPE ethylene at different temperatures (triangles T = 400 K, open circles T - 410 K, open squares 7 = 420 K, diamond T = 430 K) [44], The solid lines are...

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