Cellulose acetate parameter

Although many plasticisers have been suggested for cellulose acetate very few have been used in practice. The most important of these are dimethyl phthalate (8 = 21.4), triacetin (8 = 20.3) and triphenyl phosphate (8 = 20.0), each of which have a solubility parameter within one unit of that of cellulose diacetate (-22.0). (All in units ofMPa. )... 

Triphenyl phosphate is a crystalline solid which has less compatibility with the polymer. This may be expected from solubility parameter data. It is often used in conjunction with dimethyl phthalate and has the added virtues of imparting flame resistance and improved water resistance. It is more permanent than DMP. Triacetin is less important now than at one time since, although it is compatible, it is also highly volatile and lowers the water resistance of the compound. Today it is essential to prepare low-cost compounds to allow cellulose acetate to compete with the synthetic polymers, and plasticisers such as ethyl phthalyl ethyl glycollate, which are superior in some respects, are now rarely used. 

With particular reference to reverse osmosis systems involving cellulose acetate membranes and aqueous solutions, the membrane material has both polar and nonpolar character, and the solvent, of course, is polar. When these two components of the reverse osmosis system are kept constant, preferential sorption at the membrane-solution interface, and, in turn, solute separation in reverse osmosis, may be expected to be controlled by the chemical nature of the solute. If the latter can be expressed by appropriate quantitative physicochemical parameters representing polar-, steric-, nonpolar-, and/or ionic-character of the solutes, then one can expect unique correlations to exist between such parameters and reverse osmosis data on solute separations for each membrane. Experimental results confirm that such is indeed the case (18). 

Nonpolar Parameters. In a reverse osmosis system involving cellulose acetate membranes and aqueous solutions of hydrocarbon solutes, the adsorption of water and that of solute on the polar and nonpolar sites of the membrane surface respectively may be expected to take place essentially independently. Further, since the polymer-solute interaction forces are attractive in nature for the above case, the mobility of the solute molecules through the membrane pore is retarded, and they also tend to agglomerate... 

Figure 7. Experimental data on the effect of (a) Taft s number for alcohols and (b) interfacial free energy parameter for ionized inorganic solutes on their reverse osmosis separations in systems involving dilute aqueous solutions and cellulose acetate membranes (9 15, 37, 38 ...

Membrane Specifications. At a specified operating temperature and pressure, a cellulose acetate membrane is completely specified in terms of its pure water permeability constant A and solute transport parameter D /k6 for a convenient reference solute such as sodium chloride. A single set of experimental data on (PWP), (PR), and f at known operating conditions is enough to obtain data on the specifying parameters A and (DAM/X6)jjg(. 2 at any given temperature and pressure. 

The discussion directly following Eq (6) provides a simple, physically reasonable explanation for the preceding observations of marked concentration dependence of Deff(C) at relatively low concentrations. Clearly, at some point, the assumption of concentration independence of Dp and in Eq (6) will fail however, for our work with "conditioned" polymers at CO2 pressures below 300 psi, such effects appear to be negligible. Due to the concave shape of the sorption isotherm, even at a CO2 pressure of 10 atm, there will still be less than one CO2 molecule per twenty PET repeat units at 35°C. Stern (26) has described a generalized form of the dual mode transport model that permits handling situations in which non-constancy of Dp and Dh manifest themselves. It is reasonable to assume that the next generation of gas separation membrane polymers will be even more resistant to plasticization than polysulfone, and cellulose acetate, so the assumption of constancy of these transport parameters will be even more firmly justified. 

Figure 3.10 The porosity of cellulose acetate membranes cast from 15-wt% solutions with various solvents. The same trend of high porosity and rapid precipitation with high solubility-parameter solvents was seen with a number of other membrane materials [25]...

As previously discussed, food effects are an important parameter for enteric-coated systems, especially for drugs, that are sensitive to food. Pancreatic enzyme-containing products fail when they come in contact too early with lipids, proteins, and carbohydrates present in food. The clinical efficacy of pancreatic enzymes formulated as enteric-coated tablets was investigated in man and dog [44], The enteric materials examined were hydroxypropyl methylcellulose phthal-ate (HPMCP), cellulose acetate phthalate (CAP), and the methacrylic acid copolymer USP/NF Type C. In vivo behavior monitored by x-ray scintigraphy showed clear differences between the three coating formulations. HPMCP-coated products adhered to the gastric mucosa, whereas CAP and methacrylate copolymer... 

Figure 1. Kinetic parameters for grafting of styrene onto pre-irradiated cellulose acetate films at 50°C from a styrene/MeOH (1/1) solution. Key GR, average growth rate of graft radical PR, average termination probability DA, initiation rate of grafting. (Adapted from Figure 6 of Ref. 2.)...

Reverse osmosis properties of the membranes composed of cellulose acetate phthalate and PVPo are summarized in Table 18484. Table 18 reveals the rejection decreases as the values of solubility parameters (<5) of the membrane and of the solute approach one another. 

Brace et al. [92] investigated polymer/water interactions using SAW devices coated with either polyimide or cellulose acetate butyrate (CAB). In this study thermodynamic parameters were evaluated from the absorption isotherms, and transient responses to step changes in concentration were monitored. The transient responses observed were not consistent with Fickian diffusion, but could be described using a generalized relaxation equation containing two additive terms. Results under various conditions indicated that relaxation in the polymer system is much slower than diffusion of water. 

In another study, Wu et al. from China used a neural network to model the formulation of salbutanol sulfate osmotic pump tablets, using the amount of hydroxypropyl methyl cellulose and polyethylene glycol present in the cellulose acetate coating, in addition to the coating weight, as control factors. Using the model, the authors predicted the release parameters for 1000 formulations, from which they selected an optimum with the desired release pattern. 

Table I. Experimental Reverse Osmosis Data for the System NaCl-H O and Glycerol-H O Using Porous Cellulose Acetate Membranes and Parameters Characterizing Membranes...

Table IV. Values of Born Equation Parameters, Cellulose Acetate (Eastman E-398)...

The physicochemical criteria approach to reverse osmosis separations Involving the surface excess free energy of solvation for ionized and nonlonized solutes has been demonstrated by this work to include nonaqueous solutions. The parameters and correlations presented in this work permit the prediction of reverse osmosis separations and permeation rates for different alkali metal halides for cellulose acetate OEastman E-398) membranes of different surface porosities from only a single set of experimental data for a sodium chloride-methanol reference feed solution system. 

Reverse osmosis separations of 12 alkali meteil halides in methanol solutions have been studied using cellulose acetate membranes of different surface porosities. Data for surface excess free energy parameters for the ions and ion pairs Involved have been generated for the above mend>rane material-solution systems. These data offer a means of predicting the performance of cellulose acetate membranes in the reverse osmosis treatment of methanol solutions involving the above ions from only a single set of experimental data. 

---