Unhydrolyzed polyacrylamide

G.L. Griffith W.L. Schwoyer, USP 3457128 (1969) CA 71, 72517 (1969). Particulate self-explosive nitrated materials, such as Nitro-starch, PETN, TNT,.and their mixts, can be made relatively safe to handle and transport thru formation of a uniform nongelled aq slurry containing 10—50% water and 0.5—10% partially hydrolyzed polyacrylamide suspending agent having 0.1—50% free acid, and = 50% unhydrolyzed amide groups, and a mw of 1,000,GOO-... 

For the first non electrostatic term, such a dependence can be calculated from the classical Flory theory and the value of the theta temperature of unhydrolyzed polyacrylamide ( 0 = 265°K (22))... 

In fact this "unhydrolyzed" polyacrylamide sample is slightly charged and its low polyectrolyte character is confirmed by a slight difference of red values at pH 7 and 5, for salt free solutions. A really neutral polymer should be necessary to differentiate low effects of electrostatic interactions from non ionic interactions. coordination binding at low pH and hydrogen bonds at pH 7. Nevertheless, at this pH, the adsorption of the chain on Al(0H)3 aggregates can probably be considered as the main origin of the loss of viscosity. 

If the molecules of a polyacrylamide polymer are electrically uncharged, then the polymer is called either unhydrolyzed or non-ionic. In this case the polymer molecules are usually loose and roughly spherical coils, while showing a linear-chain configuration, in any solvent. Thus the polymer solution would exhibit a high viscosity ... 

Electrically charged (hydrolyzed) polyacrylamide molecules, on the other hand, tend to expand, similar to the unhydrolyzed polymer molecules, only in good solvents which must be free of salt, such as distilled water. Therefore, a high viscosity would be offered by the polymer solution. ... 

When an unhydrolyzed polyacrylamide polymer is dissolved in a caustic solution, a reaction starts immediately between the alkaline agent (NaOH) and the amide (NH2) groups of the polymer molecules. This reaction, which is nothing different from the hydrolyzation process (hydrolysis) of the polyacrylamide molecules, progresses continuously either for several hours or several days, depending on the nature of the polymer used. 

Now assume that the same procedure given above is repeated with a polyacrylamide polymer which is already partially hydrolyzed to a degree of 15 to 35 percent. In this case, the viscosity behavior of the PHPA polymer in caustic solution will not be very different from the repeat of the viscosity reduction part of the behavior exhibited by the unhydrolyzed polyacrylamide polymer in caustic solution. 

The adsorption of unhydrolyzed and partially hydrolyzed polyacrylamides at different adsorbent can be characterized by Langmuir I isotherms. 

Dependence of the adsorbed amount on the polymer concentration is given for a typical unhydrolyzed and a partially hydrolyzed polyacrylamide in Figures 5 and 6. Apart from the fact that the absolute values indicate considerable differences, it is characteristic that n increases with the pol3nner concentration. Within this general tendency, as it appears from these curves, the steep increase occurs after a definite pol3nner concentration in all cases. Before this critical concentration—similar to the equilibrium section of static adsorption isotherms—the adsorbed amount does not depend significantly on polymer concentration. Incidentally, the critical polymer concentration in 2 gdm" NaCl is increased twofold over values measured in distilled water for both polymers. 

The structure of the polymer solutions is influenced by cations to a higher extent in case of partially hydrolyzed polyacrylamides than in case of unhydrolyzed polymers, while the role of the average molecular mass is negligible (26). This answers the question why the adsorbed amount shows a relatively high sensitivity for salts in solutions of hydrolyzed polyacrylamides. 

The effect of pH on chain structure (post-hydrolysis) should be taken into consideration if an unhydrolyzed polyacrylamide is studied. But very small difference was found between unhydrolyzed and partially hydrolyzed polymers in this respect. 

In general, the unhydrolyzed and partially hydrolyzed polyacrylamides behave differently. The dynamic adsorption of hydrolyzed polyacrylamides is more sensitive to all factors studied than the unhydrolyzed ones. 

Fig. 13 shows that it takes about 18 days at 93°C [200 °F] for an initially unhydrolyzed polyacrylamide to reach 53% hydrolysis. Thus, the polymer would not remain in solution beyond 18 days under these conditions. 

Fig. 13—The effect of temperature on the hydrolysis of 1,000-ppm originally unhydrolyzed Polyacrylamide A in 5% NaCI solution.

Fig. 15 is a graphic representetion of the predicted precipitation times for an initially unhydrolyzed polyacrylamide as a function of temperature and hardness level presented as a contour plot. This plot was derived by comparison of a large number of calculated hydrolysis levels with the cloud-point limits shown in Fig. 14. 

The above solution-stability times are all based on an initially unhydrolyzed polyacrylamide. Partially hydrolyzed materials will precipitate sooner. Also, the hardness levels quoted are for equal amounts of calcium and magnesium. As noted above, calcium has the more serious effect. For many real oilfield waters, it is probably a good first approximation to ignore the magnesium content and to divide the above limits by two to derive maximum calcium concentrations. 

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