Drying Hygroscopic Polymers

When a hygroscopic polymer is exposed to the atmosphere, water vapor migrates into the pellet where some of the water molecules become bound to the resin s polymer chains by intermolecular forces. These forces are what makes it so difficult to dry hygroscopic polymers. 

As noted, a dry hygroscopic polymer exposed to the atmosphere will absorb water vapor until it reaches equilibrium with the surrounding air. This process may take several minutes, or several days, depending on the type of polymer and the relative humidity of the air. Even after reaching equilibrium at, say, 50% relative humidity (RH), the polymer will absorb more moisture if it is later exposed to 80% RH. Or, if the RH were reduced to 30%, some of the absorbed moisture would be given up until it again reaches equilibrium. 

The operation of RH detectors can be based on the dry- and wet-bulb reading, temperature difference, elongation of cellulose, resistance of lithium chloride (Dunmore), surface resistivity of polystyrene (Pope), and change in capacitance. The capacitors can be formed from aluminum or hygroscopic polymers. 

Since Nylon is a hygroscopic polymer, its plastic response was studied in three different states of relative humidity, namely in a 100%-dry state, in a state of 60% relative humidity, and in a water-soaked condition. The dramatic effect of different levels of relative humidity on the plastic resistance of compression-molded Nylon-6 is shown in Fig. 9.1. 

Drying TP Polyurethanes and Other Hygroscopic Polymers, DSG Report No. 20, D. S. Gilmore Lab., Upjohn, 1981. 

If, however, a nonhygroscopic polymer is compoimded with a hygroscopic pigment or additive, the resultant compound will be hygroscopic and may need to be dried differently. For more detail on drying nonhygroscopic polymers, refer to Sec. 7.15.1. 

Effective control of moisture in hygroscopic polymers almost always requires a dehumidifying air dryer (Fig. 7.36), and the processor should be armed with a good basic understanding of how the dehumidifying drying system functions, in order to keep it in good repair. 

A basic understanding of the relationships between water vapor and the polymer, and water vapor and the surrounding air, is a prerequisite to understanding the mechanics of drying. A hygroscopic polymer will absorb moisture from the air or give up some of its absorbed moisture to the air, depending on the temperature of the polymer and the air s relative humidity. 

A dehumidifying dryer is used to remove absorbed moisture from hygroscopic polymers. While a hot-air dryer heats the drying air to lower its relative humidity, a typical dehumidifying dryer first removes as much moisture as possible from the drying air by means of a desiccant, and then heats the air to obtain an extremely low relative humidity. 

Desiccant regeneration is similar to the process used in drying non-hygroscopic polymers with a single-pass hot-air dryer, and is best illustrated by referring to Fig. 7.40 ... 

Volatile traps. Occasionally, a hygroscopic polymer may contain components or additives, such as plasticizers, and fire retardants, that will vaporize at less than the recommended drying temperature. These volatiles or vapors, once driven from the polymer, will be carried into the return air circuit and will pass through the return air filter in the gaseous state. However, as the return air makes its way back to the dehumidifier and its temperature is lowered, the volatiles condense, usually into a waxy, oily substance that contaminates the desiccant. There are several common methods for contending with volatiles ... 

For an exact initial mass of the polymer, the net amount of polymer in the sample has to be determined. For hygroscopic polymers (like the water-soluble polymer PA Am), the dry content of the polymer has to be known. The dry content of the polymer can be determined via thermogravimetric methods or lyophilization. In addition to this, the salt content of the polymer (for commercial ionic polymers up to 30%) or the content of other foreign matter has to be determined, for example via reprecipitation or washing of the sample. When this is done, it is advisable to define a standard unit for the concentration. Since the unit for the intrinsic vis-... 

Drying SPS polymers are not hygroscopic and resin pre-drying is usually not required. Under adverse, extremely high humidity conditions or wide temperature fluctuations from cold to hot, surface moisture may develop on SPS resin exposed to atmospheric conditions. Drying at 82 °C (180°F) for 2 hours in a desiccant-type dryer will remove surface moisture that may develop. 

Solid Polymer. Completely dry polyacrylamide is a brittle white soHd. Commercially available dry polyacrylamide powders are typically dried under mild conditions and usually contain 5—15% water. The powders are hygroscopic, and generally become increasingly hygroscopic as the ionic character of the polymer increases. Cationic polymers are particularly hygroscopic. 

Dry polymers can be stored for up to 2 years, but are hygroscopic and absorb moisture from the air. Keep them in sealed bags in a cool, dry storeroom. Batches of polymer solutions made from dry polymers should be used within 24 hours. 

Wood is a hygroscopic material, due to the fact that the cell wall polymers contain hydroxyl groups. In an environment containing moisture, dry wood will absorb moisture until it is in equilibrium with the surrounding atmosphere. Similarly, saturated wood, when placed in an atmosphere of lower relative humidity (RH), will lose moisture until equilibrium is attained. If the wood is placed in an environment where the RH is stable, it will attain a constant moisture content (MC), known as the equilibrium moisture content (EMC). At this point, the flux of water molecules into the cell wall is exactly balanced by the outward flux into the atmosphere. 

Hygroscopicity tests were performed on slurries prepared with different dispersants after drying. Figure 1 shows that the water re-adsorption after drying is significantly reduced with the polymer called Acusol 460 N. 

Powdered polymers are hygroscopic and will absorb moisture from the air. Keep them in sealed bags in a cool, dry storeroom. 

According to these results, the conductivity and other possible contributions to the loss permittivity are more pronounced for PTHFM than for P3THFM. This phenomenon is associated to the residual water in the samples that cannot be removed after drying in vacuo at room temperature. This is an important trouble in the manipulation of polymeric materials, because it is very difficult to eliminate water and in the case of hydrophilic polymers the trouble is worst. These kind of materials are hygroscopic and the dryness condition under the experiments are performed must be strictly careful. 

Hypromellose acetate succinate should be stored in a well-closed container, in a cool, dry place. In such storage conditions, hypromellose acetate succinate is a stable material. It is stable for four years after manufacturing. Hypromellose acetate succinate is hygroscopic. It is hydrolyzed to acetic acid and succinic acid, and the hypromellose polymer starts to form if dissolved in 1 mol/L sodium hydroxide for more than two hours. The hydrolysis is the main degradation pathway that is responsible for increasing amounts of free acids in storage, especially upon exposure to moisture. 

Polycarbophil polymers are stable, hygroscopic materials. They do not undergo hydrolysis or oxidation under normal conditions. Heat aging at temperatures below 104°C for up to 2 hours does not affect the efficiency of the dry polymer. However, prolonged exposure to excessive temperatures can result in discoloration, reduced stability, and in some cases plasticization of the polymer. Complete decomposition occurs with heating for 30 minutes at 260°C. 

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