Hygroscopic polymer

Polyethylene glycol (PEG), a hygroscopic polymer, was the first molecule to be considered for modifying the surface of proteins. It has proved to be very successful. PEG, available in molecular weights ranging from a few hundred to several thousand daltons, have been used as parenteral excipients for decades with proven clinical safety. 

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. 

Water acts as a plasticizer for many of the early plastics. A sheet of casein, 4 mm thick, absorbs 5-7 per cent of its own weight in 24 hours and 30 per cent in 28 days under normal room conditions (Brydson, 1999). Polyamides, such as nylon, are the most hygroscopic polymers in common use today, containing up to 3 per cent moisture by weight under ambient conditions. In the same environment, cellulose acetate contains 0.8 per cent and poly (methyl methacrylate) and polystyrene 0.1 per cent. Plasticized PVC swells and appears opaque if stored at high relative humidity (RH) (Figure 6.6). Water vapour is an efficient plasticizer for the PVC polymer but is incompatible with commercial ester plasticizers, which are hydrophobic. As a result, the more plasticized the PVC, the less water it absorbs. Because water is only weakly bonded to the PVC polymer, it rapidly evaporates again. 

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. 

This is a visually amazing demonstration in which water is poured into an opaque disposable cup, and, when the cup is turned upside down, the water vanishes. What the students do not know is that inside the cup there is sodium polyacrylate powder, the same powder that is used in disposable baby diapers. Sodium polyacrylate is a water-loving (what chemists refer to as hygroscopic) polymer that absorbs water, and as a result, turns into a gel, which does not pour out when the cup is turned upside down. 

Hygroscopic polymers. On the other hand, most engineering-grade polymers have a chemical nature that causes them to attract and absorb moisture from the surrounding ambient air. Most engineering polymers are 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. 

Moisture absorption takes place over time, and eventually slows and stops when the pellet s moisture content reaches equilibrium with that of the surrounding air. Some hygroscopic polymers have a greater... 

To summarize, the moisture content of a hygroscopic polymer will vary, depending on... 

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. 

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. 

Pellet size is a major consideration of the material suppliers in the manufacture of hygroscopic polymers. It should also be a major consideration when reclaiming scrap regrind materials. 

A hot-air dryer may also be used successfully to remove some of the absorbed moisture from a mildly hygroscopic polymer, provided the application is not too moisture sensitive and the ambient relative humidity is not excessive. 

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-... 

Removal of water from hygroscopic polymers (e.g., ABS, PMMA, PA, PC, SAN, CA, PU, PPO, polysulfone) all polymer particles can have surface moisture left from underwater pelletizing or surface condensation from storage at varying temperatures and relative humidity. 

Removal of water from hygroscopic polymers is a common use of vented extruders. Most polymers require less than 0.2% moisture in order to properly extrude. In some polymers, this percentage is considerably lower, e.g., PMMA < 0.1%, ABS <0.1%, CA < 0.05%, PBTB < 0.05%, and PC < 0.02%. Many polymers have an equilibrium moisture content at room temperature and 50% R. H. (relative humidity) that is considerably higher than the maximum allowable percentage moisture content for extrusion. Some values of the equilibrium moisture content of hygroscopic polymers [10] are ABS 1.5% PMMA 0.8% PBTP 0.2% PC 0.2% and PA 3%. Such poly-... 

Corrosion can also occur with hygroscopic polymers, such as ABS, PA, PET, PMMA, etc., when moisture is released under high temperature and pressure, forming high-pressure steam. Braun and Maelhammer [15] found that PA 6,6 splits into various corrosive components. Calloway et al. [14] found that the corrosive attack of HIPS is dependent on the chlorine and sulfur content of the carbon blacks. Mosle et al. [16] found that degradation products of ABS can cause corrosive attack in extruders. 

Volatile removal may also be necessary in polymer compositions containing moisture, for example, with hygroscopic polymers and additives, such as wood flour, or where residual quantities of monomers or reactive by-products must be removed. Generally, in these situations, amounts of volatiles extracted are small, typically less than 3% by weight. 

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