Polyamide 6 Composites

Table 1. Rejections by Aromatic Polyamide Composite Membrane ...

Fig. 42.16 Chiral polyester-polyamide composite containing supported BINAP.

Polyamide/rubber blends, 20 361 Polyamide block copolymers, 24 704, 708 Polyamide composite membranes, 21 633 Polyamide-elastomer block copolymers, 24 698... 

Kawaguchi et al.105) in Teijin Ltd. prepared a similar polyamide composite membrane from piperazine, trimesoyl chloride, and isophthaloyl chloride on a polysulfone support. The membrane exhibited high chlorine-resistance and excellent pressure-resistance. When used for reverse osmosis of an aqueous solution of 0.5% NaCl and NaOCl (available Cl 4 5 ppm) at pH 6.5 7.0, 25 °C, and 42,5 kg/cm2, the water permeation was 1400 and 13301/m2 - day and desalination was 93.4% and 95.7% after 2 and 100 hr, respectively. 

P. Eriksson, Water and Salt Transport Through Two Types of Polyamide Composite Membranes, J. Membr. Sci. 36, 297 (1988). 

Bonin, Y. and LaBlanc, J. 1995. Fire retardant, noncorrosive polyamide composition. U.S. Patent 5,466,741. 

Since the late 1970 s, researchers in the US, Japan, Korea, and other locations have been making an effort to develop chlorine-tolerant RO membranes that exhibit high flux and high rejection. Most work, such as that by Riley and Ridgway et.al., focuses on modifications in the preparation of polyamide composite membranes (see Chapter 4.2.2).11 Other work by Freeman (University of Texas at Austin) and others involves the development of chlorine-tolerant membrane materials other than polyamide. To date, no chlorine-resistant polyamide composite membranes are commercially available for large-scale application. 

Rejection is a property of the specific feed water component and the membrane of interest. Table 3.2 lists the general rejection ability of the most common polyamide composite RO membranes. Note that ionic charge of the component of interest plays a role its rejection by an RO membrane the rejection of multi-valent ions is generally greater than for mono-valent ions. 

Table 3.2 General rejection capabilities of most polyamide composite membranes at room temperature.

Figure 4.9 Cross-section of a polyamide composite RO membrane.

Table 4.2 lists the predominant characteristics of polyamide, composite membranes. 

There have been several improvements made to polyamide, composite membranes that have enhanced their performance. Perhaps the most... 

Many varieties of spiral-wound, polyamide-composite membranes are available to suit different feed water conditions. Membranes discussed here include ... 

Polyamide, composite membranes are very sensitive to free chlorine (recall from Chapter 4.2.1 that cellulose acetate membranes can tolerate up to 1 ppm free chlorine continuously). Degradation of the polyamide composite membrane occurs almost immediately upon exposure and can result in significant reduction in rejection after 200 and 1,000-ppm hours of exposure to free chlorine (in other words after 200-1,000 hours exposure to 1 ppm free chlorine). The rate of degradation depends on two important factors 1) degradation is more rapid at high pH than at neutral or low pH, and 2) the presence of transition metals such as iron, will catalyze the oxidation of the membrane. 

Chloramines also pose a risk to polyamide, composite membranes (see Chapter 8.2.1.1). Chloramines are virtually always in equilibrium with free chlorine. Although the tolerance of the FilmTec FT30 membrane to chloramines is 300,000 ppm-hrs, FilmTec still recommends that influent water with chloramines be dechlorinated prior... 

Another note of caution with chloramines is the need for good pH control. If the pH gets up to 9, dissolved ammonia gas is present as NH3(g), which swells at least some polyamide composite membranes. This swelling can be enough to drop the salt rejection from 98% down to about 85%.10 Dropping the pH back to about 7 converts the ammonia gas to ammonium ion, which does not swell the membrane and rejection returns to nominal. 

The use of chlorine dioxide is not recommended for use with polyamide, composite membranes.4 This is because free chlorine is always present with chlorine dioxide that is generated on site from chlorine and sodium chlorate (see Chapter 8.2.1.1). 

Initially, polyamide composite membranes that have been degraded due to chlorine attack will exhibit a loss in flux.4 This drop in flux is followed by an increase in flux and salt passage. 

The sum of chlorine gas, sodium hypochlorite, calcium hypochlorite, hydochlorous acid, and hypochlorite ion is known as the free or free available chlorine. Most polyamide composite membranes have little tolerance for free chlorine they can tolerate about 200 - 1,000 ppm-hrs of exposure (e.g., 200 hours at 1 ppm of free chlorine) before rejection drops to unacceptable levels. While the pretreatment to RO should have a free chlorine residual of about 0.5 to lppm, the influent to the RO must be dechlorinated to bring the free chlorine concentration down to less than 0.02 ppm. 

In theory, the tolerance of polyamide composite membranes to chloramines is about 300,000 ppm-hrs. However, chloramines are usually in equilibrium with free chlorine, making it difficult to use chloramine in RO pretreatment, as the free chlorine will degrade polyamide composite membranes. 

Although chloramines are generally not recommended by membrane manufacturers for use with polyamide composite membranes, there is some anecdotal support for the use of chloramines if the ammonia is naturally occurring in the water to be treated.8 In such cases, there usually is an excess of ammonia. Difficulties arise when ammonia is added to chlorine to make the chloramines. These systems tend to have more free chlorine present in equilibrium with the chloramines (see Chapter 7.11 for more discussion on this topic). 

Dechlorination of feed water to polyamide composite membranes is necessary as a polyamide membrane polymer cannot tolerate oxidizers of any kind. The options for dechlorination include activated... 

Food-grade sodium metabisulfite that is free of impurities should be used in RO systems. The compound must not be cobalt-activated, as cobalt can catalyze the oxidation of the polyamide composite membrane in a manner similar to iron and manganese (see Chapter 7.6). Further, while the shelf life of solid sodium metabisulfite is 4-6 months, in solution, the shelf life depends on the concentration, as shown in Table 8.9.9... 

Non-oxidizing biocides are used on membranes to prevent microbial fouling. By definition, these products will not oxidize polyamide composite membranes and can be used directly on the membranes. There two most common, non-oxidizing biocides used with RO membranes sodium bisulfite and 2,2,dibromo-3-nitrilo-proprionamide or DBNPA. 

Exposure to high temperature at pH extremes can hydrolyze the membranes, leading to loss of membrane integrity. (See Chapter 4.2.1, Table 4.2, and Chapters 9.2,9.8, and 13.2 for more detailed discussions on the effect of temperature and pH on polyamide composite membranes.) Hydrolysis also tends to involve the entire RO skid rather than focus on only the lead membranes. Just as with oxidation of the membrane, feed water will pass into the permeate resulting in an increase in permeate flow and decrease in the product quality. 

Table 13.1 Temperature and pH limitations for dow water solutions polyamide composite membranes.1...

Table 13.2 Sample high-pH cleaning formulations for polyamide, composite membranes.

Microporous membranes are used to effect the separation by MF and UF processes. These microporous membranes differ from polyamide composite RO membranes in that they are not composites of two different polymeric materials they are usually constructed using a single membrane polymeric material. In simple terms, both UF and MF technologies rely on size as the primary factor determining which... 

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