Nylons batch polymerization

Nylon-6 is the polyamide formed by the ring-opening polymerization of S-caprolactam. The polymerization of S-caprolactam can be initiated by acids, bases, or water. Hydrolytic polymerization initiated by water is often used in industry. The polymerization is carried out commercially in both batch and continuous processes by heating the monomer in the presence of 5—10% water to temperatures of 250—280°C for periods of 12 to more than 24 h. The chemistry of the polymerization is shown by the following reaction sequence. 

Nylon-6 can be easily polymerized at atmospheric pressure. A continuous process was developed in 1940, called the VK process, which in German stands for vereinfacht kontinuierlich or simplified continuous (73). The VK process is widely used in industry in the 1990s, whereas batch processes, being less economical, are gradually phased out of use. Procedures are also available for making gram quantities of nylon-6 and nylon-6,6 in the laboratory (74). 

A process for the hydrogenation of adiponitrile and 6-aminocapronitrile to hexamethylenediamine in streams of depolymerized Nylon-6,6 or a blend of Nylon-6 and Nylon-6,6 has been described. Semi-batch and continuous hydrogenation reactions of depolymerized (ammonolysis) products were performed to study the efficacy of Raney Ni 2400 and Raney Co 2724 catalysts. The study showed signs of deactivation of Raney Ni 2400 even in the presence of caustic, whereas little or no deactivation of Raney Co 2724 was observed for the hydrogenation of the ammonolysis product. The hydrogenation products from the continuous run using Raney Co 2724 were subsequently distilled and the recycled hexamethylenediamine (HMD) monomer was polymerized with adipic acid. The properties of the polymer prepared from recycled HMD were found to be identical to that obtained from virgin HMD. 

Polymer Production. Three processes are used to produce nylon-6,6. Two of these start with nylon-6,6 salt, a combination of adipic acid and hexamethylenediamine in water they are the batch or autoclave process and the continuous polymerization process. The third, the solid-phase polymerization process, starts with low molecular weight pellets usually made via the autoclave process, and continues to build the molecular weight of the polymer in a heated inert gas, the temperature of which never reaches the melting point of the polymer. 

The polymerized product is an extremely insoluble material and must be melt-spun, as discussed later. Therefore, should a delustered or precolored fiber be desired, it is necessary to add the titanium dioxide or colored pigment to the polymerization batch prior to solidification. For ease of handling, the batch of nylon polymer may be extruded from the autoclave to form a thin ribbon, which is easily broken down into chips after rapid cooling. But, whenever possible, the liquid polymer is pumped directly to the fiber melt spinning operation (see Fig. 12.14). 

Hydrolytic polymerization [12,13] of e-caprolactam to form nylon-6 [m = 5 in Eq. (10.48)] is carried out commercially in both batch and continuous processs by heating the monomer in the presence of 5-10% water to temperatures of 250-270°C for periods of 12 hr to more than 24 hr. In the first step, the lactam is hydrolyzed to e-aminocaproic acid ... 

Nylon 6,6 may also be prepared by direct amidation of adipic acid and hexamethylene diamine. In this process, the molten monomers are added to a vessel in approximately stoichiometric proportions, but usually with a slight excess of acid. The exothermic salt formation produces sufficient heat to raise the batch temperature to approximately 200 °C, so polymerization proceeds rapidly. As the polymerization proceeds, the stoichiometry is adjusted to exactly equal molar portions of diacid and diamine, and the autoclave temperature and pressure are raised to 275 C and 250 Ib/in., respectively. After a specified heating time, pressure is reduced, and the temperature is maintained to complete polymerization. 

Although originally designed as a batch process, the direct amidation of the nylon 6,6 salt has been adapted to continuous polymerization by a wide variety of process modifications developed over the past 40 years. Many of these are quite different from an engineering standpoint, but all involve essentially the same chemistry as the batch system. 

Additional examples of polymerization processes can be found in a recently published review of fiber-forming polymerization patents by Robinson (27). A detailed comparison of batch and continuous polymerization for nylon 6,6 can be found in a review by Jacobs and Zimmerman (15). In another review Short has summarized the current state of polypropylene polymerization technology and catalyst development (28). 

Polyamides PAs are the polycondensation reaction product of diamines, of general formula NH2-R-NH2 with dicarboxylic acids of general formula HOOC-R -COOH. Batch and continuous processes for PAs have been used over the last 70 years, since Carothers synthesized the first materials in the 1930s. Nylon 6,6 is made by a multistage process by bulk polymerization of hexamethyline diamine and adipic acid. In this process, 2 CSTRs + last stage + vacuum are employed. 

Much of nylon 6 is used in producing fibers. Polycaprolactam prepared by water-catalyzed polymerizations is best suited for this purpose. It can also be used in molding, though anionically polymerized caprolactam can be used as well. The polymerizations are carried out both in batch and in continuous processes. Often, tubular flow reactors are employed. 

Caprolactam for use as fibers is hydrolytically polymerized in batches as 80%-90% aqueous solutions with 0.2%-0.5% acetic acid and ethylene diamine at 250-280° C. The acetic acid acts as chain stabilizer (see above). The ethylene diamine increases the amine equivalent of PA 6 so that mixed weaves of Perlon and wool can be evenly dyed. The water is removed as steam in the polymerization progresses. Caprolactam is also polymerized continuously by what is known as the VK process (vereinfacht-A ontinuierliche, or simplified continuous, process). This process is carried out without pressure with, for example, 6-amino caproic acid or AH salt as initiator. In contrast to the production of nylon 6,6, this process can proceed continuously the melt can be directly spun from the reactor. Polyamide 6 fibers have good properties but they yellow slowly since pyrrole structures are formed at the chain ends. 

However, for AA and BB systems, like those shown in Table 7.1, stoichiometric imbalance can occur, with serious consequences for the polymerization. The molar ratio of the two types of functional end-groups (A and B) that are available for polymerization is determined by the initial molar ratio of the two monomers in a batch reactor, and by any monomers or oligomers that might escape from the reacting mixture during the polymerization. Note that escape of volatile monomers with the resulting influence on the ratio of functional groups is a serious practical problem for some industrial polymerizations that use volatile monomers (e.g., HMD in nylon 6,6 production and diphenyl carbonate in polycarbonate production). 

In the third stage, polymerization and water removal continue in a pressurized vessel, with conditions that change with time (for batch nylon 6,6 production) or with position (for continuous nylon 6,6 production in tubular reactors [14] or in a series of back-mixed reactors) as shown in Figure 7.8. High pressure is required to maintain a sufficiently high... 

Figure 7.8 Process conditions and reaction mixture properties in the course of a typical batch or continuous melt-phase nylon 6,6 polymerization process [42].

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