Continuous Polymerization of Nylon 66 Aired Two-phase flow system speeds water-vapor removal and creates pressure gradient for control It's possible for nylon makers to use a two-phase flow system to speed watervapor removal and to establish a pressure gradient when continuously polymerizing nylon 66. A 40 lb.-per-hour, pilot-scale unit based on this type of operation has been developed by John A. Carter, principal technical officer of ICI Fibres' laboratories at Pontypool, Monmouthshire, in Great Britain. It uses a long, helically coiled tube as a flow-through reactor. ICI's pilot-scale unit was described by Mr. Carter at a symposium on reaction kinetics and process design at the Chemical Engineering Conference in London. The conference was jointly sponsored by the American Institute of Chemical Engineers and Britain's Institution of Chemical Engineers. Polymerization takes place in a two-phase turbannular flow system. The nylon/water system flows as a thin film on the inner wall of the coiled tube. Water vapor, formed as polymerization progresses, escapes as a fast-moving turbulent core. If the unit is linked to spinneret heads, polymer can be formed continuously and fiber spun directly. Nylon 66 results from the polymerization of hexamethylenediamine and adipic acid in nearly equimolecular proportions. The usual starting material is hexamethylenediamine adipate, commonly known as nylon salt. Commercial production in western Europe involves batch polymerization in an autoclave under an inert atmosphere, usually nitrogen. Pressure of about 250 p.s.i. is kept constant by allowing the steam formed during polymerization to escape. Toward the end of the reaction, the pressure is gradually lowered to atmospheric and the temperature held between 250° and 300° C. for about an hour to complete the reaction before drawing off the polymer. Pressure prevents the polymer from freezing out before it reaches a degree of polymerization—the number of amide links in the average polymer chain, in this case 100—suitable for fiber spinning. Also, separation of low-molecular-weight polymer is avoided by starting with an aqueous
solution of the nylon salt so that the dissolved water will lower further the polymer's freezing point. An efficient polymerization process calls for continuous removal of the water, careful control to avoid loss of diamine, and gradual lowering of the pressure to atmospheric. In ICI's unit, an aqueous 47% by weight nylon salt solution is pumped into the reactor under 390 p.s.i.g. The average transit time through the reactor coil is 60 min.
The reactor is a coiled stainless steel tube, heated with Dowtherm-A vapor at 290° C. The first section of the tube is 300 ft. long with an inner diameter of 3 / 8 in. In this section, all of the solution water is removed. Most of the diamine reacts in this tube, and most of the pressure drop occurs there. The first section leads into a 120-ft. length of tube with a l 1 4 /-in. inner diameter. Low-molecular-weight polymer (degree of polymerization between 10 and 20) formed in the first section is converted to polymer with a degree of polymerization of 100 to 120. At the outlet end of the reactor coil, steam escapes into the atmosphere, and the molten polymer col-
POLYMERIZER. B. W. Portus, a technician at ICI Fibres' Laboratories at Pontypool, makes an adjustment on the company's 40 lb.-per-hour pilot polymerizer before starting a test run designed to produce nylon 66 continuously JUNE
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Nylon 66 Polymerizes Continuously in Coiled-Tube Reactor
In spite of improvements in metals, corrosion continues to cost auto owners millions of dollars
•v- Dilution water
4 7 % Salt solution ^7 20° C,
_
"-« Pumps>»-«
Melt spinning head ^
lects in a small reservoir before passing through the fiber-forming spinnerets. This reservoir makes possible uninterrupted production of the polymer during intervals when the spinnerets are being serviced, Mr. Carter explains. A fixed-probe sensing device within the reservoir maintains correct relationship between the outputs of the polymerizer and of the fiber extrusion head. If the level of polymer builds up within the reservoir, it makes contact with the probe, which, in turn, activates a switch on the water injection pump at the input end of the reactor. This feeds more water to the system. This additional dilution of the nylon salt reduces the amount of polymer formed while at the same time maintaining a constant over-all mass throughout. Mr. Carter points out that the twophase system can be scaled up using relationships of length, diameter, and throughput. On the basis of constant transit time and constant pressure drop, the ratio of the lengths is directly proportional to the 0.4 power of the ratio of throughputs. On the same basis, the ratio of diameters is directly proportional to the 0.35 power of the ratio of throughputs. Mr. Carter notes, however, that very large polymerizers are probably impractical because of heat transfer and vapor velocity limitations. ICI has a big stake in nylon. Recent expansions of the company's polymer units at Wilton and Billingham have raised annual capacity to 381 million lb. Also, ICI Fibres, Ltd., is expanding its nylon fiber capacity to 300 million lb. per year by 1967, compared to the current annual level of about 150 million lb. (C&EN, April 5, page 25). 50
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Interest in Car Undercoatings Sharpens
2 8,
1965
Few people will argue that autos aren't better than ever. Better they may be, but they still rust. In spite of improvements in metals and processing methods, corrosion continues to be a major headache to the auto owner. It is a problem that costs millions of dollars annually. And with the increasing use of salts and other corrosive materials on highways to combat snow and ice, rusting is an evergrowing problem. As a result, there is a growing interest in undercoating, or rust-proofing as producers of the materials prefer to call it. These materials appear to offer the best solution to the problem. Sprayed on auto underbodies and inside enclosed areas such as rocker panels and fender wells, they inhibit "inside-out" body rusting—the major kind of corrosion plaguing cars. A variety of rust-proofing materials are available today to combat corrosion in autos. They are petroleumbased products, ranging from asphalts, greases, and waves loaded with rust inhibiting compounds to metalloor-
ganic complexes. Nearly every major oil company and several private-label formulators now offer rust-proofing products for corrosion prevention. Target of all this activity is a potential market which is huge. There are now more than 70 million autos on the road. Rust-proofing just 10% of these cars—the newer models—would amount to about $30 million worth of business to suppliers of these materials. Most suppliers feel that this is a realistic target to shoot for. In addition to the millions of cars now on the road, there is the sizable new car market. About 5% of new cars are getting rust-proofed by dealers prior to delivery to buyers. Assuming 1965 will be a 9 million-car year and that at least 10% of the new car owners can be sold on the merits of corrosion prevention, sales of rustproofing material to this segment of the market alone could add up to $5 million. Inside Out. The generally accepted thinking is that rust starts on the inside and works out—just the reverse of
UNDER CAR. A Lubrizol technician applies rust-proofing material with airless spray equipment to the under side of an auto for an 18-month, twowinter test
SOUND. The sound deadening properties of an underbody coating are tested by a Daubert technician, who is placing a test panel in Geiger sound deadening equipment
