Comparison of solution rubber finishing processes - part I.

The manufacturing process for most synthetic rubber is done by either solution or emulsion polymerization. Solution polymerization is done in a solvent at low rubber solids levels followed by steam stripping and a water slurry finishing step. The solution process offers several key advantages, such as a lower level of residuals in the final product, more flexibility and superior product properties.

The primary disadvantage of the solution process with water slurry finishing is the comparatively higher manufacturing cost. This is due to the need to purify and dry the monomer and recycle solvent before feeding them to the reactor.

Solution rubber can also be recovered from the solvent by another finishing process, direct devolatilization of the solvent from the rubber solution using a twin screw extruder. This process eliminates the steam stripping step and greatly reduces the high recycle solvent purification costs.

By comparing the manufacturing costs and economics of the two rubber finishing methods, the water slurry process and the direct devolatilizing extruder process, the economic advantage of eliminating the steam stripping step can be determined. This can influence the decision for the type of finishing process specified for new or modernized solution rubber facilities.

Solution rubber process steps

The solution rubber process covers four basic steps shown in the process flow diagrams in figures 1 and 2. They are (1) solvent and monomer purification, (2) solution polymerization and blending, (3) finishing and (4) packaging.

[Figures 1-2 ILLUSTRATION OMITTED]

The objective here is to compare the manufacturing cost of the solution rubber process using two alternatives for the finishing step, the water slurry process and the direct devolatilizing extruder process. These alternates have very different purification steps associated with them, and different costs. The polymerization step is the same for each alternate.

A packaging step, if the rubber is baled, is the same cost for each finishing alternate. We will base the comparison on a baled product from a porous pellet or crumb.

Pelletizing to a clear, non-porous product would be an extra step for the water slurry finishing process by adding a single screw pelletizing extruder. In the direct devolatilizing finishing process, a pelletizer is placed at the end of the devolatilizing extruder to make the clear product, or water is injected and a special cutter which can make a porous pellet is used.

Solvent and monomer purification

A fractionation step and molecular sieve are used to get the solvent and monomer feed dry enough for the polymerization reaction. The monomer is dried separately from the solvent.

The very important difference in the two finishing alternates is that the direct devolatilizing process recycles nearly all of the solvent directly back to the reactor, while the slurry process does not. No water is present in the devolatilizing process streams, and no separation and purification is needed. The only stream to be purified and dried is from the last stage of the devolatilizing extruder. About 10% of the recycled solvent needs purification of possible impurities.

In the water slurry finishing process, only a fraction of the solvent is directly recycled back to the reactor. The bulk of the slurry has been steam stripped and the solvent must be purified. About 95% of the solvent must be purified. Cost for the steam usage is much higher due to this.

Solution rubber polymerization

The solution polymerization process is the same for both slurry finishing and direct devolatilizing finishing. The patent references 1-3 give examples of the solution polymerization process. Monomer and solvent must have a very low level of catalyst poisons in them for the polymerization process to work. The chief poison is water. Monomers must be purified and dried by molecular sieves.

The ratio of solvent and monomer is chosen to give a viscosity to the resultant polymer low enough for the reactor to handle and pump through the system. The feeds are mixed with a catalyst and charged to the reactor. Reaction can be batch or continuous.

Rubber solution from the reactor is shortstopped, and antioxidants added. It is then pumped to the concentrating area where unreacted monomer is flashed off and recycled to the reactor. The rubber solution is then sent to the blend tank. From here it goes to the finishing step.

Water slurry finishing

The water slurry rubber finishing process is given in figure 1. After the blend tank, the rubber is steam stripped to remove all the solvent. Solvent from the steam strippers is decanted from the condensed steam and sent to a wet storage area. The rubber, in the form of a wet crumb slurry, is pumped to the drying area.

Surface water is removed from the crumb with a shaker screen, followed by two stages of extrusion drying. An expeller is used to remove most of the remaining water, and then an expander dryer gets the moisture down to about 1-3%. An additional apron type dryer may also be needed in some cases. The dry rubber is conveyed to the weigh scales and baler where bales are formed. They are then bagged or boxed for shipment.

Direct devolatilizing extruder finishing

The direct devolatilizing finishing process is given in figure 2. From the blend tank, the rubber solution is pumped through a heat exchanger, heated and then flashed under some vacuum into a flash tank. One or two flash stages are used to get the rubber solution a maximum solids level where it is still pumpable. Most times this can be about 50% solids concentration. A gear pump is used to pump out of the flash tank. The flash tank technology has been used in many polymer applications and is applicable to many rubbers and thermoplastic elastomers. Examples of flash tank technology are given in references 4-6.

A rubber solution of about 50% solids is then pumped to the main devolatilizing extruder. It is heated in a shell and tube heat exchanger before the extruder. A diagram of the devolatilizing extruder is given in figure 3. The expected flow rates of the solvent from each vent and rubber output are given in the diagram.

[Figure 3 ILLUSTRATION OMITTED]

The devolatilizing extruder consists of one rear vent port and three forward vent ports. A twin screw extruder is used since a single screw could not handle this level of input solvent. A twin screw with non-intermeshing counter-rotating screws is most common in the industry. It has superior surface renewal and vapor renewal capabilities, and is very prominent in the rubber and plastics industry for devolatilizing applications.

Much information on devolatilizing extruder technology can be found in the book "Polymer Devolatilization" (ref. 7). Detailed information on the low solids devolatilizing extruder technology is given in references 8-9. Commercial applications of devolatilizing technology can be found in patent references 10-12.

Part II of this article will appear in the April 2000 issue of Rubber World. The second part will examine a devolatilizing extruder operation, and will provide an economics comparison basis, a finishing process economics comparison, as well as the author's conclusions.

References

(1.) Halasa, A.F., Austin, L., Hsu, W., Gross, B.B., U.S. Patent 5,679,751 to Goodyear Tire and Rubber Co. (October 1997).

(2.) Smith, D.R., Zelinski, R.P., U.S. Patent 3,178,402 to Phillips Petroleum Co., (April 1965).

(3.) Van Den Berg, C., U.S. Patent 4,319,010 to Stamicarbon B.V. (March 1982).

(4.) Simon, R.H.M., Falling strand devolatilization," Polymer Devolatilization, Marcel Dekker, New York, 1996, pp. 261-290.

(5.) Hagberg, C.G., U.S. Patent 3,966,538 to Monsanto Co. (June 1976).

(6.) Fink, P., et al, U.S. Patent 4,153,501 to BASF (May 1979).

(7.) Albalak, R.J., ed., Polymer Devolatilization, Marcel Dekker, New York, 1996.

(8.) Nichols, R.J., Lindt, J.T., "Devolatilization in counter-rotating, nonintermeshing twin screw extruders," Polymer Devolatilization, Marcel Dekker, New York, 1996.

(9.) Hagberg, C.G., Bash, T.F., "Extruder devolatilization of low solids solutions," Seminar Series, NPE 1997.

(10.) Skidmore, R.H., U.S. Patent 3,963,558 to W Bar E Inc., (June 1976).

(11.) Brasz, A.J.H., et al, U.S. Patent 4,452,977 to Stamicarbon B.V. (June 1984).

(12.) Salmon, E.J., U.S. Patent 4,186,047 to Phillips Petroleum Co. (January 1980).

(13.) Cooper, E.D., "Phillips cis-polybutadiene process," Handbook of Chemicals Production Processes, McGraw-Hill Book Co., New York, 1986, p. 2.1-3.

(14.) Staff, "Stamicarbon compact polyethylene solution process," Handbook of Chemicals Production Processes, McGraw-Hill Book Co., New York, 1986, 17. 2.5-1.