GB2308596A - Removal of alkali metal from polymer cements - Google Patents

Description

REMOVAL OF LITHIUM FROM POLYMER CEMENTS This invention provides a process for separating alkali metal compounds from a polymer cement. More specifically, this invention pertains to a process for removing alkali metal compounds from a low-viscosity functionalized polymer cement.

Diene polymer cements containing highly basic lithium residues from the initiator (e.g., lithium methoxide) prove to be very difficult to hydrogenate.

High levels of nickel/aluminum catalyst are required to thoroughly hydrogenate these materials. Previously it was found that removing the lithium methoxide by gravity settling or by filtration, allowed hydrogenation of the polymers with much lower levels of catalyst. Formation of lithium methoxide precipitate and settling from these cements is very slow, and effective removal may take weeks.

US Patent 5,166,277 to Goodwin and Willis, which is incorporated hereinto by reference, teaches the difficulty encountered in hydrogenating diene polymers in the presence of basic lithium salts. The patent discloses a process of methanol termination and settling.

It has no been discovered that terminating diol polymer cements with glacial acetic acid results in the formation of large crystals which may be removed by settling, filtration or centrifugation. The crystals produced by acetic acid termination are large enough to settle within two or three hours after termination. No other terminating agent is known which produces such large crystals. Removing the crystals, which are lithium acetate, reduces the lithium content of the cement.

Prior to the above discovery, it was found that judicious choice of polymerization terminating agents enabled immediate removal of the lithium salt by centrifugation. Removal of the lithium salt has been shown to greatly enhance the hydrogenation process.

Preferred terminating agents are those which form lithium salts which are less soluble in the cement than lithium methoxide. Some examples of terminating agents which are particularly effective are 2-ethylhexanoic acid, 2,2,6,6-tetra methyl-heptane-3, 5-dione, 12-hydroxystearic acid and water.

Basic lithium salts such as lithium methoxide retard the hydrogenation of low molecular weight diene polymers. Removal of this salt has had the effect of greatly enhancing the hydrogenation process. The present invention represents an improvement over the prior art in that the salt may be removed immediately after termination, and lithium salts are removed to lower levels than is possible for lithium methoxide.

Previously it was found that removing the lithium methoxide by gravity settling or by filtration allowed hydrogenation of polymers with much lower levels of catalyst. Formation of lithium methoxide precipitate and settling from these cements is very slow and may take weeks. However, it was found that judicious choice of polymerization terminating agents enabled immediate removal of the lithium salt by centrifugation. Preferred terminating agents were those which formed lithium salts which were less soluble in the cement than lithium methoxide. Some examples of terminating agents which were found to be particularly effective are 2-ethylhexanoic acid, 2,2, 6, 6-tetramethyl-heptane-3, 5-dione, 12-hydroxystearic acid, and water. There has now been discovered a terminating agent that represents an improvement over these. Glacial acetic acid forms large crystals that start settling on their own shortly after termination.

Relatively low centrifugation speeds are required to remove these crystals.

During separation of the lithium or other alkali metal compounds, the polymer may be present in a solvent at a concentration within the range of from about 5 to about 80 weight percent, based on total solution, and the alkali metal compound, such as lithium, may be present at a concentration within the range from 0.005 to 5%wt based on the polymer. The molar ratio of acid to lithium may be within the range of from 0.6 to 2.4. Contact between the polymer and the glacial acetic acid preferably occurs at a temperature within the range of ambient to 1000C, at a pressure up to 80 psig and at a nominal holding time within the range of 0.5 hour to 20 hours. Where suitable, where the polymer is liquid, has low viscosity, and low molecular weight, the polymer may be used without a solvent.

The method of this invention can be used to separate alkali metal compounds such as lithium compounds from polymers such as low-viscosity functionalized polymer cements. The polymer may be hydrogenated or not during the alkali metal separation, but it is preferable to separate lithium compounds prior to hydrogenation.

Polymer solvents include but are not limited to hydrocarbons such as paraffins, cycloparaffins, alkyl substituted cycloparaffins, aromatics and alkyl substituted aromatics, such as benzene, toluene, cyclohexane, methyl cyclohexane, n-butane, n-hexane, n-heptane and the like. Ethers, such as diethyl ether, tetrahydrofuran, and the like can be used. Mixtures of the foregoing can be removed and another solvent substituted.

Centrifugation may be combined with other types of separation, e.g. filtration, settling and decantation.

Any alkali metal compound commonly found in a polymer solution when an alkali metal compound is used as the catalyst or initiator can be separated from the polymer. These include alkali metal hydrides, alkali metal alkoxides and alkali metal hydroxides.

Low viscosity functionalized polymers can be made in accordance with the following procedure: Anionic polymerization of conjugated diene hydrocarbons with lithium initiators is well known as described in U.S. Patents Nos. 4,039,503 and Re. 27,145 which descriptions are incorporated herein by reference. Polymerization commences with a monolithium, dilithium, or polylithium initiator which builds a living polymer backbone at each lithium site.

Typical living polymer structures containing polymerized conjugated diene hydrocarbons are: X-B-Li X-A-B-Li X-A-B-A-Li -! -3-Y-B-Li :---Y-B-A-L wherein E presents polymerized units of one or more conjugate 3lene hydrocarbons such as butadiene or isoprene. A represents polymerized units of one or more vinyl aromatic compounds such as styrene. X is the residue of a monolithium initiator such as sec-butyllithium, and Y is the residue of a lithium initiator such as the diadduct of sec-butyllithium and m-diisopropenylbenzene. Some structures, including those pertaining to polylithium initiators or random units of styrene and a conjugated diene, generally have limited practical utility although known in the art.

The anionic polymerization of the conjugated diene hydrocarbons is typically controlled with structure modifiers such as diethylether or glyme (1,2-diethoxyethane) to obtain the desired amount of 1,4-addition.

As described in Re 27,145 which is incorporated by reference herein, the level of 1,2-addition of a butadiene polymer or copolymer can greatly affect elastomeric properties after hydrogenation.

Dilithium initiation with the diadduct of sec-butyllithium (s-BuLi)and m-diisopropenylbenzene also requires presence of a non-reactive coordinating agent such as diethyl ether, glyme, or triethyl amine, otherwise monolithium initiation is achieved. Ether is typically present during anionic polymerization as discussed above, and the amount of ether typically needed to obtain specific polymeric structures has been sufficient to provide dilithium initiation.

Alternatively, anionic polymerization of conjugated dienes may be performed using protected functional initiators (PFI) as described in US Patents 5,391,663 and 5,416,168 which are incorporated herein by reference.

Anionic polymerization is often terminated by addition of water to remove the lithium as lithium hydroxide (LiOH) or by addition of an alcohol (ROH) to remove the lithium as a lithium alkoxide (LiOR) . For polymers having terminal functional groups, the living polymer chains are preferably terminated with hydroxyl, carboxyl, phenol, epoxy or amine groups by reaction with ethylene oxide, carbon dioxide, a protected hydroxystyrene monomer, ethylene oxide plus epichlorohydrin, or the amine compounds listed in U.S.

Patent 4,791,174, respectively.

The termination of living anionic polymers to form functional end groups is described in U.S. Patents 4,417,029, 4,518,753 and 4,753,991 which are herein incorporated by reference. Of particular interest for the present invention are terminal hydroxyl, carboxyl, phenol, epoxy and amine groups. Such polymers with number average molecular weights between about 1000 and 20,000 as measured by gel permeation chromatography are low viscosity functionalized polymers.

Hydrogenation of at least 90%, preferably at least 95%, of the unsaturation in low molecular weight butadiene polymers is achieved with nickel catalysts as described in U,S. Patents Re. 27,145 and 4,970,254 which are incorporated by reference herein. The preferred catalyst is a mixture of nickel 2-ethylhexanoate and triethylaluminum.

The termination step can result in release of fine particles of lithium bases as described in US Patent 5,166,277 which is incorporated by reference herein.

The lithium bases may interfere with hydrogenation of the polymer and preferably are removed, especially if the hydrogenation is to be carried out at high solids.

EXAMPLES Example 1 Termination experiments using a DiLi gel (prepared by polymerizing butadiene in cyclohexane/diethyl ether at 30% solids and capping the resulting polymer with excess ethylene oxide) and glacial acetic acid at 2 moles acetic acid/mole lithium resulted in the formation of crystals that settled within two or three hours. Microscopy show these crystals are 10 to 200 microns in size. For comparison, most of the crystals formed in a similar experiment with methanol at 1.2 mole methanol/mole lithium were at most 1 to 2 microns, with a few in the 10 to 20 micron range. The crystals in the methanol experiment remained suspended for days.

Example 2 The procedure was as follows. The cement or gel was terminated by contacting it with the terminating agent (acetic acid) for one hour in a flat bed shaker.

Settling was then allowed to occur for a prescribed period of time. After settling, the supernatant was filtered through a 20-micron filter or centrifuged in stages at 5000, 10,000, 40,000 and 140,000 g-min.

Samples of the filtered and centrifuged cements were taken for microscopy and lithium analysis. The size of the crystals was determined by microscopy. In one case the crystals were submitted for identification by X-rays.

The results show that lithium removal improves by increasing the acid to lithium molar ratio, increasing temperature, and increasing settling time. Table 4 shows exemplary results obtained with varous diols and mono-ols.

Table 1 Screening of Terminating Agents for DiLi Gel*

Terminating Terminating Cement after Cement Agent Agent/Li Termination 16 hr later (m/m) Methanol 1.2 Hazy Hazy, few solids Glacial actic 2 Cloudy Solids settled acid Water 2 Hazy Cloudy, few solids 2-Ethyl- 1 Still gel-like Still gel-like hexanoic acid * A hydrogenated diol (EB diol) is exemplary of a lowviscosity functionalized polymer (LVFP). Two approaches to making EB diols have been investigated: The DiLi approach and the PFI approach. In the DiLi process, the initiator used is diisopropenylbenzene /sec-butyllithium. This process results in a product which is a gel containing two moles of lithium per mole of polymer. The PFI approach uses a protected functional initiator, hence the name PFI. This approach introduces one mole of lithium per mole of polymer and resuls in a cement that is less viscous and easier to handle than DiLi gels.

The results of experiments using Goodwin's gel and 2 moles acetic acid/mole lithium are shown in Table 2 below. (Goodwin's gel is a DiLi gel prepared by polymerizing butadiene in cyclohexane/diethyl ether at 30% solids and capping the resulting polymer with excess ethylene oxide; the polymer is not terminated.) Upon settling at 230C, the crystal layer attained its maximum thickness after about 3.5 hours, and the top (cement) phase was completely clear after 6.5 hours.

Microscopy showed crystals ranging from 10 or 20 microns to 200 microns. Aggregates larger than 100 microns appeared to be present. Filtration produced a clear filtrate, but was slow through a 20-micron filter. Centrifugation at moderate speeds after a few hours of settling removed about 70% of the lithium.

Long settling periods (days) appeared to be the most effective way of removing lithium.

Table 2 LITHIUM (ppm) REMAINING IN DiLi GEL AFTER TERMINATION WITH 2/1 m/m ACETIC ACID/Li Contact: 1 hr; Initial Li: 1100-1400 ppm

No 5000 10000 40000 Filtra spinning g-min g-min g-min tion ** No filter No 975 900 280 settling 6.5 hr 350 336 settle; 230C 15 hr; 548 310 236 211 123 C* .

70 hr; 69 230C 70 hr; OOC 1594 70 hr; 188 230C (seeded) * X-ray of solids: Li acetate; ** slow process The effect of acetic acid/lithium ratio is shown in Table 3 for Goodwin's gel. Lithium removal seems to be somewhat more effective at higher acetic/lithium ratios, particularly at the higher centrifugation speeds.

Table 3 also shows the results of an experiment with a PFI cement (PFI defined above). This cement was a PFI diol (20% solids, 4000 MW). At low centrifugation speeds, removal of lithium from this cement is about as effective as from Goodwin's gel.

Table 3 LITHIUM (ppm) REMAINING IN CEMENT AFTER TERMINATION WITH ACETIC ACID Contact: 1 hr; Settling: 18 hr at 230C

Cement Initial Acid/Li 5000 10000 40000 Li (ppm) (m/m) g-min g-min g-min A* 1126 0.9 782 769 802 A 1126 1.8 612 452 333 B** 478 1.7 283 249 20 * Goodwin's DiLi gel ** PFI diol (20% solids, 4000 MW) Table 4 Li(ppm) REMAINING IN CEMENT AFTER TERMINATION WITH ACETIC ACID Contact: lhr

Cement Initial Acid/Li Temp Settling 5000 Li (ppm) (m/m) (C) (hr) g-min A 1126 1.8 23 18 612 B 478 1.7 23 18 283 C 521 1.6 23 1 16 D 656 1.2 60 22 less than 2 A = DiLi gel prepared by polymerizing butadiene in cyclohexane/diethyl ether at 30% solids and capping the resulting polymer with excess ethylene oxide B = PFI diol (20% solids, 4000MW) C = Mono-ol (20% solids, 4000 MW) D = Mono-ol (30% solids, 4000 MW, sec-butyllithium initiator)

Claims (4)

1. CLAIMS 1. A process for preparing a polymer cement which is relatively free of alkali metal salt comprising: i) contacting the polymer cement with glacial acetic acid which leaves alkali metal salt in the cement which is readily removable; and ii) removing the alkali metal salt from the polymer cement.
2. 2. The process of claim 1 wherein the alkali metal is lithium.
3. 3. The process of claim 1 wherein the polymer cement is a low-viscosity functionalized polymer cement.
4. 4. The process of claim 1 wherein the alkali metal salt is removed by centrifugation.