TETRAESTERS AND PROCESS FOR PRODUCTION THEREOF

The present invention relates to novel compositions containing tetraesters of pentaerythritol, and to processes by which such compositions can be produced. More specifically, the present invention relates to the production of tetra (3-alkylpropionate) esters of pentaerythritol by a novel process which affords a number of process advantages, which process also produces a novel composition from which its major component, the tetraester, can readily be recovered and purified.

Alkyl esters derived from alkylthioalkanoic acids and the like are, in general, known to be useful as stabilizers of organic materials such as polymer resins and the like which are otherwise subject to thermal and oxidative deterioration during processing, extrusion or molding, as well as during use. Esters having this general utility have in the past been prepared by various procedures. Dexter, et al . U.S. Patent No. 3,758,549, for example, basically teaches transesterification procedures for the preparation of these types of products. By such procedures, it is often difficult to obtain a product that has a tetraester content at or above 90% by weight, particularly when the transesterification is carried out on an industrial scale.

Stabilizers for enhancing the resistance of polyolefins to deterioration can also be prepared by reacting an alpha-olefin with a multi-functional ester of a mercaptocarboxylic acid. Stabilizers of this

type and the process for their preparation are disclosed in Kauder, et al . U.S. Patent No. 4,080,364. Experience with this type of addition reaction indicates the product thus formed has a tetraester content which typically does not meet or exceed 90% by weight.

Nakahara, et al . U.S. Patent No. 4,349,468 teaches the preparation of a pentaerythritol tetrakis (3-laurylthiopropionate) stabilizer for polyolefins which is produced by a process including heating an alpha-olefin such a 1-dodecene with a beta- mercaptopropionic acid or ester in the presence of an azonitrile or peroxide catalyst, followed by esterifying the resultant alkylthiopropionic acid with pentaerythritol. The resulting product is typically inferior in that the alpha-olefin reaction produces an unwanted isomer byproduct that, if not removed in a separate purification step, lowers the quality of the pentaerythritol ester. Chisholm, et al. U.S. Patent No. 5,057,622 discloses production of a pentaerythritol tetraester of 3-alkylthiopropionic acid by a process in which, first, the corresponding alkyl mercaptan is reacted with sodium acrylate under strongly basic conditions, followed by acidification and a series of steps to recover the free 3-alkylthiopropionic acid. Then, the acid is esterified with pentaerythritol in the presence of a strong acid catalyst. This overall process scheme is relatively volume inefficient and ^can present difficulties in the recovery of the tetraester from its byproducts.

In addition, Chisholm, et al . U.S. Patent

No. 5,057,622 demonstrates that attempts to produce pentaerythritol tetraesters of 3-dodecylthiopropionic acid using the processes of the aforementioned Dexter, et al. U.S. Patent No. 3,758,549, Kauder, et al . U.S. Patent No. 4,080,364, and Nakahara, et al . U.S. Patent No. 4,349,468 gave results that were unsatisfactory as to reaction time, efficiency in catalyst use, yield, presence of undesired byproducts, and/or ease of recovery.

Thus, there remains a need for a process that produces 3-alkylthiopropionate tetraesters of pentaerythritol efficiently and that does so in a manner that permits simple recovery of the tetraesters in high yield.

The present invention is directed to a process for producing a pentaerythritol tetrakis 3- alkylthiopropionate tetraester wherein each alkyl portion contains 4 to 20 carbon atoms, comprising reacting pentaerythritol and lower alkyl (preferably methyl) 3-alkylthiopropionate in the presence of an organotin catalyst for said reaction under elevated temperature conditions at which said tetraester forms. More specifically, the pentaerythritol is reacted with one or more lower alkyl 3-alkylthiopropionate esters, wherein the lower alkyl portion contains 1 to 4 carbon atoms and is unbranched, and the alkyl portion contains 4 to 20 carbon atoms.

The present invention is also directed to compositions of matter comprising at least about 90 wt.%, and more advantageously at least about 95 wt.%,

of one or more tetrakis 3-alkylthiopropionate tetraesters of pentaerythritol wherein each alkyl portion contains 4 to 20 carbon atoms, together with one or more tris (3-alkylthiopropionate) esters of pentaerythritol and with lower (C _x to C ₄ ) alkyl 3- alkylthiopropionate, i.e. methyl, ethyl, n-propyl or n-butyl 3-alkylthiopropionate, in each of which the alkyl portion contains 4 to 20 carbon atoms.

The tetraesters produced in accordance with the present invention are useful as stabilizers of, for instance, polymeric resins against thermal degradation and oxidative deterioration during processing, extrusion or molding, as well as during use of such polymeric materials. The present invention uses lower alkyl 3- alkylthiopropionate, wherein "lower alkyl" denotes an alkyl chain, preferably not branched, which when substituted with -OH is a relatively volatile alkanol. Preferred lower alkyl groups are methyl, ethyl, n- propyl and n-butyl . The most preferred lower alkyl group is methyl. The 3-alkylthiopropionate used in the process of the present invention can have been formed by any of a number of techniques, the preferred one of which is described hereinbelow. The 3-alkylthiopropionate can be made by a direct addition reaction procedure which is carried out in order to minimize the recovery of anything other than the desired 3-alkylthiopropionate. The length of the carbon chain of the alkyl group within the 3-alkylthiopropionate is selected by the carbon chain length of the mercaptan which is charged into

the reaction vessel . The selected mercaptan undergoes an addition reaction with methyl acrylate to form the 3-alkylthiopropionate. The mercaptan has the formula RSH, wherein R has a carbon chain length of between 4 and about 20 carbon atoms. Exemplary reactants in this regard include n-butylmercaptan, n- octylmercaptan, n-decylmercaptan, n-dodecylmercaptan and the like. Generally equimolar charges of this mercaptan and the ester addition reactant are incorporated into the reaction vessel, although either component may be present at a concentration slightly in excess of the equimolar level.

The other addition reactant, which may be characterized as the acrylate reactant, can be charged to the reaction vessel as the corresponding lower alkyl acrylate, wherein "lower alkyl" is as defined above. The preferred reactant in this regard is methyl acrylate. The addition reaction is run under strongly basic conditions. Any strong base can be utilized as the catalyst, provided an aqueous solution thereof will impart a pH of at least about 11. The strength of the base can be generally defined as one wherein a 1% aqueous solution thereof has a pH of at least about 13. Typically strong bases in this regard include aqueous potassium hydroxide, and aqueous sodium hydroxide. However, it is preferred to run this reaction solvent-free, so solid pellets of potassium hydroxide or other strong base are preferably used rather than an aqueous solution. It is important that the reaction composition incorporate an adequate concentration of this strong base. The

amount is to be adequate to act as a catalyst for the addition reaction. For example, the reaction composition should typically include at least about 1 to 2 mole percent of strong base per mole of acrylate charged into the reaction vessel.

After the addition reaction has progressed to the desired extent, the 3-alkylthiopropionate is isolated from the reaction composition by proceeding first with acidification of the reaction mixture, typically with a suitable aqueous mineral acid.

Aqueous and organic layers thereby defined are then separated. If necessary, depending upon the carbon chain length of the mercaptan reactant, the layers are maintained at a temperature high enough to keep the alkylthiopropionate molten. After separation has been completed, the collected organic phase is preferably vacuum stripped in order to remove (and optionally, recover) unreacted components and thereby provide the 3-alkylthiopropionate addition reaction product. The lower alkyl, and preferably methyl, 3- alkylthiopropionate, however produced or obtained, is reacted with pentaerythritol to produce the desired tetraester. It has been discovered that this reaction proceeds, at satisfactory rate and yield, when the 3- alkylthiopropionate and the pentaerythritol are reacted in the presence of an organotin catalyst for the reaction, under elevated temperature conditions. It is preferred to employ an amount of the ester representing a stoichiometric excess with respect to the amount of pentaerythritol present. Generally the amount of organotin catalyst can range up to about 5.0

wt.% based on the amount of pentaerythritol present, although higher amounts of catalyst can be used to advantage as well.

Preferred catalysts include organotin compounds, in particular monoalkyltin hydroxide, monoalkyltin chloride, monoalkyltin chlorohydroxide, and/or mixtures thereof, wherein the alkyl group contains 1 to 8 carbon atoms . Mixtures of any of the foregoing may also be used to advantage. Particularly preferred catalysts include monobutyltin chloride and monobutyltin hydroxide, and a more preferred catalyst comprises a 50:50 (by weight) mixture of monobutyltin hydroxide and monobutyltin chloride, the total combined amount of catalyst comprising about 1.5 to 2.0 wt.% based on the amount of pentaerythritol present. Preferably, at least one organotin chloride compound, and more preferably at least one alkyltin chloride, compound is present in the catalyst component. Other useful catalysts include di (C ₁ to C ₈ ) alkyl tin bis- (C ₈ to C ₁₂ ) carboxylate such as dimethyl tin bis-neodecanoate, as well as mono (C- _^ to C ₈ ) alkyl tin tris (3-alkylthiopropionate) and di (C _x to C ₈ ) alkyl tin bis (3-alkylthiopropionate) . Preferred examples of the latter include dimethyl tin bis-3- laurylthiopropionate and methyl tin tris-3- laurylthiopropionate.

The reaction of the lower alkyl, e.g. methyl, 3-alkylthiopropionate, pentaerythritol, and catalyst should be carried out solvent-free. If desired, a small but effective amount of solvent may

be employed, but it would have to be inert to the reactants, have a very high boiling point, and be relatively easily removed from the mixture formed by the reaction. The reaction is carried out at temperatures effective to enable formation of the desired tetraester. Effective reaction temperatures are typically in the range of 150°C to 250°C, and more typically in the range of about 175°C to 225°C. As those experienced in this field will recognize, it may be desirable to adjust the temperature during the course of the reaction, for instance by raising the temperature. The time required for satisfactory conversion in the reaction can typically range from 2 to 20 hours.

The fact that such reaction temperature conditions are able to achieve formation of the desired tetraester product, particularly in significant yields at acceptable rates, has been confirmed and is all the more unexpected in view of the teachings in the prior art, such as U.S. Patent No. 5,057,622, suggesting that production of the desired tetraester by transesterification would not be successful. Indeed, the process of the present invention accomplished formation of the desired tetraester at yields above about 70% even before isolation of the desired product.

It is also preferred to carry out this reaction under relatively high vacuum, which thereby provides shorter reaction times and more complete reaction. While vacuum of about 15 mm Hg is useful,

high vacuum on the order of about 2 mm Hg is preferable.

Following completion of the transesterification reaction to form the desired tetraester, the tetraester can be recovered from the reaction mixture by subsequent recovery and purification steps using procedures which are well known in this field. For instance, the catalyst should be filtered off and the product purified by one or more recrystallization steps.

It is preferred that this transesterification procedure be followed by an operation wherein the organic phase is solvent refined with an organic solvent. Preferably, the solvent refining medium is one or more organic solvents which are particularly well suited for the specific alkylthiopropionic tetraester being prepared. A preferred solvent is 2-propanol (isopropanol) . Other exemplary solvents include other low molecular weight alcohols and low molecular weight esters, including materials such as methanol, ethanol, ethyl acetate, isopropyl acetate, and the like. It has been found that a suitable solvent or blend will improve work-up purification procedures, when desired, in a manner that minimizes the expense thereof.

As an example of suitable organic solvent blends, a blend of methanol and isopropanol is generally preferred for the work-up purification of the liquid tetraester of 3-octylmercaptopropionic acid with pentaerythritol. It has been found that this solvent blend is non-miscible with this tetraester and

performs well as an extracting solvent for any triester impurity and unreacted octylmercaptopropionic acid. A typical two-component solvent blend would be at a ratio of between about 9 to 1 and about 1 to 9. In this manner, a product can be obtained which contains 90 wt.% or more, preferably 95 wt.% or more, of the desired tetra (3-alkylthiopropionate) ester of pentaerythritol . The composition of matter will also contain minor amounts of the triester, that is, a tri (3-alkylthiopropionate) ester of pentaerythritol, as well as a minor amount of unreacted lower alkyl, e.g. methyl, 3- alkylthiopropionate ester. Generally, the amounts of the triester byproduct and of the unreacted ester can comprise up to about 0.1 wt.% or even up to about 1 wt.%, although of course lesser or higher amounts may be present depending on the degree of completion of the reaction.

Esters of the type discussed herein are typically suitable for use as stabilizers for polymers. The tetraesters with pentaerythritol have been found to be especially useful as stabilizers for a class of proprietary polymers and polymer blends having a terephthalate ester component and a rubbery tyP ^e °f component. Articles extruded from these types of proprietary polymers have superior impact resistance properties and can be suitable for use as automobile bumpers and the like. The 3- dodecylthiopropionate tetraester of pentaerythritol has been observed to be generally equal in performance to similar ester stabilizers manufactured on a

commercial scale by a process believed to be more complicated than the procedure of the present invention.

Various tetraester stabilizers prepared according to this invention have different physical properties which may be particularly advantageous for different proprietary polymers. For example, esters made from dodecylmercaptan are solid at room temperature and less likely to exhibit a noticeable odor when in use as a stabilizer. Esters made from octylmercaptan are basically liquid at room temperature, are less waxy than esters having a greater molecular weight, and can be more compatible, particularly with polymer resins that tend to be liquid at room temperature. Esters prepared from decylmercaptan typically have properties the- reinbetween, and they can exhibit good compatibility without excessive volatility.

The invention is described further in the following Examples, which are present for purposes of illustration and are not intended to limit the scope of that which is regarded as the invention.

1 EXAMPLE 1

A. Production of Methyl 3-laurylthiopropionate:

In a one-liter 3 neck flask was placed 380 g lauryl mercaptan (LM) . After fitting the flask with a 5 condenser and nitrogen purge, 2.0 g KOH pellets were added. An addition funnel with 160 g methyl acrylate (MA) was placed on the flask. The contents of the flask were heated to 55°C. The MA was then slowly added over 90 minutes to maintain the temperature

10 between 50°C and 65°C. After an additional 30 minutes of mixing, the contents were analyzed. An additional 6 g MA was added and mixed for 45 minutes. After analyzing, the product was washed by adding 2.0 g concentrated H ₂ S0 ₄ , 200 g water and mixing. After

15 settling, the aqueous layer was decanted and the product was transferred to a side-arm flask. The product was dried and the residual MA was steam stripped off under vacuum at 93°C.

20 B- Production of Pentaerythritol Tetrakis (3- Laurylthiopropionate)

In a one-liter sidearm flask were placed 491.4 g of the product of Step A., 46.38 g mono- pentaerythritol, 0.34 g monobutyl tin hydroxide and

^5 0.34 g monobutyltin chloride. The mixture was heated under vacuum to 175°C for 1 hour, then the temperature was increased to 200°C for 3 hours. The product was analyzed and then filtered using Whatman #40 filter paper and Celite filter aid to remove a small amount

3° of solids. To a 2 liter beaker 477.0 g filtered product and 477.0 g isopropyl alcohol (IPA) were

35

added. The mixture was heated to 50°C and with stirring allowed to cool and crystallize. The mixture was cooled to 27°C and the crystals were collected and washed with 20 ml IPA. The crystals and mother liquor were analyzed. To a clean 2 liter beaker 342 g "wet" crystals and 313 g IPA were added. This mixture was heated to 57°C. With stirring it was allowed to cool and crystallize. After cooling to 27°C the crystals were collected and washed with 20 ml IPA. The crystals and mother liquor were analyzed.

The IPA was then stripped from the two mother liquors by heating under vacuum to 95°C. The two mother liquors were combined and analyzed.

To a 500 ml side arm flask were added 174.8 g mother liquors and 6.89 g pentaerythritol. No catalyst was added because the mother liquors contained catalyst. The mixture was heated to 200°C under vacuum and cooked for 2 hours . A dark gray product formed which was analyzed and filtered using Whatman #40 filter paper and Celite filter aid. The filtered product was light yellow.

To a beaker was added 146.9 g filtered product and 146.9 g IPA and those contents were heated to 46°C. The solution was cooled with stirring to 27°C. Crystals formed and were filtered off and washed with 20 ml IPA. Crystals and mother liquor were analyzed.

To a beaker 91.9 g "wet" crystals and 92.3 g IPA were added. The mixture was heated to 54°C and then was cooled to 27°C with stirring. The crystals were filtered off and washed twice with 20 ml IPA.

The extra wash was given because of the larger percentage of catalyst present. The crystals and mother liquor were analyzed.

Analyses of the various reaction products and fractions for methyl 3-laurylthiopropionate

("ML") , pentaerythritol tris (3-laurylthiopropionate) ("Tris") , and the desired tetraester, in wt.%, are shown in the following Table 1:

Table 1

HPLC

%ML %Tris

%Tetraester

After Cook 18.00 9.74 72.26

1st Crystals 2.92 1.86 95.22

1st Mother liquor 61.28 32.62 6.10

2nd Crystals 0.57 0.73 98.69

2nd Mother Liquor 47.10 44.15 8.75

Mother Liσuor Rework

After Cook 16.59 8.58 74.83

1st Crystals 2.25 1.34 96.41

1st Mother Liquor 53.12 34.69 22.19

2nd Crystals <.l <.l >99.8

2nd Mother Liquor 43.42 29.17 27.41

The yield based on 2nd crystals and mother liquor crystals was 84.4%.

The yield based on pentaerythritol was 96.6% (including rework crystals and known losses) .

EXAMPLE 2 The transesterification reaction to produce pentaerythritol tetrakis (3-laurylthiopropionate) was

1 carried out with varying amounts of two different catalysts to assess desirable catalyst amounts. The reactions were run for 6 hours at 205°C with mild vacuum being applied for the last two hours. The mole

5 ratio of methyl ester to pentaerythritol was 5:1. The crude reaction mixture was filtered to remove a small amount of black solids and re-crystallized twice from equal weights of IPA. It is clear from the results shown in Table 2 that catalyst type and concentration 0 are both important factors in determining the yield.

TABLE 2

Catalyst Concentration % _.

Yield - ³ Monobutyltin hydroxide 540 ppm 31.84

Monobutyltin hydroxide 1080 ppm 50.96

Monobutyltin hydroxide 1620 ppm 59.81

Monobutyltin chloro hydroxide 540 ppm 31.98

Monobutyltin chloro hydroxide 1620 ppm 68.09

Thus, the catalyst concentration is 0 preferably at least 1000 ppm (based on the total reaction mass) and more preferably at least 1500 ppm.

The process of the present invention affords a number of advantages which distinguish it further from past practices. The reaction to form the 5 tetraester can be run solvent-free, thus avoiding the expense, the additional materials handling burden, and the solvent removal burden that are imposed by the use of solvents. Indeed, even the water requirements are minimal, so contamination of product by water is 0 likewise minimized. Removal of recrystallization solvent is easier.. The tetraester has a lower acid

5

value. The mother liquors (process streams) employed in the process can be recycled with minimal requirements for purification and without requiring further reactions of the byproducts present.