Rubbery Conjugated Diene Polymers Terminated With Hydrocarbyl Phosphites

The invention pertains to the production of improved rubbery conjugated diene homopolymers and copolymers with monovinylarenes. In one aspect, the invention pertains to rubbery conjugated diene polymers containing phosphite groups. In another aspect, the invention pertains to tread stock compounds incorporating such rubbers.

Preparation of solution polymerized conjugated diene homopolymers and copolymers with monovinylarenes, such as polybutadiene and butadiene/styrene rubbery copolymers, is known. Various methods of treatment of such solution polymerized polymers are known, including termination with a variety of silicon based inorganic and organic agents. Many of these reagents effect a coupling reaction when applied to the unterminated polymer-lithium prior to other termination.

Solution-polymerized conjugated diene rubbers have been employed in tires, particularly tread stocks. Many additives have been employed in efforts to improve various properties. Needed, still, have been methods to distinctly improve the characteristics of solution-polymerized conjugated diene polymers to provide better effectiveness, particularly for use as tread stocks. Needed, still, have been methods to impart to cured compounds of conjugated diene polymers improved resilience and lower hysteresis, all of which are important at reduced rolling resistance of rubber tire treads, and, generally, improved tensile strength.

We have discovered, unexpectedly, that treatment of a hydrocarbon solution polymerized polymer-alkali metal with a hydrocarbyl phosphite or thiophosphite, at a level of at least one mole per g-atom of alkali metal in the living polymer, wherein the polymer is a rubbery conjugated diene polymer, that the treated polymer predominantly reflects phosphite or thiophosphite end-groups, rather than coupling.

Our process comprises treating a rubbery conjugated diene polymer as the polymer-alkali metal such as polymer-lithium, in which the polymer is the product of solution polymerization of a conjugated diene or of a conjugated diene plus a monovinylarene, with a hydrocarbyl phosphite or thiophosphite at a level of at least one mole per g-atom of alkali metal in the polymer.

Heretofore, one would have expected coupling, such as taught for the resinous copolymers described in U.S. Pat. No. 3,803,266. Unexpectedly, when applying such treatment to rubbery diene homopolymers and copolymers, we find that the use of the trialkylphosphites and thiophosphites results in little coupling (as indicated by gel permeation chromatogram), but rather primarily termination of the polymer with phosphite groups. The resultant polymer, after curing, still rubbery, exhibits lower hysteresis temperature buildup and generally better resilience compared to untreated stocks, and thus improved usefulness in tread stocks.

The agents which we employ as terminating agents or treating agents to provide the chemically bound functional groups into the conjugated diene homo and copolymers are hydrocarbyl phosphites or thiophosphites, represented by P(XR).sub.3 in which each X can be the same or different and is selected from the group consisting of O and S. Each R can be the same or different, and is selected from the group consisting of, alkyl, cycloalkyl, and aralkyl groups of 1 to 20 carbon atoms, preferably 1 to 5 carbon atoms.

Most preferred are the trialkylphosphites, especially P(OCH.sub.3).sub.3 (trimethyl phosphite). Other representative species include P(OC.sub.2 H.sub.5).sub.3 (triethyl phosphite), P(OC.sub.4 H.sub.9).sub.3 (tributyl phosphite), P(OC.sub.6 H.sub.11).sub.3 (tricyclohexyl phosphite), P(OC.sub.7 H.sub.7).sub.3 (tritolyl phosphite), P(SCH.sub.3).sub.3 (trimethyl trithiophosphite), P(SC.sub.4 H.sub.9).sub.3 (tributyl trithiophosphite), P(OH)(SCH.sub.3).sub.2 S,S'-(dimethyl dithiophosphite), and P(OCH.sub.3).sub.2 (SC.sub.2 H.sub.5) (O,O'-dimethyl-S-ethyl thiophosphite).

The conjugated diene polymers treated in accordance with our invention are polymers obtained by the polymerization of one or more hydrocarbon conjugated dienes alone, or with hydrocarbon monovinylarene comonomer polymerizable with alkali metal based initiators, such that the polymer has an alkali metal attached to the terminal polymer unit or the polymer chain, as is known in the art. By "with" we mean either in admixture with, or in sequential admixture with or by sequential addition polymerization to form diene homopolymers, or random and block copolymers including tapered block copolymers with one or more vinyl or substituted-vinyl group-containing comonomers, presently preferably the monovinylarenes. The polymers must have a sufficient proportion of polymerized diene to be characterized as rubbery.

The conjugated dienes ordinarily contain 4 to 12 carbon atoms per molecule; with those containing from 4 to 8 carbon atoms presently preferred. Examples of such diene monomers include 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, piperylene, 3-butyl-1,3-octadiene, 2-phenyl-1,3-butadiene, and the like. Examples of such vinylarene monomers include styrene, .alpha.-methylstyrene, 1-vinylnaphthalene, 2-vinylnaphthalene, 3-methylstyrene, 4-methylstyrene, 4-n-propylstyrene, 4-cyclohexylstyrene, 4-dodecylstyrene, 4-tertiarybutylstyrene, 2-ethyl-4-benzylstyrene, 4-tolylstyrene, 4-(4-phenyl-n-butyl)styrene, 1,2-diphenyl ethylene, and the like.

Diene monomers presently preferred are butadiene and isoprene, and the vinylarene monomer presently is styrene, due to their availability and relatively low cost.

The weight ratio of conjugated diene:monovinylarene monomers in copolymers wherein a copolymerization is employed can range broadly from about 99:1 to 50:50, but ordinarily from about 90:10 to 60:40 weight ratio, so long as the resulting copolymer can be characterized as rubbery. The number average molecular weight M.sub.n of the diene homo- and copolymers employed in our invention can range broadly from such as about 50,000 to 600,000, preferably for tire usage about 100,000 to 300,000. It should be noted that the "rubbery" characteristic is properly applied to the solution-polymerized polymer even though it is polymerized in solution and not recovered therefrom prior to our phosphite treatment step.

By rubbery we mean elastomeric, i.e., a material "capable of being extended to twice its own length at 68.degree. C. and on release of the stress returns with force to approximately its original length" (Encyclopedia of Polymer Science and Technology, Vol. 5, John Wiley and Sons, Inc., 1966, page 430).

Solution polymerization conditions known in the art can be employed for the polymerization of the described monomers with alkali metal based initiators. Hydrocarbon diluents are employed. Pressure employed should be sufficient to maintain the polymerization admixture substantially in the liquid phase. Temperatures in the range of such as about -50.degree. C. to 200.degree. C. can be employed, and depend on the monomers and initiator chosen as well as the diluent, as is known in the art. Polymerization times vary over a wide range, generally dependent on other polymerization reaction parameters. It is presently preferred that the parameters for polymerization be selected so that substantially or essentially complete conversion of monomers to polymers is obtained prior to our treating step with hydrocarbyl phosphites or hydrocarbyl thiophosphites.

Our invention generally is applicable to processes for the polymerization of the described monomers in a hydrocarbon diluent, employing an alkali metal as the initiator, or an organoalkali metal compound corresponding to the general formula R"M.sub.x as initiator. For the organoalkali metal initiators, R" is a hydrocarbyl radical and can be aliphatic, cycloaliphatic, or aromatic, x is an integer of 1 to 4, preferably 1, and M represents the alkali metal, and can be lithium, sodium, potassium, cesium, or rubidium. Presently preferred for commercial availability is a normal, secondary, or tertiary butyllithium, and most preferred is n-butyllithium (NBL).

The amount of initiator employed in producing the polymers or copolymers can be varied over a wide range, and is generally selected dependent on the ultimate molecular weight for the resulting product, since the molecular weight of the living polymer-alkali metal is inversely proportional to the amount of initiator employed. Hence, the amount employed is based on initiator level-polymer molecular weight relationships as known in the art. Typically, the amount employed is in the range of about 0.1 to 40 gram milliequivalents per 100 grams of monomers (mhm) polymerized, preferably about 0.5 to 1 mhm.

Suitable diluents for the polymerization include any of the commonly employed paraffins, cycloparaffins, and/or aromatic hydrocarbons, such as 4 to 10 carbon atoms per molecule, typically pentane, hexane, cyclopentane, cyclohexane, isooctane, benzene, toluene, and the like. Presently preferred is hexane or cyclohexane or mixtures thereof. In addition, agents that affect the vinyl content of the polymer, such as tetrahydrofuran (THF), generally are also employed.

Polymerization, of course, is conducted in the substantial absence of air or moisture, preferably under an inert atmosphere such as nitrogen, as is well known in the art.

The resulting living polymers contain one or more alkali metal terminated ends, wherein the alkali metal derived from the initiator is positioned at an end of the polymer chain. When a difunctional initiator R"M.sub.2, is employed then the living polymer will contain two ends each containing an alkali metal, and so on. Of course, all alkali metal entities do not ultimately result in a polymer-alkali metal, as there is some attrition from traces of oxygen, moisture, and the like, since such impurities present do tend to reduce the amount of alkali metal-terminated polymer formed. With the preferred NBL initiator the polymer chain contains only one lithium atom.

In accordance with our invention, the phosphite functional group-containing treating agent, selected from hydrocarbyl phosphites and hydrocarbyl thiophosphites, is introduced into the polymerization reaction admixture at the end of polymerization step. The amount of treating agent employed will depend on the amount of alkali metal initiator used and generally will be about at least one mole per gram-atom of alkali metal in the initiator, preferably from 1 to 2 moles per g-atom of alkali metal. Based on the monomer charge, amounts of hydrocarbyl phosphite or thiophosphite can range broadly from such as about 0.1 to 10 mhm. Presently preferably, amounts employed are about 0.5 to 2 mhm.

The phosphite treating agent must be added before any material such as water, acid, alcohol, antioxidant, etc. is added to inactivate and/or remove the lithium or other alkali metal atoms attached to the terminal monomer unit polymer chain.

The temperature suitable for conducting the treating reaction, i.e. reaction of the polymer-alkali metal with the phosphite treating agent, can vary over a broad range and is conveniently the same temperature used for the monomer polymerization. Typically the phosphite treating reaction is conducted at a temperature within the same range described earlier for the polymerization reaction. The time can be up to 24 hours, more usually from about 10 seconds to 30 minutes. The pressure employed for the reaction can be below or above atmospheric, and preferably is and conveniently the autogenous pressure.

Following completion of the treating reaction, the now phosphite-containing polymer is treated to remove any remaining alkali metal from the polymer, although this is not essential to the process. Suitable treating agents include such as alcohol, phenol, mild acid, water, alone or in combination. The phosphite-containing polymer can be isolated by procedures such as solvent flashing, coagulation using a non-solvent for the polymer such as isopropyl alcohol, steam-stripping, as may be convenient.

Polymers produced in accordance with our process are suitable for preparing a variety of tread stocks, but most particularly tire tread stocks, containing carbon black at a level of up to 65 parts by weight per 100 parts by weight of polymer (php) and extender oil of up to 30 (php).

For most uses, a resulting phosphorus content, expressed as P, of about 0.005 to 0.3 weight percent in the final polymer is preferred, more preferably about 0.01 to 0.03 for tire tread stocks.

The phosphite terminated rubbery polymers can be admixed with one or more additives such as fillers, pigments, extenders, other polymers, etc., as may be desired, as is known in the tread arts.

Exemplary of the additives that can be admixed with our phosphite polymers are such as calcium carbonate, iron oxide, carbon blacks, silica, calcium silicate, alumina, various dyes and pigments, titanium dioxide, reinforcing as well as non-reinforcing fillers, and various types of stabilizing materials, as known in the arts.

The phosphite-diene rubbers, along with suitable fillers, extenders, plasticizers, reinforcing agents, and the like, are employed in cured or cross-linked compounds.

The curing system can be either a sulfur-system or a peroxy-system. Presently preferred are the sulfur curing systems. The sulfur cured system comprises sulfur or sulfur-containing compounds such as organic sulfides, organic sulfenamides, and the like, as well as various combinations. Generally, the amount of sulfur in the free or combined form employed will be in the range of about 0.1 to 5 parts by weight per 100 parts by weight of the rubbery portion (php) of a tread stock compound, preferably about 1 to 3 php.

Various primary and secondary accelerators, typified by benzothiazyl disulfide or mercaptobenzothiazole or a benzothiazyl sulfenamide, benzothiazyl-2-sulfenomorpholide, or a dithiocarbamate can be incorporated. Generally, any type of commercial blending, Banbury mixer, and the like, can be employed. A generalized recipe for tire tread stocks, which tread stocks constitute a particularly desirable aspect of our invention, can include as exemplary constituents and amounts as follows:

Examples provided are intended to illustrate and demonstrate various aspects of our invention. Specific materials employed, particular relationships, species, amounts, and the like, should be considered as illustrative and not as limitative of the reasonable scope of our invention, but rather a further elucidation of our disclosure for the benefit of those skilled in the art.

This Example describes the preparation of two medium-vinyl polybutadienes suitable for use in vulcanized tire tread compounds. Each polymerization as carried out in nitrogen-purged, sealed glass bottles to which the cyclohexane, butadiene, tetrahydrofuran, and n-butyllithium were added. The bottles were heated for 75 minutes in a water bath having a temperature of about 50.degree. C. Subsequently a terminating agent and an antioxidant solution were injected into each bottle, which was heated in the 50.degree. C. water bath for several minutes. Finally the bottle contents were poured into a beaker filled with isopropanol. Each butadiene polymer coagulated and was dried in a vacuum over at 60.degree. C. for about 15 hours.

Polymerization recipes and pertinent physical properties are summarized in Table I:

These runs describe the preparation and pertinent properties of vulcanized rubber compositions employing the two control Polymers 1 and 2 and the inventive Polymer 3 prepared in Example I.

Polymer, filler, oil, and various chemical additives were blended at about 120 r.p.m. in either a C. W. Brabender Plasti-Corder with Banbury head at about 65.degree. C. (Runs 1 and 2) or in a Midget Banbury equipped with a jacket for circulating water having a temperature of about 60.degree. C. (Runs 3, 4, 4A and 4B). After mixing the polymer only for about 0.5 minute, 50% by weight of carbon black and of the chemical additives (except the oil and curing agents sulfur and Santocure NS), were added. About 1.5 minutes after start the remainder of the carbon black and chemical additives (except to oil and curing agents) were added, and after 2 minutes after start the oil also was charged. After a total mixing time of about 5 minutes, the compound was dumped at 145.degree.-155.degree. C. onto a roll mill (3" diameter) and sheeted off after about 1.5 minutes.

Then each rubber compound was remixed in the Brabender or Banbury, respectively, at about 110.degree. C. for about 2 minutes. Sulfur and Santocure NS were added at the beginning of this remilling step. Finally the rubber compounds were again sheeted off from a roll mill.

Curing of the rubber compounds was accomplished in a compression mold at about 150.degree. C. for 45 minutes. 6".times.6" slabs of each of the vulvanized rubber compounds were used for tensile and hardness measurements. Unvulcanized strips of each rubber compounds were rolled into pellets and cured separately for Mooney, scorch, and cure time measurements. Other test specimens were used as indicated in the procedures identified in Footnotes to Table II:

Data in Table II demonstrate that a rubbery polydiene terminated with a hydrocarbylphosphite (trimethyl phosphite Runs 2 and 4) exhibited lower hysteresis and higher resilience as compared to non-phosphite terminated polymers Runs 1 and 3 under commonly employed Flexometer test conditions (Method A). Under more severe conditions (blowout conditions) this advantage did not prevail.

Comparing Run 4A (inventive) and Run 3A (control) aged samples (see Footnote 1), Run 4A shows much improved hysteresis and resilience.

A strength advantage of the of the invention compound was realized in two out of three runs. Mill band ratings (a measure of processability, not shown in Table II) of both the unvulcanized control and inventive runs were poor.

This Example describes the preparation of additional medium-vinyl butadiene homopolymers terminated with trimethyl phosphite, (CH.sub.3 O).sub.3 P, essentially in accordance with the procedure of Example I. Polymerization recipes are listed in Table III. Polymers 5 and 6 were blended at a weight ratio of 1:1 so as to produce Polymer 10 (control). Polymers 7, 8 and 9 were blended at a 1:1:1 weight ratio so as to produce Polymer 11 (Invention).

Physical properties of these blends are summarized in Table III:

This Example describes the preparation and pertinent physical properties of vulcanized rubber compositions employing the two Polymer blends 10 and 11 of Example III. Blending was done in a Midget Banbury at 120 r.p.m. and a circulating water temperature of 60.degree. C., essentially in accordance with the procedure described in Example II, except that the compounds were dumped onto the roll mill (3" diameter) at 155.degree.-160.degree. C. Cure conditions were indicated to those described in Example II.

Carbon black, oil, and chemical additives, and testing procedures were the same as those listed in Example II. Compounding recipes and pertinent physical properties of cured rubber compounds are summarized in Table IV:

Data in Table IV confirm the significant hysteresis and resilience advantages of compounds containing the inventive phosphite-terminated polymer Runs 6 and 6A, as compared to compound Runs 5 and 5A containing very similarly prepared but non-phosphite terminated polymers. However, at the more severe blowout conditions this hysteresis advantage did not prevail. Processing of unvulcanized compounds (mill band rating not shown in Table II) was poor for both the control and invention runs.

This Example describes the preparation of butadiene-styrene copolymers, with and without (OCH.sub.3).sub.3 P (trimethyl phosphite) and (OC.sub.2 H.sub.5).sub.3 PS (triethyl thiophosphate) termination. The polymerization, termination, and polymer recovery of these copolymers is identical to that described in Example I. Polymerization conditions and properties of the polymer blends are summarized in Tables VA and VB. Polymer 20 is a blend of Polymers 12, 13, and 14, and Polymer 21 is a blend of Polymers 15 and 16, employing weight percentages of the polymer blend components as indicated in Table VA. Polymer 22 is a blend of Polymer 17, 18, and 19 employing weight percentages of the blend components as indicated in Table VB:

This Example describes the preparation and properties of vulcanized rubber compositions employing control Polymers 20 and 21 and invention Polymer 22. The blending and molding procedures were essentially the same as described in Example IV. Ingredients of the rubber compounds and testing procedures were the same as those listed in Table II, with one exception: the carbon black was IRB #5 (also a N 330-type black).

Compounding recipes and pertinent physical properties of the cured rubber compounds are summarized in Table VI:

Data in Table VI show that only the rubber compound of the invention Run 9 containing trimethyl phosphite-terminated butadiene-styrene copolymer exhibited improved hysteresis, resilience, and tensile strength over control Run 7 compound containing a copolymer without phosphorus. Again, under severe blowout conditions, no hysteresis advantage was observed.

Run 8 using a triethyl thiophosphate-terminated butadiene-styrene copolymer did not exhibit a significant tensile, hysteresis, or resilience advantage, thus pointing out clearly the importance of our phosphite termination.

Processing of all three unvulcanized polymer blends on the roll mill was poor.

Polymers 20, 21, and 22 were compounded according to a high carbon black/oil recipe as shown in Table VII, which also summarizes pertinent physical properties of vulcanized rubber compounds.

Data in Table VII did not show any significant tensile or hysteresis advantage of (CH.sub.3 O).sub.3 P phosphite-terminated Polymer 22 versus (C.sub.2 H.sub.5 O).sub.3 PS thiophosphate-terminated Polymer 21, or over Polymer 20, a non-phosphorus containing polymer, in the high black/oil rubber compounds. Possibly, the dilutive effect of the high oil charge masked the benefit of the phosphite-treated diene rubber. The resilience of the invention Run 12 was only slightly higher. Also at blowout conditions, Run 12 did not exhibit any hysteresis advantage. The processability (band rating) of all three unvulcanized rubber compounds was rather poor.

The disclosure, including data, has illustrated the value and effectiveness of our invention. The examples, the knowledge and background of the field of the invention and the general principles of chemistry and of other applicable sciences have formed the bases from which the broad descriptions of our invention including the ranges of conditions and the generic groups of operant components have been developed, and formed the bases for our claims here appended.