Cleavable Branching Agents in Emulsion Polymerization Systems

The invention relates to aqueous emulsion polymerization systems. In a further aspect, the invention relates to methods to reduce the Mooney value of polymers. In still another aspect, the invention relates to sponge rubbers.

Rubbery polymers of moderately low Mooney viscosities are desired in many sponge applications, such as rug underlay manufacture, since the lower viscosity polymers afford a sponge rubber of sufficient drape for such applications.

Unfortunately, polymers of moderately low Mooney viscosity are not convenient dry crumbs, but rather exhibit the characteristic of an undesirably sticky rubber crumb, making drying and handling very difficult. However, these relatively low Mooney viscosity type products are exactly what is desired so as to be able to provide a sponge that ultimately will adequately drape during rug underlay manufacture. Thus, dichotomy has existed, since the rubber manufacturer prefers to make a high enough Mooney viscosity rubber such that handling and drying of the rubber crumb product is relatively easy and economical. But, the higher Mooney viscosity rubbers are just not adequately suitable for many sponge rubber applications, as mentioned.

We have discovered that the inclusion of a bis(1,3-dienyl)dialkyltin, -germanium or -lead compound in the aqueous emulsion polymerization of a conjugated diene, optionally with a vinyl comonomer, produces a polymer of desirable Mooney viscosity that it is readily recovered, handled, and dried in emulsion polymerization production processes, and yet the polymer contains bonds readily cleaved by milling, preferably in admixture with a carboxylic acid, to provide a polymer of sufficiently low Mooney suitable for a variety of end-uses. Exemplarily, the lower Mooney values are desirable so that the rubber, in the sponge form, properly drapes in rug underlay manufacture.

We consider as one aspect of our invention the aqueous emulsion polymerization of the monomers as described with the cleavable branching agents in accordance with our invention, and the polymers so produced. We further encompass as another aspect of our invention the cleaving or milling of the so-produced polymers and the resultant cleaved polymer operation. And, we further include and encompass in a further aspect of our invention the so-cleaved polymers after blowing and curing, and the process of preparing the cured polymers.

Our invention involves the employment of a bis(1,3-dienyl)dialkylmetal compound which is a -tin, -germanium, or -lead compound in the aqueous emulsion polymerization of conjugated dienes, optionally with copolymerizable vinyl monomers. The metal compound is employed in a minor effective amount in the aqueous emulsion polymerization system as one of the comonomers during the polymerization.

The agents employed are described generically as bis(1,3-dienyl)dialkyltin, -germanium, and -lead compounds, and can be described as branching agents. Generically, the structure for the branching agent employed in the process of our invention also can be represented by: ##STR1##

In the above formula, D represents an acyclic conjugated dienyl hydrocarbyl structure or a ring-substituted styryl structure. The acyclic conjugated dienyl hydrocarbyl structure can contain as many as 8 carbon atoms per group, though presently preferred are those of 4 or 5 carbon atoms because of availability and reactivity.

The conjugated dienyl groups can be the same or different. Typical, though not limitative, dienyl structures include the following: ##STR2##

The ring-substituted styryl structures D can be represented by: ##STR3## of which the meta-structure presently is preferred because of reactivity. These ring-substituted styryl structures are presently less preferred from the standpoint of reactivity and availability, though some are feasible to employ.

In each of the formulas given, the R groups are hydrocarbyl, and each R group is individually selected from alkyl, linear or branched, or cycloalkyl, with the limitation that each R group has not more than about 12 carbon atoms per R group. R groups can be selected typically from methyl, ethyl, isopropyl, n-hexyl, 4-methylnonyl, n-dodecyl, cyclohexyl, 3-methylcyclopentyl, and the like. The total number of carbon atoms per bis(1,3-dienyl)dialkyl metal compound should be limited to about 20, for reasons of reactivity. M represents the metal atom of the branching agent and can be tin, lead or germanium.

The amount of the bis(1,3-dienyl)dialkyl metal compound employed is that minor amount effective to provide the degree of branching desired. An exemplary and effective as well as typical amount lies in the range of about 0.01 to 1 phm, parts by weight per 100 parts of monomers employed in the emulsion polymerization, presently preferably about 0.05 to 0.5 for reasons of reactivity. The minor effective and suitable amount of branching agent provides a mill-cleavable polymer.

Aqueous emulsion polymerization process in accordance with our invention involves the free radical polymerization of monomers polymerizable in an aqueous emulsion polymerization system, such as conjugated diene monomers from which elastomers are obtainable.

The polymerizable conjugated dienes include those containing 4 to 12, preferably for commercial availability 4 to 8, carbon atoms per molecule. Examples of these monomers include the presently preferred 1,3-butadiene and isoprene, as well as chloroprene, piperylene, 2-methyl-1,3-pentadiene, 2,3-dimethyl-1,3-butadiene, 1,3-octadiene, 5,6-diethyl-1,3-octadiene, and the like. The conjugated dienes can be polymerized to form homopolymers or copolymers of two or more.

Polymers of conjugated dienes include also copolymers formed by polymerizing one or more conjugated dienes with one or more copolymerizable monovinyl-substituted including substituted monovinyl-substituted aromatic compounds, such as those of 8 to 20, more particularly for commercial availability 8 to 12, carbon atoms per molecule, including styrene, various of the substituted styrenes such as the alkylstyrenes such as o-, m-, or p-methylstyrene, beta-methylstyrene, o-, m-, and p-halostyrenes such as o-chlorostyrene and 2,3-dichlorostyrene and wherein the halo can be fluorine, chlorine, bromine, or iodine; and also other copolymerizable ethylenically unsaturated monomers including the vinyl nitriles such as acrylonitrile or methacrylonitrile, esters of acrylic and methacrylic acid, e.g., ethyl acrylate and methyl methacrylate, vinyl esters such as vinyl acetate, other reactive olefins such as vinylidene chloride; and the like.

Our invention is particularly applicable in the various SBR polymerization systems, including SBR 1000 a hot rubber formulation, and also in SBR 1500 and SBR 1503 which are cold rubber formulations.

Aqueous emulsion polymerization conditions of temperature, time, pressure, and the like can range widely, depending upon particular materials and reactor conditions desired or convenient to employ, the monomers involved, relative volatility thereof, amounts of water, and equipment involved.

In aqueous emulsion polymerization, the emulsion usually is considered to be of the oil-in-water type, oil referring to the monomer, and wherein the aqueous medium is the continuous phase, with the monomer or monomers as the discontinuous phase. The ratio of aqueous medium to monomeric material usually employed is in the range of about 0.5:1 to about 3:1, parts by weight, though the range can vary widely depending on particular operations, or preferences of those skilled in the art.

A wide variety of emulsifying agents can be employed satisfactorily to produce the desired monomer emulsion in aqueous medium, including rosin acid soaps, disproportionated rosin acid soaps, fatty acid soaps, mixtures of disproportionated rosin acid soaps with fatty acid soaps, alkaryl sulfonates, and other surface active materials and mixtures thereof. Nonionic emulsifiers also can be employed including polyethers and polyols.

Amounts of emulsifying agent employed depend on the type and reaction parameters and monomer concentrations. An expedient range would be about 0.5 part to 10 phm. The relative amounts of emulsifier:modifier can range widely depending on monomers, species, reaction parameters, degree of modification desired, and the like.

Aqueous medium normally is employed with the emulsifier, though other components such as glycols may be added for lower temperature polymerizations when desired. The amount of water or water and glycol employed is a matter of convenience as to amounts of materials to handle and is not limiting as far as making the agitated admixture of emulsifier:modifier is concerned. Supplemental water for the emulsion polymerization process itself can be later added at that stage.

Aqueous emulsion polymerization commonly is carried out by continuously agitating the polymerization emulsion at polymerization temperatures expediently in the range of about -40.degree. to +100.degree. C., preferably about 0.degree. to 50.degree. C. The pH range can vary widely such as about 1 to 12, and presently preferably about 7 to 10.8.

The aqueous emulsion polymerization process can employ molecular weight modifiers or regulators generally effective in an aqueous emulsion polymerization system.

Modifiers most frequently used to control the molecular weight of polymers in an aqueous emulsion polymerization process are those with a regulating index, r, or a transfer constant, c.sub.tr, in the range of about 0.01 to 25. The regulating index is defined by d(1n S)/dx= -r, and the transfer constant by d(log S)/d(log M)= C.sub.tr, where S is the modifier concentration, x is the fractional conversion, and M is the monomer concentration. Suitable modifiers include such as the mercaptans and xanthogen disulfides. For these types, the modifier depletion can be readily measured by analysis of the remaining modifier in the polymerization medium by titration techniques as described by Bovey et al. in Emulsion Polymerization (Interscience, N.Y. 1955) Chapters IV and VIII. In instances where the modifier depletion cannot be readily followed, the transfer constant can be measured by the Mayo technique (see reference cited).

Suitable mercaptans include primary, secondary, and tertiary aliphatic, cyclic, alicyclic, and aromatic mercaptans, preferably of 8 to 20 carbon atoms per molecule. Exemplary species include n-octyl to n-octadecyl, s-octyl to s-octadecyl, and t-octyl to t-octadecyl, cyclododecyl, cyclodecenyl and cyclododecenyl, cyclododecadienyl, 1,3,5-trimethylcyclododecadienyl, B-naphthyl, t-butylphenyl, octyl to dodecyl mercaptoesters, and the like. Frequently employed disulfide modifiers include dialkyl xanthogen disulfides such as diisopropyl, di-s-butyl, difuranyl, and the like. Nonsulfur modifiers can also be used and examples of these include carbon tetrachloride, carbon tetrabromide, octyl chloroacetate, terpenoline, pentaphenylethane, and other similar free radical reactive compounds. Other useful modifiers include the dialkyl dixanthogens, diaryl disulfide, tetraalkylthiuram mono- and disulfides and mercaptothiazoles.

Examples of specific species of organosulfur compounds include n-octyl mercaptan, n-dodecyl mercaptan, t-nonyl mercaptan, t-dodecyl mercaptan, t-hexadecyl mercaptan, t-octadecyl mercaptan, t-eicosyl mercaptan, sec-octyl mercaptan, sec-tridecyl mercaptan, cyclododecyl mercaptan, cyclododecadienyl mercaptan; aryl mercaptans such as 1-naphthalenethiol, and the like; bis(tetrahydrofurfural xanthogen), diphenyl disulfide, tetramethyl thiuram disulfide, 2-mercaptobenzothiazole, and the like. Mixtures or blends of any of these organosulfur compounds can also be used. Commercially available and suitable organosulfur compound modifiers often are mixtures of organosulfur compounds, e.g., mixtures of t-dodecyl, t-tetradecyl, and t-hexadecyl mercaptans.

Presently preferred modifiers for preparing our new polymers are the tertiary alkyl mercaptans such as t-nonyl to t-dodecyl mercaptans, and also diisopropyl xanthogen disulfide.

The amount of modifier employed may vary widely, depending upon the particular compound or mixture chosen, the polymerization temperatures, the emulsifier and other recipe ingredients, and the extent of results or modification desired. Satisfactory modification can be obtained by employing on the order of 0.01 to 5 phm, parts modifier compound per 100 parts monomer by weight, with 0.05 to 2 phm presently being preferred, though greater or lesser amounts may be utilized where desired.

Free-radical initiators useful in the aqueous emulsion polymerization process include any of the broad groups of compounds and redox couples capable of generating free radicals in aqueous emulsion polymerization systems. These include the monomer-soluble organoperoxides, hydroperoxides, azo and diazo compounds. Amounts expediently employed are of the order of about 0.01 to 5 phm, more preferred about 0.03 to 1 phm.

Exemplary catalysts include di-t-butylperoxide, benzoyl peroxide, lauroyl peroxide, benzene diazothioether, t-butyl peracetate, t-butyl perbenzoate, dicumyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, t-butyl hydroperoxide, cumene hydroperoxide, p-menthane hydroperoxide, cyclopentane hydroperoxide, diisopropylbenzene hydroperoxide, p-t-butylcumene hydroperoxide, pinene hydroperoxide, 2,2'-azobis(2-methylpropionitrile), often called AIBN, potassium persulfate, and the like, and mixtures thereof.

Useful exemplary redox couples include ferrous sulfate/hydrogen peroxide, potassium persulfate/sodium bisulfite, mercaptan/potassium ferricyanide, and the like; as well as organic hydroperoxide/complexed iron reductant such as p-menthane hydroperoxide/ferrous salt ethylene dinitrilotetraacetic acid complex/sodium formaldehyde sulfoxylate (SFS); p-isopropyl cumene hydroperoxide/sodium dithionite; cumene hydroperoxide/ferrous pyrophosphate complex; t-butyl cumene hydroperoxide/triethylenetetraamine/ferrous pyrophosphate; and the like. The presently preferred initiator systems are potassium persulfate and the SFS redox couple.

After the degree or extent of polymerization desired has been accomplished, the polymerization reaction can be terminated with a shortstop, such as sodium dimethyldithiocarbamate. An antioxidant such as N-phenyl-N' -(1,3-dimethylbutyl)-p-phenylenediamine usually is added to protect the polymer. The emulsion process results in a latex from which the polymer then is recovered by a process known as creaming, i.e., the addition of an aqueous salt solution such as sodium chloride solution, and coagulation of the polymeric product by the addition of a dilute mineral acid such as sulfuric acid. The product then is collected by filtration, washed, dried in conventional drying equipment.

The milling operation can be conducted with any convenient milling means known in the art, for a time suitable to effect the Mooney decrease desired. The milling time generally is determined by the temperature desired, that is, milling is continued as necessary to reach the maximum milling temperature sought by the processor. While milling temperatures can range broadly, the only restriction needed with our invention is that the milling temperature be such as from at least about 120.degree. C. to 200.degree. C., but more usually within the range of about 140.degree. to 160.degree. C., presently preferably about 150.degree. C., to provide optimum cleavage rates and safe processing. Auxiliary heating can be employed, if desired, to shorten cleavage times.

A variety of breakdown (cleavage) procedures can be used. The polymer cleavage step can be on a hot mill, or in a mixer such as a Banbury, or the like, so long as the temperature is at least about 120.degree. C.

The cleavage reaction which takes place under milling conditions as discussed is facilitated by the employment in the rubbery product to be milled of a carboxylic acid, which can be saturated or unsaturated, and can be any of the types commonly employed in rubber vulcanization systems. Commonly, these range from about 12 to 20 carbon atoms per molecule, either as single acids, or as commercially available admixtures. Exemplary species of these acids include stearic, myristic, lauric, linoleic, oleic, ricinoleic, palmitic, and the like. These acids can be employed as such, or as the alkali metal soap thereof, such as sodium, potassium, or the like, and we consider our invention to include such soap as alternative to the acid form when we refer to the carboxylic acids.

The amount of the carboxylic acid to employ can range widely. Presently recommended is about 0.5 to 5 phr, parts by weight per 100 parts rubber, more preferably about 1 to 2 phr since these amounts are commonly used in vulcanization recipes and also are quite satisfactory in this aspect of our invention. This component generally is added during or at the onset of milling, though it should be noted that if a formulation such as an SBR formulation is employed in which the recipe for same contains a fatty acid soap, such as SBR 1503, within the suggested amount, then no additional fatty acid need be added at this stage. Furthermore, the use of the stearic acid, or similar, is optional, if one of the monomers employed itself contains a carboxylic acid group, such as acrylic acid.

It is ordinarily preferred for convenience and economy to rely upon the customary milling operation, i.e., the milling operation in which the compounding recipe ingredients are blended into the rubber preparatory for curing, to accomplish the cleavage reaction. However, it is within the scope of this invention alternatively to employ supplemental milling of the rubber optionally in the presence of a carboxylic acid prior to the addition of the remainder of the compounding ingredients for said customary milling step. This optional preliminary milling is advantageous for a branched rubber of particularly high Mooney or one with which it was desired to periodically determine the degree of cleavage accomplished. Further, this preliminary milling can be of value under circumstances as with emulsion polymerized 1,3-butadiene intended for blending with other rubbery polymers. Here, branching in accordance with this invention can be employed to initially obtain a polymer of Mooney conveniently high for handling in conventional plant operation. Then, employing preliminary milling, the polymer, such as polybutadiene, can be converted into a lower Mooney stage suitable for easy blending with other rubber polymers and curatives such as in the preparation of a tire tread stock.

It should be noted that the temperature stability of our polymers is advantageous since they undergo little or no breakdown at temperatures below at least about 120.degree. C. The higher temperature stability of our polymers is a distinct advantage in manufacturing since there is little hazard of changes in characteristics while processing.

Polymers in accordance with our invention can be compounded as commonly practiced with a variety of stabilizers, antioxidants, fillers, pigments, reinforcing agents, and other such additives as may be employed in various compounding operations.

As a guiding approximation, and not as a limitation, rubbers of a Mooney ML-4 of about 10 to 27 can be readily branched in accordance with our invention to rubbers of Mooney ML-4 of about 30 to 65. These are realistic ranges to consider with regard to many sponge rubber applications. It should be recognized, however, that the application of our invention is not necessarily limited to these values, but that these are typical applications for the primary aim of our invention, sponge rubber applications.

Rubbers that have been prepared in a polymerization process employing a branching agent in accordance with one aspect of our invention can, following isolation, be compounded with agents required for blowing and curing. Suitable blowing agents include such as urea, sodium bicarbonate, N,N'-dinitrosopentamethylene tetramine, azodicarbonamide, p,p'-oxybis(benzenesulfonyl hydrazide) and 2,2'-azobis(2-methylpropionitrile). The compounding can be conveniently carried out in an internal mixer to providing thorough mixing. The rubber compound from the internal mixer can then be banded and sheeted off employing a roll mill. The sheet thus obtained can be cut to size for laying on a grid of woven wire of, for example, the type suitable for the manufacture of waffle weave shaped rug underlay. The sheet thus cut and placed on the grid can then be cured on the grid in an oven or in a heated press or the like with provision that sufficient clearance, e.g., about 0.5 to 0.75 in., is allowed both above and below the grid to permit blowing of the rubber with draping thereof through the openings within the grid as elevated temperatures are applied for curing. Suitable curing times can range widely, such as from about 2 minutes to 30 minutes, preferably from about 5 minutes to 15 minutes. Curing temperatures can vary widely, such as from about 150.degree. to 180.degree. C., preferably from about 160.degree. to 175.degree. C.

The following examples are intended to further illustrate our invention. The polymerization steps are carried out under an inert atmosphere, e.g., nitrogen. Particular species employed, particular conditions, particular amounts used in the examples, are intended to be illustrative and not limitative of the reasonable and proper scope of our invention.

A series of runs were made employing SBR procedures and varying amounts of a bis(1,3-dienyl)dialkyltin branching agent in accordance with the following recipe:

Results obtained are shown in tabular form:

The ML-4 value obtained in comparison run 2, without the additive of our invention, was relatively high, attributable to the relatively low amount of mercaptan modifier employed as compared to the amounts employed in the other runs. Run 1 is a comparative run employing the same amount of modifier as did runs of the invention 3, 4 and 5. A comparison of control run 1 versus runs of the invention 3, 4 and 5 employing the branching agent, clearly show that the use of a branching agent, and as the amounts of branching agent increase, resulting in increases in Mooney viscosity and inherent viscosity. All polymers, despite the increased ML-4 values, were notably gel free.

Further runs were made with and without the branching agents of our invention. SBR procedures employed were in accordance with the following standard recipe:

Two runs were made in accordance with the recipe above, run 6 without branching agent, run 7 with branching agent. A portion of the product of run 7 thereafter was milled for 8 minutes at up to 150.degree. C. in an internal mixer. Results obtained are shown in Table II below:

The effectiveness of the cleavable branching agents of our invention clearly can be observed from the data summarized in Table II above. The ML-4 value of the control run 6, no branching agent, is only 14. The ML-4 value obtained for the branched polymer employing the branching agent in accordance with our invention was 35, a value readily handled in conventional recovery and drying equipment. After milling, the ML-4 value was 24, thus demonstrating cleavage of the molecular structure on the mill. The 24 is a suitable value for sponge rubber applications.

Polymer 6 above and Polymer 7 thereafter were employed in a rug underlay formulation in accordance with the following recipe:

The oleic acid, present in the curing recipe as a processing aid, can by its presence also act as a cleaving agent supplemental to the residual fatty acid present. In the above example cleaving was accomplished in the mixing step.

Three individual evaluations were made. The results obtained are summarized in Table III:

A rug underlay of serviceable, practical quality resulted from evaluation Run 3 above. Run 2 was acceptable, though marginal. It can be observed, thusly, that the preferred mode of treatment is to use a hot mix/remix procedure as described above for evaluation Run 3 (see footnote 4 above).

In further comparative runs, efforts were made to employ tetravinyltin as a branching agent in aqueous emulsion polymerizations. However, it was not effective.

The bis(1,3-dienyl)dialkyltin, -germanium, and -lead branching agents can be readily synthesized from chloroprene by a Grignard procedure. This involves the reaction of 2-chloro-1,3-butadiene with magnesium in the presence of zinc chloride and methyl iodide followed by the addition of dimethyldichlorostannane for subsequent reaction to yield bis(2-[1,3-butadienyl])dimethyltin.

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