Method of Polymerizing Chloroprene Using Conjugated Long-Chain Fatty Acids as Emulsifiers

The present invention relates to an improved method for the polymerization of chloroprene to produce polychloroprene having improved tensile strength. More particularly, the present invention relates to the discovery of new emulsifiers which produce unexpected improvements in tensile strength.

Any study of the prior art of polychloroprene, neoprene, will reveal an almost total disregard for the effect of the emulsification system on the resulting polymer. In general, most of the prior art has regarded the emulsifiers as roughly equivalent, in effect, with the primary consideration being directed to the satisfactory performance of the emulsificaton, e.g., the colloidal stability of emulsion.

It is taught in U.S. Pat. No. 3,392,134 to Apotheker, that the fatty acids such as straight-chain saturated or unsaturated carboxylic acids containing 12 to 30 carbon atoms or the dimers and trimers of unsaturated fatty acids are critical if the latex resulting during chloroprene polymerization is to be colloidally stable during post treatment.

It has now been found that the selection of a particular class of soluble salts of fatty acids as emulsifiers for the emulsion polymerization of chloroprene produces polymers having higher tensile strengths than the usual emulsifiers. Thus, it is a feature of the present invention that the same degree of emulsification efficiency may be obtained with higher tensile strength than achieved by the prior art.

Briefly stated, the present invention is an improvement in the process of emulsion polymerization of chloroprene, wherein the emulsifier comprises a soluble salt of a long-chain fatty acid having 10 to 30 carbon atoms and conjugated ethylenic unsaturation, preferably two to three conjugated ethylenic unsaturations. Fatty acid conjugated trienes have been found to be preferable. The dienes and trienes may be used in admixture with each other or in admixture with other emulsifying agents. When employed with other emulsifying agents, the diene and triene fatty acid salts preferably comprise at least 10 weight percent of the total emulsifying agents in the polymerization system, and preferably at least about 25 weight percent.

Polychloroprene may be prepared by techniques such as disclosed in U.S. Pat. Nos. 1,950,436; 2,227,517; 2,321,693; 2,371,719; 2,463,009; 2,831,842; 2,914,497; 3,042,652; 3,147,318; 3,147,317; 3,655,827; British Patents 1,237,750 and 1,158,970. Process and methods for the polymerization of chloroprene are disclosed, for example, in Encyclopedia of Polymer Science and Technology, Vol. 3, p. 705-730 Interscience, 1965.

Comonomers may be employed with the selection and quantities of comonomer being chosen to produce a polymer. In this application, the term "polymers of chloroprene" encompasses polymers in which chloroprene is the major monomer. Examples of comonomers are 2, 3-dichloro-1, 3-butadiene; acrylonitrile; methyl methacrylate, and other esters, amides, nitriles, vinyl aromatic compounds such as styrene, aliphatic conjugated diolefins such as isoprene and vinyl ethers such as methyl vinyl ether. Usually the total amount of comonomers will represent no greater than 25 mol percent of the total monomers and preferably will constitute less than 15 mol percent of the total monomers including chloroprene.

The polymerization is carried out in an aqueous systems and may be batch or continuous processes. The conjugated fatty acids may be employed as the water soluble salts, e.g., sodium, potassium, amine and the like. Suitable conjugated fatty acids include, for example, 9, 11, 13-octadecatrienoic acid, 9, 11-octadecadienoic acid and 9, 11, 13, 15-octadecatetraenoic acid. The usual emulsifiers which may be employed in conjunction with the present fatty acids are the salts of rosins and rosin derivatives such as wood rosin, or tall oil rosin and resins, disproportionated rosin or hydrogenated rosin; ammonium, sodium or potassium salts of longchain fatty acids; alkali and alkaline earth salts of the condensate of formaldehyde and naphthalene sulfonic acid; octyl sulfate salts; nonionic surface active agents such as the ethylene oxide or propylene oxide condensation products. Additional emulsifying agents are disclosed in U.S. Pat. No. 2,264,173.

The total amount of the emulsifying composition should range from about 2 to 5 parts per 100 parts of monomer used in the polymerization. Although the conjugated fatty acid salts according the present invention may comprise the entire emulsifier system it is preferred that one of the other types, preferably a resin be employed in conjunction therewith in amounts up to 90 weight % of the total emulsifier, or more preferably 70%, i.e., the conjugated fatty acid will comprise 30 to 100% of the emulsifier. The fatty acid type emulsifiers when used exclusively, tend to impart an oily texture to the polymer product, which is moderated by resin type emulsifier which serve a tackifying function.

The pH of the aqueous emulsion for polymerization may be varied and can be acidic, neutral or alkaline; however, it is peferred to have a pH in the alkaline range of about 7 to 13.5.

Conventional catalysts for chloroprene polymerization may be employed and preferred catalysts are peroxide catalysts of the organic or inorganic type. Examples of organic peroxides are benzoyl peroxide, cumene hydroperoxide, tertiary-butyl isopropylbenzene hydroperoxide, azo catalysts such as alpha-alpha-azo-bis-isobutyronitrile and the like. Suitable inorganic peroxides are salts of inorganic per acids including persulfates, perborates or percarbonates, e.g., ammonium or potassium persulfate and hydrogen peroxide. The catalyst may be used in amounts required to bring about polymerization at any desired rate with suitable ranges being from 0.001 to 0.5 parts by weight per 100 parts of polymerizable monomer.

The usual modifiers or other agents may be present in the emulsion. For instance, the polymerization may be carried out in the presence of sulfur to produce a sulfur modified polychloroprene. Also, chain-transfer agents may be employed such as the alkyl mercaptans, e.g. dodecyl mercaptan, iodoform, benzyl iodide and dialkyl xanthogen disulfides, e.g., diisopropyl xanthogen disulfide and polysulfide compounds, e.g., i-propyl (O-ethyl dithiocarbonoxy) sulfide. Preferred modifiers are mercaptans and dialkyl xanthogen disulfides with from 0.10 to 3 parts by weight per 100 parts of total monomer with the range of about 0.25 to 0.6 being especially preferred.

A particularly valuable group of chain-transfer agents are the polysulfide compounds noted above. The polysulfides have the formula ##STR1## wherein X, Y and Z are selected from the group consisting of oxygen and sulfur, R.sub.5 is a hydrocarbon radical having from 1 to 8 carbon atoms, R.sub.6 is a hydrocarbon radical having from 1 to 8 carbon atoms or is a radical

the same as ##STR2## are selected from the group consisting of hydrocarbon radicals having from 1 to 8 carbon atoms or hydrogen with at least one of R.sub.7 and R.sub.8 not being hydrogen and n is 1 to 4. Compounds of this type may be formed, e.g., as disclosed in Twiss, D., JACS 49, February 1927, p. 491-494. The hydrocarbon radicals can be acyclic, cycloaliphatic, aromatic or combinations thereof. For example, the radicals can be methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, n-pentyl, 2-ethyl hexyl, cycloaliphatic radicals having 5 or 6 carbons, aryl radicals such as phenyl, alkaryl radicals such as methylphenyl, combinations thereof and so forth. Examples of suitable polysulfide compounds are di(O-ethyl dithiocarbonoxy) sulfide, di(O-ethyl dithiocarbonoxy) disulfide, isopropyl (O-ethyl dithiocarbonoxy) disulfide and so forth. Particularly useful polysulfides are those having the general structure ##STR3## wherein R.sub.5 and n have the significance noted above. Particularly preferred are polysulfides of the structure ##STR4## wherein n is 3 to 4, such as di(O-isopropyl dithiocarbonoxy) sulfide.

The usual methods may be employed to prepare an aqueous emulsion of the monomeric material and emulsifying agent and water. The proportions are not critical, but generally, the monomer will be present in an amount such as from 30 to 60 percent by weight based on the total weight of the polymerization system. The temperature of polymerization may be varied with suitable ranges being from 0.degree. to 90.degree. C with the preferred range being between 15.degree. C and 55.degree. C. The polymerizations may be short stopped by the addition of agents such as paratertiary-butyl catechol and thiodiphenylamine. The present invention is particularly useful for the preparation of the so-called "high solids" latex. The polymers prepared according to this invention are also useful for the preparation of sol-gel blends, such as described in U.S. Pat. Nos. 3,042,652; 3,147,317; and 3,147,318. Thus, the present conversion of monomer is usually from about 60 to 90 or 100%, i.e., at least 60%.

In these examples, conventional emulsifiers are compared to a conjugated triene. The polymerizations were carried out under a nitrogen blanket at 45.degree. C. These conversions were carried to 100%.

Several latexes were prepared and tested as to both cured and uncured tensile strength.

Dipped films are prepared in the following manner. The latex is compounded in the following recipe:

The components, with the exception of the polymer, were milled in a ball mill and added to a latex of the polymer with stirring. A form, generally a glass plate, is immersed in a coagulant solution consisting of 70 parts of absolute ethanol and calcium nitrate tetrahydrate. The form is withdrawn from the coagulant solution and immersed in the compounded latex for 2 minutes. The form coated with the latex is removed from the latex and reimmersed in the coagulant solution for 2 seconds. Immerse the forms in cold water for 10 seconds to remove surface coagulant. The coagulated film is stripped from the form and leached at 40.degree. C for 2 hours in demineralized water. The leached film is dried for 4 hours at 45.degree. C (tested at this point for no-cure properties). The dried samples are cured for 30 minutes at 140.degree. C and tested for cured properties. Test results for uncured and cured samples are set out below in Table I.

In these examples tung acid was employed in a one-shot high solids latex recipe.

These examples are all run in a 5 liter glass reactor equipped with a paddle agitator and are run under a nitrogen atmosphere at 45.degree. C. In all of these examples no catalyst is charged to the reactor initially, but a mixture of 0.25% potassium persulfate, 0.05% silver salt (sodium-2-anthraquinone sulphonate) and 99.7% water is fed until the total monomer has reached 80% conversion at which time the pumped catalyst mixture is 4.5% potassium persulfate, 0.1% silver salt, 95.4% water. Polymerization is controlled to achieve about 95% conversion at 6 hours with the final percent conversion being about 99.5% after about an additional 4 hours. In the case of the surfactants or additives, the parts are based on the surfactants as received from the manufacturer.

The latexes were tested in both the cured and uncured form for tensile strength improvement. The cured samples were prepared as described in the prior examples. The results of testing are set out below in Table II.