Method of curing rubbers without forming nitrosamines

Abstract: Branched and cyclic N,N-dialkyldithiocarbamyl accelerators for the sulfur vulcanization of rubber have been found to generally perform as well as the corresponding straight chain N,N-dialkyldithiocarbamyl accelerators, but surprisingly do not produce detectable amounts of environmentally undesirable N-nitrosamines. The branched N,N-alkyldithiocarbamyl accelerators are generally represented by the formula: where R is a branched alkyl group, R' is an alkyl group which may or may not be branched or a cyclic alkyl group or an alkaryl or aryl group, X is another thiocarbamyl group of the same general structure, a primary alkylamino group, an alkylthio group, a 2-benzothiazyl group, or a metal ion, and n is an integer of from 1 to 6. The cyclic dithiocarbamyl accelerators of the invention are generally represented by the formula where X is another thiocarbamyl group of the same general structure, a primary alkylamino group, an alkylthio group, a 2-benzothiazyl group, or a metal ion, and n is an integer of from 1 to 6.

Description: FIELD OF THE INVENTION The invention relates to an improved process and accelerator system for vulcanizing rubber. BACKGROUND OF THE INVENTION Rubber compounds are typically vulcanized using various amine containing accelerators such as sulfenamides, thiocarbamyl sulfenamides, thiuram mono- or disulfides, dithiocarbamates, etc. These accelerators are generally based on secondary di-n-alkylamines and aliphatic cyclic amines which are known to generate high amounts of the corresponding N-nitrosodialkylamines. Recent worldwide environmental concerns have stimulated efforts toward finding suitable alternative accelerator systems for abating or eliminating the formation of N-nitrosodialkylamines during the vulcanization of rubber products. Although a wide variety of thiuram disulfides, based on secondary amines, have been found to be accelerators, none have been shown to provide for excellent curing characteristics without the formation of N-nitrosamines. The use of alkaline earth oxides and hydroxides in conjunction with secondary amine containing accelerators and conventional vulcanizing agents to reduce nitrosamine levels by at least 20 percent and up to as much as 95 percent has been disclosed by U.S. -A-5,070,130 to Chasar. EP-A-0 242 432 discloses sulfenamide accelerators for tire tread rubber compositions comprising at least 60 phr SBR. EP-A-0 479 526 discloses the structural formula of tetra-(2-ethyl heptyl)thiuram disulfide and rubber compositions containing said compound. EP-A-0 562 750, which constitutes a prior art disclosure according to Article 54(3)/(4) EPC discloses tetra alkyl substituted thiuram disulfides in which the alkyl groups contain 7 to 12 carbon atoms and which may be straight or branched. Furthermore rubber compositions containing these compounds as the accelerator are disclosed. SUMMARY OF THE INVENTION It has unexpectedly been discovered that dithiocarbamyl compounds derived from dialkylamines, wherein at least one of the alkyl groups is branched or wherein the two alkyl groups along with the nitrogen of the dialkylamine form a seven member ring, can be used as accelerators for the sulfur vulcanization of rubber without the formation of environmentally undesirable nitrosamine compounds. The accelerators of the invention, in addition to substantially eliminating the formation of nitrosamines, have excellent curing characteristics such as cure rate and scorch- safety and produce a vulcanizate having good physical properties such as stress-strain properties and aging properties. The branched dialkyldithiocarbamyl accelerators of the invention can be used as the sole accelerator during the vulcanization of sulfur curable rubber compositions or may be used in combination with conventional accelerators such as mercaptobenzothiazole and its derivatives to provide an accelerator system having a desirable balance of low cost, low nitrosamine formation, good curing characteristics and good vulcanizate properties. In detail the present invention relates to a composition comprising an unsaturated sulfur vulcanizable rubber, a sulfur vulcanizing agent and an accelerator selected from wherein R is a branched alkyl group having 4 to 6 carbon atoms, R' is an alkyl group having 3 to 8 carbon atoms which may or may not be branched or an alkaryl or aryl group having 6 to 12 carbon atoms or a cyclic alkyl group having 5 to 12 carbon atoms, wherein R" is a branched alkyl group having from 4 to 6 carbon atoms, and R"' is a branched alkyl group having 3 to 8 carbon atoms, a linear alkyl group having up to 8 carbon atoms, a cyclic alkyl group having from 5 to 12 carbon atoms or an alkaryl or aryl group having from 6 to 12 carbon atoms, and n is an integer of from 1 to 6; and wherein X is a second thiocarbamyl group of the same general structure and n is an integer of 1 to 6. Furthermore, the invention relates to a rubber vulcanization process comprising the steps of compounding a composition comprising a sulfur vulcanizable unsaturated rubber, sulfur vulcanizing agent, and an accelerator having the formulae mentioned above and forming and vulcanizing the composition in a desired shape. In accordance with a particularly preferred aspect of the invention, it has been unexpectedly found that N,N,N',N'-tetraisobutylthiuram monosulfide, when used with mercaptobenzothiazole or its derivatives as vulcanization accelerators, in addition to eliminating nitrosamine formation, acts as a retarder and increases scorch safety. DESCRIPTION OF THE PREFERRED EMBODIMENTS Generally, the branched dialkyldithiocarbamyl accelerators of the invention can be used in sulfur vulcanizable natural rubber, any sulfur vulcanizable synthetic rubber, or in mixtures thereof. Natural diene rubbers which can be used in association with the invention include Hevea rubber, guayule, and other naturally occurring elastomers. Synthetic polymers which can be used with the accelerators of the invention include polymers and copolymers made from conjugated dienes generally having from 4 to 12 carbon atoms, with specific examples including butadiene, isoprene, pentadiene, hexadiene, heptadiene, octadiene, 2,3-dimethyl-1,3-butadiene and 2-methyl-1,3-pentadiene, homopolymers or copolymers of conjugated diene monomers with one or more comonomers such as styrene, alphamethylstyrene, vinyl toluene, acrylonitrile, isobutylene, or carboxylic acids and esters such as maleic, fumaric, itaconic, acrylic and methacrylic acids, and methylmethacrylate. Block copolymers such as diblock and triblock, as well as various graft copolymers of the above can also be utilized such as polystyrene-butadiene and acrylonitrile-butadiene. Other synthetic polymers which can be utilized include ethylene-propylene terpolymers (EPDM) such as ethylene propylene and dicyclopentadiene, 1,4-hexadiene or ethylidene terpolymers; and chlorinated polymers which possess unsaturation such as polychloroprene. Conventional crosslinking or vulcanization agents suitable for use with the invention include elemental sulfur and certain sulfur-donating organic compounds which are capable of crosslinking vulcanizable rubber. Examples of organic sulfur-donating compounds which can be used include caprolactam disulfide, alkyl phenol disulfides, thiuram di- and polysulfides, and thiocarbamyl sulfenamides. The sulfur-donating agent is employed at levels ranging from 0.1 parts to 8 parts by weight per 100 parts by weight of sulfur vulcanizable rubber. More preferably, the level of sulfur-donating agent is from 1 to 4 parts by weight. Sulfur is the preferred crosslinking agent; however, an organic sulfur-donating agent can be used alone or with sulfur. The amount and type of sulfur-donating agent and/or sulfur used is primarily dependent on the properties desired in the vulcanized rubber and can be readily ascertained without undue experimentation by those skilled in the art. Preferably, in the first category of accelerators in accordance with the invention, i.e., the thiuram mono-, disulfides and polysulfides represented by the formula: the cyclic alkyl group has from 5 to 6 carbon atoms, the alkaryl or aryl group has from 6 to 9 carbon atoms and more preferably 6 or 7 carbon atoms, and n is an integer 1 or 2. Examples of thiuram mono- or disulfide accelerators in accordance with the invention include: N,N,N',N'-tetraisobutylthiuram disulfide, and N,N,N',N'-tetraisobutylthiuram monosulfide In accordance with a particularly preferred aspect of the invention, it has been unexpectedly discovered that, contrary to typical thiuram disulfides which, when added to benzothiazole accelerators to increase cure rate also tend to decrease scorch safety, N,N,N',N'-tetraisobutylthiuram monosulfide can be used with mercaptobenzothiazole or its derivatives to accelerate cure without forming nitrosamines and also, surprisingly, act as retarders and increase scorch safety. Preferably, in the second category of accelerators according to the invention, i.e., the alkylthiodithiocarbamates represented by the formula: the cyclic alkyl group has from 5 to 6 carbon atoms, the alkaryl or aryl group has from 6 to 9 carbon atoms and more preferably 6 or 7 carbon atoms, and n is 1 or 2. Examples of alkylthiocarbamate accelerators which can be utilized in accordance with the invention include: cyclohexyl N,N-diisobutyldithiocarbamyl disulfide and t-butyl N,N-diisobutyldithiocarbamyl disulfide. In each case, the accelerators in accordance with a first aspect of the invention are characterized as compounds containing the dithiocarbamyl moiety wherein at least one branched alkyl group, preferably isopropyl or isobutyl, is attached to the nitrogen of the dithiocarbamyl group. Surprisingly, it has been found that such branched dialkyl dithiocarbamyl compounds not only perform well as accelerators but produce either no or very low levels of N-nitrosamines during the cure, whereas it is well known that the corresponding straight chained dialkyl dithiocarbamyl accelerators produce significant quantities of environmentally undesirable N-nitrosamines. Conventional straight chained dialkyl dithiocarbamyl accelerators typically produce from 50 ppb to 100 ppb or higher levels of N-nitrosamines by weight based on the total weight of the rubber, whereas the branched alkylthiocarbamyl accelerators of the invention typically produce less than 2 ppb of N-nitrosamines by weight based on the total weight of the cured rubber. The quantity of N-nitrosamines produced depends on the amount of and the particular accelerators used, their molecular weight, and whether they are being used as accelerators in conjunction with elemental sulfur or being used as sulfur donors in the absence of elemental sulfur. The amount of accelerator used in the sulfur vulcanization of rubber depends upon the components in the rubber formulation and can be readily ascertained without undue experimentation by those skilled in the art. The amount usually is within the range of 0.1 to 10 parts by weight per 100 parts by weight of vulcanizable rubber. The preferred amount of accelerator employed is generally within the range of from 0.1 to 5 parts by weight per 100 parts by weight of vulcanizable rubber, and more preferably in the range of from 0.2 to 2 parts by weight per 100 parts by weight of vulcanizable rubber. The branched alkyl dithiocarbamyl accelerators of the invention can be used either as the sole accelerator or they may be used in combination with each other or with conventional accelerators such as 2-mercaptobenzothiazole, 2-mercaptobenzothiazole derivatives and the like to provide an accelerator system having a desired balance of low cost, low nitrosamine formation, and the desired curing and vulcanizate properties. The total amount of accelerator for the mixed accelerator system is the same as set forth above, with the amount of branched dialkyldithiocarbamyl accelerator representing at least 10 percent and preferably from 20 to 70 percent by weight of the total amount of accelerator. In the cycloalkyldithiocarbamyl accelerators characterized by the general formula: wherein X is a second thiocarbamyl group of the same general structure, and n is preferably 1 or 2. The cycloalkyldithiocarbamyl accelerators are generally similar to the branched alkyldithiocarbamyl accelerators in accordance with the first aspect of the invention, except that the R and R' groups are replaced by a hexamethylene group which forms a seven member structure with the nitrogen atom of the compound. These cycloalkyldithiocarbamyl compounds, like the alkylthiocarbamyl accelerators of the invention, have been found to perform well as vulcanization accelerators and to produce either no or very low levels of nitrosamines during the cure, such as below 2 ppb by weight based on the total weight of the cured rubber. Specific examples of accelerators in accordance with this aspect of the invention include N,N'-bis(hexamethylene)thiuram disulfide and monosulfide. Conventional compounding agents commonly used by those skilled in the art may be incorporated in conventional amounts depending on the rubber used, the application, the physical properties desired, and the curative vulcanized products. Such agents include activators, retarders, vulcanizing and curing agents; protective materials including antioxidants, anti-ozonants, anti-blocking agents, anti-flex cracking agents, flame retarders, fungicides, germicides, antistatic agents, chemical and heat stabilizers; processing materials such as plasticizers and softeners, processing aids and tackifiers; extenders, fillers and reinforcing materials; and the like. Standard compounding ingredients often include zinc oxide, carbon black in an amount of from 25 to 150, desirably from 40 to 100, and preferably from 50 to 90 phr, sulfur, organic sulfur donors, oil, fatty acids, and the like. The accelerators may be prepared in known manner by reacting the appropriate amines and other moieties with carbon disulfide. The mixture of rubber, vulcanizing agent, accelerator, and other additives may be prepared in conventional mixing equipment such as a Banbury mixer or a roll mill according to standard practice, and vulcanized in a desired shape at elevated temperature under conventional vulcanizing conditions appropriate to the particular article being fabricated such as pneumatic tires or portions thereof. While the amount of N-nitrosamine formed when vulcanizing a vulcanizable rubber utilizing the accelerators of the invention can vary depending upon a number of factors, the amount of N-nitrosamine formed will nevertheless be less with the accelerators of the invention than for the corresponding conventional accelerators, and generally the cured rubbers formed using the accelerators of the invention will have a N-nitrosamine content of less than 20 ppb or 10 ppb, and typically less than 5 ppb or 2 ppb, and often less than 1 ppb by weight based on the total weight of the cured rubber. The following examples serve to illustrate the use of the invention, but do not serve to limit it in any way. Masterbatch of SBR/BR Rubber Parts by Weight Oil Extended Styrene-Butadiene Rubber 55.0 Styrene-Butadiene Rubber 25.0 cis-Polybutadiene 35.0 Zinc Oxide 3.0 Stearic Acid 2.0 Carbon Black 70.0 Petroleum-Based Oil 20.0 Antiozonant (N-1,3-dimethylbutyl-N'-phenyl-p-phenylenediamine) Agerite ® Resin (polymerized dihydroquinoline) 2.0 214.0 The ingredients of Table I were added in the order indicated and mixed in a Banbury mixer at approximately 132°C (270°F). To the above masterbatch formulation were added various accelerators and sulfur to produce examples 1-37 as set forth in Tables II through VII. All component amounts are expressed in parts by weight. For purposes of comparison between conventional accelerators and those of the invention, approximately equimolar amounts of accelerator were used for each of the Examples 1-4 of Table II. Similarly, for purposes of making a fair comparison, equimolar amounts of accelerator were utilized for the sets of examples appearing in Table III through VII. EXAMPLES 1-6 Various accelerators and sulfur were added as shown in Table II to the masterbatch set forth in Table I. The accelerators and sulfur were added to the masterbatch on the mill at 48.9°C (120°F). The results shown in Table II demonstrate that the accelerators of the invention (tetraisobutylthiuram disulfide and N,N'-bis(hexamethylene)thiuram disulfide) exhibit good cure characteristics without generating detectable levels of nitrosamine whereas conventional accelerators (tetramethylthiuram disulfide and tetraethylthiuram disulfide) generate undesirably high levels of nitrosamines. The nitrosamines were analyzed by a gas chromatograph coupled to a thermal energy analyzer, which is specific to nitrosamine detection. Standards of nitrosamines were used to calibrate the instrument before each experimental sample. The lower detection limit for nitrosamines was about 2 ppb or less by weight based on the total weight of the rubber. Examples 7-11 were prepared in accordance with the procedures used to prepare Examples 1-6, using a masterbatch as set forth in Table I. As shown in Table III, known benzothiazole sulfenamide accelerators (examples 7 and 11) were compared to two thiuram disulfides of the invention (Examples 8 and 9) and to a conventional thiuram disulfide (Example 10). The results listed in Table III show that known benzothiazole sulfenamides and conventional thiuram disulfides produce very large quantities of nitrosamines as compared to the branched alkyldithiocarbamyl accelerators of the invention. Table III also demonstrates that when a secondary branched amine is on a sulfenamide group, it will form nitrosamine as in Example 7, but when a secondary branched amine is incorporated into a dithiocarbamyl group it will not form nitrosamines during the cure as shown in Examples 8-10. Table IV shows that a variety of dithiocarbamyl compounds in accordance with the invention do not produce detectable quantities of nitrosamines, yet exhibit good cure properties. Table V shows that the branched dialkyldithiocarbamyl compounds of the invention have excellent cure properties even when used without sulfur. Tables VI and VII show that the thiuram mono and disulfides produce orders of magnitude lower levels of nitrosamine relative to that generated by the conventional unbranched tetramethylthiuram disulfide accelerator.

Claims: A composition comprising an unsaturated sulfur vulcanizable rubber, a sulfur vulcanizing agent and an accelerator selected from wherein R is a branched alkyl group having 4 to 6 carbon atoms, R' is an alkyl group having 3 to 8 carbon atoms which may or may not be branched or an alkaryl or aryl group having 6 to 12 carbon atoms or a cyclic alkyl group having 5 to 12 carbon atoms, wherein R" is a branched alkyl group having from 4 to 6 carbon atoms, and R"' is a branched alkyl group having 3 to 8 carbon atoms, a linear alkyl group having up to 8 carbon atoms, a cyclic alkyl group having from 5 to 12 carbon atoms or an alkaryl or aryl group having from 6 to 12 carbon atoms, and n is an integer of from 1 to 6; and wherein X is a second thiocarbamyl group of the same general structure and n is an integer of 1 to 6. The composition of claim 1 wherein the branched accelerator is selected from N,N,N',N'-tetraisobutylthiuram disulfide, N,N,N',N'-tetraisobutylthiurammonosulfide, cyclohexyl-N,N-diisobutyldithiocarbamyldisulfide, t-butyl-N,N-diisobutyldithiocarbamyl disulfide, N,N'-bis(hexamethylene)thiuram disulfide or monosulfide, or combinations thereof. The composition of claim 2 further comprising 2-mercaptobenzothiazole or derivatives thereof. The composition of claim 3 wherein the amount of the accelerator is at least 10 percent by weight of the total amount of accelerators. The composition of anyone of claims 1 to 4 wherein the total accelerator amount is from 0.1 to 10 parts by weight per 100 parts by weight of vulcanizable rubber. A rubber vulcanization process comprising the steps of compounding a composition comprising a sulfur vulcanizable unsaturated rubber, sulfur vulcanizing agent, and an accelerator selected from wherein R is a branched alkyl group having 4 to 6 carbon atoms, R' is an alkyl group having 3 to 8 carbon atoms which may or may not be branched or a cyclic alkyl group having 5 to 12 carbon atoms or an alkaryl or aryl group having 6 to 12 carbon atoms, wherein R" is a branched alkyl group having from 4 to 6 carbon atoms, and R''' is a branched alkyl group having 3 to 8 carbon atoms, a linear alkyl group having up to 8 carbon atoms, a cyclic alkyl group having from 5 to 12 carbon atoms or an alkaryl or aryl group having from 6 to 12 carbon atoms, and n is an integer of from 1 to 6; and wherein X is a second thiocarbamyl group of the same general structure and n is an integer of 1 to 6; and forming and vulcanizing the composition in a desired shape. The process of claim 6 wherein the accelerator is selected from N,N,N',N'-tetraisobutylthiuram disulfide, N,N,N',N'-tetraisobutylthiuram monosulfide, cyclohexyl-N,N-diisobutyldithiocarbamyl disulfide, t-butyl-N,N-diisobutyldithiocarbamyl disulfide, N,N'-bis(hexamethylene)thiuram disulfide or monosulfide, or combinations thereof. The process of claim 7 wherein the composition further comprises 2-mercaptobenzothiazole or derivatives thereof. The process of claim 8 wherein the amount of the accelerator is at least 10 percent by weight of the total amount of accelerators. The process of anyone of claims 6 to 9 wherein the total accelerator amount is from 0.1 to 10 parts by weight per 100 parts by weight of vulcanizable rubber.

Inventors: CHASAR DWIGHT WILLIAM, LAYER ROBERT WESLEY, CHASAR, DWIGHT WILLIAM, LAYER, ROBERT WESLEY
Applicants: GOODRICH CO B F, THE B.F. GOODRICH COMPANY
Application year: 1993
Main classification: C08K5
IPC codes: C08J3, C08L21, C08K5