Dithiophosphate composition and utility in rubber

The present invention relates to polymer compositions and methods for producing the composition. More specifically, the polymer compositions comprise novel polymeric dithiophosphates.

Dithiophosphate accelerators have been used as sulfur donors in the vulcanization of diene. As compared with conventional accelerators, they offer advantages such as improved reversion of natural rubber, heat stability of the vulcanizates or rubber produced, and high solubility in rubbers of varying polarity for curing and producing a product without bleed of additive (“non blooming”).

Commercially available dithiophosphate additives are (1) “Accelerator ZIPPAC” from Akrochem. ZIPPAC is a zinc-amine dithiophosphate complex dispersed in a polymer matrix. ZIPPAC, when used in combination with thiazoles and thiurams, gives fast cure rates to ENB type EPDM's. Compared to Akrochem's other thiophosphate accelerator, Accelerator VS, ZIPPAC provides a much faster cure. ZIPPAC cures exhibit high modulus and low compression set, due to a high degree of crosslinking. Vulcanizates are usually non-blooming. Hot air aging properties are usually better than those obtained with other thiophosphate accelerators. ZIPPAC does not form harmful nitrosoamines (Zippac forms only unstable primary amines that rapidly decompose) and is a useful accelerator in low nitrosoamine generating formulas where its high crosslink density and fast cure overcome normal problems associated with safe nitrosoamine cure systems; (2) Royalac 136 from Crompton-Uniroyal Chemical is a zinc thiophosphate (zinc phosphorodithioate); (3) Meramid P a zinc dialkyl dithiophosphate under CAS number 68457-79-4; (4) Deovulc EG 187 from D.O.G. which is described as a synergistic combination of Thiazole- and basic accelerators together with a zinc dialkyldithiophosphate (ZDTP) as the main components; (5) Vocol ZBPD from Flexsys is described as Zinc O,O-Dibutylphosphorodithioate. Elastomer and utilized in NR, EPDM, LATEX rubber and non-blooming accelerator for EPDM cures and also improves reversion resistance in NR; (6) VANAX® 196 from R. T. Vanderbilt Company, which is a dithiophosphate disulfide; (7) Rhenocure AP-6 from Rhine Chemie, which is described as dithiophosphate polysulfides; (8) Rhenocure® TP/S from Rhine Chemie which is described as a Dithiophosphate, sulfur source; (9) Rhenocure® SDT/S from Rhine Chemie which is described as a phosphoryl polysulfide, sulfur source; (10) Rhenocure SDT/G from Rhine Chemie which is also is described as a phosphoryl polysulfide, sulfur source; and (12) Rhenogran® SDT-50 from Rhine Chemie which is described as a polymer bound phosphory polysulfide.

Nitrosoamine reduction in rubbers has become important due to the carcinogenic properties linked to nitrosoamines. Dithiophosphate accelerators have a renewed interest due to lack of nitrogen in the chemical structure, which is pre-requisite of nitrosoamine formation. A common expression for having better environmental properties is “green” and efforts are in way to produce more and more green additives.

One concern over dithiophosphate technology is that although it can not form harmful nitrosoamines, the technology has the ability to be a toxin. Malathion is a widely available dithiophosphate insecticide. It is not only toxic to insects but to a wide variety of animal life (The Ortho Group, “Environmental Hazards: This pesticide is toxic to fish, aquatic invertebrates, and aquatic life stage of amphibians.”). Other references to toxic dithiophosphates are provided in U.S. Pat. No. 2,725,327 Nov. 29, 1955, William R. Diveley; et al., which describes Pestisidal composition containing 2-P-Dioxanethiol S—(O,O-dialkylphophorodithioate) and U.S. Pat. No. 2,828,241 Mar. 25, 1958, Gail H. Birum; et al. which describes O,Odialkyl S-arylmercapto phosphorodithioates compositions and method of destroying insects.

It is generally accepted that polymeric materials offer improved and lower toxicity over their monomeric forms. Bonding an additive to a polymer (carrier) would afford improved environmental safety but dilutes the activity of the molecule by non active polymer (as related to rubber cure). It has been unexpectedly discovered that forming polymeric dithiophosphates via bi- or polyfunctional alcohols (utilization of di- or polyol alcohol within the reaction with phosphorous pentasulfide to form the oligomer or polymeric dithiophosphoric acid) can be oxidized to oligomeric or polymer disulfides and subsequently to polysulfides. Alternatively the dithiophosphoric acid can be reacted with metal hydroxides or oxides. The resulting product will function as an “additive” which can offer high activity, excellent vulcanization and/or stabilization properties to rubbers and offer improved handling and environmental (“green”) consideration due to their polymeric additive form (as compared to the monomeric type additives).

Dithiophosphate disulfides, dithiophosphate polysulfides, and zinc dithiophosphates are known accelerators, and their chemistry is illustrated below.

Polymers are widely and generally considered low or non-toxic as compared to monomeric or non-polymeric forms primarily due to their higher molecular weight (polymeric properties) which reduces volatility and the likelihood that they will cross biological membranes. Additionally these oligomeric or polymeric disulfides can be converted to the polysulfides by reaction with elemental sulfur and the polysulfides are part of the diversity of the invention. Dithiophosphates are utilized in many elastomers which include natural rubber (NR), ethylene propylene diene monomer rubber (EPDM), EPDM/butyl and Latex cure (NR Latex and Chloroprene Latex), polybutadiene resin and styrene-butadiene rubber stabilization.

It is known that dithiophosphates are made by the reaction of alcohols with phosphorous pentasulfide to produce the dithiophosphoric acid. It is common practice to use one or multiple alcohols to obtain the desired properties. Additionally it is common to make ammonium or zinc salts thereof or alternately to make the di- or polysulfide thereof. Typically the disulfide is made by oxidative coupling of the dithiophosphoric acid. The polysulfide analog can be made by sulfur insertion into the disulfide which elemental sulfur typically achieves in many analogs.

One embodiment of the present invention utilizes diol in combination with alcohols (mono-alcohols), reacted with the phosphorous pentasulfide to produce polydithiophosphosphoric acid with higher molecular weight. Oxidative coupling of the thiol group of the dithiophosphoric acid produces the disulfide linkage, essentially multiplying the molecular weight of the oligomer by the extent of disulfide coupling. The molecular weight of the product can be controlled by varying the ratio of the mono alcohol to the diol. It is believed to be advantageous to eliminate all of the dithiophosphoric acid which at the same time forms higher molecular weights. Additionally any part of the disulfide moieties can be converted to the di-, tri-, tetra-, or poly-sulfide (or mixtures thereof), by elemental sulfur insertion (typically done by heating with disulfide with elemental sulfur at sufficient temperature to induce the reaction).

In an another embodiment of the present invention, the mono-alcohol is substituted with a second diol of the same or different chemical composition as the first diol.

In yet another embodiment of the present invention, the polydithiophosphoric acid can be treated with a metal oxide or hydroxide to produce a polymeric metal dithiophosphate. This type of material will likely be a useful replacement for other metal dithiophosphate accelerators because of its improved safety and stability due to its polymeric structure. Suitable metal oxides or hydroxides of the present invention include oxides and hydroxides of Bi, Ca, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, Sb, Sr, Te and Ti. Preferably, Zn oxide or hydroxide is used in the reaction to form polymeric zinc dithiophosphate.

The following reactions illustrate the invention with a diol where R′ represents the residual di-radical.

Where, moles Mono Alcohol Used=2n+4 and moles Diol=n. This affords a ratio Mono Alcohol to Diol=r=(2n+4)/n, or n=4/(r−2). The length of the polymer is determined by the ratio of the mono alcohol to the diol.

It is envisioned that bis dithiophosphate salts can be made from the bis-dithiophosphoric acid shown above with alkali metal hydroxides, and would be useful as a latex additive. An example with the sodium salt is shown below.

Other types of products are also likely to be present since one phosphorous atom can also react with two diols, giving the following type of dithiophosphoric acid. In this example, 1,5-pentanediol is illustrated as the diol.

This will make each chain shorter, but will link chains together, keeping the molecular weight the same.

Polyols (versus diols) produce polymers as well and are envisioned in the subject invention but are less preferred as high crosslinking can produce handling problems as well as reduced activity.

Since the invention is utilized in vulcanization, it is particularly advantageous for the additive to be soluble in the rubber it is utilized in. Typically, for oil soluble rubber monomer, this would be done with alcohols of 4 or more carbons. However, it is also envisioned that lower carbon chain alcohols can be used in combination with higher chain alcohols. Water based polymers (i.e. latex) could either utilize the lower alcohols or an emulsion of the oil products so as to afford a material that could function in latex vulcanization. Water soluble polymers may also be made using diols alone. Of particular utility as diol in the invention, are ethylene glycol, propylene glycol, 1,4 butanediol, 1,5-pentanediol, 1,6-hexanediol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, 2-ethyl-1,3-hexanediol, 2-methyl-2,4-pentanediol, 1,8-octanediol, 1,10-decanediol, neopentyl glycol and cyclohexanedimethanol as the diol in the invention. These are economically available in bulk and produce the desired product when used in combination with the appropriate mono alcohol.

The invention may be prepared by utilization of diols in dithiophosphoric acid in known production methods (intermediate to salts or sulfides) or by methods of transesterification of either metal, amine salts, or mixtures thereof (e.g. zinc or ammonium salts are commercially available) or the sulfide (di-, tri-, tetra-, or polysulfide) with the diol (also commercially available) or mixtures of salts and sulfide here above mentioned. The invention may also be made by reaction of the diol/mono alcohol mixture with phosphorous pentasulfide to form the dithiophosphoric acid mixture. This can then be treated with known oxidizing agents to form the disulfide.

In the preferred invention represented by structure 1, R′ represents a di- or polyol, or mixtures thereof (diol only shown) where the residue of the diol or polyol may be an alkylene, alkylaryl, or aryl group that contains 1 to 22 carbon atoms. Pendant functionality does not affect the crosslinking of the diol or polyol. R1 through R6 are independently selected form an alkyl group containing 1 to 18 carbons, a hydroxyalkyl group, an alkylether, a polyalkylether, a hydroxyalkyl group, or a functional group of any of these types which is linked to a phosphorous atom in another polymer chain. Any of the disulfide may be higher sulfide (e.g. tri, tetra, etc) up to an octasulfide, and B may be 2 or more units, with 4 or more units preferred (European registration polymer exemption, is based on “4 or more” units). The value of B is only limited by the ease of handling of the product.

Study 1 of Polymeric Dithiophosphate Analogs Vs. Prior Art Dithiophosphate, VANAX® 196 in Natural Rubber

The cure characteristics of samples 2-4 and sample 6 were evaluated in Natural Rubber. The components that were used in the masterbatch are shown in Table 2. The mass percent of all the products that were tested was 1.5%. Samples 2-4 and sample 6 were all compared to VANAX 196.

As shown in Table 3, the cure properties of samples 2-4 and 6 are similar to those of VANAX 196. All of the products had similar cure rates and maximum torque values.

The Mooney Viscosity and Mooney Scorch are shown below in tables 4 and 5.

The physical properties obtained from samples 2-4 and 6 were also very similar to those of VANAX 196. Table 6 shows the tensile properties of the cured rubber that were obtained from samples 2-4 and 6. All of the cured products show a similar modulus, tensile strength, and percent elongation at break to the material that was cured with VANAX 196. The hardness of the rubber was also similar, as shown in Table 7. The rubber deterioration properties are shown in Table 8.

The polymeric additives give a low compression set in Natural Rubber which is close to the value obtained from VANAX® 196. The results are shown in Table 9.

The cure properties of the calcium, zinc, copper, and nickel dithiophosphates were also evaluated in Natural Rubber. The master batch that was used in shown in Table 10.

The cure characteristics of the metal dithiophosphate polymers are shown in Table 11. The results show that the Zn, Ca, Cu and Ni dithiophosphates all have a faster cure rates than a product without a polymeric metal dithiophosphate. In addition, based on the maximum torque obtained, the metal polymeric dithiophosphate additives all show a higher state of cure.

The additives that contained zinc or nickel show an improved compression set when compared to a product without a metal dithiophosphate. The results are shown in Table 12.

Study 2 of Polymeric Dithiophosphate Analogs Versus Prior Art Dithiophosphates VANAX® 196S and Vocol ZBPD in EPDM

The cure properties of the products were also evaluated in EPDM. Four tests were done, and the products were compared with VANAX 196 or both VANAX 196 and Vocol ZBPD. The master batch components are shown in Table 13.

Using test method ASTM D5289 at 160° C., sample 5 showed a slower cure rate than VANAX® 196 and Vocol ZBPD, but had a similar maximum torque. When sample 4 was tested using test method ASTM D5289 at 177° C., this also showed a slower cure rate than VANAX 196, but also showed a similar maximum torque value.

Samples 2 and 3 were also compared with VANAX 196. Sample 2 showed a T'90 value that was slightly longer than that of VANAX 196. Sample 3 was significantly longer. Thus, in EPDM the cure rate of the additive can be adjusted by varying alcohols that are used to make it. Both 2 and 3 showed a maximum torque value that was similar to that of VANAX 196. The results are shown in Table 14. Sample 6 showed results that came the closest to matching VANAX 196. It showed an equivalent T'90 value and only a slightly lower maximum torque value.

Table 15 shows that all of the samples other than sample 4 showed Mooney Viscosity values that were similar to the ones obtained from VANAX® 196. Sample 4 was somewhat higher.

The Mooney Scorch times are shown in Table 16. Sample 5 was between that of Vocol ZBPD and VANAX 196. Samples 2-4 and 6 were close to that of VANAX 196.

The physical properties are shown in Table 17. Sample 5 shows a similar 100% modulus and tensile strength to that of VANAX 196 and Vocol ZBPD. The percent elongation at break is also very close. Sample 4 shows a slightly lower 200% modulus, tensile strength and percent elongation at break than VANAX® 196. Sample 2 also shows a slightly lower 200% modulus and tensile strength than VANAX 196, but has a higher percent elongation at break. Sample 3 shows a 200% modulus that is very close to that of VANAX 196, but has a lower tensile strength and percent elongation at break. Sample 6 shows similar results to VANAX 196 but with slightly lower strength.

The hardness properties are shown in Table 18. Sample 5 shows similar properties to those of VANAX 196 and Vocol ZBPD. Samples 2-4 and 6 are also very similar to those of VANAX 196.

The rubber deterioration properties are shown in Table 19, and the compression set properties are shown in Table 20. The compression set properties are high when tested for 70 hours at 125° C., but drop significantly when tested for 22 hours at 100° C. The values that were obtained are similar to the ones obtained from VANAX® 196 or Vocol® ZBPD. The tear strengths of the products are shown in Table 21.

Emulsions of VANAX 196 Liquid, Lots RJT-554-54, and RJT-554-56 were used in these examples.

Films were cured at 100° C. for 20 and 30 minutes. Films were also heat-aged for 7 days at 70° C.

All the films created using VANAX® 196, RJT-554-54, and RJT-554-56 met acceptable standards both before and after aging with the exception of #7 vulcanized at 100° C. for 20 minutes. This film was under cured based on the high elongation and low tensile strength. The longer cure time of the second set of films fixed this problem.

Chloroprene Example:

Films were cured at 120° C. for 35 minutes. Films were heat-aged for 22 hrs at 100° C.

The films made using this cure system met acceptable standards when vulcanized for 35 minutes. The results at this cure time are excellent both before and after aging. Additionally films made from Compound 12 clearly show the importance of a good AO system such as VANOX SPL Slurry. These films are the only ones that showed a decrease in tensile and elongation after aging.