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Persistent antioxidants for polymers contacting extractive media.

Persistent antioxidants for polymers contacting extractive media

Antioxidants have been used to protect rubber products from elevated temperature oxidation for over fifty years. Among the gamut of modern polymer types, NBR and CR represent two of the more demanding materials from the standpoint of need for protection. Both are extensively used in applications where operating temperatures are increasing and where they are subject to the effects of oils, fuels or other fluids which can extract the antioxidants used to protect them. In 1974, J. W. Horvath and co-workers described a new technique whereby antioxidant groups could be built into NBR during polymerization [ref. 1], through the use of polymerizable antioxidant monomers [ref. 2]. This technology is still one of the most effective, practical means for protecting NBR from the effects of its usage environment.

In a paper [ref. 3] presented in 1984, P. R. Dean II and coworkers studied the volatility and solubility characteristics of antioxidants as they affect performance and introduced N,N [prime]-bis (4-anilinophenyl)-3, 3 [prime]-thiodipropionamide as an additive antioxidant for NBR compounding. Unfortunately, this material is not presently commercially available and the available grades of polymerization-bound antioxidant NBRs are not always suitable for each specific application. Therefore, the need to understand the performance characteristics of appropriate commercially available antioxidants for use in polymers intended for contact with extractive media remains today. In this article various non-volatile, reasonably non-extractable commercial materials were compared in NBR and CR.

Experimental The two basic chemical classes of antioxidants most used in rubber goods are amines and phenols. Amines are usually the choice where high performance is required. Unlike the phenols, they do not become adsorbed on carbon black and although they stain in varying degrees, it is not often a requirement for rubber goods to be non-staining if they are to be used in demanding applications in contact with fluids. Further, most of the phenolic antioxidants tend to exhibit fairly high solubilities in even aliphatic hydrocarbon fluids. As a result, only amines were considered for evaluation as "primary" antioxidants in this study.

Substituted diphenylamines, diphenylamine reaction products, quinolines and p-phenylenediamines are the four sub classes of amine antioxidants containing the greatest number of commercial products. Volatility and solubility information for selected materials from these four classes can be found in reference 3. Also included in the listings are nickel dibutyldithiocarbamate (NBC) and several mercaptoimidazoles, the latter materials finding use as secondary antioxidants or synergists.

From the four sub-classes of amines, antioxidants were selected for evaluation which possess low solubility in organic fluids and which have low volatility. Table 1 lists the acronyms used to identify the materials tested along with their trade names and suppliers. One CR and two NBR formulations comprised the basic evaluations and are identified as follows:
 Formulation Data
 "Non-black" NBR test formulation Table 3
 Black NBR test formulation Table 4
 CR test formulation Table 5

Tables 7 and 8 contain data from work done using formulations intended for specific applications. Complete formulations for all of the stocks are listed in table 2.

All compounds reported in this work were mixed in a "00" variable speed laboratory internal mixer at 65 rpm using a two pass procedure. The polymer, mineral fillers and silane coupling agent were mixed for one minute prior to adding the balance of the nonproductive compounding ingredients. The sulfur and other curatives were added to a small portion of the batch on a laboratory mill and then mixed into the whole compound during a 2-minute second stage mix. Discharge temperatures were approximately 125 [degrees] C. Standard ASTM procedures were used for determining original, oven aged, fluid and sequence aged properties. The sequence agings consisted of fluid immersions followed by an oven aging and are useful for assaying the extraction and volatility characteristics of antioxidants. A 24 hour dryout at 24 [degrees] C was used between the immersion and oven aging periods where volatile immersion fluids were used. The solubilities of the antioxidants in test fluids were determined by ASTM method D-1766. Volatility characteristics were determined by a thermal analysis system, using a thermal gravimetric analysis (TGA) unit. This testing was conducted in a nitrogen atmosphere with a flow rate of 0.0425 standard cubic meter per hour. The programmed rate of temperature increase was 10 [degrees] C/minute and sample size was approximately 25 mg.

Results and discussion

"Non-Black" NBR test formulation (table 3)

This recipe is listed as being a "non-black" test formulation even though it contains 5 phr of N-990 (MT) carbon black as a tint. From a performance standpoint, it manifests the character of a heat resistant, mineral filled stock.

Table 3 lists the data for the eleven variations of the "non-black" NBR test compound. These are identified A through L (less I) and each contained a different antioxidant system as shown. Compound A contained no antioxidant added during mixing and since the polymer used contained only a mild non-staining phenolic stabilizer, it was considered as an essentially unprotected control. Compounds B and C contained octylated and styrenated diphenylamine antioxidants respectively. Octylated diphenylamine (ODPA) is a very commonly used additive of moderate volatility while styrenated diphenylamine (SDPA) represents a highly non-volatile material from the same class [ref. 3]. Neither of the alkylated diphenylamines was expected to be particularly non-extractable based upon their relative solubilities in organic fluids [ref. 3]. Compound D contained a mixed diaryl-p-phenylenediamine (DAPD). Although the p-phenylenediamines are better known for their antiozonant activity (not in NBR where no particularly good materials are known), they are extremely powerful antioxidants as well. Unlike their activity as antiozonants where high concentrations are required [ref. 6], as antioxidants they are effective even at levels of less than 1.0 phr [ref. 5]. Because of this and the low solubility of the diaryl analogs in organic fluids, these materials become prime candidates for use in compounds intended for fluid contact.

Compounds E and F both contained polymerized quinolines, F containing the material having the higher molecular weight. Compound G contained a low temperature reaction product of acetone and diphenylamine (A/DPA) and H, a commercial blend (blend-1) of a similar amine with mercaptobenzimidazole synergist. Compound J contained N,N [prime]-bis (4-anilinophenyl)-3,3 [prime]-thiodipropionamide (TDA). This material is not a commercial product but is recognized as being extremely non-volatile and non-extractable [ref. 3]. Compound K contained the p-phenylenediamine DAPD plus PPP another recognized synergist [ref. 5].

The final compound in the study, compound M, contained no antioxidants added during mixing but utilized a commercial polymer containing 1.6 phr of polymerized-in N- (4- anilinophenyl) methacrylamide (NAPM) and BTP added during finishing. This last compound represents the best protective system available commercially for NBR, short of hydrogenation. Compound M was included as a secondary control stock to represent a target to be approached by the stocks containing the compounded-in antioxidants.

Using normal procedures, the rheometer and Mooney scorch tests were run first to establish cure times and processing limitations. Cure times for slabs and other specimens were based upon the rheometer t [prime] 90 times.

Original stress strain and other vulcanizate physical properties were measured next. In order to characterize aging qualities, as shown in figure 1, two conventional 149 [degrees] C oven agings and #1 and #3 ASTM oil immersions were run in addition to four immersion/oven sequence aging tests run to project protection following contact with extractive media. Volume changes of the vulcanizates were determined where appropriate. Aging indices, averages of the % tensile strengths and elongations retained and hardness changes, irrespective of individual test conditions, were calculated for the stocks based upon the four sequence agings as shown in figure 2. These form a convenient indicator of the overall protection provided by the various protective systems.

The following observations were made from the data in table 3:

* Effects of all of the protective systems upon cure and scorch were minimal.

* BTP synergist, being used at higher than usual antioxidant levels produced a slight plasticizing effect.

* As expected, compound M, which contained the polymerization bound antioxidant was the best protected in all of the aging tests. Compound A, with no additive antioxidant displayed the poorest protection.

* ODPA proved to be too volatile and extractable to be of great value in protecting against the severe aging conditions employed.

* SDPA protected well in the 149 [degrees] C oven agings particularly in the 7 day test. As the solubility results suggested, its performance in the sequence tests was poor.

* A/DPA and DAPD performed the best of the non-synergized commercial systems.

* Synergists, whether added separately (as BTP) or part of a blend (blend-1) significantly improved protection in nearly all of the agings.

* The performance of the polymerized quinolines was disappointing in view of their widespread use.

Black NBR test formulation (table 4)

Black loaded NBR compounds do not age as well as those which are mineral filled. However, mineral filled stocks of the type just discussed can be difficult to process. Therefore, by far the majority of working formulations are carbon black filled. Table 4 presents a comparison of four of the better persistent primary antioxidants with and without a synergist in a modified black-loaded compound. The complete base formulation is found in table 2. Even though black, this recipe also contained amorphous silica and magnesium oxide which were included to improve heat resistance. A cure system utilizing 0.30 phr of elemental sulfur was employed to provide short, heat resistant cross-links. A compound mixed using a synergized polymerization-bound antioxidant polymer was again included as a well protected target. A control stock without added protective ingredients completed the series. With the exception of the recipe containing the polymerization bound antioxidant, the rubber used as a base for all compounds was one of the newer low mold fouling, low corrosive, FDA regulated grades. Since it was stabilized with a phosphite, all antioxidant protection resulted from the materials added during compounding. The synergist used was BTP [ref. 5], the same material used in the built-in antioxidant polymer. When the recipe contained BTP, the amount of polyester plasticizer was reduced by an appropriate amount based upon the BTP added.

The four primary antioxidants compared in the black loaded NBR compound were DAPD, PTMDQ-1, SDPA and A/DPA. All except SDPA were expected to resist extraction and all are very nonvolatile. Even though it was not expected to perform well in sequence tests, the SDPA was included because not all NBR applications require resistance to exhaustive extraction. The activity of SDPA is exceptional as an antioxidant in dry heat applications. As in the prior study, the first data in table 4 are rheometer and Mooney scorch. Sheet cures were selected to be approximately t'95 from the rheometer results. Scorch times for all of the compounds proved short; adjustment would be required in order to handle the stocks in a factory environment. Vulcanizate testing included original physical properties, compression set at two temperatures, dry oven agings at three temperatures, immersion exposures in #1 and #3 ASTM oils and two sequence fuel immersion/oven tests. The two sequence tests differed only in the immersion portion. While both employed ref, fuel C, in one test the immersion time was twice as long and the fluid was changed at midterm. As before, aging indices were calculated where they might simplify comparisons.

The following observations were made from the data in table 4:

* In the dry heat, circulating air oven tests, the SDPA and the built in NAPM provided the best retention of physicals.

* Even though small differences were observed, the ASTM oil immersion tests failed to reveal any dramatic comparisons.

* As expected, the compound mixed with the bound NAPM antioxidant polymer retained its physical properties the best after the sequence tests. DAPD and A/DPA again proved to be the best additive antioxidants, DAPD exhibiting the lesser effect when the fluid was changed, suggesting that it is either less extractable or more active at a reduced concentration.

* The effectiveness of PTMDQ-1 was drastically reduced by the fluid changing sequence.

* BTP significantly activated the diphenylamine based antioxidants but had little effect upon the p-phenylene-diamine or the quinoline. It is possible that the primary/secondary antioxidant ratio was incorrect to provide effective synergism.

CR test formulation (table 5)

Another diene polymer which contacts extractive media and requires the protection of antioxidants is polychloroprene (CR). Octylated diphenylamine is the antioxidant perhaps most commonly used to protect it. DAPD is frequently used in CR as an antiozonant, but its excellent antioxidant effectiveness is not as commonly recognized.

In CR, some interesting changes in relationships occur. For example, in most diene polymers N-(1,3-dimethylbutyl)-N [prime]-phenyl-p-phenylenediamine is recognized as the more effective antiozonant [ref. 5]; in CR, DAPD is more effective [ref. 6]. It is commonly used for this purpose at concentrations of 2-4 phr. However, these dosages are higher than the accepted optimal concentration as an antioxidant, which is about 0.75 to 1.0 phr as measured in other diene polymers [ref. 5]. As a result, the decision was made to evaluate DAPD at several levels. It was tested both with and without the inclusion of 2 phr of the synergist BTP. Four compounds where styrenated diphenylamine powder (SDPA-P) was used in blends and an unprotected control completed the series.

One of the problems compounders of CR face today is that of finding a good cure system. This is because of the questionable toxicology associated with the use of ethylenethiourea. In this study the cure system used was simply zinc oxide and rigorous cure conditions. Although perhaps a better system could have been used, antioxidant comparisons were possible and the results should be comparable in a more practical compound. The complete formulation again utilized a combination loading of carbon black and amorphous silica as given in table 2. Testing consisted of a rheometer and Mooney scorch tests as mixed, and after storage at 41 [degrees] C, original physical properties, dynamic ozone exposures, an oxygen bomb exposure, oven agings at two temperatures, #1 and #3 ASTM oil immersions and two sequence fuel immersion/dry oven tests (figures 5-7). Again one of the sequence tests employed a longer immersion time and, in this case several fluid changes.

The following observations were made in studying the data in table 5:

* ODPA appears sufficiently volatile to be lost in the more severe dry oven agings. It also appears to be readily extracted in the immersion and sequence tests.

* Styrenated diphenylamine (SDPA-P), although never included by itself (an oversight) performed well in combination. As before, it was readily extracted in the sequence tests. The combination of SDPA-P and BTP performed the best of all the systems in the dry oven agings.

* DAPD, as expected, exhibited good antioxidant effectiveness. 1.0 phr was better than 2.0 or 3.0 in nearly all of the aging tests, confirming work in other polymers. At normal antiozonant concentrations of greater than 2 PHR, however, reasonably good antioxidant protection was maintained.

NBR o-ring (table 6)

The data in table 6 is based on a study run to compare the effects of two additional synergists with the extraction resistant material, DAPD. The synergists were mercaptotoluimidazole (MTI), a well known, non extractable activator and N,N [prime]-bis-beta-(3,5-di-t-buty 1-4-hydroxyphenyl) propionylhydrazide (NDBHPH), a material sold commercially as a metal deactivator. A commercial blend and the diphenylamine derivative (SDPA) were included for comparison. Duplicate unstabilized controls and duplicate compounds mixed using the polymer stabilized with polymerization bound NAPM and added BTP completed the series. These duplicated controls provide for some comparison of the precision of the various tests run. This testing included low temperature tests in addition to the standard original and aged property tests in the previous studies. The sequence tests employed #1 and #3 ASTM oils at 135 [degrees] C as the immersion media as well as a "sour" (peroxide containing) gasoline at 40 [degrees] C. Aging indices were again calculated to provide for an overall comparison.

The following observations were made:

* The compounds containing the built-in antioxidant polymer performed marginally the best of those tested. Those containing DAPD and either of the synergists were close seconds.

* As in the other studies, the styrenated diphenylamine, SDPA, provided the best dry heat resistance of any of the stabilizers, even better than that of the polymerization bound antioxidants. As before, it exhibited extraction in the sequence agings.

NBR downhole oil well cable jacket

The final evaluation was that of DAPD, SDPA and zinc mercaptotoluimidazole (ZMTI) synergist in an oil well cable jacket. This evaluation only provided one unique comparison, that of the ZMTI by itself compared with DAPD and with the two in combination. A recipe mixed using polymerization bound NAPM and added BTP was also included. The testing protocol was similar to the previous studies. Results, too, were similar in that the polymer with the polymerization bound antioxidant performed best overall, with the DAPD/ZMTI compound second. Again SDPA performed well in dry heat but was extracted in the sequence tests. As might be expected, ZMTI by itself, performed poorly but improved the performance of DAPD. It is noteworthy that the ZMTI activated the cure system in the compound.


* Various extractive media encountered by NBR and CR compounds greatly affect the oxidation resistance after exposure. The more fluid changes occur, the greater can be the problem.

* To protect against oxidation following extractive media contact, antioxidants must be non-extractable. To protect against high temperature aging they must also be nonvolatile.

* Polymerization bound antioxidants provide the best overall protection for compounds exposed to both high temperatures and extractive media.

* Persistent substituted diphenylamine antioxidants as exemplified by styrenated diphenylamine, (SDPA), provide excellent dry high temperature protection for both NBR and CR. Octylated diphenylamines (ODPA) appear to be of limited value at higher temperatures, probably due to volatility. Indications are that they are also extracted by fluid contact.

* Materials having limited solubility in extractive media such as the diaryl-p-phenylenediamines (DAPD) or the higher molecular weight acetone diphenylamine reaction products (A/DPA) are the best additive antioxidants for vulcanizates contacting extractive media. The p-phenylenediamines may have an advantage because they perform well at lower concentrations, in some cases even better than at higher levels.

* Synergists can be effective in increasing the protection attainable by the primary antioxidants. The best synergist for a given primary may vary.

* Performance of the polymerized quinolines after immersion in extractive media indicates that even though they are polymeric, they can be extracted.

* In addition to being the most effective commercial antiozonant for CR, diaryl-p-phenylenediamine (DAPD) is also one of the most effective and persistent antioxidants. Although as an antioxidant it performs better at lower dosages, reasonable protection is still maintained at (higher) antiozonant levels. It does not benefit from the inclusion of other primary antioxidants such as the alkylated diphenylamines but can be improved by synergists.

Summary The effects of more persistent members of several classes of antioxidants have been evaluated in NBR and CR vulcanizates in high temperature oven tests with and without contact with extractive media. It has been confirmed that the most effective means of protecting against these conditions, overall, is through the use of antioxidants bound to the polymer chains. Short of this, non-volatile age resisters with limited solubility in the contact fluids provide good protection. Diaryl-p-phenylenediamines and high molecular weight acetone/diphenylamine reaction products are examples of the types of materials which are more effective. Alkylated diphenylamines of lower volatility are extremely effective in protecting against dry heat but are extracted by fuels and oils. The commonly used octylated diphenylamines are both extractable and too volatile to perform well in harsher exposures. The performance of the polymerized quinolines is exceeded by several of the other classes. Synergists are of value but appear to be specific to the type of primary antioxidant and the aging/service conditions encountered. [Tabular Data 1 to 6 Omitted] [Figures 1 to 7 Omitted]

References [1]J. W. Horvath, J. R. Purdon, G. E. Meyer and F. J. Naples, "PolymerizationStabilized NBR: A Significant Advance in Age Resistance via Nonextractable Antioxidants," Applied Polymer Symposium No. 25, 187-203 (1974). [2]R. H. Kline, "Polymerizable Antioxidants in Elastomers," presented at a meeting of the Rubber Division, American Chemical Society, Toronto, Ontario, Canada, May 7-10, 1974. [3]P. R. Dean II, R. W. Dessent, R. H. Kline and J. A. Kuczkowski, "Persistance Factors Influencing Antioxidant Performance in NBR," presented at the 126th meeting of the RubberDivision, Denver, CO, 1984. [4]B. N. Leyland and R. L. Stafford, Chem Can, November 1959, p. 45. [5]P. R. Dean II and J. A. Kuczkowski, "Enhanced Polymer Oxidation ResistanceThrough the Use of Secondary Antioxidants," presented at the 127th meeting of the Rubber Division, Los Angeles, CA, 1985. [6]D. E. Miller, R. W. Dessent and J. A. Kuczkowski, "Long Term Antiozonant Protection of Tire Sidewalls," presented at the 126th meeting of the Rubber Division, Denver, CO, 1984. [7]The Goodyear Tire & Rubber Company Publications: Tech Book Facts WS-16, 1975 and WS-22, 1976.
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Author:Dean, Paul R.
Publication:Rubber World
Date:Aug 1, 1989
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