Compounding EPDM for heat resistance.

Rubber articles are susceptible to aging. Just a small percent of oxygen combined with the polymer can seriously degrade the physical properties of a rubber part. Heat can greatly increase the rate at which oxygen reacts with the polymer; the rate approximately doubles for each 10[degrees]C increase in temperature. About a 50-fold increase in reaction rate occurs between room temperature and 70[degrees]C. Said another way, compared to changes observed at a service temperature of 150[degrees]C for EPDM, it would take about 8,000 times as long to see a similar change at ambient temperature.

Mechanism of aging

The aging of polymers (and people) occurs through a free radical process. Any form of energy--heat, light or mechanical stress can break polymer bonds to form carbon radicals to start the aging process. As shown in figure 1, the carbon radicals enter into a cyclic oxidation process. In the presence of oxygen, peroxy radicals are formed. The peroxy radicals can abstract a hydrogen from the polymer backbone to generate another carbon radical and a hydroperoxide. The hydroperoxide will decompose under heat to form two more free radicals: an alkoxy radical and a hydroxyl radical. Each of these radicals can abstract hydrogen from the polymer backbone to form yet more carbon radicals.

[FIGURE 1 OMITTED]

Each time through the cycle, additional free radicals are generated, suggested by the heavier arrows. Two free radicals can combine to terminate each other, which results in a new crosslink. The creation of new crosslinks leads to hardening and embrittlement. Typically, for these rubber polymers, including EPDM, the change in elongation is a most sensitive indicator of aging. Due to space limitations in this publication, only elongation change is presented, but the complete suite of properties measured can be obtained by contacting the authors to request the references listed.

A few polymers tend to undergo reversion or chain scission in the presence of free radicals. Scission is favored in polymers that are branched or have many side groups. During aging, tertiary alkoxy or carbon radicals are generated which undergo beta scission, or unimolecular cleavage to lower molecular weight fragments, which is called reversion. Natural rubber and butyl rubber are two polymers that degrade by reversion. Following the change in tensile strength, or the tensile X elongation product, is often used as the most sensitive indicator of aging in these polymers.

Mechanism of antioxidants

Antioxidants are materials that can interfere with any of the steps in the oxidation process, as indicated in figure 2. However, some steps are easier to circumvent than others.

Initiation really depends on the strength of the carbon-carbon bond. If the polymer absorbs too much energy, the bond will break. Only the structure of the polymer determines how much energy can be tolerated.

[FIGURE 2 OMITTED]

Also, carbon radicals (C *) are highly energetic, and few materials can effectively trap out these intermediates. Diaryl p-phenylenediamines can act as carbon radical traps.

Most conventional antioxidants either trap the oxy radicals or decompose the hydroperoxides (COOH). Radical naps for peroxy (COO *) and alkoxy (CO *) radicals are the familiar amines and phenols. Examples of peroxide decomposing antioxidants are phosphites and dithiocarbamates.

For maximum antioxidant effectiveness, inhibiting the oxidative process at more than one step can be very effective. Thus, a combination of two types of antioxidants--a radical trap and a peroxide decomposer--can frequently provide better protection against oxidative aging than either type of antioxidant alone.

Examples of alkoxy and peroxy radical trap antioxidants are hindered amines and phenols. The mechanism by which phenols work is shown in figure 3. A hindered phenol, when used alone, will react with two radicals. It then is essentially "consumed" and it will no longer protect the polymer. These degradative and protective reactions are a simplified overview. Quinone formation results from the reaction of an alkoxy radical at the para position of the phenol leading to color development due the conjugated diene structure.

[FIGURE 3 OMITTED]

Peroxide decomposing antioxidants can react directly with the polymer hydroperoxide as shown at the top of figure 4 for phosphites. The phosphite antioxidant becomes oxidized to a phosphate while the peroxide is reduced to a harmless alcohol. In addition, peroxide decomposing antioxidants are thought to be capable of reacting with the consumed phenolic antioxidant to regenerate its protective action. This is another way in which a combination of two antioxidants can provide longer service than either one alone. The inclusion of phosphites as a secondary antioxidant generally provides improved color compared to the use of phenols alone.

[FIGURE 4 OMITTED]

Figure 5 illustrates that hindered amine antioxidants undergo the same two basic reactions as their fellow radical trapping phenolic counterparts. In addition, the amines can enter into a cyclic process to continuously trap radicals without being consumed. This can yield a longer lifetime at elevated temperature, provided that the amine antioxidant is not lost by volatilization, extraction or other physical means.

[FIGURE 5 OMITTED]

Sulfur cured EPDM

Many sulfur cured EPDM compounds do not call for the addition of any antioxidant beyond that incorporated by the polymer producer. These recipes, particularly low sulfur or sulfur donor compounds, have reasonably good heat resistance without any added antioxidant.

Nevertheless, for maximum heat resistance in sulfur cure, the addition of a powerful antioxidant package can further improve heat resistance. The comparison of a highly effective radical trap antioxidant to its combination with an antioxidant synergist is shown in figure 6 for elongation after various aging periods. The recipe for this sulfur cured EPDM formulation is given in table 1. Both antioxidant systems compare favorably to the control with no added antioxidant. Nevertheless, the combination of two antioxidant types provides better retained elongation after air oven aging than the radical trap antioxidant alone.

[FIGURE 6 OMITTED]

Peroxide cured EPDM

In contrast to sulfur cures, peroxide cures require the use of an antioxidant to fully utilize the inherent heat resistance of the carbon-carbon bond formed by peroxides. Peroxide-based recipes require particular attention to certain additives that can interfere with the free radical crosslinking process. Conventional radical trap antioxidants are one such consideration. Typically, a polymerized quinoline antioxidant is used with peroxide cures because it interferes least with the peroxide crosslinking reaction.

Nevertheless, a more powerful radical trap antioxidant, such as Aminox, can provide better physical properties after high temperature aging. A combination of both Aminox and the antioxidant synergist Vanox MTI provides the best overall elongation retention alter air oven aging as indicated in figure 7. The recipe for this peroxide cured EPDM formulation is given in table 1. Figure 8 shows that, compared to the sulfur cured formulation previously tested, the peroxide cured compounds exhibit much better (lower) compression set.

[FIGURE 7 & 8 OMITTED]

Other additives

Polychloroprene

In peroxide cured EPDM compounds, the addition of a small amount of a chlorinated polymer, such as polychloroprene, has long been used to improve heat resistance. Exactly why this compounding technique should help high temperature aging is not know for certain. One school of thought holds that the polar hydroperoxide intermediates migrate into the polar polychloroprene phase before they decompose to free radicals. It is also known that polychloroprene degrades at high temperature to release hydrochloric acid, and the migration of this acid into the EPDM phase could heterolytically decompose hydroperoxides to harmless byproducts before the hydroperoxide can split into free radicals that would harm the EPDM.

Figure 9 illustrates the elongation during aging with the use of polychloroprene in the mineral filled, peroxide cured EPDM compound of table 2. The additions of an antioxidant synergist, and optionally with a radical trap antioxidant, are shown as the third and fourth compounds. The use of CR provides a major improvement in retention of physical properties after aging at 158[degrees]C. The addition of an antioxidant synergist to the CR compound further improves the aging. The best results, as judged by the highest value of elongation at break after 150 days at 158[degrees]C, are obtained by the combination of CR, ZMTI and the radical trap antioxidant Naugard 445. That the antioxidant synergist ZMTI shows little benefit over polychloroprene alone supports the idea that both materials work by decomposing hydroperoxides.

[FIGURE 9 OMITTED]

Vinyl silane

An EPDM compound designed for wire and cable insulation is shown in table 3. The addition of a vinyl silane helps the retention of elongation after high temperature aging.

Nevertheless, the main reason for using a vinyl silane in electrical applications is to retain electrical insulation resistance during water immersion. Each mineral filled, peroxide cured compound was extruded onto a copper conductor, steam cured and immersed in water at 90[degrees]C with a 600 volt--60 Hertz electric potential applied. Figure 10 shows that the addition of a vinyl silane improves polymer to filler bonding to prevent water ingress and maintain good electrical properties compared to the untreated compound.

[FIGURE 10 OMITTED]

Liquid EPDM

In certain EPDM applications, the use of extractable plasticizers can be objectionable. Examples are brake parts, o-rings and gaskets. Processing usually suffers without the use of a liquid plasticizer to lower viscosity and reduce nerve. In these applications, low molecular weight EPDM that is a liquid at processing temperatures can be substituted. Example recipes are shown in table 4.

In both sulfur cured and peroxide cured formulations, the used of 10 phr Trilene 77 in place of the paraffinic oil provides a comparable 15 to 20 point decrease in Mooney viscosity without contributing to extractables or volume shrinkage in DOT #4 brake fluid.

In long term tests at high temperature, the liquid plasticizer may be expected to be lost by volatilization. In contrast, the liquid EPDM becomes permanently bound to the rubber network.

Silicone modification of EPDM

When all the foregoing approaches to improve the heat resistance of EPDM are insufficient, a significant increase in heat resistance can be obtained by the silicone modification of EPDM. The comparison of the high temperature performance of EPDM, silicone and silicone modified EPDM is shown in figure 11.

[FIGURE 11 OMITTED]

The heat resistance of silicone modified EPDM can be further improved by the addition of antioxidants, as shown in table 5. As previously shown for 100% EPDM, a combination of a radical trap and antioxidant synergist can improve the retention of physical properties of silicone modified EPDM after aging a month at 175[degrees]C.

Summary

Compression set is dramatically improved when a peroxide cure replaces a sulfur donor. High temperature aging can be greatly improved by the use of antioxidants, especially with peroxide cures. In peroxide cured wire and cable compounds, the addition of polychloroprene improves heat aging. The use of vinyl silane improves wet electrical properties of mineral filled EPDM compounds. Liquid EPDM assists the processing of dry compounds and, compared to paraffinic oil, minimizes loss by extraction. Silicone modification extends the high temperature service range of EPDM.

Table 1--sulfur cured and peroxide cured EPDM
formulations

                     Sulfur   Peroxide
                      cure      cure

Royalene 580HT        100       100
Hypalon 40             --        5
Antimony trioxide      --        5
Zinc oxide             5         20
N550 carbon black      75        50
Paraffinic oil         30        15
Zinc stearate         1.5        --
Naugex MBTS            3         --
Butazate              1.5        --
Tuex                  0.8        --
Naugex SD-1           0.8        --
Sulfur                0.7        --
SR-350 coagent         --        2
DiCup 40-KE            --        7

Antioxidant         Blank   Radical trap   Combination
variables
Aminox                --          3             1
Vanox MTI             --         --             2
Table 2--addition of CR to mineral filled,
peroxide cured EPDM

                         No AO   CR    CR/    CR/ZMTI/
                                       ZMTI     N445

Royalene 3180             100    100   100      100
Zinc oxide                15     15     15       15
Paraffinic process oil    15     15     15       15
Aluminum trihydrate       100    100   100      100
Coupling agent             1      1     1        1
SR-350 coagent             3      3     3        3
VulCup R                   3      3     3        3
Variables
Neoprene WRT              --      8     8        8
Vanox ZMTI                --     --     2        2
Naugard 445               --     --     --       2
Table 3--addition of vinyl silane to mineral
filled, peroxide cured EPDM

                              No silane   Vinyl silane

Nordel 2722                   100         100
oxide                         5           5
Naugard Q                     1.5         1.5
Translink 37                  60          60
Sunproof regular              5           5
Red lead dispersion           5           5
DiCup R                       2.6         2.6
Variable
Silquest A-172 vinyl silane   --          0.5
Table 4--liquid EPDM in sulfur cured and
peroxide cured EPDM formulations

                    Sulfur cure   Peroxide cure

Royalene 501            100            100
Zinc oxide               5              5
N774 carbon black       60             60
Naugard Q                1              1
MBTS                    1.5            --
Butazate                1.5            --
Tuex                    0.6            --
Sulfur                  1.5            --
SR-350 coagent          --              2
DiCup 40-KE             --              7

Variables           No plasticizer   Liquid EPDM   Paraffinic oil

Trilene 77                --             10              --
Paraffinic oil            --             --              10
Table 5--antioxidants in silicone modified EPDM

                                      No AO    MB     MB/N10

Royaltherm 1721                        100     100     100
DiCup 40-KE                             5       5       5
Variables
Vulkanox MB                            --       1       3
Naugard 10                             --      --       2
HVA-2                                  --      --

Cure meter at 170[degrees]C
  ML, in.-lb                           7.4     8.2     7.4
  MH, in.-lb                          36.9    38.3     32.1
  Scorch, ts1, minutes                 0.8     0.9     1.0
  Cure time, tc90, minutes             7.8     7.5     7.6

Cured 10 minutes at 170[degrees]C
Post cured 2 hours at 150[degrees]C
  300% modulus, psi                    930    1,020    780
  Tensile strength, psi               1,800   1,720   1,410
  Elongation, %                        540     490     560
  Hardness, durometer A                72      72       76

Aged 10 days at 175[degrees]C
  Tensile strength, psi               1,550   1,670   1,390
  Elongation, %                        270     270     230
  Hardness, durometer A                83      82       80

Aged 20 days at 175[degrees]C
  Tensile strength, psi                770    1,060   1,230
  Elongation, %                        80      110     140
  Hardness, durometer A                87      87       87

Aged 30 days at 175[degrees]C
  Tensile strength, psi                910     830    1,050
  Elongation, %                        10      10       50
  Hardness, durometer A                91      88       85

References

(1.) Technical Service Report on Naugard 495, Uniroyal Chemical technical literature.

(2.) A.J. Maldonado, "Effect of antioxidants on heat aging of EPDM," Rubber & Plastics News, Vol. 28, No. 13, pages 2327, (Jan. 25, 1999), based on Paper No. 27 presented at the 153rd technical meeting of the Rubber Division (May 5-8, 1998).

(3.) Improved Organosilicon Coupling System for Mineral-Filled EPDM, OSi technical literature.

(4.) Technical Note on Trilene Liquid Polymers in EPDM Molding Compounds, Uniroyal Chemical technical literature.

(5.) Royaltherm--the silicone-modified EPDM high performance elastomer, Uniroyal Chemical technical literature.

(6.) T.L. Jablonowski., Royaltherm 1721 Heat Aging Studies, Uniroyal Chemical laboratory report.