10 Antioxidants and Other Protectant Systems Sung W. Hong Crompton Corporation, Uniroyal Chemical, Naugatuck, Connecticut, U.S.A. I. INTRODUCTION To extend the service life of vulcanized rubber goods, it is very important to protect them from oxygen, ozone, light, heat, and flex fatigue. Most natural and synthetic rubbers containing unsaturated backbones—natural rubber (NR), styrene butadiene rubber (SBR), polybutadiene rubber (BR), and nitrile rubber (NBR), for example—must be protected against oxygen and ozone. Usually, internal components that are not exposed to atmosphere require only antioxidants. However, external components that are exposed to the environment require both antiozonants and antioxidants. Antioxidants react with oxygen to prevent oxidation of vulcanized rubber and react with free radicals that degrade vulcanized rubber. There are four principal theories on the mechanisms of antiozonant protection of vulcanized rubber. The first is the scavenger theory, which postulates that the antiozonant competes with the rubber for ozone. The second theory is that the ozonized antiozonant forms a protective film on the surface of the vulcanized rubber, preventing further attack. The third mechanism postulated is that the antiozonants react with elastomer ozonide fragments, relinking them and essentially restoring the polymer chain. The fourth theorized mechanism suggests that Criege zwitterions are formed from the ozonide produced. Paraffinic and microcrystalline waxes are often added to the rubber as protective agents. Usually, waxes have poor solubility in rubbers, so they 4871-9_Rodgers_Ch10_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 457 Copyright © 2004 by Taylor & Francis migrate to the surface of the vulcanized rubber and form a protective film or barrier that prevents ozone attack. During dynamic flexing, these barriers can be broken, exposing the rubber to ozone attack. Therefore, waxes can protect against ozone only under static conditions. Antiozonants alone do not prevent ozone cracking at the initial stage, because their migration rate is much slower than that of waxes owing to their better solubility. One theory supporting the combination of antiozonant with wax is that the wax would accelerate migration of the antiozonant to the surface for protection against ozone. Therefore most exterior rubber goods contain both antiozonant and wax for both static and dynamic protection from ozone cracking. Commercially available antioxidants usually protect vulcanized rubber at temperature below 120jC. Above this temperature, special polymers or types of cross-link systems provide better protection. For example, a peroxide cure system would provide carbon–carbon cross-links whose bonding energy is the strongest. Also, a monosulfuric cross-link has much higher energy than a polysulfuric bond. The lower the bonding energy, the more easily cross-links break, which would deteriorate the vulcanized rubber’s physical properties after heat aging. Cure systems with a higher level of accelerators and lower sulfur levels are known as semiefficient cure systems (semi EV cure). Such a system provides better heat aging properties than the conventional cure system, which has a smaller amount of accelerator and a higher level of sulfur. In this chapter, the protection of mechanical rubber goods through selection of antioxidants, antiozonants, waxes, and the design of the vulcan- ization system will be discussed. II. ANTIOXIDANTS A. Oxidative Degradation Generally, the greater the amount of unsaturation in the polymer, the more susceptible it is to degradation. Highly unsaturated polymers can be attacked by oxygen, especially when energy is applied. The application of energy may come from heat, shear, and ultraviolet (UV) light, which promote faster oxidation. Oxidative degradation of the polymers is a free radical process. This oxidation process, known as auto-oxidation, consists of three steps: initiation, propagation, and termination, as depicted in Figure 1. Free radicals are formed during initiation reactions. Energy from heat, mechanical shearing, or high energy radiation can dissociate the chemical bonds in the polymers (RH) resulting in the formation of free radicals (R Á ) [reaction (1)]. In auto-oxidation mechanisms of polymers, the molecular reaction of oxygen with the polymers by thermal energy has been suggested 4871-9_Rodgers_Ch10_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 458 Copyright © 2004 by Taylor & Francis for the initiation of the first free radicals in the polymer (1–4) [reaction (2)]. Hydroperoxide concentration builds up as the auto-oxidation proceeds, and consequently decomposition of the hydroperoxide eventually becomes the dominant initiation process [reactions (3) and (4)]. This is usually proceded by a short period of induction. The alkyl (R Á ) and alkylperoxyl (ROO Á ) radicals resulting from the initiation reactions are the chain-propagating species. The alkyl radicals react rapidly wth atmospheric oxygen to form alkylperoxyl radicals [reaction (5)]. The alkylperoxyl radicals abstract labile hydrogens on the polymer, regener- ate new alkyl radicals, and yield hydroperoxides (ROOH) as the primary oxidation product [reaction (6)]. Reactions (5) and (6) form a cycle in the propagation step. As the hydroperoxide concentration builds up, more alk- oxyl and alkylperoxyl radicals are formed via decomposition of the hydro- peroxide to start new cycles. Termination occurs when two free radicals, alkyl and/or alkylperoxyl radicals, react to form the stable nonradical products. The termination reactio in the solid polymers, where oxygen is limited, usually involves two alkyl radicals (R Á ), which undergo recombination to form RUR or disproportion- ation to form saturated and unsaturated products. On the other hand, in the presence of sufficient oxygen such as in liquid hydrocarbons, the hydroperoxyl Figure 1 Mechanisms of polymer degradation by oxidation under thermal energy. 4871-9_Rodgers_Ch10_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 459 Copyright © 2004 by Taylor & Francis radical (ROO Á ) concentration is much greater than the alkyl radical concen- tration (R Á ). Chain termination occurs predominantly by reaction between two hydroperoxyl radicals. Antioxidants are used to stabilize organic polymers. Antioxidants inhibit auto-oxidation by reducing the rate of auto-oxidation during process- ing, storage, and service. There are two major groups of antioxidants, commonly known as primary and secondary antioxidants. Primary antiox- idants act as chain terminators, and secondary antioxidants act as hydroper- oxide decomposers. The primary antioxidants remove the chain-carrying species (R Á and ROO Á ), while the secondary antioxidants convert the hydro- peroxides to nonradical species. A schematic chain-termination mechanism is shown in Figure 2. In reaction (10), the alkylperoxyl radical abstracts the reactive hydrogen from the antioxidant (AH). The resulting antioxidant radicals (A Á ) are stabilized via electron delocalization. Consequently, the antioxidant radicals (A Á )do not readily continue the radical chain either via hydrogen abstracting from the substrate [reaction (11a)] or via reaction with oxygen [reaction (11b)]. The resonance structures for various antioxidant radicals derived from typical antioxidants will be illustrated in the next section. Transformation products derived from typical antioxidants will also be discussed to elucidate the antioxidant mechanism. Hindered phenolics and secondary aromatic amines are the two most commonly used primary antioxidants. Examples of secondary antioxidants that act as hydroperoxide decom- posers include phosphite esters such as I and sulfur-containing compounds such as thioester II. As the hydroperoxides are removed from the organic substrates, fewer free radicals are produced via the decomposition of the hydroperoxides. Consequently, the rate of the auto-oxidation is reduced. The mechanism of converting hydroperoxides to nonradical species will be discussed in Section II.B.3. ðROÞ 3 P ðROCOCH 2 CH 2 ÞS III Figure 2 Mechanisms of reaction of peroxyl radical with antioxidant and explanation of the stabilization of antioxidant radical by electron delocation, without continuing formation of radicals. 4871-9_Rodgers_Ch10_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 460 Copyright © 2004 by Taylor & Francis B. Mechanisms of Antioxidants Commercially available antioxidants can be divided into three categories: phenolic antioxidants, aromatic amine antioxidants, and hydroperoxide- decomposing antioxidants. Each antioxidant performs in a specific way to protect polymers and rubber compounds from oxidation. The performance of each antioxidant is related to its chemical reactivity, its rate of staining or migration to the surface of vulcanized rubber or polymers, and its volatility. Therefore, it is very important to understand the mechanisms of various antioxidants before applying them for experiments. 1. Phenolic Antioxidants Hindered phenolics, which act as chain terminators, are excellent antiox- idants, Phenolic antioxidants are in general nonstaining and nondiscoloring. Many of them are approved for use in food packaging. A simplified mechanism commonly used to show the process of chain termination by a hindered phenolic is shown in Figure 3. The alkylperoxyl radicals (ROO Á ) abstract the reactive hydrogen from the phenolics. The resulting phenoxy radical 1 is stabilized through electron delocalization as indicated by the resonance structures 1 and 1a. Reaction of the alkylperoxyl radical with 1a produces the nonradical product 2. More detailed information about the antioxidant mechanism was made possible with the identification of the transformation products. The mecha- Figure 3 Chain termination by hindered phenolics. 4871-9_Rodgers_Ch10_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 461 Copyright © 2004 by Taylor & Francis nism depicted in Figure 4 illustrates the formation of transformation products using 2,6-di-t-butylhydroxytoluene (BHT) as an example. The alkylperoxyl radical attacks the reactive hydrogen from the BHT, yielding the phenoxy radical 3, which is stabilized via delocalization to the carbon-centered radical 3a. Bimolecular reaction between 3 and 3a would produce 4 and 5. Reaction of 3a with the alkylperoxyl radical forms 6, which thermally decomposes to the p-quinone 7. The phenoxy radical 3a would also be the precursor for the formation of compounds 9 and 10 (6,8). Figure 4 Antioxidation mechanism of BHT (illustrated with some transformation products). 4871-9_Rodgers_Ch10_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 462 Copyright © 2004 by Taylor & Francis 2. Aromatic Amine Antioxidants Amine antioxidants in general are better antioxidants than phenolic antiox- idants. However, most amine antioxidants are discoloring and staining and have limited approval for food contact use. The mechanism of chain termination by secondary aromatic amines is shown in Figure 5, with N,NV- dialkyldiphenylamines as an example (5,6). The alkylperoxyl radicals abstract the reactive hydrogen (NUH) from the N,NV-dialkyldiphenylamines. The Figure 5 The chain termination mechanism by N,NV-dialkyldiphenylamines. 4871-9_Rodgers_Ch10_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 463 Copyright © 2004 by Taylor & Francis resulting aminyl radical 11 is stabilized through electron delocalization as indicated by the resonance structures 11, 11a , and 11b. Reaction of the alkylperoxyl radical with 11b produces a nonradical product 12. Reaction of 11 with the primary alkylperoxyl radicals leads to a stable nitroxyl radical 13 that is capable of trapping the free alkyl radical and producing the stable product, alkoxyamine 14. Reactions of the aminyl radical 11 with secondary Figure 6 Antioxidation mechanism illustrated with the resonance structures and the transformation products from TMQ. 4871-9_Rodgers_Ch10_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 464 Copyright © 2004 by Taylor & Francis and tertiary alkylperoxyl radicals produce the hydroxylamine 15 and the nitroxyl radical 13 , respectively (17–19). Another commonly used secondary amine is polymerized 1,2-dihydro- 2,2,4-trimethylquinoline (TMQ). A mechanistic illustration with the reso- nance structures and the transformation products is shown in Figure 6. The alkylperoxyl radicals abstract the reactive hydrogen (NUH) from TMQ. The resulting aminyl radical 16 is stabilized through electron delocalization as indicated by the resonance structures 16a, 16b,and16c. However, the transformed radical 16a would be capable of trapping alkylperoxyl radicals, leading to the nonradical product 17. The aminyl radical 16 would also trap alkylperoxyl radicals to form the nitroxyl radical 18, which in turn traps alkyl radicals to form the alkoxyamine 19. N,NV-Dialkylated p-phenylenediamines are excellent antioxidants. A mechanism that illustrates chain termination by the N,NV-diphenyl-p-phenyl- enediamine 20 is shown in Figure 7 (7,8). Two alkylperoxyl radicals attack the two reactive hydrogens from N-phenyl-NV-alkyl-p-phenylenediamine, yield- ing the quinonediamine 21, which is stabilized through electron delocalization as shown by the resonance structure 21a. Further reaction of the aminyl diradical 21a with two alkylperoxyl radicals produces the dinitroxyl radical 22, which is converted via electron delocalization to the dinitrone 22a (12,15). Figure 7 Chain termination by N,NV-diphenyl-p-phenylenediamine. 4871-9_Rodgers_Ch10_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 465 Copyright © 2004 by Taylor & Francis 3. Hydroperoxide-Decomposing Antioxidants Hydroperoxide-decomposing antioxidants reduce the rate of chain initia- tion by converting hydroperoxide, ROOH, into nonradical products. Two major classes of the hydroperoxide-decomposing antioxidants are organic phosphite esters and sulfides. During the reaction with hydroperoxide, the hydroperoxide is reduced to alcohol (ROH) and the phosphite and sulfide antioxidants are oxidized to phosphates and sulfoxides, respectively (Fig. 8). Further transformation of the sulfoxides has been reported (9). In general, phosphites decompose hydroperoxides at substantially lower temperatures than the sulfides. The sulfide antioxidants are active at temperatures exceed- ing 100jC but are not active at ambient temperatures (10). An example of commonly used phosphite antioxidants is tris(nonyl- phenol) phosphite 23. Its hydroperoxide-decomposing mechanism is depicted in Figure 9. A stable trialkyl phosphoric acid ester 24 and an alkyl alcohol are formed. Thioesters are often used in combination with phenolics and are more widely used in thermoplastics, where sulfur will not interfere in the vulcani- zation process (11). A mechanistic explanation of a thioester as a hydroper- oxide decomposer is shown in Figure 10 with distearyl thiopropionate 25 as an example. A stable sulfoxide 26 of the thioester and an alkyl alcohol are formed. Thermal decomposition of the sulfoxide 26 produces sulfenic acid 27 and stearyl acrylate. Oxidation of sulfenic acid 27 by hydroperoxide yields the sulfinic acid 28.Thesulfenicacid27 could also undergo a bimolecular reaction to produce thiosulfinate ester 29 (24). Figure 8 Oxidation of phosphites and sulfides by hydroperoxide. Figure 9 Tris(nonylphenol)phosphite as hydroperoxide decomposer. 4871-9_Rodgers_Ch10_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 466 Copyright © 2004 by Taylor & Francis [...]... functions and mechanisms in the actual service life of various antiozonants Antiozonants that were employed in this study are N-(1,3-dimethylbutyl)-NV-phenyl-p-phenylenediamine (6PPD), N-isopropyl-NV-phenyl-pphenylenediamine (IPPD), N,NV-bis-(1,4-dimethylpentyl)-p-phenylenediamine (77PD), N,NV-diaryl-p-phenylenediamine (DAPD), and 2,4,6-tris (N-1, 4-dimethylpentyl-p-phenylenediamine )-1 ,3,5-triazine... Compounda NR SP-6700 N-351 black Aromatic oil Zinc oxide Stearic acid SP-1068 Antidegradant M3P TBBS TBzTD CPT 80% Insoluble sulfur 100 10 55 5 10 2 2 2 2 0.6 0.25 0.25 5.0 a SP-6700, oil-modified phenol formaldehyde two-step resin; SP-1068, alkylphenol formaldehyde resin; M3P, 1-aza-5-methylol-3, 7-dioxabicyclo[3.3.0]octane; TBBS, N-t-butyl-2-benzothiazole sulfenamide; TBzTD, N,N,NV,NV-tetrabenzylthiuram... Stearic acid Octylphenol formaldehyde R-6 Total 60 20 27 50 4 7.5 1.5 2.0 2.0 174.0 Blend Componentb MB-1 TMQ BLE AO 445 M3P MBTS DPG 80% Insoluble sulfur B- 1-1 174 a B- 1-2 174 1.0 B 1-3 174 B- 1-4 174 1.0 1.0 1.2 0.25 3.00 1.0 1.2 0.25 3.00 1.0 1.2 0.25 3.00 1.0 1.0 1.2 0.25 3.00 B- 1-5 174 0.5 0.5 1.0 1.2 0.25 3.00 R-6, resorcinol resin TMQ, polymerized 1,2-dihydro-2,2,4 trimethylquinoline; BLE, high... hardness Tear strength, % retention DeMattia flex Kilocycles to full cracking unaged Aged 70 hr at 100jC Monsanto flex fatigue Kilocycles to failure unaged Aged 70 hr at 100jC B- 1-2 B- 1-3 B- 1-4 B- 1-5 49 49 48 49 48 11. 9 11. 6 11. 6 11. 8 11. 4 0.40 3.45 0.93 1.71 2.96 0.40 3.43 0.94 1.72 3.00 0.41 3.49 0.95 1.74 3.05 0.41 3.48 0.97 1.74 3.04 19.5 450 10.50 57 40.3 18.8 460 10.2 56 43.8 19.1 470 10.1 55 40.8... evaluating phosphite, blends of phosphite– phenol antioxidants, and CG 1520 The results are shown in Table 8 The Copyright © 2004 by Taylor & Francis Figure 18 Stabilization of low-cis polybutadiene and the influence of antioxidants on oven aging performance CG 1520, 2,4-bis[(octylthio)methyl]-o-cresol; BHT, di-tbutyl-4-methylphenol; TNPP, tris(mono- and dinonylphenyl)phosphite Scale at left is for days aging... Hydrated aluminum silicate Stearic acid SP-1068 Ultramarine blue Blended wax Hindered bisphenol Zinc oxide TBBS Alkyl phenol disulfide (APD) 80% Insoluble sulfur 35 15 50 40 20 1.0 3.0 0.2 3.0 a A- 1-1 A- 1-2 15 1.5 1.0 1.0 1.5 15 1.5 1.0 1.0 EPDM, ethylene propylene diene terpolymer; CIIR, chlorobutyl rubber; SP-1068, alkyl phenol formaldehyde resin; TBBS, N-t-butyl-2-benzothiazole sulfenamide Table 2 Physical... antioxidants and sulfides because these antioxidants are reduced from ROOH to RH or ROH, which is more stable Ciba Geigy developed 2,4-bis[(octylthio)methyl]-o-cresol (CG 1520) for a polymer stabilization system (17) In this section phosphite, a phosphite-phenolic blend, and CG 1520 are evaluated in cis-BR and SBR, respectively Stabilization System for cis-BR Butadiene cement was diluted 1:1 by volume in n-hexane... 70jC B- 2-2 B- 2-3 B- 2-4 B- 2-5 2 2 2 51 49 49 52 2 50 16.0 16.4 15.9 15.1 14.9 1.05 2.50 5.10 0.34 3.40 1.04 2.51 5.09 0.34 3.31 1.06 2.55 5.13 0.35 3 .11 0.99 2.47 4.93 0.38 3.21 0.95 2.40 4.88 0.37 3.15 16.80 320 15.20 85 16.20 350 13.8 85 16.00 320 14.60 84 17.70 420 13.50 87 17.50 430 13.40 86 43 25 +5 68 34 +4 59 31 +3 74 33 +1 73 35 +2 60 31 +6 77 41 +4 73 37 +3 79 42 +2 78 43 +3 5.8 0.5 41.1 11. 7... higher molecular weight AO 445 and TMQ would provide better heat aging property 2 Bead Filler To confirm our proposed mechanisms for DAPD and N-1,3-dimethylbutylNV-phenyl-p-phenylenediamine (6PPD), they were evaluated in bead filler compound along with TMQ and BLE In both passenger vehicle and truck radial ply tires, a stiff lower sidewall construction is very important for handling performance The stiffness... Francis A- 1-1 A- 1-2 32 12.6 1.5 31 13.1 17.4 610 6.1 52 16.0 650 5.5 51 80.5 90.0 121.0 +3 452.4 Slightly yellow 87.57 95.1 120.0 +4 499.4 Very slightly yellow Table 3 Aging test—Delta Mooney Viscosity at 100jC Additivea Time (weeks) a 0.7 phr TNPP 0.7 phr TNPP/AO 0 8 17 25 35 0 1 2 3 4 None 0 3 8 10 18 0 4 5 7 14 TNPP, tris(mono- and dinonylphenyl) phosphite; AO, 2,2V-methylenebis(4-methyl-6-nonylphenol) . and TMQ and AO 445 the best Table 5 Physical Properties of Five Blends a B- 1-1 B- 1-2 B- 1-3 B- 1-4 B- 1-5 Mooney viscosity at 100jC 49 49484948 Mooney scorch at 132jC 3 Pt rise time (min) 11. 9 11. 6. 4 Naphthenic oil 7.5 Stearic acid 1.5 Octylphenol formaldehyde 2.0 R-6 2.0 Total 174.0 Blend Component b B- 1-1 B- 1-2 B 1-3 B- 1-4 B- 1-5 MB-1 174 174 174 174 174 TMQ 1.0 0.5 BLE 1.0 0.5 AO 445 1.0 M3P. mechanisms for DAPD and N-1,3-dimethylbutyl- NV-phenyl-p-phenylenediamine (6PPD), they were evaluated in bead filler compound along with TMQ and BLE. In both passenger vehicle and truck radial ply