Improved Tire Durability and Productivity Through Compound Modifications

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Paper No. 33A

Improved Tire Durability And Productivity Through Compound Modifications

Gilbert Anthoine *

Flexsys N.V

Woluwe Gardens, Woluwedal 24-26

B-1932 St. Stevens Woluwe-Zaventem

Brussels, Belgium

Frederick Ignatz-Hoover and Byron H. To

Flexsys America L.P.

260 Springside Drive

Akron, Ohio 44333

Presented at ITEC 2004 Akron, Ohio September 21 - 23, 2004

* Speaker

ABSTRACT

The Tire Industry will be facing many challenges in the 2000's. One of which is the variation of rubber performance throughout the tire service life. Maintaining properties and performance throughout a tire service life is directly related to maintaining the integrity of the vulcanizate structure under thermal and / or oxidative conditions. Historically, this has been achieved by reducing the sulfur content in the crosslinks through the use of more efficient vulcanization systems. However, as with many changes in rubber compounds, there is a trade-off which in this case is a reduction in performance characteristics such as fatigue and tear resistance. This paper presents several compounding approaches to achieve improved aged compound characteristics with minimum compromise in performance properties. In addition, recent work on increased productivity without compromise in properties using these compounding approaches will also be reviewed.

INTRODUCTION

Historically, the Rubber Industry has been described as an industry that is extremely conservative and as a result, very slow to change. This is no longer the case, and for a variety of reasons both Geopolitical and technical, the Rubber Industry is changing more quickly now than in the past and such changes could occur at an even increasing rate in the future. Because of this, it is critically important that both manufacturers and suppliers within the rubber industry work more closely to accurately define market and product performance needs and to effectively identify the most efficient solutions in a timely manner. An important technical need of the rubber industry deals with enhancing rubber product performance to meet the ever increasing expectation of the marketplace. There are three areas in which efforts must be focused to realize this improvement: • Reducing hysteresis to provide improved rolling resistance and lower heat build-up properties. • Enhancing long term durability by reducing the effects of oxidative and ozone induced degradation. • Maintaining properties throughout a product service life in order to provide consistent performance.

This paper is a review of work done at Flexsys to address this issue of tire durability through compound modifications. .

2 ENHANCED PERFORMANCE

Rubber networks rich in long polysulfidic crosslinks (i.e. 4, 5, 6 or more sulfur atoms per crosslink), provide better fatigue and tear resistance than networks rich in mono- and disulfidic crosslinks which in general, are thermally and oxidatively more stable than those of predominantly polysulfidic crosslinks. This result is primarily due to two factors. First, the polysulfidic networks are prone to reversion. Second, the polysulfidic crosslinks are susceptible to oxidative degradation. Conversely, monosulfidic and disulfidic networks provide a measure of oxidative protection while being resistant to reversion. Monosulfidic crosslinks are expected to decompose into sulfenic acid derivatives, which are postulated as powerful antioxidants.1 However, networks rich in monosulfidic and disulfidic crosslinks provide poor fatigue and tear resistance properties. Thus, compounding for severe applications (i.e. applications requiring high flex-fatigue resistance and tear strength) requires networks rich in polysulfidic crosslinks. When this is the goal, compounding with antidegradants that help to preserve the rich polysulfidic networks would also be desirable.

Reversion is the degradation of the sulfur network and results in a loss of vulcanizate physical properties. Reversion occurs upon overcure and during cure at elevated temperatures. 2 It also continues during the service life of articles where high temperatures can develop through dynamic operation. It is catalyzed by accelerator fragments complexed with zinc ions and/or zinc ions and amines. 3 Amines, in particular, have a deleterious effect on reversion resistance. In fact, even though sulfenamides provide delayed-action and fast cures compared to 2-mercaptobenzothiazole and 2- mercaptobenzothiazole disulfide, reversion is greatly accelerated. When added to model vulcanizate mixtures, amines, zinc-amine complexes or zinc-amine-accelerator complexes, likewise accelerate reversion. 4 Maintenance of the polysulfidic network upon aging would help to provide improved dynamic mechanical properties, improved fatigue resistance, and help to maintain tear strength upon aging.

The introduction of anti-reversion agents such as Duralink ®HTS (DHTS) and Perkalink ®900 (Pk 900) has allowed compounders the flexibility to optimize compound properties with minimum compromises.

DHTS, which is chemically Disodium Hexamethylene-1,6-bis-Thiosulfate Dihydrate, is a material which allows compounders to generate hybrid crosslinks, 5 containing both sulfur and carbon atoms as illustrated in Figure 1.

3 Figure 1

Hybrid Crosslinking Formation with Hexamethylene-1,6-Bis thiosulfate (DHTS)

Sx Sulfur O3SS(CH2)6SSO3 S + Accelerator (CH ) ZnO 2 6 Rubber Stearic Acid S n Sy

By controlling the sulfur/accelerator/DHTS ratio 6 the compounder can obtain good reversion resistance in natural rubber combined with excellent performance properties such as reversion and fatigue resistance as illustrated in Figure 2 & 3.

Figure 2

Reversion Resistance in NR (% torque reduction – 30 mins. @ 180°C) 45

0 Control S.EV Hi/Lo EV DHTS

Vulcanization Systems

4 Figure 3

Fatigue Resistance in NR (Kilocycles to Fail @ 100% Strain) 200 180 160 140 120 100 80 60 40 20 0 Control S.EV Hi/Lo EV DHTS

Vulcanization Systems

Figures 2 & 3 show that the DHTS compound exhibits reversion resistance which is better than the Sulfur Donor (S.EV) and Hi/Lo semi- efficient systems but not as good as the EV System. However, the DHTS system provides excellent fatigue resistance as compared to either the Semi EV or EV systems and in fact is better than the high sulfur control compound.

Another approach to preserve the total number of crosslinks in the network structure, and hence maintain the dynamic properties through a tire life, is to use anti- reversion agent such as Perkalink 900 (Pk 900) which is chemically, 1,3- bis(citraconimidomethyl)benzene,

Figure 4 – Structure of Pk 900

5 Pk 900 can substantially reduce the deterioration of rubber compound physical properties caused by reversion. It reacts to form heat stable crosslinks in sulfur cured rubber. These new crosslinks produce a vulcanized network which is resistant to overcure and provides improved high temperature performance.7 Pk 900 is unique in that it is also active after crosslinks begin to revert, therefore compound designed properties are maintained. Pk 900 has been shown to be able to compensate for the loss of polysulfidic crosslinks by scavenging the conjugated dienes, introducing thermally stable and flexible carbon – carbon crosslinks in kinetic equilibrium, hence maintaining overall crosslink density and preserving key physical properties. 8

The conventional sulfur cured network structure comprising poly-, di- and monosulfidic structures can be represented as in Figure 5. With the action of heat, some of these sulfur crosslinks degrade or revert, giving rise to conjugated dienes and trienes, cyclic sulfides and pendant groups. These new structures are no longer providing crosslinks across different polymer chains and hence the network is weakened.

S S S SxSyS z S x S1 S2 R

Figure 5 – Representation of sulfur linkage degradation during reversion .

The Pk 900 which survives the vulcanization process is available to react with these structures via a single, or preferably a double, Diels-Alder addition reaction shown in Figure 6.

O O

+N N +

O O

N N

O O

Figure 6 - Diels Alder reaction

The generation of the reactive diene & triene species is only significant after the compound is predominantly crosslinked so Pk 900 is virtually ‘dormant’ during the curing phase, becoming functional and repairing the network as the polysulfidic crosslinks start to decay as illustrated in Figure 7.

O O N

O O N O O N N O O N O O + + O O N N O O O O

N N

N O O O O

Figure 7 -The “repair” mechanism

Reversion, caused by overcure, high temperature curing, or through high temperatures encountered under service, can be significantly reduced when Pk 900 is present in a compound. This has been demonstrated through many laboratory studies and has been further confirmed through tire testings and the successful commercialization of the product. Comparison of cure characteristics in a natural rubber compound with and without Pk 900 is shown in Figure 8. It is clear that the control compound, as expected, exhibits reversion on extended cure while the compound with Pk 900 shows no reversion. Quantatively, the control has 18% reversion and the Pk 900 has none.

7 Figure 8

Curing Characteristics in Natural Rubber

150° 2

1 Control + PK 900 Torque, Nm 0 0 12 24 36 48 60 Time, min.

A heat build up test performed with a Goodrich Flexometer (Figure 9) shows that the Pk 900 containing compound attained an equilibrium temperature in less than an hour and maintained that temperature for six hours without failure, while the control compound did not survive one hour in this long duration test at 100°C.

Figure 9

Heat build up at 100°C- Long Duration Test

The use of higher accelerator, lower sulfur ratios is well known to improve the thermal oxidative resistance of unsaturated elastomers. The vulcanizing spectrum covers CV (conventional vulcanized), through semi-EV (semi efficient vulcanized), to EV (efficient vulcanized) systems, as the accelerator / sulfur ratio increases.

‘Efficient’ indicates the sulfur is used more efficiently to form the crosslinks. If the sulfur crosslink comprises multiple sulfur atoms logically there will be fewer crosslinks for the same total mass of sulfur. In most compounds there exist crosslinks ranging from mono- through disulfidic, to polysulfidic. The number of, and balance between, these is often critical to the compound performance, and as indicated above, this balance shifts as the compound is subjected to thermal energy and is subjected to thermal oxidative aging.

The bond energies for the various crosslink types are shown in Table 1.

Table 1 – Bond Dissociation Energies. Bond Type Dissociation Energy (kcal/mole) -C-C- 80 -C-S-C- 74 -C-S-S-C- 74 -C-S-S-S-C- 54 -C-S-S-S-S-C- 34

9 Therefore the lower bond energy polysulfidics are more easily cleaved compared to the mono-sulfidic and c-c bonds. The use of higher accelerator / lower sulfur ratios or sulfur donors, will result in a shift to crosslinks of a lower sulfur rank as a result providing more crosslinks with higher dissociation energies and hence better thermal oxidative resistance, and less tendency for reversion. However, the replacement of polysulfidic crosslinks with di- and monosulfidic causes a significant change in the tensile strength, tear resistance, compression set and flex properties. This trade-off of properties must be considered in the drive to better aged- property retention if semi-EV or EV systems are used.

A previous study 9 in a natural rubber compound pointed out the strengths and weaknesses of the semi-EV systems and other low sulfur / sulfur donor alternatives. It concluded that the final choice of cure system is very much dependent on the specific needs. There is no one system that could satisfy all parameters. Table 2 summarizes the results.

Table 2

Comparison of Natural Rubber Vulcanization Systems

Cure System Semi-EV Conventional Conventional Conventional Conventional plus plus plus DHTS Pk900 DHTS/Pk 900 Processing/Curing Properties Scorch safety 100 - / = /+ = = = Rheometer cure rate 100 - / = /+ = = =

Reversion Resistance Extended cure time 100 - + ++ +++ High temperature cure 100 - + ++ +++

Performance Properties Oxidative aging resistance 100 - = = = Heat build-up resistance 100 - + ++ +++ Fatigue resistance 100 + ++ = + Tear resistance 100 + + - / = +

Relative Cost Comparison 100 - - / = / + + ++

10 ENHANCED RUBBER TO BRASSED STEEL ADHESION

Adhesion between brass-coated steel cord and rubber is an important factor when determining the durability of tires. In considering rubber to brass-coated steel cord bonding much attention has been paid to the influence and efficacy of bonding agents and their effect on adhesion formation and retention has led to significant advances with regard to the durability of the rubber/steel cord composite. However, while these bonding agents provide enhanced adhesion, they can be detrimental to the long term properties of the cured rubber compound.

Adhesion characteristics can also be improved through the use of resin systems such as resorcinol/hexamethoxymethyl melamine (HMMM), often together with silica as a partial replacement for carbon black. This combination accomplishes several things. It acts as a modulus and hardness enhancer providing a more compatible system with regard to the stiffness of the steel cord compared to that of the cured skim compound. Secondly, the combination can improve resistance to the negative effects of humidity on adhesion.

The dosage of sulfur greatly influences rubber to steel cord adhesion. It is known 10 that relatively high levels of sulfur are required in order to obtain and maintain adequate adhesion. On the other hand it would be desirable to reduce the level of sulfur in order to improve compound properties such as reversion resistance and heat build up, both of which affect the durability of the rubber/steel cord composite.

DHTS has been shown to be an effective bonding promoter for natural rubber to brass coated steel cord adhesion. 11 It interposes a hexamethylene-1,6-dithiyl group within the polysulfidic crosslinks during vulcanization. The generation of such hybrid crosslinks not only increases the resistance to anaerobic aging of the rubber vulcanizates as reported earlier but also improves the adhesive strength between rubber and brass- coated steel wire. It has been reported in the literature that polysulfides are involved in bonding with the copper subsulfide layer on the brass surface of the wire cord. During service, the polysulfidic crosslinks degrade progressively and the sulfur liberated contributes to a further sulfidization of the copper subsulfide. Thus, bigger and more brittle copper sulfide crystals are formed, thereby weakening the rubber-brass layer bond. During such a process, the zinc can also be sulfidized, further weakening the bond strength. In the presence of DHTS, hybrid crosslinks are formed in the vicinity of the rubber to metal interface. Because these hybrid crosslinks maintain their polysulfidic groups chemistry longer than the classical polysulfidic crosslinks, the bonds formed between the rubber and the brass layer retain their strength longer during service. The use of DHTS provides the best maintenance of the rubber-brass bond during both steam and saline solution aging test as shown in Table 3.

11 Table 3

DHTS Vulcanization System in Rubber to Brass Coated Steel Adhesion (Effect of DHTS Loading)

Insoluble Sulfur (80%) 5.0 5.0 5.0 DCBS 1.75 1.75 1.75 DHTS 0 3 6

Wire Adhesion Test (ASTM 2229) Unaged (N) 476 (9)* 494 (9) 478 (9) Steam, 8 hrs./120°C (N) 366 (8) 417 (8) 407 (8) Salt (5%), 48 hrs./90°C (N) 184 (0) 363 (4) 467 (5)

Aged Tensile Properties (48 hrs. @ 100°C) % Tensile Retained 79 78 76 % Elongation Retained 48 46 49

* Numbers in parenthesis indicate wire coverage with (0) = no coverage and (10) = full coverage.

Other work 12 showed that the use of DHTS and Pk 900 alone or in combination can decrease heat generation and improve the aging characteristics of steel skim compounds especially under steam and salt aging conditions. Table 4 gives the test compound formulations used in this study.

Table 4

Effects of DHTS & Pk 900 on Wire Adhesion (Test Compounds)

phr Natural Rubber (SMR) 100 N-326 Black 45 Zinc Oxide 8 Stearic Acid 1.2 Resin 2 Cobalt Naphthenate 1 TMQ 1 6PPD 1 DCBS 1 CTP 0.1 Insoluble Sulfur (80%) 5

Test Compounds Control Pk900 DHTS DHTS/Pk 900

DHTS - - 2.0 1.0 Pk 900 - 0.75 - 0.5

Vulcanizate properties are shown in Figure 10. The incorporation of DHTS or Pk 900 or the combination leads to an increase in modulus, indicating that both introduce new crosslinks. Hardness, which can be considered as a measure of modulus at low strains, follows the same trend. The tensile strength data are comparable with slight reduction in elongation at break. In spite of the increased modulus, tear properties are not negatively affected. The most striking effect produced by the additives or the mixture is the reduction of heat generation during dynamic flexing.

Figure 10

Vulcanizate Properties – 12 mins. @ 150°C

300%Mod(Mpa) Tensile(Mpa) Elong.x10 Tear(kN/m) Heat BU (deg. C)

120

100

0 Control Pk 900 DHTS DHTS/Pk900

Wire adhesion data for original and aged samples are shown in Figures 11 & 12. Rubber coverage values are given on top of each bar in the charts. A trend of increased adhesion is observed for the test compounds when compared to the control. Furthermore, the compound containing both DHTS and Pk 900 exhibits significantly better performance following steam aging, both with respect to pull out force and coverage.

Figure 11

Wire Adhesion – 12 mins.@ 150°C (Pull out force, Newton)

Unaged Aged(3d/105C) Steam(2d/121C) Salt(7d/25C) 10 400 350 10 10 10 300 10 10 10 10 250 8 10

200 8 8 6 8 8 150 6 100 50 0 Control Pk 900 DHTS DHTS/Pk900

Figure 12

Wire Adhesion – 60 mins. @ 150°C (Pull out force, Newton)

Unaged Aged(3d/105C) Steam(2d/121C) Salt(7d/25C)

400 10 10 10 10 350 300 8 8 250 8 8 8 8 200 6 6 6 150 4 6 100 4 50 0 Control Pk 900 DHTS DHTS/Pk900

15 Compound viscoelastic properties are presented in Table 5. The hysteresis, as indicated by the value of tan δ, is lowest for compounds containing DHTS under unaged conditions. The control compound shows an increase in tan δ following aging whereas the compounds containing Pk 900 show significantly less change in tan δ indicating the formation of a more stable network. The compound containing both DHTS and Pk 900 exhibits superior performance, i.e. overall lower tan δ and smaller change on aging.

Table 5. Vulcanizate viscoelastic properties (cure:150°C/30 min; temperature:70°C; strain:2%; frequency:15Hz)

Stocks G’, MPa G”,MPa tan δ % increase in tan δ vs control Control Unaged 3.61 0.45 0.124 Aged 2d/100°C 5.84 0.78 0.134 Aged 3d/100°C 6.94 0.98 0.142 14

Pk 900 Unaged 4.05 0.50 0.123 Aged 2d/100°C 5.12 0.63 0.123 Aged 3d/100°C 5.74 0.72 0.125 1

DHTS Unaged 3.50 0.41 0.117 Aged 2d/100°C 4.38 0.56 0.127 Aged 3d/100°C 4.64 0.61 0.132 6

DHTS/Pk Unaged 3.86 0.45 0.116 Aged 2d/100°C 5.45 0.64 0.117 Aged 3d/100°C 5.53 0.65 0.118 - 5

16 COMPOUND STABILIZATION WITH QUINONEDIIMINE

Reversion occurs upon overcure and cure at high temperatures and also continues during dynamic service and is catalyzed by zinc ion - accelerator complexes. Amines in particular, have a deleterious effect on reversion resistance, and typical amine based antidegradants can promote reversion of the sulfur network and at the same time protecting against oxidative degradation. If the polysulfidic network can be protected during aging, this would help to provide improved dynamic mechanical properties, improved fatigue resistance, and help to maintain tear strength upon aging.

Possible methods to prevent reversion promoted by an amine derived antidegradant would be to employ non-amine antidegradants or use a polymer-bound amine based antidegradant. However, non-amine antidegradants do not sufficiently protect conventional sulfur vulcanizates from oxidation and ozone degradation. The use of only a polymer-bound antidegradant is also insufficient. Both ozone and oxygen cause degradation predominantly at the surface of the rubber, thus requiring diffusible antidegradants. 13 Ozone is so reactive that ozone degradation is considered to be a surface phenomenon while degradation due to oxygen occurs not only at the surface, but also deeper into the rubber. Thus, polymer bound antioxidants can provide a certain amount of protection, but must be used in conjunction with antiozonants/antioxidants which diffuse in rubber.

Quinonediimines are multifunctional rubber chemicals,14 which become polymer bound during mixing, processing, and during vulcanization, thereby providing non- fugitive antioxidant characteristics. Raevsky et al. found quinonediimine inhibited oxidation of the rubber. 15 The authors concluded that this material had chemically bonded to the polymer, forming relatively stable high molecular weight radicals by the reaction of the polymer radical with the quinonediimine.

Cain et al. studied black filled, sulfenamide cured NR compounds containing various quiinonediimines. 16 Thin layer chromatography and UV spectroscopy of the azeotropic extracts of the NR compounds showed 60-75% of the original quinonediimine converted to the corresponding p-phenylene diamine (PPD). The remaining quinonediimine could not be detected. In vulcanizates compounded with PPD’s, 90 - 98% of the p-phenylene diamine was recovered. As also found in Raevsky’s work the extracted (quinonediimine protected) vulcanizates showed two to three-fold increases in time to 1% oxygen uptake when compared to similarly extracted vulcanizates protected by the corresponding PPD. This additional evidence reinforced the claim that some of the quinonediimine had been bound to the rubber network and resulted in antioxidant activity.

In natural rubber compounds, 30-40% of added quinonediimine may become polymer bound during vulcanization. This bound antidegradant provides persistent antioxidant characteristics. However, since it is bound to the polymer backbone, it may not contribute to the degradation of the polysulfidic crosslinks.

Previous studies 17 show that quinonediimine (QDI) antidegradant provides better network stabilization than the corresponding para -phenylene diamine antidegradants. Figures 13 – 17 show the effects of QDI in a natural rubber compound. 2 phr of QDI was compared to a control with no antidegradant; 2 phr of 6 PPD; and 2 phr of 6 PPD/QDI at 1/1 ratio.

Figure 13

Processing & Curing Properties in NR (Effects of QDI)

Mooney t5 (min.) t90 (min.) Max.Torq. (Nm)

35 30 25 20 15 10 5 0 Control 6PPD QDI PPD/QDI

Figure 13 shows that there are no significant differences in processing and curing properties in the compounds evaluated. Figures 14, 15, & 16 show the effects of QDI on vulcanizate properties, namely modulus, tensile, and elongation. Extraction (Azeotropic mixture – Acetone/chloroform/methanol 12hrs./50°C), followed by aging (5 days/80°C), serves to mimic service conditions of the rubber. It is evident from the data presented in these figures that QDI provides an increase in modulus. This is not unexpected as it has been demonstrated that QDI does react with the polymer chains. With respect to the retention of tensile and elongation at break values, QDI provides much higher retention of the tensile and elongation at break after extraction when compared to the 6PPD control. This may be because the bound QDI is not extracted and continues to function as an antidegradant while the 6 PPD is no longer present to provide protection after extraction.

Figure 14

100% Modulus (MPa) in NR -- Effect of QDI

Unaged Aged(5d/80C) Extracted/Aged

6 5 4

3 2 1 0 Control 6PPD QDI PPD/QDI

Figure 15

Tensile Strength (MPa) in NR -- Effect of QDI

Unaged Aged(5d/80C) Extracted/Aged

30 25 20

15 10 5 0 Control 6PPD QDI PPD/QDI

Figure 16

Elongation (%) @ Break in NR -- Effect of QDI

Unaged Aged(5d/80C) Extracted/Aged

600 500 400

300 200 100 0 Control 6PPD QDI PPD/QDI

20 Positive influences on the hysteresis can be expected if QDI improves the interaction with carbon black. This viscoelastic property is shown in Figure 17. Extraction was done in water for 14 days at 50°C followed by aging in air for 3 days at 100°C. The compounds containing QDI show reduced tangent delta indicating reduced energy losses, an attribute that is expected to positively influence the rolling resistance of tires.

Figure 17

Tangent Delta in NR -- Effect of QDI

Unaged Aged (3d/100C) Extracted/Aged

0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 Control 6PPD QDI PPD/QDI

21 In order to correlate the better aging performance of the QDI vulcanizates, the networks of the vulcanizates (before and after aging) were analyzed. Total crosslinks, poly-, di- and monosulfidic crosslinks for the compounds are shown below in Table 7.

Table 7 - Crosslink Density and Distribution of Crosslink Types (Cure 150°C/t90)

XL-Density* Control 6PPD(2phr) QDI(2phr) PPD/QDI(1/1) (1) (2) (3) (4)

Total 4.95 5.01 5.20 5.05 Poly- 3.96 3.98 4.02 4.01 Di- 0.84 0.85 0.88 0.85 Mono- 0.15 0.18 0.30 0.19

Aged 5days/80°C Total 5.15 5.20 5.30 5.15 Poly- 2.31 3.10 3.76 3.61 Di- 0.64 0.70 0.77 0.72 Mono- 2.20 1.40 0.77 0.72

* Crosslink density expressed in gm.mole/gm. Rubber hydrocarbon X 10 5

The study indicates that the total crosslink densities of the networks, both before and after aging, are slightly higher than that of the control compounds containing 6PPD. After aging, the most notable characteristics observed correlate to a reduction in the extent of measurable reversion by network analysis. A higher percentage of the polysulfidic network is retained and fewer monosulfide crosslinks are measured.

Networks experiencing reversion exhibit a reduction in polysulfidic crosslink content and an increase in monosulfidic crosslink content. In addition, zinc sulfide, cyclic sulfides, and conjugated dienes and trienes form when reversion occurs. 2, 18 All of the features of reversion tend to degrade the physical properties of the compound.

The most notable feature from this study is evidence that upon aerobic aging the extent of reversion is reduced when quinonediimine antidegradants are used. When compared to compound containing 6PPD a higher percentage of polysulfides remained in the QDI protected compounds after aging. In addition, fewer monosulfidic crosslinks are formed upon aging rubber containing QDI as an additive. The data suggest that compounds containing QDI have less free amine to attack the polysulfidic crosslinks thereby leaving more crosslinks of higher sulfur rank intact. Furthermore, model compound studies show that, when QDI reacts with carbon free radicals, attachment can occur on the alkyl nitrogen atom and on the quinoidal ring as well. Once QDI becomes attached to the polymer backbone, its mobility is limited and its chemistry may well

22 change to that of a PPD containing a tert. -amine group. These tertiary amine sites could effectively complex zinc ions and zinc-accelerator fragments thereby restricting their mobility in the vulcanizate.

These proposed polymer-bound PPD-zinc complexes may account for less reversion, by preserving polysulfidic crosslinks, and provide persistent antioxidant activity. In any event, a change in the mechanism of conventional protection of a vulcanizate composition appears to occur when QDI reacts with the polymer backbone.

The quinonediimine antidegradant converts primarily to PPD or (as evidenced by previous studies) becomes bound upon vulcanization. Protection by quinonediimine antidegradant should mimic the antioxidant protection of PPD except that the bound portion offers persistent antidegradant properties when the rubber is exposed to aggressive environments (i.e. solvents, acid rain, aqueous detergents, etc.) Aging behavior in this study demonstrates that oxidative aging performance of QDI protected vulcanizates is similar to PPD protected compounds, but reversion under aerobic conditions is reduced.

IMPROVED PRODUCTION EFFICIENCIES

The rubber industry is continually striving to improve productivity and reduce unit manufacturing costs while maintaining a high level of product quality and performance. Improving production efficiencies is critically important for controlling costs and improving the profitability of the rubber industry. With today’s steadily increasing demand for tires, capacity improvements in mixing are becoming more highly valued. In addition, increasing demands for difficult to mix compounds require further increases in mixing capacity. These compounds not only include the new silica tread compounds but traditional carbon black filled materials such as steel skim, bead, chafer, apex and truck tread compounds. These compounds are generally composed of 100% natural rubber or high natural rubber content in blends with BR or SBR (tread applications.) They are filled with highly reinforcing-small particle carbon black. These finer grade carbon blacks are notoriously difficult to mix. Trends toward higher mileage truck tires shift the demand in tread grade blacks to finer particle, harder to mix carbon blacks, further exacerbating the problem.

In the production environment, the Mooney viscosity of the compound gauges the quality of the mix. Only upon achieving a predetermined Mooney viscosity can further downstream processing continue (calendaring and extrusion). Often these difficult to mix compounds require several passes through a mixer in order to achieve the desired viscosity. While pre-mastication techniques and additions of oils or soaps may reduce compound viscosities, physical and dynamic mechanical properties often suffer.19

QDI enhances mixing performance and viscosity reductions in natural rubber compounds with equal or improved vulcanizate properties. Work in large-scale mixers have shown 20 that capacities for difficult to mix tread black compounds can be increased

23 significantly when QDI is employed. In addition, performance characteristics of the mixes are favorably improved. The formulations and mixing conditions are provided in Table 8. The result from the mixing experiment using N-234 carbon black is presented in Figure 18 .

Table 8 Figure 18 Mix Formulation Control Neat First Pass RSS #2 100.00 100.00 N-234 30.00 30.00 QDI 2.00 Stearic Acid 2.00 2.00 ZnO 3.50 3.50 6 PPD 2.00 1.00 TMQ 1.00 Micro Wax 1.00 1.00 Total 139.50 139.50 2nd Pass 1st Pass MB 139.50 139.50 N-234 20 20 Total 159.50 159.50 3rd Pass 2nd Pass MB 159.5 159.5 Total 159.50 159.50 4th Pass 3rd pass MB 159.5 159.5 Sulfur 1 1 TBBS 1 1 Total 161.5 161.5

This work concluded that the use of QDI to control natural rubber compounds viscosity during mixing would result in productivity increase through the shortening of mix cycle. 20 Up to 25% in mix time reduction under factory mixing conditions have been demonstrated without penalty to physical properties. The added raw material cost of using QDI as a productivity enhancer is generally off-set by the potential savings from mix time reduction. 21

HIGH TEMPERATURE CURE

. Historically, the vulcanization process has restricted productivity improvements in many rubber manufacturing plants. Extended time at elevated temperature is required to provide a sufficient state of vulcanization throughout a rubber article. An obvious way to reduce vulcanization cycle time is by increasing the vulcanization temperatures, since the rate of sulfur vulcanization increases as the reaction temperature is elevated. However, the state of vulcanization is adversely affected with increasing temperatures. This is manifested in lower physical properties and product performance.

When sulfur vulcanized natural rubber compounds are exposed to a thermal aging environment, significant changes in physical properties and performance characteristics are observed. These changes are directly related to modifications of the original crosslink structure. Decomposition reactions tend to predominate and thus leading to reduction in crosslink density and physical properties as observed during extended cure and when using higher curing temperatures.

Over the years the rubber industry has developed several compounding approaches to address the changes in crosslink structure during thermal aging. As with many formulation changes in rubber compounding, there is a compromise that must be made when attempting to improve one performance characteristic. For example, improving the thermal stability of vulcanized natural rubber compounding by reducing the sulfur content of the crosslink through the use of the more efficient vulcanization systems will reduce dynamic performance property such as fatigue resistance. The challenge is to define a way to improving thermal stability while maintaining dynamic performance characteristics.

Earlier work 7,9 showed the use of DHTS and Pk 900 in high temperature cure applications can maintain compound properties while significantly reducing the cure cycles. Figures 19 and 20 summarize some of the key findings.

Figure 19

Effects of DHTS on High Temperature Cure @ 180°C in NR

%Mod.Ret. %Ten.Ret. Rev./Mod. Rev./Ten. Heat BU

90 80 70 60 50 40 30 20 10 0 0.6TBBS/2.5S 0.6TBBS/2.5S/2DHTS

Figure 20

Effects of Pk 900 on High Temperature Cure @ 160°C in NR

Sh. A Ten.(Mpa) %Elong.x10 Mod.(Mpa) Abrasion

180 160 140 120 100 80 60 40 20 0 0.6CBS/2.5S/150C 0.6CBS/2.5S/160C +1Pk900/160C

26 Recent studies further confirm that the use of DHTS and Pk900 would enhance the durability of compounds when cured at higher temperatures. This is illustrated in Table 9 and Figure 21. Figure 21 compares several properties of DHTS and Pk900 based cure systems in a natural rubber compound cured at 150°C and 180°C. The cure systems tested are given in Table 9. In brief, the combination of DHTS and Pk900 at the 1.5 / 0.5 level gave the best overall properties retention. Table 9

High Temperature Cure Systems Comparison in NR

Test Compound Control Semi-EV DHTS Pk900 DHTS/Pk900 Semi-EV/DHTS

TBBS 0.6 1.5 0.6 0.6 0.6 1.5

Sulfur 2.5 1.5 2.5 2.5 2.5 1.5

DHTS -- -- 2.0 -- 1.5 2.0

Pk900 ------0.75 0.5 --

Figure 21

High Temperature Cure @ 180°C In NR

Rev.@180C %Mod. Ret. %Ten.Ret. F-T-F@180C

160 140 120 100 80 60 40 20 0 Cont. S. EV DHTS Pk900 DHTS/PK SEV/DHTS

27 CONCLUSIONS

The use of DHTS and Pk 900 alone or in combinations gives significant benefits to the network through the formation of the more stable hybrid crosslinks with DHTS and or compensating effect with Pk 900 as the polysulfidic crosslinks degrade. This preserves the overall crosslink density of rubber compounds and hence maintains performance at close to the original levels. Although semi-EV cure systems can be a cost-effective mean to retain properties close to the original levels, the higher mono- and disulfidic linkages may result in a lower initial level of flex and tear properties.

Quinonediimines (QDI) offer the benefit of becoming bound antioxidants that have low extractability from the compounds, offering longer-lasting protection to the polymer backbone. Compared to amine based antidegradants such as 6PPD, they preserve the polysulfidic crosslinks and hence also preserve the dynamic properties of the compound.

These are only a few of the many alternatives available to the compounder when designing for longevity for his rubber products. No single solution is best and compromise is always necessary. The interaction between various ingredients is complex and the chemistry is still being researched to find synergies to bring about new levels of performance.

It has been demonstrated that Quinonediimine (QDI) through its attribute of NR viscosity modification with no adverse effects on compound properties provides the compounders with a cost performance approach to reduce non-productive mix cycle. High temperature curing with no loss in properties is now possible by the using of DHTS and Pk900 either alone or in combinations. These effective approaches for productivity increase give the compounders the flexibility in maximizing productivity with minimum sacrifice in compound properties.

28 NOMENCLATURE

Abbreviation Chemical Name 6 PPD N-(1,3- Dimethylbutyl)-N’-phenyl-p-phenylenediamine CBS N-Cyclohexyl-2benzothiazolesulfenamide CTP N-(Cyclohexylthio) phthalimide DCBS N,N-dicyclohexyl-2-benzothiazolesulfenamide DHTS Hexamethylene-1,6-bis Thiosulfate disodium salt, dihydrate PK 900 1,3-Bis(citraconimidomethyl) benzene QDI Quinonediimine TBBS N-Tert-butyl-2-benzothiazolesulfenamide TMQ Polymerized 1,2-dihyro-2,2,4-Trimethylquinoline

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