Relationship Among Thermal Ageing Degradation, Dynamic

Polymer-Plastics Technology and Engineering, 46: 113–121, 2007 Copyright # Taylor & Francis Group, LLC ISSN: 0360-2559 print/1525-6111 online DOI: 10.1080/03602550601152861

Relationship Among Thermal Ageing Degradation, Dynamic Properties, Cure Systems, and Antioxidants in Natural Rubber Vulcanisates Vorapong Pimolsiriphol Department of Chemistry, Faculty of Science, Mahidol University, Bangkok, Thailand

Pongdhorn Saeoui The National Metal and Materials Technology Center ðMTECÞ, Klong Luang, Pathumthani, Thailand

Chakrit Sirisinha Department of Chemistry, Faculty of Science, Mahidol University, Bangkok and Thailand and Rubber Research Unit, Faculty of Science, Mahidol University, Salaya Campus, Salaya, Nakhon Pathom, Thailand

The present work aims to study the relationship among the thermal ageing stability, dynamic properties, cure systems, and antioxidants in natural rubber (NR) vulcanisates. Thermal degradation behavior of NR vulcanisates has been investigated and correlated to the changes in cross-link density, tensile and dynamic mechanical properties. The results obtained show that thermal ageing properties of NR vulcanisates depend strongly on cross-link density, which changes during thermal oxidative ageing or the so-called postcuring effect. In addition, the increases in ageing temperature and time lead dominantly to the postcuring and linkages scission phenomena in vulcanisates cured with CV and EV systems, respectively. With increasing ageing temperature, the tensile strength shows sharp drop at ageing temperature higher than 70C and 100C for the specimens cured with CV and EV systems, respectively. The sharp drop of tensile strength of vulcanisates cured with CV system is attributed to the too high cross-link density, which is caused by the postcuring effect. In the case of the vulcanisates cured with EV system, the linkage scission causes the sharp drop of tensile strength. The addition of amine-based antioxidant appears to improve ageing properties. However, the excessive antioxidant reduces tensile properties via a decrease in cross-link density. Keywords Ageing; Cross-link density; Dynamic mechanical properties; Natural rubber; Tensile properties

1. INTRODUCTION Nowadays, natural rubber (NR) has particularly been recognized mainly in terms of its high mechanical strength, Address correspondence to Chakrit Sirisinha, Department of Chemistry, Faculty of Science, Mahidol University, Rama 6 Rd., Bangkok 10400, Thailand. Fax: þ 662-441-9322; E-mail: [email protected]

and good tack. Therefore, NR is still of great technical importance for the rubber industry, although there are attempts to replace NR with some synthetic polymers. However, NR has some disadvantages, especially, in terms of its poor thermal, oil, and chemical resistances. When sulfur-cured NR products are exposed to a thermal oxidative ageing environment, severe changes in tensile and dynamic mechanical properties are resulted[1]. These changes are directly related to changes in an original cross-link structure and=or a main-chain-scission[1–5]. To overcome this problem, chemical substances, known as antioxidants, are added to rubber mixes for prolonging a service life of rubber vulcanisates. Dynamic mechanical and tensile properties of rubber vulcanisates are considered as important properties for many engineering applications. Numerous studies on the dynamic mechanical properties of various rubber vulcanisates have been reported, including the effects of types and amounts of rubber[6,7] and fillers[8–10], the interaction between rubber and filler[11], the compounding ingredients and the state-of-cure[11,12]. It is found that changes in the cross-link structures (i.e., cross-link density and cross-link type) of NR vulcanisates cause variation in glass transition temperature (Tg), elastic modulus (G0 ) and damping factor[1]. Evidently, although there are numerous published works on thermal degradation, dynamic properties, cross-link type, and cross-link density of NR vulcanisates, no correlation among those properties has been drawn. Consequently, the present work investigates NR vulcanisates with various cure

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TABLE 1 Materials used in the present study Chemical name

Grade=supplier

Natural rubber (NR) Zinc oxide (ZnO) Stearic acid N-tert-Butyl-2-Benzothiazole sulfenamide (TBBS) Sulfur 2, 2, 4-Trimethyl-1, 2-Hydroquinoline (TMQ) N-(1, 3-Dimethylbutyl)-N0 -Phenyl-P-Phenylenediamine (6-PPD)

systems, ageing conditions, and amine-based antioxidant concentrations. Thermal degradation behavior of NR vulcanisates has been discussed and correlated to changes in cross-link density, sulfidic linkage type, and dynamic mechanical properties. 2. EXPERIMENTAL 2.1. Materials The materials used in the present study are summarized in Table 1. Notably, the sulfur to accelerator concentration ratios are 4:1 and 1:4 for the sulfur conventional vulcanization (CV) and efficient vulcanization (EV) systems to ensure the dominantly formation of polysulfidic and monosulfidic linkages in the former and latter systems, respectively. 2.2. Mixing and Vulcanization Procedures The compounding ingredients as shown in Table 2 were mixed in a laboratory-scale Brabender internal mixer equipped with cam-type rotors at mixing temperature, rotor speed and fill factor of 40C, 40 rpm and 0.65, respectively. NR was first charged into the mixing chamber and masticated for 2 min, followed by the addition of ZnO and stearic acid. After further mixing for 7 min, the remaining chemicals TABLE 2 Compounding formulation used in the present study Ingredients

CV system (phr)

EV system (phr)

NR ZnO Stearic acid TBBS TMQ 6-PPD Sulfur

100.0 5.0 2.0 1.0 2.0 2.0 4.0

100.0 5.0 2.0 4.0 2.0 2.0 1.0

STR 5L, Union Rubber Products Co., Ltd., Thailand Commercial=Chemmin Co., Ltd., Thailand Commercial grade=Petch Thai Chemical Co., Ltd., Thailand Santocure TBBS=Flexsys Co., Ltd., Belgium Commercial Grade=Chemmin Co., Ltd., Thailand Vulkanox HS=Bayer Co., Ltd., Germany Santogard 6-PPD=Flexsys Co., Ltd., Belgium

were added and mixed for 3 min. Finally, the compound was dumped, sheeted on the cold 2-roll mill and kept at room temperature for 24 h before testing. To prepare rubber vulcanisates for property measurement, uncured rubber sheets were compression molded using a hydraulic hot press (Wabash Genesis Series model G30H, Wabash, IN, USA), at 160C under pressure of 15 MPa. Cure time set for curing the sample agreed with the time to achieve 90% complete cure (tc90) as determined from the Rubber Process Analyzer or RPA 2000 (Alpha Technology model RPA 2000, Akron, OH, USA). 2.3. Mechanical Properties Tensile properties were measured per ASTM D 412-92 using the universal tensile tester (Instron model 4301) at a crosshead speed of 500 mm=min with the load cell of 1 kN. The stress-strain curves were then recorded. Dynamic mechanical properties of vulcanisates were determined with the use of dynamic mechanical thermal analyzer (Gabo Eplexor qualimeter, Germany) in tension mode of deformation. The static and dynamic strains were set at 1% and 0.1%, respectively, with the test frequency of 10 Hz. Temperature was swept from 90C to 20C at a heating rate of 2C=min. Storage modulus (E0 ) and damping factor (tan d) of NR vulcanisates as a function of temperature were recorded. 2.4. Thermal Ageing Resistance Measurement Ageing test of NR vulcanisates was performed to determine influences of elevated temperature and time on changes in tensile and dynamic mechanical properties of vulcanized rubber, compared with the unaged specimens, based on ASTM D 573-88. The dumbbell specimens were placed in a hot-air oven at various ageing temperatures of 60, 70, 80, 100, and 120C for 48 h. At the end of the ageing period, the specimens were removed from oven, cooled to room temperature on a flat surface, and allowed to rest neither less than 16 h nor longer than 96 h before performing tests for tensile

THERMAL AGEING, DYNAMIC PROPERTIES IN NR VULCANISATES

properties, percentage swelling, and dynamic mechanical properties. To investigate the ageing time effect, dumbbell specimens were aged in a hot-air oven at a given temperature, for various ageing times of 1, 2, 4, 6, and 10 days. Thereafter, the tensile properties, percentage swelling, and dynamic mechanical properties were determined, and compared with those of unaged specimens. 2.5. Cross-link Density Small pieces of vulcanisates with a total weight of approximately 0.3 g were immersed in 30-ml toluene for 7 days at room temperature. The swollen specimens were blotted with filter paper and transferred quickly to a weighing bottle. Degree of cross-link density was presented in terms of percentage swelling, which was determined from the amount of solvent uptake of rubber vulcanisates, as shown in Eq. (1), where W and Ws are weight of unswollen and swollen rubbers, respectively. Ws W % Swelling ¼ 100 ð1Þ W

3. RESULTS AND DISCUSSION 3.1. Influences of Ageing Temperature and Time on Properties of NR Vulcanisates 3.1.1. Ageing Temperature Effect Figure 1 represents the effect of ageing temperature on tensile strength of NR cured with CV and EV systems. It is clear that, at ageing temperature lower than 70C, tensile strength of both NR vulcanisates is not dependent on

FIG. 1. Effect of ageing temperature on tensile strength of NR cured with CV and EV systems.

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vulcanization system. This might be explained by the insufficient thermal history to disrupt the polysulfidic linkages of the vulcanisates cured with CV system, or monosulfidic linkages of those with EV system. The tensile strength of NR vulcanisates shows sharp drops at ageing temperatures of approximately 70C and 100C for CV and EV systems, respectively. Theoretically, the decrease in tensile strength can occur from two main mechanisms that are chainscission and too-high cross-link density of vulcanisates. The former gives a reduction in molecular weight and thus molecular entanglement while the latter results in the energy dissipation reduction via molecular mobility restriction. As for the chain-scission effect, it is known that polysulfidic linkages having relatively low bond strength are dominant in CV system while monosulfidic linkages having relatively strong bond strength are dominant in EV system. As a consequence, there are poorer thermal ageing resistances in the vulcanisates cured with CV system[13]. Apart from the linkage type, the cross-link density is known to play strong role in tensile properties. It is evident from Fig. 2 that the percentage swelling, as an indicator for cross-link density, of NR vulcanisates cured with EV system is apparently higher than that of the vulcanisates cured with CV system. In other words, the vulcanisates cured with CV system posses greater cross-link density than those cured with EV system at any given ageing temperature. The result obtained can be explained by the amount of sulfur incorporated to the vulcanisates, which is larger in the case of vulcanisates cured with CV system. The larger the sulfur concentration, the higher the magnitude of vulcanization taking place during thermal ageing (i.e., the postcuring phenomenon). Also, Fig. 2 reveals the decrease in percentage swelling

FIG. 2. Effect of ageing temperature on percentage swelling of NR cured with CV and EV systems.

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FIG. 3. Sketch of relationship between cross-link density and tensile strength properties [15].

with increasing ageing temperature in the specimens cured with CV system, indicating an increase in cross-link density as ageing temperature increases. This phenomenon is believed to be because of the postcuring effect, which tends to increase if ageing temperature increases[14]. To correlate the cross-link density to the tensile properties, it is known that the tensile properties increase with increasing amount of cross-link density until the critical cross-link density is reached. Beyond this cross-link density, the tensile properties decrease as a function of cross-link density as illustrated in Fig. 3, i.e., the too-high density of cross-link would reduce the energy dissipation via the molecular mobility[15]. As a consequence, the increase in cross-link density via the postcuring effect might be a major reason for a sharp drop in tensile strength found in vulcanisates with CV system, as illustrated earlier in Fig. 1. By contrast, in NR vulcanisates with EV system, it is seen that the percentage swelling marginally increases with an increase in the ageing temperature (Fig. 2). At high ageing temperature, the chain-scission in the vulcanisates cured with EV system seems to override the postcuring effect. Thus, the decrease in tensile strength shown earlier in Fig. 1 is attributed mainly to the chain-scission rather than the postcuring phenomena. Figure 4 exhibits the effect of ageing temperature on modulus at 100% elongation (M100) of NR cured with CV and EV systems. According to Fig. 3, M100 is found to depend strongly on cross-link density, and therefore could be another indicator for tracking the change in cross-link density after thermal oxidative ageing. It is evident that M100 of the specimens cured with CV system is greater than that of the specimens cured with EV system, which supports the proposed explanation that the NR

FIG. 4. Effect of ageing temperature on modulus at 100% elongation (M100) of NR cured with CV and EV systems.

vulcanisates cured with CV system possess higher crosslink density than those with EV system at any given ageing temperature. Moreover, the increase in M100 of the vulcanisates cured with CV system with increasing ageing temperature supports the postcuring effect as discussed previously. Unexpectedly, M100 of the specimens cured with EV system does not obviously change with increasing ageing temperature. The unexpected result is believed to be caused by the insufficient sensitivity of M100 to a low extent of cross-link density in vulcanisates with EV system. Commonly, in the rubbery region, rubber is neither purely elastic nor purely viscous in its dynamic mechanical behavior, i.e., during the deformation, elastomeric materials store some portion of energy and dissipate the other. Generally, an elastic component (E0 ) represents an immediate response to the applied force while a viscous component (E00 ) represents energy dissipated as heat. The amount of offset between the measured stress and applied strain is a direct measure of a phase lag (d). A tangent of phase lag (tan d) or damping factor is defined as the ratio of E00 to E0 , and therefore is a measure of the energy dissipated by various processes, such as molecular mobility, breakdown and reformation of the filler particles, or slippage of rubber molecules under high strain amplitudes[16–18]. In the present work, two main points of dynamic mechanical properties are of interest. The first point is a storage modulus (E0 ) at 15C, which is an elastic response corresponding directly to the cross-link density of rubber in the rubbery state. The other is a damping factor (tan dmax), which can be related to the cross-link density of rubber. In other words, the results of dynamic mechanical


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the postcuring effect, which takes place during thermal oxidative ageing, as discussed previously. By contrast, the decrease in E0 of the vulcanisates cured with EV system results from the chain-scission dominating over the postcuring effect of vulcanisates. Figure 6 demonstrates the effect of ageing temperature on tan dmax of NR cured with CV and EV systems. In general, tan dmax would decrease with increasing cross-link density because of the increase in molecular restriction via intermolecular linkages. It is evident from Fig. 6 that tan dmax of the vulcanisates cured with CV system decreases as ageing temperature increases, clearly supporting the increase in cross-link density by postcuring effect. On the contrary, the increase in tan dmax of the vulcanisates cured with EV system with ageing temperature implies the increase in molecular mobility via the chain-scission of rubber vulcanisates.

FIG. 5. Effect of ageing temperature on E0 at 15C of NR cured with CV and EV systems.

properties will be used to support the result of cross-link density determined from the swelling test. Results of E0 and tan dmax of the NR vulcanisates, as a function of ageing temperature are illustrated in Figs. 5 and 6, respectively. It is evident from Fig. 5 that E0 of the vulcanisates cured with CV system increases whereas that of the vulcanisates cured with EV system decreases with increasing ageing temperature. The increase of E0 agrees very well with

FIG. 6. Effect of ageing temperature on damping factor (tan dmax) of NR cured with CV and EV systems.

3.1.2. Ageing Time Effect In some cases, the products of NR vulcanisates are subjected to high temperature, which may change properties of NR vulcanisates. Therefore, it is necessary to study the effect of ageing time on ageing properties of NR vulcanisates. Figure 7 illustrates the influence of ageing time on tensile strength of NR cured with CV and EV systems. It is evident that the onset of sharp drops of tensile strength of vulcanisates with CV system is seen at the ageing time of approximately 4 days while that of vulcanisates with EV system drops progressively with increasing ageing time. The smaller change in tensile strength at ageing time beyond 4 days is seen in vulcanisates cured with EV system illustrating the greater thermal stability of the monosulfidic linkages. The change in tensile strength by thermal

FIG. 7. Effect of ageing time on tensile strength of NR cured with CV and EV systems.

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FIG. 8. Effect of ageing time on percentage swelling of NR cured with CV and EV systems. FIG. 9. Effect of ageing time on M100 of NR cured with CV and EV system.

ageing can be explained by the alteration in cross-link density, as mentioned previously. With CV system, the postcuring effect would increase the cross-link density and therefore the excessive cross-link density would decrease the tensile strength (see also Fig. 3). Evidence of an increase in cross-link density as a function of ageing time is shown in terms of percentage swelling (Fig. 8). It can be seen that percentage swelling of the vulcanisates cured with CV system decreases with increasing ageing time because of the postcuring effect. In the case of the vulcanisates cured with EV system, there is a slight increase in percentage swelling as ageing time increases, indicating the decrease in cross-link density and=or the increase in chain-scission of the vulcanisates. The disruption of carboncarbon bonds in NR molecules leads to a main chainscission while that of monosulfidic linkage causes the scission of carbon-sulfur bonds. Because the bond energy of carbon-carbon bonds is higher than that of carbon-sulfur bonds (i.e., 346 vs. 272 kJ=mole)[19], the degradation of NR vulcanisates should dominantly occur at the carbon-sulfur linkages. As a consequence, the progressive decrease in tensile strength with ageing time as illustrated in Fig. 7 is believed to be due mainly to the thermal degradation at monosulfidic linkages rather than the NR main-chain. Figure 9 represents M100 of NR vulcanisates cured with CV and EV systems as a function of ageing time. M100 of the vulcanisates cured with CV system increases as ageing time increases while the M100 of the specimens cured with EV system does not significantly change with increasing ageing time. This phenomenon agrees with the change in cross-link density; the higher the cross-link density, the greater the M100, as mentioned previously. However, the unchange in the M100 in the vulcanisates cured with EV

system might be because of the small change in cross-link density is insufficient to influence the M100 as previously discussed in Fig. 4. Figure 10 demonstrates the effect of ageing time on E0 at 15C as determined from dynamic mechanical properties of vulcanisates with CV and EV systems. It is clear that E0 of the specimens cured with CV system increases while that of the specimens cured with EV system decreases as ageing time increases. The change in E0 could be explained by

FIG. 10. Effect of ageing time on E0 at 15C of NR cured with CV and EV systems.


FIG. 11. Effect of ageing time on damping factor (tan dmax) of NR cured with CV and EV systems.

the postcuring and linkages scission for vulcanisates cured with CV and EV systems, respectively, as discussed earlier. Similarly, the results of tan dmax as shown in Fig. 11 exhibits a decrease in tan dmax as ageing time increases, which supports the postcuring effect. Likewise, the increase in tan dmax with ageing time of the specimens cured with EV system is in good agreement with the proposed explanation based on a monosulfidic linkages scission as discussed earlier. 3.2. Influence of Antioxidant Concentration on Properties of NR Vulcanisates In this section, the various concentrations of amine-type antioxidant, 6-PPD, is incorporated to the NR vulcanisates cured with CV system, and its effect on thermal stabilization will be discussed based on the change in cross-link density via results of swelling and dynamic mechanical properties. Figure 12 demonstrates an effect of 6-PPD concentration on tensile strength of the vulcanisates. It is obvious that, at ageing temperature lower than 60C, tensile strength of NR vulcanisates cured with CV system depends strongly on 6-PPD concentration, i.e., tensile strength markedly increases with increasing concentration of 6-PPD content up to 2 phr. This implies the protection of rubber molecules from molecular chain-scission by antioxidant. Above 2 phr of 6-PPD, a drop in tensile strength could be seen, which might be explained by the cure retardation provided by an excess of 6-PPD[20]. The excessive 6-PPD could interfere the cross-linking process by scavenging the free radicals, which are required to cross-link rubber molecules, yielding in a reduction in cross-link

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FIG. 12. Effect of 6-PPD concentration on tensile strength of aged NR vulcanisates cured with CV system.

efficiency. Thus, the optimum concentration of 6-PPD used for stabilizing these rubber vulcanisates is 2 phr. In addition, the tensile strength of aged specimens shows sharp drops at ageing temperature higher than 60C. This is because of the over cross-link density taking place from the postcuring effect, and thus a reduction in energy dissipation via molecular mobility. A clear evidence of an increase in cross-link density as a function of ageing temperature is shown in Fig. 13. It is obvious that the percentage swelling of aged NR vulcanisates decreases with increasing 6-PPD concentration up to 2 phr because of the increase

FIG. 13. Effect of 6-PPD concentration on percentage swelling of aged NR vulcanisates cured with CV system.

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changes in cross-link density, tensile and dynamic mechanical properties. According to the results obtained, the following conclusions could be drawn: Ageing properties of NR vulcanisates depend strongly on cross-link density, which changes during thermal oxidative ageing. Increases in ageing temperature and time cause dominantly the postcuring and linkages scission for vulcanisates cured with CV and EV systems, respectively. With increasing ageing temperature, tensile strength shows sharp drops which are caused by the postcuring effect and linkage scission in vulcanisates cured with CV and EV systems, respectively. The addition of amine-based antioxidant, 6-PPD, appears to significantly improve ageing properties. However, the excessive antioxidant reduces a tensile properties via a decrease in cross-link density. FIG. 14. Effect of 6-PPD concentration on M100 of aged NR vulcanisates cured with CV system.

in thermo-oxidative protection. Beyond 2 phr of antioxidant concentration, the percentage swelling appears to increase. The swelling results agree well with that of tensile strength shown earlier in Fig. 12, i.e., the decrease in percentage swelling implies an increase in cross-link density leading to greater retention of tensile strength. The increase in percentage swelling at high antioxidant concentration of 4 phr indicates a decrease in cross-link density, probably attributed to the cure retardation effect by an excess of 6-PPD, as mentioned previously. Furthermore, the sharp drop in tensile strength at the ageing temperature of about 60C is in good agreement with the drop in percentage swelling that means an excessive cross-link density caused by the postcuring effect is responsible for the drop in tensile strength at ageing temperature beyond 60C. The effect of 6-PPD content on M100 of aged NR vulcanisates is illustrated in Fig. 14. M100 appears to increase as 6-PPD concentration increases up to 2 phr because of the increase in cross-link density via the postcuring effect, according to the percentage swelling result. The drop in M100 at antioxidant concentration greater than 2 phr is certainly caused by a decrease in cross-link density. Also, a sharp rise in M100 at the ageing temperature of about 60C is in good agreement with an abrupt increase in cross-link density, as discussed in the percentage swelling result as discussed previously in Fig. 13.

4. CONCLUSIONS Thermal degradation behavior of NR vulcanisates has been investigated and experimentally correlated to the

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