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Abstract

The aim of this study was to investigate the influence that different fatty acid zinc salts had on the rheological, curing and mechanical properties of natural rubber based composites filled with silica and containing bi-functional organosilanes in the presence or absence of zinc oxide. The results demonstrated that the combination of zinc oxide and zinc soaps had a strongly pronounced anti-reversion effect. The absence of reversion in the cure curves of the rubber compounds comprising a combination of zinc oxide, stearic acid and zinc soaps, results in retention of their mechanical properties, even after overcure.

Keywords

Natural rubber Fatty acid zinc salts Vulcanization Activator Curing properties Mechanical properties

1. Introduction

The accelerated sulfur vulcanisation of rubber is a complicated chemical process. The detailed reaction scheme of the vulcanisation process is still not well studied, although the practical application of rubber vulcanisates has been known for more than 150 years¹.

The mechanical properties of rubber vulcanisates are dependent to a great extent on the type of the vulcanisation process and the chemical nature of the crosslinks. Those also determine the application of rubber composites. For instance, vulcanisates crosslinked by peroxides find application in components subjected to static loads – usually under pressure when a low compression set and good heat aging resistance are required. Sulfur-containing vulcanisates find application in articles subjected to dynamic deformation hence need to be more elastic. Having lower dissociation energy polysulfide crosslinks are destroyed easier under any deformation, but their chemical nature affords their ability to transform, hence to lower the stress^2,3.

It is known that efficient vulcanisation of rubber by elemental sulfur or by sulfur donors requires the presence of an activator. There are many substances known, mainly metal oxides (e.g. CdO, PbO, MgO etc.), activating to a different degree the accelerators during the sulfur vulcanisation⁴. Yet, zinc oxide is the most efficient and widely used in the practice inorganic activator^4–8. The activating effect of the zinc oxide is stronger expressed in the presence of fatty acids like stearic, palmitic, oleic ones⁹, etc.

The high activity of zinc oxide can be explained on the basis of the chemistry of complex formation. A preceding reaction with stearic acid forms the rubber hydrocarbon-soluble zinc stearate and releases water before the onset of cross-linking. Furthermore, ZnO is in many vulcanisation systems a precursor to zinc-derived accelerators⁴. There are other investigations supposing that Zn²⁺-ions form such active complexes with accelerators, which are more reactive than the accelerator itself¹⁰.

It seems that the complex, formed of accelerator, sulfur, divalent zinc from the zinc oxide and a fatty acid is the actual accelerating agent, whose activity controls the formation of sulfur cross-links during the vulcanisation. Action, similar to that of the fatty acids, is also performed by their zinc salts - so called zinc soaps¹¹. The zinc soaps of a short aliphatic chain (up to C₈) display an activating effect in the absence of zinc oxide¹², too. The substitution of zinc oxide by zinc soaps in the rubber compound, results in a decrease of zinc ions content, defined as eco-toxic¹³. Zinc salts of the organic acids with a longer aliphatic chain (C₁₆-C₁₈) have a less pronounced activating effect, but adding them to rubber compounds in which zinc oxide and thiazole accelerator are present, leads to improved plasticity of the compounds as well as to higher tensile strength and elasticity of their vulcanisates¹².

Depending on the fatty acids, structure, their zinc salts exhibit different effectiveness in the rubber compounds. Their properties are determined by the ratio of the organic portion to their metal content, as well as by their structure¹².

The influence of various soaps of organic acids has been investigated in natural rubber compounds in order to demonstrate their properties vs. structure dependency. The investigated rubber compound is intended for tyre production. It has been found that in some cases, the reversion resistance during vulcanisation is considerably improved¹³.

The reversion is a thermo-initiated process related mainly to the overcure. The reversion includes reactions leading to desulfurisation of the polysulfide cross-links and changes in the vulcanisation structure¹⁴. That results in worsened mechanical properties of rubber vulcanisates¹⁵. The reversion is most characteristic for natural rubber compounds vulcanised at over 155°C¹⁰.

The tendency in the production of tyres is the replacement of carbon black, used as a filler, by silica. When using silica as a filler, the presence of bi-functional organosilanes improves the filler dispersion during mixing like the zinc soaps do, but also sets up “polymer-filler” interactions during the vulcanisation. That results in an improved mechanical and dynamic properties of the vulcanisates what is obligatory for rubber compounds^16–19.

As mentioned above, a shortcoming of natural rubber is the reversion observed upon vulcanisation at temperatures over 155°C. That shortcoming can be overcome by the usage of various polyfunctional additives, e.g. different zinc salts of the fatty acids.

There are no literature data on the effect that zinc soaps have upon the reversion processes occurring during the vulcanisation of a natural rubber composite filled only with silica. A greater part of the studies dealing with the matter are on compounds filled mainly with carbon black or a combination of carbon black and a small amount of silica. Therefore, the aim of the present work is to investigate the influence of zinc soaps of different aliphatic units (fatty – zinc stearate; cyclic –zinc resinate and a mixture of fatty acids zinc salts (mainly unsaturated) – Plastikol) on the curing (including reversion resistance), rheological and mechanical properties of silica filled composites, based on natural rubber, in the presence and absence of zinc oxide.

2. Experimental

2.1 Materials

The investigations were performed using the following materials:

Natural rubber SVR 10 supplied by Hong Thanh Rubber Pty. Ltd. was used as a polymer matrix. Silica, Ultrasil 7000 GR from Evonik Industries was used as reinforcing filler. It had the following characteristics: Specific surface area (N₂) 175 m²/g; specific surface area (CTAB) 160 m²/g; loss on drying 5.2%; pore density 270 g/l; electrical conductivity ≤ 1300 μS/cm. The zinc resinate was purchased from Chemos GmbH, Germany. Plastikol was a trade product, comprising a mixture of different zinc salts of fatty acids. It was manufactured by the “Industrial Chemistry” Ltd company in Bulgaria. The acids in Plastikol were mainly unsaturated – about 50% oleic and 15% linoleic one. Plastikol incorporated also saturated fatty acids – about 30% of stearic and 5% of palmitic acid. The zinc content is about 8%. The other ingredients such as zinc oxide (Struktol WB 700A), stearic acid, zinc stearate, bis(triethoxysilylpropyl)tetrasulfide (TESPT, Si 69 – by Evonik Industries), N-tert-butyl-2-benzothiazolesulfenamide (TBBS – Vulkacit NZ/EGC, produces by Lanxess) and sulfur (Struktol SU 95AF) were also of commercial grades.

2.2 Measurements

Mooney viscosity of the investigated rubber compounds was determined according to ISO 289-1:2014. The vulcanisation characteristics of the rubber compounds were determined at 160°C on a Moving Die Rheometer (MDR 2000 Alpha Technologies) according to ISO 3417:2008.

The molecular weight of a rubber segment between two cross-links (M_c) of the investigated composites was calculated by the Flory-Rhener (1) equation after determining their equilibrium swelling degree in an organic solvent.

\frac{1}{M_{c}} = - \frac{[I n (1 - V_{r}) + V_{r} + μ V_{r}^{2}]}{V_{0} ρ (V_{r}^{1 / 3} - 0.5 V_{r})}

(1)

where: V_r – the volume fraction of rubber in equilibrium swollen vulcanisates sample; V₀ – molar volume of the used solvent; μ – Huggins parameter; ρ – density of the used solvent.

The mechanical properties of the composites studied were determined according to ISO 37:2011. The Shore A hardness of the composites studied were determined according to ISO 7619–1:2010, while the abrasion -according to ISO 4649:2010. Accelerated heat aging of the vulcanisates studied was tested according to ISO 188:2011. The coefficients of aging in terms of tensile strength (K_σ) and elongation at break (K_ε) were calculated by equations (2) and (3).

K_{σ} = \frac{σ_{2} - σ_{1}}{σ_{1}} \cdot 100, %

(2)

where: σ₁ – the tensile strength before aging; σ₂ – the tensile strength after aging.

K_{ε} = \frac{ε_{2} - ε_{1}}{ε} \cdot 100, %

(3)

where: ε₁ – the elongation at break before aging; ε₂ –the elongation at break after aging.

2.3 Preparation of rubber composites

The formulations of the compounds based on natural rubber – SVR 10 are presented in Table 1 .

Table 1
Compositions of the investigated rubber compounds (phr)

1 2 3 4 Z1 Z2 Z3 Z4

Natural rubber, NR-SVR 10 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

Silica, Ultrasil 7000 GR 60.0 60.0 60.0 60.0 60.0 60.0 60.0 60.0

Zinc oxide - - - - 4.0 4.0 4.0 4.0

Stearic acid - - - - 1.0 1.0 1.0 1.0

Zinc stearate - 5.0 - - - 5.0 - -

Zinc resinate - - 5.0 - - - 5.0 -

Plastikol - - - 5.0 - - - 5.0

TESPT, Si 69 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

TBBS 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5

Sulfur 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

	1	2	3	4	Z1	Z2	Z3	Z4
Natural rubber, NR-SVR 10	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0
Silica, Ultrasil 7000 GR	60.0	60.0	60.0	60.0	60.0	60.0	60.0	60.0
Zinc oxide	-	-	-	-	4.0	4.0	4.0	4.0
Stearic acid	-	-	-	-	1.0	1.0	1.0	1.0
Zinc stearate	-	5.0	-	-	-	5.0	-	-
Zinc resinate	-	-	5.0	-	-	-	5.0	-
Plastikol	-	-	-	5.0	-	-	-	5.0
TESPT, Si 69	5.0	5.0	5.0	5.0	5.0	5.0	5.0	5.0
TBBS	1.5	1.5	1.5	1.5	1.5	1.5	1.5	1.5
Sulfur	2.0	2.0	2.0	2.0	2.0	2.0	2.0	2.0

The rubber compounds were prepared in two stages according to the mixing protocol presented in Table 2 . At the first stage the mixing was performed on a Brabender Plasti-Corder PLE651 fitted with a 300 cm³ cam type mixer. The silane coupling agent (TESPT Si 69) was mixed with the filler studied prior to their placing into the mixing chamber. In the second stage, sulfur and the accelerator were added to the mixture compounded in an open two-roll laboratory mill L/D 320 × 160 and friction 1.27.

Table 2

Mixing protocol of the investigated rubber compounds

Stage 1, Brabender Plasti-Corder PLE651, Rotor speed 40 rpm, Temperature 140°C
Mixing order	Ingredients	Mixing time, min	Cumulative time, min
1	Natural rubber (SVR 10)	2	2
2	ZnO, Stearic acid, Zinc stearate, Zinc resinate, Plastikol	2	4
3	Silica (Ultrasil 7000 GR) and TESPT (Si 69)	5	9
Stage 2, Laboratory Two Roll Mills, Friction 1.27
1	1st stage rubber batch	2	2
2	Sulfur and TBBS	5	7

The vulcanisation process of the natural rubber based compounds was carried out in an electrically heated hydraulic press using a special homemade mould at temperature 160°C and under pressure 10 MPa.

3. Results and Discussion

3.1 Curing properties

Figure 1 presents the Mooney viscosity of the investigated rubber compounds. As seen from the figure, Mooney viscosity of the rubber compounds containing no zinc soaps (1 and Z1) was about 50 MU. In this case, the index was not affected by the presence or absence of zinc oxide and stearic acid. The presence of zinc soaps in the rubber compounds comprising no zinc oxide and stearic acid (2, 3 and 4) results in a Mooney viscosity decrease by about 15 MU. It could be noted that the plastifying effect of zinc resinate (2) is better than that of zinc stearate (1) and Plastikol (2). The plastifying effect of zinc soaps is a little bit more strongly expressed in the presence of zinc oxide and stearic acid (Z2, Z3 and Z4) (Mooney viscosity decreases by about 20 MU). In that case, the kind of soap used does not influence the viscosity of rubber compounds studied.

Figure 1

Mooney viscosity of the investigated rubber compounds

Figure 2 presents the cure curves of the investigated rubber compounds in the absence of zinc oxide, taken on a Monsanto rheometer at 160°C.

Figure 2

Cure curves of the investigated rubber compounds in the absence of zinc oxide taken at 160°C

The figure shows that the vulcanisation of all the compounds studied starts very rapidly. The scorch time (T_s2) is in the order of 30 s, which is a very short time for compounds produced with a sulfenamide accelerator. Obviously, after reaching the cure time (T₉₀), a strongly pronounced reversion occurs in the course of the cure curves for all investigated compounds. The least reversion is for the control compound (1) containing no zinc soap. According to the figure, the compounds containing zinc stearate (2) or Plastikol (4) have values of maximum torque (MH) higher than those of the compounds that were free from soap (1) or containing zinc resinate (3).

The cure curves of the compounds with various zinc soaps in the presence of zinc oxide and stearic acid, are presented in Figure 3 ; the vulcanisation of the control compound (Z1), containing only zinc oxide and stearic acid, also starts very rapidly. The maximum torque of this compound is the highest one (about 35 dNm) and a reversion in the cure curve course is also observed after reaching the cure time (T₉₀). In the case, the reversion is considerably less pronounced than that of the compounds with no zinc oxide and stearic acid (1) or containing only zinc soaps (2, 3 and 4) ( Figure 2 ). A radical change in the course of the cure curves is observed when a combination of zinc oxide, stearic acid and zinc soaps (Z2, Z3 and Z4) is present in the compounds. The scorch time (T_s2) is prolonged to 6 min, which is normal for compounds using sulfenamide accelerators. The maximum torque (MH) is lower than that of the control compound (Z1) containing no zinc soap, but no reversion in the curves course has been observed, i.e. there is a wide vulcanisation plateau. Probably the reason for that is the formation of a co-ordination complex between zinc oxide, zinc soaps, accelerator and sulfur which leads to a longer scorch time (T_s2) and to the formation of a more stable vulcanisation structure. That complex is probably also thermo-stabilising, preventing the destruction processes which at the temperature of vulcanisation proceed along the main chain of the elastomer.

Figure 3

Cure curves of the investigated rubber compounds comprising zinc oxide taken at 160°C

Table 3 summarises the basic vulcanisation characteristics of the investigated rubber compounds, determined by their cure curves presented above ( Figures 2 and 3 ).

Table 3

Curing properties of the investigated rubber compounds

	1	2	3	4	Z1	Z2	Z3	Z4
Min. torque, ML, dNm	3.7	3.3	2.2	3.0	3.1	1.3	1.4	1.3
Max. torque, MH, dNm	26.7	32.6	26.3	31.6	38.1	24.7	24.4	23.7
ΔM=MH-ML, dNm	23.0	29.3	24.1	28.6	35.0	23.4	23.0	22.4
Scorch time, T_s2, min:s	0:24	0:42	0:38	0:30	0:23	6:25	6:01	6:27
Cure time, T₉₀, min:s	3:27	5:15	4:16	4:22	8:36	14:28	14:25	15:16
Cure rate, V, %/min	33	22	29	29	13	12	13	10

The Table shows that the compounds containing no zinc oxide (1, 2, 3 and 4) have considerably higher cure rates and very short scorch times (T_s2). The presence of zinc oxide (Z1, Z2, Z3 and Z4) results in a considerable decrease of the cure rate. That is in compliance with Coran's theory¹⁰ for formation of a chelate complex between the zinc oxide, the thiazole ring of accelerator and the sulfur molecule bound to it, which results in a decrease of the cure rate and formation of a curing network in which the cross-links are of lower polysulfidity, yielding a more stable cross-link network.

Figure 4 presents the molecular weight of the rubber segment between two cross-links (M_c) of the composites of the investigated rubber compounds, obtained for different vulcanisation times. One can see from the figure that the M_c values of composites vulcanised for the cure time (T₉₀), containing zinc oxide (Z1) or a combination of zinc oxide and zinc soaps (Z2, Z3 and Z4) is lower than that of the vulcanisates containing no zinc oxide (1, 2, 3 and 4). This means that the density of the cross-link network formed in the vulcanisates containing zinc oxide is higher. Moreover, the cross-link network density of these composites increases after overcure (4xT₉₀), while that of the composites free from zinc oxide decreases. The results obtained are in compliance with the cure curves presented above. The presence of reversion in the curves course, i.e. the occurrence of destruction processes in the vulcanisates free from zinc oxide, is the reason for the lower density of their cross-link network, obtained after overcure (4xT₉₀).

Figure 4

Molecular weight of rubber segment between two cross-links (M_c) of the investigated composites

3.2 Mechanical properties

Table 4 presents the mechanical properties of the rubber composites vulcanised for cure time (T₉₀).

Table 4
Mechanical properties of the investigated composites, vulcanised for T₉₀

1 2 3 4 Z1 Z2 Z3 Z4

Modulus 100, M₁₀₀, MPa 1.9 1.9 1.7 2.0 2.2 2.2 1.9 2.3

Modulus 300, M₃₀₀, MPa 7.6 7.2 6.5 8.9 9.0 9.1 8.0 9.1

Tensile strength, σ, MPa 19.1 19.2 20.1 22.8 20.8 24.3 22.8 22.6

Elongation at break, ε₁, % 530 570 610 610 560 670 660 580

Residual elongation, ε₂, % 25 50 35 45 20 20 25 20

Shore A hardness 63 67 61 65 68 67 65 65

	1	2	3	4	Z1	Z2	Z3	Z4
Modulus 100, M₁₀₀, MPa	1.9	1.9	1.7	2.0	2.2	2.2	1.9	2.3
Modulus 300, M₃₀₀, MPa	7.6	7.2	6.5	8.9	9.0	9.1	8.0	9.1
Tensile strength, σ, MPa	19.1	19.2	20.1	22.8	20.8	24.3	22.8	22.6
Elongation at break, ε₁, %	530	570	610	610	560	670	660	580
Residual elongation, ε₂, %	25	50	35	45	20	20	25	20
Shore A hardness	63	67	61	65	68	67	65	65

As seen from Table 4 , modulus 100 values of the composites containing no zinc oxide and stearic acid (1, 2, 3 and 4) vary in the interval from 1.7 to 2.0 MPa and those of modulus 300, from 6.5 to 8.9 MPa. In such a case, it could not be said that the presence and the type of zinc soaps studied have a substantial effect on these indices. It is obvious that the residual elongation of the investigated vulcanisates is extremely high (about 35% to 50%) in the absence of zinc oxide and in the presence of zinc soaps (2, 3 and 4). As the Table shows, the presence of zinc oxide and stearic acid (Z1) or a combination of zinc oxide, stearic acid and zinc soaps (Z2, Z3 and Z4) results in an increase in the modulus 100 and modulus 300 values and in a decrease of the residual elongation.

The mechanical properties of the investigated composites, obtained after overcure (4xT₉₀) are shown in Table 5 .

Table 5

Mechanical properties of the investigated composites, vulcanised for 4xT₉₀

	1	2	3	4	Z1	Z2	Z3	Z4
Modulus 100, M₁₀₀, MPa	1.6	1.6	1.0	1.2	2.4	2.6	2.4	2.6
Modulus 300, M₃₀₀, MPa	5.4	5.5	3.1	4.6	9.0	12.1	10.8	10.8
Tensile strength, σ, MPa	14.5	14.7	12.6	14.3	15.5	22.7	23.1	20.0
Elongation at break, ε₁, %	560	560	670	630	480	510	615	520
Residual elongation, ε₂, %	30	40	35	40	25	25	30	30
Shore A hardness	63	65	56	60	70	70	67	66

According to the Table, after overcure (4xT₉₀), the values of modulus 100 and modulus 300 of the composites comprising no zinc oxide (1, 2, 3 and 4) decrease in comparison to those of the relevant composites vulcanised for the cure time (T₉₀ – Table 4 ). The values of modulus 100 and modulus 300 of the composites containing zinc oxide (Z1) or a combination of zinc oxide and zinc soaps (Z2, Z3 and Z4) increase after overcure.

Figure 5 presents the tensile strengths of the composites studied, obtained for different vulcanisation times. The Figure reveals that, for the composites containing no zinc oxide (1, 2, 3 and 4), the tensile strength of the control vulcanisate (1), obtained for cure time T₉₀ is about 19 MPa. The presence of zinc soaps in the vulcanisates free from zinc oxide, leads to an increase in the tensile strength, which is most strongly expressed for the composites containing Plastikol (4) whose tensile strength is about 22.8 MPa. It could be noted that the tensile strength of the composites studied in the presence of zinc oxide and stearic acid (Z1) as well as those with a combination of zinc oxide, stearic acid and zinc soaps (Z2, Z3 and Z4), are higher than those of the composites containing no zinc oxide. The tensile strength of the composites without zinc oxide (1, 2, 3 and 4) decreases considerably after overcure (T₉₀x4) in comparison to those of the composites vulcanised in the cure time (T₉₀). A similar tendency is also observed for the vulcanisate containing only zinc oxide and stearic acid (Z1). The composites with a combination of zinc oxide, stearic acid and zinc soap do not change their tensile strength after overcure (4xT₉₀). Moreover, the tensile strength of the composites containing a combination of zinc oxide and zinc resinate (Z3) increases insignificantly, even after overcure (4xT₉₀). The results obtained correlate entirely with the reversion observed in the cure curves of the rubber compounds without zinc oxide and zinc soap. The reversion in the course of the curves for the rubber compounds containing no combination of zinc oxide and zinc soaps (1, 2, 3, 4 and Z1) results in considerable worsening of the mechanical properties of their vulcanisates after overcure (4xT₉₀), compared to those of the relevant composites, obtained for the cure time (T₉₀). The absence of reversion in the cure curves of the rubber compounds containing a combination of zinc oxide, stearic acid and zinc soaps does not lead to changes in the mechanical properties of their vulcanisates, regardless of the duration of their vulcanisation.

Figure 5

Tensile strength of the investigated composites

The abrasion data of the investigated vulcanisates are presented in Figure 6 . It is clear that the vulcanisates made with a combination of zinc oxide and zinc soaps (Z2, Z3 and Z4) are much more abrasion resistant, i.e. their abrasion is lower than that of the vulcanisates containing no zinc oxide (1, 2, 3 and 4) or zinc oxide alone (Z1), which corresponds with their other mechanical properties.

Figure 6

Abrasion of the investigated composites

Figure 7 presents the coefficient of aging in terms of tensile strength of the composites studied, vulcanised for different vulcanisation times. As seen from the Figure, the vulcanisates containing no zinc oxide (1, 2, 3 and 4) are aging much more than those with zinc oxide (Z1) or a combination of zinc oxide and zinc soaps (Z2, Z3 and Z4). In the absence of zinc oxide, the composites obtained after overcure (4xT₉₀) are more prone to aging in comparison to those obtained for the cure time (T₉₀). That fact is the most pronounced in the case of the control sample (1) for which the tensile strength K_σ is about −40% for the composites vulcanised in (T₉₀) and about −80% for those vulcanised in 4xT₉₀. Exceptions are the composites containing zinc resinate (3), for which K_σ is almost equal, irrespective of the vulcanisation time. It could be noted that the vulcanisates made with zinc soaps (2, 3 and 4) in the absence of zinc oxide are more aging-resistant than the control vulcanisate (1) in terms of tensile strength.

Figure 7

Aging coefficient of aging of the investigated composites in terms of tensile strength (K_σ)

The vulcanisates containing zinc oxide (Z1, Z2, Z3 and Z5) are considerably more heat aging-resistant, since no change in the coefficient of aging (K_σ) according to the time of vulcanisation is to be noticed. Noteworthy on this point is that the presence of zinc soaps exerts additional positive influence on the aging resistance. Most probably, the cross-link network with lower polysulfidity of the cross-links, formed in the presence of zinc oxide or a combination of zinc oxide and zinc soaps, is the reason for the higher heat aging resistance of these vulcanisates.

Figure 8 presents the aging coefficient in terms of the elongation at break (K_ε) of the investigated composites, vulcanised for different times. The Figure shows no definite tendency of change in the aging coefficient (K_ε), depending on the presence of zinc oxide and/or zinc soap, for composites vulcanised for the cure time (T₉₀). For the overcured composites (4xT₉₀), the absence of zinc oxide results in a much more pronounced aging in terms of the elongation at break. That is again most clearly demonstrated by the control vulcanisate (1). It seems that in the presence of zinc oxide, composites vulcanised for 4xT₉₀ are more aging-resistant than those vulcanised for the cure time (T₉₀), although their aging coefficients (K_ε) values are close. The composites containing a combination of zinc oxide and Plastikol (Z4), are the most aging-resistant in terms of relative elongation at break, regardless of the vulcanisation time.

Figure 8

Aging coefficient of the investigated composites in terms of relative elongation at break (K_ε)

One can see from the results presented that the composites obtained in the presence of a combination of zinc oxide, stearic acid and zinc soaps have better mechanical properties, both before and after accelerated heat aging, even after overcure (4xT₉₀). The reason is in the absence of a reversion of the cure curves of these composites. Probably, a co-ordination complex is formed between zinc oxide, stearic acid, zinc soaps, accelerator and sulfur which yields composites with a more stable vulcanisation structure wherein cross-links of lower sulfidity prevail.

4. Conclusions

The influence that zinc soaps with different aliphatic parts (fatty – zinc stearate, cyclic – zinc resinate and a mixture of zinc salts of fatty acids (mainly unsaturated) – Plastokol) have on the rheological, curing and mechanical properties of composites based on natural rubber and filled with silica has been investigated. It was found that:

–

The presence of the investigated zinc soaps results in a considerable decrease of Mooney viscosity of the rubber compounds studied, which is much more pronounced in the presence of zinc oxide and stearic acid.

–

The combination of zinc oxide and zinc soaps has a strongly pronounced anti-reversion effect. Probably, the reason is the formation of a co-ordination complex between zinc oxide, zinc soaps, accelerator and sulfur, what produces composites of a more stable vulcanisation structure.

–

The absence of reversion results in retention of the mechanical properties of the investigated composites, regardless of the duration of their vulcanisation.

Footnotes

Acknowledgements

The present research is a result of an international collaboration program between the University of Tabuk, Tabuk, Kingdom of Saudi Arabia and the University of Chemical Technology and Metallurgy, Sofia, Bulgaria. The authors gratefully acknowledge the financial support from the University of Tabuk.

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