Impacts of Sulfur Curing Systems on Vulcanizations and Mechanical Performances of Elastomers: A Model Study Based on Sulfur Curing Systems and NR/SBR Blends

Abstract

Vulcanization is one of the most significant procedures in elastomer product processing. The components and their proportions of the curing systems significantly determined the vulcanization processes and mechanical properties of the products. To understand this effect, a model study was adopted for investigation, with sulfur curing system as the model curing system and natural rubber/styrene-butadiene rubber blends as model elastomers. By adjusting the doses of sulfur and sulfur/accelerant ratio, the entire sulfur curing systems were divided into three classifications, i.e., conventional vulcanization systems (CV), effective vulcanization systems (EV), and semi-effective vulcanization systems (SEV). Under these divisions, NR/SBR vulcanizate blends were obtained via seven specific curing systems. Upon preparation, the NR/SBR vulcanizates were thoroughly tested for mechanical properties. In a general trend, CV-based vulcanizates showed the advantages of tensile and tear strengths, EV-based vulcanizates possessed higher Young’s modulus and hardness, and SEV-based vulcanizates performed higher abrasion resistances. In addition, for each individual system, there would be an optimum sulfur/accelerant ratio by which the vulcanizates could be produced with enhanced mechanical properties.

1. Introduction

Vulcanization is a critical procedure in elastomeric product processing. During the vulcanization, the crosslinking formed between the polymer chains [1]. With appropriate specific curing systems, vulcanization could effectively improve the performances and stabilities of rubber products [1, 2]. Despite the development of novel curing systems in the lab during the past years, sulfur-based curing system is still one of the most widely used curing systems in the rubber industry [3]. Based on the sulfur usages and proportions, the sulfur curing systems could be categorized as conventional vulcanization systems (CV), effective vulcanization systems (EV) and semi-effective vulcanization systems (SEV) [3]. A major distinction among these classifications is the content and proportion of sulfur in the formula compositions, specially its ratio to vulcanization accelerator (abbreviated as sulfur/accelerant ratio). In CV systems, the sulfur/accelerant ratios are supposed to be greater than 2.5, while for EV systems, these ratios are less than 0.4. In the case of SEV systems, the sulfur/accelerant ratios are approximately around 1. It was estimated that during the vulcanization, the sulfur contents and the ratio to accelerants directly affect the formation of crosslinks, consequently affecting the configurations and/or conformations of chain segments, stereoregularities of polymer chains, and crystallization and/or orientation behaviors, thereby determining the mechanical properties of the vulcanizates [3]. Therefore, designing an appropriate curing system is a significant step for rubber processing. Some studies have indicated that the CV system is favorable for producing vulcanizates for dynamic applications, while the EV system should be considered for obtaining vulcanizates for static and hot environments [4, 5, 6, 7, 8]. However, these findings specifically focused on one type of curing system or a given example, which did not comparatively and comprehensively investigate the impacts induced by the chemical components and proportions (especially sulfur/accelerant ratios) of the curing systems, left some confusion and limitations in the applications of sulfur curing systems.

Therefore, to comprehensively understand the effects of sulfur curing systems on the vulcanization processes and related mechanical properties. A comprehensive set of vulcanizates were prepared through three sulfur curing systems. Natural rubber (NR) and styrene-butadiene rubber (SBR) were used as model elastomers due to their widespread usage. The NR and SBR were physically mixed and then vulcanized via CV, EV and SEV systems. After preparation, the vulcanizates were characterized by their mechanical properties, which included tensile strengths, tear strengths, elongations at break, Young’s modulus, hardness (Shore A) and relative volume abrasions.

2. Model study

2.1 Experiment

In this study, NR, SBR and other chemicals were purchased from Qingdao Chemical Company, two roll rubber mill was obtained from Dongguan Cfine Machinery Co. Ltd., rotorless curemeter, tensile tester (electronic mode), Shore durometer and rotating cylindrical drum device were purchased from GoTech Co Ltd., densimeter was purchased from Preciso Co Ltd. The scanning electron microscope (SEM) was purchased from JEOL.

The Formula composition of raw rubbers (in powder state), sulfur and other additives for preparations of NR/SBR vulcanizates were listed in the following Table 1.

	CV	EV	SEV
NR	360	360	360
SBR	240	240	240
High abrasion furnace black (HAF)	150	150	150
Silica	100	100	100
Oil	100	100	100
Zinc Oxide	30	30	30
Stearic acid	12	12	12
Paraffin	6	6	6
N-cyclohexyl-2-benzothiazolesulphenamide; (CBS vulcanizator)	Custom designed	Custom designed	Custom designed
isopropyl-N′-phenyl-P-phenylenediamine (4010NA, Antioxidant)	6	6	6
Tetramethylthiuram disulfide (TMTD, extra accelerator)	1.2	7.2	3.6
CaCO3	30	30	30
Sulfur	22	4	8.4

Table 1.

Formula compositions of vulcanizates in each curing system (unit: G).

NR/SBR physical mixture compound based on the ISO 2393:2014. The mixtures were vulcanized with a standard protocol ISO 3417:1991. The curing temperature was maintained at 145°C and the pressure was kept at 10 MPa, respectively. The whole curing procedures were investigated based on ISO 6502-1:2018. Upon the preparations, the vulcanizates were naturally cooled down. After that the samples were sputter coated and observed via scanning electron microscopy (SEM) to characterize the surface morphologies.

Mechanical properties were tested following the International Organization for Standardization (ISO). Specifically, tensile strengths were tested based on ISO 37:2017, tear strengths were tested based on ISO 34-1:2022, rebound resiliences were tested based on ISO 4662:2017, Young’s modulus were tested based on ISO 1827:2016 and the data were recorded at 300% of elongation, elongations (at break points) were tested based on ISO 2285:2019, Shore A hardness were tested on ISO 48-4:2018, relative volume abrasions were tested based on the ISO 4649:2017. The physical mixture of NR/SBR was also tested for tensile strength as the control.

3. Results and discussion

In this study, different NR/SBR vulcanizates were prepared under three curing systems. Due to the variations of the components in each sample, the experiment settings were adjusted individually, and Table 2 showed the optimum curing time for each sample obtained from the screening test. The NR/SBR vulcanizates were observed by SEM, as displayed in Figure 1. All the vulcanizates possessed a homogeneous phase, indicating the qualities that validate vulcanizates for mechanical tests. Figure 2 showed the tensile stress/strain properties of the vulcanizates. Compared to the physical mixture of NR/SBR, the vulcanizates under each curing system presented higher tensile stress and modulus, confirming the enhancement effect of vulcanization on rubber samples. From the stress/strain curves, it could also be found that the vulcanizates still present elasticities under initial elongation (Table 3) [9].

	Test sample #	Sulfur/accelerant ratio
CV	1	3.75
	2	3.33
	3	2.92
SEV	4	1.14
EV	5	0.42
	6	0.36
	7	0.2

Table 2.

Designs of test samples.

Figure 1.

SEM observations of NR/SBR vulcanizates. (a). From CV systems (sample 1); (b). From EV system (sample 6); (c). From SEV system (sample 4).

Figure 2.

Tensile stress/strain curves of vulcanizates under three curing systems.

Test sample #	Optimum curing time
1	6′44”
2	7′41”
3	6′15”
4	5′06”
5	7′00”
6	9′55”
7	10′18”

Table 3.

Curing time for each sulfur/accelerant ratio setting.

Mechanical properties of vulcanizates were shown from Figures 3-9. It could be found that the vulcanizates prepared by CV systems performed better in tensile and tear strengths (Figures 3 and 4). In addition, those prepared through EV systems had higher Young’s modulus and Shore A hardness (Figures 6 and 8), while those products prepared through the SEV system presented higher abrasion resistance (Figure 9).

Figure 3.

Tensile strengths of NR/SBR vulcanizates obtained from different curing systems. (unit: MPa).

Figure 4.

Tear strengths of NR/SBR vulcanizates obtained from different curing systems. (unit: MPa).

Figure 5.

Elongations at break of NR/SBR vulcanizates obtained from different curing systems. (unit: %).

Figure 6.

Young’s modulus of NR/SBR vulcanizates obtained from different curing systems. (unit: MPa).

Figure 7.

Rebound resiliences of NR/SBR vulcanizates obtained from different curing systems. (unit: % compression).

Figure 8.

Shore a hardness of NR/SBR vulcanizates obtained from different curing systems.

Figure 9.

Relative volume abrasions of NR/SBR vulcanizates obtained from different curing systems. (unit: mm³).

The impacts of sulfur curing systems could be attributed to their effects on the formations of crosslink bond crosslink densities. Based on the atoms engaged and the lengths of bonds, in sulfur curing systems, the crosslink bonds were generally divided into three types, i.e., polysulfur bond (mainly formed in CV systems), disulfide bond (usually formed in both EV and SEV systems) and monosulfur bonds (usually formed in SEV systems). The most significant differences among these types are the lengths of bonds and the bond energy. As shown in Table 4, the polysulfur bond has the lowest bond energy, bringing in the highest flexibility among all the candidates [9]. With such high flexibility, the polysulfur bonds are easy to reshape when encountering stress concentrations, and the polymer chains have enough time for orientation and crystallization. Besides, polysulfur bonds with low energy are susceptible to decomposition and rearrangement. All these effects contributed to diminishing stress concentrations. Meanwhile, with the absorbance of bond energy, the dynamic losses were reduced, accompanied by the rearrangement process. On the contrary, the disulfide and monosulfur bonds are usually short since only a few atoms were involved in forming the bonds. And high bond energies make them rigid. All these suggested that the bonds tend to decompose rapidly under stress, leaving the chains with insufficient time for orientation and crystallization. Also, both disulfide and monosulfur bonds are common chemically insert, making them less accessible to other bonds or radicals and rearrange once they are broken. The fragmented chains would result in uneven and demolished strengths. Therefore, vulcanizates equipped with high polysulfur bond contents provided better strengths in general conditions.

Types of crosslink bond	Primary existence	Average bond energy/KJ·mol^−1
Monosulfur-based	In SEV system	295.70
Disulfide-based	In EV/SEV system	266.90
Polysulfur-based	In CV system	<264.50

Table 4.

Common types of sulfur crosslink bonds and corresponding bond energies.

Young’s modulus and Shore A hardness could be affected by the stress relaxation behaviors of polymer chains. The results showed that EV-based vulcanizates possessed better Young’s modulus and Shore A hardness (Figures 6 and 8), and these would be ameliorated with the increment of the sulfur/accelerant ratio. This phenomenon could be related to the different stress relaxation behaviors in each vulcanizate under different curing systems. The Parallel Maxwell Model (shown below) could be applied to illustrate the differences in stress relaxation rates in three curing systems [10].

$$\sigma =\sum {\sigma }_{0\mathfrak{i}}{ℯ}^{\frac{-\mathcal{t}}{{\mathrm{\tau }}_{\mathcal{i}}}}E=\sum \frac{{\sigma }_{0\mathfrak{i}}}{{\epsilon }_0}{ℯ}^{\frac{-\mathcal{t}}{{\mathrm{\tau }}_{\mathcal{i}}}}$$

Specifically, σ_0i represents the stress of each independent Maxwell unit, τ_i represents the individual relaxation time of each unit, and E is the sum of the modulus as an entirety. As described before, polysulfur bonds usually have more chances to reshape and/or rearrange. Therefore, those vulcanizates composed of polysulfur bonds had faster stress relaxations than those dominated by disulfide bonds and monosulfur bonds. This means that in CV systems, τ_i could be remarkably shorter, and Young’s modulus (as well as the hardness) could not be that high. In contrast, in EV and SEV systems, due to the reshape/rearrange-retardance effects caused by disulfide and monosulfur bonds, more time was needed for stress relaxations. Therefore, as the τ_i in each unit increases, Young’s modulus and hardness would improve.

The vulcanizates’ dynamic abrasion performances, such as wear abrasions, fatigue abrasions, and curl abrasions, were affected by the mechano-chemical active chain-oxidized reactions induced by stresses [11]. In most oxidized and thermal oxidized reactions, the chains (especially the crosslinked parts) will break down and form ether bonds. In addition, the abrasion of a vulcanizate is directly proportional to its level of broken crosslink bonds [12]. As presented in Table 4, the monosulfur bond has the highest bond energy, and this keeps it from breaking down against stress. Consequently, it was inferred that under the same external conditions, the monosulfur bond-based SEV system facilitated the vulcanizates with enhanced abrasion resistances.

Figures 3-9 also proved the importance of the sulfur/accelerant ratio in any curing system by significantly determining the mechanical properties regardless of the other external factors. For example, considering a vulcanizate under CV systems, with the sulfur proportion (to accelerant) increasing, its tensile strengths, tear strengths, fractured elongations and Shore A hardness raised initially, promoted to the optimum and eventually retreated, the Young’s modulus continuously improved, the relative volume abrasions continuously decreased, and the rebound resiliences fell at the beginning and then regained. As for vulcanizate under EV systems, inputting more sulfur is a positive approach to improve tensile and tear strengths, but this does not work well on other mechanical performances, as the elongations would reduce, the rebound resiliences and Relative volume abrasions stayed within a narrow variation, the Young’s modulus as well as Shore A hardness varied randomly without regularities. By comparison, products from the SEV system presented an intermediate and moderate level of mechanical properties.

The sulfur/accelerant ratio of the vulcanizate is closely related to the its crosslink formations, including crosslink bonds distributions and densities, which further determines its mechanical behaviors, and these impacts are effective in either static or dynamic states. For example, with the augmentation of crosslink densities, polymer chains could efficiently react with each other, the structures of vulcanizates tend to become three-dimensional rather than linear, and chain segments need to overcome more restrictions to make movements [13]. Therefore, in general conditions, the Young’s modulus, hardness, strengths and elongations improve with the increment of crosslink densities. However, when crosslink density reached the peak, these properties did not further enhance. This phenomenon can be explained by vulcanizates’ crystallization and orientation characteristics.

The crystallization behaviors could be illustrated by Avrami formula: $X_{c,t}^V=X_{c,\infty }^V\left(1-e^{-{\mathit{kt}}^n}\right)$, where X repents the degrees of crystallinities of vulcanizates, k represents the crystallization rate constant of a given polymer, t represents the total time to be crystallized and n served as Avrami index [10]. For all the curing systems in this study, no additional crystallization agents were used. Therefore, it could be assumed that the crystal nucleus formed in a homogeneous condition, and the formed structures were mainly oblique crystals. With this three dimensional uniformed nucleation, the n value can be fixed as 4 [13], so only t and k would influence the crystallinity level. k is theoretically constant, but when crosslink density exceeds the level that the nucleus could endure, excessive crosslink bonds will occupy the space initially used for crystallization. The remaining space may not be enough, so the crystallization becomes more challenging, and rate constant k should be considered smaller than before. Therefore, more time would be inevitably required for vulcanizates to crystallize with external stress. Under the same processing (curing) time, the degree of crystallinity would be lowered, consequently reducing the strengths.

The level of orientation of a polymer chain could be represented by the orientation factor formula: F = (3cos2θ-1)/2, where the θ represents the average angle between the direction of the vulcanizates’ polymer chain and the direction of the orientation (0^o to 180^o) [12]. As the less crystallized polymers, the polymer chains of NR and SBR usually prefer to orient randomly. With external stress, chains will orient directly the same as direction of the stress, and the angle disappears (θ is counted as 0^o), and F will be 1 (fully oriented). However, if the crosslinks are over-concentrated, the concentration parts will cause interruptions to the orientation even under external forces. The orientation direction will be altered and no longer directly identical to the stress, and there will be a new angle. If the θ is not 0^o, the F will become lower than predicted. As a consequence, the strengths of vulcanizates would be damaged.

On the other side, the experimental settings, especially the curing time and temperature in this study, often serve a crucial role in the formation of crosslinks. Usually, the formation of crosslink bonds requires sufficient time within a complete curing process [14]. Given sufficient curing, the vulcanizates could have high crosslink densities. But if the vulcanizate underwent overdosed heating, chain breaking down could occur, during which the fragments randomly rearrange or react intramolecularly and intermolecularly. The unpredicted and uncontrolled reactions alter the appearance and qualities of vulcanizate products [15, 16]. Supported by Van’t Hoff equation and Arrhenius equation, promoting the heating temperature will reduce the curing time, but overheating also possibly causes chain breakdown and damages the mechanical properties [17]. Therefore, in this study, during the curing process the temperatures for each sample were set below 150°C.

4. Conclusions

Based on the results and analysis from this model study, it was concluded that sulfur curing systems served as decisive factor in the preparation of vulcanizates, different types of sulfur curing systems provided the vulcanizates with various features and performances. In a general trend:

1. CV system can provide vulcanizates with relatively higher strengths.

2. EV system can provide vulcanizates with relatively better Young’s modulus and Shore A hardness.

3. SEV can provide better abrasion resistances.

Another conclusion is that the mechanical properties of vulcanizates varied non-linearly according to variations of sulfur contents. Usually, an optimum sulfur/accelerant ratio could be confirmed by screening tests. Though more investigations are needed for further research, this model study could be used as guidance and for choosing the appropriate curing system for rubber products processing.

Acknowledgments

We gratefully appreciate the technical support and financial assistance from the aforementioned suppliers.

Conflict of interest

The authors declare no conflict or competition of interests.

Abbreviations

NR	Natural rubber
SBR	Styrene-butadiene rubber
CV	Conventional vulcanization
EV	Effective vulcanization
SEV	Semi-effective vulcanization