Feasibility of Utilizing Waste Natural Rubber Gloves as a Primary Rubber Matrix: Aspect of Vulcanization Systems

Abstract

In this study, the potential for re-mixing and re-vulcanizing waste natural rubber glove (WNRG) material by using it as the primary matrix was investigated. Alternative types of vulcanization systems, namely, sulfur, phenolic resin, and peroxide, were employed. The results unequivocally demonstrated that residual vulcanizing agents contained in the WNRG were not sufficient to cause crosslinking reactions without re-mixing with vulcanizing agents. Among the various vulcanization approaches, sulfur produced the greatest properties, whereas phenolic resin gave moderate performance. The WNRG vulcanized with sulfur demonstrated the highest crosslink density, tear strength, tensile strength, hardness, and strain-induced crystallization ability among the tested alternatives. The tensile strength of WNRG vulcanized with sulfur was approximately 16.23 MPa, which was 31.7% and 51.1% greater than the WNRG vulcanizates made with phenolic resin and peroxide, respectively. Because of its highest crosslink density, the WNRG vulcanizate with sulfur also offers the greatest storage modulus among the tested cases. The results clearly suggest that the WNRG can potentially be re-compounded, re-vulcanized, and used as the primary matrix. WNRG could be used as a matrix at an industrial scale, to minimize the environmental issues and increase the added value from waste gloves. The findings provide practical guidance for recycling waste rubber gloves in industrial applications, which would be a more sustainable solution for solving the problems associated with WNRG.

1. Introduction

Natural rubber gloves are an important type of personal protective equipment used for hand protection, particularly against various substances, including hazardous chemicals. Furthermore, such gloves also possess several outstanding properties, including high elasticity and excellent tear resistance [1]. Thus, they are extensively used in various sectors ranging from healthcare and laboratories to food processing and industrial operations [2]. Generally, natural rubber gloves are classified as thermoset materials since they are produced by dipping the mold in the compounded latex, applying the coagulation dipping technique, before being subjected to vulcanization [3]. The major disadvantage of thermoset materials is that they produce single-use products that cannot be remolded or reused [4].

Typically, natural rubber gloves are discarded as waste after use, causing environmental issues as they are difficult for the surroundings to break down, because waste natural rubber gloves (WNRG) are composed of crosslinked polymer chains. Traditional disposal methods, such as landfilling and open burning, remain the most common and cost-effective approaches. However, these methods have been reported to cause various environmental issues, including the emission of toxic smoke and hazardous gases, which can lead to severe health concerns [1,5]. Therefore, more sustainable methods for solving and avoiding such issues are desired.

Recycling WNRG represents an important candidate approach for suppressing and preventing such negative impacts. The recycling of WNRG helps not only to limit the environmental problems but also to increase the value of the WNRG waste stream. Various attempts have been made to reuse WNRG. Patarapaiboolchai et al. [6] utilized rejected powder and chlorinated gloves as a filler in natural rubber compounding. They found that the content of the rejected glove used as a filler affected the scorch and cure times. The recommended content of rejected gloves for maintaining high mechanical properties was at 40 phr. Nuzaimah et al. [7] investigated microstructure and mechanical properties of unsaturated polyester composites filled with waste rubber glove crumbs. They found that the incorporation of rubber crumbs enhanced the toughness of the composite but decreased the tensile strength, flexural strength, and elastic modulus due to the incompatibility between rubber crumbs and polyester. Saiwari et al. [8] investigated the use of reclaimed rubber, prepared from thermo-mechanical de-vulcanization of the WNRG, in blends with virgin natural rubber (NR) at various blend ratios. They found that as the amount of reclaimed rubber increases, scorch and cure times decreased while the maximum torque and torque difference increased. The mechanical properties of the blends depended drastically on the crosslink density. The incorporation of reclaimed rubber up to 60% did not affect the tensile strength of the blend. The rubber powder obtained from the reclamation process can also be used for the preparation of thermoplastic elastomers by blending with polypropylene [4,9]. The use of WNRG for blending with other materials has also been reported [5,10,11,12].

Although recycling of WNRG has been extensively explored, most studies have focused on converting it into powder form through devulcanization or reclamation methods and then employing the reclaimed rubber as a filler or blend component with other polymers. While WNRG has been studied in various applications, its usage as the primary rubber matrix has not been extensively investigated. Since WNRG contains high amounts of rubber component with a small crosslink density [9], applying such WNRG as a rubber matrix should be possible. Based on our preliminary study, it is suggested that WNRG can be used as the main polymer matrix for preparing rubber compounds. In this study, WNRG was processed with various vulcanization systems, i.e., sulfur, phenolic resin, and peroxide systems, in order to search for the most appropriate vulcanization system for re-mixing and re-crosslinking WNRG. The influences of different vulcanization systems on the properties of WNRG compounds and vulcanizates are discussed based on their processability, swelling, mechanical, and dynamic mechanical properties. In addition, the ability for strain-induced crystallization during stretching of the WNRG vulcanizates was also investigated to verify the structural reinforcement mechanism of the re-vulcanized WNRG.

2. Materials and Methods

2.1. Materials

WNRGs used as a polymer matrix consisted of rejected scraps from the glove manufacturing process. Stearic acid used as an organic activator in the sulfur vulcanization system was manufactured by Imperial Chemical Co., Ltd., Pathumthani, Thailand. Zinc oxide (ZnO) used as an inorganic activator in the sulfur vulcanization system was supplied by Global Chemical Co., Ltd., Samutprakan, Thailand. An antioxidant, Ionol LC, was supplied by CPM Group Co., Ltd., Bangkok, Thailand. N-cyclohexyl-2-benzothiazolesulfenamide (CBS) used as an accelerator in the sulfur vulcanization system was manufactured by Flexsys America L.P., Nitro, WV, USA. Sulfur, a vulcanizing agent, was manufactured by Siam Chemicals Co., Ltd., Samutprakan, Thailand. Dicumyl peroxide (DCP, Percumyl D) with a 99.0% active peroxide content was used as the vulcanizing agent for the peroxide system, supplied by Chemmin, Thailand. Trimethylolpropane trimethacrylate (TMPTMA), a coagent for the peroxide curing system, was manufactured by Sigma-Aldrich, St. Louis, MO, USA. Hydroxymethylol phenolic resin (HRJ-10518), a vulcanizing agent for the phenolic resin system, was manufactured by Schenectady International Inc., Schenectady, NY, USA; and stannous chloride (SnCl₂.2H₂O), a catalyst for phenolic resin system, was manufactured by Elago Enterprises Pty Ltd., Cherrybrook, NSW, Australia.

2.2. Preparation Continuous WNRG Sheet

The steps involved in continuous WNRG sheet preparation are shown in Figure 1. The obtained WNRG (Figure 1a) was initially physically reclaimed by mastication on a laboratory-size two-roll mill, model ML-D6L12 (Chareontut Co., Ltd., Bangkok, Thailand), at 30 °C for 30 min. The nip and guide of the mill were set at 0.91 mm and 121.43 mm, respectively. After 10 min of processing, the WNRG had turned to a non-uniform continuous sheet with many irregular holes (Figure 1b). The WNRG sheet became more uniform with a reduction in irregular holes when the reclaiming time was about 20 min (Figure 1c). It should be noted that some crosslinks and rubber backbone chains were cleaved during physical reclaiming due to the mechanical shear and heat generated by the two-roll mill. The reclaiming process was completed at 30 min when the WNRG formed a uniform continuous sheet (Figure 1d). The resultant continuous WNRG sheet was left at room temperature for 24 h before re-compounding with other ingredients. The characteristics of the raw WNRG material determined through thermogravimetric analysis (TGA) at a heating rate of 10 °C/min under a N₂ atmosphere, are shown in Figure 1e. It was found that the raw WNRG contains about 80% rubber hydrocarbon and 20% inorganic substances.

2.3. Compound Preparation

The WNRG compounds were prepared by re-mixing the continuous WNRG sheet with other rubber compounding chemicals, including curing activators, antioxidant, curing promoters, and a curing agent, according to the formulations shown in

Table 1. Recipes for preparation WNRG compounds with different vulcanization systems.

Ingredient\Vulcanization System	Quantity (phr 1)
	Sulfur	Phenolic Resin	Peroxide
WNRG sheet	100	100	100
Stearic acid	1	-	-
ZnO	3	-	-
Ionol LC	1	1	1
CBS	1.5	-	-
Sulfur	1.5	-	-
SnCl2.2H2O	-	1	-
Phenolic resin	-	10	-
TMPTMA	-	-	4
DCP	-	-	0.7

¹ phr refers to part(s) per hundred parts of rubber.

.

All the rubber chemicals were incorporated into the WNRG sheet sequentially, following the steps shown in Figure 2. First, the WNRG sheet was re-masticated for 2 min. Stearic acid and ZnO were then added and mixed for 2 min. After that, the Ionol LC was incorporated into the mix and the mixing was continued for 1 min. Next, the curing promotors, i.e., CBS for the sulfur system, SnCl₂.2H₂O for the phenolic resin system, and TMPTMA for the peroxide system, were added and mixed for another 1 min. Finally, a curing agent (sulfur, phenolic resin, or peroxide) was incorporated and mixed for 1 min. The total mixing time was kept constant at 8 min for all formulations. The WNRG compounds were sheeted out and left at room temperature for 24 h before vulcanization and testing.

The WNRG compounds were vulcanized at 160 °C to obtain 1 mm thick vulcanized sheets by using a hydraulic compression molding machine (SLLP-50, Siam Lab, Nonthaburi, Thailand) following their respective curing times (t₉₀) obtained from the rheometric test. The WNRG vulcanizates crosslinked with sulfur, phenolic resin, and peroxide systems were labeled as Sul. WNRG, Phen. WNRG and Per. WNRG, respectively. It should be noted here that the amounts of vulcanizing agent and vulcanizing promoter were designed according to prior publications, i.e., the sulfur system [13], peroxide system [14], and phenolic resin system [15].

2.4. Characterizations

2.4.1. Curing Characteristics

Curing characteristics of the WNRG compounds were determined according to ASTM D5289 [16] by using a moving die rheometer (MDR 3000 Basic, Montech, Buchen, Germany) at 160 °C. The curing parameter scorch time (t_s1), cure time (t₉₀), cure rate index (CRI), minimum and maximum torques (M_L and M_H), and torque difference (M_H − M_L) are reported.

2.4.2. Swelling Equilibrium Test

An equilibrium swelling test was performed to evaluate the swelling response and thereby estimate the crosslink density of the WNRG crosslinked with different vulcanizing systems. The WNRG vulcanizate samples were cut into a square shape with dimensions of 1 × 1 × 0.1 cm³. The original weight (W₀) of the sample was measured. The samples were then immersed in 30 mL of toluene at room temperature for 72 h. The soaked sample was removed from the solvent, gently surface-wiped to remove excessive solvent, and re-weighed to obtain the swollen weight of the sample (W_s). The sample was finally dried in a hot air oven at 70 °C for 24 h or until a constant weight. The degree of swelling was calculated using the following equation:

$$\mathrm{Swelling} \mathrm{degree} = {\displaystyle \frac{{\mathrm{W}}_{\mathrm{s}}- {\mathrm{W}}_0}{{\mathrm{W}}_0}} \times 100\%$$

(1)

The total crosslink density (ν) of each WNRG vulcanizate was estimated from the swelling data by utilizing the Flory–Rehner relationship shown in Equation (2) [17].

$$\mathrm{\nu }=-{\displaystyle \frac{\ln (1-{\mathrm{V}}_{\mathrm{r}})+{\mathrm{V}}_{\mathrm{r}} +\mathrm{\chi }{{\mathrm{V}}_{\mathrm{r}}}^2}{\mathrm{\varrho }{\mathrm{V}}_{\mathrm{s}}\left({{\mathrm{V}}_{\mathrm{r}}}^{1/3}-\frac{{\mathrm{V}}_{\mathrm{r}}}{2}\right)}}$$

(2)

Here, “V_r” is the volume fraction of rubber in the swollen mass, “χ” is the polymer-solvent interaction parameter (0.39 for NR-toluene), “ρ” is the rubber density (0.93 g/cm³), and “V_s” is the molar volume of the solvent (106.3 cm³/mol).

The V_r can be estimated from the following equation:

$${\mathrm{V}}_{\mathrm{r}} = -{\displaystyle \frac{\frac{{\mathrm{W}}_{\mathrm{d}}}{{\mathrm{\varrho }}_{\mathrm{r}}}}{\frac{{\mathrm{W}}_{\mathrm{d}}}{{\mathrm{\varrho }}_{\mathrm{r}}}\times \frac{\left({\mathrm{W}}_{\mathrm{s}}-{\mathrm{W}}_{\mathrm{d}}\right)}{{\mathrm{\varrho }}_{\mathrm{s}}}}}$$

(3)

where “W_d” denotes de-swollen weight, “W_s” is swollen weight, ρ_r is rubber density, and ρ_s is the solvent density.

2.4.3. Mechanical Properties

The tensile properties of all WNRG samples were measured using a universal tensile testing machine, LR5K Plus (LLOYD Instruments, Bognor Regis, UK), equipped with an extensometer, EX800 Plus (LLOYD Instruments, Bognor Regis, UK). The test was conducted at room temperature with a crosshead displacement rate of 500 mm/min according to ISO37 (type 2) [18]. The tear strength of the WNRG specimens was also tested using a tensile testing machine, operating at a crosshead speed of 500 mm/min according to the ISO34-1 [19]. A nicked-angle tear specimen was selected. Hardness was tested using a Bareiss hardness tester, model BS61 II (Bareiss Co., Ltd., Oberdischingen, Germany), in accordance with ASTM D2240 [20]. The test was performed on a mechanically unstressed sample with a diameter of 12 mm and a thickness of 6 mm. Five replicates were performed for each test, and the average is reported.

2.4.4. Wide-Angle X-Ray Scattering

The wide-angle X-ray scattering (WAXS) test was used to study the crystallinity, crystallite size and orientation parameter of the rubber chains in the vulcanized WNRG samples during stretching. The experiment was carried out at the Siam Photon Laboratory, Synchrotron Light Research Institute (SLRI), Nakhon-Ratchasima, Thailand. The data were acquired at various strains while stretching (50 mm/min) at room temperature (25 °C). The WAXS data were rectified and analyzed using the SAXSIT 4.63 program.

The degree of crystallinity (X_c) during stretching corresponding to the (200) and (120) planes was estimated using the following equation [21]:

$${\mathrm{X}}_{\mathrm{c}}(\%) = {\displaystyle \frac{{\mathrm{A}}_{\mathrm{c}\mathrm{r}}}{{(\mathrm{A}}_{\mathrm{c}\mathrm{r}}+ {\mathrm{A}}_{\mathrm{a}\mathrm{m}})}} \times 100\%$$

(4)

where A_cr and A_am are the area under crystalline peaks corresponding to the (200) or (120) plane, and the area of the amorphous halo, respectively.

The lateral crystallite sizes corresponding to (120) plane (L₁₂₀) was estimated by using the Scherrer equation [21,22]:

$$L_{hkl} = {\displaystyle \frac{\mathrm{{\rm K}}\lambda }{\beta cos\theta }}$$

(5)

where L_hkl denotes the average crystallite size in the (120) plane, K is 0.64, λ is the wavelength, β is the half-width at half-height, and θ is the Bragg angle.

2.4.5. Dynamic Mechanical Analysis

A dynamic mechanical analyzer, model DMA 1 (Mettler Toledo, Greifensee, Switzerland), was used to evaluate the dynamic mechanical properties of WNRG samples. The test was carried out in tension mode over a temperature range of −80 °C to 80 °C at a frequency of 10 Hz and a heating rate of 2 °C/min. The storage modulus, loss modulus, and damping factor (tan delta) are reported.

2.4.6. Morphological Properties

Scanning electron microscopy (SEM), with FEI Quanta 400 (FEI company, Hillsboro, OR, USA), was used to investigate the homogeneity of the WNRG matrix after re-mixing and re-vulcanization. The sample was cryo-fractured and gold-coated prior to the SEM imaging.

3. Results

3.1. Curing Characteristics

The curing characteristics of the compounds prepared from WNRG and crosslinked with various vulcanization systems are shown in Figure 3. The curing reactions in the WNRG sheet without any added chemicals (pure WNRG) are also included for reference purposes. Clearly, the torque of pure WNRG did not increase during vulcanization, revealing that the residual chemicals, such as sulfur and its accelerators in the WNRG sheet, were no longer capable of initiating crosslinking between rubber chains. However, when the vulcanizing agents had been added into the WNRG by re-mixing, the torque during vulcanization drastically increased, depending on the system used. This indicated progressing reaction extent over time. Interestingly, the development of torque with time during vulcanization is similar to other rubbers, consisting of induction period, crosslinking, and over crosslinking states [23]. The highest maximum torque was seen for the Sul. WNRG, whereas the Per. WNRG showed the lowest maximum torque. This result clearly suggests that the WNRG can be re-mixed, re-vulcanized, and recycled to form new products. The raw data on curing behavior in terms of M_L, M_H, M_H − M_L, t_s1, t₉₀, and CRI are summarized in

Table 2. Minimum torque (M_L), maximum torque (M_H), torque differential (M_H − M_L), scorch time (t_s1), cure time (t₉₀), and cure rate index (CRI) of the WNRG compounds.

Sample Name	Curing Parameter
	ML (dN·m)	MH (dN·m)	MH − ML (dN·m)	ts1 (min)	t90 (min)	CRI (min−1)
Pure WNRG	2.53 ± 0.29	N/A	N/A	N/A	N/A	N/A
Sul. WNRG	0.65 ± 0.00	10.45 ± 0.00	9.80 ± 0.00	0.83 ± 0.05	1.53 ± 0.02	142.86 ± 5.88
Phen. WNRG	0.85 ± 0.00	6.15 ± 0.00	6.15 ± 0.00	1.99 ± 0.05	17.59 ± 0.29	6.41 ± 0.18
Per. WNRG	0.91 ± 0.00	4.47 ± 0.00	3.56 ± 0.00	1.58 ± 0.09	9.99 ± 0.02	11.89 ± 0.17

. Compared to pure WNRG, the M_L associated with the initial compound viscosity of WNRG decreased after mixing with curing agents, possibly due to the plasticizing effects of stearic acid in the sulfur system [24], phenolic resin in the phenolic system [25], and TMPTMA in the peroxide system [26]. Since these chemicals worked as plasticizing agents, reductions in M_L were observed.

The M_H and M_H − M_L are indicators of vulcanizate stiffness and crosslink density in the WNRG, and these drastically increased compared to pure WNRG, to levels depending on the vulcanization system. Among the alternative vulcanization systems, the highest M_H and M_H − M_L were seen for the Sul. WNRG sample, while the Per. WNRG possessed the lowest. This result clearly indicates that the greatest stiffness and degree of crosslinking were achieved when sulfur was used as a vulcanizing agent. This suggests that the sulfur system is more effective in crosslinking the rubber chains compared to the other systems tested.

The t_s1 and t₉₀ of the WNRG compounds also varied by vulcanization system. Compared to other crosslinking systems, the sulfur system gave the shortest t_s1 and t₉₀, whereas the phenolic resin system provided the longest. A reduction in both t_s1 and t₉₀ was possibly due to the presence of unreacted accelerator in the WNRG. It was reported that the presence of unreacted accelerator in the waste glove material caused a reduction in both scorch and cure times, when using it as a filler in rubber compounds [6]. On the other hand, the longest t_s1 and t₉₀ found for the phenolic resin system revealed the slowest vulcanization reactions, starting with the lowest rate of vulcanization. This was attributed to the nature of the phenolic resin system, which generally has a slow vulcanization rate [27]. Compared to the phenolic system, the peroxide system showed shorter t_s1 and t_90, but these were still longer than the sulfur system.

Based on the curing properties, this result clearly suggests that WNRG can be re-vulcanized after re-mixing with the vulcanizing agent. To gain success in the preparation of the compounds based on WNRG, certain amounts of curing ingredients must be added. It should be noted that the only curing property available for the pure WNRG is the minimum torque, while other properties are not available due to the lack of progress in curing.

3.2. Swelling Properties and Total Crosslink Density

To further search for the most appropriate vulcanization system for crosslinking WNRG, the swelling properties and crosslink density were investigated. Figure 4 shows the equilibrium swelling (Figure 4a) and total crosslink density (Figure 4b) of WNRG vulcanizates made with the alternative vulcanizing systems.

As shown in Figure 4a, the lowest equilibrium swelling was seen for the Sul. WNRG sample, while the highest value was observed for the Per. WNRG. It is well-known that swelling capacity is inversely related to total crosslink density in rubber vulcanizates. A smaller swelling indicates a higher crosslink density [13]. Thus, the highest total crosslink density was achieved in the Sul. WNRG, whereas the Per. WNRG showed the least crosslinking (see Figure 4b). A greater crosslink density in the rubber matrix resulted in a more restricted solvent penetration. Since crosslinks are chemical linkages that join rubber chains together, more linkages would provide a denser network structure and make it more difficult for solvent molecules to penetrate and diffuse through the material. The results obtained from the equilibrium swelling test agree well with the M_H − M_L gained from the rheometric test.

3.3. Mechanical Properties

Figure 5 shows representative stress–strain curves of WNRG vulcanizates crosslinked with the various vulcanizing systems. The stress–strain curve of neat WNRG is also included for comparison purposes. Clearly, the relationships of stress with strain are similar to those of neat WNRG and other conventional NR vulcanizates. That is, in all cases, stress gradually increased at low strains and all the curves showed a steep increase in stress at high strains (i.e., over 450% strains) [28]. Such a steep response in stress was attributed to the strain-induced crystallization in the rubber matrix [15]. Both tensile strength and elongation at break of all samples remain relatively high, i.e., over 10 MPa for tensile strength and over 550% for elongation at break, with the actual values depending on the type of crosslink system used. The results clearly reveal that even though the WNRG was re-vulcanized, its capacity for strain-induced crystallization remains. The observations of strain-induced crystallization of the crosslinked WNRGs are discussed in the following Section 3.4.

It can also be seen that the development of stress with applied strain was significantly different depending on the curing system choice. The average values of 100% modulus (100% Mod), 300% modulus (300% Mod), tensile strength (TS), and elongation at break (EB) of neat WNRG and WNRG vulcanizates are tabulated in

Table 3. 100% modulus (100% Mod), 300% modulus (300% Mod), tensile strength (TS), and elongation at break (EB) of the neat WNRG and its vulcanizates made with different vulcanization systems.

Sample Type	100% Mod (MPa)	300% Mod (MPa)	TS (MPa)	EB (%)
Sul. WNRG	1.34 ± 0.10	3.98 ± 0.86	16.23 ± 1.88	569 ± 80
Phen. WNRG	0.73 ± 0.08	2.63 ± 0.49	12.32 ± 0.45	585 ± 52
Per. WNRG	0.61 ± 0.04	1.96 ± 0.60	10.74 ± 1.31	636 ± 76
Neat WNRG	1.22 ± 0.13	3.26 ± 0.22	21.03 ± 0.37	662 ± 18

. Compared to others, the Sul. WNRG sample showed the highest values of 100% Mod, 300% Mod, and TS. This was because the Sul. WNRG had the highest crosslink density, as previously discussed in the context of Figure 4b. A higher crosslink density generally provides a stronger network and thus increases the stress at a given strain. The linkages can hinder the movement of rubber chains, boosting their ability to withstand stress and deformation, which results in increased tensile strength. It should be noted that the properties of Sul. WNRG were lower than those of the neat WNRG. The tensile strength of Sul. WNRG was reduced by about 23% compared to its neat form. This reduction was attributed to the degradation of the rubber matrix during re-processing and re-vulcanization. In contrast, the Per. WNRG sample with the lowest crosslink density showed the lowest stress at various strains. The extensibility of all WNRG vulcanizates also followed the ranking by crosslink density. The sample with lower crosslink density generally had fewer linkages between rubber chains, so the rubber chains can move more freely, resulting in higher extensibility.

The tear and hardness properties of WNRG crosslinked with sulfur, phenolic, and peroxide systems are shown in Figure 6a and Figure 6b, respectively. It is generally seen that a higher crosslink density provides higher tear strength and hardness of the WNRG vulcanizate. The result is in good agreement with a prior publication [29]. The sample with higher crosslink density had more tie points holding the rubber chains together, increasing the stiffness of the material. In addition, high crosslink density also resulted in better resistance to tear propagation, resulting in both higher hardness and tear strength. Effects of crosslinking system choice on the mechanical properties have been reported in prior literature [30].

3.4. Strain-Induced Crystallization

To confirm the ability for strain-induced crystallization of the WNRG vulcanizates, WAXS measurements were performed. Figure 7 shows two-dimensional (2D) WAXS images without deformation and with a high strain level (0% and 439–446%) for the WNRG vulcanizates made with different vulcanizing systems.

It is well-known that the preferred orientation of rubber chains along the stretching direction causes the formation of crystal reflection spots in 2D WAXS patterns. A WAXS image without any reflection spots usually indicates the absence of strain-induced crystallization [31,32]. In Figure 7, no reflection spots are detected in the 2D WAXS pattern for any of the WNRG vulcanizates at 0% strain, confirming the absence of strain-induced crystallization. Without stretching, there is no preferred orientation of rubber chains. Only some residual chemicals should contribute to the pattern of WAXS. Upon stretching, all WNRG vulcanizates clearly showed the spots assigned to (200), (120), and (210) planes, implying that the rubber chains had transitioned from an isotropic amorphous state to an anisotropic semi-crystalline state [33], and the form of crystal structure in the NR is monoclinic [34]. Additionally, the stronger reflections observed in Phen. WNRG may indicate that this sample had higher levels of crystallization and chain orientation. Thus, it could be confirmed that the strain-induced crystallization phenomenon still existed even after re-vulcanization.

Figure 8 shows the degree of crystallinity versus strain for the WNRG vulcanizates made using different vulcanizing systems. The crystallinity of all WNRG samples generally increased with applied strain as the deformation caused rubber chain orientation into the stretching direction and thus reduced the disorder in the NR microstructure [21,35]. This behavior confirms the presence of deformation-induced crystallization in the WNRG even after re-vulcanization.

The onset of crystallization in the Sul. WNRG was seen at a strain of about 260%, while for Phen. WNRG and Per. WNRG, it was found at strains of about 300% and 305%, respectively. The earlier start of strain-induced crystallization found in the Sul. WNRG was possibly due to the higher crosslink density. It is presumed that strain-induced crystallization begins with a highly stretched short-chain segment located between the heterogeneously dispersed crosslinks. The shorter chains are completely stretched and serve as crystallite nucleation sites [31]. There should be more such nucleating sites in a sample with greater crosslink density. It can also be seen that the crystallinity of Phen. WNRG seemed to be higher than others at strains above 350%, possibly due to the higher stiffness and rigidity of the network structure formed by the phenolic resin crosslinking agent [36]. The stiff and rigid network structure formed by the phenolic resin in the WNRG matrix may better facilitate the strain-induced crystallization of the WNRG chains during stretching. However, the highest degree of crystallinity was observed in the Sul. WNRG. It is believed that the crystallites act as a virtual filler or crosslink, providing self-reinforcement to the rubber, such the tensile stress steeply increases after a certain amount of crystallization has formed [37].

Figure 9 shows the crystallite size corresponding to the crystallographic plane at various strains (120). The smallest crystallite size was found to be in the Sul. WNRG vulcanizates. Compared to Per. WNRG, the Phen. WNRG showed a smaller crystallite size. The result agrees well with those on crosslink density. A higher crosslink density gave smaller crystallites during deformation. This was attributed to a larger amount of crystallite nucleation in the sample with a higher crosslink density, and the crystallites then competed and ended up smaller in size [13].

3.5. Dynamic Mechanical Properties

Figure 10 shows the storage modulus, loss modulus, and tan delta of WNRG vulcanizates made with different vulcanizing systems, as a function of temperature. The storage modulus (E′) at 25 °C, glass transition temperature (T_g), and tan delta peak width and height for the WNRG vulcanizates made with three different vulcanizing systems are listed in

Table 4. Storage modulus (E′) at 25 °C, glass transition temperature (T_g), and tan delta peak width and height for the WNRG vulcanizates made with three different vulcanizing systems.

Sample Type	E′ at 25 °C (MPa)	Tg (°C)	tan delta Peak Width	tan delta Peak Height
Sul. WNRG	4.53	−43.04	19.02	1.48
Phen. WNRG	2.16	−45.52	20.74	1.57
Per. WNRG	1.29	−49.17	16.44	1.57

.

The storage modulus (E′) refers to the ability of the vulcanizate to store or return energy, while the loss modulus (E″) refers to the ability to lose or dissipate energy [38]. It can be seen that the vulcanization system choice has a significant effect on the storage modulus of the WNRG vulcanizates. Compared to the others, the Sul. WNRG showed the highest E′, particularly in the rubbery region (Figure 10a). The E′ for Sul. WNRG was about 4.53 MPa, which is about two times that for the Phen. WNRG (2.16 MPa) and about four times that for the Per. WNRG (1.29 MPa). The rank order by E′ agrees with those by crosslink density and tensile modulus, as shown previously. A greater crosslink density restricts the mobility of rubber molecules more, increasing E′. The E″ also varied by vulcanization system and the Sul. WNRG showed the highest E″ (Figure 10b). Despite the highest ability to return the energy, the Sul. WNRG sample also had the highest ability to dissipate energy.

The choice of vulcanization system seems to have a large effect on the damping factor (tan delta) of the WNRG vulcanizates. The damping factor is defined as the ratio of E″ and E′ (tan delta = E”/E′) [39]. As can be seen from (Figure 10c), the tan delta peak signifying the glass transition temperature T_g of the WNRG samples depends strongly on the system of vulcanization and on the crosslink density. The glass transitions of Sul. WNRG, Phen. WNRG, and Per. WNRG were at −43.04 °C, −45.52 °C and −49.17 °C, respectively. Again, the result is in good agreement with the rank order by crosslink density. It is well-known that T_g shifts toward higher temperatures with crosslink density, indicating that more energy is required to cause the transition of the rubber chains from a glassy to rubbery state. The increased crosslink density inhibits the movement of rubber chains, reducing elasticity, and, as a result, T_g shifts toward higher temperatures.

The width of the tan delta peak is often used to estimate the network uniformity in polymer composites [40]. A narrow peak width is an indication of a more homogeneous network structure. As can be seen in Table 4, the least tan delta peak width was seen for Per. WNRG, whereas the largest was observed for the Phen. WNRG sample. These results suggest that peroxide vulcanization promoted a more uniform network structure in the WNRG matrix, while phenolic curing resulted in a comparatively heterogeneous network. However, a narrow tan delta peak may not be attributed only to a uniform network structure formed in the WNRG matrix, since a low crosslink density also induces a narrow tan delta peak (see Figure 4). This is consistent with the well-established relationship that increasing crosslink density leads to a broader tan delta peak [41].

In addition, the heights of tan delta peaks of WNRG vulcanizates also varied by vulcanization system, corroborating the idea that chain mobility and damping properties of WNRG vulcanizates are governed by the choice of vulcanizing agent. The application of sulfur as the vulcanizing agent resulted in the least tan delta peak height, while the phenolic resin and peroxide provided similar peak heights to each other. The lowest tan delta peak height seen for the Sul. WNRG was attributed to the highest crosslink density of this case. A higher crosslink density provided greater restriction of the segmental polymer chain mobility, reducing the tan delta peak height.

3.6. Morphological Properties

Figure 11 shows SEM micrographs of re-vulcanized WNRG samples prepared with different vulcanization systems. It is seen that the morphologies of all samples consisted of the lighter appearing particles dispersed within the darker WNRG matrix: the particles are residual inorganic substances, consistent with the TGA results (Figure 1e). No phase separation in the WNRG matrix was observed in any of the samples, indicating that the re-vulcanized WNRG maintained a homogeneous matrix even after re-mixing and re-shaping.

Based on the results obtained from this study, WNRG can be recycled by re-vulcanizing, but an appropriate choice of vulcanizing system is required during re-compounding. The residual chemicals contained in the WNRG are not sufficient to support crosslinking reactions required for vulcanization. However, the residual accelerator contained in WNRG may shorten and speed up the curing process. The properties of the WNRG vulcanizates were not much different from those of pure NR vulcanizates. This supports the concept that WNRG can be recycled by utilizing it as the matrix for rubber compounds.

4. Conclusions

In this study, the possibility of utilizing WNRG as a rubber matrix for the preparation of rubber compounds was investigated. Three alternative vulcanization systems, namely sulfur, phenolic resin, and peroxide, were employed. The results clearly showed that the residual vulcanizing agents in pure WNRG were not sufficient to cause crosslinking reactions. Among the different vulcanization systems, sulfur yielded the highest M_H and M_H − M_L during vulcanization reactions, whereas phenolic resin showed a moderate result, and peroxide provided the lowest torque difference. The lowest equilibrium swelling with the greatest crosslink density, tensile, tearing, and hardness properties were also achieved when using the sulfur system. The highest tensile strength (i.e., 16.23 MPa) and ability to crystallize during stretching (i.e., 27.64%) among the WNRG vulcanizates were found when sulfur was used as the vulcanizing agent, resulting in about 31.7% and 51.1% higher crystallinity than those for phenolic resin and peroxide cases, respectively. Considering dynamic mechanical properties, the WNRG vulcanizate made with sulfur system provided the highest storage modulus (i.e., 4.53 MPa; 2.16 MPa and 1.29 MPa for Phen. WNRG and Per. WNRG, respectively) and the T_g of this sample was shifted toward higher temperatures due to the high crosslink density. The greatest property improvement was found for the WNRG vulcanizate made with the sulfur system, which is tentatively attributed to the synergistic vulcanization reactions induced by both residual and added chemical crosslinking agents. Based on the results, the WNRG can be reused so that it still provides a homogeneous matrix even after re-vulcanizing. However, to obtain the maximum properties, an appropriate choice of vulcanizing ingredients is required during re-compounding, since the residual chemicals in WNRG are not sufficient for vulcanization. Overall, the results clearly support the concept that WNRG can be re-utilized as a matrix material of rubber compounds.