Curing behaviors, mechanical properties, dynamic mechanical analysis and morphologies of natural rubber vulcanizates containing reclaimed rubber

2.1 Materials and compounding ingredients

NR, grade SVR3L, with mooney viscosity (ML[1+4]100°C of 72, relative density of 0.93 g/cm³, ash content of 0.42% and polyisoprene content of 91-94% was supplied by the Creative Polymers Co., Ltd. (Bangkok, Thailand). GTR powder, with the particle size of 30 mesh, volatile component content of 6 wt%, rubber content of 58 wt%, carbon black content of 30 wt%, and ash content of 6 wt% was purchased from the Nantong Huili Rubber Co. (China). Other curing ingredients and additives such as triethylene amine, dioctyl phthalate, petroleum resin, zinc oxide (ZnO), stearic acid (SA), N-Cyclohexyl-2-benzothiazole sulfonamide (CZ), tetramethyl thiuram disulfide (TMTD), N-1,3-dimethylbutyl-N′-phenyl-p-phenylenediamine (6PPD) and sulfur were purchased from Bayer (M) Sdn. Bhd. Using 100 phr of rubber as the reference, the amount of sulfurizing agent and reagents are as follow: 100 phr rubber, 5.0 phr ZnO, 2.0 phr SA, 0.5 phr CZ, 0.2 phr TMTD, and 2.0 phr sulfur.

2.2 Sample preparation

Reclaimed rubber was prepared as follows: 100 phr ground tire rubber, 4 phr triethylene amine as reclaiming agent, 2 phr dioctyl phthalate as reclaiming reagent, 2 phr petroleum resin as activator and 5 phr deionized water, mixed with a high speed mixer. The mixer of ground tire rubber, reclaiming agent and all reagents were activated and swelled at 220°C for 2.5 h in high pressure reactor. And then used a torque rheometer mixer at 180°C for 20 min, and finally obtain the reclaimed rubber with gel fraction of 68.6% and crosslink density of 0.33 mol/dm³.

All RR/NR or GTR/NR compounds were prepared using the compositions given in Table 1. For comparison raw NR compounds were also prepared. First, in the roll gap of 1mm, the temperature of 50°C on the two-roll mixing machine mixing NR and RR, kneading evenly after 5 times triangular bag, followed by the formula required by adding ZnO, SA, accelerator CZ and TMTD, left and right cutting knife 2 times, to be eaten completely after adding the curing system, mixing even after adjusting the roller from the film stand. And then the rubber compound in the flat vulcanizing machine molded under 150°C and 10 MPa to obtain 2 mm thick sheet.

2.3 Characterizations

3.1 Cure characteristics of RR/NR compounds

The effects of different proportions on the vulcanization process parameters of RR and NR are described by the data presented in Table 2. The torque increased continuously with increasing the amount of RR, and the cure rate also increased, whereas the scorch time and optimum cure time decreased continuously. This could be explained as due to the fact that reclaimed rubber was obtained by the desulfurization and regeneration of the ground tire rubber powder, and contains carbon black and small molecules, Additionally, the specially structured reclaimed rubber can be used as an elastic filler in NR, because reclaimed rubber and natural rubber showed better compatibility with natural rubber than the other fillers. With the increase in the RR content, the carbon black content increased as well, leading to the increase in the values of the minimum and maximum torques of the RR/NR compounds. The presence of carbon black also accelerated curing reaction rates. While the free sulfur radicals accelerated the concerted reaction with the curing system, resulting in the enhancement of the cure rate.

3.2 Mechanical properties of RR (GTR)/NR compounds

Figures 1-4 show the tensile properties, 100% modulus and hardness, respectively, of the vulcanizates of the RR/NR and GTR/NR blends plotted versus the RR and GTR concentration. The tensile strength and elongation at break decreased continuously with increasing RR and GTR content. This could be explained as due to the fact that GTR was used as an inert filler in the GTR/NR compounds, and therefore an increase in the GTR content, made the material more prone to reunite, resulting in a significant reduction in the tensile strength of the GTR/NR compounds. Unlike the ground tire rubber powders, reclaimed rubber contained carbon black and small molecules, that could reinforced the rubber compounds; however, with the increasing amount of reclaimed rubber, the amount of broken rubber networks and filler particles of reclaimed rubber also increased;

Figure 1

Tensile strength of RR(GTR)/NR compounds containing different contents.

Figure 2

Elongation-at-break for different contents of RR(GTR)/NR.

so that the rubber compounds were easily damaged due to the destructive effect of stress concentration points. Additionally, the hydrocarbon content of the reclaimed rubber increased with increasing reclaimed rubber content, so that the two phases could not form a uniform dispersion, and the interfaces between the rubber compounds acted as new failure points; Therefor, tensile strength decreases continuously with increasing RR content. On the other hand, 100% modulus and hardness increased continuously with increasing RR and GTR content. This could be explained as due to the significant breaking of the reclaimed rubber macromolecule chains, so that even though the reclaimed rubber content increased, the rubber compounds filler content increased and the rigidity of the rubber compounds filler particles increased the hardness and 100% modulus, However, a

Figure 3

Hardness for different contents of RR(GTR)/NR compounds.

Figure 4

100% moduli for different contents of RR(GTR)/NR compounds.

cross-linking network of could not be formed completely, resulting in the decrease of the elongation at break.

3.3 Dynamic properties of RR/NR compounds

DMA has proved to be useful for the investigation of viscoelastic behaviors of rubber compounds (11). Here we focus on the temperature variations of the viscoelastic properties of RR/NR compounds that are always expressed by the storage modulus, loss modulus and the tanδ parameter.

Figure 5a shows the storage modulus-temperature curves for different proportions of RR/NR vulcanizates. The storage modulus is the maximum energy that can be stored in a period of time and can provide a reference value for the rigidity of the rubber, thus reflecting the

Figure 5

Dynamic mechanical thermal properties of RR/NR vulcanizates.

elastic properties of the polymeric materials (12,13). As the temperature increases, the molecular chains begin to thaw, In the rubbery area, the molecular segments are free to move and the friction to be overcome is small, and the friction energy is reduced, The internal energy consumption is very small, resulting in no significant changes in storage modulus (14). Figure 5a shows that the RR/NR compounds filled with 90 phr of RR loading showed higher storage modulus values than compounds with other compositions. This is mainly due to the thermal-mechanical shear desulfurization of RR with linear plasticity and machinability, and the structure of RR and NR is more similar, RR can be evenly dispersed in NR matrix, and RR can re-vulcanization molding, thus, resulting in the storage modulus significantly increased of RR and NR compounds. Additionally, RR/NR compounds filled with small amount of RR show higher storage modulus values than the raw NR vulcanizates, resulting in the stronger RR-NR interactions (15).

Figure 5b shows the loss modulus-temperature curves for different proportions of RR/NR compounds, the loss modulus refers to the energy lost by the material in the event of a viscous deformation that reaction the viscous nature of polymer material (16). With the increase of the amount of RR, the loss modulus of RR/NR compounds changed significantly. The loss modulus values of the RR/NR compounds filled with 70 phr of RR content increased significantly than those for other compositions due to the hindrance of the molecular motion.

Figure 5c shows the tanδ-temperature curves for different proportions of RR/NR compounds. Tanδ is the ratio of the loss modulus to the storage modulus, and is an indicator by the dynamic behavior of RR/NR compounds (11). In general, the main causes of compound damping are: (a) the properties of the matrix and the filler; (b) the nature of the interface; (c) the friction damping or delamination resulting from the sliding of the unbounded area between the filler and the substrate; and (d) energy dissipation in the matrix crack region. The damping peaks always occur in the glass transition region and are related to the movement of pendant groups or low molecular weight units and molecular chains in the rubber. Thus, the higher damping peak corresponds to a higher degree of molecular mobility (17). With the increase of the content of RR, RR/NR compounds damping performance decreased significantly, thus, giving rise to a lower and broader damping peak. This is due to RR acting as a barrier to the mobility of the rubber molecular chains, restricting the rubber segments motion through the strong interaction between the RR and NR molecular chains, and indicating the reinforcing nature of the filler with NR (18). The temperature of the tanδ peak is the glass transition temperature (Tg), that can be used to indicate the degree of the reinforcement of the rubber (19). In this work, the amount of RR has no significant effect on Tg value of RR/NR compounds, The Tg value of the RR/NR blends filled with 90 phr of RR content became closer to the high temperature zone than the those for the other compositionss. This could be explained as follows. As the temperature increases, some RR and NR crosslinked with each other, a certain degree of infiltration reactions occurred, and the macromolecular chains became closely intertwined, thus enhancing the heat resistant properties of RR/NR composites (20).

3.4 Morphological studies of RR/NR compounds

Figures 6 and 7 show the microstructure for different proportions of GTR/NR and RR/NR compounds. Examination of the images of the fracture surfaces of the GTR/NR compounds presented in Figures 6a-c at high magnification, revealed that the GTR powder have no plasticity and secondary processing performance, and the compatibility with the NR is poor, easy to agglomerate powder together to form a defect result, therefore

Figure 6

SEM images of GTR/NR: (a) 30/70, (b) 50/50, (c) 70/30 blends.

Figure 7

SEM images of RR/NR: (a) 30/70, (b) 50/50, (c) 70/30 blends.

reduced the overall performance of the vulcanizates. Figures 7a-c show the fracture surfaces of RR/NR blends, indicating that the surface particles of RR/NR compounds were significantly decreased, and the surface was smoother. This is due to the better compatibility of RR and NR, both RR and NR underwent re-vulcanization molding and formed a continuous phase. It is clear that as the GTR and RR content increased, the GTR/NR compounds exhibits the morphology of brittle fracture, providing evidence for the inferior properties of these compositions. By contrast, the SEM images of the RR/NR blends reveal that the RR homogeneously dispersed in NR matrix, showed considerable deformability, and providing evidence for their superior mechanical properties.