Rheometer test at four temperatures
Rubbers in raw state must undergo a vulcanization process to crosslink the molecular chains and to improve the rubber properties. A rheometer measures the viscoelastic properties of rubber compounds during the vulcanization process. To evaluate the effect of different temperatures, the recycled rubber specimens in this study are subjected to rheometric test at four different temperatures: 150, 160, 170 and 180 °C. Figure 2 presents the rheometric curves of the four batches at different temperatures. In Table 3 some important rheometric parameters are summarized.
When a specimen in the rheometer is heated under pressure, the viscosity drops and the torque decreases. The lowest torque recorded on the curve is called ML (Moment Lowest). It represents the stiffness of uncured rubber at a given temperature.
As the curing begins, the torque rises. The time ts2 is the starting time of the test, when the torque has increased 2 units above ML value (the corresponding time is tML). It represents the scorch time or at which point the curing actually starts. As the curing progresses, the torque increases further. The gradient depends on the compound and curing method used. Shortly thereafter, the torque reaches a maximum value and tends to be constant. The highest recorded torque on the curve is called MH (Moment Highest). Time from the start of the test to the point where 90% of the MH value is reached is called t90. Such a description applies also for t10 and t50. As shown in Fig. 2 and Table 3, the increase of vulcanization temperature accelerates the vulcanization time. However, vulcanization at 150 °C seems to be the optimal one, because it results in the highest MH. On the other hand, the lowest MH is obtained in the vulcanization at 180 °C. Figure 3 summarizes in graphs the characteristic times (in minutes) of the rheometer curves at 150 °C and 180 °C for the 4 batches, deducing data from rheometer curves of Fig. 2. Such characteristic times are tML (time at minimum torque), ts2 (time at scorch i.e., incipient curing), t50 (time at 50% of vulcanization, corresponding to a torque that is one half the maximum torque) and t90 (time at 90% of vulcanization). Obviously tML < ts2 < t50 < t90. On the horizontal axis, the ratio between polymer and regenerate rubber is represented, so that data on the left refer to Vistalon 3666, whereas data on the right to Dutral 4038. As can be observed, Vistalon crosslinks slightly faster than Dutral 4038 and regenerated rubber RRA (batches 1 and 3) seems less reactive than RRB (batches 2 and 4). These results suggest that RRB should be preferred to RRA for the preparation of the pads. As it will be shown later in the paper, batch 4 will be identified as the best one for the preparation of the isolation pads; this notwithstanding also batch 2 exhibits excellent viscoelastic characteristics, but with a slightly worst response after ageing.
Shore A hardness
Shore (durometer) hardness is a measure of the resistance of a material towards the penetration of a spring-loaded needle-like indenter. In rubbers, the hardness is usually measured by shore A scales. In a rubber test for seismic isolation purpose, the hardness is measured before and after accelerated ageing. The accelerated aging is conditioned by storing the specimens in a oven at 70 °C for 24 × 7 h. The aged specimens are then tested after 24 h of storage at room temperature. According to EN 15129 , the variation of the hardness after aging is recommended not to exceed − 5 or + 8. Table 4 presents the measured hardness of the four recycled rubber batches before and after aging and the hardness of the virgin rubbers provided by the supplier. Batches 3A and 4B satisfy the maximum variation of rubber hardness after accelerated aging.
In compression set testing, the ability of rubber to return to its original thickness after prolonged compression at defined temperature and deflection is examined. When the rubber is compressed over time, it loses its ability to return to its original thickness. This loss of resilience may degrade the performance of rubber-based equipment such as seals or elastomeric gaskets. Compressions set results are expressed in a percentage. A rubber which exhibits a lower percentage has better resistance to permanent deformation under a defined deflection and temperature range. In the compression set, the rubber specimens as shown in Fig. 4 are subjected to compression at 70 °C for 24 h. According to EN 15129  in case of rubber material for seismic isolation, the maximum value of compression set result is 30%. In the present experimental test, the results of the compression set of rubber batches 1A, 2B, 3A, and 4B are respectively the following: 23, 12, 28, and 25%. Therefore, all four compounds satisfy the requirement of compression set for rubber seismic isolation.
Uniaxial tensile test
Rubber is well-known as an ideal example of perfectly elastic material. However, nonlinear elasticity of rubber at moderate to large strain is clearly remarkable. Such a nonlinearity is often called as hyperelasticity. To characterize the hyperelastic properties of the proposed recycled rubber, a uniaxial test based on ISO 37  is performed in this study.
For each rubber batch, three specimens in the form of dumb-bell pieces as shown in Fig. 5 are tested in the uniaxial tensile test device. The specimens are stretched to the extent of failure to define the tensile strength and the strain at failure.
Figure 6 presents the results of the uniaxial tensile tests on the four rubber batches. A single curve represents the average of three identical specimens. Results from the test of fresh specimens are presented by black curves, while the red curves present the results after aging. The accelerated aging is conditioned by storing the specimens in a oven at 70 °C for 24 × 7 h. The aged specimens are then tested after 24 h of storage at room temperature.
In the case of unaged soft rubber compounds, batch 1 exhibits larger failure strain yet lower tensile strength when compared to batch 2. Both soft batches present an identical shape of the hyperelastic curves. For hard rubber compounds, batch 3 experiences larger failure strain yet lower tensile strength compared to batch 4. The tensile strength of the 4 batches varies from 5.4 to 6.5 MPa, while the failure strain ranges from 310 to 840 MPa. In general, the values of tensile strength of 4 batches under study are significantly lower than the requirements stated in EN 15129  for commercial seismic isolators. However, the proposed recycled rubbers are intended to be used for unbonded isolators which exhibit much lower tensile stress under large shear displacement when compared to the commercial ones, as reported in the literature .
In Fig. 6, the results of tensile tests after aging are presented by red curves. In general, the aging increases the stiffness of the rubber. Table 5 shows the quantity of difference of tensile strength and strain-at-failure, before and after aging. According to EN 15129 , the maximum changes of tensile strength and failure strain are 15% and 25%, respectively. Therefore, batches 2B and 4B satisfy the aging test requirements in terms of tensile properties.
Figure 7 summarizes the experimental results obtained for the different receipts investigated at failure. On the horizontal axis the elongation-at-break (EB) in % is represented, whereas on the vertical axis the ultimate strength (TS) in MPa is depicted. The corresponding data of two virgin rubbers Vistalon 3666 and Dutral 4038 and 4 mixed regenerated rubbers are presented. In this way, the comparison with the reference compound made exclusively by virgin rubber is straightforward.
From Fig. 7, it can be deduced that the different formulations with regenerated rubber, having comparable hardness with those of the virgin materials, generally exhibit a lower ultimate strength. However, blends with type B regenerated rubber (RRB) exhibits a much better performance than type A regenerated rubber (RRA). As expected, adding regenerated rubber results into a decrease of the strength with similar elongation-at-break.
In Fig. 8, the diagrams of the tensile strength (TS) as a function of ultimate elongation-at-break (EB) after ageing at 70 °C for 128 h are presented. From Fig. 8, it is possible to notice that by ageing the elongation-at-break decreases and the tensile strength slightly increases. Considering the theoretical behavior of the observed EB and TS as a function of the vulcanization degree depicted in Fig. 9b, it can be argued that the compounds are slightly under-vulcanized. From a chemical point of view, ageing has the effect to promote an additional crosslinking, confirmed by the fact that Shore A (Hs) also increases (Fig. 8). Ageing results, therefore, into an increase of the vulcanization degree i.e., non-cured polymer that is present at the end of the vulcanization process reticulates further during ageing. The slight increase of the tensile strength shows how the initial curing condition is, however, near the optimal one i.e., not far from the point of maximum obtainable (Fig. 8).
Figure 9a shows that compound 4B (Dutral 4038 + regenerated rubber B) is that one exhibiting the best performances among all those investigated. The crosslinking density appears slightly suboptimal, because TS after aging exhibits a moderate increase, whereas EB decreases roughly from 300% to 260%. To be quantitatively conclusive about the most suitable vulcanization conditions imposed during the production phase, the authors are planning to develop ad-hoc kinetic numerical models, which first of all should account for reversion phenomenon in a rheometer chamber , then should try to help in the optimization of a real industrial production process with finite elements (FEs) [22, 23] and finally should perform automatic back analyses (assuming as objective function some expected mechanical properties), to determine the optimal production parameters by means of innovative and fast meta-heuristic approaches [24, 25]. As a matter of fact, compound 4B is the blend approximating better the behavior of the virgin material. Values of EB and TS for Dutral 4038 are scaled in the figure by a factor 26/33, because in the blends a Dutral 4038 with 26% in weight of polymer was utilized, whereas data furnished by the producer (Table 1), for the virgin material refer to an amount of polymer equal to 33%.
It is finally worth mentioning that quite satisfactory is also the behavior of blend 2B (regenerated rubber with Vistalon 3666), but with a still slightly better performance of 4B.
Production of fiber-reinforced elastomeric isolators (FREIs)
After having obtained the rubber compound with the best performance, the fiber reinforcements and the vulcanization devices are prepared. Pre-treated woven glass fibers are used in this project. The fibers are dried with a primer to improve adhesion i.e., chemically gripped (Fig. 10). The product is a commercial cover-coat adhesive (Megum 538 by Dupont), used in combination with an adhesive primer (Thixon by Dupont), commonly adopted for bonding rubber compounds to metals and other rigid substrates during vulcanization.
Figure 11a presents the preparation of rubber pads and fiber reinforcements having dimension 75 × 75 mm to be vulcanized in the mold (Fig. 11b). The vulcanization is performed at 150 °C for 40 min by compression molding. Typically the isolators are constituted by 4-5 layers of GFRP, as indicated by the cross-section in Fig. 11c. After the vulcanization, to evaluate the quality of the production process, the Shore A hardness is measured in the middle vertical section after knife cut, as can be noted in Fig. 11d. Some tribology damage of the cut surface is observed near the boundary for the utilization of a toothed knife. Five hardness measures were reported, four points near the section corners (points 2, 3, 4 and 5) and one in the center. A Shore A hardness of 58 ± 2 corresponding to the corners and of 48 ± 2 in the center is reported during measurement. On the other hand, a rubber cube without fiber sheets exhibits a homogeneous hardness from the skin to the core of 62 ± 2 Sh A.
Considering that an analogic durometer is used and due to non-flat and damaged surface obtained, the following considerations can be drawn:
Mold and vulcanization temperature (150 °C) are roughly suitable to obtain industrial items vulcanized in a proper and uniform manner.
The lower hardness registered in correspondence of the center of the section confirms that the presence of GFRP sheets is detrimental, since it isolates further the core of the samples.
Final hardness is acceptable both in the core and near the skin.