동종의 분쇄물이 충전된 고무의 가황거동 및 물리적 특성
Cure behaviors and physical properties of rubber
고무제조 반죽 혼합;
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Disposal of waste rubber products is a global problem for both environmental and economic reasons. One of the most attractive ways to recycle rubber wastes is grinding rubber vulcanizates and using ground rubber(GR) as a compounding ingredient or as a replacement of raw polymer. In the past years, considerable works have been done on the cure behaviors and physical properties of GR filled vulcanizates. From these results, it can be expected that the cure characteristics and mechanical properties of GR filled vulcanizates largely depend on the kind of rubber matrix and GR, and curatives used in rubber matrix and GR. However, the effect of curing systems for rubber matrix and GR on these properties has been rarely investigated. In this study, to investigate the effect of curing systems on cure behaviors and physical properties of GR filled vulcanizates, NR and SBR vulcanizates having three different curing systems, such as conventional(CV), semi-efficient(semi-EV) and efficient(EV) curing system, were prepared. These were ground and then mixed into parent compounds. Three EPDM vulcanizates were also prepared: one with sulfur donor curing system and the others with conventionally used (ZDMC). In addition, peroxide cured NR, SBR, and EPDM were used in the same way of sulfur-accelerated ground rubber to compare the effects on cure behaviors and physical properties. The crosslink densities and crosslink structures of GR filled vulcanizates were measured. To elucidate the variation of crosslink densities and crosslink structures of rubber matrix and GR, three layer model was used, in which core and outer layer represented GR and rubber matrix, respectively. The adhesion between rubber matrix and GR was a very important factor to determine physical properties of GR filled vulcanizates. The fractured surfaces of tensile and tear specimens were observed and the adhesion force between recured vulcanizates and fresh vulcanizates was investigated with respect to the curing systems of vulcanizates. With the addition of GR, the bound rubber contents of GR filled compounds decreased. This indicates that there is no specific interaction between GR and rubber matrix and GR acts as a non-reinforcing filler in GR filled compounds. GR increased Mooney viscosities, however, in the case of NR, the increase was much smaller than those of SBR and EPDM, which was due to the mastication of NR during extended mixing time for GR filled NR compounds. The curing systems of GR and rubber matrix have great effects on cure behaviors and physical properties of GR filled compounds and vulcanizates. For NR, the addition of GR, cured with sulfur and accelerators, decreased Mooney and rheometric scorch times, optimum cure times, and torque developments during vulcanization of GR filled compounds. Especially, compounds with GR and rubber matrix having CV curing system showed the largest changes for those characteristics. However, peroxide cured GR did not show great effects on those properties except for torque development. The cure rate for compounds with CV and semi-EV curing systems decreased and then increased a little with the addition of GR, but compounds having EV systems showed continuous increase. Modulus of GR filled vulcanizates decreased as the amount of GR increased, which was in accordance with the differences between maximum and minimum torques of cure curves. Tensile strength, tear strength, and elongations at break abruptly dropped with small amount of GR, but elongations at break, again, increased with the increase of GR contents. The largest decrease of physical properties was observed for peroxide cured GR filled vulcanizates. Considering the large particle size of GR in this research, the fractured surfaces of tensile and tear specimens were unexpectedly smooth and the interface between GR and rubber matrix was not easily differentiated. Moreover, it was observed that the fracture was propagated through GR, when GR cured with sulfur and accelerators was used. From these results, it can be inferred that the adhesion between GR and rubber matrix was enough to fracture GR. However, the surface of peroxide cured GR were observed on the fractured surfaces. For SBR, the variation of Mooney and rheometric scorch times, optimum cure times, and torque development during vulcanization of GR filled compounds was very similar with those of NR, except that the optimum cure times of GR filled vulcanizates with CV curing system showed a little changes. The changes of physical properties also showed the similar trends in the case of NR. However, peroxide cured GR filled vulcanizates showed increased modulus, which may be due to high crosslink density of peroxide cured SBR, and severe decrease of tensile strengths and elongations at break. This was thought to be caused by the low adhesion between GR and rubber matrix, attested from fractured surfaces. For EPDM, GR cured with conventionally used two accelerators, TMTD and ZDMC, showed different effects on cure behaviors of their parent compounds. GR with TMTD increased Mooney and rheometric scorch times and optimum cure times, but GR with ZDMC decreased Mooney scorch times and caused a little changes on rheometric scorch times and optimum cure times. GR with sulfur donor curing system decreased scorch times and optimum cure times. Rheometric scorch times and optimum cure times of peroxide cured GR filled compounds increased, but Mooney scorch time was not nearly changed except for ESDG compounds. The addition of GR increased cure rate and then decreased it as the amount of GR increased. Modulus and tensile strength decreased but the decrease of elongation was marginal. Tear strength of GR filled vulcanizates increased a little and then decreased. Similar to NR and SBR, vulcanizates with peroxide cured GR showed severe decrease of physical properties. It is not easy to differentiate GR and matrix rubber from SEM photographs of fractured surfaces. However, the surface of fractured specimens cured with peroxide was rougher than those of vulcanizates with sulfur and accelerators. The addition of GR decreased the crosslink densities of GR filled vulcanizates, which implied that the crosslink densities of matrix rubber dominates the crosslink densities and physical properties of GR filled vulcanizates. For NR, the decrease of crosslink densities for NC was much larger than those of NS and NE. From three layer model, it was observed that sulfur migration from outer layer to core caused the increase of crosslink densities of core and decrease of crosslink densities of outer layer. Moreover, the crosslink structures of core and outer layer also were changed. The variations of crosslink densities of outer layers decreased in the order of CV, EV and semi-EV, which was in accordance with the amount of sulfur migration. The changes of crosslink structures may be largely affected by accelerators in core and outer layer, and large variations of crosslink structures were observed in NE. For SBR, the decrease of crosslink densities and the variation of sulfur were similar to those of NR. However, the variation of crosslink structures were much smaller than that of NR. The decrease of crosslink densities of GR filled EPDM vulcanizates and the amount of sulfur migration from outer layer to core in three layer model were small compared to NR and SBR. Any large differences of crosslink densities for outer layer and core were not observed, from which it could be inferred that the differences of crosslink densities between matrix rubber and GR were not so large, compared to those of NR and SBR. Adhesion tests for recured vulcanizates and fresh NR and SBR vulcanizates, having CV, semi-EV and EV curing system showed the cohesive failure mode. Fracture occurred through recured vulcanizates. However, when peroxide cured vulcanizates were recured with fresh compounds, the adhesive failure mode was observed. From these, it can be deduced that the crosslink structures of GR have great effects on adhesion of interfaces between GR and rubber matrix. Adhesion force between recured vulcanizates and fresh vulcanizates with CV curing system showed the highest value and decreased in the order of semi-EV and EV system. From the crosslink structures of vulcanizates with respect to the curing system, it can be inferred that polysulfide links in GR and rubber matrix created new crosslinks at the interface through sulfur-sulfur bond exchanges. Based on the results from crosslink density measurements and adhesion test, it can be attested that the decrease of physical properties of GR filled NR and SBR vulcanizates, cured with sulfur and accelerators, was mainly due to not the low adhesion between GR and rubber matrix but the decrease of crosslink densities of rubber matrix. The adhesion force between GR and rubber matrix was high enough to initiate fractures through GR regardless of the condition of GR surfaces. The severe decrease of physical properties of peroxide filled vulcanizates could be accounted for the decease of crosslink densities of rubber matrix and low adhesion between two phases. In the case of EPDM, the adhesion between recured vulcanizates and fresh vulcanizates was very low compared to NR and SBR because of very low crosslink site concentration in EPDM. Except for the samples with ZDMC, in which the partial cohesive failure occurred, the adhesive failure was observed. Crosslink structures in recured vulcanizates have not any effects on adhesion of two phases. The decrease of physical properties of GR filled vulcanizates could be accounted for both the decease of crosslink densities of rubber matrix and low adhesion between GR and rubber matrix.