Elastomers rebound

To relate the physical properties of carbon black to rubber properties, we tested these tread blacks in the ASTM natural rubber recipe and in an SBR 1500 test recipe. In both elastomers, we checked standard stress/strain properties of modulus, tensile strength, and hardness. In the natural rubber recipe we also tested Firestone running temperature and rebound, and Goodyear rebound. In the SBR we checked percent swell, extrusion rate, viscosity, and laboratory abrasion. 

Three-component IPNs prepared from polyurethane, epoxy, and unsaturated polyester resin resulted in even broader tan 5 values when compared to two component (PU/E) IPN elastomers. Furthermore, the tan S values for the three component IPN systems were still high after the transitions were apparently complete, which is of enormous significance in sound energy absorption applications. IPN foams prepared by using PU/E (two-component) showed excellent energy absorbing abilities. This was reflected in rebound, hysteresis, and sound absorption studies. 

Polysulfide rubbers are manufactured by combining ethylene (CH2=CH2) with an alkaline polysulfide. The sulfur forms part of the polymerized molecule. They are also known as Thiokol rubbers. In general, these elastomers do not have great elasticity, but they do have good resistance to heat and are resistant to most solvents. Compared to nitrile rubber, they have poor tensile strength, a pimgent odor, poor rebound, high creep imder strain, and poor abrasion resistance. 

Polyurethane elastomers with higher hard segment content result in more hard segments mixed into the soft phase [25-27]. Also, polyester soft segments are more compatible with urethane hard segments. The decrease in Bashore rebound values of TG-225, TG-250 and TG-275 extended materials is associated with the lower degrees of phase... 

Reducing the monol content also improves the dynamic properties of cast elastomers. Figure 9.3 shows the DMTA response for the two polymers described above. The polymer derived from the ultra-low monol polyol has a flatter rubbery plateau region. The higher modulus in the rubbery plateau is consistent with the polymer s higher tensile modulus. The substantial reduction in tan delta (8) across the entire temperature range should also be noted. Lower tan delta translates into improved performance in dynamic applications due to lower heat buildup and improved rebound as noted above. 

The ultimate goal for MDI/BDO cured elastomers was to design a polyol that would have a combination of good processability and high rebonnd in the mainline hardness range of 80 to 95 Shore A. This was an elnsive goal as one can see from the elastomer data in Tables 9.5 and 9.6. A 90 Shore A elastomer based on the 4000-MW diol had poor processability but a high rebound, whereas, the 90 Shore A elastomer based on the 2000-MW diol was processable but had a lower rebound. 

Virtually all elastomer properties improved with lower monol contents (see Table 9.9). Of particular note, the rebound increased from 59 to 68 percent, the elongation from 470 to 550%, tensile strength from 10 to 19 MPa, tear strength from 49 to 68 kN/m, compression set decreased from 64 to 23 %, and the Taber abrasion loss (ASTM D4060-95 [26]) decreased from 220 to 80 mg loss/1000 cycles. The improvement in the stress/strain curves is shown in Eigure 9.7. Lower monol content results in significantly higher ultimate polymer MW, which results in improved mechanical properties. 

Table 9.21 illustrates this effect by increasing the PPG MW from 2000 to 4000 and the prepolymer % NCO from 6 to 8. This increases the hardness to the same level as the PTMEG-based elastomer and increases the rebound to 65%. As an aside, increasing the prepolymer % NCO from 6 to 8 represents a polyol/BDO blend of 2160 MW. 

Typical effects obtained by use of these treatments on fillers in elastomer systems can be found in the work of Dannenberg and Gotten [63]. They examined trimethylsilane treatment of a fumed silica and found effects consistent with reduced filler rubber interaction. Thus rebound resilience, modulus, tear strength and bound rubber were all reduced. Surprisingly, the treatment gave a considerable improvement in abrasion resistance, which it was believed resulted from the increased hysteresis. 

The surface forces, of van der Waals type for rubber-like materials, are able to grandly modify the stress tensor provided by the contact of a blunt asperity applied against the flat and smooth surface of a rubber sample. It will be shown how the coupling of surface adhesion properties and bulk viscoelastic behavior of rubber-like material allows us to solve adherence problems. This will be illustrated through three examples the spontaneous peeling due to the intervention of internal stresses the no-rebound of balls on the smooth surfoce of a soft elastomer and the adhesive contact and rolling of a rigid cylinder under a smooth-surfaced sheet of rubber. 

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