Tack and Green Strength of NR, SBR and NR/SBR Blends

1. A simple NR compound has superior tack compared to an SBR stock because of its greater ability to flow under compressive load and its higher green strength. The difference becomes greater as surface roughness is increased or test rate decreased. Tack differences between the two stocks can be quantitatively explained without assuming differences in the interdiffusion rate of SBR and NR molecules. NR is an “ideal” material for developing high tack. It can be processed to a low viscosity and still maintain high green strength. Furthermore, the mechanism responsible for the high green strength (strain crystallization) is not active in the bond formation step, hence does not interfere with contact and interdiffusion, but rather develops upon stressing. SBR, on the other hand, depends on entanglements, which tend to inhibit flow, to achieve green strength. Thus, if the molecular weight of the SBR is raised to enhance green strength to the level of NR, the stock will flow very poorly. Likewise, if SBR molecular weight is lowered, flow can be improved substantially, but green strength will decrease rapidly. Either of the above conditions result in poor tack. 2. For smooth, shiny surfaces brought together, contact occurs rapidly and nearly completely for both the NR and SBR stocks. This results in a relative tack near unity and hence, in this case, the tack strength is really a measure of stock green strength, i.e., tack is green strength limited. Obviously, when this condition exists, the addition to the stock of plasticizers, tackifiers, oils or other additives to facilitate stock flow (but decrease green strength) will nontheless reduce tack. Here, tack can only be increased by enhancing stock green strength, preferable without hindering flow ability. 3. The relative tack of SBR and SBR-dominated blends with moderately rough surfaces is less than unity and increases with contact pressure. Thus, tack is contact limited. In this instance, relative tack can be improved by decreasing stock viscosity, however, absolute tack will only be increased if good green strength is maintained. This may be the mechanism for the action of commencal tackifiers. 4. The tack of NR/SBR blends passes through a maximum with composition at about 80% NR. This is readily understood by noting that the green strength also reaches a maximum at this composition. For NR/SBR blends containing 60% or more of NR, green strength plays the dominant role in controlling tack. 5. Uncured adhesion of SBR to NR is less than the tack of either pure component. This suggests that interdiffusion is required to obtain high tack.