Tack and Green Strength

Since the tack and green strength of these rubbers may depend on their ability to undergo strain induced crystallization, this behavior was studied and compared with that shown by NR. Information on the strain induced crystallization behavior of these experimental rubbers was obtained from x-ray diffraction measurements and from rheo-optical studies. 

A true synthetic natural rubber was introduced in the mid-1960s with the exact same chemical structure as latex tapped from a tree. The difference is that natural rubber comes with a variety of other ingredients in the latex that can both add and detract from performance, while polyisoprene is considered relatively pure. In addition, there are some differences in molecular weight distribution that impact performance. Available in both latex and solid forms, this elastomer can be directly substituted for natural rubber in many applications. Adhesives which are not cured tend to have higher creep values than natural rubber, but also exhibit lower tack and green strength properties. Vulcanized adhesive products perform equal to cured natural rubber adhesive products. 

Like natural rubber, styrene butadiene rubber (SBR) can be blended in all proportions with bromobutyl rubber. However, SBR is less desirable for blending than natural rubber due to its low tack and green strength properties. In addition, heat, flex fatigue resistance, and weathering resistance are poorer with SBR blends than with natural rubber blends. Suggested cure systems are the same as those for bromobutyl/natural rubber blends. 

The application of the reptation model to adhesion is not limited to the adhesive bond strength at the interface. The reptation model can also be used to study tack and green strength... 

R. P. Wool, Molecular aspects of tack. Rubber Chem. Technol. 57, 307 (1984) R. P. Wool, Welding, tack and green strength of polymers, in Fundamentals of Adhesion (L. H. Lee, ed.). Chapter 7, Plenum Press, New York (1990). 

G. R. Hamed, Tack and green strength of elastomeric materials. Rubber Chem. Technol. 54, 576 (1981). 

NR show excellent tack and green strength, reduced flex crack growth rate and very low heat build-up (Table 16). 

Fig. 33. Variation of tack and green strength with /ru 5-l,4-content of HTSBRs (15% styrene).

More positively there is no substantial evidence that SBR is likely to be replaced extensively by other rubbers in non-tyre applications, this being assured largely by the relatively low cost of the polymer. No new general purpose rubbers appear to be on the horizon at the present time. There may be some replacement where up-grading of product specifications for such properties as ozone and heat resistance requires the use of ethylene-propylene rubbers. On the other hand the advent of the new SBR polymers such as the low styrene strain-crystallising materials described in the previous section with their improved tack and green strength may well, if their initial promise is realised, capture some of the markets currently held by the more expensive natural materials. 

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